Solid State NMR Studies of Paramagnetic Coordination Complexes: A Comparison of Protons and Deuterons in Detection and Decoupling

The solid state nuclear magnetic resonance (SSNMR) spectra of protons, deuterons, and carbons in the ligands of paramagnetic coordination complexes including acetylacetonate complexes, amino acid complexes, hydrates of the first row transition metals, and lanthanide acetates have been obtained. It has been known for some time that, for small paramagnetic species in solution, deuterium NMR can give better resolved spectra than proton NMR. In SSNMR of diamagnetic species in the solid state, it has also been observed that deuterium magic angle spinning (MAS) spectra are often better resolved than proton spectra. This paper reports that many paramagnetic species in the solid state can also have well-resolved deuterium MAS spectra which can be useful in chemical characterizations. In rare examples, such as certain VI1' and CU" complexes, the proton MAS spectra are also well resolved at high spinning speeds. In these cases, well-resolved carbon MAS spectra can be measured without need for simultaneous high-power proton irradiation. The MAS line widths we observed for pure microcrystalline coordination complexes are essentially unrelated to the corresponding line widths measured in solution for similar complexes. Compounds with slow electron spin-lattice relaxation times can sometimes exhibit very narrow SSNMR signals due to spin diffusion among the electrons. On the other hand, many paramagnetic compounds, including those that give narrow signals in solution, for example iron complexes and high-valent manganese complexes, can be difficult to detect with solid state carbon or proton NMR experiments. The relatively broad SSNMR signals are presumably due to large anisotropic interactions that are not averaged by MAS alone (e.g., the dipolar hyperfine interaction in the presence of a substantial g anisotropy). However, even for these complexes deuterium MAS spectra are generally very easily detected including deuterons that are within 3 8, of the metal. Because of its relatively high resolution and sensitivity, deuterium MAS NMR of paramagnetic solids lends itself for characterization of the local structure of solid paramagnetic systems. For example, well-resolved splittings are often observed and are indicative of subtle Jahn-Teller or other local distortions. Furthermore, we have demonstrated that the sensitivity is sufficient for characterization of paramagnetic biomacromolecules.