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Quinolinolato-complexes of aluminum, gallium and indium dialkyls
J. inorg,nucl.('hem.. 1973.Vol. 35, pp. 497-504. PergamonPress. Printedin Great Britain
QUINOLINOLATO-COMPLEXES GALLIUM AND INDIUM
OF ALUMINUM, DIALKYLS*
B. S E N t and G. L. WHITE~ Coates Chemical Laboratories, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 6 April 1972)
Abstract-- Monomeric complexes of the type R2MQ (R = Me, Et, iBu; M = AI, Ga, In; Q = quinoline-8-olate anion) have been isolated, and characterized by elemental analyses, and i.r., electronic, N M R and mass spectroscopy. Molecular weight of Et2GaQ was determined by vapor pressure osmometry. The compounds are generally yellow and crystalline. The reactivity of C - M bond seemed to be much reduced compared to the C - M bond in free metal alkyls. The chemical shifts in the N M R spectra could be rationalized on the assumption of delocalization of C - M bond electron density by the participation of s and d orbitals of the metal atom, which in turn reduced the C - M bond reactivity. Electronic spectra suggested ligand ~r ~ metal d (or s) transition. Metal-nonmetal vibrations a~ and t~ could be identified for iBu2MQ in the region 535-380 cm -1.
IN AN earlier communication[l] we reported the preparation and characterization of diethyl(quinolinolato)gallium; we now report the synthesis of a series of compounds of general formula R2MQ (R = Me, Et, i-Bu; M = Al, Ga, In; Q = anion quinoline-8-olate), and their characterization by elemental analyses, tool. wt determination, i.r., mass and N M R spectroscopy. Because of their novel nature, the detailed interpretation of N M R spectra of these compounds has been published elsewhere [2]. The shift of the PMR resonance peak due to the protons attached to carbon atom alpha to the metal atom (referred hereafter as alpha protons) follows the trend predicted from the study of the monomeric and dimeric complexes of these metals alkyls with piperidine [3]. The metal in these yellow colored, crystalline compounds is 4-coordinated, and obviously has essentially tetrahedral geometry. Quinoline-8-olate anion functions as a bidentate ligand via its pyridine nitrogen atom and the phenolic oxygen atom which is typical of this anion. EXPERIMENTAL All preparative and manipulative operations were carried out with the use of a dry box and/or a vacuum line. Most of the glassware was fabricated to permit evacuation and subsequent introduction of dry, oxygen-free nitrogen in order to prevent exposure to air. Materials. All solvent and reagents used in the reactions with metal alkyls were made as scru* Presented in part at the 160th ACS National Meeting, Chicago, September, 1970. tAuthor to whom all communications should be addressed. SAbstracted in part from a dissertation submitted by G. L. W. Present address: Louisiana Wildlife and Fisheries Commission, Baton Rouge, Louisiana. 1. B. Sen and G. L. White, lnorg, nucl. Chem. Lett. 7, 79 (1971). 2. B. Sen, G. L. White andJ. D. Wander, J. chem. Sot.. Dalton447 (1972). 3. B. Sen and G. L. White, To be published. 497
498
B. SEN and G. L. W H I T E
pulously free of oxygen and moisture as possible. The solvents were first dried over a series of drying agents, distilled under nitrogen, and were subsequently stored over calcium hydride and kept in the dry box. The aluminum alkyls were obtained from Alfa Inorganics and were used without further purification. Anhydrous gallium and indium trichloride were prepared by direct action of dry chlorine gas on the free metal as described by Eisch [4]. The gallium and indium alkyls were then prepared by the metathetical reaction of the appropriate aluminum alkyl with the metal trichloride according to Eisch [4]. A general description of synthesis of the complexes follows as all of them were prepared by the same procedure. General method of preparation. A weighed amount (calculated on the basis of a 1 : 1 adduct) of dried and degassed 8-quinolinol was dissolved in 20 ml of dry benzene, and the solution was then introduced into a pressure-equalizing dropping funnel. The metal alkyl was weighed and dissolved in 20 ml of dry pentane in the reaction flask. The dropping funnel containing the 8-quinolinol solution was then attached to the reaction flask. The 8-quinolinol solution was added slowly to the metal alkyl solution at such a rate that a gentle reflux of the solvent was maintained ( - 37°C). The crystalline product started to separate almost immediately. At the completion of the addition, the mother liquor was either decanted, or evaporated at reduced pressure, and the yellow crystals were washed several times with pentane. The product was finally dried under vacuum. A mixed pentane-benzene solvent system was preferred for the purification and isolation of the product because of the sparing solubility of the product in pentane. Slightly higher solubility of the product in benzene ensured greater purity of the crystals. This series of complexes appeared to be relatively stable towards oxygen and moisture. In a number of preparations the evolved alkane was trapped by freezing and was subsequently analyzed by mass spectroscopy.
