Evidence of fluoride transfer from the anion of [Zr{C5H 3[SiMe2(η1-NtBu)]2}] +[RB(C6F5)3]- complexes to the zirconocenium cation

Article abstract of DOI:10.1002/anie.200602694

(Chemical Equation Presented) C-F bond activation: In the presence of a phosphane, deactivation of cationic zirconium complexes can occur by direct fluoride migration to the zirconium center, that is, the breaking of a C-F bond - the strongest bond of organic molecules - without previous C₆F ₅ transfer to the metal atom (see picture).

Full text of DOI:10.1002/anie.200602694

Communications
olefin polymerization.[1] Weakly coordinating anions are
required to minimize the ion-pairing interactions, which are
crucial in determining the properties of the resulting poly-
meric materials and the characteristics of the polymerization
processes. These counteranions frequently contain fluoro-
organic moieties to reduce interactions by dissipating the
negative charge and decreasing the nucleophilicity. However,
[RB(C6F5)3]À groups can still be responsible for deactivation
processes.
The carbon–fluorine bond is the strongest and least
reactive bond found in organic molecules and its activation
is a chemical challenge,[2] whereas the boron–carbon bond is
more reactive and C6F5 transfer to the cationic metal center is
the most commonly observed deactivation pathway for
[L2MR’][RB(C6F5)3] catalysts.[3] Ziegler and co-workers
recently reported calculations on different competing thermal
deactivation pathways for the [L2MR’][RB(C6F5)3] ion pair.[4]
We previously reported[5] Group 4 metal complexes with
doubly silyl-amido-bridged chelating tridentate ligands. Com-
plexes of this type, when activated with methylaluminoxane
(MAO), are efficient catalysts for ethene polymerization
despite generating cationic species free of the alkyl group
required for the insertion reaction. Similar observations were
made for a related singly silyl-h-amido bridged zirconium
dicyclopentadienyl compound[6] and for cationic CoI species,[7]
which also show activity towards ethene polymerization.
We report herein the reaction pathways that are followed
when the cationic complexes [Zr{C5H3[SiMe2(h1-NtBu)]2}]
[RB(C6F5)3] ([“Zr”][RB(C6F5)3], R = CH2Ph (1), Me (2)) are
heated in the presence of triphenylphosphane. Using reported
synthetic methods,[5a] we synthesized the cationic zirconium
compounds 1 and 2[8] as barely soluble oils or oily solids by
adding one equivalent of B(C6F5)3 to C6D6 solutions of the
corresponding alkyl complexes [“Zr”R] in sealed NMR tubes.
The same products were observed when similar reactions
were carried out on a preparative scale. Complexes 1 and 2
were thermally stable upon heating them in solution up to
808C for several hours. However, the addition of one
equivalent of PPh3 to C6D6 solutions of 1 and 2 at room
temperature gave a suspension of an insoluble solid, which
upon heating at 808C for several days afforded a mixture that
contained compounds [“Zr”F] (3) and [“Zr”(C6F5)] (4). The
insoluble solid was separated by filtration, suspended in fresh
C6D6, and heated at 808C for 7 days to give compound 3.
Compounds 3 and 4 were generated by the transfer of F
and C6F5 groups, respectively, to the Zr atoms of the starting
complexes although their formation and structure were only
confirmed when they were obtained by alternative synthetic
methods. So compound 3 was prepared by treating [“Zr”-
Ion Pairs
DOI: 10.1002/anie.200602694
Evidence of Fluoride Transfer from the Anion of
[Zr{C5H3[SiMe2(h1-NtBu)]2}]+[RB(C6F5)3]À
Complexes to the Zirconocenium Cation**
Jesffls Cano, María Sudupe, Pascual Royo,* and
Marta E. G. Mosquera
Cationic Group 4 metal complexes of the type [L2MR’]+
generated by activation of the corresponding dialkyl com-
pounds with B(C6F5)3 are active homogeneous catalysts for
[9]
[*] Dr. J. Cano, M. Sudupe, Prof. P. Royo, Dr. M. E. G. Mosquera
Departamento de Química Inorgµnica
Universidad de Alcalµ
(CH2Ph)] with FSnPh3 in toluene at 808C for 5 h; the
product was characterized by elemental analysis and NMR
spectroscopy.[10] The resultant (PhCH2)SnPh3 was also iden-
Campus Universitario, 28871 Alcalµ de Henares (Spain)
Fax: (+34)918-854-683
E-mail: pascual.royo@uah.es
tified by 1H NMR spectroscopy [dCH = 2.81 ppm (1s + 2d,
2
2H, JH,Sn119 = 68, JH,Sn117 = 65 Hz].
