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

Addition of 1 equiv of HBF4 to (eta(5)-C5H4(CH2)(n)NMe2)RuH(dPPM) (n = 2, 3) gave [(eta(5-)C(5)H(4)(CH2)(n)NMe2H+)RuH(dppm)]BF4, in which the amine function was protonated. Relaxation time T-1 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 [(eta(5)-C5H4(CH2)(3)NMe2H+)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 [(eta(5)-C5H4(CH2)(3)NMe2H+)RuH(dppm)]BPh4 for X-ray study resulted in isolation of crystals of the complex, [(eta(5):eta(1)-C5H4(CH2)(3)NMe2)Ru(dppm)]BPh4, with the chelating (3-(dimethylamino)propyl)cyclopentadienyl ligand. Exposure of [(eta(5):eta(1)-C5H4(CH2)(3)NMe2)Ru(dppm)]BF4 to 60 atm of H-2 at 60 degrees C gave [(eta(5)-C5H4(CH2)(3)NMe2H+)RuH(dppm)]BF4 within 30 min. Reacting [(eta(5):eta(1)-C5H4(CH2)(3)NMe2)Ru(dppm)]BAr'(4) (Ar' = 3,5-(CF3)(2)C6H3) with Ph2SiH2 yielded [(eta(5)-C5H4(CH2)(3)NMe2H+)RuH(dppm)]BAr'(4); it is proposed that hydrolysis of the eta(2)-silane intermediate by adventitious moisture in the solvent affords an eta(2)-dihydrogen species, and heterolytic cleavage of the dihydrogen ligand by the pendant amino group gives the final product. Heating solutions of [(eta(5):eta(1)-C5H4(CH2)(n)NMe2)Ru(dppm)]BF4 under H-2/CO2 (40 atm/40 atm) at 80 degrees 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 Ha to generate [(eta(5)-C5H4(CH2)(n)NMe2H+)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 [(eta(5)-C5H4(CH2)(3)NMe2H+)RuH(dppm)]BF4 to give [(eta(5)-C5H4(CH2)(3)NMe2H+)Ru(eta(1)-SCSH)(dppm)]BF4.