4-pyridinyl and 4-pyridylidene pincer iridium cations for alkane dehydrogenation
Creators: name: Bhatti, Tariq Mehdi
Publisher: name: No Publisher Supplied
Year: 2024

description: Alkane dehydrogenation is a cornerstone of the modern chemical industry, converting our most abundant organic, natural resources—petroleum and natural gas—to valuable chemical intermediates and building blocks. Light alkanes can be selectively converted to light olefins such as propylene and isobutylene. Longer-chain alkanes tend to aromatize to mixtures of aromatic compounds. Selectively dehydrogenating higher alkanes, particularly linear alkanes, to olefins remains a challenging reaction under typical conditions in commercial refining processes.Organometallic pincer iridium complexes have emerged as the most active and selective catalysts for the conversion of alkanes to olefins, either by direct removal of hydrogen gas under non-equilibrium conditions or through (thermoneutral) transfer hydrogenation of a sacrificial olefin. Since the seminal reports on first-generation pincer iridium catalysts such as (tBu4PCP)Ir, (iPr4PCP)Ir, and (tBu4POCOP)Ir, continuing research has studied structural modifications to the ligand architecture (wingtip modifications, linker modifications, and entirely new frameworks) to improve catalytic turnover frequencies and turnover numbers.Chapter 1 of this dissertation presents a summary of the structure-reactivity relationship of pincer iridium catalysts for alkane dehydrogenation as they are understood today. Section 1 discusses catalysts that operate through low-oxidation state iridium(I/III) cycles. Sections 2 and 3 summarizes recent progress in alkane dehydrogenation by high-oxidation state iridium(III) and iridium(III/V) catalytic cycles. Section 4 discusses on-going efforts to interface pincer iridium-catalyzed alkane dehydrogenation cycles with electrochemical cells. Section 5 presents a rationale for studying 4-pyridinyl and 4-pyridylidene pincer iridium complexes for alkane dehydrogenation.Chapter 2 identifies a surprising (based on precedent) oxidatively-driven, intramolecular, sp3 C-H activation in the 4-pyridinyl complex (pN-tBu4PCP)IrHCl. Stoichiometric reactions are used to show that this cyclometallation can be performed through separated proton and electron transfer. An iridium(II) intermediate is identified through NMR monitoring and quenching with TEMPO-H. Through Density Funcitonal Theory (DFT) studies, the unexpected sensitivity of this complex is attributed to metal-ligand proton tautomerism, wherein a (4-pyridylidene-PCP)iridium(I) chloride is present in trace concentrations at equilibrium but has an oxidation potential over 1 V lower than the iridium(III) tautomer (pN-tBu4PCP)IrHCl. Importantly, calculations identify the key role of the (pN-tBu4PCP)IrCl+ cation in C-H activation. This work suggests that metal-ligand proton tautomerism can be used to sharply lower applied potentials in electrocatalysis; and also that a catalytic alkane (transfer) dehydrogenation cycle operating through cationic iridium(III) intermediates is available even to first-generation, phosphine and phosphinite-based pincer iridium catalysts.In Chapter 3, synthetic progress toward a 4-pyridylidene PCP pincer iridium catalyst is discussed. Through DFT calculations, this is predicted to be a highly active catalyst for alkane transfer dehydrogenation. It is fundamentally interesting as a test of our ability to predict highly-active catalysts through theory. Initial experimental efforts toward such a complex encountered challenges due to unselective metalation of the pro-ligand 3,5-bis(di-tert-butylphosphinomethyl)pyridine with iridium. This led to a revision of the targeted pro-ligand. The new pro-ligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine successfully formed the complex (pN-tBu4PCP)IrHCl (presented in Chapter 2), but subsequent quaternization of the pyridyl nitrogen using alkylating or silylating reagents in the presence of iridium proved unviable. Methylating the pyridyl nitrogen at an earlier stage of ligand synthesis was successful, and the resulting N-methyl functionality was robust enough to carry through subsequent synthetic steps, metalation with iridium, and ancillary ligand exchanges on the iridium center. Studies of ancillary ligand exchanges and anion metathesis reactions have identified two potential pre-catalysts (leading to the same active catalyst) for use in catalytic alkane dehydrogenation studies: (pNMe-tBu4PCP)IrCl and (pNMe-tBu4PCP)IrH3 where pNMe-tBuPCP = 3,5-bis(di-tert-butylphosphinomethyl)-1,2,6-trimethylpyridinium.