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Infrared spectra of the iodine complexes of mono N-oxides of some heterocyclic diazines
Reseeroh notea
2227
Spe&oohM~AaLctr,Vol.26A,pp.2227 to 2228.PwssmonprpY 1970.PrintedinNortbmIreland
In&wed spectra of the iodine compIexes of mono N-oxides of some heterocyclic diazines (Received7 January 1970) Al&~&-The N-O stret&ing frequenciesof the I, complexes of the mono N-oxides of some heterocyliod&&es have been observed. These data give some indicetion that in all the compounda mvestigeted, it is the oxygen end not the ring nitrogen that acts as 8 donor site. Trm electron donor abilities of both pyridine and its N-oxide have been extensively studied. Comparison of the results of these studies indio8te that the nitrogen in pyridine is 8 better donor then is the oxygen of the N-oxide towards both the hydrogen ion [l] and 8 moleoularacid such 8s I, [Z]. In the ~888 of the mono N-oxides of heterooyaliod&zi.ues,it might be expe&ed that the eleotronic h-&erectionsbetween the N-O group end the other nitrogen in the ring would 8ifeot the relative besicity of these two donor sites. Thus the question arises 8s to which of the two possible sites would be the one eatually used. Ochiai, iu his book on the chemistry of arometio N-oxides, points out that the pR, velues of phenazine and its mono N-oxide 8re too &se together to permit any decision 8s to whst is the b8sia site of the oxide [3]. As there is no other information in the litereture on this point, we would like to report on the results of an infnwed study of the oomplexes of N-oxides that 8re pertinent to this problem. We have observed their ape&r8 between 1100 and 1400 om’-l for CS, solutions of pyridine N-oxide and the mono N-oxides of the three isomeriomonocyclic.die&es both with and without I,. The N-oxides were prepared by methods given in the literature [3], 8nd the spectra of 811 solutions were obtained using 8 Beckman IR-12. First, the spectra of 8 s8tur8ted solution (No.06M) of the free donor in CS, was recorded. I, w8s then added to the solution and the speatr8 recorded 8g8in. The spectra of several solutions with I, concentrationsranging from 0.02 to 0.06iK were observed. For eeah of the oxides, it is found the addition of I, -uses two major ahangea in their epeatrom. These are a deoreasein the intensity of the peak previously assigned to the N-O stretching frequenoy [4] and the ~ppearanoeof an intense new peak in the same region. The inteneitieeof these new peaka increase88 more I, is added, however the frequenciesare independent of the I, aoncentration. Also, at the higher concentrationaof I,, the N-O peak for the free donor has completely disappeared. These new peakamay be assigned to the N-O stretching frequenaieeof the complexed donor moleaules because they appear only slightly shifted fi-om the positions of the original N-O frequenoieaend their intensities depend on the I9 oonoentr8tion. Since the fiequexmiesof these new peaks 8re independent of the 19 aonuentrations,we conclude that the oomplexes have 8 constant stoichiometry in these solutions. The positions of the N-G frequencies in both the free donors end the complexed moleoules 8re given in Tsble 1. The values given for pyridine N-oxide 8re close to those reported by KUBOTAe6uZ.[S] for CCl, solutions of the iodine pyridine N-oxide complex. From the data in Table 1, it can be seen that the effect of Is ctomplexetionis to reduce the N-O stretching frequencies in both the pyridine N-oxide and the diazine N-oxides. Since Kubota has given evidence that in the iodine:pyridiue N-oxide complex it is the oxygen that [l] (a) A. ALBERT, R. GOLDAORZ~ and J. PHILLIprJ, J. Chm. Sec. 2240 (1948). (b) H. H. Jm and G. 0. DOAIC,J. Am. Chem. Sot. 77.4441 (1966). [2] (a) w. MCKINXEY and A. I. POPOV,J. Am. Chem. Sot. Ql, 6216 (1969); and referenoes therein. (b) T. KUBOTA,ibid. 87,468 (1966). [3] E. OCEUAI,Aromatic Amine Ok&. p. 98. Elsevier (1967). [4] H. SINDO,Oh. Phuwn. Bull. 8, 33 (1960). [6] T. KW~TA, K. EZTJMI, M. YAXAU WA and Y. %fATSUI, J. Mol. &e&y 84,378 (1967). 9
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Beeearch note0 Table 1. N-O
&etching frequencies of N-oxides and N-oxide:iodine complexes
Pyridine N-oxide Pyrimidine N-oxide Pyrazine N-oxide Pyridazine N-oxide
