Strongly Phosphorescent Transition Metal π-Complexes of Boron–Boron Triple Bonds

Herein are reported the first a-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-NHS bonds, as well as large upfield shifts of the B-11 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) sr-complexes of boron boron double bonds, the Cu-n-pi-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(pi-pi*) nature, DFT studies show that the Cu' ac-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.