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Substituent effects related to the reaction of dioxygen with high spin cobalt(II) chelates in solution
INORG.
NUCL.
CHEM. LETTERS
Vol.
12,
pp.
339-343,
1976.
Pergamon
Press. Printed
in
Great
Britain
SUBSTITUENT EFFECTS RELATED TO THE REACTION OF DIOXYGEN WITH HIGH SPIN COBALT(II) CHELATES IN SOLUTION R. H. Niswander and L. T. Taylor Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 (Received 13 October 1975; in revised form 1 December 1975)
Considerable interest has been generated recently regarding the interaction of dioxygen with high spin five coordinate cobalt(ll) complexes which incorporate a linear pentadentate ligand (i.e. Co(SALDPT) and Co(SALMeDPT) (1-3).
These investigations have been devoted exclusively to solution studies
wherein, based primarily on esr results, evidence for i:i (Co:O 2) complexes has been established.
The paucity of isolable I:i complexes reported in the
literature consequently prompted us to undertake a detailed study concerning the possible isolation of some of these materials.
An x-ray structure has
been reported (4) on a 2:1 adduct [Co(SALDPT)]202"C6HsCH3 without any description of the compound's properties and the preparation of a poorly characterized material (5), Co(SALDPT)02, has been published.
Our approach in this
area has been (i) to study the effect of solvent polarity on the formation of the dioxygen complexes at different precursor concentrations and (ii) to establish the relationship between the stability of the dioxygen complexes and the electronic effect of substituents on the salicyaldehyde ring. Black solid dloxygen complexes of SALDPT derivatives have been prepared by bubbling air or oxygen through a previously filtered solution of the cobalt(ll) precursor dissolved in an appropriate solvent.
In addition to the
unsubstituted compound, the salicylaldehyde derivatives that were studied included 3-CH30 , 5-CI, 5-Br, 3,5-diCl, and 5-NO 2.
The solvents which were
used (i.e. benzene, THF, acetone, and acetonitrile) were chosen for their wide range of polarities.
All of the Co(ll) precursors form isolable 2:1 dioxygen
complexes regardless of the initial concentration of reactants or the solvent polarity.
Each complex contains solvent of crystallization, the amount of
which varies, depending both upon the solvent and the particular derivative (6).
Heating each dioxygen complex in vacuo (1-2 torr) results in a sharp
loss of both solvent and oxygen.
In most of the compounds, thermal gravimetric
analysis shows no well-deflned separation between loss of solvent and loss of oxygen.
There also doesn't appear to be any correlation between the tempera-
ture at which the weight loss occurs and the electronic effect of a particular 339
340
Substituent Effects
3300G
3400G
I
I
Figure i Esr spectrum of Co(5-NO2SALDPT)O2"I/2(CH3)2CO; solid state at 22°C.
substituent.
In most compounds the removal of solvent and oxygen occurs
between 110PC and 125°C.
The only compound that loses weight at temperatures
less than 100°C is the 5-NO 2 derivative which loses solvent and oxygen at 90°C. If it is assumed that the Co(ll) precursor is formally oxidized to Co(Ill) in the dloxygen adduct, it might be expected that electron donating groups would stabilize the cobalt(Ill) species and a more stable dioxygen adduct would be formed.
Electron withdrawing groups should, on the other hand,
destabilize the adduct.
Over a period of several days under vacuum at room
temperature, oxygen is removed much more readily from the 2:1 5-NO 2 compound than from the 3-CH30 compound as monitored via magnetic susceptibility. are other indications that an electronic effect may be operating. of all the cobalt(ll) complexes darken readily when exposed to air.
There
Solutions With the
exception of the 5-NO 2 derivative, dioxygen-adducts precipitate from these solutions in a matter of minutes. case of the 3-CH30 derivative.
This precipitation is most dramatic in the
The 5-NO 2 adduct, however, does not preclpl-
Substituent Effects
341
3300G
3400G I
I
Fisure 2 Esr spectrum of [Co(5-CISALDPT)]202"I.5(CH3)2CO ; solid state at 22°C.
tate except at very high concentrations.
