Cyclopentadiene alkylation and nickel complexes with tri-, tetra-, or pentaisopropylcyclopentadienide or an even bulkier lithium alkylcyclopentadienide

Article abstract of DOI:10.1021/om200372j

Phase transfer catalysis has been adapted to the synthesis of triisopropylcyclopentadiene isomers 1 in high yield and purity. Alkylation of sodium triisopropylcyclopentadienides with isopropyl bromide in tetrahydrofuran has been shown to be the currently most efficient way to generate tetraisopropylcyclopentadiene isomers 2, which give sodium tetraisopropylcyclopentadienide (3) upon metalation. The crucial step for the introduction of the fifth isopropyl group, the selective attack of an iminium salt at the tetraisopropylcyclopentadienide anion in the 5-position with generation of the corresponding 1,2,3,4-tetraisopropyl(6-dimethylamino)fulvene (4), has been carried out according to a published procedure. Addition of 1-naphthyllithium to the dimethylaminofulvene 4 yielded extremely bulky lithium dimethylamino(1-naphthyl)methyltetraisopropylcyclopentadienide (7). Pure 1,2,4,1′,2′,4′-hexaisopropylnickelocene (8) was obtained from sodium triisopropylcyclopentadienide and nickel(II) bromide. The tetraisopropylcyclopentadienylnickel(II) bromide [(C₅ ⁱPr₄H)Ni(μ-Br)]₂ (9) added triphenylphosphine, trimethylphosphine, trimethyl phosphite, and triisopropylphosphine with formation of the diamagnetic phosphine complexes [(C₅ⁱPr₄H)NiBr(PPh₃)] (10), [(C ₅ⁱPr₄H)NiBr(PMe₃)] (13), [(C ₅ⁱPr₄H)NiBr{P(OMe)₃}] (14), and [(C₅ⁱPr₄H)NiBr(PⁱPr₃)] (15), respectively. The triphenylphosphine derivative 10 displays a dissociation equilibrium with small amounts of 9 in deuteriobenzene solution and could be converted to the methylnickel(II) derivative [(C₅ⁱPr ₄H)NiMe(PPh₃)] (11) or to the phenyl complex [(C ₅ⁱPr₄H)NiPh(PPh₃)] (12), respectively. Substitution reactions of the bromide 9 yielded a dinuclear complex with two bridging 2,6-dimethylphenolate ligands, [(C₅ ⁱPr₄H)Ni(μ-2,6-OC₆Me₂H ₃)]₂ (16), with potassium 2,6-dimethylphenolate and methylene-bridged dinuclear [{(C₅ⁱPr₄H)Ni} ₂(μ-CH₂)] (18) with methylmagnesium chloride and concomitant methane formation. Partial hydrolysis converted the phenolate 16 to the monohydroxy derivative [{(C₅ⁱPr₄H)Ni} ₂(μ-2,6-OC₆Me₂H₃)(μ-OH)] (17). Crystal structure analyses have been carried out on complexes 8, 11, 13, 14, 16-18, and 20. Lithium pentaisopropylcyclopentadienide (6) reacted with nickel(II) bromide dimethoxyethane adduct in pentane to form the bromo-bridged [(C₅ⁱPr₅)Ni(μ-Br)]₂ (19), and the new bulky dimethylamino(naphthyl)methyl-carrying cyclopentadienide 7 reacted with the dimethoxyethane adduct of nickel(II) bromide in pentane to give the diamagnetic alkylcyclopentadienylnickel bromide monomer [{C₅ ⁱPr₄CH(NMe₂)(1-naphthyl)}NiBr] (20). The crystal structure of 20 shows no sign of electron donation from the dimethylamino nitrogen lone pair to the nickel center. The paramagnetism of 19 is in line with similar findings for the pentamethylcyclopentadienyl, tetraisopropylcyclopentadienyl, and 1,2,4-tri-tert-butylcyclopentadienyl analogues of 19. The diamagnetism of 20 is most probably due to an empty dz ² orbital, which is high in energy because of the combined electron donation from the five-membered ring and the bromo ligand on the z axis.

Full text of DOI:10.1021/om200372j

Article
pubs.acs.org/Organometallics
Cyclopentadiene Alkylation and Nickel Complexes with Tri-, Tetra-,
or Pentaisopropylcyclopentadienide or an Even Bulkier
Lithium Alkylcyclopentadienide
Daniel Weismann, Dirk Saurenz, Roland Boese, Dieter Blaser, Gotthelf Wolmershauser, Yu Sun,
̈
̈
and Helmut Sitzmann*
Fachbereich Chemie der TU Kaiserslautern, Erwin-Schrodinger-Straße 54, 67663 Kaiserslautern, Germany
̈
S
* Supporting Information
ABSTRACT: Phase transfer catalysis has been adapted to the synthesis of
triisopropylcyclopentadiene isomers 1 in high yield and purity. Alkylation of
sodium triisopropylcyclopentadienides with isopropyl bromide in tetrahydrofur-
an has been shown to be the currently most efficient way to generate
tetraisopropylcyclopentadiene isomers 2, which give sodium tetraisopropylcy-
clopentadienide (3) upon metalation. The crucial step for the introduction of
the fifth isopropyl group, the selective attack of an iminium salt at the
tetraisopropylcyclopentadienide anion in the 5-position with generation of the
corresponding 1,2,3,4-tetraisopropyl(6-dimethylamino)fulvene (4), has been
carried out according to a published procedure. Addition of 1-naphthyllithium to
the dimethylaminofulvene 4 yielded extremely bulky lithium dimethylamino-
(1-naphthyl)methyltetraisopropylcyclopentadienide (7). Pure 1,2,4,1′,2′,4′-hexaiso-
propylnickelocene (8) was obtained from sodium triisopropylcyclopentadienide and nickel(II) bromide. The
i
tetraisopropylcyclopentadienylnickel(II) bromide [(C5 Pr4H)Ni(μ-Br)]2 (9) added triphenylphosphine, trimethylphosphine,
i
trimethyl phosphite, and triisopropylphosphine with formation of the diamagnetic phosphine complexes [(C5 Pr4H)NiBr(PPh3)]
(10), [(C5 Pr4H)NiBr(PMe3)] (13), [(C5 Pr4H)NiBr{P(OMe)3}] (14), and [(C5 Pr4H)NiBr(PiPr3)] (15), respectively. The
triphenylphosphine derivative 10 displays a dissociation equilibrium with small amounts of 9 in deuteriobenzene solution and could
i
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i
i
be converted to the methylnickel(II) derivative [(C5 Pr4H)NiMe(PPh3)] (11) or to the phenyl complex [(C5 Pr4H)NiPh(PPh3)]
(12), respectively. Substitution reactions of the bromide 9 yielded a dinuclear complex with two bridging 2,6-dimethylphenolate
ligands, [(C5 Pr4H)Ni(μ-2,6-OC6Me2H3)]2 (16), with potassium 2,6-dimethylphenolate and methylene-bridged dinuclear
[{(C5 Pr4H)Ni}2(μ-CH2)] (18) with methylmagnesium chloride and concomitant methane formation. Partial hydrolysis converted
the phenolate 16 to the monohydroxy derivative [{(C5 Pr4H)Ni}2(μ-2,6-OC6Me2H3)(μ-OH)] (17). Crystal structure analyses have
i
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been carried out on complexes 8, 11, 13, 14, 16−18, and 20. Lithium pentaisopropylcyclopentadienide (6) reacted with nickel(II)
i
bromide dimethoxyethane adduct in pentane to form the bromo-bridged [(C5 Pr5)Ni(μ-Br)]2 (19), and the new bulky
dimethylamino(naphthyl)methyl-carrying cyclopentadienide 7 reacted with the dimethoxyethane adduct of nickel(II) bromide in
i
pentane to give the diamagnetic alkylcyclopentadienylnickel bromide monomer [{C5 Pr4CH(NMe2)(1-naphthyl)}NiBr] (20). The
crystal structure of 20 shows no sign of electron donation from the dimethylamino nitrogen lone pair to the nickel center. The
paramagnetism of 19 is in line with similar findings for the pentamethylcyclopentadienyl, tetraisopropylcyclopentadienyl, and 1,2,4-
2
tri-tert-butylcyclopentadienyl analogues of 19. The diamagnetism of 20 is most probably due to an empty dz orbital, which is high
in energy because of the combined electron donation from the five-membered ring and the bromo ligand on the z axis.
INTRODUCTION
SYNTHESIS OF ISOPROPYLCYCLOPENTADIENES
■
■
Ammonia as a solvent for cyclopentadiene alkylation has an old
tradition,10 because the titration of the dark blue solution of
sodium in ammonia with freshly cracked cyclopentadiene
readily provides for fast production of large amounts of sodium
cyclopentadienide from solutions of sodium in liquid ammonia.
The subsequent nucleophilic substitution of the cyclopentadie-
nide anion with isopropyl bromide is straightforward. Another
metalation−alkylation sequence can directly follow using
Pentaisopropylcyclopentadienyl chemistry started with the synthesis
of sodium pentaisopropylcyclopentadienide by a sequence of
cyclopentadiene metalation and alkylation reactions2 and with the
elegant synthesis of decaisopropylrhodocenium and pentaisopropyl-
cobaltocenium hexafluorophosphate by Astruc.3 Metallocenes with
this bulky pentaalkylcyclopentadienyl ligand are known with calcium,
strontium, barium,4 tin,5 lead,6 samarium, europium, ytterbium,7 and
rhodium;3 half-sandwich complexes with iron have shown interesting
reactivity8 and magnetic properties.8,9 The time-consuming cyclo-
pentadiene alkylation process prompted us to reevaluate different
approaches toward the introduction of the isopropyl groups.
Received: May 5, 2011
Published: November 15, 2011
© 2011 American Chemical Society
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sodium amide and isopropyl bromide. If large amounts of
diisopropylcyclopentadiene are desired, this is the method of
choice in our opinion. After distillation diisopropylcyclopenta-
diene is converted to a mixture of triisopropylcyclopentadiene
isomers 1 the same way.11 The fourth alkylation step can be
carried out conveniently in tetrahydrofuran solvent with
sodium amide for the metalation and isopropyl bromide as
an alkylating agent. While the sodium triisopropylcyclopenta-
dienide isomers are smoothly alkylated to tetraisopropylcyclo-
pentadiene isomers 2 in high yield, a problem shows up, which
is geminal dialkylation with generation of the unwanted 5,5-
diisopropyl isomers 2a,b. These isomers obviously originate
from the 1,2,4-triisopropylcyclopentadienide isomer. The
geminal dialkylated isomers 2f,g, which could be expected to
be formed by geminal dialkylation of the 1,2,3-triisopropylcy-
clopentadienide, have never been observed in our laboratory.
