"Metastable" Lu(GaMe4)3 reacts like masked [LuMe3]: Synthesis of an unsolvated lanthanide dimethyl complex

Article abstract of DOI:10.1021/om900838h

Homoleptic tetramethylgallate Lu(GaMe₄)₃ reacts selectively with superbulky (Tp^tBu,Me)H according to a methane elimination reaction, affording the quantitative formation of monomeric base-free low-coordinate (T_p

Full text of DOI:10.1021/om900838h

6
646 Organometallics 2009, 28, 6646–6649
DOI: 10.1021/om900838h
“
Metastable” Lu(GaMe ) Reacts Like Masked [LuMe ]: Synthesis of an
4 3 3
Unsolvated Lanthanide Dimethyl Complex
†
†
†
,‡
Melanie Zimmermann, Rannveig Litlabø, Karl W. T o€ rnroos, and Reiner Anwander*
†
‡
Department of Chemistry, University of Bergen, All eꢀ gaten 41, 5007 Bergen, Norway, and Institut f u€ r
Anorganische Chemie, Universit a€ t T u€ bingen, Auf der Morgenstelle 18, 72076 T u€ bingen, Germany
Received September 29, 2009
Summary: Homoleptic tetramethylgallate Lu(GaMe ) reacts
4
[LnMe(thf) ][BAr ] , which contain a nucleophilic methyl
4 2
3
n
tBu,Me
6
selectively with superbulky (Tp
elimination reaction, affording the quantitative formation of
)H according to a methane
group at a mildly Lewis acidic metal center. The permethy-
lated heterobimetallic derivatives Ln(AlMe ) (Ln = Y, Nd,
Sm), also viewed as the trimethylaluminum adducts of
4
3
tBu,Me
monomeric base-free low-coordinate (Tp
)LuMe . Weak
2
7
a,b
interaction of the strongly basic lanthanide methyl groups with
the comparatively weak Lewis acid trimethylgallium in Lu-
[
LnMe ], were first reported by Evans et al.
3
Not only
has the original synthesis been extended to encompass
(
GaMe ) is indicated by their separation into [LuMe ] and
3 n
3þ
size range, but donor-induced (Et O,
2
7c
4
3
the entire Ln
thf) aluminate cleavage of Ln(AlMe ) also allowed for the
synthesis of until then elusive homoleptic trimethylyttrium
GaMe even at -35 °C.
3
4
3
Low-coordinate rare-earth metal alkyl complexes free
from incorporated salts or coordinating solvents have been
the subject of intensive research activity over the past
8
and trimethyllutetium. As implied by the small size of
the methyl ligand, [LnMe3]n form insoluble polymeric
1
materials for the smaller rare-earth metal centers. On the
contrary, donor-cleavage reaction of La(AlMe ) , represent-
ing a large rare-earth metal center, leads to extensive methyl
decades. Particularly, unsolvated methyl complexes have
attracted considerable interest and are classified among the
4 3
2-4
most reactive organolanthanide complexes.
The large
9
group degradation and cluster formation. Indeed, salt
ionic radii of the rare-earth metals paired with the small size
and enhanced basicity of the methyl ligand clearly challenge
the synthesis of [Ln-Me] species. Thus, the accessibility of
discrete methyl complexes (L)LnMe or (L) LnMe (L =
incorporation, retained donor molecules, oligomerization,
and complex degradation are long-standing pitfalls asso-
ciated with the synthesis of rare-earth metal methyl com-
plexes. Consequently, crystallographically authenticated
2
2
monoanionic ligand) depends crucially on the availability of
suitable Ln precursors, the choice of a chemically innocent
ancillary ligand L, and efficient multistep reaction se-
(L)LnMe derivatives are only known for the smallest rare-
2
earth metal center scandium. The β-diketiminato (“nacnac”)
scaffold has been firmly established as a stabilizing environ-
ment for neutral scandium dimethyl complexes as well
3
quences. In the early 1980s, trianionic hexamethylate com-
0
0
plexes [Ln{(μ-Me) Li(donor)} ] (donor = N,N,N ,N -tetra-
2
3
1
0,11
as monocationic methyl species thereof.
