Bimetallic zirconium heterocycles supported by boron-oxygen ligands

Article abstract of DOI:10.1021/ic0200848

Reaction between bis(cyclopentadienyl)dimethylzirconium, ZrCp₂(CH₃)₂, and phenylboronic anhydride, (PhBO)₃, resulted in the formation of the heterocyclic dimer [ZrCp₂{μ-O₂BPh}]₂ (1); no reaction was observed with the mesityl derivative, (mesBO)₃. Compound 1 was also synthesized from the protonolysis reaction between ZrCp₂-(CH₃)₂ and in situ generated phenylboronic acid, PhB(OH)₂. This approach was extended to afford the analogous complexes [ZrCp₂{μ-O₂BAr}]₂ (2, Ar = mes; 3, Ar = C₆F₅) from the corresponding isolable arylboronic acids, ArB(OH)₂. The molecular structures of 1-3, determined by X-ray diffraction techniques, revealed a common, dimeric motif consisting of a central "Zr₂B₂O₄" metallacycle. Variations in the bond parameters within the heterocycle are attributed to the differing steric and electronic properties of the aryl substituents at boron.

Full text of DOI:10.1021/ic0200848

Inorg. Chem. 2002, 41, 3548−3552
Bimetallic Zirconium Heterocycles Supported by Boron−Oxygen Ligands
Jessica E. Balkwill, Sarah C. Cole, Martyn P. Coles,* and Peter B. Hitchcock
School of Chemistry, Physics and EnVironmental Science, UniVersity of Sussex,
Falmer, Brighton BN1 9QJ, U.K.
Received January 28, 2002
Reaction between bis(cyclopentadienyl)dimethylzirconium, ZrCp
resulted in the formation of the heterocyclic dimer [ZrCp {µ-O
mesityl derivative, (mesBO) . Compound 1 was also synthesized from the protonolysis reaction between ZrCp
CH and in situ generated phenylboronic acid, PhB(OH) . This approach was extended to afford the analogous
complexes [ZrCp {µ-O BAr}] (2, Ar ) mes; 3, Ar ) C ) from the corresponding isolable arylboronic acids,
ArB(OH) . The molecular structures of 1−3, determined by X-ray diffraction techniques, revealed a common, dimeric
motif consisting of a central “Zr ” metallacycle. Variations in the bond parameters within the heterocycle are
attributed to the differing steric and electronic properties of the aryl substituents at boron.
2
(CH
3
)
2
, and phenylboronic anhydride, (PhBO)
3
,
2
2
BPh}]
2
(1); no reaction was observed with the
3
2
-
(
3
)
2
2
2
2
2
6 5
F
2
2 2 4
B O
Introduction
species is an ongoing area of research. Within this context,
the perfluorophenoxide-containing salts [Ph C][Al(OC
and [Ph C][M(OC ] (M ) Nb, Ta) have recently been
investigated and their reactions with zirconocene dimethyl
3
6 5 4
F ) ]
The conversion of neutral, group 4 metallocene complexes
to the cationic alkyl species purported to be active catalysts
in the polymerization of R-olefins can occur using a number
of different cocatalyst reagents. Partially hydrated aluminum
alkyls (alumoxanes), in particular methylalumoxane (MAO),
are extremely effective in this capacity, despite detailed
structural information concerning the species responsible for
activation of the transition metal center remaining elusive.
Investigation of the model system composed of ZrCp
and the tert-butylalumoxane cage, [( Bu)Al(µ
3
6 5 6
F )
5
complexes studied. Reaction between ZrCp
Ph C][Nb(OC ] gave the phenoxide-transfer complex,
ZrCp (OC , as the only product, highlighting the pro-
2 3 2
(CH ) and
[
3
6 5 6
F )
2
6 5 2
F )
pensity for the formation of Zr-O linkages with systems
that incorporate oxygen in the activating species.
