Homoleptic iron(II) and cobalt(II) bis(phosphoranimide) complexes for the selective hydrofunctionalization of unsaturated molecules

Article abstract of DOI:10.1039/c7dt02427d

Low-coordinate homoleptic bulky M₂(NP^tBu₃)₄ (M = Fe (A), Co (B)) complexes were synthesized and characterized as dimeric structures by crystallographic studies. The iron complex A can catalyze the hydroboration reaction of aldehydes and ketones. The cobalt complex B outperformed its iron counterpart in hydrogenations of several typical alkenes and alkynes under mild conditions. Poisoning experiments indicate that the Co(ii)/HBpin catalytic system could be homogeneous.

Full text of DOI:10.1039/c7dt02427d

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Homoleptic Iron(II) and Cobalt(II) Bis (phosphoranimide)
Complexes for Selective Hydrofunctionalization of Unsaturated
Molecules
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
DOI: 10.1039/x0xx00000x
Tao Bai* , Trevor Janes and Datong Song
www.rsc.org/
t
(NP Bu
The low-coordinate homoleptic bulky M
2
3
)
4
(M = Fe
(A), Co (B)) complexes were synthesized and characterized as
dimeric structures by crystallographic studies. Iron complex A
can catalyze the hydroboration reaction of aldehydes and
ketones. Cobalt complex B outperformed its iron counterpart
in hydrogenations of several typical alkenes and alkynes
under mild conditions. Poisoning experiments indicate that
the Co(II)/HB(pin) catalytic system could be homogeneous.
Scheme 1. Tri-t-butyl-phosphoranimidometal(II) dimers.
unnecessary for the metal centre to change oxidation state
6
directly.
Phosphoranimide ligands were discovered and studied by
7
Dehnicke and Weller around 20 years ago. Stoichiometric
Hydrofunctionalization of unsaturated compounds is an
important area among organic transformations. Primary and
secondary alcohols can be easily obtained via the
hydroboration of corresponding aldehydes and ketones
reactions between transition metal halide precursors and
phosphoranimide ligands have been systematically studied as
8, 9
well. However, the catalytic reactivity is rarely studied.
Herein we wish to report the preparation and characterization
1
followed by the hydrolysis of boronate esters. Hydrogenation
of non-pincer homoleptic base metal complexes
M
2
(μ-
of C-C double or triple bonds is another well-established area
2
in fine chemical synthesis. Complexes of precious metals such
t
NP Bu ) (NP Bu ) (M = Fe (A), Co(B)) (Scheme 1). We also
t
3 2 3 2
demonstrated selective catalytic reactivity in the
hydrofunctionalization of unsaturated molecules. The Fe (II)
as rhodium, ruthenium, iridium, and palladium have been
widely used as catalysts in hydroboration, hydrogenation, or
complex
A
is an effective catalyst for the hydroboration
is
1
a, 3
transfer hydrogenation of polar and non-polar π-bonds.
reaction of aldehydes and ketones; the Co (II) counterpart
B
There are growing demands to find cheaper and more
environmentally friendly surrogates of precious metal catalysts
a hydrogenation catalyst for alkenes and alkynes under mild
conditions with the assistance of an organo borane cocatalyst.
4
for organic transformations.
Treatment of MCl
2
with nearly
PN)Li ( ) in THF resulted in the
complexes
2% yield respectively (eq 1). Both
2
eq. lithium
There are growing demands to find cheaper and more
environmentally friendly surrogates of precious metal catalysts
for organic transformations. Base metals such as iron and
cobalt are less expensive and less toxic than precious metals,
and may serve as practical replacements. Other features of
base metals, such as their small covalent radii and the narrow
energy gaps among their d orbitals, make catalyst
t
phosphoranimide salt ( Bu
3
C
t
formation of M
2
(NP Bu
3
)
4
A
and
B
with 75% and
7
A
and are very
B
hydrocarbon soluble and can be crystallized from saturated
o
pentane or hexane solutions at -35 C. An X-ray diffraction
study of
A revealed the dimeric nature of iron complex. The
structure contains two terminal and two bridging
phosphoranimide units. The distances between bridging ligand
and the metal centers are in the range of 1.998-2.006 Å (M =
Fe) (Figure 1) and 1.964-1.976 Å (M = Co) (Figure 2). The
terminal phosphoranimide ligands, approach closer to the
metal center with M-N bond lengths of 1.856, 1.863 Å (M = Fe)
and 1.811, 1.815 Å (M = Co). The intermetal distances are out
of the range of bonding (Fe(1)-Fe(2) = 2.565(6) Å; Co(1)-Co(2) =
5
development a challenging task. Therefore; ligand design is
essential for the purpose of maintaining catalytic activity. One
successful approach has been the use of redox-active ligands,
which can behave as a single electron sponge, making it
1
0a
2.514(6) Å).
