Formation of a bifunctional zirconocene complex that favours intramolecular -B(C6F5)2 addition to a Cp ring over σ-ligand abstraction

Article abstract of DOI:10.1039/b400228h

The diphenyl-ansa-zirconocene complex 2 adds HB(C₆F ₅)₂ at the C=C double bond of its pendent Cp-allyl functional group to yield 3. During 3 days at room temperature the -B(C ₆F₅)₂ group takes part in an electrophilic substitution reaction at the adjacent Cp-ring to form 5 with formation of one equivalent of benzene. Complex 5 was characterized by X-ray diffraction.

Full text of DOI:10.1039/b400228h

Formation of a bifunctional zirconocene complex that favours
intramolecular –B(C6F5)2 addition to a Cp ring over s-ligand abstraction
Michael Hill, Gerald Kehr, Gerhard Erker,* Olga Kataeva† and Roland Fröhlich†
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany.
E-mail: erker@uni-muenster.de; Fax: +49 251 83-36503
Received (in Cambridge, UK) 9th January 2004, Accepted 27th February 2004
First published as an Advance Article on the web 23rd March 2004
The diphenyl-ansa-zirconocene complex 2 adds HB(C6F5)2 at
the CNC double bond of its pendent Cp-allyl functional group to
yield 3. During 3 days at room temperature the –B(C6F5)2 group
takes part in an electrophilic substitution reaction at the
adjacent Cp-ring to form 5 with formation of one equivalent of
benzene. Complex 5 was characterized by X-ray diffraction
(Zr–C3/C4) 2.537 Å; angle C1–Si–C9 93.81(9)°]. The most
noteworthy structural feature is the presence of a newly formed B–
C(sp2) bond between the boron atom and its adjacent Cp–ring (B–
C3: 1.627(3) Å, angle C3–B–C8 106.7(2)°). The B(C6F5)2 group
has become part of a substituted borata-tetrahydroindenyl-type
ligand (see Fig. 1). Only a single s-C6H5 ligand has remained
bonded to zirconium (Zr–C14: 2.203(2) Å) with the phenyl plane
being conformationally oriented in the major s-ligand plane of the
bent metallocene framework. The other s-coordination site at Zr
has become occupied by an ortho-fluorine centre from the C6F5
substituent at boron (C14–Zr–F23: 113.0(1)°) that is axially
oriented at the half-chair shaped newly formed six-ring heterocycle
(q C6–C7–C8–B: 66.9(2)°). The resulting (C)F–Zr bond length
(Zr–F23: 2.250(1) Å) is one of the shortest encountered in such a
situation.8 The corresponding C–(mF) bond (C23–F23 1.410(2) Å)
is markedly elongated relative to the three remaining C6F5 ortho C–
F bonds (Dd > 0.05 Å). The C23–F23–Zr angle in complex 5a
amounts to 142.8(1)°.
The low temperature NMR spectra have revealed an analogous
structure of 5a in solution. The 13C NMR spectrum shows three
CH2 resonances of the newly formed anellated heterocycle at d
27.8, 22.8, and 24.3 (C6–C8). The low temperature 19F NMR
spectrum features a total of 10 different resonances: the equatorially
oriented C6F5 ring, whose B–C(aryl) rotation is “frozen” at 203 K,
shows resonances at d 2131.2/2132.1 (o), d 2160.0 (p) and d
2162.8/2164.6 (m). The other C6F5 ring is locked into a rigid
orientation by the presence of the strong (C)F–Zr interaction.
Consequently, we have observed a pair of typically differentiated
19F NMR o-(C)F signals for this ring at d 2126.2 and d 2175.4 (m-
F)2,8 in addition to signals at d 2156.8 (p) and d 2155.6/2164.2
(m). The low temperature 19F NMR spectra have also revealed the
presence of a minor conformational isomer (5b) at < ca. 280 K,
that does not show the characteristic (C)F–Zr interaction. We
assume that it is formed by equilibration between the two possible
half-chair conformations of the Cp-anellated six-membered hetero-
cyclic framework (see Scheme 2).
Abstraction of a s-alkyl ligand from zirconocene complexes
RCp2ZrR2 by strong Lewis acids such as e.g. B(C6F5)3 to generate
[RCp2ZrR+] cations constitutes a major activation pathway in
homogeneous Ziegler–Natta catalysis.1 In the literature, examples
of such s-ligand abstractions are so numerous, that alternative
competing reaction pathways of the RCp2ZrR2/B(C6F5)3 systems
may become underestimated. Addition reactions of B(C6F5)3 to p-
ligands at zirconium have been described.2,3 There are even a few
examples known where B(C6F5)3 has added to a Cp ligand at
zirconium leaving an adjacent s-ligand untouched,4 although these
rare cases have admittedly involved sterically very demanding s-
ligand environments. We have now found a system where a
strongly electrophilic boron Lewis acid has avoided abstracting a
simple s-phenyl group at zirconium in favour of entering into a
reaction sequence that is initiated by electrophilic attack at the
5
framework of a substituted h -cyclopentadienyl p-ligand.
