2509
A127 N.M.R.STUDIESOF TRIETHYLALUMINUM COMPLEXES
Aluminum-27 Nuclear Magnetic Resonance Studies of Triethylalumirium Complexes
by Harold E. Swift, Charles P. Poole, Jr., and John F. Itzel, Jr. Gulf Research and Development Co., Pittsburgh, Pennsylvania
~~
~
(Received M a r c h BO, 1064)
~~
Aluminum-27 nuclear magnetic resonance (n.m.r.) studies have been made on the complexes formed between triethylaluminum and several nitrogen, oxygen, and sulfur donor molecules. The complexes exhibited chemical shifts and greater line widths relative to uncomplexed triethylaluminurn. The chemical shift was found to depend on the atoni bonded to the aluminum and decreased in the order: sulfur > oxygen > nitrogen. The line width of the A127n.m.r. was found, in general, to broaden as the size of the donor molecule increased. This is attributed to size and possibly to steric effects on the symmetry of the molecular electric field gradient at the aluminum. The ability of a donor molecule to replace another donor molecule complexed to triethylaluminuni was also determined by A127n.m.r. The data obtained were used to arrange several donor molecules in the order of complex strength, taking into account the effects of both donor species and steric effects. The equilibrium constants for complex formation between triethylaluminum and both anisole and pyridine were estimated by A127 n.m.r. to be considerably greater than one.
Introduction Triethylaluminurrt (TEA), a Lewis acid, readily forms complexes with donor molecules such as pyridine, anisole, and thiophene. Several of these complexes have been studied potentiometrically and conductometrically1 and the heats of formation of some of them have been determined calorimetrically. In recent years it has been relported that the addition of these donor molecules to Ziegler polymerization type cata l y s t ~ can ~ , ~have pronounced effects on the rate of polymerization and on the nature of the polymer that is formed.6-11 &o, the complexing of triethylaluminum and other aluminum alkyls has been extensively studied and is still an active area of research.I2 In order to obtain more information about the nature of the complexes formed between triethylaluininuni and various donor molecudes, A127n.m.r. studies have been carried out. This technique was previously used to study the Ziegler-type catalysts formed by the reaction of aluminum alkyls with various titanium compounds, l 3 and also to study other trialkylaluminum compounds. l4
Experimental Nuclear resonances of AlZ7were observed and chemical shifts were measured with a Varian Associates V(1) E. Bonita, Chem. Ber., 88, 742 (1955). (2) G. E. Coates, “Organo-Metallic Compounds,” John Wiley and Sons, Inc., New York, N. Y., 1956, p. 137. (3) K. Ziegler, British Patent 713,081. (4) K. Ziegler, et al., Angew. Chem., 67, 541 (1955). (5) G. Geiseler and W. Knothe, Chem. B e r . , 91, 2446 (1958). (6) G. Natta, J . Polymer Sci., 48, 219 (1960). (7) R. L. McConnell, H. W. Coover, Jr., and F. B. Joyner, paper presented at the 145th Nntional Meeting of the American Chemical Society, Xew York, N . Y., September, 1963. (8) F. B. Joyner and H. W. Coover, Jr., paper presented a t the 145th National Meeting of the American Chemical Society, New York, N. Y., September, 1963. (9) A. Zambelli, J. Dipietro, and G. Gatti, J . Polymer S e i . , AI, 403 (1963). (10) E. B. Milouskoya and P. I. Dolyopolskoya, Vysokomolelcul. Soedin., 4, 1049 (1962). (11) 0. F. Solonian, P. Glineski, and E. Mikhaileskv, Dokl. Alcad. N a u k SSSR, 152, 117 (1963). (12) K. Ziegler, British Patent 936,774. (13) E. N. DiCarlo and H. E. Swift, J . Phys. Chem., 68, 551 (1964).
