Colorful 4,4-Bipyridine-Squaric Acid Multicomponent Complexes with Varying Degrees of Proton Transfer: Exploring the Nature of New Form IV in the Salt Co-Crystal Continuum

Crystallization of 4,4'-bipyridine (4BPY) with squaric acid (SQU) leads to a range of distinct, highly colored multicomponent complexes with the labile nature of the OH protons of squaric acid and varying degrees of proton transfer to the C=N nitrogen(s) of 4BPY. A new orange product (form IV) is reported, obtained from rapid crystallization that precludes single-crystal structure determination. The distinct color relative to the white starting materials and PXRD and DSC confirm a new multicomponent complex, with unique powder X-ray diffraction patterns and thermal behavior observed for all four 4BPY/SQU complexes. The nitrogen chemical shifts of core-level X-ray photoelectron and absorption spectroscopies clearly identify form I and form III formed concomitantly through slow crystallization as 1:1 salts [4BPYH](+)[SQU](-) with proton transfer from SQU to one of the 4BPY nitrogens, the high-temperature form II as a di-salt [4BPYH2](2+)[SQU](2-) with further proton transfer to the remaining 4BPY nitrogen, and the new form IV as having proton disorder across intermolecular NHO hydrogen bonds in favor of full proton transfer to 4BPY (0.82:0.18 occupancy for N+-H center dot center dot center dot O- and N center dot center dot center dot H-O). This brings the number of characterized 4BPY/SQU complexes to four, ranging in color from yellow to orange, reddish-orange, and pinkish-red with varying degrees of proton transfer.