Ind. Eng. Chem. Res. 1998, 37, 675-683
31P
675
NMR Analysis of Cyclotriphosphazenes Ho-Shing Wu* and Shang-Shin Meng Department of Chemical Engineering, Yuan-Ze University, 135 Far-East Road, ChungLi, Taoyuan 32026, Taiwan, Republic of China
The relationships of 31P chemical shift with the degree of the substitution, the different kind of substituent group, reaction activation energy, and electronegativity and of the coupling constant with the degree of the substitution were investigated. The 31P chemical shifts and the coupling constants correlate linearly with the degree of the substitution. The chemical shifts are proportional to the reaction activation energy. Observed chemical shifts are expressed and calculated with a partial chemical shift of the unit substituent group and electronegativity. These correlations can be useful for the assignment of the resonances of other related derivatives. Introduction Cyclotriphosphazene derivatives are usually prepared by the reaction of N3P3Cl6 with the corresponding substituent group, and a mixture of derivatives is obtained in the process. NMR spectroscopy, 31P NMR in particular, has been one of the main tools for the analysis of such mixtures because proton NMR data are uninformative (Krishnamurthy and Woods, 1987; Allcock, 1972). NMR data provide structural information to draw regarding the nature of the electronic interaction within the phosphazene ring as well as of the interaction between the ring and a substituent. Substituent effects on phosphorus chemical shifts are complicated and depend on an interaction of electronegativity, bond angles, and the charges in occupancy of phosphorus p and d orbitals. 31P chemical shifts and coupling constants are available for a large number of cyclotriphosphazenes containing different substituents, and the data are listed in Tables A.1-A.4 in the Appendix. In this study, we design some empirical correlations to predict and explain the phosphorus chemical shifts.
Figure 1. Correlation between the chemical shifts of halogenocyclotriphosphazene and the electronegativtiy of the halogens. (1) gem-N3P3Cl4(Ph)F; (2) gem-N3P3Cl4(Ph)Cl; (3) gem-N3P3Cl4(Ph)Br; (4) gem-N3P3Cl4(Ph)I.
Results and Discussion Relationship of Chemical Shift and Pauling Electronegativity. The shielding of a nucleus is not usually bonded directly to the electronegative elements. Nevertheless, their influence has far-reaching effects through the phosphorus skeleton of a compound. The charge density at the neighboring phosphorus atom becomes a determining factor for the resonance frequency of a substituent. Figures 1-3 display the results for the halogeno-, alkyl-, and aminocyclotriphosphazene, respectively, to illustrate the expected relationships between the chemical shift and the electronegativity of the substituents. The δPCl2 and δPClR values are found to increase proportionally with increasing electronegativity of the substituent. The shielding sequence is F > Cl > Br > I (except for F for δPClR), But > Pri > Pr > Et > Me, and NHMe > NMe2 > NH2. When the substituent type is similar, the chemical shift has a regular character. On the other hand, when the sub* To whom all correspondence should be addressed. Email:
[email protected]. Fax: (+886)-3-455-9373. Tel: (+886)-3-4638800-564.
Figure 2. Correlation between the chemical shifts of alkylcyclotriphosphazene and the electronegativtiy of the halogens. (1) N3P3Cl5(Me); (2) N3P3Cl5(Et); (3) N3P3Cl5(Pr); (4) N3P3Cl5(Pri); (5) N3P3Cl5(But).
stituent type is different, the result is not consistent with that as mentioned above (Table 1). The chemical shift does not function to the electronegativity. Relationship of Chemical Shift and Degree of Substitution. According to the data in Tables A.1-
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676 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 2. Slopes of the Linear Relationship for Different Substituent Groups of Cyclotriphosphazene
Figure 3. Correlation between the chemical shifts of aminocyclotriphosphazene and the electronegativtiy of the halogens. (1) N3P3Cl5(NH2); (2) N3P3Cl5(NHMe); (3) N3P3Cl5(NMe2)).