M--R ~R
+ RH t + Heat
+
(R = Me, Et, iBu)
(M = AI, Ga, In)
Table 1. Elemental analyses of 8-quinolinolate complexes
Compound
Color
Me2AIQ
Pale yellow
Et~AIQ
Bright yellow
iBu2AIQ
Yellow
Et2GaQ
Yellow
iBuzGaQ
Greenish yellow
iBu2InQ
Yellowish green
Mol. wt
M.P.* (°C)
%C
%H
%N
% Metal
65-67 64.64 185§ 68.12 68.37 195§ 71.58 67.92 132 57-56 56.62 61 62.39 61-15 160 54.69 54.95
5.97 5.92 6.99 6-89 8.42 8.10 5.90 5.68 7-34 6.88 6-43 6.53
6-97 6.67 6.11 6.09 4.91 4.79 5-17 5.14 4.28 4.58 3.79 3.81
13.43It 13.36 11.79" 11.74 9.47 tl 9.70 25.4(/~ 24-46 21.1~ I 20.50 30.83" 29.84
-250
2727 277~
*Uncorrected. 1Theoretical. $Experimental. §Decomposed. ~%.ineof theoretical percentages. 4. J.J. Eisch, J.Am. chem. Soc. 84, 3605 (1962).
Quinolinolato-complexes
499
The results of elemental analyses and mol. wt determination by vapor pressure osmometry, and other physical properties are given in Table 1. Maintenance of a low reflux temperature was essential to avoid formation of the dimeric, and the polymeric species, and of other undesirable products. Using benzene as the solvent, Hurley et al. obtained dimeric products [5]. Infrared spectra. The i.r. spectra were obtained with a Beckman IR-7 and/or a Beckman IR-10 Table 2. Mass spectra of 8-quinolinolate complexes Compound
m/e
Intensity
Assignment
Me2AIQ
145 171 186 201 315 144 171 200 228 229 315 145 171 172 186 228 315 69, 71
3.0 16.0 100.0 2.0 3.0 0-9 77.0 100.0 0.9 0.9 5-5 10-3 26.5 91.0 62.0 100-0 3-5 40-5
8-hydroxyquinoline AIQ MeA1Q Me2AIQ AI(Q)2 Q
EtzAIQ
iBu2A1Q
Et2GaQ
145
iBuzGaQ
iBuzGaQ
iBu2InQ
213,215 242,244 270,272 271,273 69, 71 145 213,215 214,216 228,230 247,249 270,272 284, 286 327,329 115
0.1
21-2 100.0 1.6 2.9 38.0 4.0 11.0 II.0 2.5 3'5 100.0 0-4 0.6 100-0
145
5.3
229 259 260 316 317 372 373
1.8 34-0 5.9 13-5 2-3 1"2 0-4
A1Q EtAIQ Et2AIQ - H Et2AIQ AI(Q)2 8-hydroxyquinoline AIQ AIQ+H A1CHaQ
iBuAIQ AI(Q)2 Ga 8-hydroxyquinoline GaQ EtGaQ Et2GaQ - H EtzGaQ Ga 8-hydroxyquinoline GaQ GaQ+H GaCH3Q * iBuGaQ iBuGaCH2Q iBu2GaQ In 8-hydroxyquinoline iBu21n InQ InQ + H iBulnQ iBulnQ+H iBu2InQ - H iBu2InQ
* Denotes unassigned peak. 5. J.J. Hurly, M. A. Robinson, J. A. Scruggs and S. I. Tratz, Inorg. Chem. 6, 13 l0 (1967).
500
B. SEN and G. L. W H I T E
spectrophotometer. The spectra of either a hexachlorobutadiene mull or a saturated benzene solution of the quinolinolates were obtained. Mass spectra. Mass spectra were recorded at an ionizing potential of 70 eV with a Varian M-66 cycloidal mass spectrometer using a direct insertion probe. The masses, the corresponding intensities, and the possible assignments are tabulated in Table 2. N M R spectra. The majority of the N M R spectra were measured with the Varian HA-100 N M R spectrometer; some of the N M R spectra were obtained at 60 M H z with a Varian A-60-A instrument. Benzene, the solvent in most cases, was used for locking purposes. Tetramethylsilane was used as an internal standard. All of the spectra were obtained at the ambient temperature of the probe ( - 35°C). Due to the insufficient solubility of some of the 8-quinolinolate complexes, only the alkyl portion of the N M R spectra could be recorded for the least soluble complexes. The alkyl proton resonances could be recorded because each resonance peak in the alkyl portion of the spectra resulted from the contributions of several protons; however, in the quinolinolate portion of the spectra each resonance peak generally resulted from the contribution of a single proton and, therefore, a much greater concentration was needed to make the quinofinolate resonance peaks observable. The spectral assignment are given in Table 3 and 4. RESULTS AND DISCUSSION
Elemental analyses data were jedged acceptable except for a few deviations in the values for carbon. Melting point data did not show any rational trend; this idiosyncrasy seems to be general for the group IIIA metal alkyl derivatives, and it is not well understood. The melting points reported here (cf. Table 1) for the monomers are considerably lower than those reported by Hurley et al. [5] for the corresponding dimers; the only exception is iBuzAIQ. It is quite likely that iBu2AIQ might have undergone dimerization during melting. The experimental mol. wt of Et2GaQ was in excellent agreement with the theoretical value. However, the mol. wt determination attempted for some of the other compounds yielded anomalous values. Insufficient solubility, and enhanced reactivity of the Table 3. Chemical shifts of alkyl protons of RaM and RzMQ in benzene*
Compound Me2A1Q MesA1 Et~AIQ Et3AI
iBu2A1Q iBu3AI Et2GaQ EtaGa
iBu2GaQ iBu3Ga
iBuzlnQ iBualn
Chemical shift (8) CHa CH2 CH -0.95T~: -0.36 1.33T 1-09 0.75,0.85 0.98 1.22 1"16 1.07, 1.08 0.95 1.11 1.02
0.78-~ 0-29 0.35,0-47§ 0.30 0.83 0.54 0.83 0.80 1.22 0.80
1.73 1.94
2.07 2.10 2.23 2.22
*For multiplets 8 values correspond to centers of gravity. tMeasured at 60 MHz. SHexamethylphosphorictriamide was the solvent for this determination. §Determined by.4BX analysis.
