β-Diketiminato nickel(I) complexes with very weak ligation allowing for H2 and N2 activation

Article abstract of DOI:10.1021/om9007983

Reaction of [L^tBuNiBr] (L^tBu = [HC(C(CMe ₃)NC₆H₃(iPr)₂)₂] ^- with KC₈ in toluene solution yields the complex [L ^tBuNi(toluene)], 1, where toluene

Full text of DOI:10.1021/om9007983

Organometallics 2009, 28, 6855–6860 6855
DOI: 10.1021/om9007983
β-Diketiminato Nickel(I) Complexes with Very Weak Ligation Allowing
for H2 and N2 Activation
†
‡
†
†
‡
€
Stefan Pfirrmann, Shenglai Yao, Burkhard Ziemer, Reinhard Stosser, Matthias Driess,
and Christian Limberg*,†
†
€
Institut fu€r Chemie, Humboldt-Universitat zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany, and
Institute of Chemistry: Metalorganics and Inorganic Materials, Technische Universitat Berlin, Sekr. C2,
‡
€
Strasse des 17. Juni 135, 10623 Berlin, Germany
Received September 11, 2009
Reaction of [LtBuNiBr] (LtBu = [HC(C(CMe3)NC6H3(iPr)2)2]-) with KC8 in toluene solution
yields the complex [LtBuNi(toluene)], 1, where toluene is bound in a η2 mode via a CdC unit of the
aromatic ring, as revealed by single-crystal X-ray crystallography and DFT calculations (B3LYP/6-
31G*). Performing the same reaction in hexane as the solvent did not lead to a traceable product, so
that the β-diketiminato ligand system was changed from LtBu to the less bulky LMe (LMe
=
[HC(CMeNC6H3(iPr)2)2]-). Reduction of [LMeNiBr]2 with KC8 in diethyl ether led to [LMeNi]2, 2,
with intramolecular Ni-aryl interactions, while employment of [LMeNi(μ-Br)2Li(thf)2] as a pre-
cursor for a reaction with KC8 in OEt2 led to the complex [LMeNi(μ-Br)Li(thf)2]2, 3. Both complexes 2
and 3 could be fully characterized, also with the aid of XRD, and their reactivity with respect to H2
and N2 was examined. It turned out that they oxidatively add H2 to give the known compound
[LMeNi(μ-H)]2, I, while the reaction with N2 provides the dinitrogen complex [(LMeNi)2(N2)], 4.
Introduction
can be prepared by treating [LMeNi(μ-Br)2Li(thf)2] with
KBEt3H.4 On dissolution, I does not enter into an equilib-
rium with a monomer [LMeNiH] according to the results of
spectroscopic investigations, and hence, the question arose
whether a mononuclear version of this complex can be forced
to form by analogy with the corresponding iron chemistry,5
if the steric bulk at the ligand is further increased by replacing
the methyl residues in LMe by tert-butyl residues. This pushes
the aryl rings at the N atoms further in front of the coordi-
nated metal, and consequently three-coordination is pre-
ferred. The precursor [LtBuNiBr] was thus reacted with
KBEt3H under a dinitrogen atmosphere in hexane as the
solvent. While similar conditions in the case of LMe had led to
a green solution of I, with LtBu a red-brown solution was
obtained, from which the first NiI dinitrogen complex
[(LtBuNi)2(μ-N2)], II, could be isolated (Scheme 1).6
Complex II can alternatively be prepared via reduction of
[LtBuNiBr] by KC8 in the presence of N2. Looking at
Scheme 1 of course the question arises whether the N2
triggers the H2 elimination, and what happens in a hexane
solution if it is not there. Likewise, it seems interesting to
investigate the reaction of precursors [LRNiBr] with KC8
under varying conditions: with R = Me in toluene solution
the dinuclear complex [(LMeNi)2(μ-η3:η3-C6H5Me)], III,7 is
Nickel hydride units play decisive roles in many catalytic
processes applied in industrial or academic laboratories and
in nature.1,3g For instance the central intermediate within the
catalytic cycle of the [NiFe]-hydrogenase is assumed to
contain a bridging hydride ligand between the Ni and the
Fe centers.2 Altogether this has led to an increased interest in
Ni-H coordination compounds.3 Recently we have re-
ported our results concerning the synthesis and reactivity
of the compound [(LMeNi)2(μ-H)2], I (see Scheme 1), which
*Corresponding author. Fax: (þ49) 30-2093-6966. E-mail: christian.
limberg@chemie.hu-berlin.de.
