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Transition Metal Complexes of Diazenes, XVII [1] Reactions of 1,2-Diazetines and 1,2-Diazetine-N-oxides with Iron Carbonyls A. Albini Istituto di Chimica Organica, Universitä di Pavia, 1-27100 Pavia

H. Kisch*

Max-Planck-Institut für Strahlenchemie, D-4330 Mülheim a. d. Ruhr

C. Krüger und An-Pei Chiang+ Max-Planck-Institut für Kohlenforschung, D-4330 Mülheim a. d. Ruhr Z. Naturforsch. 37 b, 463-467 (1982); received November 13, 1981 I,2-Diazetines, 1,2-Diazetine-N-oxide, Iron Complexes, X-ray

Enneacarbonyldiiron reacts with 3,3,4,4-tetramethyl-l,2-diazetine (L) or its mono-N-oxide (L') to form complexes (L)Fe(CO)4 (3), (L)Fe2(CO)7 (4), (L)Fe2(CO)6 (5), or (L')Fe(CO)4 (6), and small amounts of deoxygenated 4 and 5, respectively. Formation and stability of the complexes is compared with those of ligands with smaller and larger ring sizes and discussed in terms of the electronic properties of the ligand lone-pair Orbitals. The molecular structure of 4 is established by X-ray analysis.

1. Introduction

Mononuclear a-complexes derived from iron carbonyls and cyclic diazenes are generally not very stable, although they have been isolated in several cases [2-6]. Their initial formation is usually followed by further reactions to give more stable, binuclear complexes containing an Fe-Fe bond. The type of complex obtained depends on the ring size of the cyclic diazene. In the case of five- and six-membered rings there are formed complexes (diazene)Fe2(CO)7 and (diazene)Fe2(CO)6, contain-ing an intact diazene ring [7], whereas three-membered rings produce (R2C=N)2Fe2(CO)6 and (R2C=N)(NCO)Fe2(CO)6 by cleavage of the N = N

bond [8]. This different behaviour, as well as the stability of the mononuclear a-complexes, may be rationalized in terms of the spatial and electronic characteristics of the diazene "lone pair" orbitals and their dependence on the ring size [7]. In this connection we have studied the reactions of four-membered diazenes (1,2-diazet-l-ines) with metal carbonyls. The coordinating properties of 1,2-diazetines have not been explored until recently. One single paper reports [9] that 3,3,4,4-tetra-fluoro-l,2-diazetine is decomposed to C2F4, N2 and (CN)2 upon heating to 180 °C in the presence of Fe(CO)5. In the following we report on the reactions of 3,3,4,4-tetramethyl-l,2-diazetine (1) and its mono-N-oxide (2) with iron carbonyls.

Me H—N 11 4 - N Me

F«2(CO)9 Me yrFe (C0)4

Me

M e - h - N v J / M e (C0)3

Me _Fe(CC»3 Me 4 — N C ? I 3

Me J N I M e ^ F e ( C 0 ) 3

Scheme 1.

2. Results Irradiation of pentacarbonyhron in the presence

of diazetine 1 produces only small amounts of com-

+ Permanent address: Lanchow Institute of Chemical Physics, Academia Sinica, Lanchow, China.

* Reprint requests to Priv.-Doz. Dr. H. Kisch. 0340-5087/82/0400-0463/$ 01.00/0

plexes. Better yields are obtained if 1 and enneacar-bonyldiiron in the molar ratio 1:2, are allowed to react thermally at room temperature. Two com-plexes are formed depending on the solvent, a red-brown one (4) which is the major product (88%) in tetrahydrofuran, and a yellow one (5) being pre-dominantly obtained (50%) in the non-coordinating solvent cyclohexane. This influence of the solvent is

464 A. Albini-H. Kisch • Transition Metal Complexes of Diazenes 464

Table I. Solvent effect on the reaction of 1 and 2 with F©2(CO)9 (molar ratio 1:2) at room temperature.

Substrate Solvent Reaction time [h]

Yield [ % ] 4 5 6

1 Cyclohexane 8 33 50 -1 Tetrahydrofuran 1.5 88 7 -2 Cyclohexane 15 2 39 58 2 Tetrahydrofuran 2 2 5 67

summarized in Table I. The reactions are much faster in tetrahydrofuran than in cyclohexane or benzene. Open chain ethers like diethylether give rise to low reaction rates and small product yields.

