Intramolecular N-H...H-Ru proton-hydride interaction in ruthenium complexes with (2-(dimethylamino)ethyl)cyclopentadienyl and (3-(dimethylamino)propyl)cyclopentadienyl ligands. Hydrogenation of CO2 to formic acid via the N-H...H-Ru hydrogen-bonded complexes

Abstract

Addition of 1 equiv of HBF4 to (η5-C5H4(CH2)nNMe2)RuH(dppm) (n = 2, 3) gave [(η5-C5H4(CH2)nNMe2H+)RuH(dppm)]BF4, in which the amine function was protonated. Relaxation time T1 measurements indicated the existence of an intramolecular N−H···H−Ru hydrogen-bonding interaction in these complexes. A spin saturation transfer study and H/D exchange experiment with [(η5-C5H4(CH2)3NMe2H+)RuH(dppm)]BF4 revealed fast exchange, probably via a dihydrogen complex intermediate, between the hydride ligand and N−H. An attempt to grow single crystals of [(η5-C5H4(CH2)3NMe2H+)RuH(dppm)]BPh4 for X-ray study resulted in isolation of crystals of the complex, [(η5:η1-C5H4(CH2)3NMe2)Ru(dppm)]BPh4, with the chelating (3-(dimethylamino)propyl)cyclopentadienyl ligand. Exposure of [(η5:η1-C5H4(CH2)3NMe2)Ru(dppm)]BF4 to 60 atm of H2 at 60 °C gave [(η5-C5H4(CH2)3NMe2H+)RuH(dppm)]BF4 within 30 min. Reacting [(η5:η1-C5H4(CH2)3NMe2)Ru(dppm)]BAr‘4 (Ar‘ = 3,5-(CF3)2C6H3) with Ph2SiH2 yielded [(η5-C5H4(CH2)3NMe2H+)RuH(dppm)]BAr‘4; it is proposed that hydrolysis of the η2-silane intermediate by adventitious moisture in the solvent affords an η2-dihydrogen species, and heterolytic cleavage of the dihydrogen ligand by the pendant amino group gives the final product. Heating solutions of [(η5:η1-C5H4(CH2)nNMe2)Ru(dppm)]BF4 under H2/CO2 (40 atm/40 atm) at 80 °C for 16 h gave formic acid in low yields (n = 2, TON = 6; n = 3, TON = 8). The formation of formic acid is best explained by a mechanism involving intramolecular heterolytic cleavage of the bound H2 to generate [(η5-C5H4(CH2)nNMe2H+)RuH(dppm)]BF4, followed by CO2 insertion into the Ru−H and then N−H protonation of the formato ligand. Carbon disulfide inserted into the Ru−H bond of [(η5-C5H4(CH2)3NMe2H+)RuH(dppm)]BF4 to give [(η5-C5H4(CH2)3NMe2H+)Ru(η1-SCSH)(dppm)]BF4.