- Email: [email protected]
Some amine-interhalogen electron donor-acceptor systems
J. lnorg. Nucl. Chem., 1964, Vol. 26, pp. 1405 to 1410. P e r g a m o n Press Ltd. Printed in N o r t h e r n Ireland
SOME AMINE-INTERHALOGEN ELECTRON DONOR-ACCEPTOR SYSTEMS R. D. WHITAKER Department of Chemistry, University of South Florida, Tampa, Florida (Receiced 15 November 1963)
Abstract--Several new interhalogen-amine addition compounds have been prepared: 4-Picoline.lBr: Isoquinoline.IBr; Acridine.IBr; Isoquinoline-ICl; Acridine.lCl; Isoquinoline.ICla; Acridine.lCl:~. In addition to 1:1 type compounds, thermal studies have shown the existence of: Pyridine.21Br: lBr.2Acridine; ICI.2Acridine; IC13-2Acridine. A compound, Acridine.21Br apparently exists, but loses one mole 1Br at 110° to form the 1 : 1 compound. Trends among the melting points of the 1 : 1 addition compounds are interpreted as indicating a decrease in electron acceptor strength of the interhalogens in the order ICla ICI • IBr. Addition compounds of non-l:l stoichiometry are :also briefly discussed. A NUMBER o f f o r m a t i o n constants o f 1:1 complexes in solution between iodine, iodine m o n o b r o m i d e , a n d iodine m o m o c h l o r i d e and a variety o f electron d o n o r s 11.% have been d e t e r m i n e d s p e c t r o p h o t o m e t r i c a l l y . The results show that the strengths as electron acceptors o f these interhalogen c o m p o u n d s decrease in the o r d e r predicted by SCOTT: ('~) IC1 > i B r ~- i,,. It has not been possible to d e t e r m i n e the relative strength o f iodine trichloride as an electron a c c e p t o r using similar s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s because o f the high degree o f dissociation o f iodine trichloride into iodine m o n o c h l o r i d e and chlorine in dilute solutions. <4t Stable, solid 1 : 1 molecular a d d i t i o n c o m p o u n d s o f these interhalogen c o m p o u n d s . including iodine trichloride, with amines are well known, however./a~ On the basis o f the physical p r o p e r t i e s o f some o f these a d d i t i o n c o m p o u n d s , the suggestion has been m a d e that the c o m p o u n d s involving iodine trichloride are more ionic in c h a r a c t e r t h a n a n a l o g o u s iodine m o n o c h l o r i d e a d d i t i o n c o m p o u n d s . This suggestion implies that iodine trichloride is a stronger electron a c c e p t o r t h a n iodine m o n o c h l o r i d e . Using this idea, one would expect a d d i t i o n c o m p o u n d s involving iodine m o n o b r o m i d e to have less ionic character than their iodine chloride analogs. Molecular a d d i t i o n c o m p o u n d s between a series o f halo-substituted pyridines and iodine m o n o b r o m i d e a n d iodine m o n o c h l o r i d e "~t have been reported. The physical properties o f the a d d u c t s indicate that the iodine m o n o c h l o r i d e a d d i t i o n c o m p o u n d s are indeed m o r e ionic in character than c o r r e s p o n d i n g iodine m o n o b r o m i d e a d d i t i o n c o m p o u n d s . The present w o r k reports the p r e p a r a t i o n o f several m o r e a d d i t i o n c o m p o u n d s between the three interhalogen c o m p o u r l d s and a series o