A Series of Molybdenum(<scp>V</scp>) Complexes with the Oxalato Ligand Engaged in Different Binding Roles − An Unusual Staggered Conformation of the μ₄‐Oxalate in [{Mo₂O₄(η²‐C₂O₄)₂}₂(μ₄‐C₂O₄)]⁶⁻

Abstract

Abstract Solvothermal reactions between mononuclear halooxomolybdates( V ) and oxalic acid afforded smaller oligonuclear complexes with the oxalate ion engaged in different binding modes. The oxalate ion coordinates as a bidentate terminal ligand to each metal centre of the {Mo 2 O 4 } 2+ unit to form the anion of (MeNC 5 H 5 ) 2 [Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 py 2 ] ( 1 ; py = pyridine; MeNC 5 H 5 + = N ‐methylpyridinium cation). (MeNC 5 H 5 ) 6 [{Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 } 2 (μ 4 ‐C 2 O 4 )] ( 2 ) with a tetradentate oxalate ion connecting two dinuclear subunits is a very rare example of a coordination compound with the oxalato ligand in a completely staggered conformation: the two CO 2 entities of the μ 4 ‐oxalate ion are rotated by 90° with respect to each other. Ab initio MO calculations on [{Mo 2 O 4 (η 2 ‐C 2 O 4 ) 2 } 2 (μ 4 ‐C 2 O 4 )] 6− revealed the isomer with the staggered conformation of the μ 4 ‐oxalate ion to be of the lowest energy. The cyclic octanuclear anion of (MeNC 5 H 5 ) 2 [Mo 8 O 16 (OMe) 8 (μ 8 ‐C 2 O 4 )] ( 3 ) consists of four dinuclear subunits arranged around the central oxalate ion and additionally linked by pairs of methoxide bridges. Each oxygen atom of the oxalate ion bridges a pair of metal centres not connected by a metal−metal bond. The anionic part of [MeNC 5 H 2 (Me) 3 ] 2 [HNC 5 H 2 (Me) 3 ] 2 [Mo 2 O 3 (η 2 ‐C 2 O 4 ) 4 ] ( 4 ) [MeNC 5 H 2 (Me) 3 + = N ‐methylcollidinium cation; HNC 5 H 2 (Me) 3 + = protonated 2,4,6‐collidine] possesses an anti ‐{Mo 2 O 3 } 4+ structural fragment with a pair of bidentate oxalate ions coordinated to each molybdenum atom. The compounds were fully characterized by X‐ray structural analysis and infrared spectroscopy. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)