Structural Evidence for Lewis Acid Triggered Nitrosyl Bending in Rhenium(–I) Chloro Catalysts for Alkene Hydrogenation Reactions

Versatile Fe-Sn Bonding Interactions in a Metallostannylene System: Multiple Bonding and C-H Bond Activation

The metallostannylene Cp*(iPr2MeP)(H)2Fe-SnDMP (1; Cp* = η5-C5Me5; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*(iPr2MeP)HFe[Sn(H)DMP] intermediate, serves as a robust platform for exploration of transition-metal main-group element bonding and reactivity. Upon one-electron oxidation, 1 expels H2 to generate the coordinatively unsaturated [Cp*(iPr2MeP)Fe═SnDMP][B(C6F5)4] (3), which possesses a highly polarized Fe-Sn multiple bond that involves interaction of the tin lone pair with iron. Evidence from EPR and 57Fe Mössbauer spectroscopy, along with DFT studies, shows that 3 is primarily an iron-based radical with charge localization at tin. Upon reduction of 3, C-H bond activation of the phosphine ligand was observed to produce Cp*HFe(κ2-(P,Sn)═Sn(DMP)CH2CHMePMeiPr) (5). Complex 5 was also accessed via thermolysis of 1, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H2 from 1 to produce a stannylyne intermediate, Cp*(iPr2MeP)Fe[SnDMP] (A), is likely rate-determining. Evidence indicates that the production of 5 proceeds through a concerted C-H bond activation. DFT investigations suggest that the transition state for this transformation involves C-H cleavage across the Fe-Sn bond and that a related transition state where C-H bond activation occurs exclusively at the tin center is disfavored, illustrating an effect of iron-tin cooperativity in this system.

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

Treatment of CH₂Cl₂ solutions of Cp*M(NO)Cl₂ (Cp* = η⁵-C₅(CH₃)₅; M = Mo, W) first with 2 equiv of AgSbF₆ in the presence of PhCN and then with 1 equiv of Ph₂PCH₂CH₂PPh₂ affords the yellow-orange salts [Cp*M(NO)(PhCN)(κ²-Ph₂PCH₂CH₂PPh₂)](SbF₆)₂ in good yields (M = Mo, W). Reduction of [Cp*M(NO)(PhCN)(κ²-Ph₂PCH₂CH₂PPh₂)](SbF₆)₂ with 2 equiv of Cp₂Co in C₆H₆ at 80 °C produces the corresponding 18e neutral compounds, Cp*M(NO)(κ²-Ph₂PCH₂CH₂PPh₂) which have been isolated as analytically pure orange-red solids. The addition of 1 equiv of the Lewis acid, Sc(OTf)₃, to solutions of Cp*M(NO)(κ²-Ph₂PCH₂CH₂PPh₂) at room temperature results in the immediate formation of thermally stable Cp*M(NO→Sc(OTf)₃)(H)(κ³-(C₆H₄)PhPCH₂CH₂PPh₂) complexes in which one of the phenyl substituents of the Ph₂PCH₂CH₂PPh₂ ligands has undergone intramolecular orthometalation. In a similar manner, addition of BF₃ produces the analogous Cp*M(NO→BF₃)(H)(κ³-(C₆H₄)PhPCH₂CH₂PPh₂) complexes. In contrast, B(C₆F₅)₃ forms the 1:1 Lewis acid-base adducts, Cp*M(NO→B(C₆F₅)₃)(κ²-Ph₂PCH₂CH₂PPh₂) in CH₂Cl₂ at room temperature. Upon warming to 80 °C, Cp*Mo(NO→B(C₆F₅)₃)(κ²-Ph₂PCH₂CH₂PPh₂) converts cleanly to the orthometalated product Cp*Mo(NO→B(C₆F₅)₃)(H)(κ³-(C₆H₄)PhPCH₂CH₂PPh₂), but Cp*W(NO→B(C₆F₅)₃)(κ²-Ph₂PCH₂CH₂PPh₂) generates a mixture of products whose identities remain to be ascertained. Attempts to extend this chemistry to include related Ph₂PCH₂PPh₂ 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.