Alkyne and alkene complexes of a d0 zirconocene aryl cation

Article abstract of DOI:10.1021/ja045794m

The generation and properties of nonchelated Zr-aryl-alkyne and Zr-aryl-alkene complexes that are stabilized by the presence of β-Si-substituents in the alkyne and alkene ligands and fluorination of the aryl ligand are described. Reaction of [Cp′₂Zr(O^tBu)(ClCD₂Cl)][B(C₆F₅)₄] (1, Cp′ = C₅H₄Me) with alkyne and alkene substrates (L) generates Cp′₂Zr(O^tBu)(L)⁺ adducts (L = HC_≡CCH₂SiMe₃ (2); H₂C=CHCH₂SiMe₃ (3); HC≡CMe (4); H₂C=CHCH₂CMe₃ (5)). Equilibrium constants for substrate binding (K_eq = [Zr-L][1]^-1[L]^-1; CD₂Cl₂, -89 °C) are much larger for the β-Si-substituted compounds 2 (1.0(2) × 10⁵ M^-1) and 3 (1.7(4) × 10³ M^-1) than for hydrocarbon analogues 4 (3.6(7) × 10² M^-1) and 5 (1.9(1) M^-1), which is ascribed to β-Si stabilization of the partial positive charge on C^int of the bound substrate. [Cp₂Zr(C₆F₅)][B(C₆F₅)₄] (7, Cp = C₅H₅) was generated by the reaction of Cp₂Zr(C₆F₅)Me with [Ph₃C][B(C₆F₅)₄] in C₆D₅Cl. Reaction of 7 with alkyne and alkene substrates (L) generates Cp₂Zr(C₆F₅)(L)⁺ adducts (L = HC_≡CCH₂SiMe₃ (8); H₂=CCHCH₂SiMe₃ (10)). No insertion of the substrate into the Zr-C₆F₅ bond is observed in 8 (at -38 °C) or 10 (up to 22 °C). The allyltrimethylsilane ligand in 10 undergoes nondissociative alkene face exchange ( alkene flipping , i.e., exchange of the Cp₂Zr(C₆F₅)⁺ unit between the two alkene enantiofaces without alkene dissociation), with a first-order rate constant k_flip = 23(1) s^-1 (C₆D₅Cl, -38 °C). 10 also undergoes slower reversible decomplexation of the alkene (k_dissoc = 5.0(8) s^-1; C₆D₅Cl, -38 °C). Copyright

Full text of DOI:10.1021/ja045794m

Published on Web 08/21/2004
Alkyne and Alkene Complexes of a d0 Zirconocene Aryl Cation
Edward J. Stoebenau, III, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received July 13, 2004; E-mail: rfjordan@uchicago.edu
(C5R5)2Zr(R)(alkyne)+ and (C5R5)2Zr(R)(alkene)+ complexes are
probable key intermediates in zirconocene-catalyzed alkyne oligo-
merization1 and alkene polymerization.2 These species are chal-
lenging to study because the absence of d-π* back-bonding results
in weak Zr-substrate binding, and because insertion of the substrate
into the Zr-R bond is fast. X-ray and NMR studies of chelated
(C5R5)2Zr(OCMe2CH2CH2CHdCH2)+ complexes,3 and NMR stud-
ies of nonchelated Zr-alkoxide-alkene species,4 show that alkenes
bind to Zr(IV) unsymmetrically (d(Zr-Cterm) < d(Zr-Cint)) and
that the CdC bond is polarized with positive charge on Cint. Several
d0 metal-alkyl-alkene complexes are known,5 but except for an
yttrium system in which alkene coordination can be detected by
NMR line broadening,6 in all cases the coordinated alkene is
tethered to another ligand. Here we describe nonchelated Zr-aryl-
alkyne and Zr-aryl-alkene complexes that are stabilized by the
presence of â-Si substituents in the alkyne and alkene ligands and
fluorination of the aryl ligand.
