Iron(II) complexes of sterically bulky α-ketocarboxylates. Structural models for α-ketoacid-dependent nonheme iron halogenases

Article abstract of DOI:10.1021/ic701823y

Reaction of the sterically hindered α-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl ₂ [L = N,N,N′,N′-tetramethylpropylenediamine (Me ₄pda) or 6,6′-dimethyl-2,2′-bipyridine (dmby)] yielded LFe(Cl)(MesBF) (1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O₂CC(O)Ph)]₂ (3) instead, demonstrating the need for the sterically hindered α-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O₂CC(O)Ar)]_n (4-6) were prepared, where L′ = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-dip-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases (5 and 6) revealed dinuclear topologies (n = 2), but UV-vis and ¹H NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH₂Cl₂ solution. Although compounds 1-6 were oxidized by O₂, oxidative decarboxylation of the α-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.

Full text of DOI:10.1021/ic701823y

Inorg. Chem. 2008, 47, 1324-1331
Iron(II) Complexes of Sterically Bulky r-Ketocarboxylates. Structural
Models for r-Ketoacid-Dependent Nonheme Iron Halogenases
Seth J. Friese, Benjamin E. Kucera, Victor G. Young , Jr., Lawrence Que , Jr.,*
and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, UniVersity of Minnesota, 207
Pleasant Street SE, Minneapolis, Minnesota 55455
Received September 14, 2007
Reaction of the sterically hindered R-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes
LFeCl2 [L ) N,N,N′,N′-tetramethylpropylenediamine (Me4pda) or 6,6′-dimethyl-2,2′-bipyridine (dmby)] yielded
LFe(Cl)(MesBF) (1 or 2). X-ray crystal structures of these complexes showed that they closely model the active
site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate
with L ) dmby yielded (dmby)Fe[(O2CC(O)Ph)]2 (3) instead, demonstrating the need for the sterically hindered
R-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among
the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes
[LFe(O2CC(O)Ar)]n (4-6) were prepared, where L′ ) tris(pyridylmethyl)amine (tpa) and Ar ) 2,6-dimesitylphenyl,
2,6-dip-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases (5 and 6) revealed
dinuclear topologies (n ) 2), but UV–vis and 1H NMR spectroscopy indicated that all three complexes dissociated
in varying degrees to monomers in CH2Cl2 solution. Although compounds 1-6 were oxidized by O2, oxidative
decarboxylation of the R-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance
useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.
Introduction
glutarate (R-KG) to perform substrate oxidations, and similar
mechanisms for generation of the proposed FeIVdO active
oxidant have been proposed (Figure 1). According to the
currently favored hypotheses, a 5-coordinate FeII center with
the R-KG cofactor bound in bidentate fashion (A) reacts with
O2 to yield an FesO2 adduct (B). Subsequent intramolecular
attack by the bound O2 motiety at the R-carbon of R-KG
induces decarboxylation to yield CO2, succinate, and an
FeIVdO unit (C). Here, the pathway diverges for the different
types of enzymes. For those with the 2-His-1-carboxylate
facial triad, the FeIVdO moiety oxidizes substrate via an
oxygen rebound-type pathway, whereas for the halogenases,
transfer of the halogen ligand occurs instead of oxygen
rebound.8 The apparent link between the specific iron(II)
ligation and differing enzymatic function raises many ques-
tions about structure/function relationships in these systems
that current research aims to address.
A large and functionally diverse set of enzymes that
activate dioxygen for oxygenation of organic substrates has
a common monoiron active site supported by the so-called
“2-His-1-carboxylate facial triad” that leaves open three
coordination sites for binding of substrate and O2 during
catalysis.1–4 Recently, this class has been extended5 by the
discovery of nonheme iron halogenases in which the car-
boxylate is replaced by a halide.6,7 These halogenase
enzymes are homologous to those that use O2 and R-keto-
* To whom correspondence should be addressed. E-mail: que@
chem.umn.edu (L.Q.), tolman@chem.umn.edu (W.B.T.).
(1) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004,
104, 939–986.
(2) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Chem. ReV. 2005, 105,
2227–2252.
(3) Koehntop, K. D.; Emerson, J. P.; Que, L., Jr. J. Biol. Inorg. Chem.
2005, 10, 87–93.
Synthetic model studies have provided important insights
(4) Purpero, V.; Moran, G. R. J. Biol. Inorg. Chem. 2007, 12, 587–601.
(5) Straganz, G. D.; Nidetzky, B. ChemBioChem 2006, 7, 1536–1548.
(6) Vaillancourt, F. H.; Yeh, E.; Vosburg, D. A.; Garneau-Tsodikova, S.;
Walsh, C. T. Chem. ReV. 2006, 106, 3364–3378.
(7) Blasiak, L. C.; Vaillancourt, F. H.; Walsh, C. T.; Drennan, C. L. Nature
2006, 440, 368–371.
into the mechanisms of R-KG-dependent iron active sites.1
(8) Galonic, D. P.; Barr, E. W.; Walsh, C. T.; Bollinger, J. M.; Krebs, C.
Nat. Chem. Biol. 2007, 3, 113–116.
1324 Inorganic Chemistry, Vol. 47, No. 4, 2008
10.1021/ic701823y CCC: $40.75
2008 American Chemical Society
Published on Web 01/25/2008

Iron(II) Complexes of Sterically Bulky r-Ketocarboxylates
Figure 1. Proposed mechanisms for R-KG-dependent enzymes, showing similar oxygen activation steps (A f C) but divergent substrate oxidation routes
depending on the nature of X (halide vs carboxylate).
Pertinent examples include the structural characterization of
Scheme 1. Synthesis of Terphenyl R-Ketocarboxylates and Complexes
1 and 2
iron(II)-benzoylformate complexes supported by tri- and
tetradentate N-donor ligands that model the proposed R-KG
coordination in the enzymes9,10 and the observation of
biomimetic decarboxylation/oxidation chemistry upon oxy-
genation in some cases.9,11,12 Models of the halogenase active
site have yet to be prepared. Using a strategy involving
application of sterically hindered terphenyl-carboxylate
ligands to control complex nuclearity and facilitate isolation
of reactive intermediates through hydrophobic encapsula-
tion,13,14 we recently reported the preparation of mononuclear
zinc(II) and iron(II) complexes LM(O2CR)Cl (L ) alkylated
diamine, M ) Zn, Fe, R ) terphenyl group) that feature
flexible N,N,O(carboxylate) ligation akin to that provided
by the 2-His-1-carboxylate facial triad.15 Viewing the
complexes from another perspective, we envisioned that
replacement of the carboxylate ligand with an R-ketocar-
boxylate would generate replicas of the halogenase inter-
mediate A. Herein, we report the synthesis of sterically bulky
terphenyl-substituted R-ketocarboxylates and their successful
use for the synthesis of such accurate models of A. In
addition, we have compared the properties and O2 reactivity
of complexes of the parent benzoylformate, including previ-
ously reported ones supported by tris(pyridylmethyl)amine
(tpa),9 with analogs prepared using sterically hindered
R-ketocarboxylates. These studies have revealed important
consequences of R-ketocarboxylate substituent variation on
their biomimetic iron(II) chemistry.
