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Pentaphenylantimony (SbPh5) has been previously crystallized in either a square pyramidal or trigonal bipyramidal geometry. Investigation of the solution-state structure of SbPh5 has been hampered by the extreme fluxionality of this compound, but previous vibrational spectroscopic studies concluded that it maintains a square pyramidal geometry in solution. This non-VSEPR-compliant geometry, which is also assumed by BiPh5 in the solid state, stands in contrast to the trigonal bipyramidal geometries of PPh5 and AsPh5. A range of phenomena have been invoked to explain this discrepancy, most notably, the increased importance of relativistic effects as group 15 is descended. We present crystallographic, spectroscopic, and computational data revealing that SbPh5 in fact assumes the VSEPR-compliant trigonal bipyramidal geometry in solution. In particular, Sb X-ray absorption spectroscopy (XAS) was used to obtain geometry-sensitive spectra that do not suffer from the slow spectroscopic time scale that has prevented NMR studies from elucidating the structure of this fluxional molecule. Sb K-edge and LIII-edge XAS spectra of crystalline solids featuring SbPh5 in either a square pyramidal (nonsolvate) or trigonal bipyramidal (cyclohexane hemisolvate or THF hemisolvate) form were compared to spectra of SbPh5 in solution. The solution-state spectra agree with those from solids containing trigonal bipyramidal SbPh5. The most diagnostic spectroscopic feature was the distribution of intensity in the Sb LIII pre-edge features. These distributions were rationalized using time-dependent density functional theory calculations that take into account spin-orbit coupling. Our use of Sb XAS not only resolves a long-standing physical inorganic question but also demonstrates more widely the utility of XAS in establishing the structures of fluxional main-group compounds. This conclusion was further supported by solid- and solution-state Raman data. Finally, we note that the present high-resolution diffraction data allow τ for nonsolvated SbPh5 to be revised to 0.216.

A series of rhenium(I) fac-tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide ...

The separation of enantiomers is of considerable importance in the preparation of the compounds of biological interests, catalysis, and drug development. Here, we report a novel enantioseparation of styrene epoxides (SOs) resolved in the presence of a pair of enantio-enriched tetrahedral cages. Chiral neutral cages of formula [(Pd3X*)4(C6O4Cl2)6] ([X*]3- = RRR- or SSS-[PO(N(*CH(CH3)Ph)3]3-) are constructed from Pd3 building units supported by tris(imido)phosphate trianions and chloranilate linkers. These cages exhibit considerable enantioselective separation capabilities toward a series of styrene epoxides via a crystallization inclusion method. A highest enantiomeric excess (ee) value of up to 80% is achieved for the (R)-4-fluorostyrene oxide.

Human UDP-α-d-xylose synthase (hUXS) is a member of the extended short chain dehydrogenase/reductase (SDR) family of enzymes. Previous crystallographic studies have shown that hUXS conserves the same dimeric quaternary structure observed in other SDR enzymes. Here, we present evidence that hUXS also forms a tetramer in solution that is important for activity. Sedimentation velocity studies show that two hUXS dimers undergo a concentration-dependent association to form a tetramer with a Kd of 2.9 μM. The tetrameric complex is also observed in small-angle X-ray scattering (SAXS). The specific activity for the production of the reaction intermediate UDP-α-d-4-keto-xylose displays a hyperbolic dependence on protein concentration that is well modeled by an isotherm using the 2.9 μM Kd of the tetramer. Likewise, the rate of UDP-α-d-xylose production in the presence of increasing concentrations of the small molecule crowder trimethylamine N-oxide is consistent with the formation of a higher activity tetramer. We present several possible structural models of the hUXS tetramer based on (i) hUXS crystal packing, (ii) homology modeling, or (iii) ab initio simulated annealing of dimers. We analyze the models in terms of packing quality and agreement with SAXS data. The higher activity of the tetramer coupled with the relative instability of the complex suggests that an association-dissociation mechanism may regulate hUXS activity.

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).