The molecular and electronic structure of [FeIII 2(t-buLISQ)4(μ-O)] - A dinuclear ferric complex containing four, O,N-coordinated o-iminobenzosemiquinonate(1-) π radical anions

Article abstract of DOI:10.1039/b306764p

The synthesis and crystal structure of a dinuclear complex containing a ν-oxo(diferric) core and four O,N-coordinated o-iminobenzosemiquinonate(1-) π radical anions was described. The octahedral complex [Fe ^III(L^ISQ)₃] was found to possess a S _t = 1 ground state where a high spin ferric ion was intramolecularly antiferromagnetically coupled to three (L^ISQ)^1- radical anions. The notion of the presence of high spin ferric ions was corroborated by their Moessbauer spectra.

Full text of DOI:10.1039/b306764p

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The molecular and electronic structure of [FeIII (t-buLISQ)4(ꢀ-O)]—
2
a dinuclear ferric complex containing four, O,N-coordinated
o-iminobenzosemiquinonate(1؊) ꢁ radical anions
Soumen Mukherjee, Thomas Weyhermüller, Karl Wieghardt* and Phalguni Chaudhuri*
Max-Planck-Institute for Bioinorganic Chemistry, Stiftstrasse 34-36,
D.-45470 Mülheim an der Ruhr, Germany. E-mail: wieghardt@mpi-muelheim.mpg.de;
Chaudh@mpi-muelheim.mpg.de
Received 13th June 2003, Accepted 4th August 2003
First published as an Advance Article on the web 8th August 2003
The synthesis and crystal structure of a dinuclear complex
independent magnetic moment of 3.8 µB in the range 50–300 K
containing a ꢀ-oxo(diferric) core and four O,N-coordinated
indicating an St = 3/2 ground state8 and no population of
excited states. In contrast, similar measurements of 1 show that
in the range 30–120 K a magnetic moment of ∼2.9 µB (St = 1)
prevails but above 120 K a monotonic increase to 3.3 µB at 300
K is observed.3 From this behaviour an antiferromagnetic coup-
ling constant of J12 = J13 = J14 = Ϫ120 cmϪ1, g = 2.04 has been
o-iminobenzosemiquinonate(1؊) ꢁ radical anions, [FeIII
-
2
(ꢀ-O)(t-buLISQ)4], is described; its electronic structure is best
described as comprising two FeIII(LISQ)2 halves with S* ؍
3/2 (SFe ؍ 5/2 coupled to two organic radicals) which are
then intramolecularly antiferromagnetically coupled via
the oxo group yielding the observed S ؍ 0 ground state.
deduced by using the spin Hamiltonian J = Ϫ2JSFeSrad (SFe
=
5/2; Srad = 1/2). Apparently, the spin coupling in 1 is weaker
than in 2. This is due to the fact that the average Fe–O and
Fe–N bond distances are significantly shorter in 2 at 2.041 and
1.963 Å,8 respectively, than those in 1 at 2.099 and 2.015 Å,3
respectively.
The presence of O,N-coordinated o-iminobenzosemiquinonate
radicals in 1 and 2 has been clearly established by single crystal
In a series of recent papers1–11 we have been able to show that o-
aminophenolate ligands O,N-coordinated to transition metal
ions form in the presence of air O,N-coordinated o-iminobenz-
osemiquinonate π radical monoanions, (LISQ)1Ϫ. We have shown
that the spin of such a ligand π radical interacts intramolecu-
larly with the spins of unpaired d electrons at the transition
metal ion.3 In accordance with the Goodenough–Kanamori
rules12 for spin exchange coupling the interaction of a π radical
anion with an unpaired electron in an eg orbital of an octa-
hedral nickel() ion is strongly ferromagnetic yielding an St =
3/2 ground state. Coupling with an electron in a t2g orbital is,
in general, antiferromagnetic.13 Thus, octahedral [CrIII(LISQ)3]
is diamagnetic at ambient temperature and possesses a singlet
ground state.3
X-ray crystallography at cryogenic temperature (100(2) K). The
1Ϫ
C–C, C–N, C–O distances of the (LISQ
)
ligands in 1 and 2 are
identical within 3σ ( 0.015 Å) (see Fig. 1). As shown previously,
the aromatic ligand dianion which is the one-electron reduced
1Ϫ
form of (LISQ
)
7 and the neutral one-electron oxidized quinone
display significantly different C–C, C–O, and
1Ϫ
form of (LISQ
C–N distances.9
)
We have also shown that the octahedral complex [FeIII(LISQ)3]
(1) possesses an St = 1 ground state3 where a high spin ferric ion
(SFe = 5/2) is intramolecularly antiferromagnetically coupled to
1Ϫ
three (LISQ
)
radical anions. Similarly, [FeIII(LISQ)2Cl] (2) has
an St = 3/2 ground state8,10 comprising a high spin ferric ion and
two antiferromagnetically coupled radical anions. The com-
plexes are shown in Scheme 1
It is interesting that variable temperature (3–300 K) magnetic
susceptibility measurements of 2 revealed a temperature-
Fig. 1 Average C–C, C–N, C–O bond distances (Å) of the four O,N-
coordinated o-iminobenzosemiquinonate(1-) π radical anions in 3. The
same bond lengths have been reported for 1 and 2 in ref. 3 and 8,
respectively.
