A Family of Titanium Complexes with Catechol Ligands: Structural Investigation and Catalytic Application

Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(III) complex of a L-threonine derivative and a significant red shift of visible spectra with Al3+ salt

Four salts of an anionic iron(III) bis-complex, [Fe(LL-thr)2]1-, were synthesized from water or methanol. H2LL-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li+ (1), Na+ (2), K+ (3), and Ba2+ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li+ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba2+ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al3+ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.

Bis(catecholato-κ2 O,O′)bis(dimethyl sulfoxide-κO)titanium(IV)

Bis(catecholato)bis­(DMSO)titanium crystallizes with two crystallographically independent mol­ecules related by pseudo-glide symmetry in a distorted octa­hedral O₆ donor-atom geometry around titanium.

Bis(benzene-1,2-diolato-κ²O,O′)bis­(dimethyl sulfoxide-κO)titanium(IV), [Ti(C₆H₄O₂)₂(C₂H₆OS)₂], crystallizes with two crystallographically independent mol­ecules in the space group P2₁/c emulating ortho­rhom­bic Pbca symmetry (β = 90.0445 (9)°]. The two mol­ecules are related by pseudo-glide symmetry, broken by modulation of each one catecholate and dimethyl sulfoxide (DMSO) ligand. Twinning by pseudomerohedry was observed [twin ratio 0.5499 (7):0.4401 (7)]. Complex 3 was obtained by heating of diprotonated titanium tris-catecholate precursor 2^H in DMSO, by formal displacement of a catechol mol­ecule by two DMSO mol­ecules. Complex 3 is just the second heteroleptic, mono-nuclear, neutral bis-catecholate complex with TiO₆ metal coordination, the only other one being its bis-DMF analogue 6. The two mol­ecules of 3 exhibit a distorted octa­hedral geometry. The geometry and distortions from ideal symmetry of 3 are discussed and compared to 6 and to cationic tris-catecholate titanium complexes.

Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches

The thioester moiety is introduced as a lithium binding unit for the hierarchical formation of titanium(IV) catecholate-based lithium-bridged helicates. In solution, the coordination compounds show a monomer-dimer equilibrium which -in comparison to the oxo esters- is significantly shifted towards the monomers. In addition, the influence of the thioester side chain on the dimerization behavior is investigated and an expansible/compressible molecular switch is synthesized. In the latter case expansion and compression are performed reversibly in methanol, whereas in DMSO spontaneous expansion occurs.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Titanium phosphonate oxo-alkoxide “clusters”: solution stability and facile hydrolytic transformation into nano titania

Oligonuclear Ti(iv) oxo-alkoxide-phosphonate complexes, produced by reaction of tBuPO(OH)₂ with Ti(OR)₄, are easily topotactically hydrolyzed forming intricate nanostructures.

Suppression of the release of arsenic from arsenopyrite by carrier-microencapsulation using Ti-catechol complex

Arsenopyrite is the most common arsenic-bearing sulfide mineral in nature, and its weathering contributes to acid mine drainage (AMD) formation and the release of toxic arsenic (As). To mitigate this problem, carrier-microencapsulation (CME) using titanium (Ti)-catechol complex (i.e., Ti-based CME) was investigated to passivate arsenopyrite by forming a protective coating. Ti⁴⁺ ion dissolved in sulfuric acid and catechol were used to successfully synthesize Ti(IV) tris-catecholate complex, [Ti(Cat)₃]^2-, which was stable in the pH range of 5-12. Electrochemical studies on the redox properties of this complex indicate that its oxidative decomposition was a one-step, irreversible process. The leaching of As from arsenopyrite was suppressed by CME treatment using the synthesized Ti-catechol complex. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicate that this suppression was primarily due to the formation of an anatase (β-TiO₂)-containing coating. Based on these results, a detailed 4-step mechanism to explain the decomposition of [Ti(Cat)₃]^2- and formation of TiO₂ coating in Ti-based CME is proposed: (1) adsorption, (2) partial oxidation-intermediate formation, (3) non electrochemical dissociation, and (4) hydrolysis-precipitation.

Simple Fabrication of Titanium Dioxide/N-Doped Carbon Hybrid Material as Non-Precious Metal Electrocatalyst for the Oxygen Reduction Reaction

We report a new approach for the fabrication of hybrid titanium dioxide/carbon materials derived from a porous titanium coordination polymer composed of a catechol-substituted porphyrin and Ti⁴⁺. Titanium dioxide nanocrystals were formed distributed in a nitrogen-doped carboniferous matrix after the thermolysis of the coordination polymer. The identity of the titanium dioxide phase, i.e., anatase or rutile, could be controlled by varying the thermolysis temperature. Electrochemical performances of the composites were explored with results demonstrating that the hybrid materials are promising cathodic materials for the oxygen reduction reaction.