Paramagnetic Cluster Ions [B₆Hal<i>_n</i>Hal‘₆_-<i>_n</i>]^•^- (Hal, Hal‘ = Cl, Br, I). EPR Evidence for Radical Stabilization through Electronic Effects of the Halogen Substituents

Abstract

Monoanionic hexaborate cluster radicals [B(6)Hal(n)()Hal'(6)(-)(n)()](*)(-) with mixed halogen substitution were prepared from oxidizable dianionic precursors and were characterized by vibrational and UV-vis spectroscopy. EPR studies of these and the structurally established homoleptic species [B(6)Hal(6)](*)(-) (Hal = Cl, Br, I) reveal strongly increasing g anisotropy and relaxation rate on replacing Cl by Br and especially I substituents; the very stable B(6)I(6)(*)(-) ion (g(1) = 2.04, g(2) = 1.66, g(3) = 1.15) thus exhibits an EPR spectrum only at 4 K. The extent of these effects is attributed to the Jahn-Teller situation in [B(6)X(6)](*)(-) with only partial occupancy of a degenerate MO. Both the absence of B-H bonds and the evidently strong participation of the halogen substituents in the singly occupied MO contribute to the extraordinary stability of these cluster radicals.