Carboxylate coordination chemistry of a mononuclear Ni(II) center in a hydrophobic or hydrogen bond donor secondary environment: Relevance to acireductone dioxygenase

Article abstract of DOI:10.1021/ic061316w

A series of Ni(II) carboxylate complexes, supported by a chelate ligand having either secondary hydrophobic phenyl groups (6-Ph₂TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) or hydrogen bond donors (bnpapa, N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine), have been prepared and characterized. X-ray crystallographic studies of [(6-Ph₂TPA)Ni(O₂C(CH₂) ₂SCH₃)]ClO₄·CH₂Cl ₂ (4-CH₂Cl₂) and [(6-Ph₂TPA) Ni(O₂-CCH₂SCH₃)]ClO₄·1. 5CH₂Cl₂ (5·1.5CH₂Cl₂) revealed that each complex contains a distorted octahedral Ni(II) center and a bidentate carboxylate ligand. A previously described benzoate complex ([(6-Ph₂TPA)Ni(O₂CPh)]ClO₄ (3)) has similar structural characteristics. Recrystallization of dry powdered samples of 3, 4·0.5CH₂Cl₂, and 5 from wet organic solvents yielded a second series of crystalline Ni(II) carboxylate complexes having a coordinated monodentate carboxylate ligand ([(6-Ph₂TPA)Ni(H ₂O)(O₂CPh)]ClO₄ (6), [(6-Ph₂TPA) Ni(H₂O)(O₂C(CH₂)₂SCH ₃)]ClO₄·0.2CH₂Cl₂ (7·0.2CH₂Cl₂), [(6-Ph₂TPA)Ni(H ₂O)(O₂CCH₂SCH₃)]ClO₄ (8)) which is stabilized by a hydrogen-bonding interaction with a Ni(II)-bound water molecule. In the cationic portions of 7·0.2CH₂Cl₂ and 8, weak CH/π interactions are also present between the methylene units of the carboxylate ligands and the phenyl appendages of the 6-Ph₂TPA ligands. A formate complex of the formulation [(6-Ph₂TPA)Ni(H ₂O)(O₂CH)]ClO₄ (9) was isolated and characterized. The mononuclear Ni(II) carboxylate complexes [(bnpapa)Ni(O ₂CPh)]ClO₄ (10), [(bnpapa)Ni(O₂C(CH ₂)₂SCH₃)]ClO₄ (11), [(bnpapa)Ni(O₂CCH₂SCH₃)]ClO₄ (12), and [(bnpapa)Ni(O₂CH)]ClO₄ (13) were isolated and characterized. Two crystalline solvate forms of 10 (10·CH₃CN and 10·CH₂Cl₂) were examined by X-ray crystallography. In both, the distorted octahedral Ni(II) center is ligated by a bidentate benzoate ligand, one Ni(II)-bound oxygen atom of which accepts two hydrogen bonds from the supporting bnpapa chelate ligand. Spectroscopic studies of 10-13 suggest that all contain a bidentate carboxylate ligand, even after exposure to water. The combined results of this work enable the formulation of a proposed pathway for carboxylate product release from the active site Ni(II) center in acireductone dioxygenase.

Full text of DOI:10.1021/ic061316w

Inorg. Chem. 2007, 46, 5486−5498
Carboxylate Coordination Chemistry of a Mononuclear Ni(II) Center in a
Hydrophobic or Hydrogen Bond Donor Secondary Environment:
Relevance to Acireductone Dioxygenase
Ewa Szajna-Fuller,† Bonnie M. Chambers,† 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 July 16, 2006
A series of Ni(II) carboxylate complexes, supported by a chelate ligand having either secondary hydrophobic phenyl
groups (6-Ph2TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) or hydrogen bond donors (bnpapa,
N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been prepared and characterized.
X-ray crystallographic studies of [(6-Ph2TPA)Ni(O2C(CH2)2SCH3)]ClO4
CCH2SCH3)]ClO4 1.5CH2Cl2 (5 1.5CH2Cl2) revealed that each complex contains a distorted octahedral Ni(II) center
and a bidentate carboxylate ligand. A previously described benzoate complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3))
has similar structural characteristics. Recrystallization of dry powdered samples of 3, 4 0.5CH2Cl2, and 5 from wet
organic solvents yielded a second series of crystalline Ni(II) carboxylate complexes having a coordinated monodentate
carboxylate ligand ([(6-Ph2TPA)Ni(H2O)(O2CPh)]ClO4 (6), [(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4 0.2CH2Cl2 (7
0.2CH2Cl2), [(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]ClO4 (8)) which is stabilized by a hydrogen-bonding interaction with
a Ni(II)-bound water molecule. In the cationic portions of 7 0.2CH2Cl2 and 8, weak CH/ interactions are also
‚CH2Cl2 (4‚CH2Cl2) and [(6-Ph2TPA)Ni(O2-
‚
‚
‚
‚
‚
‚
π
present between the methylene units of the carboxylate ligands and the phenyl appendages of the 6-Ph2TPA
ligands. A formate complex of the formulation [(6-Ph2TPA)Ni(H2O)(O2CH)]ClO4 (9) was isolated and characterized.
The mononuclear Ni(II) carboxylate complexes [(bnpapa)Ni(O2CPh)]ClO4 (10), [(bnpapa)Ni(O2C(CH2)2SCH3)]ClO4
(11), [(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12), and [(bnpapa)Ni(O2CH)]ClO4 (13) were isolated and characterized. Two
crystalline solvate forms of 10 (10
distorted octahedral Ni(II) center is ligated by a bidentate benzoate ligand, one Ni(II)-bound oxygen atom of which
accepts two hydrogen bonds from the supporting bnpapa chelate ligand. Spectroscopic studies of 10 13 suggest
‚CH3CN and 10‚CH2Cl2) were examined by X-ray crystallography. In both, the
−
that all contain a bidentate carboxylate ligand, even after exposure to water. The combined results of this work
enable the formulation of a proposed pathway for carboxylate product release from the active site Ni(II) center in
acireductone dioxygenase.
Introduction
of diiron(II) carboxylate complexes that occur as a function
of the amount of water present in the reaction mixture.2-4
The Ni(II)-containing enzyme acireductone dioxygenase
(Ni(II)-ARD) catalyzes a reaction that is a shunt out of the
methionine salvage pathway in Klebsiella ATCC 8724.5-8
Carboxylate ligation is found for metal centers in a variety
of metalloenzymes.1 The chemical factors that influence the
coordination mode of a carboxylate ligand continue to be
elucidated. For example, Yoon and Lippard recently de-
scribed changes in the structural and O2 reactivity properties
(2) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 16692-16693.
(3) Yoon, S.; Kelly, A. E.; Lippard, S. J. Polyhedron 2004, 23, 2805-
2812.
(4) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 8386-8397.
(5) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1993, 268, 21466-21469.
(6) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1995, 270, 3147-3153.
(7) Dai, Y.; Wensink, P. C.; Abeles, R. H. J. Biol. Chem. 1999, 274,
1193-1195.
* Corresponding author. E-mail: berreau@cc.usu.edu. Phone: (435) 797-
1625. Fax: (435) 797-3390:
† Utah State University.
‡ University of Utah.
(1) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96,
(8) Dai, Y.; Pochapsky, T. C.; Abeles, R. H. Biochemistry 2001, 40, 6379-
6387.
2239-2314.
5486 Inorganic Chemistry, Vol. 46, No. 14, 2007
10.1021/ic061316w CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/09/2007

