Acetylenetitanium complex stabilized by aminopyridinato ligands

Article abstract of DOI:10.1002/ejic.200800132

The first acetylenetitanium complex stabilized by aminopyridinato ligands has been prepared by the reaction of Ap₂TiCl₂ (1) - Ap-H = 2-[(2,6-diisopropylphenyl)amino]pyridine - with equimolar amount of bis(trimethylsilyl)acetylene in

Full text of DOI:10.1002/ejic.200800132

SHORT COMMUNICATION
DOI: 10.1002/ejic.200800132
Acetylenetitanium Complex Stabilized by Aminopyridinato Ligands
Awal Noor[a] and Rhett Kempe*[a]
Keywords: Alkyne complex / Amido ligands / Aminopyridinato ligands / N ligands / Titanium
The first acetylenetitanium complex stabilized by aminopyr-
idinato ligands has been prepared by the reaction of Ap2TiCl2
(1) – Ap-H = 2-[(2,6-diisopropylphenyl)amino]pyridine – with
equimolar amount of bis(trimethylsilyl)acetylene in the pres-
ence of magnesium. The complex was characterized by spec-
troscopic methods as well as by X-ray single crystal structure
analysis. Reduction of 1 with KC8 under N2 in the absence of
any other stabilizing ligand leads to the ligand rearrange-
ment product Ap3Ti. In the reaction of the acetylenetitanium
complex with acetone the titanaoxacyclopentene was
formed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
atom were grown from concentrated toluene solution. The
titanium centre has a distorted octahedral coordination
(Figure 1). The Cl1–Ti–Cl2 angle [93.76(10)°] is close to the
ideal value of 90° and smaller than in similar (aminopyr-
Introduction
The stabilization of highly reactive low-valent early tran-
sition metal complexes is the key to rich and selective small-
molecule activation chemistry of these compounds. A chal-
lenge is to generate these complexes as stable and easy to
handle but nevertheless highly reactive compounds. One
strategy is the protection of the reactive sites by an easily
replaceable ligand. This strategy has been successfully ap-
plied to titanocenes and zirconocenes by using alkynes as a
protecting ligand.[1] The chemistry of these complexes, and
particularly their bonding situation, their spectroscopic
properties, their reactivity and mechanistic behavior etc.,
has been described in several reviews.[1,2] Such complexes
have been studied either using Cp (Cp = cyclopentadienyl)
or variations of Cp ligands. However, using possible alter-
natives to Cp ligands to stabilize reduced group-4 metal
centres have been investigated to a lesser extent.[3] In order
to investigate such alternatives, we decided to employ ami-
nopyridinato ligands. We have explored aminopyridinato li-
gands for many years[4] and only recently started to work
with bulky versions of these ligands.[5] Herein we report on
the synthesis and structure of a Cp-free acetylenetitanium
complex and some preliminary reactivity studies of this
compound.
idinato)chlorotitanium
complexes
[97.03(5)°[8]
and
100.54(3)°[9]]. The first acetylene complex of titanocene
without additional ligands was prepared by the reduction
of [Cp2TiCl2] with equimolar amounts of magnesium in the
presence of tolane in THF.[10,11] We have found that this
procedure can be used successfully to synthesize aminopyr-
idinato-stabilized titanium complexes as well. The reaction
of 1 with magnesium proceeds smoothly in the presence of
bis(trimethylsilyl)acetylene at room temperature, affording
the corresponding acetylene complex 2 in moderate yields
(Scheme 1).
Results and Discussion
The starting complex 1 (Scheme 1) was prepared in excel-
lent yields by reacting two equivalents of 2-[(2,6-diisopro-
pylphenyl)amino]pyridine[6] with [(CH3)2NTiCl3].[7] Crys-
tals of 1 containing two toluene molecules per titanium
Scheme 1. Synthesis of 2, 3, and 4.
[a] Lehrstuhl für Anorganische Chemie II, Universität Bayreuth,
95440 Bayreuth, Germany,
Complex 2 was isolated as dark blue crystals and charac-
terized by NMR spectroscopy. The structure was confirmed
by X-ray diffraction studies.[12] The molecular structure of 2
Fax: +49-921-55-2157
E-mail: kempe@uni-bayreuth.de
Eur. J. Inorg. Chem. 2008, 2377–2381
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Noor, R. Kempe
SHORT COMMUNICATION
cal of sp2-hybridized carbon atoms and are smaller than
what was found for the Cp stabilized complex [145.7(4) and
147.8(4)°].
