Stabilization of β-Diketiminato Nickel(I) with Alkaline Metal Halide Entities for Small Molecule Activation

Article abstract of DOI:10.1002/zaac.201800100

The reduction of the β-diketiminato nickel(II) halide complex [L^tBuNi^IIBr] [L^tBu = CH(CtBuNdipp)₂^–, dipp = 2,6-diisopropylphenyl] with potassium sources proceeds via the initial formation of [(L^tBuNi^I)_x(μ-Br)_xK_x] aggregates, which could be isolated and characterized for x = ∞ and 6. The KBr equivalents readily give way to external donors or substrates to be activated at the central nickel(I) atoms. To test, in how far the steric bulk induced by the residues at [L^tBu]^– influences the formation of the KBr adducts β-diketiminato ligands with less steric congestion, namely L^Me6 and L^Me7 [L^Me6 = CH(CMeNdmp)₂^–, L^Me7 = CMe(CMeNdmp)₂^–, dmp = 2,6-dimethylphenyl] were employed. Through deprotonation of HL^Me6 with nBuLi followed by treatment with NiBr₂(dme) the nickel(II) precursor compound [L^Me6Ni^II(μ-Br)₂Li(THF)₂] was prepared and shown to enter an equilibrium with [(L^Me6Ni^IIBr)₂] and LiBr in solution; [(L^Me6Ni^IIBr)₂] could be accessed also independently. Syntheses of the complexes [L^Me7Ni^II(μ-Br)₂Li(THF)₂] and [(L^Me7Ni^IIBr)₂] could be achieved analogously. To test the potential of nickel complexes with the L^Me6 and L^Me7 ligands for the activation of N₂ the thf-free [(L^Me6/7Ni^IIBr)₂] complexes were reduced with potassium in an N₂ atmosphere. This led neither to a KBr adduct nor to an N₂ complex but to the dimer [(L^Me6Ni^I)₂], as the smaller ligands allow an efficient interaction of the central nickel atoms with the aryl rings.

Full text of DOI:10.1002/zaac.201800100

Title: Stabilization of ꢀ-diketiminato nickel(I) with alkaline metal halide
entities for small molecule activation
Authors: Christian Limberg, Patrick Holze, Beatrice Braun-Cula, and
Stefan Mebs
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To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.201800100
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10.1002/zaac.201800100
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ARTICLE
Stabilization of -diketiminato nickel(I) with alkaline metal halide
entities for small molecule activation
Patrick Holze,[a] Beatrice Braun-Cula,[a] Stefan Mebs[b] and Christian Limberg*[a]
Dedicated to Professor Peter Comba on occasion of his 65th birthday
Abstract: The reduction of the -diketiminato nickel(II) halide
weakly coordinating ligand or a good leaving unit. Here we take a
closer look at the formation and behavior of LNiI-D in dependence
on the nature of D with a special focus on representatives where
D corresponds to an alkali metal halide entity.
–
complex [LtBuNiIIBr] (LtBu = CH(CtBuNdipp)2 , dipp = 2,6-Di-iso-
propylphenyl) with potassium sources proceeds via the initial
formation of [(LtBuNiI)x(-Br)xKx] aggregates which could be isolated
and characterized for X = ∞ and 6. The KBr equivalents readily give
way to external donors or substrates to be activated at the nickel(I)
centers. To test, in how far the steric bulk induced by the residues at
Results and Discussion
[LtBu –
ligands with less sterical congestion, namely LMe6 and LMe7
] influences the formation of the KBr adducts -diketiminato
As there are no suitable nickel(I) halide precursors, which could
form LNiI-D complexes in course of a salt metathesis reaction with
deprotonated -diketimines, such complexes are typically
prepared through the reduction of a nickel(II) complex LNiII-X
(X = halide) with alkali metals.[2,3,9,10] Hence, LNiII-X are important
–
–
(LMe6 = CH(CMeNdmp)2 ,
LMe7 = CMe(CMeNdmp)2 ,
dmp = 2,6-
dimethylphenyl) were employed. Through deprotonation of HLMe6 with
nBuLi followed by treatment with NiBr2(dme) the nickel(II) precursor
compound [LMe6NiII(-Br)2Li(THF)2] was prepared and shown to enter
an equilibrium with [(LMe6NiIIBr)2] and LiBr in solution; [(LMe6NiIIBr)2]
could be accessed also independently. Syntheses of the complexes
[LMe7NiII(-Br)2Li(THF)2] and [(LMe7NiIIBr)2] could be achieved
analogously. To test the potential of nickel complexes with the LMe6
and LMe7 ligands for the activation of N2 the thf-free [(LMe6/7NiIIBr)2]
complexes were reduced with potassium in an N2 atmosphere. This
led neither to a KBr adduct nor to an N2 complex but to the dimer
[(LMe6NiI)2], as the smaller ligands allow an efficient interaction of the
nickel centres with the aryl rings.
precursors,
and
the
bromide
[LtBuNiIIBr],
1,
–
(LtBu = CH(CtBuNdipp)2 , dipp = 2,6-Di-iso-propylphenyl) has
been accessed by us originally via refluxing of LiLtBu with
NiIIBr2(dme) for 16 h followed by filtration and crystallization. The
maximum yield of pure 1 reached by this procedure was 47%.[3]
In the meantime we have found, that better yields can be obtained
via an alternative procedure, developed along the lines of a
method reported for the synthesis of [LMe2FeIICl]2
(LMe2 = HC(CMeNdipp)2):[11] after performing the reaction as
before and removal of all volatiles the resulting residue was
heated under reduced pressure for 12 h to 90°C. Afterwards
adequate work-up involving extraction, crystallization and
washing provides 1 in yields of 74%. The compound has been
characterized previously, but we have now in addition recorded
the 1H NMR and the 1H1H COSY NMR spectrum of this
paramagnetic compound and assigned the signals.
