Design of Leaving Groups in Radical C-C Fragmentations: Through-Bond 2c–3e Interactions in Self-Terminating Radical Cascades

摘要

The reactions of [Co_2(CO)_8] with one equiv of the benzamidinate (R_2bzam) group-14 tetrylenes [M(R_2bzam)-(HMDS)] (HMDS=N(SiMe_3)_2 ; 1: M=Ge, R=iPr; 2: M=Si, R= tBu; 3: M=Ge, R=tBu) at 20 8C led to the monosubstituted complexes [Co_2{k~1M=M(R_2bzam)(HMDS)}(CO)7] (4: M=Ge, R=iPr; 5: M=Si, R=tBu; 6: M=Ge, R=tBu), which contain a terminal k~1M–tetrylene ligand. Whereas the Co_2Si and Co_2Ge tert-butyl derivatives 5 and 6 are stable at 20°C, the Co_2Ge isopropyl derivative 4 evolved to the ligand-bridged derivative [Co_2{m-k~2Ge,N-Ge(iPr_2bzam)(HMDS)}(μ-CO)(CO)_5] (7), in which the Ge atom spans the Co-Co bond and one arm of the amidinate fragment is attached to a Co atom. The mechanism of this reaction has been modeled with the help of DFT calculations, which have also demonstrated that the transformation of amidinate-tetrylene ligands on the dicobalt framework is negligibly influenced by the nature of the group-14 metal atom (Si or Ge) but is strongly dependent upon the volume of the amidinate N-R groups. The disubstituted derivatives [Co_2{k~1M-M(R_2bzam)(HMDS)}_2(CO)_6] (8: M=Ge, R=iPr; 9: M=Si, R=tBu; 10: M=Ge, R=tBu), which contain two terminal k~1M–tetrylene ligands, have been prepared by treating [Co_2(CO)_8] with two equiv of 1–3 at 208C. The IR spectra of 8–10 have shown that the basicity of germylenes 1 and 3 is very high (comparable to that of trialkylphosphanes and 1,3-diarylimidazol-2-ylidenes), whereas that of silylene 2 is even higher.