Flexibility of an Open Indenyl Ligand in Iron(II) Complexes

Flexibility of an Open Indenyl Ligand in Iron(II) ComplexesAndreas Glockner,† Thomas Bannenberg,† Kerstin Ibrom,‡ Constantin G. Daniliuc,† Matthias Freytag,†

Peter G. Jones,† Marc D. Walter,*,† and Matthias Tamm*,†

†Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,Germany‡NMR-Labor der Chemischen Institute, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

*S Supporting Information

ABSTRACT: [FeI2(thf)2] was sequentially treated withLi(C5Me5) and the potassium salt of the phenylmethallyl(“open indenyl”) ligand oIndMe, leading to selective formationof the half-open ferrocene [(η5-C5Me5)Fe(η

5-oIndMe)] (1). Avariable-temperature NMR study revealed a barrier of ca. 12kcal mol−1 for the rotation of the phenyl group. The apparentlability and susceptibility of the oIndMe ligand in 1 to undergoη5−η3 interconversion allowed the preparation of thecomplexes [(η5-C5Me5)Fe(η

3-oIndMe)(L)] (2, L = CO; 3, L= IMe; 4, L = PMe3) by addition of carbon monoxide, 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe), and trimethylphosphine, respectively. The η3-bound phenylmethallyl ligand in thesecomplexes initially adopts an anti orientation with regard to the relative positions of the phenyl and methyl substituents, followedby anti-to-syn isomerization. For L = CO, both isomers could be isolated, and the conversion of anti-2 into syn-2 was monitoredby NMR spectroscopy at 35, 50, 65, and 80 °C, affording an enthalpy of activation of ΔH⧧ = 24(1) kcal mol−1 for thisequilibrium reaction. On the basis of DFT calculations, a mechanism is proposed that proceeds via consecutive η3−η1−η3interconversions and involves η3-benzyl intermediates. In contrast, rapid equilibration was observed for L = IMe and PMe3.Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1 gave [(η5-C5Me5)Fe(η

1-oIndMe)(dmpe)] (5), containing the oIndMe

ligand bound in an η1-allyl fashion. η5-to-η3 hapticity interconversion was also observed upon reaction of 1 with methyl iodideand CH2Cl2, which formed the Fe(III) complexes [(η5-C5Me5)Fe(η

3-oIndMe)(X)] (6, X = I; 7, X = Cl); solid-state magneticsusceptibility measurements on 6 revealed an S = 1/2 ground state. The mixed indenyl−open indenyl complex [(η5-Ind″)Fe(η3-oIndMe)(CO)] (9, Ind″ = 1,3-di(tert-butyl)indenyl) was isolated from the stepwise reaction of [FeI2(thf)2] with Na(Ind″),K(oIndMe), and CO. The molecular structures of 1, anti-2, syn-2, syn-3, 5, syn-6, syn-7, and syn-9 were established by single-crystalX-ray diffraction.

■ INTRODUCTIONFollowing initial studies on CO insertion reactions inmolybdenum complexes by Mawby,1 Basolo ascribed theenhanced ligand substitution rates observed in indenyl rhodiumand manganese complexes, as compared to their cyclo-pentadienyl counterparts, to the “indenyl effect”.2 The indenylligand can relatively easily switch between a perturbed η5-coordination mode and η3-allyl-type coordination, leading to areduced electron count of two at the metal center and thusproviding the possibility for an associative ligand substitutionpathway. The origin of the indenyl effect is clearly a result ofthe rearomatization of the annulated benzene ring uponhapticity interconversion.3 However, recent theoretical studiesalso suggest that the higher stability of the (η5-Cp)−M bondversus the (η5-Ind)−M bond and the reversed order for the η3-bound intermediates play a crucial role; thereby, the activationof an η5−η3 rearrangement requires less energy for indenylcomplexes, resulting in higher reaction rates.4 Furthermore,substitution studies of [(η5-X)Mn(CO)3] complexes (X =cyclopentadienyl, indenyl, fluorenyl, hydronaphthalenyl) have

shown that further enhancement by a factor of 106 can beachieved when the indenyl ligand is replaced by a hydro-naphthalenyl ligand.5

Similarly, we have recently introduced the indenyl effect tothe pentadienyl (“open Cp”) chemistry with the preparationand coordination of a phenylmethallyl ligand, oIndMe (Figure1), which can be classified as an open indenyl ligand.6 Although

conventional pentadienyl ligands are already known to

coordinate to transition metals in various bonding modes,7

the relatively weak C−X π-bond in heteropentadienyl systems

usually results in even more facile η5−η3 haptotropic shifts.8 For

Received: April 13, 2012Published: June 6, 2012

Figure 1. Indenyl and the open indenyl, oIndMe, ligand.

Article

pubs.acs.org/Organometallics

© 2012 American Chemical Society 4480 dx.doi.org/10.1021/om3003009 | Organometallics 2012, 31, 4480−4494

pubs.acs.org/Organometallics

instance, [(η5-C5Me5)Ru(η5-2,4-C6H9O)] (C6H9O = dimethyl-

oxapentadienyl) reacts with PMe3 under reflux to yield [(η5-C5Me5)Ru(η

3-2,4-C6H9O)(PMe3)], whereas the corresponding2,4-dimethylpentadienyl complex does not.9 However, as aresult of the indenyl effect, the complex [(η5-C5Me5)Ru(η

5-oIndMe)] undergoes a hapticity shift significantly faster than itsoxapentadienyl congener upon addition of a ligand L (L = CO,PMe3, CN-o-xylyl), resulting within minutes at room temper-ature in anti-[(η5-C5Me5)Ru(η

3-oIndMe)(L)] complexes. Forthe PMe3 complex, slow isomerization to the syn isomer takesplace, which suggests that η1-oIndMe intermediates are alsoaccessible.6 As part of our continuing interest in metal−pentadienyl and metal−allyl chemistry,10 we wished to seek adeeper insight into the characteristic features of the openindenyl ligand, oIndMe, including detailed studies on the anti-to-syn isomerization and the isolation of an η1-complex. Therefore,we have extended our previous study to iron and present herethe corresponding results.

■ RESULTS AND DISCUSSIONPreparation and Characterization of a Pentamethyl-

cylopentadienyl Iron(II) Open Indenyl Complex. Thecomplex [(η5-C5Me5)Ru(η

5-oIndMe)] was readily obtained bysalt metathesis of [(η5-C5Me5)RuCl]4 with phenylmethallylpotassium, K(oIndMe),6 which is based on a commonprocedure for the preparation of half-open ruthenocenes.11 Incontrast, the synthesis of half-open ferrocenes, [(η5-Cp)Fe(η5-Pdl)] (Cp = C5H5, C5Me5; Pdl = pentadienyl or substitutedpentadienyl) appears to be more tedious and less general. Oneapproach involves the equimolar and simultaneous reaction ofNa(Cp) (Cp = C5H5, C5Me5), K(Pdl) (Pdl = 2,4-dimethylpentadienyl, 2,3,4-trimethylpentadienyl), and FeCl2or FeBr2(dme) (dme = dimethoxyethane) at low temperatures,which results in an almost statistical mixture of [(η5-Cp)2Fe],[(η5-Cp)Fe(η5-Pdl)], and [Fe(η5-Pdl)2]; subsequent fractionalcrystallization allows the separation of the half-open ferrocene,albeit in low yields.7c,12 Furthermore, for Pdl = pentadienyl orhexadienyl, [(η5-C5H5)Fe(η

5-Pdl)] can be obtained moreselectively via an η3-intermediate by the successive photolysisof [(η5-C5H5)Fe(η

1-Pdl)(CO)2], which first has to be preparedfrom [(η5-C5H5)Fe(CO)2]Na and either 1-chloro-2,4-penta-diene or 1-chloro-2,4-hexadiene.13 Since we were interested inisolating the iron analogue of [(η5-C5Me5)Ru(η

5-oIndMe)],[(η5-C5Me5)Fe(η

5-oIndMe)] (1), with pentamethylcyclopenta-dienyl as the coligand, the stable complexes [(η5-C5Me5)Fe-(NCMe3)3][PF6] and [(η5-C5Me5)Fe(acac)(L)] (acac =acetylacetonate) appeared to be the most promising candidatesfor providing the (η5-C5Me5)Fe fragment.14 Both complexesare known to yield mixed ferrocenes upon reaction withcyclopentadienide salts,15 but require time-consuming multi-step syntheses to avoid the formation of the thermodynamicallyhighly stable ferrocene molecule.14 Alternatively, [(η5-C5Me5)-FeCl(tmeda)] (tmeda = N,N,N′,N′-tetramethylethylenedi-amine) might be a suitable material,16 because it was used asan iron half-sandwich reagent in several examples.17 However,the stabilizing tmeda ligand might remain coordinated andthereby inhibit the formation of an η5-oIndMe complex.Nevertheless, other studies suggest that a nonstabilized version,[(η5-C5Me5)FeBr], is formed at low temperatures uponreaction of [FeBr2(dme)] with 1 equiv of Li(C5Me5) inTHF, but that ligand scrambling takes place after quenchingwith Na(C5H5), leading to a mixture of decamethylferrocene,pentamethylferrocene, and ferrocene.18 Notably, such iron−

halogen half-sandwich complexes are stable at room temper-ature and can be used for further transformations when bulkycyclopentadienyl ligands, such as tri(tert-butyl)-, tetra-(isopropyl)-, or penta(isopropyl)cyclopentadienyl, are em-ployed.19

