Organomanganese chemistry is the
chemistry of
organometallic compounds
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
containing a
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
to
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
chemical bond
A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
. In a 2009 review, Cahiez et al. argued that as manganese is cheap and benign (only
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
performs better in these aspects), organomanganese compounds have potential as chemical reagents, although currently they are not widely used as such despite extensive research.
Synthesis
Organomanganese compounds were first reported in 1937 by Gilman and Bailee who described the reaction of
phenyllithium
Phenyllithium or lithobenzene is an organometallic agent with the empirical formula C6H5Li. It is most commonly used as a metalating agent in organic syntheses and a substitute for Grignard reagents for introducing phenyl groups in organic synthes ...
and
manganese(II) iodide to form phenylmanganese iodide (PhMnI) and diphenylmanganese (Ph
2Mn).
Following this precedent, other organomanganese halides can be obtained by alkylation of
manganese(II) chloride
Manganese(II) chloride is the dichloride salt of manganese, MnCl2. This inorganic chemical exists in the anhydrous form, as well as the di hydrate (MnCl2·2H2O) and tetrahydrate (MnCl2·4H2O), with the tetrahydrate being the most common form. Li ...
,
manganese(II) bromide, and
manganese(II) iodide. Manganese iodide is attractive because the anhydrous compound can be prepared in situ from manganese and
iodine in
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...
. Typical alkylating agents are
organolithium
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...
or
organomagnesium Magnesium anthracenide with three thf ligands. Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-contain ...
compounds:
:
:
A variety of organomanganates (the
ate complex
In chemistry, an ate complex is a salt formed by the reaction of a Lewis acid with a Lewis base whereby the central atom (from the Lewis acid) increases its valence and gains a negative formal charge. (In this definition, the meaning of valence i ...
) are isolable:
:
:
The organomanganese compounds are usually prepared in
THF
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
where they are the most stable (via complexation) even though many of them must be handled at low temperatures. Simple dialkylmanganese compounds decompose by
beta-hydride elimination
β-Hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the β-carbon. For instance butyl groups can undergo th ...
to a mixture of alkanes and alkenes.
Derivatives of Mn2(CO)10
Many organomanganese complexes are derived from
dimanganese decacarbonyl
Dimanganese decacarbonyl is the chemical compound with the formula Mn2(CO)10. This metal carbonyl is an important reagent in the organometallic chemistry of manganese.
Synthesis
The compound was first prepared in low yield by the reduction of ...
, Mn
2(CO)
10.
Bromination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
and reduction with lithium affords
BrMn(CO)5 and LiMn(CO)
5, respectfully. 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
Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula ''η''5-C5H5)Fe(CO)2sub>2, often abbreviated to Cp2Fe2(CO)4, pFe(CO)2sub>2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crysta ...
.
The Mn(I) compound BrMn(CO)
5 is also the precursor to many pi-arene complexes:
::BrMn(CO)
5 + Ag
+ + C
6R
6 →
3(C6R6)">n(CO)3(C6R6)sup>+ + AgBr + 2 CO
These cationic half-sandwich complexes are susceptible to nucleophilic additions to give cyclohexadienyl derivatives and ultimated functionalized arenes.
133px, Sample of (BrMn(CO)5)">manganese pentacarbonyl bromide (BrMn(CO)
5)
Reactions
The chemistry of organometallic compounds of Mn(II) are unusual among the
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
s due to the high ionic character of the Mn(II)-C bond. The reactivity of organomanganese compounds can be compared to that of
organomagnesium Magnesium anthracenide with three thf ligands. Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-contain ...
and
organozinc compound
Organozinc compounds in organic chemistry contain carbon (C) to zinc (Zn) chemical bonds. Organozinc chemistry is the science of organozinc compounds describing their physical properties, synthesis and reactions.The Chemistry of Organozinc Compoun ...
s. The
electronegativity
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
of Mn (1.55) is comparable to that of Mg (1.31) and Zn (1.65), making the carbon atom (EN = 2.55)
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
. The
reduction potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
of Mn is also intermediate between Mg and Zn.
Organomanganese halides react with
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s and
ketones to the
alcohol, with
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
to the
carboxylic acid (tolerating higher
operating temperature than corresponding RLi or RMgBr counterparts),
sulfur dioxide and
isocyanate
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
s behaving like soft
Grignard reagents. They do not react with
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
s,
nitriles, or
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
s. They are more sensitive to steric than to electronic effects.
With
acyl halide
In organic chemistry, an acyl halide (also known as an acid halide) is a chemical compound derived from an oxoacid by replacing a hydroxyl group () with a halide group (, where X is a halogen).
If the acid is a carboxylic acid (), the compoun ...
s RMnX compounds form the corresponding ketones. This reaction is
chemoselective
Chemoselectivity is the preferential outcome of a chemical reaction over a set of possible alternative reactions.
In another definition, chemoselectivity refers to the selective reactivity of one functional group in the presence of others; often ...
and has been applied in
organic synthesis for this reason.
Certain manganese amides of the type RR
1NMnR
2 are used for the deprotonation of ketones forming manganese
enolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
Bonding and structure
Enolate anions are electr ...
s. Just like lithium enolates they can further react with silyl chlorides to
silyl enol ether
Silyl enol ethers in organic chemistry are a class of organic compounds that share a common functional group composed of an enolate bonded through its oxygen end to an organosilicon group. They are important intermediates in organic synthesis.
Sy ...
s, 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.
Manganese chloride is a precursor to organomanganese reagents in organic chemistry.
Activated manganese
Commercial manganese powder is not suited for the synthesis of organomanganese compounds. In 1996 Rieke introduced activated manganese (see
Rieke metal
A Rieke metal is a highly reactive metal powder generated by reduction of a metal salt with an alkali metal. These materials are named after Reuben D. Rieke, who first described the recipes for their preparation. Among the many metals that have bee ...
) obtained by reaction of anhydrous
manganese(II) chloride
Manganese(II) chloride is the dichloride salt of manganese, MnCl2. This inorganic chemical exists in the anhydrous form, as well as the di hydrate (MnCl2·2H2O) and tetrahydrate (MnCl2·4H2O), with the tetrahydrate being the most common form. Li ...
with
lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
metal in a solution of a catalytic amount of
naphthalene in THF. Other reducing agents are
potassium graphite
Graphite intercalation compounds are complex materials having a formula where the ion or is inserted ( intercalated) between the oppositely charged carbon layers. Typically ''m'' is much less than 1. These materials are deeply colored solids t ...
and magnesium. Activated manganese facilitates the Mn version of the
Barbier reaction
The Barbier reaction is an organometallic reaction between an alkyl halide (chloride, bromide, iodide), a carbonyl group and a metal. The reaction can be performed using magnesium, aluminium, zinc, indium, tin, samarium, barium or their salts. ...
and the
pinacol coupling A pinacol coupling reaction is an organic reaction in which a carbon–carbon bond is formed between the carbonyl groups of an aldehyde or a ketone in presence of an electron donor in a free radical process. The reaction product is a vicinal diol. ...
.
High-valent compounds
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
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
Mn(IV)(Me)
6−2 complex was reported in 1992, obtained by reaction of MnMe
4(PMe
3), with
methyllithium
Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in so ...
followed by addition of
TMED.
[''High-valent organomanganese chemistry. 1. Synthesis and characterization of manganese(III) and -(IV) alkyls'' Robert J. Morris, Gregory S. Girolami Organometallics, 1991, 10 (3), pp 792–799 ]
See also
*
Organorhenium chemistry
References
{{ChemicalBondsToCarbon