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Magnetostructural studies of some mono- and polynuclear FeII spin crossover compounds containing new 2,2’-dipyridylamino-substituted s-triazine ligands
thesisposted on 08.02.2017, 06:04 by Ross, Tamsyn Maree
This thesis describes the synthesis and characterisation of new iron(II) spin crossover (SCO) compounds containing new 2,2’-dipyridylamino-substituted s-triazine ligands. The compounds have been comprehensively characterised using a range of techniques, with the aim of drawing structure-function correlations between magnetic properties and crystallographic models. This work contributes significantly to the number of known FeII SCO compounds containing 2,2’-dipyridylamino-substituted s-triazine ligands, giving new insight into factors influencing magnetic character as specific to these types of compounds, and to many key aspects of current, fundamental SCO research such as the influence of lattice solvent, correlations between temperature-dependent lattice deformations, structure and the spin transition, and the effects of order-disorder transitions and polymorphism. In Chapters Two and Three the synthesis and characterisation of ten new mononuclear or 1D polynuclear FeII SCO compounds of type trans-[FeII (NCX)2(L)n]•Solvent (where n = 1 or 2, L = DPPyT (1-(4,6-bis(dipyridin-2-ylamino)-1,3,5-triazin-2-yl)pyridin-4(1H)-one), DPT (6-phenoxy-N2,N2,N4,N4-tetra-2-pyridinyl-1,3,5-triazine-2,4-diamine) or DQT (4-(4,6-bis(dipyridin-2-ylamino)-1,3,5-triazin-2-yloxy)phenol)) are presented. The compounds trans-[FeII(NCS)2(DPT)]•2CH3OH•H2O and trans-[FeII(NCS)2(DQT)]•2CH2Cl2 each undergo a ‘half’ spin transition, and contain ordered, alternating -HS-LS-HS-LS- FeII sites within 1D polymeric chains at low temperatures. The role of lattice solvent in determining the nature of the observed spin transition is investigated via careful structural analysis and, in some cases, desolvation experiments, and the molecular-scale differences in the spin transition mechanism between solvates are determined. In Chapter Four the synthesis and characterisation of ten new monomeric FeII SCO species of type cis- or trans-[FeII(NCX)2(L)2]•Solvent (where NCX = NCS- or NCSe-, and L = DBB (N2,N2,N4,N4-tetrabenzyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DDB (N2,N2,N4,N4-tetrabutyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DCCl (6-chloro-N2,N2-dicyclohexyl-N4,N4-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine), DDT (N2,N2,N4,N4-tetraphenyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), and DTT (4,6-diphenoxy-N,N-di(pyridin-2-yl)-1,3,5-triazin-2-amine)) are presented. These monomers display a wide range of magnetic properties, both in terms of T½ and the abruptness of the spin transition. Two polymorphic forms of trans-[FeII(NCS)2(DBB)2] are investigated. Large differences in T½ and in the abruptness of the observed spin transition are seen between the polymorphs. The origins of the disparity in their magnetic character are attributed largely to differences in distortion of the FeN6 octahedron, alongside electronic influences on ligands from the different number and type of intermolecular interactions in which they participate in either polymorph. Elsewhere, the effects on magnetic properties of ligand conformation and bulk are investigated, as well as the effects of loss of solvation and intermolecular interactions between complex molecules. In Chapter Five the synthesis and characterisation of a family of analogues of type cis-[FeII(NCX)2(DDE)2] (where, here, NCX = each of NCS-, NCSe- and NCBH3-, DDE = N2,N2,N4,N4-tetraethyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine are presented. Each member of this family undergoes a somewhat gradual spin transition. Variable temperature crystallographic studies on cis-[FeII(NCSe)2(DDE)2] reveal a concomitant order-disorder and crystallographic phase transition, happening approximately two thirds of the way through the spin transition. This phase transition is also found to occur for the non-SCO analogue cis-[CoII(NCSe)2(DDE)2]. The crystallographic phase transition for cis-[FeII(NCSe)2(DDE)2] and cis-[CoII(NCSe)2(DDE)2] is linked to low-temperature ordering of the diethylamino extremities of DDE, and has no readily apparent effect on their magnetic susceptibility plots. For each of the cis-[FeII(NCX)2(DDE)2] analogues, as well as cis-[CoII(NCSe)2(DDE)2], preliminary calorimentric measurements are presented and discussed. In Chapter Six four new ligands DCrCr5 (N,N-di(pyridin-2-yl)-4,6-di(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazin-2-amine), DDCr5 (N2,N2,N4,N4-tetra(pyridin-2-yl)-6-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazine-2,4-diamine), DDCr6 (6-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)-N2,N2,N4,N4-tetra(pyridin-2-yl)-1,3,5-triazine-2,4-diamine), and DCrCr6 (4,6-di(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)-N,N-di(pyridin-2-yl)-1,3,5-triazin-2-amine), which are of varying size and denticity, are presented. These ligands each contain two distinct binding sites (the N-donor bis- 2,2’-dipyridylamino-chelators and the O- and N- donor mono-aza-crown rings) and have been designed to be incorporated into novel, heterobimetallic FeII SCO compounds of varying nuclearity and composition. The new coordination compounds trans-[FeII(NCS)2(DCrCr5)2Na2](ClO4)2•4CH3CH2CH2OH, trans-[FeII(NCS)2(DDCr5)]•CH3OH, cis-[FeII(NCS)4(DDCr5)2], cis-[FeII(NCS)4(DDCr6)2] and [Ba(DCrCr6)](ClO4)2•0.25CH3CH2CH2OH are presented. The key compound trans-[FeII(NCS)2(DCrCr5)2Na2](ClO4)2•4CH3CH2CH2OH is a novel heterobimetallic FeII SCO 1D polymer; the origins of its gradual spin transition are discussed. Magnetic susceptibility measurements and single crystal studies have also been performed on the 1D polymeric SCO compound trans-[FeII(NCS)2(DDCr5)]•CH3OH. Single crystal studies are presented on cis-[FeII(NCS)4(DDCr5)2], cis-[FeII(NCS)4(DDCr6)2] and [Ba(DCrCr6)](ClO4)2•0.25CH3CH2CH2OH. Difficulty has been met in obtaining heterobimetallic FeII compounds containing desired Group 1 and Group 2 s-block guest species in the mono-aza-crown rings and the challenges presented by targeted syntheses in this area, as well as future directions for synthetic work, are discussed. Finally, Chapter Seven contains the synthetic methods and characterisation for the new 2,2’-dipyridylamino-substituted s-triazine ligands, and their precursors, that have been synthesised for the present study. An account is given of the advantages and difficulties of the five general, synthetic routes used toward the synthesis of 2,2’-dipyridylamino-substituted s-triazines for the present study, and some key aspects of the NMR spectra of this species is discussed.