Designing New Magnesium Pincer Complexes for Catalytic Hydrogenation of Imines and N-Heteroarenes: H₂ and N–H Activation by Metal–Ligand Cooperation as Key Steps

Abstract

Utilization of main-group metals as alternatives to transition metals in homogeneous catalysis has become a hot research area in recent years. However, their application in catalytic hydrogenation is less common due to the difficulty in heterolytic cleavage of the H−H bond. Employing aromatization/de-aromatization metal−ligand cooperation (MLC) highly enhances the H 2 activation process, offering an efficient approach for the hydrogenation of unsaturated molecules catalyzed by main-group metals. Herein, we report a series of new magnesium pincer complexes prepared using PNNH-type pincer ligands. The complexes were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H 2 and N−H bonds by MLC employing these pincer complexes was developed. Using the new magnesium complexes, homogeneously catalyzed hydrogenation of aldimines and ketimines was achieved, affording secondary amines in excellent yields. Control experiments and DFT studies reveal that a pathway involving MLC is favorable for the hydrogenation reactions. Moreover, the efficient catalysis was extended to the selective hydrogenation of quinolines and other N-heteroarenes, presenting the first example of hydrogenation of N-heteroarenes homogeneously catalyzed by early main-group metal complexes. This study provides a new strategy for hydrogenation of C�N bonds catalyzed by magnesium compounds and enriches the research of main-group metal catalysis.

Trendbericht Anorganik 2024: Hauptgruppen‐ und Molekülchemie

“…Damit lassen sich etliche Substrate mit C N-Funktionalität selektiv und in hohen Ausbeuten hydrieren (Abbildung 4). 5) Schlüssel ist hier das Ligandende-sign, das während Substrataktivierung und -übertragung zwischen aromatischen und nichtaromatischen Zuständen wechselt.…”

Catalytic Main‐Group Metal Complexes of Phosphine‐Based Pincer Ligands

“…Very recently, we reported an extension of the aforementioned Mg‐based catalytic hydrogenation system, employing a new set of Mg(II) complexes studied as catalysts for C=N bond hydrogenation [73] . These complexes, Mg ‐ 7 , Mg ‐ 8 , Mg ‐ 9 , Mg ‐ 10 and Mg ‐ 11 (Figure 11a), feature the anionic PNN‐type pincer ligands PNN ‐ 2 and PNN ‐ 3 .…”

“… (a) Synthesis of PNN−Mg(II) complexes Mg ‐ 7 , Mg ‐ 8 , Mg ‐ 9 , Mg ‐ 10 and Mg ‐ 11 , reported in 2023 [73] . (b) Crystal structures of Mg ‐ 7 , Mg ‐ 9 and Mg ‐ 10 .…”

“…Very recently, we reported an extension of the aforementioned Mg-based catalytic hydrogenation system, employing a new set of Mg(II) complexes studied as catalysts for C=N bond hydrogenation. [73] These complexes, Mg-7, Mg-8, Mg-9, Mg-10 and Mg-11 (Figure 11a), feature the anionic PNN-type pincer ligands PNN-2 and PNN-3. Complexes Mg-7 and Mg-8 were generated by mixing each of the corresponding protonated proligands with EtMgBr in diethyl ether, followed by treatment with THF in the case of Mg-7, and were isolated in good-to-high yields (79-85 %).…”

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

“…Greb et al reported that the calix[4]­pyrrolato aluminate ( C ) catalyzes the hydroboration of aldehydes in an aluminum–ligand cooperative fashion . Recently, Milstein’s group developed efficient magnesium and zinc catalysts ( D ) for hydrogenation reactions, which proceed by aromatization and dearomatization of ligands . In light of the previous research, further exploration of new modes of element–ligand cooperativity to broaden their catalytic applications is still highly necessary.…”