Properties, reactivity, and mechanistic aspects of anionic transition metal carbonyl hydrides

Abstract

In order to examine the effects of phosphorus donor substitution in HFe(CO)₄⁻, a series of compounds of the form cation⁺ HFe(CO)₃P⁻ (P = P(OPh)₃, P(OMe)₃, P(OEt)₃, PPh₃, PPh₂Me, PMe₃, PEt₃) have been prepared. The phosphorus donor ligand in all cases, except for P = P(OPh)₃, was axial and trans to the hydride ligand. X-ray analysis was conducted for Et₄⁺ trans -HFe(CO)₃PPh₃⁻ and PPN⁺ cis-HFe(CO)₃P(OPh)₃⁻. The anomalous structure and properties of cis-HFe(CO)₃P(OPh)₃⁻ are discussed. Thorough spectral characterization of these anions was carried out with the observation that NMR parameters (δ, J[subscript PH], J[subscript HC], and J[subscript PC]) were temperature and solvent dependent. Variable temperature FTIR indicated that changes in the H-Fe-CO[subscript eq] angle were responsible for this observed phenomenon. Fenske-Hall molecular orbital calculations supported the conclusion that the angle changes were enforced by solvent stabilization of an internal dipole in the anion. It was observed that HFe(CO)₃P⁻ derivatives had greater chemical reactivity than the parent HFe(CO)₄⁻. To study the charge distribution in these anions, ion pairing studies were conducted. For salts of trans-HFe(CO)₃P⁻ it was shown that in addition to an equatorial CO[dot dot dot]cation⁺ interaction, also seen in HFe(CO)₄⁻, a direct cation⁺ interaction was observed. Analysis of these results in combination with known isolable compounds and theoretical results led to the proposal that these group 8 hydrides are metal nucleophilic in character. In contrast, using similar experimental and theoretical data, hydridic nucleophilic behavior is suggested in reactions with electrophiles for the octahedral group 6 hydrides, HM(CO)₄L⁻ (M = Cr, W; L = CO, PR₃). In addition to the ionic mechanisms for hydride transfer to a substrate, radical reduction mechanisms were indicated. Radical probe substrates which undergo skeletal rearrangements upon forming radical intermediates during reduction were employed. The combination of using two radical probes, one sterically hindered and one unhindered toward S[subscript N]2 displacement, provided a means of evaluating the radical mechanism as well as quantifying for group 6 metal hydrides competing S[subscript N]2 and radical pathways in the reduction of primary alkyl bromides. For these hydrides it was found that the one-electron reduction pathway was a radical chain, S[subscript N]2, mechanism where the anionic hydride served as a hydrogen atom door.