Coordination of alkenes and alkynes to a cationic d0 zirconocene alkoxide complex

Article abstract of DOI:10.1021/ja029963j

This paper describes the synthesis of base-free (C₅R₅)₂Zr(O^tBu)⁺ cations, the direct observation of nonchelated alkene and alkyne adducts of these cations, and studies of the thermodynamic and dynamic properties of these novel species. Reaction of Cp₂′ZrMe₂ (Cp′ = C₅H₄Me) with tert-butyl alcohol followed by [Ph₃C][B(C₆F₅)₄] in benzene yields [ Cp₂′Zr(O′Bu) ][B(C₆F₅)₄] (1), which exists as Cp₂′Zr(O′Bu)(CIR)⁺ solvent adducts in C₆D₅Cl and CD₂Cl₂ solutions. Addition of ligands L (L = ethylene, propylene, propyne, 2-butyne, CO, phenylacetylene, allene, 1-hexene, cis-2-butene) to 1 in CD₂Cl₂ at -89 °C results in reversible formation of Cp₂′Zr(O′Bu)(L)⁺ adducts. NMR data for Cp₂′Zr(O′Bu)(H₂C=CHMe)⁺ (4) indicate that the propylene coordinates unsymmetrically and is polarized with positive charge buildup at C_int. Equilibrium constants, defined by K_eq = [Zr-L][1]^-1[L]^-1, vary in the order CO > propyne > 2-butyne > phenylacetylene > ethylene > allene > propylene > 1-hexene > cis-2-butene > vinyl chloride. Loss of L from Cp²′Zr(O′Bu)(L)⁺to give 1 appears to proceed via associative displacement by CD₂Cl₂ in most cases. Copyright

Full text of DOI:10.1021/ja029963j

Published on Web 02/21/2003
Coordination of Alkenes and Alkynes to a Cationic d0 Zirconocene Alkoxide
Complex
Edward J. Stoebenau, III and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received December 30, 2002 ; E-mail: rfjordan@uchicago.edu
Table 1. Equilibrium Constants for Eq 2 for Selected Ligandsa
Zirconium(IV) alkene and alkyne complexes, (C5R5)2Zr(R)-
(substrate)+, are probable key intermediates in zirconocene-
K
K
eq
eq
catalyzed alkene polymerization1 and alkyne oligomerization,2 but
little is known about their structures and properties. These species
may exhibit unusual features due to the absence of d-π* back-
bonding. Several d0 group 5 and 6 metal alkene3 and alkyne4
complexes have been observed by NMR, and chelated d0 group 3
and 4 metal alkene5 and alkyne2,6 complexes have been studied.7
In particular, X-ray and NMR studies of chelated (C5R5)2Zr(OCMe2-
CH2CH2CHdCH2)+ complexes show that the alkene binds to
Zr(IV) unsymmetrically (d(Zr-Cterm) < d(Zr-Cint)) and suggest
that the CdC bond is polarized with positive charge on Cint.5a,b
However, it is not yet clear how the properties of these species are
influenced by the chelation, and therefore nonchelated analogues
are highly desirable. Here we report the synthesis of base-free
(C5R5)2Zr(OtBu)+ cations, the observation of nonchelated alkene
and alkyne adducts of these cations, and studies of the thermody-
namic and dynamic properties of these novel species.
-1
-1
L
(−89 °C, M
)
L
(−89 °C, M )
CO
propyne (5)
2-butyne (6)
phenylacetylene
ethylene (3)
>1500
allene
propylene (4)
1-hexene
cis-2-butene
vinyl chloride
6.7(3)
5.4(2)
4.8(8)
2.2(1)
<0.1
360(70)
52(3)
22(1)
7.0(6)
a Keq ) [Zr-L][1]-1[L]-1
.
Table 2. Thermodynamic Data for Equilibria in Eq 2 and
Activation Parameters for Decomplexation (k-1) of L from Adducts
3-6
q
q
q
∆H°
(kcal/mol)
∆S°
(eu)
∆H
∆S
∆G
adduct
(kcal/mol)
(eu)
(kcal/mol)a
3
4
5
6
-3.6(1)
-3.8(2)
-4.1(3)
-3.6(3)
-16(4)
-17(1)
-11(1)
-11(1)
7.7(5)
8.1(9)
8.5(3)
-15(2)
-12(4)
-14(1)
4(2)
11.1(1)
11.0(1)
11.8(1)
12.9(1)
13.8(5)
The alkoxide complex [Cp′Zr(OtBu)][B(C6F5)4] (1, Cp′ )
2
a At -39 °C.
