Naked (C5Me5)2M cations (M = Sc, Ti, and V) and their fluoroarene complexes

Article abstract of DOI:10.1021/ja054544i

The ionic metallocene complexes [Cp*₂M][BPh₄] (Cp* = C₅Me₅) of the trivalent 3d metals Sc, Ti, and V were synthesized and structurally characterized. For M = Sc, the anion interacts weakly with the metal center

Full text of DOI:10.1021/ja054544i

Published on Web 09/23/2005
Naked (C5Me5)2M Cations (M ) Sc, Ti, and V) and Their
Fluoroarene Complexes
Marco W. Bouwkamp,† Peter H. M. Budzelaar,‡,| Jeroen Gercama,†
Isabel Del Hierro Morales,† Jeanette de Wolf,† Auke Meetsma,† Sergei I. Troyanov,§
Jan H. Teuben,† and Bart Hessen*,†
Contribution from the Center for Catalytic Olefin Polymerization, Stratingh Institute for
Chemistry and Chemical Engineering, UniVersity of Groningen, Nijenborgh 4,
9747 AG, Groningen, The Netherlands, Molecular Materials, Radboud UniVersity Nijmegen,
P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands, and Department of Chemistry, Moscow
State UniVersity, VorobjoVy Gory, 119899 Moscow, Russia
Received July 20, 2005; E-mail: B.Hessen@chem.rug.nl
Abstract: The ionic metallocene complexes [Cp*2M][BPh4] (Cp* ) C5Me5) of the trivalent 3d metals Sc,
Ti, and V were synthesized and structurally characterized. For M ) Sc, the anion interacts weakly with the
metal center through one of the phenyl groups, but for M ) Ti and V, the cations are naked. They each
contain one strongly distorted Cp* ligand, with one (V) or two (Ti) agostic C-H‚‚‚M interactions involving
the Cp*Me groups. For Sc and Ti, these Lewis acidic species react with fluorobenzene and 1,2-
difluorobenzene to yield [Cp*2M(κF-FC6H5)n][BPh4] (M ) Sc, n ) 2; M ) Ti, n ) 1) and [Cp*2M(κ2F-1,2-
F2C6H4)][BPh4], the first examples of κF-fluorobenzene and κ2F-1,2-difluorobenzene adducts of transition
metals. With the perfluorinated anion [B(C6F5)4]-, both Sc and Ti form [Cp*2M(κ2F-C6F5)B(C6F5)3] contact
ion pairs. The nature of the metal-fluoroarene interaction was studied by density functional theory (DFT)
calculations and by comparison with the corresponding tetrahydrofuran (THF) adducts and was found to
be predominantly electrostatic for all metals studied.
Introduction
nucleophilic anions used in conjunction with (often highly
electron deficient) cationic transition-metal catalysts. This is
As a result of the relative inertness of the C-F bond1 and
the high electronegativity of fluorine,2 and the fact that the radius
of fluorine is not much larger than that of hydrogen,3 fluorinated
groups are widely applied in transition-metal-based homoge-
neous catalysis. They can be used as substituents on the ancillary
ligand set, for example, to modify the electronic properties of
the metal center4 or the solubility of the catalyst.5 Another
important application of fluorinated moieties is found in weakly
particularly important in catalytic olefin polymerization,6 where
the counterion can strongly influence catalyst productivity and
selectivity.7,8
Despite their importance to catalysis, interactions of fluori-
nated groups with highly electrophilic transition-metal centers9
have not been studied in a systematic fashion, and structurally
characterized examples of transition-metal complexes with direct
C-F‚‚‚M interactions involve either intramolecular interactions,
where the fluorinated group forms an integral part of the ligand
system,10 or the closest contacts between a cationic transition-
metal complex and its fluorinated organoborate counterion.11
In this paper, we describe a combined experimental and
† University of Groningen.
‡ Radboud University Nijmegen.
| Current address: Department of Chemistry, University of Manitoba,
Winnipeg, Manitoba R3T 2N2, Canada.
§ Moscow State University.
(1) Eggar, K. W.; Cocks, A. T. HelV. Chim. Acta 1973, 56, 1516.
(2) The electronegativity of fluorine on the Pauling scale: 3.98. The Nature
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Introduction to Modern Structural Chemistry; Pauling, L., Ed.; Cornell
University Press: Ithaca, NY, 1960.
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Rapid Commun. 2001, 22, 1095.
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Organometallics 1993, 12, 1491. (b) Burlakov, V. V.; Pellny, P.-M.; Arndt,
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9
14310
J. AM. CHEM. SOC. 2005, 127, 14310-14319
10.1021/ja054544i CCC: $30.25 © 2005 American Chemical Society

(C5Me5)2M Cations and Their Fluoroarene Complexes
A R T I C L E S
Figure 1. ORTEP representation of 1a,b and the major fraction of 1c showing 50% probability ellipsoids. The anions of 1b,c are omitted for clarity.
Scheme 1
Table 1. Selected Bond Distances (Å) and Angles (deg) for 1a
density functional theory (DFT) theoretical study of the electron-
deficient decamethylmetallocene cations of the trivalent 3d metal
ions of scandium (d0), titanium (d1), and vanadium (d2), as well
as their interaction with fluorobenzene, 1,2-difluorobenzene, and
the [B(C6F5)4] anion. For comparison, the interaction of the
cations with the “classic” Lewis base tetrahydrofuran (THF)
was studied. Here, we report the unusual structural behavior of
the “naked” decamethylmetallocene cations, as well as the first
examples of isolated κF-fluorobenzene and κ2F-1,2-difluoroben-
zene adducts of transition metals. Experimental and theoretical
results suggest that the metal-fluoroarene interaction in these
complexes is predominantly electrostatic. Some aspects of the
titanium system were communicated previously.12
Sc(1)-Cp*(1)a
Sc(1)-Cp*(2)b
Sc(1)-H(24)
Sc(1)-H(25)
Sc(1)-C(24)
Sc(1)-C(25)
2.1516(10) C(21)-C(22)
1.420(3)
1.406(3)
1.379(3)
1.400(3)
1.397(3)
1.400(3)
2.1587(9)
2.35(2)
2.67(2)
2.679(2)
2.864(2)
C(21)-C(26)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(25)-C(26)
Cp*(1)-Sc(1)-Cp*(2) 140.94(4)
C(21)-B(2)-C(27)
C(21)-B(2)-C(213)
(C21)-B(2)-C(219)
C(27)-B(2)-C(213)
C(27)-B(2)-C(219)
115.44(17)
100.45(16)
109.91(16)
112.36(16)
106.00(16)
Cp*(1)-Sc(1)-H(24)
Cp*(1)-Sc(1)-H(25)
Cp*(2)-Sc(1)-H(24)
Cp*(2)-Sc(1)-H(25)
Sc(1)-H(24)-C(24)
Sc(1)-H(25)-C(25)
128.5(6)
117.3(5)
87.2(6)
95.2(4)
99.6(16)
92.2(15)
C(213)-B(2)-C(219) 112.86(16)
{BC-Ph}c
9.02(12)
58.0(9)
(ScH2,PhB)d
a Cp*(1) is the centroid of the C(11)-C(15) ring. b Cp*(2) is the centroid
c
of the C(111)-C(115) ring.
{B(2)-C(213),C(213) > C(218)} is defined
Results
as the angle between the B(2)-C(213) bond and the phenyl ring including
d
C(213)-C(218).
(ScH2,PhB) is defined as the angle between the Sc(1)-
Synthesis of Base-Free Decamethylmetallocene Cations.
The base-free decamethylmetallocene cations can be prepared
as their tetraphenylborate salts, [Cp*2M][BPh4] (1a-c), by
reaction of Cp*2ScMe,13 Cp*2TiMe,14 or Cp*2VMe15 with the
Brønsted acid [PhNMe2H][BPh4]16 in toluene (Scheme 1). This
reaction results in a yellow precipitate in the case of scandium
and a brown precipitate in the case of titanium or vanadium.
Recrystallization of the base-free metallocene cations is impeded
by their poor solubility in aliphatic or aromatic solvents and by
their reactivity toward most other solvents. Nevertheless, when
H(24)-H(25) and PhB planes.
these reactions are performed without stirring, the generation
of the scandocene and titanocene cations 1a,b in toluene results
in precipitation of yellow (1a) or red (1b) crystals, suitable for
X-ray diffraction, directly from the reaction mixture. The base-
free decamethylvanadocene cation 1c could be successfully
recrystallized from 1,2-difluorobenzene/cyclohexane. The com-
pounds were characterized by X-ray diffraction, by elemental
analysis, and by their reaction with THF, which cleanly afforded
the corresponding THF adducts 5a-c (vide infra).