f structurallv related amines in o r d e r that a more nearly c o m p l e t e c o m p a r i s o n o f the properties o f these c o m p o u n d s m a y be made. ~ R. D. WrlIFAKERand H. H. SISLER,J. Plo's. Chem. 67, 523 (1963). ~e~A. 1. PoPov and R. H. RvGc., J. Amer. Chem. Soc. 79, 4622 (1957). ~31 R. L. Scorr, J. Amer. Chem. Soc. 75, 1550 (1953). '~ R. D. WHIrAKERand G. B. FROZZARD, Va. J. Sci. 14, 6 (1963). "~ R. D. Wm~AKER, J. R. AMBROSEand C. W. HICKAM,J. Ino~:g. Nttcl. Chem. 17, 254 ([961). ~G~M. T. ROGERSand W. K. MEYER,J. Phys. Chem. 66, 1397 (1962). 1405
1406
R . D . WHITAKER
Some o f these systems have been studied by the construction of phase diagrams. The results for systems involving both o f the iodine chlorides with pyridine and quinoline have been reported/7, 8) In addition to 1:1 adducts, c o m p o u n d s o f the type B.2IC1 and B.2IClz, where B is a molecule o f pyridine, form in the solid state. M o r e interesting, however, was the discovery o f c o m p o u n d s o f the type ICI3.2B , where B is a molecule either o f pyridine or quinoline. N o analogous c o m p o u n d s were f o u n d to exist in the solid state for iodine monochloride. In an attempt to elucidate this p h e n o m e n o n , phase diagrams have been constructed for binary systems consisting o f iodine m o n o b r o m i d e with pyridine, quinoline, and acridine. Portions o f the binary systems, iodine monochloride-acridine and iodine trichloride-acridine were also investigated. TABLEI.--MELTING POINTSOF SOME 1:1 INTERHALOGEN-AMINECOMPOUNDS Amine
m.p. IBr compd. (°C)
Pyridine 2-Picoline 4-Picoline 2,6-Lutidine Quinoline Isoquinoline Acridine
m.p. ICI compd. (°C)
115~ 67-68 I~ 78"0-78.5 106-108121 132c6~ 143-144 168-171
m.p. IC13compd. (°C)
133I~l 77.5t21 107-108 tSI 112-113c~1 150-152 c5~ 155-156 178-180
175-17T5~ 131-133t51 133-134 t~) 103-10515 134-139 ~ 148-158" 212-213
* Melting point apparatus pre-heated to 140° before insertion of capillary. TABLE2.--ANALYTICALDATAFOR PREVIOUSLYUNREPORTEDCOMPOUNDS Compound 4-Picoline.IBr Isoquinoline.IBr Acridine.IBr Isoquinoline.IC1 Acridine.ICl Isoquinoline.ICl3 Acridine'ICl3
Calculated I.E.* 150 I.E. 168 I.E. 193 CI, 12-16 CI, 10.38 C, 29.82; H, 1-95; N, 3"87; C1, 29'35 CI, 25'79
Found I.E. 151 I.E. 171 I.E. 199 C1, 12-10 CI, 10.81 C, 29.73; H, 2.07 N, 3'85; Cl, 30"63 CI, 25.79
* Iodometric equivalent. RESULTS
Comparison o f 1 : 1 molecular addition compounds Table 1 presents the pertinent data regarding the stable, solid 1 : 1 addition comp o u n d s between the interhalogen c o m p o u n d s and a corresponding series of heterocyclic amines. Analytical data are presented in Table 2 for the previously unreported c o m p o u n d s .
Thermal studies IBr-pyridine system. The phase diagram (Fig. 1) shows the existence of two c o m p o u n d s , C~H~N.2IBr which melts at 53 °, and the k n o w n 1:1 c o m p o u n d which ~7~R. D. WnlTAKERand J. R. AMBROSE,J. Inorg. Nucl. Chem. 24, 285 (1962). 181YA. A. FIALKOVand I. D. MUZYKA,J. Gen. Chem. Moscow, 18, 1205 (1948).
amine-interhalogen electron donor-accepto, systems
Some
1407
12'0 II0 J ~,0 9O
8O
60
,q© o) c~ o2 o_ E
O
~--
C
\
C -. I -!©
I 50 - -~-0,
r
i
iC,
20
5(
4C 50 Mole pyridine,
G~
7'2
~2',
-~
:
~%
l:lG. 1. The system lBr-pyridine.
