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(η3-CH2CHCH2)(PyUr)] (6). The crystal structure of this complex is also reported.

Graphical abstractThe synthesis of a new palladium complex containing a pyridine-urea ligand is reported. The urea substituent on the ligand provides a weakly coordinating group (generating hemilability) and hydrogen bonding functionalities that could interact with incoming substrates. The reactivity of the complex towards CO and olefins has been studied. The complex has shown to be a good catalyst for CO/styrene copolymerization.Figure optionsDownload full-size imageDownload as PowerPoint slide