Structures of the Base-Free Decamethylmetallocene Cat-
ions. ORTEP representations of the metallocene cations 1a-c
are depicted in Figure 1 (see Table 1 and Table 2 for a selection
of bond lengths and angles). The [Cp*2Sc] cation adopts a
normal bent-metallocene geometry (Cp*(1)-Sc(1)-Cp*(2) )
140.94(4)°). One phenyl group of the [BPh4] anion in 1a shows
a close contact with the metal center (Sc(1)-H(24) ) 2.35(2),
Sc(1)-H(25) ) 2.67(2) Å; Sc(1)-C(24) ) 2.679(2), Sc(1)-
(11) (a) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840.
(b) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910. (c) Yang,
X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015.
(12) Bouwkamp, M. W.; De Wolf, J.; Del Hierro Morales, I.; Gercama, J.;
Meetsma, A.; Troyanov, S. I.; Hessen, B.; Teuben, J. H. J. Am. Chem.
Soc. 2002, 124, 12956.
(13) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Noltemeyer,
M.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 203.
(14) Luinstra, G. A.; Ten Cate, L. C.; Heeres, H. J.; Pattiasina, J. W.; Meetsma,
A.; Teuben, J. H. Organometallics 1991, 10, 3227.
(15) Curtis, C. J.; Smart, C.; Robbins, J. L. Organometallics 1985, 4, 1283.
(16) Eshuis, J. J. W.; Tan, Y. Y.; Meetsma, A.; Teuben, J. H.; Renkema, J.;
Evens, G. G. Organometallics 1992, 11, 362.
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14311

A R T I C L E S
Bouwkamp et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1b,c
of a Cp* methyl group with the metal center. This methyl group
is directed toward the metal center in a manner similar to that
observed in the base-free titanocene cation, 1b, with an angle
of 26.1(3)° between the Cp* plane and the C(11)-(C16) bond.
The Cp* ligand with the agostic interaction is again slipped
back, though the intraligand C-C bond distances are normal
for a Cp* ligand.
1b
1c
M(1)-Cp*(1)a
M(1)-Cp*(2)b
M(1)-C(111)
M(1)-C(112)
M(1)-C(113)
M(1)-C(114)
M(1)-C(115)
M(1)-H(119′′)
M(1)-H(120′)
C(114)-C(119)
C(115)-C(120)
C(114)-(C115)
1.9596(14)
1.9796(14)
2.352(3)
2.471(3)
2.353(3)
2.159(3)
2.167(3)
2.16(3)
1.9228(14)
1.8845(14)
2.219(3)
2.260(3)
2.303(3)
2.303(3)
2.258(3)
2.06(3)
Reaction of Base-Free Metallocene Cations with Fluoro-
arenes. Dissolution of the decamethylmetallocene cations of
scandium and titanium (1a,b) in fluorobenzene results in clear
solutions from which, respectively, yellow or green crystals
precipitated upon layering with aliphatic solvents. The com-
pounds were characterized by single-crystal X-ray diffraction
as the bis- and mono-κF-fluorobenzene adducts [Cp*2Sc(κF-
FC6H5)2][BPh4] (2a) and [Cp*2Ti(κF-FC6H5)][BPh4] (2b),
respectively (Scheme 1). Under similar conditions, reaction of
the decamethylscandocene and -titanocene cations with 1,2-
difluorobenzene results in the κ2F-1,2-difluorobenzene adducts
[Cp*2Sc(κ2F-1,2-F2C6H4)][BPh4] (3a) and [Cp*2Ti(κ2F-1,2-
F2C6H4)][BPh4] (3b). The coordinated molecules of fluoroben-
zene and 1,2-difluorobenzene are readily displaced by THF; the
1H and 19F NMR spectra of complexes 2a,b and 3a,b in THF-
d8 correspond to a mixture of the THF adducts 5a,b (vide infra)
and free fluorobenzene or 1,2-difluorobenzene, respectively. The
[Cp*2V] cation does not react with fluorobenzene or 1,2-
difluorobenzene. Precipitation of the decamethylvanadocene
cation from fluorobenzene or recrystallization from 1,2-difluo-
robenzene results in recovery of base-free 1c.
Structure of the Fluorobenzene and 1,2-Difluorobenzene
Adducts. ORTEP representations of the κF-fluorobenzene
adducts 2a,b are depicted in Figure 2 (see Table 3 for pertinent
bond distances and angles). The M-F bond distances (M )
Sc, 2.2725(17) and 2.2884(16) Å; M ) Ti, 2.151(2) Å) are
longer than those in the neutral, trivalent metallocene fluoride
complexes (M ) Sc, 2.05(2) Å;21 M ) Ti, 1.845(4) Å22) but
shorter than those in, e.g., the contact ion pair [(nacnac)ScMe]-
[(µ-Me)B(C6F5-κF1)(C6F5)2] ((nacnac ) (2,6-(CHMe2)2C6H3)-
NC(t-Bu)CHC(t-Bu)N(2,6-(CHMe2)2C6H3)), Sc-F ) 2.390(4)
Å)23 or the zwitterionic titanium(III) complex Cp*[η5-C5Me4-
CH2B(κF-C6F5)2(C6F5)Ti] (Ti-F ) 2.406 Å). The C-F bonds
in the coordinated fluorobenzene (2a ) 1.408(3) and 1.414(3)
Å; 2b ) 1.402(3) Å) are elongated relative to free fluorobenzene
(C-F ) 1.3640(16) Å).24 The M-F-C angles in both 2a
(172.21(16) and 165.27(15)°) and 2b (168.8(2)°) are close to
linear.
2.20(3)
1.502(5)
1.486(5)
1.434(4)
1.501(4)
Cp*(1)-M(1)-Cp*(2)
151.00(6)
24.9(3)
23.6(3)
155.00(7)
26.1(3)
{Cp*(1),C(114)-C(119)}c
{Cp*(1),C(115)-C(120)}d
a Cp*(1) is the centroid of the C(111)-C(115) ring. b Cp*(2) is the
c
centroid of the C(11)-C(15) ring.
{Cp*(1),C(114)-C(119)} is defined
as the angle between the Cp*(1) plane and the C(114)-C(119) bond.
d
{Cp*(1),C(115)-C(120)} is defined as the angle between the Cp*(2)
plane and the C(115)-C(120) bond.
C(25) ) 2.864(2) Å). This interaction does not lead to a
significant distortion of the phenyl group, but some deviation
from the ideal tetrahedral surrounding of the boron atom is
evident from the C(21)-B(2)-C(213) angle of 100.45(16)°.
A comparable interaction with phenyl groups of the tetraphen-
ylborate anion is observed in the salts [Cp*2M(η2-Ph)2BPh2]
(M ) Sm and U),17 where the larger ionic radius of the Sm3+
and U3+ cations allows for the binding of two phenyl groups.18
For [{C2H4(Ind)2}ZrMe(η2-Ph)BPh3] (Ind ) indenyl), the
coordination of a single phenyl group of the anion was proposed
on the basis of solution NMR data.19
The molecular structures of 1b and 1c show no interactions
between the anion and the metal center. Instead, the naked
cations both contain one highly distorted Cp* ligand. The
titanium complex 1b (Figure 1, Table 2) has one normal η5-
bound Cp* ligand and one having two agostic C-H‚‚‚Ti
interactions of adjacent methyl groups with the metal center
(Ti(1)-H(119′′) ) 2.16(3) Å; Ti(1)-H(120′) ) 2.20(3) Å). The
relevant CCp-CMe distances represent single bonds and clearly
rule out formation of a metalated Cp* ligand.20 The two agostic
methyl groups are directed toward the metal center (deviation
of the C-Me bonds from the Cp* plane: 24.9(3) and 23.6-
(3)°), and the Cp* ligand with the agostic interaction is slipped
back so that Ti(1)-C(114) and Ti(1)-C(115) are the shortest
Ti-C bond distances in the molecule. Nevertheless, the intrali-
gand C-C bond distances are normal for a Cp* ligand.
The vanadium complex 1c crystallized in the P1h space group
with two molecules of [Cp*2V][BPh4] in the asymmetric unit.
