Dedicated to Professor Christoph Elschenbroich on the occasion of his 70th birthday Spin crossover and valence tautomerism are examples of processes that can be utilized as a basis for achieving molecular switches. [1] Whereas the spin-crossover process is characterized by a temperature-, pressure-, or light-induced change of the electronic state of the metal ion to one with a different spin multiplicity, [2] valence tautomerism entails an intramolecular redox reaction between a metal ion and a coordinated ligand, which, in a few instances, is accompanied by a change in the spin state of the metal ion. [3] Various reported low-spin cobalt(III) catecholate complexes, which can be transformed into high-spin cobalt(II) semiquinonate complexes by raising the temperature, provide excellent examples of the latter process. In contrast, spin-crossover chemistry is dominated by octahedral iron(II) complexes with a FeN 6 coordination sphere; [2] however, there are only very few known octahedral cobalt(II)-containing spin-crossover complexes. [4] Herein we describe the first cobalt(II) semiquinonate complex that displays spin-crossover properties rather than valence tautomerism.The starting point of our investigation was the olive-green cobalt(III) 3,5-di-tert-butylcatecholate (dbc 2À ) complex Me 2 )(dbc)](BPh 4 )·0.8 MeCN·0.2 Et 2 O (1) containing the dimethyl derivative of the tetraazamacrocyclic ligand 2,11diaza[3.3](2,6)pyridinophane (L-N 4 Me 2 ) as coligand. This complex was obtained in 42 % yield by oxidation of the red cobalt(II) catecholate complex Me 2 )(dbc)] (prepared in situ from equimolar solutions of cobalt(II) perchlorate, L-N 4 Me 2 , and 3,5-di-tert-butylcatecholate) with ferrocenium tetrafluoroborate ([Fe(Cp) 2 ](BF 4 ); Cp = cyclopentadienyl), followed by a metathesis reaction with sodium tetraphenylborate (Scheme 1). In accordance with the description of 1 as a cobalt(III) catecholate complex, solutions and solids of this substance are diamagnetic. X-ray structure analysis of 1 also supports this assignment. [6] Figure 1 shows a perspective view of the complex cation in 1. Because of the small size of the macrocyclic ring, the coordinated ligand L-N 4 Me 2 is folded along the N amine -N amine axis, thereby rendering a distorted cis-octahedral coordina-Scheme 1. Preparation of compounds 1 and 2. Figure 1. Perspective view of the complex cation in 1 showing 50 % thermal ellipsoids; selected bond lengths []:

β-Asarone (1) belongs to the group of naturally occurring phenylpropenes like eugenol or anethole. Compound 1 is found in several plants, e.g., Acorus calamus or Asarum europaeum. Compound 1-containing plant materials and essential oils thereof are used to flavor foods and alcoholic beverages and as ingredients of many drugs in traditional phytomedicines. Although 1 has been claimed to have several positive pharmacological effects, it was found to be genotoxic and carcinogenic in rodents (liver and small intestine). The mechanism of action of carcinogenic allylic phenylpropenes consists of the metabolic activation via cytochrome P450 enzymes and sulfotransferases. In vivo experiments suggested that this pathway does not play a major role in the carcinogenicity of the propenylic compound 1 as is the case for other propenylic compounds, e.g., anethole. Since the metabolic pathways of 1 have not been investigated and its carcinogenic mode of action is unknown, we investigated the metabolism of 1 in liver microsomes of rats, bovines, porcines, and humans using (1)H NMR, HPLC-DAD, and LC-ESI-MS/MS techniques. We synthesized the majority of identified metabolites which were used as reference compounds for the quantification and final verification of metabolites. Microsomal epoxidation of the side chain of 1 presumably yielded (Z)-asarone-1',2'-epoxide (8a) which instantly was hydrolyzed to the corresponding erythro- and threo-configurated diols (9b, 9a) and the ketone 2,4,5-trimethoxyphenylacetone (13). This was the main metabolic pathway in the metabolism of 1 in all investigated liver microsomes. Hydroxylation of the side chain of 1 led to the formation of three alcohols at total yields of less than 30%: 1'-hydroxyasarone (2), (E)- and (Z)-3'-hydroxyasarone (4 and 6), with 6 being the mainly formed alcohol and 2 being detectable only in liver microsomes of Aroclor 1254-pretreated rats. Small amounts of 4 and 6 were further oxidized to the corresponding carbonyl compounds (E)- and (Z)-3'-oxoasarone (5, 7). 1'-Oxoasarone (3) was probably also formed in incubations with 1 but was not detectable, possibly due to its rapid reaction with nucleophiles. Eventually, three mono-O-demethylated metabolites of 1 were detected in minor concentrations. The time course of metabolite formation and determined kinetic parameters show little species-specific differences in the microsomal metabolism of 1. Furthermore, the kinetic parameters imply a very low dependence of the pattern of metabolite formation from substrate concentration. In human liver microsomes, 71-75% of 1 will be metabolized via epoxidation, 21-15% via hydroxylation (and further oxidation), and 8-10% via demethylation at lower as well as higher concentrations of 1, respectively (relative values). On the basis of our results, we hypothesize that the genotoxic epoxides of 1 are the ultimate carcinogens formed from 1.

