1

Discoveries from a Study across Molecular Syntheses and Framework Materials: a Summary of Research Achievements

Zhengtao Xu (September 2015)

Our most notable contribution resides in the opening of the field of sulfur-enabled

porous frameworks (SPF), wherein the powerful sulfur functions are starting to usher in a broad horizon of research in catalysis, semiconductors, sensors, heavy metal removal, energy storage and proton conductivity. Porous framework materials (e.g., metal-organic frameworks) stand at a highly competitive forefront of research that engages experts from topflight institutions across the globe. Our seminal work emanated from the observation of a glaring gap in the field: among the vast array of open structures reported, the sulfur groups (e.g., the thiol and thioether) had been curiously left out as functionalizing units. Such a gap could be ascribed partly to the inorganic backgrounds of the major players (i.e., making organic molecules was not high on their agenda), and partly to the extra skill and care entailed in handling the reactive thiol groups (e.g., it is prone to oxidation in the synthetic process).

The execution of the research plan had been greatly facilitated by our unique grounding across the traditionally divided fields of discrete molecules (i.e., organic synthesis) and infinite structures (i.e., solid state materials): whereas organic synthesis has remained an unremitting obsession since my high school years--e.g., as a national chemistry Olympian, my expertise in the solid state was nurtured in my PhD and postdoc studies (with Stephen Lee at Cornell and David Mitzi at IBM, respectively). Thus in a paper published back in 2009 (see Figure 4 below; J. Solid State Chem., 2009, 182, 1821), we inaugurated the hard-and-soft carboxyl-thiol combination for framework construction, with a systematic account on the strategic scope and further derivations of this unique Yin-Yang paradigm. Specifically, we identified two key directions: free-standing thiols for functionalization and metal thiolates for electronic properties.

Subsequent developments have been closely tracking the roadmap thus delineated, with additional framework materials serving to expand on the three prototypal systems reported therein. The year 2013 witnessed two major follow-ups. First, we achieved a highly stable SPF material using the harder Zr(IV) ion, paving the way for heavy metal removal and other broad-based applications (see Figure 5 below); later in the year, Cohen (UC San Diego; J. Am. Chem. Soc. 2013, 135, 16997) reported another Zr(IV)-based SPF meant for hydrogen generation applications. Second, Dinca (MIT; J. Am. Chem. Soc. 2013, 135, 8185) utilized the same molecule (i.e., DMBD in Figure 4) and reported another metal-thiolate system, which feature distinct porous properties. More recent follow-ups include: 1) Hong’s report on high protonic conductivity (Korea Univ; Angew. Chem., Int. Ed. 2015, 54, 5142) derived from the ZrDMBD system (Figure 4); 2) Cohen’s report (UCSD; J. Am. Chem. Soc. 2015, 137, 2191)on a SPF for catalysis application; 3) our work on leach-free, environmentally friendly solid state catalysis (Figure 8; Chem. Commun., 2015, 51, 6917).

2

This later breakthrough from our group is especially worthy of note, because it stands as a rigorously verified leach-free Pd catalyst—whereas numerous other claimed heterogeneous Pd catalyst solids actually operate through leached species (e.g., Pd), which are often highly illusive to detect. Besides its practical importance as a highly reusable, leach-free catalyst, the methodological insight is also refreshing: i.e., it shatters the long-held notion of thiols as a catalyst poison; instead, within our SPF host net, spatial constraint prevents the thiol groups from scrambling onto and poisoning the Pd(II) center. The very obstinate Pd-thiolate bond is therefore turned on its head, and rendered a robust anchor that not only prevents Pd leaching, but also enable broader-scope, novel reactivities to be opened up around the anchored metal center.

The growing visibility of our work is also reflected in the attracted citations. For

example, our work on sulfur-enabled MOFs was highlighted by the journal Nature chemistry (2012, 4, 147), and resulted in an invited chapter in Metal-Organic Framework Materials (John Wiley & Sons, Ltd., 2014), in which we systematically described the sweeping methodological breakthroughs enabled by our SFP systems for semiconductive networks—please see section(I) for a personal account of the story. Also, our work on leach-free MOF catalysts (Figure 8) was featured by Chemistry World on 7 April 2015.

Besides porous electronic materials and heterogeneous catalysts, our notable successes

encompass 1) The sensing and separation of precious metals and carcinogenic agents (sections II-IV); 2) The discovery of a novel type of self-similar molecules of the Sierpinski fractal (section III); 3) Single-component white-emitting materials for lighting applications (Figure 10).

