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

A novel family of N-heterocyclic carbene (NHC)-incorporated Se-Fe-Cu compounds, bis-1,3-dimethylimidazol-2-ylidene (bis-Me 2 -imy)-containing compound [(μ 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 [(μ 4 -Se)Fe 3 (CO) 9 {Cu(Me 2 -bimy)} 2 ] ( 3 ) or [(μ 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 [(μ 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 [(μ 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 Cu-Cu 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 Fe-Fe 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 Fe-Fe edges each were further bridged by a Cu(Me 2 -Cl 2 -imy) fragment. Due to the nonclassical C-H O(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 Se-Fe-Cu NHC compounds. Importantly, all of these synthesized Se-Fe-Cu NHC compounds 2-5 had pronounced catalytic activities for the homocoupling of arylboronic acids with high catalytic yields. Finally, these Se-containing Fe-Cu 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 Se-Fe-Cu-NHC catalysts was synthesized, which were