Production of Transition Metal Oxidants through Design and Synthesis of Polyanionic Chelating Ligands

Citation

Treco, Brian G. R. T. (1988) Production of Transition Metal Oxidants through Design and Synthesis of Polyanionic Chelating Ligands. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R8X9-3306. https://resolver.caltech.edu/CaltechTHESIS:03132013-111018743

Abstract

Ligand design criteria for stabilization of high valent metals have been developed and applied to osmium, copper, and cobalt. Ligands have been designed to have favorable coordination properities, be noninnocent, and be resistant to oxidative degradation.

A new subset of ligands has been synthesized, these being called Class II polyanionic chelating (PAC) ligands. These ligands possess two N-amido donors and two alkoxy donors. Both units have pKa's of approximately 18 and so would be expected to be very powerful donors. A number of ligands of this type have been developed with 5,5,5; 5,6,5; 6,5,6; and 6,6,6 coordination geometries. The key steps in all of the syntheses are coupling of highly hindered carboxylic acids with highly hindered diamines.

It is found that the 5,5,5; 5,6,5; and 6,5,6 coordinating ligands all stabilize Os(VI) monooxo compounds. The 5,6,5 and 6,5,6 coordinating ligands also stabilize Os(IV) complexes of octahedral geometry. The 6,6,6 ligands of this type do not coordinate to osmium. Of the 5,6,5 ligands, the most interesting results have been obtained 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 distorted square pyramidal geometry with the metal atom 0.8 Å above the plane formed by the four donor atoms of the PAC ligand. The distortion appears to occur by pyramidalization at the amide nitrogen atoms, as is shown by the χ_N values (17.0° and 9.1°). Os(HMPA-DMP)O undergoes a number of interesting reactions. It is cleanly converted to the trans-K₂Os(η⁴-HMPA-DMP)O₂ species with base; it can be electrochemically 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 complexes where L can be a wide variety of pyridines. These Os(IV) compounds may be oxidized chemically or electrochemically to produce Os(V) compounds. The Os(V)/(IV) potential may be precisely controlled by appropriate choices of L allowing production of tunable oxidants. It is also observed that these Os(IV) compounds react with molecular oxygen to regenerate the Os(VI) monooxo species. The compounds catalytically reduce molecular oxygen and oxidize triphenylphosphine to triphenylphosphine oxide. Kinetic studies are consistent with a rate-determining step 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 of air oxidation decreases with increased electron donor ability of substituents on the pyridine ring, k_obs being 1.9(1) X 10⁻⁶ min⁻¹ for the compound synthesized from 4-bromopyridine and k_obs for compounds synthesized with dialkylaminopyridines being virtually 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 to similar 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 corresponding Class I compounds. While no Cu(III)-Class I PAC complexes were stable (although reversible Cu(III) /(II) were observed in these cases), the Class II PAC ligands led to stable Cu(III) complexes, exhibiting Cu(III) /(II) potentials as low as -1.06 V vs Fe⁺ /Fe. Such potentials are much lower than have been seen for any complexes synthesized 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 ligand complement. In this case, resonance structures from lower oxidation states are not possible thus enabling the oxidation state in this complex to be unambiguously assigned as +III.

The Class II PAC ligands stabilize cobalt in the rare trivalent, square planar form. These compounds, while paramagnetic with two unpaired spins, may be characterized by proton NMR. One such complex, Na[Co(III)(HMPA-B)] has been characterized by X-ray crystallography. These compounds catalyze the epoxidation of styrene using iodosoarenes as the stoichiometric oxidants.

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