Here we report the synthesis, the electronic and molecular
structure of the first dinuclear µ-oxo bridged ferric complex
[FeIII2(µ-O)(t-buLISQ)4] (3) which contains four O,N-coordinated
o-iminobenzosemiquinonate(1Ϫ) π radical anions (see Scheme
2).
The ligand 2-(p-tert-butyl)anilino-4,6-di-tert-butylphenol,
H[t-buLAP], has been obtained from the reaction 4-tert-
butylaniline, triethylamine and 3,5-di-tert-butylcatechol in
n-heptane.†2 A solution of the ligand in methanol reacts with
[FeII(OH2)6](ClO4)2 and NEt3 in the presence of air at 60 ЊC to
give a deep green solution from which green–black crystals of 3
were obtained.‡ Recrystallization from CH2Cl2/CH3CN (3 : 1
vol) mixture produced single crystals of 3ؒ2.75CH2Cl2 suitable
for X-ray crystallography.§
Scheme 1 Complexes.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 4 8 3 – 3 4 8 5
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Table 1 Selected bond lengths (Å)a and angles (Њ) for 3ؒ2.75CH2Cl2
Fe(1)–O(40)
Fe(1)–O(31)
Fe(1)–O(1)
Fe(1)–N(37)
Fe(1)–N(7)
1.776(2)
1.968(2)
1.970(2)
2.051(3)
2.057(3)
Fe(2)–O(40)
Fe(2)–O(131)
Fe(2)–O(101)
Fe(2)–N(107)
Fe(2)–N(137)
1.775(2)
1.958(2)
1.961(2)
2.066(3)
2.069(3)
Fe(1)–O(40)–Fe(2)
175.2(2)
a See Fig. 1 for average C–C, C–O, C–N distances of the (LISQ
)
1Ϫ anions.
Table 2 Zero-field Mössbauer data of complexes 1–3 at 80 K
δ/mm sϪ1 b
∆EQ/mm sϪ1 c
Ref.
a
Compound
St
1
2
3
1
3/2
0
0.54
0.45
0.38
1.035
1.26
1.105
3
8
This work
a Ground state. b Isomer shift vs. α-Fe at 298 K. c Quadrupole splitting.
The two halves FeIII(LISQ)2 in dinuclear 3 are bridged by a
nearly linear oxo group (Fe–O–Fe 175.2(2)Њ). The two Fe–Ooxo
distances are short at 1.775(2) Å and indicative of a strongly
covalent bond with double bond character. This bonding
situation has been observed in many dinuclear µ-oxo bridged
complexes containing two high spin ferric ions (SFe = 5/2).14
Table 1 summarizes Fe–O and Fe–N bond lengths in 3. The
average Fe–Orad distance is 1.964 Å whereas the average Fe–N
bond length is 2.061 Å. They are both nearly identical in five-
coordinate [FeIIICl(LISQ)2]8 but significantly longer at 2.014 Å
(Fe–O) and 2.099 (Fe–N) Å, respectively, in six-coordinate [FeIII-
(LISQ)3].3 These data are in excellent agreement with the notion
that complexes 1, 2, and 3 contain high spin ferric ions. In
[FeIII(LISQ)2I]8 which possesses an St = 1/2 ground state these
Fe–N and Fe–Orad bonds are much shorter at 1.885 Å and 1.875
Å, respectively; they indicate the presence of an intermediate
spin ferric ion (SFe = 3/2) with two antiferromagnetically
coupled O,N-coordinated π radicals.
Scheme 2 Ligands and abbreviations.
Fig. 2 shows the structure of a neutral dinuclear molecule of
the complex in crystals of 3ؒ2.75CH2Cl2. Both ferric ions are
coordinated to two o-iminobenzosemiquinonate(1Ϫ) π radical
anions. This is conclusively deduced from the average C–N,
C–C, and C–O distances which are identical within experi-
mental error in each of the four ligands (Fig. 1). They are the
same as those reported previously for 1,3 2,8 and [CrIII(LISQ)3],3
[CoIII(LISQ)3],1 and [PtII(bpy)(LISQ)](PF6).7
The electronic spectrum of 3 has been measured in CH2Cl2
solution: λmax(ε/L molϪ1 cmϪ1): 305(3.8 × 104), 350sh(3.1 × 104),
430(2.2 × 104), 540sh(1.1 × 104), 690(0.9 × 104). Interestingly,
the spectra of 1 and 2 are very similar3,8 and indicate the pres-
ence of FeIII(LISQ)x units (x = 1, 2, or 3).