1
The patent literature has reported that introduction of
boron-oxygen compounds into metallocene/MAO polymer-
2
(CH
3 2
)
t
3
-O)] , has
6
6
izations enhances the activity of the system. It has also been
shown that the reaction proceeds to afford an equilibrium
mixture between the starting materials and the methyl transfer
product, with concomitant formation of a zirconium-oxygen
demonstrated that ill-defined B/Al hybrid materials are active
7
as cocatalysts, although little work in determining the
structure of the activator has been undertaken. The recently
2
bond. This observation suggests that simple alkyl group
t
t
2 2
reported, well-defined boroalumoxane, Al( Bu Al)( BuAl) -
abstraction and formation of discrete ion pairs may not be
the only process occurring within metallocene/alumoxane
systems.
During the past decade, many structurally well-defined
cocatalysts have been developed, predominantly based on
(
4) Li, L.; Stern, C. L.; Marks, T. J. Organometallics 2000, 19, 3332.
Williams, V. C.; Dai, C.; Li, Z.; Collins, S.; Piers, W. E.; Clegg, W.;
Elsegood, M. R. J.; Marder, T. B. Angew. Chem., Int. Ed. 1999, 38,
3695. Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.;
3
Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244. Chase,
P. A.; Piers, W. E.; Patrick, B. O. J. Am. Chem. Soc. 2000, 122, 12911.
Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
4
group 13 elements incorporating perfluoroaryl substituents.
As many of the important factors associated with a particular
catalyst system are reliant on cation-anion interactions (e.g.,
activity, thermal stability), the development of new activating
3623. Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P.
N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39, 1312. Chen, E.
Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 6287. Chen, E. Y.-X.; Stern, C. L.; Yang, S.; Marks,
T. J. J. Am. Chem. Soc. 1996, 118, 12451.
*
To whom correspondence should be addressed. E-mail: m.p.coles@
sussex.ac.uk.
(5) Sun, Y.; Metz, M. V.; Stern, C. L.; Marks, T. J. Organometallics 2000,
19, 1625.
(1) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413.
(
2) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995, 117,
(6) Welborn, H. C. World Patent WO9201005, 1992. Langhauser, F.; Lux,
M.; M u¨ lhaupt, R.; Fischer, D. World Patent WO9316116, 1992.
Kristen, M. O.; Fischer, D. World Patent WO9840418, 1998.
6465.
(3) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391.
3548 Inorganic Chemistry, Vol. 41, No. 13, 2002
10.1021/ic0200848 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/25/2002

Bimetallic Zirconium Heterocycles
,8 was postulated to possess latent Lewis acidity2
and, hence, be suitable as a cocatalyst in olefin polymeri-
zation. Accordingly, it was demonstrated that combination
with ZrCp (CH ) afforded a system that will polymerize
2 3 2
ethylene, although details concerning the nature of the active
species were not reported.
(O
2
BAr)
4
time a white precipitate formed. Removal of the volatile components
afforded crude 1 as a white powder (0.21 g, 60%). Portions of the
crude material were crystallized from a warm CH
give analytically pure 1 as colorless crystals. H NMR (CD
2
Cl
2
solution to
Cl
6 5
H ), 6.35 (s, 10H,
1
2
2
,
2
C
98K): δ 7.71 (m, 2H, C
6
H
5
), 7.43 (m, 3H, C
, 298 K): 135.5 (C
), 127.8 (C ), 113.5 (C
13
5 5
H
). C NMR (CD
2
Cl
2
6
H
5
), 129.8 (C
). MS (EI , m/z)
Zr: C, 56.30; H,
6 5
H ),
+
1
6
4
28.0 (br, ipso-C
15 [M(dimer) - Cp] . Anal. Calcd for C16
.43. Found: C, 56.19; H, 4.37.
