R ~ O R R T SL. YIELSENA N D ,J. V. PVSTINGER, ,JIL
152
Phosphorus-31 Nuclear Magnetic Resonance Studies of Phosphorus-Nitrogen Compounds
by Morris L. Nielsen and J. V. Pustinger, Jr. Dayton Laboratory, Monuanto Research Corporation, Dayton, Ohio
(Reccived Auyuat 16, 1908)
Correlations between PSI n.m.r. chemical shifts and structures of 66 phosphorus--nitrogen compounds are made for use in qualitative prediction. Substitution of -OH (-OSa) or OPli by -SI32 displaces the chemical shift to lower field, with somewhat grcatcr effect for covalent compounds. Substitution of -KH- for -0- in chain and ring compounds also generally results in negative displacement, with the exception of R2P(0)SHP(O)R2. The effects of substituting -NRz for C1 or -Ar groups on phosphorus are shown. Slcthyl groups on nitrogen attached to phosphorus generally give chemical shifts at lower field than do phenyl groups. In phosphonitrilic compounds, the trimeric derivatives have chemical shifts at lower field than the tetrameric ones Spin spin splittings are observed for 1’- K--P and 1’- N-H systems, in confirmation of their expected structures.
Phosphorus nuclear magnetic resonance is playing an ever-increasing role in structure characterization and qualitative identification. Chemical shift data for several hundred inorganic- and organophosphorus compounds have been published, --R and attempts have been made to correlate these data with bond propertips und electronic structures of - 4 ~ 6 ~lo9 ~ Comparatively few data have heen reported for phosphorus- nitrogen compounds. To cxttnd the uacfulness of the phosphorus magnetic resonance data, we have measured the chemical shifts far some 66 quadruply connected phosphorus-nitrogen compounds. Empirical correlations describing the relative effcct of different substituent groups on the phosphorus atom and substitution on the nitrogen atom are presented. Measurements of spin-spin coupling between phosphorus nuclci arid protons in the aromatic phosplioramidstes establish the prescncc of considcrable covalent bonding in the 1’-N-I3 bond networks. Thc observation of these spin-spin splittings shows minimal electrical quadrupolar iritcractions arising from the nitrogen nucleus.
Experimental The n.m.r. spectra were obtained on a Varian Model 1’-4:300-2 high resolution spectrometer with a radiofrequmcy of 16.2 51c. arid a magnetic field of approximately 9400 gauss, using a Variari magnet, Model VThe Journal of Physical Chemistry
4012-A. Chemical shift,s, reported in parts per million (p.p.m.) of the applied field, are based upon 85% H3P04 as the standard (zero shift). Upfield shifts are denoted by a plus sign, downfield shifts by a minus sign. The samples were generally contained in a 15-mm. o.d. Pyrex tube, with a Iiarrow (1-2 mm.) tube containing the HaP04inserted concentrically through thc stopper. Accuracy is approximately &0,5 p.p.rn. For mcasuririg spin. spin coupling constants, samples were coiltained in 6 m m . o.d. t,ubes and spun in order to minimize field inhomogeneities. All data were obtained with samples of pure compounds identified by physical constants or elernental TI. 8. Gutowsky, D. W. McCall. and (1. 1’. Slichtcr, J . Chem. Phys., 21, 279 (1953). (2) 11. 9. Gutow8ky and D. W . M d h l l , ibid., 22, 162 (1954). (3) N. Muller, P. C. 1,auterbur. and J. (:oldenson. J . Am. Chcm. Yoc., 78, 3557 (1956). (4) J. It. Van Waeer, C. F. Callis, J. N. Shoolcry, and It. C . Jones, ibid., 78, 5716 (1956). (5) 1%.Finegold, Ann. N . Y . Acad. Sci., 70, 875 (1958). (fi) C. F. Callis. J. R. Van Wazer, J. N. Shoolery, and W. A. Anderson, J . Am. Chem. Soc., 79, 2719 (1957). (7) K. Moedritzer, L. Maier, and L. C. D. Grmnweghe, J . Chem. Eng. Data, 7, 307 (1962). (8) R. A. Y . Jones and A. R. Katritzky, J . I n o r g . Nucl. C’hem., 15, 193 (1961). (9) J. It. Parks. J. Am. Chem. Soc., 79, 757 (1957). (10) L. C. D. Groenweghe, L. l l a i e r , and K. Moedritzer, J . Phya. Chem., 66, 901 (1962).