Quinolinolato-complexes
501
Table 4. 100 M H z N M R spectral parameters for the 8quinolinolate protons of Re MQ
Compound
H-2
H-3
Chemical shifts* H-4 H-5 H-6
H-70-H
8-Quinolinol iBu2AIQ Et2GaQ iBuzGaQ
8.50 8-44 7.61 7.68 8-25
6.74 6.71 6-53 6.53 6.72
7-51 7.46 7-51 7.50 7-56
6.95 6-85 6.69 6.69 6-85
iBu2InQ
7.05 7-79 7.23 7.20 7.28
7-30T 7.20 7.33t 7.33t 7.50t
9.10
Apparent coupling constants~t Compound
J2,3 ,13,4 J 2 , 4 J 5 . 6 J 6 , 7 Js,7
8-Quinolinol iBueAIQ Et2GaQ iBu2GaQ iBu21nQ
4-0 5.0 4.5 4.5 4.5
8.0 8.0 8.0 8.0 8-0
1.5 1.5 1.5 1.5 1.5
§ 7.5 § § §
6.0 8.5 8.0 6.0 8.5
3.0 1.0 4.5 4.0 4.5
* In ppm (6). -tUnresoived multiplet H5 and He. eFirst-order values in Hz. §Not determined.
compounds in dilute solutions were possibly responsible for the anomalous molecular weights. It should also be mentioned in this connection that of all the compounds reported here, EtzGaQ seemed to be the most stable, and in fact Et2GaQ did not show any visible sign of decomposition on exposure to the laboratory atmosphere for several months. The heaviest ion in the mass spectrum of every compound, except R2AIQ, was RzMQ which strongly suggested that the compounds were monomeric. It is likely that traces of AIQ3 were formed along with RzAIQ as this was the most exothermic reaction. However, if these were formed, their concentration was so low that they did not influence the elemental analysis, and could be observed only in the mass spectra. In the mass spectra of RzGaQ, all fragments containing one gallium atom were observed as doublets in the ratio of 3 : 2 in consonance with the isotopic abundance of 6 9 G a ( 6 0 % ) and 71Ga(40%). The absence of triplets and multiplets in the spectra of R2GaQ ruled out any fragment containing more than one gallium atom. This was considered as a very strong evidence that the quinolinolates R2MQ reported here were all monomeric. Doublet structure was not observed in the spectrum of iBuzInQ at the intensity employed due to the low abundance of 11aln(4" 16%); it could be observed at a higher intensity. Oliver and Worrall [6] have reported the mass spectra of some Group III alkoxides. The fragmentation patterns of AI(OEt)z, Ga(OEt)3, and Ga(OiPr)3 were assigned, and mass peaks corresponding to dimers, tetramers, pentamers, and hexamers were observed. Chambers, Coates, Glockling and Weston[7] reported a molecular ion for (Et2A1OEt)z. Dimeric fragments were also observed 6. J. G. Oliver and I. J. Worrall, J. chem. Soc. (A) 234 (1970). 7. D. B. Chambers, G. E. Coates, F. Glockling and M. Weston, J. Chem. Soc. (A) 1712 (1970).
502
B. SEN and G. L. W H I T E
in the mass spectra of the dimeric piperidine complexes of the group IIIA metal alkyl[3]. All evidence indicated that the quinolinolates were more stable than any of the foregoing complexes. Consequently, if the quinolinolates were polymeric, the mass spectra would have indicated fragments larger than monomers. The absence of the 8-quinolinol ( O - H ) stretch frequency at 3415 cm -1 was one important aspect of the i.r. spectra. The quality of the far i.r. spectra in the region 700-200 cm -1 was rather poor; nevertheless, two bands definitely attributable to the complexes were observed in the case of iBu~AIQ, iBu2GaQ, and iBu2InQ. These bands were observed at 535 cm -1 and 420 cm -1 in the case of the aluminum complex, at 522 cm -~ and 415 cm -~ in the case of the gallium complex, and at 502 cm -1 and 380 cm -~ in the case of the indium complex. If these bands were due to the metal-nonmetal stretching frequencies, then the frequency shifts are definitely in the fight direction. It should also be recalled that the molecules of genuine Td symmetry should have two i.r. active bands (a~) and (t2) in this region. The R~ MQ complexes have pseudo-tetrahedral symmetry. All the quinolinolate complexes studied were colored strongly yellow. Most of the intensely colored complexes owe their color to strong charge-transfer bands [8]. The most convenient pathway for this transfer is from the ring pzr orbital of the iigand to the empty outer s or d orbitals of the metal. The charge-transfer bands, observed with the Cary 14 spectrophotometer, were as follows: iBu2AIQ, 3800/~; iBu2GaQ, 4100/k; and iBu~InQ, 3800 ,~. Charge-transfer bands have been reported for some four-coordinate beryllium alkyl complexes of bipyridyl [9]. These complexes range in color from yellow to red. The N M R spectra were valuable both for the characterization of the 8quinolinolate complexes as wel as for determining C - M bond polarities. Assignment of the alkyl protons resonances in RaM as well as in R2MQ was straightforward because of the first-order splitting pattern, coupling constants (7 Hz) remained constant throughout (cf Table 3). The only exception was iBu~A1Q (cf. Fig. 1), which gave an eight-line, AB(X) pattern for the methylene protons. The A B X pattern resulted from the magentic non-equivalence of the diastereotopic methylene protons, Ha and HB of each isobutyl group, which caused each of the non-equivalent methylene protons to couple with the other, as well as with the methine proton Hx. The geminal methyl groups designated CH3~ and CH3 z
HB'~H x ~'s~zN ~":'M~C H3z Fig. 1. i-Bu~MQ. 8. L. E. Orgel, .4 n Introduction to Transition-Metal Chemistry, p. 99. Methuen, London (1961). 9. G. E. Coates and S. I. E. Green, J. chem.Soc. 33411(1962).
