Pentadentate Schiff-base ligand

Schiff bases provide useful mixed donor sets. The carbonyl function of the most frequently used ligands is derived from either 1,3-dicarbonyl compounds or salicylaldehyde. Favourable combinations involve O-, N- and S-donor atoms. A range of technetium and rhenium complexes exist with bi-, tri-, tetra- and pentadentate ligands. The geometry of these complexes depends on the number and type of coordinating atoms as well as on the chain length between the donor atoms in the SchifF-base ligands. 

Fig. 17 Pentadentate N302-donating Schiff base ligands...

The potentially pentadentate Schiff bases of Figure 19a actually act as bis-tridentate ligands with respect to a single metal centre, giving dinuclear Cu11 complexes as in Figure 19b.132,133 Furthermore,... 

Figure 19 Some pentadentate Schiff bases (a) which act as bis-tridentate ligands in copper(ll) complexes (b) pentadentate...

The stability of the D-I-I complex and the cleavage of the I-I bond depend on the donor activity of the ligand and the solvent. The formation of a complex between I2 and three pentadentate Schiff bases, 1,3-bis (salicylideneamino)-2-propanol, 1,3-bis (2-hydroxy-l-naphthylideneamino)-2-propanol, and 1,3-bis [l-(pyridine-2-yl) methylideneamino]-2-propanol results only in polarization of the I-I bond [13]. The formation of a complex between pyridine and molecular iodine (I2) leads to the polarization of the I-I bond in some non-polar solvents such as liquid alkanes and is accompanied by cleavage of the I-I bond in low-density polyethylene [14]. 

In the only instances where spin crossover has been observed for a system involving a pentadentate ligand this has been an N302-donating Schiff base and the sixth coordination site has been occupied by a nitrogen donor, giving rise to an FeN402 coordination core. Most examples involve salten (Fig. 17). 

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)]. 

Mn2 (0)2(02)(Me3tacn)2] +, and [Mn2 0(E)2] with a Schiff-base pentadentate monoanionic ligand (F = N,N-bis(2-pyridyhnethyl)-iV -salicyliden-1,2-diaminoethane). The one-electron oxidized complex [Mn2 0(F)2] + has been electrochemically prepared and crystallized and is the only example of a structurally characterized mono-oxo bridged Mn2(III,IV) complex. 

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