(1) (a) Kokes, R. J.; Emmett, P. H. J. Am. Chem. Soc. 1959, 81, 5032.
(b) Keim, W.; Behr, A.; Limbacker, B.; Kruger, C. Angew. Chem., Int. Ed.
Engl. 1983, 22, 503. (c) Keim, W.; Appel, R.; Gruppe, S.; Knoch, F. Angew.
Chem., Int. Ed. Engl. 1987, 26, 1012. (d) Nomura, K. Recent Res. Dev.
Pure Appl. Chem. 1998, 2, 473. (e) Vogt, D. In Aqueous-Phase Organo-
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(2) (a) Brecht, M.; van Gastel, M.; Buhrke, T.; Friedrich, B.; Lubitz,
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Angew. Chem., Int. Ed. 2007, 46, 8942. (c) Barton, B. E.; Whaley, C. M.;
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(3) (a) Clement, N. D.; Cavell, K. J.; Jones, C.; Elsevier, C. J. Angew.
Chem., Int. Ed. 2004, 43, 1277. (b) Steinke, T.; Gemel, C.; Cokoja, M.;
Winter, M.; Fischer, R. A. Angew. Chem., Int. Ed. 2004, 43, 2299. (c) Chen,
W.; Shimada, S.; Tanaka, M.; Kobayashi, Y.; Saigo, K. J. Am. Chem. Soc.
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2008, 27, 3082. (e) Laird, M. F.; Pink, M.; Tsvetkov, N. P.; Fan, H.; Caulton,
K. G. Dalton Trans. 2009, 1283. (f) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.;
Twamley, B.; Fraze, K.; DuBois, M. R.; DuBois, D. L. J. Am. Chem. Soc.
2009, 131, 5935. (g) Chakraborty, S.; Krause, J. A.; Guan, H. Organome-
tallics 2009, 28, 582, and references therein. (h) Tran, B. L.; Pink, M.;
Mindiola, D. J. Organometallics 2009, 28, 2234.
(4) Pfirrmann, S.; Limberg, C.; Ziemer, B. Dalton Trans. 2008, 6689.
(5) (a) Smith, M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc.
2003, 125, 15752. (b) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.;
Cowley, R. E.; Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T.
R.; Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6624.
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r
2009 American Chemical Society
Published on Web 10/28/2009
pubs.acs.org/Organometallics

6856 Organometallics, Vol. 28, No. 24, 2009
Pfirrmann et al.
Scheme 1. Synthesis of I by Reaction of [LMeNi(μ-Br)2Li(thf)2]
with KBEt3H and Formation of II via a Potential Hydride as Well
as by Reduction of [LtBuNiBr] with KC8
Figure 1. Molecular structure of 1. Thermal ellipsoids are shown
at 50% probability. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Ni1-N1 1.9281(19),
Ni1-N2 1.9121(18), Ni1-C1 2.154(2), Ni1-C2 2.437(3), Ni1-
C6 2.8773(27); N1-Ni1-N2 98.61(8), N1-Ni1-C1 129.34(10),
N2-Ni1-C1 124.61(10).
Results and Discussion
After suspension of [LtBuNiBr] in toluene the reaction with
KC8 leads to a color change from green to yellow-brown.
Appropriate workup allowed for the isolation of the reaction
product as a yellow-brown solid in 70% yield. Crystals could
be obtained via slow evaporation of the volatiles from
toluene solutions at room temperature, and an X-ray crystal
structure analysis revealed the product constitution as
[(LtBuNi)(η2-C6H5Me)] (Scheme 2), 1. Figure 1 shows the
molecular structure of 1.
