Synthesis, structures, and reactivity of chelating bis-N-heterocyclic- carbene complexes of iron(II)

Article abstract of DOI:10.1021/om200605b

Distorted tetrahedral (bis-carbene)FeX₂ complexes can be synthesized by direct metalation of chelating bisimidazolium salts, containing CH₂ or phenylene linkers, with Fe(N(SiMe₃) ₂)₂(THF). For methylenebis(N-R-imidazole-2-ylidene) ((^RC)₂CH₂), the high-spin complexes (( ^RC)₂CH₂)FeX₂ (R = ⁱPr or 2,6-di-isopropylphenyl (DIPP), X = I or Br) are isolated in good to excellent yield. In contrast the phenylene-linked congener ((^iPrC) ₂Ph)FeI₂ cannot be separated from two co-products, one of which was characterized by X-ray crystallography and zero-field ⁵⁷Fe Moessbauer spectroscopy as the square-planar complex [((^iPrC) ₂Ph)₂Fe[I₂. The disparate reactivity toward Fe(N(SiMe₃)₂)₂(THF) is due to the increase in linker length and decrease in linker flexibility of the phenylene-linked bis-carbene ligand relative to the methylene analogue. The short, flexible -CH₂- linker projects the azole rings into the xy plane (defined as the carbene-Fe bond directions) and away from the pseudoaxial anionic substituent, whereas the inflexible phenylene linker ((^RC) ₂Ph) orientates both azole rings into the z direction, which in ((^RC)₂Y)FeX₂ species (Y = CH₂ or phenylene) has the greatest steric crowding. Metallacycle distortion in tetrahedral ((^RC)₂Ph)FeX₂ is necessary to project the azoles into the xy plane, destabilizing this geometry. Addition of CO to ((^DIPPC)₂CH₂)FeI₂ forms cis-((^DIPPC)₂CH₂)FeI₂(CO) ₂, in which the carbene ligands are strong σ donors based on ν(CO). Halide metathesis of ((^DIPPC)₂CH ₂)FeI₂ with NaSMe cleanly forms ((^DIPPC) ₂CH₂)FeI(SMe), while attempts to exchange the second iodide for ^-SMe led to carbene dissociation.

Full text of DOI:10.1021/om200605b

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Structures, and Reactivity of Chelating Bis-N-Heterocyclic-
Carbene Complexes of Iron(II)
†
,‡
†
†
,§
,†
Sergey Zlatogorsky, Christopher A. Muryn, Floriana Tuna, David J. Evans,* and Michael J. Ingleson*
†
Department of Chemistry, University of Manchester, Manchester, M13 9PL, U.K.
§
Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, U.K.
S Supporting Information
b
ABSTRACT: Distorted tetrahedral (bis-carbene)FeX com-
2
plexes can be synthesized by direct metalation of chelating
bisimidazolium salts, containing CH or phenylene linkers, with
2
Fe(N(SiMe ) ) (THF). For methylenebis(N-R-imidazole-2-
3
2 2
R
R
ylidene) (( C) CH ), the high-spin complexes (( C) CH )-
FeX (R = Pr or 2,6-di-isopropylphenyl (DIPP), X = I or Br) are
2
2
2
2
i
2
isolated in good to excellent yield. In contrast the phenylene-
iPr
linked congener (( C) Ph)FeI cannot be separated from two
2
2
57
co-products, one of which was characterized by X-ray crystallography and zero-field Fe M €o ssbauer spectroscopy as the square-planar
iPr
complex [(( C) Ph) Fe][I] . The disparate reactivity toward Fe(N(SiMe ) ) (THF) is due to the increase in linker length and
2
2
2
3 2 2
decrease in linker flexibility of the phenylene-linked bis-carbene ligand relative to the methylene analogue. The short, flexible ꢀCH ꢀ
2
linker projects the azole rings into the xy plane (defined as the carbeneꢀFe bond directions) and away from the pseudoaxial anionic
R
R
substituent, whereas the inflexible phenylene linker (( C) Ph) orientates both azole rings into the z direction, which in (( C) Y)FeX
2
2
2
R
species (Y = CH or phenylene) has the greatest steric crowding. Metallacycle distortion in tetrahedral (( C) Ph)FeX is necessary to
project the azoles into the xy plane, destabilizing this geometry. Addition of CO to ((
CO) , in which the carbene ligands are strong σ donors based on ν(CO). Halide metathesis of ((
2
2
2
DIPP
DIPP
C) CH )FeI forms cis-((
C) CH )FeI -
2
2
2
2 2 2
DIPP
(
C) CH )FeI with NaSMe
2
2 2 2
DIPP
ꢀ
cleanly forms ((
C) CH )FeI(SMe), while attempts to exchange the second iodide for SMe led to carbene dissociation.
2 2
1
7,18
’
INTRODUCTION
Individually the application of iron complexes
center.
More generally ligand dissociation is highly undesir-
1
ꢀ3
able, often leading to irreversible catalyst deactivation. Over-
coming this allows access to the useful reactivity manifold
afforded by redox-active Fe centers ligated by strong carbene σ
donor ligands. For example Smith and Meyer prevent carbene
dissociation by using chelating tripodal tris-carbene ligands,
containing either an anionic backbone or a coordinating amine
and N-het-
4
ꢀ6
erocyclic carbenes (NHCs) in organic synthesis and catalysis
is becoming increasingly important. Despite the mutually attrac-
tive properties of Fe (low toxicity and high abundance) and
NHCs (tunable sterics and the ability to stabilize a range of
oxidation states due to strong σ donor and moderate π acceptor
ability), catalytic applications using Fe-NHC complexes are
currently rare. Notable exceptions have demonstrated that
23ꢀ27
7
Lewis base.
These ligands have enabled the isolation of
4
complexes containing iron in oxidation states ranging from +5 to
1, with the tris-carbene-Fe complex found to be extremely
reducing, rapidly transforming an azido moiety to an Fe-imido.
For subsequent use as a catalytic precursor a facile synthetic
route to robust carbene complexes of the general formula
carbene) FeX (X = halide) was desired. While Grubbs et al.
have previously reported a simple synthesis using monodentate
NHCs and FeX , the propensity of monocarbenes to dissociate
from electron-rich Fe centers led us to investigate the chelation of
I
+
(NHC)Fe complexes are effective for catalyzing cross-coupling
8ꢀ11 12 13
reactions,
direct arene borylation, hydrosilylation, and
14,15
polymerization.
tively unsaturated Fe center during the catalytic cycle. However,
NHC)Fe complexes with coordination numbers less than 6 are
These processes each require a coordina-
(
(
2 2
relatively rare and generally contain NHCs appended with
additional chelating ligands and/or anionic moieties. This scar-
city of low-coordinate (NHC)Fe coincides with a growing body
of evidence that the binding of carbenes to first-row transition
metals can be weak, particularly in sterically crowded systems
containing electron-rich transition metal centers.
iron even multidentate carbene-containing ligands have been
2
8,14
bis-carbenes to Fe.
Bidentate carbenes will provide the
entropically enhanced binding strength afforded by chelation
while maintaining the flexibility inherent in an Fe precursor that
is coordinatively unsaturated and ligated by two exchangeable
anionic ligands, X (essential for example in radical rebound cross-
8,16ꢀ22,61
For
16
reported to undergo dissociation from electron-rich Fe centers.
Recently Grubbs et al. have utilized this phenomenon to catalyze
2
8
coupling). To the best of our knowledge, no complexes
the oligomerization of Fe(COT) (COT = cyclo- octatetraene)
with a monodentate carbene; the requirement for only catalytic
carbene clearly indicates a reversible interaction with the Fe
2
Received: July 8, 2011
Published: August 26, 2011
r 2011 American Chemical Society
4974
dx.doi.org/10.1021/om200605b
|Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
Figure 1. Dicationic components of the bis-imidazolium precursors
Figure 2. ORTEP representation of complexes 4ꢀ6. Thermal ellip-
soids are at 50%, and hydrogens are omitted for clarity. Inset, right:
space-filling diagrams for 4 and 5 viewed along the CꢀCꢀI “equatorial
plane”. Nitrogen = blue, carbon = black, hydrogen = white, chloride =
green, bromide = brown, and iron = red.
