Contrasting behaviour under pressure reveals the reasons for pyramidalization in tris(amido)uranium(III) and tris(arylthiolate) uranium(III) molecules

A range of reasons has been suggested for why many low-coordinate complexes across the periodic table exhibit a geometry that is bent, rather a higher symmetry that would best separate the ligands. The dominating reason or reasons are still debated. Here we show that two pyramidal UX₃ molecules, in which X is a bulky anionic ligand, show opposite behaviour upon pressurisation in the solid state. UN″₃ (UN3, N″ = N(SiMe₃)₂) increases in pyramidalization between ambient pressure and 4.08 GPa, while U(SAr)₃ (US3, SAr = S-C₆H₂-^tBu₃−2,4,6) undergoes pressure-induced planarization. This capacity for planarization enables the use of X-ray structural and computational analyses to explore the four hypotheses normally put forward for this pyramidalization. The pyramidality of UN3, which increases with pressure, is favoured by increased dipole and reduction in molecular volume, the two factors outweighing the slight increase in metal-ligand agostic interactions that would be formed if it was planar. The ambient pressure pyramidal geometry of US3 is favoured by the induced dipole moment and agostic bond formation but these are weaker drivers than in UN3; the pressure-induced planarization of US3 is promoted by the lower molecular volume of US3 when it is planar compared to when it is pyramidal.

The reasons for which many low-coordinate complexes exhibit bent geometry, rather than a higher symmetry, are still under debate. Here, the authors use high-pressure crystallography to examine whether low-coordinate f-block molecules become more planar or pyramidal under pressure; which happens is dictated by the dipole moment of the complex and the volume of the planar form.

Effects of alkylthio groups on phase transitions of organic molecules and liquid crystals: a comparative study with alkyl and alkoxy groups

The effects of the alkylthio groups on the phase transition behavior of organic liquid crystal molecules were examined by comparing them with the effects of alkyl and alkoxy groups.

C-HS interaction exhibits all the characteristics of conventional hydrogen bonds

This is a tale of a pair of a hydrogen bond donor and acceptor, namely the CH donor and sulphur acceptor, neither of which is a conventional hydrogen bond participant. Sulfur (S), being less electronegative (2.58) compared to its first row analogue oxygen (3.44), has not been considered as a potential HB acceptor for a long time. The C-HY (Y = HB acceptor) interaction has its own history of exhibiting omnidirectional shifts in the CH stretching frequency upon complex formation. Therefore, a systematic investigation of the C-HS interaction was the primary goal of the work presented here. Together with gas-phase vibrational spectroscopy and ab initio quantum chemical calculations, the nature and strength of the C-HS hydrogen bond (HB) have been investigated in the complexes of 1,2,4,5-tetracyanobenzene (TCNB) with various sulfur containing solvents. Despite the unconventional nature of both HB donor and HB acceptor (C-H and S, respectively), it was found that the C-HS hydrogen bond exhibits all the characteristics of the conventional hydrogen bond. The binding strength of the C-HS H-bond in these complexes was found to be comparable to that of the conventional hydrogen bonds. The unusual stabilities of these HBs have been mainly attributed to the attractive dispersion interaction.

Synthetic analogs for evaluating the influence of N-H...S hydrogen bonds on the formation of thioester in acetyl coenzyme A synthase

