Transition with diazenes

It should be mentioned that consecutive two-electron reduction of dinitrogen is still usually considered likely in the literature, with diazene, N2H2, as an intermediate but stabilized by complexation with transition metals. At the same time there is no doubt that both 7r-bonds of N2 can be used simultaneously for four d-electrons (entering two degenerate 7ig -orbitals) from one, two, or several metal atoms forming a complex with dinitrogen, and separate cleavage of each 71-bond seems to be unnecessary, more so because it is thermodynamically unfavorable. Therefore, for-... 

From Table IV the relative magnitudes of the reactant state "sensitivity factor" (N) are 4>1>2=3= zero. From this analysis the decomposition rates of traiw-phenyl, alkyl diazenes (2) and iert-butyl peresters (3) can be predicted by assuming a dependence only on transition state effects, with no need to incorporate the back strain of the reactants into the equation. 

Mdssbauer spectra of bonding and structure in, 15 184-187 reactions with diborane, 16 213 stabilization of, 5 17, 18-19 cyanates, 17 297, 298 cyanide complexes of, 8 143-144 cyclometallated bipyridine complex, 30 76 diazene complexes, 27 231-232 dinitrogen complexes, 27 215, 217 diphosphine complexes of, 14 208-219 dithiocarbamates, 23 253-254 -1,2-dithiolene complexes, 22 323-327 hydrogen bonding, 22 327 halide complexes with phosphine, etc., 6 25 hexaflouride, structure, 27 104 hydride complexes, 20 235, 248-281, see also Transition metal-hydride complexes... 

The redox reactions of hydrazine toward main-group and transition metal oxidants have been reviewed (73). Different stoichiometries have been found, with N2 appearing as the N-containing oxidized product, sometimes accompanied by the formation of NH3 and/or HN3. The mechanisms have been analyzed in terms of the one- or two-electron nature of the oxidants, and imply both outer-and inner-sphere routes, depending on the oxidant. The very reactive, key intermediate, diazene (diimide), N2H2, has been proposed in most of these reactions. 

To provide a specific example of die method, near UV experiments have led to assignments of the vertical and adiabatic excitation energies for die I B PAg transition in A-diazene (HN=NH), where the Bg state is open-shell. Table 14.4 compares sum-method predictions at the UHF and BLYP levels of theory to diese experimental values, and also to published results at the MRCI level of theory. For diis system, die HF results are systematically too high, and the DFT too low (cf. the sum method prediction for A2 phenylnitrene in Table 14.1), but are competitive with the much more expensive MRCI results. Note that all three levels do quite well at predicting the difference in verdcal and adiabatic excitation energies. 

The reaction of phenylhydrazine with copper(II) chloride in aqueous solution gives [Cu4Cl4(PhNNH)] (13) and the reactions of 1,2-disubsti-tuted hydrazines with copper(II) salts give complexes such as [Cu2-Cl2(MeNNMe)l (46, 142). The reaction of substituted hydrazines (or lithiated, substituted hydrazines) with halido complexes of the transition metals can yield diazene complexes. Complexes of Ni, Pd, Pt (147, 199,213), and Rh (209) have been prepared in this manner [Eq. (25)]. 

The reaction of transition metal oxo complexes with substituted hy-drazines yields the diazene complexes, containing the M—NRNC(R)0 chelate. 

The relative instability of free diazenes (215) often precludes the direct reaction of these species with transition metal complexes. However azobenzene reacts with Ni° (264) species to yield materials such as [Ni(tBuNC)2(PhN2Ph)] (140, 265). Similarly, ditolyldiazene reacts with [Ni(cyclooctadiene)2] in the presence of tritolylphosphine to give [Ni(N2Ar2)(PAr3)2] (Ar = p-CH3C6H4) (200). 

Relevant ab initio calculations with the 6-316 basis set on the thiaziridine-related E- and Z-thiadiaziridine, E- and Z-diazene A-sulfide and the transition structures connecting them, gave the corresponding energies (in kJ mol-1) relative to the most stable structure—Z-diazene A-sulfide—as depicted in Scheme 4 <85PC 113-01). 

Oxidation of tricarbonylcyclobutadieneiron in the presence of l,2-bis-(p-tosyl-ethoxycarbonyl)diazene gave the diazabicyclo[2,2,0]hexene (232a), which was easily converted into 2,3-diazabicyclo[2,2,0]hexene (232b).Mild oxidation of (232) was expected to give diaza-Dewar-benzene (233). This compound could not be isolated, as it loses nitrogen to give cyclobutadiene, which was detected by trapping with dienophiles or by the formation of its dimer. This sequence affords a useful route to cyclobutadiene from transition-metal-free precursors. 

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