Synthesis, structure, electrochemistry, and metal-atom dynamics of cyclopentadienyl ferracarboranes

A series of metallacarboranes, incorporating the CpFe fragment, were studied by electrochemical techniques, temperature-dependent Mössbauer effect (ME) spectroscopy, and X-ray diffraction. The compounds studied include the parent dicarbollide complex CpFeC2B9H11 (1) and its reduced form [1]-, the charge-compensated ferradicarbollides 1-Cp-4-L-1,2,3-FeC2B9H10 [L = SMe2 (2a), NMe3 (2b), py (2c)] and their methylated analogs 1-Cp-2,3-Me2-4-SMe2-1,2,3-FeC2B 9H8 (2d) and 1-Cp*-4-SMe2-1,2,3-FeC 2B9H10 (2e), the isomeric ferratricarbollides CpFeC3B8H11 (3a-c), and the amino-substituted derivative 1-Cp-12-tBuNH-1,2,4,12-FeC3B8H10 (3d). The ferradicarbollides 2a-e were synthesized by reactions of the charge-compensated dicarbollide anions [9-L-7,8-R2-7,8-C 2B9H8]- (R = H, Me) with [(C 5R5)-Fe(MeCN)3]+ cations. The structures of 1, [NMe3Ph][1], and 2b were investigated by X-ray diffraction. The ME spectroscopic study elucidated the relationship between the nature of the five-membered carborane face coordinated to the metal center and the hyperfine interaction parameters of the Fe atom. Temperature-dependent recoil-free fraction studies yielded the root-mean-square-amplitude-of-vibration (rmsav) of the metal atom over a wide temperature range, which proved to be in good agreement with cristallographic Ui,j data for 1 and 3a,b. Electrochemistry shows that the isomeric ferratricarbollides 3a-c undergo reversible oxidation to the corresponding FeIII derivatives at potential values higher, on average, by about 0.4 V than those of the charge-compensated complexes 2a-c and by about 0.8 V than that of the parent ferradicarbollide [1]-. This result indicates the strong electron-withdrawing ability of the extra carbon atom relative to that of the replaced boron atom. As a consequence of the shift of the HOMO-LUMO frontier orbitals to the high energy levels, the FeII/FeI reduction becomes accessible. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.