A family
of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R1)-4-(R2)-pyrrolide anions (dppR1,R2)−, varying in the nature of the R1 and R2 substituents [R1, R2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of
neutral [CoII(dppR1,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
[CoII(dppMe,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 CoIII/II redox potentials are predictably tuned
by choice of substituents R1 and R2, from −0.95
(Me,Me) to −0.45 (CN,Ph) V vs Fc+/Fc, with a linear
correlation observed between
E
1
/
2(CoIII/II) and the Swain–Lupton
parameters of the pyrrolide substituents.
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