Synthesis and structures of ansa-titanocene complexes with diatomic bridging units for overall water splitting

Article abstract of DOI:10.1002/chem.201300223

The synthesis of a series of ansa-titanocene dichlorides [Cp' ₂TiCl₂] (Cp′=bridged η⁵- tetramethylcyclopentadienyl) and the corresponding titanocene bis(trimethylsilyl)acetylene complexes [Cp′₂Ti(η ²-Me₃SiC₂SiMe₃)] is described. The ethanediyl-bridged complexes [C₂H₄(C₅Me ₄)₂TiCl₂] (2-Cl₂) and [C ₂H₄(C₅Me₄)₂Ti(η ²-Me₃SiC₂SiMe₃)] (2-btmsa; btmsa=η²-Me₃SiC₂SiMe₃) can be obtained from the hitherto unknown calcocenophane complex [C₂H ₄(C₅Me₄)₂Ca(THF)₂] (1). Furthermore, a heterodiatomic bridging unit containing both, a dimethylsilyl and a methylene group was introduced to yield the ansa-titanocene dichloride [Me₂SiCH₂(C₅Me₄) ₂TiCl₂] (3-Cl₂) and the bis(trimethylsilyl) acetylene complex [Me₂SiCH₂(C₅Me ₄)₂Ti(η²-Me₃SiC ₂SiMe₃)] (3-btmsa). Besides, tetramethyldisilyl- and dimethylsilyl-bridged metallocene complexes (structural motif 4 and 5, respectively) were prepared. All ansa-titanocene alkyne complexes were reacted with stoichiometric amounts of water; the hydrolysis products were isolated as model complexes for the investigation of the elemental steps of overall water splitting. Compounds 1, 2-btmsa, 2-(OH)₂, 3-Cl₂, 3-btmsa, 4-(OH)₂, 3-alkenyl and 5-alkenyl were characterised by X-ray diffraction analysis. Copyright

Full text of DOI:10.1002/chem.201300223

DOI: 10.1002/chem.201300223
Synthesis and Structures of ansa-Titanocene Complexes with Diatomic
Bridging Units for Overall Water Splitting
Monty Kessler, Sven Hansen, Christian Godemann, Anke Spannenberg, and
Torsten Beweries*[a]
Abstract: The synthesis of a series of
ansa-titanocene dichlorides [Cp’2TiCl2]
(Cp’=bridged h5-tetramethylcyclopen-
tadienyl) and the corresponding titano-
cene bis(trimethylsilyl)acetylene com-
plexes [Cp’2Ti(h2-Me3SiC2SiMe3)] is de-
scribed. The ethanediyl-bridged com-
a heterodiatomic bridging unit contain-
ing both, a dimethylsilyl and a methyl-
ene group was introduced to yield the
dimethylsilyl-bridged metallocene com-
plexes (structural motif 4 and 5, respec-
tively) were prepared. All ansa-titano-
cene alkyne complexes were reacted
with stoichiometric amounts of water;
the hydrolysis products were isolated
as model complexes for the investiga-
tion of the elemental steps of overall
water splitting. Compounds 1, 2-btmsa,
2-(OH)2, 3-Cl2, 3-btmsa, 4-(OH)2, 3-al-
kenyl and 5-alkenyl were characterised
by X-ray diffraction analysis.
