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A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

preprint
submitted on 08.04.2020 and posted on 09.04.2020 by Marc Lehr, Tobias Paschelke, Eicke Trumpf, Anna-Marlene Vogt, Christian Näther, Frank Sönnichsen, Anna McConnell

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. In contrast to the large number of standard NMR methods for diamagnetic compounds, the number of paramagnetic NMR methods is limited and spectral assignment often relies on theoretical models. We report a toolbox of 1D (1H, proton-coupled 13C, selective 1H‑decoupling 13C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods for the straightforward structural characterisation of paramagnetic complexes in solution and demonstrate its general applicability for fields from coordination chemistry to spin‑crossover complexes and supramolecular chemistry through the characterisation of CoII and high-spin FeII mononuclear complexes as well as a Co4L6 cage. The toolbox takes advantage of the reduced signal overlap, decreased instrument time and greater sensitivity from the presence of the paramagnetic centre while overcoming the loss of structural information from the wide chemical shift dispersion and broad signals. In some circumstances, more structural information was revealed in the COSY spectra than would be observable for a diamagnetic analogue; as well as the expected through-bond cross-peaks, through-space and exchange cross-peaks were also observed for mononuclear complexes with multiple ligand environments and fast ligand exchange. With this toolbox, the standard characterisation of paramagnetic complexes and cages is now possible using NMR spectroscopic methods.


History

Email Address of Submitting Author

amcconnell@oc.uni-kiel.de

Institution

Otto Diels Institute of Organic Chemistry, Christian-Albrechts-Universität zu Kiel

Country

Germany

ORCID For Submitting Author

0000-0001-7329-4319

Declaration of Conflict of Interest

No conflict of interest

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