1
ꢀ3. All are iodide salts.
containing bidentate carbenes chelating to a metal center in a
tetrahedral geometry (as preferred by L FeI complexes) have
2
2
been reported to date. Previous work utilizing bis-carbenes has
concentrated on square-planar and octahedral-based geometries,
where the length and flexibility of the linker combined with the
steric bulk of the N-substituents are key parameters controlling
bulk inward toward the equatorial FeꢀI. In comparison the
aromatic rings of the DIPP wingtips are orthogonal to the
equatorial plane, projecting less steric bulk into the equatorial
plane (Figure 2 inset right).
2
9,30
chelating ability.
Herein we describe the synthesis of (bis-
carbene)FeX complexes containing phenylene and ꢀCH ꢀ
Compounds 4ꢀ6 are however still grossly similar, with bite
2
2
carbene linkers. Key factors controlling the accessibility and
stability of these distorted tetrahedral complexes were again
found to be the linker length and flexibility. Initial reactivity
angles all in the region of 90°, comparable to other metal
2
9,35
bis-carbene complexes containing a CH
backbone.
The
2
CꢀFeꢀC angle in 4ꢀ6 is reduced compared to monodentate
studies demonstrate ꢀCH -linked bis-carbenes remain coordi-
(carbene) FeX compounds, while there is a concomitant in-
2
2
2
2
9
nated to iron during the ligation of further Lewis bases and halide
salt metathesis, important for future catalytic applications.
crease in the XꢀFeꢀX angle and the yaw angles (θ) for 4ꢀ6
(Table 1, includes the monodentate carbene complexes
(
1
IPr) FeCl and (IEt) FeMe for comparison, IPr and IEt =
2 2 2 2
i
,3-R -imidazole-2-ylidene, R = Pr and Et, respectively). The
2
’
RESULTS AND DISCUSSION
closely comparable yaw angles for compounds 4ꢀ6 are smaller
tBu
Precursor Syntheses. Ligand precursors containing two differ-
than those of (( C)
2
CH
2
)M complexes (θ = 13ꢀ14°), where
R
t
ent ligand backbones, CH and phenylene (termed ( C) CH and
the significant steric bulk of the Bu-N-substituent results in
2
2
2
R
(
C) Ph, respectively), were targeted to investigate the different
considerable steric crowding.
2
binding propensities engendered by the formation of six- and seven-
membered metallacycles, particularly how chelation is effected by
Comparison of key structural metrics of 4ꢀ6 with the more
R
prevalent square-planar (( C)
CH
)M complexes is informa-
2
2
ring strain and orientation of N-substituents (termed wingtips). For
tive. Steric interactions in the square-planar structures are mini-
mized by the imidazole rings aligning out of the xy plane (defined
as the CꢀMꢀC plane). This projects the wingtip bulk into the z
planes and significantly reduces the unfavorable steric interac-
tions to other ligands in the xy plane. The degree of distortion of
i
the CH backbones an Pr derivative and a bulkier analogue (2,6-di-
2
isopropyl phenyl (DIPP)) were synthesized as the bis-imidazolium
salts. Compounds 1ꢀ3 were isolated as the iodide salts using
previously reported synthetic routes or modifications thereof
31ꢀ33
8
(Figure 1).
the imidazole ring out of the xy plane is termed the α angle and
R
Ligation of Fe with CH Backbone Bis-carbene. The com-
is in the range 40ꢀ57° for ( C) CH ligands in square-planar
2
2
2
R
i
plexes (( C) CH )FeI (R = Pr, 4, and R = DIPP, 5) were
compounds. Compounds 4ꢀ6 with distorted tetrahedral geom-
etries have reduced ligand bulk in the xy plane (defined as the
CꢀFeꢀC plane) but increased in the z planes compared to
square-planar complexes. Consequentially 4ꢀ6 have smaller α
angles (30.7°, 31.0°, and 31.7°, respectively) than observed in
square-planar complexes. The lower α angle flattens the ligand
and reduces the projection of the wingtip steric bulk into the
z planes.
2
2
2
readily synthesized in good to excellent yield by aminolysis of
Fe(N(SiMe ) ) (THF) with stoichiometric bis-imidazolium salt
3
2 2
1
6,34
DIPP
in THF.
The dibromide ((
C) CH )FeBr , 6, was also
2 2 2
readily accessible in moderate isolated yield (40%) by initial
deprotonation of the bis-imidazolium salt 2 with KH, addition of
the free carbene to FeBr , and subsequent heating in toluene
2
(
80 °C, 20 h). The molecular structures of each (4 and 5 were
recrystallized from THF/Et O and 6 from toluene) confirmed
Compounds 4 and 6 are extremely poorly soluble in ether and
hydrocarbon solvents, while slowly decomposing in chlorinated
and protic solvents, frustrating complete characterization. Com-
pound 5 however exhibited enhanced solubility, being partially
2
similar distorted tetrahedral geometries at Fe with FeꢀC bond
lengths (in the range 2.08 to 2.11 Å) consistent with other high-
8,14
spin four-coordinate Fe(II) carbene complexes.
The highly
1
distorted tetrahedral geometry at Fe allows for classification of
one halide as pseudoapical (with CꢀFeꢀX angles ≈ 100°) and
one as pseudoequatorial (CꢀFeꢀX angles approaching 120°),
particularly for 5 and 6. The different degrees of deviation from
ideal tetrahedral is quantified by the smaller displacement of Fe
soluble in arenes and readily soluble in THF and MeCN. The H
NMR spectrum of 5 (Figure 3, left) displays eight resonances,
i
including two inequivalent Pr methyl resonances of intensity 12,
iPr
indicating restricted rotation around the NꢀC and CarylꢀC
aryl
bonds, resulting in four methyl groups orientated in toward the
from the CꢀCꢀX
plane for 5 and 6 compared to 4 (4 =
.846 Å, 5 = 0.752 Å, 6 = 0.776 Å). This disparity can be
FeꢀI core. The absence of any intensity 1 resonances confirms
equatorial
0
the CH protons are equivalent on the NMR time scale,
2
attributed to the more sterically crowded equatorial plane
present in 4, arising from the N- Pr substituents, which direct
indicating a fluxional boat to boat conversion of the six-
membered metallacycle. The solution magnetic moment
i
37
4
975
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
Table 1. Structural Data for 4ꢀ6 Compared to Nonchelating Analogues (NHC) FeX
2
2
b
linker (compound no.)
wingtip
X
CꢀFeꢀC angle (deg)
yaw angle (deg)
MꢀNHC (Å)
XꢀMꢀX (deg)
i
CH
CH
CH
2
2
2
(4)
(5)
(6)
Pr
I
I
91.1(3)
89.03(17)
88.2(5)
9.3
8.7
8.0
4.0
2.3
2.106(9), 2.109(8)
2.075(4), 2.092(5)
2.107(12), 2.092(12)
2.136(2), 2.130(2)
2.132(3), 2.140(3)
119.33(5)
118.28(2)
121.29(8)
DIPP
DIPP
Br
a
2
i
c
(
IPr)
2
FeCl
Pr
Cl
102.05(6)
112.95(2)
a
d
(
IEt)
2
FeMe
2
Et
Me
100.8(1)
111.1(1)
a
b
Monodentate carbenes (1,3-R
constraints, is defined as the difference between the two MꢀCꢀN angles divided by 2. From ref 14. From ref 8.
2
-imidazole-2-ylidene). Yaw angle, which is the in-plane distortion of the NHC imposed by metallacycle steric
c
d
1
57
Figure 3. H NMR of compound 5 in d
3
-MeCN, * = peaks due to residual protio MeCN and THF. Inset: zero-field Fe M €o ssbauer spectrum of 5.
36
Right: temperature dependence of χM and χ T for 5 (χ = molar magnetic susceptibility). The red solid lines represent calculated values.
M
M
(
Evans method) at 294 K in d -THF of 5.2 μ is consistent with a
closely comparable solid-state and solution μ values suggest a similar
structure to that observed by X-ray diffraction for 5persists in solution.
Ligation of Fe with Phenylene Backbone Bis-carbenes.
