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NMR Spectroscopy of Paramagnetic Complexes.

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NMR Spectroscopy of Paramagnetic Complexes
By Karl E. Schwarzhans~*1
Serviceable N M R spectra can, with a few exceptions [1,61, be recorded for paramagnetic
complexes in solution. These spectra provide information about the structure of the
complexes and the disrribution of the unpaired electrons, and hence also about reactive
centers in the molecule. The elucidation of intermolecular and intramolecular exchange
phenomena, e.g. the determination of Iigand exchange rate constants, the determination
of rotation barriers, and the detection of contact complexes in solution, or even of
occupation equilibria of the electrons, is possible in this way. It can be seen, therefore,
that N M R studies on paramagnetic complexes can be a rich source of’ information.
1. Introduction
The investigation of paramagnetic compounds with
the aid of nuclear magnetic resonance was ignored
during the first decade of the application of this
spectroscopic method to chemical problems. It is
now known that N M R spectra can be recorded for
nearly all paramagnetic complexes 111; exceptions are
rare. In general, however, the spectra of paramagnetic
metal complexes are not so well resolved as those of
corresponding diamagnetic compounds; nucleusnucleus coupling is rarely observed is such spectrarz. 31.
The half-widths of the N M R signals are between
1 Hz and several thousand Hz for dissolved paramagnetic species. Like the half-widths, the shifts of
the signals also extend in part over a range that is
several orders of magnitude greater than for the analogous diamagnetic compounds “+]. Roughly speaking, the shifts behave in the same manner as the
susceptibilities of these compounds.
The first report of an investigation on the 1H-NMR
spectra of (dissolved) paramagnetic complexes was
published in 1958 [51. Since then, the number of publications on this subject has increased to about 400.
The theoretical principles of the application of N M R
spectroscopy to the study of paramagnetic substances
have been described by various authors (cf. e.g. r6-101).
[ * ] Doz. Dr. K. E. Schwarzhans
Anorganisch-chemisches Laboratorium
der Technischen Universitat
8 Miinchen 2, Arcisstrasse 21 (Germany)
[l] H . J . Keller: NMR-Basic Principles and Progress. Springer,
Berlin 1970.
[2] D. R . Eaton and W . D . Phillips, Advan. Magn. Resonance
I , 103 (1965).
[ 3 ] E. W . Randalf and D . Shaw, J. Chem. SOC.A 1969, 2867.
[4] D. R . Eaton: Physical Methods in Advanced Inorganic
Chemistry. Interscience New York 1968.
[ 5 ] H . M . McConnell and C . H . Holm, J. Chem. Phys. 28, 749
(1958).
[6] H . J . Keller and K . E . Schwarzhans, Angew. Chem. 82, 227
(1970); Angew. Chem. internat. Edit. 9, 196 (1970).
[7] H . M . McConnell et a[., J. Chem. Phys. 27, 314 (1957); 28,
107 (1958); 29, 1361 (1958); 34, 696 (1961).
[ 8 ] E . DeBoer and H . van Wiiiigen, Progr. Nucl. Magn. Resonance Spectrosc. 2, 111 (1967).
[91 M. Bose, Progr. Nucl. Magn. Resonance Spectrosc. 4 , 335
(1969).
946
Several mechanisms have been detected by which the
nuclei examined by N M R spectroscopy in metal
complexes can be influenced by unpaired electrons.
The effects can be roughly divided into two groups,
i.e. signal shifts due to Fermi contact interactions and
those due to pseudo-contact interactions. Since it is
only possible occasionally to distinguish between the
contributions of the various mechanisms to the total
shift of a given N M R signal, it has recently been suggestedC31 that signal shifts in the N M R spectra of
paramagnetic compounds should be referred to
generally as “Knight shifts” [Ill. This name has the
advantage that it does not specify any mechanism
that contributes to o r is alone responsible for the
shift, as is the case in the terms “contact shift” and
“pseudo-contact shift”. The Knight shift is thus
defined as the difference in shift between the signals
of the paramagnetic complex and the corresponding
diamagnetic model compound. This definition naturally excludes any effect of ligand exchange on the
shift of the signal.
The mechanisms of electron-nucleus interaction that
may be regarded as established for Paramagnetic
metal complexes will be outlined below.
1.1. Fermi Contact Interaction
The Fermi contact interaction is defined as the interaction of the nucleus in question with the density of
unpaired electrons on the nucleus.
If this Fermi contact interaction essentially causes
shielding of the nucleus in question, and hence a shift
of its signal, “unpaired electron density” (also known
as spin density) could conceivably reach the position
of the resonating nucleus, o r could in general reach
the ligands of a paramagnetic metal complex, in the
following ways:
(a) Unpaired d electrons of the metal fill unoccupied
antibonding orbitals of the ligands that have o
:;r
x
character. This mechanism is involved in particular
[lo] J . W . Emsley, J . Feeney, and L. H . Sutciiffe, High Resolution Nucl. Magn. Resonance Spectrosc. I , 115 (1965); 2, 826
(1966).
I l l ] W . D . Knight, Phys. Rev. 76, 1259 (1949).
