A cobalt templated outer sphere hydrogen bond donor catalyst derived from 2-guanidinobenzimidazole: Synthesis, applications in carbon–carbon bond forming reactions, structure

Abstract

A lipophilic tricationic cobalt tris(chelate) complex of 2-guanidinobenzimidazole (GBI), [Co(GBI)₃]³⁺ 3BAr_f^– (2³⁺ 3BAr_f^–; BAr_f = B(3,5-C₆H₃(CF₃)₂)₄), is prepared from the corresponding hydrophilic trichloride salt and Na⁺ BAr_f^– or Ag⁺ BAr_f^– under aqueous/organic biphasic conditions. This racemic chiral complex, which is obtained as a mer isomer and a tetradecahydrate as assayed by NMR and TGA, is an excellent catalyst for additions of dimethyl malonate and indole to trans-ß-nitrostyrenes (10 mol%), and cycloadditions of CO₂ to epoxides (1 mol%; solvent-free). Average yields (84 %, 91 %, 81 %) and rates are slightly greater than those obtained with a monocationic ruthenium GBI complex [(η⁵-C₅H₅)Ru(CO)(GBI)]⁺ BAr_f^– prepared earlier. Attempted crystallization of 2³⁺ 3Cl^– gives a tetramethanol solvate of a salt derived from loss of two molecules of HCl. This can be represented as [Co(GBI)(GBI_–H)₂]⁺ Cl⁻, and bond lengths and hydrogen bonding motifs are carefully analyzed, especially in the context of dominant chelate resonance forms.