The synthesis of isomerically pure multicentred species is an important task for preparative chemists. Only stereochemically well-defined structures of polynuclear metal complexes lead to photophysical results that are clearly interpretable. We have developed several strategies to synthesise such compounds that have a predetermined chirality (Δ- or Λ-helix) at each metal center. The approximate local symmetry of the described metal complexes is D3 (octahedral, tris-bidentate). The methods are the following: (a) resolving a racemic building block followed by substitution of the two labile monodentate ligands by a bidentate diimine ligand under total retention of configuration. (b) Synthesis of an optically pure precursor complex in which two chelating bipyridine ligands are bridged and each contain an optically active pinene unit. The remaining coordination sites are then replaced by a bidentate diimine ligand. (c) Use of ligands that have sterically demanding substituents perpendicular to the molecular plane. The use of such ligands leads direct to optically active compounds. Examples of these methods will be given, while considering their photophysical application.

In the crystal structure of the title complex, [Pt(C9H6NS)(2)], although the aromatic ligands are coordinated to a central heavy metal stem. T-shaped and shifted pi-stacked arrangements of the aromatic moieties are preferred, leading to a sandwich herring-bone type of crystal-packing motif. The crystal structure is therefore consistent with the view that the arene-arene interactions are determined by electrostatics (multipole-multipole).

Chiroptical absorption and luminescence spectra are reported for a novel Os(II)-polypyridyl complex in which Os(II) is coordinated to one achiral bidentate ligand and a chiral quadridentate ligand. The achiral ligand is 4,4'-dimethyl-2,2'-bipyridine (DMbpy), and the chiral ligand is a bisbipyridine-type molecule formed by linking two optically active (-)-[4,5]-pineno-2,2'-bipyridine molecules together via a (CH2)(6) chain bridge. This chiral ligand is one of a family of ligands often referred to as chiragens, and here it is denoted by CG[6], where [6] identifies the number of carbon atoms in the chain bridge. The coordination core of the complex has a three-bladed propeller-like structure formed by three Os(II)-bipyridyl chelate rings. This tris(bipyridyl)Os(II) core structure has approximate trigonal-dihedral (D-3) symmetry, and the propeller-like assembly of its three chelate rings has a right-handed screw sense (or helicity). The overall complex, denoted by Delta-[Os(CG[6])(DMbpy)](2+), has C-2 point-group symmetry, with a 2-fold symmetry axis that bisects both the CG[6] and DMbpy ligands and coincides with one of the digonal (C-2) axes of the tris(bipyridyl)Os(II) core structure. The overall structural chirality of the complex is dictated by the inherent chirality of the CG[6] ligand, which in turn is dictated by the absolute configuration about each of the six stereogenic carbon atoms in the CG[6] ligand structure. The chiroptical absorption and luminescence measurements performed in this study were carried out on solution samples of Delta-[Os(CG[6])(DMbpy)](PF6)(2) dissolved in acetonitrile. These measurements yielded circular dichroism and absorption dissymmetry factor data over the 12 500-30 000 cm(-1) spectral range, and circularly polarized luminescence and emission dissymmetry factor data over the 11 100-16 000 cm(-1) spectral region. Comparisons between the absorption and emission dissymmetry factor data indicate chirality-related structural differences between the ground and emitting states of Delta-[Os(CG[6])(DMbpy)](2+). Both the unpolarized and circularly polarized absorption spectra of Delta-[Os(CG[6])(DMbpy)](2+) are qualitatively similar to the corresponding spectra reported for Delta-[Os(bpy)(3)](2+) (in acetonitrile) throughout the 12 500-30 300 cm(-1) spectral region. However, there are significant quantitative differences between the spectra observed for these two complexes, and these differences are most apparent in the absorption dissymmetry factor data. This indicates that ligand structure outside the tris(bipyridyl)Os(II) coordination core of Delta-[Os(CG[6])(DMbpy)](2+) exerts a significant influence on the chiroptical absorption properties of this complex over the 12 500-30 300 cm(-1) spectral region, although all the optical absorption processes that occur within this spectral region may be assigned to one-electron d-pi*-type metal-to-ligand charge-transfer (MLCT) transitions localized within the tris (bipyridyl)Os(II) chelate structure. It appears that the noncoordinated chiral moieties of the CG[6] ligand exert a direct perturbative influence on both the dipole strengths and rotatory strengths of the MLCT transitions in Delta-[Os(CG[6])(DMbpy)](2+).

