Ancillary ligand and ketone substituent effects on the rate of ketone insertion into Zr-C bonds of zirconocene-l-Aza-l,3-diene complexes

Article abstract of DOI:10.1021/om900142c

Zirconocene-l-aza-l,3-diene complexes, [Me₂Si(C ₅H₄)₂]Zr[N(Ar)CH=CHCH(Ph)] (Ar = Ph, 2a; Ar = /J-MeOC₅H₄, 2b) and Cp₂Zr[N(Ar)CH=CHCH(Ph)] (Ar = Ph, 3a; Ar = p-MeOC₆H

Full text of DOI:10.1021/om900142c

2938
Organometallics 2009, 28, 2938–2946
Articles
Ancillary Ligand and Ketone Substituent Effects on the Rate of
Ketone Insertion into Zr-C Bonds of Zirconocene-1-Aza-1,3-diene
Complexes
Jie Zhang,† Jeanette A. Krause,† Kuo-Wei Huang,*,‡ and Hairong Guan*,†
Departments of Chemistry, UniVersity of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, and
National UniVersity of Singapore, Singapore 117543
ReceiVed February 22, 2009
Zirconocene-1-aza-1,3-diene complexes, [Me2Si(C5H4)2]Zr[N(Ar)CHdCHCH(Ph)] (Ar ) Ph, 2a; Ar
) p-MeOC6H4, 2b) and Cp2Zr[N(Ar)CHdCHCH(Ph)] (Ar ) Ph, 3a; Ar ) p-MeOC6H4, 3b), have been
synthesized and characterized by NMR spectroscopy. X-ray crystal structure determinations of compounds
2a and 3a,b reveal folded five-membered-ring moieties for the zirconacycles. DFT calculations and
variable-temperature NMR experiments for complex 2a establish a rapid ring-flipping process at room
temperature, with the conformation bearing a pseudoequatorial Ph group more stable by 5.6 kcal/mol.
Kinetic studies on ketone insertion into these zirconocene complexes show second-order reactions, and
the insertion is more favorable in the presence of a [Me2Si] ansa bridge, a less electron-rich substituent
on the nitrogen, and a more basic ketone. One of the insertion products, namely Cp2Zr-
[N(Ph)CHdCHCH(Ph)CPh2O] (5a), has also been characterized by X-ray crystallography.
Scheme 1
Introduction
Zirconocene complexes, particularly those derived from (η5-
C5R5)2ZrX2, play important roles in transition-metal-catalyzed
and -promoted reactions.1 One notable feature of these com-
plexes is their ability to reverse the polarity (or induce reactivity
umpolung) of carbon chains,2 which is typically imposed by
heteroatoms such as oxygen and nitrogen.3 In the case of an
R,ꢀ-unsaturated imine, the electrophilic ꢀ-carbon becomes
nucleophilic upon formation of a five-membered zirconacycle
(Scheme 1). Evidently, the electropositive zirconium outweighs
the influence of nitrogen, and the zirconocene-1-aza-1,3-diene
complexes (or azazirconacyclopentenes) can be viewed as
homoenolate equivalents. Such an umpolung strategy has been
successfully used in the synthesis of various N-substituted
pyrroles4 and oxygen-containing heterocycles.5 However, broader
applications of this synthetic methodology are hampered by the
limited electrophiles that are available for insertion into Zr-C
Scheme 2
* To whom correspondence should be addressed. E-mail: hairong.guan@
uc.edu (H.G.); hkw@nus.edu.sg (K.-W.H.).
† University of Cincinnati.
‡ National University of Singapore.
bonds of the zirconacycles; only isocyanides4 and ketones5,6
have been identified as viable substrates for insertion.
(1) (a) Negishi, E.; Takahashi, T. Synthesis 1988, 1–19. (b) Metallocenes
in Regio- and Stereoselective Synthesis. In Topics in Organometallic
Chemistry; Takahashi, T., Ed.; Springer: Berlin/Heidelberg, 2005; Vol. 8,
pp 1-236. (c) New Aspects of Zirconium Containing Organic Compounds.
In Topics in Organometallic Chemistry; Marek, I., Ed.; Springer: Berlin/
Heidelberg, 2005; Vol. 10, pp 1-166.
(2) For a recent review, see: Guan, H. Curr. Org. Chem. 2008, 12, 1406–
1430.
(3) (a) Seebach, D.; Enders, D. Angew. Chem., Int. Ed. 1975, 14, 15–
32. (b) Seebach, D. Angew. Chem., Int. Ed. 1979, 18, 239–258.
(4) Davis, J. M.; Whitby, R. J.; Jaxa-Chamiec, A. J. Chem. Soc., Chem.
Commun. 1991, 1743–1745.
One advantage of using transition-metal complexes for
organic synthesis is that the reactivity of metal reagents can be
easily modulated by altering ancillary ligands. Recent studies
have shown that the incorporation of an ansa bridge may
increase the reactivity of metallocenes by creating a more Lewis
acidic metal center and providing a more spacious coordination
site.7 However, quantitative data explaining the enhanced
(5) Enders, D.; Kroll, M.; Raabe, G.; Runsink, J. Angew. Chem., Int.
Ed. 1998, 37, 1673–1675.
(6) Scholz, J.; Nolte, M.; Kru¨ger, C. Chem. Ber. 1993, 126, 803–809.
10.1021/om900142c CCC: $40.75
2009 American Chemical Society
Publication on Web 04/23/2009

Zirconocene-1-Aza-1,3-diene Complexes
Organometallics, Vol. 28, No. 10, 2009 2939
Figure 1. Four possible stereoisomers for complex 2a.
reactivity in well-defined systems are scarce in the literature.
The Parkin group has shown that rotation of the Zr-Ph bond
in [Me2Si(C5Me4)2]ZrPh2 is about 400 times faster than that of
Cp*2ZrPh2.7b Their studies on H2 elimination from {[Me2Si-
(C5Me4)2]Mo(η2-H2)(H)}+ and [Cp*2MoH3]+ have also sug-
gested that the [Me2Si] ansa bridge facilitates the elimination
by a factor of ∼300 in rate constant.7c For nonunimolecular
processes, direct rate comparisons between ansa-bridged and
unbridged Cp systems are even less known. Quijada, DuPont,
and co-workers have reported that the overall rate of ethylene
polymerization by [Me2Si(C5H4)2]ZrCl2/MAO (MAO ) me-
thylalumoxane) is slightly faster than the catalytic reaction with
Cp2ZrCl2/MAO;8 however, no detailed kinetic study has been
conducted to further address the ansa effect.
