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

Abstract

The first examples of well-defined, tetrasubstituted bis-guanidinate-supported, air- and moisture-stable, monomeric boron complexes – namely, L^1–3Bpin (1a–1c), L¹BCat (2), 3, 4a, 4b and 5 (where L^1–3 = {(ArHN)(ArN) CN 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.