Reaction of (C5Me5)2Yb(ER)(NH3) with Me3Al. Formation of (C5Me5)2Yb(AlMe4) and [(C5Me5)2Yb(ER)(AlMe3)2</sub

Article abstract of DOI:10.1021/om020472h

Addition of 3 molar equiv of AlMe₃ to (Me₅C₅)₂Yb(ER)(NH₃) gives two types of products depending on the identity of the ER group. When ER is OSiMe₃ or TePh, the isolated metallocene is [(Me<

Full text of DOI:10.1021/om020472h

Organometallics 2003, 22, 627-632
627
Articles
Rea ction of (C5Me5)2Yb(ER)(NH3) w ith Me3Al. F or m a tion
of (C5Me5)2Yb(AlMe4) a n d [(C5Me5)2Yb(ER)(AlMe3)2]2
David J . Berg and Richard A. Andersen*
Department of Chemistry and Chemical Science Division of Lawrence Berkeley
National Laboratory, University of California, Berkeley, California 94720
Received J une 17, 2002
Addition of 3 molar equiv of AlMe3 to (Me5C5)2Yb(ER)(NH3) gives two types of products
depending on the identity of the ER group. When ER is OSiMe3 or TePh, the isolated metallo-
cene is [(Me5C5)2Yb(Me4Al)]2, which exists as an equilibrium between monomeric and dimeric
forms in toluene solution. When ER is OCMe3, SPh, S-p-tolyl, or SePh, the isolated metallo-
1
cenes have the stoichiometry (Me5C5)2Yb(ER)(Me3Al)2. The H NMR spectra of these mole-
cules show that several species are present in solution. The crystal structure of the S-p-
tolyl derivative shows that two (Me5C5)2Yb fragments are bridged by two Me3Al(S-p-tolyl)-
AlMe3 units by way of nearly linear Yb‚‚‚H3C-Al bonds.
In tr od u ction
and Al centers. Addition of a stoichiometric amount of
pyridine in toluene results in the formation of sparingly
soluble (C5H5)2YbMe and soluble Me3Al(py). The low
solubility of the ytterbium complex is apparently the
key to the success of these synthetic routes.
The synthetic methods used to prepare the base-free
ytterbocene methyl derivatives Cp′2YbMe, where Cp′
represents either C5H5 or a substituted cyclopentadienyl
group, depends on the nature of the starting materials,
the substituents on the cyclopentadienyl ligand, the
methyl transfer reagent, and the solvent. This complex-
ity is not apparent from the synthesis of the simplest
methyl derivative, (C5H5)2YbMe, which was described
28 years ago.1 In this synthesis (C5H5)2YbCl, which is
a chloride-bridged dimer in the solid state, is allowed
to react with methyllithium in a mixture of tetrahydro-
furan-diethyl ether solvent.2 The orange (C5H5)2YbMe
is sparingly soluble in aromatic hydrocarbons but the
degree of association in solution was not determined.
Evans reproduced this synthetic route and extended it
to the C5H4Me derivatives.3,4 These authors noted that
careful attention to experimental details is important
in order to get pure material. An alternate synthetic
route to (C5H5)2YbMe involving the cleavage of the
tetramethylaluminate, (C5H5)2Yb(µ-Me)2AlMe2, was de-
veloped by Lappert. The base-free tetramethylaluminate
is prepared from the reaction of LiAlMe4 with (C5H5)2-
YbCl in toluene, even though both reagents are very
sparingly soluble in that solvent.5 The tetramethyl-
aluminate is a monomer in the solid state and the
structure shows two methyl groups that bridge the Yb
(C5H5)2YbMe crystallizes from warm toluene and it
is a dimer in the solid state with bridging methyl
groups.6 The geometry of the bridging methyl groups is
similar to those in Me2Al(µ-Me)2AlMe2. The t-butyl sub-
stituted derivative, (C5H4CMe3)2YbMe, was also pre-
pared by addition of MeLi to the chloride in a tetrahy-
drofuran-diethyl ether mixed solvent system and the
base-free methyl complex was crystallized from toluene.
This complex is also assumed to be dimeric because a
dimeric molecular ion was observed in the mass spec-
trum.7 The related cerium methyl complex is a dimer
in the solid state and the structure consists of two metal-
locene fragments bridged by two methyl groups, analo-
gous to the ytterbium methyl complex mentioned above.8
When synthetic routes similar to those outlined above
are applied to the pentamethylcyclopentadienyl deriva-
tives, the base-free methyl derivatives are not obtained.
