Crystallographic elucidations of indium(III) porphyrin conformations, morphology and aggregation behaviour: Comparative optical study of free base porphyrins and their indium(III) derivatives at varying pH

Abstract

The supramolecular structural architectures of 5, 10, 15, 20-Tetrakis-(4-methoxyphenylporphyrinato) indium (III) chloride (I) and 5, 10, 15, 20-Tetrakis-(4-methylphenylporphyrinato) indium (III) chloride (II) have been determined from their three dimensional X-ray diffraction data. Complex I crystallizes in the orthorhombic space group, Pbca with a = 23.0083 (4) Å, b = 15.2447 (3) Å, c = 23.3737 (4) Å, α = β = γ = 90°, V = 8198.5 (3) Å3 and Z = 8, whereas complex II crystallizes in the triclinic space group, P-1 with a = 12.0737 (5) Å, b = 12.1000 (5) Å, c = 14.1682 (6) Å, α = 88.560 (3)°, β = 78.730 (3)°, γ = 78.280 (3)° V = 1987.40 (14) Å3 and Z = 2. The geometries around In+3 ions in both the complexes are square pyramidal giving dome shaped architectures to the metalloporphyrins. In their respective crystal lattices, the monomeric units of complex I and II are packed in J-type (head-to-tail) supramolecular aggregation. These monomers are linked into their three dimensional supramolecular networks by various non covalent forces. Further the packing of monomers of each complex in their respective unit cells leaves significant void volumes (porosities). The optical absorption spectra of complex I and II and their free base analogues i.e. 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP and 5, 10, 15, 20-Tetrakis-(4-methylphenyl) porphyrin, H2TTP have also been investigated in CH2Cl2 at different ionic strengths of the media. The investigations revealed that both the free bases exist as simple monomers, j-aggregates and dianionic monomers at neutral (pH = 7), acidic (pH = 4 and lower) and basic (pH = 10) media conditions. Also at high ionic strength of the media, the free bases undergo charge transfer electronic transitions. The two In3+ porphyrin exists as simple monomers at pH = 7. However at pH = 5 and lower, the In3+ ions in complex II is remarkably easily replaced by the protons of the acid to generate the corresponding protonated free base monomers which simultaneously aggregate in J-type stacking mode. In contrast to complex II that exhibits low stability in acidic media, the complex I remain intact in such media reflecting extremely high stability of the complex. The insertion of In3+ ions into the porphyrinic cores of free bases causes bathochromic shift followed by increase in the intensities of absorption bands. The appearance of two set of doublets for ortho phenyl protons in the 1H NMR spectra of each complex is attributed to the magnetic inequivalence of their ortho phenyl protons caused by the pulling of In+3 ions above the mean porphyrin planes.