The reaction of the same benzyl complex with the Lewis
acid Al(C6F5)3 proceeded at 258C with abstraction of the
benzyl ligand to give the cationic zirconium complex, [“Zr”]-
[(PhCH2)Al(C6F5)3], with simultaneous formation of a small
amount of 4. Quantitative C6F5 transfer was completed after
[**] Financial support by the Spanish MEC (project MAT2004-02614)
and DGUI/CM (program S/0505/PPQ-0328) is acknowleged.
R=Me, CH2Ph.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
12 h to give 4 as the unique reaction product,[11] which was
isolated as a crystalline solid.
zirconocenium cation to give 3.[16] Supporting this proposal,
we note that 4 is not converted into 3. In addition, we were
able to isolate a colorless single crystal of the m-F complex
[(“Zr”)2(m-F)][(PhCH2)B(C6F5)3] (5) from the mixture of
compounds obtained when an equimolar mixture of 1 and
PPh3 was warmed at 508C for 7 days.[17] An X-ray diffraction
study (Figure 1) revealed that 5 consists of a separate
dinuclear cation, [(“Zr”)2F]+, and the free [(PhCH2)B-
(C6F5)3]À anion.
The chemical shifts in the 1H NMR spectra for complexes
3 and 4 were identical to those observed for the same products
isolated by thermal transformation of 1 and 2 in the presence
of PPh3, which demonstrates that they were formed by F and
C6F5 transfer to the metal center, respectively (Scheme 1). A
Scheme 1. Thermal transformations observed for complexes 1 and 2.
significant upfield shift of the signals arising from the o-
fluorine atoms was observed in the 19F NMR spectrum of 4
(d = 109.1 ppm) with respect to the corresponding signals
observed for B(C6F5)3 (d = 132.1 ppm). The molecular struc-
ture of 4 was determined by X-ray diffraction.[3d,12] The
structure agrees with the spectroscopic data and shows clearly
the C6F5 moiety coordinated to the metal center (see
Supporting Information). The Zr coordination sphere is
completed by the {C5H3[SiMe2(h1-NtBu)]2} ligand with no
other groups bonded to the Zr center. The precision of this
study was limited by the poor quality of the crystal set.
Formation of an alkyl-bridged ion-pair system,[3b] [“Zr”{m-
Figure 1. ORTEP view of 5 with thermal ellipsoids set at 30%
probability. The complex crystallizes with two independent cations in
the unit cell {Zr1-F1-Zr1#} and {Zr2-F2-Zr2#} (not shown). Only half
of each cation is present in the asymmetric unit, the other half (Zr#)
is generated in each case by crystallographic inversion. The separa-
tions Zr1-F1 and Zr2-F2 (2.1393(4), 2.1416(5) Š) are within the range
À
for a Zr F bond (2.02–2.29 Š). The Zr-F-Zr moiety is linear
(180.000(8)8 Zr1-F1-Zr1# and 180.000(1)8 Zr2-F2-Zr2#), which agrees
with results from other complexes with bridging fluorine atoms. The
Cp rings are mutually trans. The anion is omitted for clarity.
Further confirmation of this reaction pathway was
obtained by the isolation of the zwitterionic compound [4-
Ph3P]+[C6F4{BMe(C6F5)2}]À (6).[18] Crystals of 6 suitable for X-
ray diffraction studies were obtained when equimolar
amounts of 2 and PPh3 were heated at 808C for 7 days. The
molecular structure of compound 6 (Figure 2) demonstrates
À
RB(C6F5)3}], could be responsible for the intramolecular B
C6F5 bond activation and the C6F5 transfer to the Zr center
with elimination of the neutral borane, RB(C6F5)2.[13] This
reaction is slow when the cationic complexes 1 and 2 are
heated at 808C for 7 days to give complex 4 (Scheme 1).
In the presence of a phosphane, a different deactivation
process occurs by transfer of a p-fluoro substituent of one of
the pentafluorophenyl borate rings to the Zr center. The
decomposition of the zirconocenium cation [(1,2-
Me2C5H3)2ZrMe][RB(C6F5)3] has been reported[3a,14] and
two possible pathways for fluoride transfer,[3a] both based on
the same migration of C6F5 to the metal center with
subsequent activation of the o-fluorine atom or direct fluoride
migration to the zirconium cation, were proposed although no
experimental evidence of the reaction mechanism was given.
