Sitting-atop complex

The geometry of this initial complex 1 can be described in terms of a sitting-atop complex (SAT) (Fig. 2), the formation of which has been proposed for a variety of reactions between porphyrins and metal ions in solution (See Section IV.A) (92). 

The reaction of porphyrins with metal ions Regular and sitting-atop complexes... 

With fully water-soluble porphyrins, the metal-incorporation rate has been found to be first order in free base, metal, and anion. However, it has been suggested that during incorporation of metals in poorly soluble porphyrins, which would correspond to the situation of polymeric porphyrins, intermediate ( sitting-atop complexes) can be formed. Incorporation kinetics were suggested to be second order in metal and first order in porphyrin. The number of monolayers of polymeric porphyrins occupied by Ni(II) increases with its initial concentration. At trace concentration levels, film thickness is not important because only the first few layers are occupied, and the analyte signal does not depend on film thickness. The concentration levels (10 -10 M) and preconcentration times (40-100 seconds) are not sufficient to exceed the film capacity of monolayer film to incorporate Ni(II). However, for high Ni(II) concentra-... 

Metal ion incorporation into porphyrin molecules is believed to occur through an intermediate, called the sitting-atop-complex, SAT. The metal ion rests on top of the porphyrin before it is completely incorporated. This mechanism has been verified by UV and IR spectroscopy. The active species involved in metal... 

Hsiao Y-W, Ryde U (2006) Interpretation of EXAFS spectra for sitting-atop complexes with the help of computational methods, hiorg Chim Acta 359 1081-1092... 

The formation of the complex between the free base (PH2) and the metal ion has been extensively studied. It was found that the complicated metal incorporation mechanism can be best interpreted for the reaction of the dipositive metal ion and the neutral chelating ligand. However, this does not mean that the anionic porphyrin species (PH and P ) are not reactive. In fact, they are present in very small concentrations, so small that they can hardly be detected. On the other hand, the cationic porphyrin species are definitively unreactive. Earlier works, of some twenty years ago, preferred the mechanism of metal incorporation which assumes a pre-equilibrium between the free base (PH2) and the dissociated protons of pyrrole nitrogens, so that the metal cation would actually react with the P " dianion. Later works, however, indicate the existence of the so-called SAT (sitting-atop) complex intermediate, which was already foreseen in 1960 by Fleischer and Wang. It was later shown that the SAT metal-ion intermediate can deform porphyrin, thus... 

Is this the rds There is striking correlation of the exchange rate constants for MS. and the values of k in (4.81). In addition the volumes of activation are positive for solvent exchange and interaction of M(dmf).+ (M = Mn, Co, Ni, Zn and Cd) with N-Metpp in dmf (compare Table 4.4). In spite of these two facts however, it is considered that one of two further steps, probably the first, controls the overall rate. A sitting-atop (SAT) complex is formed in which metal is attached by two bonds to the porphyrin and two N - H bonds remain intact. 

Several metal insertion mechanisms have been proposed, but none of them is conclusive.18 The rate of metallation varies from square root to second order in metal salt from one system to another, and apparently there exists more than one pathway. Where the rate law is second order in metal salt, a so called sitting-atop metal ion-porphyrin complex intermediate or metal ion-deformed porphyrin intermediate, which then incorporates another metal ion into the porphyrin centre, has been postulated (Figure 3).19 For the reactions with the square root dependence on the metal salt concentration, the aggregation of metal salts is suggested.18 Of course, there are many examples which follow simple kinetics, i.e. d[M(Por)]/df = k[M salt][H2Porj. 

The reaction between porphycene, and second and third row transition-metal carbonyls has also been investigated. Here, it was found, for instance, that the reaction between decacarbonyldirhenium (Re2(CO)io) and tetrapropylporphycene 3.20 in decalin leads to the formation of the bis[tricarbonylrhenium(I)] complex 3.86 (Scheme 3.1.13), wherein the two metal centers are bound in a sitting-atop fashion (as judged from a single crystal X-ray diffraction analysis Figure 3.1.17). On the other hand, when porphycene 3.20 is reacted with Ru3(CO)i2 in... 

Figure 28.20. Complex Formed by TATA-Box-Binding Protein and DNA. The saddlelike structure of the protein sits atop a DNA fragment that is both significantly unwound and bent.

A related substance, another sitting-atop model, is a bischloromercury(II) complex of N-tosylaminooctaethylporphyrin. The two Hg ions are on opposite sides of the porphyrin plane and are in quite different coordination environments. One Hg ion is coordinated to the tosylamino nitrogen and a chloride ion in a linear fashion. The second Hg ion is coordinated to the three other porphyrin nitrogen atoms and one chloride ion. (See structure F). 

The question then arises is this the rate determining step of the reaction A sitting -atop (SAT) complex 1.3 is formed in which the metal is attached by two bonds to the porphyrin and the two NH bonds remain intact... 

---