The red-brown complex 4 has the composition 1 •Fe2(CO)7 as indicated by the analytical data. The mass spectrum exhibits successive loss of seven CO groups from the molecular ion. In addition to four

Table II. Physical properties of complexes 3-6.

Complex IR [cm -1]a U Y [Amax, k K ( lg e ) ] a m NMR [<5]b MSC

3d 2056 s, 1974 s, 1955 vs, 1937 s _ _ _ 4 2063 s, 2014 vs, 1996 s, 1976 s, 1831 m 35.5 (4.18), 28.9 (4.0), 17.4 (3.2) 0.45e 420 5 2072 s, 2024 vs, 1997 vs, 1974 s, 1964 s 33.5 (4.44), 28.6 (3.42) 0.7e 392 6 2052 s, 1980s, 1963 vs, 1941 vs, 1580 mr 37.8 (3.68), 25.0 (3.11) 0.658 296

a In n-hexane; b in benzene-de; c peak of highest m/e-ratio, corresponds to M+; d not obta ined analytical pure; e free ligand signal at 0.95 <5; 1 free ligand absorption at 1545 cm - 1 in KBr; ® free ligand signal at 0.9 <3.

Table III. Cell data, bond lengths, and bond angles of 4*. a) Cell data (CH3)4CN=NCFe2(CO)7 a — b = c —

9.816(1) Ä 14.046(1) 12.936(1)

ß = 98.631(6)° V = 1763.364 Ä3 b) Distances in Ä

Space group — P 2i/n Z = 4 D c = 1.582 gem"1

3978 Reflections, of which 3077 observed (2 a) R = 0.03 (7?w = 0.034)

Fe 1-Fe 2 2.623(1) C 5 - 0 5 1.135(3) Fe1-C1 1.989(2) C6 -0 6 1.135(3) Fe2-C1 2.051(2) C7 -0 7 1.133(4) Fe1-C2 1.834(3) Fe 1-N1 1.903(2) Fe1-C3 1.792(3) Fe 2-N 2 1.910(2) Fe1-C4 1.801(2) N1-N2 1.278(2) Fe2-C5 1.793(3) N 1-C 8 1.502(3) Fe 2-C6 1.802(3) N 2-C11 1.517(3) Fe 2-C7 1.818(3) C8-C9 1.522(4) C l - O l 1.165(1) C8-C10 1.517(4) C2 -0 2 1.135(4) CI 1-C 12 1.519(4) C3 -0 3 1.131(3) C11-C 13 1.517(4) C 4 - 0 4 1.132(3) C11-C 8 1.595(3) c) Bond angles in Fe 1-C 1-Fe 2 N 1-Fe 1-C 1 N 2-Fe 2-C1 N l - F e 1-C 2 N 2-Fe 2-C 7 C 1-Fe 1-C 3 C 1-Fe 2-C 5 C 2-Fe 1-C 4 C3-Fe 1-C 4 C 5-Fe 2-C 6 C 6-Fe 2-C 7 Fe 1-C 1 - 0 1 Fe 2-C l - O l

80.9(1) 83.7(1) 84.2(1) 92.7(1) 93.2(1) 85.7(1) 85.2(1)

108.3(1) 91.7(1) 96.0(1)

104.8(1) 140.8(2) 138.0(2)

Fe l - C - 0 (av) Fe 2-C-O (av) Fe 1 -N1-N2 Fe 2-N 2-N1 N2-N 1-C 8 N1-N2-C11 N1-C8-C9 C9-C8-C10 C12-C11-C13 N1-C8-C11 N 2-C 11-C 8 N 2-C 11-C 12

179.0(9) 178.6(5) 110.8(1) 110.5(1) 96.4(2) 95.5(1)

110.4(2) 111.8(2) 111.8(2) 84.1(1) 83.8(1)

112.2(2)

Further details are obtainable from Fachinforma-tionszentrum Energie-Physik-Mathematik, D-7514 Eggenstein-Leopoldshafen.

v(CO) absorptions of terminal metal carbonyl groups at 2063-1976 cm - 1 the IR spectrum contains one band in the region of bridging CO groups at 1831 cm - 1 . A symmetrical coordination of the diazetine to the Fe2(CO)7 moiety is indicated by the XH NMR spectrum containing only one signal for the four methyl groups (see Table II), shifted by 0.5 6 to high field compared to free 1. Although complexes of this type have been obtained with six- and five-membered diazenes [10], we have ascertained the structure of this first four-membered derivative by a single crystal x-ray analysis.