R
SPCl2
SPClR
F Br Me Et Prn Pri Bu Ph NH2 NHMe NHEt NHPr NHCH2COOEt NHC6H5 NHC6H4Me-p NHC6H4OMe-p NC2H4 NMe2 NC4H8 NC4H8O NC5H10 OCHCH2 OCH2CF3 OCH2CF2CF2H OCH2C2F5 OCH(CF3)2 OCH2C3F7 OCH2(CF2)3CF2H OC6H5 OC6H4CH2CHCH2 OC6H4Me-p OC6H4Cl-m OC6H4Cl-p OC6H4Cl-o OC6H4Br-m OC6H4Br-p OC6H4Br-o OC6H4CHO-p OC6H4CH3-p
3.75 -1.85 -1.44 -2.5 -2.8 -2.9 -3.0 -1.15 -2.1 -0.4 0.5 0.6 1.41 0.9 0.6 0.76 0.92 0.6
3.95 -1.58
-0.65
2.9 0.6 1.96 2.43 2.20 2.00 1.83 1.675 1.4 2.05 1.95 1.9 2.00 1.95 1.55 1.95 1.70
0.9 2.1 2.05 1.6 1.47 2.1 2.89 2.84 3.23 2.47 2.82 2.73 2.13 2.48 4.60 4.60 3.40 2.55 2.46 2.38 2.65 2.29 2.47 2.28 2.22 2.47 2.21 1.78 2.52
SPR2 -1.8 -2.45
-1.02 2.25 1.6 2.23 1.6 1.6 1.78 1.18 0.94 0.85 0.95 1.46 2.95 2.51 2.40 1.75 2.35 2.44 2.32 2.68 2.08 2.39 2.29 2.08 2.39 1.69 2.19 2.32
as
Figure 4. Relationship between the chemical shifts and the degree of substitution. (O) δPCl2; (0) δPClR; (4) δPR2. (a) N3P3Cl6-nBrn; (b) N3P3Cl6-nPhn; (c) N3P3Cl6-n(NMe2)n; (d) N3P3Cl6-n(OC6H5)n. Table 1. 31P Chemical Shift on Electronegativity on Different Substituent Types for Cyclotriphosphazene compound
δPCl2
δPXCl
electronegativity
N3P3Cl5(F) N3P3Cl5(OMe) N3P3Cl5(OPh) N3P3Cl5(NH2) N3P3Cl5(Ph) N3P3Cl5(Me)
22.8 22.5 22.8 20.4 21.2 21.3
14.5 16.7 12.7 19 28.9 39.2
4 3.543 3.525 2.992 2.717 2.472
A.4, increasing substitution on a phosphorus atom for the electron-withdrawing substituent group leads to upfield shift (δPCl2 > δPClR > δPR2) and for the electrondonating substituent group leads to downfield shift (δPCl2 < δPClR < δPR2), except for the substituent groups of Ph, NC2H4, and NMe2 (δPClR > δPR2 > δPCl2). Figure 4 shows the relationships between 31P chemical shift and the degree of substitution for cyclotriphosphazene reacting with substituents (Br, Ph, NMe2, and OC6H5). The three types (δPCl2, δPClR, and δPR2) of 31P chemical shift are proportional to the degree of substitution, n. The three lines have almost identical slopes. The finding corresponds to the report of Krishnamurthy and Woods (1987). The three linear relationships can be expressed
δPCl2 ) SPCln + IPCl
(1)
δPR2 ) SPR2n + IPR2
(2)
δPClR ) SPClRn + IPClR
(3)
where S and I denote the slope and intercept of the linear relationships, respectively. Figure 4 shows four different substituent groups for cyclotriphosphazene. The slopes for the substituents of OC6H5 and NMe2 are positive; those for the substituents Br and Ph are negative. Table 2 lists the slopes of the linear relationships between chemical shift and the degree of substitution for different substituent types of cyclotriphosphazene. The finding reveals that the slope value is positive when the electronegativity of the substituent (F or alkoxy) is more than that of Cl; on the other hand, the slope value is negative. The presence of an electrondonating substituent group in which the electronegativity is below that of Cl, at one end of P-N-P, causes the drift of electron density from the phosphorus carrying the substituent group toward the other end. This electron drift effectively deshields the phosphorus nucleus. The order of magnitude for the slope of chemical shift is SPClR > SPR2 > SPCl2 expect for R ) NC4H8O, OCHCH2, OCH2CF3, OC6H4Me-p, OC6H4Clo, and OC6H4CHO-p in this study.
