X-RAY ANALYSIS OF PHTHALOCYANINS 



81 



synthetic compounds, the phthalocyanins (Linstead, 1751), and this 

 evidence has also put beyond doubt the completely planar structure 

 of the porphin nucleus. Phthalocyanin is a tetrabenzotetrazaporphin 

 (c/. Section 8), i.e., a porphin in which the four methene groups 

 combining the pyrrole rings are replaced by tertiary nitrogen and 

 the four pyrrole rings are condensed with benzene rings in their 

 jS-positions (Fig. 19). 



Fig. 19. Phthalocyanin. 



Phthalocyanin was the first organic structure to yield to an absolute 

 direct x-ray analysis from a double series of absolute intensity meas- 

 urements of reflections with and without a central metal atom, e.g., 

 nickel, by which the hydrogen atoms in the center of the molecule 

 can be replaced as in the porphyrins {2282-228 If). This alters little 

 the position of the other atoms in the molecule. 



Phthalocyanin is a large, strictly flat molecule, 9.9 X 12.5 X 4.7 A, 

 i.e., of the thickness of flat aromatic molecules. The four benzene 

 rings are regular hexagons with 120° angles and a carbon-to-carbon 

 distance of 1.39 A. There is no indication of the quinoid ring (I in 

 Fig. 19) and the whole molecule is centrosymmetrical. In the closed 

 inner ring system of eight carbon and eight nitrogen atoms, the dis- 

 tances (1-16) are all equal, of 1.34 A length, which indicates resonance 

 character. The eight carbon-to-carbon bonds combining the benzene 

 rings with their inner rings {a-h) are again all equal, and their length 

 (1.49 A) indicates only weak double bond character. There is no 

 true difference between pyrrole and pyrrolene nuclei. There is a 

 distinct deviation however, from tetragonal symmetry in free phthalo- 

 cyanin, two pairs of pyrrole nitrogen atoms being much closer in one 

 direction than in the other perpendicular to it, the distance in the 

 former direction being 2.65 A, corresponding to the distance of the 