Refinement was complicated by a disorder problem (see
Supporting Information). An ORTEP representation of the major
fraction of one of the crystallographically independent cations
of 1c is depicted in Figure 1; pertinent bond distances and angles
are listed in Table 2. Compound 1c has one normal η5-bound
Cp* ligand and one having a single agostic C-H‚‚‚V interaction
The X-ray structures of the 1,2-difluorobenzene adducts 3a,b
are represented in Figure 3 (Table 4 contains pertinent bond
distances and angles). The overall geometries of the two cations
are very similar even though the compounds are not isomor-
phous. In the scandium compound, both M-F bond distances
are equal (2.330(2) Å) and are longer than those in the κF-
fluorobenzene adducts 2a (2.2725(17) and 2.2884(16) Å). In
the Ti compound, the two M-F distances differ significantly
(21) Bottomley, F.; Paez, D. E.; White, P. S. J. Organomet. Chem. 1985, 291,
35.
(17) (a) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120,
6745. (b) Calderazzo, F.; Ferri, I.; Pampaloni, G. Organometallics 1999,
18, 2452. (c) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W.
Organometallics 2002, 21, 1050.
(22) Lukens, W. W., Jr.; Smith, M. R., III; Anderson, R. A. J. Am. Chem. Soc.
1996, 118, 1719.
(23) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002, 124,
2132.
(18) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(24) It should be noted that the C-F bond length observed in the structure of
fluorobenzene itself is elongated as well because of C-F‚‚‚H interactions
in the solid state. Thalladi, V. L.; Weiss, H.-C.; Bla¨ser, D.; Boese, R.;
Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702.
(19) Horton, A. D.; Frijns, J. H. G. Angew. Chem., Int. Ed. 1991, 30, 1152.
(20) Kupfer, V.; Thewalt, U.; Tisˇlerova´, I.; Sˇtepnicˇka, P.; Gyepes, R.; Kubisˇta,
J.; Hora´cˇek, M.; Mach, K. J. Organomet. Chem. 2001, 620, 39.
9
14312 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

(C5Me5)2M Cations and Their Fluoroarene Complexes
A R T I C L E S
Figure 2. ORTEP representation of the cations of 2a,b showing 50% probability ellipsoids. The anions and the hydrogen atoms (and the cocrystallized
fluorobenzene solvent molecule in 2b) are omitted for clarity.
Figure 3. ORTEP representation of the cations of 3a,b showing 50% probability ellipsoids. The anions and the hydrogen atoms (and the cocrystallized
1,2-difluorobenzene solvent molecule in 3b) are omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles(deg) of 2a,b
Table 4. Selected Bond Distances (Å) and Angles (deg) of the
Cations of 3a,b
2a
2b
3a
3b
M(n1)-Cp*(1)a,b
M(n1)-Cp*(2)c
M(n1)-F(n1)
M(n1)-F(12)
C(n21)-F(n1)
C(127)-F(12)
2.1543(12)
2.1475(13)
2.2725(17)
2.2884(16)
1.408(3)
2.0093(12)
2.0133(13)
2.151(2)
M(n)-Cp*(1)a,b
M(n)-Cp*(2)c
M(n)-F(n1)
2.1380(18)
2.1341(18)
2.330(2)
2.330(2)
1.389(4)
1.396(4)
2.0452(12)
2.0372(12)
2.3528(13)
2.2832(15)
1.378(3)
M(n)-F(n2)
1.402(3)
C(n21)-F(n1)
1.414(3)
C(n2m)-F(2)d
1.383(2)
Cp*(1)-M(n1)-Cp*(2)
F(1)-M(n1)-F(2)
M(n1)-F(n1)-C(n21)
M(n1)-F(2)-C(127)
Σ{angles M(n1)}d
141.29(5)
75.28(6)
172.21(16)
165.27(15)
146.53(5)
168.8(2)
359.70(9)
Cp*(1)-M(n)-Cp*(2)
F(n1)-M(n)-F(n2)
(Cp*2M,MF2)e
144.07(7)
68.034(7)
87.82(8)
3.76(11)
144.19(5)
68.58(5)
87.60(13)
2.21(8)
(MF2,C6H4F2)f
(Cp*2M,MF2)e
89.47(10)
a M ) Sc, n ) 1; M ) Ti, n ) 0. b Cp*(1) is the centroid of the C(n1)-
a M ) Sc, n ) 1; M ) Ti and V, n ) 0. b Cp*(1) is the centroid of the
C(1)-C(5) ring (2b) or the C(11)-C(15) ring (2a,c). c Cp*(2) is the centroid
of the C(11)-C(15) ring (2b) or the C(111)-C(115) ring (2a,c). d Σ{angles
M(n1)} is defined as the sum of the angles around M(n1). (Cp*2M,FC6H5)
is defined as the angle between the Cp*(1)-M(n1)-Cp*(2) and F(11)-
M(1)-F(12) planes.
C(n5) ring. c Cp*(2) is the centroid of the C(n11)-C(n15) ring. d M ) Sc,
e
m ) 6; M ) Ti, m ) 2.
(Cp*2M,F2C6H4) is defined as the angle between
f
the Cp*(1)-M(1)-Cp*(2) and F(n1)-M(n)-F(n2) planes. (MF2,C6H4F2)
is defined as the angle between the F(n1)-M(n)-F(n2) and 1,2-difluo-
robenzene planes.
single-crystal X-ray diffraction. Initially, in the case of scandium,
yellow crystals of the methyl-bridged dimer [(Cp*2Sc)2(µ-Me)]-
[B(C6F5)4] (4a′) were obtained, which were characterized by
X-ray analysis (see Supporting Information). In the presence
of additional [PhNMe2H][B(C6F5)4], dimer 4a′ is converted to
two equivalents of [Cp*2Sc(κ2F-C6F5)B(C6F5)3] (1a′). The
formation of a dimeric compound such as 4a′ was not observed
in the case of titanium or vanadium.
The reaction of Cp*2TiMe with [PhNMe2H][B(C6F5)4] per-
formed in fluorobenzene, rather than in toluene, resulted in
isolation of either the base-free compound [Cp*2Ti(κ2F-C6F5)B-
(C6F5)3] (1b′) or the fluorobenzene adduct [Cp*2Ti(κF-FC6H5)]-
[B(C6F5)4] (2b′), depending on the workup procedure. Simple
evaporation of the fluorobenzene and washing with pentane
yielded 1b′ as a brown powder. A solution in THF-d8 showed
1H NMR resonances of the THF adduct 5b′, but no liberated
fluorobenzene. Layering cyclohexane on top of a fluorobenzene
solution of 1b′ yielded green crystals of the fluorobenzene
adduct 2b′, as characterized by X-ray diffraction. Redissolving
from each other (2.3528(13) vs 2.2832(15) Å). The fact that
there is no significant difference in the mean M-F bond distance
between 3a and 3b (although Sc3+ is larger than Ti3+ 18
) suggests
that the 1,2-difluorobenzene molecule is more tightly bound in
the scandium complex than in the titanium complex. This is in
accordance with RIDFT calculations (vide infra). The elongation
of the C-F bonds relative to free 1,2-difluorobenzene (M )
Sc, 0.040 and 0.047 Å; M ) Ti, 0.029 and 0.034 Å) is
comparable to that observed for the fluorobenzene adducts (M
) Sc, 0.044 and 0.050 Å; M ) Ti, 0.038 Å).
Synthesis of Cationic Decamethylmetallocene Cations with
the [B(C6F5)4] Anion. Having established that decamethylmet-
allocene cations can bind neutral fluorobenzenes, we also
prepared the cationic metallocenes with the fluorinated “weakly”
coordinating [B(C6F5)4] anion. Reaction of Cp*2MMe with
[PhNMe2H][B(C6F5)4] in toluene (or, for M ) V, in fluoroben-
zene) resulted in the corresponding ionic complexes [Cp*2M]-
[B(C6F5)4] (1a′-c′, Scheme 2), which were characterized by
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14313

A R T I C L E S
Bouwkamp et al.
Figure 4. ORTEP representation of 1a′,b′ showing 10% probability ellipsoids. The hydrogen atoms are omitted for clarity.
Scheme 2
Table 5. Selected Bond Distances (Å) and Angles (deg) of 1a′,b′
there is no direct cation-anion interaction; the cation in 1c′ is
virtually identical to that in the tetraphenylborate salt 1c, with
a single agostic interaction of the metal center with a Cp* methyl
group. The structure of the cation in the fluorobenzene adduct
[Cp*2Ti(FC6H5)][B(C6F5)4] (2b′) is very similar to that of [Cp*2-
Ti(FC6H5)][BPh4] (2b) and will not be discussed in detail here
(see Supporting Information).
