Reactions of group 4 metallocene alkyne complexes with carbodiimides: Experimental and theoretical studies of the structure and bonding of five-membered hetero-metallacycloallenes

Article abstract of DOI:10.1021/ja111479j

The reaction of the low-valent metallocene(II) sources Cp ₂Ti(n²-Me₃SiC₂SiMe₃) (7) and Cp₂Zr(py)(n²-Me₃SiC₂SiMe ₃) (11, Cp = n⁵-cyclopentadienyl, py = pyridine) with carbodiimides RN=C=NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp₂M{Me₃SiC=C= C[N(SiMe₃)(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.(Figure Presented)

Full text of DOI:10.1021/ja111479j

ARTICLE
pubs.acs.org/JACS
Reactions of Group 4 Metallocene Alkyne Complexes with
Carbodiimides: Experimental and Theoretical Studies of the Structure
and Bonding of Five-Membered Hetero-Metallacycloallenes
Katharina Kaleta,† Martin Ruhmann,†,‡ Oliver Theilmann,†,‡ Torsten Beweries,† Subhendu Roy,§
Perdita Arndt,† Alexander Villinger,‡ Eluvathingal D. Jemmis,*,§,^ Axel Schulz,*,†,‡ and Uwe Rosenthal*,†
†Leibniz-Institut f€ur Katalyse e.V. an der Universit€at Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
‡Abteilung Anorganische Chemie, Institut f€ur Chemie, Universit€at Rostock, Albert-Einstein-Str. 3a, D-18059 Rostock, Germany
§Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
^Indian Institute of Science Education and Research Thiruvananthapuram, CET campus, Thiruvananthapuram 695016, India
S Supporting Information
b
ABSTRACT: The reaction of the low-valent metallocene(II)
sources Cp2Ti(η2-Me3SiC2SiMe3) (7) and Cp2Zr(py)(η2-
Me3SiC2SiMe3) (11, Cp = η5-cyclopentadienyl, py =
pyridine) with carbodiimides RNdCdNR (R = Cy, i-Pr, p-
Tol) leads to the formation of five membered hetero-metalla-
cycloallenes Cp2M{Me3SiCdCdC[N(SiMe3)(R)]-N(R)}
(9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol).
Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest)
was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the
structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.
fragments, was studied in detail.8 Moreover, these complexes
1. INTRODUCTION
have shown to be relevant for stoichiometric and catalytic C-C
Metallacycles (cycloallenes, dienes, alkynes, cumulenes, etc.)
are an important class of organometallic compounds. Their
importance for various catalytic applications was demonstrated
coupling and cleavage reactions of unsaturated molecules such as
alkynes, olefins, acetylides, and vinylides.3a,3c,9
Both metallacyclocumulenes and metallacyclopentynes dis-
on several occasions,1 e.g. in the chromium mediated tetramer-
play strongly bent geometries at the sp-hybridized central carbon
ization of ethylene to 1-octene.2 In our group we focused on the
atoms, which would not be stable in the corresponding hydro-
synthesis, formation, reaction, and computational studies of
carbon analogues.4,5 The remarkable stability of these complexes
three-, four-, and five-membered titana-, zircona-, and hafna-
(both metallacycles can be kept at room temperature under
argon) is attributed to the in-plane interaction of the bent
cycles.3 For these purposes, bis(trimethylsilyl)acetylene contain-
ing group 4 metallocene complexes of the type Cp02M(L)(η2-
metallocene unit with the central unsaturated C-C bond invol-
ving the vacant acceptor orbitals of the Cp2M moiety and the
filled orbitals of the C4 ligands. Alternatively, these complexes
can be described in an η4-π,π-coordination mode (3M0 and 4M0,
Scheme 1); these mesomeric forms also contribute to the
resonance hybrid of these systems.
As an extension to these unique bonding strategies, very
recently, Suzuki and co-workers and Erker et al. have isolated
and characterized the first examples for five-membered all-carbon
zircona- and hafnacycloallenes (6, Scheme 2) spectroscopically,
by X-ray diffraction and also by DFT computations.10,11 Similarly
as for the above-mentioned species 1 and 2, the carbocyclic
analogue of a five-membered metallacycloallene (5) would also
not be stable due to the angle strain.5
Me3SiC2SiMe3) (Cp0 = Cp, Cp*, etc.; M = Ti, Zr, Hf; L =
pyridine, THF, acetone) were found to be excellent starting
materials due to their ability to release the alkyne under genera-
tion of the “free” metallocene fragment under mild conditions.
The formation of saturated and unsaturated metallacycles from
these precursor complexes was widely investigated in the past.
Especially the unsaturated, highly strained metallacycles are
unusual because the corresponding carbocyclic analogues, i.e.
cyclic cumulene 1 and alkyne 2, are unstable due to the angle
strain on incorporation of linear unsaturated units, i.e.
CdCdCdC and C-CtC-C into the small ring systems.4,5
As shown recently for metallacyclopenta-2,3,4-trienes (3M)6 and
metallacyclopentynes (4M),7 replacing the CH2 group by the
isolobal Cp2M (M = Ti, Zr, and Hf) moiety in these molecules
results in a decrease of the ring strain to give stable, isolable
metallocene complexes (Scheme 1), which could be fully char-
acterized; their reactivity, e.g. toward Lewis acids or other metal
Received: December 21, 2010
Published: March 17, 2011
r
2011 American Chemical Society
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Scheme 1. Schematic Representation of Cyclocumulene 1, Cyclopentyne 2, Metallacyclocumulene 3M and Metallacyclopentyne
4M and Their Mesomeric Forms (3M0 and 4M0, Respectively)
In this contribution we present the synthesis of the first
examples for five-membered hetero-metallacycloallenes as well
as theoretical studies on the structure and bonding of these
complexes.
Scheme 2. Schematic Representation of Cycloallene 5,
Erker’s Five-Membered Hafnacycloallene 6 and Its
11a
Mesomeric Form 60
2. RESULTS AND DISCUSSION
The reaction of the titanocene alkyne complex Cp2Ti(η2-
Me3SiC2SiMe3) (7) with 1,3-N,N0-diisopropylcarbodiimide (8-i-Pr)
at low temperatures revealed the formation of a hitherto un-
known hetero-metallacycloallene 9Ti-i-Pr (Scheme 4). Perform-
ing the reaction at ambient temperature results in the formation
of a dinuclear titanocene complex; details on this reaction will be
published in due course.
