Synthesis and structures of dimeric iron(III)-Oxo and -imido complexes containing intramolecular hydrogen bonds

Article abstract of DOI:10.1016/j.ica.2006.12.020

Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate ligand bis[(N′-tert-butylureido)-N-ethyl]-N-methylaminato ([H₂1]^2-) that contains two hydrogen bond donors to examine the oxidation of the Fe^II-acetate complex, [Fe^IIH₂1(η²-OAc)]^- with dioxygen, amine N-oxides, and xylyl azide. A complex with Fe^III-O-Fe^III core results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the [H₂1]^2- ligand. A distinctly different hydrogen bonding network was found in Fe^III dimer isolated from the reaction with the xylyl azide: a rare Fe^III-N(R)-Fe^III core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.

Full text of DOI:10.1016/j.ica.2006.12.020

Inorganica Chimica Acta 360 (2007) 2397–2402
www.elsevier.com/locate/ica
Synthesis and structures of dimeric iron(III)-Oxo and -imido
complexes containing intramolecular hydrogen bonds
*
Matthew K. Zart, Douglas Powell, A.S. Borovik
Department of Chemistry, University of Kansas, 2010 Malott Hall, 1251 Wescoe Dr., Lawrence, KS 66045, USA
Received 2 November 2006; accepted 3 December 2006
Available online 16 December 2006
Abstract
Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination
spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate
ligand bis[(N0-tert-butylureido)-N-ethyl]-N-methylaminato ([H21]2ꢀ) that contains two hydrogen bond donors to examine the oxidation
of the FeII–acetate complex, [FeIIH21(g2-OAc)]ꢀ with dioxygen, amine N-oxides, and xylyl azide. A complex with FeIII–O–FeIII core
results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the
[H21]2ꢀ ligand. A distinctly different hydrogen bonding network was found in FeIII dimer isolated from the reaction with the xylyl azide:
a rare FeIII–N(R)–FeIII core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond
networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Iron complexes; Hydrogen bonds; Oxo/imido ligands
1. Introduction
rical tris[(N0-tert-butylureido)-N-ethyl]aminato ligand
[H3buea]3ꢀ, which favor formation of monomeric iron
complexes with terminal chalcogenido [5], amido [6],
and hydroxo ligands [7]. We recently introduce the
related tridentate ligand bis[(N0-tert-butylureido)-N-
ethyl]-N-methylaminato ([H21]2ꢀ) [8], in which one of
the urea arms has been replaced with a methyl group.
The molecular structure of the FeII–OAc, [FeIIH21(g2-
OAc)]ꢀ, showed a five-coordinate structure with intra-
molecular H-bonds involving the bidentate acetate
ligand. The H-bond cavity surrounding the Fe–acetate
moiety is relatively open, suggesting that polynuclear
species could be formed upon oxidation. Reported herein
are examples of dimeric FeIII–(l-O)–FeIII and FeIII–(l-
NR)–FeIIIcomplexes formed upon the oxidation of
[FeII(H21)(g2-OAc)]ꢀ with O2 and xylyl azide, respec-
tively. These complexes have different H-bond networks
that reflect the nature of the bridging atom between
the iron centers.
Oxo and imido complexes of transition metal ions are
actively studied as species involved in the oxidative
group transfer of oxo (O2ꢀ) and imido (NR2ꢀ) species
to substrates such as alkanes and alkenes [1–3]. We have
been investigating the effects of non-covalent interactions
on the metal-mediated transformation involving oxo and
imido units. As part of this program, we have examined
structure–function relationships in synthetic systems with
hydrogen bonds (H-bonds) [4], which has led to the
development of urea-based ligands that provide intramo-
lecular H-bond donors proxmal to coordinatively unsat-
urated metal centers. Most of these systems are
sterically-bulky tripodal ligands, such as the C3-symmet-
*
Corresponding author. Present address: Department of Chemistry,
University of California, Irvine, CA, USA. Tel.: +1 949 824 4097; fax: +1
949 824 475.
