Ruthenium , coordination sphere

More recently, Taqui Khan and co-workers (70) introduced the potentially tetradentate ethylenediaminetetraacetate ligand in the ruthenium coordination sphere in order to obtain an efficient water-soluble catalyst precursor. Indeed, starting from the ruthenium(III) aquo EDTA species [Ru(EDTA)(H20)] , carbonylation gives the paramagnetic carbonyl complex [Ru(EDTA)(CO)] which is able to induce the heterolytic activation of dihydrogen (Scheme 3). The hydroformylation of hex-l-ene performed at 50 bar (CO/H2= 1/1) and 130°C in a 80/20 ethanol-water solvent... 

These observations may be rationalized by assuming that the polar functional group coordinates to the metal center in one or more intermediates along the RCM pathway (Scheme 17) [30b]. Such a Lewis-acid/Lewis-base interaction may assemble the reacting sites within the coordination sphere of the ruthenium and hence provide internal bias for cyclization (e.g. structure I). However, if such an... 

For example, the substituted aniline Ar-NH2 (Ar = />-CH3OC6H4) reacts with the ruthenium nitrosyl complex Ru(bpy)2(Cl)(NO)2+ (bpy = 2,2 -bipyridine) to give a complex of the diazo ligand, namely Ru(bpy)2(Cl)(NNAr)2+ (57). Upon employing the 15N labeled nitrosyl complex Ru(bpy)2Cl(15NO)2+ this reaction resulted in the 15N coordinated product, Ru(bpy)2Cl(15NNAr)2+, demonstrating that the reaction occurs within the metal complex coordination sphere. When the reactions were conducted in non-protic solvents, these nucleophile-nitrosyl adducts could be isolated. 

Having established that there were no significant structural perturbations in the coordination spheres of the ruthenium centers in the polymer films we investigated the effect of oxidation of the ruthenium to the 3+ state. This was performed in acetonitrile/0.1M TBAP by holding the potential at +1.6 V for 5 minutes to ensure oxidation of the film. A change in the color of... 

Ways of changing the coordination sphere and properties of osmium and ruthenium complexes (see, for example, Ref. (183)) are much broader... 

Fig. 12.14 Stereoviews <4 the inner coordination spheres about the central ruthenium atom in the Orange (top) and violet (bottom) isomers of [(QH,> J,RiiftCF,bCiS2 -(CD). [From Bernal, I. Clearfield. A Rica. 1. S.. Jr J. Cryst. Mvt. Struct. 1974.4, 43-54. Reproduced with perrmssion.J...

The kinetics of intramolecular electron transfer from Ru(II) to Fe(III) in ruthenium-modified cytochrome c has been studied [77-80]. In these studies electron transfer from electron-excited Ru(II) (bpy)3, which was added to the protein solution, to ruthenium-modified horse heart cytochrome c, (NH3)5Ru(III) (His-33)cyt(Fe(III)), was found to produce (NH3)5Ru(II) (His-33)cyt (Fe(III)) in fivefold excess to (NH3)5Ru(III) (His-33)cyt(Fe(II)). As in refs. 72 and 73, in the presence of EDTA the (NH3)5Ru(II)(His-33)cyt(Fe(III)) decays mainly by intramolecular electron transfer to (NH3)5Ru(III)(His-33)cyt(Fe(II)). The rate constant k — 30 3s 1 at 296 K and does not vary substantially over the temperature range 273-353 K. Above 353 K intramolecular Ru(II) - Fe(III) electron transfer was not observed owing to the displacement of methionine-80 from the iron coordination sphere. The distance of intramolecular electron transfer in this case is also equal to 11.8 A (see Fig. 19). 

An interesting compound is (N-phenyloctaethylporphyrinato) phenyl-ruthenium(II) (entry 10) in which an agostic hydrogen of the N-phenyl group completes the heavily distored octahedral coordination sphere of the Ru(II) ion. The Ru ion protrudes by 14 pm towards the axial phenyl anion. 

Reaction of ruthenium cyclopentadienyl bisacetonitrile carbene (22) with electron-poor acetylenes yields the allylcarbene (23).28 As evidenced from DFT calculations, the reaction is likely to proceed by NHC insertion into the ruthenium-carbene bond in the metallacyclopentatriene (24). An unexpected reaction of NHC in the coordination sphere of a metal has been disclosed.29 In this example, NHC is not coordinating the metal but is linked to a phosphorus atom with a shift of the carbene centre as shown in (25). 

For both reactivity and regioselectivity, however, a compromise must be found between the bulkiness of the reagents (alkyne and carboxylic acid) and the steric hindrance of the diphosphine ligand, all of which are present in the coordination sphere of the ruthenium center during the catalytic process. Thus, with the more bulky trimethylsilylacetylene, the less hindered bis(diphenylphosphino)ethane ligand provides an efficient ruthenium catalyst (Ru(methallyl)2(dppe)) for producing silylated enol esters. Better reactivity is also observed with Ru(methal-lyl)2(dppe) as catalyst precursor when propargylic ethers are used as acetylenic substrates (Scheme 5) [10]. 

It is, thus, important that the ruthenium(II) complexes that are to be used as building blocks of the future machines contain sterically hindering chelates so as to force the coordination sphere of the metal to be distorted from the perfect octahedral geometry. We will discuss the photochemical reactivity of rotaxanes and catenanes of this family as well as non-interlocking systems like scorpionates since the lability of bulky monodentate ligands could also lead to useful photosubstitution reactions. 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

In the first step, the precursor, typically a ruthenium or osmium bis(2,2,-bipyridyl) (bpy) complex, reacts with solvent (S) to produce a solvated complex. When solvents such as dry methanol and ethanol are used, only one chloride is exchanged and the species [Ru(bpy)2(PVP) Cl]+ is obtained as the sole product. The nature of the coordination sphere around the metal center can be determined by UV-visible (UV/Vis) spectroscopy (Xmax, 496 nm) and by its redox potential, (about 0.65 V (vs. SCE), depending on the electrolyte being used). By a systematic variation of the ratio of monomer units to redox-active centers, the loading of the polymer backbone ( n) can be varied systematically. (Here, n stands for the number of monomer units in the polymer per redox-active center, e.g. in a PVP-based, n = 10 polymer, there are 10 pyridine units for every redox center. 

Fig. 5.4 depicts some results obtained in the first stages (high nuclearity complexes formation) of the synthesis in xylene solvent which leads to the formation of nanostructured powders, RuxSey, from tris-ruthenium dodeca-carbonyl (Ru3(CO)i2) and elemental selenium dissolved in an organic solvent (xylene). After 40 minutes of reaction, l3C-NMR spectrum (Fig. 5.4 (c)) puts in evidence the formation of a new polynuclear chemical precursor with a chemical shift 8 of 198.89 ppm (i.e., Ru4Se2(CO)n)- Selenium takes part in the coordination sphere. The peak intensity with the chemical shift of 199.67 ppm, corresponds to the initial chemical precursor which decreases as a function of the synthesis reaction time (Fig. 5.4(a)). Other chemical shifts (with minor peak intensities) on both sides of the 13C-NMR spectrum, which put in evidence the complex interplay of the reaction, are also observed. 

A variety of NHC-ruthenium catalysts exhibiting different coordination spheres were synthesized, for example Schiff base (see complex 28) [77], arene (see complex 29) [78], and metallic moieties (see complexes 30) [7,79, 80] (Fig. 5) and show activity in RCM and ROMP. 

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