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A New Linear Tricobalt Compound with Di(2-pyridyl)amide (dpa) Ligands:  Two-Step Spin Crossover of [Co3(dpa)4Cl2][BF4]

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posted on 18.02.2000 by Rodolphe Clérac, F. Albert Cotton, Kim R. Dunbar, Tongbu Lu, Carlos A. Murillo, Xiaoping Wang
Chemical oxidation of the linear tricobalt compound Co3(dpa)4Cl2 (1) with 1 equiv of NOBF4 yields the one-electron oxidized species [Co3(dpa)4Cl2]+ (1+), which crystallizes from dichloromethane as a solvate [Co3(dpa)4Cl2][BF4]·2CH2Cl2 (2·2CH2Cl2). The crystal structure of 2·xCH2Cl2 (with x varying from 1.125 to 2.0) has been determined at 100, 213, and 300 K, showing very little variation and an essentially symmetrical arrangement of three Co atoms in the cation 1+ with Co−Co distances of 2.3168(8) and 2.3289(8) Å. The terminal cobalt to axial chloride distances are 2.350(1) and 2.376(1) Å at 100 K. The cation maintains its symmetrical structure at ambient temperature (300 K) with only a slight lengthening (0.01−0.02 Å) of Co−Co and Co−N distances. Temperature studies of the magnetic properties for 2 in both solution and solid state have revealed a spin-crossover behavior occurring in two gradual steps, indicating that 2 exhibits a diamagnetic ground state and thermally accessible triplet and quintet spin states. An ideal solution model taking into account the two equilibria (S = 0 ↔ S = 1 and S = 1 ↔ S = 2) has been used to fit the magnetic data. The derived enthalpy and the entropy changes associated with the two steps have been found in solution to be ΔH1 = 8.7 kJ mol-1, ΔS1 = 43 J mol-1 K-1 and ΔH2 = 22.8 kJ mol-1, ΔS2 = 81 J mol-1 K-1, and in the solid state to be ΔH1 = 3.6 kJ mol-1, ΔS1 = 11 J mol-1 K-1 and ΔH2 = 17.4 kJ mol-1, ΔS2 = 53 J mol-1 K-1. The characteristic temperatures of the spin crossover can be estimated from the above thermodynamic parameters (Tsc = ΔHS) as Tsc1 = 201 K and Tsc2 = 281 K in solution and Tsc1 = 315 K and Tsc2 = 330 K in the solid state. The shape of the magnetic curves do not include a plateau for an intermediate spin state, and characteristic temperatures are very close to each other. These facts point to the conclusion that the thermal population of the triplet and quintet states occurs quasi-simultaneously. To the best of our knowledge, this paper reports the first example of a two-step spin crossover in a Co compound.