Synthesis of a Base-Stabilized Silicon(I)-Iron(II) Complex for Hydroboration of Carbonyl Compounds

Article abstract of DOI:10.1021/acs.inorgchem.8b01760

The reaction of the amidinatosilicon(I) dimer [LSi:]₂ (1; L = PhC(NtBu)₂) with FeBr₂ in tetrahydrofuran (THF) at ambient temperature afforded the silicon(I)-iron(II) dimer [LSi(FeBr₂·THF)]₂ (2) after

Full text of DOI:10.1021/acs.inorgchem.8b01760

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis of a Base-Stabilized Silicon(I)−Iron(II) Complex for
Hydroboration of Carbonyl Compounds
†
†
†
Sabrina Khoo, Jiajia Cao, Fiona Ng, and Cheuk-Wai So*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
*
S Supporting Information
using any strong reducing agents because of participation of the
noninnocent ligand in catalysis.
In the past few years, we showed that an amidinatosilicon(I)
6
ABSTRACT: The reaction of the amidinatosilicon(I)
dimer [LSi:]2 (1; L = PhC(NtBu) ) with FeBr in
2
2
tetrahydrofuran (THF) at ambient temperature afforded
the silicon(I)−iron(II) dimer [LSi(FeBr ·THF)] (2)
dimer [PhC(NtBu) Si:] can serve as a ligand to coordinate with
2
2
7
2
2
iridium and rhodium complexes. These results aroused our
interest in investigating whether the silicon(I) dimer can
coordinate with first-row transition metals. In addition, because
the silicon(I) dimer comprises two silicon donors, it could be a
noninnocent ligand and participate in catalysis. Herein, we
report the synthesis of an amidinatosilicon(I)−iron(II) complex
and its application toward the catalytic hydroboration of
carbonyl compounds in the absence of any strong reducing
reagents.
after 40 h. Compound 2 can catalyze hydroboration of
aliphatic and aromatic ketone compounds with HBpin in
the absence of any strong reducing agent. Mechanistic
studies show that complex 2 reacts with ketone
compounds to form a zwitterionic intermediate in the
first step of catalysis. Subsequent reaction with HBpin
affords the corresponding boron esters and then
regenerates complex 2.
The treatment of 1 with FeBr in tetrahydrofuran (THF) at
2
ambient temperature afforded the silicon(I)−iron(II) dimer
[LSi(FeBr ·THF)] [2; L = PhC(NtBu) ; Scheme 1] after 40 h.
2 2 2
atalytic hydroboration of carbonyl compounds to alcohols
C
is an important process for the synthesis of chemicals that
1a
are essential to our daily life. Such processes usually rely on
catalysts containing ruthenium, osmium, or other precious
metals. The high cost and limited availability of these metals
necessitate the development of catalysts based on more
abundant and less expensive first-row transition metals, such
Scheme 1. Synthesis of the Iron Complex 2
1
b
as iron. In addition, it is essential to identify ligand systems that
can increase the thermal stability and reactivity of iron catalysts,
along with avoiding one-electron pathways in catalysis. This
synthetic challenge has been tackled by numerous research
groups, who have demonstrated that a series of formally iron(0)
complexes are capable of catalyzing hydroboration of unsatu-
rated compounds with exceptional selectivity, turnover
In the reaction, 1 acted as a Lewis base to coordinate to Lewis
acidic FeBr , together with concomitant coordination by a THF
2
2
frequencies, and turnover numbers. For example, Driess and
molecule, to generate the product. It should be noted that
reducing the reaction time (ca. 16 h) or employing equimolar
co-workers illustrated that amidinatosilylene ligands can support
iron(0) centers to catalyze the hydrosilylation or hydrogenation
FeBr only compromised the yield of 2 with the recovery of
2
3
of carbonyl compounds. In addition, low-coordinate and low-
precursor 1. This indicates that any mononuclear iron complex
oxidation-state iron species can be in situ generated by reacting
more robust iron(II/III) precursors with strongly reducing
could not be formed in the reaction. The solution-state magnetic
susceptibility measurement (Evans method) of 2 in THF-d
room temperature shows that its effective magnetic moment
(μeff) is 6.47(1) μ , which is remarkably higher than those of
