Diazene-coordinating complex

Fig. 4. Curve of rotation of trares-diazene coordinated in complex 1(N2H2) using the BP86/RI (left) and B3LYP (right) methods (TZVP basis set). The zero point of rotational energy is fixed arbitrarily. Note that the hydrogen atoms are not aligned with the S-Fe-S axes since they always point into the direction of the sulfurs lone pairs.

We demonstrated how the photoisomerization hypothesis can be supported by accurate quantum chemical calculations (103). The experimental infrared and resonance Raman study of complex 5 led to the first determination of normal modes and force constants of diazene coordinated to a metal fragment. Isotope substitution yielding 15N- and 2H-isotopomers permitted the assignment of diazene normal modes in the experimental spectrum. Moreover, the spectra of these three isotopomers indicated that a laser-induced photoisomerization occurred in the Raman sample. However, a detailed assignment of the split bands was not possible in the experiment. 

A,TV-Dimethyl-A -phenylthiourea has been shown to coordinate to Rh111 as an N—S bidentate involving four-membered chelate ring formation.154 N-Substituted thioamides also may bond in this manner.155 156 l-Amidino-2-thioureas (44) may behave either as N—S or as N—N bidentates, with this donor choice being dependent mainly on pH and the nature of the metal ion.157 As N—S donors they are known to stabilize lower oxidation states.158 As part of a study on Mo—S-containing complexes as models for redox-active molybdoenzymes, Dilworth et al. have shown that some p-(substituted)phenylhydrazines may coordinate as N—S bidentates in three different ways to one metal atom.159 The three diazenido, diazene and hydrazonido forms vary in their degree of deprotonation and therefore their anionic nature. 

Binuclear diazene complexes have been prepared in the meticulous and elegant work of Sellman and co-workers. The basis of the preparative route used in these studies is the oxidation of coordinated hydrazine by copper(II)-hydrogen peroxide mixtures] to yield dinitrogen complexes (291, 292). 

The synthesis of diazene complexes has been described (Sections V,B, C, and D). It is worth reiterating that no diazene complex has been prepared via reactions of coordinated dinitrogen and no stoichiometric conversion to hydrazine or ammonia has been demonstrated. 

Diazenes with carbonyl groups like bis(benzoyl)diazene prefer coordination at oxygen and nitrogen76 affording metallocycles 25. However, (CO)2Mo(S2CNEt2)2 probably also forms a a-complex.68 ... 

The total electronic coordination energy of frans-diazene is —73.1 kJ/mol including the estimated total hydrogen bond energy of (—)20 kJ/mol for the mononuclear complex irons-2(N2H2) (see above). Thus, the stabilization of the unstable diazene ligand by hydrogen... 

The bond strength of these N— H S bonds is not directly accessible by experiment. However, it has been estimated that they contribute 70 kJ mol-1 to the total stabilization of coordinated N2H2 (47, 126). This estimate is corroborated by recent DFT calculations (134), which support the assumption that N—H S bridges are a significant factor for stabilizing diazene in [M(S )]—N2H2 complexes. 

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