Angew. Chem. internat. Edit.
1 Vol. 9 (1970) 1 NO.
12
in the signal shifts in the NMR spectra of titanium(~rr)
and vanadium(1rr) complexes 1121. Typical features
pointing to an important contribution from this
electron delocalization mechanism can also be found
i n the spectra of pyriaine, pyridine N-oxide, and
picoline adduct complexes of many 3d metals r13-15J.
A particularly good example of the delocalization of
spin density into t h e n system of the ligand is provided
by the bis(2-amino-2,4,6-cycloheptatrieniminato~nickel(rr) complexes 1161 (cf. Fig. 1).
c?
L
Nc
+
rm
c-)
I
sterically favorable arrangement, but which are not
bonded directly (in the conventional sense of a
chemical bond) to the metal atom o r ion. This “direct
interaction” is probably responsible e.g. for the
strong negative shifts of the signals in the 1H-NMR
spectra of substituted and unsubstituted bis(cyc1opentadienyl) complexes of vanadium(I1) and chromium(I1) [171. In these compounds, positive spin density
is transferred directly from the transition metal ion
to the protons of the cyclopentadienyl rings, with the
result that the signals of these protons are displaced
by up to 360 ppm toward lower fields. Another very
instructive example of the occurrence of a “direct
interaction” is found in the 1H-NMR spectra
of i minodi(trimethyleniminomethy1- o - pheno1ato)cobalt(1r) (Fig. 2) and of the corresponding nickel(r1)
complex. GeminaI protons in the alkyl chains of
these compounds exhibit differences of up to 86 ppm
in their shifts [18,191.
A
1679121
Fig. 1. 1H-NMR spectrum of a bis(2-amino-2,4,6-cycloheptatrieniminato)nickei(rr) complex in CDCll (40 MHz).
These complexes are characterized by a very fast
configuration change in solution between a squareplanar diamagnetic form and a tetrahedral paramagnetic form. All the following mechanisms that
can contribute to the Knight shift are averaged out
by this process. The electrons of the C-H bonds are
polarized by the free spin density [*I in the TF system
of the ligand in such a way that the spin density on the
proton of the aromatic ligand is negative (shift t o
higherfie1ds)for even numbers and alternately negative
and positive (shift to lower fields) for odd numbers.
(b) Electrons in the highest occupied orbitals of the
ligand o r ligands are partly transferred into unoccupied o r unfilled d orbitals of the metal. This
transfer mechanism is possible in particular for the
more highly charged ions of the 3d metals. A positive
spin density also remains in the ligand orbitals, so
that it is often practically impossible to distinguish
between mechanisms (a) and (b).
(c) Partly occupied d orbitals of the metal overlap
with occupied orbitals of ligand atoms having a
[12] D . R . Eaton, W. R . McCldlan, and J . F. Weiher, Inorg.
Chem. 7 , 2040 (1968).
[13] J . A . Happe and R . L . Ward, J. Chem. Phys. 39, 1211
(1963).
[14] R . H . Holm, G. W. Everettjr., and W. De W. Horrocksjr.,
J. Amer. Chem. SOC.88, 1071 (1966).
[15] G. N . LaMar and G. R . van Hecke, J. Amer. Chem. SOC.
91, 3442 (1969).
[16] D . R . Eaton, A . D . Josey, W . D . PhiNips, and R. E . Benson,
J. Chem. Phys. 37, 347 (1962).
[*] T h e absolute probability of finding a n unpaired electron at
a given point is known in general a s the “free spin density”.
Angew. Chem. iriternuf. Edit.
1 Vol. 9 11970) J No. 12
Fig. 2. Structural formula
phenolato)cobal t(I1).
of
H
irninodi(trimethyleniminomethy1-o-
Several reports dealing with the separation of the
contributions of the types (a) t o (c) to the Fermi
contact shift for certain classes of complexes have
been published during the past few years [12,20-261.
The clarification of this problem would enable information about the metal-ligand bond to be gained
from the signal shifts of certain ligand nuclei.
1.2. Pseudo-Contact Interaction
The investigation of the N M R spectra of paramagnetic compounds is frequently made difficult by the
fact that intramolecular (and also intermolecular in
studies o n solids) dipole-dipole interactions influence
1171 H. P . Fritz, H . J . Keller, and K . E. Schwarzhans, J. Organometal. Chem. 7, 105 (1967).
[18] G. N . LaMar and L . Sacconi, J. Amer. Chem. SOC.89,2282
(19 67).
[19] W . Gretner, Dissertation, Technische Hochschule Miinchen 1969.
1201 M . R . Rettig and R . S . Drago, J. Amer. Chem. SOC.91,
1361, 3432 (1969).
[21] K . E . Schwarzhans, Habilitationsschrift, Technische Hochschule Munchen 1968.
[221 H. P. Fritz, H . J . Keller, and K. E. Schwarzhans, Z . Naturforsch. 236, 298 (1968).
1231 G. N . LaMar and L . Sacconi, J. Amer. Chem. SOC. 90,
7216 (1968).
1241 J . R . Hutchison, G. N . LaMar, and W. De W. Horrocksjr.,
Inorg. Chem. 8, 126 (1969).
