Metal alkoxides models for metal oxides. 5. Coupling of alkyne ligands in reactions involving ditungsten hexaalkoxides: an alternative to the metathesis reaction M.tplbond.M + C.tplbond.C- .fwdarw. 2M.tplbond.C

Abstract

W2(0-Z-Bu)6 and W2(OR)6(py)2, where R = z'-Pr or CH2-z-Bu, react in hydrocarbon solvents at ambient temperatures with ethyne (>3 equiv) to give the compounds W2(0-Z-Bu)6(/i-C4H4) and W2(OR)6(g-C4H4)(C2H2), respectively. The same compounds are produced from reactions involving the ethyne adducts W2(0-r-Bu)6(py)(ju-C2H2) and W2(0-z'-Pr)6^-C2H2)(py)2 and ethyne, and an intermediate W2(OCH2-Z-Bu)6(M-C4H4)(py) has been isolated and shown to be isomorphous and isostructural with its previously characterized molybdenum analogue. The nature of the C-C coupling has been investigated by using the labeled ethynes 13C2H2 and 12C2D2 and found to occur without C-C or C-H rupture. Analogous reactions employing MeC^CMe lead to the alkylidyne complex (z-BuO)3W=CMe or W2(OR)6(^-C4Me4)(C2Me2) compounds, where R = z'-Pr and CH2-z-Bu. These new compounds provide models for the cyclotrimerization and/or polymerization of alkynes at a dimetal center, though the elimination of the hydrocarbon fragment does not readily occur from the ditunsten center. These observations are contrasted with earlier findings in the chemistry of Mo2(OR)6 compounds. Single-crystal X-ray studies reveal the connectivity (RO)3W(m-íj*,i;4-C4R,4)(OR)W(OR)2 (tz2-C2R,2) for the compounds where R = z'-Pr and R' = H and Me. One tungsten atom is in a distorted octahedral environment being coordinated to three terminal OR ligands, a bridging OR ligand, and forming two W-C a bonds (W-C = 2.13 (2) Á, averaged) to the g-C4R'4 ligand. This tungsten atom lies in a plane with the four carbon atoms of the m-C4 ligand. The other tungsten atom is coordinated to one bridging and two terminal OR ligands, the ?j2-alkyne (W-C = 2.07 (2) A, averaged, and with C-C-C angle =136 (3)°, averaged) and the four carbon atoms of the

Competitive carbon-carbon bond formation and cleavage and cluster formation in the reaction between 3-hexyne and hexaisopropoxyditungsten

“…Reactions between I and C2H2, 2 equiv (also in separate experiments * 1C2H2, 2 equiv, where *C represents 92 mol % 13C), show formation of W 2 (µ-C4Et2H 2) (r;2 3-C 2H 2) (0 -) -Pr)6 in which the carbon atoms of the ethyne are scrambled over all four sites of the µ-04 ring, as might have been anticipated for a reversible reaction, (6i), followed by either (6ii) or (7).…”

“…The proposed molecular structure for III was confirmed by a single-crystal X-ray study8 for the compound W2(0-i-Pr)6(p-C4Et2H2)(? ;2-C2H2) (see below).8 Reaction 5 is without precedent and could occur as a result of reaction sequences áhown in either (6) or (7).…”

“…NMR data for W3(M3-CEt)(p2-0-i-Pr)3(0-i-Pr)6 (22 °C, C6D6): 5(OCH(CH3)2) 1.48, 1.42, 1.28 (d, JHH = 6 Hz) in a 1:1:1 integral ratio; 5(OCH(CH3)2 5.01, 3.50 (sept, Jhh = 6 Hz) in a 2:1 integral ratio, respectively; 5(CCH2CH3) 1.81 (t, Jhh = 7 Hz); <¡(CCH2CH3) 4.09 (q with 1$3W satellites, JHH = 7 Hz, 3JW" = 6 Hz). (6) Chisholm, .…”

Possible surface intermediates in alkane reactions on metallic catalysts

“…On the other hand, in heterogeneous catalysis, carbyne recombination was postulated for the selective isomerization of methylpentanes on iridium catalysts7 before such species had been isolated. Finally, Schrock et al17 and Chisholm et al18,19 have seen evidence for this dicarbyne recombination reaction represented as 2M=CR ^(RC=CR) + M=M Many bimetallic complexes contain an acetylene bonded with the two metal atoms. 20 The most commonly observed mode of acetylene bonding is the tetrahedral M2C2 core.17 This type of reaction was also postulated on large platinum particles and on iridium catalysts for the selective dehydrocyclization of alkanes.5,7 At this stage it is not intended to propose new mechanisms, rather, it seems more appropriate to try to combine the various past proposals with the intention of improving our understanding of hydrocarbon reactions over metallic catalysts.…”

Metal-metal multiple bonds in dinuclear clusters

“…All of the isolable simple alkyne addition products are reactive toward further alkyne molecules to give new products in which two or more alkyne molecules have been coupled [163][164][165]. Two of these are shown in Fig.…”