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Ferrimagnetic Behavior of Multiple Phases and Solvates of (meso-Tetrakis(4-chlorophenyl)porphinato)manganese(III) Tetracyanoethenide, [MnTClPP]+[TCNE]•-. Enhancement of Magnetic Coupling by Thermal Annealing

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journal contribution
posted on 11.06.1998, 00:00 by Erik J. Brandon, Durrell K. Rittenberg, Atta M. Arif, Joel S. Miller
(meso-Tetrakis(4-chlorophenyl)porphinato)manganese(III) tetracyanoethenide, [MnTClPP][TCNE], has been prepared and structurally characterized as the toluene and dichloromethane disolvates, and the magnetic and thermal properties of these solvates, as well as their corresponding desolvates, have been determined. The ditoluene solvate (1) has a triclinic unit cell:  P1̄, a = 10.171(4) Å, b = 10.189(3) Å, c = 14.522 (3) Å, α = 107.51(2)°, β = 85.58(2)°, γ = 111.51(3)°, Z = 1. The bis(dichloromethane) solvate (2) belongs to the monoclinic unit cell:  P21/n, a = 9.894(2) Å, b = 10.697(2) Å, c = 23.560(5) Å, β = 101.34(2)°, Z = 2. The cation is typical with average Mn−N distances of 2.012 Å for both the toluene and dichloromethane solvates. The bonding distances for both planar anions are characteristic of [TCNE]•-. Both solvates have an uniform linear chain (1-D) coordination-polymer structure comprised of alternating cations and anions. Each [TCNE]•- binds to two MnIII's in a trans-μ-N-σ-bound manner with Mn−N spacings of 2.267 (1) and 2.276 Å (2). The Mn−N−C angles are 167.2 and 143.1°, while intrachain Mn···Mn separations are 10.189 and 9.894 Å, and the dihedral angle between the MnN4 and [TCNE]•- mean planes are 86.8 and 52.4° for 1 and 2, respectively. The νCN absorptions for the toluene and dichloromethane solvates occur at 2201 m and 2160 s cm-1 and 2195 m and 2138 s cm-1, respectively. Upon thermolysis at 175 °C 1 desolvates to α-[MnTClPP][TCNE] with νCN absorptions at 2201 m and 2159 s cm-1. In contrast, desolvation of 1 in refluxing n-octane leads to β-[MnTclPP][TCNE] with νCN absorptions at 2190 m and 2132 s cm-1. Upon facile desolvation of 2 to form γ-[MnTClPP][TCNE] the nitrile absorptions remain essentially unchanged (2195 m and 2137 s cm-1). For 1 and α the susceptibilities can be fit by the Curie−Weiss expression with ϑ = −60 K (T > 210 K) and −10 K (T > 250 K) and an effective ϑ, ϑ‘ of +13 (50 < T < 120 K) and 29 K (60 < T < 160 K), respectively. ϑ is not observed for the 2 or the β- or γ-phase; however, ϑ‘ for the 2 and the β- and γ-phases are 58, 92, and 86 K, respectively. The magnetic data are consistent with linear chain ferrimagnets comprised of antiferromagnetically coupled S = 2 MnIII sites and S = 1/2 [TCNE]•- sites with the antiferromagnetic intrachain coupling, J/kB (kB = Boltzmann's constant) determined from fits to the Seiden expression, of −33, −160, −65, −267, and −265 K for the 1, 2, and α-, β-, and γ-phases, respectively. Hysteresis with a coercive field of 5.8 kOe is observed for 1 at 2 K. Metamagnetic behavior below 5 K is observed for the 1, 2, and β- and γ-phases with critical fields of 10, 27, 27, and 27 kOe, respectively. The ordering temperatures, Tc, determined from the maxima in the χ‘(T) data taken at 10 Hz, are 8.8, 6.7, 11.1, 14.1, and 11.4 K for the phases 1, α, β, 2, and γ, respectively. Desolvation of 1 and 2 increases the magnetic disorder and the magnetic coupling.

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