Iron complexes of a bidentate picolyl-NHC ligand: Synthesis, structure and reactivity

Article abstract of DOI:10.1039/c6dt02792j

The synthesis, structure and reactivity of bidentate picolyl N-heterocyclic carbene (NHC) iron compounds were studied. Compounds [FeBr(HL)₂]Br (1), [FeBr(HL)(HMDS)] (2) and [FeBr₂(HL)] (3) (HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-1-ylidene, HMDS = hexamethyldisilazide) were prepared from H₂LBr with suitable amounts of Fe(HMDS)₂ or in situ prepared [Fe(HMDS)Br]. The deprotonation of 1 with 2 eq. of LiHMDS gave [FeL₂] (4), featuring dearomatized pyridine moieties with exocyclic C-C double bonds. The protonation of 4 with 2 eq. of PPh₃·HBr results in the formation of 1. Attempted deprotonation of 3 using benzyl Grignard as the base resulted in transmetalation products [FeBnBr(HL)] (5) and [FeBn₂(HL)] (6). Exposure of 6 to CO resulted in the formation of diamagnetic compound [Fe(CO)₃(HL)] (7) and dibenzyl ketone. Prolonged exposure of 7 to CO with heating induces pyridine dissociation, affording [Fe(CO)₄(HL-κC)] (8). Treatment of compound 6 with an equimolar amount of p-methoxybenzyl bromide yielded homo- and cross-coupling products.

Full text of DOI:10.1039/c6dt02792j

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DOI: 10.1039/C6DT02792J
Dalton
ARTICLE
Iron Complexes of a Bidentate Picolyl-NHC Ligand: Synthesis,
Structure and Reactivity
Qiuming Liang, Trevor Janes, Xhoana Gjergji and Datong Song*
Received 00th January 20xx,
Accepted 00th January 20xx
The synthesis, structure and reactivity of bidentate picolyl N-heterocyclic carbene (NHC) iron compounds were studied.
Compounds [FeBr(HL)2]Br (1), [FeBr(HL)(HMDS)] (2) and [FeBr2(HL)] (3) (HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-1-
ylidene, HMDS = hexamethyldisilazide) were prepared from H2LBr with suitable amounts of Fe(HMDS)2 or in situ prepared
[Fe(HMDS)Br]. The deprotonation of 1 with 2 eq. of LiHMDS gave [FeL2] (4), featuring dearomatized pyridine moieties with
exocyclic C−C double bond. The protonation of 4 with 2 eq. of PPh3·HBr results in the formation of 1. Attempted
deprotonation of 3 using benzyl Grignard as the base resulted in transmetalation products [FeBnBr(HL)] (5) and [FeBn2(HL)]
(6). Exposure of 6 to CO resulted in the formation of diamagnetic compound [Fe(CO)3(HL)] (7) and dibenzyl ketone. Prolonged
exposure of 7 to CO with heating induces pyridine dissociation, affording [Fe(CO)4(HL-C)] (8). Treatment of compound 6
with an equimolar amount of p-methoxybenzyl bromide yielded homo- and cross-coupling products.
DOI: 10.1039/x0xx00000x
www.rsc.org/
unfunctionalized alkenes.14,15 The dearomatization of the
pyridine moiety of the pincer ligand from the migration of
cobalt-hydride or cobalt-alkyl to the 4-position of pyridine ring
Introduction
The use of actor ligands in metal complexes, where the
ligands can participate in chemical transformation directly, has
garnered increasing attention recently as the participation of
actor ligands has enabled fascinating reactivity patterns in both
stoichiometric and catalytic fashions.1,2 The participation of
actor ligands in chemical reactions can be through ligand-based
reactivity3,4 or metal−ligand cooperation.5 Depending on the
relative positions of the metal centre and the ligand reactive site,
metal−ligand cooperation can be proximal2,5−7 or distal.2
Milstein and co-workers have demonstrated an interesting
is identified as a catalyst deactivation pathway.15
In contrast to the pincer complexes mentioned above,
complexes of bidentate picolyl-containing ligands rarely show
parallel reactivity where the removal of pyridylic proton
dearomatizes the pyridine ring16−19. Waterman and co-workers
have recently reported the Zn(II) complexes of bidentate
picolyl−phosphine ligands, where the pyridylic protons were
removed causing the dearomatization of the pyridine rings prior
to coordination to Zn(II).19 To the best of our knowledge,
complexes of the analogous deprotonated/ dearomatized
bidentate picolyl−NHC ligands have not been reported to date.
The charge neutral bidentate picolyl−NHC ligands have been
incorporated in several transition metal complexes, most of
which involve noble metals.20 Despite the increasing interest in
base metal complexes, only a few examples with bidentate
picoyl−NHC ligands have been reported.21−23 Danopoulos et al.
have synthesized the mono and bis(picolyl−NHC) nickel
dibromide complexes by transferring the ligands from the
corresponding silver complexes.21 Similar bis(picolyl−NHC)
nickel dichloride complexes have been synthesized by Jin et al.
and applied to olefin polymerization.22 In the case of iron, only
mode of distal
metal−ligand cooperation involving
dearomatization−rearomatization of pyridine/acridine based
tridentate ligands and applied such reactivity to many
stoichiometric and catalytic processes.8−9 Our group and others
have reported CNN-pincer complexes toward catalytic
hydrogenation.10−13
Through
deuterium
scrambling
experiments, we have shown that the dearomatization−
rearomatization of the pyridine moiety of the (NHC)NN-ligand
can involve the deprotonation−reprotonation of the methylene
groups on both the amine and NHC arms, although the
intermediate with deprotonated amine arm could not be
observed directly.10 Recently, Chirik and co-workers have
reported the (CNC)Fe(N2)2 and (CNC)CoR complexes and their
high catalytic hydrogenation activity of hindered,
the
piano-stool
iron
carbonyl
complexes
[Fe(CO)1−2(Cp)(picolyl−NHC)X] (X = I, BF4) have been reported.23
In fact, nitrogen/oxygen donor substituted NHC bidentate
ligands have received less attention compared to the extensive
study on NHC iron complexes.24−26 The first examples with
anionic aryloxo-functionalized NHCs, were reported by Shen et.
al.27,28 Lavoie and co-workers reported the complexes with
Davenport Chemical Research Laboratories, Department of Chemistry, University of
Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada
E-mail: dsong@chem.utoronto.ca
† Electronic Supplementary Information (ESI) available: NMR spectra and X-ray
crystallographic experimental details. CCDC 1493390-1493397. For ESI and neutral imine-functionalized NHCs.29,30 The group of Hahn and
crystallographic
data
in
CIF
or
other
electronic
format.
