Reactivity of N-heterocyclic carbene–pyridine palladacyclopentadiene complexes toward halogen addition. The unpredictable course of the reaction

Abstract

aAs an extension of a previously published work we have reacted some palladacyclopentadiene complexes stabilized by bidentate N-heterocyclic carbene-pyridine or monodentate N-heterocyclic carbene-pyridine and isocyanide ligands with the halogens I 2 and Br 2 . All the bidentate and monodentate complexes react with halogens to give at first the expected σ-coordinated butadienyl fragment. However, two of the less hindered NHC carbene-pyridine bidentate butadienyl iodo derivatives undergo a further rearrangement and novel Pd(II) complexes characterized by a ten term coordinative ring were isolated and characterized. In the most favorable case we were able to carry out the kinetics of rearrangement and measure its reaction rate. Moreover, we have surmised a plausible mechanism on the basis of a dedicated computational approach and in one case the surprising structure characterized by the ten term coordinative ring was resolved by X-ray diffraction.

Improved Synthesis, Anticancer Activity and Electrochemical Characterization of Unusual Zwitterionic Palladium Compounds with a Ten‐Term Coordinative Ring.

“…This experimental evidence is in good agreement with the energies, estimated by DFT calculations, of the different species involved in the mechanism proposed previously for this process (Scheme ) …”

“…An improved protocol for the synthesis of the zwitterionic palladium complexes is reported in Scheme . In our previous paper, compounds 3 a‐b were isolated after the slow isomerization of the σ‐butadienyl derivatives 2 a‐b which required 3 days (for R=Me) or 4 weeks (for R= Mesityl) at RT in dichloromethane …”

“…Our research group has recently synthesized an original class of zwitterionic palladium compounds, with two iodides in a mutual trans configuration and an unusual ten‐term coordinative ring (Figure ) . These species are the products of the slow isomerization of palladium(II) butadienyl derivatives stabilized by N ‐Heterocyclic Carbene‐pyridine ligands, which in turn were synthesized by addition of iodine to the corresponding palladacyclopentadienyl complexes.…”

Reactions of proteins with a few organopalladium compounds of medicinal interest

“…In fact, it can only be attacked by halogens or reactive alkyl/aryl halides leading to the corresponding Pd( ii )-butadienyl complexes. 4 The butadienyl fragment can then be released from the metal center by a further oxidative addition and consecutive reductive elimination. However, no studies have been performed so far on the reactivity of this particular palladacyclic fragment towards proteins.…”