In fact, solutions of low concentra-
tion (10-2M) actually deoxygenate upon sitting. observable via esr.
This deoxygenation is readily
A signal produced by a superoxo adduct which in solution
is in equilibrlumwlth
the ~-peroxo species gradually loses intensity but is
immediately restored upon aerating the solution in the esr tube. behavior is not observed with the 3-CH30 derivative.
This
In addition, oxygen
uptake at 760 torr 02 pressure is about twice as fast for the 3-CH30 as for the 5-NO 2 derivative. The electronic effect of the substltuents can be thought of in terms of the way in which it affects the following equilibria: colI(xsALDPT) + 02 ~ colII(xsALDPT)02 colII(xsALDPT)02--+ colI(xsALDPT) ~ [colII(xsALDPT)]202 -2 where X is the particular substituent.
A highly electron withdrawing group
should shift these equilibria to the left relative to the situation where X
342
Substituent Effects
is an electron donating group thus reducing the solution concentration of 2:1 adduct and its subsequent precipitation.
The second reaction,s further
minimized by carrying the oxygenation out at high 02 concentration, in solvents of high polarity and at low concentrations of starting material.
We have
utilized these ideas to prepare a i:i adduct of the 5-NO 2 derivative, Co(5NO2SALDPT)O2"I/2(CH3)2CO (Calcd: (7); Found:
C, 47.16; H, 4.38; N, 12.79; T.G.A., ii.15,
C, 47.56; H, 4.74; N, 12.56; T.G.A., 11.15).
may be responsible for the isolation of this i:i species.
Additional factors Perhaps the electron
withdrawing properties of the substituent affect the superoxo ligand, making it a weaker sigma donor and reducing the reactivity of the i:i adduct (8).
In
contrast to the 2:1 adducts which have magnetic moments ranging from 0 to .79 B.M., the i:i adduct has a magnetic moment of 1.51 B.M.
This magnetic
susceptibility, however, is dependent on the magnetic field strength.
02
uptake measurements in acetone are in agreement with the i:i formulation.
An
0-O stretch in the ir of the i:i adduct is obscured due to ligand vibrational modes.
On the other hand, we have obtained a well defined solid state esr
spectra of this compound (Figure i).
Such an observation in the solid state
is onusual and may be attributed to minimal interaction of the par-m~gnetic centers as a result of the solvent present in the crystal lattice. The magnetic moment of the compound is low, but is consistent with other i:I adducts (9). 2:1 impurity.
This may possibly arise from a small amount of a diamagnetic
In fact, slight paramgnetism in 2:1 adducts has been associated
with the presence of a i:i impurity (i0). in the 2:1 adducts of Co(XSALDPT).
We have observed this paramagnetism
Esr spectra of the 2:1 adducts in the
solid state show a signal which may be interpreted as a i:i adduct (Figure 2). The signal is not resolved, however, and furthermore, it is still present in the spectra of the deoxygenated species.
If this signal were due to a super-
oxo impurity, it would be expected to be less stable than a ~-peroxo adduct and therefore should have been removed upon heating.
We feel this signal
which is not wide enough to be a B-superoxo or low spin cobalt(ll) species, is produced by some irreversibly oxidized compound.
Moreover, the magnetic
moments of the deoxygenated compounds are all low (~eff = 3.14-3.89 B.M.). It is known that oxygenation of cobalt(ll) complexes results in production of some irreversibly oxidized compoonds which are not well characterized (ii). Perhaps the ultimate interpretation of this esr signal could throw some light on the nature of these irreversibly produced products and the side reactions that promote them. Acknowledgment: of this study.
We thank Dr. James Burness for discussing with us the results This research was supported by the Research Corporation.
Substituent Effects
343
References i.
B. S. Tovrog and R. S. Drago, J. Amer. Chem. Soc., 96, 6765 (1974).
2.
