- Email: [email protected]
The system gallium trichloride-methanol
~. inorg, nucl. Chem., 1966, Vol. 28. pp. 2599 to 2602. PergamonPress Ltd. Printedin NorthernIreland
THE
SYSTEM
GALLIUM
TRICHLORIDE-METHANOL
P. G. PERKINS Department of Inorganic Chemistry, University of Newcastle-upon-Tyne (Received 2 March 1966) Abstract--Investigation of the system GaCIs-CHBOH reveals two complexes of stoichiometry 1 : 1 and 1 : 3. Studies of the electrical conductivity indicate the existence of ions in the former.
Tim gallium trichloride-methanol system was investigated during an attempt to discover a synthetic route to the, as yet unknown, alkyl esters derived from gallium halides, i.e. (RO)=GaX~_~. The corresponding 1 : 1 complexes of boron and aluminium trichlorides lose hydrogen chloride readily; the former undergoes complete alcoholysis to trimethylborate at room temperature when treated with excess methanol. COASt.S and HAYTER(1) observed that elimination of methane from Me3Ga-MeOH is rapid and results in the dimeric compound (MeOGaMez)2. RESULTS
AND
DISCUSSION
A colourless 1 : 1 compound was prepared when the theoretical quantity of methanol was distilled onto gallium trichloride in a vacuum. During the reaction no HC1 was evolved and the resultant complex melted without decomposition as witnessed by its reproducible melting point (35.5°). The phase diagram of the system, determined by the melting point method, is illustrated in Fig. 1. 80 60
-•
I:1
h2
1:3
40 20 0 4
-20
o
-40
E
~0 I--
-60 -80 -IO0 - 120
0
1
I
[
I
T
I
1
I
I
10
20
30
40
50
60
70
80
90
Methanol,
mole
%
Fro. 1.--Phase diagram of the system GaCIa.MeOH. ~t~ G. E. COATESand R. G . H A Y E R , J. chem. Soc. 2519 (1953). 2599
too
2600
P . G . PE.RgrNs
The 1:1 adduct appears clearly as a sharply defined maximum on the diagram. There is evidence for a second complex, GaCI3.3MeOH , melting near --68 ° although observations of melting point are difficult to make in this temperature range. No complex of 1:2 stoichiometry was detected although this type are formed with other ligands. (~) The 1 : 1 compound was stable at room temperature but it decomposed at 130° over a period of several days evolving HC1 and leaving a heterogeneous residue. The latter has not yet been analysed but is expected to contain methyl esters of gallium. The stability of the 1 : 1 adduct towards solvolysis contrasts markedly with that of BCla'MeOH and AICIa.MeOH and is comparable to that of other complexes of gallium trichloride with oxygen-donor molecules, e.g. acetone forms an isolable complex with GaCla but this decomposes at its melting point (a) whereas diethylether gives a stable low melting 1 : 1 complex (4) which does not eliminate ethyl chloride on moderate heating. The instability of the acetone complex is not due to the weakness of the Ga---O bond since M%CO is a stronger ligand than ether but it is susceptible to resinification under the influence of a strong Friedel-Crafts catalyst like gallium trichloride. The loss of HCI from boron trichloride complexes with oxygen donors producing B--O bonds is thermodynamically favoured and it may therefore be concluded that the Ga--O and Ga---CI bonds are of similar strength. Hence the latter should generally be more resistant to solvolytic cleavage than either B---C1 or A1--CI bonds. It is curious that co-ordination complexes of B, A1 and Ga trihalides with nitrogen donors (e.g. pyridine) generally have melting points > 100° whereas when oxygen is the donor atom the complexes are low melting (e.g. etherates).
Specific conductivity A sample of the 1 : 1 complex was purified by fractional freezing until the maximum melting point (36.8 °) was obtained. The specific electrical conductivity of the molten complex was measured over the range 19-54°C in both directions using a previously described method (5) and a cell with constant 3-539 cm-L In the region between 19 and 36 ° the compound supercooled. The results are listed in Table 1. The plot of log K against 1/T ° Kis linear in both the molten and supercooled regions and has a sharp discontinuity in slope at ,~1 ° above the melting point. The specific conductivity at the melting point is 2.854 × 10-a fl-x cm-X and is similar in magnitude to the values measured for GaC13.EhO c4) and the analogous complex of boron trifluoride, c6) In the true liquid region the activation energy for conductance, E~, is 3.69 kcal mole -x whilst in the supercooled region E~ is slightly higher (4.05 kcal mole-X). No viscosity measurements were made on the complex and so the reduced conductivity and the degree of ionic dissociation were not calculated. The compound had a well-defined d.c. decomposition potential (1.95 V). Many donor-acceptor complexes of the group III trihalides have been found to conduct electricity and mechanisms of conductance have been discussed at length. is~ N. t*~ N. c,~ N. ¢6) N. ce~ N.
