Generation of Dicoordinate Boron(I) Units by Fragmentation of a Tetra-Boron(I) Molecular Square

Reduction of carbene-borane adduct [(cAAC) BBr2-(CN)] (cAAC= 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin- 2-ylidene) cleanly yielded the tetra(cyanoborylene) species [(cAAC)B(CN)](4) presenting a 12-membered (BCN)(4) ring. The analysis of the Kohn-Sham molecular orbitals showed significant borylene character of the B-I atoms. [(cAAC)B(CN)](4) was found to reduce two equivalents of AgCN per boron center to yield [(cAAC)B(CN)(3)] and fragmented into two-coordinate boron(I) units upon reaction with IMeMe (1,3,4,5-tetramethylimidazol-2-ylidene) to yield the corresponding tricoordinate mixed cAAC-NHC cyanoborylene. The analogous cAAC-phosphine cyanoborylene was obtained by reduction of [(cAAC)BBr2(CN)] in the presence of excess phosphine.