Constructing reactive Fe and Co complexes from isolated picolyl-functionalized N-heterocyclic carbenes

Article abstract of DOI:10.1039/c8dt02621a

We report the isolation of free picolyl-functionalized N-heterocyclic carbenes (NHCs), which serve as versatile precursors to access low coordinate iron and cobalt complexes. The reactivities of these new iron and cobalt complexes towards catalytic hydrosilylation of ketones have also been explored. For example, low loadings (0.05-1 mol%) of a four-coordinate iron complex bearing two deprotonated picolyl-NHC ligands can effect the fast catalytic reduction of ketones using the inexpensive industrial byproduct polymethylhydrosiloxane (PMHS) as the reductant at ambient temperature.

Full text of DOI:10.1039/c8dt02621a

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Constructing Reactive Fe and Co Complexes from Isolated Picolyl-
Functionalized N-Heterocyclic Carbenes†
Qiuming Liang, Nina Jiabao Liu and Datong Song*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the isolation of free picolyl-functionalized N-heterocyclic carbenes (NHCs), which serve as versatile precursors
to access low coordinate iron and cobalt complexes. The reactivities of these new iron and cobalt complexes towards
catalytic hydrosilylation of ketones have also been explored. For example, low loadings (0.05–1 mol%) of a four-coordinate
iron complex bearing two deprotonated picolyl-NHC ligands can effect the fast catalytic reduction of ketones using the
inexpensive industrial byproduct polymethylhydrosiloxane (PMHS) as the reductant at ambient temperature.
www.rsc.org/
the carbene transfer reaction is much less generic for 3d
metals. Moreover, silver-NHC complexes are generally light-
sensitive. Occasionally, the silver induced degradation of the
Introduction
The N-heterocyclic carbenes (NHCs) are widely used ancillary
ligands in coordination chemistry.1 Their strong
-donating
and weak -accepting nature leads to electron rich and stable
imidazolium precursors7a,b or the oxidation of the metal
precursors7c may occur. The use of free carbenes in their pure
form is a more desirable method and offers more flexibility in
choosing metal precursors and solvents. However, the
instability of the free carbene form of the N-picolyl-NHC ligand
originated from the high acidity of the bridging CH2 and
basicity of the carbene carbon presents a serious hurdle to the
free NHC route.4o,p Herein, we report the syntheses and
isolation of the free N-picolyl-NHCs and their reactivities
towards [M(HMDS)2] precursors (where M = Fe, Co; HMDS =
hexamethyldisilazide), along with the preliminary data on the
hydrosilylation of ketones catalyzed by the new Fe and Co
complexes.
σ
π
complexes.1 Recently iron– and cobalt–NHC complexes have
gained considerable attention, due to their potential
applications as inexpensive and environmentally benign
catalysts. Most of these complexes bear monodentate NHC
ligands.2 The incorporation of NHCs in a chelating scaffold can
enhance complex stability and enable bifunctional or
hemilabile ligand designs.3 The N-picolyl-NHC ligands can be
easily synthesized with different substitution patterns on the
pyridine ring and the NHC, allowing fine-tuning of the steric
and electronic properties. As such, the bidentate N-picolyl-
NHC ligands have been used to coordinate with many
transition metals.4 The lability of the picolyl arm and the
bifunctional behavior of the ligand have been described in
several stoichiometric and catalytic processes.5,6
Results and discussion
Synthesis of free N-picolyl-NHCs
The reported N-picolyl-NHC metal complexes were prepared
via one of the following two methods: (1) reacting a metal
precursor with the in situ generated carbene from the
corresponding imidazolium salt and a strong base, and (2)
transferring the ligand from a silver complex to a metal
precursor.4-6 However, both methods have their limitations.
The coordination chemistry using in situ generated NHCs can
be problematic, in that, the counteranion of the imidazolium
salt, the cation of the base, and even the conjugate acid of the
base may cause unwanted reactions. The silver-NHC route is
useful for preparing NHC complexes of 4d and 5d metals, but
The free N-picolyl-NHCs (HL) 2a
deprotonating the corresponding imidazolium salts (H2LBr)
1a4l 1b4m in diethyl ether using KOtBu at ambient temperature
(Scheme 1). The free ligands 2a 2b were isolated in good yields
/2b were prepared by
/
/
as crystalline solids, where the prompt workup after 1 h is
critical. Crystallizations from concentrated diethyl ether
solutions allowed for structural characterization by X-ray
crystallography (Fig. 1). The 13C NMR shifts of the carbene
carbons are at
2a 2b slowly decompose into complicated mixtures at ambient
temperature, but are stable for weeks at −35 °C.
