Trimethylyttrium and trimethyllutetium

Article abstract of DOI:10.1002/anie.200463109

(Chemical Equation Presented) Complexes in disguise: Homoleptic [Ln(AlMe₄)₃] (Ln = Y, Lu) complexes are effectively masked [LnMe₃]_n species, which are produced reversibly upon equimolar addition of a Lewis base to the former. Compounds [LnMe ₃]_n are thermally stable and readily form peralkylated heterobimetallic complexes with alkylaluminum, -gallium, and -magnesium reagents (see scheme).

Full text of DOI:10.1002/anie.200463109

Angewandte
Chemie
Organolanthanides
DOI: 10.1002/anie.200463109
Trimethylyttrium and Trimethyllutetium**
H. Martin Dietrich, Gabriele Raudaschl-Sieber, and
Reiner Anwander*
Permethylated transition-metal complexes, as represented by
neutral M(CH3)x and anionic [M(CH3)y]nꢀ, have attracted
considerable interest not only for displayingthe simplest truly
organometallic derivatives but also for their intrinsic bonding
phenomena.[1,2] While structural and theoretical investiga-
tions have led to enormous progress for thermally labile
homoleptic Group 4, 5, 6, and 7 derivatives, monometallic
Group 3 and lanthanide congeners have remained elusive.[2,3]
For example, the synthesis of rare-earth-metal–alkyl com-
plexes such as LnEt3 (Ln = Sc, Y) was claimed as early as
1938, however, strikingevidence of their existence, be it
spectroscopic data or clarifyingfollow-up chemistry, has
never been reported.[4] Moreover, the ease of b-hydride
elimination in such early transition-metal–alkyl complexes
led to controversial discussions.[5,6] Nevertheless, a consider-
able number of silylalkyl complexes includinghomoleptic,
donor-coordinated, donor-functionalized, and ate-type deriv-
atives have been unambiguously identified.[7–11] In the early
1980s, Schumann et al. reported the synthesis of thermally
stable ate complexes of the type [Li3(donor)3][Ln(CH3)6]
(donor= TMEDA (N,N,N’,N’-tetramethylethylenediamine),
DME (1,2-dimethoxyethane)), which featured an octahedral
coordination geometry, for the entire series of rare-earth
metals.[12] Approximately ten years later, Evans et al. de-
scribed permethylated derivatives [Nd(GaMe4)3] and [Ln-
(AlMe4)3] (Ln = Y, Nd, Sm) which can be viewed as
trimethylgallium and trimethylaluminum adduct complexes,
respectively, of the elusive LnMe3.[13,14] Meanwhile, hetero-
bimetallic derivatives [Ln{(m-Me)2AlMe2}3] are available for
the entire Group 3 and 4f series except for scandium and
promethium.[15,16] Herein, another ten years later, we describe
the synthesis and derivatization of homoleptic trimethyl-
yttrium and trimethyllutetium.
[*] H. M. Dietrich, Dr. R. Anwander
Department of Chemistry
University of Bergen
AllØgaten 41, 5007 Bergen (Norway)
Fax: (+47)5558-9490
E-mail: reiner.anwander@kj.uib.no
G. Raudaschl-Sieber
Anorganisch-chemisches Institut
Technische Universität München
Lichtenbergstraße 4, 85747 Garching (Germany)
[**] Financial support by the Deutsche Forschungsgemeinschaft (SPP
1166) and the Fonds der Chemischen Industrie is gratefully
acknowledged. We thank M. Barth and co-workers for performing
the elemental analyses.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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[22]
Donor (Do)-induced cleavage of tetraalkylaluminates
amongthe most volatile of rare-earth metal compounds.
(Do = THF, diethyl ether, pyridine) has been previously
applied to convert heteroleptic lanthanidocene and half-
lanthanidocene complexes, [Cp02Ln(AlR4)] and [Cp’Ln-
(AlR4)2] (Cp’ = substituted cyclopentadienyl), into the corre-
sponding s-bonded alkyl derivatives [Eqs. (1) and (2)].[17–20]
Such optimized reaction conditions were crucial for the
synthesis of pure [LuMe3]n (2b). The precursor [Lu(AlMe4)3]
(1b) was most efficiently separated from the coproduct of the
synthesis, [Me2Al(m-NMe2)], by fractional sublimation (ca.
908C at 10ꢀ3 mbar; > 20% yield).[23] (Caution! Compound 2b
which is also obtained quantitatively as a white powder like
the yttrium derivative 2a detonates spontaneously when
exposed to air; it also decomposes spontaneously at elevated
temperature under high vacuum (ca. 808C at 10ꢀ4 mbar) to
yield a black powder.) In addition to the C,H microanalytical
data, unambiguous evidence for the formation of compounds
2a,b stemmed from the complete reversibility of this donor-
induced cleavage reaction (Scheme 1).[18] That is, treatment of
the precipitated compounds 2 (non-isolated, as formed from
the initial reaction mixture) and likewise the isolated
compounds 2, which were resuspended in hexane, with
3 equivalents of AlMe3 both led to quantitative re-formation
of the homoleptic complexes 1. Visually, thus was
indicated by the immediate formation of a clear
solution.
