Eugene G. Rochow Harvard University Cambridge, Massachusetts
The Direct Synthesis of Organometdhc Compounds
Frankland (1) produced the first metal alkyl, diethylzinc, in 1849 by the action of ethyl iodide on metallic zinc, followed by thermal disproportionation of the resulting ethylzinc iodide:
Ever since then, most chemists have preferred to prepare organometallic derivatives directly from the corresponding metal, whenever that is possible. In those instances where the metal is insufficiently reactive, or the chemist is insufficiently clever, recourse is had to a prime reagent such as an organomagnesium halide or a lithium alkyl, which in turn is prepared from the corresponding metal. When we consider the vast quantities of tetraethyllead which are prepared from metallic sodium and lead, or the volume of dimethylsiloxane made from elementary silicon, or the amount of triethylaluminum made from metallic aluminum, we see that this type of synthesis is firmly established as a commercial production method as well as a laboratory preparative method. Moreover, the practice seems to be increasing, as a glance a t recent hooks on organometallic chemistry will show.' Such expansion of the art of direct synthesis since Frankland's time should not surprise us, considering that 4/5 of the elements are metals or metalloids, and that almost everything we want to learn about the metallic elements leads us a t one time or another to or through an organometallic compound. It is more to wonder, perhaps, that there are still some metals which we cannot yet convert to organic derivatives by this method, but would l i e to. This might be an appropriate time to look a t the method in its broader aspects, and to draw whatever conclusions we may be fortunate enough to find concerning what makes a direct synthesis successful or unsuccessful. Frederick Stanley Kipping Award address, delivered at Detroit, Michigan, ACS meeting, April 5, 1965. 1 Those who wish to pursue the topic of organometallic chemistry more generally may find the following references useful: G. E., "Organometallic Compounds," 2nd ed., John COATEB, Wiley & Sons, Inc., New York, 1960. R o c ~ o w E. , G . , "Organometallic Chemistry," Selected Topics in Modern Chemistry, Reinhold Publishing Co., New York, 1964. STONE,F. G . A., AND WEST, R. C., editom, "Advances in Organometallic Chemistry," Vol. 1, 1964, et seg. SEYFERTH, D., editor, Journal of Organomelallic Chemist%!, Vol. 1, 1963, et sep.
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Why o "Direct" Synthesis?
We might begin by asking what special appeal lies in a direct synthesis (meaning the preparation of some useful organometallic compound directly from the pertinent metallic element, rather than through a sequence of reactions starting with some appropriate compound such as a chloride or a carbonyl). Aside from the obvious advantages of simplicity, availability, and potentially greater yield, there is the aesthetic satisfaction of converting a clean, pure metal directly to a much-desired compound. There also is the matter of low cost; since the common metals are made on a huge scale by some of the oldest and hest-developed chemical reactions known to man, such metals are economical and convenient reagents. The greatest advantage of a metal as starting material, however, is its free energy content. We may look upon pieces of silicon or magnesium or aluminum (or whatever else) as chunks of storedup chemical free energy, obtained by huge expenditure of electrical or thermal energy. I n other words, the hard work has already been done by the metallurgist, and the chemist can make use of all that stored energy to let his reactions run themselves. Table 1 shows how much energy actually is required to obtain some of the chemically useful metals, and hence how much is stored up in one gram-atomicweight of the element with respect to its common source (that is, heat of formation of the source, disregarding any other product of the electrolysis or reduction). Since the chlorides of the first three metals are in equilibrium with their natural surroundings a t the outset (except anhydrous MgCl,, which would have to be considered as the hexahydrate in solution in sea water and would have the much higher figure of 596 kcal attached), the metal obviously stands a t a higher energy level than its source, as indicated. If the metal then is caused to react with methyl chloride, for example, in such a way as to establish metal-carbon and metal-chlorine bonds, Toble 1 .
Energy Stored in 1 Mole of Free Metal with Respect to Its Usual Source
Metal
Stored energy, kcal
Startine from
Na Li w3 A1 Si Sn
98 97 153 195 201 138
NaCl LiCl MgCL anhydr. ALOa SiOl SnOt
The figures of Table 1 represent minimum values, since they do not take into account res~stivelosses in the cell, etc.
