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

Abstract

Treatment of CHCl solutions of Cp*M(NO)Cl (Cp* = η-C(CH); M = Mo, W) first with 2 equiv of AgSbF in the presence of PhCN and then with 1 equiv of PhPCHCHPPh affords the yellow-orange salts [Cp*M(NO)(PhCN)(κ-PhPCHCHPPh)](SbF) in good yields (M = Mo, W). Reduction of [Cp*M(NO)(PhCN)(κ-PhPCHCHPPh)](SbF) with 2 equiv of CpCo in CH at 80 °C produces the corresponding 18e neutral compounds, Cp*M(NO)(κ-PhPCHCHPPh) which have been isolated as analytically pure orange-red solids. The addition of 1 equiv of the Lewis acid, Sc(OTf), to solutions of Cp*M(NO)(κ-PhPCHCHPPh) at room temperature results in the immediate formation of thermally stable Cp*M(NO→Sc(OTf))(H)(κ-(CH)PhPCHCHPPh) complexes in which one of the phenyl substituents of the PhPCHCHPPh ligands has undergone intramolecular orthometalation. In a similar manner, addition of BF produces the analogous Cp*M(NO→BF)(H)(κ-(CH)PhPCHCHPPh) complexes. In contrast, B(CF) forms the 1:1 Lewis acid-base adducts, Cp*M(NO→B(CF))(κ-PhPCHCHPPh) in CHCl at room temperature. Upon warming to 80 °C, Cp*Mo(NO→B(CF))(κ-PhPCHCHPPh) converts cleanly to the orthometalated product Cp*Mo(NO→B(CF))(H)(κ-(CH)PhPCHCHPPh), but Cp*W(NO→B(CF))(κ-PhPCHCHPPh) generates a mixture of products whose identities remain to be ascertained. Attempts to extend this chemistry to include related PhPCHPPh compounds have had only limited success. 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.

Effects of Coordinating a Hemilabile Ligand to 14e Cp*M(NO) Scaffolds (M = Mo, W)

“…As in 5, the P-terminus of the κ 2 -P,N-PCH-2-(3-Me-C 5 H 3 N) ligand formally derived from i Pr 2 PN is oriented such that it lies trans to the nitrosyl ligand. The H atom belonging to C11 has been located in a difference map, and its position is consistent with the existence of an sp 2 shown in Figure S32. The labeled bond lengths are in agreement with the alternating single and double C−C bonds of the resonance structure drawn in Scheme 3.…”

“…For instance, the 31 P{ 1 H} NMR spectrum shows a singlet at 28.0 ppm with 183 W satellites ( 1 J PW = 427.9 Hz; Figure S15), a feature that is similar to that previously reported for Cp*W(NO)(κ 2 -dppe). 2 The IR spectrum of 3 exhibits a strong ν(NO) absorption at 1525 cm −1 , which is a manifestation of the significant electron density extant at the metal center that results, in turn, in significant W → NO backbonding.…”

“…1 Consequently, we then extended our studies to 18e Cp*W(NO)(κ 2 -P-P) complexes such as Cp*W(NO)(κ 2 -dppe) (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) in hopes of overcoming these limitations. 2 Interestingly, the Cp*W-(NO)(κ 2 -dppe) compounds are rendered prone to effecting C−H activation reactions not by thermolyses but rather by treatment with equimolar amounts of appropriate Lewis acids (LA), which form adducts with the basic O-termini of their nitrosyl ligands. Unfortunately, these systems only promote intramolecular C−H activations presumably because of the steric congestion extant at the metal center bringing one of the phenyl substituents of the dppe ligand into a position in which it can readily undergo ortho metalation.…”

“…However, an undesirable feature of these latter activations is that they are often accompanied by unwanted side reactions such as disproportionation, isomerization, or rearrangement of the organometallic reactants . Consequently, we then extended our studies to 18e Cp*W­(NO)­(κ 2 -P-P) complexes such as Cp*W­(NO)­(κ 2 -dppe) (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) in hopes of overcoming these limitations . Interestingly, the Cp*W­(NO)­(κ 2 -dppe) compounds are rendered prone to effecting C–H activation reactions not by thermolyses but rather by treatment with equimolar amounts of appropriate Lewis acids (LA), which form adducts with the basic O-termini of their nitrosyl ligands.…”

Hydroamination of Dihapto‐Coordinated Benzene and Diene Complexes of Tungsten: Fundamental Studies and the Synthesis of γ‐Lycorane

“…We speculate that interaction of the TMS with the nitrosyl may enhance the electrophilicity of the allyl cation (through the tungsten), which would enhance its reactivity with the nitrile solvent. Previously we have shown that the nitrosyl ligand of WTp(NO)(PMe 3 )(cyclohexene) can be methylated with MeOTf, [14] and independently the Legzdins group has demonstrated this reactivity with similar tungsten‐nitrosyl systems and Lewis acids [21] . In absence of the TMS group, a purported protonation of the nitrosyl is apparently reversed by the addition of triethylamine and cyclization does not occur.…”