EPR study of o-semiquinone-catecholate cobalt complexes with bis(diphenylphosphanyl)ethane

Article abstract of DOI:10.1002/ejic.200200670

Three novel cobalt compounds with bis(diphenylphosphanyl)ethane (dppe) and semiquinone and catecholate ligands were synthesized and investigated by EPR spectroscopy (1), in solution. They are: [(dppe)Co(3,6-DBSQ)(Cat_ox^Cl)] (1), [(dppe)Co(3,6-DBCat)(phen-SQ)] (2), and [(dppe)Co(3,6-DBSQ)₂] (3), where 3,6-DBSQ and 3,6-DBCat are the radical anion and dianion of 3,6-di-tert-butyl-o-benzoquinone, respectively, Cat_ox^Cl is the dianion of perchloroxanthrenequinone-2,3, and phen-SQ is the radical anion of phenanthrenequinone. According to the ratio of the redox potentials of the o-quinone fragments, 1 and 2 are mixed semiquinone-catecholate complexes, and the 3,6-di-tert-butyl-o-quinone fragment appears as a semiquinone in 1 and as a catecholate in 2. Compound 3 exists as a mixture of redox isomers, namely semiquinone-catecholate and bis-semiquinone species. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200200670

FULL PAPER
EPR Study of o-Semiquinone-Catecholate Cobalt Complexes with
Bis(diphenylphosphanyl)ethane
Michael P. Bubnov,*[a] Irina A. Teplova,[a] Vladimir K. Cherkasov,[a] and
Gleb A. Abakumov[a]
Keywords: Chelates / Cobalt / EPR spectroscopy / Quinones / Tautomerism
Three novel cobalt compounds with bis(diphenylphosphanyl)-
ethane (dppe) and semiquinone and catecholate ligands
were synthesized and investigated by EPR spectroscopy
in solution. They are: [(dppe)Co(3,6-DBSQ)(CatCoxl)] (1),
[(dppe)Co(3,6-DBCat)(phen-SQ)] (2), and [(dppe)Co(3,6-
DBSQ)2] (3), where 3,6-DBSQ and 3,6-DBCat are the radical
quinone. According to the ratio of the redox potentials of the
o-quinone fragments, 1 and 2 are mixed semiquinone-cate-
cholate complexes, and the 3,6-di-tert-butyl-o-quinone frag-
ment appears as a semiquinone in 1 and as a catecholate in
2. Compound 3 exists as a mixture of redox isomers, namely
semiquinone-catecholate and bis-semiquinone species.
anion and dianion of 3,6-di-tert-butyl-o-benzoquinone, re-
Cl
spectively, Cat is the dianion of perchloroxanthrenequi- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ox
none-2,3, and phen-SQ is the radical anion of phenanthrene- Germany, 2003)
Introduction
The equilibrium position and the rate of isomerization de-
pend on the nature of the o-semiquinone, the neutral li-
gands, and the solvent. In all cases, decreasing the tempera-
ture shifts the equilibrium position towards the cat-
echolate isomer.
In 1980, Pierpont first observed a reversible intramolecu-
lar metalϪligand electron transfer of a cobalt bis-semiqui-
none complex in solution.[1]
All of the complexes [LnM(‘‘Q’’)2] that were investigated
to date contain ‘‘Q’’ fragments that are derivatives of the
same quinone and so their initial electronic ground state a
priori is degenerate. The aim of this work was to obtain six-
coordinate cobalt complexes containing different o-quin-
one fragments.
Most of publications listed above are dedicated to com-
plexes containing nitrogen atom donors. We have not found
in the literature any bis-quinone complexes with dppe.
Moreover dppe contains two phosphorous atoms (31P, I ϭ
1/2, 100%), the hyperfine splitting of which in the EPR
spectrum provides information regarding the spin distri-
bution in complex molecules.
