Iron(ii) complexes of ditopic carbanionic carbenes

Article abstract of DOI:10.1039/c3dt52638k

Reaction of dimesityliron(ii) (Fe₂(mes)₄) with the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexahydropyrimidin-2-ylidene (6-Xyl) afforded the novel trigonal planar complexes [Fe(IPr)(mes)₂] (1) and [Fe(6-Xyl)(mes)₂] (2), respectively. Both species were structurally characterized by single crystal X-ray diffraction and display structures and magnetic responses consistent with a quintet ground state (S = 2). Reaction of 1 with KC₈ in THF afforded K⁺ salts of the anionic complex [{:C[N(2,6-ⁱPr₂C₆H₃)] ₂(CH)C}₂Fe(mes)]^- (3) and the homoleptic organometallic anion [Fe(mes)₃]^- (4). By contrast, reduction of 2 resulted in extensive decomposition and intractable product mixtures. Complex 3 is coordinated by two ditopic carbanionic carbenes via the C4/C5 position while the C2 position retains unquenched carbenic character and remains vacant for further coordination. This was corroborated by reacting solutions of 3 with one and two equivalents of triethylaluminium (AlEt ₃) which resulted in the formation of [{Et₃Al:C[N(2,6- ⁱPr₂C₆H₃)]₂(CH)C}{:C[N(2, 6-ⁱPr₂C₆H₃)]₂(CH)C} Fe(mes)]^- (5) and [{Et₃Al:C[N(2,6-ⁱPr ₂C₆H₃)]₂(CH)C}₂Fe(mes)] ^- (6), respectively. Both of these species were structurally characterized as [K(2,2,2-crypt)]⁺ salts.

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Iron(II) complexes of ditopic carbanionic carbenes†
Rebecca A. Musgrave,a Robert S. P. Turbervill,a Mark Irwin,a Radovan Herchelb and
Cite this: Dalton Trans., 2014, 43,
4335
Jose M. Goicoechea*a
Reaction of dimesityliron(II) (Fe2(mes)4) with the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexahydropyrimidin-2-ylidene (6-Xyl) afforded
the novel trigonal planar complexes [Fe(IPr)(mes)2] (1) and [Fe(6-Xyl)(mes)2] (2), respectively. Both species
were structurally characterized by single crystal X-ray diffraction and display structures and magnetic
responses consistent with a quintet ground state (S = 2). Reaction of 1 with KC8 in THF afforded K+ salts
of the anionic complex [{:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (3) and the homoleptic organometallic
anion [Fe(mes)3]− (4). By contrast, reduction of 2 resulted in extensive decomposition and intractable
product mixtures. Complex 3 is coordinated by two ditopic carbanionic carbenes via the C4/C5 position
while the C2 position retains unquenched carbenic character and remains vacant for further coordi-
nation. This was corroborated by reacting solutions of 3 with one and two equivalents of triethyl-
aluminium (AlEt3) which resulted in the formation of [{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}{:C[N(2,6-iPr2C6H3)]2-
(CH)C}Fe(mes)]− (5) and [{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (6), respectively. Both of these species
were structurally characterized as [K(2,2,2-crypt)]+ salts.
Received 23rd September 2013,
Accepted 14th October 2013
DOI: 10.1039/c3dt52638k
www.rsc.org/dalton
1. Introduction
Despite their extensive use as supporting ligands in catalysis,1
it is well established that N-heterocyclic carbenes (NHCs) can
undergo a variety of bond activation reactions (C–H, C–C and
C–N) at the nitrogen atom functionalities when in the presence
of transition metals.2–4 By contrast, chemical activation of the
ligand backbone is much rarer and, to the best of our knowl-
edge, limited to systems with unsaturated alkenic backbones.
The first example of such a transformation was reported by
Whittlesey and co-workers who isolated a complex of an acti-
vated ditopic carbanionic carbene capable of bridging two
metal centres via the C4 and C5 positions simultaneously
(Fig. 1, Type A).5 Similar reactions affording formally anionic
activated NHC ligands capable of bridging metal centres via
the C2 and C4/C5 positions were reported soon thereafter
(Fig. 1, Type B).6 The latter species can also be accessed in the
absence of a transition metal; direct chemical reduction of
lanthanide NHC complexes or of the ligands themselves has
been found to yield alkali metal salts of the activated ligand.7,8
Fig. 1 Bimetallic ditopic carbanionic carbenes (anionic dicarbenes) and
their resonance canonicals: derived from the activation of alkenic C–H
bonds in unsaturated imidazolylidene N-heterocyclic carbene complexes.
