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.