8
B
eff
high-spin S = 2 ground state, as expected for a distorted
tetrahedral L FeX environment. M €o ssbauer spectroscopy
on 5 (Figure 3 inset, isomer shift (i.s.) = 0.70 mm s and
quadrupole shift splitting (q.s.) = 3.74 mm s at 80 K, zero field)
confirmed bulk purity and the presence of a single Fe(II) species
in a distorted tetrahedral environment, with i.s. and q.s. para-
meters comparable to distorted tetrahedral (bis-phosphine)FeI
complexes.
ceptibility measurements were made on a polycrystalline sample
of 5 in an applied magnetic field of 0.1 T (Figure 3, right). The
room-temperature χ T value of 3.4 cm K mol (μ = 5.2 μ )
is in good agreement with the presence of a high-spin (S = 2)
Fe(II) d center in 5. Upon cooling, χ T remains almost
constant until 60 K, when it starts to decrease smoothly and
38
2
2
ꢀ
1
Structurally characterized examples of metal complexes chelated
ꢀ
1
40,41
with ortho-phenylene linked bis-carbenes are rare.
The
2
nBu
previously reported octahedral complex trans-(( C) Ph)RhI -
2
2
(k -OAc) has key structural metrics comparable to compounds
4
0
2
4ꢀ6, including a bite angle of 92.2° and an α angle of 33.4°,
3
8,39
Variable-temperature (2ꢀ300 K) magnetic sus-
while a yaw angle of only 2.0° implies there is no significant strain
in the seven-membered metallacycle. However, attempts to
36
R
access (( C) Ph)FeI by aminolysis of Fe(N(SiMe ) ) (THF)
2
2
3 2 2
3
ꢀ1
M
eff
B
with stoichiometric bis-imidazolium salt 3 in THF (or MeCN)
1
were more complex than that using 1 or 2. H NMR spectra
6
M
contained diamagnetic resonances overlapped with eight obser-
vable paramagnetic resonances; the relative integrals of the latter
were not consistent with a single complex. Single crystals from
3
ꢀ1
then more rapidly below 26 K to reach 1.82 cm K mol at 2 K3.
At the same time, χ (T) increases on cooling from 0.011 cm
this mixture could be isolated, and characterization by X-ray
M
ꢀ
1
3
ꢀ1
iPr
mol at 300 K to 0.92 cm mol at 2 K. Fitting the suscept-
diffraction revealed the formation of compound 7, [(( C) -
2
ibility data above 30 K to the CurieꢀWeiss law (χ = C/T ꢀ θ)
Ph) Fe]I , a dicationic species with a ligand:metal ratio of 2:1.
M
2
2
3
ꢀ1
gave C = 3.43 cm K mol and θ = ꢀ0.57 K. The value of the
Curie constant, C, implies an average g factor near 2.13, which is
in good agreement with the distorted tetrahedral environment of
Fe(II). The small and negative value of θ is consistent with weak
intermolecular antiferromagnetic interactions in 5. Analysis of
The structure of 7 was disordered over two sites (with 57:43%
relative occupancy), both possessing similar structural metrics,
and as such, only the major component is discussed. The iron
center in 7 is square planar (Figure 4, left, angles at Fe ∑ =
359.94°) with disparate ligand bite angles of 82.6(6)° and
88.5(6)°. The FeꢀC distances (1.920(10) and 2.010(9) Å)
are shorter than in high-spin Fe-carbene compounds but longer
than in the diamagnetic Fe(II) square-planar complex
[{bis(pyridylimidazol-2-ylidene)methane}Fe][PF ] (FeꢀC =
the low-temperature χ T(T) and magnetization data (ESI) gave
M
a large value of D, implying considerable orbital angular momentum
6
within the d configuration of Fe(II) as expected, and this was
confirmed by EPR spectroscopy, which did not show any EPR
transitions at 35 GHz microwave frequency, even at T = 5 K. The
6
2
42
1.801(9) Å). The two bis-carbenes linkers adopt a “trans”
4
976
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
Figure 4. Left: ORTEP representation of the major disordered cationic component (57%) of complex 7. Thermal ellipsoids are at 50%, and hydrogens
are omitted for clarity. Inset center: space-filling diagrams (100% van der Waals) of 7, parallel and perpendicular to the xy plane (denoted in blue). Right:
5
7
ꢀ1
ꢀ1
Fe M €o ssbauer spectrum (80 K zero field, all parameters in mm s σ e ( 0.01 mm s unless indicated otherwise in parentheses, individual doublets
marked above spectra); component 1: i.s. = 0.68, q.s. = 3.82, hwhm = 0.16 (38% of total Fe), component 2: i.s. = 0.43, q.s = 0.94, hwhm = 0.31 (51 5%
of total Fe), component 3: i.s. = 0.18(2) q.s. = 4.16(3), hwhm = 0.19 (11% of total Fe).
arrangement with one phenylene backbone orientated above and
one below the square plane (Figure 4 inset center). The high α
angle of 62.0° projects the majority of the wingtip steric bulk into
the z planes, essential to attain a stable square-planar arrange-
ment. This ligand arrangement results in sterically hindered axial
sites (Figure 4 inset center), destabilizing ligand (e.g., iodide/
MeCN) coordination. In contrast analogous (PꢀP) FeX struc-
disfavored due to the steric shielding of the axial sites of 7 and the
39
observed paramagnetism of the related (PꢀP) FeI complex.
2
2
Squid magnetometry on a sample with elemental analysis consistent
with pure 7 was also inconsistent with the presence of only a square-
planar Fe(II) S = 1 compound and resembled that of 5. The results
from M €o ssbauer spectroscopy imply ca. 40% of the sample is
consistent with a tetrahedral high-spin Fe(II) compound and ca.
50% of the sample might be expected to be a low-spin (diamagnetic)
octahedral Fe(II) compound of unknown molecular mass. This
unknown compromises numerical analysis of the magnetic data,
which are presented in the Supporting Information as mass sus-
ceptibility. Both analytical methods unambiguously confirm that the
2
2
tures containing chelating phosphines adopt octahedral or square-
based pyramid structures with no evidence for analogous
2
+
39,43
[
(PꢀP) Fe] square-planar complexes.
2
Attempts to produce bulk quantities of pure 7 for further study
were complicated by variable elemental analyses across different
batches despite identical reaction conditions. We attribute this to
the isolation from solution (by precipitation) of different ratios of
II
formation of a number of Fe species is not a solution phenomena.
Compound 7, with a ligand:Fe ratio of 2:1, has to be produced
1
the complexes observed by H NMR spectroscopy. Bulk material
by carbene dissociation from an Fe complex, a process that may be
iPr
39,43
could be repeatedly produced that possessed elemental analysis
consistent with 7 by extensive washing of isolated material with
THF (in which 7 is extremely poorly soluble). However, on
initiated by solvent or anion coordination to (( C) Ph)FeI .
2 2
No free carbene is present in the synthesis of 7 until deprotonation
of the imidazolium precursor 3 by Fe(N(SiMe ) ) (THF), a
3
2 2
1
dissolution, the H NMR spectra of this material was identical to
process that invariably generates a MꢀC bond. Confirmation that
DIPP
iPr
II
the crude product, indicating numerous products. Mass spec-
(( C) Ph) will readily dissociate from Fe was provided by the
2
troscopy in acetonitrile confirmed the presence of a number of
addition of one equivalent of (( C) CH ) to the mixture
2
2
II
iPr
Fe complexes, with positive ion peaks consistent with compound
containing 7 and (( C) Ph)FeI in THF. This resulted in the
slow (due to the poor solubility of (( C) Ph)-ligated complexes in
2
2
iPr
+
iPr
7
present along with ions attributable to [(( C) Ph) FeI] ,
2
2
2
iPr
+
iPr
+
{
[(( C) Ph)FeI ] H} , and [(( C) Ph)FeI] . Solid-state
Fe M €o ssbauer spectroscopy (80 K, zero field, Figure 4, right)
THF) formation of 5 as the major iron-containing product. Related
2
2
3
2
5
7
tBu
substitution of the ethylene backbone bis-carbene (( C) Et) by
2
tBu
revealed the presence of three ironcomplexes, two major products,
(( C) CH ) has been previously reported in a square-planar
2
2
one with i.s. and q.s. consistent with a tetrahedral high-spin Fe(II)
nickel complex and attributed to a reduction in ring strain on
formation of the six-membered metallacycle relative to the seven-
membered metallacycle.
iPr
complex (assigned as (( C) Ph)FeI ) and one consistent with a
2
2
ꢀ1
21
low-spin octahedral complex (i.s. = 0.68 mm s q.s. = 3.82 mm
s , and i.s = 0.43 mms q.s = 0.94 mm s , respectively).
