Acetone and acetophenone adducts of the zwitterionic zirconocene Cp*[η5-C5Me4CH2B(C 6F5)3]ZrC6H5

Article abstract of DOI:10.1021/om970066b

The recently reported zwitterionic complex Cp*[η⁵-C₅Me₄CH₂B(C ₆F₅)₃]ZrC₆H₅, 1, reacts rapidly with 1 equiv of acetone or acetophenone to form the adducts Cp*[η⁵-C₅Me₄CH₂B(C ₆F₅)₃]Zr(C₆H ₅)[O=C(CH₃)R] (R = CH₃, 2a; C₆H₅, 2b) in ～85% yield. Spectroscopic evidence in favor of this assignment is presented; the adduct nature of the complexes was also confirmed in each case via an X-ray structural analysis. The ketone ligands are bound in an η¹ end-on fashion (the Zr-O=C angles are near linear at 174.2° and 166.1° for 2a and 2b, respectively), allowing for a π-component to the Zr-O bonding. Unlike other fleetingly observed ketone adducts of nonzwitterionic cationic metallocenes, the ketone ligands in compounds 2 do not undergo insertion into the Zr-C bond neither upon thermolysis nor in the presence of excess ketone.

Full text of DOI:10.1021/om970066b

Organometallics 1997, 16, 2509-2513
2509
Aceton e a n d Acetop h en on e Ad d u cts of th e Zw itter ion ic
Zir con ocen e Cp *[η5-C5Me4CH2B(C6F 5)3]Zr C6H5
Yimin Sun,† Warren E. Piers,*,† and Glenn P. A. Yap‡
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta T2N 1N4, Canada, and Windsor Molecular Structure Center, Department of
Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Received J anuary 30, 1997X
The recently reported zwitterionic complex Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5, 1, reacts
rapidly with 1 equiv of acetone or acetophenone to form the adducts Cp*[η5-
C5Me4CH2B(C6F5)3]Zr(C6H5)[OdC(CH3)R] (R ) CH3, 2a ; C6H5, 2b) in ∼85% yield. Spectro-
scopic evidence in favor of this assignment is presented; the adduct nature of the complexes
was also confirmed in each case via an X-ray structural analysis. The ketone ligands are
bound in an η1 end-on fashion (the Zr-OdC angles are near linear at 174.2° and 166.1° for
2a and 2b, respectively), allowing for a π-component to the Zr-O bonding. Unlike other
fleetingly observed ketone adducts of nonzwitterionic cationic metallocenes, the ketone
ligands in compounds 2 do not undergo insertion into the Zr-C bond neither upon thermolysis
nor in the presence of excess ketone.
In tr od u ction
Lewis acid capable of activating carbonyl functions
through adduct formation. Collins et al. have shown
that such adducts are key participants in Diels-Alder
additions,6 Mukaiyama aldol reactions,7 and the group
transfer polymerization of methacrylates.8 For both X
) R and OR’, the metallocene/carbonyl compound ad-
ducts have been observed spectroscopically but not
isolated.
The body of knowledge concerning the chemistry of
cationic group 4 metallocenes1 has grown rapidly in the
last decade, primarily due to the role these complexes
play in the homogeneous polymerization of olefins.2
Efforts have also been directed toward finding use for
this family of compounds in other arenas, such as other
types of polymerizations3 and synthetic organic chem-
istry.4 In both of these areas, the behavior of cationic
metallocenes in the presence of carbonyl functions has
been of interest. Since the cations are strong Lewis
acids, adduct formation with a carbonyl group is facile
(eq 1). When the X ligand in the metallocene wedge is
Recently, we prepared a family of zwitterionic zir-
conocenes via reaction of “tuck-in” complexes with the
10
highly electrophilic boranes XB(C6F5)2 (X ) H,9 C6F5
)
and examined their effectiveness as olefin polymeriza-
tion catalysts.11 Apart from the olefin chemistry of
these zwitterions, we observed that reaction of Cp*[η5-
C5Me4CH2B(C6F5)3]ZrC6H5, derived from the tuck-in
12
phenyl complex Cp*(η5-η1-C5Me4CH2)ZrC6H5
and
B(C6F5)3, with simple ketones CH3C(O)R (R ) CH3,
C6H5) yielded stable 1:1 adducts resistant to insertion.
