Multistep spin-crossover in Hofmann frameworks containing functionalised ligands

This thesis focuses on the design, synthesis and characterisation of 2-D and 3-D Hofmann-like spin crossover (SCO) frameworks that incorporate functionalised 1,2,4-triazole or pyridine ligands. The overall aim is to strategically induce structural distortion through the inclusion of ligand functional groups with competing supramolecular interactions and explore the consequence to Hofmann structural and SCO properties. By this approach, this thesis is intended to contribute to enabling a deeper understanding of the structural drivers of diverse SCO characters, in particular multistep SCO profiles. This thesis is divided into two parts. In Part I (Chapters 3-5), a family of 2-D Hofmann-type frameworks are presented that include 1,2,4-triazole ligands functionalised to explore the impact of ligand length and supramolecular interaction capacity (including hydrogen bonding and -stacking) on the structure and SCO properties. In Part II (Chapters 6-7), 2-D and 3-D Hofmann-type frameworks are presented which include pyridyl ligands functionalised to study the effect of ligand length, steric bulk, and supramolecular interaction capacity (including hydrogen bonding and -stacking) on the structure and SCO properties. All of the materials presented in this thesis show notable structural distortion and undergo thermally induced SCO transitions, including many cases of multistep SCO character. Structure-function relationships are explored across all of the materials presented using a range of variable temperature structural and physical measurements. In Chapter 3, other SCO perturbations are explored, including, light-induced SCO, pressure-induced SCO and/or guest-modified SCO. Part I: Hofmann frameworks containing functionalised 1,2,4-triazole ligands Chapter 3 presents the 2-D Hofmann-type framework [Fe(cintrz)2Pd(CN)4]∙guest, where cintrz = (1E,2E)-3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine and guest = 3H2O, 2H2O, ∅. The framework shows structural distortion away from a regular Hofmann network driven by the presence of competing host-host and host-guest interactions which arise from the use of a 1,2,4-triazole group. The presence of the long cinamalidene group at the periphery of the ligand provides pore space between the layers to enable to exploration of guest exchange and removal. The analogues with guest = 3H2O, 2H2O, ∅ have been prepared and show minor structure change but dramatic change to SCO properties, i.e., ‘half’ one-step, two-step, and one-step SCO transitions, respectively. The SCO properties of guest = 3H2O are invested in detail using a broad array of external stimuli, including variable temperature, light- and pressure-induced SCO. Detailed structural analyses over these methods provide new insight into the relative importance of structural features on multistep SCO. Chapter 4 presents the 2-D Hofmann framework material [Fe3(N-cintrz)6(Pd(CN)4)3]·6H2O, where N-cintrz: (1E,2E)-3-(2-nitrophenyl)-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imin. The N-cintrz ligand contains a 1,2,4-triazole group and an additional nitro-group to provide a wider range of host-host and host-guest interaction sites than for the cintrz analogue presented in Chapter 3. As a result of the additional interactions, the 2-D framework shows extreme distortion, including multiple FeII sites and ligand conformations, and intra- and interlayer contacts dominated by hydrogen-bonding interactions rather than the typical π-stacking arrays characteristic of layered Hofmann frameworks. The temperature-dependence of the structure and SCO properties of this novel framework are investigated by a range of variable temperature experimental techniques. The results contained in this Chapter are published in “M. Ahmed, H. E. A. Brand, V. K. Peterson, J. K. Clegg, C. J. Kepert, J. R. Price, B. J. Powell, S. M. Neville, Dual-supramolecular contacts induce extreme Hofmann framework distortion and multi-stepped spin-crossover, Dalton Trans., 50, 1434-1442, 2021” (see Appendix A). Chapter 5 presents the 2-D Hofmann framework [Fe(furtrz)2Pd(CN)4]·EtOH,H2O, where furtrz = (E)-1(furan-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine. The furtrz ligand provides contrast to cintrz and N-cintrz presented in Chapters 3 & 4, being shorter in length and with similar interaction capacity to cintrz. The Hofmann framework structure shows moderate distortion, but most importantly shows weak interlayer communication via aromatic contacts. Variable temperature magnetic susceptibility data shows a three-step SCO transition. Variable temperature structure analyses (single crystal & powder diffraction) reveal the short-range order of spin-states over the three-step transition; long-range spin-state ordering is absent due to the lack of efficient interlayer communication pathways. The three-step SCO transition is likely present due to a temperature-dependent order-disorder transition of the ligand. Part II: Hofmann frameworks containing functionalised pyridyl ligands Chapter 6 presents the 2-D Hofmann-like frameworks [Fe(furpy)2Pd(CN)4]·guest, where furpy = N-(pyridin-4-yl)furan-2-carboxamide and guest = H2O,EtOH and H2O. The ligand furpy is the pyridyl analogues of furtrz (presented in Chapter 5) but contain amide functional groups in place of imine groups. The framework structures of the solvated and partially solvated species show distorted 2-D Hofmann frameworks, dominated by host-host and host-guest interactions as driven by the amide functional group. The guest loaded species shows an asymmetric multistep SCO transition and the partially desolvated an abrupt one-step SCO transition. The structure-property relationship is studied in detail via a range of variable temperature experimental techniques. Chapter 7 presents the 3-D FeII Hofmann-type framework material [Fe(Tbenpy)(Au(CN)2)2]·2H2O, DMF, where Tbenpy = N1,N3,N5-tri(pyridin-4-yl)benzene-1,3,5-tricarboxamide. The use of a three-connecting ligand, compared to the ligands used in all of the previous chapters, results in the formation of a 3-D Hofmann framework. The collective steric bulk and supramolecular interaction propensity of Tbenpy drives the formation of a unique 3-D framework structure, most significantly including a rare FeII(py)3(N≡C···)3 coordination environment. Through a range of variable temperature experimental techniques, this coordination environment is shown for the first time to be conducive to SCO. The results contained in this Chapter are published in “M. Ahmed, Z. Xie, S. Thoonen, C. Hua, C. J. Kepert, J. R. Price, S. M. Neville, A new spin crossover FeII coordination environment in a two-fold interpenetrated 3-D Hofmann-type framework material, Chem. Comm., 57, 85-88, 2021” (see Appendix A)