Thus, addition of 1 or 2 molar equiv of MeLi to (Me5C5)2-
YbCl2Li(OEt2)2 in diethyl ether gives the anionic deriva-
tives (Me5C5)2Yb(Cl)(Me)Li(OEt2)2 or (Me5C5)2Yb(Me)2Li-
(OEt2)2, respectively. When the reaction solvent is
tetrahydrofuran, (Me5C5)2Yb(Me)(THF) is isolated.9 The
inability to isolate the base-free (Me5C5)2YbMe is sur-
prising since the corresponding lutetium methyl com-
* Address correspondence to this author at the Department of
Chemistry, University of California, Berkeley, CA 94720. E-mail:
raandersen@lbl.gov.
(1) Ely, N. M.; Tsutsui, M. Inorg. Chem. 1975, 14, 2680.
(2) Lueken, H.; Schmitz, J .; Lamberts, W.; Hannibal, P.; Handrick,
P. Inorg. Chim. Acta 1989, 156, 119.
(3) Evans, W. J .; Dominguez, R.; Hanusa, T. P. Organometallics
1986, 5, 263.
(4) Zinnen, H. A.; Pluth, J . J .; Evans, W. J . J . Chem. Soc., Chem.
Commun. 1980, 810.
(5) Holton, J .; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood
J . L.; Hunter, W. E. J . Chem. Soc., Dalton Trans. 1979, 45.
(6) Holton, J .; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood,
J . L.; Hunter, W. E. J . Chem. Soc., Dalton Trans. 1979, 54.
(7) Voskoboynikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin,
K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J . A. K. Organome-
tallics 1997, 16, 4041.
(8) Stults, S. D.; Andersen, R. A.; Zalkin, A. J . Organomet. Chem.
1993, 462, 175.
(9) Watson, P. L. J . Chem. Soc., Chem. Commun. 1980, 652.
10.1021/om020472h CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/17/2003

628 Organometallics, Vol. 22, No. 4, 2003
Berg and Andersen
plex is isolable.10,11 In the solid state, the lutetium
methyl complex is dimeric but it is not isostructural
with the C5H5 or C5H4Me derivatives mentioned above
since the two (Me5C5)2Lu fragments are bridged by a
single methyl group.12 The unusual and fascinating
reactions of the lutetium methyl complex make the
ytterbium analogue an important synthetic target.
In this paper we describe experiments aimed at
preparing (Me5C5)2YbMe. The strategy is to use Me3Al
as the methyl transfer reagent since it is ether-free and
soluble in hydrocarbon solvents, in contrast to common
methyl transfer reagents such as MeLi, MeMgX, or
MgMe2.13 The ytterbocene starting materials, (C5Me5)2-
Yb(ER)(NH3), were chosen because they are readily
prepared and soluble in hydrocarbons and the ER
leaving group can potentially exchange with one of the
methyl groups in AlMe3.14 Additionally, the coordinated
ammonia can be removed as the aliphatic hydrocarbon
soluble Me3Al(NH3).15 Although the base-free methyl
complex was not successfully obtained by this route,
some unusual reaction patterns and an unusual struc-
ture was observed for one of the products.
samarium derivative is a dimer in which the two
(Me5C5)2Sm fragments are bridged by Me4Al groups.18
The bridging Me4Al groups are rather unusual since two
of the methyl groups on each Me4Al fragment are
terminal and two form nearly linear bridges between
the two different metals. In solution these three tet-
ramethylaluminate complexes exist in a dimer-mono-
mer equilibrium that is observed by variable-tempera-
1
ture H NMR spectroscopic studies.
