Hydride reactivity of NiII-X-NiII entities: Mixed-valent hydrido complexes and reversible metal reduction

Article abstract of DOI:10.1002/chem.201203201

After the lithiation of PYR-H₂ (PYR^2-=[{NC(Me)C(H) C(Me)NC₆H₃(iPr)₂}₂(C ₅H₃N)]^2-), which is the precursor of an expanded β-diketiminato ligand system with two binding pockets, its reaction with [NiBr₂(dme)] led to a dinuclear nickel(II)-bromide complex, [(PYR)Ni(μ-Br)NiBr] (1). The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(μ-SEt)NiBr] (3). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt₃. This treatment afforded [(PYR)Ni(μ-H)Ni] (2), which is a mixed valent Ni^I-μ-H-Ni^II complex, and [(PYR-H)Ni(μ-SEt)Ni] (4), in which two tricoordinated Ni^I moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the Ni^I centres. The addition of a mild acid to complex 4 leads to the elimination of H₂ and the formation of a Ni^IINi^II compound, [(PYR)Ni(μ-SEt)NiOTf] (5), so that the original Ni^II(μ-SEt) Ni^IIX core of compound 3 is restored. All of these compounds were fully characterized, including by X-ray diffraction, and their molecular structures, as well as their formation processes, are discussed. Copyright

Full text of DOI:10.1002/chem.201203201

FULL PAPER
DOI: 10.1002/chem.201203201
II
II
ꢀ ꢀ
Hydride Reactivity of Ni X Ni Entities: Mixed-Valent Hydrido
Complexes and Reversible Metal Reduction
Henrike Gehring,[a] Ramona Metzinger,[a] Christian Herwig,[a] Julia Intemann,[b]
Sjoerd Harder,[b] and Christian Limberg*[a]
I
II
ꢀ
ꢀ
Abstract: After the lithiation of PYR-
H2
(PYR2ꢀ =[{NC(Me)C(H)C(Me)-
NC6H3A(iPr)2}2A
(C5H3N)]2ꢀ), which is the
precursor of an expanded b-diketimina-
to ligand system with two binding
mixed valent Ni m-H Ni complex,
and [(PYR-H)Ni(m-SEt)Ni] (4), in
by one of the binding pockets, thereby
converting it into an olefin ligand on
one of the NiI centres. The addition of
a mild acid to complex 4 leads to the
elimination of H2 and the formation of
AHCTUNGTRENNUNG
G
G
E
which two tricoordinated NiI moieties
are strongly antiferromagnetically cou-
pled. Compound 4 is the product of an
initial salt metathesis, followed by an
intramolecular redox process that sepa-
rates the original hydride ligand into
two electrons, which reduce the metal
centres, and a proton, which is trapped
pockets, its reaction with [NiBr
led to a dinuclear nickel(II)–bromide
complex, [(PYR)Ni(m-Br)NiBr] (1).
2A
a NiIINiII compound, [(PYR)Ni
NiOTf] (5), so that the original NiII
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
SEt)NiIIX core of compound 3 is re-
stored. All of these compounds were
fully characterized, including by X-ray
diffraction, and their molecular struc-
tures, as well as their formation proc-
esses, are discussed.
The bridging bromide ligand could be
selectively exchanged for a thiolate
ligand to yield [(PYR)NiACHTNUTRGNE(NUG m-SEt)NiBr]
(3). In an attempt to introduce hydride
ligands, both compounds were treated
with KHBEt3. This treatment afforded
Keywords: bridging ligands · hy-
drides · mixed-valent compounds ·
nickel · reduction
[(PYR)NiACHTUNGTRENNUNG(m-H)Ni] (2), which is a
Introduction
(see below). We used the b-diketiminato ligand system pre-
viously because it is known to stabilise both high and low
oxidation states[5] and, furthermore, it can be employed to
access complexes with low coordination numbers, which
often show interesting reactivities.[6] In this context, we have
The combination of nickel and hydride ligands leads to
highly reactive systems that are widely utilised in academic
and industrial laboratories, such as in the field of homogene-
ous catalysis for the transformation of organic or inorganic
substrates and in the electrolytic generation of H2.[1,2] The
nickel–hydride functionality also plays a key role in nature.
For instance, the central intermediate in the catalytic cycle
of [NiFe] hydrogenase is assumed to contain a bridging hy-
dride ligand between the Ni and Fe centres.[3] Altogether,
recently reported the synthesis of [LNi
ACHTUNGRTEN(NUNG m-H)2NiL] (I, L=
[HC(CMeNC6H3A
A
CHTUNGTRENNUNG
be very reactive: On the addition of simple donors (Do), H2
was immediately reductively eliminated with the concomi-
tant formation of [LNiI(Do)] complexes.[7]
these factors have led to great interest in investigations on
[4]
ꢀ
the Ni H coordination compounds.
With this background in mind, we are interested in the
fundamental hydride/dihydrogen chemistry of dinuclear
nickel cores, the results of which may also have some rele-
vance to certain aspects of [NiFe] hydrogenase reactivity
[a] H. Gehring, R. Metzinger, C. Herwig, Prof. Dr. C. Limberg
Institut fꢀr Anorganische Chemie
Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax : (+49)030-2093-6966
E-mail: christian.limberg@chemie.hu-berlin.de
[b] J. Intemann, Prof. Dr. S. Harder
Stratingh Institute of Chemistry
University of Groningen
Nijenborgh 4, 9747 AG Groningen (Netherlands)
Scheme 1. Synthesis of [LNiACHTNUTRGNE(NUG m-H)2NiL] (I) through two different reaction
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203201.
pathways: A metathesis reaction, starting from a nickel(II) bromide, or
oxidative addition of H2 to a nickel(I) precursor.
Chem. Eur. J. 2013, 00, 0 – 0
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Starting from LNiI complexes in the complete absence of
suitable donor molecules, the reverse reaction, that is, the
oxidative addition of H2, has also been observed
(Scheme 1).[8,9] This result is particularly valuable with
regard to the mechanistic discussion on H2 activation at the
[NiFe] hydrogenase active site: Interestingly, a mechanism
that is based on the oxidative addition of H2 to a NiI inter-
mediate has been derived for these enzymes from theoreti-
cal results[10] and this proposal now has some experimental
support. Moreover, it was shown that a nickel(II)–hydride
dimer (I) can accept one electron to give a mixed-valent
as a dark-green solid. By cooling a solution in toluene/
n-hexane to ꢀ308C, crystals that were suitable for X-ray dif-
fraction could be obtained.
The molecular structure of complex
1 is shown in
Figure 1. One nickel ion is in an almost square-planar coor-
dination environment, whereas the other is surrounded by
NiINiII–hydride compound, K
ACTHNUGTRNENUG[LNiACHTUNGTNER(NUGN m-H)2NiL], with localised
spins.[11] However, further reduction triggers a complicated
reaction sequence that reproducibly—and in good yields—
leads to the complex K2ACHTNUTRGENN[UG (LNi)2NiACHTUNGTRENN(GUN m-H)4], which features an
unprecedented Ni3H4 core.[11] Altogether, these results
showed that b-diketiminato-ligated nickel–hydride moieties
can offer versatile redox chemistry, thereby leading to inter-
esting electronic situations and reactivities.