1a′
1b′
M-Cp*(1)a
M-Cp*(2)b
M-F(2)
2.115(4)
2.138(3)
2.341(3)
2.392(4)
1.377(7)
1.383(7)
2.052(7)
2.039(5)
2.325(5)
2.370(5)
1.365(10)
1.346(10)
M-F(3)
C(23)-F(2)
C(24)-F(3)
Synthesis of Cationic Decamethylmetallocene THF Ad-
ducts. For comparison with their behavior toward fluoroarenes,
the interaction of the decamethylmetallocene cations with the
classic Lewis base THF was studied. The reaction of the
decamethylmetallocene methyl complexes Cp*2MMe with
[PhNMe2H][BPh4] in THF readily generates the THF adducts
of the [Cp*2M] cations. Slow diffusion of aliphatic solvents such
as pentane or cyclohexane into the THF solutions thus formed
afforded yellow crystals of the bis-THF adduct [Cp*2Sc(THF)2]-
[BPh4] (5a) or green crystals of the mono-THF adducts [Cp*2-
Ti(THF)][BPh4] (5b) and [Cp*2V(THF)][BPh4] (5c).
Cp*(1)-M-Cp*(2)
F(2)-M-F(3)
(Cp*2M,MF2)c
(MF2,C6F5B)d
144.67(15)
68.00(12)
89.3(3)
144.9(2)
67.62(18)
89.5(5)
21.31(15)
20.3(2)
a Cp*(1) is the centroid of the C(1)-C(5) ring. b Cp*(2) is the centroid
c
of the C(11)-C(15) ring.
(Cp*2M,MF2) is defined as the angle between
d
the Cp*(1)-Sc(1)-Cp*(2) and M-F(2)-F(3) planes.
(MF2,C6F5B) is
defined as the angle between the Cp*(1)-M-Cp*(2) and BC6F5 planes.
crystals of 2b′ in fluorobenzene, followed by evaporation of
the solvent and washing with pentane, again yielded the
fluorobenzene-free complex 1b′.
1
The H NMR spectra of 5a-c in THF-d8 show resonances
Structure of Decamethylmetallocene Cations with the
[B(C6F5)4] Anions. The X-ray structures of [Cp*2M(κ2F-
C6F5)B(C6F5)3] (M ) Sc, 1a′; M ) Ti, 1b′) are depicted in
Figure 4 (Table 5 contains pertinent bond distances and angles).
The anions are coordinated to the metal center in an κ2F-fashion
with two adjacent C-F bonds, comparable to the 1,2-difluo-
robenzene molecules in the adducts 3a,b. The structures of 1a′,b′
are very similar, with angles between the planes defined by
M-F(2)-F(3) and the coordinated C6F5 moiety of 21.31(15)
and 20.3(2)°, respectively. These angles are significantly larger
than those in the 1,2-difluorobenzene adducts 3a,b (3.76(11)
and 2.21(8)°). The M-F bond lengths in 1a′,b′ (M ) Sc, 2.341-
(3) and 2.392(4) Å; M ) Ti, 2.325(5) and 2.370(5) Å) are
slightly larger than those in 3a,b (M ) Sc, 2.330(2) and 2.330-
(2) Å; M ) Ti, 2.3528(13) and 2.2832(15) Å).
for the tetraphenylborate anion. In addition, resonances for the
Cp* ligands are observed for diamagnetic 5a (δ 1.93 ppm) and
d1 paramagnetic 5b (δ 11.7 ppm with ∆ν1/2 ) 402 Hz). For the
d2 compound 5c, no resonance for the Cp* ligand could be
observed in the chemical shift range -150 to 150 ppm. This
indicates that the d2 center in 5c has an S ) 1 configuration, as
is usual for electronically unsaturated V(III) compounds.15,25
Structure of the Cationic Decamethylmetallocene THF
Adducts. The three THF adducts 5a-c were characterized by
single-crystal X-ray diffraction (Figure 5, pertinent geometric
data in Table 6). The structure of the scandium bis(THF-d8)
adduct, 5a-d16 (crystallized from THF-d8/pentane), is very
similar to that of the known lanthanide species [Cp*2Ln(THF)2]-
(25) (a) Nieman, J.; Teuben, J. H.; Huffman, J. C.; Caulton, K. G. J. Organomet.
Chem. 1983, 255, 193. (b) Hessen, B.; Meetsma, A.; Teuben, J. H. J. Am.
Chem. Soc. 1989, 111, 5977. (c) Choukroun, R.; Couziech, B.; Pan, C.;
Dahan, F.; Cassoux, P. Organometallics 1995, 14, 4471.
An X-ray structure determination of the vanadium analogue
1c′ (see Supporting Information) shows that in this compound
9
14314 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

(C5Me5)2M Cations and Their Fluoroarene Complexes
A R T I C L E S
Figure 5. ORTEP representation of the cations of 5a-d16 and 5b,c showing 50% probability ellipsoids. The anions and hydrogen atoms (and the deuterium
atoms in 5a-d16) are omitted for clarity.
Table 6. Selected Bond Distances (Å) and Angles (deg) or 5a-d16
and 5b,c
ences in M-Cp* bonding can be accounted for by the relative
size of the Ti3+ and V3+ ions (effective ionic radii are 0.670
and 0.640 Å, respectively).18 In that respect, the V(1)-O(1)
bond distance in 5c (2.1189(8) Å) is relatively long compared
to the Ti-O bond distance in 5b (2.116(3) Å), suggesting that
the THF molecule is more tightly bound in the case of titanium
(vide infra).
5a-d16
5b
5c
M(n)-Cp*(1)a,b
M(n)-Cp*(2)c
M(n)-O(1)
2.2228(9)
2.2302(10)
2.2964(14)
2.2652(14)
2.0479(14)
1.9810(6)
1.9907(6)
2.1189(8)
2.116(3)
M(n)-O(2)
Cp*(1)-M(n)-Cp*(2)
O(1)-M(n)-O(2)
Σ{angles O(1)}d
Σ{angles O(2)}e
Σ{angles M(n)}f
(Cp*2M,MOCC)g
(Cp*2M,OMO)h
137.21(4)
91.59(5)
358.9(2)
360.0(2)
142.11(6)
360.0(4)
146.51(2)
Computational Studies. The base-free decamethylmetal-
locene cations, their THF, fluorobenzene, and 1,2-difluoroben-
zene adducts were studied by DFT calculations (RI-bp86/SV(P);
see Experimental Section for details and Supporting Information
for complete results). Also included were the parent metallocene
cations, [Cp2M]+, and their corresponding adducts. Results are
summarized in Table 7 and are graphically represented in Figure
6. The energies are relative to the base-free metallocene cations
and do not include zero-point energies or thermal corrections,
and the anions have been omitted.
359.98(13)
360.01(8)
73.11(17)
360.00(5)
80.43(7)
89.69(15)
a M ) Sc, n ) 0; M ) Ti and V, n ) 1. b Cp*(1) is the centroid of the
C(n1)-C(5) ring (M ) Sc and Ti) or the C(11)-C(15) ring (M ) V).
c Cp*(2) is the centroid of the C(11)-C(15) ring (M ) Sc and Ti) or the
C(111)-C(115) ring (M ) V). d Σ{angles O(1)} is defined as the sum of
the angles around atom O(1). e Σ{angles O(2)} is defined as the sum of
the angles around atom O(2). f Σ{angles M(n)} is defined as the sum of
g
the angles around atom M(n).
(Cp*2M,MOCC) is the angle between
The optimized structures of all adducts adopt a bent-
metallocene geometry and are in reasonable agreement with the
X-ray structures described above (when available), the main
difference being the M-X bond lengths, which are longer in
the case of the calculated structures by 0.08-0.13 Å. The energy
associated with the shortening of the M-X bond distances to
the values found in the X-ray structures, however, is small (<1.5
kcal/mol, see Supporting Information). We therefore think that
this shortening of the bond lengths is a result of crystal packing
forces, reducing the distance between the cation and anion, hence
increasing Coulombic stabilization. Similar to those found in
the X-ray structures, the C-F bond lengths in the calculated
structures of the fluorobenzene adducts are longer than those
in the free fluoroaromatics. The M-F-C angles in the
calculated structures of the decamethylmetallocene fluoroben-
zene adducts are close to 180° and are thus similar to those
found in the X-ray analyses. In the case of the cationic [Cp2M]
monofluorobenzene adducts, this angle is much smaller (139.66-
149.63 °).
the Cp*(1)-M(1)-Cp*(2) and Ti(1)-O(1)-C(11)-C(11a) or V(1)-O(1)-
h
C(121)-C(124) planes.
(Cp*2M,OMO) is the angle between the Cp*(1)-
M(1)-Cp*(2) and O(1)-M(1)-O(2) planes.
