Asymmetric Ligand Field Effects in Electron-Rich Heterometallic Extended Metal Atom Chain Compounds

In heterometallic systems, electron donation from the ligands may influence the metal atoms in either a symmetric or asymmetric way, with the expected case being that the more electronegative metal is favored. Here, we describe a systematic study of heterometallic compounds where this expectation is not observed. In this study, we use a modification of the symmetric 2,2'-dipyridylamine (dpa) ligand with electron donating ethyl groups, the 4,4'-diethyl-2,2'-dipyridylamine ligand (dedpa), to prepare heterometallic extended metal atom chain (HEMAC) complexes with formula Mo₂M'(dedpa)₄Cl₂ (M' = Cr, Mn, Fe, Co, Ni). The effects of the electron donating substituents were studied through techniques including crystallography, magnetometry, cyclic voltammetry, EPR, Mössbauer, electronic absorption spectroscopy, and DFT calculations. We find that the new HEMACs are indeed more electron rich, easier to oxidize, and, most interestingly, the impact of the ethyl substituents is not applied equally to all the metals in the chain. The ligand field is stronger at the Mo₂ site, but is surprisingly weaker at the M' center when compared to Mo₂M'(dpa)₄Cl₂ complexes. We also find that changing the ligand field allows for previously unassigned electronic transitions to become visible, including excitations tentatively assigned to a triplet δ-δ* state within the Mo₂ unit.