The work described herein
builds upon these results. It sets
out with
[{NC(Me)C(H)C(Me)NC6H3-
(iPr)2}2A
(C5H3N)]2ꢀ (PYR2ꢀ), in
which two diketiminato units
are prearranged by a pyridine
a
ligand system,
Figure 1. Molecular structure of complex 1; ellipsoids are set at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢃ] and angles [8]: N1 Ni1 1.884(2), N2 Ni1 1.837(3), N3 Ni1
CTHUNGTRENNUNG
ꢀ
ꢀ
ꢀ
ꢀ
1.950(2), Ni1 Br1 2.3568(6), Ni1 Ni2 3.4290(7), Ni2 Br1 2.4455(5), Ni2
ꢀ
ꢀ
ꢀ
Br2 2.3544(6), Ni2 N4 1.944(2), Ni2 N5 1.949(2), Ni2 N3 3.059(8); N1-
Ni1-N2 93.87(11), N1-Ni1-Br1 98.73(8), Br1-Ni1-N3 98.67(7), N3-Ni1-N2
68.72(11), N4-Ni2-N5 94.71(10), Br1-Ni2-Br2 108.42(2), Br2-Ni2-N4
117.81(8), N5-Ni2-Br1 104.43(7).
linker for the complexation of
two metal centres in a fashion
that allows for their coopera-
tion.
Therefore, this ligand system is ideally suited for studies
on multimetallic complexes; for example, several zinc, calci-
um, and magnesium–amidoboACTHNUTRGENUGNrane and –hydride complexes,
have successfully been accessed by using PYR or related li-
gands.[12] Herein, we report the results that were obtained by
employing this ligand for the investigation of dinuclear
nickel–hydride chemistry.
its ligand in a slightly distorted tetrahedral fashion; there is
one bridging bromide ligand between the Ni ions and one
additional terminal bromide ligand. The Br1-Ni2-Br2, Br2-
Ni2-N4, N4-Ni2-N5, and N5-Ni2-Br1 angles are 108.42(2),
117.81(8), 94.71(10), and 104.43(7)8 and, thus, are close to
the valence angles for a tetrahedral coordination geometry
ꢀ
ꢀ
(109.478). The Ni2 N4 and Ni2 N5 distances (1.994(2) and
ꢀ
ꢀ
1.949(2) ꢃ) are somewhat longer than N1 Ni1 and N2 Ni1
(1.884(2) and 1.837(3) ꢃ), but similar to the corresponding
distances in related tetrahedrally coordinated nickel com-
plexes that contain b-diketiminato ligands.[8] In addition, Ni1
has a contact with the N3 atom, at a distance of 1.950(2) ꢃ,
although the corresponding pyridyl unit is considerably mis-
aligned; therefore, donation has to proceed through a “bent
bond”. Bearing in mind that some complexes are known
that feature a bridging pyridyl ligand unit between two
metal centres,[13] in principle, such a binding mode could be
relevant here, too. However, considering that the distance
between the Ni2 and N3 atoms is 3.059(8) ꢃ, this effect
doesn’t seem to apply.
Results and Discussion
Similar to our earlier investigations (Scheme 1), our first
target was the synthesis of a suitable nickel–bromide precur-
sor. Hence, PYR-H2 was lithiated with nBuLi and treated
with [NiBr
to yield the dinickel(II) complex [(PYR)NiACHTUNGTRENNUNG
(dme)] (dme=1,2-dimethoxyethane; Scheme 2)
(m-Br)NiBr] (1)
The magnetic moment for compound 1 in solution at
room temperature (meff =3.03 mB), as determined by using
the Evans method, is in accordance with the spin only (s.o.)
value for two unpaired electrons on one nickel centre with a
total spin quantum number of S=1 (ms.o. =2.83 mB). Because
the average magnetic moment for tetrahedral nickel(II)
complexes is m=(3.20ꢁ0.20),[14] it is reasonable to assume
II
II
ꢀ
Scheme 2. Synthesis of Ni Ni Br complex 1.
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Hydride Reactivity of NiII-X-NiII Entities
FULL PAPER
that the magnetic moment of complex 1 results from the tet-
rahedrally coordinated Ni2 atom, whilst all of the spins on
the Ni1 atom with a distorted square-planar ligand arrange-
ment are paired.
1
The H NMR resonances are relatively sharp, but they are
paramagnetically shifted over a wide range from d=+50 to
ꢀ50 ppm. A resonance set was observed that was character-
istic of a symmetrical complex (see the Supporting Informa-
tion), which may be explained by a dynamic behaviour of
the terminal bromide ligand, that is, by its fast shift on the
NMR timescale. Cooling an NMR sample to ꢀ708C
([D8]toluene) led to several changes. However, because
these changes included a significant broadening and shifting
of resonances, a straightforward interpretation in terms of
decoalescence was not possible.
Dinickel complex 1 represents a suitable starting material
for the synthesis of hydride compounds by its reaction with
KHBEt3 or salt metathesis with other metal hydrides. A di-
I
II
I
I
ꢀ
ꢀ
ꢀ
Scheme 3. Synthesis of Ni Ni H complex 2 and Ni m-SEt Ni complex
4 (LuHOTf=lutidinium triflate).
nickel(II)–dihydride compound, similar to complex
I
(Scheme 1), was principally conceived as the product, but,
the fact that, within complex 1, one of the bromide ligands
remained terminal, already indicated that a Ni
G
dride ligand between the two nickel centres was found in
ꢀ
might not be accessible. Furthermore, complex I had been
found to be unstable with respect to H2 elimination in the
presence of donors, such as pyridines. Because PYR-H2 fea-
tures a built-in pyridine unit, we anticipated that the use of
this ligand could induce a partial loss of H2. Simple replace-
ment of the two bromide ligands by hydride ligands would
the difference Fourier map. The Ni1 H1 distance
ꢀ
(1.50(2) ꢃ) is similar to previously reported Ni H distances
II
ꢀ
for Ni H units in a square-planar coordination environ-
ment (e.g., 1.49(5), 1.43(5), and 1.505(20) ꢃ),[11,17] whilst the
ꢀ
ꢀ
Ni2 H1 bond is somewhat longer (1.62(2) ꢃ). The Ni Ni
distance (2.5966(3) ꢃ) is much shorter than the correspond-
ing one in complex 1 (3.4290(7) ꢃ). However, DFT calcula-
tions (B3LYP/Def2-SVP/TZVPD,[18] see the Supporting In-
formation) excluded a Ni···Ni bonding interaction and re-
vealed that the nature of the Ni m-H Ni moiety is not as
straightforward as Figure 2 may suggest. Rather, it should
be viewed as a Ni H unit that is coordinated to a Ni ion,
that is, as a Ni H!Ni moiety, which also explains the ob-
II
II
ꢀ
ꢀ
ꢀ
lead to a Ni m-H Ni H moiety, which resembles the
II
III
ꢀ
ꢀ
ꢀ
Fe m-H Ni H intermediate that has been proposed for
the [NiFe] hydrogenase in a theoretical study.[10,15] This
II
I
ꢀ
ꢀ
latter structure represents an energy maximum within the
II
ꢀ
system and releases a proton and an electron to give Fe m-
II
II
I
ꢀ
ꢀ
H Ni in an exothermic process. There is not much prece-
II
II
II
I
ꢀ
ꢀ
ꢀ
ꢀ
dent for Ni m-H Ni H entities in molecular chemistry, al-
ꢀ
though a few examples are known: [(dippe)2Ni2H3]
[PPh4],
served Ni H bond lengths.