Details are shown in Scheme 1.
tadienide more than doubles the yield of the desired 1,2,3,4,5-
isomer to 11%, which is still unsatisfactory.
A closer look at the alternative process of cyclopentadiene
alkylation under phase transfer conditions will show us why a
traditional metalation−alkylation procedure is the better way to
generate tetraisopropylcyclopentadienes 2 from triisopropylcy-
clopentadienes 1.
A one-pot reaction converting cyclopentadiene to tetraiso-
propylcyclopentadiene is the phase transfer process developed
by Dehmlow and Bollmann.12 With dibenzo-18-crown-6 as a
phase transfer catalyst, cyclopentadiene is multiply metalated
and alkylated by potassium hydroxide and isopropyl bromide in
tetrahydrofuran, resulting in a mixture of tetraisopropylcyclo-
pentadiene isomers 2. The 70% yield of 2 boils down to a
13.3% yield of sodium tetraisopropylcyclopentadienide, how-
ever, because 81% of the isomeric mixture consists of unwanted
5,5-diisopropylated species. The mass of dibenzo-18-crown-6
catalyst needed for the phase transfer process is about equal to
the yield of potassium tetraisopropylcyclopentadienide ob-
tained after metalation with potassium hydride. This procedure
offers the fastest and most convenient access to small quantities
of tetraisopropylcyclopentadienide (Scheme 3).
Scheme 1. Tetraisopropylcyclopentadiene by Multiple
Metalation and Alkylation of Cyclopentadiene
Scheme 3. One-Pot Phase Transfer Process of Dehmlow and
Bollmann, Recommended for Quick and Easy Preparation of
Small Amounts of Tetraisopropylcyclopentadiene from
Cyclopentadiene
Another phase transfer process was introduced in 1989 by
Venier and Casserly13 and adapted to isopropylcyclopenta-
dienes by Hanusa.14 Stirring of concentrated aqueous solutions
of potassium hydroxide with cyclopentadiene and an excess of
isopropyl bromide in the presence of methyltrioctylammonium
chloride as a phase transfer catalyst results in multiple
cyclopentadiene alkylation. The 1991 publication of Hanusa
outlines a separation procedure where mixtures of potassium
triisopropylcyclopentadienide and tetraisopropylcyclopentadie-
nide as well as different mixtures of tri- and tetraisopropylcy-
clopentadienes have to be separated by fractional crystallization
(Scheme 4). Hanusa reported a 19% yield of tetraisopropylcy-
Geminal dialkylation converts 40−50% of the triisopropylcy-
clopentadienide isomeric mixture to cyclopentadienes, which
cannot be metalated to cyclopentadienyl anions. This is even
worse for the introduction of the fifth isopropyl substituent
(Scheme 2). Direct alkylation of sodium tetraisopropylcyclo-
Scheme 4. Synthesis of Tetraisopropylcyclopentadiene As
Described by Hanusa13
Scheme 2. Pentaisopropylcyclopentadiene Isomers by
Alkylation of Tetraisopropylcyclopentadienide with
Isopropyl Bromide
pentadienide gave 5% 1,2,3,4,5-pentaisopropylcyclopentadiene
and a 95% combined yield of the unwanted 1,2,3,5,5- and
1,2,4,5,5-isomers. Alkylation of lithium tetraisopropylcyclopen-
clopentadienide, which could be increased to 23% by recycling
of mother liquors.14
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In comparison to the 35% overall yield of sodium
tetraisopropylcyclopentadienide obtained by stepwise alkyla-
tion,11,15 the low yield of tetraisopropylcyclopentadienide
obtained by this phase transfer procedure may be due to
production of large amounts of the unwanted 5,5-diisopro-
pylcyclopentadienes 2a,b. Although phase transfer obviously is
not well suited for the fourth alkylation step, it may still be a
time-saving approach for the synthesis of triisopropylcyclopen-
tadienes. We therefore modified the procedure in a search for
conditions avoiding the formation of tetraisopropylcyclopenta-
dienes.
Instead of addition of 5 equiv of isopropyl bromide all at once,
the reaction was started with addition of 2.5 equiv of isopropyl
bromide to the reaction mixture of potassium hydroxide
solution, methyltrialkyl(C8−C10)ammonium chloride, and cyclo-
pentadiene. The remaining 2.5 equiv of isopropyl bromide
was added dropwise, and an almost quantitative yield of
triisopropylcyclopentadiene isomers was obtained (Scheme 5).
regioselective attack at the tetraisopropylcyclopentadienide ion,
despite its steric hindrance, were explored. This search finally
arrived at a three-step procedure based on the synthesis of
6-dimethylaminopentafulvene reported by Hafner.16−18 In the
first step tetraisopropylcyclopentadienide is attacked selectively
at the 5-position by the dimethylaminomethoxycarbenium ion
generated by alkylation of dimethylformamide with dimethyl
sulfate. 1,2,3,4-Tetraisopropyl-6-dimethylaminofulvene (4) is
obtained as a crystalline solid, which contains varying amounts
of tetraisopropylcyclopentadiene and requires purification by
crystallization or sublimation (Scheme 6).
Scheme 6. Introduction of the Fifth Isopropyl Group in
Three Steps via the Dimethylaminofulvene 4
Scheme 5. Selective Synthesis of Triisopropyl-
cyclopentadienes by Modified Phase Transfer Catalytic
Cyclopentadiene Alkylation, Followed by Conversion to
Tetraisopropylcyclopentadienide
In the second step methyllithium is added, followed by
chlorodimethylsilane. By spontaneous dimethylaminodimethylsi-
lane elimination the fulvene skeleton is regenerated and 1,2,3,4-
tetraisopropyl-6-methylfulvene 5 is formed, which can be con-
verted to the lithium pentaisopropylcyclopentadienide diethyl
ether adduct 6 by addition of methyllithium in diethyl ether.1
The dimethylaminofulvene 4 has now been found to be
capable of 1-naphthyllithium addition despite its steric bulk
(Scheme 7).
Column chromatography was omitted; distillation gave a clean
mixture of triisopropylcyclopentadiene isomers. A subsequent
metalation−alkylation procedure in tetrahydrofuran furnished
tetraisopropylcyclopentadiene isomers 2 in 88% yield containing
only traces of triisopropylcyclopentadiene. The isomeric mixture
consisted of about 50% of the desired isomers 2c−e. Metalation
of the oily liquid with sodium amide in tetrahydrofuran gave
sodium tetraisopropylcyclopentadienide (3), which was isolated
by evaporation of solvent from the filtered solution and washed
with petroleum ether in order to remove the 5,5-dialkylation
products. The overall yield of sodium tetraisopropylcyclopenta-
dienide from cyclopentadiene was 42%.
Scheme 7. Formation of 7 by Addition of 1-Naphthyllithium
to the Dimethylaminofulvene 4
The choice of the procedure for tetraisopropylcyclopentadiene
synthesis depends on the desired amount of product. For small
amounts the procedure of Dehmlow12 is the most convenient. If
larger amounts are needed, the modified phase transfer catalyzed
cyclopentadiene alkylation permits the synthesis of 75 g amounts
of triisopropylcyclopentadienes from 0.4 mol of cyclopentadiene
in a 2 L flask, which is then converted to about half that weight
of sodium tetraisopropylcyclopentadienide. Even larger amounts
are accessible in one run by sequential alkylation in liquid
ammonia. Here, 4 mol of sodium cyclopentadienide can be
generated in 2 L of liquid ammonia and converted to about
350 g of sodium tetraisopropylcyclopentadienide in four
alkylation−metalation steps.11,15
The pale yellow salt is quite soluble in all common organic
solvents, including pentane, and stable at room temperature as a
solid or in solution under inert gas. Upon heating decom-
position was observed at temperatures exceeding 60 °C. The
13C NMR spectrum of 7 displays a characteristic high-field signal
at 2.3 ppm, which has been assigned to a localized carbanion.
This localization is probably a consequence of lithium
coordination to the nitrogen donor in the dimethylamino side
chain.
On a search for selective bond formation at the 5-position of
tetraisopropylcyclopentadienide, different approaches known
from the literature in order to find a reaction capable of
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Scheme 8. Formation of Hexaisopropylnickelocenes from Nickel(II) Bromide and a Mixture of Sodium
Triisopropylcyclopentadienide Isomers
parent nickelocene.24 The rings are planar and the isopropyl
groups do not create much strain, because the isopropyl group
at the 4-position with its two hydrogen neighbors is able to turn
its methyl groups outside and stretch one methyne hydrogen
NICKEL COMPLEXES
■
The contrast between the melting point of 1,1′,2,2′,4,4′-
hexaisopropylferrocene (172 °C11) and the value of 107−110 °C
mentioned for 1,1′,2,2′,4,4′-hexaisopropylnickelocene (8) in a
toward the other ring, where it finds ample space between the
chapter on melting points of metallocenes within a review
article19 drew our attention to this nickelocene derivative.20
two adjacent isopropyl groups rotated away from each other.
The two adjacent isopropyl groups of the other ring are
The nickelocene was formed in a smooth reaction from
rotated apart from each other in order to leave as much space as
nickel(II) bromide dimethoxyethane adduct and a solution of
possible for the 4-isopropyl substituent. The same conforma-
the two sodium triisopropylcyclopentadienide isomers (1,2,4-
tion has been found for all crystal structures of metallocenes
and 1,2,3-isomers in an approximate 4:1 ratio) (Scheme 8).
with this substituent pattern known so far. The metal−ring
The color observed during sodium triisopropylcyclopentadie-
nide addition was dark brown in the beginning, which is
centroid distance of 1.827 Å is slightly longer than the value of
1.794 Å found for the parent nickelocene.24
probably due to a mono(triisopropylcyclopentadienyl) complex
i
The triphenylphosphine complex [(C5 Pr4H)NiBr(PPh3)]
and the color of the metallocene, which mixed in as a green
(10) can be synthesized in a one-pot reaction from nickel(II)
component while the second equivalent of the sodium salt was
added. The workup was not optimized for high yield, but
bromide dimethoxyethane adduct by introduction of sodium
tetraisopropylcyclopentadienide 3, followed by addition of
triphenylphosphine (Scheme 9). The red-violet needles can be
crystallization of the product was carried out in order to obtain
the pure main isomer [(1,2,4-iPr3C5H2)2Ni] (8), while leaving
the two other possible isomers [(1,2,3-iPr3C5H2)2Ni] and
handled in air without visible change and are thermally stable
[(1,2,4-iPr3C5H2)(1,2,3-iPr3C5H2)Ni] in the mother liquor.
and sparingly soluble in petroleum ether.
