Nuclear magnetic resonance spectroscopy of paramagnetic metal-acetylacetonate complexes in the solid state

Abstract

This work is addresses the issue of signal acquisition in solid-state Nuclear Magnetic Resonance (NMR) of paramagnetic coordination complexes and provides a comprehensive overview of a methodology of signal assignment. Model coordination complexes include a series of bis/tris(acetylacetonate)metals, where the metal is varied across the transition metals from titanium to copper. Additionally, a coupled spin system of copper acetate is studied to demonstrate the effect of the magnetic coupling on the solid-state NMR signal. 13C and 1H NMR spectra of these compounds are acquired and interpreted in terms of molecular orbital interactions and valence bond theory with the aid of Density Functional Theory (DFT) calculations. The influence of electronic configuration on the detectability of 13C and 1H resonances is addressed. Based on these results, a general methodology for signal acquisition and assignment emerges, which allows chemists to probe magnetically unquenched electronic environments and potentially exploit paramagnetic interactions for molecular structural elucidation. Finally, the electronic characteristics of paramagnetic solids favorable for the acquisition and interpretation of high-quality, structurally informative NMR spectra are identified.