Hydrazone-based 3d/4f Complexes:synthesis And Magneto-structural Investigation

As a result of their potential applications ranging from medicine,such as magnetic resonance imaging(MRI),to next-generation molecular spintronic materials,comprising quantum information processing and high-density data storage,magnetic molecules offer an intriguing future.In particular,compounds exhibiting singlemolecule magnets(SMMs)and spin crossover(SCO)phenomena are one of the leading and extensively explored classes of magnetic molecules.While molecules possessing the capacity to retain magnetization in the absence of an external magnetic field are known as SMMs,compounds exhibiting magnetic bi-stability as a result of spin transition(ST)between the high spin(HS)and the low spin(LS)states are called SCO.Therefore,to assemble such magnetic molecules,transition and rare earth metals have been employed as spin centers due to their intrinsic electronic structure suitability.In this regard,in this thesis,ten new Dy(Ⅲ)and Fe(Ⅱ/Ⅲ)compounds using hydrazonebased ligands have been successfully synthesized.Furthermore,the magneto-structural dynamics of all compounds presented are fully characterized and for some selected compounds a theoretical calculation is performed to investigate their magnetic properties as follows:1.Initially,four new mono-nuclear Dy(Ⅲ)compounds(1-4)have been synthesized using pyrazolone-based aryloxide ligands.Interestingly,investigations into magneto-structural and ab initio calculations showed that all compounds displayed zero-field SMMs behavior.The given compounds magnetic properties were subtly tuned via chemical modifications to the principal crystal-field(CF)interaction,demonstrating that the magnetic features of the compounds vary depending on the ligands substituent and counterions.In comparison to neutral complexes,the compounds with the halide counter anions(1 and 2)displayed a relatively larger effective energy barrier(Ueff)as compared to the neutral compounds(3 and 4).Furthermore,ab initio calculations showed that the quantum tunneling of the magnetization(QTM)between the ground-state Kramers doublets(KDs)is very small or even vanishing for these combinations,indicating that the slow relaxation might proceed through higher excited states.2.Secondly,two new Dy(Ⅲ)compounds(5 and 6)has been assembled with the help of multi-dentate bis-pyrazolone carbohydrazide-based hydrazone ligands.Under eight coordination environment,the ligands structurally coordinated in two different ways with the Dy(Ⅲ)spin centers.The[2 × 2]grid 5 with asymmetric adjacent centers under D2d symmetry is realized by the deprotonated ligands in which carbonyl oxygen(μ2-Ocarbonyl)and hydroxyl(μ2-OH-)acting as bridging motifs.Whereas,the defected helicate-like compound 6 possessing symmetric adjacent centers with Czv symmetry is assembled by the partially and fully deprotonated ligands as well as coordinated solvents.Compound 5 showed a zero-field SMM behavior with a temperaturedependent multi-step relaxation of the magnetization that is consistent with the presence of independent centers having μ2-Ocarbonyl and μ2-OH-bridging motifs with slightly variable coordination geometry.Contrarily,the absence of the aforementioned bridging motifs,along with the coordinated additional third anionic pyrazolone oxygen(Opyrazolone)and solvents creating a centrosymmetric centers,result in compound 6 being magnetically unfavorable that exhibits only zero-field slow magnetic relaxation.3.Thirdly,the synthesis,crystal structure and magnetic property of two new Fe(Ⅲ)compounds(7 and 8)formulated from the pyrazolone-based thiosemicarbazone ligand at room temperature have been reported.Structurally,the Fe(Ⅲ)spin center was chelated under N2O2S2 coordination environment and then completed by chargeneutralizing counter ions for charge balance,i.e.the anion chloride(Cl-)for 7 and cation triethylammonium(NH(C2H5)3+)for 8.Magnetic measurements revealed that both compounds are high-spin(HS)at all the measurement temperature ranges(2-300 K)where the experimental bond distances(at 300 K)were also in good agreement suggesting a HS state.4.In the last,three new Fe(II)compounds(9-11)have been designed employing flexible glutarohydazide ligands featuring two helical-like and zigzag-like arrays.The two chelating ligands of the compounds were arranged differently,i.e.,an inverted Ushaped conformation for 9 and 11 as well as a twisted spiral arrangement for 10 were observed respectively.Interestingly,the observed structural variations of all the compounds are a result of the ligands substituents and counter ions variations.Magnetic investigations showed that both compounds 9 and 10 are in the HS state while 11 was found almost diamagnetic at 2-300 K ranges.Further ligand fine-tunings in the pyridylaldehyde moiety for the helical-like arrays bearing relatively strong field strength of the presented flexible ligands are suggested for achieving SCO event.