Novel (R₂PC₂H₄PR₂)M⁰−COT Complexes (M = Pd, Pt) Having Semiaromatic η²-COT or Dianionic η²(1,4)-COT Ligands

The complexes (R2PC2H4PR2)Pd(C2H4) (R = iPr, tBu) react with COT by displacement of the ethene ligand to afford the novel mononuclear complexes (R2PC2H4PR2)Pd(η2-C8H8) (R = iPr (1a), tBu (1b)). The COT ligand of 1a,b is semiaromatic; i.e., it is planar with alternating C-C and C=C bonds as expected for the monoanion [C8H8]-. Treatment of 1a,b with (R2PC2H4PR2)Pd-alkene complexes generates the dinuclear derivatives {(R2PC2H4PR2)Pd}2(μ-η2,η2-C8H8) (R = iPr (2a), tBu(2b)). The complexes 2a,b are labile; elimination of the COT ligand from 2a gives rise to homoleptic Pd2(μ-dippe)2. Reaction of (dippe)PtCl2 with (tmeda)2Li2COT or displacement of the alkene ligands in {(dippe)Pt}2(µ-η2,η2-C8H12) (4) or (dippe)Pt(C2H4) (5) by COT produces (dippe)Pt(η2-C8H8) (6a), which in the solid state also has a semiaromatic COT ligand. In solution, complex 6a is in equilibrium with the novel isomeric form (dippe)PtII{η2(1,4)-C8H8} (6b). The complexes are characterized by IR, MS, and solution and solid-state NMR