di(aryl)iodobenzene via lithiation, reaction with ethyl chlo-
rooxoacetate, and basic hydrolysis of the resulting esters
(Scheme 1).16,17 Addition of thallium acetate to aqueous
suspensions of the R-ketoacids according to an established
procedure18 yielded the thallium salts Tl[2,6-di(mesityl)ben-
zoylformate] (Tl-MesBF) and Tl[2,6-di(p-tolyl)benzoylfor-
mate] (Tl-p-tolylBF) as analytically pure solids in overall
yields of 58% and 57%, respectively. Reasoning that its high
degree of steric encumbrance would favor formation of
mononuclear iron(II) complexes containing a single R-ke-
tocarboxylate, we added 1 equiv of Tl-MesBF to (Me4pda)-
FeCl2 (Me4pda ) N,N,N′,N′-tetramethylpropylenediamine)19
or (dmby)FeCl2 (dmby ) 6,6′-dimethyl-2,2′-bipyridine)20 in
CH2Cl2. Removal of precipitated TlCl and recrystallization
provided purple or red complexes 1 (90%) or 2 (45%),
Results and Discussion
(a) Synthesis. The sterically bulky 2,6-di(aryl)benzoic
acids (aryl ) mesityl or p-tolyl) were prepared from 2,6-
(9) Chiou, Y.-M.; Que, L., Jr. J. Am. Chem. Soc. 1995, 117, 3999–4013.
(10) Hikichi, S.; Ogihara, T.; Fujisawa, K.; Kitajima, N.; Akita, M.; Moro-
oka, Y. Inorg. Chem. 1997, 36, 4539–4547.
(11) Mehn, M. P.; Fujisawa, K.; Hegg, E. L.; Que, L., Jr. J. Am. Chem.
Soc. 2003, 125, 7828–7842.
(12) Ha, E. H.; Ho, R. Y. N.; Kisiel, J. F.; Valentine, J. S. Inorg. Chem.
1995, 34, 2265–2266.
(16) Nimitz, J. S.; Mosher, H. S. J. Org. Chem. 1981, 46, 211–213.
(17) Chen, Y. T.; Seto, C. T. J. Med. Chem. 2002, 45, 3946–3952.
(18) Müller, R.; Hübner, E.; Burzlaff, N. Eur. J. Inorg. Chem. 2004, 2151–
2159.
(13) Que, L., Jr.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2002, 653–
660.
(14) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987–1012.
(15) Friese, S. J.; Kucera, B. E.; Que, L.; Tolman, W. B. Inorg. Chem.
2006, 45, 8003–8005.
(19) Handley, D. A.; Hitchcock, P. B.; Lee, T. H.; Leigh, G. J. Inorg. Chim.
Acta 2001, 314, 14–21.
Inorganic Chemistry, Vol. 47, No. 4, 2008 1325

Friese et al.
Scheme 2. Synthesis of Complex 3
Scheme 3. Synthesis of Complexes 4–6
Figure 2. X-ray structural representations of (a) 1 and (b) 2, with all atoms
shown as 50% thermal ellipsoids and hydrogen atoms omitted for clarity.
(b) X-ray Structures. Representations of the X-ray crystal
structures of complexes 1-3 and 6 are shown in Figures
2-5, while that of 5 (which is quite similar to 6) is presented
in Figure S1 in the Supporting Information. Selected bond
distances and angles for all structures are listed in Table 1.
In 1 and 2, the R-ketocarboxylate ligand binds in a
bidentate fashion to a 5-coordinate Fe(II) ion, with a chloride
and the bidentate N-donor ligand completing the coordination
sphere (Figure 2). The geometries are distorted from square
pyramidal to differing extents, as reflected by τ values of
0.13 and 0.38 for 1 and 2, respectively (values of 0 and 1
being associated with ideal square pyramidal or trigonal
bipyramidal geometries, respectively).21 The greater proxim-
ity of 1 to square pyramidal may be traced to the larger
chelate ring size for Me4pda than for dmby. As reported
previously for other complexes that feature bidentate R-ke-
tocarboxylate coordination,1,9,10 the Fe-O1(carboxylate)
bond distances (2.014(3) for 1 and 1.995(2) Å for 2) are
shorter than the Fe-O2(keto) distances (2.292(2) for 1 and
2.282(2) Å for 2). Because of steric interactions involving
the mesityl rings, the central aryl ring of the terphenyl unit
is oriented far from coplanarity with the R-ketocarboxylate
plane (∼70°). As a result, the appended mesityl rings
encapsulate the R-ketocarboxylate group in a manner that
we postulate has important implications for O2 reactivity (see
below).
respectively. These compounds were fully characterized by
X-ray crystallography, spectroscopy (UV–vis, NMR, IR) and
elemental analysis (C,H,N). The steric bulk of Tl-MesBF is
key to the syntheses, as indicated by the finding that reaction
of thallium benzoylformate with (dmby)FeCl2 yields the
bis(benzoylformate) complex 3 (Scheme 2). Use of 1:1 molar
ratios of the reagents in this synthesis rather than the
optimized 2:1 ratio led to a mixture comprised mostly of 3
and unreacted (dmby)FeCl2, with only a small amount
(<10%) of another species (tentatively identified as an analog
1
of 1 and 2) observable by H NMR spectroscopy.
In order to draw comparisons among the reactivity of
structurally related iron(II) complexes of benzoylformates
of varying steric properties, in particular to the previously
published complex (tpa)Fe(BF) (BF ) benzoylformate),9 we
targeted analogs by mixing tpa, Fe(ClO4)2·6H2O, and Tl-
MesBF, Tl-p-toylBF, or Tl[2,4,6-trimethylbenzoylformate]
(Tl-Me3BF) (Scheme 3). The products isolated (4–6, 79–89%
yields) are drawn as equilibrating mixtures of mono- and
dinuclear complexes on the basis of X-ray crystallographic
data for 5 and 6 that show dinuclear structures and UV–vis
and electrospray mass spectrometry data that suggest that
monomeric species with bidentate R-ketocarboxylate coor-
dination form to some extent in solution (see below).
