The Search for Molecular-Based Magnets: Synthesis and Magnetic Properties of New Low-Dimensional Magnetic Materials.
Abstract
Magnetism has interested physicists for decades, but magnetism through molecules is quite recent. One of the active areas of molecular magnetism is the design of new materials with unusual, but predictable magnetic properties. We show how simple orbital considerations permit the description of unpaired electrons in molecular complexes, allow us to foresee their interactions through molecular bridges, and to obtain molecular-based magnetic systems. The synthesis, structure and magnetic properties of linear chain 4-cyanopyridine complexes: (Cu(4-CNpy) _4H_2O) (ClO _4)_2, Cu(4-CNpy) _2Br_2 and M(4-CNpy) _2 Cl_2 (M = Cu, Ni, Co, Fe and Mn) are reported. All compounds in the 4-CNpy series crystallize in the monoclinic system. Crystallographic data for { (Cu(4-CNpy) _4H_2O) (ClO _4)_2})n Space Group P2 _1/c with a = 13.225(3) A, b = 10.567(2) A, c = 20.522(3) A; beta = 99.71(2) ^circ; V = 2826.7(9) A ^3; Z = 4, D_{rm cal} = 1.640Mg/m^3; mu = 3.430 mm^{ -1}. The compound contains planar CuL _4 units with the ligands bonded to copper ions through the ring nitrogen. These units are weakly linked into chains through semicoordinate bonds between the nitrile nitrogen and an adjacent copper ion via the axial sites. The magnetic data shows no interactions down to 2.0 K. Crystallographic data for (Cu(4-CNpy) _2Cl_2) n: Space Group P2_1/n with a = 3.779(2) A, b = 25.711(12) A, c = 7.104(4) A; beta = 95.98(4)^circ; V = 686.5(4) A^3; Z = 2, D_{ rm cal} = 1.658 Mg/m^3 ; mu = 25.177 mm^ {-1}. Crystallographic data for (Mn(4 -CNpy)_2Cl_2) n: Space Group P2_{rm 1} /c with a = 3.700(2) A, b = 7.198(2) A, c = 26.520(5) A; beta = 92.13(4)^ circ; V = 705.8(4) A^3; Z = 2, D_{rm cal} = 1.572 Mg/m^3; mu = 1.303 mm^{-1}. Both compounds have similar structure, consisting of chains of metal ions bibridged by chlorides with the 4-CNpy ligands coordinated to the metal ions in the axial sites through the pyridine nitrogens. Infrared spectra and powder X-ray diffraction patterns indicate that the remaining compounds form an isostructural series. However, the different space group indicate the the chains pack together in different patterns. Antiferromagnetic exchange (Cu,Mn) or ferromagnetic exchange (Ni,Co,Fe) exists within the chains and antiferromagnetic interactions between the chains are observed. The copper data have been fit to the model of an S = 1/2 Heisenberg antiferromagnetic linear chain with an exchange constant of J/k = -13.7(2) K. The nickel data have been fit to the model of an S = 1 ferromagnetic Heisenberg linear chain with an exchange constant J/k = +4.8(2) K. The cobalt data have been fit to the model of an S = 1/2 ferromagnetic Ising linear chain with an exchange constant J/k = +6.6(2) K. The iron data have not been successfully to fit to any model. The manganese data have been fit to an S = 5n antiferromagnetic Heisenberg linear chain model with an exchange constant J/k = -0.57(1) K. The Ni, Co and Fe compounds order antiferromagnetically at 7.2(2) K, 2.1(2) K, and 4.6(2) K respectively. (Abstract shortened by UMI.).
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1995
- Bibcode:
- 1995PhDT........24Z
- Keywords:
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- Physics: Molecular