Acireductone dioxygenase- (ARD-) type reactivity of a Nickel(II) complex having monoanionic coordination of a model substrate: Product identification and comparisons to unreactive analogues

Article abstract of DOI:10.1021/ic061177i

A mononuclear Ni(II) complex ([(6-Ph₂TPA)Ni(PhC(O)C(OH)C(O)Ph)] ClO₄ (1)), supported by the 6-Ph₂TPA chelate ligand (6-Ph₂TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl) amine) and containing a cis-β-ketoenolate ligand having a C2 hydroxyl substituent, undergoes reaction with O₂ to produce a Ni(II) monobenzoate complex ([(6-Ph₂TPA)Ni(O₂CPh)]ClO₄ (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized by X-ray crystallography, ¹H NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-β-keto-enolate Ni(II) complexes supported by the 6-Ph₂TPA ligand ([(6-Ph ₂TPA)Ni(PhC(O)CHC(O)Ph)]ClO₄ (4), [(6-Ph ₂TPA)Ni(CH₃C-(O)CHC(O)CH₃)]ClO₄ (5), and [(6-Ph₂TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized. While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O₂. The results of this study are discussed in terms of relevance to Ni(II)-containing acireductone dioxygenase enzymes, as well as in the context of recently reported cofactor-free, quercetin, and β-diketone dioxygenases.

Full text of DOI:10.1021/ic061177i

Inorg. Chem. 2007, 46, 5499−5507
Acireductone Dioxygenase- (ARD-) Type Reactivity of a Nickel(II)
Complex Having Monoanionic Coordination of a Model Substrate:
Product Identification and Comparisons to Unreactive Analogues
Ewa Szajna-Fuller,† Katarzyna Rudzka,† Atta M. Arif,‡ and Lisa M. Berreau*,†
Department of Chemistry and Biochemistry, Utah State UniVersity, Logan, Utah 84322-0300, and
Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112
Received June 27, 2006
A mononuclear Ni(II) complex ([(6-Ph
ligand (6-Ph TPA N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) and containing a cis-
enolate ligand having a C2 hydroxyl substituent, undergoes reaction with O to produce a Ni(II) monobenzoate
complex ([(6-Ph TPA)Ni(O CPh)]ClO (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified
phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized
by X-ray crystallography, 1H NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-
-keto-enolate Ni(II)
complexes supported by the 6-Ph TPA ligand ([(6-Ph TPA)Ni(PhC(O)CHC(O)Ph)]ClO (4), [(6-Ph TPA)Ni(CH C-
O)CHC(O)CH )]ClO (5), and [(6-Ph TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized.
While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O . The results
of this study are discussed in terms of relevance to Ni(II)-containing acireductone dioxygenase enzymes, as well
as in the context of recently reported cofactor-free, quercetin, and -diketone dioxygenases.
2
TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO
4
2
(1)), supported by the 6-Ph TPA chelate
2
)
â
-keto-
2
2
2
4
â
2
2
4
2
3
(
3
4
2
2
â
Introduction
Scheme 1
Acireductone dioxygenases (ARDs) are enzymes found
associated with the methionine salvage pathway in species
ranging from bacteria to mammals.1-8 Two ARD enzymes
having the same amino acid sequence (polypeptide chain)
but which differ in the active site metal present (Fe(II) or
3
Ni(II)) have been identified in Klebsiella ATCC 8724.
Interestingly these enzymes catalyze the oxidation of 1,2-
dihydroxy-3-keto-5-(methylthio)pentene to give different
*
To whom correspondence should be addressed. E-mail: berreau@
cc.usu.edu. Phone: 435-797-1625. Fax: 435-797-3390.
†
Utah State University.
University of Utah.
‡
(
(
(
1) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1993, 268, 21466-21469.
2) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1995, 270, 3147-3153.
3) Dai, Y.; Wensink, P. C.; Abeles, R. H. J. Biol. Chem. 1999, 274,
products (Scheme 1). Specifically, whereas the Fe(II)-ARD
catalyzed reaction yields an R-keto acid and formic acid,
the Ni(II)-containing ARD enzyme instead catalyzes a shunt
out of the methionine salvage pathway and the formation of
a methylthiocarboxylic acid, formic acid, and CO. Results
of anaerobic X-ray absorption and H NMR spectroscopic
studies of the Ni(II)-ARD from Klebsiella ATCC 8724
1
193-1194.
4) Dai, Y.; Pochapsky, T. C.; Abeles, R. H. Biochemistry 2001, 40, 6379-
387.
5) Hirano, W.; Gotoh, I.; Uekita, T.; Seiki, M. Genes Cells 2005, 20,
(
(
6
1
565-574.
(
(
6) Lin, T.; He, X.; Yang, L.; Shou, H.; Wu, P. Gene 2005, 360, 27-34.
7) Oram, S.; Jiang, F.; Cai, X.; Haleem, R.; Dincer, Z.; Wang, Z.
Endocrinology 2004, 145, 1933-1942.
suggest that a dianionic form of the substrate coordinates to
(8) Al-Mjeni, F.; Ju, T.; Pochapsky, T. C.; Maroney, M. J. Biochemistry
the Ni(II) center in a bidentate fashion prior to reaction with
2002, 41, 6761-6769.
8
O
2
. This notion is supported by UV-vis studies of an
(9) Ju, T.; Goldsmith, R. B.; Chai, S. C.; Maroney, M. J.; Pochapsky, S.
S.; Pochapsky, T. C. J. Mol. Biol. 2006, 393, 823-834.
alternative substrate (1,2-dihydroxy-3-keto-1-hexene) and its
1
0.1021/ic061177i CCC: $37.00
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 14, 2007 5499
Published on Web 02/13/2007

Szajna-Fuller et al.
Scheme 2
Figure 1. Protonation levels and spectroscopic properties of ARD-type
4
substrate analogues. In Klebsiella ATCC 8724 there is one common
substrate for ARD enzymes, 1,2-dihydroxy-3-keto-5-methylthiopentene.
