what is Porphyrin Ring ?

 Porphyrin Ring

large macrocyclic rigid system , composed of 4-modified pyrrole  subunits interconnected  with carbon atoms via methine (-CH=) bridges at their alpha position.

various position in porphyrin ring :

Some Features of Porphyrin ring :

• Total Number of double bond = 11
• Number double bond participate in aromaticity = 9 ( because we count only peripheral double bond) 
• Huckle rule of aromaticity :  
• 9 double bond = 18 pi electron.
•  (4n + 2) = (4×4 + 2) , Thus n = 4
• very rigid system , rigidity more than crown ethers.
• more selective for 3d- transition metal ion.
• porphyrin ring with metal is called metalloporphyrin. (e.g.. Hemoglobin , myoglobin) 
• porphyrin ring bind with metal (e.g. Fe) only in Di-anionic form.
• Stability of Ring with metal ion :
• Mn⁺² < Zn⁺² < Fe⁺² < Co⁺² < Cu⁺² < Ni⁺² 
• Geometry : Square planner (4 pyrrole ring joined by C-H unit)
• If metal size higher than the cavity size of ring then metal placed above the ring because aromatic ring can't undergo distortion.
• color of porphyrin ring is due to intra-ligand transition ( 丌→丌^✻)

porphyrin ring react with acid and base :

Spectral Features of Porphyrin ring :

1. Proton NMR :   
• Hydrogen                    chemical shift
• 4- meso hydrogen          4 ppm
• 8- beta hydrogen            8 ppm (Aromatic ring, and H- outside- De-shielded)
• 2 - NH hydrogen            2 ppm (H- inside the ring shielded)
2.   IR-spectra :
• NH- peak  -  3300 cm^-1 -  3400 cm^-1
• OH- peak -   3300 cm^-1  - 3400 cm^-1 (*** this Data not so much useful as NH and OH have stretching frequency.)

1. UV-vis spectra : 
• more intense Soret Band (or B-Band) (400-450 nm) due to pi-pi* transition.
• less intense Q-band (500-600 nm) due to nb-pi* transition.

• When porphyrin ring coordinate with metal Q-band disappear as non bonding electron involve in coordination thus nb-pi* transition not possible.

• How to disappear soret Band instead of Q-band ?

• some-how if we create disturbance in planarity then conjugation stopped and soret band disappear. 

Preparation :

color of ring (source)