More recently, the synthesis of bimetallic complexes of such
ligands has become possible via the alkali–metal-mediated
zincation of a carbene (IPr).9 These carbanionic carbenes can
be represented using a variety of resonance structures some of
which imply a significant degree of carbenic character at both
of the metallated positions. This has led to such species also
being discussed as “dicarbenes”.8 As such they are reminiscent
of the neutral 1,2,4-triazole-3,5-diylidenes first identified in
silver(I) coordination polymers.10
aDepartment of Chemistry, University of Oxford, Chemistry Research Laboratory,
Mansfield Road, Oxford OX1 3TA, UK. E-mail: jose.goicoechea@chem.ox.ac.uk
bDepartment of Inorganic Chemistry, Faculty of Science, Palacký University,
17. listopadu 12, 77146 Olomouc, Czech Republic
†Electronic supplementary information (ESI) available: PXRD diffractograms,
single crystal X-ray diffraction and magnetic data, computational details and
ESI-MS spectra are also available. CCDC 962247–962254. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt52638k
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In both kinds of backbone-activated imidazol-2-ylidene
ligands, one of the metal atoms is bonded to the carbanionic
C4 position (which has significant vinylic character). This
bonding mode is similar to that observed for transition metal
complexes of abnormally-bonded carbenes, first reported by
Crabtree and co-workers (these species are also termed
mesoionic because they cannot be represented by a Lewis
structure without invoking two opposing charges on the
ligand).11 While the number of transition metal complexes of
such ligands has steadily increased since they were first
reported,12 the first metal-free mesoionic carbene was not
reported until 2009.13
We have recently observed that chemical reduction of tran-
sition metal NHC complexes, e.g. [Mn(IPr)(mes)2] (mes = 2,4,6-
trimethylphenyl), can be used to access complexes of anionic
backbone-activated ligands such as K[{:C[N(2,6-iPr2C6H3)]2-
(CH)C}2Mn(mes)(THF)]·THF.14 This reaction can be extended
to other post-transition metal compounds such as [M(IPr)-
(mes)2] (M = Zn, Cd) affording complexes which also contain
carbanionic carbenes (or dicarbenes).15 Herein we expand on
these findings by demonstrating that chemical reduction of
the trigonal planar iron complex [Fe(IPr)(mes)2] (1) can be
used to access an iron(II) complex bearing two carbanionic
carbene ligands, [{:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (3),
alongside the previously reported anion [Fe(mes)3]− (4). By
contrast, attempts to chemically reduce the saturated ring-
Fig. 2 Molecular structure of 1 (anisotropic displacement ellipsoids pictured
at 50% probability level). Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (°): Fe1–C1: 2.115(1); Fe1–C28: 2.076(2);
Fe1–C37: 2.059(1); C1–N1: 1.362(2); C1–N2: 1.361(2); N1–C2: 1.387(2); N2–C3:
1.389(2); C2–C3: 1.346(2); N1–C4: 1.447(2); N2–C16: 1.447(2); C1–Fe1–C28:
122.6(1); C1–Fe1–C37: 117.8(1); C28–Fe1–C37: 119.6(1); N1–C1–N2: 103.3(1).
expanded carbene complex [Fe(6-Xyl)(mes)2] (2) resulted in solutions of the solids. Elemental analyses and powder X-ray
diffraction (PXRD) were used to confirm the compositional
vation nor in the generation of an isolable iron(I) complex. The purity of the solids (see ESI†).
extensive decomposition and did not result in ligand acti-
unquenched carbenic properties of the two carbanionic
carbene ligands in 3 were established by reacting the complex
The single crystal X-ray structures of 1 and 2 (Fig. 2 and 3)
confirm their trigonal planar geometries (selected data collec-
with one and two equivalents of triethylaluminium resulting tion and refinement parameters for all of the structures
in the formation of the novel complexes [{Et3Al:C- described in the manuscript are provided in Table 1). The sum
[N(2,6-iPr2C6H3)]2(CH)C}{:C[N(2,6-iPr2C6H3)]2(CH)C}Fe(mes)]− (5)
of C–Fe–C bond angles is 360.0° for both 1 and 2 (the deviation
and [{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (6), respecti- from planarity of the iron centre and bonded carbon atoms is
vely. The relative ease with which metal complexes of 0.005 and 0.000 Å, respectively). Trigonal planar iron com-
plexes of N-heterocyclic carbenes are rare and only a handful
backbone-activated carbenes can be accessed is surprising con-
sidering the significant role imidazol-2-ylidenes have played in of structurally authenticated species are reported in the chemi-
the isolation of low oxidation state main-group compounds cal literature (all of them containing imidazole based NHC sys-
tems).4d,23 No structurally authenticated trigonal planar iron
systems are known for expanded ring carbene systems; in fact,
of all of the transition metals, the only examples of such com-
plexes reported to date are limited to nickel(I).24
(similarly accessed by chemical reduction of preformed NHC
compounds).16–22
The structure of 1 is closely related to the isomorphous
manganese(II) complex previously reported by our research
group, [Mn(IPr)(mes)2],14 with some notable differences in
2. Results and discussion
2.1. Formation of neutral carbene complexes
Reaction of Fe(mes)2 with one equivalent of the N-heterocyclic bond metric parameters. The Fe–CNHC bond in 1 (2.115(1) Å) is
carbenes IPr or 6-Xyl yields the trigonal planar Lewis acid–base significantly shorter (Δd = 0.08 Å) than that of its manganese
homologue (2.195(2) Å). This observation is consistent with
adducts [M(L)(mes)2] (L = IPr (1), 6-Xyl (2)), respectively. The
resulting white solids display poor solubility in the majority of the different covalent radii published by Alvarez and co-workers
common laboratory solvents. 1 and 2 are sparingly soluble in for high-spin compounds of manganese and iron, reported to
be 1.61(8) and 1.52(6) Å, respectively.25 The metal–mesityl
THF and highly soluble in pyridine giving rise to clear red
solutions (presumably a result of pyridine coordination to the bonds are also shorter for 1 (2.059(1) and 2.076(2) Å) than for
metal centre and metal–ligand charge transfer transitions). the manganese analogue (2.122(2) and 2.125(2) Å). Geometric
parameters for 1 are consistent with other previously reported
Single crystal X-ray diffraction quality colourless crystals of 1
and 2 could be obtained by slow diffusion of hexane into trigonal planar iron(II) NHC complexes such as [Fe(IPr2Me2)-
4336 | Dalton Trans., 2014, 43, 4335–4344
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(mes)2] (IPr2Me2 = 2,5-diisopropyl-3,4-dimethylimidazol-1-yli- similar: 2.115(1) Å in 1 compared to 2.125(3) Å in [Fe(IPr2Me2)-
dene).23a Both complexes display trigonal planar geometries (mes)2]. The same is true of the Fe–Cmes distances which are
with similar bond metrics. The Fe–CNHC distances are very
2.059(1) and 2.076(2) Å in 1 (cf. 2.074(3) and 2.072(3) Å in
[Fe(IPr2Me2)(mes)2]).