Zirconocene-1-aza-1,3-diene complexes bearing an ansa bridge
may be more likely to react with electrophiles. On the other hand,
a more electrophilic Zr center may also increase its binding affinity
for the CdC bond within the zirconacycle. X-ray crystal structure
analysis of related Ti systems has revealed no π-interaction in
Cp2Ti[N(c-C6H11)CHdC(Me)CH(Ph)] but a remarkably strong Ti/
CdC interaction in Cp(Cl)Ti[N(t-Bu)CHdC(Me)CH(Ph)], which
presumably has a more electrophilic Ti center.9 If the CdC bond
dissociation step indeed precedes electrophilic insertion in the ansa-
bridged Zr system (Scheme 2), the rate at which the zirconacycle
unfolds the ring will be critical in determining the efficiency of
the overall insertion.
a Zr/CdC interaction), one would expect four possible stere-
oisomers: one pair of enantiomers A with the Ph group adopting
the pseudoequatorial position and the other pair B with the Ph
1
group adopting the pseudoaxial position (Figure 1). The H
NMR spectrum of 2a in benzene-d6 showed multiple peaks for
each of the eight inequivalent Cp hydrogens and two doublets
for both PhNCH (δ 5.90) and ZrCHPh (δ 1.59) resonances,
which are coupled to each other with a long-range coupling
constant of JH-H ) 1.4 Hz.10 In a selective homonuclear
4
1
decoupling H NMR experiment, saturating the ZrCHPh reso-
4
nance resulted in the disappearance of JH-H coupling and the
PhNCH resonance became one doublet with the olefinic
coupling constant of 3JH-H ) 6.7 Hz unchanged (Figure 2). This
result implies only one set of resonances observable for the
zirconacycle, which could be explained by one of the following
three scenarios: (1) the ring-flipping or A T B interconversion
is fast on the NMR time scale and the average of A and B
resonances has been observed, (2) the equilibrium mixture of
A and B is established, but one pair is highly populated, or (3)
there are high kinetic barriers for A T B interconversion, and
one of the two pairs is kinetically formed during the synthesis.
To distinguish these hypotheses experimentally, variable-
temperature NMR experiments with 2a were performed, but
no dynamic change was observed in the temperature range from
-60 to +70 °C.
For the comparison between [Me2Si] ansa-bridged and bis-
Cp systems, zirconocene-1-aza-1,3-diene complexes with two
plain Cp ligands were prepared from Negishi’s reagent (eq 2).11
Again, the stability of the zirconacycles was dependent on the
nitrogen substituents. Complex 3c, with an electron-withdrawing
CF3 group, was observed in the crude product along with other
unidentified Zr species, but it quickly decomposed during
purification. From the more electron-deficient imine 1d, there
was no evidence that the zirconium complex 3d formed.
In this paper we report the synthesis and characterization of
new zirconocene-1-aza-1,3-diene complexes containing a
[Me2Si] ansa bridge, their reactivities toward various electro-
philes, and comparative kinetic data for ketone insertion into
zirconacycles with different ancillary ligands (Me2Si(C5H4)2 vs
Cp2, NPh vs N(p-MeOC6H4)) and different ketone substrates
(Ph2CdO vs (p-MeOC6H4)2CdO). We also report the compu-
tational studies on the conformational change of zirconocene-
1-aza-1,3-diene complexes and its implications for the mech-
anism of ketone insertion into the zirconacycles.
Results and Discussion
Synthesis of Zirconocene-1-Aza-1,3-diene Complexes. The
Me2Si-bridged zirconocene-1-aza-1,3-diene complexes 2a,b
were prepared in one-pot syntheses from [Me2Si(C5H4)2]ZrCl2
and the corresponding R,ꢀ-unsaturated imines, following a
procedure developed by Whitby (eq 1).4 The 1H NMR and 13
C
{1H} NMR spectra of the zirconium products in C6D6 confirmed
the formation of Zr-C bonds; the characteristic proton (δ 1.59
for 2a and δ 1.68 for 2b) and carbon resonances (δ 69.8 for 2a
and δ 69.3 for 2b) are consistent with those for zirconium alkyl
species. To fully evaluate the electronic effect of nitrogen
substituents on the reactivity of zirconacycles, we attempted to
synthesize zirconium complexes with an electron-withdrawing
group (CF3 in 2c and NO2 in 2d) on the para position of the
N-bound aryl ring. Surprisingly, no desired zirconium products
were observed under reaction conditions similar to those used
for the preparation of 2a,b.
X-ray Structures of Zirconocene-1-Aza-1,3-diene Com-
plexes. To further study the zirconocene-1-aza-1,3-diene com-
plexes, X-ray-quality crystals of 2a and 3a,b were grown by
Assuming that zirconacycle 2a has an envelope-shaped
structure in solution (regardless of the absence or presence of

2940 Organometallics, Vol. 28, No. 10, 2009
Zhang et al.
Figure 2. Selective homonuclear decoupling 1H NMR experiment
1
for the complex 2a in benzene-d6 (the normal H NMR spectrum
1
shown at the bottom and the ZrCHPh proton-decoupled H NMR
spectrum shown at the top).
Figure 4. ORTEP diagram (50% probability level) of the molecular
structure of Cp2Zr[N(Ph)CHdCHCH(Ph)] (3a). Selected bond
lengths (Å) and angles (deg): Zr-N1 ) 2.129(2), Zr · · · C1 )
2.589(3), Zr · · · C2 ) 2.700 (3), Zr-C3 ) 2.321(3), C1-C2 )
1.376(4), C2-C3 ) 1.452(4); N1-Zr-C3 ) 84.79(9), Zr-N1-C1
) 92.25(16), C1-N1-C4 ) 118.7(2), Zr-N1-C4 ) 143.27(19),
(C3-Zr-N1)-(C3-C2-C1-N1) ) 125.4(2).
Figure 3. ORTEP diagram (50% probability level) of the molecular
structure of [Me2Si(C5H4)2]Zr[N(Ph)CHdCHCH(Ph)] (2a). Selected
bond lengths (Å) and angles (deg): Zr-N1 ) 2.133(5), Zr · · · C1
) 2.570(7), Zr · · · C2 ) 2.576 (7), Zr-C3 ) 2.320(6), C1-C2 )
1.364(10), C2-C3 ) 1.446(10); N1-Zr-C3 ) 85.5(2), Zr-N1-C1
) 90.7(4), C1-N1-C4 ) 118.5(5), Zr-N1-C4 ) 146.1(5),
(C3-Zr-N1)-(C3-C2-C1-N1) ) 124.6(4).
adding pentane to saturated toluene solutions of the zirconium
complexes. Thermal ellipsoid plots of the solid-state structures
of 2a and 3a,b are shown in Figures 3-5, respectively, and the
crystallographic data are summarized in Table 1. Consistent with
the structures of related compounds,6,12-14 the 1-aza-1,3-diene
moiety in all three structures is strongly bent with respect to
the N1-Zr-C3 plane. The dihedral angles between the planes
defined by N1-C1-C2-C3 and N1-Zr-C3 are 55.4(4),
54.6(2), and 57.2(2)° for 2a and 3a,b, respectively. The through-
space distances for Zr · · · C1 (2a, 2.570(7) Å; 3a, 2.589(3) Å;
3b, 2.568(3) Å) and Zr · · · C2 (2a, 2.576(7) Å; 3a, 2.700(3) Å;
3b, 2.583(3) Å) are much longer than typical Zr-C bonds,
although they are relatively close to the sum of Zr and C
covalent radii.15 The short distances for C1-C2 bonds (2a,
1.364(10) Å; 3a, 1.376(4) Å; 3b, 1.379(5) Å) confirm that they
are CdC bond units and the zirconium complexes are best
described as metallacyclopentenes, rather than η4-1-aza-1,3-
Figure 5. ORTEP diagram (50% probability level) of the molecular
structure of Cp2Zr[N(p-MeOC6H4)CHdCHCH(Ph)] (3b). Selected
bond lengths (Å) and angles (deg): Zr-N1 ) 2.129(3), Zr · · · C1
) 2.568(3), Zr · · · C2 ) 2.583(3), Zr-C3 ) 2.329(3), C1-C2 )
1.379(5), C2-C3 ) 1.460(5); N1-Zr-C3 ) 84.33(11), Zr-N1-C1
) 91.57(19), C1-N1-C4 ) 118.2(3), Zr-N1-C4 ) 143.9(2),
(C3-Zr-N1)-(C3-C2-C1-N1) ) 122.8(2).
diene complexes or olefin complexes stabilized by imine N-atom
(7) (a) Lee, H.; Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.; Parkin,
G. J. Am. Chem. Soc. 1998, 120, 3255–3256. (b) Lee, H.; Bridgewater,
B. M.; Parkin, G. Dalton Trans. 2000, 4490–4493. (c) Janak, K. E.; Shin,
J. H.; Parkin, G. J. Am. Chem. Soc. 2004, 126, 13054–13070.
(8) Quijada, R.; DuPont, J.; Silveira, D. C.; Miranda, M. S. L.; Scipioni,
R. B. Macromol. Rapid Commun. 1995, 16, 357–362.
(9) Scholz, J.; Kahlert, S.; Go¨rls, H. Organometallics 1998, 17, 2876–
2884.

