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Hexamethylphosphoramide complexes of silver(I) in solution
Notes
791
RESULTS Shown in Table 1 are the compound solubilities expressed in m-mole of salt per 1. of saturated solution. The results from each run as well as the average values are reported. The errors appearing in the column marked "Solubility" are the errors in solubility due only to counting statistics while the error listed with each average value is the experimental standard deviation. The relative standard standard deviations generally range between 2 and 6% which is what may be expected from I N A A techniques without elaborate sample handling. DISCUSSION The solubility trend of the alkali chlorides in sulfolane parallels that for water. For the alkali metal series, Li through Cs, both solvents show a minimum in molar solubility at KCI, and similar solubilities for the potassium and ammonium chlorides. In this respect the solubilities in sulfolane do not show a resemblance to those in SO2 as suggested by Drago [5]. In liquid SO2 the solubility minima for the alkali chlorides and bromides are reached at sodium and the solubilities of the corresponding ammonium halides do not correspond closely to those of the potassium salts. The solubility trend for the series of anhydrous alkaline earth metal chlorides in sulfolane does not correspond exactly to the trend in solubility for these salts in water. A steady decrease in solubility in sulfolane is observed with increasing cation size, while a maximum in solubility is reached at calcium for this series of salts in water. However, this solubility trend in sulfolane is in agreement with the observed trend in the free energies of solution for these salts in aqueous solution [6]. The apparent difference between the order of solubilities and free energies of solution in water for this series can perhaps be attributed to the formation of a new solid magnesium chloride hydrate phase. If such a hydrate phase were to form before the equilibrium between the anhydrous salt and the solution phase were attained, magnesium chloride would appear to be less soluble than the calcium salt.
Acknowledgements-We are grateful to Professors Harry Freund, R. A. Schmitt and T. H. Norris for helpful discussions and to the Triga reactor staff of the Radiation Center for their assistance in the neutron activations. This work was supported in part by a research grant from the Pacific Northwest Pulp and Paper Association.
Department of Chemistry Oregon State University Corvallis, Oregon 97331
J. A. S T A R K O V I C H M. J A N G H O R B A N I
5. R. S. Drago and K. F. Purcell, Co-ordinating Solvents. In Non-Aqueous Solvent Systems Edited by T. C. Waddington p. 246. Academic Press, New York (1965). 6. U.S. National Bureau @Standards Circular 500. U.S. Government Printing Office, Washington, D.C. (1952).
J. inorg, nucl. Chern,, 1972, Vol. 34, pp. 791-792.
Pergamon Press.
Printed in Great Britain
Hexan~thylphosphoramide complexes of silver(I) in solution (Received 12 July 1971 ) HEXAMETHYLPHOSPHORAMIDE (HMPA) is a good coordinating solvent[l]. A number of H M P A complexes have been prepared in the solid state[l, 2], but relatively little is known of the stability of H M P A complexes of metal ions in solution. Consideration of the properties of H M P A [1,2] would indicate that such complexes should exist in solvents such as nitroethane or acetone. 1. V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions p. 159. Springer-Verlag, Vienna (1968). 2. H. Normant, Angew. Chem. Intern. edn. 6, 1046 (1967).
792
Notes
EXPERIMENTAL Research Organic/Inorganic Chemical Corp. H M P A was purified by fractional distillation under vacuum from calcium hydride at 75°C. H M P A from Aldrich Chemical Co. could not be freed of a substance which reduced silver nitrate even with repeated distillation under vacuum. The other materials and the determination of the formal reduction potential of silver ion in H M P A were as previously described [3]. Silver nitrate was the source of silver ion in HMPA. The voltammetric determination of the ferricinium/ferrocene potential was as described before[3,4], except that a Moseley 7035A X - Y recorder was used. The stability constants of the H M P A complexes of silver(I) were determined by potentiometric titration with the concentration of H M P A ranging from 0.1 to 1.00 M and the concentration of silver(I) approximately millimolar[3]. In sulfolane the ionic strength was maintained at 0.1 with tetraethylammonium perchlorate to prevent the solvent from freezing at room temperature, and the concentration of H M P A was varied from 0.01 to 1.00 M. Since no solid solvate of silver nitrate or silver perchlorate with H M P A has been reported, an attempt was made to prepare such a solvate by recrystallizing silver nitrate from HMPA. Upon gentle heating of a gram of silver nitrate with a limited amount of H M P A for a few minutes on a hot plate, a flame a foot high resulted. No further attempt was made to prepare a solid solvate. RESULTS AND DISCUSSION The formal reduction potential of silver ion in H M P A was + 0"386 ± 0.004 V vs. SCE. The oxidation of ferrocene to ferricinium ion at the rotating platinum electrode took place in H M P A at an E1,2 of + 0.514 ± 0.005 V vs. SCE with Ea~4-Eu~ of 0.056 V at a current of 0.6 ~,A. Thus, the formal reduction potential of silver ion in H M P A vs. ferriciniunffferrocene is - 0 . 