The First Lanthanide Complexes with a Redox-Active Sulfur Diimide Ligand: Synthesis and Characterization of [LnCp*2 (RN=)2 S], Ln=Sm, Eu, Yb; R=SiMe3.

The first lanthanide complexes with a redox-active sulfur diimide ligand, [LnCp*2 (Me3 SiN=)2 S] (Ln=Sm, Eu, Yb; Cp*=η5 -C5 Me5 ), are reported. The complexes were synthesized by using [LnCp*2 (THF)2 ] and (Me3 SiN=)2 S and have been thoroughly characterized by single-crystal X-ray diffraction, EPR spectroscopy, UV/Vis/NIR electronic absorption spectroscopy and SQUID magnetometry. The results, as interpreted by CASSCF/SOC-RASSI calculations providing a non-perturbative treatment of spin-orbit coupling, indicate that these paramagnetic complexes are best described as Ln3+ and [(Me3 SiN=)2 S]-. adducts. As such, these complexes contain the first isolated and structurally characterized acyclic [(RN=)2 S]-. radical anions.

Synthesis, Characterization, and Properties of Iron(II) Spin-Crossover Molecular Photoswitches Functioning at Room Temperature.

Spin-crossover molecular switches [FeII(H2B(pz)2)2L] (L = novel phenanthroline-based ligands featuring photochromic diarylethene units; pz = 1-pyrazolyl) were synthesized and thoroughly characterized by variable-temperature X-ray crystallography, Mössbauer spectroscopy, and magnetic measurements. The effect of substituents introduced into the phenanthroline backbone (L2) and into the photochromic diarylethene unit (L3) on photophysical properties of metal-free ligands and spin-crossover iron(II) complexes 2 and 3, respectively, were investigated in detail. Both ligands and complexes could be switched with light in solution at room temperature. The photocyclization of 2 was accompanied by a high-spin to low-spin photoconversion determined at 19%. The closed-ring isomers of L3 and 3 reveal the lifetimes in the range of minutes, whereas those of L2 and 2 are thermally stable for days in solutions at room temperature. The reversibility of the photoswitching can be improved by avoiding the photostationary states. Prospective introduction of anchoring groups to the phenanthroline backbone might allow the construction of chemisorbed self-assembled monolayers of spin-crossover species switchable with light at room temperature.

Reversible Photoswitching of a Spin-Crossover Molecular Complex in the Solid State at Room Temperature.

Spin-crossover metal complexes are highly promising magnetic molecular switches for prospective molecule-based devices. The spin-crossover molecular photoswitches developed so far operate either at very low temperatures or in the liquid phase, which hinders practical applications. Herein, we present a molecular spin-crossover iron(II) complex that can be switched between paramagnetic high-spin and diamagnetic low-spin states with light at room temperature in the solid state. The reversible photoswitching is induced by alternating irradiation with ultraviolet and visible light and proceeds at the molecular level.

Modulation of magnetic properties at room temperature: coordination-induced valence tautomerism in a cobalt dioxolene complex.

The valence-tautomeric six-coordinate complex [Co(tbdiox)2(4-papy)2] (1; tbdiox = redox-active 3,5-di-tert-butyl-o-dioxolene, 4-papy = 4-phenylazopyridine) was synthesized and its electronic structure examined. Whereas 1 shows regular thermally driven valence tautomerism in the solid state, it partially dissociates in solution to form the five-coordinate species [Co(tbdiox)2(4-papy)] (2) and free 4-papy. Species 1 and 2 exhibit different electronic structures-low-spin (ls) Co(III) and high-spin (hs) Co(II), respectively-in solution at room temperature and therefore different magnetic properties. Since 1 and 2 are in an equilibrium that is 4-papy-dependent, the magnetic moment of the solution species can be tuned by means of the ligand content. Thus, the concept of coordination-induced valence tautomerism (CIVT) has been introduced. The electronic structures of 1 and 2 as well as their CIVT were elucidated by X-ray crystallography, electrochemistry, titration experiments, and all variable-temperature SQUID susceptometry, NMR, EPR, and electronic absorption spectroscopy. The experimental findings are strongly supported by broken-symmetry DFT calculations. The magnetic exchange interactions in different types of valence-tautomeric cobalt complexes were explored computationally.

Microsolvation-induced quantum localization in protonated methane.

Nuclear quantum effects are responsible for vivid large-amplitude motion in protonated methane CH(5)(+), which enables so-called "hydrogen scrambling" that leads to the dynamical equivalence of all five protons even at low temperatures. But what is the impact of external perturbations on hydrogen scrambling of CH(5)(+) in this quantum fluxional ground state? We report ab initio path integral simulations of CH(5)(+)(H(2))(n), n = 1, 2, 3 that demonstrate cessation of hydrogen scrambling at low temperatures (20 K), but only slowdown at moderate temperatures (110 K). Importantly, different and unexpected mechanisms that are responsible for freezing the scrambling dynamics are revealed and traced back to distinct microsolvation patterns.

