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Phosphorus-nitrogen nuclear spin coupling in representative organo-phosphorus compounds
INORG.
NUCL.
CHEM.
LETTERS
Vol.
11:10,
pp.
719-721,
1975.
PHOSPHORUS-NITR(~3}~ ~ C L E A R S P I N
Pergamon
Press, Printed
in Great
Britain.
COUPLING
IN R~RES~TATIV'E ORGANO-PHOSPHORUS CCMPOUN~. W.McFarlane
and B.Wrackmeyer
Department of Chemistry, Sir John Cass School of Science and Technology, City of London Polytechnic, Jewry Street, l~ndon,E.C.3, Great Britain.
(Received 22May 1975)
Nuclear spin coupling constants involving phosphorus show wide variations of magnitude and sign according to the number and nature of the substituents of the phosphorus atom (1,2).
in the cases of 1j(31p-H), 1j(31p-13C) and
1j(31p_31p) these can often be used to assess the phosphorus hybridisation and/or effective nuclear charge, and it is important to know the extent to which couplings between phosphorus and other nuclei can be used similarly.
In
this connection nitrogen is specially interesting, both for comparison with the behaviour of carbon and phosphorus, and because of the possibility of ~ - ~ bonding between phosphorus and nitrogen.
We have therefore prepared and
studied a range of typical nitrogen-containing three- and four- coordinate phosphorus compounds enriched to ca. 96% in 15N (I = ~, y ( O ) -
~{±5N~ "
and 1 H - {3Ap~
and have used
double resonance experiments to get the sigr~s and
magnitudes of 1j(3~-15N) and 2j(31p-N-H).
The results are in the table and
are based upon the assumption that 1j(15N-H) is negative (3).
Previously,
values for 1j(3~-15N) have been reported only for three molecules in which the phosphorus atom bears highly electronegative substituents, and which cannot therefore be regarded as typical (4). It is apparent that the reduced coupling constant 1K(P-N) becomes more positive when the coordination number of the phosphorus atom increases from three to four.
This is the same behaviour as is observed for the couplings
between phosphorus and hydrogen, carbon, phosphorus(V), fluorine, or tin, but is opposite to that found for phosphorus bound to phosphorus(III) or selenium (1,2). Although coupling constants involving fluorSne may involve mechanisms other than the Fermi contact interaction, this is generally considered to dominate when less electronegative elements are involved, and the coupling of phosphorus may be discussed in terms of this mechanism only (5).
The transformation from
three- to four-coordinate phosphorus is generally accompanied by an increase 719
720
Phosphorus-Nitrogen Nuclear Spin Coupling
V o l . 11, N o . IC
TABLE
ij(SIp_I~N)/Hz
IK(PN)/nm-a
2j(31PNH)/Hz
Ij(I~NH)/Hz
6 (31p)/p.p.ma.
b 6 (lS N)/p,p.m.
Me2PNSPh
+53. O
-10.76
-6.0
-81. O
+13.4
+28. 2
He2P(O) NIIPh
-0.5
+ 0.10
+11.1
-83. O
+31.2
+43.7
+11.3
- 2.29
+8.4
-82. O
+51.2
+36. 3 +82. 7 +15. 7
iMe2P(S)~ Me2P (Se)Ninth
+16. 5
- 3.35
+5.3
-83.5
+40. 2
+59. 6
-12.10
-lO. 9
-81. O
+57.4
+11.5
- 2.33
+10,5
-80.5
+58.3
+20. 6
+22. 2
-4.51
+7.8
-78.5
+91. O
+16. 0
+27, 5
-5.58
+6.0
-78. 5
+93.3
+11.5
I (a)
To low field of 85% H 3 PO 4
(b)
To low field of Me4N + I-
i n Me~ SO--c16
in the s_-character of the hybrid orbitals of phosphorus used for bond formation, and also by an increase in the effective nuclear charge of the phosphorus atom.
Either or both of these effects would lead to an increase
in the magnitude of the reduced coupling constant by affecting a~ or--~(0) 2. However, when negative or small positive couplings are encountered they may depend additionally upon nAB the mutual polarizability of the two atoms, and hence upon ~, the ~-overlap integral for the A-B bond (5)°
This ap}~ars to be
the dominant factor for the P - N couplings reported here, as it is for IK(P-C) and 1K(p-P) when the valence of phosphorus changes. ~e
changes in 1K(P-N) are parallelled by changes in 2j(31p-N-H) which
are in the opposite direction to those normally found (1,2) for 2j(31p-C-H). It is known that the signs of geminal coupling constants are very sensitive to the nature of the intermediate atom, and it appears that there has been a reversal of the sign of 2j(31F-N-H) compared with that of 2j(31p-C-H) in both the three- and the four-coordinate compounds.
