Oxyfunctionalization with Cp*Ir<sup>III</sup>(NHC)(Me)L Complexes

Lehman, Matthew C.; Boyle, Paul D.; Sommer, Roger D.; Ison, Elon A.

doi:10.1021/om5007352

Cited by 17 publications

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References 41 publications

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“…Recently, we described a rare example of C−O bond formation (oxyfunctionalization) from an Ir−methyl bond in [Cp*Ir(NHC)(Me)L][OTf] complexes where L = pyridine, THF, and CO; NHC = 1,3,-dimethylimidazol-2-ylidene; and OTf = trifluoromethanesulfonate. 10 From kinetic studies and isotope labeling experiments it was suggested that the source of oxygen was O 2 and the reaction proceeded by ligand dissociation of the L-type ligand to form a 16-electron intermediate, followed by O 2 binding and functionalization. We hypothesized that the utilization of an L-type ligand that dissociates from the metal freely at or below room temperature, could potentially allow the reaction to proceed at milder conditions and allow for the identification and isolation of relevant intermediates.…”

Section: ■ Introductionmentioning

confidence: 99%

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate

et al. 2015

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Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(μ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

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Section: ■ Introductionmentioning

confidence: 99%

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate

et al. 2015

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“…Half-sandwich complexes 132-135. Partial halide abstraction from 26b.An interesting oxyfunctionalization to produce methanol via Ir-methyl bond cleavage was described by the Ison group 89. Treatment of half-sandwich NHC-Ir(III) complexes 138 with dioxygen in water gave rise to methanol formation (Scheme 55).…”

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Iridium complexes with monodentate N-heterocyclic carbene ligands

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2018

Coordination Chemistry Reviews

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the stoichiometry, both [(IMes)Ir(COE)(N 2)Cl] and [(IMes) 2 Ir(COE)Cl] (198 in Scheme 75) were successfully isolated. 44 Finally, as Scheme 20 in Section 4.4 shows, a series of stable (NHC) 2 Ir(COE)Cl complexes were isolated either by the free carbene or the silver transmetallation method. Scheme 7. The synthesis of half-sandwich NHC-Ir(III) 24. The most commonly used precursor for the synthesis of NHC-Ir(III) complexes is [Cp*IrCl 2 ] 2. In 2000, Herrmann showed that treatment of [Cp*IrCl 2 ] 2 with 1 equivalent of ICy per iridium yielded 24 in an excellent 90% yield (Scheme 7). 45 Termaten et al. employed the same procedure for the synthesis of (IiPr Me)Ir(Cp*)Cl 2 (26a in Scheme 8). 15 An alternative synthetic route to access free NHCs was described in 1993 by Kuhn and Kratz, who prepared free carbenes via the reduction of thiones 25 by metallic potassium in THF (Scheme 8). 46 Using this method, Yamaguchi and coworkers prepared 26a-c by cannulating the solution of the in situ generated NHCs to a solution of [Cp*IrCl 2 ] 2 in THF (Scheme 8). 47 Scheme 8. Synthesis of NHC-Ir(III) from free carbenes. 4.2. Alkoxy metal precursors and base-assisted one-pot methods It was Köcher and Herrmann who first developed a one pot procedure in which [Ir(COD)(µ-OEt)] 2 was prepared in situ by reacting [Ir(COD)Cl] 2 with four equivalents of NaOEt. Subsequently, four equivalents of 1,3-bis(diphenylmethyl)imidazolium bromide were added and the air and moisture stable 27 was isolated two days later (Scheme 9). 48 Interestingly, despite the fact that a large excess of the NHC salt was employed, formation of a cationic biscarbene complex was not observed. Scheme 9. NHC-Ir complex synthesis from an in situ generated metal-alkoxide. The procedure starting with the alkoxy metal precursor gave bis-NHC substituted complexes (28) when sterically less demanding NHC salts were applied (Scheme 10). Notably, even a 2.2:1 ratio of NHC:Ir was sufficient to isolate 28a-c in good yields (67-86%). Iridium and rhodium bis-NHC complexes with chelating N-N bridged NHCs were prepared in the same way. 39 Scheme 10. Cationic biscarbene complexes. Recently, Savka and Plenio described a one-step synthesis to access (NHC)M(COD)Cl (M = Rh, Ir) as well as (NHC)M(CO) 2 Cl complexes by employing a relatively weak base (K 2 CO 3) in acetone under air (Scheme 11). 49 Scheme 11. One pot synthesis of (NHC)Ir(COD)Cl and (NHC)Ir(CO) 2 Cl complexes. 4.3. The silver transmetallation method Scheme 12. The silver transmetallation method. The silver transmetallation method for the synthesis of NHC-iridium complexes was first published by Crabtree and coworkers. 50 The NHC-silver adducts (29) were generated in excellent yields by modifying the original procedure of Wang and Lin. 51 Subsequent treatment of [Ir(COD)Cl] 2 at room temperature gave 30a and 30b in 81 and 68% yield, respectively (Scheme 12). X-ray analysis of the yellow crystals of 30a revealed square planar geometry around the iridium center. Complexes 30 smoothly reacted with CO to form bis-carbonyl compound...

