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

We 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, [Cu-II](2)[1,4-bis(carboxyphenyl ethynyl)bicyclo[2.2.2]octane](2)(H2O)(2) or [Cu-II](2)(bbcbco)(2)(H2O)(2). The interconnection of interpenetrated square lattices involves short rotor-rotor H center dot center dot center dot H interactions (1.9 to 2.4 angstrom) 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 center dot mol(-1). Variable-temperature H-1 spin-lattice relaxation (T-1) 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 Cu-II dimers. Remarkably, we reveal how the singlet-triplet spin dynamics of non-interacting Cu-II dimers is elegantly characterized by solid state NMR spectroscopic experiments yielding an exchange coupling constant, J(exp) = -365 K = -254 cm(-1), in good agreement with theoretical estimations and experimental data on related Cu-II dimer systems.