Aminopyridine iron catecholate complexes as models for intradiol catechol dioxygenases. Synthesis, structure, reactivity, and spectroscopic studies

Article abstract of DOI:10.1021/ic981236v

Four new Fe(III) catecholate complexes, [(bispicMe₂en)Fe(III)(DBC)]⁺, [(bispicCl₂Me₂en)Fe(III)(DBC)]⁺, [(trispic-Meen)Fe(III)(DBC)]⁺, and [(BQPA)Fe(III)(DBC)]⁺, which all contain aminopyridine ligands, were synthesized. The structure of [(bispicMe₂en)Fe(III)(DBC)]⁺ was determined by X-ray diffraction. It crystallizes in the triclinic space group P1 with a = 10.666(3) A, b = 13.467(5) A, c = 17.685(2) A, α = 93.46(2)°, β = 93.68(2)°, γ = 109.0(3)°, V = 2387.4 A³, and Z = 2. All of these complexes were found to be active toward oxidation of catechol by O₂ in DMF at 20 °C to afford intradiol cleavage products. The catechol was quantitatively oxidized, mainly (90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction rates were measured, and for the first three (topologically similar) complexes, a correlation of the second-order kinetic constants k with the optical parameters of the two LMCT O(DBC) → Fe(III) bands was found. In particular, k increases with the ε(max) of the charge-transfer bands. The k value of the complex [(BQPA)Fe(III)(DBC)]⁺, containing a tripodal ligand, is smaller than expected on the basis of these correlations. This discrepancy could be related to steric hindrance induced by the BQPA ligand. However, the much lower activity of the bispicen-Fe(III)-type complexes compared to that of the [(TPA)Fe(III)(DBC)]⁺ complex synthesized by Jang et al. (J. Am. Chem. Soc. 1991, 113, 9200-9204), despite similar ε(max) values, shows that a knowledge of optical and NMR parameters values is not sufficient to explain the dioxygenase activity rate. In their study of protocatechuate 3,4-dioxygenase, Orville et al. (Biochemistry 1997, 36, 10052-10066) suggested that asymmetric chelation of the catecholate to Fe(III) is of great importance in the efficiency of the intradiol dioxygenase reaction. Indeed, a comparison of the X-ray structures of [(TPA)Fe(III)(DBC)]⁺ and [(bispicMe₂en)Fe(III)(DBC)]⁺ shows that the Fe(III)-O bonds differ by 0.019 A in the former and are identical in the latter. Asymmetry could also play a role in the model complexes. An alternative explanation is the possible existence of a low-spin state for [(TPA)Fe(III)(DBC)]⁺, as recently identified in [(TPA)Fe(III)(cat)]⁺ by Simaan et al. (Angew. Chem., Int. Ed. Engl. 2000, 39, 196-198).

Full text of DOI:10.1021/ic981236v

2440
Inorg. Chem. 2000, 39, 2440-2444
Aminopyridine Iron Catecholate Complexes as Models for Intradiol Catechol Dioxygenases.
Synthesis, Structure, Reactivity, and Spectroscopic Studies
Pierre Mialane,1a Luba Tchertanov,1b Fre´de´ric Banse,1a Joelle Sainton,1a and
Jean-Jacques Girerd*,1a
Laboratoire de Chimie Inorganique, UMR CNRS 8613, Institut de Chimie Mole´culaire d’Orsay,
Universite´ Paris-Sud, 91405 Orsay, France, and Institut de Chimie des Substances Naturelles,
UPR CNRS 2301, 91198 Gif-sur-Yvette, France
ReceiVed October 21, 1998
Four new Fe(III) catecholate complexes, [(bispicMe2en)FeIII(DBC)]+, [(bispicCl2Me2en)FeIII(DBC)]+, [(trispic-
Meen)FeIII(DBC)]+, and [(BQPA)FeIII(DBC)]+, which all contain aminopyridine ligands, were synthesized. The
structure of [(bispicMe2en)FeIII(DBC)]+ was determined by X-ray diffraction. It crystallizes in the triclinic space
group P1 with a ) 10.666(3) Å, b ) 13.467(5) Å, c ) 17.685(2) Å, R ) 93.46(2)°, â ) 93.68(2)°, γ ) 109.0-
(3)°, V ) 2387.4 Å3, and Z ) 2. All of these complexes were found to be active toward oxidation of catechol by
O2 in DMF at 20 °C to afford intradiol cleavage products. The catechol was quantitatively oxidized, mainly
(90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction rates were measured, and for the first three
(topologically similar) complexes, a correlation of the second-order kinetic constants k with the optical parameters
of the two LMCT O(DBC) f Fe(III) bands was found. In particular, k increases with the ꢀmax of the charge-
transfer bands. The k value of the complex [(BQPA)FeIII(DBC)]+, containing a tripodal ligand, is smaller than
expected on the basis of these correlations. This discrepancy could be related to steric hindrance induced by the
BQPA ligand. However, the much lower activity of the bispicen-Fe(III)-type complexes compared to that of the
[(TPA)FeIII(DBC)]+ complex synthesized by Jang et al. (J. Am. Chem. Soc. 1991, 113, 9200-9204), despite
similar ꢀmax values, shows that a knowledge of optical and NMR parameters values is not sufficient to explain the
dioxygenase activity rate. In their study of protocatechuate 3,4-dioxygenase, Orville et al. (Biochemistry 1997,
36, 10052-10066) suggested that asymmetric chelation of the catecholate to Fe(III) is of great importance in the
efficiency of the intradiol dioxygenase reaction. Indeed, a comparison of the X-ray structures of [(TPA)FeIII-
(DBC)]+ and [(bispicMe2en)FeIII(DBC)]+ shows that the Fe(III)-O bonds differ by 0.019 Å in the former and
are identical in the latter. Asymmetry could also play a role in the model complexes. An alternative explanation
is the possible existence of a low-spin state for [(TPA)FeIII(DBC)]+, as recently identified in [(TPA)FeIII(cat)]+
by Simaan et al. (Angew. Chem., Int. Ed. Engl. 2000, 39, 196-198).
