A computational study of hydrogen bonding motifs in halide, tetrafluoroborate, hexafluorophosphate, and tetraarylborate salts of chiral cationic ruthenium and cobalt guanidinobenzimidazole hydrogen bond donor catalysts; acceptor properties of the “BArf” anion

Abstract

The chiral complexes [(η5-C5H5)Ru(CO)(GBI)]+ X− and mer-[Co(GBI)3]3+ 3X−, where GBI is the chelate ligand 2-guanidinobenzimidazole (HN=C(NHH′)NHC(NHR)=NR′), are effective hydrogen bond donor catalysts for a variety of organic reactions. Salts with X− = halide, BF4−, PF6− and BArf− (B(3,5-C6H3(CF3)2)4−) are studied computationally as an initial step in rationalizing reactivity trends and enantioselectivities. A variety of cation/anion hydrogen bonding motifs are identified, some involving a dyad of synperiplanar NH linkages (HN=C(NHH′)NHC(NHR)=NR′), and others (more stable) involving a triad (HN=C(NHH′)NHC(NHR)=NR′). Within each series, the gas phase ΔG values follow the order Cl− (most negative ΔG) < BF4−< PF6− < BArf−, paralleling catalytic activities and (inversely) hydrogen bond acceptor strengths suggested experimentally. With BF4−, all hydrogen bonds involve F-B-F units; with PF6−, both cis-F-P-F and mer-F-PF-F units can bind; with BArf− the isomers feature 2–6 NH⋯FC− and 1–2 NH⋯π interactions. The ΔG values in CH2Cl2 show a few differences in relative stabilities, which are analyzed; in the ruthenium series, free or solvent separated ions are predicted to be the major species for all PF6− and BArf− salts. Crystallographically characterized BArf− salts where the cations possess NH donor groups are analyzed.