6
H
5
+
H
6 5
5
H
5
2 3
To determine how ZrCp (CH )
2
interacts with a range of
H15BO
2
boron-oxygen compounds typical of those utilized as
additional components in cocatalyst systems, we report the
investigation of the reactivity with arylboronic acids and
Synthesis of [ZrCp {µ-O Bmes}] (2). mesB(OH) (0.33 g, 2.00
2 2 2 2
mmol) was dissolved in toluene (30 mL) and added to a precooled
anhydrides, ArB(OH)
2
3
and (ArBO) , respectively. In addition,
(-78 °C) solution of ZrCp (CH ) (0.50 g, 2.00 mmol) in toluene
2
3 2
with continued interest in the support of the metallocene and/
or activator components on solid oxide phases (e.g., silica,
alumina), it is envisaged that the compounds resulting from
this study may serve as models for possible interactions
between metallocene/borate species supported on such
materials.10
(30 mL). The reaction was left to attain room temperature and stirred
for 18 h, during which time a white precipitate formed. Removal
of the volatile components in vacuo afforded 2 as a crude white
9
powder that was crystallized from a warm CH
analytically pure, colorless crystals of 2 (0.35 g, 46%). H NMR
2 2
Cl solution to afford
1
(
6
C
6
D
6
, 298 K): δ 6.93 (s, 2H, C
), 2.25 (s, 3H, 4-Me). 13C NMR (C
), 136.3 (C ), 128.3 (C ), 127.7 (C
6
H
2
), 6.04 (s, 10H, C
, 298 K): δ 139.1
), 113.0 (C ),
5 5
H ), 2.66 (s,
H, 2,6-Me
2
6 6
D
(C
6
H
2
H
6 2
6
H
2
6
H
2
5 5
H
Experimental Section
11
2
3.1 (2,6-Me
2
), 21.3 (4-Me). B NMR (C
6 6
D , 298 K): δ 48.0 (br,
+
+
General Experimental Procedures. All manipulations were
carried out under dry nitrogen using standard Schlenk and cannula
techniques or in a conventional nitrogen-filled glovebox. Solvents
were dried over appropriate drying agents and degassed prior to
∆υ1/2 ∼ 1000 Hz). MS (EI , m/z) 763 [M(dimer)] , 699 [M(dimer)
-
+
+
Cp] , 645 [M(dimer) - mes] . Anal. Calcd for C19
2
H21BO Zr: C,
59.52; H, 5.52. Found: C, 59.72; H, 5.50.
Synthesis of [ZrCp {µ-O B(C F )}] (3). A solution of ZrCp -
2
2
6
5
2
2
11
12
use. The compounds ZrCp
2
3
(CH )
2
and (mesBO)
3
were synthe-
3 2
(CH ) (0.21 g, 0.85 mmol) in toluene (20 mL) was added to a
sized according to literature procedures. (PhBO)
3
was synthesized
precooled (-78 °C) slurry of C F B(OH) (0.18 g, 0.85 mmol) in
6
5
2
by thermal dehydration of commercially available phenylboronic
acid (Aldrich) and recrystallized from petroleum ether (40:60) under
anhydrous conditions prior to storage under an inert (nitrogen)
toluene and subsequently allowed to warm to room temperature.
The mixture was stirred at ambient temperature for 15 h, followed
by removal of the volatile component in vacuo to afford an off-
white solid. Crystallization from warm CH Cl gave 3 as analyti-
atmosphere. mesB(OH)
the procedure used by Morgan and Pinhey,13 using trimethyl borate
to replace triethyl borate. C B(OH) was made by literature
2
was synthesized using a modification of
2
1
2
cally pure colorless crystals (0.23 g, 62%). H NMR (CD Cl , 298
2
2
13
F
6 5
2
5 5 2 2
K): δ 6.31 (s, 10H, C H ). C NMR (CD Cl , 298 K): δ 148.5
methods14 and recrystallized under anhydrous conditions from
benzene. Elemental analyses were performed by S. Boyer at the
University of North London. The NMR spectra were recorded using
1
1
(d, J ) 227.1 Hz, C F ), 141.3 (d, J ) 256.4 Hz, C F ), 137.5
CF
6
5
CF
6 5
1
(d, J ) 250.0 Hz, C F ), ipso carbon of C F ring not observed,
CF
6
5
6 5
19
114.0 (C H ). F NMR (CD Cl , 298 K): δ -133.3 (m, o-C F ),
-156.7 (t, J ) 19.8 Hz, m-C F ), -164.2 (m, p-C F ). B NMR
5
5
2
2
6 5
1
13
1
19
3
11
a Bruker WM-300 at 300.13 ( H), 75.43 ( C{ H}), 282.23 ( F)
FF
6
5
6 5
13
1
11
+
or AMX-500 spectrometer at 125.72 ( C{ H}), 160.42 ( B) MHz.
