Redox-Disproportionation of a Decamethyltitanocene(III) Isonitrile Alkynyl Complex

Article abstract of DOI:10.1002/chem.201701374

A mixed decamethyltitanocene(III) isonitrile alkynyl complex (7) was synthesized by the sequential introduction of the isonitrile and alkynyl ligands. Direct synthesis results in the formation of the diamagnetic decamethyltitanocene bis(isonitrile) (2) an

Full text of DOI:10.1002/chem.201701374

A Journal of
Accepted Article
Title: Redox-disproportionation of a decamethyltitanocene(III) isonitrile
alkinyl complex
Authors: Torsten Beweries, Fabian Reiss, Dirk Hollmann, Haijun Jiao,
Perdita Arndt, Anke Spannenberg, Uwe Rosenthal, and Kai
Altenburger
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Redox-disproportionation of a decamethyltitanocene(III) isonitrile
alkinyl complex
Fabian Reiß,[a] Kai Altenburger,[a] Dirk Hollmann,[a] Anke Spannenberg,[a] Haijun Jiao,[a] Perdita Arndt,[a]
Uwe Rosenthal,[a] Torsten Beweries*[a]
Dedicated to Karel Mach on the occasion of his 80th birthday.
Abstract:
A
mixed decamethyltitanocene(III) isonitrile alkinyl
(hetero)metallacycles.
complex (7) was synthesised via sequential introduction of the
isonitrile and alkinyl ligands. Direct synthesis results in the formation
of the diamagnetic decamethyltitanocene bis(isonitrile) (2) and
Results and Discussion
bis(alkinyl)
(3)
complexes.
Compound
7
undergoes
disproportionation at room temperature to give
2
and 3. All
In order to evaluate the possibility of a direct synthesis of a
mixed titanocene(III) isonitrile alkinyl, we performed a reaction of
Cp*2TiCl (1) with 2,6-dimethylphenylisonitrile (XyNC) and lithium
alkinyl compound LiC2SiMe3 (Liac) (Scheme 1). The latter was
prepared by lithiation of trimethylsilylacetylene, isolated as a
colourless solid in good yield and unequivocally identified based
on its NMR (29Si:  -30.3 ppm) and Raman data (1985 cm-1).8
After workup in n-hexane we observed precipitation of a mixture
of two different crystalline compounds, which could not be
isolated by fractional crystallisation but were successfully
separated by crystal picking. The 1H NMR spectrum of this
mixture clearly reveals the presence of two different diamagnetic
decamethyltitanocene complexes (C5Me5 resonances at  1.88
and 1.95 ppm, respectively, Figure 2a).
complexes were fully characterised by IR, MS as well as by NMR
and EPR, respectively. Molecular structures for complexes 2 and 7
are reported. The stability and reactivity of these complexes are
rationalised by DFT computations.
Introduction
The formation of metallacycles by intermolecular coupling of
unsaturated substrates or intramolecular coupling of ligands is
known in organometallic chemistry and has allowed for the
synthesis of highly strained and exotic structures.1
Heterometallacycles as well as all-C metallacycles with exocyclic
heteroatoms can be accessed by using substrates and/or
ligands such as CO2 2, phosphaalkynes3, nitriles4, isonitriles5, etc.
The versatility of isonitriles as ligands was thoroughly reviewed
only recently.6 We have demonstrated that isonitriles are
versatile ligands for the realisation of a variety of coordination as
well as C-C and C-N coupling products at zirconocene and
hafnocene fragments.5a Depending on the ratio of metallocene
precursor and isonitrile, complexes showing different unusual
structural motifs were identified and interconversion reactions
were investigated. In general, exocyclic N substitution was
predicted to be favourable for the realisation of small, highly
strained metallacycles.7 We thus considered combination of an
isonitrile fragment with an alkyne or alkinyl complex as a
promising approach to access these. In this contribution, we
Scheme 1. Reaction of complex 1 with Liac and XyNC.
