Supramolecular trans-Coordinating Phosphine Ligands

We report a new urea-functionalized phosphorus ligand and palladium complexes thereof that self-associate by hydrogen bond formation. The solution studies of a urea-based phosphine ligand {m-[EtO(CO)CH₂NH(CO)NH]C₆H₄PPh₂}, 1, and the palladium complex (1)₂PdMeCl, 2, show that intermolecular and intramolecular hydrogen-bonding, respectively, is present between the urea hydrogens and the carbonyl of a second urea moiety. Introduction of NBu₄Cl to 2 results in the disruption of the self-association and the production of {[trans-(1₂Cl)PdMeCl]-[NBu₄]⁺}, 3, with an anion-templated, bidentate phosphine ligand system. Although the ligands are hydrogen-bonding to the chloride anion, they remain in a trans configuration about the metal center. If another equivalent of 1 is added to 2, a zwitterionic palladium methyl complex ligated by three phosphine ligands is produced and the chloride anion is abstracted from the metal center into the resulting tris-urea hydrogen-bonding pocket, generating [(1₃Cl)PdMe], 4. If at -80 °C ¹³CO is introduced to 2 instead of 1, the chloride anion is once again abstracted into the bis-urea pocket and the zwitterionic CO-adduct trans-[(1₂Cl)Pd(¹³CO)(Me)], 5, results. However, upon warming to ambient temperature, CO migratory insertion occurs to generate the acetyl species and the chloride anion migrates back to the palladium center, regenerating a neutral complex. The analogous CO-adduct of 3 could not be produced since the urea pocket is already blocked with a chloride anion, stressing the subtle control the urea pocket exerts over the reactivity of the palladium center.