NMR studies of mononuclear octahedral Ni(II) complexes supported by tris((2-pyridyl)methyl)amine-type ligands

Article abstract of DOI:10.1021/ic040002a

The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand (6-Ph₂TPA, N,N-bis((6-phenyl-2- pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph₂TPA)Ni(CH ₃CN)(CH₃OH)](ClO₄)₂ (1), [(6-Ph ₂TPA)Ni(ONHC(O)CH₃)]ClO₄ (3), and [(6-Ph ₂TPA)Ni-Cl(CH₃CN)]ClO₄ (4), and one Ni(II) complex of tris-((2-pyridyl)methyl)amine, [(TPA)Ni(CH₃CN)(H ₂O)](ClO₄)₂ (2), have been characterized in acetonitrile solution using conductance methods and NMR spectroscopy. In acetonitrile solution, 1-4 have monomeric cations that exhibit isotropically shifted ¹H NMR resonances. Full assignment of these resonances was achieved using one- and two-dimensional ¹H NMR techniques and ²H NMR of analogues having deuteration of the supporting chelate ligand. COSY cross peaks were observed for pyridyl protons of the 6-Ph ₂TPA ligand in 1 and 3. This study lays the groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance to ARD.

Full text of DOI:10.1021/ic040002a

Inorg. Chem. 2004, 43, 3988−3997
NMR Studies of Mononuclear Octahedral Ni(II) Complexes Supported by
Tris((2-pyridyl)methyl)amine-Type Ligands
,†
Ewa Szajna,† Piotr Dobrowolski,† Amy L. Fuller,† 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 January 5, 2004
The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral
Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate
oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand
(6-Ph2TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph TPA)Ni(CH CN)(CH OH)](ClO )
2
3
3
4 2
(1), [(6-Ph2TPA)Ni(ONHC(O)CH )]ClO (3), and [(6-Ph2TPA)Ni−Cl(CH CN)]ClO (4), and one Ni(II) complex of tris-
3
4
3
4
((2-pyridyl)methyl)amine, [(TPA)Ni(CH CN)(H O)](ClO ) (2), have been characterized in acetonitrile solution using
3
2
4 2
conductance methods and NMR spectroscopy. In acetonitrile solution, 1−4 have monomeric cations that exhibit
isotropically shifted 1H NMR resonances. Full assignment of these resonances was achieved using one- and two-
dimensional 1H NMR techniques and 2H NMR of analogues having deuteration of the supporting chelate ligand.
COSY cross peaks were observed for pyridyl protons of the 6-Ph2TPA ligand in 1 and 3. This study lays the
groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance
to ARD.
Scheme 1
Introduction
A new nickel enzyme, acireductone dioxygenase (ARD),
was recently identified which catalyzes the oxidative break-
down of acireductone (1,2-dihydroxy-3-keto-5-methylthio-
pentene) to 2-methylthiopropionate, carbon monoxide, and
formate in Klebsiella pneumoniae (Scheme 1).1 This reaction
is a shunt out of the methionine salvage pathway, and its
biological relevance remains under investigation.2 X-ray
absorption spectroscopic studies of ARD suggest a resting
state structure having an octahedral Ni(II) center ligated by
six O/N-donors, including 3-4 histidine ligands.3 Subsequent
conserved domain homology modeling of the active site in
ARD via comparison with jack bean canavalin suggests the
presence of three histidine ligands and one glutamate donor
* Corresponding author. E-mail: berreau@cc.usu.edu. Tel: (435) 797-
1625. Fax: (435) 797-3390.
to the Ni(II) center.4 In the presence of the acireductone
substrate, the nickel center retains an octahedral coordination
environment, albeit the oxygen content of the primary
coordination environment appears to be higher. Best fits of
the EXAFS data for the enzyme/substrate complex suggest
† Utah State University.
‡ University of Utah.
(1) (a) Dai, Y.; Wensink, P. C.; Abeles, R. H. J. Biol. Chem. 1999, 274,
1193-1195. (b) Dai, Y.; Pochapsky, T. C.; Abeles, R. H. Biochemistry
2001, 40, 6379-6387.
(2) (a) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1993, 268, 21466-
21469. (b) Wray, J. W.; Abeles, R. H. J. Biol. Chem. 1995, 270, 3147-
3153.
(3) Al-Mjeni, F.; Ju, T.; Pochapsky, T. C.; Maroney, M. J. Biochemistry
2002, 41, 6761-6769.
(4) Pochapsky, T. C.; Pochapsky, S. S.; Ju, T.; Mo, H.; Al-Mjeni, F.;
Maroney, M. J. Nat. Struct. Biol. 2002, 9, 966-972.
3988 Inorganic Chemistry, Vol. 43, No. 13, 2004
10.1021/ic040002a CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/22/2004

NMR Studies of Octahedral Ni(II) Complexes
manipulations were performed in an MBraun Unilab glovebox under
an atmosphere of purified N2. The ligands TPA (tris((2-pyridyl)-
methyl)amine),11 d6-TPA,12 and 6-Ph2TPA13 and the Ni(II) com-
plexes [(6-Ph2TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1)13 and [(6-
Ph2TPA)Ni(ONHC(O)CH3)]ClO4 (3)13 were prepared according to
literature procedures.
bidentate coordination of acireductone to the mononuclear
nickel center, with displacement of one of the histidine
residues.
ARD represents the first example of a nickel-containing
dioxygenase.5,6 The chemistry of this enzyme is intriguing
for several reasons, including the fact that removal of the
Ni(II) ion, followed by replacement with Fe(II), yields an
enzyme (ARD′) that catalyzes the breakdown of acireductone
into different products, specifically the R-ketoacid precursor
of methionine and formate.1 This is the only reported
example in the literature of a metal-dependent change in
reaction products starting from the same substrate in a
biological system. The key difference between the reaction
pathways of ARD and ARD′ appears to involve the internal
site of attack (C(2) or C(3)) on the substrate by an initially
formed hydroperoxide radical or anion at C(1) of the metal-
bound acireductone. A current hypothesis is that the metal
ion present in ARD/ARD′ must influence the structure of
the substrate adduct, and thereby determine the site of attack
of the reactive hydroperoxide radical or anion.4
Physical Methods. General. Conductance measurements for
1-4 were made at 22(1) °C using a YSI model 31A conductivity
bridge with a cell having a cell constant of 1.0 cm-1 and using
[(TPA)Cu-Cl]ClO414 as a 1:1 electrolyte standard. Preparation of
the solvent (acetonitrile) and standard solutions for conductance
measurements and subsequent data analysis were performed as
previously described.15 UV-visible spectra of 2 and 4 were
collected on a Hewlett-Packard 8453 diode array spectrophotometer.
IR spectra of 2 and 4 were recorded on a Shimadzu FTIR-8400
spectrometer as KBr pellets. Low-resolution electron impact mass
spectroscopic data for (6-Ph)2-d6-TPA and (6-Ph)2-d2-TPA and fast
atom bombardment mass spectroscopic data for 2 and 4 were
obtained at the Mass Spectrometry Facility, Department of Chem-
istry, University of California, Riverside. Elemental analyses were
performed by Atlantic Microlabs of Norcross, GA.
NMR Methods. All NMR spectra were recorded on a Bruker
ARX-400 NMR spectrometer. Temperature calibration curves were
recorded for the temperature range 198-298 K using methanol and
298-358 K using ethylene glycol.16 Chemical shifts (in ppm) are
referenced to the residual solvent peak(s) in CHD2CN (1H, 1.94
In this work, we have investigated whether NMR methods
can be used for the solution characterization of mononuclear
octahedral Ni(II) complexes having tris((2-pyridyl)methyl)-
amine (TPA)-type supporting ligands. At the outset of this
work, literature precedent suggested that monomeric octa-
hedral Ni(II) complexes typically exhibit resonances that are
considerably broader (typically at least 1 order of magnitude)
than those observed for pseudotetrahedral and high-spin
pentacoordinate Ni(II) species or dimeric nickel complexes.7-9
1
(quintet) ppm). A typical one-dimensional H NMR spectrum of
1-4 consists of 16K data points, 300 scans, and a 250 ms relaxation
delay. An exponential weighting function (lb ) 30 Hz) was used
during processing. The 90° pulse (8.9 µs) was calibrated at 302 K.
