Influence of a [Me2Si] ansa bridge on the barrier to rotation about the Zr-phenyl bond and on the generation and reactivity of benzyne intermediates: Comparison of the structures and reactivity of [Me2Si(C5Me4)2]Zr(Ph)X and Cp*2Zr(Ph)X (X = H, Cl, Ph)

Article abstract of DOI:10.1039/b005117i

Comparison of the chemistry of [Me₂Si(C₅Me₄)₂]ZrPh₂ and Cp2ZrPh2 demonstrates that incorporation of a [Me₂Si] ansa bridge reduces the barrier for both (i) rotation about the Zr-Ph bond and (ii) elimination of benzene to generate a benzyne intermediate; furthermore, the [Me₂Si] ansa bridge promotes the reaction of two equivalents of MeCN with the benzyne intermediate {[Me₂Si(C₅Me₄)₂]Zr(η ²-C6H4)}, whereas insertion of only one equivalent is observed with [Cp*₂Zr(η²-C₆H₄)]. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b005117i

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Influence of a [Me2Si] ansa bridge on the barrier to rotation about
the Zr–phenyl bond and on the generation and reactivity of
benzyne intermediates: comparison of the structures and reactivity
of [Me2Si(C5Me4)2]Zr(Ph)X and Cp*2Zr(Ph)X (X ؍ H, Cl, Ph)
Hyosun Lee, Brian M. Bridgewater and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, USA
Received 26th June 2000, Accepted 28th September 2000
First published as an Advance Article on the web 20th November 2000
Comparison of the chemistry of [Me2Si(C5Me4)2]ZrPh2
and Cp*2ZrPh2 demonstrates that incorporation of a
[Me2Si] ansa bridge reduces the barrier for both (i) rotation
about the Zr–Ph bond and (ii) elimination of benzene to
generate a benzyne intermediate; furthermore, the [Me2Si]
ansa bridge promotes the reaction of two equivalents of
MeCN with the benzyne intermediate {[Me2Si(C5Me4)2]-
Zr(ꢀ2-C6H4)}, whereas insertion of only one equivalent is
observed with [Cp*2Zr(ꢀ2-C6H4)].
Zirconocene aryl complexes have attracted attention due to
their ability to generate aryne species which, as exemplified by
Buchwald’s Cp2Zr(η2-C6H4)(PMe3),1 may be isolated in favor-
able situations. Such benzyne species are not only of intrinsic
chemical interest, but have also found considerable application
as reactive intermediates in organic synthesis.2 In this paper, we
describe further studies to delineate the ansa-effect in metallo-
cene chemistry3 by reporting the influence of a [Me2Si] ansa-
bridge on (i) the barrier to rotation about a Zr–phenyl bond,
and (ii) the ability to generate a benzyne intermediate and the
impact on its subsequent reactivity.
Scheme 1
The ansa-zirconocene phenyl complexes [Me2Si(C5Me4)2]Zr-
(Ph)Cl and [Me2Si(C5Me4)2]ZrPh2 are obtained by sequential
metathesis of [Me2Si(C5Me4)2]ZrCl2 with PhLi (Scheme 1), in
an analogous manner to the pentamethylcyclopentadienyl
derivatives, Cp*2Zr(Ph)Cl and Cp*2ZrPh2.4 The molecular
structures of [Me2Si(C5Me4)2]Zr(Ph)Cl, [Me2Si(C5Me4)2]ZrPh2,
Cp*2Zr(Ph)Cl and Cp*2ZrPh2 have been determined by X-ray
diffraction (Table 1).5,6 As has been observed previously for
other pairs of [Me2Si(C5Me4)2]MX2 and Cp*2MX2 derivatives,3
the [Me2Si] ansa bridge exerts a subtle structural influence
which forces the cyclopentadienyl groups to tilt by ca. 4Њ such
that the ring normals lose coincidence with the Zr–Cpcent vector
and the range of Zr–C bond lengths increases (Table 1); the
coordination mode of the ligand thus moves from symmetric
η5,η5-coordination towards η3,η3-coordination.
