Reactions of titanium and zirconium derivatives of bis (trimethylsilyl) acetylene with tris(pentafluorophenyl)borane: A titanium(III) complex of an alkynylboranate

Article abstract of DOI:10.1002/anie.200390364

Stabilization by coordination: A long sought after intermediate is stabilized by complexation: B(C₆F₅)₃ reacts at a bis(trimethylsilyl) acetylene titanium complex with C-Si bond cleavage and B-C bond formation to give the

Full text of DOI:10.1002/anie.200390364

Communications
Olefin Polymerization Catalysts
Reactions of Titanium and Zirconium Derivatives
of Bis(trimethylsilyl)acetylene with
Tris(pentafluorophenyl)borane: A Titanium(iii)
Complex of an Alkynylboranate **
Perdita Arndt, Wolfgang Baumann, Anke Spannenberg,
Uwe Rosenthal,* Vladimir V. Burlakov, and
Vladimir B. Shur
Dedicated to D. habil. Erhard Kurras
on the occasion of his 75th birthday
Special cations, formed by the formal abstraction of a methyl
anion from zirconocene dimethyl complexes with the help of
the strong Lewis acid B(C6F5)3, introduced by Ewen and
Marks, are highly active catalysts for the polymerization of a-
olefins.[1] It has become evident that other zirconocene
complexes (for example, of olefins, alkynes, butadienes) can
also undergo a similar activation and thus may be suitable
precursors for polymerization catalysts.[2]
Recently we described[3] that titanocene and zirconocene
complexes of silylalkynes, such as [Cp02M(h2-Me3SiC2SiMe3)]
(Cp’ = substituted or unsubstituted h5-C5H5) offer a rich
chemistry of this type with boron-containing reagents. Espe-
cially interesting are the titanium complexes because disso-
À
ciation of the alkyne ligands and C H activation of the Cp
ligands are possible (Scheme 1). Cationic complexes of
type A were formed with [HNMe3][BPh4],[3a] whereas an
electrophilic substitution of an H atom of one h5-C5H5 ring or
of one methyl group of a Cp* ligand (Cp* = C5Me5) occurred
with B(C6F5)3, and paramagnetic zwitterionic TiIII complexes
of types B[3b,c] or C[3d,e] were formed.
Similar compounds,[2] such as [Cp{C5H4B(C6F5)3}Zr-
(acac)] (acac = 2,4-pentanedione (acetylacetone))[4] or
[5]
¼
[Cp{C5H4BH(C6F5)2}Zr{s-C(Et) CHEt}], were described
also. The functionalization of the Cp* ligands is the same in
reactions of Cp* silylalkyne zirconocene complexes as for the
titanium compounds of type C. However, the alkyne does not
[*] Prof. Dr. U. Rosenthal, Dr. P. Arndt, Dr. W. Baumann,
Dr. A. Spannenberg
Leibniz-Institut für Organische Katalyse an der
Universität Rostock e.V.
Buchbinderstrasse 5–6, 18055 Rostock (Germany)
Fax: (+49)381-466-9376
E-mail: uwe.rosenthal@ifok.uni-rostock.de
Dr. V. V. Burlakov, Prof. V. B. Shur
A.N. Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
Vavilov Street 28, 117813 Moscow (Russia)
[**] We thank Prof. Karel Mach, Prague, for ESR measurements. This
work was supported by the Deutsche Forschungsgemeinschaft
(No. AR 321/1-1), the Fonds der Chemischen Industrie, and the
Russian Foundation for Basic Research (code 02-03-32589).
Supporting information (syntheses and details of X-ray structure
investigations) for this article is available on the WWW under
http://www.angewandte.org or from the author.
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The molecular structure of 1[8] (Figure 1) shows the typical
(ebthi)Zr-part, which is substituted in 3-position by the
boranate. One ortho F atom coordinates to the zirconium
C6F5
–
R
R
F
F
B
L
–
+
Ti
+
F
À
center, therefore the corresponding F Cring bond is elongated.
F
BPh4
Ti
This coordination is also evident from the 19F NMR signal
which, at d = À200.2 ppm, is shifted to extremely high field.
The alkenyl group is s bonded to the Zr center and has an
additional agostic interaction,[9] indicated by the typical Zr-
C14-C13angle of 87.4 (4) 8, similar to that found in D
F
F
L
F
F
F
F
A: L = THF, Py
B: R = H
R = iPr
1
(86.58).[3d] Additionally, the small coupling constant J(C,H)
[9]
of 91 Hz at C13is characteristic for an agostic interaction.