[α,αЈ-bpyCoIII(3,5-Cat)(3,5-SQ)] [α,αЈ-bpyCoII(3,5-SQ)2]
Ǟ
ǟ
3,5-SQ and 3,5-Cat are the anion-radical and dianion of
3,5-di-tert-butyl-o-benzoquinone.
A similar equilibrium involving a two-electron transfer
was described later[2] for the manganese complex
[(py)2Mn(3,5-DBCat)2].
Ǟ
ǟ
[(py)2Mn(3,5-DBCat)2] [(py)2Mn(3,5-DBSQ)2]
Reversible electron transfer, taking place in the solid
state, was first observed by our group[3,4] in 1990 and by
Hendrickson[5] in 1993. Further development of the under-
standing of reversible metalϪligand electron transfer in se-
miquinone metal complexes was dedicated to investigation
of solid-phase transformations.[6Ϫ14]
Results and Discussion
Number of mono-semiquinone complexes of Rh,[15Ϫ18]
Ir,[19] Cu,[20] and Ni[21] of general formula [LnMSQ] demon-
strate reversible electron transfer in solution (Ln is a neutral
ligand for Rh, Ir, and Cu, or monovalent one for Ni). It
has been shown that complexes exist in solution as an equi-
librium mixture of redox isomers that interconvert rapidly
(according to the EPR time scale) between one another.
Cobalt compounds of types 1Ϫ3 can be obtained
through oxidative addition reactions of the corresponding
o-quinones to four-coordinate [(dppe)(3,6-di-tert-butylcat-
echolato)cobalt] [Equation (1)].
[LnCoII(CatЈ)] ϩ QЈЈ Ǟ [LnCoIII(CatЈ)(SQЈЈ)]
(1)
[a]
G. A. Razuvaev Institute of Organometallic Chemistry of RAS,
49 Tropinina str., Nizhny Novgorod, GSP-445, Russia 603950
Fax: (internat.) ϩ 7-8312/661-497
Stable paramagnetic adducts were studied by EPR spec-
troscopy in solution. The appearance of the EPR spectrum
E-mail: bmp@imoc.sinn.ru
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M. P. Bubnov, I. A. Teplova, V. K. Cherkasov, G. A. Abakumov
FULL PAPER
of a particular product depends on the redox potential ratio
of the quinone fragments. We have observed three situ-
ations.
The first takes place when additional quinone is quite a
stronger acceptor than is 3,6-DBQ, which has formed the
initial catecholate. We have used perchlorooxanthrenequi-
none-2,3 [Equation (2)].
(3)
The hyperfine coupling with protons of 3,6-DBSQ is ab-
sent (giso ϭ 2.0016Ϯ0.0005, Figure 2). Only hyperfine coup-
ling is observed with 59Co and one 31P2 (aCo ഠ aP2
ഠ
(2)
0.85 mT). The interaction with the protons of phen-SQ and
P1 is unresolved because of a relatively large line width. The
spectrum view changes slightly with temperature. Optimal
resolution was achieved at 250 K. An acceptable simulation
indicates an essentially larger line width than was observed
The presented EPR spectrum (Figure 1) displays one un- in the case 1 (0.20 mT for 1 and 0.73 mT for 2). Diminution
paired electron localized on the 3,6-DBSQ fragment: of the 59Co and 31P coupling constants is expected. This
a
H1(SQ) ϭ aH2(SQ) ϭ 0.30 mT, gi ϭ 2.0016Ϯ0.0005. Perchlo- feature is explained by lower acceptor properties of phen-Q
roxanthrenequinone receives two electrons and becomes a relative to 3,6-DBQ.[24,25]
catecholate dianion. The cobalt atom is oxidized from the
oxidation state ϩ2 to ϩ3. The different hyperfine coupling
constants on the 31P nuclei (aP2 ϭ 1.30 mT, aP1 ϭ 0.30 mT)
reflect planar and apical positions of the corresponding nu-
clei with respect to the plane of the o-semiquinone.[22,23]
The coupling constant on 59Co (aCo ϭ 1.00 mT) are typical
for octahedral o-semiquinone-catecholate complexes of co-
balt() with one unpaired electron localized primarily (Ͼ
99% according to ref.[15]) on the semiquinone unit.[1] Thus,
the result is a six-coordinate semiquinone-catecholate com-
plex of low-spin cobalt().
