Redox and Luminescent Properties of Robust and Air-Stable N‑Heterocyclic Carbene Group 4 Metal Complexes

Robust
and air-stable homoleptic group 4 complexes of the type
M­(L)2 [1–3; M = Ti, Zr,
Hf; L = dianionic bis­(aryloxide) N-heterocyclic carbene (NHC) ligand]
were readily synthesized from the NHC proligand 1,3-bis­(3,5-di-tert-butyl-2-hydroxyphenyl)­imidazolinium chloride (H3L,Cl) and appropriate
group 4 precursors. As deduced from cyclic voltammetry studies, the
homoleptic bis-adduct zirconium and hafnium complexes 2 and 3 can also be oxidized, with up to four one-electron-oxidation
signals for the zirconium derivative 2 (three reversible
signals). Electron paramagnetic resonance data for the one-electron
oxidation of complexes 1–3 agree
with the formation of ligand-centered species. Compounds 2 and 3 are luminescent upon excitation in the absorption
band at 362 nm with emissions at 485 and 534 nm with good quantum
yields (ϕ = 0.08 and 0.12) for 2 and 3, respectively. In contrast, the titanium complex 1 does
not exhibit luminescent properties upon excitation in the absorption
band at 310 and 395 nm. Complexes 2 and 3 constitute the first examples of emissive nonmetallocene group 4
metal complexes.