E-mail address: aborovik@uci.edu (A.S. Borovik).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.12.020

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M.K. Zart et al. / Inorganica Chimica Acta 360 (2007) 2397–2402
2. Experimental
2.1. General
2.3.2. Bis{Bis[(N0-tert-butylureaylato)-N-ethyl]-N-
methylaminatoferrate(III)}-(l-xylylimido) ([(FeIIIH21)2-
(l-N-Xylyl)])
A
solution
of
K[FeIIH21(g2-OAc)]
(99.4 mg,
All reagents were purchased from commercial sources
and used as received, unless noted otherwise. Anhydrous
solvents were purchased from Aldrich. The syntheses of
all metal complexes were conducted in a Vacuum Atmo-
spheres, Co. drybox under an argon atmosphere. Elemen-
tal analysis of all compounds was performed by Desert
Analytics, Tucson, AZ. All samples were dried in vacuo
before analysis. FT-IR spectroscopy was used to corrobo-
rate the presence of solvates. Xylyl azide was prepared
according to the literature [9].
0.213 mmol) in anhydrous N,N-dimethylacetamide
(DMA, 5 mL) was treated with xylyl azide (18.2 mg,
0.124 mmol) under an Ar atmosphere. The initially light
yellow solution rapidly turned dark purple with evolution
of gas and was stirred for 1 h. The DMA was removed
under vacuum and the residue dissolved in anhydrous
methylene chloride. The resulting purple solution was then
filtered to remove insoluble KOAc (19 mg, 0.19 mmol).
Methylene chloride was removed under vacuum and solid
isolated crude to yield 85 mg (93%). Anal. Calc. for
C38H71Fe2N11O4 ([(FeIIIH21)2(l-N-Xylyl)]): C, 53.21; H,
8.34; N, 17.96. Found: C, 52.85; H, 8.30; N, 17.71%. FTIR
(Nujol, cmꢀ1): m 3290m, 3181m (NH), 1590s, 1557s, 1377s,
1294s, 1210s, 1061m, 900m, 760m, 659w, 593w. kmax/nm
(CH2Cl2, e, Mꢀ1 cmꢀ1): 508 (5400), 417 (5000).
2.2. Physical methods
Fourier transform infrared spectra were recorded on an
ATI Mattson Genesis Series FTIR spectrometer and are
reported in wavenumbers. Solid samples were prepared
in mineral oil and run between KBr plates. Perpendicu-
lar-mode X-band electron paramagnetic resonance spectra
2.4. Crystal structure determination
Intensity data for the compounds were collected using
a Bruker APEX CCD area detector [10] mounted on a
Bruker D8 goniometer using graphite monochromated
were collected using
a Bruker EMX spectrometer
equipped with an ER4102ST cavity and ER041XG micro-
wave bridge. The instrument was previously calibrated
using DPPH. EPR spectra were collected using the fol-
lowing spectrometer settings: attenuation, 25 dB; micro-
wave frequency, 9.46 GHz; microwave power, 0.638 mW;
sweep width, 5000 G; modulation frequency, 100 kHz;
modulation amplitude, 10.02 G; gain, 1.00 · 103; conver-
sion time, 81.920 ms; time constant, 655.36 ms; resolution,
1024 points. Electronic spectra were collected on a Cary
50 spectrophotometer using 1.00 mm quartz cuvets.
˚
Mo Ka radiation (k = 0.71073 A). The data were collected
at 100(2) K. For [(FeIIIH21)]2(l-O) the intensity data were
measured as a series of x oscillation frames each of 0.25ꢁ
for 90 s/frame. Coverage of unique data was 99.4% com-
plete to 25.00ꢁ in h. For [(FeIIIH21)]2(l-N-Xylyl) Æ CH2Cl2
the intensity data were measured as a series of x oscillation
frames each of 0.25ꢁ for 20 s/frame. Coverage of unique
data was 94.1% complete to 26.00ꢁ in h.