Overall, the structures of 1 and 2 replicate key features of
the active site of the halogenase SyrB2,7 with the more square
pyramidal site of 1 more closely approximating that of the
enzyme (Figure 3). There are significant differences between
the structures with respect to Cl ligation: the Fe-Cl distances
(2.44 Å in SyrB2 vs 2.2653(10) Å in 1 and 2.2646(8) Å in
2) and the relative disposition of the chloride and R-keto-
carboxylate ligands (carboxylate O cis to chloride in SyrB2
(20) (a) Chan, B. C. K.; Baird, M. C. Inorg. Chim. Acta 2004, 357, 2776–
2782. (b) Yang, P.; Chan, B. C. K.; Baird, M. C. Organometallics
2004, 23, 2752–2761.
(21) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
1326 Inorganic Chemistry, Vol. 47, No. 4, 2008

Iron(II) Complexes of Sterically Bulky r-Ketocarboxylates
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes
Characterized in This Studya
(Me4pda)Fe(MesBF)Cl (1)
Fe1-N1
2.159(3)
2.014(3)
Fe1-N2
Fe1-O2
2.192(3)
2.292(2)
Fe1-O1
Fe1-Cl1
2.2653(10)
99.16(11)
98.20(11)
108.31(8)
74.84(9)
O1-Fe1-N1
N1-Fe1-N2
N1-Fe1-Cl1
O1-Fe1-O2
N2-Fe1-O2
O1-Fe1-N2
O1-Fe1-Cl1
N2-Fe1-Cl1
N1-Fe1-O2
Cl1-Fe1-O2
87.70(10)
149.49(8)
100.94(8)
98.78(10)
87.84(7)
157.32(10)
(dmby)Fe(MesBF)Cl (2)
Fe1-O1
1.9949(18)
2.144(2)
2.2817(19)
Fe1-N1
2.122(2)
2.2646(8)
Figure 4. X-ray structure of 3, with all atoms shown as 50% thermal
ellipsoids, noncarbon atoms labeled, and hydrogen atoms omitted for clarity.
Fe1-N2
Fe1-Cl1
Fe1-O2
O1-Fe1-N1
N1-Fe1-N2
N1-Fe1-Cl1
O1-Fe1-O2
N2-Fe1-O2
113.21(8)
O1-Fe1-N2
O1-Fe1-Cl1
N2-Fe1-Cl1
N1-Fe1-O2
Cl1-Fe1-O2
86.35(8)
137.82(6)
105.11(6)
107.96(8)
90.88(6)
77.47(8)
108.91(6)
74.41(8)
160.65(8)
(dmby)Fe(BF)2 (3)
Fe1-O4
2.0139(17)
2.166(2)
2.3343(17)
157.40(7)
95.28(7)
105.52(7)
74.03(6)
107.14(7)
87.50(6)
169.50(6)
71.55(6)
Fe1-O1
2.0190(17)
2.172(2)
2.3350(17)
102.26(7)
92.31(7)
77.14(8)
87.33(6)
166.21(7)
74.32(7)
106.65(7)
Fe1-N2
Fe1-N1
Fe1-O6
Fe1-O3
O4-Fe1-O1
O1-Fe1-N2
O1-Fe1-N1
O4-Fe1-O6
N2-Fe1-O6
O4-Fe1-O3
N2-Fe1-O3
O6-Fe1-O3
O4-Fe1-N2
O4-Fe1-N1
N2-Fe1-N1
O1-Fe1-O6
N1-Fe1-O6
O1-Fe1-O3
N1-Fe1-O3
[(tpa)Fe(p-tolylBF)]2(ClO4)2 (5)
Fe1-O2
2.0298(19)
2.139(3)
2.2008(19)
106.55(9)
150.85(10)
91.36(9)
103.64(8)
83.72(9)
164.44(8)
75.51(10)
91.46(8)
Fe1-N4
2.132(3)
2.195(2)
2.255(2)
102.09(9)
88.00(8)
94.99(9)
84.39(8)
168.32(8)
78.29(9)
77.00(8)
Figure 5. X-ray structural representation of the cationic portion of 6, with
all atoms shown as 50% ellipsoids, noncarbon atoms labeled, and hydrogen
atoms omitted for clarity.
Fe1-N2
Fe1-N3
Fe1-O1
Fe1-N1
O2-Fe1-N4
N4-Fe1-N2
N4-Fe1-N3
O2-Fe1-O1
N2-Fe1-O1
O2-Fe1-N1
N2-Fe1-N1
O1-Fe1-N1
O2-Fe1-N2
O2-Fe1-N3
N2-Fe1-N3
N4-Fe1-O1
N3-Fe1-O1
N4-Fe1-N1
N3-Fe1-N1
upon substrate binding, so 1 and 2 may in fact be better
models for the ternary enzyme–substrate-cosubstrate complex.
The X-ray structure of 3 shows bidentate coordination of
two BF ligands to an Fe(II) ion, with the additional dmby
ligand giving a geometry distorted relatively slightly from
octahedral (Figure 4). As in 1, 2, and other bidentate
R-ketocarboxylate complexes,9,10 the O(carboxylate)-Fe
distance is shorter than that for the O(keto)-Fe bond (∆ ∼
0.16 Å). Interestingly, both R-ketocarboxylate chelate rings
are distorted from planarity, apparently to accommodate
stacking of the arene substuents, both within the same
molecule and with neighbors in the crystal lattice (see
packing diagrams in Figure S2 in the Supporting Informa-
tion). In an important contrast with 1 and 2, these aryl groups
are essentially coplanar with their respective R-keto units,
leaving the latter relatively accessible to nucleophilic attack
compared to those in 1 and 2 that are almost entirely enclosed
by the mesityl substituents of the MesBF ligand.
[(tpa)Fe(Me3BF)]2(ClO4)2 (6)
Fe1 O1
Fe1 N2
Fe1 N3
2.0053(14)
2.1551(17)
2.2006(16)
Fe1 O2
Fe1 N4
Fe1 N1
O1-Fe1-N2
O1-Fe1-N4
N2-Fe1-N4
O2-Fe1-N3
N4-Fe1-N3
O2-Fe1-N1
N4-Fe1-N1
2.1522(14)
2.1574(17)
2.2289(16)
99.26(6)
108.45(6)
151.78(6)
165.75(6)
83.24(6)
O1-Fe1-O2
O2-Fe1-N2
O2-Fe1-N4
O1-Fe1-N3
N2-Fe1-N3
O1-Fe1-N1
N2-Fe1-N1
N3-Fe1-N1
a
104.09(6)
82.32(6)
85.81(6)
88.07(6)
103.33(6)
163.53(6)
76.40(6)
77.70(6)
91.19(6)
78.35(6)
Estimated standard deviations are given in parentheses.
