Synthesis, Characterization, and Some Properties of Cp*W(NO)(H)(η³-allyl) Complexes

Abstract

Sequential treatment at low temperatures of Cp*W(NO)Cl2 in THF with 1 equiv of a binary magnesium allyl reagent, followed by an excess of LiBH4, affords three new Cp*W(NO)(H)(η(3)-allyl) complexes, namely, Cp*W(NO)(H)(η(3)-CH2CHCMe2) (1), Cp*W(NO)(H)(η(3)-CH2CHCHPh) (2), and Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3). Complexes 1-3 are isolable as air-stable, analytically pure yellow solids in good to moderate yields by chromatography or fractional crystallization. In solutions, complex 1 exists as two coordination isomers in an 83:17 ratio differing with respect to the endo/exo orientation of the allyl ligand. In contrast, complexes 2 and 3 each exist as four coordination isomers, all differing by the orientation of their allyl ligands which can have either an endo or an exo orientation with the phenyl or methyl groups being either proximal or distal to the nitrosyl ligand. A DFT computational analysis using the major isomer of Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3a) as the model complex has revealed that its lowest-energy thermal-decomposition pathway involves the intramolecular isomerization of 3a to the 16e η(2)-alkene complex, Cp*W(NO)(η(2)-CH2═CHCH2Me). Such η(2)-alkene complexes are isolable as their 18e PMe3 adducts when compounds 1-3 are thermolyzed in neat PMe3, the other organometallic products formed during these thermolyses being Cp*W(NO)(PMe3)2 (5) and, occasionally, Cp*W(NO)(H)(η(1)-allyl)(PMe3). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Thermal Chemistry of Cp*W(NO)(H)(η3-allyl) Complexes

“…4 To test the preference for primary over secondary initial C(sp 3 )−H activations of the Cp*W(NO)(H)(η 3 -allyl) complexes, the thermal reaction of 1 with cyclohexane, a substrate containing only secondary C(sp 3 )−H bonds, has been effected. Thermolysis of 1 in neat cyclohexane under the same conditions as for the reactions with n-pentane and n-heptane results in only 30% conversion of 1 to Cp*W(NO)(H)(η 3 -C 6 H 9 ) (6) (Scheme 4).…”

“…(1) The hydride ligand couples to the allyl ligand to generate the 16e intermediate alkene complex Cp*W(NO)(η 2 -CH 2 CHCHMe 2 ) (A), whose existence has been previously demonstrated. 4 (2) Intermediate A effects the single, terminal C−H activation of an n-pentane molecule to form the 16e mixed bis(alkyl) complex Cp*W-(NO)(n-C 5 H 11 )(CH 2 CH 2 CHMe 2 ) (B), which contains two β C−H agostic interactions and has an open coordination site. (3) Complex B then undergoes a β-H abstraction to give a new 16e η 2 -alkene complex, Cp*W(NO)(η 2 -CH 2 CHCH 2 CH 2 Me) (C), releasing the original η 3 -CH 2 CHCMe 2 ligand as isopentane.…”

“…4 The intriguing questions that immediately come to mind are whether these intermediate η 2 -alkene species can effect the intermolecular activations of hydrocarbon C−H bonds, and, if so, how do these activations compare with those documented for the related 16e Cp*W(NO)(η 2 -allene) and Cp*W(NO)(η 2 -diene) entities. Consequently, we initiated investigations to provide answers to these questions, and in this article we report the results of these studies.…”

Cationic and Neutral Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) Complexes of Molybdenum and Tungsten: Lewis-Acid-Induced Intramolecular C–H Activation

“…This feature is comparable to that exhibited by the related complex, Cp*W(NO)(PMe3)2, which also exhibits a large 1 JPW value of 456.8 Hz. 16 For comparison, related W(II) complexes containing either the Cp*W(NO) or TpW(NO) scaffolds consistently display much smaller 1 JPW values nearer ~250 Hz. [16][17][18] Consistent with previous reports, the magnitudes of these coupling constants are functions of the tungstens' oxidation states and their coordination numbers.…”

“…16 For comparison, related W(II) complexes containing either the Cp*W(NO) or TpW(NO) scaffolds consistently display much smaller 1 JPW values nearer ~250 Hz. [16][17][18] Consistent with previous reports, the magnitudes of these coupling constants are functions of the tungstens' oxidation states and their coordination numbers. 19 Larger coupling constants in W(0) complexes indicate a stronger W-P interaction through increased W→P backbonding than is possible when the same metal exists in a higher oxidation state such as W(II).…”