N. N. N. N. N.
GREENWOOI3 and GR~NWOOD and GREENWOOD and GREENWOOD and GREENWOOD and
K. WADE, 3". chem. Soc. 1663 (1958). P. G. PERKINS, J'. chem. Soc. 356 (1960). P. G. PERKINS. Unpublished work. I. J. WORR^LL, Z inorg, nucl. Chem. 3, 357 0957). R. L. MARTIN, 3". chem. Soc. 757 (1953).
The system gallium trichloride-methanol
2601
TABLE 1.---CoNDucnvITY OF GALLIUM TRICHLORIDE-METHANOL BETWEEN 19 AND 54 ° t
10s,x~ -1 cm -1
(°c) 18"6 22"1 25"8 29"0 29"9 31"0 32"0 33'0 34"0 35"4 36'7 37'8
t
10sxf1-1 cm -1
(°c) 1'941 2'064 2"256 2"421 2"477 2"533 2"589 2'647 2"708 2"778 2"854 2-939
39.2 40'0 41.7 42"8 44.1 45.1 46.1 47"5 48'9 50.5 51.5 53"6 54.5
3"022 3"053 3"157 3"220 3"301 3"361 3'433 3"518 3"610 3"714 3"781 3"914 3"974
Spectroscopic investigations of molten complexes support the view that they are mainly molecular and, therefore, the conductance of electricity is accomplished by a small number of highly mobile charge carriers. The conclusion is that these are ions. ~z,e.7~ When the ligand co-ordinates via an oxygen atom to which is attached an "active" hydrogen (e.g. H~O, MeOH) then the cation in a 1 : 1 adduct is most likely to be a proton solvated by a further molecule of undissociated complex, le~ The possibility of a proton switch mechanism accounting for the high conductance of such complexes in which protons may be produced has previously been ruled out in favour of normal ionic mobility. 17~ In the present case, by analogy with the complex BF3"MeOH, the ions could be [GaC13MeOHz]+ [GaClaOMe]-.
NMR spectra The proton NMR spectra of the molten 1 : 1 complex and a 1 : 3 GaC18'MeOH mixture were recorded at room temperature on a Perkin-Elmer NMR spectrometer (model R10). The chemical shifts of the methyl and hydroxyl protons wereall measured relative to internal cyclohexane. The spectra exhibit several interesting features. (a) GaC13"MeOH. The methyl proton peak consisted of a sharp doublet shifted by 2.65 ppm from the standard whereas the - - O H peak was broad (~.~12 c/s) and was separated from cyclohexane by 4.50 ppm. It did not show the expected quartet stemming from spin coupling of this proton with those of the methyl group and its width is presumably due to quadrupole broadening by the adjacent gallium nucleus. Pure methanol is extensively hydrogen bonded and so the proton shift of its - - O H group appears at low field with reference to CeHI~ (4.08 ppm). On co-ordination to gallium trichloride via the oxygen atom, however, the hydrogen bonding is destroyed and the proton becomes more unshielded. This causes it to move to even lower field (0.42 ppm) as would be expected. A much larger shift is evident when the 1 : 1 complex is compared to methanol in a dilute solution in CDC18 where the - - O H peak is at higher field (~.0.5 ppm) than cyclohexane itself. (b) GaCI3.3MeOH. In this system the CHa proton appears as a sharp doublet 77 N. N. GREENWOOD,3". inorff, nucl. Chem. 5, 229 (1958).
2602
P . G . P~r~rNs
shifted by 2.23 ppm from cyclohexane. The most surprising feature is that the - - O H peak (a sharp quarte0 is shifted considerably further downfield (1.2 ppm) from that of" the I : 1 complex. This means that the acidic character of the proton on the oxygen must increase markedly. Furthermore no peaks corresponding to methanol were observed which suggests that the 1 : 3 species detected at low temperature also exists at room temperature. It is somewhat difficult to formulate a species such as this which has a proton more ionic than that of the 1:I. Presumably the two extra MeOH molecules solvate the proton of GaC13 • MeOH causing more ionic dissociation. The sharpness of the - - O H peak for this compound gives evidence of proton exchange between the three methanol molecules.
