Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide and Lanthanoid-Actinide Complexes Possessing Redox-Active Bridging Ligands

A comparative examination of the electronic interactions across a series of trimetallic actinide and mixed lanthanide-actinide and lanthanum-actinide complexes is presented. Using reduced, radical terpyridyl ligands as conduits in a bridging framework to promote intramolecular metal−metal communication, studies containing structural, electrochemical, and X-ray absorption spectroscopy are reported for (C5Me5)2An[-NC(Bn)(tpy-M{C5Me4R}2)]2 (where An = ThIV, UIV; Bn = CH2C6H5; M = LaIII, SmIII, YbIII, UIII; R = H, Me, Et) to reveal effects dependent on the identities of the metal ions and R-groups. The electrochemical results show differences in redox energetics at the peripheral “M” site between complexes and significant wave splitting of the metal- and ligand-based processes indicating substantial electronic interactions between multiple redox sites across the actinide-containing bridge. Most striking is the appearance of strong electronic coupling for the trimetallic YbIII-UIV-YbIII, SmIII-UIV-SmIII, and LaIII-UIV-LaIII complexes, [8]−, [9b]−, and [10b]−, respectively, whose calculated comproportionation constant Kc is slightly larger than that reported for the benchmark Creutz−Taube ion. X-ray absorption studies for monometallic metallocene complexes of UIII, UIV, and UV reveal small but detectable energy differences in the “white-line” feature of the uranium LIII-edges consistent with these variations in nominal oxidation state. The sum of these data provides evidence of 5f/6d-orbital participation in bonding and electronic delocalization in these multimetallic f-element complexes. An improved, high-yielding synthesis of 4′-cyano-2,2′:6′,2′′-terpyridine is also reported.