∼218 ppm. In both the solid state and solution,
/
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Fig. 2 Molecular structures of 3[Co]. The ellipsoids are shown at 30% probability. All
hydrogen atoms are omitted for clarity. Only one disordered component is shown.
Selected bond lengths (Å) and angles (°): Co(1)–C(10) 1.961(4), Co(1)–N(3) 1.971(3),
Co(1)–C(28) 2.00(1), Co(1)–N(6) 1.970(3), N(3)–C(18) 1.362(5), C(18)–C(17) 1.353(5),
C(17)–C(16) 1.408(6), C(16)–C(15) 1.341(5), C(15)–C(14) 1.440(5), C(14)–N(3) 1.394(4),
C(14)–C(13) 1.365(5), C(13)–N(2) 1.416(4), N(6)–C(36) 1.353(5), C(36)–C(35) 1.364(5),
C(35)–C(34) 1.400(7), C(34)–C(33) 1.327(7), C(33)–C(32) 1.449(6), C(32)–N(6) 1.399(5),
C(32)–C(31) 1.42(1), C(31)–N(5) 1.40(1), C(10)–Co(1)–N(3) 93.9(1), N(3)–Co(1)–C(28)
117.6(3), C(28)–Co(1)–N(6) 94.5(4), N(6)–Co(1)–C(10) 115.2(1), N(2)–C(13)–C(14)
126.3(3), N(5)–C(31)–C(32) 123.9(9).
N
N
N
R
N
Mes
Mes
Et2O, 1 hr
R.T.
+
KOtBu
N
R
N
Br
1a R = H
1b R = Mes
2a R = H, 69%
2b R = Mes, 79%
Scheme 1 Syntheses of picolyl-NHCs.
R
N
4[Fe], R = H
N
N
4[Co], R = H
5[Fe], R = Mes
Mes
5[Co], R = Mes
M
HMDS
HMDS
90 oC
R = H,
M = Fe
R = Mes
R.T.
N
N
N
N
N
N
Fe
M
7[Fe]
7[Co]
6'[Fe]
Mes
Mes
Mes
HMDS
HMDS
Fig. 1 Molecular structures of 2a (top) and 2b (bottom). Ellipsoids are shown at 50%
probability. Hydrogen atoms have been omitted for clarity.
cooling or
crystallization
Mes
N
R.T.
N
N
N
N
Mes
0.5 M(HMDS)2
toluene
N
M
N
N
N
HMDS
Mes
Fe
N
Mes
2a
N
N
3[Fe], 59%
3[Co], 67%
6[Fe]
N
N
Scheme 2 Syntheses of 3[Fe]/3[Co].
N
Mes
Fe
HMDS
N
N
M(HMDS)2
R.T.
Mes
N
N
Mes
R
N
Scheme 4 Protonolysis of complexes 4 and 5.
N
M
HMDS
HMDS
Coordination chemistry of N-picolyl-NHCs
R
4[Fe], R = H, 64%
4[Co], R = H, 56%
5[Fe], R = Mes, 70%
5[Co], R = Mes, 75%
2a R = H
2b R = Mes
We have recently reported the synthesis of 3[Fe] from the
imidazolium salt 1a, 2 equiv. of nBuLi and 0.5 equiv. of FeBr2.6a
Using the free ligand 2a and [M(HMDS)2] in a 2 : 1 ratio in
Scheme 3 Reactions of picolyl-NHCs 2a/2b with M(HMDS)2 (M = Fe, Co).
toluene at ambient temperature gave 3[Fe]/3[Co] in good
yields (Scheme 2). As shown in Fig. 2, the structure of 3[Co]
features a distorted tetrahedral Co(II) center with two L−
ligands bearing the dearomatized pyridine rings. The metric
parameters of 3[Co] are similar to those of 3[Fe]. This
synthetic route leading to 3[Fe]/3[Co] shows a versatile
complementary alternative to access low-coordinate
complexes and dearomatized N-picolyl-NHC ligand, where low
temperature handling and strong alkyllithium bases are
avoided.
The reaction of 2a
ambient temperature gave the three-coordinate complexes
4[Fe] 4[Co] and 5[Fe] 5[Co]) in good yields within 10 minutes
/2b with [Fe(HMDS)2] in a 1 : 1 ratio at
(
/
/
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(Scheme 3). The X-ray structures (Fig. 3) show that the carbon 359.86(7), 359.95(8), 360.1(2), and 360.0(1)° for 4[Fe], 4[Co],
donor coordinates to M(HMDS)2 while the pyridine N-donor is 5[Fe], and 5[Co], respectively. The Fe–C bonds are longer than
dangling. The metric parameters are similar to those found in the Co–C bonds, while the difference between Fe–N and Co–N
[M(HMDS)2(NHC)] (M = Fe, Co).8 The planarity around each bonds is less pronounced.