Longitudinal relaxation times (T1) were measured using the
inversion-recovery pulse sequence (180°-τ-90°) method. 2H
NMR spectra were obtained at 302 K on a Bruker ARX-400
operating at 61.43 MHz using an unlocked system. Magnitude 1H
COSY spectra for 1 and 3 were obtained in CD3CN at 302 K. These
spectra were typically obtained with 512 data points in the F1
dimension and 1024 data points in the F2 dimension with a delay
time of 150 ms. An unshifted sine-bell squared function and zero
filling to 2048 data points were applied prior to Fourier transforma-
tion in both dimensions. Temperature dependence data for 1-4
was processed using standard least-squares fit procedures.
CAUTION! Perchlorate salts of metal complexes with organic
ligands are potentially explosiVe. Only small amounts of mater-
ial should be prepared, and these should be handled with great
care.17
1
We demonstrate here that use of one-dimensional H and
1
2H NMR methods, as well as two-dimensional H COSY
spectra, enables the full assignment of the 1H NMR
resonances of octahedral mononuclear Ni(II) complexes
supported by tris((2-pyridyl)methyl)amine-type ligands. In
1
addition, we demonstrate that one-dimensional H NMR
spectra of mononuclear octahedral Ni(II) complexes, sup-
ported by an aryl-appended tris((2-pyridyl)methyl)amine
ligand (6-Ph2TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-
((2-pyridyl)methyl)amine), exhibit significant changes as a
function of the anion bound to the octahedral Ni(II) center.
The results of this study provide evidence that NMR
methods, including one-dimensional 1H NMR, will be
valuable tools for monitoring model reactions of relevance
to the chemistry of Ni(II)-containing acireductone dioxyge-
nase.
[(TPA)Ni(CH3CN)(H2O)](ClO4)2 (2).18 To a solution of TPA
(48 mg, 0.16 mmol) in CH3CN (∼2 mL) was added a CH3CN (∼2
mL) solution of Ni(ClO4)2‚6H2O (60 mg, 0.16 mmol). This
(10) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Boston, MA, 1996.
(11) Canary, J. W.; Wang, Y.; Roy, R., Jr. Inorg. Synth. 1998, 32, 71-72.
(12) Nanthakumar, A.; Fox, S.; Murthy, N. N.; Karlin, K. D. J. Am. Chem.
Soc. 1997, 119, 3898-3906.
Experimental Section
General Methods. All reagents and solvents were obtained from
commercial sources and were used as received unless otherwise
noted. Solvents were dried according to published procedures and
were distilled under N2 prior to use.10 Air and water sensitive
(13) 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.
(14) (a) Wei, N.; Murthy, N. N.; Tyekla´r, Z.; Karlin, K. D. Inorg. Chem.
1994, 33, 1177-1183. (b) Schatz, M.; Becker, M.; Thaler, F.; Hampel,
F.; Schindler, S.; Jacobson, R. R.; Teykla´r, Z.; Murthy, N. N.; Ghosh,
P.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg. Chem. 2001, 40, 2312-
2322.
(15) Allred, R. A.; McAlexander, L. H.; Arif, A. M.; Berreau, L. M. Inorg.
Chem. 2002, 41, 6790-6801.
(16) Braun, S.; Kalinowski, H.-O.; Berger, S. 150 and More Basic NMR
Experiments; Wiley-VCH: New York, 1998.
(5) Broderick, J. B. Essays Biochem. 1999, 34, 173-189.
(6) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624.
(7) Rosenfield, S. G.; Berends, H. P.; Gelmini, L.; Stephan, D. W.;
Mascharak, P. K. Inorg. Chem. 1987, 26, 2792-2997.
(8) Holz, R. C.; Evdokimov, E. A.; Gobena, F. T. Inorg. Chem. 1996,
35, 3808-3814.
(9) Belle, C.; Bougault, C.; Averbuch, M.-T.; Durif, A.; Pierre, J.-L.;
Latour, J.-M.; Le Pape, L. J. Am. Chem. Soc. 2001, 123, 8053-8066.
(17) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3989

Szajna et al.
produced a purple reaction mixture that was stirred at room
temperature for 1 h. An excess of Et2O (∼60 mL) was added, and
the solution was cooled at (∼-18 °C) overnight. A purple solid
that had deposited was carefully dried under vacuum in four separate
portions. The solid was then recrystallized from CH3CN/Et2O at
Ph)2TPA following synthetic methods identical to those described
for protio analogues.13
(6-Ph)2-d6-TPA. This was prepared in a manner similar to that
reported by Karlin and co-workers for d6-tris((2-pyridyl)methyl)-
amine.12 To solid (6-Ph)2TPA (250 mg, 0.576 mmol) in a round-
bottom flask was added CH3C(O)OD (25 mL, Aldrich, 98%). The
resulting solution was refluxed under nitrogen for 48 h. After
cooling to ambient temperature, 5 M NaOH (∼30 mL) and CH2-
Cl2 (∼45 mL) were added. The reaction mixture was then stirred
for an additional 3 h, at which point the organic layer was separated,
dried over sodium sulfate, and filtered, and the solvent was removed
under reduced pressure. This yielded a white powder in quantitative
yield that was used without further purification: 1H NMR (CD3-
CN, 400 MHz) δ 8.49 (d, J ) 4.8 Hz, 1H), 8.06-8.02 (m, 4H),
7.80-7.76 (m, 2H), 7.72-7.67 (m, 4H), 7.56 (d, J ) 7.6 Hz, 2H),
7.49-7.40 (m, 6H), 7.20-7.18 (m, 1H); 2H NMR (CD3CN, 61.43
MHz) δ 4.0 ppm; LREI-MS m/z (relative intensity), 449 ([M +
H]+, 100%).
room temperature, resulting in the deposition of red-purple batches
-1
of needle-type crystals (77 mg, 77%): UV-vis, nm (ꢀ, M
cm
-1) 385 (30), 531 (17), 853 (19); FTIR (KBr, cm-1) 3399 (νOH),
1100 (νClO4), 625 (νClO4); FAB-MS (CH3CN/NBA), m/z (relative
intensity), 447 ([M - ClO4 - H2O - CH3CN]+, 100%); µeff (302
K) ) 2.92 µB. Anal. Calcd for C20H25N5Cl2O9Ni: C, 39.67; H,
3.83; N, 11.57. Found: C, 39.32; H, 3.85; N, 11.56.
