The effect of Schiff base ligands on the structure and catalytic activity of cobalt complexes in hydrosilylation of olefins

Abstract

Environmental and economic aspects render the search for new nonprecious metal hydrosilylation catalysts an up-to-date challenge. Cobalt stands as an interesting alternative to the benchmark platinum-based species, nonetheless its relative infancy necessitates further studies, particularly regarding the structure of organic ligands that decorate the catalytically active metallic centre. As a continuation of our previous communication, we synthesized and characterized a series of new cobalt(II) chloride bench-stable precatalysts coordinated to a small library of structurally similar Schiff base ligands. Thus the synthesized species were evaluated for their ability to act as olefin hydrosilylation catalysts in the presence of alkali metal triethylborohydrides. From the crystal engineering point of view, it was observed that the number and arrangement of the Schiff base non-coordinating hydrogen bond donors affects the composition of formed complexes, resulting in predominant formation of either [CoLCl2] ‘open’ or [CoL2]2+ ‘closed’ species or a mixture of these. This affects their catalytic properties, with the benzimidazole/2H-imidazole ‘open’ system being the most efficient in terms of hydrosilylation selectivity and lowest catalyst loading. All in all, our work shows that seemingly similar coordination motifs do not necessarily lead to the isostructural group of catalytically open cobalt(II) coordination compounds, which is an important factor to consider in the design of new catalysts in general.