Chemically Separable Co(II) Spin-State Isomers

The phenomenon of spin crossover involves coordination complexes with switchable spin states. This spin state change is accompanied by significant geometric changes such that low and high spin forms of a complex are distinct isomers that exist in equilibrium with one another. Typically, spin-state isomers interconvert rapidly and are similar enough in polarity to prevent their independent separation and isolation. We report here the first example, to our knowledge, of cobalt(II) spin-state isomers that can be physically separated. The reaction of Mo₂(dpa)₄ (dpa = 2,2'-dipyridylamide) with CoBr₂ produces a mixture of two heterometallic compounds with a linear, metal-metal-bonded Mo[Formula: see text]Mo-Co chain. The complexes, SC-[BrMo₂(dpa)₄Co]Br (SC-<b>2</b>) and HS-[BrMo₂(dpa)₄CoBr] (HS-<b>2</b>), have identical compositions (Mo₂Co(dpa)₄Br₂) but different ground spin states and coordination geometries of the Co(II) ion. In the solid state, SC-<b>2</b> undergoes incomplete spin crossover from an <i>S</i> = 1/2 state to an <i>S</i> = 3/2 state, and HS-<b>2</b> has a high spin, <i>S</i> = 3/2, ground state, as confirmed by SQUID magnetometry and EPR spectroscopy. Crystallographic analyses of SC-<b>2</b> and HS-<b>2</b> show that SC-<b>2</b> has an elongated Co-Br distance relative to HS-<b>2</b> and is best described as the salt [BrMo₂(dpa)₄Co]Br. This limits SC-<b>2</b>'s solubility in nonpolar solvents and allows for the physical separation of the two isomers. Solution studies of SC-<b>2</b> and HS-<b>2</b> indicate that SC-<b>2</b> and HS-<b>2</b> interconvert slowly relative to the NMR time scale. Additional solution-state EPR and UV-vis absorption measurements demonstrate that the choice of solvent polarity determines the predominant isomer present in solution.