Interestingly, the subtle structural difference between the
ansa [Me2Si(C5Me4)2]Zr(Ph)X and non-ansa Cp*2Zr(Ph)X
systems is manifested by a decrease in the barrier to rotation
about the Zr–Ph bond for the ansa complexes. The latter
Table 1 Geometrical data for Cp*2Zr(Ph)X and [Me2Si(C5Me4)2]Zr(Ph)X derivatives
d(Zr–Cpcent)/Å
d(Zr–C)/Å
d(Zr–C) range/Å
α/Њ
β/Њ
γ/Њ
Cp*2Zr(Ph)H
2.233
2.224
2.250
2.234
2.279
2.263
2.519–2.543
2.472–2.590
2.509–2.589
2.466–2.620
2.541–2.599
2.485–2.644
0.024
0.188
0.080
0.154
0.058
0.159
141.1
129.9
139.5
128.7
139.3
127.9
141.0
122.5
138.1
120.6
137.7
119.1
0.05
3.7
0.18
4.1
0.8
4.4
[Me2Si(C5Me4)2]Zr(Ph)H
Cp*2Zr(Ph)Cla
[Me2Si(C5Me4)2]Zr(Ph)Cl
a
Cp*2ZrPh2
[Me2Si(C5Me4)2]ZrPh2
a Average values for two crystallographically independent molecules.
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DOI: 10.1039/b005117i

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process can be conveniently probed by dynamic 1H NMR
spectroscopy because all five phenyl protons are chemically
inequivalent in the static solid state structure and rotation
about the Zr–Ph bond results in coalescence of the signals of
the two pairs of ortho and meta protons.7 Since a knowledge of
the factors that influence the barriers to metal ligand rotations
is of relevance to understanding the structure of polymers
obtained using metallocene catalysts,8 we have sought to quan-
tify the effect of a [Me2Si] ansa bridge on rotation barriers.
The low temperature (ca. 200 K) 1H NMR spectrum of
[Me2Si(C5Me4)2]Zr(Ph)Cl is indicative of a static structure, with
five different signals for the phenyl group, but upon warming
the signals attributed to the two pairs of ortho and meta protons
coalesce.9 In contrast to the coalescence behavior observed for
[Me2Si(C5Me4)2]Zr(Ph)Cl, the non-ansa counterpart Cp*2Zr-
(Ph)Cl retains a static structure on the NMR time-scale at 350
K.10 It is, therefore, evident that the barrier for rotation about
the Zr–Ph bond in the ansa complex [Me2Si(C5Me4)2]Zr(Ph)Cl
is substantially less than that in Cp*2Zr(Ph)Cl.11 The spectro-
scopic behavior of the pair of phenyl-hydride complexes,
[Me2Si(C5Me4)2]Zr(Ph)H and Cp*2Zr(Ph)H, is similar, with
only the former exhibiting fluxionality. Likewise, the diphenyl
derivatives [Me2Si(C5Me4)2]ZrPh2 and Cp*2ZrPh2 exhibit the
same trend, but the barrier for the latter complex is sufficiently
low that a quantitative comparison can be made; at 25 ЊC, rota-
tion about the Zr–Ph bond in [Me2Si(C5Me4)2]ZrPh2 is a factor
of ca. 400 faster than that of Cp*2ZrPh2.9 Since the phenyl
ligands in the ansa zirconocene complexes [Me2Si(C5Me4)2]Zr-
(Ph)X (X = H, Cl) exhibit a more pronounced β-agostic inter-
action, thus providing an additional barrier to rotation than in
their non-ansa counterparts, Cp*2Zr(Ph)X,3f it is evident that
the more facile rotation in the ansa system may be attributed to
the reduction in steric interactions resulting from tilting (γ) of
the cyclopentadienyl rings, which increases the distance
between the ring methyl substituents and the phenyl ligand.12
As with other diphenyl zirconocene complexes,13 thermal
elimination of benzene from [Me2Si(C5Me4)2]ZrPh2 provides