See
Chen reported the octahedral complexes with pyridinyl31 and
DOI: 10.1039/x0xx00000x
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ARTICLE
pyrimidinyl32,33 substituted NHC respectively. Herein, we report The paramagnetic compound
Dalton Transactions
is soluble in THF,VtioewluAertniclee,Oanlninde
2
the syntheses and structures of a series of iron complexes of a benzene, and slightly soluble in diethylDeOtIh: 1e0r.1.0I3n9/sCo6lDuTt0io2n79,22J
bidentate picolyl−NHC ligand, along with their reactivity slowly decomposes at room temperature into intractable
including the interconversions, transmetalation, C−C bond products. When stored in the solid state at −35°C, 2 is stable for
formations, and ligand deprotonation accompanied by the weeks, i.e., without noticeable changes in the appearance of its
dearomatization of the pyridine moiety.
1H NMR spectrum. Compound
2 crystallizes in the triclinic space
̅
group P
1
with a pair of enantiomers in the asymmetric unit. As
shown in Figure 1, the Fe(II) centre adopts a distorted
tetrahedral coordination geometry with the nitrogen and
carbon donor atoms of HL, the nitrogen donor atom of HMDS,
and a bromide ligand occupying the four coordination sites. The
average Fe−CNHC and Fe−NHMDS bond lengths of the two
enantiomers are 2.095(8) and 1.945(8) Å, respectively,
Results and discussion
Synthesis and structure of 1-3 bearing neutral picolyl-NHC ligand
Compounds [FeBr(HL)2]Br (1) and [FeBr(HL)(HMDS)] (2) can be
synthesized from the reaction of H2LBr with 0.5 and 1 eq. of
Fe(HMDS)2, respectively (Scheme 1) in excellent yields,
employing the synthetic protocol by Danopoulos.34 The
paramagnetic compound
hydrocarbons, and unstable in dichloromethane and
chloroform. Compound crystallizes in the monoclinic space
group P21/n along with THF solvent molecules. As shown in
Figure 1, the Fe(II) centre adopts distorted trigonal
1 is insoluble in THF, diethyl ether, and
1
a
bipyramidal geometry with two N donor atoms from two HL
ligands occupying the apical positions, and one bromide and
two carbon donors occupying the equatorial positions, while
the other bromide is outer-sphere to balance the charge. The
N1−Fe1−N4 angle is 167.0(1)° while sum of the three bond
angles within the equatorial plane is 360.1(2)°. The average
Fe−CNHC and Fe−Npyridine bond lengths are 2.115(6) Å and 2.302(4)
Å, respectively, similar to those of analogous complexes.24,35−45
The six-membered chelate rings adopt boat conformations. The
average bite angle of HL in
1 is 82.8(2)°. Compound 1 is
structurally and magnetically distinct from the bis(picolyl−NHC)
nickel dihalide analogues,21,22 where the nickel complexes are
diamagnetic, square planar at nickel centres with both halides
outside the coordination sphere.
Figure 1. X-ray structures of 1 (top) and 2 (bottom). Ellipsoids are shown at 50%
probability. Only one enantiomer of 2 is shown. Selected bond lengths (Å) and angles (°)
for 1: Fe1−C1 2.110(5), Fe1−C19 2.120(5), Fe1−N1 2.292(4), Fe1−N4 2.312(4), Fe1−Br1
2.4114(9), C1−Fe1−C19 119.0(2), C1−Fe1−N1 83.3(2), C19−Fe1−N1 90.1(2), C1−Fe1−N4
91.0(2), C19−Fe1−N4 82.4(2), N1−Fe1−N4 167.0(1), C1−Fe1−Br1 119.1(1), C19−Fe1−Br1
122.0(1), N1−Fe1−Br1 96.8(1), N4−Fe1−Br1 96.2(1); for 2: Fe1−N4 1.943(6), Fe1−C1
2.105(8), Fe1−N3 2.146(6), Fe1−Br1 2.457(1), N4−Fe1−C1 130.5(3), N4−Fe1−N3 107.1(3),
C1−Fe1−N3 88.0(3), N4−Fe1−Br1 120.3(2), C1−Fe1−Br1 102.9(2), N3−Fe1−Br1 98.2(2).
Scheme 1. Syntheses of picolyl−NHC iron(II) complexes.
2 | Dalton Trans. 2016, 00, 1-3
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Fe−CNHC (Fe1−C1 2.065(3) Å) and Fe−Npyridine (Fe1−N3 2.157(3) Å)
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DOI: 10.1039/C6DT02792J
1 and more similar to
bond lengths are shorter than those in
those in
Synthesis and structure of 4
2
.
Complex possesses the charge neutral bidentate HL ligand and
3
two bromide ligands on a four-coordinate Fe(II) centre. It is
conceivable that the deprotonation of the CH2 group of HL
ligand of
3
could potentially generate an L− ligand with a
dearomatized pyridine ring in the form of [FeBrL] with a three-
coordinate Fe(II) centre or [FeBrL]2 with four-coordinate Fe(II)
centres and two bridging bromides. However, the
deprotonation of
3 using 1 eq. of base is often competing with
Figure 2. X-ray structures of 3. Ellipsoids are shown at 50% probability. Selected bond the substitution of the bromide ligand by the base and is also
lengths (Å) and angles (°): Fe1−C1 2.065(3), Fe1−N3 2.157(3), Fe1−Br2 2.3984(6), Fe1−Br1
2.4004(6), C1−Fe1−N3 89.1(1), C1−Fe1−Br2 118.5(1), N3−Fe1−Br2 103.96(7),
C1−Fe1−Br1 109.5(1), N3−Fe1−Br1 111.01(8), Br2−Fe1−Br1 119.85(2).
complicated by the rapid ligand redistribution of the
deprotonation product. For example, the treatment of
eq. of LiHMDS in toluene at low temperatures cleanly yields
(Scheme 1), the ligand substitution product. If this reaction is
carried out in diethyl ether or THF, the mixture of and is
3 with 1
2
2
4
often obtained under various conditions (i.e., different
temperatures, concentrations, and stoichiometries), where the
presence of
comparing with an independently synthesized sample.