ansa-titanocene dichloride [Me
(C5Me4)2ATiCl2] (3-Cl2) and the bis(tri-
methylsilyl)acetylene complex [Me2ASi-
CH2A(C5Me4)2A 3ASi 2ASiMe3)] (3-
Ti(h2-Me
2ACHTUNGERTNSNUNG iCH2-
A
C
A
CTHUNGTRENNUNG
A
E
N
CHTUNGTRENNNUG ACHTNUREGTNNUNGC CHUNTGTRENNUNG
plexes [C2H
(C5Me4)2TiCl2] (2-Cl2) and
btmsa). Besides, tetramethyldisilyl- and
[C2H4ACHTUNGTRENNUNG
(C5Me4)2Ti(h2-Me3SiC2SiMe3)] (2-
btmsa; btmsa=h2-Me3SiC2SiMe3) can
Keywords: cyclopentadienyl ligand ·
titanium · water splitting · X-ray
diffraction
be obtained from the hitherto unknown
calcocenophane
complex
[C2H4-
A
N
ACHTUNGTRENNUNG
Introduction
[Cp*2Ti(OH)2] yields the trivalent titanocene complex
[Cp*2Ti(OH)], thus demonstrating the general feasibility of
such compounds to act as model complexes for intermedi-
ates in the catalytic cycle of splitting water into its elements
(Scheme 1). However, in the mentioned case loss of a Cp*
ligand occurs as a side reaction. As a consequence, forma-
tion of hydrogen peroxide (from recombination of two OH
radicals) is not observed, instead pentamethylcyclopentadi-
Titanocene complexes proved to be a versatile material for
various applications, such as in medicinal chemistry and for
catalytic transformations, for example, polymerisation, epox-
ide ring opening, organic synthesis[1] and HAT-reactions
(HAT=hydrogen-atom transfer).[2] The latter inspired us to
investigate the reactivity of titanocene compounds towards
water regarding the elemental steps of water splitting and
hydrogen evolution. Employing the trivalent decamethyltita-
ACHUTNGRENeNUG nACHUTNGTRENoNGUN l is found to be the byproduct of the photoreduction.
This loss of a Cp* ligand leads to very stable, inactive tetra-
nuclear titanium(IV) compounds. The thermally or photo-
chemically induced elimination of cyclopentadienyl ligands
is well precedented and often prevents the use of such com-
plexes in catalytic or stoichiometric applications.[6]
nocene complex [Cp*2TiACTHNUTRGNEUNG
(OTf)] (Cp*=h5-pentamethylcyclo-
pentadienyl) we demonstrated the possibility to reduce
water to dihydrogen, accompanied by oxidation of the tita-
nium centre to [Cp*2Ti(OH)OTf].[3] More recently, we ad-
dressed the second, probably more challenging part of over-
all water splitting, that is, water oxidation, by using the hy-
droxyl-bearing titanocene [Cp*2Ti(OH)2].[4] Having in mind
that in a photo-electrochemical TiO2/Pt array oxygen evolu-
tion is known to take place at the TiO2 electrode, as it was
found by Honda and Fujishima, this approach seemed to be
most reasonable.[5] The photoreduction of the complex
In contrast, so called ansa-metallocenes exhibit increased
stability, for example, towards thermal decomposition,[7]
making them attractive for the investigation of the behav-
iour of such compounds under irradiation with light. Creat-
ing small changes in molecular geometries often yields sig-
nificant changes in reactivity.[8,9] For this reason, the investi-
gation of ansa-titanocenes with different bridging units and
thus varying Cp*-Ti-Cp* angles is an attractive target for
the elucidation of the reactivity towards small molecules.
In this context we herein describe the synthesis of hitherto
unknown permethylated ansa-titanocene alkyne complexes,
which are considered to be versatile precursors for the syn-
thesis of the corresponding dihydroxidotitanocenes, and
their reactions with water.
[a] M. Kessler, Dr. S. Hansen, C. Godemann, Dr. A. Spannenberg,
Dr. T. Beweries
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax : (+49) 381 1281 51104
E-mail: torsten.beweries@catalysis.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300223.
6350
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FULL PAPER
spectrum of this complex shows
the expected set of singlet reso-
nances for the methyl groups at
the cyclopentadienyl ligands at
d=1.76 and 2.01 ppm, respec-
tively, as well as for the bridg-
ing ethanediyl moiety at d=
2.95 ppm. The molecular struc-
ture is depicted in Figure S2 in
the Supporting Information. It
shows the calcium centre sur-
rounded by the bridged per-
ꢀ
Scheme 1. Photoreduction and Ti O bond activation of [Cp*2Ti(OH)2].
Results and Discussion
ACHTUNGERTNmUNGN ethACHTUNREGTGyNNUN lCAHTNUGTRNEcNUGN yclopentadienyl groups
and two molecules of THF in distorted tetrahedral coordina-
tion geometry.