120
i O9
) -4(
I
1
t
I0
20
30
F [ ( ; . 2.
I
I
r
40 50 60 Mole q u i n o l i n e ,
[
I
70
80
i --390
I00
%
The system [Br-quinoline.
melted at I I 0 (lit. I ~ ) ] | 5 ) . Eutectics occur at I0 ~with a composition of19 mob petcent pyridine, at 40 ° with a composition of 38 mole per cent pyridine, and at 48 with a composition of about 97 mole per cent pyridine. | B r quinoline system. Only the I :] compound was found to exist. (Fig. 2). It melted at 125<~(lit. 6 132°) and the eutectics were found at ]0 ° and about 18 mole per cent quinoline and at --37 ° with an approximate composition of 99 per cent mole quinoline. Glasses tended to form upon cooling mixtures with compositions near the
1408
R.D. WHITAKER
eutectics. It was therefore impossible to locate the eutectics with a high degree of accuracy. IBr-acridine system. The phase diagram for this system (Fig. 3) shows that a compound IBr.2C13HgN which melts at 131 ° is formed as well as the 1 : 1 compound which melted at about 160 °. The shape of the freezing curve indicates that the former compound is rather highly dissociated at the melting point. A melting point of 168171 ° for the 1 : 1 compound, which is reported in Table 1, was obtained on a pure sample prepared by precipitation from solution. The dotted line represents the 180 170 160
J50 140
1:50 120
611o
-
.__.j
/
,oo
~ 9o I-.-
70 60
50 40~
3O 20---
-
I0 m
0
JO
20
30
40
50
Mole acridine,
60
70
80
90
I00
%
FIG. 3.--The system IBr-acridine. author's conception of that portion of the diagram. There is apparently a compound of the type, CIsHgN.2IBr, in the system which loses iodine monobromide when heated to 110 ° to form the 1:1 compound. Eutectics occur at 22 ° with a composition of about 9 mole per cent acridine, 115 ° with a composition of 57.2 mole per cent acridine, and 96 ° with a composition of 91 mole per cent acridine. ICl-acridine and IC13-acridine systems. Only the acridine-rich portions of these systems were investigated in detail because of vigorous reactions which occurred upon warming mixtures in which the amount of interhalogen was large. Even with mixtures in which acridine was greater than 50 mole per cent, irreversible reactions involving the release of chlorine and iodine often occurred. It was thus found to be impossible to obtain data of sufficient precision to allow construction of the phase diagrams for either of these systems. However, enough data was accumulated to establish that compounds with a mole ratio of iodine chloride to acridine of 1 : 2 do form in both of the systems. The approximate melting points of these compounds were found to be: IC1.2CIsHgN, m.p. 120 ° and IC1s-2C1sHgN , m.p. 167 °. DISCUSSION
If one assumes that relative electron acceptor strength of the interhalogens, and hence ionic character of their adducts, decreases in the order, IC1S > IC1 > IBr,
Some amine-interhalogen electron donor-acceptor systems
1409
a good correlation is found a m o n g the melting points of the adducts listed in Table 1. In general, for a given donor, the melting points of the addition c o m p o u n d s increase x~ith increasing electron acceptor strength of the interhalogen. The data of Table I are presented in graphical form in Fig. 4. It is likely that all of the 1 : 1 addition c o m p o u n d s are best described in terms of salt-like structures, viz. BI + • • - - X (where X is Br or (Z1) and BICI2+ • • • CI--. A
200 --
r80 --
Pyridine oe 2 - Picoline 4 - Picoline [] 2, 6-Lutidine • Quinoline z~ Isoquinoline • Acridine
/ / / /
•/
•
C3 o
.2
-~ 140 o
I O0
60
IBr
ICI
ICI 3
