Cobalt-Catalyzed Hydrogenations via Olefin Cobaltate and Hydride Intermediates

Article abstract of DOI:10.1021/acscatal.9b01584

Redox noninnocent ligands are a promising tool to moderate electron transfer processes within base-metal catalysts. This report introduces bis(imino)acenaphthene (BIAN) cobaltate complexes as hydrogenation catalysts. Sterically hindered trisubstituted alkenes, imines, and quinolines underwent clean hydrogenation under mild conditions (2-10 bar, 20-80 °C) by use of the stable catalyst precursor [(^DippBIAN)CoBr₂] and the cocatalyst LiEt₃BH. Mechanistic studies support a homogeneous catalysis pathway involving alkene and hydrido cobaltates as active catalyst species. Furthermore, considerable reaction acceleration by alkali cations and Lewis acids was observed. The dinuclear hydridocobaltate anion with bridging hydride ligands was isolated and fully characterized.

Full text of DOI:10.1021/acscatal.9b01584

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Article
Cobalt-Catalyzed Hydrogenations via
Olefin Cobaltate and Hydride Intermediates
Sebastian Sandl, Thomas Maier, Nicolaas van Leest, Susanne Kröncke, Uttam
Chakraborty, Serhiy Demeshko, Konrad Koszinowski, Bas de Bruin, Franc Meyer,
Michael Bodensteiner, Carmen Herrmann, Robert Wolf, and Axel Jacobi von Wangelin
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01584 • Publication Date (Web): 10 Jul 2019
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ACS Catalysis
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Cobalt-Catalyzed Hydrogenations via Olefin
Cobaltate and Hydride Intermediates
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Sebastian Sandl,a Thomas M. Maier,b Nicolaas P. van Leest,c Susanne Kröncke,a Uttam Chakraborty,a
Serhiy Demeshko,d Konrad Koszinowski,e Bas de Bruin,c Franc Meyer,d Michael Bodensteiner,b
Carmen Herrmann,a Robert Wolfb,* and Axel Jacobi von Wangelina,*
a Department of Chemistry, University of Hamburg, Martin Luther King Pl 6, 20146 Hamburg (Germany)
b Institute of Inorganic Chemistry, University of Regensburg, 93040 Regensburg (Germany)
c van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam (The
Netherlands)
d Institute of Inorganic Chemistry, Universität Göttingen, Tammannstr. 4, 37077 Göttingen (Germany)
e Institute of Organic and Biomolecular Chemistry, Universität Göttingen, Tammannstr. 2, 37077 Göttingen (Germany)
ABSTRACT: Redox non-innocent ligands are a promising tool to moderate electron transfer processes within base-metal catalysts.
This report introduces bis(imino)acenaphthene (BIAN) cobaltate complexes as hydrogenation catalysts. Sterically hindered tri-
substituted alkenes, imines, and quinolines underwent clean hydrogenation under mild conditions (2-10 bar, 20-80°C) by use of the
stable catalyst precursor [(DippBIAN)CoBr2] and the co-catalyst LiEt3BH. Mechanistic studies support a homogeneous catalysis
pathway involving alkene and hydrido cobaltates as active catalyst species. Furthermore, considerable reaction acceleration by
alkali cations and Lewis acids was observed. The dinuclear hydridocobaltate anion with bridging hydride ligands was isolated and
fully characterized.
KEYWORDS: hydrogenation, cobalt, hydrides, metalloradicals, reaction mechanism, redox-active ligands
Introduction
of reduced forms of the catalyst by the ligand, and iv) broad
scope of hydrogenations of unsaturated C=C and C=X.
Metal-catalyzed hydrogenations of alkenes constitute one of the
key chemical transformations with numerous applications to
lab-scale syntheses and industrial manufacture.(1) The
elucidation of the underlying catalytic mechanisms by
Eisenberg, Halpern, Tolman, and others were major scientific
milestones toward the understanding of catalytic elemental
steps and the rational design of more active and selective
catalysts.(1),(2) Very recently, the dominance of hydrogenation
catalysts based on the noble metals Rh, Ru, Ir, Pd, and Pt has
been challenged by the development of highly active 3d
transition metals.(3) While the use of more abundant, cheaper,
and often less toxic base metals constitutes an important
contribution to a more sustainable chemistry, their distinct
reactivity and selectivity was often plagued by undesirable
destructive side reactions.(4) Recently, elaborate ligand design
enabled the development of highly active cobalt catalysts by the
groups of Beller, Budzelaar, Chirik, Hanson, Elsevier, de Bruin,
and others (Figure 1).(5),(6),(7),(8) In the most recent literature, the
implementation of pincer ligands (e.g. NNN; PNP; CNC)
proved pivotal to the control of high activity and selectivity.(9)
Following our previous work on metalates with redox non-
innocent arene ligands,(10),(11) we believed that an efficient 3d
metal catalyst for hydrogenation reactions would fulfill the
following criteria: i) facilitation of redox steps at the metal by a
redox-active ligand; ii) modular ligand design that allows for
convenient synthesis and easy catalyst tuning; iii) stabilization
Figure
1.
Homogeneous
cobalt
catalysts
for
hydrogenations.(8)a,b,c,(10) (Dipp = 2,6-diisopropylphenyl)
H
N
[BArF
]
4
N
N
N
N
Cy2P
PCy2
TMS
2012
this work:
- new catalyst type
Co
N
N
N
Co
R
Co
N
Ar
Ar
R
R
CH3
2013
2005
+
[Li(thf)3.5
]
[K(dme)2]+ Co
- hydrogenation of alkenes,
imines, and quinolines
Dipp N
NDipp
- concise mechanistic study
Co
- isolation of new intermediates
2014
Imine-based ligand architectures constitute a privileged class of
ligands as evidenced by the numerous applications to catalytic
reactions.(7) Simple α-diimine catalysts were first introduced by
tom Dieck and co-workers in 1977.(12) Pincer-type motifs such
as pyridinediimines(13) (PDI) have recently received great
attention. Bis(imino)acenaphthenes(14),(15) (BIANs) are another
class of ligands that fulfill the aforementioned criteria: BIANs
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can be rapidly assembled from commercial precursors on multi- In the absence of H2, 1-octene rapidly isomerized to a mixture
1
2
3
4
5
6
7
8
gram scales and are highly redox-active as they can harbor up
to four electrons.(14)b There are eight reports of BIAN cobalt
complexes with five applications to catalysis.(16) On this basis,
we investigated combinations of BIAN ligands and cobalt salts
toward their ability to form active hydrogenation catalysts.
Documented herein are the benefits of using this simple
catalytic system that presents tangible advances over the current
state-of-the-art that could not have been predicted: Clean
hydrogenations of challenging alkenes (e.g. tetra-substituted),
imines, and heteroarenes proceed under mild conditions.
Mechanistic insight was gained from the isolation of
structurally novel olefin and hydride complexes as potential
catalyst intermediates that are distinct from those of the
traditional noble metal catalysts (Figure 1, bottom).
of octene regioisomers and stereoisomers. With phenyl-
acetylene, slow cyclotrimerization to triphenylbenzene was
observed in low yield (see Supporting Information).(21)
Table 1. Selected optimization experiments.
2) 3 mol% (DippBIAN)CoBr2
procedure
Ph
3) 9 mol% LiEt3BH
A
1)
Ph
9
Ph
Ph
H
2 bar H2
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H
THF, 20 °C, 3 h
Ph
3 mol%
DippBIAN)CoBr2
B
1)
Ph
(
2) 9 mol% LiEt3BH
Ph
3)
Ph
Ph
Results and Discussion
Entry
Deviation from standard conditions
Yield (%) a
92 (93)
41 (50)
75 (75)
23 (33)
64 (65)
1 (12)
Optimization and Alkene Hydrogenation
1
2
3
4
5
6
7
8
9
A: reduction in presence of the substrate
B: substrate addition after reductant
A: 6 mol% LiEt3BH
Initially, we probed the ability of (DippBIAN)CoIIBr2 to act as
pre-catalyst for the hydrogenation of the model substrate
triphenylethylene under very mild conditions (Dipp = 2,6-diiso-
propylphenyl). High conversion was observed with lithium
superhydride (LiEt3BH) as co-catalyst at 2 bar H2 and room
temperature with only 3 mol% (DippBIAN)CoBr2 (Table 1,
procedure A). The presence of olefins during the reduction
proved beneficial for the high catalyst activity, possibly due to
transient olefin coordination and stabilization of the low-valent
catalyst.(6)d,(17),(18) The significantly lower activity of NaEt3BH
suggests a considerable alkali-cation effect (entry 5).†,(19)
Mono-, di- and tri-substituted alkenes were cleanly
hydrogenated under 2-10 bar H2 pressure at room temperature
(Scheme 1).(20) The high efficacy of the developed protocol was
demonstrated in the hydrogenation of challenging tri- and tetra-
substituted alkenes such as myrcene, α-pinene, and α,β,β-tri-
methylstyrene under mild conditions (Scheme 2). Under
standard conditions, the hydrogenation of α-methylstyrene
exhibited a turnover frequency (TOF) of 780 h–1 (Supporting
Information). To the best of our knowledge, this protocol
involves one of the most active homogeneous Co catalysts for
alkene hydrogenations.(8) Reduction-sensitive functional groups
in the alkenes required a different protocol involving addition
of the hydride co-catalyst prior to the alkene (protocol B, see
Table 1, entry 2 and Scheme 2, right). This alternative protocol
B was tolerant to chloride, bromide, ether, and ester functions.
