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 oInd^Me, leading to selective formation of the half-open ferrocene [(η⁵-C₅Me₅)­Fe­(η⁵-oInd^Me)] (<b>1</b>). 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 oInd^Me ligand in <b>1</b> to undergo η⁵–η³ interconversion allowed the preparation of the complexes [(η⁵-C₅Me₅)­Fe­(η³-oInd^Me)­(L)] (<b>2</b>, L = CO; <b>3</b>, L = IMe; <b>4</b>, 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 <i>anti</i> orientation with regard to the relative positions of the phenyl and methyl substituents, followed by <i>anti</i>-to-<i>syn</i> isomerization. For L = CO, both isomers could be isolated, and the conversion of <i>anti</i>-<b>2</b> into <i>syn</i>-<b>2</b> was monitored by NMR spectroscopy at 35, 50, 65, and 80 °C, affording an enthalpy of activation of Δ<i>H</i>^⧧ = 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 <b>1</b> gave [(η⁵-C₅Me₅)­Fe­(η¹-oInd^Me)­(dmpe)] (<b>5</b>), containing the oInd^Me ligand bound in an η¹-allyl fashion. η⁵-to-η³ hapticity interconversion was also observed upon reaction of <b>1</b> with methyl iodide and CH₂Cl₂, which formed the Fe­(III) complexes [(η⁵-C₅Me₅)­Fe­(η³-oInd^Me)­(X)] (<b>6</b>, X = I; <b>7</b>, X = Cl); solid-state magnetic susceptibility measurements on <b>6</b> revealed an <i>S</i> = 1/2 ground state. The mixed indenyl–open indenyl complex [(η⁵-Ind″)­Fe­(η³-oInd^Me)­(CO)] (<b>9</b>, Ind″ = 1,3-di­(<i>tert-</i>butyl)­indenyl) was isolated from the stepwise reaction of [FeI₂(thf)₂] with Na­(Ind″), K­(oInd^Me), and CO. The molecular structures of <b>1</b>, <i>anti</i>-<b>2</b>, <i>syn</i>-<b>2</b>, <i>syn</i>-<b>3</b>, <b>5</b>, <i>syn</i>-<b>6</b>, <i>syn</i>-<b>7</b>, and <i>syn</i>-<b>9</b> were established by single-crystal X-ray diffraction