Synthesis and structural characterization of iron(II) complexes with tris(imidazolyl)phosphane ligands: A platform for modeling the 3-histidine facial triad of nonheme iron dioxygenases

Article abstract of DOI:10.1002/ejic.201101282

Several monoiron(II) complexes containing tris(imidazolyl)phosphane (TIP) ligands have been prepared and structurally characterized by using X-ray crystallography and NMR spectroscopy. Two TIP ligands were employed: tris(2-phenylimidazol-4-yl)phosphane (4

Full text of DOI:10.1002/ejic.201101282

FULL PAPER
DOI: 10.1002/ejic.201101282
Synthesis and Structural Characterization of Iron(II) Complexes with
Tris(imidazolyl)phosphane Ligands: A Platform for Modeling the 3-Histidine
Facial Triad of Nonheme Iron Dioxygenases
Michael M. Bittner,[a] Jacob S. Baus,[a] Sergey V. Lindeman,[a] and Adam T. Fiedler*[a]
Keywords: Bioinorganic chemistry / Iron / Enzyme models / Tripodal ligands / Phosphane ligands
Several monoiron(II) complexes containing tris(imidazolyl)-
phosphane (TIP) ligands have been prepared and structur-
ally characterized by using X-ray crystallography and NMR
spectroscopy. Two TIP ligands were employed: tris(2-phenyl-
imidazol-4-yl)phosphane (4-TIPPh) and tris(4,5-diphenyl-1-
methylimidazol-2-yl)phosphane (2-TIPPh2). These tridentate
ligands resemble the 3-histidine (3His) facial triad found re-
cently in the active sites of certain nonheme iron dioxygen-
ases. Three of the reported complexes are designed to serve
as convenient precursors to species that model the enzyme–
substrate intermediates of 3His dioxygenases; thus, each
contains an [Fe(κ3-TIP)]2+ unit in which the remaining coor-
dination sites are occupied by easily displaced ligands, such
as solvent molecules and/or carboxylate groups. The viability
of these complexes as precursors was demonstrated through
the synthesis of TIP-based complexes with β-diketonate and
salicylate ligands that represent faithful models of β-diketone
dioxygenase and salicylate 1,2-dioxygenase, respectively.
GDO is very similar to those catalyzed by the extradiol cat-
echol dioxygenases and likely follows a similar mechanism,
SDO is unique in performing the oxidative cleavage of an
aromatic ring with only one electron-donating group.
Introduction
Mononuclear nonheme iron dioxygenases play a central
role in the oxidative catabolism of a wide range of biomole-
cules and pollutants.[1] Members of this enzyme family in-
clude the extradiol catechol dioxygenases,[2] Rieske dioxy-
genases,[3] homogentisate dioxygenase,[4] and (chloro)hydro-
quinone dioxygenases.[5] These enzymes feature a common
active-site motif in which the ferrous center is facially lig-
ated by one aspartate (or glutamate) and two histidine resi-
dues [the so-called 2-His-1-carboxylate (2H1C) facial
triad].[6] However, recent structural studies have shown that
the Asp/Glu ligand in some monoiron dioxygenases is re-
placed with His, resulting in the 3His facial triad.[7] Mem-
bers of this “3His family” catalyze novel transformations
that have expanded the known boundaries of Fe dioxygen-
ase chemistry. For example, cysteine dioxygenase (CDO)[8]
–
the first 3His enzyme to be structurally characterized – cat-
alyzes the initial step in l-cysteine catabolism by converting
the thiol into a sulfinic acid (Scheme 1), while β-diketone
dioxygenase (Dke1) oxidizes acetylacetone to acetic acid
and 2-oxopropanal.[9] Other 3His Fe dioxygenases include
gentisate 1,2-dioxygenase (GDO)[10] and salicylate 1,2-di-
oxygenase (SDO),[11] both of which oxidatively cleave aro-
matic C–C bonds (Scheme 1). Each of these microbial en-
zymes participates in the degradation pathways of polycy-
clic aromatic hydrocarbons. While the reaction catalyzed by
Scheme 1.
[a] Department of Chemistry, Marquette University,
P. O. Box 1881, 535 N. 14th St. Milwaukee, WI 53233, USA
E-mail: adam.fiedler@marquette.edu
Our knowledge of nonheme Fe dioxygenases has greatly
benefitted from the development of small-molecule ana-
logues that replicate important structural, spectroscopic,
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101282.
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Modeling the 3-Histidine Facial Triad of Nonheme Fe Dioxygenases
and/or functional properties of the enzyme active sites.[12] of the homoleptic [Fe(TIP)2]2+ complexes. Recently, we de-
The 2H1C triad has been suitably modeled with anionic, scribed the synthesis and structural characterization of a
tridentate supporting ligands such as tris(pyrazol-1-yl)bor- series of [Fe2+(4-TIPPh)(acacX)]OTf complexes (acacX
=
ates (Tp),[13] bis(pyrazolyl)acetates,[14] and bis(1-alkylimid- substituted β-diketonate; OTf = triflate) that serve as mod-
azol-2-yl)propionates.[15] The last two ligand sets replicate els of the Dke1 enzyme–substrate complex.[19] These models
the mixed N2O donor set of the 2H1C triad by the inclusion were prepared by directly mixing one equivalent of the so-
of carboxylate arms. Given the unique and significant reac- dium salt of the appropriate β-diketone, Na(acacX), with
tions catalyzed by the 3His family of Fe dioxygenases, it is equimolar amounts of Fe(OTf)2 and 4-TIPPh in MeOH.