A comparison of the structural features of 1 and 2 (Table 2)
confirms previous observations on the steric requirements of
expanded ring carbenes with respect to their five-membered
ring analogues.26,27 The greater N–CNHC–N angle of 6-Xyl
relative to IPr (117.1(2) vs. 103.3(1)°) makes the former
significantly more sterically demanding, pushing the
2,6-dimethylphenyl substituents towards the iron centre. This
is manifested in a longer Fe–CNHC bond, more obtuse
CNHC–Fe–Cmes angles and a more acute angle between the
mesityl substituents.
According to our density functional theory (DFT) studies,
the aforementioned structural parameters are consistent with
an S = 2 ground state for both 1 and 2. The optimized com-
puted geometries for both complexes with singlet (S = 0),
triplet (S = 1) and quintet (S = 2) electronic configurations
were calculated at the DFT level of theory (see ESI† for full
details). For both complexes, the lowest energy electronic con-
figuration is S = 2. Moreover, the optimized geometries for the
quintet states have the greatest similarity to those determined
by single crystal X-ray diffraction (M–C bond distances and
C–M–C bond angles are strongly affected by the electronic con-
figuration of the metal centre). Our calculations show that
M–C bond distances lengthen substantially on increasing the
number of unpaired electrons at the metal centre as might be
expected. Moreover, the singlet and triplet structures display a
more “T-shaped” geometry than that of the S = 2 system. These
observations were reported previously for the manganese(II)
Fig. 3 Molecular structure of 2 (anisotropic displacement ellipsoids
pictured at 50% probability level). Hydrogen atoms have been omitted
for clarity. Selected bond distances (Å) and angles (°): Fe1–C1: 2.139(2);
Fe1–C12: 2.062(1); C1–N1: 1.341(2); N1–C2: 1.479(2); C2–C3: 1.530(5);
C2’–C3: 1.469(5); N1–C4: 1.442(2); C1–Fe1–C12: 124.4(1); C12–Fe1–C12’:
111.3(1); N1–C1–N1’: 117.08(18). Symmetry operation ’: 1 − x, +y, 0.5 − z.
Table 1 Selected X-ray data collection and refinement parameters for 1, 2, K[3]·C6H14·0.5THF, [K(2,2,2-crypt)][5]·0.5THF and [K(2,2,2-crypt)][6]
1
2
K[3]·C6H14·0.5THF
[K(2,2,2-crypt)][5]·0.5THF
[K(2,2,2-crypt)][6]
Formula
C
45H58FeN2
C38H46FeN2
586.62
Monoclinic
C2/c
21.1298(3)
9.8555(1)
17.0866(3)
C71H99FeKN4O0.5
1111.49
C89H136AlFeKN6O6.5
1515.97
Monoclinic
P2(1)/n
10.8094(1)
25.7657(4)
31.1703(4)
C93H147Al2FeKN6O6
1594.08
Orthorhombic
Pna2(1)
23.1194(5)
37.1470(12)
11.0300(3)
Fw [g mol−1
]
682.78
Crystal system
Monoclinic
P2(1)/c
12.3163(1)
17.9494(1)
18.0222(1)
Triclinic
ˉ
P1
Space group
a (Å)
12.7856(3)
14.2917(5)
19.9211(7)
71.953(3)
85.004(2)
74.811(2)
3340.08(18)
2
b (Å)
c (Å)
α (°)
β (°)
98.365(1)
116.231(2)
94.291(1)
γ (°)
V (Å3)
3941.79(4)
4
3191.77(8)
4
8656.96(19)
4
9472.7(4)
4
Z
Radiation, λ (Å)
T (K)
Cu Kα, 1.54178
150(2)
ρcalc (g cm−3
)
1.151
1.221
1.105
1.163
1.118
μ (mm−1
)
3.291
3.984
2.676
2.344
2.245
Reflections collected
Independent reflections
Parameters
82 827
8283
441
0.0473
3.56/9.36
4.04/9.74
1.022
8080
31 883
11 766
724
0.0546
6.33/17.22
7.72/18.56
1.060
42 401
15 228
964
48 896
15 442
1000
0.0502
7.06/13.63
8.30/14.05
1.105
3317
208
R(int)
0.0207
3.29/8.88
3.52/9.09
1.022
0.0364
4.83/12.76
6.57/13.65
1.060
R1/wR2,a I ≥ 2σI (%)
R1/wR2,a all data (%)
GOF
a R1 = [∑||Fo| − |Fc||]/∑|Fo|; wR2 = {[∑w[(Fo)2 − (Fc)2]2]/[∑w(Fo2)2]}1/2; w = [σ2(Fo)2 + (AP)2 + BP]−1, where P = [(Fo)2 + 2(Fc)2]/3 and the A and B
values are 0.0539 and 1.19 for 1, 0.0567 and 1.73 for 2·THF, 0.1204 and 0.16 for K[3]·C6H14·0.5THF, 0.0789 and 0.80 for [K(2,2,2-crypt)][5]·0.5THF
and 0.0041 and 20.88 for [K(2,2,2-crypt)][6].