However, for mechanistic studies, where large amounts of substrate are
1
required, substrate analogues have been used.
spectroscopic properties under anaerobic conditions in the
4
presence of Ni(II)-ARD. As shown in Figure 1(left), in
aqueous solution at pH ) 7.4, an acireductone analogue
-
1
exists as a monoanion with λmax ) 305 nm (ꢀ ) 20 000 M
-
1
cm ), whereas at pH ) 13 a dianionic species is formed
-1
-1 4
with λmax ) 345 nm (ꢀ ) 14 000 M cm ). In the presence
of the Ni(II)-ARD enzyme at pH ) 7.4, under anaerobic
conditions, a λmax ) 345 nm is found for this substrate
analogue, consistent with the formation of a dianionic species
that is coordinated to the Ni(II) center.4 It is worth noting
that a phenyl-substituted acireductone-type substrate analogue
,8
(1,2-dihydroxy-3-oxo-3-phenyl-1-propene, Figure 1 (right))
exhibits similar behavior, with the monoanionic form having
-
1
-1
λ
max ) 320 nm (ꢀ ) 14 000 M cm ) and the dianionic
-
1
-1
form λmax ) 360 nm (ꢀ ) 11 000 M cm ). Introduction
of this substrate analogue to Ni(II)-ARD at pH ) 7.4 in
the absence of O results in a red-shift of the absorption
2
maximum from 320 to 360 nm. This is consistent with the
formation of a coordinated dianionic species.
We recently reported the first reactive model system for
Ni(II)-ARD enzymes, albeit using a bulky model substrate
analogue that has not been reported to be a substrate for the
enzyme.10 Using the 6-Ph
2
TPA chelate ligand, a Ni(II) cis-
â-keto-enolate complex having a coordinated monoanionic
form of the model substrate (1, Scheme 2) was prepared and
characterized. Treatment of this complex with 1 equiv of
benzoate complex, CO, benzil, benzoic acid, and trace
amounts of other unidentified phenyl-containing organic
products. The production of CO and labeled Ni(II)-
4 2 3
base (Me NOH‚5H O) in CH CN under an inert atmosphere
resulted in a red shift of the λmax from 399 to 420 nm. This
was taken as initial evidence for the formation of a new Ni-
18
coordinated benzoate (50% O incorporation starting from
18
2
O (99%)) in this reaction indicates that an ARD-type
(
II) complex having a coordinated dianionic form of the
pathway is operative in this system. However, the production
of benzil, unlabeled benzoate/benzoic acid, and other trace
phenyl-containing organic products must result from an
alternative reaction pathway.
In further work we have compared the structural and
spectroscopic properties of 1 with those of mononuclear
11
ARD-type substrate. While attempts to isolate this complex
are currently in progress, we have previously shown that
treatment of this dianionic substrate species with excess O
2
results in the formation of 2 and CO (Scheme 2).
In the work presented herein we have examined the
reaction of 1 with O in the absence of any added base. This
reaction results in the formation of a mononuclear Ni(II)
2
6-Ph TPA-ligated Ni(II) complexes having a monoanionic
2
cis-â-keto-enolate ligand derived from dibenzoylmethane,
2
,4-pentanedione, or 1,2-dibenzoylethenol. These complexes
(
10) Szajna, E.; Arif, A. M.; Berreau, L. M. J. Am. Chem. Soc. 2005, 127,
are unreactive toward O
The results of this study provide insight into factors that
influence substrate reactivity in ARD type substrates and are
2
.
17186-17187.
1
(11) Significant changes occur in the H NMR spectrum of 1 upon treatment
with 1 equiv of Me4NOH‚5H2O.
5500 Inorganic Chemistry, Vol. 46, No. 14, 2007

ARD ReactiWity InWolWing a Monoanionic Substrate
also discussed in the context of recently reported cofactor-
free, quercetin, and â-diketone dioxygenases.12-30
University of California, Riverside, CA, using a VG ZAB2SE high-
resolution mass spectrometer in a matrix of m-nitrobenzyl alcohol
(MNBA). GC-MS spectra were acquired on a Shimadzu GC-MS
QP5000. Elemental analyses were performed by Atlantic Microlabs
of Norcross, GA. CO formation was detected using the palladium
chloride method in which elemental palladium is deposited upon
Experimental Section
General Methods. All reagents and solvents were obtained from
commercial sources and were used as received unless otherwise
35
reaction with CO in the presence of water.
noted. CD
CaH and was distilled under vacuum prior to use. The 6-Ph
TPA ligand,32 Ph
dibromoethane34 were synthesized following literature procedures.
3
CN was dried according to a literature procedure from
Caution! Perchlorate salts of metal salts supported by organic
ligands are potentially explosiVe. Only small amounts of material
3
1
2
2
-
33
33
2
TPA-d
6
,
Ph
2
TPA-d10
,
and dibenzoyl-1,2-
36
should be prepared, and these should be handled with great care.
Treatment of [(6-Ph
with O . Identification of Products. Addition of O
2 mL) solution of 1 (25 mg) at room temperature results in a rapid
color change from deep orange-brown to green-yellow. Testing of
2
TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO
4
(1)
10
Compound 1 was prepared as previously described.
2
2
to a CH
3
CN
1
13
1
Physical Methods. H and C{ H} NMR spectra were recorded
on a JEOL GSX-270, JEOL ECX-300, or Bruker ARX-400
spectrometer. Chemical shifts (in ppm) are referenced to the residual
(
35
2
an aliquot of the headspace gas using the PdCl method indicated
1
13
1
2
solvent peak(s) (CD HCN: H, 1.94 (quintet); C{ H}, 1.39
the formation of CO during this reaction. After stirring of the
1
2
(
heptet) ppm). H and H NMR spectra of paramagnetic Ni(II)
solution for ∼12 h, the solvent was removed under reduced pressure.
3
3
complexes were acquired as previously described. Longitudinal
relaxation times (T ) were measured using the inversion-recovery
pulse sequence (180°-τ-90°) method. H NMR COSY spectra
for 3 and 4 were obtained in CD CN at 302 K and processed as
The remaining green-yellow solid was washed with Et
mL). The Et O wash was then brought to dryness under reduced
2
2
O (2 × 20
1
1
pressure, yielding an off-white solid. When dissolved in organic
3
solvent, this material exhibits several overlapping resonances in
previously described.33 UV-vis spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer. FTIR spectra were
recorded on a Shimadzu FTIR-8400 spectrometer as KBr pellets.
Fast atom bombardment (FAB) mass spectra were obtained at the
1
the aromatic region of the H NMR (in CD
3
CN) and a GC-MS
3
(CH CN) trace consistent with the presence of benzil (PhC(O)C-
(O)Ph), major product), benzoic acid (minor product), and trace
amounts of other unidentified phenyl-containing products. The
R-keto acid benzoylformic acid is not produced. The green solid
(
(
(
(
(
(
(
(
(
12) Bauer, I.; Beyer de, A.; Tshisuaka, B.; Fetzner, S.; Lingens, F. FEMS
Microbiol. Lett. 1994, 117, 299-304.
