Reactions of Cp3Y with benzophenone: A simple and efficient method for transformation of unsubstituted cyclopentadienyl to bridged ansa -cyclopentadienyl/alkoxyl ligand

Article abstract of DOI:10.1021/om100755s

Insertion of benzophenone into the Y-Cp (Cp = C₅H₅) bond and two new reactivity patterns of the Cp-substituted alkoxide complexes have been established, by which an efficient and convenient method for conversion of the unsubstituted cyclopentadienyl group to the single-carbon-bridged ansa-cyclopentadienyl/alkoxyl ligand by using a simple ketone as the functionalizing reagent is developed. All products including the four-center interaction precursor of the insertion have been characterized by X-ray structural analyses.

Full text of DOI:10.1021/om100755s

4606 Organometallics 2010, 29, 4606–4610
DOI: 10.1021/om100755s
Reactions of Cp3Y with Benzophenone: A Simple and Efficient Method for
Transformation of Unsubstituted Cyclopentadienyl to Bridged
ansa-Cyclopentadienyl/Alkoxyl Ligand
Xiaoqing Li,† Jianquan Hong,† Ruiting Liu,† Linhong Weng,† and Xigeng Zhou*,†,‡
†Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Fudan University, Shanghai 200433, People’s Republic of China, and ‡State Key Laboratory of
Organometallic Chemistry, Shanghai 200032, People’s Republic of China.
Received August 2, 2010
Insertion of benzophenone into the Y-Cp (Cp = C5H5) bond and two new reactivity patterns of
the Cp-substituted alkoxide complexes have been established, by which an efficient and convenient
method for conversion of the unsubstituted cyclopentadienyl group to the single-carbon-bridged
ansa-cyclopentadienyl/alkoxyl ligand by using a simple ketone as the functionalizing reagent is
developed. All products including the four-center interaction precursor of the insertion have been
characterized by X-ray structural analyses.
Controlling metal coordination environment and reactiv-
ity through modification of supporting ligation is a common
theme in organometallic chemistry. Cyclopentadienyl and
substituted cyclopentadienyl groups are ubiquitous func-
tional ligands in organometallic compounds that have been
extensively used as catalysts in organic synthesis1 and olefin
polymerizations.2 In addition, metallocene complexes are also
valuable as synthetic intermediates in organic synthesis,3 as
precursors to a wide range of functional materials,4 and as
potential pharmaceuticals;5 thus considerable effort continues
to be directed toward the development of new substituted
cyclopentadienyl ligands and new methods for their
construction.6 The major goal is to obtain new functionalized
metallocenes with required properties and develop simpler and
even cheaper methods for synthesis of metallocenes. Among
a variety of approaches for their preparation, the preparation
of functionalized cyclopentadienyl complexes directly from
simple cyclopentadienyl complexes and unsaturated substrates
is arguably one of the best possibilities from both economic and
environmental viewpoints, because it avoids the initial synthe-
sis of substituted cyclopentadienes that undergo easily dimer-
ization, an additional step that sometimes involves expensive or
toxic reagents and requires chemical separations. Although
many viable methods for ring modification of metallocene
complexes of late transition metals have been developed, includ-
ing functionalization via metallacyclopentadienylation,7 direct
C-H addition,8 and Friedel-Crafts reactions,9 they usually
require the use of additives such as Lewis acids and strong bases
and/or generate the acidic byproducts, which are not suitable
for sensitive metallocene complexes having labile metal-
cyclopentadienyl bonds.10 Thus, the development of a funda-
mental strategy capable of functionalizing the unsubstituted
*To whom correspondence should be addressed. E-mail: xgzhou@
fudan.edu.cn.
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Coord. Chem. Rev. 2006, 250, 1141. (c) Kotora., M. Top. Organomet.
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pubs.acs.org/Organometallics
Published on Web 09/30/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 20, 2010 4607
Scheme 1
the synthesis of the bifunctionalized cyclopentadienyl (C5H3(C-
(O)dCPh2)2) trianion yttrium complex. However, very poor
yield is obtained for the synthesis of the monofunctionalized
cyclopentadienyl (C5H4C(O)dCPh2) dianion complex by this
protocol. Furthermore, there is little information concerning the
insertion intermediates.
