ETHERATE FORMATION IN ORGANOALUMINUM COMPOUNDS or important as a precursor to an active species, its observation illustrates the utility of Mossbauer spectroscopy for the study of catalyst systems. Acknowledgments. We wish to express our thanks to Professor D. A. Shirley for making this work possible and for helpful discussions and to Mrs. M. 0. Faltens, who obtained the Mossbauer spectra at the Lawrence Radiation Laboratory of the University of California
3567
a t Berkeley. The help of Mr. G. Bergstrom, who took the electron micrographs at Stanford’s Center for Materials Research, is also gratefully acknowledged. We are indebted to Dr. U. Gonser for critical discussions of this paper and to Mr. R. K. Mark for sharing with us his IBM 360 version of the variable metric minimization Mossbauer data-fitting program developed a t Argonne National Laboratories.
Etherate Formation in Organoaluminum Compounds.
Complex-Formation
Tendency in a Series of Trialkylaluminum, Dialkylaluminum Chloride, Alkylaluminum Dichloride, and Aluminum Chloride Aryl Etherates by G. H. Smith and F. J. Hamilton Contribution N o . 508 o j The Goodyear Tire and Rubber Company, Research Division, Akron, Ohio
(Received M a y 9, 1968)
Cryoscopic measurements in benzene a t 5” have been used to study etherate formation within the series R3Al, = Me, Et, n-Pr, n-Bu, and i-Bu). In Ph20, PhSMe, and PhOMe systems, the order of increasing complex formation is RsAlCl < R3A1 < RAlCl2 < A1C13. Some of the etherates of R,41C12 and all A1C13 etherates show an apparent intermolecular association. For a given complex type, increasing etherate dissociation occurs as the length of the alkyl group bound to aluminum increases. The phenomena are explained in terms of Lewis acid-base effects. Neither P h 2 0 nor PhOMe forms a complex with ( ~ - B U ) ~ A ~ H .
R2AICI,RA1Cl2,and AICh (R
Introduction Much is known of aliphatic etherates of organoaluminum compounds.1$2 However, little is known about aromatic ether complexes. Distillable anisolates of Et3A1, (n-Pr),Al, and ( ~ - B U ) ~are A~ but PhORk can be distilled out of a mixture of PhOMe with (n-C6Hla)3Al,3 I n the PhOA!Ie-(n-Pr)3AlzC13 (n-propylaluminum sesquichloride*) system, ether cleavage is promoted by n-PrAIClz; phenoxyaluminum compounds are formed.’ Anisolates and phenetolates of Et3A1 and EtzAIBr formed from Al-Mg alloy, ethyl halide, and ether are stable below 120’. Ether cleavage above 120” leads to phenoxyaluminum compound^.^ 27Alnmr studies of Et3Al-PhORIe showed no evidence for the formation of complexes having more than 1 mol of anisole/mol of Et3AL5 In order to obtain further information about the complex-forming tendency within the series of R3A1, R2AlC1, RA1C12,and AlCl, with aromatic ethers, we have carried out cryoscopic studies on typical systems in benzene at 5”.
Experimental Section Throughout this paper, organoaluminum compounds
will be written as monomers for the sake of clarity. Differentiation between monomer and dimer species will be made where necessary. Reagents. MeaAl of 99+ mol % purity is available commercially. Removal of Et2AlH from Et3A1is difficult, and a high-purity sample of Et3A1was used which had been synthesized from ethyl chloride via the “sesquichloride” and subsequent dehalogenation with ~ o d i u r n . ~(n-Pr)3Al containing 10 mol yo of (n-Pr)2(1) K. Ziegler in “Organometallic Chemistry,” H. Zeiss, Ed., Reinhold Publishing Corp., New York, N. Y., 1960, p 194. (2) R. Koester and P. Binger, Advan. Inorg. Chem. Radiochem., 7, 263 (1965). (3) G. Geisler and W, Knothe, Chem. Ber., 91, 2446 (1958). (4) V. M. Mardykin, P. N. Gaponik, and V. K. Saevich, Zh. Obshch. Khim., 36, 2162 (1966). (5) H. E. Swift, C. P. Poole, Jr., and J. F. Itzel, Jr., J. Phys. Chem., 6 8 , 2509 (1964). (6) E. Bonito, Chem. Ber., 88, 742 (1955). (7) S. Pasynkiewica, W. Dahlig, and K. Starowieyski, Rocz. Chem., 36, 1583 (1962). (8) F. C. Hall and A. W. Nash, J . Inst. Petro2. Technol., 23, 679 (1937). (9) Very generously supplied by Ethyl Corp.
