Competing H2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition

Article abstract of DOI:10.1021/jacs.0c00758

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H₂ upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L^Ph]^3- with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L^Ph(Ni^II-H)₂]^- (3) are prone to intramolecular reductive H₂ elimination, and an equilibrium between 3 and orthometalated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal-metal cooperative phenyl group C(sp²)-H oxidative addition to the dinickel(I) intermediate [L^PhNi^I₂]^- (4). While Ni^I species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni^I complexes has rarely been described. The reversible H₂ and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [L^PhNi^I₂]^- intermediate 4 either via elimination of H₂ from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H₂ byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.

Full text of DOI:10.1021/jacs.0c00758

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Article
Competing H2 versus Intramolecular C-H Activation at a Dinuclear
Nickel Complex via Metal-Metal Cooperative Oxidative Addition
Hendrik Stevens, Peng-Cheng Duan, Sebastian Dechert, and Franc Meyer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00758 • Publication Date (Web): 12 Mar 2020
Downloaded from pubs.acs.org on March 12, 2020
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Nickel Complex via Metal-Metal Cooperative Oxidative Addition
Hendrik Stevens , Peng-Cheng Duan , Sebastian Dechert and Franc Meyer*
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‡
‡§
Universität Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, D-37077 Göttingen, Germany
KEYWORDS Nickel, Hydride Complexes, Bimetallic Complexes, Metal-Metal Cooperativity, C-H Activation.
ABSTRACT: Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too
unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds
Ph 3−
and reductively eliminate H
m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L (Ni -H)
reductive H elimination, and an equilibrium between 3 and orthometallated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown
to form via intramolecular metal-metal cooperative phenyl group C(sp )-H oxidative addition to the dinickel(I) intermediate [L Ni
2
upon substrate binding. Here we present a pyrazolate-based bis(-diketiminato) ligand [L ] with bulky
Ph
II
−
2
] (3) are prone to intramolecular
2
2
Ph
I −
2
] (4);
while Ni species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni complexes has rarely been
described. The reversible H and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron
I
I
2
reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of
interconversion of 2 and 3, and also for subsequent thermally driven formation of a twice orthometallated dinickel(II) complex 6. X-ray
crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows
Ph
I −
for unmasking of a highly reactive [L Ni
2
] intermediate 4 either via elimination of H
2
from dihydride 3 or via reductive C-H elimination
from monohydride 2. The latter does not release any H
2
byproduct and adds a distinct platform for metal-metal cooperative two-electron
substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.
INTRODUCTION
Scheme 1. Selected Transformations of β-Diketiminato-
Ligated Ni (II, III) and Ni Hydride (I) Complexes.
Molecular nickel hydride complexes are currently receiving much
attention because of their ability to transfer hydrides, hydrogen
atoms or protons during reactions with substrate molecules. While
I
II
17–19
1
much of the recent interest is fueled by the relevance of a Ni-H unit
N
N
in the catalytic cycle of the natural metalloenzyme [NiFe]
NiI
2–8
+ H2
2
hydrogenase and in bioinspired complexes for H generation,
NiI
nickel hydrides are considered as key intermediates also in a variety
1
of other Ni-catalyzed processes. Treatment of nickel(II) halide
N
N
complexes with borohydride reagents or oxidative addition of H
2
II
or C-H bonds to nickel(0) precursors are among the various
N
N
N
1
H
H
methods to generate the Ni-H moieties.
NiII
NiII
N
Many nickel hydrides show a propensity to lose their metal-bound
2
hydrogen, mostly in a reductive fashion giving H . While this is a
1
major cause of instability of nickel hydride complexes, the
resulting low-valent nickel species may give rise to interesting
subsequent reactivity. Nickel(I) metalloradicals are quite strong
one-electron reductants whose isolation often is challenging, and
that bear great potential for, e.g., the activation of small
I
N
toluene, 
- H2
Ni
N
N
N
Ni
9
,10
molecules. This is increasingly being exploited for the reductive
transformation of unsaturated substrates with double or triple
III
1
1
12,13
bonds, or for cross-coupling chemistry.
Though monovalent
nickel has also been implicated in catalytic C-H bond
I
functionalization, however, activation of C-H bonds at Ni has
N-aryl substituted β-diketiminato (nacnac) ligands such as the ones
shown in Scheme 1 have been extensively used for supporting
14–16
rarely been evidenced.
Similar to the oxidative addition of H
2
2
0–23
to give two Ni-H units, many of the above reactions involve two
Ni entities to enable two-electron chemistry. As an example,
reactive metal complexes in a variety of oxidation states.
Fine-
I
tuning of the ligand scaffold via modification of the peripheral
bulky aryl groups or the backbone substituents allows for
Scheme 1 shows the synthesis of nickel(II) hydride complex I from
1
7
22
the nickel(I) precursor II and H
2
,
and the thermal loss of H
2
from
stabilizing reactive intermediates, which has been favorably
I
21,24,25
I to give dinickel complex III which has been described as a
exploited in Ni chemistry.
Dinucleating scaffolds composed
I
18,19
toluene-masked Ni species.
of two -diketiminato have been developed for enabling metal-
metal cooperativity, such as a pyridine-based ligand that afforded a
I
II
hydride-bridged mixed-valent Ni (-H)Ni species IV (Scheme
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).2 Recently we introduced a trianionic dinucleating ligand
6
Ph
2
2
yields the dinuclear complex L Ni Br (1) after several washings
iPr 3−
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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5
5
5
6
[L ] that has two β-diketiminato compartments appended to a
with acetone. 1 was obtained in high purity and 56% yield (Scheme
2
7
central pyrazolate bridge. In its dinickel(II) dihydride complex V
Scheme 2) the two terminal hydrides are directed into the
bimetallic cleft, and experimental as well as computational studies
indicated low barriers for intramolecular reductive H elimination
3).
(
Scheme 3. Preparation of precursor complex 1 and its
transformation to the dinickel monohydride complex 2-K
and dihydride complex 3-K.
2
+
+
+
that depend on the cation present, viz. Na , K or [K(DB18C6]
DB18C6 = dibenzo[18]crown-6). Hence V can be viewed as a
masked form of the dinickel(I) species VI, and indeed V was found
to undergo pairwise H /D exchange and to reductively activate a
variety of small molecule substrates within the bimetallic pocket
concomitant with H release (Scheme 2; XY is, e.g., HCCPh, O
Storing reducing equivalents in two adjacent
metal hydrides and unmasking the highly reactive intermediate via
elimination upon substrate binding is an attractive option for
(
N
N
1
. 3 n-BuLi
. 2 (dme)NiBr2
N
N
N
2
2
N
H
N
N
N
2
H
H
NiII
NiII
N
N
Br PhN
THF
,
Ph
Ph Ph
Ph
2
2
0
1
2
3
4
5
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0
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2
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0
1
2
3
4
5
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7
8
9
0
Ph
-
78°C  50°C
Ph
Ph
2
7–30
NO or PhNO).
1
8 h
56%
H3LPh
1
H
2
KHBEt3
THF, rt, 2h
KC8
THF, rt, 6h
72%
circumventing the direct formation and isolation of potentially
unstable intermediates with reduced metal ions (such as a
dinickel(I) species). In fact, such scenario is reminiscent of the
operating mode of the metalloenzyme nitrogenase whose FeMo
cofactor first accommodates protons and electrons to form metal
64%
K+
K+
H2 (1 atm.)
THF, rt, <1 min
N
N
N
N
N
N
NiII
N
N
N
H
N
H
N
quant.
NiII
NiII
NiII
Ph
2 2
hydrides and then couples N reduction to the elimination of H .
N
H
no H2 atm.
Ph
Ph
Ph
Ph
Ph
Ph
-
H2
Scheme 2. Bimetallic ligand scaffolds containing N-aryl
substituted β-diketiminato sidearms and their nickel
hydrido complexes26,27
3-K
2-K
Suitable crystals for X-ray diffraction analysis were obtained by
slow evaporation of a saturated dichloromethane solution of 1. The
molecular structure is shown in Figure 1 (top), and selected metric
parameters are listed in Table 1; further bond lengths and angles
can be found in the Supporting Information. 1 features the expected
bimetallic core that has the two nickel(II) ions hosted in the ligand’s
N
N
N
N
- H2
H2
N
N
N
N
N
N
N
H
N
H
NiII
NiII
NiI
NiI
+
3
tridentate {N } compartments and bridged by the pyrazolate unit,
V
VI
with a Ni∙∙∙Ni separation of 3.781(5) Å. The exogenous bromido
ligand is found within the bimetallic cleft, which gives rise to a
distorted square-planar coordination environment of the two metal
ions. The overall structure is very similar to that of the
corresponding complex L Ni Br, though the shielding of the
dinickel core and of the central bromide is even more pronounced
than in previously reported L Ni Br (Figure 1, bottom). NMR
2
spectroscopy confirms that 1 is a diamagnetic compound in
accordance with low-spin Ni ions, and that it has apparent C2v
symmetry in solution.
