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The relative rates of reaction of trimeric and tetrameric phosphonitrilic halides with n-propylamine
J. lnorg. Nucl. Chem., 1963, Vol. 25, pp. 1397 to 1400. PergamonPress Ltd. Printedin Northern Ireland
THE RELATIVE RATES OF REACTION OF TRIMERIC A N D TETRAMERIC PHOSPHONITRILIC HALIDES WITH n-PROPYLAMINE T. MOELLERand S. G. KOKALIS* Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois
(Received 19 Alarch 1963) Abstraet--A conductimetric kinetic analysis of the aminolysis of the phosphonitrilic halides (NPF~.)a ana ~, (NPCI2)3 ~n,l ~, and (NPBr2)a ~nd 4 by n-propylamine in acetonitrile at 25 ° indicates a second-order rate-determining step in each instance. From data for the second-order constants (k.,) and the pseudo first-order constants (k'O, the comparative order of reactivity is (NPBr0~ > (NPBr~)a (NPC1..)4 > (NPC12)3 ~ (NPF2)4 > (NPF2)a.
ALTHOUGH reactions of the phosphonitrilic halides (esp. chlorides) with primary alkyl amines have been utilized for synthesis, (1 6) no quantitative correlations of rates of aminolysis have been established. Kinetic studies have been limited to the polymerization of the tetrameric chloride, tT,sl to the reaction of the trimeric chloride with aniline in ethanol, m) and to the reaction of the trimeric chloride with piperidine in toluene. I1°1 In the last of these studies, mixed second- and third-order kinetics were observed for the formation of the mono-piperidino derivative, and it was noted that the tetrameric chloride reacts much more rapidly with an amine than does the trimeric chloride. Inasmuch as the interpretation of d,-p~ aromatic character in these ring systems is of general interest, an insight into the effect of both ring complexity and electronegativity of the halogens present on the availability of d-orbitals from the phosphorus atoms for nucleophilic attack is of interest. Kinetic data essential to such an approach can be obtained conductimetrically since the aminehydrohalide product of the aminolysis reaction is ionic. For convenience, the reactions studied were limited to those of n-propylamine. EXPERIMENTAL
Materials. Commercial trimeric and tetrameric phosphonitrilic chlorides obtained from Hooker Chemicals Corporation were purified by recrystallization to materials melting at 114 ~ and 123, * Louderman Chemical Laboratories, Washington University, St. Louis, Mo. (1) S. G. KOKALIS,K. JOHN, T. MOELLERand L. F. AUDRIETH, J. Inorg. Nucl. Chem. 19, 191 (1961). t2) M. BECKE-GOEHRING, K. JOHN and E. FLUCK, Z. Anorff. Chem. 302, 103 (1959). 13) K. JOHN, T. MOELLERand L. F. AUDRIETH,J. Amer. Chem. Soc. 82, 5616 (1960); 83, 2608 (1961). 14) D. LIPKIN, U.S. Pat. No. 2192921 (1940). tsi S. K. RAY and R. A. SHAW,Chem. andlndustr. 53 (1959); Proc. Chem. Soc. 26 0960); J. Chem. Soc. 872 (1961). tG) M. YOKOYAMA,Nippon Kagaku Zasshi 81, 1453 (1960). ~7~F. PATAT and K. FROMBLING, Monatsh. 86, 718 (1955). la~ j. O. KONECNY, C. M. DOUGLASand M. Y. GRAY, J. Polymer Sei. 42, 383 (1960). tg~ j. V. BAILEYand R. E. PARKER, Chem. andlndustr. 1823 (1962). ~a0~B. CAPON, K. HILLS, and R. A. SHAW, Proe. Chem. Soc. 390 (1962). 7
1397
1398
T. MOELLER and S. G. KOKALIS