a means of generating the benzyne intermediate {[Me2Si-
(C5Me4)2]Zr(η2-C6H4)}. For example, {[Me2Si(C5Me4)2]Zr(η2-
C6H4)} is trapped by ethylene to give [Me2Si(C5Me4)2]Zr-
(η2-C6H4CH2CH2),5 as illustrated in Scheme 2.14 While Marks
has reported that the non-ansa system behaves analogously to
[Me2Si(C5Me4)2]ZrPh2 in the presence of ethylene, giving
Cp*2Zr(η2-C6H4CH2CH2),4b,15 the two systems behave very dif-
ferently in the presence of acetonitrile. Specifically, whereas
[Cp*2Zr(η2-C6H4)] is trapped by a single molecule of MeCN to
give Cp* Zr[η2-C,N-C H {C(Me)᎐N}] (Scheme 3), the ansa
᎐
2
6
4
Scheme 3
counterpart {[Me2Si(C5Me4)2]Zr(η2-C6H4)} is trapped by two
molecules of MeCN giving [Me2Si(C5Me4)2]Zr[η3-C,N,N-
C H {C(CH )NC(Me)᎐NH}] under comparable conditions
᎐
6
(Sch4eme 2). 2The molecular structures of the acetonitrile inser-
tion products, Cp* Zr[η2-C,N,N-C H {C(Me)᎐N}] and [Me Si-
᎐
(C Me ) ]Zr[η3-C,N,N-C H {C(CH )NC(Me)᎐NH}] have b2een
2
6
4
᎐
5
4
2
6
4
2
determined by X-ray diffraction (Fig. 1).5 Key spectroscopic
evidence for the characterization of [Me2Si(C5Me4)2]Zr[η3-
C,N,N-C H {C(CH )NC(Me)᎐NH}] is the observation of a
᎐
6
4
triplet resonance at283.5 ppm (1JC-H = 157 Hz) in the 13C NMR
spectrum for the CH2 group. In addition to representing an
interesting example of an ansa effect, the formation of the
double insertion product is notable since other zirconocene
benzyne species typically only insert a single RCN molecule.13,16
Kinetics studies are in accord with the above reactions of
[Me2Si(C5Me4)2]ZrPh2 proceeding via a benzyne intermediate.
Specifically, the rate constants at 40 ЊC for the reactions with
C2H4 [k = 4.3(1) × 10Ϫ6 Ϫ1] and MeCN [k = 4.0(1) × 10Ϫ6 sϪ1
s ]
are experimentally indistinguishable and are also independent
of the concentration of substrate, as would be expected for rate
determining elimination of benzene and the formation of a
benzyne intermediate. Interestingly, the rate of elimination of
benzene from [Me2Si(C5Me4)2]ZrPh2 is noticeably faster than
that from Cp*2ZrPh2 by a factor of 1.7.17 A possible origin for
the enhanced rate may be related to the more facile rotation
about the Zr–Ph bond in [Me2Si(C5Me4)2]ZrPh2. Thus, since
elimination of benzene would be most favored for a configur-
ation in which the phenyl group that abstracts the hydrogen is
perpendicular to the incipient benzyne plane, the rate constant
would be expected to be greater for the ansa system because of
the reduced steric demands.
In summary, comparison of the chemistry of [Me2Si(C5-
Me4)2]Zr(Ph)X and Cp*2Zr(Ph)X complexes provides a further
illustration of the manner in which an ansa bridge may modu-
late reactivity. Incorporation of a [Me2Si] ansa bridge thus
facilitates rotation about the Zr–Ph bond in [Me2Si(C5Me4)2]-
Zr(Ph)X derivatives and promotes elimination of benzene
from [Me2Si(C5Me4)2]ZrPh2. Finally, the benzyne complex
so obtained, {[Me2Si(C5Me4)2]Zr(η2-C6H4)}, reacts with two
Scheme 2
J. Chem. Soc., Dalton Trans., 2000, 4490–4493
4491

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(k) J. H. Shin, T. Hascall and G. Parkin, Organometallics, 1999,
18, 6.