Catechol dioxygenases catalyze the last step in the degradation
of aromatic compounds. They have been extensively studied
from both biochemical and chemical modeling points of view.2
Intradiol dioxygenases contain in the active site an Fe(III) ion
coordinated to two tyrosine groups, two histidines, and an OH-
anion. This structure has been inferred from model studies and
confirmed by X-ray diffraction.3 The group of L. Que has made
a major contribution to the modeling of intradiol dioxygenases.
In their early studies, they developed models with Fe(salen)X.4
These complexes showed a very weak dioxygenase activity,
despite a structure similar to that of the enzyme active site.
Extensive work has been done in order to increase the reaction
rate. The group of Que insisted on the importance of using an
auxiliary ligand favoring the electron transfer from the catecho-
late group to the Fe(III),5 and along this line, they demonstrated
the rapid stoichiometric oxidation of catechol in the [(TPA)-
Fe(DBC)]+ complex.6 In 1995, Koch and Kru¨ger discovered
the catalysis of the same reaction by an Fe(III) complex of N,N′-
dimethyl-2,11-diaza[3.3](2,6)pyridinophane.7 The macrocyclic
nature of that ligand certainly contributes to the stabilization of
the corresponding Fe(III) complex, even in the presence of an
excess of catecholate in catalytic conditions. Duda et al.8 used
an Fe(III) complex of a TPA analogue in which one pyridine
group was replaced by N-methylimidazole to achieve an efficient
catalysis. Funabiki et al.9 have shown that the Fe(III) complex
with TPA itself can catalyze the oxygenative cleavage of
chlorocatechols.
We concentrated our study on the synthesis, characterization,
and mechanism of catecholate degradation for a series of [LFeIII-
(DBC)]+ complexes in which L is a ligand of the bispicen
(1) (a) Universite´ Paris-Sud. (b) Institut de Chimie des Substances
Naturelles.
(2) (a) Lipscomb, J. D.; Orville, A. M. In Metal Ions in Biological Systems;
Sigel, H., Sigel, A., Eds.; Marcel Dekker: New York, 1992; Vol. 28,
pp 243-298. (b) Que, L., Jr. In Iron Carriers and Iron Proteins; Loehr,
T. M., Ed.; VCH: New York, 1989; pp 467-524.
(3) Ohlendorf, D. H.; Orville, A. M.; Lipscomb, J. D. J. Mol. Biol. 1994,
244, 586-608.
(4) (a) Lauffer, R. B.; Heistand, R. H., II; Que, L., Jr. Inorg. Chem. 1983,
22, 50-55. (b) Heistand, R. H., II; Roe, A. L.; Que, L., Jr. Inorg.
Chem. 1982, 21, 676-681.
(5) Lauffer, R. B.; Heistand, R. H., II; Que, L., Jr. J. Am. Chem. Soc.
1981, 103, 3947-3949.
(6) Jang, H. G.; Cox, D. D.; Que, L., Jr. J. Am. Chem. Soc. 1991, 113,
9200-9204.
(7) Koch, W. O.; Kru¨ger, H.-J. Angew. Chem., Int. Ed. Engl. 1995, 34,
2671-2674.
(8) Duda, M.; Pascaly, M.; Krebs, B. Chem. Commun. 1997, 835.
(9) Funabiki, T.; Yamazaki, T.; Fukui, A.; Tanaka, T.; Yoshida, S. Angew.
Chem., Int. Ed. Engl. 1998, 37, 513.
