Representation of nonplanar structures of nickel(II) 5,15-disubstituted porphyrins in terms of displacements along the lowest-frequency normal coordinates of the macrocycle

The influence of substituents with increasing steric demands on the structure of nickel(II) 5,15-disubstituted porphyrins has been investigated with X-ray crystallography, UV-visible absorption spectroscopy, resonance Raman spectroscopy, molecular energy optimization calculations, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporphyrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrins (where the alkyl group is propyl, isopropyl, and tert-butyl) are calculated to be in a predominantly gabled (gab) conformation resulting from an alpha alpha orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear combination of distortions along the lowest-frequency out-of-plane macrocycle normal coordinates of A(2u) (doming) and B-1u (ruffling) symmetry types. A higher energy stable ap conformer is also predicted for dialkylporphyrins, and its nonplanar structure can be represented as an equal combination of distortions along the x- and y-components of the lowest-frequency E(g) (waving) normal coordinate. The nonplanar structures calculated by using molecular mechanics have been structurally decomposed into the displacements alone the lowest-frequency normal coordinate of each symmetry type, and the contributions of each type to the total distortion in the calculated structures agree well with contributions obtained from structural decompositions of the available X-ray crystal structures. The predicted gab distortion is confirmed most convincingly by the X-ray crystal structure of [5,15-di-tert-butylporphinato]nickel(II) which is found to be in a gab alpha alpha conformation. Finally, INDO/s calculations show that the red shifts in the absorption spectra of the nickel disubstituted porphyrins are caused by the increasing nonplanarity resulting from increasing steric crowding within the series.