Synthesis of [ZrCp {µ-O BPh}] (1). H O (36 µL, 1.03 mmol)
was added via syringe to a solution of (PhBO) (0.11 g, 0.34 mmol)
in toluene (25 mL) and the mixture stirred at room temperature for
0 min. The resultant solution of PhB(OH) was cooled to -78
C, and a solution of ZrCp (CH (0.26 g, 1.03 mmol) in toluene
20 mL) was added dropwise. The reaction mixture was allowed
to warm to room temperature and stirred for 18 h, during which
(CD Cl , 298 K): δ 42.2 (br, ∆υ ∼ 400 Hz). MS (EI , m/z) 795
2
2
1/2
+
2
2
2
2
[M(dimer) - Cp] . Anal. Calcd for C H BF O Zr: C, 44.56; H,
16
10
5
2
3
2.34. Found: C, 44.39; H, 2.20.
X-ray Crystal Structure Determination for Compounds 1,
3
2
2, and 3. Colorless crystals of 1 formed by allowing an NMR tube
°
2
3 2
)
2 3 2 3 6 6
containing equimolar amounts of ZrCp (CH ) and [PhBO] in C D
(
(ca. 400 µL) to stand for 2 days at ambient temperature. Colorless
crystals of 2 and 3 were obtained by allowing a warm (ca. 35 °C),
saturated dichloromethane solution of the compound to slowly attain
ambient temperature. A sample of the crystals of 1, 2, and 3 was
covered in oil, and a suitable single crystal was selected under a
microscope and mounted on a Kappa CCD area detector. The
structures were refined using SHELXL-97.15
(
7) Bohnen, H. World Patent WO9906414, 1999. Geerts, R. L.; Hill, T.
G.; Kufeld, S. E. U.S. Patent 5411925, 1995. Geerts, R. L.; Hill, T.
G. U.S. Patent 5414180, 1995. Geerts, R. L. U.S. Patent 5480848,
1
996. Sugano, T.; Takahama, T. U.S. Patent 5449650, 1995.
8) Richter, B.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem. Commun.
001, 1286.
9) Chien, J. C. W. Top. Catal. 1999, 7, 23.
10) Duchateau, R.; Cremer, U.; Harmsen, R. J.; Mohamud, S. I.;
Abbenhuis, H. C. L.; van Santen, R. A.; Meetsma, A.; Thiele, S. K.-
H.; van Tol, M. F. H.; Kranenburg, M. Organometallics 1999, 18,
(
2
(
Results and Discussion
(
To determine whether ZrCp
2 3 2
(CH ) reacts with the aryl-
5
447. Duchateau, R.; Abbenhuis, H. C. L.; van Santen, R. A.;
Meetsma, A.; Thiele, S. K.-H.; van Tol, M. F. H. Organometallics
998, 17, 5663. Duchateau, R.; Abbenhuis, H. C. L.; van Santen, R.
A.; Thiele, S. K.-H.; van Tol, M. F. H. Organometallics 1998, 17,
boronic anhydrides (RBO) (R ) Ph, mes), the 1:1 reaction
3
1
was monitored by H NMR spectroscopy. The mesityl
derivative did not react at ambient temperature, and gradual
heating of the sample to 90 °C resulted in the formation of
a mixture of products, indicated by the presence of multiple
1
5
222. Duchateau, R.; van Santen, R. A.; Yap, G. P. A. Organometallics
2000, 19, 809.
(
(
11) Samuel, E.; Rausch, M. D. J. Am. Chem. Soc. 1973, 95, 6263.
12) Gibson, V. C.; Mastroianni, S.; White, A. J. P.; Williams, D. J. Inorg.
Chem. 2001, 40, 826.
5 5
signals in the C H region of the spectrum. In contrast, the
(
(
13) Morgan, J.; Pinhey, J. T. J. Chem. Soc., Perkin Trans. 1 1990, 715.
14) Frohn, H.-J.; Franke, H.; Fritzen, P.; Bardin, V. V. J. Organomet.
Chem. 2000, 598, 127.
(15) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 41, No. 13, 2002 3549

Balkwill et al.
Figure 2. ORTEP diagram of [ZrCp2{µ-O2BPh}]2 (1) with hydrogen atoms
omitted for clarity.