2.4.1. [(FeIIIH21)2(l-O)]
[(FeIIIH21)2(l-O)] crystallized in the monoclinic space
group P21/c, which was determined by systematic absences
and statistical methods and verified by subsequent refine-
ment. Cell parameters were determined from non-linear
least squares fit of 2276 peaks in the range
2.18 < h < 24.18ꢁ. A total of 21286 data were measured
in the range 2.00 < h < 25.00ꢁ. The data were corrected
for absorption by the semi-empirical method [11] from
equivalent reflections giving minimum and maximum
transmission factors of 0.7151 and 0.9839. The structure
was solved by direct methods and refined by full-matrix
least-squares methods on F2 [12]. The carbons (C2B,
C12B and C22B) around N1B were disordered and mod-
eled in two orientations with refined occupancies of
0.833(6) and 0.167(6) for the unprimed and primed atoms,
respectively. Restraints on the positional and displacement
parameters of the disordered atoms were included in the
refinement. The displacement ellipsoids were drawn at the
50% probability level. Hydrogen atom positions were ini-
tially determined by geometry and refined by a riding
model. Non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atom displacement
2.3. Preparation of the complexes
2.3.1. Bis{bis[(N0-tert-butylureaylato)-N-ethyl]-N-
methylaminatoferrate(III)}(l-oxo) ([(FeIIIH21)2(l-O)])
A
solution of K[FeIIH21(g2-OAc)] [8] (100.0 mg,
0.214 mmol) in anhydrous N,N-dimethylacetamide
(DMA, 5 mL) was treated with trimethylamine N-oxide
(8.1 mg, 0.108 mmol) under an Ar atmosphere. The ini-
tially light yellow solution rapidly turned red-orange and
was stirred for 1 h. The DMA was removed under vacuum
and the residue dissolved in anhydrous methylene chloride.
The resulting orange solution was then filtered to remove
insoluble KOAc (20 mg, 0.20 mmol). Anhydrous pentane
was layered on the methylene chloride solutions in two
vials. The orange solid that precipitated was collected by
filtration and washed with pentane to yield 67 mg (83%).
Anal. Calc. for C30H62Fe2N10O5 ([(FeIIIH21)2(l-O)]): C,
47.75; H, 8.28; N, 18.56. Found: C, 47.44; H, 7.74; N,
18.08%. FTIR (Nujol, cmꢀ1): m 3301s, 3087ms (NH),
1593s, 1564s, 1500s, 1349s, 1304s, 1217s, 1070s, 905m,
777m, 658w, 580w. kmax/nm (CH2Cl2, e, Mꢀ1 cmꢀ1): 316
(7700), 361 (6900).

M.K. Zart et al. / Inorganica Chimica Acta 360 (2007) 2397–2402
2399
parameters were set to 1.2 (1.5 for methyl) times the dis-
3. Results and discussion
placement parameters of the bonded atoms. A total of
4552 parameters were refined against 28 restraints and
6609 data to give wR(F2) = 0.1279 and S = 0.999 for
3.1. Synthesis and solution properties
weights
of
w = 1/[r2(F2) +
(0.0600P)2],
where
Scheme 1 outlines the preparative routes to [(FeIIIH21)2-
(l-X)] (X = O2ꢀ and N-xylyl2ꢀ). Addition of either 0.25
equivalent (equiv.) of O2 or 0.5 equiv. of trimethylamine-
N-oxide to a dry dimethylacetamide (DMA) solution of
K[FeIIH21(g2-OAc)] resulted in an immediate color change
form light yellow to deep orange. Removal of the solvent
after 1 h and dissolving the residue in CH2Cl2 before filter-
ing removed 1 equiv. of KOAc. [(FeIIIH21)2(l-O)] was iso-
lated as orange crystals in 80% yield after layering the
CH2Cl2 solution of the compound with pentane.