–
B(C6F5)3
The preparation of complex 1 demonstrates the impor-
tance of the proper sequence when adding reagents to achieve
catalytic activity. If the alkyne complex rac-[(ebthi)Zr-
(h2-Me3SiC2SiMe3)] is first treated with ethene to yield the
zirconacyclopentane rac-[(ebthi)Zr(C4H8)] and this is subse-
quently activated by B(C6F5)3, an active catalyst for polymer-
ization results.[10a] But if the borane is added to the alkyne
–
B(C6F5)2
H
SiMe3
F
+
Zr
+
Ti
F
F
Ph
F
F
C
D
Scheme 1. Typical structures of boron-containing titanocene and zirco-
nocene complexes
dissociate but binds the H atom under formation of an agostic
alkenyl compound of type D.[3d]
Herein we report the reactions of complexes rac-
[(ebthi)M(h2-Me3SiC2SiMe3)],[6]
(ebthi = 1,2-ethylene-1,1’-
bis(h5-tetrahydroindenyl)) with B(C6F5)3,[1] which proceed
totally differently for Ti and Zr complexes.[7] The reaction of
the Zr complex occurs as for the type D complexes with an
electrophilic substitution at an H atom of the Cp ring to give
an alkenyl compound 1 (Scheme 2). At room temperature the
1H NMR spectra of 1 display at very broad resonance signals,
which become sharp on cooling to 252 K and broaden again at
lower temperatures. The poor solubility (crystallization upon
cooling) prevents a deeper insight into the dynamics of the
molecule, but at 252 K two species in a 2:1 ratio are evident.
They undergo chemical exchange and exhibit the number of
NMR signals expected from their solid state structures (see
below). One plausible explanation of this dynamic behavior
could be a hindered rotation of the alkenyl group around the
Figure 1. ORTEP plot of 1. The thermal ellipsoids are set at 30% prob-
ability. For clarity the hydrogen atoms (with exception of H13) and one
position of the disordered groups are omitted. Selected bond
lengths [Š] and angles [8]: C1-B 1.640(9), Zr-F6 2.379(3), C21-F6
1.421(6), C26-F7 1.332 (8), C25-F8 1.377(8), C24-F9 1.361(7), C23-F10
1.343(7), Zr-C14 2.259(7); C1-B-C22 108.2(5), Zr-C14-C13 87.4(4).
À
C Zr bond. A reason for the hindered rotation could be the
agostic Zr-H-C interaction or the proximity of one silyl group
to one C6F5 group.
5
6
C6F5
F
C6F5
–
B
F
F
C6F5
B
SiMe3
F
+ B(C6F5)3
100 °C
H
+
Zr
Zr
F
F
F
Zr
2
– Me3SiC CSiMe3
H
F
F
SiMe3
SiMe3
F
SiMe3
1
Scheme 2. Formation and reaction of 1.
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Communications
compound first, complex 1 is formed, which does not react
with ethene and, accordingly, is inactive in polymerization.
SiMe3
SiMe3
SiMe3
C6F5
The similar complex D is active in ethene polymerization,
+
Ti
+
B(C6F5)3
À
–
presumably because it has no Zr F interaction.
Ti
B
C6F5
– SiMe3
If solutions of 1 are heated to 1008C, the alkyne is
F
À
F
eliminated and one B C bond is cleaved with a subsequent
F
transfer of a pentafluorophenyl group from boron to zirco-
nium, whereupon complex 2 with one m-H atom between
boron and the zirconium center is formed (Scheme 2) Such a
transfer of C6F5 from boron to zirconium[10b] is known, as is
the m-H atom type of structure.[10c] It is possible to describe
compound 2 either as ZrIV s-C6F5 hydrido complex with an
electroneutral borane ligand or alternatively as a zwitterionic
betaine-boranate.
F
F
3
Scheme 3. Conversion of rac-[(ebthi)Ti(h2-Me3SiC2SiMe3)] into 3.
