[(NHC)CoR2]: pre-catalysts for homogeneous olefin and alkyne hydrogenation

Article abstract of DOI:10.1039/c8cc08891h

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR₂] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH₂SiMe₃)₂] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η²-PhCCPh)₂].

Full text of DOI:10.1039/c8cc08891h

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[
(NHC)CoR ]: pre-catalysts for homogeneous
2
olefin and alkyne hydrogenation†
Cite this: Chem. Commun., 2018,
4, 13798
5
Andreea Enachi, Dirk Baabe, Marc-Kevin Zaretzke, Peter Schweyen,
Matthias Freytag, Jan Raeder and Marc D. Walter
Received 7th November 2018,
Accepted 19th November 2018
*
DOI: 10.1039/c8cc08891h
rsc.li/chemcomm
27
A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR
2
] is ligands and therefore offer attractive alternatives to [(py) CoR ]. In
2 2
presented. In these complexes tmeda is readily replaced by an NHC addition, [(ItBu)Co(CH SiMe ) ] (ItBu = 1,3-di-tert-butylimidazolin-
2
3 2
or a bidentate phosphine ligand to form 3- and 4-coordinate 2-ylidene; 2a-H) acts as an efficient, homogeneous olefin hydro-
compounds, respectively. [(ItBu)Co(CH SiMe ] (ItBu = 1,3-di-tert- genation pre-catalyst.
2
3 2
)
2
8
butylimidazolin-2-ylidene) serves as an efficient, homogeneous
Treatment of [(tmeda)Co(acac)
2
]
with MgR
2 2 3
(R = CH CMe ,
2
9,30
olefin hydrogenation pre-catalyst and allows the preparation of CH
2
SiMe
3
, CH
2
CMe
2
Ph)
forms the dialkyl complexes 1a–c
in high purity and good yields (Scheme 1 and see ESI†).
2
the novel cobalt bis(alkyne) complex [(ItBu)Co(g -PhCCPh)
2
].
This synthetic route significantly surpasses the previously
The replacement of precious metals in catalysis by earth- reported procedures for [(tmeda)Co(CH SiMe ) ] proceeding
2
3 2
abundant 3d-transition metals such as Fe, Co and Ni represents via [(py) Co(CH SiMe ) ], which is thermally sensitive and its
2
2
3 2
1
–10
a flourishing and highly topical field of chemical research.
yield strongly depends on the quality and purity of the CoCl
As part of this endeavour, these metals have been successfully starting material.
2
27,31
Compounds 1a–c were characterised (see ESI†)
utilised in hydrogenation, hydrofunctionalisation and cross- by NMR, EPR and UV/Vis spectroscopy, elemental analyses and
coupling reactions. Recently, the Chirik group introduced efficient solid-state magnetic susceptibility studies. Consistent with previous
olefin hydrogenation catalysts based on dialkyl cobalt complexes studies on four-coordinate, tetrahedral dialkyl Co(II) species, the
stabilised by bidentate phosphines such as [(P-P)Co(CH
2
SiMe
3
)
2
]
complexes 1a–c adopt a high-spin configuration (S = 3/2) with
3
,9–11
(
P-P = dppe, depe, dmpe, dppBz).
Alternatively, low-valent effective magnetic moments (m ) that range between m (300 K) =
eff eff
4
ꢀ
4
32–36
Co(–I) species such as [(Z -anthracene) Co] , [(Z -naphthalene)- 4.74 and 5.72 mB (Fig. 1 and see ESI†).
Co(Z -cod)] (cod = 1,5-cyclooctadiene) and [(Z -cod) Co] may significantly higher than the S = 3/2 spin-only value of 3.87 mB
also be used.
These values are
2
4
ꢀ
4
ꢀ
2
1
2
N-heterocyclic carbenes (NHCs) represent implying substantial orbital contributions to meff, which is commonly
37
an attractive alternative to ubiquitous phosphine ligands in observed for high-spin Co(II) complexes. At temperatures below
1
3–17
catalysis and small molecule activation.
For example, NHC- T = ca. 100 K, the declining magnetic moment with decreasing
stabilised cobalt complexes have been applied, e.g. in the temperature is attributed to zero-field splitting (ZFS) and was further
8
,18,19
catalytic olefin hydrosilylation with tertiary silanes,
functionalization,
N
2
2
analysed by a simplified spin-Hamiltonian approach (with rhombic
ZFS parameter E/D = 0) utilising anisotropic g-values with
2
0
or Kumada cross-coupling reactions.
Furthermore, starting from (NHC)Co fragments, NHC–silyl– g = g o g (see ESI†). This analysis reveals average g-values
x
y
z
2
1
22,23
NHC pincer ligands, cobalt(0) bis(olefin),
and cobalt between g = 2.328 and 2.701, and positive values for the axial
2
4–26
ꢀ1
imido complexes
contribution we detail a novel and high-purity synthesis of
(tmeda)CoR ] (R = CH SiMe (1a), CH CMe (1b) and CH CMe Ph butylimidazolin-2-ylidene; ItBuMe
1c)), which serve as excellent starting materials for ligand substitu- dimethylimidazolin-2-ylidene) and bidentate phosphines
were successfully prepared. In this ZFS parameter (D) in the range of D = +9.6 to +16.8 cm
.
