116563-30-5Relevant academic research and scientific papers
Tungsten-carbon, carbon-hydrogen, and carbon-carbon bond activation in the chemistry of 1,2-W2R2(OR′)4(W=W) complexes. 1. Alkyne-promoted metal-to-metal alkyl migrations, W=W/C=C bond metathesis, and metallacyclopropene formation
Chisholm, Malcolm H.,Eichhorn, Bryan W.,Folting, Kirsten,Huffman, John C.
, p. 49 - 66 (2008/10/08)
Compounds of formula 1,2-W2R2(O-i-Pr)4(W≡W) (1) (where R = Me, Et, CH2Ph, CH2SiMe3, n-Pr, i-Bu, C6H5, and C6H4-p-Me) react with internal acetylenes (MeC≡CMe, MeC≡CEt, and EtC≡CEt) in hydrocarbon solvents to form bis(alkyne) adducts of formula W2R2(R′CCR″)2(O-i-Pr)4 (2) in good yields. Similarly 1,2-W2(C6H4-p-Me)2(O-t-Bu) 4 reacts with MeC≡CMe (2 equiv) to form W2(C6H4-p-Me)2(MeCCMe) 2(O-t-Bu)4 (2′). Compounds 2 and 2′ were characterized by 1H NMR, 13C NMR, IR, and electronic absorption spectroscopy and elemental analysis. Single-crystal X-ray diffraction studies of W2(C6H5)2(MeCCMe) 2(O-i-Pr)4 and W2(CH2Ph)2(MeCCMe)2(O-i-Pr) 4 revealed puckered W2(μ-O-i-Pr)2 cores with M-M distances of 2.66 (1) and 2.67 (1) A?, respectively. The coordinated alkyne ligands display short W-C distances (2.05 (2) A?) and long C-C distances (1.31 (2) A?) and are best viewed as metallacyclopropene units. The unusual molecular geometries of compounds 2 and 2′ are attributed to optimization of M-L π bonding at the expense of M-M bonding based on the findings of a qualitative molecular orbital analysis. The barriers to alkyne rotation about the metal-alkyne vectors (ΔG≠) were calculated to be ca. 9.6 kcal mol-1 for compound 2 and 14.4 kcal mol-1 for compound 2′. The barrier to rotation about the W-C(Ar) bond (ΔG≠) in 2 (R = C6H4-p-Me, R′ = R″ = Me) was calculated to be 7.5 (3) kcal mol-1 at -130°C indicative of no significant metal-to-arene π* back-bonding. Ethyne, terminal and diphenylacetylenes do not react with compounds 1 apparently due to the poorer basicity of these alkynes relative to MeC≡CMe, MeC≡CEt, and EtC≡CEt. The Mo analogues of 1 do not form analogous bis (alkyne) compounds 2 but act as alkyne polymerization catalysts. The lack of reactivity in the Mo system is attributed to a mismatch of orbital energies between the acetylene π* orbitals and the filled Mo d orbitals. 1,2-W2(CH2-t-Bu)2(O-i-Pr)4 reacts with EtC≡CEt to form [W(≡CEt)(CH2-t-Bu)(O-i-Pr)2]2 (4) in a 64% isolated yield. This reaction represents a W≡W/C≡C bond metathesis that overrides the formation of compounds 2 when the steric demands of the system become sufficiently large. An X-ray diffraction study of 4 revealed a dimeric molecular structure in which the two TBP metal atoms are fused along a common axial-equatorial edge by virtue μ-O-i-Pr bridges with a W≡C distance of 1.76 (2) A? and a W-W separation of 3.514 (1) A?. In solution, 4 was shown to be in equilibrium with the monomeric compound W(≡CEt)(CH2-t-Bu)(O-i-Pr)2 (4′). Compound 4 reacts with pyridine (py) to form a py adduct of formula [W(≡CEt)(CH2-t-Bu)(O-i-Pr)2(py)]2 (5). The proposed structure of 5 consists of an edge-shared bioctahedral geometry in which the py ligands are trans to the CH2-t-Bu groups. 1,2-W2Me2(O-t-Bu)4(py)2 reacts with MeC≡CMe to form 1,1-W2Me2(μ-C2Me2)(O-t-Bu) 4(py) (6) representing the first fully characterized example of a metal-to-metal alkyl migration on a W2 center. In the solid state, 6 contains a slightly skewed alkyne bridge and no molecular symmetry. In solution, a turnstile rotation presumably occurs about one end of the molecule generating an apparent plane of symmetry. 1,2-W2Me2(O-t-Bu)4(py)2 reacts with ethyne to form W2Me(μ-HCCH)(O-t-Bu)5(py) in low yield presumably by intermolecular ligand group scrambling. Crystal data for W2(CH2Ph)2(O-i-Pr)4(MeCCMe) 2 at -159°C: a = 11.999 (2) A?, b = 18.650 (4) A?, c = 15.522 (3) A?, β = 97.84 (1)°, Z = 4, dcalcd = 1.727 g cm-3, and space group P21/n. For W2(Ph)2(O-i-Pr)4(MeCCMe)2 at -159°C: a = 11.854 (2) A?, b = 19.543 (4) A?, c = 14.699 (2) A?, β = 105.70 (1)°, Z = 4, dcalcd = 1.756 g cm-3, and space group P21/n. For W2Me2(O-t-Bu)4(py)(MeCCMe) at -162°C: a = 9.468 (3) A?, b = 35.968 (16) A?, c = 10.176 (3) A?, β = 113.68 (2)°, Z = 4, dcalcd = 1.724 g cm-3, and space group P21/c. For [W(≡CEt)(O-i-Pr)2(CH2-t-Bu)]2 at -155°C: a = 9.947 (2) A?, b = 10.481 (2) A?, c = 9.457 (2) A?, α = 93.40 (1)°, β = 108.07 (1)°, γ = 107.66 (1)°, Z = 1, dcalcd = 1.563 g cm-3, and space group P1.