Herein, we report the details concerning the preparation
of these adducts and their full characterization, includ-
ing their crystal structures.
an alkyl group, the adducts are unstable toward an
insertion reaction which yields a cationic alkoxide
complex, usually stabilized by a second equivalent of
carbonyl compound, L in eq 1.5 On the other hand,
when X is a heteroatom-based donor (such as an
alkoxide), the metallocene behaves simply as a strong
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Routine procedures have been
described in detail elsewhere.11 Acetone and acetophenone
† University of Calgary.
(6) (a) Collins, S.; Koene, B. K.; Ramachandran, R.; Taylor, N. J .
Organometallics 1991, 10, 2092. (b) Collins, S.; Hong, Y.; Kuntz, B.
K. Organometallics 1993, 12, 964.
(7) Hong, Y.; Norris, D. J .; Collins, S. J . Org. Chem. 1993, 58,
3591.
(8) (a) Collins, S.; Ward, D. G. J . Am. Chem. Soc. 1992, 114, 5460.
(b) Collins, S.; Ward, D. G.; Suddaby, K. H. Macromolecules 1994, 27,
7222.
(9) Parks, D. J .; Spence, R. E. v. H.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809.
‡ University of Windsor.
X Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) (a) J ordan, R. F.; Bradley, P. K.; LaPointe, R. E.; Taylor, D. F.
New J . Chem. 1990, 14, 505. (b) J ordan, R. F. Adv. Organomet. Chem.
1991, 32, 325.
(2) (a) Mohring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479,
1. (b) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c)
Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255.
(3) (a) Dioumaev, V. K.; Harrod, J . F. Organometallics 1996, 15,
3859. (b) Dioumaev, V. K.; Harrod, J . F. J . Organomet. Chem. 1996,
521, 133.
(10) (a) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2,
245. (b) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1991,
113, 3623.
(4) J ordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics
1990, 9, 1546 and references therein.
(5) J ordan, R. F.; Casher, W. E.; Echols, S. F. J . Am. Chem. Soc.
1986, 108, 1718.
(11) Sun, Y.; Spence, R. E. v. H.; Piers, W. E.; Parvez, M.; Yap, G.
P. A. J . Am. Chem. Soc., in press.
(12) Schock, L. E.; Brock, C. P.; Marks, T. J . Organometallics 1987,
6, 232.
S0276-7333(97)00066-6 CCC: $14.00 © 1997 American Chemical Society

2510 Organometallics, Vol. 16, No. 12, 1997
Sun et al.
Ta ble 1. Su m m a r y of Da ta Collection a n d
Str u ctu r e Refin em en t Deta ils for 2a a n d 2b
were dried by distillation from activated molecular sieves or
calcium hydride. Isotopically labeled ketones employed herein
were purchased from Cambridge Isotopes and dried in a
similar fashion to unlabeled reagents prior to use. B(C6F5)3
was purchased from Boulder Scientific and dried by treatment
of a toluene suspension with excess BCl3 followed by sublima-
tion.
Syn th esis of Cp *[η5-C5Me4CH2B(C6F 5)3]Zr (C6H5)[OdC-
(CH3)2], 2a . Benzene (15 mL) was condensed into an evacu-
ated flask containing Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5, 1,
(108 mg, 0.114 mmol) at -78 °C. Dry acetone (8.3 µL, 0.113
mmol) was added via syringe; an immediate orange to yellow
color change was observed. The reaction was stirred for 1 h
at -78 °C and warmed to room temperature. Benzene was
removed in vacuo, and the residue was recrystallized from
benzene (2 mL). The yellow crystals were isolated by filtration
and washed once with a portion of cold hexanes. Yield of 2a :