ꢀ
1
ꢀ1
ꢀ1
38,39,43
Surmising that ligand strain may destabilize the distorted
iPr
One minor product was also present at ∼10% and is assigned to 7,
tetrahedral (( C) Ph)FeI complex and lead to carbene dis-
2
2
ꢀ
1
ꢀ1
nBu
with parameters (i.s. = 0.18(2) mm s , q.s. = 4.16(3) mm s )
closely comparable to square-planar S = 1 Fe(II) complexes ligated
by strong σ donor and moderate π acceptor ligands, a ligand field
sociation and formation of 7, the structure of trans-(( C)2-
Ph)RhI (k -OAc), which possesses the structural metrics
expected in tetrahedral (( C) Ph)FeI , was revisited. Interest-
2
2
iPr
2
2
3
8,44
consistent with four NHCs ligands.
The identity of the octahe-
ingly despite the insignificant yaw angles, strain in trans-
nBu 2
dral compound is currently unknown, but the coordination of two
(( C) Ph)RhI (k -OAc) is indicated by the internal NꢀCꢀC
2
2
iPr
iodides (or another ligand), forming octahedral (( C) Ph) FeI , is
angles of the seven-membered metallacycle being appreciably
2
2
2
4
977
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
Figure 5. ORTEP representation of the cationic portion of complex 8. Thermal ellipsoids shown at 50% probability. Inset right: the formation of 9 from
the deprotonation of 3 in the absence of Fe salts.
R
larger than the ideal 120°. The strain in (( C )Ph) can be quan-
et al., during the alkylation of (IEt) FeCl with PhLi, and pro-
2 2
2
tified by the angle between the phenylene centroid and the two
imidazolium centroids, which in an ideal system will approach
posed to be an iron-mediated process proceeding via (IEt)
2
8
FePh(Cl). However, NꢀC
cleavage and CꢀC bond
isopropyl
6
0°. For example in the imidazolium precursor 3 this value
formation is also observed on deprotonation of 3 in the absence
3
6
Me
is 62.3°, while in the square-planar complexes (( C) Ph)Rh-
of any iron species (e.g., using two equivalents of KH or
2
(
CO) and 7 it is 65.4° and 64.3°, respectively. In contrast in
LiN(SiMe ) ); thus the decomposition of 3 appears to follow
2
3 2
nBu
2
trans-(( C )Ph)RhI (k -OAc) this intercentroid angle is
the general Brønsted base initiated mechanism, possibly pro-
2
2
49
7
1.8°, indicating greater strain. With small ligand wingtips in
ceeding via a protonated dimer. The intermediate produced on
deprotonation of 3 with KH and formation of a CdC bond will
then undergo subsequent [1,3]-sigmatropic rearrangement to
form the observed compound 9 (Figure 5, inset right), analogous
nBu
2
trans-(( C )Ph)RhI (k -OAc) this distortion can be attribu-
2
2
ted to strain in the metallacycle engendered by the axial iodide
ligands enforcing a small α angle. This is significant, as the desired
distorted tetrahedral complex (( C) Ph)FeI would require a
iPr
50
2
2
to related tetraamino-alkenes. The instability of the free bis-
nBu
2
iPr
similar α angle to trans-(( C) Ph)RhI (k -OAc) (based on
carbene (( C) Ph) is also consistent with previous synthetic
2
2
2
n
compounds 4ꢀ6) to minimize unfavorable steric interactions
routes to (( C) Ph)M complexes all proceeding by direct
2
iPr
between the pseudoaxial halide and the (( C) Ph) ligand. Thus
metalation of bis-imidazolium salts with a suitable metal pre-
cursor, with the free carbene not reported to the best of our
knowledge. The preorganized geometric arrangement enforced
by the rigid phenylene backbone is a key factor in CdC bond
2
we propose the lack of flexibility inherent in the phenylene backbone
combined with the requirement for a low α angle engendered by
axial ligation in a distorted tetrahedral environment destabilizes
iPr
51
(
( C) Ph)FeI sufficiently to result in reversible coordination of
formation, with methylene and more relevantly ethylene back-
2
2
n
21,52,53
the carbene to iron. In contrast the (( C) CH ) bis-carbenes can
bone analogues stable and isolatable as the free bis-carbenes.
2
2
5,29
iPr
adopt small α angles on chelation with little energetic penalty;
C) CH )FeI is stable to ligand dissociation even in the
presence of Lewis bases (e.g., PMe and MeCN) for 3 days (no new
complexes were observed by H NMR despite square-planar
( C) CH ) M structures being sterically accessible).
The synthesis commencing from 3 formally uses one equiva-
lent of Fe(N(SiMe ) ) (THF) only as a deprotonating agent,
The inaccessibility of the free bis-carbene (( C) Ph) combined
with the lack of pure (( C) Ph)FeI or 7 precluded these
2
DIPP
iPr
thus ((
2
2
2
2
2
3
complexes from further reactivity studies, and subsequent work
focuses on derivatizing 5.
1
n
21,45
(
Initial Reactivity Studies. Compound 5 will be useful only
2
2 2
DIPP
if the {((
C) CH )Fe} moiety persists during addition of
2 2
3
2 2
neutral ligands and/or salt metathesis. The addition of 1 atm of
producing “FeI ” and HN(SiMe ) as byproducts that can be
separated by repeated THF extraction. Attempts to form 7
cleanly by the low-temperature deprotonation of 3 with KH
CO to a degassed THF solution of 5 resulted in a slow color
change to violet. Monitoring the reaction by H NMR spectros-
2
3 2
1
copy revealed the gradual growth of a single new diamagnetic
species as the major product, which displayed two CO bands at
followed by subsequent addition of 0.5 equivalent of FeX
instead led to H and C{ H} NMR spectra indicative of a mix-
2
1
13
1
ꢀ1
2024 and 1974 cm , along with the formation of a minor
ture of products arising from migration of isopropyl from N to C.
paramagnetic byproduct. The observation of two CO bands is
54,55
NꢀC cleavage was unambiguously confirmed from the reac-
iPr
consistent with a cis CO arrangement.
complexes with chelating diphosphines produce the trans-CO
In contrast analogous
tion of compound 3 with 0.5 equivalent of FeCl and 2
2
5
6
DIPP
equivalents of LiN(SiMe ) in THF at ꢀ78 °C. On warming
cis-iodide isomer, (PꢀP)Fe(CO) I . Identification of ((
2 2
C)2-
3
2
and recrystallization from THF/pentane, a small quantity of
poor-quality crystals was obtained of the planar monocationic
compound 8 (Figure 5) as the iodide salt. The monocationic
charge and structural metrics for compound 8 are consistent with
one imidazole (C1ꢀN1 = 1.362(13) Å and C1ꢀN2 = 1.330(14)
Å, N1ꢀC1ꢀN2 = 112.5(10)°) and one imidazolium ring
CH )FeI (CO) , 10, as the cis-(CO)-cis-(I) or cis-(CO)-trans-(I)
2
2
2
isomer was frustrated by the lack of suitable crystalline material.
However, comparison of the CO stretching frequencies for 10 with
0
(2,2 -bipyridyl)FeI (CO) where both isomers have been charac-
2
2
terized (the cis-(CO)-cis-(I) and cis-(CO)-trans-(I) isomers have
ꢀ
1
54
νCO of 2044/2004 and 2077/2033 cm , respectively) demon-
0
(
C2ꢀN3 = 1.338(14) Å and C2ꢀN4 = 1.329(14) Å,
strates that 10 is significantly more electron rich than 2,2 -bipyridyl.
N3ꢀC2ꢀN4 = 108.2(9)°) linked through a CꢀC single bond
In contrast NHCs and pyridyl ligands have similar overall basicities
46ꢀ48
+
(
C1ꢀC2 = 1.418(15) Å),
with the caveat that the poor-
toward {CpFe(CO)} fragments, which has been attributed to
53
quality data lead to high uncertainties in bond lengths and angles.
greater FefNHC charge transfer compared to Fef pyridyl.