phosphinimine ligands and a tridentate [PNP ] ligand
Lately, anilido-
-
methylethylenediamine, tmeda; 1,2-dimethoxyethane, dme)
were reported by Schumann et al. for the entire series of
5
rare-earth metals. They have, however, not been considered
as a synthesis equivalent for the elusive [LnMe ]. Very
3
(
6) (a) Arndt, S.; Beckerle, K.; Zeimentz, P. M.; Spaniol, T. P.;
recently, related hexamethylate complexes [Li (thf) Ln-
3
n
Okuda, J. Angew. Chem. Int. Ed. 2005, 44, 7473. (b) Kramer, M. U.;
Robert, D.; Arndt, S.; Zeimentz, P. M.; Spaniol, T. P.; Yahia, A.; Maron, L.;
Eisenstein, O.; Okuda, J. Inorg. Chem. 2008, 47, 9265.
Me ] were prepared in the group of Okuda, giving access
6
to a series of thf-solvated rare-earth metal methyl dications
(
7) (a) Evans, W. J.; Anwander, R.; Ziller, J. W. Organometallics
1
995, 14, 1107. (b) Klooster, W. T.; Lu, R. S.; Anwander, R.; Evans, W. J.;
*
To whom correspondence should be addressed. E-mail: reiner.
anwander@uni-tuebingen.de. Fax: (þ49) 7071 292436.
1) Edelmann, F. T. In Comprehensive Organometallic Chemistry III,
Vol. 4.01; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; pp
Koetzle, T. F.; Bau, R. Angew. Chem. Int. Ed. 1998, 37, 1268. (c)
Zimmermann, M.; Frøystein, N. Å.; Fischbach, A.; Sirsch, P.; Dietrich, H.
M.; T €o rnroos, K. W.; Herdtweck, E.; Anwander, R. Chem.;Eur. J. 2007,
13, 8784.
(
1
-199.
(8) Dietrich, H. M.; Raudaschl-Sieber, G.; Anwander, R. Angew.
Chem. Int. Ed. 2005, 44, 5303.
(
2) (a) Davidson, P. J.; Lappert, M. F.; Pearce, R. Chem. Rev. 1976,
7
6
6, 219. (b) Ziegler, T.; Folga, E.; Berces, A. J. J. Am. Chem. Soc. 1993, 115,
36.
(3) (a) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (b) Watson, P.
L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (c) Thompson, M. E.;
Bercaw, J. E. Pure Appl. Chem. 1984, 56, 1. (d) Thompson, M. E.; Baxter, S.
M.; Bulls, A. R.; Burger, J. B.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W.
P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (e) Sadow, A. D.; Tilley,
T. D. J. Am. Chem. Soc. 2003, 125, 7971. (f) Evans, W. J.; Perotti, J. M.;
Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894.
(
4) Dietrich, H. M.; Grove, H.; T o€ rnroos, K. W.; Anwander, R. J.
Am. Chem. Soc. 2006, 128, 1458.
5) (a) Schumann, H.; M u€ ller, J. Angew. Chem., Int. Ed. Engl. 1978,
7, 276. (b) Schumann, H.; Pickardt, J.; Bruncks, N. Angew. Chem., Int. Ed.
(
1
Engl. 1981, 20, 120. (c) Schumann, H.; M €u ller, J.; Bruncks, N.; Lauke, H.;
Pickardt, J.; Schwarz, H.; Eckart, K. Organometallics 1984, 3, 69. (d)
Schumann, H.; Lauke, H.; Hahn, E.; Pickardt, J. J. Organomet. Chem.
1
984, 263, 29.
pubs.acs.org/Organometallics
Published on Web 11/18/2009
r 2009 American Chemical Society

Communication
Organometallics, Vol. 28, No. 23, 2009 6647
Scheme 1. Synthesis of Lu(GaMe
4
)
3
(1): (a) [Amide] f [Gallate]
route albeit being hampered by the comparable solubility of
2
0
Transformation; (b) GaMe Adduct Formation
3
colorless crystalline 1 and (Me GaNMe ) .
2 2 2
Very pure Lu(GaMe ) , in almost quantitative yield, could
3
4
be obtained by adding a slight excess of GaMe to a suspen-
3
8
sion of polymeric [LuMe ] in hexane (Scheme 1, b). The
3
n
nearly stoichiometric use of expensive trimethylgallium and
the avoidance of undesired gallium-containing byproducts
are clearly favorable attributes. However, the applicability of
this synthesis approach is so far limited to the smaller rare-
8
earth metal centers.
