Unexpected reactivity resulting from modifications of the ligand periphery: Synthesis, structure, and spectroscopic properties of iron complexes of new tripodal N-heterocyclic carbene (NHC) ligands

Article abstract of DOI:10.1016/j.ica.2010.07.039

The chelating ligand tris-[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMEN^R, R = aryl = 2,6-xylyl (xyl), mesityl (mes)) has provided access to reactive transition metal complexes. Here, two new tripodal N-heterocyclic carbene ligands of the TIM

Full text of DOI:10.1016/j.ica.2010.07.039

Inorganica Chimica Acta 364 (2010) 226–237
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Inorganica Chimica Acta
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Unexpected reactivity resulting from modifications of the ligand periphery:
Synthesis, structure, and spectroscopic properties of iron complexes of new
tripodal N-heterocyclic carbene (NHC) ligands
⇑
Carola S. Vogel, Frank W. Heinemann, Marat M. Khusniyarov, Karsten Meyer
Department of Chemistry and Pharmacy, Inorganic Chemistry, University Erlangen-Nuremberg, Egerlandstraße 1, D-91058 Erlangen, Germany
a r t i c l e i n f o
a b s t r a c t
The chelating ligand tris-[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMENR, R = aryl = 2,6-xylyl (xyl),
mesityl (mes)) has provided access to reactive transition metal complexes. Here, two new tripodal
N-heterocyclic carbene ligands of the TIMENR system (R = aryl = tolyl (tol), 3,5-xylyl (3,5xyl)), featuring
sterically less demanding aryl substituents were synthesized. With these ligands, Fe(II) precursor com-
plexes could be obtained, namely [(TIMENtol)Fe](BF4)2 (3) and [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 (7), which
showed unexpected reactivity upon reduction. Treatment of the compounds with sodium amalgam yield
Article history:
Available online 17 August 2010
Dedicated to Prof. Arnold L. Rheingold
Keywords:
Iron
Carbene ligands
C–H bond activation
Cyclometallation
Crystal structure determination
***
the tris- and bis-metallated products, [(TIMENtol )Fe] (4) and [(TIMEN3,5xyl**)Fe] (8), respectively. While
the Fe(III) complex 4 is relatively inert towards oxygen, the Fe(II) complex 8 is prone to oxidation. This
oxidation of 8 can readily be observed in chlorinated solvents, producing the Fe(III) complex [(TIME-
N3,5xyl**)Fe](PF6) (9). All new ligand imidazolium precursors and metal complexes were characterized
by single crystal X-ray structure determination.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
was achieved [8]. The first X-ray crystal structure determination of
a discrete iron nitride complex, [(TIMENR)Fe(N)]+, was recently
Coordinatively unsaturated, electron-rich metal complexes
have proven to be powerful species for small molecule activation
accomplished after photolysis of an iron-azide complex stabilized
by the N-anchored tris(carbene) ligand TIMENR [9]. Shortly after
this, the second X-ray crystallographic characterization of an iron
nitride complex with a closely related borate-anchored tris(car-
bene) ligand was reported by Smith and co-workers, namely
and functionalization [1]. Due to their
r-donor and p-accepting
properties [2–4], N-heterocyclic carbene (NHC) ligands are partic-
ularly suitable for the synthesis of a variety of low to high-valent
metal complexes; thus, perfectly suitable to act as supporting li-
gands for small molecule activation at reactive coordination com-
plexes [5]. The steric bulk of NHC ligands is highly tunable and
often the ligands can be conveniently synthesized. The anchoring
unit of polydentate NHC systems provides the chelators with addi-
tional electronic and structural flexibility. It could be shown that
sterically encumbering tripodal ligands of tris-[2-(3-aryl-imida-
zol-2-ylidene)ethyl]amine (TIMENR, with R = aryl = 2,6-xylyl (xyl),
mesityl (mes)) create a trigonal platform for metal ions that en-
ables the coordination and activation of small molecules of indus-
trial and biological importance in a protective cylindrical cavity,
such as dinitrogen [6].
[(PhBtBuIm)Fe(N)],
with
[(PhBtBuIm) = phenyltris(1-tert-buty-
limidazol-2-ylidene)borate(1-) [10], reflecting the potential of car-
bene based ligand systems, not only for the stabilization but for the
synthesis of reactive iron nitride complexes as well. Interestingly,
the reactivity of the N-anchored Fe„N appears to be fundamen-
tally different compared to the B-anchored system. While the
B-anchored Fe„N system shows reactivity towards a number of
substrates, like the triphenylmethyl radical or TEMPO-H (1-hydro-
xy-2,2,6,6-tetramethyl-piperidine), the N-anchored Fe„N com-
plexes are remarkably inert towards the same substrates [11].
This observed difference in reactivity might be a result of the indi-
vidual electronic and geometric structures of these two Fe(IV) sys-
tems. While the N-anchored complex is best described as trigonal
pyramidal, the iron ion in the B-based Fe„N is tetrahedrally
coordinated. Additionally, the steric demand of the two NHC
tripods is markedly different. In contrast to the relatively accessi-
ble Fe„N unit in [(PhBtBuIm)Fe(N)], the nitride functionality in
[(TIMENR)Fe(N)]+ is situated in a deep cylindrical cavity with no
side-access for substrates.
An iron active center with an iron nitride moiety has recently
been suggested to be present at the site of biological nitrogen
reduction [7] and significant progress in the synthesis and spectro-
scopic elucidation of iron imide, Fe@NR, and nitride species, Fe„N,
⇑
Corresponding author.
E-mail address: karsten.meyer@chemie.uni-erlangen.de (K. Meyer).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.039

C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
227
(BF4)2
N
N
N
N
R
R
N
C*
Fe
C
N
C
C*
C*
N
N
N
N
C
N
N
C
C
Fe
N
N
C
N
N
N
N
N
N
N
R
[(TIMENtol)Fe](BF4)2 (3)
[(TIMENtol***)Fe] (4)
TIMENR
(PF6)2
PF6
C*
C*
C
N
H
N
C
C*
C
C
H
N
C*
C
N
Fe
C
N
Fe
C
N
C
N
N
C
N
N
N
C
N
N
C
Fe
N
C
N
N
N
N
N
N
N
N
[(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 (7)
[(TIMEN3,5xyl**)Fe] (8)
[(TIMEN3,5xyl**)Fe](PF6) (9)
Chart 1. Overview of the tripodal TIMENR ligand and its iron complexes.
In order to synthesize N-anchored Fe„N complexes with in-
creased reactivity, we started to modify our NHC systems by vary-
ing the steric demand of the aryl substituents. Here, we present the
synthesis and characterization of two new N-anchored NHC li-
gands of the TIMENR system, namely the tolyl and the 3,5-xylyl
derivatives [TIMENtol] and [TIMEN3,5xyl]. During the synthesis of
the corresponding iron complexes, unexpected reactions were ob-
served resulting in the here presented series of new four, five, and
six-coordinate iron complexes (see Chart 1). The synthesis,
spectroscopic, and X-ray structural characterization of the iron
complexes [(TIMENtol)Fe](BF4)2 (3), [(TIMENtol***)Fe] (4), [(TIME-
N3,5xyl)Fe(CH3CN)](PF6)2 (7), [(TIMEN3,5xyl**)Fe] (8) and [(TIME-
N3,5xyl**)Fe](PF6) (9) is presented. The electronic structures of 4, 8,
and 9 were examined by density functional theory (DFT) calcula-
tions and are presented as well.
characterized by single crystal X-ray structure determination with
crystals being representative for the bulk sample.
2.2. Spectroscopic methods
1H NMR spectra were recorded on JEOL 270 and 400 MHz
instruments, operating at respective frequencies of 269.714 and
400.178 MHz with a probe temperature of 23 °C. 13C NMR spectra
were recorded on JEOL 270 and 400 MHz instruments, operating at
respective frequencies of 67.82 and 100.624 MHz with a probe
temperature of 23 °C. Chemical shifts were reported in ppm rela-
tive to the peak of SiMe4 using 1H (residual) chemical shifts of
the solvent as a secondary standard.
57Fe Mößbauer spectra were recorded on a WissEl Mößbauer
spectrometer (MRG-500) at 77 K in constant acceleration mode.
