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A Definitive Example of a Geometric “Entatic State” Effect: Electron-Transfer Kinetics for a Copper(II/I) Complex Involving A Quinquedentate Macrocyclic Trithiaether−Bipyridine Ligand
journal contribution
posted on 2007-04-25, 00:00 authored by Gezahegn Chaka, Jason L. Sonnenberg, H. Bernhard Schlegel, Mary Jane Heeg, Gregory Jaeger, Timothy J. Nelson, L. A. Ochrymowycz, D. B. RorabacherThe quinquedentate macrocyclic ligand cyclo-6,6‘-[1,9-(2,5,8-trithianonane)]-2,2‘-bipyridine
([15]aneS3bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been
synthesized and complexed with copper. The CuII/IL redox potential, the stabilities of the oxidized and
reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with
a series of six counter reagents have been studied in acetonitrile at 25 °C, μ = 0.10 M (NaClO4). The
Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of
the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k11. The
latter value has also been determined independently from NMR line-broadening experiments. The cumulative
data are consistent with a value of k11 = 1 × 105 M-1 s-1, ranking this among the fastest-reacting CuII/I
systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures
show that the geometry of the CuIIL and CuIL complexes are nearly identical, both exhibiting a five-coordinate
square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable
geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the CuIL
complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the
transition-state geometry more closely approximating the geometry of the CuIIL ground state. The
combination of a nearly constant geometry and a large self-exchange rate constant implies that this CuII/I
redox system represents a true geometric “entatic state.”