The ytterbocene derivative 1 shows similar solution
behavior to the metallocenes discussed above. At room
temperature, three resonances at δ 247, 5.2, and -26.5
ppm are observed in a relative area ratio of 6:30:6,
respectively. These resonances are assigned to the
bridging AlMe2 and (Me5C5)2Yb and terminal AlMe2
resonances in species A. In addition to these resonances,
a set of two less intense resonances is observed at δ 31.5
and 4.0 ppm in a relative area ratio of 6:30 due to the
bridging or terminal AlMe2 and (Me5C5)2Yb resonances,
respectively, of species B. The other methyl resonance
was not located for B, presumably because it is too broad
to be observed. Assuming that the largest resonances
in each set correspond to the Me5C5 groups, then the
ratio of A to B at 32 °C is 4:1. Increasing the temper-
ature to 79 °C changes the A:B ratio to 8:1. The
population change as a function of temperature is
consistent with a dimer-monomer equilibrium, similar
to that studied quantitatively in the diamagnetic com-
pounds mentioned above.16,17 The increase in the A:B
ratio with increasing temperature is consistent with A
being the monomeric species. In both A and B, bridge-
terminal methyl exchange is slow on the NMR time
scale. Heating the sample to 140 °C results in the
disappearance of the resonances assigned to the bridg-
ing and terminal aluminum methyl groups and they do
not reappear on heating to 175 °C. The resonances
assigned to the (Me5C5)2Yb groups sharpen and shift
as expected for a paramagnetic metal center. The
disappearance of the methyl resonances at high tem-
perature could be due to either intramolecular bridge-
terminal methyl site exchange or rapid dissociation and
reassociation of AlMe3. Teuben favors the latter expla-
nation in the Y case.17 However, dissociation of AlMe3
can be ruled out in the Yb case because addition of a
small amount of AlMe3 to the NMR sample results in
no change in the position or intensity of the NMR
resonances observed.
Resu lts a n d Discu ssion s
Addition of 3 molar equiv of Me3Al in pentane to a
toluene solution of either (Me5C5)2Yb(OSiMe3)(NH3) or
(Me5C5)2Yb(TePh)(NH3) gives dark blue solutions from
which the tetramethylaluminate [(Me5C5)2Yb(AlMe4)]2
(1) may be crystallized on cooling (eq 1). Complex 1 can
also be prepared by the metathetical reaction of (Me5C5)2-
YbCl(THF) with LiAlMe4. The yield from either syn-
thetic route is about 70%. The tetramethylaluminate 1
is a high-melting solid (mp 220-222 °C) that is not
volatile enough to give a molecular ion in the EI mass
spectrum.
Compound 1, and some of the others reported here,
consistently analyze 1-3% low in carbon in combustion
analysis. This is probably due to the fact that, even
though the freshly crystallized materials have well-
developed faces, exposure to vacuum results in their
collapse into a powder and probable loss of Me3Al. The
reaction in eq 2 with pyridine was monitored by NMR
spectroscopy to determine the stoichiometry.
The reaction of AlMe3 with decamethylytterbocene
derivatives containing different chalcogenides takes a
different course. Addition of 3 molar equiv of AlMe3 to
(Me5C5)2Yb(ER)(NH3), where ER is SPh, S-p-tolyl, SePh,
or OCMe3, yields 2:1 adducts that crystallize on cooling
the reaction mixture (eq 3). These adducts are high-
melting blue or purple solids that, like the tetrameth-
ylaluminates, give low combustion analyses for carbon,
with one exception, viz., the S-p-tolyl derivative. As with
the tetramethylaluminate, the 2:1 stoichiometry can be
confirmed by examining the 1H NMR spectrum of a
C6D6 sample of each compound in the presence of a
slight excess of pyridine (eq 4).
Metallocenes of identical stoichiometry to 1 have been
isolated for Y, Sm, and Lu.16-18 In the solid state, the
(10) Watson, P. L.; Roe, D. C. J . Am. Chem. Soc. 1982, 104, 6471.
(11) Watson, P. L.; Herskovitz, B. In Initiation of Polymerization;
Bailey, F. E., Ed.; ACS Symp. Ser. No. 212; American Chemical
Society: Washington, DC, 1983; p 459.
(12) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51.
(13) Coates, G. E.; Wade, K. Organometallic Compounds; Methuen
and Co. Ltd.: New York, 1967; Vol. 1.
(14) Berg, D. J .; Andersen, R. A.; Zalkin, A. Organometallics 1988,
7, 1858.
(15) Interrante, L. V.; Sigel, G. A.; Garbauskas, M.; Hejna, C.; Slack,
G. A. Inorg. Chem. 1989, 28, 252.
(16) Busch, M. A.; Harlow, R. A.; Watson, P. L. Inorg. Chim. Acta
1987, 140, 15.
(17) den Haan, K. H.; Wielstra, Y.; Eshuis, J . J . W.; Teuben, J . H.