Solutions of 10 are air-sensitive, however, and deuterioben-
zene solutions of analytically pure 10 always exhibit the broad
low-field NMR signals of very small amounts of the
paramagnetic, phosphine-free nickel bromide dimer
The crude product obtained from the petroleum ether extract
melted at 117 °C; triple recrystallization gave samples with melting
points of 146, 151, and 151 °C. Because proton NMR spectra of
the paramagnetic compound exhibit broad signals and do not
permit the detection of traces of impurities, we cannot rule out a
small contamination with isomers containing residual 1,2,3-
triisopropylcyclopentadienide in our sample, however.
X-ray crystal structure investigation of a green single crystal
of 8 (Figure 1; details regarding the crystal structure analyses
are given in Table 1) showed a centrosymmetric molecule
like that of other hexaisopropyl metallocenes21−23 or the
i
[(C5 Pr4H)Ni(μBr)]2 (9) in addition to the signals of
diamagnetic 10. These observations can be explained assuming
a dissociation equilibrium, which is shifted strongly toward the
diamagnetic phosphine complex:
A small fraction of complex 10 dissociates in deuterioben-
zene solution.
With methylmagnesium chloride 10 forms the corresponding
methyl complex [(C5iPr4H)NiCH3(PPh3)] 11, whose solubility
in petroleum ether is slightly better than that of 10. Proton NMR
spectra of 11 exhibit a doublet at −0.62 ppm with 3JP,H = 6.9 Hz
for the methyl group bound to the nickel center. The spectra
show no signs of phosphine dissociation, which may be
understood as a consequence of the slightly released steric bulk
compared to the bromo complex 10. Phosphine dissociation
could still occur below the detection limit of NMR spectroscopy.
The green complex could be crystallized from toluene
solution as monoclinic blocks. The X-ray crystal structure
analysis (Figure 2) reveals a half-sandwich complex with a Ni−
Cpcent distance of 1.77 Å, which is shorter than the Ni−Cpcent
distance of 1.794 Å in the parent nickelocene.24 The
alkylcyclopentadienyl ring plane of 11 is oriented almost
perpendicular to the plane defined by the nickel atom and its
two σ-bonded ligand atoms (phosphorus and methyl carbon).
Figure 1. ORTEP drawing of the 1,2,4,1′,2′,4′-hexaisopropylnick-
elocene 8. Distances (Å) and angles (deg): Ni1−C1 = 2.1912(16),
Ni1−C2 = 2.1910(16), Ni1−C3 = 2.1909(17), Ni1−C4 = 2.2104(16),
Ni1−C5 = 2.1867(16), Ni−Cpcent = 1.827, Ni−C(av) = 2.190, C1−
C2 = 1.428(2), C2−C3 = 1.431(2), C3−C4 = 1.422(2), C4−C5 =
1.426(2), C5−C1 = 1.432(2); Cpcent−Ni−Cpcent = 180.
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i
The phenylnickel complex [(C5 Pr4H)NiC6H5(PPh3)] (12)
was obtained in 43% yield as a greenish powder from the
bromide starting complex and phenylmagnesium bromide and is
easily soluble in petroleum ether, diethyl ether, and toluene. In
addition to the proton NMR signals of the tetraisopropylcyclo-
pentadienyl ligand with their 1:2:2:6:6:6:6 integral ratio a
multiplet between 6.8 and 7.5 ppm corresponds to 20 protons
for three phenyl rings attached to phosphorus and one to nickel.
The signals of the phenyl group bound to the nickel atom could
not be distinguished from those of the phosphine phenyl signals.
Here, as in the case of the methyl complex 11, phosphine
dissociation has not been observed by NMR spectroscopy.
Upon addition of 2 equiv of trimethylphosphine or trimethyl
Scheme 9. Synthesis of Phosphine Complexes 10−15 from
the Bromo-Bridged Dimer 9
i
phosphite the bromo complex [(C5 Pr4H)Ni(μ-Br)]2 was
converted to the red trimethylphosphine complex [(C5 Pr4H)-
i
NiBr(PMe3)] (13) or to the orange-brown trimethyl phosphite
i
complex [(C5 Pr4H)NiBr{P(OMe)3}] (14), respectively. The
methyl substituents of the trimethylphosphine ligand of 13 are
split into doublets with about the same coupling constant as the
methyl protons of the isopropyl groups, but shifted to lower
field by about 0.5 ppm. Prominent signals in the EI mass
spectrum of 13 are those of the molecular ion (44.5%), the
tetraisopropylcyclopentadienyl cation (32%), the trimethyl-
phosphinyl radical cation (83.5%), the dimethylphosphenium
cation (100%), and the isopropyl cation (70%); smaller signals
are fragmentation products of the tetraisopropylcyclopenta-
dienyl ligand. The methoxy proton signal of 14 is a doublet
3
with a JP,H coupling constant of 11.8 Hz
Complexes 13 (Figure 3) and 14 (Figure 4) show
conformations very similar to that of the triphenylphosphine
Figure 2. ORTEP drawing of the methylnickel complex 11. Distances
(Å) and angles (deg): Ni−C = 1.9936(19), Ni−C1 = 2.2020(16), Ni−
C2 = 2.1574(17), Ni−C3 = 2.1071(16), Ni−C4 = 2.1681(16), Ni−C5 =
2.1116(15), Ni-Cpcent = 1.770, C1−C2 = 1.410(2), C2−C3 =
1.471(3), C3−C4 = 1.426(2), C4−C5 = 1.430(2), C5−C1 =
1.432(2); C6−Ni−P = 89.19(7); dihedral angle between the Cp
ring plane and the plane defined by Ni, P, and C6 90.2°.
The phosphine ligand is located underneath the only ring
hydrogen substituent for steric reasons. The two isopropyl
groups on C2 and C3 are rotated away from each other in order
to provide as much space as possible for the methyl ligand
underneath. The nickel−methyl carbon distance of 1.9936(19) Å
is comparable to the values found for tetragonal-planar nickel(II)
diphosphine thiophenolate complexes of the type
[(R2PC2H4PR2)NiCH3(S-Ar′)] with Ni−CH3 distances between
1.96 and 1.99 Å, where the aryl group Ar′ is a phenyl or
pentafluorophenyl group (three examples) or carries a side chain
with an NH group forming a hydrogen bridge to the thiolate
sulfur atom (five examples).25 Also comparable is a penta-
methylcyclopentadienyl analogue of 11 with a triethylphosphine
ligand, [(C5Me5)NiCH3(PEt3)].26 In this complex a Ni−C
distance of 1.96(1) Å was observed, but the structure could not
be well refined because of a crystallographic pseudo mirror plane.
Two independent molecules of the benzyl derivative [(C5Me5)-
NiCH2Ph(PEt3)] described in the same publication show Ni−C
distances of 1.979(3) and 2.007(4) Å, and the Ni−CH3 distance
in the diphosphine nickel(II) complex [(iPr2C2H4PiPr2)-
NiCH3(CHMePh)] was found to be 1.987(4) Å.27
Figure 3. ORTEP drawing of the trimethylphosphine complex 13.
Distances (Å) and angles (deg): Ni−Br = 2.331(2)/2.324(3), Ni−P =
2.1556(9), Ni−C1 = 2.164(3), Ni−C2 = 2.167(3), Ni−C3 = 2.152(3),
Ni−C4 = 2.148(3), Ni−C5 = 2.018(3), Ni−Cpcent = 1.749, C1−C2 =
1.396(4), C2−C3 = 1.485(4), C3−C4 = 1.406(4), C4−C5 = 1.434(4),
C5−C1 = 1.435(4); Br−Ni−P = 92.69(13)/90.8(2); dihedral angle
between the Cp ring plane and the plane defined by Ni, P, and Br1
91.8°. The bromo ligand is disordered. The disorder was refined to the
Br1 position with 60% probability and the Br2 position with 40%
probability. Ellipsoids are at the 50% probability level.
derivative 11 with a methyl ligand. In all three cases the
phosphine is located underneath the only ring proton and the
bromo or methyl ligand sits between two isopropyl groups of
the cyclopentadienyl ligand, which are rotated away from each
other. Structural details for comparison are compiled in Table 2.
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Scheme 10. Synthesis of the 2,6-Dimethylphenolate
Complex 16 and Its Partial Hydrolysis Product 17
solvents and very air-sensitive both as a solid or in solution.
Proton NMR spectra are typical for a paramagnetic complex with
broad or very broad signals between 37 and −16 ppm and half-
widths from 15 to 300 Hz. According to the Eyring equation,28
the large chemical shifts provide for high signal coalescence
temperatures and enable the observation of rotamers at room
temperature. As a consequence, there are many broad signals of
varying intensity at high and low field belonging to different
rotamers, which makes the interpretation of spectra difficult. An
exemplary NMR investigation of three paramagnetic alkylcyclo-
pentadienylnickel bromides will be devoted to this problem.31
The EI mass spectrum of the 2,6-dimethylphenolate 16 did not
Figure 4. ORTEP drawing of the trimethylphosphite complex 14.
Distances (Å) and angles (deg): Ni−Br = 2.336(3)/2.333(3), Ni−P =
2.095(5)/2.115(5), Ni−C1 = 2.168(15)/2.202(15), Ni−C2 =
2.180(17)/2.118(15), Ni−C3 = 2.152(17)/2.155(16), Ni−C4 =
2.112(15)/2.149(15), Ni−C5 = 2.062(13)/ = 2.009(14), Ni-Cpcent
=
1.762/1.754, C1−C2 = 1.370(19)/1.41(2), C2−C3 = 1.47(2)/
1.51(2), C3−C4 = 1.37(2)/1.340(19), C4−C5 = 1.442(18)/
1.421(18), C5−C1 = 1.440(16)/1.396(17); Br−Ni−P = 94.43(17)/
94.71(18); dihedral angle between the Cp ring plane and the plane
defined by Ni, P, and Br 91.9/87.4.