Experimental details of the structural analysis of 4 are summarized in Table III. The molecular structure is shown in Fig. 1. In its general geometric appearence the structure of 4 is similar to that established for (pyridazine)Fe2(CO)7, an analogous complex of a six-membered 1,2-diazine [11]. Both Fe(C0)3-groups are linked by a bridging carbonyl group (CiOi), a metal-metal contact and through the two nitrogen atoms of the ligand. The trigonal bipyramidal Fe(CO)3-groups are in an eclipsed con-formation with respect to each other, as seen along the Fe-Fe vector. Planes defined by FeiFe2Ci and FeiFe2NiN2 intersect at an angle of 101.23°. The four-membered ligand ring, although slightly twisted, with deviations of the mean plane of ± 0.02 A for Ni and N2 and ±0 .018 Ä for C8 and Cn, is coplanar with the Fe2N2 ring. The Fe-Fe distance (2.623 Ä) is elongated significantly by 0.05 Ä as compared to the corresponding distance

A. Albini-H. Kisch • Transition Metal Complexes of Diazenes 465

in (pyridazine)Fe2(CO)7 (2.573(1)). This may reflect the impact of steric strain in the four-membered ring upon the directional properties of the lone pairs at both nitrogen atoms [8]. The corresponding Fe-N-distances are shortened by 0.05 Ä. All further molecular parameters are in accordance with known values.

The proposed structure of complex 5 is based on spectral and analytical data (Table II) and their comparison with the established structure of (2,3-diazanorbornene)Fe2(CO)6 [12]. The pattern of the v(CO) IR absorptions is almost identical for both compounds; from the fact that in 5 all bands are shifted to higher wavenumbers by about 4 cm -1, it may be concluded that 1 has better jr-accepting and/or poorer a-donating properties as compared with the larger-ring-size ligand 2,3-diazanorbornene.

In the search for mononuclear complexes, ennea-carbonyldiiron was reacted with an excess of 1 (molar ratio 1:2) in tetrahydrofuran at —20 °C. The reaction was stopped at low conversion to prevent formation of binuclear complexes. Low-temperature chromatography yields small amounts of 4 and a new yellow-brown complex 3. Due to its instability we have not been able to obtain 3 analytically pure. On the basis of the striking similarity of the IR spectrum with that of (2,3-diazanorbornene)Fe(CO)4 [5] and its smooth con-version into 4 and 5 upon reaction with Fe2(CO)9, we propose that 1 is bonded via one lone pair to the

Fe(CO)4 unit as shown in Scheme 1. Complex 3 is less stable than analogous complexes of this type containing diazenes of larger ring size [2, 13].

M e - L _ N M e | - N

Me H N Me-j N 1 v0 N)

Me Me

2 6 Scheme 2.

The reaction of the diazetine mono-N-oxide 2 with enneacarbonyldiiron is again faster in tetra-hydrofuran than in less coordinating solvents (Table I). As the major product one obtains a yellow complex 6 and small amounts of 4 and 5. From analytical and spectral data the structure of an N-a-bonded Fe(CO)4 complex as shown in Scheme 2 is proposed for 6 (Table II). The presence of the N 0 bond seems to have only a very small in-fluence on the nature of the Fe-N bond as indicated by pattern and position of the v(CO) absorptions almost identical with those of 3 and other (diazene)-Fe(CO)4 compounds. The N - 0 vibration is shifted only slightly to higher frequency upon coordination.

3. Discussion

These results demonstrate that 1,2-diazetines have similar coordinating properties towards iron carbonyls as five- and six-membered diazenes and support the previous rationalisation of the excep-tional behaviour of the three-membered diazirines. In the latter case, further reaction of the initially formed intermediate (diazirine)Fe(CO)4 gives rise to isolable (diazirine)[Fe(CO)4]2. The stability of these bis(tetracarbonyliron) complexes seems to arise from a large Fe-"Fe distance originating in the spatial properties of the two "lone-pair" molecular orbitals. In the case of the four-membered diazetine ring this distance should decrease and Fe-Fe bond formation should therefore be favoured. This may lead to a stability of the bis(tetracarbonyliron) complex too low to allow isolation. Instead, as in the case of five- and six-membered diazenes, the more stable Fe2(CO)7- and Fe2(CO)6-complexes are obtained. In the case of the three-membered diazirines Fe-Fe bond formation eventually may occur via ring opening and cleavage of the N = N bond [8].