Complexes 5 (Figure S1) and 6 (Figure 5) have analogous
structures that feature two octahedral Fe(II) ions bound to
tetradentate tpa ligands and bridged by the carboxylates of
their respective R-ketocarboxylates. We were unable to
obtain crystals of 4 suitable for X-ray diffraction studies.
For 5 and 6, an inversion center relates each half of the
dimeric molecule, which have similar Fe···Fe separations of
∼4.65 Å and overall topologies analogous to that reported
for [(tpa)2Fe(O2CCH3)2](BPh4)2.22 These dimeric structures
Figure 3. Comparison of the cores of (a) the active site of the halogenase
SyrB2 (Protien Databank (pdb) 2FCT7) and (b) the model compound 1.
vs trans in 1). The two model complexes also lack the
additional water ligand found in the enzyme structure.
However, assuming that the generally accepted mechanism
for R-ketoglutarate-dependent nonheme iron enzymes1,4
applies to the halogenases, this ligand is likely to be lost
(22) Ménage, S.; Zang, Y.; Hendrich, M. P.; Que, L., Jr. J. Am. Chem.
Soc. 1992, 114, 7786–7792.
Inorganic Chemistry, Vol. 47, No. 4, 2008 1327

Friese et al.
Figure 7. UV–vis spectra of 1.0 mM solutions (CH2Cl2) of 4 (solid line),
5 (dashed line), and 6 (dotted line).
Figure 6. UV–vis spectra of 1.0 mM solutions (CH2Cl2) of 1 (solid line)
and 2 (dashed line).
aryl group, which is absent in the aliphatic examples.
Similarly, this conjugation is lacking for MesBF in 1 and 2
because of the large steric profile of the mesityl groups,
which causes the central aryl ring to be almost orthogonal
to the R-ketocarboxylate plane (Figure 2). 1H NMR spectra
also indicate that intact complexes 1 and 2 are the predomi-
nate species in CD2Cl2 solution, but a small amount (∼20%)
of disproportionation to LFeCl2 and LFe(MesBF)2 (L )
Me4pda or dmby) is evident; this is most clearly illustrated
in the spectrum for 2 (Figure S3 in the Supporting Informa-
tion, where the assignment for (dmby)Fe(MesBF)2 is based
on the similarity of the peaks for the dmby ligand to those
of 3).
UV–vis spectra of 4–6 (Figure 7) also exhibit Fe(II) f
R-ketocarboxylate metal-to-ligand charge transfer (MLCT)
transitions indicative of bidentate R-ketocarboxylate coor-
dination and, thus, adoption of monomeric structures in
solution. The intensities differ, however, suggesting that
either (a) the degree of dissociation varies (4 > 5 > 6) or
(b) all of the complexes exist as monomers, but with differing
proportions of mono- and bidentate R-ketocarboxylate co-
ordination. With the data currently available, we cannot
distinguish between these possibilities. However, the fact that
the apparent extinction coefficients for the visible bands fall
within the range found for mononuclear iron(II) complexes
with bidentate R-ketocarboxylate coordination reported thus
far11 suggests that species with bidentate R-ketocarboxylate
coordination represent at least one-third of the molecules
present in solution, so there is significant dissociation of the
dinuclear structures found in the crystals. Further support
for the presence of monomers is apparent in the ESI-MS
data, exemplified for 4 in Figure S4 in the Supporting
Information, which shows the appropriate m/z and isotope
pattern for [FeII(tpa)(MesBF)]+. The 1H NMR spectrum for
each complex (Figure S5) contains a single set of peaks for
the TPA protons (paramagnetically shifted as in other
TPA-FeII compounds9) and for those associated with the
R-ketocarboxylate. Together, the UV–vis and NMR data are
best interpreted to indicate that the compounds are highly
fluxional in solution, with interconversions between mono-
and dinuclear structures and/or mono- and bidentate R-ke-
tocarboxylate coordination modes being rapid on the NMR
time scale. Importantly for O2 reactivity studies, the UV–vis
distinctly contrast with the monomeric ones reported previ-
ously for BF complexes supported by tpa and tris(6-
methylpyridylmethyl)amine (6-Me3-TPA),9 which presum-
ably results from the greater steric demands of the MesBF,
p-tolylBF, and Me3BF ligands. An apparent consequence of
the bridging carboxylate binding mode of the R-ketocar-
boxylates in 5 and 6 is positioning of the keto group out of
coplanarity with the carboxylate; in 6, the torsional angle
O1-C1-C2-O3 is 35.8(4)°, while in 5 it is 54.1(3)°.
Finally, we note that the dimeric structures for 4–6 appear
to be disrupted in solution, as discussed below.
1
(c) Properties in Solution. UV–vis and H NMR spec-
troscopic data for complexes 1 and 2 in CH2Cl2 or CD2Cl2
indicate that they maintain their structural integrity to a large
extent in solution, albeit not entirely. They exhibit purple
and red colors, respectively, that are characteristic for an
R-ketocarboxylate bound in bidentate fashion to an Fe(II)
center.9,11,23,24 In the UV–vis spectrum of 1, three peaks with
approximate λmax values of 488, 523, and 587 nm (ε ∼ 210,
200, and 150 M-1 cm-1, respectively; Figure 6) are observed,
while, for 2, closely overlapping peaks appear with λmax
values of about 458, 483, and 547 nm (ε ∼ 220, 200, and
80 M-1 cm-1, respectively). By analogy to previously
reported data,9,11,23,24 we attribute these features to Fe(II)
f R-ketocarboxylate MLCT transitions. The shift to lower
wavelength (higher energy) of the features for 2 is consistent
with the greater Lewis acidity of the Fe(II) ion in this
complex that arises from poorer electron donation from the
supporting dmby donor relative to Me4pda. The extinction
coefficients for these bands in both complexes are low
compared to those for complexes with simple benzoylfor-
mates (∼540 M-1 cm-1)11 but are similar to those of an
aliphatic R-ketocarboxylate complex (∼220 M-1 cm-1)11 and
the R-KG complexes of several proteins.24,25 The higher
extinctions for the parent BF case were attributed to
conjugation of the R-ketocarboxylate functionality with the
(23) Ho, R. Y. N.; Mehn, M. P.; Hegg, E. L.; Liu, A.; Ryle, M. J.;
Hausinger, R. P.; Que, L., Jr. J. Am. Chem. Soc. 2001, 123, 5022–
5029.