important to develop supporting ligands with specific rele- This “one-pot” approach, however, is not successful for
vance to the 3His facial triad. To this end, we have sought various combinations of supporting and “substrate” li-
to exploit the tris(imidazol-2-yl)phosphane (2-TIPR2) and gands. Thus, as described in this article, we have generated
tris(imidazol-4-yl)phosphane (4-TIPR) frameworks shown several Fe2+ complexes with κ3-TIP ligands that also con-
in Scheme 2, which accurately mimic the charge and donor tain displaceable ligands (such as solvent, triflate, benzoate,
strength of the 3His coordination environment. These li- and acetate) bound to the opposite face of the octahedron.
gands were initially generated to model the 3His ligand sets These complexes resemble the resting states of 3His Fe di-
found in the active sites of carbonic anhydrase (Zn2+) and oxygenases, which feature two or three cis-labile H2O mole-
cytochrome c oxidase (Cu2+).[16] To date, the application cules.[20] In addition, it is shown that these TIP-based com-
of the TIP framework to Fe systems has been limited to plexes serve as excellent precursors for the formation of
homoleptic [Fe(TIP)2]2+/3+ complexes[17] and carboxylate- monoiron complexes with three facial imidazole donors and
bridged diiron(III) species.[17c,18]
various bound substrates, including β-diketonates and sali-
cylates (mimics of Dke1 and SDO, respectively). Thus, the
chemistry described here establishes a valuable platform for
future synthetic modeling studies of nonheme Fe dioxygen-
ases with the 3His facial triad.
Results and Discussion
Fe2+ Complexes Containing 2-TIPPh2
The novel 2-TIPPh2 ligand was synthesized by means of
lithiation of 4,5-diphenyl-1-methylimidazole at the 2-posi-
Scheme 2.
A key advantage of the 2-TIPR2 and 4-TIPR ligands is tion at –78 °C, followed by addition of PCl3 (0.33 equiv.).
that their steric properties can be easily modified by altering Reaction of 2-TIPPh2 with Fe(OTf)2 in MeCN provided the
the R substituent(s). Thus far, we have primarily employed complex [1](OTf)2 in 60% yield (Scheme 3). Crystals suit-
the 2-TIPPh2 and 4-TIPPh ligands, as the steric bulk of the able for X-ray diffraction (XRD) analysis were obtained by
phenyl rings discourages both dimerization and formation layering a concentrated MeCN solution with diethyl ether.
Scheme 3.
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M. M. Bittner, J. S. Baus, S. V. Lindeman, A. T. Fiedler
Table 1. Selected metric parameters for [Fe2+(LN3)(MeCN)3]2+ complexes. Bond lengths in Å and angles in degrees.
FULL PAPER
[1](OTf)2·0.5Et2O[a]
[Fe(trisoxtBu)(MeCN)3]2+
(ref.[22] [b]
[Fe(tpmPh2)(MeCN)3]2+
(ref.[21] [c]
)
)
Fe1–N2
Fe1–N4
Fe1–N6
Fe1–N7
Fe1–N8
Fe1–N9
Fe–NTIP (av.)
Fe–Nsolv (av.)
2.186(1)
2.177(1)
2.182(1)
2.196(1)
2.179(1)
2.205(1)
2.181
2.257(2)
2.205(2)
2.215(2)
2.163(2)
2.131(2)
2.171(3)
2.226
2.199(2)
2.196(2)
2.205(3)
2.131(3)
2.166(2)
2.156(3)
2.200
2.193
2.155
2.151
N2–Fe1–N4
N2–Fe1–N6
N4–Fe1–N6
N7–Fe1–N8
N7–Fe1–N9
N8–Fe1–N9
88.87(5)
91.43(5)
88.32(5)
85.48(5)
82.06(5)
83.23(5)
84.62(6)
82.12(6)
86.89(7)
90.25(8)
91.33(8)
86.00(8)
84.40(9)
85.84(9)
83.27(9)
87.6(1)
86.4(1)
90.3(1)
[a] Average values for the two independent, but chemically equivalent [1]+ cations. [b] trisoxtBu = 1,1,1-tris(4-tert-butyloxazolin-2-yl)-
ethane. [c] tpmPh2 = tris(3,5-diphenylpyrazol-1-yl)methane.
The structure features two symmetrically independent [1]2+ tances of 2.20 and 2.23 Å, respectively. Conversely, the
units with nearly identical metric parameters (Table 1; de- average Fe–NMeCN distance in [1]2+ is approximately 0.04 Å
tails concerning the data collection and analysis of all X- longer than those reported for the tpmPh2 and trisoxtBu
ray structures are summarized in Table 4). As shown in Fig- complexes. Both facts suggest that 2-TIPPh2 is a somewhat
ure 1, the six-coordinate (6C) Fe2+ center is ligated by 2- stronger donor than other neutral N3 ligands that have ap-
TIPPh2 and three MeCN ligands in a distorted octahedral peared in the literature.
geometry. As expected, the 2-TIPPh2 ligand coordinates in
a facial manner. The average Fe–N distance of 2.19 Å is
indicative of a high-spin Fe2+ center (S = 2), consistent with
the measured magnetic moment of 5.2 μB. The triflate
counteranions are not bound to the metal centers, and the
asymmetric unit also contains one equivalent of noncoordi-
nated Et2O.
