Ligands, quadridentate synthesis

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

Although complexes with these ligands are common in palladium(II) chemistry, their occurrence is more scarce in platinum(II) compounds. Nevertheless these complexes can be prepared, examples being platinum(II) complexes of the optically active quadridentate Schiff base of salicylaldehyde and (R)-l, 2-diamines.1212 An alternative synthesis involves formation of the Schiff base by reaction of a complexed amino ligand on platinum(II) with amide acetates (equation 372).1213... 

The synthesis of quadridentate imine chelates usually requires the combination of two equivalents of the carbonyl compound and a diamine, as in the formation of complexes of ligands (6),17 18 (7)19 and (8).20,21 In similar fashion, thermodynamic template reactions allow the very effective synthesis of quinquedentate and sexadentate metal complexes of ligands such as (9),22 (10),23 (ll),24 (12)25 and (13).26 Condensation of 1,1,1-tris(aminomethyl)ethane with pyridine-2-carbaldehyde alone yields... 

Simple macrocyclic quadridentate complexes can be synthesized by template reactions from ethers derived from salicylaldehyde and diamines in the presence of appropriate metal ions such as nickel(II) (equation 3).35>36 However, these reactions can also be carried out quite effectively in the absence of metal ions to yield the free ligands, which can be obtained by hydrolysis of the complexes. An iron(II) macrocyclic quinquedentate chelate of this type has been produced by template synthesis (equation 4).37... 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

A class of synthetic metalloporphyrins which are conveniently studied in aqueous solution derives from coordination compounds of the quadridentate ligand a,(S,7,5-tetra(4-pyridyl)-porphine. Although a method of synthesis for the zinc complex has been published,1 the yield is often poor and the method unreliable. If the same reaction is allowed to proceed under... 

Quadridentate ligands such as (237) are prepared by what must be one of the first (1944) uses of metal ion template ligand synthesis (Figure 5). ... 

Studies of the mechanism of stereoselective synthesis of quadridentate open-chain ligands by copper(II)-directed condensation of amino acids have included molecular mechanics analysis of possible intermediates. From racemic aniline, the only condensation product detected for [Cu(2,5,8-trimethyl-5-nitro-3,7-diazanonanedioate)] was the (2R, 85) isomer. A model was proposed to explain this selectivity. With a racemic amino acid, one side is blocked, as shown in Figure 11. Molecular mechanics calculations were performed with... 

The absorption and c.d. spectra of 4,4 -(i -propylene-di-iminato)di(3-penten-2-one)-copper(ii) (133) has been interpreted to show that the complex is tetrahedrally distorted with the absolute configuration A 2f-ray diffraction has confirmed this. The synthesis of new binucleating agents (134) and their dimeric cupric complexes has been reported. The new macrocyclic quadridentate ligands of type (135) form... 

Stereoselective Synthesis of Quadridentate Ligands Utilizing a Template Reaction of Metal Complexes... 

A variety of phosphinoacetylenes has been prepared by reaction of an acetylene and a chlorophosphine in the presence of butyl-lithium. The synthesis of several quadridentate ligands containing both phosphorus and arsenic donor atoms, e.g (3), from a chlorophosphine and the appropriate lithio-derivative has been described. ... 

Chiral Homo-Quadridentate Ligands. Chiral porphirines (PORPH) com-plexed to manganese(III) have been published recently as efficient epoxidation catalysts (97). The synthesis of the ligand is based on the condensation of enan-tiomerically pure [2.2]-p-cyclophane-4-carbaldehyde and pyrrol (Fig. 11). 

Template synthesis has also been used to construct complexes of other non-symmetrical quadridentate ligands. Thus [M(L82)], where M = Ni or Cu, have been prepared following Scheme 2-6 from the corresponding 2-hydroxybenz-aldehyde 5 -alkylisothiosemicarbazone by prolonged heating with Hacac used as ligson and solvent, and triethyl orthoformate in the presence of M(acac)2, where M = Nior Cu[116, 117]. 

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