1,3-Ditungstacyclobutadienes. 1. Reactions with Alkynes. Alkyne Adducts and 1,3-Dimetallaallyl Derivatives

Malcolm H. Chisholm, John C. Huffman, Joseph A. Heppert

Research output: Contribution to journalArticle

42 Scopus citations

Abstract

[(Me3SiCH2)2W(μ-CSiMe3)]2, 1, reacts in hydrocarbon solvents with i-PrOH to give [(i-PrO)2W(μ-CSiMe3)]2, 2, with elimination of Me4Si. The molecular structure of 2 is closely related to that of 1 and contains a central O4W2C2 unit having virutal D2h symmetry. Pertinent averaged bond distances (angstroms) and angles (degrees) are W-W = 2.62 (1), W-O = 1.85 (1), W-C = 1.90 (5), O-W-O = 115 (1), W-C-W = 86 (1), and C-W-C = 94 (1). Compound 1 reacts in hydrocarbon solutions with alkynes, R1C≡CR2, where R1 = R2 = H, Me, and Ph and R1= H and R2 = Me, Ph, and Me3Si to give products of insertion of the alkyne into one of the bridging alkylidyne ligands, W2(CH2SiMe3)4(μ-CSiMe3)(μ-C3R1R2SiMe3). In all cases the reaction proceeds via initial formation of an alkyne adduct which is an intermediate in the insertion reaction. Both the rate of alkyne adduct formation and the rate of insertion are similarly affected by steric factors kRC=CH> JRC==CR. For reactions involving MeC≡CMe and PhC≡CMe, the alkyne adducts have been isolated and characterized. The solid-state molecular structure of W2(CH2SiMe3)4(μ-CSiMe3)2(PhCCMe) allows an interpretation of alkyne adduct formation based on a formal oxidative addition wherein the pair of electrons of the M-M bond in 1 is transferred to the alkyne which forms a metallacyclopropene unit: W-W = 2.915 (3) A, W-C = 2.00 (5) A, and C-C = 1.30 (5) A. One tungsten atom is in a pseudotetrahedral geometry forming two W-C single bonds to the CH2SiMe3 ligands, W-C = 2.12 (2) A, and two W-C double bonds, W-C = 1.77 (4) A (averaged), to the μ-CSiMe3 ligands. The other tungsten atom is in a pseudo-trigonal-bipyramidal geometry if the η2-alkyne is considered to occupy a single site. The two W-CH2SiMe3 ligands, W-C = 2.16 (2) A (averaged), and one W-μ-CSiMe3 ligand, W-C = 2.15 (3) A, occupy equatorial sites and the η2-alkyne and the other μ-CSiMe3 ligand, W-C = 2.19 (4) A, the axial positions. The other alkyne adducts show NMR spectra interpretable in terms of the adoption of similar structures in solution, and the 13C NMR data for the adduct involving *C2H2, where *C represents 92.5 atom % 13C, reveal that the alkyne is acting as a four-electron donor: δ(η2-C2H2) = 216.8 ppm, J183W-13c = 33 and J13C_13C = 42 Hz. Collectively the NMR data indicate that rotation about the alkyne-W vector is rapid on the NMR time scale. A qualitative MO description is presented which accounts for these observations. Studies of the insertion reaction show that the alkyne unit inserts in an intact manner into the μ-CSiMe3 ligand. The migratory insertion reaction is equivalent to a cycloaddition or a ring expansion reaction. Subsequent to the insertion step, rearrangements can occur, scrambling the CRl, CR2, and CSiMe3 positions within the μ-C3 ligand. The reaction between 2 and C2H2 proceeds similarly via formation of an ethyne adduct, detectable only at low temperatures (-60 °C), and insertion to give W2(O-i-Pr)4(μ-CSiMe3)(μ-CHCHSiMe3) which upon heating to +80 °C generates the thermodynamic isomer having a μ-CHCSiMe3CH ligand. The reaction between 2 and Me3SiO=CH generates initially the W2(O-i-Pr)4(μ-CSiMe3)(μ-CSiMe3CHCSiMe3) compound which subsequently rearranges to the isomer containing