Titanocene Silylpropyne Complexes: Promising Intermediates en route to a Four-Membered 1-Metallacyclobuta-2,3-diene?

Article abstract of DOI:10.1002/chem.201703444

Coordination of the alkyl-substituted alkynes Me₃SiC₂CH₂R (1: R=SiMe₃; 2: R=N(SiMe₃)₂) to titanocene centres [Cp′₂Ti] (Cp′=Cp, Cp*) yields stable alkyne complexes of the type Cp′₂Ti(η²-Me₃SiC₂CH₂R) (3: Cp′=Cp, R=SiMe₃; 5: Cp′=Cp, R=N(SiMe₃)₂; 6: Cp′=Cp*, R=SiMe₃) that are not prone to alkyne/allene isomerisation. When reacting alkyne 2 with Cp^*₂TiCl₂ and Mg formation of the complex Cp*₂Ti(III)(η³-Me₃SiC₂CH₂) (7) which displays a propargylic unit coordinated to the Ti^III centre takes place. All complexes were fully characterised, the molecular structures for 5, 6, and 7 are discussed.

Full text of DOI:10.1002/chem.201703444

DOI: 10.1002/chem.201703444
Communication
&
Metallacycles
Titanocene Silylpropyne Complexes: Promising Intermediates en
route to a Four-Membered 1-Metallacyclobuta-2,3-diene?
Fabian Reiß, Melanie Reiß, Anke Spannenberg, Haijun Jiao, Dirk Hollmann, Perdita Arndt,
[a]
Uwe Rosenthal, and Torsten Beweries*
nificantly, thus allowing for the isolation of small hetero-metal-
Abstract: Coordination of the alkyl-substituted alkynes
Me SiC CH R (1: R=SiMe ; 2: R=N(SiMe ) ) to titanocene
[3]
lacycles. In a computational study, we have shown that it
3
2
2
3
3 2
should in principle be possible to stabilise highly strained all-C-
centres [Cp’ Ti] (Cp’=Cp, Cp*) yields stable alkyne com-
2
1
-metallacyclobuta-2,3-dienes by introduction of N substitu-
2
plexes of the type Cp’ Ti(h -Me SiC CH R) (3: Cp’=Cp, R=
[4]
2
3
2
2
ents at the periphery of the desired metallacycle.
SiMe ; 5: Cp’=Cp, R=N(SiMe ) ; 6: Cp’=Cp*, R=SiMe )
3
3 2
3
Following up on this, we have attempted to realize this
using coordination and coupling reactions at group 4 metallo-
cene complexes with carbodiimides, nitriles, and isonitriles as
that are not prone to alkyne/allene isomerisation. When
*
reacting alkyne 2 with Cp TiCl and Mg formation of the
2
2
3
complex Cp* Ti(III)(h -Me SiC CH ) (7) which displays a
[5]
2
3
2
2
reaction partners. Alternatively, use of alkyl substituted al-
III
propargylic unit coordinated to the Ti centre takes place.
All complexes were fully characterised, the molecular
structures for 5, 6, and 7 are discussed.
kynes appears to be a promising approach as protons in a po-
sition to the CꢀC bond can in principle be removed by depro-
tonation to enable ring extension that could yield four-mem-
bered cyclic structures. Also, the relationship of allene struc-
[
6,7]
tures and propynes was discussed before. In a recent contri-
bution, Mach and co-workers discussed alkyne/allene
The chemistry of small, highly strained metallacyclic structures
has attracted considerable attention in the last decades as or-
ganometallic chemists were always motivated to push the
frontiers of structural chemistry and explore possibilities to sta-
[
8]
isomerisation in the coordination sphere of Ti, which can be
regarded as a reason for the lack of detailed studies of the co-
ordination chemistry of CH R substituted alkynes at early tran-
2
[1]
bilise new unusual compounds. For example, orbital symme-
try does not allow a significant bending of allene or cumulene
units. However, in recent years, our group and others have re-
ported on the stabilisation of a variety of exotic structural
motifs such as 1-metallacyclopent-3-ynes (A), 1-metallacyclo-
penta-2,3,4-trienes (B), and 1-metallacyclopenta-2,3-dienes (C)
sition metal centres (Scheme 1, right). In this contribution, we
present attempts to access highly unsaturated, strained four-
membered titanacycloallenes by coordination and activation of
propyne derivatives bearing methylene groups adjacent to the
coordinating alkyne triple bond.
