Zinc hydridotriphenylborates supported by a neutral macrocyclic polyamine

Article abstract of DOI:10.1039/c7dt01094j

The zinc hydridotriphenylborates [(L)Zn(TMDS)][HBPh₃] and [(L)ZnX][HBPh₃] (L = Me₄TACD, Me₄[12]aneN₄; TMDS = N(SiHMe₂)₂; X = Cl, Br, I) were synthesized by BPh₃-mediated β-SiH abstraction and salt metathesis with KHBPh₃, respectively. CO₂ is rapidly inserted into the B-H bonds. [(L)Zn(TMDS)][HBPh₃] catalyzes the hydroboration of polar substrates including CO₂.

Full text of DOI:10.1039/c7dt01094j

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DOI: 10.1039/C7DT01094J
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Zinc hydridotriphenylborates supported by a neutral macrocyclic
polyamine
Debabrata Mukherjee,# Ann-Kristin Wiegand,# Thomas P. Spaniol, and Jun Okuda*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The zinc hydridotriphenylborates [(L)Zn(TMDS)][HBPh3] and (Mg, Ca)4d,10 metals is proved to be a convenient route for
[(L)ZnX][HBPh3] (L = Me4TACD, Me4[12]aneN4; TMDS = N(SiHMe2)2; their [HBPh3]− derivatives. Stirring
THF solution of
X = Cl, Br, I) were synthesized by BPh3-mediated β-SiH abstraction [Zn{N(SiHMe2)2}2] and L with BPh3 in 1:1:2 ratio for 24 h at
and salt metathesis with KHBPh3, respectively. CO2 is rapidly room temperature provided [(L)Zn{N(SiHMe2)2}][HBPh3] ( ) as
inserted into the B−H bonds. [(L)Zn(TMDS)][HBPh3] catalyzes the colorless microcrystals in 66% yield (Scheme 1). An excess of
a
1
hydroboration of polar substrates including CO2.
BPh3 or longer reaction times did not affect the remaining
silazide group in , likely due to diminished hydridicity of the
silicon hydrides.10-11 Heating the reaction mixture to 60
caused decomposition. Notably, L apparently offers only labile
coordination to the [Zn{N(SiHMe2)2}] precursor and any adduct
between the two could not be isolated.
1
Zinc borohydride [Zn(BH4)2] is a commercially available
mild reducing agent1 that also finds interest as hydrogen
storage material,2 whereas any substituted derivative of the
type [HBR3]− (R = alkyl, aryl, alkoxy etc.) for finer selectivity
control is currently unknown.3 Lately, we have been exploiting
the moderately Lewis acidic triphenylborane (BPh3) and the
hydride anion [HBPh3]− in catalysis of polar substrates.4 The
alkali metal derivatives [M(Me6TREN)][HBPh3] (M = Li, Na, K)
catalyzed the hydroboration of carbonyls and CO2
º
C
]
[Zn{N(SiHMe2)2}2
+
N
N
BPh3
or
2
equiv.)
(1
N
N
N
N
N
N
[HBPh3]
Zn
N
THF, 24 25 °C
h,
chemoselectively,4c
while
[Mg(thf)6][HBPh3]2
reduced
−
Me
2HSiN SiMe2
Me
SiHMe2
2HSi
0.5
substrates with a wider substrate scope.4d The stronger Lewis
acidic aluminum derivative catalyzed the hydroborylative ring-
opening of cyclic ethers THF and THP.4e Homogeneous
catalysis mediated by the earth-abundant and non-toxic metal
zinc is currently of significant interest.5 As a softer Lewis acid,
divalent zinc could offer distinct reactivity when compared
with hydridotriphenylborates of group 1, 2, and 13 metals.6
Here we describe cationic hydridotriphenylborates of zinc
supported by the macrocyclic polyamine 1,4,7,10-tetramethyl-
1,4,7,10-tetraazacyclododecane (Me4TACD, Me4[12]aneN4 = L),
their reactivity towards CO2, and their catalytic activity in
hydroboration of polar substrates. L is a neutral tetradentate
donor that forms stable complexes with s-,7 d-,8 and f-block9
metals.
Me
2Si NSiHMe2
1 66%
,
=
Me
L
4TACD
Scheme 1. β-SiH abstraction from [Zn{N(SiHMe2)2}2] by BPh3 in the presence of L.
Complex
1 is soluble in THF, but insoluble in aliphatic and
aromatic hydrocarbons. The 1H and 13C{1H} NMR spectra in
THF-d8 exhibit the diagnostic resonances of L, [N(SiHMe2)2],
and the [HBPh3]− anion. The BH proton is found as a broad
quartet at
δ
3.23-3.81 ppm, while the 11B NMR spectrum
shows a characteristic doublet at δ −7.9 ppm (1JBH = 78 Hz).4c,4d
The solid-state IR spectrum (KBr) reveals several overlapping
νBH and νSiH stretching bands in the region between 2000 and
−
1
2226 cm .