Dark red crystals of compound 9Ti-i-Pr suitable for X-ray
analysis were obtained from concentrated n-hexane solutions in
moderate yields (50%); the molecular structure and structural
parameters are depicted in the Supporting Information. The
structural motif is similar to the corresponding zirconocene
complexes 9Zr-Cy and 9Zr-p-Tol (cf. Figure 1) showing a five-
membered nonplanar metallacycle with the ring nitrogen being
bent out of the plane formed by the metal atom and the ring
carbon atoms. NMR spectroscopic data nicely reveal the presence
of a hetero-metallacycloallene unit. In the 13C NMR spectrum
resonances due to the allene scaffold can be found at 110.6, 117.9,
and145.9 ppm. Moreover, lossofsymmetry in themoleculecanbe
derived from the nonequivalent signals for the SiMe3 protons in
As shown recently by our group for the synthesis of a
1-zircona-2,5-disilacyclopent-3-yne,12 insertion of another het-
eroatom such as silicon, nitrogen, oxygen, or sulfur into the
metallacycle can be another elegant way to increase the stability
of such highly strained systems. This approach was discussed in
the past;3f however, experimental support for this concept was
rare. Very recently, we reported on the reaction of the well-
known titanocene alkyne complex Cp2Ti(η2-Me3SiC2SiMe3)
(7) with 1,3-N,N0-dicyclohexylcarbodiimide (8-Cy), yielding
two different dinuclear titanocene species (Scheme 3).13 Most
interestingly, one of these complexes can be referred to as a
transient titanocene carbene-like complex, which can be regarded
as an intermediate for the formation of the final reduction-
coupling product.
The first examples for well-defined seven-membered aza-
zirconacycloallenes were presented by Xi et al.14 The authors
showed that insertion of a variety of organic nitriles into the Zr-
C bond of a zirconacyclopropene followed by intramolecular
coupling of the two cyclic moieties of the intermediate results in
ring enlargement to yield a seven-membered metallacycle.
1
the H NMR spectra (δ = 0.39 and 0.42 ppm). Further details
referring to chemical shifts in 1H, 13C, and 29Si NMR experiments
can be found in the Experimental Section (vide infra).
Most surprisingly and in contrast to the reaction of the alkyne
complex 7 with 8-i-Pr, under similar reaction conditions the use of
1,3-N,N0-di-p-tolylcarbodiimide 8-p-Tol results in the formation
of a dinuclear complex 10 bridged by a tetraaminate ligand
(Scheme 5). This complex was described before by Floriani et al.15
The synthesis of compound 10 was successfully carried out in
a one-pot reaction. A solution of 1,3-N,N0-di-p-tolylcarbodiimide
(8-p-Tol) dissolved in n-hexane was added to a solution of alkyne
complex 7 also dissolved in n-hexane resulting in a deep-red
solution. Crystallization at low temperature led to the formation
of red crystals (yield 58%) which were characterized as complex
10 by X-ray studies. X-ray analysis and properties of the isolated
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Scheme 3. Reaction of the Titanocene Alkyne Complex 7 with a Carbodiimide
Scheme 4. Formation of the Hetero-titanacycloallene 9Ti-i-Pr
i-Pr and p-Tol). If performed with stoichiometrically equal
amounts of metallocene source 11 and heteroallene substrate
8-Cy in n-hexane for 12 h, the mixture showed a red color.
After the solvent was removed, a red, oily product was isolated.
This crude product was purified by sublimation at elevated
temperatures (120 ꢀC; 10-3 mbar). During sublimation the
pure reaction product 9Zr-Cy condensed in pale-yellow
crystals which appeared to be suitable for X-ray analysis.
Compound 9Zr-i-Pr was obtained as a yellow powder after
recrystallization from mixtures of n-hexane, ether, and THF at
-78 ꢀC. Similarly, the reaction of the zirconocene complex 11
and the carbodiimide 8-p-Tol in THF for 6 h yields an orange
reaction mixture, from which the yellow crystalline product
9Zr-p-Tol can be isolated at -40 ꢀC. Crystals suitable for
X-ray analysis were obtained from THF-d8 (Scheme 6). It is
noteworthy, that for all reactions described here, the THF
stabilized zirconocene alkyne complex Cp2Zr(thf)(η2-
Me3SiC2SiMe3) can also be used as a metallocene source with
THF as solvent. Moreover, monitoring of the reactions by
means of NMR techniques has shown that almost no bypro-
ducts are formed in the reactions of the alkyne complexes with
the corresponding carbodiimides.
NMR data closely resemble those of the titanocene complex
described above; however, the 13C resonance due to the central
carbon atom C2 is found to be slightly shifted downfield (e.g.,
134.8 ppm for complex 9Zr-Cy). A detailed compilation of the
data from 1H, 13C, and 29Si NMR experiments is depicted in the
Experimental Section (vide infra). In mass spectrometry experi-
ments (CI, isobutane) characteristic signals for the molecular
ions were observed at 596 (9Zr-Cy), 516 (9Zr-i-Pr) and 612
(9Zr-p-Tol). Moreover, for complexes 9Zr-Cy and 9Zr-p-Tol,
the free ligand could be detected at 377 [Me3SiCtC-C(N-
(Me3Si)(Cy))dN(Cy) þ H]þ (9Zr-Cy) and 395 [Me3SiCt
C-C(N(Me3Si)(p-Tol))dN(p-Tol) þ H2 þ H]þ (9Zr-p-
Tol), respectively.