Elemental analysis performed with ground and dried
crystals of [1](OTf)2 indicate that at least two MeCN li-
gands are removed under vacuum. In addition, evidence for
Fe–OTf bonding in non-coordinating solvents was obtained
by using 19F NMR spectroscopy. For [1](OTf)2 in CD3CN,
the triflate counteranion gives rise to a sharp peak at δ =
–79.2 ppm, which is identical to the chemical shift observed
for [NBu4]OTf under the same conditions. The lengthy lon-
gitudinal relaxation time (T1 value) of 128 ms measured for
this feature suggests that the triflate counteranion is only
weakly associated with the [Fe(2-TIPPh2)]2+ unit in MeCN.
In contrast, the 19F NMR spectrum of [1](OTf)2 in CD2Cl2
exhibits a broad feature at δ = –60.9 ppm with a short T1
value of 14 ms (Figure S1 in the Supporting Information),
which indicates that the triflate ion is directly bound to the
Fe center.
Reaction of equimolar amounts of Fe(OTf)2, 2-TIPPh2
,
and sodium benzoate (NaOBz) in MeOH provided the col-
orless complex [Fe(2-TIPPh2)(OBz)(MeOH)]OTf ([2]OTf),
as shown in Scheme 3. X-ray-quality crystals were obtained
from a solution of [2]OTf in MeOH layered with pentane.
The resulting structure reveals a pentacoordinate (5C)
iron(II) center with a κ3-2-TIPPh2 ligand, monodentate
benzoate ligand, and bound solvent (Figure 2). In addition
to the second-sphere triflate anion, the asymmetric unit also
Figure 1. Thermal ellipsoid plot (50% probability) of [1](OTf)2·
0.5Et2O. Only one of the symmetrically inequivalent [1](OTf)2 units
is shown. Hydrogen atoms, counteranions, and noncoordinating
solvent molecules have been omitted for clarity.
Two related high-spin Fe2+ structures with [Fe(LN3)- contains four MeOH molecules that do not directly interact
(MeCN)3]2+ compositions have been reported in the litera- with the [2]+ cation. The complex adopts a distorted
ture, and their metric parameters are also provided in square-pyramidal geometry (τ = 0.25[23]) with an O2N2
Table 1. The average Fe–NTIP distance of 2.18 Å in [1]2+ is pseudobasal plane. Two phenyl rings of the 2-TIPPh2 ligand
significantly shorter than the distances observed for lie across the vacant coordination site (i.e., parallel to the
the
(tpmPh2 [21]
(trisoxtBu [22]
analogous
)
)
tris(3,5-diphenylpyrazol-1-yl)methane plane of the benzoate ligand), which prevents further sol-
and 1,1,1-tris(4-tert-butyloxazolin-2-yl)ethane vent binding. The Fe–NTIP and Fe–O distances are typical
complexes, which display average Fe–N dis- for high-spin Fe2+ centers (Table 2). The H atom of the co-
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Modeling the 3-Histidine Facial Triad of Nonheme Fe Dioxygenases
ordinated MeOH molecule was found objectively and re- with a monodentate benzoate ion linked to a neutral ligand
fined. The resulting O2···O3 distance of 2.610(2) Å and by means of an intramolecular hydrogen bond. As shown
H3···O2 distance of 1.81(1) Å are indicative of an intramo- in Table 2, the metric parameters of [2]+ and 3 are quite
lecular hydrogen bond that closes a six-membered ring.
similar; indeed, the average Fe–NTIP distance of 2.16 Å
found for [2]+ is only 0.01 Å longer than the average Fe–
N
Tp distance in 3. This result is consistent with our previous
study of [Fe2+(LN3)(β-diketonato)]+/0 complexes that found
only slight differences (on average) between Fe–NTIP and
Fe–NTp bond lengths in 5C species, despite the different
charges of the supporting ligands.[19]
Starting from either of these two Fe(2-TIPPh2) precur-
sors – [1](OTf)2 or [2]OTf – we were able to generate the
complex [Fe(2-TIPPh2)(acacPhF3)]OTf ([4]OTf; Scheme 3; in
which acacPhF3 = anion of 4,4,4-trifluoro-1-phenyl-1,3-but-
anedione). The acacPhF3 ligand was selected for two
reasons: (i) it is a viable Dke1 substrate,[25] and (ii) previous
studies in our laboratory found that [Fe(LN3)(acacPhF3)]+/0
complexes exhibit intense Fe2+ ǞacacPhF3 MLCT bands
that serve as useful spectroscopic markers.[19] For both
Fe(2-TIPPh2) precursors, reaction with Na(acacPhF3) pro-
vides a deep purple solution that displays an absorption
manifold centered at 502 nm (ε = 700 m–1 cm–1; see Figure
S2 in the Supporting Information). Not surprisingly, the
[4]OTf spectrum closely resembles the one published for
[Fe(4-TIPPh)(acacPhF3)]OTf, although the absorption fea-
Figure 2. Thermal ellipsoid plot (50% probability) derived from
[2]OTf·4MeOH. Non-coordinating solvent molecules,
counteranions, and most H atoms have been omitted for clarity. tures are blueshifted in the former by approximately
400 cm–1.[19]
The dotted line indicates the hydrogen-bonding interaction between
H3 of the MeOH ligand and O2 of the benzoate anion.
