Synthetic, Structural And Magnetic Studies Of Iron(II) Spin States Based On Pyridyl-Pyrazole Ligands

The functionalization of coordination compounds is a fast-developing trend in the fields of chemistry and materials, and the structure-property correlation lies at the foundation of understanding and tuning the properties of molecule-based materials. As known, iron(II) is a classic d6 metal suited for octahedral ligand field, which experiences the synergy between ligand field splitting energy and electron pairing energy to determine its spin states, and further to affect the magnetic property of iron(II)-based compounds. Our work mainly focused on utilizing two pyridyl-pyrazole ligands, namely, 3,5-bis(2-pyridyl)-pyrazole (HL2-2) and 3-(2-pyridyl)-5-(4-pyridyl)-pyrazole (HL2-4), whose coordination abilities had been well investigated in our group previously, to fabricate pre-designed iron(II) coordination compounds, ranging from mononuclear, dinuclear or pentanuclear oligomers to one-dimensional or three-dimensional polymers. Particularly, we were interested in the iron(II) spin states in these targeted complexes, and we were able to confirm the high-spin or low-spin states by combining crystallographic analysis and magnetic measurement. The thesis is divided into three chapters.In Chapter One, the research background and recent progresses on [FeIIN6] complexes and pyridyl-pyrazole ligands were concisely introduced, with emphasis on the ligand field theory and the ligand field splitting energy versus electron pairing energy to determine the spin states. In addition, research works on the pyridyl-pyrazole ligands in our group were briefly summarized, and the purpose and significance of this work were outlined.In Chapter Two, the bis-chelating ligand, namely, 3,5-bis(2-pyridyl)-pyrazole (HL2-2) was introduced to react with iron(II) salts under hydro(solvo)thermal conditions, yielding four oligomeric iron(II) complexes 1-4. Compound [Fe(NCS)2(HL2-2)2] (1) was a mononuclear structure with typical octahedral [FeIIN6] sphere, and structural and magnetic investigations both confirmed the high-spin state for iron(II). Compounds [Fe2(NCS)2(L2-2)2] (2) and [Fe2Cl2(L2-2)2] (3) were dinuclear structures, and the coordination mode was unusual with five-coordinated [FeIIN5] sphere. The magnetic measurements for 2 and 3 indicated antiferromagnetic coupling between the two iron(II) cores, while the coupling interaction was slightly interfered by the different co-ligands NCS- and Cl-. Compound [Fe5(L2-2)6O]X (4) was a pentanuclear cluster helicate, which was previously reported to experience antiferromagnetic coupling.In Chapter Three, the semi-chelating-semi-bridging ligand, namely, 3-(2-pyridyl)-5-(4- pyridyl)-pyrazole (HL2-4) was introduced to react with iron(II) salts under hydro(solvo)thermal conditions, yielding four polymeric iron(II) complexes 5-8. Compound [Fe(NCS)2(HL2-4)]n (5) was a zigzag chain-like coordination polymer with rare five-coordinated [FeIIN5] sphere, and structural and magnetic investigations both confirmed the high-spin state for iron(II), similar to that of 1. Compound [Fe(L2-4)2·2H2O]n (6) was a two-fold interpenetrating three-dimensional network with hexagonal channels filled with water solvent molecules. Though the classic [FeIIN6] sphere was observed in 6, the low-spin state for iron(II), contrary to that of 1 and 5, was unambiguously confirmed by both structural and magnetic evidences. Compound [Fe3(NCS)6(HL2-4)3·H2O]n (7) exihited a ring-chain structure with both [FeIIN5] and [FeIIN6] modes observed, while compound [FeCl2(HL2-4)]n (8) was a two-fold interpenetrating three-dimensional network, which was closely related to the structure of 6 but without guest molecule filling the quadrangular channels. Compounds 7 and 8 had yet to be magnetically characterized due to difficulties concerning product purity and yield.This research was based on the well investigated pyridyl-pyrazole ligand system in our group, and was conducted following the principle of crystal engineering, and hence the targeted iron(II) coordination compounds were closely related structurally and magnetically. This work offers insight for the investigation of structure-property correlation, especially for determining the spin states, by combining crystallographic analysis and magnetic measurement.