Structures of rhenium(I) complexes with 3-hydroxyflavone and benzhydroxamic acid as O,O'-bidentate ligands and confirmation of π-stacking by solid-state NMR spectroscopy.

The synthesis and crystal structures of two new rhenium(I) complexes obtained utilizing benzhydroxamic acid (BHAH) and 3-hydroxyflavone (2-phenylchromen-4-one, FlavH) as bidentate ligands, namely tetraethylammonium fac-(benzhydroxamato-κO,O')bromidotricarbonylrhenate(I), (CHN)[ReBr(CHNO)(CO)], 1, and fac-aquatricarbonyl(4-oxo-2-phenylchromen-3-olato-κO,O')rhenium(I)-3-hydroxyflavone (1/1), [Re(CHO)(CO)(HO)]·CHO, 3, are reported. Furthermore, the crystal structure of free 3-hydroxyflavone, CHO, 4, was redetermined at 100 K in order to compare the packing trends and solid-state NMR spectroscopy with that of the solvate flavone molecule in 3. The compounds were characterized in solution by H and C NMR spectroscopy, and in the solid state by C NMR spectroscopy using the cross-polarization magic angle spinning (CP/MAS) technique. Compounds 1 and 3 both crystallize in the triclinic space group P-1 with one molecule in the asymmetric unit, while 4 crystallizes in the orthorhombic space group P222. Molecules of 1 and 3 generate one-dimensional chains formed through intermolecular interactions. A comparison of the coordinated 3-hydroxyflavone ligand with the uncoordinated solvate molecule and free molecule 4 shows that the last two are virtually completely planar due to hydrogen-bonding interactions, as opposed to the former, which is able to rotate more freely. The differences between the solid- and solution-state C NMR spectra of 3 and 4 are ascribed to inter- and intramolecular interactions. The study also investigated the potential labelling of both bidentate ligands with the corresponding fac-Tc-tricarbonyl synthon. All attempts were unsuccessful and reasons for this are provided.