Zirconocene-1-Aza-1,3-diene Complexes
Organometallics, Vol. 28, No. 10, 2009 2941
Table 1. Summary of Crystallographic Data
2a
3a
3b
5a
empirical
formula
C27H27NSiZr C25H23NZr
C26H25NOZr
C38H33NOZr
formula wt
temp, K
cryst syst
484.81
150(2)
monoclinic
428.66
150(2)
monoclinic
P21/c
458.69
150(2)
triclinic
610.87
150(2)
monoclinic
P21/n
j
space group P21/n
P1
Figure 6. Zirconocene-1-aza-1,3-diene complexes that have been
previously studied by X-ray diffraction.
a, Å
b, Å
c, Å
7.7983(1)
11.6487(5)
7.8515(4)
21.7801(10)
90
99.606(1)
90
1964.07(16)
4
1.450
8.0360(4)
11.0923(6)
11.5186(6)
96.383(1)
95.822(1)
90.250(1)
1014.99(9)
2
9.3835(1)
14.7303(2)
20.8409(3)
90
95.920(1)
90
2865.30(6)
4
1.416
21.9708(4)
13.9755(2)
90
104.602(1)
90
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
2317.15(6)
4
dcalcd, g/cm3 1.390
1.501
λ, Å
1.54178
4.469
16770
0.77490
0.705
20660
0.77490
0.693
12305
1.54178
3.380
23962
µ, mm-1
no. of data
collected
no. of
3401
4970
4964
5145
unique data
R1, wR2
(I > 2σ(I))
R1, wR2
(all data)
0.0662, 0.1582 0.0477, 0.1208 0.0477, 0.1442 0.0250, 0.0654
0.0897, 0.1747 0.0543, 0.1245 0.0507, 0.1469 0.0275, 0.0669
Table 2. Geometrical Data for Zirconocene-1-aza-1,3-diene
Complexes
Figure 7. Calculated energies for the two Zr conformers of 2a and
the transition state for ring flipping.
by 0.07 Å, which may be attributed to the effect of the
substituent (alkyl vs aryl) on C3. The weaker C3-C(sp3) bond
in 3f strengthens the Zr-C3 interaction by providing a carbon
orbital with more s orbital character.18
Computational Studies on the Conformational Change
of 2a. Because the variable-temperature NMR experiments
offered little information on the solution structure of 2a, DFT
studies were conducted to gain mechanistic insights on the
conformational change of the zirconacycle. The calculated
structure for conformer A agrees well with the X-ray crystal-
lographic data of 2a, and conformer B shows similar Zr-C3
and Zr-N1 bond distances (see the Supporting Information for
details). The calculated relative free energies suggest that
conformer A is more stable than conformer B by 5.6 kcal/mol
at 25 °C (Figure 7). This energy difference translates into an
equilibrium constant of 8.6 × 10-5, disfavoring conformer B
when equilibrated with conformer A.
The calculated transition state for the conformational change
contains a flattened five-membered ring with the dihedral angle
between the N1-C1-C2-C3 and N1-Zr-C3 planes being
7.7°. The low kinetic barrier19 (15.8 kcal/mol at 298 K) for ring
flipping from conformer A establishes a rapid interconversion
process on the NMR time scale: the rate constant for A f B is
estimated to be 16 s-1. This result is in accord with our
experimental observation of only one set of resonances for the
1H NMR spectrum of 2a. Although cooling the sample to low
temperature can slow down the interconversion process, con-
former A is still anticipated to be the predominant species. On
the basis of the computed thermodynamic data for A f B (∆H
) 5.1 kcal/mol and ∆S ) -1.4 eu), one would predict that at
-60 °C the amount of conformer B is too small (0.00029 mol
% of all Zr species) to be detected.
dav(Zr-Cpcent)/Å d(Zr-N1)/Å d(Zr-C3)/Å R/deg ꢀ/deg γ/deg
2a
3a
3b
3e
3f
2.244
2.245
2.248
2.249
2.248
2.133
2.129
2.129
2.094
2.160
2.320
2.321
2.329
2.328
2.254
61.3 124.7 3.0
55.4 127.0 1.2
56.5 126.4 1.4
54.6 125.4 0.0
57.1 124.5 0.8
coordination.16 It should also be noted that, in the solid state,
all three zirconium complexes prefer the conformation with the
Ph group occupying the pseudoequatorial position.
The [Me2Si] bridge is expected to exert some influence
on the structures of the zirconacycles, which can ultimately
affect the reactivities of these zirconium complexes. The
geometrical data for the ansa complex 2a and bis-Cp complexes,
including the two known compounds 3e6 and 3f12 (Figure 6),
are summarized in Table 2. As expected for ansa complexes,
the interplanar-ring angle R for 2a (61.3°) is larger than those
in the bis-Cp system (54.6-57.1°), due to the constraint
associated with the natural bond angle preference of the bridging
Si atom.17 The Zr-Cpcent vector in 2a deviates further from
the Cp ring normal, and the tilt angle γ increases about 1.6-3.0°
compared to those for the bis-Cp complexes. These results are
consistent with the notion that the zirconium center with an
ansa-bridged ligand should be sterically more accessible.
Interestingly, the Cp ligand and the N substituent seem to have
little effect on the Zr-Cpcent, Zr-C3, and Zr-N1 bond
distances. However, in complex 3f, the Zr-C3 bond is shortened
(10) This four-bond coupling of vinyl and allylic hydrogens was also
observed for the complexes Cp2Zr[N(Ar)CHdCHCH(Ph)] (Ar ) p-MeC6H4,
o-MeC6H4).6
(11) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett.
1986, 27, 2829–2832.
(12) Michael, F. E.; Duncan, A. P.; Sweeney, Z. K.; Bergman, R. G.
J. Am. Chem. Soc. 2005, 127, 1752–1764.
(13) Scholz, J.; Kahlert, S.; Go¨rls, H. Organometallics 2004, 23, 1594–
1603.
(14) (a) Yasuda, H.; Tatsumi, K.; Nakamura, A. Acc. Chem. Res. 1985,
18, 120–126. (b) Erker, G.; Kru¨ger, C.; Mu¨ller, G. AdV. Organomet. Chem.
1985, 24, 1–39.

2942 Organometallics, Vol. 28, No. 10, 2009
Zhang et al.
Figure 9. Plots of kobs versus [PhCOPh] for CdO insertion into
Zr-C bonds at 333 K: (9) 2a; (b) 2b; (0) 3a; (2) 3b.