Complexes 1a–c were treated with NHCs (ItBu = 1,3-di-tert-
1,3-di-tert-butyl-4,5-
[
2
2
3
2
3
2
2
2
=
(
tion reactions when treated with NHC or bidentate phosphine
Technische Universit a¨ t Braunschweig, Institut f u¨ r Anorganische und Analytische
Chemie, Hagenring 30, 38106 Braunschweig, Germany. E-mail: mwalter@tu-bs.de
†
Electronic supplementary information (ESI) available: Experimental, catalytic
and crystallographic details; EPR and UV/Vis spectroscopic data; magnetic
susceptibility studies. CCDC 1863894–1863904. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c8cc08891h
Scheme 1 Synthesis of [(tmeda)CoR
[(tmeda)Co(acac) ] and [MgR ].
2
] complexes 1a–c derived from
2
2
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Fig. 1 Effective magnetic moment (meff) vs. T plots for 1a, 2a-Me, 4 and 5
recorded at temperatures between T = 2.6 and 300 K with an applied
magnetic field of Hext = 1 kOe. Symbols: experimental data; line: fit based
on a simplified spin-Hamiltonian approach (see text).
Fig. 2 Displacement ellipsoid plots (50% probability) for complexes 2a-H, 3, 4
and 5. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angles (1): complex 2a-H: Co–C1 2.0569(16), Co–C12 2.0438(19), Co–C16
2.0308(19), C1–Co–C12 111.73(8), C12–Co–C16 119.82(8), C1–Co–C16
126.79(7). Complex 3: Co–P1 2.2181(6), Co–P2 2.1938(6), Co–C1 1.970(2),
Co–C8 2.003(2), P1–Co–P2 86.08(2), P2–Co–C8 90.43(7), C1–Co–C8
0
8
2.82(9). Complex 4: Co–P 2.2152(4), Co–C1 2.0359(15), P–Co–P 86.98(2),
0
0
0
P –Co–C1 92.57(5), C1–Co–C1 88.73(9), P–Co–C1 92.57(5). Complex 5:
Co–C1 1.9678(19), Co–C7 1.9470(14), Co–C8 1.9196(13), C7–C8 1.282(2).
Scheme 2 Ligand substitution reactions based on compounds 1a–c.
Table 1 Olefin hydrogenation using 2a-H as pre-catalyst
(dippe = 1,2-bis(di-iso-propylphosphino)ethane; depe = 1,2-
Cat. loading
Conv. %
bis(diethylphosphino)ethane) (Scheme 2). In all cases, tmeda
replacement proceeded smoothly, and complexes 2–4 were
isolated in good yield and high-purity (Fig. 2 and see ESI†).
For the neophyl derivative 1c, the 5-membered metallacycle 3
was formed because of CH bond activation of the neophyl
moiety (see ESI†). Tmeda exchange by one NHC does not alter
the spin-state (S = 3/2) of the Co(II) atom in 2a-H/Me and 2b-H/Me,
whereas a replacement with bidentate phosphines such as depe
and dippe results in spin-pairing to yield the square-planar,
low-spin (S = 1/2) compounds 3 and 4, whose X-band EPR
spectra and effective magnetic moments of meff (300 K) = 2.63
Product
Entry Substrate (mol%) (time) (yield %)
a
1
00
a
1
2
3
5 (27 h)
(73)
a
1
00
a
0.5 (6 h)
0.5 (18 h)
(93)
a
1
(
00
b
100)
a
98
b
(98)
4
5
0.5 (18 h)
3 (49 h)
and 2.68 m
B
, respectively, are unexceptional (Fig. 1 and see
ESI†). For the high-spin Co(II) compounds (2a-H/Me and
b-H/Me), again, typical EPR spectra and meff (300 K) values in
the range between 4.47 and 5.04 m (Fig. 1 and see ESI†)
The analysis of the temperature-dependent
effective magnetic moment was performed analogous to that of
a
1
(
00
3
b
84)
2
a
B
22
(6)
6
7
5 (26 h)
5 (20 h)
b
3
8–41
were obtained.
a
25
a
(17)
1
a–c, revealing average g-values in the range between g = 2.162
a
ꢀ1
13
(
and 2.467, and negative D-values between D = ꢀ37 and ꢀ40 cm
a
7)
3
8–41
8
0.5 (6 h)
for 2a-H/Me and 2b-H/Me (Fig. 1, and see ESI†).
Similar to the reactivity of the related species [(P-P)CoR ]
P-P = dppe, depe, dmpe), the three-coordinate NHC-adducts
2
a
b
1
c
3
Determined by GC. Determined by H NMR spectroscopy. Product
(
[
2
1
13
1
distribution determined by H and C{ H} NMR spectroscopy.
