The study of paramagnetic compounds based on 4d and 5d transition metals is an emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of paramagnetic compounds. Among the attractive characteristics of heavier elements vis-à-vis magnetism are the diffuse nature of their d orbitals, their strong magnetic anisotropy owing to enhanced spin-orbit coupling, and their diverse structural and redox properties. This critical review is intended to introduce readers to the topic and to report recent progress in this area. It is not fully comprehensive in scope although we strived to include all relevant topics and a large subset of references in the area. Herein we provide a survey of the history and current status of research that has been conducted on the topic of second and third row transition metal molecular magnetism. The article is organized according to the nature of the precursor building blocks with special topics being highlighted as illustrations of the special role of heavier transition metal ions in the field. This paper is addressed to readers who are interested in molecular magnetism and the application of coordination chemistry principles to materials synthesis (231 references).

No abstract

A series of structurally related pseudocubic metal cyanide clusters of Re(II) and 3d metal ions [{MX}4{Re(triphos)(CN)3}4] (M = Mn, Fe, Co, Ni, Zn; X = Cl, I, -OCH3) have been prepared, and their magnetic and electrochemical properties have been probed to evaluate the effect of changing the identity of the 3d metal ion. Electrochemistry of the clusters reveals several rhenium-based oxidation and reduction processes, some of which result in cluster fragmentation. The richest electrochemistry was observed for the iron congener, which exists as the Re(I)/Fe(III) cluster at the resting potential and exhibits six clear one-electron reversible redox couples and two, closely spaced one-electron quasi-reversible processes. The [{MnIICl}4{ReII(triphos)(CN)3}4] complex exhibits single molecule magnetism with a fast tunneling relaxation process observed at H = 0 determined by micro-SQUID magnetization measurements. A comparative evaluation of the magnetic properties across the series reveals that the compounds exhibit antiferromagnetic coupling between the metal ions, except for [{NiIICl}4{ReII(triphos)(CN)3}4] that shows ferromagnetic behavior. Despite the large ground-state spin value of [{NiIICl}4{ReII(triphos)(CN)3}4] (S = 6), only manganese congeners exhibit SMM behavior to 1.8 K.

The syntheses, structures, and magnetic properties of four new complex salts, (PPN){[Mn(III)(salphen)(MeOH)]2[M(III)(CN)6]}·7MeOH (Mn2M·7MeOH) (M = Fe, Ru, Os and Co; PPN(+) = bis(triphenylphosphoranylidene)ammonium cation; H2salphen = N,N'-bis(salicylidene)-1,2-diaminobenzene), and a mixed metal Co/Os analogue (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH were undertaken. It was found that all compounds exhibit switchable single-molecule magnet (SMM) and exchange-bias behavior depending on the interstitial methanol content. The pristine (PPN){[Mn(salphen)(MeOH)]2[Os(CN)6]}·7MeOH (Mn2Os·7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (Ueff/kB), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of Ueff/kB to 42.0 K and an opening of the hysteresis loop observable at 1.8 K. Mn2Os·7MeOH shows also exchange-bias behavior with magnetic hysteresis loops exhibiting a shift in the quantum tunneling to 0.25 T from zero-field. The Fe(III) and Ru(III) analogues were prepared as reference compounds for assessing the effect of the 5d versus 4d and 3d metal ions on the SMM properties. These compounds are also SMMs and exhibit similar effects but with lower energy barriers. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their slow relaxation of the magnetization properties. The (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)(CN)6]}·7MeOH (Mn2Co·7MeOH) analogue with a diamagnetic Co(III) central atom and the mixed Co/Os (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH (Mn2Co/Os·7MeOH) "magnetically diluted" system with a 9:1 Co/Os metal ratio were prepared in order to further probe the nature of the energy barrier increase upon desolvation of Mn2Os. In addition, inelastic neutron scattering and frequency-domain Fourier-transform THz electron paramagnetic resonance spectra obtained on Mn2Os·7MeOH and Mn2Os in combination with the magnetic data revealed the presence of anisotropic exchange interactions between Mn(III) and Os(III) ions.

To expand the field of new cyanide materials of the 5d elements, we incorporated the [Os(CN)(6)](3-) anion into PB architectures in combination with the Co(II) cation. Herein, we report the first example of a photomagnetic PB analog containing Os(III) ions. In a similar vein as the prototypical CoFe PB analogs, this compound exhibits a wide variety of properties including Charge Transfer Induced Spin Transition (CTIST), Temperature Induced Excited Spin State Trapping (TIESST), and magnetic ordering.

