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Induced optical rotation — The Pfeiffer effect of some hexacoordinate group (IV) metal complexes
INORG.
NIJCL,
CHEM.
LETTERS
Vol. 6,
pp.
295-2/'18, 1970,
Pergamon
Pless.
PrinleCl
in
Great
BrHaJn,
INDUCED OPTICAL ROTATION - THE PFEIFFER EFFECT OF SOME HEXACOORDINATE GROUP (IV) METAL COMPLEXES \/era Doron* and William Durham Department of Chemistry, Rutgers University The State University of New Jersey, Newark, N .J° 07102 (Re(:eiued ~ Oc:toher 196"9: ir~ revised form 1(~ Der't'mber l~ff~9~
A considerable amount of research has been done recently on the characterlzat~on 1-7 of hexacoordlnate tln(IV) and tltanlum(IV) compounds, such as R2M(AA) 2 and M(AA)2X2, in which X is a halide, R a phenyl or alkyl group, AA a bidentate llgand, and M tltanlum(IV) and tln(IV)°
Some of these compounds have been characterized as the cls,
others as the trans species. In some cases for Sn(IV), the trans configuration apparently has been assigned erroneously 4,8 on the basis of N . M . R . evidence, even though, For example, a dipole moment of approximately 4 D was found for the compound (C6Hs)2Sn(AA)2, (AA = acetylacetonate)o
Nelson and Martln 4 suggested, however, that this moment was
due to h~gh induced polarization°
Doron and Fischer7suggested a study of the temperature
dependence of the d~pole moment to ascertain whether a permanent dipole moment was ~nvalved, and the compounds were found to have permanent d~pole moments° 9, 10, 11 In view of the strong evidence in support of the cis J configuration and the suggestion 5, 7, 12 that stereochemlcally non-rigid spec ~es (most probably configurational equilibrla) are present in these cases, it was of great interest to see whether these labile hexacoordinate racemic mixtures of optically acHve compounds, (which are as yet unresolvable* and which at least potentially could exhibit configuratlonal act~vlty or the Pfeiffer Effect 13-16) will in fact show the Effect. ;ATJ inquirles to T0~addressed to this auth-~r. **Numerous attempts by this investigator and others4 by a variety of resolution methods were unsuccessfu I. 285
286
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
Vol. 6, No. 3.
The Pfeiffer Effect is a change in optical ratation of a solution containing a stable asymmetric compound, the so-called "e nvlronment " compound", in the presence of a labile (or stereochemically non-rigid) racemic species in solution. In the present work we report the observation of the Pfeiffer Effect for [Sn(acac)2CI2], [Sn(dbzm)2CI2] and [(C6H5)2Sn(dbzm)2]*o To our knowledge this is the first time that optical activity changes have been induced in a racemic inner complex of a metal of Group (IV). In view of the fact that this effect could not be brought about by non-racemic, symmetrical species of tin(IV) under identical conditions, further evidence is offered in favor of the ci.~sconfiguration for the above species°
TABLE 1. The Pfeiffer Effect of [Sn(acac)2CI2], [Sn(dbzm)2CI2] and [(C6H5)2Sn(dbzm) 2] in DMF solutions.
Initial racemic cornpl ex
Environment comp 'd
Pfeiffer rotation* P(obsd)* (deg)
~e+c(deg)*
~e(deg)*
[Sn(acac)2CI 2] (0.03 M)
d-Malic acid (0.2M)
+0.351
+0.609
+0.258
[(C6Hs)2Sn(dbzm)2] (0.03MI
d-Malic acid (0.2 M)
+0.158
+0.418
+0.260
[Sn(acac)2CI 2] (O.03M)-
1-Mallc acid (O.2M)
+0.359
-0.628
-0.269
[Sn(dbzm)2CI ~] (0.03M) "
d-Malic acid (0.2M)
+0.312
+0.580
+0.268
"25 °, 589m~; P(obsd)18 = + (a~e+c - ~e); (~e+c, observed rotation of Pfeiffer active system, contalning both the added racemlc complex and the environment compound.~e, observed rotation of environment compound in solution before addition of racemlc complex. *acac = acetylacetonate; dbzm = dibenzoylmethanate o
Vol. 6, No. 3.
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
287
The experiments were carried out in DMF solutions employing d- a n d / - malic acids as environment compounds° Table I lists the Pfeiffer rotations of the complexes, as well as the observed rotations of the complexes in the presence of an environment compound, and of the environment compound only, prior to complex addition. The Effect is observed instantaneously, which lends support to the proposed theory of the Pfeiffer Effect, 17,18 and it is important to note that none of the systems exhibiting the Effect instantaneously have as yet been resolved by conventional means. Further studies including ORD and CD measurements on these and related hexacoordinate species of the Group (IV)5-7'9-11 elements, which were shown to be labile and found to have permanent dipole moments, are currently in progress.
