Synthesis and characterization of iron(II) quinaldate complexes

Article abstract of DOI:10.1021/ic901464b

Treatment of iron(II) chloride or iron(II) bromide with 2 equiv of sodium quinaldate (qn=quinaldate or C₁₀H₆NO₂ ^-) yields the coordinatively unsaturated mononuclear iron(II) quinaldate complexes Na[Fe^II(qn)₂Cl]-DMF and Na[Fe ^II(qn)₂Br] · DMF (DMF=N,N-dimethylformamide), respectively. When a similar synthesis is carried out using iron(II) triflate, a solvent-derived linear triiron(II) complex, [Fe^II ₃(qn)₆(DNF)₂], with two five-coordinate iron(II) centers and a single six-coordinate iron(II) center Is obtained. Each of these species has been characterized using X-ray diffraction. The vibrational features of these complexes are consistent with the observed solid-state structures. Each of these compounds exhibits an iron(II)-to-quinaldate (π^*) charge-transfer band between 520 and 550 nm. These metalto-ligand charge-transfer bands are sensitive to substitution of the quinaldates as well as alteration of the first coordination sphere ligands. However, the ¹H NMR spectra of these paramagnetic high-spin iron(II) complexes are not consistent with retention of the solid-state structures in a DMF solution. The chemical shifts, longitudinal relaxation times (T ₁), relative integrations, and substitution of the quinaldate ligands provide a means to fully assign the ¹H NMR spectra of the paramagnetic materials. These spectra are consistent with coordination equilibria between five- and sixcoordinate species in a DMF solution. Electrochemical studies are reported to place these oxygen-sensitive compounds in a broader context with other iron(II) compounds. Iron complexes of bidentate quinoline-2-carboxylate-derived ligands are germane to metabolic pathways, environmental remediation, and catalytic applications.

Full text of DOI:10.1021/ic901464b

Inorg. Chem. 2010, 49, 879–887 879
DOI: 10.1021/ic901464b
Synthesis and Characterization of Iron(II) Quinaldate Complexes
Dylan T. Houghton, Nicholas W. Gydesen, Navamoney Arulsamy, and Mark P. Mehn*
Department of Chemistry, University of Wyoming, 1000 E. University Avenue, Laramie, Wyoming 82071
Received July 23, 2009
-
Treatment of iron(II) chloride or iron(II) bromide with 2 equiv of sodium quinaldate (qn=quinaldate or C10H6NO2
)
yields the coordinatively unsaturated mononuclear iron(II) quinaldate complexes Na[FeII(qn)2Cl] DMF and Na-
3
[FeII(qn)2Br] DMF (DMF=N,N-dimethylformamide), respectively. When a similar synthesis is carried out using iron(II)
3
triflate, a solvent-derived linear triiron(II) complex, [FeII (qn)6(DMF)2], with two five-coordinate iron(II) centers and a
3
single six-coordinate iron(II) center is obtained. Each of these species has been characterized using X-ray diffraction.
The vibrational features of these complexes are consistent with the observed solid-state structures. Each of these
compounds exhibits an iron(II)-to-quinaldate (π*) charge-transfer band between 520 and 550 nm. These metal-
to-ligand charge-transfer bands are sensitive to substitution of the quinaldates as well as alteration of the first
coordination sphere ligands. However, the 1H NMR spectra of these paramagnetic high-spin iron(II) complexes are not
consistent with retention of the solid-state structures in a DMF solution. The chemical shifts, longitudinal relaxation
times (T1), relative integrations, and substitution of the quinaldate ligands provide a means to fully assign the 1H NMR
spectra of the paramagnetic materials. These spectra are consistent with coordination equilibria between five- and six-
coordinate species in a DMF solution. Electrochemical studies are reported to place these oxygen-sensitive
compounds in a broader context with other iron(II) compounds. Iron complexes of bidentate quinoline-2-carbo-
xylate-derived ligands are germane to metabolic pathways, environmental remediation, and catalytic applications.
Introduction
on the determination of binding constants were carried out in
water6-8 and provide the foundation for the use of quinaldic
acid in quantitative determinations of the metal content
via gravimetric analysis.9,10 The reaction of an iron(II) salt
with either quinaldic acid or sodium quinaldate initially
yields a red complex, [Fe(qn)2], which precipitates out of
aqueous solution and slowly converts to the dark-blue-violet
[Fe(qn)2(OH2)2].11,12 Several structures of iron(II) quinaldate
water and alcohol adducts have been reported. The majority
of these structures feature a six-coordinate iron(II) geometry
with two bidentate quinaldates and two solvent molecules
occupying apical positions of a distorted octahedron.13-18
In the central nervous system, the majority of tryptophan
catabolism occurs via the kynurenine pathway.1 Several of
the quinoline- and pyridinecarboxylic acid metabolites
formed are important signaling agents in neuronal pathways,
which are vital to the onset of certain diseases. For instance,
kynurenic acid (4-hydroxyquinoline-2-carboxylic acid) plays
a role in glutamate signaling in the brain and is found at
elevated levels in people with schizophrenia.2 Increases in
iron concentrations and picolinic acid have been observed in
Alzheimer’s disease.3,4 The number of quinoline-2-carboxylic
acids found in this pathway and their involvement in brain
tumor neuropathology led us to examine the iron(II) coordi-
nation chemistry of these metabolites.
(7) Majumdar, A. K.; Bag, S. P. Anal. Chim. Acta 1960, 22, 549–553.
ꢀ
(8) Kral, M. Collect. Czech. Chem. Commun. 1972, 37, 46–51.
(9) Vogel, A. I. Textbook of Quantitative Inorganic Analysis including
Elementary Instrumental Analysis, 4th ed.; Longman: London, 1978.
(10) Kolthoff, I. M.; Sandell, E. B. Textbook of Quantitative Inorganic
Analysis, 3rd ed.; Macmillan: New York, 1952.
Since the initial recognition that quinaldic acid (quinoline-
2-carboxylic acid, Hqn) can form complexes with iron(II),5
the coordination chemistry of quinaldate with iron(II) has
received a great deal of attention. Many of the early studies
(11) Ray, P.; Bose, M. K. Z. Anal. Chem. 1933, 95, 400–415.
(12) Bielig, H.-J.; Bayer, E. Naturwissenschaften 1953, 40, 340.
(13) Okabe, N.; Makino, T. Acta Crystallogr. 1998, C54, 1279–1280.
(14) Osawa, K.; Furutachi, H.; Fujinami, S.; Suzuki, M. Acta Crystallogr.
2003, E59, m315–m316.
*To whom correspondence should be addressed. E-mail: mmehn@uwyo.edu.
(1) Stone, T. W. Pharmacol. Rev. 1993, 45, 309–379.
(2) Erhardt, S.; Schwieler, L.; Nilsson, L.; Linderholm, K.; Engberg, G.
Physiol. Behav. 2007, 92, 203–209.
