Barman Suman K, Lloret Francesc, Mukherjee Rabindranath
Department of Chemistry, Indian Institute of Technology Kanpur , Kanpur 208 016, India.
Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMOL), Universitat de València , Polígono de la Coma, s/n, 46980 Paterna, València, Spain.
Inorg Chem. 2016 Dec 19;55(24):12696-12706. doi: 10.1021/acs.inorgchem.6b01895. Epub 2016 Dec 8.
From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [Ni(L)(HO)(CHOH)][ClO]·CHOH (1) (HL = 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 CHCN/CHOH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [Ni(HL)(μ-OH)(μ-OCCH)][ClO]·CHCOCH (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(HL)(μ-OCCH)(HO)][ClO]·4HO (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 HL with 2 equiv of Ni(OCCH)·4HO, followed by the usual workup, and recrystallization from CHCl led to the isolation of [Ni(HL)(μ-OCCH)][ClO]·CHCl·2HO (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni(OCCH)·4HO with HL is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni(HL)(μ-OCCHCH)(HO)][ClO] (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 for 1, J = -20.0(1) cm for 4, and J = -18.80(8) cm for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (J = +16.02 cm, and J = +9.02 cm) and an antiferromagnetic (J = -49.7 cm), have been identified.
从合成金属水解酶的角度出发，合成了一种苯氧桥联二镍（II）配合物[Ni(L)(HO)(CHOH)][ClO]·CHOH (1)（HL = 2,6 - 双[{{(5 - 溴 - 2 - 羟基苄基)(N',N″ - (二甲基氨基)乙基)}氨基}甲基] - 4 - 甲基苯酚）并对其进行了结构表征。一个空的配位位点和一个弱结合水分子的存在为底物结合提供了空间，使其能够作为金属水解酶模型。在298 K下，将乙酸乙酯蒸气扩散到1的CHCN/CHOH溶液中，形成了一种五核乙酸根桥联配合物[Ni(HL)(μ - OH)(μ - OCCH)][ClO]·CHCOCH (2)，首次证明了在室温下金属配位水促进的羧酸酯水解反应。当用几滴乙酸乙酯湿润的1的晶体进行乙酸乙酯蒸气扩散时，它转变为单乙酸根桥联配合物[Ni(HL)(μ - OCCH)(HO)][ClO]·4HO (3)。这种伴随着外部底物（乙酸乙酯）水解的溶剂（蒸气）诱导的单晶到单晶结构转变是前所未有的。HL与2当量的Ni(OCCH)·4HO反应，经过常规后处理，然后从CHCl中重结晶，得到了[Ni(HL)(μ - OCCH)][ClO]·CHCl·2HO (4)。配合物4在结构上与3不同，这证实了Ni(OCCH)·4HO与HL的反应与1中镍配位水促进的乙酸乙酯水解是不同的现象。配合物1也能够将丙酸乙酯水解为丙酸根桥联配合物[Ni(HL)(μ - OCCHCH)(HO)][ClO] (5)。对于上述水解现象，配位配体供体位点（酚盐和叔胺）在二镍（II）中心周围提供了一个微环境，以促进在环境条件下乙酸乙酯和丙酸乙酯的高效化学计量水解。对二聚体配合物1、4和5进行的温度依赖性磁性研究表明存在中等程度的反铁磁耦合：1的J = -25.0(1) cm，4的J = -20.0(1) cm，5的J = -18.80(8) cm。对于五核配合物2，已经确定了三种类型的磁交换相互作用，两种铁磁相互作用（J = +16.02 cm和J = +9.02 cm）和一种反铁磁相互作用（J = -49.7 cm）。
