Building Thiol and Metal Thiolate Functions into Crystalline Coordination Networks for Robust, Porous and Electroactive Materials

We propose a versatile synthetic approach to robust crystalline solids combining porous structures and highlyelectroactivemetal-thiolate functions, in order to fundamentally impact the wide-ranging technologies of sensors, heterogeneous catalysis, and photoelectrochemical cells and batteries. The metal-thiolate bond is famous for its strength and versatile electroactive properties. However, the integration of this bond into crystalline porous solids remains a challenge because of the inherent dilemma of bond strength and crystallinity: namely, the very strong metal-thiolate bonds often form too fast and irreversibly, and the resultant framework solids tend to be ill-ordered. To resolve this conflict between bond strength and crystallinity, it is especially helpful to divorce the crystallization process from the formation of the metal-thiolate bonds. For this we call on a stepwise strategy: first, we crystallize a precursor network using the more reversible metal-carboxyl coordination bonds; second, we proceed to install the stronger metal-thiolate links within this pre-assembled scaffold.The efficacy of the synthetic strategy follows naturally from our long-standing effort to synergize molecular design and network engineering. The molecule here features two sets of organic functions: one is the chemically hard carboxyl that strongly favors chemically hard metal ions (e.g., Ln3+or Zr4+); the other set is the soft thiol (-SH) function that prefers the soft (e.g., HgCl2, SnCl2) rather than the hard metal ions. This bifunctional molecule thus enables the following sequence of events. First we can react the hard carboxyls with hard metal ions to build up the precursor coordination net, while the soft sulfur groups do not bond to the hard metal ions?they remain free-standing at this stage and do not interfere with the crystallization of the precursor net; Second, we introduce into the pores of this host net relatively soft metal species (e.g., BiBr3, HgCl2), so as to react with the thiol groups and to set up the covalent (strong) metal-thiolate links throughout the networks. The installation of the metal-thiolate links not only serves to strengthen the precursor coordination scaffold, but more importantly, enhance electroactivity and charge transport throughout the cross-linked network.