Initially, we targeted a Pd2L4 MOC based on bis-pyridyl ligands with acetylene spacers due to its extensively studied host-guest binding properties and our previous successful translation of related bis-pyridine MOCs to polyMOC materials. When the bis-meta-pyridine ligand L1 is mixed with 0.5 equiv. [Pd-(MeCN)4][OTf]2 in DMSO (5mM Pd 2+ ) at room temperature, supramolecular complex MOC is formed rapidly and quantitatively (Figure ). Although no guest molecule is explicitly added, the triflate counterion is presumed to reside within the MOC cavity (vide infra). Here, we note that coordinating solvents (e.g., DMSO or acetonitrile) are necessary for MOC formation and subsequent polyMOC fabrication based on this MOC. MOCs of this structure are known to strongly bind guests with electron-rich substituents within their cavity, including (divalent) anions or quinone-type molecules; however, previous studies have largely focused on quantifying association constants within structurally similar Pd2L4 MOCs bearing non-coordinating counterions such as tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (BArF), cages in non-polar solvents (e.g., dichloromethane, nitromethane), or a combination thereof. Therefore, direct translation of previously reported extended-quinone guest molecules proved unsuccessful, as either their binding strength with MOC is too low (in the case of naphthoquinone, Ka ~ 25 M -1 ), or the guests exhibit insufficient solubility in DMSO (as was the case with anthraquinone and pentacenequinone). Thus, we identified three previously unreported guests for MOC: tetra butyl ammonium salts of hydrogen sulfate, HSO4, or nitrate, NO3, and 2,6diaminoanthraquinone, DAQ (Figure ). When 1 equiv of each guest is added to a 5 mM DMSO-d6 solution of MOC, distinct down-field peak shifts are observed via 1 H NMR spectroscopy (Figure ). In line with previous reports, the interior proton resonances of MOC (Ha and Hb based on labeling in Figure ) show the largest shifts upon guest addition, indicative of binding within the MOC cavity. Association constants, Ka, for guest binding were quantified through titration experiments and are listed in Table (see the Supporting Information section "Guest Binding Studies" for further details). The Ka measurements fit well to a 1:1 binding isotherm and span approximately three orders of magnitude, with binding strength increasing in the order: HSO4 < NO3 < DAQ. To our knowledge, the association constant of 62,000 ± 2,000 M -1 for DAQ is the largest reported for any Pd2L4 MOC in DMSO by three orders of magnitude. While we could not quantify the binding strength of the triflate anion within the cavity of this cage due to its inherent presence in the palladium salt used, we performed density functional theory calculations of its binding strength using the B3LYP-D3(BJ) method, which predict that the triflate anion will bind within a guest-free MOC with a favorable free energy difference of 128.4 kJ/mol, suggesting near quantitative guest occupancy (triflate or otherwise) under the conditions explored in this work (see Supporting Information section "DFT Calculations" for full details).