96 mg, 0.095 mmol, 84%. Once crystallized out of solution,
compound 2a was found to be only sparingly soluble in benzene
and toluene. Anal. Calcd for C47H40F15OBZr: C, 55.97; H,
4.00. Found: C, 56.37; H, 2.35.
2a
2b
formula
fw
cryst syst
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
space group
Z
C50H43BF15OZr
1046.87
orthorhombic
18.1795(4)
26.1871(4)
19.2613(4)
90
C64H54BF15OZr
1226.10
triclinic
10.9823(2)
14.4223(2)
20.7910(3)
98.523(1)
102.039(1)
100.744(1)
3103.64(8)
P1h
90
90
9169.7(3)
Aba2
8
2
F(000)
4248
1.517
0.339
0.0762
0.2479
1.135
1252
1.312
0.261
0.0804
0.204
1.495
d
calc, mg m-3
µ, mm-1
R1
wR2
gof of F2
Syn th esis of Cp *[η5-C5Me4CH2B(C6F 5)3]Zr (C6H5)[OdC-
(CH3)(C6H5)], 2b. An analogous procedure to that employed
above for 2a , using toluene (15 mL), 1 (104 mg, 0.110
mmol), and dry acetophenone (13.6 µL, 0.117 mmol), was
employed. Yield of orange 2b: 100 mg, 0.093 mmol, 85%. Once
crystallized out of solution, compound 2b was found to be
only sparingly soluble in benzene and toluene. Anal. Calcd
for C52H42F15OBZr: C, 58.38; H, 3.96. Found: C, 58.69; H,
2.81.
and unit-cell parameters were consistent with the space groups
Aba2 and Acam (Cmca). The absence of a molecular inversion
point, mirror plane, or a 2-fold axis and Z ) 8 strongly
suggested the acentric option, which yielded chemically rea-
sonable and computationally stable results. A trial application
of a semi-empirical absorption correction based on redundant
data at varying effective azimuthal angles yielded Tmax/Tmin
at unity and was ignored. A molecule of cocrystallized ben-
zene solvent was located disordered at a 2-fold axis with a
50% site occupancy distribution. Non-hydrogen atoms were
refined with anisotropic displacement coefficients; hydrogen
atoms were treated as idealized contributions. 2b: Crystals
were grown by slow diffusion of hexanes into a benzene
solution of in situ generated 2b . The diffraction data and
unit cell parameters indicated no symmetry higher than
triclinic was present. The E stastistics strongly suggested
the centric option, and solution in P1h yielded chemically rea-
sonable and computationally stable results. A semi-empirical
absorption correction based on redundant data at varying
effective azimuthal angles was applied to the data set. Two
molecules of cocrystallized benzene were located in the asym-
metric unit. Non-hydrogen and hydrogen atoms were treated
as above.
Rea ction of 2a w ith Aceton e. Acetone (0.9 µL, 1.26 ×
10-5 mol) was injected into an NMR tube containing 1 (12 mg,
1.26 × 10-5 mol) in C6D6. After the acetone and 1 were mixed,
1
a color change from orange to yellow was observed and the H
NMR spectrum showed the clean formation of 2a . A second
equivalent of acetone was injected via syringe and resulted in
an immediate precipitation of adduct 2a . The 1H NMR
spectrum was recorded and showed only the resonances for
free acetone and 2a ; no evidence of insertion of acetone into
the Zr-C6H5 bond was observed. The sample was heated at
50 °C for 0.5 h without a significant change; when heated at
80 °C, decomposition to a variety of products ensued.
Rea ction of 2a w ith Aceton e-d 6. An NMR sample of 2a
was generated as described above. Acetone-d6 (1 equiv) was
1
injected via syringe, and the H NMR spectrum was recorded.
The spectrum indicated the presence of free acetone, while the
signal for coordinated acetone was diminished in intensity
accordingly (ratio of coordinated:free ≈ 1:1).
Resu lts a n d Discu ssion
When benzene solutions of zwitterion 1 were treated
with 1 equiv of acetone or acetophenone, an immediate
orange to yellow or red color change was observed, sig-
naling the formation of the adducts Cp*[η5-C5Me4CH2B-
(C6F5)3]Zr(C6H5)[OdC(CH3)R] (R ) CH3, 2a ; C6H5, 2b),
eq 2. In more concentrated solutions, the ketone ad-
Rea ction of 2b w ith Aceton e. Acetophenone (2.3 µL, 2.0
× 10-5 mol) was injected into an NMR tube containing 1 (19
mg, 2 × 10-5 mol) in C6D6. After the acetophenone and 1 were
mixed, a color change from orange to red was observed and
the 1H NMR spectrum showed the clean formation of 2b.
Acetone (1.5 µL, 2.0 × 10-5 mol) was added to this sample via
syringe, inducing a color change from red to yellow. Formation
of 2a and free acetophenone was confirmed by 1H NMR
spectroscopy. There is no evidence of insertion of acetone or
acetophenone into the Zr-C6H5 bond. The sample was heated
at 50 °C for 0.5 h without a significant change; when heated
at 80 °C, decomposition to a variety of products ensued.