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diffraction. The crystals were identified as a K+ salt of [{:C[N-
(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (3), which contains two
C4/C5 bonded carbanionic carbenes. The formation of 3
involves the chemical reduction of one of the imidazolium
protons of the alkenic backbone to yield dihydrogen. The
deprotonation of an NHC complex followed by migration of
the C2 substituent to the C4 (or “abnormal” position) has
previously been demonstrated by Bertrand and co-workers.28
A balanced equation for the synthesis of 3 can be written by
invoking the formation of [Fe(mes)3]− as a side-product (see
Scheme 1). Addition of half of an equivalent of 2,2,2-crypt to
the mother liquor that afforded crystals of 3 allowed for the
crystallization of [K(2,2,2-crypt)][Fe(mes)3] ([K(2,2,2-crypt)][4];
this species has previously been reported by our research
group).29 Similarly, addition of 18-crown-6 affords the novel
salt [K(18-crown-6)THF][4] (see ESI† for structural data). It is
worth noting that the reaction mixtures that give rise to
species 3 and 4 are highly reactive and that the presence of
other species in solution, such as cation sequestering agents
(18-crown-6) or polydimethylsiloxane grease, can give rise to
products resulting from the chemical activation of C–O or Si–O
bonds, respectively. During the course of our studies we were
able to isolate and crystallographically characterize two such
products [{:C[N(2,6-iPr2C6H3)]2(CH)C}Fe(κ2,μ1,μ2-O(C2H4O)4-
C2H4O)K]2 (7) and [K(2,2,2-crypt)]2{Fe[Fe{κ2-O4(SiMe2)3}]2-
[κ2,μ2,μ2-O3(SiMe2)2]2} ([K(2,2,2-crypt)]2[8]). Due to the small
quantities of samples obtained the structures of these two
novel species are presented in the ESI† solely for reference
purposes.
Table 2 Selected bond distances (Å) and angles (°) for 1 and 2
1
2
Fe–CNHC
Fe–Cmes
N–CNHC–N
2.115(1)
2.076(2), 2.059(1)
103.3(1)
2.139(2)
2.062(1)
117.1(2)
124.4(1)
111.3(1)
C
C
NHC–Fe–Cmes
mes–Fe–Cmes
122.6(1), 117.8(1)
119.6(1)
complex [Mn(IPr)(mes)2] which has a sextet (S = 5/2) ground
state.14
The paramagnetic character of 1 and 2 is manifested in the
broad paramagnetically shifted resonances observed in their
1H NMR spectra. Due to the dynamic behaviour of these
species in pyridine (presumably involving the formation of a
Lewis acid–base adduct between the solvent and the metal
centre) attempts to determine the magnetic moment of the
samples using the Evans method were unsuccessful. The
chemical shift differences (Δδ) between the residual protic
solvent and the internal reference are not meaningful as each
solvent resonance experiences a different absolute shift (due
to variations in pseudo contact shifts). Attempts to use any (or
an average), of these values to determine magnetic moments
led to unphysically large values of μeff. SQUID magnetometry
measurements on solid samples of 1 and 2 were also compli-
cated by the presence of trace amounts of a paramagnetic con-
taminant and were thus run using a ferrosubtract protocol at
high fields to saturate the contaminant (this procedure is
described in the Experimental section). The results obtained
are consistent with a quintet ground state for both samples
with μeff values found to be moderately larger than the calcu-
lated spin-only value of 4.89 μB. The effect of trigonal symmetry
and spin–orbit coupling is manifested by substantial values
for the axial zero-field splitting parameters, D = −11.5 cm−1 for
1 and D = −18.9 cm−1 for 2, which resulted from the magnetic
analysis (see ESI† for more details). These values are compar-
able to others reported in the chemical literature for trigonal
planar iron(II) complexes.23b
The lack of alkenic protons in the carbene backbone of 2
led us to hypothesize that ligand activation might be circum-
vented on reduction allowing access to an iron(I) complex.
However, all attempts to reduce 2 using the methodology
described above gave rise to oily, intractable solids that could
not be satisfactorily identified.
The crystal structure of K[3]·C6H14·0.5THF (Fig. 4) contains
a single crystallographically unique anionic iron(II) complex in
the asymmetric unit (accompanied by a potassium cation and
solvent of crystallization). Species 3 is closely related to a
manganese(II) complex previously reported by our research
2.2. Synthesis of complexes containing ditopic carbanionic
carbene ligands
group,
K[{:C[N(2,6-iPr2C6H3)]2(CH)C}2Mn(mes)(THF)]·THF.14
The most notable difference is that the manganese species
adopts a distorted tetrahedral geometry due to the presence of
a THF molecule in the coordination sphere of the metal. No
such solvent is observed for 3, which is consistent with the
smaller covalent radius of iron relative to manganese (a result
A 1 : 1 stoichiometric mixture of [Fe(IPr)(mes)2] and KC8 was
stirred overnight in THF after which the black suspension was
filtered to remove graphite from the reaction mixture. Slow
diffusion of hexane into the resulting brown solution yielded
off-white crystalline blocks suitable for single crystal X-ray
Scheme 1 Synthesis of 3 and 4 by chemical reduction of 1.
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distances were notably shorter than the iron–carbene distance
(by approximately 0.05 Å), and is consistent with the vinylic
character of the iron–carbene bonds in 3.
