386
recently observed that in methanesulfonic acid-water mixtures the solvent composition at which the coil-tohelix transition of poly-L-glutamic acid occurs does not change upon changing the molecular weight of the polymer over a wide range.6 There is an additional factor which may account for the differences between the two polymers. Noncovalent bonds among side chains might contribute differently to the conformational stability of PCHA and PLP. Already it has been shown that noncovalent interactions like hydrophobic bands among side chains are responsible for the enhanced conformational stability of PLP in aqueous solution^.^ It is presently well known that nonpolar paraffin chains in aqueous solutions form hydrophobic bonds and tend to aggregate, the driving force for aggregation being the increase of entropy or decrease of order of the water structure involved in the process.21p22 Strong acids like sulfuric acid d o resemble water very closely in the fact that they possess a structure in which acid molecules are very strongly hydrogen bonded. 23,24 Long-chain fatty acids form micelles in concentrated sulfuric acid and in sulfuric acid-water mixtures. It was shown that this phenomenon is due to the favor(21) G. Nemethy and H. A. Scheraga, Angew. Chem. Intern. Ed. Engl., 6,195 (1967). (22) P. Mukerjee, Aduan. Colloid Sci., 1, 241 (1967). (23) R. J. Gillespie, E. D. Hughes, and C. K. Ingold, J . Chem. Soc., 2473 (1950). (24) R. J. Gillespie in “Physico-Chemical Processes in Mixed Aqueous Solvents,” F. Franks, Ed., Heineman Education Books Ltd., London, 1967, p 130.
able entropy changes going from individual fatty acid molecules with solvent highly organized around the paraffin chain to micelles where the aggregation decreases the order of the solvent structure.25 Micelle formation studies in methanesulfonic acid are not reported in the literature, but very probably noncovalent bonds between nonpolar solutes might form in this acid and in acid-water mixtures as well. Therefore it is possible that the attitude of the cyclohexane side chains of PCHA to form “acidophobic” bonds is different from that of the phenyl groups of PLP. In this respect one might observe that the side chains of PCHA are much less polar than the side chains of PLP. Phenyl groups can in fact be considered nonpolar only when they are viewed parallel to the plane of the ring; both faces of the ring are polar. As a consequence there is the possibility that stronger noncovalent bonds are formed in PCHA than in PLP. This fact could enhance the stability of the a-helical structure of PCHA, with respect to PLP, in spite of the unfavorable steric interference between the side chains and the peptide backbone. Acknowledgments. The authors express their gratitude to Professor Ernest0 Scoffone for continuous and stimulating discussions during this work, and to Professor Joseph Steigman, Polytechnic Institute of Brooklyn, for useful suggestions at the beginning of this research. (25) J. Steigman and N. Shane, J , Phys. Chem., 69,968 (1965).
Crystalline Salt Complexes of Macrocyclic Polyethers C. J. Pedersen
Contribution No. 209 from the Elastomer Chemicals Department, Experimental Station, E. I . du Pont de Nemours and Company, Inc., Wilmington, Delaware 19898. Received February 19, 1969 Abstract: The stoichiometry of the crystalline complexes of sodium, potassium, ammonium, rubidium, cesium, and barium salts with the subject polyethers has been investigated in greater detail. Depending on the relative sizes of the “hole” in the cyclic polyether and of the cation, complexes with po1yether:cation mol ratios of 1:1, 3: 2, or 2: 1 are obtained. The 2: 1 and 3: 2 complexes might possibly be “sandwich” structures. By preferential complex formation in methanol, potassium and cesium ions have been separated almost quantitatively.
T
he preparation and properties of a number of macrocyclic polyethers have been described previous1y.l Due to the cumbersomeness of the nomenclature of these compounds, a simplified method of naming them,’ as shown by the examples in Figure 1, will be used. Certain polyethers, particularly those containing 5 to 10 oxygen atoms each separated from the next by 2 carbon atoms, were shown to form crystalline complexes of alkali and alkaline earth salts, such as chlorides, iodides, and thiocyanates. It was originally reported that the stoichiometry of these complexes is always one molecule of polyether per ion regardless of its valence. It is ( I ) C. J. Pedersen, J . Amer. Chem. SOC.,89, 7017 (1967),
the purpose of this paper to report that more recent work indicates that the stoichiometry is not as simple as previously assumed. Some indication that the stoichiometry is not strictly 1:1 is evident in the published data.’ For example, in Table VII’ the ratio of moles of XXVIII dissolved per mole of cesium thiocyanate is given as 1.20, indicating that a mole of this salt interacts with more than a mole of the polyether in methanol. In Table XIV,’ whereas the melting points of the XXVIII complexes of sodium, potassium, ammonium, and rubidium thiocyanates are sharp, that of the cesium salt is unsatisfactory. Also, Pressman found that a rubidium ion can interact in solution with more than a single molecule of dicyclohexyl18-crown-6.
