Heterolytic activation of molecular hydrogen by transition metal complexes

Abstract

New iron hydride complexes of the type trans-[(dppe)?Fe(H)(RCN)][BF?] (RCN = nitrile) with varying degree of ?-accepting ability of the nitrile moiety have been prepared and characterized. The protonation reactions of these hydrides afforded the corresponding dihydrogen complexes. The intact nature of the H–H bond was confirmed by the observation of substantial J(H,D) in the HD isotopomers. The H? ligand in the dihydrogen complexes does not seem to be substantially elongated as a result of reduced back-donation of the electron density from the metal to the ?* orbitals of H?. The lower basicity of iron results in poor stability of the H? complexes due to reduced back-bonding in relation to the osmium analog. The complexes of the type trans-[(dppe)?Ru(?²-H?)(RCN)][BF?] (R = CH?, CH?CH?, CH?CH?CH?, CH?=CH, p-CH?C?H?CH?, C?H?, (CH?)?N)) are formulated as dihydrogen complexes based on their short T? values and the observation of substantial J(H,D) in the corresponding HD isotopomers. All of these derivatives are stable at room temperature. The relatively long H–H distances indicate substantial back-bonding from the ruthenium to the bound H? in these derivatives. The values of these complexes indicate that they are quite acidic. The donor abilities of the trans nitrile ligand influence significantly the acidity of the bound H?. The complexes trans-[(dppe)?Ru(?²-H?)(PF?)]²?, cis-[(dppm)?Ru(?²-H?)(PF?)]²? and trans-[(dppm)?Ru(?²-H?)(PF?)]? are formulated as dihydrogen complexes based on their short T? values and the observation of substantial H,D coupling in the corresponding HD isotopomers. The trans-[(dppe)?Ru(?²-HD)(PF?)]²? complex shows an interesting temperature dependence of the J(H,D). The trans-[(dppe)?Ru(?²-H?)(PF?)]²? complex was found to be stable with respect to H? loss for few hours whereas the other derivatives are unstable with respect to H? loss at room temperature. The dihydrogen complexes are formed only upon addition of excess HOTf to the precursor hydrides indicating the high acidities of these derivatives. The protonation reactions of the dihydride complex cis-[(?²-S?CH?)(PCy?)?Ir(H)?] with HBF?–Et?O resulted in a highly dynamic dihydrogen/hydride derivative cis-[(?²-S?CH?)(PCy?)?Ir(?²-H?)(H)][BF?]. Even low-temperature NMR spectroscopy did not yield the limiting spectra with separate resonances due to the dihydrogen and hydride moieties, indicating that the barrier for the H-atom site exchange process must be quite small (< 5 kcal/mol). It can be concluded that the dynamic process involves minimal movement of the co?ligands, rendering the three hydrogens equivalent. The partially deuterated isotopomers gave an average J(H,D) of 6.8 Hz from which a J(H,D) value of 20.4 Hz can be calculated for the intact H? ligand. This corresponds to an H–H distance (rHH) of 1.08 Å, falling in the category of elongated dihydrogen complexes. The protonation of the monocationic hydride complex cis-[(?²-S?CH?)(PCy?)?Ir(CH?CN)(H)][BF?] using either excess HBF?–Et?O or excess HOTf did not afford the corresponding dicationic dihydrogen complex, suggesting that a much stronger acid is required and that such a derivative, if realized, could be expected to be extremely acidic and quite unstable.