Proton Transfer in Aminocyclopentadienyl Ruthenium Hydride Complexes

A new ruthenium hydride complex of the aminocyclopentadienyl ligand (Cp-N)RuH(PPh<INF>3</INF>)<INF>2</INF> (Cp-N = C<INF>5</INF>H<INF>4</INF>CH<INF>2</INF>CH<INF>2</INF>NMe<INF>2</INF>, <BO>1</BO>) has been prepared and characterized by X-ray diffraction. Protonation of <BO>1 </BO>with excess HPF<INF>6</INF> leads to the dicationic derivative [(Cp-NH)RuH<INF>2</INF>(PPh<INF>3</INF>)<INF>2</INF>](PF<INF>6</INF>)<INF>2</INF> (<BO>2</BO>), in which both the metal and the amino substituent have been protonated. Addition of 1 equiv of HBF<INF>4</INF>·Et<INF>2</INF>O to <BO>1</BO> leads to the complex [(Cp-N)Ru(PPh<INF>3</INF>)<INF>2</INF>](BF<INF>4</INF>) (<BO>3</BO>), containing a chelating amino cyclopentadienyl ligand after elimination of H<INF>2</INF>. However, using (HNEt<INF>3</INF>)(BPh<INF>4</INF>) or (HPBu<INF>3</INF>)(BPh<INF>4</INF>) as protonating agent, it is possible to form [(Cp-NH)RuH(PPh<INF>3</INF>)<INF>2</INF>](BPh<INF>4</INF>) (<BO>4</BO>), which was isolated as yellow crystals of <BO>4</BO>·H<INF>2</INF>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 ¹H NMR, and activation energies of 11 kcal·mol^-1 were calculated for <BO>4</BO>·H<INF>2</INF>O and the product resulting from in situ addition of [HNEt<INF>3</INF>][BPh<INF>4</INF>] to <BO>1,</BO> whereas an activation energy of 10.1 kcal·mol^-1 was found for the product resulting from in situ addition of [HPBu<INF>3</INF>][BPh<INF>4</INF>] to <BO>1</BO>. A density functional study (B3PW91) was carried out, and the dihydrogen bond in the model system for <BO>4</BO> was calculated to be 1.545 Å, in excellent agreement with T<INF>1</INF> measurements (1.52 Å). The proposed mechanism for the fluxional process does not involve a proton transfer within the dihydrogen bond.