˚
The Ni-N distances of 1.9281(19) and 1.9121(18) A in 1
Scheme 2. In Dependence on the Ligand Residue R the Nickel(I)
Compounds III or 1 are Obtained by Reduction of the Corre-
sponding Nickel(II) Precursors in the Presence of Toluene
are in the range of the bond lengths found in other nickel(I)
complexes containing that β-diketiminato ligand.6 In con-
trast to the system [LMeNi(μ-Br)2Li(thf)2]/KC8/toluene,
which leads to compound III, 1 does not react with a
further LtBuNi species to generate a dinuclear complex. Its
nickel center binds asymmetrically to the toluene unit,
forming two short bonds of 2.154(2) (Ni1-C1) and
˚
˚
2.437(3) A (Ni1-C2) and one longer bond of 2.8773(27) A
(Ni1-C6) to the toluene carbon atoms. The nickel-carbon
distances in III containing the sterically less demanding
ligand LMe are shorter in general and show a smaller variance
˚
(1.980(5)-2.209(5) A). From this point of view the coordi-
nation mode of the arene unit in 1 would be described best as
η2. However, as hydrogen atoms cannot be localized un-
ambiguously by means of X-ray crystallography, there re-
mained the possibility of a σ-interaction of the metal center
with a C-H bond, especially with that belonging to C1. To
analyze the bonding situation in detail density functional
theory (DFT) calculations (B3LYP/6-31G*) were carried
out, using the molecular structure of 1 as starting geometry
(see Supporting Information). The optimized structure is
shown in Figure 2. As expected, calculations predict a
doublet ground state with the unpaired electron located at
the Ni center. A detailed natural bond orbital (NBO) analy-
sis revealed a significant stabilization of the LtBuNi molecule
(183 kJ/mol) by a donor-acceptor interaction between an
occupied Ni(d) orbital and the empty π* orbital of the
C1-C2 bond. Further stabilization (106 kJ/mol) is achieved
by a back-donation from the occupied π orbital of the
C1-C2 bond to the empty Ni(4s) orbital. Since other
obtained (Scheme 2), but what sort of complex is generated if
R corresponds to tert-butyl residues, which destabilize the
binding of larger ligands at the nickel center and thus a μ-
η3:η3-coordination mode of toluene? How does the reaction
proceed in the complete absence of suitable σ- or π-donors?

Article
Organometallics, Vol. 28, No. 24, 2009 6857
Figure 3. Molecular structure of 2. Thermal ellipsoids are shown
at 50% probability. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Ni1-N1 1.9062(13),
Figure 2. Optimized structure of 1 according to DFT calcula-
tions (B3LYP/6-31G*, doublet state). Hydrogen atoms are
˚
omitted for clarity. Selected bond lengths (A) and angle (deg):
Ni1-N2 1.9350(14), Ni1-C1 2.1337(15), Ni1-C2 2.0747(15);
N1-Ni1-N2 97.30(6), N1-Ni1-C1 149.56(6), N1-Ni1-C2
145.46(6), N2-Ni1-C1 106.45(6), N2-Ni1-C2 109.89(6).
Ni-C1 2.065, Ni-C2 2.141, Ni-C6 3.009; C1-Ni-C2 39.58.
donor-acceptor interactions involving the Ni atom are
much weaker, this makes clear that the coordination of the
toluene molecule indeed occurs in a η2 mode.