[(dcype)2Ni2H3][PPh4], and [(dippe)2Ni2(H)3][BEt4] (dippe=
A
ACHTUNGTRENNUNG
1,2-bis(diisopropylphosphino)-ethane, dcype=1,2-bis(dicy-
clohexylphosphino)ethane).[16]
Adding KHBEt3 to a solution of compound 1 in toluene
immediately led to a colour change from green to red, ac-
companied by the appearance of an absorption band at
560 nm in the UV/Vis spectrum (see the Supporting Infor-
mation). Subsequent work-up led to a dinuclear NiIINiI com-
plex, [(PYR)Ni
ACHTUNGTRENNUNG(m-H)Ni] (2), as red crystals, which featured
a
bridging hydride ligand between the metal centres
II
II
ꢀ
ꢀ
ꢀ
(Scheme 3). Perhaps, first of all, a Ni m-H Ni H complex
had formed, as suggested above. However, because the b-di-
ketiminato ligand stabilises the NiI oxidation state, the
system prefers to eliminate a H atom in the form of H2,
which could be detected (see the Supporting Information).
Crystals suitable for X-ray diffraction could be obtained
by the slow evaporation of the solvent from a solution in
Et2O. The molecular structure of compound 2 shows that
one of the two nickel centres is surrounded by its ligands in
a distorted square-planar fashion, whereas the second one is
located in a trigonal coordination sphere. The bridging hy-
Figure 2. Molecular structure of complex 2; ellipsoids are set at 50%
probability. Hydrogen atoms, beside the bridging hydride atoms, are
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: N1 Ni1
ꢀ
ꢀ
ꢀ
1.8800(13), N2 Ni1 1.8586(14), N3 Ni1 1.9049(13), Ni1 Ni2 2.5966(3),
ꢀ
ꢀ
ꢀ
ꢀ
Ni2 N4 1.9493(14), Ni2 N5 1.8946(13), Ni2 N3 2.7449(14), Ni1 H1
ꢀ
1.50(2), Ni2 H1 1.62(2); N1-Ni1-N2 94.62(6), N4-Ni2-N5 96.33(6).
Chem. Eur. J. 2013, 00, 0 – 0
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C. Limberg et al.
As in complex 1, the square-planar-coordinated Ni1 atom
can be expected to be in the low-spin state (S=0), whereas
the Ni2 atom contains one unpaired electron (as confirmed
by DFT calculations). Correspondingly, the magnetic
As in complex 1, one of the nickel centres adopts a
square-planar coordination geometry, whilst the other one is
surrounded by its ligands distorted tetrahedrally. The mag-
netic moment for its solution in [D6]benzene at room tem-
perature, meff =3.06 mB, is typical for tetrahedral S=1 nickel
systems (ms.o. =2.83 mB). DFT calculations confirmed the S=
1 ground state of complex 3. Analysis of a solution in
MeCN by mass spectrometry (ESI) showed a signal at m/z
766.2951, which corresponded to [MꢀBr]+, with a character-
istic isotopic distribution that matched the calculated isotop-
ic distribution (see the Supporting Information).
moment of complex 2 was found to be meff =1.82 mB (ms.o.
=
1.73 mB) at room temperature (Evans method). This result is
in accordance with the EPR spectrum that was recorded for
a solution after cooling to 77 K, which showed a rhombic
signal (gx =2.054, gy =2.159, gz =2.454) that is typical for NiI
compounds with one unpaired electron (S=1/2; Figure 3).[19]
Consequently, in the next step, we attempted to replace
the remaining bromide ligand in complex 3 by a hydride
ligand. Indeed, an EXD investigation on the product of the
reaction between complex 3 and KHBEt3 confirmed the ab-
sence of bromide, indicating a conversion as envisaged.
However, as subsequently revealed by single-crystal X-ray
diffraction analysis (Figure 5) the expected hydride com-
Figure 3. EPR spectrum of complex 2 in toluene at 77 K. Red: Experi-
mental spectrum with MgO/Cr3+ (*) as an internal standard; blue: Pow-
der simulation with the indicated g values.
Within the active site of [NiFe] hydrogenase, the nickel
centre is in a cysteinate environment, which presumably aids
the redox transformations. Hence, we envisaged introducing
a thiolate ligand into the [(PYR)Ni2]2+ system prior to hy-
dride transfer. This goal was achieved by the reaction of
compound 1 with KSEt, which led to a selective replace-
ment of the bridging bromide by a thiolate ligand and, thus,
to complex [(PYR)NiACTHNUGRTENUNG(m-SEt)NiBr] (3; Scheme 3), whose
molecular structure is shown in Figure 4.
Figure 5. Molecular structure of complex 4·n-hexane; ellipsoids are set at
50% probability. The non-coordinating n-hexane molecule in the unit
cell and the hydrogen atoms are omitted for clarity. Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢃ] and angles [8]: N1 Ni1 1.888(5), N2 Ni1 1.896(4), N3 Ni1
ꢀ
ꢀ
ꢀ
ꢀ
2.9174(4), Ni1 S1 2.1131(13), Ni1 Ni2 2.3619(9), Ni2 N3 1.914(4), Ni2
ꢀ
ꢀ
C15 2.059(5), Ni2 C16 2.032(5), Ni2 S1 2.1471(14); N1-Ni1-N2
93.35(14), Ni1-S1-Ni2 67.34(4).
pound, [(PYR-H)Ni
ACHTUNGTRENNUNG
formed; rather, an isomer, [(PYR-H)NiACHTUNGTRENNNUG
obtained, which could formally be derived from complex 4’
through the following process: 1) Heterolytic cleavage of
the initially formed Ni H bond into H and Ni0; 2) proto-
nation of one diketiminato binding pocket with the removal
of Ni0; 3) coordination of Ni0 onto the pyridyl unit and the
formation of a NiINiI core after interaction with the remain-
ing NiII ion; 4) saturation of the coordination sphere of the
released Ni centre by a C=C unit of the diketimine. Alto-
gether, the reaction may be compared to the reduction of a
nickel centre by a hydride, with the concomitant trapping of
H+ by a pendant base, as observed by DuBois and co-work-
ers.[2b,4g,20]
II
+
ꢀ
Figure 4. Molecular structure of complex 3·toluene; ellipsoids are set at
50% probability. The non-coordinating toluene molecule in the unit cell
and the hydrogen atoms are omitted for clarity. Selected bond lengths
ꢀ
ꢀ
ꢀ
[ꢃ] and angles [8]: N1 Ni1 1.895(3), N2 Ni1 1.858(3), N3 Ni1 1.960(3),
ꢀ
ꢀ
ꢀ
ꢀ
Ni1 S1 2.2355(10), Ni1 Ni2 3.2072(7), Ni2 S1 2.2715(10), Ni2 Br1
ꢀ
ꢀ
ꢀ
2.3764(6), Ni2 N4 1.958(3), Ni2 N5 1.952(3), Ni2 N3 2.9536(19); N1-
Ni1-N2 93.58(13), N4-Ni2-N5 94.40(12), S1-Ni2-Br1 103.99(3), Br1-Ni2-
N4 125.83(9), N5-Ni2-S1 111.26(9).
ꢀ
The Ni Ni distance in complex 4 is 2.3619(9) ꢃ, which is
much shorter than that in the other complexes (1–3). Com-
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Hydride Reactivity of NiII-X-NiII Entities
FULL PAPER
paring this nickel–nickel separation to that in other related
NiI complexes suggests an analogous interpretation of its
I
I
[21]
ꢀ
structures in terms of a Ni Ni bond.