2 ½Cp02YfAlðCH3Þ4gŠ þ 2 Do ! ½Cp20 Yðm-CH3ފ2 þ 2 AlðCH3Þ3ðDoÞ ð1Þ
3 ½Cp0YfAlðCH3Þ4g2Š þ 6 Do ! ½Cp0Yðm-CH3Þ2Š3 þ 6 AlðCH3Þ3ðDoÞ ð2Þ
We found that homoleptic [Y(AlMe4)3] (1a) and [Lu-
(AlMe4)3] (1b) are also amenable to this donor-induced
tetraalkylaluminate cleavage. Accordingly, addition of a
stoichiometric amount of THF (3 equiv) to a solution of 1a
in hexane at ambient temperature produced a white precip-
itate, which was insoluble in aliphatic and aromatic solvents
(Scheme 1). After several washings with hexane and drying
A uniform coordination environment at the
rare-earth-metal center in compounds 2 was con-
firmed by solid-state FTIR and MAS NMR spec-
troscopy. Nujol mulls of [LnMe3]n gave IR spectra
that displayed a characteristic vibration at n˜ = 1191
and 1202 cmꢀ1 for the yttrium and lutetium deriv-
atives, respectively, and a broad, intense low-
energy band featuring three maxima at n˜ = 522,
1
431, and 361 cmꢀ1 (2a). The H MAS NMR spec-
trum of 2a showed a broad resonance centered at
d = ꢀ0.3 ppm that exhibited two well-resolved
shoulders at around d = 0.1 and 1.0 ppm. The
13C CPMAS NMR spectra of compounds 2 are
shown in Figures 1a and b. For each, one signal was
detected for the methyl carbon atoms at d = 28.3
(2a) and 31.4 ppm (2b). For comparison, the
yttrium precursor complex 1a revealed two signals
at d ꢁ ꢀ10 and 18.0 ppm which can be assigned to
Y-CH3-Al bridging and Al-CH3 terminal methyl
ligands (Figure 1c), whereas the half-yttrocene
[Cp*Y(m-Me)2]3 (Cp* = C5Me5), which is discussed
later, exhibits 13C resonances at d = 11.9, 30.3, and
118.7 ppm that are attributed to carbon atoms of
CH3(Cp*) substituents, Y-CH3-Y bridging methyl
ligands, and the Cp* ring, respectively (Fig-
ure 1d).[19] Similar solid-state NMR chemical
shifts have been previously reported for the com-
Scheme 1. Synthesis and derivatization of [LnMe3]n (2). Detailed reaction conditions are
given in the Supporting Information (Cn=1,4,7-trimethyl-1,4,7-triazacyclononane).
*
plex [Cp2 Th(CH3)2] (Cp*: d = 123.1 and 12.0 ppm;
under high vacuum, the white powder was identified as
polymeric [YMe3]n (2a). Optimized conditions for this
cleavage reaction comprised the use of freshly sublimed 1a
as well as the less Lewis basic donor diethyl ether as a
cleavage reagent, and a low reaction temperature (ꢀ358C).[21]
1H NMR experiments in C6D6 confirmed Al(CH3)3(OEt2) as
the only soluble byproduct of the cleavage reaction (d =
ꢀ0.42 (s, 9H, AlCH3), 0.73 (t, 6H, CH3), 3.25 ppm (q, 4H,
OCH2)). Note that owingto its relatively low sublimation
point (808C at 10ꢀ3 mbar; > 90% yield), compound 1a is
Th-CH3(terminal): d = 68.4 ppm).[24] These findings clearly
indicate the formation of homoleptic complexes 2 (ether
donor ligands can be ruled out by the 13C CPMAS NMR
spectra) and to the absence of any methyl groups bonded to
aluminum centers.[25,26] Carbon resonances of “Al-CH3”
moieties were previously shown to appear upfield,[27] and
this is also corroborated by the spectrum of precursor
compound 1a (Figure 1c).
Preliminary investigations into the reaction behavior of
[LnMe3]n were performed usingthe yttrium derivative 2a
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ment of compound 2 with AlMe2Cl and documents the
comparatively higher halophilicity of the rare-earth elements.
Use of AlEt2Cl as a halogenating reagent afforded the mixed
alkylaluminum AlMeEt2 (7) as a coproduct.
In conclusion, we have shown that homoleptic rare-earth-
metal–methyl complexes can be obtained through donor-
induced
tetraalkylaluminate
cleavage
of
[Ln{(m-
Me)2AlMe2}3] (Ln = Y, Lu). Compounds [LnMe3]n are
highly condensed materials. Notably, those derived from the
smaller rare-earth-metal cations are stable at ambient tem-
perature and accessible to a variety of ligand-exchange and
addition reactions. Current investigations of [LnMe3]n com-
pounds are focused on 1) the accessibility of derivatives of the
large rare-earth-metal cations, 2) the feasibility of controlled
alkyl-elimination reactions, and 3) their potentials as cata-
lysts.