BPh3-mediated
β-hydride abstraction from N(SiHMe2)2
Unlike [Zn{N(SiHMe2)2}2], the dihalides ZnX2 (X = Cl, Br, I)
reacted with L in THF to give the ionic halide complexes
(TMDS) or C(SiHMe2)3 complexes of group 1 (Li, Na, K)4c and 2
[(L)ZnX][X] (X = Cl, 2; Br, 3; I, 4) in 84-89% yield (Scheme 2).
These are insoluble in THF, aliphatic and aromatic
hydrocarbons and only soluble in DMSO.
a.Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056,
Aachen (Germany). E-mail: jun.okuda@ac.rwth-aachen.de
# These authors have contributed equally.
Stirring THF suspensions of
room temperature for 16 h gave [(L)ZnX][HBPh3] (X = Cl,
; I, ) (Scheme 2). Direct reactions of L, ZnX2 and KHBPh3 in
2
-4
with KHBPh3 in 1:2 ratio at
Electronic Supplementary Information (ESI) available: Description of compounds
1–
5
; Br,
11 hydroboration catalysis, and crystallographic data files. CCDC 1539861-
,
1539863. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
6
7
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1:1:2 ratio also resulted in the formation of 5-7. The 'inner
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DOI: 10.1039/C7DT01094J
sphere' halide remained unreacted while the 'outer sphere'
halide was replaced by the [HBPh3]− anion. The 'inner sphere'
halide was not abstracted even with an excess of KHBPh3 or
longer reaction times. Pure
5-7 were isolated as colorless
crystals in 73-83% yields by using one equiv. of KHBPh3
(Scheme 2).
N
N
=
=
=
X
X
X
2
89%
87%
Cl
Br
THF
( ),
+
Figure 1. Molecular structure of 4. Only the [(L)ZnI]+ cationic part is shown.
Displacement parameters are drawn at 50% probability. Hydrogen atoms are omitted
for clarity. Selected bond distances (Å): Zn1−I1 2.5576(12); Zn1−N1 2.199(6); Zn1−N2
2.182(6); Zn1−N3 2.192(6); Zn1−N4 2.170(6).
ZnX2
L
N
N
3
( ),
[X]
or
Zn
30 min, 25 °C
I
4
84%
( ),
X
KHBPh
THF, 16 h
KX
2
equiv)
3 (1
−
N
N
KHBPh3
or
=
X
5
83%
82%
Cl
Br
( ),
2
equiv.)
(1
N
N
=
=
X
X
6
( ),
[HBPh3]
Zn
THF, 16 h
−
I
7
73%
( ),
KX
X
Scheme 2. Salt metathesis route for the L-supported zinc hydridotriphenylborates (5-7).
1
Compounds
5
-
7
are soluble in THF and the H and 13C{1H
NMR spectra in THF-d8 exhibit the diagnostic resonances of L
and those of [HBPh3]− anion. The 11B NMR spectra also show
the characteristic doublet at δ −7.8 ppm (1JBH = 77 Hz). Their
solid-state IR spectra (KBr) contain multiple νBH bands between
2000 and 2200 cm-1.
Figure 2. Molecular structure of 7. Displacement parameters are drawn at 50%
probability. Hydrogen atoms except for B-H are omitted for clarity. Selected bond
distances (Å): Zn1−I1 2.5624(7); Zn1−N1 2.159(4); Zn1−N2 2.191(4); Zn1−N3 2.153(4);
Zn1−N4 2.189(4).
A complex Zn[HBPh3]2 free of any ancillary ligands, in
analogy to the magnesium derivative4d could not be isolated.
No hydride abstraction by BPh3 resulted with
[Zn{N(SiHMe2)2}2]. The reaction of zinc dihalides ZnX2 with
KHBPh3 led to decomposition to H2, metallic zinc, and KX. The
CO2 insertion into the B−H bonds of [(Me6TREN)M][HBPh3]
(M = Li, Na, K) and [Mg(thf)6][HBPh3]2 was previously reported
to be fast.4c,4d A similar reactivity was shown for Pd and Pt
complexes.15 Complexes
1 and 5-7 reacted rapidly with CO2 (1
12
reaction of polymeric [ZnH2]∞ with BPh3 likewise gave
atm) in THF to give the respective formoxyborates 8-11
,
metallic zinc and H2. The adduct of ZnI2 with an N-heterocyclic
carbene and [Li(HBEt3)] was reported to decompose in a
similar fashion.13 [Zn{Si(SiHMe2)3}2] is described to be unstable
isolated as colorless crystals in 66-81% yields (Scheme 3).
N
N
too.14 Reactions of
2-4 with [Li(HBEt3)] or [Li{HB(sBu)3)] also
atm)
CO
2 (1
THF
<5 min, 25 °C
N
N
resulted in decomposition to give metallic zinc.
1
Zn
N
BPh3
]
[HCO2
Single crystal X-ray diffraction of the iodide derivatives
4
Me
2HSi
SiHMe2
and
The cationic zinc center in both cases has a distorted square-
pyramidal geometry with a
4-coordinated L and a terminal
iodide. The counter anions I− and [HBPh3]− are free with no
close interaction with the cationic part. The Zn N bond lengths
vary from 2.153(4) to 2.199(6) Å. The terminal Zn I distances
are 2.5576(12) Å in and 2.5624(7) Å in
7 confirmed separate ion-pair structures (Figure 1 and 2).