energy will be liberated as the metal approaches its former condition again. This energy of chlorination serves to drive the reaction toward completion, assisted by whatever energy is released by the formation of the C-M bonds. It is this aspect of stored-up free energy released during reaction which makes some examples of direct synthesis so satisfactory. Scope of ihe Reaction
The figure shows which of the 83 metals and metalloids have been shown to undergo direct syntheses, i.e., to react in the metallic state with appropriate carbon compounds to establish carbon-metal bonds and yield organometallic derivatives. Those elements marked with a black diamond shape, react with one or more hydrocarbon halides, such as CHaCI, CH3Br, CHJ, or CeH&l, to form the corresponding organometallic halide (or, as is necessary for the Group I elements, a mixture of metal alkyl and metal halide). Thosemarked with an open diamond, 0,react with the same hydrocarbon halides, but customarily in a solvent (which may range from a hydrocarbon to a strongly basic ether). Those marked with black squares react directly with carbon monoxide to form carbonyls (t), and those marked with open squares are elements known to react with a hydrocarba to form an organometallic product [magnesium dicyclopentadienide (3) from cyclopentadiene and magnesium, and ferrocene (4) from cyclopentadiene and iron catalyst powder]. The periodic table is seen to be far from completely marked, with only 31 of the 83 pertinent elements taking part in direct syntheses of the type under discussion. If we ask why so few metals participate, the answers seem to be of three kinds: (a) scarcity of some of the metals, such as Fr, Po, etc., has hindered investigation; (b) the distinctive type of bonding involved in transitionmetal organometallic compounds favors carhonyls, perfluorocarbon derivatives (6),and T-cyclopentadiene compounds, but excludes ordinary methyl and phenyl derivatives, so that we cannot make direct comparisons; or (c) there simply has not been enough research on the matter. If we concentrate on the non-transition or so-called "representative" metals, we find that 23 of the 31
*,
+
0 O
elements are known to participate in direct syntheses of organometallic derivative^.^ This seems a large enough sample, and so the remainder of this paper will be devoted to the reactions of elements marked with diamond shapes and open squares on Figure l--especially since the author can draw upon personal experience in discussing them. Again, there is no fundamental need to exclude transition elements from the discussion, but only a necessity to broaden the discussion to include other types of product. We hope that someone will do this some day, but at present the reader's attention is directed just to reactions of methyl halides with representative metals and metalloids. Alkali Metals
Of the alkali metals, lithium is by far the most versatile and the most popular in the formation and application of its organometallic compounds. As Coates points out (6),the singular combination of high renctiviiy, easy prcp~ration,and solubility in variok I~ylroc,~rt~on solreutv nl>lkrslithiuni nlkyls and nrylu of special value in synthesis. The compounds are prepared from dispersions or granules of lithium in pentane or benzene, usually a t reflux temperature (but not always). Notice, for example, the preparation of phenyllithium in ether under argon (7) a t surprisingly low temperature, indicating a very high order of reactivity on the part of lithium metal: ether
C6H6C1+ Li wire -------t 65% yield of CsH6Li 5 hrs -10'to -2n0
ether
CsHPl f Li dispersion A 95% yield of CsHsLi -15'to
-30°
Perhaps the most extreme case is the direct synthesis of perfluoro-n-propyllithium from lithium containing 2% of sodium a t -74" in ether, but not in pentane (8):
However, most preparations from lithium take place ¶The score becomes 24/32 if phosphorus is admitted to the category of metalloids.
REACT WlTH RX REACT WlTH RX IN A SOLVENT REACT WlTH CO REACT WlTH HYDROCARBON
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a t room temperature or slightly above: CxH.Br
+ 2Li
n-CaHn 25'
CnH.Li
+ LiBr
Sodium and potassium dispersions react with chlorobenzene a t 35' in a hydrocarbon solvent, and the alloy of 10% sodium and 90% potassium with vinyl chloride in tetrahydrofuran at 0". Group II
At the top of Group 11,beryllium requires a distinctly higher temperature for reaction, even with methyl iodide: Be
+ CHJ
100'
seeled tube
CHaBeI
Calcium, strontium, and barium react with methyl iodide at or near room temperature to form the normal alkyl (CH&Ca, etc., by simultaneous disproportion* tion:
thallium alloy (the counterpart of the tetraethyllead synthesis), and methyl iodide reacts with finely divided thallium to give some C2H5T1(1I). Group IV
I n Group IVB, the reactions of elementary silicon with a great many hydrocarbon halides (plus a few alcohols and ethers) have been summarized and documented. Petrov et al. (12) list reagents and products for 73 such direct syntheses, and Zuckerman also describes a great many (IS). We shall consider only the reactions of the methyl halides with silicon a t this time, and especially the reactions of methyl chloride, because the aim is to compare the conditions of reaction with those of comparable reactions of Ge, Sn, As, Sb, Bi, Li, Al, Mg, and the rest of the 23 elements pointed out above. For this purpose we may write (14, 16): 2CHtCI
+ Si
Cu 285°-3000
(CH&3iCll
+ 41 other p r o d ~ ~ c t s
and
M may be calcium, strontium or barium. The reaction of magnesium with alkyl and aryl halides in ether to produce the familiar Grignard reagents is so well known that it need not be considered here, except possibly to point out that dry magnesium requires a temperature of 100" or more to react unless a basic ether is present to add its energy of solvation to the process and to stabilize the product.
