Robust complexes

The trick is to meet EuroVI at affordable cost, while retaining the fuel economy and drivability attributes that are keys to Diesel s acceptance in the marketplace. Now the question is what is the best way to meet it A variety of options must be considered cost, robustness, complexity and application constraints. As 2007 begins, car manufacturers and suppliers are scrambling to make sure that, whatever they commit to in production, it will not be more costly than what the other companies devise. Over the past few years and for a long time, Diesel clean-up has been a very competitive and strategic field. 

The bora-2,5-cyclohexadienes 25, 43, and 44 have been used in experimental studies. Photochemical reaction of 25 and 43 with Fe(CO)s produces the robust complexes 45 (29) and 46 (92) with 7)5-divinylborane structures (92), whereas thermally induced complex formation of 44 with Fe2(CO)9 is accompanied by ligand isomerization and affords complex 47 (67). We note in passing that S(UB) = 38.8 ppm for 47 is at rather low field. The only strictly comparable boraolefin known is l-methoxy-6-(trimethylsilyl)bora-2,4-cyclohexadiene [8(nB) = 47.1 ppm] (26). On this basis, the high field shift upon complexation (only 8.3 ppm) indicates weak Fe-B interaction. 

C, high pressure CO), which necessitate extremely robust complexes for prolonged use as catalysts. 

Based on these results, it became clear that there was indeed a need for new, more robust complexes capable of withstanding the higher temperatures at which it was beheved the alkane dehydrogenation process would occur more efficiently. Consequently, the chemistry of pincer compounds appeared on the scene. In 1976,... 

In contrast to stability, lability and inertness are kinetic quantities. A labile (or robust) complex is quite reactive an inert entity is quite slow to react. A criterion has been suggested by Taube (41) "labile is applied to those systems where reactions are complete within time of mixing—i.e., within 1 minute at room temperature "inert is applied to systems in which reactions are too slow to measure or occur at measurable (by conventional techniques) rates. 

Further modification of the benzilidene moiety of 78 by isopropoxystyrene gives a highly robust complex 79, which can be easily recovered by silica gel chromatography after RCM [70]. A similar phosphine-free recyclable complex with a biaryl moiety 80 proved to be more active than 76 and 79 the relative reactivity order is 79<78<80 [71]. 

In many cases, very robust complexes are formed, being insensitive to heat, air and moisture. 

Wide polarity retention control Uniform sensitivity independent of retention Flexible control oi Rs and 4 All components eluted Larger injection volumes Methods not robust Complex method development Long cycle time Shorter column lifetime Method transfer problems... 

In the case of the octahedral robust complexes of cobalt (III) and chro-mium(III), substitution in the first sphere is hindered. This type of complex ion is, therefore, especially suitable for studying association in the second sphere. The hexammine and tris(ethylenediamine) cobalt(III) ions have especially been used for this kind of study. For the association of these ions with anions, such as sulfate and thiosulfate, the ion-pair constant is of the order of magnitude of 10 at 7 = 0, somewhat smaller for Coena" than for Co(NH3)6 21)y but strongly dependent on the ionic strength. Thus Posey and Taube 37) y from spectrophotometric measurements in the ultraviolet, obtain the following expression for the association constant of the ion pair [Co(NH3)6]S04 in solutions with y/Jvarying from 0.04 to 0.3 ... 

These ruthenium complexes react rapidly and quantitatively with ethyl vinyl ether to form a Fischer carbene that is only weakly metathesis active at elevated temperatures [86, 87]. This property can be employed to end-cap ROMP and ADMET polymers and to ensure that there are no polymeric ruthenium alkyhdenes present. Since ruthenium alkylidenes are relatively robust complexes they could survive workup procedures, although experimental evidence has yet to confirm this notion. Treatment of an ADMET polymer with ethyl vinyl ether gives the polymer well-defined terminal olefinic endgroups and should prevent backbiting metathesis upon dilution of the polymer (Scheme 6.22). 

In all these examples, both enzymic and model systems, it can be seen that the role of the metal ions is an active one. They do not merely hold the pieces together. Moreover, the model studies, especially on the preformed robust complexes, give clear indications on how the metal ion activates the substrate, provides a potent nucleophile at neutral pH, and helps the leaving group depart. 

In contrast, the carbene complexes 15 and 16 could be used very efficiently in pure liquid or supercritical CO2, giving high yields of polynorbomenamer with very similar cis trans ratios of the double bonds to those obtained with the same catalyst in CH2CI2 [6]. More recent studies showed that the robust complex 16 can be used at substrate to catalyst ratios up to 5300 without deactivation [100]. Fairly broad molecular weight distributions were observed when 16 and norbomene were placed together in the reactor, but narrow distributions could be achieved if the catalyst was injected as a solid or in solution to the supercritical mixture of alkene and CO2. The polymer morphology was very similar to samples prepared with 16 under conventional conditions. 

Some very active molybdenum carbene initiators are listed in Table 2.2. A very convenient way of generating the highly active complex Mo(=CHCMe3) (=NAr)(OR)2(Me3CNH2) in situ is to react the very robust complex Mo(=NAr) (=NCMe3)(CH2CMe3)2 with ROH in the presence of the olefinic substrate (Bell, A. 1994a). 

With more robust complexes spectral changes are more likely to result from solvent effects, rather than from ligand exchange. For example the observed shifts in the absorption maxima of spectra of neodymium acetylacetone and neodymium benzoylacetone complexes in methanol-water mixtures were due to solvent effects. ... 

Spirogermanium 11.50), a newly introduced robust complex, seems useful in treating tumours of the colon and ovary. It is one of the few carcinolytic agents that do not attack the bone-marrow (myelosuppression). Large doses show toxic effects on the central nervous system (Slavik etal., 1982). 

Kunze and coworkers have shown that the Co(lll)-coordinated cyanide group of vitamin B12 bridges to rheniiun(l) and technetium(l) centers, to yield robust complexes with the central structural feature Co - CN - Re(Tc) (12) [39]. This concept allows the direct labelUng of vitamin B12 (1) with [99mjj.(OH2)(L )(CO)3] complexes for radiodiagnosis or with rhenium as a mediator between 1 and additional biomolecules. 

A study of the optical rotatory dispersion and of circular dichroism of tris((+)j)-2,3-bn) complexes become particularly interesting in that rotatory strengths of complexes of known absolute configuration with centtal ions not forming "robust complexes such as Ni(II) and Cu(II) may thus be obtained for the first time. An example is shown in fig. 2. 

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