Crystals of [4]OTf were obtained from the reaction of
[1](OTf)2 and Na(acacPhF3
) in CH2Cl2, followed by
Table 2. Selected metric parameters for ferrous carboxylate com-
plexes [2]OTf·4MeOH, 3·2CH2Cl2, and [5]BPh4·3MeOH. Bond
lengths in Å and angles in degrees.
crystallization in CH2Cl2/pentane. The asymmetric unit
contains two independent units with virtually identical
structures. As shown in Figure 3, the 5C Fe2+ center is co-
ordinated to the 2-TIPPh and acacPhF3 ligands in a distorted
[a]
[2]OTf·4MeOH
3·2CH2Cl2
[5]BPh4·3MeOH
Fe1–N2
Fe1–N4
Fe1–N6
Fe–NLN3 (av.)
Fe1–O1
2.124(2)
2.127(2)
2.226(2)
2.158
2.129(3)
2.111(3)
2.210(3)
2.150
2.193(4)
2.195(4)
2.186(4)
2.191
2.245(4)
2.256(4)
2.077(4)
1.267(6)
1.268(6)
2.011(1)
1.977(3)
Fe1–O2
Fe1–O3(N7)
O1–Ccarboxyl
O2–Ccarboxyl
2.105(1)
1.273(2)
1.254(2)
2.144(3)
1.271(5)
1.244(4)
N2–Fe1–N4
N2–Fe1–N6
N4–Fe1–N6
O1–Fe1–N2
O1–Fe1–N4
O1–Fe1–N6
O1–Fe1–O3(N7) 87.64(6)
N2–Fe1–O3(N7) 93.34(6)
N4–Fe1–O3(N7) 100.01(6)
N6–Fe1–O3(N7) 168.29(6)
93.94(6)
85.36(6)
91.69(6)
153.44(6)
112.06(6)
88.38(6)
94.3(1)
80.7(1)
87.4(1)
152.4(1)
110.9(1)
89.2(1)
98.7(1)
89.0(1)
96.6(1)
169.2(1)
0.28
90.2(2)
90.3(2)
90.6(2)
105.9(2)
91.1(2)
163.7(2)
84.6(2)
93.9(2)
174.8(2)
92.6(2)
N/A
τ value
0.25
Figure 3. Thermal ellipsoid plot (50% probability) derived from
[a] Data from the literature.[24] The N and O atoms in the
3·2CH2Cl2 structure were renumbered to correspond to the num-
bering scheme used for the other complexes.
[4]OTf·2CH2Cl2.
Non-coordinating
solvent
molecules,
counteranions, and most H atoms have been omitted for clarity.
Only one of the two independent [5]+ units is shown. Selected bond
lengths [Å] and angles [°] for this unit: Fe1–O1 2.089(3), Fe1–O2
1.973(3), Fe1–N2 2.118(4), Fe1–N4 2.190(4), Fe1–N6 2.118(4); O1–
Fe1–O2 87.2(1), O1–Fe1–N2 91.0(2), O1–Fe1–N4 176.4(2), O1–
Fe1–N6 89.5(2), O2–Fe1–N2 120.4(2), O2–Fe1–N4 96.5(2), O2–
Complex [2]+ resembles the structure of [Fe(Ph,MeTp)-
(OBz)(Ph,Mepyz)] (3; in which Ph,Mepyz = 3-phenyl-5-methyl-
pyrazole) published by Fujisawa and co-workers.[24] Both
complexes feature a distorted square-pyramidal geometry Fe1–N6 146.3(2).
Eur. J. Inorg. Chem. 2012, 1848–1856
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M. M. Bittner, J. S. Baus, S. V. Lindeman, A. T. Fiedler
FULL PAPER
trigonal-bipyramidal geometry (τ = 0.51) with the O atom
proximal to the CF3 group (O1) in the axial position. The
metric parameters of [4]OTf are not significantly different
from those reported previously for [Fe2+ Ph2Tp)(acacPhF3)]
(
and [Fe2+(4-TIPPh)(acacPhF3)]OTf.[19]
The solution structures of [2]OTf and [4]OTf in CD2Cl2
were probed using 1H NMR spectroscopy, and the observed
chemical shifts, peak integrations, and T1 values are sum-
marized in Table 3. The three imidazole ligands are spectro-
scopically equivalent in solution due to dynamic averaging
of the ligand positions on the NMR spectroscopic time-
scale. The 2-TIPPh2-derived resonances were assigned with
the help of peak integrations and by making two assump-
tions: (i) T1 values follow the order ortho Ͻ meta Ͻ para
for each phenyl ring,[13c,19] and (ii) T1 values of the 4-Ph
protons are shorter than the corresponding protons on the
5-Ph ring. Thus, the fast-relaxing peaks (T1 ≈ 1 ms) near
–20 ppm were attributed to the ortho protons of the 4-
phenyl 2-TIPPh2 substituents, which are positioned near the
Fe2+ center. The peaks with the largest integration at
(21Ϯ1) ppm were assigned to the 1-N-Me protons. The re-
maining resonances were then identified as the benzoate
and acacPhF3 groups of [2]OTf and [4]OTf, respectively, by
using the relative T1 values to assign the phenyl resonances
of both ligands.
Scheme 4.
X-ray-quality crystals of [5]BPh4 were prepared by slowly
cooling a solution of [5]BPh4 in MeOH; the [5]+ cation is
shown in Figure 4 and the corresponding bond lengths and
angles are provided in Table 2. The high-spin Fe2+ center is
hexacoordinate with a facially coordinating 4-TIPPh ligand.
