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

The halide ligands of Fe(4)C(CO)(12)(CuCl)(2)](2-) (1) and Fe(5)C(CO)(14)CuCl](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 Fe(4)Cu/Fe(4)Cu(3) metallic frameworks. Thus, the reaction of 1 with excess dppe yielded {Fe(4)C(CO)(12)Cu}(2)(μ-dppe)](2-) (3). {Fe(4)C(CO)(12)Cu}(2)(μ-pyz)](2-) (4) was obtained by reaction of 2 with Ag(+) and pyrazine. Fe(4)C(CO)(12)Cu-py](-) (5) was formed more directly from Fe(4)C(CO)(12)](2-), Cu(NCMe)(4)](+) and pyridine. Fe(4)Cu(3)C(CO)(12)(μ-S(2)CNEt(2))(2)](-) (6) and {Fe(4)Cu(3)C(CO)(12)(μ-pz)(2)}(2)](2-) (7) were prepared by substitution of the halides of 1 with diethyldithiocarbamate and pyrazolate, in the presence of Cu(i) ions. All of 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 Cu(3) triangle with Cu-Cu distances ranging 2.63-3.13 ?. In 7, an additional unbridged cuprophilic interaction (2.75 ?) 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 back-donation from the ligand to Cu. Additionally, DFT found that, in solution, the tight ion pair NEt(4)](2)7 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 communication between the two monomeric units, as shown by electrochemistry and spectroelectrochemical studies. (dppe = PPh(2)CH(2)CH(2)PPh(2), pyz = pyrazine C(4)N(2)H(4), py = pyridine C(5)H(5)N, pz = pyrazolate C(3)N(2)H(3)(-)).