Heterotrinuclear Complexes with Palladium, Rhodium, and Iridium Ions Assembled by Conformational Switching of a Tetraphosphine Ligand around a Palladium Center

Reaction of [PdCl2(cod)] with a tetraphosphine, meso-bis­[((diphenylphosphino)­methyl)­phenylphosphino]­methane (dpmppm), afforded the mononuclear PdII complexes [PdCl­(dpmppm-κ3)]­X (X = Cl (1a), PF6 (1b)); the pincer-type dpmppm ligand coordinates to the Pd atom with two outer and one inner phosphorus atom to form fused six- and four-membered chelate rings. The remaining inner phosphine is uncoordinated and readily reacts with [Cp*MCl2]2 to give the heterodimetallic complexes [PdCl­(Cp*MCl2)­(μ-dpmppm-κ3,κ1)]­X (X = Cl, M = Rh (21a), Ir (21b); X = PF6, M = Rh (23a), Ir (23b)). Attachment of the second metal fragment to the uncoordinated phosphine caused a crucial conformational change of the six-membered chelate ring from a stable chair conformation to a twist-boat structure, which concomitantly destabilizes the four-membered ring for its opening reactions. Complexes 21 (X = Cl) were converted to [PdCl2(Cp*MCl2)­(dpmppmO)], in which the terminal P atom is dissociated and oxidized as Ph2P­(O)­CH2P­(Ph)­CH2P­(Ph)­CH2PPh2 (dpmppmO), and in the presence of another 1 equiv of [Cp*M′Cl2]2, complexes 21 were readily transformed into the heterotrinuclear complexes [PdCl2(Cp*M′Cl2)­(Cp*MCl2)­(μ-dpmppm-κ2,κ1,κ1)] (M = M′ = Rh (31a), Ir, (31b); M = Ir, M′ = Rh (31c)), where the third metal M′ is trapped by the terminal P atom with its four-membered-ring opening. Complexes 23 also reacted with another 1 equiv of [Cp*M′Cl2]2 to afford the heterotrinuclear complexes [PdCl­(μ-Cl)­(Cp*M′Cl)­(Cp*MCl2)­(μ-dpmppm-κ2,κ1,κ1)]­PF6 (M = M′ = Rh (32a), Ir, (32b); M = Ir, M′ = Rh (32c), M = Rh, M′ = Ir (32d)); the additional metal M′ is ligated by the terminal phosphine and is further connected to the Pd atom via a chloride bridge, resulting in a rather electron-deficient M′ center on the basis of cyclic voltammetry. These results exhibited that the addition of a bulky metal fragment to the uncoordinated phosphine of 1 brings about a conformational switch around the Pd center to promote the ring-opening reaction of the four-membered chelate ring, which leads to an incorporation of the third metal fragment to construct heterotrinuclear structures.