57Co/Rh was used as the radiation source. WinNormos for Igor
Pro software was used for the quantitative evaluation of the spec-
tral parameters (least-squares fitting to Lorentzian peaks). The
minimum experimental line widths were 0.23 mm sÀ1. The tem-
perature of the samples was controlled by an MBBC-HE0106
MÖSSBAUER He/N2 cryostat within an accuracy of 0.3 K. Isomer
2. Experimental
2.1. General considerations
All air- and moisture-sensitive experiments were performed
under dry nitrogen atmosphere using standard Schlenk line tech-
niques or an MBraun inert-gas glovebox containing an atmosphere
of purified dinitrogen. Anhydrous iron(II) chloride, 99.9%, was pur-
chased from Aldrich and used as received. Sodium tetraphenylbo-
rate, potassium tert-butoxide, and sodium tert-butoxide were
purchased from ACROS and were used without further purification.
Solvents were purified using a two-column solid-state purification
system (Glasscontour System, Irvine, CA) and transferred to the
glove box without exposure to air. NMR solvents were obtained
packaged under argon and stored over activated molecular sieves
and sodium (where appropriate) prior to use. Tris-(2-chloro-
ethyl)amine, 1-(3,5-xylyl)imidazole, and 1-(tolyl)imidazole were
synthesized according to literature [12,13]. Elemental analysis
were performed at the Analytical Laboratories at the Friedrich–
Alexander–University Erlangen–Nuremberg (Erlangen, Germany).
Due to poor solubility, limited stability, and difficulties in separa-
tion of the reactants during work-up of some of the compounds,
certain routine analytical methods could not be applied. Yields
were calculated and noted where possible. All compounds were
shifts were determined relative to a-iron at 298 K.
EPR measurements were performed in quartz tubes with J.
Young valves. Frozen solution EPR spectra were recorded on a JEOL
continuous wave spectrometer JESFA200 equipped with an X-band
Gunn diode oscillator bridge, a cylindrical mode cavity, and a he-
lium cryostat. For all samples,
a modulation frequency of
100 kHz and a time constant of 0.1 s were employed. All spectra
were obtained on freshly prepared solutions of 1–10 mM com-
pound in acetonitrile.
2.3. X-ray crystal structure determinations
Colorless plates of [H3TIMENtol](CF3SO3)3 (1(CF3SO3)3) were
grown by layering an n-pentane solution of the corresponding imi-
dazolium salt with diethyl ether. Colorless plates of [(TIMENtol)-
Fe](BF4)2 Á 0.8 Et2O (3 Á 0.8 Et2O) were obtained from a solution of
the complex in a mixture of acetonitrile and diethyl ether. Dark
red needles of [(TIMENtol***)Fe] Á 2 THF (4 Á 2 THF) were grown from
a THF solution of the complex at À37 °C. Colorless plates of

228
C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
Table 1
Crystallographic data, data collection, and refinement details of [H3TIMENtol](CF3SO3)3 (1(CF3SO3)3), [(TIMENtol)Fe](BF4)2 Á 0.8 Et2O (3 Á 0.8 Et2O), and [(TIMENtol***)Fe] Á 2 THF
(4 Á 2 THF).
1(CF3SO3)3 [H3TIMENtol](CF3SO3)3
3 Á 0.8 Et2O [(TIMENtol)Fe](BF4)2 Á 0.8 Et2O
4 Á 2 THF [(TIMENtol***)Fe] Á 2 THF
Empirical formula
Molecular weight
Crystal size (mm3)
T (K)
C
39H42F9N7O9S3
C39.2H47B2F8FeN7O0.8
858.51
C44H52FeN7O2
766.78
1019.98
0.37 Â 0.11 Â 0.10
0.24 Â 0.18 Â 0.07
0.20 Â 0.10 Â 0.08
100
100
150
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
monoclinic
P21/c (no. 14)
14.761(2)
13.986(2)
20.053(3)
90
100.39(2)
90
4071.9(9)
4
triclinic
ꢀ
ꢀ
P1 (no. 2)
P1 (no. 2)
10.2584(7)
12.483(2)
19.679(2)
73.265(10)
84.479(8)
67.943(2)
2236.4(5)
2
12.355(1)
13.3642(8)
13.7659(7)
106.685(4)
90.960(5)
116.281(5)
1924.7(2)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g/cm3)
1.515
0.265
1.400
0.448
1.323
0.440
l
(mmÀ1
)
F(0 0 0)
1052
1782
814
Tmin; Tmax
2h interval (°)
Collected reflections
0.897; 0.970
6.7 6 2h 6 54.2
66 332
0.846; 0.970
6.6 6 2h 6 54.2
85 162
0.842; 0.965
6.2 6 2h 6 54.2
53 280
Independent reflections (Rint
)
9846 (0.0441)
7902
671
8955 (0.0667)
7000
637
8478 (0.0371)
7410
536
Observed reflections [I P 2
No. refined parameters
wR2 (all data)
r(I)]
0.0900
0.1159
0.1223
R1 [I P 2
r
(I)]
0.0366
1.023
0.0449
1.044
0.0428
1.052
Goodness of fit (GOF) on F2
D
qmax/min
0.527/À0.358
0.495/–0.458
0.917/–0.587
[H3TIMEN3,5xyl](Cl)3 Á solv (5(Cl)3 Á solv (solv = 0.9735 CHCl3 Á 2.125
H2O)) precipitated at room temperature from a CHCl3 solution of
the hygroscopic imidazolium salt layered with diethyl ether.
Brown irregular shaped crystals of [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 Á
2 CH3CN (7 Á 2 CH3CN) were obtained by slow diethyl ether diffu-
sion into a layered acetonitrile solution of the complex. Dark red
plates of [(TIMEN3,5xyl**)Fe] (8) were grown from a THF/n-pentane
mixture. Brown plates of [(TIMEN3,5xyl**)Fe](PF6) Á 0.6 THF Á 0.4
CHCl3 (9 Á 0.6 THF Á 0.4 CHCl3) were grown from a mixture of chlo-
roform, dichloromethane and THF. For crystallographic data, data
collection, and refinement details see Tables 1 and 2. Suitable sin-
gle crystals of the compounds were embedded in protective perflu-
oropolyalkylether oil and quickly transferred to the cold nitrogen
gas stream of the diffractometer. Intensity data were collected
either on a Bruker–Nonius KappaCCD diffractometer (1(CF3SO3)3,
3 Á 0.8 Et2O, 4 Á 2 THF, 7 Á 2 CH3CN and 8) using graphite monochro-
occupancy for O200–C204 and O210–C214, respectively. SIMU,
ISOR and SAME restraints were applied in the refinement of the
disorder. The asymmetric unit of [H3TIMEN3,5xyl](Cl)3 Á solv
(5(Cl)3 Á solv) contains a total of four independent molecules of
the imidazolium salt, 3.75 molecules of CHCl3 and 8.5 molecules
of solvent water. In one of the imidazolium cations one of the three
ligand arms is disordered. Two alternative positions were refined
resulting in site occupancies of 81.4(2) and 18.6(2)% for the atoms
N306, N307, C311–C315, C332–C339 and N406, N407, C411–C415,
C432–C439, respectively. This disorder also affects some of the
CHCl3 solvate molecules while other solvate molecules are occu-
pied by approximately 50% or 75%. SIMU, ISOR, DFIX, FLAT, and
SAME restraints were applied in the refinement of 5(Cl)3 Á solv. In
[(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 Á 2 CH3CN) (7 Á 2 CH3CN), one of
the PF6 anions is disordered. Two alternative orientations were re-
fined resulting in occupancies of 61.6(6) and 38.9(6)% for atoms
P2–F26 and P2A–F26A, respectively. In [(TIMEN3,5xyl**)Fe](PF6) Á 0.6
THF Á 0.4 CHCl3 (9 Á 0.6 THF Á 0.4 CHCl3), one of the PF6 anions is
subjected to rotational disorder in the equatorial plane. Two alter-
native orientations were refined resulting in occupancies of
59.8(6)% and 40.2(6)% for atoms F13–F16 and F13A–F16A, respec-
tively. Compound 9 Á 0.6 THF Á 0.4 CHCl3 crystallizes with a mixture
of solvent molecules (0.6CHCl3 and 0.4THF) which share a common
crystallographic site. SIMU, ISOR, and SAME restraints were ap-
plied in the refinement.
matized Mo K
APEX2 Duo diffractometer (5(Cl)3 Á solv and 9 Á 0.6 THF Á 0.4 CHCl3)
equipped with an I S microsource and QUAZAR focusing optics
using Mo K radiation (k = 0.71073 Å). Lorentz and polarization ef-
a radiation (k = 0.71073 Å) or on a Bruker Kappa
l
a
fects were taken into account during data reduction, semiempirical
absorption corrections were performed on the basis of multiple
scans using SADABS [14]. All structures were solved by direct meth-
ods and refined by full-matrix least-squares procedures on F2 (with
the exception of 5 Á solv, which was refined in a block matrix) using
SHELXTL NT 6.12 [15].
