title: Influence of mixed substituents on the macrocyclic ring distortions of free base porphyrins and their metal complexes.
abstract: Crystal structures of a series of free base porphyrins, 2,3,12,13-tetra(cyano/chloro/bromo)-5,7,8,10,15,17,18,20-octaphenylporphyrin solvates [H(2)(TPP(Ph)(4)(CN)(4)) x 3 (C(2)H(4)Cl(2)), H(2)(TPP(Ph)(4)Cl(4)) x 2 (CH(3)OH), and H(2)(TPP(Ph)(4)Br(4)) x 2 THF x 1.5 (CH(3)OH)], 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 beta-pyrrole positions, and the root-mean-square deviation of the core atoms follows the order CN (0.508 A) < Cl (0.687 A) < Br (0.792 A). 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 = Cl, Br) structures. For H(2)(TPP(Ph)(4)X(4)) (X = Cl, Br) structures, the core elongation is along the antipodal pyrroles bearing halogen groups, and in the case of the H(2)(TPP(Ph)(4)(CN)(4)) structure, it is along the other antipodal pyrroles with phenyl groups. However, the average core, N...N separation along the transannular pyrrole direction follows the trend H(2)(TPP(Ph)(4)(CN)(4)) (4.134(4) A) < H(2)(TPP(Ph)(4)Cl(4)) (4.184(5) A) < H(2)(TPP(Ph)(4)Br(4)) (4.205(5) A). The bond lengths of the 24-atom core are comparable, but its bond angles showed significant differences along the antipodal direction bearing beta-pyrrole with X groups when compared to the other transannular pyrrole direction. The four-nitrogen porphyrin core (N(4)H(2)) exhibited weak intramolecular hydrogen bonding and also intermolecular interactions. Interestingly, H(2)(TPP(Ph)(4)Cl(4)) x 2 (CH(3)OH) shows an extended chain structure involving hydrogen-bonding interactions between the CH(3)OH...OHCH(3) (O...O) and CH(3)OH...core (N(4)H(2)) 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)) x C(6)H(14) x 0.5 (C(2)H(4)Cl(2)) 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)) x 3 (C(2)H(4)Cl(2)) structure. The crystal structure of the Zn(TPP(Ph)(4)Cl(4))(Py) x (C(2)H(4)Cl(2)) 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)Cl(4))(Py) exhibited mainly saddling when compared to their corresponding free base porphyrin structures. The nonplanar distortion in the Ni(TPP(Ph)(4)(CN)(4)) x (C(6)H(14)) x 0.5 (C(2)H(4)Cl(2)) complex indicates that the contracted porphyrin core and the weak intermolecular interactions are responsible for the nonplanar geometry of the macrocyclic ring.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India. pbhyrappa@hotmail.com
authors.forename: P
authors.initials: P
authors.lastname: Bhyrappa
authors.forename: C
authors.initials: C
authors.lastname: Arunkumar
authors.forename: B
authors.initials: B
authors.lastname: Varghese