Polynuclear copper(ii) pyrazolate complexes: temperature-dependent protonolysis reactions, crystal structures and high catalytic activity toward the condensation of nitriles with 2-aminoalcohol

Abstract

Reaction of Cu(OAc)₂·H₂O and 1H-pyrazole-3,5-dicarboxylic acid dimethyl ester (Hdcmpz) in MeOH at room temperature afforded one tetranuclear Cu(II)/pyrazolate complex [{Cu₂(μ-OAc)₂}₂(μ-dcmpz)₂(μ-OAc)₂] (1) in 89% yield. The similar reaction in refluxing MeOH produced a hexanuclear metallamacrocyclic Cu(II)/pyrazolate complex [{Cu(μ-dcmpz)}₂(μ-OMe)₂]₃ (2) in 85% yield. Treatment of the same components under solvothermal conditions resulted in the formation of another tetranuclear Cu(II)/pyrazolate/carboxylate complex [{Cu(MeOH)}₄(μ-mcccpz)₄] (3, Hmcccpz = 5(3)-(methoxycarbonyl)-1H-pyrazole-3(5)-carboxylic acid) in 30% yield. The mcccpz²⁻ ion in 3 was in situ generated via the hydrolysis of one of two esters on dcmpz ligand. Complexes 1–3 were characterized by elemental analysis, IR and single-crystal X-ray diffraction. An X-ray analysis revealed that 1 contains two {Cu(μ-OAc)}₂ fragments that are interconnected by two μ-η²,η²-dcmpz⁻ ligands and two μ-η¹,η¹-OAc⁻ ions, forming a unique tetrameric structure. Complex 2 is composed of three {Cu(μ-dcmpz)}₂ fragments linked by three pairs of μ-OMe⁻ anions, forming a metallamacrocyclic crown structure. 3 consists of four {Cu(MeOH)} fragments linked by two pairs of μ-η¹,η²-mcccpz²⁻ ligands, forming a tetrameric [2 × 2] grid-like structure. Complexes 1–3 displayed high catalytic activity toward the condensation of nitriles with 2-aminoalcohol under solvent-free conditions to produce various 2-oxazolines.