at
8
organometallic reagents such as Grignard reagents, NaHBEt ,
3
4
activated magnesium, and KOtBu. For example,Tamang and
B
8
Findlater reported that the iron(III) complex Fe(acac) is
other mononuclear iron carbene complexes and four-
coordinated iron phosphine complexes with a high-spin ferrous
3
9
inactive toward catalytic hydroboration, unless it reacts with
center. However, the effective magnetic moment of 2 is lower
than expected for a system with two uncoupled high-spin ferrous
centers (spin-only value for two uncoupled S = 2 iron(II)
NaHBEt to form an [Fe−H] species, in order to catalytically
3
convert carbonyl compounds into their corresponding boron
5
esters. Reduction to such metal hydride intermediates are
10
centers: 6.9 μB), suggesting the existence of antiferromagnetic
coupling between the two iron(II) centers. A similar
typically key to commencement of the catalytic cycle for
saturated or higher-oxidation-state iron species. Very recently,
Baker and co-workers showed a new catalysis strategy, where the
imine-coupled iron(II) complex can selectively catalyze the
hydroboration of aliphatic and aromatic aldehydes without
Received: June 25, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01760
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
phenomenon was observed in the halide-bridged diiron complex
and toluene to be relatively superior (entries 2 and 3) at room
temperature over 16 h. When the amount of 2 and HBpin
increased, toluene was outperformed by MeCN (entry 8 vs 7).
This could be ascribed to the relatively poor solubility of 2 in
toluene. Through evaluation of the catalyst and hydroborating
agent loadings, we surmised that the reaction with 1.5 equiv of
HBpin and 10 mol % 2 in MeCN afforded the best results (entry
8b
with N-heterocyclic carbene ligands [μ : 6.5(1) μ ].
eff
B
The X-ray crystal structure of 2 (Figure 1) shows that it is in a
gauche-bent fashion. The silicon and iron centers in 2 are all
7
).
With optimized conditions in hand, the scope of the reaction
was then evaluated (Scheme 2 and Chart 1). Both electron-
Scheme 2. Catalytic Hydroboration of 3
Figure 1. ORTEP drawing of 2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms and solvent molecules are elided for
clarity. Selected bond lengths (Å) and bond angles (deg): Si1−Si2
Chart 1. Scope of Hydroboration by Complex 2 with Ketone
Substrates
a
2
.407(4), Si1−Fe1 2.508(3), Si2−Fe2 2.490(3); Br2−Fe1−Si1
14.93(11), Si1−Fe1−Br1 106.18(10), Si2−Si1−Fe1 127.14(15).
1
four-coordinate and adopt tetrahedral geometries, albeit with
that around silicon experiencing a greater degree of distortion.
The Si−Fe interactions [Si1−Fe1 = 2.508(3) Å and Si2−Fe2 =
2
.490(3) Å] coincide well with the Si−Fe bonds in other
silylene−iron(II) complexes (ca. 2.5 Å), while being consid-
erably elongated with respect to related compounds bearing iron
in a formal oxidation state of zero (ca. 2.2 Å). The Si1−Si2
bond measures 2.407(4) Å, which is only marginally shortened
3
compared to that of 1 [2.413(2) Å]. A similar observation was
a
All reactions were carried out under the conditions outlined in Table
noted when the amidinatogermanium(I) dimer [LGe:] , reacted
2
1, entry 7; GC conversion based on the ketone starting material is
reported.
with Fe (CO) to generate [L{Fe(CO) }Ge] , whereby the
2
9
4
2
retained Ge−Ge bond is only reduced slightly from its original
11
length [2.57(5)−2.55(5) Å].
donating and -withdrawing groups on the aryl rings of the ketone
were tolerated (4a−4e). 3-Acetylthiophene (3f) could also react
well and led to the desired product 4f in 95% yield. An aliphatic
ketone, 3-methyl-2-butanone (3k), was also converted smoothly
by this methodology. It should be noted that the reaction of
propiophenone (3h) with HBpin led to 4h in only 60% yield,
and the use of benzophenone (3i) did not lead to the proposed
product because of steric effects. Interestingly, no reducing agent
was required to evoke catalysis. Hence, the mechanism of
catalysis mediated by 2 was investigated.
