Titration Reactions of Alkylaluminum Compounds Using Phenazine Indicator George W. Heunisch Research and Development Department, Continental Oil Company, Ponca City, Okla. 74601
The titration of alkylaluminum compounds with pyridine using phenazine indicator follows a complex although stoichiometric course. Triethylaluminum forms a brown colored, 1:l coordination complex with the indicator while diethylaluminum hydride produces a green colored semiquinone free radical. Titration with an appropriate Lewis base disrupts the triethylaluminum complex to indicate the first end point and provide a measure of the trialkyl compound concentration. The radical species, stable only in acid solution, disproportionates in basic solution to yield colorless products. At that change, all of the acidic species have been titrated. Triethoxyaluminum is an extremely weak acid and does not interfere.
PHENAZINE is used for an analytical end point indicator in the titration of mixtures of diethylaluminum hydride and triethylaluminum using pyridine titrant, and according to the original report ( I ) , simple complexation is involved in the reactions of both diethylaluminum hydride and triethylaluminum with the phenazine. A free radical, however, has subsequently been identified as the reaction product of diethylaluminum hydride with phenazine and suggests that reduction as well as complexation is involved in the titration. In the introductory work, the overall quantitative nature of the analytical method was the primary concern, and the microscopic stoichiometry and the mechanisms of the indicator and titrant reactions with the alkylaluminum compounds were not thoroughly studied. Those reactions comprise the purpose of this work. The color-forming reactions within the titration scheme are presently under investigation in order to more thoroughly define the quantitative nature of the titration. The titration, applicable to the quantitative determination of trialkylaluminum and dialkylaluminum hydride in a single sample aliquot, involves, briefly, addition of phenazine indicator to the mixture of alkyl compounds to produce a colored solution followed by titration of the colored solution with pyridine. The color of the solution depends upon the ratio of alkylaluminum compounds present. Trialkylaluminum produces a red-brown solution, while dialkylaluminum hydride produces a green colored solution. Titration of the colored mixture with an appropriate Lewis base such as pyridine eliminates the red-brown color to provide a measure of the trialkyl followed by degeneration of the green color to indicate complete titration of all of the acidic species. Other analysis schemes have been proposed for the determination of alkylaluminum compounds in complex mixtures, and reviews of those are available ( 2 , 3). The pyridine
( 1 ) D. E. Jordan, ANAL.CHEM., 40, 2150 (1968). (2) “Methods of Elernento-OrganicChemistry,” A. N. Nesrneyanov and K. A. Kocheshkov, Ed., World Publishing Co., New York, N.Y., 1961. (3) T. R. Crompton, “Analysis of Organoaluminurn and Organozinc Compounds,” Pergarnon Press, New York, N.Y., 1968.
titration possesses the distinct advantage of a relatively short analysis time. EXPERIMENTAL
Equipment and Reagents. Reported electron paramagnetic resonance (EPR) spectra were measured at ambient temperature, using a Jeolco Model JES-ME-1X Spectrometer. Spectra were measured at -10 “C with no enhancement in hyperfine resolution. No further temperature changes were studied. The g value (2.0033) for the phenazinyl was measured relative to potassium nitrosodisulfonate (g = 2.0057). The spin density ( p = 2.89 X lozospinslg) was measured by comparison with ar,ar’-diphenyl-P-picrylhydrazyl (DPPH, g = 2.0036, p = 1.53 x 102’ spins/g) and calculated using the integration method of Wyard (4). The Jeolco is a single cell instrument, and for that reason the samples and references were measured individually. The sensitivity of the cell cavity was normalized by use of the standard ruby line, and concentrations were adjusted so that the instrumental parameters could be held as constant as possible over all of the measurements. Nuclear magnetic resonance (NMR) spectra were measured also at ambient temperature, using a Varian Model HA-IOOD Spectrometer. Benzene-ds solvent and tetramethylsilane reference were used. An F& M Model 185 Carbon-Hydrogen-Nitrogen Analyzer was used to analyze isolated compounds for carbon, hydrogen, and nitrogen. Products containing ethylated aluminum components were not measured directly but were deliberately oxidized in dry air for at least 24 hours. Freshly distilled diethylaluminum hydride was used throughout this investigation. The distillate fraction rich in either diethylaluminum hydride or triethylaluminum, as determined both by the phenazine titration (1) and by the gas volumetric was used. The highly air sensitive material was technique (3, stored under an inert atmosphere in a bottle equipped with a rubber septum. Preparation of Phenazinyl (IV). A 2-ounce screw cap bottle equipped with a rubber septum was used for the reaction vessel, and a Teflon-coated magnetic stirring bar, 0.30 gram of phenazine, and 1 5 ml of dry xylene were added. With the rubber septum and cap in place, the solution was deaerated with nitrogen through syringe needles in the septum for at least 20 minutes. The nitrogen inlet was removed, and 0.3 ml of diethylaluminurn hydride was introduced into the phenazine solution zia a gastight syringe. The solution immediately became brown colored followed quickly by the characteristic green color of the phenazine radical. After 1-hour reaction time, the mixture was filtered under nitrogen through a medium sintered glass crucible. The green solid was washed with n-pentane and dried in a flow of nitrogen gas. For analytical purposes, portions of the solid were placed in tared screw cap vials and weighed. For EPR studies, portions of the solid were transferred directly to quartz EPR tubes, dissolved in xylene, and stoppered. In that way, contact with air was eliminated. (4) S. J. Wyard, J. Sci. Ziutrum., 42, 769 (1965). (5) F. Bonitz, C h n . Ber., 88, 742 (1955). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
741
Anal. Calcdfor C20H2sN2A12:Al, 15.3. Found: Al, 15.1. Calcd for the oxidation product (I) C32H36N4A1409: C, 52.8; H,5.0; N,7.7; Al,14.8. Found: C,52.9; H,5.1; N, 8.1; Al, 14.9.