P ¼ ½F 2o þ 2F 2cꢁ=3. The final R(F) was 0.0572 for the 4196
observed, [F > 4r(F)], data. The largest shift/s.u. was
0.001 in the final refinement cycle. The final difference
3
˚
map had maxima and minima of 1.130 and ꢀ0.318 e/A ,
respectively.
2.4.2. [(FeIIIH21)2(l-N-xylyl)] Æ CH2Cl2
[(FeIIIH21)2(l-N-xylyl)] Æ CH2Cl2 crystallized in the tri-
ꢀ
clinic space group P1, which was determined by statistical
tests and verified by subsequent refinement. Cell parame-
ters were determined from non-linear least squares fit of
5542 peaks in the range 2.50 < h < 30.00ꢁ. A total of
19783 data were measured in the range 1.94 < h < 30.03ꢁ.
The data were corrected for absorption by the semi-empir-
ical method[11] from equivalent reflections giving mini-
mum and maximum transmission factors of 0.7974 and
0.9413. The structure was solved by direct methods and
refined by full-matrix least-squares methods on F2 [12].
One side arm, N7A-C11A, of one ligand was disordered
and was modeled in two orientations with refined occupan-
cies of 0.628(9) and 0.372(9) for the unprimed and primed
atoms. The solvent molecule was disordered and was mod-
eled in two orientations with refined occupancies of
0.570(11) and 0.430(11) for the unprimed and primed
atoms. Restraints on the positional and displacement
parameters of the disordered atoms were required. The dis-
placement ellipsoids were drawn at the 50% probability
level. Hydrogen atom positions were initially determined
by geometry and refined by a riding model. Non-hydrogen
atoms were refined with anisotropic displacement parame-
ters. Hydrogen atom displacement parameters were set to
1.2 (1.5 for methyl) times the displacement parameters of
the bonded atoms. A total of 597 parameters were refined
against 64 restraints and 12512 data to give
wR(F2) = 0.1292 and S = 1.022 for weights of w = 1/
[r2(F2) + (0.0760 P)2], where P ¼ ½F o2 þ 2F 2cꢁ=3. The final
R(F) was 0.0480 for the 9371 observed, [F > 4r(F)], data.
The largest shift/s.u. was 0.001 in the final refinement cycle.
The final difference map had maxima and minima of 0.744
The bridged imido complexes [(FeIIIH21)2(l-N-xylyl)]
was prepared analogously by treating K[FeIIH21(g-OAc)]
in DMA with 0.5 equiv. of xylylazide. This immediately
resulted in a color change from light yellow to dark purple
with concomitant evolution of N2. The dark purple bridg-
ing imido complex, [(FeIIIH21)2(l-N-xylyl)], was obtained
in its purest form by simply removing volatiles after dis-
solving and filtering the crude product in CH2Cl2 [10b].
Single crystals could be obtained from CH2Cl2/pentane
or THF/pentane mixtures, but prolonged standing in the
recystallization media led to impure product.
Solutions of both complexes are intensely colored: the
absorbance spectrum of [(FeIIIH21)2(l-N-xylyl)] in CH2Cl2
has bands at kmax (eM) = 508 (5400) and 417 (5000) nm,
while bands for [(FeIIIH21)2(l-O)] in CH2Cl2 are at 361
(6900) and 316 (7700) nm. Frozen solution of
[(FeIIIH21)2(l-N-xylyl)] and [(FeIIIH21)2(l-O) were EPR
silent at 4 and 77 K.
3.2. Crystal structures
X-ray diffraction measurements reveal dimeric struc-
tures for each complex. Table 1 contains crystallographic
details, Table 2 pertinent metrical results, and Figs. 1 and
2
contain the thermal ellipsoid plots and for
[(FeIIIH21)2(l-O)] and [(FeIIIH21)2(l-N-xylyl)] Æ CH2Cl2,
respectively. Each of the metal ions has a trigonal bipyra-
midal coordination geometry. Donor atoms from the
[H21]2ꢀ ligands occupy four of the coordination sites on
the iron ions; the bridging ligand occupies the fifth site.
