Intramolecular N−H···H−Ru Proton−Hydride Interaction in Ruthenium Complexes with (2-(Dimethylamino)ethyl)cyclopentadienyl and (3-(Dimethylamino)propyl)cyclopentadienyl Ligands. Hydrogenation of CO₂to Formic Acid via the N−H···H−Ru Hydrogen-Bonded Complexes

Addition of 1 equiv of HBF 4 to (η 5 -C 5 H 4 (CH 2 ) n NMe 2 )RuH(dppm) ( n = 2, 3) gave [(η 5 -C 5 H 4 (CH 2 ) n NMe 2 H + )RuH(dppm)]BF 4, 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 [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )RuH(dppm)]BF 4 revealed fast exchange, probably via a dihydrogen complex intermediate, between the hydride ligand and N−H. An attempt to grow single crystals of [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )RuH(dppm)]BPh 4 for X-ray study resulted in isolation of crystals of the complex, [(η 5:η 1 -C 5 H 4 (CH 2 ) 3 NMe 2 )Ru(dppm)]BPh 4, with the chelating (3-(dimethylamino)propyl)cyclopentadienyl ligand. Exposure of [(η 5:η 1 -C 5 H 4 (CH 2 ) 3 NMe 2 )Ru(dppm)]BF 4 to 60 atm of H 2 at 60 °C gave [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )RuH(dppm)]BF 4 within 30 min. Reacting [(η 5:η 1 -C 5 H 4 (CH 2 ) 3 NMe 2 )Ru(dppm)]BAr‘ 4 (Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3 ) with Ph 2 SiH 2 yielded [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )RuH(dppm)]BAr‘ 4; it is proposed that hydrolysis of the η 2 -silane intermediate by adventitious moisture in the solvent affords an η 2 -dihydrogen species, and heterolytic cleavage of the dihydrogen ligand by the pendant amino group gives the final product. Heating solutions of [(η 5:η 1 -C 5 H 4 (CH 2 ) n NMe 2 )Ru(dppm)]BF 4 under H 2 /CO 2 (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 H 2 to generate [(η 5 -C 5 H 4 (CH 2 ) n NMe 2 H + )RuH(dppm)]BF 4, followed by CO 2 insertion into the Ru−H and then N−H protonation of the formato ligand. Carbon disulfide inserted into the Ru−H bond of [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )RuH(dppm)]BF 4 to give [(η 5 -C 5 H 4 (CH 2 ) 3 NMe 2 H + )Ru(η 1 -SCSH)(dppm)]BF 4 .