The reaction of the bis(6,6-dimethylcyclohexadienyl)zirconium fragment with PhC2SiMe3 - A 5 + 2 + 2 ring construction

Article abstract of DOI:10.1016/j.molstruc.2008.04.001

The reaction of the edge-bridged open zirconocene, Zr(6,6-dmch)₂(PMe₃)₂ (dmch = dimethylcyclohexadienyl) with two equivalents of PhC₂SiMe₃ leads to a 5 + 2 + 2 ring construction, resulting in a 4.3.1 bicyclodecadienyl skeleton. The resulting complex, with η³-allyl, η⁴-diene, and η⁵-dienyl coordination, has a formal 16 electron configuration, and contains close contacts between the metal center and two lengthened C-C bonds, indicative of weak agostic interactions.

Full text of DOI:10.1016/j.molstruc.2008.04.001

Journal of Molecular Structure 890 (2008) 107–111
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
The reaction of the bis(6,6-dimethylcyclohexadienyl)zirconium fragment
with PhC2SiMe3 – A 5 + 2 + 2 ring construction
*
Benjamin G. Harvey, Atta M. Arif, Richard D. Ernst
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the edge-bridged open zirconocene, Zr(6,6-dmch)2(PMe3)2 (dmch = dimethylcyclohexa-
dienyl) with two equivalents of PhC2SiMe3 leads to a 5 + 2 + 2 ring construction, resulting in a 4.3.1 bicy-
clodecadienyl skeleton. The resulting complex, with g3-allyl, g4-diene, and g5-dienyl coordination, has a
formal 16 electron configuration, and contains close contacts between the metal center and two length-
ened C–C bonds, indicative of weak agostic interactions.
Received 2 January 2008
Accepted 1 April 2008
Available online 7 April 2008
Keywords:
Ó 2008 Elsevier B.V. All rights reserved.
Open zirconocene
Agostic bonding
Coupling reaction
1. Introduction
whether this might undergo subsequent coupling(s) involving the
6,6-dmch ligand. However, given the possibility that the Zr(6,6-
Half-open metallocenes of titanium and zirconium have been
found to undergo reactions with a variety of unsaturated organic
molecules, including imines, ketones, nitriles, isonitriles, and al-
kynes [1]. In most, but not all, of these reactions, the unsaturated
substrates have undergone a coupling reaction, thus far always
involving the more strongly bound pentadienyl ligand, rather
than the aromatic cyclopentadienyl ligand. While the incorpora-
tions of the heteroatom-containing substrates have inevitably been
accompanied by such couplings with the pentadienyl ligand [2],
exceptions have been observed for reactions of alkynes with com-
plexes containing the edge-bridged 6,6-dimethylcyclohexadienyl
ligand (6,6-dmch), as a result of its properties being intermediate
between those of other pentadienyl ligands and the cyclopentadi-
enyl ligand [3]. Much of this can be traced to the relatively short
separation between the terminal carbon atoms (C1, C5) of a 6,6-
dmch ligand, which leads to enhanced overlap with metal valence
orbitals. Thus, the complexes Zr(6,6-dmch)2(PhC2SiMe3)(PMe3) (1)
and Zr(C5H5)(6,6-dmch)(Me3SiCC(Ph)C(Ph)CSiMe3) (2) have been
isolated from the respective reactions of Zr(6,6-dmch)2(PMe3)2
and Zr(C5H5)(6,6-dmch)(PMe3)2 complexes with one or two equiv-
alents of PhC2SiMe3. There is some evidence that a slow coupling,
presumably a 5 + 2 ring construction, takes place for the first spe-
cies, as has been observed in other related titanium [4], zirconium
[2b], chromium [5], manganese [6], and cobalt [7] compounds. For
the second species, the two alkynes have simply coupled to each
other, yielding a zirconacyclopentadiene complex. It is not yet clear
dmch)2 fragment might slowly couple with a single coordinated
alkyne ligand might lead one to suspect that the Zr(6,6-dmch)2
fragment would be even more prone to couple upon incorporation
of two alkynes, which could presumably first form a zirconacyclo-
pentadiene fragment. Herein we report on the reaction of Zr(6,6-
dmch)2(PMe3)2 with two equivalents of alkyne, which does indeed
lead to the desired 5 + 2 + 2 coupling process (Fig. 1).
