Palladium pincer complexes

As previously observed for the palladium complexes, pincer ligand systems coordinated to Pt(ii) centers have been described in the literature between 1995 and 2005 <2004IC725, 2004OM5432>. 

Reinhoudt and coworkers studied the synthesis of hyper-branched polymers composed of organopalladium complexes with an SCS pincer ligand [11]. Removal of acetonitrile ligands on palladium led to the self-assembly of dinu-clear palladium complex (9) to give hyper-branched polymer (10), which was... 

Palladium complexes of inexpensive, easily synthesized bis(phosphinite) PCP -pincer ligands show good activity in the Suzuki coupling of deactivated and sterically hindered aryl bromides.256... 

Fig. 6 Structure of the fluorous palladium PCP pincer complex lO-Rfs top, ORTEP representation middle, view with atoms at van der Waals radii bottom, packing diagram...

Slagt MQ, van Zwieten DAP, Moerkerk A, Gebbink R, van Koten G. NCN-pincer palladium complexes with multiple anchoring points for functional groups. Coord Chem Rev 2004 248 2275-2282. 

Random polynorbomene-based terpolymer 5 (Fig. 5.8) functionalized with SCS palladated pincer complexes, dibenzo[24]crown-8 (DB24C8) rings, and 2,6-diaminopyridine units was synthesized by ROMP (South et al. 2006). The palladium complex serves as the anchoring unit for metal coordination, 2,6-diaminopyridine as the DAD hydrogen bonding moiety, and DB24C8 as the precursor for pseudorotaxane formation, which has been smdied extensively by Stoddart and his group... 

The third reaction was related to the hydrocarboxylation of allenes with C02, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a a-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of C02 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a C02 fixation reaction, but also as a general method for the synthesis of Py-unsaturated carboxylic acids. 

Subsequent to the first PCP-pincer study, Johansson used NMR spectroscopy to investigate the reactivity of the PCP-Pd complexes in transmetallation reactions with organotin derivatives. Somewhat surprisingly, only those palladium complexes with electron-withdrawing groups and less sterically demanding substituents could react efficiently with tributylallyltin [50]. 

Carbene transfer to palladium yields the palladium complex [Pd(pincer)Me]Cl that undergoes a 1,2-methyl shift to form a pendant imidazolium salt at 150 °C in DMSO. This makes the pincer carbene ligand still relatively thermally stable with regard to reductive elimination. This is probably due to the difficulty encountered by the NHC unit in the pincer carbene ligand to orientate itself perpendicular to the pyridyl plane, the most favourable orientation for the reaction with the cix-methyl group. 

The flexibility of the pincer carbene ligand system can be seen from the crystal structures of their palladium complexes. The ligand without linker units features the central pyridine ring and the flanking NHC units strictly coplanar [431] whereas the linker unit introduces twisting of these three ring systems [490]. The flanking NHC units are now... 

The palladium(II) pincer carbene complex showed good activity in standard Heck cou-phng reactions using activated aryl bromides, but was less effective with chlorides. 

Examples come from Hahn et al. [491] who synthesised the pincer imidazolium salt from a,a -dibromolutidine and A-alkyl-benzimidazole using a standard protocol. Synthesis of the palladium(ll) complex was then achieved with palladium(ll) acetate in DMSO. The structure is similar to that of the palladium(II) pincer carbene complexes with nonannulated pincer ligands. Temperature dependant H-NMR spectroscopy... 

The palladium(II) pincer carbene complexes were employed in the Heck reaction between aryl bromides and iodides and styrene (a-methylstyrene). The robustness of the catalytic system towards air and moisture approaches levels suitable for commercialisation. 

Note Since only the C,N,C pincer ligand forms cationic complexes with palladium(II), the C,C,C palladium(II) pincer carbene complex does not possess a counter anion that could assist in the atropisomerism. 

The method of preparation is seemingly irrelevant as demonstrated by Danopoulos et al. [504]. They used three different routes to synthesise the same C,C,C palladium(II) pincer carbene complexes and showed in the process that all three methods can be applied to the linker unit free ligand as well (see Figure 3.174). 

Figure 3,174 Three routes to the same QQC palladium(ll) pincer carbene complex.

Scheme 6 Pincer palladium complexes by the oxidative addition of (lO-S-3-tetraazapenta-lene) to Pd(PPh3)4...

Palladaofcles, Pincers and Other Palladium Complexes as Precursors 323... 

In conclusion, for C-C bond forming reactions on aryl bromides and iodides there is no need for the use of complexes, palladacycles or pincers as simple ligand-free Pd(OAc)2 will perform as well. For C—C bond forming reactions on aryl chlorides some palladacycles may be the catalyst of choice. For problematic cases, the palladium complexes based on bulky electron-rich phosphines or catalysts based on carbene Hgands are likely to give the best results. 

The Role of Redox Processes in Reactions Catalyzed by Nickel and Palladium Complexes with Anionic Pincer Ligands... 

---