Pendant bases as proton relays in iron hydride and dihydrogen complexes

The complex trans-[HFe(PNP)(dmpm)(CH₃CN)]BPh₄, 3, (where PNP is Et₂PCH₂N(CH₃)CH ₂PEt₂ and dmpm is Me₂PCH₂PMe ₂) can be successively protonated in two steps using increasingly strong acids. Protonation with 1 equiv of p-cyanoanilinium tetrafluoroborate in acetone-d₆ at -80 °C results in ligand protonation and the formation of endo (4a) and exo (4b) isomers of frans-[HFe(PNHP)(dmpm)(CH ₃CN)]-(BPFu)₂. The endo isomer undergoes rapid intramolecular proton/hydride exchange with an activation barrier of 12 kcal/mol. The exo isomer does not exchange. Studies of the reaction of 3 with a weaker acid (anisidinium tetrafluoroborate) in acetonitrile indicate that a rapid intermolecular proton exchange interconverts isomers 4a and 4b, and a pK_a value of 12 was determined for these two isomers. Protonation of 3 with 2 equiv of triflic acid results in the protonation of both the PNP ligand and the metal hydride to form the dihydrogen complex [(H₂)Fe(PNHP) (dmpm)(CH₃CN)]³⁺, 11. Studies of related complexes [HFe(PNP)(dmpm)(CO)]⁺ (12) and [HFe(depp)(dmpm)(CH ₃CN)]⁺ (10) (where depp is bis(diethylphosphino)propane) confirm the important roles of the pendant base and the ligand trans to the hydride ligand in the rapid intra- and intermolecular hydride/proton exchange reactions observed for 4. Features required for an effective proton relay and their potential relevance to the iron-only hydrogenase enzymes are discussed.