Scheme 3. Synthesis of the Nickel(I) Complexes 2 and 3 and
Their Conversion into the Compounds 4 and I upon Exposure to
Dinitrogen and Dihydrogen, Respectively
The magnetic moment of 2.06 μB displayed by 1 in the solid
state at room temperature is only slightly higher than the
expected spin-only value of 1.73 μB for one unpaired electron
and thus points to the oxidation state of þI at the nickel
center, in agreement with the theoretical data. The EPR
spectrum of 1 (see Supporting Information) in the solid state
at 77 K shows a rhombic signal (gx = 2.468, gy = 2.152, gz =
2.072), which further supports the formulation of 1 as a
nickel(I) complex, by comparison with the results found for
other β-diketiminato nickel(I) complexes.4,7,8
When the same reaction was performed in hexane instead
of toluene as the solvent, an orange solid precipitated from
the reaction mixture. An attempt to separate this solid from
the concomitantly produced graphite by using more polar
solvents than hexane was unsuccessful, as only the corre-
sponding solvent adducts [LtBuNi(solv)] (with solv = diethyl
ether,6 toluene) could be isolated. When hexane suspensions
of this product were allowed to react with dinitrogen, the
dinitrogen complex II was generated. We therefore assume
the orange solid to correspond to a transient nickel(I) species
similar to the corresponding iron compound [LtBuFe(KCl)-
(solv)x] reported by Holland et al.,9 which is supported by
results obtained for the analogous system with LMe (vide
infra). Similar observations could be made when [LtBuNiBr]
suspended in hexane was treated with KBHEt3 in an argon
instead of a N2 atmosphere: an orange solid precipitated
from the reaction mixture, but again all efforts to isolate or to
crystallize the compound failed. Hence we have reduced the
sterics around the Ni centers somewhat by replacing LtBu by
LMe and found that depending on the starting materials,
different types of NiI complexes are formed: Reduction of
[LMeNiBr]2, which can be obtained from [LMeNi(μ-Br)2Li-
(thf)2] by heating it in toluene,10 with KC8 in hexane led to a
red-brown solution from which [(LMeNi)2], 2, could be
isolated in 23% crystalline yield (Scheme 3). Its molecular
structure as revealed by single-crystal X-ray diffraction
analysis is shown in Figure 3.
In 2 the nickel ions bind to the β-diketiminato-N atoms as
well as to an aryl ring of a second LMeNi unit of the dimer in a
η2-coordination mode. This constitution becomes possible
due to the T-shaped geometry around the nickel ions that is
typical for nickel(I) complexes of β-diketiminato ligands:4,7,11
The N1-Ni1-C1 and N2-Ni1-C1 angles amount to
149.56(6)° and 106.45(6)°, respectively, and this asymmetry
is also expressed in two different Ni-N bond lengths
˚
(8) (a) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.;
Warren, T. H. J. Am. Chem. Soc. 2005, 127, 11248. (b) Saraev, V. V.;
Kraikivskii, P. B.; Svoboda, I.; Kuzakov, A. S.; Jordan, R. F. J. Phys. Chem.
A 2008, 112, 12449. (c) Xiong, Y.; Yao, S.; Bill, E.; Driess, M. Inorg. Chem.
2009, 48, 7522.
of 1.9062(13) and 1.9350(14) A. In consequence the two
(10) Eckert, N. A.; Bones, E. M.; Lachicotte, R. J.; Holland, P. L.
Inorg. Chem. 2003, 42, 1720.
(9) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.;
Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.;
Holland, P. L. J. Am. Chem. Soc. 2006, 128, 756.
(11) (a) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.;
Warren, T. H. J. Am. Chem. Soc. 2005, 127, 11248. (b) Eckert, N. A.;
Dinescu, A.; Cundari, T. R.; Holland, P. L. Inorg. Chem. 2005, 44, 7702.

6858 Organometallics, Vol. 28, No. 24, 2009
Pfirrmann et al.