To evaluate the
bonding situation in complex 4, DFT calculations were per-
formed (B3LYP/Def2-SVP/TZVPD,[18] see the Supporting
Information). A broken-symmetry singlet state with two NiI
ions was found to be the ground state, wherein the two un-
paired electrons of the Ni ions are coupled antiferromagnet-
ically. The corresponding triplet state and the closed-shell
singlet state (with one NiII and one Ni0 centre) are posi-
tioned 16 and 22 kJmolꢀ1 higher in energy than the ground
state, respectively. Hence, although there is no bonding in-
teraction between both nickel ions, their unpaired electrons
exhibit strong antiferromagnetic coupling, so that compound
4 shows diamagnetic character, thus leading to a sharp set of
Figure 6. Molecular structure of complex 5; ellipsoids are set at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
1
signals in the diamagnetic range of the H NMR spectrum
ꢀ
ꢀ
ꢀ
lengths [ꢃ] and angles [8]: N1 Ni1 1.895(5), N2 Ni1 1.834(5), N3 Ni1
(see the Supporting Information). To detect the formation
of any hydride intermediates, the reaction of complex 3 with
KHBEt3 was carried out at ꢀ708C in [D8]toluene and moni-
tored by NMR spectroscopy. Indeed, the reaction began im-
mediately, but the spectrum of paramagnetic complex 3
changed into that of a further paramagnetic species, which
changed upon warming until, finally, the spectrum of dia-
magnetic complex 4 was obtained. The broad, paramagnetic
signals that were observed in between these two states did
not allow for a reasonable interpretation.
ꢀ
ꢀ
ꢀ
ꢀ
1.955(5), Ni1 S1 2.2209(18), Ni1 Ni2 3.3485(11), Ni2 N3 2.990(4), Ni2
ꢀ
ꢀ
ꢀ
N4 1.925(5), Ni2 N5 1.943(5), Ni2 S1 2.2637(16), Ni2 O1 2.008(4); N1-
Ni1-N2 93.4(2), Ni1-S1-Ni2 96.60(7), N4-Ni2-N5 95.75(19).
However, its NMR spectra in C6D6 indicated diamagnetic
behaviour and, thus, spin-pairing in solution. To rationalise
this finding, DFT calculations were also carried out for com-
plex 5: Setting out with the results of the X-ray diffraction
analysis, a single-point calculation revealed a preference for
a triplet state, whilst geometry optimisation led to a struc-
ture in which two high-spin NiII centres are strongly antifer-
romagnetically coupled so that the molecule has a singlet
ground state. This result explains the diamagnetic character
of complex 5 on the basis that the structure relaxation that
was observed in the calculations also occurs upon the disso-
lution of solid complex 5.
It should be emphasised that the formation of complexes
2, 4, and 5 proceeds in a very clean manner: The yields of
the crystalline products are all at least 60% and analysis of
the crude products revealed even higher (almost quantita-
tive) conversion. Consequently, these findings show that,
within the [(PYR)Ni2]2+ framework, the ligand that bridges
the two nickel centres sensitively influences its electronic
structure. Thus, this ligand also determines the redox chem-
istry that is triggered by the presence of hydride ligands. In
case of a thiolate ligand, this chemistry led to complex 4,
which will be in the focus of future studies: Examples of thi-
olate-bridged dinuclear nickel(I) compounds are rare[21a] and
complex 4 is highly reactive; thus, it will be interesting to ex-
plore its potential, for instance, in mediating redox coupling
reactions.
ꢀ
ꢀ
As a consequence of the short Ni Ni distance, the Ni S
ꢀ
ꢀ
bonds in complex 4 are shorter (Ni1 S1 2.1131(13) ꢃ, Ni2
ꢀ
S1 2.1471(14) ꢃ) than those in complex
2.2355(10) and Ni2 S1 2.2715(10) ꢃ). The N3 Ni1 separa-
tion is very large (2.9174(45) ꢃ), whilst the Ni2 N3 distance
(1.914(4) ꢃ) compares well with the N3 Ni1 distances in
3
(Ni1 S1
ꢀ
ꢀ
ꢀ
ꢀ
complexes 1–3 (1.950(2), 1.9049(13), and 1.960(3) ꢃ).
Having “loaded” the [(PYR)Ni
(m-SEt)Ni]+ system with
hydride, which led to internal redox chemistry, the question
was how the reduced core behaved upon protonation. After
the addition of lutidinium triflate as a mild acid to a solution
of compound 4 in THF, NiIINiII complex [(PYR)Ni
ACHTUNGTRENNUNG
SEt)NiACHTUNGTRENNUNG
tive of complex 3 (Scheme 3). It is likely that, first of all, the
proton attacked the NiINiI core, thereby leading to a
NiIINiIIH unit, based on precedence for this reaction in the
literature.[4j,22] However, the hydride ligand then combined
with the proton at the protonated b-diketiminato site to
yield H2.
Hence, altogether, a dinickel system is described in which
hydride can be stepwise separated into a proton and elec-
trons (!4) that, on the addition of a proton, are recom-
bined to give H2. The molecular structure of complex 5 is
comparable to that of the starting compound (3), but, in-
stead of the terminal bromide, a triflate ion is bound to the
Ni2 atom in complex 5 (Figure 6). As in complexes 2 and 3,
the coordination sphere of one nickel ion is almost square-
planar, whereas the other one is coordinated in a distorted
tetrahedral geometry; thus, again, a S=1 ground state was
expected and, indeed, a meff =2.50 mB magnetic moment was
determined for solid complex 5 at 295 K (ms.o. =2.83 mB).
Conclusion
We have shown that the PYR2ꢀ ligand is able to coordinate
II
ꢀ
two nickel(II)–bromide moieties, thereby affording a Ni m-
II
ꢀ
ꢀ
Br Ni Br core. In a subsequent reaction with KHBEt3, the
bromide ligands are replaced by hydride ligands, but the
Ni m-H Ni H product, which is likely formed initially, is
II
II
ꢀ
ꢀ
ꢀ
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C. Limberg et al.
ysis calcd (%) for C39H51Br2N5Ni2: C 54.02, H 5.93, N 8.01; found:
C 54.61, H 5.92, N 7.90.
unstable with respect to H2 elimination so that, finally, a
I
II
ꢀ
ꢀ
Ni m-H Ni complex is isolated. If the bridging bromide
ligand is replaced by a thiolate unit prior to the reaction
with KHBEt3, the product formation takes a different
course. It is assumed that, first of all, a bromide/hydride ex-
ACHUTNGRENUN[G (PYR)NiAHCTUNGTREN(NUGN m-H)Ni] (2): [(PYR)Ni2Br2] (400 mg, 0.461 mmol) was dis-
solved in Et2O (40 mL) and a solution of KHBEt3 (2 equiv, 127 mg,
0.923 mmol) in Et2O (10 mL) was added. After stirring for 5 min at RT,
the solvent was removed in vacuo. The residue was extracted with tol-
uene (20 mL) and evaporation of the solvent yielded complex 2 as a red
solid (205 mg, 63% yield). IR (KBr): n˜ =3573 (w), 3419 (m), 3058 (w),
2960 (s), 2925 (m), 2867 (m), 1628 (m), 1539 (s), 1457 (s), 1437 (s), 1378
(vs), 1321 (s), 1224 (m), 1187 (m), 1160 (m), 1056 (m), 760 cmꢀ1 (m);
UV/Vis (toluene): lmax (e)=455 (6830), 560 nm (1500 molꢀ1 dm3 cmꢀ1);
meff =1.82 mB (295 K, ms.o. =1.73 mB); elemental analysis calcd (%) for
C39H52N5Ni2: C 66.13, H 7.40, N 9.89; found: C 65.30, H 7.25, N 9.42.
