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• W950674382 abstract "Ligand design criteria for stabilization of high valent metals have been developedand applied to osmium, copper, and cobalt. Ligands have been designed to havefavorable coordination properities, be noninnocent, and be resistant to oxidativedegradation. A new subset of ligands has been synthesized, these being called Class II polyanionicchelating (PAC) ligands. These ligands possess two N-amido donors and twoalkoxy donors. Both units have pKa's of approximately 18 and so would be expectedto be very powerful donors. A number of ligands of this type have been developedwith 5,5,5; 5,6,5; 6,5,6; and 6,6,6 coordination geometries. The key steps in all ofthe syntheses are coupling of highly hindered carboxylic acids with highly hindereddiamines. It is found that the 5,5,5; 5,6,5; and 6,5,6 coordinating ligands all stabilizeOs(VI) monooxo compounds. The 5,6,5 and 6,5,6 coordinating ligands also stabilizeOs(IV) complexes of octahedral geometry. The 6,6,6 ligands of this type do notcoordinate to osmium. Of the 5,6,5 ligands, the most interesting results have beenobtained with H₄HMPA-DMP, or 2,4-bis-hydroxymethylpropamido-2,4-dimethyl-3-pentanone. Reaction of this with potassium osmate produces Os(VI)(η⁴-HMPA-DMP)O. This molecule has been structurally characterized. It assumes a distortedsquare pyramidal geometry with the metal atom 0.8 A above the plane formed bythe four donor atoms of the PAC ligand. The distortion appears to occur by pyramidalizationat the amide nitrogen atoms, as is shown by the χN values (17.0°and 9.1°). Os(HMPA-DMP)O undergoes a number of interesting reactions. It iscleanly converted to the trans-K₂Os(η⁴-HMPA-DMP)O₂ species with base; it can beelectrochemically reduced to a stable (although highly air-sensitive) Os(V) species;and it can be chemically reduced to produce trans-Os(IV)(η⁴-HMPA-DMP)L₂ complexeswhere L can be a wide variety of pyridines. These Os(IV) compounds maybe oxidized chemically or electrochemically to produce Os(V) compounds. TheOs(V)/(IV) potential may be precisely controlled by appropriate choices of L allowingproduction of tunable oxidants. It is also observed that these Os(IV) compoundsreact with molecular oxygen to regenerate the Os(VI) monooxo species. The compoundscatalytically reduce molecular oxygen and oxidize triphenylphosphine totriphenylphosphine oxide. Kinetic studies are consistent with a rate-determiningstep involving dissociation of a pyridine ligand from the trans-Os(IV)(η⁴-HMPA-DMP)L₂ species to generate a vacant coordination site to which oxygen may bind.The reaction has been examined with a variety of pyridine ligands. The rate ofair oxidation decreases with increased electron donor ability of substituents on the pyridine ring, kobs being 1.9(1) X 10⁻⁶ min⁻¹ for the compound synthesized from4-bromopyridine and kobs for compounds synthesized with dialkylaminopyridines beingvirtually too small to measure. Similar Os(VI) monooxo and octahedral Os(IV)compounds have been synthesized with a number of other Class II PAC ligands.The Os(IV) and Os(VI) compounds produced in this work have been compared tosimilar species produced with Class I PAC ligands. The Class II PAC ligands have also been used to stabilize trivalent copper.A number of such compounds have been synthesized (one of which, [TPP][Cu(η⁴-HMPA-B)]) has been structurally characterized) and compared with the correspondingClass I compounds. While no Cu(III)-Class I PAC complexes were stable (althoughreversible Cu(III) /(II) were observed in these cases), the Class II PAC ligandsled to stable Cu(III) complexes, exhibiting Cu(III) /(II) potentials as low as -1.06 Vvs Fe⁺ /Fe. Such potentials are much lower than have been seen for any complexessynthesized to date with first-row donor atoms. The [TMA][Cu(III)(HMPA-DMP)]complex is an example of a Cu(III) compound with a completely innocent ligandcomplement. In this case, resonance structures from lower oxidation states are notpossible thus enabling the oxidation state in this complex to be unambiguouslyassigned as +III. The Class II PAC ligands stabilize cobalt in the rare trivalent, square planarform. These compounds, while paramagnetic with two unpaired spins, may becharacterized by proton NMR. One such complex, Na[Co(III)(HMPA-B)] has beencharacterized by X-ray crystallography. These compounds catalyze the epoxidationof styrene using iodosoarenes as the stoichiometric oxidants." @default.
• W950674382 created "2016-06-24" @default.
• W950674382 creator A5070770921 @default.
• W950674382 date "1988-01-01" @default.
• W950674382 modified "2023-09-27" @default.
• W950674382 title "Production of Transition Metal Oxidants through Design and Synthesis of Polyanionic Chelating Ligands" @default.
• W950674382 doi "https://doi.org/10.7907/r8x9-3306." @default.
• W950674382 hasPublicationYear "1988" @default.
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