(1)
P31
153
N.M.R.STUDIES OF PHOSPHORUB-NITROGEN COMPOUNDS
analyses. Tabulation of data for these and related compounds will appear elsewhere. l1
Results and IHscussion High resolution nuclear magnetic resonance data provide information about the structure of molecules in two ways. The chemical shift data indicate relative degrees of electron screening of the nucleus. Observation of the electron coupled spin-spin interactions provides additional information on the structural relationships of the nonequivalent nuclei in a molecule. Chemical Shift Data.-Several theoretical treahments12-14 of electron shielding have been reported. As yet, a suitable method has not been derived for precise prediction of chemical shifts from n.m.r. theory or empirical relationships. Qualitatively, electron shielding of a nucleus can be considered as being influenced by two opposing effects, namely, electronegativity and tlhe ability to form double bonds. For quadruply connected phosphorus, the chemical shifts are believed due in significant part to changes in the distribution of ?r-bonds among the four u-bonds.15 I n spite of the difficulty in disentangling electronegativity and double bond effects, the data for the phosphorus-nitrogen systems emphasize the influence of the double-bond character of the four u-bonds. Substitution of nitrogen for oxygen on quadruply connected phosphorus results in more negative chemical shifts (Table I). Table I : Displacement of P31 N.m.r. Chemical Shift due to Substitution of the -NHZ Group for -0Na Chemioal shift, p.p.m.
( NaO)sPO (NaO)2P(0)NH2 NaOPO( NH& PO(N&)a (Na0)aPS NaOPS(NH2)Z PS(”z)s (EtO)zP(0)ONa (EtO)zP(O)NHz (PhO)zP(0)ONa (PhO)zP(0)NHe PhOPO(0Na)z PhOPO(NH:!)ONHd PhOPO(NH8)z PhPO(ONa)l PhPO(“2)s PhzP( 0 )ONa PhzP(0)NHz
See ref. 8.
”
See ref. 7.
-6.0 -8.9 -14.5 -22.0 -33.8 -54.2 -61.1 -3.8 -11.1 +9.0 -2.8 0.0 -0.5 -15.2 -13.8 -25.4 -23.6 -25.5
Displacement of shift per N atom, p.p.m,
-
It is probable that the paramagnetic portion of the chemical shift is related to the degree of hybridization of the P-?; bond as compared with that of the P-0 bond. For monomeric phosphorus compounds, there appears to be a relation between the type of compound and the extent of the displacement. The displacement is least €or compounds having a high degree of ionic character, as, for example, (Na0)2P(0)NH2. For the compounds shown, the average displacement is -4 f 1 p.p.m. per nitrogen atom. Compounds which are characteristically covalent, such as those containing alkyl or aryl groups, show tt higher displacement : approximately .- 8 f 3 p.p.m. An “oxygen-to-nitrogen shift” of - 11 $3 2 p.p.m. was reported by Van Wazer, et aL4 The nitrogen-for-oxygen displacement is also negative for the linear dimers (pyrophosphates and imidodiphosphates) and trimers (triphosphates and diimidotriphosphates) (Table 11). These displacements Table 11: Displacement of P31 N.m.r. Chemical Shift due to Substitution of -NH- for -0- in Chain Compounds Chemical shift, p.p.m. (NaO)zP(O)OP(O) (0Na)i (XaO)zP(O)NHP(O)(0Na)z (EtO)zP(O)OP(O)(0Et)z (EtO)zP(OINHP(O)(0Et)l (PhO)aP(O)OP(O)(0Ph)z (PhO)zP(O)NHP(O)(0Ph)z (NaO)Pa(O)OPp(O)(ONa)OPa(O)(ONa)z
+6.0 -2.7 -I-13.4 -2.5 +23.9 $10.7 +4 (Pa) +IS (PB)
Displacement of shift,, p.p.m.
...
(I
-8.7
...
-15.9
...
