The Syntheses, Structures And Properties Of Complexes Based On Imidazole Shiff-base Ligand

As a frontier field of molecular magnetic materials, spin-crossover complex (SCO) hasattracted extensive attention of chemists, physicists and material scientists due to its potentialapplication in temperature sensor, optical switch, display materials and high-density storagefield. Design and synthesis mulitifunctional complexes with SCO behavior and finish thestudies of their properties have important theoretical and realistic significance. This thesis isaimed at designing and synthesizing of various homochiral shiff-base ligands based onimidazole, and controlling the SCO properties of complexes through changing the strength ofligands. This thesis shall be divieded into three parts:(1) Two pairs of homochiral mononuclear iron(II) enantiomeric complexes:fac-Λ-[Fe(R-L1)3](ClO4)2·MeCN (1), fac-Δ-[Fe(S-L1)3](ClO4)2·2MeCN (2),fac-Λ-[Fe(R-L1)3](BF4)2·MeCN (3) and fac-Δ-[Fe(S-L1)3](BF4)2·MeCN (4)[L1=1-phenyl-N-(1-methyl-1H-imidazol-2-ylmethylene)ethanamine] have been designed andsynthesized successfully, detailed characterizations of ligand (R or S)-L1and complexes1-4have been accomplished by elemental analyses,1H NMR, IR, UV and CD. X-raycrystallography reveals that all the complexes crystallize in the chiral space group (P21),iron(II) center is surrounded by three bidentate ligands defining a pseudooctahedral [FeN6]coordination environment with fac configulation. Magnetic measurements reveal that all ofthem display obviously SCO behavior (LS→HS) at365K as the temperature rise. Afterdesolvated, they exhibit a reversible spin transition with a small hysteresis loop of ca.3Kappearing at about T1/2=222K and T1/2=219K. Two new complexes5(3·1/3(R)-LN)and6(3·1/3(S)-MGN) have been obtained by the cocrystallization of complexes3andracemic lactonitrile (LN) or2-methylglutaronitrile (MGN), both of them crystallize in a newchiral space group (P212121). Magnetic measurements reveal that the transition temperaturesof complexes5(3·1/3(R)-LN) and6(3·1/3(S)-MGN) are365K and333K, the introductionof LN or MGN and the removing of CH3CN have changed the SCO properties of complexe3.(2) Based on ligands1-(1-naphthyl)-N-(1H-imidazole-4-ylmethylene)ethanamine (L2)and1-(2-naphthyl)-N-(1-methyl-1H-imidazol-2-ylmethylene)ethanamine (L3), four couples ofhomochiral complexes have been synthesized and characterized successfully:fac-Λ-[Fe(R-L2)3](ClO4)2(7) and fac-Δ-[Fe(S-L2)3](ClO4)2H2O (8)、fac-Λ-[Fe(R-L2)3](BF4)2(9) and fac-Δ-[Fe(S-L2)3](BF4)2(10)、 fac-Λ-[Fe(R-L3)3](ClO4)2H2O (11) andfac-Δ-[Fe(S-L3)3](ClO4)2H2O (12)、 fac-Λ-[Fe(R-L3)3](BF4)2H2O (13) andfac-Δ-[Fe(S-L3)3](BF4)2H2O (14). X-ray crystallography reveals that all of them crystallize ina same chiral space group P213, ligands with R or S configuration induce complexes topresent a single fac-Λ or fac-Δ configuration separately. Magnetic measurements indicate thatcomplexes7-10always stay in High State (S=2) between300-2K, while complexes11-14with a stronger ligand strength show slowly SCO behaviours within this range.(3) Regarding1-bromo-hydrocarbon with different length of carbon chain as alkylatingreagents, the alkylation reactions of imidazole-2-carboxaldehyde have been completed. Afterthe condensation reaction with (R or S)-(±)-methylbenzylamine and the coordination reaction with FeX26H2O, a series complexes of fac-Λ-[Fe(R-Cn-L)3]X2and fac-Δ-[Fe(S-Cn-L)3]X2have been obtained (Cn-L=1-phenyl-N-(1-alkyl-1H-imidazol-2-ylmethylene)ethanamine; n=4、5、6、7、8、10、12、14、16、18、20and22; X=ClO4-and BF4-). X-ray crystallographyand magnetic measurements of complexes manifest they crystallize in various chiral spacegroup, form different accumulated structures in the space and display diversiform SCOproperties due to the difference of alkyl chain length. DSC thermal analysis prove that thealkylation reactions of imidazole-2-carboxaldehyde (n=14-22) bring new liquid craystalproperties to complexes successfully.