Chromium complexes cyclam

The analysis of the experimental M-B distances has also allowed the location of some examples of ionic complexes described in the literature as MLn(q1-BH4). The long Cu-B distances preclude BLC- coordination. These examples are the copper complexes [Cu(Cyclam)]+[BH4]", [Cu(TMAC)]+ [BH4] , (TMAC = CyclamMe4) (see Table 3) and the chromium complex [(py Crtq1 -BH4)]+[BH4] . 

One way to overcome the above problem would be to suppress hydrolysis of the amine ligands by working with an appropriate amine buffer medium. This strategy has been used with great success by Andersen et al. to obtain quantitative equilibrium data for the formation of mononuclear amine complexes (195, 196). Andersen et al. have also studied the formation of polynuclear complexes under similar conditions, but equilibrium was not attained with respect to these species (40, 42, 60, 87). The fact, however, that both thermal hydrolysis and charcoal/chromium(II)-catalyzed hydrolysis in such an amine buffer medium give the same polynuclear species in almost identical ratios would seem to indicate that some degree of equilibration had been achieved. It therefore seems likely that these methods could, in principle, be modified so as to also be applicable for equilibrium studies. Quite a different approach would be to study complexes with macrocy-clic amines such as cyclam, which are known to have a reduced tendency to hydrolysis. However, such systems have not as yet been studied in detail. 

DeLeo MA, Ford PC. Photoreactions of coordinated nitrite ion. Reversible nitric oxide labilization from the chromium(III) complex [trans-Cr(cyclam)(ONO )2]+. Coord Chem Rev 2000 208 47. 

Chromium(lll) complexes Pressure effects for both the emission and photoreaction quantum yields under comparable conditions have now been described for several Cr(III) complexes in fluid solution including Cr(bpy)3, Cr(NH3)6 , Cr(NH3)5(NCS) + and the cis and trans isomers of Cr(cyclam)(NH3)2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) [3, 50-57]. There is a rich and subtle literature regarding the photochemistry of hexacoordinate Cr(IlI) complexes to which such pressure studies have contributed insight. 

Equates with 72 % retention of conliguration whereas photoaquation of the cyclam complex is 100% stereoretentive. Qualitatively the behaviour is again similar to that of chromium(iu) complexes but there are differences in detail. For example, the stereochemistry of the products indicates important differences in excited-state reaction mechanisms. 

House and Nor have studied the aquation (first stage) of trans-i 55R-[CoBr2(cyclam)] and a variety of chromium(III) complexes trans-l / ,5S-[CrBr2(3,2,3-tet)], trans-R,S-[CTBr2 2,3,2-iti)], and trans-RSSR-[CrBr2(tet a)] [tet a = C-meso-Mt cyclenn = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)] (see Table 7.2). Dissociative mechanisms are proposed for both the Cr(III) and Co(III) systems. The bromo/chloro aquation rate ratio is about 4 for Co(III) and about 15 for Cr(III). 

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