A: 6 mol% NaEt3BH
A: 9 mol% NaEt3BH
A: 9 mol% HBpin + 9 mol% KOtBu
A: (DippBIAN)CoCl2
72 (72)
25 (35)
<1 (9)
A: CoCl2 + 2 DippBIAN
A: w/o reductant
Conditions: 0.2 mmol alkene (1 M, THF), 9 mol% LiEt3BH (1 M,
THF), 3 mol% (DippBIAN)CoBr2, 2 bar H2; a Yields determined by
quantitative GC-FID vs. internal n-pentadecane; conversions in
parentheses.
Methodology Extension: Hydrogenation of Imines
The homogeneous Co-catalyzed hydrogenation of imines(8)b),(10)
and quinolines(8)f,h) is still in its infancy, despite being an atom-
economic route to bioactive amines.(22) Very good conversions
were observed with the same cobalt catalyst in hydrogenations
of selected electron-deficient and electron-rich imines and
quinolines (10 bar H2, 60°C, Scheme 2). Ester functions were
tolerated. Similarly mild conditions were recently reported with
related homogeneous Co catalysts.(8)b,f,h,(10)
Scheme 1. Substrate scope of cobalt-catalyzed hydrogenation of alkenes. Bonds in blue indicate the site of complete π-bond
hydrogenation. Standard conditions: 0.2 mmol alkene/alkyne (1 M, THF), 3 mol% (DippBIAN)CoBr2, 9 mol% LiEt3BH (1 M, THF). Yields
were determined by quantitative GC-FID vs. n-pentadecane. Conversions are given in parentheses if <90%. Procedure A: Catalyst reduction
a
b
in the presence of substrate. Procedure B: Catalyst reduction in the absence of substrate. Traces of α-methylstyrene formed. Traces of
cumene formed. c Conversion <20%.
Procedure A
Procedure B
2 bar, 20°C, 3 h
R = Ph: >99%
10 bar, 60°C, 24 h
10 bar, 20°C, 24 h
10 bar, 20°C, 24 h
Ph R = Ph: 92%
R = Me: >99%
R
O
75% (78%)
(2 bar, 3 h)
R = Me: >99%
Ph
>99% (80°C)
NMe2
R
Ph
R = H: 86%
76%
O
Ph
Ph
R = OTMS: <5%c
>99%
86% (86%)
X = Cl: >99%
2
5
2
X = Br: 80% (81%)
X = I: <5%a,b,c
X = CN: <5%a,c
X = SMe: 16%c
X = OMe: >99%
95%
95%
Ph
H2N
89%
X
Ph
n = 1: >99%
n = 2: 94%
n
18% (33%)
87%
99%
92%
Ph
96%
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Scheme 2. Hydrogenation of imines and quinolines. Blue bonds
indicate the sites of double bond hydrogenation. Conditions:
Procedure A, 0.2 mmol substrate (1 M, THF), mol%
DippBIAN)CoBr2, 9 mol% LiEt3BH (1 M, THF); 10 bar H2, 60 °C,
24 h. GC-FID yields vs. internal n-pentadecane; conversions in
parentheses if <90%. a Procedure B. b 80 °C. c traces of the 5,6,7,8-
tetrahydroquinoline derivative.
The clear distinction between homogeneous and heterogeneous
catalyst species is intricate,(23) yet our observations are
consistent with a homogeneous mechanism. Reaction progress
analyses documented an immediate onset of catalytic activity
and steady conversion, which indicates a zero order for the
substrate in the rate law (Scheme 4, red curve). Thus, the rate-
determining step presumably does not include olefin
coordination. A plot of the initial rates versus catalyst
concentrations showed a first order rate in cobalt.† The absence
of any sigmoidal curvature argues clearly against initial pre-
catalyst nucleation and particle formation.(6) However, an
induction period might be not visible due to the experimental
setup (Procedure B, substrate conversion determined by gas-
uptake; H2 consumption was recorded after pre-catalyst
formation and substrate addition).† Kinetic poisoning studies
are a competent tool to ascertain the topicity of the operating
catalyst species.(23) The attempted amalgamation of the catalyst
with 300 equiv. Hg had only a minimal effect on the reaction
rate. Upon addition of sub-catalytic amounts of
trimethylphosphite (P(OMe)3, 0.3 mol%), partial catalyst
inhibition was recorded. Complete inhibition was achieved at a
catalyst/poison ratio of 1:1 which is consistent with a
homotopic catalyst (Scheme 4, green curve). The selective
homotopic catalyst poison dibenzo[a,e]cyclooctatetraene(25)
(dct, 10 equiv. per Co) resulted in catalyst inhibition which was
slightly diminished by the concomitant hydrogenation of dct as
a competing substrate (Scheme 4, violet curve, 31% conversion
of dct).† The lower efficacy of dct as poison is presumably a
consequence of the lower stability of 3d olefin complexes vs.
their heavier congeners.(18)
1
2
3
4
5
6
7
8
3
(
Ph
Ph
Ph
N
Ph
N
Ph
N
Ph
N
89%a,b
74%
70%
R
9
R = OMe: 83% (isol.)a
R = NMe2: 80% (isol.)a
R = F: 55% (isol.)a
O
10
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Ph
O
N
Ph
N
O
88% (isol.) a
73% (isol.)
R = H: >99%
R
R = Me: 87% b,c R = Me: 67% (72%) a,b,c
R = OMe: 87% b,c R = OMe: 63% (63%) a,b,c
R = COOMe: 38% (43%) a,b
N
Mechanism
The advent of 3d transition metal catalysts has gone hand in
hand with the utilization of ligands that profoundly influence
the electronic properties at the metal ions and enable redox
reactivity patterns that are distinct from those of noble metals
catalysts.(9) The reaction mechanisms of catalytic alkene
hydrogenations with 2nd and 3rd row transition metals (Rh, Ru,
Ir, Pd, Pt) are very well understood. For the classical Rh-
catalyzed hydrogenation, alkene and hydride complexes have
been determined as key catalyst intermediates and the elemental
reaction steps to involve two electron redox events at the
metal.(1),(2),(7),(8) There is much less insight into the
hydrogenation mechanisms of first row transition metals; the
nature of the key catalyst intermediates are still largely
unexplored. Chirik and coworkers reported on a bis(aryl-
imidazol-2-ylidene)pyridine cobalt hydride complex and a
radical pathway that operates in cobalt-catalyzed alkene
hydrogenations.(8)c In this work, we aimed at a concise
mechanistic study of Co-BIAN catalysts in alkene
hydrogenations that would address the following questions: Is
the BIAN ligand redox-active under the reaction conditions?(9)
Are radical pathways operating?(7) To what extent are
heterogeneous catalyst species involved?(23) Do alkene and
hydride intermediates play a similarly important role as with 4d
and 5d metal catalysts?
We commenced our mechanistic studies with a set of key
experiments that addressed the operation of radical mechanisms
and the topicity of the active catalysts species. Initially, radical
probes were evaluated. α-Cyclopropyl styrene underwent dual
alkene hydrogenation and hydrogenative ring-opening to give
2-phenylpentane in excellent yields following protocol A or B,
respectively (Scheme 3, A). This might be indicative of a
mechanism involving hydrogen atom transfer (HAT).(24)
Furthermore, this is in full accord with our observations that
non-styrenic olefins (i.e. alkenes without aryl substituents that
could stabilize potential radical intermediates in benzyl
positions) constitute more difficult substrates under the
standard conditions. Hydrogen atom transfer from the solvent
is rather unlikely as no deuterium incorporation could be
determined from reactions in THF-d8 (Scheme 3, B). The high
activity of the catalyst was further demonstrated by the
Scheme 3. Key mechanistic experiments.
3 mol% DippBIANCoBr2
9 mol% LiBEt3H
A
Ph
Ph
3 h, 20°C, THF (2 mL)
2 bar H2
Procedure A: 95%
Procedure B: >99%
1) 3 mol% DippBIANCoBr2
9 mol% LiBEt3H
2 bar H2, THF-d8, 3 h, 20 °C
0% D
B
C
Ph
Ph
Ph
2) D2O
99%
3 mol% DippBIANCoBr2
9 mol% LiBEt3H
Ph
49% (67%)
24 h, 60°C, THF (2 mL)
10 bar H2
Scheme 4. Catalyst poisoning studies with P(OMe)3, Hg, and
hydrogenation of
(Scheme 3, C).
a C-C-σ-bond in cyclopropylbenzene
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dct. Procedure B. Substrate conversion determined by gas-uptake
and quantitative GC-FID vs. n-pentadecane.
Scheme 5. Cobalt complexes 1, 3 and 4 that were isolated from
reactions of (DippBIAN)CoBr2 and LiBEt3H.