II
II
ꢀ ꢀ
ꢀ
ꢀ
change takes place, which would lead to a Ni m SEt Ni
H species. In contrast to the observation made when setting
out with an all-bromide system, further reaction does not
C
proceed through the elimination of H (!H2) but, instead,
proceeds through a process that may be formally viewed as
a reduction of the metal centres by the hydride ligand:[20] Its
two electrons reduce both NiII centres to an oxidation state
of +I, whilst the proton is trapped by a basic site on the
ligand. Interestingly, the split hydride can be removed from
compound 4 again by the addition of a mild acid, thereby
regenerating a core that is comparable to that in complex 3.
Future investigations will concern the reactivity of com-
pound 4 with H2/H+/eꢀ, that is, the substrates of [NiFe] hy-
drogenase.
A
E
A
solution of complex
1
(400 mg,
0.461 mmol) in THF (40 mL) was treated with KSEt (1 equiv, 46.0 mg,
0.461 mmol) and the mixture was stirred for 16 h at RT. After filtration
of the green–brown solution, the solvent was removed in vacuo. The resi-
due was extracted with toluene (20 mL) and evaporation of the solvent
afforded complex 3 as a brown solid (310 mg, 80% yield). IR (KBr): n˜ =
3058 (w), 2959 (s), 2924 (m), 2867 (m), 1755 (w), 1595 (s), 1543 (vs), 1526
(s), 1455 (s), 1437 (s), 1406 (vs), 1392 (vs), 1364 (vs), 1339 (s), 1319 (s),
1285 (s), 1300 (s), 1260 (m), 1229 (m), 1223 (m), 1184 (m), 1162 (m),
1099 (m), 1056 (m), 1029 (s), 957 (w), 937 (w), 859 (w), 796 (s), 761 (s),
727 (w), 713 (w), 677 (w), 629 (w), 543 (w), 519 cmꢀ1 (w); meff =3.06 mB
(295 K, ms.o. =2.83 mB); HRMS (ESI): m/z calcd for [C41H56N5Ni2S]+:
766.2963 [MꢀBr]+; found: 766.2951; elemental analysis calcd (%) for
C41H56BrN5Ni2S: C 58.05, H 6.65, N 8.26, S 3.78; found: C 58.43, H 6.63,
N 8.21, S 3.44.
Experimental Section
ACHUTNRGEN[NUG (PYR-H)NiACHTUNGTNER(NGUN m-SEt)Ni] (4): A solution of KHBEt3 (1 equiv, 15.8 mg,
General: All manipulations were carried out under an argon atmosphere
in dried glassware in a glove box or by using standard Schlenk techni-
ques. Solvents were dried by using an MBraun Solvent Purification
System (SPS). The 1H and 13C NMR spectra were recorded on a Bruker
AV 400 NMR spectrometer at 258C. 1H and 13C chemical shifts are re-
ported in ppm and were calibrated internally to the solvent signals
(C6D6: d=7.15 and 128.02 ppm, respectively). The coupling constants are
given in Hz. IR spectra were measured as KBr pellets on a Shimadzu
FTIR 8400S spectrometer. UV/Vis spectroscopy was performed on a
HP8453A diode-array spectrometer. HRMS (ESI) was recorded on an
Agilent 6210 time-of-flight instrument. Elemental analysis was performed
on a HEKAtech Euro EA 3000 elemental analyzer. The Evans method
was used to determine the magnetic moments in solution at RT;[23] the
samples were measured in C6D6 with 1% tetramethylsilane (TMS), to-
gether with a capillary tube that contained C6D6 with 1% TMS as an in-
ternal standard. Different susceptibility measurements led to different
shifts of the TMS resonances. The magnetic moment of a solid sample
was determined by using a magnetic balance (Alfa) at RT. For the dia-
magnetic correction of the susceptibility, Pascal’s constants were used.[24]
EPR spectroscopy was carried out in silica glass tubes on an ERS 300 X-
band EPR spectrometer at 9.2 GHz. The ligand PYR-H2 was prepared
according to a literature procedure.[12a]
0.114 mmol) in Et2O (5 mL) was added to a solution of complex 3
(100 mg, 0.114 mmol) in Et2O (15 mL). The mixture was stirred for
15 min at RT and the solvent was removed in vacuo. The residue was ex-
tracted with n-hexane (10 mL) and evaporation of the solvent yielded
complex
4 as a
brown–red solid (62.5 mg, 71% yield). 1H NMR
(400 MHz, C6D6, 258C): d=8.92 (s, 1H; NH), 7.15 (m, 6H; CHAr), 6.58
(t, 3J=7.6 Hz, 1H; CH4-Pyr), 5.76 (d, 3J=7.6 Hz, 1H; CH3,5-Pyr), 5.64 (d,
3J=7.6 Hz, 1H; CH3,5-Pyr), 5.02 (s, 1H; CH), 4.98 (s, 1H; CH), 4.21 (m,
1H; CH
(d, 3J=6.4 Hz, 3H; CH
1.80 (m, 2H; SCH2), 1.69 (s, 3H; CH3), 1.63 (d, 3J=6.4 Hz, 3H; CH-
(CH3)2), 1.51 (d, 3J=6.8 Hz, 3H; CH
(CH3)2), 1.44 (s, 3H; CH3), 1.18 (d,
A
ACHTUNTGRENGU(N CH3)2), 2.87 (m, 2H; CHACHUTNTGERN(NUGN CH3)2), 2.12
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3
3J=6.4 Hz, 3H; CH
A
ACHTUGNTREN(UNNG CH3)2), 1.00 (d,
3J=6.8 Hz, 3H; CH
AHCTUNGTRENNUNG
(100 MHz, C6D6, 258C): d=169.3 (1C; C), 168.6 (1C; C), 162.5 (1C; C),
154.3 (1C; C), 151.1 (1C; C), 146.5 (1C; C), 140.5 (1C; C), 139.5 (1C;
C), 138.5 (1C; CH4-Pyr), 137.0 (1C; C), 136.5 (1C; C), 124.9 (1C; CHAr),
124.1 (2C; CHAr), 123.3 (C; CHAr), 123.3 (1C; CHAr), 122.9 (1C; CHAr),
112.0 (1C; C), 109.7 (1C; CH3,5-Pyr), 106.8 (1C; CH), 103.6 (1C; CH3,5-
Pyr), 28.6 (1C; CH
(1C; CH(CH3)2), 25.3 (1C; CH3), 24.8 (1C; CH
(CH3)2), 23.9 (1C; CH(CH3)2), 23.9 (1C; CH(CH3)2), 23.7 (1C; CH3),
23.4 (1C; CH(CH3)2), 23.3 (1C; CH(CH3)2), 23.0 (1C; CH(CH3)2), 22.6
ACHTUGTNRENUNG(CH3)2), 28.6 (3C; CHACTHUNGTRENNNUG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG[(PYR)NiACHTUNGTRENNUNG(m-Br)NiBr] (1): A solution of PYR-H2 (2.64 g, 4.47 mmol) in
A
R
ACHTUNGTRENNUNG
THF (30 mL) was cooled to ꢀ788C and nBuLi (2 equiv, 3.57 mL,
(1C; CH3), 22.3 (1C; CH3), 18.1 ppm (1C; SCH3); IR (KBr): n˜ =3062
(w), 2961 (s), 2924 (m), 2865 (m), 1610 (m), 1560 (s), 1510 (m), 1437 (s),
1384 (vs), 1320 (s), 1261 (vs), 1186 (s), 1157 (s), 1099 (s), 1056 (m), 1027
(s), 986 (m), 936 (w), 865 (w), 798 cmꢀ1 (s); elemental analysis calcd (%)
for C41H57N5Ni2S: C 64.00, H 7.47, N 9.10, S 4.17; found: C 63.82, H 7.60,
N 8.56, S 3.29.
