Mandal Sukanta, Balamurugan V, Lloret Francesc, Mukherjee Rabindranath
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India.
Inorg Chem. 2009 Aug 17;48(16):7544-56. doi: 10.1021/ic8018937.
Four dinuclear nickel(II) complexes [Ni(II)(2)(L(1))(O(2)CMe)(2)(H(2)O)(2)][PF(6)].MeOH.3H(2)O (1), [Ni(II)(2)(L(1))(O(2)CMe)(2)(NCS)] (2), [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (3), and [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][BPh(4)].3MeOH.H(2)O (4) have been synthesized [HL(1): 2,6-bis[N-methyl-N-(2-pyridylethyl)amino]-4-methylphenol; HL(2): 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol]. Complexes 1, 3, and 4 are new while complex 2 was reported previously by Fenton and co-workers (the structure of 2 was presented but no physicochemical properties of this complex were reported; in this work such studies have been completed). X-ray crystallographic analyses of 1 and 4 reveal that each nickel(II) center is six-coordinate, terminally coordinated by two nitrogen donors [(pyridin-2-yl)ethylamine unit in 1 and 3-(pyridin-2-yl)pyrazole moiety in 4], and bridged by an endogenous phenolate ion. Each of the acetate ions in 1 adopts a eta(2)-coordination mode (chelating) whereas in 4 each is coordinated in a mu-eta(1):eta(1) syn-syn bridging mode. In 1 each Ni(II) center has water coordination whereas in 4 one Ni(II) center has a methanol and the other has water coordination. The X-ray structure of 3 could not be determined. The physicochemical properties (electronic spectroscopy and cyclic voltammetry) of the cation of 3 are identical to that of 4. Magnetic susceptibility measurements have revealed the occurrence of ferromagnetic coupling of spins of the nickel(II) centers in 2 [J = +9.80 cm(-1)]. The nickel(II) centers in 1 and 3 are antiferromagnetically coupled, but to different extents [J = -48.4 cm(-1) (1); J = -1.24 cm(-1) (3)]. The magnetic properties are correlated with the nature of bridges between the nickel(II) ions. The two coordinated aqua ligands in 1 and the aqua and methanol ligands in 3 have enabled these dinuclear nickel(II) complexes to function as catalysts in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Complex 1 is more effective in the conversion of substrate to product (p-nitrophenolate ion) than 3, under identical experimental conditions. Pseudo first-order kinetic treatment has been done for complexes 1 and 3. Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification reaction of HPNP and to propose a mechanistic pathway. The activation parameters are DeltaH(++) = 64 kJ mol(-1), DeltaS(++) = -104 J mol(-1) K(-1) for 1 and DeltaH(++) = 68 kJ mol(-1), DeltaS(++) = -109 J mol(-1) K(-1) for 3. A mechanism consistent with the kinetic data is presented.
已合成了四种双核镍（II）配合物：[Ni(II)(2)(L(1))(O(2)CMe)(2)(H(2)O)(2)][PF(6)].MeOH.3H(2)O（1）、[Ni(II)(2)(L(1))(O(2)CMe)(2)(NCS)]（2）、[Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)]（3）和[Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][BPh(4)].3MeOH.H(2)O（4）[HL(1)：2,6-双[N-甲基-N-(2-吡啶基乙基)氨基]-4-甲基苯酚；HL(2)：2,6-双[3-(吡啶-2-基)吡唑-1-基甲基]-4-甲基苯酚]。配合物1、3和4是新合成的，而配合物2先前已由芬顿及其同事报道过（给出了2的结构，但未报道该配合物的物理化学性质；在本工作中已完成此类研究）。对1和4的X射线晶体学分析表明，每个镍（II）中心都是六配位的，由两个氮供体末端配位（1中的[(吡啶-2-基)乙胺单元和4中的3-(吡啶-2-基)吡唑部分]，并由一个内源性酚盐离子桥连。1中的每个乙酸根离子采用η(2)-配位模式（螯合），而在4中每个乙酸根离子以μ-η(1):η(1)顺-顺桥连模式配位。在1中每个Ni(II)中心都有水配位，而在4中一个Ni(II)中心有甲醇配位，另一个有 水配位。无法确定3的X射线结构。3的阳离子的物理化学性质（电子光谱和循环伏安法）与4的相同。磁化率测量表明，2中镍（II）中心的自旋发生了铁磁耦合[J = +9.80 cm(-1)]。1和3中的镍（II）中心是反铁磁耦合的，但程度不同[J = -48.4 cm(-1)（1）；J = -1.24 cm(-1)（3）]。磁性质与镍（II）离子之间的桥的性质相关。1中的两个配位水配体以及3中的水和甲醇配体使这些双核镍（II）配合物能够作为2-羟丙基对硝基苯基磷酸酯（HPNP）水解的催化剂。在相同的实验条件下，配合物1在将底物转化为产物（对硝基苯酚盐离子）方面比3更有效。已对配合物1和3进行了准一级动力学处理。进行了温度依赖性测量以评估HPNP水解/酯交换反应的动力学/热力学参数，并提出了一种机理途径。对于1，活化参数为ΔH(++) = 64 kJ mol(-1)，ΔS(++) = -104 J mol(-1) K(-1)；对于3，活化参数为ΔH(++) = 68 kJ mol(-1)，ΔS(++) = -109 J mol(-1) K(-1)。提出了一种与动力学数据一致的机理。