8 81%
,
κ
N
N
−
−
CO
atm)
-
2 (1
THF
<5 min, 25 °C
N
N
7
BPh3
]
5
[HCO2
Zn
X
4
7.
=
=
=
X
X
X
9
66%
Cl
( ),
Br 10 66%
( ),
I
11 75%
),
(
Scheme 3. Synthesis of formoxyborates 8-11 via CO2 insertion into the B−H bonds.
The solubility of 11 decreases in THF in the following
8
-
order; (soluble) > 9, 10 (moderately soluble) > 11 (poorly
8
2 | J. Name., 2012, 00, 1-3
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Table 1. Catalytic hydroboration of polar substrates by HBpin using 10 mol% of 1.a
soluble). None of them is soluble in aromatic and aliphatic
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hydrocarbons. 1H and 13C{1H} NMR spectra in THF-d8 reveal
DOI: 10.1039/C7DT01094J
Entry
Substrates
Products
˚C
Time (h)b
signals for the formoxy groups at ca.
δ 8.33 and 169 ppm,
respectively. The 11B NMR spectra contain a broad singlet at
3.21 ppm.
δ
Bpin
Ph
O
O
1
25
<1
Ph
Ph
Ph
H
O
+
Bpin
Bpin
Bpin
2
3
CO2
60
60
16
20
MeO
Ph
H
Bpin
Ph
Ph
H
N
N
Ph
N
H
H
H
N
Bpin
4
60
60
20
35
O
Bpin
Figure 3. Molecular structure of 11. Displacement parameters are drawn at 50%
probability. Hydrogen atoms except for that of the HCO2 unit are omitted for clarity.
Selected bond distances (Å): C13−O1 1.284(6); C13−O2 1.212(6); B1−O1 1.549(6).
5c
2
O
O
O
N
6c
60
100d
The iodide derivative 11 was structurally authenticated by
N
single crystals X-ray diffraction (Figure 3). The [(L)ZnI]+ cation is
identical as in
[HCO2BPh3]− as a free anion. The boron center has a
7
and [HBPh3]− has been replaced by
a [1] 0.062 M, [substrate] 0.62 M, [HBpin] 0.65 M, 0.45 mL of solvent (THF : THF-
d8 = 2 : 1). Time for complete substrate consumption, as detected by NMR
spectroscopy. c [HBpin] 1.30 M, d 7% conversion.
b
tetrahedral geometry. The C13
distances are 1.284(6) and 1.212(6)
bond is 1.549(6)
coordinated in [(Me6TREN)M][HCO2BPh3] (M = Li, Na, K)4c and
[Mg(thf)4][HCO2BPh3]2.4d
−
Å
O1 and C13
, respectively. The B1
−
O2 bond
O1
−
Å
long. Notably, [HCO2BPh3]− was
In conclusion, coordination of L allowed to stabilize zinc
hydridotriphenylborates in the form of separate ion-pairs that
otherwise appears to be redox unstable. The zinc centers are
coordinatively over-saturated by the κ4 bonded L and the
terminal tetramethylsilazide or halides, preventing a direct
Complexes
hydroboration of polar substrates. Tetramethylsilazide
complex showed the highest activity among them but it is
less active than the group 1 and 2 metal compounds.4c,4d Table
summarises the results using 10 mol% of and
pinacolborane (HBpin) as the reducing agent.
Benzophenone was the easiest substrate to reduce and
completely reacted within 1 h at 25 C (entry 1). CO2
hydroboration was accomplished within 16 h at 60 C giving a
1 and 5-7 were tested as catalysts for the
1
Zn−HBPh3 interaction. The significantly reduced Lewis acidity
of zinc is manifested by a low catalytic hydroboration activity.
1
1
Acknowledgements
˚
We thank the Deutsche Forschungsgemeinschaft through
the International Research Training Group “Selectivity in
Chemo- and Biocatalysis” for financial support and the
Alexander von Humboldt Foundation for a fellowship to D. M.
˚
1:1 mixture of [MeOBpin] and [pinBOBpin] (entry 2).
Hydroboration of N-benzylideneaniline (entry 3) and
regioselective dearomatization of pyridine to N-borylated 1,4-
dihydropyridine (entry 4) were achieved within 20 h at 60 ˚C.
Hydroboration of ethyl acetate to 2 equiv. of EtOBpin was
slower compared to the previous entries and needed 35 h to
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TOC
Zinc hydridotriphenylborates supported by a neutral macrocyclic polyamine
Debabrata Mukherjee, Ann-Kristin Weigand, Thomas P. Spaniol, and Jun Okuda*
A neutral macrocyclic polyamine
1,4,7,10-tetramethyl-1,4,7,10-
tetraazacyclododecane (Me4TACD)
stabilizes
cationic
zinc
hydridotriphenylborates that react
with CO2 (1 atm) under insertion into
the B−H bonds.