Ni(II) Carboxylates and Acireductone Dioxygenase
Scheme 1
Scheme 2 Proposed Mechanism for Substrate Oxidation Catalyzed by
Ni(II)-ARD
This reaction results in the production of carboxylate/
carboxylic acid products via the oxidative degradation of 1,2-
dihydroxy-3-keto-5-(methylthio)pentene (acireductone, Scheme
1). While the X-ray crystal structure of Ni(II)-ARD from
Klebsiella ATCC 8724 has not been reported, the ligand
environment of the nickel center has been investigated by
X-ray absorption spectroscopy (XAS),9 as well as by NMR
and conserved domain homology modeling methods.10,11 The
resting state mononuclear Ni(II) center exhibits pseudo-
octahedral coordination, with 6 O/N donors, of which 3-4
are histidine residues. Conserved domain homology modeling
of the active site in resting state Ni(II)-ARD, via comparison
with jack bean canavalin, is consistent with three histidine
ligands and one glutamate donor to the nickel center.10 For
the enzyme-substrate complex, XAS studies suggest that
the Ni(II) center is also pseudo-octahedral, with the best fit
of the data being for a primary coordination environment
containing two short O/N donors. These two short donors
are suggested to be from bidentate coordination of a dianionic
form of the acireductone substrate. Formation of the enzyme-
substrate complex is suggested to involve displacement of
one histidine ligand and one water molecule. These inves-
tigations, coupled with additional results from spectroscopic
and mechanistic studies, enabled a proposed mechanism to
be put forth for Ni(II)-ARD (Scheme 2).9,10 Notably, there
has been nothing reported to date as to how the products of
the Ni(II)-ARD reaction may interact with the Ni(II) center.
NMR and conserved domain homology modeling studies
have also provided insight into the secondary environment
surrounding the metal center in Ni(II)-ARD.10,11 Two phe-
nylalanine residues (Phe92 and Phe142) have been identified
as being close enough to the Ni(II) center to have paramag-
netically broadened NMR resonances.10 It has been proposed
that these residues may be involved in orienting the bound
substrate properly for the reaction with O2.
Scheme 3
We recently reported the first reactive model system for
Ni(II)-containing ARD enzymes (Scheme 3).12,13 Specifically,
a synthetic Ni(II) complex, supported by the 6-Ph2TPA
ligand, and having a coordinated cis-â-keto-enolate ligand
(1), has been shown to undergo reaction with O2 under
conditions wherein the bound enolate is in either a dianionic
or monoanionic form.12,13 These reactions result, in part, in
the formation of Ni(II) carboxylate complexes (2 and 3) and
CO as products. As shown in Scheme 3, these complexes
exhibit differing coordination motifs for the 6-Ph2TPA ligand
(9) Al-Mjeni, F.; Ju, T.; Pochapsky, T. C.; Maroney, M. J. Biochemistry
2002, 41, 6761-6769.
(10) Pochapsky, T. C.; Pochapsky, S. S.; Ju, T.; Mo, H.; Al-Mjeni, F.;
Maroney, M. J. Nat. Struct. Biol. 2002, 9, 966-972.
(11) Pochapsky, T. C.; Pochapsky, S. S.; Ju, T.; Hoefler, C.; Liang, J. J.
Biomol. NMR 2006, 34, 117-127.
(12) Szajna, E.; Arif, A. M.; Berreau, L. M. J. Am. Chem. Soc. 2005, 127,
17186-17187.
(13) Szajna-Fuller, E.; Rudzka, K.; Arif, A. M.; Berreau, L. M. Inorg. Chem.
2007, 46, 1471-1480.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5487

Szajna-Fuller et al.
(κ4 versus κ3) and monodentate versus bidentate carboxylate
coordination.
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosiVe. Only small amounts of material
should be prepared, and these should be handled with great care.17
In the work described herein, we have further examined
the carboxylate coordination chemistry of the 6-Ph2TPA-
supported Ni(II) center. Efforts to independently synthesize
the bisbenzoate derivative 2 were not successful, with the
monobenzoate complex 3 instead being produced. This
indicates that the formation of 2 requires more than just
mixing of the individual components. In further examining
the coordination chemistry of monocarboxylate derivatives
(e.g., 3, Scheme 3), we have found that, within the
hydrophobic secondary environment produced by the 6-Ph2-
TPA ligand, the carboxylate ligand coordinates differently
depending on the water content of the reaction mixture.
Under relatively water-free conditions, the carboxylate ligand
is bidentate, as is found in 3 (Scheme 3). However, when
sufficient water is present, the carboxylate ligand is mono-
dentate and forms a hydrogen-bonding interaction with a Ni-
(II)-bound water molecule. This water-dependent bidentate
versus monodentate coordination behavior occurs for a
variety of carboxylate ligands, including 3-methylthiopro-
pionate, which is a product generated in the reaction
catalyzed by Ni(II)-ARD. Interestingly, when the phenyl
groups of the 6-Ph2TPA ligand are replaced by hydrogen-
bond donor secondary amine appendages in the bnpapa
chelate ligand, only bidentate carboxylate coordination has
been identified, even in the presence of water. These results
demonstrate that the secondary environment surrounding a
mononuclear Ni(II) center influences the coordination prop-
erties of a carboxylate ligand. In terms of Ni(II)-ARD, the
results presented herein enable the formulation of a proposed
pathway for carboxylate product release in the active site of
Ni(II)-ARD.
Attempted Independent Synthesis of 2. Equimolar amounts
of 6-Ph2TPA and Ni(ClO4)2‚6H2O were combined in CH3CN,
methanol, or CH2Cl2 (∼2-3 mL). Each mixture was stirred for
∼10 min. At this point, excess Na(O2CPh) (3 equiv), dissolved in
methanol/H2O or H2O, was added to the reaction mixture. The
resulting solutions were stirred for a minimum of 12 h. For each
reaction mixture, the solvent was removed under reduced pressure
1
and a H NMR spectrum was obtained of the remaining solid. In
each case, the spectrum was consistent with the clean formation of
[(6-Ph2TPA)Ni(O2CPh)]ClO4 (3).
Treatment of [(6-Ph2TPA)Ni(H2O)(O2CPh)2]ClO4 (2) with
NH4ClO4. Formation of [(6-Ph2TPA)Ni(O2CPh)]ClO4 (3). An
acetonitrile slurry of 2 (11 mg, 0.13 mmol) was purged with a
stream of N2. To this solution was added dropwise a solution of
NH4ClO4 (0.13 mmol) in CH3CN. During this addition process,
the N2 purge was continued. After ∼1 h, the purge was stopped,
and the solution was stirred overnight at ambient temperature.
Removal of the solvent under reduced pressure followed by analysis
of the remaining purple solid by 1H NMR indicated the clean
formation of [(6-Ph2TPA)Ni(O2CPh)]ClO4 (3). Recrystallization of
the solid via diethyl ether diffusion into a CH3OH/CH3CN solution
yielded the product as purple crystals (6.5 mg, 70%).
General Method for the Preparation of 4‚0.5CH2Cl2 and 5.
A solution of Ni(ClO4)2‚6H2O (0.10 mmol) in CH3CN (∼2 mL)
was added to a slurry of 6-Ph2TPA (0.10 mmol) in CH3CN (∼2
mL). The resulting mixture was stirred for 15 min at room
temperature. This solution was then transferred to a vial containing
Me4NOH‚5H2O (0.11 mmol). To this combined mixture was added
3-methylthiopropionic acid or methylthioacetic acid (0.11 mmol),
and the resulting solution was stirred overnight at room temperature.
The solvent was removed under reduced pressure. The remaining
solid was dissolved in dry CH2Cl2 (∼5 mL), and the solution was
filtered through a celite/glass wool plug. Pentane diffusion into the
CH2Cl2 filtrate yielded 4‚0.5CH2Cl2 or 5 as purple crystals suitable
for X-ray crystallography. These crystals were crushed and dried
under vacuum prior to elemental analysis.
Experimental Section
General. All reagents were obtained from commercial suppliers
and were used as received unless otherwise noted. Solvents for
water-sensitive reactions were dried according to published pro-
cedures and were distilled under N2 prior to use.14 Water-sensitive
reactions were performed in an MBraun Unilab glovebox under
an atmosphere of purified N2. The 6-Ph2TPA (N,N-bis((6-phenyl-
2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) ligand was synthe-
sized following a literature procedure.15 The benzoate complex [(6-
Ph2TPA)Ni(O2CPh)]ClO4 (3) was prepared as previously described.13
[(6-Ph2TPA)Ni(O2C(CH2)2SCH3)]ClO4‚0.5CH2Cl2
(4‚
0.5CH2Cl2). Yield: 70%. FTIR (KBr, cm-1) 1092 (νClO4), 621
(νClO4). Anal. Calcd for C34H33N4SO6ClNi‚0.5CH2Cl2: C, 54.36;
H, 4.50; N, 7.35. Found: C, 54.77; H, 4.49; N, 7.32.
[(6-Ph2TPA)Ni(O2CCH2SCH3)]ClO4 (5). Yield: 74%. FTIR
(KBr, cm-1) 1095 (νClO4), 623 (νClO4). Anal. Calcd for C33H31N4-
SO6ClNi: C, 56.15; H, 4.43; N, 7.94. Found: C, 56.05; H, 4.42;
N, 7.95.
1
2
Physical Methods. H and H NMR spectra of paramagnetic
Ni(II) complexes were collected as previously described.16 UV-
vis spectra were recorded at ambient temperature using a Hewlett-
Packard 8453 diode array spectrophotometer. Infrared spectra were
recorded on Shimadzu FTIR-8400 spectrometer as KBr pellets or
as CH3CN solutions. Mass spectrometry data was obtained at the
Mass Spectrometry Facility, Department of Chemistry, University
of California, Riverside. Atlantic Microlabs of Norcross, GA,
performed elemental analyses.
[(6-Ph2TPA)Ni(H2O)(O2CPh)]ClO4 (6). This compound was
prepared in an identical manner to 3. However, the final recrys-
tallization involved n-pentane diffusion into a CH2Cl2:H2O solution
(∼3 mL/2 drops). Yield: 80%. FTIR (KBr, cm-1) 3558 (br, νOH),
1095 (νClO4), 623 (νClO4). Anal. Calcd for C37H33N4O7ClNi: C,
60.07; H, 4.50; N, 7.57. Found: C, 59.64; H, 4.45; N, 7.40.
General Method for the Preparation of 7‚0.2CH2Cl2 and 8.
The aqua carboxylate complexes 7‚0.2CH2Cl2 and 8 were prepared
in an identical manner to 4‚0.5CH2Cl2 and 5, albeit water (∼2
drops) was added to the CH2Cl2 filtrate (∼3 mL) prior to the
diffusion of n-pentane. For each complex, blue crystals suitable
for X-ray crystallography were obtained. These crystals were
crushed and dried under vacuum prior to elemental analysis.
(14) Armarego, W. L. F; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Boston, MA, 1996.
(15) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.;
Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. Inorg. Chem. 2003,
42, 7472-7488.
(16) Szajna, E.; Dobrowolski, P. D.; Fuller, A. L.; Berreau, L. M. Inorg.
Chem. 2004, 43, 3988-3997.
(17) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.
5488 Inorganic Chemistry, Vol. 46, No. 14, 2007