[(6-Ph2TPA)Ni-Cl(CH3CN)]ClO4 (4). To a solution of Ni-
(ClO4)2‚6H2O (40 mg, 0.11 mmol) in CH3CN was added a slurry
of 6-Ph2TPA (48 mg, 0.11 mmol) in CH3CN. The resulting mixture
was stirred for ∼1 h. To this solution was added a slurry of Me4-
NCl (12 mg, 0.11 mmol) in CH3CN, and the resulting solution was
stirred for 1 h. During this time, the color of the reaction mixture
changed from brown to bright green. The solvent was then removed
under reduced pressure, and the green residue was redissolved in
CH2Cl2, filtered through a glass wool/Celite plug, and evaporated
to dryness. Recrystallization from CH3CN/Et2O at ambient tem-
[((6-Ph)2-d6-TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1-d6), [((6-
Ph)2-d6-TPA)Ni(ONHC(O)CH3)]ClO4 (3-d6), and [((6-Ph)2-d6-
TPA)Ni-Cl(CH3CN)]ClO4 (4-d6) were prepared using (6-Ph)2-
d6-TPA following synthetic methods identical to those described
for protio analogues.13
perature yielded green crystals suitable for X-ray crystallography
-1
(82 mg, 73%): UV-vis, nm (ꢀ, M
cm-1) 370 (36), 598 (8),
(6-Ph)2-d2-TPA. The preparation of (6-Ph)2-d2-TPA followed a
synthetic route similar to that reported for 6-Ph2TPA except for an
initial step involving reduction of 6-phenyl-2-pyridinecarboxyal-
dehyde.13 Specifically, d1-6-phenyl-2-pyridinemethanol was pre-
pared via the reduction of 6-phenyl-2-pyridinecarboxyaldehyde (780
mg, 4.26 mmol) with LiAlD4 (250 mg, 5.96 mmol) in dry THF
(55 mL). The LiAlD4 was placed in a 500 mL round-bottom flask
in 40 mL of THF. To this solution was carefully added a THF (15
mL) solution of 6-phenyl-2-pyridinecarboxyaldehyde. Following
stirring of the resulting mixture for ∼15 min at ambient temperature,
D2O (∼10 mL) was added. The solution was then extracted with
CH2Cl2. The organic fractions were dried over Na2SO4 and filtered,
and the solvent was removed under reduced pressure, yielding a
yellow oil (640 mg, 88%): 1H NMR (CD3CN, 400 MHz) δ 8.09-
8.07 (m, 2H), 7.85-7.82 (m, 1H), 7.74 (d, J ) 7.7 Hz, 1H), 7.52-
7.40 (m, 3H), 7.35 (d, J ) 7.5 Hz, 1H), 4.69 (s, 1H). d1-2-
(Chloromethyl)-6-phenylpyridine hydrochloride was prepared from
treatment of d1-phenyl-2-pyridinemethanol with freshly distilled
SOCl2 (yield: 99%): 1H NMR (CD3CN, 400 MHz) δ 8.42-8.38
(m, 1H), 8.09-8.05 (m, 3H), 7.90 (d, J ) 7.8 Hz, 1H), 7.67-7.58
(m, 3H), 5.36 (s, 1H). N,N-Bis((6-(phenyl)-2-pyridyl)-d1-methyl)-
N-((2-pyridyl)methyl)amine ((6-Ph)2-d2-TPA) was generated via
treatment of 2 equiv of d1-2-(chloromethyl)-6-phenylpyridine
hydrochloride with 1 equiv of 2-pyridylmethylamine in CH3CN in
the presence of sodium carbonate and tetrabutylammonium bromide.
The product obtained from this reaction was isolated and purified
as previously described (yield: 35%):13 1H NMR (CD3CN, 400
MHz) δ 8.49 (dt, J1 ) 4.8 Hz, J2 ) 1.4 Hz, 1H), 8.07-8.04 (m,
4H), 7.80-7.77 (m, 2H), 7.72-7.68 (m, 4H), 7.55 (d, J ) 7.6 Hz,
1007 (8); FTIR (KBr, cm-1) 2319 (νCH3CN), 1094 (νClO4), 623
(νClO4); FAB-MS (CH3CN/NBA), m/z (relative intensity), 535 ([M
- ClO4 - CH3CN]+, 100%). Anal. Calcd for C32H29N5Cl2O5Ni:
C, 56.76; H, 4.32; N, 10.34. Found: C, 56.53; H, 4.16; N, 9.85.
6-(d5-Ph)2TPA. The preparation of 6-(d5-Ph)2TPA followed a
synthetic route similar to that reported for 6-Ph2TPA except using
appropriately deuterated precursors.13 d5-6-phenyl-2-pyridinecar-
boxyaldehyde was prepared from 6-bromopicolylaldehyde and d5-
PhB(OH)2 (Aldrich, 98%-d) following a method reported by Canary
and co-workers for the protio analogue (yield: 37%):19 1H NMR
(CD3CN, 400 MHz) δ 10.09 (d, J ) 0.7 Hz, 1H), 8.12 (dd, J1 )
7.9 Hz, J2 ) 1.3 Hz, 1H), 8.03 (td, J1 ) 7.7 Hz, J2 ) 0.8 Hz, 1H),
7.88 (dd, J1 ) 7.5 Hz, J2 ) 0.8 Hz, 1H). d5-6-Phenyl-2-
pyridinemethanol was prepared via the reduction of d5-6-phenyl-
2-pyridinecarboxyaldehyde with NaBH4 in methanol/water (yield:
72%):13 1H NMR (CD3CN, 400 MHz) δ 7.85-7.80 (m, 1H), 7.73
(d, J ) 7.9 Hz, 1H), 7.34 (d, J ) 7.6 Hz, 1H), 4.70 (d, J ) 5.3 Hz,
2H), 3.75 (t, J ) 5.6 Hz, 1H). The halide derivative d5-2-
(chloromethyl)-6-phenylpyridine hydrochloride was prepared from
treatment of d5-phenyl-2-pyridinemethanol with freshly distilled
SOCl2 (yield: 98%): 1H NMR (CD3CN, 400 MHz) δ 8.26-8.22
(m, 1H), 7.98 (d, J ) 8.0 Hz, 1H), 7.78 (d, J ) 7.8 Hz, 1H), 5.23
(s, 2H). Preparation of N,N-bis((6-(d5-phenyl)-2-pyridyl)methyl)-
N-((2-pyridyl)methyl)amine (6-(d5-Ph)2TPA) was achieved via
treatment of 2 equiv of d5-2-(chloromethyl)-6-phenylpyridine
hydrochloride with 1 equiv of 2-pyridylmethylamine in CH3CN in
the presence of sodium carbonate and tetrabutylammonium bromide.
The product obtained from this reaction was isolated and purified
as previously described (yield: 62%):13 1H NMR (CD3CN, 400
MHz) δ 8.49 (m, 1H), 7.80-7.76 (m, 2H), 7.72-7.68 (m, 4H),
2
2H), 7.49-7.40 (m, 6H), 3.99-3.93 (br, 4H); H NMR (CD3CN,
61.43 MHz) δ 4.0 ppm; LREI-MS m/z (relative intensity), 445 ([M
2
7.55 (d, J ) 7.5 Hz, 2H), 7.20-7.17 (m, 1H), 3.96 (s, 6H); H
+ H]+, 100%).
NMR (CD3CN, 61.43 MHz) δ 8.2, 8.1, 7.5 ppm; LREI-MS m/z
[((6-Ph)2-d2-TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1-d2), [((6-
Ph)2-d2-TPA)Ni(ONHC(O)CH3)]ClO4 (3-d2), and [((6-Ph)2-d2-
TPA)Ni-Cl(CH3CN)]ClO4 (4-d2) were prepared using (6-Ph)2TPA
following synthetic methods identical to those described for protio
analogues.13
(relative intensity), 453 ([M + H]+, 40%).
[((6-(d5-Ph))2TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1-d10), [((6-
(d5-Ph))2TPA)Ni(ONHC(O)CH3)]ClO4 (3-d10), and [((6-(d5-
Ph))2TPA)Ni-Cl(CH3CN)]ClO4 (4-d10) were prepared using 6-(d5-
(18) The X-ray structure of a very similar complex, [(TPA)Ni(CH3CH2-
CN)(H2O)](ClO4)2, has been reported. Ito, M.; Sakai, K.; Tsubomura,
T.; Takita, Y. Bull. Chem. Soc. Jpn. 1999, 72, 239-247.
(19) Chuang, C.-L.; Lim, K.; Chen, Q.; Zubieta, J.; Canary, J. W. Inorg.
Chem. 1995, 34, 2562-2568.
3990 Inorganic Chemistry, Vol. 43, No. 13, 2004

NMR Studies of Octahedral Ni(II) Complexes
Scheme 2
Figure 1. Mononuclear nickel(II) complexes examined using one- and
1
two-dimensional H NMR.
[(d6-TPA)Ni(CH3CN)(H2O)](ClO4)2 (2-d6). This was prepared
using d6-TPA in a manner identical to the synthetic procedure
described for [(TPA)Ni(CH3CN)(H2O)](ClO4)2 (2).
