Syntheses, crystal structures and isomerization kinetics of a series of [Co(dtc)2{P(OMe)3 − nPhn}2]+(n = 0–2) complexes (dtc−= N,N-dimethyldithiocarbamate): role of σ-donicity, π-acidity, and cone angle of the P-ligands in the trans influence and trans effect

Abstract

A series of geometrical isomers of the new cobalt(III) complexes containing P(OMe)3 − nPhn (n = 0, 1 or 2), trans- and cis-[Co(dtc)2{P(OMe)3 − nPhn}2]+ (dtc− = N,N-dimethyldithiocarbamate), have been synthesized and the crystal structures and spectrometric properties were obtained. By comparison with reported structures of related [Co(dtc)2(P-ligand)2]+ complexes, it was shown that the Co–P bond lengths in [Co(dtc)2(P-ligand)2]+ were linearly dependent on the Tolman's cone angle, θT, of the P-ligands. On the other hand, the Co–P bond lengths as well as the transition energies of the splitting component (1A1g → a1Eg: D4h) of the first d–d band of trans-[Co(dtc)2(P-ligand)2]+ were not linearly dependent on the Tolman's electronic parameter (χ) of the P-ligands. In the trans-isomers, it was found that the mutual electronic trans influence was not significant in the [Co(dtc)2{P(OMe)3 − nPhn}2]+ species while it was far more evident in the complexes with strong σ-donating phosphines such as PMe3 and PMe2Ph. A reversible cis–trans inter-conversion was observed for the [Co(dtc)2{P(OMe)3 − nPhn}2]+ complexes: cis to trans isomerization was induced by irradiation of light while slow thermal trans to cis isomerization was observed as a dark reaction. Kinetic measurements of thermal trans to cis isomerization reactions revealed that the absorption change with time followed multi-exponential kinetics when no free P-ligand was added to the reaction mixture: by addition of excess amounts of free P-ligand, first-order kinetic traces were observed for all reactions. The apparent first-order rate constants were independent of the concentration of added free ligands for the isomerization reactions of [Co(dtc)2{P(OMe)2Ph}2]+ and [Co(dtc)2{P(OMe)Ph2}2]+, while saturation kinetics were observed for the reaction of [Co(dtc)2{P(OMe)3}2]+: all isomerizations involved pre-dissociation of one of the coordinated P-ligands. It was indicated that a significant amount of square pyramidal [Co(dtc)2{P(OMe)3}]+ exists in equilibrium with [Co(dtc)2{P(OMe)3}2]+, while merely a trace amount of such a 5-coordinate species was involved in the reactions of [Co(dtc)2{P(OMe)2Ph}2]+ and [Co(dtc)2{P(OMe)Ph2}2]+. A contradictory phenomenon observed in these complexes, a small static trans influence with a significant kinetic trans effect, was explained by the combination of (1) a lack of strong σ-donicity of P(OMe)3 − nPhn and (2) stabilization of the 5-coordinate square-pyramidal geometry through the electron sponge effect by the dtc− ligand.