1 3 _+0"01 V, the same as the value in dimethyl sulphoxide (DMSO)[3]. This was unexpected since many other properties of H M P A indicate that it has a greater tendency to coordinate than DMSO[1,2]. Perhaps steric problems result in a weaker solvation of the silver ion in H M P A than might be expected otherwise. The values of the stability constants of H M P A complexes of silver(I) in various solvents are given in Table 1. The method of calculation of stability constants from potentiometric data has been Table 1. Stability constants of H M P A complexes of silver(I) in various solvents Solvent
log fll
log fl~
log fla
Nitroethane 8.8 -+ 0.1 Sulfolane 4"04 ± 0.10 4.30 - 0-10 5"20 -4-0.05 Acetone 6.17 -+ 0.10 2-Butanol 0-68_+0-10 0.97_+0.10 0.3_+0.2 Methanol - 0 . 2 ±0-1 0.2_+0-1 Acetonitrile 0.30 ± 0.10 0.34 +--0.20 0.30 -4-0.10 Water No complexes could be detected
log f14
1.48+_0.10
described[3]. The relative stabilities of the H M P A complexes in the various solvents is similar to that found for the corresponding DMSO [3] and dimethyl formamide (DMF) complexes [5]. H M P A is known to be a powerful donor in forming hydrogen bonds [2]. In all solvents the H M P A complexes of silver(I) are more stable than the corresponding DMSO or D M F complexes.
Department of Chemistry and Chemical Engineering Michigan Technological University Houghton, Mich. 49931 U.S.A.
D E A N C. L U E H R S
3. D. C. Luehrs, R. W. Nicholas and D. A. Harem, J. electroanal. Chem. 29, 417 (1971). 4. D. C. Luehrs, R. T. Iwamoto andJ. Kleinberg, Inorg. Chem. 5,201 0966). 5. D. C. Luehrs, J. inorg, nucl. Chem. 33, 2701 (1971).
791
RESULTS Shown in Table 1 are the compound solubilities expressed in m-mole of salt per 1. of saturated solution. The results from each run as well as the average values are reported. The errors appearing in the column marked "Solubility" are the errors in solubility due only to counting statistics while the error listed with each average value is the experimental standard deviation. The relative standard standard deviations generally range between 2 and 6% which is what may be expected from I N A A techniques without elaborate sample handling. DISCUSSION The solubility trend of the alkali chlorides in sulfolane parallels that for water. For the alkali metal series, Li through Cs, both solvents show a minimum in molar solubility at KCI, and similar solubilities for the potassium and ammonium chlorides. In this respect the solubilities in sulfolane do not show a resemblance to those in SO2 as suggested by Drago [5]. In liquid SO2 the solubility minima for the alkali chlorides and bromides are reached at sodium and the solubilities of the corresponding ammonium halides do not correspond closely to those of the potassium salts. The solubility trend for the series of anhydrous alkaline earth metal chlorides in sulfolane does not correspond exactly to the trend in solubility for these salts in water. A steady decrease in solubility in sulfolane is observed with increasing cation size, while a maximum in solubility is reached at calcium for this series of salts in water. However, this solubility trend in sulfolane is in agreement with the observed trend in the free energies of solution for these salts in aqueous solution [6]. The apparent difference between the order of solubilities and free energies of solution in water for this series can perhaps be attributed to the formation of a new solid magnesium chloride hydrate phase. If such a hydrate phase were to form before the equilibrium between the anhydrous salt and the solution phase were attained, magnesium chloride would appear to be less soluble than the calcium salt.
Acknowledgements-We are grateful to Professors Harry Freund, R. A. Schmitt and T. H. Norris for helpful discussions and to the Triga reactor staff of the Radiation Center for their assistance in the neutron activations. This work was supported in part by a research grant from the Pacific Northwest Pulp and Paper Association.
Department of Chemistry Oregon State University Corvallis, Oregon 97331
J. A. S T A R K O V I C H M. J A N G H O R B A N I
5. R. S. Drago and K. F. Purcell, Co-ordinating Solvents. In Non-Aqueous Solvent Systems Edited by T. C. Waddington p. 246. Academic Press, New York (1965). 6. U.S. National Bureau @Standards Circular 500. U.S. Government Printing Office, Washington, D.C. (1952).
