COMPOUNDS' CHARLES A. KRAUS Brown University, Providence, Rhode Island
TYPICAL ORGANO-METALLIC COMPOUND-EDRMING ELEMENTS
The terms "organo-metallic" or "metallo-organic" are not properly descriptive of the compounds that are formed between hydrocarbon groups and atoms of metallic elements. In these compounds, the elements in question function in their nonmetallic rather than in their metallic capacity. There is only one series of organo-metallic compounds known whose members exhibit metallic properties; these are the monoalkyl mercury compounds (1). In compounds such as stannic or stannous chloride, for example, tin is acting in its metallic capacity, that is, it is functioning to supply electrons to chlorine to form something approximating an electropolar compound. In compounds such as tetraethyltin, the tin atom is joined to carbon atoms by covalent bonds. Tin, in tetraethyltin, functions in the same manner as does nitrogen in triethylamine, oxygen in diethylether, or chlorine in ethyl chloride. Before proceeding further, let us examine the elements that form organo-metallic compounds. It may be pointed out, here, that the hydrides of these elements are properly included among what we call organic-metallo compounds, for, in these compounds with hydrogen, the metallic elements are functioning in the same manner as in their compounds with hydrocarbon groups. The elements that form compounds with hydrocarbon groups are those of the first to the seventh groups (inclusive) of the periodic system that lack from seven to one .electrons in their outermost shell over that of the rare gas configuration. They include all the elements in these groups in the two periods of eight and the terminal elements of the long periods. While some of the transition elements, such as chromium, form comTABLE 1 Elements That Form Organo-Metallic Compounds
pounds with hydrocarbon groups, they are usually of low stability and often they are difficult to prepare, if, indeed, they can be prepared a t all. The elements wit.h which we are concerned are indicated in the following table where, to avoid confusion, we note down only the elements of the intermediate fourth group and the terminal elements of the first long period. These thirty-five elements are the most important eelments of the periodic system. Their compounds with carbon form the foundation upon which modern organic chemistry is based. The elements which we are now considering fall into three distinct groups: (1)elements of the fourth group; (2) elements of the fifth, sixth, and seventh groups; and (3) elements of the first, second, and third groups. The elements of the fourth group are characterized by their ability to form stable polyatomic negative ions whose salts are soluble in suitabIe solvents. Thus, lead forms the compound NaPbg (Z), which is readily soluble in liquid ammonia and is ionized into the ions Na+ and Ph-4. Elements of the 5th, 6th, and 7th groups also form polyatomic negative ions whose salts are soluble in liquid ammonia. But these elements also form another important class of compounds which are not formed by the elements of the fourth (or any other) group, namely, the onium compounds. Thus, we have the well-known ammonium, sulfonium, and iodonium salts. The onium groups are, in effect, synthetic metals, closely resembling the alkali metals in their electrochemical properties and exhibiting metallic properties in the free state when in solution in suitable solvents. The elements of the first, second, and third groups do not form soluble polyatomic negative ions. TYPICAL ORGANO-METALLIC COMPOUNDS AND THEIR PROPERTIES