Compositionally pure solid samples of K[3]·C6H14·0.5THF
could be obtained by successive recrystallizations from THF/
hexane. The presence of 3 in solution was confirmed by electro-
spray mass-spectrometry of a THF solution of K[3]·C6H14·
0.5THF which showed the molecular ion at an m/z value of 950.2
in the negative ion mode spectrum. The mass spectrum shows
no evidence of other reaction side-products although some
decomposition of the anion was observed during ionization.
Elemental analyses for K[3]·C6H14·0.5THF were consistently
(and reproducibly) found to give low carbon values (as pre-
viously observed for related manganese and zinc complexes of
carbanionic carbenes) presumably due to the formation of
metal carbides.14,15 A powder X-ray diffraction pattern col-
lected on a solid sample of K[3]·C6H14·0.5THF confirmed that
the crystalline material was compositionally pure (see ESI†).
Due to the high sensitivity of the compound and the presence
of decomposition products with a strong magnetic response
(colloidal iron resulting from the reduction of 1 is ferro-
magnetic), we were unable to collect satisfactory magnetic data
for K[3]·C6H14·0.5THF. Calculations at the DFT level are consist-
ent with an S = 2 or quintet ground state. The lowest energy
configuration gives rise to an S = 2 state which is lower in
energy than the singlet (S = 0) and triplet (S = 1) states by
74.4 and 23.1 KJ mol−1, respectively. Bond metric data for the
S = 2 optimized computed geometry (53DFT) are also the most
closely related to the experimentally determined values and
shows Fe–C bond distances and C–Fe–C bond angles that are
comparable to the values as determined by single crystal X-ray
diffraction.
Fig. 4 Molecular structure of K[3]·C6H14·0.5THF (anisotropic displace-
ment ellipsoids pictured at 50% probability level). Solvent of crystalliza-
tion and hydrogen atoms have been omitted for clarity. Atoms of
iPr groups are pictured as spheres of arbitrary radii. Selected bond dis-
tances (Å) and angles (°): Fe1–C3: 2.071(2); Fe1–C29: 2.064(2); Fe1–C55:
2.065(3); K1–C1: 2.811(3); C1–N1: 1.354(4); C1–N2: 1.359(3); N1–C2:
1.400(3); N2–C3: 1.406(3); C2–C3: 1.359(4); K2–C28: 2.824(3); C28–N3:
1.372(3); C28–N4: 1.350(4); N3–C29: 1.416(3); N4–C30: 1.403(3);
C29–C30: 1.369(4); C3–Fe1–C29: 122.5(1); C3–Fe1–C55: 108.4(1);
C29–Fe1–C55: 128.4(1).
of an increase in effective nuclear charge on moving from left
to right along the first transition metal series). This is also
manifested in the M–C bond lengths which are on average
0.11 Å shorter for 3 than for the manganese complex (see ESI†
for a comparison of bond metric data). The iron metal center
in 3 is coordinated by two ditopic carbanionic carbenes, in
each case through the C4/C5 position. The remaining carbenic
C2 sites are stabilized by electrostatic contacts with charge-
balancing potassium cations (dC–K: 2.811(3) and 2.824(3) Å).
The Fe–C distances in 3 are comparable to one another,
Fe–CNHC: 2.071(2) and 2.064(2) Å; Fe–Cmes: 2.065(3). This is
contrary to what was observed for 1 where the iron–mesityl
2.2. Complexation of Lewis acids to vacant carbenic sites
THF solutions of K[3] were reacted with one and two equi-
valents of triethylaluminium in the presence of an equivalent
of a cation sequestering agent (2,2,2-crypt) yielding the Lewis
acid–base adducts [{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}{:C[N(2,6-
iPr2C6H3)]2(CH)C}Fe(mes)]− (5) and [{Et3Al:C[N(2,6-iPr2C6H3)]2-
(CH)C}2Fe(mes)]− (6), respectively. A schematic representation
of the synthesis of these two novel compounds is provided in
Scheme 2. Formation of these species serves to highlight the
carbenic nature of the two C2 positions in 3 as neutral NHCs
Scheme 2 Synthesis of 5 and 6 by reaction of K[3] with one and two equivalents of triethylaluminium, respectively.
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Fig. 5 Molecular structure of [K(2,2,2-crypt)][5]·0.5THF (anisotropic dis-
placement ellipsoids pictured at 50% probability level). [K(2,2,2-crypt)]+,
solvent of crystallization and hydrogen atoms have been omitted for
i
clarity. Atoms of the Pr and Et groups are pictured as spheres of arbi-
Fig. 6 Molecular structure of [K(2,2,2-crypt)][6] (anisotropic displace-
ment ellipsoids pictured at 50% probability level). [K(2,2,2-crypt)]+ and
trary radii. Selected bond distances (Å) and angles (°): Fe1–C3: 2.110(2);
Fe1–C29: 2.074(2); Fe1–C55: 2.071(2); Al1–C1: 2.123(2); C1–N1:
1.358(3); C1–N2: 1.364(3); N1–C2: 1.398(3); N2–C3: 1.418(3); C2–C3:
1.358(4); C28–N3: 1.367(3); C28–N4: 1.366(3); N3–C29: 1.426(3); N4–
C30: 1.399(3); C29–C30: 1.359(3); C3–Fe1–C29: 124.9(1); C3–Fe1–C55:
109.6(1); C29–Fe1–C55: 125.4(1).