A
Carbene NꢀC cleavage has been recently reported by Deng
decrease in metal to ligand π back-bonding from the {Fe(CO) I }
2
2
4
978
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
’
CONCLUSIONS
Distorted tetrahedral FeX compounds ligated by chelating
2
carbenes are accessible in good yield provided the linker length is
sufficiently short and flexible to project the azole rings predomi-
nantly into the sterically less crowded equatorial plane. Utilizing a
nonflexible and longer linked bis-carbene that has a steric
preference for high α angles (and thus square-planar structures)
prevents the clean formation of (bis-carbene)FeX , leading to the
2
formation of two co-products, one identified as the unusual
iPr
square-planar S = 1 complex [(( C) Ph) Fe]I , 7. The forma-
2
2
2
DIPP
tion of 7 and the observed dissociation of (
C) CH from Fe
2
2
on addition of two equivalents of NaSMe to 5 indicate that the
NHCꢀFe bond is weaker than expected in these electron -rich
Figure 6. ORTEP representation of complex 11. Thermal ellipsoids
are shown at 50% probability.
II
Fe complexes. Thus ligand metallacycle strain and steric
crowding must both be minimized in electron-rich (bis-carbene)-
II
Fe complexes to ensure persistence of the (biscarbene)Fe
fragment to NHC (or pyridyl) can be expected due to the presence
of two CO groups (with one or two carbonyls trans to NHCs,
dependent on regioisomer). Thus the increased electron density at
Fe in 10 is due to the superior σ donor properties of the NHC
moiety in future applications.
’
EXPERIMENTAL SECTION
53
relative to pyridyls.
General Considerations. All reactions were performed using
Pleasingly, metathesis of halide in 5 for the sterically small
thiolate, SMe, is also facile by addition of one equivalent of
NaSMe in THF. Recrystallization and structural analysis con-
standard glovebox or Schlenk line techniques, unless otherwise speci-
fied. Solvents used were either purified by an Innovative Technology PS-
MD-5 solvent purification system or distilled from appropriate drying
agents and degassed. Deuterated solvents were distilled from appro-
priate drying agents and degassed. N-Dipp-imidazole was prepared
firmed the formation of the distorted tetrahedral compound
DIPP
((
C) CH )FeI(SMe), 11. Key structural metrics are com-
2 2
parable to 4ꢀ6, including a bite angle of 88.1(5)°, average yaw
31
according to a published procedure and purified by sublimation.
and α angles of 7.5° and 29.0°, and FeꢀC bond lengths of
i
Methylenebis(N- Pr-imidazolium) diiodide was prepared as described
2
.112(13) and 2.103(14) Å. The FeꢀS distance in 11 (2.274(4)
32
in the literature. o-Phenylenediimidazole was prepared according to
II
Å) is in the range expected for four-coordinate high-spin Fe
33
the published procedure. All other materials were purchased from
5
7
thiolates. It is also noteworthy that the SMe occupies the less
sterically hindered pseudoequatorial site, while the bulky DIPP
groups also prevent any dimerization through (μ-SMe). In
commercial vendors and used as received. NMR spectra were recorded
1
13
with a Bruker AV-400 spectrometer (400 MHz H; 100 MHz C; 162
1
MHz). H NMR chemical shifts are reported in ppm relative to protio
impurities in the deuterated solvents, and C NMR using the center line
of CD Cl (or other solvent as appropriate) as internal standard. Unless
1
3
solution (d -THF) 11 is paramagnetic with a magnetic moment
8
(
294 K, Evans method) of 4.3 μ , most consistent with a
B
2
2
distorted tetrahedral iron center. Compounds 5 and 11 are
ligated by strong σ donor carbenes and good π donor ligands
otherwise stated all NMR spectra are recorded at 293 K. Protio-solvent
suppression experiments were performed on Varian Inova instruments
(flame-sealed d -dmso capillaries used to lock field) using pulse
6
(iodide and thiolate) but while electron rich both show no
evidence for carbene dissociation (stable in THF for days).
The addition of a further equivalent of NaSMe to 11 however
results in the formation of one new paramagnetic product with
ligand resonances that are symmetry equivalent by H NMR
spectroscopy. This compound can be isolated in good yield (81%
sequences written in-house by Prof. Gareth Morris. Elemental analysis
of air-sensitive compounds was performed by London Metropolitan
University service. Solution magnetic moments were recorded at 25 °C
5
8,59
1
(unless otherwise stated) using the Evans method.
M €o ssbauer
spectra were recorded in zero magnetic field at 80 and/or 298 K on
an ES-Technology MS-105 M €o ssbauer spectrometer with a 100 MBq
based on Fe) as a microcrystalline solid that has an elemental
5
7
DIPP
Co source in a rhodium matrix at ambient temperature. Spectra were
composition consistent with a ((
C) CH ):Fe:SMe ratio of
2 2
referenced against 25 μm iron foil at 298 K, and spectrum parameters
were obtained by fitting with Lorentzian curves. Samples were prepared
under a dry nitrogen atmosphere by grinding with boron nitride powder
1
:2:4, indicating carbene dissociation from iron has occurred.
Dissolution of this microcrystalline material produced an iden-
tical H NMR spectrum to the in situ product, and we tentatively
assign it as (MeS)Fe(μ-MeS) (μ-((
the basis of analogy to the methylation of ((
which afforded ((Me)Cr(μ-Me) (μ-((
by loss of bis-carbene. The dissociation of (
1
DIPP
prior to mounting in the sample holder, and all errors are <(0.01 mm
C) CH ))Fe(SMe), on
2
2
2
ꢀ1
DIPP
s
unless otherwise stated. SQUID measurements were carried out on
C) CH )CrCl ,
C) CH ))Cr(Me)
2 2
DIPP
2
2
2
microcrystalline material by enclosing the sample in an O-ring-sealed
Kel-F capsule. The capsule was transferred to the sample holder in a
glovebox, transported to the SQUID magnetometer (Quantum Design
MPMS XL7) in a sealed Schlenk tube, and then rapidly transferred to the
helium-purged sample space of the magnetometer. Corrections for dia-
magnetism were made using Pascal’s constants, and the experimental
magnetic data were also corrected for the diamagnetic contribution of the
KelF capsule, which was measured in similar experimental conditions.
DIPP
2
20
C) CH2
2
implies a weakened interaction between NHCs and the
electron-rich Fe center, as the steric crowding in the targeted
DIPP
intermediate ((
C) CH )Fe(SMe) will only be marginally
2 2 2
greater than that in the stable complex 5. The latter shows no
propensity for carbene dissociation, despite iodide generally being a
better electron donor than thiolate. The stability of more crowded
i
N- Pr-imidazole. Acetonitrile was added to a mixture of imidazole
10 can be rationalized by the enhanced Lewis acidity of the Fe center
(1.36 g, 20 mmol) and potassium hydride (800 mg, 20 mmol) at 0 °C,
and the reaction allowed to warm to RT with stirring for approximately
2 h (until visible gas evolution stopped). The sample was then cooled
to 0 °C, and isopropyl iodide (2.99 mL, 5.1 g, 30 mmol) added.
on coordination of π acidic carbonyls, strengthening the NHCꢀFe
bonds. Thus an electron-rich Fe center combined with sufficient
steric crowding are both required for carbene dissociation.
4
979
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
The reaction mixture was stirred overnight at RT (further manipulations
were carried out in an open laboratory), acetonitrile removed in vacuo,
and the product extracted with dichloromethane to give a yellowish oil
volume, and pentane added (50 mL), causing precipitation of a yellow-
green powder. This was filtered, washed with pentane (2 ꢁ 25 mL), and
dried in vacuo, yielding a yellow-green powder (1.6 g, 2.05 mmol, 97%).
(
2 g, 91%). The identity of the compound was confirmed by comparison
2
X-ray quality crystals were grown by slow diffusion of Et O into a THF
60
1
with published NMR data (CDCl3).
solution of the compound. H NMR (400 MHz, THF+(CD ) SO
3 2
Methylenebis(N-Dipp-imidazolium) Diiodide, 2. All manip-
ulations were carried out in an open laboratory. N-Dipp-imidazole (1.9
g, 8.33 mmol) and methylene iodide (0.34 mL, 1.117 g, 4.17 mmol) were
heated to reflux in acetonitrile (10 mL) in a high-pressure ampule
overnight, resulting in a white suspension. This was concentrated to ca.
capillary, 293 K) δ: 64.8 (2H), 30.4 (2H), 13.2 (2H, v br), 7.1 (2H), 6
1
(4H), ꢀ1.6 (12H), ꢀ5.6 (12H), ꢀ10.6 (4H). H NMR (400 MHz,
3
CD CN, 293 K), δ: 63.2 (2H), 28.5 (2H), 21 (2H, v br), 7.3 (2H), 6.3
(4H), ꢀ1.5 (12H), ꢀ5.4 (12H), ꢀ10.7 (4H). Evans (d
-THF, toluene
8
ꢀ
1
1
capillary, concentration approximately 0.05 g mL , 400 MHz for H,
1
3
3
mL, 10 mL of diethyl ether was added, and this was briskly heated to
125 MHz for C): averaged value 5.40 μ
B
. Anal. Calcd for
reflux. The sample was cooled to room temperature, and the colorless
solid was isolated by filtration, washed with diethyl ether, and dried
in vacuo (2.36 g, 78%). If needed, further purification can be achieved by
crystallization from acetonitrile/diethyl ether. The identity of the
compound was confirmed by comparison with published NMR data
C
31
H
40FeI : C = 47.84, H = 5.18, N = 7.20. Found: C = 47.67,
2 4
N
H = 5.32, N = 7.03.