(I) The pursuit of porous metal-thiolate nets The journey begins—an early exercise on combining crystallinity and covalent links:

first, moderate Ag(I)-nitrile interactions set up a crystalline (but fragile) net; second, strong O-Si-O links were installed (Figure 1).

Figure 1. A two-step synthesis of a crystalline covalent network (part of my PhD work with Stephen Lee; Adv. Mater. 2001, 13, 637).

The above is a conceptual forerunner to these currently hot topics: covalent organic frameworks (COF), postsynthetic modifications of metal-organic frameworks (MOF), and the hard-and-soft (e.g., the carboxyl-thioether combination, see below) approach of network design.

The two-step idea, together with a postdoc study on hybrid semiconductors (with

David Mitzi at IBM), led me to propose the electronically interesting metal-dithiolene net

3

(Figure 2). This proposal helped me to land a job at George Washington University, and to secure a grant from PRF.

Figure 2. A proposed two-step synthesis of a crystalline electroactive metal-dithiolenenet: from a proposal funded by 2004 ACS Petroleum Research Fund (Type G, PRF # 41159-G10,7).

Back then we perceived the challenge to come from the strong metal-sulfur bond, which was generally deemed incompatible with crystallization. So the idea was to first convert the thiols into the weaker donors of thioesters, and to facilitate the growth of crystalline coordination nets. In the second step, we proposed to recover the metal-thiolate link using an amine molecule.

In hindsight, Figure 2 stands for an overly elaborate and risky scheme: for

example, the thioester is prone to hydrolysis, and the amine in the second step could easily attack the Cu(II) center and disrupt the whole net instead.

What we ended up discovering, is a group of close-packed hybrid semiconductors with

significant electronic interaction between the Bi(III) halide components and the aromatic thioether -systems (Figure 3).

Figure 3. Bridging the VDW gap: Bi(III)-thioether coordination bonds boost electronic communication across organic -systems. It is our long-standing effort to invent coordination networks as semiconductors with functional and processing advantages (Chem. Mater., 2005, 17, 4426; Cryst. Growth Des., 2005, 5, 423).

At City University of Hong Kong, we also dabbled with the number 2 in the form of carboxyl-sulfur combination. The advantages of such a hard-and-soft setup were systematically examined prior to 2009 using the molecule DMBD (Figure 4; J. Solid

4

State Chem., 2009, 182, 1821-1826). For example, with a soft metal ion like Cu(I), one can primarily engage the thiol groups, and build up electronically interesting metal-thiolate frameworks (Figure 4, left side); with a hard metal ion like Eu(III), one can selectively engage the carboxyl groups, leaving the thiol groups as free-standing functions decorating the framework (Figure 4, right side).

Figure 4. Left: The 2D network of CuDMBD as an early open metal-thiolate framework (coordinated ethylene diamine molecules are also shown): normal bonding distances (2.177 to 2.302 Å) are observed for all the six Cu-S contacts around the Cu(I) trimer. Right: a 3D net of EuDMBD as an early example of thiol-laced MOF; the connection of the Eu2(COO)6 units is also shown (further to the right) to highlight the free-standing thiol units (J. Solid State Chem., 2009, 182, 1821-1826).

The discovery of Zr(IV)-based MOFs by Lillerud in 2008 inspired us to make

ZrDMBD (Figure 5). The subsequent uptake of Hg completes a two-step procedure that serves to install the covalent Hg-S links within the crystalline host net. The very distinct hard-and-soft divide as embodied by the robust ZrDMBD thus provides an especially versatile system to achieve the original objective of a metal-thiolate net (as outlined in Figure 2): various carboxyl-thiol molecules can be used to set up the net with Zr(IV) in the first step, and various metal species can be entered in the second step to modify the electronic properties.

Figure 5. The synthetic scheme for the ZrDMBD network (same topology as UiO-66; simplified as a tetrahedral cage unit) and the uptake of Hg species (vapor or Hg2+ in water) to install the covalent metal-thiolate links throughout the network. The electronic properties of framework can thus be conveniently tuned by the very diverse metal guests introduced to link up the thiol donors (J. Am. Chem. Soc. 2013, 135, 7795-7798).