Let us now consider the different types of compounds formed by elements of the fourth group. Typical examples are indicated in the following table, where 4 . represents a fourth group element; R, a univalent hydrocarbon group, or hydrogen; and X, an equivalent of an electronegative element. In the case of tin, examples are known of all types of compounds indicated in the table. For other elements of the fourth group, some of the types are missing. Let us now consider the properties of the different 'This paper was presented a t the symposium on organotypes of compounds illustrated in the above table. metallic compounds a t the meeting of the American Chemical Compounds such as R4A are among the stablest of the Sooiety, Chicago, Illinois, April 20, 1948. C
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JOURNAL OF CHEMICAL EDUCATION
TABLE 2 Types of Organa-Metallic Compounds of Fourth Group Elements I. &A &-.AX. 11. RxAM RAMS 111. (RaAh IV. (R.AL
Rz. + ,An
AX, (R2ALM2
known compounds, when R is an organic group; when R is hydrogen, only the derivatives of silicon and germanium are stable under ordinary conditions. However, mixed derivatives of alkyl or aryl groups and hydrogen are more stable, particularly in the case of compounds containing only one hydrogen atom attached to the central atom. Thus, (CH3hSnH is a stable compound (3). The stability of the carbon-metal atom bond increases greatly with decreasing atomic number of the central atom. Compounds of the type R A X are somewhat salt-like in character. While the uure com~oundsin the liquid state are indifferent electrical co&uctors, their sblutions in basic solvents, such as alcohols and arnines, are good conductors (4). These compounds resemble the corresponding triarylmethyl derivative in many respects, except that with the metallic elements there is little difference between aryl and alkyl derivatives. Compounds of the type R A X are relatively stable tovard hydrolysis or ammonolysis in the case of the elements of higher atomic number, but they usually anlmonolyze in the case of silicon and, in many instances, in the case of germanium. At times, ethylamine may be employed advantageously in place of ammonia in order to avoid ammonolysis. Phenyl derivatives appear to be somewhat more stable toward ammonolysis than are corresponding alkyl derivatives. As the number of halogens attached to the central atom increases, the tendency to ammonolyze increases. The dihalides of tin are stable but the corresponding derivatives of germaninm and silicon ammonolyze readily. SALTS OF ORGANO-METALLIC GROUPS
salts can be prepared otherwise, they may be employed in solution in an inert solvent such as ether. Salts of the type (%A).M2 have thus far been obtained and used only in the case of tin (8). Salts such as R3AM may readily be prepared by reduction of the corresponding halide R3AX with two equivalents of alkali metal in liquid ammonia, if the halide does not ammonolyze. This procedure is generally operative in the case of tin and, in'a more limited way, in the case of germanium. With silicon, this procedure is successful in veryexceptional cases only. More generally, these salts are prepared by reducing the dimeric group (R8Ge)2by means of alkali metals in ammonia or the amines. The dimeric groups (R3A)% may be prepared by reducing the corresponding halide, R3AX, with alkali metals in neutral solvents, such as xylene, or, in the case of silicon, by reducing the pure, molten halide. With germanium, compounds such as (RaGe)Z, may be reduced to alkali metal salts, R3GeM, by means of alkali metals in ammonia or ethylamine. I n the case of silicon, this procedure is inapplicable because of the stability of the Si-Si bond. Here, the difficulty may, a t times, be overcome by coupling the silicon group with a similar group of an element of higher atomic number. Thus, lithium triethylsilicide may be prepared by reducing the mixed compound EtaSi,GePha by means of lithium in ethylamine (12). The Si-Ge compound may be prepared by .reacting Ph3GeNa with Et3SiRr in benzene solution. The germamide may be prepared by reducing (Ph3Ge)z with sodium in liquid ammonia. At times, unusual and useful compounds are met with. When Ph3SiBr is reduced with lithium in &thylamine (IS), the following reaction occurs: Ph,SiBr
