Decacarbonyldimanganese

Mn2(CO)10

[10170-69-2]  · C10Mn2O10  · Decacarbonyldimanganese  · (MW 389.98)

(source of a wide range of manganese complexes; catalyst or cocatalyst for isomerization, oligomerization, radical coupling, etc.)

Alternate Names: dimanganese decacarbonyl; manganese carbonyl.

Physical Data: mp 154 °C; sublimes 80 °C/0.1 mmHg, 70 °C/0.05 mmHg.

Solubility: insol H2O; mod sol common org solvents.

Form Supplied in: yellow crystalline solid; widely available but expensive.

Preparative Method: the most convenient laboratory synthesis is from tricarbonyl(h5-methylcyclopentadienyl)manganese by reduction with sodium/naphthalene under carbon monoxide to yield Sodium Pentacarbonylmanganate, acidification, and oxidation;1 highly efficient stirring is essential in the first stage for good yields, and incomplete oxidation or the loss of the volatile (toxic) intermediate HMn(CO)5 in the final stages must be carefully avoided.

Handling, Storage, and Precautions: use in a fume hood; toxic, but not sufficiently volatile to present handling problems. May be stored indefinitely in a well-stoppered bottle at rt.

Introduction.

The molecule consists of two Mn(CO)5 fragments linked by an Mn-Mn bond such that each Mn atom has octahedral coordination.2 Many uses of Mn2(CO)10 involve its initial reduction to the anion [Mn(CO)5]- or oxidation by halogens to XMn(CO)5 (X = Cl, Br) and their use in turn to produce hydrido, alkyl, acyl, h3-allyl, h5-cyclopentadienyl, and h6-arene complexes, amongst others.

Catalytic Uses.

Most catalytic systems utilizing Mn2(CO)10 require temperatures of approximately 140 °C, sufficient for cleavage of the relatively weak Mn-Mn bond (eq 1). In contrast to such thermolysis, photolysis involves two competitive processes: the same cleavage (eq 1), as well as loss of CO (eq 2).

The radicals formed by thermolysis are efficient abstractors of halide from RCCl3 (eq 3),3 for example, and hence serve to initiate selective hydrogenolysis of such halides in the presence of good hydrogen donors (eq 4)4 or addition to activated alkenes (eq 5).5

Addition to unactivated alkenes (except C2H4) is generally less efficient4b,5b and, whereas Mn2(CO)10 is more efficient and more selective than Pentacarbonyliron for reductions of the type shown by eq 4, the opposite is true for the additions (eq 5). In such reactions, 1:1 adducts tend to predominate and even in the addition of trichloroacetate to ethylene (eq 6), which gives a significant proportion of the telomers (n = 2-4), the use of DMF as solvent largely suppresses formation of all but the 1:1 adduct (n = 1).6

Similar selective hydrogenolysis and addition reactions involving hydrogen rather than halogen abstraction as the initiating step are involved when such reactive hydrogen sources as silanes, thiols, or phosphites are used with Mn2(CO)10 (eqs 7-9). Detailed study of such reductions as that of 1,3,3,5-tetrachloropentane by Et3SiH (eq 10)7 shows clearly that &bdot;Mn(CO)5 acts only as initiator with the silane (not HMn(CO)5) providing the hydrogen in the propagation step and Et3Si&bdot; abstracting the chlorine atom.

The efficiency and especially the high selectivity of Mn2(CO)10 in this and related reductions, both by silanes8 and thiols (e.g. eq 11)9 is only rarely matched or exceeded by other metal carbonyls (including Re2(CO)10, Fe(CO)5, and Mo(CO)6).

Particularly striking is the selectivity in additions to alkenes of the radicals formed according to eqs 7-9. Here, only the 1:1 adducts are significant products in the additions, exemplified in eqs 12-14,10-12 when Mn2(CO)10 is used as initiator, whereas substantial amounts of higher telomers result when peroxides or other classical initiators are employed. In a typical comparative study,11 the addition (eq 15) using Mn2(CO)10 catalysis provided the products (n = 1) and (n = 2) in 99 and 0.5% yields, Hexacarbonylmolybdenum was almost as selective (but required a long reaction time), Decacarbonyldirhenium was inactive, while 1,1-Di-t-butyl Peroxide catalysis gave 50% (n = 1) and 20% (n = 2).

Stoichiometric Reactions with Unsaturated Hydrocarbons.

Much of the synthetically interesting chemistry involving reactions of Mn2(CO)10 with alkenes and alkynes has been obscured by the fact that few of the metal complexes initially obtained have been subjected to further reactions or even to cleavage of the organic ligand from the metal. Nevertheless, the potential for specific transformations of such unsaturated hydrocarbons is clear from the reactions outlined below.

