Synthesis, structure, and reactivity of (C5H4SiMe3)2Y{(μ-FC 6F4)(μ-Me)B(C6F5)2}: Tight ion pairing in a cationic lanthanide complex

Article abstract of DOI:10.1021/om971044s

The reaction of[Cp′₂YMe]₂ (Cp′ = C₅H₅, C₅H₄-SiMe₃) with B(C₆F₅)₃ affords the complexes Cp′₂Y{MeB-(C₆F₅)₃}. The anion is coordinated in a chelating fashion via one ortho-fluorine atom and agostic interactions to two of the methyl hydrogens; the complexes are highly fluxional in solution. They act as initiators for the carbocationic polymerization of isobutene.

Full text of DOI:10.1021/om971044s

1004
Organometallics 1998, 17, 1004-1006
Syn th esis, Str u ctu r e, a n d Rea ctivity of
(C5H4SiMe3)2Y{(µ-F C6F 4)(µ-Me)B(C6F 5)2}: Tigh t Ion
P a ir in g in a Ca tion ic La n th a n id e Com p lex
Xuejing Song, Mark Thornton-Pett, and Manfred Bochmann*
School of Chemistry, University of Leeds, Leeds LS2 9TH, U.K.
Received November 25, 1997
Summary: The reaction of [Cp′2YMe]2 (Cp′ ) C5H5, C5H4-
SiMe3) with B(C6F5)3 affords the complexes Cp′2Y{MeB-
(C6F5)3}. The anion is coordinated in a chelating fashion
via one ortho-fluorine atom and agostic interactions to
two of the methyl hydrogens; the complexes are highly
fluxional in solution. They act as initiators for the
carbocationic polymerization of isobutene.
The complexes [Cp′2Y(µ-Me)]2 (1a , Cp′ ) C5H5; 1b,
Cp′ ) C5H4SiMe3) react with B(C6F5)3 in dichlorometh-
ane at -70 to 0 °C to give Cp′2Y(µ-Me)B(C6F5)3 (2);
according to the 1H and 19F NMR spectra, the conversion
is quantitative.8 If only 1 equiv of B(C6F5)3 per mole of
1 is employed, the formation of 2 together with unre-
acted 1 is observed. Surprisingly, at no stage was it pos-
sible to observe the expected binuclear cationic methyl-
bridged species [Cp′2Y(µ-Me)YCp′2]+[MeB(C6F5)3]- (3),
even at low temperature. It seems that even if 3 is an
intermediate in the formation of 2, any equilibrium be-
tween 2 and 3 lies strongly on the side of 2 (Scheme 1).
By contrast, following the reaction of 1a with 2 equiv
of [CPh3][B(C6F5)4] in CD2Cl2 by NMR at -65 °C
provides evidence for [(C5H5)2Y(µ-Me)Y(C5H5)2][B(C6F5)4]
(1H NMR: δ -0.74 (t, 3 H, J Y-H ) 3.3 Hz, µ-Me), 6.40
(20 H, C5H5)) as an unstable intermediate. Warming
to 0 °C results in the replacement of the Cp and CH3
signals by a new Cp resonance at δ 6.16, assigned to
the decomposition product [(C5H5)2YCl]2. The same
reaction carried out in toluene at room temperature
gave an oily precipitate, which, on addition of THF,
afforded crystalline [(C5H5)2Y(THF)2][B(C6F5)4] (4a ).9
Compounds 2a and 2b were isolated as colorless sol-
ids in high yields by reacting 1 with 2 equiv of B(C6F5)3
Metallocene complexes of group 3 and lanthanide
metals of the type Cp2MR have attracted considerable
attention in recent years because of their pronounced
Lewis acidic character and their catalytic activity.1
There are, however, only few reports of cationic lan-
thanide complexes, all of which are stabilized by donor
ligands, [Cp2ML2]+X- (L ) THF, N2H4, DME, or tet-
2-4,5b
rahydrothiophene; M ) Sm,2 Ce,3 La,4 Yb;5 X ) BPh4
or Co(CO)45a). In view of our recent synthesis of the
aluminocenium cation [AlCp2]+ which proved to be a
highly effective initiator for carbocationic polymeriza-
tions,6 we became interested in the possibility of syn-
thesizing “base-free” group 3 complexes [MCp2]+ which
might offer promise as cationic initiators or as activators
for group 4 metal alkyl catalyst precursors.7 We de-
scribe here the synthesis of zwitterionic yttrium com-
plexes Cp′2Y(µ-Me)B(C6F5)3 (2a , Cp′ ) C5H5; 2b, Cp′ )
C5H4SiMe3) and their use as polymerization initiators.
