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Joel S Miller

Publications and source records attributed to Joel S Miller.

At least 19 recordsLinked to original sources
PubMed2007

Control of two-electron four-center (2e-/4c) C-C bond formation observed for tetracyanoethenide dimerization, [TCNE]2(2-).

Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- dimers possessing a long 2.89 +/- 0.05 A two-electron four-center (2e-/4c) C-C bond in the solid state. This bond arises from the overlap of the b2g pi* singly occupied molecular orbital (SOMO) on each [TCNE]*- fragment, forming a filled bonding orbital of b2u symmetry, and the stabilizing effect of the cation...anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]*-...TCNE]*- motif in the solid state that lacks the 2e-/4c C-C bonding. To better understand the unusual nature of 2e-/4c C-C bonding, the genesis of the differences between their respective pi-[TCNE]*-...TCNE]*- interactions was sought. The lack of 2e-/4c C-C bond formation is attributed to the weaker radical character of the [TCNE]*- ligand, which has a total spin population of only 0.5 electron, half of that required for two S = 1/2 [TCNE]*- moieties to form a [TCNE]22- dimer. Hence, the antiferromagnetic MnII-[TCNE]*- intramolecular interaction (between the formally S = 1/2 Mn-bound [TCNE]*- and the paramagnetic Mn(II)) dominates over the intermolecular pi-[TCNE]*--[TCNE]*- spin coupling (between two S = 1/2 [TCNE]*- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that can interact with sigma-[TCNE]*-, dimerization forming [TCNE]22- can be favored or disfavored.

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PubMed2006

Magnetic ordering (Tc = 90 K) observed for layered [Fe(II)(TCNE*-)(NCMe)2]+[Fe(III)Cl4]- (TCNE = tetracyanoethylene).

[Fe(TCNE)(NCMe)2][FeCl4] is isolated from the reaction of TCNE and FeCl2(NCMe)2 and orders as a ferrimagnet below 90 K and is the initial member of a new class of magnets. It is the first metal-TCNE magnet with direct bonding between metal ion and [TCNE]*- whose structure has been determined, and it possesses a novel planar mu4-[TCNE]*- spin coupling unit bonded to four FeII's, with an axial pair of MeCNs. The [FeIIICl4]- anion occupies sites between the [FeII(TCNE*-)(NCMe)2]+ layers. [Fe(TCNE)(NCMe)2][FeCl4] has a coercive field of 1730 Oe and a remnant magnetization of 7500 emuK/mol at 50 K.

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PubMed2006

Solvent-enhanced magnetic ordering temperature for mixed-valent chromium hexacyanovanadate(II), Cr(II)0.5Cr(III)[V(II)(CN)6].zMeCN, magnetic materials.

The reaction of V(III)(THF)3Cl3 with NEt(4)CN in acetonitrile (MeCN) forms (NEt4)3[V(III)(CN)6].4MeCN (1), which after characterization was used as a molecular building block toward the synthesis of Prussian blue structured magnets. The reaction of 1 with [Cr(II)(NCMe)4](BF4)2 forms Cr(II)(0.5)Cr(III)[V(II)(CN)6].zMeCN via internal electron transfer, whose structure and magnetic properties are dependent on the degree of solvation, z. When solvated, Cr(II)(0.5)Cr(III)[V(II)(CN)6].1.2MeCN (2) is a mixture of crystalline and amorphous fractions that yield a material with two magnetic phases: bulk ferrimagnetic phase/crystalline [faced-centered-cubic lattice with a = 10.55(2) A] and cluster-glass phase/amorphous. The bulk ferrimagnetic phase exhibits a critical temperature, Tc, of 110 K, while the amorphous cluster-glass phase exhibits a freezing temperature, Tf, of approximately 25 K. Amorphous Cr(II)(0.5)Cr(III)[V(II)(CN)6].0.1MeCN (3) was determined to be the pure cluster-glass phase. This is an overall enhancement of 85 K (350%) in the magnetic ordering temperature via solvation, z. The coercivity was also increased 4-fold from 890 (2) and 3900 Oe (3) via desolvation.

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PubMed2006

Characterization of the chloranilate(*3-) pi radical as a strong spin-coupling bridging ligand.

