Tetra-carboxylate ligands

The earliest example of a coordination tetrahedron was reported by Saalfrank and coworkers [144], Their study showed the formation of the tetranuclear magnesium(II) complex 30 bridged by six tetra-carboxylate ligands 29, representing the first reported coordination tetrahedron exhibiting M4L6 stoichiometry (Fig. 21). This serendipitous discovery was followed by the formation of other, pre-designed tetrahedral architectures [19,122,145]. 

Fig. 21 Saalfrank and co-workers reported the formation of the first molecular tetrahedron 30 [144]. Modification of the original tetra-carboxylate ligand 29 allowed the formation of tetrahedron 32 based on ligand 31 [122]...

Anaerobic photooxidation of a range of aliphatic carboxylic acids using iron(III) tetra(2-N-methylpyridyl)porphyrin has been described and the primary step reported to be photoreduction of the Fe(III) atom by the axial carboxylate ligand to give a solvent caged [RCO Fe(II)porphyrin] species. The rate of the reaction is determined by competitive reactions of RCO2 in the solvent cage. 

Elimination of CO2 from carboxylate anions can be brought about by visible-light irradiation of complexes of the anions with iron(III) tetra(2-AT-methylpyr-idyl)porphyrin. Spectroscopic studies show that the primary reaction is photoreduction of iron(III) to iron(II) by the carboxylate ligands, and consequent formation of the corresponding acyloxy radicals. The reactions have been studied under both aerobic and anaerobic conditions. In the presence of air, re-oxidation of iron(II) to iron(III) can occur, allowing the system to become photocatalytic in... 

Displacement of triclycine (G3) from [CuH-aGg]- by macrocyclic tetra-amine ligands (Table 2), is much slower than the corresponding process with trien. The proposed mechanism involves initial binding at the carboxyl site followed by proton transfer to the adjacent peptide link, thus weakening the copper(ii)-peptide bond. The rate with [CuH 2G3] is 4 x 10 higher than the rate with [Cu(edta)] , which has no proton acceptor site and requires nucleophilic displacement by the macrocycle. 

In the case of manganese porphyrin catalyzed epoxidations, the axial ligands have been used alone or together with other additives like carboxylic acids (Banfi and coworkers) and soluble bases (Johnstone and coworkers). For example, Mansny and coworkers showed that in the presence of imidazole, 2-methylimidazole or 4-imidazole chloromanganese(tetra-2,6-dichlorophenylporphyrin) catalyzes the epoxidation of varions aUtenes including 1-alkenes by Under these conditions alkene conversion... 

Carboxylic esters undergo the conversion C=0 —> G=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of N,N,N, N -tetra-methylethylenediamine.469 Metal carbene complexes470 R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the 0=0 of carboxylic esters and lactones to CR2.471 It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. 

Most of the knowledge about aluminate and alkylaluminum coordination stems from X-ray crystallographic studies. The basic idea of this section is to compile a rare-earth metal aluminate library categorizing this meanwhile comprehensive class of heterobimetallic compounds. Main classification criteria are the type of homo- and heterobridging aluminate ligand (tetra-, tri-, di-, and mono alkylaluminum complexes), the type of co-ligand (cyclopen-tadienyl, carboxylate, alkoxide, siloxide, amide), and the Ln center oxidation state. In addition, related Ln/Al heterobimetallic alkoxide complexes ( non-alkylaluminum complexes) are surveyed. Emphasis is not put on wordy structure discussions but on the different coordination modes (charts) and important structural parameters in tabular form. An arbitrary collection of molecular structure drawings complements this structural report. 

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