The green compound 3 is paramagnetic (EPR in toluene
at 208C: g = 1.980, DH = 6.7 G, aTi = 11.0 G). A large number
of lines indicate an unpaired electron, which interacts with
one or two F-atoms. The IR spectrum (nujol mull) displays
The resonance signal of the hydride unit is located in the
same region of the 1H NMR spectra as the signals of the ebthi
groups and was identified only by 11B-decoupling experiments
(d = 0.82 ppm). During the thermal interconversion of 1 into 2
the high-field shifted 19F NMR signal disappeared, which
the absorption for the C C bond at 1988 cmÀ1 which indicates
ꢀ
very weak alkyne complexation. In the starting complex
which can be described as a TiII alkyne complex or alter-
natively as a TiIV metallacyclopropene, this absorption was
found at 1594 cmÀ1,[6] a result of stronger backbonding effects.
This interpretation is also supported by the X-ray crystal
structure (Figure 3).
À
indicates that there is no F Zr interaction in 2.
The molecular structure of 2[11] (Figure 2) displays the
typical (ebthi)Zr-part, substituted by the diaryl boron unit
B(C6F5)2. The Zr-H-B bridge of 2 has atomic distances typical
for this type of structure.[10c]
Figure 3. ORTEP plot of 3. The thermal ellipsoids are set at 30% prob-
ability. For clarity the hydrogen atoms and one position of the disor-
dered groups are omitted. Selected bond lengths [Š] and angles [8]:
C15-C16 1.226(5), C15-Si 1.866(4), C16-B 1.617(5), Ti-C16 2.639(4),
Ti-C15 2.515(4), Ti-F1 2.260(2), C20-F1 1.389(4); C15-C16-B 161.1(4),
C16-C15-Si 144.3(3).
Figure 2. ORTEP plot of 2. The thermal ellipsoids are set at 30% prob-
ability. For clarity the hydrogen atoms (with exception of H1) are omit-
ted. Selected bond lengths [Š]: Zr-C13 2.334(6), B-H1 1.481, Zr-H1
2.113.
The Ti complex rac-[(ebthi)Ti(h2-Me3SiC2SiMe3)] reacts,
surprisingly, in a totally different way with B(C6F5)3 to the Zr
compound (Scheme 3). Neither an electrophilic substitution
of an H atom nor an elimination of the alkyne were observed,
The molecular structure of 3[13] (Figure 3) shows the
typical (ebthi)Ti structure, but in contrast to 1 is unsubsti-
À
ꢀ
tuted. The [Me3SiC CB(C6F5)3] ligand is complexed unsym-
À
À
but after a C Si bond cleavage and a subsequent B C bond
formation the first zwitterionic titanocene(iii) h2-(trimethyl-
silyl)alkynylboranate complex rac-[{(ebthi)Ti}+{h2-Me3SiC2B-
metrically and weakly as expected. A comparably weak
coordination was also found in the dimeric s,p-alkynyl-
ꢀ
bridged titanocene(iii) complexes [{Cp2Ti(C CSiMe3)}2] (C-C
À
À
(C6F5)3} ] (3) was obtained. Similar Si C bond cleavage
reactions of silylalkynes Me3SiC CR by transition-metal
complexes under formation of s-alkynyl complexes
1.244(3), Ti-C(Ti) 2.393(1), Ti-C(Si) 2.318(2) Š; C-C-Ti
176.4(1), C-C-Si 141.5(2)8).[14] A stronger complexation is
ꢀ
2
*
found in [Cp2 Ti(h -Me3SiC2SiMe3)] (C-C 1.309(4), Ti-C
[LnM(SiMe3)(C CR)] are known for late-transition-metal
2.122(3) and 2.126(3) Š; Si-C-C 134.8(3) and 136.8(3)8).[15]
ꢀ
compounds.[12]
One ortho F atom in 3 coordinates to the titanium center and
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thus lies farther from the ring carbon atoms than all the other
F atoms.
Keywords: borate ligands · lewis acids · titanium · zirconium ·
zwitterions
.
Compound 3, to our knowledge, is the first TiIII alkyne
complex, if one leaves out the very special cases of dimeric
[14]
ꢀ
s,p-alkynyl-bridged complexes [{Cp2Ti(C CSiMe3)}2] and
[{Cp2Ti(C CSnMe3)}2].[16] In addition metallocene complexes
of alkynylboranates [RC C-BR3] have not been isolated or
characterized before, though they were assumed frequently as
reactive intermediates.[17,18] One example for this is the
ꢀ
À
[1] H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R.
