Axial ligand, effect

Organocobalt B models axial ligand effects on the structure and coordination chemistry of coba-loximes. N. Bresciani-Pahor, M. Forcohin, L. G. Marzilli, L. Randaccio, M. F. Summers and P. J. Toscano, Coord. Chem. Rev., 1985, 63,1 (263). 

Highly enantioselective epoxidation of unfunctionalized alkenes was developed by using chiral metalloporphyrin catalysts.1214-1218 Remarkable anion axial ligand effects were observed with [Fe(TPFPP)X] complexes (X = triflate, perchlorate, nitrate).1219 Hexafluoroacetone was found to be an efficient cocatalyst with H202,1220 and alkenes could be epoxidized by ozone at ambient temperature.1221... 

Kumar D, de Visser SP, Sharma PK et al (2005) The intrinsic axial ligand effect on propene oxidation by horseradish peroxidase versus cytochrome P450 enzymes. J Biol Inorg Chem 10 181-189... 

Spectroelectrochemistry [Fig. 41(a,h)] measurements showed that the reversible wave at 0.2 V involves two electrochemical processes, corresponding to the Ru (III)Ru(III)Ru(III)/Ru(III)Ru(III)Ru(II) E° = 0.21 V) and Co(III/II)P( ° = 0.07 V) redox pairs. Surprisingly, in the him, there is an inversion in the redox potentials observed in solution (Table IV), so that the peripheral clusters are reduced before the cobalt porphyrin. This fact was ascribed to axial ligand effects, in changing from the acetonitrile solution to the solid-hlm-water interface (170). This is a very important aspect since now the peripheral complexes in the reduced form can act as electron relays enhancing the catalytic activity of the cobalt porphyrin center. 

Gross, Z. and S.A. Nimri (1994). Pronounced axial ligand effect on the reaetivity of oxoiron(lV) porphyrin eation radicals. Inorg. Chem. 33, 1731-1732. 

Antony, J., M. Grodzicki, and A.X. Trautwein (1997). Local density functional study of oxo-iron(IV) porphyrin complexes and their one-electron oxidized derivatives. Axial ligand effects. J. Phys. Chem. A 101, 2692-2701. 

Recent studies (81) on the axial ligand effect on substrate hydroxylation further generalized the H-abstraction trends and showed that the trends are also applicable when one substrate and a selection of oxidants is investigated. However, the free energy of activation in that case is proportional to BDEqh rather than BDEqh, which was explained with a VB diagram. 

Recent studies of oiu-s (123) on the axial ligand effect of aromatic hydroxylation by [Fe =0(Por+ )53 with X a variable axial ligand showed that the axial ligand effect manifests itself on the kinetics of a chemical reaction, whereby the fi ee energy of activation was found to be proportional to BDEqh-... 

Lin CL, Fang MY, Cheng SH (2002) Substituent and axial ligand effects on the electrochemistry of zinc porphyrins. J Electroanal Chem 531 155-162... 

The effect of axial ligands on ground state properties of complexes with orbitally degenerate ground terms. G. A. Webb, Coord. Chem. Rev., 1969, 4,107-145 (151). 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

The photochemically active bands of methylcobalamin have been identified as the intense hands due to -n—n transitions within the conjugated corrin ring, and the following quantum yields (< ) were obtained A = 490 nm, Similar quantum yields ( = 0.3-0.5) were also obtained for the photolysis of methylcobalamin in acid, where the base has been displaced and protonated, and the complex is present as a mixture of the methylaquo and five coordinate methyl complexes (/40). The effect of varying the second axial ligand on the rate of photolysis by white light has also been studied (134). 

Most of the examples listed are pentacyanide, corrinoid, or DMG complexes. The axial ligands are not identified in the tables, but are as follows corrinoids, 5,6-dimethylbenziminazole (cobalamins), H O or none (cobinamides), (DMG)2, usually pyridine or H2O, less frequently NHj, imidazole, benzimidazole, PBuj, etc. The nature of the axial and equatorial ligands may have a striking effect on reactivity, but few direct comparisons are available these are discussed in the next section. 

Fukui et /. have found that acetylcobinamide reacts faster than the cobalamin with hydroxide (t,/2 = 4 and 30 min, respectively, in 0.05 N KOH) (67). It is not known whether the cobinamide is five- or six-coordinate, but these relative rates establish the labilizing effect of the trans-axial ligand for heterolysis to give Co(I) as HjO or none > 5,6-dimethylbenziminazole (see also Section B,3). 

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