Oxophilicity

 ( Acid–base Chemistry ) 

Oxophilicity is the tendency of certain chemical compounds to form by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early such as titanium, niobium, and tungsten. Oxophilicity is often stated to be related to the hardness of the element, within the HSAB theory (HSAB theory), but it has been shown that oxophilicity depends more on the electronegativity and effective nuclear charge of the element than on its hardness. This explains why the early transition metals, whose electronegativities and effective nuclear charges are low, are very oxophilic. Many main group compounds are also oxophilic, such as derivatives of aluminium, silicon, and phosphorus(III). The handling of oxophilic compounds often requires air-free techniques.

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Examples Complexes of oxophilic metals typically are prone to hydrolysis. For example, the high valent chlorides hydrolyze rapidly to give oxides:

TiCl₄ + 2 H₂O → Titanium dioxide + 4 HCl

These reactions proceed via oxychloride intermediates. For example, WOCl₄ results from the partial hydrolysis of tungsten hexachloride. Hydroxide-containing intermediates are rarely observed for oxophilic metals. In contrast, the anhydrous halides of the later metals tend to hydrate, not hydrolyze, and they often form .

Reduced complexes of oxophilic metals tend to generate oxides by reaction with oxygen. Typically the oxide-ligand is bridging ligand, e.g.

2 (C₅H₅)₂TiCl + 1/2 O₂ → {(C₅H₅)₂TiCl}₂O

Only in rare cases do the products of oxygenation feature terminal oxo ligands.Nugent, W. A.; Mayer, J. M. "Metal-Ligand Multiple Bonds" J. Wiley: New York, 1988. .

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Applications of oxophilicity in synthesis Oxophilic reagents are often used to extract or exchange oxygen centers in organic substrates, especially carbonyls (esters, ketones, amides) and epoxides. The highly oxophilic reagent generated from tungsten hexachloride and butyl lithium is useful for the deoxygenation of . Such conversions are sometimes valuable in organic synthesis. In the McMurry reaction, ketones are converted into alkenes using oxophilic reagents:

2 R₂CO + Ti → R₂C=CR₂ + TiO₂

Similarly, Tebbe's reagent is used in olefination reactions:

Cp₂TiCH₂AlCl(CH₃)₂ + R₂C=O → Cp₂TiO + 0.5 (AlCl(CH₃)₂)₂ + R₂C=CH₂

Oxophilic main group compounds are also well known and useful. The highly oxophilic reagent Si₂Cl₆ stereospecifically deoxygenates .David P. Sebesta "Hexachlorodisilane" in Encyclopedia of Reagents for Organic Synthesis John Wiley, London, 2001. Article Online Posting Date: April 15, 2001. Phosphorus pentasulfide and the related Lawesson's reagent convert certain organic carbonyls to the corresponding sulfur derivatives:

P₄S₁₀ + n R₂C=O → P₄S_{10− n}O_n + n R₂C=S

Owing to the high stability of carbon dioxide, many carbon compounds such as phosgene are oxophilic. This reactivity is used for recycling of triphenylphosphine oxide:

OPPh₃ + COCl₂ → Cl₂PPh₃ + CO₂

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