J . Organomet. Chem. 1987, 323, 181.
(18) Evans, W. J .; Chamberlain, L. R.; Ulibarri, T. A.; Ziller, J . W.
J . Am. Chem. Soc. 1988, 110, 6423.

Reaction of (C5Me5)2Yb(ER)(NH3) with Me3Al
Organometallics, Vol. 22, No. 4, 2003 629
There are several structural possibilities for these 2:1
adducts and fortunately the S-p-tolyl derivative 3 yields
crystals suitable for a X-ray diffraction study. An
ORTEP drawing showing the structure of 3 is given in
Figure 1; the toluene of crystallization has been omitted.
The molecule crystallizes in the triclinic space group P1h.
Some bond lengths and angles are listed in Tables 1 and
2 and crystal data are given in Table 3. The structure
is constructed from two (Me5C5)2Yb(III) fragments that
are linked by two anionic Me3Al-(S-p-tolyl)-AlMe3 units
to give a molecular unit that contains a 12-membered
ring of idealized C2h symmetry. The (Me5C5)2Yb(III)
fragment is normal with an averaged Yb-C distance of
2.62(2) Å, a Me5C5(centroid)-Yb distance of 2.35 Å, and
a Me5C5(centroid)-Yb-Me5C5(centroid) angle of 148°.
These parameters are nearly identical with those found
in (Me5C5)2Yb(S2CNEt2),19 (Me5C5)2Yb(SPh)(NH3),20 and
(Me5C5)2Yb(TePh)(NH3),14 with the exception that the
(Me5C5)(centroid)-Yb-(Me5C5)(centroid) angle in these
three metallocenes is closed by 10-12° relative to 3.
The Me3Al-(S-p-tolyl)-AlMe3 fragments are com-
posed of two Me3Al groups that are connected to the S-p-
tolyl group so that each Al center is 4-coordinate with
an average Al-S distance of 2.375(3) Å. This distance
is very similar to the Al-S distance in Me4Al2(µ-SMe)2
in the gas phase of 2.370(5) Å.21 The Al-S-Al angle of
119.8(1)° is of course opened relative to the equivalent
angle in Me4Al2(µ-SMe)2 (94.5(5)°). The difference be-
tween the average Al-C bridge (2.015(1) Å) and termi-
nal (1.989(5) Å) distances is far less (0.026 Å) in 3 than
in Al2Me6 (0.17 Å).22
F igu r e 1. An ORTEP30 drawing of [(Me5C5)2Yb(AlMe3)2-
(S-p-C6H4Me)]2‚C7H8 (3). The toluene of crystallization is
not shown. The Yb-Me5C5(ring centroid) distance, repre-
sented by CP1 which is C(1-5) and CP2 which is
C(11-15), is 2.35 Å and the Me5C5(ring centroid)-Yb-
Me5C5(ring centroid) angle is 148°. The p-tolyl ring is
represented by its ipso-carbon atom, C(21). The heavy
atoms are refined anisotropically and they are shown as
50% probability ellipsoids; the hydrogen atoms on C(28)
and C(31) are refined isotropically and they are of arbitrary
size.
Ta ble 1. Selected Bon d Dista n ces (Å) for
[(Me5C5)2Yb(AlMe3)2(S-p-C6H4Me)]2‚P h Mea
Yb-C(1)
Yb-C(2)
Yb-C(3)
Yb-C(4)
Yb-C(5)
Yb-C(6)
Yb-C(7)
Yb-C(8)
Yb-C(9)
Yb-C(10)
Yb-C(28)
Yb-C(31)
Yb-H(1)
Yb-H(2)
Yb-H(3)
Yb-H(4)
Yb-H(5)
Yb-H(6)
2.638(7)
2.639(7)
2.612(7)
2.607(7)
2.608(7)
2.608(7)
2.586(7)
2.633(7)
2.641(8)
2.614(7)
2.670(7)
2.667(6)
2.55(7)
Al(1)-C(28)
Al(1)-C(29)
Al(1)-C(30)
Al(2)-C(31)
Al(2)-C(32)
Al(2)-C(33)
Al(1)-S
2.013(7)
1.984(8)
1.986(8)
2.017(7)
1.999(11)
1.986(12)
2.381(3)
2.369(3)
0.996(12)
0.998(12)
0.992(12)
0.991(12)
0.995(12)
0.996(12)
Al(2)-S
C(28)-H(1)
C(28)-H(2)
C(28)-H(3)
C(31)-H(4)
C(31)-H(5)
C(31)-H(6)
2.61(7)
2.44(7)
2.50(7)
2.56(6)
2.56(6)
The average Yb-C-Al angle is nearly linear
(173.2(1)°)sa geometry that has been observed in a few
metallocenes of the lanthanides with bridging methyl
groups.12,16,18,23 In the present case, the hydrogen atoms
on the bridging carbon atoms, C(28) and C(31), respec-
tively, were located in the difference map and refined
isotropically. The averaged C-H distance of 0.99 Å is
normal as are the averaged H-C-H angles of 110.5°.
