Influence of Mixed Substituents on the Macrocyclic Ring Distortions of Free Base Porphyrins and Their Metal Complexes

Crystal structures of a series of free base porphyrins, 2,3,12,13-tetra(cyano/chloro/bromo)-5,7,8,10,15,17,18,20octaphenylporphyrin solvates [H-2(TPP(Ph)(4)(CN)(4))center dot 3(C2H4CI2), H-2(TPP(Ph)(4)CI4)center dot 2(CH3OH), and H-2(TPP(Ph)(4)Br-4)center dot 2THF center dot 1.5(CH3OH)], were determined to examine the influence of mixed antipodal beta-pyrrole substitution on the stereochemistry of the porphyrin macrocycle. Nonplanarity of the porphyrin skeleton increases with an increase in size of the X group at the P-pyrrole positions, and the root-mean-square deviation of the core atoms follows the order CN (0.508 angstrom) < CI (0.687 angstrom) < Br (0.792 angstrom). The normal-coordinate decomposition analysis of the free-base structures shows dramatic substituent- (X-)dependent out-of-plane distortions featuring saddling combined with a ruffled conformation in H-2(TPP(Ph)(4)(CN)(4)), while it is predominantly saddled geometry in H-2(TPP(Ph)(4)X-4) (X = CI, Br) structures. For H2(TPP(Ph)4X4) (X = Cl, Br) structures, the core elongation is along the antipodal pyrroles bearing halogen groups, and in the case of the H2(TPP(Ph)4(CN)4) structure, it is along the other antipodal pyrroles with phenyl groups. However, the average core, N center dot center dot center dot N separation. along the transannular pyrrole direction follows the trend H-2(TPP(Ph)(4)(CN)(4)) (4.134(4) angstrom) < H-2(TPP(Ph)(4)CI4) (4.184(5) angstrom) < H-2(TPP(Ph)(4)Br-4) (4.205(5) angstrom). The bond lengths of the 24-atom core are comparable, but its bond angles showed significant differences along the antipodal direction bearing P-pyrrole with X groups when compared to the other transannular pyrrole direction. The four-nitrogen porphyrin core (N4H2) exhibited weak intramolecular hydrogen bonding and also intermolecular interactions. Interestingly, H-2(TPP(Ph)(4)CI4)center dot 2(CH3OH) shows an extended chain structure involving hydrogen-bonding interactions between the CH3OH center dot center dot center dot OHCH3 (O center dot center dot center dot O) and CH3OH center dot center dot center dot core (N4H2) interactions. The nonplanar geometry of these free base porphyrin rings suggests a more predominant influence of steric crowding of the peripheral substituents rather than intermolecular interactions. The four-coordinated Ni(TPP(Ph)(4)(CN)(4)) center dot C6H14 center dot 0-5(C2H4CI2) complex shows an enhanced ruffling of the macrocycle along with the saddled conformation relative to more saddle-shaped H-2(TPP(Ph)(4)(CN)(4))center dot 3(C2H4CI2) structure. The crystal structure of the Zn(TPP(Ph)(4)CI4)(PY) center dot (C2H4CI2) complex features distorted square-pyramidal geometry with the reduction in the nonplanarity of the core in contrast to its free base porphyrin structure. Normal-coordinate-decomposition analysis for the out-of-plane displacement of the core atoms in the Ni(TPP(Ph)4(CN)4) structure showed enhanced ruffling combined with saddling of the macrocycle while Zn(TPP(Ph)(4)CI4)(Py) exhibited mainly saddling when compared to their corresponding free base porphyin structures. The nonplanar distortion in the Ni(TPP(Ph)(4)(CN)(4)) center dot (C6H14) center dot 0.5(C2H4CI2) complex indicates that the contracted porphyrin core and the weak intermolecular interactions are responsible for the nonplanar geometry of the macrocyclic ring.