Fzo. 4.--Meltingpointsof some l:I interhalogen-aminecompounds. Steric factors must be considered in the case of adducts between the relatively bulky iodine trichloride molecule and the highly hindered 2,6-1utidine molecule or the relatively unsymmetrical quinoline and isoquinoline molecules. It is not unreasonable to expect relatively smaller lattice energies for these compounds bccause of reduced symmetry of the adducts. The IC]2+ cation is a bent species, and it is reasonable to assume that the species, BICI2-'L is approximately T-shaped. Ai| of the I :i compounds are quite stable under ordinary conditions, it has been erroneously reported 16) that the compounds: Pyridine.ICl3 and 2,6-Lutidine.ICla lose chlorine at room temperature. This same report liststhe melting point of the Pyridine'lCl a c o m p o u n d as 143 °. Reference to the phase diagram for this system C7) shows clearly that the c o m p o u n d , Pyridine.21C13 was confused with the 1 : 1 compound. It is likely that the same error was made with the adduct involving 2,6-1utidine. Table 3 lists the melting points of addition c o m p o u n d s of other than 1 : l stoicheiometry. These melting points have been assembled from phase diagrams reported in the present work and in previous work.~T, s) O f the three donors considered, only for the most unhindered, pyridine, do we find stable c o m p o u n d s with two moles of interhalogen per mole of donor. It is reasonable to assume that these substances also
1410
R . D . WHITAKER
TABLE3.--MELTING POINTSOF INTERHALOGEN--AMINECOMPOUNDSOF NON-I:I S'[OICHEIOIVlETRIES* Melting Point, °C Donor(B)
Pyridine
B.2IBr
B.2ICI
B.2ICI3
53
35
140
69 (incongruent)
Unstable
81
Quinoline Acridine
Loses IBr above 110
Not investigated
Not investigated
IBr.2B
130
ICI.2B
120
ICI3'2B
167
* Blank spaces indicate compound does not exist in solid state. have considerable salt-like character, viz. Pyridine.I + . . . I X 2- (where X is CI or Br) a n d Pyridine-IC12+ • • • IC14-. It was suggested previously (r) that iodine trichloride a d d i t i o n c o m p o u n d s c o n t a i n ing two moles of d o n o r per mole of acceptor might be related to some unique b o n d i n g ability of iodine trichloride. This suggestion n o w seems less attractive in view of the fact that all three interhalogens form such 1:2 a d d u c t s with acridine. The fact that acridine molecules are paired in the solid state 19) s o m e w h a t similar to anthracene, p r o b a b l y aids in the f o r m a t i o n o f these adducts. However, it is possible that the iodine trichloride c o m p o u n d s of this type with pyridine a n d q u i n o l i n e are related to the crystal structures of the solids rather t h a n any b o n d i n g characteristics u n i q u e to iodine trichloride. EXPERIMENTAL
Chemicals. The preparation and purification of starting materials have been described. ~ Preparation of i : 1 addition compounds. These compounds were prepared by precipitation from CC14 solutions as previously described. (5~ Yields on most of the compounds prepared in this work were essentially quantitative. The colours of these compounds are varying shades of yellow with iodine trichloride adducts brightest and iodine monobromide palest. Analyses. Chlorine was determined as previously described.CS~ The iodometric equivalents for the IBr compounds were determined according to the method described by ROGERS and MEYER.(6~ The chloride analyses were used where possible because higher precision was obtained with them than with the iodometric determinations. The carbon, hydrogen and nitrogen analyses for Isoquinoline'ICl3 were done by Galbraith Laboratories, Knoxville, Tennessee. Thermal studies. Phase diagrams were constructed from cooling curves obtained by means of a calibrated copper--constantan thermocouple attached to a V.O.M. 5 B. & L. Recording Potentiometer. The binary compositions were prepared by direct weighing in an all-glass apparatus equipped with a micro-stirring bar so that the mixtures could be magnetically stirred while cooling. The entire apparatus was calibrated over the temperature range encountered in the studies by means of several standards of known melting points. The cold junction temperature was maintained at 0°. All points were determined at least twice, and the reported temperatures are believed to be accurate to within ±3 °. Acknowledgement--The author wishes to thank Mr. Y. C. FERNANDEZfor his assistance with some of the laboratory work. 19~D. C. PHILLIPS,Acta Cryst. 9, 237 (1956).