In order to obtain further insight into the insertion reac-
tivity of Cp3Y and to develop the synthetic strategy for the
one-pot synthesis of bridged ansa-cyclopentadienyl/alkoxy
complexes from aunsubstituted cyclopentadienyl complex
precursors, we became interested in the possibility of insert-
ing a ketone into the η5-Y-C5H5 bond and the in situ
transformation of the resulting insertion products to func-
tionalized cyclopentadienyl complexes and hoped that an
investigation of the difference between ketones and ketenes
might lead to the development of conditions for the inser-
tions of Cp3Y in improved yield, selectivity, substrate scope,
and avoidance of the use of strong bases. Herein, we report
an efficient direct conversion of the Cp group bound to the
yttrium into the single-carbon-bridged ansa-cyclopentadienyl/
alkoxyl dianion ligand with Ph2CO as a functionalizing
reagent.
cyclopentadienyl ligand of sensitive organometallic compounds
represents a highly desirable but significantly challenging target.
Recently, precedent for insertion of carbodiimide into the
η1-indenyl-zirconium bond of {σ:η5-(C9H6)C2B9H10}Zr{η2-
(RN)2C(NMe2)}(THF) and subsequent regeneration of the
η1-substituted indenyl-zirconium bond has shed light on
ring modification of sensitive metallocenes.11 However, at-
tempts to expand upon this class of reaction to the η5-
cyclopentadienyl system have been unsuccessful.12 In addition,
whether the resulting ring-substituted ligands could regenerate
the η5-bonding mode remains unexplored in organometallic
chemistry of main and rare earth metals, although it has proven
successful for a few large sterically crowded lanthanocenes such
as (C5Me4R)3Ln,13 (Flu)2Ln(THF)2 (Flu= fluorenyl),14 and
(Ind)2Yb(THF)2 (Ind = indenyl)15 and a variety of metallocene
complexes of main metals16 to undergo insertions with unsatu-
rated substrates. During our research into the reactivity of or-
ganolanthanides toward ketenes,17 we observed in one case that
Cp3Y can undergo unusual tandem insertion/cycloaddition/
isomerization reactions with diphenyl ketene (Scheme 1).18
These reactions have proven to be efficient as the key step in
Treatment of Cp3Y with 1 equiv of Ph2CO in toluene at
room temperature clearly indicated the formation of the
adduct Cp3Y(OCPh2) (1), albeit in a low yield with a large
amount of Cp3Y recovered (Scheme 2). Attempts to obtain
1
the H NMR information of 1 were impaired due to the
partial dissociation of coordinated Ph2CO molecules in
common lab solvents. The molecular structure of 1 is shown
in Figure 1, featuring the donating coordinated Ph2CO
˚
moiety. The C(1) C(27) distance is 3.675 A, which clearly
3 3 3
prefigures the possibility of 1 as a precursor of the well-
documented four-centered insertion transition state.19
In marked contrast to Ph2CdCdO,18 heating a 1:1 mix-
ture of Cp3Y and Ph2CO in toluene at 85 °C for 8 h, after
recrystallization in THF, gave the Cp-substituted alkoxide
complexes 2 as colorless crystals in 67% yield, indicating that
one Ph2CO molecule inserts into a Y-Cp bond to afford the
initial product (2c), which subsequently undergoes tauto-
merization via a formal 1,3- or 1,5-hydrogen shift of cyclo-
pentadiene, forming the stable alkoxide complexes 2a and
2b, as shown in Scheme 2. However, the reaction of Cp3Y
with Ph2CO in THF did not occur even with prolonged
heating at 70 °C. Furthermore, the dissociation of 1 to
Cp3Y(THF) and Ph2CO was observed when it was treated
with THF. This demonstrates that the precoordination of
ketone to the metal plays a key role in the insertion process.