Yolume 7.9, Number 10 October 1968
3568
G. H. SMITHAND F. J. HAMILTOX
A1H was easily purified by either small-batch simple fractional distillation or by molecular distillation. (i-Bu)aAl is very easily purified by fractional crystallization. R2AlCl and RAlCl2 were specially prepared high-purity samples from Texas Alkyls. Aluminum and chlorine analyses of all organoaluminum compounds used are given in Table I. Table I : Aluminum and Chlorine Analyses of Organoaluminum Compounds --%
of aluminum--
Compd
Theor
Found
MeaAl EtsAl (n-Pr ),A1 (n-Bu)BAl (i-Bu)SAl EtzAlC'l (n-Pi-)?AlCl (n-Bu)zAlCl (i-Bu)zAlCl EtAlCln (n-Pr )AlC12 (n-Bu)AlClz (i-Bu)AlClz (i-Bu)zAlH
37.4 23.6 17.3 13.6 13.6 22.4 18.2 15.3 15.3 21.3 19.1 17.4 17.4 19.0
37.1 23.6 17.3 13.75 13.6 22.1 18.2 15.5 15.0 21,l 19.4 17.4 17.4 19.0
--% Theor
of chlorine-Found
Yaqnilic Stlrrer L
29.4 23.9 20.1 20.1 55.9 50.3 45.7 45.7
29.7 24.0 20.4 19.8 55.4 50.7 45.6 45.7
The purities in mole per cent were determined from these and from hydrolysis gas compositions.10 All compounds were at least 95 mol % pure. A1C13 was 99.5% pure from Matheson Coleman and Bell. It was used without further purification. Benzene was Natheson Coleman and Bell Spectroquality purified over Na and silica gel and stored under nitrogen. PhOhIe and PhzO were from Fisher Scientific. Glpc analysis indicated a purity better than 99 mol yo. They were dried over silica gel and were stored under nitrogen. Et20 was Anhydrous grade from Allied Chemical Co., dried over silica gel and stored under nitrogen. Handling of organoaluminum compounds presents some very special problems because of their extreme reactivity with oxygen and moisture, We used hypodermic syringe techniques and Teflon-Ice1 F syringe stopcocks.11 All runs including weighings and transfers were done without any smoking of an aluminum compound. A p p a r a t u s . The accurate measurement of small temperature differences was accomplished using the Hewlett-Packard DY-2801-A quartz thermometer operating in the differential mode. The absolute accuracy is better than ~0.001°.12Although data using thermistors can be quite precise, the absolute accuracy is difficult to estimate because of occasional spurious changes in the internal resistance of thermistors. l 3 Cryoscope and Procedure. A schematic picture is given in Figure 1. It consisted of two essentially The Journal of Physical Chemistry
H I
L
Mapnelic stirrer I
Figure 1. Cryoscopic apparatus.