Figure 8. ORTEP diagram (50% probability level) of the molecular
structure of Cp2Zr[N(Ph)CHdCHCH(Ph)CPh2O] (5a). Selected
bond lengths (Å) and angles (deg): Zr-N1 ) 2.1671(15), Zr · · · C1
) 2.967(2), Zr · · · C2 ) 3.473(2), Zr · · · C3 ) 3.550(2), Zr-O1 )
1.9608(12), C1-C2 ) 1.337(3), C2-C3 ) 1.506(2), C3-C4 )
1.589(2); N1-Zr-O1 ) 96.03(5), Zr-N1-C1 ) 110.11(11),
C1-N1-C5 ) 112.90(15), Zr-N1-C5 ) 133.00(12).
Whitby and co-workers have experimentally determined the
free energy of activation (11.2 kcal/mol at -31 °C) for the ring
flipping of the complex Cp2Zr[N(Ph)CHdCHCH2] (3g),4 which
contains two diastereotopic protons distinguishable at -60 °C.
Our calculated ∆Gq values for the two isomers of 2a at -31
°C suggest that conformer A is less likely to flip the five-
membered ring (∆Gq ) 15.5 kcal/mol) than 3g, while conformer
B inverts the ring more rapidly (∆Gq ) 10.1 kcal/mol).
Reactivities of Zirconocene-1-Aza-1,3-diene Complexes.
Like other zirconocene-1-aza-1,3-diene complexes, the ansa
complexes 2a,b reacted with ketones such as benzophenone to
give the ring-expanded products 4a,b, respectively (eq 3). The
reactions with the bis-Cp complexes 3a,b gave rise to similar
seven-membered zirconacycles. Interestingly, the long-range
(4JH-H) coupling was not observed in any of the insertion
products, possibly due to a near-180° torsion angle between the
allylic C-H bond and the neighboring vinylic C-H bond.20
Indeed, the X-ray study of 5a (Figure 8) reveals a torsion angle
of 157.6° in the solid state, which is significantly larger than
the same angle in the starting material 3a (116.1°).
Figure 10. Eyring plot for the reaction between 2a and PhCOPh
in toluene-d8.
species as well as the free imine ligand 1a were observed. Acetic
anhydride and CO did not react with 2a at room temperature
but gave intractable products when heated to 100 °C. Acid
chlorides such as p-toluoyl chloride cleanly converted 2a into
[Me2Si(C5H4)2]ZrCl2 at 80 °C, although the fate of the organic
moiety could not be determined. Less polar substrates, including
ethyl vinyl ether, styrene, and diphenylacetylene, were com-
pletely unreactive with 2a at 100 °C.
Kinetic Studies on Ketone Insertion into Zirconocene-
1-Aza-1,3-diene Complexes. For a detailed evaluation of
ancillary ligand and ketone substituent effects on the rate of
ketone insertion, a series of kinetic studies were conducted.
When the zirconium complex 2a was treated with a large excess
of PhCOPh (10 equiv) in toluene-d8 at 60 °C, the CdO insertion
1
product formed with no intermediate observed by H NMR
spectroscopy. Monitoring the disappearance of 2a gave the
pseudo-first-order rate constant kobs (see the Supporting Informa-
tion). Variation of PhCOPh concentration (Table S-1 (Support-
ing Information), Figure 9) indicated that kobs was linear with
PhCOPh concentration, implying that the insertion reaction is
second order overall. Zirconium complexes 2b and 3a,b
exhibited similar kinetic behavior. Rate measurements for
complex 2a between 55 and 75 °C (Figure 10) gave activation
parameters ∆Hq ) 16.5 ( 1.2 kcal/mol and ∆Sq ) -22.5 (
3.4 eu. The negative entropy of activation is consistent with a
highly ordered transition state for CdO insertion.
In addition to benzophenone, a number of other electrophiles
were tested for insertion into complex 2a (in toluene-d8).
Acetonitrile, benzonitrile, ethyl acetate, and tert-butyl acrylate
showed no net reaction even at 100 °C. Allylic bromide, 1,2-
epoxy-3-butene, and CO2 consumed complex 2a quickly at room
temperature; however, complicated mixtures of zirconium

Zirconocene-1-Aza-1,3-diene Complexes
Organometallics, Vol. 28, No. 10, 2009 2943
Scheme 3
Direct comparison of the second-order rate constants confirms
that ansa complexes insert ketones more quickly than bis-Cp
complexes. With the same imine ligand, complex 2a is about
2.8 times more reactive than the bis-Cp complex 3a. The rate
enhancement is less pronounced with an electron-rich imine
ligand; the ansa complex 2b is only about 1.8 times more
reactive than 3b. On the other hand, the property of the im-
ine ligand also plays a key role in controlling the reactivity of
the zirconium complexes. Adding a p-methoxy group to the
N-bound aryl ring decreases the insertion rate constant by 2.8
in the bis-Cp case and by 4.2 in the [Me2Si] bridge case. The
steric effect is expected to be minimal, as the methoxy group
is not in the close proximity of the Zr center and there are
essentially no structural differences between 3a and 3b. Instead,
the electronic effect dominates and our kinetic data suggest that
an electron-deficient Zr center is more favorable for the ketone
insertion reactions. These results, in return, provide indirect
evidence that the [Me2Si(C5H4)2]-ligated Zr center is more
electron deficient than the Cp2-ligated Zr center.17a,21
The kinetics for the insertion reaction with 2a was further
explored using 4,4′-dimethoxybenzophenone as the ketone
substrate (see the Supporting Information for details). The
second-order rate constant obtained at 60 °C is approximately
2.2 times greater than the rate constant for the unsubstituted
benzophenone, which should contain a more electrophilic
carbonyl carbon. The difference in reactivity observed for
different ketone substrates does, however, correlate with the
basicity of the carbonyl oxygen.
initial η2 coordination of the alkene and then alkyl migration to
construct the C-C bond. In contrast, mechanistic information
on the insertion of aldehydes and ketones into similar
metal-carbon bonds is limited, despite its high relevance in
many catalytic and stoichiometric reactions.23,24 For the ketone
insertion into zirconocene-1-aza-1,3-diene complexes, it is
conceivable that an insertion reaction takes place following a
similar ketone coordination-alkyl migration sequence. Although
both zirconocene-η2-ketone24f,25 and zirconocene-η1-ketone
complexes26 are known in the literature,27 the absence of
electrons at the d0 metal in our zirconocene-1-aza-1,3-diene
complexes prevents stabilization of η2-ketone complexes by
back-bonding, particularly from the lateral coordination site.
Inspecting the space-filling model of these complexes also
suggests that severe steric repulsion can be anticipated if the
ketone substrate does adopt the η2 coordination mode prior to
insertion. For these reasons, the mechanism for the ketone
insertion in this case is more likely to involve initial η1
coordination of the ketone, followed by C-C bond formation
(Scheme 3).28 The fact that ethyl vinyl ether, styrene, and
diphenylacetylene do not react with 2a further supports this
Mechanism of Ketone Insertion into Zr-C Bonds of
Zirconocene-1-Aza-1,3-diene Complexes. The mechanism of
alkene insertion into group IV metal-carbon bonds has been
extensively studied, due to its importance in metallocene-
catalyzed alkene polymerization.22 It is generally accepted that
the insertion reactions occur by a two-step process involving
(23) Insertion of CdO into Ti-C bonds: (a) Blandy, C.; Gervais, D.
Inorg. Chim. Acta 1981, 52, 79–81. (b) Meinhart, J. D.; Grubbs, R. H.