13) Bauer, I.; Max, N.; Fetzner, S.; Lingens, F. Eur. J. Biochem. 1996,
3 2
was dissolved in CH CN, and excess Et O was added, which
1
resulted in the precipitation of a green-yellow solid. The H NMR
spectroscopic features of this solid match those of an independently
240, 576-583.
14) Frerichs-Deeken, U.; Ranguelova, K.; Kappl, R.; H u¨ ttermann, J.;
Fetzner, S. Biochemistry 2004, 43, 14485-14499.
generated sample of [(6-Ph
Independent Synthesis of [(6-Ph
Equimolar amounts of 6-Ph TPA (0.025 g, 0.056 mmol) and Ni-
ClO ‚6H O (0.020 g, 0.056 mmol) in CH
OH (∼2 mL) were
combined with a slight excess of NaO CPh (0.011 g, 0.079 mmol)
in CH
TPA)Ni(O
2
CPh)]ClO
(3).
2
4
2
TPA)Ni(O
2
CPh)]ClO (3).
4
15) Frerichs-Deeken, U.; Fetzner, S. Curr. Microbiol. 2005, 51, 344-
352.
2
16) Schaab, M. R.; Barney, B. M.; Francisco, W. A. Biochemistry 2006,
5, 1009-1016.
17) Gopal, B.; Madan, L. L.; Betz, S. F.; Kossiakoff, A. A. Biochemistry
005, 44, 193-201.
18) Bowater, L.; Fairhurst, S. A.; Just, V. J.; Bornemann, S. FEBS Lett.
(
4
)
2
2
3
4
2
OH (∼2 mL). The resulting mixture was stirred overnight
2
3
at room temperature. At this point, a pale blue precipitate was
present. The solvent was removed under reduced pressure, and the
2004, 557, 45-48.
19) Barney, B. M.; Schaab, M. R.; LoBrutto, R.; Francisco, W. A. Protein
Expression Purif. 2004, 35, 131-141.
2 2
remaining residue was dissolved in CH Cl . This solution was
20) Fusetti, F.; Schr o¨ ter, K. H.; Steiner, R. A.; van Noort, P. I.; Pijning,
T.; Rozeboom, H. J.; Kalk, K. H.; Egmond, M. R.; Dijkstra, B. W.
Structure 2002, 10, 259-268.
filtered through a celite/glass wool plug. The filtrate was then
brought to dryness under reduced pressure. A crystalline product
was obtained by recrystallization of the remaining residue via
(
(
(
(
(
(
(
(
(
(
(
(
21) Steiner, R. A.; Kalk, K. H.; Dijkstra, B. W. Proc. Natl. Acad. Sci.
U.S.A. 2002, 99, 16625-16630.
diethyl ether diffusion into a CH
3
3
CN-CH OH (1:2) solution at
22) Kooter, I. M.; Steiner, R. A.; Dijkstra, B. W.; van Noort, P. I.; Egmond,
M. R.; Huber, M. Eur. J. Biochem. 2002, 269, 2971-2979.
23) Steiner, R. A.; Meyer-Klaucke, W.; Dijkstra, B. W. Biochemistry 2002,
ambient temperature. This procedure yielded purple blocks suitable
for single-crystal X-ray crystallography (85%). Anal. Calcd for
37 31 4 6
C H N O ClNi: C, 61.57; H, 4.33; N, 7.76. Found: C, 61.58;
41, 7963-7968.
-1
24) Steiner, R. A.; Kooter, I. M.; Dijkstra, B. W. Biochemistry 2002, 41,
H, 4.17; N, 7.77. FTIR (KBr, cm ): 1605, 1094 (νClO4), 621 (νClO4).
7955-7962.
-1
-1
UV-vis (CH
3
CN) [nm (ꢀ, M cm )]: 395 (30), 574 (14), 1050
25) Hund, H.-K.; Breuer, J.; Lingens, F.; H u¨ ttermann, J.; Kappl, R.;
Fetzner, S. Eur. J. Biochem. 1999, 263, 871-878.
26) Oka, T.; Simpson, F. J. Biochem. Biophys. Res. Commun. 1971, 43,
(
15). LRFAB-MS (CH Cl /NBA) [m/z (relative intensity)]: 621 ([M
2
2
+
4
- ClO ] , 100%).
1-5.
1,2-Dibenzoylethenol. A solution of NaOH (1.3 g, 30 mmol)
27) Straganz, G. D.; Brecker, L.; Weber, H.-J.; Steiner, W.; Ribbons, D.
W. Biochem. Biophys. Res. Commun. 2002, 297, 232-236.
28) Straganz, G. D.; Glieder, A.; Brecker, L.; Ribbons, D. W.; Steiner,
W. Biochem. J. 2003, 369, 573-581.
29) Straganz, G. D.; Hofer, H.; Steiner, W.; Nidetzky, B. J. Am. Chem.
Soc. 2004, 126, 12202-12203.
30) Straganz, G. D.; Nidetzky, B. J. Am. Chem. Soc. 2005, 127, 12306-
in methanol (25 mL) was added to solid dibenzoyl-1,2-dibromo-
ethane (0.25 g, 0.62 mmol) at room temperature, which yielded an
orange reaction mixture. This solution was then stirred for 30 min
at 60(1) °C. Water (25 mL) was added to the warm reaction mixture,
followed by aqueous HCl (1 M) until pH ∼ 1 was reached. This
resulted in the deposition of a yellow precipitate. Dichloromethane
12314.
31) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th Ed.; Butterworth-Heinemann: Boston, MA, 1996.
32) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.;
Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. Inorg. Chem. 2003,
(
∼150 mL) was added to the solution and precipitate, at which
point the solid dissolved. The two-phase mixture was separated,
and the aqueous portion was further extracted with dichloromethane
42, 7472-7488.
(2 × ∼50 mL). The combined organic fractions were dried over
(
33) Szajna, E.; Dobrowolski, P.; Fuller, A. L.; Arif, A. M.; Berreau, L.
M. Inorg. Chem. 2004, 43, 3988-3997.
(34) Zhang, J-J.; Schuster, G. B. J. Am. Chem. Soc. 1989, 111, 7149-
(35) Allen, T. H.; Root, W. S. J. Biol. Chem. 1955, 216, 309-317.
(36) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.
7155.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5501

Szajna-Fuller et al.