4 Cp*2ZrPh2 has been previously reported. See: (a) H. S. Tung and
C. H. Brubaker, Jr., Inorg. Chim. Acta, 1981, 52, 197; (b) L. E.
Schock, C. P. Brock and T. J. Marks, Organometallics, 1987, 6, 232.
5 The molecular structures of all new compounds reported in this
paper have been determined by single crystal X-ray diffraction.
[Me2Si(C5Me4)2]Zr(Ph)Cl, C26H35ClSiZr, monoclinic, a =
9.6955(4) Å, b = 12.4482(6) Å, c = 20.4091(9) Å, β = 91.194(1)Њ, V =
2462.7(2) Å3, P21/c, Z = 4, T = 203 K, µ = 0.614 mmϪ1, R1 = 0.0320,
5689 reflections.
[Me2Si(C5Me4)2]ZrPh2, C35H43SiZr, triclinic, a = 9.899(1) Å, b =
11.497(2) Å, c = 13.928(2) Å, α = 74.67(1)Њ, β = 79.25(1)Њ, γ =
3
Ϫ1
¯
86.97(1)Њ, V = 1501.9(4) Å , P1, Z = 2, T = 298 K, µ = 0.427 mm
R1 = 0.0401, 5130 reflections.
,
Cp*2Zr(Ph)Cl, C26H35ClZr, triclinic, a = 8.4788(4) Å, b =
16.1860(8) Å, c = 17.4483(9) Å, α = 89.919(1)Њ, β = 85.415(1)Њ, γ =
3
Ϫ1
¯
83.084(1)Њ, V = 2369.5(2) Å , P1, Z = 4, T = 203 K, µ = 0.586 mm
R1 = 0.0441, 10126 reflections.
,
Cp*2ZrPh2, C32H40Zr, monoclinic, a = 9.3667(1) Å, b = 16.310(2)
Å, c = 34.803(4) Å, β = 92.765(2)Њ, V = 5310(1) Å3, P21/c, Z = 8,
T = 203 K, µ = 0.431 mmϪ1, R1 = 0.0520, 12213 reflections.
Cp*2ZrPh(H), C26H36Zr, monoclinic, a = 10.2338(5) Å, b =
13.7292(7) Å, c = 17.2395(9) Å, β = 105.576(1)Њ, V = 2333.2(2) Å3,
P21/c, Z = 4, T = 203 K, µ = 0.479 mmϪ1
reflections.
, R1 = 0.0410, 5335
[Me2Si(C5Me4)2]Zr(η2-C6H4CH2CH2), C28H38SiZr, monoclinic,
a = 10.475(1) Å, b = 13.998(1) Å, c = 16.784(2) Å, β = 100.838(2)Њ,
V = 2417.1(4) Å3, P21/n, Z = 4, T = 208 K, µ = 0.517 mmϪ1
R1 = 0.0454, 5587 reflections.
,
[Me2Si(C5Me4)2]Zr[η3-C,N,N-C6H4{C(CH2)NC(Me)᎐NH}], C30-
᎐
H40N2SiZr, triclinic, a = 9.4411(5) Å, b = 10.2964(6) Å, c =
14.6641(8) Å, α = 83.578(1)Њ, β = 78.987(1)Њ, γ = 74.495(1)Њ, V =
3
1345.5(1) Å , P1, Z = 2, T = 223 K, µ = 0.474 mmϪ1, R1 = 0.0438,
¯
5989 reflections.
Cp*2Zr[η2-C,N-C6H4{C(Me)᎐N}], C28H37NZr, triclinic, a =
᎐
8.7032(7) Å, b = 9.6822(8) Å, c = 15.812(1) Å, α = 89.326(1)Њ, β =
3
¯
79.892(2)Њ, γ = 70.091(1)Њ, V = 1231.7(2) Å , P1, Z = 2, T = 223 K,
µ = 0.460 mmϪ1, R1 = 0.0470, 5018 reflections. CCDC reference
number 186/2213. See http://www.rsc.org/suppdata/dt/b0/b005117i/
for crystallographic files in .cif format.
Fig.