Homoleptic tris(tetramethylgallate) complex 1 is soluble
in hydrocarbon and aromatic solvents. In the absence of
excess GaMe , precipitation of [LnMe ] (accompanied by
3
3 n
the release of GaMe ) immediately occurs in hexane or
3
toluene solutions of Lu(GaMe ) at ambient temperature.
4
3
-
(PNP = N[2-P(CHMe ) -4-methylphenyl] ) were reported
to support monomeric, base-free dimethylscandium com-
2 2 2
Even at -35 °C metal alkyl separation is observed within a
day. This clearly reflects the weak interaction of the
21
1
2-14
plexes.
Trofimenko’s tris(pyrazolyl)borate ligands (Tp
vide a uniquely stabilizing ligand environment for low-
strongly basic lanthanide methyl groups with the compara-
tively weak Lewis acid trimethylgallium.
Single crystals of the homoleptic lutetium tetramethyl-
R,R’ 15
)
pro-
coordinate complexes (L)LnR , as shown by Piers, Takats,
2
gallate were grown from a saturated hexane/GaMe solution
3
tBu,Me
16
)Ln(CH SiMe ) (Ln = Sc, Y) and
and us for (Tp
tBu,Me
(Tp
sent a study of homoleptic lutetium tris(tetramethylgallate),
emphasizing its propensity to act as a [LuMe ] synthon.
3
Utilizing the weak coordination of GaMe in the Lu(GaMe )
3
22
at -35 °C. The X-ray crystallographic structure determi-
2
3 2
17
)Ln(Me)(AlMe ) (Ln = Y, Lu). Herein we pre-
4
nation revealed the 2:1 inclusion complex [{Lu(GaMe ) }
4
3 2
3
(GaMe )] (1 ). As shown in Figure 1, GaMe has been
3 crystal 3
trapped in the crystal lattice, sandwiched between two
molecules of homoleptic Lu(GaMe ) . For homoleptic tetra-
methylaluminates inclusion of Al Me in the crystalline
4
4
3
tBu,Me
moiety and superbulky [Tp
dimethyl lutetium complex has been isolated.
], a monomeric base-free
2
6
lattice has been reported; however, no interaction with
neighboring Ln(AlMe ) molecules was observed (Ln = Y,
7a
Nd).
7
Like the homologous aluminate compound, Lu(Ga-
c
4
3
Me ) (1) was obtained by the original GaMe -mediated
4
3
3
1
8
[NMe ] f [GaMe ] ligand exchange reaction. Accordingly,
2 4
treatment of a suspension of [Lu(NMe ) (LiCl) ] in hexane
with excess GaMe (>6 equiv) yielded 1, (Me GaNMe ) ,
2
and LiCl (Scheme 1, a). While the ligand transformation is
clearly driven by the thermodynamic stability of the alky-
lated gallium amide byproduct, its separation from the
desired Lu(GaMe ) is delicate. The decreased volatility of
Me GaNMe ) compared with the corresponding amino-
2 2 2
aluminum compound prevents the separation in vacuo (10
Torr). Fractional crystallization was found to be a viable
The GaMe moiety is planar (sum of angles about Ga7
3
2
3
3
359.98°) and weakly coordinated to two terminal methyl
groups of neighboring tetramethylgallate ligands, with a
1
9
3
2 2
˚
Ga7 C3 distance of 3.225(3) A and a Ga7 C16 distance
3
3 3
3 3 3
˚
of 3.217(3) A. The Ga7 H separations range from 2.70
3
3 3
˚
to 3.44 A. Similar weak contacts between GaMe molecules
3
4
3
were previously found in the solid-state structures of
trimethylgallium, causing the formation of infinite GaMe3
(
-
2
23
2
shorter than in gaseous GaMe (1.967(2) A), showing that
D networks. The Ga7-C bonds are only marginally
˚
3
the intermolecular interactions do not strongly distort the
2
4
structure of the sandwiched GaMe3. The Lu-C(μ-Me)
(
12) Conroy, K. D.; Piers, W. E.; Parvez, M. J. Organomet. Chem.
008, 693, 834.
