The addition of halogens and interhalogens on palladacyclopentadienyl complexes bearing quinolyl-thioether as spectator ligands. A kinetic and computational study
Graphical abstract
We have studied the oxidative addition of halogens (I2, Br2) and interhalogens (ICl, IBr) on palladacyclopentadienyl complexes bearing heteroditopic quinolyl-thioethers as ancillary ligands. The reactions with halogens under stoichiometric conditions yield the σ-butadienyl derivatives. Br2 in excess yields the free dibromo-(E, E)-σ-butadiene and the quinolylthioether palladium(II) dibromide species, whereas the same reaction with I2 in excess does not induce the extrusion of the diene. The interhalogens react with the starting palladacyclopentadienyl complexes to give the less thermodynamically stable isomers. An interpretation based on computational and mechanistic studies is proposed.
Introduction
Owing to their potential ability to give σ-butadienyl derivatives as the products of the oxidative addition of organic halides or halogens, palladacyclopentadienyl complexes have often been studied from the catalytic [1] and stoichiometric [2] point of view. The reaction entails the Pd(II)–Pd(IV)–Pd(II) conversion which was by far less studied than that involving the Pd(0)–Pd(II) process [3]. For such a reason we think that any attempts at shedding light on the still debated Pd(IV) intermediate should be of remarkable interest. Moreover, the possibility to obtain dienes with a Z–Z configuration by further addition of organic halides or halogens to the σ-butadienyl complexes extends the range of synthetic available methodologies based on acetylene coupling [4]. In this respect we have recently published three studies of the reactivity of some palladacyclopentadienyl complexes bearing isocyanides, phosphoquinolines and pyridylthioethers as ancillary ligands with halogens and interhalogens. In any case we have obtained interesting results. In the first study we have measured the rates of intramolecular conversion of the intermediate trans-diiodo palladium(IV) into the cis-diisocyanide-tetramethyl pallada-1-iodobuta-1,3-diene-1,2,3,4-tetracarboxylate and its subsequent isomerisation to the trans-isomer [2d]. The second investigation was characterized by the peculiar evolution of the initially formed σ-butadienyl complex, consisting in an intermolecular attack of the phosphorus originally coordinated to the metal on the sp2 carbon of the σ-butadienyl fragment with the consequent widening of the phosphoquinoline coordinating ring and the unexpected final formation of a zwitterionic species (Scheme 1) [2e].
Finally, we have studied the palladacyclopentadienyl complexes bearing pyridylthioethers as spectator ligands reacting with halogens and interhalogens to give the σ-butadienyl derivatives as final products. However, in the case of interhalogens the final product was not the predictable one but rather the less thermodynamically stable species [2h].
The intriguing aspects of these studies prompted us to undertake a further investigation on palladacyclopentadienyl complexes bearing thio-quinoline based spectator ligands reacting with halogens and interhalogens. In the present study, we tried to establish how the combination between the structure of the ancillary ligand and the sulfur atom might influence the overall reactivity of the complexes themselves and investigate whether the formation of the less stable σ-butadienyl and/or zwitterionic species can be considered a general trend. Furthermore, we have studied the kinetics in detail and surmised a plausible mechanism for the reaction between the σ-butadienyl complexes and Br2 in excess leading to the extrusion of 1,4-dibromobuta-1,3-diene.
The ligands, the investigated complexes, the halogens, the interhalogens and the products of the oxidative addition are reported in Scheme 2.
Section snippets
General remarks
The ligands TMQ, TTBQ [5], TMQ-Me [6], TTBQ-Me [7], the polymer [PdC4(COOMe)4]n [8] and complex 1c [6] were obtained according to published protocols. The complexes 1a, b, d are newly synthesized species and were obtained by adding a small excess of the appropriate ligand to the polymer [PdC4(COOMe)4]n under inert atmosphere in anhydrous acetone. The formation of the complexes 1a–d is deduced from their NMR spectra which display all the signals of the ligands and carboxymethyl groups at
Conclusion
We have synthesized some palladacyclopentadienyl derivatives bearing differently substituted thioquinolines as spectator ligands. The palladacyclopentadienyl complexes were reacted with halogens (I2 and Br2) and interhalogens (ICl and IBr) to give the thioquinoline palladium σ-butadienyl species which in two cases were characterized by the diffractometric determination of their solid state structures and in all cases by 1H and 13C NMR, IR and elemental analyses. The addition of interhalogens to
Solvents and reagents
All the following distillation processes were carried out under inert atmosphere (Argon). Acetone and CH2Cl2 were distilled over 4 Å molecular sieves and CaH2, respectively. THF was carefully dried by distillation over Na/benzoquinone. All other chemicals were commercially available grade products and were used as purchased.
IR, NMR, UV–Vis measurements and elemental analysis
The IR, 1H, 13C and 31P NMR spectra were recorded on a Perkin–Elmer Spectrum One spectrophotometer and on a Bruker 300 Avance spectrometer, respectively. UV–Vis spectra were
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