Ethylenediamine labilization

Were the ethylenediamine labilization random rather than rules-based, Reactions 2, 3, 4, 5, and 6 (Figure 7) would occur with bout equal probability, and one would predict 2/3 cis- relative to trans-[Co(en)2(H20)Cl] which agrees very poorly with observation. Furthermore, in order to obtain the observed value of 0.6 for < >nh3/< ci by random nitrogen labilization, Reaction 1 would have to have a probability of about 0.6 relative to that for nitrogen labilization, or a probability of about 3 relative to Reactions 2, 3, 4, 5, or 6. One then expects about 4/5 m-relative to frans-[Co(en)2(NH3) (H20)] which is about that observed no distinction between the random and the labilization rules-based mechanism is thus possible in this case. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

Rate parameters for only a few substituted ethylenediamine chelates have been reported and, until recently, none for ethylenediamine chelates because the chemical shifts between the two forms in diamagnetic complexes (on which the analysis depends) are small and because of the extreme lability of the system. For the interconversion of the 5 and X, forms of... 

The preparation of salts containing the [Cr(en)3]3+ cation from anhydrous chromium sulfate has been described previously in Inorganic Syntheses,1 and the merits of this, and other, methods have been reviewed.9 A more rapid route to this cation involves refluxing CrCl3 6H20 in methanol with ethylenediamine and zinc metal, which allows the substitution to proceed by way of the kineti-cally labile chromium(II) species.10 All of these preparations yield hydrated salts the procedure described below leads to anhydrous [Cr(en)3] Br3. 

This is another of the very interesting contributions in Tobe s paper. Tobe has studied substitution reactions of the dichloro-bis(ethylenediamine)cobalt(III) ion in methanol, reported the preparation of the supposed solvo intermediate that would be required, and studied the rate of the chloride anion entry into this supposed solvo intermediate. He reports that the lability of methanol in this complex is insufficient to allow the complex to be an intermediate in a substitution process of the dichloro complex. Yet it is possible to obtain, in the case of the dichloro-chloride exchange, a term in the rate law for the free ion. This leads to the conclusion that, in fact, one has a genuinely unimolecular substitution process. 

Chelate complexes with two ethylenediamine rings in a cis configuration lack a plane of symmetry and thus have the potential to be separated into enantiomeric (A, A) (12) forms. Inert cis-bis(en) complexes of Co111,247 Crmn or Rh111248 can be resolved by the method of racemic modification 249 or using chromatographic techniques,35 but labile systems, such as Ni(en)2+, which occasionally crystallize in one chiral form,220 rapidly racemize in solution. 

An interesting challenge to Vanquickenborne s model of regiospecific labilization has been presented by Muir and co-workers,746 who studied the photoinduced amine aquation from complexes of the form tra .v-[RhL4Cl2]+ where L is a heterocyclic amine (pyrazine, pyrazole, pyridine, imidazole, and substituted analogs). Unlike the ammine and ethylenediamine analogs, where chloride loss dominates, both amine and chloride are stereoretentively labilized from these com-... 

The labile Co2+ ion was made to react with three molecules of ethylenediamine (en), to give the octahedral [Con(en)3]2+ complex, which was then oxidised by air to the inert [Cora(en)3]3+ (Fig. 8, step i). Then, Schiff base condensation with six... 

Among several iron chelators used, only o-phenanthroline inhibited the soluble dehydrogenase (42). It was shown by Hateff et al. (42) that incubation of the enzyme with o-phenanthroline results in the loss of labile sulfide, while pretreatment with bathophenanthroline sulfonate, Tiron (1,2-dihydroxybenzene 3,5-disulfonate) or ethylenediamine tetraacetate protects the enzyme against the loss of labile sulfide and inhibition of activity upon subsequent incubation with o-phenanthroline. The unique destructive ability of o-phenanthroline has been demonstrated by these investigators for several iron-sulfur proteins (S5,96). 

Closely related to the football ligands are the so-called sepulchrate ligands. One can be formed by the condensation of formaldehyde and ammonia onto the nitrogen atoms of tris(ethylenediamine)cobalt(llI). This results in tris(methylene)amino caps on opposite faces of the coordination octahedron. If the synthesis utilizes one of the (A, A)-enantiomers, the chirality of the complex is retained. Furthermore, the complex may be reduced to the corresponding cobalt(II) cation and reoxidized to co-balt(III) without loss of chirality. This is particularly unusual in that, as we shall see in the following chapter, cobalt(ll) complexes are quite labile in contrast to the stability of cobalt(III) complexes. Once again the extra stability of polydentate complexes is demonstrated. 

As shown in Table 1, a single ligand is labilized from the Cr(III) coordination sphere in most photosubstitutions. Consequently, the photochemical technique can be used to prepare complexes of polydentate ligands having one ligand coordination site detached, e.g., the monodentate ethylenediamine (en) complexes ... 

Reduction of epoxides. Hallsworth and Henbest (1,579,refs. 12 and 13)found that some steroidal epoxides, which were unreactive to lithium aluminum hydride, are easily reduced with a large excess of lithium in ethylamine. However, some olefin is also formed in some cases. Brown et al.1 now report that the combination of lithium and ethylenediamine at 50° is excellent for reduction of labile epoxides of bicyclic ketones, which are reduced only slowly by lithium aluminum hydride and usually with some extensive rearrangement. They chose ethylenediamine rather than ethylamine because the reduction is less vigorous in ethylenediamine than in ethylamine and thus easier to control. Also isolation of the alcohol is simplified because ethylenediamine is very soluble in water and only slightly soluble in ether, whereas ethylamine is miscible both in water and in ether. By this procedure, norbomene oxide (1) is reduced to pure ejco-norbomanol (2) in 87% yield (isolation). Analysis by glpc indicated that two rearranged alcohols (3, 4) are formed to a minor extent and that (2) is formed in 99.3% yield. 

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