We conceived an isodesmic reaction between Cp’ TiMe and
2
2
[
2]
(
Scheme 1, left). Four-membered, all-carbon based ring sys-
the allene precursors Me(Me Si)C=C=C(SiMe )Me as well as
3 3
tems of early transition metals (D) were not reported to date,
however, our group has demonstrated on several occasions
that incorporation of heteroatoms releases the ring strain sig-
Me(tBu)C=C=C(tBu)Me for comparison to get a deeper insight
into the chemistry of these titanacyclobuta-2,3-dienes
(Scheme 2).
Furthermore we considered the corresponding metallacyclo-
penta-2,3,4-trienes for the analogue isodesmic reaction
(
Table 1). The metallacyclopenta-2,3,4-triene Cp Ti(tBuC tBu) is
2 4
experimentally available, nicely expressed by its Gibbs free
À1
energy of À19.86 kcalmol , and therefore we choose it as
[
9]
benchmark value. The calculations clearly reveal the combi-
nation of decamethyltitanocene and tert-butyl substituents at
the allene unit as inappropriate due to the endergonic Gibbs
free energies for the considered reactions. Also, all remaining
cases show slightly exergonic values. With a closer look to the
Scheme 1. Schematic representation of selected metallacycles (left), [Ti]
mediated alkyne/allene isomerisation (right).
[
a] Dr. F. Reiß, Dr. M. Reiß, Dr. A. Spannenberg, Dr. H. Jiao, Dr. D. Hollmann,
Dr. P. Arndt, Prof. Dr. U. Rosenthal, Dr. 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
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201703444.
Scheme 2. Isodesmic reaction to titanacyclobuta-2,3-dienes.
Chem. Eur. J. 2017, 23, 14158 – 14162
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Communication
side-on coordination of the triple bond as it was observed
À1
Table 1. Calculated Gibbs free energies (kcalmol ) for the syntheses of
-metallacyclobuta-2,3-dienes and metallacyclopenta-2,3,4-trienes.
[
12]
before for titanocene alkyne complexes. This is further corro-
1
13
borated by the C NMR spectrum, which shows downfield sig-
nals for the coordinated alkyne at 226.8 and 206.7 ppm, both
being shifted to lower field significantly compared to the free
alkyne (CCH : 106.0, CSiMe : 83.4 ppm; Table S5). Chemical ion-
Precursor
Cp
2
TiMe
2
2 2
Cp* TiMe
[
a]
Me(Me
Me(Me
3
3
Si)C
Si)C
3
(SiMe
(SiMe
(t-Bu)Me
(t-Bu)Me
3
3
)Me
)Me
À11.77
À23.49
À3.94
À1.75
4
À10.07
+16.63
+3.53
2
3
[a]
Me(t-Bu)C
Me(t-Bu)C
3
isation mass spectrometry (CI-MS) analysis of the oily mixture
shows the molecular ion signal for the postulated alkyne com-
plex at m/z 361. Attempts to optimise the synthesis of this spe-
cies by changing the reducing agent to Mg as this was found
to be the preferred method for most of the titanocene alkyne
4
À19.86
[
a] Energy given for the more stable triplet state.
2
[12]
computed values, Cp Ti(h -Me SiC SiMe ) appeared as the most
complexes failed as also in this case formation of brown oily
2
3
3
3
promising candidate for the formation of a 1-metallacyclobuta-
mixtures containing 3 took place.