1
2
3
+
[Li(thf)3.5
]
4
5
6
7
8
9
,ls
N

Co
1,5-COD
THF
Dipp
N
Dipp
(17%
crystal
yield)
1
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Br

Br
N
,hs
N

Co
Co
3 LiBEt3H
Dipp
N
Dipp
Dipp
N
Dipp
C6H6
(64%
crystal
yield)
3
Dipp
N
Dipp
N
,hs
Co
,hs 

H
Et2O
(21% crystal yield)
Co
H H
N
N
Dipp
Dipp
Complexes and Catalyst Intermediates
4a
4b
[Li(thf)3(Et2O)]+
[Li(thf)4]+
Based on the initial mechanistic experiments, we postulate a
homotopic mechanism by molecular cobalt catalysts. The
distinct electronic properties of 3d transition metals vs. their
heavier congeners might also entail the participation of catalyst
structures that are different from the Rh(I) catalysts of
hydrogenation reactions. While the operation of alkene and
hydride pathways has been intensively studied in rhodium-
catalyzed hydrogenations, the knowledge of related catalyst
intermediates with cobalt is still rather in its infancy. In an effort
to identify potential catalyst species, we investigated reactions
of (DippBIAN)CoBr2 with 3 equiv. LiEt3BH in THF solution
(Scheme 5). LIFDI-MS (liquid injection field desorption mass
spectrometry) analyses of the crude catalyst mixture displayed
the formation of the low-valent dimer [(DippBIAN)Co]2 which is
structurally related to a complex with two direct cobalt-arene
bonding interactions prepared by Yang and coworkers using a
different diimine.(26) We surmised that the low-valent
monomeric unit (DippBIAN)Co might exhibit catalytic activity
and thus employed several arenes/olefins as labile coordination
placeholders during the reductive dehalogenation of
DippBIANCoBr2. Reduction of DippBIANCoBr2 in THF with 3
equiv. LiBEt3H and excess amounts of 1,5-cyclooctadiene (cod)
led to the formation of [Li(thf)3.5{(DippBIAN)Co(cod)}] (1)
which was isolated after recrystallization in 17% yield.(27),(16)c
This complex is the corresponding Li salt to our previously
described potassium cobaltate (2) and shows similar 1H and 13C
spectra.(16)c Based on literature precedents, the oxidation level
of BIAN in 1 can be assigned as 2– from the crystallographic
bond distances (C-C: 1.389(4) Å; C-N: 1.383(3) Å; Figure
2).(28),(29) In comparison, DippBIANCoBr2 consists of a neutral
BIAN (C-C: 1.513(7) and 1.521(6) Å); C-N: 1.277(7)–
1.286(8) Å).(16)b,(28),(29)
The analogous reduction of (DippBIAN)CoBr2 with 3 equiv.
LiEt3BH in benzene furnished the neutral complex
[(DippBIAN)Co(η6-C6H6)] (3) as dark red single crystals in 64 %
yield. (3 is also formed from the same reaction in a mixture of
1,5-cod and benzene.)(30) Single crystal structure analysis
suggests a radical anion state of the BIAN ligand (C-C:
1.433(2) Å; C-N: 1.3246(19) Å and 1.3224(19) Å, Figure 2),
which was further investigated by EPR.(28),(29) The X-band
spectrum of 3 in toluene glass at 20 K (Figure 3) shows a
rhombic symmetry and was simulated in accordance with an
unpaired electron coupled to a spin 7/2 nucleus. We attribute
this signal to a cobalt-centered radical.† Inclusion of the Euler
angles [-2.0, +90.0, 0] proved to be necessary to align the g and
ACo tensors and provided a more satisfactory simulation of the
measured spectrum. Some linear and quadratic A-strain
parameters have been included to simulate the final line shape.†
Some remaining slight deviations in the line shapes between
simulation and experiment can be attributed to non-perfect glass
formation. The provided simulation allowed for accurate
determination of the g and ACo tensors (MHz): [2.013, 2.145,
2.134] and [+185.0, +406.0, 198.4], respectively. These results
are in agreement with an effective magnetic moment µeff of
1.9 µB (Evans method, C6D6), which is only slightly higher than
the spin-only value for an S = ½ system (µeff = 1.7 µB).
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Figure 2. Molecular structures of 1, 3 and 4b. Thermal ellipsoids
crystallized in a benzene-free synthesis of 1, which might be a
1
2
3
4
5
6
7
8
at the 50% probability level; minor disordered parts, non-
coordinated solvents, selected H atoms and the [Li(thf)4]+ cation of
4b were omitted for clarity.
consequence of a solvent impurity.
Transition metal hydrides are key intermediates in many
synthetic(32) and biological(33) processes. The largest industrial
catalytic processes are hydrogenation reactions that operate via
metal hydride species. Since the landmark studies of
homogeneous Rh-catalyzed hydrogenations,(2) extensive
knowledge of hydridorhodium complexes has been collected
whereas very little is known about the nature and catalytic role
of related intermediates in Co-catalyzed reactions. From a
reaction of (DippBIAN)CoBr2 with 3 equiv. LiEt3BH in Et2O in
a closed reaction vessel, we isolated a structurally unusual
cobalt hydride complex.†,(34) Effervescence was observed
during the reduction, presumably by formation of H2.
Extraction with n-heptane and Et2O afforded the anionic
hydridocobaltate [Li(thf)3(Et2O){(DippBIAN)Co}2(µ-H)3] (4a)
as dark green microcrystals in 23% yield.(35) The closely related
[Li(thf)4]+ (4b) solvate can be isolated in an analogous fashion
by crystallization from THF/n-hexane (Figure 2).† X-ray
diffraction analysis revealed that 4a and 4b show very similar
solid-state molecular structures, thus only the structural
parameters of 4a are given below. The anion of 4a shows three
hydride ligands (located in the electron density Fourier map)
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43
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45
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47
48
49
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53
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60
that bridge two
(
DippBIAN)Co units (Figure 2). The
[Li(thf)3(Et2O)]+ counterion is solvent-separated.(14)b A very
short Co-Co distance presumably is observed due to the
presence of three bridging hydrides: 2.2640(5) Å and 2.2426(3)
for 4a and 4b, respectively. To the best of our knowledge, the
latter is the shortest Co(µ-H)nCo motif known to date (2nd
shortest: 2.249(1) Å).(35)b The Co-H bond distances are between
1.51(2) and 1.63(5) Å. The twist angle between the two CoN2
planes is 54.94(7)°. The NCCN bond lengths of BIAN are
slightly shorter than in 3 (Figure 1; C-N: 1.333(3)–1.349(3) Å,
C-C: 1.412(3)–1.419(3) Å), yet are in good agreement with the
monoanionic BIAN in the complex [(DippBIAN)2Fe] (C-N:
1.3367(15) and 1.3393(15) Å, C-C: 1.4234(18)) Å) which
contains a high-spin Fe2+ that is antiferro-magnetically coupled
to BIAN.(36) Accordingly, the observed bond lengths of the
BIAN ligands in 4 suggest a radical anion state of BIAN which
is supported by theoretical studies (vide infra).(28),(29),(37)
Figure 3. Simulated (blue) and experimental (black) X-band
EPR spectrum of 3. In toluene glass at 20 K. ν = 9.389494 GHz,
microwave power = 1.002 mW, mod. amp. = 1.000 G.
The sum formula of 4a was further verified by negative-ion
mode ESI mass spectrometry (Figure 4, m/z = 1121.4). The
compound proved highly sensitive as unsealed THF solutions
decomposed in an argon-filled glovebox within several hours to
a red-brown paramagnetic mixture presumably by formation of
H2. Direct evidence of such decomposition came from the gas-
phase fragmentation of the mass-selected anionic component of
4a in ESI-MS. Apart from dissociation into its monomeric
subunit [(DippBIAN)CoH2]−, the dinuclear cobaltate readily
underwent dehydrogenation (Figures S25 and S26).
Remarkably, multiple dehydrogenation steps were operative (≥
7). Most likely, the released H-atoms originated from the
bridging hydrides and from the isopropyl groups of the
DippBIANs. The 1H-NMR spectrum of 4a displayed a
characteristic, broad singlet resonance for the three bridging
hydrides at –75.2 ppm (see the Supporting Information for 2D
NMR analyses). This remarkable high-field shift may indicate
an open-shell structure which was further investigated by
Further analysis of 3 included elemental analysis, LIFDI-MS
(m/z = 637.2781), cyclic voltammetry (CV, one reversible
reduction, E = -2.3 V vs. Fc/Fc+) and UV-VIS (C6H6, max
=
481 nm, εmax = 14300 mol-1cm-1L). The combined data point to
a highly unusual electronic structure of complex 3 which is
described as a [(BIAN)CoI(η6-C6H6)] complex that contains a
very rare high-spin Co(I) center.(31) The BIAN radical anion is
(strongly) antiferromagnetically coupled to the S = 1 Co(I) ion,
thus resulting in an effective S = ½ system with the unpaired
electron being primarily located at Co (as detected by EPR).
The cobalt-arene coordination in 3 is not only relevant for the
catalysis protocol, as it is structurally related to
[(DippBIAN)Co]2. Moreover, substrates may coordinate in a
similar way, as most substrates involve a phenyl ring. 3 also co-
1
temperature-dependent H-NMR studies.(38) Remarkably, the
chemical shift of this resonance is temperature-dependent and
shows a strong upfield shift upon decreasing temperature
(Figure 5). A similar behavior is observed for the remaining 1H
NMR resonances.