13) Scott, J.; Fan, H.; Wicker, B. F.; Fout, A. R.; Baik, M.-H.;
Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 14439.
bond lengths of the Lu(GaMe4)3 moieties (2.465(2)-
2
(
2
(
˚
.493(2) A) are slightly longer than those observed for Lu-
˚
AlMe ) (2.455(2)-2.471(2) A), while the Lu Ga dis-
-
4 3
3 3 3
(
14) Another [PNP ] ligand was employed to stabilize a monomeric
unsolvated scandium diethyl complex, [N(SiMe CH PiPr ]ScEt
Fryzuk, M. D.; Giesbrecht, G.; Rettig, S. J. Organometallics 1996, 15,
329.
15) Trofimenko, S. Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999.
16) (a) Blackwell, J.; Lehr, C.; Sun, Y.; Piers, W. E.; Pearce-
Batchilder, S. D.; Zaworotko, M. J.; Young, V. G. Jr. Can. J. Chem.
997, 75, 702. (b) Cheng, J.; Saliu, K.; Kiel, G. Y.; Ferguson, M. J.;
McDonald, R.; Takats, J. Angew. Chem. Int. Ed. 2008, 47, 4910.
17) (a) Zimmermann, M.; Takats, J.; Kiel, G.; T o€ rnroos, K. W.;
˚
tances (2.9883(3)-3.0018(3) A) are marginally shortened
2
2
2
)
2
2
:
3
(
3 n 4 3
(21) The formation of insoluble [LnMe ] from Lu(GaMe ) is
further observed in the solid state shortly after the mother liquor is
taken off the crystalline material. The previously clear crystals become
(
opaque and brittle within minutes. Crystals of Lu(GaMe
ever, be stored under their mother liquor with some drops of added
GaMe
at -35 °C for weeks.
(22) Compound 1crystal ((C12
crystallizes from a hexane/GaMe
4 3
) can, how-
1
3
(
H
36Ga
3
Lu)
2
ꢀ(C
˚
3 9 r
H Ga), M = 1243.90)
Anwander, R. Chem. Commun. 2008, 612. (b) Litlabø, R.; Zimmermann,
M.; Saliu, K.; Takats, J.; T €o rnroos, K. W.; Anwander, R. Angew. Chem. Int.
Ed. 2008, 47, 9560.
3
mixture in the triclinic space group
˚
˚
P1 with a = 7.2253(2) A, b = 17.3978(5) A, c = 18.6916(6) A, R =
77.626(1)°, β = 86.399(1)°, γ = 85.495(1)°, V = 2285.39(12) A , and
dcalc = 1.808 g cm for Z = 2. Data were collected at 100(2) K on a
3
˚
-
3
(
18) Evans, W. J.; Anwander, R.; Doedens, R. J.; Ziller, J. W. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1641.
19) (a) Coates, G. E. J. Chem. Soc. 1951, 2003. (b) Beachley, O. T.;
Bruker-AXS 2K CCD system. The structure was solved by direct
methods, and least-squares refinement of the model based on 13 491
(all data) and 12 272 reflections (I > 2.0σ(I)) converged to a final wR2 =
0.0477 and R1 = 0.0193, respectively.
(23) (a) Mitzel, N. W.; Lustig, C.; Berger, R. J. F.; Runeberg, N.
Angew. Chem. Int. Ed. 2002, 41, 2519. (b) Boese, R.; Downs, A. J.; Greene,
T. M.; Hall, A. W.; Morrison, C. A.; Parson, S. Organometallics 2003, 22,
2450.
(
Coates, G. E.; Kohnstam, G. J. Chem. Soc. 1965, 3248. (c) Schauer, S. J.;
Lake, C. H.; Watkins, C. L.; Krannich, L. K.; Powell, D. H. J. Organomet.
Chem. 1997, 549, 31.
(
20) The GaMe
proceed as smoothly as with the more Lewis acidic AlMe
of GaMe has to be used to avoid incomplete ligand exchange, as
evidenced earlier crystallographically with Nd(NMe (GaMe and
La(NMe (GaMe (GaMe ), ref 18.
3
-mediated [NMe
2
] f [GaMe
4
] exchange does not
3
; a large excess
3
2
)
3
3
)
3
(24) McGrady, G. S.; Turner, J. F. C.; Ibberson, R. M.; Prager, M.