A similar fluoride transfer was reported very recently for the
reaction of the dinuclear hydride complex [{rac-(ebthi)ZrH-
(m-H)}2] with B(C6F5)3.[15]
The thermal fluoride transfer observed for complexes 1
À
and 2 may occur by nucleophilic addition of PPh3 to the p-C
F bond of one of the electron-deficient C6F5 rings of the
metal-coordinated alkylborate counteranion [RB(C6F5)3]À
with immediate transfer of the generated fluoride to the
Figure 2. ORTEP view of 6 with thermal ellipsoids set at 30%
À
À
probability. Both P C (1.811(3) Š) and B C (1.680(4) Š) interatomic
distances are within the range for single bonds.
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Communications
[9] H. Hatop, H. W. Roesky, T. Labahn, A. Fischer, H. G. Schmidt,
M. Noltemeyer, Organometallics 2000, 19, 937.
that the triphenylphosphonium and the methylbis(pentafluor-
ophenyl)borate fragments are the para-substituents of the
[10] 3: 1H NMR (300 MHz, C6D6): d = 0.53, 0.61 (2s, 2 ” 6H, SiMe2),
1.31 (s, 18H, NtBu), 6.69 (m, 1H, C5H3), 6.88 ppm (m, 2H,
C5H3). 13C NMR (75 MHz, C6D6): d = 2.4, 5.1 (2 ” SiMe2), 36.4
(NCMe3), 56.7 (NCMe3), 120.7, 124.1, 127.7 ppm (3 ” C5H3).
19F NMR (280 MHz, C6D6): d = À24.9 ppm. Elemental analy-
sis (%) calcd: C 47.28, H 7.70, N 6.49; found: C 47.90, H 7.11,
N 6.36.
resulting tetrafluorophenyl activated ring.[17]
In summary, we have found experimental evidence to
support a reaction pathway involving direct fluoride migra-
tion to the zirconium cation without previous transfer of C6F5.
It is noteworthy that the opening of the silyl–amido bridge
was not observed in the course of these activations. Experi-
ments with other donor ligands are in progress to investigate
the conditions under which the ion pairs 1 and 2 may follow
different decomposition routes and the possible mechanisms
of these reactions.
[11] 4: 1H NMR (300 MHz, C6D6): d = 0.45, 0.53 (2s, 2 ” 6H, SiMe2),
1.25 (s, 18H, NtBu), 6.37 (m, 1H, C5H3), 6.69 ppm (m, 2H,
C5H3). 13C NMR (75 MHz, C6D6): d = 1.9, 2.5 (2 ” SiMe2), 35.5
(NCMe3), 57.2 (NCMe3), 116.9, 124.4, 125.0 (3 ” C5H3), 135.8,
139.1, 147.9, 150.1 ppm (4 ” C6F5). 19F NMR (280 MHz, C6D6):
d = 109.1 (m, 2F, o-C6F5), 155.9 (m, 1F, p-C6F5), 162.4 ppm (m,
2F, m-C6F5). Elemental analysis (%) calcd: C 47.64, H 5.74,
N 4.83; found: C 47.92, H 5.27, N 4.63.
Received: July 6, 2006
Published online: October 20, 2006
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[14] L. Jia, X. M. Yang, C. L. Stern, T. J. Marks, Organometallics
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Keywords: homogeneous catalysis · ion pairs · polymerization ·
reaction mechanisms · zirconium
.
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[8] 2: 1H NMR (300 MHz, C6D6): d = 0.21, 0.33 (2s, 2 ” 6H, SiMe2),
0.94 (s, 18H, NtBu), not observed (BCH3), 6.31 (m, 1H, C5H3),
6.48 ppm (m, 2H, C5H3). 13C NMR (75 MHz, C6D6): d = 1.5 (2 ”
SiMe2), 34.8 (NCMe3), 58.6 (NCMe3), 127.5 (3 ” C5H3),
136.5 ppm (4 ” C6F5). 19F NMR (280 MHz, C6D6): d = 132.1 (m,
2F, o-C6F5), 163.6 (m, 1F, p-C6F5), 167.2 ppm (m, 2F, m-C6F5).
Elemental analysis (%) calcd: C 49.65, H 3.97, N 2.76; found:
C 50.51, H 3.71, N 3.35.
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Angew. Chem. Int. Ed. 2006, 45, 7572 –7574