C5H4Me) was synthesized in a one-pot procedure (eq 1). Reaction
of Cp′ZrMe2 with tert-butyl alcohol followed by [Ph3C]-
2
[Cp′Zr(OtBu)(C2H4)][B(C6F5)4] (3, eq 2). The 13C NMR spectrum
2
[B(C6F5)4] in benzene cleanly yields 1. The Ph3CMe byproduct
can be removed by washing with benzene and hexanes to
afford 1 as a bright yellow solid (80%). The analogous complex
[Cp2Zr(OtBu)][B(C6F5)4] (2) was generated in a similar manner.
of 3 contains four Cp′ CH resonances, consistent with Cs symmetry.
The 1H and 13C NMR spectra each contain one coordinated ethylene
resonance indicative of fast ethylene rotation. The ethylene 1H
resonance (δ 5.92) is shifted 0.56 ppm downfield, and the 13C
resonance (δ 119.0) is shifted 3.8 ppm upfield from the free ethylene
resonances. The 1JCH value for ethylene is virtually unchanged upon
coordination.
Compound 1 is soluble in halocarbon solvents, forms an oil
1
in benzene or toluene, and is insoluble in alkanes. The H and
13C NMR spectra of 1 in C6D5Cl at 23 and -35 °C, and in CD2Cl2
at -89 °C, contain two CH resonances for the Cp′ rings indica-
tive of a C2V-symmetric cation. These data are consistent with
either a dinuclear [Cp′Zr(µ-OtBu)]2 dication8 or a labile mono-
2+
2
nuclear ion pair or solvent adduct with fast site epimerization at
Taking the concentration of CD2Cl2 to be constant, we define
the equilibrium constant for ethylene coordination in eq 2 by Keq
) [3][1]-1[C2H4]-1 ) 7.0(6) M-1 (-89 °C, Table 1). This value
is unaffected by changing the initial concentrations of 1 (0.038-
0.080 M) or ethylene (0.13-0.29 M) or by addition of
[Ph3C][B(C6F5)4] as an excess anion source. However, addition of
30 equiv of C6H5Cl shifts the equilibrium to the left, possibly due
to C6H5Cl coordination.10b Raising the temperature also shifts the
equilibrium to the left. A van’t Hoff plot gives ∆H° ) -3.6(1)
kcal/mol and ∆S° ) -16(4) eu for substitution of CD2Cl2 by
ethylene (Table 2).11
t
1
Zr. The H NMR spectrum of a 1:1 mixture of Cp′Zr(O Bu)Me
2
and Cp2Zr(OtBu)Me after treatment with 2 equiv of [Ph3C]-
[B(C6F5)4] in C6D5Cl exhibits sharp resonances for 1 and 2 but
no resonances or line broadening effects indicative of a mixed
[Cp′2Zr(µ-OtBu)2ZrCp2]2+ dimer. The 19F NMR spectra of 1 show
no evidence of anion coordination down to -89 °C.9 Additionally,
ethylene coordination to 1 in CD2Cl2 is influenced by C6H5Cl but
not by [Ph3C][B(C6F5)4] (vide infra). These results strongly suggest
that 1 forms Cp′Zr(OtBu)(ClR)+ adducts in chlorocarbon solu-
2
tions.10
Addition of ethylene to a CD2Cl2 solution of 1 at -89 °C affords
an equilibrium mixture of 1, free ethylene, and the ethylene complex
The NMR resonances for 1, 3, and free ethylene broaden and
coalesce as the temperature is raised from -89 °C, consistent with
9
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J. AM. CHEM. SOC. 2003, 125, 3222-3223
10.1021/ja029963j CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
the exchange of 1 and 3 and of free and coordinated ethylene. The
line widths for 3 are independent of the free ethylene concentration,
which implies that coordinated ethylene is not directly displaced
by free ethylene. Analysis of VT NMR spectra provides first-order
rate constants (k-1) and activation parameters (Table 2) for ethylene
decomplexation. The negative ∆Sq value suggests that CD2Cl2
displaces the coordinated ethylene in an associative mechanism.12
Addition of propylene to 1 results in partial conversion to the
presence of 1. Finally, tert-butylacetylene, trans-2-butene, benzene,
N2, H2, and, interestingly, 1,3-butadiene do not displace CD2Cl2
from 1.