SOME AMINE-INTERHALOGEN ELECTRON DONOR-ACCEPTOR SYSTEMS R. D. WHITAKER Department of Chemistry, University of South Florida, Tampa, Florida (Receiced 15 November 1963)
Abstract--Several new interhalogen-amine addition compounds have been prepared: 4-Picoline.lBr: Isoquinoline.IBr; Acridine.IBr; Isoquinoline-ICl; Acridine.lCl; Isoquinoline.ICla; Acridine.lCl:~. In addition to 1:1 type compounds, thermal studies have shown the existence of: Pyridine.21Br: lBr.2Acridine; ICI.2Acridine; IC13-2Acridine. A compound, Acridine.21Br apparently exists, but loses one mole 1Br at 110° to form the 1 : 1 compound. Trends among the melting points of the 1 : 1 addition compounds are interpreted as indicating a decrease in electron acceptor strength of the interhalogens in the order ICla ICI • IBr. Addition compounds of non-l:l stoichiometry are :also briefly discussed. A NUMBER o f f o r m a t i o n constants o f 1:1 complexes in solution between iodine, iodine m o n o b r o m i d e , a n d iodine m o m o c h l o r i d e and a variety o f electron d o n o r s 11.% have been d e t e r m i n e d s p e c t r o p h o t o m e t r i c a l l y . The results show that the strengths as electron acceptors o f these interhalogen c o m p o u n d s decrease in the o r d e r predicted by SCOTT: ('~) IC1 > i B r ~- i,,. It has not been possible to d e t e r m i n e the relative strength o f iodine trichloride as an electron a c c e p t o r using similar s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s because o f the high degree o f dissociation o f iodine trichloride into iodine m o n o c h l o r i d e and chlorine in dilute solutions. <4t Stable, solid 1 : 1 molecular a d d i t i o n c o m p o u n d s o f these interhalogen c o m p o u n d s . including iodine trichloride, with amines are well known, however./a~ On the basis o f the physical p r o p e r t i e s o f some o f these a d d i t i o n c o m p o u n d s , the suggestion has been m a d e that the c o m p o u n d s involving iodine trichloride are more ionic in c h a r a c t e r t h a n a n a l o g o u s iodine m o n o c h l o r i d e a d d i t i o n c o m p o u n d s . This suggestion implies that iodine trichloride is a stronger electron a c c e p t o r t h a n iodine m o n o c h l o r i d e . Using this idea, one would expect a d d i t i o n c o m p o u n d s involving iodine m o n o b r o m i d e to have less ionic character than their iodine chloride analogs. Molecular a d d i t i o n c o m p o u n d s between a series o f halo-substituted pyridines and iodine m o n o b r o m i d e a n d iodine m o n o c h l o r i d e "~t have been reported. The physical properties o f the a d d u c t s indicate that the iodine m o n o c h l o r i d e a d d i t i o n c o m p o u n d s are indeed m o r e ionic in character than c o r r e s p o n d i n g iodine m o n o b r o m i d e a d d i t i o n c o m p o u n d s . The present w o r k reports the p r e p a r a t i o n o f several m o r e a d d i t i o n c o m p o u n d s between the three interhalogen c o m p o u r l d s and a series o f structurallv related amines in o r d e r that a more nearly c o m p l e t e c o m p a r i s o n o f the properties o f these c o m p o u n d s m a y be made. ~ R. D. WrlIFAKERand H. H. SISLER,J. Plo's. Chem. 67, 523 (1963). ~e~A. 1. PoPov and R. H. RvGc., J. Amer. Chem. Soc. 79, 4622 (1957). ~31 R. L. Scorr, J. Amer. Chem. Soc. 75, 1550 (1953). '~ R. D. WHIrAKERand G. B. FROZZARD, Va. J. Sci. 14, 6 (1963). "~ R. D. Wm~AKER, J. R. AMBROSEand C. W. HICKAM,J. Ino~:g. Nttcl. Chem. 17, 254 ([961). ~G~M. T. ROGERSand W. K. MEYER,J. Phys. Chem. 66, 1397 (1962). 1405