Scheme 1
between (PhBO)
surface, each resulting in the formation of small quantities
of PhB(OH) that subsequently protonate ZrCp (CH to
3
and hydroxyl functionalities on the glass
Figure 1. Space filling models of (mesBO)3 (a) and (PhBO)3 (b) (structural
data taken from refs 16 and 17, respectively).
2
2
3 2
)
form 1, is a possible explanation. The important feature of
the reaction, however, is the incorporation of a boron/oxygen
containing fragment within the product via the formation of
Zr-O linkages, illustrating a potential problem associated
with the use of such species in cocatalyst mixtures. As no
firm conclusions concerning the nature of the zirconocene/
arylboronic anhydride interactions could be deduced from
our study, we switched our attention to the reaction with
arylboronic acids as a rational synthesis of complexes of type
phenyl derivative reacted immediately upon mixing of the
reagents, indicating that formation of a new species had
occurred (ca. 50% conversion), that displayed two methyl
5 5
resonances at δ 0.94 (3H) and 0.41 (3H), with a C H peak
at δ 5.67 (10H). Analysis of the sample after 15 h at room
temperature, however, indicated that further reaction had
occurred, again resulting in the formation of a mixture of
products. Upon standing at room temperature for a further 2
days, colorless crystals formed in the NMR tube. These were
subjected to X-ray analysis, which established that formation
1
.
The literature preparation of phenylboronic acid, PhB-
OH) , involves recrystallization of the anhydride from water
with subsequent “drying” in a slow stream of air saturated
(
2
2 2 2
of the dimeric complex [ZrCp {µ-O BPh}] (1) had occurred
(
see Figure 2).
1
8
1
6
2
with H O. To prevent the possibility of hydrolysis of the
Investigation of the crystal structures of (mesBO)
3
and
offers a likely explanation for the different
(CH . The former compound
Figure 1a) adopts a propeller-like conformation of aryl
groups about the planar “B ” ring (dihedral angles between
mesityl ring and B plane ) 35-41°), which results in
shielding of the oxygen atoms by the 2,6-methyl substituents
of the mesityl group. The phenyl substituents in (PhBO)
Figure 1b) adopt a more coplanar configuration (C-C-
17
zirconocene starting material, our approach consisted of the
in situ hydration of a sample of the anhydride that had been
prepared under anhydrous conditions. Subsequent reaction
(PhBO)
3
reactivity observed with ZrCp
2
3 2
)
(
with 1 equiv of ZrCp
a white precipitate upon warming to room temperature.
Similar reactions between ZrCp (CH and the isolable
phenylboronic acid derivatives ArB(OH) [Ar ) mes, C
2 3 2
(CH ) at -78 °C in toluene afforded
3
O
3
3
O
3
2
3 2
)
2
6 5
F ]
3
in toluene also resulted in formation of insoluble white
powders (Scheme 1). In each case, removal of the volatile
component and crystallization from dichloromethane afforded
(
B-O conformation angles 3-8°), rendering the oxygen
atoms more accessible to the zirconium center. While the
precise mechanism for the formation of 1 is currently
unknown, the presence of adventitious water, or the reaction
colorless crystals of [ZrCp
mes (2); Ar ) C (3)] in moderate to good yields. Mass
spectral analysis revealed signals for either the intact dimer
2) or the dimer with loss of a cyclopentadienyl ligand (1,
2
2 2
{µ-O BAr}] [Ar ) Ph; (1); Ar
)
6 5
F
(
(16) Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J.; Suwinska,
K. Inorg. Chem. 2000, 39, 5763.
(
17) Boese, R.; Polk, M.; Bl a¨ ser, D. Angew. Chem., Int. Ed. Engl. 1987,
(18) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1998.
26, 245.