2 4
Na SO and filtered, and the filtrate was brought to dryness under
reduced pressure, which yielded an orange solid. This solid was
recrystallized from hot ethanol, which gave the final product as an
orange solid (0.11 g, 70%). This compound has been previously
prepared by an alternative synthetic route, which resulted in a
slightly higher yield.3 H NMR (CDCl
7 1
3
, 400 MHz): δ 8.14 (d, J
)
8.4 Hz, 2H), 7.99 (d, J ) 8.4 Hz, 2H), 7.68-7.56 (m, 2H), 7.55-
7.46 (m, 4H), 6.85 (s, 1H), hydroxyl proton not identified in wet
13
1
3 3
CDCl . C{ H} NMR (CDCl , 100 MHz): δ 190.7, 187.3, 182.6,
134.5, 133.7, 130.7, 129.1, 128.9, 127.8, 96.4 (12 signals expected
for inequivalent phenyl rings; 10 signals observed due to overlap
of meta- and para-carbon resonances of both rings). FTIR (KBr,
-
1
cm ): ∼3120 (br, νOH), 1682 (νCO), 1671 (νCO).
General Preparation Method for Ni(II) Complexes Supported
by the 6-Ph
cis-â-Keto-Enolate Ligand. To a solution of 6-Ph
in CH
CN (∼2 mL) was added a solution of Ni(ClO
mmol) in CH CN. The resulting mixture was stirred at ambient
temperature for approximately 15 min. At this point, â-diketone
2
TPA Ligand and Containing a Chelating Bidentate
TPA (0.1 mmol)
‚6H O (0.1
2
3
4
)
2
2
1
Figure 2. (a) H NMR spectrum of 1 in CD3CN at 25(1) °C. The sharp
resonances in the region of 30-50 ppm are assigned to â/â′-H’s of the
3
pyridyl rings.33 (b) 1H NMR spectrum of 3 in CD3CN at 25(1) °C. An
identical spectrum is produced upon treatment of 1 with O2.
(
(
2
0.1 mmol) was added to the solution, followed by Me
0.1 mmol). This heterogeneous mixture was then stirred for 15-
0 h at ambient temperature in the presence of air. The solvent
was then removed under reduced pressure, and the remaining
residue was dissolved in CH Cl . Filtration of this CH Cl solution
through a celite/glass wool plug enabled removal of the Me NClO
byproduct. The filtrate was then brought to dryness under reduced
4
2
NOH‚5H O
collected with an oscillation range of 1 deg/frame and an exposure
time of 20 s/frame. Final cell constants were determined from a
set of strong reflections from the actual data collection. These
reflections were indexed, integrated, and corrected for Lorentz,
polarization, and absorption effects using DENZO-SMN and
SCALEPAC.38 The structures were solved by a combination of
direct and heavy-atom methods using SIR 97. All of the non-
hydrogen atoms were refined with anisotropic displacement coef-
ficients. Unless otherwise stated, all hydrogen atoms were assigned
isotropic displacement coefficients U(H) ) 1.2U(C) or 1.5U(Cmethyl
and their coordinates were allowed to ride on their respective
carbons using SHELXL97.
Structure Solution and Refinement. Complex 3 crystallizes
2
2
2
2
4
4
2 2
pressure for samples of 4 and 5, while the solution of 6 in CH Cl
was treated with excess hexane.
[
(6-Ph
diffusion of Et
complex at ambient temperature (yellow-brown microcrystals,
6%). Anal. Calcd for C45 ClNi: C, 65.60; H, 4.53; N, 6.80.
Found: C, 65.43; H, 4.49; N, 7.16. FTIR (KBr, cm ): 1595, 1096
2
TPA)Ni(PhC(O)CHC(O)Ph)]ClO
4
(4). Recrystallized by
2
O into a CH CN-CH OH (1:1) solution of the
3
3
)
9
H N O
37 4 6
-1
39
-
1
-1
(
(
(
ν
ClO4), 623 (νClO4). UV-vis (CH
3
CN) [nm (ꢀ, M cm )]: 368
13 000), 559 (14), ∼1100 (12). LRFAB-MS (CH
2
Cl
2
/NBA) [m/z
in the space group P2 /c. All hydrogen atoms were located and
independently refined. The acetylacetonate complex 5 crystallizes
1
+
relative intensity)]: 723 ([M -ClO
4
] , 100%).
[
(6-Ph
lized from CH
purple prisms, 69%). Anal. Calcd for C35
H, 4.75; N, 8.01. Found: C, 59.50; H, 4.91; N, 7.73. FTIR (KBr,
2
TPA)Ni(CH
3
C(O)CHC(O)CH
3
)]ClO
4
(5). Recrystal-
O (X-ray-quality
33 4 6
H N O ClNi: C, 60.07;
in the space group Pbca. The oxygen atoms of the perchlorate anion
exhibit disorder. Each was split into two fragments, with the second
being denoted by a prime, and refined. This refinement led to a
0.51:0.49 occupancy ratio. Complex 6 crystallizes in the space group
P 1h with one molecule of methylene chloride/formula unit. All
hydrogen atoms, except those of the methylene chloride solvate,
were located and refined independently. Two oxygen atoms (O(6)
and O(7)) of the perchlorate anion exhibit disorder. Each atom was
split into two fragments, with the second being denoted by prime,
and refined. This refinement led to an 81:19 ratio in occupancy.
3
CN in the presence of excess Et
2
-
1
3
cm ): 1595, 1093 (νClO4), 622 (νClO4). UV-vis (CH CN) [nm (ꢀ,
-
1
-1
M
cm )]: 238 (22 000), 285 (17 000), 548 (22), ∼1100 (14).
LRFAB-MS (CH
ClO
2
Cl
2
/NBA) [m/z (relative intensity)]: 599 ([M -
+
4
] , 100%).
[(6-Ph
2
TPA)Ni(PhC(O)CHC(O)C(O)Ph)]ClO
4
‚1.25CH
2 2
Cl (6).
Following filtration in CH
2
Cl , excess hexane was added which
2
resulted in the deposition of yellow-orange powder (88%). Elemen-
tal analysis was performed on this powdered sample. Anal. Calcd
Results
for C46
Found: C, 59.28; H, 3.93; N, 5.13. X-ray-quality crystals of 6 were
obtained by diffusion of pentane into a CH Cl solution of the
complex at ambient temperature. FTIR (KBr, cm ): 1675 (νC)O),
H
37
N
4
O
7
ClNi‚1.25CH
2 2
Cl : C, 59.23; H, 4.16; N, 5.85.