The Fe–NTIP distances in [5]+ are quite similar to those
found for [1]2+ and [2]+, which suggests that the 4-TIPPh
and 2-TIPPh2 ligands possess comparable donor properties.
The κ2-acetate ligand coordinates in a symmetric manner
with nearly identical Fe–Oacetate distances of 2.251(6) Å.
Table 3. Summary of 1H NMR spectroscopic parameters for [2] The remaining site is occupied by a solvent molecule trans
OTf and [4]OTf in CD2Cl2.
to N4 with a relatively short Fe–OMeOH distance of
2.077(4) Å. The crystal structure of [5]BPh4·3MeOH also
features an extensive hydrogen-bonding network. As shown
in Figure 4, the coordinated acetate and MeOH moieties
participate in hydrogen-bonding interactions with three
MeOH “chaperones” that comprise a second-sphere shell
surrounding one face of the [5]+ octahedron. In addition,
the MeOH molecules that serve as hydrogen-bond donors
to the acetate ligand also act as hydrogen-bond acceptors
for two H–Nimidazole groups on adjacent [5]+ cations.
[2]OTf
[4]OTf
Resonance δ [ppm] T1 [ms] Resonance δ [ppm] T1 [ms]
o-4-Ph
m-4-Ph
p-4-Ph
–21.0
6.7
9.0
1.1
12.0
31.6
o-4-Ph
m-4-Ph
p-4-Ph
–16.0
5.2
9.3
0.4
4.7
13.3
o-5-Ph
m-5-Ph
p-5-Ph
2.6
6.3
5.2
31.5
159
238
o-5-Ph
m-5-Ph
p-5-Ph
2.4
6.1
5.0
13.7
88.2
120
N-1-Me
21.9
15.7
N-1-Me
20.2
22.6
5.9
o-OBz
m-OBz
p-OBz
34.8
19.0
10.6
3.2
37.0
67.6
acac o-Ph
1.7
acac m-Ph 9.5
20.0
45.5
0.8
acac p-Ph
acac H
17.3
39.4
Fe2+ Complexes Containing 4-TIPPh
The complex [Fe(4-TIPPh)(OAc)(MeOH)]BPh4 ([5]BPh4)
was generated by addition of NaBPh4 to a solution of
Fe(OAc)2 and 4-TIPPh in MeOH, which resulted in the im-
mediate formation of a white precipitate (Scheme 4). The
IR spectrum of the isolated solid revealed a peak at
3259 cm–1 from the ν(N–H) stretch of the 4-TIPPh ligands,
along with acetate-derived features at 1562 and 1402 cm–1.
Figure 4. Thermal ellipsoid plot (50% probability) derived from
[5]BPh4·3MeOH. The BPh4 counteranion and most H atoms have
been omitted for clarity. The dotted lines signify the hydrogen-
bonding interactions between the coordinated acetate and MeOH
ligands and three second-sphere solvent molecules. Note: Ellipsoids
are not shown for the proximal 2-Ph substituent due to disorder.
1
The 4-TIPPh-derived resonances in the H NMR spectrum
largely followed the pattern reported previously for [Fe2+(4-
TIPPh)(acacX)]+ complexes.[19] The acetate ligand of [5]BPh4
exhibited a downfield signal at δ = +105 ppm.
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Modeling the 3-Histidine Facial Triad of Nonheme Fe Dioxygenases
Significantly, we found that [5]BPh4 provides access to O2–C28 is unexpectedly longer than O1–C28 [1.273(2) vs.
iron(II) salicylate (sal) species that mimic the enzyme–sub- 1.257(2), respectively], which indicates that the negative
strate complex of SDO. The complex [Fe(4-TIPPh)(sal)] (6) charge is delocalized over the carboxylate moiety. The crys-
was prepared by mixing [5]BPh4 with salicylic acid tal also contains noncoordinating MeCN and MeOH mole-
(1 equiv.) in MeOH, followed by layering with MeCN cules (one of each); the latter serves as a hydrogen-bond
(Scheme 4). As shown in Figure 5, the X-ray crystal struc- donor to the phenolate oxygen atom (O3) of the salicylate,
ture of 6 reveals a neutral 5C Fe2+ complex with a geometry while acting as a hydrogen-bond acceptor to an imidazole
between square pyramidal and trigonal bipyramidal (τ = H–N group. Thus, in this structure, MeOH behaves in a
0.35). The dianionic salicylate ligand coordinates in a bi- manner similar to second-sphere residues in dioxygenase
dentate fashion with Fe–O bond lengths of 1.958(1) and active sites, which often play a crucial role in stabilizing
2.060(1) Å for the phenolate and carboxylate donors, metal-bound substrates through noncovalent interac-
respectively. To the best of our knowledge, 6 represents the tions.[27]
first structurally characterized iron(II) salicylate complex in
the chemical literature.[26]
Conclusion
This paper has described the synthesis and X-ray struc-
tural characterization of iron(II) complexes supported by
tris(imidazolyl)phosphane ligands (2-TIPPh2 and 4-TIPPh).