X-r a y Cr ysta llogr a p h y. Gen er a l Con sid er a tion s. The
structures were solved from data collected on the Siemens
SMART system on crystals loaded into sealed glass capillaries;
the crystal data is summarized in Table 1. In each case, unit-
cell parameters were calculated from reflections obtained from
60 data frames collected at different sections of the Ewald
sphere. The structures were solved by direct methods, com-
pleted by subsequent Fourier syntheses, and refined with full-
matrix least-squares methods. All scattering factors and
anomalous dispersion coefficients are contained in the SHELX-
TL 5.03 program library. 2a : Crystals were obtained from
benzene solutions of in situ generated 2a . The diffraction data
ducts crystallized out of the medium with one (2a ) or
two (2b) molecules of benzene; the products were
isolated in 80-85% yield. Spectroscopic data for the
new compounds is presented in Table 2.
We have observed phenyl zwitterion 1 to react rapid-
ly with PMe3 to regenerate the neutral tuck-in phen-
yl complex with concommitant formation of Me3P‚

Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5
Organometallics, Vol. 16, No. 12, 1997 2511
Ta ble 2. NMR a n d IR Sp ectr a l Da ta for
Zw itter ion ic Keton e Ad d u cts 2a a n d 2ba
F igu r e 1. ORTEP diagram for acetone adduct 2a .
a
All NMR spectra were recorded in C6D6; IR spectra were
b
obtained in KBr pellets. Room temperature spectra reflect the
average spectra of the 2b-endo and 2b-exo isomers. c Chemical
shifts at 216 K; other peaks are essentially unchanged in the low-
d
temperature spectra.
Not detected. e Other resonances are
obscured by the solvent peak in the 2-D spectra.
B(C6F5)3.11 This reaction path was, thus, also a pos-
sibility for reactions of 1 with ketones; however, we have
previously prepared the acetophenone adduct of
F igu r e 2. ORTEP diagram for acetophenone adduct 2b.
13
B(C6F5)3 and it was not observed in this reaction.
Furthermore, the 11B chemical shifts are characteristic
of anionic borate centers,14 witnessing that complexes
2 remain zwitterionic. Other data are consistent with
ketone adduct formation as opposed to insertion to form
however, the 13C NMR spectrum of 2b prepared from 1
and C6H513C(O)CH3 showed a peak at 220.4 ppm for the
carbonyl carbon and proved conclusively that 2b was
also an adduct. This peak is shifted downfield by 23.6
ppm from that of free acetophenone (196.8 ppm),
deshielded as a consequence of coordination to the Lewis
acid.
The tendency of these compounds to crystallize out
of benzene allowed us to characterize them crystallo-
graphically. ORTEP drawings of the two molecules are
shown in Figures 1 and 2, while selected bond lengths
and angles are given in Tables 3 and 4, respectively.
The structures share similar gross features. Coordina-
tion of the ketones supplant the phenyl agostic interac-
tion and the donation from the borate carbon to zirco-
nium which stabilize phenyl zwitterion 1,11 and the
distortions in the phenyl group and the metallated Cp*
ring these interactions engender are not observed in
adducts 2. The borate moiety is, thus, completely swung
away from the metal center toward the side of the
equatorial bonding region in compounds 2a and 2b, and
no close contacts between zirconium and any atoms of
the CH2B(C6F5)3 unit exist.
1
cationic alkoxide derivatives. In the H NMR spectra,
signals for protons of the same connectivity in the bor-
ate-substituted Cp* ring are inequivalent, indicative of
two different ligands in the metallocene wedge. Al-
though similar patterns might be obtained if an inser-
tion product was stabilized by a strong chelating inter-
action between the pendant borate counterion and the
zirconium center, 13C and IR spectroscopic data further
support an uninserted adduct product structure. For
complex 2a , a strong band in the IR at 1660 cm-1 was
observed for the CdO stretch of the coordinated ace-
tone, shifted 40 cm-1 to lower energy compared to free
acetone.15 The analogous band in the IR spectrum of
2b was obscured by absorbances for the Cp* ligands;
(13) Parks, D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118,
9440.
(14) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983, Vol. 2.
(15) The Aldrich Library of FTIR Spectra; Pouchert, C. J ., Ed.;
Aldrich Chemical Company: Milwaukee, WI, 1985; p 405.

2512 Organometallics, Vol. 16, No. 12, 1997
Sun et al.