Ni(II) Carboxylates and Acireductone Dioxygenase
[(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4‚0.2CH2Cl2 (7‚
0.2CH2Cl2). Yield: 72%. FTIR (KBr, cm-1) 3466 (br, νOH), 1098
(νClO4), 623 (νClO4). Anal. Calcd for C34H35N4SO7ClNi‚0.2CH2Cl2:
C, 54.42; H, 4.73; N, 7.42. Found: C, 54.54; H, 4.65; N, 7.80.
[(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]ClO4 (8). Yield: 65%.
FTIR (KBr, cm-1) 3391 (br, νOH), 1092 (νClO4), 621 (νClO4). Anal.
Calcd for C33H33N4SO7ClNi: C, 54.76; H, 4.60; N, 7.74. Found:
C, 54.73; H, 4.61; N, 7.76.
[(6-Ph2TPA)Ni(H2O)(O2CH)]ClO4 (9). A solution of Ni(ClO4)2‚
6H2O (0.014 g; 0.038 mmol) in CH3OH (∼2 mL) was added into
a slurry of 6-Ph2TPA (0.017 g; 0.038 mmol) in 2 mL of CH3OH
and stirred for ∼15 min. To this solution was added an excess of
sodium formate (0.008 g; 0.115 mmol) in 2 mL of H2O, and the
resulting mixture was stirred for ∼20 h. The solvent was removed
under reduced pressure, and the residue was dissolved in CH2Cl2
and filtered though a glass wool/Celite plug. Et2O diffusion into
the CH2Cl2 filtrate yielded pale yellow crystals suitable for X-ray
crystallography (0.020 g, 81%). FTIR (KBr, cm-1) 3470 (br, νOH),
1096 (νClO4), 623 (νClO4). Anal. Calcd for C31H29N4O7ClNi‚0.33CH2-
Cl2: C, 54.38; H, 4.32; N, 8.10. Found: C, 54.30; H, 4.46; N,
8.15. Trace CH2Cl2 was present in the elemental analysis sample
of 9. This solvate is not present in the X-ray crystallographically
characterized sample of 9.
5. Crystalline samples of 11 and 12 were obtained by dissolving
the powdered complex (derived from removal of the CH2Cl2 from
the filtrate) in a minimal amount of CH3CN (∼10 mg/0.5 mL)
followed by the addition of ∼20 mL of Et2O. Storing of these
solutions at ambient temperature for ∼12-24 h resulted in the
deposition of purple crystals, which were crushed and dried prior
to elemental analysis. In general, the yield of the bnpapa-ligated
complexes is low due to their high solubility in a variety of organic
solvents.
[(bnpapa)Ni(O2C(CH2)2SCH3)]ClO4 (11). Yield: 46%. FTIR
(KBr, cm-1) 3335 (νNH), 1090 (νClO4), 621 (νClO4). Anal. Calcd for
C32H47N6SO6ClNi: C, 52.08; H, 6.42; N, 11.39. Found: C, 52.43;
H, 6.50; N, 11.41.
[(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12). Yield: 63%. FTIR
(KBr, cm-1) ∼3300 (br, νNH), 1082 (νClO4), 621 (νClO4). Anal. Calcd
for C31H45N6SO6ClNi: C, 51.43; H, 6.27; N, 11.61. Found: C,
51.54; H, 6.38; N, 11.63.
[(bnpapa)Ni(O2CH)]ClO4 (13). This complex was prepared in
a similar manner to 9, albeit water was not added to the reaction
mixture. Yield: 54%. FTIR (KBr, cm-1) 3340 (br, νNH), 1092
(νClO4), 623 (νClO4). Anal. Calcd for C29H41N6O6ClNi: C, 52.47; H,
6.23; N, 12.66. Found: C, 52.37; H, 6.23; N, 12.63.
d6-bnpapa (Benzylic Deuteration). To bnpapa (0.19 g, 4.0 ×
10-4 mol) in a round-bottom flask was added CH3C(O)OD (9 mL,
Aldrich, 98%). The resulting solution was refluxed under nitrogen
for 12 h. After cooling to room temperature, the reaction mixture
was transferred to a 250 mL Erlenmeyer flask, and aqueous NaOH
(5 M) was carefully added dropwise until the pH > 11. To this
solution was added CH2Cl2 (40 mL), and the solution was stirred
for 1.5 h at room temperature. The CH2Cl2 layer was then removed,
and the aqueous layer was further extracted with additional CH2-
Cl2 (2 × 40 mL). The combined organic fractions were dried over
sodium sulfate, the solution was filtered, and the filtrate was brought
to dryness by rotary evaporation. The remaining dark brown oil
was used for the synthesis of Ni(II) carboxylate complexes without
N,N-Bis((6-pivalolylamido-2-pyridyl)methyl)-N-((2-pyridyl)-
methyl)amine (bppapa). A synthetic procedure for this compound
has been previously reported.18 Starting from the same precursors,
a slightly different synthetic procedure was performed using CH3-
CN and Na2CO3 as the solvent and base, respectively.19 The crude
brown oil obtained was used in the preparation of bnpapa without
further purification.
N,N-Bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)-
methyl)amine (bnpapa). A detailed synthetic procedure for this
ligand has not been previously reported. A 100 mL round-bottom
flask was charged with bppapa (0.16 g, 0.36 mmol) and dry Et2O
(30 mL). After purging this solution with N2, LiAlH4 (0.17 g, 4.4
mmol) was added in small portions. The resulting solution was
stirred overnight at ambient temperature. Water was then added
dropwise until no further reaction was observable. The organic/
aqueous mixture was then extracted with ethyl acetate (3 × 30 mL),
and the combined organic fractions were dried over Na2SO4.
Filtration, followed by removal of the solvent from the filtrate under
reduced pressure, yielded the crude product as a brown oil.
Purification of this oil via column chromatography on silica gel
(230-400 mesh; 92% 9/1 ethyl acetate/hexanes + 8% NEt3) yielded
the product as a yellow oil (72%). 1H NMR (CD3CN, 400 MHz) δ
8.47 (m, 1H), 7.74-7.69 (m, 2H), 7.35 (t, J ) 8.1 Hz, 2H), 7.19-
7.17 (m, 1H), 6.76 (d, J ) 7.1 Hz, 2H), 6.34 (d, J ) 8.2 Hz, 2H),
5.01 (br, 2H), 3.84 (s, 2H), 3.61 (s, 4H), 3.17 (d, J ) 6.3 Hz, 4H),
0.93 (s, 18H).
[(bnpapa)Ni(O2CPh)]ClO4 (10). This compound was prepared
in an identical manner to 3. Crystals suitable for single-crystal X-ray
diffraction were obtained by storing a CH3CN/Et2O (∼1/20) or a
CH2Cl2/acetonitrile/water solution of the complex at ambient
temperature for several days. Yield: 47% (from CH3CN:Et2O).
FTIR (KBr, cm-1) 3336 (νNH), 1093 (νClO4), 624 (νClO4). Anal. Calcd
for C35H45N6O6ClNi: C, 56.81; H, 6.13; N, 11.36. Found: C, 56.42;
H, 6.25; N, 11.76.
1
further purification. H NMR (CD3CN, 300 MHz) δ 8.44 (d, J )
5.2 Hz, 1H), 7.67 (d, J ) 3.8 Hz, 2H), 7.36-7.28 (m, 2H), 7.20-
7.12 (m, 1H), 6.71 (d, J ) 7.2 Hz, ∼1H), 6.32 (d, J ) 8.6 Hz,
2H), 5.03 (br s, ∼2H, N-H), 3.13 (d, J ) 6.5 Hz, 4H), 0.91 (s,
18H) ppm. Low integration of the aromatic resonance at 6.71 ppm
indicates partial deuteration of a â-H position of the amine-appended
pyridyl ring. Incorporation of deuterium at the benzylic position is
>95% as indicated by the fact that singlets found at 3.84 and 3.61
1
ppm in the H NMR of the bnpapa ligand are not observed in the
2
1H NMR of d6-bnpapa. H NMR (CD3CN, 46 MHz) δ 6.8, 3.8,
3.6 ppm.
d4-bnpapa (Neopentyl Methylene Deuteration). The amide-
containing bppapa (0.32 g, 6.5 × 10-4 mol) was combined with
LiAlD4 (12 equiv) in dry diethyl ether (50 mL). This mixture was
stirred for approximately 12 h at room temperature under a nitrogen
atmosphere. Water (∼15 mL) was carefully added dropwise until
no frothing occurred. This solution was then extracted with ethyl
acetate (3 × 50 mL). The combined organic fractions were dried
over sodium sulfate. Following filtration, the filtrate was brought
to dryness under reduced pressure which yielded a pale brown oil.
Column chromatography on silica gel using the conditions outlined
above for the bnpapa ligand yielded the d4-labeled ligand a yellow
General Method for Preparation of 11 and 12. These
complexes were prepared in a similar manner to 4‚0.5CH2Cl2 and
1
oil. H NMR (CD3CN, 300 MHz) δ 8.43 (d, J ) 4.8 Hz, 1H),
7.70-7.62 (m, 2H), 7.34-7.27 (m, 2H), 7.15-1.11 (m, 1H), 6.73
(d, J ) 7.2 Hz, 2H), 6.30 (d, J ) 8.3 Hz, 2H), 5.01 (br, 2H), 3.80
(s, 2H), 3.57 (s, 4H), 0.93 (s, 18H) ppm. H NMR (CD3CN, 46
(18) Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Einaga, H. Bull.
Chem. Soc. Jpn. 1998, 71, 1031-1038.
(19) Garner, D. K.; Fitch, S. B.; McAlexander, L. H.; Bezold. L. M.; Arif,
A. M.; Berreau, L. M. J. Am. Chem. Soc. 2002, 124, 9970-9971.
2
MHz) δ 3.14 ppm.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5489

