Titanium compounds

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See also the categories Titanium compounds and Titanium minerals
A steel colored twist drill bit with the spiral groove colored in a golden shade.
TiN-coated drill bit

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

Titanium dioxide powder

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]

Nitrides and carbides

Titanium nitride (TiN) is a refractory solid exhibiting extreme hardness, thermal/electrical conductivity, and a high melting point.[13] TiN has a hardness equivalent to sapphire and carborundum (9.0 on the Mohs scale),[14] and is often used to coat cutting tools, such as drill bits.[15] It is also used as a gold-colored decorative finish and as a barrier layer in semiconductor fabrication.[16] Titanium carbide (TiC), which is also very hard, is found in cutting tools and coatings.[17]

Halides

Titanium(III) compounds are characteristically violet, illustrated by this aqueous solution of titanium trichloride.

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]

Titanium(III) and titanium(II) also form stable chlorides. A notable example is titanium(III) chloride (TiCl3), which is used as a catalyst for production of polyolefins (see Ziegler–Natta catalyst) and a reducing agent in organic chemistry.[22]

Organometallic complexes

Main article: Organotitanium chemistry

Owing to the important role of titanium compounds as polymerization catalyst, compounds with Ti-C bonds have been intensively studied. The most common organotitanium complex is titanocene dichloride ((C5H5)2TiCl2). Related compounds include Tebbe's reagent and Petasis reagent. Titanium forms carbonyl complexes, e.g. (C5H5)2Ti(CO)2.[23]

Anticancer therapy studies

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]

See also

References

  1. ^ a b Greenwood & Earnshaw 1997, p. 958
  2. ^ Greenwood & Earnshaw 1997, p. 970
  3. ^ Greenwood & Earnshaw 1997, p. 960
  4. ^ Greenwood & Earnshaw 1997, p. 967
  5. ^ Greenwood & Earnshaw 1997, p. 961
  6. ^ "Titanium". Columbia Encyclopedia (6th ed.). New York: Columbia University Press. 2000–2006. ISBN 978-0-7876-5015-5.
  7. ^ Emsley, John (2001). "Titanium". Nature's Building Blocks: An A-Z guide to the elements. Oxford, England, UK: Oxford University Press. ISBN 978-0-19-850340-8.
  8. ^ 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.
  9. ^ Bonardi, Antonio; Pühlhofer, Gerd; Hermanutz, Stephan; Santangelo, Andrea (2014). "A new solution for mirror coating in γ-ray Cherenkov Astronomy". Experimental Astronomy. 38 (1–2): 1–9. arXiv:1406.0622. Bibcode:2014ExA....38....1B. doi:10.1007/s10686-014-9398-x. S2CID 119213226.
  10. ^ Greenwood & Earnshaw 1997, p. 962.
  11. ^ 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. PMID 16771446.
  12. ^ McKelvy, M.J.; Glaunsinger, W.S. (1995). "Titanium Disulfide". Inorganic Syntheses. Vol. 30. pp. 28–32. doi:10.1002/9780470132616.ch7. ISBN 9780470132616.
  13. ^ Saha, Naresh (1992). "Titanium nitride oxidation chemistry: An x-ray photoelectron spectroscopy study". Journal of Applied Physics. 72 (7): 3072–3079. Bibcode:1992JAP....72.3072S. doi:10.1063/1.351465.
  14. ^ Schubert, E.F. "The hardness scale introduced by Friederich Mohs" (PDF). Educational resources. Troy, NY: Rensselaer Polytechnic Institute. Archived (PDF) from the original on 3 June 2010.
  15. ^ Truini, Joseph (May 1988). "Drill bits". Popular Mechanics. Vol. 165, no. 5. p. 91. ISSN 0032-4558.
  16. ^ Baliga, B. Jayant (2005). Silicon carbide power devices. World Scientific. p. 91. ISBN 978-981-256-605-8.
  17. ^ "Titanium carbide product information". H.C. Starck. Archived from the original on 22 September 2017. Retrieved 16 November 2015.
  18. ^ Seong, S.; Younossi, O.; Goldsmith, B.W. (2009). Titanium: Industrial base, price trends, and technology initiatives (Report). Rand Corporation. p. 10. ISBN 978-0-8330-4575-1.
  19. ^ Johnson, Richard W. (1998). The Handbook of Fluid Dynamics. Springer. pp. 38–21. ISBN 978-3-540-64612-9.
  20. ^ Coates, Robert M.; Paquette, Leo A. (2000). Handbook of Reagents for Organic Synthesis. John Wiley and Sons. p. 93. ISBN 978-0-470-85625-3.
  21. ^ Greenwood & Earnshaw 1997, p. 965
  22. ^ Gundersen, Lise-Lotte; Rise, Frode; Undheim, Kjell; Méndez Andino, José (2007). "Titanium(III) Chloride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rt120.pub2. ISBN 978-0471936237.
  23. ^ Hartwig, J.F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. New York, NY: University Science Books. ISBN 978-1891389535.
  24. ^ a b c Tshuva, Edit Y.; Miller, Maya (2018). "Chapter 8. Coordination complexes of titanium(IV) for anticancer therapy". In Sigel, Astrid; Sigel, Helmut; Freisinger, Eva; Sigel, Roland K.O. (eds.). Metallo-drugs: Development and action of anticancer agents. Vol. 18. Berlin, DE: de Gruyter GmbH. pp. 219–250. doi:10.1515/9783110470734-014. ISBN 9783110470734. PMID 29394027. {{cite book}}: |journal= ignored (help)

Works cited

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