Solventless Supramolecular Chemistry via Vapor Diffusion of Volatile Small Molecules upon a New Trinuclear Silver(I)-Nitrated Pyrazolate Macrometallocyclic Solid: An Experimental/Theoretical Investigation of the Dipole/Quadrupole Chemisorption Phenomena

A comparative study on the tendency
of a new trinuclear silver­(I)
pyrazolate, namely, [N,N-(3,5-dinitropyrazolate)­Ag]3 (1), and a similar compound known previously,
[N,N-[3,5-bis­(trifluoromethyl)­pyrazolate]­Ag]3 (2), to adsorb small volatile molecules was
performed. It was found that 1 has a remarkable tendency
to form adducts, at room temperature and atmospheric pressure, with
acetone, acetylacetone, ammonia, pyridine, acetonitrile, triethylamine,
dimethyl sulfide, and tetrahydrothiophene, while carbon monoxide,
tetrahydrofuran, alcohols, and diethyl ether were not adsorbed. On
the contrary, 2 did not undergo adsorption of any of
the aforementioned volatile molecules. Adducts of 1 were
characterized by elemental analysis, IR, thermogravimetric analysis
(TGA), Brunauer–Emmett–Teller (BET) surface area, and
diffusion NMR measurements. The crystal structures of 1·2CH3CN and compound 3, derived from
an attempt to crystallize the adduct of 1 with ammonia,
were determined by single-crystal X-ray diffractometric studies. The
former shows a sandwich structure with a 1:2 stoichiometric [Ag3]/[CH3CN] ratio in which one acetonitrile molecule
points above and the other below the centroid of the Ag3N6 metallocycle. Compound 3 formed via rearrangement
of the ammonia adduct to yield an anionic trinuclear silver­(I) derivative
with an additional bridging 3,5-dinitropyrazolate and having [Ag­(NH3)2]+ as the counterion, [Ag­(NH3)2]­[N,N-(3,5-dinitropyrazolate)4Ag3]. Irreversible sorption and/or decomposition
upon vapor exposure are desirable advantages toward toxic gas filtration
applications, including ammonia inhalation. TGA confirms the analytical
data for all of the samples, showing weight loss for each adsorbed
molecule at temperatures significantly higher than the corresponding
boiling temperature, which suggests a chemical-bonding nature for
adsorption as opposed to physisorption. BET surface measurements of
the “naked” compound 1 excluded physical
adsorption in its porous cavities. Density functional theory simulation
results are also consistent with the chemisorption model, explain
the experimental adsorption selectivity for 1, and attribute
the lack of similar adsorption by 2 to significantly
less polarizable electrostatic potential and also to strong argentophilic
bonding whose energy is even higher than the quadrupole–dipole
adduct bond energy upon proper selection of the density functional.