%0 Journal Article
%T Influence of the Steric Demand of Coligands on the Catalytic Activity of Nickel(II) Complexes in the Copolymerization of Ethene and Carbon Monoxide
%A Monika M. Lindner
%A Udo Beckmann
%A Walter Frank
%A Wolfgang Kl£¿ui
%J ISRN Inorganic Chemistry
%D 2013
%R 10.1155/2013/732629
%X The copolymerization of ethene and carbon monoxide can be catalyzed with square planar arenidotriphenylphosphane nickel(II) complexes containing an N,O-chelate ligand. To examine the influence of the phosphorus ligand on the catalytic activity, five new nickel(II) complexes with P-ligands of different basicities and different steric demands were synthesized and fully characterized including the determination of the crystal structures of three of the complexes. The investigation of the catalytic activity of the new compounds showed a decisive influence of the steric properties of the P-ligand. A minimum steric demand is essential to ensure catalytic activity. 1. Introduction Phosphanes are widely used as ligands in catalysts for reactions such as hydroformylations, hydrogenations, and oligomerizations in organometallic and coordination chemistry. A large variety of substituents leads to a broad scope of different phosphorus ligands on the one hand, for example, tertiary phosphanes and phosphites, and on the other hand makes the successive variation of the substituents possible to create P-ligands with fine-tuned properties. A well-directed variation of the spectator ligands allows to control activity, selectivity, and productivity of a catalyst complex [1¨C5]. The coordination properties of P-ligands are determined by the electronic effects of the substituents on the phosphorus atom, arising from the substituents¡¯ inductive effects, and by the steric demand of the substituents. A positive inductive effect leads to a high electron density at the phosphorus atom, while a negative inductive effect reduces the electron density at the P-atom which also affects the electronic situation at the metal centre. The electronic effects thus influence the basicity of the phosphorus atom. The basicity of phosphanes has been widely documented in the literature [6¨C11]. A useful concept to assess the basicity of a P-ligand is the electronic parameter defined by Tolman in 1976 who used the frequency of the A1 CO stretching vibration in Ni(CO)3L to rank different P-ligands L in a series [12, 13]. The more basic a ligand, the lower is the CO stretching frequency and the higher is the coordination ability of the ligand. The steric properties of a P-ligand were also described by Tolman in his concept of the cone angle [12, 14] which offers a good estimation of the size of a P-ligand with small ligands showing better coordination abilities. Phosphanes are ubiquitous ligands in transition metal catalysts. By far the most popular phosphane for large scale processes is
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