Quinolinolato-complexes
503
were also magnetically non-equivalent and resonated at different fields (cf. Table 3). The N M R spectra were rather complex in the low-fieM region and it seemed expedient to examine first the spectrum of 8-quinolinol in order to provide a basis for assignment, and chemical shift evaluation of the ligand proton resonances (cf. Table 4). A detailed interpretation of the N M R spectra of these compounds is reported elsewhere [2]. A comparison of the alpha proton chemical shifts of the complexes R2 MQ and the metal alkyls RzM revealed a number of trends. First, one notices that, except in the case of the pair Me2A1Q-MeaAI, alpha protons are always deshielded in RzMQ (cf. Table 5) which as a first approximation was anticipated due to the substitution of a C - M bond by an O - M bond. The nitrogen lone pair however, would oppose this deshielding trend by inductive accumulation electron density in the C-M bond region. In fact, in the piperidine-complexes RsMPip, the alpha protons are shielded compared to the alpha protons in the alkyls R3M[3]. However, this shielding decreases in the order RsAIPip > RsGaPip > RaInPip. The alpha proton deshielding in the quinolinolato-complexes as well as in the metal alkyls follows the same trend. That is the order of deshielding for the two series are R2InQ > RzGaQ > RzA1Q, and Rain t> RsGa > RaA1 (cf. Table 6). That is, deshielding increased in descending the group. If the electronegativity of the metal atom were the decisive factor, then the order of deshielding should have been Ga > In >I AI instead of the observed sequences. The similar trend ofdeshielding in three different series is too consistent to be fortuitous, and demands a simple rationale. We postulate that 3d orbitals (in the case of Ga) and 4d orbitals (in the case In) are involved in forming the hybrid valence orbitals of the metal atom. This facilitates d-electronic contribution to the bonding sigma molecular orbitals which in turn inhibits inductive electron withdrawal from the N - M bond to the C - M bond. In going from the gallium compound to the indium compound, the increased Table 5. Comparison of a alkyl proton chemical shifts for R2 MQ and R3 M AS(ppm): + = d e s h i e l d e d ; - = shielded Me2AIQ (-0-95) * MeaA1- (-0.36)
E h G a Q (+0.83) E t s G a - (+0-54)
A8 = --0-59
A8 = +0.29
Et2AIQ(+0.78) E t s A I - (+0.29)
iBu~GaQ (+0.83) i B u a G a - (0.80)
A8 = +0.49
A8 = +0.03
iBu2AIQ (+0.41) i B u s A l - (+0.30)
iBulnQ (+ 1.22) iBuaIn-- (+0.80)
A8 = +0-11
A8 = +0.44
* Hexamethylphosphorictriamide was the solvent.
504
B. SEN and G. L. WHITE Table 6. Comparison of alpha proton chemical shifts as a function of the metal AS(ppm): + = deshielding;- = shielding EtzGaQ (+0.83) E t 2 A I Q - (+0.78) AS --- +0.05 ,]3u~InQ (+1"22) j...., A ' =+0"39 I iBu2GaQ(+0.83)_ [--~ AS = +0.81 1 "-->AS = +0"42 I iBu2AIQ(+0.41) /
interatomic distance reduces the electron density of the bonds as well as at the bonded atoms which in turn cause the alpha protons of R2InQ, RzInPip and RzIn to be more deshielded compared to corresponding gallium compounds in conformity with the observed chemical shifts. Involvement of outer s or d orbitals of the metal atom, and the aromatic or-electrons of the Q anion, as evidenced by the electronic spectra and the color of the R2MQ complexes, possibly further delocalizes the electrons in the hetero-ring containing the metal atom, assisting in further deshielding of the alpha protons. In our opinion, the foregoing argument is no more preposterous than the concept of back-donation of d-electrons to ¢r* M.O. of CO in metal carbonyls and synergic delocalization of bond electrondensity [10]. It is reasonable to assume that the ligand or-electron and the available outer s or d orbitals will encourage electron movement in the opposite direction causing synergic delocalization of bond electron density. The Me2AIQ spectrum was taken in hexamethylphosphorictriamide, and the deviation is most likely due to solvent effect. The extent of deshielding of the alpha protons depends on the molecular weight of R group also (cf. Table 5). However, the influence of the increasing tool. wt of the R group, and the influence of the metal atom parameters seemed to militate against each other (cf. Table 5 and Table 6). Acknowledgement-We would like to thank Mrs. Cheryl T. White for taking numerous mass spectra for us. 10. F. A. Cotton and G. Wilkinson, Adv. lnorg. Chem. 2rid Edn., p. 731-33. Interscience, New York (1966).
