Bond Activation and Catalysis by Ruthenium Pincer Complexes

TL;DR: The heterolytic activation of dihydrogen by NH/H2 MLC was reported by Fryzuk and co-workers in 1987 with pincer Ir and Rh complexes, and complex 73 was found to be an active dehydrogenation catalyst for ammoniaborane and hydrogenation reactions as discussed below.

Abstract: ed by an internal site in intramolecular heterolytic splitting, intermolecular activation requires an external base. Such intramolecular heterolytic splitting is also applicable to C−H or other heteroatom−H bond activation as is frequently observed with ruthenium pincer complexes and discussed in this section. 3.1. Activation of the H−H Bond As shown in Scheme 4b, formation of a dihydride complex such as [(PNP)Ru(H)2(H2)] 20 is a result of dihydrogen activation by oxidative addition due to the electron-rich ruthenium center as discussed above. Gunnnoe and co-workers reported a five-coordinate amido complex (PCP)Ru(CO)(NH2) 71, which heterolytically activates H2, as 71 has the required vacant coordination site for H2 coordination and a basic amido ligand, which abstracts intramolecularly a proton from coordinated H2, followed by dissociation of the formed ammonia (BDERu−NH3 = 12.6 kcal/mol), providing the complex (PCP)RuH(CO) 72 (Scheme 23). The dearomatized pincer complexes 3a−5a developed by Milstein react with molecular hydrogen at room temperature, undergoing rearomatization to provide the trans-dihydride complexes of 3b−5b, respectively, upon heterolysis of dihydrogen (Scheme 24). In the H NMR spectra, the magnetically equivalent trans-dihydrides of 3b and 4b resonate as a triplet at δ = −4.96 ppm (JPH = 20.0 Hz) and δ = −4.90 ppm (JPH = 17.0 Hz), respectively, whereas they display a doublet at δ = −4.06 ppm (JPH = 17.0 Hz) for complex 5b. Further, the structure of the trans-dihydride complex 5b was determined by a single-crystal X-ray analysis (Figure 2). The trans-dihydride complexes 3b−5b slowly lose H2 at room temperature to regenerate complexes 3a−5a, respectively, setting the stage for several catalytic processes (see below) including the production of molecular hydrogen from biorenewable sources. As discussed in the MLC section, the acridine pincer complex 9 also heterolytically activates dihydrogen (Scheme 16). The amido ruthenium pincer complex 73 is in equilibrium with complexes 74 and 75. Formation of amino trans-dihydride complex 74 results from the reversible heterolysis of dihydrogen (Scheme 25). The trans-dihydrides of 74, which resonate at δ = −8.00 and δ = −8.52 ppm, are magnetically inequivalent (JPH = 16.7−24.3 Hz), due to the proximity of one of the hydrides to the backbone amino proton. As observed in heterolysis of hydrogen by Ir pincer complexes, DFT studies suggest that proton transfer from ruthenium to the amido nitrogen is facilitated by the presence of water, which forms a hydrogen bond with both amido nitrogen and H2, leading to a six-membered transition state and lowering the barrier for proton transfer. Complex 73 was found to be an active dehydrogenation catalyst for ammoniaborane and hydrogenation reactions as discussed below. The heterolytic activation of dihydrogen by NH/H2 MLC was reported by Fryzuk and co-workers in 1987 with pincer Ir and Rh complexes. Using the pincer complex [RuCl(PPh3)Scheme 22. Oxidative Addition versus Heterolytic Cleavage of H−H and H−X Bonds Scheme 23. Heterolytic Activation of Hydrogen Scheme 24. Heterolytic Activation of Hydrogen by Aromatization−Dearomatization Process Figure 2. X-ray structure of complex 5b (50% probability level). Hydrogen atoms (except hydrides) are omitted for clarity. Reproduced with permission from ref 100. Copyright 2011 Nature Publishing Group. Chemical Reviews Review dx.doi.org/10.1021/cr5002782 | Chem. Rev. XXXX, XXX, XXX−XXX K (N(SiMe2CH2PPh2)2)], they reported in 1991 that reaction with H2 resulted in a mixture of Ru−H complexes, albeit the reversibility of the reaction was not observed. Recently, Koridze and co-workers reported that a metallocene-derived PCP pincer ruthenium complex also activates molecular hydrogen heterolytically. 3.2. Activation of C−H Bonds While many PCP-type ruthenium pincer complexes were formed as a result of C−H activation or even double C−H activation of the ligand, there are only a few examples of intraor intermolecular C−H activations by ruthenium pincer complexes, as discussed in this section. C−H activation of arenes is widespread with Ir-pincer PNP and PCP complexes. However, such sp C−H activation by Rupincer complexes is rarely encountered. An example of intramolecular C−H activation in a Ru pincer complex was reported by Fryzuk and co-workers. Gunnnoe and co-workers reported that the five-coordinate complexes [(PCP)Ru(CO)(NH2)] 71 and [(PCP)Ru(CO)(CH3)] 76 liberate ammonia and methane, respectively, to generate the cyclometalated PCP ruthenium pincer complex 77 as a result of intramolecular sp C−H activation (Scheme 26). The conversion rate of the methyl complex 76 to 77 is approximately 5 times faster (Kobs = 3.2(1) × 10 −4 s−1 at 50 °C) than the analogous conversion with the amido complex 71 (Kobs = 6.0(3) × 