ACKNOWLEDGEMENT We thank Professor S. Kirschner of Wayne State University for making available to us some of his research data prior to publication. We also thank the Rutgers University Research Council for financial support, and the National Science Foundation for a summer undergraduate research participancy for W.D.
REFERENCES .
Y. KAWASAKI, T. TANAKA, J. Chem. Phys., 43, 3396 (1965)
2.
J.A. SMITH, E.J. WILKINS, Chem. Comm., 381 (1965).
3.
Y. KAWASAKI, T. TANAKA, R. OKAWARA, Inorg. Nucl. Chem. Letters, 2, 9
,
(l966). W.H. NELSON, D.F. MARTIN, J. Inorg. Nucl. Chem., 27, 89 (1965).
5.
R.C. FAY, R.N. LOWRY, Inorg, Chem., 6, 1512 (1967).
6.
W.H. NELSON, Inorg. Chem., 6, 1509 (1967).
7.
V. DORON, C. FISCHER, Inorg. Chem., 6, 1917 (1967).
8.
M . M . McGRADY, R.S. TOBIAS, ,.Am. Chem. Soc., 87, 1909 (1965).
9.
C . Z . MOORE, W.H.
NELSON, Inorg, Chem., 8, 138 (1969).
288
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
10. C.Z. MOORE, W.H. HELSON, ibid.,
Vol. 6 . No. 3,
8, 143 (1969).
11. R..C. FAY, Abstr. of Meeting of the ACS, Minneapolis, Minn., April 1968, Inorg. Div. 51. 12. J.W. FALLER, A. DAVlSON, Inorg, Chem., 6, 182 (1967). 13. P. PFEIFFER,K. QUEHL, Ber., 64, 2667 (1931). 14. W.W. BRANDT, F.P. DWYER, E.C. GYARFAS, Chem. Rev., 54, 998 (1954). 15. P. PFEIFFER,Y. NAKASUKA, Bet., 6_~,410 (1933). 16. S. KIRSCHNER, K.R. MAGNELL, Advances in Chem. Series, 62, 366 (1966). 17. S. KIRSCHNER, Coord. Chem. Rev.i 3, 201 (1968). 18. S. KIRSCHNER, N. AHMAD, J. Am. Chem., Soc., 90, 1910 (1968).
NIJCL,
CHEM.
LETTERS
Vol. 6,
pp.
295-2/'18, 1970,
Pergamon
Pless.
PrinleCl
in
Great
BrHaJn,
INDUCED OPTICAL ROTATION - THE PFEIFFER EFFECT OF SOME HEXACOORDINATE GROUP (IV) METAL COMPLEXES \/era Doron* and William Durham Department of Chemistry, Rutgers University The State University of New Jersey, Newark, N .J° 07102 (Re(:eiued ~ Oc:toher 196"9: ir~ revised form 1(~ Der't'mber l~ff~9~
A considerable amount of research has been done recently on the characterlzat~on 1-7 of hexacoordlnate tln(IV) and tltanlum(IV) compounds, such as R2M(AA) 2 and M(AA)2X2, in which X is a halide, R a phenyl or alkyl group, AA a bidentate llgand, and M tltanlum(IV) and tln(IV)°
Some of these compounds have been characterized as the cls,
others as the trans species. In some cases for Sn(IV), the trans configuration apparently has been assigned erroneously 4,8 on the basis of N . M . R . evidence, even though, For example, a dipole moment of approximately 4 D was found for the compound (C6Hs)2Sn(AA)2, (AA = acetylacetonate)o
Nelson and Martln 4 suggested, however, that this moment was
due to h~gh induced polarization°
Doron and Fischer7suggested a study of the temperature
dependence of the d~pole moment to ascertain whether a permanent dipole moment was ~nvalved, and the compounds were found to have permanent d~pole moments° 9, 10, 11 In view of the strong evidence in support of the cis J configuration and the suggestion 5, 7, 12 that stereochemlcally non-rigid spec ~es (most probably configurational equilibrla) are present in these cases, it was of great interest to see whether these labile hexacoordinate racemic mixtures of optically acHve compounds, (which are as yet unresolvable* and which at least potentially could exhibit configuratlonal act~vlty or the Pfeiffer Effect 13-16) will in fact show the Effect. ;ATJ inquirles to T0~addressed to this auth-~r. **Numerous attempts by this investigator and others4 by a variety of resolution methods were unsuccessfu I. 285
286
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
Vol. 6, No. 3.