(7) Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255. (b)
Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Grubbs, R. H.;
Coates, G. W. Acc. Chem. Res. 1996, 29, 85. (d) Marks, T. J . Acc. Chem.
Res. 1992, 25, 57.
(1) Edelmann, F. T. In Comprehensive Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford,
1995; Vol. 4, p 11. For recent examples of catalytic reactions, see also:
(a) Heeres, H. J .; Teuben, J . H. Organometallics 1991, 10, 1980. (b)
Deelman, B. J .; Bijpost, E. A.; Teuben, J . H. J . Chem. Soc., Chem.
Commun. 1995, 1741. (c) Duchateau, R.; van Wee, C. T.; Teuben, J .
H. Organometallics 1996, 15, 2291. (d) Schaverien, C. J . J . Chem. Soc.,
Chem. Commun. 1992, 11. (e) Coughlin, E. B.; Bercaw, J . E. J . Am.
Chem. Soc. 1992, 114, 7606. (f) Mitchell, J . P.; Hajela, S.; Brookhart,
S. K.; Hardcastle, K. I.; Henling, L. M.; Bercaw, J . E. J . Am. Chem.
Soc. 1996, 118, 1045. (g) Yasuda, H.; Ihara, E. Macromol. Chem. Phys.
1995, 196, 2417. (h) Yasuda, H.; Ihara, E. Tetrahedron 1995, 51, 4563.
(i) Ihara, E.; Nodono, M.; Yasuda, H.; Kanehisa, N.; Kai, Y. Macromol.
Chem. Phys. 1996, 197, 1909. (j) Yang, X. M.; Seyam, A. M.; Fu, P. F.;
Marks, T. J . Macromolecules 1994, 27, 4625. (k) Giardello, M. A.;
Yamamoto, Y.; Brard, L.; Marks, T. J . J . Am. Chem. Soc. 1995, 117,
3276. (l) Roesky, P. W.; Stern, C. L.; Marks, T. J . Organometallics 1997,
16, 4705. (m) Deming, T. J .; Novak, B. M.; Ziller, J . W. J . Am. Chem.
Soc. 1994, 116, 2366. (n) Boffa, L. S.; Novak, B. M. Macromolecules
1997, 30, 3494. (o) Evans, W. J .; De Coster, D. M.; Greaves, J .
Macromolecules 1995, 28, 7929.
(2) (a) Evans, W. J .; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J .
W.; Alvarez, D. Organometallics 1990, 9, 2124. (b) Evans, W. J .; Kociok-
Kohn, W. J .; Ziller, J . W. Angew. Chem., Int. Ed. Engl. 1992, 31, 1081.
(3) Heeres, H. J .; Meetsma, A.; Teuben, J . H. J . Organomet. Chem.
1991, 414, 351.
(4) Hazin, P. N.; Bruno, J . W.; Schulte, G. K. Organometallics 1990,
9, 416.
(5) (a) Deng, D.; Zheng, X.; Qian, C.; Sun, J .; Dormond, A.; Baudry,
D.; Visseaux, M. J . Chem. Soc., Dalton Trans. 1994, 1665. (b) Yuan,
F.; Shen, Q.; Sun, J . J . Organomet. Chem. 1997, 538, 241.