Theoretical spectroscopy using molecular dynamics: theory and application to CH5(+) and its isotopologues.

Infrared spectroscopy is a powerful technique to unravel the structure and dynamics of molecular systems of ever increasing complexity. For isolated molecules in the gas phase theoretical approaches that directly rely on solving the Schrödinger equation, either approximately or quasi-exactly, are well established. A distinctly different approach to compute infrared spectra can be based on advanced molecular dynamics, itself being based on classical Newtonian dynamics, in conjunction with concurrent first principles electronic structure calculations. At variance with traditional methods, which are formulated in terms of the Schrödinger representation of quantum mechanics, the molecular dynamics approach stems from Heisenberg's representation and thus relies on computing thermal expectation values of time-correlation functions. Crucial in addition to generating the spectra themselves is their decomposition in terms of modes, which can be assigned to correlated atomic motion. This ab initio molecular dynamics route to compute infrared spectra, and its recent extension to quasiclassical techniques relying on approximate path integral dynamics, is covered in the review part of this Perspective. The usefulness of this unconventional approach, which can be generalized beyond infrared spectroscopy, is demonstrated in detail by applying the full machinery in computing and assigning the infrared spectra of protonated methane and its isotopologues. This particular molecule is often considered to be the most prominent member of the class of floppy or fluxional molecules. CH5(+) has been a longstanding challenge for theoretical infrared spectroscopy because it undergoes intricate large-amplitude motion, which is also reviewed. Molecular dynamics based infrared spectroscopy is general and can be applied to diverse systems such as molecular complexes in the gas phase, chromophores in biomolecular environments, and solute-solvent systems in the liquid phase.

Tailoring the Cu(100) work function by substituted benzenethiolate self-assembled monolayers.

The structure and electronic interface properties of five differently substituted benzenethiol based self-assembled monolayers (SAMs) on Cu(100) have been studied by means of low energy electron diffraction, thermal desorption spectroscopy, X-ray absorption spectroscopy (NEXAFS), and UV photoelectron spectroscopy. Because highly ordered SAMs are formed of which lateral density had been precisely determined, effective molecular dipole moments were derived from the measured work function shifts. These values are compared with gas phase dipole moments computed by quantum chemical calculations for the individual thiol molecules considering the molecular orientation determined from NEXAFS data. Furthermore, this comparison yields clear evidence for a coverage dependent depolarization effect of the adsorbed molecules within the SAMs.

Communications: On artificial frequency shifts in infrared spectra obtained from centroid molecular dynamics: Quantum liquid water.

Centroid molecular dynamics (CMD) is a popular method to extract approximate quantum dynamics from path integral simulations. Very recently we have shown that CMD gas phase infrared spectra exhibit significant artificial redshifts of stretching peaks, due to the so-called "curvature problem" imprinted by the effective centroid potential. Here we provide evidence that for condensed phases, and in particular for liquid water, CMD produces pronounced artificial redshifts for high-frequency vibrations such as the OH stretching band. This peculiar behavior intrinsic to the CMD method explains part of the unexpectedly large quantum redshifts of the stretching band of liquid water compared to classical frequencies, which is improved after applying a simple and rough "harmonic curvature correction."

Methanol synthesis on ZnO(0001). I. Hydrogen coverage, charge state of oxygen vacancies, and chemical reactivity.

Oxygen vacancies on ZnO(0001) have been proposed to be the catalytically active sites for methanol synthesis on pure ZnO. The charge state and thus the chemical reactivity of such vacancies on this polar O-terminated basal plane of ZnO is expected to be intimately connected to the degree of its hydroxylation in view of its Tasker type(3) unstable character. Here, the interplay between hydrogen adsorption and the thermodynamic stability of O vacancies in various charge states, corresponding formally to F(++), F(+), F(0), F(-), and F(--) centers, is investigated using electronic structure calculations. Assuming thermodynamic equilibrium of the defective surface with a hydrogen containing gas phase the thermodynamically most stable O vacancy type is determined as a function of temperature and pressure. For the adsorption of H(2) molecules at O vacancy sites it is found that the homolytic process leads to energetically more favorable structures than heterolytic adsorption and hydride formation. By homolytic adsorption and desorption one can switch between F(++), F(0), and F(--) or between F(+) and F(-), a process which is believed to occur during methanol synthesis. However, the barrier for heterolytic dissociation of H(2) at O vacancies is significantly lower compared to homolytic cleavage. Furthermore, the barrier for transforming hydridic hydrogen, i.e., ZnH species, to protonic hydrogen, i.e., OH species together with a reduction of ZnO itself, is quite high. This implies that hydridic H(-) species created as a result of heterolytic dissociation might have a long enough lifetime at O vacancies that they will be available for methanol synthesis. ZnH and OH vibrational frequencies have been computed in order to assist future experimental assignments.

