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Reactivity of molybdenum nitrosyl alkyl complexes

University of British Columbia

Hydrogenation of the bis(alkyl) complex, Cp*Mo(NO)(CH₂SiMe₃)₂,yields the bimetallic oxo bridging-nitrido complex, [Cp*Mo(NO)( CH₂SiMe₃)](μ- N)[Cp*Mo(O)(CH CH₂SiMe₃)].When the alkyl group of the bis(alkyl) precursor is changed to neopentyl (R CH₂CMe₂)or neophyl (R =CH₂CMe₂Ph),intermediate bridging nitrosyl complexes, (Cp*MoR)₂(μ-NO)₂,are isolated. These bridging nitrosyl complexes isomerize to bimetallic oxo bridging-nitrido complexes, [Cp*Mo(NO)R](μ N)[Cp*Mo(O)R]. A kinetic study of the isomerization (R = neophyl) reveals that it is first order in (Cp*MoR)₂(μ-NO)₂ with kobs( 20°C) 1.1 ± 0.3 x 10⁻⁴ s⁻¹. Furthermore, kinetic analyses at various temperatures establish that ∆H‡ = 39 ± 3 kJ mol¹ and ∆S‡ =-188 ± 6 J mol⁻¹ K⁻¹,which is consistent with the isomerizations occurring in an intramolecular manner. Hydrogenation of a mixture of Cp*Mo(NO)(CH₂SiMe₃)₂ and Cp*W(NO)(CH₂S1Me₃)₂ produces the heterobimetallic bridging nitrosyl complex, [Cp*Mo(CH₂SiMe₃)](μ-NO)₂[W(CH₂SiMe₃)],which similarly isomerizes to a 60:40 mixture of [Cp*W(NO)(CH₂SiMe₃)](μ-N)[Cp*Mo(O)(and [Cp*Mo(NO)(CH₂SiMe₃)](μ-N)[Cp*W(O)(CH₂SiMe₃)]. The products obtained, when Cp*Mo(NO)(CH₂SiMe₃)₂ is hydrogenated in the presence of added substrates, are dependent on the nature of the substrate. A transiently generated molybdenum hydride complex, Cp*Mo(NO)(CH₂SiMe₃)H, is trapped in the presence of acetone or benzaldehyde to produce alkyl alkoxide complexes, Cp*Mo(NO)(CH₂SiMe₃)(OR)(R = CHMe₂,CH₂Ph). In the presence of acyclic dienes, α, β-unsaturated ketones, PPh or PhSSPh₃, both alkyl groups undergo hydrogenolysis and complexes of the type, Cp*Mo(NO)L (L =η⁴-diene, α, β-unsaturated ketone, (PPh₃)₂, (SPh)₂),are obtained. In the presence of 1,3-COD, however, the cyclic diene undergoes initial C-H bond activation and couples in a novel way to produce, Cp*Mo(NO)( η⁴ C₁₆H₂₄). A pyridine trapped alkylidene complex, CpMo(NO)(=CHCMe₃)(py),is generated from the thermolysis of the bis(neopentyl) complex, CpMo(N0)(CH ₂CMe₃)₂,in the presence of pyridine. The α-carbon of this neopentylidene is very nucleophilic and undergoes stereoselective addition reactions with a variety of polar REH complexes (E 0, 5, N; R = alkyl, aryl) to produce the corresponding neopentyl alkoxide, thiolate and amide complexes. Similarly, it reacts with carboxylic acids to yield neopentyl carboxylate complexes, CpMo(NO)(CH₂CMe₃)(η²-OC{O}R). Dimeric species are obtained when bifunctional REH reagents are used. For example, oxalic acid produces [CpMo(NO)(CH₂CMe₃)]₂-μ-(η²-OC{O}C {O}0) and neopentyl glycol produces [CpMo(NO)(CH₂CMe₃)]₂(μ -OCH₂CMe₂CH₂O).These addition reactions probably proceed via the formation of an initial Lewis-acid Lewis-base pair, followed by proton transfer. CpMo(NO)(CH₂CMe₃)(ER)complexes readily undergo exchange reactions with RE’H (E’ = 0, S, N) reagents (pKa RE’H <REH). For example, the amide ligand of CpMo(NO)(CH₂CMe₃)(NH-p-tolyl) readily undergoes exchange with one equivalent of HOSiPh₃,p-cresol, t-butyl thiol and acetic acid to produce CpMo(NO)(CH₂CMe3)(E) (E OSiPh₃,O-p-tolyl, SCMe₃ and OC{O}Me) and free p-toluidine. An equilibrium mixture ofCpMo(N0)(CH₂CMe₃)(O-p-tolyl) and CpMo(NO)(CH₂CMe₃)(OSiPh₃)is generated when one equivalent of HOSiPh₃ is reacted with CpMo(NO)(CH₂CMe₃)(O-ptolyl) or one equivalent of HO-p-tolyl is reacted withCpMo(NO)(CH₂CMe₃)(OSiPh₃). Labeling studies are consistent with these heteroatom exchange reactions occurring with the retention of configuration at the CpMo(NO)(CH₂CMe₃)fragment.

Thesis/Dissertation

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2009-06-03

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http://hdl.handle.net/2429/8696

Chemistry

University of British Columbia

UBCV

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