cis-Bis-homoleptic platinum(II) complexes, with predetermined helical chirality at the metal center, can be obtained by using strongly sterically interacting ligands. With this aim, two new ligands, (8R,10R)-2-(2'-thienyl)-4,5-pinenopyridine, th4,5ppy (2), and (8R,10R)-2-(2'-thienyl)-5,6-pinenopyridine, th5,6ppy (4), were synthesized and coordinated to platinum. The structures of the resulting complexes, Pt(th4,5ppy)(2) (5) and Pt(th5,6ppy)(2) (6), were determined by X-ray diffraction, and it was found that they both crystallize with a Delta-cis configuration. Thermal oxidative additions (TOA) of alkyl halides were performed with both complexes leading, in the case of 5, to a mixture of isomers and, in the case of 6, to isomerically pure products. The predetermination of chirality at the metal center is therefore preserved in the octahedral (OC-6) platinum(IV) complexes. Crystals of Pt(th4,5ppy)(2) (5) are orthorhombic, of space group P2(1)2(1)2(1), with a = 12.973(1) Å, b = 13.619(2) Å, c = 17.665(2) Å, alpha = beta = gamma = 90 degrees, and Z = 4. Final R = 0.0268 and R(w) = 0.0424 for 3101 observed reflections. Crystals of Pt(th5,6ppy)(2) (6) are hexagonal, of space group P6(1), with a = 11.5465(4) Å, b = 11.5465(4) Å, c = 35.356(3) Å, alpha = beta = 90 degrees, gamma = 120 degrees, and Z = 6. Final R = 0.0424 and R(w) = 0.0845 for 2660 observed reflections. Neither molecule possesses a crystallographic C(2) symmetry.

The two enantiomers (Delta and Lambda) of Ru(chiragen[X])Cl-2, where ''chiragen'' is a tetradentate ligand with a chiral bridging unit between two bipyridine moieties, have been prepared in high yields. X is m-xylyl (m-xyl); other bridging groups (e.g., -(CH2)(5)- or -(CH2)(6)-) behave similarly. This complex can be used as an enantiomerically pure building block for the synthesis of stereochemically well defined polynuclear species. As an example, all three isomers (Delta Delta, Lambda Lambda, and Delta Lambda) of [(chiragen[m-xyl])Ru(bpym)Ru(chiragen[m-xyl])](PF6)(4) were prepared and fully characterized by NMR and various other spectroscopic methods.

