AXIAL COORDINATION AND CONFORMATIONAL HETEROGENEITY OF NICKEL(II) TETRAPHENYLPORPHYRIN COMPLEXES WITH NITROGENOUS BASES

Axial ligation of nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) w ith pyrrolidine or piperidine has been investigated using X-ray crysta llography, UV-visible spectroscopy, resonance Raman spectroscopy, and molecular mechanics (MM) calculations. By varying the pyrrolidine conc entration in dichloromethane, distinct nu(4) Raman lines are found for the four-, five-, and six-coordinate species of NiTPP. The equilibriu m constants for addition of the first and second pyrrolidine axial lig ands are 1.1 and 3.8 M-1, respectively. The axial ligands and their or ientations influence the type and magnitude of the calculated nonplana r distortion. The differences in the calculated energies of the confor mers having different ligand rotational angles are small so they may c oexist in solution. Because of the similarity in macrocyclic structura l parameters of these conformers and the free rotation of the axial li gands, narrow and symmetric nu(2) and nu(8) Raman lines are observed. Nonetheless, the normal-coordinate structural-decomposition analysis o f the nonplanar distortions of the calculated structures and the cryst al structure of the bis(piperidine) complex reveals a relationship bet ween the orientations of axial ligand(s) and the macrocyclic distortio ns. For the five-coordinate complex with the plane of the axial ligand bisecting the Ni-N-pyrrole bonds, a primarily ruffled deformation res ults. With the ligand plane eclipsing the Ni-N-pyrrole bonds, a mainly saddled deformation occurs. With the addition of the second axial lig and, the small doming of the five-coordinate complexes disappears, and ruffling or saddling deformations change depending on the relative or ientation of the two axial ligands. The crystal structure of the NiTPP bis(piperidine) complex shows a macrocycle distortion composed of wav (x) and wav(y) symmetric deformations, but no ruffling, saddling, or d oming. The difference in the calculated and observed distortions resul ts partly from the phenyl group orientation imposed by crystal packing forces. NIM calculations predict three stable conformers (ruf, sad, a nd planar) for four-coordinate NiTPP, and resonance Raman evidence for these conformers was given previously.