The [Cp′Zr(OtBu)][B(C6F5)4] system enables, for the first time,
2
direct study of alkene and alkyne coordination to a cationic Zr(IV)
center in the absence of chelation. The Cp′Zr(OtBu)(alkene)+
2
complexes are models for (C5R5)2ZrR(alkene)+ species in zir-
conocene-catalyzed alkene polymerizations.1 The use of an alkoxide
instead of an alkyl ligand is expected to decrease the metal Lewis
propylene adduct [Cp′Zr(OtBu)(H2CdCHMe)][B(C6F5)4] (4, eq
2
acidity, so alkene binding in Cp′Zr(OtBu)(alkene)+ may be weaker
2). The 13C NMR spectrum of 4 contains eight Cp′ CH and two
2
than in Cp′ZrR(alkene)+ species.5a Future studies of alkene
1
Cp′ Me resonances, consistent with C1 symmetry. The H NMR
2
coordination to a broader set of (C5R5)2Zr(OtBu)+ complexes will
enable us to probe how the (C5R5)2Zr structure influences olefin
coordination, and may provide new insights to the factors which
underlie structure/reactivity trends in zirconocene-catalyzed alkene
polymerization.
Hint resonance of the coordinated propylene (δ 7.34) is shifted far
downfield from the free propylene resonance (δ 5.79). The
propylene 13C Cint resonance (δ 153.5) is shifted 19.6 ppm
downfield, and the Cterm resonance (δ 102.6) is shifted 12.4 ppm
upfield by coordination. The propylene JHH and JCH values are
virtually unchanged by coordination. These data are very similar
to the data for chelated (C5R5)2Zr(OCMe2CH2CH2CHdCH2)+ olefin
complexes, which suggests that the propylene ligand in 4 is bound
unsymmetrically and is polarized in the same manner as proposed
for the chelated complexes.5a,b VT NMR studies show that
propylene and ethylene bind with similar strength to 1 and that the
barrier to propylene decomplexation is nearly identical to that for
ethylene decomplexation from 3 (Tables 1, 2).
Acknowledgment. We thank the NSF (CHE-0212210) for
financial support, and Frank Schaper for helpful discussions.
Supporting Information Available: Experimental procedures, data
for new compounds, and VT NMR spectra (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Similarly, addition of propyne to 1 yields an equilibrium mixture
of 1, free propyne, and [Cp′Zr(OtBu)(HCtCMe)][B(C6F5)4] (5,
2
(1) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b) Resconi, L.;
Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253.
1
eq 2). The H and 13C NMR spectra of 5 contain four Cp′ CH
resonances, consistent with Cs symmetry. The propyne 13C Cint
resonance (δ 89.1) is shifted 8.9 ppm downfield, and the Cterm
resonance (δ 64.2) is shifted 2.8 ppm upfield upon coordination.
These results suggest that the coordinated propyne is unsymmetri-
cally bound and polarized in a similar manner as the propylene
of 4.
(2) Horton, A. D. Chem. Commun. 1992, 185.
(3) (a) Humphries, M. J.; Douthwaite, R. E.; Green, M. L. H. J. Chem. Soc.,
Dalton Trans. 2000, 2952. (b) Witte, P. T.; Meetsma, A.; Hessen, B.;
Budzelaar, P. H. M. J. Am. Chem. Soc. 1997, 119, 10561. (c) Kress, J.;
Osborn, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 1585.
(4) Curtis, M. A.; Finn, M. G.; Grimes, R. N. J. Organomet. Chem. 1998,
550, 469.
The Cs-symmetric 2-butyne adduct [Cp′2Zr(OtBu)(MeCtCMe)]-
[B(C6F5)4] (6, eq 2) is generated by addition of 2-butyne to 1. At
-89 °C, the 1H and 13C NMR spectra of 6 each contain one
2-butyne CMe resonance, which is broadened due to restricted
rotation of the in-plane-bound 2-butyne ligand. These resonances
sharpen as the temperature is raised to -59 °C due to 2-butyne
rotation, and then broaden and coalesce with the free 2-butyne t
CMe resonances at higher temperatures due to the exchange in eq
2. VT NMR studies show that 2-butyne binds more weakly than
propyne, but the barrier to 2-butyne decomplexation is higher than
that for propyne decomplexation. These results, and the more
positive ∆Sq value for 2-butyne decomplexation from 6, are
suggestive of a greater degree of dissociative character in the
substitution of 2-butyne by CD2Cl2 vis-a`-vis ligand decomplexation
of 3-5. Significant steric crowding is expected in the transition
state for associative CD2Cl2 displacement of 2-butyne.