Synthesis of ansa-titanocene bis(trimethylsilyl)acetylene
complexes: One very elegant approach to couple cyclopen-
tadienyl ligands that are included in a sandwich complex is
the insertion of a bridging unit, which contains one or more
atoms. Unlike in the case of ferrocene,[8] in titanium com-
plexes this unit has to be introduced before insertion of the
metal centre.[10] To the best of
Synthesis of the ansa-titanocene dichloride[12] 2-Cl2 pro-
ceeds through a transmetallating salt metathesis from com-
plex 1 and [TiCl3ACTHNUGTRNEUNG(thf)3] as demonstrated before by Tacke
and co-workers,[16] followed by subsequent oxidation with
PbCl2 (Scheme 2). The product complex 2-Cl2 can be ob-
our knowledge, the only exam-
ple for a coupling at a titano-
ˇ
cene was described by Horꢃcek
et al., who reported an intramo-
lecular reductive coupling of si-
lanyl-substituted titanocene to
yield a tetramethyldisilyl-bridg-
ed
bridging units of the herein de-
scribed compounds where
ansa-titanocene.[11]
The
chosen to contain two atoms.
Incremental increase of the size
of the bridging moiety is ach-
ieved by subsequently substitut-
ing its atoms by the next largest
element of the same group.
Starting from the dicarbon
Scheme 2. Synthesis of complexes 1, 2-Cl2 and 2-btmsa (btmsa=h2-Me3SiC2SiMe3). a) TiCl2, THF, 508C, 3 d;
b) 1.5 equiv Ca, 2.5 mol% HgCl2, THF, ꢀ308C to RT; c) i) [TiCl
3A
d) 1.25 equiv Mg, Me3SiC2SiMe3, THF, 658C.
bridging unit, stepwise exchange of the atoms leads to sili-
con–carbon- and disilicon-bridged titanocenes, respectively.
Although the ligands for ansa-titanocenes with silicon–
carbon and disilicon bridging units are in most cases conven-
iently prepared from its metalated building blocks MCp’
(M=Li, Na, K; Cp’=substituted h5-cyclopentadienyl) and
the corresponding diatomic unit, for example, dichlorodisi-
lane, the synthesis of 1,2-ethylene-bis(tetramethylcyclopen-
tadiene) and the corresponding ansa-titanocene complex
showed to be more diverse.[12] 1,2-Difluorenylethane can be
readily obtained from 1,2-dibromoethane and lithium fluore-
nide.[13] However, for tetramethylcyclopentadienides, 1,2-
elimination at the 1,2-dibromoethane occurs. Recently, more
convenient routes towards dicarbon bridged metallocenes
were established by reductive coupling of fulvenes by using
alkali[14] or alkaline earth metals.[15]
tained as dark red, air-stable crystals in high yields. The
presence of this compound was confirmed by H NMR anal-
1
ysis; the data were found to be in good agreement with
values reported in literature.[12] Alternatively, this dichloride
complex can be readily obtained from the reaction of
1,2,3,4-tetramethylfulvene with TiCl2 in a mixture of THF
and toluene (Scheme 2, see the Supporting Information for
details), similar to a procedure published by Eisch and co-
workers.[17]
Reduction of complex 2-Cl2 with elemental magnesium in
the presence of bis(trimethylsilyl)acetylene (Me3SiC2SiMe3)
gives the hitherto unknown ansa-titanocene alkyne complex
2-btmsa in good yields. Its 13C NMR spectrum displays a res-
onance at d=251.1 ppm, corresponding to the coordinated
triple bond. The IR band for this structural motif can be
found at n˜ =1570 cmꢀ1. These values are in good agreement
with values obtained before for other titanocene alkyne
complexes (e.g., [Cp*2Ti(h2-Me3SiC2SiMe3)], IR: n˜ =1598,
1563 cmꢀ1, 13C NMR: d=248.5 ppm).
Following this procedure, conversion of elemental calcium
with tetramethylfulvene in the presence of mercury(II)
chloride was achieved to form complex 1. The 1H NMR
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T. Beweries et al.
included formation of the bridging disilane unit by reductive
ꢀ
coupling of two Si H groups in the backbone of an unbridg-
ed titanocene unit to give the corresponding ansa-titanocene
bis(trimethylsilyl)acetylene complex. Oxidation with PbCl2
gave the desired dichloride complex 4-Cl2. Alternatively,
synthesis starting from building blocks through preparation
of the complete ligand[21] offers another suitable method and
was described by Xu et al.[22] Main drawbacks to be men-
tioned are moderate yields that can be obtained through
this route.
Figure 2 shows the molecular structure of complex 3-Cl2.
