A novel, mechanically interlocked molecular device was obtained from unique supramolecular tectons -pdeficient tetraazamacrocyclic complexes of copper(II) and nickel(II). We present the synthesis of the first rotaxanes based on donor-acceptor interactions involving transition metal complexes. While spontaneous shuttling manifests itself in the variability of the NMR spectra, voltammetric experiments reveal a surprising mode of potential-controlled molecular switching, which does not employ common co-conformational changes. Significantly, it relies on reversible folding/unfolding of the rotaxane. The process is driven by the interplay between electrostatic repulsion and cohesive p-p interactiona tug of war with a critical point at 1.31 V. Although rotaxanes with equivalent stations are considered degenerate molecular shuttles, we show that this is not the case when an unusual mechanism of switching is involved.

CitationQuantum mechanical alternative to Arrhenius equation in the interpretation of proton spin-lattice relaxation data for the methyl groups in solids 2015 Phys. Chem. Chem. Phys. Theory of nuclear spin-lattice relaxation in methyl groups in solids has been a recurring problem in nuclear magnetic resonance (NMR) spectroscopy. The current view is that, except for extreme cases of low torsional barriers where special quantum effects are at stake, the relaxation behaviour of the nuclear spins in methyl groups is controlled by thermally activated classical jumps of the methyl group between its three orientations. The temperature effects on the relaxation rates can be modelled by Arrhenius behaviour of the correlation time of the jump process. The entire variety of relaxation effects in protonated methyl groups has recently been given a consistently quantum mechanical explanation not invoking the jump model regardless of the temperature range. It exploits the damped quantum rotation (DQR) theory originally developed to describe NMR line shape effects for hindered methyl groups. In the DQR model, the incoherent dynamics of the methyl group include two quantum rate, i.e., coherence-damping processes. For proton relaxation only one of these processes is relevant. In this paper, temperature-dependent proton spin-lattice relaxation data for the methyl groups in polycrystalline methyltriphenyl silane and methyltriphenyl germanium, both deuterated in aromatic positions, are reported and interpreted in terms of the DQR model. A comparison with the conventional approach exploiting the phenomenological Arrhenius equation is made. The present observations provide further indications that incoherent motions of molecular moieties in condensed phase can retain quantum character over much broader temperature range than is commonly thought. Eprint version