10.1021/ic981236v CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/18/2000

Aminopyridine Iron Catecholate Complexes
Inorganic Chemistry, Vol. 39, No. 12, 2000 2441
Table 1. Crystallographic Data for [(BispicMe2en)FeIII(DBC)]BPh4
family. To this set, we added the complex [(BQPA)FeIII-
(DBC)]+, where BQPA is the bis(2-quinolylmethyl)(2-pyridyl-
methyl)amine ligand, which is a strong π-acceptor. The goal
of this study is the elucidation of the role of electronic, steric,
and symmetry factors in the process of catecholate degradation.
formula
fw
radiation
temp, K
space group
C54H62BFeN4O2
865.74
Mo KR (0.71073 Å)
294(2)
P1
a, Å
b, Å
c, Å
10.666(3)
13.467(5)
17.685(2)
93.46(2)
93.68(2)
109.0(3)
2387.4(11)
2
0.360
11152
10958
0.0393
0.0903
Experimental Section
Materials. Aerobic conditions were used for ligand syntheses,
whereas complex syntheses were performed under anaerobic conditions.
Chemical starting materials were purchased from Aldrich and used as
received. Solvents were dried before use.
R, deg
â, deg
γ, deg
V, Å3
Z
Ligand Preparation. Bis(2-quinolylmethyl)(2-pyridylmethyl)-
amine (BQPA). To a solution of 2-aminomethylpyridine (1.06 g, 10
mmol) in 50 mL of acetonitrile was added a solution of 2-chloromethyl-
quinoline‚HCl (4.3 g, 20 mmol) in 50 mL of acetonitrile. Then, 7 g of
sodium carbonate was added, and the mixture was stirred for 2 days at
50 °C. After filtration, the solvent was evaporated under vacuum. The
orange powder collected was recrystallized in hexane to give a yellow,
microcrystalline powder (yield, 80%). 1H NMR (250 MHz): δ 8.5 (d,
1H, H-py), 8.1 (d, 2H, H-qui), 8.0 (d, 2H, H-qui), 7.7 (m, 8H, H-qui),
7.5 (m, 2H, H-py), 7.1 (t, 1H, H-py), 4.1 (s, 4H, CH2-qui), 3.9 (s, 2H,
CH2-py).
µ, mm-1
Nmeas
Nobs
R
Rw
[(TrispicMeen)FeIII(DBC)]BPh4. A solution of trispicMeen (227
mg, 1 mmol) in 10 mL of ethanol was added to a solution of anhydrous
FeCl3 (162 mg, 1 mmol) in 10 mL of ethanol. The resulting yellow
solution was stirred for 30 min at room temperature. A solution of
DBCH2 (222 mg, 1 mmol) in 5 mL of ethanol was then added, followed
by a dropwise addition of triethylamine (280 µL, 2 mmol) in 5 mL of
ethanol. The blue solution was stirred for 30 min at 50 °C and then
allowed to cool to room temperature, and sodium tetraphenylborate
(342 mg, 1 mmol) was added. Finally, 10 mL of distilled water was
very slowly added while the solution was vigorously stirred at -15 °C
for 1 h. A blue powder precipitated and was filtered and washed with
a small amount of ethanol. Anal. Found: C, 74.6; H, 6.9; N, 7.4; Fe,
5.9; B, 1.1. Calcd for C59H65N5O2FeB: C, 74.1; H, 6.9; N, 7.4; Fe,
6.1; B, 1.0.
Studies of Oxygenated Products. The following procedure was used
for all of the complexes described. A solution of 0.5 mmol of complex
in 5 mL of DMF was stirred at room temperature with bubbling oxygen.
When the solution turned brown, the mixture was dried under vacuum.
The brown oil obtained was diluted with water (pH 8), extracted with
ether, and dried under vacuum. The pH of the remaining aqueous
solution was decreased to a value of 2 by addition of a 0.01 N HCl
solution, and the second oil was extracted with ether and dried. The
N,N′-Dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine
(BispicMe2en). The ligand bispicMe2en was synthesized as previously
described.10
N,N′-Dimethyl-N,N′-bis(4-chloro-2-pyridylmethyl)ethane-1,2-di-
amine (BispicCl2Me2en). The ligand bispicCl2Me2en was a gift from
Professor J.-B. Verlhac (University of Bordeaux, Bordeaux, France).11
N-Methyl-N,N′,N′′-tris(2-pyridylmethyl)ethane-1,2-diamine
(TrispicMeen). The ligand trispicMeen was synthesized as previously
described.12 This ligand has also been described by Bernal et al.13
Preparation of Complexes. [(BQPA)FeIII(DBC)]BPh4. A solution
of BQPA (390 mg, 1 mmol) in 10 mL of ethanol was added under
argon to anhydrous FeCl3 (162 mg, 1 mmol) dissolved in 10 mL of
ethanol. The yellow mixture was stirred at room temperature for 30
min, and DBCH2 (222 mg, 1 mmol) dissolved in 5 mL of ethanol was
then added, followed by a dropwise addition of triethylamine (280 µL,
2 mmol) in 5 mL of ethanol. The solution turned blue, and sodium
tetraphenylborate (342 mg, 1 mmol) in 5 mL of ethanol was added. A
deep blue powder immediately precipitated and was filtered and washed
with a small amount of ethanol. Anal. Found: C, 78.0; H, 6.3; N, 5.7.
