The first structurally characterized cationic lanthanide-alkyl complexes

Article abstract of DOI:10.1039/b201613n

Reaction of rare earth metal-alkyl complexes [Ln(CH₂SiMe₃)₃(THF)₂] (Ln = Y, Lu) with B(C₆X₅)₃ (X = H, F) in the presence of crown ethers gives crystallographically characterized

Full text of DOI:10.1039/b201613n

The first structurally characterized cationic lanthanide–alkyl complexes†
Stefan Arndt, Thomas P. Spaniol and Jun Okuda*
Institut für Anorganische Chemie und Analytische Chemie, Universität Mainz, Duesbergweg 10-14, D-55099
Mainz, Germany. E-mail: okuda@mail.uni-mainz.de
Received (in Cambridge, UK) 12th February 2002, Accepted 13th March 2002
First published as an Advance Article on the web 2nd April 2002
Reaction of rare earth metal–alkyl complexes [Ln(CH2Si-
Me3)3(THF)2] (Ln = Y, Lu) with B(C6X5)3 (X = H, F) in the
presence of crown ethers gives crystallographically charac-
terized ion pairs [Ln(CH2SiMe3)2(CE)(THF)n]+[B(CH2Si-
[B(CH2SiMe3)Ph3]2 as anion, at 25 °C in THF-d8, contains a
1
signal at d 210.4, whilst in the H NMR spectra the BCH2
2
resonance appears as a quartet at d 0.17 with JBH = 4.7 Hz,
indicating the presence of the identical anion.
Me3)(C6X5)3]– (CE
=
[12]-crown-4,
n
=
1; CE
=
1H and 13C NMR spectroscopy have shown that the cationic
lutetium complex derived from [12]-crown-4 contains one
molecule of THF that is labile on the NMR timescale.9 In the
yttrium analogue [Y(CH2SiMe3)2([12]-crown-4)(THF)]+, the
CH2 protons of the two equivalent alkyl groups at the yttrium
[15]-crown-5 and [18]-crown-6, n = 0).
In contrast to the intensely investigated chemistry of the group
4 metal–alkyl cations in the context of homogeneous olefin
polymerization catalysis,1 related cationic alkyl complexes of
the group 3 metals and lanthanides have not attracted much
attention.2–5 Recently, yttrium–alkyl cations [Y(L)(CH2Si-
Me3)]+[B(C6F5)4]2 (L = N,NA-R2-tacn-NB-(CH2)2NtBu; tacn =
center in CD2Cl2 give rise to a doublet at d 20.86 with 2JYH
=
3.2 Hz, whilst the carbon atoms were recorded as a doublet at d
1
39.8 with JYC = 41.0 Hz. The CH2 protons of the facially
1
coordinated [12]-crown-4 appear in the H NMR spectrum as
i
1,4,7-triazacyclononane, R = Pr, Me) were reported as single-
two separate broad multiplets at d 3.27 and 3.65, suggesting
rigid coordination at the yttrium center. The lutetium complex
shows similar NMR spectra. As shown in Fig. 1, the lutetium
derivative exhibits a seven-coordinate metal center best de-
scribed as a capped trigonal prism. Two parallel trigonal planes
are formed by the atoms O(1), O(2), C(9) and O(3), O(4), C(13).
The capping atom O(5) is located over the square plane O(2),
O(3), C(9), C(13). The angles at the alkyl carbon atoms,
127.3(1) and 135.4(1)°, suggest the absence of any strong
agostic interaction.
