Steric control in the formation of Co2(CO)6-alkyne complexes from group 14 tetraalkynes and their reactions with acid

Article abstract of DOI:10.1016/s0020-1693(96)05209-7

A series of tetrakis(trimethylsilylethyne) derivatives of Group 14 metals (2-4) was prepared. Co₂(CO)₆ complexes 5-10 were synthesised by the reaction of 2-4 with Co₂(CO)₈. From the silyl and germyl based compounds 2 and 3, either one or two alkynes could be complexed with Co₂(CO)₆. In contrast, the tin derived compound 4 could accommodate up to four Co₂(CO)₆ complexes. The longest wavelength UV-Vis absorbances of the silicon and germanium-based complexes were consistent with multiple, non-conjugated Co₂(CO)₆ chromophores. The tetrakis Co₂(CO)₆ complex 10, however, absorbs at a much longer wavelength suggesting conjugation of Co₂(CO)₆ complexes through the tin. The reactivity towards protonolysis of the uncomplexed alkynes 2-4 is a consequence of the hyperconjugative stabilisation of the intermediate β-vinyl cation (the β-effect): Sn(C≡CSiMe₃)₃>SnOTf(C≡CSiMe ₃)₂>SiMe₃>Ge(C≡CSiMe ₃)₃>Si(C≡CSiMe₃)₃. The reactivity of the Co₂(CO)₆ complexes, however, was quite different from the reactions of 2-4 and from analogous all-carbon systems. Treatment of 5-10 with strong acid led neither to protiodemetallation of the complexed or non-complexed alkynes but to decomplexation of the cobalt. Similarly, ligand metathesis reactions between 10 and Ph₂SiCl₂ were not observed. The normal reactivity of silylalkynes towards electrophiles, which was expected to be enhanced by the presence of the cobalt complex, was diminished by the particular steric environment of the molecules under examination (5-10). As a result, the favoured reaction under these conditions was decomplexation of the cobalt.