Compound can be cleanly obtained from the reaction of 2 eq.
of LiHMDS with 1 eq. of in THF; although the structure of
4
can be confirmed through 1H NMR experiment by
4
1
4
could not be revealed with NMR experiments due to its
paramagnetic nature, the stoichiometry and cleanness of this
reaction suggest a formula of [FeL2]. Alternatively, compound
4
can be prepared from the reaction of 1 eq. of in situ generated
L− (from 2 eq. of nBuLi and 1 eq. of H2LBr) with 0.5 eq. of FeBr2.
The product of the latter method allows for the growth of X-ray
quality single crystals and in turn, the structural determination
Figure 3. Molecular structures of 4. Ellipsoids are shown at 30% probability. Selected
bond lengths (Å) and angles (°): Fe1−N1 2.001(2), Fe1−N4 2.011(3), Fe1−C19 2.024(3),
Fe1−C1 2.033(2), C4−C5 1.372(4), C5−C6 1.448(4), C6−C7 1.344(4), C7−C8 1.411(4),
C8−C9 1.353(4), N1−C9 1.366(3), N1−C5 1.398(3), C22−C23 1.368(5), C23−C24 1.487(5),
C24−C25 1.348(7), C25−C26 1.388(7), C26−C27 1.348(5), N4−C27 1.372(4), N4−C23
1.377(4), N1−Fe1−N4 129.4(1), N1−Fe1−C19 113.6(1), N4−Fe1−C19 93.3(1), N1−Fe1−C1
92.23(9), N4−Fe1−C1 114.7(1), C19−Fe1−C1 115.2(1).
by X-ray crystallography, which confirms the formula of
[FeL2]. Presumably the formation of from the deprotonation
of goes through a three-coordinate [FeLBr] intermediate,
which undergoes rapid ligand redistribution to form and FeBr2.
Using other bases such as alkoxides, LDA, Group 1 hydrides, and
CH3Li to deprotonate leads to complicate mixtures of products
that contain To test the reversibility of
dearomatization/aromatization, was treated with 2 eq. of
PPh3 HBr, which resulted formation of in good yields.
Compound is soluble in THF and toluene, and slightly soluble
4 as
4
3
4
3
4
.
4
comparable to those of the analogous literature complexes.45,46
·
1
The average bite angle of the HL ligand in
2
is 88.1(3)°, wider
in
4
than that in
1
. The largest peak in the 1H NMR spectrum of
2
in diethyl ether, and reacts with dichloromethane and
chloroform. It is extremely reactive toward air and moisture,
C6D6 at 13.92 ppm has been tentatively assigned to the protons
of the HMDS group. The reaction of H2LBr with an equimolar
1
but thermally robust, i.e., no noticeable change in the H NMR
amount of
2 in THF cleanly affords 1 in nearly quantitative yield
spectrum after heating a C6D6 solution sealed under N2 at 60 °C
(Scheme 1).
for several days. As shown in Figure 3, the Fe(II) centre in
4
Using the protocol by Byers,47 adding in situ prepared
adopts a distorted tetrahedral geometry with two N,C-chelates
completing the coordination sphere. The Fe−N (Fe1−N1 2.001(2)
Å and Fe1−N4 2.011(3) Å) bonds are much shorter than those in
[FeBr(HMDS)] (premixing 0.5 eq. of anhydrous FeBr2 and 0.5 eq.
of Fe(HMDS)2 in THF) to 1 eq. of H2LBr yields [FeBr2(HL)] (
an excellent yield (Scheme 1). The paramagnetic compound
3
) in
is
3
complexes
2.033(2) Å) bonds are shorter than those in
extent. The C−N bonds in the C5N rings are elongated compared
to those in and the C−C bonds of the C5N rings display
1
−3
, while the Fe−C (Fe1−C19 2.024(3) Å and Fe1−C1
sparingly soluble in diethyl ether and hydrocarbons, slightly
soluble in THF, and soluble and stable in dichloromethane and
1−3
to a lesser
chloroform. Compound
3 crystallizes in the monoclinic space
1−3
group P21/n. As shown in Figure 2, the distorted tetrahedral
geometry adopted by the Fe(II) centre is similar to those of the
methylene-bridged biscarbene iron(II) dihalide complexes.48−50
obvious alternating long−short pattern. The C−C bonds
exocyclic to the C5N rings (C22−C23 1.368(6) Å and C4−C5
1.371(4) Å) display bond lengths typical for C−C double bonds.
The bite angle of HL in
3 (89.1(1)°) is similar to that in 2. The
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The C−N bonds exocyclic to the C3N2 rings (C22−N5 1.429(5) Å
and C4−N2 1.412(3) Å) are slightly shortened compared to those
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DOI: 10.1039/C6DT02792J
in compound 1, showing the partial delocalization of the π
electrons in the six-membered chelate ring. However, the
shortening of the C−N bonds is much less significant compared
to that of the C−C bonds, suggesting that the dominant
resonance form has a C−C double bond and C−N single bond
Scheme 2. Reactions of 3 with benzyl Grignard reagent.
between the two rings of the L− ligand in
4. Each chelate ligand
is nearly planar with the Fe(II) centre sitting outside the planes
by ~0.5 Å. All these structural features suggest that the desired
L− ligand has been obtained, featuring the dearomatized
pyridine ring and an exocyclic C−C double bond, analogous to
the related CNN pincer ligand10 and picolyl−phosphine
bidentate ligand.16−19 The mesityl ring of one L− ligand in
4 is
nearly parallel with the C5N ring of the other L− ligand (dihedral
angles: 2.75 and 3.96°) with the shortest contact distance of
~3.6 Å, suggesting weak π−π stacking interactions in between.
Reaction of 3 with Grignard reagent
When 1.2 eq. of benzyl Grignard reagent is used to
deprotonate
[FeBnBr(HL)] (
Compound
ether and toluene. The reaction of
Grignard reagent in diethyl ether/dioxane gives the double-
transmetalation product [FeBn2(HL)] in good yields.
Compound is soluble in THF, toluene and diethyl ether. The
molecular structures of and have been confirmed
3
in THF/dioxane, the transmetalation product
5
) can be obtained in good yields (Scheme 2).