(3) Guillemin, G. J.; Cullen, K. M.; Lim, C. K.; Smythe, G. A.; Garner, B.;
Kapoor, V.; Takikawa, O.; Brew, B. J. J. Neurosci. 2007, 27, 12884–12892.
(4) Testa, U.; Louache, F.; Titeux, M.; Thomopoulos, P.; Rochant, H. Br.
J. Hamaetol. 1985, 60, 491–502.
ꢀ
(15) Dobrzynska, D.; Duczmal, M.; Jerzykiewicz, L. B.; Warchulska, J.;
Drabent, K. Eur. J. Inorg. Chem. 2004, 110–117.
ꢀ
(16) Dobrzynska, D.; Jerzykiewicz, L. B. J. Chem. Crystallogr. 2004, 34,
51–55.
ꢀ
(17) Dobrzynska, D.; Jerzykiewicz, L. B.; Duczmal, M. Polyhedron 2005,
24, 407–412.
(18) Jain, S. L.; Slawin, A. M. Z.; Woollins, J. D.; Bhattacharyya, P. Eur.
J. Inorg. Chem. 2005, 721–726.
(5) Skraup, H. Monatsh. Chem. 1886, 7, 210–215.
(6) Wenger, P.-E.; Monnier, D.; Epars, L. Helv. Chim. Acta 1952, 35, 569–573.
r
2009 American Chemical Society
Published on Web 12/23/2009
pubs.acs.org/IC

880 Inorganic Chemistry, Vol. 49, No. 3, 2010
Houghton et al.
There is only one instance of a cis orientation of solvent
molecules.19 Presumably, both cis and trans geometries are
accessible in solution.
sodium dihydrogen phosphate (4.35 g) in water (43.5 mL) over
5 min. The mixture was stirred at room temperature overnight,
after which the organic solvents were removed by rotary evapo-
ration. Water was added, and the residue was extracted with
CH2Cl2, dried over MgSO4, and concentrated by rotary evapo-
ration to give the acid as a white solid (0.8143 g, 85%). 1H
(CDCl3, 400 MHz): δ (ppm) 8.39 (d, 1H), 8.31 (d, 1H), 8.20 (m,
1H), 7.64 (m, 1H), 7.58 (m, 1H).
Despite over a century of experimentation on the coordi-
nation chemistry of iron and quinaldate, questions still
remain. Reported herein are the syntheses, characterization,
and physical properties of a number of iron(II) quinaldate
complexes, with the first report of quinaldate supported,
anionic, five-coordinate complexes. The single-crystal X-ray
diffraction (XRD) patterns have been solved for the coordi-
natively unsaturated iron(II) complexes 1, 2, and 4. The elec-
tronic, IR, and 1H NMR spectra of 1-4 are presented. Our
1H NMR results demonstrate that the solid-state structures
do not adequately capture the constitution of these com-
pounds in solution. Electrochemical studies of the iron(II)
compounds are presented. These studies provide a basis for
ongoing work on the reactivity of each of the iron(II) com-
plexes with a variety of oxidants.
7-Fluoroquinoline-2-aldehyde. To a hot solution of 99% SeO2
(6.73 g, 60 mmol) in 1,4-dioxane (100 mL) was added 97% 7-
fluoro-2-methylquinoline (1.66 g, 10 mmol). The mixture was
refluxed for 3 h, filtered while hot, and concentrated by rotary
evaporation. The resultant residue was extracted with CH2Cl2
and filtered. The filtrate was washed with two portions of water,
followed by a brine wash. The organic layer was dried over
MgSO4, and the solvent was removed by rotary evaporation to
give the product as a white solid (1.375 g, 78%). 1H NMR
(CDCl3, 400 MHz): δ (ppm) 10.21(s), 8.32 (d), 8.01 (d), 7.91 (m),
7.88 (m), 7.49 (m). 13C NMR (CDCl3, 100 MHz): δ (ppm) 193.7
(s), 163.7 (d, 1JCF=252 Hz), 153.5 (s), 149.1 (d, 3JCF=12 Hz),
137.6 (s), 130.1 (d, 3JCF=10 Hz), 127.3 (s), 120.0 (d, 2JCF=26
Hz), 117.0 (s), 114.1 (d, 2JCF=20 Hz).
Experimental Section
7-Fluoroquinoline-2-carboxylic Acid [H(7-F-qn)]. To a solu-
tion of 7-fluoroquinoline-2-aldehyde (0.876 g, 5 mmol) in tert-
butyl alcohol (100 mL) and 90% 2-methylbut-2-ene (25 mL) was
added a solution of 80% sodium chlorite (4.35 g) and 99%
sodium dihydrogen phosphate (4.35 g) in water (43.5 mL) over 5
min. The mixture was stirred at room temperature overnight,
after which the organic solvents were removed by rotary eva-
poration. Water was added, and the residue was extracted with
CH2Cl2, dried over MgSO4, and concentrated by rotary eva-
Materials. All manipulations were carried out using standard
Schlenk or glovebox techniques under a dinitrogen atmosphere
unless otherwise noted. All reagents and solvents were obtained
from commercial vendors and used as received unless otherwise
noted. Tetrahydrofuran (THF), toluene, benzene, and diethyl
ether were distilled under nitrogen from sodium benzophenone
and subsequently dried over activated alumina. N,N-Dimethyl-
formamide (DMF) was stirred over CaH2 overnight, filtered,
vacuum-distilled, and stored under nitrogen. Acetonitrile was
distilled from calcium hydride under nitrogen. Nonhalogenated,
aprotic solvents were typically tested with a standard purple
solution of sodium benzophenone ketyl in THF to confirm
effective oxygen and moisture removal. All chemical reactions
were performed at high-altitude conditions (∼7200 ft or ∼2200
1
poration to give the acid as a white solid (0.7834 g, 82%). H
NMR (D2O, 400 MHz): δ (ppm) 8.60 (m, 1H). 8.04 (t, 1H), 7.99
(d, 1H), 7.74 (d, 1H), 7.54 (t, 1H).
Na[Fe(qn)2(Cl)] DMF (1). All of the halide complexes were
3
formed via similar synthetic procedures. Hqn (0.346 g, 2.00
mmol) and NaH (0.0480 g, 2.00 mmol) were added to 10 mL of
THF and allowed to stir until gas evolution ceased. FeCl2 (0.127 g,
1.00 mmol) was added to the suspension and the vial rinsed with
an additional 5 mL of THF. After stirring overnight, the
resultant purple solution was evaporated to dryness in vacuo
and the resultant solids were extracted with DMF. The purple
solution was filtered and evaporated to dryness. Purple blocks
of 1, suitable for XRD, were grown via vapor diffusion of
diethyl ether into a dilute DMF solution over 3 days (yield:
0.362 g, 68.0%). Anal. Calcd for C23H19ClFeN3NaO5: C, 51.96;
H, 3.60; N, 7.90. Found: C, 52.06; H, 3.97; N, 8.24. IR (KBr,
cm-1): 1672 [(CO)DMF], 1632 [(COO)as], 1395 [(COO)s]. ESI/
m). Fe(OTf)2 2CH3CN (OTf=-OSO2CF3) was prepared ac-
3
cording to literature precedent from TMS(OTf) and FeCl2.20,21
Caution! Although we encountered no difficulties, the perchlorate
salts of metal complexes with organic ligands are potentially
explosive and should be handled with care in small quantities.