The (E,E)-1,3-bis[2-(2,2′-bipyridine-5-yl)ethenyl]adamantane (BAB) and (E,E)-3,3′-bis[2-(2,2′-bipyridine-5-yl)ethenyl]-1,1′-biadamantane (BAAB) bridging ligands made of two 2,2′-bipyridine groups (B) separated by spacers containing one and two adamantane (A) units have been synthesized. Their dinuclear complexes [(bpy)2Ru(BAB)Ru(bpy)2]4+ (RuII.BAB.RuII), [(bpy)2Os-(BAB)Os(bpY)2]4+ (OsII.BAB.OsII), [(bpy)2Ru(BAB)O9(bPY)2]4+ (ROSAB.Osn), [(bPY)2Ru(BAAB)-Ru(bpy)2]4+ (RuII.BAAB.RuII), and [(bpy)2Os(BAAB)Os(bpy)2]4+ (OsII.BAAB.OsII) have been prepared as PF6− salts. In these novel compounds each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. In the homodinuclear complexes also the luminescence properties of each unit are unaffected. In the mixed metal RuII.BAB.OsII complex electronic energy transfer takes place from the Ru-based to the Os-based unit with rate constant 5.8 × 108 s−1 at room temperature, whereas at 77 K energy transfer takes place through two distinct processes with rate constants 1.4 × 108 S−1 and 2.0 × 107 s−1, presumably because of the presence of two conformers (or families of conformers). Partial oxidation of the binuclear compounds OsII.BAB.OsII, RuII.BAB.OsII, and OsII.BAAB.OsII by CeIV in acetonitrile/water solutions leads to the mixed-valence OsII.BAB.OsIII, RuII.SAB.OsIII, and OsII.BAAB.OsIII species where the oxidized metal-based unit quenches by electron transfer the luminescent excited state of the unit that is not oxidized. At room temperature the rate constants for the excited state ∗OsII.BAB.OsIII → OsIII.BAB.OsII, ∗OsII.BAAB.OsIII → OsIII.BAAB.OsII processes are 4.0 × 109 s−1 and 8.8 × 108 si, respectively. For RuII.BAB.OsIII, the rate constants for the excited state ∗RuII.BAB.OsIII → RuIII.BAB.OsII process is 2.8 × 109 s−1 and the rate constant for the back electron transfer process RuIII.BAB.OsII → RuII.BAB.OsIII is 4.0 × 107 s−1.

Pd(2-thpy)(2) isolated in protonated or deuterated frozen n-octane (Shpol'skii matrices) exhibits highly resolved triplet emission and excitation spectra. One observes interesting differences for the two matrices: (i) The protonated matrix shows only one dominant guest site while the deuterated matrix exhibits two dominant sites. (ii) Low-energy satellites corresponding to lattice modes are distinctly shifted to lower energy due to deuteration of the matrix. (iii) 1.3 K the triplet sublevels emit independently with lifetimes being nearly equal for both matrices. However, for 1.3 < T < 5 K one observes obvious differences in the decay behavior. This is explained by substantially smaller rates of spin-lattice relaxation in the deuterated host. Different mechanisms of spin-lattice relaxation are discussed.

The cyclometalated Pt(2-thpy)(2) complex with thpy(-) as the deprotonated form of 2-(2-thienyl)pyridine shows highly resolved phosphorescence and triplet excitation spectra at low temperatures when the complex is isolated in Shpol'skii matrices, as is shown for the first time. Sharp-line Shpol'skii spectra were obtained by dissolving Pt(2-thpy)(2) in n-hexane, n-heptane, n-octane, n-nonane, and n-decane matrices. The highest resolution was reached using n-octane. In this matrix only one dominant site governs the spectra. The lowest electronic origins lie at 17 156 (I), 17 163 (II), and 17 172 cm(-1) (III) (+/-1 cm(-1)). They represent triplet sublevels that are split by the relatively large zero-field splitting of 16 cm(-1). These sublevels are assigned as pi-pi* ligand-centered (LC) with an appreciable metal-to-ligand charge transfer (MLCT) admixture. The emission from the lowest triplet sublevel \I] to the ground state \0] (origin line I) is strongly forbidden (emission lifetime at T = 1.3 K: 110 mu s), but due to vibronic (Herzberg-Teller) coupling, additional radiative deactivation paths are opened and thus a large number of ''false origins'' occur. The emission and excitation spectra corresponding to the sublevels \II] and \III] show relatively strong origin lines due to direct spin-orbit coupling. Thus, one observes a large number of vibrational satellites of the Franck-Condon type and combinations. A comparison of the highly resolved vibrational satellite structures allows one to conclude that the emitting tripIet state (all three sublevels) and the singlet ground state exhibit very similar force constants and nuclear equilibrium positions. Interestingly, a comparison to the properties of the homologous Pd(2-thpy)(2) (with triplets exhibiting only a very small MLCT or d-d* contribution) indicates that with increasing MLCT admixture the discussed distortions become less pronounced. Thus, an increase of MLCT character leads to a more pronounced covalency in the involved states.