Identification of Reaction Products upon Treatment of
1
O
with O
2
. Treatment of an acetonitrile solution of 1 with
2
2
-
1
2
results in the rapid bleaching of the orange-brown color
-
1
-1
-
1
(λmax ) 399 nm (ꢀ ∼ 6800 M cm )) to yield a green-
1
596, 1094 (νClO4), 622 (νClO4). UV-vis (CH
2
Cl
2
) [nm (ꢀ, M
-1
yellow solution (λ ∼ 570, 1050 nm). Significant changes
cm )]: 283 (30 000), 357 (8400). LRFAB-MS (CH
2
Cl /NBA) [m/z
2
max
+
1
4
(relative intensity)]: 751 ([M - ClO ] , 100%).
are also evident using H NMR (Figure 2) as a spectroscopic
X-ray Crystallography. A single crystal of 3, 5, and 6 was
monitoring method. Specifically, four sharp resonances in
the region of 30-50 ppm in the spectrum of 1 are replaced
by a series of broad resonances. Spectroscopic studies and
an independent synthesis (vide infra) confirmed the formation
mounted on a glass fiber with traces of viscous oil and then
transferred to a Nonius KappaCCD diffractometer with Mo KR
radiation (λ ) 0.710 73 Å) for data collection at 150(1) K. An initial
set of cell constants was obtained from 10 frames of data that were
(
38) Otwinowski, Z.; Minor, M. Methods Enzymol. 1997, 276, 307-326.
(
37) Lutz, R. E.; Wilder, F. N.; Parrish, C. I. J. Am. Chem. Soc. 1934, 56,
(39) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
1980-1987.
5502 Inorganic Chemistry, Vol. 46, No. 14, 2007

ARD ReactiWity InWolWing a Monoanionic Substrate
Scheme 3
Figure 3. ORTEP representation of the cationic portion of 3. Ellipsoids
are drawn at the 50% probability level. All hydrogen atoms have been
omitted for clarity.
(
1)-O(2) 2.1293(13) Å) differ by ∼0.08 Å, with the longer
being trans to the unsubstituted pyridyl donor. These
distances are longer than the average Ni-O distance in 1
(
∼1.97 Å) for the cis-â-keto-enolate moiety. Notably, the
average Ni-NPhPy distances in 1 (∼2.30 Å) and 3 (∼2.18
Å) differ considerably, with the longer average distance being
found in 1 where the Ni-O bonds of the coordinated anion
are shorter. From this limited comparison, it appears that
the Ni-NPhPy bonding in these complexes is modulated
depending on the interaction involving the bound anion (vide
infra). In this context, it important to note that the Ni(1)-
N(1) and Ni(1)-N(2) distances in 1 and 3 are generally
similar (Table 2).
of [(6-Ph
containing product. Use of
2
TPA)Ni(O
2
CPh)]ClO
4
(3) as the only Ni(II)-
in the reaction produced 3
18
O
2
With the identification of a number of organic byproducts
for the reaction outlined in Scheme 3, we reexamined the
18
having ∼50% O incorporation in the benzoate ligand.
Control experiments indicate that the presence of labeled
dioxygen reactivity of 1 in the presence of 1 equiv of Me
4
-
18
2
water (H O) in solutions of 3 does not result in the
NOH‚5H O (Scheme 2) to probe whether additional organic
2
exchange of carboxylate oxygen atoms. This indicates that
the 50% incorporation is not due to loss of the labeled oxygen
due to exchange with adventitious water. Analysis of the
byproducts are generated in this reaction. As determined by
1
TLC, H NMR, and GC-MS, benzil is also produced in this
reaction. However, no additional organic products are
generated, thus indicating that this reaction is cleaner than
the reaction involving 1 without added base (Scheme 3). This
16
2 2
headspace gas of the reaction of 1 with O using the PdCl
35
1
method indicated the formation of CO. TLC, H NMR, and
GC-MS methods were used to identify the organic products
of the reaction, which were found to be benzil (major),
benzoic acid (minor), and trace amounts of other unidentified
phenyl-containing organic products (Scheme 3). The R-keto
acid benzoylformic acid is not a product of this reaction.
Quantification of the organic products was not performed at
this time, as we have not yet found a method of cleanly
separating the inorganic and organic products of the reaction
mixture without losing some of the organic products. GC-
MS analysis indicated that the benzil does not contain a
labeled oxygen atom. Because the noncoordinated benzoic
acid is produced in low yield, and thus far is always part of
1
8
also corresponds to a higher level of O incorporation into
the benzoate product ([(6-Ph TPA)Ni(O CPh) (H O)], 67%
2
2
2
2
1
8
O-labeled).
Unreactive Analogues: Synthesis and Properties. We
have also investigated the chemistry of Ni(II) complexes
containing a cis-â-keto-enolate ligand derived from diben-
zoylmethane, 2,4-pentanedione, or 1,2-dibenzoylethenol
(Scheme 4, 4-6). The cis-â-keto-enolate ligands in these
complexes are not substrates for ARD enzymes. However,
such complexes provide additional insight toward evaluating
the structural and spectroscopic features of 1. As shown in
Scheme 4, these complexes (4-6) have been prepared by
1
8
a mixture, we cannot currently report the level of
incorporation into this product.
O
treatment of the 6-Ph
2
TPA ligand with Ni(ClO
4
)
2
2
‚6H O,
followed by addition of the â-diketone in the presence of 1
equiv of base. Each complex was isolated as a crystalline
solid in good to high yield. Complexes 5 and 6 have been
investigated using X-ray crystallography. Elemental analysis,
IR, UV-vis, and mass spectrometry have also been used to
characterize each complex.
The Ni(II) benzoate complex 3 was identified via inde-
pendent synthesis. X-ray crystallographic characterization of
4
0
3
revealed a six-coordinate Ni(II) center having bidentate
coordination of the benzoate ligand (Figure 3, Tables 1 and
). The two Ni-O distances (Ni(1)-O(1) 2.0521(13), Ni-
2
X-ray crystallographic studies of 5 and 6 revealed the
presence of a mononuclear six-coordinate Ni(II) center in
each complex (Figure 4; Tables 1 and 2). The acetylacetonate
(
40) Kleywegt, G. J.; Wiesmeijer, W. G. R.; Van Driel, G. J.; Driessen,
W. L.; Reedijk, J.; Noordik, J. H. J. Chem. Soc., Dalton Trans. 1985,
2177-2184.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5503

Szajna-Fuller et al.