On the applicability of centroid and ring polymer path integral molecular dynamics for vibrational spectroscopy.

Centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD) are two conceptually distinct extensions of path integral molecular dynamics that are able to generate approximate quantum dynamics of complex molecular systems. Both methods can be used to compute quasiclassical time correlation functions which have direct application in molecular spectroscopy; in particular, to infrared spectroscopy via dipole autocorrelation functions. The performance of both methods for computing vibrational spectra of several simple but representative molecular model systems is investigated systematically as a function of temperature and isotopic substitution. In this context both CMD and RPMD feature intrinsic problems which are quantified and investigated in detail. Based on the obtained results guidelines for using CMD and RPMD to compute infrared spectra of molecular systems are provided.

Microsolvation of protonated methane: structures and energetics of CH5(+)(H2)n.

Effects of microsolvating CH(5)(+) with up to four H(2) molecules have been investigated in terms of structures and energies. For the smaller complexes, benchmark calculations have been carried out using MP2 and CCSD(T) with basis sets up to aug-cc-pV5Z quality and energies have been extrapolated to the infinite basis set limit. It is found that MP2 calculations using the aug-cc-pVQZ basis set or better yield robust reference data for both structures and energies. More than 30 stationary points including minima and first-order as well as second-order stationary points have been characterized by this method and are discussed in terms of solvation motifs. Finally, the performance of several density functionals has been assessed for this very demanding case. Popular GGA functionals such as BLYP and PBE fail, whereas the TPSS meta-GGA functional captures many structural and energetic aspects of microsolvation satisfactorily.

Cooperative reduction by Ln(2+) and Cp*(-) ions: synthesis and properties of Sm, Eu, and Yb complexes with 3,6-di-tert-butyl-o-benzoquinone.

The first examples of samarium, europium, and ytterbium complexes with 3,6-di-tert-butyl-o-benzoquinone(3,6-dbbq) in the form of catecholate have been obtained by reactions of the quinone with the corresponding lanthanocenes, [LnCp2*(thf)n] (n = 1 or 2) in solution. In the course of the reactions lanthanide ions lose one or two Cp* ligands, which take part in reduction of a quinone molecule into a catecholate anion (dbcat, 2(-)). As a result of the reactions, Sm and Yb clearly yield dimeric complexes[(LnCp*)2(dbcat)2], where each Ln ion loses one Cp* ligand. Eu forms a trimeric complex [(EuCp*)-(Eu·thf)2(dbcat)3], in which one Eu ion is coordinated by one Cp* ligand, while two Eu ions have lost all Cp* ligands and are coordinated by THF molecules instead. Magnetic properties corroborate the assignment of oxidation states made on the basis of single-crystal X-ray diffraction: all the quinone ligands are present in the catecholate state; both Sm/Yb ions in the dimers are in the +3 oxidation state, whereas the Eu trimer contains two Eu(II) and one Eu(III) ions. Cyclovoltammetry studies show the presence of two reversible oxidation waves for all complexes, presumably concerned with the redox transitions of the dbcat ligands.

Bidirectional photoswitching of magnetic properties at room temperature: ligand-driven light-induced valence tautomerism.

Valence tautomeric (VT) metal complexes are highly promising bistable molecular compounds for applications as molecular switches in molecular electronics and spintronics. Although VT species can be switched with light, the photoswitching in all reported systems requires very low temperatures (usually below 20 K) because photoinduced states are highly unstable at room temperature. The thermal instability hinders any practical application of these complexes in genuine devices. In this report, for the first time we demonstrate photoswitching of VT species and associated magnetic properties at room temperature. The bidirectional photoswitching in solution is due to cis-trans photoisomerizable 4-styrylpyridine ligands deliberately integrated into cobalt dioxolene molecular complexes. The novel type of photoswitching has been coined Ligand-Driven Light-Induced Valence Tautomerism (LD-LIVT). The photoconversion of VT states of 28% has been achieved in solution at room temperature. The photoinduced states show extraordinary thermal stability for hours at room temperature, as compared to common nanoseconds reported previously. The switching proceeds at molecular level with the effective photoswitching rate of 3 × 1013 molecules per s under our conditions. Consequently, this work may open new horizons in applications of molecular switches based on VT metal complexes in molecular devices functioning at room temperature.

Tetraanionic N2O2-coordinating ligands as potential building blocks for supramolecular magnetic networks.

