The reductive homologation of CO has been studied since at least 1834, when Justus von Liebig reacted molten potassium and CO gas to give a dark, ill-defined solid. Later, his product—”potassium carbonyl” or (KCO)_n—was shown to contain a mixture of species, including potassium benzenehexolate, K₆C₆O₆. The mechanism of this reaction is still unknown, but it has been proposed that transition-metal impurities in the molten potassium promote the formation of K₆C₆O₆.

Cameron Jones, Monash University, Melbourne, Australia, Laurent Maron, University of Toulouse, France, and colleagues have shown that soluble, reducing magnesium(I) compounds and Mo(CO)₆ can enable the reductive hexamerization of CO and the formation of well-defined magnesium benzenehexolate compounds (reaction pictured below). The team reacted magnesium(I) compounds of the type [{(^ArNacnac)Mg}₂] (^ArNacnac = [HC(MeCNAr)₂]^–, Ar = mesityl or o-xylyl) with CO in the presence of [Mo(CO)₆]. The mesityl and the o-xylyl derivatives were obtained in yields of up to 25 % and 30 %, respectively.

Density functional theory (DFT) calculations and spectroscopic studies showed that the reaction is catalyzed by Mo(CO)₆. The work sheds light on the mechanism of Liebig’s classical reaction and could help to understand how other C-C bond forming reactions that use CO as a C₁ feedstock operate, such as the Fischer–Tropsch process.

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• Reductive Hexamerization of CO Involving Cooperativity Between Magnesium(I) Reductants and [Mo(CO)₆]: Synthesis of Well‐Defined Magnesium Benzenehexolate Complexes,
  Cameron Jones, Albert Paparo, K. Yuvaraj, Aidan J. R. Matthews, Iskander Douair, Laurent Maron,
  Angew. Chem. Int. Ed. 2020.
  https://doi.org/10.1002/anie.202009523