Nickel sepulchrate

The template synthesis of the nickel sepulchrate proved to be rather complicated because of macrocyclic and acyclic amines (Scheme 69), competitive formation reactions occurred upon refluxing ethylenediamine, formaldehyde, and ammonia in the presence of Ni + ion. 

By the template reaction of ammonia, ethane-1,2-diamine and formaldehyde, complex (400), trivial name nickel sepulchrate, was obtained in a very low yield Bond distances... 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

Nevertheless, after precipitation of the major reaction products (the nickel(II) complex with ligand 1, yield ca 90%, and a complex with azasen ligand 2 as perchlorate salts). [Ni(sep)](C104)2 sepulchrate was isolated from the solution by fractional recrystallization (yield ca 1%) [160], Attempts to obtain the... 

As mentioned above, a template synthesis is a very efficient approach to the preparation of sarcophaginates and sepulchrates of certain metals. For all other metal ions, this synthetic pathway is inapplicable or gives the desired products in low yields. For instance, the yields of nickel(II) and chromium(III) sepulchrates resulting from template condensation are only ca 1 and 10%, respectively. This problem can largely be overcome if the synthesis of a variety of metal... 

The macrobicyclic 3d-metal aza-capped l,3pn-sarcophaginates were studied by voltammetry. In contrast to sepulchrates and sarcophaginates, these complexes should favour to some degree the adoption of low oxidation states (+2 and +1). The oxidation waves are observed in DMF at 1380, 820, 227, 1180, and 1220 mV us SCE, respectively, for manganese, iron, cobalt, nickel, and copper complexes. The dependence of redox potentials on the number of d-electrons is the same as for [MCsar)] " " couple redox potentials. 

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