Studies of the Donor Properties of New Polyanionic Chelating Ligands through the Synthesis and Characterization of Osmium Complexes

• Main Author: Peake, Geoffrey Tower
• Format: Others
• Language: en
• Published: 1987
• Online Access: https://thesis.library.caltech.edu/11847/2/Peake_GT_1987.pdf; Peake, Geoffrey Tower (1987) Studies of the Donor Properties of New Polyanionic Chelating Ligands through the Synthesis and Characterization of Osmium Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/68mt-5y71. https://resolver.caltech.edu/CaltechTHESIS:10232019-094659896 <https://resolver.caltech.edu/CaltechTHESIS:10232019-094659896>

<p>The synthesis, characterization, physical and chemical properties of osmium complexes containing highly donating polyanionic chelating (PAC) ligands are discussed.</p> <p>The tetradentate tetraanionic ligand HBA-B (H₄HBA-B ≡ 1,2-bis(2-hydroxybenzamido)benzene) coordinates to osmium to form a variety of complexes. Reaction of H₄HBA-B with K₂[Os(OH)₄(O)2] gives an osmium(VI) trans-dioxo complex coordinating the PAC ligand through two deprotonated phenol groups and two deprotonated amide groups. Reduction of the dioxo species with triphenylphosphine gives the neutral osmium(IV) species <i>trans</i>-Os(η⁴-HBA-B)(PPh₃)₂ (3). This complex is an excellent starting material to make a series of osmium(IV) complexes because of the lability of the phosphine ligands. Ligand exchange reactions with nitrogen-base ligands, anionic two-electron donor ligands, mono- and bidentate phosphines, π-acid ligands, and bipyridines are observed. Most of the osmium(IV) PAC ligand complexes prepared are paramagnetic with μ_eff ~1 BM. The paramagnetic osmium(IV) species all exhibit well-resolved, paramagnetically shifted NMR spectra. The complex <i>trans</i>-Os(η⁴-HBA-B)(PPh₃)(py) (5) has twelve inequivalent sets of protons, all of which are on aromatic rings. The twelve resonances are observed in the ¹H NMR spectrum in the range +11 to -5 ppm. The results of 2D-COSY and difference NOE experiments allow the complete assignment of the ¹H NMR spectrum.</p> <p>Electrochemical studies of these osmium(IV) complexes, and the coordination chemistry observed indicate that these PAC ligands are very good electron donors to osmium. The Os(IV/III) reversible couples for the neutral osmium(IV) species have potentials m the range -0.9 to -0.7 V vs. Fc⁺/Fc. The cation/neutral reversible couples have potentials in the range +0.2 to +0.35 V vs. Fc⁺/Fc, but probably cannot be called Os(V/IV) couples. Reaction of 3 with carbon monoxide produces a rare osmium(IV) carbonyl complex, <i>cis-α</i>-Os(η⁴-HBA-B)(PPh₃)(CO) (10). Reaction of <i>t</i>-butylisocyanide with 3 gives the analogous mono-phosphine mono-isocyanide. The stability of these complexes in an indication of the strong donating nature of the PAC ligand.</p> <p>A crystal structure of 10 shows it to have a cis-α coordination geometry. Complexes with cis geometries can also be made by reacting 3 with bidentate ligands, such as dppe. Cis-α and cis-β osmium complexes of the HBA-B PAC ligand, and other PAC ligand complexes, contain the first nonplanar amide ligands. We have defined three causes for the formation of nonplanar amide ligands: (1) structural requirements of the metal center (for instance coordination of a bidentate dppe ligand forcing a cis geometry), (2) depletion of electron density from the metal center (either by oxidation or coordination of an electron withdrawing ligand), causing the amide ligands to adopt the more highly donating nonplanar structure and offset the depletion of electron density at the metal, (3) steric interactions forcing deformation of the amide ligand to a nonplanar state.</p> <p>The syntheses of osmium complexes containing the CHBA ligand (H₂CHBA ≡ 3,5-dichloro-2-hydroxybenzamide) are discussed. Several complexes, including <i>trans</i>-Os(η²-CHBA)₂(PBu₃)₂ (26) and <i>trans</i>-Os(η²-CHBA)₂(OPBu₃)₂ (27), have been prepared in attempts to make a long-lived catalyst for the electrochemical oxidation of alcohols. The cyclic voltammetric experiments with 26 and 27 in the presence of alcohol are discussed. It appears that 27 exhibits catalytic behavior and 26 does not. A detailed study probing the catalysis has not been done.</p> <p>The results of reactions between K₂[Os(OH)₄(O)₂] and ligands similar to 2-(2-hydroxyphenyl)imidazole are reported. The reactions give neutral osmium(VI) trans-dioxo complexes containing two 2-(2-phenoxy)imidazole ligands coordinated as bidentate monoanions through a phenoxy ligand and an imidazole ligand. This coordination mode contrasts with that observed for osmium complexes of 2-(3,5-dichloro-2-hydroxyphenyl)-5,6-dichlorobenzimidazole, which coordinates as a dianion.</p>