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Insertion of Acrylonitrile into Palladium Methyl Bonds in Neutral and Anionic Pd(II) Complexes

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posted on 16.02.2005, 00:00 by Laurent F. Groux, Thomas Weiss, Dastigiri N. Reddy, Preston A. Chase, Warren E. Piers, Tom Ziegler, Masood Parvez, Jordi Benet-Buchholz
The reactions of a series of Pd(II) methyl compounds of general formula LPd(NCCH3)CH3, where L is a bulky phenoxydiazene or phenoxyaldimine ligand with the polar olefin acrylonitrile (AN), are reported. The compounds react with an excess of AN to give the products of 2,1 insertion into the Pd−Me bond, yielding dimers and/or trimers which feature bridging α-cyano groups. The reactions were studied by low temperature 1H NMR spectroscopy, revealing an initial formation of compounds featuring N-bound AN, which isomerized to an (unobserved) π-bound species that rapidly underwent 2,1 insertion into the Pd−Me bond. Intermediate oligomeric complexes retaining a Pd−Me function were observed at low [AN] in these reactions. Under pseudo first-order conditions, kobs values of 8.5 × 10-5 to 2.68 × 10-3 M-1 (−22 °C to 10 °C, 100 equiv of AN) and activation parameters of ΔH = 14.4(5) kcal mol-1 and ΔS = −19(5) eu were obtained in one case. Comparison of the overall rates of insertion between two LPd(NCCH3)CH3, differing in the overall charge on the supporting ligand L, showed that the complex bearing a negatively charged ligand reacts with AN twice as fast as one with no anionic charge. The rates of insertion in both of these complexes are significantly faster than reported rates for analogous reactions in cationic Pd(II) derivatives, indicating that increasing the negative charge on the complex enhances the rate of AN insertion. These results provide fundamental mechanistic insights into a crucial reaction for incorporation of polar comonomers into alpha olefins via a coordination polymerization mechanism.