Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes

Abstract

The reactivity of amidinatotetrylenes of the type E(tBu₂ bzm)R¹ (E=Si, Ge; tBu₂ bzm=N,N'-bis(tertbutyl)benzamidinate; R¹ =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C₂ R² }(CO)₅ ] (R² =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C² atom (γ-attack), which leads to σ-allenyl complexes in which the original C_carbene atom maintains its attachment to the Cr(CO)₅ and OEt groups (compounds 3 ${}_{E\text{-}R{}^1\text{-}R{}^2}$ ), and (b) attack of the amidinatotetrylene to the C_carbene atom (α-attack), which ends in σ-allenyl complexes in which the original C_carbene atom is not attached to the metal atom and has been inserted into an E-N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ${}_{E\text{-}R{}^1\text{-}R{}^2}$ ). It has been found that compounds 3 ${}_{E\text{-}R{}^1\text{-}R{}^2}$ are thermodynamically less stable than their corresponding 4 ${}_{E\text{-}R{}^1\text{-}R{}^2}$ isomers and that some of the former (E=Ge; R¹ =CH₂ SiMe₃ ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R¹ =Mes, tBu) end in the carbene-substitution products [Cr{E(tBu₂ bzm)R¹ }(CO)₅ ].

Keywords: Fischer carbenes; allenyl complexes; chromium; germylenes; silylenes.