The geometry about the bridging carbon atoms requires
that the hydrogen atoms point toward the decameth-
ylytterbocene fragment with averaged Yb‚‚‚C and
Yb‚‚‚H distances of 2.669(1) and 2.53(5) Å, respectively.
It is useful to compare the geometry of the bridging
methyl in 3 with that of two other adducts of (Me5C5)2-
Yb(II), viz., (Me5C5)2Yb(µ-Me)Be(Me5C5)23 and (Me5C5)2-
Yb(µ-Et)AlEt2(THF).24 The Yb‚‚‚C distance in these two
a
Estimated standard deviations in parentheses.
Ta ble 2. Selected Bon d An gles (d eg) for
[(Me5C5)2Yb(AlMe3)2(S-p-C6H4Me)]2‚P h Mea
C(28)-Yb-C(31)
Yb-C(28)-Al(1)
Yb-C(31)-Al(2)
C(28)-Al(1)-C(29) 112.8(4) C(28)-Al(1)-S
C(28)-Al(1)-C(30) 111.7(4) C(31)-Al(2)-S
C(29)-Al(1)-C(30) 117.1(4) H(1)-C(28)-H(2) 109.8(17)
C(31)-Al(2)-C(32) 110.7(4) H(1)-C(28)-H(3) 110.5(18)
C(31)-Al(2)-C(33) 112.7(4) H(2)-C(28)-H(3) 110.4(17)
C(32)-Al(2)-C(33) 118.3(6) H(4)-C(31)-H(5) 110.9(17)
H(4)-C(31)-H(6) 110.6(17)
87.7(3) Al(1)-S-Al(2)
171.1(4) Al(1)-S-C(21)
175.4(4) Al(2)-S-C(21)
119.8(1)
110.1(3)
105.6(3)
110.2(3)
99.8(3)
H(5)-C(31)-H(6) 110.8
a
Estimated standard deviations in parentheses.
Yb(II) adducts is 2.746(4) and 2.85(2) Å, respectively,
both of which are longer than the equivalent Yb‚‚‚C
distance in the 2:1 adduct. This is consistent with the
idea that 3 contains a Yb(III) center because the metal
ionic radius is ca. 0.15 Å smaller than that of Yb(II) in
(19) Tilley, T. D.; Andersen, R. A.; Zalkin, A.; Templeton, D. H. Inorg.
Chem. 1982, 21, 2644.
(20) Zalkin, A.; Henly, T. J .; Andersen, R. A. Acta Crystallogr., Sect.
C 1987, C43, 233.
(21) Haaland, A.; Stokkeland, O.; Weidlein, J . J . Organomet. Chem.
1975, 94, 353.
(22) Huffman, J . C.; Streib, W. E. J . Chem. Soc., Chem. Commun.
1971, 911.
(23) Burns, C. J .; Andersen, R. A. J . Am. Chem. Soc. 1987, 109, 5853.
(24) Yamamoto, H.; Yasuda, H.; Yokota, K.; Nakamura, A.; Kai, Y.;
Kusai, K. Chem. Lett. 1988, 1963.

630 Organometallics, Vol. 22, No. 4, 2003
Berg and Andersen
shift is not linear in T-1, indicating that temperature-
dependent processes exist in solution. As a minimum,
the net reactions corresponding to the equilibrium
shown in eq 5 are occurring in solution; the complexity
of this system precludes a quantitative examination.