Waymouth, Angew. Chem. 1995, 107, 1255 – 1283; Angew. Chem.
Int. Ed. Engl. 1995, 34, 1143– 1171, and references therein.
[2] a) W. E. Piers, Chem. Eur. J. 1998, 4, 13– 18; b) G. Erker, Acc.
Chem. Res. 2001, 34, 309 – 317, and references therein.
ꢀ
À
B(C6F5)3-catalyzed C C coupling of alkynyl groups to 1,3-
butadiynes from zirconocene(iv)-bisalkynyl complexes
[3] a) A. Ohff, R. Kempe, W. Baumann, U. Rosenthal, J. Organo-
met. Chem. 1996, 520, 241 – 244; b) V. V. Burlakov, S. I. Troya-
nov, A. V. Letov, E. I. Mysov, G. G. Furin, V. B. Shur, Izv. Akad.
Nauk Ser. Khim. 1999, 48, 1022 – 1023; (Engl. Transl. ) Russ.
Chem. Bull. 1999, 48, 1012 – 1013; c) V. V. Burlakov, S. I.
Troyanov, A. V. Letov, L. I. Strunkina, M. Kh. Minacheva,
G. G. Furin, U. Rosenthal, V. B. Shur, J. Organomet. Chem.
2000, 598, 243– 247; d) V. V. Burlakov, P.-M. Pellny, P. Arndt, W.
Baumann, A. Spannenberg, V. B. Shur, U. Rosenthal, Chem.
Commun. 2000, 241 – 242; e) V. V. Burlakov, P. Arndt, W.
Baumann, A. Spannenberg, U. Rosenthal, A. V. Letov, K. A.
Lyssenko, A. A. Korlyukov, L. I. Strunkina, M. Kh. Minacheva,
V. B. Shur, Organometallics 2001, 20, 4072 – 4079.
ꢀ
[Cp2Zr(C CR)2] described by Erker et al. which was pro-
À
ꢀ
posed to proceed via the not-isolated [RC CB(C6F5)3]
complexes of zirconocene(iv) monoalkynyl cations
+ [17]
Complexes of the type [Cp2Zr{h2-
ꢀ
[Cp2Zr(C CR)] .
RC2B(C6F5)2}] were isolated by Piers et al. from neutral
alkynylboranes RC C BR2 with metallocenes.[19] Com-
ꢀ À
pounds with alkynylboranates were proposed also by Wrack-
ꢀ
meyer et al. in the reaction of stannylalkynes Me3SnC CR
with triethylborane (BEt3), where not-isolated intermediates,
+
À
ꢀ
such as [Me3Sn] [RC CBEt3] react under formation of
[18]
¼
stannylolefines Me3SnC(Et) CR(BEt2).
[4] J. Ruwwe, G. Erker, R. Fröhlich, Angew. Chem. 1996, 108, 108 –
110; Angew. Chem. Int. Ed. Engl. 1996, 35, 80 – 82.
[5] L. Somlai, W. E. Piers, unpublished results, citat [22] in ref. [2a].
[6] C. Lefeber, W. Baumann, A. Tillack, R. Kempe, H. Görls, U.
Rosenthal, Organometallics 1996, 15, 3486 – 3490.
The reaction which generates 3 may be understood as an
À
ꢀ
oxidative Si C bond cleavage in Me3SiC CSiMe3 (in analogy
to ref. [12]) under formation of the s-alkynyl complex rac-
ꢀ
[(ebthi)Ti(SiMe3)(C CSiMe3)], which gives (by reduction)
[7] P. Arndt, V. V. Burlakov, A. Spannenberg, W. Baumann, U.
Rosenthal, 50 Years Catalysis Research in Rostock, 2002, P24.
[8] Crystal structure of 1: crystal size 0.4 ” 0.3” 0.2 mm, colorless
Me3Si radicals and the dimeric s,p-alkynyl-bridged complex
ꢀ
rac-[{(ebthi)Ti(C CSiMe3)}2] (in analogy to ref. [14]). With
B(C6F5)3, this complex would generate 3. Similar reaction
ꢀ
prism, space group P1, triclinic, a = 13.186(3), b = 13.822(3), c =
ꢀ
steps were described for
Me3SnC CSnMe3,
via
ꢀ
16.300(3) Š, a = 85.44(3), b = 88.65(3), g = 83.00(3)8, V=
2939.0(11) Š3, Z = 2, 1calcd = 1.270 gcmÀ3, 7137 reflections meas-
ured, 7137 were independent of symmetry and 3913 were
observed (I > 2s(I)), R1 = 0.058, wR2 (all data) = 0.178, 633
parameters.
