Dicationic dihydrogen complexes of iron with almost no Fe-η²-H₂ back-bonding

Abstract

Transition metal hydride complexes of the type trans-[(dppe)₂Fe(H)(RCN)][BF₄] (dppe = Ph₂PCH₂CH₂PPh₂; R = CH₃, CH₃CH₂, CH₃CH₂CH₂, CH₂=CH, C₆H₅, p-CH₃C₆H₄CH₂) have been prepared by substitution of the η²-H₂ ligand in trans-[(dppe) ₂Fe(H)( η²-H₂)][BF₄] with the corresponding nitriles. Protonation of these hydride complexes using HBF₄·Et₂O gives the dicationic dihydrogen complexes trans- [(dppe)₂Fe(η²-H₂)(RCN)][BF₄]₂.The intact nature of the H-H bond in these derivatives has been established by the observation of substantial H-D coupling constants for the η²-HD isotopomers and the short spin lattice relaxation times ( T₁ ) for the η²-HD ligand. The H₂ ligand in these complexes is not substantially elongated indicating reduced or almost no Fe-H₂ back-bonding. The bound H₂ ligand is quite labile. upon its loss, one of the BF₄- counterions binds with the metal to afford trans-[(dppe)₂Fe(BF₄)(RCN)][BF₄] derivative. Reactivity behaviour of the dihydrogen complexes toward Lewis bases such as acetonitrile, triethylamine, and piperidine has been studied. The X-ray structure of trans- [(dppe)₂Fe(H)(CH₃CH₂CH₂CN)][BF₄] has been determined.