XY
iPr
27
N
N
N
N
N
2
N
N
N
X
N
Y
NiII
NiII
Ni
Ni
iPr
N
H
N
II
IV
VII
In the present contribution we now report a modified pyrazolate-
Ph 3−
based compartmental ligand [L ] in which the peripheral 2,6-
iPr 3−
diisopropyl substituents of [L ] are replaced by bulky m-
terphenyl groups, and we evidence that this new ligand system can
mask its reactive dinickel(I) species either via H addition to give a
2
dinickel(II) dihydride or via an unusual reversible intramolecular
C-H oxidative addition across the bimetallic core. Dramatic
counteraction effects are also demonstrated. This offers new
prospects for the metal-metal cooperative two-electron activation
of challenging substrates within a bimetallic cleft, complementing
bimetallic nickel chemistry that exploits metal-metal bonds and/or
31–33
redox active ligands for storing multiple redox equivalents.
RESULTS AND DISCUSSION
Dinickel Potassium Monohydride and Dihydride Complexes.
Drawing on the synthesis of the pyrazole-based dinucleating ligand
iPr
H
3
L
used in V (Scheme 2) a multistep route towards the new
Ph
34
proligand H
3
L
was developed, using 2,6-diphenylaniline
instead of 2,6-di-isopropylaniline; details of the synthetic protocols
Ph
and characterization data for H
Information. Threefold deprotonation of H
and subsequent addition of (dme)NiBr (dme = dimethoxyethane)
3
L
are provided in the Supporting
3
L with n-butyllithium
2
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Figure 1. Top: Molecular structure (30% probability thermal
ellipsoids) of 1. Solvent molecules and hydrogen atoms are omitted
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
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5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ph
2
for clarity. Bottom: Space filling models of the new L Ni Br (1;
iPr
27
left) and previously reported L Ni
2
Br (right).
The dinickel dihydride complex V was obtained via the reaction of
iPr
27
L Ni
2
Br with KHBEt
3
or NaHBEt
3
.
Applying the same protocol
to a
Figure 2. Top: Molecular structure (30% probability thermal
ellipsoids) of the anion of 2-K. Bottom: 1D arrangement in the
crystal lattice showing interactions with the K cation. Solvent
molecules and hydrogen atoms are omitted for clarity, except for
the Ni-H.
to 1, viz. adding a THF solution of 2.5 equivalents of KHBEt
3
+
suspension of 1 in THF, resulted in a similar dark red colored
1
solution within 40 mins. However, H NMR spectroscopy after
workup indicated the formation of a product of lower symmetry.
Red tabular crystals of the product 2-K were obtained by vapor
diffusion of pentane into the THF solution at room temperature.
The molecular structure determined by X-ray diffraction analysis
is shown in Figure 2 and reveals that orthometallation at one of the
winglet aryl group has occurred, giving a monohydride complex
1H- and 13C-NMR spectra of 2-K in THF-d
8
reflect the C
1
symmetry of the complex (Figures S16 and S17), confirming that
its molecular structure is retained in solution. Specifically, the four
diastereotopic methylene protons (4.41, 4.15, 4.09 and 3.97 ppm)
aryl
with a Ni-C bond at the adjacent second subunit. The Ni-H
1
show a distinct pattern in the H-NMR spectrum typical of two AB-
subunit exhibits the same square planar environment previously
seen in V, which is typical for low-spin nickel(II). In contrast, at
the orthometallated site the metal ion environment is drastically
distorted from square planar because of the geometric constraints
imposed by the chelate arrangement. Here, the two diketiminato N-
donors are in plane with the pyrazole-N and the Ni center, while
the Ni-bound phenyl-C is bent out of that plane by 21.4(9)°. As a
spin systems. A second indicator for the orthometallation of one
phenyl group is the two high-field shifted triplet signals at 6.29 and
6
.38 ppm for the protons in meta- and para-position with respect to
the Ni-bound C atom. The Ni-H shows a sharp discrete signal at
26.12 ppm, consistent with typical chemical shifts for Ni-hydride
-
1
+
species reported in the literature; the K salt of dihydride complex
V showed a singlet at -24.16 ppm in THF-d
To explore a possible reversibility of the C-H activation, a degassed
THF-d solution of 2-K was treated with an atmosphere of
dihydrogen (~1 atm.). In the H-NMR spectrum the hydride signal
at -26.12 ppm vanishes while a new downfield signal at -23.80 ppm
arises, with doubled intensity. Furthermore, the diastereotopic
8
.
pz
consequence of this distortion, the N -Ni-C angle is widened to
98.58(16)° and the Ni∙∙∙Ni distance is elongated to 4.266(7) Å. The
potassium cation is found above the pyrazole ring close to the
pyrazole-N atoms3 and is additionally involved in an
intermolecular cation- interaction with a phenyl moiety of a
8
1
5
+
nearby molecule (d(K ∙∙∙C) = 2.78-3.28 Å), which results in a
proton signals for the two CH
3
3
2
groups of 2-K in the range 4.41 –
.97 ppm converge to a singlet at 4.04 ppm for the product complex
-K, and the two triplets at 6.29 and 6.38 ppm shift downfield into
chain-like 1D arrangement in the crystal (Figure 2, bottom).
Selected bond lengths and angles for 2-K are included in Table 1.
in the region typical for aryl signals. The absolute number of signals
decreases, indicative of C2v symmetry of the product 3-K. The
combined NMR data imply formation of the dihydride species 3-K
upon reaction of 2-K with dihydrogen (Scheme 3). Further
characterization of 3-K was hampered by its instability, because it
loses H
removed. However, when 2-K was treated with [2,2,2]cryptand
before exposure to H and crystallization was performed under an
atmosphere, single crystals of 3-K(crypt) grew from the THF
2 2
and transforms back to 2-K when the H atmosphere is
2
H
2
solution at room temperature and could be analyzed by X-ray
diffraction. The molecular structure of the anion is depicted in
Ph
Figure 3 and confirms formation of the dihydride complex [L (Ni-
−
H)
2
] ; selected bond lengths and angles are included in Table 1.
Each subunit of 3-K(crypt) is similar to the Ni-H part of 2-K, and
the overall structure is similar to that of dihydride complex V. Both
metal ions are found in close to square planar donor environment
with angles close to the expected 180° (∠(Ntrans-Ni-Npz) = 177.67(7)
and 178.71(7)°). The Ni∙∙∙Ni distance is 4.0478(8) Å, i.e.,
significantly shorter than in 2-K and also slightly shorter than in
+
+
+
the [K(DB18C6)] or K or Na salts of V, which were found in the
2
7
range 4.0636(7) - 4.1636(7) Å. This suggests that release of H
2
from 3-K(crypt) (or 3-K) via intramolecular reductive elimination
akin to the scenario for V shown in Scheme 2 should be facile.
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type direct hydrogenation of the Ni-C bond and involves only one
subunit of the dinickel core (alternative monometallic oxidative
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
IV
addition/reductive elimination pathways via
intermediate seem unlikely). The ESXY NMR results that show a
correlation between the signals for free H and the Ni-H of 3-K, but
a formal Ni
2
not between the Ni-H signals of 3-K and 2-K, are in line with
scenario A. To further probe the mechanism, the reaction of 2-K
1
2
2
with D was studied by both H and H NMR spectroscopies
1
(Figure 5). The H NMR spectrum of the product does not show
any signal for a Ni-H unit that would be expected for 3 -K (Figure
HD
5
2
2
, middle) while Ni-D formation is evidenced by the singlet at -
2
3.77 ppm in the H-NMR spectrum (cf. the Ni-H resonance at -
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3.62 ppm in the H-NMR spectrum of dihydride complex 3-K).
Furthermore, deuteration of a phenyl ring is not observed, based on
the absence of any signal in the typical range 6.5 – 8.5 ppm in the
Figure 3. Molecular structure (30% probability thermal ellipsoids)
of the anion of 3-K(crypt); solvent molecules, the
[K([2,2,2]cryptand)] cation and hydrogen atoms except for the Ni-
2
H-NMR spectrum (Figure 5, bottom). All these findings strongly
+
D2
suggest that 3 -K is formed and that mechanism A is operative.
H are omitted for clarity.
At moderate D pressures, trace amounts of 2-K (with a single Ni-
2
D2
H unit resonating at -26.12 ppm) are present besides 3 -K,
1
The variable temperature (VT) H-NMR spectrum of 3-K(crypt)
providing further support for mechanistic pathway A.
8 2 2
in THF-d under H atmosphere is shown in Figure 4. At low H
2
Scheme 4. Possible pathways for D activation by 2-K. Left:
pressures (1 atm.), both 3-K(crypt) and 2-K(crypt) are present,
reflected by the hydride signals at -23.66 ppm and -26.28 ppm at
243 K. The ratio shifts towards 2-K(crypt) with increasing
reductive elimination. Right: Ni-C bond hydrogenation.
A
B
temperature, in accordance with entropically favored loss of H
room temperature, the signals for the pyrazole-H of 3-K(crypt)
2
. At
N
N
N
N
N
N
4
NiII
NiII
(
(
5.30 ppm), free dihydrogen (4.56 ppm) and Ni-bound hydride
-23.66 ppm) are broadened beyond detection. Such behavior has
H
Ph
Ph
Ph
C-H
oxidative
addition
also been observed for V, due to dynamic exchange of free
dihydrogen with the dihydride species via the dinickel(I)
intermediate VI (Scheme 2). Dynamic H /Ni-H exchange is also
2
consistent with ESXY NMR measurements for 3-K(crypt) at 243
K that show a clear correlation between the signals for free
dihydrogen and the Ni-bound hydrides of 3-K(crypt) (Figure 4,
right). In contrast, no EXSY correlation is observed between the
hydride signals of 2-K(crypt) and 3-K(crypt).
reductive
elimination
D2
2-K
N
N
N
N
D
NiII
N
N
N
N
N
D
Ni I
NiI
NiII
N
N
N
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
4-K
2-K
D2
N
N
N
N
N
N
N
N
N
N
N
NiII
NiII
Ph
NiII
NiII
DN
D
D
H
D
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3D2-K
3HD-K
I
I
The conclusion that 2-K forms from the Ni Ni intermediate 4-K
via bimetallic intramolecular C-H oxidative addition suggested an
alternative synthesis of 2-K that does not involve hydride reagents
such as KHBEt
3
. The cyclic voltammogram of the precursor
p,c
complex 1 shows a quasireversible reduction with E = -2.85 V vs.
