The Design Multianionic Chelating Ligands for the Production of Inorganic Oxidizing Agents, Synthesis, Structure and Reactivity of Osmium Complexes Derived from a Tetradentate Ligand

The design of multianionic chelating ligands for use in high valent transition metal chemistry is discussed. Possible application of such ligands to problems in inorganic oxidation chemistry is addressed. A class of potentially tetradentate tetraanionic ligands was synthesized. The ligand 1,2-bis(3,5-dichloro-2-hydroxybenzamido)ethane (H4CHBA-Et), 1, was found to coordinate to osmium as a tetradentate tetraanion in the oxidation states (II), (III), (IV), (V), and (VI). X-ray crystal structures of two osmium(IV) complexes of this ligand are reported. The µ-oxo dimer, K2({Os(η4-CHBA-Et)(OPPh3)}2-O], 3, features octahedral osmium with the tetradentate tetraanionic ligand 1 coordinated to the equatorial positions and the potassium ions in unusual coordination environments. The structure of octahedral Os(η4-CHBAEt)(py)2,5, shows the ligand 1 coordinated in the same fashion with pyridines in the axial positions. Compound 5 and all of the osmium(IV) compounds reported here exhibit well-resolved paramagnetically shifted NMR spectra. The 1H NMR data suggest the possibility of an unusual π-backbonding interaction from osmium(IV) to pyridine. Electrochemical data indicate that ligand 1 and related ligands have a profound effect on the osmium redox couples which are found at significantly lower potentials than with other ligands. Controlled potential oxidation of 5 in the presence of water or alcohol was found to trigger a series of irreversible chemical and electrochemical transformations in which the ethane backbone of the ligand is oxidized in a selective and stepwise fashion. Several key intermediates have been isolated, independently synthesized and characterized. The first intermediate isolated, Os(η4-CHBA-ethylene)(py)2, 7, results from dehydrogenation of the ligand bridge. An X-ray crystal structure of this material is reported and significant features are discussed. In the second isolated intermediate the unsaturated ethylene bridge of 7 has been oxidized to a trans-1,2-diether in compound 8. A crystal structure of the related trans-1,2-hydroxy-alkoxy complex, 8*, has been performed. Cleavage of the bridge carbon-carbon bond in 8 yields the final oxidation products cis-α and trans-Os(η2-Fo-CHBA)2-(py)2, 9 and 9', in which the amide nitrogen of each bidentate ligand is substituted with a formyl group. t-Bupy derivatives of both isomers have been characterized by X-ray crystal structure determinations. The diastereomeric distribution is determined by the nature of the alcohol in solution during the electrolysis. Mechanistic aspects of the ligand oxidation process are addressed. Compounds 9 and 9' can be converted to catalysts for the electrochemical oxidation of alcohols. The catalysts were found to be the osmium(IV) compounds cis-α- and trans-Os(η2-CHBA)2(t-Bupy)2, 11 and 11', which are formed by a selective stepwise hydrolysis of the two formyl groups in 9/9'. A crystal structure of 11' shows the primary amide group of 3,5-dichloro-2-hydroxybenzamide, H2CHBA, to be coordinated through nitrogen. Chemical synthesis of the catalysts was pursued by coordination of H2CHBA to osmium. Two of the ligands coordinate to osmium(VI) as bidentate dianions in the complex K2 (Os(η2-CHBA)2(O)2), 12, which can be converted to 11', 15 and the compounds trans and cis-Os(η2-CHBA)2(t-Bupy)(Ph3P=O), 13 and 14. Compound 15 has been formulated as the dimer Os2(η2-CHBA)4(t-Bupy)4. Compounds 11, 11', 13 and 14 are catalysts for the electrochemical oxidation of alcohols. The catalytic system selectively oxidizes benzyl alcohol to benzaldehyde without further oxidation to benzoic acid or benzyl esters. Approximately 150 molecules of benzyl alcohol are oxidized during the lifetime of the catalyst, 11, but activity with other alcohols is quite low. The synthesis, characterization and properties of the above osmium compounds and significant features of the catalytic system are reported.