B. M. Hoffman,
T. Szyamanski,
and F. Basolo, J. Amer. Chem. Soc., 97, 673
(1975). 3.
Co(SALDPT) (II).
= N,N'-(3,3'-bis(propyl)amlne)bis(salicylideneimlnato)-cobalt-
Co(SALMeDPT)
= N,N'-(3,3'-bls(propyl)methylamine)-bls(salicylidene-
iminat o) cobalt (II). 4.
L. A. Lindblom, W. P. Schaefer and R. E. Marsh, Acta Cryst., B27, 1461 (1971).
5.
C. Floriani and F. Calderazzo,
6.
Duplicate
J. Chem. Soc. (A), 946 (1969).
CH + N analyses coupled with thermal gravimetrlc
analysis
weight loss data are in excellent agreement with the following formulations : [Co(3-CH3OSALDPT) ]202 •THF [Co (5-NO2SALDPT) ]202 .THF [Co (5-CISALDPT) ]202 •I. 5 (CH 3) 2CO [Co(3,5 -dICISALDPT) ]202 •i. 5(CH 3) 2CO [Co (5-BrSALDPT) ]202 • i. 5 (CH 3) 2C0 [Co (5-BrSALDPT) ]202 • C6H 6 [Co (5-CISALDPT) ]202 . I. 5 C6H 6 [Co(3-CH3OSALDPT) ]202 • CH3CN [Co(SALDPT) ]202 • 1.5 C6H 6 7.
Weight loss calculated for removal of mass equivalent
to one 02 and one-
half acetone. 8.
No other i:i adducts have been prepared in this study. prepare the previously
9.
B. M. Hoffman,
reported compound
D. L. Diemente,
All attempts to
Co(SALDPT)O 2 were unsuccessful.
and F. Basolo, J. Amer. Chem. Soc., 92, 61
(1970). i0.
D. Diemente,
ii.
R. Caraco, D. Braun-Steinle, (1975).
B. M. Hoffman,
and F. Basolo, Chem. Commun.,
467 (1970).
and S. Fallab, Coord. Chem. Revs., 16, 147
NUCL.
CHEM. LETTERS
Vol.
12,
pp.
339-343,
1976.
Pergamon
Press. Printed
in
Great
Britain
SUBSTITUENT EFFECTS RELATED TO THE REACTION OF DIOXYGEN WITH HIGH SPIN COBALT(II) CHELATES IN SOLUTION R. H. Niswander and L. T. Taylor Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 (Received 13 October 1975; in revised form 1 December 1975)
Considerable interest has been generated recently regarding the interaction of dioxygen with high spin five coordinate cobalt(ll) complexes which incorporate a linear pentadentate ligand (i.e. Co(SALDPT) and Co(SALMeDPT) (1-3).
These investigations have been devoted exclusively to solution studies
wherein, based primarily on esr results, evidence for i:i (Co:O 2) complexes has been established.
The paucity of isolable I:i complexes reported in the
literature consequently prompted us to undertake a detailed study concerning the possible isolation of some of these materials.
An x-ray structure has
been reported (4) on a 2:1 adduct [Co(SALDPT)]202"C6HsCH3 without any description of the compound's properties and the preparation of a poorly characterized material (5), Co(SALDPT)02, has been published.
Our approach in this
area has been (i) to study the effect of solvent polarity on the formation of the dioxygen complexes at different precursor concentrations and (ii) to establish the relationship between the stability of the dioxygen complexes and the electronic effect of substituents on the salicyaldehyde ring. Black solid dloxygen complexes of SALDPT derivatives have been prepared by bubbling air or oxygen through a previously filtered solution of the cobalt(ll) precursor dissolved in an appropriate solvent.
In addition to the
unsubstituted compound, the salicylaldehyde derivatives that were studied included 3-CH30 , 5-CI, 5-Br, 3,5-diCl, and 5-NO 2.
The solvents which were
used (i.e. benzene, THF, acetone, and acetonitrile) were chosen for their wide range of polarities.