Time-resolved phosphorescence spectra from the lowest electronic triplet of Pd(2-thpy)(2) (with 2-thpy(-) = ortho-C-deprotonated form of 2-(2-thienyl)pyridine) (see the inset of Figure 2) are presented. The complex was isolated in a Shpol'skii matrix to obtain high resolution. The emitting triplet lies at 18 418 +/- 1 cm(-1) (electronic origin). Its zero-field splitting is less than 1 cm(-1) and could not be resolved optically. However, at 1.3 K, when the spin-lattice relaxation is slow compared to the emission lifetimes of the sublevels (130, 235, 1200 mu s), the individual sublevels emit independently. Thus, by time-resolved spectroscopy it is possible to separate a fast-decaying emission spectrum from a slow-decaying one. A highlight of this investigation is that these spectra exhibit different vibrational satellite structures. This shows that different spin-orbit coupling mechanisms (direct spin-orbit coupling and Herzberg-Teller coupling) govern the radiative deactivation of the sublevels. In particular, it is found that specific vibrational modes couple very selectively to individual sublevels. For example, the 528 cm(-1) mode couples only to the slow-decaying sublevel. Thus, these optically well resolvable vibrational satellites display directly properties of the individual sublevels, which are unresolvable by conventional optical spectroscopy. This effect is observed for the first time for transition metal complexes.

We report the synthesis and the properties of some dinuclear ruthenium and/or osmium complexes bridged by rigid ligands. We focus on the nature of the bridging ligand and on its effect on the electrochemical, structural, and photophysical behavior of the complexes. Intercomponent energy and electron transfer processes are discussed.

The chemical and structural properties of chromia on titania have been investigated after different pretreatments and after use for the low temperature selective catalytic reduction (SCR) of NO by NH3. A series of catalysts containing 0.5 to 30 wt% chromia was prepared by impregnation with aqueous chromium nitrate solution. The catalysts exhibited high activity for SCR in the low temperature range (T < 470 K). The NO conversion rate was proportional to the chromia loading up to 5 wt%. Higher loadings resulted in a less efficient use of the chromia species. Selectivities to N2 and N2O were not influenced by the amount of deposited chromia at higher loadings (≥5 wt%), but depended markedly on the pretreatment of the catalysts. Oxidative pretreatment at 573 K for 3 h led to enhanced formation of undesired N2O, whereas previous exposure to a H2-containing atmosphere at T < 720 K yielded selectivities of N2 exceeding 90%. The overall conversion of NO was comparatively little influenced by these pretreatments. Thermal analysis (TG, DTA) and temperature-programmed reduction indicated the formation of Cr(III) species upon reduction in the H2-containing atmosphere at 720 K. These species were reoxidizable in air at 670 K and probably highly hydroxylated, as indicated by the substantial formation of water during the thermoanalytical run. Exposure to higher temperatures (>720 K) in an inert or a hydrogen-containing atmosphere led to the formation of less active α-Cr2O3 which was not reoxidizable under similar conditions. Electron paramagnetic resonance (EPR) measurements of catalysts exposed to SCR conditions indicated uniformly sized clusters of Cr(III) together with Cr(V) surface species. After oxidative pretreatment and SCR use, EPR revealed the existence of Cr(I) species, probably present as {CrNO}2+ surface complexes. Such species were not observed after reductive pretreatment of the catalysts.

A general account is given of the synthetic research carried out in the field of coordination chemistry in one group in the Institute of Inorganic Chemistry in Fribourg. With the goal of synthesizing artificial structures, which can function as photochemical molecular devices, several synthetic strategies for multicenter coordination species are mentioned. It is reported, that the molecules, which have been designed to form coordination compounds with predefined chiralities of the central metal, can also be used as ligands in enantioselective catalysis.

