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

摘要

The chiral complexes [(eta(5)-C5H5)Ru(CO)(GBI)](+) X and mer-[Co(GBI)(3)](3+) 3X, where GBI is the chelate ligand 2-guanidinobenzimidazole (H(N)under bar=C(NHH')NHC(NHR)=(N)under barR'), 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)under bar=C((NH)under barH')NHC(NHR)=NR'), and others (more stable) involving a triad (HN=C((N)under barH(H)under bar')(NH)under barC ((NH)under barR)=NR'). Within each series, the gas phase Delta G values follow the order Cl (most negative Delta G) < 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 center dot center dot center dot FC and 1-2 NH center dot center dot center dot pi interactions. The Delta G 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. (C) 2020 Elsevier Ltd. All rights reserved.