Table 2. Structural Details of the Tetraisopropyl-
cyclopentadienylnickel(II) Phosphine Complexes
[(C5iPr4H)NiCH3(PPh3)] (11), [(C5iPr4H)NiBr(PMe3)] (13),
and [(C5iPr4H)NiBr{P(OMe)3}] (14)
i
show a molecular ion but instead the monomeric [(C5 Pr4H)-
Ni(OC6Me2H3)]+ cation with m/z 412 and 92% intensity in
addition to a parent peak for an oxidation product of the
composition [(C5 Pr4H)Ni(OC6Me2H3)2NiO]+ and a small
i
signal for [(C5iPr4H)Ni(OC6Me2H3)2]+. 16 did not crystallize
readily from solution. After several manipulations and solvent
changes finally a few orange crystals could be obtained by slow
evaporation of a mixture of pentane and cyclopentene, which
are most probably a result of partial hydrolysis of 16 in solution.
16 could be crystallized, however, by vacuum sublimation in a
sealed glass tube at 60 °C over a couple of weeks. The partial
a
b
11
1.770
13
1.746
14
Ni−Cpcent
Ni−Br
1.762 (A)
1.754 (B)
2.331(2)
2.324(3)
2.1556(9)
2.336(3) (A)
2.333(3) (B)
2.095(5) (A)
2.115(5) (B)
Ni−P
2.1256(5)
i
hydrolysis product [(C5 Pr4H)Ni(μ-2,6-OC6Me2H3)(μ-OH)Ni-
Ni-CH3
1.9936(19)
2.1492
(C5iPr4H)] (17) was characterized by an X-ray crystal structure
analysis only.
Ni−C(av) Cp ring
2.1298
2.135 (A)
2.127 (B)
91.9 (A)
c
The Ni···Ni distances of both dinuclear phenolate complexes
(3.134(2) and 3.099(2) Å, respectively; Figures 5 and 6) are
clearly nonbonding, and the Cpcent−Ni distances are very
similar to those of the nickelocene 8 but longer than those
observed for the diamagnetic phosphine complexes 11, 13, and
14 (see Table 2), for the methylene-bridged dinickel complex
18 (1.753/1.752 Å for Ni1 and Ni2, respectively), and for the
monomeric bromide 20 (1.729, 1.729, 1.751, and 1.757 Å for
four independent molecules A−D).
dihedral angle
90.2
91.8 (Br1)
84.1 (Br2)
87.4 (B)
a
b
Disorder of the bromo ligand, positions Br1 and Br2. Two
c
independent molecules A and B. Dihedral angle between the cyclo-
pentadienyl plane and the plane containing nickel, phosphorus, and
the second σ-bonded ligand (bromide or methyl carbon atom).
The violet crystals of the triisopropylphosphine complex
[(C5 Pr4H)NiBr(PiPr3)] (15) are easily soluble in pentane and
i
other organic solvents. The solutions are very air-sensitive and
change from violet to brown and green upon contact with air.
The broadened proton and phosphorus NMR signals of 15 as
well as the signals of small amounts of the bromo-bridged
dimer 9 in the proton NMR spectrum of 15 indicate phosphine
dissociation similar to the triphenylphosphine derivative 10, but
more pronounced.
Solid methylmagnesium chloride diethyl etherate causes a
solution of the bromo-bridged dimer 9 to turn black. Bubbles
indicating concomitant gas evolution may be seen, if the
Grignard compound is added quickly. The black powder
obtained from the reaction mixture is easily soluble in pentane
and other organic solvents. In addition to the signals expected
for the tetraisopropylcyclopentadienyl ligand, the 13C NMR
1
The bromo complex 9 has also been used for substitution
reactions with anionic nucleophiles.
spectrum shows a triplet at 65.9 ppm with JC,H = 145 Hz.
Proton NMR spectra show a singlet at 3.64 ppm with the same
intensity as that of the tetraisopropylcyclopentadienyl ring
proton. The hypothesis of a dinickel complex with a methylene
A red-brown tetrahydrofuran solution of 9 slowly turned
orange-brown upon stirring with potassium 2,6-dimethylpheno-
i
i
late (Scheme 10). The orange, solid product [(C5 Pr4H)Ni(μ-
bridge [{(C5 Pr4H)Ni}2(μ-CH2)] (18) could be confirmed by a
2,6-OC6Me2H3)]2 (16) is soluble in pentane and other organic
crystal structure analysis.
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The Ni−Cpcent distances of 1.753 and 1.752 Å for Ni1 and
Ni2, respectively, are shorter than in the phenolate-bridged
dinuclear complexes and compare well with the valued found
for the diamagnetic phosphine complexes (see Table 2). The
Ni−CH2 distances between 1.87 and 1.89 Å as well as the Ni−
Ni distance of 2.316 Å are quite short (Figure 7). In accordance
Figure 5. Crystal structure of the bis(2,6-dimethylphenolate) complex
16. Distances (Å) and angles (deg): Ni1···Ni1A = 3.134(2), Ni1−O1 =
1.981(2), Ni1−O2 = 1.986(2), Ni1−C1 = 2.207(4), Ni1−C2 =
2.210(3), Ni1−C3 = 2.230(3), Ni1−C4 = 2.179(3), Ni1−C5 =
2.161(4), Ni1−Cpcent = 1.828, C1−C2 = 1.447(5), C2−C3 =
1.448(5), C3−C4 = 1.430(5), C4−C5 = 1.413(6), C5−C1 =
1.425(6); Ni1−O1−Ni1A = 104.52(16), Ni1−O2−Ni1A =
104.20(15), O1−Ni1−O1A = 75.42(11); dihedral angle between
two five-membered-ring planes 180 by symmetry, dihedral angle
between both aryl planes and the Cp plane 7.2, dihedral angle between
the planes defined by Ni1, Ni1A, O1 and Ni1, Ni1A, O2 8.0.
Figure 7. Crystal structure of the methylene complex 18. Distances (Å)
and angles (deg): Ni1−Ni2 = 2.3158(10), Ni1−C18 = 1.869(12), Ni1−
C18A = 1.886(14), Ni2−C18 = 1.892(11), Ni2−C18A = 1.869(13),
Ni1−C1 = 2.138(5), Ni1−C2 = 2.138(5), Ni1−C3 = 2.109(5), Ni1−
C4 = 2.131(5), Ni1−C5 = 2.138(5), Ni2−C21 = 2.131(5), Ni2−C22 =
2.117(5), Ni2−C23 = 2.127(5), Ni2−C24 = 2.130(5), Ni2−C25 =
2.144(5); C18−Ni1−C18A = 30.3(5), Ni1−C18−Ni2 = 76.0(4), Ni1−
C18A−Ni2 = 76.1(5), C18−Ni1−Ni2 = 52.4(3), C18−Ni2−Ni1 =
51.5(4), C18A−Ni1−Ni2 = 51.5(4), C18A−Ni2−Ni1 = 52.3(4). The
methylene bridge is disordered and was refined assuming an equal
distribution over both positions C18 and C18A. Attempted refinement
of an occupation parameter for these positions did not improve the
quality of the structure solution.
with these observations, the 15-valence-electron CpNi frag-
ments call for a NiNi double bond and two Ni−C single
bonds.
The formation of the dinuclear complex 18 obviously
involves α-hydride abstraction, which could produce a
mononuclear methylene complex with 18 valence electrons as
has been postulated by Pasynkiewicz.28 The hypothetical,
highly reactive intermediate [(iPr4C5H)NiH(CH2)] could be in
an equilibrium with the 16-valence-electron species [(iPr4C5H)-
NiCH3] and the tetrahydrofuran adduct thereof (Scheme 11).
Two such molecules are necessary for the formation of one
molecule of complex 18 together with 1 equiv of methane as a
plausible byproduct. A very similar hypothesis has been put
forward by Pasynkiewicz et al.30 for the reaction of nickelocene
with methyllithium in the presence of 2-hexene, where the
olefin takes the role of the additional donor ligand. The
dinuclear complex [CpNi(μ-CH2)NiCp] was postulated as an
intermediate, but analogues of 18 with less bulky cyclo-
pentadienyl ligands could not be detected or isolated before,
which is probably due to follow-up reactions with formation of
higher nuclearity complexes (cf. ref 30).
Figure 6. Crystal structure of the partial hydrolysis product 17. Distances
(Å) and angles (deg): Ni1···Ni2 = 3.099(2), Ni1−O1 = 1.9824(19), Ni2−
O1 = 1.986(2), Ni1−O2 = 1.925(11), Ni1−O3 = 1.948(4), Ni2−O2 =
1.902(11), Ni2−O3 = 1.948(5), Ni1−C1 = 2.200(3), Ni1−C2 =
2.184(3), Ni1−C3 = 2.211(3), Ni1−C4 = 2.189(3), Ni1−C5 =
2.161(3), Ni2−C6 = 2.178(3), Ni2−C7 = 2.174(3), Ni2−C8 =
2.207(3), Ni2−C9 = 2.198(3), Ni2−C10 = 2.171(3), Ni1−Cpcent
=
1.820, Ni2−Cpcent = 1.824, C1−C2 = 1.428(4), C2−C3 = 1.447(4), C3−
C4 = 1.429(4), C4−C5 = 1.428(4), C5−C1 = 1.420(4), C6−C7 =
1.468(5), C7−C8 = 1.438(5), C8−C9 = 1.401(4), C9−C10 = 1.373(5),
C6−C10 = 1.390(5); Ni1−O1−Ni2 = 102.72(9), Ni1−O2−Ni2 =
108.2(6), Ni1−O3−Ni2 = 105.4(3), O1−Ni1−O2 = 72.9(5), O1−Ni1−
O3 = 75.45(14), O1−Ni2−O2 = 73.3(5), O1−Ni2−O3 = 75.36(14);
dihedral angle between two five-membered-ring planes 8.7°, dihedral angle
between aryl plane and Cp plane (C1−C5) 3.5, dihedral angle between
aryl plane and Cp plane (C6−C10) 5.2.
Since the diethyl ether adduct of lithium pentaisopropylcy-
clopentadienide is soluble in pentane, its reaction with the
dimethoxyethane adduct of nickel(II) bromide was carried out
in pentane solution at ambient temperature and proceeded
smoothly within a few hours (Scheme 12). The product 19 was
obtained in 69% yield as a crystalline solid, which is soluble in
pentane and very soluble in other organic solvents such as
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Scheme 11. Hypothesis on the Formation of the Dinuclear Methylene Complex 18
toluene, diethyl ether, tetrahydrofuran, and methylene chloride.
19 is very sensitive toward air or moisture but can be stored
under inert gas at room temperature and even sublimed at
70 °C and 10−2 mbar and decomposes upon heating above
80 °C. Combustion analysis results are in accord with the
expected pentaisopropylcyclopentadienylnickel(II) bromide.