In the solid state 2 is stable under argon at room tem-
perature but rapidly decomposes in the presence of air and
moisture.
1
It is noteworthy that in the H NMR spectrum of 2 at
room temperature there is a singlet corresponding to SiMe3
at δ = 0.16 ppm, four doublets for the four diastereotropic
methyl groups and two separate septets for CH protons of
the aminopyridinato ligand at δ = 2.86 and 3.42 ppm. The
13C NMR spectrum shows, in addition to other signals, the
characteristic signal for carbon atoms of acetylene coordi-
nated to Ti (δ = 209.7 ppm) which is comparatively upfield
than what has been observed for other titanium-stabilized
alkyne complexes indicating a weakly bound acetylene li-
gand.[2c,15] Reduction of 1 with KC8 under N2 atmosphere
in the absence of any supporting ligand leads to a homolep-
tic tris(aminopyridinato)titanium complex 3, the only prod-
uct that could be isolated (Scheme 1) owing most probably
due to ligand rearrangement. Since the paramagnetic nature
of 3 prevents structural characterization by NMR spec-
troscopy, X-ray analysis was performed (Figure 3).[12] Only
very weakly diffracting crystals of 3 could be obtained re-
sulting in a low-quality structure, therefore detailed dis-
cussion of bond lengths and angles is waived. However, the
connectivity was established.
Figure 1. Molecular structure of 1 (ellipsoids correspond to the
50% probability level). Hydrogen atoms and two toluene molecules
are omitted for clarity; selected bond lengths (Å) and angles (°):
N1–Ti1 2.179(8), N2–Ti1 1.954(6), N3–Ti1 2.124(7), N4–Ti1
2.036(7), Cl1–Ti1 2.310(3), Cl2–Ti1 2.247(3); N4–Ti1–N3 63.2(3),
N2–Ti1–N1 63.8(3), Cl2–Ti1–Cl1 93.76(10).
is shown in Figure 2. The small N–Ti–N angle [64.22(18)°]
expresses the strained binding situation of the aminopyridi-
nato ligands and the shorter Namido–Ti [2.058(5) Å] dis-
tance compared to Npyridine–Ti [2.152(5) Å] indicates the lo-
calization of the anionic function at the amido N-atom.[13]
The carbon atoms of the acetylene, together with the silicon
atoms bonded to them, form almost a planar system. The
torsion angle SiCϵCSi is 2.0(2)°. The coordinated CϵC
bond of the acetylene ligand [1.338(13) Å] is much longer
than the normal CϵC bond (1.181 Å), its length being close
to the value of 1.331 Å, typical for the C=C bond.[14]
Furthermore, this value is significantly higher than that ob-
served for [Cp2Ti(Me3SiCϵCSiMe3)] [1.283(6) Å].[2c] The
distances between the titanium atom and the carbon atoms
of the coordinated alkyne are 2.090(7) Å which are slightly
shorter than the values found for [Cp2Ti(Me3Si-
CϵCSiMe3)] [2.136(5) and 2.139(4) Å]. The CCSi angles of
138.9(2)° differ from 180° to approach a value of 120°, typi-
Figure 3. Molecular structure of 3 (ellipsoids correspond to the
50% probability level).
NMR-tube experiments indicate that 2 did not react with
1,4-bis(trimethylsilyl)-1,3-butadiyne even when harsh con-
ditions were applied (a C6D6 sample of the mixture was
heated at 110 °C for 48 hours). This experiment also indi-
cates that 2 itself is rather stable in solution at high tem-
perature since no decomposition was observed. Complex 2
is capable of insertion reactions as it reacts with one equiva-
lent of acetone in hexane and gives 4 in quantitative yield.
Complex 4 has been isolated as red crystals and X-ray
analysis[12] reveals two independent molecules per asymmet-
ric unit along with one hexane solvate molecule. The molec-
ular structure of 4 is shown in Figure 4. The 1H NMR spec-
Figure 2. Molecular structure of 2 (ellipsoids correspond to the
50% probability level). Selected bond lengths (Å) and angles (°):
C1–C1A 1.338(13), C1–Ti1 2.090(7), Ti1–N1 2.058(5), Ti1–N2
2.152(5); C1–C1A–Si1 138.9(2), C1–C1A–Ti1 71.33(18), N1–Ti1–
N1A 121.4(3), C1–Ti1–C1A 37.3(4), N1–Ti1–N2 64.22(18).
