Chromium macrocyclic complex

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

In comparison with some other metal ions, limited synthetic work has been carried out on chromium(III) complexes with macrocyclic ligands. The vast majority of reported complexes involve tetraazamacrocycles. 

Dillon CT, Lay PA, Bonin AM, et al. 1993. In vitro DNA damage and mutations induced by a macrocyclic tetraamide chromium(V) complex implications for the role of Cr(V) peptide complexes in chromium-induced cancers. Carcinogenesis 14(9) 1875-1880. 

Table 64 Some Properties of Macrocyclic-chromium Nitrosyl Complexes ...

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

R. Kumar and R. Singh, Chromium(III) complexes with different chromospheres macrocyclic ligand, synthesis and spectroscopic studies, Turkish Journal of Chemistry, vol. 30, no. 1, pp. 77-87, 2006. 

When hydrated chromium]III) chloride reacts with L739 in ethanol, the compound [Cr(L739)Cl2]Cl is formed (Eq. 3.51). Treatment of the latter with 2,6-dfp imder conditions of the transient template reaction mentioned above gives rise unexpectedly to [Cr(L742)(H20)2]Cl3-2H20, the structure of which was established by X-ray diffraction methods. In this compound the central atom is in a regular pentagonal-bipyramidal environment, with the axial positions occupied by water molecules. This is a rare example of a seven-coordinate macrocyclic complex of chromium(III) [133],... 

The use of base-catalysed reactions for the template synthesis of co-ordinated, often macrocyclic ligands was discussed in the introduction to this chapter. " Chromium(m) Complexes.—Studies of the base hydrolysis of chromium(ra) complexes at high pH are relatively rare, probably because of the ease with which polymeric hydroxy-complexes can be precipitated. Studies of aqua-chromium(m) complexes even at low pH invariably show that conjugate-base formation is important owing to the acidity of the co-ordinated water molecules. Conjugate-base formation is apparent when the observed pseudo-first-order rate constant, k, varies with acidity according to the equation A =A o+ -x/[H+]. Recent examples include studies of the [Cr(HaO)6(NHs) + and [Cr(ox)2(N3)(H20)]2- ions." ... 

Mock MT, Chen S, O Hagan M, et al. Dinitrogen reduction by a chromium(O) complex supported by a 16-membered phosphoms macrocycle. J Am Chem Soc. 2013 135 11493-11496. 

The stereochemistry of the ligands can also significantly affect the thermally activated relaxation of the E state of chromium(III). From the observed difference in lifetime between cis- and traiw-Cr([14]aneN4)(NH3)2 it has been suggested that ligand stereochemistry is the cause of this difference (Fig. 2.3). Further work with other similar macrocyclic complexes has substantiated this suggestion. 

Berkessel et al. have used an asymmetric Nozaki-Hiyama-Kishi reaction for the synthesis of several laulilamide analogs. (/ ,/ )-DlANANE ligand 14 was used for the key NHK coupling of the macrocyclic aldehyde 61 and iodide 62. Ten mol% of the chromium(ll) complex of 14 overcame the substrate selectivity and afforded the desired configuration in a dr of 78 22 and yield of 43% (Scheme 12.40). 

The literature available from the end of the last report (December 1989) to September 1991 is covered in this chapter. A complete revision of the International Union of Pure and Applied Chemistry (lUPAC) " Nomenclature for Inorganic Chemistry has appeared and lUPAC-recommended ligand abbreviations will be used wherever possible. Research activity in chromium chemistry continues at about the same level as in the past, but there are odd surges as new techniques " or complexes become available. As in previous years, the general chemistry of chromium has been reviewed. l Other, more specialist reviews include the spectroscopy of Cr(VI), organochromium(III) chemistry,and macrocyclic complexes of chromium in various oxidation states.Closer to the mechanistic area is a review of the photophysics of chromium(III) complexes and, more specifically, the photochemical water-exchange process in chromium(III) complexes. A summary of new insights into the mechanism of spontaneous and base-catalyzed substitution reactions of inert-metal amine complexes has also appeared. ... 

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