respectively. The phosphonitrilic bromides were prepared as previously describedm~ and were recrystallized from n-heptane. The fluorides were obtained by reaction of the chlorides with sodium fluoride in nitrobenzene, t12~ Purified n-propylamine was refluxed over and then distilled from potassium hydroxide pellets. Acetonitrile was distilled four times from fresh phosphorus (V) oxide. Its conductivity was within the range 5-9 × 10-8 f] cm -~ at 25 ° cited for samples containing less water than is detectable with Karl Fischer reagent2 TM Apparatus. All conductance measurements were made at 25'0 4- 0.1 ° with an Industrial Instruments, Inc., bridge, Type RC-16B, operating at 1000 c/s. The conductance cell contained platinized 2.0 × 3.0 cm platinum electrodes and had a capacity of 65 ml. A ground-glass stopper excluded moisture. The cell constant was 0.147 cm -1. Procedure. Since it was established independently that no complicating reactions occur and that freshly prepared solutions of the reactants ((NPX2)~ and n-CsHTNH2) in anhydrous acetonitrile have negligible conductance, only the amine hydrohalide, n-C3H7NHa+X -, produced during aminolysis was responsible for observed conductance. Standard 10-2 or 10-3 M solutions of the reactants were used. These were brought to thermal equilibrium in the thermostat before mixing. Mixing was effected in the conductance cell by adding a measured volume of one reactant with a syringe to a measured volume of the other. A stopwatch was started at the instant of mixing, and resistance was then measured at suitable time intervals. It was found desirable to record resistance each minute for the first 10 min and then at increasingly long intervals up to 100 min. The resistance observed after 100 hr was considered to be that for complete reaction ( R , ) . Successive dilutions of the original stock solutions to one-tenth the original concentration were used to provide additional reaction systems. Consistency in procedural details permitted correlation of reaction rates among the phosphonitriles studied.
RESULTS AND DISCUSSION T h e overall o r d e r o f the aminolysis r e a c t i o n was d e t e r m i n e d in each instance b y the v a r i a t i o n - i n - r a t i o - o f - r e a c t a n t s method314) I t was f o u n d essential to use 10 - 5 10-6 M solutions o f the reactants to achieve less t h a n 10 p e r cent r e a c t i o n within the first 5-10 min o f study. Based u p o n the generalized rate expression dx d-7 = k[(NPX2)3A]m[n-C3H7NH2]n
(1)
the o r d e r as so d e t e r m i n e d for each r e a c t a n t is s u m m a r i z e d in Table 1. The values o f m a n d n a p p r o a c h u n i t y in all cases within e x p e r i m e n t a l error. The g r a p h i c a l m e t h o d o f PESSENc15~ was used to d e t e r m i n e the t o t a l o r d e r o f the reaction, N. W h e n 10-2, 10 -3 o r 1 0 - 4 M solutions were used, r e a s o n a b l y linear plots o f the q u a n t i t y R t / ( R t - - R~o) vs. time were o b t a i n e d for all o f the p h o s p h o n i t r i l i c halides studied. These are in a g r e e m e n t with a s e c o n d - o r d e r r a t e - d e t e r m i n i n g step for the aminolysis reaction. Pertinent d a t a are s u m m a r i z e d in T a b l e 2. R a t e constants were derived f r o m half-life d a t a (Table 3) b y PF_SSEN'S graphical m e t h o d a n d a p s e u d o first-order calculation, t~6~ The p s e u d o k ( values (Table 3) were based u p o n the first 3 m i n o f r e a c t i o n with the a m i n e in 30--40 m o l e r a t i o excess. T h e k ( values were o b t a i n e d b y dividing k~' values b y the c o n c e n t r a t i o n s o f the amine, a. ~llJ K. JOHN and T. MOELLER,J. Inorg. NucL Chem. 22, 199 (1961). (12) T. MOELLER,K. JOHN and F. Y. TSANG,Chem. and Industr. 347 (1961). <13~W. S. MONEY and J. F. COETZEE,J. Phys. Chem. 66, 89 (1962). tx41 F. DANIELSand R. A. ALBERTY,Physical Chemistry, p. 330. J. Wiley, New York (1955). ~15~H. PESSEN,Science 134, 676 (1961). tie) F. BASOLO,J. CHATT, I-'I.B. GRAY, R. G. PEARSONand B. L. SHAW,d. Chem. Soc. 2207 (1961).