(6) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl.
1996, 35, 2226.
(8) 1b: Prepared from 2.25 g (2.8 mmol) of [(C5H4SiMe3)2YCl]2 and
MeLi (4.0 mL, 5.6 mmol) in 200 mL of diethyl ether at 0 °C. White
microcrystals were obtained from light petroleum (3.0 g, 70%).1H NMR
(C6D6, 25 °C): δ -0.70 (t, J Y-H ) 3.1 Hz, 6H, µ-CH3), 0.32 (s, 36 H,
SiMe3), 6.35 (t, J ) 2.4 Hz, 8 H, Cp′), 6.77 (t, J ) 2.4 Hz, 8 H, Cp′). 13
C
NMR (C6D6, 25 °C): δ 0.62 (SiMe3), 24.7 (t, J Y-C ) 25.0 Hz, µ-Me),
114.7 (Cp′), 118.8 (s, ipso-C of Cp′), 120.7 (Cp′). Anal. Calcd (found)
for C17H29Si2Y: C, 53.98 (53.90); H, 7.67 (7.35). 2a : Toluene (30 mL)
was added to a mixture of 0.42 g (0.9 mmol) of [(C5H5)2YMe]2 and 0.92
g (1.8 mmol) of B(C6F5)3 at room temperature. The mixture was stirred
for 2 h, pumped to dryness, and washed with 3 × 10 mL of light
petroleum to give a white powder (1.1 g, 80%). 1H NMR (C6D6, 25 °C):
δ 0.90 (br, 3 H, µ-Me), 5.90 (s, 10 H, Cp). 13C NMR (C6D6, 25 °C): δ
15.0 (br, µ-Me), 113.7 (Cp). 11B NMR (C6D6, 25 °C): δ -14.4. 19F
NMR: (C6D6, 25 °C) δ -134.1 (d, J ) 22.9 Hz, 6 F, o-F), -159.8 (t, J
) 21.0 Hz, 3 F, p-F), -164.7 (t, J ) 19.5 Hz, 6 F, m-F); (CD2Cl2, -20
°C) δ -134.8 (d, J ) 25.9 Hz, 6 F, o-F), -162.0 (t, J ) 20.7 Hz, 3 F,
p-F), -166.1 (t, J ) 20.7 Hz, 6 F, m-F). Anal. Calcd (found) for C29H13-
BF15Y: C, 46.65 (46.35); H, 1.74 (1.95). 2b: Prepared from 0.60 g (0.80
mmol) of 1b and 0.85 g (1.66 mmol) of B(C6F5)3 in toluene (20 mL) at
room temperature; isolated at -20 °C from toluene/light petroleum as
colorless crystals (1.0 g, 70%). 1H NMR (C6D6, 25 °C): δ -0.06 (s, 18
H, SiMe3), 1.10 (br, 3 H, µ-Me), 6.00 (t, 4 H, J ) 2.4 Hz, Cp′), 6.58 (t,
4 H, J ) 2.4 Hz, Cp′). 13C NMR (CD2Cl2, -20 °C): δ -0.31 (SiMe3),
14.0 (br, µ-Me), 118.8, 123.9, 127.8 (Cp′). 11B NMR (C6D6, 25 °C): δ
-15.5. 19F NMR (CD2Cl2, -80 °C): δ -134.8 (d, J ) 20.7 Hz, 6 F, o-F),
-160.5 (t, J ) 20.7 Hz, 3 F, p-F), -164.8 (t, J ) 20.7 Hz, 6 F, m-F).
Anal. Calcd (found) for C35H29BF15Si2Y: C, 47.20 (46.75); H, 3.26 (3.30);
F, 32.03 (31.65).
S0276-7333(97)01044-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/24/1998

Communications
Organometallics, Vol. 17, No. 6, 1998 1005
Sch em e 1
in toluene at room temperature. While 2a is a micro-
crystalline solid that is only poorly soluble in toluene
once isolated, 2b is readily recrystallized from toluene.