Clearly, in the present case the coordination of a metal center
to Ph2CO not only promotes the polarization of the carbonyl
but also places the Cp group and the inserting molecule into
close proximity, which facilitates the formation of the four-
membered insertion transition state.19
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1
The H NMR spectrum indicates that 2 exists mainly as
two isomers, 2a and 2b (ca. 1.38:1), of the three possible
isomers due to double-bond migration in the cyclopenta-
diene ring. Furthermore, the structure of 2a (Figure 2) was
unambiguously determined via X-ray crystallography anal-
ysis. The pseudotetrahedral coordination sphere about Y in
2a is composed of two centers of η5-Cp rings, one oxygen
atom from the newly formed alkoxide ligand and one THF
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4608 Organometallics, Vol. 29, No. 20, 2010
Li et al.
Scheme 2
Figure 1. Thermal ellipsoid (30%) plot of 1. Hydrogen atoms
˚
are omitted for clarity. Selected bond distances (A) and angles
(deg): C(1) C(27) 3.675, Y(1)-O(1) 2.342(3), O(1)-C(1)
3 3 3
1.222(4), Y(1)-C(24) 2.682(5), Y(1)-C(25) 2.686(5), Y(1)-C-
(26) 2.715(5), Y(1)-C(27) 2.688(5), Y(1)-C(28) 2.680(5), C-
(1)-O(1)-Y(1) 170.3(3).
Figure 2. Thermal ellipsoid (30%) plot of 2a. Hydrogen atoms
˚
are omitted for clarity. Selected bond distances (A) and angles
(deg): O(2)-C(11) 1.408(5), Y(1)-O(2) 2.046(3), Y(1)-O(3)
2.345(3), C(11)-O(2)-Y(1) 171.6(3).
˚
oxygen. The 1.408(5) A O(2)-C(11) distance is in the normal
range of C-O single-bond distances. The bond angles
around C(18) are consistent with sp2 hybridization, while
those around C(11) are consistent with sp3 hybridization.
To our delight, reaction of 2 with n-BuLi in THF gave
selectively the ansa-bridged cyclopentadienyl/alkoxyl yttrium
derivative [CpY(μ-η1:η1:η5-OCPh2C5H4)]2 (3) in 71% isolated
yield. The formation of complex 3 might be interpreted as
metalation of the free cyclopentadiene moiety of 2 followed
by intramolecular ligand exchange between Y3þ and Liþ, as
shown in Scheme 2. The eliminated CpLi is structurally
confirmed by X-ray diffraction analysis.20 A view of the
crystallographic structural data for 2a (Figure 2) indicates
that during crystal packing there is no positional preference for
the pendent cyclopentadienyl group and the second phenyl
group, and both are disordered over two sites, related by a
rotational displacement.
Assembling the anionic substituent on the cyclopenta
dienyl ring judiciously has become an attractive strategy for
the fine-tuning of stability, catalytic activity, selectivity, and
reactivity of organometallic compounds.21 Despite signifi-
cant recent advances in this field, examples of metal com-
plexes bearing linked ansa-cyclopentadienyl/alkoxyl dianion
ligands remain scarce.22 This is somewhat surprising, con-
sidering both the extensive use of the amido analogues2a,b,21
in organometallic chemistry and the expectation that the
nature of the related ligand system may be adjusted by
variation of either the cyclopentadienyl moiety, the alkoxyl
(21) Braunschweig, H.; Breitling, F. M. Coord. Chem. Rev. 2006, 250,
2691.
(22) (a) Cauo, J.; Kunz, K. J. Organomet. Chem. 2007, 692, 4411. (b)
Siemeling, U. Chem. Rev. 2000, 100, 1495.
(20) The structural parameters are identical to those previously
reported: Dinnebier, R. E.; Behrens, U.; Olbrich, F. Organometallics
1997, 16, 3855.