i
hydrogen atoms have been omitted for clarity. Atoms of the Pr and Et
groups are pictured as spheres of arbitrary radii. Two of the Et groups
bonded to Al1 exhibit positional disorder, only the major components
are shown for clarity. Selected bond distances (Å) and angles (°): Fe1–
C3: 2.085(5); Fe1–C29: 2.085(5); Fe1–C55: 2.049(5); Al1–C1: 2.092(5);
C1–N1: 1.371(6); C1–N2: 1.365(6); N1–C2: 1.391(6); N2–C3: 1.414(6);
C2–C3: 1.359(7); Al2–C28: 2.105(5); C28–N3: 1.359(6); C28–N4:
1.369(6); N3–C29: 1.423(6); N4–C30: 1.396(6); C29–C30: 1.339(7);
C3–Fe1–C29: 106.4(2); C3–Fe1–C55: 136.1(2); C29–Fe1–C55: 116.5(2).
are known to react with a variety of main-group based Lewis
acids such as AlEt3.30 Both anionic complexes could be isolated
as crystallographically pure samples by slow diffusion of hexane
into THF solutions of the crude reaction mixtures.
Table 3 Selected bond distances (Å) and angles (°) for 3, 5 and 6
Complexes 5 and 6 were characterized by single crystal X-ray
diffraction in [K(2,2,2-crypt)][5]·0.5THF and [K(2,2,2-crypt)][6],
respectively (Fig. 5 and 6). Each species displays a trigonal
planar moiety consisting of an iron(II) centre coordinated by
two ditopic carbanionic carbenes (via the C4/C5 positions) and
a mesityl substituent. These cores are identical to those
observed for 3. Bond metric data for these complexes are very
similar and summarized in Table 3. One of the two vacant C2
carbene positions present in 3 is bonded to a triethylalumi-
nium moiety in the case of 5 (dC–Al = 2.123(2) Å) while both of
the available sites form carbene–AlEt3 adducts in the case of 6
3
5
6
Fe–C3
Fe–C29
Fe–C55
C1–Al1
C28–Al2
C3–Fe–C29
C3–Fe–C55
C29–Fe–C55
2.071(2)
2.064(2)
2.065(3)
N/A
2.110(2)
2.074(2)
2.071(2)
2.123(2)
N/A
124.9(1)
109.6(1)
125.4(1)
2.085(5)
2.085(5)
2.049(5)
2.092(5)
2.105(5)
106.4(2)
136.1(2)
116.5(2)
N/A
122.5(1)
108.4(1)
128.4(1)
(dC–Al = 2.095(5) and 2.105(5) Å). The remaining bond metric meaningful with regard to the behaviour of the anion in
data for the complexes are generally very similar and do not solution. Analogous zinc species have been shown by multi-
merit a detailed analysis. Perhaps the most interesting obser- element NMR spectroscopy to exhibit free rotation about the
vation is the relative orientation of the two carbanionic car- Zn–CNHC bonds.15
benes in 6. While in 3 and 5 these two ligands are oriented in
Species 5 and 6 were further characterized in solution by
the same manner (with the imidazolium protons pointing means of electrospray mass-spectrometry. The negative ion mode
away from one another), in 6 these two protons point directly spectra of THF solutions of the samples show the anions as the
at one another (conferring greater C2v character to the anion). molecular ions at 1063.0 and 1177.7 Da. In the positive ion
However, while this observation is structurally noteworthy, it mode ion-coupled peaks for [K2(5)]+ and [K{K(2,2,2-crypt)}2(6)]+
presumably arises due to crystal packing effects and is not were also observed at m/z values of 1141.1 and 2049.2.
4340 | Dalton Trans., 2014, 43, 4335–4344
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Fe(IPr)(mes)2 (1). Fe2(mes)4 (500 mg, 0.85 mmol) and IPr
(660 mg, 1.70 mmol) were dissolved in diethyl ether (15 mL)
3. Conclusions
We have shown that the reaction of Fe(mes)2 with one equi- and the reaction mixture allowed to stir for 3 hours. A grey-
valent of the NHC ligands IPr and 6-Xyl can be used to access white solid was found to immediately precipitate from the
the trigonal planar iron(II) complexes [Fe(IPr)(mes)2] (1) and dark brown solution. The solid was isolated by filtration,
[Fe(6-Xyl)(mes)2] (2). Chemical reduction of the imidazol-2- washed with 1,2-difluorobenzene (2 mL) and subsequently
ylidene complex, 1, affords an iron(II) compound containing dried under vacuum, affording 1.04 g of a grey-white powder
carbanionic carbenes, [{:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− Fe(IPr)(mes)2 (89% yield). Off-white crystals suitable for single
(3). This reaction mixture gives rise to the known side-product crystal X-ray diffraction were grown by slow diffusion of
[Fe(mes)3]− (4), but was also found to activate 18-crown-6 and hexanes into a THF solution of the product (CCDC 962247). A
polydimethylsiloxane grease when present, highlighting the powder X-ray diffraction pattern was obtained which matched
highly reactive nature of these mixtures. By contrast, chemical the simulated diffraction pattern based on the single crystal
reduction of 2 was found to give rise to complex and intract- X-ray diffraction data (see ESI† for details). Anal. Calcd for
able product mixtures. The unquenched Lewis basic character C45H58FeN2 (Fw = 682.31): C 79.14%, H 8.57%, N 4.10%.
of the coordinated carbanionic carbene ligands in 3 was Found: C 78.94%, H 8.44%, N 3.97%.