2
,3-diene structure with the Gibbs free energy for its formation
As addition of the alkyne 1 to the in situ generated [Cp Ti]
2
À1
of À11.8 kcalmol being exergonic but challenging. It should
furnished the alkyne complex 3, we modified the reaction pro-
tocol for the coordination of 2 and first added nBuLi to the
formal allene precursor, followed by addition of Cp TiCl to this
be noted that we found triplet states in two cases for [Cp* Ti]
2
complexes, which are lower in energy than its corresponding
singlet states (marked with [a] in Table 1). This is in good
agreement with our previously reported results about the reac-
tions of 2-substituted pyridines with titanocenes and zircono-
2
2
mixture. Workup gave a yellow oily residue and NMR analysis
showed the absence of Cp resonances as well as characteristic
signals due to an n-butyl fragment at 0.82–2.23 ppm. Further
analysis indicated the presence of the five-membered hetero-
cycle 4 (Scheme 4), which was formed by addition of nBuLi to
[
10]
cenes.
On the basis of these results we have chosen two trimethyl-
silyl substituted propynes 1 and 2, for this study. We consid-
[11]
ered bis(trimethylsilyl)amino propyne (2) as a very promising
candidate as this contains an amino functionality, which was
computed to have a strong stabilising effect for a 1-metallacy-
clobuta-2,3-diene structure. Furthermore, the reported synthe-
sis of bis(trimethylsilyl)propyne 1 proceeds from the tetrakis(tri-
methylsilyl)allene with two equivalents of trifluoroacetic acid
[15]
Scheme 4. Formation of 4 by activation of 2 (left), comparable to E (right).
(
TFA), nicely demonstrating the isomerisation of the allene into
[
6]
dÀ
d+
propyne. The inverse reaction was described by Barton et al.
the Me SiC ꢀC fragment, considering the partial charges,
3
who transformed 1 into the 1,3-di-litho-1,3-bis(trimethylsilyl)al-
lene in situ with two equivalents nBuLi. This further encour-
aged us to use 1 and 2 as starting materials.
followed by formal MeLi group elimination and intramolecular
cyclisation to 4-buthyl-2,2-dimethyl-1,3-bis(trimethylsilyl)-1-aza-
2-silacyclopent-3-ene (4). A similar nBuLi addition to the CꢀC
bond of octa-3,5-diyne was described by Becker and Klein in
[7]
For coordination and activation of alkyne 1 to [Cp Ti] we en-
2
[
13]
visioned use of nBuLi for reductive activation of the titanocene
1973. Also, Fritz and Bçttinger reported a MeMgCl triggered
ring opening of a 1,3-disila-4-trimethylsilyl-cyclopentene to a
linear ethyne derivative, formally the inverse reaction of the
and deprotonation of the CH group. Reaction of Cp TiCl with
2
2
2
1
in the presence of nBuLi in THF (Scheme 3) gave brown oily
[
14]
residues after workup with n-hexane and removal of all vola-
tiles in vacuum. Formation of significant amounts of a dark-
green paramagnetic by-product, identified as [Cp TiCl] , was
herein observed cyclisation to 4.
After optimisation of the reaction sequence without the use
of any metallocene source, 4 was obtained in good yields.
Apart from the 1,2,2-trimethyl-5-phenyl-3,4-bis(trimethylsilyl)-1-
2
2
observed.
[
15]
aza-2-sila-cyclopent-3-ene (E)
synthesised by Seyferth and
co-workers by insertion of C=N bond into the SiC ring of a sili-
2
rene, compound 4 is a new member of the small group of
these unusual azasilacyclopentenes. (Scheme 4).
Based on these observations, we decided to avoid the use
of nBuLi in order to first prepare the well-known side-on tita-
nocene alkyne complexes. Therefore we evaluated the feasibili-
ty of ligand exchange reactions at the corresponding alkyne
Scheme 3. Synthesis of 3. Method a: nBuLi, THF, À788C to RT, 12 h; Meth-
od b: Mg, THF, RT, 4 h.