Ta ble 3. Su m m a r y of Cr ysta llogr a p h ic Da ta for
[(Me5C5)2Yb(AlMe3)2(S-p-C6H4Me)]2‚P h Me
formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C40H64Al2SYb
804.02
triclinic
P1h (no. 2)
12.894(5)
15.356(6)
11.187(6)
97.13(4)
103.98(4)
86.45(4)
2131.6
2
Z
F(calcd) (g cm-3
)
1.253
22.97
Mo KR, 1.542
296
µ (cm-1
)
Although the 2:1 adduct when ER ) OCMe3 (5) is
stoichiometrically identical with those described above,
the 1H NMR spectrum of this compound is much
simpler. At 32 °C, only three resonances are observed
at δ 42, 4.5, and -26 ppm in a ratio of 9:30:9 in C6D6.
None of these resonances are due to (Me5C5)2Yb(AlMe4)
at this temperature. Addition of Me2AlOCMe3 does not
alter the appearance of the paramagnetic species and
mixing equimolar amounts of this compound with
(C5Me5)2Yb(AlMe4) does not generate the 1H NMR
spectrum of 5. These results rule out a rapid equilibrium
of the type shown in eq 5. However, the observation of
three resonances of 9:30:9 ratio is inconsistent with a
2:1 adduct, whether undergoing fast bridge-terminal
exchange or not because these structures should give
either three resonances in 9:30:18 ratio or four signals
in 9:30:6:12 ratio, respectively. One possible explanation
for this is that 5 dissociates in a different way from 2-4
(eq 6). This would explain the lack of exchange with free
Me2AlOCMe3 or (Me5C5)2Yb(AlMe4) and it is also con-
sistent with the greater bond strength expected for an
Yb-O bond when compared with Yb-S and Yb-Se
linkages.
radiation, λ (CC)
T (K)
2θmax (deg)
100.2
11885
7557
0.042
no. of obsd reflcns
no. of unique reflcns
Ra
b
Rw
0.057
R (all data)
0.074
a
b
2
R ) ∑(|Fo| - |Fc|)/∑|Fo|. Rw ) [∑w(|Fo| - |Fc| )/∑w(|Fo|)2]1/2
.
an equivalent coordination number.25 Recently, the
crystal structure of polymeric (Et4Al)2Yb has been
reported in which the ethyl groups form bridge bonds
between the aluminum and ytterbium metals.26 The
averaged Yb-C distance is 2.67 Å and the methylene
group has C-H‚‚‚Yb secondary contacts. In the (Me5C5)-
BeMe adduct, the averaged Yb‚‚‚H distance of 2.59 Å
is comparable to those found in the 2:1 adduct. The
similarity of the geometric parameters in these two
adducts suggests that the bonding is similar, i.e., that
donation of electron density from the C-H bonds to the
metal center occurs. Thus, 3 may be viewed as an adduct
between a (Me5C5)2Yb(III)+ cation and an [Me3Al(S-p-
tolyl)AlMe3]- anion.
The 1H NMR spectra of the four 2:1 adducts (2-5)
are quite complex and inconsistent with the solid-state
1
structure determined for 3. The H NMR spectra of the
SPh (2), S-p-tolyl (3), and SePh (4) derivatives are quite
similar to each other but differ markedly from that of
the OCMe3 adduct 5. In the case of the sulfur or
selenium adducts 2-4, resonances due to (Me5C5)2Yb-
(AlMe4) were observed at room temperature. This
observation severely complicates the spectra since this
species itself exists in solution as an equilibrium
between the monomer and dimer (vide supra). In
solutions of 2, 3, and 4, the chemical shift and relative
integration of the (Me5C5)2Yb(AlMe4) resonances change
with temperature, as observed for the pure compound.