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Figure 4. ESI-MS of 4a. Negative-ion mode ESI mass spectrum
µeff = 2.1 µB per dimer was determined in solution at 293 K
(Evans method, THF-d8).
The solid-state magnetic behavior of crystalline sample of 4b
1
of 4a (5 mM in THF). Inset: Experimental (black) and simulated
(blue line) isotope pattern of [{(DippBIAN)Co}2H3]−.
2
3
was investigated in the 2–250
K range by SQUID
magnetometry (Figure 6). The MT product was 1.76 cm3 mol–
1 K (or 3.75 µB) at 250 K and decreased to almost zero by
lowering the temperature, indicating overall antiferromagnetic
coupling and a diamagnetic ground state of 4b. The best fit was
achieved using a model of four antiferromagnetically coupled
centers: two BIAN radical anions with S = ½ and two S = 3/2
cobalt(II) ions. The best fit parameters were: g(BIAN) = 2.0
(fixed), g(Co(II)) = 2.08, J(BIAN-Co) = –427 cm–1 and J(Co-
Co) = –17 cm–1.
4
5
6
7
8
9
10
11
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13
14
15
16
17
18
19
20
21
22
23
24
25
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27
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30
31
32
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34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Temperature‐dependence of the product χMꢀT of 4b.
Figure 5. Variable temperature 1H-NMR spectrum of 4a.
Figure 7. Energy diagram of 4 in various spin states. Anion
optimized with BP86/def2-TZVP. Left: Spin density (isosurface
value: 0.001) of the corresponding spin state. Right: Local charges
(light/dark grey: positive/negative) and spins (blue/red: α/β) on
specific fragments of the molecule in different spin states. A local
spin of ½ corresponds to one unpaired electron. * From single-point
calc. on optimized mol. structure in css state (BP86/def2-TZVP).
signals. The observed non-Curie behavior indeed points to an
antiferromagnetic coupling of the cobalt centers with the
diamagnetic ground state at low temperatures. Signal fitting
provided a ratio of the coexisting configurations. Hence, the
paramagnetic configurations are 27% of the singlet at 293 K
(Figure 5).†,(39) The ratio decreased to 0.3% at 193 K
(ΔEtriplet-singlet = 21.4 kJ/mol). An effective magnetic moment
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ACS Catalysis
[Co], poisoning studies).†,(41) Based on the collected synthetic,
Additional analyses of the cobaltate 4a include elemental
analysis (EA), UV-VIS spectroscopy (C6H6, Imax=474 nm,
max=1200 mol-1cm-1L),
and cyclic voltammetry
1
2
spectroscopic, and theoretical data, we propose a homotopic
reaction mechanism that involves cobaltate complexes as active
catalyst species. Rate acceleration by Lewis acids and an alkali-
cation effect were observed.
ε
(THF/[nBu4N]PF6; one reversible reduction was observed E =
3
4
5
–2.4 V vs. Fc/Fc+). The combined data are strongly indicative
of
a
highly unusual electronic structure of the
trihydridodicobaltate anion of 4a,b, which is best described as
[{(DippBIAN)CoII}2(µ-H)3]1, assuming that DippBIAN and each
bridging hydride atom are singly negatively charged,
respectively. DFT calculations suggest a charge of –0.33 for
each of the hydrides and of 0.85 for each of the cobalt centres
(Figure 7). Since charge distributions are typically less
polarized than formal oxidation numbers suggest, this is
compatible with a CoII assignment, even though it does not
exclude CoI. Importantly, DFT confirms the singlet ground
state, this state being both lowest in energy and showing the best
agreement with the X-ray crystallographic structure.(37)
6
7
8
9
Table 2. Hydrogenations with isolated complexes and pre-
catalyst mixtures.
3 mol% cat.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
2 bar H2, THF, 20°C, 3h
Yield [%] a
Entry
Catalyst mixture
cyclo- cyclo-
octene
octane
1b
2
3
4
5
6
7
8
9
(
DippBIAN)CoBr2 + 9 mol% LiBEt3H
96
5
1
61
44
17
33
1
[Li(thf)3.5{DippBIAN)Co(cod)}] 1
1 + 3.5 mol% 12-crown-4
1+ 3 mol% [Fc]PF6
Hydrogenation Activities of Complexes 1-4 and Mechanistic
Proposal
4
2
We evaluated the catalytic activities of the isolated cobalt
complexes 1-4 and various pre-catalyst mixtures in a
hydrogenation model reaction (Table 2). The cobaltate complex
[Li(thf)3.5{DippBIAN)Co(cod)}] (1) was found to be active for
the hydrogenation of 1,5-cyclooctadiene (cod), albeit exhibiting
slightly lower activity than the in situ formed catalyst (entry 2).
Interestingly, 1 could be further activated by addition of 3
equiv. Et3B (entry 5), which may indicate Lewis acid-assisted
catalysis.(40) The borane could facilitate the cleavage of H2 as
demonstrated by Peters and coworkers with a borylcobalt
complex.(40)a The catalytic inactivity of the corresponding
potassium derivative [K(thf){(DippBIAN)Co(cod)}] (2, Table 2,
entry 6) manifested the observed alkali cation effect during our
preliminary optimization experiments (Table 1, entries 1 and
5).†,(19) One possible explanation for this effect is an attractive
non-covalent cationπ interaction. As mainly electrostatic
interaction, the association free enthalpy (ΔH°) for the alkali
metals with benzene follows the trend: Li+ > Na+ > K+. Hence,
the alkali cation can stabilize transition states or bind substrates
(i.e. alkenes, arenes) in proximity to the catalyst.(19) Moreover,
alkali metals are able to tune the redox activity of the ligand.
Mazzanti and coworkers reported on ligand- or metal-based
reduction of cobalt salophen complexes dependent on the alkali
metal.(19)f The group of Holland reported reduced iron dimers
with redox-active formazanate ligands. The dimer, which is
stabilized by cationπ interactions, rearranged in THF solution
to form a five-membered metallacycle with a reactivity order of
Na+ > K+ < Rb+ < Cs+.(19)h
1+ 9 mol% BEt3
[K(thf){(DippBIAN)Co(cod)}] 2
66
-
2+ 3 mol% [Fc]PF6
1
-
2+ 30 mol% LiBr + [2.2.2]Cryptand
2+ 30 mol% LiCl + 3 mol% 18-crown-6
-
2
-
1
10 2+ 9 mol% BEt3
1
1
11 2+ 9 mol% BEt3+ 30 mol% LiBr
12 b [Li(thf)3(Et2O){(DippBIAN)Co}2(µ2-H)3] 4a
13 b 4a + 3 mol% Et3B
14 b 4a + 1.5 mol% LiEt3BH
15 [(DippBIAN)Co(η6-C6H6)] 3
16 c 3+ 9 mol% LiBEt3H
3
38
7
6
22
57
-
22
43
-
4
29
Conditions: 0.2 mmol alkene, 0.1 M in THF, 3 mol% cat., 2 bar
H2. a Yields determined by quantitative GC-FID vs. internal
n-pentadecane; ane = cyclooctane, ene = cyclooctene; b 1.5 mol%
4a; c Reduction in presence of the substrate.
Scheme 6. Related reactivity of 1, 2 and 4b.
2 bar H2
A
1 or 2
+
THF, r.t., 10 min
1: 51% conversion
2: 27% conversion
16%
2%
35%
25%
OH
H
OH
O
19 mol%
4b
+
Ph
B
C
Ph
Ph
Ph
THF-d8, r.t., 1 h
Ph
H
The neutral (benzene)cobalt complex [(DippBIAN)Co(η6-C6H6)]
3 was only active after additional reduction with LiEt3BH
(Table 2, entry 16). Notably, the related 17 valence electron
OH
27%*
*ref. to PhCHO
Ph Ph
55% conversion
28%
33 mol% 4b
+
Ph
Ph
(VE)
complex
[(dppe)Co(cod)]
(dppe
=
THF, r.t., 12 h
Ph
51% (>51%)
traces
1,2-diphenylphosphinoethane) bearing a redox-innocent ligand
is indeed an active pre-catalyst for hydrogenations.(7)d The
hydridocobaltate [Li(thf)3(Et2O){(DippBIAN)Co}2(µ-H)3] (4a)
showed moderate hydrogenation activity which was
significantly enhanced by further reduction with 0.5 equiv. of
LiEt3BH (entries 12 and 14). It may be speculated that the
hydridocobaltate anion present in 4a,b (or related derivatives)
act as catalyst reservoirs for mononuclear hydrides as indicated
by in situ NMR studies.† A catalytic mechanism via multi-
nuclear metal complexes can likely be ruled out (1st order in
The observed alkali cation effect was also evident in the more
effective stoichiometric hydrogenation of 1 vs. 2 (Scheme 6, A).
