Influence of the Cone Angles and the π-Acceptor Properties of Phosphorus-Containing Ligands in the Chemistry of Dihydrogen Complexes of Ruthenium

A series of new dicationic dihydrogen complexes of ruthenium of the type trans-[(dppe)2Ru(η2-H2)(L‘)][BF4]2 (dppe = Ph2PCH2CH2PPh2; L‘ = PF(OMe)2, PF(OEt)2, PF(OiPr)2) have been
prepared by protonating the precursor hydride complexes trans-[(dppe)2Ru(H)(L‘)][BF4] using
HBF4·Et2O. The precursor hydride complexes have been obtained from trans-[(dppe)2Ru(H)(L)][BF4] (L = P(OMe)3, P(OEt)3, P(OiPr)3) via the substitution of a −OR group on the
trans phosphorus ligand with a fluoride in the presence of HBF4·Et2O. Coupling of the
dihydrogen ligand with the trans phosphorus moiety has been observed. In addition to the
complexes bearing trans phosphite groups, the precursor hydrides containing trans phosphine
ligands, viz., PMe3 and PMe2Ph, have also been prepared and characterized. It was found
that the binding ability of the trans phosphorus ligand in both the hydride and dihydrogen
complexes decreases with an increase in the steric congestion of the trans phosphorus moiety.
This indicates that the stability of this series of complexes depends on the cone angles of
the trans phosphorus ligand. The protonation reactions of the hydride precursors trans-[(dppe)2Ru(H)(L)][BF4] (L = P(OMe)3, P(OEt)3, P(OiPr)3) (under certain experimental
conditions in the case of P(OiPr)3) result in mixtures of the new hydride complexes trans-[(dppe)2Ru(H)(L‘)][BF4] (L‘ = PF(OMe)2, PF(OEt)2, PF(OiPr)2) and the trans-[(dppe)2Ru(η2-H2)(L‘)][BF4]2 derivatives. There is a strong dependence on the quantity of the acid used for
the isolation of either the new hydrides or the corresponding dihydrogen complexes. Both
dihydrogen and the new hydride complexes have been isolated and characterized. On the
other hand, the protonation reactions of the starting hydrides that have trans-PMe3, PMe2Ph,
or P(OiR)3 (under certain experimental conditions) ligands gave a hydride dihydrogen
complex, the structure formulation of which could not be established with certainty. The
roles of the steric as well as the π-accepting properties of the trans phosphorus ligands in
this series of complexes are discussed. X-ray crystal structures of trans-[(dppe)2Ru(H)(P(OMe)3)][BF4], trans-[(dppe)2Ru(H)(PF(OMe)2)][BF4], and trans-[(dppe)2Ru(η2-H2)(PF(OEt)2)][BF4]2 have been determined. trans-[(dppe)2Ru(H)(L)][BF4] (L = PMe3, PMe2Ph,
P(OiPr)3) undergoes substitution of the trans phosphorus ligand with H2 to give trans-[(dppe)2Ru(H)(η2-H2)][BF4] reversibly under very mild conditions.