Metallocenes, Strong Electron Donors. A Mechanistic Review

The great stability of a-metallocenyl carbocations is well known. There is general agreement that the electrons from the region between the two cyclopentadienyl rings are most effective in accounting for this stability. Electron transport may involve conjugation with the 7t-system of the pentadienyl rings or direct participation of metal electrons. The study of the secondary a-deuterium kinetic isotope effect (a-D KIE) can help, under certain circumstances, in solving the problem. We have recently determined the a-D KIEs in acetolysis and for- molysis of dideuterioferrocenylmethyl benzoate and found that, in the presence of LiClO.^ (which prevents reversion from the solvent- separated to contact ion-pairs), the ratios k^lkD at 25 °C are 1.53 ± 0.02 (acetolysis) and 1.48 ± 0.03 (formolysis). The solvolyses exhibited a special salt effect, indicating the presence of solvent- separated ion-pairs and the return to contact ion-pairs. The high values of the KIE strongly suggest that both solvolyses are limiting dissociation processes with a carbenium ion-like transition state, which is stabilized mainly by conjugation with the 7t-system of the pentadienyl rings. For both solvolyses, the ratios of Arrhenius pre-exponential factors AR / AD are significantly less than unity; whether tunnelling plays a role in causing this effect is discussed. Consistent with a dissociative (Sjjl) mechanism, in similar solvolyses a general tendency has been observed for faster reactions in solvents of higher ionizing power. However, we observed even a somewhat lower solvolysis rate in formic acid than in acetic acid, though the ionizing power of formic acid is much higher than that of acetic acid. This phenomenon is, at least partially, explained by the reaction being of the protonated ester, or protonate

Mehanizam formolize i acetolize ferocenilmetil benzoata

The formolysis and acetolysis of ferrocenylmethyl benzoate are proton catalyzed and accelerated by addition of perchloric acid. Our earlier experiments have shown that at a temperature of about 20 °C addition of a common benzoate ion in formolysis of 3.2 x 10–4 mol dm–3 ferrocenylmethyl benzoate suppresses the formolysis rate only slightly but the formolysis is significantly slower at 40 °C. Such rate lowering is not observed in acetolysis of the same substrate. Decrease of the formolysis rate at increased temperatures (20–40 °C) is most probably caused by the decrease of the hydrogen ion concentration, because the stronger formic acid displaces the weaker benzoic acid from sodium benzoate, thus reducing the formic acid concentration in this acid-catalyzed system. This, however, does not occur in the weaker acetic acid. Besides, the acetolysis rates are equal in the presence of 0.1 and 0.2 mol dm–3 sodium benzoate, while the formolysis rates decrease with the addition of benzoate. The strong temperature dependence of the formolysis rates is probably connected with the formation of formic acid dimers and the temperature dependent hydrogen bonding. Hydrogen bonding and the dimer formation in acetic acid are less pronounced than in formic acid.Formoliza i acetoliza ferocenilmetil benzoata katalizirane su protonima, a ubrzava ih dodatak perklorne kiseline. Naši raniji pokusi pokazali su da na temperaturi od približno 20 °C dodatak zajedničkog iona benzoata u formolizi 3,2 x10–4 mol dm–3 ferocenilmetil benzoata neznatno smanjuje brzinu formolize, dok je na 40 °C formoliza bitno sporija. Takovo smanjenje brzine ne zamjećujemo u acetolizi istog kompleksa. Smanjenje brzine formolize na povišenoj temperaturi (20–40 °C) vrlo je vjerojatno posljedica smanjenja koncentracije vodikovih iona, jer jača mravlja kiselina istiskuje slabiju benzojevu kiselinu iz natrijeva benzoata, što dovodi do smanjenja koncentracije mravlje kiseline u tom kiselinom kataliziranom sustavu. To se me|utim ne doga|a u slabijoj octenoj kiselini. Nadalje, brzine acetolize jednake su u prisutnosti 0,1 i 0,2 mol dm–3 benzoata, dok se brzine formolize smanjuju dodatkom benzoata. Izgleda da je jaka temperaturna ovisnost brzine formolize povezana sa stvaranjem dimera mravlje kiseline pri čemu je dimerizacija jako ovisna o temperaturi. Vodikove veze i stvaranje dimera u octenoj kiselini od manjeg su značaja nego u mravljoj kiselini

Metalloporphyrins. The nature of ligand bonding and the mechanism of replacements

The imidazole ring is an essential component of many biological systems (hemoglobin and myoglobin, nucleic acids, vitamin B^, cytochromes, metalloenzymes, etc.). The nature of the bond between imidazole-nitrogen and the metal is therefore of biological interest. Imidazole is an electron-donating ligand in both a and n sense, much more electron donating than the majority of nitrogen heterocycles. As ligands, most imidazoles are good a donors and moderate n donors. They can also function as n acceptors if the imidazole ring has one or more electron-withdrawing substituents. Our recent study of bonding modes of pyridine and imidazole type ligands in the transition state of a Conl(protoporphyrin IX) model complex enabled us to conclude that stabilization of the reaction transition state is more sensitive to the change in the strength of n bonding than in that of a bonding. This observation is important for some metallo-enzymatic reactions. Mechanisms of replacements in porphyrin and in corrin rings are discussed