Treatment of the allyl-functionalized ansa-zirconocene di-
chloride (1)5 with two molar equivalents of phenyl lithium in ether
gave the corresponding diphenyl zirconocene complex 2 (96%
6
isolated). Subsequent treatment with HB(C6F5)2 resulted in a
selective hydroboration reaction of the pendent a-olefin moiety to
give 3. The bifunctional product shows three 13C NMR signals of
the connecting trimethylene unit (C6–C8: d 32.3, 26.9, and 31.8)
and a 11B NMR resonance at d 79.2, which is typical of
tricoordinate boron of a RB(C6F5)2 unit [corresponding 19F NMR
signals at d 2129.7 (o), 2147.5 (p), and 2160.9 (m)]. Complex 3
is not stable for a prolonged time at room temperature. During 3
days it reacted further with liberation of one equivalent of benzene
to yield 5a (89% isolated).7
Complex 5a was characterized by X-ray diffraction. In the
crystal it features a slightly strained Me2Si-bridged ansa-zircono-
cene system with typical general structural parameters [averaged
proximal Zr–C(Cp) distances (Zr–C9/C10/C13) 2.459 Å; (Zr–C1/
C2/C5) 2.428 Å; distal Zr–C(Cp) distances (Zr–C11/C12) 2.550 Å,
Complex 5 adds one equivalent of PMe3 to form the adduct 6 as
a single isomer (ca. 90% isolated).9 The 11B NMR spectrum of 6
features a typical tetracoordinated borate resonance at d 213.0 and
Scheme 1 i) + PhLi, 0 °C, Et2O, – LiCl, 96% yield; ii) + HB(C6F5)2, toluene,
r.t.; iii+iv) toluene, 3d, r.t., –C6H6, 89% yield; v) + PMe3, toluene, r.t.
† X-ray crystal structure analyses.
Fig. 1 Molecular structure of compound 5a.
1020
C h e m . C o m m u n . , 2 0 0 4 , 1 0 2 0 – 1 0 2 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Engl., 2003, 42, 1414–1418) and references cited therein. G. Erker, G.
Kehr and R. Fröhlich, J. Organomet. Chem., 2004, in press.
5 J. Cano Sierra, D. Hüerländer, M. Hill, G. Kehr, G. Erker and R.
Fröhlich, Chem. Eur. J., 2003, 9, 3618–3622; M. Hill, G. Kehr, R.
Fröhlich and G. Erker, Eur. J. Inorg. Chem., 2003, 3583–3589.
6 D. J. Parks, R. E. v. H. Spence and W. E. Piers, Angew. Chem., 1995,
107, 895–897 (Angew. Chem., Int. Ed. Engl., 1995, 34, 809–811); R. E.
v. H. Spence, D. J. Parks, W. E. Piers, M. A. MacDonald, M. J.
Zaworotko and S. J. Rettig, Angew. Chem., 1995, 107, 1337–1340
(Angew. Chem., Int. Ed. Engl., 1995, 34, 1230–1234); W. E. Piers and
T. Chivers, Chem. Soc. Rev., 1997, 26, 345–354.
Scheme 2
a 31P NMR signal of the [Zr]–PMe3 unit at d 29.4. Below 213 K the
rotation of both C6F5 rings at boron is slow on the NMR time scale
[19F NMR: d 2105.0/2110.0 (o), 2157.4 (p), 2162.6/2162.7 (m
of ring A), d 2120.7/129.8 (o), 2157.8 (p), 2162.8/2164.2 (m of
ring B).
We must assume that the strongly Lewis acidic –B(C6F5)2 group
in the bifunctional diphenylzirconocene complex 3 undergoes an
intramolecular addition to its adjacent substituted Cp ring system10
to form the reactive intermediate 4 (see Scheme 1). Addition of the
–B(C6F5)2 functional group from the outside consequently results
in an orientation of the remaining ipso-(Cp)C–H vector towards the
central [Zr]Ph2 moiety, thus enabling one of the zirconium bound
phenyl groups to act as an internal base. Deprotonation with
formation of one equivalent of benzene then reforms the (substi-
7 A mixture of 2 (125 mg, 265 mmol) and HB(C6F5)2 (92 mg, 265 mmol)
was dissolved in toluene and stirred for 3 days at room temperature.
