THE REACTIONS OF ORGANOMETALLIC REACTIONS OF ORGANOMETALLIC COMPOUNDS OF TRANSITION METALS WITH MOLECULAR NITROGEN AND CARBON DIOXIDE M. E. V0L'PIN Institute of Organo-Element Compounds

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  • THE REACTIONS OF ORGANOMETALLICCOMPOUNDS OF TRANSITION METALS WITH

    MOLECULAR NITROGEN AND CARBON DIOXIDE

    M. E. V0L'PIN

    Institute of Organo-Element Compounds Academy of SciencesMoscow, USSR.

    ABSTRACT

    A development of organometallic chemistry has revealed wide possibilitiesfor activating various unsaturated and saturated molecules for numerous novelreactions. Many nitrogen fixating systems based on the low-valent transitionmetal derivatives and reducing agents have been discovered. Depending on thereaction conditions, complexed nitrogen reduction occurs to form either hydra-zine derivatives or ammonia. An investigation of the nitrogen fixating systemsinvolving Lewis acids, to regenerate the catalyst, has led to catalytic nitrogenfixation. Nitrogen activated by complex formation with transition metalcompounds is capable of insertion reactions into metalcarbon bonds to formorganic amines. In this connection the possibility arises of forming aminesfrom the reaction of nitrogen with hydrocarbons in the presence of transitionmetal compounds. Transition metal nitrogen compounds seem to be inter-mediates in the 'reverse' reactions as well, i.e. decompositions of nitrogen-compounds with nitrogen evolution (Sandmeyer reaction, hydrazine oxidation.etc).

    Unlike nitrogen, carbon dioxide is rather reactive and undergoes insertionreactions into non-transition metal-carbon boids. Carbon dioxide alsoappeared to be capable of direct or indirect complex formation with low-valenttransition metal compounds (Ru, Rh, Pt, etc.). Investigation of the stabilityof such complexes permits novel synthetic and catalytic reactions to be found.Thus the formation of MeH bonds (e.g. in the formic acid decompositionwith CO2 evolution) can be utilized for catalytic reduction of olefins and otherunsaturated compounds by means of formic acid. When the intermediatecarbon dioxide complexes with MeC bonds (carboxylates) are formed. thecompetitive reaction occurs, i.e. olefin insertion into MeH or MeC bondswith elimination of the respective carboxylic acid. Another interesting reaction,insertion of CO2 into MeH or MeC bonds, can follow either 'normal'(formation of a carbon-carbon' bond) or 'reverse' (formation of the metallo-

    acid ester) pathways.

    A development of organometallic chemistry has provided unusualpossibilities for activating various unsaturated and saturated moleculesand introducing them into the various reactions.

    The present lecture deals with the problem of activation by transition607

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  • M. E. VOL'PIN

    metal compounds of the two rather simple molecules, molecular nitrogen(N2) and carbon dioxide (C02). A common feature of these so differentsubstances is, perhaps, their exclusive biological significance. The mainsource of terrestrial nitrogen is the atmospheric molecular nitrogen, which,through biological fixation or the industrial ammonia synthesis, providesalmost the only source of nitrogen-containing compounds. On the otherhand, carbon dioxide is practically the only source of carbon for all livingmatter. Photosynthesis and biological nitrogen fixation are the two processeswhich provide the life on Earth.

    The main part of terrestrial carbon is in the form of carbon dioxide andthe carbonates. It is possible that carbon dioxide and the natural carbonateswill replace petroleum and coal as the main sources fir the industrialorganic synthesis of the future.

    The exceptional chemical inertness of nitrogen is well known. It is themost inert of the unsaturated compounds with multiple bonds. This propertyof the nitrogen molecule is due to not only the high ionization potential,low affinity, and high dissociation energy but also the unusually highstrength of the third bond (if one may say so) with respect to the second

    Table 1.

    NN. Dissociation energy 225 kcal mol'

    lonisation potential 367 kcal mo11Electron affinity 84 kcal mol'

    Bond energies, kcal mol'

    CC 80 CN 73 NN 37

    CC 198 = CN 210 = NN 225

    and first bonds (Table 1). Thus until recently, all methods of nitrogen fixa-tion, including the known Haber process, employed high temperatures andenergies for activating the nitrogen molecule.

    Approximately ten years ago, Shur and myself began to attempt nitrogenfixation by means of organometallic compounds. Our work was based onthe hypothesis that complex formation between nitrogen and the transitionmetal compounds via formation of both the donor-acceptor and dativebonds may be an effective means of activating the nitrogen molecule. Wedid not attempt to isolate the nitrogen complexes (this was soon achievedby Allen and Senoff in Canada) but instead we searched for possible reactionproducts of activated nitrogenammonia, hydrazine, amines and othersubstances. At first, the numerous experiments either failed or aflordedpoorly reproducible results.