+ Li + CIHsNHl = Ph,Si.C2HJTH~+ LiBr
(1)
The complex of the triphenylsilicyl group with ethylamine is stable, volatilizing in a high vacuum a t 150°C. without decomposition and melts a t 45% It is reduced to Ph3SiLi by lithium in ethylamine and the,salt reacts with phenyl bromide to yield tetraphenylsilicon; with Me3SnCl in liquid ammonia, it yields Ph3Si.SnMea. Compounds of the type R A M are valuable reagents for coupling the group R3A with other groups. As an example, we may consider the reaction of Ph3GeNa with silicochloroform, HWC13. The reaction carried out in ether solution proceeds as follows (14):
The compounds of Type I1 are among the most interesting and useful in the whole field of metallo-organic chemistry; they are the reagents by means of which we are able to couple various groups with other groups or elements (5-7). Normally, these compounds are strong electrolytes (9-11). They are usually very soluble, not only in solvents of higher dielectric constant, such as liquid ammonia, but also in solvents of 3PhGeNa + HSiCI, = (Ph,Ge)aSiH + 3NaC1 (2) low dielectric constant, such as ether or benzene. At The product tri-triphenylgermanylsilicane is a stable times, solutions in solvents of the latter type are of great importance in the synthetic chemistry of organo- compound. The hydrogen may be replaced by halogens or reduced by alkali metals. Salts of higher type, metallic compounds. Comnonnds such as RxAM are stable in liauid am- such as U M e z or (R2A),M2, have, with rare excepmonia solution in the case of tin and usually inthe case tion, been obtained only in the case of tin (8). The of germanium. In the case of silicon, ammonolysis hydrolysis of the polyhalide derivatives makes it diffifrequently occurs, particularly with alkyl derivatives. cult to prepare the desired compounds in the case of In some instances, the cBiculty may be overcome by germanium and silicon. So, also, compounds of the-type (U), have been employing ethylamine as solvent. More often, if the
JANUARY, 1949
47
obtained only with tin (a),if we except a similar compound in the case of the cermaniurn hydrides (15). The compound &Sn, when-treated with-two equivalents of sodium in liquid ammonia, yields the ternary salt NaSnSNa; when treated with one equivalent, it yields the compound NaSnRa.SnRzNa, which is useful in synthesizing chains of tin atoms (8).
quantitative, proceeds in a very d i f f e ~ nfashion t (27). We have, for exam~le: NaGeH8
+ C&Br
= C6He
+ GeH, + NaBr
(9)
The affinity of the phenyl group for hydrogen is such as to take a hydrogen atom away from the GeH, group, leaving a GeHz group which may be looked upon as a germanoethylene. The remarkable fact about the SOME REACTIONS Of HYDRIDES GeHz group is its solubility in liquid ammonia. The We shall now proceed to a discussion of the reactions compound GeHz may be obtained as a white solid on of the hydrides of germanium and related compounds evaporation of ammonia but it has a very low stability; and reactions. Germanium tetrahydride is ~articu- it decomposes according to the equation: 3GeHz = GeH, ZGeH (10) lady suited for the investigation of hydrides; it is readily prepared and is stable in air. The monohydride is identical with that obtained on When monogermane is passed through a solution of treating NaGe with ammonium bromide, The dihysodium in liquid ammonia, reaction takes place quantidride reacts with one equivalent of sodium to form a tatively according to the equation (16): stable solution of reddish brown color.
+
Ge14
+ Na = NSG~HI+ '/zHz
(3)
