Comparative CO₂ Hydrogenation Catalysis with MACHO-type Manganese Complexes

Abstract

A pair of manganese complexes containing MACHO-type pincer ligands bearing a secondary amine, [HN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2, which can participate in pathways involving metal–ligand cooperation (MLC), and a tertiary amine, [MeN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2, which cannot participate in pathways involving MLC, are compared for the hydrogenation of CO2 to formate in the presence of a base. Lewis acid cocatalysts are crucial for increasing the activity of both catalysts, with [MeN­{CH2CH2(P i Pr2)}2]­MnH­(CO)2 reaching TONs of up to 18,300 and yields of up to 73% in the presence of lithium triflate. This productivity is far greater than for the MLC capable secondary amine MACHO-supported manganese catalyst. Preliminary mechanistic experiments indicate that CO2 insertion into the Mn–H of each catalyst affords a stable manganese formate complex. In situ NMR spectroscopy and comparative catalytic experiments are consistent with the intermediacy of these manganese formate complexes in the catalytic cycle, likely representing the catalyst resting states. Our findings suggest that the tertiary amine ligated system gives greater productivity due to a combination of longer catalyst lifetime and greater enhancement from Lewis acid additives.

Synthesis, Characterization, and Reactivity of a (PPP) Pincer-Ligated Manganese Carbonyl Complex: Polarity Reversal Imparted by the Electrophilic Nature of a Planar Mn-P(NR2)2 Fragment

“…By solid-state IR spectroscopy, two carbonyl stretching modes were observed at 1906 and 1836 cm –1 (Figure S17). Although these stretching frequencies are comparable to the range of those in dicarbonyl Mn I complexes (2000–1800 cm –1 ), ,,,,,− the electronic description of the Mn center and P NHP atom remains ambiguous as the electron-withdrawing effects of a cationic NHP + ligand toward a Mn –I center may yet account for the observation of higher stretching frequencies. In solution, both of the stretches observed in the solid-state IR spectrum are present at 1906 and 1837 cm –1 , but a second set of stretches is observed at 1912 and 1850 cm –1 (Figure S18).…”

“…Tremendous progress has been made in recent years toward the development of base metal catalysts, as both economic and environmental pressures have limited the sustained use of noble metals in homogeneous catalysis. − To this end, much attention has been given to employing Earth-abundant manganese in transition metal complexes with applications in small molecule activation and catalysis. , In particular, pincer-ligated manganese complexes have been used as dehydrogenation and hydrogenation catalysts to achieve a variety of chemical transformations, − with some notable examples including dehydrogenative coupling reactions to afford industrially relevant building blocks like imines, , amides, and N-heterocycles such as pyrroles and pyrimidines, − the reduction of CO 2 , − and H–E (E = B, P, and Si) hydroelementation reactions . The coordination of a pincer ligand scaffold to these transition metal catalysts has been popularized to impart stability in well-defined structures through chelation as well as provide steric and electronic tunability via selection and modification of the coordinating groups and backbone linkers …”

PCP Pincer Complexes of Titanium in the +3 and +4 Oxidation States

“…Since the work of Shaw in 1976, pincer complexes have been a powerful weapon in the organometallic chemist’s arsenal . Formally defined as tridentate ligands which usually hold a fixed meridional geometry, their high thermal stability and extraordinary tunability has resulted in their use in myriad applications. − Pincer complexes have been applied extensively in catalysis, particularly for hydrogenation/dehydrogenation reactions. − They have also been applied in the area of small-molecule activationfor example, in the cleavage and reduction of dinitrogen. − …”

PCP Complexes of Titanium in the +3 and +4 Oxidation State

“…[2][3][4] Pincer complexes have been applied extensively in catalysis, particularly for hydrogenation/dehydrogenation reactions. [5][6][7] They have also been applied in the area of small molecule activation, notably in recent work by Nishibayashi using Molybdenum pincer complexes for record-breaking dinitrogen reduction. 8 The bulk of research carried out with pincer complexes has focused on the late transition metals.…”

“…The [BAr F 4]− anion, hydrogen atoms and methyl groups on the tert-butyl substituents of the phosphines are omitted for clarity. Selected bond lengths (Å) and angles (°): Ti(1)-Cl(1) 2.2591(7), Ti(1)-Cl(2) 2.4758(7), Ti(1)-P(1) 2.6357(7), Ti(1)-P(2) 2.6199(7), Ti(1)-C(1) 2.173(2), Ti(1)-Ti(2) 4.8972(6), Ti(2)-Cl(2) 2.4378(7), Ti(2)-Cl(3) 2.2719(7), Ti(2)-P(3) 2.5924(7), Ti(2)-P(4) 2.6374(7), Ti(2)-C(25) 2.187(2); Cl(1)-Ti(1)-C(1) 128.19(7), P(1)-Ti(1)-P(2) 151.03(2), Cl(1)-Ti(1)-Cl(2) 105.77(3), P(1)-Ti(1)-Cl(2) 95.56(2), P(2)-Ti(1)-Cl(2) 100.51(2), C(1)-Ti(1)-Cl(2) 126.02(7), Ti(1)-Cl(2)-Ti(2) 170.62(3), Cl(3)-Ti(2)-C(25) 147.76(7), P(3)-Ti(2)-P(4) 146.80(2), Cl(2)-Ti(2)-Cl(3) 104.82(3), Cl(2)-Ti(2)-P(3) 101.05(2), Cl(2)-Ti(2)-P(4) 106.10(2), Cl(2)-Ti(2)-C(25) 107.10(7) The two titanium centres both adopt a square-based pyramid geometry, although one, Ti(1), is more distorted (τ = 0.067 and 0.016, Ti(1) and Ti(2) respectively). The PCP ligands are almost perpendicular to one another, demonstrated by the angles between the C(1)-Ti(1)-Cl(2) and C(25)-Ti(2)-Cl(2) planes: 79.58°.…”