I
I
0
I t7 AI203
I Preparation of Triethylaluminum-Phenazine Adduct. The triethylaluminum-phenazine complex was prepared in the
same manner as was the phenazinyl, and purple, needlelike crystals were obtained. Anal. Calcd for C18H23N2A1: AI, 9.2. Found: Al, 9.3. Calcd for the oxidation product (11) G4HI6O3N4Al2:C, 62.3; H,?.5; N, 12.1. Found: C,61.8; H,4.2; N, 11.9.
o:D +
'/2AI,OS
n NMR spectrum: A2B2 pattern, 7.24 and 8.17 ppm. NMR spectrum after titration with pyridine: A2B2pattern, 7.36 and 8.15; multiplet, 6.8, 7.15, 8.5; triplet, 1.31; quartet, 0.26 PPm. Aluminum Determination. Accurately weighed samples (ea. 0.1 gram) were hydrolyzed in covered, 250-ml beakers by dropwise addition of water, followed by decomposition with 5 ml of sulfuric acid. The organic carbon residue was removed by heating the acid mixture to fumes of sulfuric acid and adding concentrated nitric acid dropwise until the solution cleared. The nitrate ion was removed by fuming the sulfuric acid for several minutes. Upon cooling, the sample was diluted to approximately 30 ml, heated to dissolve the solid, and titrated (6) with cyclohexanediaminetetraacetic acid disodium salt (Hexaver) using dithizone indicator. A semiquantitative aluminum determination was made by atomic absorption spectrometry to ensure that no impurity ions were titrated. In all cases, the comparisons showed that only aluminum was titrated. Hydrolysis of Phenazinyl. Hydrolysis of the solid was performed in a 1-ounce screw cap bottle fitted with a rubber septum. Approximately 0.05 gram of solid was dissolved in 2 ml of benzene-d6 and 10% sodium hydroxide solution was added carefully and slowly while venting the liberated gases. The mixture was stirred, and upon separation of the layers, an aliquot of the benzene was measured by NMR. Filtration of the organic gave a dark blue solid: mp 215-216 "C [Lit. (7) mp for phenazhydrin: 216-217 "C]. Anal. Calcd for CzaHI8N4: C, 79.5; H, 5.0; N, 15.5. Found: C,79.4; H,4.7; N , 15.9. NMR spectrum,jiltrate: A2B2pattern, 7.25 and 8.19 ppm; 5.70 and 6.41 ppm. Hydrolysis of Soluble Alkylaluminum-Phenazine Adducts. Reaction mixtures containing 0.05 gram of phenazine and (6) E. Wanninen and R. Ringbom, Anal. Chem. Acta, 12, 308 (1955). (7) C. Dufraisse, A. Etienne, and E. Toromanoff, C. R. Acad. Sci., 232, 2379 (1951). 742
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
0.08 gram of the alkylaluminum in 1.5 ml of benzene-de were allowed to react with stirring for 1 hour at room temperature. At that time, pure (99 %) pyridine was added dropwise until the solution was colorless, and 2 ml of 10% sodium hydroxide was carefully and slowly added to the solution while venting the liberated gases. Upon separation of the layers, an aliquot was taken for measurement of NMR spectra. The NMR tube was flushed with nitrogen to avoid oxidation of any air sensitive compounds. NMR spectra: Triethylaluminum adduct: A2B2 pattern, 7.36 and 8.19; weak A2B2pattern, 6.18 and 6.40; multiplets, 6.8, 7.1, 8.5 ppm. Diethylaluminum hydride adduct: A2B2 pattern, 7.25 and 8.19; A2B2pattern, 5.70 and 6.41 ; multiplet, 6.8,7.2,8.5 ppm. Preparation of 5,lO-Dihydrophenazine. Dihydrophenazine was prepared by reduction of phenazine in benzene-diethylether solution with lithium aluminum hydride as described by Bohlmann (8). Anal. Calcd for C12H10N2: C, 79.1; H, 5.5; N, 15.4. Found: C,78.8; H, 5.6; N, 15.6. Quantitative Nature of the Titration. The titration procedure described by Jordan ( I ) was used to determine if any loss could be detected by reaction of the alkylaluminum compounds with the phenazine indicator. A mixture of 0.848 mmole of diethylaluminum hydride was allowed to react with stirring for 1 hour. The total activity was measured by titration with pyridine to be 0.846 mmole. The triethylaluminum end point was obscured by the extremely dark solution and could not be determined. The reaction with triethylaluminum was evaluated using a solution estimated to contain 29.2 triethylaluminum and 94.2 total activity. After 1 hour, the solution was titrated to yield 9 3 , 4 x total activity. Only a slight loss of triethylaluminum is indicated.