[BPh4] (Ln ) Sm26 and Yb27). The two Sc-O distances in 5a-
d16 differ slightly (by 0.03 Å), and the O-Sc-O angle of
91.59(1)° is noticeably larger than the F-Sc-F angle of 75.28-
(6)° in the bis-fluorobenzene adduct 2a.
The asymmetric unit of the titanium mono-THF adduct 5b
contains two independent [Cp*2Ti(THF)] cations, each located
on a crystallographic C2 axis, that are virtually identical. Data
of only one of these are included in Table 6. Only one THF
molecule is bound to the metal center, contrasting with the bis-
THF adduct observed for the parent metallocene cation [Cp2-
Ti(THF)2]+. As expected for a complex with a lower coordi-
nation number, the Ti(1)-O(1) bond distance in 5b of 2.116(3)
Å is shorter than those in [Cp2Ti(THF)2]+ (2.19-2.24 Å).28 The
THF adducts 5a,b have a geometry similar to those of the
corresponding fluorobenzene adducts 2a,b with slightly smaller
Cp*-M-Cp* angles (M ) Sc, 137.21(4) vs 141.29(5)°; M )
Ti, 142.11(6) vs 146.53(5)°).
The base-free metallocene cations were studied not only as
a reference to the adducts described here but also to investigate
the nature of the agostic interactions observed in the base-free
metallocene cations 1b,c. For [Cp*2Sc]+, the similarity between
the observed and calculated structures indicates that the close
The geometry of the cationic vanadium mono-THF adduct
in 5c closely resembles that of the titanium analogue. Differ-
(26) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvares,
D. Organometallics 1990, 9, 2124.
(27) Schumann, H.; Winterfeld, J.; Keitsch, M. R.; Herrmann, K.; Demtschuk,
J. Z. Anorg. Allg. Chem. 1996, 622, 1457.
(28) (a) Merola, J. S.; Campo, K. S.; Gentile, R. A.; Modrick, M. A. Inorg.
Chim. Acta 1989, 195, 87. (b) Ohff, A.; Kempe, R.; Baumann, W.;
Rosenthal, U. J. Organomet. Chem. 1996, 520, 241. (c) Ahlers, W.; Temme.
B.; Erker, G.; Fro¨hlich, R.; Zippel, F. Organometallics 1997, 16, 1440. (d)
Plecnik, C. E.; Liu, F. C.; Liu, S. M.; Liu, J. P.; Meyers, E. A.; Shore, S.
G. Organometallics 2001, 20, 3599.
-
contact between the cation and the BPh4 anion observed in
the solid state does not represent a strong chemical interaction.
For the titanocene cation, the optimized structure shows no
agostic interactions, but a structure in which these interactions
are present is just 2.42 kcal/mol higher in energy. Furthermore,
in the case of the decamethylvanadocene cation, two local
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14315

A R T I C L E S
Bouwkamp et al.
Table 7. RIDFT/bp86 Results for [Cp2MLn]+ and [Cp*2MLn]+
Systems (M ) Sc, Ti, and V; L ) C6H5F, 1,2-C6H4F2, and THF)a
in the X-ray structures of the 1,2-difluorobenzene adducts 3a,b
and of the THF adducts 5b,c. Furthermore, the binding energies
of fluorobenzene and 1,2-difluorobenzene are smaller than that
of THF, which is a much better σ donor.
b
Erel
M
−X
C
−X
M−X−C
(kcal/mol)
(Å)
(Å)
(deg)
[Cp2Sc(FC6H5)]+
[Cp2Sc(FC6H5)2]+
[Cp2Sc(F2C6H5)]+
[Cp2Sc(THF)]+
[Cp2Sc(THF)2]+
[Cp2Ti(FC6H5)]+
[Cp2Ti(FC6H5)2]+
[Cp2Ti(F2C6H5)]+
[Cp2Ti(THF)]+
-21.68 2.22
1.42
142.15
Discussion
-33.79 2.27, 2.28 1.41, 1.41 163.79, 158.55
-28.35 2.37, 2.37 1.39, 1.39
-36.50 2.20
Considering the molecular orbital (MO) scheme of the bent-
metallocene fragment,29 there are three orbitals suitable for the
binding of additional substrates, with C2V symmetry labels 1a1,
b2, and 2a1 in order of increasing energy. For the d0 metal ion
Sc(III), all three orbitals are, in principle, available for the
binding of additional ligands. For the d1 ion Ti(III) and the d2
(S ) 1) ion V(III), orbitals are occupied by unpaired electron-
(s), leaving two (Ti) or one (V) free valence orbitals, respec-
tively.
-42.73 2.27, 2.28
-19.85 2.18
1.42
142.22
-28.14 2.32, 2.32 1.40, 1.40 155.99, 151.44
-26.53 2.34, 2.34 1.38, 1.38
-35.14 2.15
[Cp2Ti(THF)2]+
[Cp2V(FC6H5)]+
[Cp2V(FC6H5)2]+
[Cp2V(F2C6H5)]+
[Cp2V(THF)]+
-51.17 2.29, 2.27
-18.10 2.22
1.41
139.66
-19.90 2.25, 3.26 1.41, 1.37 142.67, 149.63
-18.92 2.26, 3.00 1.39, 1.35
-35.12 2.12
[Cp2V(THF)2]+
-24.61 2.18, 2.22
In addition to the number of available valence orbitals, the
coordination chemistry of the [Cp*2M] cations is determined
by the accessibility of the metal center. For [Cp*2Ti]+, the space
in the wedge between the ligands only allows the binding of a
single THF molecule, whereas the electronic configuration
would allow the binding of two THF molecules, as seen in the
unsubstituted metallocene cation [Cp2Ti(THF)2]+.28 The d2 S
) 1 [Cp*2V] cation can bind only one molecule of THF,
irrespective of the substituents on the cyclopentadienyl ligand.30
The vanadocene cation can only accommodate two additional
ligands in the case of a strong field ligand such as CO. In that
case, the binding is accompanied by spin pairing, as seen in
the bis-CO adducts [Cp2V(CO)2][Co(CO)4] and [Cp2V(CO)2]-
[BPh4].31 From the DFT calculations, it can clearly be seen that
in the decamethylmetallocene cations the THF binding energies
have a significant steric component; in the [Cp2M(THF)] cations,
the THF molecule is bound equally strong, irrespective of the
metal center, whereas the THF binding energy in the corre-
sponding [Cp*2M(THF)] cations differs for the three metals
corresponding to the relative accessibility of the metal center
(Sc > Ti > V).
The interplay of steric and electronic effects is also seen in
the structures of the base-free [Cp*2M][BPh4] compounds.
Whereas for scandium the metallocene wedge is sufficiently
open to allow interaction with the anion, the titanium and
vanadium cations cannot accommodate this. Instead, these
metallocene cations adopt (at least in the solid state) structures
with agostic interactions with Cp*-Me groups. It is presently
unclear whether these observed distortions are due to an increase
in Coulombic lattice energy (e.g., by allowing the anion to
approach the cation more closely) or due to true electronic
stabilization by these intramolecular interactions. The DFT
calculations on the isolated cations indicate no (Ti) or very small
(V) stabilizations upon distortion. Nevertheless, the fact that
the number of agostic interactions corresponds to the number
of free valence orbitals on the metal center (two for Ti, one for
V) suggests that these interactions are at least enabled by
the electronic configuration of the metal. Even though agostic
C-H‚‚‚M interactions are well-known in early transition-metal
chemistry,32 intramolecular interactions with methyl substituted
Cp ligands are, to our knowledge, unprecedented.
[Cp*2Sc(FC6H5)]+
-14.44 2.29
1.41
165.71
[Cp*2Sc(FC6H5)2]+ -20.54 2.39, 2.40 1.40, 1.40 168.88, 167.54
[Cp*2Sc(F2C6H5)]+ -17.17 2.45, 2.45 1.38, 1.38
[Cp*2Sc(THF)]+
[Cp*2Sc(THF)2]+
[Cp*2Ti]+
-23.05 2.29
-27.83 2.38, 2.36
0.00
1.12, 1.13
1.41
[Cp*2Ti(FC6H5)]+
-11.58 2.27
164.61
[Cp*2Ti(FC6H5)2]+ -13.94 2.47, 2.51 1.39, 1.39 165.40, 164.73
[Cp*2Ti(F2C6H5)]+ -15.37 2.40, 2.40 1.38, 1.38
[Cp*2Ti(THF)]+
[Cp*2Ti(THF)2]+
[Cp*2V]+
-19.19 2.25
-15.73 2.37, 2.48
0.28
0.00 2.10
-6.55 2.33
-9.28 2.37, 4.00 1.40, 1.37 165.24, 152.76
-6.44 2.39, 3.19 1.38, 1.35
-13.79 2.23
[Cp*2V]+ (agostic)
[Cp*2V(FC6H5)]+
[Cp*2V(FC6H5)2]+
[Cp*2V(F2C6H5)]+
[Cp*2V(THF)]+
1.16
1.40
163.28
a Energies (kcal/mol) are for the reaction Cp′2M + 2L f Cp′2MLn +
(2-n)L. Distances are measured in Å and angles are measured in deg. X
denotes the ligand heteroatom or (for the agostic species) the Cp*H atom.
b Relative to the base-free metallocene cation.
minima are found, one of which does show such an agostic
interaction (the energy difference between the two structures is
0.28 kcal/mol, with the agostic structure being the lowest in
energy).