Dinuclear [(TPA)Co(II)(CA2-)Co(II)(TPA)](BF4)2.2MeOH (1) [TPA = tris(2-pyridylmethyl)amine] and [(TPA)CoII(CA*3-)CoII(TPA)](BF4).2Et2O (2) with a bridging chloranilate radical ligand formed by reduction of 1 are crystallographically and magnetically characterized. 1 has shown a weak antiferromagnetic coupling within the Co(II) dimer [J/kB = -0.65 K (-0.45 cm(-1))], while 2 has a 2 orders of magnitude stronger antiferromagnetic interaction between the Co(II) ion and a radical [J/kB = -75 K (52 cm(-1))].

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PubMed2006

Solid-state NMR spectra and long intradimer bonds in the pi-[TCNE]22- dianion.

The principal (13)C chemical-shift values for the pi-[TCNE](2)(2-) dimer anion within an array of counterions have been measured to understand better the electronic structure of these atypical chemical species in several related TCNE-based structures. The structure of pi-[TCNE](2)(2-) is unusual as it contains two very long C-C bond lengths (ca. 2.9 Angstroms) between the two monomeric units and has been found to exist as a singlet state, suggestive of a (1)A(1g) (b(2u)(2)b(1g)(0)) electronic configuration. A systematic study of several oxidation states of [TCNE](n) (n = 0, 1-, 2-) was conducted to determine how the NMR chemical-shift tensor values change as a function of electronic structure and to understand the interactions that lead to spin-pairing of the monomer units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. Such theoretical methods provide information on the tensor magnitudes and orientations of their principal tensor components with respect to the molecular frame. Both theoretical and experimental ethylenic chemical-shielding tensors reveal high sensitivity in the component, delta(perpendicular), lying in the monomer molecular plane and perpendicular to the pi-electron plane. This largest shift dependence on charge density is observed to be about -111 ppm/e(-) for delta(perpendicular). The component in the molecular plane but parallel to the central C=C bond, delta(parallel), exhibits a sensitivity of approximately -43 ppm/e(-). However, the out-of-plane component delta'(perpendicular) shows a minimal dependence of -2.6 ppm/e(-) on the oxidation state (n) of [TCNE](n). These relative values support the claim that it is changes within the ethylenic pi-electrons and not the sigma-electrons that best account for the dramatic variations in bonding and shift tensors in this series of compounds. Concerning the intraion bonding, relatively weak Wiberg bond orders between the two monomeric components of the dimer correlate with the long bonds linking the two [TCNE(*)](-) monomers. The chemical-shift tensors for the cyano group, compared to the ethylene shifts, exhibit a reduced sensitivity on the TCNE oxidation state. The experimental principal chemical-shift components agree (within typical errors) with the calculated quantum mechanical shieldings used to correlate the bonding. The embedded ion model (EIM) was used to investigate the typically large electrostatic lattice potential in these ionic materials. Chemical-shielding principal values calculated with the EIM model differ from experiment by +/-3.82 ppm on average, whereas in the absence of an electrostatic field model, the experimental and theoretical results agree by +/-4.42 ppm, which is only a modest increase in error considering the overall ionic magnitudes associated with the tensor variations. Apparently, the effects of the sizable long-range electrostatic fields cancel when the shifts are computed because of lattice symmetry.

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PubMed2006

Structure and magnetic properties of a hydroxo-bridged copper(II) distorted cubane stabilized via supramolecular hydrogen bonding with an ionic hexafluoroacetylacetonate.

Tetranuclear [Cu(II)4(OH)4(aib)4)](hfac)4 (1; aib = 2-methyl-2-amino-4-iminopentane; hfac = hexafluoroacetylacetonate) forms from the reaction of aqueous ammonia and Cu(hfac)2.2H2O in acetone. The structure of 1 reveals that four noncoordinating hfac- counterions stabilize the distorted cubane complex via multiple H-bonding contacts. Magnetic susceptibility studies reveal that cubane-like 1 is best described as a pair of independent antiferromagnetically coupled dimers with g = 2.10 and J/kB = -298 K (207 cm(-1)) (H = -2JS1.S2).

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PubMed2006

Magnetically ordered molecule-based assemblies.