demonstrated by reacting the anion with one and two equi-
Fe(6-Xyl)(mes)2 (2). Fe2(mes)4 (150 mg, 0.51 mmol) and
valents of triethylaluminium affording the novel complexes 6-Xyl (150 mg, 0.26 mmol) were dissolved in THF (5 mL). A
[{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}{:C[N(2,6-iPr2C6H3)]2(CH)C}Fe- grey-white solid immediately precipitated from the dark brown
(mes)]− (5) and [{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)]− (6). solution and the resulting mixture was stirred for 24 hours at
These findings show that formation of carbanionic carbenes is room temperature. The solid was isolated by filtration and dried
a common result on reduction of organometallic complexes of under vacuum, affording 59 mg of a grey-white powder identi-
imidazol-2-ylidene type ligands and expand the increasing fied as Fe(6-Xyl)(mes)2 (58% yield). Colourless crystals suitable
number of backbone activated NHC complexes capable of for single crystal X-ray diffraction were grown by slow diffusion
acting as ditopic donors between metal centers.
of hexanes into a red solution of the product in a 1 : 1 mixture
(by volume) of THF–pyridine (CCDC 962248). A powder X-ray
diffraction pattern was obtained which matched the simulated
diffraction pattern based on the single crystal X-ray diffraction
data (see ESI† for details). Anal. Calcd for C38H46FeN2 (Fw =
586.21): C, 77.79%; H, 7.91%; N, 4.78%. Found: C, 76.87%; H,
Experimental section
General synthetic methods
All reactions and product manipulations were carried out 7.94%; N, 4.70%. The % C values obtained for this sample were
under an inert atmosphere of argon or dinitrogen using stan- found to be consistently higher than anticipated, perhaps due
dard Schlenk-line or glovebox techniques (MBraun UNIlab to the presence of a solvate containing THF in the lattice.
glovebox maintained at <0.1 ppm H2O and <0.1 ppm O2).
K[{:C[N(2,6-iPr2C6H3)]2(CH)C}2Fe(mes)] (K[3]) and K[Fe-
Fe2(mes)4 (mes = trimethylphenyl), 1,3-bis(diisopropylphenyl)- (mes)3] (K[4]). A mixture of 1 (100 mg, 0.15 mmol) and KC8
imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexa- (20 mg, 0.15 mmol) was weighed into a glass vial under di-
hydropyrimidin-2-ylidene (6-Xyl) were prepared according to nitrogen inside a glovebox. THF (5 mL) was added and the
previously reported literature procedures.31,32,27a KC8 was pre- resulting black mixture stirred for 48 hours. The mixture was
pared by heating a stoichiometric mixture of K (99.95%, filtered through a Celite plug and layered with hexane to yield
Strem) and graphite (99.99+%, Sigma-Aldrich) at 200 °C under off-white crystals of K[3]·C6H14·0.5THF suitable for single
an inert atmosphere over the course of several days. 2,2,2- crystal X-ray diffraction (26% crystalline yield; CCDC 962249).
Crypt
(4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]-hexa- Samples of K[3]·C6H14·0.5THF suitable for powder X-ray dif-
cosane; VWR, 99%) and 18-crown-6 (1,4,7,10,13,16-hexaoxa- fraction were obtained by multiple recrystallizations from
cyclooctadecane; 99%, Alfa Aesar) were used as received after THF–benzene mixtures, and a final recrystallization from
careful drying under vacuum. Hexane (hex; HPLC grade, THF–hexane. ESI (−ve ion mode): m/z 950.2 [M]−. Anal. Calcd
Sigma-Aldrich) and diethylether (Et2O; pesticide residue grade, for C63H81FeKN4 (Fw = 988.59): C, 76.47%; H, 8.26%; N,
Fisher) were purified using an MBraun SPS-800 solvent system. 5.67%. Found: C, 69.38%; H, 8.14%; N, 5.86%. The sample
Tetrahydrofuran (THF; HPLC grade, Sigma-Aldrich) was dis- was found to consistently burn low for carbon as previously
tilled over a sodium metal–benzophenone mixture. 1,2-Difluoro- observed for the manganese analogue.
benzene (DFB; 98%, Fluorochem) and pyridine (py; 99%, Alfa
The mother liquor that afforded K[3]·C6H14·0.5THF was iso-
Aesar) were distilled over CaH2. d5-Pyridine (99.5%, Cambridge lated and reduced to dryness under a dynamic vacuum. The result-
Isotope Laboratories, Inc.) was dried over CaH2 and vacuum ing dark solid was redissolved in THF and 2,2,2-crypt (28 mg,
distilled. All dry solvents were stored under argon in gas-tight 0.075 mmol) was added. Filtration of the THF solution followed
ampoules. Additionally hexanes, Et2O, diflurorobenzene and by its slow diffusion into hexane afforded crystals of the pre-
THF were stored over activated 3 Å molecular sieves. Celite 545 viously-reported compound [K(2,2,2-crypt)][4]. A similar procedure,
(Sigma-Aldrich) was used as received after careful drying under employing 18-crown-6 as a cation sequestering in place of 2,2,2-
a dynamic vacuum.
crypt, yielded the [K(18-crown-6)THF][4] salt (CCDC 962250).