1
Molecular structures of Cp*2Zr[η2-C,N-C6H4{C(Me)᎐N}]
᎐
(top) and [Me2Si(C5Me4)2]Zr[η3-C,N,N-C6H4{C(CH2)NC(Me)᎐NH}]
᎐
6 For the molecular structure of Cp2ZrPh2, see: W. Clegg, L.
Horsburgh, D. M. Lindsay and R. E. Mulvey, Acta Crystallogr.,
Sect. C, 1998, 54, 315.
(bottom).
7 For other studies concerned with M–aryl rotational barriers as
equivalents of MeCN, in contrast to the one equivalent that
1
studied by H NMR spectroscopy, see: (a) P. Courtot, R. Pichon,
reacts with the non-ansa counterpart [Cp*2Zr(η2-C6H4)].
J. Y. Salaun and L. Toupet, Can. J. Chem., 1991, 69, 661; (b) J.
Jeffery, M. F. Lappert, N. T. Luong-Thi, J. L Atwood and W. E.
Hunter, J. Chem. Soc., Chem. Commun., 1978, 1081; (c) P. R. Sharp,
D. Astruc and R. R. Schrock, J. Organomet. Chem., 1979, 182, 477;
(d) W. D. Jones and F. J. Feher, Inorg. Chem., 1984, 23, 2376.
8 See, for example: J. C. W. Lohrenz, M. Bühl, M. Weber and W. Thiel,
J. Organomet. Chem., 1999, 592, 11 and references therein.
Acknowledgements
We thank the U. S. Department of Energy, Office of Basic
Energy Sciences (#DE-FG02-93ER14339) for support of this
research.
9 [Me2Si(C5Me4)2]Zr(Ph)H: ∆G‡ = 12.8(2) kcal molϪ1
103
5(1) × 101
,
k = 3(1) ×
s
Ϫ1. [Me2Si(C5Me4)2]Zr(Ph)Cl: ∆G‡ = 15.2(1) kcal molϪ1, k =
s
Ϫ1. [Me2Si(C5Me4)2]ZrPh2: ∆G‡ = 13.9(1) kcal molϪ1
,
Notes and references
1 S. L. Buchwald, B. T. Watson and J. C. Huffman, J. Am. Chem. Soc.,
1986, 108, 7411.
k = 4(1) × 102 sϪ1. Cp*2ZrPh2: ∆G‡ = 17.5(1), k = 9(2) × 10Ϫ1 sϪ1. All
values at 25 ЊC determined from a ∆G‡ versus T fit using KINPAR
(J. R. Norton, personal communication).
2 (a) S. L. Buchwald and R. D. Broene, Comprehensive Organometallic
Chemistry II, E. Abel, F. G. A. Stone and G. Wilkinson, (eds),
Pergamon, New York, 1995, pp. 771–784; (b) S. L. Buchwald and
R. D. Broene, Science, 1993, 261, 1696; (c) S. L. Buchwald and
R. B. Nielsen, Chem. Rev., 1998, 88, 1047; (d) S. L. Buchwald
and R. A. Fisher, Chem. Scr., 1989, 29, 417; (e) J.-P. Majoral,
P. Meunier, A. Igau, N. Pirio, M. Zablocka, A. Skowronska and
S. Bredeau, Coord. Chem. Rev., 1998, 178–180, 145.
3 For lead articles, see: (a) J. C. Green and C. N. Jardine, J. Chem.
Soc., Dalton Trans., 1999, 3767; (b) J. C. Green, Chem. Soc. Rev.,
1998, 263; (c) S. L. J. Conway, T. Dijkstra, L. H. Doerrer, J. C.
Green, M. L. H. Green and M. L. H. Stephens, J. Chem. Soc.,
Dalton Trans., 1998, 2689; (d) L. Labella, A. Chernega and M. L. H.
Green, J. Chem. Soc., Dalton Trans., 1995, 395; (e) A. Chernega,
J. Cook, M. L. H. Green, L. Labella, S. J. Simpson, J. Souter and
A. H. H. Stephens, J. Chem. Soc., Dalton Trans., 1997, 3225; ( f ) H.
Lee, P. J. Desrosiers, I. Guzei, A. L. Rheingold and G. Parkin, J. Am.