2378
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Eur. J. Inorg. Chem. 2008, 2377–2381

Acetylenetitanium Complex Stabilized by Aminopyridinato Ligands
trum of 4 shows two sets of signals for the non-equivalent
SiMe3 groups at δ = 0.26 and 0.42 ppm and two sets of
signals for the aminopyridinato ligands. The C(1)–C(2) dis-
tance of 1.370(4) Å and the C3–O1 distance [1.432(3) Å] are
in the expected ranges for C–X (X = C, O) bonds.[14]
Scheme 2. Atom numbering in Ap ligand of Ap complexes.
Synthesis of 1: Ether (15 mL) was added to 2-[(2,6-diisopro-
pylphenyl)amino]pyridine (0.508 g, 2 mmol) and (CH3)2NTiCl3
(0.198 g, 1 mmol) at ambient temperature. The resulting purple
solution was stirred overnight and then filtered in order to separate
a dark product. The filtrate was kept at low temperature to afford
crystalline material; yield (0.598 g, 95.67%). C34H42Cl2N4Ti·C7H8
(717.64): calcd. C 68.62, H 7.02, N 7.81; found C 68.50, H 7.12, N
1
7.68. H NMR (250 MHz, C6D6): δ = 0.92 (d, J = 6.8 Hz, 12 H,
H15,16,17,18), 1.21 (d, J = 6.8 Hz, 12 H, H15,16,17,18), 3.28 (br. s, 2 H,
H13,14), 5.52 (d, J = 8.4 Hz, 1 H, H3), 5.96 (m, 2 H, H5), 6.70 (m,
2 H, H4), 7.10–7.18 (m, 6 H, H9,10,11), 7.78 (d, J = 5.1 Hz, 2 H,
H6) ppm. 13C NMR (63 MHz, C6D6): δ = 24.5 (C15,16,17,18), 25.7
(C15,16,17,18), 28.3 (C13,14), 105.2 (C3), 113.9 (C5), 124.7 (C9,11),
137.8 (C4), 141.8 (C4), 142.2 (C8,12), 143.9 (C7), 145.2 (C6), 169.6
(C2) ppm.
Figure 4. Molecular structure of 4 (ellipsoids correspond to the
50% probability level). Selected bond lengths (Å) and angles (°):
C1–C2 1.370(4), C1–Ti1 2.178(3), C3–O1 1.432(3), Ti1–N1
2.263(3), Ti1–N2 2.008(3), O1–Ti1 1.776(2); O1–Ti1–C1 79.83(11),
N2–Ti1–N1 61.99(10).
Synthesis of 2: Bis(trimethylsilyl)acetylene (0.44 mL, 1.92 mmol)
was added to 1 (1.2 g, 1.92 mmol) and Mg (0.066 g, 2.72 mmol) in
THF (20 mL). The solution was stirred overnight at room tempera-
ture during which time the colour changed from purple to blue.
The solvent was evaporated in vacuo and the product extracted
with hexane (2ϫ20 mL). The filtrate was kept at low temperature
to afford dark blue crystals of 2; yield 0.632 g (45.4%).
C42H60N4Si2Ti (724.99): calcd. C 59.58, H 8.34, N 7.73; found C
Conclusions
Complex 2 can be prepared from [Ap2TiCl2] via re-
duction with magnesium in the presence of bis(trimethylsi-
lyl)acetylene. The spectroscopic data of 2 justifies it having
a weakly bound acetylene complex. Compound 2 is not
only quite stable at room temperature but also at high tem-
perature under argon atmosphere. The reduction of [Ap2-
TiCl2] using KC8 in the absence of any protecting ligand
leads to a homoleptic tris(aminopyridinato)titanium com-
plex. Complex 2 is capable of insertion reactions as it reacts
with one equivalent of acetone. In future attempts we are
interested in exploring the reactivity of 2 and related tita-
nium as well as zirconium aminopyridinates.