Ta ble 3. Selected Bon d Len gth s (Å) for 2a a n d 2b
Ta ble 4. Selected Bon d An gles (d eg) for 2a a n d 2b
2a
2b
2a
2b
Zr-O
2.132(5)
2.231(11)
2.499(7)
2.518(7)
2.519(8)
2.554(8)
2.571(7)
1.658(11)
1.241(10)
1.461(14)
1.494(14)
1.419(10)
1.421(10)
1.405(11)
1.420(10)
1.429(9)
1.501(10)
1.483(11)
1.501(11)
1.485(10)
1.510(9)
2.250
Zr-O
2.134(5)
2.288(7)
2.593(6)
2.496(6)
2.532(7)
2.570(7)
2.577(6)
1.674(9)
1.252(7)
1.466(9)
1.490(9)
1.412(8)
1.423(8)
1.435(9)
1.415(10)
1.418(9)
1.503(9)
1.526(10)
1.520(9)
1.492(9)
1.500(8)
2.262
O-Zr-C(6)
C(7)-O-Zr
97.2(4)
174.2(6)
110.6(12) C(1)-C(6)-C(5)
118.9(9)
130.3(10) C(5)-C(6)-Zr
119.2(9) O-C(7)-C(16)
120.5(10) O-C(7)-C(8)
120.3(10) C(16)-C(7)-C(8)
O-Zr-C(6)
C(7)-O-Zr
97.4(2)
166.1(4)
115.1(7)
114.8(5)
129.8(6)
119.0(6)
119.8(6)
121.2(6)
Zr-C(6)
Zr-C(6)
Zr-C(35)
Zr-C(41)
C(1)-C(6)-C(5)
Zr-C(31)
Zr-C(42)
C(5)-C(6)-Zr
C(1)-C(6)-Zr
Zr-C(34)
Zr-C(43)
C(1)-C(6)-Zr
Zr-C(33)
Zr-C(44)
O-C(7)-C(8)
Zr-C(32)
Zr-C(45)
O-C(7)-C(9)
B-C(45)
B-C(9)
C(8)-C(7)-C(9)
O-C(7)
O-C(7)
C(32)-C(31)-C(35) 108.9(6)
C(33)-C(32)-C(31) 107.6(7)
C(32)-C(33)-C(34) 108.7(6)
C(33)-C(34)-C(35) 108.0(6)
C(31)-C(35)-C(34) 106.8(6)
C(32)-C(31)-C(41) 124.3(7)
C(35)-C(31)-C(41) 126.2(7)
C(33)-C(32)-C(42) 127.4(8)
C(31)-C(32)-C(42) 124.3(8)
C(32)-C(33)-C(43) 125.4(8)
C(34)-C(33)-C(43) 124.8(8)
C(33)-C(34)-C(44) 124.4(7)
C(35)-C(34)-C(44) 125.9(7)
C(31)-C(35)-C(45) 126.3(6)
C(34)-C(35)-C(45) 126.8(6)
C(42)-C(41)-C(45) 108.9(5)
C(41)-C(42)-C(43) 106.9(5)
C(44)-C(43)-C(42) 108.2(6)
C(45)-C(44)-C(43) 108.3(5)
C(44)-C(45)-C(41) 107.4(5)
C(42)-C(41)-C(9)
C(45)-C(41)-C(9)
C(7)-C(8)
C(7)-C(9)
C(31)-C(32)
C(31)-C(35)
C(32)-C(33)
C(33)-C(34)
C(34)-C(35)
C(31)-C(41)
C(32)-C(42)
C(33)-C(43)
C(34)-C(44)
C(35)-C(45)
Zr-Cp*cent
Zr-Cp*Bcent
C(7)-C(16)
C(7)-C(8)
C(41)-C(42)
C(41)-C(45)
C(42)-C(43)
C(43)-C(44)
C(44)-C(45)
C(42)-C(52)
C(43)-C(53)
C(44)-C(54)
C(45)-C(55)
C(41)-C(9)
Zr-Cp*cent
Zr-Cp*Bcent
127.3(5)
123.6(5)
C(41)-C(42)-C(52) 125.5(6)
C(43)-C(42)-C(52) 125.7(6)
C(44)-C(43)-C(53) 123.7(7)
C(42)-C(43)-C(53) 126.3(7)
C(45)-C(44)-C(54) 125.5(7)
C(43)-C(44)-C(54) 124.5(7)
C(44)-C(45)-C(55) 124.0(6)
C(41)-C(45)-C(55) 128.2(6)
2.227
2.250
C(35)-C(45)-B
119.3(5)
C(41)-C(9)-B
C(15)-C(16)-C(11) 118.9(7)
116.4(5)
The phenyl ligands are twisted out of the plane
bisecting the two Cp* donors away from the endo group
of the ketone ligands by about 12-14°. The ketone
ligands of compounds 2 reside in the other metallocene
coordination site and are also twisted out of the afore-
mentioned bisecting plane. Steric interactions between
the phenyl ligand and the endo ketone substitutent are
also minimized as a consequence of the end-on coordi-
nation mode of the ketone ligands. The Zr-O-C(7)
angles are near linear at 174.2° and 166.1° for 2a and
2b, respectively. While there is a significant amount
of latitude in the M-O-C angles for carbonyl com-
pounds bonded to Lewis acids, usually η1-bound ketone
