Controlling the Lability of Square-Planar PtII Complexes through Electronic Communication between π-Acceptor Ligands
✍ Scribed by Deogratius Jaganyi; Andreas Hofmann; Rudi van Eldik
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 88 KB
- Volume
- 40
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
on the occasion of his 65th birthday
Studies on the substitution mechanism of low-spin d 8 square-planar complexes for many years centered around the s trans influence or trans effect. For Pt II complexes this has involved detailed systematic studies of different trans groups using a range of different nucleophiles. Mechanistic studies established that ligand-substitution reactions of Pt II complexes mainly occur by an associative process involving a trigonal-bipyramidal intermediate. In recent years, volumes of activation (DV = ) obtained from high-pressure kinetic measurements, have been used in distinguishing mechanistic pathways of substitution reactions; negative values indicating degree of bond cleavage in the transition state is significantly higher for complex 3 than in 1 and 2.
Returning to the motivation for this work, our results on the water-exchange mechanism of the porphyrins studied are in excellent agreement with the mechanistic interpretation offered by Ford and co-workers for the complex-formation reactions of 1 and 3 with NO. Their reported activation volumes of 8.3 AE 1.5 and 13 AE 1 cm 3 mol À1 for these reactions, respectively, are almost identical to those reported for the water-exchange reactions in the present study. Their conclusion that the observed DV = for the ªonº reaction with NO mainly represents DV = (k 1 ) for reaction (1), is perfectly correct as shown by the data reported here. Thus the formation of 1 and 3 is not only controlled by the rate but also by the mechanism of the water-exchange process. Depending on the structural and electronic situation this process tends to occur according to an I d or D mechanism, the complex-formation reactions with nucleophiles such as NO follow the same mechanism.
The water-exchange rate and associated activation enthalpy of iron(iii) porphyrins are significantly affected by the charge on the porphine and to a lesser degree by steric compression. The mechanism of the process, however, is controlled by steric factors and varies between a dissociative interchange and a limiting dissociative mechanism. Thus the lability of the axialbound solvent molecules in these systems plays a key role in the mechanism and substitution behavior of porphyrin-and heme-based systems. High-pressure NMR spectroscopic techniques present a powerful tool to add to the mechanistic understanding of such processes, which could lead to a more profound understanding of the reactions and processes in biologically relevant macrocyclic systems such as metmyoglobin and cytochrome P-450.
Experimental Section
Na 3 [Fe III (TPPS)(H 2 O) 2 ] (Na 3 -1) was synthesized as described elsewhere. Fe III (TMPyP)(H 2 O)(OH) 4 (2-pts 4 ), where pts p-toluenesulfonate, and Na 3 [Fe III (TMPS)(H 2 O) 2 ] (Na 3 -3) were purchased from Frontier Scientific Ltd. Fine Chemicals Utah, USA, and used without further purification. Ca. 20 % enriched 17 O-labeled water (D-Chem Ltd. Tel Aviv, Israel) was used for the 17 O NMR water-exchange measurements. NaClO 4 (Aldrich) was used to adjust the ionic strength to 0.5 m, and HClO 4 (1 and 3) and tosylic acid (2) were used to adjust the pH of the solution. No salt was added in the case of 2 to avoid precipitation. The porphyrin samples were prepared by combining weighted amounts of salt, perchloric or tosylic acid, and water. The resulting solution was transferred to the NMR tube. The pH was determined on identical samples prepared in ordinary water. The water exchange measurements were performed at pH 3 (for 1 and 3) and pH 1.1 (for 2), where only the monomeric aqua forms of the porphyrins are present in solution. The complex concentrations were 3.4 Â 10 À2 m (1), 2.0 Â 10 À2 m (2), and 3.0 Â 10 À2 m (3).