Tertiary Phosphine and Arsine Complexes of Phosphorus Pentafluoride: Synthesis, Properties, and Electronic Structures.

The reaction of PMe3 or PPh3 with PF5 in anhydrous CH2Cl2 or hexane forms the white, moisture-sensitive complexes [PF5(PR3)] (R = Me, Ph). Similar reactions involving the diphosphines o-C6H4(PR2)2 afford the complexes [PF4{o-C6H4(PR2)2}][PF6]. The X-ray structures of [PF5(PR3)] and [PF4{o-C6H4(PMe2)2}][PF6] show pseudo-octahedral fluorophosphorus centers. Multinuclear NMR spectra (1H, 19F{1H}, 31P{1H}) show that in solution in CH2Cl2/CD2Cl2 the structures determined crystallographically are the only species present for [PF5(PMe3)] and [PF4{o-C6H4(PMe2)2}][PF6] but that [PF5(PPh3)] and [PF4{o-C6H4(PPh2)2}][PF6] exhibit reversible dissociation of the phosphine at ambient temperatures, although exchange slows at low temperatures. The complex 19F{1H} and 31P{1H} NMR spectra have been analyzed, including those of the cation [PF4{o-C6H4(PMe2)2}]+, which is a second-order AA'XX'B2M spin system. The unstable [PF5(AsMe3)], which decomposes in a few hours at ambient temperatures, has also been isolated and spectroscopically characterized; neither AsPh3 nor SbEt3 forms similar complexes. The electronic structures of the PF5 complexes have been explored by DFT calculations. The DFT optimized geometries for [PF5(PMe3)], [PF5(PPh3)], and [PF4{o-C6H4(PMe2)2}]+ are in good agreement with their respective crystal structure geometries. DFT calculations on the PF5-L complexes reveal the P-L bond strength falls with L in the order PMe3 > PPh3 > AsMe3, consistent with the experimentally observed stabilities, and in the PF5-L complexes, electron transfer from L to PF5 on forming these complexes also follows the order PMe3 > PPh3 ≈ AsMe3.

Systematics of BX3 and BX2(+) Complexes (X = F, Cl, Br, I) with Neutral Diphosphine and Diarsine Ligands.

The coordination chemistry of the neutral diphosphines, R2P(CH2)2PR2 (R = Me or Et) and o-C6H4(PR'2)2 (R' = Me or Ph), and the diarsine, o-C6H4(AsMe2)2, toward the Lewis acidic BX3 (X = F, Cl, Br, and I) fragments is reported, including several rare complexes incorporating BF3 and BF2(+). The studies have revealed that the flexible dimethylene linked diphosphines form [(BX3)2{µ-R2P(CH2)2PR2}] exclusively, confirmed by multinuclear NMR ((1)H, (11)B, (19)F{(1)H}, and (31)P{(1)H}) and IR spectroscopy and microanalytical data. Crystallographic determinations of the four BX3 complexes with Et2P(CH2)2PEt2 confirm the 2:1 stoichiometry and, taken together with the spectroscopic data, reveal that the Lewis acid behavior of the BX3 fragment toward phosphine ligands increases in the order F ≪ Cl ∼ Br < I. The first diphosphine- and diarsine-coordinated dihaloboronium cations, [BX2{o-C6H4(EMe2)2}](+) (E = P, As), are obtained using the rigid, preorganized o-phenylene linkages. These complexes are characterized similarly, and the data indicate that the complexes with o-C6H4(AsMe2)2 are much more labile and readily decomposed than the phosphine analogues. X-ray crystallographic studies on [BX2{o-C6H4(PMe2)2}][BX4] (X = Cl, Br), [BI2{o-C6H4(PMe2)2}][I3], and [BCl2{o-C6H4(AsMe2)2}][BCl4] confirm the presence of distorted tetrahedral coordination at boron through a chelating diphosphine or diarsine and two X ligands, with d(B-P) revealing a similar increase in Lewis acidity down group 17. Comparison of d(B-P) and d(B-As) reveals an increase of ca. 0.08 Å from P to As. Reaction of BCl3 with the diphosphine dioxide Ph2P(O)CH2P(O)Ph2 gives the ligand-bridged dimer [(BCl3)2{Ph2P(O)CH2P(O)Ph2}], while using either BF3 gas or [BF3(SMe2)] gives a mixture containing both [(BF3)2{µ-Ph2P(O)CH2P(O)Ph2}] and the unexpected difluoroboronium salt, [BF2{Ph2P(O)CH2P(O)Ph2}][B2F7] containing a chelating phosphine oxide. The structure of the latter was confirmed crystallographically.

