Direct access to a cAAC-supported dihydrodiborene and its dianion

Abstract

The two-fold reduction of (cAAC)BHX (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene; X = Cl, Br) provides a facile, high-yielding route to the dihydrodiborene (cAAC)BH. The (chloro)hydroboryl anion reduction intermediate was successfully isolated using a crown ether. Overreduction of the diborene to its dianion [(cAAC)BH] causes a decrease in the B-B bond order whereas the B-C bond orders increase.

Facile Synthesis of a Stable Dihydroboryl {BH2}− Anion

“…Plots of the frontier molecular orbitals (MOs) of 3 show a HOMO corresponding to the B−C π‐bond, with a small π antibonding contribution from the C1−N1 bonds (Figure b), similar to other CAAC‐supported boryl anions . Furthermore, the B−C Mayer bond order amounts to 1.664, which is less than for the analogous [(CAAC Me )BHCl] − anion (1.703), but still indicative of substantial π backbonding from boron to CAAC Me . The calculated Hirshfeld partial charges of −0.161 for B1 and −0.077 for C1 confirm the negative charge accumulation on the boron atom, which is the opposite charge distribution calculated for [(CAAC Me )BHCl] − (−0.086 for B1 and −0.102 for C1) .…”

“…Plots of the frontier molecular orbitals (MOs) of 3 show a HOMO corresponding to the B−C π‐bond, with a small π antibonding contribution from the C1−N1 bonds (Figure b), similar to other CAAC‐supported boryl anions . Furthermore, the B−C Mayer bond order amounts to 1.664, which is less than for the analogous [(CAAC Me )BHCl] − anion (1.703), but still indicative of substantial π backbonding from boron to CAAC Me .…”

“…Since then, CAACs have been successfully employed to stabilize a wide range of ever more reactive borylenes and boryl anions . The most recent examples of these include the CAAC‐CO‐stabilized derivative IV , which under photolytic conditions liberates CO and generates a dicoordinate borylene synthon, the highly reactive chloro(hydro)boryl anion V , which was isolated as a potassium crown ether complex, and most recently, the N 2 activation compound VI , which may be viewed as a N 2 ‐bridged borylene dimer …”

“…Furthermore, the B−C Mayer bond order amounts to 1.664, which is less than for the analogous [(CAAC Me )BHCl] − anion (1.703), but still indicative of substantial π backbonding from boron to CAAC Me . The calculated Hirshfeld partial charges of −0.161 for B1 and −0.077 for C1 confirm the negative charge accumulation on the boron atom, which is the opposite charge distribution calculated for [(CAAC Me )BHCl] − (−0.086 for B1 and −0.102 for C1) . This should make 3 / 3′ particularly nucleophilic at boron.…”

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

“…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 ).…”