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 677
Figure 5. Relationship between the chemical shift differences and the activation energy differences. (O) δPCl2; (0) δPClR; (4) δPR2. (a) N3P3Cl6 ) 5.9 mmol, HOCH2CF3 ) 70 mmol, NaOH ) 75 mmol, H2O ) 20 mL, C6H5Cl ) 50 mL (Wang and Wu, 1991); (b) N3P3Cl6 ) 5.9 mmol, HOCH2CF3 ) 70 mmol, NaOH ) 75 mmol, H2O ) 20 mL, CH2Cl2 ) 50 mL (Wang and Wu, 1991); (c) N3P3Cl6 ) 5.9 mmol, HOCH2CF3 ) 70 mmol, NaOH ) 75 mmol, H2O ) 20 mL, C6H5Cl ) 50 mL (Wang and Wu, 1991).
Table 3 R
PCSCl
F Br Me Et Prn Pri Bu Ph NH2 NHMe NHEt NHPr NHCH2COOEt NHC6H5 NHC6H4Me-p NHC6H4OMe-p NC2H4 NMe2 NC4H8 NC4H8O NC5H10 OCHCH2 OCH2CF3 OCH2CF2CF2H OCH2C2F5 OCH(CF3)2 OCH2(CF2)3CF2H OC6H5 OC6H4CH2CHCH2 OC6H4Me-p OC6H4Cl-m OC6H4Cl-p OC6H4Cl-o OC6H4Br-m OC6H4Br-p OC6H4Br-o OC6H4CHO-p OC6H4CH3-p
9.5 + 1.87n 9.9 - 0.93n 12.2 - 0.72n 12.1 - 1.25n 12.1 - 1.4n 12.3 - 1.45n 12.2 - 1.5n 9.7 - 0.57n 11.2 - 1.1n 11.1 - 0.2n 9.9 + 0.25n 9.9 + 0.3n 9.6 + 0.7n 10 + 0.45n 10.5 + 0.3n 10.3 + 0.38n 10.7 + 0.46n 10 + 0.3n 8.1 + 1.45n 10.1 + 0.3n 10.5 + 0.98n 10 + 1.2n 10.2 + 1.1n 10.3 + 1n 10.5 + 0.92n 10.6 + 0.84n 10.5 + 0.7n 10.2 + 1.03n 10.3 + 0.96n 10.1 + 0.95n 10.2 + 1n 10.3 + 0.98n 10.5 + 0.77n 10.3 + 0.98n 10.3 + 0.85n
PCSR -17 - 0.9n 20.4 - 1.23n 30.5 - 3.2n 28.6 - 3.4n 37.4 - 4.4n 29.1 - 3.4n 10.8 - 0.5n 2.3 + 1.1n 3.5 + 1.34n 3.0 + 2.5n 1.5 + 0.8n 2.7 + 1.1n -2.9 + 0.8n -2.4 + 0.8n -2.2 + 0.89n 16 + 0.59n 9.5 + 0.47n 6.4 + 0.43n 7.1 + 0.47n 6.2 + 0.73n -3 + 1.48n 0.69 + 1.25n 1.34 + 1.2n 17.7 + 1.55n 1.65 + 0.88n 1.4 + 1.2n -2.4 + 1.22n -2.3 + 1.16n -3.3 + 1.34n -1.94 + 1.04n -2.58 + 1.2n -2.7 + 1.15n 1.97 + 1.04n -2.58 + 1.2n 1.2 + 0.85n -2.84 + 1.1n 2.32 + 1.18n
Relationship of Chemical Shift and Type of Substituent Group. Harris (1983) has analyzed the 31P chemical shifts of a series of alkyl- and arylsubstituted cyclotriphosphazenes and evaluated the partial contributions of alkyl groups to express an
Figure 6. Relationship between the coupling constant and the degree of substitution. Table 4. Slopes of the Linear Relationship for Different Substituent Groups of Cyclotriphosphazene R
SJAB
Me Ph NHPr NHCH2COOEt NHC6H4OMe-p NC2H4 NMe2 NC4H8O NC5H10 OCHCH2 OCH2CF3 OCH2CF2CF2H OC6H5 OC6H4CH2CHCH2 OC6H4Me-p OC6H4Cl-m OC6H4Cl-p OC6H4Cl-o OC6H4Br-m OC6H4Br-p OC6H4Br-o OC6H4CHO-p OC6H4CH3-p
-1.58 -3.27 0.95 4.71 1.11 -2.6 -3.21 -1.95 -2.46 5.42 5.89 6.1 6.27 5.85 6.2 15.4 14.67 16.5 15.27 14.67 16.53 16.24 13.58
equation for the observed chemical shift.