10 −5 s−1 at 50 °C). However, attempted intermolecular C−H activation of methane by complex 71 was not successful and led only to intramolecular C−H activation to provide 77. Gusev and co-workers reported a double C−H activation reaction in a pincer complex. Caulton’s square-planar Ru(II) pincer complex 63a reacted with 1 equiv of MeLi at −78 °C to afford the hydrido−carbene complex 79 as a result of double C−H activation of a single methyl group (Scheme 27). Apart from the characteristic spectroscopic features, DFT calculations indicate that the Ru−C bond length (2.08 Å) in complex 78 decreases to 1.84 Å in 79, in line with formation of a Ru− carbene. The intermediate complex 78 facilitates the C−H agostic interaction and the observed loss of methane. The hydrido−carbene complex 79 reacts with excess of pyridine, immediately forming the diamagnetic η-pyridyl complex 80 as a result of ortho sp C−H activation. Recently, Milstein reported the intramolecular sp C−H activation of expanded PNN ruthenium pincer complexes. Upon deprotonation of the saturated Ru(II) PNN pincer complexes (7 and 8), containing alkyl or cycloalkyl groups on one of the methylene arms, the dearomatized pincer complexes 7a and 8a were formed (Scheme 28). Attempted synthesis of 7a also resulted in intramolecular C−H activation and quantitative formation of the cyclometalated 7b after 4 h. Independent synthesis of 7a was possible using the base KHMDS (potassium hexamethyldisilazide) at −35 °C in toluene-d8, although upon workup only 7b was obtained as a single diastereoisomer, implying that 7a is indeed an intermediate that undergoes cyclometalation and concomitant aromatization to provide 7b. The X-ray structure of 7b exhibits a highly distorted octahedron with the PNC donors coordinated in a pseudomeridional manner (Figure 3). The PNN complex 8a, analogous to 7a, was relatively stable due to steric rigidity, and the conversion to cyclometalated aromatized complex 8b occurred slowly over 5 days. Dearomatized pincer complexes, particularly the bipyridine derived [(PNN)Ru(H)(CO)] 6a and its parent complex 6, are involved in the sp CH activation/exchange of αand βpositions of alcohols with D2O, as will be discussed in section 4.3.1. 3.3. Activation of N−H Bonds As in H2 and C−H activation reactions, MLC of pyridinederived pincer complexes also enables N−H activation of amines and ammonia. As described by Milstein, upon reaction of the dearomatized PNP complex 4a with electron-poor anilines, the unsaturated ligand arm was protonated, and the pyridine ring underwent rearomatization as a result of N−H activation (Scheme 29). The saturated amide complexes 82a Scheme 25. Heterolytic Activation of Hydrogen by Amide−Amine Interconversion Scheme 26. Intramolecular Activation of sp C−H Bonds Scheme 27. Double C−H Activation Leading to the Formation of a Hydrido Carbene Complex Chemical Reviews Review dx.doi.org/10.1021/cr5002782 | Chem. Rev. XXXX, XXX, XXX−XXX L and 82b were obtained in pure form from the reaction of complex 4a with 4-nitroaniline and 2-chloro-4-nitroaniline, respectively, and their symmetrical structures were evident from the P NMR (single signal for each complex) and H NMR spectra. However, the reaction of 2-bromoaniline and 3,4dichloroaniline with complex 4a provided equilibrium mixtures of aromatized N−H activated amide complexes 82c and 82d, and the starting 4a, despite the presence of excess of haloanilines. This observation of reversible N−H bond activation at room temperature was unique, and indicated the low barrier for these reactions. For electron-rich amines and ammonia, the amine-coordinated unsaturated complexes of type 81 are thermodynamically favored. Upon reaction of complex 4a with ND3, formation of the deuterated complex 83 was observed after 5 min at room temperature, showing N−D activation (Scheme 30). The H NMR spectrum of 83 confirmed that deuteration at the methylene arm is stereospecific as only one of the two CH2 arm protons disappeared. No exchange occurred with vinylic protons, and such high selectivity indicated that the activation process on these types of pincer systems occurs only intramolecularly on one face of the ligand with the coordinated ND3 ligand. This observation indicated that other activation processes could also be stereoselective. The trend observed in the experiment was in agreement with DFT studies carried out on the N−H activation reactions (Figure 4). In the reaction of 2-bromoaniline with 4a, the unbound state (I) and the activated state (III) have similar energies with a connecting barrier of 20 kcal/mol. The N−H activated complex of isopropylamine is 16.8 kcal mol−1 above the unbound state, and experimentally it reacted with complex 4a upon warming to 80 °C, whereas in the case of the amines that underwent irreversible N−H activation at room temperature the calculated activated complexes (III) are energetically below the unbound state (I) as expected; barriers for the exchange between amine coordinated (II) and activated (III) states are accessible at room temperature. PNP Rh(I) pincer complexes also activate N−H bonds by similar MLC. N−H bond acti