5
is soluble in THF, and slightly soluble in diethyl
with 2.4 eq. of benzyl
3
(6)
6
5
6
crystallographically (Figure 4). Each Fe(II) centre adopts a
distorted tetrahedral geometry displaying typical bond
lengths.42,51,52 The bite angle of the HL ligand in
5
(88.3(2)°) is
, while the bite angle of HL in
(86.05(7)°) is slightly smaller. Both and are thermally
similar to those of
2
and
3
6
5
6
unstable and decompose in solution into intractable products at
room temperature over a few days.
Since complex
base, adding
6
has Fe-bound two benzyl groups as the built-in
π-acceptor ligands to the metal centre could not
only force one of the benzyl ligands closer to the to-be-
deprotonated CH2 group of the HL ligand, but also increase the
acidity of the CH2 group, facilitating the deprotonation. To test
this possibility, a toluene solution of
6 was exposed to a CO
Figure 4. X-ray structures of complexes 5 (top) and 6 (bottom). Ellipsoids are shown at
50% probability. Selected bond lengths (Å) and angles (°) for 5: Fe1 C8 2.060(5), Fe1−C1
2.061(5), Fe1−N1 2.174(4), Fe1−Br1 2.451(1), C8−Fe1−C1 121.9(2), C8−Fe1−N1 88.3(2),
C1−Fe1−N1 107.7(2), C8−Fe1−Br1 108.3(1), C1−Fe1−Br1 120.9(2), N1−Fe1−Br1 102.6(1);
for 6: Fe1−C15 2.087(2), Fe1−C8 2.090(2), Fe1−C1 2.115(2), Fe1−N1 2.210(2),
C15−Fe1−C8 126.81(9), C15−Fe1−C1 114.40(8), C8−Fe1−C1 112.16(9), C15−Fe1−N1
86.05(7), C8−Fe1−N1 95.42(8), C1−Fe1−N1 116.38(8).
atmosphere at ambient temperature, which resulted in an
immediate colour change from yellow-brown to red-purple and
the clean formation of dibenzyl ketone and a diamagnetic
compound,
colour change to yellow, accompanied by the conversion of
. At ambient temperature, the 13C NMR spectrum of
in CD2Cl2
displays only one carbonyl peak at 222.53 ppm, while the IR X-ray crystallography reveals that compounds
7
(Scheme 3); longer reaction time caused further
7
to
8
7
7
and
C)], respectively
7 adopts a distorted trigonal
8 have
spectrum shows three CO stretches at 1953, 1868 and 1832 formula of [Fe(CO)3(HL)] and [Fe(CO)4(HL-

cm−1, respectively. The singlet at 4.98 ppm with an integration (Figure 5). The Fe(0) centre in
1
of two protons in its H NMR spectrum is diagnostic for the bipyramidal coordination geometry with the carbene donor of
methylene group of the HL ligand. Compared to those of
7, the HL and a carbonyl ligand occupying the two axial positions and
carbonyl signal of
8
is upfield shifted to 215.94 ppm in its 13C the pyridine nitrogen donor and the other two carbonyl ligands
NMR spectrum, while the proton signal of the CH2 group of the occupying the three equatorial positions. The angle between
HL ligand is downfield shifted to 5.78 ppm in its 1H NMR two axial bonds (C4−Fe1−C2) is 176.7(1)° and the sum of the
spectrum. The IR spectrum of
8 in THF has four CO stretches at three bond angles within equatorial plane is 359.9(1)°.
2035, 1946, 1924, and 1906 cm−1, respectively.
Compound 8 is a ligand substitution product from 7, where the
4 | Dalton Trans. 2016, 00, 1-3
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Dalton Transactions ARTICLE
pyridine moiety is replaced by an additional carbonyl ligand, presumable [Fe(CO)4(Me2IPr)]58 and the analogue methylene
View Article Online
leaving the pyridine donor of HL dangling. The coordination bridged
biscarbene
iron
tricaDrbOoI:n1y0l.1039/Cco6DmTp0o27u9n2dJ
sphere of the Fe(0) centre of is similar to those of [Fe(CO)3(DippC)2CH2)] ((DippC)2CH2 = bis(N-Dipp-imidazole-2-
8
[Fe(CO)4(NHC)]53-56 and [Fe(CBA)(CO)4] (CBA = cyclic bent ylidene methane),50 compound
allene)57 compounds where the NHC occupies one of the axial under a dinitrogen atmosphere.
positions. Instead of triggering the desired deprotonation, the
7 is readily isolable and stable
Scheme 3. Reaction of 6 with CO.
Scheme 4. Reaction of 6 with an equimolar amount of p-methoxybenzyl bromide. The
percentage yields given in parentheses are the averages of two parallel runs (see ESI†)
Compound
of at 60 °C under CO atmosphere for three hours (Scheme 3).
Compound is light-sensitive pale yellow crystals. The pale
yellow solution of slowly turns red under a dinitrogen
atmosphere and ambient light at room temperature, forming
this reaction is slow even at elevated temperatures. However,
8 can also be prepared by heating a toluene solution
7
8
8
7
;
the irradiation of a solution of
in the complete conversion to
8
7
in C6D6 under a UV lamp results
within three hours at ambient
interconversion between
temperature.
Similar
[Fe(CO)1−2(picolyl−NHC)X] (X = I, BF4) compounds have been
reported, but the resulting compounds with dangling pyridine
moiety could not be cleanly isolated.23
Although we were unable to trigger the deprotonation of HL
ligand in
6 by the benzyl ligands on the Fe centre, compound 6
is still intriguing due to its relevance to the proposed key
intermediate [Fe(NHC)(R)2] (where R is a hydrocarbyl group) in
NHC−Fe-catalyzed Kumada-type coupling reactions.42,61,62 It has
been proposed that [Fe(NHC)(R)2] undergoes halogen atom
abstraction from the electrophile R′−X to generate a radical (R′•)
and the Fe(III) species [Fe(NHC)(R)2X]; the dissociated R′• can
then rebound to generate the cross-coupling product R’−R
(major) or self-couple to form R’−R’ (minor).42 As shown in
Figure 5. Molecular structure of complexes 7 (top) and 8 (bottom). Ellipsoids are shown
at 50% probability level. Selected bond lengths (Å) and angles (°) for 7: Fe1−C1 1.751(2),
Fe1−C3 1.753(3), Fe1−C2 1.766(2), Fe1−C4 1.957(2), Fe1−N1 2.074(2), O1−C1 1.167(3),
O2−C2 1.157(3), O3−C3 1.157(3), C1−Fe1−C3 115.1(1), C1−Fe1−C2 85.1(1), C3−Fe1−C2
91.8(1), C1−Fe1−C4 95.72(9), C3−Fe1−C4 90.8(1), C2−Fe1−C4 176.7(1), C1−Fe1−N1
129.2(1), C3−Fe1−N1 115.6(1), C2−Fe1−N1 91.82(9), C4−Fe1−N1 85.21(8); for 8: Fe1−C3
1.784(2), Fe1−C1 1.785(2), Fe1−C2 1.794(2), Fe1−C4 1.822(2), Fe1−C5 1.998(2), O1−C1
1.147(2), O2−C2 1.150(2), O3−C3 1.154(2), O4−C4 1.144(2), C3−Fe1−C1 90.61(9),
C3−Fe1−C2 123.85(9), C1−Fe1−C2 91.81(9), C3−Fe1−C4 118.58(9), C1−Fe1−C4 86.72(8),
C2−Fe1−C4 117.57(9), C3−Fe1−C5 85.02(8), C1−Fe1−C5 174.72(8), C2−Fe1−C5 88.18(8),
C4−Fe1−C5 97.97(8).