[25] F. Rohrscheid, R . E . Ernst, and R . H . Holm, J. Amer.
Chem. SOC.89, 6472 (1967).
[26] H . P . Fritz, W. C . Gretner, H . J . Keller, and K . E. Schwarzham, Z . Naturforsch. 236, 906 (1968).
947
the magnitude and sign of the shifts of the resonance
signals in an extremely complicated manner that can
only be determined in particularly simple cases. In
complexes with extensive delocalization of electrons,
the pseudo-contact contribution to the total shift of
the resonance signals should be small, since the
pseudo-contact model 1271 is based on the assumption
of a point charge localized on the central metal ion.
It is not yet known, however, how much zero-field
splitting, which is a property of all complexes with
more than one unpaired electron, contributes to the
pseudo-contact component of the shift. Information
on this point should be obtainable from ESR experiments at very low temperatures 1281.
2. Application and Results
exchange process on addition of small quantities of
pyridine or picoline in excess. If the exchange frequency is very much greater than the difference in
shift in Hz between the signals of the protons of the
pyridine ligand in the paramagnetic complex and of
free pyridine, an averaged signal appears in the N M R
spectrum [311 at a position that depends on the ratio
of the concentrations of the free ligand and of the
complex. The ring protons of the salicylaldehyde
ligands were assigned by comparison with the spectra
of the derivatives with methyl substituents in position
3, 4, 5 , o r 6 of the ringc191. The assumption that the
symmetry of the complex changes only slightly on
substitution appears to be confirmed by the invariance of the signal shifts of the pyridine and phenylene
protons in the substituted and unsubstituted complexes.
?Y 5
Hetero-NMR measurements on paramagnetic complexes in solution have so far been confined to a few
investigations on simple systems. For example, 1 7 0 and 14N-NMR spectra of hexaaquo and hexaammine
complexes respectively have been recorded for the
determination of ligand exchange rates in these compounds 129,301. N M R experiments in which different
types of atoms in a paramagnetic molecule are detected are therefore particularly interesting.
2.1. Investigations on Pyridine and Picoline Adducts of
Bis(salicylaldehydato)iron(II), -cobalt(II), and
-nickel(lI)
Pyridine and simple methyl derivatives of pyridine
form 2 :1 adducts with paramagnetic bis(salicyla1dehydato)metal(II) complexes, and these adducts are
particularly suitable for N M R studies. They have a
simple ligand system in a pseudo-octahedral arrangement, and are very readily soluble in organic solvents. The pyridine and - picoline ligands readily
undergo exchange with excess ligands, and this
greatly facilitates the assignment of the resonance
signals. Kinetic data for this exchange can also be
determined from N M R studies.
2.1.1. I H - N M R S p e c t r a
The data from the 1H-NMR spectra of such adducts
(Fig. 3) are presented in,Tables 1 to 3. The assignment for the pyridine protons was possible on the
basis of the signal intensities, by comparison of the
spectra of the pyridine adducts with those o f the
P-picoline and y-picoline adducts, and finally on the
basis of the changes produced in the spectra by an
1271 N . Bloembergen and W. C. Dickinson, Phys. Rev. 79, 179
(1950).
[28] J . P . Jesson, J. Chem. Phys. 47, 579, 582 (1967).
[29] H . H . Glaeser, H. W. Dodgen, and J . P . Hunt, Inorg.
Chem. 4, 1061 (1965).
[30] H . H. Glaeser, G . A. Lo, H . W. Dodgen, and J . P. Hunt,
Inorg. Chem. 4, 206 (1965).
5
3
I
-500
1679131
-400
- 300
- 200
6lppmI-
-100
0 TMS
Fig. 3. 1H-NMR spectra of bis(salicylaldehydato)bis(pyridine)metal(II)
complexes in CDCI, (100 MHz).
The large shifts of the aldehydic protons, which
change only slightly on substitution in the aromatic
ring of the salicylaldehyde ligand are a characteristic
feature of all the spectra.
Except in the case of complexes substituted in position 6 of the salicylaldehyde ligand (Fig. 4), the equivalent protons of the ligands generally give only one
N M R signal. Methylation in position 6 evidently
leads to moderate distortion of the complex. Separate
resonance signals are found both for the two CH3
groups and for the two aldehydic protons. A much
greater distortion of the complex should be observed
on replacement of the pyridine ligand by a-picoline.
[31] J . A . Pople, W. G . Schneider, and H . J . Bernstein: Highresolution Nuclear Magnetic Resonance. McGraw-Hill, New
York 1959, p. 218.
Angew. Chem. internat. Edit. 1 Vol. 9 (1970)
1 NO. 12
l a b l e I . 1 H - N M R data for paramagnetic pyridine and picoline adducts of bjs(salicyIaldehydato)iron(II) complexes. Shifts in Hz against internal
T M S at 100 MHz. Saturated solutions in CDC13 at 32-C.
