High tetraalkylaluminate fluxionality in half-sandwich complexes of the trivalent rare-earth metals

Article abstract of DOI:10.1039/b212754g

Steric factors govern the formation of half-sandwich complexes (C₅Me₄R)Ln[N(SiHMe₂)₂] ₂ according to acid-base reactions utilising Ln[N(SiHMe₂)₂]₃-(thf)₂ ar

Full text of DOI:10.1039/b212754g

High tetraalkylaluminate fluxionality in half-sandwich complexes of the
trivalent rare-earth metals†
Reiner Anwander,*a Michael G. Klimpel,a H. Martin Dietrich,a Dmitry J. Shorokhovb and Wolfgang
Schererb
a Anorganisch-chemisches Institut, Technische Universität München, Lichtenbergstraße 4, D-85747
Garching, Germany. E-mail: reiner.anwander@ch.tum.de; Fax: 49 89 28913473; Tel: 49 89 28913096
b Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany.
E-mail: wolfgang.scherer@physik.uni-augsburg.de; Fax: 49 821 5983227; Tel: 49 821 5983350
Received (in Cambridge, UK) 2nd January 2003, Accepted 10th March 2003
First published as an Advance Article on the web 24th March 2003
Steric factors govern the formation of half-sandwich com-
plexes (C5Me4R)Ln[N(SiHMe2)2]2 according to acid–base
reactions utilising Ln[N(SiHMe2)2]3(thf)2 and substituted
cyclopentadienes. Subsequent trimethylaluminium-pro-
moted silylamide elimination produces the first half-sand-
wich bis(tetramethylaluminate) complexes (C5Me4R)Ln-
(AlMe4)2.
silyl groups in solution. The use of silyl-substituted tetra-
methylcyclopentadienes HC5Me4(SiR3) gave the half-sandwich
complexes in excellent yields which can be attributed to the
lower pKa values of the ligand precursor (3b: Y/SiHMe2, 93%;
3c: Y/SiMe3, 93%; 3d: SiHPh2, 83%; 4b: Lu/SiHMe2, 90%; 4c:
Lu/SiMe3, 87%; yields determined by NMR only).†,‡
We found that a trimethylaluminium-promoted alkylation of
complexes 3a and 4a is a viable route for the synthesis of mono-
Cp Ln(III) hydrocarbyl complexes (Scheme 1).†,‡ Remarkably,
complete silylamide elimination occurred without ligand redis-
tribution as evidenced by spectroscopic and microanalytical
data. The bis(aluminate) complexes Cp*Ln(AlMe4)2 (5a, Ln =
Y; 6a, Ln = Lu) could be efficiently separated from the co-
product {Me2Al[m-N(SiHMe2)2]}2 by fractional crystallization
from hexane solution. Enhanced steric unsaturation in these
half-sandwich complexes was clearly revealed by 1H NMR
spectroscopy. Even at 290 °C only a single proton resonance
appeared for the tetramethylaluminate ligands which is con-
sistent with rapid exchange of the bridging and terminal methyl
groups.
New synthetic pathways toward bis(amido) and bis(hydro-
carbyl) half-sandwich complexes are a prominent target in
organolanthanide(III) chemistry.1 Such mono(cyclopentadienyl)
derivatives is ascribed a high potential in homogeneously
catalyzed reactions, e.g. in the polymerization of styrene or
acrylonitrile.2 This potential applicability was probably ration-
alised on the basis of the superb performance of structurally
related CpTiX3-type compounds in syndiotactic styrene polym-
erization when combined with MAO.3–5 However, compared to
the ubiquitous “Cp2Ln(III)X” lanthanidocene complexes, deriv-
atives CpLn(III)X2 display enhanced electronic and steric
unsaturation, and hence, are more difficult to handle.6–9 As a
consequence, extensive Lewis base complexation and ligand
redistribution reactions due to high Cp and X ligand mobility
are commonly observed. Noteworthy, Cp*LnR2 (Ln = La, Ce,
Complex 6a was also characterized by an X-ray structure
analysis (Fig. 1).¶ The most striking feature of the solid state
structure is the differently bonded tetramethylaluminate moie-
ties. One AlMe42 ligand coordinates in the routinely observed
R
= CH(SiMe3)2; Ln = Ce, R =
N(SiMe3)2)6 and
Cp*Gd(CH2Ph)2(thf)7 and ate complexes [Li(thf)4][(C5H4t-
Bu)Yb(NPh2)3] and [Li(dme)3][(C5H4Me)La(NPh2)3]9 repre-
sent the only structurally characterized mononuclear mono-
(cyclopentadienyl) complexes featuring one type of additional
hydrocarbyl and amide ligand, respectively.