[Methylenebis(N-Dipp-imidazole-2-ylidene)]dibromoir-
on(II), 6. Methylenebis(N-Dipp-imidazole-2-ylidene) (250 mg, 0.53
mmol) and iron(II) bromide (105 mg, 0.48 mmol) were heated at 70 °C
in toluene (15 mL) for 20 h. The light brown precipitate was filtered,
19
for the analogous dibromide salt. Anal. Calcd: C = 51.39, H = 5.84, N =
7
.73. Found: C = 51.49, H = 5.92, N = 7.63.
o-Phenylenebis(N- Pr-imidazolium) Diiodide, 3. All manip-
washed with Et O (2 ꢁ 10 mL), and dried in vacuo to yield a light brown
2
i
powder (145 mg, 44%). X-ray quality crystals were grown by slow
1
ulations were carried out in an open laboratory. o-Phenylenediimidazole
3 g, 14.29 mmol) and isopropyl iodide (5.7 mL, 9.7 g, 57.14 mmol)
cooling of a refluxing C
(400 MHz, THF+(CD
7.3 (2H), 6.7 (2H), 5.5 (4H), ꢀ1.4 (12H), ꢀ3.7 (12H), ꢀ8.9 (4H).
Anal. Calcd for C31 FeN : C = 54.41, H = 5.89, N = 8.19. Found:
C = 54.08, H = 6.06, N = 8.08.
Compound 3 + Fe(N(SiMe ) ) (THF). A. MeCN (50 mL) was
D
6
6
solution of the compound to RT. H NMR
(
3 2
) SO capillary, 293 K) δ: 61.7 (2H), 25.9 (2H),
were heated to reflux in acetonitrile (15 mL) in a high-pressure ampule
overnight, and the resulting yellow solution was concentrated and kept
overnight at 3 °C. Colorless crystals were filtered off and dried in vacuo
H40Br
2
4
(4.3 g, 55%). X-ray quality crystals were grown by slow cooling of a
3
2 2
i
1
saturated MeCN solution of compound 3 from reflux to RT. H NMR
300 MHz, CD
CN, 293 K) δ: 9.60 (s, 2H, NCHN), 7.93ꢀ7.84 (m, 2H
2H, C ), 7.61 (app. s, 2H, NCH CH N), 7.51 (app. s, 2H,
NCH CH N), 4.74 (sept, 2H, CH(CH ) , J = 6.69 Hz), 1.58 (d,
added to a mixture of o-phenylene(N- Pr-imidazoline) diiodide (1 g, 1.82
mmol) and bis(trimethylsilylamido)(tetrahydrofuran)iron(II) (815 mg,
1.82 mmol) at RT and stirred overnight to yield a dark brown solution.
This was filtered through a plug of Celite, MeCN was removed, and the
resulting solid was washed with pentane (3 ꢁ 30 mL), then with THF
(80 mL), and finally dried in vacuo to afford a pale brownish-green
powder (490 mg, 60% yield based on ligand). X-ray quality crystals were
(
+
3
6
H
4
2
2
2
2
3 2 HꢀH
13
1
12H, CH(CH
3
)
2
, JHꢀH = 6.78 Hz). C{ H} NMR (75 MHz, CD
3
CN,
2
(
93 K): not observed (2C, N-C-N), δ 136.84 (2C, C
2 4 4
C H ), 133.02
0
2C, C C H ), 128.82 (2C, C C H ), 123.84 (2C, NCHCHN),
2 4 4 4 4
00
2
1
22.11 (2C, NCHCHN), 54.56 (2C, CH(CH ) ), 22.26 (4C, CH-
grown by slow diffusion of Et O into a MeCN solution of the com-
3
2
2
(
CH
3
)
2
. Anal. Calcd: C = 40.37, H = 4.81, N = 9.91. Found: C = 39.44,
pound at RT.
H = 4.13, N = 10.01.
Evans (CD CN, 293 K toluene capillary, concentration approxi-
3
1
13
Methylenebis(N-Dipp-imidazole-2-ylidene). THF (25 mL)
was added at ꢀ78 °C to a mixture of methylenebis(N-Dipp-imidazole)
diiodide (2 g, 2.76 mmol) and potassium hydride (221 mg, 5.52 mmol),
and the reaction allowed to warm to RT overnight with stirring
mately 0.028 g/mL, 400 MHz for H, 125 MHz for C): average value
6.28 μ . M/S: 604.9 (+ve CI, LigandFeI ), 477.1 (+ve EI, LigandFeI),
322.2 (+ve NSI, Ligand Fe ). Representative elemental analysis:
B
2
2
+
2
found: C = 47.71, H = 4.84, N = 12.32 (for pure 7 C36
H
44FeI
2
N
8
(
CAUTION: hydrogen evolution). The resultant brown suspension was
calculated: C = 48.13, H = 4.94, N = 12.47).
i
removed by filtration, and the THF solution removed under reduced
B. A J. Young’s NMR tube was charged with o-phenylene(N- Pr-
imidazoline) diiodide (24.6 mg, 0.045 mmol), bis(trimethylsilylamido)-
(tetrahydrofuran)iron(II) (20 mg, 0.045 mmol), and THF (0.6 mL),
and the contents were mixed for 4 days, at which point THF was
pressure. Pentane (30 mL) was then added, and cooling to ꢀ78 °C for
1
h resulted in the formation of a precipitate that was filtered and dried to
give a cream powder (1 g, 77%, 95% pure by NMR). The identity of the
compound was confirmed by comparison with published NMR data.
1
9
1
removed and MeCN added. The H NMR spectrum was analogous to
i
Methylenebis(N- Pr-imidazole-2-ylidene)]diiodoiron(II),
[
that observed for the product from method A.
i
4
. THF (50 mL) was added at ꢀ78 °C to methylenebis(N- Pr-
imidazole) diiodide (488 mg, 1 mmol) and bis(trimethylsilylamido)-
tetrahydrofuran)iron(II) (448 mg, 1 mmol), and the reaction mixture
Compound 8. A Schlenk flask was charged with o-phenylene-
i
(N- Pr-imidazoline) diiodide (200 mg, 0.37 mmol), FeCl
2
(23 mg, 0.18
(
mmol), and 10 mL of THF and cooled to ꢀ78 °C. A THF solution of
was allowed to slowly warm to RT overnight with stirring. The resultant
yellow-brown solution was filtered, and the solvent was removed in
vacuo. The resulting solid was washed with pentane (2 ꢁ 20 mL) and
dried in vacuo to give a yellow-brown powder (310 mg, 57%). X-ray
quality crystals were grown by slow diffusion of Et
solution of the compound at RT. H NMR (400 MHz, THF+(CD ) SO
LiN(SiMe (122 mg, 0.72 mmol dissolved in 10 mL of THF) was
3 2
)
added dropwise over 10 min. The resultant yellow solution was stirred at
ꢀ78 °C for 2 h and warmed to 0 °C over 30 min with a gradual color
change to red. Warming to 20 °C and stirring overnight resulted in a
further color change to yellow. Filtration and slow diffusion of pentane
yielded a significant quantity of yellow-brown powder and a limited
number of small crystals, which on analysis by X-ray diffraction were the
migration product 8.
2
O into a THF
1
3
2
capillary, 293 K) δ: 57.5 (2H), 16.3 (2H), 13.8 (2H), 12.1 (2H), 1.4
6H), 0.5 (6H). Anal. Calcd for C13 FeN : C = 28.70; H = 4.08;
N = 10.30. Found: C = 28.61, H = 4.03, N = 10.17.