, we may call sodium ~l~~compoun~N ~ G ~ Bwhich gemlanyl, is a truesalt ((17)and very soluble in ammonia. It crystallizes from solution with six molecules of alnmonia. Ammoniates containing 41/%and 2 molecules of ammonia are also known, as is, also, the anammoniate, NaGeH3. All are white, crystalline substances. The an-amoniate, NaGeHs, decomposes slowly a t -33% and much more rapidly a t higher ternperatures. Reaction occurs as follows:
It is an interesting fact that while compounds of the type RAM react quantitatively with the alkyl halides with coupling of the organjc group, i t is the exception that coupling occurs in the case of aryl halides. Usually, ammonia takes part in the reaction which, in a simple proceeds as RaAM
+ CsH5X + NHa = C6H6+ RIANHz+ MX (11)
Thus, when p-bromobenzylchloride is reacted with triphenylgermanide, there is obtained beneyltriphenylgermanium; the aromatic ring is hydrogenNaGeH, = NaGe S/lH1 (4) ated and an amine is formed (18). Having monoalkylgermanes, the hydrogens may be The brown solid of composition NaGe may be looked successively substituted by an alkali metal and the upon as a germano-acetylide. When treated with germanium coupled to alkyl groups. However, the ammonium bromide in liquid ammonia, we have the substitution of hydrogen by metals in the mono- and reaction : poly-alkylgermanes is, as a rule, not quantitative (15). NaGe N a B r = GeH + NaBr + NH3 (5) For example, in the reaction which might be expected to The resulting brownish compound, which is stable at proceed as follows: -33"C., loses hydrogen at higher temperatures accordCHzGeH8+ Na = NaGeH2.CHs + l/nHn (121 ing to the equation: the amount of hydrogen evolved is much greater than GeH = Ge '/,HZ (6) one equivalent per atom of metal and the amount of Potassium reacts with Ge& in a manner similar to germane required to complete the reaction is also that of sodium (17). The resulting compound, KGeH3, greater than required by this equation. There is is exceedingly soluble in liquid ammonia, 1 mole in 4.6 reason for believing that a side reaction occurs, somemoles of ammonia. It crystallizes from solution, what as follows: .ammonia-free, and its properties closely resemble those RGeHa Na NH8 = RGeH2(NH2) H2 Na (13) of the corresponding sodium salt. When a solution of sodium germanyl is electrolyzed The precise role of the sodium remains uncertain but in liquid ammonia with a mercury cathode, the anode we shall return to an analogous reaction of triethylsilicane in which all reactants are accounted for. reaction is as follows (17 ): The reduction of alkyl germanes proceeds somewhat 6GeHx- 2NHs = 6GeI4 Nn 6e(7) more efficiently with lithium in ethylamine than in The affiity of the GeH, group for hydrogen is suffi- ammonia. With monomethylgermane, two atoms of hydrogen are evolved for one atom of lithium; on addiciently high to reduce ammonia to nitrogen. The compounds NaGeHs and KGeH, are useful and tion of ammonium bromide, approximately 60 per cent convenient reagents for the purpose of coupling alkyl of the original germane is recovered (15). On treating monoethylgermane with lithium in ethylgroups to germanium. The reaction mine, ethane is produced as well as hydrogen (15). NaGeHs AlkX = AlkGeHa NaX (8) Dependmg somewhat upon conditions, the amount of hydrogen is less than one atom per atom of lithium and is quantitative (17). In the case of the aryl halides the reaction, although the hydrogen and ethane, together, are equi~ralentto
+
+
+
+
+
+
+ +
+
+
+ +
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JOURNAL OF CHEMICAL EDUCATION
the lithium used. The amount of ethylgermane reacted is very nearly equivalent to the lithium employed. Yields bf products of reaction of alkyl halides with the lithium salt show that some, if not all, of the ethane is derived from.ethylamine. In the reduction, about 3.5 atoms of hydrogen are evolved per mole of ethane. Isoamylgermane and ethylisoamylgermane appear to react with lithium in substantially quantitative proportion in ethylamine.