Free donor %O (cm-1)
I, complex *No (cm-l)
Av* (cm-l)
Av/v
1207 1281 1319 1342
1223 1241 1286 1316
44 40 33 27
0.034 0.031 0.026 0.020
* Difference between vNo in free donor and iodine complex. is the donor site [2b, 51, it is likely that for the oxides of the diaxines, oxygen is again the donor site. Further support for this view can be obtained from an examination of some of the representetive reeonanoeforma of the N-oxides. Those for pyrazine N-oxide are shown as an example:
The double bond character and thus the position of the N-O stretching frequency will be governed by the relative contribution of forma similar to B. Therefore, if the ring nitrogen is the basic site, the contributionof B would increaseand cause the frequency of the N-O stretch to increase. On the other hand, if oxygen is the basic site, then the contributions of forms A and C would increase and the N-O frequency might be expected to decrease. As it is the latter result which is observed for all of the complexes reportedhere, we can deduce that oxygen is the basic site in these adducts. The order of b&city obtained from a considerationof the frequency shifts (Av in Table 1) is another piece of evidence for oxygen acting aa the basic site. If one assumesthat the order of basicity towards a common acid is the same 88 the order of Av and AVIV,then the basicity is pyridine N-oxide > pyrimidine N-oxide > pyraxine N-oxide > pyridaxine N-oxide. This sequencecan be rationalizedby a considerationof the resonanceand inductiveeffectsof the ring nitrogen on the N-O group, with the oxygen acting a~ the donor site. The use of Av or Aviv to determinethe orderof donor abilitiesis not completely adequate, but may be justiiledby the fact that for the complexes of titanium tetrafluoridewith a series of substituted pyridine N-oxides the v NO of the complexes donors have been used to obtain linear Hammet plots [S]. We are currently determining thermodynamic data for the association of these & complexes which should clarify thia point. UnCertity of North Dakota Abbott Hall Departwwnt of C%e&&y Grand Fork.9, North Dakota 68201
rNo~~ KULEVSKY ROLAND G. SEVERSON,JR.
[f3] F. E. DEOKSON,E. Q. GOWLINCIand F. F. BENTLY, Inorg. Chmn. 6, 1099 (1967).
2227
Spe&oohM~AaLctr,Vol.26A,pp.2227 to 2228.PwssmonprpY 1970.PrintedinNortbmIreland
In&wed spectra of the iodine compIexes of mono N-oxides of some heterocyclic diazines (Received7 January 1970) Al&~&-The N-O stret&ing frequenciesof the I, complexes of the mono N-oxides of some heterocyliod&&es have been observed. These data give some indicetion that in all the compounda mvestigeted, it is the oxygen end not the ring nitrogen that acts as 8 donor site. Trm electron donor abilities of both pyridine and its N-oxide have been extensively studied. Comparison of the results of these studies indio8te that the nitrogen in pyridine is 8 better donor then is the oxygen of the N-oxide towards both the hydrogen ion [l] and 8 moleoularacid such 8s I, [Z]. In the ~888 of the mono N-oxides of heterooyaliod&zi.ues,it might be expe&ed that the eleotronic h-&erectionsbetween the N-O group end the other nitrogen in the ring would 8ifeot the relative besicity of these two donor sites. Thus the question arises 8s to which of the two possible sites would be the one eatually used. Ochiai, iu his book on the chemistry of arometio N-oxides, points out that the pR, velues of phenazine and its mono N-oxide 8re too &se together to permit any decision 8s to whst is the b8sia site of the oxide [3]. As there is no other information in the litereture on this point, we would like to report on the results of an infnwed study of the oomplexes of N-oxides that 8re pertinent to this problem. We have observed their ape&r8 between 1100 and 1400 om’-l for CS, solutions of pyridine N-oxide and the mono N-oxides of the three isomeriomonocyclic.die&es both with and without I,. The N-oxides were prepared by methods given in the literature [3], 8nd the spectra of 811 solutions were obtained using 8 Beckman IR-12. First, the spectra of 8 s8tur8ted solution (No.06M) of the free donor in CS, was recorded. I, w8s then added to the solution and the speatr8 recorded 8g8in. The spectra of several solutions with I, concentrationsranging from 0.02 to 0.06iK