Seventeen-electron chromium(i)tricarbonyltris(phosphine) complexes supported by tris(phosphinomethyl)phenylborates

Abstract

The inaugural crystallographic characterization of chromium(I)tricarbonyltris(phosphine) radicals has been achieved. Oxidation of [PPN][Cr(CO)₃(PhBP^Ph₃)] (PhBP^Ph₃ = [PhB(CH₂PPh₂)₃]⁻) and analogous Cr(0) complexes featuring 3,5-dimethylphenyl and 3,5-bis(trifluoromethyl)phenyl borate substituents affords charge-neutral Cr(CO)₃(PhBP^Ph₃) zwitterions, containing the first fully characterized [Cr(CO)₃P₃]⁺ units. The stabilization affected by the intramolecular charge separation established by PhBP^Ph₃ ligands dramatically increases the robustness of these seventeen-electron Cr(I) complexes. Previous attempts to isolate salts of mer/fac-[Cr(CO)₃P₃]⁺ were frustrated by the thermal instability of these cations. The EPR spectroscopic data of Cr(CO)₃(PhBP^Ph₃) supports Rieger's hypothesized low temperature preparation of fac-[Cr(CO)₃{CH₃C(CH₂PPh₂)₃}]⁺. The robust [Cr(CO)₃P₃]⁺ unit of Cr(CO)₃(PhBP^Ph₃) motivated the preparation of structurally characterized Cr(0)/Cr(I) (Cr(CO)₃{η⁶-(PhBP^Ph₃)Cr(CO)₃}, Cr(CO)₃{η⁶-(((3,5-CH₃)C₆H₃)BP^Ph₃)Cr(CO)₃}) and W(0)/Cr(I) (W(CO)₃{η⁶-(((3,5-CH₃)C₆H₃)BP^Ph₃)Cr(CO)₃}) complexes. While these bimetallics feature classical κ³-phosphine and η⁶-arene metal-binding, they are noteworthy since all other reported mixed-valent Cr(0)/Cr(I) complexes exhibit (a) significant thermal instability that has precluded their isolation and (b) greater uncertainty regarding the presence of distinct Cr(I) and Cr(0) centers. This work illustrates the utility of tris(phosphino)borates for the stabilization of cationic metal fragments within zwitterions that are inaccessible or difficult to characterize independently.