0
/+
Fc in THF (Supporting Information), indicating that 4-K might
be accessible from 1 when using strong reductants. Upon treatment
1
Figure 4. Left: VT H-NMR spectra of 3-K(crypt) in THF-d
under H atmosphere from 303 – 203 K; only the N-H region is
shown. Right: part the EXSY spectrum of 3-K(crypt) in THF-d
showing exchange between the Ni-H of 3-K(crypt) and free H
8
8
of 1 in THF with 2.2 eq. of KC at rt, the green suspension turned
2
into a red solution over 6 hours, and analysis of the crude material
8
1
2
.
by H NMR spectroscopy showed formation of 2-K as the major
product. Crystallization via layering of hexane resulted in red
tabular crystals of 2-K in a yield of 72 % (Scheme 3). This synthetic
Two mechanisms are conceivable for the interconversion of
orthometallated 2-K and dihydride 3-K. In scenario A implying
metal-metal-cooperativity (Scheme 4), reductive C-H elimination
I
I
route provides further indirect evidence that 2-K forms via a Ni Ni
intermediate; the same method was also utilized for the selective
synthesis of the sodium derivative 2-Na (vide infra).
from 2-K or reductive H
dinickel(I) intermediate 4-K that can undergo either bimetallic C-
H or H oxidative addition. Scenario B proceeds via -metathesis
2
elimination from 3-K generates a
2
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Journal of the American Chemical Society
benzaldehyde substrate is twofold reduced in 5-K. In view of the
1
2
3
4
5
6
7
8
9
redox balance (Scheme 5) and the roughly square planar
coordination environment of the two nickel ions (∠(Ntrans-Ni-O) =
1
68.50(8)° and ∠(Ntrans-Ni-C) = 168.43(9)°) 5-K appears to be best
described as a low-spin dinickel(II) complex with a doubly
deprotonated benzyl alcohol species in the bimetallic cleft. This
6 6
assumption is supported by NMR data for 5-K in C D (Supporting
1
13
Information) which show that the H and C NMR signals of the
"aldehyde-CH" are dramatically shifted to higher field and are
found at 2.03 ppm and 86.10 ppm, respectively. The findings
corroborate that 2-K serves as a masked dinickel(I) species and as
a powerful two-electron reductant, operating via intramolecular C-
H reductive elimination and intermediate 4-K. However, no
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
evidence for oxidative addition of external substrates C
toluene-d has been obtained when these were added in excess to a
solution of 2-K in THF-d at rt or 60°C, likely reflecting that the
6 6
D or
8
8
high local concentration of the ligand phenyl groups favors an
intramolecular reaction. Further experiments with external
substrates having lower C-H bond dissociation energies (BDEs) are
in progress.
1
Figure 5. Top: H-NMR spectrum of 3-K under H
2
atmosphere in
atmosphere in
2
atmosphere in THF.
1
D2
THF-d
THF-d
8
. Middle: H-NMR of 3 -K under D
2
. Bottom: H-NMR of 3 -K under D
The asterisks indicate THF signals.
2
D2
8
I
I
Since the Ni Ni intermediate 4-K could not be isolated but was
assumed to form via the equilibrium with 2-K, a trapping
experiment with an unsaturated substrate featuring a double bond
was pursued, anticipating twofold substrate reduction akin to the
2
7–30
reactivity of the dinickel(I) complex VI (Scheme 2).
addition of benzaldehyde to 2-K in THF the color of the solution
changed from red to brown, and brown crystals of the new product
-K could be isolated in 68% yield (Scheme 5).
Upon
5
Scheme 5. Reactivity of 2-K, shown by trapping of the Ni NiI
intermediate 4-K with benzaldehyde.
I
K+
N
N
N
N
K+
N
N
N
N
N
N
N
N
NiII
NiII
Ni I
NiI
H
Ph
Ph
Ph
Ph
Ph
O
Ph
Ph
Figure 6. Molecular structure (30% probability thermal ellipsoids)
of 5-K. Solvent molecules and hydrogen atoms (except for the one
bound to C52) omitted for clarity.
2-K
4-K
H
Dinickel Sodium Monohydride and Dihydride Complexes. In
+
+
case of dihydride complex V, the cation (either K or Na or
+
K+
[K(DB18C6] ) and its specific interaction with the pyrazolate-
N
N
N
N
N
N
O
NiII
NiII
bridged dinickel dihydride core was found to play a major role
regarding the stability of the dihydride and the release of H2.27
Ph
Ph
Ph
Ph
Therefore, the effect of the cation was also probed for the present
H
Ph
+
+
system by using Na instead of K . Reaction of 1 with 2.5 eq. of
5-K
1
3
NaHBEt resulted in a mixture of two species according to H NMR
The molecular structure of 5-K determined by X-day diffraction
(Figure 6) reveals that the aldehyde substrate is hosted in C,O-
bridging mode within the bimetallic pocket, the Ni∙∙∙Ni distance
3.9594(4) Å) being similar to the one in the dihydride complex 3-
K(crypt) (4.0478(8) Å). The K is located above the central six-
membered ring of 5-K and is additionally ligated by two THF
spectroscopy, which were identified as the monohydride 2-Na and
dihydride complex 3-Na (Scheme 6). Increasing the amount of
NaHBEt up to 100 equiv. or longer reaction times resulted in only
3
(
minor changes in the product ratio. Similar to 2-K, however, the
+
+
Na salt 2-Na could be selectively synthesized in high yields (95%)
by direct reduction of 1, using the strong reductant sodium
+
0
0/+ 38
molecules, complemented by two K ∙∙∙arene contacts. Selected
naphtalide in this case (E = −3.10 V vs Fc ).
bond lengths and angles for 5-K are included in Table 1.
Importantly, the C-O bond of the bound substrate is substantially
elongated to dC-O = 1.388(3) Å, much longer than in free
benzaldehyde (1.212(3) Å) and closer to the C-O bond length of
benzyl alcohol (1.424(6) Å). Furthermore, the angle C -C-O of
14.58(19)° is significantly smaller than in benzaldehyde
+
Scheme 6. Synthetic transformations for the Na salts of the
mono- and dihydride dinickel complexes
3
6
3
7
ar
1
(
3
6
123.6(4)°) and approaches the tetrahedral angle found for benzyl
alcohol (108.9(3)°). These geometric changes confirm that the
37
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Journal of the American Chemical Society
Page 6 of 13
signal shifts back to higher fields (Figure 8). We assume that
NaHBEt3
THF, rt, 2h
+
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
N
N
N
N
N
N
interactions between 2-Na and additional Na play a role and
NiII
NiII
2-Na + 3-Na
+
become dominant at higher Na concentrations. These conclusions
Br
4
Ph
Ph
are consistent with trends for the pyrazole-H signal where the
Ph
Ph
same chemical shifts are found for 2-Na and 2-K (5.70 ppm at 298
K) but a clear shift to higher field is observed upon addition of >10
equiv. NaOTf (δH4 = 5.55 ppm after addition of 100 equiv. NaOTf;
see Figure S54).
1
Na[C10H8]
THF, rt, 6h
H2 (1 atm.)
THF, rt
<1 min
quant.
Na+
Ph
Na+
N
N
N
N
N
N
N
N
H
N
N
H
N
H
NiII
Ph
NiII
NiII
NiII
Ph
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
N
no H2 atm.
- H2
slow
Ph
Ph
Ph
Ph
2
-Na
3-Na

THF, 60°C, 2-3 d
Na+
N
N
N
N
N
NiII
NiII
N
Ph
Ph
6
-Na
Single crystals were obtained by vapor diffusion of pentane into a
THF solution of 2-Na. Overall, the molecular structure of 2-Na in
solid state (Figure 7) is very similar to the one of 2-K, featuring
orthometallation at a winglet aryl group at one side of the bimetallic
core and a Ni-H unit on the other side. Again, the alkali metal cation
is found above the pyrazole ring close to the pyrazole-N atoms with
close contacts to the Ni-H unit and nearby aryl rings, but in 2-Na
the Na is additionally bound to a THF and thus lacks any
intermolecular M ∙∙∙arene interactions as found for 2-K. In the
ATR-IR spectrum of solid 2-Na the nickel-bound hydride gives rise
to a broad band for the Ni-H stretch at ῦ = 1897 cm .
1
Figure 8. High-field range of the H-NMR spectra of 2-K (top) and
2
-Na (bottom) in THF-d
8
at rt and effect of titration of NaOTf into
the solution of 2-K (second from top to second from bottom).
2
Upon reaction with H (~1 atm), 2-Na converts fully to the
dihydride complex 3-Na; crystals suitable for X-ray diffraction
analysis could be obtained from THF/diethyl ether solutions.