The calculations corroborate the observations that the scan-
docene cation prefers the formation of a bis-THF adduct, in
the case of both the [Cp2Sc] and the [Cp*2Sc] cation. They
furthermore agree with the observation that the parent titanocene
cation binds two molecules of THF, whereas a mono-THF
adduct is obtained in the case of the decamethyltitanocene
cation, and with the fact that the vanadocene cation binds only
one molecule of THF irrespective of the substitution pattern of
the cyclopentadienyl ligand (in fact, the second molecule of THF
dissociates when trying to optimize the structure of the [Cp*2V-
(THF)2] cation).
In the case of fluorobenzene, the binding of one or two
molecules of fluorobenzene is enthalpically favorable in the case
of all metallocene cations considered. Nevertheless, for the
compounds that were not accessible in the experiments described
above, the binding energy is very low (lower than the expected
loss of entropy on complex formation, which is estimated at 10
kcal/mol). For example, the energy associated with the binding
of fluorobenzene to the decamethylvanadocene cation is only
6.55 kcal/mol.
(29) (a) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729. (b)
Green, J. C. Chem. Soc. ReV. 1998, 27, 263.
The binding energies decrease with an increasing number of
unpaired electrons, except for the mono-THF adducts of the
[Cp2M] cations. This parallels the relative M-X bond lengths
(30) Choukroun, R.; Douziech, B.; Pan, C.; Dahan, F.; Cassoux, P. Organo-
metallics 1995, 14, 4471.
(31) (a) Calderazzo, F.; Bacciarelli, S. Inorg. Chem. 1963, 2, 721. (b) Calderazzo,
F.; Ferri, I.; Pampaloni, G. Organometallics 1999, 18, 2452.
9
14316 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

(C5Me5)2M Cations and Their Fluoroarene Complexes
A R T I C L E S
Figure 6. Relative energies of C6H5F, C6H4F2, and THF adducts of metallocenium cations. Note the different vertical scale for the THF adducts.
Reaction of the base-free decamethylmetallocene cations with
fluorobenzenes resulted in the isolation of the first κF-
fluorobenzene and κ2F-1,2-difluorobenzene adducts of transition
metals. There are a limited number of κX-halobenzene (X )
Cl, Br, and I)33 and κ2I-1,2-diiodobenzene adducts known to
date.34 In the κX-halobenzene adducts, the M-X-C angle
(95.361-116.406°) is generally much smaller than the ca. 180°
observed in our κF-fluorobenzene adducts. To our knowledge,
the only other example of a halobenzene adduct with a linear
M-X-C bond is the lithium fluorobenzene adduct Li2[µ-
N(SiMe3)2]2(C6H5F).35 It appears that the metal-fluorobenzene
interaction in the cationic metallocene adducts described here
is also predominantly electrostatic in nature. The calculated
ligand binding energies corroborate the poor σ donor character
of the fluorine atom compared to that of the oxygen atom of
THF. π Back-bonding to the fluorobenzene ligand (necessarily
absent in the case of the d0 metal ion Sc3+) is negligible in the
case of the titanium and (hypothetical) vanadium complexes;
the spin density in these complexes is mainly located in metal-
centered orbitals (see Supporting Information).
The metal-fluoroarene interactions in the κ2F-1,2-difluo-
robenzene adducts and the [Cp*2Ti(κ2F-C6F5)B(C6F5)3] com-
plexes are comparable, though the M-F bond distances in the
B(C6F5)4 adducts are slightly larger than those in the corre-
sponding 1,2-difluorobenzene adducts. As the Cp* ligands have
a large degree of freedom, as seen by their rotational disorder
in the crystal structure, it is unlikely that these longer M-F
bonds are the result of steric repulsion of the Cp* ligands with
the C6F5 groups of the anion. Instead, it might be that, due to
the strong electron-withdrawing nature of all other fluorine
substituents in the anion, the amount of negative charge at the
two coordinating fluorines is actually smaller than in the
difluorobenzene complexes. The relative weakness of the
metal-B(C6F5)4 interaction is also underlined by the observation
of an equilibrium between the contact ion pair [Cp*2Ti(κ2F-
C6F5)B(C6F5)3] and the fluorobenzene adduct [Cp*2Ti(κF-
FC6H5)][B(C6F5)4] in fluorobenzene solution.
Conclusions
The decamethylmetallocene cations [Cp*2M]+ (M ) Sc, Ti,
and V), with d0-d2 metal electronic configurations, were
generated for use as scaffolds to probe the interactions of the
cationic metal center with the C-F bonds of fluoroarenes. The
coordination chemistry of the metal centers in these species is
governed by the number of free valence orbitals and by the
spatial accessibility of the metal center through the wedge
formed by the Cp* ligands. The balance of these factors is
illustrated, for example, by the observed stoichiometries of
complexes with Lewis bases such as THF, but also by the
structures of the base-free species. For the largest metal,
scandium, [Cp*2M][BPh4] forms a contact ion pair in the solid
state, whereas for the smaller titanium and vanadium metals,
the anion cannot be accommodated in the metallocene cleft.
Instead, the cations in these compounds adopt unprecendented
distorted metallocene structures with agostic C-H‚‚‚M interac-
tions to one (V) or two (Ti) of the Cp* methyl groups. These
substantial deformations seem to be associated with surprisingly
small energy changes.
The decamethylmetallocene cations of scandium and titanium
react with fluorobenzenes to give isolable, fluorine-bound
adducts [Cp*2M(κF-FC6H5)n][BPh4] (M ) Sc, n ) 2; M ) Ti,
n ) 1) and [Cp*2M(κ2F-1,2-F2C6H4)][BPh4], whereas for the
smallest and least electropositive metal vanadium, the interaction
with fluoroarenes is too weak to allow isolation of adducts. The
bonding of the fluoroarene molecules in these complexes appears
to be governed mainly by electrostatics. In view of these
observations, it comes as no surprise that the perfluorinated
[B(C6F5)4] anion coordinates to the Sc and Ti cations in much
the same way as 1,2-difluorobenzene. Nevertheless, it is
remarkable that neutral fluorobenzene can compete with the
[B(C6F5)4] anion for bonding to the metal center in the Ti
system. Apparently, the 20 fluoro atoms in the anion can
(32) (a) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. Chem.
Commun. 1982, 802. (b) Jordan, R. F.; Bradley, P. K.; Baenziger, N. C.;
LaPointe, R. E. J. Am. Chem. Soc. 1990, 112, 1289. (c) Hyla-Krypsin, I.;
Gleiter, R.; Kruger, C.; Zwettler, R.; Erker, G. Organometallics 1990, 9,
517. (d) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Abdul Malik,
K. M. Organometallics 1994, 13, 2235. (e) Pindado, G. J.; Thornton-Pett,
M.; Bouwkamp, M. W.; Meetsma, A.; Hessen, B.; Bochmann, M. Angew.
Chem., Int. Ed. 1997, 36, 2358. (f) Nakamura, H.; Nakayama, Y.; Yasuda,
H.; Maruo, T.; Kanehisa, N.; Kai, Y. Organometallics 2000, 19, 5392. (g)
Schmitt, O.; Wolmersha¨user, G.; Sitzmann, H. Eur. J. Inorg. Chem. 2003,
3105.
(33) (a) Kullawiek, R. J.; Faller, J. W.; Crabtree, R. H. Organometallics 1990,
9, 745. (b) Butts, M. D.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc.
1996, 118, 11831. (c) Powell, J.; Horvath, M. J.; Lough, A. J. Chem. Soc.,
Dalton Trans. 1996, 1669. (d) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am.
Chem. Soc. 2004, 126, 15360.