Table 1. Crystal Data and Experimental Parameters for the
Crystal Structure Analyses of 1, 2, and 3
1
2
3
formula
C
42H61N2Ni
C58H82N4Ni2 C74H114Br2-
Li2N4Ni2O4
weight, g mol-1
temp, K
cryst syst
652.64
100(2)
monoclinic
P21/n
9.9120(4)
16.9945(5)
22.2217(9)
90
96.818(3)
90
3716.8(2)
4
1.166
952.70
100(2)
monoclinic
C2/c
1414.81
100(2)
monoclinic
P21/c
space group
˚
a, A
17.0249(8)
18.1902(9)
16.8559(7)
90
21.8673(6)
19.8655(5)
17.5029(5)
90
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
90.302(4)
90
91.549(2)
90
3
˚
V, A
5220.0(4)
4
1.212
7600.6(4)
4
1.236
Z
Figure 4. Molecular structure of 3. Thermal ellipsoids are shown
at 50% probability. Hydrogen atoms are omitted for clarity.
density, g cm-3
μ(Mo KR), mm-1
F(000)
0.552
1420
0.761
2056
1.593
3000
˚
Selected bond lengths (A) and angles (deg): Ni1-N1 1.910(5),
Ni1-N2 1.887(5), Ni1-Br1 2.3652(10), Br1-Li1 2.595(11), Br1-
Li10 2.465(11); N1-Ni1-N2 98.8(2), N1-Ni1-Br1 114.28(15),
N2-Ni1-Br1 146.66(16).
GoF
R1 [I > 2σ(I)]
wR2 [all data]
0.830
0.0402
0.0760
0.913
0.0282
0.0654
0.819
0.0685
0.1241
-3
˚
ΔFmin/ΔFmax, e A
0.539/-0.472 0.494/-0.338 0.880/-0.577
coordinating arene units are nearly parallel to each other with a
dihedral angle of 7.177(60)° between the two aromatic planes
and distances between the C atoms of one aromatic unit and
complex [(LMeNi)2(μ-η3:η3-C6H5Me)], III, is formed imme-
diately, as confirmed by 1H NMR spectroscopy. Conse-
quently it appears plausible that 3 represents an interme-
diate species in the formation of III from [LMeNi(μ-Br)2Li-
(thf)2] and KC8. Additionally, 3 seems to be only metastabile
and readily eliminates LiBr to give [(LMeNi)2], 2: If concen-
trated solutions of 3 in hexane are stored at room tempera-
ture for several days, the precipitation of a white solid, which
presumably corresponds to Li(thf)2Br, and the formation of
crystalline [(LMeNi)2], 2, can be observed.
At room temperature complex 3 exhibits a magnetic
moment in solution of μeff = 2.58 μB (Evans’ method,14 pen-
tane-d12), which points to the presence of two uncoupled
spins of S1 = S2 = 1/2 (the spin-only value expected for two
uncoupled NiI ions amounts to 2.45 μB). The EPR spectrum
of 3 at 77 K in the solid state shows a rhombic signal (gx =
2.441, gy = 2.237, gz = 2.085), which further supports that
formulation (see Supporting Information).
Having learned that in the absence of donors I is thermo-
dynamically stable in hexane solution at room temperature,
naturally the question occurred, whether 2 or 3 can be
reacted with H2 to give I in a binuclear oxidative addition.
For this purpose a hexane solution of 3 was stirred under an
atmosphere of hydrogen. In the course of 16 h a color change
from red to green could be observed with concomitant
precipitation of a white solid (Li(thf)2Br). Filtration and
removal of the solvent afforded the hydride I in good yields
(53%). In the case of 2 an immediate color change from
brown to red-orange could be observed when a hexane
solution was exposed to a dihydrogen atmosphere. Subse-
quently, this reaction mixture turns to green within 1 h, and
from such solutions again I could be isolated in 86% yield.
The initial color change is suggestive of an intermediate
species between 2 and I, but so far all attempts to isolate
that intermediate were unsuccessful. Reactions of 2 and 3
with H2 are possible only due to their strained structures and
weak ligation. III, for instance, seems to be far more stable
(although the toluene ligand is also readily displaced by
˚
the plane of the second one ranging from 3.68 to 4.03 A, which
may indicate extra stabilization due to π-π interactions.12
Preliminary measurements concerning the magnetic mo-
ments of 2 in the solid state as well as in solution point to two
basically uncoupled NiI centers at room temperature with
spins of S1 = S2 = 1/2, which is probably due to the thermal
energy antagonizing the coupling: single-point DFT calcula-
tions (B3LYP/6-31G*) based on the molecular structure of 2
predict an antiferromagnetic coupling between the two metal
centers, leading to a broken symmetry singlet ground state
(see Supporting Information). The coupling may be medi-
ated by π-π stacking interactions of the ligand aryl units,
similarly to that in a case recently described by Wieghardt
et al.,13 and the diamagnetism of III is also due to antiferro-
magnetic coupling via an aryl unit.7 Consistently, compound
2 is EPR silent at 77 K in frozen hexane solutions as well as in
the solid state.