Journal of the American Chemical Society / 92:2 / January 28, 1970
387
A systematic overestimation of the diameters of the "holes" in the cyclic polyethers, based on van der Waals radii, decisively influenced the earlier conclusions. The original estimates and the more correct values for the diameters are shown in Table 1.
D.
BENZO-15-CROWN-5
Po? Table I.
Diameters of Holes,
A
~~
Polyet hers All All All All
14-crown-4 (e.g., XV) 15-crown-5 fe.g., IV) 18-crown-6 (e.g., XXVIII) 21-crown-7 (e.g., XXXIII)
Original estimates 1.8 2.7 4.0 Over 4
Revised 1.2"-1.5b 1.7-2.2 2.6-3.2 3.4-4.3
According to Corey-Pauling-Koltun atomic models. * According to Fisher-Hirschfelder-Taylor atomic models. The ionic diameters of the cations- dealt with in this gaper are: sodium, 1.90 A ; pota!sium, 2.66 A ; ~mmonium,2.84 A ; rpbidium, 2.96A; cesium, 3.34 A ; silver, 2.52 A ; and barium, 2.70 A.
The diameter of the "hole" in XXVIII, the compound with which mqst of the work had been done, was believed to be 4 A and this is large enough to accommodate any uncomplexed metal cation. Since the cationic portion of the complexes was thought to be a cyclic polyether with a cation nestling in the center of the hole, it did not seem plausible that a second molecule of the polyether could be involved in complex formation. Hence, the crystalline complexes described in the previous paper' were prepared by reacting a mole of polyether with a mole of salt, or with an excess of salt, but never with an excess of polyether. This omission has now been corrected and several new examples of crystalline complexes show that the stoichiometric ratio of polyether to cation can be as high as 2: 1 .
Results and Discussion Methods of Preparation of Crystalline Complexes. The complexes were prepared in methanol but, unfortunately, they could not all be prepared under identical conditions. In general, three different methods were used, the first yielding the most reliable results and the third one the least. Method 1. Different proportions of polyether and salt are reacted in methanol at about 60", filtered hot, and allowed to stand at room temperature. Crystals that form either on cooling or on the loss of some solvent, but never approaching dryness, are recovered, washed with cold methanol, and dried. Method 2. A solution of the salt in methanol is mixed with different proportions of a suspension of polyether in methanol and the resulting slurry is stirred at about 60" for 5-10 min. The crystals, after cooling, are recovered as above. Method 3. Different proportions of polyether and salt are mixed in methanol and the clear solution is allowed to evaporate at room temperature until solids separate, often requiring the removal of almost all the solvent. The crystals, if recovered by filtration, are either not washed with methanol or only sparingly. (2) B. C. Pressman, Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pa., private communication.
Xxnm
XXH
I:B E N Z O 0 :
OIBENZO-10-CROWN-6
CYCLOHEXYL D I S Y C L O H E X Y L - 1 8 - C R O W N ' 6
Figure 1. Structural formulas of cyclic polyethers: first number in the name = total number of atoms in the polyether ring; second number in the name = number of oxygen atoms in the polyether ring.
In order to give greater significance to the melting points of the complexes, the melting points of the uncomplexed compounds are listed in Table 11. Table II. Melting Points of Uncomolexed Comoounds
