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Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case Cu I . Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metalation of the B-B triple bonds causes lengthening of the B-B and B-C NHC bonds, as well as large upfield shifts of the 11 B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne/alkali metal cation complexes. In contrast to previously reported fluorescent copper(I) π-complexes of boron-boron double bonds, the Cu n -π-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. While the Cu diborene bond is dominated by electrostatic interactions, giving rise to S 1 and T 1 states of pure IL(π-π*) nature, DFT studies show that the Cu I π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S 1 and T 1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S 1 → T n and phosphorescence T 1 → S 0 .

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.