Synthesis, Theoretical Analysis, and Experimental pKa Determination of a Fluorescent, Nonsymmetric, In-Out Proton Sponge

Herein, we report the synthesis and theoretical investigation of a nonsymmetric bis(diisopropylamino)cyclopropenimine (DAC)-functionalized proton sponge derivative, coined the "Janus" sponge. The reported sponge was isolated as a monoprotonated salt, though no intramolecular hydrogen bond was observed. Homodesmotic equations supported the absence of a N-HN intramolecular hydrogen bond and a relatively low freebase strain, while DFT calculations and X-ray crystallography revealed the presence of a hydrogen bond to the Cl(-) counterion. Associated with this fact was the rare in-out geometry of the basic nitrogens, which represents the first such instance in a proton sponge not having an ortho-substituent and/or being in a protonated state. Furthermore, NLP donation into the cyclopropenium cation was found to stabilize this unprecedented in-out geometry. The measured pKa was determined to be 23.8, in good agreement with the computed value of 23.9. Lastly, the Janus sponge was found to have fluorescent properties both in the solid state and in solution, which notably represents the first example of a cyclopropenimine-based fluorescent organic compound.