title: Reduction of a redox-active ligand drives switching in a Cu(I) pseudorotaxane by a bimolecular mechanism.
abstract: The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR(+)) into the reduced [3]pseudorotaxane ([3]PR(+)) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR(+) and [3]PR(2+) are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M(+)) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR(2+). The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR(+) to 1088 nm for [3]PR(2+). Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K(1) = 8.9 x 10(8) M(-1) and K(2) = 3.1 x 10(6) M(-1), indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR(+), and [3]PR(2+) shows the one-electron reductions at E(1/2) = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu(+) ion. CVs of [2]PR(+) show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR(0) + [2]PR(+) = [3]PR(+) + Me(2)BPTZ(0) (K(D)*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR(+):Me(2)BPTZ. The driving force (DeltaG(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR(+) over reduced [2]PR(0) by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M(-1)). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.
authors.affiliation: Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, USA.
authors.forename: Kristy A
authors.initials: KA
authors.lastname: McNitt
authors.forename: Kumar
authors.initials: K
authors.lastname: Parimal
authors.forename: Andrew I
authors.initials: AI
authors.lastname: Share
authors.forename: Albert C
authors.initials: AC
authors.lastname: Fahrenbach
authors.forename: Edward H
authors.initials: EH
authors.lastname: Witlicki
authors.forename: Maren
authors.initials: M
authors.lastname: Pink
authors.forename: D Kwabena
authors.initials: DK
authors.lastname: Bediako
authors.forename: Christina L
authors.initials: CL
authors.lastname: Plaisier
authors.forename: Nga
authors.initials: N
authors.lastname: Le
authors.forename: Lee P
authors.initials: LP
authors.lastname: Heeringa
authors.forename: Douglas A
authors.initials: DA
authors.lastname: Vander Griend
authors.forename: Amar H
authors.initials: AH
authors.lastname: Flood
mesh_terms: D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D043862:Rotaxanes / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D008024:Ligands; D043862:Rotaxanes; D003300:Copper