Volume 6 8 , Number 0
September, 1964
2510
4200-A n.m.r. spectrometer, Varian 25-cm. electromagnet, and V-2100 power supply. A BC-221-D Bendix Radio Corp. frequency meter was used in the calibration of the V-4280 helipot used for scanning the magnetic field. Line widths and shifts were measured a t 7.2 A4c.p.s. and a magnetic field strength near 6490 gauss. The first derivative of the resonance absorption was recorded using audio modulation (40 c.P.s.) of the static field, and the peak to peak modulation amplitude employed was about 0.3 gauss. The absorption mode resonances were recorded a t a radiofrequency field strength of approximately 100 mgauss. The chemical shifts were measured by recording a spectrum of trimethylaluminum before and after the complex and determining the shift in the crossover point of the resonances. All of the n.m.r. measurements were made approximately 15-30 min. after the samples were prepared. Research grade hexane, obtained from the Phillips Petroleum Co., was distilled over sodium and then stored over Linde 5-A Molecular Sieves in a nitrogen atmosphere. Triethylaluminum was obtained from Texas Alkyls lnc. and was used as received without further purification. All of the other materials were vacuum distilled and stored over Linde 5-A molecular sieves in a nitrogen atmosphere with the exception of diethyl ether. Anhydrous diethyl ether was obtained from the Mallinckrodt Chemical Co. and was used as received. Thiophene, pyridine, triethylamine, quinoline, and diethyl sulfide were obtained from the Eastman Kodak Co. Tetrahydrofuran was obtained from Matheson Coleman and Bell. Anisole was obtained from the Fisher Scientific Co. Methyl phenyl sulfide was obtained from the Columbia Organic Chemicals Co. Pyrrolidine and pentamethylene sulfide were obtained from the Aldrich Chemical Co. Pyrex tubes, 15 em. in length and 15 mm. o.d., held the samples in the radiofrequency probe. Each tube was equipped with a side arm and a stopcock for the admission of nitrogen. The tubes were baked in an oven at 130" for 24 hr. and then were cooled in a nitrogen atmosphere prior to use. Hexane was subsequently addedand the tubes were sealed with rubber syringe caps. The triethylalumirium and other compounds were added by means of a hypodermic syringe and needle. In all cases the triethylaluminum was added first, followed by the donor compound with which it complexed.
H. E. SWIFT,C.. P. POOLE, JR., AND J. F. ITZEL, JR.
donor molecules in hexane solution. The generalized reaction can be represented by the equation &(C2H6)6 -I- 2D 2DAl(CzH6)3 D = donor molecule
(I)
Triethylaluminum exists mainly as a dimer a t room temperature and the addition of donor molecules such as amines, ethers, etc., supposedly dissociates this dimer and fornis the addition complex.2 The AlZ7n.m.r. data showed that significant changes in the chemical shift and the line width of the AlZ7 absorption occurred on the addition of various donor molecules to triethylaluminum (see Table I). The values reported in Table J for the chemical shifts were obtained by the addition of an excess of donor to triethylaluminum. It was found that the addition of donor molecules in excess of a mole to mole ratio of donor molecule to triethylaluminum had very little effect on the line width (see Fig. 1 and 2). It was interesting to note that at rooiii temperature only one resonance was observed regardless of the mole ratio of donor molecule to triethylaluminum (see Fig. 3). All of the spectra were Lorentzian in shape, indicating that only one resonance was being observed. The calculated Lorentzian points are shown by X in Fig. 3. These results suggest the operation of an exchange reaction between complexed and uncomplexed triethylaluminum. At a low mole ratio of donor molecule to triethylaluminum in the absence of exchange one expects to observe two resonances, that is, uncomplexed triethylaluminuni and complexed triethylaluminum.
0.5
I
1.5
2
MaMOLES PYRIDINE ADDED [Aid
MOLES AI ICzHd,
25
3
Results and Discussion
Figure 1. The line width of the triethylaluminum-pyridine complex as a function of the pyridine-triethylaluminum mole ratio. The circles represent the experimental points and the solid curves are the calculated widths for two values of the equilibrium constant k, l./mole.
The complexes studied were formed by the reaction of triethylaluminum with nitrogen, sulfur, and oxygen
for publication.
The Journal of Physical Chemistry
3.5
ADDED
(14) C. P. Poole, H. E. Swift, and J. F. Itzel, Jr., to be submitted
AlZ7K.M.R.STUDIES OF TRIETHYLALUMINUM COMPLEXES
2511
Table I : Chemical Shifts and Line Widths of Complexes Formed with Triethylaluminum"
p.p.m.
Size or steric effect
+I4 -9 -20 -11 N-8 -22 --20 N-20 --48 --48 - 65 (+a
Small Small Small Large Small Small Large Large Large Large Small
Chemical
System
TEA TEA TEA TEA TEA TEA TEA TEA TEA TEA TEA TEA
shift?
Complexing atom
+ triethylamine* + pyrrolidine + pyridine* + quinoline* + diethyl etber* + tetrahydrofuran + anisole* + methyl phenyl sulfide* + pentamethylene sulfide + diethyl sulfide + thiophene*
N N N N 0 0 0
S S S S
- AHf,
Line width,c
kcal./ moled
gauss
0.87 1.0
...
1.1 1.3 ~ 2 . 3 1.05 1.25 -1.9 -2.0 -1.9 1.7 0.95
19.4 20.1 11.2 14.0 2.3
...