x
x
RESULTS AND DISCUSSION
Indicator Reactions. DIETHYLALUMINUM HYDRIDE. The radical nature of the phenazine cation is not new knowledge and has been suggested for many years, stemming from the electrochemical work of Michaelis (9). In recent years, EPR has confirmed the radical character of the phenazine cation (IO, II), which in acid solution produces a rather simple spectrum of seven principal hyperfine lines. The species generally attributed to the radical is (111). I
HI
+
H
m The first derivative EPR spectrum given in Figure l a was obtained by measurement of the product solution resulting from the reaction of phenazine with diethylaluminum hydride. Each line of the basic five-line pattern is split further by additional hyperfine components. (8) F. Bohlmann, Chem. Ber., 85, 390 (1952). (9) L. Michaelis and E. S . Hill, J. Amer. Chem. SOC.,55, 1481 (1933). (10) K. H. Hausser, A. Haebich, and V. Franzen, 2.Naturforsch., 16a, 836 (1961). (11) Y . Matsurraga and C. A. McDowell, Proc. Chem. SOC. London, 1960, 175.
considering two nitrogen atoms (nuclear spin = l), eight equivalent hydrogen atoms (nuclear spin = 1/2), and a single, dissimilar, ninth hydrogen atom with splitting constants of 5.30, 0.75, and 2.25 gauss, respectively, and line widths of 0.65 gauss. The spectra are nearly identical, suggesting that the measured spectrum is generated by the same components as calculated in the computer simulation. The seven-line spectrum generally obtained can be measured from methanolic hydrochloric acid solutions treated either with ultraviolet light as suggested by Dufraisse et ai. (12) or with sodium borohydride. The absence of more complex splitting-Le., by hydrogen atoms not directly attached to the nitrogen atoms-immediately suggests assignment of the hydrogen splitting to interactions with hydrogens associated with the aluminurn moieties of the complex. The hydrogen atoms around the ring appear t o have little effect on the free
+ /L !-\Il la
m;n AI (C2H512 H
ic
\
0
or
Ip One aluminum atom is covalently bonded to one nitrogen, while the other is coordinated t o the second nitrogen. The character of the A1-N bonds alternates as the free electron oscillates through the A1-N-N-AI structure. The A1-H bond character necessarily alternates a t the same time causing the hydrogen to effectively exchange. However, whether the exchange is intramolecular or intermolecular is not known. The broadening of the lines can be related to that exchange by the alternate line width broadening phenomenon (13,14). The splitting associated with eight equivalent hydrogen atoms corresponds to the methylene hydrogens, while the ninth dissimilar hydrogen corresponds t o that bound directly to the aluminum atoms and which effectively exchanges between the aluminum atoms. No aluminum interaction is observed, probably because the lines cannot be resolved. The existing measured lines would be split into eleven lines with line widths of 0.065 gauss. Compound IV can be isolated from solution and provides a convenient source of information. Relatively concentrated solutions of phenazine in dry xylene (ca. 2%), when reacted with excess diethylaluminum hydride, will precipitate compound IV. The deep green solid is filtered through sintered glass under a n inert atmosphere. Dissolving the solid in dry xylene produces a weakly green solution, the color of which fades within about one hour. A weak EPR spectrum is also produced that shows little fine structure. (12) C. Dufraisse, A. Etienne, and E. Toromanoff, C. R., Acad. Sci., 235, 759 (1952). (13) G. K. Fraenkel, J . Phys. Chem., 71, 139 (1967). (14) H. S. Gutowsky and C . H. Holm, J . Chem. Phys., 25, 1228 ( 1956).