Figure 1. Molecular structure of 9Zr-Cy. Thermal ellipsoids are drawn
at the 30% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: Zr1-N1 2.183(1), N1-
C3 1.361(2), C2-C3 1.395(2), C1-C2 1.290(2), Zr1-C1 2.301(1),
Zr1-C2 2.333(1), Zr1-C3 2.529(1), N1-C10 1.466(2), N2-C3
1.420(2), Si1-C1 1.856(1), Si2-N2 1.764(1), N2-C16 1.486(2);
N1-Zr1-C1 91.88(5), C3-N1-Zr1 87.87(8), N1-C3-C2 114.0(1),
C1-C2-C3 153.6(1), C2-C1-Zr1 75.22(8), C10-N1-Zr1
149.35(8), C3-N1-C10 122.1(1), N1-C3-N2 126.3(1), C2-C3-
N2 119.8(1), C2-C1-Si1 138.6(1), Si1-C1-Zr1 144.72(7), C3-N2-
Si2 117.16(9), C16-N2-Si2 123.72(9), C3-N2-C16 118.1(1).
crystals of 10 have already been published by Floriani and co-
workers.15 This complex displays a different structural motif than
9Ti-i-Pr, which could be attributed to the presence of alkyl vs aryl
substituents at the substrate. The scope of carbodiimide substit-
uents in this respect is under investigation; details will be
published in due course.
Similarly, the reaction of the zirconocene precursor Cp2Zr-
(py)(η2-Me3SiC2SiMe3) (11, py = pyridine) with the carbodiimides
RNdCdNR 8-R (R = Cy, i-Pr and p-Tol) resulted in the
formation of the hetero-zirconacycloallenes 9Zr-R (R = Cy,
The mononuclear complex 9Zr-Cy crystallizes in the triclinic
space group P1 with two molecules per unit cell, whereas
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Scheme 5. Formation of the Dinuclear Complex 10
Scheme 6. Formation of the Hetero-zirconacycloallenes 9Zr-R
compound 9Zr-p-Tol crystallizes in the orthorhombic space group
Pccn with eight molecules per unit cell. The molecular structures are
shown in Figures 1 and S1 (Supporting Information). The bond
lengths and angles of the two complexes are comparable. Small
deviations can be found in the bond angles due to the different
electronic and steric properties of the Cy and p-Tol substituents.
Especially the most interesting structural features of the metalla-
cycle such as the conformation along the C-C-C-N unit or the
metal ligand distances and C-C and C-N are almost identical.
Due to these observations the molecular structure of 9Zr-Cy is
discussed exemplarily for both compounds.
In compound 9Zr-Cy the zirconium center adopts a strongly
distorted tetrahedral geometry (Ct1-Zr1-Ct2 125.65(6)ꢀ,
N1-Zr1-C1 91.88(5)ꢀ; Ct = centroid of the Cp fragment).
This is in agreement with other zirconocene-containing com-
plexes with highly unsaturated ligands.3d,3f,7a,7d,10,11b The Zr1-
N1 bond (2.183(1) Å) is distinctly shorter than the Zr1-C1
bond (2.301(1) Å). The metallacycle Zr1-C1-C2-C3-N1
does not appear planar but shows an envelope conformation in
which the nitrogen atom is bent out of the plane (Zr1-N1-
C3-C2 38.9ꢀ). On the contrary, Zr1, C1, C2, and C3 form a
plane with a deviation from planarity smaller than 2ꢀ (Zr1-C1-
C2-C3 1.85ꢀ). Furthermore, in this metallacycle unit the found
C1-C2 bond length is the shortest with 1.290(2) Å and
concerns more to a double bond in an allene unit (1.307 Å17)
than to an elongated C-C triple bond (Σrcov for a triple bond:
1.20 Å16). The N1-C3 bond is somewhat longer (1.361(2) Å)
and can be referred to as a typical Csp2-Nsp2 single bond (1.336 Å)
or a weak C-N double bond (1.279 Å).17 The central C2-
C3 bond displays a double bond rather than single bond
character (1.395(2) Å); however, this distance is elongated
compared to literature values (1.34 Å16). In principle, two
mesomeric Lewis formulas may be mainly involved to character-
ize this compound, namely a zirconocene(II)-bis-π-complex (F)
and a zirconocene(IV)-metallacycloallene (G) (Scheme 8).
However, the data from X-ray analysis suggest the presence of
a mesomeric form which corresponds more to the description as
a metallacycloallene with the metal center in the oxidation state
þ4 (G). The complex is stabilized by means of intramolecular
coordination, which can be derived from the relatively short
Zr1-C2 bond (2.333(1) Å).
The mechanism of theformationof the titaniumcomplex 9Ti-i-
Pr is unclear; however, it is known that in the first reaction step the
titanocene fragment [Cp2Ti] is formed by dissociation of the
alkyne.3b This can then react with one molecule of a 1,3-
carbodiimide to yield a Ti(III) radical species (A), which should
be capable of cleaving the Si-C bond of the free alkyne. Such
formal insertions of carbenes into Si-C bonds are known in
literature.18 Subsequently, the newly formed radical intermediate
(B and C, respectively) can form the isolated product by recom-
bination of the radical moieties and ring closure (Scheme 7).
In contrast, the mechanism of the formation of hetero-
zirconacycloallenes 9Zr-R should include a Si-C bond cleavage
and a C-C bond-coupling step. Most likely, loss of the stabiliz-
ing ligand L is accompanied with a Si-C bond rupture to form a
zirconocene silyl-alkynyl species (D). Precedent for this reaction
motif was reported on several occasions, e.g. for nickel and
platinum compounds19 as well as for hafnocene complexes.20
Subsequently, the carbodiimide inserts into the Zr-Si bond by
forming a zirconocene heteroene-alkynyl (E). Finally, this inter-
mediate experiences an intramolecular C-C coupling by gen-
erating a heteroene-yne ligand coordinated to the zirconocene
fragment. This product is supposed to be a zirconocene(II) bis-
π-complex F or, in another mesomeric description, a
zirconocene(IV) metallacycle G (Scheme 8).
This structural motif was predicted to be existent earlier;3c the
synthesis of compounds 9M-R (M = Ti, R = i-Pr; M = Zr, R = Cy,
i-Pr, p-Tol), which closely resemble the so-called 1-metallacy-
clopenta-2,3-dienes follows the results published by Erker et al.
for Zr and Hf11 and by Suzuki and co-workers for Zr.10 In principle,
there are two reaction pathways possible to obtain such 1-metal-
lacyclopenta-2,3-dienes: mixed σ-bounded alkynyl-alkenyl
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Scheme 7. Proposed Mechanism for the Formation of the Hetero-Titanacycloallene 9Ti-i-Pr
Scheme 8. Proposed Mechanism for the Formation of Hetero-Zirconacycloallenes 9Zr-R
complexes [Cp02M(-CtCR)(-CRdCR2)] form metal-coor-
dinated but-1-ene-3-ynes (R2CdCR(-CtCR)). The zircono-
cene forces the 1-ene-3-yne ligand into two mesomeric
coordination modes, the first one being a η4-π-attachment (F0)
of the unsaturated ligand, the second mode being an asymmetric
1-zirconacyclopenta-2,3-diene (G0) (Scheme 9).
