Kinetic studies on the reversible ring opening–closure reaction of the triazine ligand and structural properties of palladium(II) complexes

Abstract

A Pd(II) complex containing didentate triazine ligand L 1 (2-(2-methylphenyl)-3-(2-pyridyl)-2,3-dihydronaphtho[2,1- e][1,2,4]triazine) [PdCl 2(L 1)] ( 1) was prepared, and the structure was determined by X-ray crystallography. The absorption spectrum of complex 1 in dichloromethane changed gradually with isosbestic points when methanol was added to the solution, and [PdCl(L 2)] ( 2) (L 2 = N-[methoxy(2-pyridyl)methyl]-1-(2-methylphenylazo)-2-naphthylamide) was obtained from the resulting solution after the reaction was completed. Addition of hydrogen chloride to the solution of complex 2 led to the recovery of complex 1. Thus, a reversible ring opening and closure reaction of the triazine ligand was observed. The progress of the ring opening reaction was monitored by observing the absorbance changes at several wavelengths of the visible spectra as a function of the concentration of methanol. The absorbance plots were fitted successfully with a mechanism that includes the consumption of two methanol molecules and the release of HCl, whose concentration is equivalent to that of the produced Pd complex ( [ PdCl 2 ( L 1 ) ] + 2 CH 3 OH ⇌ [ PdCl ( L 2 ) ] + CH 3 OH 2 + · Cl - ) . In dichloromethane with a low dielectric constant, the polar HCl molecule will be stabilized by forming an adduct with methanol. The equilibrium constant was determined as K = [ PdCl ( L 2 ) ] [ CH 3 OH 2 + · Cl - ] [ PdCl 2 ( L 1 ) ] - 1 [ CH 3 OH ] - 2 = ( 2.2 ± 0.4 ) × 10 - 4 M - 1 at T = 25.0 °C. The kinetics of the reaction of [PdCl 2(L 1)] with methanol was investigated by monitoring the absorbance change of the reacting solution with time. We obtained rate constant values of k 1 = (2.40 ± 0.07) × 10 −3 s −1 and k 2 = (5.8 ± 0.1) × 10 −3 M −1 s −1 at T = 25.0 °C on the observed pseudo-first-order rate constant of the forward reaction, k f = k 1 + k 2 [CH 3OH].