Molecular-based magnets constructed from cyanometalate building blocks

Entley, William Robert

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Description

Title

Molecular-based magnets constructed from cyanometalate building blocks

Author(s)

Entley, William Robert

Issue Date

1995

Doctoral Committee Chair(s)

Girolami, Gregory S.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Inorganic

Language

eng

Abstract

Solids related to Prussian blue (FeII,3lFen(CN)6]2,JcH20) are shown to be ideal candidates for the preparation of molecular-based magnets with high magnetic ordering temperatures: they can be easily prepared from well-characterized cyanometalate building blocks, the metal centers are linked covalently into a 3D network, and a wide range of metals with different spin states and oxidation states can be substituted into the lattice. The manganese compounds K2Mnn[Mnn(CN)6l, CsMnn[Mnm(CN)6]«V2H20, and Mnn3[Mnn(CN)6]»12H20 crystallize in face-centered cubic lattices with cell constants of 10.15, 10.69, and 10.62 A, respectively. Susceptibility and magnetization measurements show that these compounds are ferrimagnets at low temperatures: the Neel points of these compounds are 41,31, and 37 K, respectively. K2Mnn[Mn!I( CN)6] exhibits magnetic hysteresis at 24 K with a coercive field of ca. 370 Gauss and a remnant magnetization of 4.1 x 103 Gauss cm3 mol'1. CsMnII[Mnin(CN)6],1/2H20 also exhibits magnetic hysteresis at 4.5 K, with a coercive field of ca. 1100 Gauss and a remnant magnetization of 8.4 x 103 Gauss cm3 mol'1. A superexchange mechanism is presented which qualitatively accounts for the relative magnitude of the TN'S. The magnetic ordering temperature of CsMnn[Mnin(CN)6],1/2H20 is about 40 % lower than expected; this is attributed to the effects of cyanide linkage isomerism, which is evident in the IR spectrum of this compound but not the other two, and which causes a type of spin frustration. The compounds Ni%[Mnra(CN)6]2«12H20 and CsNin[Mnra(CN)6]«H20 crystallize in face-centered cubic lattices with cell constants of 10.29 and 10.42 A, respectively. Low field magnetization measurements reveal that these compounds exhibit ferromagnetic transitions at Tc = 30 and 42 K, respectively. Nin3[Mnra(CN)6]2*12H20 and CsNin[Mnra(CN)6>H20 both exhibit hysteresis below their magnetic phase transition temperatures. Coercive fields of He = 48 G and He « 39 G and remnant magnetizations of Mr = 760 G cm3 mol*1 and Mr ~ 630 G cm3 mol*1 are observed at 20 and 34 K for these compounds, respectively. The local ferromagnetic interactions are accounted for on the basis of an orbital symmetry model and the relative magnetic ordering temperatures are interpreted in terms of mean field theory: vacancies in the lattice reduce the magnetic ordering temperature. As the antiferromagnetic exchange interactions in Prussian blue analogues propagate principally through the empty it* orbitals of the cyanide ligands, higher magnetic ordering temperatures should result upon substituting into the structure metals that have high-energy (and more radially expanded) t2g orbitals, viz., early transition metals in lower oxidation states. Accordingly, we have prepared the first vanadium-substituted Prussian blue analogues Cs2Mnn[Vn(CN)6] and [NEu]o.4Mnn[Vn(CN)4Oo.5]o.8*l-2H20. Whereas the cesium salt crystallizes in a face-centered cubic lattice (a = 10.66 A), the NEu salt crystallizes in a non-cubic space group. These salts are ferrimagnets with Neel temperature of 125 and 230 K; only two other molecular magnets have higher magnetic ordering temperatures. Both compounds exhibit hysteresis loops characteristic of soft magnets at low temperatures, but the cesium salt shows anomalous variable-temperature magnetization behavior owing to the very small magnitude of Hc above 80 K. The magnetic properties of the rubidium salt Rbi.4MnII[VII(CN)6]o.85,l.lH20 and the potassium salt Ki.4MnH[vn(CN)6]o.85,l-3H20 resemble those of Cs2Mnn[Vn(CN)6]: their Neel temperatures are 151K and 162 K, respectively. The reaction of NEuCN with TiCMeCNMOgSCFs)] yields orange prisms of the first authentic cyanotitanate: [NE%]3[Tini(CN)6]. Crystallographic studies reveal that the anion in this salt adopts a nearly ideal octahedral geometry in the solid state (Ti-C = 2.202 A). This solid has been characterized by infrared (VCN = 2071 enr1) and UV-vis spectroscopy (AQ = 22800 cm'*), and its variable-temperature EPR spectra and magnetic susceptibility have been studied

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Copyright 1995 Entley, William Robert

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