The same kind of compound is formed if in LMe the
isopropyl residues are replaced by ethyl residues (see Sup-
porting Information, complex 2a).
By contrast, reaction of the “ate-complex” [LMeNi(μ-
Br)2Li(thf)2]7 with KC8 in diethyl ether led to the isolation
of the orange-red complex [LMeNi(μ-Br)Li(thf)2]2, 3, in 29%
yield, as revealed by X-ray diffraction analysis (Scheme 3).
The molecular structure of 3 is shown in Figure 4. Two
L
MeNiI moieties are coordinated to a Li2Br2 unit, so that,
again, a T-shaped coordination sphere results for the Ni
centers: The N1-Ni1-Br1 and N2-Ni1-Br1 angles
amount to 114.28(15)° and 146.66(16)°, respectively. The
lithium ions are additionally coordinated by two thf mole-
cules, thus yielding a distorted tetrahedral coordination
geometry around lithium. Hence, the structure of 3 is similar
to the one displayed by the iron compound [LtBuFe(KCl)(18-
crown-6)]5a and represents;to our knowledge;the first
example of a complex where an alkali metal halide segment
is coordinated to a NiI center. If 3 is dissolved in toluene, the
(12) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885.
(14) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Schubert, E. M. J.
Chem. Educ. 1992, 69, 62. (c) Ayers, T.; Turk, R.; Lane, C.; Goins, J.;
Jameson, D.; Slattery, S. J. Inorg. Chim. Acta 2004, 357, 202.
€
(13) Milsmann, C.; Patra, G. K.; Bill, E.; Weyhermuuller, T.; DeBeer,
G. S.; Wieghardt, K. Inorg. Chem. 2009, 48, 7430, and references therein.

Article
Organometallics, Vol. 28, No. 24, 2009 6859
stronger donors)7,8c,15 and remains unchanged in contact
with H2 (16 h stirring of a hexane suspension under an H2
atmosphere). III is also stable in a dinitrogen atmosphere,
while, consistently, 2 and 3 react instantaneously: If hexane
solutions of 2 and 3 are exposed to N2, the immediate
precipitation of the dinitrogen complex [(LMeNi)2(N2)], 4
(the LMe derivative of III, see Scheme 1), which is poorly
soluble in this solvent, can be observed. In turn, the reaction
of 4 with dihydrogen also results in the formation of I. This
finding is in contrast to the analogous iron chemistry where
the corresponding dinitrogen complex [(LtBuFe)2(N2)] does
not react with dihydrogen,16,17 probably due to the stronger
activation of the N2 unit within the iron compound as
compared to that within 4. However, treatment of a mixture
of [LtBuFeCl] and KC8 in diethyl ether with dihydrogen
yielded the hydride complex [LtBuFe(μ-H)]2.16
Materials. The complexes [LtBuNiBr],6 [LMeNi(μ-Br)2Li-
10
(thf)2],7 and [LMeNi(μ-Br)]2 were prepared according to the
literature methods.