As we considered the formation of the bis(isonitrile) complex
2 and the bis(alkinyl) complex 3, we next aimed for the synthesis
and characterisation of both complexes individually. Reaction of
the alkyne complex Cp*2Ti(Me3SiC2SiMe3) with XyNC in toluene
cleanly yields the bis(isonitrile) complex 29 in good yield
(Scheme 2). Indeed, 1H NMR analysis shows that 2 matches
one of the species mentioned before (singlets at  1.88 and 2.44
ppm, respectively). This reaction is computed to be exergonic by
29.36 kcal/mol, indicating the easy formation and high stability of
complex 2.
report the synthesis and characterisation of
a
mixed
decamethyltitanocene isonitrile alkinyl complex. In solution this
complex shows an equilibrium with the corresponding
bis(isonitrile) and bis(alkinyl)complexes via disproportionation.
These complexes can be regarded as potential precursors for
the design of small, highly strained and exotic four membered
[a]
Dr. F. Reiß, Dr. K. Altenburger, Dr. D. Hollmann, Dr. A.
Spannenberg, PD Dr. H. Jiao, Dr. P. Arndt, Prof. Dr. U. Rosenthal,
Dr. habil. T. Beweries
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
E-mail: torsten.beweries@catalysis.de
Scheme 2. Synthesis of 2.
Supporting information for this article is given via a link at the end of
the document.
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Single crystals suitable for an X-ray analysis were obtained
from saturated n-hexane solution at -78 °C. In the solid state two
inequivalent isonitrile ligands coordinate to the titanium center
(Figure 1). The C-N bond distances (C1-N1 1.196(2) and C2-N2
1.183(2) Å) are in the same range and are slightly elongated due
to complexation (c.f. C-N 1.160(3)/1.161(2) Å in the free isonitrile
XyNC).10 Both isonitrile fragments in 2 display a significant
deviation from linearity of the C≡N-C(Xy) unit [C1-N1-C3
149.23(16), C2-N2-C11 171.08(17)°]. This observation was
reported before for the complex [Cp*2Ti(1-CNMes)2] [C-N-
reaction is exergonic by 12.02 kcal/mol, indicating
thermodynamically very favoured reaction.
a
ATR-IR data for the triple bond in 3 resemble those reported
before by Mach (2016 cm-1). Also, the NMR data are in the
expected range with the signal for the -C atom showing a 13C
resonance at  182.7 ppm. In the 29Si NMR spectrum a slight
downfield shift is found compared to Liac ( -25.3 ppm). The
optimised structure of complex 3 has C2 symmetry and two
equivalent alkinyl ligands. In contrast to complex 2, the C-C-Si
moiety in complex 3 is only slightly bent (178.4°). This is in line
with the d0 Ti(IV) center, which does not have free valance
electrons to donate back to the ligands.
C(Mes) 142.4(2)/176.1(2)°; Mes = 2,4,6-trimethylphenyl].11
A
possible explanation was given by Filippou et al., who reported
that a very strong back donation from the metallocene fragment
to the isonitrile ligand induced the bending.12 The
cyclopentadienyl ligand seems to have a strong influence as on
one hand the complex [{Me2Si(5-C5H4)2}Ti(1-CNXy)2]13 shows
only
small
deviations
from
linearity
[C-N-C(Xy)
176.3(5)/170.3(5)8] on the other hand the complex rac-
(ebthi)Ti(1-CNXy)2]5a exhibits two bent isonitrile units [C-N-
C(Xy) 154.14/155.24°]. The optimised gas-phase structure of
complex 2 has C2 symmetry and two equivalent isonitrile ligands.
It also shows the deviation of the linear C≡N-C moiety in the free
ligand to the bent complexed ligand by 156.3°; and the C-N triple
bond is slightly elongated from 1.185 Å in the free ligand to
1.214 Å in 2, revealing the strong back donation from the formal
d2 Ti(II) center to ligands.
Scheme 3. Synthesis of 3.