SiMe3
Ph
SiMe3
Ph
Zr
PMe3
Zr
Ph
SiMe3
1
2
2. Experimental
All reactions were carried out under an atmosphere of nitrogen,
using Schlenk apparatus. Solvents were dried by passing through
* Corresponding author.
E-mail address: ernst@chem.utah.edu (R.D. Ernst).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.04.001

108
B.G. Harvey et al. / Journal of Molecular Structure 890 (2008) 107–111
C20
C19
C18
C35
C13
C34
Si2
C14
C17
C12
C15
C9
C28
C30
C29
C11
C33
C16
C24
C10
C27
Si1
C32
Zr1
C31
C25
C26
C2
C3
C1
C4
C5
C6
C7
C8
Fig. 1. Solid state structure of the Zr(6,6-dmch)2-(PhC2SiMe3)2 coupling product, 3. The methyl substituents on Si1 and Si2 have been omitted for clarity.
columns of activated alumina, under a nitrogen atmosphere. Deu-
terated solvents were vacuum transferred from sodium. Zr(6,6-
dmch)2(PMe3)2 was prepared as previously described [8].
or H4, J = 7.7, 3.3 Hz), 3.53 (dt, 1H, H2 or H4, J = 7.7, 2.2 Hz), 3.19
(dt, 1H, H9 or H13, J = 8.1, 2.3 Hz), 3.06 (td, 1H, H1 or H5, J = 7.7,
2.2 Hz), 3.02 (dd, 1H, H1 or H5, J = ?(obscd.), 3.3 Hz), 2.83 (ddd,
1H, H9 or H13, J = 9.9, 6.8, 2.5 Hz), 0.98 (s, 3H, Me), 0.85 (s, 3H,
Me), 0.60 (s, 3H, Me), 0.50 (s, 3H, Me), 0.01 (s, 9H, SiMe3), ꢀ0.18
(s, 9H, SiMe3).
2.1. Bis(phenyltrimethylsilylacetylene)/Zr(6,6-dmch)2 coupling
product, Zr(dmch)2(PhCCSiMe3)2 (3)
13C NMR (benzene-d6, ambient): d 148.6 (s, 1C, Ph), 146.8 (s, 1C,
Ph), 137.4 (d, 1C, Ph), 133.3 (d, 1C, Ph), 132.9 (d, 1C, Ph), 129.5 (d,
1C, Ph), 128.24 (d, 1C, Ph), 128.17 (d, 1C, Ph), 127.6 (d, 1C, Ph),
127.1 (d, 1C, Ph), 127.0 (s, 1C, C15 or C16), 124.5 (d, 1C, Ph),
123.3 (d, 1C, Ph), 116.71 (d, 1C, C1 or C5, J = 157 Hz), 116.65 (d,
1C, C1 or C5, J = 157 Hz), 115.4 (s, 1C, C15 or C16), 104.1 (d, 1C,
C2 or C4, J = 155 Hz), 94.8 (d, 1C, C11, J = 169 Hz), 92.0 (d, 1C, C3,
J = 162 Hz), 86.8 (d, 1C, C2 or C4, J = 158 Hz), 56.1 (d, 1C, C10 or
C12, J = 168 Hz), 53.5 (s, 1C, C14 or C17), 52.6 (s, 1C, C14 or C17),
44.1 (d, 1C, C9 or C13, J = 139 Hz), 43.5 (d, 1C, C9 or C13,
J = 137 Hz), 40.6 (d, 1C, C10 or C12, J = 171 Hz), 34.5 (q, 1C, Me,
J = 125 Hz), 33.4 (s, 1C, C6 or C18), 33.3 (q, 1C, Me, J = 125 Hz),
31.5 (s, 1C, C6 or C18), 31.1 (q, 1C, Me, J = 124 Hz), 24.9 (q, 1C,
Me, J = 125 Hz), 7.3 (q, 3C, SiMe3, J = 120 Hz), 3.3 (q, 3C, SiMe3,
J = 118 Hz).