    It should be noted that the study of nitrogen fixation requires that theresults be rigorously checked, with necessary nitrogen labelling, since mostreagents contain the admixtures of nitrates, nitrites and ammonium salts.

    After a rather long time we at last found the conditions in which nitrogenreacting at room temperature with transition metal compounds gives anoticeable quantity of ammonia, after the action of water. We checked theseresults, manipulating with labelled 15N until we were convinced that

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    molecular nitrogen itself was an actual source of the combined nitrogen.Thus in 1964 we decided to publish our first communication1. We reportedthat nitrogen readily reacts at room temperature with the low-valentorganometallics of titanium, chromium, molybdenum, tungsten (see Table2). These low-valent compounds were obtained from the reactions of

    Table 2.

    RMgXRLi (H20)N2+MXfl+

  • M. E. VOL'PIN

    Table 4.

    -LM -* L,,,

    1LMN -1-M or

    LLmM2MLmJ

    -[2NN2] -

    1H20H2NNH2

    [N3]j H20NH3

    state. Then it reacts with nitrogen to form the complexes. Such complexesare known after the pioneering work of Allen and Senoff2 who were thefirst to isolate nitrogen complexes of ruthenium.

    Table4a

    RuCI3 + N2H4 * ERu(NH3)5N2JC12 (Ref. 2)RuCl3nH2O + Zn/Hg + N21 RuCl2(N2)(H2O)2THF (Reti 3)

    Co(acac)3 + R3AJ + PPh3 + N2 * (Ph3P)3CoH(N2) (ReL 4)

    MNN (Cr) (Mn) Fe Co Ni1MNNM Mo Ru Rh PdNEEN_M_NN W Re Os Jr

    Sbilov and his collaborators3 and Yamomoto et al.4 and then manyother workers have shown the possibility of preparing stable complexesdirectly from molecular nitrogen. Nowadays nitrogen complexes (or dinitro-gen according to the new nomenclature) are known for a great number oftransition metals (Table 4a). Those for which the structure has been deter-mined are linear. Both mono- and binuclear complexes and those with twonitrogen ligands have been synthesized. The complexes of rhenium and theGroup VIII metals are most stable and have been studied in detail. However,until recently, the numerous attempts to reduce the nitrogen in these stablecomplexes were unsuccessful. In view of this it may be interesting to note thatin the stable complexes such as Yamamoto's cobalt complex, nitrogen couldbe reduced in the presence of compounds of other transition metals5. Thisaffords considerable amounts of ammonia (Table 5). The role of the second

    Table 5. Reduction of (Ph3P)3CoH(N2) (yield of NH3 in mol %)5

    TiC]4 MoC]5 FeC]2 C:C12

    C10H8Na 25 4 3 2Li + C10H8Li 73 25 7 14Mg+Mg12

    LmMNN +21

    MX - L

    mMNN 1

    M'X2))e NH3

    transition metal may be the formation of binuclear complexes in whichnitrogen reduction takes place. However, the mechanism of such reductionrequires special investigation.

    Let us again discuss the general scheme of nitrogen fixation. The firststeps of the process involve formation of the nitrogen complexes. Then theelectron transfer to the complexed nitrogen takes place with rupture of the

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    first two NN bonds and formation of hydrazine derivatives. The hydrazinecan be isolated if the reaction is carried out under sufficiently mild conditions.More drastic conditions or the presence of more active reducing agents cleavethe last NN bond to give the ammonia derivatives.

    It should be noted that until recently all the known nitrogen fixatingsystems were not catalytic. Stoicheiometric reaction of a transition metalcompound with nitrogen was observed. As a result no more than one nitro-gen molecule per one atom of transition metal was reduced in spite of agreat excess of reducing agent. In our opinion the two main factors responsiblefor the absence of catalysis are (i) in some cases the instability of a low-valenttransition metal compound active towards nitrogen and (ii) the high stabilityof the transition metalnitrogen nitride bond in the product of nitrogenfixation. Therefore, in order to realize a catalytic fixation of nitrogen it isnecessary to introduce the appropriate ligands for stabilizing the low-valentmetal compound and then to add a reagent, such as a Lewis or protic acid,which cleaves the metalnitrogen bond in the reaction product and re-generates the catalyst. On the basis of this hypothesis we started to searchfor catalytic systems.

    The first nitrogen fixating catalytic system was discovered at our laboratorythree years ago6. It consisted of the reducing agent, metallic aluminiumand the Lewis acid, aluminium bromide. Compounds of titanium wereused as catalysts. In the absence of titanium compounds nitrogen is notreduced by aluminium or by a mixture of aluminium and aluminiumbromide. However, even in the presence of one-tenth per cent of titaniumtetrachioride, nitrogen is rapidly reduced to give lithium nitrides, whichproduce ammonia and a small amount of