h -fashion forming an almost planar heterobimetallic Lu(m-
2
CH3)2Al(CH3)2 moiety with a torsion angle •C–Lu–C–Al1 of
6°.14 However, the second AlMe4 ligand features an unusual
2
2
h -coordination, which is seen by (i) an atypically bent
heterobimetallic Lu(m-CH3)2Al(CH3)2 moiety (•C–Lu–C–Al2
28°), (ii) elongated Lu–(m-CH3) bond lengths (DLu–C = ca. 0.08
Å), (iii) a shortened Lu–Al distance (DLu–Al = ca. 0.15 Å), and
(iv) an additional Lu–(m-CH3) contact of 3.447 Å. For
comparison, the Lu–C(m) bond lengths in homometallic
asymmetrically bridged Cp*2Lu(m-Me)LuCp*2Me with 7- or
8-coordinate lutetium centres are 2.440(9) and 2.756(9) Å,
Depending on the size of the metal centre and the steric bulk
of the cyclopentadienyl ligand, complexes (C5Me4R)Ln[N-
(SiHMe2)2]2 (3: Ln = Y, 4: Ln = Lu; R = H, Me, SiHMe2,
SiMe3, SiHPh2) can be selectively obtained by the extended
silylamide route (Scheme 1).10†‡ The silyl-substituted tetra-
methylcyclopentadienes were synthesized by standard meth-
ods.11
For example, the combination of small rare-earth metal
centres and Cp* (R
= Me) affords such half-sandwich
complexes in moderate crystallised yields (3a: Y/Cp*, 73%;12
4a: Lu/Cp*, 40%). Interestingly, the THF donor molecules are
completely displaced during the acid–base ligand exchange.
The corresponding La/Cp*-reaction gave a mixture of com-
plexes Cp*La[N(SiHMe2)2]2 and Cp*2La[N(SiHMe2)2].13§
The
IR
spectra
(Nujol
mull)
of
complexes
Cp*Ln[N(SiHMe2)2]2 (3a, Ln = Y; 4a, Ln = Lu) reveal the
presence of asymmetrically coordinated silylamide ligands as
evidenced by two strong, well-resolved n(SiH) bands at 2066
(2069) and 1833 (1820) cm21, respectively. The lower-energy
frequencies indicate strong Ln–H–Si b-agostic interactions in
the solid state. A variable temperature NMR study in the range
of 25 to 290 °C showed only a single SiH signal at 4.49 (3a)
and 4.59 (4a) ppm, which is in agreement with highly fluxional
Scheme 1 Synthesis of half-sandwich rare-earth metal amide and tetra-
alkylaluminate complexes according to the extended silylamide route (3: R
= Me (a), SiHMe2 (b), SiMe3 (c), SiHPh2, (d); 4: R = Me (a), SiHMe2 (b),
SiMe3 (c).
† Electronic supplementary information (ESI) available: complete synthesis
and characterization data. See http://www.rsc.org/suppdata/cc/b2/
b212754g/
1008
CHEM. COMMUN., 2003, 1008–1009
This journal is © The Royal Society of Chemistry 2003

extracted with hexane and the hexane fraction crystallized at 245 °C. 4a
was isolated as colourless plates (2.80 mmol, 1.60 g, 40%): IR (Nujol, n/
cm21): 2069vs, 1820s, 1732m, 1460vs, 1377s, 1246s, 1044vs, 900vs, 841s,
790s, 765s, 721m, 646w, 623w, 603w, 409w. 1H NMR (400 MHz, C6D6, 25
°C): d 4.59 (sp, 3JH,H = 3.0 Hz, 4H, SiH); 2.15 (s, 15H, Cp–Me); 0.26 (d,
3JH,H = 3.0 Hz, 24H, NSiMe). 13C NMR (100.6 MHz, C6D6, 25 °C): d
119.3 (Cp–C), 11.4 (Cp-Me), 3.0 (NSiMe). 4a (0.87 mmol, 500 mg) was
dissolved in 10 mL of hexane and an excess of AlMe3 (6.9 mmol, 501 mg)
added. After stirring for 2 h at ambient temperature, the slightly cloudy
reaction mixture was centrifuged. The clear solution yielded colourless
crystals of 6a at 245 °C (0.62 mmol, 300 mg, 71%): IR (Nujol, n/cm21):
1462vs, 1377vs, 1258w, 1237m, 1213m, 1193m, 1023w, 913w, 855m,
721s, 579m, 499w, 458w. 1H NMR (400 MHz, C6D6, 25 °C): d 1.75 (s, 15H,
Cp–Me); 20.18 (s, 24H, AlMe). 13C NMR (100.6 MHz, C6D6, 25°C): d
120.9 (Cp-C), 11.8 (Cp-Me), 1.49 (br s, AlMe). Satisfactory elemental
analyses were obtained for 4a and 6a (C, H, N).