(24) Pavel, E. G.; Zhou, J.; Busby, R. W.; Gunsior, M.; Townsend, C. A.;
Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 743–753.
(25) Ryle, M. J.; Padmakumar, R.; Hausinger, R. P. Biochemistry 1999,
38, 15278–15286.
1328 Inorganic Chemistry, Vol. 47, No. 4, 2008

Iron(II) Complexes of Sterically Bulky r-Ketocarboxylates
data confirm that each complex is able to access (to a
significant extent) a structure with the bidentate R-ketocar-
boxylate coordination thought to be required for oxidative
decarboxylation.
O2 on the R-keto group, resulting only in the autoxidation
of the iron(II) center. The crystal structures of 1 and 2 show
that substitution at the 2 and 6 positions of the benzoate
phenyl ring causes the aromatic ring to rotate out of the plane
of the iron-coordinated R-ketocarboxylate. This rotation
places the 2- and 6-substituents in the space above and below
this plane, thereby hindering O2 binding to the iron center
and attack of the R-ketocarboxylate. Thus, while the use of
bulky ortho-substituents on the R-ketocarboxylate has fa-
cilitated isolation and crystallization of suitable structural
models for the active site of the iron halogenases, this very
strategy ironically prevents them from acting as functional
models.
(d) Reactivity with O2. All complexes studied react with
dioxygen. In the case of 1, the reactions in CH2Cl2, MeCN,
THF, toluene, and benzene all resulted in a fading of the
iron(II) chromophore followed by the formation of an
insoluble red-brown precipitate (presumably iron-oxides). In
contrast, when complex 2 (0.5 mmol solution in CH2Cl2)
was exposed to dioxygen, no precipitate formed and the
initially red solution became dark orange. We have been
unable to isolate or characterize the final oxidized iron-
containing products, but the organic ligands were recovered
for analysis by adding 1 M aq. H2SO4 to the orange oxidized
solution. The ligands were then extracted with Et2O, washed
with water, and dried with MgSO4. Removal of the solvent
Conclusion
In summary, by extending the strategy for constructing
novel metal complexes using sterically hindered terphenyl
carboxylate ligands13–15 to one using similarly hindered
R-ketocarboxylates, mononuclear complexes (1 and 2) that
closely model the nonheme iron halogenase active site
structure7 have been prepared. The steric bulk of the
R-ketocarboxylate substituents is key to the successful
isolation of these structural model compounds, as shown by
the isolation of the bis(R-ketocarboxylate) complex 3 when
the unsubstituted BF was used instead. However, the steric
bulk inhibits biomimetic decarboxylation by O2, as revealed
in studies of 1, 2, and tpa-supported diiron(II) complexes
4–6. From these results it is evident that further tuning of
R-ketocarboxylate structure will be required in order to access
complexes that both structurally and functionally model
nonheme R-ketoacid-dependent enzyme active sites.
1
under vacuum enabled analysis by ESI-MS and H NMR
spectroscopy. Only MesBF was observed, and there was no
evidence for its oxidative decarboxylation to form 2,6-
(dimesityl)benzoic acid. Similar to 2, when complex 3 (0.5
mmol in CH2Cl2 or MeCN) was exposed to dioxygen, the
solution turned from purple to orange without formation of
a precipitate. While the final oxidized iron products could
not be isolated or characterized, the ligands were recovered
via the same method as that for complex 2. Analysis by ESI-
1
MS and H NMR spectroscopy indicated formation of the
decarboxylation product benzoic acid in 34% yield (for
reaction of one BF ligand), a different outcome from that
seen for complexes 1 and 2. Unfortunately, attempts at
trapping any intermediates with the exogenous substrates
PPh3, dihydroanthracene, and cumene were not successful.
Complexes 4-6 are bulky analogues of the complexes
[(tpa)Fe(BF)(MeOH)]ClO4 and [(6-Me3-TPA)Fe(BF)]ClO4
reported previously to undergo decarboxylation upon expo-
sure to dioxygen in MeCN solvent at room temperature over
a period of days.9 The observed product of the reaction of
[(tpa)Fe(BF)]ClO4 with O2 was {[FeIII2O-(tpa)2-(O2CPh)2-
(ClO4)}+ as monitored by FAB-MS. It was proposed that the
reaction with O2 resulted in the decarboxylation of the R-ke-
tocarboxylate to form [(tpa)-Fe(O2CPh)]-ClO4 initially, which
subsequently underwent further oxidation to give rise to the
observed product. In contrast, the reaction of 4, 5, or 6 with O2
in MeCN solvent for 2 days only resulted in the oxidation of
FeII to FeIII and did not give the expected oxidative decarboxy-
lation product. Notably, ESI-MS data indicated formation of
the [FeIII2-O(tpa)2-(MesBF)2]2+, [FeIII2-O(tpa)2-(p-tolylBF)2]-
2+, and [FeIII2O-(tpa)2-(2,4,6-Me3BF)2]2+ ions, but there was
no evidence for the formation of the carboxylate analogs that
would result from oxidative decarboxylation (see Figure S6
in the Supporting Information for the experimental and
simulated parent ion envelope for the production of the
oxygenation of 4).
Experimental Section
General Considerations. Manipulations were performed under
an atmosphere of nitrogen using a Vacuum Atmospheres nitrogen
box or employing standard Schlenk techniques, unless otherwise
stated. Pentane, diethyl ether, CH2Cl2, and toluene were passed
through a solvent purification column (Glass Contour, Laguna,
CA). THF was vacuum distilled from sodium/benzophenone.
Acetonitrile was vacuum distilled from calcium hydride. N,N,N′,N′-
Tetramethylpropylenediamine (Me4pda) was distilled from so-
dium metal. Commercial FeCl2 was dried by treatment with neat
Me3SiCl. (dmby)-FeCl2,20 2,6-dimesityliodobenzene,26 2,6-di(p-
tolyl)iodobenzene,26a tris-(pyridylmethyl)-amine (tpa),27 and
thallium benzoylformate18 were prepared as described in the
literature. The preparation of (Me4pda)FeCl2 was carried out at
room temperature instead of reflux as stated in the literature.19
1H NMR spectra were recorded on a Varian 300 MHz or Varian
500 MHz instrument. Spectra were referenced to the residual
solvent peak. Infrared spectra were obtained on a Nicolet Avatar
370 FT-IR. Elemental analyses were performed by Robertson
Microlit. Inc. Data collection and structure solution for crystal
structures were conducted at the X-Ray Crystallographic Labora-
tory, S146 Kolthoff Hall, Department of Chemistry, University
of Minnesota; details are available in the CIFs.
