Use of acetylacetonate‐based paramagnetic metalloligands in the construction of supramolecular magnetic coordination capsules

Abstract

In molecular magnetism, rational design and serendipity have played complementary roles in the synthesis of complexes which display a breadth of interesting physical characteristics. These range from the basic understanding of magneto‐structural correlations, to more complicated phenomena such as slow relaxation of the magnetisation, spin frustration effects, and tuning magnetic interactions with a view to spintronics. The inherent physical properties of these complexes has already afforded molecules which can behave as single‐molecule magnets, singlechain magnets, single‐ion magnets, magnetic metal‐organic frameworks, magnetic refrigerants, and molecular qubits. Even when the building blocks are well known, the rational design of magnetic clusters can be extremely difficult, with the shape and nuclearity often dominated by several internal and external factors. Metallosupramolecular processes proffer an attractive strategy to the rational design of these clusters by making use of structurally‐rigid precursors which, when combined in the correct stoichiometric ratio, can be used to construct various predefined discrete two‐ and three‐dimensional polygons and polyhedra. In particular, the use of metalloligands as structurally‐rigid precursors is appealing, not only because of their often‐straightforward synthesis, but because of their ability to be easily modified in order to create comparable building blocks with different chemical and physical properties. It is therefore surprising that there are limited examples of magnetic architectures built through this approach. Each chapter of this thesis aims to exploit the use of acetylacetonate‐based paramagnetic metalloligands for the synthesis of structurally analogous magnetic coordination capsules, with inherently different magnetic properties. Chapter 2 describes the structural and magnetic studies of fourteen tetradecanuclear coordination cubes, synthesised using the paramagnetic metalloligand [MIIIL3] (MIII = Cr, Fe; HL = 1‐(4‐pyridyl)butane‐1,3‐dione). The heterometallic [MIII8MII6L24]n+ (MII = Co, Ni, Cu, and Pd; n = 0‐ 12) cubes formed from the reaction of [MIIIL3] and a “naked” MII salt are all topologically similar, with the MIII ions occupying the corners of the cubes and the MII ions occupying the faces. Excluding the PdII‐based cube, all of the complexes display magnetic exchange interactions at low temperatures. Due to the enormous size of these clusters and their resulting matrices, the magnetic fitting was done using the process of statistical spectroscopy. Chapter 3 describes the structural and magnetic studies of five [MIII2MII3L6]n+ (MIII = Cr, Fe, and Al; MII = Co, Zn, and Pd; HL = 1‐(4‐pyridyl)butane‐1,3‐dione; n = 0‐6) trigonal bipyramids, built using the diamagnetic and paramagnetic metalloligands [MIIIL3]. [FeIII2CoII3L6Cl6] represents the first magnetic trigonal bipyramid synthesised through the pyridyl‐based metalloligand approach. SQUID magnetometry studies show a weak antiferromagnetic exchange interactions between the FeIII and CoII ions, while EPR spectroscopy measurements demonstrate a small increase in the zero‐field splitting parameter of the FeIII ion upon coordination of [FeIIIL3] to a MII ion. Complete active space self‐consistent field (CASSCF) calculations show the axial zero‐field splitting parameter of CoII to be ≈‐14 cm‐1, which is consistent with the magnetothermal and spectroscopic data. Chapter 4 describes the synthesis and characterisation of six magnetic trigonal bipyramids, synthesised through dynamic covalent reactions of the metalloligand [FeIIILNH23] (HLNH2 = 1‐(4‐ aminophenyl)butane‐1,3‐dione) with either a dialdehyde or diacyl dichloride. The three [FeIII2MII3Lim3]n+ (MII = Co, Ni; n = 0‐6) imine‐based cages are formed from the reaction of the metalloligand with 2,6‐pyridinedicarboxaldehyde in the presence of a templating MII salt and a catalytic amount of acid, whereas the three [FeIII2Lam3] amide‐based cages are formed from the reaction of the metalloligand with isophthaloyl chloride in the presence of a base. The [FeIII2NiII3Lim3]n+ trigonal bipyramid displays weak antiferromagnetic interactions between FeIII and NiII ions, with JFe‐Ni = ‐0.12 cm‐1 and DNi = 8.93 cm‐1, while the [FeIII2Lam3] amide‐based cages display interesting configurational features dominated by the enthalpic gain from a series of intermolecular interactions.