or aldehyde ligands are bent at oxygen.16 In addition
to the steric considerations already mentioned, linear,
end-on coordination allows for π-donation from the
ketone (I) to the empty 1a1 frontier orbital of the
metallocene.17 The dihedral angles between the planes
C(15)-C(16)-C(7)
C(11)-C(16)-C(7)
Cpcent-Zr-Cpcent
121.1(6)
120.0(6)
139.0
Cpcent-Zr-Cpcent
137.3
ligands1b than those wherein the π component is
attenuated.1b,18 The Zr-O bonds in adducts 2 are also
closer in length to the covalent Zr-O bonds found in
η2-bound ketone and aldehyde complexes of zirconium
(which fall in the range of 2.11-2.18 Å19) than the dative
Zr-O linkages in neutral (2.20-2.29 Å20) and cationic
(2.242(7) Å in Cp*2Zr(η2-COCH3)CO21) η2-acyl com-
plexes. These observations suggest that the distances
in compounds 2 may be viewed as being short for a
dative interaction, because of the π component depicted
in I. Since both the σ and π donation from the ketone
ligands to zirconium occurs from orbitals which are
nonbonding with respect to C(7)-O, these bonds are
only slightly elongated to 1.241(10) and 1.252(7) Å from
a typical CdO distance of 1.210Å.22
Evidence of a dynamic behavior associated with the
1
ketone ligands is apparent in the H NMR data (Table
2). For example, at room temperature, only one singlet
is observed for the methyl groups of the acetone ligand
in 2a ; the solid state structures, if static, would give
rise to two singlets for inequivalent methyl groups.
Furthermore, the proton resonance for the acetophenone
methyl group in 2b is considerably broadened at room
temperature. These observations suggest a process
which exchanges the two ketone substituent sites. In
the case of 2a , this process was frozen out at low
temperature, giving two signals at 1.91 and 1.75 ppm
for the now inequivalent exo and endo methyl groups
defined by C(6), Zr, and O and the four coplanar atoms
of the ketone ligands are ∼12° and ∼18° for 2a and
2b, respectively, deviating only slightly from the copla-
narity expected for such a σ-π bonding mode. The
near linearity also allows for optimal alignment of the
CdO dipole toward the positively charged zirconium
center.
Direct comparison data are unavailable, but the Zr-O
bond distances of 2.132(5) and 2.134(5) Å in 2a and 2b
are more comparable to the Zr-O bond lengths of ∼2.12
Å found in cationic zirconocenes with σ-π bound THF
(18) Piers, W. E.; Koch, L.; Ridge, D. S.; MacGillvray, L. R.;
Zaworotko, M. Organometallics 1992, 11, 3148.
(19) Erker, G.; Dorf, U.; Czisch, P.; Petersen, J . F. Organometallics
1986, 5, 668 and references therein.
(20) (a) Fachinetti, G.; Fochi, G.; Floriani, C. J . Chem. Soc., Dalton
Trans. 1977, 1946. (b) Marsella, J . A.; Huffman, J . C.; Caulton, K.
G.; Longato, B. C.; Norton, J . R. J . Am. Chem. Soc. 1982, 104, 6360.
(c) Campion, B. K.; Falk, J .; Tilley, T. D. J . Am. Chem. Soc. 1987, 109,
2049. (d) Moore, E. J .; Santarsiero, B. D. Acta Crystallogr., Sect. C
1989, 45, 579.
