We report single-molecule-transistor measurements on devices incorporating magnetic molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that sequential electron tunneling may enhance the magnetic relaxation of the magnetic molecule.

The substitution of Mo(III) for Cr(III) in metal-cyanide clusters is demonstrated as an effective means of increasing the strength of the magnetic exchange coupling and introducing magnetic anisotropy. Synthesis of the octahedral complex [(Me(3)tacn)Mo(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) is accomplished with the addition of precisely 3 equiv of LiCN to a solution of [(Me(3)tacn)Mo(CF(3)SO(3))(3)] in DMF. An excess of LiCN prompts formation of a seven-coordinate complex, [(Me(3)tacn)Mo(CN)(4)](1)(-), whereas less LiCN produces multinuclear species such as [(Me(3)tacn)(2)Mo(2)(CN)(5)](1+). In close parallel to reactions previously performed with [(Me(3)tacn)Cr(CN)(3)], assembly reactions between [(Me(3)tacn)Mo(CN)(3)] and [Ni(H(2)O)(6)](2+) or [(cyclam)Ni(H(2)O)(2)](2+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) afford face-centered cubic [(Me(3)tacn)(8)Mo(8)Ni(6)(CN)(24)](12+) and linear [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) clusters, respectively. Generation of the former involves a thermally induced cyanide linkage isomerization, which rapidly leads to a low-spin form of the cluster containing diamagnetic Ni(II) centers. The cyclic voltammagram of this species in DMF reveals a sequence of six successive reduction waves spaced approximately 130 mV apart, suggesting class II mixed-valence behavior upon reduction. The magnetic properties of the aforementioned linear cluster are consistent with the expected ferromagnetic coupling and an S = 4 ground state, but otherwise vary slightly with the specific conformation adopted (as influenced by the packing of associated counteranions and solvate molecules in the crystal). Magnetization data indicate an axial zero-field splitting parameter with a magnitude falling in the range [D] = 0.44-0.72 cm(-1), and fits to the magnetic susceptibility data yield exchange coupling constants in the range J = 17.0-17.6 cm(-1). These values represent significant increases over those displayed by the analogous Cr(III)-containing cluster. When perchlorate is used as a counteranion, [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) crystallizes from water in a dimeric form with pairs of the linear clusters directly linked via hydrogen bonding. In this case, fitting the magnetic susceptibility data requires use of two coupling constants: one intramolecular with J = 14.9 cm(-1) and another intermolecular with J' = -1.9 cm(-1). Reacting [(Me(3)tacn)Mo(CN)(3)] with a large excess of [(cyclam)Ni(H(2)O)(2)](2+) produces a [(Me(3)tacn)(2)(cyclam)(3)(H(2)O)(2)Ni(3)Mo(2)(CN)(6)](6+) cluster possessing a zigzag structure that is a simple extension of the linear cluster geometry. Its magnetic behavior is consistent with weaker ferromagnetic coupling and an S = 6 ground state. Similar reactions employing an equimolar ratio of reactants afford related one-dimensional chains of formula [(Me(3)tacn)(cyclam)NiMo(CN)(3)](2+). Once again, the ensuing structure depends on the associated counteranions, and the magnetic behavior indicates ferromagnetic coupling. It is hoped that subs...

We report the synthesis of the first well-documented example of a cyano-bridged single-molecule magnet. An assembly reaction parallel to that employed in producing the trigonal prismatic [(Me(3)tacn)(6)MnCr(6)(CN)(18)](2+) (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) cluster affords K[(Me(3)tacn)(6)MnMo(6)(CN)(18)](ClO(4))(3) (1), containing an analogous molybdenum(III)-substituted cluster. Fits to the DC magnetic susceptibility and magnetization data for 1 show that the MnMo(6) cluster possesses weak antiferromagnetic coupling (J = -6.7 cm(-1)), leading to an S = (13)/(2) ground state with significantly enhanced magnetic anisotropy (D = -0.33 cm(-1) and E = -0.018 cm(-1)). Consistent with these results, AC magnetic susceptibility measurements show the molecule to exhibit slow magnetic relaxation indicative of a single-molecule magnet with an energy barrier of 10 cm(-1) for spin reversal.

