Binuclear iron and ruthenium complexes with bis(diazene) or bis(diazenido) bridging ligands: Synthesis, characterization, X-ray crystal structure, and electrochemical studies

Binuclear complexes [(MHP(4))(2)(mu-HN=NAr-ArN=NH)](BPh(4))(2) [M = Fe, Ru; P = P(OMe)(3), P(OEt)(3), PPh(OEt)(2); Ar-Ar = 4,4'-C6H4-C6H4, 4,4'-(2-CH3)C6H3-C6H3(2-CH3), 4,4'-C6H4-CH2-C6H4] were prepared by allowing hydride species MH(2)P(4) to react with bis(diazonium) salts (N2Ar-ArN2)(BF4)(2) in a 2:1 ratio at low temperature. The compounds were characterized by H-1 and P-31{H-1} NMR spectroscopy (including N-15 isotopic substitution) and an X-ray crystal structure determination. The complex [{FeH(P(OEt)(3))(4)}(2){mu-4,4'-HN=N(2-CH3)C6H3-C6H3(2-CH3)N=NH}](BPh(4))(2) crystallizes in the space group P (1) over bar with a 16.105(5), b = 17.551(6), c = 11.539(3) Angstrom; alpha = 96.41(2), beta = 84.49(2), gamma = 97.97(2)degrees; and Z = 2 The structure is centrosymmetric and consists of binuclear cations with the two iron atoms in an octahedral environment having the hydride and the diazene ligands in mutually cis positions. The [(MHP(4))(2)(mu-HN=NAr-ArN=NH)](2+) cations reacted with mono(diazonium) (RN(2))BF4 salts to give the binuclear bis(diazene) derivatives [{M(RN=NH)P-4}(2)(mu-HN=NAr-ArN=NH)](BPh(4))(4) (R = C6H5, 4-CH3C6H4, 4-CH3OC6H4). Binuclear bis(diazenido) complexes [(MP(4))(2)(mu-N2Ar-ArN2)](BPh(4))2 [M = Fe, Ru; P P(OEt)(3); Ar-Ar = 4,4'-C6H4-C6H4, 4,4'-C6H4-CH2-C6H4] were also prepared by deprotonation with triethylamine of the diazene-nitrile [M{4-CH3C6H4CN)P-4}(2)(mu-HN=NAr-ArN=NH)](BPh(4))(4) derivatives. Characterization of all the complexes by infrared and H-1 and P-31 NMR spectroscopy is discussed. Cyclic voltammetry of the binuclear [(MHP(4))(2)(mu-HN=NAr-ArN=NH)](2+) and of the related mononuclear [MH(RN=NH)P-4](+) derivatives was also investigated. Electrochemical reduction of binuclear compounds, where conjugation between the metal centers is possible, proceeds via two well-separated one-electron reduction steps whereas, when conjugation is hindered, the two consecutive reductions tend to merge into a onestep reduction process.