The geometry of both metals is planar and
three-coordinate.
complexes on
There is no obvious peak for both
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a
Table 1. Hydroboration of aldehydes and ketones catalyzed by iron(II) complex A.
t
2 3 4
Figure 1. ORTEP drawing of complex Fe (NP Bu ) A at 50% probability ellipsoids.
The hydrogen and co-crystallized solvent is omitted for clarity. Selected bond
o
lengths (Å) and angles ( ): Fe(1)-N(1) 1.863(2), Fe(1)-N(2) 2.006(3), Fe(1)-N(4)
.000(2), Fe(2)-N(2) 1.998(3), Fe(2)-N(3) 1.856(3), Fe(2)-N(4) 2.001 (3), Fe(1)-
Fe(2) 2.565 (6); N(1)-Fe(1)-N(4) 134.2(1), N(1)-Fe(1)-N(2) 133.4(1), N(4)-Fe(1)-
N(2) 91.9(1), N(4)-Fe(2)-N(2) 92.1(1), N(4)-Fe(2)-N(3) 131.9(1), N(3)-Fe(2)-N(2)
35.8(1).
2
1
a
2 2
The reaction was set up in glove box with H O and O <0.1 ppm; the reaction
scale was 0.2 mmol for aldehydes and ketones
on the carbonyl group can afford the alcohol in 65-95% yield
(2j-2n,
t
Table 1). Ketones without conjugation to a phenyl ring are also
2 3 4
Figure 2. ORTEP drawing of complex Co (NP Bu ) B at 50% probability ellipsoids.
The hydrogen and co-crystallized solvent is omitted for clarity. There are
suitable substrates (2o and 2p, Table 1); the cyclic aliphatic
alcohol 2q can be obtained by employing cyclic ketone 1q as
substrate.
disordered atoms shown in the crystal structure. Selected bond lengths (Å) and
o
angles ( ): Co(1)-N(1) 1.964(4), Co(1)-N(2) 1.815(5), Co(1)-N(4) 1.976(4), Co(2)-
N(1) 1.966(4), Co(2)-N(3) 1.811(5), Co(2)-N(4) 1,970(4), Co(1)-Co(2) 2.514 (6);
N(1)-Co(1)-N(2) 135.4(2), N(2)-Co(1)-N(4) 134.3(2), N(1)-Co(1)-N(4) 89.5(4), N(1)-
Co(2)-N(4) 89.6(2), N(1)-Co(2)-N(3) 136.1(2), N(3)-Co(2)-N(4) 133.7(2).
In order to study the chemoselectivity of hydroboration
3
1
1
employng iron catalyst A, competition experiments between
P NMR and several broad peaks in H NMR spectra due to the
paramagnetic characteristics (see ESI for detail).
aldehydes and ketones were conducted both inter- and
intramolecularly (Scheme 2). Equimolar amounts of aldehyde,
ketone and HBpin were mixed together with complex A as
t
Inspired by the reaction of HNP Bu
HBpin) to generate and pinB-NP Bu
We envisioned that HBpin may undergo
3
with pinacolborane
t
(
H
2
3
under mild
1
3
11
catalyst at room temperature (eqs. 1-3, Scheme 2) . In all
conditions.
three examples, the reactions afforded the boronate esters of
aldehydes with the NMR yields of 72-87% as major products
regardless of the electronic effect on the phenyl ring. The
intramolecular chemoselective hydroboration reactions of
aldehydes and ketones were also studied with 2-formylphenyl
acetate (1r) and 4-(2-oxo-2-phenylethoxy)benzaldehyde (1s) as
substrates, which gave the hydroboration products of
aldehydes as well with 63% (2r) and 80% (2s) isolated yields
after hydrolysis.
metathesis with A to yield a catalytically active metal hydride
intermediate. To test this hypothesis, we attempted
1
hydroboration of C=O π bonds using Fe(II) complex
2
A.
Fortunately, the desired reduction compounds can be
obtained via hydroboration followed by hydrolysis of the
boronate ester (Table 1). Aldehydes and ketones were suitable
to produce the corresponding primary or secondary alcohols.
For the aryl aldehyde substrates, the primary alcohol can be
obtained in 74-85% isolated yield. Different electron donating
or withdrawing groups on the phenyl ring can be tolerated in
this reaction (2a-2f, Table 1). The non-conjugated aldehyde 1g
can also be reacted to form 2g in 95% yield; the α, β-
unsaturated aldehyde 1h could be reduced to form the alcohol
When allyl benzene was employed as the hydroboration
substrate, no desired product formed. However, the
hydrogenation product 4a can be observed from the proton
NMR analysis of crude reaction mixture. Therefore, we realized
that the iron (II) complex and the organo-borane compound
could be hydrogenation co-catalysts. This hypothesis was
tested on the hydrogenation of allyl benzene (Table 2). The
2h without forming the 1,4- reduction product. Also, alkyl
substituted aldehyde 1i can form the fatty alcohol 2i in 82%
yield. Several ketones were applied to generate the secondary
alcohols: employing methyl, isopropyl and phenyl substituents
iron complex
A
could only show 8% hydrogenation product.
