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

Abstract

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.

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…42 The steric hindrance of the NHC moiety appears crucial to the stability of such bis-chelates since less hindering N-substituents (such as Mes and 2,6-Et2-C6H3) led to the unexpected formation of 6) with preliminary studies of their redox and optical properties. 45 The Zr(IV) and Hf(IV) chelates 50b and 50c readily undergo ligand-centered multiple oxidations in a controlled manner. They are also luminescent upon excitation at 362 nm (in the absorption band) with emissions at 485 and 534 nm with good quantum yields (ϕ = 0.08 and 0.12, respectively).…”

Redox-Active Bis(phenolate) N-Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States

“…63,68−72 Most of these reports formulate the ligand as a bis(phenolate) dianion, but two previous papers discussed an expanded redox capacity. 70,72 Bercaw and co-workers described one-and two-electron oxidations of an [(OCO)Ir III Cl(PCy 3 ) 2 ] complex and suggested the possibility of a ligand-centered radical in the oxidized materials. 72 However, the solution electron paramagnetic resonance (EPR) data did not permit definitive assignment of the oxidized species as metal-centered (OCO 2− )Ir IV/V or ligand radical complexes (OCO •− )Ir III/IV .…”

Photoluminescence and mechanoluminescence of solid‐state zirconocene dichlorides

“…1 and 2, the spectra of complexes 1-10 exhibited low-energy absorption bands, which correspond to the excited states formed as a result of ligand-to-metal charge transfer (LMCT) transitions usually observed in the d 0 complexes of titanium group metals. [4][5][6][7][8][9][10] The PL spectra (Figures 1 and 2) of the zirconocenes showed a red shift in the emission maximum with the increasing complexity of the molecular structure, in particular, in the appearance of substituents in the ligand, a bridging group linking the π-ligands (ansa-complexes) or going to a substituted bis-indenyl complex 10. The PL of the zirconocenes was observed in the visible region of the spectrum (430-570 nm).…”

“…Currently, photoluminescence (PL) properties have been studied for Ti-, Zr-, and Hf-based metallocene and post-metallocene complexes. [4][5][6][7][8][9][10] These complexes are used widely as catalysts in the chemistry of organoaluminium and organomagnesium compounds, [11][12][13][14][15][16][17][18][19][20] as well as in the synthesis of polyolefins. [21][22][23] As a rule, a moderate concentration of metal complexes in the catalytic system and, therefore, low content of catalytically active sites precludes the use of common physicochemical methods for studying reaction mechanisms.…”