2CHaBr
Cu + Si + (CH.)*SiBr. etc. 325-
and also 2CsHEI
+ Si
CU ------4
3O0'J5O0
(C2Hs)23iChetc.
and
+
Ag
~CGHBCI Si A (CsHs)~SiClz etc Group Ill
Boron is diffcult to get in a pure crystalline form, and because of its strange structure of tightly-bound icosahedra it is singularly unreactive. Powdered amorphous boron (prepared by reduction of Bz03) is far more reactive to oxygen than the pure crystalline material, and yet it has frustrated all attempts to bring about a direct synthesis of organohoron chlorides. At about 600" it begins to react with methyl chloride, but the only product which can be isolated is boron trichloride (9). There is a merest trace of substance with boroncarbon IR absorption, and nothing more. It must be conceded that elementary boron lies just outside of the practical limit for direct synthesis, which is a great pity. Aluminum reacts with methyl iodide without a solvent a t 90" to 120" if the reaction is started with a little trimethylaluminum or some iodine or aluminum chloride (I0) :
4W0-430"
The role of the copper or silver catalyst has been the subject of a great deal of controversy which we need not go into here, but all will agree that without the catalyst a much higher reaction temperature is required to get anything at all, and that under no conditions can a direct synthesis be run satisfactorily with silicon without the metallic catalyst. I t is interesting that the same catalysts are not capable of facilitating a comparable direct synthesis from boron, but are decidedly beneficial in the reactions of Ge, Sn, As, and Sh with methyl chloride. Hence we may write 2CHaC1
cu + Ge ----r (CHs)nGeC19+ smaller amount of other 320' products (16)
and
On the other hand, the established procedure for making tetraethyllead makes use of a sodium-lead alloy reacting with ethyl chloride without any copper:
When an alloy of 70% aluminum and 30% magnesium was used with methyl chloride, external heating still was necessary and the reaction proceeded best at 90" to 120". Gallium and indium have not been tried very much; nothing is available about their use in direct syntheses. Low yields of triethylthallium result from the action of ethyl chloride on sodium60
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The finely-divided lead which is liberated from this reaction is "active lead," and is capable of reaction with methyl bromide or iodide (but not methyl chloride) a t 100" to 130" (18). Since the reactivities of selected halides are given as CHJ > CIHJ > CHaBr > CHaCl > C2H5Br> C2H5Cl,the temperature of reaction for CH3C1 would probably be 130-160" for satisfactory rates of reaction.
Group V
Turning to Group V, red phosphorus reacts with methyl chloride with the help of copper powder as catalyst a t the same temperature as arsenic and antimony do, although the same phosphorus reacts with CFJ in a hydrocarbon solvent at 250'. Perhaps it would be best to cite just the temperatures given for satisfactory rapid reaction: CHaCl
Cu + red P -350' +
CH8Br
(SO)
-
+
(SO)
Cu
CH&Br.
