Metal ions salen

Recently, several groups reported molecular squares prepared by an alternative approach, in which bifunctional metalloligands serve as the linkers thereby placing the metal ions into the walls of the container molecules (114—116). Hupp and coworkers, for example, have designed square-shaped macrocycles based on salen-type components (117). Thus, the molecular square 15 was prepared by the directed assembly of cis- [(PEt3)2Pt(OTf)2] and... 

The solubility properties of salen-type complexes can be tailored by employing substituents such as t-butyl and triphenylphosphonium-methyl (—CFl2PPh3+), with its associated chloride, in the aromatic rings.The stability constant for A,A -o-phenylenebis(salicylideneimine)-iron(III) in 80% (w/w) DMSO-water has been compared with stability constants for complexes of six metal ions with this ligand. 

The salen-Ni(II) complex 39a derived from (lR,2R)-[N,N -bis(2 -hydroxybenzyl-idene)]-l,2-diaminocyclohexane was also equally effective (Table 7.3, entry 4). In contrast to earlier reports on salen-metal complexes, where the introduction of a bulky tert- butyl substituent increased enantioselectivity [31], the use of complex 39b exhibited a significant decrease in enantioselectivity (entry 5). The presence of a bulky tert-butyl group obstructed the chelation of alkali metal ions by phenolic oxygen atoms. A dramatic increase in selectivity could be achieved when nickel was replaced with copper, and a salen-Cu(II) complex 39c afforded 85% ee (entry 6). Although screening of other bases or 50% NaOH were not advantageous, the use of 3 equiv. NaOH improved the enantiomeric excess to 92% (entry 9) and after recrystallization of a-methylphenylalanine optical purity was increased to 98% ee. 

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

This can be achieved by cyclization (crown ethers), by the formation of an intramolecular hydrogen bond (Taddol and Nobin), or by use of a second metal ion to correctly position the chelating atoms within its coordination sphere (metal (salen) complexes). 

A large number of reactions that mimic these biological oxidations have been characterised and this is an area of active study by those attempting to understand the ways in which metal ions act in metalloproteins. A very simple model reaction is seen in the oxidation of phenol to 1,4-benzoquinone by dioxygen in the presence of cobalt(n) complexes (Fig. 9-27). A particularly useful catalyst for a variety of such oxygen transfer reactions is the complex [Co(salen)], 9.13. 

Other dinuclear cyanide-containing complexes with trivalent metal ions include the homodinuclear species A[(NS3)2V 2(CN)] A=TEA, TEA" NS3=[N(CH2CH2S)3] (54), (TBA)[(salen)2Cr 2(CN)3] (55), (PI14P)... 

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