Rhenium alkoxides

This section is actually devoted to the description of rhenium alkoxides, as the technetium ones are rarely studied. The latter are reviewed in a short appendix at the end. The chemistry ofrhenium in lower oxidation states is much alike that of ruthenium and therefore even for rhenium, the lower oxidation state complexes with jt-acceptor ligands are described in this chapter. 

The main route to rhenium alkoxides is the interaction of halids and oxy-halids with alkali alkoxides or alcohols in presence of amines (method 5). As the important starting reagents can serve also Re2(CO),0 and Re207 (method 3) The preparation ofrhenium (V) and (VI) oxoderivatives by the anodic oxidation of metal in alcohols has also been described (method 2) (see Table 12.22). The bimetallic alkoxides ofrhenium and heavy transition metals can most efficiently be obtained by interaction ofrhenium (VII) oxide with the alkoxides ofthese elements in refluxing toluene ... 

The other feature that enriches but also complicates the chemistry of rhenium alkoxides is their ability to use different kinds of decomposition processes, not leading to the changes in the oxidation state. That is first and foremost the ether elimination from the rhenium (V-VII) derivatives. It leads to the formation of oxocomplexes, as, for example [168] ... 

The epimerization of secondary alcohols catalyzed by rhenium alkoxide complexes proceeds via the C-H bond activation the first step of the reaction is the hydrogen atom migration from alcohol to the metal ion (Scheme lV-16) [30],... 

Hevia E, Perez J, Riera L, et al. Insertion of imsaturated organic electrophiles into molybdenum-akoxide and rhenium-alkoxide bonds of neutral, stable carbonyl complexes. Chem Eur J. 2002 8 4510-4521. 

R.D. Simpson, R.G. Bergman - Synthesis, Structure, and Exchange-Reactions of Rhenium Alkoxide and Aryloxide Complexes. Evidence for Both Proton and H-Atom Transfer in the Exchange Transition State, Organometallics 12,781,1993. 

Rhenium(VI) complexes, 4,194 alkoxides, 4,196 amides, 4,194 amines, 4,199 carboxylates, 4,199 dimethylformamide, 4,199 dioxane, 4,198 halides, 4,195,199 2-hydroxypyridine, 4,199 imides, 4,194 magnetic behavior, 1,271 mixed sulfur-nitrogen compounds, 4,196 N heterocycles, 4,199 nitrides, 4,194 oxide halides, 4, 195 oxoanions,4,196 pyridine, 4,199 sulfates, 4,198 sulfur compounds, 4,196 tellurates, 4,198... 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

We could show that the modification of transition metal alkoxides is a versatile tool to adjust the reactivity of precursors for the needs in lyotropic crystalline templating processes. In case of high surfactant concentrations where the liquid crystalline template is formed prior to the addition of the precursor the use of a modifier may become unnecessary. The synthesis of nanostructured rhenium dioxide and the utilization of MTO as precursor for this purpose clearly shows that in some cases the use of unusual specialized compounds is imperative. First promising results in the synthesis of nanostructured chromium oxide surfactant composites have been displayed although hydrolysis of the precursor seems to be still uncompleted within the nanostructure. The possibility of tailoring the d-values in a desired way besides the synthesis of certain particle morphologies encourages for further work in the future. 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

Processes of this kind are very characteristic of the alkoxides of rhenium [168], molybdenum, and to a minor extent in tungsten of all the Mo(OR)6... 

The complex formation of tungsten alkoxides with the alkoxides of other metals than the alkali, bismuth, and rhenium (see Section 12.21) has not been investigated in detail, and therefore the major attention below will be paid to the description of the bimetallic alkoxides of molybdenum. 

The observed E0 values in this case indicate the highest stability of rhenium (V) derivatives in this series. The comparison of the preparative data published indicates that these are the +5 and +6 oxidation states that appear to be most stable for the alkoxocomplexes of rhenium. The low-valent (+1 — +3) complexes should either be stabilized by JT-acceptor ligands (CO, PR3, NO, unsaturated hydrocarbons) or contain multiple M M bonds [321, 586, 729, 762,]. The compounds of rhenium (VII) are very unstable and decompose at room temperature in several minutes when isolated. They can be isolated and kept for several days as the complexes with N-donor ligands such as Tmeda or Py [533, 519, 1358]. The decomposition products of rhenium (VI) and (VIT) alkoxides are often described in literature as a black tar. The compound with this kind of appearence turned to be the major product of the anodic oxidation of rhenium in methanol (at high current density) and was shown by the X-ray single crystal study to be Re402(0Me)16 [906]. 

In addition to the bimetallic complexes of rhenium and alkaline metals formed as byproducts in the exchange reactions of rhenium halids with alkali alkoxides (such as, for example, LiReO(OPr )5 xLiCl(THF)2 [519]) there has been recently prepared a number ofbimetallic complexes ofrhenium and molybdenum, rhenium and tungsten, and rhenium and niobium [904, 1451]. The latter are formed either due to the formation of a metal-metal bond, arising due to combination of a free electron pair on rhenium (V) and a vacant orbital of molybdenum (VI) atom or via insertion of molybdenum or tungsten atoms into the molecular structure characteristic of rhenium (V and VI) oxoalkox-ides. The formation of the compounds with variable composition becomes possible in the latter case. 

S. Cai, D. M. Hoffman, and D. A. Wierda, Alkoxide and Thiolate Rhenium(VII) Oxo-Alkyl Complexes and Re205(CH2CMe3)4, a Compound with a [02Re—O—Re02]4+ Core, Inorg. Chem. 28, 3784-3786 (1989). 

The red rhenium(I) complex [Re(CO)4(phen)]Co(CO)4(compound A) on warming to 90°C affords the compound (phen)(CO)sRe-Co(CO)4 which contains a rhenium to cobalt bond. The irradiation of a THF solution of the latter material gives the yellow binuclear compound [Re(CO)3(phen)]2 (443, 444). Compound A (above) undergoes the followuag reversible reaction with various alkoxide species RO (446) ... 

Rhenium(VI) complexes, 194 alkoxides, 196 amides, 194 amines, 199 carboxylates, 199 dimethylformamide, 199 dioxane, 198 halides, 195,199 2-hydroxypyridine, 199 imides, 194... 

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