C3-H
C-'--CH>
C4- H
C4-CH3
Cs-H
C5-CH3
C6-H
C6-CH3
Ald-H
3-sal
=
Fe(5-sal)z . 2 py
Fe(4-sal)z . 2 py
Fe(3-sal)~. 2 py
-9 I60
-3230
-9 330
-3121
- 8 600
-3080
-8900
-3140
-I 460
-1920
-1458
- I 340
+ 1 300
1300
-'
j
-9070
-3 300
I
.
Fe(sal)z 2 y-pic
Fe(6-sal)z ' 2 py
-1602
+ 1 290
+1100
-4950
-5646
-7 130
-3050
-7 800
-3200
-1 130
-1750
+ 50
+ 720
-t 720
-5 440
-5 390
-t 310
+ 350
- 1 740
-4460
-4810
t 1560
+ 862
+986
1-956
-3241
-2860
-3270
+ 750
-2745
-3360
+ 340
-4010
-3950
-46600
-48 200
+ 1430
-46700
-43400
-45 150
3-methylsalicylaldehydate, py - pyridine, pic
=
-47 300
-48 100
-45 300
picoline.
Table 2. 1H-NMR data for paramagnetic pyridine and picoline adducts of bis(salicylaldehydato)cobalt(Ii) complexes. Shifts in Hz against internal
TMS at 100 MHz. Saturated solutions in CDCI, at 32 "C.
Co(sal)t . 2 py
-8 300
-1 540
C3-H
C3-CH3
C4-H
C4-CH3
C5-H
CS-CH,
C6-H
C6-CH3
Ald-H
I
~o(3-sa1)2
. 2 py
-11200
-1770
1
Co(sa1)z . 2 @pic
~ o ( 4 - s a 1.)2~ py
-8 300
-1940
-8750
- 1 150
-9 330
-1 720
I- 384
+ 1060
+510
-8250
-8200
-8360
-6670
-5580
-1 150
-1 350
-790
-130
-8150
-1460
+ 1634
-8940
-8460
-7700
-8150
-5530
-6450
-5720
-5480
-1 250
-1 040
- 1060
-704
-760
-2050
-1610
-1 520
-700
-895
Co(sal)z . 2 y-pic
-7700
-1 620
+170
1464
-1 720
-310
-36520
-35700
-36200
-36900
-43 100
-40 600
-33800
-34 120
Table 3. 1H-NMR data for paramagnetic pyridine and picoline adducts of bis(salicylaldehydato)nickel(II) complexes. Shifts in Hz against internal
TMS a t 100 MHz. Saturated solutions in CDCl3 at 32 "C. The absorptions marked ? could not be assigned because of excess line width or overlap.
Ni(3-sal)l . 2
a
P
Ni(4-sal)z . 2 py
I
Ni(5-sal)z ' 2 p
Ni(6-sal)z . 2 py
-8650
-3 370
-9 I30
-3 180
-10450
-3440
-9730
-3230
-I 440
-1 360
-1470
--I 330
P-CH3
Y
.
Ni(sal)* 2 p-pic
-9 350
-3350
-682
- I250
Ni(sa1)Z
-10700
-3640
+703
Y-CH~
C3-H
C'--CHa
C4-H
C4-CH3
Cs-H
C5-CH3
0-H
C6-CH3
Ald-H
. 2 y-pic
-647
-840
-3080
-2 630
-805
?
-590
I
-650
-2610
-50
-690
-560
-632
-674
-2760
-2460
-2700
-690
-746
-810
-84
-81
-100
-97
-4 I 200
?
?
-120
-36 100
!
I -46720
However, n o 2 : l adduct ofa-picoline with any of the
bis(salicylaldehydato)metal(II) complexes has yet
been obtained. Axial entry of the a-picoline is presumably impossible in these compounds on steric
grounds. If, however, measurements are carried out
on an exchange system with a-picoline in solution
together with bis(salicylaldehydato)bis(pyridine)cobalt(rr), all the proton resonance signals of the
salicylaldehyde ligands split into doublets. The two
Angew. Chern. internal. Edit.
Vol. 9 (1970)1 No. 12
-IS0
-360
0
-39400
?
aldehyde ligands in the complex are thus n o longer
magnetically equivalent, i.e. the symmetry of the dissolved complex is reduced. NMR studies on paramagnetic compounds can thus also serve as a sensitive test for changes in the geometry of the arrangement of the ligands, since the Knight shifts of the
resonance signals, as was mentioned earlier, are
strongly dependent on the arrangement of the ligands
with respect to the central metal ion.
949
Hz) “91. The fall in rate constant from iron to nickel
for exchange reactions on pseudo-octahedral complexes is a well-known phenomenon, and is associated
with the ligand field stabilization energy in the trigonal-bipyraniidal transition state 119,341.
HY
H
I
/-H5
2.2. ‘H-NMR Studies on a Dimethylacetamide Adduct
of Uranium Tetrabromide
Fig. 4. Scheme of the structure of bis(salicylaldehydato)bis(pyridine)
metal(n) complexes.
In a purely qualitative consideration of the 1H-NMR
shifts of this type of complex, the highest spin density
is to be expected on the aldehyde grouping of the
salicylaldehyde ligand. This position in the molecule
should be particularly suitable for a free-radical reaction. The complexed salicylaldehyde is in fact
oxidized by air to salicylic acid 1321.
