Mechanism of alkyne insertion of a cationic zirconocene aryl complex

Article abstract of DOI:10.1021/ja0760067

The reaction of Cp₂Zr(4-F-o-tolyl)(CD₂Cl_2M)⁺ (A) with 2-butyne at -85 °C in CD₂Cl₂ proceeds by reversible formation of the alkyne complex Cp₂Zr(4-F-o-tolyl)(2-butyne)⁺

Full text of DOI:10.1021/ja0760067

Published on Web 10/04/2007
Mechanism of Alkyne Insertion of a Cationic Zirconocene Aryl Complex
Orson L. Sydora, Stefan M. Kilyanek, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois, 60637
Received August 9, 2007; E-mail: rfjordan@uchicago.edu
Chain growth in metallocene-catalyzed alkene polymerization
and alkyne oligomerization proceeds by coordination of substrate
to the active d0 LnMR species followed by migratory insertion
(eq 1).1 The substrate adducts are transient species due to weak
coordination and low insertion barriers and have not been directly
observed.2,3 Casey showed that Cp*2YR alkyl complexes react with
alkenes by fast reversible coordination followed by insertion.4 The
formation of Cp*2Y(R)(alkene) species was inferred from NMR
spectra of mixtures of Cp*2YR and alkene, which contained one
Figure 1. 1H NMR spectra (CD2Cl2, -85 °C, Me region) of the reaction
set of exchange-averaged alkene resonances shifted in the same
direction from the free alkene positions as for chelated yttrium
of Cp2Zr(4-F-o-tolyl)(CD2Cl2)+ (A) and excess 2-butyne, obtained im-
mediately (a), 25 min (b), and 80 min (c) after thermal equilibration.
Resonances of A, Cp2Zr(4-F-o-tolyl)(2-butyne)+ (C), and Cp2Zr{CMed
CMe(4-F-o-tolyl)}+ (D) are indicated (see text for assignments). The δ 2.16
peak is from Cp2Zr(CMedCMeCH2CtCMe)+ formed by a minor C-H
activation pathway. The δ 2.28 peak is from m-F-toluene formed in the
synthesis of A and the C-H activation pathway.
alkene complexes. Similarly, Erker showed that Cp′2ZrCH2-
CHCHCH2B(C6F5)3 (Cp′ ) C5H4Me) and related betaine species
react with alkenes by fast reversible coordination followed by
insertion; however, again the intermediate adducts were not
observed.5 Here we report a Cp2Zr(aryl)+ system for which alkyne
binding and insertion can be observed and quantified, enabling a
description of the energy profile for the overall reaction.
-85 °C, thermally equilibrated, and monitored by NMR. Initially
following the equilibration period, a mixture of A, the 2-butyne
adduct Cp2Zr(4-F-o-tolyl)(2-butyne)+ (C), and the insertion product
Cp2Zr{CMedCMe(4-F-o-tolyl)}+ (D) was observed (eq 3). After
2 h, the formation of D was 90% complete. Complex D is thermally
unstable and was not isolated.
We reported that d0 (C5H4R)2Zr(C6F5)+ (R ) H, Me) species
react with H2CdCHCH2SiMe3 and HCtCCH2SiMe3 to form
(C5H4R)2Zr(C6F5)(substrate)+ adducts.6 These species are stabilized
by the â-Si effect and the low nucleophilicity of the C6F5 group
and do not undergo insertion at temperatures up to 22 °C. These
results suggested that modification of the aryl group and substrate
might give species that form observable substrate adducts that are
more reactive for insertion.
The complex [Cp2Zr(4-F-o-tolyl)][B(C6F5)4] (1, Cp ) C5H5) was
synthesized by the reaction of a 1/1 mixture of Cp2Zr(4-F-o-tolyl)2
and Cp2ZrMe2 with 2 equiv of [Ph3C][B(C6F5)4] at room temper-
ature in the dark (eq 2).7 This reaction proceeds by successive
methyl abstraction and aryl exchange steps. In chlorocarbon
solvents, 1 forms Cp2Zr(4-F-o-tolyl)(RCl)+ solvent adducts which
contain â-H-Zr agostic interactions involving the aryl ortho C-H
bond. DFT calculations for Cp2Zr(4-F-o-tolyl)(CD2Cl2)+ (A) show
that the “endo” isomer in which the â-agostic interaction occupies
the central coordination site is 6 kcal/mol more stable than the “exo”
isomer in which the â-agostic interaction occupies a lateral site.
The methyl region of the 1H NMR spectra of a reacting mixture
of A and B is shown in Figure 1. The o-Me resonance of A appears
at δ 2.32. The key resonances for C are the o-Me signal at δ 2.42
and the 2-butyne signal at δ 2.12, which is similar to that for Cp′2-
Zr(OtBu)(2-butyne)+ (δ 2.03).2a The presence of a single 2-butyne
resonance for C indicates that 2-butyne rotation is fast on the NMR
time scale, as observed for Cp′2Zr(OtBu)(2-butyne)+ and other d0-
metal alkyne complexes. The o-H resonance for C appears at high
field (δ 5.98) indicative of an agostic interaction as in A. The DFT
structure of C (Figure 2) contains a symmetrically bound alkyne
(Zr-C ) 2.85 Å) and an agostic o-C-H unit (Zr-H ) 2.48 Å).
The 1H spectrum of D contains two Cp resonances indicating that
the Cp rings are inequivalent, and two low field aryl resonances
(δ 7.80, 7.51). The DFT structure of D contains close Zr-aryl
contacts which may explain these features (Zr-C6, 2.56; Zr-H6,
2.65; Zr-C1, 2.92 Å). The o-Me signal of D appears at δ 2.56.
Quenching D with MeOD gives DMeCdCMe(4-F-o-tolyl).
Several observations provide insight to the mechanism of eq 3.
First, when 2-butyne (B) is present in >10 fold excess relative to
