A Crystalline Hybrid of Paddlewheel Copper(II) Dimers and Molecular Rotors: Singlet‐triplet Dynamics Revealed by Variable‐temperature Proton Spin‐lattice Relaxation

Abstract

AbstractWe report on the synthesis and application of a 2 nm long, curved ditopic biscarboxylic ligand with a 1,4‐bis(ethynyl)bicyclo[2.2.2]octane rotator core and an helical twist to the construction of an extended single‐crystalline framework solid with paddlewheel hinges, [CuII]2[1,4‐bis(carboxyphenyl ethynyl)bicyclo[2.2.2]octane]2(H2O)2 or [CuII]2(bbcbco)2(H2O)2. The interconnection of interpenetrated square lattices involves short rotor–rotor H···H interactions (1.9 to 2.4 Å) such that the moving parts are expected to rub onto each other in the lattice in a Brownian rotational motion with a calculated rotational barrier of 3.7 kcal·mol–1. Variable‐temperature 1H spin‐lattice relaxation (T1) experiments carried out on a static crystalline sample did not provide however a value of this rotational barrier because the relaxation proved to be dominated by the coupling of the moving protons to the electronic spins of the CuII dimers. Remarkably, we reveal how the singlet‐triplet spin dynamics of non‐interacting CuII dimers is elegantly characterized by solid state NMR spectroscopic experiments yielding an exchange coupling constant, Jexp = –365 K = –254 cm–1, in good agreement with theoretical estimations and experimental data on related CuII dimer systems.