THE
SYSTEM
GALLIUM
TRICHLORIDE-METHANOL
P. G. PERKINS Department of Inorganic Chemistry, University of Newcastle-upon-Tyne (Received 2 March 1966) Abstract--Investigation of the system GaCIs-CHBOH reveals two complexes of stoichiometry 1 : 1 and 1 : 3. Studies of the electrical conductivity indicate the existence of ions in the former.
Tim gallium trichloride-methanol system was investigated during an attempt to discover a synthetic route to the, as yet unknown, alkyl esters derived from gallium halides, i.e. (RO)=GaX~_~. The corresponding 1 : 1 complexes of boron and aluminium trichlorides lose hydrogen chloride readily; the former undergoes complete alcoholysis to trimethylborate at room temperature when treated with excess methanol. COASt.S and HAYTER(1) observed that elimination of methane from Me3Ga-MeOH is rapid and results in the dimeric compound (MeOGaMez)2. RESULTS
AND
DISCUSSION
A colourless 1 : 1 compound was prepared when the theoretical quantity of methanol was distilled onto gallium trichloride in a vacuum. During the reaction no HC1 was evolved and the resultant complex melted without decomposition as witnessed by its reproducible melting point (35.5°). The phase diagram of the system, determined by the melting point method, is illustrated in Fig. 1. 80 60
-•
I:1
h2
1:3
40 20 0 4
-20
o
-40
E
~0 I--
-60 -80 -IO0 - 120
0
1
I
[
I
T
I
1
I
I
10
20
30
40
50
60
70
80
90
Methanol,
mole
%
Fro. 1.--Phase diagram of the system GaCIa.MeOH. ~t~ G. E. COATESand R. G . H A Y E R , J. chem. Soc. 2519 (1953). 2599
too
2600
P . G . PE.RgrNs
The 1:1 adduct appears clearly as a sharply defined maximum on the diagram. There is evidence for a second complex, GaCI3.3MeOH , melting near --68 ° although observations of melting point are difficult to make in this temperature range. No complex of 1:2 stoichiometry was detected although this type are formed with other ligands. (~) The 1 : 1 compound was stable at room temperature but it decomposed at 130° over a period of several days evolving HC1 and leaving a heterogeneous residue. The latter has not yet been analysed but is expected to contain methyl esters of gallium. The stability of the 1 : 1 adduct towards solvolysis contrasts markedly with that of BCla'MeOH and AICIa.MeOH and is comparable to that of other complexes of gallium trichloride with oxygen-donor molecules, e.g. acetone forms an isolable complex with GaCla but this decomposes at its melting point (a) whereas diethylether gives a stable low melting 1 : 1 complex (4) which does not eliminate ethyl chloride on moderate heating. The instability of the acetone complex is not due to the weakness of the Ga---O bond since M%CO is a stronger ligand than ether but it is susceptible to resinification under the influence of a strong Friedel-Crafts catalyst like gallium trichloride. The loss of HCI from boron trichloride complexes with oxygen donors producing B--O bonds is thermodynamically favoured and it may therefore be concluded that the Ga--O and Ga---CI bonds are of similar strength. Hence the latter should generally be more resistant to solvolytic cleavage than either B---C1 or A1--CI bonds. It is curious that co-ordination complexes of B, A1 and Ga trihalides with nitrogen donors (e.g. pyridine) generally have melting points > 100° whereas when oxygen is the donor atom the complexes are low melting (e.g. etherates).
Specific conductivity A sample of the 1 : 1 complex was purified by fractional freezing until the maximum melting point (36.8 °) was obtained. The specific electrical conductivity of the molten complex was measured over the range 19-54°C in both directions using a previously described method (5) and a cell with constant 3-539 cm-L In the region between 19 and 36 ° the compound supercooled. The results are listed in Table 1. The plot of log K against 1/T ° Kis linear in both the molten and supercooled regions and has a sharp discontinuity in slope at ,~1 ° above the melting point. The specific conductivity at the melting point is 2.854 × 10-a fl-x cm-X and is similar in magnitude to the values measured for GaC13.EhO c4) and the analogous complex of boron trifluoride, c6) In the true liquid region the activation energy for conductance, E~, is 3.69 kcal mole -x whilst in the supercooled region E~ is slightly higher (4.05 kcal mole-X). No viscosity measurements were made on the complex and so the reduced conductivity and the degree of ionic dissociation were not calculated. The compound had a well-defined d.c. decomposition potential (1.95 V). Many donor-acceptor complexes of the group III trihalides have been found to conduct electricity and mechanisms of conductance have been discussed at length. is~ N. t*~ N. c,~ N. ¢6) N. ce~ N.
