Directed Assembly of Single-Molecule and Single-Chain Magnets: From Mononuclear High-Spin Iron(II) Complexes to Cyano-Bridged Chain Compounds

The work herein describes the design, synthesis, and characterization of magnetic molecules and chain compounds, with an emphasis on probing slow magnetic relaxation. Chapter 1 presents an extensive survey of the literature of cyano-bridged single-molecule and single-chain magnets, focusing on a building block approach wherein simple cyanometalate precursor complexes direct the assembly of larger architectures. Specific synthetic strategies to obtain multinuclear clusters and chain compounds of desired structure and magnetic properties are outlined in detail. Finally, perspectives on the future directions in the field are presented. Chapter 2 exemplifies the utility of the building block approach in generating high-nuclearity cyano-bridged clusters. It describes the design and synthesis of the facially-capped tricyanide building unit, [TpCr(CN)3]−, as its incorporation into the face-centered cubic cluster [Tp8(H2O)6Cu6Cr8(CN)24]4+. Ferromagnetic exchange between CrIII and CuII ions gives rise to an S = 15 spin ground state, one of the highest yet observed for a cyano-bridged cluster. In addition, the formation of this cluster is accompanied by a linkage isomerism of 12 of the 24 cyanide ligands, providing the first example of a molecule undergoing partial cyanide isomerism. Chapter 3 presents a survey of actinide-containing molecules that have demonstrated evidence of magnetic exchange coupling. The strong magnetic anisotropy characteristic of these elements marks them as promising candidates for single-molecule magnets, however the spin-orbit coupling that gives rise to this anisotropy also complicates analysis of exchange interactions. Current methods for extracting coupling information in these systems are outlined in detail. In addition, molecules that bear exchange-coupled centers but as of yet have not been thoroughly characterized are presented. Chapter 4 describes a detailed investigation of a series of iron(II) pyrrolide complexes of formulae [(tpaR)Fe]−, representing the first examples of transition metal-based mononuclear single-molecule magnets. Static magnetic measurements and high-field EPR spectroscopy reveal the presence of exceptionally strong uniaxial anisotropy in the complexes. Moreover, dynamic magnetic measurements carried out in a small dc field demonstrate that this anisotropy leads to slow relaxation in the complexes. In addition, this relaxation dynamics is probed through Mössbauer spectroscopy, which reveals that the phenomenon occurs in zero applied field in at least two complexes. Chapter 5 describes the synthesis and characterization of a series of cyano-bridged single-chain magnets. Reaction of the S = 3/2, high-anisotropy building unit [ReCl4(CN)2]2− with [M(DMF)6]2+ (M = Mn, Fe, Co, Ni) is shown to direct the formation of the chain compounds (DMF)4MReCl4(CN)2. Dc susceptibility measurements uncover the presence of intrachain antiferromagnetic (Mn) and ferromagnetic (Fe, Co, Ni) exchange. Most importantly, ac susceptibility measurements reveal that all of the chain compounds exhibit slow magnetic relaxation at low temperature. Notably, the Fe congener displays significant magnetic hysteresis at low temperatures, thus demonstrating classical magnet-like behavior in a one-dimensional system. Chapter 6 describes the synthesis the incorporation of [ReCl4(CN)2]2− into the zig-zag chain compound (Bu4N)[TpCuReCl4(CN)2], which is found to demonstrate the strongest ferromagnetic exchange yet observed through cyanide. The strong coupling arises from judicious selection of ReIV and CuII ions, whose molecular orbitals interact through the cyanide bridge such that orbital overlap is minimized. Moreover, the compound is shown to display metamagnetic behavior, and the complete magnetic phase diagram is elucidated through a combination of experimental and theoretical analysis. Finally, the anisotropy tensors of the ReIV centers are shown to cancel, leading to a small effective chain anisotropy and thus the absence of single-chain magnet behavior. Chapter 7 concludes this work by demonstrating that [ReCl4(CN)2]2− can also be employed in the assembly of molecular magnets, as it presents the synthesis and characterization of two linear trinuclear clusters of formulae [(PY5Me2)2M2ReCl4(CN)2]2+ (M = Mn, Ni; PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)-pyridine). Dc susceptibility measurements reveal the presence of antiferromagnetic exchange in the Mn congener, while ferromagnetic exchange is observed in the Ni analogue. In addition, dc magnetization experiments show the presence of axial magnetic anisotropy in both clusters