THE KINETIC INFLUENCE OF INCREASING STERIC HINDRANCE TO METALLATION BY ZINC(II) WHICH ACCOMPANIES THE INCLUSION OF 5,10,15,20-MESO(TETRAPHENYL)PORPHYRIN INTO CAPPED AND QUADRUPLY BRIDGED CLOSELY SPACED BIS-PORPHYRIN STRUCTURES

摘要

The solvent system toluene:acetic acid:methanol = 5:20:75% (vol./vol.) has been developed for metallation of porphyrins. This system is quite suitable for kinetic studies because it affords stable homogeneous solutions of porphyrins plus metal salts. In this solvent system, Zn2+ insertion into TPP (IH2) is more than 10(4) times faster than observed in DMF. Kinetics of Zn2+ incorporation into other porphyrins, C4-capped porphyrin (IIH2), cofacial bis-porphyrin (IIIH4) and spheroidal bis-porphyrin (IVH4), were then studied in this mixed solvent at 30.0 degrees C. It is found that Zn2+ insertion into IIIH4 is first order in both [Zn2+] and [IIIH4]. The final kinetic product was characterized as IIIZn2. The mono Zn2+ insertion product (IIIZnH2) was separately synthesized and characterized. The kinetics of the stepwise Zn2+ insertion leading to IIIZn2 was followed by HPLC. The second order rate constants for the first and second insertion reactions are comparable (0.59 and 0.21 s(-1) M(-1)). Because both sides of the bis-porphyrin IVH4 are fully protected, the metallation was extremely slow. The relative rates of Zn2+ insertion into the four porphyrins (IH2, IIIH4, IIH2, IVH4) are 32 000 000:480 000:3 000:1. [References: 27]