metal center is evidenced by the sum of bond angle data of
Fig. 3 Molecular structures of 4[Fe] (top left), 4[Co] (bottom left), 5[Fe] (top right) and 5[Co] (bottom right). The ellipsoids are shown at 30% probability. All hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°), for 4[Fe]: Fe(1)–N(4) 1.949(2), Fe(1)–N(5) 1.966(2), Fe(1)–C10 2.138(2), N(4)–Fe(1)–N(5), 126.16(7), N(4)–Fe(1)–C(10),
122.32(7), N(5)–Fe(1)–C(10) 111.38(7); for 4[Co]: Co(1)–N(4) 1.930(2), Co(1)–N(5) 1.956(2), Co(1)–C(10) 2.077(2), N(4)–Co(1)–N(5) 126.06(8), N(4)–Co(1)–C(10) 122.19(8), N(5)–
Co(1)–C(10) 111.66(8); for 5[Fe]: Fe(1)–N(4) 1.961(4), Fe(1)–N(5)–1.970(4), Fe(1)–C(10) 2.145(5), N(4)–Fe(1)–N(5) 127.0(2), N(4)–Fe(1)–C(10) 121.7(2), N(5)–Fe(1)–C(10) 111.4(2);
for 5[Co]: Co(1)–N(4) 1.929(3), Co(1)–N(5) 1.961(2), Co(1)–C(10) 2.078(3), N(4)–Co(1)–N(5) 125.7(1), N(4)–Co(1)–C(10) 122.5(1), N(5)–Co(1)–C(10) 111.8(1).
At ambient temperature, the pale-yellow solution of 4[Fe] turns reddish brown, accompanied by the appearance of a set
turns into a dark green colour in a few hours. Crystallization of new signals in the 1H NMR spectrum. Such changes are fully
from pentane at −35 °C yields red crystals of the dimeric reversible. This phenomenon makes us believe that although
complex 6[Fe] (Scheme 4). It is conceivable that the formation 6[Fe] is dimeric in the solid-state, it exists in solution (C6D6,
of 6[Fe] may start with the coordination of pyridine N-donor toluene-d8) as a monomer [Fe(HMDS)L] (6’[Fe]) at ambient
followed by the deprotonation of the pyridylic CH2 group by temperature. At −35 °C in toluene-d8, the 6[Fe]
one of the HMDS ligands. The resulting three-coordinate 3:1. Our attempted synthesis of the analogous cobalt
[Fe(HMDS)L] intermediate 6’[Fe] dimerizes in a head-to-tail complex led to complicated mixture of products.
:6’[Fe] ratio is
∼
a
fashion with iron atoms Fe(1) and Fe(2) bound to the pyridylic Recrystallization of the mixture afforded green brown crystals
carbon atoms C(37) and C(13), respectively (Fig. 4). of an unexpected di-Co structure, i.e., a [CoL2] unit bound to a
Interestingly, the X-ray structure reveals an approximate C2 [Co(HMDS)2] unit via the pyridylic carbon (see ESI†). We were
symmetry instead of a centre of inversion (i.e., the two unable to obtain a pure sample of this di-Co compound.
stereocentres C(37) and C(13) within any given molecule have
the same configuration), presumably due to the stacking slower, due to the steric hindrance around the pyridine N-
interaction between the two pyridine rings (e.g., the centroid– donors. Heating 5[Fe] 5[Co] at 90 °C in toluene overnight leads
centroid distance is 3.6 Å, the shortest contact distance is to the full conversion to 7[Fe] 7[Co] (Scheme 4). The X-ray
3.0 Å, and the dihedral angle is 21°). The Fe(1)–C(37) and structures of 7[Fe] 7[Co] reveal three-coordinate
Fe(2)–C(13) bond distances of 2.218(4) Å are longer than pyramidalized metal centers (Fig. 5), with the metal centres
typical Fe–alkyl bonds reported.9 Similar dimeric structures sitting outside the C(10)–N(3)–N(4) planes by
0.3 Å due to
The conversions of 5[Fe]/5[Co] to 7[Fe]/7[Co] are much
π–π
/
∼
/
∼
∼
/
∼
have been reported for [Fe{Ph2PC(H)py}(HMDS)]2 and [Zn(bis- the steric. The sums of bond angles around the metal centres
NHC*)(R)]2 (where bis-NHC* = methylidyne-3,3’-bis(N-tert- are 353.96(9) and 352.8(1)° for 7[Fe] and 7[Co], respectively.