Therefore, we attempted the synthesis of [(η5-C5Me5)Fe(η5-

oIndMe)] (1) by the stepwise addition of Li(C5Me5) andK(oIndMe) to [FeI2(thf)2] at −78 °C (Scheme 1). The

successive addition of reagents is a slight, but crucial,modification of the original procedure for the synthesis ofhalf-open ferrocenes,7c because the 1H NMR spectrum of thecrude product clearly shows the selective formation of 1;neither [(η5-C5Me5)2Fe)] nor [Fe(η5-oIndMe)2] can beobserved (see the Supporting Information for the 1H NMRspectrum in C6D6). In agreement with earlier studies,18 thissuggests the presence of [(η5-C5Me5)FeI] as a stableintermediate as long as the yellow solution is kept at −78°C; the same color was observed when the structurallycharacterized red dimer [(η5-C5H2tBu3)Fe(μ-I)]2 was dissolvedin THF.19d The use of [FeI2(thf)2] instead of [FeBr2(dme)]might actually lead to an enhanced kinetic stability of the [(η5-C5Me5)FeI] intermediate because of the larger size of thehalide, which then reacts with K(oIndMe) in a second saltmetathesis step to yield the desired product. Usually, an oilyproduct is obtained after extraction with pentane; purificationcan be achieved by crystallization from concentrated pentanesolutions at −30 °C, affording 1 as a dark green solid in 44−64% yield. Because of the extremely high solubility even inhydrocarbon solvents, reaction yields are probably considerablyhigher than isolated yields. Complex 1 is indefinitely stableunder an inert atmosphere (N2), but appears to be extremelyair-sensitive in solution. Sometimes, the precipitation of acolorless solid was observed in subsequent reactions with 1. Wesuspect that 1 may occasionally be contaminated with LiI if theextraction during workup is not performed carefully. ReplacingLi(C5Me5) with Na(C5Me5) still yields 1, and the byproductNaI can be removed more easily because of its lower solubilityin comparison to LiI. However, this is at the expense of thepurity; additional unidentified resonances at 1.75, 1.65, and1.15 ppm (in C6D6) were always observed in the 1H NMRspectrum of 1 after crystallization.NMR spectroscopy suggests the presence of oIndMe in 1 as

an η5-pentadienyl ligand, but there are striking differencesrelative to the ruthenium analogue, for which we did notobserve dynamic behavior on the NMR time scale at ambienttemperature.6 The 1H NMR spectrum of 1 in THF-d8 showstwo individual sharp signals for the protons of the CH2 group,showing that the rotation about the C1−C2 axis is slow (seeScheme 1 for the numbering of 1). The two ortho protons ofthe phenyl ring (H5/H9), however, give rise to only one very

Scheme 1. Synthesis of [(η5-C5Me5)Fe(η5-oIndMe)] (1) via

an Iron Half-Sandwich Intermediate

Organometallics Article

dx.doi.org/10.1021/om3003009 | Organometallics 2012, 31, 4480−44944481

broad resonance at δ ≈ 4.49 ppm (width at half-height ca. 500Hz at 400 MHz), suggesting moderately fast rotation of thephenyl ring about the C3−C4 bond. Upon lowering thetemperature, the signal separated into two resonances, which, at221 K, were sharp enough to allow the full assignment of all 1Hand 13C NMR signals by standard 1D and 2D NMR procedures(1H,1H-COSY, 1H,1H-NOESY, 1H,13C-HSQC, 1H,13C-HMBC). The other 1H and the 13C NMR chemical shifts atroom temperature and at 221 K exhibit only minor temperaturedependence; chemical shifts and assignments are given inTables 1 and 2. At room temperature, the δ values of the

chemically equivalent ortho (H5/H9) and meta (H6/H8)protons and also of the meta-carbon atoms (C6/C8) equal theaverage shifts of the corresponding positions at 221 K. As forthe ortho protons H5 and H9, the large chemical shift differencebetween the ortho carbons C5 and C9 at 221 K (Δδ = 63.8

ppm) should entail a very broad signal at room temperature,which, for this reason, could not be detected at the predictedposition of 99.3 ppm. The chemical shifts of C1−C4 andespecially C5 (δ = 67.4) at 221 K indicate that the oIndMe

ligand is bound to Fe in an η5-manner, which also explains thealternation of the size of the ortho spin−spin coupling constantsbetween the phenyl ring protons. Interestingly, the HMBCspectrum shows a distinct cross peak between C5 and anti-H1,which are separated by five bonds. Although the exact 13C,1Hspin−spin coupling constants cannot be extracted from aconventional magnitude HMBC spectrum where the crosspeaks are obscured by the proton−proton couplings, theremarkable splitting of the cross peak in the indirect dimensionof about 22 Hz suggests an additional coupling pathwaybetween the nuclei, presumably via the Fe atom.Further low-temperature 1D and 2D 1H NMR measure-

ments were carried out to study the dynamics of the phenylring rotation. Bandshape analysis of proton spectra measured atsix temperatures evenly distributed over a temperature range of197−256 K yielded free enthalpies of activation (ΔG⧧)between 11.7 and 12.1 kcal mol−1.20 These are, if at all, onlyvery slightly temperature-dependent and to a lesser extent thanthe error of the analysis, which is estimated to be ±0.2 kcalmol−1. A similar barrier for phenyl rotation was found for therelated benzyl complex [(η5-C5H5)Fe(η

3-CH2Ph)(CO)] (ΔG⧧

= 11.8(4)−12.6(2) kcal mol−1),21 whereas a smaller value wasdetermined for Li(oIndMe) (ΔG⧧ = 10.0(2) kcal mol−1).22

The large chemical shift difference between the two orthoprotons of 1 (Δδ = 6.04 ppm) causes very severe signalbroadening near the coalescence temperature. In combinationwith signal overlap, this does not permit a reliable line-shapeanalysis at temperatures higher than 256 K. Thus, rateconstants for the chemical exchange of the two ortho and ofthe meta positions, respectively, were only obtained within atemperature range of about 60 K. The value for the entropy ofactivation (ΔS⧧) is close to zero, and therefore, ΔH⧧ ≈ ΔG⧧.This is consistent with an intramolecular process, but the errorassociated with ΔS⧧ is disproportionally high. Therefore, weprefer to refrain from giving an exact value.The molecular structure of 1 in the solid state was

determined by X-ray diffraction analysis (Figure 2) and provedto be isostructural with the ruthenium analogue.6 It should benoted that the phenylmethallyl ligand has two enantiotopic

Table 1. 1H and 13C NMR Data of 1 at Room Temperaturein THF-d8

a

position δC δH HMBC

1 43.4, t 2.39, dd, Janti‑H1,syn‑H1 = 3.5, Jsyn‑H1,H3 ≥ 0.7Hz, 1H, syn-H1; −0.78, d, 1H, anti-H1

5.66, 1.91,(−0.78)b,c

2 96.4, s (5.66), (2.39),1.91,(−0.78)

CH3 25.3, q 1.91, s, 3H 5.66, 2.39,(1.91),c

−0.783 82.1, d 5.66, s, 1H (5.66),c 2.39,

1.91, −0.784 103.8, s 6.93, (5.66),

(−0.78)5/9 n. o.d ≈ 4.49, v br s, Δν1/2 ≈ 500 Hz, 2H n. o.d

6/8 133.9,br d

6.93, m, 2H n. o.,d broad13C signal

7 121.1, d 6.83, m (“t”, splitting 7.3 Hz), 1H (6.83)c

C5Me5 80.1, s 1.39

C5Me5 10.0, q 1.39, s, 15H 1.39c

aReferenced to δH = 1.72 and δC = 25.3 ppm, respectively. bShifts inparentheses indicate weak crosspeaks. c1JCH correlation. dNotobserved.

Table 2. 1H and 13C NMR Data of 1 at 221 K in THF-d8a

position δC δH HMBC

1 43.5, t 2.33, d, Janti‑H1,syn‑H1 ≥ 3.4 Hz, 1H,syn-H1; −0.91, d, 1H, anti-H1

5.64, 1.95

2 96.3, s (2.33),b 1.95CH3 25.3, q 1.95, s, 3H 5.64, 2.33,

(1.95),c −0.913 82.4, d 5.64, s, 1H 7.50, (5.64),c

2.33, 1.95,(−0.91)

4 103.2, s 7.00, 6.90, 5.64,(1.46)

5 67.4, d 1.46, d, JH5,H6 = 5.8 Hz, 1H 7.50, 6.83, 5.64,(−0.91)

6 139.5, d 6.90, “t” (dd), JH6,H7 = 8.6 Hz, 1H 7.007 121.0, d 6.83, “t” (dd), JH7,H8 = 5.7 Hz, 1H 7.50, 1.468 127.8, d 7.00, “t” (dd), JH8,H9 = 8.5 Hz, 1H (6.90)9 131.2, d 7.50, d, 1H 6.83, 5.64, 1.46C5Me5 80.1, s 1.37C5Me5 10.2, q 1.37, s, 15H 1.37c

aReferenced to δH = 1.72 and δC = 25.3 ppm, respectively. bShifts inparentheses indicate weak crosspeaks. c1JCH correlation. Figure 2. ORTEP diagram of 1 with thermal displacement parameters

drawn at 50% probability. Selected bond lengths (Å) and angles (deg):Fe−C(C5Me5) 2.058(2)−2.100(2), Fe−C1 2.086(2), Fe−C22.062(2), Fe−C3 2.055(2), Fe−C4 2.119(2), Fe−C5 2.254(2), C1−C2 1.426(3), C2−C3 1.417(3), C3−C4 1.430(3), C4−C5 1.436(3),C5−C6 1.427(3), C6−C7 1.362(3), C7−C8 1.425(3), C8−C91.346(3), C4−C9 1.444(3); C1−C2−C3 122.2(2), C2−C3−C4127.0(2), C3−C4−C5 123.2(2).



faces, and on coordination of the (η5-C5Me5)Fe fragment, aracemic mixture is formed. The oIndMe ligand exhibits theexpected η5-coordination mode, clearly identifying 1 as a half-open ferrocene with a typical sandwich structure. Both ligandsare reasonably parallel, as shown by a small tilt angle of 6.4°between the least-squares planes of the metal-bound carbonatoms. The open indenyl ligand plane approaches the ironatom much closer than C5Me5 (1.486 vs 1.688 Å) in order toachieve similar Fe−C bond distances (Figure 2).7 Incomparison to the only other structurally characterized half-open ferrocene, [(η5-C5H5)Fe(η

