The previously inferred intramolecular [C-H-CIhydrogen bond in ferrocenophanyllithium from 1H NMR chemical shifts in solution is found to be absent in the solid state.We report results of an X-ray crystallographic study of ferrocenophanyllithium 1 (Fig. l), prepared from [l. llferrocenophane by abstraction of the C-1 proton by n-butyllithium and crystallized from a 2,5-dimethyltetrahydrofuran (DMTHF)-hexane solution. Crystals of lt are dark-red needles that decompose rapidly when isolated at ambient temperature, even under inert atmosphere. Special low temperature handling techniques2 have therefore been employed for isolating, selecting and mounting crystals of 1 for X-ray analysis.Previously, Mueller-Westerhoff and coworkers have, based on solution 1H NMR studies of the title compound and derivatives in tetrahydrofuran, claimed the 'first evidence for the existence of C-H-C hydrogen bonds'.3 Now, the solid state structure shows a long distance between the endo bridge

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[1.1]Ferrocenophanyllithium (1) is shown by dynamic NMR (DNMR) and isotopic perturbation to undergo a rapid intramolecular 1,12-proton transfer coupled with 1,12-lithium ion transfer. The first [C−H−C]- hydrogen bond previously reported to be present in 1 is shown by X-ray crystallography to be absent in the solid state. The 13C NMR spectrum of the bridge-labeled compound [1,12-13C2][1.1]ferrocenophanyl[1-6Li]lithium (6) in 2,5-dimethyltetrahydrofuran (DMTHF) displays a 1:1:1 triplet (1 J(13C,6Li) = 4.0 Hz) showing that 6Li is bonded to C-1 and that 1 is monomeric. Transient nuclear Overhauser effects (tNOE's) measured in DEE-d 10 in bridged monodeuterated 1 show that the bridges in 1 are syn and not anti to each other and that lithium is exo-coordinated to the anionic bridge carbon. The 1 J(13C,1H) coupling constants show that the hybridization of the carbanionic carbon is intermediate between sp2 and sp3. Thus the solution and solid state structures of 1 are similar. UV−vis spectroscopy shows that 1 exists mainly as contact ion pairs (CIPs) in the temperature range −100 to +25 °C in THF, DMTHF, and diethyl ether (DEE). In dimethoxyethane (DME), 1 is mainly CIPs above +25 °C but is mainly separated ion pairs (SIPs) below −30 °C.

Insight into the detailed mechanism of carbon lithiation by an organolithium reagent and of carbon sodiation by an organosodium reagent has been obtained using [1.1]ferrocenophanyllithium (1) and [1.1]ferrocenophanylsodium (3), respectively. In tetrahydrofuran (THF) 1 and 3 undergo rapid 1,12-proton transfer reactions which are coupled with 1,12-lithium ion and 1,12-sodium ion transfers, respectively. It is concluded that the degenerate rearrangement of 1 does not make use of a pseudorotation mechanism, but occurs by direct conversion of a syn-conformer to another syn-conformer. Activation parameters (ΔH ⧧ = 19 kJ mol-1 and ΔS ⧧ = −93 J K-1 mol-1) for the degenerate reaction of 1 in THF have been measured by dynamic NMR spectroscopy. The primary isotope effect (k H/k D) of the 1,12-hydron transfer is 7.4 ± 1.5 at 320 K. The degenerate rearrangement of 1 shows strong solvent dependence, e.g. the reaction is 4 × 103 times faster in THF than in dimethyltetrahydrofuran (DMTHF). Thus, the rearrangement may be catalyzed by THF in DMTHF. The catalysis is first order in THF at low concentrations of THF. The results show that in the rate-limiting activated complex the lithium ion is coordinating one solvent molecule more than in the initial state. It is paired with a carbanion in which the proton is symmetrically located between the bridge carbons. Compound 3 shows a behavior similar to that of 1, but it is more fluxtional. It also shows a solvent catalysis that is weaker than for 1. It is concluded that 3 also exchanges using an activated complex that contains one solvent molecule more than the initial complex.

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