Bull. Chem. Soc. Jpn. 1988, 61, 171–180. (c) Hill, J. E.; Balaich, G.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1993, 12, 2911–2924. (d)
Huang, Y.; Etkin, N.; Heyn, R. R.; Nadasdi, T. T.; Stephan, D. W.
Organometallics 1996, 15, 2320–2330. (e) Steinhuebel, D. P.; Lippard, S. J.
J. Am. Chem. Soc. 1999, 121, 11762–11772. (f) Ozerov, O. V.; Parkin, S.;
Brock, C. P.; Ladipo, F. T. Organometallics 2000, 19, 4187–4190. (g) Stroot,
J.; Beckhaus, R.; Saak, W.; Haase, D.; Lu¨tzen, A. Eur. J. Inorg. Chem.
2002, 1729–1737. (h) Kingston, J. V.; Ozerov, O. V.; Parkin, S.; Brock,
C. P.; Ladipo, F. T. J. Am. Chem. Soc. 2002, 124, 12217–12224. (i)
Kingston, J. V.; Sarveswaran, V.; Parkin, S.; Ladipo, F. T. Organometallics
2003, 22, 136–144.
(15) For covalent radius data of Zr (1.75 ( 0.07 Å) and C (C(sp3), 0.76
( 0.01 Å; C(sp2), 0.73 ( 0.02 Å), see: Cordero, B.; Go´mez, V.; Platero-
Prats, A. E.; Reve´s, M.; Echeverr´ıa, J.; Cremades, E.; Barraga´n, F.; Alvarez,
S. Dalton Trans. 2008, 2832–2838.
(16) For a discussion of the bonding description for related complexes,
see: Thomas, D.; Baumann, W.; Spannenberg, A.; Kempe, R.; Rosenthal,
U. Organometallics 1998, 17, 2096–2102.
(17) (a) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin,
G.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525–9546. (b) Shapiro, P. J.
Coord. Chem. ReV. 2002, 231, 67–81. (c) Prashar, S.; Antin˜olo, A.; Otero,
A. Coord. Chem. ReV. 2006, 250, 133–154. (d) Wang, B. Coord. Chem.
ReV. 2006, 250, 242–258.
(24) Insertion of CdO into Zr-C bonds: (a) Buchwald, S. L.; Watson,
B. T.; Wannamaker, M. W.; Dewan, J. C. J. Am. Chem. Soc. 1989, 111,
4486–4494. (b) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A. L. J. Am.
Chem. Soc. 1990, 112, 7994–8001. (c) Cope´ret, C.; Negishi, E.; Xi, Z.;
Takahashi, T. Tetrahedron Lett. 1994, 35, 695–698. (d) Hanna, T. A.;
Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 3292–3293. (e) Hanna, T. A.; Baranger, A. M.; Bergman, R. G. J.
Org. Chem. 1996, 61, 4532–4541. (f) Peulecke, N.; Ohff, A.; Tillack, A.;
Baumann, W.; Kempe, R.; Burlakov, V. V.; Rosenthal, U. Organometallics
1996, 15, 1340–1344. (g) Scott, M. J.; Lippard, S. J. J. Am. Chem. Soc.
1997, 119, 3411–3412. (h) Schenk, W. A.; Gutmann, T. J. Organomet.
Chem. 1998, 552, 83–89. (i) Shao, P.; Gendron, R. A. L.; Berg, D. J.;
Bushnell, G. W. Organometallics 2000, 19, 509–520. (j) Gade, L. H.;
Renner, P.; Memmler, H.; Fecher, F.; Galka, C. H.; Laubender, M.;
Radojevic, S.; McPartlin, M.; Lauher, J. W. Chem. Eur. J. 2001, 7, 2563–
2580. (k) Zhou, S.; Yan, B.; Liu, Y. J. Org. Chem. 2005, 70, 4006–4012.
(25) Cp2Zr(η2-Ph2CO) and its dimer: (a) Rosenfeldt, F.; Erker, G.
Tetrahedron Lett. 1980, 21, 1637–1640. (b) Erker, G.; Rosenfeldt, F. J.
Organomet. Chem. 1982, 224, 29–42. (c) Erker, G.; Dorf, U.; Czisch, P.;
Petersen, J. L. Organometallics 1986, 5, 668–676.
(18) The C2-C3-alkyl angle in 3f is 112.0°, while the C2-C3-Ph
angle in 3a is 121.3°.
(19) Calculated activation parameters for A f B: ∆Hq ) 14.3 kcal/
mol and ∆Sq ) -5.0 eu.
(20) (a) Garbisch, E. W., Jr. J. Am. Chem. Soc. 1964, 86, 5561–5564.
(b) Barfield, M.; Chakrabarti, B. Chem. ReV. 1969, 69, 757–778.
(21) Other evidence has been reported to support a more electron-
deficient Zr center in [Me2Si(C5H4)2]Zr as compared to that in Cp2Zr. IR
spectroscopic data of (CpR)2Zr(CO)2: (a) Atwood, J. L.; Rogers, R. D.;
Hunter, W. E.; Floriani, C.; Fachinetti, G.; Chiesi-Villa, A. Inorg. Chem.
1980, 19, 3812–3817. (b) Bajgur, C. S.; Tikkanen, W. R.; Petersen, J. L.
Inorg. Chem. 1985, 24, 2539–2546. Electrochemical studies on (CpR)2ZrCl2:
ref 21a. Zr (3d5/2, 3d3/2) binding energies in (CpR)2ZrCl2: (c) Siedle, A. R.;
Newmark, R. A.; Lamanna, W. M.; Schroepfer, J. N. Polyhedron 1990, 9,
301–308.
(22) For reviews of metallocene-catalyzed alkene polymerization, see:
(a) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325–387. (b) Brintzinger,
H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. 1995, 34, 1143–1170. (c) Marks, T. J. Acc. Chem. Res.
1992, 25, 57–65. (d) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996,
255–270. (e) Alt, H. G.; Ko¨ppl, A. Chem. ReV. 2000, 100, 1205–1221. (f)
Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100,
1253–1345. (g) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391–
1434. (h) Rappe´, A. K.; Skiff, W. M.; Casewit, C. J. Chem. ReV. 2000,
100, 1435–1456.
(26) Cp*[η5-C5Me4CH2B(C6F5)3]Zr(Ph)(η1-OdC(Me)R): Sun, Y.; Piers,
W. E.; Yap, G. P. A. Organometallics 1997, 16, 2509–2513.
(27) Reviews on aldehyde and ketone complexes: (a) Huang, Y.-H.;
Gladysz, J. A. J. Chem. Educ. 1988, 65, 298–303. (b) Shambayati, S.;
Crowe, W. E.; Schreiber, S. L. Angew. Chem., Int. Ed. 1990, 29, 256–272.
(28) We cannot rule out a concerted mechanism where ketone activation
and C-C bond formation take place simultaneously in the transition state,
but the Zr/O interaction should play a major role.

2944 Organometallics, Vol. 28, No. 10, 2009
Zhang et al.