3550 Inorganic Chemistry, Vol. 41, No. 13, 2002

Bimetallic Zirconium Heterocycles
Table 1. Crystallographic Data for 1, 2, and 3
1
2
3
formula
C32H30B2O4Zr2
C38H42B2O4Zr2‚ C32H20B2F10O4Zr2
CH2Cl2
2
fw
temp (K)
wavelength (Å) 0.71073
682.62
173(2)
936.63
173(2)
0.71073
862.54
173(2)
0.71073
space group
a (Å)
b (Å)
P212121 (No. 19) P 1h (No. 2)
P 1h (No. 2)
7.6289(5)
10.7594(7)
10.7876(8)
116.618(4)
104.393(3)
92.155(4)
755.35(9)
1
1.90
0.79
R1 ) 0.037,
wR2 ) 0.086
R1 ) 0.043,
wR2 ) 0.091
8.2070(2)
17.2306(3)
20.8110(5)
90
90
90
9.0114(4)
10.0372(5)
12.5709(6)
69.285(2)
87.880(3)
76.756(3)
1034.02(8)
1
c (Å)
R (deg)
â (deg)
γ (deg)
3
V (Å )
2942.9(1)
4
Z
3
Dcalcd (Mg/m )
abs coeff (mm
final R indices
1.54
1.50
-
1
)
0.74
0.80
a,b
R1 ) 0.033,
wR2 ) 0.085
R1 ) 0.035,
wR2 ) 0.087
R1 ) 0.043,
wR2 ) 0.096
R1 ) 0.056,
wR2 ) 0.102
Figure 3. ORTEP diagram of [ZrCp2{µ-O2Bmes}]2 (2) with hydrogen
atoms and dichloromethane solvate molecules omitted for clarity.
[
I > 2σ(I)]
R indices
(all data)
a
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo - Fc ) ]/∑[w(Fo ) ]}1/2.
b
2
2 2
2 2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[ZrCp2{µ-O2BPh}]2 (1)
Zr(1)-O(1)
Zr(1)-O(4)
B(1)-O(1)
1.984(2)
1.979(2)
1.341(4)
1.366(4)
1.587(5)
2.548(4)
2.550(4)
2.522(4)
2.525(4)
2.535(4)
2.555(4)
2.555(4)
2.536(4)
2.518(4)
2.527(4)
Zr(2)-O(2)
Zr(2)-O(3)
B(2)-O(3)
1.980(2)
1.968(2)
1.351(4)
1.349(4)
1.573(4)
2.538(4)
2.527(4)
2.521(5)
2.514(4)
2.542(5)
2.520(6)
2.535(5)
2.504(4)
2.507(4)
2.493(5)
B(1)-O(2)
B(2)-O(4)
B(1)-C(1)
B(2)-C(7)
Zr(1)-C(13)
Zr(1)-C(14)
Zr(1)-C(15)
Zr(1)-C(16)
Zr(1)-C(17)
Zr(1)-C(18)
Zr(1)-C(19)
Zr(1)-C(20)
Zr(1)-C(21)
Zr(1)-C(22)
Zr(2)-C(23)
Zr(2)-C(24)
Zr(2)-C(25)
Zr(2)-C(26)
Zr(2)-C(27)
Zr(2)-C(28)
Zr(2)-C(29)
Zr(2)-C(30)
Zr(2)-C(31)
Zr(2)-C(32)
Figure 4. ORTEP diagram of [ZrCp2{µ-O2B(C6F5)}]2 (3) with hydrogen
atoms omitted for clarity.