butylimidazol-2-ylidene) and R = Et, H, SiH2Ph, O(CH)Ph2, The bite angle of the chelate ligand in 7[Co] (94.4(1)°) is similar
OCHO, or N(iPr)CH(iPr)).10 Upon cooling under an argon to the bite angles of both ligands in 3[Co] (94.5(4) and
atmosphere, the dark green solution of 6[Fe] in toluene-d8 93.9(1)°), while the bite angle of the chelate ligand in 7[Fe]
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(92.36(9)°) is similar to that of one of the two ligands in 3[Fe] temperature in the presence of 0.015 mol% of iron(N-
(92.23(9)°) and smaller than the other (93.3(1)°). The phosphinoamidinate)(HMDS) and 1 equiv. of PhSiH3.13d
alternating long–short bond distances in the C5N rings support
Our initial findings prompted us to test the catalytic
using the industrial byproduct
a dearomatized pyridine structure. Complexes 7[Fe]/7[Co] are reaction
thermally robust, i.e., no noticeable change in their 1H NMR polymethylhydrosiloxane (PMHS), an inexpensive and less
spectra after heating their solutions in C6D6 under N2 at 90 °C reactive silane, in lieu of PhSiH3. Several iron-catalyzed
for several days.
hydrosilylation reactions using PMHS have been reported.14
However, high catalyst loadings (≥1 mol% of Fe) or
temperatures are required due to the lower activity of
PMHS.14 At a 0.05 mol% loading of 3[Fe] in neat PMHS, the
complete consumption of acetophenone was achieved within
2 h at ambient temperature. The corresponding alcohol was
obtained after hydrolysis (Table 1). In comparison, a 5%
conversion was observed when Fe(HMDS)2 was used under
Catalytic hydrosilylation of ketones
The hydrosilylation of carbonyl groups is an important
synthetic transformation that is widely employed in the
Fig. 4 Molecular structures of 6[Fe]. The ellipsoids are shown at 30% probability. All
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–
N(3) 2.147(3), Fe(1)–C(10) 2.077(4), Fe(1)–N(4) 1.980(3), Fe(1)–C(37) 2.218(4), Fe(2)–
N(7) 2.140(3), Fe(2)–C(34) 2.082(4), Fe(2)–N(8) 1.983(3), Fe(2)–C(13) 2.218(4), N(3)–
Fe(1)–C(10) 89.1(1), C(10)–Fe(1)–N(4) 119.3(1), N(4)–Fe(1)–N(3) 109.6(1), N(3)–Fe(1)–
C(37) 96.0(1), C(37)–Fe(1)–C(10) 109.9(1), C(37)–Fe(1)–N(4) 123.8(1), N(2)–C(13)–C(14)
115.8(3), N(2)–C(13)–Fe(2) 113.5(2), C(14)–C(13)–Fe(2) 108.3(2), N(7)–Fe(2)–C(34)
88.2(1), C(34)–Fe(2)–N(8) 119.7(1), N(8)–Fe(2)–N(7) 110.8(1), N(7)–Fe(2)–C(13) 97.3(1),
C(13)–Fe(2)–C(34) 110.2(1), C(13)–Fe(2)–N(8) 122.2(1), N(6)–C(37)–C(38) 115.9(3),
N(6)–C(37)–Fe(1) 114.1(2), C(38)–C(37)–Fe(1) 107.3(2).
laboratory and in industry.11 In recent years, carbonyl
hydrosilylation catalysts based upon inexpensive first-row
transition metals, such as iron12-14 and cobalt,15 have been
developed.16 Despite the recent contributions made toward
the development of iron-catalyzed carbonyl hydrosilylation,
there is still a great demand for such transformations at room
temperature employing low loadings (≤1 mol% of Fe).13 Having
Fig. 5 Molecular structures of 7[Fe] (top) and 7[Co] (bottom). The ellipsoids are shown
at 30% probability. All hydrogen atoms are omitted for clarity. Only one disordered
component is shown. Selected bond lengths (Å) and angles (°), for 7[Fe]: Fe(1)–N(3)
2.001(2), Fe(1)–C(10) 2.053(2), Fe(1)–N(4) 1.925(2), N(2)–C(13) 1.408(3), C(13)−C(14)
1.361(3), C(14)−C(15) 1.447(4), C(15)–C(16) 1.340(4), C(16)–C(17) 1.419(4), C(17)–C(18)
1.354(4), C(18)–N(3) 1.380(3), N(3)–C(14) 1.404(3), N(3)–Fe(1)–C(10) 92.36(9), C(10)–
Fe(1)–N(4) 129.13(9), N(4)–Fe(1)–N(3) 132.48(9), N(2)–C(13)–C(14) 126.9(2); for 7[Co]:
Co(1)–N(3) 1.962(2), Co(1)–C(10) 1.982(3), Co(1)–N(4) 1.902(2), N(2)–C(13) 1.408(4),
C(13)−C(14) 1.365(4), C(14)−C(15) 1.439(4), C(15)–C(16) 1.341(4), C(16)–C(17) 1.414(4),
C(17)–C(18) 1.355(4), C(18)–N(3) 1.380(4), N(3)–C(14) 1.394(4), N(3)–Co(1)–C(10)
prepared the iron and cobalt complexes 3–7, we explored their
catalytic activity towards the hydrosilylation of ketones. Our
initial screening using PhSiH3 (1.2 equiv.) and acetophenone,
3[Fe] proved most effective, i.e., the complete conversion was
achieved within 2 h with a 0.05 mol% catalyst loading at
ambient temperature (Table S4). Such a catalytic performance
is comparable to that of the best performing catalyst in the 94.4(1), C(10)–Co(1)–N(4) 130.2(1), N(4)–Co(1)–N(3) 128.2(1), N(2)–C(13)–C(14)
literature.13c,d For example, Yang and Tilley reported that a
126.3(3).
