58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Alkyne metathesis is an emerging dynamic covalent reaction that provides rigid and robust ethynylene linkages. Although alkyne metathesis has been applied to the synthesis of various shape-persistent 2-D macrocycles, 3-D covalent organic polyhedrons as well as natural products, it has never been explored in the preparation of porous organic materials. Various poly(aryleneethynylene) frameworks have previously been prepared via Sonogashira cross coupling reactions. However, moderate surface areas typically in the range of 500-800 m²/g-1 were observed, with highest 1018 m²g^-1 for frameworks containing 1,3,5-trisubstituted benzene monomers, and 1213 m²g^-1 for frameworks containing tetrahedral monomers. Continuing our efforts to develop new synthetic tools for organic porous materials, particularly through reversible covalent bond formation, herein, we report the first example of utilizing reversible alkyne metathesis in the preparation of porous poly(aryleneethynylene) frameworks, which exhibit specific surface areas exceeding the previously reported similar frameworks.

Framework synthesis:

Our group has recently developed a series of highly active multidentate molybdenum carbyne catalysts for alkyne metathesis. They exhibit high catalytic activity, broad substrate scope, fast reaction rate, and long lifetime. Additionally, these catalysts showed excellent catalytic performance with the aid of 5  molecular sieves as small alkyne by-product scavengers. We envisioned that our catalysts could enable the efficient preparation of poly(aryleneethynylene) frameworks through alkyne metathesis. Catalyzed by a Mo(VI) carbyne catalyst (1), we prepared rigid ethynylene-linked frameworks, PPF-8-RAM, and PPF-9-RAM, through reversible alkyne metathesis (RAM) of 2 or 3. Alternatively, the frameworks with the same chemical connectivity, PPF-8-ICC, and PPF-9-ICC, were also prepared through irreversible cross-coupling (ICC) of 4 and 5 or 6 and 7 (Scheme 1). We found the frameworks prepared through alkyne metathesis at three different temperatures (40 ^oC, 55 ^oC, and 75 ^oC) consistently exhibit higher specific surface area, and higher thermal stability.

Scheme 1. Synthesis of PPF-8 and PPF-9 frameworks through reversible alkyne metathesis and irreversible cross coupling reactions.

Gas adsorption isotherm characterization:

To evaluate the permanent porosity of such as-synthesized porous polymer frameworks (PPFs), nitrogen gas (N₂) adsorption/desorption studies were conducted on the freshly activated samples. As shown in Figure 1, the N₂ adsorption isotherms at 77 K show a rapid uptake at low relative pressure (P/P₀ below 0.1) and typical type I adsorption behavior, indicating the permanent micro-porous nature of the framework. PPF-8-RAM also shows a slight hysteresis loop, indicating the presence of some mesopores. The gradual nitrogen adsorption of the frameworks under relatively high pressure (P/P₀ > 0.1) may be due to the presence of interparticular void and some meso- as well as macro-pores. The Brunauer-Emmett-Teller (BET) specific surface area within the pressure range of P/P₀ = 0.01-0.1 was found to be 1373 m²g^-1, which surpasses those of poly(aryleneethynylene) frameworks previously reported.

Figure 1. N₂adsorption (solid) and desorption (hollow) isotherms of frameworks at 77 K.

Alkyne metathesis vs. cross-coupling:

Most of the dynamic covalent reactions applied in COF synthesis deal with reversible unsymmetrical bond formations and require hetero-complimentary building blocks, e.g. aldehyde and amine, boronic acid and diol. Therefore, the corresponding irreversible chemistry that provides the same type of bond formation is not readily available to compare with the outcome of the reversible chemistry. In that sense, the ethynylene linkage, which can be formed through both reversible and irreversible chemistry, is clearly unique. Accordingly, we explored how the frameworks obtained through alkyne metathesis and irreversible cross coupling reactions are different in terms of gas adsorption properties. We therefore synthesized the frameworks PPF-8-ICC and PPF-9-ICC through irreversible palladium-catalyzed Sonogashira cross-coupling reactions. Although the frameworks (PPF-8-RAM and PPF-9-RAM) constructed through alkyne metathesis are amorphous, at each reaction temperature (40 ¡C, 55 ^oC, and 75 ^oC), they showed significantly higher BET surface areas than PPF-8-ICC and PPF-9-ICC. Specifically, almost two-fold higher BET surface area was observed for PPF-9-RAM at 55 ¡C, compared to PPF-9-ICC obtained through Sonogashira coupling at the same temperature (Figure 2). The cross coupling reactions were also conducted at elevated temperature (90 ^oC). However, only little increase in framework porosity was observed for both PPF-8-ICC and PPF-9-ICC. Clearly, alkyne metathesis is advantageous in the preparation of high porosity materials. However, the nature of how reversibility of alkyne metathesis contributes to the higher porosity still needs further investigation.

Figure 2. Comparison of BET surface areas of frameworks prepared through alkyne metathesis (PPF-8-RAM, and PPF-9-RAM) with the frameworks (PPF-8-ICC, and PPF-9-ICC) constructed through Sonogashira cross-coupling reactions.

Synthesis of phenanthroline-based covalent organic polyhedron (COP):

By utilizing dynamic imine chemistry, which represents another type of dynamic covalent chemical approach, we successfully synthesized two phenanthroline-based shape-persistent COP (10, R = I, or C₁₄H₂₉) from triamine 8 and dialdehyde 9 (Figure 3). Cage 10 (R = I) exhibits a CO₂ adsorption capacity of 14.5 cc/g and a selectivity of 16.5/1 in CO₂/N₂ adsorption. We also prepared metallated COP 11 by reacting 10 with PdCl₂ or AgOTf salt, and they exhibited a selectivity of 1.5-2.1/1 in adsorption of ethylene over ethane. Currently we are further optimizing the metal-coordinated cage preparation by varying the ratio of metal ions to the phenanthroline moieties. Eventually we will integrate COP 11 into membranes to achieve highly energy-efficient light hydrocarbon separation.

Figure 3. Synthesis of phenanthroline-based, metal-coordinated COP 11.

Summary.

We have successfully synthesized poly(aryleneethynylene) frameworks with high BET surface area through dynamic alkyne metathesis. The frameworks with the same chemical connectivity were also prepared through irreversible cross-coupling reactions. Porous polymer frameworks prepared through alkyne metathesis consistently displayed higher specific surface area and higher thermal stability compared to those frameworks prepared through Sonogashira cross-coupling under similar reaction conditions at various temperatures. Although the frameworks prepared through alkyne metathesis are amorphous at this stage, it is conceivable that such dynamic covalent approach could generate ordered materials in the future.