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.
The 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 degrees C, mu = 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 x 10(5) 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."
Chaka
Gezahegn
G
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
Sonnenberg
Jason L
JL
Schlegel
H Bernhard
HB
Heeg
Mary Jane
MJ
Jaeger
Gregory
G
Nelson
Timothy J
TJ
Ochrymowycz
L A
LA
Rorabacher
D B
DB
D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D004579:Electron Transport; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010084:Oxidation-Reduction; D013440:Sulfides / Q000138:chemical synthesis / Q000737:chemistry*
D008024:Ligands; D013440:Sulfides; C520936:cyclo-6,6'-(1,9-(2,5,8-trithianonane))-2,2'-bipyridine; D015082:2,2'-Dipyridyl; D003300:Copper