>98% conversion of acetophenone was achieved within 18 h at
Table 1. Hydrosilylation of ketones catalyzed by 3[Fe].a
ambient temperature in the presence of 0.03 mol% Fe(HMDS)2
and 1.6 equiv. of Ph2SiH2;13c Sydora, Stradiotto, Turculet and
co-workers reported that the complete conversion of
acetophenone was achieved within
4
h
at ambient
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All reactions were carried out in a dinitrogen-filled glovebox or
using the standard Schlenk techniques under dinitrogen. Glassware
was dried in a 180 °C oven overnight. Diethyl ether, hexanes,
pentane, and toluene solvents were dried by a Grubbs−type solvent
purification system manufactured by Innovative Technology and
degassed prior to use. THF solvent was dried by refluxing and
distilling over sodium benzophenone ketyl under dinitrogen. C6D6
was degassed through three consecutive freeze−pump−thaw cycles.
All solvents were stored over 3 Å molecular sieves prior to use.
Unless otherwise noted, all NMR spectra were recorded on an
Agilent DD2 600 MHz spectrometer at 25 °C. Chemical shifts are
referenced to the solvent signals. NMR assignments were made
based on 1H-COSY, 1H-13C-HSQC, and 1H-13C-HMBC NMR
spectroscopy. Solution magnetic moments were measured at 25 °C
by the method originally described by Evans with stock and
94, 0.05, 2 h
96, 0.05, 4.5 h
90, 0.05, 11 h
81, 0.05, 5 hb
95, 0.05, 3 h
96, 0.05, 4 h
98, 1, 22 hb
84, 0.5, 4 h
89, 0.05, 22 h
90, 0.05, 1.5 h
86, 0.1, 4.5 hb
98, 0.05, 5 h
a General reaction conditions: ketone (3.34 mmol) in PMHS (0.5 mL) for liquid
ketones or in a mixture of PMHS (0.2 mL) + THF or toluene co-solvent (0.3 mL) for
solid ketones, ambient temperature. Under each substrate, the isolated yield in
percentage, the catalyst loading in mol%, and the reaction time are given
sequentially. b 50 °C.
experimental solutions containing
a known amount of a
cyclohexane standard.17 Elemental analyses were carried out by
ANALEST at the University of Toronto. KOtBu was purchased from
Sigma-Aldrich. Fe(HMDS)2,18 Co(HMDS)2,19 1a4l and 1b4m were
prepared according to literature procedures.
the otherwise same conditions (Table S4). Sydora, Stradiotto,
Turculet, and coworkers reported negligible reactivity using
their iron complex of N-phosphinoamidinate as the pre-
catalyst and PMHS as the reducing reagent.13d
We then surveyed the substrate scope with other ketones
(Table 1). Acetophenone derivatives bearing various functional
groups, the pyridyl and furyl variants, and benzophenone were
Synthesis of 2a (1-mesityl-3-(picolyl)imidazol-2-ylidene). To the
mixture of 1a (716.6 mg, 2.00 mmol) and KOtBu (224.4 mg, 2.00
mmol) was added 10 mL of diethyl ether. The mixture was stirred at
room temperature for 1 h before filtered through Celite. The filtrate
was concentrated to ~2 mL and cooled to −35 °C to afford beige X-
ray quality crystals of 2a. The supernatant was decanted off and the
crystals were washed with cold pentane (3 × 1 mL), dried under
reduced
efficiently.
Sterically
hindered
2,4,6-
trimethylacetophenone was successfully converted to the
desired alcohols, using a higher catalyst loading at 50 °C. The
cyclic and dialkyl ketones were suitable substrates as well. The
alkene functionality remote from the carbonyl group remains
intact during the reduction.
