Pincer rhodium

The pincer rhodium(III) complex was successfully employed in the transfer hydrogenation of cyclohexanone, acetophenone and benzophenone with isopropyl alcohol as... 

The presence of a C-C-H T -agostic interaction was proposed as the intermediate for oxidative addition of an SCS-pincer rhodium complex on the basis of the DFT calculation (Scheme 1.7) [14]. Both the C-C and C-H c-bonds donate bonding electrons to the d orbital of rhodium (Figure 1.4). The olefin ligand located at the fraws-position accepts electrons, which reinforces electron donation from C-C and C-H o-bonds. The agostic complex is the intermediate leading to both C-C and C-H activations. 

Thus, as is the case for the st pincer derivatives with nickel, Moulton and Shaw also reported the first PCP pincer rhodium and iridium derivatives and studied their reactivity toward carbon monoxide [4]. Hence, the (PCP) rhodium derivative (38) affords complex (40), as a product of the decomposition of the unstable hydrido chloro carbonyl intermediate (39) via a hydrodechlorination process, although this compound is obtained impure. To favor the hydrodechlorination process, an alternative route was attempted employing EtONa as base. This approach afforded exclusively compound (40) (Scheme 2.21). It is noteworthy that analogous reactions with the iridium analogous to complex (38) only afford impure samples of the carbonyl species analogous to complex (40). 

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. 

Following this observation, a general approach for the synthesis of pincer-type methylene arenium compounds was developed (Scheme 3.4). Upon reaction of the methyl rhodium (or iridium) complexes 5 with a slight excess of triflic acid, dihydrogen (not methane ) was evolved to form the methylene arenium complexes 4a.11 Thus, the methylene arenium form is clearly preferred over the benzylic M(III) form, in which the positive charge is localized at the metal center. 

Iridium pincer complexes-and to a larger extent also the corresponding rhodium complexes-have provided useful insights into the mechanism of Caryi—Ca yi bond... 

Figure 3.153 Coordination geometries in rhodium pincer carbene complexes.

Figure 3.154 The pincer carbene ligand acting as a bridging ligand to two rhodium (I) moieties.

Simons et al. [487] used these pincer carbene ligands in the synthesis of silver(I), palladium(II) and rhodium(l) pincer carbene complexes (see Figure 3.167). Similar to the Anker-free pincer carbene ligands, [Rh(cod)Cl]2 proved to be an unsuitable starting material to synthesise a rhodium(l) pincer carbene complex [462]. The ligand acted as bridging ligand to two [Rh(cod)Cl] units instead. 

Note The 1,5-cod ligand did not prevent the formation of the chelate complex as was observed for the corresponding C,N,C rhodium(I) pincer carbene complex. 

Figure 3.175 Synthesis of the bridging and chelating rhodium(l) C,C,C pincer carbene complexes.

Hollis and coworkers reported also on a transmetalla-tion process from zirconium to rhodium. As illustrated in Scheme 33, imidazolium salt (209) reacted instantaneously with Zr(NMe2)4 to generate the biscarbene pincer (210) with orthometallation of the phenyl group. The biscarbene pincer was not isolated, but reacted with [Rh(COD)Cl]2. The Rh(III) complex (211) was isolated in equilibrium with the dimer (212). 

Some other intermolecular C-H activations involving the NHC ligand have been observed during the synthesis of particular NHC-containing pincer -type complexes also called CCC-NHC complexes. In addition to zirconium- and rhodium-based complexes (210) and (211)/ several examples involving palladium of general structure (271) have been synthesized. Whereas Faller... 

A phosphine-amine pincer ligand reacts with a rhodium olefin complex more easily than diphosphine pincer ligands to give a C-C bond activated complex in minutes at room temperature. In this case, a C-H activated complex was not observed upon monitoring the reaction even at -50°C [55]. 

Alkane dehydrogenation took a step forward in 1996 with the report of rhodium and iridium pincer complexes that could catalyze transfer hydrogenation. While the rhodium complex was found to be active but unstable, the iridium complex was stable even after a week at 200 °C. This permitted it to efficiendy catalyze the transfer hydrogenation of cyclooctane to cyclooctene (12 t.o./min, Scheme The reaction is inhibited by high concentrations of olefin, either the... 

As mentioned above in Section 1.25.5.2, rhodium and iridium pincer complexes have been used to catalytically dehydrogenate alkanes, giving terminal olefins as the kinetic products. In a recent report by Goldman and Brookhart, the iridium Pincer complexes were combined with Schrock s alkylidene metathesis catalyst... 

Scheme 8.20 Difference in interaction of carbon dioxide and dihydrogen with rhodium and iridium pincer complexes.

The presence of a MCH3 (M = Ir or Rh) moiety in new iridium and rhodium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)-3,5-diphenylpyrazine (PONOP) (PONOP)RhMc3 and (PONOP)IrMc3, has been confirmed by Brookhart and co-workers by the observation of /hp coupling of 4.9 Hz for the rhodium complex also Vhrii of 3.1 and VpRji of 171 Hz have been reported. 

Tridentate bis(oxazolinyl)pyridinyl rhodium and ruthenium pincer complexes are useful as catalysts for hydrosilylations and cyclopropanations. These NNN-type inorganic pincer complexes are not as stable, however, as phosphine or salen-type pincer complexes. On the other hand, an organometallic tridentate bis(oxazolinyl) phenyl NCN-type complex is stable. These optically active NCN-type pincer complexes act as efficient catalysts for enantioselective hetero Diels-Alder reactions of Danishefsky s diene with glyoxylates [26]. 

Recently, pincer metal compounds, such as those with rhodium [36], palladium [37,38], and platinum [39], are used as catalysts for Michael addition reactions. For example, the addition of an a-cyanopropionate to acrolein under mild, neutral conditions in the presence of a bis(oxazolinyl)phenylstannane-derived rhodium complex 8.44 proceeds enantioselectively with a high yield and high TON, as shown in Eq. (8.9) [36]. 

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