A series of square planar methylnickel(II) complexes, (dppe)Ni(Me)(SAr) (dppe = 1,2-bis(diphenylphosphino)ethane); 2. Ar = phenyl; 3. Ar = pentafluorophenyl; 4. Ar = o-pivaloylaminophenyl; 5. Ar = p-pivaloylaminophenyl; (depe)Ni(Me)(SAr), (depe = 1,2-bis(diethylphosphino)ethane); 7. Ar = phenyl; 8. Ar = pentafluorophenyl; 9. Ar = o-pivaloylaminophenyl; 10. Ar = p-pivaloylaminophenyl), were synthesized via the reaction of (dppe)NiMe(2) (1) and (depe)NiMe(2) (6) with either the corresponding thiol or disulfide. These complexes were characterized by various spectroscopic methods including (31)P NMR, (1)H NMR, (13)C NMR and infrared spectroscopies and in most cases by X-ray diffraction analyses. Solid state and solution measurements establish that 4 and 9 contain intramolecular N-H...S bonds. Carbonylation of the complexes 2-4, 7-10 leads to (dRpe)Ni(CO)(2) and MeC(O)SAr via the intermediacy of the acylnickel adducts, (dRpe)Ni(C(O)Me)(SAr), detected at low temperature by (31)P NMR spectroscopy. Consistent with experimental observations, density functional theory results reveal that the intramolecular hydrogen bond in 9 stabilizes the acylnickel adduct compared with its non-hydrogen-bonded adduct, 10. Oxidative addition of MeC(O)SC(6)F(5) to (dRpe)Ni(COD) followed by spontaneous decarbonylation proceeds in variable yields generating 3 and 8.

N-H···S Hydrogen Bonds in a Ferredoxin Model

The Fe4S4 complex {(CH3)3NCH2CONH2}2[Fe4S4((tBuS)4] (1) was synthesized to replicate the ferredoxin active site with a subset of its N-H...S hydrogen bonds. The two cationic counterions mimic the polypeptide backbone of ferredoxin (Fd) as amide hydrogen-bond donors to sulfur atoms of the iron-sulfur cluster. X-ray crystallographic data show that the organic sulfur (Sgamma) of one tert-butylthiolate ligand and one inorganic sulfur of the cluster core serve as N-H...S hydrogen-bond acceptors. The cluster core of complex 1 is tetragonally elongated in contrast to that of Fd, which is tetragonally compressed. This is the first observation of an elongated [Fe4S4]2+ cluster core. Additionally, this is the first synthetic Fd model in which N-H...S hydrogen bonding to a cluster has been achieved.

Synthetic modeling of zinc thiolates: quantitative assessment of hydrogen bonding in modulating sulfur alkylation rates

A series of mononuclear zinc thiolate complexes have been prepared and fully characterized. The reactions of the complexes with alkyl halides, leading to zinc halides and the corresponding thioethers, have been examined by kinetic methods. In toluene, the reactions obey a second-order rate law displaying activation parameters consistent with a S(N)2 attack of the zinc-bound thiolate on the carbon electrophile. Intramolecular hydrogen bonding of an amide NH to the thiolate sulfur reduces the nucleophilicity and consequently, the rate of alkylation more than 30-fold at 25 degrees C. The H-bonding shows an inverse HD isotope effect of 0.33 (60 degrees C) ascribed to differential H-bonding for the two isotopomers due to zero point energy differences. These model studies provide quantitative evaluation of H-bonding on reaction rates relevant to zinc thiol-activating proteins.

NH-S hydrogen bonding in zinc enzyme model complex with S2N2 binding set studied by normal coordinate analysis of vibrational spectra

In order to provide theoretical evidence for the existence and effect of NH-S hydrogen bonding in the zinc enzyme model complex [Zn(S-2-C6H4NHCOC6H5)2(1-MeIm)2] (1-MeIm = 1-methylimidazole) in addition to the spectroscopic and crystallographic investigations, normal coordinate analysis (NCA) was carried out using a modified Urey-Bradley force field. The vibrational frequencies of the complex with and without the NH-S hydrogen bonding as well as the corresponding internal coordinates were obtained. The good agreement found between the calculated and observed frequencies in the presence of the NH-S hydrogen bond, supports the reliability of the analysis. The stretching force constant of the NH-S hydrogen bond obtained from the calculation is 0.18 mdyne per A. The calculation shows that the N-H bond is weakened by formation of the NH-S hydrogen bond. The results are indicative of the existence of the NH-S hydrogen bond in the complex.