QUINOLINOLATO-COMPLEXES GALLIUM AND INDIUM
OF ALUMINUM, DIALKYLS*
B. S E N t and G. L. WHITE~ Coates Chemical Laboratories, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 6 April 1972)
Abstract-- Monomeric complexes of the type R2MQ (R = Me, Et, iBu; M = AI, Ga, In; Q = quinoline-8-olate anion) have been isolated, and characterized by elemental analyses, and i.r., electronic, N M R and mass spectroscopy. Molecular weight of Et2GaQ was determined by vapor pressure osmometry. The compounds are generally yellow and crystalline. The reactivity of C - M bond seemed to be much reduced compared to the C - M bond in free metal alkyls. The chemical shifts in the N M R spectra could be rationalized on the assumption of delocalization of C - M bond electron density by the participation of s and d orbitals of the metal atom, which in turn reduced the C - M bond reactivity. Electronic spectra suggested ligand ~r ~ metal d (or s) transition. Metal-nonmetal vibrations a~ and t~ could be identified for iBu2MQ in the region 535-380 cm -1.
IN AN earlier communication[l] we reported the preparation and characterization of diethyl(quinolinolato)gallium; we now report the synthesis of a series of compounds of general formula R2MQ (R = Me, Et, i-Bu; M = Al, Ga, In; Q = anion quinoline-8-olate), and their characterization by elemental analyses, tool. wt determination, i.r., mass and N M R spectroscopy. Because of their novel nature, the detailed interpretation of N M R spectra of these compounds has been published elsewhere [2]. The shift of the PMR resonance peak due to the protons attached to carbon atom alpha to the metal atom (referred hereafter as alpha protons) follows the trend predicted from the study of the monomeric and dimeric complexes of these metals alkyls with piperidine [3]. The metal in these yellow colored, crystalline compounds is 4-coordinated, and obviously has essentially tetrahedral geometry. Quinoline-8-olate anion functions as a bidentate ligand via its pyridine nitrogen atom and the phenolic oxygen atom which is typical of this anion. EXPERIMENTAL All preparative and manipulative operations were carried out with the use of a dry box and/or a vacuum line. Most of the glassware was fabricated to permit evacuation and subsequent introduction of dry, oxygen-free nitrogen in order to prevent exposure to air. Materials. All solvent and reagents used in the reactions with metal alkyls were made as scru* Presented in part at the 160th ACS National Meeting, Chicago, September, 1970. tAuthor to whom all communications should be addressed. SAbstracted in part from a dissertation submitted by G. L. W. Present address: Louisiana Wildlife and Fisheries Commission, Baton Rouge, Louisiana. 1. B. Sen and G. L. White, lnorg, nucl. Chem. Lett. 7, 79 (1971). 2. B. Sen, G. L. White andJ. D. Wander, J. chem. Sot.. Dalton447 (1972). 3. B. Sen and G. L. White, To be published. 497
498
B. SEN and G. L. W H I T E
pulously free of oxygen and moisture as possible. The solvents were first dried over a series of drying agents, distilled under nitrogen, and were subsequently stored over calcium hydride and kept in the dry box. The aluminum alkyls were obtained from Alfa Inorganics and were used without further purification. Anhydrous gallium and indium trichloride were prepared by direct action of dry chlorine gas on the free metal as described by Eisch [4]. The gallium and indium alkyls were then prepared by the metathetical reaction of the appropriate aluminum alkyl with the metal trichloride according to Eisch [4]. A general description of synthesis of the complexes follows as all of them were prepared by the same procedure. General method of preparation. A weighed amount (calculated on the basis of a 1 : 1 adduct) of dried and degassed 8-quinolinol was dissolved in 20 ml of dry benzene, and the solution was then introduced into a pressure-equalizing dropping funnel. The metal alkyl was weighed and dissolved in 20 ml of dry pentane in the reaction flask. The dropping funnel containing the 8-quinolinol solution was then attached to the reaction flask. The 8-quinolinol solution was added slowly to the metal alkyl solution at such a rate that a gentle reflux of the solvent was maintained ( - 37°C). The crystalline product started to separate almost immediately. At the completion of the addition, the mother liquor was either decanted, or evaporated at reduced pressure, and the yellow crystals were washed several times with pentane. The product was finally dried under vacuum. A mixed pentane-benzene solvent system was preferred for the purification and isolation of the product because of the sparing solubility of the product in pentane. Slightly higher solubility of the product in benzene ensured greater purity of the crystals. This series of complexes appeared to be relatively stable towards oxygen and moisture. In a number of preparations the evolved alkane was trapped by freezing and was subsequently analyzed by mass spectroscopy.
M--R ~R
+ RH t + Heat
+
(R = Me, Et, iBu)
(M = AI, Ga, In)
Table 1. Elemental analyses of 8-quinolinolate complexes
Compound
Color
Me2AIQ
Pale yellow
Et~AIQ
Bright yellow
iBu2AIQ
Yellow
Et2GaQ
Yellow
iBuzGaQ
Greenish yellow
iBu2InQ
Yellowish green
Mol. wt
M.P.* (°C)
%C
%H
%N
% Metal
65-67 64.64 185§ 68.12 68.37 195§ 71.58 67.92 132 57-56 56.62 61 62.39 61-15 160 54.69 54.95
5.97 5.92 6.99 6-89 8.42 8.10 5.90 5.68 7-34 6.88 6-43 6.53
6-97 6.67 6.11 6.09 4.91 4.79 5-17 5.14 4.28 4.58 3.79 3.81
13.43It 13.36 11.79" 11.74 9.47 tl 9.70 25.4(/~ 24-46 21.1~ I 20.50 30.83" 29.84
-250
2727 277~
*Uncorrected. 1Theoretical. $Experimental. §Decomposed. ~%.ineof theoretical percentages. 4. J.J. Eisch, J.Am. chem. Soc. 84, 3605 (1962).