Calcd for C64H64N4O2FeB: C, 77.5; H, 6.3; N, 5.9.
[(BispicMe2en)FeIII(DBC)]BPh4. A solution of bispicMe2en (270
mg, 1 mmol) in 10 mL of methanol was added to a solution of
anhydrous FeCl3 (162 mg, 1 mmol) in 25 mL of methanol. The mixture
was stirred at room temperature for 30 min, after which DBCH2 (222
mg, 1 mmol) and 2 mmol of triethylamine (280 µL, 2 mmol) dissolved
in 10 mL of methanol were added. The mixture was stirred at room
temperature for 30 min, and then sodium tetraphenylborate (342 mg, 1
mmol) in 5 mL of methanol was added. A purple powder precipitated,
which was filtered and washed with 10 mL of methanol and then
recrystallized in acetonitrile under a nitrogen flow. Crystals of
[(bispicMe2en)FeIII(DBC)]BPh4 suitable for X-ray diffraction studies
were obtained. Anal. Found: C, 74.8; H, 7.2; N, 6.5; Fe, 6.4. Calcd
for C54H62N4O2FeB: C, 74.0; H, 7.2; N, 6.5; Fe, 6.4.
1
two oils were studied by H NMR and infrared spectroscopy.
NMR Studies. NMR spectra were recorded on a Bruker AM250
spectrometer using CDCl3 or CD3CN as the internal lock. T1 values
were determined using an inversion-recovery pulse sequence (π-τ-
π/2-ACQ).
UV-Vis Studies. Spectra and kinetic studies were recorded at 20
°C in DMF on a Varian Cary 5E spectrophotometer.
Crystallographic Data Collection and Refinement of Structure.
[(BispicMe2en)FeIII(DBC)]BPh4. A single crystal of dimensions 0.33
× 0.15 × 0.10 mm3 was mounted on a four-circle Philips PW1100
diffractometer with graphite monochromated Mo KR radiation (λ )
0.71073 Å). The unit cell was obtained by least-squares procedures
based on the 2θ values of the 25 reflections measured (8.4° < 2θ <
14.4°). All reflection intensities were collected at room temperature in
the range 4° < 2θ < 40° for -14 < h < 13, -17 < k < 17, and 0 <
l < 23. A total of 10 958 independent reflections was collected. The
structure was solved by direct methods (SHELXS86)14 and refined by
full least-squares approximation based on F2. Crystallographic data are
presented in Table 1. Refinement was anisotropic for all non-H atoms.
Hydrogen positions were calculated by assuming geometrical positions
and were included in the structural model. Clusters of electronic residual
appeared in a subsequent Fo - Fc synthesis. Anisotropic full-matrix
least-squares refinement of this structural model was continued until
convergence when R ) 0.0393 and Rw ) 0.0903 for all 10 958
reflections. The final difference map showed the largest residual peaks
of 0.331 and -0.234 eÅ-3 (the latter being on a symmetry center).
[(BispicCl2Me2en)FeIII(DBC)]BPh4‚CH3OH. Synthesis of this com-
plex is identical to that described for [(bispicMe2en)FeIII(DBC)]BPh4.
Anal. Found: C, 69.3; H, 6.5; N, 6.0; Cl, 7.6. Calcd for C54H60N4O2-
Cl2FeB: C, 68.8; H, 6.5; N, 5.9; Cl, 7.9.
(10) Toftlund, H.; Pedersen, E.; Yde-Andersen, S. Acta Chem. Scand., Ser.
A 1984, 38, 429.
(11) Delroisse, M. Thesis, Universite´ de Bordeaux I, Bordeaux, France,
1995. Delroisse, M.; Rabion, A.; Chardac, F.; Te´tard, D.; Verlhac,
J.-B.; Fraisse, L.; Se´ris, J.-L. J. Chem. Soc., Chem. Commun. 1995,
949.
(12) Nivorozhkin, A. L.; Anxolabe´he`re-Mallart, E.; Mialane, P.; Davydov,
R.; Guilhem, J.; Ce´sario, M.; Audie`re, J.-P.; Girerd, J.-J.; Styring, S.;
Schussler, L.; Seris, J.-L. Inorg. Chem. 1997, 36, 846-853.
(13) Bernal, I.; Jensen, I. M.; Jensen, K. B.; McKenzie, C. J.; Toftlund,
H.; Tuchagues, J. P. J. Chem. Soc., Dalton Trans. 1995, 3667.
(14) Sheldrick, G. M. SHELXS86: Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1986.

2442 Inorganic Chemistry, Vol. 39, No. 12, 2000
Mialane et al.
Figure 3. UV-visible spectra in DMF at 20 °C of the complexes (a)
[(BQPA)FeIII(DBC)]+, (b) [(trispicMeen)FeIII(DBC)]+, (c) [(bispicCl2-
Me2en)FeIII(DBC)]+, and (d) [(bispicMe2en)FeIII(DBC)]+.
