Synthesis and Characterization of 3,5-Bis(di-<i>tert</i>-butylphosphinito)pyridine Pincer Complexes

Swisher, Nicholas A.; Grubbs, Robert H.

doi:10.1021/acs.organomet.0c00270

Cited by 5 publications

(4 citation statements)

References 41 publications

(59 reference statements)

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“…14,15 Quaternizing the nitrogen results in a significant increase of positive charge at the metal center, including examples with "P(C-pyridinyl)P"-pincer complexes. 16,17 Milstein and co-workers have studied P(Cpyridinyl)P-pincer ruthenium complexes as platforms for metal−ligand cooperative aromatization/dearomatization, enabling diverse reactivity including dihydrogen activation, alcohol dehydrogenation, and alcohol-amine dehydrogenative coupling. 18 In the case of lactate racemase, mechanistic studies of the enzyme 19,20 and synthetic model complexes 21−23 have arrived at a proton-coupled hydride transfer mechanism 24 for the racemization of lactate.…”

Section: Introductionmentioning

confidence: 99%

“…Pyridinyl and especially pyridylidene ligands can be viewed as a class of Fischer carbenes ,− in which p(π) electrons of the pyridyl nitrogen and the backbonding d-electrons of the metal compete to populate a vacant p orbital on the ipso carbon . The remote nitrogen therefore strongly influences the electronics at the metal center, enabling electronic switchability and proton-responsiveness. , Quaternizing the nitrogen results in a significant increase of positive charge at the metal center, including examples with “P(C-pyridinyl)P”-pincer complexes. , Milstein and co-workers have studied P(C-pyridinyl)P-pincer ruthenium complexes as platforms for metal–ligand cooperative aromatization/dearomatization, enabling diverse reactivity including dihydrogen activation, alcohol dehydrogenation, and alcohol-amine dehydrogenative coupling …”

Section: Introductionmentioning

confidence: 99%

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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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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

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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“…End-on coordination through one of the aromatic carbon atoms ( i.e. , pyridinyl fragments) has been used to construct anionic pincer ligands, where the unligated pyridine N atom can be used for further functionalization …”

Section: Introductionmentioning

confidence: 99%

Diazines and Triazines as Building Blocks in Ligands for Metal-Mediated Catalytic Transformations

Doll,

Becker,

Roşca

2023

ACS Org. Inorg. Au

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Pyridine is a ubiquitous building block for the design of very diverse ligand platforms, many of which have become indispensable for catalytic transformations. Nevertheless, the isosteric pyrazine, pyrimidine, and triazine congeners have enjoyed thus far a less privileged role in ligand design. In this review, several applications of such fragments in the design of new catalysts are presented. In a significant number of cases described, diazine- and triazine-based ligands either outperform their pyridine congeners or offer alternative catalytic pathways which enable new reactivities. The potential opportunities unlocked by using these building blocks in ligand design are discussed, and the origin of the enhanced catalytic activity is highlighted where mechanistic studies are available.

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“…[14][15] Quaternizing the nitrogen results in a significant increase of positive charge at the metal center, including examples with "P(C-pyridinyl)P"-pincer complexes. [16][17] Milstein and co-workers have studied P(C-pyridinyl)P-pincer ruthenium complexes as platforms for metal-ligand cooperative aromatization/dearomatization, enabling diverse reactivity including dihydrogen activation, alcohol dehydrogenation, and alcohol-amine dehydrogenative coupling. 18 In the case of lactate racemase, mechanistic studies of the enzyme [19][20] and synthetic model complexes [21][22][23] have arrived at a proton-coupled hydride transfer mechanism 24 for the racemization of lactate.…”

Section: Introductionmentioning

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

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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.

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Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

Cited by 5 publications

References 41 publications

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

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

Diazines and Triazines as Building Blocks in Ligands for Metal-Mediated Catalytic Transformations

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

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Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

Cited by 5 publications

References 41 publications

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

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

Diazines and Triazines as Building Blocks in Ligands for Metal-Mediated Catalytic Transformations

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

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