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“…However, introduction of oxygen/air to the mixture of 1+ MTO or 3 at −78 °C resulted in the formation of complex 4 (Scheme ) in 78% yield along with the formation of some methanol (20%). This is one of the rare examples MeOH formation from O 2 as oxidant. − Complex 4 was characterized by 1 H NMR, 13 C{ 1 H} NMR, and 195 Pt{ 1 H} NMR as well as by single-crystal X-ray (Figure ). X-ray structure of complex 4 features the rhenium metal center with tetrahedral geometry, typical for the perrhenate anion, whereas the platinum center has octahedral geometry, which is characteristic for Pt(IV).…”

Section: Resultsmentioning

confidence: 99%

Mechanism of Me–Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt–Re Bimetallic Center

et al. 2017

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Unusual cis-oxidative addition of methyltrioxorhenium (MTO) to [PtMe(bpy)], (bpy = 2,2'-bipyridine) (1) is described. Addition of MTO to 1 first gives the Lewis acid-base adduct [(bpy)MePt-Re(Me)(O)] (2) and subsequently affords the oxidative addition product [(bpy)MePtReO] (3). All complexes 1, MTO, 2, and 3 are in equilibrium in solution. The structure of 2 was confirmed by X-ray crystallography, and its dissociation constant in solution is 0.87 M. The structure of 3 was confirmed by extended X-ray absorption fine structure and X-ray absorption near-edge structure in tandem with one- and two-dimensional NMR spectroscopy augmented by deuterium and C isotope-labeling studies. Kinetics of formation of compound 3 revealed saturation kinetics dependence on [MTO] and first-order in [Pt], complying with prior equilibrium formation of 2 with oxidative addition of Me-Re being the rate-determining step. Exposure of 3 to molecular oxygen or air resulted in the insertion of an oxygen atom into the platinum-rhenium bond forming [(bpy)MePtOReO] (4) as final product. Density functional theory analysis on oxygen insertion pathways leading to complex 4, merited on the basis of Russell oxidation pathway, revealed the involvement of rhenium peroxo species.

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Oxyfunctionalization with Cp*Ir^III(NHC)(Me)L Complexes

Cited by 17 publications

References 41 publications

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate

Iridium complexes with monodentate N-heterocyclic carbene ligands

Mechanism of Me–Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt–Re Bimetallic Center

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Oxyfunctionalization with Cp*IrIII(NHC)(Me)L Complexes

Cited by 17 publications

References 41 publications

Oxyfunctionalization with Cp*IrIII(NHC)(Me)(Cl) with O2: Identification of a Rare Bimetallic IrIV μ-Oxo Intermediate

Oxyfunctionalization with Cp*IrIII(NHC)(Me)(Cl) with O2: Identification of a Rare Bimetallic IrIV μ-Oxo Intermediate

Iridium complexes with monodentate N-heterocyclic carbene ligands

Mechanism of Me–Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt–Re Bimetallic Center

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Oxyfunctionalization with Cp*Ir^III(NHC)(Me)L Complexes

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV μ-Oxo Intermediate