Table 1. Summary of X-ray Data Collection and Refinementa
param
3
5
6
empirical formula
fw
C37H31ClN4NiO6
721.82
C35H33ClN4NiO6
699.81
C47H39Cl3N4NiO7
936.88
cryst system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
10.8226(2)
19.9370(5)
15.7971(4)
90
103.1594(14)
90
3319.04(13)
4
orthorhombic
Pbca
18.1012(3)
15.7016(3)
22.6629(2)
90
triclinic
P1h
11.2375(4)
13.4086(5)
16.0769(6)
107.4617(18)
102.107(2)
103.424(3)
2143.36(14)
2
R (deg)
â (deg)
90
90
ø (deg)
3
V (Å )
6441.20(17)
8
1.443
Z
-
3
d(calcd), Mg m
1.445
1.452
temp (K)
150(1)
150(1)
150(1)
cryst size (mm)
diffractometer
0.38 × 0.20 × 0.18
Nonius KappaCCD
0.719
0.35 × 0.35 × 0.33
Nonius KappaCCD
0.738
0.25 × 0.15 × 0.03
Nonius KappaCCD
0.697
abs coeff (mm-1)
2
θ max (deg)
54.98
99.5
13 079
54.96
99.8
14 018
50.08
99.4
12 473
completeness to 2θ (%)
reflcns collcd
indep reflcns
7571
7375
7516
variable params
R1/wR2
566
0.0373/0.0825
1.019
464
0.0476/0.1204
1.014
717
0.0561/0.1258
1.043
b
2
goodness-of-fit (F )
-
3
largest diff (e Å
)
0.294/-0.361
0.502/-0.620
0.956/-0.760
a
Radiation used: Mo KR (λ ) 0.710 73 Å). b R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo - Fc ) ]/[∑(Fo ) ]] , where w ) 1/[σ (Fo ) + (aP) + bP].
2
2 2
2 2 1/2
2
2
2
Table 2. Selected Bond Distances (Å) and Angles (deg)a
Scheme 4
param
3
5
6
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(4)
2.0521(13)
2.1293(13)
2.0282(16)
2.0691(16)
2.1458(16)
2.2134(16)
2.0052(18)
1.9640(16)
2.051(2)
2.088(2)
2.217(2)
2.221(2)
1.976(2)
2.004(3)
2.055(3)
2.071(3)
2.209(3)
2.333(3)
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-N(2)
O(2)-Ni(1)-N(2)
N(3)-Ni(1)-N(4)
O(2)-Ni(1)-N(1)
O(1)-Ni(1)-N(1)
N(1)-Ni(1)-N(3)
N(1)-Ni(1)-N(4)
N(1)-Ni(1)-N(2)
N(2)-Ni(1)-N(3)
N(2)-NI(1)-N(4)
63.30(5)
168.33(6)
106.49(6)
149.83(6)
170.84(6)
108.59(6)
101.75(6)
97.48(6)
81.99(7)
77.80(6)
82.16(6)
91.56(7)
94.21(8)
171.69(8)
150.41(8)
93.30(8)
174.71(8)
96.52(8)
99.05(8)
81.19(8)
81.70(8)
76.05(8)
91.87(11)
174.56(12)
92.61(12)
158.40(11)
173.21(11)
93.03(11)
98.13(12)
79.62(12)
82.77(12)
78.15(11)
80.26(11)
a
Estimated standard deviations in the last significant figure are given in
parentheses.
derivative 5 exhibits a shorter average Ni(1)-NPhPy distance
2.22 Å) than 1 and 6 (2.30 and 2.27 Å, respectively). Thus,
(
the NPhPy distance is longer in complexes having a bulkier
cis-â-keto-enolate ligand. The Ni(1)-N(1) and Ni(1)-N(2)
distances associated with the pyridyl and tertiary amine
-
1 41
perchlorate anion (∼1095 and 625 cm ). The infrared
spectrum of 6 also contains a vibration at 1675 cm
consistent with the presence of a noncoordinated ketone
-
1
2
donors of the 6-Ph TPA ligand exhibit only minimal differ-
4
2
carbonyl group.
ences (<0.025 Å) in this series of complexes. Similarly, the
average Ni-O distance involving the cis-â-keto-enolate
ligand shows little variability (1.96-1.99 Å). Therefore, the
overall coordination environments of the Ni(II) centers in
The UV-vis spectra of 4 and 6 contain an intense
absorption feature at 368 and 357 nm (Table 3), respectively.
These features are at higher energy and exhibit stronger
intensities than a similar absorption band present in the
spectrum of 1 at 399 nm. These absorptions are tentatively
1, 5, and 6 are similar, with subtle perturbation of the average
Ni(1)-NPhPy distance as a function of cis-â-keto-enolate
ligand.
(41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 5th ed.; Wiley & Sons: New York, 1997.
The IR spectra of 4-6 are generally similar and all contain
(42) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of
the vibrations expected for the noncoordinated tetrahedral
Organic Compounds, 6th ed.; Wiley & Sons: New York, 1998.
5504 Inorganic Chemistry, Vol. 46, No. 14, 2007

ARD ReactiWity InWolWing a Monoanionic Substrate
Figure 5. 1H NMR spectra of mononuclear Ni(II) cis-â-keto-enolate
complexes in CD3CN at 25(1) °C: (a) [(6-Ph2TPA)Ni(PhC(O)C(OH)C-
(O)Ph)]ClO4 (1); (b) [(6-Ph2TPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4); (c) [(6-
Ph2TPA)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5); (d) [(6-Ph2TPA)Ni(PhC(O)-
CHC(O)C(O)Ph)]ClO4 (6).
appended pyridyl donors. This plane of symmetry is evident
in the X-ray structures of 1, 5, and 6 wherein the bound
cis-â-keto-enolate ligand is positioned in a sandwich-type
motif between the two phenyl-appended pyridyl groups. The
â/â′ proton resonances were assigned to either pyridyl ring
protons or phenyl pyridyl ring protons on the basis of
integration. For example, in the spectrum of 4 (Figure 5b),
the signal at 34.1 ppm integrates to two protons relative to
the integration of the upfield-shifted cis-â-keto-enolate proton
resonance at -13.3 ppm. This signal is assigned as a â′
proton resonance of the phenyl-appended pyridyl rings
Figure 4. ORTEP representations of the cationic portions of 5 and 6.
Ellipsoids are drawn at the 35% probability level. All hydrogen atoms have
been omitted for clarity.