Table 1. Summary of X-ray Data Collection and Refinement for
4‚2CH3CNa
empirical formula
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
C36H35N7Cl2O4Ni
759.32
monoclinic
P21/n
15.4913(4)
10.0769(3)
23.3121(5)
X-ray Crystallography. A crystal of 4 was mounted on a glass
fiber with traces of viscous oil and then transferred to a Nonius
KappaCCD diffractometer with Mo KR radiation (λ ) 0.71073 Å)
for data collection at 150(1) K. An initial set of cell constants was
obtained from 10 frames of data that were 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. For the data set collected for 4,
each reflection was indexed, integrated, and corrected for Lorentz,
polarization, and absorption effects using DENZO-SMN and
SCALEPAC.20 All of the non-hydrogen atoms were refined with
anisotropic displacement coefficients.
Structure Solution and Refinement. Complex 4 crystallizes
in the space group P21/n. Two noncoordinated acetonitrile molecules
were found in the asymmetric unit. All hydrogen atoms except those
on C(34) and C(36) of the noncoordinated CH3CN solvent
molecules were located and refined independently using SHELXL97.
R (deg)
â (deg)
99.4683(15)
γ (deg)
V (Å3)
3589.54(16)
4
1.405
Z
density (calcd), Mg m-3
temp (K)
150(1)
crystal size (mm3)
diffractometer
0.33 × 0.25 × 0.09
Nonius KappaCCD
0.739
abs coeff (mm-1
)
2θ max (deg)
54.96
13801
8095
570
0.0543/0.1302
1.038
0.811/-0.612
reflections collected
indep reflections
variable parameters
R1/wR2b
GOF (F2)
Results and Discussion
largest diff (e Å-3
)
Preparation and Characterization of Mononuclear
Octahedral Ni(II) Complexes. We are pursuing a project
directed at the development of synthetic model systems
relevant to the nickel-containing enzyme acireductone di-
oxygenase (ARD). As a portion of our work in this area, we
have reported the synthesis and structural characterization
of [(6-Ph2TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1) and [(6-Ph2-
TPA)Ni(ONHC(O)CH3)]ClO4 (3) (Figure 1).13 Notably, the
isolation of 3 provides evidence that the 6-Ph2TPA ligand
can support octahedral Ni(II) complexes having a chelating
bidentate anion akin to a proposed substrate-bound form of
ARD.
Two additional complexes have been prepared and char-
acterized for NMR studies. First, an analogue of 1, supported
by the tris((2-pyridyl)methyl)amine ligand, [(TPA)Ni(CH3-
CN)(H2O)](ClO4)2 (2), was prepared in 73% yield via
treatment of TPA with an equimolar amount of Ni(ClO4)2‚
6H2O, followed by recrystallization from CH3CN/Et2O. A
new mononuclear nickel chloride complex [(6-Ph2TPA)Ni-
a Radiation used: Mo KR (λ ) 0.71073 Å). b R1 ) ∑||Fo| - |Fc||/∑|Fo|;
wR2 ) [∑[w(Fo - Fc )2]/[∑(Fo )2]]1/2 where w ) 1/[σ2(Fo ) + (aP)2
bP].
+
2
2
2
2
Cl(CH3CN)]ClO4 (4) was prepared via the synthetic pathway
shown in Scheme 2. Both 2 and 4 were characterized by
UV-vis, FTIR, FAB-MS, conductance and elemental analy-
sis. The chloride derivative 4 was also characterized by
single-crystal X-ray crystallography (Table 1). The cationic
portion of 4 is shown in Figure 2. Overall, the mononuclear
Ni(II) cation exhibits a slightly distorted octahedral geometry
with the chloride ion positioned in the plane of the pyridyl
nitrogen donors of the 6-Ph2TPA ligand. A coordinated
acetonitrile solvent molecule is positioned between the two
phenyl substituents of the 6-Ph2TPA chelate ligand as has
been previously observed for 1. The average Ni-Npy bond
length (Table 2) for the phenyl-substituted pyridyl donors
in 4 (Ni-NPhPy: 2.24 Å) is slightly longer than the analogous
average distance for 1 (2.21 Å). This elongation could be
due to reduced Lewis acidity for the Ni(II) center in 4
(relative to 1) due to coordination of the chloride anion. The
(20) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3991

Szajna et al.
Figure 2. ORTEP representation of the cationic portion of 4‚2CH3CN.
Ellipsoids are plotted at the 50% probability level. All hydrogen atoms
omitted for clarity.
Figure 3. Onsager plots for 1-4 and the 1:1 standard [(TPA)Cu-Cl]-
ClO4.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
4‚2CH3CNa
four nitrogen donors from the chelate ligand as an initial
motif for modeling the protein contribution to the ligand
environment in the enzyme active site. In 1 and 2, two cis
coordination positions are occupied by solvent molecules,
akin to the proposed metal-coordinated waters in resting state
ARD.4 In 3, a bidentate ligand (hydroxamate) is coordinated
to the Ni(II) center, a structure that has some relevance to a
proposed substrate adduct in the enzyme.4 Notably, three of
these complexes (1, 3, and 4) have been characterized by
X-ray crystallography,13 and the X-ray structure of a close
analogue of 2 has been reported in the literature.18 Conduc-
tance measurements, performed over a range of concentra-
tions for 1-4, yielded data that could be fit to the Onsager
law, revealing 1:2 electrolyte behavior for 1 and 2, and 1:1
Ni-N(1)
Ni-N(2)
Ni-N(3)
Ni-N(4)
Ni-N(5)
Ni-Cl(1)
2.088(2)
2.272(2)
2.081(3)
2.213(2)
2.034(3)
2.364(1)
N(3)-Ni(1)-N(5)
N(4)-Ni(1)-N(2)
N(1)-Ni(1)-Cl(1)
N(5)-Ni(1)-N(1)
N(5)-Ni(1)-N(2)
N(5)-Ni(1)-Cl(1)
N(5)-Ni(1)-N(4)
N(1)-Ni(1)-N(2)
N(3)-Ni(1)-N(1)
N(1)-Ni(1)-N(4)
174.63(10)
157.67(10)
169.73(7)
93.79(10)
103.94(10)
90.21(8)
98.35(10)
79.62(9)
83.04(10)
97.64(9)
a Estimated standard deviations in the last significant figure are given in
parentheses.
remaining Ni-N distances in 4 (Ni-NPy 2.088(2), Ni-Namine
2.081(3), Ni-NCH3CN 2.034(3) Å) are within experimental
error of those found in 1 (Ni-NPy 2.065(4), Ni-Namine 2.082-
(4), Ni-NCH3CN 2.044(4) Å). It is worth noting that a nickel
chloride complex of the TPA ligand ([(TPANi)2(µ-Cl)2]-
(ClO4)2) has been previously reported in the literature. The
solid-state structure of this complex contains a dinuclear
cation with two bridging chloride anions.21
(22) Four-coordinate aminotroponeimineate- and salicylaldimine-ligated
complexes: (a) Eaton, D. R.; Josey, A. D.; Phillips, W. D.; Benson,
R. E. J. Chem. Phys. 1962, 37, 347-360. (b) Eaton, D. R.; Josey, A.
D.; Benson, R. E.; Phillips, W. D.; Cairns, T. L. J. Am. Chem. Soc.
1962, 84, 4100-4106. (c) LaLancette, E. A.; Eaton, D. R.; Benson,
R. E.; Phillips, W. D. J. Am. Chem. Soc. 1962, 84, 3968-3970. (d)
Eaton, D. R.; Josey, A. D.; Phillips, W. D.; Benson, R. E. Discuss.
Faraday Soc. 1962, 77-87. (e) Eaton, D. R.; Josey, A. D.; Phillips,
W. D.; Benson, R. E. J. Chem. Phys. 1963, 39, 3513-3518. (f) Eaton,
D. R.; Phillips, W. D.; Caldwell, D. J. J. Am. Chem. Soc. 1963, 85,
397-406.