(34) (a) Crabtree, R. H.; Faller, J. W.; Mellea, M. F.; Quirck, J. M. Organo-
metallics 1982, 1, 1361. (b) Powell, J.; Lough, A.; Saeed, T. J. Chem.
Soc., Dalton Trans. 1997, 4137.
(35) Williard, P. G.; Liu, Q. Y. J. Org. Chem. 1994, 59, 1596.
9
J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005 14317

A R T I C L E S
Bouwkamp et al.
[Cp*2V][BPh4] (1c). To a mixture of 201 mg (0.60 mmol) of Cp*2-
VMe and 242 mg (0.59 mmol) of [PhNMe2H][BPh4] was added 5 mL
of toluene. After 1 h, the toluene was evaporated at reduced pressure
and the product was washed with pentane (3 × 3 mL). Evaporation of
the volatiles at reduced pressure yielded 263 mg (0.42 mmol, 71%) of
[Cp*2V][BPh4] as a brown powder. Recrystallization of the compound
from 1,2-difluorobenzenze/cyclohexane afforded reddish crystals that
were used in an X-ray diffraction study. Crystals thus obtained were
also submitted for elemental analysis. Anal. Calcd for C44H50BV: C,
82.49; H, 7.87. Found: C, 80.86; H, 7.86.37
[Cp*2Sc(κF-FC6H5)2][BPh4] (2a). [Cp*2Sc][BPh4] (25.9 mg, 41
µmol) was dissolved in 1 mL of fluorobenzene. Slow evaporation of
the solvent at reduced pressure afforded 8.9 mg (9 µmol, 21%) of yellow
crystals of [Cp*2Sc(FC6H5)2][BPh4]‚2C6H5F. Anal. Calcd for C56H60-
BF2Sc‚2C6H5F: C, 79.85; H, 6.93. Found: C, 80.22; H, 7.86.36
[Cp*2Ti(κF-FC6H5)][BPh4] (2b). Fluorobenzene (40 mL) was added
to a frozen mixture of 660 mg (1.98 mmol) of Cp*2TiMe and 874 mg
(1.98 mmol) of [PhNMe2H][BPh4]. The frozen mixture was allowed
to warm to ambient temperature, and the reaction mixture was stirred.
The color darkened while gas evolution was observed. After 3 h, the
solution was decanted and concentrated. Crystallization by slow
diffusion of pentane into the fluorobenzene solution yielded 869 mg
(1.04 mmol, 53%) of [Cp*2Ti(FC6H5)][BPh4](C6H5F). Anal. Calcd for
C50H55BFTi‚C6H5F: C, 81.06; H, 7.29; Ti, 5.77. Found: C, 80.36; H,
7.40; Ti, 5.62.37
dissipate the negative charge sufficiently to allow this competi-
tion, a tribute to the weakly nucleophilic nature of this anion.
An interesting aside is the observation that, despite its weak
nucleophilicity, the [B(C6F5)4] anion shows a stronger interaction
with the Ti center than the [BPh4] anion that is usually
considered to be more strongly interacting. In this case, this is
probably dictated by geometric factors; the most favorable
coordination mode of the [BPh4] anion (through one of the CHd
CH moieties of the phenyl group) is not accessible because of
the steric demand of the two Cp* ligands bound to the relatively
small Ti center.
Experimental Section
General Considerations. All reactions and manipulations of air-
and moisture-sensitive compounds were performed under a nitrogen
atmosphere using standard Schlenk, vacuum line, and glovebox
techniques. Reagents were purchased from commercial suppliers and
used as received, unless stated otherwise. Deuterated solvents were dried
over Na/K alloy prior to use or degassed and stored over molecular
sieves (4 Å) (fluorobenzene-d5). Other solvents were dried by percola-
tion over columns of aluminum oxide (THF and toluene), R3-11
supported Cu-based oxygen scavengers (THF, pentane and toluene),
and molecular sieves (pentane and toluene) or by distillation from Na/K
alloy (cyclohexane). Fluorobenzene and 1,2-difluorobenzene were dried
and stored over molecular sieves. The compounds Cp*2ScMe,13 Cp*2-
TiMe,14 Cp*2VMe,15 and [PhNMe2H][BPh4]16 were prepared according
[Cp*2Sc(κ2F-F2C6H4)][BPh4] (3a). 1,2-Difluorobenzene (0.5 mL)
was added to 24.8 mg (39 µmol) of [Cp*2Sc][BPh4]. On top of the
solution, 3 mL of cyclohexane was layered carefully. Slow mixing of
the two layers resulted in the precipitation of 12.9 mg (17 µmol, 44%)
of [Cp*2Sc(F2C6H4)][BPh4]. Anal. Calcd for C50H54BF2Sc: C, 80.30;
H, 7.28. Found: C, 80.03; H, 7.11.36
1
to literature procedures. H and 19F NMR spectra were recorded on a
Varian Unity 500 or Varian Gemini 200 spectrometer. Chemical shifts
are reported in ppm and are referenced to residual solvent resonances.
IR spectra were recorded on a Mattson 4020 Galaxy Fourier transform
infrared (FT-IR) spectrophotometer. The elemental analyses were
performed by the Microanalytical Department of the University of
Groningen or by H. Kolbe, Mikroanalytisches Laboratorium, Mu¨lheim
an der Ruhr. In the case of the analyses performed at the University of
Groningen, each value is the average of two independent determinations.
Some of these analyses show low, but reproducible, carbon values.
This is a common observation for organometallic complexes with a
high carbon content and is associated with the formation of inert carbide
species. In all cases, V2O5 was added to the samples to reduce the
formation of such species. The analysis performed under different
conditions by Kolbe does not suffer from these low carbon values.
The values reported in this case are single measurements. An extended
experimental section including alternative methods of preparation,
Toepler pump experiments, and a detailed description of the X-ray
diffraction studies can be found in the Supporting Information.
[Cp*2Sc][BPh4] (1a). Toluene (5 mL) was added to a mixture of
507 mg (1.5 mmol) of Cp*2ScMe and 627 mg (1.5 mmol) of
[PhNMe2H][BPh4]. After 1 h, the solvent was evaporated and the
resulting yellow solid was washed with pentane (3 × 5 mL), affording
631 mg (1.0 mmol, 67%) of [Cp*2Sc][BPh4]. Anal. Calcd for C44H50-
BSc: C, 83.39; H, 7.95. Found: C, 83.16; H, 7.95.36 Compound 1a is
slightly soluble in C6D6, but insufficiently to record 1H and 13C NMR
spectra.
[Cp*2Ti(κ2F-F2C6H4)][BPh4] (3b). [Cp*2Ti][BPh4] (54.2 mg, 85
µmol) was dissolved in 0.5 mL of 1,2-difluorobenzene. The green
solution was layered with 2.5 mL of cyclohexane. Slow diffusion of
the cyclohexane into the 1,2-difluorobenzene solution afforded 58.6
mg (68 µmol, 80%) of blue-green crystals of [Cp*2Ti(1,2-F2C6H5)]-
1
[BPh4]‚(C6H4F2). The H NMR spectrum of the compound in THF-d8
was identical to that of the mono-THF adduct 5b and showed resonances
for 1,2-difluorobenzene. Anal. Calcd for C50H54BF2Ti‚C6H4F2: C, 77.69;
H, 6.75; Ti, 5.33. Found: C, 77.68; H, 6.85; Ti, 5.53.36 In a separate
experiment, 34 mg (55 µmol) of [Cp*2Ti][BPh4] was dissolved in 0.5
mL of 1,2-difluorobenzene. From this solution, 40.2 mg (46 µmol, 84%)
of blue-green crystals of [Cp*2Ti(1,2-F2C6H5)][BPh4]‚(C6H4F2) were
obtained after slow diffusion of cyclohexane into the solution.
[Cp*2Sc(κ2F-C6F5)B(C6F5)3] (1a′). Toluene (1 mL) was added to a
mixture of 30.6 mg (93 µmol) of Cp*2ScMe and 73.0 mg (91 µmol) of
[PhNMe2H][B(C6F5)4]. The resulting solution was layered with cyclo-
hexane resulting in a brownish, oily precipitate. Upon standing for one
week, the compound crystallized to yield 55.3 mg (56 µmol, 62%) of
[Cp*2Sc][B(C6F5)4] after decanting of the toluene solvent and washing
with pentane (2 × 2 mL). Anal. Calcd for C44H30BF20Sc: C, 53.19; H,
3.04. Found: C, 53.16; H, 3.32.37
[Cp*2Ti(κ2F-C6F5)B(C6F5)3] (1b′) from Cp*2TiMe and [PhNMe2H]-
[B(C6F5)4]. To a mixture that was frozen in liquid nitrogen of 122 mg
(0.37 mmol) of Cp*2TiMe and 320 mg (0.35 mmol) of [PhNMe2H]-
[B(C6F5)4] was added 10 mL of fluorobenzene. The reaction mixture
was stirred for 1 h, resulting in a green solution. The solvent was
removed in a vacuum, and the resulting green oil was washed 5 times
with 10 mL of pentane. After drying in a vacuum, this afforded 0.30
g (0.3 mmol, 86%) of [Cp*2Ti][B(C6F5)4]. The 1H and 19F NMR spectra
of the compound in THF-d8 show resonances for the THF adduct 5b′
and for a diamagnetic impurity with a resonance at 2.05 ppm (17%
assuming the impurity involves a [Cp*2M] fragment). Hence, no
satisfactory elemental analysis could be obtained.