[Me2Si(C5H4)2]Zr[N(p-MeOC6H4)CHdCHCH(Ph)] (2b). This
mechanistic hypothesis, as CdC and CtC bonds lack the ability
to coordinate to Zr in an end-on fashion. In view of the observed
second-order kinetics for the ketone insertion, the coordination
step is proposed to be the rate-determining step, which is
consistent with our experimental results that reactions are
favored by more electron-deficient metal centers as well as more
basic ketone substrates.
compound was prepared in 78% yield by a procedure similar to
1
that used for 2a. H NMR (400 MHz, CD2Cl2, δ): 7.31-6.83
(m, Ph, 9H), 6.42-6.41 (m, C5H4, 1H), 6.22-6.20 (m, C5H4,
3
4
1H), 6.09 (dd, JH-H ) 6.6 Hz, JH-H ) 1.4 Hz, NCHdCHCH,
1H), 5.92-5.90 (m, C5H4, 1H), 5.89-5.87 (m, C5H4, 1H),
3
5.86-5.84 (m, C5H4, 1H), 5.64 (dd, JH-H ) 9.2 and 6.6 Hz,
NCHdCHCH, 1H), 5.53-5.52 (m, C5H4, 1H), 5.35-5.34 (m,
C5H4, 1H), 4.64-4.62 (m, C5H4, 1H), 3.82 (s, OCH3, 3H), 1.50
Conclusions
3
4
(dd, JH-H ) 9.2 Hz, JH-H ) 1.4 Hz, NCHdCHCH, 1H), 0.78
(s, SiCH3, 3H), 0.39 (s, SiCH3, 3H). 1H NMR (400 MHz,
benzene-d6, δ): 7.24-7.04 (m, Ph, 9H), 6.39-6.38 (m, C5H4,
1H), 6.32-6.30 (m, C5H4, 1H), 6.09 (dd, 3JH-H ) 6.5 Hz, 4JH-H
) 1.1 Hz, NCH ) CHCH, 1H), 5.79-5.77 (m, C5H4, 1H),
5.68-5.66 (m, C5H4, 1H), 5.60-5.52 (m, C5H4 and
NCHdCHCH, 3H), 5.21-5.19 (m, C5H4, 1H), 4.76-4.74 (m,
C5H4, 1H), 3.55 (s, OCH3, 3H), 1.68 (dd, 3JH-H ) 9.2 Hz, 4JH-H
) 1.1 Hz, NCHdCHCH, 1H), 0.58 (s, SiCH3, 3H), 0.14 (s,
SiCH3, 3H). 13C{1H} NMR (101 MHz, CD2Cl2, δ): 155.47,
146.53, 145.48, 128.40, 122.41, 121.79, 121.52, 120.09, 117.36,
114.14, 114.00, 111.07, 109.42, 108.44, 106.88, 106.36, 105.79,
103.20, 102.25, 100.93, 69.30 (NCHdCHCH), 55.53 (OCH3),
-4.50 (SiCH3), -6.27 (SiCH3). Anal. Calcd for C28H29NOSiZr:
C, 65.32; H, 5.68; N, 2.72. Found: C, 65.07; H, 5.82; N, 2.68.
Cp2Zr[N(Ph)CHdCHCH(Ph)] (3a). The synthesis of this
complex has been previously reported with limited characteriza-
tion data.4 It was prepared in 73% yield by a procedure similar
A new class of zirconocene-1-aza-1,3-diene complexes
with a [Me2Si] ansa bridge has been synthesized. Kinetic
studies on the reactions between benzophenone and these
zirconocene complexes, including their bis-Cp equivalents,
demonstrate that the incorporation of a [Me2Si] ansa bridge
reduces the kinetic barrier for insertion. The reactions are
faster when a more basic ketone is used but are slower when
a more electron-rich substituent is introduced to the imine
nitrogen. The kinetic data in combination with other reactivity
studies support a two-step process for the overall insertion:
initial coordination of the ketone in an η1 fashion followed
by formation of the C-C bond. Ring flipping of the
zirconacycles, which has been studied by DFT calculations,
is too fast to affect the insertion reactions.
Experimental Section
1
to that used for 2a. H NMR (400 MHz, CD2Cl2, δ): 7.34-6.92
General Comments. All air-sensitive compounds were pre-
pared and handled under an argon atmosphere using standard
Schlenk and inert-atmosphere box techniques. Toluene and THF
were deoxygenated and dried over an Innovative Technology
solvent purification system. Toluene-d8 and benzene-d6 were
distilled from Na and benzophenone under an argon atmosphere.
CD2Cl2 wasdriedoverCaH2,degassedbythreefreeze-pump-thaw
3
4
(m, Ph, 10H), 6.09 (dd, JH-H ) 6.7 Hz, JH-H ) 1.4 Hz,
NCHdCHCH, 1H), 6.05 (s, Cp, 5H), 5.68 (dd, 3JH-H ) 9.2 and
6.7 Hz, NCHdCHCH, 1H), 5.51 (s, Cp, 5H), 1.57 (dd, JH-H
3
)
9.2 Hz, 4JH-H ) 1.4 Hz, NCHdCHCH, 1H). 13C{1H} NMR (101
MHz, CD2Cl2, δ): 152.16, 147.34, 128.64, 128.48, 122.97,
121.93, 121.84, 121.76, 119.90, 107.23, 104.72, 103.83, 68.37
(NCHdCHCH). Anal. Calcd for C25H23NZr: C, 70.04; H, 5.41;
N, 3.27. Found: C, 69.93; H, 5.41; N, 3.25.
cycles, and then purified by vacuum transfer at room temperature.
21b
[Me2Si(C5H4)2]ZrCl2
and R,ꢀ-unsaturated imines (1a-d)29
Cp2Zr[N(p-MeOC6H4)CHdCHCH(Ph)] (3b). This compound
was prepared in 85% yield by a procedure similar to that used
for 2a. 1H NMR (400 MHz, CD2Cl2, δ): 7.32-6.84 (m, Ph, 9H),
were prepared as described in the literature.
[Me2Si(C5H4)2]Zr[N(Ph)CHdCHCH(Ph)] (2a). Under an ar-
gon atmosphere n-BuLi (0.69 mL, 1.6 M in hexanes, 1.10 mmol)
was added dropwise to a solution of [Me2Si(C5H4)2]ZrCl2 (174
mg, 0.50 mmol) in THF (20 mL) at -78 °C. The mixture was
stirred at this temperature for 1 h before a solution of 1a (104
mg, 0.50 mmol) in THF (10 mL) was added via cannula. The
resulting mixture was then slowly warmed to room temperature
over a period of 5 h and stirred for an additional 1 h, at which
time a bright orange solution formed. The volatiles were removed
under vacuum, and the residue was treated with toluene and
filtered under argon. Removal of toluene under vacuum afforded
the crude product, which was subjected to recrystallization in
toluene/pentane to give 2a as an orange solid (170 mg, 70%
yield). 1H NMR (400 MHz, benzene-d6, δ): 7.32-7.26 and
7.04-6.94 (m, Ph, 10H), 6.28-6.26 (m, C5H4, 1H), 6.17-6.15
3
4
6.08 (dd, JH-H ) 6.6 Hz, JH-H ) 1.2 Hz, NCHdCHCH, 1H),
3
6.01 (s, Cp, 5H), 5.62 (dd, JH-H ) 9.2 and 6.6 Hz,
NCHdCHCH, 1H), 5.49 (s, Cp, 5H), 3.83 (s, OCH3, 3H), 1.49
3
4
(dd, JH-H ) 9.2 Hz, JH-H ) 1.2 Hz, NCHdCHCH, 1H).
