Studies of various pentamethylcyclopentadienyl nitrosyl complexes of molybdenum and tungsten

Cp*W(NO)(H)(η³-CH₂CHCHMe) (2.1) [Cp* = η⁵-C₅Me₅] effects the formation of unsymmetrical ketones from hydrocarbons and CO via C−H activation, C−H functionalization, and C−C coupling. Cp*W(NO)(CO)₂ is also formed during the reactions, and can be conveniently converted back into complex 2.1, thereby completing a full synthetic cycle for the conversion of hydrocarbons into ketones. cis-Cp*W(NO)(H)(κ²-PPh₂(C₆H₄)) (3.1) has been previously shown to effect the thermal C−H activation of benzene. In an effort to extend this investigation, novel Cp*W(NO)(R)(H)(L) [R = alkyl, L = phosphine or phosphite] complexes have been synthesized and characterized, and their ability to effect C−H activation has been investigated. Upon heating, complex 3.1 in various neat hydrocarbon solutions forms trans-Cp*W(NO)(H)(κ²-PPh₂(C₆H₄)) (3.2), an isomerization product, and Cp*W(NO)(PPh₃)₂, a disproportionation product, both of which are inactive species. In an attempt to avoid the problems of isomerization and disproportionation, the novel complexes trans-Cp*W(NO)(CH₂CMe₃)(H)(P(OPh)₃) (3.3) and trans-Cp*W(NO)(H)₂(P(OPh)₃) (3.4) have been synthesized. Unfortunately, complex 3.3 does not effect the thermal C−H activation of hydrocarbon substrates in neat solutions. Instead, the elimination of neopentane, release of free P(OPh)₃, and subsequent decomposition of the organometallic fragment occur, even at room temperature. The use of bidentate bisphosphine ligands in order to circumvent the problems experienced in the previous section has been investigated. The complex Cp*Mo(NO)(κ²-dmpe) (4.1) [dmpe = 1,2-bis(dimethylphosphino)ethane] does not effect the thermal C−H activation of hydrocarbons, but it does display some interesting reactivity towards electrophiles. Reaction of 4.1 with elemental sulfur results in the formation of either (μ-S)[Cp*Mo(NO)(κ¹-dmpeS)]₂ (4.3) or Cp*Mo(NO)(η²-S₂)(κ¹-dmpeS) (4.4) depending on the ratio of reactants used. Complex 4.3 is, to the best of my knowledge, the only example of an organometallic bimetallic complex that contains a M=S=M linkage, with no other bridging linkages between the two metal centres. Reaction of 4.1 with benzyl bromide results in (μ-dmpe)[Cp*Mo(NO)Br₂]₂ (4.5) and bibenzyl. This product suggests radical processes wherein bibenzyl is formed via homolytic coupling. In addition, reaction of 4.1 with bromoalkanes results in [Mo(NO)(Br)₂(κ²-dmpe)]₂ (4.6), alkane, olefin, and Cp*H. This transformation most likely also involves radical processes and is a rare example of the liberation of Cp*H from a transition-metal organometallic complex.Science, Faculty ofChemistry, Department ofGraduat