Synthesis and Characterization of Palladium Complexes Containing 3,3'-Annelated 2,2'-Bipyridines and 3,3'-Annelated 2,2'-Biquinolines

Abstract

The synthesis and characterization of a number of PdCl2(N⁀N) and Pd(N⁀N)(fn) complexes (N⁀N = 3,3'-annelated 2,2'-bipyridines and 3,3'-annelated 2,2'-biquinolines, fn = fumaronitrile) have been described. Despite the variation in dihedral angle and distance between the nitrogen lone pairs in these series of ligands, they are all capable of forming stable chelates. An investigation was made as to whether the 3,3'-trimethylene-2,2'-biquinoline (3-bq) and 3,3'-tetramethylene-2,2'-biquinoline (4-bq) ligands could serve as C2-chiral auxiliaries. The ligand 3-bq in Pd(3-bq)(fn) turned out to be conformationally rigid on the NMR time scale, but the diastereomers appeared to be indistinguishable, as could be concluded from the 1H NMR spectra. However, Pd(4-bq)(fn) exists in two diastereomeric forms and their ratio (57 : 43) could be determined from 1H NMR spectra. Assignment of the major and minor isomers was done with a 2D NOESY experiment. Epimerization of the diastereomers occurred on the laboratory time scale which was deduced from a spin saturation transfer experiment and also from the observation that diastereomerically enriched crystalline material of Pd(4-bq)(fn) at low temperature showed a ratio of 80 : 20 which deteriorated to a ratio of 57 : 43. From the spin saturation transfer experiment, the thermodynamic parameters for the inversion could be determined (∆H≠ = 29.1 kJ mol-1 and ∆S≠ = -136.3 J K-1 mol-1). This large negative entropy of activation favours an epimerization process via inversion of the coordinated 4-bq ligand over a mechanism via alkene dissociation/recoordination. The structures of the complexes have been studied in solution and in the solid state and the following X-ray crystal structures have been determined: (i) [dichloropalladium(II)](3,3'-trimethylene-2,2'-bipyridine-κ2N), monoclinic crystals, space group P21/n, with a = 8.4230(7) Å, b = 16.425(2) Å, c = 9.5836(7) Å, β = 106.316(7)°, Z = 4, final R = 0.033 for 2 309 reflections, (ii) [3,3'-bis(methoxycarbonyl)-2,2'-bipyridine-κ2N][dichloropalladium(II)], triclinic crystals, space group P1, with a = 7.340(4) Å, b= 10.012(3) Å, c= 11.954(3) Å, α = 98.10(2)°, β = 104.03(3)°, γ = 107.33(5)°, Z = 2, final R = 0.039 for 4 273 reflections, (iii) [chloromethylpalladium(II)](4,5-diazafluorene-κ2N), orthorhombic crystals, space group Pmcn, with a = 6.713(1) Å, b = 10.120(1) Å, c = 17.187(3) Å, Z = 4, final R = 0.048 for 949 reflections.