Double Cyclometalation: Implications for C—H Oxidative Addition with PCP Pincer Compounds of Iridium

Mohammad, Hani A. Y.; Grimm, Jost C.; Eichele, Klaus; Mack, Hans‐Georg; Speiser, Bernd; Novak, Filip; Kaska, William C.; Mayer, Hermann A.

doi:10.1021/bk-2004-0885.ch014

Cited by 8 publications

(7 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…22 kcal/mol below that at 1 atm. Under such conditions, those states shown in Figure that include free H 2 would be relatively much lower in energy; cyclo-1 plus H 2 and C 2 H 6 would be the single lowest-free-energy state of those shown in Figure . As noted above, elimination of ethane from 1-H 2 (C 2 H 4 ) (in the absence of a trap such as H 2 or CO) at ca.…”

Section: Resultsmentioning

confidence: 99%

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

et al. 2018

View full text Add to dashboard Cite

Several complexes of ( tBu 4 PNP)Os (1) ( tBu 4 PNP = C 5 NH 3 -2,6-(CH 2 P t Bu 2 ) 2 ) are reported. 1 is isoelectronic with ( tBu 4 PCP)Ir ( 2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the ( tBu 4 PNP)Os complexes were investigated in this context. ( tBu 4 PNP)OsH 4 (1-H 4 ) is analogous to ( tBu 4 PCP)IrH 4 (2-H 4 ), but while 2-H 4 has some character of a dihydrogen dihydride, 1-H 4 is unambiguously a tetrahydride. Ethylene reacts with 1-H 4 to afford trans-DFT calculations indicate that alkane C−H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C−H or H−H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H 2 addition to the perfectly isoelectronic [( tBu 4 PNP)Ir(I)] + cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H 2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H 2 (to give tetrahydrides) is much more favorable for ( tBu 4 PNP)Os. NBO analysis indicates that the MH 2 /MH 4 additions are oxidative, whereas the M/MH 2 transformations are reductive.

show abstract

Section: Resultsmentioning

confidence: 99%

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

et al. 2018

View full text Add to dashboard Cite

show abstract

“…In general, the tridentate coordination mode of pincers results in strong binding to the metal center and engenders high stability of the pincer−metal unit. The rigidity of the framework probably tends to inhibit, though certainly not always prevent, , cyclometalation at the terminal groups, such as dialkylphosphino. The high thermal stability of pincer complexes allows them to act as catalysts for highly endothermic reactions; this is certainly one of the key properties responsible for their success as dehydrogenation catalysts, the topic of this review.…”

Section: Introductionmentioning

confidence: 99%

Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes

et al. 2011

View full text Add to dashboard Cite

“…48 Mayer and coworkers undertook cyclic voltammetry studies of ( tBu PCP)IrHCl and (MeO-tBu PCP)IrHCl, in which they found that oxidation leads to cyclometallation with loss of H2. 42,[63][64] They proposed that upon one-electron oxidation, cyclometalation to yield H2 (Scheme 10, eq b) becomes thermodynamically favorable. Our calculations, however, strongly indicate that the thermodynamics of such dehydrogenative cyclometalation reactions are substantially endergonic and strikingly unaffected by single-electron oxidation (Scheme 10).…”

Section: Considerations Of Selectivity and Implications For Catalysismentioning

confidence: 99%

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

Bhatti

Kumar

Parihar

et al. 2023

Preprint

View full text Add to dashboard Cite

The para-N-pyridyl-based PCP pincer ligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners (tBuPCP)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(sp3)-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; DFT calculations of direct one-electron oxidations are in good agreement with 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 (negative charge on Ir) and a p-N-pyridylidene (remote NHC) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (G° = 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 complexes would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n alkanes. The resulting iridium alkyl complexes could undergo facile -H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation that is driven by one-electron oxidation and deprotonation, enabled by MLPT.

show abstract

Double Cyclometalation: Implications for C—H Oxidative Addition with PCP Pincer Compounds of Iridium

Cited by 8 publications

References 0 publications

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes

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

Contact Info

Product

Resources

About

Double Cyclometalation: Implications for C—H Oxidative Addition with PCP Pincer Compounds of Iridium

Cited by 8 publications

References 0 publications

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units

Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes

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

Contact Info

Product

Resources

About

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units