N. N. N. N. N.
GREENWOOI3 and GR~NWOOD and GREENWOOD and GREENWOOD and GREENWOOD and
K. WADE, 3". chem. Soc. 1663 (1958). P. G. PERKINS, J'. chem. Soc. 356 (1960). P. G. PERKINS. Unpublished work. I. J. WORR^LL, Z inorg, nucl. Chem. 3, 357 0957). R. L. MARTIN, 3". chem. Soc. 757 (1953).
The system gallium trichloride-methanol
2601
TABLE 1.---CoNDucnvITY OF GALLIUM TRICHLORIDE-METHANOL BETWEEN 19 AND 54 ° t
10s,x~ -1 cm -1
(°c) 18"6 22"1 25"8 29"0 29"9 31"0 32"0 33'0 34"0 35"4 36'7 37'8
t
10sxf1-1 cm -1
(°c) 1'941 2'064 2"256 2"421 2"477 2"533 2"589 2'647 2"708 2"778 2"854 2-939
39.2 40'0 41.7 42"8 44.1 45.1 46.1 47"5 48'9 50.5 51.5 53"6 54.5
3"022 3"053 3"157 3"220 3"301 3"361 3'433 3"518 3"610 3"714 3"781 3"914 3"974
Spectroscopic investigations of molten complexes support the view that they are mainly molecular and, therefore, the conductance of electricity is accomplished by a small number of highly mobile charge carriers. The conclusion is that these are ions. ~z,e.7~ When the ligand co-ordinates via an oxygen atom to which is attached an "active" hydrogen (e.g. H~O, MeOH) then the cation in a 1 : 1 adduct is most likely to be a proton solvated by a further molecule of undissociated complex, le~ The possibility of a proton switch mechanism accounting for the high conductance of such complexes in which protons may be produced has previously been ruled out in favour of normal ionic mobility. 17~ In the present case, by analogy with the complex BF3"MeOH, the ions could be [GaC13MeOHz]+ [GaClaOMe]-.
NMR spectra The proton NMR spectra of the molten 1 : 1 complex and a 1 : 3 GaC18'MeOH mixture were recorded at room temperature on a Perkin-Elmer NMR spectrometer (model R10). The chemical shifts of the methyl and hydroxyl protons wereall measured relative to internal cyclohexane. The spectra exhibit several interesting features. (a) GaC13"MeOH. The methyl proton peak consisted of a sharp doublet shifted by 2.65 ppm from the standard whereas the - - O H peak was broad (~.~12 c/s) and was separated from cyclohexane by 4.50 ppm. It did not show the expected quartet stemming from spin coupling of this proton with those of the methyl group and its width is presumably due to quadrupole broadening by the adjacent gallium nucleus. Pure methanol is extensively hydrogen bonded and so the proton shift of its - - O H group appears at low field with reference to CeHI~ (4.08 ppm). On co-ordination to gallium trichloride via the oxygen atom, however, the hydrogen bonding is destroyed and the proton becomes more unshielded. This causes it to move to even lower field (0.42 ppm) as would be expected. A much larger shift is evident when the 1 : 1 complex is compared to methanol in a dilute solution in CDC18 where the - - O H peak is at higher field (~.0.5 ppm) than cyclohexane itself. (b) GaCI3.3MeOH. In this system the CHa proton appears as a sharp doublet 77 N. N. GREENWOOD,3". inorff, nucl. Chem. 5, 229 (1958).
2602
P . G . P~r~rNs
shifted by 2.23 ppm from cyclohexane. The most surprising feature is that the - - O H peak (a sharp quarte0 is shifted considerably further downfield (1.2 ppm) from that of" the I : 1 complex. This means that the acidic character of the proton on the oxygen must increase markedly. Furthermore no peaks corresponding to methanol were observed which suggests that the 1 : 3 species detected at low temperature also exists at room temperature. It is somewhat difficult to formulate a species such as this which has a proton more ionic than that of the 1:I. Presumably the two extra MeOH molecules solvate the proton of GaC13 • MeOH causing more ionic dissociation. The sharpness of the - - O H peak for this compound gives evidence of proton exchange between the three methanol molecules.