Inversion and Proton Exchange at Asymmetric Nitrogen

Coordination of ammonia or a substituted ammonia to a metal ion alters markedly the N — H dissociation rate (see See. 6.4.2). Since also proton dissoeiation of complexed ammines is base-catalyzed, then exchange can be made quite slow in an aeid medium. Thus, in a eoordinated system of the type 12, containing an asymmetric nitrogen atom (and this is the only potential souree of optical activity), there is every chance for a successful resolution in acid conditions, since inversion is expected only after deprotonation. It was not until 1966 that this was suc-eessfully performed, however, using the complex ion 12. A number of Co(III), Pt(II) and Pt(IV) complexes containing sarcosine or secondary amines have been resolved and their raeemizations studied.Asymmetrie nitrogen centers appear eonfined to d and d  

Molecules which contain a chiral cobalt as well as an asymmetric nitrogen exist in four possible optical isomeric forms. These are represented for Co(sar)(hbg) +, hbg = NH2C( = NH)NHC(=NH)NH2 in Fig. 7.12. All four optically-active isomers have been isolated and characterized by cd, nmr and vis/uv absorption spectroscopy. The kinetics of 

For all systems studied so far it is found that the rates of racemization R and proton exchange E (measured either by DjO exchange (Sec. 3.9.5) or by nmr line coalescence methods (Sec. 3.9.6)) are both first-order in [OH ]. 

Since proton exchange is usually measured by nmr methods in D2O (Secs. 3.9.5 and 3.9.6), the more appropriate rate law is 

Since k k, the loss of a proton from the N — H group must rarely lead to racemization. The results can be accommodated by a scheme (M, contains the metal center)  

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Inversion and Proton Exchange at Asymmetric Nitrogen Centres A number of metal complexes contain chiral N-H groups. One of the first examples to be studied was the cobalt(UI) complex containing coordinated sarcosine N-metl lglycine, (5.7). The complex has been resolved and its racemisation in baac solution studied. 

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