8.93 mmol, 2.5m in n-hexane) was added. After stirring for 15 min at RT,
the solution was added to a suspension of [NiBr2ACTHNUTRGNEUNG(dme)] (2 equiv, 2.76 g,
8.93 mmol) in THF (20 mL) and the mixture was stirred for 16 h. After
filtration of the dark-green solution, the solvent was removed in vacuo.
The residue was extracted with toluene (15 mL) and layered with n-
hexane (10 mL). Cooling to ꢀ308C afforded complex 1 as dark-green
crystals (2.15 g, 56% yield). 1H NMR (400 MHz, C6D6, 258C): d=42.15
AHCTUNGTERGUN[NN (PYR)NiACTHUGNTREN(NUNG m-SEt)NiAHCTUNGTRENN(GUN OTf)] (5): A solution of [(PYR-H)NiACHTUNGTRENNUNG(m-SEt)Ni]
(25.0 mg, 0.033 mmol) in THF (5 mL) was treated with a solution of luti-
dinium triflate (8.40 mg, 0.033 mmol) in THF (2 mL) and the reaction
mixture was stirred for 5 min at RT. The solvent was removed in vacuo
and the residue was extracted with Et2O. Evaporation of the solvent
yielded compound 5 as a dark-red solid (18.5 mg, 60% yield). 1H NMR
(400 MHz, C6D6, 258C): d=7.04 (m, 6H; CH3,5-Pyr, CHAr), 6.85 (t, 3J=
(1H; CH4-Pyr), 19.35 (4H; CHAr), 1.73 (12H; CH
(CH3)2), ꢀ0.05 (12H;
CH
ACHTUNGTRENNUNG
CH3), ꢀ19.67 (6H; CH3), ꢀ41.48 ppm (2H; CH); IR (KBr): n˜ =3056 (w),
2962 (s), 2926 (m), 2865 (m), 1596 (s), 1573 (s), 1550 (vs), 1517 (s), 1455
(s), 1437 (s), 1410 (vs), 1394 (vs), 1372 (vs), 1339 (s), 1319 (s), 1302 (s),
1282 (vs), 1254 (m), 1228 (s), 1186 (m), 1158 (s), 1107 (w), 1055 (w), 1026
(m), 934 (w), 837 (w), 797 (m), 762 (m), 731 (w), 632 (w), 532 cmꢀ1 (w);
UV/Vis (toluene): lmax (e)=470 (8150), 612 (450), 662 nm
(315 molꢀ1 dm3 cmꢀ1); meff =3.03 mB (295 K, ms.o. =2.83 mB); elemental anal-
3
3
7.8 Hz, 1H; CHp-Ar), 6.57 (t, J=7.8 Hz, 1H; CH4-Pyr), 6.36 (d, J=7.2 Hz,
3
2H; CHm-Ar), 5.48 (d, J=6.9 Hz, 1H; CH3,5-Pyr), 4.97 (s, 1H; CH), 4.95 (s,
1H; CH), 3.95 (m, 1H; CHACHTUNGTRENNU(G CH3)2), 3.73 (m, 1H; CHCAHTUNTGREN(NUGN CH3)2), 3.11 (m,
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Hydride Reactivity of NiII-X-NiII Entities
FULL PAPER
1H; CH
CH3), 1.81 (m, 2H; SCH2), 1.45 (d, 3J=6.9 Hz, 3H; CH
9H; CH3, CH(CH3)2), 1.13 (m, 6H; CH
(CH3)2), 1.08 (d, 3J=6.3 Hz, 3H;
CH(CH3)2), 0.92 (d, J=6.6 Hz, 3H; CH
G
N
100 K; Z=4; 18121 reflections measured; 21882 unique reflections
(Rint =0.1607); final R indices [I>2s(I)] R1=0.0700 and wR2=0.1011; R
indices (all data) R1=0.1395 and wR2=0.1180.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
3
N
ACHTUNGTREN(NNUG CH3)2), 0.76 ppm (t, J=7.5 Hz,
3H; SCH3); IR (KBr): n˜ =3129 (w), 3058 (w), 2963 (s), 2927 (m), 2867
(m), 1594 (s), 1541 (vs), 1459 (s), 1437 (s), 1409 (s), 1391 (vs), 1364 (vs),
1339 (s), 1317 (vs), 1299 (s), 1281 (s), 1254 (m), 1231 (s), 1210 (s), 1181
(s), 1164 (s), 1108 (m), 1097 (m), 1055 (m), 1026 (s), 958 (w), 942 (w),
933 (w), 834 (w), 793 (m), 761 (m), 732 (w), 715 (w), 680 (w), 634 (s), 586
(w), 515 cmꢀ1 (w); meff =2.50 mB (solid, 295 K, ms.o. =2.83 mB); HRMS
(ESI): m/z calcd for [C41H56N5Ni2S]+: 766.2963 [MꢀOTf]+; found:
766.2965; elemental analysis calcd (%) for C42H56F3N5Ni2O3S2 : C 54.98,
H 6.15, N 7.63, S 6.99; found: C 55.69, H 6.27, N 8.08, S 5.51.
Acknowledgements
We are grateful to the Cluster of Excellence “Unifying Concepts in Cat-
alysis”, which is funded by the Deutsche Forschungsgemeinschaft (DFG),
the Fonds der Chemischen Industrie, and the Humboldt-Universitꢁt zu
Berlin for financial support.
Crystallographic studies: Crystallographic data were collected on
a
STOE IPDS 2T diffractometer at 100 K by using MoKa radiation (l=
0.71073 ꢃ). The structures were solved by using direct methods with
SHELXS-97 and refined by using full-matrix least-squares with
SHELXL-97.[25] All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were added geometrically and refined by using a riding
model; in complex 2, the bridging hydride atom was located in the differ-
ence Fourier map. Numerical absorption correction was applied for com-
plexes 1 and 3, based on a Gaussian algorithm, which was implemented
in the X-RED program (Stoe, 2002). Owing to thermal motion, the
ethyl-thiolate group in complex 5 was found to be disordered over two
sites. The 1–2 and 1–3 distances of the disordered parts were restrained
to be similar by using the EADP and DFIX commands. For the disor-
dered toluene molecule in the unit cell of complex 3, the SADI instruc-
tion was applied and all of the atoms of the disordered solvent molecule
were forced into a plane by using the FLAT command. The restraints
DELU and EADP were applied to the solvent molecule that was incor-
porated into the unit cell of complex 4.
[1] For example, see: a) R. Kokes, J. Am. Chem. Soc. 1959, 81, 5032–
5037; b) M. Sakai, N. Hirano, F. Harada, Y. Sakakibara, N. Uchino,
Bull. Chem. Soc. Jpn. 1987, 60, 2923–2926; c) E. Kogut, A. Zeller,
T. H. Warren, T. Strassner, J. Am. Chem. Soc. 2004, 126, 11984–
11994; d) S. Chakraborty, J. Zhang, J. Krause, H. Guan, J. Am.
Chem. Soc. 2010, 132, 8872–8873; e) E. E. Smith, G. Du, P. E. Fan-
wick, M. M. Abu-Omar, Organometallics 2010, 29, 6527–6533; f) S.