Our efforts were then focused on the catalytic hydroboration
of ketone compounds. Condition optimization was conducted
with acetophenone (3a) and pinacolborane (HBpin), which was
selected based on the foundation of its practicality and
versatility. First, insignificant conversion occurred in the absence
of 2 (entry 1, Table S1). Second, only low conversion was
achieved when only FeBr (entries 3 and 4, Table S3) or FeBr ·
2
2
2
THF (entries 5 and 6, Table S3) was employed, highlighting
that the presence of the amidinatosilicon(I) dimer in 2 plays a
crucial role in activating possible catalytic intermediates. Third,
solvent screening (entries 1−4, Table 1) with 3 mol % 2 as the
catalyst and equimolar HBpin suggested acetonitrile (MeCN)
First, there is no reaction between compound 2 and HBpin in
11
a stoichiometric amount over 16 h, which was confirmed by B
NMR spectroscopy. This indicates that the initial step of
catalysis should involve compound 2 and carbonyl substrates
only.
Table 1. Condition Optimization for Hydroboration of
a
Acetophenone Mediated by Complex 2
Second, a combination of 1 (10 mol %) and FeBr ·2THF (20
2
b
mol %) is capable of catalyzing aromatic and nonaromatic
entry
HBpin (equiv)
2 (mol %)
solvent
conv (%)
12
ketones, as well as a series of aldehydes 5 (Chart 2). This
modified protocol is found to be capable of overcoming steric
constraints, affording 90% conversion of 3h to 4h (60%; Chart
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1.5
1.5
3
3
3
3
5
10
10
10
THF
42
61
63
35
71
90
97
90
MeCN
toluene
dioxane
MeCN
MeCN
MeCN
toluene
1
) and 13% conversion of 3i to 4i (previously not detected;
Scheme 2). These indicate that carbonyl compounds could
insert into the Si−Fe bond in 2-catalyzed hydroboration.
Selectivity for the carbonyl functionality over olefins (3l and
5
m) and esters (5n) was also observed.
In this context, it is proposed that carbonyl compounds could
a
insert into the Si−Fe bonds in 2 to form zwitterionic
intermediate 2A in the first step of catalysis (Scheme 3).
There is another possibility that compound 2 could dissociate
All reactions were carried out at room temperature for 16 h in 0.2
b
mL of solvent. Conversion determined by gas chromatography (GC)
with n-dodecane as the internal standard; based on 3a.
B
DOI: 10.1021/acs.inorgchem.8b01760
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Chart 2. Scope of Carbonyl Substrates Hydroborated by a
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01760.
■
a
Mixture of Compound 1 and FeBr ·2THF
2
*
S
Condition optimization, experimental section, and NMR
spectra (PDF)
Accession Codes
CCDC 1846543 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: CWSo@ntu.edu.sg.
■
*
ORCID
Cheuk-Wai So: 0000-0003-4816-9801
Author Contributions
†
S.K., J.C., and F.N. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research is supported by ASTAR SERC PSF and AcRF Tier
1 grants.
■
a
GC conversion based on a carbonyl starting material is reported.
b
Catalytic loading for ketone substrates: 1 (10 mol %); FeBr ·2THF
2
(20 mol %).
REFERENCES
■
(
1) (a) Chong, C. C.; Kinjo, R. Catalytic Hydroboration of Carbonyl
Scheme 3. Proposed Mechanism for the Hydroboration of
Carbonyl Compounds Mediated by 2
Derivatives, Imines, and Carbon Dioxide. ACS Catal. 2015, 5, 3238−
3259. (b) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Nickel
and Iron Pincer Complexes as Catalysts for the Reduction of Carbonyl
Compounds. Acc. Chem. Res. 2015, 48, 1995−2003.