identical cryoscopes (C) each containing a probe (P) of the quartz thermometer. The cryoscopes were enclosed in evacuated flasks and kept in place by an 0 ring (0). The flasks were held in a shielded cooling bath (A) having 1 in. of foam rubber (F) on all sides, the top, and the bottom. The bath temperature was kept at 4.5 ==I 0.5". Each cryoscope contained a magnet (AI) and a few glass beads (G). These created enough nucleates to ensure no more than 0.1' supercooling. A typical procedure was as follows. Each cryoscope, containing the magnet and glass beads, was dried at 120" for 12 hr, then cooled under nitrogen. Serum caps (SI and S,) were attached. When cool, cap S1 was put in tightly and the nitrogen stream was diverted to a bubbler. The tubing (T) was pinched off and cut. The cryoscope was weighed and then almost filled with pure dry benzene by siphon action from a freshly filled vessel. The cryosope was reweighed. The serum cap (SI) was removed and the probe (P), which was threaded through a rubber stopper, was inserted against a flow of nitrogen. The flow was reduced and the cryoscopes were cooled to ca. 6'. Then they mere put into the flasks and secured with 0 rings. The cooling rate was ca. 0.02"/min. The difference between the temperature ( A T ) of the benzene in the two cryoscopes was approximately f0. l o . After a very small amount of supercooling (usually less than O . l o ) , the benzene froze, and when crystals of frozen benzene appeared in each cryoscope, the temperature difference rapidly became constant at 0.0000". This temperature remained constant for at least 30 min, as did the (10) "Aluminum Alkyls, Specifications, Physical Properties, and Analytical Procedures," Texas Alkyls Inc, Deer Park, Texas. (11) Available from the Hamilton Co., Whittier, Calif. (12) From linearity and short-term stability data from HewlettPaclcard, Palo Alto, Calif. (13) A. Beck, J . Sci. Instrum., 33, 16 (1956).
ETHERATE FORMATION IN ORGANOALUMINUM COMPOUNDS absolute temperature reading for either mass of benzene. Only when AT = 0.0000" was a run started. A weighed amount of the organoaluminum compound was now injected into the cryoscope containing the weighed amount of benzene, and both cryoscopes were warmed slightly until the last crystal of benzene had melted and the AT was adjusted to be close to that expected. It was desirable that the reference benzene (which had the longest constant temperature-time plateau) should freeze first. A good approximation to the final AT was obtained when the organoaluminum compound was added to the cryo~ope.'~After the solutions were frozen, a steady AT ( A T I )was observed which remained ( =tOo.OO31")for 5 min. Great care was taken to ensure that the over-all time between the reference-benzene freezing and the attainment of AT1 was less than 10 min. AT1 could be reproduced to &0.0002" (as determined on several occasions) and was recorded. Then an amount of an ether 1'1 Of ether/mo1 Of A1R3) was added and the melting and freezing process was repeated to give ATz. ATz was corrected for the depression due to excess ether. Finally, the corrected ATz was used in the calculation of the per cent dissociation according to eq 1. A value of 5.069 was used for the cryoscopic constant of benzene. This was determined using Spectroquality toluene in benzene. It agrees well with the literature value. l 6
3569
The total moles of all species present will thus be
P
=
p{ 1
+
XD
- (n/2)1]
and if this value is substituted into the expression for freezing point depression
T = -lOOOkP W where k is the cryoscopic constant, P is the number of moles of solute, and W is the weight of solvent, one obtains
AT =
100Okp
-( 1
W
+ ~ [-l ( n / 2 ) ] ) (1)
or x =
ATW - lOOOkp lOOOkp[l - (n/2)]
Values of n may be obtained from similar freezing point depression measurementon the trialkylaluminum itself, whereby n ~ ( -1 nI(R3Al) + f- p
a (Ro-412)
(2)
and
p
=
P P - (n/2)l
or
ATW
Results Cryoscopic measurements offer a particularly good method for investigating complex formation between ethers and alkylaluminum compounds. A sensitive temperature-sensing device is necessary for precise work, and, in the case of alkylaluminum compounds, so is a means of handling the pyrophoric compounds with the total exclusion of air and moisture. Cryoscopic measurements on trialkylaluminumether systems are complicated because of the two competing equilibria R3Al-ether 2R3A1
R3A1
+ ether
ReAlz
Consideration of both these equilibria is necessary for an accurate interpretation of cryoscopic results. If one considers an initial p mol of etherate, of which a fraction x dissociates to free trialkylaluminum, and further considers that a fraction n of the trialkylaluminum associates, the following summarizes the moles of each species present p ( l - x)(R3Al--ether) px(1 - n)(R3A1) pz(ether)
+
There are three cases to consider: (1) n = 0, i.e., for completely monomeric trialliylaluminum compounds, ~~
~-
Table I1 : Concentration Dependence of Monomer-Dimer Equilibrium in Tri-n-propylaluminum Mole fraction of (n-Pr)sBl ( X 109
1.3 1.5 3.0 3.3 4.8 5.5 6.3 7.2 9.4 11.5 14.2
na
0.52 0.59 0.71 0.78 0.80 0.83 0.83 0.83 0.88 0.90 0.91
% of monomer
65 58 44 37 33 29 29 29 22 19 17
n is the fraction of monomeric species associated to dimer.