To a magnetically stirred solution of 0.50 g of Zr(dmch)2
(PMe3)2 (1.09 mmol) in 50 mL of hexane at ꢀ78° was added
0.38 g of 1-phenyl-2-trimethylsilylacetylene (2.2 mmol) via syr-
inge. The dark red solution was allowed to warm to room
temperature and was stirred overnight. The solution had taken
on a bright red hue within ca. 1 h at ambient temperatures and
no further color change was observed. The solvent was removed
in vacuo and the remaining dark red, sticky solid was subjected
to dynamic high vacuum (10ꢀ3 Torr) for an additional 2–3 h.
The residue was then extracted with three 20 mL aliquots of pen-
tane, and filtered through a coarse frit with a Celite pad. The fil-
trate was concentrated to ca. 2 mL and placed in a ꢀ90° freezer
for several days. After 1–3 days, a sticky, dark red solid separated
from the supernatant. The solid was isolated by removal of the
supernatant via syringe and dried in vacuo. Typically, this first
solid was too oily to be easily manipulated in a glove box. A fur-
2.2. Crystallographic study
ther recrystallization from
a concentrated pentane solution
yielded 0.35 g of small, bright red crystals (49% yield). Crystals
suitable for an X-ray structural analysis were grown from a
concentrated, insulated pentane solution at ꢀ60°. The compound
is stable at room temperature under an inert atmosphere in both
solution and the solid state.
Single crystals of the compound were examined under Paratone
oil, and a suitable one subsequently was transferred to an Enraf–
Nonius Kappa CCD diffractometer for unit cell determination and
data collection. Pertinent data collection parameters are given in
Table 1. The structure was solved using SIR97, and refinements car-
ried out using SHELXL97 [9]. All non-hydrogen atoms were refined
anisotropically, while most hydrogen atoms of the primary mole-
cule could be refined isotropically. Those that could not be refined
satisfactorily were allowed to ride on their attached carbon atoms
and were assigned idealized thermal parameters. A hexane mole-
1H NMR (benzene-d6, ambient): d 7.82–7.75 (m, 1H, Ph), 7.51 (d,
1H, Ph, J = 8.1 Hz), 7.27 (t, 1H, Ph, J = 7.7 Hz), 7.14–7.05 (m, 6H, Ph),
6.99–6.90 (m, 1H, Ph), 5.81 (t, 1H, H3, J = 7.2 Hz), 5.56 (ddd, 1H,
H11, J = 8.2, 6.2, 1.8 Hz), 5.22 (ddd, 1H, H10 or H12, J = 8.2, 6.2,
1.8 Hz), 4.90 (tt, 1H, H10 or H12, J = 6.2, 1.8 Hz), 3.55 (dd, 1H, H2

B.G. Harvey et al. / Journal of Molecular Structure 890 (2008) 107–111
109
Table 1
Crystallographic parameters for the Zr(6,6-dmch)2/bis(PhC2SiMe3) coupling product
SiMe3
Ph
Formula
C41H57Si2Zr
697.27
150(1)
0.71073
Triclinic
P1
Formula weight
Temperature (K)
k (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
a (°)
b (°)
Ph
Ph
SiMe3
Ph
SiMe3
SiMe3
Zr
Zr
9.3762(2)
10.5562(3)
20.0704(5)
103.6972(17)
90.0676(17)
105.5801(16)
1854.64(8); 2
1.249
3.88
2.0–27.5
ꢀ12 6 h 6 11
ꢀ13 6 k 6 12
ꢀ25 6 ‘ 6 26
13399
c (°)
Volume (Å3); Z
Dcalc
Absorption coefficient (cmꢀ1
h Range (°)
Limiting indices
)
3
4
The first point of interest for 3 focuses on the formal metal oxi-
dation state. The metal center is coordinated by two unambigu-
ously monoanionic ligands, an g3-allyl (C10–12) and an g5-dienyl
(C1–5). The remaining four carbon atoms, C14–C17, could be pres-
ent either as a neutral g4-diene (5) or as a formally dianionic g4-
enediyl (6). The carbon–carbon bond lengths provide only a weak
indication in favor of the latter, but the average Zr–C distances of
2.37 Å for the ‘‘diene” termini and 2.44 Å for the internal ‘‘diene”
carbon atoms (Table 2), provide more solid support for the enediyl
formulation, favoring the presence of Zr(IV) over Zr(II).