2.1.2. 1 4 N - N M R S p e c t r a
The 14N-NMR spectra of the bis(salicyla1dehydato)bis(pyridine)metal(xr) complexes of iron, cobalt, and
nickel could not be recorded with the purecompounds
in solution. An important reason for this is the fact
that the spectral sensitivity of the 14N nucleus is
lower by a factor of lo3 than that of the proton. The
complexes contain only a little nitrogen (about 6.1 %),
so that even concentrated samples contain only a few
14N nuclei. Moreover, because of the undoubtedly
high spin density on the nitrogen atoms to be considered, the 14N resonance signal is expected t o be very
broadr191. It was possible to avoid both of these difficulties by measurements on the complex under the
conditions of the exchange with excess pyridine. If
the 14N-NMR spectrum of pyridine is recorded with
the addition of various quantities of the bis(salicy1aldehydato)bis(pyridine)metal(II) complex, the shift
of the *4N signal varies linearly with the concentration of the complex in the case of the iron and cobalt
compounds; for the analogous nickel compound, on
the other hand, the ligand exchange rate constant
must be much smaller. A Knight shift of -7800 ppm
can be extrapolated for the Fe” complex, and -5400
ppm for the CoII complex, from the concentration
dependence of the 14N signals.
The ligand exchange rate constants found from these
spectra, which merely indicate a lower limit for the
exchange rate1331, differ only slightly for the two
complexes ( k p , ~ =
~ 2.0 x 105, ~ C ~ =
I I 1.8 x l o 5
[32] H. G. Biedermann and K . E . Schwarrhans, Angew. Chem.
82, 640 (1970); Angew. Chem. internat. Edit. 9, 640 (1970).
. J . Swift and R . E . Connik, J. Chem. Phys. 37, 307
‘
[33] 7
(1962).
950
Dimsthylacetarnide forms a 4 : l adduct with uranium
tetrabrornide. It was uncertain at first whether the
bonding to the central metal ion occurs via the nitrogen or the oxygen atom of the ligand. Rotation about
the C-N bond in free dirnethylacetamide is hindered
at room temperature; the two methyl groups on the
nitrogen give different 1H-NMR signals, showing
that their chemical environments are different. If the
spectrum is recorded at various temperatures the
two signals of the N-CH3 groups fuse at 49.8”C,
from which the height of the rotation barrier is found
to be 12 i 2 kcal1351.
Exactly the same behavior is found in the spectrum
of the adduct. The two 1H-NMR signals of the
N-CH3 groups, which are separated at lower temperatures, unite at 50°C (Fig. 5). The height of the
10
20
30
LO
50
Ti”CIFig. 5 . Temperature dependence of the ‘H-NMR signal shifts of the
N-CH3 groups of dimethylacetamide in (C4H90N)4UBr4 (dissolved in
CHZBrz; 100 MHz).
rotation barrier is therefore unchanged by the complex formation, and it follows that the dimethylacetamide cannot be bonded to the uranium ion via the
nitrogen atom [361.
___.~
[34] F. Basolo and R . G . Pearsorz: Mechanisms of Inorganic
Reactions. Wiley, New York 1967.
[35] H . S. Gutowsky and C . H . Holm, J. Chem. Phys. 25, 1228
(1956).
[ 3 6 ] F. Lux, G . Wirtli, K . W . Bagnall, and D . Brown, 2. Naturforsch. 246, 214 (1969).
Angew. Chem. internat. Edit. ,’ Vol. 9 (1970) No. 12
2.3. Investigations on Paramagnetic Ion Associates
Because of the greater signal shifts in the IH-NMR
spectra of the diamagnetic tetrabutylammonium and
tetraphenylarsonium cations in solutions containing
paramagnetic complex anions, such as hexacyanoferrate(1rr) or tetrachloroniccolate(II), a number of
authors 137-421 postulated the presence of "ion pairs".
The unusual signal shifts were attributed to pseudocontact interaction, which depends on the g-factor
anisotropy of the unpaired electrons and on the
distance an3 the fixed steric relation between the
ions 161.
phanecobaltate(r1) at various concentrations) can be
satisfactorily explained only if one postulates a
direct transfer of spin density from the central metal
ion of the anionic complex e.g. t o the ammonium
nitrogen atom.
Table 4. 'H and 14N Knight shifts (in ppm) of (C4H9)4Nt in
[ ( C I H ~ ) ~ N ] [ C O I ~ ( C ~(dissolved
H ~ ) ~ P ] in CDCI3; 100 and 7.22 MHz).
c
1.07
0.88
0.71
0.53
0.26
0.15
0.075
0.034
0.010
0.005
0.002
0.001
However, there are three serious objections to such
an interpretation of the 1H-NMR spectra on the
basis of ion associates of this nature:
(a) All the paramagnetic anions and diamagnetic
cations investigated have approximately spherical
symmetry. There is therefore no reason to insist on a
preferred steric arrangement for the ions in solution
over a long measuring period, which would be fundamentally necessary in order to be able to observe
pseudo-contact shifts. Without a sufficiently longlasting, definite relation between the paramagnetic
ion and the nucleus that is interacting magnetically
with it, the dipole-dipole contribution to the signal
shift is averaged out. It is clear from the I H - N M R
spectra of the ion associates, however, that a very
much faster ion exchange takes place in the solution
when the diamagnetic cation is present in excess.
