Synthesis, Structure, Electrochemistry, and Metal‐Atom Dynamics of Cyclopentadienyl Ferracarboranes

Abstract

Abstract 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 CpFeC 2 B 9 H 11 ( 1 ) and its reduced form [ 1 ] – , the charge‐compensated ferradicarbollides 1‐Cp‐4‐L‐1,2,3‐FeC 2 B 9 H 10 [L = SMe 2 ( 2a ), NMe 3 ( 2b ), py ( 2c )] and their methylated analogs 1‐Cp‐2,3‐Me 2 ‐4‐SMe 2 ‐1,2,3‐FeC 2 B 9 H 8 ( 2d ) and 1‐Cp*‐4‐SMe 2 ‐1,2,3‐FeC 2 B 9 H 10 ( 2e ), the isomeric ferratricarbollides CpFeC 3 B 8 H 11 ( 3a – c ), and the amino‐substituted derivative 1‐Cp‐12‐ t BuNH‐1,2,4,12‐FeC 3 B 8 H 10 ( 3d ). The ferradicarbollides 2a – e were synthesized by reactions of the charge‐compensated dicarbollide anions [9‐L‐7,8‐R 2 ‐7,8‐C 2 B 9 H 8 ] – (R = H, Me) with [(C 5 R 5 )Fe(MeCN) 3 ] + cations. The structures of 1 , [NMe 3 Ph][ 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 crystallographic U i , j data for 1 and 3a , b . Electrochemistry shows that the isomeric ferratricarbollides 3a – c undergo reversible oxidation to the corresponding Fe III 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 Fe II /Fe I reduction becomes accessible. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)