Quinolinolato-complexes
499
The results of elemental analyses and mol. wt determination by vapor pressure osmometry, and other physical properties are given in Table 1. Maintenance of a low reflux temperature was essential to avoid formation of the dimeric, and the polymeric species, and of other undesirable products. Using benzene as the solvent, Hurley et al. obtained dimeric products [5]. Infrared spectra. The i.r. spectra were obtained with a Beckman IR-7 and/or a Beckman IR-10 Table 2. Mass spectra of 8-quinolinolate complexes Compound
m/e
Intensity
Assignment
Me2AIQ
145 171 186 201 315 144 171 200 228 229 315 145 171 172 186 228 315 69, 71
3.0 16.0 100.0 2.0 3.0 0-9 77.0 100.0 0.9 0.9 5-5 10-3 26.5 91.0 62.0 100-0 3-5 40-5
8-hydroxyquinoline AIQ MeA1Q Me2AIQ AI(Q)2 Q
EtzAIQ
iBu2A1Q
Et2GaQ
145
iBuzGaQ
iBuzGaQ
iBu2InQ
213,215 242,244 270,272 271,273 69, 71 145 213,215 214,216 228,230 247,249 270,272 284, 286 327,329 115
0.1
21-2 100.0 1.6 2.9 38.0 4.0 11.0 II.0 2.5 3'5 100.0 0-4 0.6 100-0
145
5.3
229 259 260 316 317 372 373
1.8 34-0 5.9 13-5 2-3 1"2 0-4
A1Q EtAIQ Et2AIQ - H Et2AIQ AI(Q)2 8-hydroxyquinoline AIQ AIQ+H A1CHaQ
iBuAIQ AI(Q)2 Ga 8-hydroxyquinoline GaQ EtGaQ Et2GaQ - H EtzGaQ Ga 8-hydroxyquinoline GaQ GaQ+H GaCH3Q * iBuGaQ iBuGaCH2Q iBu2GaQ In 8-hydroxyquinoline iBu21n InQ InQ + H iBulnQ iBulnQ+H iBu2InQ - H iBu2InQ
* Denotes unassigned peak. 5. J.J. Hurly, M. A. Robinson, J. A. Scruggs and S. I. Tratz, Inorg. Chem. 6, 13 l0 (1967).
500
B. SEN and G. L. W H I T E
spectrophotometer. The spectra of either a hexachlorobutadiene mull or a saturated benzene solution of the quinolinolates were obtained. Mass spectra. Mass spectra were recorded at an ionizing potential of 70 eV with a Varian M-66 cycloidal mass spectrometer using a direct insertion probe. The masses, the corresponding intensities, and the possible assignments are tabulated in Table 2. N M R spectra. The majority of the N M R spectra were measured with the Varian HA-100 N M R spectrometer; some of the N M R spectra were obtained at 60 M H z with a Varian A-60-A instrument. Benzene, the solvent in most cases, was used for locking purposes. Tetramethylsilane was used as an internal standard. All of the spectra were obtained at the ambient temperature of the probe ( - 35°C). Due to the insufficient solubility of some of the 8-quinolinolate complexes, only the alkyl portion of the N M R spectra could be recorded for the least soluble complexes. The alkyl proton resonances could be recorded because each resonance peak in the alkyl portion of the spectra resulted from the contributions of several protons; however, in the quinolinolate portion of the spectra each resonance peak generally resulted from the contribution of a single proton and, therefore, a much greater concentration was needed to make the quinofinolate resonance peaks observable. The spectral assignment are given in Table 3 and 4. RESULTS AND DISCUSSION
Elemental analyses data were jedged acceptable except for a few deviations in the values for carbon. Melting point data did not show any rational trend; this idiosyncrasy seems to be general for the group IIIA metal alkyl derivatives, and it is not well understood. The melting points reported here (cf. Table 1) for the monomers are considerably lower than those reported by Hurley et al. [5] for the corresponding dimers; the only exception is iBuzAIQ. It is quite likely that iBu2AIQ might have undergone dimerization during melting. The experimental mol. wt of Et2GaQ was in excellent agreement with the theoretical value. However, the mol. wt determination attempted for some of the other compounds yielded anomalous values. Insufficient solubility, and enhanced reactivity of the Table 3. Chemical shifts of alkyl protons of RaM and RzMQ in benzene*
Compound Me2A1Q MesA1 Et~AIQ Et3AI
iBu2A1Q iBu3AI Et2GaQ EtaGa
iBu2GaQ iBu3Ga
iBuzlnQ iBualn
Chemical shift (8) CHa CH2 CH -0.95T~: -0.36 1.33T 1-09 0.75,0.85 0.98 1.22 1"16 1.07, 1.08 0.95 1.11 1.02
0.78-~ 0-29 0.35,0-47§ 0.30 0.83 0.54 0.83 0.80 1.22 0.80
1.73 1.94
2.07 2.10 2.23 2.22
*For multiplets 8 values correspond to centers of gravity. tMeasured at 60 MHz. SHexamethylphosphorictriamide was the solvent for this determination. §Determined by.4BX analysis.