Reactions between K(3)[M(CN)(6)] and [Mn(5-Brsalen)(H(2)O)(2)](+) (5-Brsalen = N,N'-ethylenebis(5-bromosalicylidene)aminato dianion) in a mixture of methanol and water afford the compounds K[(5-Brsalen)(2)(H(2)O)(2)Mn(2)M(CN)(6)].2H(2)O, with M = Cr (1) or Fe (2). The two compounds are isostructural, each containing a molecular cluster with a linear Mn(III)-NC-M(III)-CN-Mn(III) core and tetragonally elongated coordination about the Mn(III) centers. Magnetic data indicate the presence of weak exchange interactions within the clusters, giving rise to ground states of S = (5)/(2) and (9)/(2) with significant zero-field splitting. Despite the proximity of spin-excited states, ac susceptibility data reveal frequency-dependent out-of-phase signals characteristic of single-molecule magnets with spin-reversal barriers of U(eff) = 16 and 25 cm(-)(1), respectively.

The synthesis of high-nuclearity metal-cyanide clusters presents a possible means of controlling magnetic properties in the design of new single-molecule magnets. Previous work employed tridentate blocking ligands in directing the assembly of a cubic [(tacn)8Co8(CN)12]12+ (tacn = 1,4,7-triazacyclononane) cluster; an improved crystal structure now confirms the lack of a guest water molecule inside the cluster cage. The ability to generate larger clusters by using a blocking ligand on only one of the mononuclear reaction components is demonstrated with the synthesis of a fourteen-metal [(Me3tacn)8Cr8Ni6(CN)24]12+ cluster. The geometry of this cluster consists of a cube of eight Me3tacn-ligated CrIII ions connected via bridging cyanide ligands to six square-planar NiII ions situated just above the center of each cube face. Surprisingly, no guest species are evident within the 284 Å3 cavity defined by the rigid metal-cyanide cage. Assembly of the cluster in boiling aqueous solution involves a linkage isomerization wherein the carbon end of each cyanide ligand reorients from binding a CrIII center in the reactant to binding the softer NiII center in the product. Consequently, the NiII ions become diamagnetic, resulting in magnetic behavior at high temperatures that is consistent with eight isolated CrIII (S = 3/2) ions per cluster. However, below 30 K, a drop in the χ M T is attributed to weak antiferromagnetic coupling between CrIII ions through the LUMO orbitals of the [Ni(CN)4]2--like units centering each cluster face. Carrying out the assembly reaction in methanol at −40 °C forestalls the linkage isomerization, yielding a high-spin green form of the cluster. Reaction of [(Me3tacn)8Cr8Ni6(CN)24]12+ with [Ni(CN)4]2- affords an aggregate species with a tetracyanonickelate ion capping each face of the cluster through a mean Ni···Ni contact of 3.00(1) Å, an interaction that destroys the long-range antiferromagnetic coupling between CrIII ions. Efforts to construct a larger cluster with an edge-bridged cubic geometry produced a linear [(Me3tacn)2(cyclam)NiCr2(CN)6]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) fragment exhibiting an S = 4 ground state. The weak ferromagnetic coupling (J = 10.9 cm-1) within this cluster leads to a more rapid decrease in the magnetization with increasing temperature at higher magnetic fields as a result of the Zeeman splitting and population of low-lying excited states.

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.

The use of 1,3,5-triaminocyclohexane (tach) as a capping ligand in generating metal-cyanide cage clusters with accessible cavities is demonstrated. The precursor complexes [(tach)M(CN)(3)] (M = Cr, Fe, Co) are synthesized by methods similar to those employed in preparing the analogous 1,4,7-triazacyclononane (tacn) complexes. Along with [(tach)Fe(CN)(3)](1)(-), the latter two species are found to adopt low-spin electron configurations. Assembly reactions between [(tach)M(CN)(3)] (M = Fe, Co) and [M'(H(2)O)(6)](2+) (M' = Ni, Co) in aqueous solution afford the clusters [(tach)(4)(H(2)O)(12)Ni(4)Co(4)(CN)(12)](8+), [(tach)(4)(H(2)O)(12)Co(8)(CN)(12)](8+), and [(tach)(4)(H(2)O)(12)Ni(4)Fe(4)(CN)(12)](8+), each possessing a cubic arrangement of eight metal ions linked through edge-spanning cyanide bridges. This geometry is stabilized by hydrogen-bonding interactions between tach and water ligands through an intervening solvate water molecule or bromide counteranion. The magnetic behavior of the Ni(4)Fe(4) cluster indicates weak ferromagnetic coupling (J = 5.5 cm(-)(1)) between the Ni(II) and Fe(III) centers, leading to an S = 6 ground state. Solutions containing [(tach)Fe(CN)(3)] and a large excess of [Ni(H(2)O)(6)](2+) instead yield a trigonal pyramidal [(tach)(H(2)O)(15)Ni(3)Fe(CN)(3)](6+) cluster, in which even weaker ferromagnetic coupling (J = 1.2 cm(-)(1)) gives rise to an S = (7)/(2) ground state. Paralleling reactions previously performed with [(Me(3)tacn)Cr(CN)(3)], [(tach)Cr(CN)(3)] reacts with [Ni(H(2)O)(6)](2+) in aqueous solution to produce [(tach)(8)Cr(8)Ni(6)(CN)(24)](12+), featuring a structure based on a cube of Cr(III) ions with each face centered by a square planar [Ni(CN)(4)](2)(-) unit. The metal-cyanide cage differs somewhat from that of the analogous Me(3)tacn-ligated cluster, however, in that it is distorted via compression along a body diagonal of the cube. Additionally, the compact tach capping ligands do not hinder access to the sizable interior cavity of the molecule, permitting host-guest chemistry. Mass spectrometry experiments indicate a 1:1 association of the intact cluster with tetrahydrofuran (THF) in aqueous solution, and a crystal structure shows the THF molecule to be suspended in the middle of the cluster cavity. Addition of THF to an aqueous solution containing [(tach)Co(CN)(3)] and [Cu(H(2)O)(6)](2+) templates the formation of a closely related cluster, [(tach)(8)(H(2)O)(6)Cu(6)Co(8)(CN)(24) superset THF](12+), in which paramagnetic Cu(II) ions with square pyramidal coordination are situated on the face-centering sites. Reactions intended to produce the cubic [(tach)(4)(H(2)O)(12)Co(8)(CN)(12)](8+) cluster frequently led to an isomeric two-dimensional framework, [(tach)(H(2)O)(3)Co(2)(CN)(3)](2+), exhibiting mer rather than fac stereochemistry at the [Co(H(2)O)(3)](2+) subunits. Attempts to assemble larger edge-bridged cubic clusters by reacting [(tach)Cr(CN)(3)] with [Ni(cyclam)](2+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) complexes instead generated extended one- or two-d...