Figure 1. EPR spectra: a. 5-diethylaluminum-lO-(diethylaluminum hydride) phenazinyl; b. computer-simulated spec-
trum
Considering (IV) and the equilibrium that must exist between the complexed diethylaluminum hydride and the phenazine nucleus suggests that addition of excess hydride will stabilize the radical in solution. Diethylaluminum hydride is a weak acid (15) and in solution probably dissociates from the phenazine molecule. Addition, then, of diethylaluminum hydride should affect the equilibrium by forming more of the associated product. The symmetry of the molecule is increased, adding to the stability of the radical. Upon solution of the isolated solid in dry xylene containing diethylaluminum hydride, a stable paramagnetic species is produced, yielding a n EPR spectrum identical to that in Figure la. The radical appears stable indefinitely if maintained under a n inert atmosphere. Because the phenazine radical has been shown to be unstable in basic solution (16), the weak Lewis acid nature of diethylaluminum hydride is a stabilizing feature of this radical species. Hydrolysis has been used to characterize radical (IV) which can be considered to proceed by either of two routes. The coordinated hydride molecule should be eliminated, followed by either a direct coupling to form a n N-N bonded dimer or by disproportionation t o yield a dialuminum compound, which will produce dihydrophenazine after hydrolysis, and (15) E. G. Hoffman and G. Schomburg, Z . Elektroclrem., 61, 1101 (1957). (16) R. C. Kay and H. I. Stonehill, J. Chern. SOC.Lorrdor?., 1952, 3240. ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
0
743
*
phenazine. The two pathways are illustrated below: H
I
NH ”-
The diethylaluminum hydride is titrated after both the excess triethylaluminum and the phenazine-complexed triethylaluminum indicating its relatively weak Lewis acid nature and is shown in Equations 3, 4,and 5 .
Y A I (C2H5)2 H
t
XI
Al(CIH&H
&
AI(CaHe)rH
(31
AI (CIHdn H
k It should be mentioned that while N-N dimers have been proposed, none have been confirmed (27). That structure, then, is considered unlikely. Hydrolysis of the paramagnetic solid yields dihydrophenazine and phenazine as well as phenazhydrin, a bimolecular compound of the two, and suggests the disproportionation route. By hydrolyzing the solid in benzene-d6 solution, phenazine and dihydrophenazine are detected by NMR. The N M R spectrum consists of two ABBPpatterns, and standard NMR spectra of phenazine and dihydrophenazine compare directly with that of the mixture. Filtration permits isolation of the phenazhydrin. Basic hydrolysis has been used in order to solubilize any liberated aluminum. TRIETHYLALUMINUM. While the reaction of diethylaluminum hydride involves both complexation and reduction, triethylaluminum forms a single coordination complex with phenazine. The phenazine-triethylaluminum product is not a radical as indicated by measurement of only a very weak EPR spectrum, and N M R spectra have been obtained on solutions of triethylaluminum and phenazine as well as the isolated reaction product and suggest the coordination complex (V). The measured EPR spectrum is considered to result from a slight (ca. 2 %) diethylaluminum hydride impurity.
py
b
I py~AI(CnHi)r
The point within the titration sequence at which the adduct in Step 4 is formed is not known, but that step is proposed to discount any loss of diethylaluminum hydride by its reaction with phenazine. Disposition of the radical (VI) in solution as given in Step 5 should proceed by either the disproportionation or coupling route mentioned earlier. N M R spectra of hydrolyzed solutions of the pyridine-titrated diethylaluminum hydridephenazine adduct show two AzB2 patterns corresponding to phenazine and dihydrophenazine just as was found upon hydrolysis of the solid phenazinyl. Standard N M R spectra of phenazine and dihydrophenazine compare directly with the mixture, and those products, again, suggest the disproportionation route. The disproportionation probably occurs through a phenazhydrin as suggested by (VII). C2H7
,C
2 H5
P Solution of the isolated material and measurement by N M R produces a spectrum of free phenazine, the dissociation product of the complex. Triethylaluminum is not observed because of its very high field (