The relative rates of reaction of trimeric and tetrameric phosphonitrilic halides
1399
TABLE I . - - O R D E R IN REACTANT MOLECULES
Phosphonitrile
m
NaP3F6
0.68-1"15 0'86-1 "09 0'72-1 '21 0'67-1 "20 0"77-1'25 0-68-1 "20
N4P4F8
N3P3CI~ N4P4CIs NaP3Br6 N~PaBr8
n 0.62-1.30 0"68-1"19 0'73-1 "38 0'65-1"17 0"80-1-25 0'70-1 '25
TABLE 2.--PESSEN METHOD OF ANALYSIS
Phosphonitrile N3P3F6 N~P4F8 NsPsC16 NaP4CIs N3P3Br6 N4P4Br8
N 2.14 2-24 2.10 2-20 2.00 2.22
ks (min-~mole -~ 1.)
Reaction completed in first 10 min.* (%)
25-300 200-1,000 2,000-6,700 8,000-10,000 20,000-80,000 20,000-160,000
15 26 36 40 38 68
*(NPXs),: RNHs= l : t 2 f o r n
3,1:16 for n = 4. Allat 10-4M.
TABLE 3.--DATA FROMHALF-LIFE*AND PSEUDOkl" CALCULATION Phosphonitrile NsP3F6 N4P4F8 N3P3CI6 N4P4CIs NsPsBra N4P4Brs
ks (min-~mole -~ 1.) 17-300 133-1,880 1,300-6,600 3,000-6,670 2,400-20,000 20,000-180,000
kl' min-~ 0'0354)'050 0"041-0"037 0"070-0'152 0'129-0"246 0"085-0"220 0"195-0.274
ks' (min-lmole-11.) 35-500 41-366 152-700 246-1,290 220-850 195-2,740
* k2 = l / a t ½
Although the rate-determining step is indicated to be second order in all instances, simultaneous fast first-order processes may not be excluded from mechanistic considerations since DOSTROVSKY and HOLMANNO7) have shown that an amino substituent on a phosphorus atom enhances the loss of halogen from that atom by an SN1 process. It is essential, of course, to extend these investigations to other solvents to determine whether the SN2 rate constants change and thus to establish whether or not a more complex rate-determining step involving interaction with the solvent exists. In that successive aminolysis reactions are occurring, only the slowest step is being detected. A complete interpretation of mechanism will require evaluation of rates of aminolysis of the partially substituted derivatives, N3P3(NHR)6_~X n. A promising beginning has been made by SHAW/1°) The minimal reactivity of the trimeric fluoride suggests the least availability of d-orbitals from the phosphorus atoms for nucleophilic attack with this compound. A higher degree of aromaticity for this compound through involvement of the phosphorus d orbitals with the nitrogen p-orbitals in d~-p, ring interaction would reduce (17)I. DOSTROVSKYand M. HOLMANN,J. Chem. Soc. 502 (1953).
1400
T. MOELLERand S. G. KOKALIS
orbital availability. That the trimeric chloride reacts completely with trimethylamine whereas the trimeric fluoride undergoes no reaction ~18~ supports the concept of greater resonance stability in the latter compound and greater availability of dorbitals for SN2 attack in the former. Data for the aminolysis of the phosphorohalidates (RzPOF < RzPOCI < R2POI ) show the same trend in halide reactivity, howeverJ 17~ Since no ring resonance can be involved with these compounds, it is reasonable to assume that the steric environment of the phosphorus atom may also influence the reactivity of substituted groups. If the formation of a transition-state intermediate with penta-substituted phosphorus is required, the greatest difficulty in its formation should be noted with the most rigid planar ring, i.e., that of the trimeric fluoride. The less rigorous conformational requirements about phosphorus in the trimeric chloride and bromide might then allow the formation of a transition-state intermediate with less difficulty. The greater reactivity of the less rigidly structured tetramers may be a consequence of the same general situation since greater steric flexibility about a phosphorus atom more readily allows the conformational changes required in the formation of intermediates. The data cited above do not agree with the suggestion that the bulk effects of the amine ~5~ are of primary importance in accounting for a greater rate of aminolysis with the tetrameric chloride than with the trimeric chloride. The polarizability of the halogen atom being replaced will also influence the overall rate of substitution, other factors being equal. This would suggest the order I > Br > CI. It is apparent from these preliminary observations that extensive kinetic investigations will be necessary before the solvolytic reactivities of the phosphonitrilic halides can be completely understood. That reactions of this type proceed at measurable rates should be helpful in studies of this type. Acknowledgement--Support received by S. G. K. from the Ethyl Corporation and E. I. du Pont de
Nemours and Company in the form of graduate fellowshipsis gratefully acknowledged. (xs)A. B. BURGand A. P. CARRON,J. Amer. Chem. Soc. 81, 836 (1959).