The only comparable instance to
this is that the direction of the change in 2j(31p-C-H) is also reversed in certain vinyl phosphorus compounds (6) where the intermediate carbon atom is sp 2 rather than sp3 hybridised. This is in conformity with the views of Jameson (7), but it should be noted that 2j(31p-N-H) is very sensitive to rotation about the P-N bond (6) and we cannot allow completely for this factor at present.
There is no evident correlation between 1j(15N-H) and the
effective electronegativity of the substituents of phosphorus; attempted correlations involving this coupling constant have generally had limited success(8).
Vol. 11, No. 10
Phosphorus-Nitrogen Nuclear Spin Coupling
721
Replacement of the two P-methyl groups hy tert-butyl ,hakes 1K(P-N) more negative in each case by ca. 2 nm -3.
This implies that ~PN has become
more negative and hence that ~ is smaller;
this comld be due to the bulk of
the tert-butyl groups which would lead to higher~-character and lower in the P-N bond.
in the P-C bonds
In the absence of bulky groups on nitrogen it
seems unlikely that rotation about the P-N bond would be seriously affected. It is important to note that 1j(3~-15N)
varies over a wide range in
derivatives of four-coordinate phosphorus according to the nature of the substituents of the phosphorus atom, and it is not possible to exclude the possibility of an overlap with the range normally associated with threecoordinate species.
This may place some limitations upon the diagnostic use
of measurements of this coupling constant. Acknowledgements
We thank the Deutsche Forschungsgemeinschaft,
the S.R.Co, and the
University of London Central Research Fund for support.
Re ferences 1.
W.MCFARLANE,
Proc.Roy.Soc.,
306A, 185 (1968).
2.
G.MAVEL, Ann.Rep.N.M.R.Spectroscopy,
3.
L.PAOLILLO and E.D.BECKER, J.Mag.Resonance,
4.
J.R.SCHEIGER,
5 B, 1 (1972). 3, 200 (1970).
A.H.COWL~Wf, E.A.CO~D~N, P.A.GROON and S.L.MANATT, J.Amer.Chem.
Soc., 96, 7122 (1974). 5.
J.A.POPLE and D.P.S~J~TRY, Mol.Phys., 8, 1 (1964).
6.
R.M.LEQUAN and M.P.SIMONNIN,
7.
C.J.J@~ESON, J.Amer.Chem.Soc.,
8.
T.AX~INROD, N.M.R.Spectroscopy Wiley, New York (1974).
Bull.Soc.Chim.France,
2365 (1973).
91, 6232 (1969). of Nuclei Other than Protons, p. 81. John
NUCL.
CHEM.
LETTERS
Vol.
11:10,
pp.
719-721,
1975.
PHOSPHORUS-NITR(~3}~ ~ C L E A R S P I N
Pergamon
Press, Printed
in Great
Britain.
COUPLING
IN R~RES~TATIV'E ORGANO-PHOSPHORUS CCMPOUN~. W.McFarlane
and B.Wrackmeyer
Department of Chemistry, Sir John Cass School of Science and Technology, City of London Polytechnic, Jewry Street, l~ndon,E.C.3, Great Britain.
(Received 22May 1975)
Nuclear spin coupling constants involving phosphorus show wide variations of magnitude and sign according to the number and nature of the substituents of the phosphorus atom (1,2).
in the cases of 1j(31p-H), 1j(31p-13C) and
1j(31p_31p) these can often be used to assess the phosphorus hybridisation and/or effective nuclear charge, and it is important to know the extent to which couplings between phosphorus and other nuclei can be used similarly.
In
this connection nitrogen is specially interesting, both for comparison with the behaviour of carbon and phosphorus, and because of the possibility of ~ - ~ bonding between phosphorus and nitrogen.
We have therefore prepared and
studied a range of typical nitrogen-containing three- and four- coordinate phosphorus compounds enriched to ca. 96% in 15N (I = ~, y ( O ) -
~{±5N~ "
and 1 H - {3Ap~
and have used
double resonance experiments to get the sigr~s and
magnitudes of 1j(3~-15N) and 2j(31p-N-H).
The results are in the table and
are based upon the assumption that 1j(15N-H) is negative (3).
Previously,
values for 1j(3~-15N) have been reported only for three molecules in which the phosphorus atom bears highly electronegative substituents, and which cannot therefore be regarded as typical (4). It is apparent that the reduced coupling constant 1K(P-N) becomes more positive when the coordination number of the phosphorus atom increases from three to four.
This is the same behaviour as is observed for the couplings
between phosphorus and hydrogen, carbon, phosphorus(V), fluorine, or tin, but is opposite to that found for phosphorus bound to phosphorus(III) or selenium (1,2). Although coupling constants involving fluorSne may involve mechanisms other than the Fermi contact interaction, this is generally considered to dominate when less electronegative elements are involved, and the coupling of phosphorus may be discussed in terms of this mechanism only (5).
