Syntheses and structures of copper complexes of 3-(6-(1H-pyrazol-1-yl)pyridin-2-yl)pyrazol-1-ide and their excellent performance in the syntheses of nitriles and aldehydes

Abstract

Reactions of a pincer ligand 2-(1H-pyrazol-1-yl)-6-(1H-pyrazol-3-yl)pyridine (pzpypzH) with Cu(NO₃)₂, Cu(ClO₄)₂, CuSO₄, CuCl₂ or CuI produced three dinuclear Cu(II) complexes [{Cu(NO₃)}(μ-pzpypz)]₂ (1), [{Cu(ClO₄)}(μ-pzpypz)]₂ (2), [Cu₂(μ-SO₄)(μ-pzpypz)₂]·2MeOH (3·2MeOH), one mononuclear Cu(II) complex [CuCl₂(pzpypzH)] (4) and one trinuclear Cu(I)/Cu(II) complex [(ICu)(μ-I)₂Cu₂(μ-pzpypz)₂] (5), respectively. Treatment of 4 with two equiv. of AgNO₃ in DMF also gave rise to 1. Complexes 1–5 were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Complex 1 or 2 has a dimeric structure in which two {Cu(X)} (X = NO₃, ClO₄) fragments are interconnected by two μ-pzpypz⁻ ligands. 3 also adopts a dimeric structure in which two Cu(II) centers are interconnected by a pair of μ-pzpypz⁻ ligands and one μ-SO₄²⁻ ion. The Cu(II) center in 4 is five-coordinated by three N atoms of the pzpypzH ligand and two Cl atoms. In 5, two Cu(II) centers are bridged by two μ-pzpypz⁻ ligands and one CuI₃²⁻ unit, forming a unique trinuclear structure. Complexes 1–5 displayed high catalytic activity toward the ammoxidation of alcohols to nitriles and the aerobic oxidation of alcohols to aldehydes in H₂O. The nitrile or aldehyde products could be readily separated from the catalytic system by extraction and the residual aqueous solution containing 1 retained good activity for several cycles.