Chemistry of pincer-ligated transition metals: A new paradigm for N2 fixation by pincer-molybdenum complexes and reactivity of triphos-ruthenium complexes

Ammonia production is critical to the well-being of society. Ammonia is the source of synthetic fertilizer, upon which 50% of global food production is based. On a scale even larger, ammonia also has great potential for energy storage and transportation, as well as use as fuel. Unfortunately, the only commercial process for ammonia synthesis is the Haber-Bosch process (HB) which is responsible for 2% of global fossil fuel consumption and co-production of commensurate amount of CO2 with the attendant contribution to climate change. In contrast, electrochemical nitrogen reduction (ENR) to ammonia using renewable energy would be effectively carbon-free, taking water and nitrogen (78% of the atmosphere) to ammonia and oxygen (the only byproduct). In this context we are investigating the synthesis of ammonia by the reduction of N2 utilizing molecular catalysts, an approach that has gained significant attention since Schrock’s seminal report in 2003. Our focus has been on dinitrogen reduction proceeding through bimetallic N2 cleavage, with our ultimate goal to utilize electrocatalysis and allow a fully sustainable approach. In the first part of this thesis, we report development of thioXantphos-derived (PSP), pincer-ligated molybdenum complexes that undergo facile N₂ splitting to afford a stable nitride. Our studies show that protonation of this nitride is more favorable than a conventional proton-coupled electron transfer (PCET) or hydrogen-atom transfer (HAT) pathway for NH3 formation. We isolated and fully characterized the corresponding imide complex, a key intermediate in the N2-to-NH3 conversion. Through experiment and DFT calculations, we elucidated a catalytic mechanism which had not previously been proposed, for the PSP system as well as Nishibayashi’s well-established pyridine-based PNP system, and likely for catalysts of this class more generally. We identified the first N–H bond formation of the Mo nitride as the highest barrier to catalysis; this can be attributed to the stability of the nitride and the fact that the single non-bonding orbital is filled. On this basis, we designed a Mo complex bearing Ozerov’s formally anionic PNP ligand. Under an N2 atmosphere, this complex forms a μ-N2–bridged dimer that undergoes only slow thermal N2 splitting at 110 °C, yet still functions catalytically to produce NH3 at room temperature. Extensive DFT studies reveal that protonation of the N2-bridged dimer lowers the kinetic barrier to N2 scission, facilitating the overall NH3-formation cycle. While we were exploring the N2 bridged complex bearing (PNP)Mo, we found that under argon (rather than dinitrogen) atmosphere, the two-electron reduction of (PNP)MoBr3 leads to formation of a diamagnetic species which is identified as Mo-Mo quadruply bonded dimer (characterized by SC-XRD, NMR). This species does not react with any other L type ligand. Depending on the conditions of the reduction, a paramagnetic compound is also produced, suspected to be (PNP)MoBr(THF)n. In support of this proposed formulation, although the compound does not undergo reaction with N2, it reacts with CO or PMe3 to form (PNP)MoL2Br. Detailed NBO analysis of the Mo≣Mo compound was performed to identify the sigma, pi, and delta bonding orbitals. The second part of this thesis is focused on organometallic reactivity in late transition metal chemistry.Metal ligand cooperativity (MLC) has unveiled a broad range of unusual reactivity in catalysis in the past two decades. Since Milstein’s report on pyridine-based ruthenium pincer complexes exhibiting aromatization-dearomatization of the pincer backbone, Ru in particular has exhibited a rich MLC-based chemistry. Our group recently reported the (tBuPPHP)Ir complex to be the fastest known catalyst for alkane-transfer dehydrogenation. In this context, we explored metal ligand proton tautomerism in a ruthenium system. During metalation of (tBuPPHP) with a RuCl2 precursor, we isolated only (PPClP)Ru(H)Cl, indicating hydride migration from P to Ru and chloride migration from Ru to P. We showed that P-H migration is driven by a strong thermodynamic preference for a five-coordinate Ru(H)Cl complex. In six-coordinate complexes, however, the migration depends on the electronic nature of L: strong π-acceptors (CO, isonitriles, NO⁺) promote migration of H to P, whereas with N2, CH3CN, or PMe3, the thermodynamically most stable isomers of (PPClP)Ru(H)Cl(L) are isolated. While this unusual P–H/Ru–Cl migration intrigued us, our primary objective has always been to develop a Ru-based alkane dehydrogenation catalyst. Whereas many alkane-functionalization precatalysts are Ir-hydride complexes, our effort to access the analogous (PPHP)RuH4 succeeded in NMR scale, but isolation remains challenging. Reaction with propene affords a Ru-allyl–hydride complex that is stable and isolated under argon atmosphere.In parallel, we observed rapid fluorination of the pincer ligand: (PPHP)RuCl2 or (PPClP)Ru(H)Cl reacts with [NO]BF4 to give (PPFP)RuCl2. Formation of the P–F bond is confirmed by a large value of J(P–F) ≈ 900 Hz, observed in both the 31P and 19F NMR spectra. We then screened several fluorinating reagents; tetrabutylammonium fluoride (TBAF) gave the best outcome, affording (PPFP)Ru(H)Cl. The different reagent appear to operate via different mechanisms: [NO]BF4 appears to abstract hydride from Ru, whereas TBAF promotes halide exchange, replacing Cl⁻ with F⁻.