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Structural Determination of a [Zn(L)2]2+ Complex (L = Acetamide-Pyridine Ligand) From Liquid NMR Measurements and DFT Computations

Asymmetric tridentate ligands L (L= acetamide- pyridine) of the type A-B-C, (Figure 1a) enrich the isomerism of bis-tridentate A-B-A ligands [1] to six possible isomers of [M(A-B-C)2] complexes (M=metal, Figure 1b). Ligand L and metal complex [Zn(L)2](BF4)2 have been synthesized and characterized by solution-state NMR (1H and 13C). It is generally known that ligand coordination to the metal atom destroys NMR magnetic equivalences of nuclei near the metal atom, therefore increasing the number of signals in NMR spectra and appearance of hidden J-couplings (Figure 1c). Magnetic equivalences originate from fast nuclei- exchange processes (faster than NMR timescale), due to rotational/inversion motions of molecules in solution. Therefore, the DFT simulation of solution-state NMR spectra is a computational challenge and only recently satisfactory procedures have been suggested [2]. Herein, the conformational space of ligand L and complex [Zn(L)2]2+ has been explored by the CREST program [3]. Ensembles of conformations were optimized on the DFT theory level and free energies were calculated. For conformers with Boltzmann population > 1% the NMR parameters (shifts and J- couplings) have been calculated by the DFT/GIAO theory using the ORCA program [4]. Finally, from differences between measured and calculated NMR data, the most probable isomer of [Zn(L)2]2+ complex in solution is obtained.

DFT ; NMR ; metal complexes

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