Scheme 4, the reaction of
bromide results in the formation of the cross-coupling product
II in 50% yield and the homo-coupling products and III in 35%
and 28% yield, respectively. The homo-coupling product might
6 with 1 eq. of p-methoxybenzyl
I
I
originate from reductive elimination, while the homo-coupling
product III might arise from the self-coupling of p-
methoxybenzyl radicals.
addition of CO to compound
insertion and reductive elimination to form
ketone. Similar processes have been reported in the
literature.57−60 Deng and co-workers have reported the isolation
of (Mes)2CO from the reaction of [Fe(Me2IPr)(Mes)2] (Me2IPr =
1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) with CO
6
appears to induce sequential 1,1-
and dibenzyl
Conclusions
7/8
In summary, we have synthesized 1−3 from the reactions of
H2LBr and appropriate amounts of Fe(HMDS)2 and the in situ
prepared [Fe(HMDS)Br], respectively. While the direct
deprotonation of the charge neutral HL ligand in
eq. of base has been unsuccessful, the deprotonation of
2 eq. of LiHMDS has successfully furnished where the iron(II)
2
and
3
with 1
1
with
where
the
presumable
iron
containing
product
4
[Fe(CO)4(Me2IPr)] could not be isolated.58 Unlike the
This journal is © The Royal Society of Chemistry 2016
Dalton Trans. 2016, 00, 1-3 | 5
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ARTICLE
Dalton Transactions
centre is coordinated to two L− ligands featuring the C36H38N6FeBr2: C, 56.13; H, 4.97; N, 10.91. FoundV:ieCw, A5rt5ic.l8e 6O;nliHne,
DOI: 10.1039/C6DT02792J
dearomatized pyridine moieties and exocyclic C−C double 4.92; N, 10.45.
bonds. Alternatively,
situ generated L− ligand and 0.5 eq. of FeBr2. Treating
eq. of PPh3 HBr affords protonated compound . Attempts to mixture was stirred overnight to afford a yellow suspension. All
deprotonate compound using benzyl Grignard as the base volatiles were removed under reduced pressure to afford a light
cleanly afford the transmetalation products and . The yellow solid of , which was then washed with THF (3 × 1 mL)
dibenzyl complex reacts rapidly with CO affording and and pentane (3 × 1 mL), dried under vacuum (70.9 mg, 92%).
dibenzyl ketone. The prolonged exposure of to an atmosphere From : To the mixture of (60.9 mg, 0.1 mmol) and PPh3 HBr
of CO causes the substitution of the pyridine N-donor by a CO (68.6 mg, 0.2 mmol) was added 3 mL of THF. The resulting
ligand, leading to the formation of where the pyridine moiety mixture was stirred overnight to afford a yellow brown
of the HL ligand is dangling. Compound
irradiating under UV light. Compound
4
can be synthesized from 1 eq. of the in From
2: To the mixture of 2 (57.3 mg, 0.10 mmol) and H2LBr
4
with 2 (35.8 mg, 0.10 mmol) was added 3 mL of THF. The resulting
·
1
3
5
6
1
6
7
7
4
4
·
8
7
can be reformed by suspension. All volatile was removed under reduced pressure to
8
6
is reactive toward p- afford a yellow solid, which was then washed with THF (3 × 1
methoxybenzyl bromide affording homo- and cross-coupling mL) and pentane (3 × 1 mL) and dried under vacuum to afford
products. The deprotonation of complexes and and their (59.4 mg, 77 %).
bulkier analogues and the reactivity of toward various small [FeBr(HL)(HMDS)], 2
1
2
3
4
molecules are under investigation in our laboratory.
To the mixture of H2LBr (179.2 mg, 0.50 mmol) and Fe(HMDS)2
(188.5 mg, 0.50 mmol) was added 3 mL of THF. The reaction
mixture turned red-orange immediately and slowly became
homogenous. The mixture was stirred for 3 h at room
temperature before all volatiles were removed under vacuum.
The oily residue was dissolved in THF and filtered through Celite.
The filtrate was concentrated to ~1 mL, top-layered with 5 mL
Experimental Section
General Remarks
All reactions were performed in a nitrogen glovebox or using the
standard Schlenk techniques. Glassware was dried in a 180 °C
oven overnight. Solvents were dried by a Grubbs-type solvent
purification system manufactured by Innovative Technology
(dichloromethane, diethyl ether, hexanes, pentane and toluene)
or dried by refluxing and distilling over sodium benzophenone
ketyl (benzene, dioxane, DME, THF, and C6D6) under dinitrogen.
CDCl3 was distilled over P2O5 and CD2Cl2 was distilled over CaH2,
degassed through three consecutive freeze−pump−thaw cycles.
All solvents were stored over 3 Å molecular sieves prior to use.