A general strategy for obtaining 19F-19F and 13C-19F residual dipolar couplings in perfluorocarbons from the NMR spectroscopy of liquid crystalline samples.

A two-dimensional Fluorine Detected Local Field (FDLF) NMR experiment is demonstrated on a sample of perfluoropropyl iodide dissolved in the nematic solvent ZLI1132. In analogy to the proton detected local field (PDLF) technique, for each resolved site of the carbon spectrum, a simple map of the heteronuclear coupling network is obtained in the indirect dimension. A full analysis of the FDLF spectrum was achieved with the aid of two-dimensional (19)F-(13)C HETCOR and (13)C, D-resolved spectra (with D representing the anisotropic spin-spin coupling). A one-dimensional (19)F spectrum was recorded on the same sample at intermediate resolution, and values of the residual spin-spin couplings T(CF) and T(FF) obtained from both experiments were combined and used to provide starting parameters for the analysis of a very high resolution (19)F spectrum, including the weak satellite lines from single-(13)C isotopomers. The high-precision, residual, anisotropic couplings were used to explore whether they have an appreciable contribution from the anisotropic electron-mediated spin-spin couplings.

The conformation and orientational order of a 1,2-disubstituted ethane nematogenic molecule (I22) in liquid crystalline and isotropic phases studied by NMR spectroscopy.

The structure, conformation and orientational order of the mesogen I22 have been studied by proton, carbon-13 and deuterium 1D and 2D-NMR spectroscopies at natural abundance and at various magnetic fields when in the nematic phase, the isotropic phase close to the nematic-isotropic phase transition, and as a solute in the chiral nematic solution comprised of the polypeptide PBLG dissolved in chloroform. It is concluded that 95% of conformers have a trans arrangement about the central C-C bond of the ethane fragment in all phases.

A comparison of the conformational distributions of the achiral symmetric liquid crystal dimer CB7CB in the achiral nematic and chiral twist-bend nematic phases.

The sets of residual dipolar couplings between carbon and hydrogen nuclei obtained from the proton-encoded (13)C 2D NMR experiment are used to investigate the conformational changes which occur when the achiral symmetric liquid crystal dimer CB7CB changes from the achiral nematic to the chiral twist-bend nematic phase. It is found that these changes are a consequence of the chirality of the twist-bend nematic phase, rather than being the driving force for the stability of this phase.

Chiral solutes can seed the formation of enantiomorphic domains in a twist-bend nematic liquid crystal.

The twist-bend nematic, an enantiomorphic liquid-crystalline phase, exhibited by the structurally symmetric liquid-crystal dimer CB7CB is induced to form a single domain of uniform handedness, in the bulk, by the addition of the dopant chiral solute (S)-1-phenylethanol. Addition of a nonracemic (or scalemic) mixture of both R and S enantiomers of this solute produced equal volumes of P and M chiral domains for the twist-bend nematic phase. This seeding of the domains in an enantiomorphic nematic conglomerate is revealed using deuterium NMR spectroscopy.

Alignment of druglike compounds in lipid bilayers analyzed by solid-state (19)F-NMR and molecular dynamics, based on dipolar couplings of adjacent CF3 groups.

Solid-state (19)F-NMR spectroscopy is frequently used to analyze the structure and dynamics of lipophilic drugs and peptides embedded in biomembranes. The homonuclear dipolar couplings of trifluoromethyl (CF3) labels can provide valuable parameters such as orientational constraints and/or distances. To characterize the complex dipolar patterns of multiple (19)F spin interactions, three different model compounds carrying two CF3 groups in meta-position on a phenyl ring were incorporated in macroscopically aligned DMPC bilayers. The dipolar patterns obtained with the CPMG (Carr-Purcell-Meiboom-Gill) multipulse sequence were analyzed to yield simultaneously the intra-CF3 and intergroup dipolar coupling values. The fluorine-fluorine distances were predicted by a density functional calculation, and the alignment of the labeled molecular segment could be determined from these distances and the dipolar coupling values. The different compounds were found to align in the lipid bilayer according to their amphiphilic properties, though with a weak anisotropic preference that is typical of solutes in liquid crystals. The residual dipolar couplings were used to calculate Saupe order parameters. For the least complex molecule, (CF3)2-BA, an orientational probability function for the solute in the lipid matrix could be derived. The overall description of how (CF3)2-BA is embedded in the bilayer was independently assessed by molecular dynamics simulations, and compared in structural and dynamical terms with the results of the NMR experiments.

