PubMed Health⌕ Search

Biomedical subjects

Joel S. Miller

Publications and source records attributed to Joel S. Miller.

6 recordsLinked to original sources
PubMed1999

Observation of Ferromagnetic and Antiferromagnetic Coupling in 1-D and 2-D Extended Structures of Copper(II) Terephthalates.

The reaction between CuCl(2).2H(2)O and disodium terephthalate, Na(2)tp, in aqueous solution simultaneously produces chain, bis(aqua)[&mgr;-(terephthalato-kappaO:kappaO')]copper(II), monohydrate, Cutp(OH(2))(2).H(2)O (1), and layered, bis(aqua)[&mgr;-(terephthalato-kappaO)]copper(II), Cutp(OH(2))(2) (2), structured materials. 1 (C(8)H(10)CuO(7)) belongs to the orthorhombic P2(1)2(1)2 space group [a = 6.3015(4) Å, b = 6.8743(4) Å, c = 22.9972(14) Å, and Z = 4] and incorporates tp in a bridging bis-monodentate binding mode and Cu(II) in a tetragonally elongated octahedron. 2 (C(8)H(10)CuO(6)) which belongs to the orthorhombic Pmc2(1) space group [a = 10.7421(8) Å, b = 7.2339(10) Å, c = 5.7143(13) Å, and Z = 2] incorporates tp in a mono-bidentate binding mode and Cu(II) in a distorted square pyramid. 1 and 2 exhibit axial X-band powder EPR spectra with g( perpendicular) = 2.08, g( parallel) = 2.29 (1) and g( perpendicular) = 2.07, g( parallel) = 2.29 (2) at 300 K. 1 obeys the Curie-Weiss law at high temperatures (theta = -7.2 K) and at low temperatures behaves as 1-D magnetic chains with an exchange-coupling constant of J/k(B)= -9.15 K (H = -2JS(1).S(2)). This material displays a spontaneous moment below 2 K under small applied magnetic fields, consistent with the presence of spin canting. 2 exhibits ferromagnetic interactions with theta = +0.8 K. Along the 1-D chain where coordinated water forms the bridge between metal centers, the coupling between Cu(II) is J/k(B) = +0.6 K. The fit of the magnetic susceptibility for 2 using a molecular field correction, which takes into consideration antiferromagnetic interactions between chains via the tp ligand, yields J'/k(B) = -0.13 K.

Journal Article↗
PubMed1999

Observation of Magnetic Ordering as High as 28 K for meso-Tetrakis(4-halophenyl)porphinatomanganese(III) Tetracyanoethenide, [MnTXPP][TCNE] (X = F, Br, I).

The toluene solvates of meso-tetrakis(4-halophenyl)porphinatomanganese(III) tetracyanoethenide, [MnTFPP][TCNE] (1F), [MnTBrPP][TCNE] (1Br), and [MnTIPP][TCNE] (1I) have been prepared, and the magnetic and thermal properties have been determined and compared to those of [MnTClPP][TCNE] (1Cl). 1Br and 1I form uniform 1-D chains with each [TCNE](*)(-) being trans-&mgr;-N-sigma-bound to Mn(III) with Mn-N distances of 2.293 (1Br) and 2.276 (1I), which are a bit longer than 2.267 Å observed for 1Cl. The Mn-N-C angles are 167.2, 168.1, and 158.7, while intrachain Mn.Mn separations are 10.189, 10.277, and 10.101 Å. The magnetic susceptibilities for 1Br and 1I can be fit by the Curie-Weiss expression with high-temperature (T > 200 K) theta and low-temperature (T < 110 K) effective theta' values of -53 and 13 K and -79 and 30 K, respectively, compared to -60 and 13 K in 1Cl. theta is not observed for 1F; however, theta' is 70 K. The magnetic data are consistent with linear chain ferrimagnets composed of antiferromagnetically coupled S = 2 Mn(III) sites and S = (1)/(2) [TCNE](*)(-) sites with the antiferromagnetic intrachain coupling, J(intra/)k(B) (k(B) = Boltzmann's constant) determined from fits to the Seiden expression, of -225, -33, -30, and -53 K for the 1F, 1Cl, 1Br, and 1I, respectively. Hysteresis with coercive fields at 2 K of 20.0, 6.7, 4.0, and 15.9 kOe was observed for the 1F, 1Cl, 1Br, and 1I, respectively. Metamagnetic behavior is observed below 10 K for 1F, and below 5 K for both 1Br and 1I. The observed critical fields of 21.8, 6.8, 4.1, and 15.8 kOe were observed for 1F, 1Cl, 1Br, and 1I at 2 K, respectively. The ordering temperatures, T(c), determined from the maxima in the chi'(T) data taken at 10 Hz, are 28.0, 8.8, 8.0, and 6.5 K for 1F, 1Cl, 1Br, and 1I, respectively. The 28 K T(c) for 1F is the highest reported for this family of magnets.

