Organomanganese chemistry is the chemistry of organometallic compounds containing a carbon to manganese chemical bond. In a recent review Cahiez et al. argue that as manganese is cheap and benign (only iron performs better in these aspects), organomanganese compounds have potential as chemical reagents, although currently they are not widely used as such despite extensive research.
The first organomanganese compounds were synthesised in 1937 by Gilman and Bailee who reacted phenyllithium with manganese(II) iodide to form phenylmanganese iodide (PhMnI) and diphenylmanganese (Ph2Mn).
The reactivity of organomanganese compounds can be compared to that of organomagnesium compounds and organozinc compounds. The electronegativity of Mn (1.55) is comparable to that of Mg (1.31) and Zn (1.65) making the carbon atom (EN = 2,55) nucleophilic. The reduction potential of Mn is also intermediate between Mg and Zn. Key disadvantage of organomanganese compounds is that they can be obtained directly from the metal only with difficulty.
General methods for the synthesis of organomanganese compounds exist. Organomanganese halides can be obtained by reaction of manganese halides (manganese(II) bromide, manganese(II) bromide) with organolithium or organomagnesium compounds in transmetallation:
- RM + MnX
2→ 2RMnX + MX
- 2RM + MnX
2Mn + 2MX
Organomanganates (the ate complex) are the most stable compounds:
- 3RM + MnX
3MnX + 2MX
- 4RM + MnX
The organomanganese compounds are usually prepared in THF where they are the most stable (via complexation) even though many of them must be handled at low temperatures. Simple dialkylmanganese decompose by beta-hydride elimination to a mixture of alkanes and alkenes.
Derivatives of Mn2(CO)10
Many organomanganese complexes are derived from dimanganese decacarbonyl, Mn2(CO)10. Bromination and reduction affords BrMn(CO)5 and LiMn(CO)5. These species are precursors to alkyl, aryl, and acyl derivatives:
- BrMn(CO)5 + RLi → RMn(CO)5 + LiBr
- LiMn(CO)5 + RC(O)Cl → RC(O)Mn(CO)5 + LiCl
- RMn(CO)5 + CO → RC(O)Mn(CO)5
The general pattern of reactivity is analogous to that for the more popular cyclopentadienyliron dicarbonyl dimer.
The Mn(I) compound BrMn(CO)5 is also the precursor to many pi-arene complexes:
- BrMn(CO)5 + Ag+ + C6R6 → [Mn(CO)3(C6R6)]+ + AgBr + 2 CO
These cationic half-sandwich complexes are susceptible to nucleophilic additions to give cyclohexadienyl derivatives and ultimated functionalized arenes.
Organomanganese halides react with aldehydes and ketones to the alcohol, with carbon dioxide to the carboxylic acid (tolerating higher operating temperature than corresponding RLi or RMgBr counterparts), sulfur dioxide and isocyanates behaving like soft Grignard reagents. They do not react with esters, nitriles, or amides. They are more sensitive to steric than to electronic effects.
Certain manganese amides of the type RR1NMnR2 are used for the deprotonation of ketones forming manganese enolates. Just like lithium enolates they can further react with silyl chlorides to silyl enol ethers, with alkyl halides in alpha-alkylation and with aldehydes and ketones to beta-keto-alcohols. Manganese enolates can also be obtained by transmetalation of manganese halides with Li, Mg, K or Na enolates.
Manganese halides are catalysts in several homo- and crosscoupling reactions involving stannanes and Grignards in which organomanganese intermediates play a part. Likewise coupling reactions involving organomanganese halides are catalysed by Pd, Ni, Cu and Fe compounds
Commercial manganese powder is not suited for the synthesis of organomanganese compounds. In 1996 Rieke introduced activated manganese (see Rieke metal) obtained by reaction of anhydrous manganese(II) chloride with lithium metal in a solution of a catalytic amount of naphthalene in THF. Other reducing agents are potassium graphite and magnesium. Activated manganese facilitates the Mn version of the Barbier reaction and the pinacol coupling.
Several organomanganese compounds with valency +3 or +4 are known. The first one discovered (1972) was Mn(nor)4 with four norbornyl units. An octahedral Mn(IV)(Me)6−2 complex was reported in 1992, obtained by reaction of MnMe4(PMe3), with methyllithium followed by addition of TMED.
Methylcyclopentadienyl manganese tricarbonyl is a half-sandwich compound used as a gasoline additive.
Higher group 7 organometallics
In the same group 7 elements technetium is a radioactive synthetic element and little explored. Organorhenium compounds can be found with oxidation states +4 and +5. An important starting material is dirhenium decacarbonyl, which can be synthesized by carbonylation of rhenium(VII) oxide. Methylrhenium trioxide is used as a catalyst.
- Chemical bonds of carbon with other elements in the periodic table:
|Compounds of carbon with other elements in the periodic table
- Chemistry of Organomanganese(II) Compounds Gerard Cahiez, Christophe Duplais, and Julien Buendia Chem. Rev. 2009 doi:10.1021/cr800341a
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- Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997