Introduction
-diketiminato nickel(I) complexes have demonstrated unique
potential in the area of small molecule activation.[1] They have
been shown to react, for instance, with the group 16 elements,
dinitrogen,[2-4] phosphorous,[5] but also with SF6,[6] CO2,[7] and
dihydrogen,[4,8] weakening bonds through transfer of electron
density or even cleaving them reductively. However, the co-ligand
saturating the residual coordination sphere of the nickel(I) center
is also decisive. Obviously, LNiI-D complexes (L = -diketiminate,
D = donor) show the highest reactivity when D corresponds to a
Setting out from 1, nickel(I)-donor complexes can be obtained via
reaction with K or KC8 in the presence of the chosen donor;
naturally the strongest donor available will coordinate. Hence,
isolation of the labile N2 complex [(LtBuNiI)2(-1-1-N2)], 2,
required hexane as the solvent.[3] As the N2 ligand is bound only
very weakly, 2 in turn proved an excellent precursor for the
activation of other small molecules.[6,7,9] Based on the results of
our investigations and those obtained by Holland and co-workers
for the formation of [(LtBuFeI)2(-1-1-N2)] the following
mechanism may be postulated for the formation of 2:[12,13] After
reduction with KC8 in a non-coordinating solvent like hexane a di-,
oligo- or even polymeric nickel(I) complex [(LtBuNiI)x(-Br)xKx], (3)x
(x = 1, 2…n), is formed, where KBr units are saturating the
coordination spheres of the nickel centers. This hypothesis is
fueled by the fact that a dinuclear compound of that type, namely
[(LMe2NiI)2(3-Br)2Li2(THF)] – with LiBr units and the smaller ligand
[LMe2]– – has been isolated before (the Li+ cations were not derived
from the reductant though), and this complex reacted even with
N2, which replaced the LiBr.[3] Donor molecules are expected to
[a]
Dr. Patrick Holze, Dr. Beatrice Braun-Cula, Prof. Dr. Christian
Limberg,
Institut für Chemie
Humboldt-Universität zu Berlin
Brook-Taylor Str. 2 in 12489 Berlin
E-mail: christian.limberg@hu-berlin.de
Dr. Stefan Mebs
[b]
Institut für Physik
Freie Universität Berlin
Kaiserswerther Str. 16-18 in 14195 Berlin
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Scheme 1. Formation of (3)x in course of reduction of 1, subsequent reaction
with donor molecules like dinitrogen.
first of all coordinate to the Ni centers in (3)x to give
[(LtBuNiI)x(D)x(-Br)xKx], (4)x (D = donor molecule), before the
elimination of KBr leads to the corresponding mononuclear
adduct [LtBuNiI-D], 5-D (Scheme 1).
before the temperature was lowered to 60°C to allow for the
aggregation and solidification of the potassium (m.p. = 63°C).
Afterwards the solution containing (3)x could be easily transferred
through a pre-warmed cannula into a pre-warmed Schlenk tube,
the temperature of which was lowered every 24 h by 20 K
(hexane) or 10 K (heptane), respectively (Scheme 1).
An interesting question concerns the nuclearity of (3)x, in
particular considering the findings of Holland and co-workers that
more than one iron(I) center is required for the activation of N2
[13]
and that the degree of activation increases with increasing
number of cooperating iron(I) centers.[14] Initial indications on the
existence of (3)x were obtained investigating the reaction between
1 and KC8 in hexane solution. An orange colored solid was
isolated, which, however, could not be separated from the
graphite generated simultaneously, so that it could not be
characterized adequately. (3)x thus generated readily reacted
with N2 to give 2 after work-up, which does not cause an EPR
signal as a solid. Investigation of a hexane solution of 2 showed
a characteristic signal for LNiI, though, from which it could be
derived that mononuclear [LNiI-N2] exists in solution as part of an
equilibrium.[3]
To obtain more information on (3)x the synthetic procedure was
modified, avoiding KC8. Instead, elemental potassium was used
that had been sublimed into the reaction vessel before a hexane
or heptane solution of 1 was added. Simple stirring of the mixture
led to the precipitation of (3)x as an amorphous powder within one
day, which again posed problems with regards to a separation
from the reductant. Hence the mixture was heated to 68 °C
(hexane) or 98 °C (heptane), respectively, for 2 h – thus
increasing the reaction rate and keeping the product dissolved –
This procedure also led to the crystallization of (3)x in form of
orange-red crystals, which were suitable for X-ray diffraction and
the result of a corresponding analysis of crystals grown from
heptane is shown in Figure 1. The molecular entities of (3)x,
[LtBuNiI(-Br)K], are arranged in form of chains where [LtBuNiI-Br]–
units are connected by potassium ions, that is, each bromido
ligand connects three metals. The potassium ions further undergo
electrostatic interactions with two aryl residues of two different -
diketiminato ligands, that primarily involve the closest meta and
the para C atoms (K–C 3.016(18) – 3.066(11) Å), so that they may
be regarded as 2- interactions. While comparable complexes
often exhibit T-shaped ligand arrangement, the coordination
spheres of the nickel centers in (3)x can be described best as
trigonal planar as the differences between the larger and the
smaller N–Ni–Br angles amount to only 5.3(3) –7.4(2) °. This is
probably due to the aggregation of the molecular units to a
coordination polymer. Compared to [(LMe2NiI)2(3-Br)2Li2(THF)]
the Ni–Br distances in (3)x are enlarged (2.373(2) – 2.3794(15) Å),
as one could expect upon the presence of a bulkier -diketiminato
ligand.[4]
Figure 1. Molecular structure of crystalline (3)x. Half a molecule of heptane,
hydrogen atoms and i-Propyl groups of the N-Aryl groups are not depicted for
clarity. Selected bond lengths (/Å): Ni1–N1 1.904(8), Ni1–N6 1.910(8), Ni2–N2
1.902(7), Ni2–N3 1.918(7), Ni3–N4 1.895(9), Ni3–N5 1.906(7), Ni1–Br1
2.3727(15), Ni2–Br2 2.3746(14), Ni3–Br3 2.3795(15), K1–Br1 3.167(3), K1–Br2
3.174(2), K2–Br2 3.183(2), K2–Br3 3.190(2), K3–Br3 3.180(3), K3–Br1’
3.167(3). Selected atom distances (/Å): Ni1–Ni2 8.337(3), Ni2–Ni3 9.063(2).
