Tris((4-BMes₂)phenyl)methanide: a carbanion with a delocalised triple quinoidal structure

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 structure 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 (18 400 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.