The fluorinated quinaldic acids were prepared under aerobic
conditions following modifications of literature precedents.22,23
6-Fluoroquinoline-2-aldehyde. To a hot solution of 99% SeO2
(6.66 g, 60 mmol) in 1,4-dioxane (100 mL) was added 97%
6-fluoro-2-methylquinoline (1.66 g, 10 mmol). The mixture was
refluxed for 3 h, filtered while hot, and concentrated by rotary
evaporation. The resultant residue was extracted with hot CHCl3
and filtered. The filtrate was washed with two portions of water,
followed by a brine wash. The organic layer was dried over MgSO4,
and the solvent was removed by rotary evaporation to give the pro-
duct as a white solid (1.258 g, 72%). 1H NMR (CDCl3, 400 MHz):
δ (ppm) 10.21 (s, 1H), 8.27 (m, 2H), 8.06 (d, 1H), 7.60 (m, 1H),
7.53 (m, 1H). 13C (CDCl3, 100 MHz): δ (ppm) 193.5 (s), 162.2 (d,
MS (DMF, 200 °C): m/z 496 ([M - Cl]þ, i.e., Naþ[Fe(qn)2]
3
DMF), 423 ([M - Cl - DMF]þ, i.e., Naþ[Fe(qn)2]).
Na[Fe(4-MeO-qn)2(Cl)] DMF (4-MeO-1). H(4-MeO-qn)
3
(0.2033 g, 1.00 mmol) and NaH (0.0240 g, 1.00 mmol) were
added to 5 mL of DMF and allowed to stir until gas evolution
ceased. FeCl2 (0.0638 g, 0.50 mmol) was added to the suspension
and the vial rinsed with an additional 5 mL of DMF. After
stirring overnight, the resultant dark red/purple solution was
filtered and evaporated to dryness. The solid was dissolved in
DMF and filtered, and dark-red crystals of 4-MeO-1 were
obtained by vapor diffusion of diethyl ether into the solution
(yield: 0.249 g, 84.3%). IR (KBr, cm-1): 1653 [(COO)as], 1390
[(COO)s]. ESI/MS (DMF, 200 °C): m/z 556 ([M - Cl]þ, i.e.,
1JCF=253 Hz), 152.3 (d, 4JCF=3 Hz), 145.2 (s), 136.9 (d, 4JCF
6 Hz), 133.3 (d, JCF=10 Hz), 131.2 (d, JCF=10 Hz), 121.2
(d, 2JCF=27 Hz), 118.3 (s), 111.2 (d, 2JCF=22 Hz).
=
3
3
6-Fluoroquinoline-2-carboxylic Acid [H(6-F-qn)]. To a solu-
tion of 6-fluoroquinoline-2-aldehyde (0.876 g, 5 mmol) in tert-
butyl alcohol (100 mL) and 90% 2-methylbut-2-ene (25 mL) was
added a solution of 80% sodium chlorite (4.35 g) and 99%
Naþ[Fe(4-MeO-qn)2] DMF), 483 ([M - Cl - DMF]þ, i.e.,
3
Naþ[Fe(4-MeO-qn)2]).
Na[Fe(6-F-qn)2(Cl)] DMF (6-F-1). H(6-F-qn) (0.1914 g, 1.00
(19) Okabe, N.; Muranishi, Y. Acta Crystallogr. 2003, E59, m220–m222.
(20) Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B. Inorg. Chem. 2000, 39,
6086–6090.
3
mmol) and NaH (0.0240 g, 1.00 mmol) were added to 5 mL of
DMF and allowed to stir until gas evolution ceased. FeCl2
(0.0636 g, 0.50 mmol) was added to the suspension and the vial
rinsed with an additional 5 mL of DMF. After stirring over-
night, the resultant purple solution was filtered and evaporated
(21) Hagen, K. S. Inorg. Chem. 2000, 39, 5867–5869.
(22) Bu, X.; Deady, L. W.; Finlay, G. J.; Baguley, B. C.; Denny, W. A.
J. Med. Chem. 2001, 44, 2004–2014.
ꢀ
(23) Contour-Galcera, M.-O.; Sidbu, A.; Plas, P.; Roubert, P. Bioorg.
Med. Chem. Lett. 2005, 15, 3555–3559.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 881
to dryness. The solid was dissolved in DMF and filtered, and
purple crystals of 6-F-1 were obtained by vapor diffusion of
diethyl ether into the solution (yield: 0.258 g, 91.0%). IR (KBr,
cm-1): 1650 [br, (COO)as], 1358 [(COO)s]. ESI/MS (DMF,
Table 1. Summary of Crystallographic Data for 1, 2, and 4
1
2
4
200 °C): m/z 532 ([M - Cl]þ, i.e., Naþ[Fe(6-F-qn)2] DMF), 517
chemical formula
C
23H19Cl-
FeN3NaO5
C23H19Br-
FeN3NaO5
576.16
P2(1)/n
9.9868(2)
15.3065(3)
15.3134(3)
90
94.076(1)
90
2334.93(8)
4
1.639
C78H78-
3
Fe3N12O18
1639.07
P1
([M - Cl - DMF]þ, i.e., Naþ[Fe(6-F-qn)2]).
Na[Fe(6-F-qn)2(Cl)] DMF (7-F-1). H(6-F-qn) (0.1912 g, 1.00
fw (g/mol)
space group
531.70
P2(1)/n
9.9545(2)
15.2180(3)
15.1842(3)
90
94.6620(10)
90
2292.61(8)
4
1.540
3
˚
mmol) and NaH (0.0240 g, 1.00 mmol) were added to 5 mL of
DMF and allowed to stir until gas evolution ceased. FeCl2
(0.0637 g, 0.50 mmol) was added to the suspension and the vial
rinsed with an additional 5 mL of DMF. After stirring over-
night, the resultant purple solution was filtered and evaporated
to dryness. The solid was dissolved in DMF and filtered, and
purple crystals of 7-F-1 were obtained by vapor diffusion of
diethyl ether into the solution (yield: 0.262 g, 92.3%). IR (KBr,
cm-1): 1650 [br, (COO)as], 1360 [(COO)s]. ESI/MS (DMF,
a (A)
10.4573(2)
13.2512(3)
14.1832(3)
85.557(1)
87.301(1)
73.295(1)
1876.16(7)
1
1.451
150(2)
0.652
0.0378
0.0948
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
Dcalcd (mg/m3)
temp (K)
100(2)
0.834
0.0481
0.1127
150(2)
2.146
0.0431
0.1068
200 °C): m/z 532 ([M - Cl]þ, i.e., Naþ[Fe(7-F-qn)2] DMF),
abs coeff (mm-1
)
3
517 ([M - Cl - DMF]þ, i.e., Naþ[Fe(7-F-qn)2]).
