Abstract
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~]·2CH~3~CN (7). In the presence of methanol, dinuclear {(C~6~H~5~)~4~P}~2~[(MoOBr~4~)~2~(Pyz)]·2CH~3~CN (4) was transformed directly into tetranuclear (PyH)~4~[(Mo~2~O~4~Br~4~)~2~(Pyz)~2~]·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)]·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.