The notion of the presence of high spin ferric ions in 1, 2,
and 3 is corroborated by their Mössbauer spectra. Table 2 gives
isomer shift, δ, and quadrupole splitting, ∆EQ, parameters at
80 K and Fig. 3 exhibits the zero-field Mössbauer spectrum of 3
at 80 K. The fact that the isomer shift decreases from 0.54 mm
s
Ϫ1 for octahedral 1 to 0.45 mm sϪ1 for five-coordinate 2 and to
0.38 mm sϪ1 for five-coordinate 3 is rationalized by an increas-
ingly covalent character of the iron-to-ligand bonds; in 3 it is
dominated by the covalently bound oxo bridge.14
Fig. 2 Structure of the neutral molecule [FeIII2(µ-O)(t-buLISQ)4] in
crystals of 3ؒ2.75CH2Cl2 (top) and of the [(FeIIIN2O2)2(µ-O)] core
(bottom).
Fig. 3 Zero-field Mössbauer spectrum of 3 at 80 K.
D a l t o n T r a n s . , 2 0 0 3 , 3 4 8 3 – 3 4 8 5
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turned deep red. The solution was heated to reflux for 1 h in the pres-
Finally, Fig. 4 exhibits the temperature dependence of the
magnetic moment of dinuclear 3 in the range 3–300 K. It has
been possible to fit the behaviour successfully by using a model
which invokes very strong intramolecular antiferromagnetic
coupling between high spin ferric ions (SFe = 5/2) and—in each
case—two o-iminosemiquinonate π radicals yielding a fictitious
S* = 3/2 state at each iron ion. This is fully in accord with the
magnetism of mononuclear 2 which possesses an St = 3/2
ground state with no population of higher excited states up to
300 K.8 The two halves of 3 are then coupled through the
µ-oxo group. Thus, using the spin Hamiltonian H = Ϫ2JS1S2
(S1 = S2 = 3/2) yields the following parameters: J = Ϫ123(5)
cmϪ1, g = 2.0 (fixed). A mononuclear paramagnetic impurity of
0.6% (S = 5/2) was included. Thus, a strong antiferromagnetic
coupling between the two halves in 3 prevails yielding the
observed singlet ground state.
ence of air whereupon a color change to deep green was observed.
From the cooled and filtered solution black microcrystals crystallized
upon slow evaporation of the solvent within 2 d. The crude material
was recrystallized from a CH2Cl2/CH3CN (3 : 1) mixture. Yield: 0.12 g
(63%). Anal. calcd. for (C24H33NO)4OFe2: C, 75.15; H, 8.68; N, 3.65; Fe,
7.30. Found: C, 75.0; H, 8.6; N, 3.5; Fe, 7.4.
§ X-Ray crystallography: diffraction data were collected at 100(2) K
with Mo-Kα radiation (λ = 0.71073 Å) on a Nonius Kappa-CCD dif-
fractometer equipped with a Mo-target rotating-anode X-ray source
and a graphite monochromator (2θmax = 55Њ). Data collection was per-
formed by a full-sphere run taking frames at 1.0Њ in ω up to a data
completeness of 90.6% (R(int) = 0.0482). Crystal faces were determined
and the intensity data were corrected for absorption effects by using
the Gaussian-type routine embedded in XPREP15 giving min/max
transmission factors of 0.927 and 0.953, respectively. The Siemens
ShelXTL15 software package was used for solution and artwork of the
structure; ShelXL9716 was used for refinement. The structure was
solved by Patterson and subsequent difference Fourier techniques. All
non-hydrogen atoms were refined anisotropically and hydrogen atoms
were placed at calculated positions and refined as riding atoms with
isotropic displacement parameters. The CH2Cl2 solvent molecules were
found to be poorly defined. A split atom model with occupation factors
of 0.8 (C(80)) and 0.2 (C(85)) was employed to account for disorder of
one CH2Cl2 molecule. Another solvent molecule was found to be fully
occupied (C(70)) and two more positions were refined with occupation
factors of 0.5 (C(60)) and 0.25 (Cl(91) and Cl(92)). It has not been
possible to locate the corresponding carbon atom. C–Cl and Cl–Cl
distances were restrained to be equal within certain error limits using
the SADI option of ShelXL97.
