A comparative study on NHC-functionalized ternary Se/TeFeCu compounds: synthesis, catalysis, and the effect of chalcogens

A novel family of N-heterocyclic carbene (NHC)-incorporated SeFeCu compounds, bis-1,3-dimethylimidazol-2-ylidene (bis-Me 2 -imy)-containing compound [( 4 -Se)Fe 3 (CO) 9 {Cu(Me 2 -imy)} 2 ] ( 2 ), bis- N -methyl- or bis- N -isopropyl-substituted benzimidazol-2-ylidene (bis-Me 2 -bimy or bis- i Pr 2 -bimy)-incorporated compounds [( 4 -Se)Fe 3 (CO) 9 {Cu(Me 2 -bimy)} 2 ] ( 3 ) or [( 4 -Se)Fe 3 (CO) 9 {Cu( i Pr 2 -bimy)} 2 ] ( 4 ), and a bis-1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (bis-Me 2 -Cl 2 -imy)-containing compound [( 3 -Se)Fe 3 (CO) 9 {Cu(Me 2 -Cl 2 -imy)} 2 ] ( 5 ), were synthesized in moderate yields in facile one-pot reactions of the ternary pre-designed compound [( 3 -Se)Fe 3 (CO) 9 {Cu(MeCN)} 2 ] ( 1 ) with the corresponding imidazolium salts and KO t Bu in THF in an ice-water bath. Single-crystal X-ray analyses revealed that the Me 2 -imy compound 2 or the Me 2 -bimy compound 3 each exhibited a trigonal bipyramidal SeFe 3 (CO) 9 Cu geometry with an Fe 2 Cu plane further capped by a Cu(Me 2 -imy) or Cu(Me 2 -bimy) fragment, respectively, with one long CuCu covalent bond. In addition, compound 4 also comprised a trigonal bipyramidal SeFe 3 (CO) 9 Cu core structure, but the second Cu( i Pr 2 -bimy) group bridged the equatorial FeFe edge with two unbonded Cu atoms, due to the presence of a sterically bulky i Pr 2 -bimy fragment. On the other hand, the strong electron-withdrawing chloro-containing NHC compound 5 showed a comparatively open tetrahedral SeFe 3 (CO) 9 metal core, where two FeFe edges each were further bridged by a Cu(Me 2 -Cl 2 -imy) fragment. Due to the nonclassical CHO(carbonyl) hydrogen bonds between the CO groups of the SeFe 3 (CO) 9 Cu 2 core and CH moieties of the neighboring NHC ligands, both compounds 2 and 3 comprised a one-dimensional network, while compounds 4 and 5 each were made up of a two-dimensional framework in the solid state, which efficiently enhanced the stability of these SeFeCu NHC compounds. Importantly, all of these synthesized SeFeCu NHC compounds 25 had pronounced catalytic activities for the homocoupling of arylboronic acids with high catalytic yields. Finally, these Se-containing FeCu NHC compounds further represented excellent models for studying chalcogen effects in comparison to their Te analogs, as demonstrated by their catalytic performances and electrochemical behaviors, and by DFT calculations. A novel family of highly efficient SeFeCuNHC catalysts was synthesized, which were suitable models for t