Palladium(ii) complexes with electron-poor, 4,5-disubstituted diimidazol-2-ylidene ligands: synthesis, characterization and catalytic activity

Abstract

Diimidazolium salts featuring different bridges between the imidazolium groups, as well as electron-withdrawing groups (chloride, cyanide) at the 4- and 5-position of the heterocyclic rings, have been successfully prepared. The diimidazolium salts serve as convenient precursors of di(N-heterocyclic carbene) ligands, which coordinate in a chelating fashion to palladium(ii) centres. The effect of the newly introduced electron-withdrawing groups on the spectroscopic and structural characteristics of the resulting complexes as well as on their reactivity as catalysts in a model alkyne hydroarylation reaction has been investigated and is discussed herein.

Group 11 Metal Complexes with Electron‐poor, 4,5‐Disubstituted Diimidazol‐2‐ylidene Ligands

“…The carbene carbon in complex [ 3a ](BF 4 ) 2 is the most deshielded in the series [ δ = 187.7 ppm vs. 181.1 ppm in the case [ 2a ](BF 4 ) 2 with –Cl substituents], confirming the large electron withdrawing character of the –CN group; by contrast, the presence of the –Cl substituents in the backbone has only a negligible effect in the carbene carbon shift, which results to be comparable to the one recorded for the corresponding unsubstituted complex ( δ = 180.9 ppm, for X = H) 9d. This trend has been already observed in the corresponding diazolium salts and palladium(II) complexes 10…”

“…The choice of one synthetic procedure rather than the other mainly depends on the counter anion of the starting bis(azolium) salt; in fact, with non‐coordinating anions, such as OTf – , PF 6 – , or BF 4 – , the transmetalation is generally preferred, while with halides (Cl – , Br – , or I – ) the second procedure is adopted. In this work we have employed mainly the first synthetic procedure, considering that the bis(imidazolium) precursors could be obtained with BF 4 – as counter anions 10. The dinuclear silver(I) complexes [ 1a ](BF 4 ) 2 – [ 3a ](BF 4 ) 2 were synthesized by reaction of the corresponding bis(azolium) salts with excess Ag 2 O in acetonitrile at 60 °C for 24 h (Scheme ).…”

“…The reagents were purchased from Aldrich as high‐purity products and generally used as received; all solvents were of technical grade and used as received. All the diazolium salts were prepared according to literature procedures 10,15. NMR spectra were recorded at 25 °C with a Bruker Avance 300 MHz (300.1 MHz for 1 H and 75.5 MHz for 13 C); chemical shifts ( δ ) are relative to the residual solvent signals.…”

“…We have an ongoing research program on the synthesis, characterization and application of late transition metal complexes with poly‐NHC ligands;9 in a recent contribution, we have reported on the successful preparation of diimidazolium salts featuring electron withdrawing groups in the positions 4 and 5 of the imidazolium heterocycles (Scheme ), and we have demonstrated that these salts can be employed as precursors of chelating dicarbene ligands in the preparation of mononuclear palladium(II) complexes 10. In this contribution, we extend our investigations to the reactivity of these diimidazolium salts with group 11 metal centers, and we disclose novel dinuclear complexes of the corresponding dicarbene ligands with silver(I) and gold(I).…”

Advances in Transition‐Metal‐Catalysed Alkyne Hydroarylations

“…Another possibility was to introduce suitable electron‐withdrawing substituents in the 4,5‐position of the carbene heterocyclic rings 40. The particular problem in this case regards the preparation of the functionalized dicarbene ligands; eventually, this problem was solved and methods were developed for the preparation of a small library of complexes, among which complexes XVI – XVIII (Scheme ) were tested as catalysts in hydroarylations.…”