PCP‐ and SCS‐Pincer Palladium Complexes Immobilized on Mesoporous Silica: Application in CC Bond Formation Reactions

Abstract

À , E= PPh 2 and SPh} tethered to a trialkoxysilane moiety through a carbamate linkage were immobilized on ordered mesoporous silicas SBA-15 and MCM-41 using a grafting process. The resulting hybrid materials were characterized by IR spectroscopy, solid-state CP/MAS NMR ( Si), and elemental analyses. These analyses showed the integrity of the pincer-metal complexes on the supports, which highlights their stability under the applied immobilization conditions. An H-bonding interaction between the carbamate carbonyl group of the complex and free silanol groups on the silica surface was also established. The hybrid materials were found to act as Lewis acid catalysts in the aldol reaction between methyl isocyanoacetate and benzaldehyde. SBA-15 modified with the PCP-pincer Pd complex was used in up to five runs without loss of activity. Control experiments showed the true heterogeneous nature of the catalyst in this reaction. Nitrogen physisorption data, XRD, and TEM/EDX analyses of the hybrid materials revealed that the mesoporous structure of these materials was retained during the immobilization process as well as during catalysis.

Self-assembly of para-OH functionalized ECE-metalated pincer complexes

“…This process is currently under investigation as it may give rise to the formation of unique organometallic polymers in which the organometal phenoxide is the repetitive unit. (16)(17)(18)(19) The current interest in the catalytic activity of ECE-pincer Pd-complexes in C-C and C-X cross-coupling reactions [5,11,15,20,22,31] combined with the possibility to recycle these complexes via the para-OH functionality make the present complexes highly interesting. Immobilization of these complexes on silica has a twofold advantage; separation of the catalyst from the product can be achieved as well as recycling.…”

“…Ample evidence has been gathered to show that catalytic properties of the pincer complex can be affected via the nature of the Z-substituent [2,11,12]. Furthermore, Z can be used to attach tethering groups which are suited to immobilize pincer-metal complexes to various supports, like polymers [3,[13][14][15], silica [16][17][18], dendrimers [19,20], and bucky-balls [21], or peptides [6,22], and enzymes [23].…”

“…They are even soluble in nonpolar solvents such as benzene or toluene, which allowed grafting of these complexes on silica in these solvents [39]. Details on the grafting procedures and on the use of modified silica's in catalysis were reported by our group [16,17].…”

“…Found: C,41.28;H,5.59; N, 7.95%. 16,144.94,145.94,154.44 (ArC) (16) Triethoxysilylpropyl isocyanate (0.54 g, 2.2 mmol) was added to a solution of 13a (0.7 g, 2 mmol), triethylamine (0.22 g, 2.2 mmol), and 4-(dimethylamino)pyridine (27 mg, 0.1 mmol) in dry dichloromethane (15 mL). The resulting reaction mixture was refluxed for 16 h after which a clear yellowish colored solution had formed.…”

Palladium Pincer Complex‐Catalyzed Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives. Dependence of the Stereoselectivity on the Ligand Effects

“…Formation of similar complexes was postulated in condensation reactions of 3 with aldehydes in pincer complexcatalyzed processes. [49,50] After 1a was completely converted to 6a the solvent and the excess of 3 were evaporated. Complex 6a proved to be surprisingly stable, as the NMR shift values of 6a after evaporation and dissolution in CDCl 3 were identical to the corresponding data observed under the stoichiometric reaction.…”