Na[Fe(qn)2(Br)] DMF (2). 2 was prepared in a manner
R1 [I > 2σ(I)]a
wR2 [I > 2σ(I)]b
3
P
P
P
P
similar to that of 1 using FeBr2 (0.216 g, 1.00 mmol). Purple
blocks of 2, suitable for XRD, were grown via vapor diffusion of
diethyl ether into a dilute DMF solution over 3 days (yield: 0.501
g, 87%). Anal. Calcd for C23H19BrFeN3NaO5: C, 47.95; H,
3.32; N, 7.29. Found: C, 47.91; H, 3.19; N, 7.34. IR (KBr, cm-1):
1677 [(CO)DMF], 1632 [(COO)as], 1393 [(COO)s]. ESI/MS
a R1= ||Fo| - |Fc||/ |Fo|. b wR2=[ w(Fo2 - Fc2)2]/ w(Fo2)2]]1/2
where w=q/σ2(Fo2) þ (a*P)2 þ b*P.
configuration with a glassy carbon working electrode, a Ag/
AgNO3 (0.1 M in CH3CN) reference electrode, and a platinum
wire auxiliary electrode was used. The potential values were
referenced to an internal ferrocenium/ferrocene couple, which
is reported to be þ0.47 V vs SCE in 0.1 M (nBu4N)(ClO4) in
DMF.24 The peak separations are reported with a scan rate of
200 mV/s (the Fcþ/0 peak separation was 90 mV under these
conditions). The electrospray ionization mass spectral (ESI/
MS) data for the compounds were obtained using an LCQ
mass spectrometer (Finnigan MAT) on DMF solutions that
were directly infused into the spectrometer via a syringe
pump. The heated capillary was set at 200 °C.
(DMF, 200 °C): m/z 496 ([M - Br]þ, i.e., Naþ[Fe(qn)2] DMF),
3
423 ([M - Br - DMF]þ, i.e., Naþ[Fe(qn)2]).
Fe(qn)2(DMF) (3). 3 was prepared in a manner similar to that
of 1 using FeI2 (0.3097 g, 1.00 mmol). A powder of 3 was
obtained via vapor diffusion of diethyl ether into a saturated
DMF solution over 3 days (yield: 0.3607 g, 76.2%). Anal. Calcd
for C23H19FeN3O5: C, 58.37; H, 4.05; N, 8.88. Found: C, 58.56;
H, 4.57; N, 8.80. IR (KBr, cm-1): 1657 [(CO)DMF], 1612
[(COO)as], 1384 [(COO)s]. ESI/MS (DMF, 200 °C): m/z 496
([M þ Na]þ, i.e., Naþ[Fe(qn)2] DMF), 423 ([M - DMF þ
XRD Analysis. 1, 2, and 4 were characterized using XRD. A
purple diamond-shaped crystal of 1, a purple rectangular plate
of 2, and a purple rectangular plate of 4 were glued to a
MiTeGen micromount using Paratone N oil and mounted on
a Bruker Smart Apex II CCD area detector for data collection
at either 100 or 150 K using graphite-monochromated Mo KR
3
Na]þ, i.e., Naþ[Fe(qn)2]).
[Fe3(qn)6(DMF)2] 4DMF (4). Hqn (0.346 g, 2.00 mmol) and
3
NaH (0.0480 g, 2.00 mmol) were dissolved in 10 mL of THF and
allowed to stir for 2 min (before they were fully dissolved).
Fe(OTf)2 2CH3CN (0.412 g, 1.00 mmol) was added to the
3
˚
(λ=0.710 73 A) radiation. A summary of the crystallographic
solution and the vial rinsed with an additional 5 mL of THF.
After stirring overnight, the resultant deep-red solution was
filtered and evaporated to dryness in vacuo, yielding a deep-red
powder. IR (KBr, cm-1): 1669 [(COO)as], 1383 [(COO)s]. Purple
blocks of 4, suitable for XRD, were grown via vapor diffusion
of diethyl ether into a saturated DMF solution of 4 over
5 days (yield: 0.3720 g, 68.1%). Anal. Calcd for C78H78Fe3-
N12O18: C, 57.16; H, 4.80; N, 10.25. Found: C, 57.51; H, 4.86; N,
10.01. IR (KBr, cm-1): 1657 [(COO)DMF], 1612 [(COO)as], 1338
[(COO)s]. ESI/MS (DMF, 200 °C): m/z 496 ([M þ Na]þ, i.e.,
details is given in Table 1. All non-hydrogen atoms were refined
anisotropically, whereas the hydrogen atoms were placed in
ideal positions and refined as riding atoms with relative isotropic
displacement parameters. The final full-matrix least-squares
refinement converged to R1=0.0481 and wR2=0.1290 for 1,
R1=0.0431 and wR2=0.1224 for 2, and R1=0.0378 and wR2=
0.1134 for 4.
Results and Discussion
Naþ[Fe(qn)2] DMF), 423 ([M - DMF þ Na]þ, i.e., Naþ[Fe-
3
Mononuclear iron(II) quinaldate complexes were anaero-
bically synthesized by the addition of 2 equiv of sodium
quinaldate to a variety of simple iron salts in THF
(Scheme 1). Single crystals suitable for XRD analysis were
grown via the slow diffusion of diethyl ether into DMF
solutions of 1 and 2. We have only obtained needles of 3,
which proved unsuitable for XRD analysis from the iron(II)
iodide synthesis. Interestingly, when similar reaction condi-
(qn)2]).
trans-[Fe(qn)2(PrOH)2]. This compound was prepared
according to literature precedent.15
Physical Methods. Elemental analyses were performed at
Columbia Analytical Services Inc., Tuscon, AZ. 1H and 13C
NMR were collected on a Bruker Avance DRX-400 NMR
spectrometer at room temperature unless otherwise noted. For
paramagnetic samples, special care was taken to ensure that the
delay between pulses was greater than 5 times the longest proton
longitudinal relaxation time (T1) for proper integration of
peaks. UV-vis spectra were recorded on an Agilent 8453
diode-array spectrophotometer at room temperature. A Varian
800 FT-IR spectrometer with a resolution of 1 cm-1 was used to
record IR spectra of the KBr pellets of the samples. Electro-
chemical measurements were carried out in a drybox under
dinitrogen in a DMF solution with 0.1 M (nBu4N)(ClO4) as the
supporting electrolyte using a model CS-1200 computer-
controlled potentiostat (Cypress Systems). A three-electrode
tions are employed with [Fe(OTf)2] 2CH3CN, a deep-red
3
powder is obtained, which we believe to be analogous to the
previously reported [Fe(qn)2].12,17 Although the elemental
analysis is convincing,12 the ambiguous magnetic properties
and absence of XRD data of the proposed complex did not
provide conclusive evidence for this formulation.17 Unlike
the previously reported red-black species, our complex is not
(24) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.