One urea arm from each [H21]2ꢀ ligand bridges between
3
˚
and ꢀ0.368 e/A , respectively.
Scheme 1. Conditions: (a) 0.5 equiv. TMANO or 0.25 equiv. O2 DMA, rt, Ar; (b) 0.5 equiv. N3Xylyl, DMA, rt, Ar.

2400
M.K. Zart et al. / Inorganica Chimica Acta 360 (2007) 2397–2402
Table 1
Table 2
Crystallographic data for [(FeIIIH21)2(l-O)] and [(FeIIIH21)2(l-N-
xylyl)] Æ CH2Cl2
Selected bond distances (A) and angles (ꢁ) for [(FeIIIH21)2(l-O)] and
˚
[(FeIIIH21)2(l-N-xylyl)] Æ CH2Cl2
a
[(FeIIIH21)2(l-O)]
[(FeIIIH21)2(l-
N-xylyl)] Æ CH2Cl2
Bond lengths or angles [(FeIIIH21)2(l-O)]
[(FeIIIH21)2(l-N-
xylyl)] Æ CH2Cl2
Molecular formula
Formula weight
T (K)
C30H62Fe2N10O5
754.60
100(2)
C39H73Cl2Fe2N11O4
942.68
100(2)
Fe1A–X
Fe1B–X
1.818(3)
1.827(3)
2.260(3)
2.045(3)
2.007(3)
1.973(3)
2.302(3)
2.047(4)
1.997(3)
1.984(3)
3.062(4)
2.935(5)
3.005(3)
1.872(2)
1.867(3)
2.266(2)
2.034(2)
2.011(2)
2.057(1)
2.288(2)
2.037(2)
2.013(20
2.051(2)
Fe1A–N1A
Fe1A–N4A
Fe1A–N14A
Fe1A–O6A
Fe1B–N1B
Fe1B–N4B
Fe1B–N14B
Fe1B–O6A
N17Aꢂ ꢂ ꢂX
N17Bꢂ ꢂ ꢂX
Fe1Aꢂ ꢂ ꢂFe1B
Fe1A–X–Fe1B
N1A–Fe1A–X
N1A–Fe1A–N4A
N1A–Fe1A–N14A
N1A–Fe1A–O6B
N4A–Fe1A–X
N4A–Fe1A–N14A
N4A–Fe1A–O6B
N14A–Fe1A–X
N14A–Fe1A–O6B
O6B–Fe1A–X
ꢀ
Space group
P21/c
P1
˚
a (A)
11.9219(10)
20.3802(17)
15.9341(13)
90
103.349(2)
90
4 (Z0 = 1)
3781.9(5)
1.325
11.9975(7)
13.5221(8)
16.3671(9)
78.507(2)
73.002(2)
72.102(2)
2 (Z0 = 1)
2398.9(2)
1.305
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
Z
3
˚
V (A )
dcalc (Mg/m3)
2.9741(4)
Ra
0.0572
0.1279
0.999
0.00480
0.1292
1.022
111.02(2)
174.4(1)
76.9(1)
77.7(1)
89.5(1)
97.9(1)
129.9(1)
114.8(1)
104.3(1)
110.7(1)
94.6(1)
172.7(1)
77.1(1)
79.0(1)
88.7(1)
95.7(1)
126.1(1)
123.3(1)
106.5(1)
103.4(1)
94.5(1)
105.37(8)
101.91(7)
74.84(7)
79.01(7)
151.34(6)
105.99(7)
133.96(7)
89.27(7)
116.15(7)
96.45(7)
105.37(6)
105.81(7)
75.83(7)
79.25(7)
147.13(6)
105.53(7)
136.67(7)
86.09(7)
115.14(7)
96.65(6)
105.37(7)
b
Rw
Goodness-of-fitc
P
P
a
b
c
R = [ jDFj/ jFoj].
P
P
2
Rw ¼ ½ xðDF Þ = xF 2oꢁ.
