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Figure 8.10 Ramachandran plot for dihedral angles at amino acid residues of a series of com-
plexes Li 2 [(13) 2 (OR) 2 Ti 2 ].
In conclusion, the thermodynamically most favored isomer is the one with a parallel
orientation of the ligands with both N-termini binding to a L and the C-termini binding to
a D-configured titanium center. Those studies impressively show how the interplay of
experiment and theory together with conformational considerations can help to under-
stand relatively simple stereochemical processes [31].
Substitution of the amino acid in the spacer of the dicatechol ligands 13 by short pep-
tides results in ligands 14 and 15 . Now the binding sites are far separated and no binding
of small bridging coligands can occur. The formation of triple-stranded helicates should
be possible with titanium(IV).
Accordingly, ligand 14 -H 4 coordinates to titanium(IV) ions and forms the triple-
stranded helicate [ 14 3 Ti 2 ] 4 which has been characterized by ESI MS. 1 H NMR spectros-
copy shows that initially a mixture of isomers is formed. In DMSO it slowly transforms
into only one thermodynamically favored species with all three ligand strands orientated
parallel (Figure 8.11).
Use of the longer tripeptide ligand 15 -H 4 in coordination studies with titanium(IV)
affords a mixture of compounds. Triple-stranded helicates are only one component of the
mixture. Due to the flexibility of the relatively big ligand, loop type structures are addi-
tionally formed in which both catechols of a ligand bind to the same metal center.
This is also observed, if dipeptides with less sterically demanding side chains are used
instead of the Val-Val spacer of 14 . It is obvious that ligand 14 represents a special case in
which the ligand has to adopt a linear arrangement, enforced by the short chain length in
concert with repulsion of the bulky groups. Reduction of the constraints by using longer
peptides or less bulky groups enables back-binding of the chelating units to bind to only
one metal center [32].
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