Selected bond and torsion angles (/°): N1–Ni1–N6 99.8(4), N2–Ni2–N3 99.5(4),
N4–Ni3–N5 99.2(3), N1–Ni1–Br1 132.8(4), N6–Ni1–Br1 127.5(3), N2–Ni2–Br2
133.6(3), N3–Ni2–Br2 126.9(2), N4–Ni3–Br3 134.1(2), N5–Ni3–Br3 126.7(2),
Br1–K1–Br2 126.27(8), Br2–K2–Br3 152.34(11), Br1’–K3–Br3 122.62(10), N2–
N3–N4–N5 -140.3(5), N1–N2–N3–N6 -15.32(9), N1–N4–N5–N6 4.88(13).
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Upon dissolution the chains are probably broken up and
oligomers are formed. This assumption is corroborated by the fact
that a hexamer (3)6 could be crystallized from hexane, the
molecular structure of which is shown in Figure 2; the quality of
appropriate work-up. 6 corresponds to an aggregate of the
desired [LMe6Ni-Br] complex and one equivalent of LiBr, which was
generated during its synthesis but did not leave the coordination
sphere of the nickel center, as it is often observed when less bulky
Figure 2. Molecular structure of (3)6. For clarity, hydrogen atoms, i-propyl and
t-butyl groups are not depicted. The molecule shown is one of two structural
isomers in the unit cell of the crystal. The data quality does not allow for
discussion of the bond parameters. Additionally, the cell parameters of the unit
cell could not be determined to full certainty which allows for the possibility that
the unit cell is half the size featuring only one hexamer.
Scheme 2. Synthesis of nickel(II) complexes with sterically less congested -
diketiminato ligands.
the data does not allow a discussion of bond lengths and angles,
though. An EPR spectrum recorded for solid (3)6 at 77 K showed
a signal of unusually low anisotropy, the simulation of which gave
g values of gx = 2.277, gy = 2.197 and gz = 2.164. Observation of
this signal indicates only weak coupling of the nickel(I) centers
even at these low temperatures. Since the hexamer, once
crystallized is only sparingly soluble in hexane or heptane a
solution spectrum could not be recorded.
substituents are chosen at the -diketiminate framework. Upon
dissolution of 6 in benzene LiBr is partly eliminated and a mixture
between 6 and [(LMe6NiIIBr)2], 7, is formed (ratio 2:1 after 12 h) as
1
revealed by an NMR spectrum (1H NMR, H,1H-COSY), where
two signal sets became visible. Unequivocal identification of the
second signal set as arising from 7 became possible through
development of an independent synthesis for 7: When the
deprotonation of HLMe6 was performed with KH rather than n-butyl
lithium, 7 was formed and could be isolated in 54% yield.
Alternatively, 7 could be generated from 6 through warming to
110 °C in toluene followed by filtration at 80 °C (87% yield) and a
third possibility was warming of solid 6 for 3 d to 120 °C under
reduced pressure followed by extraction with a non-coordinating
solvent (quantitative yield). Previously, Zhang et al. had claimed
that 7 can be synthesized in 67% yield through treatment of HLMe6
The structural characterization of 3 in two different levels of
aggregation supports the previous proposal that the reduction of
1 with potassium first of all leads to an [LtBuNiI] complex, where
KBr entities are saturating the coordination spheres of the nickel
centers, as depicted in Scheme 1; similar complexes have been
proposed by Holland and co-workers to form with iron(I) as the
central metal but these could never be isolated.[12,13] Considering
that in both molecular structures the nickel centers are separated
from each other by more than 8 Å, an added donor like N2 will
interact initially with just one nickel center, and apparently this
contact is sufficient to trigger KBr elimination and activation.
The next question was, how this situation changes if the steric
bulk induced by the residues at [LtBu]– is reduced, bearing in mind
that in case of iron changing from LtBu to LMe7
1
with n-butyl lithium and then with NiBr2(dme).[16] In the H NMR
spectrum they had found two signal sets of paramagnetically
shifted resonances for the -diketiminate ligand, which they
proposed to be due to an equilibrium of 7 with a monomeric
species, similarly to [LMe2NiIICl]2, which has been shown to enter
into such an equilibrium by Holland and co-workers.[17] However,
our results suggest that the inferences of Zhang et al. are not
correct: isolated, pure 7 does not enter into an equilibrium in
benzene and just one signal set is observed. The chemical shifts
of these signals are rather similar to those assigned to monomeric
7 by Zhang et al., while their second signal set resembles the one
we were able to assign to 6.
–
(LMe7 = CMe(CMeNdmp)2 , dmp = 2,6-Dimethylphenyl) finally
allowed even for the cleavage of N2 as four iron(I) centers could
approach sufficiently for a cooperative action.[14]
Hence, we have employed -diketiminato ligands with less
sterical
congestion,
namely
see Scheme 2).
LMe6
and
HLMe6
LMe7
was
–
(LMe6 = CH(CMeNdmp)2 ,
deprotonated with n-BuLi and then treated with NiBr2(dme),
analogously to reported protocols for [LMe2] complexes,[15] which
led to the complex [LMe6NiII(-Br)2Li(THF)2], 6, in 87% yield after
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position (above) and the second one in special position. Hydrogen atoms are
omitted for clarity. Selected bond lengths for the molecule in general position
(/Å): Ni1–N1 1.9244(17), Ni1–N2 1.8812(17), Ni1–C29 2.074(2), Ni1–C30
2.0806(19), Ni2–N3 1.8840(17), Ni2–N4 1.9232(17), Ni2–C17 2.065(2), Ni2–
C18 2.0881(19), C29–C30 1.402(3), C17–C18 1.405(3). Selected bond angles
(/°): N1–Ni1–N2 97.45(7), N1–Ni1–C29 108.75(8), N1–Ni1–C30 105.88(8), N2–
Ni1–C29 145.16(8), N2–Ni1–C30 150.63(8), N3–Ni2–N4 97.55(8), N3–Ni2–
C17 151.47(9), N3–Ni2–C18 144.40(8), N4–Ni2–C17 105.04(8), N4–Ni2–C18
109.37(8).