466 A. Albini-H. Kisch • Transition Metal Complexes of Diazenes 466

The stability of the mononuclear a- complex 3 lies between those of the analogous complexes of three- and five- or six-membered diazenes. In the case of the diazirines this type of complex is ob-servable as an intermediate only, in the case of diazetines it is isolable in an impure state but with five- and six-membered diazenes it is obtained analytically pure. This sequence of growing stability parallels the increasing lone-pair character of the highest occupied molecular orbital (ni) (Table IV). This wi-MO is localized [14] on the diazene group to 32, 42 and 67% in diazirine, 1,2-diazetine and eis-dimethyldiazene (as model for a six-membered ring), respectively. Accordingly, the strength of the Fe-N bond should increase due to an increasing inter-action of ni with an empty metal orbital.

Table IV. Composition of frontier molecular orbitals in diazenes [14].

Diazene MOa Bond character [%] N = N N-C C-H LP

cis-Dimethyl- bi (m) 0 29 4 67 diazene b2(?r) 85 0 15 0

ai (n2) 9 0 22 69 1,2-Diazet-l-ine bi (ni) 0 50 8 42

ax (n2)b 12 0 15 37 b2(jr) 80 0 20 0

Diazirine bi (ni) 0 68 0 32 b8(jr) 74 0 26 0 ai (n2) 16 6 0 78

a Arranged in sequence of decreasing energy; b has also 36% C-C bond character.

The tetracarbonyliron complex 6 is, to our knowledge, the first metal carbonyl derivative of an aliphatic azoxy compound. From the striking simi-larity of the pattern and position of the four v(CO) bands to that of analogous diazene complexes, it is concluded that the N = N ( 0 ) group bonds to the metal like an undisturbed N = N group and not like an amine, in which case one should expect the appearance of only three v(CO) bands [5, 7]. From this one may generalize that cis-azoxy compounds are able to form a-complexes with transition metals in the same way as ds-diazenes.

Formation of 4 and 5 from the N-oxide 2 does not proceed via intermediate 6 which is stable under the reaction conditions. Although the corresponding N,N'-dioxide is essentially unreactive in the pres-ence of Fe2(C0)g (only traces of 5 are formed after

prolonged reaction) it cannot be decided if deoxy-genation occurs via initial coordination at oxygen or nitrogen.

Experimental

All experiments with metal carbonyls were per-formed in an argon atmosphere. THF and diethyl -ether were distilled from LiAlH,*, hydrocarbon solvents from sodium before use. 3,3,4,4-Tetra-methyl- 1,2-diazetine [15], its mono N-oxide [15] and its N,N'-dioxide [16] were prepared and purified according to the literature. IR spectra were measured on Perkin-Elmer 257 and 680 instruments, NMR spectra on Perkin-Elmer R 12, mass spectra on Du Pont 492-B and MATCH 5. Melting points were determined in a closed cappillary. For spectral data see Table II.

Preparation of 4-6 General procedure: 1.5 mmol of the ligand (1 or 2)

was added to a suspension of 1.1 g (3 mmol) of Fe2(CO)9 in the appropriate solvent (Table I). The suspension was stirred at room temperature to complete dissolution of the iron carbonyl and filtered. Evaporation of the solvent at < 5 °C was followed by chromatography on silicagel (50 g) at —20 °C using petroleum ether-toluene mixtures. The frac-tions were collected at —20 °C and solvents evaporated at the same temperature. Analytically pure compounds were obtained by crystallisation from toluene-petroleum ether at —70 °C. Yields of isolated products are summarized in Table I.

4: dark green plates, m.p. = 84 °C (complete conversion to 5).

Elemental analysis for CizH\2FezNz07

Found C 37.14 H 2.88 N 6.52, Calcd C 37.14 H 2.86 N 6.67.

5: yellow needles, m.p. = 156 °C. Elemental analysis for CiiHiiFeiNzOs

Found C 36.59 H 3.11 N 6.93, Calcd C 36.73 H 3.06 N 7.14.

6: yellow needles, m.p. = 154 °C. Elemental analysis for CioH\iFeNiOs

Found C 40.41 H 4.03 N 9.40, Calcd C 40.54 H 4.05 N 9.46.

Preparation of 3: As described above but the molar ratio of l/Fe2(CO)9 was reversed and the experiment was conducted in THF at —20 °C. 3 was obtained as an impure yellow powder decomposing at room temperature to 4 and 5.

We are indebted to Consiglio Nazionale delle Ricerche, Roma, for support of this work and to Dr. G. Olbrich for performing the m.o. calculations.

A. Albini-H. Kisch • Transition Metal Complexes of Diazenes 467

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