Saturated tetraaza macrocycles

In the presence of a saturated tetraaza macrocycle such as cyclam, disproportionation of Ag(i) occurs to produce a silver mirror and a stable Ag(n) complex of the macrocycle (Kestner Allred, 1972 Barefield Mocella, 1973). In some cases the Ag(n) complexes so formed may then be oxidized further to Ag(m) species either electrochemically or chemically [using nitrosyl (NO+) salts]. 

In contrast to nickel(II) complexes with saturated tetraaza macrocycles, which exhibit a variety of coordination numbers and geometries, the ligands being in either a planar or a folded coordination, the majority of the complexes formed by unsaturated tetraaza macrocycles are square planar. Few six-coordinate complexes have been prepared with 14- and 16-membered macrocycles. It is expected that imino groups present in the chelate rings reduce the possibility of the folded coordination of the macrocycles and the conformational possibilities of the chelate rings. 

Interestingly, the presence of methyl substituents on the saturated tetraaza macrocycles also affects the properties of their iron(II) complexes. Whilst [Fe(Me6[14]aneN4)(NCS)2] shows a i i) = 57 2k spin transition, [Fe([l4]aneN4)(NCS)2] is low-spin. Furthermore, [Fe([14]aneN4)(MeCN)2](PF6)2 does not lose MeCN to form a five-coordinate species which is in contrast to the analogous complex [Fe(Me6[14]aneN4)(MeCN)2](BF4)2. A further example of the... 

Typical examples of the rich coordination chemistry of saturated tetraaza macrocycles are the complexes given by 1,4,8,11-tetraazacyclotetradecane, also known as cyclam, with nickel(II) (Table 103). Isomerism is expected to occur in the Ni-cyclam system due principally to the different configurations about the asymmetric coordinated secondary amines which, in turn, influence the possible conformations which can be adopted by the chelate rings. In the tram octahedral complexes [NiX2(cyclam)] (373 X = Cl, and [NiI(cyclam)]I-H20 ... 

A summary of nickel(H) complexes formed by representative saturated polyaza macrocycles is reported in Table 103, together with a concise description of the synthetic procedures and some of their physicochemical properties. Most studies are concerned with nickel(II) complexes with tetraaza macrocycles, but examples of complexes with triaza and pentaaza macrocycles are not rare. 

A remarkable dependence of pK values on macrocyclic ring size has been shown for a series of complexes of general formula [Zn(L)(H20)], where L represents a saturated tri- or tetraaza macrocycle (Fig. 4). Kimura et al. (24) reported that complexes of the 12-membered macrocycles [12]aneN3 and iso[12]aneN3 show a marked reduction in pK, to values almost identical to that observed for carbonic anhydrase... 

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