The titanocene centre is coordinated by two chloride ligands
and the bridged cyclopentadienyl fragment in a distorted
tetrahedral geometry. The extent of bending of the metallo-
cene fragment is slightly smaller than in the ethandiyl-bridg-
ed titanocene dichloride 2-Cl2 (2-Cl2: 131.08;[12] 3-Cl2:
134.8)8), owing to the substitution of one CH2 group by a
larger dimethylsilyl moiety.
Figure 1. Molecular structure of complex 2-btmsa. Thermal ellipsoids cor-
respond to 30% probability. Hydrogen atoms and the second position of
the disordered bridged cyclopentadienyl ligand are omitted for clarity.
The molecular structure of complex 2-btmsa (Figure 1)
shows the titanium centre coordinated by a h2-bis(trimethyl-
silyl)acetylene and the bridged cyclopentadienyl ligand. The
ꢀ
C C bond length of the coordinated alkyne (1.310(2) ꢄ) is
the same as in the corresponding unbridged titanocene
alkyne complex [Cp*2Ti(h2-Me3SiC2SiMe3)] (1.308(3) ꢄ)[18]
and in the range of a C=C double bond.[19] Moreover, com-
ꢁ
parable C C-Si angles, which can be regarded as another in-
dicator for metal–alkyne binding strength, are observed (2-
btmsa: 138.24(4)8, [Cp*2Ti(h2-Me3SiC2SiMe3)]: 134.8(3)8).
Similar ansa-titanocene complexes with silicon–carbon
and disilicon bridging units are accessible from conventional
synthetic routes, that is, coupling of the corresponding, com-
mercially available building blocks.[13,20] Reaction of potassi-
um tetramethylcyclopentadienide with (bromomethyl)-
ACHTUNGTRENNUNGchloroACHTUNGTRENNUNGdimethylsilane yields the bridged cyclopentadiene
(Cp’-SiMe2-CH2-Cp’, Cp’=C5Me4H) as a yellow oil. At-
tempts to purify this crude product were of limited success.
However, similarly as the tetramethyl-1,2-bis(tetramethylcy-
clopentadienyl)disilane ligand, which was obtained from salt
metathesis of potassium tetramethylcyclopentadienide and
1,2-dichlorotetramethyldisilane as a white crystalline sub-
stance, the crude product can be used for further reactions
without purification. Dilithiation of the ligands and subse-
Figure 2. Molecular structure of complex 3-Cl2. Thermal ellipsoids corre-
spond to 30% probability. Hydrogen atoms are omitted for clarity.
The corresponding alkyne complex 3-btmsa is obtained in
analogy to complex 2-btmsa as yellow crystals in good yields
(see Scheme 3). Similarly as described above for complex 2-
btmsa, a strong downfield shift of the 13C NMR signal of the
coordinated alkyne (d=248.9 ppm) as well as a red shift of
the respective IR band (n˜ =1559 cmꢀ1) are observed. The
molecular structure is depicted in Figure 3. Compared to the
dichloride complex 3-Cl2 it shows only very small changes
(e.g., CE-Ti-CE (CE=centroids of the bridged Cp rings)
136.5 in 3-btmsa vs. 134.88 in 3-Cl2).
quent addition of [TiCl3ACHTNUTRGNE(UNG thf)3] and PbCl2 have proven to be
a convenient route to the corresponding titanocenophane di-
chlorides 3-Cl2 and 4-Cl2 (see Scheme 3). Both complexes
can be obtained in moderate yields as dark red crystals.
Noteworthy, the disilyl ansa-titanocene dichloride 4-Cl2 was
reported before. However, the described synthetic protocol
A brief comparison of the IR spectroscopic data of the
herein described titanocene bis(trimethylsilyl)acetylene
complexes shows that the bands due to the stretching mode
of the alkyne triple bond only differ to a very small degree
(Table S1 in the Supporting Information). Taking into ac-
count the 13C NMR values for the alkyne carbon atoms,
which are also very similar, one could conclude that the
changes in the electronic structure caused by the bridging of
the cyclopentadienyl rings are only minimal. Hence, the re-
activity with respect to alkyne dissociation should be compa-
rable and only steric effects should be considered for the re-
Scheme 3. Synthesis of complexes 3-Cl2 and 3-btmsa. a) KH, Me2Si-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
THF, 658C.
