Chemically Separable\nCo(II) Spin-State Isomers

The phenomenon of spin crossover involves coordination\ncomplexes\nwith switchable spin states. This spin state change is accompanied\nby significant geometric changes such that low and high spin forms\nof a complex are distinct isomers that exist in equilibrium with one\nanother. Typically, spin-state isomers interconvert rapidly and are\nsimilar enough in polarity to prevent their independent separation\nand isolation. We report here the first example, to our knowledge,\nof cobalt(II) spin-state isomers that can be physically separated.\nThe reaction of Mo<sub>2</sub>(dpa)<sub>4</sub> (dpa = 2,2′-dipyridylamide)\nwith CoBr<sub>2</sub> produces a mixture of two heterometallic compounds\nwith a linear, metal–metal-bonded MoMo–Co chain. The complexes,\nSC-[BrMo<sub>2</sub>(dpa)<sub>4</sub>Co]Br (SC-<b>2</b>) and\nHS-[BrMo<sub>2</sub>(dpa)<sub>4</sub>CoBr] (HS-<b>2</b>), have\nidentical compositions (Mo<sub>2</sub>Co(dpa)<sub>4</sub>Br<sub>2</sub>) but different ground spin states and coordination geometries of\nthe Co(II) ion. In the solid state, SC-<b>2</b> undergoes incomplete\nspin 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\nand EPR spectroscopy. Crystallographic analyses of SC-<b>2</b> and HS-<b>2</b> show that SC-<b>2</b> has an elongated\nCo–Br distance relative to HS-<b>2</b> and is best described\nas the salt [BrMo<sub>2</sub>(dpa)<sub>4</sub>Co]Br. This limits SC-<b>2</b>’s solubility in nonpolar solvents and allows for\nthe 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\nHS-<b>2</b> interconvert slowly relative to the NMR time scale.\nAdditional solution-state EPR and UV–vis absorption measurements\ndemonstrate that the choice of solvent polarity determines the predominant\nisomer present in solution.