In the computations of the crystal structure determinations of
compounds 1 to 9, all non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were
placed in positions of optimized geometry, their isotropic displace-
ment parameters were tied to those of the corresponding carrier
atoms by a factor of either 1.2 or 1.5.
In [H3TIMENtol](CF3SO3)3 (1(CF3SO3)3), one of the imidazolium
rings is disordered. Two preferred orientations were refined result-
ing in occupancies of 86.9(3)% for atoms N2, N3, C1–C5 and
13.1(3)% for N2A, N3A, C1A–C5A. SIMU and ISOR restraints were
applied. In [(TIMENtol)Fe](BF4)2 Á 0.8 Et2O (3 Á 0.8 Et2O) both BF4 an-
ions are subjected to rotational disorder around one B–F bond. Two
preferred orientations were refined each resulting in occupancies
of 57(2) and 43(2)% for atoms F12–F14 and F12A–F14A, respec-
tively and of 52(5) and 48(5)% for atoms F22–F24 and F22A–
F24A, respectively. [(TIMENtol***)Fe] Á 2THF (4 Á 2THF) crystallizes
with two molecules of THF per formula unit one of which is disor-
dered showing two alternative orientations of 46.4(7) and 53.6(7)%
2.4. Syntheses and characterization
2.4.1. [H3TIMENtol](BF4)3 (1(BF4)3)
A 100 mL flask was charged with tris-(2-chloroethyl)amine
(4.06 g, 19.86 mmol) and 1-(tolyl)imidazole (9.43 g, 59.59 mmol),
the mixture was heated to 150 °C for 2 days. The resulting brown

C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
229
Table 2
Crystallographic data, data collection, and refinement details of [H3TIMEN3,5xyl](Cl)3 Á solv (5(Cl)3 Á solv), [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 Á 2 CH3CN (7 Á 2 CH3CN),
[(TIMEN3,5xyl )Fe] (8), and [(TIMEN3,5xyl**)Fe](PF6) Á 0.6 THF Á 0.4 CHCl3 (9 Á 0.6 THF Á 0.4 CHCl3).
**
5(Cl)3 Á solv [H3TIMEN3,5xyl
]
7 Á 2 CH3CN [(TIMEN3,5xyl)Fe-
8
9 Á 0.6 THF Á 0.4 CHCl3
(Cl)3 Á solv
(CH3CN)](PF6)2 Á 2 CH3CN
[(TIMEN3,5xyl**)Fe] [(TIMEN3,5xyl**)Fe](PF6) Á 0.6 THF Á 0.4 CHCl3
Empirical formula
Molecular weight
Crystal size (mm3)
T (K)
C39.9375H53.1875Cl5.8125N7O2.125 C45H54F12FeN10P2
C39H43FeN7
665.65
C41.8H48.2Cl1.2F6FeN7O0.6
901.63
P
871.39
1080.77
0.28 Â 0.16 Â 0.04
0.50 Â 0.23 Â 0.18
0.24 Â 0.08 Â 0.08 0.20 Â 0.06 Â 0.02
100
150
150
100
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
triclinic
triclinic
monoclinic
P21/c (no. 14)
14.001(2)
13.587(2)
23.134(4)
90
102.443(3)
90
4297(2)
4
ꢀ
ꢀ
ꢀ
P1 (no. 2)
P1 (no. 2)
P1 (no. 2)
17.660(4)
21.259(5)
26.505(6)
106.629(4)
104.166(4)
94.311(4)
9131(4)
11.488(2)
14.968(2)
15.908(2)
74.453(7)
88.036(8)
70.889(7)
2485.7(4)
2
11.5632(7)
11.9611(7)
12.3184(9)
81.472(5)
71.432(5)
82.705(5)
1591.4(2)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
8
qcalc (g/cm3)
1.341
0.406
1.444
0.457
1.389
0.516
1.394
0.530
l
(mmÀ1
)
F(0 0 0)
3661
1116
704
1873
Tmin; Tmax
0.665; 0.746
6.0 6 2h 6 51.4
134 232
0.739; 0.921
6.5 6 2h 6 54.2
70 427
0.818; 0.960
6.7 6 2h 6 54.2
45 758
0.506; 0.421
5.1 6 2h 6 54.2
33 178
2h interval (°)
Collected reflections
Independent
34 579 (0.0905)
10 940 (0.0725)
7010 (0.0711)
9448 (0.0697)
reflections (Rint
Observed reflections
[I P 2 (I)]
No. refined
parameters
wR2 (all data)
R1 [I P 2 (I)]
)
17 923
2248
8332
704
5529
430
6073
612
r
0.2667
0.0892
1.341
0.1269
0.0467
1.039
0.0901
0.0397
1.025
0.1285
0.0506
1.010
r
Goodness of fit (GOF)
on F2
D
qmax/min
1.811/–1.203
0.696/–0.577
0.368/–0.369
0.506/–0.421
solid was dissolved in 20 mL of methanol and the hygroscopic
chloride salt was converted to the corresponding stable tetrafluo-
roborate salt, [H3TIMENtol](BF4)3, by addition of a solution of NaBF4
(6.41 g, 60.00 mmol) in 50 mL of methanol. The tetrafluoroborate
salt precipitated immediately, was collected by filtration, washed
with methanol and diethyl ether, and dried in vacuo.
Yield, 9.19 g (11.02 mmol, 56%). Anal. Calc. for C36H44B3F12N7O:
C, 50.80; H, 5.21; N, 11.52. Found: C, 50.90; H, 5.08; N, 11.18%. 1H
NMR (270 MHz, RT, DMSO-d6): d [ppm] = 9.66 (s, 3H), 8.16 (d, 3H,
3J = 1.5 Hz), 7.85 (d, 3H, 3J = 1.5 Hz), 7.54 (d, 6H, 3J = 8.6 Hz), 7.37
(d, 6H, 3J = 8.0 Hz), 4.36 (t, 6H, 3J = 6.5 Hz), 3.15 (s, 6H,
3J = 6.8 Hz), 2.38 (s, 9H). 13C{1H} NMR (100 MHz, RT, DMSO-d6): d
[ppm] = 214.93 (3C), 135.17 (3C), 132.17 (3C), 130.52 (6C),
123.38 (3C), 121.32 (6C), 120.77 (3C), 51.32 (3C), 46.13 (3C),
20.57 (3C).
2.4.3. [(TIMENtol)Fe](BF4)2 (3)
Method A: [H3TIMENtol](BF4)3 (1000 mg, 1.21 mmol) and so-
dium tert-butoxide (400 mg, 1.21 mmol) in 15 mL THF were stirred
for 2 h, filtered and added to a suspension of FeCl2 (135 mg,
1.21 mmol) in 5 mL pyridine. The reaction mixture was allowed
to stir overnight, during which time an off-white precipitate
formed. The precipitate was collected by filtration, washed with
pyridine, diethyl ether and n-pentane, and dried in vacuo.
Method B:
A
mixture of [H3TIMENtol](BF4)3 (1000 mg,
1.21 mmol), sodium tert-butoxide (400 mg), NaBPh4 (410 mg,
1.21 mmol) and FeCl2 (135 mg, 1.21 mmol) in THF was allowed to
stir overnight, during which time an off-white precipitate formed.
The precipitate was collected by filtration, washed with THF,
diethyl ether and n-pentane, yielding the crude product. Recrystal-
lization can be achieved by dissolving the crude product in acetoni-
trile and layering with diethyl ether at room temperature.
Anal. Calc. for C36H39B2F8FeN7: C, 54.10; H, 4.92; N, 12.27.
Found: C, 53.69; H, 5.07; N, 12.29%. In the 1H NMR spectrum, 11
signals are expected for the complex, four are observed; the
remaining signals are likely shifted and broadened into the base-
line due to the paramagnetism of the compound. 1H NMR
(270 MHz, RT, acetonitrile-d3): d [ppm] = À0.94, 10.61, 12.16,
2.4.2. [TIMENtol] (2)
A solution of potassium tert-butoxide (340 mg, 3.03 mmol) in
10 mL THF was added to a suspension of [H3TIMENtol](BF4)3
(722 mg, 0.87 mmol) in 5 mL of THF and stirred for 3 h. The solu-
tion was then evaporated to dryness and the solid residue was dis-
solved in 15 mL of diethyl ether. The resulting solution was filtered
through celite and evaporated to dryness in vacuo.