Removal of the solvent in vacuo gave 196 mg of 5 (89%) as a yellow
solid. 1H NMR (293 K, d8-toluene, 600 MHz): d 7.21, 7.20, 7.04 (m, 5H,
o-, m-, p-Ph), 6.44 (m, 2H, 4A-H, 5-H), 6.35 (m, 1H, 5A-H), 5.95 (m, 1H,
3A-H), 5.80 (m, 1H, 2-H), 5.79 (m, 1H, 2A-H), 2.32/1.68 (m, each 1H,
6-H, 6-HA), 1.56/1.23 (m, each 1H, 8-H, 8-HA), 1.46/1.30 (m, each 1H,
7-H, 7-HA), 0.65/0.61 (s, each 3H, Si(CH3)2). 13C{1H} NMR (293K, d8-
toluene, 150 MHz): d 189.3, 128.6, 128.3, 127.6 (ipso-, o-, m-, p -Ph),
151.9 (C3), 125.0 (C5A), 121.6 (C4A), 119.8 (C5), 118.8 (C2), 115.2
(C2A), 112.4 (C3A), 105.0 (C1), 99.9 (C1A), 27.8 (C6), 24.3 (C8), 22.8
(C7), 25.5/25.6 (Si(CH3)2), (13C6F5 signals not cleanly resolved). X-
ray crystal structure analysis of complex 5a (single crystals from toluene
at 220 °C): Crystal data for C33H23BF10SiZr * C7H8, M, = 831.77,
5
tuted) h -cyclopentadienyl p-ligand system to yield the observed
product 5. Our study has shown that the addition of a strongly
electrophilic borane to a Zr-coordinated cyclopentadienide can
successfully compete with or even be favoured over the ubiquitous
s-ligand abstraction reaction. We will see whether electrophilic
attack at such nucleophilic p-ligand systems may follow similar
selectivity rules as they were previously established for the
complementary addition of nucleophilic reagents to the p-ligands
of strongly electrophilic transition metal complexes (the “Davies,
Green, Mingos rules”).11
¯
triclinic, space group P1 (No. 2), a = 9.766(1), b = 10.662(1), c =
18.675(1) Å, a = 104.46(1), b = 96.15(1), g = 109.47(1)°, V =
1736.2(3) Å3, Dc = 1.591 g cm23, m = 4.36 cm21, Z, = 2, l = 0.71073
Å, T, = 198 K, 16169 reflections collected (±h, ±k, ±l), [(sinq)/l] =
0.67 Å21, 8376 independent (Rint
=
0.037) and 7283 observed
reflections [I, 4 2 s(I)], 480 refined parameters, R, = 0.035, wR2
=
0.086. CCDC 229733. See http://www.rsc.org/suppdata/cc/b4/
b400228h/ for crystallographic data in .cif or other electronic format.
8 M. Dahlmann, G. Erker, R. Fröhlich and O. Meyer, Organometallics,
2000, 19, 2956–2967; J. Karl, G. Erker and R. Fröhlich, J. Am. Chem.
Soc., 1997, 119, 11165–11173; N. Kleigrewe, T. Brackemeyer, G. Kehr,
R. Fröhlich and G. Erker, Organometallics, 2001, 20, 1952–1955 and
references cited therein.
Financial support from the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie, and NATO (grant PST.EV.
980416 for O. K.) is gratefully acknowledged.
9 A sample of 2 (162 mg, 344 mmol) was reacted with HB(C6F5)2 (119
mg, 344 mmol) in toluene for 3 days at room temperature to generate 5.
A slight excess of PMe3 was then added. Removal of all volatiles in
vacuo gave 250 mg (89%) of 6 as a yellow solid, mp = 168 °C
(decomp.). Anal. calcd. for C36H32BF10PSiZr (815.7): 53.01% C,
3.95% H, found: 52.53% C, 3.69% H. 1H NMR (293 K, d8-toluene, 600
MHz): d 7.15, 7.12, 7.00 (m, 5H, Ph), 6.71 (m, 1H, 4A-H), 6.09 (m, 1H,
5-H), 5.97 (m, 1H, 3A-H), 5.79 (m, 1H, 2-H), 5.74 (m, 1H, 5A-H), 5.53
(m, 1H, 2A-H), 2.30/1.96 (m, each 1H, 6-H, 6-HA), 1.66/0.96 (m, each
1H, 7-H, 7-HA), 1.31/0.85 (m, each 1H, 8-H, 8-HA), 0.35/0.31 (s, each
3H, Si(CH3)2, 0.22 (d, 2JPH = 10 Hz, 9H, PMe3). 13C{1H} NMR (293
K, d8-toluene, 150 MHz): d 186.7, 131.4, 127.0, 126.0 (Ph), 149.0 (1JCF
= 238 Hz), 139.5 (1JCF = 261 Hz), 137.7 (1JCF = 263 Hz, o-, p-, m- of
C6F5) 144.3 (C3), 120.2 (C4A), 125.0 (C3A), 119.4 (C2), 116.3 (C5),
115.4 (C2A), 109.6 (C5A), 99.7 (C1), 97.4 (C1A), 28.5 (C6), 24.3 (C7),
16.2 (C8), 8.9 (d, 1JCP = 30 Hz, PMe3), 24.9/25.8 (Si(CH3)2), (13C6F5
resonances not cleanly resolved). 11B{1H} NMR (293K, d8-toluene, 64
MHz): d 213.0 (n1/2 = 224 Hz), 31P{1H} NMR (293 K, d8-toluene, 81
MHz): d 29.4.
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C h e m . C o m m u n . , 2 0 0 4 , 1 0 2 0 – 1 0 2 1
1021