REACTIONS OF ORGANO-METALLIC SALTS WITH ORGANIC POLYHALIDES
The reaction of salts of the .type MAR8 with organic polyhalides is often a complex one, whether carried out in a neutral solvent or in liquid ammonia. In the latter case, ammonia frequently enters the reaction. The nature of the central element A, as well as that of the suhstituents, R, has a marked influence on the reactions (18 1 9 Thus, tri-triphenylstannylmethane is obtained when chloroform reacts with sodium triphenvltin (19). while the corresponding compounds are not obtained & the case of triphenylgermanium (22) or trimethyltin ($0). We have seen that in reacting phenylbromide with sodium germanyl, benzene and the unsaturated germane (GeH,) are obtained. A somewhat similar reaction occurs on treating chloroform with sodium trimethyltin in liquid ammonia (80). The end products accord with the over-all reaction:
due. When NaGePha reacts with aliphatic polyhalides, with the halogens on the same or adjacent carbon atoms, the dimer (GePh3), is obtained. When, however, NaGePHs reacts with a dihalide in which the halogen atoms are separated by two or more CH, groups, coupling takes place (18). As was noted above, the alkali metals do not, as a rule, react quantitatively with the partially alkylated germanium hydrides with substitution of a hydrogen atom. Seemingly, reaction also occurs according to the equation: R8GeH
+ Na + NH? = RaGeNH. + HZ+ Na
EtsSiH
+ Li + EtNH, = EtnSi.NHEt + H2 + Li
(18)
Reactions of this type are often met with in the case of silicon and germanium and they not infrequently accompany other reactions. It is an interesting fact that we are here dealing with a reaction which is catalyzed by electrons in solution. REACTIONS WITH ALKALI METAL AMIDES
When a hydride is treated with sodium or potassium amide in liquid ammonia, we should expect that the following reaction might occur: &AH
The products may be accounted for by the breaking down of ali initial tertiary substitution product according to the equation:
(17)
This type of reaction has been well established in the case of triethylsilane (12). When this componnd is treated with lithium in ethylarnine, reaction occurs quantitatively as follows:
+ KNHl = R,AK + NH,
(19)
A reaction of this type occurs in many instances but never quantitatively. It is accompanied by a second reaction which, in some instances, is quantitative. Thus, in the case of triethylsilane, we have the quantitative reaction (12) :
ZEhSiH + KNH? = (EbSi)lNK + ZH, (20) Methylene chloride reacts quantitatively with sodium trimethyltin and sodium triphenyltin to yield the ex- COUPLING REACTIONS pected disubstituted tin derivative (21). With sodium triphenyltin and chloroform, the trisubstituted product The reagents of the type of are for the case of germanium, twosimul- ling atoms and groups of different kinds. We have is obtained (19). taneous reactions occur in one of which ammonia is already mentioned the coupling of three triphenylgermanium groups with silicon. In the same way, it concerned (28). Thus, we have: has been found possible to couple three triphenylgerzPhsGeNa + CH~C1n = (PhrGe)nCHs + 2NaC1 manium groups with boron through the reaction (18): ZPh,GeNa
+ CH?Cln + NH1 = PhsGeCHa + PhaGeNH. + ZNaCl (16b)
SLiGePh,
+ BCI, = B(GePh3, + 3LiCl
(21) '
In a similar manner, compounds such as EtbSi,GePha In the second reaction, a carbon atom is hydrogenated by one atom of hydrogen from ammonia, the residual ( I S ) , PhaSi8nMe3(IS), Ph3Ge.GeEta (Zd), and GeaPhs NH, group reacting with a triphenylgermanium group (83), have been prepared. to yield an amine. With chloroform, a trisubstituted REACTION OF TERNARY ORGANO-METALLIC SALTS product is not obtained. Compounds of the type M,ARa are valuable as reWith aromatic halides in ammonia, or the amines, the aromatic group is invariably hydrogenated and the agents in building up chains of organo-metallic groups. amine, RaANH2,is