Figure 1. Ligands used in this study.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[(BispicMe2en)FeIII(DBC)]BPh4
Fe-O(2)
Fe-O(1)
Fe-N(11)
Fe-N(21)
Fe-N(22)
Fe-N(12)
1.914(4)
1.914(2)
2.163(6)
2.176(7)
2.205(4)
2.210(4)
C(1)-O(1)
C(6)-O(2)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(1)
1.344(3)
1.346(2)
1.391(3)
1.384(3)
1.399(3)
1.393(3)
1.403(3)
1.418(3)
O(2)-Fe-O(1)
85.72(6)
94.69(13)
94.1(2)
94.05(9)
91.1(2)
170.15(7)
90.85(8)
166.01(6)
99.7(2)
O(2)-Fe-N(11)
O(1)-Fe-N(11)
O(2)-Fe-N(21)
O(1)-Fe-N(21)
N(11)-Fe-N(21)
O(2)-Fe-N(22)
O(1)-Fe-N(22)
N(11)-Fe-N(22)
N(21)-Fe-N(22)
O(2)-Fe-N(12)
O(1)-Fe-N(12)
N(11)-Fe-N(12)
N(21)-Fe-N(12)
N(22)-Fe-N(12)
Figure 2. Structure of the [(bispicMe2en)Fe(DBC)]+ dication showing
the 50% probability thermal ellipsoids and atom-labeling scheme.
Hydrogen atoms are omitted for clarity.
Results and Discussion
75.6(2)
The ligands used in this study are represented in Figure 1.
The ligands bispicMe2en and bispicCl2Me2en are of the linear
tetradentate type. The chloropyridine derivative was used to
study the electronic effect of the chloro substituent. The in-
fluence of an extra pyridine group was studied with the pen-
tadentate ligand trispicMeen. The ligand BQPA is a tetradentate
tripodal analogue of TPA. Elementary analyses and spectro-
scopic data show that every compound contains the [LFeIII-
(DBC)]+ entity. In the case of [(bispicMe2en)FeIII(DBC)]BPh4,
the structure was determined by X-ray diffraction.
Structure of [(BispicMe2en)FeIII(DBC)]BPh4. The [(bispic-
Me2en)FeIII(DBC)]+ cation is represented in Figure 2. It has a
distorted octahedral geometry, the catecholate dianion being
chelated to Fe(III). As usual, the bispicMe2en ligand has the
cis R configuration. This structure has already been noted by
Glerup et al.15 and by Arulsamy et al.16 The metal-ligand bond
distances (Table 2) are Fe-N(pyridine) ) 2.163(6) and 2.176-
(7) Å, Fe-N(amine) ) 2.210(4) and 2.205(4) Å, and Fe-O )
1.914(4) and 1.914(2) Å. As expected, the Fe-O distances are
similar to each other for [(bispicMe2en)FeIII(DBC)]+ and [(L-
N4Me2)FeIII(Cat)]+ [1.915(3) and 1.903(3) Å, respectively].7 In
[(TPA)FeIII(DBC)]+,6 the chelation of catecholate is slightly
166.83(7)
103.51(7)
75.52(11)
95.12(8)
82.32(8)
asymmetric [1.917(3) and 1.898(2) Å], but the asymmetry is
more pronounced for [(BPG)FeIII(DBC)]17 [1.989(2) and 1.889-
(2) Å]. In [(bispicMe2en)FeIII(DBC)]+, the catecholate dianion
presents C-O distances of 1.344(3) and 1.346(2) Å and C-C
distances between 1.384(3) and 1.418(3) Å. The analogous
distances in [(TPA)FeIII(DBC)]+ are very close.6 These values
for the catecholato dianion are quite different from those
observed for the semiquinonato DBSQ°- anion in the [LFeIII-
(DBSQ)] complex, where LH2 ) N,N′-bis(4-methyl-6-tert-butyl-
2-methylphenolato)-N,N′-bismethyl-1,2-diaminoethane.18
UV-Visible Spectroscopy. The electronic absorption spectra
of the [LFeIII(DBC)]+ complexes in DMF solution (Figure 3)
exhibit two prominent bands, one in the visible (λ1 ) 550-
582 nm) region and one in the NIR (λ2 ) 925-957 nm) (Table
3). The same type of spectrum has been observed in [(BPG)-
FeIII(DBC)],17,19 [(TPA)FeIII(DBC)]+,6 [(NTA)FeIII(DBC)]2-,17,19
[(L-N4Me2)FeIII(DBC)]+,7 and [(SS-CTH)FeIII(DBC)]+.20 The
(15) Glerup, J.; Goodson, P. A.; Hazell, A.; Hazell, R.; Hodgson, D. J.;
McKenzie, C. J.; Michelsen, K.; Rychlewska, U.; Toftlund, H. Inorg.
Chem. 1994, 33, 4105-4111.