THE RELATIVE RATES OF REACTION OF TRIMERIC A N D TETRAMERIC PHOSPHONITRILIC HALIDES WITH n-PROPYLAMINE T. MOELLERand S. G. KOKALIS* Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois
(Received 19 Alarch 1963) Abstraet--A conductimetric kinetic analysis of the aminolysis of the phosphonitrilic halides (NPF~.)a ana ~, (NPCI2)3 ~n,l ~, and (NPBr2)a ~nd 4 by n-propylamine in acetonitrile at 25 ° indicates a second-order rate-determining step in each instance. From data for the second-order constants (k.,) and the pseudo first-order constants (k'O, the comparative order of reactivity is (NPBr0~ > (NPBr~)a (NPC1..)4 > (NPC12)3 ~ (NPF2)4 > (NPF2)a.
ALTHOUGH reactions of the phosphonitrilic halides (esp. chlorides) with primary alkyl amines have been utilized for synthesis, (1 6) no quantitative correlations of rates of aminolysis have been established. Kinetic studies have been limited to the polymerization of the tetrameric chloride, tT,sl to the reaction of the trimeric chloride with aniline in ethanol, m) and to the reaction of the trimeric chloride with piperidine in toluene. I1°1 In the last of these studies, mixed second- and third-order kinetics were observed for the formation of the mono-piperidino derivative, and it was noted that the tetrameric chloride reacts much more rapidly with an amine than does the trimeric chloride. Inasmuch as the interpretation of d,-p~ aromatic character in these ring systems is of general interest, an insight into the effect of both ring complexity and electronegativity of the halogens present on the availability of d-orbitals from the phosphorus atoms for nucleophilic attack is of interest. Kinetic data essential to such an approach can be obtained conductimetrically since the aminehydrohalide product of the aminolysis reaction is ionic. For convenience, the reactions studied were limited to those of n-propylamine. EXPERIMENTAL
Materials. Commercial trimeric and tetrameric phosphonitrilic chlorides obtained from Hooker Chemicals Corporation were purified by recrystallization to materials melting at 114 ~ and 123, * Louderman Chemical Laboratories, Washington University, St. Louis, Mo. (1) S. G. KOKALIS,K. JOHN, T. MOELLERand L. F. AUDRIETH, J. Inorg. Nucl. Chem. 19, 191 (1961). t2) M. BECKE-GOEHRING, K. JOHN and E. FLUCK, Z. Anorff. Chem. 302, 103 (1959). 13) K. JOHN, T. MOELLERand L. F. AUDRIETH,J. Amer. Chem. Soc. 82, 5616 (1960); 83, 2608 (1961). 14) D. LIPKIN, U.S. Pat. No. 2192921 (1940). tsi S. K. RAY and R. A. SHAW,Chem. andlndustr. 53 (1959); Proc. Chem. Soc. 26 0960); J. Chem. Soc. 872 (1961). tG) M. YOKOYAMA,Nippon Kagaku Zasshi 81, 1453 (1960). ~7~F. PATAT and K. FROMBLING, Monatsh. 86, 718 (1955). la~ j. O. KONECNY, C. M. DOUGLASand M. Y. GRAY, J. Polymer Sei. 42, 383 (1960). tg~ j. V. BAILEYand R. E. PARKER, Chem. andlndustr. 1823 (1962). ~a0~B. CAPON, K. HILLS, and R. A. SHAW, Proe. Chem. Soc. 390 (1962). 7
1397
1398
T. MOELLER and S. G. KOKALIS
respectively. The phosphonitrilic bromides were prepared as previously describedm~ and were recrystallized from n-heptane. The fluorides were obtained by reaction of the chlorides with sodium fluoride in nitrobenzene, t12~ Purified n-propylamine was refluxed over and then distilled from potassium hydroxide pellets. Acetonitrile was distilled four times from fresh phosphorus (V) oxide. Its conductivity was within the range 5-9 × 10-8 f] cm -~ at 25 ° cited for samples containing less water than is detectable with Karl Fischer reagent2 TM Apparatus. All conductance measurements were made at 25'0 4- 0.1 ° with an Industrial Instruments, Inc., bridge, Type RC-16B, operating at 1000 c/s. The conductance cell contained platinized 2.0 × 3.0 cm platinum electrodes and had a capacity of 65 ml. A ground-glass stopper excluded moisture. The cell constant was 0.147 cm -1. Procedure. Since it was established independently that no complicating reactions occur and that freshly prepared solutions of the reactants ((NPX2)~ and n-CsHTNH2) in anhydrous acetonitrile have negligible conductance, only the amine hydrohalide, n-C3H7NHa+X -, produced during aminolysis was responsible for observed conductance. Standard 10-2 or 10-3 M solutions of the reactants were used. These were brought to thermal equilibrium in the thermostat before mixing. Mixing was effected in the conductance cell by adding a measured volume of one reactant with a syringe to a measured volume of the other. A stopwatch was started at the instant of mixing, and resistance was then measured at suitable time intervals. It was found desirable to record resistance each minute for the first 10 min and then at increasingly long intervals up to 100 min. The resistance observed after 100 hr was considered to be that for complete reaction ( R , ) . Successive dilutions of the original stock solutions to one-tenth the original concentration were used to provide additional reaction systems. Consistency in procedural details permitted correlation of reaction rates among the phosphonitriles studied.