5-2,4-C7H11)] (C7H11 =dimethylpentadienyl),23,24 the opening of the acyclic ligand in1 is wider, with a larger separation between the terminal carbonatoms C1 and C5 (2.861 vs 2.746 Å). The open nature of theoIndMe ligand may be the cause of its more distorted η5-coordination mode compared to that of conventional group 8indenyl complexes.3b,25 Some slippage of the Fe atom towardthe C1−C3 allyl moiety is observed, with Fe−C bond distancesranging from 2.055(2) to 2.086(2) Å, while the values for C4and C5 (2.119(2)/2.254(2) Å) are markedly longer. Never-theless, significant bonding interaction of the metal with C4and C5 is indicated by a noticeable bending of the phenylgroup toward iron, as revealed by a fold angle between the C1−C3 and C4−C9 planes of 11.7°. Furthermore, the phenylcoordination is supported by a distinct long-short-long-short-long pattern of the C−C bond lengths within the C5-C6-C7-C8-C9-C4 moiety, associated with a perturbed electrondelocalization within the six-membered ring. Overall, thestructural features of 1 resemble those found for [(η5-C5Me5)Ru(η

5-oIndMe)], although the metal−carbon bonddistances are naturally shorter in 1.η5-to-η3 Hapticity Interconversion by Ligand Addi-

tion. The reduction of the electron count of the metal by twoupon η5−η3 hapticity interconversion, which then provides avacant orbital for ligand coordination or substrate activation, isa known process for pentadienyl ligands.26 This proposalrather than an intimate associative mechanismis furthersubstantiated by the fact that a rapid interconversion betweenthe η5- and η3-coordination modes is already observed for 1 insolution (vide supra). One reason for the easier adoption of anη3-coordination mode of pentadienyl in comparison tocyclopentadienyl ligands is the smaller loss of resonanceenergy.7b,d However, recent studies suggest that the reactivitycan further be enhanced by using heteropentadienyl ligands,which, according to photoelectron spectroscopy and theoreticalstudies,27 interact more weakly with late metals than doconventional pentadienyl ligands.8 Our studies on [(η5-C5Me5)Ru(η

5-oIndMe)] have shown that haptotropic shiftscan be achieved even more readily for the open indenyl ligand;here, the rearomatization can be considered as the main drivingforce for facile η5−η3 interconversion.6To extend our previous studies, potential reactions of the 18-

electron sandwich complex [(η5-C5Me5)Fe(η5-oIndMe)] (1)

were explored utilizing various two-electron donor ligands(Scheme 2). Notably, a review article mentions the reaction of[(η5-C5H5)Fe(η

5-C10H9)] (C10H9 = hydronaphthalenyl) withCO, resulting in [(η5-C5H5)Fe(η

3-C10H9)(CO)], whereas thecorresponding η5-hexadienyl complex appears to be inert.28 Inour case, carbon monoxide was passed through a green pentanesolution of 1 at room temperature, which gave an immediatecolor change to orange. Coordination of CO is clearly indicatedby a resonance at 223.8 ppm in the 13C NMR spectrum.Additionally, the aromatic region in the 1H NMR spectrum

now integrates for five hydrogen atoms, which is in agreementwith an uncoordinated phenyl group. Similar to the anti-η3-oIndMe complexes of ruthenium,6 the downfield resonance forH3 at 4.10 ppm suggests that H3 adopts a syn orientation withrespect to the methyl group. Consequently, the open indenylligand has undergone an η5−η3 hapticity interconversionresulting in anti-[(η5-C5Me5)Fe(η

3-oIndMe)(CO)] (anti-2),which was also confirmed by X-ray diffraction analysis (Figure3, Table 3). The phenyl group still resides in the anti position

as in 1, albeit significantly tilted out of the allyl plane with anincrease of the absolute C1−C2−C3−C4 torsion angle from2.0° (synperiplanar) in 1 to 49.5° in anti-2. CO coordination toiron from the open edge of the allyl ligand corresponds to anexo conformation,29 which is usually preferred to the endoconformation in d6-[(η5-C5H5)M(η3-allyl)(L)] complexes (L =CO, PR3).

30 The Fe−CO bond distance of 1.7464(14) Åcompares well to the values reported for similar complexes,such as [(η5-C5H5)Fe(η

3-hexadienyl)(CO)] (1.717(6) Å) and[(η5-C5H5)Fe{η

3-C3H4(CO)OCH3}(CO)] (1.717(6)Å).13b,31

Photolysis of a pentane solution of anti-2 for 6 h resulted inpartial CO loss and reformation of 1 by an η3−η5interconversion process. In contrast, thermal activation byrefluxing anti-2 in hexane does not cause any CO loss, butnevertheless yields a new product, which can be identified assyn-[(η5-C5Me5)Fe(η

3-oIndMe)(CO)] (syn-2) by NMR spec-troscopy (see the Experimental Section for details).32 Inparticular, the syn orientation of the phenyl and methylsubstiutents is evidenced by ortho-phenyl/CH3 and anti-H1/H3cross peaks in the 1H−1H NOESY NMR spectrum and a

Scheme 2. Syntheses of 2−4 and Their Isomerizations to thesyn Isomers

Figure 3. ORTEP diagram of anti-2 with thermal displacementparameters drawn at 50% probability.



dramatic upfield shift from 4.10 to 1.83 ppm for H3,corresponding to its change to the anti position. X-raydiffraction studies reveal that the structural parameters of syn-2 and anti-2 are very similar (Figure 4, Table 3). Apart from the

altered position of the phenyl group, there are only two othermajor differences: (1) For syn-2, the C1−C2−C3−C4 torsionangle is almost antiperiplanar (176.9°), whereas the corre-sponding angle in anti-2 (49.5°) is far apart from synperi-planarity. (2) The allyl moiety adopts an endo conformation,which is accompanied by a significant deviation from a mutuallyparallel orientation of the π-ligands; the angle τ increases from6.9° for anti-2 to 64.5° for syn-2 (Table 3). In the present case,DFT studies (vide infra) suggest that the exo conformation isdestabilized by ΔΔG298 = 3.2 kcal mol−1 in comparison to theobserved endo conformation, which contrasts with earlierstudies.30

At room temperature, the anti−syn isomerization proceedsslowly and can conveniently be followed by NMR spectrosco-py. Accordingly, sealed samples of anti-2 in C6D6 were heatedto 35, 50, 65, and 80 °C, respectively, and the isomerization wasmonitored by 1H NMR spectroscopy (see the SupportingInformation for details). The C5Me5 ligands in the two isomershave distinct chemical shifts; while the signal at 1.54 ppm foranti-2 progressively decreased, a new resonance at higher field(1.43 ppm), assigned to syn-2, increased with time. Eventually,no further change in the syn/anti ratio was observed, and wesuspect that the system had reached its equilibrium at thechosen temperature. The presence of an equilibrium was alsoverified by the observation that pure syn-2, obtained byrecrystallization, slowly reconverts to anti-2 at room temper-ature until the mixture is equilibrated (syn/anti = 93:7). For afirst-order reaction proceeding to equilibrium, a plot ofln(Asyn‑2,∞ − Asyn‑2,t) against time gives a straight line.33 Asexpected for an intramolecular process, this was indeed foundfor the anti−syn isomerization of 2 at all four temperatures(Figure 5), and the corresponding rate constants and half-livesare listed in Table 4 and can be used to determine theactivation parameters for the reaction. An Eyring plot (see theSupporting Information) affords an enthalpy of activation ofΔH⧧ = 24(1) kcal mol−1 and an entropy of activation of ΔS⧧ =−7(3) eu; the latter value probably results from a loss of somedegree of freedom while the transition state is approached.Notably, the barrier for the isomerization of a planar chiralopen ferrocene, [Fe(η5-2,3-C7H11)2], via the partial decoordi-T

able

3.Selected

Bon

dLengths(Å

)andAngles(deg)fortheη3-oIndM

eCom

plexes

anti-2

syn-2

syn-3

syn-6

syn-7

syn-9

Fe−C(C

5Me 5)

2.0795(13)−2.1401(13)

2.0941(19)−2.134(2)

2.0901(12)−2.1423(12)

2.116(3)−2.185(3)

2.115(3)−2.164(3)

2.0596(15)−2.2413(15)

Fe−Cp p

lanea

1.719

1.728

1.731

1.772

1.763

1.763

Fe−C1

2.1028(13)

2.076(2)

2.0709(13)

2.102(3)

2.154(4)

2.0967(16)

Fe−C2

2.0408(13)

2.070(2)

2.0267(12)

2.092(3)

2.061(4)

2.0984(15)

Fe−C3

2.1021(13)

2.114(2)

2.1736(12)

2.132(3)

2.111(4)

2.1321(16)

Fe−allyl planea

1.600

1.648

1.722

1.678

1.750

1.644

C1−

C2−

C3

121.04(12)

115.5(2)

114.93(11)

115.2(3)

114.2(3)

116.72(15)

C1−

C2−

C3−

C4(abs.value)

49.5(2)

176.9(2)

177.4(1)

176.3(3)

178.9(4)

179.0(2)

Fe−L

1.7464(14)

1.737(2)

1.9644(12)

2.6001(5)

2.3087(8)

1.7226(17)

τb6.9

64.5

17.8

68.8

63.0

62.6

aCp p

laneandallyl planereferto

theleast-squaresplanes

oftheη5-andη3-ligands,respectively.bτ=anglebeweenthetwoligandplanes.