Quinolinolato-complexes
501
Table 4. 100 M H z N M R spectral parameters for the 8quinolinolate protons of Re MQ
Compound
H-2
H-3
Chemical shifts* H-4 H-5 H-6
H-70-H
8-Quinolinol iBu2AIQ Et2GaQ iBuzGaQ
8.50 8-44 7.61 7.68 8-25
6.74 6.71 6-53 6.53 6.72
7-51 7.46 7-51 7.50 7-56
6.95 6-85 6.69 6.69 6-85
iBu2InQ
7.05 7-79 7.23 7.20 7.28
7-30T 7.20 7.33t 7.33t 7.50t
9.10
Apparent coupling constants~t Compound
J2,3 ,13,4 J 2 , 4 J 5 . 6 J 6 , 7 Js,7
8-Quinolinol iBueAIQ Et2GaQ iBu2GaQ iBu21nQ
4-0 5.0 4.5 4.5 4.5
8.0 8.0 8.0 8.0 8-0
1.5 1.5 1.5 1.5 1.5
§ 7.5 § § §
6.0 8.5 8.0 6.0 8.5
3.0 1.0 4.5 4.0 4.5
* In ppm (6). -tUnresoived multiplet H5 and He. eFirst-order values in Hz. §Not determined.
compounds in dilute solutions were possibly responsible for the anomalous molecular weights. It should also be mentioned in this connection that of all the compounds reported here, EtzGaQ seemed to be the most stable, and in fact Et2GaQ did not show any visible sign of decomposition on exposure to the laboratory atmosphere for several months. The heaviest ion in the mass spectrum of every compound, except R2AIQ, was RzMQ which strongly suggested that the compounds were monomeric. It is likely that traces of AIQ3 were formed along with RzAIQ as this was the most exothermic reaction. However, if these were formed, their concentration was so low that they did not influence the elemental analysis, and could be observed only in the mass spectra. In the mass spectra of RzGaQ, all fragments containing one gallium atom were observed as doublets in the ratio of 3 : 2 in consonance with the isotopic abundance of 6 9 G a ( 6 0 % ) and 71Ga(40%). The absence of triplets and multiplets in the spectra of R2GaQ ruled out any fragment containing more than one gallium atom. This was considered as a very strong evidence that the quinolinolates R2MQ reported here were all monomeric. Doublet structure was not observed in the spectrum of iBuzInQ at the intensity employed due to the low abundance of 11aln(4" 16%); it could be observed at a higher intensity. Oliver and Worrall [6] have reported the mass spectra of some Group III alkoxides. The fragmentation patterns of AI(OEt)z, Ga(OEt)3, and Ga(OiPr)3 were assigned, and mass peaks corresponding to dimers, tetramers, pentamers, and hexamers were observed. Chambers, Coates, Glockling and Weston[7] reported a molecular ion for (Et2A1OEt)z. Dimeric fragments were also observed 6. J. G. Oliver and I. J. Worrall, J. chem. Soc. (A) 234 (1970). 7. D. B. Chambers, G. E. Coates, F. Glockling and M. Weston, J. Chem. Soc. (A) 1712 (1970).
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B. SEN and G. L. W H I T E
in the mass spectra of the dimeric piperidine complexes of the group IIIA metal alkyl[3]. All evidence indicated that the quinolinolates were more stable than any of the foregoing complexes. Consequently, if the quinolinolates were polymeric, the mass spectra would have indicated fragments larger than monomers. The absence of the 8-quinolinol ( O - H ) stretch frequency at 3415 cm -1 was one important aspect of the i.r. spectra. The quality of the far i.r. spectra in the region 700-200 cm -1 was rather poor; nevertheless, two bands definitely attributable to the complexes were observed in the case of iBu~AIQ, iBu2GaQ, and iBu2InQ. These bands were observed at 535 cm -1 and 420 cm -1 in the case of the aluminum complex, at 522 cm -~ and 415 cm -~ in the case of the gallium complex, and at 502 cm -1 and 380 cm -~ in the case of the indium complex. If these bands were due to the metal-nonmetal stretching frequencies, then the frequency shifts are definitely in the fight direction. It should also be recalled that the molecules of genuine Td symmetry should have two i.r. active bands (a~) and (t2) in this region. The R~ MQ complexes have pseudo-tetrahedral symmetry. All the quinolinolate complexes studied were colored strongly yellow. Most of the intensely colored complexes owe their color to strong charge-transfer bands [8]. The most convenient pathway for this transfer is from the ring pzr orbital of the iigand to the empty outer s or d orbitals of the metal. The charge-transfer bands, observed with the Cary 14 spectrophotometer, were as follows: iBu2AIQ, 3800/~; iBu2GaQ, 4100/k; and iBu~InQ, 3800 ,~. Charge-transfer bands have been reported for some four-coordinate beryllium alkyl complexes of bipyridyl [9]. These complexes range in color from yellow to red. The N M R spectra were valuable both for the characterization of the 8quinolinolate complexes as wel as for determining C - M bond polarities. Assignment of the alkyl protons resonances in RaM as well as in R2MQ was straightforward because of the first-order splitting pattern, coupling constants (7 Hz) remained constant throughout (cf Table 3). The only exception was iBu~A1Q (cf. Fig. 1), which gave an eight-line, AB(X) pattern for the methylene protons. The A B X pattern resulted from the magentic non-equivalence of the diastereotopic methylene protons, Ha and HB of each isobutyl group, which caused each of the non-equivalent methylene protons to couple with the other, as well as with the methine proton Hx. The geminal methyl groups designated CH3~ and CH3 z
HB'~H x ~'s~zN ~":'M~C H3z Fig. 1. i-Bu~MQ. 8. L. E. Orgel, .4 n Introduction to Transition-Metal Chemistry, p. 99. Methuen, London (1961). 9. G. E. Coates and S. I. E. Green, J. chem.Soc. 33411(1962).
