TpRu Hydride and Dihydrogen Complexes Bearing Bidentate Phosphinoamine Ligands. NMR Study of Proton Transfer to [TpRuH(L)] (L = R,R-dippach, dippae; Tp = Hydrotris(pyrazolyl)borate)

Abstract

The diastereomerically pure complexes [TpRuX(R,R-dippach)] (X = Cl (1a), H (2a); R,R-dippach = (R,R)-1,2-bis((diisopropylphosphino)amino)cyclohexane) as well as the nonchiral derivatives [TpRuX(dippae)] (X = Cl (1b), H (2b); dippae = 1,2-bis((diisopropylphosphino)amino)ethane) have been prepared and characterized. The reaction of either 1a or 1b with H2 and NaBAr‘4 (Ar‘ = 3,5-bis(trifluoromethyl)phenyl) in fluorobenzene yields the corresponding stable dihydrogen complexes [TpRu(H2)(R,R-dippach)][BAr‘4] (3a) and [TpRu(H2)(dippae)][BAr‘4] (3b). No significant intramolecular interaction between the amino protons and the hydrogen atoms bound to the metal has been observed in any of these compounds. Both 3a and 3b react readily with dinitrogen, furnishing the complexes [TpRu(N2)(R,R-dippach)][BAr‘4] (4a) and [TpRu(N2)(dippae)][BAr‘4] (4b). The X-ray crystal structures of 3b and 4b were determined. The proton-transfer processes over the monohydrides 2a and 2b with HBF4·OEt2 have been studied by NMR spectroscopy. Dicationic dihydrogen complexes [TpRu(H2)(R,R-dippachH)]2+ (5a) and [TpRu(H2)(dippaeH)]2+ (5b) result from the protonation of 2a and 2b at one of the NH groups of the respective phosphinoamine ligands by an excess (>10 equiv) of HBF4. These species undergo slow tautomerization to their monohydride isomers [TpRuH(R,R-dippachH2)]2+ (5a) and [TpRuH(dippaeH2)]2+ (5b). Intermediates having short hydride (T1)min values have been detected in the course of the proton transfer reactions, which possibly involve the formation of dihydrogen-bonded complexes and/or contact ion pairs between the monohydride complex and the proton donor. Finally, the reaction of 2a with less than 1 equiv of HBF4 at −80 °C generates a hydride species having an extremely short (T1)min value of 0.5 ms at −62 °C and 400 MHz. The causes for this fast relaxation phenomenon are so far unknown.