Synthesis and Structure of Pentacoordinate Hypervalent Boron Compounds Bearing a 1,8-Dimethoxy-10-methylacridinium Skeleton

Pentacoordinate hypervalent boron compounds 4a and 4b with a newly prepared 1,8-dimethoxy-10-methylacridinyl ligand were synthesized. X-ray crystallography revealed that the distances between the central boron and both oxygen atoms of the tridentate ligand were in the range of 2.37–2.53 Å, showing the pentacoordinate structure. Cyclic voltammetry of 4b showed that the potential of the reduction (E1⁄2=−0.57 V in CH2Cl2, vs. SCE) is much lower than that of the corresponding Gabbaï’s system 5 (−0.28 V), probably due to the difference in coplanarity between the boryl group and the acridinyl skeleton in 4b and 5 (twist angle = 44.8° in 5 and 88.2° in 4b).