Table 3. πfπ* Transitions of Ni(II) Complexes Having an
Aryl-Containing cis-â-Keto-Enolate Ligand (in CH3CN)
1
anion or complex
λmax (ꢀ, M- cm-1
)
1
4
6
399 (6800)
368 (13 000)
357 (8400)
(Figure 2 (inset)). A resonance of similar intensity is found
at 44.6 ppm, albeit it cannot individually be integrated due
to overlap with another resonance. The two resonances at
assigned as being derived from an intraligand π f π*
4
3,44
transition (Table 3) in the cis-â-keto-enolate ligand.
worth noting that Na(PhC(O)CHC(O)Ph) and Me
It is
N(PhC-
4
6.3 and 43.8 ppm are assigned as the â-H resonances of
4
the unsubstituted pyridyl donor. The larger downfield shift
for the â′ (phenyl pyridyl) resonances relative to the â
(
3
O)CHC(O)C(O)Ph) exhibit a π f π* transition at 352 and
CN solution, respectively.43
35 nm in CH
3
(pyridyl) proton resonances is consistent with differences in
Consistent with the solid-state structural similarities out-
1
the ligand field and spin distribution for these two different
types of donors. The phenyl-appended pyridyl donors have
longer Ni-N distances than the pyridyl appendage. As the
hyperfine shifts of the resonances in these types of complexes
lined above for 1, 5, and 6, the H NMR features of these
complexes are highly similar (Figure 5), albeit a resonance
upfield of 0 ppm is present in 4-6. This resonance, which
has been definitively assigned using acetylacetone-d
precursor for the preparation of [(6-Ph TPA)Ni(CH C(O)-
CDC(O)CH )]ClO (4-d ), is assigned to the proton on the
2
as a
3
3
have a significant contact contribution, the protons of the
unsubstituted pyridyl ring, which are closer to the paramag-
netic Ni(II) center, exhibit a slightly greater hyperfine shift.
A similar description is appropriate for the spectral features
of 1, 5, and 6 in the region of 30-50 ppm.
2
3
3
4
1
cis-â-keto-enolate moiety. In 1 the hydroxyl group replaces
this proton. The four relatively sharp resonances in the range
of 30-50 ppm for 1 and 4-6 are assigned to the â/â′-H’s
(
inset, Figure 2) of the pyridyl rings of 6-Ph
basis of chemical shift, T values, relative integration, and
comparison with the spectra of previously reported Ni(II)
2
TPA on the
Discussion
1
The results outlined herein, together with our previously
1
0
33
reported studies, indicate that Ni(II)-ARD-type products
carboxylates and CO) are generated in the reaction of O
complexes supported by this chelate ligand. The identifica-
tion of a total of four pyridyl â/â′ proton resonances is
consistent with an effective plane of symmetry in these
complexes which yields spectroscopically equivalent phenyl-
(
2
with a Ni(II) complex having a bulky ARD model substrate
coordinated as a monoanion or dianion. The formation of 3
in the reaction of 1 with O requires the production of one
2
equivalent of benzoate. If this reaction were going exclusively
by an ARD-type pathway, CO generation would be ac-
companied by the production of 2 equiv of benzoate/benzoic
(
43) Halcrow, M. A.; Sun, J.-S.; Huffman, J. C.; Christou, G. Inorg. Chem.
995, 34, 4167-4177.
44) Eistert, B.; Weygand, F.; Csendes, E. Chem. Ber. 1951, 84, 745-
80.
1
(
7
Inorganic Chemistry, Vol. 46, No. 14, 2007 5505

Szajna-Fuller et al.
acid and a high level of 18O incorporation would be
Scheme 5
2
expected. On the basis of our identification of the production
1
8
of CO and 1 equiv of benzoate in 3 (with only 50%
O
incorporation), we propose that only a portion of the benzoate
(
∼50%) is coming from an ARD-type reaction. The benzil,
unlabeled benzoate, benzoic acid, and other phenyl-contain-
ing products must be coming from an alternative reaction
pathway(s). This pathway may involve the generation of
benzoyl radicals, which could be the precursor to benzil
45
formation. Notably, the radical decomposition of R-hydro-
peroxy ketones has been demonstrated to produce benzoyl
•
radicals (e.g., PhC(O) ) and benzoic acid. We also note that
the dianionic species formed by treatment of 1 with 1 equiv
of Me
. Specifically, while benzil is generated, the benzoate
ligands in 2 (which is isolated in 75% yield as a crystalline
4 2 3
NOH‚5H O in CH CN exhibits a cleaner reaction with
O
2
18
solid) contain ∼67% O incorporation.
The reaction of the Ni(II)-coordinated monoanion
-
PhC(O)C(OH)C(O)Ph with O
2
has some precedent in the
chemical literature, as carbanions have been shown to react
with oxygen to form peroxide anions.4
6-48
These reactions
4
9
may occur by a single electron transfer (SET) process,
which would result in the generation of an organic radical
and superoxide, or via a two-electron step between the
anionic substrate and electrophilic O . Studies of the reactiv-
2
ity of Ni(II)-ARD suggest that a radical reaction may be
occurring in the enzyme-catalyzed reaction, albeit from a
Scheme 6
4
Ni(II)-coordinated dianionic form of the substrate. The
evidence for this radical chemistry comes from studies
involving a cyclopropyl-containing substrate analogue. The
enzyme/substrate complex formed using this analogue is not
EPR active, indicating that no radical is present. However,
in the presence of O
analogue slowly and irreversibly inhibits Ni(II)-ARD after
100 turnovers, suggesting that radical chemistry may be
occurring. Involvement of a radical pathway is also sup-
ported by recent computational studies.50
2
the cyclopropyl-containing substrate
∼
Mn(II)) as a monoanion via the hydroxyl moiety in the
4
20-24
substrate.
via internal redox involving the metal center (e.g., Cu(II)).
This radical can then undergo reaction with O to yield a
This monoanion may take on radical character
CO-producing reactions similar to that catalyzed by Ni-
2
(
II)-ARD are catalyzed by the cofactor-free 1H-3-hydroxy-
reactive peroxide moiety. Alternative mechanistic pathways
have been proposed wherein the presence of a reduced metal
center may also participate in oxygen activation via formation
12-26
4
-oxoquinaldine2,4-dioxygenaseandbyquercetindioxygenase.
The former cofactor-free enzyme has been shown to catalyze
the reaction of the substrate dianion with O to form a
resonance-stabilized radical anion.14 Specifically, in the
ternary complex (enzyme/substrate/O ) single electron trans-
fer occurs from the substrate dianion to O , resulting in the
2
1
6-19
of a metal-superoxide species.
Quercetin and other
in the
This type
flavonol-type substrates are also reactive with O
absence of metal ions under basic conditions.
