Classical and non-classical iron hydrides: synthesis, NMR characterisation, theoretical investigation and X-ray crystal structure of the iron(IV) dihydride 〚Cp*Fe(dppe)(H) 2〛 +BF 4–

Abstract

Protonation of Cp*Fe(dppe)H ( 1; Cp* = η 5-C 5Me 5, dppe = Ph 2PCH 2CH 2PPh 2) by HBF 4Et 2O at –80 °C in diethylether affords the dihydrogen complex 〚Cp*Fe(dppe)( η 2-H 2)〛 +BF 4 – ( 2 +BF 4 –) in 90% yield. Its PF 6 – salt analogue ( 2 +PF 6 –) is obtained in 94% yield by reaction between the 16-electron derivative 〚Cp*Fe(dppe)〛 +PF 6 – ( 3 +PF 6 –) with H 2 gas at –80 °C. The presence of a bound dihydrogen ligand in 2 + is indicated by a short T 1 minimum values consistent with a H–H distances of 0.98(1) Å. For the partially deuterated derivative 2 +–d 1, the observed J HD value of 27.0 Hz confirms the presence of the coordinated dihydrogen ligand, which displays an H–H separation of 0.97(1) Å, in complete agreement with the distance calculated using the T 1 static rotation model. Variable temperature NMR study shows the gradual, complete and irreversible transformation of the dihydrogen complex into its classical dihydride isomer trans-〚Cp*Fe(dppe)(H) 2〛 + ( 4 +). Thermal solid state reaction (–20 °C, 48 h) of 2 +BF 4 – gives quantitatively 4 +BF 4 –, whereas 4 +PF 6 – is obtained by simple contact of H 2 with a solution of 3 +PF 6 – in THF at room temperature. The crystal structure of 4 +BF 4 – has been determined and shows a transoid arrangement of hydride ligands, consistent with the formulation of 4 + as an iron(IV) dihydride. DFT calculations on both dihydride and dihydrogen isomers of 〚Cp*Fe(dppe)H 2〛 + indicate that 4 + is more stable than 2 + by 0.19 eV, while this energy difference is reversed in the case of 〚CpFe(dpe)H 2〛 + (dpe = H 2PCH 2CH 2PH 2). The preference for the dihydride form in the case of 〚Cp *Fe(dppe)H 2〛 + and of the dihydrogen one in the case of 〚CpFe(dpe)H 2〛 + is due to the larger π-donor and σ-acceptor abilities of the 〚Cp*Fe(dppe)〛 + fragment, as compared to the 〚CpFe(dpe)〛 + unit.