Proton NMR spectra in deuteriobenzene show three broad
signals with an intensity ratio of 3:3:1. Two signals of equal
intensity at 38.3 and 34.5 ppm with a half-width of 195 Hz
originate from the methyl protons and can be attributed to five
methyl groups oriented outward and five methyl groups close
to the nickel center. The smaller resonance of the isopropyl
methyne protons was found at 18.6 ppm with a half width of
250 Hz. A discussion of NMR spectra and dynamic properties
sublimation. 20 can be stored at room temperature under
inert gas but decomposes quickly upon contact with air.
1H NMR spectra of 20 show two sets of sharp and well-
resolved signals within a spectral window between 0.9 and
7.9 ppm. 20 is diamagnetic and seems to consist of two
diastereomers. Each isopropyl methyl group of each diaster-
3
eomer has its own doublet with JH,H coupling constants
between 6.35 and 7.56 Hz, which gives 16 doublets between
0.4 and 1.56 ppm. The full data set can be found in the
Experimental Procedures. 13C NMR spectra exhibit 20 signals,
which have been assigned to the naphthyl moieties of 2 species
with very similar spectra and interpreted as the signals of 2
diastereomers.
The reason for diastereomer formation has to be the addition
of the naphthyl substituent to the former CH(NMe2) group
with formation of a center of chirality. Since the five
interlocking substituents at the five-membered ring are too
sterically hindered for rotation, the combination of a chirality
center at one substituent and axial chirality caused by the
directionality of the isopropyl and the dimethylamino-
(naphthyl)methyl substituent around the five-membered ring
creates two diastereomers visible in the NMR spectra, with each
diastereomer consisting of two enantiomers.
Slow evaporation of a pentane solution of 20 gave crystals
suitable for X-ray diffraction. The unit cell belongs to the space
group Cc and contains four independent molecules. All of them
are monomeric half-sandwich complexes and carry the nickel
bromide fragment in the vicinity of the dimethylamino group.
The diastereomers with nickel in the vicinity of the naphthyl
group must be present in solution but do not show up in the
crystal picked for X-ray diffraction. The ring−nickel distance
4
5
of the Cp ligand has been published. The paramagnetism of
[(C5HiPr4)Ni(μ-Br)]2 (19), the tetraisopropylcyclopentadienyl
analogue of 8, has been investigated by magnetic susceptibility
1
measurements in the solid state and by H and 13C NMR
spectroscopy in solution at variable temperature.31 The dimeric
structure of 9 in the solid state was determined by X-ray
crystallography and mentioned in refs 20, 32, and 33. From
these findings and their parallels we infer that the paramagnetic
pentaisopropylcyclopentadienylnickel bromide 19 is a dimer
i
with bromo bridges according to the formula [(C5 Pr5)Ni(μ-
Br)]2 (Scheme 12) and possesses two unpaired electrons at
each nickel center, although it must be viewed as an 18 valence
electron complex. Possible reasons for this behavior will be
discussed after the introduction of complex 20, which is an
example of the contrary.
Lithium tetraisopropyldimethylamino(naphthyl)methyl-
cyclopentadienide (7) and the dimethoxyethane adduct of
nickel bromide in pentane also gave an intense red solution,
from which the intense red nickel complex 20 could be isolated
(Scheme 13). The crystalline compound was heated at 10−2
mbar and decomposed above 85 °C without noticeable
Scheme 13. Reaction of the Pentane-Soluble Lithium
Dimethylamino(1-naphthyl)methyltetraisopropylcyclopentadienide
Diethyl Ether Complex with Nickel(II) Bromide
Dimethoxyethane Complex To Form the Monomeric
Complex [{R4C5CH(1-C10H7)(NMe2)}NiBr] (20)
Scheme 12. Reaction of the Pentane-Soluble Lithium
Pentaisopropylcyclopentadienide Diethyl Ether Adduct
with Nickel(II) Bromide Dimethoxyethane Complex To
Form the Dimeric Bromide [5CpNi(μ-Br)]2 (19)
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varies between 1.729 and 1.757 Å, and the angle between the
Ni−Br bonds and the Cp ring planes varies between 5.9 and
7.0°. The terminal nickel−bromine distance is shorter than
the Ni−Br distances in the bromo-bridged dimer 9 (average
2.425 Å) and even shorter than in the phosphine complexes 13
and 14 (average 2.330 Å; for details see Table 2). More data are
given along with Figure 8.
Furthermore, the ligands do not provide a strong ligand field.
For these reasons the splitting between the two d orbitals
highest in energy is too small to enforce a diamagnetic ground
state for complexes 9 and 19.
The monomeric half-sandwich complex 20 happens to be
diamagnetic. In this case there must be one d orbital
significantly higher in energy than the second highest one in
2
order to make spin pairing attractive. This could be the dz
orbital, whose energy is increased not only by the
pentaalkylcyclopentadienyl ligand on one side but also by the
only bromo ligand on the other side.
There are, of course, many diamagnetic nickel(II) complexes
with 16 valence electrons, which are planar. The geometry of
complex 20 resembles more a distorted tetrahedron, where
diamagnetic species of this type are rare. One example for a
geometrically similar and diamagnetic nickel(II) complex with
a distorted-tetrahedral ligand sphere is the bromo derivative
of a nickel(II) complex with a bulky tris(3,5-diphenyl)-
pyrazolylborate ligand.34
CONCLUSION AND OUTLOOK
■
Several routes for tetraisopropylcyclopentadienide and a quite
efficient three-step procedure for lithium pentaisopropylcyclo-
pentadienide from tetraisopropylcyclopentadienide pave the way
toward transition-metal complexes of sterically demanding
alkylcyclopentadienyl ligands and facilitate the synthesis of
dinuclear mono(alkylcyclopentadienyl) complexes. The para-
magnetism of cyclopentadienylnickel complexes with 18 valence
electrons is not as well established as that of nickelocenes with
20 valence electrons. The dinuclear nickel complex 16 with 2,6-
dimethylphenolate bridges and the bromo-bridged dimer 19 not
only exhibit broad NMR signals in a wide spectral window but
also their nickel(II)−Cpcent distances are markedly longer than
those found for the diamagnetic 18-valence-electron nickel(II)
phosphine complexes 11, 13, and 14, the methylene-bridged
dinuclear complex 18, and the mononuclear 16-valence-electron
cyclopentadienylnickel(II) bromide 20. While these observa-
tions can be explained by the stronger ligand field exerted by
phosphines or the methylene bridge as compared to that of π
donor ligands such as phenolate or bromide, the difference
between the dinuclear bromide 19 and the mononuclear
bromide 20 is more subtle and is presumably due to the
geometry of the complex. The almost linear Br−Ni−Cpcent
arrangement provides for a relatively strong combined ligand
Figure 8. ORTEP drawing of the nickel complex 20. Distances (Å) and
angles (deg): Ni1−C1 = 2.083(10), Ni1−C2 = 2.141(8), Ni1−C3 =
2.122(9), Ni1−C4 = 2.142(7), Ni1−C5 = 2.124(7), Ni1−Cpcent.
=
1.729, Ni1−Br1 = 2.2483(19), Ni2−C9 = 2.094(10), Ni2−C19 =
2.118(8), Ni2−C29 = 2.126(8), Ni2−C39 = 2.114(9), Ni2−C49 =
2.141(8), Ni2−Cpcent. = 1.729, Ni2−Br2 = 2.244(2), Ni3−C101 =
2.178(11), Ni3−C102 = 2.131(7), Ni3−C103 = 2.122(11), Ni3−C104 =
2.124(8), Ni3−C105 = 2.128(8), Ni3−Cpcent = 1.751, Ni3−Br3 =
2.242(2), Ni4−C109 = 2.182(11), Ni4−C119 = 2.152(7), Ni4−C129 =
2.139(8), Ni4−C139 = 2.105(9), Ni4−C149 = 2.116(8), Ni4−Cpcent
=
1.757, Ni4−Br4 = 2.236(2); dihedral angles between Cp ring planes
and the Ni−Br vectors within each of the four independent molecules
Ni1−Br1 84.1, Ni2−Br2 83.5, Ni3−Br3 83.4, Ni4−Br4 83.0.
With respect to the steric bulk in the proximity of the metal
atom this five-membered ring is the bulkiest cyclopentadienyl
ligand known to us. The dimethylamino group extends far into
the narrow space on the metal side of the ring, so that even a
bromo-bridged dimer is unfavorable for steric reasons. Our
initial idea of nitrogen lone pair coordination to the nickel
center being responsible for monomer formation proved
wrong. Not only is the lone pair not pointing toward the
nickel atom but also the small displacement of the bromo
ligand out of the axial position tells us that nitrogen
coordination is not a valid assumption in this case. The
dimethylamino group seems to act as a steric obstacle in the
proximity of the central atom and is bulky enough to hinder
dimer formation.
The bromo-bridged dimer 19 is thought to be monomeric in
the gas phase, because a sublimation temperature of 70 °C at a
pressure of 0.01 mbar is very low for a complex with more than
1400 atomic mass units.
The paramagnetism of 19 was noted in ref 33. A diamagnetic
ground state is expected for nickel(II) with a d8 configuration,
if the two d orbitals highest in energy are separated by a gap
exceeding the spin pairing energy. The coordination number at
nickel in complexes 9 or 19 is 5, the coordination geometry is
not very symmetric, and there is no pair of ligands exerting
their electron-repelling influence on the same nickel d orbital.
2
influence on the dz orbital despite the weak ligand field of only
one bromo ligand in addition to the five-membered ring.
The methylene-bridged complex 18 is an example of a
species never before encountered with less bulky cyclo-
pentadienyl ligands and supports the postulation of a C5H5
analogue of 18 as an unstable intermediate during reactions of
nickelocene with methyllithium.30
The observation of the facile sublimation of bis[bromo-
(pentaisopropylcyclopentadienyl)nickel] (19) but even more
the monomeric structure of complex 20 with an even bulkier
alkylcyclopentadienyl ligand suggest that a critical bulk has been
achieved, where transition-metal complexes with still lower
coordination number can be stabilized as electronically
unsaturated monomers, whose reactivity will have to be
investigated further.