Three of the complexes – [1](OTf)2, [2]OTf, and [5]BPh4 –
feature easily displaced ligands, such as solvent molecules
and/or carboxylates, in the coordination sites trans to the
TIP chelate. These complexes exhibit variability in their co-
ordination numbers (5C or 6C) and carboxylate binding
modes (κ1 or κ2). Intra- and intermolecular hydrogen-bond-
ing interactions between the ligands and solvent are evident
in the solid-state structures of each complex {with the ex-
ception of [1](OTf)2} In particular, the presence of unpro-
tected imidazole groups in [5]BPh4 gives rise to an extensive
Figure 5. Thermal ellipsoid plot (50% probability) derived from
hydrogen-bonding network in which second-sphere MeOH
6·MeOH·MeCN. The noncoordinating MeCN and most H atoms
molecules form bridges between acetate ligands and
have been omitted for clarity. The dotted line represents the hydro-
H–Nimid groups from neighboring [5]+ units.
gen-bonding interaction between the salicylate ligand and MeOH.
Like the resting states of the enzymatic active sites, these
“precursor” complexes are intended to serve as scaffolds
that permit various substrate ligands to coordinate to the
iron(II) center. The versatility of this approach was demon-
strated by the formation of the Dke1 model [4]OTf from
the reaction of Na(acacPhF3) with the 2-TIPPh2-based com-
plexes [1](OTf)2 and [2]OTf. Similarly, the SDO model 6
was generated through the direct reaction of [5]BPh4 with
salicylic acid. The facile formation of [4]OTf and 6 indicates
that the TIP ligands are resistant to displacement by strong,
anionic ligands. This is significant because half-sandwich
ferrous complexes with neutral LN3 ligands, such as trispyr-
azolylmethanes, have been shown to suffer from high la-
bility and a tendency to decompose to the more stable bis-
ligand species.[21] The relatively short Fe–NTIP bond lengths
found in our series of complexes suggest that the TIP li-
gands bind tightly to the iron centers. Thus, the precursor
complexes described here provide a robust platform for the
development of synthetic models of dioxygenases with the
3His facial triad.
Selected bond lengths [Å] and angles [°]: Fe1–O1 2.060(1), Fe1–O3
1.958(1), Fe1–N2 2.135(1), Fe1–N4 2.150(1), Fe1–N6 2.183(1), O1–
C28 1.257(2), O2–C28 1.273(2), O3–C30 1.327(2), O3···O1S
2.725(1); O1–Fe1–O3 86.29(4), O1–Fe1–N2 92.17, O1–Fe1–N4
96.90(4), O1–Fe1–N6 168.66(4), O3–Fe1–N2 147.75(4), O3–Fe1–
N4 117.46(4), O3–Fe1–N6 91.71(4).
As with [5]BPh4·3MeOH, the lattice of 6 exhibits numer-
ous hydrogen-bonding interactions (see Scheme 5). The un-
coordinated oxygen atom of the carboxylate (O2) forms hy-
drogen bonds with two H–N groups belonging to adjacent
4-TIPPh ligands. These interactions account for the fact that
Experimental Section
General Procedures: All reagents and solvents were purchased from
commercial sources and used as received unless otherwise noted.
MeCN and CH2Cl2 were purified and dried using a Vacuum Atmo-
Scheme 5. Hydrogen-bonding network in the solid-state structure
of 6.
Eur. J. Inorg. Chem. 2012, 1848–1856
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1853

M. M. Bittner, J. S. Baus, S. V. Lindeman, A. T. Fiedler
FULL PAPER
spheres solvent purification system. The compounds 4,5-diphenyl-
1-methylimidazole[28] and 4-TIPPh[16h] were prepared according to
literature procedures. The synthesis and handling of air-sensitive
materials were carried out under an inert atmosphere using a Vac-
uum Atmospheres Omni-Lab glovebox equipped with a freezer set
to –30 °C. Elemental analyses were performed at Midwest Micro-
lab, LLC in Indianapolis, IN. Infrared (IR) spectra of solid samples
were measured with a Thermo Scientific Nicolet iS5 FTIR spec-
trometer equipped with the iD3 attenuated total reflectance access-
ory. UV/Vis spectra were obtained with an Agilent 8453 diode ar-
ray spectrometer. NMR spectra were recorded on a Varian
400 MHz spectrometer. 19F NMR spectra were referenced using
the benzotrifluoride peak at –63.7 ppm. 31P NMR spectra were ref-
erenced to external H3PO4 (δ = 0 ppm). Magnetic susceptibility
measurements were carried out using the Evans NMR method.
(32 mg, 0.59 mmol) in THF was stirred for 30 min, after which the
solvent was removed under vacuum to give white Na(acacPhF3).
Na(acacPhF3) was then dissolved in CH3CN (5 mL) and slowly
added to a solution of [1](OTf)2 (704 mg, 0.583 mmol) in CH2Cl2
(5 mL). The purple solution was stirred overnight and the solvent
was removed under vacuum. The residue was dissolved in CH2Cl2
(5 mL), filtered, and layered with pentane to yield deep red crystals
suitable for X-ray crystallography (457 mg); yield 68%. The X-ray
structure revealed uncoordinated CH2Cl2 molecules in the asym-
metric units, and elemental analysis suggests that a small amount
of solvent (≈0.7 equiv.) remains after vacuum drying.