Figure 4. ORTEP diagram of syn-2 with thermal displacementparameters drawn at 50% probability.



nation of the pentadienyl ligand is comparable (ΔH⧧ = 22 kcalmol−1).7d

From a mechanistic point of view, experimental andtheoretical studies on various metal−allyl complexes suggestthat anti−syn isomerizations occur via an η1-intermediate,which connects the opposite isomers via a C−C bondrotation.34,35 However, starting from the kinetic product ofthe ligand addition to 1, anti-[(η5-C5Me5)Fe(η

3-oIndMe)(L)] inthe exo conformation (highlighted in gray in Scheme 3), two16-electron η1-intermediates can be envisaged: The first,intermediate A1 (Scheme 3), can be expected to bethermodynamically favored and, therefore, more easilyaccessible than B1, because it is less sterically congested andadditionally has the maximum conjugation in a styrene-type π-system. However, the anti orientation of the methyl and thephenyl groups is preserved, and rotation around the C1−C2bond, followed by η1−η3 interconversion, leads to A2, nowadopting an endo conformation. If a concomitant rotationaround the Fe−C1 bond takes place,30e,34a A3 is formed, whichis indistinguishable from the original complex, although the twohydrogen atoms at C1 have formally exchanged their positions.In contrast, pathway B is productive, because rotation aroundthe C2−C3 bond is possible and finally results in the syn isomerB2 or B3, depending on the thermodynamic preference foreither the exo or the endo conformation. Unique for pathway Bis the possibility of stabilizing the 16-electron η1-intermediate(or transition state) B1 as an 18-electron η3-benzylintermediate B1′, which still allows for productive rotationaround C2−C3. As an alternative to the η3−η1−η3 switch forthe η3-allyl to η3-benzyl interconversion, a concerted mecha-nism proceeding directly to B1′ can also be envisaged. It isworth mentioning that stable iron η3-benzyl complexes havebeen isolated and their fluxional process studied.21,36 If anelectron-deficient 16-electron intermediate (or transition state),such as A1 or B1, is involved, the choice of the ligand L shouldsubstantially affect the stability and, therefore, the rate ofisomerization.35,37 Higher stability can be anticipated if strongσ-donor ligands are used, whereas good π-acceptors, such asCO, should lead to higher activation barriers and consequently

slower rates, while steric effects are also likely to play animportant role.We then turned our attention to N-heterocyclic carbenes

(NHCs), which are strong σ-donor ligands with a diminishedπ-acceptor capability (relative to carbon monoxide) associatedwith a high-lying p(π) orbital;38 in some cases, they have evenbeen considered as pure donor ligands.39 The reaction of1,3,4,5-tetramethylimidazolin-2-ylidene (:C[N(Me)C(Me)]2,IMe) with [(η5-C5Me5)Fe(η

5-oIndMe)] (1) in pentaneimmediately gave a red solution, and crystallization wasachieved upon storage at −30 °C. The molecular structurewas determined by X-ray diffraction analysis and is shown inFigure 6. As in the reaction with CO, hapticity interconversionwas induced by the coordination of the carbene to iron; thecorresponding Fe−C bond distance is at 1.9644(12) Å, inagreement with [(η5-C5H5)FeI(CO)(IMes)] (1.980(5) Å,IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene),40

but shorter than that in NHC adducts of [Fe{N(SiMe3)2}2].41

Surprisingly, the X-ray study shows that the anti−synisomerization has already taken place at room temperature,resulting in syn-[(η5-C5Me5)Fe(η

3-oIndMe)(IMe)] (syn-3). Incontrast to syn-2, an exo conformation is adopted, mostprobably for steric reasons.NMR spectroscopy, however, did not indicate the presence

of only one isomer in solution. Instead, both syn-3 and anti-3are found in a ratio of ca. 68:32 as judged by the integration ofthe corresponding methyl groups; the resonances for theC5Me5 ligands are not sufficiently resolved for individualintegration. Nevertheless, two-dimensional NMR spectroscopyhelps to distinguish the resonances assigned to the major (syn-

Figure 5. First-order kinetic plots for the conversion of anti-2 to syn-2in C6D6 at four different temperatures.

Table 4. Kinetic Data for the anti−syn Isomerization

T k (10−6 s−1) t1/2 (h) anti/syn ratio

35 °C (308.15 K) 1.43(3) 134.4 10:9050 °C (323.15 K) 6.96(8) 27.6 8:9265 °C (338.15 K) 51.8(5) 3.7 8:9280 °C (353.15 K) 227(1) 0.8 9:91

Scheme 3. Proposed Mechanism for the anti−synIsomerization



3) or minor (anti-3) isomers. This is particularly useful for theallyl moiety, because exchange peaks in the 1H−1H NOESYNMR spectrum give further evidence for the proposed anti-to-syn isomerization mechanism (Scheme 3). Consistent with therelatively slow exchange rates at room temperature, nosignificant line broadening is observed in the standard 1HNMR spectrum. Exchange peaks for anti-H1/syn-H1 and anti-H1′/syn-H1′ (Scheme 4, the apostrophe denotes the minor

isomer) support the unproductive pathway A (Scheme 3) thatdoes not lead to the opposite isomer, but represents an(indistinguishable) exchange of both hydrogen atoms at C1.The most important exchange peak is anti-H3/syn-H3′; thisexchange stems from the productive anti-to-syn, and vice versa,isomerization, which requires pathway B. Additionally, a secondslow fluxional process can be identified: The methylsubstituents on the nitrogen atoms of IMe give two separateresonances for each isomer in the 1H NMR spectrum, whichare all interconnected by exchange peaks in the 1H−1HNOESY spectrum, consistent with hindered carbene rotation atroom temperature.To underline further that the choice of the ligand L has an

influence on the rate of isomerization, we have usedtrimethylphosphine (PMe3) as a third example. Reaction of 1with 1 equiv of PMe3 resulted in an immediate color changefrom green to red-brown. After stirring for 30 min, the solventwas removed, and a brown oil was obtained, which resistedfurther purification and crystallization because of its extremelyhigh solubility even in hexamethyldisiloxane. However,coordination of PMe3 is clearly evidenced by a downfieldshift from −61.9 ppm for the free phosphine to 32.8 ppm in the

31P{1H} NMR spectrum, which exactly matches the reportedshift for [(η5-C5Me5)Fe(η

3-C3H5)(PMe3)],42 and couplings to

the 31P nucleus in the 1H and 13C NMR spectra. Additionally,the 1H NMR spectrum shows three resonances in a 2:2:1 ratioin the aromatic region, confirming the presence of anuncoordinated phenyl group. Supported by 1H−1H NOESYNMR spectroscopy and also by the upfield shift for the H3hydrogen atom to 0.79 ppm, the product of the reaction isidentified as syn-[(η5-C5Me5)Fe(η

3-oIndMe)(PMe3)] (syn-4).43

Clearly, the kinetic product, anti-4, quickly rearranges to the synisomer at room temperature, and syn-4 is almost exclusivelyobserved (Scheme 2).44 If the 31P NMR spectrum is directlyrecorded after mixing 1 and PMe3, an additional resonance at25.1 ppm is found, which can be tentatively assigned to anti-4,because it disappears with time, while the resonance at 32.8ppm increases. It should be noted that only slow conversionwas observed at room temperature for the analogous rutheniumcomplex, and a rate constant of 6.57(2)·10−6 s−1 was deducedfrom a kinetic study at 50 °C.6 The higher rate observed for theFe−PMe3 complex, and also for its CO analogue (vide infra),compared with that for the corresponding rutheniumcomplexes agrees with the general trend that first-row transitionmetals generate weaker metal−ligand bonds.45

Since η1-oIndMe intermediates are proposed in themechanism of the anti-to-syn isomerization (Scheme 3), theisolation of such species was desirable. Therefore, [(η5-C5Me5)Fe(η

5-oIndMe)] (1) was treated with an excess (20equiv) of PMe3 to enforce the formation of [(η5-C5Me5)Fe(η

1-oIndMe)(PMe3)2]. However, the formation of this complexcould not be detected by NMR spectroscopy, and only theresonances associated with syn-4 were found. Bis-(dimethylphosphino)ethane (dmpe) can be expected to be ofsimilar size to two PMe3 ligands, but the chelate effect shouldfacilitate the coordination of the second phosphorus atom assoon as the η3-oIndMe complex is formed. Indeed, the reactionof 1 with 1 equiv of dmpe instantaneously gave a brownsolution, which eventually turned red (Scheme 5). Crystal-

lization of the highly soluble product from pentane/hexamethyldisiloxane afforded red crystals of [(η5-C5Me5)Fe-(η1-oIndMe)(dmpe)] (5). In contrast to 1−4, the hydrogenatoms at C1 give rise to only one resonance at 1.22 ppm in the1H NMR spectrum, which suggests that they are bonded to ansp3-hybridized carbon atom; otherwise, a shift to lower fieldwould be expected. Instead, a low-field resonance at 6.32 ppmcan be assigned to H3. Furthermore, the correspondingresonances in the 13C NMR spectrum for C1 and C3 arefound at 10.3 and 115.6 ppm, respectively, which indicates thatthe open indenyl ligand in 5 is bound to iron via C1 rather thanC3. This is further substantiated by an X-ray diffraction studythat establishes a Fe−C1 bond length of 2.160(2) Å and also an


Scheme 4. Relevant 1H−1H NOESY Exchange Peaks for theMajor, syn-3, and the Minor Isomer (anti-3)

Scheme 5. Synthesis of the η1-oIndMe Complex 5



anti orientation of the phenyl and the methyl group across theC2−C3 double bond (Figure 7). The formation of 5 can be

rationalized by pathway A (Scheme 3), which is unproductivewith respect to the anti-to-syn isomerization, but results in theless sterically encumbered η1-oIndMe complex with η1-coordination via C1. The coordination around iron is bestdescribed as a distorted three-legged piano-stool geometry withthe chelating phosphine occupying two positions. The Fe−Pbond distances, at 2.1761(7) and 2.1548(6) Å, resemble thosefound in other bis(phosphino) iron complexes, for example,2.225(2) Å in [(η3-C5H7)2Fe(depe)] (depe = bis-(diethylphosphino)ethane) or 2.176(3) and 2.179(3) Å in[(η5-C5H4)SiMe2(η