EXPERIMENTAL PROCEDURES
Synthetic work has been carried out under inert gas in rigorously dried
and deoxygenated solvents using Schlenk line techniques for the
■
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preparation of fulvenes and alkali-metal cyclopentadienide salts and an
argon-filled glovebox from MBraun, Garching, Germany, for
preparation and handling of the nickel complexes. Solvents have
been dried under inert gas with molten alkali metals such as potassium
(tetrahydrofuran) or sodium/potassium alloy (diethyl ether, petro-
leum ether, pentane) and distilled prior to use. NMR spectra were
obtained on Bruker DPX 400 and Avance 600 NMR spectrometers
and referenced to the residual solvent signals: chemical shifts δ are
given in ppm and coupling constants J in Hz. Elemental analyses have
been carried out in the analytical laboratory of the chemistry
department of the TU Kaiserslautern. Gas chromatograms have
been taken on a Shimadzu GC-17A with flame ionization detector
equipped with a column of 30 m length, 0.32 mm inner diameter, and
a poly(5% diphenyl/95% dimethylsiloxane) layer of 0.25 μm thickness.
The injector temperature and the column temperature in the
beginning was 70 °C and was increased by 50 °C/min to a limit of
280 °C; the detector temperature was 280 °C. The synthesis of lithium
pentaisopropylcyclopentadienide followed the protocol developed by
Dezember in our research group.1
Triisopropylcyclopentadiene Isomers 1. A 2 L four-necked
round-bottom flask equipped with a mechanical stirrer, dropping
funnel, reflux condenser, thermometer, and heating mantle was
charged with potassium hydroxide solution (50%, 1 L solution made
from 750 g of potassium hydroxide and 750 mL of water),
methyltrialkyl(C8−C10)ammonium chloride (Adogen 464, 13.5 g),
and freshly cracked cyclopentadiene (32 mL, 0.4 mol). The mixture
was stirred vigorously, while 2-bromopropane was admitted within 2
min (90 mL, 117.9 g, 0.96 mol). Another portion of 2-bromopropane
(98 mL, 128.4 g, 1.04 mol) was added dropwise within 1 h. The inner
temperature went up to 52−55 °C and was raised to 65 °C within 3−4
h and kept for another 9 h. Stirring was maintained throughout this
procedure. The mixture was cooled to ambient temperature, and the
phases were separated. A solid precipitate of potassium bromide was
discarded. The aqueous phase was extracted with petroleum ether
(ca. 150 mL) and set aside for a second run after addition of potassium
hydroxide (112 g, 2 mol). From the combined organic extracts
petroleum ether was removed by rotary evaporation. The remaining oil
was distilled in vacuo without prior use of drying agents, because the
water comes out together with a small forerun (ca. 5 mL) consisting of
diisopropylcyclopentadiene with traces of triisopropylcyclopentadiene.
The pale yellow main fraction was collected in a boiling range of
75−95 °C at a pressure of 6 mbar and found to contain
triisopropylcyclopentadiene isomers 1 and only traces of di- and
tetraisopropylcyclopentadiene and amounted to a yield of 75 g (0.389
mol, 97%).
GC data: several peaks between 5.9 and 6.4 min retention time.
Tetraisopropylcyclopentadiene Isomers 2. To a stirred
mixture of sodium amide (15.1 g, 0.4 mol) and tetrahydrofuran
(300 mL) in a 1 L three-necked round-bottom flask equipped with a
mechanical stirrer, dropping funnel, reflux condenser, and heating
mantle were added triisopropylcyclopentadiene isomers (76.2 g, 0.4
mol) and the suspension was slowly heated to a gentle reflux. After 8 h
the evolution of ammonia had ceased and the concentrated, viscous,
brown solution of sodium triisopropylcyclopentadienide isomers was
cooled to 40 °C. Upon dropwise addition of 2-bromopropane (48.7 g,
0.4 mol) the temperature rose again and a colorless solid precipitated
from the solution. Gradually the brown color faded away and the
viscosity was gradually reduced. After heating to reflux for 1 h, stirring
was continued while the mixture was cooled to ambient temperature.
Cautious addition of water (15 mL) and removal of most of the
solvent by distillation left a mixture, which was extracted with diethyl
ether (200 mL). From the extract the product was isolated by solvent
evaporation and distillation, yielding a main fraction of tetraisopro-
pylcyclopentadiene isomers (81.3 g, 0.346 mol, 88%) in a boiling range
of 70−85 °C at a pressure of 0.1 mbar.
ammonia was observed and the solution turned brown. When the gas
evolution had ceased, the solution was filtered through a medium-
porosity glass frit and the solvent was removed from the filtrate in
vacuo. The residual sludge was treated with petroleum ether (250 mL)
and filtered through a medium-porosity glass frit. The solid product
was washed thoroughly with petroleum ether, dried in vacuo, and
stored under inert gas. The yield was 38.6 g (0.15 mol, 50%) of ivory
powder.
GC data: a small sample was hydrolyzed with a mixture of diethyl
ether and water and gave two signals at 7.9 and 8.0 min.
Lithium Tetraisopropyl-5-{(dimethylamino)(naphth-1-yl)-
methyl}cyclopentadienide (7). A solution of 1-naphthyllithium
(926 mg, 6.91 mmol) in diethyl ether (20 mL) was added dropwise to
a solution of tetraisopropyl-6-dimethylaminofulvene (2.00 g, 6.91
mmol) in diethyl ether (20 mL) cooled in an ice bath. The ice bath
was thawed to ambient temperature, and stirring was continued for
48 h. During this period the color of the solution changed from intense
red to pale orange. Evaporation of the solution, stirring of the residue
with pentane, and fractional crystallization of the product from
pentane solution yielded the pure product in moderate yield (1.58 g,
3.7 mmol, 54%). Anal. Calcd for C30H42LiN: C, 85.06; H, 9.99. Found:
1
C, 84.93; H, 9.21. H NMR (600 MHz, 298 K, C6D6): δ 7.65−7.85
(br, 7H, naphthyl), 7.35−7.55 (br, 7H, naphthyl), 3.61 (sep, 1H,
3JH,H = 7.33), 3.42 (sep, 1H, 3JH,H = 7.33), 3.22 (sep, 1H, 3JH,H = 6.35),
3
2.74 (sep, 1H, JH,H = 7.33), 2.25 (s, 1H, CHNMe2C10H7), 1.62 (d,
3
3
3H, JH,H = 7.33, CH(CH3)2), 1.50 (d, 6H, JH,H = 7.33, CH(CH3)2),
3
3
1.40 (d, 6H, JH,H = 7.33, CH(CH3)2), 1.38 (d, 3H, JH,H = 7.33,
3
CH(CH3)2), 1.3−1.5 (br, 6H, N(CH3)2), (1.03 (d, 3H, JH,H = 6.35,
CH(CH3)2), 0.99 (d, 3H, JH,H = 6.35, CH(CH3)2). 13C{1H} NMR
3
(C6D6): δ 150.9 (C3 of Cp ring), 147.9 (C2 of Cp ring), 146.9 (C2 of
Cp ring), 139.3 (naphthyl), 137.8 (naphthyl), 136.7 (naphthyl), 134.7
(naphthyl), 134.2 (naphthyl), 132.3 (naphthyl), 130.6 (naphthyl),
129.5 (naphthyl), 129.3 (naphthyl), 128.5 (naphthyl), 128.4
(naphthyl), 127.1 (naphthyl), 126.5 (naphthyl), 126.3 (naphthyl),
126.2 (naphthyl), 125.8 (naphthyl), 125.4 (naphthyl), 124.4
(naphthyl), 34.6 (CHMe2), 33.4 (isopropyl CH3), 33.3 (isopropyl
CH3), 27.6 (CHMe2), 27.5 (CHMe2), 27.1 (CHMe2), 25.0 (NCH3),
24.5 (NCH3), 23.3 (isopropyl CH3), 22.9 (isopropyl CH3), 21.8
(isopropyl CH3), 21.0 (isopropyl CH3), 14.4 (isopropyl CH3), 2.3
(Li−C).
1,1′,2,2′,4,4′-Hexaisopropylnickelocene (8). A suspension of
nickel(II) bromide dimethoxyethane complex (800 mg, 2.59 mmol) in
tetrahydrofuran (30 mL) was stirred at −30 °C, and a solution of
sodium triisopropylcyclopentadienide isomers consisting of ca. 78%
1,2,4- and 22% 1,2,3-triisopropylcyclopentadienide (1.11 g, 5.18
mmol) was added dropwise. The mixture turned brown immediately
and changed to olive green when more than half of the sodium salt
solution had been added. The mixture was thawed to ca. 10 °C, and
then the solvent was removed in vacuo. The olive green residue was
extracted with petroleum ether (60 mL), and the insoluble material
was separated by centrifugation. The volume of the organic extract was
reduced in vacuo to ca. 10 mL, at which point incipient crystallization
was observed. At −40 °C green crystals were obtained within 1 day
(200 mg, 0.45 mmol, 17.5%), which showed a melting point of
117 °C. After repeated recrystallization from petroleum ether melting
points of 146, 151, and 151 °C could be observed and crystals suitable
for X-ray diffraction could be isolated. Anal. Calcd for C28H46Ni: C,
1
76.20; H, 10.51. Found: C, 75.50; H, 10.42. H NMR (400 MHz, 298
K, C6D6): δ (half width) 29.43 (Δν1/2 = 220 Hz, 6H, CH3), 10.91
(Δν1/2 = 210 Hz, 6H, CH3), 0.47 (Δν1/2 = 150 Hz, 6H, CH3).
Bromo(1,2,3,4-tetraisopropylcyclopentadienyl)-
(triphenylphosphine)nickel(II) (10). To a suspension of nickel(II)
bromide dimethoxyethane adduct (7.57 g, 24.5 mmol) in tetrahy-
drofuran (100 mL) stirred at −20 °C was added a solution of sodium
tetraisopropylcyclopentadienide (6.10 g, 23.8 mmol) in tetrahydrofu-
ran (80 mL) via addition funnel within a couple of minutes. The
reaction mixture turned dark brown quickly. Solid triphenylphosphine
(6.55 g, 25.0 mmol) was added immediately thereafter, and the
mixture was stirred for 1 h at that temperature and thawed to +10 °C.
After removal of volatiles in vacuo the red residue was extracted with
GC data: several peaks between 7.8 and 8.3 min retention time.
Sodium Tetraisopropylcyclopentadienide (3). A suspension
of sodium amide (16 g, 0.41 mol) in a solution of tetraisopropylcy-
clopentadiene isomers (70 g, 0.30 mol) in tetrahydrofuran (350 mL)
was heated to reflux for 60 h. During this time the slow evolution of
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Organometallics
Article
toluene (200 mL) at room temperature. Filtration and removal of the
toluene gave a brown-red solid, which was washed with four portions
of petroleum ether (20 mL each). The first washings turned brownish;
the last portion of petroleum ether had a light, transparent red color.