Conductance Measurements. To confirm that the mono-
nuclear structures of 1-4 are retained in CH3CN solution, a
series of conductance measurements were performed. On-
sager plots for data collected for 1-4 are shown in Figure 3
along with the 1:1 standard [(TPA)Cu-Cl]ClO4.14 Two
distinct groups of slopes are observed, consistent with 1:1
electrolyte behavior of 3, 4, and [(TPA)Cu-Cl]ClO4, and
1:2 behavior of 1 and 2 in CH3CN solution.
(23) [Ni(acac)2] + pyridine-type bases: Happe, J. A.; Ward, R. L. J. Chem.
Phys. 1963, 39, 1211-1218.
(24) [Ni(acac)2] + substituted anilines: Kluiber, R. W.; Horrocks, W. D.,
Jr. Inorg. Chem. 1967, 6, 430-431.
(25) Ni(Py)62+: (a) Cramer, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970,
92, 66-70. (b) Wicholas, M.; Drago, R. S. J. Am. Chem. Soc. 1969,
91, 5963-5970.
(26) Tetrahedral bis-pyridyl, -picolyl, or -â-iminoamino complexes: (a)
Holm, R. H.; Everett, G. W., Jr.; Horrocks, W. D., Jr. J. Am. Chem.
Soc. 1966, 88, 1071-1073. (b) Parks, J. E.; Holm, R. H. Inorg. Chem.
1968, 7, 1408-1416.
NMR Studies. General. Examples of the use of NMR
methods for the solution characterization of paramagnetic
Ni(II) complexes have previously appeared in the litera-
ture.22-31 However, these studies focused on synthetic
complexes that are not structurally relevant to the nickel
center in ARD or for which mononuclear solid-state struc-
tures have not been reported.8,9,22-31 In this study, using 1D
(27) [Ni(bipy)3]2+ complexes: (a) Wicholas, M.; Drago, R. S. J. Am. Chem.
Soc. 1968, 90, 2196-2197. (b) Wicholas, M.; Drago, R. S. J. Am.
Chem. Soc. 1968, 90, 6946-6950.
(28) [Ni(phen)3]2+ complexes: (a) Wicholas, M. Inorg. Chem. 1971, 10,
1086-1087. (b) La Mar, G. N.; Van Hecke, G. R. Inorg. Chem. 1970,
9, 1546-1557.
(29) Heteroatom-substituted [Ni(acac)2] + pyridine: La Mar, G. N. Inorg.
Chem. 1969, 8, 581-586.
1
2
and 2D H NMR and H NMR, we have fully assigned the
proton NMR resonance of four mononuclear octahedral Ni-
(II) complexes of relevance to the active site nickel center
in acireductone dioxygenase (ARD). These complexes have
(30) Schiff base-type ligands: (a) La Mar, G.; Sacconi, L. J. Am. Chem.
Soc. 1967, 89, 2282-2291. (b) Thwaites, J. D.; Sacconi, L. Inorg.
Chem. 1966, 5, 1029-1035. (c) Thwaites, J. D.; Bertini, I.; Sacconi,
L. Inorg. Chem. 1966, 5, 1036-1041. (d) Campbell, T. G.; Urbach,
F. L. Inorg. Chem. 1973, 12, 1840-1846.
(31) (a) Yonezawa, T.; Morishima, I.; Ohmori, Y. J. Am. Chem. Soc. 1970,
92, 1267-1274. (b) Morishima, I.; Okada, K.; Yonezawa, T. J. Am.
Chem. Soc. 1972, 94, 1425-1430.
(21) Zhang, Z. H.; Bu, X. H.; Zhu, Z. A.; Jiang, Z. H.; Chen, Y. T.
Transition Met. Chem. 1996, 21, 235-237.
3992 Inorganic Chemistry, Vol. 43, No. 13, 2004

NMR Studies of Octahedral Ni(II) Complexes
Table 3. 1H NMR Chemical Shifts for 1-4 in CD3CN at 302 K
assignment
chem shift (ppm)a ∆ν1/2 (Hz)b
T
1exp (ms)c rel area
[(6-Ph2TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1)
R
∼176
4000
280, 270
370, 290
71
d
d
â
58.0, 50.4
53.0, 39.9
15.5
3, 3
3, 2
9
1, 1
2, 2
1
2
d, d
d
â′
γ
γ′
8.5
d
7
CH2 (PhPy)e
CH2 (Py)
Ph
∼158, ∼36e
3000
2900
d, 700
d, d
<1
8, <1
∼77
8.4, 6.6
6, 4
[(TPA)Ni(CH3CN)(H2O)](ClO4)2 (2)
R
∼142
52, 50
14.3
∼79
4900
720, 710
100
d
2, 3
7
d
3, 3
3
â
γ
CH2
7200
d
d
[(6-Ph2TPA)Ni(ONHC(O)CH3)]ClO4 (3)
R
160
900
d
∼1
1, 1
2, 2
1
1
2, 2
2
2, 4, 4
1
3
Figure 4. 1H NMR spectrum of 1 in CD3CN at 302 K. Spectrum was
referenced to the residual proton solvent signal of CHD2CN at 1.94 ppm.
â
47.5, 41.8
38.8, 29.3
14.8
6.4
143, 45
51
80, 80
100, 100
46
49
600, 500
370
60, 55, 220
500
83
10, 11
13, 7
29
25
d, <1
1
25, 19, 2
d
â′
γ
γ′
CH2 (PhPy)e
CH2 (Py)
Ph
8.2, 8.0, 4.7
N-H
CH3
128f
9.0
7
[(6-Ph2TPA)Ni-Cl(CH3CN)]ClO4 (4)
R
∼170
∼10000
220, 390
780, 720
72
360
d, 1750
1900
d
d
d
d
1
2
d
d
d
â
54.6, 37
∼52, ∼42
14.8
6, 3
a, 3
13
â′
Figure 5. Labeling scheme for (6-Ph2)TPA ligand.
γ
γ′
10g
12
in CH3CN solution 1 has an effective plane of symmetry
that yields equivalent phenyl-appended pyridyl substituents.
A sharp resonance at 15.5 ppm (T1 ) 9 ms), integrating to
1H, is assigned as the γ-H of the unsubstituted pyridyl ring.
A resonance at 8.5 ppm, having a similar T1 value (7 ms),
that could be integrated in the 1H NMR spectrum of the d10-
analogue [((6-(d5-Ph))2TPA)Ni(CH3CN)(CH3OH)](ClO4)2
(1-d10, Figure 6), is assigned as the γ′ resonance. Confirma-
tion of these assignments was achieved using two-dimen-
sional 1H NMR techniques. A magnetic COSY spectrum of
1 (Figures S5 and S6, Supporting Information), recorded at
29 °C, shows clear cross peaks between the resonances at
53.0/39.9 ppm (â′) and 8.5 ppm (γ′), and 58.0/50.4 ppm (â)
and 15.5 ppm (γ).
CH2 (PhPy)
CH2 (Py)
Ph
∼192,e ∼21
<1
<1
d, 17, 7
∼76
d, 8.6, 6.9
d, 50, 250
a Chemical shifts in ppm relative to the residual solvent peak of CHD2CN
(1H, 1.94 (quintet) ppm). b Line widths are full width at half-maximum.
c T1 values were obtained at 400 MHz. d Could not be observed/determined.
e Located using complex supported by (6-Ph2)-d2-TPA ligand. f Peak
disappears upon addition of D2O to a CD3CN solution of 3. g Located using
complex supported by 6-(d5-Ph)2TPA ligand.
behavior for 3 and 4.32 Thus, all of the complexes reported
in this study are mononuclear in CH3CN solution.
1H NMR spectra of 1-4 were collected at 29 °C in d3-
acetonitrile solution. Chemical shifts, relative areas, line
widths, and T1 values for 1-4 are given in Table 3. All of
the complexes exhibit several isotropically shifted 1H NMR
resonances spread over a chemical shift range of ∼150-
180 ppm.