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ansa-Titanocene Complexes for Overall Water Splitting
FULL PAPER
Figure 3. Molecular structure of complex 3-btmsa. Thermal ellipsoids cor-
respond to 30% probability. Hydrogen atoms are omitted for clarity.
Figure 4. Molecular structure of complex 4-(OH)2. Thermal ellipsoids cor-
respond to 30% probability. Hydrogen atoms (except H1 and H1A) are
omitted for clarity.
action with water and potential photochemical investiga-
tions.
ꢀ
ꢀ
138.08). The Ti O distances (Ti1 O1 1.923(2) ꢄ) are well in
line with the values in the unbridged complex
[23]
ꢀ
ꢀ
Reactions of ansa-titanocene bis(trimethylsilyl)acetylene
complexes with water: In order to access new titanocene hy-
droxido and oxido complexes for the investigation of the
second half reaction of water splitting, the described ansa-ti-
tanocene bis(trimethylsilyl)acetylene complexes were react-
ed with water. For the unbridged [Cp*2Ti(h2-Me3SiC2SiMe3)]
this approach cleanly yielded the corresponding dihydroxido
complex [Cp*2Ti(OH)2] in good yields.[23]
[Cp*2Ti(OH)2] (Ti1 O1 1.892(3), Ti1 O2 1.891(3) ꢄ).
To our surprise, in the hydrolysis of complex 4-btmsa a
second well-defined titanocene species was present in solu-
tion. However, we were unable to isolate this species from
this reaction. Fortunately, a similar species was observed in
hydrolysis reactions of the ethanediyl-bridged alkyne com-
plex 2-btmsa and the heterodiatomically bridged alkyne
complex 3-btmsa (Scheme 5).
Reaction of the silanediyl-bridged bis(trimethylsilyl)acety-
For the latter case, an unprecedented s-alkenyl complex
lene complex 4-btmsa with
a
slight excess of water
of the type [Cp’2TiACHTGUNTERNNU(G OSiMe3)(CH=CHSiMe3)] was isolated as
(2.5 equiv) gave the bright yellow dihydroxido complex 4-
(OH)2 in moderate yields (Scheme 4). The H NMR spec-
the byproduct. Some evidence for the formation of the cor-
responding dihydroxido complex 3-(OH)2 was found by
1H NMR spectroscopy (broad singlet at d=5.30 ppm) but
this species could not be isolated in pure form. The
1H NMR spectrum of complex 3-alkenyl displays eight dif-
ferent resonances for the methyl groups of the cyclopenta-
dienyl ring, thus, indicating the unsymmetrical constitution
of the complex. Characteristic doublets at d=5.82 and
7.56 ppm (3J=20.0 Hz) due to the alkenyl unit are reminis-
cent of the signals in similar titanocene alkenyl species [for
1
Scheme 4. Hydrolysis of complex 4-btmsa to give 4-(OH)2. a) 5 equiv
H2O, THF, RT.
example,
ACTHNGUTERNNUG
[(h5-C5Me4H)2Ti(h1-(E)-CH=CHSiMe3)(h1-
trum of this complex shows the
expected signals, including the
characteristic singlet resonance
at d=5.44 ppm, which can be
assigned to the OH groups.
The molecular structure of
complex 4-(OH)2 is depicted in
Figure 4. The titanocene centre
is coordinated by the bridged
cyclopentadienyl fragment and
the two OH groups in a distort-
ed tetrahedral geometry (O1-
Ti1-O1A 98.4(1), CE-Ti1-CE complexes 2-btmsa and 3-btmsa.
Scheme 5. Formation of different hydrolysis products from the bridged titanocene bis(trimethylsilyl)acetylene
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T. Beweries et al.
Figure 5. Molecular structure of complex 3-alkenyl. Thermal ellipsoids
correspond to 30% probability. Hydrogen atoms (except H1 and H2) are
omitted for clarity.
Figure 6. Molecular structure of complex 2-(OH)2. Thermal ellipsoids cor-
respond to 30% probability. Hydrogen atoms (except H1 and H2) are
omitted for clarity.
