Tungsten isomeric forms

A toluene solution of La Cs2 and l,l,2,2-tetramesityl-l,2-disilirane (13) was photoirradiated with a tungsten halogen lamp (cutoff <400 nm) (equation 7)17. The MALDITOF mass spectrum of the product shows the presence of La Cg2(Mes2Si)2CH2 (21). The EPR spectrum of the adduct (21) in 1,2,4-trichlorobenzene shows the presence of at least two species, presumably two positionally isomeric forms of the disilirane... 

Methyl-3,5-diphenyl-l//-thiopyran 1-oxide forms red air-stable complexes with carbonyls of chromium, molybdenum and tungsten. (78CB1709). X-Ray diffraction measurements on these compounds confirm the non-planarity of the thiabenzene oxide nucleus and reinforce the evidence for high inversion barriers at sulfur. These complexes are found in isomeric forms with either the sulfur-oxygen bond axial to the half-chair conformation adopted by the ring, or the S- methyl group axial the complexes cannot be interconverted. 

Two crystal stmctures of dimolybdates with different cations have been described. Both are of the type of 36 (O Fig. 10) and could be considered a dihydroxyethyl derivative of the ery-thritol complex 14 (O Fig. 2), whereas D-mannitol is only triply deprotonated in 36 similarly to an eiythritol complex described above [24,41,42]. The structure of 36 (O Fig. 10) is retained in solution in two isomeric forms for molybdenum as well as for tungsten in a way similar to D-arabitol whose configuration matches the one of the coordination site of D-mannitol [25,26,37,43]. 

The heterometallacyclic alkyne-containing complex 85 (M = Mo or W, R = Pr or /7-MeQH4, R = Me or Ph) isomerizes into 86 above 20°C (90). At 55°C the tungsten compound with R being Pr and R Ph rearranges, with incorporation of the alkyne ligand into a metallacycle and extrusion of the thiolate from the chelate ring, to form 87. 

The photochemical cis-trans isomerization of bisphosphine-substituted carbyne complexes [Eq. (57), Section IV,B], which was studied in our group, was postulated to proceed via pentacoordinate metal ketenyl intermediates (97). Support for this postulate was found in the low-temperature characterization of ketenyl complex 208 on irradiation of complexes 207 in the presence of PMe, [Eq. (164)]. In the presence of phenylacetylene the ketenyl alkyne tungsten complex W(/ -PhCCO)CI(PhC2H)(CO)(PMe3)2 forms at low temperatures. 

Molybdenum(VI) and tungsten(VI) are found to form only one of two possible isomeric ery-r/zro-type dimetallates analogously to 14 (O Fig. 2) with fourfoldly deprotonated ribitol. One of the terminal hydroxyl functions is not part of the coordination site. Even the formed isomer proves to have low stability due to unfavorable steric interaction between the terminal carbon atoms of the sickle conformer [25,26,37]. 

The active constituent of the catalyst is an oxide of tungsten prepared by partial reduction of WO - Unsupported WOj was found to have activity but better specific activity is obtained by depositing the oxide on a support. A number of grades of alumina and silica were tested. Gamma alumina was found to react with WOj at 400°C to a considerable extent to form aluminium tungstate. This was identified by XPS and tests on AljfWO ) showed it to be inactive as a catalyst for the isomerization reaction. 

The activity for skeletal isomerization exhibited by these tungsten catalysts is developed only under specific conditions of treatment and operation. The source of tungsten oxide and the method of support are not critical. The HT-alumina favoured in these tests had the advantage of good dispersion of WO and limited loss by reaction with the support to form aluminium tungstate. 

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