Realizing thermometric control of Cobalt-59 spin-based probes via ligand design

Abstract

Cobalt-59 is an exemplary nucleus for the design of NMR thermometers through its highly responsive nuclear spin properties such as its temperature-driven chemical shift (Δδ/ΔT) and relaxation dynamics (ΔT1/ΔT and ΔT2/ΔT). Investigated through a series of low-spin d6 Co3+ octahedrally coordination complexes, the temperature dependences of the 59Co nuclear spin properties are readily affected by molecular features such as coordination geometry, ligand identity, and local environmental factors. However, precise control of these thermometric properties (i.e., Δδ/ΔT, ΔT1/ΔT, and ΔT2/ΔT) via ligand design is absent. While it is known that molecular identity, defined by the coordinating ligand, ultimately dictates the thermometric properties of the 59Co-containing complex, it is neither known how it is governed nor how it may be improved. Thus, the goal of this dissertation aims to explore fundamental design principles that inform on the synthetic control of thermometric properties of 59Co via molecular features. Presented herein is the first comprehensive collection of experimental and computational investigations on the temperature-dependence of 59Co in a series of structurally similar coordination complexes with progressively encapsulating ligands. Through a suite of spectroscopic and theoretical techniques, a critical lens has been applied to establish a variety of key structural implications. These key points include the enhancement of thermometric properties via multidentate ligand encapsulation, temperature-driven structure, symmetry, and lastly, mass-induced changes to ligand-specific vibrational modes. The identified design principles pave way for future studies of either new ligand systems, coordination geometries, or other NMR-active transition metal nuclei.