(b) The octahedral and tetrahedral symmetries of
the hexacyanoferrate(m), hexacyanochromate(rrI),
and tetrachloroniccolate(r1) anions C431 d o not allow
any anisotropy of the g factor, which is also an assumption for pseudo-contact interaction.
(c) In complexes with strong delocalization of electrons, as was mentioned in Section 1.2, the pseudocontact contribution to the signal shift should be
small or zero 1271. At least for the complex anions triiodotriphenylphosphanecobaltate(i1) and triiodotriphenylphosphaneniccolate(II), however, the large
shifts of the I H - N M R signals of the aromatic protons
of the triphenylphosphane ligands point to strong
delocalization of electrons in these compounds.
2.3.7.
1H-, 1 4 N - ,
and
31P-NMR
(mole/l)
4
4
4.2
4.4
5.0
5.5
6.0
7.3
8.1
8.5
8.4
8.0
1.4
1.4
1.4
1.4
1.4
1.4
1.6
2.0
2.5
1.5
1.7
1.8
0.4
0.4
0.5
0.4
0.5
0.6
3.0
3.7
4.3
4.5
4.5
4.3
2.0
2.3
2.7
2.8
2.8
2.6
0.7
0.8
0.9
0.9
0.9
0.9
-14.4
-13.5
-12.8
-13.0
-10.2
- 7.8
- 5.7
-
The monotonic decrease in the spin density transmitted along the o bond skeleton of the carbon chain
from the x- to the a-Catom is typical of alkyl chains
in paramagnetic transition metal complexes 1441. In
( C 4 H 9 ) 4 N + , the highest density of unpaired electrons
should be localized on the nitrogen. On correct application of the pseudo-contact model 15,271, a signal shift
toward higher fields is to be expected for the nitrogen
of the diamagnetic tetrabutylammonium ion 1371. In
the case of a direct transfer of unpaired electrons
from the central metal ion to the nitrogen atom, however, the 1 4 N resonance signal must be displaced to
lower fields (negative sign) 1421. The positive spin
density on the nitrogen atom would then pass over
to the adjacent x-CH2 group by configuration interaction r61.
Only shifts with negative signs were found for the
I 4 N and 3 1 P signals in the spectra of the ion associates
with ammonium and phosphonium cations (Table 5 ) .
Table 5. 14N and 3IP Knight shifts of the diamagnetic cation in the ion
associate (dissolved in CDCI,; 7.22 and 40.5 MHz).
Compound
Spectra
The 1 H - N M R spectra recorded for the diamagnetic
cations of the ion associates (given in Table 4 for the
tetrabutylammonium salt of triiodotriphenylphos1371 G . N . LaMar, J . Chem. Phys. 41,2992 (1964).
[381 R . J . Fitzgerald and R. S . Drago, J. Amer. Chem. S O C . 90,
2523 (1968).
1391 D . W . Larsen and A . C . Wahl, Inorg. Chem. 4,1281 (1965),
I401 W . De W . Horrocks j r . , R . H . Fischer, J. R . Hutchison.
and G . N . LaMar, J . Amer. Chem. S O C . 88, 2436 (1966).
[411 G . N . LaMar, 3. Chem. Phys. 43, 235 (1965).
[42] R. If. Fischer and W . De W . Horrocks j r . , Inorg. Chem. 7,
2659 (1968).
I431 I . M . Walker and R . S . Drago, J. Amer. Chem. S O C . 90,
6951 (1968).
Angew. Chein. internat. Edit. / Vol. 9 (1970)
1 No. I 2
2.3.2. D i s c u s s i o n
The negative signal shifts that are always found in the
1 4 N - and 3 1 P - N M R spectra of the cations (Table 5)
clearly show the transfer of positive spin density from
the central metal ion of the anionic complex to the
nitrogen or phosphorus atom. It can also be seen
from Table 4 that the position of the *H-NMRsignals
of the cation is strongly dependent on the concentration, whereas the Knight shifts of the anion remain
[443 H . P . Fritz, H . J . Keller, and K . E . Schwarzhans, J. Organometal. Chem. 6, 652 (1966).
95 1
constant over the entire concentration range investigated. This is true of all the ion associates studied. In
Figure 6, there are three easily recognizable regions
a t concentrations below 0.2 mole/l, in which the
ratio of the change in shift to the change in concentration assumes different values.
ample of such an anomaly is known as yet for t h e
electron configuration d4, while an iron(r1) complex
will be described here as a typical and confirmed example with the configuration d6.
Though the anomalous magnetic properties of several
bis(o-phenanthroline)iron(II) complexes r481 have
been interpreted as a tempzrature-depeildent quintetsinglet occupation equilibrium, the sudden change in
the effective magnetic moment of these compounds
at a certain temperature is definitely due to a phase
change, i . ~to
. an intermolecular effect.