All NMR spectra were recorded on an Agilent DD2 600 MHz
of pentane, and stored in a
resulting precipitate was collected by filtration, washed with
pentane (3 × 2 mL) and dried under vacuum to afford as a
−35 °C freezer overnight. The
2
yellow solid (252.7 mg, 88%). X-ray quality crystals were
1
obtained by cooling its toluene/hexanes solution at
NMR (C6D6):
−
35 °C. H

94.53 (1H), 47.96 (1H), 44.96 (1H), 38.70 (1H),
23.48 (1H), 13.92 (18H), 8.31 (1H), 2.13 (2H), 0.47 (3H), −3.28
(2H). Not all proton signals were observed. Anal. Calcd. for
C24H37N4Si2FeBr: C, 50.26; H, 6.50; N, 9.77. Found: C, 50.32; H,
6.35; N, 9.56.
spectrometer at 25 °C. Chemical shifts are referenced to the
From
toluene), was slowly added LiHMDS (1 M in hexanes, 25
mol) at −80 °C. The reaction mixture was allowed to warm to
3
To a suspension of
3
(12.3 mg, 25
µmol, in 2 mL of
solvent signals. IR spectra were recorded on a Bruker ALPHA
spectrometer equipped with an ATR sampling unit. Elemental
analyses were carried out by ANALEST at the University of
Toronto. GC/MS analyses were performed on an Agilent 7890A
GC System and 5975C inert XL MSD with Triple-Axis Detector.
Anhydrous FeBr2 and benzylmagnesium bromide solution were
purchased from Strem Chemicals Inc. and Acros Organics,
respectively. Lithium hexamethyldisilazide and n-butyl lithium
were purchased from Sigma Aldrich. Grade 2.5 carbon
monoxide was purchased from Linde. H2LBr,63 Fe(HMDS)2,64
µL, 25
µ
room temperature slowly and stirred overnight. The resulting
mixture was filtered through Celite and concentrated to dryness
to afford 2 as a light orange solid (12.8 mg, 89%).
[FeBr2(HL)], 3
To the mixture of anhydrous FeBr2 (107.5 mg, 0.50 mmol) and
Fe(HMDS)2 (188.5 mg, 0.50 mmol) was added 3 mL of THF. The
mixture was stirred at room temperature for 2 h whereupon all
solids were dissolved. The resulting solution was added to H2LBr
(358.3 mg, 1.00 mmol) and stirred at room temperature
overnight. After the removal of all volatiles, the solid residue
was extracted into dichloromethane and filtered through Celite.
The filtrate was concentrated to dryness under reduced
PPh3·
HBr65 and p-methoxybenzyl bromide66 were prepared
according to literature procedures.
[FeBr(HL)2]Br, 1
To the mixture of H2LBr (179.1 mg, 0.50 mmol) and Fe(HMDS)2
(103.7 mg, 0.28 mmol) was added 3 mL of THF. The resulting
yellow-orange suspension was stirred overnight at room
temperature. The precipitate was collected by vacuum filtration,
washed with THF (3 × 2 mL) and pentane (3 × 2 mL), dried under
pressure to give
compound can be further purified if necessary by top-layering a
dichloromethane solution with pentane at 35 °C overnight. X-
ray quality crystals were obtained by vapour diffusion of
hexanes into a DME solution at −35 °C. 1H NMR (CDCl3):
98.88
3 as a yellow solid (445.7 mg, 90%). This
−
vacuum to afford
1 as a yellow solid (180.6 mg, 94%). X-ray

quality crystals were obtained from the reaction of H2LBr and
Fe(HMDS)2 in THF with no stirring at room temperature. No
NMR spectrum was recorded due to the low solubility and
instability in various NMR solvents. Anal. Calcd. for
(1H), 58.12 (1H), 50.09 (1H), 33.03 (1H), 18.00 (3H), 4.42 (2H),
2.56 (4H), −7.16 (6H). Anal. Calcd. for C18H19N3FeBr2: C, 43.85; H,
3.88; N, 8.52. Found: C, 43.58; H, 3.91; N, 8.33.
6 | Dalton Trans. 2016, 00, 1-3
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Dalton Transactions
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Dalton Transactions ARTICLE
[FeL2], 4
1.2 mmol). The resulting dark red-brown mixtureVwiewasArtaicllleoOwnleinde
to warm to room temperature slowly, stirred overnight, and
DOI: 10.1039/C6DT02792J
To a suspension of H2LBr (114.6 mg, 0.32 mmol, in 3 mL of THF)
was slowly added nBuLi (1.6 M in hexanes, 0.40 mL, 0.64 mmol)
at −80 °C. The resulting mixture was allowed to warm to room
temperature slowly and further stirred for 5 h. The resulting
dark orange solution was cooled to −80 °C and slowly added into
a pre-cooled (−80 °C) slurry of anhydrous FeBr2 (41.3 mg, 0.19
mmol, in 2 mL of THF). The mixture was allowed to warm to
room temperature slowly, stirred overnight, and concentrated
under reduced pressure. The dark oily residue was extracted
into toluene, filtered through Celite and concentrated under
reduced pressure. The residue was then extracted into benzene
and filtered through Celite. The filtrate was lyophilized to afford
concentrated to dryness under reduced pressure. The solid
residue was extracted into diethyl ether and filtered through
Celite. The filtrate was concentrated to ~1 mL and top-layered
with 2 mL of pentane, and cooled to −35 °C to yield dark red
crystals of 6. The supernatant was decanted off and the crystals
were washed with cold pentane (3 × 2 mL), and dried under
vacuum (204.3 mg, 79%). Crystals suitable for X-ray
crystallography were obtained by cooling a concentrated
1
diethyl ether solution at −35 °C. H NMR (C6D6):
 72.31 (1H),
58.27 (1H), 41.52 (1H), 38.73 (1H), 35.91 (4H), 28.24 (1H), 22.58
(2H), 5.12 (2H), 2.76 (3H), −9.00 (6H), −12.98 (1H), −65.25 (3H),
−84.41 (2H). Not all proton signals were observed. Due to the
thermal instability of this compound, satisfactory elemental
analysis results could not be obtained. Best results: Anal. Calcd.
for C32H33N3Fe: C, 74.56; H, 6.45; N, 8.15. Found: C, 73.63; H,
6.47; N, 8.01.
