Synthetic, redox and coordination chemistry of bis(pentachlorophenyl)boryl ferrocene, FcB(C6Cl5)2

• Synthesis of the novel Lewis acid FcB(C6Cl5)2 from FcLi and Cl B(C6Cl5)2.
• Demonstration of a synthetically facile new route to the related system, FcB(C6F5)2.
• Demonstration through CV that FcB(C6Cl5)2 is more electron deficient than FcB(C6F5)2.
• Demonstration via Gutmann analysis that FcB(C6Cl5)2 is the stronger Lewis acid.

Two synthetic approaches to bis(pentachlorophenyl)boryl ferrocene have been explored. One mirrors that used in a novel approach to FcB(C6F5)2 from FcBBr2, but is less selective than its perfluorinated counterpart on account of the greater steric bulk of LiC6Cl5 over LiC6F5. This approach does, however, provide a viable route to unsymmetrical mono(pentachlorophenyl) derivatives of the type FcB(C6Cl5)Ar through the intermediacy of the mono-substituted species FcB(C6Cl5)Br. FcB(C6Cl5)2 itself is best synthesized from ferrocenyllithum and ClB(C6Cl5)2 and is a violet–blue species featuring an extremely electron deficient Fe(II) centre (E1/2 = +550 mV with respect to ferrocene/ferrocenium). A combination of structural, spectroscopic and reactivity studies of these and related ferrocenylboranes allow some general comments to be made concerning the relative steric and electronic properties of the C6Cl5 group. Thus, in terms of their relative capabilities as electron-withdrawing groups the substituents examined can be ranked C6Cl5 > C6F5 > Mes, while steric properties are ordered Mes > C6Cl5 > C6F5.

Violet–blue bis(pentachlorophenyl)boryl ferrocene, FcB(C6Cl5)2, can be synthesized from ferrocenyllithum and ClB(C6Cl5)2, and features an extremely electron-deficient Fe(II) centre (E1/2 = +550 mV vs. FcH/FcH+). Studies of this and related ferrocenylboranes allow the relative steric and electronic properties of the C6Cl5 group to be ranked against other aryl substituents (shown below).Figure optionsDownload as PowerPoint slide