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-Me₂-imy)-containing compound [(μ₄-Se)Fe₃(CO)₉{Cu(Me₂-imy)}₂] (2), bis-N-methyl- or bis-N-isopropyl-substituted benzimidazol-2-ylidene (bis-Me₂-bimy or bis-ⁱPr₂-bimy)-incorporated compounds [(μ₄-Se)Fe₃(CO)₉{Cu(Me₂-bimy)}₂] (3) or [(μ₄-Se)Fe₃(CO)₉{Cu(ⁱPr₂-bimy)}₂] (4), and a bis-1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (bis-Me₂-Cl₂-imy)-containing compound [(μ₃-Se)Fe₃(CO)₉{Cu(Me₂-Cl₂-imy)}₂] (5), were synthesized in moderate yields in facile one-pot reactions of the ternary pre-designed compound [(μ₃-Se)Fe₃(CO)₉{Cu(MeCN)}₂] (1) with the corresponding imidazolium salts and KO^tBu in THF in an ice-water bath. Single-crystal X-ray analyses revealed that the Me₂-imy compound 2 or the Me₂-bimy compound 3 each exhibited a trigonal bipyramidal SeFe₃(CO)₉Cu geometry with an Fe₂Cu plane further capped by a Cu(Me₂-imy) or Cu(Me₂-bimy) fragment, respectively, with one long Cu-Cu covalent bond. In addition, compound 4 also comprised a trigonal bipyramidal SeFe₃(CO)₉Cu core structure, but the second Cu(ⁱPr₂-bimy) group bridged the equatorial Fe-Fe edge with two unbonded Cu atoms, due to the presence of a sterically bulky ⁱPr₂-bimy fragment. On the other hand, the strong electron-withdrawing chloro-containing NHC compound 5 showed a comparatively open tetrahedral SeFe₃(CO)₉ metal core, where two Fe-Fe edges each were further bridged by a Cu(Me₂-Cl₂-imy) fragment. Due to the nonclassical C-H⋯O(carbonyl) hydrogen bonds between the CO groups of the SeFe₃(CO)₉Cu₂ 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.