Ru(CHdCHFc)Cl(CO)(P i Pr 3) 2 (Fc = ferrocenyl, (η 5-C 5 H 4)Fe(η 5-C 5 H 5)), 1, has been prepared by hydroruthenation of ethynylferrocene and characterized by NMR, IR, ESI-MS, and Moessbauer spectroscopy and by X-ray crystallography. Complex 1 features conjoined ferrocene and (vinyl)ruthenium redox sites and undergoes two consecutive reversible oxidations. Pure samples of crystalline, monooxidized 1 •+ have been prepared by chemical oxidation of 1 with the ferrocenium ion. Structural comparison with 1 reveals an increase of Fe-C and Fe-Cp centr. bond lengths and ring tilting of the Cp decks, as is typical of ferrocenium ions, but also a discernible lengthening of the Ru-C(CO) and Ru-P bonds and a shortening of the Ru-C(vinyl) bond upon oxidation. This supports the general idea of charge delocalization over both redox sites in 1 •+. Band shifts of the charge-sensitive IR labels (ν(CO) for Ru, ν(C-H, Cp) for Fc), the rather small g-anisotropy in the ESR spectrum of 1 •+ , and the results of quantum chemical calculations indicate that in solution the positive charge partly resides on the vinyl ruthenium moiety. Comparison of IR shifts in the solid state and in solution and the quadrupole splitting in the Moessbauer spectrum of powdered 1 •+ point to a larger extent of charge localization on the ferrocenyl site in solid samples. This is probably due to CH 3 3 3 F hydrogen bonding interactions between the cyclopentadienyl hydrogen atoms of the radical cations and the PF 6 counterions. Monooxidized 1 •+ displays low-energy electronic absorption bands at 1370 and 2150 nm. According to quantum chemical calculations, the underlying transitions are largely localized on the ferrocene part of the molecule with only little charge transfer into the vinyl ruthenium subunit. The second oxidation is more biased toward the (vinyl)ruthenium site.

A combination of spectroscopic and electrochemical methods--XANES, EXAFS, X-ray, (1)H NMR, EPR, Mössbauer, and cyclic voltammetry--demonstrate that the most efficient Pd catalysts for the asymmetric rearrangement of allylic trifluoroacetimidates unexpectedly possess in the activated oxidized form a Pd(III) center bound to a ferrocene core which remains unchanged (Fe(II)) during the oxidative activation. These are the first recognized Pd(III) complexes acting as enantioselective catalysts.