2
2
5
1-54
2
of reaction yields the same types of products (a phenolic
carboxylic acid ester and CO, Scheme 6) as those produced
by quercetin dioxygenase enzymes. In DMF, reaction
formation of a radical pair (Scheme 5). Radical recombina-
tion is proposed to yield a peroxide species that then proceeds
via intramolecular attack to yield the reaction products.
In quercetin dioxygenase enzymes, the flavonol-type
substrate binds to a metal center (either Cu(II), Fe(II), or
2
between potassium flavonolate and O proceeds by a single-
electron-transfer mechanism (SET) as evidenced by the
4
9,52
identification of a flavonoxyl radical by EPR.
DMSO-H O solution at pH ) 6.4-10.8 the same reaction
is suggested to proceed via direct reaction of the deprotonated
In 50%
(
(
(
(
(
45) Sawaki, Y.; Ogata, Y. J. Org. Chem. 1976, 41, 2340-2343.
46) Barton, D. H. R.; Jones, D. W. J. Chem. Soc. 1965, 3563-3570.
47) Gersmann, H. R.; Bickel, A. F. J. Chem. Soc., B 1971, 2230-2237.
48) Abell, L. M.; Schloss, J. V. Biochemistry 1991, 30, 7883-7887.
49) Smith, M. B.; March, J. AdVanced Organic Chemistry, 5th ed.; Wiley-
Interscience: New York, 2001; pp 402-404.
2
5
3
2
flavonol and O in a two-electron step. Thus, depending
(52) Rajananda, V.; Brown, S. B. Tetrahedron Lett. 1981, 22, 4331-4334.
(50) Borowski, T.; Bassan, A.; Siegbahn, P. E. M. J. Mol. Struct. 2006,
(53) Barh a´ cs, L.; Kaizer, J.; Speier, G. J. Org. Chem. 2000, 65, 3449-
7
72, 89-92.
3452.
(
51) Nishinaga, A.; Tojo, T.; Tomita, H.; Matsuura, T. J. Chem. Soc., Perkin
Trans. 1 1979, 2511-2516.
(54) Balogh-Hergovich, E.; Speier, G. J. Org. Chem. 2001, 66, 7974-
7978.
5506 Inorganic Chemistry, Vol. 46, No. 14, 2007

ARD ReactiWity InWolWing a Monoanionic Substrate
1
a
on conditions, it appears that this reaction can proceed via
two reaction pathways.
Table 4. Assignment of Selected H NMR Resonances of 1 and 4
chem shift (ppm)b
∆ν1.2 (Hz)c
T1exp (ms)d
assgnt
rel area
The role of the Ni(II) ion in Ni(II)-ARD may be to act
as a Lewis acid to ensure formation of a dianionic form of
the substrate. As noted in the Introduction, there is spectro-
scopic evidence in support of the formation of an enzyme/
[
(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1)
â
â
γ
γ
45.8, 42.6
43.3, 32.9
15.0
100, 80
120, 110
42
6, 7
7, 5
21
1, 1
2, 2
1
5.2
64
17
2
substrate adduct in Ni(II)-ARD wherein the substrate is
[(6-Ph2TPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4)
8
coordinated to Ni(II) as a dianion prior to reaction with O
2
.
â
46.3, 43.8
44.6, 34.1
15.0
82, 69
100, 86
33
6, 8
6, 4
18
1, 1
2, 2
1
2
1
â
For nonenzymatic reactions involving the model substrate
,2-dihydroxy-3-oxo-hexene (Scheme 1), the rate of reaction
between the substrate dianion (at pH ) 13) and O is
γ
1
γ
5.4
40
15
2
CH
-13.3
140
3
approximately 66× faster than the reaction involving the
a
1
The H NMR features of 5 and 6 are similar to those exhibited by 4
(see Figure 5). b Chemical shifts in ppm relative to the residual solvent peak
4
substrate monoanion (at pH ) 7.5). Formation of a Ni(II)-
1
c
of CHD2CN ( H, 1.94 (quintet) ppm). Line widths are full width at half-
coordinated substrate dianion may be essential to producing
an appropriate rate of reaction involving O . On the basis of
2
maximum. d T1 values were obtained at 300 MHz.
the results presented herein, formation of a Ni(II)-coordinated
dianionic species may also be important toward limiting
alternative oxidation pathways.
from a strain of Acinetobacter johnsonii and requires Fe(II)
for activity. Replacement of Fe(II) with other metal ions,
including Ni(II), yields inactive enzyme. A mechanistic
pathway for acetylacetone oxidation has been proposed
wherein the substrate binds to the Fe(II) center as a bidentate
monoanionic cis-â-keto-enolate ligand. The Fe(II) center is
proposed to couple the oxidation of the bound substrate with
dioxygen reduction. An important feature of the Dke1 active
site is the presence of only three amino acid ligands (all
histidines) bound to the Fe(II) center. In the presence of the
bidentate coordinated substrate, this leaves one metal coor-
The preparation and characterization of 4-6 has enabled
an examination of how the structural and electronic properties
of the Ni(II) center are influenced by changes in the cis-â-
keto-enolate monoanion. The UV-vis spectra of 1 and 4
exhibit differences in terms of the energy and molar
absorptivity of the π f π* transition (Table 3). The lower
energy of the π f π* transition in 1 indicates either
destabilization of the π orbitals or stabilization of the π*
orbital due to the presence of the hydroxyl substitutent. The
2
dination site available for interaction with O , which may
be required for the oxidation of cis-â-keto-enolate-type
substrates that lack a C2-hydroxyl moiety.
reactive nature of 1 toward O
2
correlates with the presence
. The
similarity of the H NMR spectral features of 1 and 4-6
Table 4) suggests similar unpaired spin delocalization
properties in this family of complexes.
With regard to the O reaction of metal-bound cis-â-keto-
enolate ligands, it is worth noting that a non-heme iron
dioxygenase was recently reported which catalyzes the O
dependent oxidative aliphatic carbon-carbon bond cleavage
of the hydroxyl group, as 4 is unreactive with O
2
1
(
Acknowledgment. We thank the National Institutes of
Health (Grant 1R15GM072509) for financial support of this
work and Sara Huefner for technical assistance.
2
Supporting Information Available: X-ray crystallographic
2
-
(CIF) files for 3, 5, and 6. This material is available free of charge
2
7-30
via the Internet at http://pubs.acs.org.
of acetylacetone into methylglyoxal and acetate.
This
enzyme, termed diketone dioxygenase (Dke1), was obtained
IC061177I
Inorganic Chemistry, Vol. 46, No. 14, 2007 5507