Cooling of a saturated solution of 7 in hexane to –30 °C led to
single crystals that were suitable for an X-ray diffraction analysis
and the result is shown in Figure 3. The overall geometry is quite
similar to the one found for [LMe2NiIICl]2.[17] The nickel centers of 7
are located somewhat more deeply in the binding pockets of LMe6
though.
,
slow cooling of a concentrated hexane solution to –30 °C yielded
single crystals which were subjected to an X-ray diffraction study,
and the result is shown in Figure 4. The respective complex
represents a dimer [(LMe6NiI)2], 10, where each nickel ion binds to
the β-diketiminate N atoms as well as to an aryl ring of a second
[LMe6NiI] unit of the dimer in a η2-coordination mode. This
constitution becomes possible due to the T-shaped ligand
geometry around the nickel ions that is typical for nickel(I)
complexes with β-diketiminate ligands: The N1(N4))–Ni–C and
N2(N3)–Ni–C angles amount to 105.04(8)-109.37(8) and
144.40(8)-151.47(9)°, respectively, and this asymmetry is also
expressed in different Ni–N bond lengths of 1.8812(17)-
1.8840(17) and 1.9232(17)-1.9244(17) Å. In consequence the two
coordinating arene units are nearly parallel to each other with a
dihedral angle of 3.88(10)° between the two aromatic planes, and
the distance between the gravity centers is 4.415(13) Å which
may indicate extra stabilization due to π-π stacking interactions.
The respective aryl rings are not significantly influenced by the
nickel contacts: all C–C bonds are very similar and also compare
well with those of the uncoordinated aryl rings. For the second
independent molecule in the asymmetric unit which lies on the
inversion center, the N6–Ni–C and N6–Ni–C angles amount to
103.75(8)- 113.54(9) and 141.67(8)- 154.13(9) and the
coordinating arene rings are parallel due to symmetry. The
distance between the centers of gravity is 4.441(13). Altogether
the structure is similar to the one of [(LMe2NiI)2] which features
somewhat longer Ni-C bonds and larger separation of the parallel
aryl residues which is not surprising considering the bulkiness of
its LMe2 ligand.[4]
Figure 3. Molecular Structure of 7. Hydrogen atoms are not depicted. Selected
bond lengths (/Å): Ni1-N1 1.924(3), Ni1-N2 1.911(3), Ni1-Br1 2.4210(6), Ni1-
Br1′ 2.4265(6). Selected bond angles (/°): N1-Ni1-N2 94.97(13), Ni1-Br1-
Ni1′ 88.886(19), Br1-Ni1-Br1′ 91.11(2).
Syntheses of the complexes [LMe7NiII(-Br)2Li(THF)2], 8, and
[(LMe7NiIIBr)2], 9, could be achieved analogously to the syntheses
of 6 and 7 (see Scheme 2).
To test the potential of nickel complexes with the LMe6 and LMe7
ligands for the activation of N2 7 and 9 appeared more suitable
than 6 and 8, as the latter contain Lewis basic thf ligands which
can compete with N2 for coordination sites. Upon treatment of 7
dissolved in hexane with KC8 in an N2 atmosphere the original
dark blue suspension changed color to intense red. After filtration
Although, the reduction that had led to 10 and the subsequent
crystallization of the product had been generated and crystallized
in an N2 atmosphere, as in case of the reduction of 1 that had led
to the N2 complex 2, 10 does not contain N2, and unlike [(LMe2NiI)2]
it was not reactive towards N2. To test whether 10 only selectively
crystallized from a mixture of compounds, the reduction of the
bromide precursor with KC8 was performed again in the presence
and absence of N2 and the two raw products were investigated
1
EPR, IR and H NMR spectroscopically. As can be seen from
Figure 5 both EPR spectra were identical and the same was true
for the IR and NMR spectra suggesting that 10 was formed in both
cases as the main product.
Figure 4. Molecular structure of 10. The crystal structure contains two
independent molecules in the asymmetric unit: one of them lies in general
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towards N2 and H2 which can be attributed to the weak electron
donating character of the [LiBr]2(THF)2 core. Although III is stable
enough for isolation, it is best described to be metastable as upon
storage it slowly eliminates LiBr forming the dimer [(LMe2NiI)2], IV.
Surprisingly, IV remains reactive towards N2 and H2 whereas in
case of 10, no N2 reactivity was observed. The inert character
towards N2 can be ascribed to the decreased sterical demand of
LMe6 and LMe7 as compared to LMe2, which allows for an ideal
interaction of the nickel(I) centers with the aryl units and thus a
strong entanglement of the two LNiI moieties in 10. In case of LMe2
the core of IV cannot reach a similar arrangement, as the iso-
propyl residues of the aryl units do not permit a sufficiently close
approach. Hence, the aryl units can be readily displaced even by
weak electron donating molecules like N2.
Finally, in (3)x the aryl rings can hardly compete with the [KBr] unit
as the tBu residues push them further towards the nickel bound
donors forming a smaller binding pocket so that a dimeric complex
[(LtBuNiI)2] as compared to [(LNiI)2] (L = LMe2, LMe6, LMe7) is not
stable. Consequently, the [KBr] complexes (3)x and (3)6 are
accessible and can even be stored. They remain very reactive
towards added donors, though, even if they are weak like N2, so
that they can only be isolated in the absence of any donor
molecules.
Figure 5. Comparison of EPR spectra (9.45 GHz, 77K) recorded for the frozen
crude product solutions in hexane of the reduction of 7 with potassium graphite
in an Ar (black) and an N2 (red) atmosphere, respectively.
That 10 apparently is inert towards N2 at first sight seems
surprising considering that [(LMe2NiI)2] reacts with N2 to form
[(LMe2NiI)2(-N2)].[4] However, it is plausible that the less sterically
demanding methyl groups of the aryl unit allow for a stronger
clamping and interaction of the two monomeric [LMe6NiI] units than
the more bulky iso-propyl groups of the aryl groups do in
[(LMe2NiI)2], comparing the structural parameters.