2
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Scheme 2. Competing Hydroboration reaction between Aldehydes and ketones.
hydrogenation reaction of phenylacetylene, as evidenced by
Table 3. Substrate scope of hydrogenation of C-C π-bonds catalyzed by Co(II) complex
B.
Entry
1
Substrates 3
Products 4
NMR yields
of 4 (%)
82
2
3
4
5
6
7
81
83
Table 2. Catalytic reactivity comparison of iron and cobalt complexes by the
hydrogenation of allylbenzene.
>99
83
a
NMR yield of products (%)
Entry
M
3
a
3aa
43
0
3ab
4
4a
8
1
2
3
Fe
Co
14.5
0
92
0
82
0
b
Co
71
0
0
a
b
80
Mesitylene was used as the internal standard; there is
no reaction occur without the participation of HB(pin) at
room temperature.
a
The reaction scale is 0.2 mmol; NMR yield was measured using mesitylene as
The major products are the isomerization products 3aa and
internal standard.
3
ab
can be accomplished by using the cobalt complex
NMR yield without any contamination of internal alkenes
(
entry 1, Table 2). Surprisingly, the hydrogenation process
B
with 82%
the maintenance of product yields, which implies that Co
nano-particles are not formed during catalysis. (entries 1-3
Table 4). The catalytic system could also be tolerated under
normal monophosphine ligand (PPh ), even more nucleophilic
phosphine ligand (PMe entries 4 and 5, Table 4);
diphosphine and bipyridine ligands as poisoning additive
entries 6 and Table 4). Based on the observations above,
homogeneous hydrogenation under catalysis of Co(II) complex
is most likely.
,
1
4
(entry
2, Table
2);
a catalytic amount of HBpin is very
essential for this reaction. There will be starting material
3
recovered in 71% yield without the assistance of HBpin (entry
3
)
(
3, Table 2).
Other alkenes and alkynes were also tried under these mild
(
7,
hydrogenation conditions. The results are summarized in Table
3. The terminal alkyne 3b and 3e can also afford the fully
B
hydrogenated product 4b and 4e with high NMR yield (entries
Table 4. Poisoning effect of Co (II) B catalyzed hydrogenation reaction.
2
3
and 5, Table 3). The alkyl substituted terminal alkynes 3f and
can also be hydrogenated to give the saturated
g
hydrocarbon products with 92% and 80% yield respectively
entries 6 and 7, Table 3); the internal alkynes were also
suitable for the transformation with excellent yields (entries 3
and 4, Table 3). The stoichiometric reactions of complexes or
with 2 eq. of HB(pin) ( /[ ] = 1:2) were also conducted to
(
A
B
M B
t
give the Bu P=N-B(pin) as only phosphorus containing
3
product,
3
1
which was shown on P NMR spectrum (see ESI for detail).
Furthermore, in order to understand the properties of the
hydrogenation catalytic system, we studied the poisoning
effect by adding different reagents to the catalytic system.
1
5
(
Table 4). Different equivalents of mercury cannot poison the
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Johnson, I. C. Lennon, P. H. Moran, J. Ramsden, A. Acc. Chem.
Res., 2007, 40, 1291; (d) C. S. Schultz, S. W. Krska, Acc. Chem.
NMR
NMR
yield of
4b (%)
80
NMR
yield of
4c (%)
<1
Poisoning
reagent
Equivalent
(mol %)
Conclusions
Entry
yield of
3
b (%)
<1
Novel homoleptic three coordinate base metal complexes
have been synthesized and characterized. These non-pincer
dimers are effective catalysts under mild conditions for the
hydrofunctionalization of π-bonds. The iron (II) complex is an
effective hydroboration catalyst for aldehydes and ketones;
the cobalt (II) counterpart is a hydrogenation catalyst for C-C π
bonds. It’s the simple and effective approach to accomplish
the different type of reduction reaction by the same ligand
environment. It is noteworthy that different types of reduction
reactions can be accomplished by the same ligand
environment. The design of new highly active Fe and Co based
catalysts presents an opportunity for constructing other base
metal catalyst systems.
1
2
3
4
Hg
Hg
Hg
20
50
100
5
<1
75
<1
<1
63
<1
PPh
PMe
dppe
3
<1
78
4
5
6
3
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5
<1
<1
74
80
<1
3
2
,2’-
7
5
<1
88
<1
bipyridine
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We would like to acknowledge the overseas high-level talents
program of Shanxi University (112545005 and 112545012) and
Shanxi Scholarship Council of China (112639901001) for
financial support of this research. We also thank Dr. Cao Wei
and Prof. Chao Jianbin in Scientific Instrument Center, Shanxi
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