The use of â-Si-substituted alkynes and alkenes should favor
the formation of stable d0 metal-substrate complexes, due to â-Si
stabilization of the positive charge on Cint.7,8 To test this idea, we
compared the coordination of â-Si-substituted and non-Si-
substituted substrates to [Cp′2Zr(OtBu)(ClCD2Cl)][B(C6F5)4] (1; Cp′
) C5H4Me).4 Propargyltrimethylsilane (PTMS) and allyltrimeth-
ylsilane (ATMS) react with 1 to give robust [Cp′2Zr(OtBu)(L)]-
[B(C6F5)4] adducts (L ) HCtCCH2SiMe3 (2); H2CdCHCH2SiMe3
(3); eq 1). The NMR resonances for the alkyne and alkene units of
Incorporation of fluorine substituents in the Zr-R group should
stabilize a (C5R5)2Zr(R)(substrate)+ species against insertion, due
to the resulting decreased nucleophilicity of the Zr-R group.9 To
test this idea, we investigated alkyne and alkene binding to the
Cp2Zr(C6F5)+ cation.
The reaction of Cp2Zr(C6F5)Me (6, Cp ) C5H5)10 with 1 equiv
of [Ph3C][B(C6F5)4] for 2 days (C6D5Cl, 22 °C) yields a 1:1 mixture
of Ph3CMe and [Cp2Zr(C6F5)][B(C6F5)4] (7, 97%, eq 2). At earlier
reaction times, mixtures of 7, Ph3CMe, Ph3C+, and [{Cp2Zr(C6F5)}2-
(µ-Me)][B(C6F5)4]10 were observed. Compound 7 is stable for 3
weeks in C6D5Cl at 22 °C, but decomposes in seconds in CD2Cl2
at -78 °C. The -38 °C 19F NMR spectrum of 7 contains two o-F
resonances, one at δ -118.2 that is typical for Zr(C6F5) compounds,
and a second at δ -140.0, ca. 20 ppm upfield of the normal range
(δ -116 ( 10).10,11 These results show that the sides of the C6F5
ligand are inequivalent, and suggest that one o-F is coordinated to
Zr.11a Additionally, complex 7 may be further stabilized by solvent
coordination. VT 19F NMR spectra reveal that the sides of the C6F5
ligand exchange as the temperature is raised, due to a lateral pivot
of the C6F5 ligand and/or Zr-C6F5 rotation.
Addition of PTMS to 7 (C6D5Cl, -38 °C) yields the alkyne
adduct [Cp2Zr(C6F5)(HCtCCH2SiMe3)][B(C6F5)4] (8, eq 3).12 The
19F NMR spectrum of 8 contains two o-F resonances in the normal
range (δ -114.6, -121.0), which are broadened due to Zr-C6F5
rotation. The alkyne Cint 13C NMR resonance is more strongly
shifted in 8 (∆δ ) +62.5) than in 2 (∆δ ) +21.7), which may
indicate a greater degree of polarization of the alkyne in 8. The
J
CH values for the alkyne unit in 8 (1JCH ) 232; 2JCH ) 34 Hz) are
2 and 3 are more strongly shifted from free substrate values
than those for the non-Si-containing analogues [Cp′2Zr-
(OtBu)(HCtCMe)][B(C6F5)4] (4, eq 1) and [Cp′2Zr(OtBu)-
(H2CdCHCH2CMe3)][B(C6F5)4] (5, eq 1), suggesting a greater
degree of substrate polarization.5a For example, the Cint 13C NMR
resonance of 3 shifts far downfield (∆δ ) δcoord - δfree ) +31.6),
the Cterm resonance shifts upfield (∆δ ) -19.7), and the Hint
resonance shifts far downfield (∆δ ) +1.80), compared to the free
ligand values. In contrast, much smaller coordination shifts are
observed for 5 (∆δ: Cint +18.8, Cterm -11.7, Hint +1.46). The
10-15 Hz smaller than the values for free PTMS, 2, and other
terminal alkyne complexes of 1,4 but are within the range for sp-
hybridized carbons and inconsistent with insertion products in which
these carbons would be sp2-hybridized.13 PTMS binds strongly to
7 with Keq ) [8][7]-1[HCtCCH2SiMe3]-1 ) 9.1(6) × 102 M-1
(C6D5Cl, -38 °C). THF displaces PTMS from 8 to give [Cp2Zr-
(C6F5)(THF)][B(C6F5)4] (9, 100%). Compound 8 is stable for 8 h
at -38 °C (C6D5Cl).