Organometallics 2000, 19, 4398.
2
)
2
3
)
2
4

6
648 Organometallics, Vol. 28, No. 23, 2009
Zimmermann et al.
tBu,Me
Scheme 2. Synthesis of (Tp
“
)LuMe
Alkylgallate Route”
2
(3) According to the
2
8
Figure 1. Molecular structure of 1crystal ([{Lu(GaMe4)3}2 (Ga-
3
Me
Hydrogen atoms are omitted for clarity. Selected bond distances
3
)]) (atomic displacement parameters set at the 50% level).
˚
[
A] and angles [deg]: Lu1-C1 2.488(2), Lu1-C2 2.480(2),
Lu1-C5 2.483(2), Lu1-C6 2.465(2), Lu1-C9 2.479(2),
Lu1-C10 2.480(2), Lu2-C13 2.493(2), Lu2-C14 2.476(2),
Lu2-C17 2.477(2), Lu2-C18 2.475(2), Lu2-C21 2.480(2),
8,26
presence of donor solvents is not essential. The reverse
reaction of [LuMe ] with 3 equiv of GaMe quantitatively
Lu2-C22 2.479(2), Lu1 Ga1 2.9939(3), Lu1 Ga2 2.9883-
3
3 3
3 3 3
3
n
3
(
3), Lu1 Ga3 3.0010(3), Lu2 Ga4 3.0018(3), Lu2 Ga5
27
yields 1 (Scheme 1, b). (CAUTION: Volatiles containing
3
3 3 3 3 3 3 3 3
2
1
3
.9924(3), Lu2 Ga6 2.9986(3), Ga7-C25 1.960(3), Ga7-C26
3 3 3
trimethylgallium react violently when exposed to air).
tBu,Me
Lu(GaMe ) (1) dissolved in hexane reacts with (Tp
.963(4), Ga7-C27 1.954(3), Ga7 C3 3.225(3), Ga7 C16
3 3 3 3 3
3
)H
4
3
.217(3), C1-Lu1-C2 88.41(8), C5-Lu1-C6 88.89(8),
according to an alkane elimination reaction, as evidenced
C9-Lu1-C10 88.28(8), C13-Lu2-C14 87.98(8), C17-Lu2-
C18 88.91(8), C21-Lu2-C22 88.27(8), C25-Ga7-C26
2
8
by instant methane evolution (Scheme 2). Concentration of
the reaction mixture under reduced pressure resulted in the
formation of a white precipitate, which upon separation and
1
18.24(16), C25-Ga7-C27 121.14(19), C26-Ga7-C27 120.6-
(
2), C3
3 3 Ga7 3 3 3 C16 170.68(6).
3
evaporation of the volatiles afforded a powdery white com-
tBu,Me
7
c
˚
pound identified as (Tp
of [LuMe ] in aliphatic solvents, direct reaction with
)LuMe (3). Due to the insolubility
2
(
Lu Al 3.0062(8)-3.0176(5) A). This shortening is
3
3 3
further reflected in less acute C-Lu-C angles. However,
these features are not as pronounced as in the respective
3 n
tBu,Me
tBu,Me
(Tp
)H in hexane to form (Tp
)LuMe (3) does not
2
29 1
13
1
8,25
occur. H NMR and C NMR spectra of 3 at ambient
temperature show only resonances for equivalent pyrazolyl
groups. The resonance assignable to the two methyl ligands
appears as a narrow singlet at -0.08 ppm, indicating the highly
yttrium and neodymium congeners.
Samples for the NMR spectroscopic investigation of Lu-
(
GaMe ) (1) were prepared by removing the supernatant of
4
3
crystalline 1crystal followed by drying in vacuo for a short time
CAUTION: Extended drying in vacuo leads to decomposi-
1
fluxional nature of complex 3. Variable-temperature H NMR
(
tion of 1). The H NMR and C NMR spectra of 1
1
13
spectroscopy of 3 in toluene-d revealed beginning decoale-
8
scence of the methyl signal only at temperatures below -90 °C.
