Single and double nitroxide labeled bis(terpyridine)-copper(II): influence of orientation selectivity and multispin effects on PELDOR and RIDME.

A rigid, nitroxide substituted terpyridine ligand has been used to synthesize hetero- and homoleptic bis-terpyridine complexes of copper(II). The homoleptic complex represents a three-spin system, while the metal ion in the heteroleptic complex is in average bound to one nitroxide bearing ligand. Both complexes are used as model systems for EPR distance measurements using pulsed electron-electron double resonance (PELDOR or DEER) and relaxation induced dipolar modulation enhancement (RIDME) sequences. The results of both methods are analyzed using detailed geometric data obtained from the crystal structure of the homoleptic complex as well as information concerning ligand scrambling and the electronic structure of the copper center. In addition, both methods are compared with respect to their sensitivity, the extent of orientation selectivity and the influence of multispin effects.