Di(2-pyridyl)-amides and -phosphides: Syntheses, reactivity, structures, Raman-experiments and calculations

The 2-pyridyl containing compounds (2-Py)(2)NH1, (2-Py)(2)PH 2, Me2Al(2-Py)(2)N 3, Me2Al(2-Py)(2)P 4, Et2Al(2-Py)(2)N 5, Et2Al(2-Py)(2)P 6 and Et2Al(2-Py)(2)NAlEt3 7 have been synthesized and analyzed by solid state structure determination, FT-Raman spectroscopy and theoretical calculations in order to elucidate the charge density distribution. All di(2-pyridyl) amides and -phosphides coordinate the R2Al+ fragment via both ring nitrogen atoms, but the Lewis basicity of the central two-coordinated nitrogen atom in 5 is high enough to coordinate a second equivalent AlEt3 to form the Lewis acid base adduct Et2Al(2-Py)(2)NAlEt3 7. Several density functionals (BLYP, B3LYP, BPW91) have been examined in relation to various basis sets (6-31G, 6-31+G, 6-31G(d), 6-31+G(d)). This computational tool facilitates the unambiguous assignment of the Raman ring vibration frequencies. The shift to higher wavenumbers proceeding from the parent di(2-pyridyl)amine 1 and di(2-pyridyl)-phosphane 2 to the metal complexes 3 and 4 indicates partial double bond localization in the ring positions 3 and 5. This effect is more pronounced in the di(2-pyridyl)amide complexes than in the phosphide. Due to the higher electronegativity of the central nitrogen atom in 3, 5 and 7 compared to the bridging two-coordinated phosphorus atom in 4 and 6 the di(2-pyridyl) amide is the harder Lewis base. In the phosphides nearly all charge density couples into the rings leaving the central phosphorus atom only attractive for soft metals.