Resonances found at 8.4 (T1 ) 8 ms) and 6.6 (T1 ) <1
1
ms) ppm in the one-dimensional H NMR spectrum of 1
were conclusively identified as belonging to the phenyl
appendages via deuterium substitution using [((6-(d5-Ph))2-
TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1-d10). The H NMR
Assignment of Resonances: [(6-Ph2TPA)Ni(CH3CN)-
(CH3OH)](ClO4)2 (1). Initial assignment of the resonances
for 1 (Figure 4) was made on the basis of integrated intensity
and T1 values. Four resonances found in the region of 39-
58 ppm are assigned as the â-H’s of the pyridyl and phenyl-
substituted pyridyl rings (Figure 5). Two of these resonances
(53.0 and 39.9 ppm) integrate to two protons each, and thus
are associated with the phenyl-appended pyridyl moieties.
The remaining two signals (58.0 and 50.4 ppm) in this region
each integrate to one proton and are assigned to the
unsubstituted pyridyl group. The T1 values for the â-H’s of
1 are all similar, falling in the range of 2-3 ms. The
observation of only four pyridyl â-resonances indicates that
2
spectrum of 1-d10 displays two overlapping resonances (8.5
and 8.4 ppm) and a third broad resonance at 6.6 ppm. On
the basis of peak intensity and T1 values, we assign the
resonance at ∼6.6 ppm to the ortho hydrogens of the phenyl
groups.
The presence of an effective plane of symmetry for 1 in
CD3CN solution, as suggested by observation of equivalent
PhPy donors (vida supra), indicates that a total of three
methylene proton resonances should be observed. At 29 °C,
three broad resonances are identifiable at ∼176, 158, and
77 ppm, each of which has a peak width of g2900 Hz. These
resonances were conclusively identified as methylene protons
of the chelate ligand via examination of the 1H and 2H NMR
(32) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81-122.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3993

Szajna et al.
consistent with the close proximity of this proton to the
paramagnetic Ni(II) center (Ni(I)‚‚‚H 3.17 Å).
A broad resonance in the 1H NMR spectrum of 1 in CD3-
CN, found at ∼3.5 ppm, is assigned to the methyl protons
of methanol. The observed chemical shift of this resonance,
which is slightly downfield of the expected chemical shift
of free methanol (3.28 ppm) in CD3CN solution under
diamagnetic conditions, indicates that the Ni(II)-coordinated
methanol undergoes exchange with the deuterated solvent.
A second broad peak found at ∼2.4 ppm indicates the
presence of trace water in the sample (2.13 ppm in CD3CN
under diamagnetic conditions). A clear resonance for CH3-
CN, which is a ligand to the Ni(II) center in the solid-state
structure of 1, could not be identified and may be overlapping
with the above-mentioned 2.4 ppm resonance.
[(TPA)Ni(CH3CN)(H2O)](ClO4)2 (2). Five broad hyper-
fine-shifted resonances are observed in the 1H NMR spectrum
of 2 in CD3CN. Two resonances at 52 (T1 ) 2 ms) and 50
(T1 ) 3 ms) ppm, respectively, each of which integrates to
three protons, are assigned as the pyridyl â-protons. The
sharpest signal in the spectrum (∆ν1/2 ) 100 Hz, T1 ) 7
ms) is found at 14.3 ppm and integrates to three protons.
This resonance is assigned as the pyridyl γ-protons. The
observation of only one set of pyridyl â- and γ-protons
suggests a rearrangement of the unsymmetrical ligand
environment that is rapid on the NMR time scale and results
in similar chemical shifts for all the pyridyl protons of 2.
Consistent with this hypothesis is the observation of a single
broad methylene proton resonance centered at ∼79 ppm
(∆ν1/2 ∼7200 Hz). The 2H NMR spectrum of the d6 analogue
complex [(d6-TPA)Ni(CH3CN)(H2O)](ClO4)2 (2-d6) contains
two broad resonances centered at ∼79 and 68 ppm, sug-
gesting similar chemical environments for the methylene
deuterons. A broad resonance in the 1H NMR spectrum of 2
at ∼142 ppm is assigned as the signal for the pyridyl
R-proton. Finally, a broad peak at ∼2.46 ppm is tentatively
assigned as being for H2O and CH3CN, which are displaced
from the Ni(II) center via exchange with CD3CN.
Figure 6. Deuterated ligands.
spectra of the deuterated analogue [((6-Ph)2-d6-TPA)Ni(CH3-
CN)(CH3OH)](ClO4)2 (1-d6, Figure 6) in which all of the
methylene protons were replaced by deuterons. Specifically,
resonances at ∼163, 75, and ∼39 ppm are observed in the
2H NMR spectrum of 1-d6. We attribute the subtle differences
in chemical shift between the 2H NMR resonances and those
1
in the H NMR spectrum to the broad nature of the peaks,
1
particularly in the H NMR spectrum. On the basis of
literature precedent,33,34 we assign the most downfield shifted
1
(∼158 ppm in the H NMR spectrum) of the methylene
proton resonances to Heq, the methylene proton on each PhPy
arm that has a dihedral angle (Ni-N-C-H) closer to 180°.
A dramatic difference in chemical shift for Heq and Hax is
expected for the PhPy geminal methylene protons due to
chelate ring formation, which results in different hyperfine
contact shift and relaxation times for each proton. Use of a
d2-functionalized ligand having a racemic 50:50 H:D content
for the PhPy methylene units (Figure 6) confirmed the
assignment of the resonances at ∼163 and 39 ppm in the 2H
NMR spectrum of 1-d6 as being the two independent signals
expected for the geminal PhPy methylene deuterons. The
[(6-Ph2TPA)Ni(ONHC(O)CH3)]ClO4 (3). Of all the
1
complexes in this study, the sharpest H NMR resonances
at 29 °C are observed for 3 (Figure 7 (top)), a complex which
has some structural relevance to a proposed substrate adduct
for ARD.4 Whereas the Py and PhPy â-proton resonances
in 1 have peak widths at half-height of ∼280-370 Hz,
analogous resonances in 3, found at 47.5, 41.8, 38.8, and
29.3 ppm, have ∆ν1/2 ) 80-100 Hz. The γ- and γ′-proton
resonances are easily identifiable for 3 at 14.8 and 6.4 ppm,
respectively. This complex was amenable to study by two-
1
signal at ∼39 ppm corresponds to a peak observed in H
NMR spectrum of 1 at ∼36 ppm when the sample is heated
2
to g65 °C. The final signal in the H NMR spectrum of
1
dimensional H COSY NMR (Figure 8), which confirmed
1-d6 at 75 ppm is assigned as the expected methylene
resonance for the Py linker deuterons. Finally, the signal at
∼176 ppm (∆ν1/2 ∼4000 Hz) in the 1H NMR spectrum of 1
is assigned as the R-H of the unsubstituted pyridyl ring. The
large isotropic shift and broad nature of this resonance is
the assignment of all of the â- and γ-hydrogens. Unlike 1,
all three resonances of the phenyl substituents are easily
observed at 29 °C (8.2 (T1 ) 25 ms), 8.0 (T1 ) 19 ms), and
2
4.7 (T1 ) 2 ms) ppm). H NMR studies of 3-d10 confirmed
the assignment of these resonances. The 4.7 ppm resonance
in 3 exhibits the shortest T1 value, suggesting that this signal
is for the ortho protons of the phenyl rings.35 Three broad
(33) Ming, L.-J.; Jang, H. G.; Que, L. Jr. Inorg. Chem. 1992, 31, 359-
364.
(34) Holm, R. H.; Hawkins, C. J. In NMR of Paramagnetic Molecules:
Principles and Applications; La Mar, G. N., Horrocks, W. D., Jr.,
Holm, R. H., Eds.; Academic Press: New York, 1973; pp 243-332.
resonances (143 ppm (∆ν1/2 ) 600 Hz), 51 ppm (∆ν1/2
)
1
370 Hz), and 45.4 ppm (∆ν1/2 ) 500 Hz) ppm) in the H
3994 Inorganic Chemistry, Vol. 43, No. 13, 2004

NMR Studies of Octahedral Ni(II) Complexes
Figure 8. 1H COSY spectrum of 3 obtained at 400 MHz at 302 K in
CD3CN solution.
peak widths of the 1H NMR resonances of 4 over a 10-fold
concentration range (∼1.5-15 mM) yields no change in the
spectral features, suggesting that the observed peak broaden-
ing is not due to formation of multinuclear species. This
conclusion is supported by conductance measurements of 3
in CH3CN solution, which indicate that the complex behaves
as a 1:1 electrolyte.