2
Compared to 3-K, complex 3-Na is less prone to H loss and
+
+
-
1
orthometallation but is stable under dinitrogen atmosphere for
several days. The molecular structure of 3-Na is similar to that of
+
3
-K(crypt), with very similar metric parameters. The Na is
positioned above the pyrazolato-bridged dinickel core and appears
to interact with both hydrides and one of the ligand’s wingtip
phenyl groups; additionally it has two bound THF molecules
(Figure 9).
Figure 7. Molecular structure (30% probability thermal ellipsoids)
of 2-Na. Solvent molecules and hydrogen atoms omitted for clarity,
except for the Ni-H.
1
The effect of the alkali metal ion is most evident in the H-NMR
spectra of 2-Na and 2-K recorded in THF-d
ppm for 2-Na and -26.12 ppm for 2-K at rt (Figure 8). Addition of
equiv. of KOTf to the solution of 2-Na in THF-d
equiv. of NaOTf to the solution of 2-K, gives rise to a single
resonance at -26.10 ppm, very close to the signal for 2-K (Figures
and S55). This indicates that interaction of K with the bimetallic
complex is much favored. Titrating further equivalents of NaOTf
into the solution of 2-K in THF-d gradually shifts the hydride
8
, with δhydride = -25.81
1
1
8
, or addition of
Figure 9. Molecular structure (30% probability thermal ellipsoids)
of 3-Na. Solvent molecules and hydrogen atoms omitted for clarity,
except for the Ni-H.
+
8
Addition of D
2
(1 atm.) to 2-Na in THF results in the formation of
8
D2
3
-Na as evidenced by the characteristic signal at -23.58 ppm in
signal to lower field, towards the one expected for 2-Na, but
beyond 10 equiv. of NaOTf the trend reverses and the hydride
2
the H-NMR spectrum (Figure 10, top). After replacing the D
2
1
2
atmosphere by H (to give 3-Na; see H NMR spectra in Figure
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Journal of the American Chemical Society
S59), free D
2
is observed at 4.57 ppm while no signal for HD is
Ni-C bonds forming an angle of 26.503(15)° and 26.859(24)° with
+
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
detected (Figure 10, bottom). This finding is in accordance with
pairwise exchange of the two hydrides (or deuterides, respectively)
and with scenario A shown in Scheme 4.
the respective {NiN
3
} plane. The Na is located above one of the
+
orthometallated phenyl rings (distance between Na and the phenyl
plane ~2.66 Å) and is additionally bound to three THF molecules.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 10. H-NMR spectrum of 3D2-Na in THF (prepared from
2
exposing the solution of 2-Na to an atmosphere of D
2
) (top) and
atmosphere with H
bottom); corresponding H-NMR spectra are shown in Figure S59.
2
H-NMR spectrum after replacing the D
2
2
1
(
The asterisks indicate THF signals.
Twofold orthometallation at the dinickel core. To study the
conversion of 3-Na to 2-Na and to probe further reactivity of 2-Na,
a mixture of 2-Na and 3-Na was heated at 60°C in THF-d and
8
changes were monitored by H NMR spectroscopy (Scheme 6,
Figure 11). While the ratio between 2-Na and 3-Na remained
nearly constant, both compounds gradually disappeared and
formation of a new species 6-Na was observed, evident from a new
characteristic signal for the pyrazole-H and two new signals in the
aromatic region of the H NMR spectrum (Figure 10). 6-Na does
not show any hydride signal, but the methylene groups in 3- and 5-
positions of the pyrazolate backbone are diastereotopically split
into two doublets characteristic for an AB spin system. The overall
1
Figure 12. Molecular structure of 6-Na; hydrogen atoms omitted
for clarity.
8
4
When 2-K in THF-d was subjected to the same reaction conditions
1
and heated to 60°C, the second C-H activation to give 6-K is also
observed but the reaction takes much longer than in case of 2-Na/3-
Na; full conversion is not reached even after 2 weeks at 70°C and
multiple degassing processes. To further study the effect of the
cation, DB18C6 was added to the mixture of 2-Na and 3-Na in
1
H NMR spectrum indicates C
2
symmetry of the new species 6-Na
8
THF-d before heating the dark red solution to 60°C for multiple
in solution.
days. Interestingly, almost no conversion to 6-Na(DB18C6) was
observed even after 3 days at 70°C (Supporting Information). This
indicates that the cation has a dramatic effect on the second
+
orthometallation reaction, and that Na promotes the C-H
activation to enable formation of 6-Na.
2
Figure 13. H NMR spectrum of the reaction of 6-Na with D
2
(1
8
atm.) in THF-d after heating to 65°C for 10 d.
1
Figure 11. H NMR spectra in THF-d
8
of a mixture of 2-Na and
-Na at rt (top) and after heating to 60°C for 24 hours (middle) and
8 hours (bottom).
To investigate the reversibility of the second orthometallation,
which obviously cannot proceed via a dinickel(I) intermediate as
3
4
proposed for the first orthometallation, 6-Na was treated with H
2
1
and the reaction progress was monitored by H NMR spectroscopy
over a period of several days (Figure S57). The reaction back to
dihydride complex 3-Na is fairly slow, requires elevated
temperatures (65°C), and full conversion to 3-Na was not observed.
Interestingly, only very small amounts of the monohydride
complex 2-Na were detected during the reaction, indicating that the
After heating to 60°C for 48 hours and slow vapor diffusion of
pentane into the dark red THF solution at rt, single crystals suitable
for X-ray crystallography were obtained. The molecular structure
of 6-Na (Figure 12) shows that wingtip phenyl groups at both
subunits have undergone C-H activation to furnish two Ni-C bonds.
Steric congestion and distortion of the two nickel coordination
planes away from square planar is substantial, and the
orthometallated phenyl rings are constrained to lie above and below
the plane defined by the pyrazolato-bridged dinickel core with the
reaction of 2-Na with H
reaction of 6-Na with H , and suggesting different mechanisms for
the two steps. Even at higher H pressures (3 atm.) the conversion
2
to give 3-Na is much faster that the
2
2
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Journal of the American Chemical Society
Page 8 of 13
of 6-Na to 3-Na is incomplete and requires heating to 65°C. These
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
observation are in line with challenging direct hydrogenation of the
first Ni-C bond in 6-Na akin to mechanism B shown in Scheme 4,
in contrast to facile C-H reductive elimination followed by
bimetallic H oxidative addition for the second step, viz. conversion
2
of monohydride 2-Na to dihydride 3-Na according to mechanism
A (Scheme 4). Apparently, the activation energy for the direct Ni-
C hydrogenation via heterolytic H
The mechanistic scenario is supported by a deuteration experiment:
heating 6-Na with D for prolonged times not only gave rise to the
2
splitting is significantly higher.
2
characteristic signals for the Ni-D unit of the dideuteride (major)
and monodeuteride (minor) complexes, but also a new set of signals
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
in the aromatic region of the H-NMR spectrum, indicative of D
incorporation in the phenyl groups (Figures 13 and S58).
Table 1. Selected atom distances and bond angles of 1, 2-K, 2-Na, 3-K(crypt), 3-Na, 5-K and 6-Na.
D [Å] / ∠ [°]
1
2-K
2-Na
3-K(crypt)
3-Na
5-K
6-Na
Ni···Ni
Ni-H
3.7813(7)
-
4.266(8)
1.291(3)
4.345(5)
1.336(3)
4.0478(8)
1.39(2)
4.057(6)
3.9594(4)
-
4.1840(8)
-
1.403(2)
1.454(5)
1
.42(2)
1
1
.841(2)
.836(2)
1.878(3)
1.936(4)
1.881(2)
1.925(2)
1.8651(16)
1.8672(17)
1.859(2)
1.914(3)
1.8526(18)
1.8919(18)
1.8963(18)
1.9046(16)
Ni-Npz
Ni-Nnac
Ni-C
1
.877(2)-
1.877(4)-
1.936(4)
1.8810(15)-
1.9178(15)
1.8645(17)-
1.9179(17)
1.859(3)-
1.914(2)
1.8813(18)-
1.9709(18)
1.8630(18)-
1.9310(18)
1
.892(2)
1
.8854(18)
1.900(2)
-
1.901(5)
-
1.901(5)
-
-
-
-
-
2.015(2)
1.8299(15)
-
Ni-O
-
-
-
-
-
2.354(3)
+
M -H
2.226(6)
2.227(3)
2
.449(4)
1
1
77.90(10)
78.75(10)
167.15(14)
177.22(15)
164.96(7)
176.70(8)
177.67(7)
178.71(7)
175.91(11)
178.19(11)
172.90(8)
178.36(8)
166.14(7)
166.55(6)
N
N
N
trans-Ni-Npz
trans-Ni-C
trans-Ni-O
1
1
53.14(7)
53.50(8)
-
-
-
158.55(17)
153.94(8)
-
-
-
-
-
-
168.43(9)
168.50(8)
114.58(19)
-
-
-
-
-
-
Ni-Cald-O
2
The reversible H and C-H reductive elimination/oxidative addition
equilibrium allows to smoothly unmask the reactive dinickel(I)
intermediate 4 in the presence of added substrate, with 4 serving as
a powerful 2-electron reductant. As a first example presented in this
work, addition of benzaldehyde to a solution of 2 rapidly gives
complex 5 with a twice reduced benzaldehyde, viz. a doubly
deprotonated benzyl alcohol, as C,O-bridging ligand within the
CONCLUSIONS
The new compartmental pyrazolato-bridged bis(nacnac) ligand
Ph 3−
with bulky peripheral m-terphenyl substituents [L ] is shown to
Ph
−
form a dinickel(II) dihydride complex [L (Ni-H)
the two hydride ligands are in rather close proximity within the
bimetallic pocket and are prone to facile pairwise reductive H
2
] (3) in which
2
iPr
−
bimetallic pocket. While the analogous [L (Ni-H)
only via reductive H elimination to access the dinickel(I) species
] (VI), the present system now offers both elimination
from 3 or reductive C-H elimination from 2 for unmasking
2
] (V) operates
elimination. However, the putative dinickel(I) intermediate
2
Ph
I −
[
[
2
L Ni ] (4) is not observed, in contrast to previously reported
iPr
I
−
27
[L Ni
of H
2
iPr
I
−
27
2
L Ni ] (VI) equipped with peripheral 2,6-diisopropylphenyl
2
aryl
substituents, because of rapid intramolecular C -H oxidative
Ph
I −
[
L Ni
cofactor of nitrogenase that stores redox equivalents in two
adjacent metal hydrides, and for which exothermic H release
contributes significantly to providing the driving force for N
2
] (4). The former process is known from the FeMo
addition that gives the orthometallated monohydride complex 2.