Addition of 4 equiv of ATMS to 7 (C6D5Cl, -38 °C) re-
sults in partial formation of the alkene adduct [Cp2Zr(C6F5)-
(H2CdCHCH2SiMe3)][B(C6F5)4] (10, eq 3). The Hint 1H NMR
equilibrium constant for the formation of 2, Keq ) [2][1]-1
-
[HCtCCH2SiMe3]-1 ) 1.0(2) × 105 M-1 (CD2Cl2, -89 °C, eq
1), is 280 times larger than that for coordination of propyne to 1 to
give 4,4 even though propyne is smaller than PTMS. Similarly, the
equilibrium constant for ATMS binding to 1 (Keq ) 1.7(4) × 103
M-1, CD2Cl2, -89 °C) is 900 times larger than that for coordination
of 4,4-dimethyl-1-pentene to 1 to give 5. These results show that
the â-Si substituents in 2 and 3 greatly enhance substrate binding.
9
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C O M M U N I C A T I O N S
resonance of the ATMS ligand is shifted far downfield, and the
13C NMR resonances for Cint and Cterm are divergently shifted
(∆δ: Hint +1.89, Cint +51.2, Cterm -15.4), as expected for
Cp′2Zr(OtBu)(alkene)+ complexes.4 Similar nondissociative alkene
face exchanges have been deduced to occur during chain end
epimerization in propylene polymerization with Zr catalysts through
studies with isotopically labeled propylenes.17a,b Alkene flipping
likely occurs via an alkene C-H σ-complex intermediate or
transition state.17d
These results show that nonchelated d0 Zr-aryl-alkyne and Zr-
aryl-alkene complexes can be generated using â-Si-substituted
alkynes and alkenes to strengthen substrate coordination and the
poorly nucleophilic -C6F5 group to inhibit insertion. Both tactics
are required: non-â-Si-substituted substrates such as propyne and
2-butyne do not coordinate to 7, and Cp2ZrMe+, Cp2ZrCH2Ph+,
and Cp2HfMe+ rapidly insert and oligomerize or polymerize ATMS
even at -78 °C.19 Neither 8 (at -38 °C) nor 10 (up to 22 °C)
undergoes insertion. The availability of stabilized d0 metal-carbyl-
alkene species should enable direct study of their structures and
dynamics to probe important issues in catalytic alkene polymeri-
zation.2,17 With further adjustment of the nucleophilicity of the Zr-R
group, it should be possible to access (C5R5)2Zr(R)(alkene)+ systems
in which both alkene coordination and insertion can be directly
observed and quantified.
1
unsymmetrical alkene coordination.3-5 The JCH values for the
alkene carbons (Cint 161, Cterm 150 Hz) are typical for an alkene
coordinated to a d0 metal,3-5 and are inconsistent with insertion
products in which those carbons would be sp3-hybridized.14 Addition
of THF to 10 (C6D5Cl, -38 °C) gives 9 (100%) and free ATMS.
The equilibrium constant for ATMS binding to 7 at -38 °C in
C6D5Cl, Keq ) [10][7]-1[H2CdCHCH2SiMe3]-1 ) 8.2(1.4) M-1
,
is 2.8 times larger than the Keq for ATMS binding to 1, which under
these conditions has a value of 2.9(7) M-1. VT NMR gives ∆H°
) -5.3(2) kcal/mol and ∆S° ) -18(1) eu for binding of ATMS
to 7.
When a solution of 7, 10, and free ATMS is warmed from -38
to +2 °C over 4 h, 10 and ATMS are gradually consumed and the
ATMS dimer 6,6-dimethyl-4-((trimethylsilyl)methyl)-6-silahept-1-
ene (11)15 is formed. 11 results from a Lewis acid-mediated
dimerization of ATMS15 due to 7 or trace Ph3C+ in solution.16 NMR
and GC/MS analysis of the organic products from a 7/ATMS
mixture maintained at 22 °C for 3 days shows the presence of
dimers and trimers of ATMS; while the exact structures of these
products have not been determined, none contain C6F5 groups. The
trimers likely form by a Lewis acid-mediated allylsilylation of 11.15
There is no evidence for ATMS insertion in 10.
Acknowledgment. We thank the NSF (CHE-0212210) for
support and the University of Chicago for a William Rainey Harper
Fellowship (E.J.S.).
1
Supporting Information Available: Experimental procedures, data
for new compounds, and selected NMR spectra (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
VT NMR and H EXSY studies show that 10 undergoes two
dynamic processes. First, 10 undergoes reversible alkene decom-
plexation (eq 3). This process broadens all of the NMR signals of
10. The rate constant for ATMS decomplexation from 10 found
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1
2
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1
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1
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