A new linear tricobalt compound with di(2-pyridyl)amide (dpa) ligands: Two-step spin crossover of [Co-3(dpa)(4)Cl-2][BF4]

Chemical oxidation of the linear tricobalt compound Co-3(dpa)(4)Cl-2 (1) wi th 1 equiv of NOBF4 yields the one-electron oxidized species [Co-3(dpa)(4)C l-2](+) (1(+)), which crystallizes from dichloromethane as a solvate [Co-3( dpa)(4)Cl-2][BF4]. 2CH(2)Cl(2) (2.2CH(2)Cl(2)). The crystal structure of 2. xCH(2)Cl(2) (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 symmetrica l arrangement of three Co atoms in the cation 1(+) with Co-Co distances of 2.3168(8) and 2.3289(8) A. The terminal cobalt to axial chloride distances are 2.350(1) and 2.376(1) A at 100 K. The cation maintains its symmetrical structure at ambient temperature (300 K) with only a slight lengthening (0. 01-0.02 A) of Co-Co and Co-N distances. Temperature studies of the magnetic properties for 2 in both solution and solid stare have revealed a spin-cro ssover behavior occurring in two gradual steps, indicating that 2 exhibits a diamagnetic ground state and thermally accessible triplet and quintet spi n 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 Delta H-1 = 8.7 kJ mol(-1), Delta S-1 = 43 J mol(-1) K-1 and Delta H-2 = 22.8 kJ mol(-1). Delta S-2 = 81 J mol(-1) K-1, and in the solid state to be Delta H-1 = 3.6 kJ mol(-1), Delta S-1 = 1 1 J mol(-1) K-1 and Delta H-2 = 17.4 kJ mol(-1), Delta S-2 = 53 J mol(-1) K -1. The characteristic temperatures of the spin crossover can be estimated from the above thermodynamic parameters (T-sc = Delta H/Delta S) 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 c lose to each other. These facts point to the conclusion that the thermal po pulation of the triplet and quintet states occurs quasi-simultaneously. To the best of our knowledge, this paper reports the first example of a two-st ep spin crossover in a Co compound.