Seth N. Brown

Metalation of the trans-spanning 1,2-ethanediyldianthranilate-bridged bis(iminoquinone) C₂H₄[O₂CC₆H₄-2-(NC₆H₂-3,5-^tBu₂-2-O)]₂ (Egan) with bis(cyclooctene)iridium(I) chloride dimer results in the formation of a mixture of iridium compounds containing one or no chlorides per iridium. The monochloride product is a six-coordinate monomer, with one ester carbonyl of the bridge coordinated to give a mer, κ³ iminoxolene linkage. This compound is formed exclusively as a cis-α isomer with the two iminoxolene nitrogens mutually trans, but it isomerizes upon heating to form an equilibrium mixture with the cis-β isomer. The iridium complex with no chlorides is iridium-iridium bonded dimeric (A,C)-(Egan)₂Ir₂, which is formed as a single S₄-symmetric stereoisomer. The iridium-iridium distance, 2.5584(4) Å, is extremely short for an unsupported iridium-iridium bond. This is attributed to donor–acceptor interactions between filled metal dπ orbitals on one iridium and empty metal-iminoxolene π* orbitals on the other iridium fostering a significant degree of metal-metal π bonding. The dimer can be reduced to a monomeric four-coordinate anion which is alkylated by iodomethane to form a five-coordinate methyliridium complex. In both of these complexes, the ester groups in the bridge are not bound to iridium.

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 (∆G° = 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 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.

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.

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.

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.

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.