Amphiphilicity in Oxygen Atom Transfer Reactions of Oxobis(iminoxolene)osmium Complexes

Bis(iminoxolene)iridium complexes over four oxidation states: from sodium-iridium bonding to ligand-centered radicals

The novel iminoquinone N-(2,6-dibromophenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone (Briq), with electron-withdrawing and only mildly bulky 2,6-dibromophenyl substituents, is metalated by the iridium(I) complex {(coe)2IrCl}2 to give an inseparable mixture of (Briq)2Ir, (Briq)2IrCl, and (Briq)2IrCl2. This mixture can be funneled into a single pure compound either by exhaustive oxidation with iodobenzene dichloride to give (Briq)2IrCl2, or by exhaustive reduction by sodium naphthalenide to give (Briq)2IrNa(THF)2. The latter compound can be oxidized by one electron to give four-coordinate (Briq)2Ir, or undergo two-electron oxidative addition reactions with iodine or iodomethane to yield (Briq)2IrI or (Briq)2IrCH3. Structural data in the four different oxidation states indicate that each one-electron redox step is delocalized across the metal and iminoxolene ligand, with the redox changes becoming progressively more ligand-centered as the overall oxidation state increases. In the solid state, (Briq)2IrNa(THF)2 has a covalent bond between sodium and iridium (d = 2.9754(8) Å). The iridium-sodium bond partially heterolyzes in solution (ΔG° = 1.3 kcal mol-1 in THF-d8), and the four-coordinate anion has been characterized in the solid as the cobaltocenium salt. The iridium-sodium bond dissociation free energy is 74.3 kcal mol-1.

Optically active bis(aminophenols) and their metal complexes

Optically active C2-symmetric bis(aminophenols) based on (R)-2,2'-diaminobinaphthyl (BiniqH4) and (R,R)-2,3-butanediyldianthranilate (BdanH4) have been prepared by condensation of the diamines with 3,5-di-tert-butylcatechol. Group 10 bis(iminosemiquinone) complexes (R)-(Biniq)M (M = Pd, Pt) and (C,R,R)-(Bdan)Pd have been prepared by oxidatively metalating the corresponding ligands. In (R)-(Biniq)M, the C2 axis passes through the approximate square plane of the bis(iminosemiquinone)metal core, while in (C,R,R)-(Bdan)Pd the C2 axis is perpendicular to this plane. In the latter compound, the (R,R)-butanediyl strap binds selectively over one enantioface of the metal complex in a conformation where the methyl groups are anti to one another. Osmium oxo complexes with the intrinsically chiral OsO(amidophenoxide)2 chromophore are obtained by metalation of OsO(OCH2CH2O)2 with (R,R)-BdanH4. Both the (A,R,R) and (C,R,R) diastereomers can be observed, with metalation in refluxing toluene selectively giving the latter isomer. The electronic structures of the complexes are illuminated by the circular dichroism spectra, in conjuction with the optical spectra and TDDFT calculations.

Slip to π Ru: structural distortions due to metal-iminoxolene π bonding

Both pseudo-octahedral and pseudo-square pyramidal bis-iminoxolene complexes trans-(Diso)2RuCl2 and trans-(Diso)2Ru(PPh3) are structurally distorted, with the ruthenium atom slipping off the twofold axis of the idealized coordination polyhedra. These distortions take place because they allow or enhance π interactions between ruthenium and the iminoxolene π orbitals.

Modulation of Isomerization and Ligand Exchange Rates by π Bonding in Bis(iminoxolene)iridium Pyridine Complexes

The bis(iminoxolene)iridium complex (Diso)₂IrCl (Diso = N-(2,6-diisopropylphenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone) reacts with pyridine to give trans-(Diso)₂Ir(py)Cl as the kinetic product, with cis-(Diso)₂Ir(py)Cl formed as the exclusive thermodynamic product upon heating. Electronic spectra and density functional theory calculations point to very similar electronic structures for the cis and trans isomers, with a nonbonding iminoxolene-centered HOMO and a metal-iminoxolene π* LUMO. The triplet states of cis-(Diso)₂Ir(py)Cl and cis-[(Diso)₂Ir(py)₂]⁺ (but not trans-(Diso)₂Ir(py)Cl) are unusually low in energy (1000-1500 cm^-1 above the singlets), as shown by variable-temperature NMR spectroscopy. The low-energy triplets are attributed to a change in dihedral angle in the iminoxolenes, which allows a partial π interaction that cannot be achieved in the trans octahedral compounds. Mechanistic studies of the trans-cis isomerization in toluene indicate that the reaction proceeds via isomerization of the five-coordinate species to a form with cis iminoxolene ligands and an apical oxygen. This form is high in energy due to the loss of a secondary iminoxolene-to-iridium π-donor interaction that is possible in the trans form but not in the cis form for the square pyramidal structures. This stereoelectronic effect, combined with the poorer binding of pyridine in trans-(Diso)₂Ir(py)Cl due to the interactions of the N-aryl substituents with the pyridine, makes the pyridine dissociate faster from the trans isomer by a factor of 10⁸ at room temperature.

Mono- and Bis(iminoxolene)iridium Complexes: Synthesis and Covalency in π Bonding

N-(2,6-Diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Diso) reacts with the (cyclooctadiene)iridium chloride dimer to form a monoiminoxolene complex, (Diso)Ir(cod)Cl. Reaction of 2 equiv of the iminoquinone with chlorobis(cyclooctene)iridium dimer affords the bis-iminoxolene (Diso)₂IrCl. This five-coordinate complex adopts a distorted square pyramidal structure with an apical chloride ligand and undergoes halide exchange to form an air-stable iodide complex. (Diso)₂IrCl can be reduced by one electron to form neutral, square planar (Diso)₂Ir, while oxidation with PhICl₂ gives octahedral trans-(Diso)₂IrCl₂. The cis isomer can be prepared by air oxidation of (Diso)₂IrCl; cis/trans isomerization is not observed even on prolonged heating. Structural and spectroscopic features of the complexes are consistent with the presence of strong, covalent π bonding between the metal and the iminoxolene ligands, with structural data suggesting between 45 and 60% iridium character in the π bonding orbitals, depending on the ancillary ligands. The spectroscopic similarity of (Diso)₂Ir and (Diso)₂IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.

Nonclassical oxygen atom transfer reactions of an eight-coordinate dioxomolybdenum(vi) complex

Eight-coordinate MoO₂(DOPO^Q)₂ can donate two oxygen atoms to substrates such as phosphines in a four-electron nonclassical oxygen atom transfer reaction.