-
(14) E.G.Hoffmann, A.nn. Chem., 629, 104 (1960). (15) B. C. Barton and C. A. Kraus, J . Amer. Chem. Soc., 73, 4561 (1961); K. Ziegler, W. R. Kroll, W. Larbig, and 0. W. Steudel, Ann. Chem., 629, 82 (1960)
Volume 78,Number i0 October 1968
G. H. SMITHA N D F. J. HAMILTON
3570
Table 111: Dissociation of Organoaluminum and Aluminum Chloride Anisolates, Thioanisolates, and Phenyl Etherates 7 -
Aluminum compd
NIesAl EtaAl EtzAlCl EtAlClz AIC13 (n-Pr)aAl (n-Pr)zAIC1 (n-Pr)AlCh AlCla (n-Bu)sAl (n-Bu)zAlCl (n-Bu)AlClz AlCla (i-Bu)BAl (i-Bu)zAlCl i-Bu AlClz AlCla
Etherate mole fractiona ( x 103)
8 10
7 10 5 7 6 7 5 3 4 6 5 6 4 6 5
----
Anisole-----
Thioanisole-------
Dissociation,
Etherate mole fractionQ
Dissociation,
70
( X 108)
%
2 2 -2 - 14 - 10 4 10 -6 - 10 6
Phenyl ether----
7---
Errorb
Etherate mole fractiona ( x103)
7 8 8 10
6 6 6 4
9
-3 - 10
10 17
-4
- 10
7 68 6 -.13
1 5
4 6 7 8 4 7 5 7 4 6 5 4 4
7
Dissociation,
%
Errorb
82 64 94
+lo, -5
1 5 +10
-2 -9
57 98 5 -9 55 100 14 -9 50 108 14 -9
+lo, -5
+10
f10, - 5 $10
+lo, -5
+10
a Concentrations are not equivalent because it was felt to be more important to minimize handling losses. However, variations in concentrations are not felt to materially affect the general results. The error is compounded from experimental error and from uncertainty in the value of n (unless shown it is 1 2 ) .
e . g . , (i-Pr)BAl; (2) n = 1, i e . , for completely dimeric trialkylaluminum compounds, e.g., 11e3Al; (3) 0 < n < 1, e.g., Et3A1, (n-Pr)BAl, and (n-Bu)BAl. In the latter case, n varies with concentration.16 Data on this effect are shown in Table 11. These data are in good agreement with those of earlier workers. 16,17 For trialkylaluminums other than (n-Pr),Al, the equilibria are different.14~16~18 The values of n were used to obtain19 values shown in Table I11 for (n-Pr)3Al complexes. Thus for strong anisolates, e.g., of (n-Pr),Al, the concentration of free monomeric trialkylaluminum is very small. Hence, n approaches 0. The phenyl etherate of (n-Pr)aAl, on the other hand, has a large concentration of free monomeric trialkylaluminum, and n approaches 0.8. The results of cryoscopic measurements on complexes between PhOlle, PhSMe, and PhzO and a series of aluminum compounds are given in Table 111. Negative values for the dissociations of the complexes, calculated from eq l (in the Discussion) are attributed to intermolecular dipolar interactions. 2o This results in an apparent association of the etherate molecules to species of higher molecular weight than that based on a 1: 1 complex. The aluminum compounds show, quite clearly, the following apparent order of complex-formation tendency with the aromatic ethers: A1C13 > RAlCI, > R3Al > R2A1C1. This effect is most noticeable with PhzO. RzAlCl compounds do not complex at all with PhzO, whereas R3A1 compounds form weak dissociated complexes. RAIClz and AlC& form strong phenyl etherates. I n the only PhSMe series studied, The Journal of Physical Chemistry
