Remarkably robust mono-n-butyl group IV dicyclohexylamido complexes {(Cy)2N}3M(n -butyl) (Cy: Cyclohexyl [C6H11], M: Ti, Zr)

Article abstract of DOI:10.1021/om5008105

The syntheses of alkyl titanium and zirconium dicyclohexylamido complexes from (Cy₂N)₃MCl (Cy: cyclohexyl, M: Ti 1a, M: Zr 1b) are presented. Particularly the β-H-containing n-butyl derivatives (Cy₂N)₃Mⁿ

Full text of DOI:10.1021/om5008105

Communication
pubs.acs.org/Organometallics
Remarkably Robust Mono‑n‑butyl Group IV Dicyclohexylamido
Complexes {(Cy)2N}3M(n‑butyl) (Cy: cyclohexyl [C6H11], M: Ti, Zr)
Christian Adler, Gabriele Tomaschun, Marc Schmidtmann, and Rudiger Beckhaus*
̈
Institut fur Chemie, Carl von Ossietzky Universitat Oldenburg, Carl von Ossietzky-Straße 9-11, D-26129 Oldenburg, Federal
̈
̈
Republic of Germany
S
* Supporting Information
ABSTRACT: The syntheses of alkyl titanium and zirconium
dicyclohexylamido complexes from (Cy2N)3MCl (Cy: cyclohexyl,
M: Ti 1a, M: Zr 1b) are presented. Particularly the β-H-
containing n-butyl derivatives (Cy2N)3MnBu (M: Ti 3a, M: Zr 3b)
become available as isolable, thermally stable complexes by
reactions of 1 with n-BuLi. The solid-state structures of 3a and 3b
are presented. No hints for C−H agostic interactions are found. The behavior of 3 is discussed in comparison to the
corresponding methyl complexes (Cy2N)3MMe (M: Ti 2a, M: Zr 2b) in the solid state, as well as in solution.
arly transition metal alkyl complexes containing β-
hydrogen atoms are of general interest, particularly as
(Cy2N)2ZrCl2. Regardless of the stoichiometry, (Cy2N)3ZrCl
(1b) was formed, analogous to a known process.25 The use of
ZrCl4 and n-hexane as solvent results in an improved yield. We
suspect that the usage of THF leads to the formation of
octahedral-coordinated anionic zirconium complexes as a
byproduct. A similar phenomenon was reported by Petersen
et al. for the synthesis of Zr(NMe2)4 from ZrCl4 and LiNMe2.31
In contrast to Verkade et al. we were able to obtain single
crystals suitable for X-ray diffraction analysis of 1b from a
saturated n-hexane solution at −30 °C.25 The molecular
structure of 1b is shown in Figure 1. The tetrahedral-
coordinated complex crystallizes in the monoclinic space
group Cc. The dicyclohexylamide ligands are coordinated
trigonal planar (sum of all angles: N1: 359.5°, N2: 359.6°, N3:
359.4°). All Zr−N bonds are of comparable length (Zr1−N1
2.041(2) Å, Zr1−N2 2.042(3) Å, Zr1−N3 2.048(3) Å). The
Zr−N bonds are shorter than a typical single bond, which
indicates an attractive N(pπ)−Zr(dπ) interaction.17,32 However,
these bond lengths correspond well to known zirconium
complexes with secondary amido ligands.16,17,32,33 The Zr1−
Cl1 bond is within the expected range (2.4375(8) Å) for similar
compounds.16,17,32 The crystallographic properties of complex
1b are similar to the analogous titanium complex (Cy2N)3TiCl
(1a).24
E
models for the growing alkyl chain in olefin polymerization
processes.1 Generally, dominant β-H elimination processes are
found for titanium as well as zirconium σ-alkyls (ethyl and
homologues), even at low temperatures.2 Up to now only a
small number of n-butyl complexes of titanium3−5 or
zirconium6−14 characterized by X-ray diffraction analysis can
be found in the CCDC database.
The early transition metal chemistry with cyclopentadienyl-
type ancillary ligands is widely developed. In the last decades
the reactivity of early transition metal complexes with amido
ligands is an area of growing interest.15,16 Amido ligands are
generally able to stabilize electron-deficient transition metal
complexes through N(pπ)−M(dπ) interactions.17 The diverse
possibilities of organic substituents attached to the nitrogen
atom allow an adjustment of electronic features and steric
hindrance around the transition metal center.18 These proper-
ties make amido ligands as versatile as Cp-type ligands, and
their complexes are alternatives for various catalytic pro-
cesses.19−21 The utilization of dicyclohexylamine as a ligand in
particular has many advantages.22,23 The bulky substituents
result in a steric demand similar to cyclopentadienyl ligands.
