A Hypervalent Pentacoordinate Boron Compound with an N−B−N Three-Center Four-Electron Bond

For the purpose of synthesizing and characterizing hypervalent boron compounds with strong hypervalent interaction, we have prepared a boron compound with a tridentate ligand bearing two pyrimidine rings as nitrogen donors. X-ray analysis and molecular orbital calculations suggested that the boron compound was of hypervalent pentacoordinate structure with an N−B−N hypervalent bond. Thus, we have prepared the first hypervalent second row element compound with apical N coordination. A breakdown of energy contributions by DFT calculations revealed that the N−B−N bond energy of the pentacoordinate state ground state (13) was 2.8 kcal mol−1. Implications were that the conjugation energy difference of 6.6 kcal mol−1 (14.2−7.6 kcal mol−1) with the tetracoordinate state was a crucial factor for shifting stability toward the pentacoordinate structure.