14.59. Mößbauer parameters: d = 0.51(1) mm sÀ1
, DEQ = 1.54(1)
mm sÀ1 CFWHM = 0.28(1) mm sÀ1
,
1H NMR (270 MHz, RT, benzene-d6): d [ppm] = 7.74 (d, 6H,
3J = 8.5 Hz), 6.90 (d, 6H, 3J = 8.5 Hz), 6.82 (d, 3H, 3J = 2.1 Hz), 6.43
(d, 3H, 3J = 2.1 Hz), 3.87 (t, 6H, 3J = 6.2 Hz), 2.70 (t, 6H,
3J = 6.2 Hz), 2.00 (s, 9H). 13C{1H} NMR (100.5 MHz, RT, benzene-
d6): d [ppm] = 214.61 (3C), 140.71 (3C), 135.28 (3C), 129.88 (6C),
121.22 (3C), 120.91 (6C), 116.55 (3C), 56.41 (3C), 49.73 (3C),
20.71 (3C).
2.4.4. [(TIMENtol***)Fe] (4)
A solution of [(TIMENtol)Fe](BF4)2 (155 mg, 0.19 mmol) in THF
was stirred over an excess of sodium amalgam (3300 mg, Na
0.8 wt.%) overnight. The resulting red solution and precipitate were
separated from the sodium amalgam with the help of a Pasteur
pipette, the THF was removed and after addition of CHCl3 the

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C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
resulting solution was filtered through celite. Removal of the sol-
vent yields the crude product. Recrystallization can be achieved
by dissolving the crude product in a mixture of CHCl3 and benzene
and cooling the filtered solution to À35 °C.
Na 0.8 wt.%). The solution was filtered through celite, concentrated
in vacuo and cooled to À35 °C to yield the product as dark red
crystals.
Mößbauer parameters: d = 0.14(1) mm sÀ1
,
DEQ = 2.29(2)
EPR (acetonitrile, 7.1 K, 8.987523 GHz, ModWidth = 1.0 mT,
Power = 1.00 mW) g|| = 2.29, g\ = 1.94. Mößbauer parameters:
, .
mm sÀ1 CFWHM = 0.93(3) mm sÀ1
d = 0.03(1) mm sÀ1 EQ = 2.37(1) mm sÀ1 CFWHM = 0.26(1) mm sÀ1
, D , .
2.4.9. [(TIMEN3,5xyl**)Fe](PF6) (9)
Method A: [(TIMEN3,5xyl**)Fe] (200 mg, 0.30 mmol) was dis-
solved in 10 mL CHCl3 to form a green solution. Excess of NaPF6
was added and diethyl ether was then diffused into the filtered
solution at room temperature to give green crystals overnight.
Method B: Addition of silver triflate (77 mg, 0.30 mmol) to a sus-
pension of [(TIMEN3,5xyl**)Fe] (200 mg, 0.30 mmol) yielded a green
solution and a dark grey precipitate of elemental silver. The solu-
tion was filtered through celite and the solvent was removed at re-
duced pressure to yield the green product.
2.4.5. [H3TIMEN3,5xyl](PF6)3 (5(PF6)3)
A 50 mL flask was charged with tris-(2-chloroethyl)amine
(8.49 g, 41.50 mmol) and 1-(3,5-xylyl)imidazole (22.00 g,
128.00 mmol), the mixture was heated to 150 °C for 2 days. The
resulting brown solid was dissolved in 30 mL of methanol and
the hygroscopic chloride salt was converted to the corresponding
stable hexafluorophosphate salt, [H3TIMEN3,5xyl](PF6)3, by addition
of a solution of NH4PF6 (22.00 g, 135.00 mmol) in 150 mL of meth-
anol. The white hexafluorophosphate salt precipitated immedi-
ately, was collected by filtration, washed with methanol and
diethyl ether, and the solid was dried in vacuo.
EPR (acetonitrile, 86 K, 8.981690 GHz, ModWidth = 1.0 mT,
Power = 1.00 mW) g1 = 2.42, g2 = 2.20, g3 = 1.94.
Yield, 28.39 g (27.00 mmol, 65%), 1H NMR (400 MHz, RT, DMSO-
d6): d [ppm] = 9.72 (s, 3H), 8.16 (s, 3H), 7.88 (s, 3H), 7.34 (s, 6H),
7.25 (s, 3H), 4.39 (t, 6H, 3J = 7.48 Hz), 3.20 (t, 6H, 3J = 7.48 Hz),
2.5. Theoretical calculations
The program package ORCA 2.7 revision 0 was used for all
calculations [16]. The geometry optimization calculations were
performed by the spin-unrestricted DFT method with the BP86
[17–19] functional. The single point calculations and calculations
of Mößbauer parameters were performed with the B3LYP func-
tional [20–22]. The triple-f basis sets with one-set of polarization
functions [23] (TZVP) were used for iron ions and the double-f ba-
sis sets with one-set of polarization functions [23] (SVP) were used
for all other atoms. For calculation of Mößbauer parameters, the
‘‘core” CP(PPP) basis set for iron [24,25] was used. This basis is
based on the TurboMole DZ basis, developed by Ahlrichs and
co-workers and obtained from the basis set library under ftp.
chemie.uni-karlsruhe.de/pub/basen. Molecular orbitals were
visualized via the program MOLEKEL [26].
2.35 (s, 18H). 13C{1H} NMR (100.5 MHz, RT, DMSO-d6):
d
[ppm] = 139.80 (6C), 135.12 (3C), 134.29 (3C), 130.92 (3C),
123.26 (3C), 120.60 (3C), 118.83 (6C), 50.85 (3C), 48.02 (3C),
20.56 (6C).
2.4.6. [TIMEN3,5xyl] (6)
A solution of potassium tert-butoxide (0.33 g, 2.95 mmol) in
THF was added to a suspension of [H3TIMEN3,5xyl](PF6)3 (1.00 g,
0.95 mmol) in 5 mL of THF and stirred for 1 h. The solution was
then evaporated to dryness and the solid residue was dissolved
in 15 mL of diethyl ether. The resulting solution was filtered
through celite and the filtrate was evaporated to dryness in vacuo.
1H NMR (270 MHz, RT, benzene-d6): d = 7.54 (s, 6H), 6.88 (m,
3H), 6.67 (m, 6H), 6.63 (d, 3H, 3J = 1.32 Hz), 3.99 (t, 6H
3J = 6.24 Hz), 2.84 (t, 6H, J = 6.24 Hz), 2.12 ppm (s, 18H). 13C{1H}
NMR (100.5 MHz, RT, benzene-d6): d = 212.70 (3C), 142.46 (3C),
138.55 (6C), 128.33 (3C) 121.07 (3C), 118.95 (6C), 116.48 (3C),
56.15 (3C), 49.38 (3C), 21.02 (6C) ppm.
3. Results and discussion
The tripodal ligand system TIMENR (R = mesityl and 2,6-xylyl)
has been successfully employed for the stabilization of metal
centers in high and low oxidation states [9]. A general property
of N-heterocyclic carbenes (NHCs) is the ability to act as both, a
2.4.7. [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 (7)
[H3TIMEN3,5xyl](PF6)3 (1.00 g, 0.95 mmol) and potassium tert-
butoxide (0.33 g, 2.95 mmol) in 15 mL THF were stirred for 2 h, fil-
tered and added to a suspension of FeCl2 (0.12 g, 0.95 mmol) in
5 mL pyridine. The reaction mixture was allowed to stir overnight,
during which time an off-white precipitate formed. The precipitate
was collected by filtration, washed with pyridine, diethyl ether and
n-pentane, and dried in vacuo. Recrystallization of [(TIMEN3,5xyl)-
Fe](PF6)2 from a solvent mixture of acetonitrile and diethyl ether
yielded the compound [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 with a coor-
dinated acetonitrile molecule.
r
-donor, stabilizing high oxidation states, and as
providing the possibility for -backbonding, thus also stabilizing
low oxidation states at electron-rich metal ions. While for a long
time NHCs were regarded as pure -donor ligands, it is now widely
accepted that this class of ligand can also act as -acceptor [2,3].
p-acceptor,
p
r
p
Recently, the synthesis of a discrete high-valent iron nitride
complex was accomplished using a sterically encumbering N-an-
chored tris(carbene) ligand TIMENR, (R = aryl = 2,6-xylyl (xyl),
mesityl (mes), Scheme 1) [9]. The reaction route toward these iron
nitrides proceeds via deprotonation of the imidazolium salt
[H3TIMENR]3+ with a base, like potassium tert-butoxide, yielding
the N-heterocyclic carbene. Subsequent treatment of the free tripo-
dal carbenes with FeCl2 yields the four-coordinated Fe(II) complex
[(TIMENR)Fe(Cl)]Cl, which can be reduced over sodium amalgam to
afford the Fe(I) species [(TIMENR)Fe]BPh4. Addition of TMS-N3 to
the Fe(I) complex results in formation of Me6Si2 and the divalent
complex [(TIMENR)Fe(N3)]BPh4 which – after photolysis with UV
light – yields the deeply purple colored Fe(IV) nitride complex
[(TIMENR)Fe(N)]BPh4. This N-anchored Fe„N complex is relatively
unreactive and air- as well as moisture-stable in solid-state.