... 0.0
+
a Concn. of samples: 1.82 X mole of TEA 3.64 X mole of donor molecule in 4 cc. or 3.1 X mole of hexane. The systems marked with * are arranged in the order of decreasing complex strength as determined by n.m.r. Pyrrolidine and tetrahydrofuran are listed in the order of increasing line width among the nitrogen and oxygen donors, respectively, while pentamethylene sulfide and diethyl sulfide are somewhat arbitrarily placed in the middle of the sulfur groups. * Chemical shifts measured relative to a sample of pure trimethylalluminum. A positive sign indicates a shift to higher field. The chemical shifts are estimated to be accurate The heats of formation are from the 1iLeraThe line widths are estimated to be accurate t o within k0.05 gauss. to & I 0 p.p.m. ture.
0.5
I
1.5
LD., MOLES
I
I
I
2
25
3
[AI71 MOLES AI(CIHI~,
ADDED
Figure 2. The line width of the triethylaluminum-anisole complex as a function of the anisole-triethylaluniinum mole ratio. The circles represent the experimental points and the solid curves are the calculated widths for two values of the equilibrium constant k , l./mole.
The requirement for the appearance of only one resonant line is that the exchange rate Vexoh exceed the line width AH expressed in frequency units y A H / 2 s where y is the gyroniagnetic ratio. For our case
>> 3 kc.p.s.
yexoh
I.o
35
A N I S O L E ADDED
(2)
The ratio of the amount of complexed to uncoiiiplexed triethylaluniinum in accordance with eq. 1 niay be ob-. tained from the equilibrium constant k of the reaction.
I
,
0
1
I
i
2 3 GAUSS
,
4
Figure 3. A127 n.m.r. spectra of the triethylaluminumpyridine complex. The top spectrum is of triethylaluminum in hexane a,nd the others are the spectra of 1 : 3 and 1 : 1 mole ratios of pyridine to triethylaluminum.
The triethylaluiiiinuni exists aliilost entirely in the dimeric state and so the concentration of the monomer is too low to influence the n.m.r. spectra.I5 The observed line width AH should be related to the line width A H A ,of the triethylaluiiiinuiii alone
AHA, = 0.87 gauss ~~
(4) ~~
~
(15) K. S. Pitzer and H. S. Gutowsky, J . Am. Chem. SOC.,64, 316 (1942).
vo'olume 68, Number 9 September, 1964
H. E. SWIET, C. 1’. POOLE, JR.,A N D J .
2512
and the line width of the compex AHDAIby the equation‘$
+
AH = PAIAHAI PDAIAHDA~ ( 5 ) where AHDAI= 1.9 and 1.47 for anisole and pyridine, rcspectively, and PAl and PDAI are the mole fractions of aluminum in triethylaluminum and the complex, respectively. The quantities PAI and PDAI are both unknown; and thus cq. 3 was rewritten in the form IC =
____
[DAl(Cz&)31
{ [ A ~ T] [DAl(C:H,)3IJ {
TI - IDAl(CzHd,I 1 (6)
where [ A h ] and [DT] are the total concentrations of TEA (monomer) and donor added to the solution. Equation 6 was solved for [DA1(Et)3]over the range
(7) for k = 25 l./mole and k = m . The results were used to evaluate AH over this range with the aid of the relations [ DAl( CzHd 3 I PA*= 1 - __--____ (8) [AlT1 and
(9) The two theoretical curves of AH us. [DT]/[A~T] are compared with the experimental data in Fig. 1 and 2. There is sonie scatter in the data, but it is believed that this coniparison indicates k >> 1 l./mole, with the equilibrium highly in favor of the formation of the complex. It was found that the line width AHA\ of TEA in hexane is only slightly dependent on the concentration of TEA.’* I t is believed that the principal line broading mechanism of these cornplexes arises from the large, noncubic molecular electric field gradients around the AlZ7 nucleus.” The line width data in Table I indicate that the line width increases with the size of the donor niolecule. This effect is particularly striking if one considers the following two pairs of molecules. C~H~-O-C~HS
C H s - O a
Diethyl ether
Anisole
AH = 1.07gauss
hH=1.89 gauss
Q Pyridine
AFI =1.32 gauss
Quinoline
AH = 2.25 gauss
IC.
ITZEL,JR.
