Acrylonitrile Insertion Reactions of Cationic Palladium Alkyl Complexes

The reactions of acrylonitrile (AN) with “L2PdMe+” species were investigated; (L2 = CH2(N-Me-imidazol-2-yl)2 (a, bim), (p-tolyl)3CCH(N-Me-imidazol-2-yl)2 (b, Tbim), CH2(5-Me-2-pyridyl)2 (c, CH2py‘2), 4,4‘-Me2-2,2‘-bipyridine (d), 4,4‘- t Bu2-2,2‘-bipyridine (e), (2,6- i Pr2−C6H3)NCMeCMeN(2,6- i Pr2−C6H3) (f)). [L2PdMe(NMe2Ph)][B(C6F5)4] (2a − c) and [{L2PdMe}2(μ-Cl)][B(C6F5)4] (2d − f) react with AN to form N-bound adducts L2Pd(Me)(NCCHCH2)+ (3a − f). 3a − e undergo 2,1 insertion to yield L2Pd{CH(CN)Et}+, which form aggregates [L2Pd{CH(CN)Et}] n n + (n = 1−3, 4a − e) in which the Pd units are proposed to be linked by PdCHEtCN- - -Pd bridges. 3f does not insert AN at 23 °C. 4a − e were characterized by NMR, ESI-MS, IR and derivatization to L2Pd{CH(CN)Et}(PR3)+ (R = Ph (5a − e), Me (6a − c)). 4a,b react with CO to form L2Pd{CH(CN)Et}(CO)+ (7a,b). 7a reacts with CO by slow reversible insertion to yield (bim)Pd{C(O)CH(CN)Et}(CO)+ (8a). 4a − e do not react with ethylene. (Tbim)PdMe+ coordinates AN more weakly than ethylene, and AN insertion of 3b is slower than ethylene insertion of (Tbim)Pd(Me)(CH2CH2)+ (10b). These results show that most important obstacles to insertion polymerization or copolymerization of AN using L2PdR+ catalysts are the tendency of L2Pd{CH(CN)CH2R}+ species to aggregate, which competes with monomer coordination, and the low insertion reactivity of L2Pd{CH(CN)CH2R}(substrate)+ species.