Article
Organometallics, Vol. 29, No. 20, 2010 4609
Figure 4. Thermal ellipsoid (30%) plot of 4. Hydrogen atoms
˚
are omitted for clarity. Selected bond distances (A) and angles
Figure 3. Thermal ellipsoid (30%) plot of 3. Hydrogen atoms
˚
are omitted for clarity. Selected bond distances (A) and angles
(deg): Y(1)-O(2) 2.207(4), Y(1)-O(3) 2.386(4), Y(1)-O(1)
2.444(4), Y(2)-O(2) 2.183(4), Y(2)-O(1) 2.222(4), O(1)-C(1)
1.430(6), C(1)-C(14) 1.511(7); O(2)-O(4) 2.90(1); O(2)-Y-
(1)-O(3) 78.2(2), O(2)-Y(1)-O(1) 70.6(2), O(3)-Y(1)-O(1)
138.6(2), O(2)-Y(2)-O(1) 75.4(2), C(1)-O(1)-Y(2) 142.8(4),
Y(2)-O(1)-Y(1) 102.1(2), Y(2)-O(2)-Y(1) 111.7(2), O(1)-C-
(1)-C(14) 104.2(5).
(deg): Y(1)-O(1) 2.326(2), Y(1)-O(1A) 2.262(2), O(1)-C(11)
1.436(4), C(6)-C(11) 1.530(6), O(1A)-Y(1)-O(1) 74.7(1), O-
(1)-C(11)-C(6) 105(1).
group, the ansa-bridge, or a combination thereof. There does
not appear to be a large effort to expand upon this class of
compounds, which is probably due to the lack of viable
synthetic methodologies to access these complexes. The
anionic side chains of cyclopentadienyl ligands are mainly
introduced before application in the synthesis of constrained
geometry complexes (CGCs). The approach of introducing
an ansa-bridge by a reaction in the ligand sphere of a suitable
metallocene precursor to create CGCs is much less developed
and gives access only to a limited variety in terms of prefunc-
tionalized cyclopentadienyl complexes.23 To the best of our
knowledge, there are as yet no generally satisfactory routes
to the direct introduction of the anionic substituent on the
unsubstituted Cp ring. Obviously, the present reaction pro-
vides an efficient means of obtaining CGCs containing the
bridged ansa-cyclopentadienyl/alkoxyl ligands.
of 2 by intramolecular cyclopentadiene abstraction. Gratify-
ingly, heating 2 in refluxing toluene, after recrystallization in
THF, gave one new bridged cyclopentadienyl/alkoxyl complex,
Cp3Y2(μ-OH)(μ-η1:η1:η5-OCPh2C5H4)(THF)2 (4), accompa-
nied with the liberation of diphenylfulvene and cyclopenta-
diene. The formation of 4 might be interpreted as the tandem
intra- and interligand hydrogen migration of 2, for which the
former led to the elimination of diphenylfulvene, while the
latter led to deprotonation and recoordination of the pendant
cyclopentadiene substituent, as shown in Scheme 2. Complex 4
can also be obtained by directly heating a 1:1 mixture of Cp3Y
and Ph2CO in refluxing toluene followed by recrystallization in
THF. The results demonstrate that the resulting cyclopentadiene-
substituted alkoxyl ligand can undergo intramolecular proton
exchange with the unsubstituted cyclopentadienyl ligand, and
thus direct functionalization of unsubstituted cyclopentadienyl
ligand with ketone without the presence of any additive is also
feasible. In addition, the reaction provides the first authentic
structural evidence for the transformation of an alkoxide ligand
to a hydroxide one.
An X-ray crystallographic study unambiguously con-
firmed 3 to be a dimer structure (Figure 3). The newly formed
ansa-bridged cyclopentadienyl/alkoxyl (C5H4CPh2O) ligand
interacts with the two yttrium atoms in a μ-η1:η1:η5-bonding
˚
mode. The average Y-O distance of 2.294(2) A is compa-
rable to the corresponding value in Cp4Y3[C5H3(COCPh2)2-
1,2][C5H3(COCPh2)2-1,2]Li(THF), 2.301(3) A.18 The O-Y-O
˚
and Y-O-Y angles are in the normal range.24
The formation of diphenylfulvene and cyclopentadiene
was identified by GC-MS analysis of the organic products.
The X-ray analysis shows that complex 4 is a dinuclear
structure (Figure 4), and one Ph2C(O)- anion functionality
is appended to a cyclopentadienyl ring via a formal hydrogen
substitution. The structural features of the newly formed
ansa-bridged cyclopentadienyl/alkoxide ligand are essen-
tially identical to those of complex 3. The presence of the
hydroxyl is also confirmed by the hydrogen-bond interaction
between it and the THF molecule.