ꢀ
[Cp2Ti(SnMe3)(C CR)], to Me3Sn radicals and [{Cp2Ti(C C-
SnMe3)}2].[16] An NMR-spectroscopy investigation of the
reaction was prevented by the presence of paramagnetic
complexes, and the fate of the Me3Si group remains unknown.
Pure complex 3 crystallizes from the mixture but after
evaporation of all the n-hexane, only amorphous mixtures
of undefined compounds remained.
[9] D. Thomas, N. Peulecke, V. V. Burlakov, B. Heller, A. Spannen-
berg, R. Kempe, U. Rosenthal, R. Beckhaus, Z. Anorg. Allg.
Chem. 1998, 624, 919 – 924.
[10] a) S. Mansel, D. Thomas, C. Lefeber, D. Heller, R. Kempe, W.
Baumann, U. Rosenthal, Organometallics 1997, 16, 2886 – 2890;
b) X. Yang, C. L. Stern, T. J. Marks, J. Am. Chem. Soc. 1994, 116,
10015 – 10031, and references therein; c) Y. Sun, R. E. v. H.
Spence, W. E. Piers, M. Parvez, G. P. A. Yap, J. Am. Chem. Soc.
1997, 119, 5132 – 5143.
[11] Crystal structure of 2: crystal size 0.4 ” 0.3” 0.2 mm, yellow
prism, space group P21/n, monoclinic, a = 9.331(2), b =
21.371(4), c = 18.188(4) Š, b = 95.04(3)8, V= 3612.9(13) Š3,
Z = 4, 1calcd = 1.635 gcmÀ3, 12779 reflections measured, 4514
were independent of symmetry and 2629 were observed (I >
2s(I)), R1 = 0.044, wR2 (all data) = 0.112, 515 parameters.
[12] C. Müller, R. J. Lachicotte, W. D. Jones, Organometallics 2002,
21, 1190 – 1196, and references therein.
[13] Crystal structure of 3: crystal size 0.5 ” 0.4 ” 0.4 mm, dark green
prism, space group P21/n, monoclinic, a = 13.373(3), b =
12.206(2), c = 24.584(5) Š, b = 102.72(3)8, V= 3914.4(13) Š3,
Z = 4, 1calcd = 1.564 gcmÀ3, 11406 reflections measured, 5892
were independent of symmetry and 3358 were observed (I >
2s(I)), R1 = 0.048, wR2 (all data) = 0.112, 546 parameters.
CCDC-195461 (1), CCDC-195462 (2), CCDC-195463( 3) con-
tain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
With utmost exclusion of moisture,[20a] the green complex
3 was formed in a yield of approximately 35% along with
trace amounts of blue crystals, which are a mixture of rac-
[{(ebthi)Ti}+{HO-B(C6F5)3}]À (4) and rac-[{(ebthi)Ti}+{H-B-
(C6F5)3}]À (5).[20b] If water is present (as the Brönsted acid
B(C6F5)3·H2O[21]), 4 and 5 were isolated as the only pro-
ducts,[20b] and bis(trimethylsilyl)acetylene was detected in the
reaction solution by NMR spectroscopy. Complex 3 was not
formed in the reaction of rac-[(ebthi)Ti(h2-Me3SiC2SiMe3)]
with a mixture of 4 and 5 in n-hexane.
The compounds described here form a series of complexes
+
À
ꢀ
of the type rac-[{(ebthi)Ti} {X-B(C6F5)3} ] (X = C CSiMe3
(3), OH (4) or H (5)). To date, we failed to synthesize similar
complexes by the reaction of the recently published stable
*
ꢀ
permethyltitanocene monoalkynyl complexes [Cp2 Ti-C C-
tBu][22] with B(C6F5)3. Whether this is because of the specific
properties of the Cp ligands (Cp* vs. ebthi) or of the alkynyl
substituents (Me3Si vs. tBu) or because of the reaction path
itself is being investigated.
Received: October 23, 2002
Revised: November 29, 2002 [Z50421]
Angew. Chem. Int. Ed. 2003, 42, No. 12
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Communications
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