Dedicated to Professor Christoph Elschenbroich on the occasion of his 70th birthdaySpin crossover and valence tautomerism are examples of processes that can be utilized as a basis for achieving molecular switches.[1] Whereas the spin-crossover process is characterized by a temperature-, pressure-, or light-induced change of the electronic state of the metal ion to one with a different spin multiplicity, [2] valence tautomerism entails an intramolecular redox reaction between a metal ion and a coordinated ligand, which, in a few instances, is accompanied by a change in the spin state of the metal ion.[3] Various reported low-spin cobalt(III) catecholate complexes, which can be transformed into high-spin cobalt(II) semiquinonate complexes by raising the temperature, provide excellent examples of the latter process. In contrast, spin-crossover chemistry is dominated by octahedral iron(II) complexes with a FeN 6 coordination sphere; [2] however, there are only very few known octahedral cobalt(II)-containing spin-crossover complexes.[4] Herein we describe the first cobalt(II) semiquinonate complex that displays spin-crossover properties rather than valence tautomerism.The starting point of our investigation was the olive-green cobalt(III) 3,5-di-tert-butylcatecholate (dbc 2À ) complex [Co-(L-N 4 Me 2 )(dbc)](BPh 4 )·0.8 MeCN·0.2 Et 2 O (1) containing the dimethyl derivative of the tetraazamacrocyclic ligand 2,11-diaza[3.3](2,6)pyridinophane (L-N 4 Me 2 ) as coligand. This complex was obtained in 42 % yield by oxidation of the red cobalt(II) catecholate complex Me 2 )(dbc)] (prepared in situ from equimolar solutions of cobalt(II) perchlorate, L-N 4 Me 2 , and 3,5-di-tert-butylcatecholate) with ferrocenium tetrafluoroborate ([Fe(Cp) 2 ](BF 4 ); Cp = cyclopentadienyl), followed by a metathesis reaction with sodium tetraphenylborate (Scheme 1). In accordance with the description of 1 as a cobalt(III) catecholate complex, solutions and solids of this substance are diamagnetic. X-ray structure analysis of 1 also supports this assignment.[6] Figure 1 shows a perspective view of the complex cation in 1. Because of the small size of the macrocyclic ring, the coordinated ligand L-N 4 Me 2 is folded along the N amine -N amine axis, thereby rendering a distorted cis-octahedral coordina-

The influence of a coordinated π-radical on the spin crossover properties of an octahedral iron(II) complex was investigated by preparing and isolating the iron(II) complex containing the tetradentate N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane and the radical anion of N,N'-diphenyl-acenaphtene-1,2-diimine as ligands. This spin crossover complex was obtained by a reduction of the corresponding low-spin iron(II) complex with the neutral diimine ligand, demonstrating that the reduction of the strong π-acceptor ligand is accompanied by a decrease in the ligand field strength. Characterization of the iron(II) radical complex by structural, magnetochemical, and spectroscopic methods revealed that spin crossover equilibrium occurs above 240 K between an S=1/2 ground state and an S=3/2 excited spin state. The possible origins of the fast spin interconversion observed for this complex are discussed.

A missing link: A superoxovanadium(V) complex is the first reaction intermediate in the oxidative conversion of a peroxovanadium(V) complex into a vanadyl(IV) complex and molecular oxygen (see scheme). The superoxo species appears also to play an essential role in the formation of the peroxovanadium(V) complex from the vanadyl(IV) complex and molecular oxygen.

A macrocyclic pseudopeptide 3 is described featuring three amide groups and three 1,4-disubstituted 1,2,3-triazole units along the ring. This pseudopeptide was designed such that the amide NH groups and the triazole CH groups converge toward the cavity, thus creating an environment well suited for anion recognition. Conformational studies in solution combined with X-ray crystallography confirmed this preorganisation. Solubility of 3 restricted binding studies to organic media such as 5 vol% DMSO/acetone or DMSO/water mixtures with a water content up to 5 vol%. These binding studies demonstrated that 3 binds to a variety of inorganic anions in DMSO/acetone including chloride, nitrate, sulfate, and dihydrogenphosphate anions. In the more competitive DMSO/water mixtures, only affinity to the more strongly coordinating oxoanions is retained. Quantitative binding studies showed that dihydrogen phosphate complexation in DMSO/water involves the dimer of the HPO anion. By contrast, sulfate and hydrogenpyrophosphate complexation involves a stepwise process comprising formation of a 1 : 1 complex followed by a 2 : 1 complex in which two molecules of 3 (R) bind to a single anion (A). While the second binding equilibrium is associated with a much smaller stability constant in comparison with the first one in the case of sulfate complexation, the two binding constants are of similar magnitude in the case of the hydrogenpyrophosphate anion. Formation of the 2 : 1 complex was attributed to the fact that the cavity size and rigidity of 3 prevents saturation of all hydrogen acceptor sites on the anionic guests.