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Close Encounters of the Weak Kind: Investigations of Electron–Electron Interactions between Dissimilar Spins in Hybrid Rotaxanes

Lockyer, Selena J., Fielding, Alistair J., Whitehead, George F. S., Timco, Grigore A., Winpenny, Richard E. P., McInnes, Eric J. L.
Journal of the American Chemical Society 2019 v.141 no.37 pp. 14633-14642
X-ray diffraction, copper, electron paramagnetic resonance spectroscopy, ions, moieties, rotaxanes
We report a family of hybrid [2]rotaxanes based on inorganic [Cr₇NiF₈(O₂CᵗBu)₁₆]⁻ (“{Cr₇Ni}”) rings templated about organic threads that are terminated at one end with pyridyl groups. These rotaxanes can be coordinated to [Cu(hfac)₂] (where Hhfac = 1,1,1,5,5,5-hexafluoroacetylacetone), to give 1:1 or 1:2 Cu:{Cr₇Ni} adducts: {[Cu(hfac)₂](py-CH₂NH₂CH₂CH₂Ph)[Cr₇NiF₈(O₂CᵗBu)₁₆]}, {[Cu(hfac)₂][py-CH₂NH₂CH₂CH₃][Cr₇NiF₈(O₂CᵗBu)₁₆]}, {[Cu(hfac)₂]([py-CH₂CH₂NH₂CH₂C₆H₄SCH₃][Cr₇NiF₈(O₂CᵗBu)₁₆])₂}, {[Cu(hfac)₂]([py-C₆H₄-CH₂NH₂(CH₂)₄Ph][Cr₇NiF₈(O₂CᵗBu)₁₆])₂}, and {[Cu(hfac)₂]([3-py-CH₂CH₂NH₂(CH₂)₃SCH₃][Cr₇NiF₈(O₂CᵗBu)₁₆])₂}, the structures of which have been determined by X-ray diffraction. The {Cr₇Ni} rings and Cuᴵᴵ ions both have electronic spin S = 1/2, but with very different g-values. Continuous-wave EPR spectroscopy reveals the exchange interactions between these dissimilar spins, and hence the communication between the different molecular components that comprise these supramolecular systems. The interactions are weak such that we observe AX or AX₂ type spectra. The connectivity between the {Cr₇Ni} ring and thread terminus is varied such that the magnitude of the exchange interaction J can be tuned. The coupling is shown to be dominated by through-bond rather than through-space mechanisms.