The synthesis and fabrication of nanoscale materials for new types of electronic and magnetic devices is a central theme in materials science research in this second decade of the 21st century. Given that conventional storage materials are estimated to approach their miniaturization limit by 2016, [1] heightened efforts are being directed at the design and synthesis of new types of bistable nanoscale materials, including those capable of undergoing a change from low to high resistance under the application of an electric field. Such nonvolatile memory devices are capable of operating at increased speeds and require less energy than conventional memory devices. Among the materials being investigated for resistance-based memory are materials that contain organic components and whose properties are influenced by magnetic or electric fields. [2] Materials that respond to the application of an electric field or changes in light, pressure, or temperature are being sought for incorporation into electronic devices with ultrafast operating speeds. [3,4] Examples of molecule-based materials that exhibit fascinating properties are the spin-crossover complex [Fe(picolylamine) 3 Cl 2 (C 2 H 5 OH)], [5,6] the neutralionic transition system TTF-chloranil (TTF = tetrathiafulvalene), [7][8][9] the metallo-organic conductor Cu(DM-DCNQI) 2 , [10][11][12][13][14][15][16] (DM-DCNQI = dimethyl-N,N'-dicyanoquinonediimine) and the salt (EDO-TTF) 2 PF 6 , [17,18] (EDO-TTF = ethylenedioxytetrathiafulvalene). These materials provide compelling evidence for the contention that molecular solids may eventually be useful in device applications.In terms of electric-field-induced behavior, the most extensively studied examples are the organocyanide-based materials Cu(TCNQ) (TCNQ = 7,7,8,, which exhibits reversible switching from a highresistance state to a conducting state promoted by the application of an electric field or upon irradiation, [19][20][21] and the current-driven conductor K(TCNQ) salt.[22] The latter material is a key member of the binary series of alkali-metal salts of TCNQ that behave as so-called "Mott insulators" at high temperatures, in which the fully reduced radical anions are arranged in columns with evenly spaced TCNQ units. At lower temperatures, these "soft" materials undergo a phase transition in which the TCNQ units are brought into close proximity as a result of p dimerization. The electrons are then trapped in the dimers, the conductivity drops, and the materials pass into the spin-Peierls insulating state.An approach that we have adopted for discovering conducting TCNQ phases is to capitalize on the rich chemistry of alkali metals while circumventing some issues that hinder their conductivity. In this vein, thallium is an interesting element, since it can behave as a pseudo-alkali metal. In contrast to other Group 13 elements, Tl prefers the 1 + oxidation state (although Tl 3+ is known), and many similarities between the chemistry of alkali-metal ions and Tl + have been noted.[23] The electronegativity of Tl (2.04)...

Derivatives of the single molecule magnet (SMM) {[Re(triphos)(CN)(3)](4)[MnCl](4)} (Re(4)Mn(4)) {[Re(triphos)(CN)(3)](4)[Mn(CH(3)CN)](4)}(ClO(4))(4) (2) and {[Re(triphos)(CN)(3)](4)[Mn(ClO(4))](3)[Mn(CH(3)OH)(CH(3)O)]} (3), were prepared by reacting [Et(4)N][Re(triphos)(CN)(3)] (1) with Mn(ClO(4))(2)·6H(2)O. Reaction of sodium dicyanamide, NaN(CN)(2), with compound 2 in CH(2)Cl(2) results in the product {[Re(triphos)(CN)(3)](4)[Mn(N(CN)(2))](4)} (4), in which each Mn(II) site is terminated by a dicyanamide ligand. A similar reaction of compound 3 in CH(2)Cl(2)-MeOH solution with LiTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) leads to the formation of {[Re(triphos)(CN)(3)](4)[Mn(TCNQ-TCNQ)(3)(Mn(HTCNQ)(CH(3)O))]} (5) whose structure is best described as a chain of dimers of Re(4)Mn(4) cubes bridged by sigma-bonded (TCNQ(2))(2-) linkers with one capping HTCNQ ligand.

Reactions between the [M(III)(CN)(6)](3-) (M = Fe and Co) anions and the mononuclear complex [Co(II)(dppe)(2)(H(2)O)][BF(4)](2) result in the formation of two isostructural trinuclear clusters {[Co(II)(dppe)(2)](2)[M(III)(CN)(6)]}(BF(4)). Surprisingly, reactions of [Co(dppe)(2)(H(2)O)](BF(4))(2) and [Co(triphos)(CH(3)CN)(2)](BF(4))(2) with [Cr(CN)(6)](3-) yield the mononuclear complexes [Co(dppe)(2)(CN)](BF(4)) and [Co(triphos)(CN)(2)], respectively. In the former case, an unusual pentanuclear intermediate complex {[Co(II)(3)(dppe)(4)(MeCN)][Cr(III)(CN)(6)](2)} was isolated. The reaction was probed by solution IR spectroscopy, which revealed a gradual conversion of the nu(C[triple bond]N) stretches of the starting materials to those of the CN-bridged intermediate and eventually to the single nu(C[triple bond]N) stretch of the final mononuclear product. The loss of carbon-bound CN(-) ligands from [Cr(CN)(6)](3-) occurs on a sufficiently slow time-scale for observation of varying degrees of cyanide linkage isomerism in the trigonal bipyramidal complex {[Co(tmphen)(2)](3)[Cr(CN)(6)](2)}; the study was aided by the use of different Co(II) starting materials. Results obtained by a combination of X-ray crystallography, infrared spectroscopy, and magnetometry provide unequivocal evidence that the presence of certain Lewis acids (e.g., Co(II) in this work and Fe(II) ions and BPh(3) in previously reported studies) promote the process of cyanide linkage isomerism, which, in the case of Co(II) species, leads to facile labilization of cyanide ligands from the [Cr(CN)(6)](3-) anion.