1
vacuum (380.6 mg, 69%). H NMR (600 MHz, C6D6) δ 8.43 (ddd, J =
4.8, 1.9, 0.9 Hz, 1H, py-H), 7.13 (dt, J = 7.8, 1.1 Hz, 1H, py-H), 7.02
(td, J = 7.6, 1.8 Hz, 1H, py-H), 6.77 (s, 2H, Mes-H), 6.76 (d, J = 1.5 Hz,
1H, im-H), 6.58 (ddd, J = 7.5, 4.8, 1.1 Hz, 1H, py-H), 6.37 (d, J = 1.5
Hz, 1H, im-H), 5.50 (s, 2H, CH2), 2.13 (s, 3H, CH3), 2.10 (s, 6H, CH3).
13C NMR (151 MHz, C6D6) δ 218.29 (im-C2), 159.46 (py-C), 149.44
(py-C), 139.26 (Mes-C), 137.18 (Mes-C), 136.38 (py-C), 135.41 (Mes-
C), 129.11 (Mes-C), 122.23 (py-C), 121.84 (py-C), 121.28 (im-C),
119.43 (im-C), 57.24 (CH2), 21.02 (CH3), 18.17 (CH3). Anal. Calcd for
C18H19N3: C, 77.95; H, 6.90; N, 15.15. Found: C, 77.30; H, 7.10; N,
15.35.
Conclusions
In summary, we have isolated and structurally characterized
two picolyl-functionalized NHC ligands in the free-carbene
form for the first time. These free picolyl-NHCs are thermally
unstable at ambient temperatures but stable at low
temperatures in the solid state. The isolation of the free
carbene form of these ligands enables the easy entry to the
corresponding rich coordination chemistry, avoiding all the
issues with the indirect and in situ synthetic routes towards
metal complexes. We have also further demonstrated the
catalytic activity of the resulting base-metal complexes
towards the hydrosilylation of ketones, featuring low catalyst
loadings, mild conditions, and a broad substrate scope. It is
worth noting that even the sterically demanding substrates
can be converted into the corresponding alcohols in excellent
yields using PMHS under relatively mild conditions. Further
synthetic applications of these free carbenes and the reactivity
chemistry of the resulting complexes are being investigated in
our laboratory.
Synthesis of 2b (1-mesityl-3-(6-mesityl-picolyl)imidazol-2-ylidene).
To the mixture of 1b (476.5 mg, 1.00 mmol) and KOtBu (112.2 mg,
1.00 mmol) was added 5 mL of diethyl ether. The mixture was
stirred at room temperature for 1 h before filtered through Celite.
The filtrate was slowly concentrated to dryness under vacuum to
afford 2b as a white crystalline solid, which was washed with
pentane (3 × 1 mL) and dried under high vacuum (311.4 mg, 79%).
Crystals suitable for X-ray crystallography were obtained by cooling
1
a concentrated diethyl ether solution at −35 °C. H NMR (600 MHz,
C6D6) δ 7.15 – 7.10 (overlapping, 2H, py-H), 6.87 (overlapping, 2H,
Mes-H), 6.81 (d, J = 1.6 Hz, 1H, im-H), 6.78 (overlapping, 2H, Mes-
H), 6.76 (dd, J = 7.3, 1.3 Hz, 1H, py-H), 6.39 (d, J = 1.5 Hz, 1H, im-H),
5.52 (s, 2H, CH2), 2.20 (s, 3H, CH3), 2.14 (s, 3H, CH3), 2.11 (s, 6H,
CH3), 2.08 (s, 6H, CH3). 13C NMR (151 MHz, C6D6) δ 218.35 (im-C2),
160.06 (py-C), 159.18 (py-C), 139.31 (Mes-C), 138.63 (Mes-C),
137.27 (Mes-C), 137.17 (Mes-C), 136.81 (Mes-C), 135.88 (py-C),
135.44 (Mes-C), 129.12 (Mes-C), 128.73 (Mes-C), 123.43 (py-C),
Experimental Section
General remarks
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Synthesis of 6[Fe]. To the cold (−35 °C) mixture of Fe(HMDS)2
(150.6 mg, 0.4 mmol) and 2a (110.9 mg, 0.4 mmol) was added 5 mL
121.17 (im-C), 119.73 (im-C), 119.54 (py-C), 57.34 (CH2), 21.19
(CH3), 21.02 (CH3), 20.51 (CH3), 18.17 (CH3). Anal. Calcd for C27H29N3:
C, 81.99; H, 7.39; N, 10.62. Found: C, 81.65; H, 7.23; N, 10.25.
of cold (
−35 °C) pentane. The reacꢀon mixture was sꢀrred overnight
at room temperature, filtered. The filtrate was concentrated to ~1
mL and cooled to −35 °C to yield red X-ray quality crystals of 6[Fe],
which was washed with cold pentane (1 mL) and dried under high
vacuum (120.4 mg, 61%). Anal. Calcd for C48H72N8Si2Fe2: C, 58.52; H,
7.37; N, 11.37. Found: C, 58.24; H, 7.11; N, 11.16. Compound 6[Fe]
is dimeric in solid state and monomeric in solution at room
temperature. μeff (Evans) for monomer: 5.0 μB.