Syntheses. [(LtBuNi)(η2-C6H5Me)] (1). [LtBuNiBr] (600 mg,
0.94 mmol) and KC8 (165 mg, 1.22 mmol, 1.3 equiv) were
suspended in 20 mL of toluene, and the reaction mixture was
stirred for 12 h at room temperature. After filtration from the
graphite the solvent was removed under vacuum. The resulting
yellow-brown residue was extracted with 10 mL of hexane, and
the solvent was removed again, affording 1 (430 mg, 0.66 mmol,
70%) as a brown solid. Crystals suitable for X-ray crystal-
lography could be obtained by slow evaporation of a toluene
solution at room temperature. Anal. (%) Calcd for C42H61N2Ni
(652.64 g mol-1): C 77.29, H 9.42, N 4.29. Found: C 75.67, H
9.55, N 4.65 (consistently low C analyses may result from
formation of NiC upon combustion); μeff = 2.06 μB (295 K,
μ
s.o. = 1.73 μB);
[LMeNi]2 (2). [LMeNiBr]2 (232 mg, 0.42 mmol) and KC8 (85
An overview of the various synthetic routes to 4 as well as I
starting from different nickel(I) complexes is given in
Scheme 3.
mg, 0.63 mmol, 1.5 equiv) were suspended in 20 mL of diethyl
ether, and the reaction mixture was stirred for 16 h. The solvent
was removed under vacuum, and the brown residue was ex-
tracted with 8 mL of hexane. Cooling of the brown solution
to -30 °C afforded 2 as brown crystals (47 mg, 0.05 mmol,
23%). Anal. (%) Calcd for C58H82N4Ni2 (952.69 g mol-1): C
73.12, H 8.68, N 5.88. Found: C 72.42, H 8.87, N 5.58 (con-
sistently low C analyses may result from formation of NiC upon
combustion).
Conclusions
Three novel complexes are reported that contain LRNiI
complex metal fragments (R = Me, tBu): In 1 the coordina-
tion sphere of LtBuNi is saturated by a toluene ligand binding
in a η2-coordination mode, while the aryl rings of LMe are
serving as intramolecular donors in the dimer [LMeNi]2, 2.
[LMeNi(μ-Br)Li(thf)2], 3, contains a Li2Br2 diamond core
unit between two LMeNi units, which is bound only very
loosely, considering that it is eliminated continuously on
storing of complex solutions at room temperature with
concomitant formation of 3. 2 and 3 are therefore very
reactive sources of transient LMeNiI: They even react with
H2 and N2 to give [LMeNi(μ-H)]2 and [(LMeNi)2(N2)], re-
spectively, while the known LMeNiI precursor [(LMeNi)2-
(toluene)], III, is inert. These compounds are thus the most
reactive representatives of this substance class, and future
research will now further exploit its chemistry.
[LMeNi(μ-Br)Li(thf)2]2 (3). [LMeNi(μ-Br)2Li(thf)2] (1.5 g,
1.90 mmol) and KC8 (320 mg, 2.38 mmol, 1.25 equiv) were
suspended in 20 mL of diethyl ether, and the reaction mixture
was stirred for 16 h. The solvent was removed under vacuum,
and the red solid was extracted with 5 mL of hexane. Cooling of
the solution to -30 °C afforded 3 as dark red crystals (387 mg,
0.27 mmol, 29%). Anal. (%) Calcd for C74H114Br2Li2N4Ni2O4
(1414.80 g mol-1): C 62.82, H 8.12, N 3.96, Br 11.30. Found: C
62.35, H 8.15, N 4.06, Br 11.65 (consistently low C analyses may
result from formation of NiC upon combustion); μeff = 2.58 μB
(Evans method, pentane-d12, 297 K, μs.o. = 2.45 μB).