The complex bis[4-(2-pyridyl)-2-(2-pyridylamino)thiazolato]iron(~~)
exhibits a characteristic decrease
in its effective magnetic moment with falling temperature 1491. peff is 4.96 p~ at room temperature and
1.55 PB at -170 ‘C, and decreasessteadily in the intervening range. If this behavior is due t o an intramolecular effect, it can only be explained by the existence of
a singlet-quintet occupation equilibrium, naturally
with participation of interjacent triplet states in ac0
005
0.10
015
0.20
0 25
cordance with their energy levels. On the basis of the
1119101
c Imoleill1 R spectra recorded at various temperatures, it was
Fig. 6. Concentration dependence of the Knight shifts of the % and’i3
possible to show that the magnetic anomalies of bisprotons of the tetrabutylammonium ion in [C~H~)~N][COI,(C~H~)JP]
[4-(2-pyridyl)- 2-(2-pyridylamino)thiazolato]iron(11)
(dissolved in CDC13).
are not due to any intermolecular effect [SO]. Where a
temperature-dependent occupation equilibrium exists
This observation points to the presence of higher agbetwzen the singlet ground state and the lowest
gregates (associates), and is not compatible with the
quintet state, the N M R spectra of thecompound must
existence of “ion pairs” in solution. Such associates
show characteristic changes with temperature. The
may be expected in solvents having low dielectric
Knight shifts of the resonance signals of a paramagconstants at relatively high salt concentrations c451.
netic compound are directly proportional to the
There appear to be several definite aggregates, which
density of the unpaired electrons on the nucleus in
evidently differ in their stability, and each of which is
question, and inversely proportional to the absolute
stable in a given concentration range.
temperature c6-101. Thus the signal shifts in the N M R
spectra of paramagnetic substances normally increase
with falling temperature. I n our example, the
2.4. IH-NMR Studies on the Occupation Equilibrium
effective
magnetic moment decreases slowly with
of the Electrons in Bis[4-(2-pyridyl)-2-(2-pyridylamino)falling temperature, i.e. the singlet state becomes
thiazolatoliron(11)
more highly populated and the free spin density in
the molecule as a whole decreases. Two opposing
Complexes in which a temperature-dependent oceffects are therefore operative in the spectrum of such
cupation equilibrium exists between the electronic
a complex. In bis[4-(2-pyridyl)-2-(2-pyridylamino)ground state and nearby excited states are of interest
thiazolato]iron(~~)
a t room temperature, the quintet
in biochemistry. Examples of compounds belonging
state is predominantly populated ( p e ~= 4.96 p ~ ) .
to this class are in the iron-porphyrin complexes,
The Knight shifts of the resonance signals will therewhose term diagrams show a change in the ground
fore initially become larger with falling temperature,
state with the nature of the other ligands as a result
as is to be expected for any magnetically normal paraof crossover of two terms. The NMR-spectroscopic
magnetic complex. Ultimately, however, the occupainvestigation of the anomalous magnetic behavior of
tion of the singlet ground state becomes so high that
such complexes is frequently found to be very difficult
the direction of the signal shift in the N M R spectrum
because of their extremely low solubility.
is reversed; the Knight shifts thus pass through a
This anomalous magnetic behavior has been observed
maximum on cooling. The shifts then decrease
and studied by N M R spectroscopy for the electron
rapidly with further population of the singlet ground
configuration d5 in dithiocarbamatoiron(II1) comstate. This effect is shown in Table 6 for seven
plexes[461 and for the configuration d7 in some
IH-NMR signals of the complex in question in the
chelate complexes of cobalt(I1) iodide [471. N o extemperature range between +30 and -60 “C.
.- .__
[45] C. W. Davies: Ion Association. Butterworth, London
1962.
[46] R . M . Golding, W . C. Tennant, C. R . Kanekar, R. L .
Martin, and A . H . White, J . Chem. Phys. 45, 2688 (1966).
[471 R . C. Stoufer, D . H . Busch, and W. B. Hadky, J. Amer.
Chem. SOC. 83, 3732 (1961).
952
[48] E . K6nig and K . Madeja, J. Amer. Chem. SOC. 88, 4528
(1966); Inorg. Chem. 6, 48 (1967).
[49] R . N . Sylva and H . A . Goodwin, Australian J. Chem. 21,
1081 (1968).
[ S O ] H . J . KeNer, K . E . Schwarrhans, H . A . Goodwin, and R. N.
Svlva, Z. Naturforsch. 246, 1058 (1969)
Angew. Chern. internat. Edir. J VoI. 9 (1970)
/ No. 12
Table 6. Shifts of the ' H - N M R signals 1 t o 7 of bis[4-(2-pyridyl)-2-(2pyridylamino)thiazolato]~ron(~~)
in relation t o T M S (internal) toward
lower fields with falling temperature (100 MHz).
__
'r
( 'C)
I
-
~
30
20
10
0
-I0
-20
30
-40
-50
-60
~
4630
4980
5120
5150
5220
5245
5120
4970
4600
4200
4410
4700
4800
4850
4880
4900
4815
4700
4300
4200
2810
2930
3020
3040
3050
3050
3000
2920
2645
2350
2580
2695
2740
2780
2820
2840
2790
2780
2565
2350
5 I6
7
__
2210
2280
2380
2405
2450
2450
2410
2405
2210
2020
865
860
915
915
(960?)
