A PCP Pincer Ligand for Coordination Polymers with Versatile Chemical Reactivity: Selective Activation of CO₂ Gas over CO Gas in the Solid State

Abstract

A tetra(carboxylated) PCP pincer ligand has been synthesized as a building block for porous coordination polymers (PCPs). The air- and moisture-stable PCP metalloligands are rigid tetratopic linkers that are geometrically akin to ligands used in the synthesis of robust metal-organic frameworks (MOFs). Here, the design principle is demonstrated by cyclometalation with Pd(II) Cl and subsequent use of the metalloligand to prepare a crystalline 3D MOF by direct reaction with Co(II) ions and structural resolution by single crystal X-ray diffraction. The Pd-Cl groups inside the pores are accessible to post-synthetic modifications that facilitate chemical reactions previously unobserved in MOFs: a Pd-CH3 activated material undergoes rapid insertion of CO2 gas to give Pd-OC(O)CH3 at 1 atm and 298 K. However, since the material is highly selective for the adsorption of CO2 over CO, a Pd-N3 modified version resists CO insertion under the same conditions.

Isolating reactive metal-based species in Metal–Organic Frameworks – viable strategies and opportunities

“…A related approach to include reactive organometallic complexes was pursued by Humphrey et al 112,113 Seeking to achieve air and moisture stability, the researchers used a tetra(carboxylated) P-C-P pincer ligand to target highly connected, robust MOFs. Cyclometalation of the P-C-P pincer with Pd 2+ gave a metalloligand which could be used to prepare a crystalline 3D MOF (PCM-36, Phosphorus Coordination Material) with Co 2+ ions.…”

Stabilization of reactive Co 4 O 4 cubane oxygen-evolution catalysts within porous frameworks

“…Here, we report the greatly improved stability of a [Co 4 O 4 ] molecular cluster by covalent immobilization in a porous metal-organic framework. This strategy has allowed (i) significantly improved stability for a molecular OER catalyst (under practical, high pH conditions), and (ii) observation of a reactive, proposed (and otherwise unstable) intermediate in the OER mechanism (23)(24)(25)(26)(27)(28)(29)(30)(31). The organic linkers of these frameworks were optimized both for protection of the [Co 4 O 4 ] units from aggregation, and to provide electronic and structural tuning of reactivity (32)(33)(34).…”

A Metal–Organic Framework with Cooperative Phosphines That Permit Post‐Synthetic Installation of Open Metal Sites

“…Instead, we explored the use of pre‐formed phosphine coordination complexes decorated with ancillary carboxylic acids (namely, approach 3). We showed that Pd II or Pt II complexes of carboxylated bis(phosphines) and PCP‐pincers, could be used to prepare porous PCMs with unique solid‐state reactivity. However, the multi‐step routes to prepare the precursors are complicated and large quantities are usually required in the search for optimal MOF‐forming conditions.…”