Chiral oxorhenium(V) complexes as candidates for the experimental observation of molecular parity violation: a structural, synthetic and theoretical study

Valence-shell photoelectron circular dichroism of ruthenium(III)-tris-(acetylacetonato) gas-phase enantiomers

Chiral transition-metal complexes are of interest in many fields ranging from asymmetric catalysis and molecular materials science to optoelectronic applications or fundamental physics including parity violation effects. We present here a combined theoretical and experimental investigation of gas-phase valence-shell photoelectron circular dichroism (PECD) on the challenging open-shell ruthenium(III)-tris-(acetylacetonato) complex, Ru(acac)₃. Enantiomerically pure Δ- or Λ-Ru(acac)₃, characterized by electronic circular dichroism (ECD), were vaporized and adiabatically expanded to produce a supersonic beam and photoionized by circularly-polarized VUV light from the DESIRS beamline at Synchrotron SOLEIL. Photoelectron spectroscopy (PES) and PECD experiments were conducted using a double imaging electron/ion coincidence spectrometer, and compared to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The open-shell character of Ru(acac)₃, which is not taken into account in our DFT approach, is expected to give rise to a wide multiplet structure, which is not resolved in our PES signals but whose presence might be inferred from the additional striking features observed in the PECD curves. Nevertheless, the DFT-based assignment of the electronic bands leads to the characterisation of the ionized orbitals. In line with other recent works, the results confirm that PECD persists independently on the localization and/or on the achiral or chiral nature of the initial orbital, but is rather a probe of the molecular potential as a whole. Overall, the measured PECD signals on Ru(acac)₃, a system exhibiting D₃ propeller-type chirality, are of similar magnitude compared to those on asymmetric-carbon-based chiral organic molecules which constitute the vast majority of species investigated so far, thus suggesting that PECD is a universal mechanism, inherent to any type of chirality.

The self-disproportionation of enantiomers (SDE): a menace or an opportunity?

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied.

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied. The SDE has implications ranging from the origins of prebiotic homochirality to unconventional enantiopurification methods, though the risks of altering the enantiomeric excess (ee) unintentionally, regrettably, remain greatly unappreciated. While recrystallization is well known as an SDE process, occurrences of the SDE in other processes are much less recognized, e.g. sublimation and even distillation. But the most common process that many workers seem to be completely ignorant of is SDE via chromatography and reports have included all manner of structures, all types of interactions, and all forms of chromatography, including GC. The SDE can be either a blessing – as a means to obtain enantiopure samples from scalemates – or a curse, as unwitting alteration of the ee leads to errors in the reporting of results and/or misinterpretation of the system under study. Thus the ramifications of the SDE are relevant to any area involving chirality – natural products, asymmetric synthesis, etc. Moreover, there is grave concern regarding errors in the literature, in addition to the possible occurrence of valid results which may have been overlooked and thus remain unreported, as well as the potential for the SDE to alter the ee, particularly via chromatography, and the following concepts will be conveyed: (1) the SDE occurs under totally achiral conditions of (a) precipitation, (b) centrifugation, (c) evaporation, (d) distillation, (e) crystallization, (f) sublimation, and (g) achiral chromatography (e.g. column, flash, MPLC, HPLC, SEC, GC, etc.). (2) The SDE cannot be controlled simply by experimental accuracy and ignorance of the SDE unavoidably leads to mistakes in the recorded and reported stereochemical outcome of enantioselective transformations. (3) The magnitude of the SDE (the difference between the extremes of enantioenrichment and -depletion) can be controlled and used to: (a) minimize mistakes in the recorded experimental values and (b) to develop unconventional and preparatively superior methods for enantiopurification. (4) The magnitude of the SDE cannot be predicted but can be expected for compounds possessing SDE-phoric groups or which have a general tendency for strong hydrogen or halogen bonds or dipole–dipole or aromatic π–π interactions. (5) An SDE test and the rigorous reporting and description of applied physicochemical processes should become part of standard experimental practice to prevent the erroneous reporting of the stereochemical outcome of enantioselective catalytic reactions and the chirooptical properties of scalemates. New directions in the study of the SDE, including halogen bonding-based interactions and novel, unconventional enantiopurification methods such as pseudo-SDE (chiral selector-assisted SDE resolution of racemates), are also reported.

