Bridging linker installation strategy in the engineering of copper(ii) complexes: activation of nitriles, efficient catalysts for the cyanosilylation of aldehydes

Abstract

A series of bridging linkers such as pyrazine (L¹), 4,4′-bipyridine (L²), (E)-1,2-di(pyridin-4-yl)ethene (L³), 1,2-di(pyridin-4-yl)ethane (L⁴), 1,2-di(pyridin-4-yl)disulfane (L⁵), 1,2-di(pyridin-4-yl)diazene (L⁶) and 1,4-di(pyridin-4-yl)benzene (L⁷) has been used in a Cu(II)-mediated cyano group activation in sodium 2-(2-(dicyanomethylene)hydrazineyl)benzenesulfonate (NaHL), leading to [{Cu(H₂O)(L^a)}₂(μ-L¹)]_n (1) (L^a = (Z)-2-(1-cyano-2-imino-2-methoxyethylidene)-1-(2-sulfonatophenyl)hydrazin-1-ide), [{Cu(H₂O)(L^a)}₂(μ-L²)]·2H₂O (2), [{Cu(X)(L^a)}₂(μ-L³)]·3H₂O (3) (X = H₂O or CH₃OH for the two independent units), [{Cu(CH₃OH)(L^a)}₂(μ-L⁴)]·2H₂O (4), [{Cu(L^a)}₂(μ-L⁵)₂]_n (5), [{Cu(μ-L^a)}₂(μ-L⁶)Cu(H₂O)(L^a)]₂·4H₂O (6) and [{Cu(H₂O)(L^a)}₂(μ-L⁷)] (7), respectively. Depending on the length of the respective linkers, the distance between the two copper(II) centres in 1–7 can be expanded/adjusted from 6.762 to 15.138 Å. The catalytic activity of the synthesized compounds in the cyanosilylation reaction of aldehydes with trimethylsilyl cyanide in methanol was performed. No linear correlation among the length of the bridging ligands and the yield of 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile was observed in 1–7 catalysed cyanosilylation of benzaldehyde with trimethylsilyl cyanide, but the copper complexes show a higher catalytic activity in comparison to the monomer [Cu(H₂O)₂(L^a)]. The higher catalytic activity exhibited by complex 5 was further investigated evaluating the effects of reaction time, catalyst amount, temperature and the activity with different aldehydes.