Syntheses, X-ray Structures, and Physicochemical Properties of Phenoxo-Bridged Dinuclear Nickel(II) Complexes: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate

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• 作者：Sukanta Mandal ; V. Balamurugan ; Francesc Lloret ; Rabindranath Mukherjee
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：August 17, 2009
• 年：2009
• 卷：48
• 期：16
• 页码：7544-7556
• 全文大小：1020K
• 年卷期：v.48,no.16(August 17, 2009)
• ISSN：1520-510X

文摘

Four dinuclear nickel(II) complexes [Ni^II₂(L¹)(O₂CMe)₂(H₂O)₂][PF₆]·MeOH·3H₂O (1), [Ni^II₂(L¹)(O₂CMe)₂(NCS)] (2), [Ni^II₂(L²)(O₂CMe)₂(MeOH)(H₂O)][ClO₄] (3), and [Ni^II₂(L²)(O₂CMe)₂(MeOH)(H₂O)][BPh₄]·3MeOH·H₂O (4) have been synthesized [HL¹: 2,6-bis[N-methyl-N-(2-pyridylethyl)amino]-4-methylphenol; HL²: 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 η²-coordination mode (chelating) whereas in 4 each is coordinated in a μ−η¹:η¹ 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 ΔH = 64 kJ mol⁻¹, ΔS = −104 J mol⁻¹ K⁻¹ for 1 and ΔH = 68 kJ mol⁻¹, ΔS = −109 J mol⁻¹ K⁻¹ for 3. A mechanism consistent with the kinetic data is presented.