| Popis: |
There are a very small number of structurally-characterized sandwich compounds of the type M2L2 where M = Mo, W and L = a (CH2)n carbocyclic ring. This is in contrast to the large number of mononuclear compounds of the same type. This interesting disparity may be a result of insufficient synthetic attempts to make such compounds rather than some inherent instability of the M2(carbocycle)2 species. A new family of half-sandwich complexes of ditungsten alkoxides is defined as structures having the formula W2(carbocycle)(OR)4. Attempts to make such compounds via the alcoholysis of 1,2-W2Cp2(NMe2)4 were unsuccessful. 1,2-W2Cp2(NMe2)4 shows a unique resistance among dimethylamide compounds of tungsten towards alcoholysis with the typical aliphatic alcohols used in such reactions. Strongly acidic alcohols, such as CF3CH2OH, are required to react with this dimethylamide. These reactions lead to over-alcoholysis products such as 1,1 W2Cp(η3-C5H7)(OCH2CF3)6. Unlike W2Cp2(NMe2)4, W2COT(NMe2)4 undergoes facile alcoholysis to form [W2COT(OR)4]n species where n = 1 for R = tert butyl, isopropyl, and neopentyl and n = 2 for R = Me, Et, and Pr. The dinuclear alkoxides exhibit a fluxional COT ligand which rotates about the W-W bond via a 1,2-alkyl exchange mechanism. Two strong and symmetrical W-OR-W alkoxide bridges between two W2COT(OR)3 fragments are found within the tetranuclear compounds. Several attempts to make sandwich compounds of ditungsten of stoichiometry W2COT2 were unsuccessful. Reactions of K2COT with quadruply-bonded W2Cl4(PR3)4 species lead to ill-defined species. Reactions of the Mo2(O2CCH3)4 paddlewheel compound with two equivalents of K2COT″ where COT″ = 1,4-bis-trimethylsilyl-1,3,5,7-cyclooctatetraene resulted in the formation of M2COT″3 compounds, despite the use of a bulky cyclooctatetraene ligand. Attempts to react the W4(OPr)16 cluster with two equivalents of Li2COT lead to reduction of the cluster and formation of two equivalents of Li2W2(OPr)8. DFT computational studies are used to explain the instability of W2COT2 sandwich compounds having both D8h and D2h symmetry. The same studies predict that (M2COT2)2+ species might be quite stable. DFT computational studies of W2(benzene)2 sandwich compounds indicate that the W-W bridging, antifacial D2h structure is more stable than the D8h nonbridged and D2h synfacial structures. W2(benzene)2 compounds are expected to be stable. A suitable synthetic route to these compounds might be the reaction of metal atoms with immobilized arenes or M(arene)2 compounds. |
