Experimental Assessment of the Strengths of B–B Triple Bonds

Diborynes, molecules
containing homo­atomic boron–boron
triple bonds, have been investigated by Raman spectroscopy in order
to determine the stretching frequencies of their central BB
units as an experimental measure of homoatomic bond strengths. The
observed frequencies between 1600 and 1750 cm–1 were
assigned on the basis of DFT modeling and the characteristic pattern
produced by the isotopic distribution of boron. This frequency completes
the series of known stretches of homoatomic triple bonds, fitting
into the trend established by the long-known stretching frequencies
of CC and NN triple bonds in alkynes and dinitrogen,
respectively. A quantitative analysis was carried out using the concept
of relaxed force constants. The results support the classification
of the diboryne as a true triple bond and speak to the similarities
of molecules constructed from first-row elements of the p block. Also
reported are the relaxed force constants of a recently reported diborabutatriene,
which again fit into the trend established by the vibrational spectroscopy
of organic cumulenes. As part of these studies, a new diboryne with
decreased steric bulk was synthesized, and a computational study of
the rotation of the stabilizing ligands indicated alkyne-like electronic
isolation of the central B2 unit.