Whereas cationic group 4 complexes such as [Cp2ZrMe]+
tend to decompose readily in dichloromethane at >0 °C
by halide abstraction from the solvent,10 solutions of 2
are remarkably stable in chlorinated solvents over a
period of hours at room temperature. According to the
NMR data, the structures of 2a and 2b in solution are
1
essentially identical.8 The H and 13C chemical shifts
of the methyl group of the [CH3BC6F5)3]- anion as well
as the 19F chemical shift differences for the p-F and m-F
atoms of >4 ppm11 suggest that the compounds exist
as tight ion pairs both in toluene and dichloromethane
solutions. The addition of THF to these solutions gives
the THF adducts [(C5H5)2Y(THF)2][MeB(C6F5)3] (5a )
and [(C5H4SiMe3)2Y(THF)2][MeB(C6F5)3] (5b), which, by
contrast, are ionic and show the spectroscopic charac-
teristics of the uncoordinated anion.
Crystals of 2b suitable for X-ray diffraction were
obtained from toluene at room temperature. The struc-
ture is shown in Figure 1. The yttrocenium cation is
coordinated to the [MeB(C6F5)3]- anion via one ortho-F
atom of one of the C6F5 substituents as well as agostic
interactions to two of the three hydrogen atoms of the
boron-methyl group, with Y-H contacts of 2.18(5) and
2.43(4) Å while the distance to the third hydrogen atom
is 3.28 Å. The shorter of these contacts falls within the
range of Y-H distances to bridging hydrides, as in
[(C5H4Me)2Y(µ-H)(THF)]2 and [(C5H5)2Y(µ3-H)(µ-H)AlH2-
(THF)]2.12 The solid-state structure of the yttrium
F igu r e 1. Crystal structure of (C5H4SiMe3)2YMeB(C6F5)3
(2b), showing the atomic-numbering scheme. Ellipsoids are
drawn at 40% probability. Selected bond distances (Å) and
angles (deg): Y-F(132), 2.366(3); Y-C(14), 2.853(7);
Y-H(81), 2.43(4); Y-H(82), 2.18(5); Y-C(1), 2.603(5);
Y-C(2), 2.592(5); Y-C(3), 2.586(5); Y-C(4), 2.572(5);
Y-C(5), 2.572(5); B-C(14), 1.647(8); B-C(111), 1.649(8);
B-C(121), 1.636(7); B-C(131), 1.660(7); B-Y-C(14), 134.6-
(4); Y-F(132)-C(132), 158.2(3); C(14)-H(81)-Y, 111(4);
C(14)-H(82)-Y, 116(4); H(81)-C(14)-H(82), 96(4); C(14)-
B-C(111), 104.3(5); C(14)-B-C(121), 109.5(5); C(14)-B-
C(131), 112.8(5); C(131)-C(132)-F(132), 120.8(4).
complex may be compared to those of the zwitterionic
zirconocene derivatives (1,2-C5H3Me2)2ZrMe(µ-Me)B-
(C6F5)3 (6)13 and Cp2Zr{η3-C4H6B(C6F5)3} (7)14 but re-
veals significant differences in the coordination of the
anion. Although yttrium and zirconium have nearly
identical covalent radii and the C5H4SiMe3 ligands do
not exert significant steric hindrance, the Y-C(14)
distance to the bridging methyl group of 2.853(7) Å is
too long to be regarded as a bonding interaction, in
(9) The reaction between 1b and [CPh3][B(C6F5)4] in CD2Cl2 only
proceeds at g-10 °C and leads to decomposition. Toluene solutions at
room temperature give a yellow product consistent with the formation
of [(C5H4SiMe3)2Y][B(C6F5)4]. Once isolated, the compound proved
insufficiently soluble in toluene to allow recrystallization. 1H NMR
(C7D8, 25 °C): δ 0.04 (SiMe3), 5.93 (br, Cp′), 6.35 (br, Cp′). 13C NMR
(C7D8, 25 °C): δ -0.97 (SiMe3), 120.3, 124.2, 125.6 (Cp′). 11B NMR
(C7D8, 25 °C): δ -15.9. 19F NMR (C7D8, 25 °C): δ -133.4 (br, 6 F,
o-F), -160.4 (br, 3 F, p-F), -165.3 (br, 6 F, m-F).
