Catecholate, titanium complex

Tire early-late transition metal complex of Raymond et al. is interesting in that it requires both metal atoms to form the basic C3 structure (ideally D i, may form). Titanium complexation to three catechol units leads to a tridentate ligand, and, only when palladium bromide is added, trans coordination to palladium gives the agglomerate 31 [78]. 

This can be achieved by using the cis molybdenum(VI)dioxo ion instead of titanium(IV). In this ion two comers of the octahedron at the molybdenum are already blocked by oxygen atoms and only two catechol units can be bound to the metal. Thus, reaction of ligand 3-H4 with MoC>2(acac)2 in the presence of potassium carbonate leads to a mononuclear macrocydic complex [3Mo0212 in which a loop-type conformation is stabilized at the peptide (Scheme 1.3.3) [23]. (Similar... 

Within the cage M8[Ti4(L21)4] (M = Li+, Na+ or K+) four counterions are bound to the internal oxygen atoms of the titanium tris(catecholate) units together with three DMF molecules per cation. Those counterions could be exchanged successively with primary ammonium ions as shown in Fig. 18. This exchange could be monitored by hi NMR spectroscopy, e.g. for the Lig[Ti4(L21)4] complex [139]. 

Other studies involving the self-assembly of tri-stranded, non-chiral complexes composed of three bis(catecholate) ligands wrapped around two titanium(IV) ions have been reported. The self-assembly of gallium(III) catecholamide triple helices has also been investigatedalong with the assembly and racemisation of helicates incorporating linked catechol derivatives and either titanium(IV) or... 

Fig. 5.21. Top Self-assembly drives the formation of helical, homochiral dimeric titanium tris-catecholate complexes. Dimerization is only mediated by LC, while Na and K" " do not lead to comparable products. Bottom Crystal structures of the dimers formed from the aldehyde (left, R = H), the ethyl ketone (centre, R = C2H5), and the methyl ester (right, R = OCH3). Shown is a side view in space-filling representation and a ball-and-stick model with a view along the Ti—Ti...

SCHEME 2.2 Catechol complexes with different ligand metal ratios and solid state structure of a dinuclear titanium(IV) helicate with ethylene-bridged dicatechol ligands. 

Crystallization leads only to the dinuclear titanium(IV) helicates. However, the monomer-dimer equilibrium in solution can be investigated easily by using different NMR techniques. In the dimer, the protons of R near the carbonyl unit of one complex moiety are neighbored by an aromatic catecholate of the second and thus experience an anisotropic shift to high field. In the monomers, the enantiomeric A and A complexes equilibrate very quickly, generating only one signal of CH2 groups at the substituent R. In the dimer, the stereochemistry is locked and no fast racemization... 

Figure 21-5. The UV-Vis absorption spectrum of surface titanium-catecholate complexes (reprinted from Rodriguez, 1996, with permission from Elsevier).

Phthalic Acid. While catechol and sahcylate could chelate individual titanium sites, phthaUc acid isomers are more likely to inteact with two diffeent surface Ti sites, due to the position of the two carboxylic add groups. Figure 21 -7 shows a proposed ball and stick three-dimensional molecular model of surface Ti complexation by phthalic add (Moser, 1991). 

Rodriguez R., Blesa M., Regazzoni A. Surface complexation at the TiOi (anatase)/aqueous solution interface Chemisorption of catechol. J. CoUoid Interface Sci. 1996 177 122-131 Rossetti R., Brus L. Time-resolved Raman scattering study of adsorbed, semioxidized eosin Y formed by excited-state electron transfer into colloidal titanium(IV) oxide particles. J. Am. Chem. Soc. 1984 106 4336-4340... 

The titanium(IV) tris-catecholate 408 has been used in [213] as a complex syntone for the synthesis of a heterometallic M2 M3 L6 cage framework 808 by its reaction with palladium(II) ions (Scheme 4.205). The coordination capsule 808 has been also obtained by reversible conversion from the metallamacrocyclic precursor 409... 

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