The Pfeiffer Effect is a change in optical ratation of a solution containing a stable asymmetric compound, the so-called "e nvlronment " compound", in the presence of a labile (or stereochemically non-rigid) racemic species in solution. In the present work we report the observation of the Pfeiffer Effect for [Sn(acac)2CI2], [Sn(dbzm)2CI2] and [(C6H5)2Sn(dbzm)2]*o To our knowledge this is the first time that optical activity changes have been induced in a racemic inner complex of a metal of Group (IV). In view of the fact that this effect could not be brought about by non-racemic, symmetrical species of tin(IV) under identical conditions, further evidence is offered in favor of the ci.~sconfiguration for the above species°
TABLE 1. The Pfeiffer Effect of [Sn(acac)2CI2], [Sn(dbzm)2CI2] and [(C6H5)2Sn(dbzm) 2] in DMF solutions.
Initial racemic cornpl ex
Environment comp 'd
Pfeiffer rotation* P(obsd)* (deg)
~e+c(deg)*
~e(deg)*
[Sn(acac)2CI 2] (0.03 M)
d-Malic acid (0.2M)
+0.351
+0.609
+0.258
[(C6Hs)2Sn(dbzm)2] (0.03MI
d-Malic acid (0.2 M)
+0.158
+0.418
+0.260
[Sn(acac)2CI 2] (O.03M)-
1-Mallc acid (O.2M)
+0.359
-0.628
-0.269
[Sn(dbzm)2CI ~] (0.03M) "
d-Malic acid (0.2M)
+0.312
+0.580
+0.268
"25 °, 589m~; P(obsd)18 = + (a~e+c - ~e); (~e+c, observed rotation of Pfeiffer active system, contalning both the added racemlc complex and the environment compound.~e, observed rotation of environment compound in solution before addition of racemlc complex. *acac = acetylacetonate; dbzm = dibenzoylmethanate o
Vol. 6, No. 3.
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
287
The experiments were carried out in DMF solutions employing d- a n d / - malic acids as environment compounds° Table I lists the Pfeiffer rotations of the complexes, as well as the observed rotations of the complexes in the presence of an environment compound, and of the environment compound only, prior to complex addition. The Effect is observed instantaneously, which lends support to the proposed theory of the Pfeiffer Effect, 17,18 and it is important to note that none of the systems exhibiting the Effect instantaneously have as yet been resolved by conventional means. Further studies including ORD and CD measurements on these and related hexacoordinate species of the Group (IV)5-7'9-11 elements, which were shown to be labile and found to have permanent dipole moments, are currently in progress.
ACKNOWLEDGEMENT We thank Professor S. Kirschner of Wayne State University for making available to us some of his research data prior to publication. We also thank the Rutgers University Research Council for financial support, and the National Science Foundation for a summer undergraduate research participancy for W.D.
REFERENCES .
Y. KAWASAKI, T. TANAKA, J. Chem. Phys., 43, 3396 (1965)
2.
J.A. SMITH, E.J. WILKINS, Chem. Comm., 381 (1965).
3.
Y. KAWASAKI, T. TANAKA, R. OKAWARA, Inorg. Nucl. Chem. Letters, 2, 9
,
(l966). W.H. NELSON, D.F. MARTIN, J. Inorg. Nucl. Chem., 27, 89 (1965).
5.
R.C. FAY, R.N. LOWRY, Inorg, Chem., 6, 1512 (1967).
6.
W.H. NELSON, Inorg. Chem., 6, 1509 (1967).
7.
V. DORON, C. FISCHER, Inorg. Chem., 6, 1917 (1967).
8.
M . M . McGRADY, R.S. TOBIAS, ,.Am. Chem. Soc., 87, 1909 (1965).
9.
C . Z . MOORE, W.H.
NELSON, Inorg, Chem., 8, 138 (1969).
288
HEXACOORDINATE GROUP (IV) METAL COMPLEXES
10. C.Z. MOORE, W.H. HELSON, ibid.,
Vol. 6 . No. 3,
8, 143 (1969).
11. R..C. FAY, Abstr. of Meeting of the ACS, Minneapolis, Minn., April 1968, Inorg. Div. 51. 12. J.W. FALLER, A. DAVlSON, Inorg, Chem., 6, 182 (1967). 13. P. PFEIFFER,K. QUEHL, Ber., 64, 2667 (1931). 14. W.W. BRANDT, F.P. DWYER, E.C. GYARFAS, Chem. Rev., 54, 998 (1954). 15. P. PFEIFFER,Y. NAKASUKA, Bet., 6_~,410 (1933). 16. S. KIRSCHNER, K.R. MAGNELL, Advances in Chem. Series, 62, 366 (1966). 17. S. KIRSCHNER, Coord. Chem. Rev.i 3, 201 (1968). 18. S. KIRSCHNER, N. AHMAD, J. Am. Chem., Soc., 90, 1910 (1968).