Inversion nitrogen ligands

Trans- complex is obtained only with Cu11 which is coordinated to four oxygen atoms of two hfac ions and two nitrogen atoms of two TTF—CH=CH py ligands. Cu11 lies on inversion center and therefore the TTF—CH=CH py ligands are in trans- conformation. The copper ion adopts a Jahn-Teller distorted octahedral... 

The 4,4 -bipy and bpe derivatives display a similar crystal structure to that of Cd(4,4 -bipy)2[Ag(CN)2]2 reported by Iwamoto et al. [89]. It consists of the interpenetration of two identical 3D networks. The knots of the networks are defined by the iron(II) and silver(I) atoms. Each iron(II) atom located on an inversion centre defines an elongated octahedron whose axial positions are occupied by the nitrogen atoms of two 4,4 -bipy ligands. In addition, each 4,4 -bipy ligand binds a silver atom so that it is three-coordinated. This is the reason why the [Ag(CN)2] group is bent (see Fig. 18). 

Fig. 4j6 Stepwise eomplexing of Cu(OH)4 by a tetradentate macrocyclic ligand. The first Cu(II)-N bond is formed by replacement of an axial solvent molecule (k ) followed by a Jahn-Teller inversion (Ar, ) which brings the coordinated nitrogen into an axial position. Second-bond formation follows a similar pattern (k2 and 2b)- Reproduced with permisson from J. A. Drumhiller, F. Montavon, J. M. Lehn and R. W. Taylor, Inorg. Chem. 25, 3751 (1986). (1986) American Chemical Society.

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

Figure 3 represents the only dimer formed in this series of metallacycles. The coordination number of the indium atom is increased by interaction with the nitrogen atom of a second molecule. The dimer 41 has a center of inversion and three edge-fused four-membered rings. This type of structure is very common in metallacycles that are derived from the ligand 25 (5(5). The central N2In2 ring is almost square-planar [In-N = 2.27(1) A] and the In-C dis-... 

The metal ion does, however, introduce a new subtlety into these reductions. The reduction of the two imine groups in the nickel(n) complex 4.10 is readily achieved with Na[BH4], The free tetraamine ligand would be expected to exhibit a facile pyramidal inversion at each nitrogen atom, whereas in the nickel(n) complex this inversion is not possible without significant weakening (or breaking) of the Ni-N bonds. In macrocyclic complexes it is very often found that the complex obtained by the reduction of a co-ordinated imine does not possess the same stereochemistry as that obtained by the direct reaction of the free amine with metal ion. 

Figure 6-16. Co-ordination of the four nitrogen atoms of a tetraaza macrocyclic ligand to a metal results in a restricted inversion at each nitrogen. The filled circles represent a substituent lying above the plane of the paper, and the open circles one lying below the plane. In the case of cyclam, with hydrogen substituents, the barriers to inversion are relatively low, but with bulkier substituents on the nitrogen the different diastereomers are readily isolable. Note also that the isomers labelled II and V are chiral and will exist in two enantiomeric forms.

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