J. inorg, nucl. Chern,, 1972, Vol. 34, pp. 791-792.
Pergamon Press.
Printed in Great Britain
Hexan~thylphosphoramide complexes of silver(I) in solution (Received 12 July 1971 ) HEXAMETHYLPHOSPHORAMIDE (HMPA) is a good coordinating solvent[l]. A number of H M P A complexes have been prepared in the solid state[l, 2], but relatively little is known of the stability of H M P A complexes of metal ions in solution. Consideration of the properties of H M P A [1,2] would indicate that such complexes should exist in solvents such as nitroethane or acetone. 1. V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions p. 159. Springer-Verlag, Vienna (1968). 2. H. Normant, Angew. Chem. Intern. edn. 6, 1046 (1967).
792
Notes
EXPERIMENTAL Research Organic/Inorganic Chemical Corp. H M P A was purified by fractional distillation under vacuum from calcium hydride at 75°C. H M P A from Aldrich Chemical Co. could not be freed of a substance which reduced silver nitrate even with repeated distillation under vacuum. The other materials and the determination of the formal reduction potential of silver ion in H M P A were as previously described [3]. Silver nitrate was the source of silver ion in HMPA. The voltammetric determination of the ferricinium/ferrocene potential was as described before[3,4], except that a Moseley 7035A X - Y recorder was used. The stability constants of the H M P A complexes of silver(I) were determined by potentiometric titration with the concentration of H M P A ranging from 0.1 to 1.00 M and the concentration of silver(I) approximately millimolar[3]. In sulfolane the ionic strength was maintained at 0.1 with tetraethylammonium perchlorate to prevent the solvent from freezing at room temperature, and the concentration of H M P A was varied from 0.01 to 1.00 M. Since no solid solvate of silver nitrate or silver perchlorate with H M P A has been reported, an attempt was made to prepare such a solvate by recrystallizing silver nitrate from HMPA. Upon gentle heating of a gram of silver nitrate with a limited amount of H M P A for a few minutes on a hot plate, a flame a foot high resulted. No further attempt was made to prepare a solid solvate. RESULTS AND DISCUSSION The formal reduction potential of silver ion in H M P A was + 0"386 ± 0.004 V vs. SCE. The oxidation of ferrocene to ferricinium ion at the rotating platinum electrode took place in H M P A at an E1,2 of + 0.514 ± 0.005 V vs. SCE with Ea~4-Eu~ of 0.056 V at a current of 0.6 ~,A. Thus, the formal reduction potential of silver ion in H M P A vs. ferriciniunffferrocene is - 0 . 1 3 _+0"01 V, the same as the value in dimethyl sulphoxide (DMSO)[3]. This was unexpected since many other properties of H M P A indicate that it has a greater tendency to coordinate than DMSO[1,2]. Perhaps steric problems result in a weaker solvation of the silver ion in H M P A than might be expected otherwise. The values of the stability constants of H M P A complexes of silver(I) in various solvents are given in Table 1. The method of calculation of stability constants from potentiometric data has been Table 1. Stability constants of H M P A complexes of silver(I) in various solvents Solvent
log fll
log fl~
log fla
Nitroethane 8.8 -+ 0.1 Sulfolane 4"04 ± 0.10 4.30 - 0-10 5"20 -4-0.05 Acetone 6.17 -+ 0.10 2-Butanol 0-68_+0-10 0.97_+0.10 0.3_+0.2 Methanol - 0 . 2 ±0-1 0.2_+0-1 Acetonitrile 0.30 ± 0.10 0.34 +--0.20 0.30 -4-0.10 Water No complexes could be detected
log f14
1.48+_0.10
described[3]. The relative stabilities of the H M P A complexes in the various solvents is similar to that found for the corresponding DMSO [3] and dimethyl formamide (DMF) complexes [5]. H M P A is known to be a powerful donor in forming hydrogen bonds [2]. In all solvents the H M P A complexes of silver(I) are more stable than the corresponding DMSO or D M F complexes.
Department of Chemistry and Chemical Engineering Michigan Technological University Houghton, Mich. 49931 U.S.A.
D E A N C. L U E H R S
3. D. C. Luehrs, R. W. Nicholas and D. A. Harem, J. electroanal. Chem. 29, 417 (1971). 4. D. C. Luehrs, R. T. Iwamoto andJ. Kleinberg, Inorg. Chem. 5,201 0966). 5. D. C. Luehrs, J. inorg, nucl. Chem. 33, 2701 (1971).