I
While Ni species have been implicated in catalytic C-H bond
2
I
functionalization, discrete activation of C-H bonds at Ni
2
1
4,16
complexes has rarely been described in literature.
In the present
reductive
39
reduction. In contrast, reductive C-H elimination from 2 does not
release any byproduct (such as H ), and hence it adds a distinct
system the reversible 2-electron
C
aryl-H
2
elimination/oxidative addition chemistry is enabled by metal-metal
cooperativity of the highly preorganized dinickel core. Dihydride
complex 3 and orthometallated monohydride complex 2 are in fast
equilibrium via the intermediate dinickel(I) species 4, the amount
platform (likely with a different thermochemistry) for providing
reducing equivalents via the dinickel(I) intermediate while
circumventing the isolation of any unstable superreduced form of
the bimetallic complex. Studies towards exploiting this scenario for
the activation of challenging substrates are in progress in our
laboratory.
of 3 depending on the applied H
2
pressure. Hence, in accordance
Ph
with the proposed mechanism, addition of D
2
rapidly gives [L (Ni-
2
] (3 ) as the sole product, without H/D scrambling.
−
D2
D)
ACS Paragon Plus Environment

Page 9 of 13
Journal of the American Chemical Society
An interesting aspect is the pronounced cation effect on the
1.0 eq.) in triethylamine (10 mL). The resulting suspension was
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
interconversion between 2 and 3, and also on the formation of the
twice orthometallated 6. E.g., the dihydride complex 3-K(crypt)
stirred for 19 h at 40°C. After removal of the solvent under reduced
pressure, the crude solid was extracted multiple times with toluene
(2 x 30 mL). After another removal of the solvent the crude material
loses H
slowly and is stable under N
may reflect differences in ground state stabilization of the
2
if not kept under H
2
atmosphere, while 3-Na loses H
2
only
2
atmosphere for several days. This
was suspended in ethanol, cooled down to -20°C and filtered. After
Ph
multiple washings with pentane, H
3
L
was obtained as a pale
+
+
1
individual complexes 2 or 3 by interaction with Na , K (or the lack
yellow solid (2.1 g, 2.8 mmol, 59 %). H-NMR (400 MHz, THF-
+
nacnac
of such interaction in case of [K(crypt)] ), or an influence of the
d
8
) = 10.79 (s, 1H, NH
), 7.38 (d, JH-H = 8 Hz, 8H, Ph), 7.24 (d,
Lewis acidic cation on reaction trajectories. The roles that Lewis
acids can play in transition-metal hydride reactivity are currently
JH-H = 8 Hz, 4H, Ph), 7.19 (d, JH-H = 8 Hz, 8H, Ph), 7.12 – 7.07 (m,
pz
6H, Ph), 5.85 (s, 1H, CH ), 4.33 (s, 2H, CHCNCH
CH Pz), 1.81 (s, 6H, CH N=CMe). 1.23 (s, 6H, ArN=CMe) ppm.
C-NMR(101 MHz, THF) = 166.41 (ArN=C), 156.41 (CH N=C),
3
), 4.22 (s, 4H,
4
0
receiving much attention. It will be particularly interesting to
2
2
1
3
investigate how different Lewis acids can work in concert with this
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ph
Ar
Ar
Ar
{
L Ni
2
}
platform to impart selectivity during substrate
148.13 (C ), 147.70 (3,5-Pz), 140.20 (CH ), 132.66 (CH ),
128.22 (CH ), 126.67 (CH ), 125.21 (CH ), 121.82 (CH ),
Ar
Ar
Ar
Ar
transformations.
Pz
9
9.96 (CH ), 93.70 (CHCNCH
(CH ) ppm. MS (ESI(+), MeCN) m/z: calcd. [M+H] 745.40 exp.
51 48 6
[M+H] C H N
3 2 3
), 38.84 (CH ), 21.36 (CH ), 18.07
+
3
EXPERIMENTAL SECTION
+
745.93.
Anal.
Calcd.
for
Materials and Methods. Manipulations involving air- and
moisture-sensitive compounds were conducted under an
atmosphere of dried (phosphorous pentoxide on solid support
(744.39 g/mol) = C 82.25, H 6.57, N 11.17; Found C 82.21, H
6.50, N 11.28.
Complex 1. H
4 mL) and the clear yellow solution cooled to -78°C. n-Butyl
3
LPh (0.75 g, 1.0 mmol, 1.0 eq.) was dissolved in THF
[
Sicapent, Merck]) dinitrogen or argon using standard Schlenk
techniques, or in a glovebox (O < 0.5 ppm and H O < 0.5 ppm)
filled with dinitrogen atmosphere. THF, THF-d , diethyl ether and
(
2
2
lithium (2.6 M in hexane, 1.2 mL, 3.2 mmol, 3.2 eq.) was added
slowly via a syringe to give a clear orange solution. Under stirring
the reaction mixture was warmed to room temperature over 15
8
pentane were dried over sodium or sodium/potassium in the
presence of benzophenone. Triethylamine and dichloromethane
were dried over calcium hydride (ABCR) and freshly distilled
before use. Hydrogen gas was purchased from Messer and
2
minutes to yield a dark red solution. (dme)NiBr (512 mg, 1.8
mmol, 2.0 eq.) was then added. The resulting dark-brown mixture
was stirred overnight at 50°C. The suspension was separated by a
centrifuge and the THF phase discarded. The solid material was
washed several times with acetone until the solvent remained
colorless. After removing residual solvent under reduced pressure,
34
deuterium gas from Sigma-Aldrich. 2,6-Diphenylaniline and
41
sodium naphtalide were synthesized according to known
literature procedures. All other chemicals used were either present
in the working group or purchased from commercial sources, or
their synthesis is described in the Supporting Information.
1
was obtained as a green fine powder. (0.53 mg, 0.56 mmol, 56
%). Slow evaporation of a concentrated dichloromethane solution
yielded cube-shaped crystals of 1, suitable for XRD analysis. H-
NMR samples for H
atmosphere. Reactants were dissolved in THF-d
2
experiments were prepared under an inert
and transferred
1
8
2 2 H-H
NMR (400 MHz, CD Cl ) = 7.92 (d, J = 8 Hz, 8H, Ph), 7.45 –
into a J. Young NMR tube. The solution was degassed by means of
a pump-freeze-thaw procedure using liquid nitrogen, which was
7.37 (m, 12H, Ph), 7.09 (s, 6H, Ph), ), 5.46 (s, 1H, CHpz), 4.49 (s,
2H, CHCNCH ), 3.93 (s, 4H, CH Pz), 1.84 (s, 6H, CH N=CMe).
3
2
2
repeated three times before H
2
was added. Because all new
(µ-Br)] are extremely sensitive to
moisture (ultimately giving the hydroxido-bridged dinickel(II)
1.36 (s, 6H, ArN=CMe) ppm. 13C-NMR(101 MHz, THF) = 160.82
Ph
pz
Ar
compounds except [L Ni
2
(CH N=C), 159.30 (ArN=C), 154.02 (C ), 148.58 (C ), 141.73
2
(CAr), 137.92 (CAr), 131.02 (CHAr), 130.51 (CHAr), 128.16 (CHAr),
Ph
II
Ar
Ar
Pz
complex [L Ni (µ-OH)]), gases such as H
2
and D
2
were dried with
127.16 (CH ), 126.98 (CH ), 97.50 (CHCNCH ), 92.39 (CH ),
3
a cooling trap, cooled with liquid dinitrogen, or stirred above
concentrated sulfuric acid before use.
