Group 6 Complexes with Iron and Zinc Heterometals: Understanding the Structural, Spectroscopic, and Electrochemical Properties of a Complete Series of MM···M′ Compounds

Binuclear quadruply bonded complexes Cr2(dpa)4 (1, dpa = 2,2′-dipyridylamide), Mo2(dpa)4 (2), and W2(dpa)4 (3) react with anhydrous FeCl2, yielding heterometallic compounds CrCrFe(dpa)4Cl2 (4), MoMoFe(dpa)4Cl2 (5), and WWFe(dpa)4Cl2 (6). These molecules are structurally similar, having a linear MM···Fe chain that is axially capped by chloride ions and is equatorially supported by the helically twisted dpa ligands. A structurally related zinc analog, CrCrZn(dpa)4Cl2 (7), can be prepared upon metalation of 1 with ZnCl2. This reaction also persistently produces a 2:1 adduct of ZnCl2 with 1, [Cr2(dpa)4](ZnCl2)2 (8), which is in equilibrium with 7 and has the two zinc ions bound externally to the Cr2 core and axial bridging chloro ligands attached to each Cr ion. The sole isolable product of the addition of ZnCl2 to 3 is a 1:1 adduct, [W2(dpa)4]ZnCl2 (9). The structurally related chain complexes 4, 5, 6, and 7 are characterized by X-ray crystallography, UV–vis spectroscopy, cyclic voltammetry, and 57Fe Mössbauer spectroscopy for the iron complexes in order to gain insights into the nature of heterometallic interactions, electronic excited states, and redox properties of these compounds, which have implications for all other MM···M′ molecules. Additionally, NMR spectroscopy has been used to gain insight into the mechanism of the metalation of 1 by Zn(II).