[(LMeNi)2(N2)] (4). Method A, starting from 2: A solution of 2
(50 mg, 0.05 mmol) in 5 mL of hexane was stirred for 12 h in a
dinitrogen atmosphere, during which the precipitation of a
brown solid occurred. Removal of the solvent under vacuum
afforded 4 as a brown solid (41 mg, 0.04 mmol, 80%). Method B,
starting from 3: A solution of 3 (50 mg, 0.04 mmol) in 5 mL of
hexane was stirred in a dinitrogen atmosphere for 12 h, during
which the precipitation of a brown solid occurred. Filtration and
exhaustive extraction with hexane (approximately 100 mL) and
removal of the solvent under vacuum afforded 4 as a brown solid
(31 mg, 0.03 mmol, 89%). Anal. (%) Calcd for C58H82N6Ni2
(978.53 g mol-1): C 71.03, H 8.43, N 8.57. Found: C 69.64,
H 8.49, N 7.58 (due to the rather high reactivity and sensitivity
of 4, elemental analyses always showed deviations higher than
Experimental Section
General Procedures. All experiments were carried out in a dry
nitrogen/argon atmosphere using a glovebox and/or standard
Schlenk techniques. Solvents were purified employing an MBraun
SPSsolvent purification system. IRspectra wererecorded on solid
samples prepared as KBr pellets with a Shimadzu FTIR-8400S
spectrometer. Microanalyses were performed on a Leco CHNS-
932 elemental analyzer. Magnetic measurements of the solids were
performed with an Alfa magnetic susceptibility balance at RT.
Solution magnetic susceptibilities were determined by the Evans
method with a Bruker AV 400 NMR spectrometer (1H 400.13
MHz).14 EPR spectra were recorded at the X-band spectrometer
ERS 300 (ZWG/Magnettech Berlin/Adlershof, Germany)
equipped with a fused quartz Dewar for measurements at liquid
nitrogen temperature. The g-factors were calculated with respect
to a Cr3þ/MgO reference (g = 1.9796).
commonly accepted). IR (KBr): 2170 (vw, νNN) cm-1
.
[LMeNi(μ-H)]2 (I). (alternative route starting from 3). A solu-
tion of 3 (50 mg, 0.07 mmol) in hexane (7 mL) was stirred in a
hydrogen atmosphere for 14 h. The resulting green solution was
filtered off from the white precipitate, and the solvent was
removed under vacuum to afford I (18 mg, 0.04 mmol, 53%)
as a dark green solid. The synthesis of I starting from 2 and 4 was
performed in an identical fashion (86% and 75% yield, re-
spectively).
Crystal Structure Determinations. The crystal data were col-
lected on a Stoe IPDS 2T diffractometer using Mo KR radiation,
(15) (a) Yao, S.; Bill, E.; Milsmann, C.; Wieghardt, K.; Driess, M.
Angew. Chem., Int. Ed. 2008, 47, 7110. (b) Yao, S.; Milsmann, C.; Bill, E.;
Wieghardt, K.; Driess, M. J. Am. Chem. Soc. 2008, 130, 13536. (c) Yao, S.;
Xiong, Y.; Zhang, X.; Schlangen, M.; Schwarz, H.; Milsmann, C.; Driess, M.
Angew. Chem., Int. Ed. 2009, 48, 4551.
˚
λ = 0.71073 A. In all cases, the structures were solved by direct
methods (SHELXS-97)18 and refined versus F2 (SHELXL-97)19
(16) Dugan, T. R.; Holland, P. L. J. Organomet. Chem. 2009, 694,
2825.
(18) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; University of Gottingen, 1997.
(19) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
(17) Whether [(LMeFe)2(N2)] also is inert toward dihydrogen was not
reported. However, its reactivity should be higher than that of
[(LtBuFe)2(N2)].
€
€
Refinement; University of Gottingen, 1997.

6860 Organometallics, Vol. 28, No. 24, 2009
Pfirrmann et al.
with anisotropic temperature factors for all non-hydrogen
atoms. All hydrogen atoms were added geometrically and
refined by using a riding model. Relevant crystallographic data
are collected in Table 1.
support. We thank C. Herwig for DFT calculations as
well as P. Neubauer for crystal structure analyses.
Supporting Information Available: CIF file containing full de-
tails of the structural analysis of complexes 1, 2, 2a, and 3, experi-
mental procedure for 2a, crystal structure of 2a (Figure S1), EPR
spectra of 1 and 3, text describing the DFT method including a full
citation of the Gaussian 03 program. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful to the Cluster of
Excellence “Unifying Concepts in Catalysis” funded
by the Deutsche Forschungsgemeinschaft for financial