(
2) (a) Chirik, P. J. Iron- and Cobalt-Catalyzed Alkene Hydro-
genation: Catalysis with Both Redox-Active and Strong Field Ligands.
Acc. Chem. Res. 2015, 48, 1687−1695. (b) Obligacion, J. V.; Chirik, P. J.
Highly Selective Bis(imino)pyridine Iron-Catalyzed Alkene Hydro-
boration. Org. Lett. 2013, 15, 2680−2683. (c) Tondreau, A. M.;
Atienza, C. C. H.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.;
Chirik, P. J. Iron Catalysts for Selective Anti-Markovnikov Alkene
Hydrosilylation Using Tertiary Silanes. Science 2012, 335, 567−570.
(d) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. Preparation and Molecular
and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes
and Their Application to Catalytic Hydrogenation and Hydrosilation. J.
Am. Chem. Soc. 2004, 126, 13794−13807.
(3) (a) Luecke, M.-P.; Porwal, D.; Kostenko, A.; Zhou, Y.-P.; Yao, S.;
Keck, M.; Limberg, C.; Oestreich, M.; Driess, M. Bis(silylenyl)-
substituted Ferrocene-stabilized η6-Arene Iron(0) Complexes: Syn-
thesis, Structure and Catalytic Application. Dalton Trans 2017, 46,
16412−16418. (b) Blom, B.; Enthaler, S.; Inoue, S.; Irran, E.; Driess, M.
Electron-Rich N-Heterocyclic Silylene (NHSi)-Iron Complexes:
Synthesis, Structures, and Catalytic Ability of an Isolable Hydridosi-
lylene-Iron Complex. J. Am. Chem. Soc. 2013, 135, 6703−6713.
into 1 and FeBr (solvent) during the catalysis, which function
2
x
as Lewis base and acid, respectively, to react with carbonyl
compounds to form 2A. Subsequent reaction with HBpin
affords the corresponding boron esters and regenerates complex
2
. Such a mechanism is not known in both silicon and iron
(
c) Gallego, D.; Inoue, S.; Blom, B.; Driess, M. Highly Electron-rich
chemistry as yet. The isolation of intermediates in the catalysis is
currently under investigation.
Pincer-type Iron Complexes Bearing Innocent Bis(metallylene)-
pyridine Ligands: Syntheses, Structures, and Catalytic Activity.
Organometallics 2014, 33, 6885−6897.
In conclusion, the amidinatosilicon(I)−iron(II) dimer 2 was
capable of catalyzing various carbonyl compounds by HBpin
without using any reducing reagents. Mechanistic studies
showed that carbonyl compounds could be inserted into the
Si−Fe bond in 2, followed by reaction with HBpin to form boron
esters.
(
4) (a) Cabrera-Lobera, N.; Rodriguez-Salamanca, P.; Nieto-
Carmona, J. C.; Bunuel, E.; Cardenas, D. J. Iron-Catalyzed Hydro-
borylative Cyclization of 1,6-Enynes. Chem. - Eur. J. 2018, 24, 784−788.
(b) Chen, C.; Shen, X.; Chen, J.; Hong, X.; Lu, Z. Iron-Catalyzed
Hydroboration of Vinylcyclopropanes. Org. Lett. 2017, 19, 5422−5425.
C
DOI: 10.1021/acs.inorgchem.8b01760
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(
c) Gorgas, N.; Alves, L. G.; Stoeger, B.; Martins, A. M.; Veiros, L. F.;
Kirchner, K. Stable, Yet Highly Reactive Nonclassical Iron(II)
Polyhydride Pincer Complexes: Z-selective Dimerization and Hydro-
boration of Terminal Alkynes. J. Am. Chem. Soc. 2017, 139, 8130−8133.
