A New Trigonal‐Bipyramidal [CuII(pytBuN3)Cl2] Complex: Synthesis, Structure and Ligand Substitution Behaviour

Abstract

AbstractThe synthesis, structure and ligand substitution mechanism of a new five‐coordinate copper(II) complex with a sterically constrained pytBuN3 chelate ligand [pytBuN3 = 2,6‐bis(3,5‐di‐tert‐butylphenyliminomethyl)pyridine] are reported. In the crystal structure of the complex the [Cu(pytBuN3)Cl2] chromophore possesses a distorted trigonal‐bipyramidal coordination geometry. The kinetics and mechanism of chloride substitution by thiourea (TU) and N,N,N′,N′‐tetramethylthiourea (TMTU) were studied in detail as a function of nucleophile concentration, temperature and pressure in methanol as solvent. The kinetics showed that the substitution reaction of [Cu(pytBuN3)Cl2] is a biphasic process that involves the subsequent displacement of both chloride ligands. The substitution of the first chloride by TU, k1296 = 918 ± 30 M–1 s–1, is 570 times faster than the substitution of the second chloride, k2296 = 1.62 ± 0.06 M–1 s–1. Substitution of chloride by TU is characterized by the activation parameters: ΔH# = 42 ± 2 and 58 ± 2 kJ mol–1, ΔS# = –46 ± 7 and –47 ± 6 J K–1 mol–1, and ΔV# = –6.5 ± 0.2 and –5.3 ± 0.7 cm3 mol–1, for the first and second substitution reactions, respectively. It is concluded from the activation parameters that both reactions follow an associative interchange (Ia) mechanism. When the substitution reaction was carried out with the sterically hindered nucleophile TMTU, the rate constant for the displacement of the first chloride, k2296 = 6.9 ± 0.5 M–1 s–1, was more than 133 times slower than for the reaction with TU, which further supports the Ia nature of the substitution mechanism.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)