Synthesis and Study of Bidentate Benzimidazolylidene−Group 10 Metal Complexes and Related Main-Chain Organometallic Polymers

Abstract

A series of group 10 bis(benzimidazolylidene) complexes featuring chelating N-(o-phenol) moieties were synthesized and characterized. The ligand was prepared in 85% overall yield from 1-fluoro-2-nitrobenzene using a short SNAr/reductive cyclization/alkylation reaction sequence. Direct metalation of the respective benzimidazolium precursor with Ni(II), Pd(II), and Pt(II) salts under ambient atmosphere at 50−80 °C provided the discrete chelating complexes in excellent yields (≥91%). Whereas chelation occurred spontaneously in the case of the Ni complex, an intermediate displaying ligation from the NHCs without chelation from pendent phenol groups was isolated and characterized when Pd was used. This complex was subsequently converted to its chelated form upon treatment with base. The effect of chelation was measured via thermogravimetric analysis and found to enhance the stability of the complex by 24 °C. The chelation also did not significantly effect overall electronic characteristics. A similar reaction sequence was observed when Pt was used, but the respective intermediate could not be isolated. Chelated Ni, Pd, and Pt complexes were characterized by X-ray crystallography and found to exhibit cis configurations about their respective square-planar metal centers. Based on these model systems, a new class of main-chain organometallic polymers comprised of a benzobis(imidazolylidene) ligand with chelating phenolate moieties and group 10 transition metals was synthesized and characterized. The respective bis(bidentate) ligand was prepared from 1,5-dichloro-2,4-dinitrobenzene in three chromatography-free steps in 78% overall yield from commercially-available starting materials and used as monomer. Consistent with the model studies, direct metalation of the monomer was accomplished by addition of a stoichiometric amount of metal(II) salt under basic conditions, which resulted in excellent yields (≥95%) of the respective organometallic polymers with molecular weights up to 363,000 g/moL (relative to polystyrene standards). The polymers were found to be exceptionally air- and moisture-stable and displayed thermal stabilities exceeding 350 °C (under nitrogen), as measured by thermogravimetric analysis. Electronic absorption measurements indicated the λmax values of these polymers ranged between 312 and 322 nm, depending on the incorporated transition metal, and were bathochromically shifted by up to 27 nm relative to their corresponding model complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

“…The reaction of the benzobis(imidazolium) salts with suitable metal precursors allows the preparation of organometallic polymers. [100] To vary the electronic situation at the carbene carbon atom a number of carbo-and heterocycle-annulated imidazolin-2-ylidenes have been prepared and studied. The doubly pyrido[a]-annulated carbene 28 d (Scheme 37) can be considered formally as the complex of 2,2'-bipyridine with a singlet carbon atom.…”

The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials

“…[10][11][12] These organic building blocks offer two metal binding sites which are mutually connected via a conjugated π system. Due to the mutual trans arrangement of the binding sites, bimetallic complexes are conveniently accessible.…”

“…4). [12] The analogous palladium-and platinum-containing polymers display an increased thermal stability compared to their non-chelated congeners poly-7. These results emphasize the strength of Bielawski's modular synthetic approach for N-substitution in the ligand precursor.…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…This was followed by the addition of sodium carbonate to the intermediate carbene complexes, and finally by their cyclization in a mixture of THF and methanol. 217 Alternatively, bіsbenzіmіdazolіum salts in DMSO could be used to form chelated organometallic polymers. Alkali metal complexes of this type have not yet been synthesized.…”