Question

In addition to cyclopentadienyl, indenyl and fluorenyl can also act as η5 ligands. For which η5-ligand would you expect that the substitution of (η5-ligand)Mn(CO)3 with P(n-Bu)3 to occur the fastest? slowest? (Hint: A review by Casey in 1987 may help)

Answer 1

[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)] (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 oInd^Me ligand in 1 to undergo η⁵–η³ interconversion allowed the preparation of the complexes [(η⁵-C₅Me₅)Fe(η³-oInd^Me)(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-oInd^Me)(dmpe)] (5), containing the oInd^Me 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-oInd^Me)(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-oInd^Me)(CO)] (9, Ind″ = 1,3-di(tert-butyl)indenyl) was isolated from the stepwise reaction of [FeI₂(thf)₂] with Na(Ind″), K(oInd^Me), 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.