Interestingly, preliminary attempts to oxidize the Fe(IV) nitride
complexes to Fe(V) species, unexpectedly lead to reduced bis-car-
bene imine species that formed via insertion of the terminal nitride
Yield, 0.64 g (0.66 mmol, 70%). Anal. Calc. for C41H48F12FeN8P2:
C, 49.31; H, 4.84; N, 11.22. Found: C, 49.46; H, 4.85; N, 11.42%. In
the 1H NMR, 12 signals are expected for the complex, five are ob-
served, the remaining signals are likely shifted and broadened into
the baseline due to the paramagnetism of the compound. 1H NMR
(270 MHz, RT, DMSO-d6): d = 23.99, 20.82, 12.88, 10.29, 4.50.
Mößbauer
0.52(1) mm sÀ1
Mößbauer parameters for [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2: d =
0.73(1) mm sÀ1 EQ = 1.08(1) mm sÀ1 CFWHM = 0.36(1) mm sÀ1
parameters
for
[(TIMEN3,5xyl)Fe](PF6)2:
CFWHM = 0.50(1) mm sÀ1
d =
,
D
EQ = 1.12(1) mm sÀ1
,
.
,
D
,
.
2.4.8. [(TIMEN3,5xyl**)Fe] (8)
A solution of [(TIMEN3,5xyl)Fe](PF6)2 (0.30 g, 0.31 mmol) in THF
was stirred overnight over an excess of sodium amalgam (5.60 g,

C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
231
X3
R
NH2
N
N
N
Cl
R
R
N
R
N
1.) 150°C, 2d
O
O
H
H
2.) NH4X, MeOH
HOAc
O
+
N
3
70°C, 1d
N
N
N
Cl
Cl
NH4
N
-
-
R = aryl
X = BF4 , PF6
R
KOtBu
THF
R
X2
N
R
N
N
N
C
R
R
N
R
N
N
ferrous salt
N
Fe
N
C
C
THF
N
N
N
N
Fe(II) precursor
TIMENR
N
R
Scheme 1. Synthesis of TIMENR and the Fe(II) precursor complexes.
ligand into one of the iron – carbene bonds of the TIMENR ligand.
This chemistry will be reported in due time and appears to be rem-
iniscent to the insertion chemistry observed for the Co(III) imide
complex, [(TIMENR)Co(NR)]BPh4 [27]. The latter complex can be
isolated at À35 °C but at room temperature forms similar bis-car-
bene imine species in solution.
Even in the presence of substrates, these examples of observed
intramolecular insertion chemistry showed that the methyl groups
in ortho position of the mesityl and 2,6-xylyl aryl rings of the NHC
ligand play an important role in the reactivity of the corresponding
iron complexes. Due to a strong steric shielding of the substituents
on these positions, side-access of potential substrates to the axial
M„L functional group likely is efficiently suppressed, thereby pre-
venting further reactivity of the Fe„N unit in [(TIMENR)Fe(N)]BPh4
and its oxidized Fe(V) species.
than typically observed for four-coordinate Fe(II) high-spin com-
plexes of this system (d ꢀ 0.65–0.75 mm sÀ1) [9].
The molecular structure of the cation [(TIMENtol)Fe]2+ (Fig. 1) in
crystals of [(TIMENtol)Fe](BF4)2Á0.8Et2O (3Á0.8Et2O), obtained from
a solvent mixture of acetonitrile and diethyl ether, exhibits a
four-coordinated iron center with three equivalent iron carbene
distances (average Fe–Ccarbene distance 2.051 Å) and an Fe–Nanchor
distance of 2.204(2) Å, clearly indicating a coordinated N-anchor.
Aiming at high-valent metal complexes with axially bound ter-
minal p-donor ligands, such as nitride, imides, and oxo ligands for
atom and group transfer catalysis, we therefore synthesized two
new tripodal carbene ligands with sterically less demanding aryl
substituents, namely TIMENR (R = aryl = tolyl (tol), 3,5-xylyl
(3,5xyl), Scheme 1). These ligands were employed in the synthesis
of the corresponding Fe(II) precursor complexes in order to inves-
tigate their potential for the preparation of new low and high-va-
lent iron complexes with enhanced reactivity.
Scheme 1 shows the general synthetic route to the tripodal li-
gand system. The first step is a one-pot reaction employing glyoxal,
formaldehyde, ammonium salt, and an amine producing the
derivatized imidazole [13]. The imidazoles can be linked to the
framework anchor in an S2N-type reaction with tris-(2-chloro-
ethyl)amine [12]. Exchange of the ClÀ counterions to PF6À or BFÀ4
anions leads to non-hygroscopic imidazolium salts [28]. Deproto-
nation of the imidazolium salt with a strong base, like potassium
tert-butoxide, yields the N-heterocyclic carbene. Reaction of the
free carbene with a ferrous salt like FeCl2 or FeOTf2 delivers the
Fe(II) precursor complexes. The cationic Fe(II) precursor [(TIME-
Ntol)Fe]2+ was obtained via convenient one-pot synthesis, a new
and viable alternative route for the synthesis of Fe(II) precursors
of the TIMEN system. In this one-pot synthesis, a mixture of
[H3TIMENR](BF4)3, NaOtBu, NaBPh4, and FeCl2 in THF is stirred
overnight, followed by recrystallization of the crude product.
The [(TIMENtol)Fe](BF4)2 complex exhibits a Mößbauer isomer
Fig. 1. Molecular structure of the complex cation of [(TIMENtol)Fe](BF4)2 (3) in
crystals of 3Á0.8Et2O (50% probability ellipsoids). Hydrogen atoms (except H28A),
BFÀ4 anions, and co-crystallized solvents are omitted for clarity. The dotted line
indicates the weak anagostic FeÁ Á ÁH interaction.
shift, d, of 0.51(1) mm sÀ1 EQ = 1.54(1) mm sÀ1), slightly smaller
(D

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C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
The metal center is located 0.197(2) Å below the trigonal plane of
the three NHC carbene carbon atoms. It is remarkable that,
although present in the reaction mixture, no axially bound ligand,
like chloride or acetonitrile, is observed in the crystallographically
determined structure of 3. This is in contrast to the Fe(II) com-
plexes of the related TIMENmes and TIMENxyl system with deep
cylindrical cavities formed by the aryl substituents. For these com-
plexes, axial coordination is always observed in the presence of
coordinating anions or solvents. A possible reason for this could
be the lack of methyl groups in 2- and 6-position of the aryl sub-
stituents in complexes of [(TIMENtol)Fe]2+, allowing free rotation
of the tolyl arms of the ligand, and thereby, preventing the forma-
tion of an accessible ligand cavity for axial coordination in the
TIMENtol system. This rotational freedom is efficiently suppressed
in ligands of the mesityl and 2,6-xylyl ligand derivatives. Further-
more, a close inspection of the structurally determined tolyl hydro-
gen atoms reveals that at least one of the ortho aryl C–H hydrogen
atoms is pointing towards the center of the cavity and is thereby
additionally blocking ligand access. The Fe1Á Á ÁH28A distance of
2.90 Å and the corresponding Fe1Á Á ÁH28A–C28 angle of 122° is
indicative for a weak and largely electrostatic interaction (anago-
stic interaction) [29].
Attempts to reduce the Fe(II) ion in complex 3 with sodium
amalgam yield a different product than observed for the mesityl
and 2,6-xylyl derivatives (Scheme 2). Instead of the expected for-
mation of the reduced Fe(I) species, the oxidized, threefold cyclo-
metallated
r
-aryl Fe(III) complex [(TIMENtol***)Fe] (4) was
isolated (Fig. 2).
The EPR spectrum of a frozen solution of 4 shows an axial-sym-
metrical signal with resonances centered at g|| = 2.29 and g\ = 1.94.
This is in agreement with the complexes’ axially distorted octahe-
dral symmetry and a ground state of S = 1/2. As expected for a low-
spin Fe(III) complex, the Mößbauer isomer shift of
d = 0.03(1) mm sÀ1 EQ = 2.37(1) mm sÀ1) is considerably smaller
(D
than that of the Fe(II) precursor complex 3 (0.51 mm sÀ1) and
larger than that of the Fe(IV) nitride species (d4, low-spin, S = 0,
d = À0.27 mm sÀ1). This confirms, that, despite the different coordi-
nation geometries and spin states of the complexes, the trend of
decreasing isomer shifts with increasing oxidation states in 57Fe
Mößbauer spectroscopy is confirmed within this series of
complexes.