1406
R . D . WHITAKER
Some o f these systems have been studied by the construction of phase diagrams. The results for systems involving both o f the iodine chlorides with pyridine and quinoline have been reported/7, 8) In addition to 1:1 adducts, c o m p o u n d s o f the type B.2IC1 and B.2IClz, where B is a molecule o f pyridine, form in the solid state. M o r e interesting, however, was the discovery o f c o m p o u n d s o f the type ICI3.2B , where B is a molecule either o f pyridine or quinoline. N o analogous c o m p o u n d s were f o u n d to exist in the solid state for iodine monochloride. In an attempt to elucidate this p h e n o m e n o n , phase diagrams have been constructed for binary systems consisting o f iodine m o n o b r o m i d e with pyridine, quinoline, and acridine. Portions o f the binary systems, iodine monochloride-acridine and iodine trichloride-acridine were also investigated. TABLEI.--MELTING POINTSOF SOME 1:1 INTERHALOGEN-AMINECOMPOUNDS Amine
m.p. IBr compd. (°C)
Pyridine 2-Picoline 4-Picoline 2,6-Lutidine Quinoline Isoquinoline Acridine
m.p. ICI compd. (°C)
115~ 67-68 I~ 78"0-78.5 106-108121 132c6~ 143-144 168-171
m.p. IC13compd. (°C)
133I~l 77.5t21 107-108 tSI 112-113c~1 150-152 c5~ 155-156 178-180
175-17T5~ 131-133t51 133-134 t~) 103-10515 134-139 ~ 148-158" 212-213
* Melting point apparatus pre-heated to 140° before insertion of capillary. TABLE2.--ANALYTICALDATAFOR PREVIOUSLYUNREPORTEDCOMPOUNDS Compound 4-Picoline.IBr Isoquinoline.IBr Acridine.IBr Isoquinoline.IC1 Acridine.ICl Isoquinoline.ICl3 Acridine'ICl3
Calculated I.E.* 150 I.E. 168 I.E. 193 CI, 12-16 CI, 10.38 C, 29.82; H, 1-95; N, 3"87; C1, 29'35 CI, 25'79
Found I.E. 151 I.E. 171 I.E. 199 C1, 12-10 CI, 10.81 C, 29.73; H, 2.07 N, 3'85; Cl, 30"63 CI, 25.79
* Iodometric equivalent. RESULTS
Comparison o f 1 : 1 molecular addition compounds Table 1 presents the pertinent data regarding the stable, solid 1 : 1 addition comp o u n d s between the interhalogen c o m p o u n d s and a corresponding series of heterocyclic amines. Analytical data are presented in Table 2 for the previously unreported c o m p o u n d s .
Thermal studies IBr-pyridine system. The phase diagram (Fig. 1) shows the existence of two c o m p o u n d s , C~H~N.2IBr which melts at 53 °, and the k n o w n 1:1 c o m p o u n d which ~7~R. D. WnlTAKERand J. R. AMBROSE,J. Inorg. Nucl. Chem. 24, 285 (1962). 181YA. A. FIALKOVand I. D. MUZYKA,J. Gen. Chem. Moscow, 18, 1205 (1948).
amine-interhalogen electron donor-accepto, systems
Some
1407
12'0 II0 J ~,0 9O
8O
60
,q© o) c~ o2 o_ E
O
~--
C
\
C -. I -!©
I 50 - -~-0,
r
i
iC,
20
5(
4C 50 Mole pyridine,
G~
7'2
~2',
-~
:
~%
l:lG. 1. The system lBr-pyridine.
120
i O9
) -4(
I
1
t
I0
20
30
F [ ( ; . 2.
