Synthesis and reactions of mono- and dipalladium complexes containing 2-thienyl and 2,5 (2,5') - bridged thienylene (of dithienylene) ligand: new dipalladium complex formation connected by thioketene part via C-S bond cleavage of thiophene

Synthesis and reactions of mono- and dipalladium complexes containing 2-thienyl and 2,5 (2,5') - bridged thienylene (of dithienylene) ligand: new dipalladium complex formation connected by thioketene part via C-S bond cleavage of thiophene

Oxidative addition reactions of 2-iodothiophene, 2-bromothiophene, and 2,5-dibromothiophene with equimolar Pd(CH₂=CHPh)L₂(L=PMe₃and PEt₃) at room temperature in THF produce mononuclear α-thienyl palladium(Ⅱ) complexes trans-(2-C₄H₃S)PdX(PMe₃)₂(X=1,1;X=Br,2) and trans-(5-Br(2-C₄H₃S))PdBrL₂(L=PMe₃,3;L=PEt₃.4). Reactions using 2,5-dibromothiophene, 2,5-diiodothiophene, and 5,5-dibromo-2,2'-bithiophene under 2:1 molar ratio in THF give thienylene(or dithienylene) bridged dipalladium complexes trans, trans-L₂XPd(μ-2,5-C₄H₂S)PdXL₂(L=PMe₃,X=I,5;Br,6;L=PEt₃;X=Br,7) and trans, trans-(Me₃P)₂BrPd(μ-2,5'-C₄H₂S-C₄H₂S)PdBr(PMe₃)₂(8), in high yields. However, the equimolar reaction of 2,5-dibromothiophene with the Pd(0) complex results in formation of a dinuclear complex cis, trans-[(Me₃P)₂Pd(n²-S=C=CH-CH=CBr)PdBr(PMe₃)₂(9), in 56% yield via C-S bond cleavage of the thiophene ring. All the complexes provided satisfactory IR and NMR (¹H, ³P{³H}, and ¹³C{ ¹H}) spectra as well as elemental analyses. Molecular structures of 1,7, and 9 were confirmed by X-ray crystallography. Treatment of 5 and 7 with CO(1 atm) results in CO insertion into one of the Pd-C bonds to give trans, trans-(Me₃)₂IPdC(O)C₄H₂SPdI(PMe₃)₂(10) in 94% and trans, trans-(Et₃)₂BrPdC(O)C₄H₂SPdBr(PEt₃)₂(11) in 23%, respectively. Alsom treatments of 5 with n-butyl, tert-butyl, and cyclohexyl isocyanides result in its insertion into both of the Pd-C bonds to form iminoacyl Pd complexes trans, trans-(Me₃P)₂IPd-C(=N-R)C₄H₂SC(=N-R)-PdI(PMe₃)₂(R=n·Bu, 12;t-Bu, 13; cyclohexy1, 14), respectively.