13C{1H} NMR (101 MHz, CD2Cl2, δ): 155.57, 147.52, 146.07,
128.46, 122.96, 122.46, 121.74, 120.71, 113.94, 107.03, 103.93,
103.61, 68.12 (NCHdCHCH), 55.55 (OCH3). Anal. Calcd for
C26H25NOZr: C, 68.08; H, 5.49; N, 3.05. Found: C, 68.02; H,
5.48; N, 3.06.
Cp2Zr[N(p-CF3C6H4)CHdCHCH(Ph)] (3c). This compound
was detected from the crude product by following a procedure
similar to that used for 2a; however, attempted recrystallization
in toluene/pentane led to the decomposition of the zirconium
1
3
4
complex. H NMR (400 MHz, CD2Cl2, δ): 7.57-6.96 (m, Ph,
(m, C5H4, 1H), 5.90 (dd, JH-H ) 6.7 Hz, JH-H ) 1.4 Hz,
NCHdCHCH, 1H), 5.67-5.65 (m, C5H4, 1H), 5.54-5.52 (m,
C5H4, 1H), 5.46-5.41 (m, NCHdCHCH and C5H4, 3H),
5.07-5.05 (m, C5H4, 1H), 4.62-4.60 (m, C5H4, 1H), 1.59 (dd,
3
4
9H), 6.08 (s, Cp, 5H), 6.03 (dd, JH-H ) 6.8 Hz, JH-H ) 1.4
3
Hz, NCHdCHCH, 1H), 5.78 (dd, JH-H ) 9.2 and 6.8 Hz,
3
NCHdCHCH, 1H), 5.54 (s, Cp, 5H), 1.70 (dd, JH-H ) 9.2 Hz,
4JH-H ) 1.4 Hz, NCHdCHCH, 1H).
4
3JH-H ) 9.2 Hz, JH-H ) 1.4 Hz, NCHdCHCH, 1H), 0.44 (s,
SiCH3, 3H), 0.01 (s, SiCH3, 3H). 13C{1H} NMR (101 MHz,
benzene-d6, δ): 152.00, 146.77, 129.00, 128.67, 128.44, 127.92,
127.68, 127.33, 122.63, 122.24, 121.17, 119.11, 117.63, 114.55,
111.36, 109.65, 108.97, 107.37, 106.62, 105.79, 104.61, 102.33,
100.81, 69.78 (NCHdCHCH), -4.65 (SiCH3), -6.60 (SiCH3).
Anal. Calcd for C27H27NSiZr: C, 66.89; H, 5.61; N, 2.89. Found:
C, 66.94; H, 5.68; N, 2.76.
[Me2Si(C5H4)2]Zr[N(Ph)CHdCHCH(Ph)CPh2O] (4a). Under
an argon atmosphere, benzophenone (115 mg, 0.63 mmol) was
added to a solution of 2a (305 mg, 0.63 mmol) in toluene (30
mL). The resulting mixture was stirred at 80 °C for 8 h, cooled
to room temperature, and passed through a short plug of dry
Celite under argon. The solvent was removed under vacuum and
the residue was subjected to recrystallization in toluene/pentane
to give 4a as a light yellow solid (365 mg, 87% yield). 1H NMR
(400 MHz, benzene-d6, δ): 7.77-6.75 (m, Ph and C5H4, 23H),
6.21-6.15 (m, C5H4 and NCHdCHCH, 3H), 6.02-5.99 (m,
(29) Tomioka, K.; Shioya, Y.; Nagaoka, Y.; Yamada, K. J. Org. Chem.
2001, 66, 7051–7054.

Zirconocene-1-Aza-1,3-diene Complexes
Organometallics, Vol. 28, No. 10, 2009 2945
C5H4, 1H), 5.84 (br s, C5H4, 1H), 5.73-5.71 (m, C5H4, 1H),
(SiCH3), -6.67 (SiCH3). Anal. Calcd for C42H41NO3ZrSi: C,
69.38; H, 5.68; N, 1.93. Found: C, 69.00; H, 5.23; N, 2.08.
NMR Measurements of Rate Constants for Ketone Inser-
tion into Zirconocene-1-Aza-1,3-diene Complexes. In a typical
experiment, benzophenone or 4,4′-dimethoxybenzophenone (10-60
mg) was dissolved in toluene-d8 (0.5-1 mL) with added toluene
(2-5 µL) as an internal standard. The resulting solution was
transferred into a resealable NMR tube, followed by the addition
of the zirconocene-1-aza-1,3-diene complex. The first 1H NMR
spectrum was recorded (on a Bruker AMX 400 MHz NMR
spectrometer) within 5 min, and spectra were taken every 10-20
min for 3-5 half-lives. The integration for the higher field MeSi
resonances of ansa complexes (or the integration for the lower
field Cp resonances of bis-Cp complexes) was compared to the
integration for the toluene methyl resonance (δ 2.11). The probe
temperature was calibrated using 100% ethylene glycol.30
Computational Details. DFT calculations on the zirconium
complexes were performed by employing the Gaussian 03
program (Revision D.02).31 The B3LYP gradient corrected
exchange hybrid DFT method32 was applied. Geometry optimi-
zations were performed in the gas phase using the default
convergence criteria without any constraints. The LANL2DZ
ECP basis set was used for the zirconium atom,33 while the all-
electron 6-31G(d) Pople basis set was employed for H, C, N,
and Si atoms.34 Frequency calculations were employed to
characterize the stationary points as minima or transition-state
structures, as well as to determine zero-point energies (ZPEs)
for free-energy calculations at 298.15 K and 1 atm. The Cartesian
coordinates of the optimized structures and the corresponding
thermodynamic data are summarized in the Supporting Informa-
tion.
X-ray Structure Determinations. Crystal data collection and
refinement parameters are summarized in Table 1. For complexes
3a,b, data were collected on a Bruker APEX2 CCD detector at
Beamline 11.3.1 at the Advanced Light Source (Lawrence
Berkeley National Laboratory) using synchrotron radiation tuned
to λ ) 0.77490 Å. For complexes 2a and 5a, data were collected
at 150 K on a Bruker SMART6000 CCD diffractometer using
graphite-monochromated Cu KR radiation (λ ) 1.54178 Å). The
data frames were collected using either APEX2 or SMART and
integrated using SAINT. The data were corrected for decay,
Lorentz, and polarization effects as well as absorption and beam
corrections on the basis of the multiscan technique used in
SADABS. The structures were solved by a combination of direct
methods in SHELXTL and difference Fourier techniques and
refined by full-matrix least-squares procedures. Non-hydrogen
atoms were refined with anisotropic displacement parameters.
3
5.40 (dd, JH-H ) 9.5 and 7.8 Hz, NCHdCHCH, 1H), 4.13 (d,
3JH-H ) 9.5 Hz, NCHdCHCH, 1H), 0.62 (s, SiCH3, 3H), 0.44
(s, SiCH3, 3H). 13C{1H} NMR (101 MHz, toluene-d8, δ): 156.21,
148.61, 146.85, 142.37, 136.88, 136.82, 130.45, 129.49, 129.32,
127.24, 126.82, 126.73, 126.54, 125.97, 122.58, 121.29, 120.39,
117.11, 114.88, 112.97, 112.39, 110.77, 109.65, 109.08, 99.54,
57.26 (NCHdCHCH), -3.84 (SiCH3), -6.71 (SiCH3). Anal.
Calcd for C40H37NOZrSi: C, 72.02; H, 5.59; N, 2.10. Found: C,
72.13; H, 5.37; N, 2.17.
[Me2Si(C5H4)2]Zr[N(p-MeOC6H4)CHdCHCH(Ph)CPh2O] (4b).