4
as a brown solid (65.2 mg, 67%). Crystals suitable for X-ray
crystallography were obtained by cooling a concentrated THF
1
solution to −35 °C. H NMR (C6D6):
 83.64 (2H), 63.73 (2H),
21.94 (4H), 4.05 (2H), 1.36 (8H), 0.39 (2H), −7.13 (6H), −15.23
(6H), −54.79 (2H), −121.59 (2H). Satisfactory elemental analysis
result could not be obtained due to the extreme sensitivity
[Fe(CO)3(HL)] (7).
toward air and moisture. Best result: Anal. Calcd for C36H36N6Fe: A yellow-brown solution of
6 (51.5 mg, 0.1 mmol, in 5 mL of
C, 71.05; H, 5.96; N, 13.81. Found: C, 68.76; H, 5.84; N, 13.44. toluene) was subjected to a freeze−pump−thaw cycle before 1

eff (Evans67,68) = 5.1 B
From : To a suspension of
was slowly added LiHMDS (1 M in hexanes, 50
−80 C. The reaction mixture was allowed to warm to room done to remove CO from the headspace. After the flask was
.
atm of CO was introduced into the flask, giving a red-purple
mol, in 2 mL of THF), solution. The solution was allowed to stand for 5 min at room
1
1
(19.3 mg, 25
µ
µ
L, 50 µmol) at temperature, before another freeze−pump−thaw cycle was
°
temperature slowly and stirred overnight before it was refilled with dinitrogen gas, the solution was filtered,
concentrated under vacuum. The solid residue was extracted concentrated to ~1 mL, top-layered with 5 mL of pentane, and
with diethyl ether/hexanes (1:1 by volume), filtered through cooled to −35 °C to yield X-ray quality red crystals of
7
. The
Celite and concentrated to dryness to afford
(12.1 mg, 80%).
[FeBnBr(HL)], 5
4 as a brown solid supernatant was decanted off and the crystals were washed
with pentane (3 × 5 mL), and dried under vacuum (30.2 mg,
1
72%). H NMR (CD2Cl2)
δ
9.05 (ddd, 1H, J = 5.6, 1.5, 0.8 Hz, py-
H), 7.66 (td, 1H, J = 7.6, 1.6 Hz, py-H), 7.44 (dt, 1H, J = 7.7, 1.1
Hz, py-H), 7.36 (d, 1H, J = 1.9 Hz, im-H), 7.05 (ddd, 1H, J = 7.3,
5.5, 1.5 Hz, py-H), 6.97 (s, 2H, Mes-o-H), 6.88 (d, 1H, J = 1.9
Hz, im-H), 4.98 (s, 2H, CH2), 2.34 (s, 3H, p-CH3), 1.98 (s, 6H, o-
To a −35 °C suspension of 3 (49.3 mg, 0.1 mmol, in 2 mL of THF
and 0.5 mL of dioxane) was slowly added benzylmagnesium
chloride solution (90.6 mg, 20 wt% in THF, 0.12 mmol). The
resulting red-orange mixture was allowed to warm to room
temperature slowly, stirred overnight, and then concentrated
to dryness under vacuum. The solid residue was extracted into
THF and filtered through Celite. The filtrate was concentrated
to ~2 mL, top-layered with 3 mL of pentane, and stored in a
CH3). 13
C
NMR
(CD2Cl2):
δ
222.53 (CO),
197.08 (im-C2),
156.92 (py-C), 156.29 (py-C), 139.29 (Mes-C), 136.95 (Mes-C),
136.61 (Mes-C), 136.22 (py-C), 129.10 (Mes-C), 124.17 (py-C),
123.12 (im-C), 122.77 (py-C), 121.83 (im-C), 55.72 (CH2),
21.25 (o-CH3), 17.95 (p-CH3). IR (neat solid): 푣̃ (CO) 1953, 1868,
1832 cm−1. Anal. Calcd for C21H19N3O3Fe: C, 60.45; H, 4.59; N,
10.07. Found: C, 60.09; H, 4.51; N, 9.97.
The supernatant and pentane washes were combined and
concentrated under vacuum. The residue was subjected to flash
column chromatography with ethyl acetate as eluent. Dibenzyl
ketone was isolated as a light yellow oil (16.0 mg, 76%). 1H NMR
−35 °C freezer overnight to give 5 as an orange crystalline solid.
The supernatant was decanted off and the solid was washed
with cold diethyl ether (3 × 1 mL) and pentane (3 × 1 mL), dried
under vacuum (37.6 mg, 75%). Crystals suitable for X-ray
crystallography were obtained by concentrating a THF−toluene
1
(ca. 1/1) solution under vacuum. H NMR (C6D6):
 57.55 (1H),
42.17 (1H), 35.00 (1H), 34.60 (2H), 22.02 (1H), 4.00 (2H), 1.78
(4H), −1.12 (3H), −5.86 (1H), −14.26 (2H), −73.52 (1H), −88.20
(1H). Not all proton signals were observed. Due to the thermal
instability of this compound, satisfactory elemental analysis
results could not be obtained. Best results: Anal. Calcd. for
C25H26N3FeBr: C, 59.55; H, 5.20; N, 8.33. Found: C, 58.48; H, 4.97;
N, 7.99.
(CDCl3):
 7.33−7.30 (m, 4H), 7.28−7.25 (m, 2H), 7.16−7.14 (m,
4H), 3.72 (s, 4H, CH2). 13C NMR (CDCl3):
 205.77, 134,12, 129.64,
128.86, 127.20, 49.25. MS: m/z calcd for C15H14O+ 210, found
210. NMR data are consistent with literature.69
From 8: In a J. Young NMR tube, 8 (6.1 mg, 14 mol) was
dissolved in C6D6 (0.5 mL). The tube was sealed and irradiated
for 3 h using a 450 W Ace Glass medium-pressure mercury lamp
[FeBn2(HL)] (6).
1
inside a photochemical reaction cabinet. H NMR experiment
To a −35 °C suspension of
3 (246.5 mg, 0.5 mmol, in 5 mL of
showed the complete consumption of 8 and formation of 7.
[Fe(CO)4(HL-C)] (8).
diethyl ether and 1 mL of dioxane) was slowly added
benzylmagnesium chloride solution (905.3 mg, 20 wt% in THF,
This journal is © The Royal Society of Chemistry 2016
Dalton Trans. 2016, 00, 1-3 | 7
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ARTICLE
Dalton Transactions
13 Kim, D.; Le, L.; Drance, M. J.; Jensen, K. H.; Bogdanovski, K.;
Cervarich, T. N.; Barnard, M. G.; PudalDoOv,I: N10..1J0.;3V9Kie/nwCaA6prDtpicT,l0eS2O7.n9Mli2nJe.
M.; Chianese, A. R. Organometallics 2016, 35, 982.
