Crystal Engineering and Organometallic Architecture.

Magnets composed of molecular components that provide both electron spins and spin-coupling pathways can stabilize bulk magnetic ordering. This was first reported for the ionic, zero-dimensional (0-D) electron transfer salt [Fe(C5Me5)2]+[TCNE]˙− (TCNE = tetracyanoethylene), which orders as a ferromagnet at Tc = 4.8 K. Later V[TCNE]x (x ∼ 2) was characterized to order above room temperature at 400 K (127 °C). Subsequently, numerous examples of organic- and molecule-based magnets have been characterized. In this critical review, after a discussion of the important aspects of magnetism pertaining to molecule-based magnets, including the determination of the magnetic ordering temperature (Tc) these magnetically ordered materials are reviewed from a perspective of the structural dimensionality (208 references).

Magnetically ordered molecule-based materials

Organometallic- and organic-based materials exhibiting the technologically important property of bulk magnetism have been designed, prepared, and studied. These magnets are prepared via conventional organic chemistry methodologies and unlike conventional inorganic-based magnets do not require metallurgical processing. Furthermore, these magnets are frequently soluble in conventional solvents, and examples have saturation magnetizations exceeding that of iron metal on an Fe or mole basis. Also, magnets with critical temperatures exceeding room temperature, magnets with coercive fields exceeding that of Co5Sm, and thin-film magnets have been prepared. This article highlights the collective joint research executed in our, as well as Arthur J. Epstein's, laboratories.

Organometallic- and organic-based magnets: New chemistry and new materials for the new millennium

Unusual dimers with wide interplanar separations, that is, very long bonds with dD ≈ 3.05 A, are common to the spontaneous self-association of various organic π-radicals in solution and in the crystalline solid state, independent of whether they are derived from negatively charged anion radicals of planar electron acceptors (TCNE-•, TCNQ-•, DDQ-•, CA-•), positively charged biphenylene cation-radical (OMB+•), or neutral phenalene radical (PHEN•). All dimeric species are characterized by intense absorption bands in the near-IR region that are diagnostic of the charge-transfer transitions previously identified with intermolecular associations of various electron-donor/acceptor dyads. The extensive delocalizations of a pair of π-electrons accord with the sizable values of (i) the enthalpies (−ΔHD) and entropies (−ΔSD) of π-dimerization measured by quantitative UV−vis/EPR spectroscopies and (ii) the electronic coupling element Hab evaluated from the strongly allowed optical transitions, irrespective of whether...

Stable (Long-Bonded) Dimers via the Quantitative Self-Association of Different Cationic, Anionic, and Uncharged π-Radicals: Structures, Energetics, and Optical Transitions

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.

https://science.sciencemag.org/content/sci/240/4848/40.full.pdf

Molecular/Organic Ferromagnets

The charge-transfer reaction of decamethylferrocene, Fe(C/sub 5/Me/sub 5/)/sub 2/, Fc*, and 7,7,8,8-tetracyano-p-quinodimethane, TCNQ, has been characterized. Three major reaction products of varying stoichiometry, conductivity, and magnetism are formed: a 1-D metamagnetic 1:1 salt, (Fc*)/sup +/(TCNQ)/sup -/; a paramagnetic (Fc*)/sup 2/ /sup +/(TCNQ)/sup 22 -/; 1:1; dimeric salt, (Fc*)/sup 2/ /sup +/(TCNQ)/sup 22 -/; and a conducting 1:2 salt of (Fc*) /sup +/(TCNQ)/sup 2 -/ composition. The crystal and molecular structures of the 1:1 paramagnetic dimeric and metamagnetic one-dimensional salts were solved. The ions in the dimeric phase crystallize in the centrosymmetric monoclinic space group P2/sub 1//c with a = 9.708 (1) A, b = 12.211 (2) A, c = 23.585 (4) A, ..beta.. = 95.01 (1)/sup 0/, Z = 4, and R = 0.058 for 3665 independent reflections. The magnetic susceptibility of powder samples of (Fc*)/sup 2/ /sup +/(TCNQ/sub 2/)/sub 2//sup 2 -/ obeys the Curie expression above 1.5 K suggesting very little magnetic coupling between the intradimer S = 1/2 Fe(III)'s which are separated by approx.14 A. The (TCNQ)/sub 2//sup 2 -/ dimer is strongly antiferromagnetically coupled. Ions in the linear chain metamagnetic phase crystallize in the centrosymmetric triclinic space group P1 vector. The unit cell at -106 /supmore » 0/C is a = 8.635 (4) A, b = 9.384 (6) A, c = 10.635 (9) A; ..cap alpha.. = 116.76 (5)/sup 0/, ..beta.. = 112.58 (5)/sup 0/; ..gamma.. = 72.49 (4)/sup 0/, V = 701.8 A/sup 3/, Z = 1, and R/sub w/ = 0.060 for 1156 reflections. The one-dimensional phase exhibits metamagnetic properties with a Neel temperature of 2.55 K. The intrachain Fe/sup III/-Fe/sup III/separation is 10.540 A. The calculated structure and scaled vibrational frequencies are in excellent agreement with the observed values.« less

Characterization of the charge-transfer reaction between decamethylferrocene and 7,7,8,8-tetracyano-p-quinodimethane (1:1): the 57Fe Mössbauer spectra and structures of the paramagnetic dimeric and the metamagnetic one-dimensional salts and the molecular and electronic structures of [TCNQ]n (n=0, −1, −2)

Crystal and molecular structure of the paramagnetic 1:1 decamethylferrocenium 7,7,8,8-tetracyano-p-quinodimethanide dimer salt: {[Fe(C5Me5)2]+.cntdot.}2(TCNQ)22-

L. D. Preston

Papers

Crystal and molecular structure of the paramagnetic 1:1 decamethylferrocenium 7,7,8,8-tetracyano-p-quinodimethanide dimer salt: {[Fe(C5Me5)2]+.cntdot.}2(TCNQ)22-