§ Similar observations were made by Teuben et al. when studying
corresponding acid–base reactions of homoleptic complexes
Ln[N(SiMe3)2]3 and Ln[CH(SiMe3)2]3 with Cp*H. However, due to an
increased thermal lability of the hydrocarbyl ligand and a higher kinetic
Fig. 1 Molecular structure of 6a. Selected bond lengths (Å) and angles (°):
Lu–Al1 3.0612(9), Lu–Al2 2.9138(9), Lu–C(Cp*) 2.566(3)–2.603(3), Lu–
C11 2.501(3), Lu–C12 2.509(3), Lu–C15 2.597(3), Lu–C16 2.572(3), Lu–
H11A 2.41(3), Lu–H12A 2.44(3), Lu–H15B 2.46(4), Lu–H16B 2.41(3),
Al1–C11 2.082(3), Al1–C12 2.072(3), Al1–C13 1.962(3), Al1–C14
1.972(3), Al2–C15 2.065(3), Al2–C16 2.067(4), Al2–C17 1.959(3), Al2–
C18 1.963(3); Al1–Lu–Al2 112.01(2), C11–Lu–C12 84.4(1), C15–Lu–C16
80.0(1), C11–Lu–C16 84.5(1), C12–Lu–C15 85.0(1), Lu–C11–Al1 83.3(1),
Lu–C12–Al1 83.3(1), Lu–C15–Al2 76.4(1), Lu–C16–Al2 77.0(1), Lu–
C11–H11A 73(2), Lu–C11–H11B 162(2), Lu–C11–H11C 90(2), Lu–C12–
H12A 74(2), Lu–C12–H12B 161(2), Lu–C12–H12C 93(2), Lu–C15–H15A
118(2), Lu–C15–H15B 71(2), Lu–C15–H15C 139(3), Lu–C16–H16A
137(2), Lu–C16–H16B 69(2), Lu–C16–H16C 116(2).
barrier for Cp* introduction [steric bulk: CH(SiMe3)2 ≈ N(SiMe3)2
>
N(SiHMe2)2], well-defined half-sandwich complexes of the smaller rare-
earth elements could not be obtained.13 Also for steric reasons, putative
Cp*Lu[CH(SiMe3)2]2 could not be obtained by a salt metathesis reaction
starting from Cp*Lu[CH(SiMe3)2]2Cl2Li(thf)2 and LiCH(SiMe3)2]2.6b
¶ Crystallographic data for 6a: C18H39Al2Lu, M = 484.42, orthorhombic,
space group Pbca, a = 17.2295(1), b = 14.3690(1), c = 17.9638(1) Å, V
= 4447.31(5) Å3, Z = 8, rcalc = 1.447 gcm23, F(000) = 1952, m(Mo–Ka)
= 4.513 mm21, l = 0.71073 Å, T = 173 K. The 48726 reflections
measured on a Nonius Kappa CCD system yielded 4063 unique data (Qmax
= 25.4°, Rint = 0.033) [3617 observed reflections (I > 2s(I)]. R1 = 0.0208,
wR2 = 0.0499. CCDC reference number 200967. See http://www.rsc.org/
suppdata/cc/b2/b212754g/ for crystallographic data in cif or other electronic
format.
respectively.15 A considerably smaller deviation of the four-
membered metallacycle from planarity was also reported for
complex Cp2Y(m-CH3)2Al(CH3)2 (•C–Y–C–Al1 10–13°).16 In
complex 6a, the hydrogen atoms of the bridging methyl groups
were located and refined. The bridging 5-coordinate carbon
atoms display severely distorted trigonal bipyramidal geome-
tries with one hydrogen atom and the lutetium metal in the
apical positions (•Lu–C11–H11B 162(2)°). Moreover, one of
the equatorial hydrogen atoms each forms a close contact to the
lutetium centre (2.41(3)–2.46(4) Å) involving angles •Lu–C–
H as acute as 69(2)°. These solid-state structural features can be
interpreted in terms of Lu–H–C interactions, which, however,
cannot be observed by any spectroscopic method.
∑ An associative mechanism is also found by a variable temperature NMR
investigation of rac-[Me2Si(2-Me-C9H5)2]Y(AlMe4) in the temperature
range of 25 to 100 °C. Line shape treatment led to an entropy barrier DS‡
of 2130(8) J K21mol21 which is in agreement with a highly ordered
3
transition state favouring an h -bonded tetramethylaluminate moiety (cf.,
DS‡ = +123.1 J K21mol21 for Al2Me6).15
1 S. Arndt and J. Okuda, Chem. Rev., 2002, 102, 1953.
2 (a) H. Yasuda and E. Ihara, Bull. Chem. Soc. Jpn., 1997, 70, 1745; (b)
H. Yasuda, J. Polym. Sci. Part A: Polym. Chem., 2001, 39, 1955.
3 G. W. Coates, Chem. Rev., 2000, 100, 1223.
4 (a) T. Ishihara, T. Seimiga, M. Kuramoto and M. Uoi, Macromolecules,
1986, 19, 2464; (b) T. Ishihara, M. Kuramoto and M. Uoi, Macromole-
cules, 1988, 21, 3256.
