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.
Rittenberg
Durrell K.
DK
Department of Chemistry, 315 South 1400 East RM Dock, University of Utah, Salt Lake City, Utah 84112-0850.
Miller
Joel S.
JS