Research Article Cite This: ACS Catal. 2018, 8, 5515−5525
pubs.acs.org/acscatalysis
Full-Range Interconversion of Nanocrystals and Bulk Metal with a Highly Selective Molecular Catalyst Verena Goldbach, Marina Krumova, and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany S Supporting Information *
ABSTRACT: Catalysis by soluble metal complexes often encompasses the occurrence of nanoparticles and aggregated inactive states, but the role of the different species is unclear. For the generation of highly active catalysts, it is crucial to know the relations between active and inactive states and the reversibility of such interconversions. This is further complicated by the question of the true nature of the active species, in particular for reactions that use dispersed nanoparticles as a catalyst (precursor), which can interconvert to soluble species. We show that a molecular catalyst can interconnect completely up to the step of bulk metal in an isomerizing methoxycarbonylation converting an unsaturated fatty acid to a linear diester with a very characteristic reactivity. The active species, a diphosphine-coordinated Pd hydride, decomposes to the diprotonated diphosphine ligand and a Pd0 species that agglomerates and precipitates as Pd black. This reaction is completely reversible, as shown for several examples of Pd0 precursors. Precise Pd nanocrystals enabled imaging of the dissolution process with the diprotonated diphosphine ligand via transmission electron microscopy. The characteristic selectivity of the isomerizing methoxycarbonylation is a clear indicator of the molecular nature of the active species. This was further demonstrated by NMR spectroscopy via capture of the active Pd hydride formed from Pd nanocrystals. CO, often a problematic reductant, acts as a stabilizer of the molecular oxidized catalyst species. The activation of Pd0 was also realized from macroscopically separated bulk species, like Pd black, sponge, and wire, evidencing that even highly agglomerated states of Pd can be converted to the molecular catalytically active species. KEYWORDS: palladium, nanoparticles, isomerizing methoxycarbonylation, carbonylation, fatty acids, deactivation, reactivation
■
INTRODUCTION Catalysis by soluble transition metal complexes often involves interconversions of the soluble catalyst to nanoparticles and further to highly aggregated and inactive states. Understanding these deactivation pathways of the active catalyst and the ultimate transformation to a completely inactive species is of great interest.1 Research on palladium-catalyzed C−C couplings has shown that molecular catalyst species can interconvert to metal clusters and nanoparticles and vice versa.2−16 However, the true nature of the active species mononuclear metal complexes or surface atoms of small clusters or dispersed nanoparticleshas remained unclear throughout the discussion and seems to be highly dependent on the reaction conditions and parameters.17,18 These C−C couplings are also particular in that the substrates, namely aryl halides, undergo oxidative addition as part of the catalytic cycle. Furthermore, in the systems studied selectivity is not an issue. Investigations of hydroformylation, as one of the most prominent and important homogeneously catalyzed reactions, showed that Rh nanoparticles can be used instead of molecular precursors.19−26 A typical Rh-catalyzed hydroformylation of an olefin, also with nanoparticles as precursors, results in a linear to branched product ratio of approximately 3:1. Examples of a © XXXX American Chemical Society
highly selective catalyst, e.g., reaching a selectivity for the linear product above 90%, are rare, and in these cases it is particularly doubtful whether nanoparticles are the true active species or a precursor for a homogeneous molecular catalyst.24,27 Widegren and Finke28 and Crabtree29 have reviewed various tests and methods for distinguishing between homogeneous and heterogeneous catalyst species. Notwithstanding this, it remains a challenge to make a clear distinction between catalyst species, and it is possible that the true nature of the active species is a “cocktail of catalysts” in many cases.30 The formation, agglomeration, and precipitation of such particles to form macroscopically separated bulk metal, e.g., the formation of “palladium black” from molecular Pd precursors, has been considered as the final inactive state of many catalysts that terminates the reaction.31−33 Formation of zero-valent species and agglomeration is particularly an issue for catalytic reactions that are catalyzed by a transition metal species in a positive oxidation state in a reducing environment, e.g., PdIIcatalyzed carbonylation reactions in the presence of CO. Received: March 12, 2018 Revised: May 4, 2018
5515
DOI: 10.1021/acscatal.8b00981 ACS Catal. 2018, 8, 5515−5525
Research Article
ACS Catalysis
highly selective reaction that takes place in the presence of CO, which here acts as a stabilizing agent rather than a reductant.
