Novel Methanol‐Containing Oxomolybdate(<scp>V</scp>) Complexes: Synthesis and Structural Characterisation of Intermediates in the Formation of {Mo₂O₄}²⁺ Clusters from [MoOCl₄(H₂O)]^– and [MoOBr₄]^– Precursors

Abstract

Abstract A series of oxomolybdate( V ) complexes with methanol was prepared by the reaction of (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 ( 1 ) or (PyH)[MoOBr 4 ] (PyH + = pyridinium cation, C 5 H 5 NH + ) in methanol. (PyH) 2 [MoOCl 4 (MeOH)]Cl ( 2 ) is obtained upon substitution of coordinated water for methanol in 1 . Small amounts of weak bases such as pyridine or trifluoroacetate promote further substitution chemistry at the labile sites, followed by dimerisation to the well‐known {Mo 2 O 4 } 2+ core. A dinuclear anion with coordinated methanol can be isolated as its pyridinium salt, (PyH) 2 [Mo 2 O 4 Cl 4 (MeOH) 2 ], in two crystalline modifications, either as a triclinic ( 3 ) or as an orthorhombic polymorph ( 4 ). After prolonged reaction times further assembly of dinuclear fragments takes place and tetranuclear [Mo 4 O 8 (OMe) 2 (MeOH) 2 Cl 4 ] 2– is formed. The products of the analogous reactions with (PyH)[MoOBr 4 ] as the starting material depend largely upon the reaction temperature. Dinuclear (PyH) 2 [Mo 2 O 4 Br 4 (MeOH) 2 ] ( 5 ), which crystallises in an orthorhombic unit cell and whose structure is isotypic to that of 4 , is obtained only if the reaction takes place at low temperature (5 °C); under ambient conditions (PyH) 2 [Mo 4 O 8 (OMe) 2 (MeOH) 2 Br 4 ] ( 6 ) is obtained instead. The tetranuclear anion of 6 consists of a commonly adopted assembly of two {Mo 2 O 4 } 2+ units. The identity of the products was determined by infrared spectroscopy and X‐ray structure analyses. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)