Proton Transfer in Aminocyclopentadienyl Ruthenium Hydride Complexes

A new ruthenium hydride complex of the aminocyclopentadienyl ligand (Cp-N)RuH(PPh 3 ) 2 (Cp-N = C 5 H 4 CH 2 CH 2 NMe 2, 1 ) has been prepared and characterized by X-ray diffraction. Protonation of 1 with excess HPF 6 leads to the dicationic derivative [(Cp-NH)RuH 2 (PPh 3 ) 2 ](PF 6 ) 2 ( 2 ), in which both the metal and the amino substituent have been protonated. Addition of 1 equiv of HBF 4 ·Et 2 O to 1 leads to the complex [(Cp-N)Ru(PPh 3 ) 2 ](BF 4 ) ( 3 ), containing a chelating amino cyclopentadienyl ligand after elimination of H 2 . However, using (HNEt 3 )(BPh 4 ) or (HPBu 3 )(BPh 4 ) as protonating agent, it is possible to form [(Cp-NH)RuH(PPh 3 ) 2 ](BPh 4 ) ( 4 ), which was isolated as yellow crystals of 4 ·H 2 O upon addition of undistilled methanol and characterized by X-ray crystallographic analysis. A fluxional process exchanging the ammonium proton and the hydride without changing the thermodynamic state of the system could be established by 1 H NMR, and activation energies of 11 kcal·mol - 1 were calculated for 4 ·H 2 O and the product resulting from in situ addition of [HNEt 3 ][BPh 4 ] to 1, whereas an activation energy of 10.1 kcal·mol - 1 was found for the product resulting from in situ addition of [HPBu 3 ][BPh 4 ] to 1 . A density functional study (B3PW91) was carried out, and the dihydrogen bond in the model system for 4 was calculated to be 1.545 Å, in excellent agreement with T 1 measurements (1.52 Å). The proposed mechanism for the fluxional process does not involve a proton transfer within the dihydrogen bond.