Considering that the use of n-butyllithium as a deprotonating
reagent should set limit for the utility of this reaction when the
system contains another functional group that is sensitive to
nBuLi, together with the inspiration of the intramolecular
thermal proton migration between the guanidinate ligands,25
we decided to try to deprotonate the pendant cyclopentadiene
ꢀ
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In summary, the insertion of benzophenone into the η5-
Ln-Cp bond and two new reactivity patterns of Cp-substituted
alkoxyl complexes have been revealed, by which an efficient and
convenient conversion of an unsubstituted cyclopentadienyl
ligand into a single-carbon-bridged ansa-cyclopentadienyl/
alkoxyl ligand is developed. The results described here not
only provide good insight into the versatility of insertions of
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4610 Organometallics, Vol. 29, No. 20, 2010
Li et al.
Cp3Ln but also demonstrate that the tandem insertion and
hydrogen transfer reaction of Cp3Ln with unsaturated sub-
strates is a practical and versatile strategy for the direct
introduction of the ansa-bridge on the cyclopentadienyl ring
in the synthesis of CGCs, which are difficult to achieve by
other means, from the viewpoints of operational simplicity
and assembly efficiency.18 Furthermore, this work presents
authentic structural evidence for the transformation of an
alkoxide to a hydroxide. Further investigations on the scope
and limitations of the functionalization of the cyclopentadienyl
and related rings of sensitive lanthanocenes and synthetic
applications of this reaction are underway in our laboratory.
of toluene. Toluene was then removed under vacuum, and the
resulting solid was redissolved in 3 mL of THF. Diffusion of
hexane into the solution afforded complex 3 as colorless crystals.
The residue was dissolved in THF, and diffusion of hexane to
the solution afforded colorless crystals of [C5H5Li]n. Yield:
1
0.455 g (71%). H NMR (400 MHz, C6D6, 25 °C): δ 7.65-
7.55 (m, 4H, Ph), 7.11-6.91 (m, 16H, Ph), 6.43 (s, 10H, C5H5),
5.93 (s, 2H, -C5H4), 5.69(s, 2H, -C5H4), 5.47 (s, 2H, -C5H4),
4.69(s, 2H, -C5H4). Anal. Calcd for C46H38O2Y2 (%): C, 69.01;
H, 4.78. Found: C, 68.89; H, 4.63.
Preparation of Cp3Y2(μ-OH)(μ-η1:η1:η5-OCPh2C5H4)]-
(THF)2 (4). Ph2CO (0.166 g, 0.91 mmol) was added to a stirred
solution of Cp3Y (0.258 g, 0.91 mmol) in toluene (20 mL). After
stirring at 110 °C for 8 h, all volatile substances were removed under
vacuum, and the residue was redissolved in THF and allowed to
stand at -35 °C for several days. Colorless crystals of 4 were
separated. Yield: 0.231 g (65%). 1H NMR (400 MHz, C6D6,25°C):
δ 7.14-7.01 (m, 10H, Ph), 6.52 (s, 5H, C5H5), 6.24 (s, 1H, -C5H4),
6.07 (s, 5H, C5H5), 5.91(s, 1H, -C5H4), 5.86 (s, 1H, -C5H4), 5.75
(s, 1H, -C5H4), 5.60 (s, 5H, C5H5), 3.45 (t, 8H, THF), 1.40-1.33
(m, 8H, THF), 0.82 (s, 1H, OH). Anal. Calcd for C41H46O4Y2 (%):
C, 63.08; H, 5.94. Found: C, 62.89; H, 5.82.
Experimental Section
General Procedures. All manipulations were conducted under
a nitrogen atmosphere using standard Schlenk or glovebox
techniques. All organic solvents (including deuterated solvents
for the NMR measurements) were predried over sodium wire
and distilled from sodium/benzophenone under dinitrogen prior
to use. Benzophenone and n-BuLi (2.5 M in n-hexane) were
purchased from Aldrich Chemical Co. and used as received.
