A family of rhodium and iridium complexes with semirigid benzylsilyl phosphines: from bidentate to tetradentate coordination modes

The synthesis of a new trisbenzylsilanephosphine P{(o-C6H4CH2)SiMe2H}3 (1) is shown to proceed with high yields from P(o-tolyl)3. Compound 1 coordinates to the Rh and Ir dimers [MCl(COD)]2 (M = Rh, Ir) in a tetradentate or tridentate fashion, depending on the strict exclusion of water. The dimeric compounds [ClM(SiMe2CH2-o-C6H4)2P(o-C6H4-CH2SiMe2H)]2, 2Rh and 2Ir, feature a tetradentate coordination of the starting ligand with P and two Si atoms as well as a non-classical agostic Si–H group. The presence of adventitious water in the solvents leads to the formation of two new complexes [(μ2-Cl)2M2(SiMe2CH2-o-C6H4)2P(o-C6H4-CH2SiMe2OSiMe2CH2-o-C6H4–)P(SiMe2CH2-o-C6H4)2], 3Rh and 3Ir, which feature a siloxane bridge through Si–H bond breaking in 2. Reaction of [RhCl(COD)]2 with the bisbenzylsilanephosphine PhP{(o-C6H4CH2)SiMe2H}2 leads to the formation of compound 4Rh which features also a dimeric structure with the SiPSi ligand coordinated through the two silicon atoms, one of which occupies the apical position of a square-pyramidal geometry in the solid state, while the second is disposed equatorially trans to π-donor Cl. Finally, bidentate coordination of a PSi ligand is achieved by reaction of [RhCl(COD)]2 with Ph2P{(o-C6H4CH2)SiMe2H} which leads to the monometallic species [RhCl(SiMe2CH2-o-C6H4-PPh2)2], 5Rh, incorporating two chelating PSi ligands and maintaining a Cl ligand.