Spin Crossover Supramolecular Coordination Compounds

Spin Crossover Supramolecular Coordination Compounds
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Total Pages : 244
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ISBN-10 : OCLC:986629383
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Book Synopsis Spin Crossover Supramolecular Coordination Compounds by : Mohanad D. Darawsheh

Download or read book Spin Crossover Supramolecular Coordination Compounds written by Mohanad D. Darawsheh and published by . This book was released on 2016 with total page 244 pages. Available in PDF, EPUB and Kindle. Book excerpt: The results obtained in this thesis demonstrate the importance of supramolecular chemistry for the design of potential new SCO clusters. Starting from synthesizing the suitable ligands with suitable functional groups, novel host-guest systems could be obtained where the guest play a major role in tuning the physical properties of the guest. Two bis-pyrazolylpyridine lignads, H2L4 and H2L6 were designed and prepared to achieve the assembly of transition metal ions in a triple-stranded helicate fashion where the central cavity can encapsulate different counterions depending on the size of this cavity. The N-H groups found in these ligands which usually directed toward the internal cavity help in the encapsulation of hydrogen acceptor anions. 13 iron-based compounds have been crystallized and studied adding significantly to the helical SCO compounds in the literature. The crystal structure for all the compounds were resolved, which allowed for an extensive study of supramolecular interactions and the influence of these interactions on the magnetic properties of the compounds. The first part of the thesis deals with spin-crossover dinuclear triple-stranded helicates compounds with encapsulated halide ions using H2L4. Six of such helicates with different encapsulated halide or counter ions were synthesized: Cl2"Fe2(H2L4)3]Cl(PF6)2·5.7CH3OH (1) Br2"Fe2(H2L4)3]Br(PF6)2·4CH3OH (2) Cl2"Fe2(H2L4)3]Cl(PF6)2·3CH3OH·1H2O (1a) Br2"Fe2(H2L4)3]Br(PF6)2·1CH3OH·1H2O (2a) Cl2"Fe2(H2L4)3](I3)3·3(Et2O) (3) Br2"Fe2(H2L4)3](I3)3·3(Et2O) (4) These isostructural compounds consist of triple-stranded helicates that encapsulate halide ion inside their cavity. The main difference is the kind of outer counterions and lattice solvents which affect dramatically the magnetic properties of these compounds as a result of changing the supramolecular interactions. Changing the halide ion from chloride to bromide in 1 and 2, respectively, shift the spin transition by 30 K. The SCO occur here from [HS-LS] to [HS-HS] upon heating. Compounds 1a and 2a are the water solvate helicates that produced from single-crystal to single-crystal exchange by exposing the crystals to the ambient water. This exchange leads to important changes; the {X2"[Fe2(H2L4)3]}3+ helicate are now symmetric and the two Fe centers are crystallographically identical. This change in the solvent affects dramatically the SCO behavior of the helicates. Two-step SCO from [LS-LS] à [HS- LS]à [HS-HS] states was observed in the bulk magnetic studies. Compounds 3 and 4 consists of {X2"[Fe2(H2L4)3]}3+ triple-stranded helicates similar to the one seen in previous helicates where the counter ions are now three triodide (I3- ) linear ions occupying the outer space formed between the helical strands and the solvent is ether. In these complexes the iron centers remain in the HS state through all the temperatures. The change of the solvents used in the reaction yielded different supramolecular compounds using the same ligands H2L4. Dimerized mononuclear helicates {X2"Fe(H2L4)3]2}3+ was prepared where a halide ion is encapsulated inside the cavity formed by the intercalating dimers. Five dimerized triple-stranded helicates are presented in this thesis: Cl2"Fe(H2L4)3]2(OH)(PF6)2·H2O (5). Cl2"Fe(H2L4)3]2(FeCl4)3·2C3H6O·4C7H8 (6). Br2"Fe(H2L4)3]2(OH)(PF6)2·H2O (7). I2"Fe(H2L4)3]2(PF6)2.23(I)0.21(I3)0.56·2CH3OH (8) I2"Fe(H2L4)3]2(I)2(I3)0.6(OH)0.4·0.6H2O·2CH3OH·2C3H6O (9) In every mononuclear helicate, one pyrazolyl-pyridine side of each ligand is not coordinated to any metal ion. The SCO behavior of the dimer is also affected by the nature of the halide ions which make hydrogen bonding with the N-H groups of the pyrazole rings. Using the ligand H2L6, the encapsulation of [M(III)(ox)3]3- (M = Fe and Cr; ox = oxalate) metal complexes inside the helical cavity of [Fe2(H2L6)3]4+ was achieved. Two of such triple-stranded helicates are presented in this thesis: Fe(C2O4)32"Fe2(H2L6)3](BF4)·4CH3OH·3.7H2O (10). Cr(C2O4)32"Fe2(H2L6)3](BF4)·1.4CH3OH·6H2O (11). The Fe(II) ions of the helicate exhibit SCO behavior and LIESST effect in the case of the encapsulated chromium oxalate complex. Interestingly, the guest [Cr(III)(ox)3]3- exhibits SIM-like behavior at low temperatures. This is the first example where a host- guest system exhibits both LIESST effect and SIM behavior.


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