Synthesis and Spectroscopic Characterization of Low- to High-Valent Weak-Field Lanthanide Complexes

Redox chemistry and valence electronic structure of the lanthanides in molecular complexes is a rapidly expanding field of research. The contemporary understanding of the accessible oxidation states of the lanthanide elements and the variability in their electronic structure is the result of several groundbreaking fundamental discoveries. While the lanthanide elements have already found widespread use in technical and consumer applications, the continued reevaluation of basic redox properties is a central chemical concern to establish a more complete description of periodic properties. This continuous development of understanding of valence electronic structure and its connection to oxidation state and coordination environment is essential for the continued development of lanthanides in quantum information science and quantum materials research. Due, in part, to the minimal extension of the valence 4f orbitals in lanthanide complexes, covalent bonding and electronic communication between metal centers, in particular lanthanide-lanthanide metal centers, is nearly non-existent and unexplored. This thesis details the development of new methodology for lanthanide triiodide starting materials for salt metathesis reactions. This thesis also outlines the development of novel lanthanide complexes that exhibit unique electronic structure properties such as vibronic coupling in neutral divalent complexes and intervalence charge transfer in mixed-valent, homobimetallic complexes. Additionally, the first isostructural molecular valences series spanning three oxidation states (Eu2+, Gd3+, and Tb4+) is synthesized and interrogated through high-field and -frequency electronic paramagnetic resonance. This work correlates formal charge, zero-field splitting, and covalency in these lanthanide complexes.Ph.D

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes.

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system