The fact that oxidative decarboxylation is not observed
for any of the complexes with sterically bulky R-ketocar-
boxylates, despite evidence for the formation of bidentate
R-ketocarboxylates, suggests that the substituents introduced
into the 2 and 6 positions are preventing attack of the bound
(26) (a) Du, C. J. F.; Hart, H.; Ng, K. K. D. J. Org. Chem. 1986, 51, 3162–
3165. (b) Smith, R. C.; Protasiewicz, J. D. Eur. J. Inorg. Chem. 2004,
998–1006.
(27) Britovsek, G. J. P.; England, J.; White, A. J. P. Inorg. Chem. 2005,
44, 8125–8134.
Inorganic Chemistry, Vol. 47, No. 4, 2008 1329

Friese et al.
Ethyl 2,6-Dimesitylbenzoylformate. To a diethyl ether solution
(150 mL) containing 2,6-dimesityliodobenzene (2.11 g, 4.80 mmol),
n-BuLi (2.5 M in hexanes, 1.95 mL) was added dropwise at room
temperature and stirred for 30 min. The pale yellow solution was
then cooled to -78 °C, upon which ethyl chlorooxoacetate (0.59
mL, 5.3 mmol) was added. Once the addition was complete, the
yellow solution was warmed to room temperature and stirred for
18 h. A white precipitate was formed, which was removed by
filtration with a glass frit. The yellow filtrate was then concentrated
(ca. 10 mL) and EtOH (ca. 20 mL) was added. This procedure
was repeated two more times before cooling in an ice bath,
subsequently collecting the white precipitate on a glass frit, and
washing twice with cold methanol (0 °C). The product (1.60 g,
81% yield) was obtained as a white solid after drying under vacuum.
1H NMR (CD2Cl2, 300 MHz) δ 7.61 (t, 1H, JHH ) 7.5 Hz), 7.17
(d, 2H, JHH ) 7.5 Hz), 6.89 (s, 4H), 3.65 (q, 2H, JHH ) 7.2 Hz),
2.30 (s, 6H), 2.00 (s, 12) 0.97 (t, 3H, JHH ) 7.2 Hz) ppm. 13C
NMR (CD2Cl2, 75 MHz) δ 13.1, 20.0, 20.5, 61.7, 127.7, 128.9,
131.0, 135.2, 136.0, 136.5, 137.3, 140.5, 161.8, 189.0 ppm. IR
(Neat): 850, 1024, 1194, 1303, 1456, 1705 (CO), 1726 (CO), 2858,
2920, 2977 cm-1. Anal. calcd for C28H30O3: C, 81.13; H, 7.29.
Found: C, 80.91; H, 7.06.
30 mL of the acetone/water mixture was added again and stirred
for an additional 2 h. This process was performed one more time
before washing the remaining solids with ether and drying under
vacuum to yield a white solid (411 mg, 0.70 mmol; 75% yield).
1H NMR (DMSO-d6, 500 MHz) δ 7.42 (t, 1H, JHH ) 7.5 Hz),
6.93 (d, 2H, JHH ) 7.5 Hz), 6.76 (s, 4H), 2.20 (s, 6H), 1.98 (s, 12)
ppm. 13C NMR (DMSO-d6, 75 MHz) δ 21.6, 21.7, 128.5, 129.0,
129.9, 136.8, 137.1, 137.8, 139.5, 141.6, 166.6, 201.3 ppm. IR
(neat): 850, 1213, 1362, 1453, 1592, 1676, 2849, 2916, 2946, 3048
cm-1. Anal. calcd for C26H25O3Tl: C, 52.94; H, 4.27. Found: C,
52.68; H, 4.15.
Thallium 2,6-Dip-tolylbenzoylformate (Tl-p-tolylBF). This
compound was prepared analogously to thallium 2,6-dimesitylben-
zoylformate. The product was obtained as a white solid (2.12 g,
1
92% yield) after drying under vacuum. H NMR (DMSO-d6, 300
MHz) δ 7.45 (t, 1H, JHH ) 7.5 Hz), 7.24 (d, 2H, JHH ) 7.5 Hz),
7.23 (d, 4H, JHH ) 7.5 Hz), 7.13 (d, 4H, JHH ) 7.5 Hz), 2.30 (s,
6H) ppm. 13C NMR (DMSO-d6, 75 MHz) δ 21.7, 129.1, 129.4,
129.7, 130.0, 137.1, 138.5, 140.4, 142.1, 166.5, 204.3 ppm. IR
(neat): 794, 976, 1213, 1372, 1453, 1514, 1633, 1682, 2861, 2915,
3031, 3058, 3085 cm-1. Anal. calcd for C22H17O3Tl: C, 49.51; H,
3.21. Found: C, 49.28; H, 2.95.
Ethyl 2,6-Dip-tolylbenzoylformate. This compound was pre-
pared analogously to ethyl 2,6-dimesitylbenzoylformate. The
product (6.75 g, 72% yield) was obtained as a white solid after
drying under vacuum. 1H NMR (CD2Cl2, 500 MHz) δ 7.59 (t, 1H,
Thallium 2,4,6-Trimethylbenzoylformate (Tl-Me3BF). This
compound was prepared analogously to thallium 2,6-dimesitylben-
zoylformate. The product was obtained as a white solid (1.51 g,
89% yield) after drying under vacuum yielding a white solid (1.51
1
JHH ) 7.5 Hz), 7.40 (d, 2H, JHH ) 7.5 Hz), 7.20 (dd, 8H, JHH
)
g, 89% yield). H NMR (DMSO-d6, 300 MHz) δ 6.81 (s, 2H),
8.0 Hz), 3.82 (q, 2H, JHH ) 7.0 Hz), 2.39 (s, 6H), 0.98(t, 3H, JHH
) 7.0 Hz) ppm. 13C NMR (CD2Cl2, 125 MHz) δ 13.9, 21.4, 62.8,
129.6, 129.7, 129.8, 131.0, 136.2, 137.1, 138.4, 142.4, 162.3, 191.1
ppm. IR (Neat): 713, 802, 825, 954, 1013, 1195, 1450, 1515, 1709
(CO), 1726 (CO), 2921, 2972, 3027 cm-1. Anal. calcd for C24H22O3:
C, 80.32; H, 6.19. Found: C, 80.32; H, 6.09.