This compound was prepared in 88% yield by a procedure similar
to that used for 4a. 1H NMR (400 MHz, benzene-d6, δ):
7.83-6.83 (m, Ph and C5H4, 21H), 6.73-6.71 (m, C5H4, 1H),
6.22-6.20 (m, C5H4 and NCHdCHCH, 2H), 6.14-6.12 (m,
C5H4, 1H), 6.06-6.05 (m, C5H4, 1H), 5.84 (br s, C5H4, 1H),
3
5.79-5.77 (m, C5H4, 1H), 5.41 (dd, JH-H ) 9.5 and 9.4 Hz,
NCHdCHCH, 1H), 4.17 (d, 3JH-H ) 9.4 Hz, NCHdCHCH, 1H),
3.55 (s, OCH3, 3H), 0.65 (s, SiCH3, 3H), 0.48 (s, SiCH3, 3H).
13C{1H} NMR (101 MHz, toluene-d8, δ): 154.83, 149.50, 148.81,
147.00, 142.61, 138.04, 136.88, 136.82, 130.47, 129.29, 129.20,
127.24, 126.74, 126.68, 126.60, 125.90, 121.46, 116.23, 115.54,
113.72, 113.32, 112.06, 111.30, 109.74, 109.53, 100.11, 57.24
(NCHdCHCH), 54.68 (OCH3), -3.98 (SiCH3), -6.52 (SiCH3).
No satisfactory elemental analysis results were obtained for this
compound, despite repeated crystallizations to purify the product.
Anal. Calcd for C41H39NO2ZrSi: C, 70.65; H, 5.64; N, 2.01.
Found: C, 69.92; H, 5.17; N, 2.13.
Cp2Zr[N(Ph)CHdCHCH(Ph)CPh2O] (5a). This compound
was prepared in 91% yield by a procedure similar to that used
for 4a. X-ray-quality crystals of 5a were grown by adding
pentane to a saturated toluene solution of the zirconium complex.
1H NMR (400 MHz, CD2Cl2, δ): 7.57-6.59 (m, Ph, 20H), 6.51
3
(s, Cp, 5H), 6.17 (d, JH-H ) 7.6 Hz, NCHdCHCH, 1H), 5.96
3
(s, Cp, 5H), 4.91 (t, JH-H ) 8.4 Hz, NCHdCHCH, 1H), 3.80
3
(d, JH-H ) 9.2 Hz, NCHdCHCH, 1H). 13C{1H} NMR (101
MHz, CD2Cl2, δ): 156.12, 147.90, 146.19, 142.47, 138.24,
130.39, 129.44, 129.33, 127.97, 127.83, 127.38, 126.92, 126.75,
126.45, 125.94, 122.16, 121.25, 119.48, 112.12, 111.53, 98.31,
56.75 (NCHdCHCH). Anal. Calcd for C38H33NOZr: C, 74.71;
H, 5.44; N, 2.29. Found: C, 74.51; H, 5.39; N, 2.21.
Cp2Zr[N(p-MeOC6H4)CHdCHCH(Ph)CPh2O] (5b). This com-
pound was prepared in 90% yield by a procedure similar to that
used for 4a. 1H NMR (400 MHz, benzene-d6, δ): 7.69-6.62
3
(m, Ph, 19H), 6.23 (s, Cp, 5H), 6.16 (d, JH-H ) 7.5 Hz,
3
NCHdCHCH, 1H), 5.80 (s, Cp, 5H), 5.11 (t, JH-H ) 8.4 Hz,
NCHdCHCH, 1H), 3.97 (d, 3JH-H ) 9.2 Hz, NCHdCHCH, 1H),
3.45 (s, OCH3, 3H). 13C{1H} NMR (101 MHz, benzene-d6, δ):
154.64, 150.12, 148.46, 146.68, 142.79, 138.84, 130.67, 129.76,
129.58, 127.93, 127.69, 127.48, 126.93, 126.68, 126.08, 122.79,
122.44, 113.81, 112.05, 111.21, 99.54, 57.04 (NCHdCHCH),
55.02 (OCH3). Anal. Calcd for C39H35NO2Zr: C, 73.08; H, 5.50;
N, 2.18. Found: C, 72.79; H, 5.43; N, 2.08.
(30) Raiford, D. S.; Fisk, C. L.; Becker, E. D. Anal. Chem. 1979, 51,
2050–2051.
(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G., Jr.; Stratmann, R. E.; Burant,
J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain,
M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci,
B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg,
J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2004.
(32) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(33) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b)
Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (c) Wadt, W. R.;
Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.
(34) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971,
54, 724–728. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys.
1972, 56, 2257–2261. (c) Harihara, P. C.; Pople, J. Theor. Chim. Acta 1973,
28, 213–222.
[Me2Si(C5H4)2]Zr[N(Ph)CHdCHCH(Ph)C(p-MeOC6H4)2O]
(4h). This compound was prepared in 89% yield by a procedure
1
similar to that used for 4a. H NMR (400 MHz, toluene-d8, δ):
7.60-6.71 (m, Ph and C5H4, 20H), 6.62-6.60 (m, C5H4, 1H),
6.13-6.08 (m, C5H4 and NCHdCHCH, 3H), 5.99-5.97 (m,
C5H4, 1H), 5.84 (br s, C5H4, 1H), 5.65-5.64 (m, C5H4, 1H),
3
5.29 (dd, JH-H ) 9.5 and 9.4 Hz, NCHdCHCH, 1H), 3.99 (d,
3JH-H ) 9.4 Hz, NCHdCHCH, 1H), 3.44 (s, OCH3, 3H), 3.33
(s, OCH3, 3H), 0.58 (s, SiCH3, 3H), 0.41 (s, SiCH3, 3H). 13C{1H}
NMR (101 MHz, toluene-d8, δ): 158.86, 158.66, 155.95, 142.83,
140.66, 139.08, 130.62, 130.42, 130.40, 128.10, 127.93, 127.08,
125.9, 124.27, 122.41, 121.18, 120.26, 120.11, 117.06, 114.89,
113.09, 112.51, 112.44, 111.94, 110.83, 109.69, 109.15, 98.90,
57.56 (NCHdCHCH), 54.33 (OCH3), 54.23 (OCH3), -3.84

2946 Organometallics, Vol. 28, No. 10, 2009
Zhang et al.
The H atoms were either located or calculated and subsequently
treated with a riding model. For complex 2a, high-resolution
data were very weak, which was also indicated by the higher
value of Rint (not diffracting very well, beginning at ap-
proximately 1 Å). Data used in the refinement were truncated
at 0.89 Å resolution.
Crystallography at Advanced Light Source) program at
Beamline 11.3.1 at the Advanced Light Source (ALS),
Lawrence Berkeley National Laboratory (supported by the
U.S. Department of Energy, Office of Energy Sciences
Materials Sciences Division, under contract DE-AC02-
05CH11231).
Acknowledgment. This work was supported by start-
up funds from the University of Cincinnati (H.G.) and
the National University of Singapore (K.-W.H.). We thank
Dr. Keyang Ding (University of Cincinnati) for assistance
with the selective homonuclear decoupling NMR experi-
ment. Crystallographic data were collected on a Bruker
SMART6000 diffractometer (funded by an NSF-MRI
grant; CHE-0215950), or through the SCrALS (Service
Supporting Information Available: Figures, tables, and CIF
files giving complete details of the crystallographic study, DFT
computational results, kinetic data, and plots of these data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
OM900142C