Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes

A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R-1)-4-(R-2)-pyrrolide anions (dpp(R1,R2))(-), varying in the nature of the R-1 and R-2 substituents [R-1, R-2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of neutral [co(II)(dpp(R1,R2))(2)] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [co(II)(dpp(Me,Me))(2)], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl3 solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn-Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible Co-III/II redox potentials are predictably tuned by choice of substituents R-1 and R-2, from -0.95 (Me,Me) to -0.45 (CN,Ph) V vs Fc(+)/Fc, with a linear correlation observed between E-1/2(Co-III/II) and the Swain-Lupton parameters of the pyrrolide substituents.