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[K(2,2,2-crypt)][{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}{:C[N(2,6- air- and moisture-sensitivity of the complexes they were flame-
iPr2C6H3)]2(CH)C}Fe(mes)] ([K(2,2,2-crypt)][5]). A mixture of sealed under vacuum in Suprasil tubes. In order to distinguish
K[3] (44 mg, 0.04 mmol) and 2,2,2-crypt (15 mg, 0.04 mmol) the intrinsic susceptibility of 1 and 2 from that of ferromagnetic
was weighed into a glass vial in a glovebox. AlEt3 (5 µL, impurities, the magnetization was measured at five applied
0.04 mmol) and THF (5 mL) were added and the resulting fields between 4 and 5 T after an initial zero-field cooling to 2 K.
mixture stirred for 24 hours at room temperature. The mixture The sample was warmed to 302 K in small increments and the
was filtered through a Celite plug and layered with hexane to magnetization measured at each step at the same five fields.
yield colourless crystals of [K(2,2,2-crypt)][5]·0.5THF suitable The slope of the resulting linear plot of the molar magnetiza-
for single crystal X-ray diffraction (CCDC 962251). After the tion as a function of applied field was used to obtain the molar
SXRD experiment a small number of crystals (∼5 mg) were magnetic susceptibility at each temperature. Diamagnetic
able to be isolated for mass spectrometric analysis. ESI (−ve corrections, obtained from tables of Pascal’s constants,36 have
ion mode): m/z 1063.0 [M]−; (+ve ion mode): m/z 1141.1 [K2M]+. been applied to the measured molar magnetic susceptibilities.
[K(2,2,2-crypt)][{Et3Al:C[N(2,6-iPr2C6H3)]2(CH)C}Fe(mes)]
Computational methods
([K(2,2,2-crypt)][6]). A mixture of K[3] (100 mg, 0.09 mmol)
and 2,2,2-crypt (34 mg, 0.09 mmol) was weighed into a glass
vial in a glovebox under dinitrogen. AlEt3 (24 µL, 0.18 mmol)
and THF (5 mL) were added and the resulting mixture stirred
for 24 hours at room temperature. The mixture was filtered
through a Celite plug and layered with hexane to yield colour-
less crystals of [K(2,2,2-crypt)][6] suitable for single crystal
X-ray diffraction (56% crystalline yield; CCDC 962252). ESI
(−ve ion mode): m/z 1177.7 [M]−. ESI (+ve ion mode): m/z
2049.2 {K[K(2,2,2-crypt)]2[6]}+.
All calculations described in this paper were performed with
the Amsterdam Density Functional package (ADF2013.01).37
The TZ2P Slater-type basis set of triple-ζ quality, extended
with two polarization functions, was used to describe Fe (the
DZP basis set was employed for Al, C and N and the DZ basis
set used for H). Electrons in orbitals up to and including 2p
for Fe were considered part of the core and treated in accord-
ance with the frozen core approximation (Fe 2p). All calcu-
lations employed the local density approximation (LDA)38 to
the exchange potential, along with the local exchange-corre-
lation potential of Vosko, Wilk and Nusair (VWN)39 and
gradient corrections to non-local exchange and correlation pro-
posed by Becke and Perdew (BP86).40 All calculations were
unrestricted (the doublet, quartet and sextet states were com-
puted for 1 and 2). Relativistic effects were incorporated using
the zeroth order relativistic approximation (ZORA).41 The pres-
ence of cations in the crystal lattice was modelled by surround-
ing the clusters with a continuum dielectric model (COSMO).42
The chosen dielectric constant ε = 7.58 corresponds to tetra-
hydrofuran, although structural parameters are not strongly
dependent on this choice. All structures were optimized using
the gradient algorithm of Versluis and Ziegler.43
Single crystal X-ray structure determination
Single crystal X-ray diffraction data were collected using an
Oxford Diffraction Supernova dual-source diffractometer
equipped with a 135 mm Atlas CCD area detector. Crystals
were selected under Paratone-N oil, mounted on micromount
loops and quench-cooled using an Oxford Cryosystems open
flow N2 cooling device.33 Data were collected at 150 K using
mirror monochromated CuKα radiation (λ = 1.5418 Å) and pro-
cessed using the CrysAlisPro package, including unit cell para-
meter refinement and inter-frame scaling (which was carried
out using SCALE3 ABSPACK within CrysAlisPro).34 Equivalent
reflections were merged and diffraction patterns processed
with the CrysAlisPro suite. Structures were subsequently solved
using direct methods, and refined on F2 using the SHELXL
97-2 package.35
Acknowledgements
We thank the EPSRC and the University of Oxford for financial
support of this research (DTA studentships R.S.P.T. and M.I.)
and the University of Oxford for access to Chemical Crystallo-
graphy and Oxford Supercomputing Centre facilities. We also
thank Elemental Microanalysis (Devon) for performing the
elemental analyses. R.H. acknowledges the financial support
from the Operational Program Research and Development for
Innovations (CZ.1.05/2.1.00/03.0058) of the Ministry of Edu-
cation, Youth and Sports of the Czech Republic.
Other characterization techniques
Positive and negative ion mode electrospray mass spectra were
recorded on THF solutions of the compounds (10–20 mM) on
a Masslynx LCT Time of Flight mass spectrometer with a
Z-spray source (150 °C source temperature, 200 °C desolvation
temperature, 2.4 kV capillary voltage and 25 V cone voltage).
The samples were made up inside a glovebox under an inert
atmosphere and rapidly transferred to the spectrometer in an
air-tight syringe. Samples were introduced directly with a 1 mL
SGE syringe and a syringe pump at 0.6 mL h−1
.
1H NMR spectra were acquired at 500 MHz on a Varian
Unity 500 MHz NMR spectrometer and referenced to the most
downfield residual solvent resonance (d5-pyridine: 8.74 ppm).
The magnetic properties of all complexes were measured on
unanchored polycrystalline samples with a Quantum Design
MPMS 5 or MPMS XL SQUID magnetometer. Due to the high
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