Probing the Dynamic Covalent Chemistry Behavior of Nitrogen-Centered Di- and Triurazole Radicals

Abstract

Dynamic covalent chemistry (DCvC) describes systems in which readily reversible bond formation allows for control of product distributions by straightforward manipulation of reaction conditions (e.g., changes in temperature, solvent, concentration, etc). Nitrogen-centered 1-aryl urazole radicals reversibly form tetrazane dimers in solution via N–N bond formation. When two such urazole units are attached to a single, appropriately substituted benzene ring, the resulting diradical system engages in DCvC. At room temperature, a polymeric network of units is created that exhibits gel-like properties, while at higher temperatures, near quantitative dimerization to form a molecular cage is observed.However, attaching three such urazole units to a single appropriately substituted benzene ring inhibits DCvC behavior.

Evaluating the Effectiveness of Tethered Bis(urazolyl) Diradicals as Molecular Building Blocks for Dynamic Covalent Chemistry

“…Indeed, TLC analysis in 100% ethyl acetate failed to budge the majority of the product mixture from the baseline, which was consistent with a very polar polymeric product. The initial formation of a plastic-like material had similarly been observed from reaction of diurazolyl radical 1 . However, heating of this plastic in boiling CHCl 3 for 24 h was sufficient to convert the initially formed polymeric product to a single cage-like compound resulting from the dimerization of two of the diradical species .…”

“…We have demonstrated previously that thiophenol is a sufficiently strong hydrogen atom donor to quench urazolyl radicals . Addition of an excess of thiophenol to a solution of the oligomeric product in CDCl 3 resulted in reaction within 2 h to form starting diurazole 6 (isolated in quantitative yield) in addition to the oxidized byproduct, diphenyl disulfide.…”

“…Tethered bis­(ether) compounds 7 with varying methylene chain lengths were treated with the potent electrophile N -methyl-1,2,4-triazoline-3,5-dione (MeTAD) in CH 2 Cl 2 in the presence of TFA to form diurazoles 8 in good yields. The corresponding diradicals were then generated via oxidation of the bis­(urazoles) using the heterogeneous oxidant Ni 2 O 3 as has been described previously …”

“…Imine, boronic acid ester, and disulfide bonds have proven especially robust. − Radical species have also shown promise as molecular building blocks for DCvC applications. , Indeed, we recently reported our findings on the behavior of bis- and tris­(urazolyl) di- and triradicals 1 and 2 , respectively (Figure ). Note that the bis- ortho substitution on the benzene rings to which the urazole rings are attached was determined to be a necessary structural requirement for the urazolyl radicals to prefer to form intermolecular N–N covalent bonds (required for DCvC applications) rather than to predominantly exist in radical form. While diradical 1 exhibited some borderline reversible behavior consistent with DCvC, triradical 2 did not, although in both cases interesting cage compounds were formed as major products.…”

“…6. Linear dependence of ln K for dimer/monomer equilibrium on 1/T as measured by1 H NMR spectroscopy of equilibrated solutions at various temperatures.…”