Cyclic trimers of phosphinic acids in polar aprotic solvent: symmetry, chirality and H/D isotope effects on NMR chemical shifts

Abstract

The hydrogen-bonded self-associates of dimethylphosphinic (1), diphenylphosphoric (2), phenylphosphinic (3), and bis(2,4,4-trimethylpentyl)phosphinic (4) acids have been studied by using liquid-state NMR down to 100 K in a low-freezing polar solvent, CDF/CDClF. The H/D isotope effects on H NMR chemical shifts caused by partial deuteration of hydroxyl groups unambiguously reveal the stoichiometry of the self-associates and the cooperativity of their hydrogen bonds. In all cases, cyclic trimers are the dominant form, while cyclic dimers are present as a minor form for 1 and 2. Due to the asymmetry of substituents, cyclic trimers of 3 exist in two isomeric forms, depending on the orientation of the phenyl groups with respect to the plane of the hydrogen bonds. The racemic mixture of 4 leads to the coexistence of up to 64 isomers of cyclic trimers, many of which are chemically equivalent or effectively isochronous. The mole fractions of such isomers deviate from the statistically expected values. This feature could provide information about the relative stabilization energies of hydrogen-bonded chiral self-associates. The complexation of 4 with SbCl (complex 5) suppresses the self-association and 5 exists exclusively in the monomeric form with chemically non-equivalent P nuclei in RS, SR and RR/SS forms.

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

“…Again, the average coefficient refers to the fitting of the entire data set shown in Figure 8. It should be noted that despite the generally high sensitivity of δP to the molecular structure and to non-covalent interactions [71], in the literature there are only few attempts to use δP for the solution of the reverse spectroscopic problem for non-covalent complexes, i.e., for the finding of the complex's energy and structure based on the phosphorous chemical shift value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to δP from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

“…value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to P from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids

“…Under these conditions the proton and molecular exchange processes slowdown in the NMR time scale, resulting in the observation of resolved signals from complexes of different stoichiometry. Previously it has been shown that in freonic solutions acids 1-4 self-associate into cyclic dimers and cyclic trimers ( Figure 1b) [14,15]. Meanwhile, the stoichiometry, structure, and properties of the mixed complexes-heterodimers and heterotrimers ( Figure 1c)-are unknown.…”

“…Perhaps the most classical and well-studied example of such complexation is provided by carboxylic acids, which form cyclic dimers in the gas phase [1], in dilute aprotic solutions [2] and in crystals [3,4], except for formic [5] and acetic [6] acids that crystallize as infinite chains. A somewhat different situation is observed for POOH-containing acids, namely, phosphinic (R 2 POOH) and phosphoric acids ((RO) 2 POOH), which also 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 our previous works the stoichiometry of the phosphinic and phosphoric acid cyclic trimers has been established by the use of H/D isotope effects on 1 H-NMR chemical shifts: three hydrogen bonds are mutually coupled and deuteration in one of them changes the geometry and, thus, the NMR parameters of the remaining ones.…”

“…The number of such H/D isotope effects allows one to enumerate the coupled hydrogen bonds in a complex; the signs of the H/D isotope effects enable one to distinguish between cooperative and anticooperative coupling schemes [17]. It has been demonstrated that in the cyclic trimeric self-associates of the phosphinic and phosphoric acids hydrogen bonds are cooperative (mutually strengthening each other), which one would expect for the head-to-tail linking motif [14,15]. While the fact of trimerization is well grounded, the reasons for it are less clear.…”