Compound Benzo-15-crown4 (IV) Dibenzo-15-crown4 (XXV) Benzo-18-crown-6 (X) Dibenzo-18-crown-6 (XXVIII) Bis(butylbenzo)-l8-crown-6(XXIX) Dibenzo-21-crown-7 (XXXIII) Dibenzo-24-crown-8 (XXXV) Dicyclohexyl- 18-crown-6 (XXXI) (a mixture of 2 isomers) Sodium thiocyanate Potassium thiocyanate Ammonium thiocyanate Rubidium thiocyanate Cesium thiocyanate Barium thiocyanate Silver nitrate Cesium iodide
Formula
MP, "C
79-79.5 C18HzoOj 113.5-1 15 Cl6H2406 43-44 C20H2406 164 C2&4[106 135-137 CzzH2807 106.5-107.5 Cz4H& 113-1 14 C Z O H ~ GBetween O~ 36 and 56 NaCNS 287 KCNS 173 N H K N S 150 RbCNS 188-190 CSCNS 204-205 Ba(CNS)z Over 290 AgN03 212 CSI 621 C14H2006
Resuits The compositions and the melting points of the welldefined crystalline complexes and the methods of their preparation are given in Table 111. Except for no. 12, these are considered to be complexes of proven composition and comprise 1:1 and 2: 1 complexes. When equal moles of benzo- 15-crown-5 and potassium thiocyanate were treated according to method 1, a 38% yield of crystalline 2:1 complex was obtained. Apparently there is no tendency for a 1 :1 complex of these components to crystallize out of methanol. When 3 mol of dibenzo-18-crown-6 and 1 mol of cesium thiocyanate, were treated according to method 1, unreacted polyether and a 70 % yield of crystalline 2: 1 complex were obtained, and no evidence was found for the formation of crystalline complexes of a ratio above two polyethers per cation. Unusual complexes or those with less satisfactory melting point and/or composition are shown in Table IV. Number 1 is a mixture of benzo-15-crown-5 and its 1 :1 complex with sodium thiocyanate, and no. 4 is a mixture of dibenzo-18-crown-6 with its 1 :1 complex with potassium thiocyanate. Pedersen
Salt Complexes of Macrocyclic Polyethers
388 Table 111. Well-Defined Crystalhe Complexes (Mol polyether)/ (mol salt) Crystalline Reac- Comcomplex Method tants plex
No,
Benzo-15crown-5 NaCNS KCNS NH4CNS CsCNS AgN03
1 2 3 4 5
6 Ba(CNS)? Dibenzo-15crown-5 7 KCNS Dibenzo- 18crown-6 8 KCNS 9 KCNS 10 RbCNS 11 RbCNS 12 CsCNS 13 CsCNS 14 CsI
Mp, "C
Yield,
-
Calcd, Z H N
--
Z
Formula
50 77 81
C16HzoNOsSNa CzgHroNOioSK GoH44NzOioS CzilHaoNOioSCs CirHzoNOsAg
51.6 55.0 56.9 47.9 38.3
51.5 55.0 56.8 48.0 38.3
C
S
C
--
Found, Z H N
S
1 2 2 1 1
1:1 2:l 2:l 2:l 2:l
1:l 2:l 2:l 2:l 1:l
162-165 176 131-132 127 134-135
59
3
2:l
2:l
169-171
17
5.7 4.0 9 . 2 6 . 3 2 . 2 5.1 7 . 2 4 . 6 5.2 5 . 5 1.9 4.4 4 . 6 3 . 2 Ag = 24.7 C a o H a o N 2 0 1 0 ~ B45.6 a 5 . 1 3.5 8.1
3
2:1
2:l
143-144
27
Ca7HaaNOioSK
60.9 5 . 5 1 . 9
4.4
60.2 5 . 7 1.8
5.0
1 1 1 1 1 1 1
1:1 2:1 1:1 2:1 1:l" 2:1 2:1
1:l 1:l 1:l 2:l
245-246 246-247 184-185 175-176
83 33 >37 21
G~HZ~NOOSK GiHzaN06SK C2iHz4NO6SRb C41H48N01zSRb
55.1 55.1 50.0 57.0
7.0 7.0 6.4 3.7
55.6 55.6 50.1 56.5
7.0 6.6 6.4 3.9
2:l 2:l
146-147 115-116*
-~
58 53
5.2 3.1 5 . 2 3.1 4 . 8 2.8 5.6 1.6
C ~ ~ H ~ ~ N O I Z S54.0 C S 5 . 3 1.5 3.5 CtoH4sOizIC~ 49.0 4 . 8 I 13.0
5.7 6.0 7.0 5.3 4.4
4 . 1 9.5 2.3 5.0 4.5 5.4 2.3 4.8 3.2 Ag = 24.0 45.5 5.1 3.7 8 . 5
4.9 5.2 4.6 5.4
3.2 3.0 3.2 1.7
53.2 5 . 3 1 . 7 3.9 48.2 5 . 0 I = 13.0 ~
The spurious 1 :1 complex described previously1was found to be a mixture of (dibenz0-18-crown-6)~-CsCNS complex mixed with CsCNS. On recrystallization from methanol, 2 :1 crystals of satisfactory composition were obtained which melted at 149-150". All melting points are uncorrected. * A portion did not melt even at 270'. a
Table IV. Less Well-Defined Crystalline Complexes
No.
