5,10,15,20-Tetra-2-furylporphyrin

Molecules of the title macrocycle, C36H22N4O4, are located on an inversion center. The porphyrin ring shows a wave-like conformation with adjacent pyrrole rings tilted above the porphyrin plane and the interporphyrin distance is 3.584 (3) Å. The dihedral angles between the meso-furyl groups and the porphyrin plane are 38.87 (7) and 48.29 (7)°; these are much smaller than those observed for meso-tetraphenylporphyrin, indicating that the meso-furyl groups are more inclined towards the porphyrin plane. The decrease in the dihedral angle is due to the presence of intramolecular hydroden bonding between the meso-fury O atom and the β-pyrrole CH group. Intramolecular N—H⋯N hydrogen bonds are also present.

Financial support from the DST and CSIR, Goverment of India, to MR is gratefully acknowledged. The aromatic nature of (I) is evident from the observation that the Cα-Cβ distance is greater than the Cβ-Cβ bond distance.
The four inner pyrrole N atoms are almost in plane with four meso carbons. The bond distances and bond angles of (I) are altered relative to those of (II) revealing replacing phenyl groups with furyl groups at meso positions changes the porphyrin π-electron delocalization pathway. The dihedral angles of meso-furyl groups with respect to porphyrin plane in (I) are 38.87 (7)° and 48.29 (7)° and those of meso-phenyl groups in (II) are 61.0° and 61.3°. This significant decrease in the dihedral angle in case of (I) is due to presence of intramolecular hydroden bonding between meso-furyl "O" and β-pyrrole "CH".
As is clear from Figure 1, the four meso-furyl"O" are involved in hydrogen bonding with two β-pyrrole "CH" which are opposite to each other. This bonding helps in the significant reduction of dihedral angle of meso-furyl groups with the plane of the porphyrin. As a result the meso-furyl groups are inclined more towards the porphyrin plane resulting in extension of π-delocalization of the porphyrin ring to the furyl groups. The observed spectroscopic properties of (I), such as large red shifts in absorption and emission maxima and significant downfield shifts of NH and \b-pyrrole protons in NMR as compared to (II) also in agreement with the enhanced π-delocalization in (I). Thus, the crystal structure presented here indicates that the porphyrin (I) adopts more planar structure as compared to porphyrin (II).

Experimental
In a 500 ml one necked round bottom flask fitted an with argon bubbler, furan-2-aldehyde (286 mg, 2.98 mmol) and pyrrole (210 ml, 2.98 mmol) in 300 ml of CH 2 Cl 2 were condensed in the presence of BF 3 .OEt 2 (120 ml of 2.5 M stock solution) under argon atmosphere for 1 h followed by oxidation with DDQ (674 mg, 2.98 mmol) in open air for additional 45 min. The solvent was removed under reduced pressure and the crude compound was purified by silica gel column chromatography using CH 2 Cl 2 (62 mg, 12%). M. P. 300°C. Single crystals of (I) suitable for X-ray analysis were obtained by slow evaporation of a dichloromethane/n-hexane solution over a period of one week.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.