Unusual Reduction of Ammonium Heptamolybdate to Novel Molybdenum(IV)-Stabilized Azo Anion Radical Complexes

In an unusual reaction of the polyacid, ammoniumheptamolybdate tetrahydrate ((NH₄)₆[Mo₇O₂₄]·4H₂O), and the ligand, 2-[(arylamino)phenylazo]pyridine (general abbreviation HL), in the presence of PPh₃ afforded the brown oxo free molybdenum complexes of type [Mo(L)₂] in high yields (ca. 80%). The reaction occurs smoothly in ethanol. It is slow on a steam bath (25 h) but is complete in about an hour in a microwave oven. X-ray structures of two representatives are reported. In these complexes the ligand acted as a tridentate ligand using its pyridyl(N), azo(N), and the deprotonated amine(N), respectively. The geometry is meridional, and the relative orientations within the coordinated pairs of nitrogens are cis, trans, and cis, respectively. Bond length data of the coordinated ligands are consistent with a Mo^IV[L^•]₂ ([L^•]^2- = azo dianion radical formed by one electron reduction of the deprotonated anionic ligand, [L]^-) description. For example, the N−N lengths (1.349(5)−1.357(2) Å) in these complexes are appreciably longer than that (1.246(3) Å) in the uncoordinated and protonated salt of a representative ligand, [H₂L^d]ClO₄. The N−N lengths, however, correspond well with metal complexes of the ligand containing azo ion radical. The complexes are diamagnetic and showed highly resolved ¹H NMR and ¹³C NMR spectra. The two coordinated ligands in these are magnetically equivalent, and resonances for only one ligand were observed in their spectra. Diamagnetism in the present molybdenum complexes is attributed to strong antiferromagnetic coupling between Mo^IV(4d²) and the two planar radical [L^•]^2- ligands. The complexes display multiple redox responses. The ESR spectrum of electrogenerated [1a]^- showed a characteristic spectrum for Mo(III) with weak hyperfine lines due to the presence of molybdenum isotopes having nonzero nuclear spin. Visible range multiple charge transfer transitions in these complexes are ascribed to ligand-to-metal transitions.