1
58.97, H 8.21, N 7.76. H NMR (300 MHz, C6D6): δ = 0.16 (s, 18
H, SiMe3), 0.84 (d, J = 6.9 Hz, 6 H, H15,16,17,18), 1.04 (d, J =
6.9 Hz, 6 H, H15,16,17,18), 1.17 (d, J = 6.9 Hz, 6 H, H15,16,/7,18), 1.20
(d, J = 6.9 Hz, 6 H, H15,16,/7,18), 2.86 (sept, J = 6.9 Hz, 2 H, H13,14),
3.42 (sept, J = 6.9 Hz, 2 H, H13,14), 5.48 (d, J = 8.4 Hz, 2 H, H3),
5.86 (t, J = 6.1 Hz, 2 H, H5), 6.63 (t, J = 7.2 Hz, 2 H, H4), 7.21
(m, 6 H, H9,10,11), 7.68 (d, J = 5.1 Hz, 2 H, H6) ppm. 13C NMR
(100 MHz, C6D6): δ = 1.5 [C2(SiMe3)2], 24.3 (C15,16,17,18), 25.2
(C15,16,17,18), 25.7 (C15,16,17,18), 27.8 (C13,14), 28.5 (C13,14), 105.0
(C3), 111.2 (C5), 124.5 (d, C9,11), 126.5 (C9,11), 140.2 (C8,12), 141.3
(C10), 144.8 (C4), 145.7 (C7), 150.4 (C6), 160.3 (C2), 209.7
[C2(SiMe3)2] ppm.
Synthesis of 3: THF (30 mL) was added to 1 (1.251 g, 2.00 mmol)
and KC8 (1.083 g, 8.00 mmol) at ambient temperature. Warming
up was observed and suddenly the colour changed to black. The
reaction mixture was stirred under N2 for 24 hours. The solvent
was evaporated in vacuo and the product extracted with hexane.
The dark filtrate was kept at room temperature to afford red crys-
tals of the product; yield 0.205 g (12.7%). C51H63N6Ti (807.95):
calcd. C 75.81, H 7.86, N 10.40; found C 74.94, H 7.86, N 10.86.
1H NMR (400 MHz, C6D6): δ = –11.79 (s), –8.33 (s), 0.38 (s), 0.86–
1.23 (br. m), 1.40 (d), 1.62 (s), 2.98 (s), 3.57 (s), 4.05 (br. s), 5.78
(d), 6.11 (br. s), 6.94 (d), 7.02 (d), 7.96 (s), 8.38 (br. s), 9.82 (s)
ppm.
Experimental Section
General Procedures: All manipulations were performed with rigor-
ous exclusion of oxygen and moisture in Schlenk-type glassware on
a dual manifold Schlenk line or in a N2-filled glove box (mBraun
120-G) with a high-capacity recirculator (Ͻ0.1 ppm O2). Solvents
were dried by distillation from sodium wire/benzophenone. Deuter-
ated solvents were obtained from Cambridge Isotope Laboratories
and were degassed, dried and distilled prior to use. NMR spectra
were recorded with Bruker (250 MHz), Varian Inova (300 MHz)
or Varian Inova (400 MHz) spectrometers at ambient temperature.
Atom labelling is shown in Scheme 2. The chemical shifts are refer-
Synthesis of 4: Acetone (0.02 mL, 0.28 mmol) was added to 2
(0.2 g, 0.28 mmol) in hexane (5 mL) at room temperature; the col-
our of the mixture changed from blue to dark red. The solution
was stirred for five minutes and then kept at low temperature to
1
enced to internal TMS for H and 13C.
Eur. J. Inorg. Chem. 2008, 2377–2381
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2379

A. Noor, R. Kempe
SHORT COMMUNICATION
Nikiforov, P. Credson, S. Gambarotta, I. Korobkov, P. H. M.
Budzelaar, Organometallics 2007, 26, 48–55.
For review articles on aminopyridinato ligands see: a) R.
Kempe, Angew. Chem. 2000, 112, 478–504; Angew. Chem. Int.
Ed. 2000, 39, 468–493; b) R. Kempe, H. Noss, T. Irrgang, J.
Organomet. Chem. 2002, 647, 12–20; c) R. Kempe, Eur. J. Inorg.
Chem. 2003, 791–803.
a) N. M. Scott, T. Schareina, O. Tok, R. Kempe, Eur. J. Inorg.
Chem. 2004, 3297–3304; b) N. M. Scott, R. Kempe, Eur. J. In-
org. Chem. 2005, 1319–1324; c) W. P. Kretschmer, A. Meetsma,
B. Hessen, T. Schmalz, S. Qayyum, R. Kempe, Chem. Eur. J.
2006, 12, 8969–8978; d) W. P. Kretschmer, A. Meetsma, B.