54.99 (CH ), 24.22 (CH pz), 21.68 (CH3Ar) ppm. IR (ATR) ν/cm-1
2
3
= 3025 (w), 2922 (m), 1898 (w), 1597 (w), 1549 (m), 1529 (s), 1496
(w), 1456 (w), 1428 (w), 1392 (s), 1314 (w), 1280 (w), 1261 (w),
1206 (w), 1179 (w), 1085 (w), 1072 (w), 1029 (w) 1016 (m), 957
1
2
13
Instrumentation. H-, H- and C-NMR spectra were recorded on
1
Bruker Avance spectrometers (300 and 500 MHz for H) at room
(
6
w), 946 (w), 912 (w), 872 (w), 802 (m), 752 (s), 732 (m), 695 (vs),
11 (m), 589 (m), 539 (w), 504 (w). Anal. Calcd. for
C H N Ni Br∙4CH Cl (1278.0 g/mol) = C 51.65, H 4.18, N 6.57;
temperature unless otherwise noted. Chemical shifts are reported in
parts per million relative to residual proton and carbon signals of
42
the solvent. Electron ionization (EI) and electrospray ionization
ESI) mass spectra were collected on a Bruker HCTultra
51 45
6
2
2
2
Found C 51.89, H 4.20, N 6.61.
(
instrument. Moisture- or oxygen-sensitive samples were prepared
in a glovebox (MBRAUN UNIlab) under a dinitrogen atmosphere
and injected into the Bruker HCTultra instrument via a direct PEEK
tubing connection. Elemental analyses were carried out with an
Elementar 4.1 vario EL 3 instrument by the Analytical Laboratory
of the Department of Chemistry at the Georg-August-University.
IR spectra of solid samples were measured with a Cary 630 FTIR
spectrometer equipped with a DialPath and Diamond ATR
accessory (Agilent) placed in a glovebox. IR bands were labeled
according to their relative intensities with vs (very strong), s
Complex 2-K. The green suspension of 1 (0.10 g, 0.11 mmol,
1.0 eq.) in THF (2 mL) was treated with potassium triethyl
borohydride in THF (1 M, 0.30 mL, 0.30 mmol, 3.0 eq.). After
stirring for 1 hour the suspension changed to a dark red color. The
solution was filtered, and the solvent was removed under reduced
pressure. The remaining solid was washed several times with
hexane (3 x 10 mL) and the remaining solvent was removed. Red
tabular crystals of 2-K (69 mg, 0.064 mmol, 64%) were obtained
by vapor diffusion of pentane into a prepared THF solution.
Alternative Synthesis: The green suspension of 1 (100 mg,
0.11 mmol, 1.0 eq.) in THF (2 mL) was treated with KC (32 mg,
8
(
strong), m (medium), w (weak), and very weak (vw).
LPh. 2-[(2,6-Diphenylphenyl)imido]-pent-2-en-4-one V (3.3 g,
0 mmol, 2.0 eq.) was dissolved in dichloromethane (30 mL) to
H
1
3
0.24 mmol, 2.2 eq.). The reaction mixture was stirred for 6 hours at
room temperature. After removal of the solvent, the crude product
was washed with hexane (3 x 4 mL) and dried in vacuo.
Crystallization by layering a THF solution with hexane, resulted in
give a slight yellow solution. To this solution triethyloxonium
tetrafluoroborate (3.1 g, 16 mmol, 3.2 eq.) was added and the
reaction mixture stirred at room temperature for 19 h. The pale
yellow solution was then added dropwise to a suspension of
bis(3,5-aminomethyl)pyrazole hydrochloride III (1.0 g, 5.1 mmol,
1
red tabular crystals of 2-K (71 mg, 0.080 mmol, 72%).
H-NMR
(400 MHz, THF-d ) = 8.55 (d, 2H, J = 8 Hz, Ar), 7.75 (d, 2H,
8
H-H
JH-H = 8 Hz, Ar), 7.67 (d, 1H, JH-H = 8 Hz, Ar), 7.47 (d, 2H, JH-H
=
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 13
8
Hz, Ar), 7.32 – 6.96 (m, 16H, Ar), 6.37 (t, 1H, JH-H = 8 Hz, Ar),
CHCNCH
3 2
), 4.18-3.94 (m, 3H, CH Pz), l.92 (s, 3H, Me), 1.74 (s,
pz
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6.29 (t, 2H, JH-H = 8 Hz, Ar), 5.70 (s, 1H, CH ), 4.48 (s, 1H,
3H, Me), l.23 (s, 3H, Me), 1.06 (s, 3H, Me), -25.81 (s, 1H, Ni-H)
1
3
q
q
CHCNCH
3
), 4.43 (d, JH-H = 24 Hz, 1H, CH
), 4.21 (d, JH-H = 24 Hz, 1H, CH Pz), 4.06 (d, JH-H = 24
Pz), 3.99 (d, , JH-H = 24 Hz, 1H, CH Pz), 1.92 (s, 3H,
Me), 1.05 (s, 6H, 2xllMe), -26.12 (s, 1H, Ni-H) ppm. C-
2
Pz), 4.24 (s, 1H,
8
ppm. C-NMR(75 MHz, THF-d ) = 160.59 (C -Me), 160.16 (C -
q
q
Ar
Ar
CHCNCH
3
2
Me) 159.43 (C -Me), 158.89 (C -Me), 157.96 (C ), 157.72 (C ),
Ar
Ar
Ar
Ar
Hz, 1H, CH
2
2
157.71 (C ), 155.84 (C ), 155.82 (C ), 151.30 (C ), 144.17
1
3
Ar
Ar
Ar
Ar
Ar
(C ), 143.56 (C ), 143.53 (C ), 142.91 (C ), 142.61 (C ),
q
pz
Ar
Ar
Ar
Ar
NMR(101 MHz, THF-d
8
) = 160.72 (C -Me), 159.57 (C ) 159.34
139.20 (C ), 137.77 (C ), 137.02 (C ), 136.66 (C ), 134.04
q
q
q
pz
Ar
Ar
Ar
Ar
Ar
(C -Me), 158.90 (C -Me), 157.14 (C -Me), 157.04 (C ), 156.46
(C ), 131.83 (C ), 130.81 (C ), 130.24 (C ), 130.15 (C ),
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
(C ), 154.69 (C ), 143.26 (C ), 143.21 (C ), 142.97 (C ),
142.20 (C ), 141.75 (C ), 140.07 (C ), 137.31 (C ), 136.85
128.93 (C ), 128.58 (C ), 128.30 (C ), 128.01 (C ), 127.42
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
(C ), 126.58 (C ), 126.54 (C ), 126.33 (C ), 125.02 (C ),
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
(C ), 135.87 (C ), 132.46 (C ), 131.19 (C ), 131.05 (C ),
124.39 (C ), 124.29 (C ), 123.32 (C ), 120.85 (C ), 103.90
Ar
Ar
Ar
Ar
pz
1
30.14 (C ), 130.09 (C ), 129.88 (C ), 128.80 (C ), 128.58
(CHCNCH
3 3 2
), 98.01 (CHCNCH ), 93.55 (CH ), 52.41 (CH ),
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ar
Ar
Ar
Ar
Ar
(C ), 128.38 (C ), 128.26 (C ), 127.78 (C ), 126.84 (C ),
52.07 (CH ), 23.72 (Me), 23.10 (Me), 21.02 (Me), 20.03 (Me) ppm.
2
Ar
Ar
Ar
Ar
-1
1
26.74 (C ), 126.43 (C ), 126.29 (C ), 125.70 (C ), 125.37
IR (ATR) ν/cm = 3049 (w), 2952 (w), 2918 (w), 2858 (w), 1897
(w), 1557 (m), 1519 (s), 1496 (m), 1450 (m), 1425 (s), 1386 (vs),
1355 (m), 1314 (m), 1274 (s), 1250 (m), 1208 (m), 1181 (w), 1156
(w), 1107 (w), 1090 (w), 1073 (w), 1046 (w), 1021 (m), 965 (w),
Ar
Ar
Ar
Ar
Ar
(C ), 124.33 (C ), 123.45 (C ), 123.02 (C ), 120.88 (C ),
pz
103.80 (CHCNCH
(CH
3
), 97.74 (CHCNCH
3
), 92.97 (CH ), 52.65
2
), 52.22 (CH
) ppm. IR (ATR) ν/cm = 3051 (w), 3023 (w), 2984 (w), 2908
2
), 23.49 (Me), 23.19 (Me), 20.98 (Me), 20.21
-1
(CH
3
45 6 2
916 (w), 869 (w), 803 (w). Anal. Calcd. for C51H N NaNi ∙2THF
(
1
(
(
w), 2858 (w), 2821 (w), 1561 (m), 1511 (s), 1422 (s), 1387 (vs),
306 (m), 1267 (s), 1248 (s), 1204 (s), 1156 (m), 1091 (m), 1068
m), 1022 (s), 996 (m), 906 (m), 853 (m), 799 (m), 745 (vs), 692
2
vs), 605 (s), 541 (s). Anal. Calcd. for 2-K ∙ THF C55 Ni O
(1026.55 g/mol) = C 69.42, H 5.99, N 8.19; Found C 68.64, H
5.84, N 8.36.
H
53KN
6
Complex 3-Na. The mixture of 2-Na and 3-Na after washing with
hexane was dissolved in a minimal amount of THF. The red
solution was degassed three times before dry dihydrogen gas (~1
atm.) was added. Needle-shaped crystals of 3-Na for XRD analysis
were obtained after layering of diethyl ether on top of the
(970.50 g/mol) = C 68.06, H 5.50, N 8.66; Found C 67.97, H 5.47,
N 8.69.
Complex 3-K. 2-K was dissolved in minimal amounts of THF.