Enyl and Dienyl Complexes.

Many different dienes are known to react with Mn2(CO)10 to give complexes. In the case of open-chain dienes, h3-enyl complexes are isolated as initial products of irradiation (eq 16),13 but these may be smoothly converted to h5-dienyl complexes (eq 17).

For cyclopentadiene14a (or its pentamethyl derivative14b), cyclohexadiene,15 and cycloheptadiene,16 thermal conditions have been used that lead to dienyl complexes (eq 18). These reactions occur at temperatures which enable cleavage of the Mn-Mn bond, but it is not established whether the mechanism involves initial H abstraction by &bdot;Mn(CO)5 or a concerted process. Since the temperatures also suffice for decomposition of HMn(CO)5 to dihydrogen and Mn2(CO)10, all of the manganese can be utilized in complex formation.

The synthetic potential of such dienyl complexes is perhaps most strikingly illustrated by the diene photoaddition to the open-chain complexes leading to nine-membered rings (still attached to the metal), as in eq 19.17 Cyclohexadiene adds in a different manner, involving formation of one C-C bond and H-migration (eq 20); incorporation of a second diene unit occurs in some cases.13a

As open-chain dienyl and cyclohexadienyl tricarbonylmanganese complexes are more usually prepared from halopentacarbonylmanganese, XMn(CO)5 (X = Cl, Br), and (alkyl)cyclopentadienyltricarbonylmanganese from MnCl2 (via (RC5H4)2Mn), only the useful reactions of the cycloheptadienyl complex are discussed further here.

Tricarbonylcycloheptadienylmanganese (formed according to eq 18, n = 3) is smoothly converted16 to the cationic triene complex by hydride abstraction (eq 21) and the latter adds a wide range of nucleophiles to give exo-substituted cycloheptadienyl complexes (eq 22; Y = OR, NMe2, CN, CH(CO2Me)2, Me, Ph, etc.).16,18 If an exo-hydrogen could be specifically abstracted as hydride (eq 23), the product should undergo further nucleophilic addition to give vicinally (i.e. 6,7-) disubstituted cycloheptadienyl complexes. However, trityl either abstracts Y (e.g. when Y = OR, NMe2, or CN) or fails to react (R = Me), apparently for steric reasons, and other reagents tried also fail to react or cause complete disruption (e.g. NBS to give [Mn(CO)4Br]2).

Regeneration of a cationic system, but at the dienyl level, has been accomplished using nitrosonium hexafluorophosphate (eq 24) and the products then add nucleophiles specifically to give 5,7-disubstituted cycloheptadiene complexes (eq 25).18,19 Analogous methods have been used to convert exo-substituted cyclohexadienyl complexes to vicinally (i.e. 5,6-) disubstituted cyclohexadienemanganese complexes.19a,b

Photolysis of Mn2(CO)10 with allenes leads chiefly to (m-allene)Mn2(CO)8 complexes, but is accompanied by allene di- and even trimerization in apparently regioselective fashion (eqs 26 and 27).13b,20

A unique ligand dimerization and ring-opening process occurs in the reaction of Mn2(CO)10 with tropone (eq 28).21

Alkyne Complexes.

Reactions of alkynes with manganese carbonyl do not appear to have been studied systematically, but have yielded synthetically interesting products in several cases. None more so than the formation of a dihydropentalenyl complex from acetylene itself (eq 29).22

The reaction of diethylaminopropyne (eq 30) yields four complexes whose sequential formation involves two proton migrations, the first of which converts the alkyne to an allene ligand.23 More obvious synthetic potential can be seen in the reaction of ethoxy- and dimethylamino-alkynes (with Mn2(CO)10 activated by prior displacement of one CO); thus ethoxyacetylene itself gives a product with a divinyl ketone ligand (eq 31).24a,b Similar reactions with diethyl acetylenedicarboxylate have been described.24c

Finally, the lactone synthesis from Mn2(CO)10, a terminal alkyne, and iodomethane (eq 32) probably involves reaction of the alkyne with initially formed acetyl(pentacarbonyl)manganese rather than directly with the decacarbonyl.25

Related Reagents.

Pentacarbonylmethylmanganese; Pentacarbonylchloromanganese; Pentacarbonylphenylmanganese; Sodium Pentacarbonylmanganate; Pentacarbonyl(trimethylsilyl)manganese.


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Peter L. Pauson

University of Strathclyde, Glasgow, UK



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