Palladium(0)-tetracyanoethylene complexes of diphosphines and a dipyridine with large bite angles, and their crystal structures

Complexes of Pd(tcne) (tene=tetracyanoethylene) containing bidentate ligands with large bite angles, bis[2-(diphenylphosphino)phenyl]ether (L-1), 4,6-bis(diphenylphosphino)-10, 10-dimethyl-10H-dibenzo[b,c][1,4]oxasiline (L-2), 4,6-bis(diphenylphosphino) 2,8-dimethylphenoxathiine (L-3), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (L-4) and trans-5,6-bis(2-pyridyl)bicyclo[2.2.1]hept-2-ene (L-6), were prepared and characterized. The compound 4,6-bis(diphenylphosphino)dibenzo[b,d]furan (L-5) did not form chelating palladium complexes, owing to its large natural bite angle of 138-degrees. The crystal structures of L-6, [PdL1(tcne)]-center-dot-2,5CH(2)Cl(2) 1, [PdL2(tcne)]-center-dot-4CH(2)Cl(2) 2, [PdL4(tcne)]-center-dot-2CH(2)Cl(2) 4, and [PdL6(tcne)] 5 have been determined. The similarity of electronic effects induced by the free diphosphines was demonstrated by MOPAC calculations. The geometries of the ligands, however, were most accurately predicted by molecular mechanics (MM2) calculations for the diphosphines, and MNDO for L-6. THe largest P-Pd-P angle in the zerovalent palladium complexes was found to be 104.6-degrees. A further increase in the natural bite angle of the ligand results in elongation of the Pd-P bond length in the complex rather than enlargement of the P-Pd-P bite angle. The ligand L-6 assumed a bite angle of 99.5(2)-degrees in complex 5, which is considerably smaller than its calculated value of 117-degrees.