Metal fragment isomerisation upon grafting a d2 ML4 perhydrocarbyl Os complex on a silica surface: origin and consequence

Grafting of Os bisalkylidene complexes, [Os(=CHtBu)(2)(CH(2)tBu)(2)], on a silica partially dehydroxylated at 700 degrees C selectively yields an alkylidyne complex [( SiO) Os( CtBu)(CH(2)tBu)(2)] according to mass balance analysis, IR and solid state NMR spectroscopies, but 70% of the silanols remains unreacted. Grafting corresponds to a replacement of one alkyl by a siloxy ligand and induces the isomerisation of the metal fragment from a bis-alkylidene to an alkyl alkylidyne. Molecular (B3PW91) and periodic DFT-calculations show that the bis-alkylidene is the energetically favoured isomer for the perhydrocarbyl complex, while the alkyl alkylidyne isomer is more stable when one of the alkyl ligands is replaced by a siloxy ligand. The change of the nature of the ligand is accompanied with a change of geometry: from a distorted tetrahedral structure for [Os(=CHtBu)(2)(CH(2)tBu)(2)] to a butterfly-geometry with apical siloxy and alkylidyne ligands for [( SiO) Os( CtBu)(CH(2)tBu)(2)]. Finally, DFT calculations show that grafting occurs via a sigma-bond metathesis between the silanol and a metal-alkyl bond and not through the typical addition of the silanol onto the alkylidene ligand.