Tris((4-BMes 2 )phenyl)methanide: a Carbanion with a Delocalized Triple Quinoidal Structure

Abstract

Herein, we report that incorporation of strongly electron-accepting BMes2 (Mes = 2,4,6-trimethylphenyl) groups at the three para-positions of a trigonal [CPh3]– framework leads to extensive delocalisation of the negative charge over the three aryl branches and onto the boron centres. Single-crystal X-ray analysis reveals that all three branches of CB-Mes-1 exhibit short C(center)–C(phenylene) and C(phenylene)–B bonds, and significant C–C bond length alternation within the central phenylene ring, indicative of a unique triple quinoidal structure in a single system. The HOMO of CB-Mes-1 is delocalised over the whole molecule, further indicating its quinoidal structure. Upon deprotonation of the neutral HC[(C6H4)-4-B(Mes)2]3 precursor (CB-Mes), the resulting CB-Mes-1 shows a 439 nm (18400 cm-1) red shift in its absorption from 316 nm in the UV to 755 nm in the NIR. Compared with trigonal N-centred analogues and [CPh3]–, CB-Mes-1 displays a significantly raised HOMO energy level, accompanied by a dramatic red shift of the maximum absorption band with a large oscillator strength.