Trans-diazene

In the free state diazene can occur in three isomeric forms cis- and trans-diazene (12 and 13, respectively), and iso-diazene (14). 

Although trans-diazene is thermodynamically unstable with respect to its decomposition, the rate of this reaction is surprisingly slow. This is because of orbital symmetry and the sterically unfavorable trans configuration. Thermal conversion to the more reactive cis isomer is necessary for the reaction to occur. It has also been noted that trans-diazene reacts surprisingly slowly with HC1 in the gas phase (167). 

Iron carbonyl complexes are easily prepared by the reaction of Fe2(CO)9 with a cyclic diazene. An excess of the diazene prevents the more favoured formation of binuclear compounds29 3°). Trans-diazenes like trans-dimethyldiazene3 or azo-... 

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 . 

Aromatic trans-diazenes easily form orthometallated complexes, sometimes accompanied by cleavage of the N=N bond to give the o-semidine containing products. These reactions have been recently reviewed by Bruce and Goodall2a ... 

B. Different Stabilization Effects in Coordinated cis- and trans-Diazene... 

For a discussion of the complicated rotational energy curve of trans-diazene in complex 2(N2H2), the reader is referred to the original publication (82). 

N2 is stabilized by only 11.9 kJ/mol upon coordination to the metal fragment. This electronic stabilization effect is quenched by the zero-point vibrational level differences of 10.8 kJ/mol and becomes unfavorable in view of the reverse entropic effect. trans-Diazene experiences the largest stabilization by —60.3 kJ/mol, while the a-donating ligands are weakly bound. 

Although only the trans-diazene has been characterized experimentally, other four structures corresponding to two minima and two transition structures in the potential energy surface have been postulated by several theoretical investigations. As shown in Figure 5, trans-diazene can isomerizes into cis and sin isomers. 

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... 

An alternative type of diazene is typified by azobenzene (Eq. 16.70). Photolysis now does not lead to N2 extrusion, because the phenyl radicals that would be formed are too unstable. Instead, a fairly efficient cis-trans isomerization occurs. This process can be repeated m any ti mes, and since the cis and trans diazenes usually have substantially different absorption spectra, wavelengths can be chosen that strongly favor the cis or the trans form in the photostationary state. As discussed in a Connections highlight on photochromism on page 969, these factors have made azobenzene and derivatives favorites for the development of systems that are photochemically switchable between two forms. 

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