A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions

From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel­(II) complex [NiII2(L)­(H2O)2­(CH3OH)]­[ClO4]·CH3OH (1) (H3L = 2,6-bis­[{{(5-bromo-2-hydroxybenzyl)­(N′,N″-(dimethyl­amino)­ethyl)}­amino}­methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH3CN/CH3OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [NiII5(H2L)2­(μ3-OH)2­(μ-O2CCH3)4]­[ClO4]2·CH3CO2C2H5 (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [NiII2(HL)­(μ-O2CCH3)­(H2O)2]­[ClO4]·4H2O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H3L with 2 equiv of NiII(O2CCH3)2·4H2O, followed by the usual workup, and recrystallization from CH2Cl2 led to the isolation of [NiII2(H2L)­(μ-O2CCH3)2]­[ClO4]·CH2Cl2·2H2O (4). Complex 4 is structurally different from 3, confirming that the reaction of NiII(O2CCH3)2·4H2O with H3L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by NiII-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [NiII2(HL)­(μ-O2CCH2CH3)­(H2O)2]­[ClO4] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel­(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = −25.0(1) cm–1 for 1, J = −20.0(1) cm–1 for 4, and J = −18.80(8) cm–1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (Ja = +16.02 cm–1, and Jb = +9.02 cm–1) and an antiferromagnetic (Jc = −49.7 cm–1), have been identified.