were observed. For eeah of the oxides, it is found the addition of I, -uses two major ahangea in their epeatrom. These are a deoreasein the intensity of the peak previously assigned to the N-O stretching frequenoy [4] and the ~ppearanoeof an intense new peak in the same region. The inteneitieeof these new peaka increase88 more I, is added, however the frequenciesare independent of the I, aoncentration. Also, at the higher concentrationaof I,, the N-O peak for the free donor has completely disappeared. These new peakamay be assigned to the N-O stretching frequenaieeof the complexed donor moleaules because they appear only slightly shifted fi-om the positions of the original N-O frequenoieaend their intensities depend on the I9 oonoentr8tion. Since the fiequexmiesof these new peaks 8re independent of the 19 aonuentrations,we conclude that the oomplexes have 8 constant stoichiometry in these solutions. The positions of the N-G frequencies in both the free donors end the complexed moleoules 8re given in Tsble 1. The values given for pyridine N-oxide 8re close to those reported by KUBOTAe6uZ.[S] for CCl, solutions of the iodine pyridine N-oxide complex. From the data in Table 1, it can be seen that the effect of Is ctomplexetionis to reduce the N-O stretching frequencies in both the pyridine N-oxide and the diazine N-oxides. Since Kubota has given evidence that in the iodine:pyridiue N-oxide complex it is the oxygen that [l] (a) A. ALBERT, R. GOLDAORZ~ and J. PHILLIprJ, J. Chm. Sec. 2240 (1948). (b) H. H. Jm and G. 0. DOAIC,J. Am. Chem. Sot. 77.4441 (1966). [2] (a) w. MCKINXEY and A. I. POPOV,J. Am. Chem. Sot. Ql, 6216 (1969); and referenoes therein. (b) T. KUBOTA,ibid. 87,468 (1966). [3] E. OCEUAI,Aromatic Amine Ok&. p. 98. Elsevier (1967). [4] H. SINDO,Oh. Phuwn. Bull. 8, 33 (1960). [6] T. KW~TA, K. EZTJMI, M. YAXAU WA and Y. %fATSUI, J. Mol. &e&y 84,378 (1967). 9
2228
Beeearch note0 Table 1. N-O
&etching frequencies of N-oxides and N-oxide:iodine complexes
Pyridine N-oxide Pyrimidine N-oxide Pyrazine N-oxide Pyridazine N-oxide
Free donor %O (cm-1)
I, complex *No (cm-l)
Av* (cm-l)
Av/v
1207 1281 1319 1342
1223 1241 1286 1316
44 40 33 27
0.034 0.031 0.026 0.020
* Difference between vNo in free donor and iodine complex. is the donor site [2b, 51, it is likely that for the oxides of the diaxines, oxygen is again the donor site. Further support for this view can be obtained from an examination of some of the representetive reeonanoeforma of the N-oxides. Those for pyrazine N-oxide are shown as an example:
The double bond character and thus the position of the N-O stretching frequency will be governed by the relative contribution of forma similar to B. Therefore, if the ring nitrogen is the basic site, the contributionof B would increaseand cause the frequency of the N-O stretch to increase. On the other hand, if oxygen is the basic site, then the contributions of forms A and C would increase and the N-O frequency might be expected to decrease. As it is the latter result which is observed for all of the complexes reportedhere, we can deduce that oxygen is the basic site in these adducts. The order of b&city obtained from a considerationof the frequency shifts (Av in Table 1) is another piece of evidence for oxygen acting aa the basic site. If one assumesthat the order of basicity towards a common acid is the same 88 the order of Av and AVIV,then the basicity is pyridine N-oxide > pyrimidine N-oxide > pyraxine N-oxide > pyridaxine N-oxide. This sequencecan be rationalizedby a considerationof the resonanceand inductiveeffectsof the ring nitrogen on the N-O group, with the oxygen acting a~ the donor site. The use of Av or Aviv to determinethe orderof donor abilitiesis not completely adequate, but may be justiiledby the fact that for the complexes of titanium tetrafluoridewith a series of substituted pyridine N-oxides the v NO of the complexes donors have been used to obtain linear Hammet plots [S]. We are currently determining thermodynamic data for the association of these & complexes which should clarify thia point. UnCertity of North Dakota Abbott Hall Departwwnt of C%e&&y Grand Fork.9, North Dakota 68201
rNo~~ KULEVSKY ROLAND G. SEVERSON,JR.
[f3] F. E. DEOKSON,E. Q. GOWLINCIand F. F. BENTLY, Inorg. Chmn. 6, 1099 (1967).