site polymerization catalysts for ethylene.2 However, these and
other alkyl cations of the rare earths reported in the literature,
such as [Y(L)(CH2SiMe3)2]+ (L
=
deprotonated aza-
[18]-crown-6) were generated in situ and only characterized by
NMR spectroscopy.2,4 The thermal sensitivity coupled with the
extreme electrophilicity of the metal center due to its low
valence electron count were cited as reasons for the difficulty in
isolating such species. We present here, surprisingly robust rare
earth metal–dialkyl cations supported by crown ethers, which
allow full crystallographic characterization. Crown ethers have
long been known to coordinate the cations of lanthanide
chlorides and nitrates,6 but their use in organolanthanide
chemistry has only been described recently.7
[15]-Crown-5 stabilizes the formally four-electron dialkyl
cation fragment without an additional molecule of THF.9 The
CH2 resonance of the alkyl groups at the lutetium center of
1
[Lu(CH2SiMe3)2([15]-crown-5)]+ in CD2Cl2 appear in the H
When a THF solution of the lutetium–trialkyl complex
[Lu(CH2SiMe3)3(THF)2]8 was treated at 25 °C with one
equivalent of the borane B(C6X5)3 (X = H, F), followed by one
equivalent of crown ether CE, analytically pure, colorless
crystals of the ion pair [Lu(CH2SiMe3)2(CE)(THF)n]+[B(CH2-
SiMe3)(C6X5)3]2 were isolated in practically quantitative yield
(Scheme 1). In contrast to cationic d0 metal–benzyl com-
plexes,1,5 the anions were shown to be non-coordinating.
Irrespective of the anion, the LuCH2 resonances shift to higher
field in the 1H and 13C NMR spectra with increasing size of the
crown ether; this indicates enhanced shielding in this series. The
ion pairs are soluble in THF and CH2Cl2, but insoluble in
aliphatic or aromatic hydrocarbons. Surprisingly, THF solu-
tions of the ionic complexes are stable for more than 12 h. As
alkyl abstraction with triphenylborane resulted in complete
conversion at ambient temperature, there was no obvious
advantage in using the more electrophilic perfluorinated
triphenylborane. The 11B{1H} NMR spectra of all cations with
NMR spectrum as a sharp singlet at d 21.17. The diastereotopic
methylene protons of the [15]-crown-5 give rise to two broad
multiplets at d 4.46 and 3.75. Again, the coordination
polyhedron around the seven-coordinate metal center can be
regarded as a capped trigonal prism (Fig. 2). Two parallel
trigonal planes are formed by the atoms O(2), O(3), C(15) and
O(4), O(5), C(11), with the capping atom O(1) located over the
square plane O(2), O(5), C(11), C(15).
Scheme 1 Conditions: i. THF, 25 °C, 20 min; ii. crown ether (CE), 25 °C,
5 min. CE = [12]-crown-4, n = 1; CE = [15]-crown-5, n = 0; CE =
[18]-crown-6, n = 0.
Fig.
1
ORTEP diagram of [Lu(CH2SiMe3)2([12]-crown-4)-
(THF)]+[B(CH2SiMe3)Ph3]–. Anion and hydrogen atoms omitted for
clarity, thermal ellipsoids drawn at 30% probability level. Selected bond
lengths (Å): Lu–O(1) 2.438(1), Lu–O(2) 2.451(1), Lu–O(3) 2.503(1), Lu–
O(4) 2.406(1), Lu–O(5) 2.307(1), Lu–C(9) 2.340(2), Lu–C(13) 2.354(2).
† Electronic supplementary information (ESI) available: experimental and
spectroscopic details. See http://www.rsc.org/suppdata/cc/b2/b201613n/
896
CHEM. COMMUN., 2002, 896–897
This journal is © The Royal Society of Chemistry 2002

1
contains three THF molecules. Comparison of its H and 13C
NMR data with that of [Lu(CH2SiMe3)2(THF)n]+[B(CH2Si-
Me3)(C6F5)3]2 shows that the LuCH2 resonances appear at
significantly higher field (d 21.03 vs. 20.92 and d 39.8 vs.
40.4, respectively). This finding strongly suggests that the
[B(CH2SiMe3)Ph3]2 anion is loosely coordinating in the
absence of crown ethers.11
In conclusion, we have shown that in the presence of oxygen
donors, thermally robust lutetium and yttrium alkyl cations
[Ln(CH2SiMe3)2(CE)(THF)n]+ become easily available. Pre-
liminary experiments with these extremely oxygen- and
moisture-sensitive cationic lanthanide alkyl complexes indicate
that they not only react with Broensted acids such as
t
HOC6H2 Bu2-2,6-Me-4, but exhibit significant ethylene polym-
erization activity.