882 Inorganic Chemistry, Vol. 49, No. 3, 2010
Houghton et al.
Scheme 1
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1, 2, and 4
4
1
2
5C-Fe1
6C-Fe2a
Fe-O1
Fe-O3
Fe-N1
Fe-N2
Fe-X
2.0413(11)
2.0705(11)
2.1895(14)
2.1890(13)
2.2788(5)
2.030(2)
2.061(2)
2.197(2)
2.198(2)
2.4309(5)
2.0079(11)
2.0166(11)
2.1704(13)
2.1803(12)
2.0459(12)
2.1541(11)
2.2550(12)
2.0738(11)
O1-Fe-X
125.08(4)
124.86(7)
119.57(5)
92.78(5),
87.22(5)
74.90(4)
O1-Fe-N1
O3-Fe-N2
O1-Fe-N2
O1-Fe-O3
O3-Fe-X
O3-Fe-N1
N1-Fe-N2
N1-Fe-X
77.67(5)
76.60(4)
86.60(5)
115.04(5)
119.77(4)
89.92(5)
152.70(5)
104.86(4)
77.74(8)
76.94(7)
87.22(8)
115.61(9)
119.45(6)
90.77(8)
154.27(8)
103.86(6)
78.84(5)
78.18(5)
97.74(5)
128.96(5)
111.40(5)
92.32(5)
164.74(5)
97.90(5)
105.10(4)
180.00(5)
92.78(5)
105.10(4)
180.0
96.06(4),
83.94(4)
N2-Fe-X
102.43(4)
0.55
101.86(6)
0.58
96.70(5)
0.60
τ25
a Comparable distances are compared although the numbering of the
individual sites differs.
Structures of Iron(II) Quinaldates. All of the previously
crystallographically characterized iron(II) quinaldate
compounds contain coordinatively saturated iron centers
and are neutral complexes. The crystal structure of 1
reveals a five-coordinate iron(II) center with two quinal-
date chelates and a single chloride, to yield the complex
anion (Figure 1, top left, and Table 2). The iron(II) center
is best described as a distorted trigonal bipyramid (τ =
0.55).25 The principal axis through the iron(II) center is
defined by the nitrogen atoms of the two quinaldates
[N1-Fe-N2=152.70(5)°], while the more tightly bound
carboxylates and the chloride define the trigonal plane.
Figure 1. ORTEP plot of the anions of 1 and 2 (top) and the triiron
complex 4 (bottom) showing 50% probability thermal ellipsoids and the
labeling scheme for selected atoms. Only half of the symmetric trimer has
been labeled. Hydrogen atoms, cocrystallized DMF, and the sodium
cation from 1 and 2 are omitted for clarity.
soluble in chloroform or dichloromethane.12,17 When
dissolved in DMF, a color change to dark purple is imme-
diately observed. Prolonged exposure of the purple solution
to vacuum yields a dark-red powder consistent with a
reversible equilibrium. Diffusion of diethyl ether into a
saturated DMF solution yields purple crystals of the triiron
complex 4.
(25) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 883
Table 3. Summary of Physical Properties and Comparisons with Literature Precedents
ν
sym(COO)
(cm-1
ν
asym(COO)
(cm-1
ν
DMF(CdO)
)
λ (nm)
E (mV)
vs Fcþ/0
color
purple
)
)
(cm-1
[ε (M-1 cm-1)]
ref
1
1395
1390
1632
1672
546 [730]
381 [307]
-334
(113)
-393 (96)
a
a
4-MeO-1
dark red
1653 (br)
530 [596]
370 [592];
345 [882]
371 [309]
368 [314]
375 [384]
368 [274]
6-F-1
7-F-1
2
purple
purple
purple
purple
dark red
red-black
purple
1358
1360
1393
1384
1383
1377
1338
1393
1361
1399
1387
1650 (br)
1650 (br)
1632
1612
1669
1652
1612
1647
1652
1619
1635
1628
1628
549 [590]
550 [592]
534 [980]
527 [775]
-288 (84)
-300 (75)
-216 (99)
-167 (75)
a
a
a
a
a
17
a
17
18
17
18
14
1677
1657
3
precursor to 4
[Fe(qn)2]
4
Na(qn)
trans-[Fe(qn)2(pyr)2]
trans-[Fe(qn)2(H2O)2]
trans-[Fe(qn)2(MeOH)2]
trans-[Fe(qn)2(EtOH)2]
trans-[Fe(qn)2(PrOH)2]
556
530 [1180]
1657
376 [373]
-170 (88)
purple
deep red
brown
red-violet
purple
539
527 [800]
528
1394
371 [326]a
-160 (83)a
[1010]a
a This work.
The iron-carboxylate distances are consistent with those
observed in other high-spin iron(II) complexes.20,26,27
Properties of Iron(II) Complexes. The IR spectra of the
iron(II) quinaldate complexes were collected as KBr
pellets (Table 3 and Figure S1 in the Supporting In-
formation). In 1, three strong features at 1672, 1632,
and 1395 cm-1 can be identified. On the basis of the
observed spectra and literature precedents,14,15,18,28,29 we
tentatively assign the band at 1672 cm-1 to cocrystallized
DMF retained in the KBr pellets, and the features at 1395
and 1632 cm-1 to symmetric and asymmetric carboxylate
stretches. As shown in Table 3, the separation between the
symmetric and asymmetric bands ranges from 220 to 291
cm-1. These values are consistent with the carboxy-
late coordination modes observed in the solid state.28
Attempts to attain vibrational information for the com-
plexes in solution have been unsuccessful because of
intense DMF solvent features. The complexes are in-
soluble in other solvents.
The iron(II) quinaldate complexes undergo no notice-
able change in color upon dissolution of the crystals in
DMF. [Fe(qn)2(Cl)]- exhibits an intense feature at 546
nm (18 300 cm-1) with a less intense band at 381 nm
(26 250 cm-1; Figure 2A and Table 3). Upon a change in
the fifth ligand to bromide (Figure 2B), a shift to higher
energy is observed for each feature, namely, 534 nm
(18 730 cm-1) and 375 nm (26 670 cm-1). Furthermore,
the substituted quinoline complexes provide additional
evidence for an iron(II)-to-quinaldate charge-transfer
assignment. While 6-F-qn and 7-F-qn substitutions per-
turb the spectrum relatively little, [Fe(4-MeO-qn)2(Cl)]-
exhibits a distinct electronic spectrum (Figure S2 in the
Supporting Information). A clear change in the position
of the charge-transfer band to 530 nm (18 900 cm-1), as
A
comparison of geometric parameters in all known iron(II)
quinaldates is provided in the Supporting Information
(Table S1). The presence of a sodium cation confirms the
binding of the quinaldates in the deprotonated state. The
crystal structure of quinaldic acid exhibits a greater dispa-
rity between the carboxylate bond distances [1.314(2) and
16
˚
1.221(2) A], which further supports the deprotonation
of the carboxylate in this compound. The five-membered
chelate rings of the iron quinaldate complexes are nearly
planar, exhibiting no torsion angles over 11°. The structure of
the bromide 2 is similar to that of the chloride (Figure 1, top
right). The longer iron-bromide distance is accompanied by
a shortening of the Fe-O bonds in the trigonal plane.