A comparison of 1H NMR spectra of isolated 2 and 3 with the
1H NMR spectrum of the mixture as obtained from the reaction
shown in Scheme 1 shows that – as assumed – the proposed
Ti(II) bis(isonitrile) and Ti(IV) bis(alkinyl) complex were indeed
formed in a redox disproportionation reaction of the Ti(III)
precursor Cp*2TiCl (Figure 2). Similar reactions were described
before for titanocenes, e.g. by Bercaw for the system
[Cp*2TiH]/[Cp*2Ti]/[Cp*2TiH2]15; however, well-defined examples
are rare. Ti(III) halides are known to undergo disproportionation
reactions to Ti(II) and Ti(IV) species under drastic conditions.16
Figure 1. Molecular structure of 2 in the solid state. Hydrogen atoms are
omitted for clarity. The thermal ellipsoids correspond to 30% probability.
ATR-IR spectra of complex 2 shows the expected sharp
signal at 2115 cm-1 due to the C≡N vibration, which is in good
agreement with the observed signal in rac-(ebthi)Ti(1-CN-Xy)2.
In addition, broad signals at 3367 and 1813 cm-1 were observed,
which point towards extreme moisture sensitivity of these
complexes.
Next, we prepared the proposed bis(alkinyl) complex 3,
which – to our surprise – was not isolated in pure form to date.
However, it should be noted that such complexes were
investigated before by the groups of Rosenthal and Mach.14
Indeed, Mach and co-workers identified 3 as an intermediate by
means of IR spectroscopy.14a Reaction of Cp*2TiCl2 with Liac at
low temperature furnishes the desired product in moderate yield
(Scheme 3). To avoid the quantum chemical problem of LiCl
formation/solvation, we computed the exchange of two chlorides
by two alkinyl anions resulting in complex 3; and this exchange
Figure 2. 1H NMR spectra of crystal mixture (a) as obtained from the reaction
shown in Scheme 1 and isolated complexes 2 (b) and 3 (c) (*: [D6]benzene).
For this reason, we changed our strategy to prepare a mixed
Ti isonitrile-alkinyl complex. Addition of one equivalent of XyNC
to Cp*2TiCl gives the Ti(III) coordination product 6, which can be
isolated as a brownish solid in good yields (Scheme 4). As in
complex 2, the optimised structure of complex 6 has the bent C-
C-N moiety (168.9°). Under stoichiometric condition, the
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formation of complex 6 from complex 1 via isonitrile coordination
is only slightly endergonic by 1.88 kcal/mol, indicating a ligand
mediated equilibrium between complexes 1 and 6.
(168.4°) and C-C-Si (175.2°) moieties. As found experimentally,
the C-N bond in 7 is shorter than in 2 (1.195 vs. 1.214 Å).
However, both C-N bonds in 2 and 7 are longer than that in the
free ligand (1.185 Å). Indeed, such structural changes in C-N as
well as C-Ti bond lengths and C-N-C bond angle from
complexes 2 to 7 is associated with the formal oxidation state of
the Ti center. The Ti(II) in 2 with d2 configuration can have
stronger back donation than Ti(III) in 7 with d1 configuration.
Scheme 4. Synthesis of complex 7 via 6.
The paramagnetic nature of 6 was investigated by EPR
spectroscopy. 6 shows an EPR signal of an isolated Ti(III) centre
with a g value (g = 1.9911) comparable with previously isolated
[Cp*2Ti(III)] complexes (Figure 3).17 Furthermore the signal
shows the typical hyperfine structure (hfs) splitting (ATi = 10.3 G)
which results from the coupling of the single electron of Ti(III) to
the nuclear spin of the isotopes 47Ti (I = 5/2, 7.44 % natural
abundance) and 49Ti (I = 7/2, 5.41 % natural abundance).
Figure 4. Molecular structure of 7 in the solid state. Hydrogen atoms and co-
crystallised n-hexane are omitted for clarity. The thermal ellipsoids correspond
to 30% probability.
