Structural variation, dynamics, and catalytic application of palladium(ii) complexes of di-N-heterocyclic carbene–amine ligands

Abstract

A series of palladium(II) complexes incorporating di-NHC-amine ligands has been prepared and their structural, dynamic and catalytic behaviour investigated. The complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))PdCl(2)] (12) and [trans-(kappa(2)-(Mes)CN(H)C(Mes))PdCl(2)] (13) do not exhibit interaction between the amine nitrogen and palladium atom respectively. NMR spectroscopy between -40 and 25 degrees C shows that the di-NHC-amine ligand is flexible expressing C(s) symmetry and for 13 rotation of the mesityl groups is prevented. In the related C(1) complex [(kappa(3)-(tBu)CN(H)C(tBu))PdCl][Cl] (14) coordination of NHC moieties and amine nitrogen atom is observed between -40 and 25 degrees C. Reaction between 12-14 and two equivalents of AgBF(4) in acetonitrile gives the analogous complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))Pd(MeCN)(2)][BF(4)](2) (15), [trans-(kappa(2)-(Mes)CN(H)C(Mes))Pd(MeCN)(2)][BF(4)](2) (16) and [(kappa(3)-(tBu)CN(H)C(tBu))Pd(MeCN)][BF(4)](2) (17) indicating that ligand structure determines amine coordination. The single crystal X-ray structures of 12, 17 and two ligand imidazolium salt precursors (tBu)C(H)N(Bn)C(H)(tBu)][Cl](2) (2) and [(tBu)C(H)N(H)C(H)(tBu)][BPh(4)](2) (4) have been determined. Complexes 12-14 and 15-17 have been shown to be active precatalysts for Heck and hydroamination reactions respectively.

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

“…Beyond cis ‐chelating configurations, trans ‐chelating Pd–bis(NHC) complexes 29 – 31 (Figure ) were obtained by Douthwaite and co‐workers through transmetalation by using [PdCl 2 (CH 3 CN) 2 ] . Bulky Mes wingtips prevented associative substitution, which led to the low activity of complex 30 .…”

“…The coupling reaction between 4‐bromoacetophenone and tert‐ butyl acrylate can proceed under a low catalyst loading (7×10 −5 mol %) with precatalyst 31 , and a high TOF value of 3.47×10 4 h −1 was calculated for a reaction time of 24 h when the product yield was 58 % . This observation led the authors to assume the generation of colloidal palladium(0) during the catalytic process.…”

Bis(N‐heterocyclic carbene) Dipalladium Complexes: Synthesis, Solid‐State Conformational Studies and Solution Behaviour

“…[8] Furthermore, palladium complexes of the type Pd(pyridine)Cl 2 (NHC) and PdCl 2 (NHC) 2 have been found to exhibit potent anticancer activity. [9] Various bis-NHC ligands have been reported, which are usually linked by alkenic chains [7,[10][11][12][13] or an N-donor such as a pyridyl derivative or amine, [14][15][16] and tend to form chelating complexes with metals. NHC ligands containing a xylylene-bridge are surprisingly rare.…”

Donor‐Functionalised N‐Heterocyclic Carbene Complexes of Group 9 and 10 Metals in Catalysis: Trends and Directions

“…More recently Douthwaite et al have investigated the preparation of amino-bridged bis-NHC ligands, and Pd complexes, synthesised by a Ag-transfer route. [56] The amino functionality was labile, and complexes in which the ligand behaved as a CNC pincer, and others in which the amino functionality was non-coordinating were prepared; examples of both types of complex were tested in Heck coupling, low loadings were used, and performance with aryl bromides was good, however, activity with aryl chloride was low. It appeared likely that colloidal Pd was involved in the catalysis, particularly at low loadings.…”