Complex Re

The chemistry of technetium(II) and rhenium(II) is meagre and mainly confined to arsine and phosphine complexes. The best known of these are [MCl2(diars)2], obtained by reduction with hypophosphite and Sn respectively from the corresponding Tc and Re complexes, and in which the low oxidation state is presumably stabilized by n donation to the ligands. This oxidation state, however, is really best typified by manganese for which it is the most thoroughly studied and, in aqueous solution, by far the most... 

Re Complexes for Treating Painful Bone Cancers - Re-HEDP, MDP... 

In the bifunctional radiopharmaceuticals, Tc and Re complexes that are suitable for coupling to antibodies are almost square-pyramidal oxo complexes of tetradentate ligands, as mentioned earlier. Spies et al. [65] recently studied the syntheses and structures of rhenium complexes with a tetradentate NS3 tripod ligand 2, 2,, 2"-nitrilotris(ethanethiol) (l in Fig. 9) as an interesting alternative to the oxo complex. Such ligands could lead to weakly polar, trigonal-bipyramidal complexes in which the metal atom is more strongly shielded than in the square-pyramidal oxo complexes. Schemes for the syntheses... 

From the results described above, C02 seems to be reduced to CO in the presence of Re complexes in most cases, while RH complexes give formic acid and Ru complexes give both formic acid and CO, depending on the conditions. 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

With the low-valence iron pentacarbonyl, vinylcyclopropanes 22 are thermally transformed to the diene re-complexes, the (1,3-trans-pentadiene)iron carbonyl complexes 23, through bond fission, 1,2-hydrogen shift, and stereoselective coordination [15]. (Scheme 9)... 

Dicyclopropylethylene 29 does not behave as a six 7t-electron ligand, producing the diene re-complex 31 via ring opening and carbonylation together with the similar -complex 30 as mentioned above [16]. The complex 30 is not a precursor to 31. (Scheme 10)... 

Only molecular S7 and Se7 are firmly known, yet only the former has been characterized by X-ray analysis. However, Re complexes of Se7 are known55,56 and the neutral Se7 homocycle cocrystallized with salts of polyselenides.57,58 S7+ is thought to be present in S02 solutions of sulfur cations however, this is an unproven but likely hypothesis. Other than that, only the structures of Sig2+ and Se172+ contain E7 homocycles (see there). 

The reversibly formed re-complexes precursor to the hydration of olefins have the proton imbedded in the re-electron cloud of the double bond somewhere between the two carbon atoms. They are therefore... 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

The shift correlates in magnitude with the separation of each particular group distance-wise from the aromatic moiety of the substrate or product this points to the formation of an intermediate -complex, for which the rate of formation and the rate of decay can be determined. The 1H-PHIP-NMR spectrum, as well as the anticipated intermediate product-catalyst-re-complex observed during the hydrogenation of styrene, is outlined in Figure 12.19. 

Fig. 12.19 The H-PHIP-NMR spectrum and the anticipated intermediate product-catalyst-re-complex observed during the hydrogenation of styrene.

Table 12.3 Rates of formation and of decay of the interim product-catalyst-re-complexes [44].

Furthermore, the qualitative influence of substituents on the symmetry and electronic structure of the substrate and its hydrogenation product on the efficiency of the transfer of polarization to the 13C-nuclei have been discussed, as well as the feasibility of a polarization transfer to other heteronuclei. Evidence in the form of a shift of the aromatic 13C resonances has been found for an initial attachment of hydrogenation products containing aromatic segments to the metal center of the cationic hydrogenation catalyst - probably in the form of a re-complex. 

In addition to the observed polarization transfer, attachment of the hydrogenated product to the catalyst - most likely in the form of a re-complex between the aromatic portion of a product and the cationic catalyst - has also been observed in the 13C-PHIP-NMR spectra. The associated larger shift range of the affected 13C will make it possible to characterize the nature of this attachment as well as the associated binding energies of the hydrogenation product to the catalysts metal center more precisely and effectively. 

AT-Alkyl methanesulfonamides, 23 685 Alkyl monoperoxycarbonic acid, 18 466 Alkylnaphthalenes, 17 84-85 dispersant moieties, 8 706t Alkyl naphthalene sulfonates, 24 146 Alkylnickel, re-complexes of, 17 116 Alkyl nitrites, formation of, 17 165-166 Alkylonium salt hydrates, 14 171... 

We performed a computational study [69] to assess which interaction (H bonding, metal-alcoholate formation, or metal-alcohol coordination between the allylic hydoxyl moiety and the Re complex) affects the TS and to determine which oxygen of the Re peroxo moiety acts as H-bond acceptor in the case of an H-bonded TS. A summary of the results with propenol as model allylic alchohol is presented in the following. 

The metal-alcoholate mechanism is well established for allylic alcohol epoxidation in the presence of Ti and V catalysts. [41, 51, 52, 111-113], In principle, it can provide a viable pathway also for catalysis by a Re complex. In fact, allylic alcohols may add, at least formally, to either an oxo-Re or peroxo-Re moiety (e.g. of 5a or 5b) in a process which is referred to as metal-alcoholate binding this mechanism gives rise to metal-alcoholate intermediates. We identified four intermediates of alcohol addition to di(peroxo) complexes two resulting transition states, S-8 and S-9b, are shown in Figure 11. All metal-alcoholate intermediates he significantly higher in energy (by 10-22 kcal/mol) than 5b + propenol, except the... 

Figure 12. Syn transition structures 10-12 for the epoxidation of propenol by the unsoivated di(peroxo) Re complex 5 and the mono(peroxo) Re complex 4. Bond lengths in A.

With regard to the competition between mono(peroxo) and di(peroxo) Re complexes, our calculation confirm the deduction from experimental kinetics that the epoxidation rate of allylic alcohols by the mono(peroxo) Re complex (e.g. TS 12) is negligible compared to that by the di(peroxo) complex [69]. It is interesting to note that this observation is unique and does not hold for any other substrate studied so far. 

The computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

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