Pincer-pyridine complexes

Tripincer template-directed cyclization of a dienic pyridine precursor leads to the formation of the 69-membered ring trimeric heterocycle 112 (Figure 2.6). Decomplexation of the tripincer from the product is realized with NaCl [72]. The synthesis of a [2]rotaxane by M-RCM using a similar pincer-pyridine complex for pre-assembly was also reported by Leigh et al. [73]. 

Pyridine complexes of Pd- and Pt-pincer ligands are also suitable substrates for olefin metathesis [116]. The first-generation catalyst 9 efficiently mediates the RCM of diallylphosphines and diallyl sulfide when the heteroatom is com-plexed by a cationic [C5H5(NO)(PPh3)Re] moiety [117]. This principle has been exploited in the same study for tungsten, rhodium, and platinum complexes. 

Danopoulos and coworkers [473] then developed a ruthenium(II) pincer carbene complex exactly on the lines described above starting from the Grubbs catalyst. Reaction with the free carbene ligand results in displacement of the phosphane ligands and coordination of the central pyridine unit changes the coordination geometry from square pyramidal to octahedral (see Figure 3.157). 

Figure 4.11 Metallo-supramolecular polymers based on (a) SCS-Pd" prncer-pyridine or SCS-Pd" pincer-nitrile complexes and (b) Ru(bpy)3 + side chains.

Pd2(dba)3 92 and 93 are generated by the reaction of Pd(OAc)2 with the ligand precursor. Aspects of their structure, fluxional behavior, and catalytic reactivity have been reported. The conformer interconversion for the cationic carbene-pyridine-carbene C-N-C) pincer 93 and the neutral C-C-C dicarbene Pd pincer 94 complexes were investigated, and it was found that, for cationic complexes, the choice of anion can affect the mechanism for... 

Milstein D (2010) Discovery of environmentally benign catalytic reactions of alcohols catalyzed by pyridine-based pincer Ru complexes, based on metal-ligand cooperation. Top Catal 53 915... 

Pyridine-bridged bisimidazolium salts also afforded the preparation of dimetallic Rh(l) species by transmetallation from a silver(l)-NHC complex and deprotonation with a weak base. A pincer pyridine-bis-NHC Rh(lll) complex 330 was obtained from the reaction of the corresponding bisimidazolium salt and [Rh(/r-Cl)(cod)]2 in the presence of... 

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

The de- and re-aromatization of the pyridine moiety of the pincer ligand appears to be crucial for this process. This is also the key underlying feature in the oxidative addition of H2 by complex 19 in an apparent iridium(111) oxidation state, which results in the formation of the dihydride complex 20 (Scheme 12.9). Similarly, the addition of CO to the iridium(I) complex 18 formally results in oxidation of the metal center and provides the iridium(III) complex 21 (Equation 12.9). 

Pincer (tridentate-mer) coordination from a pyridine-bisimidazolium (28) salt can be achieved using NEt3 and the addition of [RhCl(cod)]2 [92] or [RuCl2(cod)]2 [93] as metal complexes, affording compounds 29 and 30, respectively (Scheme 23). 

Poyatos et al. did precisely that, they synthesised a mono-pincer and a di-pincer ruthenium(II) complex starting from [Ru(cod)Clj]j as the precursor complex (see Figure 3.155) [468]. Not surprisingly, the CO ligand coordinates trans to the pyridine ring of the pincer ligand. 

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... 

Reaction of UX4 (X=C1, BH4) and two equivalents of [Li(Et20)] [SPS ], the lithium salt of an anionic SPS pincer ligand composed of a central hyper-valent Z4-phosphinine ring bearing two orf o-positioned diphenylphosphine sulfide side arms, formed complexes of the type [UX2(SPS )2] (Fig. 36). Crystals of the chloride derivative were obtained as the pyridine adduct, and... 

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