1-C5H4)Fe(dmpe)].46

DFT Studies. To shed further light on the formation of[(η5-C5Me5)Fe(η

3-oIndMe)(L)] complexes (2: L = CO; 3: L =IMe; 4: L = PMe3) and their anti-to-syn isomerization, densityfunctional theory (DFT) calculations were performed employ-ing the M06-L functional.47 First, the molecular structures of 1,anti-2, syn-2, and syn-3 were optimized; the calculated structuralparameters agree very well with the experimentally derived data(see the Supporting Information for details) and served asstarting points for the calculation of the remaining anti and syncomplexes (Table 5). Since, for syn-2, the endo conformer wasunexpectedly observed (vide supra), the syn isomers werecalculated in both the exo and the endo conformations. Table 5lists the thermodynamic data for the adduct formation from 1and the corresponding free ligand L. In all cases, breaking thecomparatively labile Fe−C4 and Fe−C5 bonds in [(η5-C5Me5)Fe(η

5-oIndMe)] (1), followed by Fe−L bond formation,is exothermic, and a substantial contribution probably comesfrom the rearomatization of the phenyl group. The PMe3complexes 4, however, were calculated to be significantly lessstable, which agrees with the experimental observation that 4loses PMe3 and reforms 1 when heated to 65 °C under vacuum

for prolonged times. In line with the experimental results, theendo conformation is only more stable for syn-2, while the exoconformation is preferred for syn-3 as well as for syn-4. The ΔΔvalues (Table 5) give information about the energy differencesbetween the anti and syn isomers and, therefore, about theequilibrium constants. In 2 and 4, an equilibrium between antiand syn isomers is experimentally observed, favoring the antiisomer; this result is at least qualitatively consistent with theDFT calculations. However, in agreement with the experiment,3 behaves differently, because the ΔΔG298 value is endergonicwith +2.6 kcal mol−1.Despite the importance of anti−syn isomerizations in allyl

chemistry,34 this fluxional behavior was addressed in only fewcomputational studies involving group 10 transition metal−allylcomplexes (M = Ni, Pd, Pt),34j,35 but similar investigations withiron or ruthenium remain, to the best of our knowledge,unprecedented. For this study, we have chosen the COcomplex 2, [(η5-C5Me5)Fe(η

3-oIndMe)(CO)], for which wehave acquired the most detailed experimental and structuraldata (vide supra). As mentioned above (Scheme 3), the startingpoint of the isomerization is anti-2 in the exo conformation(Figure 8), which is the kinetic product from the reaction of 1with CO. The productive pathway requires breaking the Fe−C1 rather than the Fe−C3 bond, leading to an η1-benzyltransition state (TS-1). Although we were not able to locateTS-1 on the potential energy surface, the identification of TS-3(vide infra) suggests its existence.48 The second step representsthe η1−η3 shift to the first intermediate (IM-1), an η3-benzylcomplex with Fe−C bond lengths of 2.063, 2.130, and 2.303 Åfor C3, C4, and C5, respectively; the C5−C4−C3−C2 torsionangle is 51.6° (Figure 9). IM-1 is equivalent to the intermediateB1′ proposed in Scheme 3 and is also in accord with thestability of iron−benzyl complexes.21,36 Rotation around theuncoordinated C2−C3 bond has a comparatively small energybarrier (ΔΔG298 = 6.1 kcal mol−1) with respect to IM-1 andchanges the relative positioning of the methyl and phenylgroups from anti to syn. A potential energy surface scan (PES)connects IM-1 through the transition state TS-2 with the syn-η3-benzyl intermediate (IM-2), and their freely optimizedstructures are displayed in Figure 9. The final change from η3-

Figure 7. ORTEP diagram of one of the two independent moleculesof 5 with thermal displacement parameters drawn at 50% probability.Selected bond lengths (Å) and angles (deg): Fe−C(C5Me5)2.114(2)−2.135(2), Fe−C1 2.160(2), Fe−P1 2.1761(7), Fe−P22.1548(6), C1−C2 1.460(3), C2−C3 1.355(3), C3−C4 1.473(3);P1−Fe−C1 95.71(6), P1−Fe−P2 83.29(2), P2−Fe−C1 87.09(6).Values for the second molecule are similar (see the SupportingInformation for details). A least-squares fit of both molecules gave anrms deviation of 0.16 Å.

Table 5. M06-L Energies and Enthalpies (in kcal mol−1) for2, 3, and 4a

complex L ΔEel ΔE298 ΔH298 ΔG298

formation ofanti

2 (exo)b CO −33.3 −30.7 −31.2 −22.03 (exo) IMe −33.3 −30.7 −31.3 −17.54 (exo) PMe3 −13.0 −10.5 −11.1 1.9

formation ofsyn

2 (exo) CO −32.9 −30.0 −30.6 −20.92(endo)b

CO −35.6 −33.1 −33.7 −24.1

3 (exo)b IMe −31.7 −28.8 −29.4 −14.93 (endo) IMe −28.8 −26.5 −27.1 −14.74 (exo) PMe3 −14.2 −11.6 −12.2 1.04 (endo) PMe3 −7.5 −4.9 −5.5 7.8complex L ΔΔEel ΔΔE298 ΔΔH298 ΔΔG298

anti-to-synisomerizationc

2 CO −2.3 −2.5 −2.5 −2.13 IMe 1.6 1.9 1.9 2.64 PMe3 −1.3 −1.1 −1.1 −0.9

aΔEel: zero-point uncorrected energies. ΔE298: relative energies at 298K. ΔH298: enthalpies at 298 K. ΔG298: Gibbs free energies at 298 K.bX-ray data used as start geometry. cCalculated for the most stableconformer.



benzyl to η3-allyl coordination is associated with two hapticityinterconversions (η3−η1 and vice versa). Supported by intrinsicreaction coordinate (IRC) calculations, the singlet transitionstate (TS-3, equivalent to B1 in Scheme 3) with η1-coordination shows that the iron atom stops at C3 and thenmoves on to the η3-coordination mode via C1−C3, affordingsyn-2 with an endo conformation. The energy differencebetween TS-3 and syn-2 amounts to ΔΔG = 34.2 kcal mol−1,and it can be anticipated that approximately the same activationenergy is required for the η3−η1 hapticity interconversion ongoing from anti-2 to TS-1, which would then constitute therate-determining step of the overall process. The involvementof η1-transition states nicely agrees with our observation thatgood σ-donor ligands, such as IMe or PMe3, support fasteranti−syn isomerizations than π-acceptor ligands.35,37 Further-more, our kinetic NMR study (vide infra) provides a smalleractivation barrier (ΔG⧧

298 = 26 kcal mol−1) than that derivedcomputationally, probably resulting from stabilizing solventeffects, in particular, for η1-intermediates, which are notproperly addressed by calculations under pseudo-gas-phaseconditions. As a final comment, it should be noted that exo−

endo interconversions for d6 complexes also proceed via theη3−η1−η3 pathway and can, therefore, accompany the anti-to-syn isomerization, as observed for 3 and 4.30e,34a

η5-to-η3 Hapticity Interconversion Induced by Oxida-tive Addition. For the ruthenium complex [(η5-C5Me5)Ru(η

5-2,4-C6OH9)] (C6OH9 = dimethyloxapentadienyl), it wasreported that the oxidative addition of SnCl4, I2, or CHCl3results in Ru(IV) η3-oxapentadienyl complexes, whereas noreaction with CH3I was observed, even when a 100-fold excesswas applied under reflux.9 Since our earlier studies demon-strated a greater reactivity for the open indenyl ligand,6 thereaction of [(η5-C5Me5)Fe(η

5-oIndMe)] (1) with CH3I wasinvestigated. At room temperature, no immediate reactionsimilar to that with PMe3 or CO was observed, but heating of 1with 2 equiv of methyl iodide in hexane to 50 °C for 1 h yieldeda dark red, paramagnetic solid after workup (Scheme 6). X-raydiffraction analysis revealed this compound to be syn-[(η5-C5Me5)Fe(η

3-oIndMe)(I)] (6), which was also confirmed bymass spectroscopy and elemental analysis (Figure 10).

Similarly, dichloromethane does not behave as an inertsolvent toward 1. A gradual color change from green to redoccurs when 1 is dissolved in CH2Cl2, and the product wasidentified as syn-[(η5-C5Me5)Fe(η

3-oIndMe)(Cl)] (7). Both 6and 7 are rare examples of iron(III) allyl complexes with anelectron count of 17 and formally stem from a one-electronoxidation of Fe(II). Similarly to the reaction of [(η5-C5Me5)Fe(η

3-C3H5)(C2H4)] and allyl chloride, which delivered[(η5-C5Me5)Fe(η

3-C3H5)(Cl)] and hexadiene,49 the formationof 6 and 7 presumably also involves a radical process to giveethane and 1,2-dichloroethane as the byproducts.The molecular structure of 7 was established by X-ray

diffraction analysis, and an ORTEP diagram is shown in Figure11, while pertinent bonding parameters are listed in Table 3.

Figure 8. Reaction profile for the anti-to-syn isomerization of the COcomplex 2; for exo,anti-2 and endo,syn-2, the calculated structures arevery similar to those established by X-ray diffraction (Figures 3 and 4).

Figure 9. Calculated structures for IM-1 and TS-2 (top row) and forIM-2 and TS-3 (bottom row).

Scheme 6. Reaction of 1 with CH3I or CH2Cl2 Resulting inComplexes 6 and 7




Besides [(η5-C5Me5)Fe(η3-C3H5)(X)] (X = Cl, CH3),

49 6 and7 represent, to the best of our knowledge, the only otherstructurally characterized Fe(III) allyl complexes to date. Inboth cases, isomerization has already taken place, because thephenyl group is found in the syn orientation with respect to themethyl group. In contrast to [(η5-C5Me5)Fe(η

3-C3H5)(CH3)],the allyl ligand adopts an endo conformation, which results inlarge interplanar angles of τ = 68.8° and 63.0° for 6 and 7,respectively. For the former, the larger atomic radius of theiodide substituent causes a greater shift of the allyl ligand and isalso reflected in the longer Fe−X bond distance (Fe−I =2.6001(5) Å) in comparison to that in the chloride derivative(Fe−Cl = 2.3087(8) Å). The Fe−C bond lengths resemblethose found in the Fe(II) complexes 2 and 3. Indeed, structures7 and syn-2 are isotypic, whereby the chloro and carbonylligands play a similar structural role as acceptors of a “weak”hydrogen bond from C6−H6, with H6···Cl = 2.76 Å in 7 andH6···O = 2.49 Å in syn-2. Structure 6 is, however, not isotypicto 7 and syn-2.The iodo complex 6 was selected for solid-state magnetic

susceptibility measurements using the “quartz tube technology”,which is particularly useful for highly air-sensitive compounds.50

Curie behavior is observed, because the mangnetic moment isreasonably temperature-independent throughout the measuredrange of 2−300 K, and a plot of χ−1 vs T gives a straight line(Figure 12). The magnetic moment of μeff ≈ 2.0 μB isconsistent with one unpaired electron (S = 1/2) in the 17-

electron complex 6; the slightly higher value in comparison tothe spin-only value (μeff ≈ 1.73 μB) can be ascribed to thecontribution of some spin−orbit coupling.