(10) Bochmann, M.; Lancaster, S. J . Angew. Chem., Int. Ed. Engl.
1994, 33, 1634.
(11) Horton, A. D.; de With, J .; van der Linden, A. J .; van de Weg,
H. Organometallics 1996, 15, 2672.
(13) (a) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1991,
113, 3623. (b) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc.
1994, 116, 10015. (c) Bochmann, M.; Lancaster, S. J .; Hursthouse, M.
B.; Malik, K. M. A. Organometallics 1994, 13, 2235.
(14) Temme, B.; Erker, G.; Karl, J .; Luftmann, H.; Fro¨hlich, R.;
Kotila, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1755.
(12) (a) Evans, W. J .; Drummond, D. K.; Hanusa, T. P.; Doedens,
R. J . Organometallics 1987, 6, 2279. (b) Bel’skii, V. K.; Bulychev, B. M.;
Erofeev, A. B.; Soloveichik, G. L. J . Organomet. Chem. 1984, 268, 107.

1006 Organometallics, Vol. 17, No. 6, 1998
Communications
Ta ble 1. Isobu ten e P olym er iza tion s In itia ted by
[Y(C5H4SiMe3)2]+
contrast to a Zr-C distance of 2.549(3) Å in 6. By
comparison, yttrium-alkyl bond lengths in neutral
cyclopentadienyl complexes are considerably shorter,
ranging from 2.468(7) Å in mononuclear (C5Me5)2YCH-
(SiMe3)2 to 2.67(2) Å for bridging methyl ligands in [(C5-
Me5)2Y(µ-Me)2AlMe2]2, with an average value of 2.545
Å for [(C5H5)2YMe]2 being typical.15
amount
T
time yield
Mw
Mw/
Mn
initiator
(µmol) (°C) (min) (g) (×10-3
)
1b
50
50
50
42
42
-78 5.0
-78 5.0 0.15 1180
513
0
2ba
2ba
2.2
2.0
2.4
2.6
-50 8.0 0.34
1b + [CPh3][B(C6F5)4]b
1b + [CPh3][B(C6F5)4]b
-78 5.0 0.30 1380
Both 2b and 6 contain agostic interactions to the
methyl hydrogens. It has been argued that in com-
pounds of type 6 the anion is primarily bonded via the
methyl hydrogens, with possibly negligible bonding to
carbon,13b although it is noteworthy that in all cases the
Zr-C distances fall well within a range consistent with
a metal-carbon bonding interaction.13b,c By contrast,
in 2b, agostic metal-hydrogen interactions are the only
bonding contributions between the metal center and the
methyl group. Structures A and B illustrate the dif-
ference in the methyl bonding in 2b and 6, respectively.
On the other hand, the Y-F(134) bond length of
2.366(3) Å is quite short compared to the Zr-F(42)
distance of 2.423(3) Å in the electronically more satu-
rated η3-allyl complex 7. However, whereas in 7 ex-
change of the C6F5 substituents is sufficiently slow at
-86 °C to allow the pattern of coordinated and unco-
ordinated fluorine atoms to be resolved, cooling solutions
of 2b to -95 °C resulted merely in broadening of the
signals for the ortho-, meta-, and para-fluorine atoms.
-50 5.0 0.43 1180
a
Conditions: 50 µmol of 2b dissolved in 2 mL of CH2Cl2 at -78
°C was injected into 10 mL of isobutene condensed into a flame-
dried all-glass reactor and equilibrated at the given temperature.
