Ruthenium-based bioconjugates: Synthesis and X-ray structure of the mixed ligand sandwich compound RuCpiPr(p-(CO2H)C6H4Tp) and labelling of amino acids and the neuropeptide enkephalin

Four differently substituted mixed ligand sandwich complexes CpRu(p-BrC6H4)Tp (3), CpRu(p-BrC6H4)TpMe (4), Cp∗Ru(p-BrC6H4)Tp (5), CpiPrRu(p-BrC6H4)Tp (6), incorporating cyclopentadienyl (Cp) and functionalized tris(pyrazolyl)borate (Tp) ligands, have been synthesized and characterized. Air-stable 6 has been converted to benzoic acid-functionalized CpiPrRu(p-(CO2H)C6H4)Tp (7), which has been structurally characterized in the solid state by X-ray diffraction. Compound 7 may be readily coupled to biomolecules as exemplified by the coupling to phenylalanine-methylester to give CpiPrRu(p-(CO-Phe-OMe)C6H4Tp) (8). In a solid phase peptide synthesis (SPPS), 7 has been coupled to the pentapeptide Enkephalin, to provide CpiPrRu(p-(CO-Tyr-Gly-Gly-Phe-Leu-OH)C6H4Tp) (9) as the first example of a mixed ligand sandwich ruthenium bioconjugate.