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

Abstract

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) 2 IrCl. 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) 2 IrCl can be reduced by one electron to form neutral, square planar (Diso) 2 Ir, while oxidation with PhICl 2 gives octahedral trans-(Diso) 2 IrCl 2 . The cis isomer can be prepared by air oxidation of (Diso) 2 IrCl; 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) 2 Ir and (Diso) 2 IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.

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

“…A π interaction similar to that proposed in isomer 3 has been described, and its spectroscopic implications noted, for (Diso) 2 IrCl. 18 The results of this study suggest that the energetic value of this π interaction also exerts a significant effect on the mechanism of pyridine addition to (κ 2 ,η 2 -Tipsi)(3,5-t Bu 2 Cat)IrCl by stabilizing the trans isomer 3 relative to its cis counterparts.…”

“…This intermediate is tentatively identified as square planar (Tipsi)-IrCl(coe) based on its symmetry as judged by its 1 H NMR spectrum. This behavior contrasts with that of the iminoquinone Diso (N-(2,6-diisopropylphenyl)-4,6-di-tertbutyl-2-imino-o-benzoquinone), which produces the bis-(iminoquinone) complex (Diso) 2 IrCl, 18 although traces of an analogous mono-ligated species can be observed in the metalation of Diso at very short times before binding of a second Diso ligand takes place.…”

“…For iridium iminoxolenes, (Diso) 2 IrCl and (Diso) 2 IrI, which have the same nominal oxidation state as (Tipsi)(3,5-t Bu 2 Cat)IrCl, afford Scheme 2. Synthesis of (Tipsi)(3,5-t Bu 2 Cat)IrCl (1a) (11), respectively, in the bis(iminoxolenes), 18 with π bond orders of approximately 0.5, the bonding orbital can be estimated to be approximately 70% metal-centered. It is more difficult to find a good iridium dioxolene comparison to compound 1a, as the only reported iridium bis(dioxolene) structure is that of (3,6-t Bu 2 Cat) 2 Ir(cod), 27 which is overall one electron more reduced than 1a.…”

“…This is consistent with the experimentally determined structure of the fivecoordinate bis(iminoxolene) complex (Diso) 2 IrCl, which is square pyramidal with apical chloride. 18 Subsequent Berry pseudorotations convert 3 N and 3 O to the more stable 3.…”

“…SiCH),18.55 (SiCH(CH 3 ) 2 ),18.58 (SiCH(CH 3 ) 2 ),18.75 (SiCH(CH 3 ) 2 ), 18.87 (SiCH(CH 3 ) 2 ), 28.41 (C(CH 3 ) 3 ), 28.48 (C(CH 3 ) 3 ), 30.90 (C(CH 3 ) 3 ), 31.72 (C(CH 3 ) 3 ), 33.75 (C(CH 3 ) 3 ), 34.00 (C(CH 3 ) 3 ), 34.52 (C(CH 3 ) 3 ),35.10 (C(CH 3 ) 3 ), 76.37 (bound C�CSi), 101.81 (free C�CSi), 101.89 (ArC�C), 102.82 (ArC� C), 108.12, 113.79, 114.90, 118.21, 122.48, 122.26, 128.61, 134.36, 134.59, 136.49, 139.48, 142.96, 150.22, 152.87, 159.48, 165.02 (CO), 170.16 (CO), 174.83 (CO). IR: 2957 (m), 2943 (m), 2862 (m), 2143 Cl 2): 370 nm (ε = 9600 L mol −1 cm −1 ), 420 nm (9300 L mol −1 cm −1 ), 1060 nm (7500 L mol −1 cm −1 ), 1170 nm (8700 L mol −1 cm −1 ).…”