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

Abstract

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.

Pincer-Ligated Iridium Complexes with Low-Field Ancillary Ligands: Complexes of (iPrPCP)IrCl2 and Comparison with (iPrPCP)IrHCl

“…The ( i Pr PCP)­Ir­(I) fragment has been found in many instances to be catalytically more active than the iconic ( t Bu PCP)­Ir­(I) analogue, , presumably due to lesser crowding at the iridium center. In the case of Ir­(III) complexes, with the metal center bearing ancillary anionic ligands in addition to the pincer, we would expect that the less sterically demanding i Pr PCP ligand would confer an even greater advantage with respect to intermolecular reactivity . Accordingly, we chose to investigate ( i Pr PCP)­Ir­(III) complexes, and we began our study with the synthesis of ( i Pr PCP)­IrHCl ( 1-HCl ; 1 = ( i Pr PCP)­Ir).…”

Pincer-ligated Iridium Complexes with Low-Field Ancillary Ligands: Complexes of (iPrPCP)IrCl2 and Comparison with (iPrPCP)IrHCl

“…License: CC BY-NC-ND 4.0 12/17/23 confer an even greater advantage with respect to intermolecular reactivity. 59 Accordingly, we began our study with the synthesis of ( iPr PCP)IrHCl 57 (1-HCl; 1 = ( iPr PCP)Ir). Although this complex is well known, previously reported synthetic routes in our experience lead to mixtures of 1-HCl and 1-HBr, where the bromide is derived from 1,3-bis(bromomethyl)benzene starting material 58 .…”

CuI–Amidobis(phosphine)-Catalyzed Direct Amidation of Unactivated Alkanes via C(sp3)–H Activation

“…The formation of C–N bonds holds paramount significance in the synthesis of medicines, pharmaceutical molecules, and organic group functionalization. − In this context, the C–H functionalization approach offers a sustainable and atom-economical route for the synthesis of nitrogen-containing compounds. − Although direct functionalization of C–H bonds to create C–N bonds has been achieved, often, using expensive transition metal complexes, , C­(sp 3 )–H functionalization presents a notable challenge due to the low polarity and poor reactivity of these bonds. − As a result, reports on the inter- and intramolecular functionalization of unactivated C–H bonds are relatively scarce …”