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, <i>meso</i>-bis­[((diphenylphosphino)­methyl)­phenylphosphino]­methane (dpmppm), afforded the mononuclear Pd^II complexes [PdCl­(dpmppm-κ³)]­X (X = Cl (<b>1a</b>), PF₆ (<b>1b</b>)); 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 (<b>21a</b>), Ir (<b>21b</b>); X = PF₆, M = Rh (<b>23a</b>), Ir (<b>23b</b>)). 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 <b>21</b> (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 <b>21</b> were readily transformed into the heterotrinuclear complexes [PdCl₂(Cp*M′Cl₂)­(Cp*MCl₂)­(μ-dpmppm-κ²,κ¹,κ¹)] (M = M′ = Rh (<b>31a</b>), Ir, (<b>31b</b>); M = Ir, M′ = Rh (<b>31c</b>)), where the third metal M′ is trapped by the terminal P atom with its four-membered-ring opening. Complexes <b>23</b> 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 (<b>32a</b>), Ir, (<b>32b</b>); M = Ir, M′ = Rh (<b>32c</b>), M = Rh, M′ = Ir (<b>32d</b>)); 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 <b>1</b> 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