In contrast to this, for the lighter congener titanium, Mach and
co-workers suggest a coordination of exclusively the alkyne
moiety to the smaller titanocene fragment resulting in a η2-π-
coordination mode.21
in more detail by theoretical methods. The results described here are
from BP86 calculations with a BS2 basis set (see Computational
Details in the Experimental Section). The nature of bonding of
complexes 9Zr-Cy and 9Zr-p-Tol is studied using NBO analysis.22
Nucleus-independent chemical shifts (NICS)23 are calculated at the
geometrical center point of the ring to get an understanding of the
extent of cyclic delocalization of electrons in the complexes. This also
gives an idea about the possible interaction between the Zr center and
the C-C-C-N moiety in complexes 9Zr-Cy and 9Zr-p-Tol. The
correlation diagram in Figure 4 is drawn from single-point calculations
of the structures. The results of the analysis for complexes 9Zr-Cy and
9Zr-p-Tol are similar in nature. Thus, only the analysis of complex
9Zr-Cy is described here in detail.
3. STRUCTURE AND BONDING ANALYSIS OF THE FIVE-
MEMBERED HETERO-METALLACYCLOALLENE
Our experimental observations prompted us to investigate the
structure and bonding of the five-membered hetero-metallacycloallene
The computed structure of complex 9Zr-Cy is in excellent
agreement with the data obtained from X-ray crystal structure
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Scheme 9. Coordination of a 1-Ene-3-Yne to a Metallocene Fragment
Figure 2. Schematic representation (left) and calculated structure
(right) of complex 9Zr-Cy.
elucidation (Table S1 and Figures 1 and 2, respectively). It shows
the characteristic nonplanarity of the metallacyclic unit of com-
plex 9Zr-Cy. The deviation from planarity (angle N1-
C3 C1-Zr1 = 33.0ꢀ) matches well with the calculated
3 3 3
deviation (N1-C3 C1-Zr1 = 32.0ꢀ). The Zr1-C1 bond
3 3 3
length is 2.319 Å which is close to the experimental value of
2.301(1) Å and in the same range as found in other compounds
with Zr-C σ-bonds (e.g., 2.277 Å in Cp2Zr(CH3)2).24 The
Zr1-C2 bond length is 2.359 Å, which is only slightly longer
than the Zr1-C1 bond (2.319 Å). This might indicate the
presence of a strong interaction between the metal center and
C2. The Zr1-N1 bond length is 2.241 Å which is longer than the
experimental value of 2.183(1) Å. The C1-C2 bond length is
1.309 Å which is very close to the C-C bond length of a typical
allene (C-C = 1.307 Å).17 The larger bond length of the C2-
C3 bond (1.404 Å) than the C1-C2 bond is attributed to the
ineffective interaction between the C2 and C3 carbon p-orbitals
because of orthogonal orientation. The C3-N1 bond length is
1.365 Å and it is close to a typical Csp2-Nsp2 bond (1.336 Å17).
The Wiberg bond indices of C1-C2 and C2-C3 are 2.041 and
1.287, respectively (Table S2), which also supports the cycloal-
lenic character of complex 9Zr-Cy. The lower value of the
Wiberg bond index for the C2-C3 bond is already explained
in terms of ineffective π-orbital interaction between the C2 and
C3 carbon atoms. It is worthwhile to note that the Zr1-C3
distance is 2.607 Å, which is somewhat larger than those found
for Zr1-C1 and Zr1-C2. However, it is not far from the Zr-C
distance in the complex Cp2Zr(CH3)2 (vide supra)24 and thus
indicates the existence of an interaction between the metal center
and the carbon atom C3.
Figure 3. Important MOs of complex 9Zr-Cy (R = SiMe3, R0 = Cy).
between the C2 and C3 carbon atoms. Thus, the metal is in the
formal oxidation state of þ4 in complex 9Zr-Cy. The orthogonal
orientation of the C2 and C3 carbon orbital makes the π-bonding
interaction between the C2 and C3 carbon atoms less effective.
As a result, the C2-C3 bond does not shorten as much as
expected. This is reflected in the relatively larger bond length of
the C2-C3 bond compared to that of the C1-C2 bond. The
substituents, SiMe3 (R) and Cy (R0) at the C1 and N1 atoms,
respectively, are tilted in a way to make the C1 and N1 orbitals
interact more effectively with the metal orbitals. The other
stabilizing interaction (HOMO-14) involves donation of elec-
trons from the ligand orbital to the vacant metal orbital. It is
important to note that the metal center interacts effectively with
both the C1 and C2 carbon atoms (HOMO and HOMO-14)
and this is supported by the existence of a 3c-2e bond among the
Zr1-C1-C2 unit of complex 9Zr-Cy in the NBO analysis. This
is also indicated by the strong Zr1-C1 and Zr1-C2 interac-
tions, which lie in the range of a typical Zr-C single bond.24 The
Moreover, we have carried out the molecular orbital (MO)
analysis of bonding of complex 9Zr-Cy. In the isolated Cp2Zr
fragment the metal is in the formal oxidation state þ2 with two
valence electrons. The HOMO of complex 9Zr-Cy, which
involves donation of electrons from the metal orbital
(HOMO) to the vacant ligand orbital (LUMO), is bonding
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Figure 4. Correlation diagram between the planar structure and the nonplanar structure of complex 9Zr-Cy. The variation of the HOMO and LUMO
of complex 9Zr-Cy (Cy =R0) is plotted as a function of the nonplanarity of complex 9Zr-Cy (on both sides).
relatively weak interaction between Zr1 and C3 results indirectly
from the strong interaction of C3 and N2. There is one localized
perpendicular π-bond on the C1 and C2 (HOMO-2) carbon
atoms. The short C1-C2 bond length displays the existence of a
π-bond. The metal does not have any significant interaction with
this perpendicular π-orbital. So, the orbital is available for a
further interaction with another moiety. The HOMO-1 is a
perpendicular p-orbital localized on N1 carrying the lone pair of
the nitrogen atom. So, the molecular orbital analysis supports a
cycloallenic electronic structure for complex 9Zr-Cy. The
HOMO, LUMO, HOMO-1, HOMO-2, HOMO-8, and
HOMO-14 of complex 9Zr-Cy are shown in Figure 3.
