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Conformational Analysis of Ferrocene-Containing Alcohols. A Density Functional Study of Weak OH· · · Fe Interactions

Optimized geometries and OH stretching frequencies are reported for a set of monosubstituted ferrocenes, Fe(C5H5)(C5H4R) [R = (CH2)nOH (n = 1-4), CH(Me)OH, CH(tBu)OH], at the BP86 level of density functional theory. In addition, NMR chemical shifts have been computed at the GIAO-B3LYP level. In all species studied, the most stable conformer is characterized by an OH· · · Fe moiety with Fe· · · H distances in the region between 2.61 and 2.95 Å , followed by conformers with OH· · · interactions involving the C(ipso) atoms of the cyclopentadienyl ring. According to population and topological (Bader) analyses of the electron density, these conformers are stabilized by weak electrostatic interactions, rather than by true intramolecular hydrogen bonds. The OH stretching frequencies are a very sensitive probe for the OH· · · Fe interaction, and the observed red-shift of this band relative to isomers with "free" OH bonds, which can exceed 100 cm-1, is well reproduced computationally. When other H-bond acceptors are present, the intramolecular OH· · · Fe interaction cannot compete with intermolecular H-bond formation, as has been explicitly shown in a Car-Parrinello molecular dynamics (CPMD) simulation of Fe(C5H5)-(C5H4CH2OH) in water. Compared to these unconstrained ferrocene-containing alcohols, somewhat stronger OH· · · Fe interactions can be present in ansa derivatives, e.g., in a [2]ferrocenophane derivative with a CH2CH2OH group, for which a bond path between Fe and the alcoholic H atom is found.

ferrocene; hydrogen bond; alcohol; DFT

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