A Radical Anion of Structurally Constrained Triphenylborane

Chemical
reduction of the structurally constrained triphenylborane 1 with K produced a radical anion. EPR analysis demonstrated the delocalization
of the unpaired electron spin density over the entire π skeleton
to a greater extent than is the case for a nonconstrained Ph3B radical anion. DFT calculations indicated that the coplanarization
of the benzene rings with the boron plane and the shortening of the
B–C bond lengths by the structural constraint are responsible
for the spin delocalization. The geometry optimization also suggested
a bowl-shaped conformation as a viable local minimum structure in
addition to the planar conformation. X-ray crystal structure analysis
indeed revealed the nonplanar structure of the radical anion 1•–.