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.