We also sidetracked into the surging field of porous polymers (Figure 6)—only this

time, simply one step is needed, and we can settle with little or no crystallinity, as porosity is often imposed by the rigid building blocks.

5

Figure 6. Synthesis of a porous polymer framework from cyanuric chloride (CC) and a pyrazole linker. One key design here is the well-defined local chelation motif for metal uptake (J. Am. Chem. Soc., 2014, 136, 2818).

The Aha moment: our exercise on porous polymers relaxes our obsession with

reversible interactions and crystals—in a simple and ironic twist of event, we directly reacted PtCl2 with the molecule HTT (Figure 7), and achieved what we originally set out to achieve a whole decade ago (as outlined in Figure 2)!

Figure 7. Bridging porous polymer frameworks (PPF) and metal-organic frameworks (MOF): the direct reaction between the HTT molecule and PtCl2 forms covalent metal-organic framework (CMOF) of porosity, redox activity and ion exchange capability; the structure of the HTT-Pt net was derived from powder X-ray diffraction data (Chem. Commun., 2014, 50, 3986).

(II) More highlights from the carboxyl-sulfur combo

 Figure 8. Highly reusable, leach-free solid state catalysis! By spacing the thiol groups far from each other, we prevent their poisoning effect, and achieve highly recyclable, bona fide heterogeneous catalysis by dangling thiol-palladium functions within a porous metal-organic solid (Chem. Commun., 2015, 51, 6917-6920). This work was featured by Chemistry World on 7 April 2015.

6

Figure 9. Extracting palladium from nuclear waste--from strong acids: the soft allyl and sulfur donors, integrated in a robust Zr(IV) MOF, does the job. The framework is isostructural with UiO-66 and is simplified as an octahedral cage (Journal of Materials Chemistry A, 2015, 3, 3928).

Figure 9. Convenient detection of palladium by a metal-organic framework with sulfur and olefin functions (J. Am. Chem. Soc., 2013, 135, 7807-7810).

Figure 10. Side chains chip in: white light emission and second harmonic generation from secondary group participation (SGP) in a coordination network (J. Am. Chem. Soc., 2012, 134, 1553−1559). This work was highlighted by Nature Chemistry (2012, 4, 147).

Figure 11. An early example of metalating MOF: sulfur picks up HgCl2 from inside a porous coordination network. Besides cleaning up mercury, uploaded metal species (Cu, Pd, Rh…) impart catalysis, H2 storage and other rich chemistries offered by metal ions and atoms—in the pore (Chem. Commun., 2009, 5439-5441).

7

Figure 12. Imbed AgCl first, then Ag2S (by H2S treatment): this two-step approach opens the opportunity to transplant the (almost) all-encompassing semiconductor systems of sulfides (and selenides, tellurides, pnictides) into well-defined organic-based host nets (Inorg. Chem., 2010, 49, 7629–7631).

(III) Backfolded molecules for building networks

Figure 13. Like so many fractals, they look pretty: moreover, backfolded dendrimers open new vistas for network engineering, and create crystalline materials with most sophisticated organic functionalities (Cryst. Growth Des. 2009, 9, 1663; Chem. Commun. 2007, 4779).

We are now exploring the backfolded shape using the more versatile carboxyl donors (Figure 14).

8

Figure 14. Reconcile the opposites for solid design: the rigid arms (shown in green color) regulate the motility of the soft core, making for large framework dynamics. The resultant framework also exhibits distinct amphoteric properties, capable of taking up PdCl2 (e.g., by the S donor) as well as H2S (e.g., by the open-site Pb2+; Angew. Chem., Int. Ed., 2014, 53, 14438).

(VI) More on sensing applications and crystal engineering

Figure 15. Sensing the carcinogen nitrobenzene from a nonporous but dynamic coordination net. Plus, this one absorbs C6H6 but NOT C6F6: two molecules difficult to separate by distillation or crystallization (Chem. Mater., 2009, 21, 541).

Figure 16. Inorganic chains imposes face-to-face stacking--stacking that favors charge transport across organic molecules. Impact: molecular packing, organic electronics (Cryst. Growth Des., 2008; 8, 1468).

9

Figure 17. Pendant groups prevent/control interpenetration--molecular design does the trick. Impact: preserving porosity, accessing noncentrosymmetric crystals/non-linear optical materials (Cryst. Growth Des., 2007; 7, 2542; Inorg. Chem., 2006, 45, 1032).