Aromatic PCN Palladium Pincer Complexes. Probing the Hemilability through Reactions with Nucleophiles

Abstract

A series of unsymmetrical PCN pincer ligands (1-(3-((di-tert-butylphosphino)­methyl)­phenyl)-N,N-dialkylmethanamine) were cyclometalated with palladium to generate a series of new PCN supported Pd­(II) chloro complexes, (PCN)­PdCl (4–6), where alkyl = methyl, ethyl, and n-propyl, which were fully characterized by NMR spectroscopy and X-ray crystallography. The N,N-dimethyl complex 4 reacts with methyl lithium to give the corresponding methyl and dimethyl complexes (PCN)­PdMe (12) and Li­[(PCN)­PdMe2] (13), which could not be isolated but were characterized in solution. The substitution reactions of (PCN)­PdCl (4–6) with iodide to form the corresponding iodo complexes (PCN)­PdI (7–9) were investigated by use of UV–vis stopped-flow spectrophotometry. The experiments were performed in methanol over a temperature range from 293 to 325 K. The reactions are reversible and were shown to proceed exclusively via the solvento complex in two reversible consecutive steps. Activation parameters for both the forward and reverse reactions were determined, and they, together with reactivity trends, support an associative pathway. No displacement of the nitrogen donor was detected, and overall this points to a limited hemilability of the ligands on palladium.

Rationalization of the mechanism of in situ Pd(0) formation for cross-coupling reactions from novel unsymmetrical pincer palladacycles using DFT calculations

“…Table 3 also show that there is no significant difference in the catalytic activity of unsymmetrical and symmetrical pincer palladacycles. This may be because the trans-influence [30] and/or hemilability [69,70] of the pincer ligands (and complexes) do not affect the Pd-Cl and/or Pd-C bonds in the key TM and RE steps of the pre-catalyst activation for catalysis.…”

The Trans Influence in Unsymmetrical Pincer Palladacycles: An Experimental and Computational Study

“…This can lead to different physical and chemical properties of the donor ligand arms, resulting in preferential decoordination of one of the ligand arms, providing the opportunity to fine tune the catalytic activity of unsymmetrical pincer palladacycles [18][19][20][21]. An excellent example by Wendt and co-workers reported the hemilabile nature of nitrogen and phosphorus donor atoms by reacting with a strong nucleophile (MeLi) [22]. The results showed that the nitrogen donor atom arm decoordinated from the Pd center, while the phosphorus donor atom arm remained coordinated to the Pd center (Scheme 1).…”

“…Scheme 1. Reaction of unsymmetrical pincer palladacycles with MeLi showing the hemilability of the nitrogen donor atom arm by Wendt et al [22].…”

Enhancing the Stability of Aromatic PCN Pincer Nickel Complexes by Incorporation of Pyridine as the Nitrogen Side Arm

“…Despite the fast expansion of the field of pincer complexes, [1][2][3][4][5][6][7][8][9] those with unsymmetric pincer ligands have received limited attention compared to the symmetric ones in part as a result of the long synthetic routes used to prepare the unsymmetric ligand scaffolds. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] However, unique reactivity has sometimes been documented for these ligands, and for PCN ligands such reactivity includes selective C-C vs. C-H bond activation and hemilability through de-coordination of the weak side arm of the unsymmetric pincer ligand. [10][11][12]18] Also increased reactivity in CO 2 insertion has been reported.…”