Synthesis and characterisation of [(triphos)Fe(CO)H2] and its protonation to a dihydrogen complex via an unconventional hydrogen-bonded intermediate

Abstract

The complex [(triphos)Fe(CO)H₂] 1 has been prepared and characterised by ¹H and ³¹P NMR and IR spectroscopies, and by single-crystal X-ray diffraction. Complex 1 is fluxional at ambient temperatures in solution; NMR spectral simulation shows that both a Bailar twist of the triphos ligand and pairwise hydride exchange contribute to this fluxionality, with the latter process apparently more facile. DFT-calculated vibrational frequencies for 1 indicate that both these processes are thermally accessible at ambient temperatures. The X-ray crystal structure of complex 1 shows that the triphos ligand adopts a fac arrangement in a distorted octahedral structure, with the two hydride ligands mutually cis, and a non-linear Fe–C–O moiety. Protonation of complex 1 with hexafluoroisopropanol (HFIP) initially produces a hydrogen-bonded intermediate; reaction proceeds further to give the cationic complex [(triphos)Fe(CO)H(η²-H₂)]⁺2. Complex 2 decomposes above 250 K in solution, but has been characterised by NMR spectroscopy. T_{1 min} and J_HD measurements indicate that 2 contains a stretched dihydrogen ligand, with r_HH = 1.03 Å, in apparent disagreement with DFT calculations that suggest that the equilibrium H–H distance is 0.85 Å, indicative of a conventional dihydrogen moiety. However, the very shallow nature of the potential energy surface for the Ru(η²-H₂) moiety can account for this difference between experimental and calculated values. The experimental and computed structural and spectroscopic features deduced for 1 and 2 are compared with those of their known ruthenium analogues, and are discussed in the context of other hydride and dihydrogen complexes of the Group 8 metals.