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-κ2O,O′)bromido­tri­carbonyl­rhenate(I), (C8H20N)[ReBr(C7H6NO2)(CO)3], 1, and fac-aqua­tricarbon­yl(4-oxo-2-phenyl­chromen-3-olato-κ2O,O′)rhenium(I)–3-hy­droxy­flavone (1/1), [Re(C15H9O3)(CO)3(H2O)]·C15H10O3, 3, are reported. Furthermore, the crystal structure of free 3-hy­droxy­flavone, C15H10O3, 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 1H and 13C NMR spectroscopy, and in the solid state by 13C NMR spectroscopy using the cross-polarization magic angle spinning (CP/MAS) technique. Compounds 1 and 3 both crystallize in the triclinic space group P\overline{1} with one mol­ecule in the asymmetric unit, while 4 crystallizes in the ortho­rhom­bic space group P212121. 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 13C 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-99mTc-tricarbonyl synthon. All attempts were unsuccessful and reasons for this are provided.