Lin, M. W. Day, T. Agapie, J. Am. Chem. Soc. 2011, 133, 3828–3831;
g) N. Castellanos-Blanco, M. Flores-Alamo, Organometallics 2012,
31, 680–686.
[2] a) T. James, L. Cai, M. Muetterties, Inorg. Chem. 1996, 35, 4148–
4161; b) A. D. Wilson, R. K. Shoemaker, A. Miedaner, J. T. Mucker-
man, D. L. DuBois, M. R. DuBois, Proc. Natl. Acad. Sci. USA 2007,
104, 6951–6956; c) I. Dance, Dalton Trans. 2010, 39, 2972–2983;
d) S. Canaguier, M. Fontecave, V. Artero, Eur. J. Inorg. Chem. 2011,
1094–1099; e) T. Matsumoto, T. Nagahama, J. Cho, T. Hizume, M.
Suzuki, S. Ogo, Angew. Chem. 2011, 123, 10766–10768; Angew.
Chem. Int. Ed. 2011, 50, 10578–10580; f) M. Vogt, B. de Bruin, H.
Berke, M. Trincado, H. Grꢀtzmacher, Chem. Sci. 2011, 2, 723–727.
[3] a) M. Brecht, M. van Gastel, T. Buhrke, B. Friedrich, W. Lubitz, J.
Am. Chem. Soc. 2003, 125, 13075–13083; b) C. Mealli, T. B. Rauch-
fuss, Angew. Chem. 2007, 119, 9100–9102; Angew. Chem. Int. Ed.
2007, 46, 8942–8944.
[4] a) T. Steinke, C. Gemel, M. Cokoja, M. Winter, R. Fischer, Angew.
Chem. 2004, 116, 2349–2352; Angew. Chem. Int. Ed. 2004, 43, 2299–
2302; b) N. D. Clement, K. J. Cavell, C. Jones, C. J. Elsevier, Angew.
Chem. 2004, 116, 3933; Angew. Chem. Int. Ed. 2004, 43, 3845; c) W.
Chen, S. Shimada, M. Tanaka, J. Am. Chem. Soc. 2004, 126, 8072–
8073; d) Y. Journaux, V. Lozan, J. Klingele, B. Kersting, Chem.
Commun. 2006, 83–84; e) L. She, X. Li, H. Sun, J. Ding, M. Frey,
Organometallics 2007, 26, 566–570; f) L. Liang, P. Chien, Organo-
metallics 2008, 27, 3082–3093; g) J. Y. Yang, R. M. Bullock, W. J.
Shaw, B. Twamley, K. Fraze, M. R. DuBois, D. L. DuBois, J. Am.
Chem. Soc. 2009, 131, 5935–5945; h) M. F. Laird, M. Pink, N. P.
Tsvetkov, H. Fan, K. G. Caulton, Dalton Trans. 2009, 1283–1285;
i) S. Chakraborty, J. A. Krause, H. Guan, Organometallics 2009, 28,
582–586, and references therein; j) B. E. Barton, C. M. Whaley,
T. B. Rauchfuss, D. L. Gray, J. Am. Chem. Soc. 2009, 131, 6942–
6943; k) S. Canaguier, M. Field, Y. Oudart, J. Pꢄcaut, M. Fontecave,
V. Artero, Chem. Commun. 2010, 46, 5876–5878; l) T. He, N. P.
Tsvetkov, J. G. Andino, X. Gao, B. C. Fullmer, K. G. Caulton, J. Am.
Chem. Soc. 2010, 132, 910–911; m) M. T. Mock, R. G. Potter, M. J.
OꢅHagan, D. M. Camaioni, W. G. Dougherty, W. S. Kassel, D. L.
DuBois, Inorg. Chem. 2011, 50, 11914–11928; n) C. Tsay, J. C.
Peters, Chem. Sci. 2012, 3, 1313–1318.
CCDC-891189 (1), CCDC-891190 (2), CCDC-891191 (3), CCDC-891192
(4), and CCDC-891193 (5) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif.
Crystal data for complex 1. C39H51Br2N5Ni2; Mw =867.09; monoclinic;
space group P21/c; a=13.9183(17), b=12.0355(11), c=23.810(2) ꢃ; a=
90, b=100.251(9), g=908; V=3924.8(7) ꢃ3; T=100 K; Z=4; 37161 re-
flections measured; 7303 unique reflections (Rint =0.1347); final R indices
[I>2s(I)] R1=0.0436 and wR2=0.1101; R indices (all data) R1=0.0497
and wR2=0.1136.
Crystal data for complex 2. Slow evaporation of a solution in Et2O yield-
ed complex 2 as red crystals. C39H52N5Ni2; Mw =708.28; triclinic; space
group P-1; a=8.8433(5), b=12.2811(7), c=17.3724(10) ꢃ; a=105.382(4),
b=95.104(4), g=102.044(4)8; V=1758.15(17) ꢃ3; T=100 K; Z=2;
18557 reflections measured; 6888 unique reflections (Rint =0.0498); final
R indices [I>2s(I)] R1=0.0277 and wR2=0.0658; R indices (all data)
R1=0.0340 and wR2=0.0681.
Crystal data for complex 3. Brown crystals of complex 3 were obtained
by layering a solution in toluene with n-hexane and cooling to ꢀ308C.
C89H120N10Ni4S2; Mw =1788.73; triclinic; space group P-1; a=10.3265(14),
b=12.0614(19), c=18.755(4) ꢃ; a=97.814(16), b=103.849(15), g=
103.516(12)8; V=2159.2(7) ꢃ3; T=100 K; Z=1; 20469 reflections meas-
ured; unique reflections (Rint =0.1419); final R indices [I>2s(I)] R1=
0.0501 and wR2=0.1442; R indices (all data) R1=0.0579 and wR2=
0.1504.
Crystal data for complex 4. Red–brown crystals of complex 4 were ob-
tained after slow evaporation of a solution n-hexane. C47H70N5Ni2S; Mw =
854.56; triclinic; space group P-1; a=9.1306(6), b=12.6216(12), c=
20.4084(15) ꢃ; a=90.694(7), b=102.437(6), g=98.116(7)8; V=
2271.5(3) ꢃ3; T=100 K; Z=2; 18121 reflections measured; 8176 unique
[5] For oxidation states in the [NiFe] hydrogenase, compare: a) R. P.
Happe, W. Roseboom, S. P. J. Albracht, Eur. J. Biochem. 2001, 259,
602–608; b) M. Stein, W. Lubitz, J. Inorg. Biochem. 2004, 98, 862–
877.
[6] For example, see: a) J. M. Smith, R. J. Lachicotte, K. A. Pittard,
T. R. Cundari, G. Lukat-Rodgers, K. R. Rodgers, P. L. Holland, J.
Am. Chem. Soc. 2001, 123, 9222–9223; b) L. Bourget-Merle, M. F.
Lappert, J. R. Severn, Chem. Rev. 2002, 102, 3031–3066; c) N. W.
Aboelella, S. V. Kryatov, B. F. Gherman, W. W. Brennessel, V. G.
Young, R. Sarangi, E. V. Rybak-Akimova, K. O. Hodgson, B.
reflections (Rint =0.1155); final
R indices [I>2s(I)] R1=0.0661 and
wR2=0.1415; R indices (all data) R1=0.1164 and wR2=0.1632.
Crystal data for complex 5. Black crystals of compound 5 were obtained
after the evaporation of a solution in Et2O. C42H56F3N5Ni2O3S2; Mw =
917.46; monoclinic; space group P21/c; a=13.8476(11), b=12.2518(8),
c=25.368(3) ꢃ; a=90, b=97.774(9), g=908; V=4264.3(7) ꢃ3; T=
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C. Limberg et al.