An account of the strategies for the synthesis of coordination compounds with interesting photophysical and photochemical properties is given. Ru(II)-diimine complexes have been developed with various absorption and emission characteristics and for special chemical stability of the excited state species. The isomer problem, encountered in the synthesis of polynuclear complexes, which are candidates for Photochemical Molecular Devices (PMD), is attacked by using enantiomerically pure chiral building blocks or chiragen ligands, which form metal complexes with predetermined helical chirality. Cyclometallating ligands with Pt(II) and Pd(II) can be designed, so that the photochemically induced oxidative addition reactions lead to compounds of special interest, especially from the stereochemical point of view.

Alfred Werner conjectured as early as 1899 that octahedrally coordinated metal complexes should occur in nonidentical mirror image isomers. For such objects, Lord Kelvin, in 1893, had coined the adjective "chiral", a term never used by Werner. It can be proved by examination of the original sample of [Co(NO2)(2)(en)(2)]Br, prepared by Edith Humphrey, a Ph.D. student of Werner's, that crystals of optically pure samples were obtained in Werner's laboratory as early as 1899 or 1900. However, Werner did not publish the first successful resolution of an octahedral metal complex until 1911. Presently, interest in chirality in coordination compounds is booming, mainly because of the importance of coordination compounds in enantioselective homogeneous catalysis. Other interesting applications are enantioselective interactions of chiral coordination species with biomolecules, and the stereoselective synthesis of multicenter systems.

The homo- and hetero-nuclear complexes of Ru(II) and Os(II) containing a bis(bipyridine) rigid bridging ligand (A) have been prepared. Their absorption spectra, electrochemical behavior and luminescence properties have been investigated. Luminescence intensity and lifetime measurements indicate that in the mixed-metal complex [(bpy)2RuAOs(bpy)2]4+ efficient energy transfer takes place from the Ru-based component to the Os-based one.

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The tris(bipyridine) tripod ligands 1,3,5-tris[4-(((2,2'-bipyridyl-5-yl)carbonyl)benzylamino)methyl]benzene (1), 1,3,5-tris[4-(((2,2'-bipyridyl-5-ylcarbonyl)benzylamino)methyl)phenyl]benzene (2), and 1,3,5-tris[4-((((2,2'-bipyridyl-5-ylcarbonyl)benzylamino)methyl)phenylyl)phenyl]benzene (3) have been synthesized and their complexes 1[Ru(bpy)2]2+, 1[Ru(bpy)2]2(4+), 1[Ru(bpy)2]3(6+), 1[Os(bpy)2]3(6+), 1[Ru(bpy)2]2[Os(bpy)2]6+, 2[Ru(bpy)2]3(6+), 2[Os-bpy)2]3(6+), 2[Ru(bpy)2]2[Os(bpy)2]6+, and 3[Ru(bpy)2]3(6+) have been prepared. All the complexes display very intense, ligand centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer bands in the visible. Electrochemical oxidation of each Ru(II) or Os(II) metal center occurs always at the same potential (+1.30 V for Ru(II), +0.87 V for Os(II)), regardless of tripod ligand and number and type of metal-based units that are present in the supramolecular structure. The five homometallic Ru(II) species exhibit the same luminescence properties, and this is also the case for the two homometallic Os(II) species. The luminescence data obtained for the two mixed-metal species show that electronic energy transfer takes place from the Ru-based to the Os-based components. The efficiency of energy transfer decreases in going from 1[Ru(bpy)2]2[Os(bpy)2]6+ to 2[Ru(bpy)2]2[Os(bpy)2]6+, i.e., as the size of the spacer which links the three arms of the bridging ligand increases. Oxidation of 1[Ru(bpy)2]3(6+), 2[Ru(bpy)2]3(6+), 1[Os(bpy)2]3(6+), and 2[Os(bpy)2]3(6+) by Ce(IV) leads to mixed-valence species where the oxidized metal-based units quench the luminescent excited state of the units that are not oxidized. The quenching efficiency decreases as the size of the spacer increases. The mechanisms of the quenching processes are discussed. The results obtained indicate that at least in the case of the trinuclear species of 2, there are conformers in which the quenching can take place very rapidly and conformers where it does not take place at all.