Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications

Abstract

The activation of weakly polarized bonds represents a challenging, yet highly valuable process. In this context, precious metal catalysts have been used as reliable compounds for the activation of rather inert bonds for the last several decades. Nevertheless, base-metal complexes including cobalt, iron, or nickel are currently promising candidates for the substitution of noble metals in order to develop more sustainable processes. In the past few years, manganese(I)-based complexes were heavily employed as efficient catalysts for (de)hydrogenation reactions. However, the vast majority of these complexes operate via a metal-ligand bifunctionality as already well implemented for precious metals decades ago. Although high reactivity can be achieved in various reactions, this concept is often not applicable to certain transformations due to outer-sphere mechanisms. In this Account, we outline the potential of alkylated Mn(I)-carbonyl complexes for the activation of nonpolar and moderately polar E-H (E = H, B, C, Si) bonds and disclose our successful approach for the utilization of complexes in the field of homogeneous catalysis. This involves the rational design of manganese complexes for hydrogenation reactions involving ketones, nitriles, carbon dioxide, and alkynes. In addition to that, the reduction of alkenes by dihydrogen could be achieved by a series of well-defined manganese complexes which was not possible before. Furthermore, we elucidate the potential of our Mn-based catalysts in the field of hydrofunctionalization reactions for carbon-carbon multiple bonds. Our investigations unveiled novel insights into reaction pathways of dehydrogenative silylation of alkenes and trans-1,2-diboration of terminal alkynes, which was not yet reported for transition metals. Due to rational catalyst design, these transformations can be achieved under mild reaction conditions. Delightfully, all of the employed complexes are bench-stable compounds. We took advantage of the fact that Mn(I) alkyl complexes are known to undergo migratory insertion of the alkyl group into the CO ligand, yielding an unsaturated acyl intermediate. Hydrogen atom abstraction by the acyl ligand then paves the way to an active species for a variety of catalytic transformations which all proceed via an inner-sphere process. Although these textbook reactions have been well-known for decades, the application in catalytic transformations is still in its infancy. A brief historical overview of alkylated manganese(I)-carbonyl complexes is provided, covering the synthesis and especially iconic stoichiometric transformations, e.g., carbonylation, as intensively examined by Calderazzo, Moss, and others. An outline of potential future applications of defined alkyl manganese complexes will be given, which may inspire researchers for the development of novel (base-)metal catalysts.

Scheme 1. Synthesis Routes toward Manganese Alkyl and Aryl Carbonyl Complexes

Scheme 2. Ligand-Induced Migratory Insertion of an Alkyl or Aryl Group in the Adjacent CO Ligand

Scheme 3. Reaction of Manganese Carbonyl Alkyl Complexes [Mn(CO)₅R] (R = n-Alkyl Groups) with Triphenylphosphine

Scheme 4. Hydrogenation of Nitriles and Ketones Catalyzed by Mn1

Scheme 5. Catalyst Design and E–H Bond Activation by Manganese Carbonyl Alkyl Complexes

Scheme 6. Additive-Free Hydrogenation of Nitriles Catalyzed by Mn2

Scheme 7. Hydrogenation of α,β-Unsaturated Ketones Catalyzed by Mn4

Scheme 8. Simplified Reaction Mechanism for the Reduction of Ketones

Scheme 9. Lewis Acid Assisted Hydrogenation of Carbon Dioxide to Formate Catalyzed by Mn4

Scheme 10. Hydrogenation of Mono- and Disubstituted Alkenes Catalyzed by Mn4

Scheme 11. Simplified Catalytic Cycle of the Hydrogenation of Monosubstituted Alkenes

Scheme 12. Semihydrogenation of Alkynes Catalyzed by Mn4

Scheme 13. Simplified Reaction Mechanism for the Semihydrogenation of Alkynes

Scheme 14. Efficient Hydroboration of Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by Mn4

Scheme 15. Simplified Reaction Mechanism for the trans-1,2-Diboration of Terminal Alkynes

Scheme 16. Dehydrogenative Silylation of Alkenes Yielding Vinyl- and Allyl Silanes Catalyzed by Mn4

Scheme 17. Simplified Reaction Mechanism for DS of Aromatic Alkenes Following Two Parallel Pathways

Scheme 18. Dimerization and Cross-Coupling of Terminal Alkynes Catalyzed by Mn4

Scheme 19. Simplified Catalytic Cycle for the Dimerization of Aromatic Alkynes