Transition metal complexes with this ligand have excellent
crystallization properties. Solid-state structures of dicyclohex-
ylamido titanium complexes reveal a close intramolecular
contact between the metal center and one ipso carbon of the
ligand.24,25
We recently reported on the chemistry of titanium
dicyclohexylamido complexes, which are able to activate
C−H bonds under mild conditions, forming titanaaziridines
or azatitanacyclobutanes.24 Generally, α-, β-, or γ-C−H bond
activation processes are of fundamental significance for the
understanding of organometallic reactions.26−30 Due to these
results, we were interested in transferring this ligand system to
zirconium. By the reaction of zirconium(IV) chloride with
LiNCy2 it was not possible to synthesize (Cy2N)ZrCl3 or
The remaining chlorine of complex 1b can easily be
substituted with salt metathesis reactions, forming alkylated
compounds (Scheme 1). The reaction of 1b with one
equivalent of methyllithium in n-hexane yields the methylated
complex (Cy2N)3ZrMe (2b). After subsequent purification,
single crystals, suitable for X-ray diffraction analysis, could be
obtained from a saturated n-hexane solution at −30 °C.
Complex 2b crystallizes in the monoclinic space group Cc. The
ORTEP plot is given in Figure 2. All nitrogen atoms are
Received: August 7, 2014
Published: December 8, 2014
© 2014 American Chemical Society
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Figure 1. Molecular structure of complex 1b. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms, the second
position of the disordered part of the molecule, and the second
molecule of the asymmetric unit are omitted for clarity. Selected bond
lengths [Å] and angles [deg]: Zr1−N1 2.041(2), Zr1−N2 2.042(3),
Zr1−N3 2.048(3), Zr1−Cl1 2.4375(8), N1−C1 1.485(4), N1−C7
1.472(4), N2−C13 1.482(4), N2−C19 1.470(4), N3−C25A 1.574(8),
N1−C31 1.475(7), N1−Zr1−N2 111.73(10), N1−Zr1−N3
110.33(10), N1−Zr1−Cl1 107.03(7), N2−Zr1−N3 112.33(11),
Zr1−N1−C1 105.90(17), Zr1−N1−C7 138.5(2), C7−N1−C1
115.1(2), Zr1−N2−C13 105.33(18), Zr1−N2−C19 139.4(2), C13−
N2−C19 114.90(17), Zr1−N3−C25A 104.4(3), Zr1−N3−C31
136.2(2), C25A−N1−C31 118.8(4).
Figure 2. Molecular structure of complex 2b. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms, the second
position of the disordered part of the molecule, and the second
molecule of the asymmetric unit are omitted for clarity. Selected bond
lengths [Å] and angles [deg]: Zr1−N1 2.048(2), Zr1−N2 2.054(2),
Zr1−N3 2.064(3), Zr1−C73 2.286(3), N1−C1 1.482(4), N1−C7
1.472(4), N2−C13 1.480(4), N2−C19 1.467(4), N3−C25A 1.567(8),
N1−C31 1.465(4), N1−Zr1−N2 113.16(10), N1−Zr1−N3
111.89(10), N1−Zr1−C73 105.92(11), N2−Zr1−N3 113.52(10),
Zr1−N1−C1 106.54(17), Zr1−N1−C7 137.86(19), C7−N1−C1
115.3(2), Zr1−N2−C13 105.76(17), Zr1−N2−C19 139.0(2), C13−
N2−C19 114.9(2), Zr1−N3−C25A 104.2(3), Zr1−N3−C31
135.9(2), C25A−N3−C31 119.5(3).