Szajna-Fuller et al.
[(d6-bnpapa)Ni(O2CPh)]ClO4 (10-d6), [(d6-bnpapa)Ni(O2C-
(CH2)2SCH3)]ClO4 (11-d6), and [(d6-bnpapa)Ni(O2CH)]ClO4 (13-
d6) were prepared using d6-bnpapa following identical synthetic
methods as those outlined for the preparation of 10, 11, and 13.
[(d4-bnpapa)Ni(O2CPh)]ClO4 (10-d4), [(d4-bnpapa)Ni(O2C-
(CH2)2SCH3)]ClO4 (11-d4), and [(d4-bnpapa)Ni(O2CH)]ClO4 (13-
d4) were prepared using d4-bnpapa following identical synthetic
methods as those outlined for the preparation of 10, 11, and 13.
X-ray Crystallography. Single crystals of selected compounds
were mounted on a glass fiber with viscous oil and then transferred
to a Nonius KappaCCD diffractometer (Mo KR, λ ) 0.71073 Å)
for data collection at 150(1) K. For each compound, an initial set
of cell constants was obtained from 10 frames of data that were
collected with an oscillation range of 1°/frame and an exposure
time of 20s/ frame. Indexing and unit cell refinement based on all
observed reflections from those 10 frames indicated monoclinic P
lattices for 4‚CH2Cl2, 5‚1.5CH2Cl2, 9, and 10‚CH3CN, a mono-
clinic C lattice for 7‚0.5CH2Cl2, triclinic P lattices for 6‚
1.75CH2Cl2 and 8, and an orthorhombic P lattice for 10‚CH2Cl2.
Final cell constants for each complex were determined from a set
of strong reflections from the actual data collection. For each data
set, these reflections were indexed, integrated, and corrected for
Lorentz, polarization, and absorption effects using DENZO-SMN
and SCALEPAC.20 The structures were solved by a combination
of direct methods and heavy atom using SIR97. All non-hydrogen
atoms were refined with anisotropic displacement coefficients.
Unless otherwise noted, 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 SHELXL97.21
Figure 1. 1H NMR spectra of (a) [(6-Ph2TPA)Ni(O2CPh)]ClO4 (3) and
(b) [(6-Ph2TPA)Ni(H2O)(O2CPh)2] (2) in CD3CN at 25(1) °C. Inset:
Labeling of proton positions for the 6-Ph2TPA ligand.
Structure Solution and Refinement. The methylthiopropionate
complex 4‚CH2Cl2 crystallizes in the monoclinic crystal system in
the space group P21/c. All hydrogen atoms except those of a
methylene chloride solvate molecule were located and refined
independently. The carbon atom and one chlorine atom of this
solvate molecule exhibit disorder over two positions.
Complex 5‚1.5CH2Cl2 crystallizes in the monoclinic crystal
system in the space group P21/c. All hydrogen atoms except those
of the methylene chloride solvate molecules were located and
refined independently. The perchlorate anion exhibits disorder,
which was addressed by splitting the O(4) and O(6) atoms into
two fragments (O(4)/O(4′), O(6)/O(6′)), followed by refinement.
This led to a 0.88:0.12 ratio in occupancy over the two positions
for each oxygen atom. The half-occupied methylene chloride is
positioned near an inversion center, and the atoms of this solvate
exhibit disorder.
Complex 6‚1.75CH2Cl2 crystallizes in the triclinic crystal system
in the space group P1h. There are two independent formula units
per asymmetric unit, with the second being denoted by “A”.
Methylene chloride (1.75 molecules per formula unit) is present in
the asymmetric unit. One methylene chloride solvate molecule is
disordered.
Complex 7‚0.5CH2Cl2 crystallizes in the monoclinic crystal
system in the space group C2/c. All hydrogen atoms except those
on C(21), C(34), and the methylene chloride solvate molecule were
located and refined independently. The disordered, half-occupied
molecule of methylene chloride is positioned near an inversion
center.
Figure 2. ORTEP representations of the cationic portions of (a) 4‚CH2Cl2
and (b) 5‚1.5CH2Cl2. All ellipsoids are drawn at the 35% probability level.
Hydrogen atoms have been omitted for clarity.
those on C(32) and C(33) were located and refined independently.
The methylene carbon (C(32)), sulfur (S(1)), and methyl carbon
(C(33)) atoms of the methylthioacetate ligand exhibit disorder. Each
was split into two fragments (C(32)/C(32A), S(1)/S(1A), C(33)/
C(33A)) followed by refinement. This resulted in a 70:30 ratio in
occupancy for each atom.
The aqua methylthioacetate complex 8 crystallizes in the triclinic
crystal system in the space group P1h. All hydrogen atoms except
(20) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(21) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Complex 9 crystallizes in the monoclinic crystal system in the
space group P21/n. All hydrogen atoms were located and refined
5490 Inorganic Chemistry, Vol. 46, No. 14, 2007

Ni(II) Carboxylates and Acireductone Dioxygenase
Scheme 4
independently. Three oxygen atoms of the perchlorate anion (O(5),
O(6), and O(7)) exhibit disorder. Each was split into two fragments
(with the second being denoted by “A”) and was refined. This
refinement resulted in a 80:20 ratio in occupancy for each atom.
Complex 10‚CH3CN crystallizes in the monoclinic crystal system
in the space group P21/n with one molecule of acetonitrile in the
lattice. Complex 10‚CH2Cl2 crystallizes in the orthorhombic crystal
system in the space group P212121. The secondary amine hydrogen
atoms were located and refined independently in both structures.
Results
Bis- and Monobenzoate Ni(II) Complexes of the
6-Ph2TPA Ligand. In the O2-dependent reaction catalyzed
by the Ni(II)-containing acireductone dioxygenase from
Klebsiella ATCC 8724, carboxylate/carboxylic acid products
are generated from the oxidative degradation of 1,2-dihy-
droxy-3-keto-5-(methylthio)pentene (acireductone, Scheme
1).5,6 These products do not inhibit the enzyme at concentra-
tions up to 1 mM.8 As a means of elucidating the chemical
factors that govern the O2 reactivity and carboxylate coor-
dination chemistry of the active site metal center in Ni(II)-
ARD, we have developed a reactive synthetic model complex
(1, Scheme 3) that exhibits single turnover Ni(II)-ARD-type
reactivity.12 This complex, which contains a coordinated cis-
â-keto-enolate ligand as a model for the ARD substrate,
undergoes reaction with O2 under varying conditions to yield
Ni(II) carboxylate complexes (2 and 3, Scheme 3) and CO.
In the work described herein, we have further examined the
coordination chemistry of the Ni(II) carboxylate products.
After identifying the bisbenzoate complex [(6-Ph2TPA)-
Ni(H2O)(O2CPh)2] (2) as the product of the reaction of 1
and O2 in the presence of 1 equiv of base, we attempted to
independently synthesize this complex. Admixture of equimo-
lar amounts of 6-Ph2TPA and Ni(ClO4)2‚6H2O in acetonitrile,
methanol, or CH2Cl2 followed by the addition of excess
NaO2CPh (in methanol/H2O or H2O) and stirring for a
minimum of 12 h at room temperature, resulted only in the
isolation of [(6-Ph2TPA)Ni(O2CPh)]ClO4 (3). Thus, 2 cannot
be generated via a spontaneous self-assembly type process.
We hypothesize that the reaction pathway involving 1 and
O2 in the presence of base (Scheme 3) by which 2 is formed
must provide the conditions under which κ3-coordination of
the 6-Ph2TPA ligand is accessed.
Scheme 5
dry acetonitrile, followed by treatment with Me4NOH‚5H2O
and 3-methylthiopropionic acid (Scheme 5). The methylth-
ioacetate complex [(6-Ph2TPA)Ni(O2CCH2SCH3)]ClO4 (5)
was prepared in a similar fashion. These complexes were
characterized by X-ray crystallography, elemental analysis,
1H NMR, FTIR, and FAB-MS.
The cationic portions of the X-ray structures of 4‚CH2Cl2
and 5‚1.5CH2Cl2 are shown in Figure 2. Details of the X-ray
data collection and refinement for these complexes are given
in Table 1. Selected bond distances and angles are given in
Tables 2 and 3, respectively. For comparison, included in
the latter tables are bond distances and angles for [(6-Ph2-
TPA)Ni(O2CPh)]ClO4 (3).
The bisbenzoate complex 2 is readily transformed into the
monobenzoate derivative 3 and benzoic acid via the addition
of 1 equiv of a proton donor (NH4ClO4, Scheme 4). This
reaction is straightforward to monitor using 1H NMR (Figure
1). Although both complexes exhibit broad, paramagnetically
shifted resonances, the features found in the 30-50 ppm
region of the spectrum are characteristic of each complex.
Addition of slightly greater than 1 equiv of NH4ClO4 to 2
results in the formation of a mixture of 3 and [(6-Ph2TPA)-
Ni(CH3CN)2](ClO4)2.15,16
Methylthioalkyl Ni(II) Carboxylate Complexes of the
6-Ph2TPA Ligand. An analogue of 3 having a coordinated
3-methylthiopropionate ligand, [(6-Ph2TPA)Ni(O2C(CH2)2-
SCH3)]ClO4 (4‚0.5CH2Cl2), was prepared via admixture of
equimolar amounts of 6-Ph2TPA with Ni(ClO4)2‚6H2O in
The Ni(II) carboxylate derivatives 3, 4‚CH2Cl2, and 5‚
1.5CH2Cl2 each contain a mononuclear Ni(II) center ligated
by a bidentate carboxylate ligand and the four available
nitrogen donors of the 6-Ph2TPA ligand. The Ni(II) center
in each complex coordinates the 6-Ph2TPA ligand with
Inorganic Chemistry, Vol. 46, No. 14, 2007 5491