Elemental analyses for C and H were carried out on a Rapid
Crystal Structure Determination. Suitable single crystals were
sealed under N2 in thin-walled glass capillaries. X-ray diffrac-
tion data were collected on a SMART APEX CCD diffracto-
meter (graphite-monochromated Mo KR radiation, φ-ω scan
1
CHN-O analyzer. H NMR data were obtained on a Bruker
DRX-400 NMR spectrometer.
˚
technique, λ = 0.71073 A). The intensity data were integrated by
Preparation of Cp3Y(OCPh2) (1). Ph2CO (0.169 g, 0.93 mmol)
was added to a solution of Cp3Y (0.264 g, 0.93 mmol) in toluene
(20 mL). The color changed from colorless to red immediately.
After stirring for 24 h at room temperature, the solution was
concentrated to ca. 8 mL under reduced pressure and allowed to
stand at -15 °C for several days. Red crystals of 1 (0.023 g, 5%)
were separated with a large amount of Cp3Y recovered. Anal.
Calcd for C28H25OY (%): C, 72.10; H, 5.40. Found: C, 71.92; H,
5.23.
Preparation of Cp2Y[(OCPh2(C5H4)](THF) (2). Ph2CO (0.182 g,
1.00 mmol) was added to a solution of Cp3Y (0.284 g, 1.00 mmol)
in toluene (20 mL). After stirring at 85 °C for 8 h, THF (10 mL) was
added and the solution continued to stir for another 2 h. Then the
solvents were removed under vacuum. The residue was washed with
hexane and was redissolved in THF. Diffusion of hexane into the
solution afforded complex 2 as colorless crystals. Yield: 0.361 g
(67%). 1H NMR (400 MHz, C6D6, 25 °C): δ 7.59-7.18 (m, 10H,
Ph), 6.76-6.38(m, 3H, C5H5uncoord), 6.14 (s, 0.58 ꢀ 10H, C5H5),
6.12(s, 0.42 ꢀ 10H, C5H5), 3.34(t, 4H, THF), 3.10 (br s, 0.42 ꢀ 2H,
C5H5uncoord), 2.85 (br s, 0.58 ꢀ 2 H, C5H5uncoord), 1.26-1.23 (m,
4H, THF). Anal. Calcd for C32H33O2Y (%): C, 71.37; H, 6.18.
Found: C, 71.21; H, 6.05.
means of the SAINT program.26 SADABS27 was used to per-
form area-detector scaling and absorption corrections. The
structures were solved by direct methods and were refined
against F2 using all reflections with the aid of the SHELXTL
package.28 All non-hydrogen atoms were found from the dif-
ference Fourier syntheses and refined anisotropically. All hy-
drogen atoms were assigned in idealized positions with isotropic
thermal parameters related to those of the supporting carbon
atoms but were not included in the refinement. All calculations
were performed using the Bruker Smart program.
Acknowledgment. We thank the National Natural
Science Foundation of China, 973 Program (2009CB825300),
Shanghai Science and Technology Committee (No.
08dj1400100), and Shanghai Leading Academic Discipline
Project (B108) for financial support.
Supporting Information Available: X-ray data for 1-4 in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
Preparation of [CpY(μ-η1:η1:η5-OCPh2C5H4)]2 (3). n-BuLi
(2.5 M in hexane, 0.64 mL, 1.60 mmol) was added dropwise to a
THF solution (20 mL) of 2 (0.862 g, 1.60 mmol) at -30 °C. After
stirring for 30 min at -30 °C, the mixture was slowly warmed to
room temperature and stirred overnight. THF was removed
under vacuum, and the resulting solid was extracted with 20 mL
(26) SAINTPlus Data Reduction and Correction Program, v. 6.02 a;
Bruker AXS: Madison, WI, USA, 2000.
(27) Sheldrick, G. M. SADABS, A Program for Empirical Absorption
€
€
Correction; University of Gottingen: Gottingen, Germany, 1998.
(28) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
€
€
Crystal Structures; University of Gottingen: Gottingen, Germany, 1997.