Figure 2. X-Band EPR spectrum (top) of [(dppe)Co(3,6-
DBCat)(phen-SQ)] (2) (toluene, 250 K) and (bottom) its simulation
(EPR line width ϭ 0.73 mT)
Figure 1. Room temperature x-band EPR spectrum (top) of
Thus, additional phenanthrenequinone accepts one elec-
tron and becomes a semiquinone, the cobalt() ion loses
one electron and becomes a cobalt() ion, and the 3,6-
DBSQ unit remains a catecholate. The resulting six-coordi-
nate complex of cobalt() contains one unpaired electron
Cl
[(dppe)Co(3,6-DBSQ)(Cat )] (1) (toluene, 290 K) and (bottom) its
ox
simulation (EPR line width ϭ 0.2 mT)
The second situation occurs when the additional quinone localized on the phenanthrenesemiquinone and the 3,6-
is a quite-weaker acceptor than is quinone, which has DBQ fragment in the catecholate form.
formed the initial catecholate; e.g., when it is phen-
anthrenequinone [Equation (3)].
The third situation takes place when both the quinone
fragments are the same [Equation (4)].
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EPR Study of o-Semiquinone-Catecholate Cobalt Complexes
FULL PAPER
can occur though the intermediacy of a bis-semiquinone
adduct of cobalt() (b). The presence of this intermediate
containing at least three unpaired electrons could be the
reason for the broadening and temperature dependence of
the EPR spectrum.
(4)
One can imagine the EPR spectrum in the absence of this
intermediate. In the case of slow interconversion (relative to
the timescale of EPR spectroscopy) of a and c, the EPR
spectrum should reflect, as was observed for 1, a hyperfine
interaction with the cobalt nucleus of two different phos-
phorus atoms (one planar and one apical with respect to
the plane of the semiquinone unit) and two protons of one
semiquinone ring.
In the case of fast exchange, the EPR spectrum would
show an interaction of the unpaired electron with the cobalt
nucleus, the two different phosphorus atoms with equal
constants, and the four protons of 3,6-DBSQ with one half-
constant (ഠ 0.15 mT).[27]
The EPR spectrum consists of one broad, unresolved
line, the form and intensity of which depends on the tem-
perature (giso ϭ 2.0020Ϯ0.0005, Figure 3). No hyperfine
structure is observed. A bearable simulation of the spec-
trum at 210 K indicates a line width of 5.0 mT. Based on
literature data,[1,26] we propose that line broadening takes
place because of the following equilibrium [Equation (5)]:
In the case of a medium rate of interconversion, i.e., when
the lifetime of this intermediate is close to the characteristic
time of EPR spectroscopy, an EPR spectrum presenting
slow exchange phenomena will transform to depict fast ex-
change with an increase in temperature, according to
known rules.[28] Only some components of the spectrum can
undergo broadening.
(5)
This equilibrium is a tautomeric interconversion of iso-
mers a and c followed by a reversible electron transfer. It
Total EPR spectroscopic line broadening can take place
in the case where one of the exchanging particles has a very
broad (maybe infinite) line width and/or a very short life-
time. The particles a and c presented above have equal life-
times because of their total identity; the EPR spectra corre-
sponding to fast and slow exchange between them have al-
ready been described. A complex of cobalt() with two se-
miquinone units (particle b) having at least three unpaired
electrons should display a very broad line. Interconversion
of a to c passing through form b leads to lines broadening.
