Preparation, crystal structures and isomerization kinetics of cis- and trans-[Co(dtc)2(PHPh2)2]+: thermodynamically and kinetically stable cobalt(iii)–P bonds through interplay of σ-donicity, π-acidity, and steric bulkiness

Abstract

Novel cobalt(III)–diphenylphosphine complexes, cis- and trans-[Co(dtc)2(PHPh2)2]+, were synthesized and structurally characterized by X-ray crystallographic analyses and spectroscopic methods. The Co–P bond lengths in both isomers were shorter than those in the analogous cobalt(III)–bis(tertiary phosphine) complexes with sterically less bulky but more basic phosphine ligands: Co–P(1) = 2.2340(6) and Co–P(2) = 2.2258(7) A for the cis-isomer, and Co–P = 2.276(1) A for the trans isomer. The title complexes also exhibited a unique dynamic behavior: cis to trans isomerization was induced by irradiation with visible light, while thermal trans to cis isomerization took place at elevated temperatures. The absorbance change for the trans to cis isomerization reaction exhibited multi-exponential kinetic traces when no free PHPh2 was present in the solution. Such a complicated kinetic behavior was explained either by the slow dissociation of coordinated PHPh2 or by the abstraction of a P–H proton from coordinated PHPh2 through an acid–base interaction with trace water in the bulk solvent. By addition of an excess amount of PHPh2, the dissociation of coordinated PHPh2 as well as the basicity of impure water was suppressed, and a first-order kinetic trace was observed. Kinetic studies with excess free PHPh2 in acetonitrile revealed that the isomerization reaction takes place via an intramolecular twist mechanism: ΔH* = 120 ± 1 kJ mol−1 and ΔS* = 50 ± 18 J mol−1 K−1. AOM calculations indicate that the twist mechanism involves a spin state change (1A1g to 5A1′) during the activation process. The importance of the π-acidity of PHPh2 together with the cooperative effect of the spectator ligand (dtc−) was suggested to explain the thermodynamic and kinetic behaviors of these complexes.