The transformation from
three- to four-coordinate phosphorus is generally accompanied by an increase 719
720
Phosphorus-Nitrogen Nuclear Spin Coupling
V o l . 11, N o . IC
TABLE
ij(SIp_I~N)/Hz
IK(PN)/nm-a
2j(31PNH)/Hz
Ij(I~NH)/Hz
6 (31p)/p.p.ma.
b 6 (lS N)/p,p.m.
Me2PNSPh
+53. O
-10.76
-6.0
-81. O
+13.4
+28. 2
He2P(O) NIIPh
-0.5
+ 0.10
+11.1
-83. O
+31.2
+43.7
+11.3
- 2.29
+8.4
-82. O
+51.2
+36. 3 +82. 7 +15. 7
iMe2P(S)~ Me2P (Se)Ninth
+16. 5
- 3.35
+5.3
-83.5
+40. 2
+59. 6
-12.10
-lO. 9
-81. O
+57.4
+11.5
- 2.33
+10,5
-80.5
+58.3
+20. 6
+22. 2
-4.51
+7.8
-78.5
+91. O
+16. 0
+27, 5
-5.58
+6.0
-78. 5
+93.3
+11.5
I (a)
To low field of 85% H 3 PO 4
(b)
To low field of Me4N + I-
i n Me~ SO--c16
in the s_-character of the hybrid orbitals of phosphorus used for bond formation, and also by an increase in the effective nuclear charge of the phosphorus atom.
Either or both of these effects would lead to an increase
in the magnitude of the reduced coupling constant by affecting a~ or--~(0) 2. However, when negative or small positive couplings are encountered they may depend additionally upon nAB the mutual polarizability of the two atoms, and hence upon ~, the ~-overlap integral for the A-B bond (5)°
This ap}~ars to be
the dominant factor for the P - N couplings reported here, as it is for IK(P-C) and 1K(p-P) when the valence of phosphorus changes. ~e
changes in 1K(P-N) are parallelled by changes in 2j(31p-N-H) which
are in the opposite direction to those normally found (1,2) for 2j(31p-C-H). It is known that the signs of geminal coupling constants are very sensitive to the nature of the intermediate atom, and it appears that there has been a reversal of the sign of 2j(31F-N-H) compared with that of 2j(31p-C-H) in both the three- and the four-coordinate compounds.
The only comparable instance to
this is that the direction of the change in 2j(31p-C-H) is also reversed in certain vinyl phosphorus compounds (6) where the intermediate carbon atom is sp 2 rather than sp3 hybridised. This is in conformity with the views of Jameson (7), but it should be noted that 2j(31p-N-H) is very sensitive to rotation about the P-N bond (6) and we cannot allow completely for this factor at present.
There is no evident correlation between 1j(15N-H) and the
effective electronegativity of the substituents of phosphorus; attempted correlations involving this coupling constant have generally had limited success(8).
Vol. 11, No. 10
Phosphorus-Nitrogen Nuclear Spin Coupling
721
Replacement of the two P-methyl groups hy tert-butyl ,hakes 1K(P-N) more negative in each case by ca. 2 nm -3.
This implies that ~PN has become
more negative and hence that ~ is smaller;
this comld be due to the bulk of
the tert-butyl groups which would lead to higher~-character and lower in the P-N bond.
in the P-C bonds
In the absence of bulky groups on nitrogen it
seems unlikely that rotation about the P-N bond would be seriously affected. It is important to note that 1j(3~-15N)
varies over a wide range in
derivatives of four-coordinate phosphorus according to the nature of the substituents of the phosphorus atom, and it is not possible to exclude the possibility of an overlap with the range normally associated with threecoordinate species.
This may place some limitations upon the diagnostic use
of measurements of this coupling constant. Acknowledgements
We thank the Deutsche Forschungsgemeinschaft,
the S.R.Co, and the
University of London Central Research Fund for support.
Re ferences 1.
W.MCFARLANE,
Proc.Roy.Soc.,
306A, 185 (1968).
2.
G.MAVEL, Ann.Rep.N.M.R.Spectroscopy,
3.
L.PAOLILLO and E.D.BECKER, J.Mag.Resonance,
4.
J.R.SCHEIGER,
5 B, 1 (1972). 3, 200 (1970).
A.H.COWL~Wf, E.A.CO~D~N, P.A.GROON and S.L.MANATT, J.Amer.Chem.
Soc., 96, 7122 (1974). 5.
J.A.POPLE and D.P.S~J~TRY, Mol.Phys., 8, 1 (1964).
6.
R.M.LEQUAN and M.P.SIMONNIN,
7.
C.J.J@~ESON, J.Amer.Chem.Soc.,
8.
T.AX~INROD, N.M.R.Spectroscopy Wiley, New York (1974).
Bull.Soc.Chim.France,
2365 (1973).
91, 6232 (1969). of Nuclei Other than Protons, p. 81. John