A correlation diagram between the planar structure and
nonplanar structure of complex 9Zr-Cy is drawn in Figure 4
in order to understand why the complex adopts a nonplanar
structure. The HOMO and LUMO of the planar complex (9Zr-
CyP, Figure 4) are very close in energy (ΔE = 0.04 eV). As a
result, the planar complex (9Zr-CyP) undergoes structural
distortion, allowing mixing of HOMO and LUMO and thus
stabilizes the nonplanar complex 9Zr-Cy to a greater extent.
The HOMO-LUMO gap increases with the increase in the
extent of nonplanarity as can be seen from the correlation
diagram (from ΔE = 0.04 eV to ΔE = 2.5 eV, Figure 4). The
structural distortion takes place due to a second-order energy
change in the HOMO. So, this is a second-order or pseudo-
Jahn-Teller distortion.25
Thus, the near-planar structure (9Zr-CyTS, Scheme 10) of
complex 9Zr-Cy turns out to be the transition state with a barrier
of 18.3 kcal/mol for the conformational isomerism of complex
9Zr-Cy, where the C3 carbon atom moves up and down to get to
the nonplanar geometry as the stable structure.
The C1-C2-C3 allene arrangement (<C1-C2-C3 =
155.7ꢀ) in complex 9Zr-Cy falls in the category of a bent
allene.26 This moiety is effectively stabilized by the Cp2Zr
fragment, thus reconfirming the tendency of the bent metallo-
cene (Cp2M) unit to internally stabilize strained π systems.
The complex 9Zr-Cy may be described as shown before in
Scheme 9. It is a long-standing question of whether to describe a
system such as complex 9Zr-Cy as a coordinated ene-yne
complex (F) or a cycloallene complex (G). The NICS values
of the isolated ligand and the complex are shown in Table 1. The
increase in the negative NICS values, when the isolated ligand
forms the complex, implies a significant interaction between the
Cp2Zr moiety and the allenic ligand moiety. In a similar way,
complex 9Zr-p-Tol is also shown to have a good interaction
between the Cp2Zr moiety and the allenic ligand moiety. The
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Scheme 10. Schematic Representation of the Transition State of Complex 9Zr-Cy
Table 1. Calculated NICS (x) Values (in ppm) for Complexes 9Zr-Cy and 9Zr-p-Tol (at the BP86/BS2 level of theory)
9Zr-Cy
(bent-[Me3SiHCCC(NSiMe3(Cy)NHCy])b
concave side convex side
NICS(x)a
concave side
convex side
NICS (0)
NICS (0.5)
NICS (1.0)
NICS (1.5)
-19.7
-9.5
-16.6
-12.0
-8.0
-19.5
-12.5
-7.2
-7.4
-4.0
-1.7
-9.9
-5.9
-2.4
9Zr-p-Tol
(bent-[Me3SiCCC(NSiMe3(p-Tol)NHp-Tol])b
concave side convex side
NICS(x)a
concave side
convex side
NICS (0)
-21.5
-17.4
-12.2
-8.0
-9.7
-20.9
-12.8
-7.0
NICS (0.5)
NICS (1.0)
NICS (1.5)
-6.8
-10.2
-3.2
-1.1
-5.5
-1.8
a Calculated at the ring center (0) and above the ring center (0.5 to 1.5 Å). b Using the same structure by deleting Cp2Zr and satisfying the two dangling
valencies of the bent-[Me3SiHCCC(NSiMe3(R)NHR] (R = Cy, 9Zr-Cy; R = p-Tol, 9Zr-p-Tol) by two H atoms.
stabilization of a bent allene unit, as mentioned above, by the
Cp2Zr moiety in complex 9Zr-Cy also supports this.
important role. Further studies on the reactivity of these com-
plexes in ring-opening and insertion reactions as well as coordi-
nation of Lewis acidic fragments to the metallacycle are currently
undertaken and will provide useful insights regarding the role of
the ring heteroatom.
Thus, from MO and NBO analysis as well as NICS calcula-
tions, the complex 9Zr-Cy is better described as a σ-complex
with cycloallenic character (G, Scheme 9) rather than a coordi-
nated ene-yne complex (F). Pseudo-Jahn-Teller distortion plays
a major role in stabilizing the nonplanar structure of the complex.
5. EXPERIMENTAL SECTION
4. CONCLUSION
General Information. All manipulations were carried out in an
oxygen- and moisture-free argon atmosphere using standard Schlenk and
drybox techniques. Nonhalogenated solvents were dried over sodium/
benzophenone and freshly distilled prior to use. The carbodiimides were
purchased from Sigma-Aldrich and sublimed or distilled prior to use. The
alkyne complexes Cp2Ti(η2-Me3SiC2SiMe3) (7) and Cp2Zr(py)(η2-
Me3SiC2SiMe3) (11) were synthesized as described in literature
previously.3b,27,28 1H, 13C, and 29Si NMR spectra were recorded on
The reaction of group 4 titanocene and zirconocene alkyne
complexes 7 and 11 with cyclohexyl-, isopropyl-, or p-tolyl-
substituted carbodiimides revealed the formation of five-mem-
bered metalla(IV)cycles exhibiting allene moieties. These me-
tallacycles may be also described as metallocene(II) bis-π-
complexes. In each case, a Si-C bond-cleavage step forming a
metallocene silyl-alkynyl compound has to be supposed. Subse-
quently, insertion of the hetero-allene substrate into the newly
formed Si-Zr bond with a following intramolecular C-C
coupling was expected. The molecular structures of these com-
plexes confirmed a contribution of both mesomeric structures
(metallacycle and bis-π-complex). Interestingly, theoretical stud-
ies show the hetero-metallacycloallene form of complex 9Zr-Cy
to be the major contributor to the resonance hybrid of the
complex. The herein described complexes are the first examples
for heteroatom-containing metallacyclopenta-2,3-dienes. In re-
spect to the described reactions, the choice of metallocene
source, allene substrate, and further substituents plays an
1
Bruker AV250, AV300, and AV400 instruments, respectively. The H
and 13C chemical shifts were referenced to the solvent signals: benzene-d6
(δH 7.16, δC 128.0), CDCl3 (δH 7.26, δC 77.2), and THF-d8 (δH 3.58 and
1.73, δC 67.4 and25.2). IR spectrawere recordedona Nicolet6700 FT-IR
spectrometer with a smart endurance attenuated total reflection
(ATR) device. Mass spectra were recorded on a Finnigan MAT 95-XP
(Thermo-Electron). Elemental analysis were performed on a Leco
CHNS-932 elemental analyzer. Melting points are uncorrected and were
measured in sealed capillaries using an E/Z-Melt, Standford Research
Systems.