Scheme 1. Preparation of (Cy2N)3MCH3 and (Cy2N)3MnBu
Scheme 2. Subsequent Reactions of Alkylated Titanium and
Zirconium Dicyclohexylamido Complexes
coordinated trigonal planar (sum of all angles: N1: 359.7°, N2:
359.7°, N3: 359.6°). The Zr−N bonds are of comparable
length (Zr1−N1 2.048(2) Å, Zr1−N2 2.054(2) Å, Zr1−N3
2.064(3) Å) and in agreement with comparable known
complexes, as well as the Zr1−C73 bond (2.286(3) Å).18,34−37
In general, 4d- and 5d-transition metal alkyl complexes are
more stable than the corresponding 3d-complexes due to a
more efficient orbital overlap between the ligand and the metal
center.38−41 The analogous titanium complex (Cy2N)3TiMe
(2a) is not stable in solution at ambient temperature. A C−H
bond elimination occurs in α-position to the nitrogen atom,
methane and cyclohexene are released, and a binuclear titanium
imido complex is formed (Scheme 2), as a subsequent product
of an imine intermediate.24 In contrast, the formation of
degradation products of the analogous zirconium complex (2b)
could be observed above 90 °C using temperature-dependent
1H NMR spectroscopic experiments. After 16 h at 100 °C the
characteristic signals of 2b completely disappeared. However,
only the signals of dissolved methane and cyclohexene are
detected, whereas the nature of the subsequent zirconium-
containing product could not be solved.
a saturated n-hexane solution at −30 °C. Complex 3b
crystallizes in the monoclinic space group P21/n and has a
slightly distorted tetrahedral coordination geometry. The
molecular structure of 3b is shown in Figure 3.
Analogous to 1b and 2b all Zr−N bonds are of comparable
length (Zr1−N1 2.0651(8) Å, Zr1−N2 2.0612(8) Å, Zr1−N3
2.0616(8) Å), and all nitrogen atoms are coordinated trigonal
planar (sum of all angles: N1: 359.8°, N2: 359.9°, N3: 359.9°).
The Zr1−C37 bond (2.2938(10) Å) is slightly elongated
compared to that in complex 2b, but corresponds well to other
known Zr−nBu bond lengths.8,11,14 The molecular structures of
complexes 1b−3b exhibit no great differences except for the
Zr−N bond lengths, which increase from compound 1b to 3b.
This effect can be attributed to the exchange of the halide to
alkyl ligands. All complexes are slightly distorted tetrahedral
coordinated, and compounds 1b and 2b are similar to the
Treatment of an n-hexane solution of 1b with one equivalent
of n-butyllithium leads to butylated complex (Cy2N)3ZrnBu
(3b) in excellent yields. Crystals suitable for X-ray diffraction
analysis could be obtained in the form of colorless needles from
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reaction of (Cy2N)3TiCl with n-butyllithium in n-hexane at
−30 °C produces the butylated complex (Cy2N)3TinBu (3a) in
moderate yield. Single crystals of 3a suitable for X-ray
diffraction analysis could be obtained from a saturated n-
hexane solution at 0 °C.
Complex 3a crystallizes in the triclinic space group P1. The
̅
ORTEP plot is given in Figure 4. Analogous to the zirconium
Figure 3. Molecular structure of complex 3b. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths [Å] and angles [deg]: Zr1−N1
2.0651(8), Zr1−N2 2.0612(8), Zr1−N3 2.0616(8), Zr1−C37
2.2938(10), N1−C1 1.4790(12), N1−C7 1.4709(12), N2−C13
1.4791(12), N2−C19 1.4684(12), N3−C25 1.4784(12), N1−C31
1.4683(12), N1−Zr1−N2 112.10(3), N1−Zr1−N3 113.63(3), N1−
Zr1−C37 105.55(3), N2−Zr1−N3 111.48(3), Zr1−N1−C1
108.27(5), Zr1−N1−C7 136.59(6), C7−N1−C1 114.95(7), Zr1−
N2−C13 104.88(5), Zr1−N2−C19 137.91(6), C13−N2−C19
117.13(7), Zr1−N3−C25 106.72(5), Zr1−N3−C31 136.98(6),
C25−N3−C31 116.23(7).
Figure 4. Molecular structure of complex 3a. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms and the second
position of the disordered part of the molecule are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: Ti1−N1 1.9150(10),
Ti1−N2 1.9184(10), Ti1−N3 1.9101(8), Ti1−C37 2.1464(10), N1−
C1 1.4795(3), N1−C7A 1.503(2), N2−C13 1.4759(13), N2−C19A
1.5162(19), N3−C25 1.4803(12), N1−C31 1.4693(12), N1−Ti1−N2
111.27(4), N1−Ti1−N3 109.58(4), N1−Ti1−C37 111.16(4), N2−
Ti1−N3 113.22(4), Ti1−N1−C1 113.58(6), Ti1−N1−C7A
117.79(11), C7A−N1−C1 128.63(12), Ti1−N2−C13 108.85(6),
Ti1−N2−C19A 139.95(10), C13−N2−C19A 111.18(11), Ti1−N3−
C25 114.37(6), Ti1−N3−C31 124.48(6), C25−N3−C31 121.14(7).
