Abstract
The synthesis, structure and ligand substitution mechanism of a new five‐coordinate copper(II) complex with a sterically constrained py__t__BuN~3~ chelate ligand [py__t__BuN~3~ = 2,6‐bis(3,5‐di‐tert‐butylphenyliminomethyl)pyridine] are reported. In the crystal structure of the complex the [Cu(py__t__BuN~3~)Cl~2~] 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(py__t__BuN~3~)Cl~2~] is a biphasic process that involves the subsequent displacement of both chloride ligands. The substitution of the first chloride by TU, k~1~^296^ = 918 ± 30 M^–1^ s^–1^, is 570 times faster than the substitution of the second chloride, k~2~^296^ = 1.62 ± 0.06 M^–1^ s^–1^. Substitution of chloride by TU is characterized by the activation parameters: Δ__H__^#^ = 42 ± 2 and 58 ± 2 kJ mol^–1^, Δ__S__^#^ = –46 ± 7 and –47 ± 6 J K^–1^ mol^–1^, and Δ__V__^#^ = –6.5 ± 0.2 and –5.3 ± 0.7 cm^3^ mol^–1^, for the first and second substitution reactions, respectively. It is concluded from the activation parameters that both reactions follow an associative interchange (I~a~) mechanism. When the substitution reaction was carried out with the sterically hindered nucleophile TMTU, the rate constant for the displacement of the first chloride, k~2~^296^ = 6.9 ± 0.5 M^–1^ s^–1^, was more than 133 times slower than for the reaction with TU, which further supports the I~a~ nature of the substitution mechanism.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)