Drying in vacuo yielded red, microcrystalline product 9 (8.80 g, 13.9
mmol, 57.5%), which melted at 212 °C. Anal. Calcd for C35H44BrNiP:
C, 66.40; H, 6.99. Found: C, 66.28; H, 7.00. 1H NMR (400 MHz, 298
K, C6D6): δ 7.93 (br, 3H, phenyl H), 7.33 (m, 3H, phenyl H), 7.15 (m,
9H, phenyl H), 3.61 (d, 1H, JP,H = 3.5 Hz, Cp ring H), 3.29 (sep, 2H,
JH,H = 6.9 Hz, CHMe2), 2.61 (sep, 2H, JH,H = 6.9 Hz, CHMe2), 1.87
(sep, 6H, JH,H = 6.9 Hz, CH3), 1.28 (d, 6H, JH,H = 7.0 Hz, CH3), 1.02
(d, 6H, JH,H = 7.0 Hz, CH3), 0.91 (d, 6H, JH,H = 6.9 Hz, CH3). 31P
NMR (160 MHz, 298 K, C6D6): δ 32.5 ppm.
trimethyl phosphite (0.11 mL, 0.94 mmol). The solution turned
orange-brown and stirring was continued for 2 h; then the solvent was
evaporated and the residue was extracted with petroleum ether (50
mL). After centrifugation the orange-brown solution was concentrated
until incipient crystallization was observed. At −20 °C orange-red
crystals were formed in 300 mg (0.60 mmol, 64%) yield. Single crystals
for X-ray diffraction were grown from petroleum ether solution at
−20 °C. Anal. Calcd for C20H38BrNiO3P: C, 48.42; H, 7.72. Found: C,
1
48.20; H, 7.76. H NMR (400 MHz, 298 K, C6D6): δ 3.63 (d, 9H,
J
P,H = 11.8 Hz, OCH3), 3.61 (d, 1H, JP,H = 2.6 Hz, ring H), 2.99 (sep,
2H, CHMe2), 2.60 (sep, 2H, CHMe2), 1.61 (d, 6H, JH,H = 6.9 Hz,
CH(CH3)2), 1.25 (d, 6H, JH,H 6.9 Hz, CH(CH3)2), 1.23 (d, 6H, JH,H
=
6.9 Hz, CH(CH3)2), 1.03 (d, 6H, JH,H = 6.9 Hz, CH(CH3)2). 31P
NMR (160 MHz, 298 K, C6D6): δ 35.7 (s).
Methyl(1,2,3,4-tetraisopropylcyclopentadienyl)-
(triphenylphosphine)nickel(II) (11). To a solution of 9 (1.641 g,
2.59 mmol) in diethyl ether (80 mL) was added crystalline
methylmagnesium chloride diethyl ether adduct (407 mg, 2.73
mmol) in one portion at room temperature. The red solution turned
brown quickly and green after 12 h of stirring. The solvent was
evaporated, and the residue was extracted with petroleum ether (50
mL). After centrifugation the deep green solution was concentrated to
20 mL and stored at −85 °C to yield a green, microcrystalline powder
(700 mg, 1.30 mmol, 48%), which melted at 172 °C. Crystals suitable
for X-ray diffraction were obtained by slow evaporation of a toluene
solution of 10. Anal. Calcd for C36H47NiP: C, 75.93; H, 8.32. Found:
Bromo(1,2,3,4-tetraisopropylcyclopentadienyl)-
(triisopropylphosphine)nickel(II) (15). To a solution of bis{μ-
bromo(tetraisopropylcyclopentadienyl)nickel} (350 mg, 0.47 mmol)
in tetrahydrofuran (20 mL) stirred at −15 °C was added
triisopropylphosphine (0.18 mL, 0.94 mmol) dropwise. The solution
turned reddish within 1 min. The solution was warmed to room
temperature, and then the solvent was evaporated and the violet
residue was extracted with petroleum ether (50 mL). After
centrifugation the violet solution was concentrated until the formation
of seed crystals was observed. At −20 °C violet product could be
crystallized (284 mg,(0.53 mmol, 57%). Anal. Calcd for C26H50BrNiP:
C, 58.67; H, 9.47. Found: C, 58.57; H, 9.20. 1H NMR (400 MHz, 298
K, C6D6): δ 3.90 (br, 1H, ring H), 3.30 (sep, 2H, CHMe2), 2.57 (sep,
2H, CHMe2), 1.76 (br, 3H, PCHMe2), 1.29 (br, 6H, CH(CH3)2), 1.20
(br, 6H, CH(CH3)2), 1.16 (br, 6H, CH(CH3)2), 1.05 (br, 6H,
CH(CH3)2). The broad methyl signals of the isopropyl substituents of
the phosphine ligand are partially hidden under the isopropyl signals of
the cyclopentadienyl ring. 31P NMR (160 MHz, 298 K, C6D6): δ 44.4
(br).
1
C, 74.96; H, 8.05. H NMR (400 MHz, 298 K, C6D6): δ 7.68−7.78
(m, 6H, phenyl H), 6.99−7.11 (m, 9H, phenyl H), 4.16 (d, 1H, JP,H
=
2.2 Hz, Cp ring H), 3.25 (sep, 2H, JH,H = 6.9 Hz, CHMe2), 2.54 (sep,
2H, JH,H = 6.9 Hz, CHMe2), 1.66 (sep, 6H, JH,H = 6.9 Hz, CH3), 1.37
(sep, 6H, JH,H = 6.9 Hz, CH3), 1.18 (sep, 6H, JH,H = 6.9 Hz, CH3), 1.02
(sep, 6H, JH,H = 6.9 Hz, CH3), −0.62 (d, 3H, JP,H = 6.9 Hz, Ni−CH3).
31P NMR (160 MHz, 298 K, C6D6): δ 48.5 ppm.
Phenyl(1,2,3,4-tetraisopropylcyclopentadienyl)-
(triphenylphosphine)nickel(II) (12). To a solution of 9 (0.50 g,
0.79 mmol) in tetrahydrofuran (20 mL) was added a solution of
phenylmagnesium bromide diethyl ether adduct (142 mg, 0.55 mmol)
at room temperature. The red solution turned dark, and stirring was
continued for 12 h. The solvent was evaporated, and the residue was
extracted with petroleum ether (40 mL). After centrifugation the olive
green solution was evaporated until seed crystals appeared. At −85 °C
green, microcrystalline 11 (150 mg, 0.237 mmol, 43%) was obtained,
which melted between 107 and 109 °C. Anal. Calcd for C41H49NiP: C,
77.97; H, 7.82. Found: C, 76.65; H, 7.54. 1H NMR (400 MHz, 298 K,
C6D6): δ 6.82−7.55 (m, 20H, phenyl H), 4.56 (s, 1H, Cp ring H), 2.99
(sep, 2H, CHMe2), 2.83 (sep, 2H, CHMe2), 1.50 (sep, 6H, JH,H = 7.0
Hz, CH3), 1.22 (sep, 6H, JH,H = 6.6 Hz, CH3), 1.13 (sep, 6H, JH,H = 7.2
Hz, CH3), 1.08 (sep, 6H, JH,H = 7.3 Hz, CH3). 31P NMR (160 MHz,
298 K, C6D6): δ 42.4 ppm.
Bromo(1,2,3,4-tetraisopropylcyclopentadienyl)-
(trimethylphosphine)nickel(II) (13). To a solution of bis{μ-
bromo(tetraisopropylcyclopentadienyl)nickel} (440 mg, 0.59 mmol)
in tetrahydrofuran (15 mL) stirred at −10 °C was added
trimethylphosphine (0.12 mL, 1.18 mmol). A deep red solution was
obtained and warmed to room temperature. Stirring was continued for
12 h, and then the solvent was evaporated and the residue was
extracted with petroleum ether (20 mL). After centrifugation the
orange-brown solution was concentrated until incipient crystallization
was observed. At −20 °C orange-red crystals were formed in 310 mg
(0.69 mmol, 59%) yield. Mp: 159−161 °C. Anal. Calcd for
C20H38BrNiP: C, 53.61; H, 8.55. Found: C, 53.57; H, 8.74. 1H
NMR (400 MHz, 298 K, C6D6): δ 3.56 (d, 1H, JP,H = 4.8 Hz, ring H),
Bis{(μ-2,6-dimethylphenolato)(1,2,3,4-tetraisopropylcyclo-
pentadienyl)nickel(II)} (16). Solid bis{μ-bromo(tetraiso-
propylcyclopentadienyl)nickel} (340 mg, 0.457 mmol) was mixed
with a solution of potassium 2,6-dimethylphenolate (155 mg, 0.967
mmol) in tetrahydrofuran (20 mL) and stirred at room temperature
overnight. The solvent was evaporated, and the orange-brown residue
was extracted with petroleum ether (30 mL). After centrifugation the
volume of the orange extract was reduced to 10 mL by evaporation
and the concentrate was stored at −40 °C. The product was obtained
as an orange, crystalline solid (273 mg, 0.33 mmol, 72%). The
extremely air sensitive solid could not be analyzed, because the
compound decomposed during sample manipulation. Crystals for
X-ray diffraction have been obtained by vacuum sublimation in a
1
sealed glass tube under vacuum over a couple of weeks at 60 °C. H
NMR (400 MHz, 298 K, C6D6): signals of high intensity, δ (half
width) 23.65 (Δν1/2 = 110 Hz), 21.64 (Δν 1/2 = 15 Hz), 16.90
(Δν1/2 = 240 Hz), 15.05 (Δν 1/2 = 190 Hz), 12.80 (superposition),
11.60 (superposition); signals of low intensity, δ (half width) 37.06
(Δν1/2 = 35 Hz), 36.01 (Δν1/2 = 300 Hz), 34.89 (superposition),
33.61 (Δν1/2 = 15 Hz), 30.47 (superposition), 26.46 (Δν 1/2 = 25 Hz),
−9.52 (Δν1/2 = 20 Hz), −14.27 (Δν1/2 = 15 Hz), −15.59 (Δν1/2
15 Hz).
=
Crystallization from solution was unsuccessful. Repeated attempts at
crystal growth from solutions in different solvents and solvent mixtures
finally produced crystals from a mixture of pentane and cyclopentene,
which were suitable for X-ray diffraction. These orange crystals
consisted of the partial hydrolysis product (μ-2,6-dimethylphenolato)-
(μ-hydroxo)bis{(1,2,3,4-tetraisopropylcyclopentadienyl)nickel(II)}
(17), which was apparently formed during the crystallization
experiments with traces of water.