The reaction was quenched by injecting 2 mL of methanol. b A 0.19
g (0.25 mmol) amount of 1b in toluene (20 mL) was added to 0.46
g (0.50 mmol) of [CPh3][B(C6F5)4] in 100 mL of toluene at room
temperature. The solution was stirred for 2.5 h, during which time
the color of [CPh3][B(C6F5)4] disappeared. Initiation was carried
out as stated above by injecting a solution containing 42 µmol of
the yttrium complex.
In view of the activity of [AlCp2]+ as an initiator for
carbocationic polymerizations,6 the activity of 2b in
isobutene polymerizations was tested.16 Dichloromethane
solutions were used for 2b, whereas 1b/[CPh3][B(C6F5)4]
mixtures are unstable in chlorinated solvents and were
prepared in toluene. The results are collected in Table
1. Both the polymer yields and molecular weights are
comparable to those found for [AlCp2][MeB(C6F5)3] and
considerably higher than those reported for (C5Me5)-
TiMe3/B(C6F5)3.17 There was no isobutene polymeriza-
tion with 1a or 1b in the absence of B(C6F5)3 or
[CPh3][B(C6F5)4].
The results provide the first examples of cationic
group 3 metallocene cations [MCp′2]+ stabilized by flu-
oroarylborate association rather than N- or O-donor lig-
ands. These complexes have significant ionic character,
which results in weak agostic bonding and highly flux-
ional anion coordination. As expected, the yttrocenium
cation behaves as a strong Lewis acid, and its reactivity
is exemplified by its effectiveness as an initiator for the
carbocationic polymerization of isobutene. Catalytic
applications of these complexes are currently being
explored.
Although anion displacement by a weakly coordinating
solvent such as CD2Cl2 might possibly be expected, the
data, such as the 19F chemical shift differences between
the m-F and p-F atoms, are consistent with anion
coordination over the temperature range from -95 to
-20 °C and suggest facile rotation of the anion and
rapid exchange of all six o-F atoms within the metal
coordination sphere, even at low temperature.
To allow for fluorine coordination, the Y-C(14)-B
moiety significantly deviates from linearity (angle 134.6-
(4)°), compared to the corresponding Zr-C-B angle of
161.8(2)° in 6. Both crystallographic and NMR data
support the notion that 2b is more ionic in character
than related zwitterionic complexes of group 4 metals
and is best regarded as a tight ion pair.
Ack n ow led gm en t. This work was supported by the
Engineering and Physical Sciences Research Council.
We are grateful to Professor M. B. Hursthouse (Crystal-
lographic National Service, University of Cardiff) for the
collection of the data set of 2b.
Su p p or tin g In for m a tion Ava ila ble: Text giving general
preparative details and spectroscopic data for 4 and 5 and
tables of crystal data, atomic coordinates, anisotropic displace-
ment parameters, hydrogen atom coordinates, interatomic
distances, and bond angles (12 pages). Ordering information
is given on any current masthead page.
OM971044S
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Atwood, J . L.; Hunter, W. E. J . Chem. Soc., Dalton Trans. 1979, 54.
(b) Den Haan, K. H.; de Boer, J . L.; Teuben, J . H.; Spek, A. L.; Kojic-
Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726. (c)
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J . H. Organometallics 1993, 12, 3531. (f) Evans, W. J .; Boyle, T. J .;
Ziller, J . W. J . Organomet. Chem. 1993, 462, 141.
(16) The only earlier report on the possible function of a lanthanide
metallocene as
a cationic initiator appears to be on the use of
[(t-BuC5H4)2Yb(THF)2]+ to effect the polymerization of styrene at
elevated temperatures (80-100 °C), although under these conditions
the reaction is difficult to distinguish from the noncatalyzed thermally
induced styrene polymerization (ref 5b).
(17) Barsan, F.; Baird, M. C. J . Chem. Soc., Chem. Commun. 1995,
1065.