2.22 (s, 3H), 2.20 (s, 6H) ppm. 13C NMR (DMSO-d6, 75 MHz) δ
20.3, 21.7, 128.9, 135.1, 138.5, 139.3, 170.0, 205.2 ppm. IR (neat):
786, 980, 1223, 1365, 1585, 1686, 2856, 2919, 2950 cm-1. Anal.
calcd for C11H11O3Tl: C, 33.40; H, 2.80. Found: C, 33.29; H, 2.38.
(Me4pda)Fe(MesBF)Cl (1). Thallium dimesitylbenzoylformate
(100 mg, 0.17 mmol) was added to a CH2Cl2 solution (ca. 10 mL)
containing (tmpda)FeCl2 (43.6 mg, 0.17 mmol). The resulting purple
solution was stirred for 10 min at room temperature. The solution
was filtered and condensed to ca. 2 mL. Pentane (ca. 5 mL) was
added and the solution was cooled to –20 °C overnight, which gave
2,6-Dimesitylbenzoylformic Acid. To a MeOH suspension
containing ethyl 2,6-dimesitylbenzoylformate (1.57 g, 4.0 mmol),
2.5 M NaOH (6 mL) was added and stirred at 50 °C for 2 h. The
suspension was then cooled and conc HCl was added until the pH
was about 2. The white suspension was then filtered and washed
with H2O (5 × 5 mL) and cold (0 °C) MeOH (2 × 2 mL). The
product (2.60 g, 96%) was obtained as a white solid after drying
1
purple crystals (2 crops; 92 mg, 90% yield). H NMR (CD2Cl2,
500 MHz) δ 166 (2H), 113 (6H), 98 (2H), 92 (6H), 8.7 (2H), 7.2
(1H), 3.2 (4H) 1.7 (12H), -0.8 (6H), -10 (1H), -16 (1H) ppm.
UV–vis [δmax, nm (δ, M-1 cm-1) in CH2Cl2 at 20 °C]: 488 (210),
523 (200), 587 (160). IR (neat): 705, 738, 815, 854, 1222, 1327,
1610, 1683, 2840, 2916, 2962, 3009, 3052 cm-1. Anal. calcd for
C33H43ClFeN2O3: C, 65.30; H, 7.14; N, 4.62. Found: C, 65.06; H,
6.88; N, 4.46.
(dmby)Fe(MesBF)Cl (2). This compound was prepared analo-
gously to (Me4pda)Fe(MesBF)Cl. The product (45 mg, 45%) was
obtained as red crystals, which became a powder after drying under
vacuum. 1H NMR (CD2Cl2, 500 MHz) δ 66 (2H), 51.0 (2H), 10.3
(2H), 7.68 (1H), 5.0 (12H), 2.50 (4H), -2.7 (6H), -12.6 (2H),
-36 (6H) ppm. UV–vis [δmax, nm (δ, M-1 cm-1) in CH2Cl2 at 20
°C]: 458 (220), 483 (200), 547 (80). IR (neat): 705, 738, 815, 854,
1222, 1327, 1610, 1683, 2840, 2916, 2962, 3009, 3052 cm-1. Anal.
calcd for C38H37ClFeN2O3 [0.2 CH2Cl2(NMR)]: C, 67.67; H, 5.56;
N, 4.13. Found: C, 67.62; H, 5.59; N, 4.06.
(dmby)Fe(BF)2 (3). Addition of thallium benzoylformate (227
mg, 0.64 mmol) to a pale yellow CH2Cl2 (ca. 5 mL) solution
containing (dmby)FeCl2 (100 mg, 0.32 mmol) led to a dark red
solution, which was stirred for 30 min. This solution was filtered
and condensed (ca. 2 mL) before diethyl ether (ca. 10 mL) was
added, which resulted in the immediate formation of red micro-
crystalline solids. These solids were collected and dried under
vaccum (1.77 mg, 98% yield). Single crystals suitable for X-ray
under vacuum. 1H NMR (CD2Cl2, 300 MHz) δ 7.61 (t, 1H, JHH
)
7.5 Hz), 7.17 (d, 2H, JHH ) 7.5 Hz), 6.89 (s, 4H), 5.9 (br s, 1H),
2.30 (s, 6H), 2.00 (s, 12) ppm. 13C NMR (CD2Cl2, 75 MHz) δ
19.9, 20.5, 127.7, 128.8, 131.0, 134.8, 135.2, 136.5, 137.6, 140.2,
160.8, 190.1 ppm. IR (Neat): 656, 847, 1208, 1376, 1453, 1698,
1709, 2400–3100 (br), 2853, 2916, 2969 cm-1. Anal. calcd for
C26H26O3: C, 80.80; H, 6.78. Found: C, 80.64; H, 6.78.
2,6-Dip-tolylbenzoylformic Acid. This compound was prepared
analogously to 2,6-dimesitylbenzoylformic acid. The product (3.82
g, 86%) was obtained as a white solid after drying under vacuum.
1H NMR (CD2Cl2, 300 MHz) δ 7.64 (t, 1H, JHH ) 7.5 Hz), 7.43
(d, 2H, JHH ) 7.5 Hz), 7.21 (d, 4H, JHH ) 4.5 Hz), 7.13 (d, 4H,
JHH ) 4.5 Hz), 6.60 (br s, 1H) 2.38 (s, 6H) ppm. 13C NMR (CD2Cl2,
75 MHz) δ 21.4, 129.6, 129.7, 129.9, 131.4, 134.2, 137.0, 138.6,
142.5, 160.6, 192.5 ppm. IR (neat): 711, 795, 820, 954, 963, 1202,
1344, 1456, 1515, 1705 (br), 2922, 3025, 2500–3300 (br) cm-1
.
Anal. calcd for C22H17O3: C, 79.98; H, 5.49. Found: C, 79.83; H,
5.68.
Thallium 2,6-Dimesitylbenzoylformate (Tl-MesBF). Using a
precedented procedure,18 a MeOH/water (ca. 30 mL in 1:2 ratio)
suspension containing 2,6-dimesitylbenzoylformic acid (358 mg,
0.93 mmol) and thallium acetate (285 mg, 0.93 mmol) was added
and stirred for 2 h. After the volatiles were removed in vacuo, ca.
