Weak Intramolecular Proton−Hydride and Proton−Fluoride Interactions:  Experimental (NMR, X-ray) and DFT Studies of the Bis(NBH₃) and Bis(NBF₃) Adducts of 1,3-Dimethyl-1,3-diazolidine

Abstract

Bis(NBH(3)), bis(NBF(3)), and NBF(3)/NBH(3) adducts 1-3 were prepared from 1,3-dimethyl-1,3-diazolidine and characterized by the (1)H, (13)C, (11)B, (19)F, 2D (1)H(-13)C HETCOR and NOESY NMR spectra. The structures and conformations of the adducts were established by the variable-temperature (1)H NMR spectra, the X-ray diffraction method (adduct 2A), and density functional calculations at different theoretical levels. The experimental and theoretical data have revealed that bis adducts 1-3 prefer trans orientations of the borane groups (1A, 2A, 3A) in solution, the solid state, and the gas phase. The studies have shown that the energetic preference of trans adducts with respect to cis compounds, decreasing as 2A (2.9 kcal/mol) > 3A (2.7 kcal/mol) > 1A (1.4 kcal/mol), is dictated by spatially repulsive interactions between the CH(3), BH(3), and BF(3) groups. The results of DFT calculations agree well with an experimental trans/cis isomeric ratio of 9:1 determined in solutions of adduct 1. The calculated geometry and energy, as well as the topological analysis of electronic densities, show that trans adducts 1-3 should exist in gas phase as twist conformations T-2 stabilized by the intramolecular C-H(delta+)...(-delta)H-B or C-H(delta+)...(-delta)F-B interactions. These interactions are characterized as closed-shell. The energy of one proton-hydride and proton-fluoride intramolecular contact, estimated as 1.9 (1A-T-2) and 0.7 (2A-T-2) kcal/mol, respectively, classifies the "elongated" intramolecular interactions CH(delta+)...(-delta)HB and CH(delta+)...(-delta)FB as weak ones. It has been established that, on going from gas phase to a condensed phase (solution and solid), the twist-conformations T-2 transform to conformations T-1, probably by intermolecular dipole-dipole interactions. The data presented in this work show that despite a weakness of the "elongated" proton-hydride and proton-fluoride interactions, they can play a significant role in the stabilization of conformational molecular states, especially when cooperativity is in action.

Medium and high oxidation state metal/non-metal fluoride and oxide–fluoride complexes with neutral donor ligands

“…29) with BF 3 . 291 BF 3 has been used to model BH 3 in adducts of the fused carbocyclic azafenestranes to probe the effect of coordination of the amine N-donor atom on the planarity of the central quaternary C atom. These also incorporate distorted tetrahedral coordination.…”

Polysulfonate Cappings on Upconversion Nanoparticles Prevent Their Disintegration in Water and Provide Superior Stability in a Highly Acidic Medium

“…iiNMR measurements combined with DFT theoretical calculations have revealed the existence of intramolecular hydrogen bonds in organofluorine-substituted derivatives of different classes of molecules (ref (27)). In addition, it has been suggested that proton–fluoride interactions can play a significant role in the stabilization of conformational molecular states ,especially via cooperativity(ref (28))…”

Beyond excitation NMR relaxation

“…It has been demonstrated (theoretically and experimentally) that bis(NBH 3 ) and bis(NBF 3 ) adducts of 1,3-dimethyl-1,3-diazolidine exhibit weak proton-hydride and proton-fluoride interactions, respectively (see Scheme 12). Nevertheless, the weak interactions can play a significant role in the stabilization of conformational molecular states, especially when cooperativity is involved (119). Intramolecular contacts of the type C-H Recently, a strategy to measure the transfer rates within the 13 C Zeeman order was proposed; this approach considers the cross-correlated relaxation rates (CCRRs), the longitudinal and transversal relaxations, and the nOe in highly symmetric compounds (121).…”

“…It has been demonstrated (theoretically and experimentally) that bis(NBH 3 ) and bis(NBF 3 ) adducts of 1,3-dimethyl-1,3-diazolidine exhibit weak proton-hydride and proton-fluoride interactions, respectively (see Scheme 12). Nevertheless, the weak interactions can play a significant role in the stabilization of conformational molecular states, especially when cooperativity is involved (119). Intramolecular contacts of the type C-H dþ ÁÁÁ dÀ H-B and C-H dþ ÁÁÁ dÀ F-B (2.2-2.5 Å ), which are established in some cyclic borane adducts in the solid state, can affect the conformational states of such molecules in solution (120).…”