corresponding titanium complexes (Cy2N)3TiCl (1a) and
(Cy2N)3TiMe (2a) as well.24 Another interesting feature of
the solid-state structures is a short intramolecular contact
between the metal center and one ipso carbon of each
dicyclohexylamine ligand (see Supporting Information, Table
S5). They are significantly shorter (on average 0.85 Å) than the
sum of the van der Waals radii of zirconium and carbon. These
close contacts are typical for complexes with this bulky
secondary amine ligand.24,25 Similar to the methylated complex
2b, temperature-dependent 1H NMR spectroscopic experi-
ments demonstrate that the monobutyl complex 3b is rather
stable. First indications for a degradation process can be noticed
at 80 °C. After 16 h at 100 °C the characteristic signal of the
ipso protons has completely disappeared, but cyclohexene is
released. The remaining decomposition products could not be
identified yet. Especially dialkyl complexes of zirconium are
known for β-H elimination reactions at low temperatures. The
Negishi system Cp2ZrCl2/n-butyllithium has to be prepared at
−78 °C and eliminates 1-butene and butane to yield a
zirconocene intermediate, which can undergo various reac-
tions.42−45 To our knowledge there are only two comparable
monobutylated complexes, reported by Jia, Brennessel, et al.11
These compounds exhibit an n-butyl group and three nitrogen-
containing ligands like complex 3b. Solid-state structures reveal
indications for weak β-agostic interactions. The Zr−Cα−Cβ
angle of 95.7° and 97.1°, respectively, is much smaller than the
corresponding angle in nonagostic compounds (108−126°),
but larger than angles in known β-agostic complexes (about
85°).46−48 The Zr−Cα−Cβ angle in complex 3b is 125.26(7)°.
We assume that the electron deficiency of the metal center is
compensated due to the good N(pπ)−Zr(dπ) interactions
between the secondary amine ligands and the metal.
Furthermore, the solid structure of 3b indicates that there is
no space for an interaction between the metal center and the
butyl group because of the high steric hindrance achieved by
the bulky cyclohexyl groups.
compound (3b) the central titanium atom is slightly distorted
tetrahedral coordinated. The titanium−nitrogen distances are
of comparable length (Ti1−N1 1.9150(10) Å, Ti1−N2
1.9184(10) Å, Ti1−N3 1.9101(8) Å). The Ti−N bonds and
the Ti1−C37 bond length of 2.1464(10) Å are within the range
for comparable complexes.5,24 All nitrogen atoms are trigonal
planar coordinated (sum of all angles: N1: 360°, N2: 360°, N3:
360°).The solid-state structure gives no evidence for a β-agostic
interaction. The angle Ti1−C37−C38 is 123.34(7)°, which is a
typical value for known nonagostic compounds.46−48 As
explained earlier for the corresponding zirconium complex,
we suspect that the sterical hindrance of the amido ligands and
the reduced Lewis acidity because of the N(pπ)−Ti(dπ)
interactions are responsible for this effect.
However, the titanium compound 3a is considerably less
1
stable compared to the analogous zirconium complex 3b. H
NMR spectroscopic experiments show a complete degradation
in solution at ambient temperature within 1 day, while
cyclohexene is released. Furthermore, the formation of the
binuclear imido-bridged titanium complex could be found after
a certain time. This indicates a similar degradation process for
the methylated (2a) and the butylated complexes (3a)
(Scheme 2).24 There are no hints for a butyl-specific β-C−H
elimination reaction at the alkyl chain.
In summary, we demonstrated the preparation of mono-
alkylated dicyclohexylamido titanium and zirconium complexes
from the corresponding chlorines by the use of methyl- and
butyllithium, respectively. The molecular structures of the
chlorine 1b and the alkylated complexes 2b, 3a, and 3b could
Due to the unexpected stability of complex 3b, we
transferred the synthetic procedure to titanium. The equimolar
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be determined by single-crystal diffraction analysis. The solid
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the zirconium complexes but at room temperature for the
titanium complex. The amido ligands are able to stabilize the
alkyl function at the metal center.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, relevant NMR spectra, X-ray crystallo-
graphic information, and CIF files for compounds 1b, 2b, 3a,
and 3b. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ruediger.beckhaus@uni-oldenburg.de.
Notes
■
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemein-
schaft (BE 1400/7-1).
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