CꢁCSiMe3)]: d=5.73 and 6.22 ppm (3J=20.5 Hz)].[24] The
molecular structure of compound 3-alkenyl is shown in
Figure 5. It displays the typical bent titanocene unit along
with the OSiMe3 group and the alkenyl unit in distorted tet-
rahedral coordination geometry.
good yields. The NMR spectra of this complex nicely cor-
roborate the aforementioned structural motif of a s-alkenyl
complex (doublets at d=5.94 and 7.33 ppm for CHSiMe3
Degradation of complex 3-alkenyl in the course of reac-
3
tion could be identified by emerging resonances of tri
N
and CHTi, respectively; J=20.1 Hz). The structural motif
A
R
(Figure 7) of this complex is the same as found before for
ꢀ
6.15 ppm, respectively) as well as evolution of the aforemen-
tioned broad singlet signal at d=5.30 ppm. However, this
resonance could not be attributed to either of the hydroxido
complex 3-alkenyl. The bond length C1 C2 (1.329(3) ꢄ)
clearly indicates the presence of a double bond, similarly as
found before in other titanocene alkenyl complexes (e.g.,
1.347(2) ꢄ in [Cp*2Ti(CPh=CHSiMe3)OH][25]).
(h5-C5Me4)2Ti
complexes [(H2CSiMe2)ACTHUNGTRENNUNG ACTHUNGTRENNUNG
[(H2CSiMe2)
certainty.
ACHTUNGTRENNUNG
In the case of the ethanediyl-bridged alkyne complex 2-
btmsa reaction patterns resemble those observed in the hy-
drolysis reaction of 3-btmsa. Formation of dihydrogen, free
bis(trimethylsilyl)acetylene, the alkenyl complex 2-alkenyl
and trimethylvinylsilane takes place, however, in this case
the corresponding dihydroxido complex 2-(OH)2 was isolat-
ed in low yield (10%). Its proton resonance due to the ter-
minal OH groups can be found at d=5.05 ppm, which is in
the same range as found before for other titanocene dihy-
droxido complexes (d=5.42 or for [Cp*2Ti(OH)2] or 4-
(OH)2, respectively). The molecular structure of 2-(OH)2 is
depicted in Figure 6. The titanium centre is coordinated by
the ethanediyl-bridged ligand and two hydroxyl groups in a
distorted tetrahedral fashion (O1-Ti1-O2 99.85(7), CE-Ti1-
CE 130.28). All other bond lengths are similar to that in
Figure 7. Molecular structure of complex 5-alkenyl. Thermal ellipsoids
correspond to 30% probability. Hydrogen atoms (except H1 and H2) are
omitted for clarity.
ꢀ
ꢀ
akin complexes (e.g., Ti1 O1 1.897(2), Ti1 O2 1.906(2) ꢄ).
In order to check, whether this reactivity pattern is de-
pending on the nature of the bridging motif, we also exam-
ined the hydrolysis of a permethylated titanocene bis(tri-
A
E
silyl)acetylene complex bearing a monoatomic con-
The ready generation of the titanocene silanolate–alkenyl
complexes from the corresponding alkyne complexes was
unexpected and without precedent for the unbridged sys-
ACHTUNGTRENNUNG
this case, cleavage of the alkyne ligand takes place to give
tems [Cp*2Ti(h2-Me3SiC2SiMe3)]/
ACHTUNGTRENNUNG
the alkenyl complex 5-alkenyl as a yellow crystalline solid in
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ansa-Titanocene Complexes for Overall Water Splitting
FULL PAPER
that water first coordinates to the metal centre to induce a
ring-opening of the metallacyclopropene unit, which yields
an alkenyl complex of the type [Cp’2Ti(OH)(h1-CSiMe3=
CHSiMe3)]. Stable complexes of this type were isolated
before.[25] Subsequent shifts of the proton as well as of the
SiMe3 group along with rotation of the alkenyl unit would
then yield the product silanolate–alkenyl complexes. As
mentioned before for 2-alkenyl and 3-alkenyl, these com-
pounds are of limited stability in the presence of further
water and decompose in solution to yield mixtures of titano-
cene hydroxido species and trimethylvinylsilane. We pro-
ꢀ
pose hydrolysis of the Ti C bond to generate the latter as
Scheme 6. Intermediate-calculated structures in the first hydrolysis step.
well as a species of the type [Cp’2Ti(OH)(OSiMe3)]. Evi-
G
dence for further hydrolysis to furnish the dihydroxido com-
1
plex was not found by H NMR spectroscopy because sig-
ꢀ
nals due to Me3SiOH were absent in all spectra.
plainable by lengthening of the Ti C bonds (compare,
ꢀ
ꢀ
207.8 pm (e1) and 213.0 pm (Ti C1, i1) 210.9 pm (Ti C2,
i1), Table 1) and thus allowing H2O to coordinate and de-
DFT analysis: The assumed hydrolysis and decomposition
pathways were investigated by means of DFT analysis. Ge-
ometry optimisations were conducted by using the Gauss-
Table 1. Selected computed bond lengths in [pm].
