Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

Abstract

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.

Synthesis, structure and catalytic activity of dicarbene dipalladium complexes with different substituents

“…Over the last years, several chelating palladium complexes with alkane-bridged bidentate carbenes have been reported [4][5][6][7][8][9][10]. However, bimetallic palladium complexes in which the bidentate ligand bridges the two metal centers are not very much [11][12][13]. The design and synthesis of binuclear complexes with di-NHC are of considerable interest because the adjacent metals could function in a synergic manner in their interactions with substrate molecules [14].…”

Homogeneous and Semi‐Heterogeneous Magnetically Retrievable Bis‐N‐Heterocyclic Carbene Rhodium(I) Based Catalysts for Selective Hydroaminomethylation Reactions

“…3,3Ј-(2,4,6-Trimethyl-1,3-phenylene)bis(methylene)bis(1-mesityl-1H-imidazolium) Bromide (6): [26] A mixture of 1-mesityl-H-imidazole (5.88 g, 30 mmol) and 2,4-bis(bromomethyl)-1,3,5-trimethylbenzene (4.6 g, 15 mmol) in toluene (90 mL) was heated at reflux for 10 h. 16.73, 17.77, 20.30, 21.05, 48.84, 123.06, 124.09, 125.44, 129.97, 130.69, 132.15, 135.37,138.00, 139.12, 140.43, 141.24, 141.40 ppm. 3Ј-[1,2-Phenylenebis(methylene)]bis{1-[3-(triethoxysilyl)propyl]-4,5-dihydro-1H-imidazolium} Chloride (7): A mixture of triethoxy-3-(2-imidazolin-1-yl)propylsilane (10 g, 36.6 mmol) and α,αЈ-dichloro-o-xylene (3.2 g, 18.3 mmol) in dry toluene (45 mL) was heated at reflux for 18 h. Progress of the reaction was checked by 1 H NMR spectroscopy.…”

“…3,3Ј-(2,4,6-Trimethyl-1,3-phenylene)bis(methylene)bis(1-methyl-1H-imidazolium) Bromide (5): [26] Prepared by following the procedure outlined for the preparation of 1 by using N-methylimidazole (0.2 mol) and pre-prepared 2,4-bis(bromomethyl) -1,3,5- 25, 20.04, 36.40, 47.84, 122.74, 124.22, 128.96, 131.72, 136.48, 139.56, 140.22 ppm.…”

Crystalline Hybrid Solid Materials of Palladium and Decamethylcucurbit[5]uril as Recoverable Precatalysts for Heck Cross‐Coupling Reactions

“…It has long been known that the judicious selection of organic ligands in palladium complexes is crucial for CÀC coupling reactions because the catalytic activities, selectivity, and stability of metal complex catalysts are dictated by the steric and electronic characters of ligands as well as the properties of the metal-ligand coordination. [4][5][6][7] Recently, Nheterocyclic carbenes (NHCs) have been used as alternative substitution of phosphine ligands to construct palladium complexes that exhibit high catalytic activities and chemical selectivity in the CÀC coupling reaction. Such palladium complexes were air and moisture stable and could be recycled a reasonable number of times.…”

“…[11] Over the past decade, a wide range of metal ions, including alkali, alkaline earth metals, first row transition metals, and some lanthanides, has been found to be successful in the preparation of solid-state materials with CB[n]. [12,13] The outstanding recognition properties Abstract: A series of MÀPdÀ Me 10 CB [5] (M = Li, Na, K, Rb, and Cs; Me 10 CB [5] = decamethylcucurbit [5]uril) hybrid solid materials have been successfully synthesized for the first time through a simple diffusion method. These as-prepared hybrid solids have been applied as phosphine-free precatalysts for Heck cross-coupling reactions with excellent catalytic performance and good recyclability.…”