Awareness of the asymmetric nature of numerous dinuclear metallobiosites and of the ability of the individual metal ions to have quite distinct roles in the functioning of the metalloenzyme concerned has led to a search for carefully designed unsymmetric dinucleating ligands that will give dinuclear complexes capable of acting as models for the metallobiosites. This review surveys progress made in the design and synthesis of complexes capable of serving as models for donor atom, coordination number and geometric asymmetries found at dinuclear metal centres.There is now an abundance of information available in the literature concerning the structures and roles of the transition-metal-derived dinuclear centres present in certain metalloproteins and metalloenzymes. Such centres may be homodinuclear or heterodinuclear in nature and may also contain metals in different oxidation states. The metals may have different roles to play in the overall functioning of the site and numerous sites have been found to be asymmetric in character. The search for designed unsymmetric dinucleating ligands which would provide model complexes for donor atom, coordination number and geometric asymmetries at dinuclear metal centres is well under way and this microreview draws attention to recent progress made in the area.
Site Asymmetry at MetallobiositesHomodinuclear sites may be exemplified by those structurally characterized in non-heme manganese catalase [Mn,Mn]
IntroductionIt would not be a total exaggeration to state that macrocyclic complexes lie at the centre of life, particularly when regarding the roles of such systems as the ironporphyrin core in haemoglobin, the cobalt-corrin of vitamin B,, and the magnesium-hydroporphyrin in chlorophylls. Recognition of the importance of complexes containing macrocyclic ligands, where a macrocycle is defined as a cyclic compound having nine, or more, heteroatomic members and with three, or more, ligating atoms,, has led to a considerable effort being invested in developing reliable syntheses for these compounds. Ideally the complex is formed by adding the required metal ion to a preformed macrocycle but there are potential disadvantages to this approach: the synthesis of the macrocycle often results in a low yield of the desired product with side reactions such as polymerization predominating. In order to circumvent this problem the ring-closure step in the synthesis may be carried out under conditions of 'high d i l ~t i o n ' , ~ or 'rigid groups' may be introduced to restrict rotation and internal entropy losses in the open-chain precursor^^-^ and so facilitate cyclization.One effective method for the synthesis of macrocyclic complexes involves an in situ approach wherein the presence of a metal ion in the cyclization reaction markedly increases the yield of the cyclic product. The metal ion plays an important role in directing the steric course of the reaction and this effect has been termed the metal-template effect.' The first example of a deliberate synthesis of a macrocycle using this procedure was described by Thompson and Busch (Scheme 1),8 although Curtis had previously demonstrated the potential of template assembly through his observations that the reaction of Ni(en),(ClO,), (en = 1,2diaminoet hane) and acetone yields isomeric tetra-azamacrocyclic complexes of '
Schiff-base lateral macrobicycles containing two different binding units, a rigid and unsaturated N(2)X set (X: N, O) and a flexible and cyclic N(2)O(n)() set, linked by two aromatic bridges, have been prepared by reaction of the appropriate bibracchial diamines N,N'-bis(aminobenzyl)-diaza-crown and diformyl precursors in the presence of Ba(II) as templating agent. The expected cryptands do not form in the absence of the cation; the presence of this metal ion is necessary to orient the diamine precursor in a syn conformation. Comparison of the X-ray crystal structures of the barium complex of the bibracchial diamine N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and the barium complex of the cryptand derived from it incorporating a pyridine unit indicates that the encapsulation of the metal ion is clearly more effective in the case of the cryptand. The coordination of Ba(II) ion to the pyridine nitrogen atom promotes the displacement of the metal into the cavity of the cryptand as well as important changes in the conformation of the crown unit, although the fold of the pendant arms remains practically unchanged. The cryptands reported constitute the first example of a novel family of macrobicycles.
A diversity of reaction products have been found in the reactions of two related unsymmetrical Schiff base dinucleating ligands, HL 1 and HL 2 , derived from 3-chloromethyl-5-methylsalicylaldehyde 1, with copper() and nickel() salts. The ligands remain intact in the copper() complexes to give the homodinuclear complexes [Cu 2 L 1 Br 3 ] 2 and [Cu 2 L 2 -(OH)(ClO 4 )]ClO 4 3, the crystal structures of which have been solved. The reactions of HL 1 and HL 2 with nickel perchlorate led to hydrolysis of the imine bond. With HL 1 the homodinuclear complex [NiL A (OH 2 )] 2 [ClO 4 ] 2 ؒ4H 2 O was formed and with HL 2 hydrolysis was followed by elimination of C 2 H 4 from the terminal NEt 2 of the iminic side arm to leave an NHEt group and the dinuclear complex [NiL C (OH 2 )] 2 [ClO 4 ] 2 ؒ3CH 3 OH. The crystal structures of the two nickel complexes are also reported.
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