(16) Arulsamy, N.; Hodgson, D. J.; Glerup, J. Inorg. Chim. Acta 1993,
209, 61.
(17) Cox, D. D.; Que, L., Jr. J. Am. Chem. Soc. 1988, 110, 8085-8092.
(18) Mialane, P.; Anxolabe´he`re-Mallart, E.; Blondin, G.; Nivorozhkin, A.
L.; Guilhem, J.; Tchertanova, L.; Cesario, M.; Ravi, N.; Bominaar,
E.; Girerd, J.-J.; Mu¨nck, E. Inorg. Chim. Acta 1997, 263, 367.

Aminopyridine Iron Catecholate Complexes
Inorganic Chemistry, Vol. 39, No. 12, 2000 2443
Table 3. Spectroscopic and Kinetic Data
λ1,a ꢀ1
ppm with T1 values of 0.78 ms. The pyridine ortho proton is
broadened because of its proximity to the metal center and is
shifted further downfield at 190 ppm. The pyridine para proton
resonates in the diamagnetic region at 1.3 ppm. Additional peaks
arise from the methylene protons, which appear as a broad band
at 107 ppm with a T1 value of 0.44 ms. The similarity with
[(TPA)FeIII(DBC)]+ is striking. The quinoline protons 3-H and
8-H appear at 48 and 51 ppm although we did not assign these
two resonances more precisely. We tentatively assign the other
quinoline protons as 4-H (1.2 ppm), 5-H and 6-H (15 ppm,
which integrates for four protons), and 7-H (43 ppm). Additional
peaks arise from the methylene protons, which appear as a broad
band at 74 ppm with a T1 value of 0.58 ms. The DBC 6-H and
4-H protons exhibit shift at -5 and -89 ppm, respectively. The
DBC 5-t-Bu and 3-t-Bu resonances are found at 9.9 (6.0 ms)
and 5.5 ppm, respectively.
As noted by Jang et al.,6 the more upfield proton in this type
of complex corresponds to the 4-H DBC proton. This proton is
recognized in this series of complexes at the following shifts:
bispicMe2en, -59.5 ppm; bispicCl2Me2en, -70.0 ppm; trispic-
Meen, -82.0 ppm; and BQPA, -89.0 ppm. For the DBC 5-t-
Bu resonance, a similar order was observed: bispicMe2en, 9.0
ppm; bispicCl2Me2en, 9.7 ppm; BQPA, 9.9 ppm; and trispic-
Meen, 9.95 ppm. As noted by Jang et al.,6 these paramagnetic
shifts reflect the amount of the Fe(II)(SQ) form in the ground-
λ2, ꢀ2
δ (1H)b
k[O2]c
(s-1
kc
(nm, mM-1 cm-1
)
5-t-Bu, 4-H
)
(M-1 s-1
)
TPA
548 (1.76)
874 (2.45)
560 (1.85)
935 (2.35)
557 (1.30)
941 (1.90)
582 (2.23)
957 (3.45)
550 (1.25)
925 (1.85)
8.7, -55.4 0.0465d
15d
trispicMeen
bispicCl2Me2en
BQPA
9.95, -82.0 8.68 × 10-4
9.7, -70.0 4.96 × 10-4
9.9, -89.0 4.03 × 10-4
9.0, -59.5 8.06 × 10-5
0.28
0.16
0.13
0.026
bispicMe2en
a In DMF. b In CD3CN. c In DMF at 20 °C. d In DMF at 25 °C.
two bands have been attributed to ligand-to-metal charge
transfers O(DBC) f Fe(III).17,20,21 Their characteristics depend
on the nature of the auxiliary ligand L. The similarity between
the electronic absorption spectra of the complexes [(bispicMe2-
en)FeIII(DBC)]+, [(BQPA)FeIII(DBC)]+, [(bispicCl2Me2en)FeIII-
(DBC)]+, and [(trispicMeen)FeIII(DBC)]+ indicates that, in each
case, the catecholato dianion is chelated to Fe(III). The bispicen
moiety must present a cis R configuration in the [(bispic-
Cl2Me2en)FeIII(DBC)]+ complex, as it does in [(bispicMe2en)-
FeIII(DBC)]+. The ligand trispicMeen has been extensively
studied, and it has been shown that this ligand can act as a
tetradentate12 or pentadentate ligand.13,22 As depicted below for
the tetradentate case, two types of structure for the [(trispic-
Meen)FeIII(DBC)]+ complex can be envisioned: either one with
the ligand trispicMeen adopting a linear cis R mode of
coordination or one in which the ligand plays the role of a
tripodal ligand. Analogy with the previously characterized
compound [(trispicMeen)ClFeIIIOFeIIICl(trispicMeen)]2+ 12 is in
favor of a linear cis R geometry.