Computational Details. The complexes 9Zr-Cy and 9Zr-p-Tol
were optimized at the BP86 level of theory using Gaussian 03 and
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Gaussian 09 program packages.29 BP86 includes the exchange function
of Becke30,31a,31b and the correlation functional of Perdew.30,32 For the
calculations, two types of basis sets were used: (i) BS1: LANL2DZ for all
the atoms and (ii) BS2: the 6-311þG(d,p) basis set for C, H, N, Si
atoms, and the LANL2DZ basis set for the Zr atom. The LANL2DZ
basis set uses the effective core potentials (ECP) of Hay and Wadt.33
Both the complexes are found to be minima on the potential energy
surface.
(ATR 16 scans, cm-1): 3097 (w), 2958 (w), 2923 (m), 2849 (m), 2664
(w), 1766 (m), 1586 (m), 1463 (w), 1446 (m), 1358 (w), 1338 (m),
1257 (s), 1245 (w), 1234 (w), 1181 (m), 1094 (s), 1011 (s), 979 (w),
938 (m), 882 (m), 837 (m), 780 (s), 752 (w), 685 (m), 648 (m), 623
(w), 595 (w), 532 (w). MS (CI, i-butane): m/z (%) 596 (100)
[C31H50N2Si2Zr]þ, 427 (9) [C23H32N2Zr
þ
H]þ, 377 (83)
[C21H40N2Si2 þ H]þ, 303 (17) [C18H31N2Si þ H]þ, 207 (14)
[C13H22N2 þ H]þ, 171 (29) [C8H18Si2 þ H]þ, 73 (9) [C3H9Si]þ.
Synthesis of Complex 9Zr-i-Pr. To a solution of the zircono-
cene complex 11 (0.235 g, 0.500 mmol) in 10 mL of n-hexane was added
dropwise 1,3-N,N0-diisopropylcarbodiimide (8-i-Pr) (0.063 g, 0.500
mmol). After addition, the reaction mixture turned from dark violet to
orange and was allowed to stand for 16 h to complete the reaction. The
solution was filtered and the solvent was removed in vacuum. By
recrystallization from a mixture of n-hexane, ether, and THF at -78 ꢀC a
yellow powder was obtained which was isolated, washed with cold
n-hexane, and dried in vacuum. Yield: 0.224 g (86%). Mp: 109 ꢀC dec.
Synthesis of Complex 9Ti-i-Pr. The titanocene complex 7
(0.300 g, 0.861 mmol) was dissolved in 5 mL of n-hexane and cooled
to -30 ꢀC. To this solution was added 1,3-N,N0-diisopropylcarbodii-
mide (8-i-Pr) (0.110 g, 0.861 mmol). The solution was kept at -30 ꢀC
for five days. During this time the color of the reaction mixture changed
from light brown to dark red. The solution was concentrated in vacuum
and stored at -30 ꢀC. The product crystallized at this temperature and
was isolated by decanting of the mother liquor. Yield: 0.185 g (50%).
Anal. Calcd for C25H42N2Si2Ti (474.65 g mol-1): C, 63.26; H, 8.92; N,
3
Anal. Calcd for C25H42N2Si2Zr (518.01 g mol-1): C, 57.97; H, 8.17;
1
5.90. Found: C, 63.16; H, 8.80; N, 5.77. H NMR (benzene-d6, 300
3
1
N, 5.41. Found: C, 58.28; H, 8.12; N, 5.34. H NMR (benzene-d6,
MHz, 298 K): δ = 0.39 (s, 9H, SiMe3), 0.42 (s, 9H, SiMe3), 1.15 (d,
6H, CH3, i-Pr-N-Si), 1.26 (d, 6H, CH3, i-Pr-N-Ti), 3.07 (m, 1H,
CH-N-Si), 3.97 (m, 1H, CH-N-Ti), 5.25 (s, 10H, Cp). 13C NMR
(benzene-d6, 75 MHz, 297 K): δ = -0.6 (Me3Si-C-Ti), 0.6 (Me3Si-
N), 25.0 ((CH3)2CH-N-Si), 25.3 ((CH3)2CH-N-Ti), 49.3 (CH-
N-Si), 50.2 (CH-N-Ti), 104.0 (Cp), 110.6 (Ti-C-Si), 117.9
(CdC-(N)2), 145.9 (CdCdC). 29Si NMR (benzene-d6, 60 MHz,
297 K): δ = -7.88 (SiMe3), 4.34 (SiMe3). MS (CI, i-butane): m/z (%)
474 (37) [C25H42N2Si2Ti]þ, 344 (13) [C19H26NSiTi]þ, 279 (13)
[C14H21NSiTi]þ, 167 (42) [C9H19NSi2]þ.