A, the ratio [A]/[C] is constant throughout the reaction. This result
The reaction of A with 2-butyne (B) was studied by low-
temperature NMR and DFT calculations (BP86/LANL2DZ,6-
31G*). A CD2Cl2 solution of 1 and excess 2-butyne was prepared
at low temperature, transferred to a precooled NMR probe at
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J. AM. CHEM. SOC. 2007, 129, 12952-12953
10.1021/ja0760067 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
rings. These effects contribute to the large insertion barrier. The
Zr-Cipso (2.34 Å) and CtC (1.26 Å) distances are only slightly
lengthened from the corresponding distances in C (2.23, 1.24 Å).
The 2-butyne is bound unsymmetrically (Zr-Calkyne ) 2.41, 2.78
Å). These results are consistent with an early transition state similar
to that found for the insertion of acetylene into Cp2ZrCH3 .
+ 10 NBO
analysis of the insertion transition state shows significant overlap
between the Zr-C σ bond and an alkyne π* orbital, but nearly no
overlap between the Cipso p orbital and the alkyne π* orbitals. In
addition, NBO analysis reveals an aryl-Zr interaction involving
overlap of the Cipso p orbital with a Zr acceptor orbital. This
interaction is analogous to the R-agostic interactions in alkene
insertion transition states for d0 metal alkyls.11
This work provides a quantitative picture of alkyne insertion in
a Cp2Zr(aryl)+ system. Alkyne insertion proceeds by reversible
alkyne binding and rate-limiting insertion. Observation of the alkyne
adduct is possible because the insertion barrier is high, owing to
the presence of an agostic interaction that stabilizes the alkyne
adduct and steric crowding between the aryl o-Me group and the
Cp rings that destabilizes the insertion transition state. This approach
may provide an opportunity to probe how the properties of the Cp2M
unit influence alkyne binding and insertion in Cp2M(aryl)+ species.
Figure 2. Free energy profile for conversion of A to D at -85 °C.
Experimental ∆G and ∆Gq values (kcal/mol) are: ∆Geq ) -1.2; ∆Gq
coord
) ca. 10.6 to 11.5; ∆Gqinsert ) 13.6. ∆Ginsert ) -29.3 kcal/mol (DFT).
implies that the exchange of bound and free alkyne (which is slow
on the NMR time scale) is much faster than alkyne insertion.
Second, the disappearance of the total of A and C in eq 3 obeys
first-order kinetics. These results are consistent with a pre-
equilibrium kinetic system, for which the rate law is given by eqs
4-6, where Keq ) [C][A]-1[B]-1 is the equilibrium constant for
alkyne binding,8 kobs is the first-order rate constant for the
disappearance of the total of A and C, and kinsert is the rate constant
for the insertion step. kobs equals kinsert scaled by the fraction of
metallocene that is in the reactive form ([C]/{[A] + [C]}; eq 5).
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-0516950). We acknowledge the
Burroughs Welcome Fund Interfaces Cross-Disciplinary Training
Program for partial support of the computer cluster.
d([A] + [C])
Supporting Information Available: Experimental and computa-
tional details. This material is available free of charge via the Internet
at http://pubs.acs.org.
rate )
) -kobs([A] + [C])
(4)
(5)
dt
Keq[B]kinsert
[C]kinsert
kobs
)
)
References
Keq[B] + 1 [A] + [C]
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[A] + [C]
[A]0 + [C]0
ln
) -kobs
t
(6)
(
)
The alkyne binding constant determined by NMR integration is
Keq ) 1.4(1) M-1 at -85 °C, which, when solvent is taken into
account, corresponds to ∆Geq ) -1.2 kcal/mol.8 The ∆Geq
estimated by DFT is -1.69 kcal/mol. This Keq value is lower than
that for 2-butyne coordination to Cp′2Zr(OtBu)+ (Keq ) 60 M-1
-85 °C, CD2Cl2).2a
,
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concentration versus time data to eq 6. The rate constant for
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10-4 s-1, which corresponds to an insertion barrier of ∆Gqinsert
)
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13.5(4) kcal/mol. The ∆Gqinsert determined by DFT is 13.6 kcal/
mol.
The barrier to alkyne coordination/decoordination was not
determined. However, the absence of exchange line broadening in
the NMR spectra of A and C, and the fact that the equilibrium
between A and C is maintained throughout the reaction to produce
D, imply that the barrier to conversion of A to C is between ca.
10.6-11.5 kcal/mol.9 For comparison, the barrier to coordination
of 2-butyne to Cp′2Zr(OtBu)(CD2Cl2)+ (CD2Cl2, -85 °C) is
10.5 kcal/mol.2a
A free energy diagram based on these results is given in Figure
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rotated 72° out of the metallocene bonding plane, which breaks
the agostic interaction and brings the o-Me group close to the Cp
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[solvent], this expression reduces to eq 5. ∆Geq is based on K′eq
.
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times faster than the conversion of C to D.
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