Cis-diazene

Dialkyldiazenes (15, R—alkyl) are sources of alkyl radicals. While there is dear evidence for the transient existence of diazcnyl radicals (17 Scheme 3.18) during the decomposition of certain unsymmetrieal diazenes49 51 and of cis-diazenes,54 all isolable products formed in thermolysis or photolysis of dialkyldiazenes (15) are attributable to the reactions of alkyl radicals. 

As in the case of (cis-diazene)Fe(C0)4 complexes the diazeno ligand disturbs the local symmetry (C4v) in L-M(CO)s and gives rise to four or five v(CO) bands instead of three. The low-energy absorption band of (benzo[c]cinnoline) M(CO)s (M=Cr, Mo, W) was assigned to an rt- -n transition5 ). However, in view of the assignments made for the spectra of LCr(CO)s compounds (L = amines, phosphines)52) this transition may be rather of "d-d" or CTML character. 

Bridged binuclear a-complexes containing the cis-diazene group fixed in a three-membered ring were prepared from diazirines by reaction with Fe2(CO)9 19 or... 

Reinvestigation30 of the reaction of diazanorbomene with Fe(CO)5 led to the isolation of an interesting (cis-diazene) Fe3(CO)9 cluster of proposed structure 26. The same type of clusters may be prepared from Ru3(CO)12 and cyclic diazenes81. ... 

From the fact that trans-diazenes do not form any of the complexes described, it may be concluded that the initial interaction of the N=N bond with the metal is largely determined by the symmetry of the HOMO (n i)89. Both trans and cis-diazenes possess similar ionisation potentials but only the latter have a HOMO of proper symmetry to overlap with the LUMO (dxy, d, dyz or dx2. y ) of the reacting iron carbonyl species. 

Here again the scheme applies only for cis-diazenes. However, as already mentioned, trans bis(cyclohexyl)diazene yields the corresponding (trans-diazene) Cr(CO)5 complex by reaction with CrfCO)sTHF45). Another interesting complex containing two different metals bound to one N=N group (VIII, M = M1, M = Cr, M = Mo) was prepared from bis(diazanorbornene)Cr(CO)4 and (nor-C Hg )Mo(CO)4 by Acker-mann and Kou41 . 

We find that cis-diazene is more strongly bound in both model complexes than irans-diazene. This is apparent in view of the Fe-N bond distances For 1(N2H2) we find a about 4 pm shorter Fe-N distance for the cis-isomer when compared with the trans-form, while this distance is 2-3 pm shorter in cis-2(N2H2) than in trans-2(N2H2). 

The three diazene isomers are all stabilized significantly upon coordination to Fe by the direct coordination energy as well as by hydrogen bonding. They should all be taken into account as possible stable intermediates in the FeMoco mechanism. Different open FeMoco structures could favor different isomers while traras-diazene needs a larger Fe-Fe distance than cis-diazene, cis-diazene, and especially NNH2 will become important if FeMoco opens only little. 

Isomerization mechanisms of isolated trans- to cis-diazene were studied and transition states for two possible interconversion routes were found to be more than 200 kJ/mol unstable than traras-diazene (98). The relative stability of trans- to cis-diazene was calculated to be 21 to 29 kJ/mol using different quantum chemical methods and basis sets [see Ref. (98) and references cited therein]. The NNH2 isomer is about 87 kJ/mol (almost independent of the density functional) higher in energy than traras-diazene. 

Bonds to Nitrogen.—The N2H2 Molecule. Three possible configurational isomers can be envisaged for diazene (di-imine), viz. trans-diazene (1), cis-diazene (2), and isodiazene (3). Two methods have hitherto been available for the preparation... 

Figure 7.27 (a) Stereoelectronics are outweighed by sterics and electrostatics in the ground state conformation of diazene. (b) cis-hydrogenations occur via a concerted cyclic transition state from the less stable cis-diazene. 

The stereochemical isomerization implies a competition between ring closure and bond rotation in the biradical reactive intermediate. As with tetramethylene, a competition is supported by studies of appropriately substituted diazenes which, on thermolysis, lose N2 and presumably produce the biradical (Eq. 11.84). However, in an early indication that things are not as simple as with tetramethylene, the diazene experiments show a "cross-over" effect. The cis diazene preferably produces trans cyclopropane, and vice versa. 

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