Exploring tetra-coordinate bis-guanidinate-supported boron complexes: synthesis, characterization, and photophysical properties

The first examples of well-defined, tetrasubstituted bis-guanidinate-supported, air- and moisture-stable, monomeric boron complexes - namely, L1-3Bpin (1a-1c), L1BCat (2), 3, 4a, 4b and 5 (where L1-3 = {(ArHN)(ArN) C 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 N C(NAr)(NHAr)}; DepBG = L1; Ar = 2,6-Et2-C6H3, MesBG = L2; Ar = 2,4,6-Me3-C6H2, XylBG = L3; Ar = 2,6-Me2-C6H3) - are reported. A reaction between pinacolborane (HBpin) and catecholborane (HBcat) with free bis-guanidine ligands yielded compounds 1a-1c and 2, respectively. Additionally, the reaction of in situ generated 2,2,-biphenoxyborane, naphtho[2,3-d]-1,3,2-dioxaborolane and naphtho[1,8-de]-1,3,2-dioxaborinane with 1.0 equivalent of L1H or L3H afforded compounds 3, 4a, 4b (with L3H) and 5, respectively. All the newly synthesized compounds (1a-1c, 2, 3, 4a, 4b and 5) were characterized by multinuclear NMR, and HRMS. Additionally, compounds 1a-1c, 2, 3, 4a and 4b were characterized by single-crystal X-ray diffraction studies. The solid-state structures reveal that all these boron complexes are monomeric. Furthermore, photophysical studies were conducted for boron complexes 1a, 2, 3, and 4a, which showed a maximum relative fluorescence quantum yield of 9.2% in THF for 4a. These photophysical properties were further evaluated using theoretical calculations.