Reaction with dioxygen of a Cu(I) complex of 1-benzyl-[3-(2′-pyridyl)]pyrazole triggers ethyl acetate hydrolysis: acetato-/pyrazolato-, dihydroxo- and diacetato-bridged Cu(II) complexes

A copper(I) compound [(L²)Cu(MeCN)₂][ClO₄] (1) containing a new bidentate N-donor ligand L², 1-benzyl-[3-(2′-pyridyl)]pyrazole, derived from the condensation of HL¹ [HL¹ = 3-(2-pyridyl)pyrazole] and benzyl chloride, has been synthesized. Structural analysis reveals that in 1 the copper(I) centre is coordinated by a pyridine and a pyrazole nitrogen from L² and two MeCN molecules, providing a distorted tetrahedral geometry. Reaction of 1 with dioxygen in N,N′-dimethylformamide (dmf) at 25 °C and subsequent workup with MeCO₂Et afforded an acetato-/pyrazolato-bridged polymeric copper(II) compound [(µ-L¹)Cu(µ-O₂CMe)]_n (2). Notably, the deprotonated form of HL¹ and MeCO₂⁻ have originated from debenzylation of L² and hydrolysis of MeCO₂Et, respectively. The structural analysis of 2 reveals a near-planar {Cu₂(µ-L¹)₂}²⁺ core unit in which two adjacent Cu(II) ions are bridged by the deprotonated N,N-bidentate pyridylpyrazole units of two L¹ and each such {Cu₂(µ-L¹)₂}²⁺ unit is bridged by MeCO₂⁻ in a monodentate bridging mode [Cu⋯Cu separations (Å): 3.9232(4) pyrazolate bridge; 3.3418(4) acetate bridge], providing a polymeric network. Careful oxygenation of 1 in MeCN led to the isolation of a dihydroxo-bridged dicopper(II) compound [{(L²)Cu(µ-OH)(OClO₃)}₂] (3). Interestingly, complex 3 brings about hydrolysis of MeCO₂Et under mild conditions (dmf, ca. 60 °C), generating a bis-µ-1,3-acetato-bridged dicopper(II) complex, [{(L²)Cu(dmf)(µ-O₂CMe)}₂][ClO₄]₂·dmf·0.5MeCO₂H (4). Compounds 3 and 4 have {Cu₂(µ-OH)₂}²⁺ [Cu⋯Cu separation of 2.8474(9) Å] and {Cu₂(µ-O₂CMe)₂}²⁺ cores [Cu⋯Cu separation: 3.0988(26) and 3.0792(29) Å (two independent molecules in the asymmetric unit)] in which each Cu(II) centre is terminally coordinated by L². A rationale has been provided for the observed debenzylation of L² and hydrolysis of MeCO₂Et. The intramolecular magnetic coupling between the Cu^II (S = 1/2) ions was found to be ferromagnetic (2J = 82 cm⁻¹) in the case of 3, but antiferromagnetic for 2 (2J = −158 cm⁻¹) and 4 (2J = −96 cm⁻¹). Absorption and EPR spectroscopic properties of the copper(II) compounds have also been investigated.