Palladium complexes containing ligands with hydrogen-bonding functionalities. Reactivity and catalytic studies with CO and olefins |
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Authors: | Gareth R Owen Ramón Vilar Andrew JP White |
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Institution: | a Department of Chemistry, Imperial College London, South Kensington, London SW7 2AY, UK b ICREA - Institució Catalana de Recerca i Estudis Avançats, Spain c ICIQ - Institute of Chemical Research of Catalonia, Avgda. Paisos Catalans s/n, 43007 Tarragona, Spain |
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Abstract: | The synthesis of the new bidentate N-N ligand 1-(2-(1-(pyridin-2-yl)ethylideneamino)ethyl)-3-ethylurea (PyUr) with a urea substituent attached to the imine nitrogen is reported. This ligand has been used to form palladium complexes and study the potential influence of the urea group (as a hydrogen bonding unit and a hemilabile ligand) in the insertion of CO and olefins into Pd-C bonds. The reaction of PyUr with Pd(CH3)(Cl)(COD)] to yield Pd(CH3)Cl(PyUr)] (1) is reported. A crystallographic study of this complex was carried out showing that the urea moieties are involved in a series of intermolecular hydrogen bonding interactions. Upon removal of the chloride from the coordination sphere of 1 (by addition of AgBF4) the urea group of PyUr coordinates to the palladium centre stabilizing an otherwise coordinatively unsaturated complex. The reaction of these complexes with CO to yield Pd{C( O)CH3}Cl(PyUr)] (3) and Pd{C( O)CH3}(PyUr)]BF4] (4) is also discussed. Following on from these reactions, the copolymerization of CO and styrene using 1 as a catalyst was studied and is herein reported. The copolymers synthesized using 1 as a catalyst were obtained in moderate yields and showed to have a narrow size distribution. The same reaction was performed using a palladium complex coordinated by an analogous pyridine ligand but without a hydrogen bonding substituent. The results of the copolymerization reactions showed that, although slightly better yields and larger molecular weights were obtained with the PyUr-containing catalyst, the hydrogen bonding groups in PyUr have little influence on the course of the reaction. To explore further the reactivity of the palladium complexes, the reaction between Pd(CH3)Cl(PyUr)]BF4] (2) and CH2 CHCH2OH was carried out to yield the allyl complex Pd(η3-CH2CHCH2)(PyUr)] (6). The crystal structure of this complex is also reported. |
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Keywords: | Palladium Catalysis Hydrogen bonding Olefin-CO copolymerization |
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