300 MHz, 297 K): δ = 0.44 (s, 9H, SiMe3) 0.45 (s, 9H, SiMe3), 1.04-
1.12 (m, 9H, CH3), 1.40 (m, 3H, CH3), 3.07 (m, 1H, CH), 3.97 (m, 1H,
CH), 5.37 (s, 5H, Cp), 5.59 (s, 5H, Cp). 13C NMR (benzene-d6, 75
MHz, 297 K): δ = 1.8 (SiMe3), 4.1 (SiMe3), 24.6 (CH3, i-Pr), 24.7
(CH3, i-Pr), 25.8 (CH3, i-Pr), 26.0 (CH3, i-Pr), 47.4 (CH, i-Pr), 48.8
(CH, i-Pr), 103.5 (Cp), 105.5 (Cp) 106.8 (Zr-C(Me3Si)d), 111.0
(dC-(N)2), 134.2 (dCd). 29Si NMR (benzene-d6, 80 MHz, 297 K)
δ = -7.92 (SiMe3), 3.16 (SiMe3). IR (Nujol mull, cm-1): 3103 (w),
2959 (m), 1770 (m), 1578 (w), 1306 (m), 1264 (m), 1174 (m), 1152
(m), 1020 (m), 841 (m), 784 (m). MS (EI, 70 eV): m/z (%) 516 (36)
[M]þ, 473 (21) [M - C3H7]þ, 289 (24) [(C5H5)2Zr(CNiPr)]þ, 220
(100) [(C5H5)2Zr]þ, 155 (91) [(C5H5)Zr]þ.
Synthesis of Complex 1015. 1,3-N,N0-Di-p-tolylcarbodiimide
(8-p-Tol) (0.063 g, 0.287 mmol) was dissolved in 10 mL of n-hexane.
The solution was cooled to -78 ꢀC and a solution of the titanocene
complex 7 (0.100 g, 0.287 mmol) in 10 mL of n-hexane was added. The
color of the reaction mixture changed from light brown to red, and after
several minutes a red precipitate was formed, which was isolated by
filtration. Crystals suitable for X-ray analysis were obtained from
saturated n-hexane solutions. Yield 0.130 g (58%). Mp: 115 ꢀC dec.
Due to the paramagnetic properties of this titanocene(III) complex
(10), no NMR data could be obtained. IR (ATR 16 scans, cm-1): 3021
(w), 2952 (m), 2919 (w), 2895 (w), 1814 (m), 1573 (s), 1504 (s), 1441
(w), 1405 (w), 1313 (m), 1303 (w), 1243 (s), 1213 (w), 1172 (w), 1104
(m), 1014 (m), 951 (m), 922 (m), 833 (s), 790 (m), 750 (m), 716 (w)
681 (m), 630 (m).
Synthesis of Complex 9Zr-p-Tol. 1,3-N,N0-Di-p-tolylcarbodi-
imide (8-p-Tol) (0.111g, 0.500mmol) wasdissolvedin5mLofTHF. This
solution was added to a solution of the zirconocene complex 11 (0.232 g,
0.500 mmol) in 5 mL of THF. After addition, the reaction mixture
turned from dark violet to orange and was allowed to stand for
6 h to complete the reaction. The solution was filtered and cooled to
-40 ꢀC. The resulting yellow precipitate was isolated and washed with
cold THF. Recrystallization from THF gave yellow crystals which where
isolated and dried in vacuum. Crystals suitable for X-ray analysis were
obtained from THF-d8. Yield: 0.198 g (65%). Mp: 217 ꢀC dec. Anal.
Calcd for C33H42N2Si2Zr (614.09 g mol-1): C, 64.54; H, 6.89; N, 4.56.
3
Synthesis of Complex 9Zr-Cy. 1,3-N,N0-Dicyclohexylcarbodi-
imide (8-Cy) (0.044 g, 0.213 mmol) was dissolved in 5 mL of n-hexane.
This solution was added to a suspension of the zirconocene complex 11
(0.100 g, 0.213 mmol) and 15 mL of n-hexane. After addition, the color
of the reaction mixture turned from dark violet to orange, and the
mixture was stored overnight (12 h) at 60 ꢀC to complete the reaction.
After cooling to room temperature the solvent was removed and a deep
red, oily solid was obtained. As attempts of recrystallization failed due to
the high solubility of the product, the complex was sublimed under
reduced pressure (10-3 mbar) at 120 ꢀC. During the sublimation the
pure reaction product condensed in yellow crystals and appeared to be
suitable for X-ray analysis. Yield: 0.102 g (80%). Mp: 157 ꢀC dec. Anal.
Found: C, 61.13; H, 6.94; N, 4.52. Despite the use of V2O5 no accurate
1
carbon content could be detected. H NMR (THF-d8, 300 MHz, 300
K): δ = 0.26 (s, 9H, C-Me3Si), 0.33 (s, 9H, N-Me3Si), 2.19 (s, 3H,
CH3, p-Tol), 2.34 (s, 3H, CH3, p-Tol), 5.36 (s, 5H, Cp), 5.47 (s, 5H,
Cp), 6.68 (m, 2H, CH, p-Tol), 6.82 (m, 2H, CH, p-Tol), 6.95 (m, 2H,
CH, p-Tol), 7.10 (m, 2H, CH, p-Tol). 13C NMR (THF-d8, 75 MHz, 300
K): δ = 1.5 (C-Me3Si), 1.9 (N-Me3Si), 20.8 (CH3, p-Tol), 20.8 (CH3,
p-Tol), 105.1 (Cp), 106.2 (Cp), 114.6 (qC), 121.9 (CH, p-Tol), 129.0
(CH, p-Tol), 129.8 (CH, p-Tol), 130.5 (qC), 131.4 (CH, p-Tol), 133.3
(qC), 135.3 (qC), 141.8 (qC), 143.9 (qC), 150.4 (qC). 29Si NMR (THF-
d8, 80 MHz, 300 K): δ = -7.13 (SiMe3), 7.04 (SiMe3). IR (ATR 16
scans, cm-1): 3020 (w), 2960 (m), 2919 (w), 2897 (w), 2860 (w), 2728
(w), 1775 (m), 1726 (m), 1588 (m), 1549 (m), 1502 (s), 1440 (m),
1408 (w), 1357 (m), 1302 (w), 1282 (w), 1258 (s), 1203 (w),
1168 (w), 1091 (m), 1012 (s), 833 (w), 786 (s), 751 (w), 719 (w),
687 (m), 631 (m), 615 (m). MS (CI, i-butane): m/z (%) 612 (11)
[C33H42N2Si2Zr]þ, 395 (100) [C23H32N2Si2 þ H2 þ H]þ, 393 (73)
[C23H32N2Si2 þ H]þ, 323 (15) [C20H24N2Si þ H2 þH]þ, 321 (9)
[C20H24N2Si þ H]þ, 297 (11) [C18H24N2Si þ H]þ, 225 (41)
[C15H16N2 þ H]þ.
