Syntheses, X-ray structures, and physicochemical properties of phenoxo-bridged dinuclear nickel(II) complexes: kinetics of transesterification of 2-hydroxypropyl-p-nitrophenylphosphate

Abstract

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.