CH3SbBr% (CH&SbBr
350"
Even the two Group VI elements which have metalloidal allotropes, selenium and tellurium, react with alkyl halides to form organometallic derivatives, though in a specialized and rather peculiar way ($1): Se
Ha0 + RI + NaOH + CHs0.NaHSOa +
R%Se(88% yield of (CHs)&)
Te
+ ZRX
-
Distance (A)
Reaction with CHCI
A1-A1 SiSi
3.04 3.20 2.86 2.35
25' in solvent 35' in solvent 120° 300'
As-As Sb-Sb B-B
2.49 2.90 1.59
Bond Li-Li
me% sn-sn
a xi
---
21.60
350' 372' Above 500'
(19)
+ (CH&AsBr
Cu
+
+ (CHdlPCI
-
+ As 250-280'
C H I B ~ Sb
CHsPClr
Table 3
.-
NaOH ---r &Te
H,O In their way, these reactions are combinations of the element with alkyl halides in a solvent, and are so indicated on the figure. Reaction Conditions
At this point we run out of metals and metalloids, so the listing must stop. The next step is to consider the conditions of reaction, and to try to find some relationship between the required conditions and some thermodynamic quantities or some recognized features of structure. Any attempt to apply thermodynamic data must immediately meet the objection that we are not dealing with systems in equilibrium, or even approaching it. That is true; the temperatures which appear on the equations are temperatures for satisfactory rate of reaction, and are not even threshold temperatures. Nevertheless, there is a strong tendency
in the conduct of all direct syntheses to keep the temperature as low as possible, principally because fewer organic radicals are destroyed while being transferred and there is less decomposition of product. Furthermore, if we select one alkyl halide, such as methyl chloride, we might expect that in its reactions with all metals it would display similar behavior. I n other words, some comparisons might be made between systems as closely similar as possible. We can only use what data we have, and these should not be condemned before even t & i g a look a t them. The concept of a free metal like lithium or aluminum being a bundle of available chemical energy gives us a clue, and so we might start by comparing temperatures of reaction of methyl chloride with the metals versus heats of formation of the metal chlorides (Table 2). No correlation is found here! Nor is the situation any more promising if free energies of formation of the chlorides are substituted for enthalpies. Obviously it is not enough to consider only what is being formed, or part of what is being formed; one also must consider the process of taking apart the crystal lattice of the metal. The interatomic distances in the metals might possibly be pertinent in this respect (Table 3). Here we find a t least a general trend: Those metals which have widely-spaced atoms react with methyl chloride a t the lowest temperatures, and the most tightly bound ones react at the highest temperatures. Seeking to change over from a static dimension to an energy function, we might take the heats of fusion of the metals, but fusion usually leaves us with dense liquid rather than isolated atoms. A better measure of the work required to take apart the metal lattice would be the heat of formation of gaseous atoms from one gramatomic-weight of metal in its standard state ($29) (Table 4). Here there is a steady trend, except for the two nonconformists called tin and arsenic. It might be tempting a t this point to dismiss tin and arsenic and claim some fundamental relationship, but actually any such conclusion would have to be tempered by two more considerations. The first of these is that any measurement of temperature "at which satisfactory reaction occurs" is really a kinetic measurement, and hence will be influenced markedly by the presence or Table 4
Table 2
Metal
Temperature of reaction with CHaCl
AHf of chloride
Li Mg A1 Si Sn As Sb
25" in solvent 35' in solvent 120° 300' 315" 350' 372'
9 7 . 4 kcal 153 167 149 127 71.4 88.3
Metal
Li Mg A1 Si Sn A.3
R
AH gaseous atoms
Reaction with CH&l
38 36 75 105 72 60
25' in solvent 35' in solvent 120" 300" 315" 350' A hnva iinn"
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absence of any catalysts. It is clear that direct syntheses using very pure metals and unassisted alkyl or aryl halides are notoriously sluggish, and that some form of activation or catalysis is almost universally required. Activation by free halogen is a favorite way, and works nicely with magnesium, aluminum, and even silicon. Solvation of the product is another way of assisting the reaction, as hss been pointed out. However, neither of these methods is suitable for silicon, germanium, tin, amenic, antimony, or bismuth. For these, copper catalysis is universally applicable. So, if we are to make any comparisons between the metals listed in Tables 3 and 4, we must set aside Li, Mg, and Al as being solvent and/or halogen assisted, and take Si, Ge, Sn, As, and Sb as representative of the coppercatalyzed lot. Having done so, we ask whether Sn and As are anomalous, or whether any explanation is available for their behavior. Up to this point we have considered only the difficulty of taking apart the solid metal or metalloid. There should be some energy credit for what is being formed, as well. Taking the accepted bond formation energies from Cottrell (B), we arrive a t some net values in Table 5.