Journal Article↗
PubMed1998

Structure and Magnetic Properties of Antiferromagnetic Manganese(III) Tetrakis(4- methoxyphenyl)porphyrin Tetracyanoethenide, [MnTOMePP][TCNE].2PhMe, and Manganese(III) Tetrakis(2-fluorophenyl)porphyrin Tetracyanoethenide, [MnTFPP][TCNE].2PhMe.

The tetracyanoethylene (TCNE) electron-transfer salts of meso-tetrakis(4-methoxyphenyl)- and meso-tetrakis(2-fluorophenyl)porphinatomanganese(II), [MnTOMePP][TCNE].2PhMe, 1, and [MnTFPP][TCNE].2PhMe, 2, respectively, have been structurally and magnetically characterized. 1 and 2 belong to the triclinicP&onemacr; (No. 2) space group [1, a = 9.896(2) Å, b = 10.256(3) Å, c = 14.447(2) Å, alpha = 82.64(2) degrees, beta = 92.40(1) degrees, gamma = 109.07(2) degrees, T = -80 degrees C, Z = 1, and R(F(o)) = 0.0295; 2, a = 10.185(5) Å, b = 11.081(3) Å, c = 12.378(3) Å, alpha = 107.55(2) degrees, beta = 82.88(3) degrees, gamma = 111.09(3) degrees, T = -80 degrees C, Z = 1, and R(F(o)) = 0.0421]. 1 and 2 are coordination polymers with the Mn(III) sites bridged by trans-&mgr;-sigma-[TCNE](*)(-). The [TCNE](*)(-) is orientationally disordered in 2, with the minor form being rotated by 90 degrees with respect to the major form and in the same plane as the major form in an approximate 5:1 ratio. The Mn-N and intrachain Mn.Mn distances are 2.289 and 2.313 Å and 10.256 and 10.185 Å, and the Mn-N-C and the dihedral angle between the mean MnN(4)-porphyrin and the mean [TCNE](*)(-) planes are 165.53 and 78.1 degrees and 148.6 and 55.4 degrees for 1 and the occupancy-weighted average of 2, respectively. For 1 above 250 K the susceptibility can be fit by the Curie-Weiss expression with theta = -65 K, while between 75 and 190 K an effective theta of +21 K is observed; for 2 above 225 K, theta is -71 K, while between 40 and 100 K the effective theta is +45 K. The initial negative theta values along with minima in the chiT(T) data at 134 K (X = OMe) and 240 K (X = F) are consistent with antiferromagnetic coupling. One chi"(T) and two chi'(T) peaks are observed for both 1 and 2. The higher temperature [ approximately 10 K (1) and 12.5 K (2)] chi'(T) peaks are frequency independent, lack a corresponding chi"(T) absorption, and are assigned to a transition to an antiferromagnetic state. In contrast, the lower-temperature frequency dependent [5.6 K (1) and 7.5 K (2) at 10 Hz] chi'(T) peaks have a corresponding frequency dependent chi"(T) peaks and are assigned to the transition to a spin-glass or superparamagnetic state. The T(c)'s are defined as chi'(T) data taken at 10 Hz and are listed above.

Journal Article↗
PubMed1996

Weak Effect on T(c) with Increased Interchain Distances. Structure and Magnetic Properties of (meso-Tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphinato)manganese(III) Tetracyanoethenide, [Mn(III)TP'P](+)[TCNE](*-).

(meso-Tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphinato)manganese(III) tetracyanoethenide, [MnTP'P][TCNE], has been structurally and magnetically characterized. [MnTP'P][TCNE] (C(96)H(108)MnN(8)O(4)) belongs to the triclinicP&onemacr; (No. 2) space group with a = 8.597(2) Å, b = 14.756(4) Å, c = 17.573(5) Å, alpha = 101.16 (2) degrees, beta = 100.56(2) degrees, gamma = 96.37(2) degrees, and Z = 1. Due to the oxidative instability of the phenoxy groups, [Mn(III)TP'P][TCNE] was prepared from the reaction of [Mn(III)TP'P]OAc with the strong acid H(2)TCNE (pK(a) = 3.6) in the presence of TCNE to form acetic acid and the product. [MnTP'P][TCNE] is a coordination polymer with the Mn(III) sites bridged by trans-&mgr;(2)-bound [TCNE](*)(-) with relatively short (8.587 Å) intrachain and long (>/=14.756 Å) interchain Mn.Mn separations. The magnetic data above 210 K obey the Curie-Weiss expression with an effective &THETAV; value of 90.0 K, the largest yet reported for a soluble molecule-based magnet. In addition to a 15 K T(c) hysteretic behavior with a coercive field of 100 Oe is observed at 5 K. Despite the significant steric bulk leading to the substantially decreased interchain interactions that are crucial for magnetic ordering, the T(c) is unexpectedly high and suggests that other linear chain systems may be expected to exhibit magnetic ordering at higher temperatures.

Journal Article↗