formed (18). Because of the ease The sodium and potassium salts of the dialkyl and with which the latter compound is hydrolyzed, the diary1 tin groups are readily obtained. They have metallo-organic group is ordinarily recovered as oxide, been employed in building up chains of up to five tin When aromatic halides are reacted with NaGePha in atoms. Even in the case of tin, the Sn-Sn bond is a neutral solvent, the dimer (GePh& is obtained (18). sufficiently stable so that compounds of the type NaIt is not known what hecomes of the hydrocarbon resi- &Sn. SnRzNa and NaSn&.Sn&.Sn&Na may be pre-
JANUARY. 1949
49
pared (8). Without doubt, such coupling would prove even more stable in the case of germanium and silicon. The existence of the germanium salts Na(GePh8). and NaGePha ($3) has been established. Thus far, little use has been made of t,bese compounds, chiefly because of the difficulty of obtaining the RzGe group. In the foregoing discussion, we have considered only cases in which organic groups are coupled to metal atoms by means of a univalent bond. There is, at present, no convenient method for effecting a divalent coupling. In thin connection, it is of interest to consider the reaction of methylene chloride with disodium dimethyl tin (8). Thereaction takesplacein twosteps:
+ CHnC1z= NaSnMeaCHIISnMe2Na (NaSnMeJrCHZ
+ CHUln
-c
+ 2NaC1 (22)
[(SnMep)2(CHp)l]x+ 2NaC1 (23)
The compound as initially obtained is a liquid at, room temperature, which becomes very viscous a t low temperature. On standing, its viscosity increases and the material finally is converted to a resin, resembling rosin in many respects. Like rosin, the resin melts a t higher temperature, The lowest molecular weight was 1 0 0 h h i c h corresponds wprohdmately to six MeaSnCXL groups per molecule. CONCLUSION
It will be evident, from the foregoing discussion, that the chemistry of the organo-metallic compounds of the fourth group elements is quite varied. ~h~ type of reaction that occurs On the "Ivent medium, the nature of the ~nhstit~uent groups a.nd the central
element itself. On occasion, unexpected reactions occur involving carbon as well as hydrogen. As yet, our stock of knowledge with respect to the organometallic compounds and their reactions is too limited t o permit extensive generalization. It would seem that further work in this interesting and somewhat neglected field should be in order. LITERATURE CITED ( 1 ) KRAUS.C. A,, J . Am. Chem. Soc., 35,1732 (1913). ( 2 ) K ~ u s C. , A., ibid., 29, 1563 (1907); SMYTH,ibid., 39, 1299 (1917). (3) KRAUS,C. A., AND W. N. GREER,ilid., 44,2629 (1922). (4) K ~ C. A,, ~ ~C. C.~ cALLIS, , ibid.. 45, 2624 (1923); KRaUs, C. A,, and W. N. GREER, ibid., 45,2946 (1923). ( 5 ) KRAUS,C. A,, AND W. V. SESSIONS, ibid., 47,2361 (1925). ( 6 ) KRAUS,C. A., AND A. M. NEAL,ibid.. 51,2403 (1929). (7) KRAUS,C. A,, AND R. H. BULLARD, ibid.,48,2131 (1926). ( 8 ) KRAUS,C. A., AND W. N. GREER,ibid., 47,2568 (1925). ( 9 ) KRAUS.C. A,, AND W. H. KAHLER, ibid., 55,3537 (1933). ibid., 55,3542 (1933). (10) Kaaus, C. A,, m n W. C. JOHNSON, (11) K,, C. A,, AND p. B. B ~i&d.. ~ 55,3609 ~ , (1933). (12) K m u s , C. A,, AND W. K. NELSON,ibid., 56,195 (1934). (13) KRAWS, C. A.. AND H. EATOUGH, ibid.. 55,5008 (1933). (14) MILLIGAN. J. G., Thesis, ~ r o w nuniversity, M ~ Y1934. , S. N., Thesis, Brown University, May, 1933. (15) GLARUM, (16) KRAUS,C. A,, AND E. S. CARNEY,J . A*. them. 56, 765 (1934). (17) TEAU,G. K., Thesis, Brown University, May, 1931. . (18) SMITH,F . B., Thesis, Brown University, May. 1934. J . A m . Chem.Soc., 55,5014 (19) KRAUS,C. A,, ANDH.EATOUGH, (1933). (20) KRAUS,C. A., AND A. M. NEAL,ibid., 52,4426 (1930). (21) KRAUS,C. A., AND A. M. NEAL.ibid.. 52,695 (1930). ibid., 54, 1622 (1932). (22) KRAUS,C. A,, AND H. S. NUTTING. (23) K ~ a u sC. , A,, AND C. A. BROWN, ibid., 52,4031 (1930). (24) KRAUS,C. A,, AND C. SHERMAN, ibid.. 55,4694 (1933).
see..