Reflections collected
Independent reflections; n: Iinr(I)
8450; 2
0.0473
0.1015
1.31
R(F)
Rw(F2)
Maximum difference Fourier peak (eÅꢀ3
)
cule is present in the lattice, situated on an inversion center. Its
carbon atoms were refined anisotropically, while the hydrogen
atoms were assigned fixed isotropic thermal parameters. The
methylene hydrogen atom positions were refined, while the
methyl hydrogen atoms were placed in idealized positions and al-
lowed to ride on their attached carbon atoms.
Zr
Zr
3. Results and discussion
5
6
The reaction of Zr(6,6-dmch)2(PMe3)2 (dmch = dimethylcyclo-
hexadienyl) with two equivalents of PhC2SiMe3 leads to the
4.3.1 bicyclic complex 3, in which the dienyl ligand’s termini each
have coupled to the Me3Si-substituted ends of separate alkynes,
whose phenyl-substituted ends have coupled together. As the
phenyl-substituted alkyne ends seem invariably to couple before
the more hindered Me3Si-substituted ends, this clearly implicates
alkyne–alkyne coupling as occurring prior to any alkyne–dienyl
coupling. This is in accord with the isolation of a metallacyclo-
pentadiene containing uncoupled 6,6-dmch ligands, but is at odds
with indications that for general, unbridged pentadienyl ligands,
the alkyne-dienyl couplings occur upon incorporation of the al-
kyne. For the unbridged ligands, the longer separations between
their dienyl termini lead to poorer bonding to the metal center,
and hence to generally higher reactivities to coupling reactions;
additionally, their coupling reactions would not be hindered by
the steric influences of edge-bridges. Notably, from a reaction of
the edge-bridged half-open zirconocene Zr(C5H5)(6,6-dmch)-
(PMe3)2 with two equivalents of PhC2SiMe3, the metallacyclo-
pentadiene complex 2 was isolated, in which the sterically less
hindered (phenyl-substituted) alkyne ends preferentially coupled
together, although spectroscopic evidence revealed the minor
presence of a second isomer, in which the phenyl-substituted
end of one alkyne coupled to the Me3Si-substituted end of the
other. It is likely that such a minor isomer, 4, also occurs in this
case, and may be responsible for the oily nature of the initially
isolated product. The fact that a metallacyclopentadiene product
was not isolated in the case of 3 may be attributed to the pres-
ence of two dmch ligands and as well to the greater steric crowd-
ing accompanying the second 6,6-dmch ligand [10], vs. the C5H5
ligand in 2.
