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• 102828-97-7 LiOCMe₂CF₃ 

Relevant academic research and scientific papers

Dimers that contain unbridged W(IV)/W(IV) double bonds

Compounds that contain an unbridged W=W bond can be prepared through bimolecular decomposition of various imido alkylidene complexes. One type of W= W species is [W(NR)(CH2-t-Bu)(OR′)]2 (R = 2,6-dimethylphenyl (Ar′) or 2,6-diisopropylphenyl (Ar); R′ = pentafluorophenyl or hexafluoroisopropyl), which is found in both heterochiral and homochiral forms. The other type is [W(NR)(OR′)2]2 (R′ = trifluoro-tert-butyl or hexafluoro-tert-butyl). All species contain an unbridged W(IV)/W(IV) double bond with a W=W bond distance of 2.4-2.5 A, a 90° angle between the imido ligands and the W=W bond, and an anti relationship of the imido ligands. There is no indication that W=W bonds are broken spontaneously; for example, heterochiral and homochiral [W(NR)(CH 2-t-Bu)(OR′)]2 species do not interconvert.

Further studies of imido alkylidene complexes of tungsten, well-characterized olefin metathesis catalysts with controllable activity

An alternative synthesis of W(CH-t-Bu)(NAr)(dme)Cl2 (Ar = 2,6-C6-H3-i-Pr2) consists of the five steps WCl6 → W(O)Cl4 → W(NAr)Cl4 → W(NAr)(O-t-Bu)2Cl2(THF) → W(NAr)(O-t-Bu)2(CH2-t-Bu)2 → W-(CH-t-Bu)(NAr)(dme)Cl2, in which tert-butoxide "protecting groups" are replaced by chlorides in the last step upon addition of PCl5. The easiest synthesis to a catalyst precursor consists of the three steps WO2Cl2 → W(NAr)2Cl2(dme) → W(NAr)2(CH2R)2 → W(CHR)(NAr)(OTf)2(dme) (R = t-Bu, CMe2Ph; OTf = OSO2CF3), in which an imido ligand protecting group is ultimately replaced by two triflate ligands upon addition of triflic acid in the last step. An X-ray study of W(CH-t-Bu)(NAr)(O-t-Bu)2 shows it to be a pseudotetrahedral complex in which the tert-butyl group points toward the imido ligand (syn conformation; space group P1, a = 14.050 (5) ?, b = 18.885 (5) ?, c = 11.123 (5) ?, α = 92.22 (3)°, β = 108.30 (3)°, γ = 79.25 (2)°, V = 2752 (2) ?3, Z = 4, Mr 572.46, ρ = 1.381 g cm-3, μ = 43.03 cm-1; R = 0.039, Rw = 0.043). Complexes of the type W(CH-t-Bu)(NAr′)(OR)2 (NAr′ = N-2,6-C6H3Me2; OR = O-t-Bu, OCMe2(CF3), OCMe(CF3)2, OC(CF3)2(CF2CF2CF3)) were prepared by methods analogous to those used originally to prepare NAr complexes. Reactions between NAr′ complexes and olefins in general yield less stable organometallic products than when the NAr ligand is present. In one case (addition of internal olefins to W(CH-t-Bu)(NAr′)[OCMe(CF3)2]2) a product was isolated that was consistent with the formation {W(NAr′)-[OCMe(CF3)2]2}2. Some of the W(CH-t-Bu)(NAr)X2 variations that were prepared include X = OAr, OCEt3, OCMe2Ph, SAr, and CH2-t-Bu. Other variations include W(CHEt)(NAr)X2 complexes (X = OCEt3, NPh2), W(CHSiMe3)(NAr)X2 complexes (X = OAr, OCMe2(CF3), OCMe(CF3)2), and W[CHSi(OMe)3](NAr)X2 complexes (X = OAr, OCMe2(CF3), OCMe(CF3)2). Syn and anti rotamers of W(CHSiMe3)(NAr)(OAr)2 were observed and found to interconvert on the NMR time scale (ΔG?298 = 15.0 (1) kcal mol-1). None of the variations have any obvious advantage over known alkoxide/NAr complexes for metathesis of ordinary or strained cyclic olefins. An attempt to prepare a derivative containing the OC(CF3)2(tolyl) ligand yielded W[OC(C6H3Me)(CF3)2](NAr)[OC(CF 3)2(tolyl)](CH2-t-Bu), formed by addition of an ortho CH bond to the W=C bond (space group P21/c, a = 16.821 (2) ?, b = 11.951 (1) ?, c = 19.455 (4) ?, β = 93.852 (8)°, V = 3920.5 ?3, Z = 4, Mr 943.6, ρ(calcd) = 1.606 g cm-3, μ = 31.09 cm-1; R = 0.038, Rw = 0.043).