the same general trend is apparent, although the complexes are uniformly stronger than the corresponding phenyl etherates. I n the PhOl\le series the same general trend is apparent particularly in the isobutyl series. However, PhOMe forms much stronger complexes than PhSMe. The minimum in the complex-formation tendency of PhzO at RzAlCl prompted us to try to separate (nPr)3Al&13 into its components by adding 0.5 mol of PhzO to 1 mol of the sesquichloride. Table IV shows the results of this experiment. A separation into nPrA1Cl2-Ph2O and (n-Pr)zAIC1clearly occurred. There is an apparent decrease in the strength of the complexes as the length of the alkyl group bound to aluminum increases. This is more noticeable in the case of PhOlIe than in the case of Ph,O. Thus Et3A1PhOl\Ie (2% dissociation) is a stronger complex than (i-B~)~Al-PhOnle (lo%), and EtzAIC1-PhOMe (-2%) is stronger than (i-Bu),AlCl-PhOl\!Ie (17%). The behavior of (i-Bu)2A1Hwith PhzO, PhORle, and EtzO is shown in Table V. Our data confirm earlier (16) K. S. Pitaer and H. S. Gutowsky, J. Amer. Chem. Soc., 6 8 , 2204 (1946). (17) W. P. Neumann, Ph.D. Thesis, Max Planck Institute, Mulheim, Germany, 1959. (18) K. Ziegler, Proceedings of the International Conference on Coordination Chemistry, Special Publication No. 13, The Chemical Society, London, 1959. (19) An equilibrium constant calculated from n values shows some variance. This has been observed previously.le (20) H. Ulich, 2. Phys. Chem. (Leipaig), Bodenstein Festband, 423 (1931).
ETHERATE FORMATION I N ~
Table IV : Distillation Fractions from “Sesquichloride” Component Separation Using PhzO“
ml
Bp, ‘C (10-3 mm)
3 4 6 19
25-55 55-60 60-70 60-70
8
70-75
VOl,
Fraction
1
2 3 (upper) 3 (lower) 4
3571
ORGANOALUbfINUlt COMPOUNDS
State at -196O
% of aluminumb
% of chlorine
% of Phi0
Glass Glass Glass Crystalline Crystalline
18.1 18.7 18.3 10.0
27 6 29.2 26.9 22.0
14 23 12 54
9.3
21.2
64
Calcd for Pr2AlC1: Al, 18.2; C1, 23.9. Calcd for PrAlCLPh20: Al, 8.7; C1, 22.8; Ph20, 54.7. h After correction for excess PhzO. Q
Table V : Dissociation in Diisobutylaluminum Hydride-Ether Systems Mole fraction of etherate
%
Ether
( X 109
dissociation
Phenyl ether Anisole Ethyl ether
6 9 5
100 97 24
Max error
f 5
rt5 rt2
spectroscopic data21 in showing that there is no complex formation for the two aromatic ethers. Et20 does form a weak complex.
