Coordination Dynamics and Reactivity of Palladium(II) Complexes Containing the N-Thienylidene- l / d -methionine Methyl Ester Ligand

Metathesis reactions of the N -thienylidene- l / d -methionine methyl ester ligand (th-metMe) with suitable palladium starting complexes afforded coordination complexes of the type PdX 2 (th-metMe), PdX(Me)(th-metMe), and [Pd(Me)(th-metMe)(L‘‘)](O 3 SCF 3 ) (X = Cl, Br, I; and L‘‘ = MeCN, pyridine, picoline, lutidine) which were studied by NMR with respect to the fluxional behavior of the diastereomeric PdNS six-membered ring chelates. The structure of PdCl 2 (th-metMe) in the solid state ( a = 13.318(5) Å, b = 11.365(2) Å, c = 13.579(5) Å, β = 96.66(4)°) showed a square planar coordination complex in which the ligand is chelating via the imine nitrogen (Pd−N = 2.030(7) Å) and the methionine sulfur donor atom (Pd−S 1 = 2.283(2) Å). The square planar geometry is completed by the chlorides (Pd−Cl 1 = 2.275(3) Å ( trans to the imine nitrogen) and Pd−Cl 2 2.322(2) Å ( trans to the sulfur donor)). The dimeric [PdCl(L‘)] 2 complex (L‘ = anionic α-methoxycarbonyl, α-(2-thienylmethylidene)amine, α‘‘-(methylthio)ethane) was formed by C−H activation of the chiral carbon atom of the α-amino acid moiety. The structure determination ( a = 8.1887(7) Å, b = 20.507(2) Å, c = 34.079(3) Å, β = 91.220(7)°) revealed two stretched out ligands of which one is coordinating to Pd(1) via the methionine sulfur donor (2.268(2) Å) and σ-bonded via the stereogenic chiral carbon atom (2.068(6)) and to Pd(2) via the imine nitrogen (2.074(5) Å). The second ligand coordinates to Pd(1) via the imine donor (2.074(5) Å) and to Pd(2) via the methionine sulfur (2.276(1)) and the former stereogenic carbon atom (2.060(6)). Both square planar coordination sites are occupied by the chlorides which are positioned trans to the carbon atom (2.390(2) and 2.414(2) Å for Cl(1A) and Cl(2A)). Complexes of the type [PdX(C(O)Me)(th-metMe)] and [Pd(C(O)Me)(L‘‘)(th-metMe)](O 3 SCF 3 ) were obtained by reaction of CO with the corresponding methyl complexes. The rates of CO insertion into the methyl−palladium bond were investigated, and it was found that the rate decreases in the order [Pd(Me)(th-metMe)(CF 3 SO 3 )] > [Pd(Me)(I)(th-metMe)] > [Pd(Me)(Br)(th-metMe)] > [Pd(Me)(th-metMe)(MeCN)](CF 3 SO 3 ) > [Pd(Me)(th-metMe)(2,6-lutidine)] (CF 3 SO 3 ) > [Pd(Me)(Cl)(th-metMe)] > [Pd(Me)(th-metMe)(pyridine)](CF 3 SO 3 ).