The large increase in line width is a t least partly a result of the increase in size since the rotational correlation time is inversely proportional to the cube of the effective radius of the I” I n addition it is possible that the large groups exhibit a strong steric effect by bending the ethyl groups of the triethylaluriiiriurii away from the donor, with the results that the elcctric field symmetry around the aluniinuni is lowered, the axial component of the electric field gradient increases, and the line broadens. The relative strengths of several of the complexes listed in Table I wcre determined by adding equimolar mixtures of two donors to triethylalurriiriuin in hexane and determining the resulting line widths, as showri in Table 11. In every case the line width from the mixture was intermediate between the widths with each donor alone. All of the line widths except the last wcrc close to the width of one of the two donors which indicates that most of the triethylaluminuni is complexed with one of the two donors, and only a small per cent is coniplexed with the other. The methyl phenyl sulfidethiophene mixture produced an intermediate line width which suggests that their complexing strength is almost equal. Using an equation of the forni ( 5 ) ; the percentage of the first donor in Table I1 that corn! plexed with triethylaluniinum was calculated; and the results are listed in the last column of the table. ,411 of the values except the last are about 86%, and cach one
Table I1 : Line Widths Obtained with Triethylaluminum and the Equimolar Addition of Two Donors“ Ime width,
Mixture
+ + + + + + +
gauss
Pyridine (1.3) quinoline (2.3) anisole (1.9) Triethylamine (1.0) diethyl ether (1.05) Quinoline (2.3) thiophene (0.95) Quinoline (2.3) anisole (1.9) Diethyl ether (1.05) methyl phenyl Diethyl ether (1.05) sulfide (2.0) Anisole (1.9) thiophene (0.95) Methyl phenyl sulfide (2.0) thiophene (0.95)
+
1 4
% of fimt donor
90 89
1 1 -2 1 -2 1 1 2 1 2
-86 -85 83 84
8 1 6
-90 62
-1
‘ T h e width of each donor complex alone is indicated in parentheses.
(16) J. A. Pople, W. G . Schneider, and H. J . Bemutein, “High Resolution Nuclear Magnetic Resonance,” McGruw-Hill Book C o . , Inc., New York. X. Y.. 1959; see Chapter 10. (17) D. E. O’Reilly, J. Chem. Phys.. 32, 1007 (1980). (18) A. Abragam, “The Principles of Nuclear Mngnetisrn,” The Clurendon Press, Oxford, 1961; see pp. 300, 314.
A127 N.M.R.STUDIESOF TRIETHYLALUMINUM COMPLEXES
obtained from a mixture with a narrow line is subject to an error of about lo%>. The data for the mixtures with broad lines are accurately enough known to deduce that complexing with the “large” donors is dominant. The data in Table I1 were used to arrange the donors in the order of decreasing strength as shown in Table I. The complexes marked by an asterisk in column I of Table I were ordered by n.m.r. The positions of the other nitrogen and oxygen donors were estimated from their line widths. The data indicate that the donor atom (nitrogen, oxygen, or sulfur) plays the dominant role in determining tlhe coniplexing strength, while the size effect plays a lesser role. The coniplexing strength of the two sulfur-containing donors appears to be influenced very little b,y the size effect. The n.1~i.r.data indicate that nitrogen donor molecules replace oxygen donor molecules, and oxygen donor molecules replace sulfur donor molecules. These results are in agreement with the findings of Davidson and Brown,lg who reported that the coordination strength, of a series of donor molecules, with trimethylaluniinuni decreases in the order trimethylamine > dimethyl ether > dimethyl sulfide. The heats of formation shown in the last coluinn of Table I correlate well with the donor strength determined by n.ii1.r. It was interesting to note that the only donor niolecules that formed colored complexes with triethylaluminum were the nitrogen-containing aromatic molecules, such as pyridine and quinoline. These complexes were yellow, whereaij the complexes formed between triethylaluniinuni and the other donor molecules studied in this work were colorless. The latter may have exhibited blue shifts which remained in the ultraviolet region. The chemical shifts shown in Table I tend to increase
2513
as the complex strength decreases. Two exceptions to this rule are thiophene and pyrrolidine.
Conclusions The triethylaluiiiinuni-donor complexes exhibited characteristic chemical shifts and line widths. The chemical shift was found to depend on the atom that bonded to the aluminum in the complex, and it decreased in the order sulfur > oxygen > nitrogen. The line width was found to increase with the size of the donor molecule. This may be due to either an increase in the rotational correlation time or to steric effects on the molecular electric field gradient at the aluniinum. The equilibrium constants for complex formation between triethylaluminuni and both anisole and pyridine were estimated to be considerably greater than one. The n.1ii.r. data on mixtures of donor molecules with triethylaluminuni enabled the donors to be arranged in the order of decreasing strength for forming complexes. It was found that the nitrogen-containing donors formed the strongest coniplexes followed by oxygen and then by sulfur. This order agreed with the heat of formation data from the literature* and the results reported by Davidson and Brown.lg Also, it was found that the nature of the donor atom appears to play a more important role in complex formation than size effects. There appears to be a rapid exchange reaction between coinplexed and noncomplexed triethylaluininum a t rooni temperature. Studies of this apparent exchange reaction are now in progress using low-temperature techniques. (19) N. Davidson and H . C . Brown, J . Am. Chem. SOC.,64, 316
(1942).
Volume 68, Number 9 September, 1061,
