A new linear tricobalt compound with di(2-pyridyl)amide (dpa) ligands: Two-step spin crossover of [Co₃(dpa)₄Cl₂][BF₄]

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 CoCo 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 mutually implies S = 1 and S = 1 mutually implies 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 (T(sc) = ΔH/ΔS) as T(sc1) = 201 K and T(sc2) = 281 K in solution and T(sc1) = 315 K and T(sc2) = 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.