The X-ray structure analysis performed on colorless single
crystals grown from a saturated toluene solution revealed a
monomeric lutetium complex featuring two terminal methyl
in benzene-d show only one narrow signal at -0.15 ppm
Δν1/2 = 8 Hz) and 1.3 ppm (Δν1/2 = 4 Hz), respectively,
indicative of a very fast exchange of bridging and terminal
6
(
methyl groups. Dissolving of Lu(GaMe ) in benzene-d
6
4
3
30
tBu,Me
ligands (Figure 2). To the best of our knowledge (Tp )-
LuMe is the first example of a structurally characterized
leads to the immediate formation of a [LnMe ] precipitate
3
n
and GaMe . A rapid dissociation-association equilibrium
2
3
between solid [LuMe ] , GaMe , and Lu(GaMe ) must be
anticipated, since no separate GaMe signal or significant
3
n
3
4 3
3
(
for the isolation of extremely pure [LuMe ] , as monitored by elemental
27) Repeated cycles of [LuMe
3 n 4 3 3 n
] f Ln(GaMe ) f [LuMe ] allow
line broadening is observed in the NMR spectra (see Sup-
porting Information, Figure S1). Considering the weak
interaction of GaMe with the [LuMe] moiety, compound
3
n
analysis after every cycle.
28) Dietrich, H. M.; T o€ rnroos, K. W.; Herdtweck, E.; Anwander, R.
Organometallics 2009, 28, DOI: 10.1021/om9007168.
(
3
1
tively low stability of [(LuMe )(GaMe ) ] can be utilized to
is best accounted for as [(LuMe )(GaMe ) ]. The compara-
tBu,Me
3
3 3
(29) Performing the [LuMe ] -(Tp
)H reaction in aromatic
3
n
solvents results in the formation of a multitude of products, which could
not be identified. Initial C-H activation reactions followed by a cascade
of degradation reactions are anticipated.
3
3 3
form very pure [LuMe ] by simply drying solutions thereof
3
n
in vacuo (Scheme 1, b). Contrary to the cleavage reaction
reported for rare-earth metal tetramethylaluminates, the
(
30) Compound 3 (C26
˚
toluene in the orthorhombic space group Pnma with a = 18.0766(6) A,
6 r
H46BLuN , M = 628.47) crystallizes from
3
˚
˚
˚
b=16.7735(6) A, c=9.8459(4) A, V = 2985.35(19) A , and dcalc = 1.398
-3
g cm for Z = 4. Data were collected at 123(2) K on a Bruker-AXS 2K
CCD system. The structure was solved by direct methods, and least-
squares refinement of the model based on 4553 (all data) and 4165
reflections (I > 2.0σ(I)) converged to a final wR2 = 0.0461 and R1 =
0.0175, respectively.
(
25) Dietrich, H. M.; Meermann, C.; T o€ rnroos, K. W.; Anwander, R.
Organometallics 2006, 25, 4316.
26) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.;
Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54.
(

Communication
Organometallics, Vol. 28, No. 23, 2009 6649
tBu,Me
Owing to the steric protection by the bulky Tp ligand,
the terminal methyl groups are kinetically protected and
3
complex 3 shows remarkable thermal stability. For com-
2
tBu,Me
parison, the aluminum congener (Tp
)Lu(Me)(AlMe4)
was obtained in ca. 61% only due to the simultaneous
formation of significant amounts of byproduct, most likely
originating from organoaluminum-assisted C-H bond activa-
tBu,Me
þ
tion, as proven in the case of yttrium for {(Tp
)AlMe} -
-
17a
{
Y(AlMe )[(μ-CH )(μ-Me)AlMe ] (AlMe )} . It has been
4 2 2 2 2
shown previously that Lu-AlMe moieties also willingly en-
4
gage in C-H abstraction reactions at ancillary ligand alkyl
3
3
groups. The absence of any Lu(III) f Ga(III) Tp ligand
transfer further corroborates the selectivity of the “alkylgallate”
route shown in Scheme 2.
