Catalytic transformations of small molecules by transition metal pincer complexes
Creators: name: Malakar, Santanu
Publisher: name: No Publisher Supplied
Year: 2022

description: This thesis dissertation has been divided into two parts: (1) Study of nitrogen splitting and reduction to ammonia by pincer ligated molybdenum complexes, and (2) Study of pincer based catalytic systems capable of affecting acceptorless and transfer alkane dehydrogenation, and dehydrogenation with secondary reactions such as olefin coupling. Part-1We study the key mechanistic steps of N–N bond cleavage and nitride protonation. The role of halides (Cl⁻, Br⁻, and I⁻) bound to the molybdenum has been investigated in the N–N bond cleavage. An unanticipated sequence of addition of protons and electrons to the molybdenum-nitride core has been discussed for six (PYP)Moᴵⱽ(≡N)X systems (Y = N and S, and X = Cl⁻, Br⁻, and I⁻). We also calculate that the molybdenum trihalide complex is an in-cycle species in the catalytic cycle for reduction of N₂. We have studied, in great details, the reduction of (PNP)MoᴵᴵᴵX₃ complexes to (PNP)MoᴵX active catalytic species responsible for cleaving N₂. The role of halides (Cl⁻, Br⁻, and I⁻) bound to the molybdenum has been investigated for the reduction of molybdenum(III) cores. Based on mechanistic and structural insights we have proposed improved catalysts that can potentially function with milder sources of reducing agents. Synthetic efforts to experimentally characterize the new catalyst is underway. We also discuss the development of new molybdenum(III) complexes that can activate and cleave N₂ in the presence of bases. The ligand design, harnesses the acidic protons on the ligand periphery for N₂ cleavage and taps into a Mo(III)/Mo(VI) cycle of N₂ reduction, essentially bypassing the reduction of Mo(III).Part-2Di-isopropylphosphino substituted pincer-ligated iridium catalysts are found to be significantly more effective for the dehydrogenation of simple tertiary amines to give enamines than the previously reported di-t-butylphosphino substituted species. It was reported that the di-isopropylphosphino substituted complexes catalyze dehydrogenation of several β-functionalized tertiary amines to give the corresponding 1,2-difunctionalized olefins. The di-t-butylphosphino substituted species are ineffective for such substrates; presumably the marked difference is attributable to the lesser crowding of the di-isopropylphosphino substituted catalysts. Experimentally determined kinetic isotope effects in conjunction with DFT-based analysis support a dehydrogenation mechanism involving initial pre-equilibrium oxidative addition of the amine α C-H bond followed by rate-determining elimination of the β-C-H bond. Iridium complexes bearing PCP-type pincer-ligands are the most effective catalysts reported to date for the low-temperature (≤ ca. 200 °C) dehydrogenation of alkanes. The (ⁱᴾʳxanPSP)Ru complex has been reported to catalyze alkane transfer dehydrogenation of the benchmark cyclooctane/t-butylethylene (COA/TBE) couple with turnover frequencies up to ca. 1 s⁻¹ at 150 °C and 0.2 s⁻¹ at 120 °C, the highest rates for alkane dehydrogenation ever reported at such temperatures. Dehydrogenation of n-octane, however, was repored to be much less effective. DFT calculations allow us to explain why (ⁱᴾʳxanPSP)Ru is more effective than (ⁱᴾʳPCP)Ir for dehydrogenation of COA while the reverse is true for dehydrogenation of n-alkanes. Considering only in-cycle species and simple olefin complexes, the (ⁱᴾʳxanPSP)Ru fragment is calculated to be much more active than (ⁱᴾʳPCP)Ir for dehydrogenation of both COA and n-alkanes. However, the resting state in the (ⁱᴾʳxanPSP)Ru-catalyzed transfer dehydrogenation of n-alkane is a very stable linear-allyl hydride complex, whereas the corresponding cyclooctenyl hydride is much less stable. We also report the synthesis and characterization of (ᵗᴮᵘ⁴PP<sup>Cl</sup>P)RuHCl and (ᵗᴮᵘ⁴PPᴴP)RuH₄ complexes. The formation of this species is from a rather unusual metalation reaction of ᵗᴮᵘ⁴PPᴴ ligand and [(p-cymene)RuCl₂]₂ Ru(II) precursor. The thermodynamic favorability of initially formed (ᵗᴮᵘ⁴PPᴴP)RuCl₂ undergoing a net hydride chloride exchange, to form the new complex, highlights the potential of an unusual metal-ligand cooperativity that this ligand was designed for. A priori, the thermodynamic drive for this unusual reactivity comes from the stability of a Ru(II) hydride chloride fragment rather than the formation of an otherwise strong Ru–H bond. Work is currently underway to synthesize the (ᵗᴮᵘ⁴PPᴴP)Ru(C₂H₄) complex as catalyst that will be explored for transfer dehydrogenation of alkanes with sacrificial olefins, including the metal-ligand cooperativity that has been identified in this work. We report an iridium acetate complex with a fluorinated Phebox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3,5-bis(trifluoromethyl)phenyl) that is a highly effective catalyst for acceptorless dehydrogenation of alkanes. Under typical acceptorless dehydrogenation conditions a high turnover frequency is obtained, which is limited by the rate of expulsion of H₂ from the reaction solution. Rates and turnover numbers for acceptorless dehydrogenation are significantly greater than found for the non-fluorinated analogue. As in the case of the non-fluorinated analogue, Na⁺ acts as a co-catalyst with the fluorinated catalyst again yielding greater rates and total turnovers. Computational studies shed light on the possible mechanistic pathways. The initial alkane activation is a net Ir-H/C-H bond metathesis leading to formation of an Ir-alkyl bond and loss of H₂; this is the slowest chemical step in the cycle. The lowest energy pathway is calculated to proceed via concerted metalated deprotonation (CMD) of the alkane. Pathways proceeding via transition states with oxidative addition (Ir(V)) character, however, are calculated to be only slightly higher in energy. These transition states can lead either to Ir(V) intermediates, which then lose H₂, or connect directly to a dihydrogen complex. The role of Na⁺ is largely to promote dechelation by coordinating to an acetate oxygen, opening a vacant coordination site which allows reaction with the alkane. This coordination by Na⁺ prevents the CMD mechanism from operating, but it significantly lowers the energy of the Ir(V) TSs. NBO analysis shows a net transfer of charge from the alkane atoms to the metal complex in the Ir(V) TSs, with and without coordinated Na⁺. Thus the oxidative addition is actually reductive in nature, driven in part by electrophilicity of the metal center. The Na⁺ cation further increases electrophilicity in addition to promoting dechelation. The greater activity of the fluorinated catalyst compared with the parent complex can also be explained in terms of the electrophilic nature of the reaction. The fluorinated catalyst is also more resistant to decomposition than the non-fluorinated analogue.