-13.2
. . .u
(NaO)zP(O)NHP(O)(ONa)NHP(O)(ONa)z (PhO)zPa(O)OPg(O)(OPh)OPa(O)(0Ph)z
-1.6 - 5 . 6 ; -19.6 t26.6(Pa) +35.2 (Pa) (PhO)zPa(O)NHP@(O) (OPh)NHPa(O)(OPh)n 1 2 , 5 (Po) - 1 3 . 1 ; - 2 9 . 3 $ 5 . 9 (PP)
+
a
See ref. 4.
. . .a -2.9 -5.6 -7.5
. . .b -9.2 -6.9
... -7.3
... -11.8
are greater for the esters than for the corresponding salts because of the covalent ligands. A comparison of tetraethyl imidodiphosphate with the sodium salt shows that the influence of the imido group is strong enough to offset the displacement caused by substituting EtO- for NaO-. As a consequence, the net difference is zero. An interesting effect on the chemical shift is produced by the two nitrogens in the pentaphenyl diimidotri-
... -0.5 -7.4
... -5.8 1 . .
-1.9
-
~~
(11) M. L. Nielsen, J. V. Pustinger, and J. Strobel, t o be published. (12) N. F. Ramsey, Phys. Re3., 78, 699 (1950); 83, 450 (1951); 8 5 , BO (1952);86,243 (1952). (13) J. A. Pople, Proc. Roy, 9 o c . (London), A239, 541,550 (1957). (14) W. G. Schneider, H. J. Bernstein, and J. A. Pople, J. Chem. Phys., 28, BO1 (1959). (15) J. R. Van Wazer, “Phosphorus a n d Its Compounds, Vol. I. Chemistry,” Interscience Publishers, Inc., N e w York, N . Y.; 1958,p. 50.
Volume 68, Number 1
January, 1964
MORRISL. NIELSENAND J. V. PUSTINGER, JR.
154
phosphate ester, where the triplet due to the middle 0
I
Table IV: Displacement of Pal N.m.r. Chemical Shift due to Substitution of -NH- for -0- in Ring Compounds Chemical shift, p.p.m.
-NH-P-NH- group (see also Tabli? XI) is a t lower I I field (+5.9 p.p.m.) than the doublet due to the end 0
1 -PNHl
groups (+12.5 p.p.m.).
ONa
op/
d
This can be attrib-
Displaaoment in shift, Pam.
'0
....
4-21.4
uted to the strong influence of two -NH- groups on the P-phosphorus. By comparison, the spectrum of pentaphenyl triphosphate ester is similar to that of sodium t r i p h o ~ p h a t e , in ~ ! ~which the triplet for the middle group is a t higher field than the doublet for the end groups. In contrast to the esters, if two phenyl groups are attached directly to phosphorus, the displacement in chemical shift resulting from replacement of -0- with -NH- is positive (Table 111). This is noted to some degree in the phosphinic compounds in Table I where the difference in the chemical shifts of PhzP(0)ONa and PhzP(0)NH2is only - 1.9 p.p.m.
f10.4 (Pa) f 2 0 . 2 (PP)
-11.0
f 4 . 2 (Pa) f 7 . 9 (PPI
-6.2 -12.3
Table 111: Displacement of Pal N.m.r. Chemical Shift due to Substitution of -NH- for -0- in Diphosphinic Compounds Chemical shift, p.p.m.
Me2P( O)OP( 0)Mez MezP(O)NHP(O)Mez PhzP(O)OP( 0)Phz PhzP(O)NHP(O)Ph, a See ref. 7.
-52.6 -43.4 -33.1 -14.5
. . .a f9.2
...
5
\NH
I
OP
/
NaO
\*/ H
/
-6.4
f1.5
PO
\
ONa
i19.1
I n ring compounds (Table IV) the effect due to substituting -"for -0- is somewhat greater than in the linear dimers and trimers. Substitution of -NH2 for -0Ph on phosphorus causes a negative displacement in the chemical shift (Table V). Substitution of -KRz for 4 1 , where R is aliphatic, in the class (MezN),P(0)C13-., generally results in a negative displacement (Table VI). The chemical shift for the trisubstituted phosphoric amide is not as far in the negative direction as expected. I n contrast, substitution of -K