1330 Inorganic Chemistry, Vol. 47, No. 4, 2008

Iron(II) Complexes of Sterically Bulky r-Ketocarboxylates
1
crystallography were grown by slow evaporation of solvent from
a toluene-CH2Cl2 mixture. H NMR (CD2Cl2, 500 MHz) δ 61.0
solid after drying under vacuum. H NMR (CD2Cl2, 500 MHz) δ
1
139, 65, 51, 49, 16.7, 8.8, 5.1, 4.9, 4.1, 2.6 ppm. UV–vis [λmax, nm
(ε, M-1 cm-1) in CH2Cl2 at 20 °C]: 521 (160), 581 (130). IR (neat):
621, 768, 1089, 1442, 1604, 1643, 1699, 2844, 2915, 2950, 3020,
3052 cm-1. Anal. calcd for C58H58Cl2Fe2N8O14: C, 54.69; H, 4.59;
N, 8.80. Found: C, 54.79; H, 4.44; N, 8.76. HRMS(ESI) m/z calcd
for [(TPA)Fe(2,4,6-trimethylBF)]+: 537.1584; found 537.1609.
Reactivity with O2. In a typical reaction for compounds 1 and
2, dioxygen was bubbled through 2.5 mL solutions (0.5 mmol in
CH2Cl2) containing compounds 1 and 2 for 10 min and allowed to
react for 2 h. The organic ligands were then recovered for analysis
by adding 1 M aq. H2SO4 (1 mL) to the orange oxidized solution.
The ligands were then extracted with Et2O (2 mL × 2), washed
with water (2 mL), dried with MgSO4, and filtered. The solvent
was removed from the filtrate under vacuum, and the residue was
analyzed by ESI-MS and 1H NMR spectroscopy. Only the MesBF
ligand was observed, and there was no evidence for decarboxylation
of it to form 2,6-(dimesityl)benzoic acid.
Reaction of dioxygen with complex 3 was carried out under
conditions similar to those used for complexes 1 and 2, except that
trimethoxybenzene was added as an internal standard. The formation
of benzoate was observed in 34% yield (relative to the decarboxy-
lation of one benzoylformate per complex) as indicated by 1H NMR
spectroscopic analysis (75% material recovery). The presence of
benzoate was also confirmed by ESI-MS analysis.
In a typical reaction for 4-6, a 1.0 mmol solution (2.5 mL) of
compounds 4, 5, or 6 in MeCN was reacted with dioxygen by
slowly bubbling O2 into the solution for 10 min with monitoring
by UV–vis spectroscopy. After the reactions were complete (within
2 days), the crude solutions were injected into the ESI mass
spectrometer (positive ion mode) for analysis. Signals corresponding
to[FeIII2O(tpa)2(MesBF)2]2+,[FeIII2O(tpa)2(p-tolylBF)2]2+,and[FeIII2O-
(tpa)2(2,4,6-Me3BF)2]2+ with m/z values of 545, 683, or 739,
respectively, were observed, but peaks for species containing the
respective products arising from oxidative decarboxylation were
not apparent. Simulations of the isotope pattern for all of these
ions closely matched the experimental spectra, as illustrated for
the case of [FeIII2O(tpa)2(MesBF)2]2+ in Figure S5 in the Supporting
Information.
(2H), 49.3 (2H), 15.4 (4H), 12.0 (2H), 8.42 (4H), -8.91 (2H).
-23.9 (6H) ppm. IR (neat): 679, 785, 818, 1178, 1233, 1450, 1463,
1597, 1632, 1667, 2923, 3066, 3103 cm-1. UV–vis [λmax, nm (ε,
M-1 cm-1) in CH2Cl2 at 20 °C]: 485 (600), 535 (650), 585 (580).
Anal. calcd for C28H22FeN2O6: C, 62.47; H, 4.12; N, 5.20. Found:
C, 62.19; H, 4.29; N, 5.47.
[(tpa)Fe(p-tolylBF)]2(ClO4)2 (5). Caution! Perchlorate salts are
prone to rapid exothermic decomposition, and appropriate care
should be taken to handle carefully in small amounts. The ligand
tpa (163 mg, 0.562 mmol) was added to a MeCN solution (ca. 15
mL) containing Fe(ClO4)2·6H2O (204.3 mg, 0.563 mmol) and stirred
at room temperature for 10 min. Thallium 2,6-di-p-tolylbenzoyl-
formate (300 mg, 0.562 mmol) was then added, and the mixture
was stirred for an additional 18 h. The solvent was removed in
vacuo. The remaining residue was extracted with CH2Cl2 and
filtered. The volatiles were removed again in vacuo before
recrstalization by vapor diffusion of ether into an MeCN solution.
The product (346 mg, 79% yield) was obtained as an orange solid
1
after drying under vacuum. H NMR (CD2Cl2, 500 MHz) δ 129,
71. 53, 47, 14.1, 12.1, 11.3, 10.0, 4.93, 4.39, 1.99, -0.6 ppm.
UV–vis [λmax, nm (ε, M-1 cm-1) in CH2Cl2 at 20 °C]: 530 (320),
586 (310). IR (neat): 763, 799, 1090, 1442, 1604, 1651, 1698, 2860,
2919, 3021, 3060 cm-1. Anal. calcd for C80H70Cl2Fe2N8O14: C,
61.99; H, 4.46; N, 7.23. Found: C, 61.73; H, 4.46; N, 7.36.
HRMS(ESI) m/z calcd for [(TPA)Fe(p-tolylBF)] (M+): 675.2054;
found 675.2054.
[(tpa)Fe(MesBF)]2(ClO4)2 (4). Caution! Perchlorate salts are
prone to rapid exothermic decomposition, and appropriate care
should be taken to handle carefully in small amounts. This
compound was prepared analogously to [(tpa)-Fe-(p-tolylBF)]2-
(ClO4)2. The product, however, was precipitated by addition of ether
into a methanol solution, which yielded the product (126 mg, 89%
yield) as a green solid after drying under vacuum. 1H NMR (CD2Cl2,
500 MHz) δ 131, 64, 54, 47, 22, 10.2, 9.5, 5.0, 4.6, 0.1 ppm.
UV–vis [λmax, nm (ε, M-1 cm-1) in CH2Cl2 at 20 °C]: 528 (400),
572 (380). IR (neat): 622, 735, 766, 1096, 1223, 1445, 1606, 1644,
1675, 2855, 2920, 2949, 3060 cm-1. HRMS-(ESI) m/z calcd for
[(TPA)-Fe-(MesBF)]+: 731.2680; found 731.2712.
[(tpa)Fe(Me3BF)]2(ClO4)2 (6). Caution! Perchlorate salts are
prone to rapid exothermic decomposition, and appropriate care
should be taken to handle carefully in small amounts. This
compound was prepared analogously to [(tpa)Fe(p-tolylBF)]2-
(ClO4)2. The product (435 mg, 87% yield) was obtained as a orange
Supporting Information Available: Figures S1-S6 and CIF
files. This material is available free of charge via the internet at
http://pubs.acs.org.
IC701823Y
Inorganic Chemistry, Vol. 47, No. 4, 2008 1331