The +4 oxidation state dominates titanium chemistry,[1] but compounds in the +3 oxidation state are also numerous.[2] Commonly, titanium adopts an octahedral coordination geometry in its complexes,[3][4] but tetrahedral TiCl4 is a notable exception. Because of its high oxidation state, titanium(IV) compounds exhibit a high degree of covalent bonding.[1]
Oxides, sulfides, and alkoxides
The most important oxide is TiO2, which exists in three important polymorphs; anatase, brookite, and rutile. All three are white diamagnetic solids, although mineral samples can appear dark (see rutile). They adopt polymeric structures in which Ti is surrounded by six oxide ligands that link to other Ti centers.[5]
The term titanates usually refers to titanium(IV) compounds, as represented by barium titanate (BaTiO3). With a perovskite structure, this material exhibits piezoelectric properties and is used as a transducer in the interconversion of sound and electricity.[6] Many minerals are titanates, such as ilmenite (FeTiO3). Star sapphires and rubies get their asterism (star-forming shine) from the presence of titanium dioxide impurities.[7]
A variety of reduced oxides (suboxides) of titanium are known, mainly reduced stoichiometries of titanium dioxide obtained by atmospheric plasma spraying. Ti3O5, described as a Ti(IV)-Ti(III) species, is a purple semiconductor produced by reduction of TiO2 with hydrogen at high temperatures,[8] and is used industrially when surfaces need to be vapor-coated with titanium dioxide: it evaporates as pure TiO, whereas TiO2 evaporates as a mixture of oxides and deposits coatings with variable refractive index.[9] Also known is Ti2O3, with the corundum structure, and TiO, with the rock salt structure, although often nonstoichiometric.[10]
The alkoxides of titanium(IV), prepared by treating TiCl4 with alcohols, are colorless compounds that convert to the dioxide on reaction with water. They are industrially useful for depositing solid TiO2 via the sol-gel process. Titanium isopropoxide is used in the synthesis of chiral organic compounds via the Sharpless epoxidation.[11]
Titanium forms a variety of sulfides, but only TiS2 has attracted significant interest. It adopts a layered structure and was used as a cathode in the development of lithium batteries. Because Ti(IV) is a "hard cation", the sulfides of titanium are unstable and tend to hydrolyze to the oxide with release of hydrogen sulfide.[12]
Titanium tetrachloride (titanium(IV) chloride, TiCl4[18]) is a colorless volatile liquid (commercial samples are yellowish) that, in air, hydrolyzes with spectacular emission of white clouds. Via the Kroll process, TiCl4 is used in the conversion of titanium ores to titanium metal. Titanium tetrachloride is also used to make titanium dioxide, e.g., for use in white paint.[19] It is widely used in organic chemistry as a Lewis acid, for example in the Mukaiyama aldol condensation.[20] In the van Arkel–de Boer process, titanium tetraiodide (TiI4) is generated in the production of high purity titanium metal.[21]
Following the success of platinum-based chemotherapy, titanium(IV) complexes were among the first non-platinum compounds to be tested for cancer treatment. The advantage of titanium compounds lies in their high efficacy and low toxicity in vivo.[24] In biological environments, hydrolysis leads to the safe and inert titanium dioxide. Despite these advantages the first candidate compounds failed clinical trials due to insufficient efficacy to toxicity ratios and formulation complications.[24] Further development resulted in the creation of potentially effective, selective, and stable titanium-based drugs.[24]
^Liu, Gang; Huang, Wan-Xia; Yi, Yong (26 June 2013). "Preparation and Optical Storage Properties of λTi3O5 Powder". Journal of Inorganic Materials. 28 (4): 425–430. doi:10.3724/SP.J.1077.2013.12309 (inactive 2024-06-22).{{cite journal}}: CS1 maint: DOI inactive as of June 2024 (link)
^Ramón, Diego J.; Yus, Miguel (2006). "In the arena of enantioselective synthesis, titanium complexes wear the laurel wreath". Chem. Rev. 106 (6): 2126–2308. doi:10.1021/cr040698p. PMID16771446.