RESULTS AND DISCUSSION T h e overall o r d e r o f the aminolysis r e a c t i o n was d e t e r m i n e d in each instance b y the v a r i a t i o n - i n - r a t i o - o f - r e a c t a n t s method314) I t was f o u n d essential to use 10 - 5 10-6 M solutions o f the reactants to achieve less t h a n 10 p e r cent r e a c t i o n within the first 5-10 min o f study. Based u p o n the generalized rate expression dx d-7 = k[(NPX2)3A]m[n-C3H7NH2]n
(1)
the o r d e r as so d e t e r m i n e d for each r e a c t a n t is s u m m a r i z e d in Table 1. The values o f m a n d n a p p r o a c h u n i t y in all cases within e x p e r i m e n t a l error. The g r a p h i c a l m e t h o d o f PESSENc15~ was used to d e t e r m i n e the t o t a l o r d e r o f the reaction, N. W h e n 10-2, 10 -3 o r 1 0 - 4 M solutions were used, r e a s o n a b l y linear plots o f the q u a n t i t y R t / ( R t - - R~o) vs. time were o b t a i n e d for all o f the p h o s p h o n i t r i l i c halides studied. These are in a g r e e m e n t with a s e c o n d - o r d e r r a t e - d e t e r m i n i n g step for the aminolysis reaction. Pertinent d a t a are s u m m a r i z e d in T a b l e 2. R a t e constants were derived f r o m half-life d a t a (Table 3) b y PF_SSEN'S graphical m e t h o d a n d a p s e u d o first-order calculation, t~6~ The p s e u d o k ( values (Table 3) were based u p o n the first 3 m i n o f r e a c t i o n with the a m i n e in 30--40 m o l e r a t i o excess. T h e k ( values were o b t a i n e d b y dividing k~' values b y the c o n c e n t r a t i o n s o f the amine, a. ~llJ K. JOHN and T. MOELLER,J. Inorg. NucL Chem. 22, 199 (1961). (12) T. MOELLER,K. JOHN and F. Y. TSANG,Chem. and Industr. 347 (1961). <13~W. S. MONEY and J. F. COETZEE,J. Phys. Chem. 66, 89 (1962). tx41 F. DANIELSand R. A. ALBERTY,Physical Chemistry, p. 330. J. Wiley, New York (1955). ~15~H. PESSEN,Science 134, 676 (1961). tie) F. BASOLO,J. CHATT, I-'I.B. GRAY, R. G. PEARSONand B. L. SHAW,d. Chem. Soc. 2207 (1961).
The relative rates of reaction of trimeric and tetrameric phosphonitrilic halides
1399
TABLE I . - - O R D E R IN REACTANT MOLECULES
Phosphonitrile
m
NaP3F6
0.68-1"15 0'86-1 "09 0'72-1 '21 0'67-1 "20 0"77-1'25 0-68-1 "20
N4P4F8
N3P3CI~ N4P4CIs NaP3Br6 N~PaBr8
n 0.62-1.30 0"68-1"19 0'73-1 "38 0'65-1"17 0"80-1-25 0'70-1 '25
TABLE 2.--PESSEN METHOD OF ANALYSIS
Phosphonitrile N3P3F6 N~P4F8 NsPsC16 NaP4CIs N3P3Br6 N4P4Br8
N 2.14 2-24 2.10 2-20 2.00 2.22
ks (min-~mole -~ 1.)