Competition Experiment: η5-to-η3 Hapticity Intercon-version in an Indenyl Iron Open Indenyl Complex. Thusfar, we have only studied the η5−η3 hapticity interconversion in[(η5-C5Me5)M(η5-oIndMe)] complexes (M = Fe, Ru). In thesesystems, the open indenyl ligand is much more likely to switchto an η3-coordination mode than the strongly boundpentamethylcyclopentadienyl ligand, although η3-Cp complexesare known, but rare.51 The question remains as to how theoIndMe ligand would behave in comparison to an indenyl ligandin an internal competition experiment. To investigate this, acomplex of the type [(η5-Ind)M(η5-oIndMe)] is required.Consequently, the preparation of [(η5-C9H7)Fe(η

5-oIndMe)]was attempted by the sequential addition of Na(C9H7) andK(oIndMe) to [FeI2(thf)2] at −78 °C, mimicking the proceduresuccessfully applied for the synthesis of 1. Unfortunately,bis(indenyl)iron was the only isolated product in severalattempts.25b We attribute this fact to the inherent instability ofthe half-sandwich intermediate [(η5-C9H7)FeI], and its directdismutation to [(η5-C9H7)2Fe] and [FeI2(thf)2].

52 However,iron half-sandwich complexes can be stabilized by bulkycyclopentadienyl ligands,19 and a similar concept should alsobe applicable to indenyl ligands. Recently, we have evaluatedthe size of differently substituted cyclopentadienyl and indenylligands using cone angle measurements on [(η7-C7H7)Zr(η

5-Cp)] and [(η7-C7H7)Zr(η

5-Ind)] complexes.53 These studieshave provided a valuable tool to assess the steric demand ofthese π-ligands and gave a cone angle of 102° for C9H7,whereas 122° was found for C5Me5. This implies that theunsubstituted indenyl ligand is significantly smaller than C5Me5,which is probably the reason for the instability of [(η5-C9H7)FeI] versus [(η5-C5Me5)FeI] even at low temperatures.1,3-Disubstitution of C9H7 with tert-butyl groups increases thecone angle to 131°, and the corresponding (1,3-di(tert-butyl)indenyl)sodium, Na(Ind″), can be obtained by animproved protocol from indene in three steps.53

Thus, the reaction of [FeI2(thf)2] with 1 equiv of Na(Ind″) at−78 °C afforded a red solution, which was subsequently reactedwith 1 equiv of K(oIndMe). Because of the extremely highsolubility of the product, we refrained from a full character-ization and directly added carbon monoxide (Scheme 7). NMRspectroscopy indicates the formation of syn-[(η5-Ind″)Fe(η3-oIndMe)(CO)] (9), since, for instance, only one resonance, at7.30 ppm, for the ortho-hydrogen atoms of the phenyl ringattached to oIndMe is observed in the 1H NMR spectrum.Furthermore, the high-field shift of 0.22 ppm for H3 is typicalfor anti-H atoms and, consequently, leads to the assignment ofa syn orientation for the phenyl group. As expected for amolecule with C1 symmetry, all positions of the Ind″ ligand areinequivalent, giving rise to seven peaks in the 1H NMRspectrum. Complex 9 is one of the very few metal complexesinvolving the 1,3-di(tert-butyl)indenyl ligand,53,54 and the firstexample for iron.55 The synthesis of 9 clearly demonstrates thatstabilization of a half-sandwich intermediate can be achieved bytaking advantage of sterically encumbered indenyl ligands,although low temperatures are required,56 since otherwisebis(indenyl)iron formation takes place.57 In addition, the moreready tendency of the open indenyl ligand, rather than anindenyl ligand, to undergo an η5−η3 rearrangement isillustrated. Similar to the arguments applied for comparativelyfacile hapticity interconversions of pentadienyl versus cyclo-


Figure 12. Plot of μeff and χ−1 vs T for [(η5-C5Me5)Fe(η3-oIndMe)(I)]

(6).



pentadienyl ligands,7b,d this behavior can be attributed to thesmaller loss of resonance energy for oIndMe.The identity of syn-9 was unambiguously confirmed by X-ray

diffraction analysis (Figure 13), which also shows the allyl

moiety in an endo conformation, similar to syn-2. Otherbonding parameters differ only marginally from those of syn-2(Table 3). The fold angle for the Ind″ ligand is, at 4.6°, lesspronounced than that for oIndMe in 1, but the Fe−C14 andFe−C15 bond distances are still slightly longer than the otherthree.

■ CONCLUSIONThis study provides further evidence for the ability of thephenylmethallyl ligand oIndMe to coordinate to transitionmetals as an η5-bound open indenyl ligand, as shown by theisolation of the half-open ferrocene [(η5-C5Me5)Fe(η

5-oIndMe)] (1). In contrast to the corresponding half-openruthenocene,6 1 shows fluxional behavior already at roomtemperature on the NMR time scale, indicating that the oIndMe

ligand can easily shuttle between the η5- and η3-coordination

modes. Consequently, 1 can easily provide an emptycoordination side, which was probed by the addition of CO,PMe3, and IMe to afford the cyclopentadienyl−allyl ironspecies 2−4. The resulting complexes undergo anti-to-synisomerization with the rate depending on the σ-donor/π-acceptor capability of the additional ligand. As expected, thisprocess is slowest for the carbonyl complex 2, and a kineticstudy afforded an enthalpy of activation of ΔH⧧ = 24(1) kcalmol−1. DFT calculations suggest that this isomerizationproceeds via consecutive η3−η1−η3 interconversions andinvolves η3-benzyl intermediates. Stabilization of an η1-allylcomplex could be achieved by addition of dmpe to 1. Thisflexibility shows that the open indenyl ligand is a pentadienylligand with enhanced reactivity and, therefore, nicely adds tothe growing family of unconventional open cyclopentadienylligands.8,58 Future work aims at exploiting this concept for thepreparation of labile transition-metal complexes for potentialuse in homogeneous catalysis.

■ EXPERIMENTAL PROCEDURESAll synthetic and spectroscopic manipulations were carried out underan atmosphere of purified nitrogen, either in a Schlenk apparatus or ina glovebox. Solvents were dried and deoxygenated either by distillationunder a nitrogen atmosphere from sodium benzophenone ketyl(THF) or by an MBraun GmbH solvent purification system (all othersolvents). NMR spectra were obtained on a Bruker Avance II 600, aBruker DRX 400, a Bruker Avance III 400, or a Bruker Avance II 300spectrometer at 600, 400, or 300 MHz (1H); 151, 101, or 75 MHz(13C); and 162 or 121 MHz (31P). Unless stated otherwise, the spectrawere recorded at ambient temperature using C6D6 as solvent. Itsresidual solvent signal was used as a chemical shift reference (δH =7.16) for the 1H spectra and the solvent signal (δC = 128.04 ppm) forthe 13C spectra. 31P spectra were referenced to virtual external 85%phosphoric acid (δP = 0). The number of protons attached to eachcarbon was determined by 13C-DEPT135 experiments. If required,signal assignment was achieved by two-dimensional H,H-COSY, H,H-NOESY, H,C-HSQC, and H,C-HMBC spectra. They were recordedusing standard Bruker pulse programs; sweep widths, digitalresolution, and pulse delays were optimized for the samples underinvestigation. Mixing times of 1000 and 2000 ms were used for theH,H-NOESY experiments. For the low-temperature measurements of1, the temperature display of the spectrometer was calibrated againstthe standard methanol sample (4% methanol in methanol-d4).

59

Bandshape analysis of the low-temperature 1H NMR spectra of 1 wasperformed with the DNMR module implemented in BrukerTopSpin2.1pl6.20 The quality of the iterative analysis is defined bythe program parameter “maximum overlap”, for which values between97.8 and 98.4% could be achieved. A Bruker Vertex 70 spectrometerwas used for recording IR spectra. Elemental analyses were performedby combustion and gas chromatographical analysis with an ElementarvarioMICRO instrument. [FeI2(thf)2],

60 Li(C5Me5), Na(C5Me5),61

Na(Ind″),53 and K(oIndMe)6 were prepared according to the literature;Li(C5Me5) was synthesized from equimolar amounts of n-butyllithiumand pentametylcyclopentadiene in pentane.62 All other reagents wereobtained commercially and used as received.

X-ray Diffraction Studies. Data were recorded at 100 K onOxford Diffraction diffractometers using monochromated Mo Kα ormirror-focused Cu Kα radiation (Table 6). The structures were refinedanisotropically using the SHELXL-97 program.63 Hydrogen atomswere either (i) located and refined isotropically (H1A, H1B, H3, andfor 1 H5; in some cases, with constraints to C−H bond lengths); (ii)included as idealized methyl groups allowed to rotate, but not tip; or(iii) placed geometrically and allowed to ride on their attached carbonatoms. Compound 5 crystallized with two independent molecules perasymmetric unit.

Theoretical Calculations. All computations were performed usingthe density functional method M06-L as implemented in the Gaussian

Scheme 7. One-Pot Synthesis of syn-[(η5-Ind″)Fe(η3-oIndMe)(CO)] (syn-9)

Figure 13. ORTEP diagram of syn-9 with thermal displacementparameters drawn at 50% probability; the hydrogen atoms of the tert-butyl groups have been omitted for clarity.