(BF4)2
The pseudo-octahedral Fe center of [(TIMENtol***)Fe] (4) in crys-
tals of 4Á2THF coordinates three NHC donors and three
r-aryl car-
*
banions (denoted with an asterisk in Scheme 2) with a facial
arrangement of each of the two different sets of C donor atoms.
In this complex, the iron center is coordinated by three bidentate
C*
N
C
Fe
C*
C*
N
C
Fe
Na/Hg
N
*
N
N
C
ligand arms with every NHC carbon situated trans to a C carbon
N
N
C
THF
C
N
C
N
atom of an aryl ring. It is remarkable, however, that in 4 one of
the Fe–C bonds, Fe–C24 = 2.366(2) Å, is significantly longer, by
N
N
*
N
N
3
N
4
ꢁ0.35 Å, than the other two Fe–Ccarbene bonds. The shortest bond
(Fe–C3 = 1.888(2) Å) is trans to this longest bond (Table 3). In con-
trast, a perfectly symmetrical arrangement is observed for fac-
[Ir{CN(C6H4Me-p)(CH2)2NC6H3Me-p}3] [30], a homoleptic complex
exhibiting exactly the same coordination mode with three NHC do-
nors (Ir–Cavg 2.03(3) Å) and three trans coordinated aryl carbanions
(Ir–Cavg 2.09(3) Å). It is reasonable to assume that the three
individual bidentate ligands, coordinating the iridium in
Scheme 2. Cyclometallation of [(TIMENtol)Fe]2+ under reductive conditions.
Table 3
Selected bond distances (Å) and angles (°) for molecular structures of 3, 4, 7, 8 and 9
(e.s.d’s in parentheses).
Compounds
3
4
7
8
9
Fe–C3
Fe–C8
Fe–C13
Fe–N1
2.046(2)
2.055(2)
2.052(2)
2.204(2)
1.888(2)
1.923(2)
1.943(2)
3.947(2)
–
2.101(3)
2.108(3)
2.107(3)
2.545(3)
1.913(2)
1.870(2)
1.941(2)
3.936(2)
–
1.972(3)
1.895(3)
1.999(3)
3.935(3)
–
Fe–N8
–
2.193(2)
Fe–C17
–
–
–
1.994(2)
2.366(2)
2.014(2)
100.01(8)
97.59(9)
97.46(9)
–
–
–
–
2.442(2)
2.050(2)
2.074(2)
101.10(8)
93.73(8)
95.53(8)
–
2.525(3)
2.032(3)
2.037(3)
103.4(2)
91.3(2)
93.9(2)
–
Fe–C24#1
Fe–C31#2
C3–Fe–C8
C3–Fe–C13
C8–Fe–C13
C3–Fe–N1
C8–Fe–N1
C13–Fe–N1
C3–Fe–C17
C8–Fe–C17
C13–Fe–C17
C3–Fe–C24#1
C8–Fe–C24#1
C13–Fe–C24#1
C3–Fe–C31#2
C8–Fe–C31#2
C13–Fe–C31#2
121.73(9)
118.81(8)
116.73(9)
96.67(8)
95.34(8)
94.45(8)
–
–
–
–
–
–
–
–
–
117.9(2)
120.1(2)
120.2(2)
85.0(1)
85.6(1)
86.0(1)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
82.12(9)
89.77(8)
172.69(8)
178.01(8)
78.86(8)
84.20(8)
89.12(8)
170.87(8)
91.69(8)
78.52(8)
176.17(7)
80.71(8)
87.40(8)
82.53(8)
177.92(8)
169.64(8)
88.44(8)
81.22(8)
86.6(2)
82.1(2)
174.9(2)
169.1(2)
85.9(2)
82.2(2)
Fig. 2. Molecular structure of [(TIMENtol***)Fe] in crystals of 4Á2THF (50% probability
#1
#2
C24 corresponds to C25 in complexes 8 and 9.
C31 corresponds to C33 in complexes 8 and 9.
ellipsoids). Hydrogen atoms and co-crystallized solvents omitted for clarity.

C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
233
(PF6)2
C*
Fe
C
C
N
H
N
C
C*
C
N
Na/Hg
N
C
N
N
THF
N
N
C
Fe
C
N
N
N
N
N
8
N
7
PF6
C*
C
N
H
C*
N
Fe
C
CHCl3
N
C
C
N
N
N
N
9
Scheme 3. Cyclometallation of [(TIMEN3,5xyl)Fe]2+ under reductive conditions.
average Fe–Ccarbene distance of 2.105 Å, and a coordinated acetoni-
trile ligand (Fe–NCH
distance of 2.193(2) Å). The iron center is
3CN
situated 0.164(2) Å above the trigonal plane formed by the three
NHC carbene donor atoms. Due to the axial coordination of an ace-
tonitrile molecule, the Fe–Nanchor distance of 2.545(3) Å is consid-
erably longer than that found in [(TIMENtol)Fe]2+ and shows that
the N-anchor is no longer coordinated in 7.
Fig. 3. Molecular structure of the complex cation of [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2
(7) in crystals of 7Á2CH3CN (50% probability ellipsoids). Hydrogen atoms of the
TIMEN3,5xyl ligand, the PFÀ6 anions, and co-crystallized solvents are omitted for
clarity.
The ability of the [(TIMEN3,5xyl)Fe]2+ moiety to accommodate an
axial ligand (as was observed earlier for the corresponding mesityl
and 2,6-xylyl derivatives) prompted us to attempt the synthesis of
a high-valent iron nitride complex. In order to synthesize the re-
quired Fe(I) precursor complexes [(TIMENR)Fe]+, reduction of 7
using the sodium amalgam protocol used for the reduction of
[(TIMENmes/xyl)Fe(Cl)]+ was performed. However, different from
the complexes of the mesityl- and 2,6-xylyl ligand derivatives,
and reminiscent to the observations made for the tolyl derivative
3 (vide supra), the reduction of the Fe(II) complex 7 yielded yet an-
other Fe(II) metallation product (Scheme 3).
fac-[Ir{CN(C6H4Me-p)(CH2)2NC6H3Me-p}3], are free to arrange in a
symmetrical fashion around the metal center, while the flexibility
of the tripodal TIMENR ligand with three N-tethered carbene aryl
entities is reduced, resulting in the observed non-symmetrical
binding mode to the smaller iron ion. This may also result in ligand
distortions. For instance, two of the ligand arms in 4, the five-mem-
bered NHC ring and the adjacent aryl ring, are almost coplanar,
whereas the two rings of the third ligand arm with a considerably
longer Fe–C* bond exhibit a significant deviation from co-planarity.
The torsion angles C*–Caryl–N–Ccarbene are a good measure of the
co-planarity of the five-membered imidazol-2-ylidene and aryl
ring systems and amount to only 0.6(2)° for C3–N3–C16–C17 and
2.4(3)° for C13–N7–C30–C31, but to 17.1(3)° for C8–N5–C23–C24.
In an effort to suppress the observed intramolecular metallacy-
cle formation, an alternative approach was sought off. The intro-
duction of 3,5-xylyl substituents into the TIMEN–ligand system
(instead of mesityl and 2-6-xylyl groups) is expected to slightly in-
crease the distance of the ortho C–H groups from the Fe center. This
should effectively prevent metallation of the reactive ortho C–H
bonds, and thus, provide a chelating ligand with increased side-ac-
cess to the M„E functional group.
The Mößbauer spectrum of a solid sample of this Fe(II) complex
shows a doublet with an isomer shift of 0.14(1) mm sÀ1 and a
quadrupole splitting,
parameters are remarkably different from its Fe(II) precursor
(d = 0.73(1) mm sÀ1 EQ = 1.08(1) mm sÀ1), and are indicative of
D
EQ, of 2.29(2) mm sÀ1. These Mößbauer
(D
an Fe(II) low-spin (S = 0) species in an octahedral ligand environ-
ment, as opposed to the four-coordinate Fe(II) high-spin (S = 2)
precursor. However, the metallated Fe(II) complex does not give
1H NMR spectra, suggesting that this complex is high-spin, S = 2,
in solution.
The results of an X-ray crystal structure determination con-
firmed the formation of a pseudo-octahedrally coordinated bis-
metallated Fe(II) complex, namely [(TIMEN3,5xyl**)Fe] (8) (Fig. 4),
rather than the tris-metallated Fe(III) species 4.