[Cp*2Ti][BPh4] (1b). To a mixture of 0.75 g (2.2 mmol) of Cp*2-
TiMe and 0.95 g (2.2 mmol) of [PhNMe2H][BPh4] was added 40 mL
of toluene at -30 °C. Gas evolution was observed as the solution was
gradually warmed to room temperature. After 1 h, the toluene was
removed at reduced pressure and the crystalline solid was washed with
pentane (3 × 20 mL). Drying the product in a vacuum afforded 1.2 g
(1.8 mmol, 86%) of [Cp*2Ti][BPh4] as red-brown crystals, suitable for
single-crystal X-ray diffraction. Anal. Calcd for C44H50BTi: C, 82.89;
H, 7.90. Found: C, 79.62; H, 8.06.37
(36) Measured at the Microanalytical Department of H. Kolbe (Mu¨lheim an
der Ruhr).
(37) Measured at the Microanalytical Department of the University of Groningen.
[Cp*2Ti(κ2F-C6F5)B(C6F5)3] (1b′) from Cp*2TiMe and [(CnH2n+2)2-
NMeH][B(C6F5)4] (n ) 16-18). Cyclohexane (0.5 mL) and 32.6 mg
9
14318 J. AM. CHEM. SOC. VOL. 127, NO. 41, 2005

(C5Me5)2M Cations and Their Fluoroarene Complexes
A R T I C L E S
1
(98 µmol) of Cp*2TiMe in another 0.5 mL of cyclohexane was layered
on top of 1.04 g of a 10.8 wt % ISOPAR solution (94 µmol) of
[(CnH2n+2)2NMeH][B(C6F5)4] (n ) 16-18). The two layers mixed
slowly, resulting in a brown, oily precipitate. Greenish-brown crystals
that were suitable for X-ray analysis precipitated overnight. Washing
of the crystals with pentane (2 mL) resulted in 75 mg (75 µmol, 77%)
of [Cp*2Ti][B(C6F5)4]. The 1H and 19F NMR spectra of the compound
in THF-d8 are similar to those of 5b′. Anal. Calcd for C44H30BF20Ti:
C, 52.99; H, 3.03. Found: C, 50.75; H, 3.35.37
[Cp*2Ti(FC6H5)][B(C6F5)4] (2b′). To a mixture of 33.0 mg (99
µmol) of Cp*2TiMe and 75.6 mg (94 µmol) of [PhNMe2H][B(C6F5)4]
was added 1 mL of fluorobenzene. Slow diffusion of 3 mL of
cyclohexane into the fluorobenzene resulted in 79.3 mg (80 µmol, 80%)
of green crystals. The compound was recrystallized from fluorobenzene/
cyclohexane to afford crystals that were suitable for a single-crystal
X-ray analysis. Crystals thus obtained were also submitted for elemental
analysis. Anal. Calcd for C50H35BF21Ti: C, 54.92; H, 3.23. Found: C,
53.84; H, 3.34. 37
supernatant and drying at reduced pressure. H NMR (THF-d8, 200
MHz, room temperature): δ 11.70 (br, ∆ν1/2 ) 402 Hz, Cp*), 7.6-
6.6 (BPh4). Anal. Calcd for C52H66BO2Ti: C, 79.89; H, 8.51; Ti, 6.13.
Found: C, 79.94; H, 8.43; Ti, 6.33.
[Cp*2V(THF)][BPh4] (5c). THF (1 mL) was added to a mixture of
58.2 mg (0.17 mmol) of Cp*2VMe and 70.2 mg (0.16 mmol) of
[PhNMe2H][BPh4]. The resulting green solution was carefully layered
with 3 mL of cyclohexane, affording 94.4 mg (83%) of green crystals
of [Cp*2V(THF)][BPh4].1H NMR (THF-d8, 200 MHz, room temper-
ature): δ 7.7-6.5 (PhB). Anal. Calcd for C48H58BOV: C, 80.89; H,
8.20; V, 7.15. Found: C, 80.76; H, 8.26; V, 7.10.
Computational Studies. Calculations were performed using the
Turbomole program,38a,b coupled to a PQS Baker optimizer.39 Geom-
etries were fully optimized at the bp8640/RIDFT41 level using the
Turbomole SV(P) basis set38c,d on all atoms. The energies are relative
to the base-free metallocene cations and do not include zero-point
energies or thermal corrections, and the anions have been omitted.
Acknowledgment. This investigation was supported by The
Netherlands Foundation for the Chemical Sciences (CW) with
financial aid from The Netherlands Organization for Scientific
Research (NWO).
[Cp*2V][B(C6F5)4] (1c′). Fluorobenzene (1.5 mL) was added to a
mixture of 55.0 mg (0.16 mmol) of Cp*2VMe and 125.7 mg (0.16
mmol) of [PhNMe2H][B(C6F5)4]. The resulting solution was layered
with cyclohexane (3 mL), which afforded 140.2 mg (0.14 mmol, 89%)
of red crystals after slow diffusion of the two solvents, removal of the
supernatant, and washing with pentane (2 × 1 mL). Anal. Calcd for
C44H30BF20V: C, 52.83; H, 3.02. Found: C, 52.14; H, 3.04.37
[Cp*2Sc(THF)2][BPh4] (5a). THF (1 mL) was added to a mixture
of Cp*2ScMe (61.3 mg, 0.19 mmol) and [PhNMe2H][BPh4] (76.1 mg,
0.17 mmol). Pentane (3 mL) was carefully layered on top of the
resulting yellow reaction mixture, affording 85.2 mg (0.11 mmol, 65%)
of yellow crystals of [Cp*2Sc(THF)2][BPh4] after slow mixing of the
two solvents by diffusion. 1H NMR (THF-d8, 200 MHz, room
temperature): δ 7.29 (m, 8H, BPh4), 6.87 (t, 7.3 Hz, 8H, BPh4), 6.72
(t, 7.3 Hz, 4H, BPh4), 3.58 (t, 6.1 Hz, THF), 1.93 (s, 30H, Cp*), 1.77
(t, 6.1 Hz, THF). Anal. Calcd for C52H66BO2Sc: C, 80.28; H, 8.55;
Sc, 5.66. Found: C, 79.98; H, 8.61; Sc, 5.70.
[Cp*2Ti(THF)][BPh4] (5b). THF (2 mL) was added to a mixture
of 52.4 mg (0.16 mmol) of Cp*2TiMe and 68.4 mg (0.15 mmol) of
[PhNMe2H][BPh4]. When no further gas evolution was observed (after
15 min), cyclohexane (3 mL) was layered carefully on top of the green
reaction mixture. Diffusion of the two layers resulted in the deposition
of [Cp*2Ti(THF)][BPh4]. THF was obtained as green crystals. The
product was isolated (57.3 mg, 0.07 mmol, 47%) by decanting the
Supporting Information Available: Full experimental pro-
cedures and characterization data for all new compounds;
complete ref 38a; computational details (extended geometric
and energy data and representations of HOMO/SOMO and spin
density); and X-ray crystallographic data (details of the refine-
ments and CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA054544I
(38) (a) Ahlrichs, R. et al. Turbomole, version 5; Theoretical Chemistry Group,
University of Karlsruhe, January 2002. (b) Treutler, O.; Ahlrichs, R. J.
Chem. Phys. 1995, 102, 346. (c) Turbomole basis set library. Turbomole,
version 5; see 38a. (d) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys.
1992, 97, 2571.
(39) (a) PQS, version 2.4; Parallel Quantum Solutions: Fayetteville, AR, 2001;
the Baker optimizer is available separately from PQS upon request. (b)
Baker, J. J. Comput. Chem. 1986, 7, 385.
(40) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3089. (b) Perdew, J. P. Phys. ReV.
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