Hessen, N. M. Scott, S. Qayyum, R. Kempe, Z. Anorg. Allg.
Chem. 2006, 632, 1936–1938; e) W. P. Kretschmer, B. Hessen,
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692, 4569–4579; f) S. M. Guillaume, M. Schappacher, N. M.
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45, 3611–3619; g) A. M. Dietel, O. Tok, R. Kempe, Eur. J. In-
org. Chem. 2007, 4583–4586; h) R. Kempe, Chem. Eur. J. 2007,
13, 2764–2773; i) G. G. Skvortsov, G. K. Fukin, A. A. Trifonov,
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562.
afford red crystals of 4; yield 0.186 g (80.4%). C45H66N4OSi2Ti·0.5
hexane (826.16): calcd. C 69.78, H 8.91, N 6.78; found C 70.19, H
8.87, N 6.95. 1H NMR (400 MHz, C6D6): δ = 0.26 (s, 9 H, -SiMe3),
0.42 (s, 9 H, -SiMe3), 0.72 (d, J = 6.6 Hz, 6 H, H15,16,17,18), 0.87 (d,
J = 6.6 Hz, 3 H, H15,16,17,18), 1.12 (s, 3 H, CH3), 1.18 (d, J = 6.6 Hz,
3 H, H15,16,17,18), 1.20 (d, J = 6.6 Hz, 3 H, H15,16,17,18), 1.28 (s, 3
H, HCH3), 1.33 (d, J = 6.9 Hz, 3 H, H15/16/17/18), 1.50 (d, J = 6.9 Hz,
3 H, H15/16/17/18), 2.22 (sept, J = 6.6 Hz, 1 H, H13/14), 2.35 (sept, J
= 6.6 Hz, 1 H, H13/14), 3.87 (sept, J = 6.9 Hz, 1 H, H13/14), 4.06
(sept, J = 6.9 Hz, 1 H, H13/14), 5.60 (d, J = 8.5 Hz, 1 H, H3), 5.73
(d, J = 8.7 Hz, 1 H, H3), 5.90 (t, J = 6.4 Hz, 1 H, H5), 6.06 (t, J =
6.0 Hz, 1 H, H5), 6.72 (m, 1 H, H4), 6.87 (m, 1 H, H4), 7.04–7.31
(m, 6 H, H9,10,11), 7.69 (dd, 1 H, H6), 8.20 (dd, 1 H, H6) ppm.
13C NMR (100 MHz, C6D6): δ = 4.5 (-SiMe3), 6.0 (-SiMe3), 25.0
(C15,16,17,18), 25.2 (C15,16,17,18), 25.7 (C15,16,17,18), 26.3 (C15,16,17,18),
27.3 (C15,16/17,18), 27.6 (C15,16/17,18), 28.2 (C13/14), 28.4 (C13/14), 28.6
(C13/14), 28.7 (C13/14), 29.2 (CCH3), 31.9 (CCH3), 92.8 (CO), 106.0
(C3), 107.3 (C3), 108.7 (C5), 111.4 (C5), 123.2 (C9/11), 124.0 (C9/11),
124.6 (C9/11), 125.4 (C9/11), 125.7 (C9/11), 126.6 (C9/11), 139.9 (C8,12),
140.8 (C8,12), 143.7 (C4), 143.9 (C4), 144.6 (C10), 144.7 (C7), 145.1
(C7), 145.4 (C6), 145.7 (C6), 170.2 (C2), 173.0 (Ti-C=C*), 186.9
(Ti-C*=C) ppm.
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0.2251, wR2 (obs.) = 0.1781; 4: space group: P1, a =
¯
10.4280(8) Å, b = 13.532(11) Å, c = 18.1300(15) Å; α =
95.933(6), β = 106.305(6), γ = 101.491(6) °, V = 2371.5(3) Å3,
z = 2, reflections collected/unique/observed: 8950/8950/3513,
parameters: 511, R (obs.) = 0.0425, wR2 (all data) = 0.0805,
wR2 (obs.) = 0.0692; X-ray crystal structure analysis was car-
ried out with a STOE IPDS II diffractometer equipped with
an Oxford Cryostream low-temperature unit. CCDC-676560
(for 1), -676561 (for 2), -676562 (for 3) and -676563 (for 4)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_r-
equest/cif.
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Received: February 1, 2008
Published Online: April 9, 2008
Eur. J. Inorg. Chem. 2008, 2377–2381
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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