Reduction of a Redox-Active Ligand Drives Switching in a Cu(I) Pseudorotaxane by a Bimolecular Mechanism | doi.page

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Reduction of a Redox-Active Ligand Drives Switching in a Cu(I) Pseudorotaxane by a Bimolecular Mechanism

Journal of the American Chemical Society2009Vol. 131(3), pp. 1305–1313

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Kristy A. McNitt, Kumar Parimal, Andrew I. Share, Albert C. Fahrenbach, Edward H. Witlicki, Maren Pink, D. Kwabena Bediako, Christina Plaisier, Nga Phi Le, Lee P. Heeringa, Douglas A. Vander Griend, Amar H. Flood

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]PR2+ are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me2BPTZ), and a preformed copper-macrocycle moiety (Cu-M+) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR2+. 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]PR2+. Quantitative analysis of the UV−vis−NIR titration shows the stepwise formation constants to be K1 = 8.9 × 108 M−1 and K2 = 3.1 × 106 M−1, indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me2BPTZ, [2]PR+, and [3]PR2+ shows the one-electron reductions at E1/2 = −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]PR0 + [2]PR+ = [3]PR+ + Me2BPTZ0 (KD*, kd). UV−vis−NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR+:Me2BPTZ. The driving force (ΔGD* = −5.1 kcal mol−1) for the reaction is based on the enhanced stability of the reduced [3]PR+ over reduced [2]PR0 by 365 mV (8.4 kcal mol−1). Digital simulations of the CVs are consistent with a bimolecular pathway (kd = 12 000 s−1 M−1). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.

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Abstract

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