A red-purple solution of
7 (20.9 mg, 0.05 mmol, in 5 mL of
toluene) was subjected to a freeze−pump−thaw cycle, before 1
atm of CO was introduced into the Schlenk bomb. The bomb
was then sealed and heated at 60 °C for 3 h. The resulting yellow
solution was filtered and concentrated to dryness under
vacuum to afford pale yellow crystals of
Crystals suitable for X-ray were grown from a concentrated
14 Yu, R. P.; Darmon, J. M.; Hoyt, J. M.; Margulieux, G. W.;
Turner, Z. R.; Chirik, P. J. ACS Catal. 2012, 2, 1760.
15 Yu, R. P.; Darmon, J. M.; Milsmann, C.; Margulieux, G.
W.; Stieber, S. C. E.; DeBeer, S.; Chirik, P. J. J. Am. Chem.
Soc. 2013, 135, 13168.
8 (20.7 mg, 93%).
16 Murso A.; Stalke, D. Dalton Trans., 2004, 16, 2563.
1
diethyl ether solution at −35 °C. H NMR (CD2Cl2):
δ
8.62 (ddd,
17 Objartel, I.; Ott, H.; Stalke, D. Z. Anorg. Allg. Chem., 2008, 634
,
1H, J = 4.8, 1.9, 1.0 Hz, py-H), 7.72 (td, 1H, J = 7.7, 1.9 Hz, py-H),
7.38 (d, 1H, J = 2.0 Hz, im-H), 7.28 (ddd, 1H, J = 7.7, 4.8, 1.0 Hz,
py-H), 7.22 (dt, 1H, J = 7.9, 0.8 Hz, py-H), 7.01 (s, 2H, Mes-o-H),
6.99 (d, 1H, J = 2.0 Hz, im-H), 5.78 (s, 2H, CH2), 2.35 (s, 3H, p-
CH3), 2.00 (s, 6H, o-CH3). 13C NMR (CD2Cl2): δ 215.94 (CO),
185.05 (im-C2), 156.24 (py-H), 150.04 (py-C), 140.35 (Mes-C),
137.42 (Mes-C), 137.31 (py-C), 136.71 (Mes-C), 129.43 (Mes-C),
124.44 (im-C), 124.19 (im-C), 123.32 (py-C), 122.73 (py-C),
2373.
18 de Boer, S. Y.; Gloaguen, Y.; Reek, J. N. H.; Lutzb, M.; van der
Vlugt, J. I. Dalton Trans., 2012, 41, 11276.
19 G. I. McGrew, P. A. Khatri, W. E. Geiger, R. A. Kemp and R.
Waterman, Chem. Commun., 2015, 51, 15804.
20 Cheng Y., Sun L., Chin. J. Org. Chem., 2012, 32, 511.
21 Winston, S.; Stylianides, N. D.; Tulloch, A. A. D.; Wright, J. A.;
Danopoulos, A. A. Polyhedron 2004, 23, 2813.
22 Wang, X.; Liu, S.; Jin, G.-X. Organometallics 2004, 23, 6002.
23 Mercs, L.; Labat, G.; Neels, A.; Ehlers, A.; Albrecht, M.
Organometallics 2006, 25, 5648.
57.21 (CH2), 21.27 (o-CH3), 17.89 (p-CH3). IR (in THF): 푣̃ (CO)
2037, 1962, 1933, 1916 cm−1. Anal. Calcd for C22H19N3O4Fe: C,
59.35; H, 4.30; N, 9.44. Found: C, 59.50; H, 4.13; N, 9.31.
Reaction of 6 with p-methoxybenzyl bromide.
24 Riener, K.; Haslinger, S.; Raba, A.; Hogerl, M. P.; Cokoja. M.;
Herrmann, W. W.; Kuhn, F. E. Chem. Rev. 2014, 144, 5215 and
references therein.
25 Bezier, D.; Sortais, J.-B.; Darcel, C. Adv. Synth. Catal. 2013, 355
,
To a solution of
6
(17.7 mg, 34.4
mol, in 4 mL of diethyl ether),
19.
was added p-methoxybenzyl bromide (5

L, 34.3 mol) at room

26 Ingleson, M. J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579.
27 Wang, Y.; Sun, H.; Tao, X.; Shen, Q.; Zhang, Y. Chin. Sci. Bull.
2007, 52, 3193.
28 Chen, M. Z.; Sun, H. M.; Li, W. F.; Wang, Z. G.; Shen, Q.; Zhang,
Y. J. Organomet. Chem. 2006, 691, 2489.
29 Thagfi, J.; Lavoie, G. G. Organometallics 2012, 31, 7531.
30 Thagfi, J.; Lavoie, G. G. Organometallics 2012, 31, 2463.
31 Kaufhold, O.; Hahn, F. E.; Pape, T.; Hepp, A. J. Organomet.
Chem. 2008, 693, 3435.
temperature. The solution was stirred for 1 h prior and
quenched with 1 drop of 1 N HCl and filtered through a pad of
anhydrous MgSO4. The filtrate was subjected to GC/MS analysis
using hexamethylbenzene as the internal standard (see ESI†).
Acknowledgements
32 Zhang, Y.; Liu, B.; Wu, H.; Chen, W. Chin. Sci. Bull. 2012, 57
,
We thank NSERC of Canada for funding. We also acknowledge
the Canadian Foundation for Innovation Project #19119, and
the Ontario Research Fund for funding the CSICOMP NMR lab at
the University of Toronto enabling the purchase of several new
spectrometers.
2368.
33 Liu, B.; Zhang, Y.; Xu, D.; Chen, W. Chem. Commun. 2011, 47
,
2883.
34 Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166.
35 Dunsford, J. J.; Evans, D. J.; Pugh, T.; Shah, S. N.; Chilton, N. F.;
Ingleson, M. J. Organometallics 2016, 35, 1098.
36 Ouyang, Z.; Meng, Y.; Cheng, J.; Xiao, J.; Gao, S.; Deng, L.
Organometallics 2016, 35, 1361.
37 Pal, K.; Hemming, O. B.; Day, B. M.; Pugh, T.; Evans, D. J.;
Layfield, R. A. Angew. Chem. Int. Ed. 2016, 55,1690.
38 Ouyang, Z.; Du, J.; Wang, L.; Kneebone, J. L.; Neidig, M. L.;
Deng L. Inorg. Chem. 2015, 54, 8808.
39 Wang, X.; Zhang, J.; Wang, L.; Deng, L. Organometallics 2015,
34, 2775.
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Graphic Abstract
Reversible deprotonation–reprotonation of a bidentate picolyl-NHC ligand on Fe(II)