Influence of the Electronics of the Phosphine Ligands on the H−H Bond Elongation in Dihydrogen Complexes

Five new monocationic dihydrogen complexes of ruthenium of the type trans-[RuCl(η2-H2)(PP)2][BF4] (PP = bis-1,2(diarylphosphino)ethane, aryl = p-fluorobenzyl, 1a, benzyl, 2a, m-methylbenzyl, 3a, p-methylbenzyl, 4a,
p-isopropylbenzyl, 5a) have been prepared by protonating the precursor hydride complexes trans-[RuCl(H)(PP)2]
using HBF4·OEt2. The dihydrogen complexes are quite stable and have been isolated in the solid state. The intact
nature of the H−H bond in these derivatives has been established from the short spin−lattice relaxation times (T1,
ms) and observation of substantial H, D couplings in the HD isotopomers. The H−H bond distances (dHH, Å)
increase systematically from 0.97 to 1.03 Å as the electron-donor ability of the substituent on the diphosphine
ligand increases from the p-fluorobenzyl to the p-isopropylbenzyl moiety. The dHH in trans-[Ru(η2-H2)(Cl)((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4], 2a, was found to be 1.08(5) Å by X-ray crystallography. In addition, two new
16-electron dicationic dihydrogen complexes of the type [Ru(η2-H2)(PP)2][OTf]2 (PP = (ArCH2)2PCH2CH2P(CH2Ar)2, Ar = m-CH3C6H4−, 6a, p-CH3C6H4−, 7a) have also been prepared and characterized. These derivatives were
found to possess elongated dihydrogen ligands.