Chem. Soc., 1998, 120, 3255; (g) J. H. Shin and G. Parkin, Chem.
Commun., 1999, 887; (h) D. Churchill, J. H. Shin, T. Hascall, J. M.
Hahn, B. M. Bridgewater and G. Parkin, Organometallics, 1999, 18,
2403; (i) D. G. Churchill, B. M. Bridgewater and G. Parkin, J. Am.
Chem. Soc., 2000, 122, 178; (j) H. Lee, J. B. Bonanno, J. Cordaro,
J. M. Hahn and G. Parkin, J. Chem. Soc., Dalton Trans., 1999, 1365;
10 It should be noted, however, that although Cp*2Zr(Ph)Cl exhibits
no coalescence behavior, a small temperature dependence of the
chemical shifts is observed.
11 For further comparison, the rotational barrier (∆G‡) in (C5Me4H)2-
Zr(Ph)Cl is 20.3 kcal molϪ1 (unspecified temperature). See reference
7a.
12 It is also worth noting that π-interactions may play an additional
role in influencing the barrier to rotation. Specifically, π-donation to
Zr would be a maximum when the phenyl ring is perpendicular to
the equatorial plane, and this stabilizing interaction would be
expected to be more influential for the more electron deficient ansa
system. It would also be expected to provide a more important
contribution to lowering the barrier for [Me2Si(C5Me4)2]Zr(Ph)H
than for [Me2Si(C5Me4)2]Zr(Ph)Cl, since π-donation from Cl in the
latter complex would effectively compete with that from the phenyl.
13 See, for example: (a) S. L. Buchwald, A. Sayers, B. T. Watson and
J. C. Dewan, Tetrahedron Lett., 1987, 28, 3245; (b) S. L. Buchwald,
B. T. Watson, R. T. Lum and W. A. Nugent, J. Am. Chem. Soc.,
1987, 109, 7137.
14 An additional distinction between the two systems is that
thermolysis of Cp*2ZrPh2 in the absence of a trap generates the
fulvene complex Cp*(C5Me4CH2)ZrPh (reference 4b), whereas
only small quantities of [Me2Si(C5Me4)(C5Me3CH2)]ZrPh may be
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J. Chem. Soc., Dalton Trans., 2000, 4490–4493

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observed under comparable conditions. The titanium counterpart
has, nevertheless, been isolated (reference 3j).
15 For other examples of zirconocene–benzyne trapping by C2H4, see:
(a) G. Erker and K. Kropp, J. Organomet. Chem., 1980, 194, 45;
(b) G. Erker and K. Kropp, J. Am. Chem. Soc., 1979, 101, 3659;
(c) K. Kropp and G. Erker, Organometallics, 1982, 1, 1246.
16 The mechanism for formation of [Me2Si(C5Me4)2]Zr[η3-C,N,N-
See: S. A. Cohen and J. E. Bercaw, Organometallics, 1985, 4,
1006.
17 Rate constants for elimination of benzene from Cp*2ZrPh2 at 40 ЊC:
C2H4 trap, k = 2.4(1) × 10Ϫ6 Ϫ1; MeCN trap, k = 2.4(1) × 10Ϫ6 sϪ1
s .
Marks has reported rate constants for elimination of benzene from
Cp*2ZrPh2 over the range 50–104 ЊC (reference 4b). These data
predict a rate constant of 3.6(3) × 10Ϫ6 sϪ1 at 40 ЊC (KINPAR). The
rate constant comparisons between [Me2Si(C5Me4)2]ZrPh2 and
Cp*2ZrPh2 reported here are for “side-by-side” kinetics samples in
order to minimize systematic error.
C6H4{C(CH2)NC(Me)᎐NH}] may involve a 1,3-hydrogen shift
᎐
within the mono insertion product [Me2Si(C5Me4)2]Zr[η2-C,N-
C6H4{C(Me)᎐N)]. 1,3-Hydrogen shifts of this type have precedence.
᎐
J. Chem. Soc., Dalton Trans., 2000, 4490–4493
4493