Compound 1 forms adducts with a wide variety of other ligands,
and equilibrium constants for eq 2 for selected cases are summarized
in Table 1. Certain trends can be noted. First, CO binds quite
strongly, and NMR studies show that free CO directly displaces
coordinated CO.13 Second, alkynes coordinate more strongly than
alkenes of similar structure, but steric crowding can strongly inhibit
alkyne binding. Third, ethylene and R-olefins bind with similar
strength, which suggests that electronic and steric effects cancel
for this series, because R-olefins are expected to be stronger
σ-donors on the basis of their higher HOMO energies.14 The
disubstituted olefin cis-2-butene binds weakly. Vinyl chloride binds
very weakly, and the coordination mode (via CdC vs Cl) is not
yet established; in this case, the only evidence for coordination is
line broadening of the vinyl chloride 13C NMR signals in the
(5) (a) Carpentier, J.-F.; Wu, Z.; Lee, C. W.; Stro¨mberg, S.; Christopher, J.
N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750. (b) Wu, Z.; Jordan,
R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, 5867. (c) Carpentier,
J.-F.; Maryin, V. P.; Luci, J.; Jordan, R. F. J. Am. Chem. Soc. 2001, 123,
898. (d) Galakhov, M. V.; Heinz, G.; Royo, P. Chem. Commun. 1998,
17. (e) Brandow, C. G.; Mendiratta, A.; Bercaw, J. E. Organometallics
2001, 20, 4253. (f) Casey, C. P.; Carpenetti, D. W., II; Sakurai, H.
Organometallics 2001, 20, 4262 and references therein. (g) Casey, C. P.;
Klein, J. F.; Fagan, M. A. J. Am. Chem. Soc. 2000, 122, 4320 and
references therein.
(6) Temme, B.; Erker, G.; Fro¨hlich, R.; Grehl, M. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1480.
(7) The existence of an yttrium-propylene adduct has been deduced from NMR
line broadening effects. Casey, C. P.; Lee, T.-Y.; Tunge, J. A.; Carpenetti,
D. W., II. J. Am. Chem. Soc. 2001, 123, 10762.
(8) Martin, A.; Uhrhammer, R.; Gardner, T. G.; Jordan, R. F.; Rogers, R. D.
Organometallics 1998, 17, 382.
(9) (a) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910. (b)
Goodman, J. T.; Schrock, R. R. Organometallics 2001, 20, 5205.
(10) (a) A base free Cp2′Zr(OtBu)+ cation is very unlikely in RCl solution. (b)
The related chlorobenzene adducts [Cp2Zr(CH2Ph)(ClPh)][B(C6F5)4]
and [(C5Me5)2ZrCl(ClPh)][B(C6F5)4] have been crystallographically char-
acterized. Wu, F.; Jordan, R. F., unpublished results.
(11) If the solvent term is included, the equilibrium constant for eq 2 is K′
)
eq
Keq[CD2Cl2], where Keq is defined as in the text. If the solvent concentra-
tion is assumed to be independent of temperature, the value of ∆H° is
not affected, but the entropy term becomes ∆S°′ ) ∆S° + R(ln[CD2-
Cl2]), where R(ln[CD2Cl2]) ≈ 5.5 eu.
(12) For associative ligand substitution of Cp2Zr(X)(L)+ species, see: (a)
Schaper, F.; Geyer, A.; Brintzinger, H. H. Organometallics 2002, 21, 473.
(b) Collins, S.; Koene, B. E.; Ramachandran, R.; Taylor, N. J. Organo-
metallics 1991, 10, 2092.
(13) For other examples of cationic Zr(IV) CO complexes, see: Guo, Z.;
Swenson, D. C.; Guram, A. S.; Jordan, R. F. Organometallics 1994, 13,
766 and references therein.
(14) von Schenck, H.; Stro¨mberg, S.; Zetterberg, K.; Ludwig, M.; Åkermark,
B.; Svensson, M. Organometallics 2001, 20, 2813.
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