Szajna-Fuller et al.
Table 1. Summary of X-ray Data Collection and Refinement for 6-Ph2TPA-supported Ni(II) Carboxylate Complexes
4‚CH2Cl2 5‚1.5CH2Cl2 6‚1.75CH2Cl2 7‚0.5CH2Cl2
empirical formula C35H35N4Cl3O6SNi C34.5H34N4Cl4O6SNi C38.75H36.5N4Cl4.5O7Ni C34.5H36N4Cl2O7SNi C33H33N4ClO7SNi
8
9
C31H29N4ClO7Ni
663.74
monoclinic
P21/n
14.6776(5)
13.8506(6)
15.1681(4)
90
107.6192(2)
90
fw
804.79
monoclinic
P21/c
10.5995(2)
19.2209(7)
18.1175(6)
90
105.0839(19)
90
833.23
monoclinic
P21/c
10.7355(2)
18.8996(4)
18.5336(4)
90
101.9763(11)
90
888.45
triclinic
P1h
780.34
monoclinic
C2/c
32.2278(9)
13.1954(4)
18.9364(3)
90
120.5857(15)
90
6932.5(3)
8
723.85
triclinic
P1h
cryst syst
space group
a (Å)
b (Å)
c (Å)
12.7841(2)
17.5239(4)
19.5938(4)
77.7264(12)
77.8212(12)
70.5710(10)
3997.84(14)
4
11.9609(3)
11.9728(3)
13.5445(3)
92.1279(10)
108.8245(13)
116.0983(11)
1611.70(7)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
3563.94(19)
4
3678.55(13)
4
2938.92(18)
4
d
calcd, Mg m-3
1.500
1.505
1.476
1.495
1.492
1.500
temp (K)
150(1)
150(1)
150(1)
150(1)
150(1)
150(1)
cryst size (mm3)
diffractometera
abs coeff. (mm-1
2θ max (deg)
reflns collected
indep reflns
0.30 × 0.20 × 0.08 0.35 × 0.25 × 0.20 0.28 × 0.25 × 0.18
0.35 × 0.25 × 0.10 0.40 × 0.35 × 0.25 0.40 × 0.25 × 0.20
Nonius Kappa CCD Nonius Kappa CCD Nonius Kappa CCD
Nonius Kappa CCD Nonius Kappa CCD Nonius Kappa CCD
)
0.879
0.925
0.840
0.829
0.805
0.807
50.68
54.96
54.96
55.02
54.96
55.24
12031
14362
27379
13167
10742
11390
6489
8396
18207
7905
7268
6719
variable params
592
0.0529/0.1245
1.051
614
0.0468/0.1069
1.026
1023
0.0569/0.1378
1.028
578
0.0625/0.1554
1.032
560
0.0411/0.0949
1.038
526
0.0563/0.1030
1.095
R1/wR2b
GOF (F2)
largest diff (e Å-3
)
1.016/-0.810
1.017/-0.538
1.565/-1.070
0.985/-1.537
1.112/-0.503
0.421/-0.484
2
-
2
2
2
a Radiation used: Mo KR (λ ) 0.71073 Å). b R1 ) Σ||Fo| - |Fc||/Σ|Fo|; wR2 ) [Σ[w(Fo
Table 2. Selected Bond Distances for 6-Ph2TPA-Supported Ni(II) Carboxylate Complexesa
Fc )2]/[Σ(Fo )2]]1/2 where w ) 1/[σ2(Fo ) + (aP)2 + bP].
3
4‚CH2Cl2
5‚1.5CH2Cl2
6‚1.75CH2Cl2
7‚0.5CH2Cl2
8
9
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(4)
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-O(3)
2.0282(16)
2.0691(16)
2.1458(16)
2.2134(16)
2.0521(13)
2.1293(13)
2.024(3)
2.075(3)
2.233(3)
2.193(3)
2.183(2)
2.014(2)
2.054(2)
2.063(2)
2.198(2)
2.165(2)
2.1868(17)
2.0370(18)
2.062(3)
2.089(3)
2.216(2)
2.241(2)
2.001(2)
2.072(3)
2.080(3)
2.240(3)
2.186(3)
1.994(2)
2.0537(18)
2.0796(18)
2.1884(17)
2.2050(17)
1.9932(15)
2.1182(16)
2.074(3)
2.080(3)
2.194(3)
2.218(3)
2.001(2)
2.062(2)
2.080(3)
2.088(3)
a Estimated standard deviations in the last significant figure are given in parentheses.
Table 3. Selected Bond Angles for 6-Ph2TPA-supported Ni(II) Carboxylate Complexesa
3
4‚CH2Cl2
5‚1.5CH2Cl2
6‚1.75CH2Cl2
7‚0.5CH2Cl2
8
9
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-O(3)
O(1)-Ni(1)-N(1)
O(1)-Ni(1)-N(2)
O(1)-Ni(1)-N(3)
O(1)-Ni(1)-N(4)
O(2)-Ni(1)-N(1)
O(2)-Ni(1)-N(2)
O(2)-Ni(1)-N(3)
O(2)-Ni(1)-N(4)
O(3)-Ni(1)-N(1)
O(3)-Ni(1)-N(2)
O(3)-Ni(1)-N(3)
O(3)-Ni(1)-N(4)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(3)
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(3)
N(2)-Ni(1)-N(4)
N(3)-Ni(1)-N(4)
63.30(5)
62.76(9)
62.37(7)
91.65(6)
93.76(9)
92.18(9)
171.64(9)
103.30(9)
98.34(9)
94.08(11)
91.08(11)
172.47(11 )
99.75(11)
93.47(10)
90.26(10)
171.35(10 )
101.12(10 )
99.77(10)
108.59(6)
168.33(6)
94.84(6)
100.81(6)
170.84(6)
106.49(6)
83.74(5)
80.59(6)
161.06(11 )
118.49(10 )
80.24(10)
83.55(10)
98.31(11)
173.00(11 )
97.75(10)
105.33(10 )
171.30(8)
107.29(8)
81.53(7)
81.31(7)
109.09(8)
168.37(8)
100.32(8)
94.56(8)
90.74(7)
171.57(7)
100.90(6)
101.99(6)
177.47(7)
96.22(7)
78.55(6)
83.17(7)
100.72(11 )
172.99(10 )
92.19(10)
93.03(9)
83.59(9)
82.31(10)
81.98(9)
99.23(9)
82.23(10)
76.48(9)
174.50(12 )
93.14(12)
93.92(11)
81.82(11)
81.79(12)
83.28(11)
99.14(11)
81.87(12)
78.20(12)
159.33(12 )
174.46(11 )
94.51(11)
92.21(11)
81.18(10)
82.02(11)
83.04(10)
102.23(10 )
81.89(11)
78.21(10)
158.42(11 )
81.99(7)
101.75(6)
97.48(6)
77.80(6)
82.16(6)
149.83(6)
80.29(11)
103.71(11 )
102.85(11 )
76.04(11)
81.66(11)
141.57(10 )
81.36(8)
99.10(8)
101.78(8)
82.52(8)
77.93(9)
148.73(8)
81.35(7)
100.20(7)
97.12(7)
77.81(7)
82.01(7)
150.97(7)
158.28(10 )
a Estimated standard deviations in the last significant figure are given in parentheses.
similar Ni(1)-N(1), Ni(1)-N(2), and average Ni(1)-N(3)/
Ni(1)-N(4) distances. The average Ni-O(carboxylate)
distance in 3, 4‚CH2Cl2, and 5‚1.5CH2Cl2 is also similar
(∼2.10 Å). The carboxylate ligand in each cation is
positioned between the phenyl substituents of the 6-Ph2TPA
ligand in a sandwich type motif.
In d3-acetonitrile, 4‚0.5CH2Cl2 and 5 (Figure 3) exhibit
1H NMR spectral features that are generally similar to those
exhibited by 3 (Figure 1a), including a set of broad
resonances in the range 30-50 ppm. These resonances are
assigned as â/â′-H’s of the pyridyl and phenyl-appended
pyridyl rings (inset, Figure 1).13,16 Relatively sharp resonances
5492 Inorganic Chemistry, Vol. 46, No. 14, 2007

