Investigating the Cluster Chemistry of α-Methyl-2-pyridine methanol (mpmH) with Select 3d Ions

Abstract

This thesis describes an investigation of the coordination chemistry of the potentially chiral bridging, chelating ligand, α-methyl-2-pyridinemethanol (mpmH) with select 3d ions for the discovery of polynuclear clusters with single molecule magnet (SMM) properties. Chapter 1 introduces the theory of molecular magnetism, SMMs and the concepts of chiral SMMs, magnetochiral dichroism and multiferroics. In Chapter 2, two NiII clusters, {Ni8} and {Ni18} prepared from rac-mpmH are reported. The {Ni8} cluster crystallizes in a trapezoidal prismatic topology and contains tetrazolate ligands that are formed via a metal-assisted click reaction. The molecular structure of the second {Ni18} cluster is highly disordered comprising of eight edge-sharing cubane subunits. Dc magnetic susceptibility measurements reveal dominant ferromagnetic interactions down to ~18 K, stabilizing spin states with large values, whereas at T < 18 K the antiferromagnetic contribution results in the population of smaller, but appreciable non-zero spin states. Ac magnetic susceptibility measurements confirm the presence of two relaxation processes at two temperature regimes that is extremely rare for a 3d-metal based SMM. The first at low temperature (5 K) is attributed to conventional SMM behavior with τ0 = 3.26 × 10-10 s and Ueff = 11 K. The origin of high temperature (15 K) relaxation process with a large Ueff = 381 K and τ0 = 2.7 × 10-15 s is less clear, but tentatively assigned to spin-glass properties. In Chapter 3, the synthesis and structure of a large mixed-valence [MnII2MnIII28MnIV] polynuclear cluster with a closed cage-like conformation is presented. Ac magnetic susceptibility measurements show the compound is an SMM with Ueff of 58 K, that is large for a 3d cluster, and a τ0 = 3 × 10−8 s. Chapter 4 describes the coordination chemistry of racemic and chiral-mpmH with CuII and FeIII, where the synthesis and magnetostructural properties of a chiral {Cu4} tetramer, a non-chiral 1-D chain, as well as a chiral {Fe6} and a non-chiral{Fe8} cluster are reported. Dc magnetic susceptibility measurements on all four complexes reveal the presence of dominant antiferromagnetic exchange interactions affording S = 0 spin ground states at low temperature that precludes the observation of any SMM behavior.