All of the Co(ll) precursors form isolable 2:1 dioxygen
complexes regardless of the initial concentration of reactants or the solvent polarity.
Each complex contains solvent of crystallization, the amount of
which varies, depending both upon the solvent and the particular derivative (6).
Heating each dioxygen complex in vacuo (1-2 torr) results in a sharp
loss of both solvent and oxygen.
In most of the compounds, thermal gravimetric
analysis shows no well-deflned separation between loss of solvent and loss of oxygen.
There also doesn't appear to be any correlation between the tempera-
ture at which the weight loss occurs and the electronic effect of a particular 339
340
Substituent Effects
3300G
3400G
I
I
Figure i Esr spectrum of Co(5-NO2SALDPT)O2"I/2(CH3)2CO; solid state at 22°C.
substituent.
In most compounds the removal of solvent and oxygen occurs
between 110PC and 125°C.
The only compound that loses weight at temperatures
less than 100°C is the 5-NO 2 derivative which loses solvent and oxygen at 90°C. If it is assumed that the Co(ll) precursor is formally oxidized to Co(Ill) in the dloxygen adduct, it might be expected that electron donating groups would stabilize the cobalt(Ill) species and a more stable dioxygen adduct would be formed.
Electron withdrawing groups should, on the other hand,
destabilize the adduct.
Over a period of several days under vacuum at room
temperature, oxygen is removed much more readily from the 2:1 5-NO 2 compound than from the 3-CH30 compound as monitored via magnetic susceptibility. are other indications that an electronic effect may be operating. of all the cobalt(ll) complexes darken readily when exposed to air.
There
Solutions With the
exception of the 5-NO 2 derivative, dioxygen-adducts precipitate from these solutions in a matter of minutes. case of the 3-CH30 derivative.
This precipitation is most dramatic in the
The 5-NO 2 adduct, however, does not preclpl-
Substituent Effects
341
3300G
3400G I
I
Fisure 2 Esr spectrum of [Co(5-CISALDPT)]202"I.5(CH3)2CO ; solid state at 22°C.
tate except at very high concentrations.
In fact, solutions of low concentra-
tion (10-2M) actually deoxygenate upon sitting. observable via esr.
This deoxygenation is readily
A signal produced by a superoxo adduct which in solution
is in equilibrlumwlth
the ~-peroxo species gradually loses intensity but is
immediately restored upon aerating the solution in the esr tube. behavior is not observed with the 3-CH30 derivative.
This
In addition, oxygen
uptake at 760 torr 02 pressure is about twice as fast for the 3-CH30 as for the 5-NO 2 derivative. The electronic effect of the substltuents can be thought of in terms of the way in which it affects the following equilibria: colI(xsALDPT) + 02 ~ colII(xsALDPT)02 colII(xsALDPT)02--+ colI(xsALDPT) ~ [colII(xsALDPT)]202 -2 where X is the particular substituent.
A highly electron withdrawing group
should shift these equilibria to the left relative to the situation where X
342
Substituent Effects
is an electron donating group thus reducing the solution concentration of 2:1 adduct and its subsequent precipitation.
The second reaction,s further
minimized by carrying the oxygenation out at high 02 concentration, in solvents of high polarity and at low concentrations of starting material.
We have
utilized these ideas to prepare a i:i adduct of the 5-NO 2 derivative, Co(5NO2SALDPT)O2"I/2(CH3)2CO (Calcd: (7); Found:
C, 47.16; H, 4.38; N, 12.79; T.G.A., ii.15,
C, 47.56; H, 4.74; N, 12.56; T.G.A., 11.15).
may be responsible for the isolation of this i:i species.
Additional factors Perhaps the electron
withdrawing properties of the substituent affect the superoxo ligand, making it a weaker sigma donor and reducing the reactivity of the i:i adduct (8).
In
contrast to the 2:1 adducts which have magnetic moments ranging from 0 to .79 B.M., the i:i adduct has a magnetic moment of 1.51 B.M.
This magnetic
susceptibility, however, is dependent on the magnetic field strength.