energy boost if solvation of the organometallic compound takes place simultaneously, still further lowering the temperature of reaction. The more basic the solvent, the greater the heat of solvation and the lower the minimum reaction temperature. 4. Those metals which are tightly bound and difficult to vaporize, such as Si, Ge, and Sn, respond to Cu catalysis but still require a much higher reaction temperature (in the range 280-360') with the same alkyl halides. Even though Si is more tightly bound in its diamond latt,ice than the isomorphous Ge or the metallic form of tin, its energy of formation of Si-C1 and Si-C bonds is higher, so that the net energy of reaction to form R,MCI, is much the same for all three elements. 5. The copper-catalyzed reactions of As and Sb fall into the general trend established by Si, Ge, and Sn. 6. Boron is so tightly bound in its elementary form that it is just beyond reaction with methyl halides in a direct synthesis. Some form of complexing would provide the necessary extra driving force. 7. These rules may be useful as guides for bringing still unused metals into reaction with hydrocarbon halides to form organometallic derivatives. Literature Cited
Table 5
Element
(A) AH atoms
Si
105 89 72 60 61
Ge
Sn As Sh
(1) FRANKLAND, E., J. Chem. Soc., 2,263 (1848-49). (2) STONE, F. G. A., unpublished Lecture notes for NSF program in inorganic chemistry at Reed College, July, 1962. W. A., Inw'g. Syn., 6, 11 (1960). (3) BARBER, (4) MILLER,S. A., TEBBOTH, J. A,, AND TREMAINE, J. F., J. Chem. Soc., 632 (1952). P. M., AND STONE,F. G. A,, "Fluorocarbon (5) TREICHEL, Derivatives of Metals," in "Advances in OrganometaUic Chemistry," F. G . A. STONE AND ROBERT WEST, editor^, Academic Press., Inc.., New York. 1964. Vol. 1. o. 143. (6) COATES,G. E., "Organo-Metallic Compounds," 2nd ed., John Wiley &Sons, Ine., New York, 1960, p. 3. (7) EsMnu, D. L., "Metal-Organic Compounds," No. 23 of Advane- in Chemistry Series, ACS, Washington, 1959, p. 46. P.M., AND STONE, F. G. A., o p . c i t . , ~146. . (8) TREICHEL, E., AND ROCHOW, E. G., unpublished mformation. (9) KRAHE, (10) MARSEL,C. J., ET AL., "Metal-Organic Compounds," No. 23 of Advances in Chemistry Series, ACS, Washington,
Tempers, (C) Net (B) M-C energy ture of M-CL energy energy kcal/mole reaction 91 81 76 70 74
72 51 54 55 51
221a 17P 188. 135* 13ab
300' 320' 315" 350' 372'
..
+
;net energy = (2B 2C) - A net energy = (2B + C) - A
The point that becomes apparent here is that although elementary silicon is more tightly bonded than germanium or tin, it releases more energy forming Si-CI and Si-CHa bonds and so compensates for the energy of atomization. I n fact, it gives off more energy than either Ge or Sn, and so need not be heated as hot by external means. Arsenic and antimony (calculated for the formation of CH,MCI,) evolve still less net energy, and so require still more external heating-all despite their less tightly bound elementary forms.
1090 n 172. r. - - - - 2
(11) COATES, G. E., op. eit., p. 156. A. D., ET AL.,"Synthesis of Orgsnosilicon Mono(12) PETROV, mers," Consultants Bureau, New Yark, 1964. J. J.. 9 d ~ Inorq. . Chem. Radioehem.,. 6,. 383 1131 . . Z~CKERMAX. (1964). (14) Rocnow, E. G., J. Am. Chem. Soc., 67,963(1945); U.S. Patents 2,380,995 and 2,447,873. W. F., J. Am. Chem.Soc.,67, (15) Rocnow, E. G., AND GILLIAM, 1772 (1945). (16) Rocnow, E. G., J. Am. Chem. Soc., 69, 1729 (1947); 72, 198 (1950); U. S. Patents 2,444,276 and 2,451,871. (17) Rocnow, E. G . , U. S. Patent 2,679,506; SMITE,A. C., AND Rocnow, E. G., J . Am. Chem. Soc., 75, 4103, 4105 (1953). (18) SHAPIRO,H., "Metal-Organic Compounds," No. 23 of Advances in Chemistry Series, ACS, Washington, 1959, p. 290. L., Angew. Chem., 71, No. 18,574,575 (1959). (19) MAIER, W. C., J . (20) MAIER,L., Rocnow, E. G., AND FERNELIUS, Inorg. Nucl. Chem., 16, 213 (1961). (21) Rocnow, E. G., HURD,D. T., AND LEWIS,R. N., "The Chemistry of Organometdlic Compounds, John Wiley & Sons, Inc., 1957, p. 225-9. T. L., "Strengths of Chemical Bonds," 2nd ed., (22) COTTRELL, Butterworths, London, 1958.
Conclusions
Using these data with caution, but with due regard for the logical expectations about what is taking place in the direct synthesis of an organometallic compound, we come to these conclusions: 1. The direct synthesis of organometallic compounds from their respective metals is a reluctant reaction which needs activation or catalysis. 2. Those metals in which the atoms are widely spaced and which are easily vaporized, such as Li, Na, and AIg, are capable of reaction with alkyl halides at a temperature of 150" or less, especially if there is preliminary or simultaneous activation by free halogen. 3. The metals described in (1) will receive a further
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