Structures of rhenium(I) complexes with 3-hydroxyflavone and benzhydroxamic acid as O,O'-bidentate ligands and...

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-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.