This work was supported by the Deutsche Forschungsge-
meinschaft and the Fonds der Chemischen Industrie.
Fig. 2 ORTEP diagram of [Lu(CH2SiMe3)2([15]-crown-5)]+[B(CH2Si-
Me3)Ph3]2·0.5(CH2Cl)2. Anion, non-coordinating solvent molecule and
hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 30%
probability level. Selected bond lengths (Å): Lu–O(1) 2.359(5), Lu–O(2)
2.419(5), Lu–O(3) 2.376(5), Lu–O(4) 2.406(5), Lu–O(5) 2.421(5), Lu–
C(11) 2.364(7), Lu–C(15) 2.345(7).
Notes and references
1 C. Pellecchia, A. Grassi and A. Immirzi, J. Am. Chem. Soc., 1993, 115,
1160; R. F. Jordan, Adv. Organomet. Chem., 1991, 32, 325; W.
Kaminsky and M. Arndt, Adv. Polym. Sci., 1997, 127, 144.
2 S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen and J. H. Teuben,
Chem. Commun., 2001, 637.
3 C. J. Schaverien, Organometallics, 1992, 11, 3476.
4 L. Lee, D. J. Berg, F. W. Einstein and R. J. Batchelor, Organometallics,
1997, 16, 1819.
5 L. W. M. Lee, W. E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez,
Organometallics, 1999, 18, 2947.
Use of [18]-crown-6 makes further expansion of the
coordination sphere of the dialkyl–lanthanide complex possi-
ble.9 The NMR spectroscopic features are similar to those of the
related cations with smaller crown ethers. As Fig. 3 shows, the
lutetium center adopts a coordination polyhedron of a doubly
capped trigonal prism, where all six oxygen atoms of
[18]-crown-6 are coordinated to the eight-coordinate lutetium
ion. The two parallel trigonal planes are formed by the atoms
O(1), O(6), C(13) and O(3), O(4), C(17). The oxygen atoms
O(2) and O(5) cap the square planes formed by O(1), O(3),
C(13), C(17), and by O(4), O(6), C(13), C(17), respectively.
In the absence of crown ethers, the reaction of [Lu(CH2Si-
Me3)3(THF)2] with B(C6F5)3 in THF-d8 gives [Lu(CH2Si-
Me3)2(THF)n]+[B(CH2SiMe3)(C6F5)3]2, whose 19F NMR
spectrum exhibits meta/para chemical shift differences Dd of
2.2 ppm, consistent with solvent-separated ion pairs.10 How-
ever, reaction between [Lu(CH2SiMe3)3(THF)2] and BPh3 in
THF at ambient temperature resulted in colorless crystals of
6 R. D. Rogers and C. B. Bauer, in Comprehensive Supramolecular
Chemistry, ed. G. W. Gokel, Pergamon, Oxford, vol. 1, 1996; J.-C. G.
Bünzli, B. Klein and D. Wessner, Inorg. Chim. Acta, 1980, 44, 147; J.
D. J. Backer-Dirks, J. E. Cooke, A. M. R. Galas, J. S. Ghotra, C. J. Gray,
F. A. Hart and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 2191,
1980; J.-C. G. Bünzli, B. Klein, D. Wessner and N. W. Alcock, Inorg.
Chim. Acta, 1982, 59, 269; J.-C. G. Bünzli, B. Klein, G. Chapuis and K.
J. Schenk, Inorg. Chem., 1982, 21, 808.
7 Y. K. GunAko, P. B. Hitchcock and M. F. Lappert, Chem. Commun.,
1998, 1843.
8 M. F. Lappert and R. Pearce, Chem. Commun., 1973, 126. For the
crystal structure of the tris(THF) adduct, see: W. J. Evans, J. C. Brady
and J. W. Ziller, J. Am. Chem. Soc., 2001, 123, 7711.