2
complexes Cp ’Ti(h -Me SiC SiMe ) (Cp’=Cp, Cp*) as common
2
3
2
3
[
12]
2
All attempts to purify this mixture by crystallisation did not
in this chemistry. In the case of Cp Ti(h -Me SiC SiMe ) ex-
2 3 2 3
1
lead to satisfactory results. The H NMR spectrum of this mix-
change equilibria were observed which are in favour of the
starting materials. Even at elevated temperatures no significant
ture shows the expected resonances for Cp and SiMe protons
3
as the main component. An additional singlet at d=1.34 ppm
conversion was observed; instead decomposition of the de-
2
suggests the presence of a CH group of the intact alkyne 1,
sired complexes 3 and 5 was evident. In the case of Cp *Ti(h -
2
2
thus pointing to the presence of an alkyne complex 3 with
Me SiC SiMe ) no exchange reaction was observed even at ele-
3
2
3
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Communication
vated temperatures and longer reaction times (further details
see Supporting Information). These results are supported by
the low values of the calculated Gibbs free energies for these
exchange reactions (cf. DG values for the combinations Cp and
To our surprise, a different outcome was obtained in the
similar reaction of the alkyne 2 (Scheme 6, right), where a dark
green paramagnetic species was isolated after workup and
crystallisation from n-hexane. The paramagnetic nature of 7
and its structural features were examined by EPR spectroscopy.
1
: À1.65; Cp and 2: À0.33; Cp* and 1: À2.16; Cp* and 2:
À1
III
+
4.95 kcalmol ).
Compound 7 shows an EPR signal of an isolated Ti centre
Hence, using Mg as the reducing agent and isolation of the
with a g value (g=1.9920) comparable with previously report-
[
10,18]
alkyne complexes might exclude the formation of these during
synthesis of the desired four-membered 1-metallacyclobuta-
ed [Cp* Ti(III)] complexes (Figure 1).
2
2
,3-dienes. However, it should be noted that in all cases de-
III
scribed hereafter, the formation of unidentified Ti species as
the side products was observed, thus giving comparably low
[16]
yields of the reported products.
Reaction of bis(trimethylsilyl)amino propyne 2 with Cp TiCl2
2
in the presence of Mg gave the alkyne complex 5 as a yellow
1
solid (Scheme 5). The H NMR spectrum displays characteristic
signals for Cp and CH protons, suggesting the presence of an
2
13
alkyne coordination compound. This is supported by C NMR
Figure 1. EPR spectrum of 7 at 278C in toluene and simulated spectrum
(
g=1.9920; ATi =2.9 G, A2xH =2.2 G, DB=1.6 G).
Scheme 5. Formation of 5.
Furthermore the signal shows the hyperfine structure split-
ting (A =2.9 G) which results from coupling of the single elec-
Ti
III
47
tron of Ti to the nuclear spin of the isotopes Ti (I=5/2,
49
data, showing downfield resonances for the coordinated
7.44% natural abundance) and Ti (I=7/2, 5.41% natural
abundance). This ATi value is remarkably lower compared to
alkyne at 225.8 (CCH ) and 201.2 ppm (CSiMe ). Strong coordi-
2
3
[
18]
nation of the ligand is evident from a comparison with values
the typical values in the range A =8–10 G. Intriguingly, the
Ti
for free 2 (CCH : 109.3, CSiMe : 86.5 ppm). Furthermore, the
small downfield shift of the N resonance in 5 compared to
free 2 confirmed the absence of a direct N···Ti interaction in 5
signal features a super hyperfine splitting by coupling of two
2
3
1
5
CH protons (A =2.2 G), revealing a different coordination
2
2”H
mode of the alkyne compared to the complexes 3, 5 and 6.
Furthermore the CI-MS analysis of the green solid shows a mo-
lecular ion signal at m/z 429 instead of m/z 589 for the expect-
ed side-on alkyne complex. The difference of Dm/z 160 be-
tween these values suggests the loss of the [N(Me Si) ] frag-
[
17]
(
Dd=À11 ppm). In addition the shift to lower wavenumbers
of the CꢀC IR vibration compared to the free alkyne of about
À1
D v˜ (CꢀC)=443 cm and the structural features (see below,
Figure 2, Table S5) unambiguously confirm the classical side-on
coordination mode without any agostic interactions of the CH2
protons to the titanium centre.
