Synthesis, and structural and photophysical studies of mono- and dinuclear alkali metal 2,6-dibenzhydryl-4-methylphenolates

Abstract

A series of dinuclear alkali metal 2,6-dibenzhydryl-4-methylphenolates [Ar*OM(THF)]₂ (Ar* = 2,6-(Ph₂CH)₂-4-MeC₆H₂-; M1, M = Li, Na, K) were isolated from the reaction of metal bis(trimethylsilyl)amides with Ar*OH phenol in the THF medium. Recrystallization of M1 from pyridine yields [Ar*OM(py)]₂ (M2). In addition to the aryloxide oxygens and solvent donor atoms, the metal cations in both M1 and M2 interact with the phenyl rings of the ligands. The hapticity of these interactions gradually changes from η² (Li⁺) to η⁶ (K⁺). Treatment of Li1 with 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn); Na1 with 12-crown-4 and 18-crown-6; and K1 with 18-crown-6 affords mononuclear complexes [Ar*OLi(Me₃tacn)] (Li3), [Na(12-crown-4)₂][OAr*] (Na4), [Ar*ONa(18-crown-6)] (Na6) and [Ar*OK(18-crown-6)] (K6), respectively. Ionic pairs [Na(12-crown-4)₂][H(OAr*)₂] (Na5) and [K(18-crown-6)(THF)₂][H(OAr*)₂] are formed upon interaction of Na4 and K6 with Ar*OH. The studied phenolates exhibit broadband luminescence spanning 370–800 nm, arising from the competition between π*–π transitions and emission from charge transfer states. In the solid state, mononuclear complexes display longer lifetimes due to suppression of non-radiative relaxation. This effect along with noticeable bathochromic shifts of the emission maxima may be attributed to formation of strong short hydrogen bonds.