Pyrazine-Assisted Dimerization of Molybdenum(V): Synthesis and Structural Characterization of Novel Dinuclear and Tetranuclear Complexes

Reactions of pyrazine (Pyz = C(4)H(4)N(2)) with [MoOCl(4)(H(2)O)](-) or [MoOBr(4)](-) afforded a series of complexes, dinuclear [(MoOX(4))(2)(Pyz)](2-) (X = Cl, Br), [(MoOCl(3)(Pyz)](2)O](2-) and tetranuclear [(Mo(2)O(4)Cl(4))(2)(Pyz)(2)](4-) anions, with N-donor ligands engaged either in monodentate or bidentate bridging coordination. A dinuclear [(MoOX(4))(2)(Pyz)](2-) ion, which may be viewed as a linkage of two mononuclear subunits with a bidentate pyrazine, crystallizes either as a PyH(+) (pyridinium cation, C(5)H(5)NH(+); compound 1), a {(C(2)H(5))(4)N}(+) (compound 2) or a ((C(6)H(5))(4)P}(+)] salt (compounds 3 and 4). In (PyH)(2-){(MoOCl(3)(Pyz)}(2)O] (5) with an anti-{Mo(2)O(3)}(4+) core, pyrazine is bound to the metal through only one nitrogen atom. The tetranuclear [(Mo(2)O(4)Cl(4))2(Pyz)2]4- ion, a linkage of two metal-metal bonded {Mo(2)O(4)[(2+) cores with a pair of bidentate pyrazines, is found in two compounds, (MeNC(5)H(5))(2)(PyzH(2))-[(Mo(2)O(4)Cl(4))(2)(Pyz)(2)] (6) [MeNC(5)H(5)(+) = N-methylpyridinium cation, PyzH(2)(2+) = pyrazinium(2+) cation] and (PyH)(4)[(Mo(2)O(4)Cl(4))2(Pyz)(2)]center dot 2CH(3)CN (7). In the presence of methanol, dinuclear ((C(6)H(5))(4)P)(2)[(MoOBr(4))(2)(Pyz)]center dot 2CH(3)CN (4) was transformed directly into tetranuclear (PyH)(4)[(Mo(2)O(4)Br(4))(2)-(Pyz)(2)]center dot 2CH(3)CN (8). All compounds were fully characterized by X-ray crystallography and IR spectroscopy. The compounds containing the [(MoOX(4))2(Pyz)]2- ions display an irreversible electrochemical reduction behaviour. Temperature-dependent magnetic measurements on [(C(6)H(5))(4)P)(2-)[(MoOX(4))(2)(Pyz)]center dot 2CH(3)CN (X = Cl for 3, X = Br for 4) reveal a weak ferromagnetic coupling across the pyrazine bridge between a pair of Mo(v) S = 1/2 spins. The magnetic properties of ((C(2)H(5))(4)N)(2)[(MoOCl(4))(2)(Pyz)] (2), another compound with dinuclear complex anions, are dominated by antiferromagnetic intermolecular interactions.