(16) (a) Huang, Y.-H.; Gladysz, J . A. J . Chem. Educ. 1988, 65, 298.
(b) Shambayati, S.; Crowe, W. E.; Schreiber, S. L. Angew. Chem., Int.
Ed. Engl. 1990, 29, 256.
(21) Guo, Z.; Swenson, D. C.; Guram, A. S.; J ordan, R. F. Organo-
metallics 1994, 13, 766.
(17) Lauher, J . W.; Hoffmann, R. J . Am. Chem. Soc. 1976, 98,
1729.
(22) International Tables for Crystallography; Wilson, A. J . C., Ed.;
Kluwer Academic Publishers: London, 1992; Vol. C.

Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5
Organometallics, Vol. 16, No. 12, 1997 2513
Sch em e 1
in the low-temperature limit. For 2b, the single set of
peaks for the various ligands observed in the room
temperature 1H NMR spectra transformed into two sets
of peaks as the temperature was lowered. The isomers
of 2b were present in a 4.8:1 ratio at 247 K; a NOESY
experiment confirmed that the isomer with the C6H5
group in the endo position (i.e., that found in the
solidstate, 2b-endo) is the major isomer in solution
under these conditions.
We propose that the exchange of exo and endo ketone
substitutents occurs by the process shown in Scheme
1. In this process, the σ-π bonding that the solid state
structures indicate relaxes to a “σ-only” Zr-O bond,
which allows for rotation of the ketone ligand and
exchange of the exo and endo sites in the ligand. The
difference of over 8° in the observed Zr-O-C(7) angles
for adducts 2 suggests that the linear σ-π to bent σ-only
transformation has a rather shallow potential surface.
The barrier found for the process exchanging the methyl
groups in 2a (by analysis of the coalescence behavior of
the signals23) was 11.1(4) kcal mol-1
.
Although fluxional, the acetone and acetophenone
ligands in adducts 2 are not prone to insertion into the
Zr-C6H5 bond. Gentle heating (50-80 °C) of solutions
of either complex resulted in decomposition to a variety
of products; similar observations were made even in the
presence of an excess of ketone. While insertion may
be occurring to a limited degree, competitive processes
involving dissociation of the ketone and decomposition
of phenyl zwitterion 1, which was observed to be
thermally unstable in solution,11 are dominant. The
lability of the ketone ligands was demonstrated by
exchange experiments, such as those shown in eq 3.
necessary for insertion due to severe steric interactions
with the bulky Cp donors, as depicted in II.24
In conclusion, we have prepared acetone and aceto-
phenone adducts of a zwitterionic zirconocene complex.
Such adducts have been observed spectroscopically in
nonzwitterionic zirconocene alkyl cations prior to ketone
insertion into the Zr-C bond; the adducts described
herein are unable to undergo insertion due to steric
destabilization of the insertion transition state. The
structures of these adducts and their dynamic behavior
in solution give some insight into how carbonyl functions
interact with these Lewis acids.
Ack n ow led gm en t. This work was generously sup-
ported by the NOVA Research & Technology Corpora-
tion of Calgary, Alberta, NSERC of Canada’s Research
Partnerships office (CRD program), and the University
of Calgary. The helpful comments of a referee regarding
the dynamic behavior of the adducts in solution are also
acknowledged. WEP thanks the Alfred P. Sloan Foun-
dation for a Research Fellowship (1996-98).
Treatment with acetone-d6 resulted in a ∼1:1 mixture
of 2a and 2a -d6, and the more basic acetone displaces
acetophenone rapidly to generate 2a from 2b (Keq > 95
1
by H NMR spectroscopy). Alternatively, the presence
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, atomic parameters, hydrogen parameters,
atomic coordinates, anisotropic thermal parameters, and
complete bond distances and angles for 2a and 2b (24 pages).
Ordering information is given on any current masthead page.
of significant amounts of benzene in the thermalized
samples of 2a and 2b suggests that enolization of the
ketone may also be occurring to some extent. It is
entirely likely that the barrier to ketone insertion is
higher than the barriers to these other processes since
the ketone is unable to assume the η2-bonding mode
OM970066B
(24) It should be noted that the phenyl ligand would also be required
to rotate perpendicular to the metallocene plane, also a sterically
problematic proposition.
(23) Sandstrom, J . Dynamic NMR Spectroscopy; Academic Press:
New York, 1982.