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

Abstract

Oxobis(iminoxolene)osmium(VI) compounds ( R ap) 2 OsO ( R ap = 2-(4-RC 6 H 4 N)-4,6-t Bu 2 C 6 H 2 O) are readily deoxygenated by phosphines and phosphites to give five-coordinate ( R ap) 2 Os(PR′ 3 ) or six-coordinate ( R ap) 2 Os(PR′ 3 ) 2 . Structural data indicate that this net two-electron reduction is accompanied by apparent oxidation of the iminoxolene ligands due to their greater ability to engage in π donation to the reduced deoxy form of the osmium complex. In ( R ap) 2 Os(PR′ 3 ) 2 , the HOMO is a ligand-based combination of the iminoxolene redox-active orbitals, while the LUMO is a highly covalent metal-iminoxolene π* orbital. In the trans isomer, the HOMO is required to be ligand-localized by symmetry, while in the cis isomer, the ligands adopt a conformation that minimizes metal−ligand π* interactions in the HOMO. Kinetic studies indicate that the deoxygenations involve the ratedetermining attack of the phosphorus(III) reagent on the five-coordinate oxo complexes. Varying the substituents of the aryl groups on the iminoxolene ligands or on the triarylphosphines has little effect on the rate of oxygen atom transfer, with the best correlation shown between oxygen atom transfer rates and the HOMO−LUMO gap of the oxo complexes. This suggests that the osmium oxo group shows a balance between electrophilic and nucleophilic character in its oxygen atom transfer reactions with phosphorus(III) reagents.

Slicing the π in Three Unequal Pieces: Iridium Complexes with Alkyne, Iminoxolene, and Dioxolene Ligands

“…Previous studies of osmium bis(iminoxolene) complexes have shown that π acceptor ancillary ligands cause a noticeable positive shift in MOS values. 4 The similarity of the MOS values in 1a and 2b is thus consistent with the characterization of the alkyne as a Polonius-type 35 ligand. It is conceivable that the replacement of alkyne by pyridine did significantly affect the electronic structure, if the change in the geometry from cis to trans exerted an opposite effect of comparable magnitude.…”

“…The kinetically preferred isomer 1b is assigned as having the same overall geometry as 1a , but with the opposite orientation of the dioxolene ligand (oxygen meta to the tert -butyl groups trans to chloride) based on the negligible changes in the optical spectrum when 1b isomerizes to 1a (Figure S29). Were the central chromophore different in the two isomers, significant changes in the optical spectrum would be expected, as seen for example in previous examples of cis- and trans- bis­(iminoxolene) , or -bis­(dioxolene) metal complexes. In principle, the alkyne could be free in 1b and the compound could adopt a five-coordinate, square pyramidal structure analogous to that observed for the bis­(iminoxolene) complex (Diso) 2 IrCl.…”

“…Iminoxolenes and dioxolenes, ligands that can assume a continuum of oxidation states from fully oxidized (iminoquinone and benzoquinone, respectively) to fully reduced (amidophenoxide and catecholate, respectively), are particularly good candidates for comparison. The π bonding in these ligands involves their so-called redox-active orbitals − (RAO, shown in the B 2 symmetry dioxolene interaction in Figure ). The RAOs are unusually high in energy for oxygen or nitrogen lone pairs because of a π* interaction between the in-phase combination of the heteroatom lone pairs and the filled benzene π orbital.…”

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

“…The cis isomer has a similar electronic structure, despite the fact that in the cis geometry both RAO combinations can overlap with metal dπ orbitals, because the iminoxolene ligand can pivot to minimize the overlap of the B -symmetry combination with the dπ orbital, weakening the antibonding interaction. In osmium and ruthenium compounds where this orbital is occupied, typical X–M–N1–C12 dihedral angles are about 115°, compared to the ∼70° dihedral angles seen in compounds where the orbital is empty and hence greater overlap leads to enhanced bonding . The values observed experimentally in cis -(Diso) 2 IrCl 2 (104.5° avg) and computationally in cis -(ap) 2 IrCl 2 (93.6°) are intermediate between these two extremes, as expected for a compound where this orbital is singly occupied and indicates that this orbital is weakly Ir-iminoxolene π antibonding in character.…”

“…In osmium and ruthenium compounds where this orbital is occupied, typical X−M−N1−C12 dihedral angles are about 115°, compared to the ∼70°dihedral angles seen in compounds where the orbital is empty and hence greater overlap leads to enhanced bonding. 31 The values observed experimentally in cis-(Diso) 2 IrCl 2 (104.5°avg) and computationally in cis-(ap) 2 IrCl 2 (93.6°) are intermediate between these two extremes, as expected for a compound where this orbital is singly occupied and indicates that this orbital is weakly Iriminoxolene π antibonding in character.…”