915
905
875
750
660
The signal shifts of the protons, which are not assigned, are measured in Hz. All the resonance signals
pass through a maximum at -20°C.The variation
of the shifts (measured i n solution) with falling temperature is directly parallel to the molar susceptibility
(measured in the solid state 1491). These experimentally
observed anomalies of bis[4-(2-pyridyl)-2-(2-pyridylamino)thiazolato]iron(lr) are thus not due to an
intermolecular effect, but they can be plausibly explained by a temperature-dependent occupation
equilibrium of the electrons between the 'A1 ground
state a nd the energetically nearby excited 5Tz state,
i.r. by an intramolecular effect.
I am gratejitl to Dr. W. Gretner .for his assistance with
the work carried out by our own research group.
I thank Dr. H . J . Keller,for many valuable discussions,
and the Principals of the Inorganic Chemistry Laboratories of the Technische Universitat Munchen,
Professor H. P. Fritz and Professor E. 0. Fischer,
for the opportunity to use the N M R spectrometer.
Thanks are due to the Deutsche Forschungsgenteinschaft
f o r financial support.
Received: February 4, 1970
[A 797 IE]
German version: Angew. Chem. 82, 975 (1970)
Translated by Express Translation Service, London
C 0 M M U N ICAT I 0 N S
PhotoeIectron Spectrum of Cyciobucanetl J
Hy Peter BischoA Edwin Haselbricii, a n d
Edrar Heilbronner [*I
The vertical ionization potentials Iv of cyclobutane C 4 H 8 ,
as determined from the photoelectron spectrum 121, are
listed in Table 1. An assignment to orbitals of particular
symmetry can be performed, assuming the validity of
Koopmans' theorem 131, on the basis of the orbital energies
E =~ -Iv obtained for cyclobutane by a S C F ab initio techniqueL41, by the MINDOj2 methodrsl, or by the extended
Huckel theory (EHT) [61 (see Table 1).
The ab initio and E H T results, which were made available
to us by L. Salein and J . S. Wright (cf. ref. [71), are based on
the following structural parameters: Symmetry D 4 h ; c- c
=: 1.556 A; C-H
= 1.095 A, 0: H C H = 116"[8l. In the
MINDOj2 method the total energy of the cyclobutane
molecule was minimized without any restriction of the
topography except for C- - H = 1.093 8, and H C H = 120'.
In this case, the calculation yields a D 4 h structure with
C-C -= 1.534 8, within the convergence criteria of the minimization process. I t should be noted that a DZd structure is
found experimentally for cyclobutane 191. For this reason
the classification of the orbitals according to Dzd symmetry
is also included in Table 1.
Table I .
butane
Vertical ionization potentials and orbital energies of cyclo-
Calculated orbital energies (eV)
nb inirio
Drh
4e
4al
lbl
3e
3ai
3b2
10.7 [b];
I I .3 (JT) [cl
11.7
12.5
13.4;
13.6 (JT) lcl
15.9
18.2
1I
Orbital
character
MINDC
EHT
-10.50
- 9.88
--12.83
-11.82
- 12.47
-14.38
-10.35
-13.92
-11.93
-14.09
-13.17
-15.36
mixed;
Walsh type
C H I (n)
C C (0)
CH2 (x)
-16.05
--15.23
-17.94
-15.68
--16.81
CH*(u) tCC(o)
C H L(xi)
[a1
--- 17.53
Ial See ref. [41.
[bl Electron iiiipact value: 10.58 eV, cf. [I31
[cl Jahn-Teller splitting.
Angew. Chem. internat. Edit. / Vol. 9 (1970) / No. 12
Fig. I . Coinparison of the calculated S C F orbital energies
vertical ionization potential Iv = -E of cyclobutane.
E
with the
As shown by the correlation diagram (Fig. l), the experimental valucs are in excellent agreement with the calculated
ob initio and M I N D 0 / 2 data. The reasonable results obtained from the E H T model are probably due to the high
degree of symmetry of the molecule. As predicted, the first
band in the PE spectrum (at 11 eV) is split into two partial
bands separated by about 0.6 eV. Since the photoelectron
vacates a degenerate orbital (3e, or 4e), the radical cation
C4Hi must be subject to a Jahn-Teller distortion in its
electronic ground state, which leads to the observed splitting. (Compare the analogous behavior of cyclopropane [lo].)
Similarly, the form of the band at 13.5 eV seems to indicate
a small splitting of 0.2eV, which would agree with the
theoretical prediction that in this case the photoelectron is
ejected from the leg (or 3e) orbital. I t should be noted that
the PE spectroscopic findings confirm the prediction [7J
that, by analogy to the situation prevailing in cyclopropane[lo. 111, the highest occupied orbital in cyclobutane is
also degenerate and can consequently be regarded as a kind
of Walsh orbital.
The structure of the radical cation C4Hi (in the electronic
ground state) has been studied in the course of a n investi-
953
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