A bisoxamate ligand containing three different types of coordination sites was designed and synthesized. The developed synthetic strategy was adopted to prepare a related 1,2-bis(2-hydroxybenzamido)benzene-derived ligand. Nickel(II) complexes of both the novel ligands were obtained and characterized by X-ray crystallography, NMR, electronic absorption spectroscopy, and theoretical calculations.

Quantum-induced symmetry breaking explains infrared spectra of CH(5)(+) isotopologues.

For decades, protonated methane, CH(5)(+), has provided new surprises and challenges for both experimentalists and theoreticians. This is because of the correlated large-amplitude motion of its five protons around the carbon nucleus, which leads to so-called hydrogen scrambling and causes a fluxional molecular structure. Here, the infrared spectra of all its H/D isotopologues have been measured using the 'Laser Induced Reactions' technique. Their shapes are found to be extremely dissimilar and depend strongly on the level of deuteration (only CD(5)(+) is similar to CH(5)(+)). All the spectra can be reproduced and assigned based on ab initio quantum simulations. The occupation of the topologically different sites by protons and deuterons is found to be strongly non-combinatorial and thus non-classical. This purely quantum-statistical effect implies a breaking of the classical symmetry of the site occupations induced by zero-point fluctuations, and this phenomenon is key to understanding the spectral changes studied here.

Autologous-allotopic ileum mucosa transplantation for small bowel elongation. A morphological study.

INTRODUCTION: Since a standard therapy for short bowel syndrome does not yet exist, every search for new surgical methods would be worthwhile. In previous studies we could show that autologous-allotopic ileum mucosa transplantation is feasible. After a modification of our technique a vascularized colon muscle coat lined completely with transplanted ileum mucosa could be engendered. METHOD: In 12 young beagles autologous ileum mucosa was transplanted in a demucosed vascularized transverse colon segment. The colon coat-ileum mucosa complex was anastomosed with the small bowel immediately after transplantation, 4 weeks later the animals were sacrificed and histology specimens were harvested from the colon coat-ileum mucosa complex, normal ileum and normal colon. After fixation in 2.5% glutaraldehyde the samples were frozen (-40 degrees C) and 14 micro m sections were stained with hemalaun and eosin. The lumen diameter, the mucosa, submucosa and colon muscle coat thickness, as well as the mucosal crypt depth were evaluated. RESULTS: The diameter of the colon coat-ileum mucosa-complex was smaller than the diameter of normal ileum and colon with no significant stenosis. There were no marked differences in thickness of mucosa and depth of the mucosal crypts compared to the controls, but the transplanted mucosa showed a slightly higher rate of shortened villi. The submucosal layer was thicker following transplantation and showed good neovascularization. The circular muscle layer of the transplanted colon coat was up to 178% thicker and the thickness of the transplanted longitudinal muscle layer differed between 58% and 143% in comparison to normal colon. CONCLUSIONS: Only a few histologic differences between transplanted and normal ileum mucosa could be observed after autologous-allotopic ileum mucosa transplantation. Therefore a nearly normal function of the colon coat-ileum mucosa complex has to be expected. Long term experiments of the histologic changes as well as further functional studies are on-going in order to finally apply autologous-allotopic ileum mucosa transplantation clinically.

Histologic changes in neuronal innervation of the ileum mucosa after autologic-allotopic ileum mucosa transplantation.

INTRODUCTION: After successful experimental autologic-allotopic ileum mucosa transplantation, we investigated the remodeling of the transplanted submucous and mucous plexus, which is essential for the motility of the created colon coat-ileum mucosa complex. METHOD: In 8 beagles we transplanted ileum mucosa in a demucosed vascularized transverse colon segment, which was reanastomosed with the small bowel immediately after transplantation. Four weeks later the animals were sacrificed and histology specimens taken from the anastomosis site of the colon coat-ileum mucosa complex, allowed comparison between transplanted and normal mucosa in the same section. After fixation in 4% formaldehyde and PBS the samples were embedded in paraffin and 7 micro m sections were prepared. The distribution of nerve fibers and submucous ganglia were examined in dewaxed sections, using antisera against protein gene product (PGP9.5), a general neuronal marker. RESULTS: The submucosal ganglia were prominent in all samples but they were smaller and the submucous nerve cells within the ganglia were less numerous compared to the controls. The innervation of the transplanted ileum mucosa was reduced as the number of nerve fibers in the mucosal villi was decreased. Besides these neuromorphologic changes the transplanted mucosa showed a slightly higher rate of shortened villi compared to normal ileum mucosa. CONCLUSIONS: After ileum mucosa transplantation the submucosal ganglia are smaller and less numerous. Furthermore there is a considerable loss of nerve fibers in the mucosal layer. Additionally a loss of microvilli in the transplanted ileum mucosa was found. Whether these findings represent a state of remodeling or a slow atrophy of the enteric nervous system in the transplanted areas is currently under investigation.