Analysis of the dark-purple crystals of 4 reveals a
symmetric triiron complex containing both five- and
six-coordinate iron(II) centers (Figure 1, bottom, and
Table 2). Two quinaldate chelates support each iron
center; however, the carboxylates from the central iron
center also contribute to the coordination sphere of the
terminal five-coordinate iron centers. The geometry of
these capping iron centers is best described as a distorted
trigonal bipyramid (τ=0.60). As in the chloride complex
1, the principal axis of the trigonal bipyramid is defined
by the two nitrogen donors [N1-Fe-N2=164.74(5)°],
while the carboxylate oxygen atoms reside in the trigonal
plane. The slightly distorted octahedral geometry of the
central iron center contains two quinaldate chelate rings
in the equatorial plane. Two DMF molecules complete
the coordination sphere of the central iron. Although the
chelate angles do not change significantly (N3-Fe-O5=
77°), the bond distances of the iron ligands appreciably
lengthen when changing from five- to six-coordinate
iron(II) centers. Furthermore, the change in the C-O
bond distances supports a fully delocalized carboxylate
for the central quinaldates (Table S1 in the Supporting
Information).
well as two high-energy features at 370 nm (27 030 cm-1
)
and 342 nm (29 240 cm-1), is observed. When the addi-
tional ligand(s) are solvent molecules, a further shift to
higher energy is seen. 3, 4, and trans-[Fe(qn)2(PrOH)2] all
exhibit bands at ∼530 nm (18 870 cm-1) and ∼372 nm
(26 881 cm-1) in DMF.
(26) Friese, S. J.; Kucera, B. E.; Young, V. G., Jr.; Que, L., Jr.; Tolman,
W. B. Inorg. Chem. 2008, 47, 1324–1331.
(27) LeCloux, D. D.; Barrios, A. M.; Mizoguchi, T. J.; Lippard, S. J.
J. Am. Chem. Soc. 1998, 120, 9001–9014.
(28) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227–250.
(29) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
ordination Compounds Part B: Applications in Coordination, Organometallic
and Bioinorganic Chemistry, 5th ed.; John Wiley & Sons, Inc.: New York, 1997.

884 Inorganic Chemistry, Vol. 49, No. 3, 2010
Houghton et al.
Figure 2. Electronic spectra of iron(II) quinaldate complexes recorded
at room temperature in DMF: (A) 1 (;); (B) 2 (- -); (C) 3 ( ); (D) 4
3
3 3 3
(- -); (E) trans-[Fe(qn)2(PrOH)2] (- -).
3 3
Figure 4. 1H NMR spectrum of (A) 1, B) 2, (C) 3, (D) trans-[Fe(qn)2-
(PrOH)2], and (E) 4 in DMF-d7 at room temperature. Spectra are
referenced to the residual protic solvent peak at 8.03 ppm. Dashed lines
identify the peaks associated with DMF resonances.
substitution, and variable-temperature studies. A rapid
inspection of the room temperature 1H NMR spectrum of
freshly dissolved crystals of 1 reveals at least 12 peaks
(Figure 3A), several more than expected considering that
the quinaldate ligands are in a symmetric environment.
Furthermore, the resonances can be grouped into two
groups of six with notable differences in their relative
intensities (1.00:0.16). This is consistent with a major
species being in equilibrium with another minor species.
The addition of [Me3PhN]Cl to samples of 1 does not
affect the relative intensities of these peaks. Increasing the
temperature results in changes in the chemical shifts of the
peaks and the relaxation times (i.e., the breadth of the
peaks) as well as a coalescence of the resonances observed
for the two species (Figures S3-S6 in the Supporting
Information). Therefore, we believe the best candidate is
an equilibrium between a five-coordinate species and a
six-coordinate [Fe(qn)2(Cl)(DMF)] species.
Figure 3. 1H NMR spectrum of (A) 1, B) 6-F-1, (C) 7-F-1, and (D)
4-MeO-1 in DMF-d7 at room temperature. Spectra are referenced to the
residual protic solvent peak at 8.03 ppm. Dashed lines identify the peaks
associated with DMF resonances.
The high extinction coefficients and observed shifts are
consistent with literature precedent of assigning these
features to metal-to-ligand charge-transfer bands.14,17
Kral reported three bands in the electronic spectrum of
trans-[Fe(qn)2(H2O)2] in an aqueous acetate buffer at 440
nm (500 M-1 cm-1, π f eg), 513 nm (500 M-1 cm-1, t2g
f
π*), and 800 nm (19 M-1 cm-1, 5T2g f 5Eg).8,30 Guided
by Kral’s assignments, we would expect a ligand-field
band at 670 nm, which we do not observe. Even when a
10 cm cell is utilized, no d-d transitions are detected in
the near-IR for any of these iron(II) quinaldate complexes
(out to 1100 nm or ∼9100 cm-1). A very weak shoulder is
observed at low energy for trans-[Fe(qn)2(PrOH)2] in
DMF.
-
-
½FeðqnÞ2ðClÞꢀ þ DMFT½FeðqnÞ2ðClÞðDMFÞꢀ
ð1Þ
The assignments for the six major peaks in 1 will be
discussed as an example of the process used for the other
complexes. In the spectrum of 1, the very broad peak
observed at -58 ppm with the very fast relaxation time
(0.8 ms) is assigned to the H8 position. This is consistent
with the average Fe---Hn distances taken from the re-
ported crystallographic data showing H8 being in closest
proximity to the iron(II) center both through space and
through bond (Scheme 2). The peak with the largest
downfield shift (71 ppm) has an integration of one proton
and a short T1 of 3.8 ms. It has been assigned to the 3
position of the quinaldate ring, which is also quite near
the metal center (Scheme 2). The substituted complexes
1
The H NMR spectra of the paramagnetic high-spin
iron(II) complexes have been analyzed in DMF-d7
(Figures 3 and 4 and Table 4). Assignments have
been made on the basis of chemical shifts, longitudinal
relaxation measurements (T1), relative integration,
(30) Independent synthesis of the iron acetate complex results in a dark-
blue solution, which by NMR and CV is a mixture of multiple species. The
presence of acetate may confound Kral’s assignments.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 885
Table 4. Summary of 1H NMR Parametersa
H3-qn δ (ppm)
71 (3.8)
69
74
75
75 (3.2)
61(2)
60 (2.9)
59 (2.4)
H4-qn δ (ppm)
H5-qn δ (ppm)
H6-qn δ (ppm)
H7-qn δ (ppm)
H8-qn δ (ppm)
1
4-MeO-1
6-F-1
7-F-1
21 (14)
4[OMe]
23
32(24)
30
30
6 (28)
14
--
6
4 (21)
4
-2 (8.3)
-58 (0.3)
-64
-56
-60
-49 (0.2)
-25 (0.2)
-23 (0.2)
-23 (0.1)
-5
-2
--
-2
0
22
30
2
33 (12)
34 (12)
34 (8.9)
34 (8.7)
33 (12)
31 (19)
31 (15)
31 (15)
3
trans-[Fe(qn)2(PrOH)2]
4
4
4 (15)
2
1 (19)
a All spectra are recorded in DMF-d7. Numbers in parentheses are relaxation times (T1) in milliseconds.