Experimental Section
2a: In a glovebox, a solution of diethyl ether (3 equiv; 222 mg,
3.00 mmol) in hexane (5 mL) at ꢀ358C was added under vigorous
Figure 1. 13C cross-polarization magic-angle-spinning (CPMAS) NMR
spectra of a) neat [YMe3]n (2a), b) neat [LuMe3]n (2b), c) neat
[Y(AlMe4)3] (1a), and d) neat [Cp*YMe2]3. Resonances due to C5Me5
stirringto a solution of Y(AlMe )3 (1a; 350 mg, 1.00 mmol) in cold
4
(ꢀ358C) hexane (10 mL). Instant formation of a white precipitate
occurred. After stirringthe reaction mixture for 10 min, the product
was separated by centrifugation and washed three times with hexane
to completely remove any soluble byproducts. After dryingfor 6 h
under high vacuum, [YMe3]n (2a) was obtained as a white powder in
quantitative yield (133 mg). IR (Nujol): n˜ = 1191 (vs), 522 (s), 431 (s),
361 (s), 320 cmꢀ1 (w); far-IR (polyethylene): n˜ = 186 cmꢀ1 (w);
1H MAS NMR (300.13 MHz, 258C): d = 1.0, 0.1, ꢀ0.3 ppm.
13C MAS NMR (75.46 MHz, 258C): d = 28.3 ppm; elemental analysis
(%): calcd for C3H9Y (134.010 gmolꢀ1): C 26.89, H 6.77; found: C
26.88, H 6.51.
Typical procedure for addition reactions, as shown for the
synthesis of 6: In a glovebox, a solution of GaMe3 (3 equiv; 128 mg,
1.12 mmol) in hexane (5 mL) cooled to ꢀ358C was added under
vigorous stirring to a suspension of [YMe3]n (2a, 50 mg, 0.37 mmol) in
hexane (5 mL) also cooled to ꢀ358C. The reaction mixture became
clear after 15 min stirring. After a further 15 min, the reaction mixture
was filtered, and the filtrate was retained. Evaporation of the solvent
in vacuo yielded compound 6 as a white crystalline product (169 mg,
95%). IR (Nujol): n˜ = 1302 (w), 1202 (s), 967 (w), 571 (s), 530 (s),
416 cmꢀ1 (m); 1H NMR (400 MHz, C6D6, 258C): d = ꢀ0.10 ppm;
13C{1H} NMR (100 MHz, C6D6, 258C): d = 4.0 ppm (v br); elemental
analysis (%): calcd for C12H36Ga3Y (478.492 gmolꢀ1): C 30.12, H 7.58;
found: C 31.30, H 7.34, N < 0.2.
(Cp*) are indicated by asterisks ( ).
*
(Scheme 1). Although the isolation of simple ether adduct
complexes, such as [YMe3(thf)x], was not successful, addition
of a stoichiometric amount of 1,4,7-trimethyl-1,4,7-triazacy-
clononane (Cn) to a suspension of [YMe3]n in hexane gave
[CnYMe3] (3) in almost quantitative yield.[28] The solid-state
13C NMR spectrum of complex 3 (not shown) was similar to
its spectrum in solution (C6D6) and gave signals at d = 55.2
(NCH2), 48.7 (NCH3), and 20.9 ppm (YCH3). In the presence
of TMEDA as an N-donor solvent, treatment of [YMe3]n with
3 equivalents of MeLi gave Schumannꢀs ate complex [Li3-
(tmeda)][Y(CH3)6] (4) in moderate yield.[12] Given the
“reversibility” of the formation of [LnMe3]n, we anticipated
that other strongLewis acids might redissolve [LnMe 3]n as
well. Accordingly, AlEt3 and GaMe3 form heterobimetallic/
heteroleptic [YAl3Me3Et9] (5) and heterobimetallic/homolep-
tic [YGa3Me12] (6) in almost quantitative yields
(Scheme 1).[29] Whereas compound 5 was obtained as an oily
residue, tetramethylgallate complex 6 yielded colorless crys-
tals from solutions in hexane. Complex 6 shows a very broad
1H NMR signal in C6D6 which is indicative of a decreased
fluxional behavior of the bridging and terminal methyl ligands
relative to tetramethylaluminate 2a.
Full experimental and physicochemical details for complexes 1–7
are available in the SupportingInformation.
Received: December 31, 2004
Revised: March 1, 2005
Published online: July 22, 2005
Instantaneous evolution of methane was observed when
compound [YMe3]n was treated with Brønsted acidic sub-
strates such as HN(SiMe3)2, HOCHtBu2, and HC5Me5
(HCp*) in hexane. The reactions of 3 equivalents of either
the silylamine or the alcohol with 2a gave the homoleptic
complexes [Y{N(SiMe3)2}3] and [Y(OCHtBu2)3], respectively,
in high yields (see Supporting Information).[30,31] Owingto
ligand scrambling, half-yttrocene complex [Cp*Y(m-Me)2]3
did not form selectively upon addition of 1 equivalent of
HCp* to compound 2a, however, its presence could be
Keywords: aluminum · lanthanides · NMR spectroscopy ·
.
yttrium
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Communications
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