The development of molecules and assemblies of molecules exhibiting technologically important bulk properties, such as magnetic ordering, is an important worldwide research focus. Organic- and molecule-based magnets have been discovered and several families have been reported with magnetic ordering temperatures exceeding room temperature and as high as approximately 125 degrees C. Examples of both hard and soft magnets have been reported with coercivities as high at 27 000 Oe (and exceeding commercially available magnets) have been reported. Several examples are based on the radical anion of tetracyanoethylene, S = 1/2 [TCNE].-. The include ionic zero-dimensional (0-D) [FeCp*2]*+[TCNE]*- (Cp* = pentamethylcyclopentadienide), 1-D [MnTPP]+[TCNE]*- [TPP = meso-tetraphenylporphinato] coordination polymers, and 3-D extended network structured M[TCNE](x).ySolvent (M = V, Mn, Fe, Co, Ni, Dy). This Perspective focuses on work in our laboratory that will be discussed at the Dalton Discussion 9 meeting entitled "Functional Molecular Assemblies." In addition to the overview of TCNE-based organic magnets, the targeted design, preparation, and chemical as well as magnetic characterization of a new family of magnets based on S = 3/2 mixed-valent [Ru(II/III)2(O2CR)4]+ (R = Me, Bu(t)) is described. In particular [Ru2(O2CMe)4]3[Cr(CN)6] prepared from aqueous media possess two interpenetrating cubic lattices and magnetically orders at 33 K. In contrast, [Ru2(O2CBu(t))4]3[Cr(CN)6] forms a 2-D layered lattice and orders at 37.5 K. Both exhibit hysteretic behavior, however, this is quite anomalous for the former cubic lattice. This as well as other anomalous magnetic behaviors is attributed to the presence of the second interpenetrating lattice.

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PubMed2006

Tetracyanoethylene (TCNE): the characteristic geometries and vibrational absorptions of its numerous structures.

Tetracyanoethylene (TCNE) undergoes numerous reactions and is reported to exist in many structural motifs. Identification of these forms and motifs can be challenging. Nonetheless, the number of nu(CN) absorptions and their frequencies provide insight with respect to the specific forms and charge on the TCNE fragment. Particularly informative is the average of the fundamental nu(CN) bands, as well as the length of the central C-C bond. This Review discusses the assignment of structure and formal charge for TCNE-containing compounds. Scrutiny of previous assignments reveals some discrepancies which are discussed, and provides a basis for further study of TCNE structure-function relationships. Several multimetal complexes with bridging [TCNE](z) units exhibit mixed valency and extensive delocalization. The scarcity of suitable model compounds, especially those with M(d(pi-pi*)) backbonding to the CN groups, have thwarted the detailed description of these valence ambiguous compounds; thus, new well-characterized polynuclear compounds are needed.

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PubMed2006

Synthesis of layered (2-D) V-based bimetallic oxalates from non-aqueous media that cannot be synthesized from aqueous media.

The reaction of (NBu4)3[V(III)(ox)3] (1, ox = oxalate) and M(II) (M = Fe, Co, Ni, Cu) ions in MeCN, leads to the isolation of V-based coordination polymers of [N(n-Bu)4][Fe(II)V(II)I(ox)3].0.30[[N(n-Bu)4](BF4)] (2), [N(n-Bu)4][Co(II)V(III)(ox)3].0.75[[N(n-Bu)4](BF4)] (3), [N(n-Bu)4][Ni(II)V(III)(ox)3].0.20[[N(n-Bu)4](BF4)].0.20MeCN (4), and [N(n-Bu)4][Cu(II)V(III)(ox)2](BF4)2 (5) composition. Due to the lability of [V(III)(ox)3]3- to dissociate ox2-, these compounds cannot be prepared from aqueous media. 5 is best described as [N(n-Bu)4][V(III)Cu(II)(ox)2](BF4)2, and 2, 3, 4, and 5 are proposed to have a layered (2-D) motif for the MM(ox)x (x = 2, 3) extended framework. The [V(III)Cu(II)(ox)2] composition of 5 is reported for the first time for a bimetallic oxalate. 2 shows a weak antiferromagnetic interaction between Fe(II), S = 2 and V(III), S = 1 ions (theta = -9.4 K) within the 2-D layers. 3 and 5 do not magnetically order above 2 K. 4 magnetically order as ferromagnets below 2.55 K [taken as the onset of magnetization in chi''(T)], and has a glass transition temperature (chi'(max) at 1000 Hz) at 2.26 K.