Analogous observations as in case of 7 were made investigating
the reduction of 9 containing the LMe7 ligand, although in this case
no crystals were grown.
Conclusions
The results described above show that the reduction of the
-diketiminato nickel(II) halide complex [LtBuNiIIBr] with potassium
sources proceeds via the initial formation of [(LtBuNiI)x(-Br)xKx]
aggregates. The KBr entities are readily released in the presence
of donors – as has been observed previously also for LiBr adducts
– and the fact that even the rather weak donor N2 replaces the
alkali metal halide moieties shows that the latter represent rather
labile leaving groups with low donor strength. They are thus
suitable as placeholders in complexes that activate small
molecules, and this recently has been exploited in nickel(II)
chemistry.[20] Furthermore, the presence of stoichiometric
amounts of a Lewis acid – namely potassium ions – may influence
the activation process or allow for the trapping of intermediates.
The findings made varying the substituents in L further show that
an undisturbed interaction with the  system of an aryl ring is
preferred over N2 ligation.
A comparison of these results with those reported previously is
displayed in Scheme 3.[3,4,18,19] Stephan et al. first characterized
the toluene complex [(LMe2NiII)2(-3-3-C6H5Me)], I, which is inert
towards N2 and H2.[4,18] However, I can be used as nickel(I)
synthon for the coordination of stronger electron donating
molecules like nitriles and is in fact reactive towards stronger
oxidants like N2O.[19] The comparatively inert behavior of I is likely
an indication of the stronger electron donation from nickel to the
toluene ligand which in fact is best described as being
dianionic.[18]
I
is synthesized by the reduction of
[LMe2NiII(-Br)2Li(THF)2], II, in presence of toluene. We have
previously shown, that in absence of any donating molecules like
toluene [(LMe2NiI)2(3-Br)2Li2(THF)], III, is formed.[4] III is reactive
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10.1002/zaac.201800100
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Scheme 3. Comparison of our findings to previously reported work. Upon increased bulkiness of the nacnac ligand, nickel(I) complexes with metal halide entities
can be isolated. These prove to be key to the activation of very weak donating molecules like N2.[3,4,18,19]
(3)6 were solved by the intrinsic phasing method (SHELXT-2013)[26] the
structures 7 and 10 were solved by direct methods (SHELXS-2013)[26]. All
structures were refined by full-matrix least-squares procedures based on
Experimental Section
F2 with all measured reflections (SHELXL-2017)[27] with anisotropic
temperature factors for all non-hydrogen atoms. All hydrogen atoms were
added geometrically and refined by using a riding model. The structure of
(3)x suffers of disorder. Some atoms involved in disorder could only be refined
isotropically. (3)x and (3)6 are heptane and hexane solvates, respectively.
Part of the solvent could be refined. Smeared low remaining electron
density, which could not be properly refined, was squeezed out.
Additionally, for (3)6 a twin refinement has been performed. CCDC
1830051- 1830053 contains 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.
General procedures. All experiments were carried out in a dry argon or
dinitrogen atmosphere using standard Schlenk techniques and/or an
MBraun glovebox. Solvents were purified using a Solvent Purification
System SPS. When necessary, further purification of hexane and heptane
was achieved using KC8. [LtBuH],[21] [LMe6H][22] and [LMe7H][14] were
prepared according to known procedures. Deuterated solvents were
stored in an MBraun glovebox and further purified by the use of appropriate
molecular sieves. NMR samples were prepared in an argon or dinitrogen
atmosphere in an MBraun glovebox. The tubes were sealed with a
J. YOUNG valve. 1H spectra were recorded on a Bruker DPX 300
spectrometer (1H 300.1 MHz) or on a Avance II 400 (1H 400 MHz) at room
temperature (r.t.) without rotating the tube. Paramagnetic samples were
measured with a higher number of scans and with a wider sweep area
(e. g. ns = 256, o1p = –20 ppm, sw = 140 ppm). Chemical shifts are
reported in ppm relative to the residual proton signals. Microanalyses were
performed with a HEKA Euro 3000 elemental analyzer. EPR spectra were
recorded at an ESR 300 X-Band EPR spectrometer (BRUKER), equipped
with a quartz dewar (for measurements at 77 K) in cooperation with Dr. A.
Schnegg from Helmholtz-Zentrum Berlin für Materialien und Energie.
Complex (3)x, Formula: C231H364Br6K6N12Ni6, molecular mass 4375.64
g/mol , crystal system monoclinic, Space group Cc, Z= 2, a= 32.946(4) Å,
b= 23.350(4) Å, c= 23.219(4) Å, = 130.678(7)°, V= 13546(4) Å3,
size/color/habit: 0.12x0.28x0.32/ orange/ fragment. Density (calc.) 1.073
g/cm3, F000= 4652, Rint = 0.0500, total nr. of refl. 265744, indep. refl.
24222, Parameters 1210, R1= 0.0613 (0.0711 all data), wR2= 0.1590
(0.1631 all data), GooF= 1.108 (1.110 all data), data completeness 99%,
largest diff. peak / hole [e/Å3 ] 1.04/ -0.50. CCDC 1830053
Complex 7: Formula: C42H50Br2N4Ni2, molecular mass 888.10 g/mol ,
crystal system monoclinic, Space group P 21/n, Z= 2, a= 12.7814(4) Å, b=
9.0306(3) Å, c= 17.4435(6) Å, = 103.469(3)°, V= 1958.02(11) Å3,
size/color/habit: 0.11x0.25x0.39/ brown/ plate. Density (calc.) 1.506 g/cm3,
F000= 912, Rint = 0.0216, total nr. of refl. 5221, Parameters 232, R1=
The crystallographic data were collected at 100 K on a BRUKER D8
VENTURE area detector for (3)x and (3)6 and STOE IPDS-2T
diffractometer for 7 and 10 and, Mo Kα radiation (λ = 0.71073 Å). Multiscan
absorption correction[25] was applied to the data. The structures (3)x and
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10.1002/zaac.201800100
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ARTICLE
0.0600 (0.0693 all data), wR2= 0.1638 (0.1756 all data), GooF= 1.032
(1.032 all data), data completeness 99%, largest diff. peak / hole [e/Å3 ]
1.70/ -2.04. CCDC 1830052
formed which were isolated by filtration and washed with 5 mL hexanes.