state wave function, which can be written as ΦFe(III)(Cat)
+
RΦFe(II)(SQ). We observed a correlation for the δ (4-H) with λ1
values. This correlation is similar for δ (4-H) and λ2, but an
inversion is found for [(BQPA)FeIII(DBC)]+ and [(trispicMeen)-
FeIII(DBC)]+. The correlation between δ and λmax is in keeping
with the idea that the lower in energy the excited Fe(II)(SQ)
form is, the larger is the contribution of this excited form in
the ground state. This is true because simple perturbation theory
gives R ) |HAB|/hνmax, where HAB is the transfer integral
between the d orbital and the implied π orbital and hνmax is the
energy gap between the states. The existence of this correlation
suggests that HAB does not change much in the series of
complexes that we investigated. We have identified another
correlation. The intensity of a CT transition is related to R2,23
so we can expect a relation between δ and ꢀmax. Indeed, we
found that the δ (4-H) values are strictly correlated to the ꢀ1
and ꢀ2 values, as depicted in Figure 4.
It must be noticed that [(TPA)FeIII(DBC)]+ does not follow
this correlation: the molar absorptivity ꢀ2 ) 2.45 mM-1 cm-1
is much larger than expected on the basis of the NMR shift δ
(4-H) ) -55.4 ppm, which is the lowest in this series of
complexes.
The structure of the [(BQPA)FeIII(DBC)]+ complex must be
different from that of the complexes described above but similar
to that of [(TPA)FeIII(DBC)]+.6 However, we cannot make a
conclusion about the cis or trans configuration of the two
quinoline groups.
Reactivity Studies. The reactivity of each complex with O2
in saturating conditions was studied in DMF at 20 °C. The
reaction was stopped after 6 h for [(trispicMeen)FeIII(DBC)]+,
24 h for [(BQPA)FeIII(DBC)]+, and 48 h for [(bispicMe2en)-
FeIII(DBC)]+ and [(bispicCl2Me2en)FeIII(DBC)]+. A quantitative
analysis of the reaction products was made by NMR. In every
case, neither 3,5-di-tert-butylcatechol nor 3,5-di-tert-butylquino-
ne was observed. The catechol was then quantitatively degraded.
The major and almost only product (90%) is 3,5-di-tert-butyl-
5-(carboxymethyl)-2-furanone. The muconic anhydride is al-
ways observed as a minor product (<5%). Jang et al.6 ob-
served the same products, although with a larger muconic
anhydride quantity. The difference could be due to the longer
reaction times used here. Indeed, the furanone is the hydrolysis
NMR Studies. The 1H NMR spectrum of [(BQPA)FeIII-
(DBC)]+ in CD3CN shows large contact shifts. By comparison
with the results by Jang et al.6 on [(TPA)FeIII(DBC)]+ and by
the use of T1 values and integrations, it is possible to partially
assign the resonances observed for [(BQPA)FeIII(DBC)]+. The
pyridine meta protons appear as relatively sharp features at 91
(19) Cox, D. D.; Benkovic, S. J.; Bloom, L. M.; Bradley, F. C.; Que, L.,
Jr.; Wallick, D. E. J. Am. Chem. Soc. 1988, 110, 2026.
(20) Dei, A.; Gatteschi, D.; Pardi, L. Inorg. Chem. 1993, 32, 1389.
(21) Salama, S.; Stong, J. D.; Neilands, J. B.; Spiro, T. G. Biochemistry
1978, 17, 3781.
(22) Mialane, P.; Nivorojkine, A.; Pratviel, G.; Aze´ma, L.; Slany, M.;
Godde, F.; Simaan, A.; Banse, F.; Kargar-Grisel, T.; Bouchoux, G.;
Sainton, J.; Horner, O.; Guilhem, J.; Tchertanov, L.; Meunier, B.;
Girerd, J.-J. Inorg. Chem. 1999, 38, 1085-1092.
(23) Hush, N. S. In Progress in Inorganic Chemistry; Cotton, F. A., Ed.;
Wiley: New York, 1967; Vol. 8, p 437.

2444 Inorganic Chemistry, Vol. 39, No. 12, 2000
Mialane et al.
FeIII(DBC)]+. For the latter, the two amino groups are trans to
the two oxygens of DBC2-, leading to two equivalent Fe-O
bonds. In contrast, for the former, one oxygen of the substrate
is trans to an amino group, and the other is trans to the nitrogen
of a pyridine. This asymmetry could influence the reactivity,
as pointed out recently for the corresponding enzymes by Orville
et al.25
An alternative explanation could be related to the recent
observation by Simaan et al. of a spin equilibrium in [(TPA)-
FeIII(cat)]+ 26 in the solid state. It could be that such a spin
equilibrium exists in solution for [(TPA)FeIII(DBC)]+, leading
to the presence of a small fraction at room temperature of low-
spin [(TPA)FeIII(DBC)]+, and that this low-spin species is at
the origin of the high reactivity observed. The problem of the
spin state of Fe(III)/aminopyridine/catecholate systems must be
explored in detail.
