Palladium complexes containing ligands with hydrogen-bonding functionalities. Reactivity and catalytic studies with CO and olefins

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 CH₂CHCH₂OH was carried out to yield the allyl complex [Pd(?³-CH₂CHCH₂)(PyUr)] (6). The crystal structure of this complex is also reported.