Using CS₂ to Probe the Mechanistic Details of Decarboxylation of Bis(phosphinite)-Ligated Nickel Pincer Formate Complexes

The reaction of the formate complex {2,6-(R₂PO)₂C₆H₃}­Ni­(OCHO) (R = ^tBu, 5; R = ⁱPr, 6) with CS₂ shows first-order kinetics in nickel concentration and zero-order in [CS₂] when CS₂ is used in large excess. Rate measurement at different temperatures gives activation parameters ΔH^⧧ = 22.6 ± 0.9 kcal/mol and ΔS^⧧ = −5.2 ± 3.0 eu for the decarboxylation of 5 and ΔH^⧧ = 22.6 ± 1.0 kcal/mol and ΔS^⧧ = −4.3 ± 3.2 eu for the decarboxylation of 6. Comparing the decarboxylation rate constants for 6 and {2,6-(ⁱPr₂PO)₂C₆H₃}­Ni­(OCDO) (6-d) yields KIE values of 1.67–1.90 within the temperature range 30–45 °C. On the basis of these experimental results and DFT calculations, an ion pair mechanism has been proposed for the decarboxylation process. The CS₂ insertion products {2,6-(R₂PO)₂C₆H₃}­Ni­(SCHS) (R = ^tBu, 3; R = ⁱPr, 4) have been characterized by X-ray crystallography.