Carbonyl pincer complexes

Carbonyl pincer complexes that are obtained not from the direct reaction of pincer species with carbon monoxide but from the reaction of pincer ligands with metal-carbonyl as starting materials and those that are obtained from decarbonylative processes of organic substrates are out of the scope of this chapter and have not been discussed. 

Sacco and coworkers [18] also made an important contribution to the chemistry of rhodium carbonyl pincer complexes. Thus, reactions of the Rh derivative (PNP)Rh(I) (41) were performed with carbon monoxide to yield the corresponding carbonyl complex (42) (Scheme 2.22). 

Moreover, the stability of the QM pincer complexes allows selective modifications of both the metal center and the carbonyl part of the quinone methide moiety, still with... 

The analogous iridium formate complex was synthesized by Kaska and coworkers by reacting CO2 with a Ir(lll) dihydride complex. However, in this case, the formate complex proved to be unstable, undergoing disproportionation to form the hydrogen carbonate complex and the carbonyl dihydride, overall corresponding to the reverse water gas shift reaction CO2 -H H2 - CO-P H20[45j. Reduction of CO2 to the methanol level has since been effected using an aromatic nickel pincer complex and a cascade reaction involving a ruthenium pincer complex in one step [46]. 

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). 

However, new reactions of carbon monoxide with nickel pincer complexes were not reported until 1980. Thus, Sacco and coworkers [5] were able to identify pincer carbonyl species important in water gas shift reaction using (PNP) pincer complexes of type (3) as catalysts (Scheme 2.2). 

A unique case of the chemistry of pincer-type complexes was the synthesis of the first carbonyl hemilabile pincer complex (PCN)Pt(CO) reported by Milstein and coworkers [15]. Hence, the carbonyl derivatives (32) and (33) were synthesized from the direct reaction of CH2CI2 solutions of the corresponding cationic complexes (30) and (31) with a slight excess of CO. Here, the weakly coordinated Bp4 counteranion is easily displaced by the better coordinating ligand CO (Scheme 2.17). 

As a ligand for trapping unstable or transient species, carbon monoxide worked well for Goldman and coworkers [33]. They reacted the pentacoordinated (PCP)Ir(III) pincer complex (75) with CO in benzene at 6 °C affording compound (76) quantitatively (Scheme 2.35). Unlike (75), carbonyl compound (76) does not undergo arene exchange and is stereochemically rigid at ambient temperature. 

Aiming at studying the reactivity of ammonia with Ir(III) PCP pincer complexes, Goldman and Hartwig reported [37] the synthesis of the anilide hydride complex (91) addition of carbon monoxide to this species produces neither carbamoyl derivative nor free formamide, but gives CO adduct (92) that is unstable at room temperature and undergoes reductive elimination to give the known carbonyl (PCP) Ir(I) compound (85) (Scheme 2.42). 

The only study on the direct reaction of carbon monoxide with osmium pincer complexes was reported by Milstein and coworkers in 2001 [52]. In this case, the pincer (PCP)Os(II) complex (131) was reacted with CO in benzene at room temperature to afford quantitatively the carbon monoxide adduct (132). This species further reacted with an additional equivalent of CO to yield the bis-carbonyl (PCP)Os(II) pincer complex (133) with the elimination of PPhs. Additionally, complex (133) can be directly generated by pressurizing the pincer compound (131) with 3 bar of CO or from the reaction of complex (134) with CO under mild conditions. The structure of... 

A hydrido carbonyl tetrahydridoborate iron pincer complex has been synthesized and characterized. This complex acts as a precatalyst for the hydrogenation of ketones under... 

The IR spectrum of the reported metal-coordinated QM compounds depends on the nature of the metal fragment. For example, pincer-type p-QMs exhibit a carbonyl stretch at about 1595 cm-1 for Rh complexes6 and 1629 cm-1 for Os complexes.14 r 2-Coordinated p-QMs of Pd give rise to signals at 158719 and 159818cm while the C=0 stretches of r 4-bent o-QMs coordinated to Ir and Rh appear at 1631-164325 cm-1. 

Haynes and associates carried out a mechanism study for Rh/Xantphos-cata-lyzed methanol carbonylation based on the combination of structural, spectroscopic, kinetic, and theoretical methods. The Rh(III) acetyl complex [Rh(Xantphos)-(COMe)l2], as the catalyst resting state, was isolated and shown to adopt a nearly octahedral geometry with the Xantphos ligand coordinated in a pincer k -P,0,P fashion, which differs from related acetyl complexes with cis-chelating diphosphines that adopt square-pyramidal structures. 

Similarly to cyclopropanes, four-membered carbocyclic compounds undergo oxidative addition to low-valent transition metals to form five-membered metallacycles. Rhodium(I) inserts into C-C bonds next to the carbonyl group of ketones to form a rhodacycloalkanone species [49]. The C-C bond of cyclobutanone was cleaved, even at room temperature, by oxidative addition to a rhodium(I) complex having a PBP pincer ligand [50]. In the case of cyclobutanone 70, catalytic decarbonylation was possible and afforded the alkene 71 and cyclopropane 72 (Scheme 3.40). 

Bis-carbene complexes of Rh 57 were applied to the TH of alkenes and arylalkyl, cyclic and dialkyl ketones at 82 °C in NaOH/z-PrOH (Figure 13.8). Whereas TH of alkenes failed, ketones were converted almost quantitatively to the corresponding alcohols within 2h, with TON>19 000. Neopentyl-sub-stituted NHC complexes 58 were also very efficient for reductions of carbonyl groups in refluxing K2C03/z -Pr0H. For CNC-pincer-containing 59, the rate of TH was found to be faster for aromatic ketones than for aliphatic ketones. " ... 

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