Effective stabilization of an intermediate species, such as nanoclusters and particles, can prevent further agglomeration. This effect presumably enhances the activity of a molecular Pd catalyst in a carbonylation reaction.34−36 To date, there are only a few examples where agglomerates, precipitates, or bulk materials have been positively identified as precursors for a homogeneously catalyzed reaction. Crudden and co-workers37 and Del Zotto and co-workers38 successfully showed that Pd bulk metal (foil, sponge, or wire) can be used as a precursor for the active Pd0 species in Suzuki−Miyaura couplings. Using a reactor that allows for a local heating of Pd foil, Crudden and co-workers were able to demonstrate the dissolution of the bulk Pd, the formation of a soluble species, and the redistribution from the reactive zone to the nonheated zone. However, both of these examples are particular in that the oxidative addition of an aryl halide to a metal atom is a key reaction step in the catalytic cycle.39 Further, Del Zotto and coworkers showed that palladium oxide impurities on the surfaces can act as the catalyst precursor in these Suzuki−Miyaura couplings and that no catalytic activity was observed after hydrogenation of the bulk material.38 In terms of generating highly active catalysts that operate as single-site and homogeneously dissolved species, it is important to know at what stage the conversion to larger structures and agglomerates becomes irreversible (Figure 1). The existing
■
RESULTS AND DISCUSSION Isomerizing Methoxycarbonylation as a Single-SiteCatalyzed/Homotopic Reaction. When investigating interconversions between an active catalyst, reservoir species, and inactive agglomerates, it is crucial to work with a reaction that is run exclusively by one defined and well-characterized species. For the following discussion, we will follow Crabtree’s definition for differentiating between homotopic catalysis (single-site catalysts) and heterotopic catalysis (multiple active sites on a catalyst) rather than differentiating between homogeneous and heterogeneous catalysis, which is based on a phase distinction.29 This definition is particularly reasonable when discussing nanoparticles or small nanoclusters that could act as heterotopic catalysts and are on the border between being solubilized or a phase-separated species, situated between the classic definitions of homogeneous and heterogeneous catalysis. Highly selective reactions, such as isomerizing carbonylation reactions, are assumed to run exclusively from a single-site/ homotopic catalyst species. In Pd-catalyzed carbonylations of unsaturated substrates, including unsaturated fatty acid esters as a renewable feedstock (Scheme 1), a Pd hydride species has been recognized as the active species.40−44 The selectivity for the terminal functionalized product starting from methyl oleate is over 90% when a sterically demanding and electron-rich diphosphine ligand such as bis(di-tert-butylphosphino)xylene (dtbpx) is used, even though the position functionalized is eight carbon atoms away from the original double bond. The high selectivity is unique and an indicator of a homotopic catalysis reaction and is in line with the nature of the assumed species. This is further attested by computational studies that have identified a homotopic species and major differences in the energetic barriers of the rate-determining methanolysis step that match well with the experimental results.29,45−48 Here, only the linear Pd acyl species reacts with methanol and forms the linear ester product, while all other branched acyl species do not react in methanolysis.47−49 At the same time, while olefin insertion into a Pd−H bond occurs rapidly and chain walking is facile, no observable olefin insertion into the formed Pd alkyls or Pd acyls occurs. This very specific overall selectivity profile is related to the environment of a PdII center entwined by a very bulky diphosphine ligand (e.g., the remaining opening angle for an approaching substrate is 50f 89 75
c
a Reaction conditions unless specified otherwise: 5.9 mmol of technical grade methyl oleate (Dakolub MB9001, 92.5% methyl oleate), specified amount of (dtbpxH2)(OTf)2 (equiv relative to Pd), 8 mL of methanol, 20 bar CO (initial pressure), 90 °C. bConversion and selectivity for the linear 1,19-diester were determined by gas chromatography of the crude reaction mixtures. cPd black was isolated from a previous isomerizing methoxycarbonylation with [(dtbpx)Pd(OTf)2] as a catalyst precursor. The precipitated Pd black and deposits on the glass inlet were washed with MeOH and CH2Cl2, dried, and weighed prior to their reuse in the methoxycarbonylation reaction (7 equiv of (dtbpxH2)(OTf)2 relative to Pd, 6.43 mmol of technical grade methyl oleate, 8.7 mL of methanol). d“Blank reactions” were run two or three times prior to the shown long-term experiments to ensure that pressure reactors were Pd-free: entry 2, 210 h (1% conversion) and 258 h (