The bonding of the g3-allyl and g5-dienyl fragments is also con-
sistent with the above. Thus, for the allyl ligand one finds a slightly
shorter average Zr–C bond distance for the allyl termini, which
bear the negative charge of the formal allyl anion. Additionally,
the Zr–dienyl bonding displays the well-established pattern for a
higher valent metal center [8,11], in which one finds a short
Table 2
Pertinent bonding parameters for the Zr(6,6-dmch)2 bis(PhC2SiMe3) coupling product
Bond distances (Å)
Zr–C1
Zr–C2
Zr–C3
Zr–C4
Zr–C5
Zr–C9
Zr–C10
Zr–C11
Zr–C12
Zr–C13
Zr–C14
Zr–C15
Zr–C16
Zr–C17
2.645(3)
2.518(3)
2.479(3)
2.502(3)
2.583(3)
2.744
2.465(3)
2.455(3)
2.392(3)
2.697
C1–C2
C2–C3
C3–C4
C4–C5
C9–C10
C9–C17
C10–C11
C11–C12
C12–C13
C13–C14
C14–C15
C15–C16
C16–C17
1.375(5)
1.414(5)
1.411(5)
1.385(5)
1.532(4)
1.567(4)
1.390(4)
1.397(4)
1.534(4)
1.574(4)
1.448(4)
1.439(4)
1.445(4)
2.358(3)
2.432(3)
2.450(3)
2.382(3)
Bond angles (°)
C1–C2–C3
C2–C3–C4
120.4(3)
C11–C12–C13
C12–C13–C14
C13–C14–C15
C14–C15–C16
C15–C16–C17
C9–C17–C17
120.1(3)
117.4(3)
120.8(3)
105.1(2)
119.9(3)
121.4(3)
105.6(2)
124.4(2)
128.2(2)
128.6(2)
124.7(2)
C3–C4–C5
C10–C9–C17
C9–C10–C11
C10–C11–C12

110
B.G. Harvey et al. / Journal of Molecular Structure 890 (2008) 107–111
M–C3 interaction (2.479(3) Å), with successively longer average
bond lengths for the 2,4 (2.510(2) Å) and 1,5 positions (2.624 Å).
This pattern reflects the inability of the contracted metal orbitals
to overlap effectively with the p-molecular orbitals of an electron-
ically open pentadienyl ligand, leading to a significant contribution
from resonance structure 7, with a short-long-long-short pattern
on the C(1–5) bond lengths. The at least partial formal sp3 hybrid-
ization resulting on C3 presumably also is reflected by the smaller
C–C–C angle about C3 (117.4(3)°) vs. C2 and C4 (120.4(3)° and
120.8(3)°).
these distances also show no apparent lengthening, however, aver-
aging 1.521(4) Å. It is clear that the agostic interaction in the zirco-
nium complex is significantly greater than any that might be
present in 8, which would be expected given that zirconium tends
to form stronger bonds than titanium. However, it is possible that
the CMe2 edge bridge may play a role by bringing the appropriate
C–C bonds closer to the zirconium center. The bridge may thus be
responsible both for promoting the agostic interaction and for pre-
venting it from leading to C–C activation. In any event, the Zr–
C(9,13) distances are relatively short, at 2.744 and 2.697 Å (cf.,
2.645(3) Å for C1), while the analogous Ti–C distances average
2.716(3) Å. It can be expected that the J(13C–13C) values for the for-
mer two Zr–C bonds would be less than 30 Hz, based on observa-
tions for other (C–C) ? M agostic systems [12]. For the present,
one can at least observe increased C–H coupling constants of 137
and 139 Hz for C9 and C13, vs. 129 Hz for the heptadiyne-derived
version of 8 (X = C3H6) [4a], consistent with the occurrence of C–C
agostic interactions [12].
NMR spectroscopic data provide some other useful comparisons
with the structural results. In accord with the C–C bond couplings
observed between the alkynes and the dienyl termini, the J (13C–H)
values for C9 and C13 are markedly lower than those for C10–C12
(137–139 vs. 168–171 Hz), consistent with formal sp3 hybridiza-
tion for the former but sp2 hybridization for the latter. Further-
more, both the 1H and the 13C NMR data reveal the lack of any
nontrivial symmetry in the ground state conformation. Simple
rotation about the Zr–dmch center of mass axis would lead to
apparent mirror plane symmetry. As this is not observed, it is clear
that there is a significant barrier to oscillation about this axis. Such
would not be the case for an analogous coupling product arising
from the Zr(C5H5)(6,6-dmch) fragment).