ACHTUNGTRENNUNG
ian 09 suite of programs[26] along with an analytical gradient
Parameter
e1
i1
i2
i2_2
i3
137.6
(270.2)
(272.1)
167.9
p1
calculation for the determination of the nature of the sta-
tionary point. Visualisations were produced by using Gauss-
View 5.[27] The hybrid functional BP86 of Becke and
Perdew[28] was used along with the split-valence basis set 6-
31G(d) for all non-metal atoms and the Los Alamos double-
z basis set LANL2DZ[29] with an effective core potential for
Ti. The bridging ligand was simplified to Cp’2 (Cp-SiH2-Cp)
for the sake of saving computational cost. Firstly the ener-
getics of the reaction of [Cp’2Ti(h2-Me3SiC2SiMe3)] with
water were investigated yielding the 5-alkenyl-like complex
p1. This reaction path is clearly shown to be exergonic and
exothermic (Figure 8).
ꢀ
C1 C2
132.4
207.8
207.8
–
131.6
213.0
210.9
236.7
136.5
(305.6)
216.8
187.2
136.7
(313.9)
216.7
186.9
135.7
ACHTUNGTRENNUNG(312.6)
213.0
184.5
ꢀ
Ti C1
R
U
ꢀ
Ti C2
ꢀ
Ti O
creasing the electron deficiency. Secondly, decoordination
and protonation of C1 occurs leading to intermediate i2. Af-
terwards, cis–trans isomerisation of the bissilylethene occurs
(i2!i2_2), which is slightly exothermic by ꢀ0.2 kcalmolꢀ1.
Protonation of C2 leads to the Ti–oxido intermediate i3 ex-
hibiting a p interaction to the trans-silylalkene that reacts to
p1 after migration of the SiH3 group to the oxygen atom.
Furthermore, two plausible mechanisms for the second
hydrolysis step as shown in Scheme 7 were investigated,
showing the reaction forming silanol (H3SiOH) to be clearly
energetically disfavoured, which is in agreement with the ex-
periments, as the silanol Me3SiOH could not be detected as
product.
It should be noted that an NMR spectroscopic re-investi-
gation of the hydrolysis reaction of the unbridged alkyne
Figure 8. Relative free energies (top) and enthalpies (bottom) of the step-
wise hydrolysis and rearrangement of e1 (Scheme 6).
In the first step the TiII complex e1 is coordinated by
water to yield i1 (Scheme 6). This is, in principle, an unusual
reaction mode for Ti species, as it is known that typically for
Zr and Hf analogues a further coordination site is present
due to their size and electron deficiency, which allows coor-
dination of donating solvent molecules as pyridine or THF.
In fact this step is shown to be slightly endergonic, but is ex-
Scheme 7. Intermediate-calculated structures in the second hydrolysis
step.
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6355

T. Beweries et al.
complex [Cp*2Ti(h2-Me3SiC2SiMe3)][23] also revealed the for-
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this reaction scheme is of general applicability and the
extent of formation of dihydroxido or silanolate-alkenyl
complex is dependent on the nature of the cyclopentadienyl
ligands.
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We have reported the synthesis and full characterisation of
a series of bridged permethyltitanocene complexes by using
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ˇ
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Acknowledgements
We would like to thank our technical and analytical staff for assistance.
Dr. Wolfgang Baumann and Dr. Haijun Jiao (both LIKAT) helped with
the interpretation of the NMR data and computational results, respec-
tively. T.B. would like to thank Prof. Uwe Rosenthal (LIKAT) for his
support and for fruitful discussions regarding titanocene chemistry. Finan-
cial support by the BMBF (project “Light2Hydrogen”) is gratefully ac-
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Received: January 21, 2013
Published online: March 27, 2013
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www.chemeurj.org
6357