Methylenebis(N-Dipp-imidazole-2-ylidene)]diiodoiron-
II), 5. THF (40 mL) was added at 0 °C to methylenebis(N-Dipp-
imidazole) diiodide (1.527 g, 2.11 mmol) and bis(trimethylsilylamido)-
tetrahydrofuran)iron(II) (945 mg, 2.11 mmol), and the reaction
(
H
22
I
2
4
Compound 9. THF (15 mL) was added at ꢀ78 °C to a mixture of
i
[
o-phenylenebis(N- Pr-imidazolium) diiodide (55 mg, 0.1 mmol) and
(
potassium hydride (8 mg, 0.2 mmol, analogous reactivity is observed
using LiN(SiMe ) ), and the reaction mixture was allowed to slowly
3
2
(
warm up to RT overnight with stirring to yield an orange-yellow
solution. Filtration followed by solvent removal afforded an oily yellow
residue (this could be crystallized by slow evaporation of a DCM/hexane
mixture was allowed to slowly warm to RT overnight with stirring.
The dark yellow-brown solution was concentrated to 1/4 of the initial
4
980
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
1:5) solution to give yellow needles, unsuitable for single-crystal XRD
ARTICLE
(
assuming the solid is [methylenebis(N-Dipp-imidazole-2-ylidene)]bis-
due to their small size).
(μ -thiomethoxido)bis(thiomethoxido)diiron(II)).
2
1
H NMR (400 MHz, C
6
D
6
, 293 K, assignments by 2D spectroscopy)
Crystallization could be achieved by slow diffusion of pentane into a
THF solution (unsuitable for single-crystal XRD due to very poor
diffraction properties); redissolving this crystalline material resulted in
3
3
δ: 7.24 (1H, d, JHH = 1.6 Hz, imidazole CꢀH), 7.00 (1H, d, JHH = 1.6
3
Hz imidazole), 6.90 (1H, m, phenylene CꢀH), 6.83 (1H, dd, J = 8.4
HH
1
3
1
Hz J = 1.6 Hz, phenylene CꢀH), 6.74 (1H, dd, J = 8.0 Hz J
=
HH
an identical NMR spectrum to the yellow powder.
HH
HH
1
1
.6 Hz, phenylene CꢀH), 6.58 (1H, m, phenylene CꢀH), 5.96 (1H, d,
3 2
H NMR (300 MHz, THF+(CD ) SO capillary, 293 K) δ: 65.8, 40.0,
3
3
J
HH = 2.8 Hz) 5.57 (1H, d, JHH = 2.8 Hz), 4.73 (1H, pseudoseptet,
37.0, 35.3, 34.0, 0.0, ꢀ1.4, ꢀ3.4, ꢀ9.1 (v br). Anal. Calcd for C35
52
H -
3
3
JHH = 6.4 Hz, NC ꢀH), 2.31 (1H, pseudoseptet, J
= 7.2 Hz,
Fe N S : C = 54.68, H = 6.82, N = 7.29. Found: C = 54.83, H = 6.69, N = 7.48.
2 4 4
iPr
3
HH
3
CCiPrH), 1.14 (3H, d, JHH = 6.4 Hz, NCHCH
3
) 1.05 (3H, d, JHH = 7.2
3
Hz, CCHCH
3
) 0.83 (3H, d, JHH = 7.2 Hz, CCHCH ) and 0.66 (3H, d,
3
’
ASSOCIATED CONTENT
3
13
1
J
HH = 6.4 Hz, NCHCH
3
) ꢀ8.9 (4H). C{ H} NMR (128 MHz, C
6 6
D ,
2
1
93 K) δ: 140.4, 135.3, 130.6, 127.0, 122.4, 118.9, 116.2, 115.8, 115.7,
S
Supporting Information. Full experimental and crystal-
b
12.7, 112.2, 83.4, 47.7, 42.0, 22.4, 21.1, 16.7, 16.3. M/S (+ve ES): 301
lographic details are available free of charge via the Internet at
+
ꢀ
+
+
(
9 + Li ), 287 (9 ꢀ Me + H Li ).
Methylenebis(N-Dipp-imidazole-2-ylidene)]dicarbonyldi-
iodoiron(II), 10. A J. Young’s NMR tube was charged with [methylenebis-
N-Dipp-imidazole-2-ylidene)]diiodoiron(II) (20 mg, 0.026 mmol) and
http://pubs.acs.org.
[
’
AUTHOR INFORMATION
(
Corresponding Author
*
THF (0.6 mL). The resulting yellow-greenish solution was degassed by
three freezeꢀpumpꢀthaw cycles, and the contents were exposed to 1 atm of
CO gas. Rotating the reaction mixture in the NMR tube overnight under
E-mail: michael.ingleson@manchester.ac.uk.
Present Addresses
Department of Chemistry, University of Liverpool, Liverpool,
1
atm of CO resulted in formation of a brown-violet solution, at which point
‡
the contents were degassed and put under a dinitrogen atmosphere.
1
L69 3BX, U.K.
H NMR (300 MHz, THF+(CD ) SO capillary, 293 K) δ: 8.09ꢀ6.83
3
2
(
2
overlapping multiplets, 12H, 2 ꢁ C
H
6 3
+ 2 ꢁ NCHCHN + NCH
2
N),
)),
.72 (app. s, 2H, CH(CH
3
)
2
), 1.23 (app. s, 6H, 2 ꢁ CH(CH
3
)(CH
3
’
ACKNOWLEDGMENT
1
.14 (app. s, 6H, 2 ꢁ CH(CH )(CH )) (major diamagnetic components,
3
3
We gratefully acknowledge the Royal Society (M.J.I. for a
these resonances could be assigned to [methylenebis(N-Dipp-imidazole-2-
ylidene)]dicarbonyldiiodoiron(II)); ꢀ6.8 (v br, 2H), ꢀ8.7 (v br, 2H)
University Research Fellowship), the EPSRC (S.Z.), and the
BBSRC (D.J.E., Core Strategic Grant to the John Innes Centre)
and the University of Manchester for funding. The EPSRC
National Mass Spectrometry Service Centre (NMSSC) is
thanked for analysis of 7. Prof. G. Morris (The University of
Manchester) is gratefully acknowledged for assistance with
solvent suppression NMR spectroscopy. We also acknowledge
the EPSRC Research Facility and Service at the University of
Manchester for analysis of compounds 5 and 7 and Prof. D.
Collison for useful discussions.
(unidentified minor paramagnetic compound). IR (THF solution against
ꢀ1
background of dry THF saturated with N , cm ): 2024, 1974.
Reaction of Compound 7 with Methylenebis(N-Dipp-
imidazole-2-ylidene). A J. Young’s NMR tube was charged with a
2
iPr
mixture containing 7 and (( C) Ph)FeI (20 mg, 0.022 mmol),
2
2
methylenebis(N-Dipp-imidazole-2-ylidene) (10.5 mg, 0.022 mmol),
and THF (0.6 mL). Rotating the reaction mixture in the NMR tube
for 2 days resulted in the formation of a brown suspension. The
precipitate was allowed to settle, and the yellow-brown solution was
carefully decanted into another J. Young’s NMR tube and was subject to
1
H NMR analysis, which revealed [methylenebis(N-Dipp-imidazole-2-
’
REFERENCES
ylidene)]diiodoiron(II) as the only product.
(
1) Plietker, B. Iron Catalysis in Organic Chemistry; Wiley-VCH:
Weinheim, 2008.
2) Junge, K.; Schroder, K.; Beller, M. Chem. Commun. 2011, 47,
849.
[
Methylenebis(N-Dipp-imidazole-2-ylidene)](thiometho-
xido)iodoiron(II), 11. THF (40 mL) was added at RT to a mixture of
methylenebis(N-Dipp-imidazole-2-ylidene)]diiodoiron(II) (250 mg, 0.32
(
[
4
mmol) and sodium thiomethoxide (22.4 mg, 0.32 mmol), and the reaction
mixture stirred overnight. THF was removed in vacuo from the resulting
dark yellow solution, and the solid residue washed with toluene (70 mL) to
give a yellow-brown powder (150 mg). This was dissolved in THF (20 mL)
(
(
3) Sun, C.-L.; Li, B.-J.; Shi, Z. J. Chem. Rev 2011, 111, 1293.
4) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009,
109, 3612.
(5) Corberan, R.; Mas-Marza, E.; Peris, E. Eur. J. Inorg. Chem.
followed by addition of Et O (10 mL) with mixing. On standing overnight,
2009, 1700.
2
the precipitate that formed was filtered and discarded. The remaining
dark yellow solution was dried in vacuo to afford a yellow-brown powder
(6) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(7) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003,
2, 612.
2
(
70 mg, 31%).
1
(
(
8) Xiang, L.; Xiao, J.; Deng, K. Organometallics 2011, 30, 2018.
9) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.;
H NMR (400 MHz, THF+(CD ) SO capillary, 293 K) δ: 60.5, 47.0,
3
2
6
.9, ꢀ3.5, ꢀ6.7, ꢀ10.8, ꢀ12.9, ꢀ20.4, ꢀ31.4. Evans (d
8
-THF, toluene
1
Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104.
10) Gao, H.; Y, C.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.; Zhang,
Y. Organometallics 2010, 29, 4189.
11) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K.; Nakamura, M.
J. Am. Chem. Soc. 2009, 131, 11949.
capillary, concentration approximately 0.017 g/mL, 300 MHz for H):
(
averaged value 4.27 μ . Anal. Calcd for C H FeI N S : C = 55.02, H =
B
32 43
1 4 1
6.20, N = 8.02. Found: C = 54.85, H = 6.08, N = 7.89.
(
Compound 5 + 2 equiv of NaSMe. THF (50 mL) was added at
RT to a mixture of [methylenebis(N-Dipp-imidazole-2-ylidene)]diiodo-
iron(II) (250 mg, 0.32 mmol) and sodium thiomethoxide (45 mg, 0.64
mmol), and the reaction mixture stirred overnight. THF was removed
from the yellow suspension, the resulting bright yellow solid was washed
with toluene (2 ꢁ 50 mL) followed by THF (25 mL), and the solid was
then dried in vacuo to give a yellow powder (100 mg, 81% based on Fe
(
(
12) Hatanaka, T.; Ohki, Y.; Tatsumi, K. Chem. Asian J. 2010, 5, 1657.
13) Kandepi, V. V. K. M.; Cardoso, J. M. S.; Peris, E.; Royo, B.
Organometallics 2010, 29, 2777.
(14) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479.
(15) Chen, M.-Z.; Sun, H.-M.; Li, W.-F.; Wang, Z.-G.; Shen, Q.;
Zheng, Y. J. Organomet. Chem. 2006, 691, 2489.
4
981
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982

Organometallics
ARTICLE
(
16) Danopoulos, A. A.; Pugh, D.; Smith, H.; Sassmannshausen, J.
(52) Wells, K. D.; Ferguson, M. J.; McDonald, R.; Cowie, M.
Organometallics 2008, 27, 691.
Chem.—Eur. J. 2009, 15, 5491.
(
(
17) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559.
18) Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew.
(53) Mercs, L.; Labat, G.; Neels, A.; Ehlers, A.; Albrecht, M.
Organometallics 2006, 25, 5648.
Chem., Int. Ed. 2011, 50, 268.
19) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics
006, 25, 4670.
20) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Chem. Commun.
007, 1510.
21) Douthwaite, R. E.; Haussinger, D.; Green, M. L. H.; Silcock,
P. J.; Gomes, P. T.; Martins, A. M.; Danopoulos, A. A. Organometallics
999, 18, 4584.
(54) Li, B.; Liu, T.; Popescu, C. V.; Bilko, A.; Darensbourg, M. Y.
Inorg. Chem. 2009, 48, 11283.
(55) Breuer, J.; Fruhauf, H.-W.; Smeets, W. J. J.; Spek, A. L. Inorg.
(
2
2
(
Chim. Acta 1999, 291, 438.
(56) Zhang, Z.-Z.; Zhang, J.-K.; Zhang, W.-D.; Xi, H.-P.; Cheng, H.;
Wang, H.-G. J. Organomet. Chem. 1996, 515, 1.
(57) Pohl, S.; Opitz, U.; Haase, D.; Saak, W. Z. Anorg. Allg. Chem.
1995, 621, 1140.
(
1
(
(
22) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247.
23) Vogel, C. S.; Heinemann, F. W.; Khusniyarov, M. M.; Meyer, K.
(58) Evans, D. F. J. Chem. Soc. 1959, 2003.
(59) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
Inorg. Chim. Acta 2010, 364, 226.
24) Vogel, C.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer, K.
Angew. Chem., Int. Ed. 2008, 47, 2681.
25) Scepaniak, J. J.; Vogel, C. S.; Khusniyarov, M. M.; Heinemann,
F. W.; Meyer, K.; Smith, J. M. Science 2011, 331, 1049.
26) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. N.
Angew. Chem., Int. Ed. 2009, 48, 3158.
27) Nieto, I.; Ding, F.; Bontchev, R. P.; Wang, H.; Smith, J. M. J. Am.
Chem. Soc. 2008, 130, 2716.
28) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.; Fujiwara, Y.; Seike,
H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc.
(60) Perry, M. C.; Cui, X.; Powell, M. T.; Hou, D.-R.; Reibenspies,
J. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 113.
(61) Layfield, R. A.; McDouall, J. J. W.; Scheer, M.; Schwarzmaier,
C.; Tuna, F. Chem. Commun. DOI: 10.1039/C1CC14576B.
(
(
(
(
(
2
010, 132, 10674.
(
(
29) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev 2009, 109, 3677.
30) Leung, C. H.; Incarvito, C. D.; Crabtree, R. H. Organometallics
2
006, 25, 6099.
31) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. Synthesis
003, 17, 2661.
32) Venkatachalam, G.; Heckenroth, M.; Neels, A.; Albrecht, M.
Helv. Chim. Acta 2009, 92, 1034.
33) Zhang, S.; Yang, S.; Lan, J.; Yang, S.; You, J. Chem. Commun.
008, 6170.
34) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166.
35) Morvan, D.; Capon, J.-F.; Gloaguen, F.; Le-Goff, A.; Marchivie,
(
2
(
(
2
(
(
M.; Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J.-J. Organo-
metallics 2007, 26, 2042.
(
36) See Supporting Information.
(
37) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007,
2
51, 841.
38) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.;
Bill, E.; Chirik, P. J. Inorg. Chem. 2005, 44, 3103.
39) Barclay, J. E.; Leigh, G. J.; Houlton, A.; Silver, J. J. Chem. Soc.,
Dalton Trans. 1988, 2865.
40) Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem. Commun.
002, 32.
41) Canac, Y.; Lepetit, C.; Abdalilah, M.; Duhayon, C.; Chauvin, R.
J. Am. Chem. Soc. 2008, 130, 8406.
(
(
(
2
(
(42) Liu, B.; Xia, Q.; Chen, W. Angew. Chem., Int. Ed. 2009, 48, 5513.
(43) Barclay, J. E.; Hills, A.; Hughes, D. L.; Leigh, G. J. J. Chem. Soc.,
Dalton Trans. 1988, 2871.
44) Muller, G.; Sales, J.; Vinaixa, J.; Tejada, J. Inorg. Chim. Acta
982, 60, 227.
45) Heckenroth, M.; Neels, A.; Stoeckli-Evans, H.; Albrecht, M.
Inorg. Chim. Acta 2006, 359, 1929.
(
1
(
(46) Athey, P. S.; Kiefer, G. E. J. Org. Chem. 2002, 67, 4081.
(47) Ostermeier, M.; Limberg, C.; Ziemer, B.; Karunkaran, V.
Angew. Chem., Int. Ed. 2007, 46, 5329.
48) Heckenroth, M.; Neels, A.; Garnier, M. G.; Aebi, P.; Ehlers,
A. W.; Albrecht, M. Chem.—Eur. J. 2009, 15, 9375.
49) Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F.;
Schutz, J. Angew. Chem., Int. Ed. 2004, 43, 5896.
(
(
(
(
50) Chamizo, J. A.; Lappert, M. F. J. Org. Chem. 1989, 54, 4684.
51) Taton, T. A.; Chen, P. Angew. Chem., Int. Ed. Engl. 1996, 35,
1011.
4
982
dx.doi.org/10.1021/om200605b |Organometallics 2011, 30, 4974–4982