02
uptake measurements in acetone are in agreement with the i:i formulation.
An
0-O stretch in the ir of the i:i adduct is obscured due to ligand vibrational modes.
On the other hand, we have obtained a well defined solid state esr
spectra of this compound (Figure i).
Such an observation in the solid state
is onusual and may be attributed to minimal interaction of the par-m~gnetic centers as a result of the solvent present in the crystal lattice. The magnetic moment of the compound is low, but is consistent with other i:I adducts (9). 2:1 impurity.
This may possibly arise from a small amount of a diamagnetic
In fact, slight paramgnetism in 2:1 adducts has been associated
with the presence of a i:i impurity (i0). in the 2:1 adducts of Co(XSALDPT).
We have observed this paramagnetism
Esr spectra of the 2:1 adducts in the
solid state show a signal which may be interpreted as a i:i adduct (Figure 2). The signal is not resolved, however, and furthermore, it is still present in the spectra of the deoxygenated species.
If this signal were due to a super-
oxo impurity, it would be expected to be less stable than a ~-peroxo adduct and therefore should have been removed upon heating.
We feel this signal
which is not wide enough to be a B-superoxo or low spin cobalt(ll) species, is produced by some irreversibly oxidized compound.
Moreover, the magnetic
moments of the deoxygenated compounds are all low (~eff = 3.14-3.89 B.M.). It is known that oxygenation of cobalt(ll) complexes results in production of some irreversibly oxidized compoonds which are not well characterized (ii). Perhaps the ultimate interpretation of this esr signal could throw some light on the nature of these irreversibly produced products and the side reactions that promote them. Acknowledgment: of this study.
We thank Dr. James Burness for discussing with us the results This research was supported by the Research Corporation.
Substituent Effects
343
References i.
B. S. Tovrog and R. S. Drago, J. Amer. Chem. Soc., 96, 6765 (1974).
2.
B. M. Hoffman,
T. Szyamanski,
and F. Basolo, J. Amer. Chem. Soc., 97, 673
(1975). 3.
Co(SALDPT) (II).
= N,N'-(3,3'-bis(propyl)amlne)bis(salicylideneimlnato)-cobalt-
Co(SALMeDPT)
= N,N'-(3,3'-bls(propyl)methylamine)-bls(salicylidene-
iminat o) cobalt (II). 4.
L. A. Lindblom, W. P. Schaefer and R. E. Marsh, Acta Cryst., B27, 1461 (1971).
5.
C. Floriani and F. Calderazzo,
6.
Duplicate
J. Chem. Soc. (A), 946 (1969).
CH + N analyses coupled with thermal gravimetrlc
analysis
weight loss data are in excellent agreement with the following formulations : [Co(3-CH3OSALDPT) ]202 •THF [Co (5-NO2SALDPT) ]202 .THF [Co (5-CISALDPT) ]202 •I. 5 (CH 3) 2CO [Co(3,5 -dICISALDPT) ]202 •i. 5(CH 3) 2CO [Co (5-BrSALDPT) ]202 • i. 5 (CH 3) 2C0 [Co (5-BrSALDPT) ]202 • C6H 6 [Co (5-CISALDPT) ]202 . I. 5 C6H 6 [Co(3-CH3OSALDPT) ]202 • CH3CN [Co(SALDPT) ]202 • 1.5 C6H 6 7.
Weight loss calculated for removal of mass equivalent
to one 02 and one-
half acetone. 8.
No other i:i adducts have been prepared in this study. prepare the previously
9.
B. M. Hoffman,
reported compound
D. L. Diemente,
All attempts to
Co(SALDPT)O 2 were unsuccessful.
and F. Basolo, J. Amer. Chem. Soc., 92, 61
(1970). i0.
D. Diemente,
ii.
R. Caraco, D. Braun-Steinle, (1975).
B. M. Hoffman,
and F. Basolo, Chem. Commun.,
467 (1970).
and S. Fallab, Coord. Chem. Revs., 16, 147