The zwitterionic 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium (bp4pc) has been synthesized and crystals of its hydrated form bp4pc·2H 2 O and of its protonated reduced form H-bp4pc have been obtained. Upon heating, bp4pc·2H 2 O undergoes partial dehydration, leading to bp4pc·H 2 O at 160°C, together with a color change from yellow (room temperature) to green (140°C) and finally to brown (160-180°C). Analysis of bond lengths in the solid state reveals the expected short (d = 1.425 Å) and long (d = 1.485 Å) C-C central bond lengths in the all-radical salt H-bp4pc and bp4pc·2H 2 O, respectively, whereas the distance of 1.475 Å in bp4pc·H 2 O does not allow a conclusion to be drawn regarding the presence of radicals in this com-[a] MOLTECH
We show how to record and analyze solid-state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu-cyclam complex as an example. Assignment of the (13)C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the (1)H signals was done by means of (1)H-(13)C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct (1)H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such paramagnetic (1)H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording (1)H-(1)H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the (1)H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu-cyclam by means of (13)CR2 relaxation experiments combined with the electronic relaxation determined by EPR.
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