Zr
7
Some of the asymmetry in the Zr–C bonding parameters noted
above may be due to the twisted orientation of the 6,6-dmch li-
gand relative to the bicyclic fragment. Another factor that could
play a role in altering some bonding parameters could be geomet-
ric constraints placed upon the allyl and diene bonding by their
simultaneous presence in the bicyclic fragment. As one indication
of this, one can note the smaller than expected values of the
C10–C9–C17 and C12–C13–C14 bond angles (105.1(2), 105.6(2)°),
and the larger than expected values of the C13–C14–C15, C9–
C17–C16, C14–C15–C16, and C15–C16–C17 angles (124.7(2)°,
124.4(2)°, 128.2(2)°, and 128.6(2)°), reflecting a substantial degree
of distortion by the bicyclic fragment in order to optimize the me-
tal–ligand bonding.
4. Conclusions
Also of note is the fact that similar 9-membered ring systems
have been isolated from the reactions of the Ti(C5H5)(2,4-C7H11
fragment with 1,5-, 1,6-, and 1,7-diynes (Eq. (1)) [4].
)
The 5 + 2 + 2 reaction reported herein is analogous to reactions
reported between Ti(C5H5)(Pdl)(PR3) complexes (Pdl = C5H7, 3-
C6H9, 2,4-C7H11) and a,x-diynes. Just as it has proven possible to
release the diyne-derived organic fragments from their metal com-
plexes, one can expect the same to be true for the 5 + 2 + 2 ana-
logues reported herein, thereby opening up potential applications
in organic synthesis. Additionally, as it has been found that single
alkynes can be incorporated into metal 6,6-dmch complexes, alter-
native 5 + 2 coupling products should also become accessible. In-
deed, 5 + 2 reactions between pentadienyl ligands and alkynes
have been known for some time, with other pentadienyl ligands
such as 2,4-C7H11 and c-C8H11, but generally multiple equivalents
of alkynes were incorporated, leading to more complicated prod-
ucts [1].
X
Ti
Ti(C5H5)(2,4-C7H11)(PEt3) + X(C≡CH)2
8
X = C2H4, CH2ZCH2, C4H8
Z = O, S, NMe, N(n-C4H9), CH2, C(CO2Me)2, C(CO2Et)2
ð1Þ
Acknowledgements
Interestingly, these products were observed to undergo slow
rearrangements on standing at room temperature to tricyclic spe-
cies, via C–C bond activation reactions. Conceivably, that could also
occur for 3 under the right conditions; quite possibly, the dmch
edge bridging CMe2 unit may retard this process by restraining
the approach of the C9–C17 and C13–C14 bonds to the metal cen-
ter, and by favoring a reconnection of any broken C–C bonds. In
fact, the data do provide some reason to anticipate such an event,
given the bond lengthenings observed for C9–C17 and C13–C14 vs.
C9–C10 and C12–C13 (1.567(4) and 1.574(4) vs. 1.532(4) and
1.534(4) Å). Notably, no significant lengthening of the correspond-
ing C–C bonds in the titanium complexes is observed, despite the
fact that they do undergo C–C activations. Thus, in 8, for
X = C2H4, these distances average 1.512(3) Å. However, the C–C
bond undergoing activation in the titanium complexes is actually
not attached to the diene but to the allyl ligand. For 8 (X = C2H4)
Acknowledgement is made to the Donors of the American
Chemical Society Petroleum Research Fund and the University of
Utah for partial support of this work.
Appendix A. Supplementary data
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Centre, CCDC,
No. 671731. Copies of this information may be obtained free of
charge from The Director, CCDC, 12, Union Road, Cambridge CB2
1EZ [Fax: +44 1223-336-033] or e-mail deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.molstruc.2008.04.001.

B.G. Harvey et al. / Journal of Molecular Structure 890 (2008) 107–111
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