NMR spectra of the isolated crystals of 7 show apart from a
broad paramagnetic resonance due to the compound ( 2.5 - 7.5
ppm) signals that can be assigned to complexes 2 and 3 which
points towards redox equilibrium between 7 and 2/3. (Figure S3).
Similarly, when NMR samples of a mixture of 2 and 3 were
heated to T = 60 °C for two days, a significant decay of the
signals of the diamagnetic species 2 and 3 was observed
(Figure 5) due to the formation of the paramagnetic species 7
which unambiguously was confirmed by EPR spectroscopy
(Figure S4).
Figure 3. Comparison of EPR spectra of 6 and 7 at T = 300 K in toluene
(black: experimental spectrum, red: simulation).
However, the reaction from complex 6 to complex 7 via
chloride exchange by alkinyl anion is exergonic by 3.34 kcal/mol.
Reaction of 6 with Liac results in the formation of the Ti(III)
complex 7, which can be obtained in yield up to 73% after
workup using n-hexane. Again EPR spectroscopy revealed the
paramagnetic nature of an isolated Ti(III) complex (Figure 3, g =
1.9941, ATi = 7.9 G).
The molecular structure (Figure 4) shows the titanium centre
in distorted tetrahedral coordination environment with two Cp*
ligands and the alkinyl and the isonitrile moieties. Bond lengths
are in the expected range (C1-C2 1.209(2), C6-N1 1.162(2) ),
however, also in this case, the C-N-C(Xy) fragment is slightly
bent (C6-N1-N7 175.03(18)°). The C-N bond in 7 is significantly
shorter compared to those in 2, on the other hand the C-Ti
bonds in 7 (C1-Ti1 2.135(2), C6-Ti1 2.130(2) Å) are significant
longer compared to those in 2 (C1-Ti1 2.025(2), C2-Ti1 2.071(2)
Å). To the best of our knowledge, 7 represent the first
structurally characterised mixed metallocene isonitrile alkinyl
complex. The optimised structure of complex 7 has bent C-N-C
Figure 5. 1H NMR spectra of the synproportionation reaction of complexes 2
(+) and 3 (#) to yield the paramagnetic complex 7 (*: [D8]toluene).
Indeed, such equilibrated redox property has been confirmed
by computation. The disproportionation of the mixed isonitrile
alkinyl complex 7 into complexes 2 and 3 is exergonic by 0.70
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[1]
[2]
[3]
[4]
[5]
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kcal/mol and has an equilibrium constant of 3.26 in favour of the
formation of 2 and 3. In turn, this can also explain the observed
synproportionation from complexes 2 and 3 to complex 7 at
elevated temperature and elongated reaction time.
Selected examples: a) P.-M. Pellny, F. G. Kirchbauer, V. V. Burlakov,
W. Baumann, A. Spannenberg, U. Rosenthal, J. Am. Chem. Soc. 1999,
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Scheme 5. Disproportion of complex 7 into 2 and 3.
In previous studies, light-induced intramolecular coupling of
bis(alkinyl)
compounds
gave
unusual
five-membered
Selected examples: a) K. Altenburger, P. Arndt, L. Becker, F. Reiß, V. V.
Burlakov, A. Spannenberg, W. Baumann, U. Rosenthal, Chem. Eur. J.
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Zhang, Y. Yang, Y. Hua, G. Liu, Z. Lin, H. Xia, J. Am. Chem. Soc. 2017,
139, 1822.
metallacyclopentatri-2,3,4-ene (i.e. metallacyclocumulene)[2a]
structures. In order to evaluate if this is also possible for the
novel isonitrile alkinyl complex 7 we irradiated using UV and
visible light. However, apart from the aforementioned
disproportionation reaction, no further reactions were observed.
Further studies on the reactivity of 7 and related compounds are
ongoing in our group.
[6]
[7]
V. P. Boyarskiy, N. A. Bokach, K. V. Luzyanin, V. Yu. Kukushkin, Chem.
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[8]
[9]
For detailed information see the Supporting Information.
