Selective CO Homologation to Acyclic [C _n O _n ] ^2– ( <i>n</i> = 2–4) Motifs through Mg/Ni Cooperative Strategy

Reductive homologation of carbon monoxide (CO) serves as a pivotal process in C1 chemistry, mimicking the chain-growth mechanisms of industrial Fischer-Tropsch synthesis. Herein, the nickel-catalyzed selective homologation of CO with dipyrromethene ligand supported magnesium alkyl compounds through a Mg/Ni cooperative strategy is reported. Reaction of magnesium alkyl compounds <b>L</b>^<b>1</b>MgR·Et₂O [<b>L</b>^<b>1</b> = 1,9-di(isopropylphenyl)-5-mesityldipyrromethene; R = ^<i>n</i>Bu, Et] with CO (1 bar) in the presence of 20 mol % Ni(COD)₂ (COD = 1,5-cyclooctadiene), selectively yielded the acyclic CO tetramerization products (<b>L</b>^<b>1</b>Mg)₂(μ-C₄O₄)R₂. By tuning the steric hindrance of the ancillary ligands, the reaction pathways can be directed to form the linear CO trimerization compound (<b>L</b>^<b>2</b>Mg)₂(μ-C₃O₃)^<i>n</i>Bu₂ (<b>L</b>^<b>2</b> <b>=</b> 1,5,9-trimesityldipyrromethene) and the CO dimerization compound (<b>L</b>^<b>3</b>Mg)₂(μ-C₂O₂)^<i>n</i>Bu₂ [<b>L</b>^<b>3</b> = 1,9-di(1-adamantyl)-5-mesityldipyrromethene], respectively. Moreover, the Mg/Ni-cooperative CO insertion species [<b>L</b>^<b>2</b>Mg(CO)^<i>n</i>Bu]₃[Ni₄(CO)₇] was isolated from the stoichiometric reaction of <b>L</b>^<b>2</b>Mg^<i>n</i>Bu·Et₂O with Ni(COD)₂ under a CO atmosphere. This species further reacted with CO to form the trimerization product (<b>L</b>^<b>2</b>Mg)₂(μ-C₃O₃)^<i>n</i>Bu₂, confirming its active role in the CO homologation process. The formation mechanism of the acyclic CO tetramerization compound was elucidated by density functional theory calculations, which mainly involved the CO insertion, C-C coupling, and carbene dimerization processes, verifying the critical role of the Mg/Ni cooperative strategy in the reaction.