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).