Bis(isonitrile) complexes were described before. Examples: a) L. B.
Kool, M. D. Rausch, H. G. Alt, H. E. Engelhardt, M. Herberhold, J.
Organomet. Chem. 1986, 317, C38. b) Y. A. Wanniarachchi, L. M.
Slaughter, Chem. Commun. 2007, 3294.
Conclusions
We have reported the synthesis of a mixed isonitrile alkinyl
complex (7). Direct reaction of the isonitrile and the alkinyl
ligands with the Ti(III) precursor results in the formation of two
[10] For XyNC two independent molecules were found in the asymmetric
unit: T. Mathieson, A. Schier, H. Schmidbaur, J. Chem. Soc., Dalton
Trans. 2001, 1196.
diamagnetic
species
which
were
identified
as
[11] M. Hähnel, M. Ruhmann, T. Beweries, S. Roy, T. Beweries, P. Arndt, A.
Spannenberg, A. Villinger, E. D. Jemmis, A. Schulz, U. Rosenthal, J.
Am. Chem. Soc. 2012, 134, 15997.
decamethyltitanocene(II)
bis(isonitrile)
(2)
and
decamethyltitanocene(IV) bis(alkinyl) complex (3). The formation
can be explained by a rare example of disproportionation of the
paramagnetic Ti(III) complex. The mixed isonitrile alkinyl
complex 7 can, however, be accessed by sequential addition of
the ligands starting from Cp*2TiCl by coordination of one XyNC
ligand, followed by an addition of the alkinyl precursor LiC2SiMe3.
Interestingly mixed isonitrile alkinyl complex 7 undergoes a
redox disproportionation under comparable mild conditions to
give the bis(isonitrile) complex 2 and the bis(alkinyl) complex 3.
The redox relationship between these complexes was examined
by NMR and EPR experiments as well as in a computational
study. Regarding structures, stability and reactivity of the
compounds full interplay between theory and experiment has
been clearly demonstrated.
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Chem. 1993, 455, 129.
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[15] J. E. Bercaw, J. Am. Chem. Soc. 1974, 96, 5087.
[16] For TiCl3 temperatures in the range of 600-800 K are necessary to form
TiCl2 and TiCl4 (cf. B. S. Sanderson, G. E. MacWood, J. Phys. Chem.
1956, 60, 316).
[17] Selected examples: a) P. T. Witte, R. Klein, H. Kooijman, A. L. Spek, M.
Polášek, V. Varga, K. Mach, J. Organomet. Chem. 1996, 519, 195. b) R.
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́
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Inorg. Chem. 2011, 627. d) C. Godemann, L. Dura, D. Hollmann, K.
Grabow, U. Bentrup, H. Jiao, A. Schulz, A. Brückner, T. Beweries,
Chem. Commun. 2015, 51, 3065. e) F. Reiß, K. Altenburger, L. Becker,
K. Schubert, H. Jiao, A. Spannenberg, D. Hollmann, P. Arndt, U.
Rosenthal, Chem. Eur. J. 2016, 22, 3361.
Acknowledgements
We thank our technical and analytical staff for assistance.
Financial support by the DFG (RO 1269/9-1) is gratefully
acknowledged.
Keywords: titanium • metallocenes • X-ray diffraction • EPR
spectroscopy • DFT calculations
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Entry for the Table of Contents
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Titanocenes*
F. Reiß, K. Altenburger, D. Hollmann, A.
Spannenberg, H. Jiao, P. Arndt, U.
Rosenthal, T. Beweries*
Page No. – Page No.
Stepwise introduction of isonitrile and alkinyl ligand gives a mixed
decamethyltitanocene(III) isonitrile alkinyl complex, which undergoes redox
disproportionation at room temperature to yield two well-defined diamagnetic Ti
complexes.
Redox-disproportionation of a
decamethyltitanocene(III) isonitrile
alkinyl complex
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