Ring Opening and Bidentate Coordination of Amidinate Germylenes and Silylenes on Carbonyl Dicobalt Complexes: The Importance of a Slight Difference in Ligand Volume

The reactions of [Co2(CO)8] with one equiv of the benzamidinate (R2bzam) group-14 tetrylenes [M(R2bzam)(HMDS)] (HMDS=N(SiMe3)2; 1: M=Ge, R=iPr; 2: M=Si, R=tBu; 3: M=Ge, R=tBu) at 20 °C led to the monosubstituted complexes [Co2{κ1MM(R2bzam)(HMDS)}(CO)7] (4: M=Ge, R=iPr; 5: M=Si, R=tBu; 6: M=Ge, R=tBu), which contain a terminal κ1M–tetrylene ligand. Whereas the Co2Si and Co2Ge tert-butyl derivatives 5 and 6 are stable at 20 °C, the Co2Ge isopropyl derivative 4 evolved to the ligand-bridged derivative [Co2{μ-κ2Ge,N-Ge(iPr2bzam)(HMDS)}(μ-CO)(CO)5] (7), in which the Ge atom spans the CoCo 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 NR groups. The disubstituted derivatives [Co2{κ1MM(R2bzam)(HMDS)}2(CO)6] (8: M=Ge, R=iPr; 9: M=Si, R=tBu; 10: M=Ge, R=tBu), which contain two terminal κ1M–tetrylene ligands, have been prepared by treating [Co2(CO)8] with two equiv of 1–3 at 20 °C. 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.