Quinolinolato-complexes
503
were also magnetically non-equivalent and resonated at different fields (cf. Table 3). The N M R spectra were rather complex in the low-fieM region and it seemed expedient to examine first the spectrum of 8-quinolinol in order to provide a basis for assignment, and chemical shift evaluation of the ligand proton resonances (cf. Table 4). A detailed interpretation of the N M R spectra of these compounds is reported elsewhere [2]. A comparison of the alpha proton chemical shifts of the complexes R2 MQ and the metal alkyls RzM revealed a number of trends. First, one notices that, except in the case of the pair Me2A1Q-MeaAI, alpha protons are always deshielded in RzMQ (cf. Table 5) which as a first approximation was anticipated due to the substitution of a C - M bond by an O - M bond. The nitrogen lone pair however, would oppose this deshielding trend by inductive accumulation electron density in the C-M bond region. In fact, in the piperidine-complexes RsMPip, the alpha protons are shielded compared to the alpha protons in the alkyls R3M[3]. However, this shielding decreases in the order RsAIPip > RsGaPip > RaInPip. The alpha proton deshielding in the quinolinolato-complexes as well as in the metal alkyls follows the same trend. That is the order of deshielding for the two series are R2InQ > RzGaQ > RzA1Q, and Rain t> RsGa > RaA1 (cf. Table 6). That is, deshielding increased in descending the group. If the electronegativity of the metal atom were the decisive factor, then the order of deshielding should have been Ga > In >I AI instead of the observed sequences. The similar trend ofdeshielding in three different series is too consistent to be fortuitous, and demands a simple rationale. We postulate that 3d orbitals (in the case of Ga) and 4d orbitals (in the case In) are involved in forming the hybrid valence orbitals of the metal atom. This facilitates d-electronic contribution to the bonding sigma molecular orbitals which in turn inhibits inductive electron withdrawal from the N - M bond to the C - M bond. In going from the gallium compound to the indium compound, the increased Table 5. Comparison of a alkyl proton chemical shifts for R2 MQ and R3 M AS(ppm): + = d e s h i e l d e d ; - = shielded Me2AIQ (-0-95) * MeaA1- (-0.36)
E h G a Q (+0.83) E t s G a - (+0-54)
A8 = --0-59
A8 = +0.29
Et2AIQ(+0.78) E t s A I - (+0.29)
iBu~GaQ (+0.83) i B u a G a - (0.80)
A8 = +0.49
A8 = +0.03
iBu2AIQ (+0.41) i B u s A l - (+0.30)
iBulnQ (+ 1.22) iBuaIn-- (+0.80)
A8 = +0-11
A8 = +0.44
* Hexamethylphosphorictriamide was the solvent.
504
B. SEN and G. L. WHITE Table 6. Comparison of alpha proton chemical shifts as a function of the metal AS(ppm): + = deshielding;- = shielding EtzGaQ (+0.83) E t 2 A I Q - (+0.78) AS --- +0.05 ,]3u~InQ (+1"22) j...., A ' =+0"39 I iBu2GaQ(+0.83)_ [--~ AS = +0.81 1 "-->AS = +0"42 I iBu2AIQ(+0.41) /
interatomic distance reduces the electron density of the bonds as well as at the bonded atoms which in turn cause the alpha protons of R2InQ, RzInPip and RzIn to be more deshielded compared to corresponding gallium compounds in conformity with the observed chemical shifts. Involvement of outer s or d orbitals of the metal atom, and the aromatic or-electrons of the Q anion, as evidenced by the electronic spectra and the color of the R2MQ complexes, possibly further delocalizes the electrons in the hetero-ring containing the metal atom, assisting in further deshielding of the alpha protons. In our opinion, the foregoing argument is no more preposterous than the concept of back-donation of d-electrons to ¢r* M.O. of CO in metal carbonyls and synergic delocalization of bond electrondensity [10]. It is reasonable to assume that the ligand or-electron and the available outer s or d orbitals will encourage electron movement in the opposite direction causing synergic delocalization of bond electron density. The Me2AIQ spectrum was taken in hexamethylphosphorictriamide, and the deviation is most likely due to solvent effect. The extent of deshielding of the alpha protons depends on the molecular weight of R group also (cf. Table 5). However, the influence of the increasing tool. wt of the R group, and the influence of the metal atom parameters seemed to militate against each other (cf. Table 5 and Table 6). Acknowledgement-We would like to thank Mrs. Cheryl T. White for taking numerous mass spectra for us. 10. F. A. Cotton and G. Wilkinson, Adv. lnorg. Chem. 2rid Edn., p. 731-33. Interscience, New York (1966).