After degassing the dark red solution three times, an atmosphere of
dry dihydrogen was applied (~1 atm.). H-NMR (400 MHz, THF-
d
4
4
2
1
1
dihydrogen saturated THF solution. Crystals of 3-Na were only
8
) = 7.78 (d, 8H, JH-H = 8 Hz, Ar), 7.20-7.19 (m, 14H, Ar), 7.07 (t,
1
stable under a H
2
atmosphere over a long period of time. H-NMR
pz
H, JH-H = 8 Hz, Ar), 5.42 (s, 1H, CH ), 4.24 (s, 2H, CHCNCH
3
),
(
(
(
6
=
400 MHz, THF-d
m, 14H, Ar), 7.11 (t, 4H, JH-H = 8 Hz, Ar), 5.46 (s, 1H, CH ), 4.29
s, 2H, CHCNCH ), 4.06 (s, 4H, CH Pz), 1.68 (s, 6H, Me). 1.34 (s,
8
) = 7.71-7.69 (d, 8H, JH-H = 8 Hz, Ar), 7.23-7.19
.04 (s, 4H, CH Pz), 1.65 (s, 6H, Me), 1.30 (s, 6H, Me), -23.80 (s,
2
pz
13
q
8
H, Ni-H) ppm. C-NMR(101 MHz, THF-d ) = 158.86 (C -Me),
57.18 (C ), 156.98 (C -Me), 156.64 (C ), 143.79 (C ), 136.71
3
2
13
Ar
q
pz
Ar
8
H, Me), -23.61 (s, 2H, Ni-H) ppm. C-NMR(101 MHz, THF-d )
Ar
Ar
Ar
Ar
Ar
(C ), 131.42 (C ), 130.99 (C ), 130.15 (C ), 128.26 (C ),
1
q
pz
q
Ar
159.25 (C -Me), 157.38 (C ), 157.35 (C -Me), 157.20 (C ),
Ar
Ar
Ar
Ar
28.21 (C ), 126.31 (C ), 125.71 (C ), 124.12 (C ), 97.82
Ar
Ar
Ar
Ar
143.69 (C ), 131.42 (C ), 131.38 (C ), 131.04 (C ), 128.41
(C ), 128.18 (C ), 126.63 (C ), 124.37 (C ), 98.04 (CHCNCH ),
3
9
Pz
(CHCNCH
3
), , 92.31 (CH ), 53.42 (CH
2
), 23.54 (CH
3
), 20.61
Ar
Ar
Ar
Ar
(CH ) ppm. Crystallization attempts of 3-K even in a dihydrogen
3
Pz
2 3 3
2.94 (CH ), 53.48 (CH ), 23.51 (CH ), 20.64 (CH ) ppm.
saturated solution always yielded a mixture of 2-K and 3-K.
However, crystals of 3-K(crypt) could be obtained as follows: 2-
K (5.0 mg, 5.6 µmol, 1.0 eq.) was dissolved in minimal amounts of
Complex 5-K. 2-K (20 mg, 0.020 mmol, 1.0 eq.) was dissolved in
THF. Benzaldehyde (2.2 µL, 2.3 mg, 0.020 mmol, 1.0 eq.) was
added to the red solution. After 2 hours of stirring the solution
changed to a dark brown color. Crystallization via vapor diffusion
into the THF solution, gave 5-K as brown crystals (15 mg,
8
THF-d (0.5 mL). [2.2.2]Cryptand (2.1 mg, 5.6 µmol, 1.0 eq.) was
added as a solid and the resulting solution was stirred for 5 mins.
After degassing the dark red solution three times, an atmosphere of
dry dihydrogen was applied (~1 atm.). The product 3-K(crypt) was
used in situ for further NMR investigations, due to the instability in
solution over a longer period of time. Single crystals were obtained
after cooling the solution to -30°C.
1
0
.014 mmol, 68%). H-NMR (400 MHz, C
6 6
D ) δ = 8.83 (d, 2H, JH-
H
= 8 Hz Ar), 8.03 (d, 2H, JH-H = 8 Hz , Ar), 7.71 (d, 2H, JH-H
=
4 Hz, Ar), 7.54 (d, 2H, JH-H = 8 Hz, Ar), 7.50 (d, 1H, JH-H = 8 Hz,
Ar) 7.31 – 7.21 (m, 6H, Ar), 7.13 – 7.05 (m, 6H, Ar) 6.97 – 6.87
(
m, 6H, Ar), 6.78 – 6.69 (m, 4H, , Ar), 5.83 (s, 1H, CHpz), 4.61 (s,
H, CHCNCH ), 4.36 (s, 1H, CHCNCH3), 4.13 (d, 1H, JH-H = 16
Hz, CH Pz), 4.03 (d, 1H, JH-H = 16 Hz, CH Pz), 3.68 (s, 2H,
CH Pz), 2.03 (s, 1H, PhCHO), 1.61 (s, 3H, Me), 1.56 (s, 3H, Me),
.36 (s, 3H, Me), 1.22 (s, 3H, Me) ppm. C-NMR(101 MHz,
C6D6) δ = 159.92 (C -Me), 159.62 (C -Me), 158.68 (C -Me),
57.93 (C -Me), 156.76 (Cpz), 153.62 (Cpz ), 150.38 (CAr), 148.28
Complex 2-Na. and Na[LNi
1
2 2
(H) ] (3-Na). The green suspension of
(0.10 g, 0.11 mmol, 1.0 eq.) in THF (2 mL) was treated with
1
3
2
2
sodium triethyl borohydride in THF (1 M, 0.30 mL, 0.30 mmol, 3.0
eq.). After stirring for 1 hour the suspension changed to a dark red
color. The solution was filtered, and the solvent was removed under
reduced pressure. The remaining solid was washed several times
with hexane (3 x 10 mL) and the residual solvent was removed. Red
tabular crystal mixture of 2-Na and 3-Na were obtained by vapor
diffusion of pentane into a THF solution.
2
13
1
q
q
q
1
q
(
1
C
Ar), 148.14 (CAr), 146.64 (CAr), 143.88 (CAr), 142.92 (CAr),
40.72 (CAr), 140.68 (CAr), 140.44 (CAr), 138.52 (CAr), 135.24
Ar), 134.34 (CAr), 132.07 (CAr), 131.23 (CAr), 131.03 (CAr),
(C
Alternative Synthesis for 2-Na: The green suspension of 1 (0.20 g,
0
of sodium naphtalide in THF (0.50 mL, 0.50 mmol, 2.4 eq.). After
2 hours, the red solution was filtered, and the solvent was
evaporated. After washing with hexane (3 x 5 mL) 2-Na was
130.86 (CAr), 130.73 (CAr), 130.03 (CAr), 129.52 (CAr), 129.21
(CPh-CHO), 127.47 (CAr), 127.22 (CAr), 126.35 (CAr), 125.95 (CAr),
125.59 (CAr), 125.33 (CAr), 125.20 (CAr), 124.39 (CAr), 124.04
(CAr), 97.63 (CHCNCH3), 97.27 (CHCNCH3), 93.15 (4-Pz), 86.70
.21 mmol, 1.0 eq.) in THF (4 mL) was treated with a 1 M solution
2 2
(CHO), 51.97 (CH Pz), 50.09 (CH Pz), 27.98 (Me), 24.41 (Me),
1
-1
isolated as a red solid (0.18 g, 0.20 mmol, 95 %). 2-Na: H-NMR
22.12 (Me), 21.40 (Me) ppm. IR (ATR) ν/cm = 3050 (vw), 3025
(vw) 2973 (vw), 2912 (vw), 2863 (vw), 1560 (m), 1519 (s), 1493
(m), 1446 (m), 1426 (s), 1386 (vs), 1313 (m), 1274 (s), 1212 (m),
1091 (m), 1045 (m), 1023 (s), 915 (w), 893 (w), 834 (w), 802 (w),
741 (vs), 721 (s), 698 (vs), 639 (m), 601 (s), 542 (m), 480 (m), 457
(400 MHz, THF-d
8
) = 8.58 (d, , JH-H = 8 Hz, 2H, Ar), 7.85 (d, JH-
= 8 Hz 1H, Ar), 7.49 (d, , JH-H = 8 Hz, 2H, Ar), 7.43 (d, , JH-H = 8
H
Hz, 1H, Ar), 7.16-7.07 (m, 17H, Ar), 6.38 (t, , JH-H = 8 Hz, 1H, Ar),
6.27 (t, ,JH-H = 8 Hz, 1H, Ar), 5.69 (s, 1H, CH ), 4.49 (s, 1H,
CHCNCH ), 4.41 (d, , JH-H = 18 Hz, 1H, CH Pz), 4.32 (s, 1H,
3 2
pz
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(
6 2
m), 417 (w). Anal. Calcd. for C58H51KN Ni O ∙ 2THF (1092.3
ACKNOWLEDGMENT
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
g/mol) = C 69.13, H 5.71, N 7.33; Found C 68.61, H 5.64, N 7.37.
P.-C.D. thanks the China Scholarship Council for a PhD
scholarship.
