Iridium complexes bearing a PNP ligand, favoring facile C(sp3)–H bond cleavage

Lokare, Kapil S.; Nielsen, Robert J.; Yousufuddin, Muhammed; Goddard, William A.; Periana, Roy A.

doi:10.1039/c1dt10577a

Cited by 13 publications

(5 citation statements)

References 19 publications

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“…Complex 3 results from cyclometalation of an isopropyl substituent of the phosphine. A few cyclometalations at terminal alkyl groups have been recently reported . Cyclometalation implies the concerted cis addition of a C–H bond of one of the methyl groups of an isopropyl substituent to the metal center of a square-planar IrCl{xant(P i Pr 2 ) 2 } intermediate counterpart of 2 .…”

Section: Resultsmentioning

confidence: 99%

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

2013

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Treatment of the dimer [Rh(μ-Cl)(C8H14)2]2 (1a) with 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene [xant(P(i)Pr2)2] leads to the d(8) square-planar complex RhCl{xant(P(i)Pr2)2} (2), whereas reaction of the iridium counterpart [Ir(μ-Cl)(C8H14)2]2 (1b) gives the d(6) octahedral compound IrHCl{xant(P(i)Pr2)[(i)PrPCH(Me)CH2]} (3) as a result of the intramolecular C-H bond activation of one of the isopropyl substituents of the phosphine. Stirring 2 and 3 in 0.5 N KOH solutions of 2-propanol gives rise to the formation of hydrides RhH{xant(P(i)Pr2)2} (4) and IrH3{xant(P(i)Pr2)2} (5), respectively. In n-octane at 60 °C, complex 2 is stable. However, compound 3 activates the alkane to give the cis-dihydride IrH2Cl{xant(P(i)Pr2)2} (6) and a mixture of 3- and 4-octene. Complex 6 can be also obtained by the reaction of 3 with H2. Under the same conditions, 2 affords the rhodium analogue RhH2Cl{xant(P(i)Pr2)2} (7). Compounds 2-4 react with triflic acid (HOTf) to give RhHCl(OTf){xant(P(i)Pr2)2} (8), IrHCl(OTf){xant(P(i)Pr2)2} (9), and RhH2(OTf){xant(P(i)Pr2)2} (10), respectively. The related iridium derivative IrH2(OTf){xant(P(i)Pr2)2} (11) has also been prepared by the reaction of 6 with Tl(OTf). Complexes 2, 6, and 9 have been characterized by X-ray diffraction analysis. The {xant(P(i)Pr2)2}M skeleton is T-shaped with the metal center situated in the common vertex.

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Section: Resultsmentioning

confidence: 99%

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

2013

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“…Previous work has shown that large tert-butyl groups in the wings of the pincer can sterically destabilize an octahedral complex, encouraging the formation of square-planar complexes in iridium pincer complexes. 57 Using a smaller metal sych as cobalt will enhance this effect, raising the potential for reduction to the squareplanar cobalt analogue of Mol 6. The reduction to Co I is more difficult, however, than reduction to Ir I .…”

Section: Acs Catalysismentioning

confidence: 99%

Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

2016

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It has been demonstrated experimentally that electrochemical CO2 reduction catalyzed by (POCOP)IrH2 ([C6H3-2,6-[OP(tBu)2]2]IrH2) produces formate without significant H2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from IrIII dihydride explains the selectivity for formate over H2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)2 + at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of IrIII(H)(NCMe)2 +) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both IrIII(H)2 and IrI–H– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H2/H+ and HCOO–/CO2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.

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“…The presence of two overlapping signals at 10.66 and 10.57 ppm in a near 1:1 ratio in the 1 H NMR spectrum indicated the new products are both protonated at the pyridinyl nitrogen. These spectral features are characteristic of a cyclometalated pincer complex, − in which one of the t -butyl groups of 2-H has undergone C–H activation to form H + 4 (Scheme ). Thus, coupled with two one-electron oxidations and Ir–C bond formation, the hydride proton and a t -butyl proton of 2-H have undergone net transfer to the pyridinyl nitrogen atoms of the now-cyclometalated complex 4 and another molecule of 2-H .…”

Section: Resultsmentioning

confidence: 99%

Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp³)–H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

Bhatti,

Kumar,

Parihar

et al. 2023

J. Am. Chem. Soc.

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The para-N-pyridyl-based PCP pincer proligand 3,5-bis(di-tertbutylphosphinomethyl)-2,6-dimethylpyridine (pN-tBu PCP-H) was synthesized and metalated to give the iridium complex (pN-tBu PCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., ( tBu PCP)IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp 3 )−H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal−ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG°= −31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2 • which is observed by NMR spectroscopy. 2 • can be further oxidized to give cationic Ir(III) complex, 2 + , which can oxidatively add a phosphino-t-butyl C−H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2 + would preferentially undergo intermolecular addition of C(sp 3 )−H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile β-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.

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Iridium complexes bearing a PNP ligand, favoring facile C(sp3)–H bond cleavage

Cited by 13 publications

References 19 publications

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp³)–H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

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Iridium complexes bearing a PNP ligand, favoring facile C(sp3)–H bond cleavage

Cited by 13 publications

References 19 publications

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

Xantphos-Type Complexes of Group 9: Rhodium versus Iridium

Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2

Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp3)–H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

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Product

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About

Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp³)–H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex