Compounds with Macrocyclic Ligands

IX. COMPOUNDS WITH MACROCYCLIC LIGANDS A. Porphyrin Complexes of Group 14 Elements (see also Sections XIV and XV)... 

The first example of a square pyramidal Ge(II) complex with macrocyclic ligands is the red crystalline compound Ge(TMTAA), which can be prepared by treating the dilithium salt of TMTAA with an equimolar amount of GeCl2 dioxane in Et20 at — 78 °C94. In that compound the Ge atom is symmetrically coordinated to the four N-atoms of the macrocycle, which means that the GeN4 subunit is square pyramidal, and the Ge atom is situated 0.909(4) A above the N4 plane (Figure 16). 

In a similar manner the so-called Zintl salts composed of alkali metal cations and clusters of metals as anions (see Chapter 16) were known in liquid ammonia solution but proved to be impossible to isolate Upon removal of the solvent they reverted to alloys. Stabilization of the cations by complexation with macrocyclic ligands allowed the isolation and determination of the structures of these compounds. 

The two main methods currently used in computational and combined computational/experimental studies in the general area of transition metal coordination compounds, and specifically also with macrocyclic ligands, are DFT and MM. While DFT yields structural data, energies and molecular vibrations, as well as electronic information (the ground state wave function, spin density, charge distribution etc ), the latter is missing in MM. 

Reaction of alloys such as NaSnx with macrocyclic ligands in amine solvents gives compounds containing anionic clusters such as [Sn5]2-, [Sn9]4— and [Pb5]2-. These have multicentre metal-metal bonding, which can often be rationalised by Wade s rule. 

Lanthanide complexes formed with macrocyclic ligands in which the usual coordination number varies from 10 to 12 along with a few exceptions of coordination number 9, are given in Table 5.14. Some other lanthanide compounds are listed in Table 5.15. 

An example of crown ether applied in selective separation of alkali metals is dibenzo-18-crown-6 (formula 1.15). Extractive separations of metal ions are also performed with macrocyclic ligands containing nitrogen or oxygen atoms, as well as macrocycles with combinations of oxygen, nitrogen, and sulphur atoms (N-0, S-0, N-S) [45,48]. A macrocyclic compound with only nitrogen hetero-atoms (formula 1.16) is selective for copper. 

There are many examples of the stabilization of unstable 3+ oxidation states by complex formation with macrocyclic ligands. and Cu are stabilized by complex formation with tetraazamacrocyclic ligands. The most universal ligand for the stabilization of unstable oxidation states is [9]aneS3 (L), which forms coordination compounds of general type ML with many unstable ions, e.g., Ag Pd % and Pt. ... 

A similar binuclear product can be obtained by transmetallation of the La compoimd with L649, but the reaction time must be increased [23]. Thus coordination compounds of the rare earth elements with macrocyclic ligands based on pyridine-2,6-dicarbonyl compounds and lower diaminoalkanes, being... 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

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