Ruthenium complexes pincer complex

Rozenel SS, Arnold J. Bimetallic ruthenium PNP pincer complex as a platform to model proposed intermediates in dinitrogen reduction to ammonia. Inorg Chem. 2012 51 9730-9739. 

Ruthenium hydride pincer complex [Ru(PNP)(H)2(H2)j [PNP=l,3-bis(di-terf-butyl-phosphinomethyl)pyridine] and its borane analog [Ru(PNP)(H)2(HBpin)] (HBpin=pinacolborane) catalyze the hydroboration of terminal alkynes to give selectively Z-vinylboronates in high yields (Scheme 32) [146]. Mechanistic studies... 

Scheme 32 Hydroboration of terminal alkynes to Z-vinylboronates using a ruthenium hydride pincer complex as catalyst...

Scheme 6 Synthesis of new PNP ligand and ruthenium(II) pincer complex based on acridine...

Symmetrical pyrazines may be accessed by the dimerisation of vicinal amino alcohols (Scheme 12.35). Milstein discovered that a ruthenium PNP pincer complex catalyses the acceptorless dehydrogenation of amino alcohols in toluene at reflux, likely inducing double condensation to a dihy-dropyrazine which may undergo an additional dehydrogenation to provide the observed products. A similar Ru-PNN pincer complex, possessing a hemi-labile amine arm, was found to catalyse the conversion of amino alcohols to diketopiperazines (see Section 12.2.2)... 

Figure 3.155 Mono-pincer and di-pincer complexes of ruthenium(ll).

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). 

Again, utilisation of the N,N,C pincer framework results in the corresponding ruthenium complex. The C,N,C ruthenium(ll) pincer carbene complex showed some activity in the olefin metathesis of norbomene, norbomadiene (catalyst loading 1 mol %), 1,5-cod, cyclooctene and diallylmalonate (catalyst loading 10 mol %). The N,C,N ruthenium(II) pincer carbene complex was not tested. 

Figure 3.157 Synthesis of a ruthenium(ll) pincer carbene complex with the Crubbs catalyst as starting point.

For example, the pyridine-based pincer complexes 1, 4, 7, and 10 undergo smooth deprotonation to provide complexes 2, 5, 8, and 11 (Schemes 1.2 and 1.3). NMR studies of 2, 5, 8, and 11 indicate dearomatization, as the pyridine protons are shifted to lower frequency (olefinic region). Moreover, the structure of complex 2 is unequivocally corroborated by single-crystal X-ray diffraction studies [23]. Importantly, the dearomatized complexes of 2, 5, and 8 activate dihydrogen by cooperation between the rathenium center and the deprotonated phosphine arm, resulting in aromatization to quantitatively yield the ruthenium traws-dihydride complexes of 3, 6, and 9, respectively (Schemes 1.2). The magnetically equivalent... 

Amide formation plays a very important role in chemical synthesis [1, 46]. Preparation of amides under neutral conditions and without generation of waste is a challenging goal [46-48]. Applying our pyridine-based ruthenium pincer complexes as catalysts, amides were synthesized directly from amines and alcohols or esters polyamides were obtained from diols and diamines, as delineated in this section. 

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]. 

In 2007, synthesis and complexation of a PSiP-pincer hgand, in which a sihcon atom and two phosphorus atoms are tethered by a phenylene group, was first reported by Turculet and coworkers [11-19]. They reported that the PSiP-ruthenium complex exhibited catalytic activity for transfer hydrogenation of ketones [11], and the PSiP-platinum and -palladium complexes efficiently catalyzed reduction of COj to methane by silanes [18[. Shortly after the Turculefs first report in 2007, we reported the first example of utilization of the phenylene-bridged PSiP-pincer complex in carbon-carbon bond formation reactions of unsaturated hydrocarbons [20[. 

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]. 

Ruthenium complexes also catalyze the anti-Markovnikov hydroamination of vinylarenes. In this case, the combination of l,5-bis(diphenylphosphino)pentane (DPPPent), triflic acid, and a ruthenium(II) precursor generates a catalyst for the additions of secondary amines to vinylarenes (Equation 16.72). This mixture of catalyst components has been shown to generate a cationic Ti -arene complex of a PCP pincer ligand generated from the DPPPent ligand. The mechanism of this reaction involves nucleophilic attack of the amine on an Ti -vinylarene complex, as described in more detail in the section on the mechanisms of hydroamination. 

Figure 6 Various supramolecular cross-linking strategies based on (a) the three-point CA-triazine couple and the six-point CA-isophthalamide (Hamilton wedge) couple (b) ruthenium-terpyridine coordination complexes (c) and (d) the platinum- or palladium-based pincer complexes. ...

Pyridines and quinolines are prepared via dehydrogenative coupling of y-aminoalcohols and secondary alcohols in the presence of a bipyridyl-based ruthenium pincer complex (Scheme 6) (13CC6632). A number of cyclic and acyclic secondary alcohols were tolerated under these conditions. 

Ye X, Plessow PN, Hofmann P (2014) Alcohol amination with ammonia catalyzed by an acridine-based ruthenium pincer complex a mechanistic study. J Am Chem Soc 136(16) 5923-5929... 

Agrawal S, Lenormand M, Martm-Matute B (2012) Selective alkylation of (hetero)aromatie amines with alcohols catalyzed by a ruthenium pincer complex. Org Lett 14(6) 1456 1459... 

Amines react with primary alcohols to give imines under the influence of a pincer complex, ruthenium(II)-PNP [PNP=2,6-bis(di-f-butylphosphanylmethyl)pyridine]. DFT has been used to identify the mechanistic steps, and in particular the factors that favour imine as product, as closely related complexes yield amides. 

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