In summary, we have presented two complementary proto-
cols for the synthesis of homoleptic rare-earth metal tetra-
methylgallates. The weak interaction of GaMe with the
3
Figure 2. Molecular structure of 3 (atomic displacement para-
meters set at the 50% level). Hydrogen atoms except for H1B are
˚
omitted for clarity. Selected bond distances [A] and angles [deg]:
strongly basic lanthanide-methyl groups leads to an equilib-
rium in solution between [LnMe ] , GaMe , and Ln-
3
n
3
(GaMe ) , which can be completely shifted toward the
formation of [LnMe ] by removing GaMe in vacuo. Taking
0
4 3
Lu-C1 2.364(3), Lu-C2 2.375(2), Lu-N2 2.3388(15), Lu-N2
3
n
3
2
.3389(15), Lu-N4 2.483(2), C1-Lu-C2 95.55(9), B-Lu-C1
58.80(8), B-Lu-C2 105.65(8), N4-Lu-C2 155.56(8) [symm.
advantage of the “metastability” of [Lu(μ-Me) GaMe ]
x
y
1
moieties (x þ y = 4);originating from the enhanced
code: x, -yþ3/2, z].
monomeric, unsolvated, and salt-free lanthanide dimethyl com-
-
covalency of GaMe and/or decreased basicity of “GaMe
3
”
4
compared to the aluminum analogues;we prepared a dis-
crete, base- and salt-free lutetium complex with two terminal
methyl ligands. Preliminary reactivity investigations re-
vealed highly efficient methylation of carbonylic functional-
ities and formation of catalytically active mono(cationic)
species. Ongoing studies are aimed at exploiting the general
applicability of Ln(GaR ) as lanthanide alkyl/methyl pre-
pound. In the solid state, complex 3 adopts a distorted trigonal-
0
bipyramidal coordination geometry, with N2, N2 , and C1
occupying the equatorial positions, and N4 and C2 the apical
sites. The apical Lu-N bond is elongated compared to the
equatorial metal-nitrogen bonds. A strong interaction of the
terminal methyl ligands with the five-coordinate lutetium metal
center is illustrated by the short Lu-C(Me) bonds of 2.364(3)
4
3
tBu,Me
cursors and the potential of (Tp
activation reactions.
)LuMe in C-H bond
2
˚
and 2.375(2) A. For comparison, the Lu-C(Meterminal) dis-
tances in dimeric [(C Me ) Lu(μ-Me)Lu(Me)(C Me ) ]
5
5 2
5
5 2
˚
2.424(3) A) and in ionic [(C Me )LuMe ][Li(tmeda) ] (av
5 5 3 2
(
Acknowledgment. Financial support from the Norwegian
Research Council (Project No. 185206/V30) is gratefully
acknowledged. We thank Joe Takats for many helpful and
stimulating discussions.
3f,31
˚
2
.519 A) are significantly longer.
Monitoring the reaction between Lu(GaMe ) (1) and
4
3
tBu,Me
1
(Tp
vealed the initial formation of (Tp
)H by H NMR spectroscopy in benzene-d re-
6
tBu,Me
)Lu(Me)(GaMe ) (2)
4
1
Scheme 2; for H NMR spectrum see Supporting Informa-
(
tion, Scheme S2), the gallate analogue to the earlier reported
Supporting Information Available: Text and figures giving
detailed procedures for the synthesis and characterization of com-
plexes 1-3 and CIF files giving full crystallographic data for
complexes 1 and 3. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data of 1 and 3
have also been deposited at the Cambridge Crystallographic Data
Centre under CCDC 746740 and 746741 and can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif.
tBu,Me
17a
(Tp
ing species appears to be stable, even at elevated tempera-
)Lu(Me)(AlMe4).
While the aluminum-contain-
ture, complex 2 slowly releases GaMe under formation of
3
dimethyl complex 3. The tetramethylgallate cleavage can
further be triggered by applying vacuum, which leads to the
tBu,Me
very clean and quantitative formation of (Tp
)LuMe2.
Remarkably, the cleavage reaction does not require the
presence of donor molecules and thus prevents the formation
of solvated complexes or (solvent) degradation reactions.
Me,Me
Me,
Me(32) Complex (Tp
)YMe (thf) supported by the less bulky Tp
2
1
9
ligand was observed only in the H NMR spectrum and reported to be
thermally unstable even at low temperatures: Long, D. P.; Bianconi, P.
A. J. Am. Chem. Soc. 1996, 118, 12453.
(33) Zimmermann, M.; Estler, F.; Herdtweck, E.; T o€ rnroos, K. W.;
Anwander, R. Organometallics 2007, 26, 6029.
(31) Schumann, H.; Albrecht, I.; Pickardt, J.; Hahn, E. J. Organomet.
Chem. 1984, 276, C5.