I
I
r
40 50 60 Mole q u i n o l i n e ,
[
I
70
80
i --390
I00
%
The system [Br-quinoline.
melted at I I 0 (lit. I ~ ) ] | 5 ) . Eutectics occur at I0 ~with a composition of19 mob petcent pyridine, at 40 ° with a composition of 38 mole per cent pyridine, and at 48 with a composition of about 97 mole per cent pyridine. | B r quinoline system. Only the I :] compound was found to exist. (Fig. 2). It melted at 125<~(lit. 6 132°) and the eutectics were found at ]0 ° and about 18 mole per cent quinoline and at --37 ° with an approximate composition of 99 per cent mole quinoline. Glasses tended to form upon cooling mixtures with compositions near the
1408
R.D. WHITAKER
eutectics. It was therefore impossible to locate the eutectics with a high degree of accuracy. IBr-acridine system. The phase diagram for this system (Fig. 3) shows that a compound IBr.2C13HgN which melts at 131 ° is formed as well as the 1 : 1 compound which melted at about 160 °. The shape of the freezing curve indicates that the former compound is rather highly dissociated at the melting point. A melting point of 168171 ° for the 1 : 1 compound, which is reported in Table 1, was obtained on a pure sample prepared by precipitation from solution. The dotted line represents the 180 170 160
J50 140
1:50 120
611o
-
.__.j
/
,oo
~ 9o I-.-
70 60
50 40~
3O 20---
-
I0 m
0
JO
20
30
40
50
Mole acridine,
60
70
80
90
I00
%
FIG. 3.--The system IBr-acridine. author's conception of that portion of the diagram. There is apparently a compound of the type, CIsHgN.2IBr, in the system which loses iodine monobromide when heated to 110 ° to form the 1:1 compound. Eutectics occur at 22 ° with a composition of about 9 mole per cent acridine, 115 ° with a composition of 57.2 mole per cent acridine, and 96 ° with a composition of 91 mole per cent acridine. ICl-acridine and IC13-acridine systems. Only the acridine-rich portions of these systems were investigated in detail because of vigorous reactions which occurred upon warming mixtures in which the amount of interhalogen was large. Even with mixtures in which acridine was greater than 50 mole per cent, irreversible reactions involving the release of chlorine and iodine often occurred. It was thus found to be impossible to obtain data of sufficient precision to allow construction of the phase diagrams for either of these systems. However, enough data was accumulated to establish that compounds with a mole ratio of iodine chloride to acridine of 1 : 2 do form in both of the systems. The approximate melting points of these compounds were found to be: IC1.2CIsHgN, m.p. 120 ° and IC1s-2C1sHgN , m.p. 167 °. DISCUSSION
If one assumes that relative electron acceptor strength of the interhalogens, and hence ionic character of their adducts, decreases in the order, IC1S > IC1 > IBr,
Some amine-interhalogen electron donor-acceptor systems
1409
a good correlation is found a m o n g the melting points of the adducts listed in Table 1. In general, for a given donor, the melting points of the addition c o m p o u n d s increase x~ith increasing electron acceptor strength of the interhalogen. The data of Table I are presented in graphical form in Fig. 4. It is likely that all of the 1 : 1 addition c o m p o u n d s are best described in terms of salt-like structures, viz. BI + • • - - X (where X is Br or (Z1) and BICI2+ • • • CI--. A
200 --
r80 --
Pyridine oe 2 - Picoline 4 - Picoline [] 2, 6-Lutidine • Quinoline z~ Isoquinoline • Acridine
/ / / /
•/
•
C3 o
.2
-~ 140 o
I O0
60
IBr
ICI
ICI 3
