A New Trigonal‐Bipyramidal [Cu II (py t BuN 3 )Cl 2 ] Complex: Synthesis, Structure and Ligand Substitution Behaviour

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)