Treatment of the free 3,5-xylyl derivatized tris(carbene) with
ferrous chloride in a mixture of THF and pyridine, followed by
recrystallization from acetonitrile, yields the corresponding Fe(II)
precursor complex [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 (7) (Fig. 3).
Complex 7 exhibits a Mößbauer isomer shift of d = 0.73(1) mm sÀ1
The pseudo-octahedral iron center in 8 coordinates to three fa-
cially arranged N-heterocyclic carbene donors and two
r-aryl car-
(D
EQ = 1.08(1) mm sÀ1), characteristic of a four-coordinate Fe(II)
banions (denoted with an asterisk à in Scheme 3). The Fe ion’s
distorted octahedral coordination environment is completed with
an agostic hydrogen interaction to one of the aryl’s ortho CH
groups, namely H17A. The position of the hydrogen atom of this
non-dehydrogenated aryl CH group could be experimentally deter-
mined from the difference Fourier map. However, for reasons of
consistency, H17A was placed in an idealized position of optimized
geometry during refinement.
high-spin (S = 2) species of this system.
In contrast to [(TIMENtol)Fe]2+ (3) with rotatable tolyl substitu-
ents, rotation of the aryl substituents in 7 is hindered by the
methyl groups in 3,5-position. Therefore, the steric demand of
the 3,5-substituted aryl arm in 7 is expectedly higher and results
in an axial cavity large enough to accommodate an axially bound
acetonitrile ligand. The Fe(II) center in crystals of [(TIMEN3,5xyl)-
Fe(CH3CN)](PF6)2Á2CH3CN is coordinated in a trigonal pyramidal
fashion with three equivalent Fe–NHC carbon bonds, with an
The Fe–C bond distances in 8 are similar to those observed in 4
and the shorter NHC–Fe bond distances are compensated by longer

234
C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
**
Fig. 5. Molecular structure of the complex cation of [(TIMEN3,5xyl )Fe]+ in crystals
of 9Á0.6THFÁ0.4CHCl3 (50% probability ellipsoids). Hydrogen atoms (except H17A),
PFÀ6 anion, and co-crystallized solvents are omitted for clarity. The dotted lines
indicate the agostic FeÁ Á ÁH interaction.
Fig. 4. Molecular structure of [(TIMEN3,5xyl**)Fe] (8) (50% probability ellipsoids).
Hydrogen atoms (except H17A) omitted for clarity. Dotted lines indicate the agostic
FeÁ Á ÁH interaction.
other two NHC carbene aryl moieties remain coplanar (the corre-
sponding torsion angles C8–N5–C24–C25 and C13–N7–C32–C33
amount to 0.1(4)° and 0.4(4)°, respectively).
In general, the overall coordination environment with two al-
most ideally coplanar arranged aryl–carbene moieties, short iron
NHC distances, and one elongated iron CHaryl distance, correspond-
ing to the tilted ligand arm trans to the shortest Fe – NHC bond,
does not change between the bis-metallated Fe(II) and Fe(III) cen-
ters in complexes 8 and 9.
*
Fe–C carbanion distances. The shortest iron carbene distance of
1.870(2) Å for Fe–C8 is observed trans to the CH of the non-dehy-
drogenated ring (FeÁ Á ÁC17 = 2.442(2) Å and FeÁ Á ÁH17A = 1.71 Å,
Fe1Á Á ÁH17A–C17 = 131°), which also exhibits the strongest devia-
tion from co-planarity of the NHC and the 3,5-xylyl rings. When
comparing the distortion between the NHC and aryl moieties in
both, the tolyl and the 3,5-xylyl derivatives, it is obvious that the
3,5-substitution of the aryl ring is increasing the steric demand.
As observed in the molecular structure of 4, two of the NHC–aryl
systems of 8 are almost perfectly coplanar with corresponding tor-
sion angles of 4.3(2)° for C8–N5–C24–C25 and 1.7(2)° for C13–N7–
C32–C33. The N-heterocyclic carbene and 3,5-xylyl moieties of the
third pendant ligand arm, however, are significantly tilted, re-
flected by the C3–N3–C16–C17 torsion angle of 27.4(3)° (as com-
pared to 17.1(3)° found in the sterically less crowded tolyl
derivative 4).
4. Theoretical considerations
The electronic structures of 4, 8, and 9 were additionally exam-
ined by density functional theory (DFT) calculations. The geome-
tries of the complexes have been fully optimized by using the
BP86 functional (Table 4). The optimized structures of 8 and 9
are in excellent agreement with the available X-ray structures
showing the presence of agostic FeÁ Á ÁH interactions (Table 4). In
contrast, the experimental structure of 4 possesses an axially dis-
torted octahedral geometry, whereas the calculated structure is
The bis-metallated complex [(TIMEN3,5xyl**)Fe] (8) can be easily
oxidized to yield a green colored Fe(III) complex (Scheme 3). This
oxidation occurs readily and even during simple work-up of 8 in
halogenated solvents. A straightforward synthesis was accom-
plished by treatment of 8 with silver salts, like AgOTf, in acetoni-
trile. The molecular structure of the resulting complex
[(TIMEN3,5xyl**)Fe](PF6) (9) is depicted in Fig. 5.
In analogy to 4, the EPR spectrum of a frozen acetonitrile solu-
tion of 9, with g-values of g1 = 2.42, g2 = 2.20, g3 = 1.94, confirms
the S = 1/2 ground state of this low-spin Fe(III) complex.
*
symmetrical, showing a set of three similar Fe–C and a set of three
similar Fe–CNHC bonds.
Attempts to optimize the molecular structure of 4 by using lar-
ger basis sets, by including relativistic effects, a conductor like
screening model (COSMO), and by including empirical van der
In 9, the Fe(III) center is pseudo-octahedrally coordinated and
all metric parameters of the molecular structure are very similar
to those observed for the Fe(II) complex 8. Slight differences are
observed only for the Fe–C distances with longer iron carbene
bonds and shorter iron carbanion bonds compared to those found
in the Fe(II) species (Table 3). Compared to 8, the Fe(III)–CHaryl
bond distance in 9 is slightly longer and was determined to
be FeÁ Á ÁC17 = 2.525(3) Å. The other relevant distances are
FeÁ Á ÁH17A = 1.79 Å and Fe1Á Á ÁH17A–C17 = 131° and are again rep-
resenting an agostic interaction. This elongation of bond distances
in 9 is most likely due to the smaller ionic radius of the Fe(III) ion,
compared to the divalent Fe center in 8. The third arm of the ligand
shows a significant tilt between the NHC and the 3,5-xylyl moiety
evidenced by the C3–N3–C16–C17 torsion angle of 25.7(4)°. The
Table 4
Principal bond distances of the optimized structures as obtained from the spin-
unrestricted BP-DFT calculations.
4
8
9
Fe–CNHC
Fe–C
Fe–CNHC
Fe–C
1.964 (1.943)a
2.016 (1.994)
1.962 (1.923)
2.014 (2.014)
1.962 (1.888)
2.015 (2.366)
–
1.897 (1.913)
2.092 (2.074)
1.940 (1.940)
2.074 (2.050)
1.872 (1.870)
2.468 (2.442)
1.782 (1.708)
2.011 (1.990)
2.041 (2.032)
1.983 (1.972)
2.055 (2.037)
1.882 (1.895)
2.593 (2.524)
1.883 (1.795)
*
b
*
Fe–CNHC
*
Fe–C
FeÁ Á ÁHc
a
b
c
The experimental values are given in parentheses.
trans to CNHC
Agostic interactions.
.

C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
235
Fig. 6. Molecular representations of complexes 4 (left), 8 (middle), and cation 9 (right), the axial distortion highlighted in bold.
Waals corrections all gave very similar, symmetrical structures.
Therefore, based on the DFT calculations, it appears that – in prin-
ciple – the ligand is sufficiently flexible to coordinate the Fe ion in a
symmetrical fashion with three facially coordinating NHC carbon
and three metallated aryl carbon ligands. It is, however, quite
remarkable that the tris-metallated Fe(III) complex 4 and the bis-
metallated Fe(II) and Fe(III) complexes 8 and 9 show a similar
effect is expected. On the other hand, while there is no apparent
steric hindrance in 4, it appears reasonable to assume that steric
pressure, exerted by the aryl methyl groups in the 3,5-xylyl-deriv-
atized species, effectively prevents tris-metallation in complex 8,
which results in one weak FeÁ Á ÁCH agostic interaction and one
short Fe–CNHC bond of 1.870 Å trans to it. This may also explain
the observed torsion angles within the five-membered NHC and
the aryl substituent, which are more pronounced in the [(TIME-
departure from octahedral symmetry with an axial distortion
*
(one short Fe – CNHC and one long Fe–C
bond and agostic
aryl
FeÁ Á ÁCH interaction trans to it, respectively, see Fig. 6 and Table 5).