Discussion Monomer-Dimer Equilibria in Trialkylalunzinurns. The dependence of the calculated value for the dissociation of a trialkylaluminum etherate upon n (the fraction of monomer associated to dimer ReAl2) requires that the dependence of n upon concentration be known for a particular system. Our data in Table I1 parallel those of Pitzer and Gutowsky.16 We used eq 2 and neglected activity effects. Quite clearly (n-Pr)@A12should not be considered monomeric in benzene, except perhaps a t extremely low concentrations. It is not possible, from our data, to conclude that the undiluted compound is completely dimeric. The values of n used in calculating the etherate dissociations listed in Tables I11 and V are derived from Table I1 (for (n-Pr),Al systems) and from published data. 14,16 For the anisolate systems having very small dissociations, we consider any free (n-Pr)3Al, ( ~ - B U ) ~ and A ~ , (i-Bu)sAl to be monomeric and any free Me3A1 arid Et3A1 t o be dimeric. For the phenyl etherates, we consider the ( ~ - B U ) ~toA be ~ completely monomeric at the concentrations used and both Me3Al and Et3A1 to be completely dimeric. Complex-Forming Tendency within the Homologous Series of Trialkylalurninum Etherates. It has been generally assunned that within the homologous series
of aliphatic trialkylaluminum compounds increasing chain length leads to progressively lower complexing abilities.2 Indeed, the synthesis of trialkylaluminums directly in PhOMe provides good evidence for this p h e n ~ m e n o n . ~Increasing the alkyl group to hexyl gave such a weak anisolate that free PhOMe could be distilled from its admixture with (n-C+&&Al. Our cryoscopic data for trialkylaluminum anisolates in Table I11 substantiate this general trend. Increasing complex dissociation occurs as the length of the alkyl group increases beyond n-propyl. The monomerdimer equilibrium in the parent trialkylaluminum is known to be quite rapidJZ2 and free RBAl species must be competing with themselves or Lewis base molecules to form the R3Al dimer or R3Al-ether, respectively. The tendency to form anisolates is strongest with those trialkylaluminums which form the least dissociated dimers. This suggests that the Lewis acid strength of the monomeric R3A1is the determining factor in anisolate formation. The behavior of PhzO, on the other hand, is strikingly different. The phenyl etherate of Me3A1here shows the largest dissociation of the series. A steric factor, coupled with the strong tendency of Me3Al to dimerize, must be considered in the case of phenyl etherates (although it is probably small in the case of Rle3A1). The relatively large Ph2O molecule effectively blocks the approach of the R3A1 species, and, regardless of their Lewis acidity, a “leveling” effect is exerted on complex formation. Me3A1 is a stronger Lewis acid than the other trialkylaluminums, but the dimer-formation tendency is so strong that very little free etherate has a separate existence. PhzOis a weaker Lewis base than PhOMe, which factor also contributes to the etherate’s tendency toward high dissociation. The fact that (iBu)sAl can be assumed to be completely monomeric at these concentrations and yet it shows a large dissociation lends support to the above conclusions. Because commercial unpurified RaAl compounds are known to contain relatively high levels of RzAlH species, it was of interest to show, cryoscopically, whether aromatic ethers were capable of forming complexes with the hydride compounds. We find that the hydrogen-bridged trimeric ( ~ - B U ) ~ A1H (other RzAlHl compounds would be expected to behave similarly) does not form a complex with either PhOR/le or Phz021(Table V). Et20 forms a partially dissociated complex, in contrast to the distillable complexes formed from Et20 and R d . In this case the hydrogen bridge bonds are considerably stronger than those of methyl or higher alkyl groups, and the tendency toward association outweighs the tendency toward complex formation.22 (21) E, G. Hoffmann and G. Sohomberg, Z. Electrochem., 61, 1101
(1967). (22) K. C. Ramey, J. P. O’Brien, I. Hasegawa, and A. E. Borchert, J. Phys. Chem., 69, 3418 (1965).