(
d) Chen, X.; Cheng, Z.; Lu, Z. Iron-Catalyzed, Markovnikov-Selective
Hydroboration of Styrenes. Org. Lett. 2017, 19, 969−971. (e) McNeill,
E.; Ritter, T. 1,4-Functionalization of 1,3-Dienes With Low-Valent Iron
Catalysts. Acc. Chem. Res. 2015, 48, 2330−2343. (f) Gilbert-Wilson, R.;
Chu, W.-Y.; Rauchfuss, T. B. Phosphine-iminopyridines as Platforms
for Catalytic Hydrofunctionalization of Alkenes. Inorg. Chem. 2015, 54,
5596−5603. (g) Chen, J.; Xi, T.; Lu, Z. Iminopyridine Oxazoline Iron
Catalyst for Asymmetric Hydroboration of 1,1-Disubstituted Aryl
Alkenes. Org. Lett. 2014, 16, 6452−6455. (h) Jones, A. S.; Paliga, J. F.;
Greenhalgh, M. D.; Quibell, J. M.; Steven, A.; Thomas, S. P. Broad
Scope Hydrofunctionalization of Styrene Derivatives Using Iron-
Catalyzed Hydromagnesiation. Org. Lett. 2014, 16, 5964−5967.
Greenhalgh, M. D.; Thomas, S. P. Chemo-, Regio-, and Stereoselective
Iron-Catalyzed Hydroboration of Alkenes and Alkynes. Chem.
Commun. 2013, 49, 11230−11232. (j) Zhang, L.; Peng, D.; Leng, X.;
Huang, Z. Iron-Catalyzed, Atom-Economical, Chemo- and Regiose-
lective Alkene Hydroboration with Pinacolborane. Angew. Chem., Int.
Ed. 2013, 52, 3676−3680. (k) Li, L.; Liu, E.; Cheng, J.; Zhang, G.
Iron(II) Coordination Polymer Catalysed Hydroboration of Ketones.
Dalton Trans 2018, 47, 9579−9584.
(
5) Tamang, S. R.; Findlater, M. Iron Catalyzed Hydroboration of
Aldehydes and Ketones. J. Org. Chem. 2017, 82, 12857−12862.
6) (a) Das, U. K.; Higman, C. S.; Gabidullin, B.; Hein, J. E.; Baker, R.
(
T. Efficient and Selective Iron-Complex-Catalyzed Hydroboration of
Aldehydes. ACS Catal. 2018, 8, 1076−1081.
(7) Khoo, S.; Yeong, H.-X.; Li, Y.; Ganguly, R.; So, C.-W. Amidinate-
Stabilized Group 9 Metal-Silicon(I) Dimer and -Silylene Complexes.
Inorg. Chem. 2015, 54, 9968−9975.
(8) (a) Xiang, L.; Xiao, J.; Deng, L. Synthesis, Structure, and Reactivity
of Organo-Iron(II) Complexes with N-Heterocyclic Carbene Ligation.
Organometallics 2011, 30, 2018−2025. (b) Przyojski, J. A.; Arman, H.
D.; Tonzetich, Z. J. Complexes of Iron(II) and Iron(III) Containing
Aryl-Substituted N-Heterocyclic Carbene Ligands. Organometallics
2
(
012, 31, 3264−3271.
9) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.;
Bill, E.; Chirik, P. J. Square Planar vs Tetrahedral Geometry in Four
Coordinate Iron(II) Complexes. Inorg. Chem. 2005, 44, 3103−3111.
(10) (a) Zhang, Y.; Mei, T.; Yang, D.; Zhang, Y.; Wang, B.; Qu, J.
Synthesis and Reactivity of Thiolate-bridged Multi-iron Complexes
Supported by Cyclic (Alkyl)(amino)carbene. Dalton Trans 2017, 46,
1
(
5888−15896. (b) Kahn, O. Molecular Magnetism; Wiley, 1993.
11) Sen, S. S.; Kratzert, D.; Stern, D.; Roesky, H. W.; Stalke, D.
Reactivity studies of a GeI-GeI compound with and without cleavage of
the Ge-Ge bond. Inorg. Chem. 2010, 49, 5786−5788.
(12) The catalytic hydroboration of benzaldehyde 5a with HBpin can
be activated by compound 1 and FeBr ·2THF alone, albeit without full
2
conversion (entries 2 and 5, Table S4).
D
DOI: 10.1021/acs.inorgchem.8b01760
Inorg. Chem. XXXX, XXX, XXX−XXX