Fzo. 4.--Meltingpointsof some l:I interhalogen-aminecompounds. Steric factors must be considered in the case of adducts between the relatively bulky iodine trichloride molecule and the highly hindered 2,6-1utidine molecule or the relatively unsymmetrical quinoline and isoquinoline molecules. It is not unreasonable to expect relatively smaller lattice energies for these compounds bccause of reduced symmetry of the adducts. The IC]2+ cation is a bent species, and it is reasonable to assume that the species, BICI2-'L is approximately T-shaped. Ai| of the I :i compounds are quite stable under ordinary conditions, it has been erroneously reported 16) that the compounds: Pyridine.ICl3 and 2,6-Lutidine.ICla lose chlorine at room temperature. This same report liststhe melting point of the Pyridine'lCl a c o m p o u n d as 143 °. Reference to the phase diagram for this system C7) shows clearly that the c o m p o u n d , Pyridine.21C13 was confused with the 1 : 1 compound. It is likely that the same error was made with the adduct involving 2,6-1utidine. Table 3 lists the melting points of addition c o m p o u n d s of other than 1 : l stoicheiometry. These melting points have been assembled from phase diagrams reported in the present work and in previous work.~T, s) O f the three donors considered, only for the most unhindered, pyridine, do we find stable c o m p o u n d s with two moles of interhalogen per mole of donor. It is reasonable to assume that these substances also
1410
R . D . WHITAKER
TABLE3.--MELTING POINTSOF INTERHALOGEN--AMINECOMPOUNDSOF NON-I:I S'[OICHEIOIVlETRIES* Melting Point, °C Donor(B)
Pyridine
B.2IBr
B.2ICI
B.2ICI3
53
35
140
69 (incongruent)
Unstable
81
Quinoline Acridine
Loses IBr above 110
Not investigated
Not investigated
IBr.2B
130
ICI.2B
120
ICI3'2B
167
* Blank spaces indicate compound does not exist in solid state. have considerable salt-like character, viz. Pyridine.I + . . . I X 2- (where X is CI or Br) a n d Pyridine-IC12+ • • • IC14-. It was suggested previously (r) that iodine trichloride a d d i t i o n c o m p o u n d s c o n t a i n ing two moles of d o n o r per mole of acceptor might be related to some unique b o n d i n g ability of iodine trichloride. This suggestion n o w seems less attractive in view of the fact that all three interhalogens form such 1:2 a d d u c t s with acridine. The fact that acridine molecules are paired in the solid state 19) s o m e w h a t similar to anthracene, p r o b a b l y aids in the f o r m a t i o n o f these adducts. However, it is possible that the iodine trichloride c o m p o u n d s of this type with pyridine a n d q u i n o l i n e are related to the crystal structures of the solids rather t h a n any b o n d i n g characteristics u n i q u e to iodine trichloride. EXPERIMENTAL
Chemicals. The preparation and purification of starting materials have been described. ~ Preparation of i : 1 addition compounds. These compounds were prepared by precipitation from CC14 solutions as previously described. (5~ Yields on most of the compounds prepared in this work were essentially quantitative. The colours of these compounds are varying shades of yellow with iodine trichloride adducts brightest and iodine monobromide palest. Analyses. Chlorine was determined as previously described.CS~ The iodometric equivalents for the IBr compounds were determined according to the method described by ROGERS and MEYER.(6~ The chloride analyses were used where possible because higher precision was obtained with them than with the iodometric determinations. The carbon, hydrogen and nitrogen analyses for Isoquinoline'ICl3 were done by Galbraith Laboratories, Knoxville, Tennessee. Thermal studies. Phase diagrams were constructed from cooling curves obtained by means of a calibrated copper--constantan thermocouple attached to a V.O.M. 5 B. & L. Recording Potentiometer. The binary compositions were prepared by direct weighing in an all-glass apparatus equipped with a micro-stirring bar so that the mixtures could be magnetically stirred while cooling. The entire apparatus was calibrated over the temperature range encountered in the studies by means of several standards of known melting points. The cold junction temperature was maintained at 0°. All points were determined at least twice, and the reported temperatures are believed to be accurate to within ±3 °. Acknowledgement--The author wishes to thank Mr. Y. C. FERNANDEZfor his assistance with some of the laboratory work. 19~D. C. PHILLIPS,Acta Cryst. 9, 237 (1956).