3
), consistent with the metallacyclic structures illustrated in
O(1)-Zr(1)-O(4)
O(1)-B(1)-O(2)
O(1)-B(1)-C(1)
O(2)-B(1)-C(1)
B(1)-O(1)-Zr(1)
B(2) -O(4)-Zr(1)
100.90(11)
122.4(3)
119.6(3)
118.0(3)
144.7(2)
148.3(2)
O(2)-Zr(2)-O(3)
O(3)-B(2)-O(4)
O(3)-B(2)-C(7)
O(4)-B(2)-C(7)
B(1)-O(2)-Zr(2)
B(2)-O(3)-Zr(2)
100.98(11)
121.2(3)
119.7(3)
119.1(3)
144.4(2)
153.0(3)
Scheme 1. To confirm the bonding in 2 and 3 and investigate
the steric and electronic influences that the different aryl
substituents confer to the heterocyclic core of the molecule
in comparison with 1, X-ray structural analyses were
performed (Figures 2-4). Crystal data are summarized in
Table 1, and selected bond lengths and angles are collected
in Tables 2-4.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
ZrCp2{µ-O2Bmes}]2 (2)
[
Zr-O(1)
Zr-O(2′)
B-O(1)
B-O(2)
B-C(11)
Zr-C(1)
Zr-C(2)
Zr-C(3)
1.971(2)
Zr-C(4)
Zr-C(5)
Zr-C(6)
Zr-C(7)
Zr-C(8)
Zr-C(9)
Zr-C(10)
2.523(7)
2.513(9)
2.539(3)
2.538(3)
2.556(3)
2.529(3)
2.529(3)
Compounds 1-3 form an analogous series of dimeric
2.008(2)
1.356(3)
1.352(3)
1.591(3)
2.527(8)
2.545(6)
2.542(6)
complexes, consisting of two “ZrCp
bridging [µ-O BAr] ligands, to afford a central eight-
membered “Zr ” metallacycle. The metal centers are
positioned above and below the approximate plane defined
by the boron and oxygen atoms, generating a “chair” like
conformation within the heterocycle. In all cases, the
zirconium atoms adopt distorted tetrahedral geometries, with
Cp-Zr-Cp angles typical for the zirconocene fragment
2
” fragments linked by
2
2 2 4
B O
O(1)-Zr-O(2′)a
B-O(1)-Zr
B-O(2)-Zr′
100.51(7)
156.7(2)
141.8(2)
O(1)-B-O(2)
O(1)-B-C(11)
O(2)-B-C(11)
121.1(2)
120.3(2)
118.6(2)
a
[1 ) 128.8° (av); 2 ) 128.7(2)°; 3 ) 129.6(1)°] and the
Symmetry transformations used to generate equivalent atoms (′): -x,
VI
19
-y, -z.
Zr-C distances in the range expected for [Zr Cp
The O-Zr-O angles [1 ) 100.95° (av); 2 ) 100.51(7)°; 3
98.86(9)°] are similar to those observed in the related
macrocyclic structures [ZrCp {µ-OCH C(CH CH O}] and
ZrCp {µ-OCH CH O}] of 101.4(3)° and 99.4(1)°,
2
] bonds.
respectively,20 and the monomeric aryloxide derivatives
)
[
ZrCp
2
(OAr)
2 2 6 3
] where Ar ) 2,6-Me C H [98.6(2)°] and Ar
2
2
)
3 2
2
2
21
)
2,4,6-Me C H [99.0° (av)].
3 6 2
[
2
2
C
6
H
4
2
2
(
20) Stephan, D. W. Organometallics 1990, 9, 2718.
(19) Cardin, D. J.; Lappert, M. F.; Raston, C. L. Chemistry of Organo-
(21) Benetollo, F.; Cavinato, G.; Crosara, L.; Milani, F.; Rossetto, G.;
Zirconium and Hafnium Compounds; Wiley: New York, 1986.
Scelza, C.; Zanella, P. J. Organomet. Chem. 1998, 555, 177.
Inorganic Chemistry, Vol. 41, No. 13, 2002 3551

Balkwill et al.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
[ZrCp2{µ-O2B(C6F5)}]2 (3)
Zr-O(1)
Zr-O(2′)
B-O(1)
B-O(2)
B-C(1)
Zr-C(7)
Zr-C(8)
Zr-C(9)
1.996(2)
Zr-C(10)
Zr-C(11)
Zr-C(12)
Zr-C(13)
Zr-C(14)
Zr-C(15)
Zr-C(16)
2.522(4)
2.526(4)
2.527(4)
2.528(4)
2.517(4)
2.516(4)
2.531(4)
2.006(2)
1.349(4)
1.339(4)
1.613(5)
2.534(4)
2.533(4)
2.541(4)
O(1)-Zr-O(2′)a
B-O(1)-Zr
B-O(2)-Zr′
98.86(9)
146.3(2)
140.8(2)
O(1)-B-O(2)
O(1)-B-C(1)
O(2)-B-C(1)
123.6(3)
118.0(3)
118.4(3)
a
Symmetry transformations used to generate equivalent atoms (′): -x,
-
y, -z.
