Toward Alkane Functionalization Effected with Cp*W(NO)(alkyl)(η³-allyl) Complexes

Abstract

Cp*W(NO)(n-alkyl)(η3-allyl) complexes result from the selective activations of the terminal C−H bonds of alkanes. Consequently, the reactions of prototypical members of this family of complexes with a range of electrophiles and nucleophiles have been explored with a view to developing methods for functionalizing the newly formed alkyl ligands. The two principal complexes investigated in this regard have been Cp*W(NO)(CH2SiMe3)(η3-CH2CHCHMe) (1) and Cp*W(NO)(CH2C6H5)(η3-CH2CHCHMe) (2). It has been found that treatment of 1 and 2 with the oxidant I2 at −60 °C produces Cp*W(NO)I2 and terminally functionalized ICH2SiMe3 and ICH2C6H5, respectively. Oxidation of 1 by H2O2 also results in the loss of the allyl ligand and production of the known oxo peroxo complex Cp*W(O)(η2-O2)(CH2SiMe3). Treatment of 1 and 2 with electrophiles affords the products resulting from addition of the electrophile to the electron-rich terminus of the σ−π distorted allyl ligands in the reactants. Thus, reagents of the type E-X (E = triphenylcarbenium, H, catecholborane; X = Cl, BF4) liberate CH3CHCHCH2E and form the organometallic products Cp*W(NO)(X)(CH2SiMe3) and Cp*W(NO)(X)(CH2C6H5), respectively. Exposure of the tungsten alkyl allyl complexes to isocyanide reagents leads to the formation of complexes bearing β,γ-unsaturated η2-iminoacyl ligands that apparently arise from the migratory insertion of isocyanide into the tungsten−allyl linkages. For instance, reaction of 1 with 2,6-xylylisocyanide produces Cp*W(NO)(CH2SiMe3)(η2-CH3CHCHCH2CNC6H3Me2) (4a). Interestingly, gentle heating or chromatography of 4a causes isomerization of the olefin and conversion to the conjugated product Cp*W(NO)(CH2SiMe3)(η2-CH3CH2CHCHCNC6H3Me2) (4b). A similar reaction of 2 with 2,6-xylylisocyanide affords both unconjugated and conjugated isocyanide insertion products, while treatment of 2 with n-butylisocyanide produces primarily the conjugated product. Finally, exposure of these tungsten alkyl allyl complexes to 1000 psi of CO gas generally results in the desired migratory insertion of the CO into the metal−alkyl linkages to form acyl compounds. Hence, 1 is first converted into Cp*W(NO)(C(O)CH2SiMe3)(η3-CH2CHCHMe), which then subsequently transforms into isolable Cp*W(NO)(C(O)CH3)(η3-CH2CHCHMe) (7). Interestingly, 2 does not react with CO under these experimental conditions. Nevertheless, the generality of this mode of reactivity is established by the fact that similar treatment of four other Cp*(W)(NO)(CH2CMe3)(η3-allyl) complexes with CO gas at elevated pressures does afford the corresponding acyl products (8−11). All new organometallic complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of several compounds have been established by X-ray crystallographic analyses.

Functionalization of Methane Initiated by Cp*W(NO)(CH2CMe3)(η3-CH2CHCMe2)

“…The solid-state molecular structure of 4 is shown in Figure and confirms the 1,1-CO insertion into the W–C bond of the neopentyl ligand. The acyl carbonyl has a typical C(16)–O(2) bond length of 1.220(5) Å . In addition, the W(1)–C(16)–O(2) and O(2)–C(16)–C(17) bond angles are consistent with an sp 2 -hybridized C atom.…”

“…Compound 4 is isolated in high yield (93%) as a yellow solid that is pure by elemental analysis without any workup beyond removing the solvent from the final reaction mixture in vacuo. The presence of an acyl ligand in 4 is supported by a carbonyl-stretching frequency at 1618 cm –1 in its Nujol mull IR spectrum and a signal at δ 273.5 ppm attributable to an acyl CO in its 13 C NMR spectrum in C 6 D 6 .…”

“…The functionalization of transition-metal–carbon σ bonds by insertion of CO is a well-established method for the generation of new C–C bonds. − Prior to this work it had been demonstrated that the complexes Cp*W­(NO)­(CH 2 CMe 3 )­(η 3 -CH 2 CHCHR) (R = Ph, Me) convert slowly to the corresponding acyl complexes when they are subjected to CO pressure . Consequently, similar functionalization of Cp*W­(NO)­(R)­(η 3 -CH 2 CHCMe 2 ) [R = CH 2 CMe 3 , Me] complexes has been investigated.…”

“…22−24 Prior to this work it had been demonstrated that the complexes Cp*W(NO)(CH 2 CMe 3 )(η 3 -CH 2 CHCHR) (R = Ph, Me) convert slowly to the corresponding acyl complexes when they are subjected to CO pressure. 25 4) (Scheme 3). Compound 4 is isolated in high yield (93%) as a yellow solid that is pure by elemental analysis without any workup beyond removing the solvent from the final reaction mixture in vacuo.…”

Chromium, molybdenum and tungsten

“…80 A number of intriguing nitrosyl complexes have been reported. [81][82][83][84][85] A new and improved synthetic route to WSF 4 was developed from the reaction of WF 6 and Sb 2 S 3 in anhydrous HF. 8 Tungsten sulfide tetrafluoride was characterised by Raman and 19 F NMR spectroscopy for the first time in HF solvent.…”