(NHC)CoR
2
] were probed in hydrogenation reactions. Complex
a-H was exposed to H in the presence of various olefins at
2 3 2
pressure (ca. 4 bar) (Table 1 the four-coordinate complexes [(P-P)Co(CH SiMe ) ], were explicitly
2
ambient temperature and moderate H
and see ESI†). In addition, substrates, difficult to hydrogenate with included (see entries 1, 2 and 6 in Table 1). For entries 1–5 efficient
2
3
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Fig. 3 Poisoning studies performed with pre-catalyst 2a-H in the hydro-
genation of COD by addition of dct (6 equiv.). Lines are only drawn as
visual guides.
Chart 1 Proposed catalytic cycle for the hydrogenation of olefins using
pre-catalyst 2a-H.
hydrogenation is achieved, and for substrates 6–8 improved
after 40 min of COD hydrogenation effectively stops the cata-
lytic activity (Fig. 3). In analogy to [(dppe)Co(Z -cod)], we
3
conversions compared to [(P-P)Co(CH SiMe ) ] are realised.
4
3
2
3 2
3
In contrast to [(dppe)Co(CH SiMe ) ], full hydrogenation of
2
3 2
assume that upon addition of DCT under catalytic conditions
cyclooctadiene (COD) to cyclooctane is realised within 6 h with
only 0.5 mol% loading of 2a-H. Nevertheless, the first hydro-
genation step of COD to yield cyclooctene (COE) is significantly
slower than the second one, and only after complete consump-
tion of COD rapid hydrogenation of COE occurs. This observa-
tion is consistent with the formation of a relatively stable COD
adduct, which hampers further hydrogenation and acts as a
dormant species outside the catalytic cycle (Chart 1). However,
after complete substrate consumption, cobalt particles form in
the reaction mixture.
4
[
(ItBu)Co(Z -dct)] is formed, but despite repeated attempts we
have so far been unable to isolate this species.
Nevertheless, the reactions of the difficult to hydrogenate
substrates (entries 7 and 8 in Table 1) were also probed and in
these cases adduct formation was established by EPR spectro-
scopy (see ESI† for details), which further strengthens our
proposed reaction mechanism (Chart 1). In both cases a spin-
state change from S = 3/2 to S = 1/2 is observed. Furthermore,
for tolane (PhCCPh) the corresponding bis(alkyne) complex
2
[
(ItBu)Co(Z -PhCCPh) ] (5) was isolated (Scheme 3), which
2
To exclude heterogeneous hydrogenation, poisoning experi-
adopts a low-spin (S = 1/2) configuration with meff (300 K) =
.38 m (Fig. 1 and see ESI†).
The characteristic CC bond distance of the coordinated
12,42,43
ments are required.
For this purpose mercury is frequently
2
B
employed, since Hg covers the metal particle surface, which
reduces the catalytic activity, but this process may require a
significant induction period. An alternative and complementary
approach represents the addition of dibenzo[a,e]cyclooctatetraene
alkyne (1.282(2) Å) in 5 (Fig. 2) is moderately elongated relative
45
to the CRC bond distance in diphenylacetylene (1.210(3) Å)
approaching the value of a typical CC double bond (1.331 Å),
46
(
DCT), that binds efficiently to homogeneous catalysts and stops
which is consistent with p-backbonding from the Co atom into
the p*-orbitals of the coordinated alkyne. No reactivity of
complex 5 was observed upon addition of H in the presence
2
or absence of COD, establishing that 5 is not a catalytically
active species.
42
catalytic activity with no or only a very short induction period.
Hg addition to 2a-H under an inert atmosphere (N
degradation, but in the presence of Hg and H
2
) induces no
the catalytic
2
efficiency in COD hydrogenation is significantly reduced. While
this observation may imply heterogeneous catalysis, we instead
suggest that the catalytically active (homogeneous) species formed
In conclusion, we demonstrated that the four-coordinated
2
[(tmeda)CoR ] (1a–c) compounds are excellent starting materials for
44
after H addition to 2a-H is sensitive to Hg (Chart 1). In our
2
ligand substitution reactions with NHCs or bidenate phosphines.
2
proposed mechanism the highly reactive [(NHC)CoH ] species is
formed, which readily undergoes olefin insertion and reductive
elimination to yield [(NHC)Co], which can either coordinate
Furthermore, the three-coordinate complex [(ItBu)Co(CH SiMe ) ]
2
3 2
(
2a-H) serves as a good homogeneous olefin hydrogenation pre-
catalyst showing superior activity to the four-coordinate com-
pound [(dppe)Co(CH SiMe ]. Reaction of 2a-H with PhCCPh
2
olefins or undergo H addition (Chart 1).
2
3 2
)
We also noted that when 2a-H is exposed to H
2
in the absence
of olefins rapid decomposition to cobalt particles occurs,
but these particles are not active in olefin hydrogenations.
Nevertheless, DCT as strongly chelating ligand efficiently shifts
4
the equilibrium to the catalytically inactive [(ItBu)Co(Z -dct)]
species. Indeed, when DCT (6 equiv.) was added to reaction
2
mixture containing COD prior to H addition negligible conver-
sions to COE are observed, whereas the addition of DCT (6 equiv.) Scheme 3 Preparation of complex 5.
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in the presence of H
2
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We acknowledge the financial support by the Deutsche
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