Preliminary explorations of the reactivity of relevant hydrides
were performed with 4b as model compound: Protolysis
occurred with the strong Brønsted acid HCl in dioxane to give
H2 evolution (2.3 ± 0.1 eq. H2). In the presence of
benzaldehyde, 4b reacted to give 28% benzyl alcohol and 27%
pinacol coupling product (Scheme 6, B). This may indicate the
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competing operation of hydride transfer and single-electron the high electron density in this complex is stabilized by the
1
2
3
4
5
6
7
8
transfer processes from 4b. In the absence of dihydrogen,
incomplete isomerization of (Z)-stilbene to (E)-stilbene was
observed (51%, Scheme 6, C). 4a represents a conceivable
intermediate in our recently published (BIAN)Co-catalyzed
amine-borane dehydrogenation reaction as it affords the same
redox non-innocent BIAN. It is reasonable to assume that the
anion in 4a,b constitutes a catalytically competent off-cycle
intermediate of (BIAN)Co-catalyzed (de)hydrogenation
reactions.(16)e
ASSOCIATED CONTENT
† Supporting Information.
reaction
products
(borazine,
cyclotriaminoborane,
cyclodiaminoborane, H3BNH2-cyclo-B3N3H11, polyborazine
and polyaminoborane).(16), †
The Supporting Information (experimental procedures, analytical
and crystal data of compounds, mechanistic studies, spectra) is
available free of charge via the Internet on the ACS Publication
website at DOI:
9
Conclusion
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11
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Crystal data for (DippBIAN)CoBr2, 1, 3, 4a, 4b with CCDC
1909828, 1909827, 1909829, 1909830, 1909831, respectively.
(CIF).
In summary, this report has established reduced cobalt
complexes as competent catalysts in
a
user-friendly
hydrogenation protocol for challenging alkenes under mild
conditions. The obtained reactivity suggests bidentate BIANs
as interesting alternatives to well-established pincer-type motifs
possessing comparably high activities in cobalt-catalyzed
alkene and imine hydrogenations. Mechanistic studies revealed
considerable alkali cation and Lewis-acid effects. Synthetic,
kinetic, and spectroscopic experiments indicate a mechanism
involving homotopic cobaltate catalysts. Catalytically relevant
cobalt complexes were isolated that document the redox non-
innocence of the BIAN ligand. Especially, the isolation of
trihydridocobaltates 4a,b represents a tangible advance over the
current state-of-the-art of transition metal hydrides. Their
molecular structures show the first reported anionic cobalt
complex with bridging hydride ligands. In contrast to the vast
majority of reported transition metal hydrides bearing
multidentate phosphines, cyclopentadienyl, or carbonyl ligands,
AUTHOR INFORMATION
Corresponding Authors
* E-mail: axel.jacobi@uni-hamburg.de
* E-mail: robert.wolf@ur.de
ACKNOWLEDGMENT
We thank Prof. Jürgen Heck, Prof. Peter Burger, and Dr. Dieter
Schaarschmidt for helpful discussions. Technical assistance by
Max Völker, Felix Seeberger and Ursula Otterpohl is gratefully
acknowledged. This work was financed by the Deutsche
Forschungsgemeinschaft (DFG, JA 1107/6-1, WO 1496/6-1, KK
2875/8-1), the European Research Council (ERC, CoG 683150)
and the Fonds der Chemischen Industrie (T.M.M.)
Chen, Y.; Wang, J.; Su, D.; Tang, M.; Mao, S.; Wang, Y. Cobalt
Encapsulated in N-Doped Graphene Layers: An Efficient and Stable
Catalyst for Hydrogenation of Quinoline Compounds. ACS Catal.
2016, 6, 5816–5822; (c) Jagadeesh, R. V.; Murugesan, K.;
Alshammari, A. S.; Neumann, H.; Pohl, M.-M.; Radnik, J.; Beller, M.
MOF-Derived Cobalt Nanoparticles Catalyze a General Synthesis of
Amines. Science 2017, 358, 326–332; (d) Sandl, S.; Schwarzhuber, F.;
Pöllath, S.; Zweck, J.; Jacobi von Wangelin, A. Olefin-Stabilized
Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations. Chem.
Eur. J. 2018, 24, 3403–3407; (e) Büschelberger, P.; Reyes-Rodriguez,
E.; Schöttle, C.; Treptow, J.; Feldmann, C.; Jacobi von Wangelin, A.;
Wolf, R. Recyclable Cobalt(0) Nanoparticle Catalysts for
Hydrogenations. Catal. Sci. Technol. 2018, 8, 2648–2653.
(7) Reviews: (a) Chirik, P. J. Iron- and Cobalt-Catalyzed Alkene
Hydrogenation: Catalysis with Both Redox-Active and Strong Field
Ligands. Acc. Chem. Res. 2015, 48, 1687–1695; (b) Zell, T.; Langer,
R. From Ruthenium to Iron and Manganese-A Mechanistic View on
Challenges and Design Principles of Base-Metal Hydrogenation
Catalysts. ChemCatChem 2018, 10, 1930–1940; (c) Mukherjee, A.;
Milstein, D. Homogeneous Catalysis by Cobalt and Manganese Pincer
Complexes. ACS Catal. 2018, 8, 11435–11469; (d) Papa, V.; Junge, K.;
Beller, M. Cobalt-Pincer Complexes in Catalysis. Chem. Eur. J. 2019,
25, 122–143; (e) Alig, L.; Fritz, M.; Schneider, S. First-Row Transition
Metal (De)Hydrogenation Catalysis Based On Functional Pincer
Ligands. Chem. Rev. 2019, 119, 2681–2751; (f) Ai, W.; Zhong, R.; Liu,
X.; Liu, Q. Hydride Transfer Reactions Catalyzed by Cobalt
Complexes. Chem. Rev. 2019, 119, 2876–2953.
REFERENCES
(1) (a) Nishimura, S. Handbook of Heterogeneous Catalytic
Hydrogenation for Organic Synthesis; Wiley: New York, 2001. (b) The
Handbook of Homogeneous Hydrogenation. de Vries, J. G.; Elsevier,
C.J., Eds.; Wiley-VCH: Weinheim, 2007.
(2) (a) Meakin, P.; Jesson, J. P.; Tolman, C. A. The Nature of
Chlorotris(triphenylphosphine)Rhodium in Solution and Its Reaction
with Hydrogen. J. Am. Chem. Soc. 1972, 94, 3240–3242; (b) Tolman,
C. A.; Meakin, P. Z.; Lindner, D. L.; Jesson, J. P. Triarylphosphine,
Hydride, and Ethylene Complexes of Rhodium(I) Chloride. J. Am.
Chem. Soc. 1974, 96, 2762–2774; (c) Halpern, J. Mechanistic Aspects
of Homogeneous Catalytic Hydrogenation and Related Processes.
Inorg. Chim. Acta 1981, 50, 11–19; (d) Halpern, J. Mechanism and
Stereoselectivitiy of Asymmetric Hydrogenation. Science 1982, 217,
401-407; (d) Duckett, S. B.; Newell, C. L.; Eisenberg, R. Observation
of New Intermediates in Hydrogenation Catalyzed by Wilkinson’s
Catalyst, RhCl(PPh3)3, Using Parahydrogen-Induced Polarization. J.
Am. Chem. Soc. 1994, 116, 10548–10556.
(3) Bullock, R. M. Catalysis without Precious Metals, Wiley-VCH,
Weinheim, Germany, 2010.
(4) (a) Hess, W.; Treutwein, J.; Hilt, G. Cobalt-Catalysed Carbon-
Carbon Bond-Formation Reactions. Synthesis 2008, 40, 3537–3562;
(b) Holzwarth, M. S.; Plietker, B. Biorelevant Metals in Sustainable
Metal Catalysis-A Survey. ChemCatChem 2013, 5, 1650–1679; (c)
Röse, P.; Hilt, G. Cobalt-Catalysed Bond Formation Reactions; Part 2.
Synthesis 2016, 48, 463–492.
(5) Non-Noble Metal Catalysis: Molecular Approaches and
Reactions. Klein Gebbink, R. J. M.; Moret, M.-E., Eds.; Wiley-VCH:
Weinheim, Germany, 2019.
(6) Efficient heterogeneous Co catalysts have been prepared by
chemical reduction, pyrolysis, or solvothermal synthesis: (a) Chen, F.;
Topf, C.; Radnik, J.; Kreyenschulte, C.; Lund, H.; Schneider, M.;
Surkus, A. E.; He, L.; Junge, K.; Beller, M. Stable and Inert Cobalt
Catalysts for Highly Selective and Practical Hydrogenation of C≡N and
C=O Bonds. J. Am. Chem. Soc. 2016, 138 8781–8788; (b) Wei, Z.;
(8) Selected examples: (a) Knijnenburg, Q.; Horton, A. D.; Van Der
Heijden, H.; Kooistra, T. M.; Hetterscheid, D. G. H.; Smits, J. M. M.;
De Bruin, B.; Budzelaar, P. H. M.; Gal, A. W. Olefin Hydrogenation
Using Diimine Pyridine Complexes of Co and Rh. J. Mol. Catal. A
Chem. 2005, 232, 151–159; (b) Zhang, G.; Scott, B. L.; Hanson, S. K.
