Heterobimetallic alkoxysilyl cationic complexes: investigations into the displacement of a µ-η2-Si,O bridge by functional phosphine ligands

Abstract

The lability of the SiO→M interaction unique to some bimetallic complexes may confer hemilabile properties to the Si(OR)₃ ligand and various bifunctional phosphines P–Z have been used in order to evaluate the possible competition for co-ordination between the bridging SiO→M interaction and P–Z chelation. Thus, treatment of the heterobimetallic complexes [(OC)₃Fe{µ-Si(OMe)₂(OMe)}(µ-dppm)MCl] (M = Pd 4 or Pt 6; dppm = Ph₂PCH₂PPh₂) and [(OC)₃Fe{µ-Si(OMe)₂(OMe)}(µ-dppa)PdCl] 5 (dppa = Ph₂PNHPPh₂) with TlPF₆ in the presence of P–Z afforded the corresponding cationic compounds [(OC)₃Fe{µ-Si(OMe)₂(OMe)}(µ-dppm)M(P–Z)PF₆ (M = Pd 1 or Pt 3) and [(OC)₃Fe{µ-Si(OMe)₂(OMe)}(µ-dppa)Pd(P–Z)]PF₆2 (P–Z = Ph₂PC₆H₄(o-OMe), a P{C₆H₄(o-OMe)}₃, b Ph₂PCH₂C(O)Ph, c Ph₂PCH₂CHCH₂, d Ph₂P(CH₂)₂CN, e or Ph₂PCH₂C(O)NPh₂, f). These complexes are stabilized by the occurrence of a Fe–Si-O→M four-membered ring and the pre-existent SiO→M interaction in 4–6 was not displaced by the donor function of the incoming P–Z ligand. Complexes 1b and 1f were obtained as mixtures of two isomers, the P–Z ligands acting either as monodentate or as a P,O chelate. In the latter cases formation of five-co-ordinated palladium species is proposed. Displacement of the SiO→Pd interaction originally present in 4 was observed when the diphosphines (Ph₂P)₂NR (R = Me or (CH₂)₃Si(OEt)₃) were used, since the two phosphorus atoms co-ordinate to the Pd. Surprisingly these diphosphine ligands show different co-ordination modes, depending on whether dppa or dppm is used as the assembling ligand. In the former case, chelation to the Pd is observed, which leads to complex [(OC)₃{(MeO)₃Si}Fe(µ-dppa)Pd{Ph₂PN(Me)PPh₂}]PF₆2g, whereas in the latter case, oligomeric entities of the type [{(OC)₃{(MeO)₃Si}Fe(µ-dppa)Pd[Ph₂PN(R)PPh₂]}_n][PF₆]_n1g,1h (n probably equals two) were formed. The molecular structures of [PdCl(dppm-P,P′){Ph₂PC₆H₄(o-OMe)}]PF₆ g and [Pd₂Cl₂(µ-CO)(µ-dppm)₂] 10, obtained during this work, have been determined by X-ray diffraction.