Volume 72, Number 10 October 1968
G. H. SMITHAND F. J. HAMILTON
3572
Complex-Fomning Tendency within the Series R3Al, RzAlCl and RAlClz undergo rapid bridge and terminal RzAIC1,RAlCl,, and AlCls. It is apparent from Table group exchange.28 It is unlikely that free monomeric I11 that the tendency toward complex formation folspecies exist. Complex formation is then controlled by > RzAIC1. The simple Lewis acid-base strengths of the components. lows the order A1C13 > RA1ClZ> In the case of PhzO, the strongly acidic A1C13complexes effect is most marked in the case of phenyl etherates and completely and to such an extent that apparent interthioanisolates. Although not so obvious from Table 111, it is also quite real for anisolates. Thus it is posmolecular association occurs. With RA1C12 a strong sible to distil all the PhONe out of an equimolar mixLewis acid-base interaction still occurs; the strongest ture of PhOJIe with (n-Pr)zA1C1,23whereas an equicomplex is formed with the strongest Lewis acid EtAlCl, molar mixture of PhORIe and (n-Pr)3Al distils unand the weakest complex is formed by BuA1Cl2. With R2AlCl essentially no complex formation occurs wit)h ~ h a n g e d . It ~ ~is~also ~ possible to separate the comPh20 because it is too weak a base to complex with the ponents of (n-Pr)3Al&13 by adding half a molar equivaweakly acidic R2AlC1. R3Al compounds are not lent of P h 2 0 and distilling the resulting mixture (Table such strong Lewis acids as RzAICl but have more monoIV) . This minimum in the complex-forming tendency at meric character. Complex formation can occur quite readily. Steric factors may have some influence on RzAICl is unexpected from purely Lewis acid-base the system but are not considered to be the most imconsiderations, although it is not without precedent. portant. Thus certain alkali metal halide and hydride comThe same argument holds for PhSlIe and PhOMe, plexes of ethylaluminum compounds, e.g., Et3A1, but whereas the effect is still seen clearly with PhSMe EtzAIOEt, and EtA1(OEt)Zz4and some Et20 complexes complexes of n-propylaluminum compounds, the of (RCE=C)~A~,(RC=C)zAIR’, and (RC=C)A1R2’ greater Lewis basicity of PhOlIe tends to mask the efshow the same trendeZ5However, no plausible exfect, since uniformly stronger complexes are formed. planation has been offered for the phenomenon. With the restrictions imposed by cryoscopic techThe complex-forming tendency can be considered in niques, we have shown that there is a definite minimum two ways. The series of decreasing complex strength in the complex-formation tendency at RzAICl in the is either AlC13 > RAlClz > RzAICl with AIR3 being series R3A1, RzAlC1, RAlC12, and A1C13 with PhOMe, anomalous or AlC13 > RAIClz > AlR3 with AlRzCl being PhSJIe, and PhzO. Other systems will undoubtedly anomalous. We prefer the former for the following be shown to exhibit this phenomenon. An explanation reasons. The bridge bonds in RZAlCl, RAIC12, and in terms of Lewis acid-base theory seems in order, alAICls are formed by chlorinez6and are so strong that though the mechanisms by which complex formation AlC13shows evidence of an apparent association higher occurs are undoubtedly more complex. than a dimer.20 On the other hand, all trialkylaluminums have been shown to be, at least partially, disAcknowledgments. The gracious assistance of Resociated into monomeric species under the conditions search Personnel at Ethyl Corp. and of Dr. Scott Eidt used in our cryoscopic s t ~ d i e s . ~ ~ J ~ ~ ~ ~ ~ ~ ’ of Texas Alkyls is acknowledged. Our thanks are due I n order t o form a Lewis acid-base complex, these to Hewlett-Packard for the use of a quartz thermomebridge bonds must be broken either simultaneously or ter. Thanks are also due The Goodyear Tire and Rubsequentially, and there is then competition for bond refber Co. for permission to report the data contained in ormation both from the other monomeric unit, acting this article. as a Lewis base, and the ether molecule. Cryoscopic measurements are unable to distinguish these paths, of course. The equilibria, as far as cryoscopy is con(23) T. L. Hanlon, unpublished data. cerned, are (24) H. Lehmkuhl, Angew. Chem., 75, 1090 (1963). 2RzO-AlR3
2A1R3
.lr
AlzRa
The Journal of Physical Chemistry
+ 2Rz0
(25) G. Wilke and W. Schneider, B d l . SOC.Chem. Fr., 1462 (1963). (26) J. Schmidt, M. P. Groenwege, and H. de Vries, Rec. Trav. Chim., 81, 729 (1962). (27) XI, B. Smith, J . Phys. Chem., 71, 364 (1967). (28) J. Brandt and E. G. Hoffmann, Brennst.-Chem., 45, 201 (1964)
