Structural variations, electrochemical properties and computational studies on monomeric and dimeric Fe-Cu carbide clusters, forming copper-based staple arrays

The halide ligands of [Fe4C(CO)12(CuCl)2]2- (1) and [Fe5C(CO)14CuCl]2- (2) can be displaced by N-, P- or S-donors. Beside substitution, the clusters easily undergo structural rearrangements, with loss/gain of metal atoms, and formation of Fe4Cu/Fe4Cu3 metallic frameworks. Thus, the reaction of 1 with excess dppe yielded [{Fe4C(CO)12Cu}2(-dppe)]2- (3). [{Fe4C(CO)12Cu}2(-pyz)]2- (4) was obtained by reaction of 2 with Ag+ and pyrazine. [Fe4C(CO)12Cu-py]- (5) was formed more directly from [Fe4C(CO)12]2-, [Cu(NCMe)4]+ and pyridine. [Fe4Cu3C(CO)12(-S2CNEt2)2]- (6) and [{Fe4Cu3C(CO)12(-pz)2}2]2- (7) were prepared by substitution of the halides of 1 with diethyldithiocarbammate and pyrazolate, in the presence of Cu(I) ions. All these products were characterized by X-ray analysis. 3 and 4 and 5 are square based pyramids, with iron in the apical sites, the bridging ligands connect the two copper atoms in 3 and 4. 6 and 7 are octahedral clusters with an additional copper ion held in place by the two bridging anionic ligands, forming a Cu3 triangle with Cu-Cu distances ranging 2.63-3.13 Å. In 7, an additional unbridged cuprophilic interaction (2.75 Å) is formed between two such cluster units. DFT calculations were able to reproduce the structural deformations of 3-5, and related their differences to the backdonation from the ligand to Cu. Additionally, DFT found that, in solution, the tight ion pair [NEt4]27 is almost isoenergetic with the monomeric form. Thus, 3, 4 and 7 are entities of nanometric size assembled either through conventional metal-ligand bonds, or weaker electrostatic interactions. None of them allows electronic comunication between the two monomeric units, as shown by electrochemistry and spectroelectrochemical studies.