C59H45F6FeN6O5PS·0.7CH2Cl2 (1210.36): calcd. C 59.24, H 3.86,
N 6.94; found C 59.25, H 3.99, N 6.75. UV/Vis (MeCN): λmax (ε,
m
–1 cm–1) = 519 (720), 494 (730) nm. IR (neat): ν = 3058, 2955,
˜
1602 [ν(C=O)], 1572, 1462, 1443, 1253, 1141, 1029, 981 cm–1. 19F
NMR (376 MHz, CD2Cl2): δ = –44.9 (acacPhF3), –77.7 (OTf) ppm.
2-TIPPh2: 4,5-Diphenyl-1-methylimidazole (6.81 g, 29.1 mmol) was
dissolved in THF (175 mL) and the solution was purged with argon
for 25 min. The flask was cooled to –78 °C and nBuLi (32.0 mmol)
was added dropwise. The solution was stirred for 30 min at –78 °C
and then for 30 min at room temperature. The reaction was cooled
again to –78 °C and PCl3 (0.850 mL, 9.74 mmol) was added slowly.
The mixture was allowed to slowly warm to room temp. over the
course of several hours, and then 30% NH4OH (75 mL) was added
and stirred for 1 h. The layers were separated and the aqueous layer
was extracted with THF (2ϫ35 mL). The combined THF layers
were washed with H2O and brine (50 mL each), dried with MgSO4,
and the solvent was removed under vacuum. The orange residue
was triturated with pentane and washed with methanol, thereby
providing a fine white powder (1.66 g); yield 24%. C48H39N6P
(730.8): calcd. C 78.88, H 5.38, N 11.50; found C 78.05, H 5.83, N
11.03. The disagreement indicates that small amounts of impurities
[Fe(4-TIPPh)(OAc)(MeOH)]BPh4 ([5]BPh4): Fe(OAc)2 (488 mg,
2.81 mmol) and 4-TIPPh (1.28 g, 2.79 mmol) were stirred in MeOH
(10 mL) for 10 min while the solution became clear. A solution of
NaBPh4 (956 mg, 2.79 mmol) in MeOH was then added dropwise
and the mixture was stirred for 5 h. During this time, a white pre-
cipitate developed. The white solid was collected and recrystallized
from MeOH at –30 °C; yield 48%. Elemental analysis indicates that
the bound MeOH ligand only partially (50%) occupied the com-
plex in the ground, vacuum-dried solid. C53H44BFeN6-
O2P·0.5MeOH (910.6): calcd. C 70.57, H 4.98, N 9.23; found C
70.69, H 5.08, N 8.95. IR (neat): ν = 3304, 3259 [ν(N–H)], 3054,
˜
2999, 2993, 2928, 1562 [νas(OCO)], 1478, 1402 [νs(OCO)],
1341 cm–1.
[Fe(4-TIPPh)(sal)] (6):
A
suspension of [5]BPh4 (142 mg,
1
0.159 mmol) and sodium salicylate (28.0 mg, 0.175 mmol) was
stirred overnight in MeOH (5 mL). The resulting yellow solution
was layered with MeCN to provide X-ray-quality crystals of 6;
yield 32%. C34H25FeN6O3P (652.4): calcd. C 62.59, H 3.86, N
12.88; found C 62.19, H 3.98, N 12.52. UV/Vis (MeOH): λmax (ε,
are present. H NMR (400 MHz, CDCl3): δ = 7.48 (m, 12 H, Ar–
H), 7.40 (m, 6 H, Ar–H), 7.17 (m, 12 H, Ar–H), 3.64 (s, 9 H, CH3)
ppm. 13C NMR (100 MHz, CDCl3): δ = 140.5, 140.0, 139.9, 134.8,
133.6, 131.1, 131.0, 129.2, 129.0, 128.2, 126.9, 126.5, 33.4 ppm. 31
P
NMR (162 MHz, CDCl ): δ = –56.6 ppm. IR (neat): ν = 3053,
˜
3
m
–1 cm–1) = 440 (150) nm. IR (neat): ν = 3133, 3052, 2900, 1598,
˜
2940, 2863, 1601, 1503, 1442, 1363, 1071, 1024, 961 cm–1.
1563, 1521, 1476, 1458, 1439, 1386, 1314 cm–1.
[Fe(2-TIPPh2)(MeCN)3](OTf)2 {[1](OTf)2}: 2-TIPPh2 (1.32 g,
1.81 mmol) and Fe(OTf)2 (670 mg, 1.90 mmol) were mixed in
CH3CN (20 mL) and stirred until the solution had become clear
(about 3 h). The solution was filtered and layered with excess Et2O;
X-ray-quality crystals formed after one day. The white crystals were
collected and dried under vacuum to provide 1.31 g of material;
yield 60%. Elemental analysis showed that at least two of the
coordinated CH3CN ligands are removed upon drying.