Crystalline complex
Benzo-15-crown-5 1 NaCNS
(Mol polyether)/ (mol salt) Reac- ComMethod tants plex 3
Benzo-18-crown-6 2 KCNS 3 3 CsCNS SpeciaP Dibenzo-18crown-6 4 KCNS Speciala 5 CsCNS 1 Bis(butylbenz0)18-crown-6 6 CsCNS 1 Dibenzo-21crown-7 7 CsCNS 1 Dibenzo-24crown-8 8 CsCNS 1 Dicyclohexyl-18crown-6 9 KII Speciala
Mp, "C
Yield,
z
Formula
S
-Found, C H
%N
S
2:l
Softat 80 150-164
38 G9HaoNOlaSNa 56.4 6.5 2 . 3
5.2 56.9 6 . 4 2.3 5.7
2:l 2:l
1:1 3:2
112-132 103-104
68 C I ~ H Z ~ N O ~ S K49.9 5.9 3.4 77 C~oH~zNgOisSzCS2 45.5 5 . 5 2.1
7.8 50.4 5.5 3.6 7.8 4.9 45.2 5 . 5 2.4 5 . 3
2:l 1 :1
2: lb 3:2
164-233 145-146
64 C ~ I H ~ ~ N O I Z S K60.2 5.9 1 . 7 50.9 S Z 4.9 1.9 85 C ~ Z H ~ Z N ~ ~ & C
3.9 61.6 5.8 1.5 3.5 4.4 50.7 5.1 1.9 4 . 4
2:l
2:l
108-116
2:l
2:1
2:l
>44 Cs7H8oNOizSCs
60.3 7 . 1 1.2
2.8 59.6 6.7 1 . 3 3.0
S o f t a t 4 0 ca. 20 C&&OlrSCS
54.0 5.6 1 . 4
3.2 54.1 5.1 1 . 6 3.2
1:l
194-195
76 GoHaeo~IsK
30.3 4.6 28.6 4.3
2:l
Special"
2:l
1:l
193-195
68 CzoH360eI3Rb
11 CSIs
Speciala
2:l
3:2
112-114
54 CaoHlosOisI&h
See the Experimental Section.
%N
2:l
10 RbI3
Q
-Calcd, C H
I K
= = = = =
I Rb 33.6 5.0 1 CS =
48.1 30.8 4.6 I = 46.0 4.9 K = 5.4 45.4 28.6 4.4 I = 41.5 Rb = 11.0 10.2 35.5 33.5 4.9 1 = 34.8 12.4 CS = 11.5
Pure 1 :1 complex crystallized out of the filtrate on standing at room temperature,
Table IV, no. 3, 9, 10, and 11 had to be prepared by evaporating the solutions to dryness and carefully removing the unreacted polyethers without decomposing the complexes with a solvent in which the complexes are not _ ... significantly soluble at room temperature. ThLre are tcree examples of 3 :2 complexes listed in Table Iv. Their melting points are sharp and their analyses are acceptable. Their molecular weights were ~
Journal of the American Chemical Society I 92:2
not determined because of probable lack of significance until more is known about the behavior of these complexes in solution in regard to stability and degree of ionization. These complexes will be discussed later. The powder X-ray diffraction patterns3 of dibenzo-18(3) We thank J. S. Proctor, Jackson Laboratory, Organic Chemicals Deoartment. E. I. du Pont de Nemours and Co., for kindly obtaini n l r n e patterns,
I January 28, 1970
389
crown-6 and the complexes of the following thiocyanates showed that: (1) they are all free of uncomplexed polyether and salt, (2) the patterns of 1 :1 potassium, 1 :1 and 2 : 1 rubidium, and 2: 1 cesium are distinctly different, but (3) that of 3 :2 cesium is basically the same as that of 2 : 1 cesium except for intensity differences. The spectrum of dibenzo-18-crown-6 (KBr pellet) obtained with a Perkin-Elmer grating infrared spectrophotometer Model 221 has an absorption band at 10.02 p. This band is not evident in the spectra of 1 : 1 complexes of this polyether with sodium, potassium, rubidium, and barium thiocyanates, the 2: 1 complexes of cesium thiocyanate and iodide, and the 3 :2 complex of cesium thiocyanate. However, the so-called 2 : 1 complex of potassium thiocyanate (Table IV, no. 4) and the 2 : 1 complex of rubidium thiocyanate (Table 111, no. 11) still show this band, the relative absorbance of the complexes being about 50% of the polyether’s.
Discussion It must be stated at the outset that nothing definitive can be said about the structure of any of these crystalline complexes because single-crystal X-ray analysis has not as yet been successfully applied to this problem. However, sufficient evidence supports the conclusion that the dibenzo-18-crown-6 complexes of the following thiocyanates are of proven composition and integrity: 1 :1 potassium, 1 :1 and 2 : 1 rubidium, and 2 :1 and 3 :2 cesium. The 2:l complex of rubidium cannot be a mixture of the 1 :I complex with dibenzo-18-crown-6 because its X-ray pattern does not contain the elements of the polyether pattern. The infrared spectrum of this complex, however, suggests that the two polyether molecules might not be made complex in an identical way. The 3:2 complex of cesium is an interesting case. The infrared spectrum seems to indicate that the polyether molecules are complexed as in the 2:l complex, and the similarity of the two X-ray patterns also suggests that the polyether molecules and the cations are arranged in a similar manner. In the absence of experimental proof of structures, the 2: 1 and 3:2 complexes invite speculation concerning them. The cationic portion of the 2:l complex has a structure consisting, in some way, of (crown)-(cation)(crown), and that of the 3 :2 complex a (crown)