09 program.47,64 For all main-group elements (C, H, N, O, and P), theall-electron triple-ζ basis set (6-311G**) was used,65 whereas for iron,a small-core relativistic ECP together with the corresponding double-ζvalence basis set was employed (Stuttgart RSC 1997 ECP).66

[(η5-C5Me5)Fe(η5-oIndMe)] (1). [FeI2(thf)2] (2.0 g, 4.407 mmol)

was completely (!) dissolved in THF (60 mL); sometimes, ultrasoundand heating were employed to accelerate the dissolution process. Thebrown solution was cooled to −78 °C, efficiently (!) stirred, and a paleyellow suspension of Li(C5Me5) (0.627 g, 4.407 mmol) in THF (30mL) was slowly added with a syringe. The resulting yellow solutionwas stirred for 10 min at −78 °C, followed by the slow addition ofK(oIndMe) (0.750 g, 4.407 mmol) in THF (15 mL), which resulted ina color change to brown. During the slow warm-up, the color turnedto green, and the solvent was removed after 3.5 h. Pentane (∼100 mL)was added, and the mixture was cooled to −30 °C. While still cold,filtration through a pad of Celite gave a green solution, which wasconcentrated until saturation was achieved. The solution wastransferred to the freezer (−30 °C), which resulted in thecrystallization of large, round, green plates (0.794 g); furtherconcentration of the mother liquor afforded a second crop. Totalyield after drying: 0.914 g (64%). Single crystals were obtained from aconcentrated pentane solution at −20 °C.

1H NMR (400 MHz): δ = 6.94 (m, 2H, m-phenyl), 6.85 (m (“t”),splitting 7.2 Hz, 1H, p-phenyl), 5.60 (s, 1H, H3), ≈ 4.62 (v br s, ν1/2 ≈340 Hz, 2H, o-phenyl), 2.56 (d, Janti‑H1,syn‑H1 ≥ 3.4 Hz, 1H, syn-H1),1.91 (s, 3H, CH3), 1.41 (s, 15H, C5Me5), −0.46 (d, 1H, anti-H1). 13CNMR (101 MHz): δ = 133.2 (br, m-phenyl) 120.7 (p-phenyl), 103.2(C4), 95.9 (C2), 81.8 (C3), 79.7 (C5Me5), 43.4 (C1), 25.2 (2-CH3),9.9 (C5Me5), the o-C atoms of the phenyl ring were not observed. 1HNMR, 13C NMR (400 and 101 MHz, respectively, THF-d8): seeTables 1 and 2 in the Results and Discussion section. Anal. Calcd forC20H26Fe: C, 74.54; H, 8.13. Found: C, 74.20; H, 8.20.anti-[(η5-C5Me5)Fe(η

3-oIndMe)(CO)] (anti-2). Compound 1(0.170 g, 0.528 mmol) was dissolved in pentane (10 mL). CO wasthen passed through the solution for half a minute, which resulted in acolor change to orange. The solution was kept under CO for 20 min,the solvent was removed, and the resulting orange-red oil was

redissolved in a minimum amount of pentane. Filtration andcrystallization at −30 °C afforded an orange-red solid (0.116 g,63%). Single crystals were obtained from a concentrated pentanesolution at −15 °C.

1H NMR (400 MHz): δ = 7.09−7.03 (m, 2H, phenyl), 6.94−6.87(m, 3H, phenyl), 4.10 (s, 1H, H3), 2.27 (m, 1H, syn-H1), 1.91 (s, 3H,CH3), 1.83 (d, 2JHH = 2.0 Hz, 1H, anti-H1), 1.54 (s, 15H, C5Me5).13C{1H} NMR (101 MHz): δ = 223.8 (CO), 149.5 (ipso-phenyl),128.6 (phenyl), 125.1 (phenyl), 123.5 (phenyl), 89.9 (C5Me5), 87.3(C2), 58.2 (C3), 39.3 (C1), 25.8 (CH3), 10.2 (C5Me5). Anal. Calcdfor C21H26FeO: C, 72.01; H, 7.48. Found: C, 71.93; H, 7.41. IR(ATR): ν(CO/cm−1) = 1924.

syn-[(η5-C5Me5)Fe(η3-oIndMe)(CO)] (syn-2). Compound 1 (0.123

g, 0.382 mmol) was dissolved in hexane (15 mL). CO was then passedthrough the solution for half a minute. The resulting orange solutionwas refluxed overnight, and the solvent was removed, which gave aslightly oily, orange solid. Recrystallization from a filtered pentanesolution at −30 °C afforded orange-red crystals (0.076 g, 57%).

1H NMR (400 MHz,): δ = 7.48 (m (“d”), splitting 7.7 Hz, 2H, o-phenyl), 7.18−7.14 (m, 2H, m-phenyl), 7.06 (m (“t”), splitting 7.5 Hz,1H, p-phenyl), 2.46 (s, 1H, syn-H1), 2.00 (s, 3H, CH3), 1.85 (s, 1H,H3), 1.43 (s, 15H, C5Me5), 0.40 (s, 1H, anti-H1).

13C{1H} NMR (101MHz): δ = 220.1 (CO), 142.6 (ipso-phenyl), 130.8 (phenyl), 128.0(phenyl), 125.5 (phenyl), 108.0 (C2), 91.9 (C5Me5), 65.6 (C3), 42.6(C1), 21.4 (CH3), 10.0 (C5Me5). Anal. Calcd for C21H26FeO: C,72.01; H, 7.48. Found: C, 72.29; H, 7.64. IR (ATR): ν(CO/cm−1) =1876.

[(η5-C5Me5)Fe(η3-oIndMe)(IMe)] (3). Tetramethylimidazolin-2-yli-

dene (IMe) (0.047 g, 0.372 mmol) was dissolved in pentane (10 mL),and 1 (0.120 g, 0.372 mmol) in pentane (6 mL) was added. The colorchanged immediately to red, and the solvent was removed after 30 minof stirring. The red, voluminous solid was redissolved in a minimumvolume of pentane for crystallization at −30 °C, which gave dark redcrystals overnight (0.101 g, 61%). Single crystals were grown from aconcentrated pentane solution at −20 °C.

1H NMR (400 MHz, resonances for the minor isomer are denotedwith ′): δ = 7.48−7.42 (m, 2H, phenyl), 7.34−7.28 (m, 2H, phenyl),

Table 6. Crystallographic Data

1 anti-2 syn-2 syn-3 5 syn-6 syn-7 syn-9

empiricalformula

C20H26Fe C21H26OFe C21H26OFe C27H38N2Fe C26H42P2Fe C20H26IFe C20H26ClFe C28H34OFe

formula weight 322.26 350.27 350.27 446.44 472.39 449.16 357.71 442.40temperature (K) 100(2) 100(2) 100(2) 100(2) 100(2) 100(2) 100(2) 100(2)wavelength λ(Å)

1.54184 1.54184 1.54184 0.71073 0.71073 1.54184 0.71073 1.54184

cryst syst monoclinic monoclinic monoclinic monoclinic triclinic monoclinic monoclinic monoclinicspace group P21/c P21/n P21/c P21/n P1 P21/c P21/c P21/ca [Å] 13.5328(8) 10.0070(2) 12.3722(2) 14.6168(2) 9.8454(2) 8.5902(2) 12.2535(2) 14.1221(4)b [Å] 6.8730(4) 16.1642(2) 9.2362(2) 8.2424(2) 15.7886(4) 9.1456(2) 9.2674(2) 9.8383(3)c [Å] 17.8642(12) 10.9946(2) 15.8669(2) 19.4457(4) 16.6900(4) 23.3844(6) 15.9496(4) 17.0165(5)α [°] 90 90 90 90 76.568(2) 90 90 90β [°] 104.134(6) 96.813(2) 97.385(2) 91.592(2) 85.622(2) 95.068(2) 97.815(2) 98.689(3)γ [°] 90 90 90 90 87.456(2) 90 90 90volume [Å3] 1611.27(17) 1765.87(5) 1798.10(5) 2341.86(8) 2515.10(10) 1829.96(7) 1794.38(7) 2337.10(12)Z 4 4 4 4 4 4 4 4reflns collected 22 699 28 820 28 475 92 882 87 159 27 352 43 233 63 517independentreflns

3319 [Rint =0.0567]

3678 [Rint =0.0349]

3415 [Rint =0.0349]

5363 [Rint =0.0344]

11 098 [Rint =0.0639]

3807 [Rint =0.0590]

3398 [Rint =0.0363]

4878 [Rint =0.0715]

goodness of fiton F2

1.042 1.113 1.023 1.047 1.026 1.048 1.110 1.062

ρcalcd [g cm−3] 1.328 1.317 1.294 1.266 1.248 1.630 1.324 1.257μ [mm−1] 7.402 6.843 6.720 0.660 0.737 19.785 0.984 5.279R(Fo), [I >2σ(I)]

0.0369 0.0254 0.0342 0.0261 0.0401 0.0363 0.0475 0.0321

Rw (Fo2) 0.0990 0.0709 0.0887 0.0672 0.0885 0.0957 0.1198 0.0789

Δρ [e Å−3] 0.623/−0.314 0.235/−0.404 0.551/−0.223 0.315/−0.273 0.573/−0.255 1.873/−0.946 0.914/−0.423 0.382/−0.329



6.78−6.71 (1H, m, phenyl′), 6.69−6.58 (2H, br m, phenyl′), 3.85 (s,1H, H3′), 3.75 (s, 3H, N-CH3 NHC′), 3.67 (s, 3H, N-CH3 NHC),3.67 (s, 3H, N-CH3 NHC), 3.06 (s, 3H, N-CH3 NHC′), 2.69 (s, 1H,syn-H1′), 2.58 (s, 3H, CH3), 2.43 (s, 3H, CH3′), 2.28 (s, 1H, syn-H1),1.58 (s, C5Me5′), 1.57 (s, C5Me5), 0.42 (s, 1H, anti-H1′), −0.10 (s, 1H,H3), −0.65 (s, 1H, anti-H1), two resonances for the phenyl groups arehidden under the residual solvent peak, and the C-CH3 NHC groupsoverlap with the resonances for C5Me5.

