Nickel pincer complexes

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

When we go from palladium to nickel, we stay within group 10 and retain the square planar geometry common to most d -complexes. We do not need to concern ourselves with the synthesis and structure of nickel(II) pincer carbene complexes. They are analogous to the palladium(II) homologues. However, in their applications, the Kumada-Corriu reaction [291,292] takes a prominent place besides the Heck and Suzuki reactions encountered also with palladium. 

Note Since the pincer geometry calls for meridonal coordination, tetrahedral nickel(II) pincer carbene complexes are not expected to exist. 

Figure 3.157 Use of a nickel (II) pincer carbene complex in Kumada-Corriu, Heck and Suzuki coupling reactions.

One of the earliest applications of pincer complexes in catalysis is the ATRA of polyhaloalkanes (mainly CCI4, but also CBt4 or CF3CCI3) to alkenes, also known as the Kharasch reaction (Eq. (2.8)) [33, 87]. Nickel complexes of type 10 (containing... 

Scheme 5.2 Synthesis and reactivity of nickel PNP-pincer complexes.

Figure 5.1 Diphosphine-derived nickel PCP-pincer complexes.

Figure 5.10 Carbene-based nickel CNC-pincer complexes.

Bis(oxazoline)pyridine (Pybox) and terpyridine (terpy) belong to the family of tridentate ligands that are often excluded from the discussions on pincer chemistry. However, recent studies have shown that in some cases they are redox-active [52, 53] and exhibit properties closely resembling those of pincer ligands. Thus, cross-coupling reactions catalyzed by nickel complexes containing these ligands are also discussed in this chapter. 

As illustrated by other chapters of this book, a diverse array of pincer and pincer-type ligands are known in the literature and can be incorporated to make new nickel complexes. Their catalytic activity in various cross-coupling reactions is likely to be the focus of future research. Another research direction that deserves more attention is carbon-heteroatom bond-forming reactions very few nickel pincer complexes have been studied for these catalytic applications. 

Fu et al. showed that a combination of NiCla and bis(oxazoline) 11 or 17 catalyses asymmetric Kumada coupling of racemic a-bromoketones with aryl Grignard reagents, giving a-arylketones in good yields and enantiomeric excess (Scheme 14.8). The reactions proceeded at low temperature, which enabled the asymmetric synthesis of racemisation-prone a-arylketones. The Pincer-nickel complex [(N2N)Ni-Cl] (18) catalyses Kumada reaction of primary allqrl bromides/iodides or secondary all l iodides with alkyl, (hetero)aryl or all nyl Grignard reagents (Scheme 14.9). The reactions... 

Unexpected Formation of Chiral Pincer CNN Nickel Complexes with P-Diketiminato-Type Ligands via C-H Activation. Synthesis, Properties, Structures, and Computational Studies... 

Nickel complexes with NNN nitrogen pincer ligands showed good efficiency in allyl- and arylthiolation of iodobenzene [52, 53] ... 

Regarding the use of well-defined nickel complexes as catalysts for reduction of carbonyl groups, only three examples are described in the literature. In 2009, Guan and coworkers [77] described the efficiency of a nickel PCP-pincer complex performing the hydrosilylation of aldehydes. In the same year, the catalytic hydrosilylation of ketones via a transient Ni-H complex supported by a monoanionic bidentate amidophosphine ligand was reported by Mindiola [78]. Later, Jones investigated well-defined PNP nickel pincer complexes, which catalyzed the hydrosilylation of aldehydes [79] (Fig. 10.16). 

There is now a growing literature of nickel organometallic complexes that contain carbon dioxide or related cumulene ligands that result from reactions with carbon monoxide. The first structurally characterized complex of carbon dioxide was the nickel complex Ni(G02)(PCy3)2 reported in 1975. A more recent study of this complex provides the complete assignments of the vibrational spectra and theoretical calculations of different isomers in support of a mechanism for CO2 fluxionality that involves end-on coordination. The tridentate pincer ligand 2,6-bis((diiso-propylphosphino)methyl)phenyl (PGP) has been used to form the square-planar Ni(ii) hydroxide complex Ni(OH)(PGP). The complex Ni(OH)(PCP) reacts with CO to give a binuclear /X-GO2 complex (Equation (2)). 

As is the case for pincer nickel complexes, their palladium and platinum counterparts exhibit very similar reactivity. Hence, similar carbamoyl (10) and alkoxycarbonyl (11) products were obtained by Sacco and coworkers in 1985 [7] (Scheme 2.7), when the Pd(II) (8) and Pt(II) (9) (PNP) pincer derivatives reacted under similar conditions as their Ni analogues (see Schemes 2.5 and 2.6). 

Inamoto and Doi have also worked on pincer-type biscarbene complexes of nickel(II). Pre-catalyst 20 was successfully applied to the Suzuki-Miyaura reaction... 

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