Hedman, E. I. Solomon, C. J. Cramer, W. B. Tolman, J. Am. Chem.
Soc. 2004, 126, 16896–16911; d) W. H. Monillas, G. P. A. Yap, L. A.
MacAdams, K. H. Theopold, J. Am. Chem. Soc. 2007, 129, 8090–
8091; e) A. R. Sadique, W. W. Brennessel, P. L. Holland, Inorg.
Chem. 2008, 47, 784–786; f) T. R. Dugan, P. L. Holland, J. Organo-
met. Chem. 2009, 694, 2825–2830; g) D. J. Mindiola, Angew. Chem.
2009, 121, 6314–6316; Angew. Chem. Int. Ed. 2009, 48, 6198–6200;
h) Y.-C. Tsai, Coord. Chem. Rev. 2012, 256, 722–758; i) S. Yao, M.
Driess, Acc. Chem. Res. 2012, 45, 276–287.
Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr.,
J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N.
Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghava-
chari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Mo-
rokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannen-
berg, S. Dapprich, A. D. Daniels, ꢈ. Farkas, J. B. Foresman, J. V.
Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09, Revision C.01;
Gaussian, Inc., Wallingford CT, 2010.
[7] a) S. Pfirrmann, C. Limberg, B. Ziemer, Dalton Trans. 2008, 6689–
6691; b) S. Pfirrmann, C. Limberg, C. Herwig, Angew. Chem. 2009,
121, 3407–3411; Angew. Chem. Int. Ed. 2009, 48, 3357–3361.
[8] S. Pfirrmann, S. Yao, B. Ziemer, R. Stçsser, M. Driess, C. Limberg,
Organometallics 2009, 28, 6855–6860.
[9] A further example was reported only recently; see: S. J. Tereniak,
E. E. Marlier, C. C. Lu, Dalton Trans. 2012, 41, 7862–7865.
[10] S. O. N. Lill, P. E. M. Siegbahn, Biochemistry 2009, 48, 1056–1066.
[11] S. Pfirrmann, C. Limberg, C. Herwig, C. Knispel, B. Braun, E. Bill,
R. Stçsser, J. Am. Chem. Soc. 2010, 132, 13684–13691.
[12] a) D. F.-J. Piesik, S. Range, S. Harder, Organometallics 2008, 27,
6178–6187; b) J. Spielmann, D. F.-J. Piesik, S. Harder, Chem. Eur. J.
2010, 16, 8307–8318; c) S. Harder, J. Spielmann, J. Intemann, H.
Bandmann, Angew. Chem. 2011, 123, 4242–4246; Angew. Chem. Int.
Ed. 2011, 50, 4156–4160.
[13] M. Al-Anber, B. Walfort, S. Vatsadze, H. Lang, Inorg. Chem.
Commun. 2004, 7, 799–802.
[14] R. Knorr, H. Hauer, A. Weiss, H. Polzer, F. Ruf, P. Lçw, P.
Dvortsꢆk, P. Bçhrer, Inorg. Chem. 2007, 46, 8379–8390.
[15] A. Pardo, A. L. De Lacey, V. M. Fernꢆndez, H.-J. Fan, Y. Fan, M. B.
Hall, J. Biol. Inorg. Chem. 2006, 11, 286–306.
[16] a) M. Jimꢄnez Tenorio, M. C. Puerta, P. Valerga, J. Chem. Soc.
Dalton Trans. 1996, 1305–1308; b) M. G. Crestani, M. MuÇoz-
Hernꢆndez, A. Arꢄvalo, A. Acosta-Ramꢇrez, J. J. Garcꢇa, J. Am.
Chem. Soc. 2005, 127, 18066–18073.
[19] a) G. Bai, P. Wei, D. W. Stephan, Organometallics 2005, 24, 5901–
5908; b) E. Kogut, H. L. Wiencko, L. Zhang, D. E. Cordeau, T. H.
Warren, J. Am. Chem. Soc. 2005, 127, 11248–11249; c) V. V. Saraev,
P. B. Kraikivskii, I. Svoboda, S. Kuzakov, R. F. Jordan, J. Phys.
Chem. A 2008, 112, 12449–12455.
[20] a) K. Fraze, A. D. Wilson, A. M. Appel, M. R. DuBois, D. L.
DuBois, Organometallics 2007, 26, 3918–3924; b) M. L. Helm, M. P.
Stewart, R. M. Bullock, M. R. DuBois, D. L. DuBois, Science 2011,
333, 863–866; c) D. L. DuBois, R. M. Bullock, Eur. J. Inorg. Chem.
2011, 1017–1027; d) S. Raugei, S. Chen, M.-H. Ho, B. Ginovska-
Pangovska, R. J. Rousseau, M. Dupuis, D. L. DuBois, R. M. Bullock,
Chem. Eur. J. 2012, 18, 6493–6506.
[21] a) M. Ito, T. Matsumoto, K. Tatsumi, Inorg. Chem. 2009, 48, 2215–
2223; b) T. Li, J. J. Garcꢇa, W. W. Brennessel, W. D. Jones, Organo-
metallics 2010, 29, 2430–2445; c) A. Velian, S. Lin, A. J. M. Miller,
M. W. Day, T. Agapie, J. Am. Chem. Soc. 2010, 132, 6296–6297.
[22] a) X. Zhao, I. P. Georgakaki, M. L. Miller, J. C. Yarbrough, M. Y.
Darensbourg, J. Am. Chem. Soc. 2001, 123, 9710–9711; b) F. Gloa-
guen, J. D. Lawrence, T. B. Rauchfuss, J. Am. Chem. Soc. 2001, 123,
9476–9477; c) F. Gloaguen, J. D. Lawrence, T. B. Rauchfuss, M.
Bꢄnard, M.-M. Rohme, Inorg. Chem. 2002, 41, 6573–6582; d) F.
Gloaguen, T. B. Rauchfuss, Chem. Soc. Rev. 2009, 38, 100–108.
[23] a) D. F. Evans, J. Chem. Soc. 1959, 2003–2005; b) E. M. Schubert, J.
Chem. Educ. 1992, 69, 62; c) T. Ayers, R. Turk, C. Lane, J. Goins, D.
Jameson, S. Slattery, Inorg. Chim. Acta 2004, 357, 202–206.
[17] J. Breitenfeld, R. Scopelliti, X. Hu, Organometallics 2012, 31, 2128–
2136.
[18] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A.
Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F.
Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,
[24] G. A. Bain, J. F. Berry, J. Chem. Educ. 2008, 85, 532–536.
[25] G. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
Received: September 8, 2012
Published online: && &&, 0000
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Hydride Reactivity of NiII-X-NiII Entities
FULL PAPER
Ni–H in the claw: An investigation of
the chemistry of nickel hydride within
a ligand framework that is composed
of two b-diketiminato binding sites has
led to the observation of remarkable
reactivities and rare structural features,
including a mixed-valent dinickel–
hydride core (see figure) and a thio-
late-bridged NiINiI complex.
Mixed-Valent Compounds
H. Gehring, R. Metzinger, C. Herwig,
J. Intemann, S. Harder,
C. Limberg* ..................... &&& — &&&
Hydride Reactivity of NiII X Ni
II
ꢀ ꢀ
Entities: Mixed-Valent Hydrido
Complexes and Reversible Metal
Reduction
Chem. Eur. J. 2013, 00, 0 – 0
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These are not the final page numbers!