The average Zr-O distances [1 ) 1.98 Å; 2 ) 1.99 Å; 3
2.00 Å] are shorter than the sum of the covalent radii for
)
zirconium and oxygen (2.21 Å), previously attributed to
either pπ-dπ donation from the oxygen lone pairs into the
vacant orbitals on zirconium or an ionic contribution to the
bonding.22 The angles at oxygen vary considerably within
the structures [1 ) 144.4(2)-153.0(3)°; 2 ) 141.8(2)-
156.7(2)°; 3 140.8(2)-146.3(2)°], and although the general
trend is that larger Zr-O-B angles result in shorter Zr-O
bonds, caution must be exercised in interpreting this in terms
of increased π-character in the Zr-O bond due to the
additional presence of boron adjacent to the oxygen that may
also be involved in π-interactions.23 Indeed, the B-O bond
distances in 1 [1.341(4)-1.366(4) Å] are generally shorter
2
4
Figure 5. Compounds 1, 2, and 3 viewed along the B‚‚‚B axis illustrating
the different orientations of the C6 ring of the aryl substituents with respect
to the metallacycle. The cyclopentadienyl ligands and ring substituents have
been omitted for clarity.
than in the parent phenylboronic acid [1.362-1.378 Å],
suggesting possible delocalization throughout the complete
metallacycle for this particular compound. The inclusion of
an electron-withdrawing, perfluorophenyl substituent in
complex 3 has little overall effect on the B-O distances
In summary, the prototypical group 4 metallocene dialkyl,
ZrCp
mesBO)
ultimately afford the dimeric species [ZrCp
via an unknown reaction mechanism. The reactions between
ZrCp (CH and the phenylboronic acids ArB(OH) [Ar )
Ph, mes, C ], however, proceed smoothly via protonolysis
of the zirconium methyl groups to afford the dimeric
complexes [ZrCp {µ-O BAr}] [Ar ) Ph; (1); Ar ) mes
2); Ar ) C (3)] as white powders that are insoluble in
toluene but may be crystallized from CH Cl . X-ray structural
2
(CH
3
)
2
, does not react with the bulky boronic anhydride
but reacts rapidly with the phenyl derivative to
{µ-O BPh}] (1)
[B-O(1) ) 1.349(4) Å; B-O(2) ) 1.339(4) Å].
(
3
Viewing each compound through the axis defined by the
2
2
2
two boron atoms, one can observe the different orientations
of the C ring with respect to the metallacycle (Figure 5).
6
2
)
3 2
2
As may be expected, the mesityl groups in 2 are most
perturbed from coplanar [angle between the planes defined
by the C ring and the BO Cipso moiety ) 68.56°] in order
6 2
6 5
F
2
2
2
to accommodate the 2,6-methyl substituents. The corre-
sponding angles are greatly reduced in 1 [C1-C6 ) 1.61°;
C7-C12 ) 1.55°] and 3 [21.60°], reflecting the reduction
in the steric demands of the aryl substituent. The near
coplanar arrangement found in 1 generates the correct orbital
alignment for overlap between the π-system of the aryl ring
and the empty p-orbital of the boron atom. However, the
B-Cipso bond distances [1.587(5) Å; 1.573(4) Å] are found
to be greater than in phenylboronic acid [1.563 Å; 1.562
(
6 5
F
2
2
analyses of 1-3 reveal a common eight-membered metal-
lacyclic core, with variations in the bond parameters arising
from both steric and electronic properties of the aryl
substituent.
Acknowledgment. We thank the EPSRC for a research
studentship (S.C.C.) and University of Sussex for additional
financial support. Dr. Anthony G. Avent is thanked for the
24
Å], illustrating that the disposition of the aryl groups about
the metallacycle is likely a consequence of both steric and
electronic interactions.
11
19
B and F NMR measurements.
Supporting Information Available: X-ray crystallographic files
for 1, 2, and 3 in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
(
22) Howard, W. A.; Trnka, T. M.; Parkin, G. Inorg. Chem. 1995, 34, 5900.
23) Cayton, R. H.; Chisholm, M. H.; Davidson, E. R.; DiStasi, V. F.; Du,
P.; Huffman, J. C. Inorg. Chem. 1991, 30, 1020.
(24) Rettig, S. J.; Trotter, J. Can. J. Chem. 1977, 55, 3071.
IC0200848
3552 Inorganic Chemistry, Vol. 41, No. 13, 2002