Figure 7. Top: 1H NMR spectrum of 3. Bottom: A portion of the 1H
NMR spectrum of 4. Spectra were obtained in CD3CN at 302 K and were
referenced to the residual proton solvent signal of CHD2CN at 1.94 ppm.
The resonances for the â-H’s in 4 appear as two relatively
sharp (54.6 and 37 ppm) and two broad resonances (∼52
and ∼42 ppm) at 29 °C. The γ-H’s for the pyridyl and
phenyl-appended pyridyl rings can be assigned on the basis
of integration to peaks at 14.8 and ∼10 ppm, respectively.
Two resonances associated with the phenyl appendages are
present in the 1H NMR spectrum of 4 at 8.6 and 6.6 ppm. A
2H NMR spectrum of 4-d10 revealed peaks at 11.2, 8.6, and
6.8 ppm. The most downfield of these resonances is very
NMR spectrum of 3 disappear upon deuteration of the
methylene linkers (using 3-d6). Correspondingly, a 2H NMR
spectrum of 3-d6 has three resonances at ∼141, 50, and 45
ppm, consistent with three methylene signals as is observed
for 1. A 2H NMR spectrum of 3-d2 indicates that the signals
at ∼141 and 45 ppm are the expected geminal methylene
deuteron resonances of the PhPy appendages.
Two broad downfield-shifted resonances (∼160 and 128
ppm) are present in the 1H NMR spectrum of 3-d6. The signal
at 128 ppm disappears when 3 is treated with D2O in dry
CD3CN, consistent with assignment of this peak as the
hydroxamate N-H proton resonance. A resonance at 9.0 ppm
is assigned, on the basis of integration, as the methyl
resonance for the hydroxamate ligand. Finally, the signal at
∼160 ppm in the 1H NMR spectrum of 3 is assigned as the
pyridyl R-proton resonance.
2
broad in the H NMR spectrum of 4-d10. Thus, it is likely
that the corresponding signal in the 1H NMR spectrum of 4
is broadened into the baseline and/or is unobservable due to
overlap with the γ′-proton resonance. Two methylene
resonances have been assigned in the 1H NMR spectrum of
4 at ∼76 and ∼21 ppm. These assignments were confirmed
via examination of the 4-d6 analogue using 2H NMR, where
signals are present at 74 ppm and ∼25 ppm. In the 2H NMR
spectrum of 4-d6 a third very broad resonance is present at
∼192 ppm. The assignment of the resonance as Heq was
confirmed via examination of the deuterated analogue [((6-
Ph)2-d2-TPA)Ni-Cl(CH3CN)]ClO4 by 2H NMR, which
revealed a very broad feature at ∼190-200 ppm, and a
second signal at ∼26 ppm (Hax). Similar to the assignments
made for 1 and 3, these resonances correspond to the geminal
[(6-Ph2TPA)Ni-Cl(CH3CN)]ClO4 (4). Resonances in the
1H NMR spectrum of 4 at 29 °C are considerably broader
than those observed for 1 and 3 under identical conditions
(Figure 7 (bottom)). Comparison of the chemical shifts and
(35) Jensen, M. P.; Lange, S. J.; Mehn, M. P.; Que, E. L.; Que, L., Jr. J.
Am. Chem. Soc. 2003, 125, 2113-2128.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3995

Szajna et al.
Table 4. Temperature Dependence of Chemical Shifts
Curie plotb polynomial plotc
deuterons of the PhPy methylene linkages. The pyridyl R-H
in 4 is assigned to a very broad feature (∆ν1/2 g 10000 Ηz)
at ∼170 ppm. A broad peak at ∼2.14 ppm is assigned as
protio acetonitrile, which has been displaced from the metal
center by CD3CN.
a × 10-3 δdia a′ × 10-3 b′ × 10-5
assignment δ (ppm)a
δ
int (ppm) (ppm‚K) (ppm) (ppm‚K) (ppm‚K2)
[(6-Ph2TPA)Ni(CH3CN)(CH3OH)](ClO4)2 (1)
R
∼176
-58.67
68.9
14.8
10.3
12.4
10.0
2.4
8.49 25.2
7.70 14.4
7.18 14.8
7.70 13.7
70.8
0.6
-6.9
-2.0
1.9
0.4
-1.2
-1.4
Temperature Dependence of Chemical Shifts. Examina-
tion of the 1H NMR spectra of 1-4 over a temperature range
of 259-346 K revealed that the majority of resonances
sharpened and moved toward the diamagnetic region with
increasing temperature, thus showing typical Curie law
behavior (Figures S1-S4, Supporting Information). Fits for
1/T vs δ (ppm) plots for the resonances of 1-4 gave
correlation coefficients in the range of 0.95-0.99. Selected
resonances for 1-4 exhibit a small degree of anti-Curie
behavior.36 These include two aromatic resonances for 1 (γ′
and a Ph resonance), one phenyl resonance of 3, and the γ′
resonances of 3 and 4. Intercepts for selected resonances on
the 1/T vs δ (ppm) plots differ significantly from the expected
diamagnetic shift at 0 K (Table 4; e.g. R, â, and CH2(Ph)
resonances in 1, â/â′, Ph, CH2(PhPy), and N-H resonances
in 3, and R, â/â, γ′, CH2(PhPy), CH2Py, and Ph resonances
in 4) suggesting small deviations from the Curie law. For
pseudo-octahedral Ni(II) centers, zero-field splitting (D),9,37
wherein D , thermal energy kBT, may influence both 1/T
(contact) and 1/T2 (dipolar) contributions to the temperature
dependence of the hyperfine chemical shift.38 In an attempt
to probe the role of D in the deviations from the Curie Law
observed for 1-4, the temperature dependence of the
chemical shifts was also fit using a second-order polynomial
equation (Table 4). However, similar to the results reported
by Belle and co-workers for dimeric Ni(II) complexes having
pseudo-octahedral metal centers,9 no significant improvement
in fit quality, as evidenced by comparison of correlation
coefficients (∼0.95-0.99), was observed in the polynomial
plots, thus precluding further interpretation of the influence
of zero-field splitting on the observed chemical shifts.
