The chemistry of the hydrazido(1–)-ligand. Preparations and crystal structures of [Mo(NHNHCO2Me)(NNCO2Me)(S2CNMe2)2], [Mo(NHNMePh)(NNMePh)(S2CNMe2)2]BPh4, and [ReCl2(NHNHCOPh)(NNHCOPh)(PPh3)2]. Mechanism of formation of substituted hydrazines from [Mo(NNRPh)2(S2CNMe2)2](R = Me or Ph)

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Abstract
The preparation of three complexes containing a hydrazido(1–)-ligand is described, together with a kinetic study of the intermediacy of such a species in the hydrazine-forming reaction of acid with [Mo(NNRPh)2(S2CNR′2)2](R = R′= Me; R = Ph, R′= Et). Addition of 1 equivalent of HCl to [Mo(NNMePh)2(S2CNMe2)2] gives [Mo(NHNMePh)(NNMePh)(S2CNMe2)2]+ isolated as its BPh4 – salt [crystal data: space group P21/n, a= 16.102(3), b= 15.669(5), c= 18.752(2)Å, α= 90.00, β= 112.23(1), γ= 90.00°, Z= 4; 3 060 independent reflections gave an R value of 0.051]. Crystal data for [Mo(NHNHCO2Me)(N2CO2Me)(S2CNMe2)2], space group C2/c, a= 19.963(1), b= 18.855(1), c= 13.127(1)Å, α=γ= 90.0, β= 114.41(1)°, Z= 8; 1 585 independent reflections gave R 0.066. Both complexes show distorted pentagonal-bipyramidal geometries with an NN′-bonded hydrazido(1–)-ligand in the pseudo-pentagonal plane. Reaction of [ReCl2(N2COPh)(PPh3)2] with benzoylhydrazine gives [ReCl2(NHNHCOPh)(NNHCOPh)(PPh3)2][crystal data: space group P, a= 12.276(2), b= 14.147(2), c= 15.284(2)Å, α= 86.33(1), β= 72.04(1), γ= 83.10(1)°, Z= 2; 4 995 independent reflections gave R 0.042]. Initial protonation of [Mo(NNRPh)2(S2CNR′2)2] by HX (X = Cl or Br) gives [Mo(NHNRPh)(NNRPh)(S2CNR′2)2]+. Then, depending on R, ring closure occurs to give [Mo(NHNRPh)(NNRPh)(S2CNR′2)2]+ which competes with further protonation to give [Mo(NHNHRPh)(NNRPh)(S2CNR′2)2]2+. Solvent attack on the latter, followed by halide substitution, leads to further protonation of the hydrazide ligand leading to loss of hydrazinium salt and formation of [MoX2(NNRPh)(S2CNR′2)2]. The complex [Mo(NHNRPh)(NNRPh)(S2CNR′2)2]+ undergoes ring opening at too slow a rate for it to be an intermediate en route to hydrazine formation in excess of acid. The kinetics of the reactions of [Mo(NHNRPh)(NNRPh)(S2CNR′2)2]+ with a variety of bases suggests the formation of the transient NN′-bonded hydrazido(2–)-complex [Mo(NNRPh)(NNRPh)(S2CNR′2)2](R′= R = Me) prior to regeneration of the bis[hydrazido(2–)]-complex.