Flexibility of an Open Indenyl Ligand in Iron(II) Complexes

[FeI2(thf)2] was sequentially treated with Li(C5Me5) and the potassium salt of the phenylmethallyl (“open indenyl”) ligand oIndMe, leading to selective formation of the half-open ferrocene [(η5-C5Me5)Fe(η5-oIndMe)] (1). A variable-temperature NMR study revealed a barrier of ca. 12 kcal mol–1 for the rotation of the phenyl group. The apparent lability and susceptibility of the oIndMe ligand in 1 to undergo η5–η3 interconversion allowed the preparation of the complexes [(η5-C5Me5)Fe(η3-oIndMe)(L)] (2, L = CO; 3, L = IMe; 4, L = PMe3) by addition of carbon monoxide, 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe), and trimethylphosphine, respectively. The η3-bound phenylmethallyl ligand in these complexes initially adopts an anti orientation with regard to the relative positions of the phenyl and methyl substituents, followed by anti-to-syn isomerization. For L = CO, both isomers could be isolated, and the conversion of anti-2 into syn-2 was monitored by NMR spectroscopy at 35, 50, 65, and 80 °C, affording an enthalpy of activation of ΔH⧧ = 24(1) kcal mol–1 for this equilibrium reaction. On the basis of DFT calculations, a mechanism is proposed that proceeds via consecutive η3–η1–η3 interconversions and involves η3-benzyl intermediates. In contrast, rapid equilibration was observed for L = IMe and PMe3. Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1 gave [(η5-C5Me5)Fe(η1-oIndMe)(dmpe)] (5), containing the oIndMe ligand bound in an η1-allyl fashion. η5-to-η3 hapticity interconversion was also observed upon reaction of 1 with methyl iodide and CH2Cl2, which formed the Fe(III) complexes [(η5-C5Me5)Fe(η3-oIndMe)(X)] (6, X = I; 7, X = Cl); solid-state magnetic susceptibility measurements on 6 revealed an S = 1/2 ground state. The mixed indenyl–open indenyl complex [(η5-Ind″)Fe(η3-oIndMe)(CO)] (9, Ind″ = 1,3-di(tert-butyl)indenyl) was isolated from the stepwise reaction of [FeI2(thf)2] with Na(Ind″), K(oIndMe), and CO. The molecular structures of 1, anti-2, syn-2, syn-3, 5, syn-6, syn-7, and syn-9 were established by single-crystal X-ray diffraction.