C50H39F6FeN6O6PS2·CH3CN (1125.9): calcd. C 55.47, H 3.76, N
X-ray Structure Determination: XRD data were collected at 100 K
with an Oxford Diffraction SuperNova kappa-diffractometer (Ag-
ilent Technologies) equipped with dual microfocus Cu/Mo X-ray
sources, X-ray mirror optics, Atlas CCD detector, and low-tem-
perature Cryojet device. Crystallographic data for particular com-
pounds are summarized in Table 4. The data were analyzed with
the CrysAlis Pro program package (Agilent Technologies, 2011)
typically using a numerical Gaussian absorption correction (based
on the real shape of the crystal), followed by an empirical multiscan
correction using the SCALE3 ABSPACK routine. The structures
were solved using the SHELXS program and refined with the
SHELXL program[29] within the Olex2 crystallographic package.[30]
B-, H-, and C-bonded hydrogen atoms were positioned geometri-
cally and refined using appropriate geometric restrictions on the
corresponding bond lengths and bond angles within a riding/rotat-
ing model (torsion angles of methyl hydrogen atoms were rota-
tionally optimized to better fit the residual electron density). The
positions of the methanolic hydrogen atoms (H3) in [2]-
OTf·4MeOH and [5]BPh4·3MeOH were refined freely. The remain-
ing OH groups were refined using geometrical restrictions and rota-
tionally optimized to better fit the residual electron density. Crys-
tals of [4]OTf·2CH2Cl2 represent pseudo-orthorhombic quasi-
merihedral twins (β ≈ 90°). Crystals of [5]BPh4·3MeOH are system-
8.71; found C 55.02, H 3.90, N 8.68. IR (neat): ν = 3048, 2932,
˜
2283 [ν(CϵN)], 1466, 1444, 1257, 1222, 1145, 1028, 983 cm–1.
[Fe(2-TIPPh2)(OBz)(MeOH)]OTf ([2]OTf): 2-TIPPh2 (779 mg,
1.07 mmol), NaOBz (155 mg, 1.07 mmol), and Fe(OTf)2 (378 mg,
1.07 mmol) were combined in MeOH (12 mL). After stirring for
several hours the precipitate was removed by filtration and the fil-
trate was reduced to about 5 mL in volume. Layering with pentane
afforded the desired product as a white crystalline material
(116 mg); yield 11%. X-ray diffraction analysis revealed four unco-
ordinated MeOH molecules in the resulting structure, and elemen-
tal analysis indicated that two solvent molecules remain after dry-
ing under vacuum. C57H47F3FeN6O6PS·2CH3OH (1152.0): calcd.
C 61.51, H 4.81, N 7.30; found C 61.35, H 4.48, N 7.07. IR (neat):
ν = 3043, 2953, 1598, 1551, 1443, 1370, 1258, 1153, 1029, 981 cm–1.
˜
[Fe(2-TIPPh2)(acacPhF3)]OTf ([4]OTf): A solution of 4,4,4-trifluoro- atic twins grown together along a common bc plane. The chiral
1-phenyl-1,3-butanedione (126 mg, 0.584 mmol) and NaOCH3 space group (P21) of [5]BPh4·3MeOH does not result from the mo-
1854
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1848–1856

Modeling the 3-Histidine Facial Triad of Nonheme Fe Dioxygenases
Table 4. Summary of the X-ray crystallographic data collection and structure refinement.
[a]
[1](OTf)2·0.5Et2O
[2]OTf·4MeOH
[4]OTf·2CH2Cl2
[5]BPh4·3MeOH
6·MeCN·MeOH
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C58H53F6FeN9O6.5PS2
1245.03
C61H64F3FeN6O10PS C61H49Cl4F6FeN6O5PS C57H60BFeN6O6P C37H32FeN7O4P
1217.06
triclinic
P1
1320.77
monoclinic
Pc
1022.74
monoclinic
P21
725.52
monoclinic
P21/n
triclinic
¯
¯
P1
13.3432(3)
15.8007(3)
27.8621(6)
76.994(2)
88.757(2)
87.690(2)
5718.4(2)
4
14.9470(5)
15.1921(5)
16.4349(6)
90.642(3)
113.784(3)
115.873(4)
2992.3(3)
2
16.0689(5)
20.6668(5)
19.6239(4)
90
90.088(2)
90
13.8829(3)
11.6385(4)
16.5130(4)
90
91.591(2)
90
13.6187(7)
14.9164(9)
17.5278(8)
90
102.190(5)
90
γ [°]
V [Å3]
6516.9
4
2667.1(2)
2
3480.4(3)
4
Z
Dcalcd. [gcm3]
λ [Å]
1.446
1.5418
3.749
1.351
1.325
1.274
1.385
1.5418
0.7107
1.5418
1.5418
μ [mm–1]
3.205
0.489
2.996
4.328
θ range [°]
Reflections collected
Independent reflections
7 to 149
64740
22662
7 to 148
39217
11891
7 to 59
56565
28249
4 to 148
32914
10151
4 to 149
26690
6954
(Rint = 0.0315)
22662/19/1591
1.025
0.0332/0.0855
0.0374/0.0886
(Rint = 0.0299)
11891/2/784
1.028
0.0413/0.1094
0.0449/0.1128
(Rint = 0.0339)
28249/12/1550
1.061
0.0608/0.1541
0.0666/0.1613
(Rint = 0.1419)
10151/87/643
1.025
0.0682/0.1778
0.0872/0.1963
(Rint = 0.0278)
6954/0/454
1.037
0.0273/0.0710
0.0306/0.0731
Data/restraints/parameters
GOF (on F2)
R1/wR2 [IϾ2σ(I)][b]
R1/wR2 (all data)
[a] One of the solvates in [4]OTf·2CH2Cl2 is partially occupied by a pentane molecule. [b] R1 = Σ ||Fo| – |Fc||/Σ|Fo|; wR2 = [Σw(F2o – Fc2)2/
Σw(F2o)2]1/2
.
[6]
[7]
[8]
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however, annihilates the chirality on the macroscopic level since the
components of the twin are of opposite chirality.
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FULL PAPER
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Received: November 17, 2011
Published Online: February 27, 2012
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