Manganese(I) Tricarbonyl Complexes with Bidentate Pyridine-Based Actor Ligands: Reversible Binding of CO₂ and Benzaldehyde via Cooperative C–C and Mn–O Bond Formation at Ambient Temperature

Abstract

We report manganese(I) tricarbonyl complexes decorated with imino-and amino-pyridine ligands [Mn(impy)(CO) 3 Br] and [Mn(ampy)(CO) 3 Br], respectively. Both compounds can be transformed either via two-electron reduction for the former or double deprotonation for the latter into anionic species with a disturbed ("dearomatized") πelectron system of the pyridine ring M[Mn(amidopy*)(CO) 3 ] (M = alkali metal). The newly formed five-coordinated complex is anionic and encompasses a nucleophilic carbon center within its metalla cycle. This leads to noteworthy reactivity: [Mn(amidopy*)(CO) 3 ] − readily reacts with C�O double bonds. Specifically, CO 2 and benzaldehyde can bind to the complex via a metal−ligand cooperative [1,3]-addition under C−C and Mn−O bond formation and concomitant rearomatization of the pyridine ring. Remarkably, we found that this addition is reversible. Exchange reactions using isotopically labeled 13 CO 2 indicate reversible C−C and Mn−O bond formation at ambient temperature. Likewise, bonded benzaldehyde is exchanged from the complex under a CO 2 atmosphere. Density functional theory calculations suggest a significant role for the cationic counter ion in the bond activation reactions that can make this bond activation feasible.

H2 Pressure Dependence in the Homogeneously Catalyzed Guerbet Reaction of Ethanol to Butanol using Mn(I) and Ru(II)

“…The isolation of this complex opens the way for an alternative, previously unconsidered catalytic pathway via attack of the nucleophile in the ligand back-bone followed by hydride abstraction by the ruthenium atom under reformation of the acridine. Similar modes of action have been observed for non-innocent ligands before, [50,51] but commonly the ligand backbones react as nucleophiles e. g. with CO 2 and not as electrophiles as we now observed for complexes containing ligands of the type 2-LÀ R 2 . Compounds with a related acridine motif have, however, shown radical-based reactivity at C16 when combined with first row transition metals.…”