Mild and Homogeneous Cobalt-Catalyzed Hydrogenation of C=C,
C=O, and C=N Bonds. Angew. Chem., Int. Ed. 2012, 51 12102–12106;
(c) Yu, R. P.; Darmon, J. M.; Milsmann, C.; Margulieux, G. W.;
Stieber, S. C. E.; Debeer, S.; Chirik, P. J. Catalytic Hydrogenation
ACS Paragon Plus Environment

Page 9 of 12
ACS Catalysis
Activity and Electronic Structure Determination of Bis(arylimidazol-
(d) Formenti, D.; Ferretti, F.; Topf, C.; Surkus, A.-E.; Pohl, M.-M.;
Radnik, J.; Schneider, M.; Junge, K.; Beller, M.; Ragaini, F. Co-Based
Heterogeneous Catalysts from Well-Defined α-Diimine Complexes:
Discussing the Role of Nitrogen. J. Catal. 2017, 351, 79–89; (e) Maier,
T. M.; Sandl, S.; Shenderovich, I. G.; Jacobi von Wangelin, A.;
Weigand, J. J.; Wolf, R. Amine‐Borane Dehydrogenation and Transfer
Hydrogenation Catalyzed by α‐Diimine Cobaltates. Chem. – A Eur. J.
2019, 25, 238–245; (f) Ziegler, C. G. P.; Maier, T. M.; Pelties, S.;
Taube, C.; Hennersdorf, F.; Ehlers, A. W.; Weigand, J. J.; Wolf, R.
Construction of Alkyl-Substituted Pentaphosphido Ligands in the
Coordination Sphere of Cobalt. Chem. Sci. 2019, 10, 1302–1308.
(17) Recent studies from our group showing the beneficial effect of
olefins: (a) Gülak, S.; Jacobi von Wangelin, A. Chlorostyrenes in Iron-
Catalyzed Biaryl Coupling Reactions. Angew. Chem., Int. Ed. 2012, 51,
1357–1361; (b) Gülak, S.; Gieshoff, T. N.; Jacobi von Wangelin, A.
Olefin-Assisted Iron-Catalyzed Alkylation of Aryl Chlorides. Adv.
Synth. Catal. 2013, 355, 2197–2202; (c) Gülak, S.; Stepanek, O.;
Malberg, J.; Rad, B. R.; Kotora, M.; Wolf, R.; Jacobi von Wangelin, A.
Highly Chemoselective Cobalt-Catalyzed Biaryl Coupling Reactions.
Chem. Sci. 2013, 4, 776–784.
(18) (a) Johnson, J. B.; Rovis, T. More than Bystanders: The Effect of
Olefins on Transition-Metal-Catalyzed Cross-Coupling Reactions.
Angew. Chem., Int. Ed. 2008, 47, 840–871; (b) Defieber, C.;
Grützmacher, H.; Carreira, E. M. Chiral Olefins as Steering Ligands in
Asymmetric Catalysis. Angew. Chem., Int. Ed. 2008, 47, 4482–4502.
(19) Cation effects: (a) Collman, J. P.; Finke, R. G.; Cawse, J. N.;
Brauman, J. I. Lewis Acid Catalyzed [RFe(CO)4]- Alkyl Migration
Reactions. A Mechanistic Investigation. J. Am. Chem. Soc. 1978, 100,
4766–4772; (b) Hartmann, R.; Chen, P. Noyori’s Hydrogenation
Catalyst Needs a Lewis Acid Cocatalyst for High Activity. Angew.
Chem., Int. Ed. 2001, 40, 3581–3585. (c) Macchioni, A. Ion Pairing in
Transition-Metal Organometallic Chemistry. Chem. Rev. 2005, 105,
2039–2073; (d) Kennedy, C. R.; Lin, S.; Jacobsen, E. N. The Cation-π
Interaction in Small-Molecule Catalysis. Angew. Chemie Int. Ed. 2016,
55, 12596–12624; (e) Kita, M. R.; Miller, A. J. M. An Ion-Responsive
Pincer-Crown Ether Catalyst System for Rapid and Switchable Olefin
Isomerization. Angew. Chem., Int. Ed. 2017, 56, 5498–5502; (f)
Andrez, J.; Guidal, V.; Scopelliti, R.; Pécaut, J.; Gambarelli, S.;
Mazzanti, M. Ligand and Metal Based Multielectron Redox Chemistry
of Cobalt Supported by Tetradentate Schiff Bases. J. Am. Chem. Soc.
2017, 139, 8628–8638; (g) Neel, A. J.; Hilton, M. J.; Sigman, M. S.;
Toste, F. D. Exploiting Non-Covalent π Interactions for Catalyst
Design. Nature 2017, 543, 637–646; (h) Broere, D. L. J.; Mercado, B.
Q.; Bill, E.; Lancaster, K. M.; Sproules, S.; Holland, P. L. Alkali Cation
Effects on Redox-Active Formazanate Ligands in Iron Chemistry.
Inorg. Chem. 2018, 57, 9580–9591; (i) Yamada, S. Cation−π
Interactions in Organic Synthesis. Chem. Rev. 2018, 118, 11353–
11432; (j) Mahmudov, K. T.; Gurbanov, A. V.; Guseinov, F. I.; Guedes
da Silva, M. F. C. Noncovalent Interactions in Metal Complex
Catalysis. Coord. Chem. Rev. 2019, 387, 32–46.
2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes. J. Am.
Chem. Soc. 2013, 135, 13168–13184; (d) Friedfeld, M. R.; Margulieux,
G. W.; Schaefer, B. A.; Chirik, P. J. Bis(phosphine)cobalt Dialkyl
Complexes for Directed Catalytic Alkene Hydrogenation. J. Am.
Chem. Soc. 2014, 136, 13178–13181; (e) Rösler, S.; Obenauf, J.;
Kempe, R. A Highly Active and Easily Accessible Cobalt Catalyst for
Selective Hydrogenation of C=O Bonds. J. Am. Chem. Soc. 2015, 137,
7998–8001; (f) Xu, R.; Chakraborty, S.; Yuan, H.; Jones, W. D.
Acceptorless, Reversible Dehydrogenation and Hydrogenation of N-
Heterocycles with a Cobalt Pincer Catalyst. ACS Catal. 2015, 5, 6350–
6354; (g) Korstanje, T.J.; van der Vlugt, J.I.; Elsevier, C.J.; de Bruin,
B. Hydrogenation of Carboxylic Acids with a Homogeneous Cobalt
Catalyst. Science 2015, 350 (6258), 298‑302; (h) Adam, R.; Cabrero-
Antonino, J. R.; Spannenberg, A.; Junge, K.; Jackstell, R.; Beller, M.
A General and Highly Selective Cobalt-Catalyzed Hydrogenation of N-
Heteroarenes under Mild Reaction Conditions. Angew. Chem., Int. Ed.
2017, 56, 3216–3220.
(9) (a) Lyaskovskyy, V.; de Bruin, B. Redox Non-Innocent Ligands:
Versatile New Tools to Control Catalytic Reactions. ACS Catal. 2012,
2 270–279; (b) Luca, O. R.; Crabtree, R. H. Redox-Active Ligands in
Catalysis. Chem. Soc. Rev. 2013, 42, 1440–1459.
(10) (a) Gärtner, D.; Welther, A.; Rad, B. R.; Wolf, R.; Jacobi von
Wangelin, A. Heteroatom-Free Arene-Cobalt and Arene-Iron Catalysts
for Hydrogenations. Angew. Chem., Int. Ed. 2014, 53, 3722–3726; (b)
Büschelberger, P.; Gärtner, D.; Reyes-Rodriguez, E.; Kreyenschmidt,
F.; Koszinowski, K.; Jacobi von Wangelin, A.; Wolf, R. Alkene
Metalates as Hydrogenation Catalysts. Chem. Eur. J. 2017, 23, 3139–
3151.
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(11) Schnöckelborg, E. M.; Khusniyarov, M. M.; De Bruin, B.; Hartl,
F.; Langer, T.; Eul, M.; Schulz, S.; Pöttgen, R.; Wolf, R. Unraveling
the Electronic Structures of Low-Valent Naphthalene and Anthracene
Iron Complexes: X-Ray, Spectroscopic, and Density Functional
Theory Studies. Inorg. Chem. 2012, 51, 6719–6730.
(12) tom Dieck, H.; Bruder, H. Bis(diazadiene)Iron Complexes,
(R1N=CR2–CR2=NR1)2Fe. J. Chem. Soc., Chem. Commun. 1977, 24–
25.
(13) Flisak, Z.; Sun, W. H. Progression of Diiminopyridines: From
Single Application to Catalytic Versatility. ACS Catal. 2015, 5, 4713–
4724.
(14) (a) Hill, N. J.; Vargas-Baca, I.; Cowley, A. H. Recent
Developments
in
the
Coordination
Chemistry
of
Bis(imino)acenaphthene (BIAN) Ligands with s- and p-Block
Elements. Dalton Trans. 2009, 0, 240–253; (b) Fedushkin, I. L.;
Skatova, A. A.; Chudakova, V. A.; Fukin, G. K. Four-Step Reduction
of Dpp-Bian with Sodium Metal: Crystal Structures of the Sodium Salts
of the Mono-, Di-, Tri- and Tetraanions of Dpp-Bian. Angew. Chem.,
Int. Ed. 2003, 42, 3294–3298.