Figure 4. Correlation between δ (4-H) and ꢀ1 or ꢀ2 for the complexes
[(BQPA)FeIII(DBC)]+ (δ ) -89.0 ppm), [(trispicMeen)FeIII(DBC)]+
(δ ) -82.0 ppm), [(bispicCl2Me2en)FeIII(DBC)]+ (δ ) -70.0 ppm),
and [(bispicMe2en)FeIII(DBC)]+ (δ ) -59.5 ppm). The second-order
rate constant k in DMF at 20 °C is represented as a function of δ (4-
H). For the three topologically similar complexes, k increases when
the chemical shift decreases. The k value for [(BQPA)FeIII(DBC)]+ (δ
) -89.0 ppm) is lower than expected (see text).
Conclusion
We isolated new molecules containing the Fe(III) catecholate
motif. These complexes all presented an intradiol-type dioxy-
genase activity. The structure of [(bispicMe2en)FeIII(DBC)]+ was
determined. The NMR and optical properties indicate that, for
the three complexes presenting similar topological character-
istics, the reactivity is correlated with the λmax value, and better
with the ꢀmax value, of the LMCT O(DBC) f Fe(III) band and
with the chemical shift of the 4-H proton of the DBC2- ligand.
These results elucidate previous suggestions by the group of
Que. However, these observations cannot be extrapolated to the
[(BQPA)FeIII(DBC)]+ complex. The large steric hindrance at
the metal center because of the quinoline arms must considerably
decrease the rate of substrate oxidation by O2.
product of the anhydride, and the larger amount of anhydride
was obtained for the shorter reaction time {[(trispicMeen)FeIII-
(DBC)]+}.
Kinetic measurements were made for all of the complexes
in DMF at 20 °C. The pseudo-first-order constants were deduced
from simulation, and the second-order constants were calculated
using [O2] ) 3.1 × 10-3 M in DMF at 20 °C (Table 3).24 The
series does not follow that deduced from UV-vis and NMR
(Figure 4) data. Nevertheless, if we focus on the topologically
similar complexes [(bispicMe2en)FeIII(DBC)]+, [(bispicCl2Me2-
en)FeIII(DBC)]+, and [(trispicMeen)FeIII(DBC)]+, the correlation
is verified (Figure 4). A decrease of the δ (4-H) value or an
increase of the ꢀmax value, both of which signify a larger
contribution of the Fe(II)(SQ) form to the ground state, leads
to better reactivity. The introduction of a σ-attractor group on
the pyridine ring leads to an improvement of the reactivity. The
k value found for the complex [(BQPA)FeIII(DBC)]+ is smaller
than expected on the basis of the large paramagnetic shift
observed for the 4-H proton. We propose that this low reactivity
is due to the large steric hindrance of the quinolines to the metal
center, which can affect the formation of the µ-peroxo inter-
mediate postulated in the mechanism of intradiol dioxygenases.25
The complex [(TPA)FeIII(DBC)]+, synthesized by the group
of L. Que, remains the most active model complex leading to
degradation of catechol. This observation is not in agreement
with the correlation found here for the bispicen-type ligands.
The most striking disagreement is that the [(TPA)FeIII(DBC)]+
complex has the smallest NMR shift δ (1H, 4-H) ) -55.4 ppm
but nevertheless presents the fastest reaction. Steric consider-
ations cannot explain this phenomenon. The high reactivity of
[(TPA)FeIII(DBC)]+ is intriguing.
The complex [(TPA)FeIII(DBC)]+ remains the most active
species synthesized to date. The remarkable activity of this
complex cannot be explained by a consideration of its optical
and NMR properties. Asymmetric chelation of the catecholate
to Fe(III), such as that noted in the enzyme by Orville et al.,25
could explain the high reactivity of [(TPA)FeIII(DBC)]+. An
alternative explanation could be a large electronic difference
between this complex and the series studied here. From recent
data, it seems possible that [(TPA)FeIII(DBC)]+ presents a spin
equilibrium such as that observed recently for [(TPA)FeIII(cat)]+
in the solid state. The nature of the spin states of these Fe(III)/
catecholate systems must be studied in greater detail.
Acknowledgment. This work was supported by a grant from
the Elf company.
Supporting Information Available: Tables of crystal data, atomic
coordinates, selected bond lengths and angles, bond lengths and angles,
thermal parameters, H-atom coordinates and values of 10|Fo| and 10|Fc|
for [(bispicMe2en)FeIII(DBC)]BPh4. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC981236V
One possible explanation could rely on subtle structural
(25) Orville, A. M.; Lipscomb, J. D.; Ohlendorf, D. H. Biochemistry 1997,
36, 10052-10066.
(26) Simaan, J.; Boillot, M.-L.; Rivie`re, E.; Boussac, A.; Girerd, J.-J. Angew.
Chem., Int. Ed. Engl. 2000, 39, 196-198.
differences between [(TPA)FeIII(DBC)]+ and [(bispicMe2en)-
(24) James, H. J.; Broman, R. F. Anal. Chim. Acta 1969, 48, 411.