Substitution chemistry of MM quadruply bonded complexes (M = Mo or W) supported by the anion of 2-hydroxy-6-methylpyridine

Abstract

The compounds M₂(mhp)₄, where M = Mo or W and mhp is the anion formed from deprotonation of 2-hydroxy-6-methylpyridine, are shown to react with carboxylic acids RCOOH to give an equilibrium mixture of products M₂(O₂CR)_n(mhp)_4−n where R = 2-thienyl and phenyl. The equilibrium can be moved in favor of M₂(O₂CR)₄ by the addition of excess acid or by the favorable crystallization of these products. The latter provides a facile synthesis of the W₂(O₂CR)₄ compound where R = 9-anthracene. Reactions involving 2,4,6-triisopropyl benzoic acid, TiPBH, yield M₂(TiPB)₂(mhp)₂ compounds as thermodynamic products. Reactions involving Me₃OBF₄ (1 and 2 equiv.) yield the complexes Mo₂(mhp)₃(CH₃CN)₂BF₄ and Mo₂(mhp)₂(CH₃CN)₄(BF₄)₂, respectively. The latter compound has been structurally characterized and shown to have mirror symmetry with two cis mhp ligands: MoMo = 2.1242(5) Å, Mo–O = 2.035(2) Å, Mo–N(mhp) = 2.161(2) Å, and Mo–N(CH₃CN) = 2.160(3) and 2.170(3) Å. Reactions involving Mo₂(mhp)₃(CH₃CN)₂²⁺ and Mo₂(mhp)₂(CH₃CN)₄²⁺ with ⁿBu₄NO₂CMe (1 and 2 equiv.) yield the complexes Mo₂(mhp)₃(O₂CMe) and Mo₂(mhp)₂(O₂CMe)₂ which are shown to be kinetically labile to ligand scrambling. Reactions between Mo₂(mhp)₃(CH₃CN)₂⁺BF₄⁻ (2 equiv.) and [ⁿBu₄N⁺]₂[O₂C–X–CO₂]²⁻ yielded dimers of dimers [Mo₂(mhp)₃]₂(µ-O₂C–X–CO₂] where X = nothing, 2,5- or 3,4-thienyl and 1,4-C₆H₄. Reactions between Mo₂(mhp)₂(CH₃CN)₄²⁺(BF₄⁻)₂ and tetra-n-butylammonium oxalate and terephthalate yield compounds [Mo(mhp)₂bridge]_n which by MALDI-TOF MS are proposed to be a mixture of molecular squares (n = 4) and triangles (n = 3) along with minor products of [Mo₂(mhp)₃]₂(bridge) and Mo₂(mhp)₄ that arise from ligand scrambling.