Engineered design of double metal cyanides as catalysts for organic reactions

Double metal cyanides (DMCs) are coordination polymers consisting of two different metal centers bonded by a cyanide group. Within this class of materials, we find Prussian blue and its analogues, known simply as Prussian blue analogues or PBAs. This subdivision of DMCs is the most common with a structural formula M’u[M’’(CN)6]v; they typically crystallize in the cubic system with a rock salt array of the M’N and M’’C octahedra. Prussian blue is also the oldest known inorganic coordination polymer. It was discovered in the 18th century and since then, it has found applications as pigment, molecular magnet, gas adsorbent, cathode for batteries, ion exchanger, etc. In the 1960s, DMCs were first studied as catalysts for the ring-opening polymerization of epoxides. This represented the start of a new field of study of DMCs. In this manuscript, we will not only focus on their performance as catalysts for two known applications: ring-opening polymerization and intermolecular hydroamination, but also in a new potential application, namely, the synthesis of propargylamines via A3 coupling, an important organic transformation. DMCs are microporous materials (pore size ~ 5 Å), which results beneficial in the majority of the aforementioned applications. In spite of this, in catalysis, it has been disclosed that for some organic reactions the pores are too narrow to allow the diffusion of reactants to the catalytic sites. Therefore, the reaction is limited to the external surface, which decreases the overall activity of the material. In view of this, the main objective of this research is to design catalytic systems that effectively deal with this issue, thus enhancing the catalytic performance of DMCs.  

First, in Chapter 1, the pertinent literature regarding DMCs is summarized. Starting from their discovery (Prussian blue) and their first use as pigment, all the way to their more recent applications. Special attention is given to their potential utilization as catalyst. Then, as a first strategy to increase their catalytic activity, in Chapter 2, the dispersion of Zn3[Co(CN)6]2-type DMCs on silica as a way of increasing the accessibility and availability of the active sites is investigated. Furthermore, the effect of the presence of water during the grinding (liquid-assisted grinding, LAG) on the physicochemical properties and on the activity of the catalysts is studied. Two reactions were investigated: the hydroamination of phenylacetylene with 4-isopropylaniline and the ring-opening polymerization of 1,2-epoxyhexane. Highly active, stable and reusable supported DMC catalysts can be prepared by physical mixing or ball milling. The characterization of the supported DMCs indicated that the presence of water during the grinding step (LAG) preserves the structural and textural properties of the DMC and enables the dispersion of the active phase onto the support, resulting in more active samples than those prepared under dry grinding conditions.

Next, in Chapter 3, the reaction scope of DMCs is expanded by studying a series of M’‑Co DMCs catalysts (M’[Co(CN)6]2/3 PBAs) for the synthesis of propargylamines via the A3 coupling of phenylacetylene, benzaldehyde and piperidine. A series of catalysts was synthesized based on earth-abundant transition metals. For the bimetallic samples, the highest reaction rate was obtained with the Cu3[Co(CN)6]2 PBA (Cu-Co PBA), while the Zn3[Co(CN)6]2 PBA (Zn-Co PBA) was the most selective towards propargylamines. In light of this, we synthesize a new series of multi-metal PBAs (Cu and Zn as M’). A synergistic effect was revealed when these two metals were combined in the same crystalline framework, with the Cu0.86Zn0.14‑Co PBA proving to be an active, selective and recyclable heterogeneous catalyst for the A3 coupling reaction. However, in our PBA-catalyzed system, where the two metals cooperate in the reaction cycle, there is no need to introduce the Cu in the bulk of the crystals. Therefore, in Chapter 4, the occurrence of post-synthetic metal ion exchange (PSE) in Zn-Co and Cu-Co PBAs is investigated for the first time. PSE represents an interesting preparation method of catalytically active multi-metal PBAs, considering the fact that the metal atoms at the outer surface are, on average, coordinated to fewer N atoms and more likely to be released from the structure first. This allows a more selective incorporation of the desired second metal (Cu in this case) on the external surface. The benefits of PSE as a preparation method of PBAs were clearly demonstrated in the A3 coupling reaction of benzaldehyde, piperidine and phenylacetylene. The catalytic activity of the exchanged PBAs was higher than that of the Zn‑Co and Cu‑Co PBAs (bimetallic samples) and that of multi-metal PBAs of similar composition prepared by the standard co‑precipitation procedure. 

Finally, looking for new structures and motifs that would display a high external surface area and thus have more accessible active sites, in Chapter 5, a new two-dimensional DMC phase consisting of positively charged Zn-Co DMC layers connected through acetate ions is synthesized. This phase, referred to as L-DMC, was compared to the benchmark DMC catalyst in terms of their activity in the hydroamination of phenylacetylene with 4-isopropylaniline and the ring-opening polymerization of 1,2‑epoxyhexane. In the two reactions probed, L‑DMC displayed an activity superior to that of the benchmark DMC. These results are particularly interesting in the case of the hydroamination reaction, where there is still a need for a catalyst exhibiting high stability and activity. Furthermore, L-DMC could also allow a further enhancement of its catalytic performance by means of post-synthetic modifications, like delamination.

In conclusion, in this research we developed and described three effective strategies to deal with the microporosity issue of DMCs and to increase the accessibility of their active sites with the purpose of enhancing their catalytic performance. These strategies were investigated not only in two reactions where DMCs have already been used as catalyst, but also in a potentially new application, the synthesis of propargylamines. We expect that the results presented in this manuscript will pave the way to new studies regarding the design of these interesting materials.