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

Abstract

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."

Electrochemical and homogeneous electron transfers to the Alzheimer amyloid-β copper complex follow a preorganization mechanism

“…Several artificial systems possessing similar entatic characteristics have been proposed (25,28,(50)(51)(52)(53). These entatic structures are the result of strong constraints exerted on the coordination sphere.…”

Copper in diseases and treatments, and copper‐based anticancer strategies

“…Both Solomon 42 and Ryde 43 have carried out theoretical calculations from which they concluded that the Cu atom can be viewed as existing in an electronic entatic state rather than a simple geometric constraint physically imposed upon the active site by the protein matrix. 44 …”

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

“…It has been postulated that state M 1 L 0 W 2 of S 3 represents the ''entatic state'' (for details on ''entatic states'' see, Williams 1995, a nice illustrative experimental example for the true geometric ''entatic state'' has been reported for Cu complexes, see Chaka et al 2007) for photosynthetic O 2 formation (Renger 2001(Renger , 2004. The generation of state Y Z OX is assumed to shift the populations in the multistate equilibrium of S 3 towards M 1 L 0 W 2 that is subsequently oxidized by Y Z OX thus readily leading to the formation of M 2 L 0 W 2 , i.e., a state that formally corresponds with a peroxide complexed at the cluster in redox state S 2 .…”