13C{1H} NMR (101 MHz): δ= 204.8 (Fe-C), 202.4 (Fe-C′), 155.5 (ipso-phenyl′), 149.2 (ipso-phenyl), 129.1 (phenyl), 128.0 (phenyl), 126.7 (phenyl′), 125.0 (CqNHC), 124.9 (Cq NHC), 121.7 (phenyl), 119.9 (phenyl′), 88.7 (C2′),86.3 (C2), 82.7 (C5Me5′), 82.2 (C5Me5), 52.1 (C3), 50.7 (C3′), 41.3(C1′), 38.0 (C1), 36.7 (N-CH3 NHC), 36.4 (N-CH3 NHC), 36.2 (N-CH3 NHC), 36.1 (N-CH3 NHC), 27.6 (CH3′), 20.7 (CH3), 11.0(C5Me5′), 10.6 (C5Me5), 9.4 (C-CH3 NHC), 9.3 (C-CH3 NHC), 9.1(C-CH3 NHC), one phenyl′ is probably hidden under the solvent, andthe two Cq NHC′ and one C-CH3 were not observed. Anal. Calcd forC27H38FeN2: C, 72.64; H, 8.58; N, 6.27. Found: C, 72.61; H, 8.58; N,6.23.syn-[(η5-C5Me5)Fe(η

3-oIndMe)(PMe3)] (syn-4). Trimethylphos-phine (PMe3) (0.025 g, 0.326 mmol) dissolved in pentane (4 mL)was added to a pentane solution (10 mL) of 1 (0.100 g, 0.310 mmol).The resulting red-brown solution was stirred for 30 min at roomtemperature, and the solvent was subsequently removed undervacuum. The red-brown oil was used without further purification forNMR studies.

1H NMR (400 MHz): δ = 7.57 (m (“d”), splitting 7.7 Hz, 2H, o-phenyl), 7.24 (m (“t”), 2H, m-phenyl), 7.10 (m (“t”), splitting 7.3 Hz,1H, p-phenyl), 2.65 (s, 3H, CH3), 1.85 (s, 1H, syn-H1), 1.46 (s, 15H,C5Me5), 0.92 (d, 2JPH = 6.4 Hz, PMe3), 0.79 (d, 3JPH = 18.1 Hz, 1H,H3), −0.47 (d, 3JPH= 22.5 Hz, 1H, anti-H1). 13C{1H} NMR (101MHz): δ = 148.3 (ipso-phenyl), 129.9 (phenyl), 127.8 (phenyl), 122.9(phenyl), 83.5 (C5Me5), 83.1 (C2), 44.2 (d, 2JPC = 5.5 Hz, C3), 30.9(d, 2JPC = 10.8 Hz, C1), 23.3 (CH3), 18.5 (d, 1JPC = 22.1 Hz, PMe3),10.8 (C5Me5).

31P{1H} NMR (162 MHz): δ = 32.8.[(η5-C5Me5)Fe(η

1-oIndMe)(dmpe)] (5). Complex 1 (0.120 g, 0.372mmol) was dissolved in pentane (8 mL). The addition of dmpe (0.056g, 0.372 mmol) in pentane (3 mL) initially resulted in a brownsolution, which eventually turned to red within 2 h. After solventremoval, the remaining red oil was taken up in a mixture of pentaneand hexamethyldisiloxane, filtered, and stored at −30 °C overnight,which resulted in the formation of red crystals (0.106 g, 60%). Singlecrystals for X-ray diffraction analysis were obtained from a pentanesolution at −20 °C.

1H NMR (400 MHz): δ = 7.38−7.33 (m, 2H, phenyl), 7.29−7.23(m, 2H, phenyl), 7.05 (m (“t”,) splitting = 7.3 Hz, 1H, p-phenyl), 6.32(s, 1H, H3), 2.01 (s, 3H, CH3), 1.61 (s, 15H, C5Me5), 1.53−1.47 (m,2H, P-CH2), 1.27−1.24 (m, 6H, P-CH3), 1.23−1.20 (m, 2H, H1),1.19−1.11 (m, 2H, P-CH2), 1.05−1.00 (m, 6H, P-CH3).

13C{1H}NMR (101 MHz): δ = 158.8 (C2 or ipso-phenyl), 141.9 (C2 or ipso-phenyl), 128.5 (phenyl), 128.3 (phenyl), 123.8 (phenyl), 115.6 (C3),84.3 (C5Me5), 28.9 (P-CH2), 22.5 (P-CH3), 22.3 (CH3), 14.8 (P-CH3), 11.1 (C5Me5), 10.3 (C1). 31P{1H} NMR (162 MHz): δ = 76.0.Anal. Calcd for C26H42FeP2: C, 66.10; H, 8.96. Found: C, 65.83; H,8.93.syn-[(η5-C5Me5)Fe(η

3-oIndMe)(I)] (6). Complex 1 (0.145 g, 0.450mmol) was dissolved in hexane (10 mL), followed by the addition ofCH3I (0.900 mmol, 0.56 mL). The color changed to red within 20 minupon stirring at 50 °C. After a total time of 1 h, the red solution wasfiltered, and the solvent was concentrated to ∼3 mL, accompanied bythe precipitation of the product. The supernatant was removed, andthe dark red microcrystalline solid was dried under vacuum (0.142 g,72%).The EI mass spectrum showed a molecular ion at m/z = 449 amu.

The parent ion isotopic cluster was simulated: (calcd %, observed %)451 (4, 4), 450 (26, 26), 449 (100, 100), 448 (2, 2), 447 (7, 7). Anal.Calcd for C20H26FeI: C, 53.48; H, 5.83. Found: C, 53.64; H, 5.88.syn-[(η5-C5Me5)Fe(η

3-oIndMe)(Cl)] (7). Dichloromethane (8 mL)was added to 1 (0.120 g, 0.372 mmol). The green solution graduallychanged to red within 50 min at room temperature. Filtration and

solvent removal gave a slightly oily solid, which was layered withpentane (5 mL) and stored at −30 °C overnight. The supernatant wasremoved, and the remaining dark red solid was dried (0.075 g, 56%).

The EI mass spectrum showed a molecular ion at m/z = 357 amu.The parent ion isotopic cluster was simulated: (calcd %, observed %)360 (10, 10), 359 (36, 36), 358 (35, 35), 357 (100, 100), 356 (2, 2),355 (7, 7). Anal. Calcd for C20H26ClFe: C, 67.15; H, 7.33. Found: C,67.36; H, 7.35.

syn-[(η5-Ind″)Fe(η3-oIndMe)(CO)] (9). [FeI2(THF)2] (1.0 g, 2.203mmol) was completely (!) dissolved in THF (30 mL), and Na(Ind″)(0.552 g, 2.203 mmol) in THF (10 mL) was slowly added with asyringe at −78 °C. The resulting red solution was stirred for 10 min at−78 °C, followed by the slow addition of K(oIndMe) (0.375 g, 2.203mmol) in THF (10 mL). During the slow warm-up, the color changedfrom brown to red, and the solvent was removed under vacuum.Extraction with pentane, followed by filtration, gave a red solution. COwas then passed through the solution for 1 min, which was notaccompanied by any obvious color change. After stirring for 15 minunder a CO atmosphere, the solvent was removed. Crystallization at−30 °C from a pentane/hexamethyldisiloxane mixture yielded 0.442mg (45%) of a red-brown solid. Single crystals were obtained bycooling a concentrated hexamethyldisiloxane solution to −30 °C forseveral days.

1H NMR (600 MHz): δ = 7.39 (m, (“d”), splitting 7.9 Hz, 2H, o-phenyl), 7.22 (m (“d”), splitting 8.9 Hz, 1H, indenyl), 7.14−7.06 (m,4H, m- and p-phenyl + indenyl), 6.71 (m, 1H, indenyl), 6.63 (m, 1H,indenyl), 4.87 (s, 1H, indenyl), 3.33 (s, 1H, syn-H1), 1.91 (s, 3H,CH3), 1.44 (s, 9H, tert-butyl), 1.18 (s, 9H, tert-butyl), 0.22 (s, 1H,H3), 0.15 (s, 1H, anti-H1). 13C{1H} NMR (151 MHz): δ = 222.2(CO), 143.5 (ipso-phenyl), 129.6 (phenyl), 128.3 (phenyl), 126.5 (CHindenyl), 125.8 (phenyl), 125.7 (CH indenyl), 125.1 (CH indenyl),124.7 (CH indenyl), 108.6 (C2), 107.9 (Cq indenyl), 104.8 (Cq

indenyl), 97.8 (Cq indenyl), 95.6 (Cq indenyl), 84.6 (CH indenyl),71.0 (C3), 50.3 (C1), 33.2 (Cq tert-butyl), 33.0 (Cq tert-butyl), 32.2(tert-butyl), 31.8 (tert-butyl), 20.0 (CH3). Anal. Calcd for C28H34FeO:C, 76.01; H, 7.75. Found: C, 75.14; H, 7.75. IR (Nujol): ν(CO/cm−1)= 1909.

■ ASSOCIATED CONTENT

*S Supporting InformationCrystallographic information files (CIF), NMR spectra of 1,details of the kinetic NMR study of 2, details of the conversionof syn-4, details of the DFT calculations and mol2-files of allcalculated structures, and comparison of experimental andtheoretical structural data. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (M.D.W.), [email protected](M.T.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Richard D. Ernst (University of Utah) for helpfuldiscussions and Richard A. Andersen and Greg Nocton(University of California, Berkeley) for SQUID measurements.This work was supported by the Fonds der ChemischenIndustrie (AG) and the Deutsche Forschungsgemeinschaft(DFG) through the Emmy-Noether program (MDW, WA2513/2-1). A.G. gratefully acknowledges the German AcademicExchange Service for a travel scholarship.



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Flexibility of an Open Indenyl Ligand in Iron(II) Complexes