Ni(II) Carboxylates and Acireductone Dioxygenase
Figure 3. 1H NMR spectra of (a) [(6-Ph2TPA)Ni(O2C(CH2)2SCH3)]ClO4
(4‚0.5CH2Cl2) and (b) [(6-Ph2TPA)Ni(O2CCH2SCH3)]ClO4 (5) in CD3CN
at 25(1) °C.
Table 4. FAB-MS AND UV-vis Spectroscopic Properties of Selected
6-Ph2TPA-Supported Ni(II) Carboxylate Complexes
FAB-MS m/z
UV-vis [nm] (ꢀ, [M-1 cm-1])
[M - ClO4]+ (100%)
(CH2Cl2)
3
621
619
605
395(39), 562(15), 1047(18)
405(25), 570(12), 1050(15)
400(29), 574(13), 1053(16)
395(36), 561(15), 1048(18)
400(45), 569(31), 1056(38)
4‚0.5CH2Cl2
5
6
9
545
1
at ∼15 ppm in the H NMR spectra of 4‚0.5CH2Cl2 and 5
are assigned as being for the γ-H’s in these complexes.
The carboxylate C-O vibrations for 3, 4‚0.5CH2Cl2, and
5 are not easily identifiable in solid-state IR spectra of the
complexes due to overlap with vibrations of the 6-Ph2TPA
ligand in the region of 1620-1380 cm-1. Typically, chelating
bidentate carboxylate ligands exhibit two vibrations (νa and
-
-
νs) with a ∆ value (νa (CO2 ) - νs(CO2 )) less than 200
cm-1.22 The FAB-MS and UV-vis properties of 3, 4‚
0.5CH2Cl2, and 5 are summarized in Table 4.
Figure 4. ORTEP representations of (a) 6‚1.75CH2Cl2, and the cationic
portions of (b) 7‚0.5CH2Cl2 and (c) 8. All ellipsoids are drawn at the 35%
probability level. Hydrogen atoms other than the water protons omitted for
clarity.
With the same series of carboxylate ligands as is found in
3, 4‚0.5CH2Cl2, and 5, in the presence of water, mononuclear
6-Ph2TPA-ligated Ni(II) complexes having a monodentate
carboxylate ligand and one coordinated water molecule have
been isolated. These complexes, [(6-Ph2TPA)Ni(H2O)(O2-
CPh)]ClO4 (6), [(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4
(7‚0.2CH2Cl2), and [(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]-
ClO4 (8), were characterized by X-ray crystallography,
the average Ni(1)-O(1) distance (∼2.1 Å) present in the
bidentate analogues shown in Figure 2 and previously
discussed. The Ni(1)-O(H2O) distance in this series of
complexes ranges from 2.06 to 2.12 Å. A moderately strong
hydrogen-bonding interaction is found between a proton of
the Ni(II)-bound water molecule and the noncoordinated
oxygen atom of the carboxylate ligand.23 For example, in
6‚1.75CH2Cl2, this interaction is characterized by an O(2)‚
‚‚O(3) distance of 2.54 Å and a O(3)-H‚‚‚O(2) angle of
165°. A summary of the hydrogen-bonding interactions in
the cationic portions of 6‚1.75CH2Cl2, 7‚0.5CH2Cl2, and 8
is given in Table 5.
1
elemental analysis, H NMR, and FTIR.
ORTEP representations of the cationic portions of 6‚
1.75CH2Cl2, 7‚0.5CH2Cl2, and 8 are shown in Figure 4.
Details of the X-ray data collection and refinement are given
in Table 1. Selected bond distances and angles are given in
Tables 2 and 3, respectively. A shorter Ni(1)-O(1) interac-
tion (∼2.0 Å) is found in these complexes as compared to
(22) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Complexes, 5th ed.; Wiley: New York, 1997.
(23) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University
Press: New York, 1997.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5493

Szajna-Fuller et al.
Table 5. Parameters of Secondary Interactions in 6-Ph2TPA-Supported
Ni(II) Complexes Containing Monodentate Carboxylate Ligands
O(carb)‚‚‚O(H2O) O(carb)-H‚‚‚O(H2O) CH‚‚‚arene
complex
(Å)
(deg)
centroid (Å)
a
6‚1.75CH2Cl2
2.54/2.56
2.57
2.62
165/165
166
167
b
7‚0.5CH2Cl2
3.58/3.57c
3.65/3.73d
b
8
9
2.63
164
a Two independent molecules are present in the asymmetric unit of
6‚1.75CH2Cl2. b Not applicable. c The methylene carbon (C(32)), sulfur
(S(1)), and methyl carbon (C(33)) atoms of the methylthioacetate ligand
exhibit disorder. The CH‚‚‚arene centroid distances listed are for
C(32a)‚‚‚centroid (C(13)-C(18)) and C(32)‚‚‚centroid (C(25)-C(30)),
respectively. d CH‚‚‚arene centroid distances listed are for C(32)‚‚‚centroid
(C(13)-C(18)) and C(32)‚‚‚centroid (C(25)-C(30)), respectively.
Figure 6. ORTEP representation of [(6-Ph2TPA)Ni(H2O)(O2CH)]ClO4 (9).
Ellipsoids are drawn at the 35% probability level. Hydrogen atoms other
than the formyl and water protons omitted for clarity.
A 6-Ph2TPA-Supported Ni(II) Formate Complex. As
formic acid is a product of the Ni(II)-ARD-catalyzed reaction
shown in Scheme 1, a formate complex (9) has been prepared
and isolated by combining equimolar amounts of 6-Ph2TPA,
Ni(ClO4)2‚6H2O, and NaO2CH in methanol/water. We were
unable to prepare a water-free Ni(II) formate complex of
the 6-Ph2TPA ligand. This could be due in part to the
insolubility of NaO2CH in dry organic solvents.
Figure 5. Region of the infrared spectra of 3 and 6.
An ORTEP representation of 9 is shown in Figure 6. A
summary of the data collection and refinement parameters
for this structure is given in Table 1. Selected bond distances
and angles are given in Tables 2 and 3. The coordination
environment of the Ni(II) center in 9 is similar to that found
in 6‚1.75CH2Cl2, 7‚0.5CH2Cl2, and 8. The Ni(II)-bound
water molecule of 9 is involved in two hydrogen-bonding
interactions. One of these interactions involves the nonco-
ordinated oxygen atom of the Ni(II)-bound formate. This
interaction, which may be classified as a moderate hydrogen-
bonding interaction, involves a O(H2O)‚‚‚O(formate) het-
eroatom distance of 2.63 Å (Table 5).23
In acetonitrile, 9 exhibits a 1H NMR spectrum with several
broad resonances in the 30-50 ppm range (Figure 7). The
spectral features are similar to those described above for other
monocarboxylate Ni(II) complexes of the 6-Ph2TPA ligand.16
A sharp signal at 14.6 ppm is tentatively assigned to the γ
hydrogen (see Figure 1 (inset)). Using this signal as a
standard integrating to one hydrogen, the signal at 33.7 ppm
integrates to two hydrogens and is assigned as a â′-H signal.
The overlapping signals in the region 42-50 ppm integrate
to a total of four hydrogens and are assigned to the â/â′
protons.
Weak CH/π interactions are present between the aliphatic
methylene protons of the methylthioalkyl carboxylates
in 7‚0.5CH2Cl2 and 8 and the phenyl appendages of the
6-Ph2TPA ligand (Table 5).24 These interactions are in-
dicated by the fact that the distance between an aliphatic
carbon (C(32)) of the bound carboxylate and the arene
centroid of one or both phenyl groups is less than ∼3.7 Å.
This distance is the sum of the van der Waals radii of a
methyl group and an sp2 hybridized carbon. Notably, in the
bidentate carboxylate derivative 4‚0.5CH2Cl2 there is only
one short CH‚‚‚centroid distance (3.77 Å, C(32)‚‚‚arene
centroid (C25-C30)), and in 5 there are no CH/π interac-
tions. Therefore, the number of CH/π interactions is higher
in complexes having a monodentate methylthioalkyl car-
boxylate ligand.
In dry acetonitrile, the 1H NMR features of 6, 7‚
0.5CH2Cl2, and 8 are identical to those found for the
bidentate carboxylate analogues 3, 4‚0.5CH2Cl2, and 5,
respectively, indicating that 1H NMR is not a viable method
for determining the carboxylate coordination mode. Com-
parison of the solid-state (KBr) spectra for the benzoate
derivatives 3 and 6 (Figure 5) revealed identifiable differ-
ences in the region 1620-1380 cm-1. While the vibrations
in this region have not been definitively assigned, the
identification of spectroscopic differences does suggest a
change in the carboxylate coordination mode.
The solid-state infrared spectral features of 9 in the region
1620-1380 cm-1 are less complicated than those exhibited
by 6‚1.75CH2Cl2, 7‚0.5CH2Cl2, and 8. However, assignment
of the carboxylate C-O vibrations is still not possible due
to the presence of vibrations of the 6-Ph2TPA ligand in the
(24) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction: EVidence,
Nature, and Consequences; Wiley-VCH: New York, 1998.
5494 Inorganic Chemistry, Vol. 46, No. 14, 2007