3
2
ment, the exact nature of the by-product could not be verified.
This observation is in good agreement with two CꢀC vibra-
À1
To evaluate the influence of the cyclopentadienyl ligand on
the coordination mode of the alkynes 1 and 2, we next investi-
gated reactions of Cp* TiCl . Magnesium reduction of the latter
tions at 1931 and 1903 cm for 7 indicating a much weaker
III
coordination to the Ti centre (Table S5). These values are in
good agreement to the reported CꢀC stretch vibrations of the
2
2
3
3
in the presence of 1 resulted in the formation of a similar side-
on alkyne complex 6 without further activation of the alkyne
early transition metal h -propargylic complexes Cp Zr(Me)(h -
2
[
19]
À1
3
[20]
À1
PhC CH )
1924 cm
and Cp* Y(h -PhC CH )
1923 cm .
2
2
2
2
2
13
(
Scheme 6, left). The C NMR spectrum shows two distinct res-
With all these observations in hand we suggest the formation
III
3
onances for the coordinated carbon atoms at 223.8 (CCH ) and
of a Cp* Ti (h -Me SiC CH ) complex with a coordinated prop-
2
2
3
2
III
2
1
92.9 ppm (CSiMe ). Here again the shift of the CꢀC vibration
argylic unit to the Ti centre for 7. This was unambiguously
confirmed by X-ray crystallography (see below, Figure 2,
Table S5).
3
À1
of D v˜ (CꢀC)=483 cm and the structural features (see below,
Figure 2, Table S5) confirm 6 as a metallacyclopropene.
Single crystals suitable for X-ray analysis of 5, 6 and 7 were
obtained from saturated n-hexane solutions (Figure 2). In all
three complexes the titanium centre is surrounded by the two
Cp’ ligands to give a bent metallocene. Complexes 5 and 6
show the titanium centre in the expected distorted tetrahedral
coordination mode with the bent CꢀCCH unit in asymmetric
2
side-on coordination. This is evident from differences in the
Scheme 6. Formation of complexes 6 and 7.
bond lengths and angles (see Table S5, 5: Ti1ÀC1 2.1204(17),
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Figure 2. Molecular structures of complexes 5 (left), 6 (middle) and 7 (right). Thermal ellipsoids correspond to 30% probability. Hydrogen atoms except H
atoms of the central C CH fragments are omitted for clarity.
2 2
Ti1ÀC2 2.0894(17) Š, Ti1-C1-C2 70.96(10), Ti1-C2-C1 73.59(11)8;
moieties and found that formation of alkyne complexes with
side-on coordination takes place. These complexes do not un-
dergo alkyne/allene isomerisation. In case of Cp* substituents
6
: Ti1ÀC1 2.1407(15), Ti1ÀC2 2.0855(15) Š, Ti1-C1-C2 69.87(9),
Ti1-C2-C1 74.53(9)8). The larger steric demand of the silyl
group leads to significantly longer Ti1ÀC1 bonds compared to
the Ti1ÀC2 bond length. This observation is in best agreement
with the structural parameters for the non-symmetric titano-
III
and alkyne 2 an unusual Ti propargylic complex is formed by
activation of the alkyne and coordination of the Me SiC CH
2
3
2
unit. Further reactions of the alkyne complexes with bases to
result in ring extension and formation of highly strained four-
2
cene silylpropyne complex Cp Ti(h -Me SiC CMe ) where also
2
3
2
3
the TiÀC(SiMe ) bond is significantly longer than the TiÀ membered metallacycles were so far not successful. We are
3
[
21]
C(CMe ) bond (cf. 2.103(3) vs. 2.018(3) Š). Comparison of the
currently investigating this approach and others for the realisa-
tion and stabilisation of 1-metallacyclobuta-2,3-diene struc-
tures.