Crystal data for 3ؒ2.75CH2Cl2: C96H132Fe2N4O5ؒ2.75CH2Cl2,
¯
M = 1767.3, triclinic, space group P1, a = 11.9826(6), b = 15.9612(9),
c = 27.532(2) Å, α = 84.48(1), β = 82.47(1), γ = 77.64(1)Њ, V = 5086.7(5)
Å3, Z = 2, Dc = 1.154 g cmϪ1, µ = 0.479 mmϪ1. Full-matrix least
squares refinement on F 2 (21829 unique data, 1065 variables) converged
to R1 = 0.068, wR2 = 0.180 for all data. CCDC reference number
211465. See http://www.rsc.org/suppdata/dt/b3/b306764p/ for crystal-
lographic data in CIF or other electronic format.
Fig. 4 Temperature dependence of the magnetic moment of 3 (per
dinuclear unit): experimental values; the solid line represents a best fit
of the data by using parameters given in the text.
1 C. N. Verani, S. Gallert, E. Bill, T. Weyhermüller, K. Wieghardt and
P. Chaudhuri, Chem. Commun., 1999, 1747.
2 P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyhermüller and
K. Wieghardt, J. Am. Chem. Soc., 2001, 123, 2213.
3 H. Chun, C. N. Verani, P. Chaudhuri, E. Bothe, E. Bill,
T. Weyhermüller and K. Wieghardt, Inorg. Chem., 2001, 40, 4157.
4 D. Herebian, P. Ghosh, H. Chun, E. Bothe, T. Weyhermüller and
K. Wieghardt, Eur. J. Inorg. Chem., 2002, 1957.
Complex 3 provides the first example of a structurally char-
acterized µ-oxo(diferric) complex containing four organic
ligand radicals.
5 H. Chun, P. Chaudhuri, T. Weyhermüller and K. Wieghardt,
Inorg. Chem., 2002, 41, 790.
6 H. Chun, E. Bill, E. Bothe, T. Weyhermüller and K. Wieghardt,
Inorg. Chem., 2002, 41, 5091.
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F. Neese and K. Wieghardt, Inorg. Chem., 2002, 41, 4295.
8 H. Chun, T. Weyhermüller, E. Bill and K. Wieghardt, Angew. Chem.
Int. Ed., 2001, 40, 2489.
9 K. S. Min, T. Weyhermüller and K. Wieghardt, Dalton Trans., 2003,
1126.
10 H. Chun, E. Bill, T. Weyhermüller and K. Wieghardt, Inorg. Chem.,
in press.
11 S. Mukherjee, E. Rentschler, T. Weyhermüller, K. Wieghardt and
P. Chaudhuri, Chem. Commun., 2003, 1828.
12 (a) O. Kahn, Molecular Magnetism, Wiley-VCH, New York, 1993;
(b) A. P. Ginsberg, Inorg. Chim. Acta Rev., 1971, 5, 45.
13 (a) C. G. Pierpont and C. W. Lange, Prog. Inorg. Chem., 1994, 41,
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Grant:
Priority program ChIII/2-1), the Max-Planck Society and the
Fonds der Chemischen Industrie for financial support.
Notes and references
† The ligand 2-(p-tert-butyl)anilino-4,6-di-tert-butylphenol, H[t-buLAP],
has been prepared as follows. 3,5-Di-tert-butylcatechol (11.1 g; 50
mmol) and 4-tert-butylaniline (7.5 g; 50 mmol) were dissolved in
n-heptane (60 mL) and triethylamine (0.5 mL). The solution was heated
to reflux for 3 h in the presence of air. After cooling and reduction
of the reaction volume by evaporation of the solvent (∼30 mL) a
colorless microcrystalline solid precipitated (yield: 12 g; 68%) which
1
was collected by filtration and washed with n-pentane. H NMR (400
MHz, CDCl3): δ 1.33 (s, 9H), 1.38 (s, 9H), 1.56 (s, 9H), 4.95 (s, 1H), 6.70
(d, 2H), 7.14 (d, 1H), 7.33 (1H), 7.34 (2H).
‡ [FeIII2(µ-O)(t-buLISQ)4] 3: the ligand H[t-buLAP] (0.36 g; 1.0 mmol)
and [FeII(H2O)6](ClO4)2 (0.06 g; 0.25 mmol) were dissolved in methanol
(25 mL). Upon addition of NEt3 (0.5 mL) the color of the solution
14 D. M. Kurtz Jr., Chem. Rev., 1990, 90, 585.
15 ShelXTL V.5, Siemens Analytical X-Ray Instruments, Inc.1994 .
16 ShelXL97 G. M. Sheldrick, University of Göttingen, 1997 .
D a l t o n T r a n s . , 2 0 0 3 , 3 4 8 3 – 3 4 8 5
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