Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes

Abstract

Mixed-base-stabilised chloroborylenes are easily accessed by twofold reduction of a cyclic (alkyl)(amino)carbene-supported trichloroborane in the presence of a second Lewis base, thus enabling fine-tuning of the electronic properties of the electron-rich borylene centre.

Reduction of a dihydroboryl cation to a boryl anion and its air-stable, neutral hydroboryl radical through hydrogen shuttling

“…Attempts to reduce 2-L, 3-L and 4-L under various conditions all resulted in unselective reactions, except for 2-CAAC Me , which was readily reduced with excess KC 8 to the red-coloured (alkyl) hydroboryl anion 5 by 1,2-migration of one hydrogen atom from boron to CAAC Me (Scheme 2a). The 11 B NMR spectrum of 5 7, B1-C21 1.607 7, B1-H1 1.11 (6), B1-H2 1.16(6), C1-N1 1.316(6), C21-N2 1.310(6); for 3-DMAP B1-C1 1.619(4), B1-N2 1.585(3), B1-N4 1.597(3), B1-H2 1.10(2), C1-N1 1.490(3). (Fig.…”

“…This is indicative of strong p backdonation from the lone pair of the boryl anion to the p-accepting CAAC Me ligand, as found in all CAAC-stabilised boryl anions. 6,[14][15][16][17] According to DFT calculations carried out at the uB97XD/6-31+G* level of theory, the HOMO of 5 possesses p-bonding character between B1 and C1A, with a nodal plane located at the C1A-N1 0 bond region (Fig. 4).…”

“…Unlike in 5 and 5-thf, the B1-C1 and C1-N1 bonds at the neutral CAAC Me ligand (1.5174(18) and 1.4601(18)Å, respectively) are within the range typical of partial double bonds, as is typical for CAAC-stabilised boryl radicals due to the delocalisation of the unpaired electron over the N1-C1-B1 p framework. [5][6][7][8][9]21,22,[31][32][33] The IR spectrum of 6 shows a B-H stretching band at 2533 cm À1 (calc. : 2558 cm À1 at uB97XD/6-31+G*), ca.…”

“…I, Fig. 1a), [5][6][7][8][9][10] boryl radical cations ([(CAAC)LBY]c + , L ¼ Lewis donor) [10][11][12][13] and boryl anions ([(CAAC)BXY] À , e.g. II), [14][15][16][17] as well as boron(I) species such as borylenes ((CAAC)LBX, e.g.…”

“…III, and (CAAC)BNR 2 ). [6][7][8][11][12][13]16,[18][19][20] In all these compounds, the accumulation of negative charge on the low-valent boron centre is stabilised through p backbonding to the CAAC ligand(s) ( Fig. 1a), making many of them surprisingly stable under inert conditions.…”