Methylenebis{(1,2,3,4-tetraisopropylcyclopentadienyl)-
nickel(II)} (18). To a magnetically stirred, red-brown solution of
bis{μ-bromo(1,2,3,4-tetraisopropylcyclopentadienyl)nickel(II)} (519
mg, 0.697 mmol) in tetrahydrofuran (15 mL) at room temperature
was added solid methylmagnesium chloride diethyl ether adduct (220
mg, 1.48 mmol) at once. The solution turned black, and stirring was
continued for 14 h. The solvent was evaporated, and the black residue
3.16 (sep, 2H, CHMe2), 2.59 (sep, 2H, CHMe2), 1.72 (d, 9H, JP,H
=
7.1 Hz, PCH3), 1.29 (d, 6H, JH,H = 7.2 Hz, CH(CH3)2), 1.20 (d, 6H,
JH,H not observed because of superposition, CH(CH3)2), 1.16 (d, 6H,
JH,H = 6.7 Hz, CH(CH3)2), 1.05 (d, 6H, JH,H = 6.8 Hz, CH(CH3)2).
31P NMR (160 MHz, 298 K, C6D6): δ −11.9 (br).
Bromo(1,2,3,4-tetraisopropylcyclopentadienyl)(trimethyl
phosphite)nickel(II) (14). To a solution of bis{μ-bromo-
(tetraisopropylcyclopentadienyl)nickel} (350 mg, 0.47 mmol) in
tetrahydrofuran (15 mL) stirred at room temperature was added
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was extracted with petroleum ether (80 mL). After centrifugation the
solvent was removed in vacuo and the crude black product was stirred
with acetonitrile (3−4 mL). The solution was discarded, and the
insoluble product was dried to yield 250 mg (0.418 mmol, 60%) of a
black powder. Anal. Calcd for C35H60Ni2: C, 70.26; H, 10.10. Found:
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for 8, 11, 13, 14, 16−18,
and 20. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
C, 69.11; H, 9.77. H NMR (400 MHz, 298 K, C6D6): δ 4.64 (s, 2H,
ring H), 3.64 (br, 2H, CHMe2), 3.23 (br, 2H, CHMe2), 2.28 (d, 6H,
JH,H = 6.2 Hz, CH(CH3)2), 1.71 (d, 6H, JH,H = 6.8 Hz, CH(CH3)2),
1.57 (d, 6H, JH,H = 6.7 Hz, CH(CH3)2), 1.41 (s, 2H, CH2 bridge), 1.29
(d, 6H, JH,H = 6.6 Hz, CH(CH3)2). 13C NMR (100 MHz, 298 K,
C6D6): δ 72.8 (d, 2C, 1JC,H 171 Hz, ring CH), 65.9 (t, 1C, 1JC,H = 145
AUTHOR INFORMATION
■
Corresponding Author
*Tel: (49)-631-2054399. Fax: (49)-631-2054676. E-mail:
sitzmann@chemie.uni-kl.de.
1
Hz, bridging CH2), 36.5 (q, 4C, JC,H = 125 Hz, CH3), 35.6 (q, 4C,
1JC,H = 125 Hz, CH3), 32.6 (q, 4C, 1JC,H = 125 Hz, CH3), 30.5 (q, 4C,
1
1JC,H = 125 Hz, CH3), 22.5 (d, 4C, JC,H = 121 Hz, CHMe2), 19.4 (d,
4C, 1JC,H = 121 Hz, CHMe2). Ring CiPr signals could not be assigned
because of a combination of low signal intensity and high noise.
Bromopentaisopropylcyclopentadienylnickel Dimer 19. To
a stirred suspension of nickel(II) bromide dimethoxyethane adduct
(260 mg, 0.84 mmol) in pentane (5 mL) was added a solution of
lithium pentaisopropylcyclopentadienide diethyl ether adduct (300
mg, 0.84 mmol) in pentane (15 mL). Over 3 days the stirred mixture
changed from a pale salmon to an intense red. Insolubles were settled
by centrifugation, stirred up with pentane (10 mL), and subjected to
centrifugation again. Upon evaporation to dryness the combined
pentane extracts yielded 240 mg (0.58 mmol, 69%) of a dark red,
crystalline solid. Anal. Calcd for C40H70Br2Ni2: C, 58.01; H, 8.52.
ACKNOWLEDGMENTS
■
H.S. is grateful to Professor Scherer for many years of generous
support. We also thank Prof. Dr. E. Dormann, Dr. H. Winter,
and Dr. M. Kelemen for magnetic susceptibility measurements
carried out in the physics department of the Technical
University Karlsruhe.
REFERENCES
■
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(Δν1/2 = 195 Hz, 15H, CH3), 34.5 (Δν1/2 = 195 Hz, 15H, CH3), 18.6
(Δν1/2 = 250 Hz, 5H, CH).
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Karcher, J.; Dormann, E.; Winter, H.; Wachter, C.; Kelemen, M.
Bromotetraisopropyl-5-(dimethylaminonaphth-1-yl)-
methylcyclopentadienylnickel (20). To a stirred suspension of
nickel(II) bromide dimethoxyethane adduct (150 mg, 0.49 mmol) in
pentane (20 mL) was added a solution of lithium tetraisopropyl-5-
dimethylamino(naphth-1-yl)methylcyclopentadienide (206 mg, 0.49
mmol) in pentane (10 mL) dropwise at ambient temperature. Within
24 h the mixture changed from pale salmon to an intense red, while
the amount of insolubles decreased. The solids were removed by
centrifugation, and the solvent was evaporated in vacuo to leave
behind 114 mg (0.21 mmol, 42%) of a red, crystalline solid. Anal.
Calcd for C30H42BrNNi: C, 64.89; H, 7.62. Found: C, 64.12; H, 7.59.
1H NMR (600 MHz, 298 K, C6D6): δ 7.65−7.85 (br, 7H, naphthyl),
̈
Angew. Chem. 1996, 108, 3013−3016; Angew. Chem., Int. Ed. Engl.
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(9) Weismann, D.; Sun, Y.; Lan, Y.; Wolmershauser, G.; Powell, A.
̈
K.; Sitzmann, H. Chem. Eur. J. 2011, 17, 4700−4704.
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1991, 113, 4843−4851.
7.35−7.55 (br, 7H, naphthyl), 5.18 (s, 1H, CHNMe2C10H7), 5.13 (s,
3
1H, CHNMe2C10H7), 3.69 (sep, 1H, JH,H = 7.00, CH(CH3)2), 3.52
3
(sep, 1H, JH,H = 7.32, CH(CH3)2), 3.44 (s, 3H, NCH3), 3.32 (s, 3H,
NCH3), 2.55 (br, 4H, CH(CH3)2), 2.46 (s, 6H, NCH3), 1.72 (sep, 1H,
3JH,H = 7.56, CH(CH3)2), 1.56 (d, 3H, 3JH,H = 7.00, CH(CH3)2), 1.55
3
3
(d, 3H, JH,H = 7.00, CH(CH3)2), 1.50 (d, 6H, JH,H = 7.33,
CH(CH3)2), 1.46 (d, 3H, 3JH,H = 7.14, CH(CH3)2), 1.42 (d, 3H, 3JH,H
3
= 7.56, CH(CH3)2), 1.40 (d, 3H, JH,H = 7.32, CH(CH3)2), 1.39 (d,
3H, JH,H = 7.32, CH(CH3)2), 1.38 (d, 3H, JH,H = 6.60, CH(CH3)2),
1.37 (d, 3H, JH,H = 7.00, CH(CH3)2), 1.31 (d, 3H, JH,H = 6.35,
3
3
3
3
(15) Sitzmann, H. In Synthetic Methods of Organometallic and
Inorganic Chemistry (Herrmann/Brauer); Herrmann, W. A., Salzer, A.,
Vol. Eds.; Georg Thieme Verlag: Stuttgart, New York, 1996; Vol. 1, pp
57−60.
3
CH(CH3)2), 1.30 (d, 3H, JH,H = 6.70, CH(CH3)2), 1.26 (d, 3H,
3JH,H = 7.00, CH(CH3)2), 1.22 (d, 3H, 3JH,H = 7.00, CH(CH3)2), 1.00
3
3
(d, 3H, JH,H = 7.00, CH(CH3)2), 0.98 (d, 3H, JH,H = 7.38,
3
(16) Hafner, K.; Vopel, K. H.; Ploss, G.; Konig, C. Liebigs Ann. Chem.
̈
̈
CH(CH3)2), 0.97 (d, 3H, JH,H = 6.35, CH(CH3)2), 0.94 (d, 3H,
3JH,H = 6.35, CH(CH3)2). 13C{1H} NMR (C6D6): δ 138.9 (naphthyl),
134.4 (naphthyl), 134.2 (naphthyl), 133.8 (naphthyl), 132.8
(naphthyl), 129.0 (naphthyl), 128.9 (naphthyl), 127.0 (naphthyl),
126.7 (naphthyl), 126.0 (naphthyl), 125.8 (naphthyl), 125.7
(naphthyl), 125.6 (naphthyl), 125.5 (naphthyl), 125.4 (naphthyl),
125.4 (naphthyl), 125.1 (naphthyl), 125.0 (naphthyl), 124.9
(naphthyl), 124.0 (naphthyl), 72.0 (C3 of Cp ring), 64.1 (C2 of Cp
ring), 58.5 (C1 of Cp ring), 58.2 (C1 of Cp ring), 44.9 (NCH3), 44.6
(NCH3), 34.2 (CHMe2), 33.0 (isopropyl CH3), 32.9 (isopropyl CH3),
28.0 (CHMe2), 27.7 (CHMe2), 27.6 (CHMe2), 27.0 (CH(NMe2)-
(naphthyl)), 26.9 (CHMe2), 26.6 (CHMe2), 26.2 (CHMe2), 23.7
(isopropyl CH3), 22.9 (isopropyl CH3), 22.8 (isopropyl CH3), 22.7
(isopropyl CH3), 22.6 (isopropyl CH3), 22.5 (isopropyl CH3), 22.1
(isopropyl CH3), 21.9 (isopropyl CH3), 21.2 (isopropyl CH3), 20.8
(isopropyl CH3), 14.0 (isopropyl CH3), 13.9 (isopropyl CH3).
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