5 For a further example, see: P. Foster, J. C. W. Chien and M. D. Rausch,
Organometallics, 1996, 15, 2404 and references therein.
6 (a) H. van der Heijden, C. J. Schaverien and A. G. Orpen,
Organometallics, 1989, 8, 255; (b) H. van der Heijden, P. Pasman, E. J.
M. de Boer, C. J. Schaverien and A. G. Orpen, Organometallics, 1989,
8, 1459; (c) H. J. Heeres, A. Meetsma, J. H. Teuben and R. D. Rogers,
Organometallics, 1989, 8, 2637; (d) W. T. Klooster, L. Brammer, C. J.
Schaverien and P. H. M. Budzelaar, J. Am. Chem. Soc., 1999, 121,
1381.
3
2
We have recently detected truly h -coordinating AlEt4
ligands in the solid state structure of homoleptic [Yb(AlEt4)2]n
involving the large Yb(II) centre.17 This previous structural
evidence combined with the highly fluxional behaviour of 6a in
2
3
2
solution, implicating transient h /h -coordinating AlMe4
moieties, gives striking evidence for an associative methyl
group exchange at sterically unsaturated rare-earth metal
18
centres (Scheme 2).∑ Note that intramolecular methyl group
exchange in trimethylaluminium, Al2Me6, occurs via a dis-
sociative mechanism.19
7 A. Mandel and J. Magull, Z. Anorg. Allg. Chem., 1996, 622, 1913.
8 W. J. Evans, J. C. Brady and J. W. Ziller, J. Am. Chem. Soc., 2001, 123,
7711.
9 (a) J. Guan, S. Jin, Y. Lin and Q. Shen, Organometallics, 1992, 11,
2483; (b) L. Mao, Q. Sheng and S. Jin, Polyhedron, 1994, 13, 1023.
10 R. Anwander, Top. Organomet. Chem., 1999, 2, 1.
11 (a) J. Eppinger, M. Spiegler, W. Hieringer, W. A. Herrmann and R.
Anwander, J. Am. Chem. Soc., 2000, 122, 3080; (b) M. G. Klimpel, W.
A. Herrmann and R. Anwander, Organometallics, 2000, 19, 4666.
12 M. G. Klimpel, H. W. Görlitzer, M. Tafipolsky, M. Spiegler, W. Scherer
and R. Anwander, J. Organomet. Chem., 2002, 647, 236.
13 M. Booij, N. H. Kiers, H. J. Heeres and J. H. Teuben, J. Organomet.
Chem., 1989, 364, 79.
14 J. Holton, M. F. Lappert, D. G. H. Ballard, R. Pearce, J. L. Atwood and
W. E. Hunter, J. Chem. Soc., Dalton Trans., 1979, 45.
15 P. L. Watson and G. W. Parshall, Acc. Chem. Res., 1985, 18, 51.
16 G. R. Scollary, Aust. J. Chem., 1978, 31, 411.
17 M. G. Klimpel, R. Anwander, M. Tafipolsky and W. Scherer,
Organometallics, 2001, 20, 3983.
18 Jörg Eppinger, Ph.D. Thesis, Technische Universität München, 1999.
19 M. E. O’Neill and K. Wade, Aluminum in Comprehensive Organome-
tallic Chemistry (eds. G. Wilkinson, F. G. A. Stone and E. W. Abel),
Pergamon Press, New York, 1982, p. 593.
Scheme 2 Proposed associative methyl group exchange in rare-earth
tetraalkylaluminate complexes.
We thank the Deutsche Forschungsgemeinschaft, the Fonds
der Chemischen Industrie, and Prof. Wolfgang A. Herrmann for
generous support. We are also grateful to Prof. Robert M.
Waymouth for his hospitality, technical assistance and stimulat-
ing discussions.
Notes and references
‡ All operations were performed with rigorous exclusion of air and water,
using high-vacuum and glovebox techniques. Representative synthesis for
4a and 6a: A solution of Cp*H (21.00 mmol, 2.861 g) in 10 mL of toluene
was added to a solution of 1b (7.00 mmol, 5.013 g) in toluene and refluxed
for 70 h. After removal of solvent and released silylamine, the residue was
CHEM. COMMUN., 2003, 1008–1009
1009