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 ] · 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.