Since resonances due to the tetramethylaluminate 1
were observed it seemed reasonable that resonances due
to Me2AlER should also be observed. This was shown
to be the case for Me2AlSPh, since addition of a small
amount of this compound to a solution of the 2:1 adduct
increased the intensity of the resonances assigned to
this compound at room temperature (see Experimental
Section for details). In addition, when the 1H NMR
spectrum of a sample of 3 in C7D8 was examined as a
function of temperature, the (Me5C5)Yb resonances did
not follow Curie-Weiss behavior since the chemical
Con clu sion s
The products isolated from addition of AlMe3 to
(Me5C5)2Yb(ER)(NH3) in hydrocarbon solvents depend
on the identity of the ER group. When ER is TePh or
OSiMe3, the tetramethylaluminate [(Me5C5)2Yb(AlMe4)]2
(1) is isolated but when ER is SPh, S-p-tolyl, or SePh,
adducts with the stoichiometry (Me5C5)2Yb(ER)(AlMe3)2
(2-4) are obtained. It seems reasonable to assume that
the isolated solid compounds represent the thermody-
namically favorable forms of several structures that are
likely to be present in solution (cf. eqs 5 and 6). A
postulate that helps in understanding this situation
qualitatively is outlined below. The first step is removal
of NH3 as the Me3Al(NH3) adduct and formation of the
transient species (Me5C5)2Yb(ER)(AlMe3). This adduct
can react with another equivalent of AlMe3 in two
ways: by eliminating Me2Al(ER) to form (Me5C5)2Yb-
(AlMe4) (ER ) TePh or OSiMe3) or by adding AlMe3 to
form (Me5C5)2Yb(ER)(AlMe3)2, which rearranges to the
solid-state structure observed for 3 and assumed for ER
) SPh and SePh. This postulate predicts that the
difference in net reaction is related to the A-ER bond
(25) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32A, 751.
(26) Klimpel, M. G.; Anwander, R.; Tafipolsky, M.; Scherer, W.
Organometallics 2001, 20, 3983.

Reaction of (C5Me5)2Yb(ER)(NH3) with Me3Al
Organometallics, Vol. 22, No. 4, 2003 631
and filtered by cannula and the filtrate was cooled at -78 °C
for several days. Blue needles were isolated from the mother
liquor by filtration. Yield: 0.25 g (75%). Mp 213-215 °C. IR
(Nujol, CsI): 2720 (vw), 1581 (w), 1260 (vw), 1188 (s), 1156
(vw), 1086 (m), 1070 (vw), 1024 (m), 941 (s), 920 (s), 797 (br
vs), 745 (s), 729 (m), 696 (vs), 650 (vs), 586 (s), 528 (m), 484
strength, which is likely to fall in the order O > S > Se
. Te.27 Thus, only when the donor strength is high will
a 2:1 AlMe3 adduct form; weaker donors such as TePh
cannot support this structure and collapse to the tet-
ramethylaluminate (1). The case of OSiMe3 relative to
OCMe3 is rationalized by these arguments because
SiMe3 is electron withdrawing relative to CMe3 so
OSiMe3 is a poorer donor than OCMe3.28
1
(vw), 464 (vw), 429 (vw), 386 (w), 362 (m), 314 (vs) cm-1. H
NMR (toluene-d8, 32 °C): δ 247 (6H, ν1/2 > 500 Hz), 80.1 (4H,
ν1/2 ) 350 Hz), 31.6 (2H, ν1/2 ) 33 Hz), 29.0 (1H, ν1/2 ) 125
Hz), 20.5 (16H, ν1/2 ) 175 Hz), 14.2 (4H, ν1/2 ) 95 Hz), 6.2
(78H, ν1/2 ) 65 Hz), 5.3 (30H, ν1/2 ) 60 Hz), 4.1 (7H, ν1/2 ) 35
Hz), -0.4 (2H, ν1/2 ) 5 Hz), -2.6 (2H, ν1/2 ) 35 Hz), -13.0
(3H, ν1/2 ) 37 Hz), -27.0 (6H, ν1/2 ) 30 Hz) ppm. Relative
integrated intensities change with temperature. Anal. Calcd
for C64H100Al4S2Yb2: C, 55.2; H, 7.67; S, 4.60. Found: C, 53.3;
H, 7.70; S, 4.28. The stoichiometry of the complex was verified
Exp er im en ta l Section
[(Me5C5)2Yb(AlMe4)]2 (1). (a ) F r om (Me5C5)2YbCl(THF )
a n d LiAlMe4. (Me5C5)2YbCl(THF) (1.80 g, 3.27 mmol) and
LiAlMe4 (0.38 g, 4.0 mmol) were weighed into a Schlenk tube
and dissolved in 150 mL of toluene with vigorous stirring. The
purple solution was stirred at room temperature overnight.
After the reaction mixture was allowed to settle, the blue-violet
mother liquor was filtered and the filtrate was concentrated
to ca. 100 mL. Cooling at -10 °C afforded blue crystals of 1.