9 See electronic supplementary information (ESI) for experimental and
spectroscopic
details.
[Ln(CH2SiMe3)2([12]-crown-4)(THF)]+-
[B(CH2SiMe3)Ph3]– (Ln
=
Lu, Y) and [Lu(CH2SiMe3)2-
1
[Lu(CH2SiMe3)2(THF)3]+[B(CH2SiMe3)Ph3]2. The H NMR
([18]-crown-6)(THF)]+[B(CH2SiMe3)Ph3]2 were prepared according
to the method used for the synthesis of [Lu(CH2SiMe3)2([15]-crown-
5)]+[B(CH2SiMe3)Ph3]–: A solution of Lu(CH2SiMe3)3(THF)2 (200
mg, 344 mmol) and BPh3 (83 mg, 344 mmol) in THF (1 mL) was stirred
for 20 min at 25 °C, and treated with [15]-crown-5 (75 mL, 344 mmol).
After evaporation of the solvent, pentane (3 mL) was added to give a
colorless powder which was washed with pentane (3 3 2 mL). Yield:
284 mg, 92%. 1H NMR (CD2Cl2, 25 °C), d 21.17 (s, 2 3 2 H, LuCH2),
–0.44 (s, 9 H, BCH2SiCH3), –0.07 (s, 2 3 9 H, LuCH2SiCH3), 0.18 (br,
2 H, BCH2), 3,45, 3.75 (br m, 10 H, OCH), 6.87 (t, 3JHH 7.0 Hz, 3 H,
spectrum in CD2Cl2 shows that the cation of this complex
3
4-Ph), 7.03 (t, JHH 7.1 Hz, 3 3 2 H, 3-Ph), 7.45 (br 3 3 2 H, 2-Ph).
13C{1H} NMR (CD2Cl2, 25 °C), d 2.4 (BCH2SiCH3), 4.2 (LuCH-
2SiCH3), 36.7 (LuCH2), 69.2 (OCH), 122.0 (4-Ph), 126.0 (3-Ph), 135.3
(2-Ph), 167.5 (q, 1JBC 48.3 Hz, 1-Ph). The signal of the BCH2 group was
not detected. 11B{1H} NMR (THF-d8, 25 °C), d 210.4. Anal. Calc. for
C40H68BLuO5Si3: C, 53.44; H, 7.56. Found: C, 52.56; H, 8.32.
Crystallographic data: CCDC reference number 179961, [Lu(CH-
2SiMe3)2([12]-crown-4)(THF)]+[B(CH2SiMe3)Ph3]2;
[Lu(CH2SiMe3)2([15]-crown-5)]+[B(CH2SiMe3)Ph3]2·0.5(CH2Cl)2;
179962,
179963,
Me3)Ph3]2·0.5(CH2Cl)2.
[Lu(CH2SiMe3)2([18]-crown-6)]+[B(CH2Si-
See http://www.rsc.org.suppdata/cc/b2/
b201613n/ for crystallographic data in CIF or other electronic format.
10 19F chemical shift differences between meta and para F atoms of less
than 3 ppm are reported to be characteristic of a non-coordinating anion:
A. D. Horton, J. de With, A. J. van der Linden and H. van de Weg,
Organometallics, 1996, 15, 2672.
Fig. 3 ORTEP diagram of [Lu(CH2SiMe3)2([18]-crown-6)]+[B(CH2Si-
Me3)Ph3]2·0.5(CH2Cl)2. Anion, non-coordinating solvent molecule and
hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 30%
probability level. Selected bond lengths (Å): Lu–O(1) 2.532(5), Lu–O(2)
2.422(5), Lu–O(3) 2.431(5), Lu–O(4) 2.433(5), Lu–O(5) 2.399(5), Lu–O(6)
2.524(5), Lu–C(13) 2.366(8), Lu–C(17) 2.371(8).
n
11 A strong h coordination of the aryl groups is unlikely because of the
high fluxionality. Furthermore, no reduction of the anion’s C3v
symmetry is observed on the NMR timescale.
CHEM. COMMUN., 2002, 896–897
897