High-nuclearity metal ± cyanide clusters may ultimately provide a vehicle for the design of new single-molecule magnet molecules possessing an energy barrier for magnetic moment reversal. [1] This contention is partly supported by recent work in which an understanding of the factors influencing superexchange interactions across a bridging cyanide ligand has led to the synthesis of Prussian blue type solids [2] with magnetic ordering temperatures as high as 373 K. [3] Typically, such solids are obtained from aqueous assembly reactions between octahedral [M(H 2 O) 6 ] x and [M'(CN) 6 ] yÀ complexes. The synthesis of molecular clusters with similarly adjustable magnetic properties is expected to require inhibition of some reactive sites on the precursor complexes through substitution of inert blocking ligands. For example, the use of 1,4,7-triazacyclononane (tacn) as a facecapping tridentate ligand on each metal complex can direct the formation of [(tacn) 8 M 4 M' 4 (CN) 12 ] z clusters with core structures consisting of a single cubic unit excised from the Prussian blue type framework. [4,5] However, to produce the exceptionally large spin states desiredÐalong with magnetic anisotropyÐfor increasing the spin reversal barrier in single-molecule magnets, it is necessary to develop methods for constructing larger cluster geometries in which more metal centers can be magnetically coupled. [6] A simple strategy for achieving higher nuclearities involves the use of a blocking ligand on just one of the reaction components, thereby permitting cluster growth to propagate further before a closed structure forms. Accordingly, the reaction between [Ni(H 2 O) 6 ] 2 and [(Me 3 tacn)-Cr(CN) 3 ] (Me 3 tacn N,N',N''-trimethyl-1,4,7-triazacyclononane) in aqueous solution generates [(Me 3 tacn) 8 -Cr 8 Ni 6 (CN) 24 ] 12 , a 14-metal cluster featuring a cube of eight Cr 3 ions with each square face spanned by a [Ni(CN) 4 ] 2À unit. [7] In further pursuing reactions of this type, we have discovered two new cluster geometries, including a 19-metal species that represents the largest metal ± cyanide cluster reported to date.Reaction of NiI 2 with [(Me 3 tacn)Cr(CN) 3 ] in aqueous solution does not lead to the face-centered cubic [(Me 3 tacn) 8 -Cr 8 Ni 6 (CN) 24 ] 12 cluster obtained with chloride, bromide, nitrate, or perchlorate as counteranions. [7] Instead, crystallographic analysis [8] of a red-brown crystal isolated from the reaction mixture revealed a product of composition 1, [(Me 3 tacn) 8 Cr 8 Ni 5 (CN) 24 ]I 10 ´27 H 2 O 1 featuring the open-cage cluster depicted in Figure 1. The core structure of this [(Me 3 tacn) 8 Cr 8 Ni 5 (CN) 24 ] 10 cluster most notably differs from the complete face-centered cubic geometry by having a Ni 2 ion missing from one of the cube faces.