the μ-CHCSiMe3CSiMe3 ligand. It is proposed that the insertion step always involves C-C bond formation between the μ-CSiMe3 ligand and the least sterically encumbered carbon atom of the alkyne, but in most instances this is not detected because of subsequent facile site exchange within the μ-C3 ligand. Rate studies involving the alkyne adducts of 1 and Me3SiG≡CH and MeC=CMe show ΔH* = 15.2 ± 1.2 and 20.5 ± 0.7 kcal mol-1, respectively. The entropies of activation are negative and of medium magnitude, ΔS* = -13 ± 3 eu (MeCCMe) and -15 ± 5 eu (Me3SiCCH), implying a highly ordered transition state. The μ-C3 ligand may be viewed as a dimetallated allyl, a 3- ligand, which taken together with the presence of the μ-CSiMe3 ligand, 3-, and four uninegative ligands, CH2SiMe3 or O-i-Pr, leads to a (W-W)10+ center. The structural characterizations of W2(CH2SiMe3)4(μ-CSiMe3)(μ-CPhCPhCSiMe3) and W2(O-i-Pr)4(μ-CSiMe3)(μ-CHCHCSiMe3) reveal W-W distances of 2.548 (1) and 2.658 (1) A, respectively, consistent with the presence of a W-W bond. In both molecules, one tungsten atom is contained within the μ-C3 plane of the allyl ligand while the other tungsten atom is π-bonded. The molecules may be viewed as the sum of two fragments: a d° L/L2W(C3R3) metallacyclobutadiene unit is π-bonded to a d2 L2L'W, center, where L = CH2SiMe3 or O-i-Pr and L' = μ-CSiMe3. In each molecule, there is a roughly planar X2WWX2 unit, where X = C or O, and the W2(μ-C) plane of the alkylidyne ligand is at right angles to the former. Variable-temperature NMR studies reveal a very low-energy process wherein the μ-C3R3 ligand flips back and forth between the two tungsten centers. This makes the two tungsten centers equivalent on the NMR time scale, and only for W2(O-i-Pr)4(μ-CSiMe3)(μ-CHCHCSiMe3) has this been frozen out at low temperatures. The second energy process that is sometimes observed to be rapid on the NMR time scale involves a scrambling of CR sites within the μ-C3R3 ligand. This does not involve a reversible deinsertion process. A general scheme for the scrambling of CR positions within the μ-C3R3 ligand is proposed based on the reversible formation of a W2(μ-η12-cyclopropene) intermediate by formation of a C-C bond between the a and a' carbon atoms of the allyl moiety. By a sequence of 60 °C rotations followed by ring openings, all possible isomers of the μ-CR1CR2CR3 ligand are accessible. These findings are discussed briefly in the context of related findings involving mono-, di-, and trinuclear compounds. The summary of crystal data is as follows: for (i) W2(O-/-Pr)4(μ-CSiMe3)2 at -69 °C, a = 15.335 (4)Å, b = 11.515 (3) Å, c = 10.652 (3) Å, β= 122.52 (1)°, γ = 99.96 (1)°, Z = 2, dcalcd = 1.659 g cm-3, and space group P1; (ii) for W2(CH2SiMe3)4(μ-CSiMe3)22-PhCCMe) at -157 °C, a = 20.322 (13) Å, b = 12.344 (6) Å, c = 17.936 (10) Å, = 91.16 (3)°, Z = 4, dcalcd = 1.487 g cm-3, and space group P21/a; for W2(CH2SiMe3)4(μ-CSiMe3)(μ-CPhCPhCSiMe3) at -160 °C, a = 20.884 (9) A, b = 13.734 (5) A, c = 16.842 (6) A, Z = 4, dcalcd= 1.465 g cm-3, and space group P212121; for W2(O-i-Pr)4(μ-CSiMe3)(μ-CHCHCSiMe3) at -158 °C, a = 17.827 (7) Å, b = 17.872 (8) Å, c = 10.025 (3) Å, a = 106.07 (2)°, Z = 4, dcalcd = 1.732 g cm-3, and space group P21/c.

Original languageEnglish
Pages (from-to)5116-5136
Number of pages21
JournalJournal of the American Chemical Society
Volume107
Issue number18
DOIs
StatePublished - Sep 1985

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