Oxidative Addition of Propargyl Halides, Chloroacetonitrile, and Ethyl Chloroacetate to a Dimethylplatinum(II) Complex: Kinetic and DFT Studies

Oxidative addition reaction of propargyl
halides, XCH2CCH (X = Cl, Br), with the platinum­(II)
complex [PtMe2(bipy)] (1), in which bipy =
2,2′-bipyridine, gave a mixture of the propargyl
halide complex [PtXMe2(CH2CCH)­(bipy)],
(X = Cl, trans isomer 2a and cis isomer 2a′; X = Br, trans isomer 2b and cis isomer 2b′) and the allene complex
[PtXMe2(CHCCH2)­(bipy)], (X =
Cl, trans isomer 3a and cis isomer 3a′; X = Br, trans isomer 3b and cis isomer 3b′). The reaction involving propargyl bromide followed second-order
kinetics (first order in both reactants), while that involving propargyl
chloride showed third-order kinetics (first order in 1, second order in ClCH2CCH) in acetone or benzene
solvent. Similar reactions of chloroacetonitrile (ClCH2CN) and ethyl chloroacetate (ClCH2COOEt) reagents
with complex 1 yielded [PtClMe2(CH2CN)­(bipy)] (4) and [PtClMe2(CH2COOEt)­(bipy)] (5), respectively, each as a mixture
of cis and trans isomers, proceeding
with second-order kinetics. Unpredictably, the reaction rates of complex 1 with propargyl bromide and chloroacetonitrile in benzene
solvent are faster than those in acetone solvent. Consistent with
our 1H NMR experimental results, DFT studies showed that
the cis addition products of allene tautomers (3a′,b′) are more stable than their
corresponding trans isomers (3a,b).