Synthesis of 3[Fe]/3[Co]. To a stirring solution of 1a (138.7 mg, 0.50
mmol) in 2 mL of toluene was slowly added M(HMDS)2 (0.25 mmol)
in 2 mL of toluene. The reaction mixture was stirred overnight, and
then concentrated to dryness under vacuum. The residue was
extracted into diethyl ether, filtered and slowly concentrated to
dryness to afford a solid of 3[Fe]/3[Co], which was washed with
cold ether (1 mL) and pentane (3 × 1 mL) and dried under high
vacuum.
Reaction of 2a with Co(HMDS)2. Reaction of 2a and Co(HMDS)2. To
the cold (−35 °C) mixture of Co(HMDS)2 (75.9 mg, 0.2 mmol) and 2a
(55.5 mg, 0.2 mmol) was added 5 mL of cold (−35 °C) pentane. The
reaction mixture was stirred overnight at room temperature,
filtered. The filtrate was concentrated to ~1 mL and cooled to −35
°C to yield green brown X-ray quality crystals of [Co2(HMDS)2L2],
which were characterized by X-ray crystallography. Attempted
isolation of pure sample for elemental analysis was unsuccessful.
3[Fe]. The product was a brown solid (89.8 mg, 59%). NMR data are
consistent with previously reported.6a
3[Co]. The product was a dark green microcrystalline solid (102.7
mg, 67%). Crystals suitable for X-ray crystallography were obtained
1
by cooling a concentrated diethyl ether solution at −35 °C. H NMR
(600 MHz, C6D6) δ 79.02, 54.03, 20.49, 1.72, 0.22, −0.84, −6.21,
−7.32, −10.88, −64.67, −120.85. μeff (Evans): 4.0 μB. Anal. Calcd for
C36H36N6Co: C, 70.69; H, 5.93; N, 13.74. Found: C, 70.42; H, 5.86; N,
13.43.
Synthesis of 7[Fe]/7[Co]. To a stirring solution of M(HMDS)2 (0.5
mmol) in 5 mL of toluene was added 2b (197.8 mg, 0.5 mmol) in 5
mL of toluene. The resulting mixture was stirred at 90 °C overnight,
then concentrated to dryness under vacuum. The solid residue was
extracted into diethyl ether and filtered. The filtrate was slowly
Synthesis of 4[Fe]/4[Co]. To the mixture of M(HMDS)2 (0.25 mmol)
and 2a (69.3 mg, 0.25 mmol) was added 5 mL of pentane. The
reaction mixture was stirred for 10 minutes at room temperature,
filtered. The filtrate was concentrated to ~1 mL and cooled to −35
°C to yield X-ray quality crystals of 4[Fe]/4[Co], which were washed
with cold pentane (1 mL) and dried under high vacuum.
4[Fe]. The product was a beige crystal (104.3 mg, 64%). 1H NMR
(600 MHz, C6D6) δ 41.91, 37.70, 10.78, 4.84, 4.58, 3.64, 1.50, 1.33,
0.12, −0.70, −7.51. μeff (Evans): 4.9 μB. Anal. Calcd for C30H55N5Si4Fe:
C, 55.10; H, 8.48; N, 10.71. Found: C, 54.86; H, 8.52; N, 10.73.
4[Co]. The product was a pine green crystal (91.5 mg, 56%). 1H NMR
(600 MHz, C6D6) δ 116.69, 105.52, 15.74, 12.97, 3.56, 0.09, −0.40,
−3.65, −6.28, −8.02, −8.94, −17.43. μeff (Evans): 4.6 μB. Anal. Calcd
for C30H55N5Si4Co: C, 54.84; H, 8.44; N, 10.66. Found: C, 54.70; H,
8.31; N, 10.50.
concentrated to ∼1 mL, cooled to −35 °C to afford
a
microcrystalline solid of 7[Fe]/7[Co]. The supernatant was decanted
off and the solid was washed with cold ether (1 mL) and cold
pentane (3 × 1 mL) and dried under vacuum. Crystals suitable for X-
ray crystallography were obtained by cooling a concentrated diethyl
ether solution at −35 °C.