3
bond lengths of C1ÀC2 in metallacyclopropenes 5 and 6 with
[
22]
data for the free alkyne Me SiC SiMe
3
and the well-known
3
2
2
[12]
complexes Cp’ Ti(h -Me SiC SiMe )
reveals the presence of
2
3
2
3
triple bonds (Table S5, 5: 1.282(3); 6: 1.293(2) Š). It should be
mentioned, that 5 and 6 are the second and third member of
Acknowledgements
2
1
2
the titanacyclopropenes which consist of a Cp’ Ti(h -R C CH R )
2
2
2
We thank our technical and analytical staff, in particular PD Dr.
Wolfgang Baumann for support. Financial support by LIKAT is
gratefully acknowledged.
motif. Mach et al. reported the synthesis and structure of
2
Cp* Ti(h -PhCꢀCCH ) (Ti1ÀC1 2.0968(16), Ti1ÀC2 2.0899(17),
2
3
C1ÀC2 1.296(3) Š) and gives, supported by further experi-
[8]
ments, an explanation for the lack of such structures. Alkylat-
ed alkynes (H CC R, R=Et, iPr and tBu) undergo an alkyne/
3
2
Conflict of interest
allene isomerisation in the coordination sphere of the Ti by
,3-proton shift, thus forming allene complexes (Scheme 1
1
The authors declare no conflict of interest.
right). This CH activation is in good agreement with our earlier
experiments using 1,4-bis(diphenylphosphanyl)but-2-yne and
should be considered as another possible reaction pathway in
Keywords: alkynes · metallacycles · titanium · titanocenes · X-
ray analysis
[
23,21]
addition to coordination and coupling at the metal centre.
The X-ray analysis of 7 unambiguously reveals the presence
3
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of
a
decamethyltitanocene h -propargyl complex (C1ÀC2
1
.368(2), C2ÀC3 1.258(2) Š; Figure 2, right). The additional coor-
dination of the triple bond to the metal centre results in a
slight elongation of this bond but the observed elongation is
lower than in the metallacyclopropenes 5 and 6, which confirm
a weaker interaction with the metal centre. The coordinated
2007, 36, 719; d) L. Becker, U. Rosenthal, Coord. Chem. Rev. 2017, 345,
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[
2] Selected examples: a) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A.
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Kehr, R. Frçhlich, S. Grimme, G. Erker, Angew. Chem. Int. Ed. 2008, 47,
propargylic C unit in 7 shows features reminiscent of previ-
3
3
ously reported early transition-metal complexes Cp Ti(h -
2
[
24]
3
[19]
3
H CC CH B(C F ) ),
Cp Zr(Me)(h -PhC CH )
and Cp* Y(h -
2
2
2
2
6
5 3
2
2
2
[
20]
PhC CH ) (Table S5). In all four complexes C1ÀC2 can be best
2
2
described as double bonds whereas the C2ÀC3 bonds show a
[25]
larger triple bond character. Therefore the best bonding de-
scription of the CH C R unit in these complexes is a combina-
2622; Angew. Chem. 2008, 120, 2662; g) J. Ugolotti, G. Kehr, R. Frçhlich,
2
2
3
3
S. Grimme, G. Erker, J. Am. Chem. Soc. 2009, 131, 1996; h) G. Bender, G.
Kehr, C. G. Daniliuc, B. Wibbeling, G. Erker, Dalton Trans. 2013, 42,
tion of h -propargyl and h -allenyl resonance structures.
In summary, we have demonstrated the coordination
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chemistry of CH R substituted alkynes 1 and 2 to titanocene
Organometallics 2014, 33, 3481.
2
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Communication
[
3] a) O. Theilmann, M. Ruhmann, A. Villinger, A. Schulz, W. W. Seidel, K.
Kaleta, T. Beweries, P. Arndt, U. Rosenthal, Angew. Chem. Int. Ed. 2010,
[15] D. Seyferth, S. C. Vick, M. L. Shannon, Organometallics 1984, 3, 1897.
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Manuscript received: July 25, 2017
Accepted manuscript online: August 28, 2017
Version of record online: September 18, 2017
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