37 mg (54 µmol, 22%) (3)6 were isolated as orange prisms. Method 2: 100
mg (0.16 mmol) 1 and 100 mg (2.56 mmol, 16.4 eq) elemental potassium
were refluxed in 150 mL freshly dried heptane at 99 °C. Aftler two hours
the temperature was reduced to 60 °C. It was stirred gently to ensure a
complete agglomeration of the potassium in large agglomerates. 75% of
the solution was transferred quickly into a preheated vessel at 60 °C.
Subsequently, the solution was heated to 90 °C and concentrated to
30 mL while remaining at 90 °C. Afterwards, the solution was stored at
90 °C for 24 hours. Every 24 hours the temperature was reduced by 10 K
until room temperature was reached. During the process, large orange
crystals were formed which were isolated by filtration and washed with
5 mL hexanes. 40 mg (59 µmol based on one LNi entity, 37%) (3)6 were
Complex 10: Formula: C21H25N2Ni, molecular mass 364.14 g/mol , crystal
system monoclinic, Space group P -1, Z= 6, a= 9.5181(3) Å, b= 9.5181(3)
Å, c= 18.7375(6) Å, = 80.597(3)°, = 103.469(3)°, = 74.782(3)°, V=
2673.42(16) Å3, size/color/habit: 0.09x0.19x0.29/ red/ plate. Density (calc.)
1.357 g/cm3, F000= 1158, Rint = 0.0560, total nr. of refl. 47800, indep. refl.
14932, parameters 667, R1= 0.0436 (0.0687 all data), wR2= 0.0927
(0.0999 all data), GooF= 1.026 (1.026 all data), data completeness 99%,
largest diff. peak / hole [e/Å3 ] 0.88/ -1.04. CCDC 1830051
isolated. EPR (powder, 77 K, 9.46 GHz): gx = 2.277, gy = 2.197,
g
⊥
= gz = 2.164. Elemental analysis (%) calculated for polymeric
(3)6·heptane[23] (4177.23 g·mol–1): C 62.39, H 8.06, N 4.02; found: C 62.01,
H 7.92, N 3.98.
–
Improved synthesis of [LtBuNiIIBr], 1,[3] (LtBu = CH(CtBuNdipp)2 , dipp = 2,6-
The synthesis [LMe6Li(THF)] was performed according to a protocol for
[LMe6Li(OEt2)] developed by Roesky and co-workers.[24] At -90 °C 4 mL of
an n-butyl lithium solution in hexane (10 mmol, 1.1 eq, 2.5 M) were added
carefully to 2.79 g (9.09 mmol, 1 eq) LMe6H in 30 mL THF. After the
addition, the pale yellow solution was allowed to warm to room
temperature. After one hour, all volatile components were removed under
reduced pressure and the yellow residue was extracted three times with
30 mL hexane. The combined extract was concentrated to 30 mL and
stored for three days at -30 °C. The suspension was filtered and the filtrate
was concentrated to 5 mL and stored at -30 °C for two weeks resulting in
the precipitation of a powder which was isolated by filtration and washed
at -30 °C using 5 mL cold hexane. 2.76 g (7.2 mmol, 79%) [LMe6Li(THF)]
were recovered as pale yellow polycrystalline powder. δ (400.1 MHz, C6D6
Di-iso-propylphenyl). 6.9 g (13.7 mmol) [LtBuH] were dissolved in 40 mL
THF. At -78 °C 6.0 mL (15.0 mmol, 1.1 eq) of a solution of a n-butyl lithium
solution in hexanes (2,5 M) were slowly added. After 10 minutes the pale
yellow solution was allowed to warm to room temperature and after
another two hours, the yellow solution was added to a violet suspension of
4.2 g (13.7 mmol, 1 eq) NiBr2·dme in 60 mL thf and the resulting dark
suspension was heated to 66 °C for 16 hours. The resulting dark green
suspension was filtered at room temperature. The filtration residue was
extracted with thf until the extract was colorless. All volatiles of the
combined thf solutions were removed in vacuo and the remaining residue
was solidified by freeze-drying. The green powder was heated to 90 °C in
a dynamic vacuo for 12 hours and subsequently it was extracted three
times with 30 mL dichloromethane. The combined extracts were
concentrated to 20 mL at 35 °C under reduced pressure and carefully
layered with 30 mL diethyl ether followed by storage at -30 °C overnight.
Black block-shaped crystals (4.30 g) grew over night and were isolated by
filtration, washed with 15 mL hexanes and 15 mL diethyl ether. The
collected filtrate was combined with the diethyl ether wash liquid and all
volatiles were removed in vacuo. The green powder was dissolved in
20 mL dichloromethane. The solution was concentrated to 5 mL and
layered with 10 mL diethyl ether. After storing the solution overnight
at -30 °C, another 2.20 g of crystalline material was isolated by filtration,
washed with 5 mL hexanes and 5 mL diethyl ether. 6.50 g (10.1 mmol,
74%) of crystalline 1 was isolated. The collected analytical data was
identical to the previously published data. Additionally, the 1H NMR
spectrum of paramagnetic 1 was recorded and the signals assigned:
δ (400.1 MHz, C6D6 /ppm) = 32.9 (brm, 4 H, m-ArH), 22.1 (brs, 4 H,
CHMe2), 6.4 (d, 12 H, Me2CH), 5.7 (d, 12 H, Me2CH), 2.4 (s, 18 H, t-Bu),
–18.5 (brm, 2 H, p-ArH), –140.8 (brm, 1 H, α-HC(C(t-Bu)NAr)2).