Goodness of fit on F2.
the iron centers to form two l-1,3-(jN:jO)-ureate linkages.
In this binding mode, the deprotonated urea Nꢀ atom
coordinates to one iron ion, while its carbonyl oxygen atom
binds to the other metal center.
These similarities in structural orientation are also
reflected in their metrical parameters. In [(FeIIIH21)2-
N1B–Fe1B–X
N1B–Fe1B–N4B
N1B–Fe1B–N14B
N1B–Fe1B–O6A
N4B–Fe1B–X
N4B–Fe1B–N14B
N4B–Fe1B–O6A
N14B–Fe1B–X
N14B–Fe1B–O6A
O6A–Fe1B–X
˚
(l-O)], the average Fe–Namine distance is 2.281(3) A, signif-
icantly longer than the average Fe–Nureate distance of
2.024(3) A. For [(FeIIIH21)2(l-N-xylyl)], these average dis-
˚
˚
tances are 2.277(2) and 2.024(2) A, respectively. This differ-
ence in bond length between amine and deprotonated urea
nitrogen atoms to the metal center is similar to that seen in
the Fe(II) precursor, FeIIH21(g2-OAc)]ꢀ. The average Fe–
a
X denotes the bridging atom for each species: O1 for [(FeIIIH21)2(l-O)]
and N1C for [(FeIIIH21)2(l-N-xylyl)] Æ CH2Cl2.
Oureate distances are 1.979(3) A for [(FeIIIH21)2(l-O)]
˚
compared with 2.054(2) A in the case of [(FeIIIH21)2(l-N-
˚
xylyl)].
In [(FeIIIH21)2(l-O)], the average Fe–Ooxo bond length
˚
is 1.823(3) A. This value lies towards the longer side for
known l-oxo dimers of FeIII where the range is 1.73–
˚
˚
1.82 A and the average is 1.77 A [13]. This may be an effect
of the H-bond donation by the two urea arms. The Fe–Fe
˚
separation is 3.005(3) A and is slightly shorter compared to
distances found in other di- and tri-bridged l-oxo dimers
˚
that range from 3.05 to 3.39 A. Similarly, the Fe1A–O1–
Fe1B angle is 111.0(2)ꢁ, falling at the acute end of the range
for known l-oxo dimers (114–180ꢁ) [6].
In [(FeIIIH21)2(l-N-xylyl)], the average Fe–Nimido bond
III
˚
length is 1.870(2) A. Imido bridge Fe dimers are rare:
the only other known example is (l-p-tolylimido)
bis[(N,N0-ethane-1,2-diyl- bis(salicylaldiminato))iron(III)],
[(FeIIIsalen)2(l-N-p-tolyl)] [14]. The average bond length
for [(FeIIIsalen)2(l-N-p-tolyl)] is comparable at 1.88(1) A.
˚
Note that FeIII complexes with terminal organoimido
Fig. 1. Molecular structure of [(FeIIIH21)2(l-O)]. The ellipsoids are drawn
at the 50% probability level. Non-urea hydrogen atoms and the methyl
groups of the tert-butyl functionalities have been removed for clarity.
ligands have considerably shorter Fe–N lengths (less than
˚
1.7 A), reflecting their multiple bond character that is

M.K. Zart et al. / Inorganica Chimica Acta 360 (2007) 2397–2402
2401
The oxidation chemistry observed for [FeIIH21-
(g-OAc)]ꢀ contrasts that found for iron(II) complexes of
the tripodal ligand, [H3buea]3ꢀ, which yield monomeric
Fe(III) complexes with terminal oxo or hydroxo ligands.
The more protective cavity of the tripodal ligands in con-
junction with the extensive H-bond network surrounding
the Fe(III)–O(H) units stabilizes the monomeric species.