(15) Selected examples of hydrogenations with BIAN: (a) van Asselt,
R.; Elsevier, C. J. Homogeneous Catalytic Hydrogenation of Alkenes
by Zero-Valent Palladium Complexes of Cis-Fixed Dinitrogen
Ligands. J. Mol. Catal. 1991, 65, L13–L19; (b) Van Laren, M. W.;
Elsevier, C. J. Selective Homogeneous Palladium(0)-Catalyzed
Hydrogenation of Alkynes to (Z)-Alkenes. Angew. Chem., Int. Ed.
1999, 38 (24), 3715–3717; (c) Villa, M.; Miesel, D.; Hildebrandt, A.;
Ragaini, F.; Schaarschmidt, D.; Jacobi von Wangelin, A. Synthesis and
Catalysis of Redox-Active Bis(imino)acenaphthene (BIAN) Iron
Complexes. ChemCatChem 2017, 9, 3203–3209.
(16) Selected (BIAN)Co reports: (a) Khusniyarov, M. M.; Harms, K.;
Burghaus, O.; Sundermeyer, J. Molecular and Electronic Structures of
Homoleptic Nickel and Cobalt Complexes with Non-Innocent Bulky
Diimine Ligands Derived from Fluorinated 1,4-Diaza-1,3-Butadiene
(DAD) and Bis(arylimino)acenaphthene (BIAN). Eur. J. Inorg. Chem.
2006, 2985–2996; (b) Rosa, V.; Carabineiro, S. A.; Avilés, T.; Gomes,
P. T.; Welter, R.; Campos, J. M.; Ribeiro, M. R. Synthesis,
Characterisation and Solid State Structures ofꢀ α-Diimine Cobalt(II)
Complexes: Ethylene Polymerisation Tests. J. Organomet. Chem.
2008, 693, 769–775; (c) Pelties, S.; Maier, T.; Herrmann, D.; De Bruin,
B.; Rebreyend, C.; Gärtner, S.; Shenderovich, I. G.; Wolf, R. Selective
P4 Activation by a Highly Reduced Cobaltate: Synthesis of Dicobalt
Tetraphosphido Complexes. Chem. - A Eur. J. 2017, 23, 6094–6102.
(20) α-Methylstyrene was hydrogenated under standard conditions in
the presence of various functional additives (1 equiv., protocol B, 3 h,
20 °C, THF (2 mL)): No decrease in hydrogenation activity was
observed with added PhNH2, whereas reduced activity was observed
with 4-Tol-CH2OH, PhC(O)Ph, respectively. No conversion was
obtained in the presence of PhCN, PhC(O)H, PhNO2, respectively.†
(21) Brenna, D.; Villa, M.; Gieshoff, T. N.; Fischer, F.; Hapke, M.;
Jacobi von Wangelin, A. Iron-Catalyzed Cyclotrimerization of
Terminal Alkynes by Dual Catalyst Activation in the Absence of
Reductants. Angew. Chem., Int. Ed. 2017, 56, 8451–8454.
(22) Muthukrishnan, I.; Sridharan, V.; Menéndez, J. C. Progress in the
Chemistry of Tetrahydroquinolines. Chem. Rev. 2019, 119, 5057–
5191.
(23) (a) Widegren, J. A.; Finke, R. G. A review of the problem of
distinguishing true homogeneous catalysis from soluble or other metal-
particle heterogeneous catalysis under reducing conditions. J. Mol.
Catal. A 2003, 198, 317–341; (b) Astruc, D.; Lu, F.; Aranzaes, J. R.
Nanoparticles as Recyclable Catalysts: The Frontier between
Homogeneous and Heterogeneous Catalysis. Angew. Chem., Int. Ed.
2005, 44, 7852–7872; (c) Crabtree, R. H., Resolving Heterogeneity
Problems and Impurity Artifacts in Operationally Homogeneous
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Transition Metal Catalysts. Chem. Rev. 2012, 112, 1536–1554. (d)
(33) Reviews: (a) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E.
Drost, R.M.; Rosar, V.; Dalla Marta, S.; Lutz, M.; Demitri, N.; Milani,
B.; de Bruin, B.; Elsevier, C.J., Pd-Catalyzed Z-Selective
Semihydrogenation of Alkynes: Determining the Type of Active
Species. ChemCatChem, 2015, 7, 2095–2107.
Hydrogenases. Chem. Rev. 2014, 114, 4081–4148; (b) Schilter, D.;
Camara, J. M.; Huynh, M. T.; Hammes-Schiffer, S.; Rauchfuss, T. B.
Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal
Hydrides. Chem. Rev. 2016, 116, 8693–8749.
(34) It is noteworthy that the reaction needs to be performed in a
closed reaction vessel as reported by Finke and co-workers: Laxson,
W. W.; Özkar, S.; Folkman, S.; Finke, R. G. The Story of a Mechanism-
Based Solution to an Irreproducible Synthesis Resulting in an
Unexpected Closed-System Requirement for the LiBEt3H-Based
Reduction: The Case of the Novel Subnanometer Cluster, [Ir(1,5-
COD)(μ-H)]4, and the Resulting Improved, Independently Repeatable,
Reliable Synthesis. Inorg. Chim. Acta 2015, 432, 250–257.
(35) Related hydrides: (a) Fryzuk, M. D.; Ng, J. B.; Rettig, S. J.;
Huffman, J. C.; Jonas, K. Nature of the Catalytically Inactive Cobalt
Hydride Formed upon Hydrogenation of Aromatic Substrates.
Structure and Characterization of the Binuclear Cobalt Hydride
[{iPr2P(CH2)3PPri2}Co]2(H)(µ-H)3. Inorg. Chem. 1991, 30, 2437–
2441; (b) [{(Cp*)Co}2(µ2-H)3]: Kersten, J. L.; Rheingold, A. L.;
Theopold, K. H.; Casey, C. P.; Widenhoefer, R. A.; Hop, C. E. C. A.
“[Cp*Co=CoCp*]” Is a Hydride. Angew. Chem., Int. Ed. 1992, 31,
1341–1343; (c) [(NacNac)Co(µ2-H)]2 and K2[(NacNac)Co(µ-H)]2:
Ding, K.; Brennessel, W. W.; Holland, P. L. Three-Coordinate and
Four-Coordinate Cobalt Hydride Complexes That React with
Dinitrogen. J. Am. Chem. Soc. 2009, 131, 10804–10805; d)
LiEt3BH∙[(dippe)Rh(µ2-H)]2: Swartz, B. D.; Ateşin, T. A.;
Grochowski, M. R.; Oster, S. S.; Brennessel, W. W.; Jones, W. D.
Unusual Lithium Coordinated Platinum and Rhodium Hydride Dimers.
Inorg. Chim. Acta 2010, 363, 517–522.
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(24) (a) Kinetics and mechanism of the hydrogenation of α-
cyclopropylstyrene: Rate constant of the ring-opening rearrangement
of the corresponding radical: 3.6 × 105 s–1 at 22 °C in hexane solution:
Bullock, M. R.; Samsel, E. G. Hydrogen Atom Transfer Reactions of
Transition-Metal Hydrides by Metal Carbonyl Hydrides. J. Am. Chem.
Soc. 1990, 112, 6886–6898; (b) Choi, J.; Tang, L.; Norton, J. R.
Kinetics of Hydrogen Atom Transfer from (η5-C5H5)Cr(CO)3H to
Various Olefins: Influence of Olefin Structure. J. Am. Chem. Soc. 2007,
129, 234–240. (c) de Bruin, B.; Dzik, W.I.; Li, S.; Wayland, B.B.
Hydrogen-Atom Transfer in Reactions of Organic Radicals with
[CoII(por)]. (por=Porphyrinato) and in Subsequent Addition of
[Co(H)(por)] to Olefins. Chem. Eur. J. 2009, 15, 4312-4320.
(25) (a) Anton, D. R.; Crabtree, R. H. Dibenzo[a,e]cyclooctatetraene
in a Proposed Test for Heterogeneity in Catalysts formed from Soluble
Platinum-group Metal Complexes. Organometallics 1983, 2, 855-859;
(b) Franck, G.; Brill, M.; Helmchen, G. Dibenzo[a,e]cyclooctene:
Multi-gram Synthesis of a Bidentate Ligand. Org. Synth. 2012, 89, 55-
65; (c) Sandl, S; Jacobi von Wangelin, A. Dibenzo[a,e]cyclooctatetra-
ene in Encyclopedia of Organic Reagents. John Wiley & Sons, New
York, USA, 2019.
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(26) Yang, X.-J.; Fan, X.; Zhao, Y.; Wang, X.; Liu, B.; Su, J.-H.;
Dong, Q.; Xu, M.; Wu, B. Synthesis and Characterization of Cobalt
Complexes with Radical Anionic α-Diimine Ligands. Organometallics
2013, 32, 6945–6949.
(27) Examples of α-diimine cobaltates: (a) Li[(α-diimine)Co(cod)]:
Döring, M.; Uhlig, E.; Taldbach, T. Zur Reaktion von [Li(TMED)2]
[Co(COD)2] mitꢀπ-Akzeptorliganden. Z. Anorg. Allg. Chem. 1991, 600,
163–167; (b) K[(bipy)Co(cod)]: Brennessel, W. W.; Ellis, J. E.