δ (300.1 MHz, CD2Cl2 /ppm) = 24.7 (brm, 4 H, m-ArH), 16.4 (brs, 4 H,
CHMe2), 4.8 (d, 12 H, Me2CH), 4.5 (d, 12 H, Me2CH), 2.1 (s, 18 H, t-Bu),
–10.6 (brm, 2 H, p-ArH), –98.8 (brm, 1 H, α-HC(C(t-Bu)NAr)2). 1H1H
COSY cross peaks: δ1/δ2 (400.1 MHz, C6D6 /ppm/ppm) = 32.9/-18.5
(m-ArH/p-ArH), 22.1/6.4 (CHMe2/Me2CH), 22.1/5.7 (CHMe2/Me2CH).
3
3
/ppm) = 7.09 (d, JHH = 7.5 Hz, 4 H, m-ArH), 6.93 (t, JHH = 7.5 Hz, 2 H,
p-ArH), 4.98 (s, 1 H, α-HC(CMeNAr)2), 2.84 (m, 4 H, α-CH2 (THF)), 2.23
(s, 12 H, o-MeAr), 1.82 (s, 6 H, MeCNAr), 0.86 (m, 4 H, β-CH2 (THF)).
Synthesis of [LMe6NiII(-Br)2Li(THF)2], 6. A solution of 2.46 g (6.42 mmol)
[LMe6Li(THF)] in 30 mL THF was added to a solution of 1.98 g (6.42 mmol,
1 eq) NiBr2·dme in 30 mL THF and refluxed at 66 °C. After 22 hours the
suspension was allowed to cool to room temperature while removing all
volatile components under reduced pressure. The blue solid material was
extracted three times with 20 mL dcm. All volatile components of the
combined extracts were removed under reduced pressure and the solid
was washed with 5 mL hexane and 5 mL THF. 3.8 g (5.6 mmol, 87%) of 6
were isolated as blue powder. δ (400.1 MHz, C6D6 /ppm) = 50.6 (s, 12 H,
o-MeAr), 47.0 (brm, 4 H, m-ArH), 0.83 (brm, 8 H, THF), –0.51 (brm, 8 H,
THF), –24.9 (brm, 2 H, p-ArH), –67.8 (s, 6 H, MeCNAr), –191.0 (brs, 1 H,
α-HC(CMeNAr)2). Note: in C6D6, 6 enters into an equilibrium with the
dinuclear nickel complex 8 and LiBr. Analytical data for 8 are listed below.
Elemental
analysis
(%)
calculated
for
C29H39Br2LiN2NiO2
(M = 673.09 g·mol–1): C 51.75, H: 5.84, N: 4.16; found: C: 52.08, H: 5.94,
N: 4.02.
Synthesis of [LMe7NiII(-Br)2Li(THF)2], 7. At –78 °C 2 mL of a solution of n-
butyl lithium in hexane (2.5 M, 5 mmol, 1.05 eq) was carefully added to a
solution of 1.53 g (4.77 mmol) [LMe7H] in 30 mL THF. After complete
addition the pale yellow solution was allowed to warm to room temperature
and added to a suspension of 1.47 g (4.77 mmol, 1 eq) NiBr2·dme in
40 mL THF after one hour. Subsequent to refluxing for 22 hours at 66 °C,
the blue suspension was filtrated and the solid residue extracted with thf
until the extract was colorless. Subsequently, all volatile components of
the combined blue solutions were removed in vacuo and the residue was
washed with 10 mL hexanes and 5 mL THF. 2.07 g (3.01 mmol, 63%) 7
were isolated as blue powder. δ (300.1 MHz, C6D6 /ppm) = 170.9 (brs, 3 H,
α-MeC(CMeNAr)2), 52.2 (s, 12 H, o-MeAr), 48.4 (s, 4 H, m-ArH), 0.7 (m,
Synthesis of 3 as hexamer (method 1) or polymer (method 2). Method 1:
150 mg (0.23 mmol) 1 and 100 mg (2.56 mmol, 10.9 eq) elemental
potassium were heated to 68 °C in freshly dried hexanes. After 2 hours,
the temperature was cooled to 60 °C and the suspension was gently stirred
to allow for aggregation of the remaining elemental potassium. 75% of the
solution was transferred quickly into a preheated vessel at 60 °C.
Subsequently, the solution was concentrated to 30 mL while remaining at
60 °C. Afterwards, the solution was stored at 60 °C. After 24 hours the
temperature was decreased to 40 °C. After another 24 hours, the solution
was allowed to cool to room temperature. After one week, crystals were
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10.1002/zaac.201800100
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ARTICLE
8 H, THF), –0.5 (m, 8 H, THF), –30.3 (s, 2 H, p-ArH), –73.2 (s, 6 H,
MeCNAr). Note: in C6D6, 7 enters into an equilibrium with the binuclear
nickel complex 9 and LiBr. δ (300.1 MHz, THF-d8 /ppm) = 160.7 (brs, 3 H,
α-MeC(CMeNAr)2), 50.7 (s, 12 H, o-MeAr), 47.6 (s, 4 H, m-ArH), 3.5
(THF), –1.7 (THF), –28.7 (s, 2 H, p-ArH), –69.5 (s, 6 H, MeCNAr). In
THF-d8 the resonances of the THF molecules bound to LiBr could not be
integrated due to the exchange equilibrium of bound THF with THF-d8.
Elemental analysis (%) calculated for C42H50Br2N4Ni2 (M = 687.10 g·mol–
1): C 52.44, H: 6.01, N: 4.08; found: C: 52.34, H: 6.21, N: 4.23. ν (KBr, cm–
1) = 3432 (s), 3062 (w), 3015 (m), 2975 (s), 2916 (s), 2873 (s), 2731 (vw),
1653 (m), 1528 (s), 1462 (s), 1443 (s), 1368 (s), 1326 (vs), 1305 (vs), 1293
(s), 1253 (w), 1192 (vs), 1161 (w), 1136 (w), 1096 (m), 1083 (w), 1045 (vs),
996 (s), 911 (m), 893 (m), 868 (s), 804 (m), 766 (vs), 761 (vs), 705 (w),
668 (w), 650 (w), 620 (vw), 497 (w).
Synthesis of [(LMe6NiI)2], 10. A suspension of 200 mg (225 mol) 8 and
183 mg (1.35 mmol, 6 eq) KC8 in 40 mL hexane were reacted for 36 hours.
