Structural and theoretical studies of mono and di-insertion of symmetric alkynes into the Pd–C σ bond of cyclopalladated secondary (tert-butyl and ethyl) benzylamines

The reactivity of the dinuclear cyclopalladated complexes derived from secondary benzylamines, [Pd2{(C,N)–C6H4CH2NH(R)}2(μ-X)2] [X = Br, R = t-Bu (1a); X = Cl, R = t-Bu (1b); X = Cl, R = Et (1c)] is reported. 1a reacted with dimethyl acetylenedicarboxylate (DMAD) to yield the mono-inserted complex [Pd2{(C,N)–C(CO2Me)double bond; length as m-dashC(CO2Me)C6H4CH2NH(t-Bu)}2(μ-Br)2] (2a). 1a, 1b and 1c reacted with (R′)Ctriple bond; length of mdashC(R′) (R′ = CO2Me, Ph) to give [Pd{(C,N) C(R′)double bond; length as m-dashC(R′)C(R′)double bond; length as m-dashC(R′)C6H4CH2NH(R)}X] [X = Br, R = t-Bu, R′ = CO2Me (3a); X = Cl, R = t-Bu, R′ = Ph (2b); X = Cl, R = Et, R′ = Ph (2c)] through a double insertion of the alkyne into the Pd–C σ bond. 2c also reacts with Ag(CF3SO3) and pyridine to give mononuclear cationic complex [Pd{(C,N)–C(Ph)double bond; length as m-dashC(Ph)C(Ph)double bond; length as m-dashC(Ph)}C6H4CH2NH(Et)(py)]CF3SO3 (3c). The crystal structures of di-inserted complexes 2b, 2c and 3c have been determined by X-ray diffraction studies. Their molecular structures showed that the conformation of the Cdouble bond; length as m-dashC double bonds within the butadienyl group attached to the aromatic ring was trans–cis. Density functional theory (DFT) calculations also indicated that this arrangement is more stable than other possible conformations.