It can be speculated that the observed structural distortion in 4
originates from electronic (Jahn–Teller distortion), steric (ligand
flexibility) or even crystal packing effects. The phenomenon ap-
pears too pronounced, however, to be entirely due to crystal pack-
ing effects. It also seems unlikely that such a packing effect can be
observed in three different crystal structures. Alternatively, one
could also argue that d5 low-spin complexes (S = ½), such as 4
and 9, are subject to a Jahn–Teller distortion, and hence, the ob-
served structural distortion could be the result of the Jahn–Teller
effect, leading to a non-degenerate ground state. However, com-
plex 8 is a low-spin Fe(II) complex (S = 0), for which no Jahn–Teller
Table 5
Principal bond distances of the observed structures as obtained from the X-ray crystal
structure determination
Fig. 7. Occupied frontier iron-based MOs for
unrestricted B3LYP-DFT calculations: quasi-restricted orbitals are shown, all three
MOs are mixtures of dxy, dxz, and dyz iron orbitals.
4 as obtained from the spin-
4
8
9
Fe–CNHC
Fe–C
1.943a
1.994
1.923
2.014
1.888
2.366
–
1.913
2.074
1.941
2.050
1.870
–
1.999
2.032
1.972
2.037
1.895
–
*
Fe–CNHC
*
Fe–C
Fe–CNHC
Fe–C
FeÁ Á ÁCHa
2.053
2.138
a
These values refer to the distance between Fe and the middle of the CH bond.
Table 6
Mößbauer parameters of 4, 8, and 9 obtained from the spin-unrestricted B3LYP-DFT
calculations.
4
8
9
d (mm sÀ1)
EQ (mm sÀ1)
À0.11 (+0.03)a
+0.05 (+0.11)
+2.34 (2.29)
À0.01b
D
+2.12 (2.37)
1.30b
a
The experimental values are given in parentheses.
No experimental data available.
Fig. 8. Occupied frontier iron-based MOs for
unrestricted B3LYP-DFT calculations: canonical orbitals are shown.
8 as obtained from the spin-
b

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C.S. Vogel et al. / Inorganica Chimica Acta 364 (2010) 226–237
As predicted, introduction of sterically less demanding aryl sub-
stituents bearing potential reactive C–H bonds in ortho position of
the aromatic rings changes the reactivity of the Fe(II) precursor
complexes. Introduction of the tolyl group reduced the steric pres-
sure in a way that the aryl groups are now free to rotate thereby
preventing coordination of an axial ligand in the corresponding
Fe(II) complex 3. In contrast, replacing the tolyl with 3,5-xylyl
groups resulted in the formation of an Fe(II) precursor complex 7
with an accessible axial coordination cavity. From the structure
determinations of the Fe(II) precursor complexes 3 and 7 it is evi-
dent that the steric demand is significantly reduced. However,
these Fe(II) precursor complexes exhibit unpredicted reactivities
that differ significantly from the mesityl and 2,6-xylxl derivatives.
Attempts to reduce the Fe(II) precursors over sodium amalgam did
not yield the expected Fe(I) complexes. Instead, cyclometallation of
the aryl carbene moieties leading to pseudo-octahedrally coordi-
nated, tris-metallated (in the case of the tolyl system) or bis-met-
allated (for the 3,5-xylyl system) Fe(II) and Fe(III) complexes.
While the tris-metallated species [(TIMENtol***)Fe] (4) is relatively
stable towards oxidation, the bis-metallated [(TIMEN3,5xyl**)Fe]
(8) is easily oxidized to yield the corresponding Fe(III) compound
[(TIMEN3,5xyl**)Fe](PF6) (9). The observed structural parameters
demonstrate that this ligand framework is well suited to coordi-
nate iron centers in a trigonal pyramidal fashion with three equiv-
alent ligand arms. An octahedral coordination of the central metal
requires a twist of the third ligand arm that features large twist an-
gles between the five-membered NHC ring and the adjacent aryl
ring of 17.1(3)° (for 4), 27.4(3)° (for 8) and of 25.7(4)° (for 9).
Reducing the steric demand of the TIMENR ligand system by intro-
ducing aryl rings with no substituents in the 2,6-position was suc-
cessfully achieved. However, the two new ligands TIMENtol and
TIMEN3,5xyl yielded Fe(II) precursor complexes 3 and 7 that again
showed unexpected reactivities upon reduction, very different
than observed for the mesityl and 2,6-xylyl derivatives. Obviously,
relatively small changes in the periphery of the ligand have drastic
effects on the reactivity of the resulting complexes. Future work
will aim at fine-tuning the TIMENR ligand system even further by
introducing other aryl substituents for the synthesis of Fe„N com-
plexes with enhanced reactivity.
Fig. 9. Occupied frontier iron-based MOs for
unrestricted B3LYP-DFT calculations: quasi-restricted orbitals are shown.
9 as obtained from the spin-
N3,5xyl**)Fe]0/+ complexes 8 and 9 compared to the tolyl-derivatized
species 4.
In summary, whereas the axially distorted, pseudo-octahedral
structures of 8 and 9 can be theoretically reproduced and ex-
plained on the basis of steric pressure, the reason for the axial dis-
tortion in 4 and its discrepancy to the computationally calculated
structure remains unclear.
The Mößbauer parameters for 4, 8, and 9 were also calculated at
the B3LYP level of DFT (Table 6). Interestingly, and despite the fail-
ure to reproduce the experimentally determined structure of 4, all
calculated parameters reproduce the experimentally determined
values accurately. The calculated complexes exhibit isomer shifts
typical for low-spin ferrous and low-spin ferric ions.
The calculated electronic structures confirm the presence of a
low-spin ferrous ion in 8 and a low-spin ferric ion in 4 and 9. Sev-
eral attempts to calculate intermediate- and high-spin species re-
sulted in significant discrepancy between the calculated and the
experimental molecular structures. This further confirms the
low-spin ground states for 4, 8, and 9. All doubly and singly occu-
pied molecular orbitals (MOs) with significant iron d-character
were identified among the sets of canonical or quasi-restricted
MOs (Figs. 7–9). The unoccupied d-orbitals are highly delocalized
and are not shown. The approximate C3 symmetry of the complex
4 results in strong mixing of d-orbitals within the t2g set. However,
d-orbital mixing in 8 and 9 is lower than in 4 due to the less sym-
metrical molecular structures of 8 and 9. Hence, the one-electron
oxidation 8 ? 9 can computationally be shown to result in the loss
of one-electron from mainly the dxy metal orbital.
Appendix A. Supplementary material
CCDC 776777, 776778, 776779, 776780, 776781, 776782 and
776783 contain the supplementary crystallographic data for
1(CF3SO3)3, [H3TIMENtol](CF3SO3)3), 3 Á 0.8 Et2O, [(TIMENtol)Fe]-
(BF4)2 Á 0.8 Et2O), 4 Á 2 THF, [(TIMENtol***)Fe] Á 2 THF), 5(Cl)3 Á solv,
[H3TIMEN3,5xyl](Cl)3 Á solv), 7 Á 2 CH3CN, [(TIMEN3,5xyl)Fe(CH3CN)]-
(PF6)2 Á 2 CH3CN), 8, [(TIMEN3,5xyl**)Fe]) and 9 Á 0.6 THF Á 0.4 CHCl3,
[(TIMEN3,5xyl**)Fe](PF6) Á 0.6 THF Á 0.4 CHCl3). These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
5. Conclusion
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.07.039.
The reduced reactivity of the first generation complexes [(TIME-
NR)Fe] was attributed to the steric demand exerted by the mesityl
and 2,6-xylyl substituents. In an effort to enhance the reactivity of
the various low- and high-valent iron complexes of the TIMENR li-
gand system (R = aryl = mesityl (mes), 2,6-xylyl (xyl)), two new li-
gands and their corresponding iron complexes were synthesized
and spectroscopically characterized. Throughout the syntheses of
the iron complexes, unexpected reactivities were observed with
these new ligand systems, resulting in a series of new four, five,
and six-coordinate complexes. The divalent iron precursor com-
plexes 3 and 7, tris-metallated 4, and the bis-metallated complexes
8 and 9, as well as the imidazolium salts of the ligands (see depos-
ited data) were characterized by single crystal X-ray structure
determination and spectroscopic methods.
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