7[Fe]. The product was a brown microcrystalline solid (212.6 mg,
69%). 1H NMR (600 MHz, C6D6) δ 63.60, 57.80, 34.84, 30.19, 7.16,
3.54, 2.42, −1.02, −3.50, −6.12, −13.54, −14.16, −26.99, −33.67,
−117.22. μeff (Evans): 5.3 μB. Anal. Calcd for C33H46N4Si2Fe: C, 64.90;
H, 7.59; N, 9.17. Found: C, 64.11; H, 7.52; N, 8.88.
7[Co]. The product was a dark green microcrystalline solid (220.4
mg, 72%). 1H NMR (600 MHz, C6D6) δ 90.43, 66.55, 16.67, 15.38,
10.53, 7.16, −5.10, −5.43, −7.02, −11.69, −18.34, −40.40, −51.44. μeff
(Evans): 4.4 μB. Anal. Calcd for C33H46N4Si2Co: C, 64.57; H, 7.55; N,
9.13. Found: C, 63.78; H, 7.56; N, 9.06.
Synthesis of 5[Fe]/5[Co]. To the mixture of M(HMDS)2 (0.25 mmol)
and 2b (98.9 mg, 0.25 mmol) was added 5 mL of diethyl ether. The
reaction mixture was stirred for 10 minutes at room temperature,
filtered. The filtrate was slowly concentrated to ~1 mL and cooled
to −35 °C to yield a crystalline solid of 5[Fe]/5[Co], which was
washed with cold ether (1 mL) and pentane (1 mL) and dried under
high vacuum. Crystals suitable for X-ray crystallography were
obtained by cooling a concentrated diethyl ether solution at −35 °C.
5[Fe]. The product was a beige microcrystalline solid (134.2 mg,
70%). 1H NMR (600 MHz, C6D6) δ 45.63, 11.07, 7.16, 5.29, 4.73,
4.04, 4.04, 3.60, 1.26, 0.74, 0.60, 0.12, −0.70, −3.49, −11.27. μeff
(Evans): 5.0 μB. Anal. Calcd for C39H65N5Si2Fe: C, 60.66; H, 8.48; N,
9.07. Found: C, 60.41; H, 8.45; N, 8.87.
General Procedure for the Hydrosilylation Catalysis with 3[Fe] and
PMHS. In a nitrogen glovebox, catalyst (0.05–1 mol%), ketones
(3.34 mmol), PMHS (0.5 mL, 8.25 mmol) and a stirbar were charged
in a 2-dram borosilicate glass vial. The vial was fitted with a PTFE-
lined rubber septum screw cap. The reaction was left stirring at
ambient temperature for indicated time. The vial was then removed
from the glovebox and the contents were hydrolyzed with 2M
NaOH (5 mL) and stirred for 90 minutes. The mixture was extracted
with diethyl ether (3 × 10 mL). The combined organic layer was
washed with brine (2 × 10 mL), dried over MgSO4, and concentrated
under reduced pressure to afford the desired alcohol.
5[Co]. The product was a cyan microcrystalline solid. (145.8 mg,
1
75%). H NMR (600 MHz, C6D6) δ 117.13, 106.97, 14.21, 5.90, 1.66,
0.34, 0.10, −0.39, −3.97, −3.98, −6.21, −7.25, −8.71, −9.64, −17.41.
μeff (Evans): 4.5 μB. Anal. Calcd for C39H65N5Si2Co: C, 60.42; H, 8.45;
N, 9.03. Found: C, 61.15; H, 8.13; N, 8.49.
Conflicts of interest
There are no conflicts to declare.
6 | J. Name., 2012, 00, 1-3
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Organometallics 2011, 30, 5493; (f) Z. Lu, I. Demianets, R.
Hamze, N. J. Terrile, T. J. Williams, ACS Catal. 2016, , 2014;
(g) J. P. Morales-Cerón, P. Lara, J. López-Serrano, L. L. Santos,
Acknowledgements
6
We thank Natural Science and Engineering Research Council
(NSERC) of Canada for funding. Q.L. thanks the Ontario government
for an Ontario Graduate Scholarship. We also acknowledge the
Ontario Research Fund for funding the CSICOMP NMR lab at the
University of Toronto enabling the purchase of several new NMR
spectrometers.
V. Salazar, E. Álvaréz, A. Suárez, Organometallics, 2017, 36
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Dalton Transactions
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DOI: 10.1039/C8DT02621A
The Fe(II) and Co(II) complexes prepared from isolated free carbene form of the picolyl-NHC ligands
display excellent catalytic activity towards the hydrosilylation of ketones.