Similarly, we have found that monomeric Fe(III)-amido
complexes are formed when Fe(II) tripodal complexes are
treated with aryl azides. Our studies with the iron tripodal
complexes suggest that high valent FeIV species are the
likely intermediates during the activation of dioxygen or
arylazides [6]. For [FeII(H21)(g2-OAc)]ꢀ, where the metal
center is more exposed, a dimerization process would be
likely and lead to the formation of the observed products.
Numerous examples of heme and non-heme complexes
with FeIII–O–FeIII unit are know and proceed through a
similar dimerization route [16]. In addition, a analogous
pathway with arylazides has been proposed in the forma-
tion of [(FeIIIsalen)2(l-N-p-tolyl)] [14].
Fig. 2. Molecular structure of [(FeIIIH21)2(l-N-xylyl)]. The ellipsoids are
drawn at the 50% probability level. Non-urea hydrogen atoms and the
methyl groups of the tert-butyl functionalities have been removed for
clarity.
4. Summary
We have reported the preparations and structural pro-
perties for [(FeIIIH21)2(l-X)] complexes formed upon the
oxidation of [FeII(H21)(g2-OAc)]ꢀ with dioxygen and xylyl
azide. Both dimers display bridging l-1,3-(jN:jO)-ureate
linkages, which are becoming more prevalent in coordina-
tion chemistry [17]. [(FeIIIH21)2(l-N-xylyl)] exhibits an
unusual bridging imido ligand with this being only the sec-
ond example reported. The l-oxo FeIII dimer contains two
intramolecular H-bonds to the bridging oxo, which is rare
in synthetic complexes, but is common in metalloproteins
with Fe–(O)n–Fe cores [18]. The intramolecular H-bond
donors within these dimers can ‘‘sense’’ the presence of a
bridging species and rearrange to its size and electron
density.
absent in the dimer complexes [15]. The Fe–Fe separation
in [(FeIIIH21)2(l-N-xylyl)] is 2.974(4) A while that in
˚
[(FeIIIsalen)2(l-N-p-tolyl)] is significantly longer at
3.399(3) A. The shorter distance in [(FeIIIH21)2(l-N-xylyl)]
˚
is most likely a result of three bridging groups versus the
single bridging group in [(FeIIIsalen)2(l-N-p-tolyl)]. The
Fe1A–N1C–Fe1B angle is 105.37(8)ꢁ compared with
129.6(6)ꢁ in [(FeIIIsalen)2(l-N-p-tolyl)].
3.3. H-bonding networks
The non-bridging urea arms from each ligand form
intramolecular hydrogen bonds in each of the two com-
plexes. For [(FeIIIH21)2(l-O)], the two arms containing
N17A and N17B are H-bonded to the bridging oxo group.
˚
Acknowledgments
The Nꢂ ꢂ ꢂO distances are 2.935(5) and 3.062(4) A, which are
consistent with H-bonds. This mode of interaction is not
observed for [(FeIIIH21)2(l-N-xylyl)] where intramolecular
H-bonds involving the bridging imido ligand might be
anticipated. However, the nitrogen atom of the bridging
arylimido ligand is not expected to have as much electron
density as a l-oxo ligand. Moreover, the larger xylyl imido
group may affect the intramolecular H-bond network.
Therefore, the urea arms are found to H-bond with the
oxygen atoms O6A and O6B of the bridging ureate, form-
ing NHꢂ ꢂ ꢂO@C H-bonds at distances of 2.975(3) and
We thank the NIH (GM50781) for financial support of
this work. The National Science Foundation (CHE-
0079282) and the University of Kansas funded the pur-
chase of the X-ray instrumentation.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2006.
12.020.
˚
3.023(2) A. In addition, the methyl groups of the tertiary
amines, N1A and N1B, have different orientations in the
two complexes: in [(FeIIIH21)2(l-O)] they are positioned
anti to the dimer core, whereas in [(FeIIIH21)2(l-N-xylyl)]
the methyls are syn, flanking the xylyl group. Taken
together, these results suggest that during complex forma-
tion, the H-bond donating arms can adjust to the nature of
the bridging species within the molecule.
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