Reaction completed in first 10 min.* (%)
25-300 200-1,000 2,000-6,700 8,000-10,000 20,000-80,000 20,000-160,000
15 26 36 40 38 68
*(NPXs),: RNHs= l : t 2 f o r n
3,1:16 for n = 4. Allat 10-4M.
TABLE 3.--DATA FROMHALF-LIFE*AND PSEUDOkl" CALCULATION Phosphonitrile NsP3F6 N4P4F8 N3P3CI6 N4P4CIs NsPsBra N4P4Brs
ks (min-~mole -~ 1.) 17-300 133-1,880 1,300-6,600 3,000-6,670 2,400-20,000 20,000-180,000
kl' min-~ 0'0354)'050 0"041-0"037 0"070-0'152 0'129-0"246 0"085-0"220 0"195-0.274
ks' (min-lmole-11.) 35-500 41-366 152-700 246-1,290 220-850 195-2,740
* k2 = l / a t ½
Although the rate-determining step is indicated to be second order in all instances, simultaneous fast first-order processes may not be excluded from mechanistic considerations since DOSTROVSKY and HOLMANNO7) have shown that an amino substituent on a phosphorus atom enhances the loss of halogen from that atom by an SN1 process. It is essential, of course, to extend these investigations to other solvents to determine whether the SN2 rate constants change and thus to establish whether or not a more complex rate-determining step involving interaction with the solvent exists. In that successive aminolysis reactions are occurring, only the slowest step is being detected. A complete interpretation of mechanism will require evaluation of rates of aminolysis of the partially substituted derivatives, N3P3(NHR)6_~X n. A promising beginning has been made by SHAW/1°) The minimal reactivity of the trimeric fluoride suggests the least availability of d-orbitals from the phosphorus atoms for nucleophilic attack with this compound. A higher degree of aromaticity for this compound through involvement of the phosphorus d orbitals with the nitrogen p-orbitals in d~-p, ring interaction would reduce (17)I. DOSTROVSKYand M. HOLMANN,J. Chem. Soc. 502 (1953).
1400
T. MOELLERand S. G. KOKALIS
orbital availability. That the trimeric chloride reacts completely with trimethylamine whereas the trimeric fluoride undergoes no reaction ~18~ supports the concept of greater resonance stability in the latter compound and greater availability of dorbitals for SN2 attack in the former. Data for the aminolysis of the phosphorohalidates (RzPOF < RzPOCI < R2POI ) show the same trend in halide reactivity, howeverJ 17~ Since no ring resonance can be involved with these compounds, it is reasonable to assume that the steric environment of the phosphorus atom may also influence the reactivity of substituted groups. If the formation of a transition-state intermediate with penta-substituted phosphorus is required, the greatest difficulty in its formation should be noted with the most rigid planar ring, i.e., that of the trimeric fluoride. The less rigorous conformational requirements about phosphorus in the trimeric chloride and bromide might then allow the formation of a transition-state intermediate with less difficulty. The greater reactivity of the less rigidly structured tetramers may be a consequence of the same general situation since greater steric flexibility about a phosphorus atom more readily allows the conformational changes required in the formation of intermediates. The data cited above do not agree with the suggestion that the bulk effects of the amine ~5~ are of primary importance in accounting for a greater rate of aminolysis with the tetrameric chloride than with the trimeric chloride. The polarizability of the halogen atom being replaced will also influence the overall rate of substitution, other factors being equal. This would suggest the order I > Br > CI. It is apparent from these preliminary observations that extensive kinetic investigations will be necessary before the solvolytic reactivities of the phosphonitrilic halides can be completely understood. That reactions of this type proceed at measurable rates should be helpful in studies of this type. Acknowledgement--Support received by S. G. K. from the Ethyl Corporation and E. I. du Pont de
Nemours and Company in the form of graduate fellowshipsis gratefully acknowledged. (xs)A. B. BURGand A. P. CARRON,J. Amer. Chem. Soc. 81, 836 (1959).