UBC Theses and Dissertations
Activation of small molecules by a tungsten acetylene complex Lumb, Sean A.
A series of η¹- and η²-vinyl-containing complexes of the form Cp*W(NO)(ηx- CPh=CH2)(E) [x = 1 or 2, E = CH2SiMe3, Cl, η²-02CPh, OTf, NHfBu, and (H)(PPh3)] has been prepared, and the solid-state metrical parameters and solution NMR spectroscopic properties of these complexes have been examined. The hapticity of the vinyl ligand in these complexes is dependent of the electron donicity of the ancillary ligand E. Ligands which function as three-electron (3e) donors (such as η²-02CPh and NH'Bu) enforce a monohapto bonding mode for the vinyl ligand, whereas ligands that function as 1e donors (such as CH2SiMe3) permit the dihapto coordination of the vinyl ligand. A search of the Cambridge Structural Database (CSD) reveals a paucity of structurally characterized η²-vinyl complexes of tungsten. Comparisons of the solid-state metrical parameters determined for the η²-vinyl complexes described in this Thesis and those found in the CSD are made which shed light on the nature of the vinyl interaction in the nitrosyl-containing complexes described herein. Thermolysis of the alkyl vinyl complex Cp*W(NO)(CH2SiMe3)(CPh=CH2) (1) at 54 °C in the presence of unsaturated, heteroatom-containing compounds such as esters, nitriles or acetone quantitatively affords metallacyclic products of subtrate-alkyne coupling. The nature of these metallacycles is consistent with the intermediacy of the acetylene complex Cp*W(NO)(η²- CPh=CH) (A) derived in situ from the reductive elimination of SiMe4 from 1. With esters ROAc [R = Me, Et], coupling and C-O bond cleavage yield the alkoxide-containing oxametallacyclopentadiene complexes Cp*W(NO)(η²-O=C(Me)CH=CPh)(OR) R = Me, Et]. Thermolysis of 1 in RCN [R = Me, Et, 'Pr] containing trace amounts of R'OH yields the respective hydroxide or alkoxide compounds Cp*W(NO)(η²-NH=C(R)CH=CPh)(OR') [R = Me, R' = H; R - Et, R' = H; R = 'Pr, R' = H; R = Me, R' = C3H5]. Utilization of cyclopentadiene (CpH) as the trapping agent in MeCN affords the aminopentafulvene complex Cp*W(NO)(HNC(=C(C4H4))(Me))(η²-NH=C(Me)CH=CPh). Thermolysis of 1 in RCN [R = Me, iPr] containing trace amounts of acetone gives the bicyclic species Cp*W(NO)(η³- OC(Me)2N=C(R)CH=CPh). In the absence of added trapping reagent, thermolysis of 1 in RCN [R = Me, Et] yields the vinyl amidinate complexes Cp*W(NO)(η³-NHC(R)=NC(=C(R¹)(R²))CH=CPh) [R = Me, R¹ = R² = H; R = Et, R¹ = H, R² = Me]. The results of labelling studies corroborate mechanistic proposals that account for the observed chemistry. The results of a kinetic study involving several of these transformations substantiate the proposal for a rate-limiting generation of acetylene intermediate A in a dissociative mechanism under saturation conditions (ie. a "saturation" mechanism) via SiMe4 elimination followed by the rapid trapping of A in coupling reactions with organic substrates. A qualitative molecular-orbital overlap rationale is given to account for the observed chemistry. The quantitative decomposition of Cp* W(NO)(η²-CPh=CH2)(CH2SiMe3) (1) at 54 °C in neat hydrocarbon solutions transiently generates Cp*W(NO)(η²-CPh=CH) (A) which activates solvent C-H bonds in situ. For example, the thermolysis of 1 in benzene solution quantitatively generates Cp* W(NO)(η²-CPh=CH2)(Ph). The thermolysis of 1 in solutions of methyl-substituted arenes such as toluene and p-, m-, or o-xylene affords mixtures of aryl and benzyl vinyl complexes of the general formulae Cp*W(NO)(η²-CPh=CH)(aryl), Cp*W(NO)(η²-CPh=CH2)(η¹-benzyl), or Cp*W(NO)(η¹-CPh=CH2)(η²-benzyl). During these conversions, no products of ortho-C-H bond activation are observed. The thermolysis of 1 in (Me3Si)2O under identical conditions quantitatively affords Cp*W(NO)(η²- CPh=CH2)(CH2SiMe2OSiMe3). The mechanism by which RH is eliminated from Cp*W(NO)(R)(CPh=CH2) to generate the proposed acetylene-containing intermediate A is considered in detail. Deuterium-labelling experiments support the intermediacy of A along the reaction pathway. The results of kinetic studies are indicative of a mechanism involving the rate-limiting generation of A under saturation conditions via vinyl-H elimination and RH extrusion, in accord with those of the kinetic study described above. Mechanistic studies provide evidence for the transiency of hydrocarbon σ-complexes along the reaction coordinate prior to both hydrocarbon elimination and substrate C-H bond activation. Competition studies employing mixtures of hydrocarbon substrates reveal a lack of selectivity by A towards the nature of the activated C-H bond, consistent with the proposed mechanism. An MO scheme depicting the W-acetylene valence orbital interactions in A aids in rationalizing the activity of acetylene-containing A towards substrate C-H bonds. Dual C-H bond activation of the aliphatic substrate occurs during the thermal generation of A from 1 in aliphatic hydrocarbon solutions. Consequently, metallacycles of the form Cp*W(NO)(η²-CH(η²-Ph)CH2CH(R)CH2) [R = nPr, nBu, fBu, OEt] result from the dehydrogenation of η-pentane, η-hexane, 2,2-dimethylbutane and diethyl ether, respectively. This dual C-H activation process displays a selectivity for linear substrates that contain an ethyl substituent. Dual C-H bond activation of 2,3-dimethyl-2-butene in the presence of 1 under thermolysis conditions regioselectively affords Cp*W(NO)(η³-endo- CH2C(Me)C(Me)CH2(η¹-CPhMe)). Plausible mechanisms are proposed for the formation of these metallacyclic complexes, and a rationale for the regioselectivities extant in these conversions is also presented. [Scientific formulae used in this abstract could not be reproduced.]
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