Metal ions coordination modes

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Multiple metal ion coordination of anions, or cascade binding, has been an area of intense recent investigation for coordination chemists. The role of the metals in these receptors is threefold. They provide the source of interaction with anions through the formation of coordinate bonds. They fulfill a structural role, the separation of the metal ions being used to impart anion selectivity or different modes of anion binding, and in some cases they have taken a functional role. In particular, the use of this type of receptor for biocatalytic mimicking and rate enhancement has already been established and could prove particularly fruitful. 

Beside Hg, other late transition metal ions (e.g., Ag and Au ) can coordinate to DNA nucleobases to form interstrand cross-links. Several DNA coordination modes have been suggested for Ag ions, including coordination to the purine N1 and the pyrimidine N3 (86). At Ag /base pair ratios <0.5, the metal ion coordinates to the N7 position of purines, in particular to guanine, while at a Ag /bp ratio of 0.5, Ag forms cross-links between the two strands of the duplex by binding to two nucleobases (89). The Au " " ion was also shown to coordinate to solvent-exposed GC base pairs in RNA duplexes to form a G-Au-C bridge that resembles T-Hg-T (88). 

The binding of the carbonyl oxygen of amides to zinc(II) of carboxypeptidase is based on X-ray studies of enzyme-inhibitor complexes. Of course, the amides studied may be inhibitors because they bind to the enzyme incorrectly. An alternative mechanism, which makes chemical sense, could involve metal-ion coordination to the amide nitrogen which would not only stabilise the tetrahedral intermediate (XVI) but also facilitate carbon-nitrogen bond cleavage. Such a mode of catalysis... 

Luminescence spectroscopy is an extremely powerful tool for probing the intricacies of the Ln(III) coordination sphere of complexes in solution. Information on properties that are crucial to Ln(III) solution chemistry, including the hydration number (number of bound water ligands), the number of different species present in solution, the mode of interaction of the complex with anions, and the formation of dinuclear complexes, dimers and aggregates can be obtained [1-3]. It is litde wonder that Ln(III) ions are considered to be among the most versatile spectroscopic probes of metal ion coordination sphere. 

Figure 9.6. Structural models for complexes between Cu(ll) and peptides derived from the A/3 peptide sequence, (a) Proposed model for Cu(ll) coordination with a peptide mode for which Cu(ll) binding inhibits fibril formation, (b) Crystal structure for a Cu(ll)-peptlde complex for which Cu(ll) binding promotes fibril formation, (c) Structural model for peptide arrangements in fibrils. With the exception of one histidine side chain, only the peptide backbone is shown. The Cu(ll) ions are depicted as small spheres. The dotted lines indicate hydrogen bonding between peptides arranged in a parallel fi-sheet. Adapted from Dong, J. Canfield, J. M. Mehta, A. K. Shakes, J. , Tian, B. Childers, W. S. Simmons, J. A. Mao, Z Scott, R. A. Warncke, K. Lynn, D. G. Engineering Metal Ion Coordination to Regulate Amyloid Fibril Assembly and Toxicity. Proceedings of the National Academy of Sciences of the United States of America 2007, 104, 13313-13318.

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

A good example is the excited state of the tris(bipyridine)ruthenium(2+) ion, Ru(bpy)5+. This species results from the transfer of an electron from the metal to a ligand. In the language of localized valences, it is a ruthenium(3+) ion, coordinated to two bipyridines and to one bipyridyl radical anion in other words, [Ru3+(bpy)2(bpy )]2+. This excited state is a powerful electron donor and acceptor.17 The following equations show an example of each quenching mode ... 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Other coordination modes of trans-diammac have been identified where one (154) or both (155) primary amines are free from the metal.721 725 An extension of this concept involves attachment of active functional groups such as crown ethers selectively at one primary amine to generate ditopic ligands capable of electrochemically sensing alkali metal ions through their inductive effect on the Co11111 redox potential. One example is provided by (156) further, the 15-crown-5 and 18-crown-6 analogs were also prepared.726... 

---