Complex 6-Na. 2-Na (0.10 g, 0.11 mmol, 1.0 eq.) was dissolved in
a minimal amount of THF (1 mL). The red solution was heated for
at least three days at 60 °C in a closed vessel. 6-Na (95 mg, 0.11
mmol, 96%) was isolated after evaporation of the solvent as a red
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1
on top of the THF solution. H-NMR (400 MHz, THF-d
8
) = 7.76
(d, 4H, JH-H = 8 Hz Ar), 7.51 (d, 2H, JH-H = 8 Hz , Ar), 7.30 (d, 2H,
(
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H-H = 8 Hz, Ar), 7.09 (t, 2H, JH-H = 8 Hz, Ar), 6.97 – 6.87 (m, 4H,
Ar), 6.69 (d, 2H, JH-H = 8 Hz, Ar), 6.29 (t, 2H, JH-H = 8 Hz, Ar),
pz
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5.94 (t, 2H, JH-H = 8 Hz, Ar), 5.70 (s, 1H, CH ), 4.30 (d, 2H, JH-H =
(
1
CH
6 Hz CH
2
Pz), 4.23 (s, 2H, CHCNCH
3
), 4.12 (d, 2H, JH-H = 16 Hz
1
3
2
Pz), 1.81 (s, 6H, Me). 1.02 (s, 6H, Me) ppm. C-
(4)
Ogo, S. H
Insights from Model Complexes. Coord. Chem. Rev.
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2 2
and O Activation by [NiFe]Hydrogenases –
q
q
8
NMR(101 MHz, THF-d ) = 158.69 (C -Me), 158.39 (C -Me),
1
Ar
pz
Ar
Ar
56.54 (C ), 156.27 (C ), 144.94 (C ), 143.55 (C ), 140.00
2
Ar
Ar
Ar
Ar
Ar
(C ), 139.07 (C ), 137.47 (Ni-C ), 135.39 (C ), 130.49 (C ),
(5)
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.
Ar
Ar
Ar
Ar
127.81 (C ), 126.65 (C ), 126.58 (C ), 125.04 (C ), 122.68
(C ), 121.66 (C ), 120.85 (C ), 120.38 (C ), 101.84
Ar
Ar
Ar
Ar
Pz
(
(
2
1
CHCNCH
CH ), 20.56 (CH
973 (vw), 2912 (vw), 2863 (vw), 1560 (m), 1519 (s), 1493 (m),
446 (m), 1426 (s), 1386 (vs), 1313 (m), 1274 (s), 1212 (m), 1091
3
), 91.05 (CH ), 52.69 (CH
2
1
), l23.72 (CH
3
), 24.14
-
3
3
) ppm. IR (ATR) ν/cm = 3050 (vw), 3025 (vw)
(6)
Brazzolotto, D.; Gennari, M.; Queyriaux, N.; Simmons, T.
R.; Pécaut, J.; Demeshko, S.; Meyer, F.; Orio, M.; Artero,
V.; Duboc, C. Nickel-Centred Proton Reduction Catalysis
in a Model of [NiFe] Hydrogenase. Nat. Chem. 2016, 8,
(
m), 1045 (m), 1023 (s), 915 (w), 893 (w), 834 (w), 802 (w), 741
(vs), 721 (s), 698 (vs), 639 (m), 601 (s), 542 (m), 480 (m), 457 (m),
417 (w). Anal. Calcd. for NaNi ∙3THF
1096.64 g/mol) = C 69.00, H 6.16, N 7.66; Found C 68.73, H
1
054–1060.
C
51
H
43
N
6
2
(
7)
8)
Brazzolotto, D.; Wang, L.; Tang, H.; Gennari, M.;
Queyriaux, N.; Philouze, C.; Demeshko, S.; Meyer, F.;
Orio, M.; Artero, V.; Hall, M. B.; Duboc, C. Tuning
Reactivity of Bioinspired [NiFe]-Hydrogenase Models by
Ligand Design and Modeling the CO Inhibition Process.
ACS Catal. 2018, 8, 10658–10667.
(
6
.03, N 7.74.
ASSOCIATED CONTENT
Supporting Information
(
Yang, J. Y.; Bullock, R. M.; DuBois, M. R.; DuBois, D.
L. Fast and Efficient Molecular Electrocatalysts for H
2
The Supporting Information is available free of charge on the ACS
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IR spectra.
Production: Using Hydrogenase Enzymes as Guides. MRS
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Yao, S.; Driess, M. Lessons from Isolable Nickel(I)
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Zimmermann, P.; Limberg, C. Activation of Small
Molecules at Nickel(I) Moieties. J. Am. Chem. Soc. 2017,
Corresponding Author
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39, 4233–4242.
E-mail: franc.meyer@chemie.uni-goettingen.de.
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Yoo, C.; Lee, Y. A T-Shaped Nickel(I) Metalloradical
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Weix, D. J. Methods and Mechanisms for Cross-
ORCID
(
2
Franc Meyer: 0000-0002-8613-7862
Peng-Cheng Duan: 0000-0003-2516-3618
Sebastian Dechert: 0000-0002-2864-2449
Electrophile Coupling of Csp Halides with Alkyl
Electrophiles. Acc. Chem. Res. 2015, 48, 1767–1775.
Milligan, J. A.; Phelan, J. P.; Badir, S. O.; Molander, G.
A. Alkyl Carbon-Carbon Bond Formation by
Nickel/Photoredox Cross-Coupling. Angew. Chem. Int.
Ed. 2019, 58, 6152–6163.
(13)
Present Address
(14)
Keen, A. L.; Doster, M.; Johnson, S. A. 1,4-Shifts in a
Dinuclear Ni(I) Biarylyl Complex: A Mechanistic Study
of C−H Bond Activation by Monovalent Nickel. J. Am.
Chem. Soc. 2007, 129, 810–819.
§
P.-C.-D.: Center for Catalysis and Florida Center for Heterocyclic
Compounds, Department of Chemistry, University of Florida,
Gainesville, Florida, 32611-7200, United States of America.
(
15)
16)
Pfirrmann, S.; Limberg, C.; Herwig, C.; Knispel, C.;
Braun, B.; Bill, E.; Stösser, R. A Reduced β-Diketiminato-
Ligated Ni H Unit Catalyzing H/D Exchange. J. Am.
3 4
Author Contributions
‡
H.S. and P.-C.D.: These authors contributed equally.
Chem. Soc. 2010, 132, 13684–13691.
(
Zarate, C.; Yang, H.; Bezdek, M. J.; Hesk, D.; Chirik, P.
J. Ni(I)–X Complexes Bearing a Bulky α-Diimine Ligand:
Synthesis, Structure, and Superior Catalytic Performance
in the Hydrogen Isotope Exchange in Pharmaceuticals. J.
Am. Chem. Soc. 2019, 141, 5034–5044.
Funding Sources
This work was financially supported by the University of Göttingen
and the China Scholarship Council.
Notes
(17)
Pfirrmann, S.; Yao, S.; Ziemer, B.; Stösser, R.; Driess, M.;
Limberg, C. β-Diketiminato Nickel(I) Complexes with
The authors declare no competing financial interests.
2 2
Very Weak Ligation Allowing for H and N Activation.
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Organometallics 2009, 28, 6855–6860.
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Mindiola, D. J. Nacnac … Are You Still There? The
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(18)
(19)
(20)
(21)
(22)
(35)
A similar situation has been found previously in anionic
+
pyrazolato-bridged bimetallic complexes with
K
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
countercations; see, e.g.: (a) Röder, C. J.; Meyer, F.;
Kaifer, E. Bifunctional Cp∩N Complexes – Unusual
Structural Features and Electronic Coupling in Highly
Preorganized Bimetallic Systems. Angew. Chem. Int. Ed.
2002, 41 (13), 2304-2306.
It is also a common binding motive in potassium
tris(pyrazol-1-yl)borates, see for example: Bieller, S.;
Bolte, M.; Lerner, H.-W.; Wagner, M. Unusual
Coordination Modes for Tris(pyrazol-1-yl)borates:
(23)
(24)
Synthesis
and
Solid
State
Structures
of
Sodium[(dimethylamino)tris(pyrazol-1-yl)borate],
Potassium[(dimethylamino)tris(pyrazol-1-yl)borate], and
Potassium[phenyltris(pyrazol-1-yl)borate]. Z. Anorg.
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Harder, S.; Limberg, C. Hydride Reactivity of Ni -X-Ni
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629–1636.
Manz, D.-H.; Duan, P.-C.; Dechert, S.; Demeshko, S.;
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2
/
D
2
2
(38)
(39)
Dinickel(II) Complex Featuring Two Terminal Hydrides.
J. Am. Chem. Soc. 2017, 139, 16720–16731.
Duan, P.-C.; Manz, D.-H.; Dechert, S.; Demeshko, S.;
(28)
Meyer, F. Reductive O
Leading to Redox Interconvertible μ-1,2-Peroxo and μ-
,2-Superoxo Dinickel(II) Intermediates. J. Am. Chem.
2
Binding at a Dihydride Complex
1
N Reduction. Proc. Natl. Acad. Sci. 2018, 115, E10521–
2
Soc. 2018, 140, 4929–4939.
E10530.
(
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Maity, A.; Teets, T. S. Main Group Lewis Acid-Mediated
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(
Ferretti, E.; Dechert, S.; Meyer, F. Reductive Binding and
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015, 137, 6104–6110.
(32)
Hartline, D. R.; Zeller, M.; Uyeda, C. Catalytic
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FOR TABLE OF CONTENTS ONLY:
1
2
3
4
5
6
7
8
9
substrate
N
N
N
N
N
N
+
H2
Ni I
Ph
NiI
- H2
Ph
Ph
Ph
N
N
N
N
N
N
N
N
N
N
N
N
NiII
NiII
NiII
NiII
PhCHO
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
K+
N
N
N
N
O
N
NiII
Ph
NiII
Ph
N
Ph
Ph
H
Ph
1
3
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