Formation of Iridium(III) Allene Complexes via Isomerization of Internal Alkynes

Abstract

Syntheses of the alkyne complexes (POCOP)Ir(η2-RCCR) (1-DPA, R = Ph; 1-Oct, R = C3H7; 1-Hex, R = C2H5; POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) are reported. 1-DPA is stable in both the solid state and as a benzene solution; however, both 1-Oct and 1-Hex slowly isomerize to afford the corresponding allene complexes 2-Oct and 2-Hex, respectively. A single-crystal X-ray structure determination of 2-Oct confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, in contrast to prior reports, initial mechanistic studies have indicated the isomerization process is not catalyzed by the presence of an acid.

Ligand survey results in identification of PNP pincer complexes of iridium as long-lived and chemoselective catalysts for dehydrogenative borylation of terminal alkynes

“…The phenomenon might be related to the fact that Rh analog 10-Rh-H 2 has been shown to be an alkyne dimerization catalyst 65 and PCP/POCOP iridium complexes reacted with alkynes to form a variety of allene or enyne complexes. 66 , 67 …”

“…Such downfield chemical shift is characteristic of internal aromatic alkyne π-complexes. 67 , 70 , 71 However, the isomerization from 10-Ir-p-tol to 10-Ir-v-tol proceeded at an appreciable rate at ambient temperature (about 50% after 15 h) and precluded the isolation of pure 10-Ir-p-tol . We surmised that a more electron-poor alkyne should be thermodynamically less predisposed to form a vinylidene, 68 and that the alkyne isomer may also be kinetically more long-lived.…”

Unexpected Solvent Effects in the Isomerization of iPrPCPIr(η2‐PhC≡CPh) to a 1‐Iridaindene

“…[19] As well, tBu POCOPIr was discovered to direct the isomerization of internal alkynes to allenes. [20] Intriguingly, the formation of tBu POCOPIr(h 2 -PhC CPh) was reported in this study, though no information on further investigations of reactivity were given. In the present work we report the synthesis and characterization of p-bound R PCPIr-diphenylacetylene species, and isomerization of iPr PC-PIr(h 2 -PhCCPh) to the 1-iridaindene iPr PCPIr[C 8 H 5 (2-Ph)] upon heating in aromatic solvents.…”

“…Notably, Ozerov and coworkers have demonstrated the dehydrogenative borylation of terminal alkynes by SiNN and PNP type pincer iridium complexes, while the species tBu PCPIr was found to promote the dimerization of phenylacetylene to the corresponding ene‐yne . As well, tBu POCOPIr was discovered to direct the isomerization of internal alkynes to allenes . Intriguingly, the formation of tBu POCOPIr(η 2 ‐PhC≡CPh) was reported in this study, though no information on further investigations of reactivity were given.…”

Synthesis, characterization and reactivity of iron- and cobalt-pincer complexes

“…[13][14] Given the high cost of precious metals such as Ir and Rh, the application of such ligands in base metal catalysis is no surprise and a number of reports have appeared in which such base-metal complexes were deployed in fields ranging from energy science to asymmetric catalysis. [15][16] Given our interest in the hydrosilylation of carbonyl moieties using iron, [17] we turned our attention to the synthesis and reactivity of first-row metal PONOP and PNP pincer complexes. These types of PONOP and PNP ligands are readily available via salt metathesis reactions between 2,6-dihydroxypyridines or 2,6bis(bromomethyl)pyridines and dichlorophosphines to afford either PONOP or PNP pincer ligands, respectively.…”