From Ordinary to Extraordinary: Insights into the Formation Mechanism and pH-Dependent Assembly/Disassembly of Nanojars

Nanojars are large
(2 nm wide) anion-incarcerating coordination complexes of the composition
[anion⊂{Cu­(μ-OH)­(μ-pz)}n] (n = 27–36), formed by the self-assembly
of simple Cu2+, HO–, and pyrazolate (pz– = C3H3N2–) ions in the presence of certain anions with large hydration energy
(e.g., CO32–, SO42–, PO43–, HPO42–). Nanojars display spectacular chemical properties, such as unparalleled
anion binding strength and, as shown herein, extraordinary resistance
to extreme alkalinities (10 M NaOH). To shed light on the mechanism
of the self-assembly process leading to these distinctive constructs,
we employed an array of complementary techniques including mass spectrometry,
pH titration, UV–vis and NMR spectroscopies, chemical synthesis,
and single-crystal X-ray diffraction. In the reaction of Cu­(NO3)2, pyrazole, NaOH, and Na2CO3 in tetrahydrofuran (THF), the first major intermediate is a trinuclear
copper pyrazolate complex, [Cu3(μ3-OH)­(μ-pz)3(NO3)2(H2O)], which was separately
isolated and characterized. As the THF-insoluble NaOH slowly reacts,
the nitrate ions are gradually precipitated out as NaNO3 and replaced by hydroxide ions. The resulting species, [Cu3(μ3-OH)­(μ-pz)3(OH)x(NO3)3–x]− (x = 1–3), have unstable
terminal Cu–OH groups and react with each other to yield OH-bridged
units, such as [Cu3(μ3-OH)­(μ-pz)3(NO3)2]2(μ-OH) and
then [{Cu3(μ3-OH)­(μ-pz)3(μ-OH)2}x(NaNO3)y(Na2CO3)z] oligomers. The Cu3(OH)3(pz)3 repeating units of these oligomers have the same
composition as the [Cu­(OH)­(pz)]n (n = 3x) nanojars and rearrange to the final
products, Na2[CO3⊂{Cu­(μ-OH)­(μ-pz)}n] (n = 27, 29, 31), while
eliminating the last amounts of NaNO3. pH titration, UV–vis
monitoring, and chemical synthesis also confirm the formation of the
trinuclear intermediate, followed by its clean transformation to nanojars.
While displaying an unusual stability to high pH, nanojars are sensitive
to acids stronger than water, a property exploitable for the recovery
of the incarcerated anion. On lowering the pH, nanojars first break
down to trinuclear complexes and finally to copper ions and pyrazole.
This process is fully reversible, and nanojars are reassembled as
pH is increased.