Pyridine-amide-type ligands

Acetonitrile and benzonitrile, aliphatic amines, pyridine, amides (especially DMF), alcohols, and propylene-carbonate are used as solvents in the syntheses of this type. All of them have properties of ligands. Moreover, such classic N-donors as 2,2 -bipy and 1,10-phen were used as ligands, as well as compounds with P-(triphenylphosphine) and O- donor centers (urea, pyridine-N-oxide, triphenyl-phosphinoxide) and chelating ligand systems ((3-diketones, 8-hydroxyquinoline, dimethylglioxime) [202]. 

Bis(pyrrole-2-aldiminato)nickel(II) complexes (125) are diamagnetic in the solid state when R = H, Pr, Pr and Et, and paramagnetic pseudotetrahedral when R = Bu. 1001-1003 In solution there exists an equilibrium between square planar and tetrahedral species when R = Pr1, Bus and Bu Such equilibria were also investigated for complexes of the type (126) obtained from the condensation reaction in basic media of o-aminobenzaldehyde and a number of diamines in the presence of nickel(II).1004-1007 Square planar complexes (127)1008,1009 and (128)1010 were obtained with deprotonated pyridinecarboxamide ligands. In these complexes the Ni—N (amide) bond distance (184-187 pm) is shorter than the Ni—N (pyridine) distance (192-195 pm). 

The solubility of NdX3 catalysts is improved by the addition of electron donors (D). Catalyst activity is remarkably increased without substantial deterioration of the cis- 1,4-content. Typical donor ligands applied in NdX3 Dn/AlR3 type systems are alcohols such as EtOH [92,112,113], 2-ethylhexanol [114] or various pentanol isomers [115]. Furthermore, tetrahydrofuran (THF) [35], tributyl phosphate (TBP) [116-119], alkyl sulfoxides [116,117,120,121], propion amide [122,123], B(0-CH2-CH2-0-CH2-CH2-0H)3/B(0-CH2-CH2-0-C2H5)3 [124,125], pyridine [126] and dioxides [127,128] are applied as donors. The increase in catalyst activity by donor ligands is attributed to the improved solubility of the active species in hydrocarbon solvents [129,130]. 

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