Involvement of Nickel Carbonites in the Formation of Oxalate: a Combined Experimental and Computational Study

With the background of C-C coupling reactions to oxalate observed in the reaction of [LtBuNi(N2)NiLtBu] with CO2, or starting from [LtBuNiOOCH] through formate deprotonation with an potassium amide base, the latter conversion was investigated more closely. It was found that the nickel(II) carbonite produced upon deprotonation of formate with the amide readily reacts with the precursor [LtBuNiOOCH], however, not via an electron transfer but via salt metathesis, driven by the low solubility of KOOCH, to accumulate the tentative intermediate [LtBuNi(CO2)NiLtBu]. The latter can be assumed to be rather unstable towards elimination of CO2, which according to the results of DFT calculations, can react with unreacted [LtBuNi(CO2)NiLtBu] to form oxalate. This, however, happens only with potassium amides, as in case of Na and Li due to the more soluble formate salts, the metathesis reactions are slow and thus the intermediate does not accumulate. A mechanism via salt metathesis, an intermediate [LtBuNi(CO2)NiLtBu] and subsequent CO2 elimination is also in agreement with the findings made for the system [LtBuNi(N2)NiLtBu]/CO2, as the same intermediate can be assumed to be generated there, and it is also supported by the fact that oxalate formation was observed, too, when a Cs amide was used, leading to insoluble CsOOCH.