Iron(0) arene and iron(II) hydride complexes from the hydrogenation or thermolysis of high-spin iron(II) alkyls

Abstract

The reactions of hydrogen with the 14-electron high-spin iron(II) dialkyls of stoichiometry [FeR₂(dippe)][dippe = 1,2-bis(diisopropylphosphino)ethane] have been studied. The p-methylbenzyl complex in diethyl ether yielded the zerovalent p-xylene complex [Fe(η⁶-C₆H₄Me₂-1,4)(dippe)], where the arene ligand has been formed by addition of a hydrogen atom to the α-carbon of the p-methylbenzyl group. Hydrogenation of [Fe(CH₂C₆H₄Me-p)₂(dippe)] in toluene gave a mixture of several products: the iron(0) arene complexes [Fe(η⁶-C₆H₄Me₂-1,4)(dippe)] and [Fe(η⁶-C₆H₅Me)(dippe)], the iron (II) monohydride [Fe(η⁵-C₆H₅Me₂-1,4)H(dippe)], where the 1,4-dimethylcyclohexadienyl ligand arises from addition of two hydrogen atoms to a p-methylbenzyl group, and the iron(II) dihydride [FeH₂(dippe)₂]. Further experiments suggest that these products do not interconvert under the reaction conditions, but instead are probably formed from a common intermediate. Several of the products formed in the hydrogenolysis reactions can be independently prepared. Reaction of [FeBr₂(dippe)] with MgEt₂ or MgBu^t₂ in an arene solvent provides an alternative route to [Fe(η⁶-arene)(dippe)] species, the complexes [Fe(η⁶-C₆H₆)(dippe)] and [Fe(η⁶-C₆H₆)(dippe)] and [Fe(η⁶-C₆H₅Me)(dippe)] being prepared in this fashion. Presumably, the alkyl complexes ‘FeEt₂(dippe)’ and ‘FeBu^t₂(dippe)’ are generated, but they rapidly undergo β elimination to give unstable iron(II) hydrides; these intermediates then reductively eliminate H₂ to yield an iron(0) centre which is trapped by the arene solvent. The iron(II) dihydride [FeH₂(dippe)₂] has been prepared independently by sodium-amalgam reduction of FeBr₂ in the presence of an excess of dippe and 4 atm of H₂. The NMR spectra of all the new complexes are described.