observed chemical shift ) (partial shift contribution for alkyl) × (bulk coefficient) + partial shift contribution for halogen (4) Equation 4 was only applied to calculate the first and second degree of substitution. The significance of the bulk coefficient was not explained. In this study, PCSCl and PCSR represent the partial chemical shift (PCS) of the Cl and R group, respectively. According to Tables 2 and A.1-A.4 and eq 4, the partial chemical shifts of the Cl and R group are listed in Table 3. The results can be described by three parts: (i) The electronegativity of the substituent is less than that of Cl (e.g., Br, alkyl):
δPCl2 ) 2PCSCl
(5)
δPR2 ) 2PCSR
(6)
()
δPClR ) PCSCl
eCl eR
3
+ PCSR
(7)
where e represents the electronegativity of the substituent.
678 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table A.1. Chemical Shift and Coupling Constant of δPCl2
δPClR
N3P3Cl5F
22.8
cis-N3P3Cl4F2 trans-N3P3Cl4F2 N3P3F6 N3P3Cl5Br nongem-N3P3Cl4Br2 nongem-N3P3Cl3Br3 gem-N3P3Cl3Br3 nongem-N3P3Cl2Br4 N3P3ClBr5 N3P3Br6
compound
31P
NMR for Halogenocyclotriphosphazenes δPR2
JAB (Hz)
refs
14.5
79.0
26.7 26.4
18.5 18.4
100.2 97.6
17.7 16.1
-7.8 -8.7 -9.8 -10.0 -12.1 -14.0
Paddock and Patmore, 1976; Heatley and Todd, 1966 Clare et al., 1975 Clare et al., 1975 Allen et al., 1968 Engelhardt et al., 1966 Engelhardt et al., 1966 Coxon and Sowerby, 1967 Engelhardt et al., 1966 Engelhardt et al., 1966 Engelhardt et al., 1966 Engelhardt et al., 1966
14.0
Table A.2. Chemical Shift and Coupling Constant of compound
δPCl2
δPClR
N3P3Cl5Me gem-N3P3Cl4Me2 gem-N3P3Cl2Me4 N3P3Me6 N3P3Cl5Et gem-N3P3Cl4Et2 N3P3Cl5(Prn) gem-N3P3Cl4(Prn2) N3P3Cl5(Pri) gem-N3P3Cl4(Pri2) N3P3Cl5(Bun) gem-N3P3Cl4(Bun2) N3P3Cl5(But) N3P3Cl5Ph gem-N3P3Cl4Ph2 cis-N3P3Cl3Ph3 trans-N3P3Cl3Ph3 gem-N3P3Cl2Ph4 cis-N3P3Cl2Ph4 trans-N3P3Cl2Ph4 N3P3ClPh5 N3P3Ph6
21.3 18.0 16.6
39.2
21.7 19.2 21.3 18.5 21.7 18.8 21.4 18.4 21.8 21.2 17.1
46.0
13.9
-39.8 -41.3 -42.5 -45.4 31P
NMR for Alkyl- or Arylcyclotriphosphazenes
δPR2 35.7 31.6 25.9 48.1
43.7 43.6 51.8 56.6 44.2 44.5 57.1 28.9 19.5 29.6 30.2
14.8 30.2 28.5 28.6
5.4 17.1 19.0 19.0 16.8 15.2
(ii) The electronegativity of the substituent approximates to that of Cl (e.g., amino):
δPCl2 ) 2PCSCl
(8)
δPR2 ) 2PCSR
(9)
δPClR ) 1.35(PCSCl + PCSR)
(10)
(iii) The electronegativity of the substituent is more than that of Cl (e.g., alkoxy, aryloxy):
δPCl2 ) 2PCSCl
(11)
δPR2 ) 2PCSR
(12)
()
δPClR ) PCSCl
eR eCl
1.5
+ PCSR
(13)
Although all values are quoted with reference to aqueous 85% phosphoric acid (external standard) in Tables A.1-A.4, the chemical shift shows little difference because the NMR condition is different. For instance, the chemical shift of N3P3Cl6 is in the range of 19-20.5 ppm. Hence, the maximum error for the chemical shift calculated is 15%. Most regression factors of the linearsquares regression are larger than 0.98. Relationship between Chemical Shift and Activation Energy. While most cyclotriphosphazenes have been characterized by the use of 31P NMR spectroscopy, no attempt has been made to interpret these data in
JAB (Hz) 7.8 8.9 3.6 2.0