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PubMed2006

Tris(chloranilato)ferrate(III) anionic building block containing the (Dihydroxo)oxodiiron(III) dimer cation: synthesis and characterization of [(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4)0.5.1.5MeOH.H2O [TPA = tris(2-pyridylmethyl)amine; CA = chloranilate].

[(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4)0.5.1.5MeOH.H2O (1) which possesses both the [FeIII(CA)3]3- (CA= chloranilate) and hydroxooxoiron(III) ions has had its structure determined by single-crystal X-ray diffraction. The 2-300 K magnetic susceptibility of 1 provides the magnetic parameters, g = 2.07, J/kB = -165 K (115 cm-1), theta = -1 K, and the spin impurity, rho = 0.05, which indicates a strong antiferromagnetic interaction between iron(III) ions via the oxo anion.

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PubMed2005

Synthesis, structure, and magnetic ordering of layered (2-D) V-based tris(oxalato)metalates.

The reaction of K3[M(III)(ox)3].3H2O [M = V (1), Cr; ox = oxalate], Mn(II)/V(II), and [N(n-Bu)4]Br in water leads to the isolation of 2-D V-based coordination polymers, [[N(n-Bu)4][Mn(II)V(III)(ox)3]]n (2), [[N(n-Bu)4][V(II)Cr(III)(ox)3]]n (3), [[N(n-Bu)4][V(II)V(III)(ox)3]]n (4), and an intermediate in the formation of 4, [[N(n-Bu)4][V(II)V(III)(ox)3(H2O)2]]n.2.5H2O (4a), while 1-D [V(II)(ox)(H2O)2]n (5) is obtained by using Na2ox and [V(OH2)6]SO4 in water. The structures of 1-5 have been investigated by single crystal and/or powder X-ray crystallography. In 1, V(III) is coordinated with three oxalate dianions as an approximately D3 symmetric, trigonally distorted octahedron. 1 is paramagnetic [mu(eff) = 2.68 mu(B) at 300 K, D = 3.84 cm(-1) (D/k(B) = 5.53 K), theta = -1.11 K, and g = 1.895], indicating an S = 1 ground state. 2 exhibits intralayer ferromagnetic coupling below 20 K, but does not magnetically order above 2 K, and 3 shows a strong antiferromagnetic interaction between V(II), S = 3/2 and Cr(III), S = 3/2 ions (theta = -116 K) within the 2-D layers. 4 and 4a magnetically order as ferrimagnets at T(c)'s, taken as the onset of magnetization, of 11 and 30 K, respectively. The 2 K remanent magnetizations are 2440 and 2230 emu.Oe mol(-1) and the coercive fields are 1460 and 4060 Oe for 4 and 4a, respectively. Both 4 and 4a clearly show frequency dependence, indicative of spin-glass-like behavior. The glass transition temperatures were at 6.3 and 27 K, respectively, for 4 and 4a. 1-D 5 exhibits antiferromagnetic coupling of -4.94 cm(-1) (H = -2Jsigma(i=1)n.S(i-1) - gmu(B)sigma(i=0)(n)H.S(i)) between the V(II) ions.

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PubMed2005

Structure and magnetic properties of (meso-tetraphenylporphinato)manganese(III) bis(dithiolato)nickelates.