The orange red suspension was filtrated and the solid residue was
extracted with hexane until the extract was colorless. The combined
solutions were concentrated to 10 mL at 50 °C under reduced pressure
and slowly (over the course of several hours) allowed to cool to room
temperature. After storing for
2 weeks at room temperature and
subsequent filtration 68 mg (93 mol, 42%) 10 were isolated as red
crystals that were suitable for X-ray diffraction analysis. δ (400.1 MHz,
C6D6, /ppm) = 26 – 23 (brs), 2 – 1 (brs), 0.5 – –0.5 (brs). Elemental analysis
calculated for C42H50N4Ni2 (M = 728.26 g·mol–1): C 69.27, H: 6.92, N: 7.69;
gef.: C: 68.61, H: 6.90, N: 7.46.
Reduction of 9 in N2/Ar. 30 mg (32.7 µmol) 9 und 10 mg (72.0 µmol,
2.2 eq) KC8 in hexane were reacted for 22 hours. After subsequent
filtration all volatile components of the orange red filtrate were removed
under reduced pressure and the red brown polycrystalline material was
analyzed. In absence and presence of N2 identical analytical data were
obtained. δ (300.1 MHz, C6D6 /ppm) = 30 – 20 (brs), 19 – 17 (brs), 2.8 –
1.5 (brm), –15 – –18 (brs). ν (KBr, cm–1) = 3026 (vw), 3031 (w), 3009 (w),
2962 (s), 2913 (s), 2853 (m), 2724 (vw), 1651 (m), 1591 (w), 1557 (w),
1530 (vs), 1468 (m), 1461 (m), 1450 (m), 1447 (m), 1440 (m), 1395 (w),
1368 (vs), 1356 (vs), 1314 (w), 1200 (vs), 1087 (vs), 1057 (s), 1021 (vs),
998 (s), 871 (w), 788 (s), 762 (vs), 698 (vw), 689 (vw), 498 (vw).
Synthesis of [(LMe6NiII)2(-Br)2], 8. The previously reported[16] synthesis of
8 did not lead to 8 but to a mixture of two compounds. According to our
findings, 8 can be obtained via 3 different methods. Method 1: 1.00 g
(1.49 mmol) [LMe6NiII(-Br)2Li(THF)2] (6) were suspended in 25 mL toluene
and refluxed at 111 °C for one hour. After filtration at 100 °C all volatile
components of the blue filtrate were removed under reduced pressure.
571 mg (0.64 mmol, 87%) 8 were isolated as blue green solid. Method 2:
A solution of 2.00 g (6.53 mmol) [LMe6H] in 30 mL THF was added to a
suspension of 262 mg (6.53 mmol, 1 eq) potassium hydride in 20 mL THF
and refluxed at 66 °C for 1 day. After cooling to room temperature, 2.02 g
(6.53 mmol) NiBr2·dme and 50 mL THF were added and subsequently
refluxed at 66 °C. After 22 h the suspension was filtrated and the solid
residue was extracted with THF until the extract was colorless. All volatile
components of the combined solutions were removed in vacuo. The green
blue residue was solidified by freeze-drying and further dried in high
vacuum at 80 °C for 3 hours. The solid was extracted three times with
30 mL toluene at 60 °C. After evaporation of all volatile components in
vacuo, 1.57 g (1.77 mmol, 54%) 8 were isolated as blue green powder.
Method 3: 1.00 g (1.49 mmol) 6 in form of a fluffy powder was dried for 3
days at 120 °C in high vacuum and subsequently extracted with toluene
until the extract was colorless. After evaporation of all volatile components
in vacuo, 647 mg (0.73 mmol, 98%) of 8 were isolated as a blue green
solid. δ (400.1 MHz, C6D6 /ppm) = 46.2 (brm, 8 H, m-ArH), 42.5 (s, 24 H,
o-MeAr), –24.9 (brm, 4 H, p-ArH), –80.6 (s, 12 H, MeCNAr), –216.8 (brs,
2 H, α-HC(CMeNAr)2). Elemental analysis (%) calculated for
C42H50Br2N4Ni2 (M = 888.07 g·mol–1): C 56.80, H: 5.67, N: 6.31; found:
C: 56.82, H: 5.70, N: 6.20. Single crystals of 8 that were suitable for X-ray
diffraction analysis were grown by storing a saturated hexane solution of
8 at -30 °C for 1 week.
Acknowledgements
We are grateful to the Humboldt-Universität zu Berlin for financial
support and the cluster of excellence UniCat for valuable
discussions. We thank Dr. A. Schnegg from “Helmholtz-Zentrum
Berlin für Materialien und Energie” for the possibility to measure
EPR spectra.
Keywords: nickel(I) • small molecule activation • low-
coordination • weak ligands • alkali metals • dinitrogen activation
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Synthesis of [(LMe7NiII)2(-Br)2], 9. 9 can be obtained following 2 different
protocols. Method 1: A suspension of 1.00 g (1.46 mmol) 7 in 25 mL
toluene was refluxed at 111 °C for 3 hours and filtrated at 100 °C. After
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isolated as a blue solid. δ (300.1 MHz, C6D6 /ppm) = 200.9 (brs, 3 H,
α-MeC(CMeNAr)2), 47.4 (s, 4 H, m-ArH), 43.0 (s, 12 H, o-MeAr), –43.3 (s,
2 H, p-ArH), –86.9 (s, 6 H, MeCNAr). Elemental analysis calculated for
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C: 58.21, H: 6.38, N: 5.76.
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10.1002/zaac.201800100
Zeitschrift für anorganische und allgemeine Chemie
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KBr entities formed in the course of
reduction of nickel(II) halides in the
ligand sphere of resulting nickel(I)
complexes are readily released in the
presence of donors as they represent
rather labile leaving groups with low
donor strength. They are thus
suitable as placeholders in
Patrick Holze, Beatrice Braun-Cula,
Stefan Mebs and Christian Limberg*[
Page No. – Page No.
Stabilization of -diketiminato
nickel(I) with alkaline metal halide
entities for small molecule activation
complexes that activate small
molecules.
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10.1002/zaac.201800100
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ARTICLE
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