Ligand modifications modulate the mechanism of binuclear phosphatase biomimetics

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 [Ni^II₂(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 [Ni^II₅(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 [Ni^II₂(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 Ni^II(O₂CCH₃)₂·4H₂O, followed by the usual workup, and recrystallization from CH₂Cl₂ led to the isolation of [Ni^II₂(H₂L)(μ-O₂CCH₃)₂][ClO₄]·CH₂Cl₂·2H₂O (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni^II(O₂CCH₃)₂·4H₂O with H₃L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni^II-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni^II₂(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 (J_a = +16.02 cm^-1, and J_b = +9.02 cm^-1) and an antiferromagnetic (J_c = -49.7 cm^-1), have been identified.

Metallohydrolase biomimetics with catalytic and structural flexibility

The phosphatase activity of zinc complexes containing six- and seven-dentate ligands was evaluated through kinetic and³¹P NMR studies.

Role of solvent in the phosphatase activity of a dinuclear nickel(ii) complex of a Schiff base ligand: mechanistic interpretation by DFT studies

A biomimetic metallohydrolase has been synthesized and its catalytic mechanism and the related solvent effect were elucidated using theoretical modeling.

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

An enhanced understanding of the metal ion binding and active site structural features of phosphoesterases such as the glycerophosphodiesterase from Enterobacter aerogenes (GpdQ), and the organophosphate degrading agent from Agrobacterium radiobacter (OpdA) have important consequences for potential applications. Coupled with investigations of the metalloenzymes, programs of study to synthesise and characterise model complexes based on these metalloenzymes can add to our understanding of structure and function of the enzymes themselves. This review summarises some of our work and illustrates the significance and contributions of model studies to knowledge in the area.