However, the molar fraction of this cobalt() adduct ap-
pears to be relatively low because it is possible to observe
the spectrum of 3 and its g factor is the same as it is in the
cases of 1 and 2.
In a publication describing a similar situation for com-
plex [(α,αЈ-bpy)Rh(3,6-DBCat)(3,6-DBSQ)], EPR spectro-
scopic line broadening occurs upon warming it in solution
as well as in the solid state at a very small high-spin fraction
that cannot be detected by magnetic susceptibility.[26]
A very small impurity of a CoII species seems to be pre-
sent in the cases of the complexes 1 and 2. This impurity
might be the reason for the relatively large line widths. By
comparing the EPR spectroscopic line widths of complexes
1 and 2 with the values of E1/2 of the corresponding qui-
nones [E1/2(phen-Q) ϭ Ϫ0.66 V, E1/2(3,6-DBQ) ϭ Ϫ0.39 V,
Cl
E1/2(Q ) ϭ Ϫ0.03 V] it becomes clear why the EPR spec-
ox
trum of 2 is poorly resolved in comparison with 1. The dif-
Cl
ference between the values of E1/2 of Q and 3,6-DBQ
ox
(∆ ഠ 0.36 V) is larger than between 3,6-DBQ and phen-Q
(∆ ഠ 0.27 V). Based on these general considerations it be-
comes clear that the energy separation between the
Figure 3. Temperature dependence of the x-band EPR spectrum
(top) of [(dppe)Co(3,6-DBCat)(3,6-DBSQ)] (3) (g ϭ 2.002Ϯ0.0005)
and (bottom) its simulation at T ϭ 210 K (aCo ϭ aP2 ϭ 1.3 mT;
line width ϭ 5.0 mT).
CoIII(CatЈ)(SQЈЈ) and CoII(SQЈ)(SQЈЈ) states is larger in the
Cl
Q Ϫ3,6-DBQ pair than in the 3,6-DBQϪphen-Q pair, and
ox
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FULL PAPER
so the amount of an impurity of a CoII species is, respec-
tively, smaller. As a consequence, the line widths in the EPR
spectrum of 1 are smaller than they are in the case of 2.
Of course, these comments are qualitative and approximate
because they are based on the assumption that the first val-
ues of E1/2 of quinones correlate with the second ones.
Complex 3 was isolated and characterized in the solid
state. Moreover, it was obtained in another way [Equa-
tion (6)].
amalgam was shaken until the color of the suspension became
bright yellow. The light suspension of thallium catecholate was
carefully decanted to another evacuated ampoule. Thallium amal-
gam was carefully washed several times with THF (5 ϫ 10 mL)
until the solvent became colorless. (Attention! Thallium catecholate
is extremely air sensitive.) Thallium catecholate was added to a mix-
ture of dppe (0.4 g, 1 mmol) and CoCl2 (0.13 g, 1.0 mmol) in THF
(70 mL). The reaction mixture was heated under reflux for 1 h, and
then the solvent was changed to toluene. The brown solution was
filtered, partly evaporated, diluted with n-hexane, and then kept at
Ϫ10 °C overnight. The resulting air-stable, brown, crystalline solid
was filtered, washed with cold light petroleum, and then dried un-
der vacuum. Yield 0.2 g (33%). IR (Nujol): ν˜ ϭ 515 s, 525 (C3P
skeleton vibrations); 735, 695 v.s. δЌ CϪH (Ph); 1105 (νPϪC); 1260,
[Co(3,6-DBSQ)3] ϩ dppe Ǟ
(6)
[(dppe)Co(3,6-DBCat)(3,6-DBSQ)] ϩ 3,6-DBQ
1250 (νCϪO, Cat); 940, 980, 1395, 1375, 1440 (3,6-DBCat) cmϪ1
.
A magnetic susceptibility investigation indicates a mag-
netic moment of ca. 1.76 µB, which corresponds to one un-
paired electron per complex. The magnetic moment as a
function of temperature remains constant down to ca.