Ni(II) Carboxylates and Acireductone Dioxygenase
Table 7. Selected Bond Distances and Angles for 10‚CH3CN and
a
10‚CH2Cl2
10‚CH3CN
10‚CH2Cl2
Ni(1)-N(2)
2.125(3)
2.043(3)
2.136(3)
2.028(3)
2.145(3)
2.124(4)
2.051(4)
2.129(3)
2.021(4)
2.135(3)
Ni(1)-N(3)
Ni(1)-N(4)
Ni(1)-N(6)
Ni(1)-O(1)
Ni(1)-O(2)
2.099(3)
2.081(3)
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-N(2)
O(1)-Ni(1)-N(3)
O(1)-Ni(1)-N(4)
O(1)-Ni(1)-N(6)
O(2)-Ni(1)-N(2)
O(2)-Ni(1)-N(3)
O(2)-Ni(1)-N(4)
O(2)-Ni(1)-N(6)
N(2)-Ni(1)-N(3)
N(2)-Ni(1)-N(4)
N(2)-Ni(1)-N(6)
N(3)-Ni(1)-N(6)
N(4)-Ni(1)-N(6)
62.70(10)
92.53(11)
106.75(11)
91.04(11)
167.64(11)
100.24(10)
169.24(11)
97.27(11)
105.04(11)
81.67(12)
161.78(12)
88.06(12)
85.56(12)
92.24(12)
62.99(10)
94.68(13)
106.81(13)
89.81(12)
167.70(12)
99.71(12)
169.77(13)
97.94(12)
104.73(13)
81.34(14)
161.91(13)
87.89(14)
85.46(14)
91.43(14)
a Estimated standard deviations in the last significant figure are given in
parentheses.
Figure 7. 1H NMR spectrum of [(6-Ph2TPA)Ni(O2CH)(H2O)]ClO4 (9)
in CD3CN at 25(1) °C.
the presence of water does not induce a shift in the
carboxylate coordination mode. Complex 10 has been
isolated in two different crystalline forms (10‚CH3CN and
10‚CH2Cl2). In a dry powdered form, 10 has also been
Table 6. Summary of X-ray Data Collection and Refinement for
10‚CH3CN and 10‚CH2Cl2
10‚CH3CN
10‚CH2Cl2
empirical formula
fw
C37H48N7ClO6 Ni
780.98
C36H47N6Cl3O6 Ni
824.86
1
characterized by H NMR, FTIR, mass spectrometry, and
elemental analysis.
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/n
9.05470(10)
16.9928(4)
25.0648(6)
90
93.3772(14)
90
3849.89(14)
4
1.347
150(1)
0.35 × 0.35 × 0.15
Nonius Kappa CCD
0.627
54.98
14094
8675
659
0.0612/0.1480
1.066
1.548/-1.406
orthorhombic
P212121
9.1450(2)
19.5880(3)
21.9762(4)
90
90
90
3936.65
4
1.392
Details of the X-ray data collection and refinement for
10‚CH3CN and 10‚CH2Cl2 are given in Table 6. Selected
bond distances and angles are given in Table 7. An ORTEP
representation of the cationic component of 10‚CH2Cl2 is
shown in Figure 8. The Ni(II) center in both crystalline forms
is six-coordinate with the benzoate ligand bound in a
bidentate fashion. In 10‚CH3CN the Ni(1)-O(2) bond is
∼0.14 Å shorter than the Ni(1)-O(1) bond. This is interest-
ing as the O(2) atom also acts as a hydrogen bond acceptor
for the two secondary amine moieties involving the sup-
porting chelate ligand. Typically, involvement of a metal-
bound atom in a hydrogen-bonding interaction results in
elongation of the metal-atom bond.25 In the cationic portion
of 10‚CH2Cl2, the difference in bond lengths of the Ni(1)-
O(1) and Ni(2)-O(2) bonds is smaller (∼0.05 Å). The Ni-
(1)-N distances are very similar in 10‚CH3CN and 10‚
CH2Cl2. It is important to note that the average Ni(1)-
O(carboxylate) bond length in 3, 10‚CH3CN, and 10‚CH2Cl2
all fall in the range 2.08-2.11 Å. Thus, the change in
secondary environment from hydrophobic in 3 to a hydrogen
bond donor environment in 10‚CH3CN and 10‚CH2Cl2 does
not produce a notable change in the nickel/carboxylate or
nickel/chelate ligand bond distances.
Z
d
calcd, Mg m-3
temp (K)
150(1)
cryst size (mm3)
diffractometera
abs coeff (mm-1
2θ max (deg)
reflns collected
indep reflns
0.33 × 0.15 × 0.08
Nonius Kappa CCD
0.748
54.98
8749
8749
498
0.0581/0.1304
1.021
0.648/-0.591
)
variable params
R1/wR2b
GOF (F2)
largest diff (e Å-3
)
a Radiation used: Mo KR (λ ) 0.71073 Å). b R1 ) Σ||Fo| - |Fc||/Σ|Fo|;
wR2 ) [Σ[w(Fo - Fc )2]/[Σ(Fo )2]]1/2 where w ) 1/[σ2(Fo ) + (aP)2
bP].
+
2
2
2
2
same region. The mass spectral and UV-vis features of 9
are given in Table 4.
Ni(II) Carboxylate Chemistry in a Hydrogen Bond
Donor Secondary Environment. To investigate the influ-
ence of the secondary environment on the chemistry of Ni-
(II) carboxylate species of relevance to acireductone diox-
ygenase, a series of complexes was prepared using the
hydrogen bond donor bnpapa chelate ligand (Scheme 6).
Notably, the bidentate benzoate derivative 10 can be isolated
under a variety of conditions, including from water-contain-
ing solutions. This indicates that, unlike the chemistry of
the 6-Ph2TPA-ligated Ni(II) benzoate complexes (3 and 6),
The hydrogen-bonding interactions in 10‚CH3CN and 10‚
CH2Cl2 involving the secondary amine appendages and one
oxygen atom of the bound carboxylate can be described as
moderate hydrogen bonds, with heteroatom N‚‚‚O distances
of ∼2.9-3.0 Å.23 It is noteworthy that this heteroatom
(25) Berreau, L. M. Eur. J. Inorg. Chem. 2006, 273-283.
Inorganic Chemistry, Vol. 46, No. 14, 2007 5495