The chirality of a twist-bend nematic phase identified by NMR spectroscopy.

One of the defining characteristics of the twist-bend nematic phase, formed by the methylene-linked liquid crystal dimer 1″,7″-bis(4-cyanobiphenyl-4'-yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D∞h symmetry is lost in a chiral phase because its symmetry is reduced to D∞ on removal of the mirror plane. Recording proton-enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C-H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the twist-bend nematic. Strikingly, this equivalence is not lost for the central methylene group where the two protons are homotopic. We also show how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4-octyl-4'-cyanobiphenyl-d2, perdeuteroacenaphthene-d10, and acenaphthene-d4 as examples. For the standard nematic phase deuterium, NMR shows that the deuterons in these methylene groups are equivalent but, as expected, in the twist-bend nematic phase this equivalence is lost. The deuterium NMR spectra of these probe molecules dissolved in CB7CB have been recorded from the isotropic phase, through the nematic and deep into the supercooled twist-bend nematic.

Magnetic field induced dipolar couplings in the pretransitional region of a nematic liquid crystal.

13C and 1H NMR spectroscopy have been used to study the orientational order which develops when a nematogenic compound, 4-pentyl-4'-cyanobiphenyl (5CB), approaches the transition from the isotropic to the nematic phase at T(NI). The experiments yield values of field induced dipolar couplings, (1), between all of the directly bonded carbon and hydrogen nuclei in the molecule, and 2D(HH)B, the geminal dipolar coupling between protons in the first methylene group in the alkyl chain. The temperature dependence of these couplings shows that in each case they follow a divergence behavior governed by (T - T*)(-1),where T* is a temperature determined from the experimental data and which is close to but less than T(NI). Experiments performed at spectrometer field strengths of 11.75 and 18.79 T confirm the prediction that the induced couplings should depend on the square of the applied field strength. It was found that, within experimental error, T* is the same for each field-induced coupling, and that T(NI) - T* is the same at 11.75 and 18.79 T. It is shown that the set of field-induced couplings 1D(CHi)B obtained at a temperature close to T(NI) can be used to derive a conformer distribution for 5CB in the isotropic phase.

The structure and conformations of 2-thiophenecarboxaldehyde obtained from partially averaged dipolar couplings.

The proton NMR spectra of samples of 2-thiophenecarboxaldehyde dissolved in a nematic liquid crystalline solvent, including those from all five singly labelled 13C isotopomers, have been obtained. These have been analysed to yield sets of partially averaged dipolar couplings which have been used to determine the structure and the relative amounts of the cis and trans forms, which are the two minimum-energy structures generated by rotation about the ring-aldehyde bond. A procedure for applying vibrational corrections to the dipolar couplings in the presence of large amplitude motions is discussed.

Is styrene planar in liquid phases?

The proton NMR spectra of two (13)C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, phi(0), in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of phi(0) is found to be 18.0 degrees +/-0.2 degrees for the two solutions, compared with a value of 27 degrees calculated by the molecular method MP2/6-31G(*).

2D-NMR strategy dedicated to the analysis of weakly ordered, fully deuterated enantiomers in chiral liquid crystals.

Methods for the assignment of the quadrupolar doublets in the deuterium NMR spectra of weakly ordered, perdeuterated or partially deuterated enantiomers dissolved in chiral liquid crystals are described which use robust 2D correlation NMR experiments. To overcome a lack of resolution in deuterium tilted Q-COSY 2D spectra in such materials, we propose and explore a correlation 2D sequence which is based on deuterium-carbon 2D correlation spectroscopy. The technique results in a (13)C-(2)H contour plot and allows the full resonance assignment of overcrowded deuterium 1D spectra using carbon-deuterium correlations. The (2)H autocorrelation and (13)C-(2)H correlation experiments are applied in the case of a racemic mixture of 2-ethylhexanoic acid-d(15) dissolved in a polypeptidic chiral oriented solvent. The performance and the limits of both techniques are presented and discussed. For the last step of the assignment procedure, we propose a simple method for obtaining two coherent sets of quadrupolar splittings, one for each enantiomer.