10 K.
The EPR spectrum of 3 (powder), which displays one
broad, unresolved line, is quite similar to those reported[26]
to have been observed for [(α,αЈ-bpy)Rh(3,6-DBCat)(3,6-
DBSQ)], and no temperature changes were detected. Thus,
complex 3 appears to be a ‘‘CoIII(SQ)(Cat)’’ species in the
solid state that lacks the ‘‘CoII(SQ)2’’ impurity.
C40H44CoO2P2 (677.67) calcd. C 70.90, H 6.50, Co 8.71; found C
70.95, H 6.49, Co 8.75.
Sample Preparation: Equimolar amounts of [(dppe)Co(3,6-DBCat)]
and the corresponding quinone were placed into a special ampoule
for EPR spectroscopic investigations. The ampoule was evacuated
and purified toluene was condensed inside.
[Bis(diphenylphosphanyl)ethane)(3,6-di-tert-butyl-o-benzosemiquin-
ono)(3,6-di-tert-butylcatecholato)cobalt (3): Dppe (0.4 g, 1 mmol) in
THF (50 mL) was added to [Co(3,6-DBSQ)3] (0.716 g, 1.0 mmol)
in THF (80 mL) and then the mixture was carefully warmed. The
solvent was evaporated and the residue was washed with n-hexane
(3 ϫ 50 mL). The solid was dissolved in CH2Cl2, and then diluted
with n-hexane. Slow evaporation led to a dark-blue microcrystalline
solid, which was filtered, washed by cold light petroleum, and dried
under vacuum. Yield: 0.5 g (ca. 50%). An additional crop of crys-
tals was obtained after cooling the mother solution. IR (Nujol):
ν˜ ϭ 480, 495, 535 (C3P skeleton vibrations); 695 v.s., 740 m δЌ
CϪH (Ph); 1105 (νPϪC); 1270, 1280 (νCϪO, Cat); 1410 s (νCϪO, SQ)
cmϪ1. C54H64CoO4P2 (897.98) calcd. C 72.24, H 7.13, Co 6.58;
found C 72.32, H 7.18, Co 6.60.
Conclusion
Naturally, the charge on the quinone fragment Ϫ ‘‘3,6-
DBQ’’ in complex [(dppe)Co(‘‘3,6-DBQ’’)(‘‘Q’’)] Ϫ de-
pends on the redox potential of the neighboring quinone
Cl
fragment ‘‘Q’’. If ‘‘Q’’ is a stronger acceptor, such as ‘‘Q ’’,
ox
then ‘‘3,6-DBQ’’ becomes a semiquinone unit, but if
‘‘Q’’ is a weaker acceptor (‘‘phen-Q’’) then it remains a
catecholate unit. When ‘‘Q’’ ligands are identical, the com-
plex remains in the CoIII(Cat)(SQ) state, in general, but a
very small impurity of a CoII species essentially causes lines
Acknowledgments
The authors are thankful to Dr. V. N. Ikorskii and Prof. V. I.
Ovcharenko who have made the magnetochemical measurements.
to broaden in the EPR spectrum. Complex [(dppe)Co(3,6- This work was supported by the Russian Foundation of Basic Re-
search, Grants No. 00-15-97336 and 01-03-33065. EPR spectro-
DBCat)(3,6-DBSQ)] in the solid state exists as a ‘‘CoIII
(Cat)(SQ)’’ species without any ‘‘CoII(SQ)2’’ impurity.
-
scopic investigations were carried out in the Analytical Center of
the G. A. Razuvaev Institute of Organometallic Chemistry of RAS,
which is supported by the Russian Foundation of Basic Research
(grant No. 00-03-40116).
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EPR Study of o-Semiquinone-Catecholate Cobalt Complexes
FULL PAPER
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Received November 28, 2002
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