Naphthalene and Anthracene Cobaltates(1-): Useful Storable Sources
of an Atomic Cobalt Anion. Inorg. Chem. 2012, 51, 9076–9094; (c)
Na/Co/α-diimine/polyarene complexes: Wang, X.; Zhao, Y.; Gong, S.;
Liu, B.; Li, Q.-S.; Su, J.-H.; Wu, B.; Yang, X.-J. Mono- and Dinuclear
Heteroleptic Cobalt Complexes with α-Diimine and Polyarene
Ligands. Chem. Eur. J. 2015, 21, 13302–13310.
(28) Ray, K.; Petrenko, T.; Wieghardt, K.; Neese, F. Joint
Spectroscopic and Theoretical Investigations of Transition Metal
Complexes Involving Non-Innocent Ligands. Dalton Trans. 2007,
1552–1566.
(29) For average bond distances of BIAN from the literature, see ref.
(16)c: BIAN0: C–N 1.28, C–C 1.49; BIAN1–: C–N 1.34, C–C 1.44;
BIAN2–: C–N 1.39, C–C 1.40.
(36) Fedushkin, I. L.; Skatova, A. A.; Khvoinova, N. M.; Lukoyanov,
A. N.; Fukin, G. K.; Ketkov, S. Y.; Maslov, M. O.; Bogomyakov, A.
S.; Makarov, V. M. New High-Spin Iron Complexes Based on
Bis(Imino)Acenaphthenes (BIAN): Synthesis, Structure, and Magnetic
Properties. Russ. Chem. Bull. 2013, 62, 2122–2131.
(37) Based on molecular structure optimizations of the anion of 4a,b
with the BP86 functional and Ahlrichs’ def2-TZVP basis set, the
closed-shell singlet (css) represents the lowest energy state, with the
triplet (t) being the next highest in energy (ΔE(t-css) = 30.8 kJ/mol),
followed by the quintet state (q) (ΔE(q-css) = 50.2 kJ/mol).† Single-
point energy calculations of the open-shell singlet state (oss) on the
triplet molecular structure converged to the closed-shell singlet state
(css). The highest energy state is the septuplet (sp) (ΔE(sp-css) = 297.2
kJ/mol), which due to convergence issues is evaluated as a single-point
energy calculation on the optimized molecular structure of the css.
Molecular structure optimizations with the TPSSH functional and
Ahlrichs’ def2-TZVP basis set predict the triplet state to be the lowest
energy state (ΔE(t-css) = 22.5 kJ/mol), followed by the quintet state
(ΔE(q-css) = 10.7 kJ/mol).† From local charges and spins obtained
from BADER population analysis, the cobalt ions in 4 are predicted to
be singly positively charged for both the BP86 and TPSSH functional.†
The three bridging hydrides summed up are singly negatively charged
in total for either functional. The oxidation state of the BIAN core
remains ambiguous, as its charge ranges between –1.15 (css) and –2.23
(t), and between –2.19 (oss) and –2.85 (t) for the BP86 and the TPSSH
functional, respectively. However, the overall ligand fragment
(30) Similar (η6-arene)Co complexes: (a) [(NacNac)Co(arene)]: Dai,
X.; Kapoor, P.; Warren, T. H. [Me2NN]Co(η6-Toluene): O=O, N=N,
and O=N Bond Cleavage Provides β-Diketiminato Cobalt μ-Oxo and
Imido Complexes. J. Am. Chem. Soc. 2004, 126, 4798–4799; (b)
[(α-diimine)Co(arene)] and [(α-diimine)Co]2: see ref. (26); (c) Cobalt
α-diimine and polyarene complexes: see ref.: (27)c; (d)
[(NacNac)Co(arene)]: Chen, C.; Hecht, M. B.; Kavara, A.; Brennessel,
W. W.; Mercado, B. Q.; Weix, D. J.; Holland, P. L. Rapid,
Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt
Catalyst. J. Am. Chem. Soc. 2015, 137, 13244–13247; (e)
[(α-diimine)Fe(arene)] Complexes as Inactive Olefin Hydrogenation
precatalysts: Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J.
Low-Valent α-Diimine Iron Complexes for Catalytic Olefin
Hydrogenation. Organometallics 2005, 24, 5518–5527.
(31) Krzystek, J.; Ozarowski, A.; Zvyagin, S.A.; Telser, J. High Spin
Co(I): High-Frequency and -Field EPR Spectroscopy of CoX(PPh3)3
(X = Cl, Br). Inorg. Chem., 2012, 51, 4954–4964.
(32) Reviews: (a) Moore, D. S.; Robinson, S. D. Hydrido Complexes
of the Transition Metals. Chem. Soc. Rev. 1983, 12, 415–452; (b)
Darensbourg, M. Y.; Ash, C. E. Anionic Transition Metal Hydrides.
Adv. Organomet. Chem. 1987, 27, 1–50; (c) Robinson, S. J. C.;
Heinekey, D. M. Hydride & Dihydrogen Complexes of Earth Abundant
Metals: Structure, Reactivity, and Applications to Catalysis. Chem.
Commun. 2017, 53, 669–676.
[
DippBIAN] turns out to be roughly singly negatively charged in all
cases except for the septuplet.
(38) Knijnenburg, Q.; Hetterscheid, D.; Martijn Kooistra, T.;
Budzelaar,
P.
H.
M.
The
Electronic
Structure
of
(Diiminopyridine)cobalt(I) Complexes. Eur. J. Inorg. Chem. 2004,
1204–1211.
(39) Bachmann, B.; Hahn, F.; Heck, J.; Wünsch, M. Cooperative
Effects In π-Ligand Bridged Binuclear Complexes. 8. Cyclic
Voltammetric, NMR, and ESR Spectroscopic Studies of Electron-Poor
Synfacial
Bis((η5-cyclopentadienyl)metal)
μ-Cyclooctatetraene
Complexes of Chromium and Vanadium. Organometallics 1989, 8,
2523–2543.
(40) Lewis-Acid Effects in Hydrogenation Reactions: (a) Lin, T.-P.;
Peters, J. C. Catalytic Boryl-Mediated Reversible H2 Activation at
Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer
Hydrogenation. J. Am. Chem. Soc. 2013, 135, 15310–15313; (b) Maity,
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ACS Catalysis
A.; Teets, T. S. Main Group Lewis Acid-Mediated Transformations of
Ltd, Amsterdam, Netherlands, 2001; pp 299–327; (b) Siedschlag, R.
B.; Bernales, V.; Vogiatzis, K. D.; Planas, N.; Clouston, L. J.; Bill, E.;
Gagliardi, L.; Lu, C. C. Catalytic Silylation of Dinitrogen with a
Dicobalt Complex. J. Am. Chem. Soc. 2015, 137, 4638–4641; (c)
Gieshoff, T. N.; Chakraborty, U.; Villa, M.; Jacobi von Wangelin, A.
Alkene Hydrogenations by Soluble Iron Nanocluster Catalysts. Angew.
Chem., Int. Ed. 2017, 56, 3585–3589; (d) Powers, I. G.; Uyeda, C.
Metal-Metal Bonds in Catalysis. ACS Catal. 2017, 7, 936–958; (e)
Chakraborty, U.; Reyes-Rodriguez, E.; Demeshko, S.; Meyer, F.;
Jacobi von Wangelin, A. A Manganese Nanosheet: New Cluster
Topology and Catalysis. Angew. Chem., Int. Ed. 2018, 57, 4970–4975.
Transition-Metal Hydride Complexes. Chem. Rev. 2016, 116, 8873–
8911; (c) Tokmic, K.; Jackson, B. J.; Salazar, A.; Woods, T. J.; Fout,
A. R. Cobalt-Catalyzed and Lewis Acid-Assisted Nitrile
Hydrogenation to Primary Amines: A Combined Effort. J. Am. Chem.
Soc. 2017, 139, 13554–13561; (d) Léonard, N. G.; Chirik, P. J. Air-
Stable α-Diimine Nickel Precatalysts for the Hydrogenation of
Hindered, Unactivated Alkenes. ACS Catal. 2018, 8, 342–348.
(41) (a) Oro, L. A.; Sola, E. Mechanistic Aspects of Dihydrogen
Activation and Catalysis by Dinuclear Complexes in Recent Advances
in Hydride chemistry; Peruzzini, M.; Poli, R., Eds.; Elsevier Science
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Table of Contents (TOC)
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9
Lewis-acid
effect
effect
Br
Br
N
cation

Alkali
R2
Co
R2
Dipp
N Dipp
H
R3
R1
R1
R3
3 mol%
H2
+
H
R4
R4
Dipp = 2,6-diisopropylphenyl
1,5-COD
9 mol% LiBEt3H
Et2O
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C6H6
Dipp
Dipp
N
N
,hs
,hs

Co

H
,hs
N

Co
,ls

Co
Co
Dipp
N
Dipp
H H
N
N
Dipp
N
N Dipp
Dipp
Active
Dipp
+
]
[Li(thf)3.5
[Li(solv)]+
BIAN1–: BP86/def2-TZVP; XRD
Co(II, hs): SQUID; XRD
Antiferromagnetic coupling Co–Co; Co–BIAN
Active?
Active
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