Comparison of the NMR Features of 1-4. The most
â
58.0
6.99
50.4
53.0
39.9
15.5
8.5
14.54
9.67
5.43
7.24
9.19
â′
7.56
7.70
7.77
8.8
2.1
0.5
γ
γ′
-0.3
44.6
CH2 (PhPy) ∼158
5.68
3.96 45.6
∼36d
CH2 (Py)
Ph
∼77
21.16
15.7
3.96 26.7
-17.6
0.4, 0.1
0.3
8.4
6.6
7.88, 7.28 0.2, 0.3 7.45
7.70 -0.3 8.04 -0.6
0.4, 0.2
[(TPA)Ni(CH3CN)(H2O)](ClO4)2 (2)
R
∼142
52
50
14.3
∼79
7.25
4.55
4.53
6.60
7.70
41.2
14.3
13.7
2.3
8.48 40.7
7.58 12.5
7.20 12.1
0.6
2.7
2.5
1.0
-4.1
â
γ
CH2
7.71
1.7
22.0
3.84 24.6
[(6-Ph2TPA)Ni(ONHC(O)CH3)]ClO4 (3)
R
160
47.5
41.8 -2.29
38.8 12.47
29.3 -3.85
14.8
6.4
143
45
51
8.2
8.0
4.7
128
8.92
5.79
45.6
12.6
12.4
8.9
10.0
2.4
-1.5
51.6
0.7
15.1
0.2
8.49 45.8
7.70 11.5
-0.3
1.7
8.4
-4.2
10.0
0.8
-3.1
27.7
-35.5
2.4
â
7.18
6.8
â′
7.70 11.7
7.56
7.70
7.77
3.2
1.9
0.6
γ
γ′
6.85
11.27
-27.01
43.07
1.27
7.44
6.76
11.47
-19.81
CH2 (PhPy)
3.96 33.0
3.96 24.3
3.96 13.4
CH2 (Py)
Ph
7.45
7.45 -0.04
8.04 0.007
8.98 27.5
1.76 1.1
0.2
0.009
0.4
0.6
-3.0
25.0
3.3
-2.0
44.5
3.3
N-H
CH3
9.0 -1.99
[(6-Ph2TPA)Ni-Cl(CH3CN)]ClO4 (4)
R
∼170
14.46
1.82
-19.07
33.75
13.52
8.63
30.03
17.79
7.57
45.9
15.6
18.1
5.3
7.0
1.8
-6.2
38.2
4.0
8.49 49.1
7.70 12.1
7.18
7.70 20.9
7.56 10.6
7.70
7.77
3.96 45.8
3.96 6.1
3.96 24.4
7.45 0.5
8.04 -3.4
-4.2
5.2
23.3
â
54.6
37
∼52
∼42
2.4
â′
-23.1
-5.3
-0.8
-20.0
-10.3
-2.8
-9.3
-0.5
9.1
γ
γ′
14.8
2.4
7.2
10
CH2 (PhPy) ∼192
∼21
1
striking difference between the H NMR spectral features
CH2 (Py)
Ph
∼76
8.6
14.51
7.98
18.1
0.2
of the 6-Ph2TPA- and TPA-ligated complexes is the observa-
tion of distinct â and γ resonances for the pyridyl rings of
the Py and PhPy appendages in 6-Ph2TPA derivatives. In 1,
3, and 4, the â′ and γ′ resonances of the PhPy donors appear
upfield of the analogous Py ring protons, with the hydrox-
amate complex 3 exhibiting the smallest PhPy â′ and γ′
hyperfine shifts. The larger shifts observed for the Py versus
PhPy ring protons indicates differences in ligand field and
spin distribution for these two types of donors in the chelate
ligands. In this regard, it is important to note that the length
of the Ni-Npy bond in 1, 3, and 4 is ∼0.15-0.19 Å shorter
than the average Ni-NPhPy distance in these complexes, with
the greatest difference in NPy and NPhPy(av) bond lengths
(∼0.19 Å) being identified for 3. Thus, the smaller hyperfine
6.9 -2.15
2.7
a Observed chemical shift (ppm) at 302 K. b Fit of change in chemical
shift (ppm) with temperature (K) according to the Curie law (δobs ) δint
+
a/T), where δint is the intercept at T ) 0 K and a is the Curie slope. c Fit
of change in chemical shift (ppm) with temperature (K) according to a
polynomial function (δobs ) δint + a′/T +b′/T2), where δint is the chemical
shift of the diamagnetic resonance of the free ligand. d Variable temperature
chemical shift data not obtained.
shifts for the PhPy pyridyl ring protons, which involve a
significant contact contribution, is consistent with the longer
distance of these hydrogen atoms from the paramagnetic
Ni(II) center.
1
Comparison of H NMR Properties of 1-4 with Iron
Complexes Ligated by Tris((2-pyridyl)methyl)amine-
Type Ligands. Extensive paramagnetic 1H NMR studies on
tris((2-pyridyl)methyl)amine-ligated iron complexes have
been reported by Que and co-workers.35,39 Similar to the
octahedral Ni(II) complexes described here, six-coordinate
Fe(II) complexes of TPA exhibit a pattern of isotropically
shifted pyridine resonances in the order R-H > â-H > γ-H
(36) (a) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-
17804. (b) Banci, L.; Bertini, I.; Luchinat, C.; Pierattelli, R.; Shokhirev,
N. V.; Walker, F. A. J. Am. Chem. Soc. 1998, 120, 8472-8479.
(37) Sacconi, L.; Mani, F.; Bencini, A. In ComprehensiVe Coordination
Chemistry; Wilkinson, S. R., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon Press: Oxford, 1987; Vol. 5, pp 56-60.
(38) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286-301.
3996 Inorganic Chemistry, Vol. 43, No. 13, 2004

NMR Studies of Octahedral Ni(II) Complexes
consistent with a σ delocalization mechanism. NMR char-
acterization of iron complexes of monoaryl-appended tris-
((2-pyridyl)methyl)amine ligands (6-PhTPA and (6-(4-OCH3-
identified. No evidence for PhPy dissociation in 1-4 was
found in the NMR studies outlined herein.
Summary and Perspective
1
Ph)-TPA)) have also been reported.35,40 Similar to the H
NMR spectroscopic properties of 1, 3, and 4, the six-
coordinate Fe(II) cation [(6-PhTPA)Fe(CD3CN)2]+ in CD3-
CN solution exhibits a 2:1 pattern of â-H resonances,
indicating an effective mirror plane within the cation in
solution. Mandon and co-workers have characterized the
Fe(II) complex [(6-Ph2TPA)FeCl2] in which one PhPy arm
is dissociated from the metal both in the solid state and in
CH3CN solution. In this case, two resonances at 2.1 and 1.8
ppm, respectively, were assigned to protons of the uncoor-
dinated pyridyl ring. A distinct set of resonances for the
phenyl substituent of the uncoordinated PhPy donor was not
Examples of mononuclear octahedral Ni(II) complexes for
which both the solid-state structure is known and the
paramagnetic 1H NMR resonances have been fully assigned
are rare. In this work, we have completely assigned the
proton resonances of synthetic mononuclear octahedral
nickel(II) complexes of relevance to ARD using 1D and 2D
2
1H NMR methods, as well as H NMR. Our success in this
endeavor suggests that NMR methods will be viable for
monitoring reactions of 1 and 2 with model acireductone
substrates. As the active site nickel center in ARD is ligated
by four protein-derived residues, including three histidine
donors, initial model studies involving Ni(II) complexes of
tris((2-pyridyl)methyl)amine-type ligands are appropriate.
(39) (a) Me´nage, S.; Zang, Y.; Hendrich, M. P.; Que, L., Jr. J. Am. Chem.
Soc. 1992, 114, 7786-7792. (b) Zang, Y.; Jang, H. G.; Chiou, Y.-
M.; Hendrich, M. P.; Que, L., Jr. Inorg. Chim. Acta 1993, 213, 41-
48. (c) Dong, Y.; Me´nage, S.; Brennan, B. A.; Elgren, T. E.; Jang, H.
G.; Pearce, L. L.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 1851-
1859. (d) Borovik, A. S.; Hendrich, M. P.; Holman, T. R.; Mu¨nck,
E.; Papaefthymiou, V.; Que, L., Jr. J. Am. Chem. Soc. 1990, 112,
6031-6038. (e) Zang, Y.; Que, L., Jr. Inorg. Chem. 1995, 34, 1030-
1035. (f) Kojima, T.; Leising, R. A.; Yan, S.; Que, L., Jr. J. Am. Chem.
Soc. 1993, 115, 11328-11335. (g) Jang, H. G.; Cox, D. D.; Que, L.,
Jr. J. Am. Chem. Soc. 1991, 113, 9200-9204.
Acknowledgment. This work was supported by the
Herman Frasch Foundation (501-HF02) and the National
Science Foundation (CAREER Award CHE-0094066).
Supporting Information Available: Isotropic shift vs 1/T plots
1
for 1-4 (Figures S1-S4); magnitude H COSY spectra for 1 and
3 (Figures S5 and S6); X-ray crystallographic CIF file for 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(40) (a) Lange, S. J.; Miyake, H.; Que, L., Jr. J. Am. Chem. Soc. 1999,
121, 6330-6331. (b) Mandon, D.; Nopper, A.; Litrol, T.; Goetz, S.
Inorg. Chem. 2001, 40, 4803-4806. (c) Mandon, D.; Machkour, A.;
Goetz, S.; Welter, R. Inorg. Chem. 2002, 41, 5364-5372.
IC040002A
Inorganic Chemistry, Vol. 43, No. 13, 2004 3997