rearrangement may also result in two sets of quinaldate
signals. The relative integrations of the major and minor
peaks do not support two inequivalent quinaldates bound
to the same metal center. The concentration of the analyte
does not noticeably affect the ratio of the peaks, so we do
not favor the formation of a coordination polymer
(similar to 4) in a DMF solution.
Scheme 2. Numbering and Average XRD Fe---Hn Distances
The 1H NMR spectrum of 2 (Figure 4B) was assigned
based on its similarity to that of 1. One notable exception
is the peak at 33 ppm, which integrates for two protons.
allow us to watch for systematic absences in the 1H NMR
spectra and to unambiguously assign the remaining re-
sonances. The spectrum of 6-F-1 displays no resonance at
6 ppm (Figure 3B); therefore, we assign this peak to H6.
Similarly, 7-F-1 exhibits no peak at -2 ppm (Figure 3C).
This leads us to assign the H7 resonance to the peak at -2
ppm, consistent with its shorter T1 value. 4-MeO-1
eliminates the H4 resonance at 21 ppm in 1 with the
appearance of a new methyl group resonance (integra-
ting for three protons) at 4 ppm (Figure 3D). Therefore,
the best candidate for the H4 resonance in 1 is the peak at
21 ppm (T1=14 ms). This substitution also shifts the H6
proton resonance from 6 to 14 ppm (integrating for one
proton). The single remaining resonance of sufficient
integration is the peak at 32 ppm, which we assign to
H5. The longer T1 value of H5 relative to H4 is also
consistent with the increased through-space distance to
the iron center. It is noteworthy that neither an σ-spin nor
a π-spin delocalization pathway appears to dominate in
this aromatic quinoline ring system. The short relaxation
times of each of these resonances give insufficient time for
spin mixing before equilibrium is restored precluding the
observation of COSY cross-peaks.
With the major peaks assigned, the assignment of the
minor resonances should be addressed. On the basis of the
substituted complexes (Figure 3), we can confidently
ascribe the peaks to two sets of quinaldate rings from
two complexes in which the respective ligands are in
magnetically inequivalent environments. The 4-MeO-qn
complex shows an inversion in the intensities of the peaks
corresponding to the major and minor species (eq 1). The
electron-donating substituent presumably gives rise to a
less Lewis acidic metal center.31 Therefore, we believe the
five-coordinate geometry is the major species in [Fe-
(4-MeO-qn)2Cl]- (Figure 3D), while the six-coordinate
[Fe(qn)2(Cl)(DMF)] is the major species in solutions of 1
(Figure 3A). This is not the sole possibility for the
observed equilibria. For instance, a cis/trans isomeriza-
tion, a different coordination equilibrium, or a ligand
1
The variable-temperature H NMR spectra give rise to
two separate peaks of different widths for the H4 and H5
resonances, providing further support for the assignment
of these spectral features (Figure S5 in the Supporting
Information). On the basis of its similarity to 1 and the
variable-temperature experiments, we assign the feature
at 33 ppm in the room temperature spectrum to the H4
and H5 protons. Notably, the H6 resonance is shifted to 4
ppm and broadened in 2, making it harder to distinguish
from the solvent peak; however, a T1 of 21 ms was
measured, akin to that observed in 1. Also, the H7
resonance is greatly broadened in 2, prohibiting a T1
measurement of this peak.
The 1H NMR spectra of 3, trans-[Fe(qn)2(PrOH)2], and
the triiron complex 4 are essentially indistinguishable
(Figure 4C-E and Table 4). The spectrum of the complex
formed upon dissolving 3 in DMF exhibits four para-
magnetically shifted peaks for the quinaldate ligands,
with the remaining two very broad peaks at 4 and 2 ppm
(Figure 4C). These assignments are summarized in
Table 4.32 Interestingly, a similar set of peaks is observed
after dissolving the coordination polymer 4 in DMF.
trans-[Fe(qn)2(PrOH)2] exhibits the same paramagneti-
cally shifted peaks. Additionally, peaks for propanol are
observed in the diamagnetic region without paramagnetic
broadening. This is consistent with DMF displacing the
propanol ligands in solution. The similarity in the NMR
responses leads us to propose that the same moiety is
being formed in each solution. Furthermore, when
DMF solutions of 3, 4, and/or trans-[Fe(qn)2(PrOH)2]
1
are mixed, no spectral changes are observed in the H
NMR spectra.
On the basis of literature precedent and the coordina-
tion of the central iron center in the trimer, [Fe(qn)2-
(DMF)2] is a likely candidate for the species formed in
(32) Although Jain and co-workers18 did report NMR spectra for
[Fe(qn)2(MeOH)2] in DMF-d7, we believe that the quickly relaxing peaks
(60 and -23 ppm) were inadvertently omitted from their spectra. We do not
observe peaks at 12, 4.4, or 2.3 ppm. Furthermore, our independent synthesis
of the methanol adduct returns a purple complex, which turns brown upon
oxidation.
€
(31) Xue, G.; Fiedler, A. T.; Martinho, M.; Munck, E.; Que, L., Jr. Proc.
Natl. Acad. Sci. U.S.A. 2008, 105, 20615–20620.

886 Inorganic Chemistry, Vol. 49, No. 3, 2010
Houghton et al.
in the Supporting Information). The other iron(II)
quinaldate complexes do not exhibit this behavior. Of
the many FeII/III couples known,33,34 these quinaldate
species are most similar to FeSOD35 and iron complexes
of tetraazamacrocycles.36
Summary and Implications
The synthesis and physical properties of a series of iron(II)
quinaldates are described. The structures of three coordina-
tively unsaturated iron(II) quinaldates are reported. Yet, the
1H NMR spectra of these complexes in DMF (i.e., a
coordinating solvent) provide evidence for the formation of
six-coordinate DMF adducts in the anionic complexes.
Therefore, the results strongly suggest that the solid-state
structures do not reflect the composition of these samples in
solution. At this stage, both geometric isomers (cis and trans)
are possible, but the ligands must be in symmetric environ-
ments. On basis of a comparison with 4-MeO-1, the major
species in 1 is the six-coordinate DMF adduct. In the solvent
adducts, the major species may be either five-coordinate or
six-coordinate.
