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

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journal contribution
posted on 28.09.1999, 00:00 by Durrell K. Rittenberg, Joel S. Miller
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-μ-N-σ-bound to MnIII 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) θ and low-temperature (T < 110 K) effective θ‘ values of −53 and 13 K and −79 and 30 K, respectively, compared to −60 and 13 K in 1Cl. θ is not observed for 1F; however, θ‘ is 70 K. The magnetic data are consistent with linear chain ferrimagnets composed of antiferromagnetically coupled S = 2 MnIII sites and S = 1/2 [TCNE]•- sites with the antiferromagnetic intrachain coupling, Jintra/kB (kB = 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, Tc, determined from the maxima in the χ‘(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 Tc for 1F is the highest reported for this family of magnets.