Pentadentate macrocycles

In the ternary complexes [Fe(127)(CN)2] the normally pentadentate macrocyclic aza-terimine ligand is only tetradentate, thanks to the particularly advantageous combined ligand field of two cyanide ligands and four nitrogen donor atoms in octahedral geometry. ... 

Pentadentate macrocyclic ligands L13-L15 accommodate Ni(II) ion with simultaneous dissociation of the two amide protons to form the square-pyramidal high-spin complex of [NiH 2L]°, 27. These complexes are air-sensitive and show a very low Ni(II)/Ni(III) oxidation potential of +0.24 V vs SCE (72, 73). They form 1 1 dioxygen adducts, 27-02, at room temperature in aqueous solution, which are formulated as Nilu-02 based on the EPR spectral data. The magnetic moment of the 1 1 02 adduct is 2.83 BM, which is interpreted in terms of weak interactions of Ni(III) with the superoxide where the spin coupling is weak. The oxygen uptake reaction is first-order with respect to both [02] and [NiH 2L]° in aqueous solutions and yields a second-order rate constant... 

So far, we have concentrated upon reactions resulting from discrepancies between the size of the metal ion and the size of the macrocyclic cavity. However, it is not only the size of the metal ion that may result in a mismatch what happens if the favoured conformation of the ligand does have an arrangement of donor atoms that matches with the preferred co-ordination geometry of the metal ion This is exactly the situation that we observe with metal complexes of some pentadentate macrocycles. We have previously observed the formation of tetraaza macrocycles from the template condensation of 2,6-diacetylpyridine with diamines in the presence of a transition metal ions. We also noted that if the size of the metal ion were incorrect, it was possible to get [2+2] or other products. Now let us look at this topic in a little more detail. 

The first structurally characterized expanded porphyrin system to be reported in the literature was the so-called superphthalocyanine ligand [112]. This compound, which represents the first example of a well-characterized pentaligated complex prepared from any aromatic pentadentate macrocycle ligand, was obtained as an outgrowth of early efforts to prepare uranyl phthalocyanine and not as the product of a directed step-by-step synthesis. As such, the early literature associated with this species remains somewhat clouded and incomplete. 

A number of similar trinuclear complexes were prepared with Mn(II) using the pentadentate macrocycle maodp as a planar blocking ligand. The reaction between [Mn°(maodp)(H20)2l and [Fe CN)6]" results in the neutral product [Mn maodp)(H20)]2[Fe CN)6] (101). As in the case of the complexes formed with Mn(III) Schiff base building blocks, the H2O molecule coordinated to each peripheral Mn(II) center is arranged trans to the bridging cyanide [Fig. 16(a)]. 

Reviews of synthetic procedures can be found fortridentate and pentadentate macrocyclic ligands with nitrogen donors, mixed nitrogen donors, and sullur donor macrocycles, the techniques of which can be expanded to other ring sizes. The general procedures will be summarized below. 

Much of the work in this area has been reported by Lindoy and co-workers, who have performed extensive studies on the role of hole size in complex stability and rates of complex formation. 4 Bradshaw, Krakowiak, and Izatt have published an extensive text on the synthesis of aza crowns. A review of tri- and pentadentate macrocyclic ligands also includes mixed donor results as well as the influence of pendant arms. Due to the numerous ramifications of this area, a few key findings will be cited for the simplest systems. 

A suggests that this ion (as a template) is ideally suited for constructing pentadentate macrocyclic ligands such as phthalocyanine and porphyrin analogs. On a more general level, the present findings indicate that... 

Pentadentate macrocyclic ligands with the topology shown in Scheme 18 are flexible enough to wrap around five- or six-coordinated metal centers. However, they often appear in a planar conformation in pentagonal bipyramidal seven-coordinate complexes with a trans conformation. 

Tetradentate and pentadentate macrocyclic systems based on o-aminobenzaldehyde and its derivatives can be obtained by metal-template reactions of diaminodicarbonyl compounds with diamines (Scheme 2-24(a)), and also by interaction of monoaminodicarbonyl compounds with primary diamines containing additional supporting nitrogen atoms in the chain (see Scheme 2-24(b)) [391],... 

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