Calcd for C31H50N2Si2Zr (598.14 g mol-1): C, 62.25; H, 8.34; N, 4.68.
3
Found: C, 55.13; H, 8.29; N, 4.68. Despite the use of V2O5 no accurate
1
carbon content could be detected. H NMR (benzene-d6, 300 MHz,
300 K): δ = 0.46 (s, 9H, SiMe3), 0.48 (s, 9H, SiMe3), 1.81-0.85 (m,
22H, Cy), 5.42 (s, 5H, Cp), 5.61 (s, 5H, Cp). 13C NMR (benzene-d6,
75 MHz, 300 K): δ = 1.4 (N-Me3Si), 1.9 (C-Me3Si), 26.2 (CH2, Cy),
26.6 (CH2, Cy), 26.8 (CH2, Cy), 27.2 (CH2, Cy), 27.4 (CH2, Cy), 27.9
(CH2, Cy), 35.9 (CH, Cy), 37.4 (CH, Cy), 103.5 (Cp), 105.5 (Cp),
109.4 (Zr-C(Me3Si)d), 112.4 (dC-(N)2), 134.8 (dCd). 29Si NMR
(benzene-d6, 60 MHz, 300 K) δ = -8.07 (SiMe3), -3.50 (SiMe3). IR
Structure Elucidation. Crystals of the complexes 9Ti-i-Pr, 10,
9Zr-Cy, and 9Zr-p-Tol suitable for X-ray determination (Table 2), were
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Table 2. Crystal Structure Data
9Ti-i-Pr
10
9Zr-Cy
9Zr-p-Tol
chem. formula
form. weight [g mol-1
C
25H42N2Si2Ti
C50H48N4 Ti2
800.72
brown
C31H50N2Si2Zr
598.13
yellow
C33H42N2Si2Zr
614.09
yellow
]
474.69
dark red
triclinic
P1
3
color
crystal system
space group
a [Å]
triclinic
P1
triclinic
P1
orthorhombic
Pccn
10.3611(11)
11.5566(13)
13.457(2)
96.926(6)
106.836(6)
116.529(4)
1319.8(3)
2
8.1040(6)
10.3858(7)
12.3515(9)
94.436(3)
94.238(3)
103.346(3)
1003.96(12)
1
10.4688(3)
12.2660(4)
13.3377(4)
78.808(1)
85.741(1)
69.627(1)
1574.99(8)
2
16.8952(6)
31.7271(9)
11.7933(3)
90.00
b [Å]
c [Å]
R [deg]
β [deg]
γ [deg]
V [Å3]
Z
90.00
90.00
6321.6(3)
8
F
calc. [g cm-3
]
1.194
1.324
1.261
1.290
3
μ [mm-1
Mo KR [Å]
T [K]
]
0.429
0.439
0.447
0.447
λ
0.71073
173(2)
19446
0.71073
173(2)
12735
0.71073
133(2)
15194
0.71073
200(2)
32846
no. of rflns collected
no. of indep. rflns
5665
4905
8866
6193
no. of rflns with I > 2σ(I)
3678
2644
7837
4174
Rint.
0.0613
512
0.0491
420
0.0189
636
0.0330
2576
F(000)
R1 (R [F2 > σ2 (F2)])
wR2(F2)
0.0504
0.1076
0.989
0.0536
0.1328
0.907
0.0713
0.0740
1.043
0.0255
0.0506
0.825
GooF
parameters
281
283
331
351
selected in Fomblin YR-1800 oil (Alfa Aesar) at room temperature. All
samples were cooled during measurement. The data were collected on a
Bruker Apex Kappa-II CCD diffractometer (9Ti-i-Pr, 10 and 9Zr-Cy)
and a STOE IPDS II diffractometer (9Zr-p-Tol). The radiation was
graphite monochromated Mo KR (λ = 0.71073 nm). The structures
were solved by direct methods and refined by least-squares full matrix
methods.34 For compounds 9Ti-i-Pr, 10, and 9Zr-Cy semiempirical
absorption corrections (SADABS)35 were utilized; complex 9Zr-p-Tol
was treated with numerical absorption correction (X-Shape and
X-Red32).36 All non-hydrogen atoms were refined anisotropically, all
H atoms were positioned geometrically and refined using a riding model.
Diamond was used for graphical representations.37
9Zr-p-Tol. Financial support by the DFG (RO 1269/7-2) is
gratefully acknowledged. E.D.J. and S.R. acknowledge the SERC
of IISc and CMSD of UoH for computations. The Department of
Science and Technology, New Delhi, is thanked for funding this
research (J. C. Bose Fellowship).
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Arndt, P.; Baumann, W.; Spannenberg, A. Organometallics 2003, 22, 884.
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U.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A. Organo-
metallics 2005, 24, 456. (e) Rosenthal, U. In Acetylene Chemistry;
Diederich, F., Stang, P. J., Tykwinski, R. R., Eds.; VCH: Weinheim,
2005; p 139. (f) Rosenthal, U.; Burlakov, V. V.; Bach, M. A.; Beweries, T.
Chem. Soc. Rev. 2007, 36, 719. (g) Rosenthal, U.; Burlakov, V. V.; Arndt,
P.; Spannenberg, A.; J€ager-Fiedler, U.; Klahn, M.; Hapke, M. In
Activating Unreactive Substrates; Bolm, C., Hahn, E., Eds.; VCH:
Weinheim, 2009; p 165. (h) Jemmis, E. D.; Phukan, A. K.; Giju, K. T.
Organometallics 2002, 21, 2254. (i) Jemmis, E. D.; Phukan, A. K.; Jiao,
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M. A.; Parameswaran, P.; Jemmis, E. D.; Rosenthal, U. Organometallics
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic details of
b
complexes 9Ti-i-Pr, 10, 9Zr-Cy, and 9Zr-p-Tol; details of the
DFT calculations on complexes 9Zr-Cy and 9Zr-p-Tol as well as
the full reference 29.This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jemmis@iisertvm.ac.in; axel.schulz@uni-rostock.de; uwe.rosenthal@
catalysis.de
’ ACKNOWLEDGMENT
We thank our technical and analytical staff for assistance, in
particular Dr. Anke Spannenberg for X-ray analysis of compound
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Journal of the American Chemical Society
ARTICLE
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