The crystal structures of [MnTPP]{Ni[S2C2H(CN)]2} [MnTPP = (meso-tetraphenylporphinato)manganese(III)] and [MnTPP]{Ni[S2C2(CN)2]2} have been determined. These salts possess trans-mu-coordination of S = 1/2 {Ni[S2C2H(CN)]2}*- and {Ni[S(2)C(2)(CN)(2)](2)}*- to Mn(III) and form parallel 1-D coordination polymer chains exhibiting nu(CN) at 2210 and 2200 and 2220 and 2212 cm(-1), respectively. The bis(dithiolato) monoanions are planar and bridge two cations with MnN distances of 2.339(16), and 2.394(3) A, respectively, which are comparable to related MnN distances observed for [MnTPP][TCNE].x(solvates). In addition, [MnTP'P]{Ni[S2C2(CN)2]2} {H2TP'P = meso-tetrakis[3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin] and [MnTP'P(OH2)]{Ni[S2C2(CN)2]2} were prepared. The latter forms isolated paramagnetic ions. The room-temperature values of chiT for 1-D [MnTPP]{Ni[S2C2H(CN)]2}, [MnTPP]{Ni[S2C2(CN)2]2}, and [MnTP'P]{Ni[S2C2(CN)2]2} are 2.55, 3.28, and 2.86 emu K/mol, respectively. Susceptibility (chi) measurements between 2 and 300 K reveal weak antiferromagnetic interactions with theta= -5.9 and -0.2 K for [MnTPP]{Ni[S(2)C(2)H(CN)](2)} and [MnTPP]{Ni[S2C2(CN)2]2}, respectively, and stronger antiferromagnetic coupling of -50 K for [MnTP'P]{Ni[S2C2(CN)2]2} from fits of chi(T) to the Curie-Weiss law. The 1-D intrachain coupling, J(intra), of [MnTPP]{Ni[S2C2H(CN)]2} and [MnTPP]{Ni[S2C2(CN)2]2} was determined from modeling chiT(T) by the Seiden expression (H = -2JSi.Sj) with J/kB = -8.00 K (-5.55 cm(-1); -0.65 meV) for [MnTPP]{Ni[S2C2H(CN)]2}, J/kB = -3.00 K (-2.08 cm(-1); -0.25 meV) for [MnTP'P]{Ni[S2C2(CN)2]2}, and J/kB = -122 K (-85 cm(-1)) for [MnTP'P]{Ni[S2C2(CN)2]2}. These observed negative J(intra)/kB values are indicative of antiferromagnetic coupling. These materials order as ferrimagnets at 5.5, 2.3, and 8.0 K, for [MnTPP]{Ni[S2C2H(CN)]2}, [MnTPP]{Ni[S2C2(CN)2]2}, and [MnTP'P]{Ni[S2C2(CN)2]2}, respectively, based upon the temperature at which maximum in the 10 Hz chi'(T) data occurs. [MnTP'P]{Ni[S2C2(CN)2]2} has a coercivity of 17,700 Oe and remanent magnetizations of 7250 emu Oe/mol at 2 K and 17 Oe and 850 emu Oe/mol at 5 K; hence, upon cooling it goes from being a soft magnet to being a very hard magnet.

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PubMed2005

Cyano-bridged hexanuclear Fe4M2 (M = Ni, Co, Mn) clusters: spin-canted antiferromagnetic ordering of Fe4Ni2 cluster.

Heterobimetallic hexanuclear cyano-bridged complexes, [{Fe(Tp)(CN)3}4{M(MeCN)(H2O)2}(2)].10H2O.2MeCN [M = Ni (1), Co (2), Mn (3); Tp = hydrotris(1-pyrazolyl)borate], have been synthesized in H2O-MeCN solution. Complexes 1-3 are isostructural and hexanuclear with [{Fe(Tp)(CN)3}4{M(MeCN)(H2O)2}2] units linked by hydrogen bonds to form a 2D-structure in the solid state. Complex 1 is a canted antiferromagnet that undergoes a field-induced spin-flop-like transition at approximately 1 T and 2 K. At 4.45 K 1 has a transition to paramagnetic state of noninteracting S = 4 magnetic clusters. However, 2 and 3 show antiferromagnetic intracluster coupling. Facile loss of solvent from 2 alters the local symmetry resulting in changing the intracluster interaction from antiferro- to ferromagnetic.

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PubMed2004

Observation of hysteretic bistability in [Co(III)Cp(2)](+)[Fe(III)I(4)](-).

[Co(III)Cp(2)](+)[Fe(III)I(4)](-) (Cp = cyclopentadienyl) prepared by the double oxidation of FeI(2) and CoCp(2) with iodine exhibits a 30 degrees C thermal hysteresis in magnetic susceptibility between 134 and 164 K that is attributed to a phase transition to a disordered triclinic unit cell from an ordered monoclinic unit cell upon cooling.

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