An oxorhenium complex bearing a chiral cyclohexane-1-olato-2-thiolato ligand: Synthesis, stereochemistry, and theoretical study of parity violation vibrational frequency shifts

In our effort towards measuring the parity violation energy difference between two enantiomers, a simple chiral oxorhenium complex 5 bearing enantiopure 2-mercaptocyclohexan-1-ol has been prepared as a potential candidate species. Vibrational circular dichroism revealed a chiral environment surrounding the rhenium atom, even though the rhenium is not a stereogenic center itself, and enabled to assign the (1S,2S)-(-) and (1R,2R)-(+) absolute configuration for 5. For both compound 5 and complex 4, previously studied by us and bearing a propane-2-olato-3-thiolato ligand, relativistic calculations predict parity violating vibrational frequency differences of a few hundreds of millihertz, above the expected sensitivity attainable by a molecular beam Ramsey interferometer that we are constructing.

Strong enhancement of parity violation effects in chiral uranium compounds

A new generation of molecular candidates for parity violation measurements. The chiral UNXYZ compounds are predicted to exhibit strong parity violating effects which are up to an order of magnitude larger than for any of the previously suggested candidates.

Dissipative geometric phase and decoherence in parity-violating chiral molecules

Within a generalized Langevin framework for open quantum systems, the cyclic evolution of a two-level system is analyzed in terms of the geometric phase extended to dissipative systems for Ohmic friction. This proposal is applied to the dynamics of chiral molecules where the tunneling and parity violating effects are competing. The effect of different system-bath coupling functions in the dissipated energy is shown to be crucial to understand the behavior of the geometric phase as well as the decoherence displayed by the corresponding interference patterns.

Progress toward the first observation of parity violation in chiral molecules by high-resolution laser spectroscopy

Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.

Analysis of parity violation in chiral molecules

In order to guide the experimental search for parity violation in molecular systems, in part motivated by the possible link to biomolecular homochirality, we present a detailed analysis in a relativistic framework of the mechanism behind the tiny energy difference between enantiomers induced by the weak force. A decomposition of the molecular expectation value into atomic contributions reveals that the effect can be thought of as arising from a specific mixing of valence s(1/2) and p(1/2) orbitals on a single center induced by a chiral molecular field. The intra-atomic nature of the effect is further illustrated by visualization of the electron chirality density and suggests that a simple model for parity violation in molecules may be constructed by combining pre-calculated atomic quantities with simple bonding models. A 2-component relativistic computational procedure is proposed which bridges the relativistic and non-relativistic approaches to the calculation of parity violation in chiral molecules and allows us to explore the single-center theorem in a variational setting.

Theoretical investigations into the enantiomeric and racemic forms of α-(trifluoromethyl)lactic acid

There are many different hypotheses on the origin of biomolecular homochirality. One possible scenario concerns the enantiomeric enrichment of a nearly racemic solid via self-disproportionation of enantiomers. In particular, in a recent paper Soloshonok and co-workers showed a first example of optical self-purification of α-(trifluoromethyl)lactic acid by sublimation [V. A. Soloshonok et al., J. Am. Chem. Soc. 2007, 129, 12112]. Here we present detailed theoretical studies of α-(trifluoromethyl)lactic acid in the solid state as well as in the gas-phase dimeric form. The calculations of energy differences between dimers show that in the solid state the enantiomeric pure compound is energetically preferred, while in the gas phase the equilibrium shifts towards the racemic mixture although thermodynamic corrections cannot be neglected, thus providing a detailed microscopic explanation for the enantio-purification process for the first time.