Szajna-Fuller et al.
Scheme 6
distance is ∼0.3 Å longer than that involving the Ni(II) bound
water and carboxylate ligand in 6‚1.75CH2Cl2, 7‚0.5CH2Cl2,
8, and 9.
39 ppm relative to the intensity of this resonance in the fully
protio parent complexes. Measurement of the H NMR
2
spectra of the same series of complexes yielded a relatively
sharp signal at ∼38 ppm for each complex. This signal is
due to the deuterium incorporation at a â′ position in the
As shown in Scheme 6, two methylthioalkyl carboxylate
complexes, [(bnpapa)Ni(O2C(CH2)2CCH3)]ClO4 (11) and
[(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12), and a formate com-
plex [(bnpapa)Ni(O2CH)]ClO4 (13) of the bnpapa ligand,
were also prepared and characterized. Complexes 11-13
were obtained as microcrystalline solids that were not suitable
for X-ray crystallography. However, elemental analysis
results confirmed the analytical formulations of these com-
plexes and indicate that no water is present in the isolated
crystalline samples.
2
d6-bnpapa ligand as noted above. Additionally, in the H
NMR spectra of 10-d6, 11-d6, and 13-d6 a small signal is
found at ∼64-65 ppm. This chemical shift matches that of
a resonance found in the fully protio parent complexes and
suggests minor incorporation of deuterium in the other â′
position of the amine-appended pyridyl ring (inset, Figure
9). Resonances for the benzylic -CH2- protons of 10-d6,
2
11-d6, and 13-d6 can be identified only in the H NMR
spectra. For 10-d6, two broad 2H NMR resonances are found
at ∼49 and 25 ppm. These resonances are likely too broad
1
The H NMR features of 10-13 are shown in Figure 9.
Similar to what is found for 6-Ph2TPA-supported complexes,
a series of broad resonances is present in the region 35-70
ppm. Although these have been conclusively assigned, we
believe that some of the signals are due to â and â′ hydrogens
of the pyridyl rings (inset, Figure 9). A signal at ∼15 ppm
in the spectrum of each complex (10-13) is tentatively
assigned as the γ proton of the unsubstituted pyridyl ring.
Use of deuterium-labeled derivatives of the bnpapa ligand
for the preparation of analogues of 10-13 has yielded
1
to be observable in the H NMR spectrum of the parent
complex 10. On the basis of the symmetry features of the
cationic portions of 10‚CH3CN and 10‚CH2Cl2, three
benzylic proton resonances are expected.16 For 10-d6, the
third resonance must be too broad to be observable. However,
for the methylthiopropionate complex 11-d6, three broad
signals are observable at ∼49, 24, and ∼-96 ppm. For the
formate derivative 13-d6, only one broad -CD2- feature is
2
1
identifiable at ∼62 ppm in the H NMR spectrum.
additional insight into the H NMR spectroscopic features
of these complexes. The ligands employed, d6-bnpapa and
d4-bnpapa (Figure 10), contain deuterium atoms at the
benzylic or neopentyl methylene positions, respectively. The
synthetic procedure used for the preparation of d6-bnpapa
also results in the partial incorporation of deuterium at a â′
position. The 1H NMR spectral features of 10-d6, 11-d6, and
13-d6 are generally similar to those shown in Figure 9, with
a notable decrease in the intensity of a resonance at ∼38-
Incorporation of deuterium at the neopentyl methylene
position in 10-d4, 11-d4, and 13-d4 yielded two signals at
2
∼1.4 and ∼1.1 ppm in the H NMR spectrum of each
1
complex. These resonances are not identifiable in the H
NMR spectrum of the parent 10, 11, and 13 complexes due
to overlap with the intense tert-butyl methyl proton signal
at ∼1.5 ppm. The identification of two neopentyl methylene
resonances in these complexes is consistent with the “inner”
5496 Inorganic Chemistry, Vol. 46, No. 14, 2007

Ni(II) Carboxylates and Acireductone Dioxygenase
Table 8. Selected Spectroscopic Properties of bnpapa-supported Ni(II)
Carboxylate Complexes
FAB-MS m/z
UV-vis [nm]
[M - ClO4]+ (100%)
(ꢀ, [M-1 cm-1]) (CH3CN)
10
11
12
13
639
637
623
563
556(6), 928(14)
553(15), 921(22)
553(19), 921 (22)
554(18), 884(20)
(II) center via the hydrogen-bonding interaction involving
the metal-bound carboxylate oxygen.
The solid state (KBr) infrared spectral features of 10-13
are similar. There is a broad, intense νN-H vibration at
∼3300-3350 cm-1. This vibration is found at lower energy
and exhibits a stronger intensity than the νN-H vibration for
the free bnpapa ligand, which is consistent with the involve-
ment of the secondary amine N-H donors in hydrogen-
bonding interactions.23 A solution FTIR spectrum of 10 in
wet acetonitrile contains features in the region of 1620-
1500 cm-1 that are identical to those found in the solid-
state infrared spectrum of the complex. On the basis of this
data, we suggest that the carboxylate ligand in 10-13 is
bidentate in both the solid-state and in wet acetonitrile
solution.
Figure 8. ORTEP representation of the cationic portion of 10‚CH2Cl2.
Ellipsoids are drawn at the 35% probability level. Hydrogen atoms other
than the secondary amine N-H atoms are omitted for clarity.
A summary of the mass spectral and UV-vis features of
10-13 is given in Table 8. The energy and molar absorptivity
of the UV-vis features of 10-13 are consistent with d-d
type transitions for a pseudo-octahedral Ni(II) center.
Discussion
The Ni(II)-containing enzyme acireductone dioxygenase
(Ni(II)-ARD) catalyzes the oxidation of 1,2-dihydroxy-3-
keto-5-(methylthio)pentene using O2 to produce methylthi-
opropionate, formic acid, and CO. In order to investigate
mechanistic details of this type of reaction, a synthetic model
complex of relevance to a possible enzyme/substrate adduct
in Ni(II)-ARD has been prepared, characterized, and evalu-
ated in terms of O2 reactivity.12 This complex, [(6-Ph2TPA)-
Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1), which undergoes a
Ni(II)-ARD-type reaction with O2 either in the presence or
absence of an extra equivalent of base, is supported by a
tetradentate chelate ligand containing two hydrophobic
phenyl appendages. Inclusion of these phenyl appendages
was done to mimic the possible influence of two hydrophobic
phenylalanine residues that are positioned near the active
site metal center in Ni(II)-ARD.10 Reaction of 1 with O2 in
the presence of 1 equiv of base results in the formation of a
Ni(II) biscarboxylate complex ([(6-Ph2TPA)Ni(H2O)(O2-
CPh)2] (2), Scheme 7) having κ3-coordination of the 6-Ph2-
TPA chelate ligand. Treatment of 2 with 1 equiv of NH4ClO4
produces a monocarboxylate species [(6-Ph2TPA)Ni(O2CPh)]-
ClO4 (3) and free benzoic acid. In the presence of water,
the benzoate ligand in 3 shifts from bidentate to monodentate,
with the latter being stabilized in [(6-Ph2TPA)Ni(H2O)(O2-
CPh)]ClO4 (6) by hydrogen-bonding involving a Ni(II)-
bound water molecule. Notably, the carboxylate shift chem-
istry is also found for 6-Ph2TPA-ligated Ni(II) methylthioalkyl
carboxylate complexes. Comparative studies of bnpapa-
ligated complexes demonstrated that water-dependent car-
Figure 9. 1H NMR spectra of (a) [(bnpapa)Ni(O2CPh)]ClO4 (10), (b)
[(bnpapa)Ni(O2C(CH2)2SCH3)]ClO4 (11), (c) [(bnpapa)Ni(O2CCH2SCH3)]-
ClO4 (12), and (d) [(bnpapa)Ni(O2CH)]ClO4 (13). Spectra were collected
at 25(1) °C in CD3CN.
Figure 10. Deuterium-labeled bnpapa ligands: (a) d6-bnpapa, (b) d4-
bnpapa.
and “outer” chemical environment of these protons in the
solid-state structures of 10‚CH3CN and 10‚CH2Cl2.
Addition of D2O to a CD3CN solution of 13 results in the
disappearance of a resonance at 48 ppm. We attribute this
resonance to being for the secondary amine protons as these
are the only easily exchangeable hydrogen atoms in the
cationic portion of 13. The downfield shift of this signal
suggests delocalization of unpaired spin density from the Ni-
Inorganic Chemistry, Vol. 46, No. 14, 2007 5497

Szajna-Fuller et al.
Scheme 7
Scheme 8
a hydrogen-bonding interaction involving a Ni(II)-OH2
moiety and/or via CH/π interactions involving surrounding
aromatic amino acid side chains (e.g., Phe92 and Phe142).10
This is important, as maintaining a monodentate carboxylate
may be key toward ensuring facile release of this ligand. If
the carboxylate in the B structure (Scheme 8) were instead
coordinated bidentate, this may inhibit displacement of this
product and reduce the efficiency of the catalytic cycle.26
boxylate shift chemistry does not occur in similar complexes
having a hydrogen bond donor secondary environment.
The results of the studies presented herein enable the
formulation of a possible pathway for stepwise carboxylate
product release in Ni(II)-ARD. For the enzyme-catalyzed
reaction, release of one carboxylate product from the active
site Ni(II) center may be promoted by the protein via a
reaction involving a protonated histidine residue. Specifically,
EXAFS studies are consistent with the release of a histidine
ligand from the Ni(II) center upon coordination of the
dianionic form of the substrate (Scheme 8).9 Following the
O2 reaction, a biscarboxylate Ni(II) species (A, Scheme 8)
akin to 2 may be formed. The dissociated, protonated
histidine residue may provide a proton to promote the release
of one caboxylate product (e.g., formate) as the correspond-
ing acid. This reaction has been modeled by treatment of 2
with NH4ClO4 to yield 3 and benzoic acid. On the basis of
the chemistry found for 6, 7‚0.2CH2Cl2, 8, and 9, in a
monocarboxylate Ni(II) species such as B (Scheme 8),
monodentate carboxylate coordination could be stabilized by
Acknowledgment. We thank the National Institutes of
Health (1R15GM072509) for financial support of this work.
We thank Thomas Pochapsky for helpful discussions and
Casey Winger for technical assistance.
Supporting Information Available: X-ray crystallographic
(CIF) files for 4‚CH2Cl2, 5‚1.5CH2Cl2, 6‚1.75CH2Cl2, 7‚0.5CH2Cl2,
8, 9, 10‚CH3CN, and 10‚CH2Cl2. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC061316W
(26) Bergquist, C.; Fillebeen, T.; Morlok, M. M.; Parkin, G. J. Am. Chem.
Soc. 2003, 125, 6189-6199.
5498 Inorganic Chemistry, Vol. 46, No. 14, 2007