Tetrapodal pentadentate coordination modules

Whereas the classification of the previous section was by ligand type and donor set, the present section presents the material grouped by the identity of the central metal, in order to facilitate comparisons of reactivity. The discussion will concentrate on the metal ions Zn, Cu, Ni, Co, and Fe, and on ligands with a predominance of nitrogen donor atoms. More elaborate ligand structures derived from 1 by templated condensation reactions are not covered by this review (24-28). The same applies to ligands with N04 (29), NS4, and NP4 donor sets (30), for which the reader is referred to the literature. 

The study of zinc-aqua complexes as synthetic carbonic anhydrase models has shown a low coordination number and a hydrophobic environment to be prerequisites for a low pK value of the aqua ligand, which is essential for efficient enzyme function. The pK value in the case of 13 is expected to be greater than 10.7, which is the value determined for pentacoordinate [Zn(tren)(H20)](C104)2, in which the water ligand is more strongly bound and thus expected to be more acidic (cf. the discussion of bond lengths, above). Thus, 13 is not expected to show carbonic anhydrase-like reactivity. However, in related tetraazamacrocyclic systems it has been shown that upon 

Copper(II) polyamine complexes are substitutionally labile, in a manner similar to the corresponding nickel(II) complexes. This means that individual donor atoms may at times decoordinate and thus be available for derivatization reactions, e.g., with suitable carbonyl compounds. More complex ligands may thus be constructed, including macrocycles (35), polymacrocycles (36 38), and concave chelators (35). The copper(II) complex of 1 was synthesized as a starting material for reactions aiming at the derivatization of the pentaamine ligand (24-28). 

In the case of the purely aliphatic ligand 2,2,6,6-tetrakis(amino-methyl)-4-azaheptane (12), complex formation with copper appears to proceed in two steps, as elucidated by titration experiments with the fully protonated ligand (12 5 HC1). Three and two protons from (Hr,12) + are sequentially abstracted, and the predominant species after full deprotonation appears to be a dinuclear complex in which two copper(II) ions are coordinated, each in square planar fashion, by the l,3-diaminoprop-2-yl units of two molecules of pentaamine ligand, thus forming a macrocyclic complex of composition [Cu2(12)2]4+ (23). The UV/vis spectral data show an interesting solvent dependence, suggesting an equilibrium between [Cu2(12)2]4 + and two equivalents of mononuclear complex [Cu(12)]2+ under suitable conditions. ESR spectroscopic data are also compatible with the formulation of a dinuclear species. Further addition of base to an aqueous solution of [Cu2(12)2]4+ gives the mononuclear hydroxo complex [(12)Cu(OH)]+, as inferred from the UV/vis spectroscopic data. 

Nickel (II) Complexes of the Pentaamine Ligand 1 Obtained by Recrystallization of [(l)Ni(OH2)]Cl2 (17) in the Presence of the Reagents Listed in Some Cases, Added NH4PF6 Produces the Highly Crystalline Hexafluorophosphate Salts 

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Zinc, tetrapodal pentadentate coordination modules, 183-184 Zinc-aqua complexes, 183... 

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