Inner hydrogen atom transfer in benzo-fused low symmetrical metal-free tetraazaporphyrin and phthalocyanine analogues: Density functional theory studies

Density functional theory (DFT) calculations were carried out to study the inner hydrogen atom transfer in low symmetrical metal-free tetrapyrrole analogues ranging from tetraazaporphyrin H2TAP (A0B0C0D0) to naphthalocyanine H2Nc (A2B2C2D2) via phthalocyanine H2Pc (A1B1C1D1). All the transition paths of sixteen different compounds (A0B0C0D0–A2B2C2D2 and A0B0CmDn, m ≤ n ≤ 3) are fully optimized at the B3LYP/6-31G(d) level and vibration analyses have been conducted to verify the optimized structures. It is revealed that the number and position of fused benzene rings onto the TAP skeleton have significant effect on the potential energy barrier of the inner hydrogen atom transfer. Introducing fused benzene rings onto the hydrogen-releasing pyrrole rings can increase the transitivity of inner hydrogen atom and thus lower the transfer barrier of this inner hydrogen atom while fusing benzene rings onto the hydrogen-accepting pyrrole rings will increase the hydrogen transfer barrier to this pyrrole ring. The transient cis-isomer intermediate with hydrogen atoms joined to the two adjacent pyrrole rings with less fused benzene rings is much stable than the others. It is also found that the benzene rings fused directly onto pyrrole rings have more effect on the inner hydrogen atom transfer than the outer benzene rings fused onto the periphery of isoindole rings. The present work, representing the first effort towards systematically understanding the effect of ring enlargement through asymmetrical peripheral fusion of benzene ring(s) onto the TAP skeleton on the inner hydrogen transfer of tetrapyrrole derivatives, will be helpful in clarifying the N–H tautomerization phenomenon and detecting the cis-porphyrin isomer in bio-systems.