para‐Functionalized NCN‐Pincer Palladium(II) Complexes: Synthesis, Catalysis and DFT Calculations

Abstract

Several para-substituted NCN-pincer palladium(II) complexes (1a−g and 6a−g) {NCN = [C 6 H 3 (CH 2 NMe 2 ) 2 -2,6] − , para = 4-position} have been prepared and the electronic influences of the para substituents were studied in catalysis as well as by DFT calculations (B3LYP/LANL2DZ). From DFT calculations, it was found that the para substituent exerts only a minor effect on the partial charge, investigated by means of the Mulliken population analysis, at the palladium(II) center. Also, when the para-functionalized, cationic NCN−Pd II complexes 6a−g were applied as Lewis acid catalysts in the double Michael reaction between methyl vinyl ketone and ethyl α-cyanoacetate, only small differences in the activities of the various catalysts were observed. These results, when translated to immobilized multipincer catalysts,

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

“…À , E = NMe 2 , PPh 2 and SPh) with a para-functionality (Z) on the aromatic ring of the pincer anion are well known [1][2][3][4][5][6][7][8][9]. Electronic properties of the metal center can be tuned by inductive and mesomeric effects of this substituent [10].…”

“…Electronic properties of the metal center can be tuned by inductive and mesomeric effects of this substituent [10]. 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].…”

Recyclable Nanosize Homogeneous Catalysts

“…This moiety was chosen because of its chemical and heat stability preventing deactivation and decomposition, as well as its high activity as a Lewis acid catalyst in Michael-type reactions [16]. Also, a series of mononuclear parasubstituted [Pd II X(NCN)] complexes was synthesized to study the electronic influence of the para substituent, the anchoring point for immobilization, on the Lewis-acidic palladium centres by means of catalysis and DFT calculations [17]. This was done to estimate beforehand any uncontrolled influences from these para functionalities on the catalytic activity of the pincer moiety when incorporated in a macromolecular complex.…”

“…4, M = Pd, L = OH 2 (BF 4 )) [16,17]. Compared to the catalytic activity of the reference monometallic catalyst 7b, it became clear that the catalytic activities per Pd II centre for the multi(NCNPd) catalysts 3b-5b were similar, indicating that all Pd II centres act as independent catalytic sites [17]. This implies an efficient use of all catalytic sites during catalysis and once again underlines that the Lewis acid catalytic activity is almost independent of the para functionality of the pincer moiety.…”

“…Interestingly, dodecakis(NCN-Pd) 6b showed an almost threefold increase in catalytic activity per Pd II centre which is proposed to be due to cooperative effects between different catalytic sites, since the palladium centres are The following formula was used for determining the retention rates: R = 1 + (V t /V r ) ln(C f /C 0 ), V t = total volume flushed through membrane, V r = reactor volume, C f = concentration in filtrate, C 0 = initial concentration in the reactor. much closer together than in the other shape-persistent compounds [17,20]. Dodecakis(NCN-Pd) complex 6b was applied as a homogeneous catalyst in the double Michael reaction between MVK and ethyl a-cyanoacetate under continuous reaction conditions in a nanofiltration membrane reactor [20].…”