Controlled synthesis of copper halide-incorporated triiron carbonyl sulfide clusters: Synthesis, electrochemistry, and computational studies

The reactions of sulfur-capped triiron carbonyl cluster [Et ₄N]₂[SFe₃(CO)₉] with 1-6 equiv of copper halides in tetrahydrofuran (THF) at 0 °C led to the formation of a series of CuX-, Cu₂X₂-, or Cu₄X ₂-incorporated mono- or di-SFe₃-based clusters, [Et ₄N]₂[SFe₃(CO)₉CuX] (X = Cl, [Et ₄N]₂[1a]; Br, [Et₄N]₂[1b]; I, [Et₄N]₂[1c]), [Et₄N]₄[{SFe ₃(CO)₉CuX}₂Cu₂X₂] (X = Cl, [Et₄N]₄[2a]; Br, [Et₄N]₄[2b]), and [Et₄N]₂[{SFe₃(CO)₉} ₂Cu₄X₂] (X = Cl, [Et₄N] ₂[3a]; Br, [Et₄N]₂[3b]). X-ray analysis showed that each SFe₃ core in 1a-1c was bridged by the incoming CuX across the Fe-Fe bond (X = Cl, 1a; Br, 1b; I, 1c); in the case of 2a and 2b, the two SFe₃ cores each were bridged by two CuX fragments across the Fe-Fe and S-Fe bonds to form a Cu₂X₂-linked di-SFe ₃CuX cluster (X = Cl, 2a; Br, 2b). In contrast to 2a and 2b, cluster 3a displayed a different di-SFe₃-based geometry, in which the two SFe₃ moieties were connected by a Cu₄Cl₂ hexagon. In addition, complexes 1a and 1b were found to undergo cluster expansion reactions to yield 2a and 2b or 3a and 3b upon the treatment of 1 equiv or 3 equiv of CuX (X = Cl, Br), respectively. Furthermore, clusters 2a and 2b could be converted to 3a or 3b upon the addition of 4 equiv of CuX (X = Cl, Br). The nature, formation, and electrochemistry of these copper halide-incorporated mono- or di-SFe₃ clusters are elucidated in detail with the aid of molecular calculations at the B3LYP level of the density functional theory in terms of the effects of chalcogens and halides.