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

The inaugural crystallographic characterization of chromium(i)tricarbonyltris(phosphine) radicals has been achieved. Oxidation of [PPN][Cr(CO) 3(PhBP)] (PhBP = [PhB(CH 2PPh 2) 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) zwitterions, containing the first fully characterized [Cr(CO) 3P 3] + units. The stabilization affected by the intramolecular charge separation established by PhBP ligands dramatically increases the robustness of these seventeen-electron Cr(i) complexes. Previous attempts to isolate salts of mer/fac-[Cr(CO) 3P 3] + were frustrated by the thermal instability of these cations. The EPR spectroscopic data of Cr(CO) 3(PhBP) supports Rieger's hypothesized low temperature preparation of fac-[Cr(CO) 3{CH 3C(CH 2PPh 2) 3}] +. The robust [Cr(CO) 3P 3] + unit of Cr(CO) 3(PhBP) motivated the preparation of structurally characterized Cr(0)/Cr(i) (Cr(CO) 3{η 6-(PhBP)Cr(CO) 3}, Cr(CO) 3{η 6-(((3,5-CH 3)C 6H 3)BP)Cr(CO) 3}) and W(0)/Cr(i) (W(CO) 3{η 6-(((3,5-CH 3)C 6H 3)BP)Cr(CO) 3}) complexes. While these bimetallics feature classical κ 3-phosphine and η 6-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.