The electronics of the metal center can be tuned via
substitution of the aromatic quinaldate ring or the ancillary
ligands. The electrochemical potentials and electronic spectra
point to cursory descriptions of the electronic structure.
Assuming that the energy levels of the π* orbitals of the
quinaldate ligands are stationary, the electronic transition of
the metal-to-ligand charge-transfer bands are indicative of
the energy levels of the metal orbitals. The charge-transfer
transition for the chloride occurs at lower energy than that of
the bromide. Therefore, a lower oxidation potential of the
chloride species is expected, and one is indeed observed.
Introduction of the 4-methoxy group in the quinaldate ring
shifts the charge-transfer bands to higherenergy than those in
the unsubstituted complex 1. This indicates a widening in the
gap between the metal orbitals and the ligand π* orbitals, due
to either a lowering of the energy of the metal orbitals or an
increase in energy of the π* orbitals (or a combination of the
two). The more negative shift for the Fe2þ/3þ couple for the 4-
MeO-1 complex relative to 1 indicates that the methoxy
group causes a destabilization (increase in energy) of the
metal orbitals, consistent with a stronger donor. Similar
effects have been observed in other pyridine-rich ligands.31
The DMF adduct has a potential that is less positive than the
halides, consistent with its electronic spectrum and its in-
creased air stability.
Figure 5. Cyclic voltammograms of 1 mM 1 (;) and 2 (- -) in DMF
with 0.1 M (nBu4N)(ClO4) as the supporting electrolyte at a scan rate of
200 mV/s.
solution. DMF solutions of the solvent adducts also give
identical positive-ion ESI patterns with notable peaks at
m/z 496 (i.e., Naþ[Fe(qn)2] DMF) and 423 (i.e., Naþ[Fe-
3
(qn)2]). Despite our best efforts, we see no evidence for
[Fe(qn)2(DMF)2] in the gas phase. These results only pro-
vide constitution of the ions observed and yield no informa-
tion on the connectivity of the nuclei in the gas phase.
1
The H NMR of the solvent adducts also exhibit a
second set of quinaldate peaks with significantly lower
integration (Figure 4C,D). The intensity of these peaks is
insensitive to the precursor (i.e, 3, 4, or propanol adduct)
used to generate the sample as well as to the concentration
of the complex in DMF. As we hypothesized in the
chloride and bromide complexes, the presence of an
equilibrium in the solvent adducts cannot be ruled out.
On the basis of the data provided here, we cannot
definitively state the species involved in this equilibrium.
The electrochemical properties of the iron(II) precur-
sors have been examined in DMF (Figures 5 and S7 in the
Supporting Information and Table 3). Each of the halide
complexes exhibits a quasi-reversible one-electron oxida-
tive wave. The oxidation potentials of the various com-
plexes shift depending upon the identity of the ancillary
ligands. Examination of the chloride and bromide com-
plexes shows a positive potential shift of 118 mV for the
bromide species (i.e., the chloride is more easily oxidized).
Upon a change of the fifth ligand from a halide to a
solvent molecule(s), a further positive shift is observed.
The average of the potentials for the Fe2þ/3þ couple for
the complexes 3, 4, and trans-[Fe(qn)2(PrOH)2] is -166
mV (vs Fcþ/0), a 168 mV shift in the positive direction
from the chloride complex 1. The voltammograms of 3, 4,
and trans-[Fe(qn)2(PrOH)2] in DMF all exhibit behavior
consistent with a chemical step after an initial oxidation
(Figure S8 in the Supporting Information). This reduc-
tion peak does not grow larger with repeated electroche-
mical cycles.
Quinaldic acid has been utilized as an additive in both
Fenton and Gif hydrocarbon oxidation systems.37-40 While
the typical solvent for Fenton chemistry is water, pyridine is
characteristically used in Gif systems. Elemental iron reacts
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(39) Bianchi, D.; Bortolo, R.; Tassinari, R.; Ricci, M.; Vignola, R. Angew.
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Substitution of the quinoline ring also affects the
potential of the Fe2þ/3þ couple as it appears at -393 mV
(vs Fcþ/0) for 4-MeO-1, making it more easily oxidized by
59 mV. The introduction of the methoxy group at the 4
position of the quinoline ring introduces ligand non-
innocence within the potential range studied (Figure S7

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 887
with quinaldic acid in pyridine to yield trans-[Fe(qn)2-
(pyr)2].18 Although iron(II) salts are typically employed as
the starting materials in these oxidations, the identity of the
species responsible for hydrocarbon oxidation remains
elusive. Furthermore, the identity of the active oxidant and
the product distributions can be significantly altered by the
presence of either a weakly or strongly coordinating ligand.41
This was one of the reasons why we are interested in the
constitution of iron(II) quinaldates.
oxidant for organic substrates,56-58 metal peroxycarbonate
species have been postulated in a number of catalytic
schemes,59 green oxidations,60 and biological processes.61-64
Early proposals invoked various roles for bicarbonate at the
oxygen-evolving cluster of photosystem II;65,66 however, later
studies discount the role of bicarbonate in water oxidation.67
Our laboratory is exploring alternate routes for first-row
transition-metal peroxycarbonate generation.
In a recent report, a quinaldate-supported bis(μ-hydroxy)-
diiron(III) species reacts with hydrogen peroxide in the
presence of a base to yield a mononuclear iron(III) peroxo.42
Reaction with carbon dioxide then yields an iron(III) peroxy-
carbonate adduct, which, upon warming, reversibly cleaves
the O-O bond.42,43 Few examples of first-row transition-
metal complexes that catalyze O-O bond formation and
cleavage are known.44-47 While second- and third-row tran-
sition-metal peroxycarbonates have been known for some
time,48-52 first-row transition-metal peroxycarbonates are
far more rare.53-55 Suzuki’s peroxycarbonate is the only first-
row peroxycarbonate adduct that has been structurally
characterized. Although peroxycarbonate itself is a potent
Acknowledgment. M.P.M. thanks the University of
Wyoming for start-up funds to support this work. Finan-
cial support by the NSF (Grant CHE 0619920) for the
purchase of the Bruker Apex II diffractometer is grate-
fully acknowledged. D.T.H. was supported by an NSF-
GK-12 (Grant DGE-0538642) fellowship and a graduate
fellowship from NCRR and Wyoming INBRE (Grant
P20RR016474) during separate portions of this work. We
gratefully thank Teresa Lehmann for assistance with the
set-up of the T1 measurements.
Supporting Information Available: Table S1 with detailed
metrics for all known iron(II) quinaldates; Figure S1 with IR
data; Figure S2 with an electronic spectrum of 4-MeO-1;
Figures S3-S6 with variable-temperature 1H NMR data,
Figure S7 with CV data for substituted quinaldate complexes,
Figure S8 with CV data for 4, and crystallographic data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
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