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

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posted on 2016-12-15, 09:47 authored by Qiang-Qiang Ma, Ting Liu, Anubendu Adhikary, Jie Zhang, Jeanette A. Krause, Hairong Guan
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.