Iron(o) Arene and Iron(ii) Hydride Complexes from the Hydrogenation or Thermolysis of High-spin Iron(ii) Alkyls

The reactions of hydrogen with the 14-electron high-spin iron( 1 1 ) dialkyls of stoichiometry [ FeRJdippe)] [dippe = 1,2-bis(diisopropylphosphino)ethane] have been studied. The p-methylbenzyl complex in diethyl ether yielded the zerovalent p-xylene complex [Fe(q6-C,H4Me,-1 ,4) (dippe)], where the arene ligand has been formed by addition of a hydrogen atom to the a-carbon of the p-methylbenzyl group. Hydrogenation of [ Fe(CH,C,H,Me-p),(dippe)] in toluene gave a mixture of several products: the iron (0) arene complexes [Fe(qe-C,H,Me,-l ,4) (dippe)] and [Fe(qs-C,H,Me) (dippe)], the iron(l1) monohydride [ Fe(q5-C,H,Me,-l ,4) H (dippe)], where the 1.4-dimethylcyclohexadienyl ligand arises from addition of two hydrogen atoms to a p-methylbenzyl group, and the iron ( 1 1 ) dihydride [ FeH,(dippe),]. 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 [FeBrJdippe)] with MgEt, or MgBu', in an arene solvent provides an alternative route to [ Fe(qs-arene) (dippe)] species, the complexes [ Fe(q6C,H,)(dippe)] and [Fe(qs-C,H,Me) (dippe)] being prepared in this fashion. Presumably, the alkyl complexes 'FeEtJdippe)' and 'FeBu'Jdippe)' are generated, but they rapidly undergo p elimination to give unstable iron (11) hydrides; these intermediates then reductively eliminate H, to yield an iron(o) centre which is trapped by the arene solvent. The iron(l1) dihydride [FeH,(dippe),] has been prepared independently by sodium-amalgam reduction of FeBr, in the presence of an excess of dippe and 4 atm of H,. The NMR spectra of all the new complexes are described.