Abstract
Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric 1, dimethylphosphoric 2, diphenylphosphinic 3, and dimethylphosphinic 4) are studied by the low-temperature (100 K) 1H-NMR and 31P-NMR using liquefied gases CDF3/CDF2Cl as a solvent. Formation of cyclic dimers and cyclic trimers consisting of molecules of two different acids is confirmed by the analysis of vicinal H/D isotope effects (changes in the bridging proton chemical shift, δH, after the deuteration of a neighboring H-bond). Acids 1 and 4 (or 1 and 3) form heterotrimers with very strong (short) H-bonds (δH ca. 17 ppm). While in the case of all heterotrimers the H-bonds are cyclically arranged head-to-tail, ···O=P–O–H···O=P–O–H···, and thus their cooperative coupling is expected, the signs of vicinal H/D isotope effects indicate an effective anticooperativity, presumably due to steric factors: when one of the H-bonds is elongated upon deuteration, the structure of the heterotrimer adjusts by shortening the neighboring hydrogen bonds. We also demonstrate the formation of cyclic tetramers: in the case of acids 1 and 4 the structure has alternating molecules of 1 and 4 in the cycle, while in case of acids 1 and 3 the cycle has two molecules of 1 followed by two molecules of 3.
Highlights
The compounds that have both proton-donating and proton-accepting groups are prone to self-association via hydrogen bonds, depending on the spatial orientation of interacting groups and various steric factors
A somewhat different situation is observed for POOH-containing acids, namely, phosphinic (R2 POOH) and phosphoric acids ((RO)2 POOH), which crystallize as cyclic dimers [7] or infinite chains [8] and form cyclic dimers in the gas phase [9,10,11,12,13], though in some polar aprotic solutions the dominant self-associates are cyclic trimers [14,15]
In this work we considered POOH-containing acids 1–4, in particular, the mixed hydrogen bonded complexes formed by molecules of two different acids in the polar aprotic solution
Summary
The compounds that have both proton-donating and proton-accepting groups are prone to self-association via hydrogen bonds, depending on the spatial orientation of interacting groups and various steric factors. We note that the cyclic trimerization of small molecules via hydrogen bonds is rather a rare phenomenon in nature; only pyrazoles [19], 1-amino-3-iminopropenes [20], and possible imidazoles [21] can be mentioned as examples (again, in these cases the structures are governed by the relative orientations of proton-donating NH groups and lone pairs on proton-accepting N atoms) Another distinctive feature of the phosphinic and phosphoric acids self-associates is the unusually high stability: in the gas phase the energies up to 60 kcal/mol per dimethylphosphinic acid self-associate (presumably, cyclic dimer) have been reported previously [9,22]. The high stability might be linked to the following: (i) phosphinic acids are reasonably strong proton donors (the pKa value is 3.08 for Me2 POOH [24], 2.32 for Ph2 POOH [25], 1.85 for (PhO) POOH [25], and 1.25 for (MeO) POOH [25]); (ii) the semipolar P=O group makes phosphinic acids reasonably strong proton acceptors; and (iii) the internal mobility within a self-associate reduces the entropic penalty upon complexation, which is another possible reason for the trimerization in solution
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