Several subsequent crops of crystals were isolated for a total
yield of 1.10 g (64%). Mp 220-222 °C. IR (Nujol, CsI): 2720
(w), 1259 (w), 1178 (m), 1119 (vw), 1097 (w), 1061 (w), 1022
(m), 877 (br vs), 801 (br m), 783 (br m), 743 (s), 698 (vw), 677
(s), 623 (s), 592 (vw), 568 (s), 543 (s), 511 (vw), 495 (vw), 385
(m), 314 (vs), 309 (m) cm-1. 1H NMR (toluene-d8): resonances
due to the C5Me5 rings and the terminal CH3 groups of both
monomer and dimer are observed in solution. Resonances due
to the bridging CH3 groups are observed for the monomer
but could not be detected for the dimer. The monomer:dimer
ratio changes from 4:1 at 32 °C to 8:1 at 79 °C. 32 °C:
monomer, δ 247.0 (µ-CH3, 6H, ν1/2 ) 600 Hz), 5.2 (C5Me5, 30H,
ν1/2 ) 45 Hz), -26.5 (Al-CH3, 6H, ν1/2 ) 30 Hz); dimer, δ 31.5
(Al-CH3, 6H, ν1/2 ) 40 Hz), 4.0 (C5Me5, 30H, ν1/2 ) 28 Hz). 79
°C: monomer, δ 204 (µ-CH3, 6H, ν1/2 ) 544 Hz), 4.5 (C5Me5,
30H, ν1/2 ) 65 Hz), -21.2 (Al-CH3, 6H, ν1/2 ) 56 Hz); dimer,
δ 27.2 (Al-CH3, 6H, ν1/2 ) 55 Hz), 4.5 (C5Me5, 30H, overlaps
monomer resonance). Anal. Calcd for C48H84Al2Yb2: C, 54.3;
H, 7.98. Found: C, 52.8; H, 8.14.
1
by H NMR after addition of pyridine to a toluene-d8 solution
of 2: δ 1.7 (30H, (Me5C5)2Yb(SPh)(py)), -0.2 (18H, Me3Al(py))
ppm.
The addition of a 5-fold excess of PhSAlMe2 (prepared as
described below) to a solution of 2 in toluene-d8 produced a
1
new H NMR spectrum: δ 80.3 (2H, ν1/2 ) 450 Hz), 28.9 (1H,
ν1/2 ) 325 Hz), 20.7 (2H, ν1/2 ) 110 Hz), 19.4 (2H, ν1/2 ) 90
Hz), 15.3 (4H, ν1/2 ) 95 Hz), 6.3 (30H, ν1/2 ) 110 Hz), -2.9
(3H, ν1/2 ) 33 Hz), -13.7 (4H, ν1/2 ) 85 Hz). Addition of 1 equiv
of PhSAlMe2 to a solution of 1 in toluene-d8 generated a 1H
NMR spectrum very similar to that of 2 reported above: δ 248
(3H, ν1/2 > 500 Hz), 80.0 (3H, ν1/2 ) 380 Hz), 27.6 (3H, ν1/2
155 Hz), 20.3 (8H, ν1/2 ) 192 Hz), 14.2 (6H, ν1/2 ) 175 Hz), 6.2
(60H, ν1/2 ) 120 Hz), 5.2 (15H, ν1/2 ) 65 Hz), -0.1 (6H, ν1/2
)
)
10 Hz), -2.8 (4H, ν1/2 ) 40 Hz), -12.3 (6H, ν1/2 ) 58 Hz), -27.3
(3H, ν1/2 ) 34 Hz).
(a ) P r ep a r a tion of P h SAlMe2. AlMe3 (5.5 mmol) was
diluted with 150 mL of pentane and the solution was cooled
to -78 °C. Thiophenol (5.4 mmol) was added via syringe with
vigorous stirring. After allowing the solution to slowly warm
to room temperature, the solvent was removed under reduced
pressure and the white residue was redissolved in a minimum
of hexane. The solution was filtered and the filtrate cooled at
-10 °C overnight. White crystals of PhSAlMe2 were isolated
from the mother liquor by cannula filtration. Mp 130-132 °C.
1H NMR (toluene-d8, 21 °C): δ 7.47 (2H, m, o-PhH), 6.90 (3H,
m, m- and p-PhH), -0.28 (6H, s, AlMe2).
The stoichiometry of this complex was also established by
1
examining the H NMR spectrum of a toluene-d8 solution of 1
to which a few drops of pyridine was added. The resulting red-
orange solution clearly showed resonances at δ 3.8 (30H) and
-0.27 (9H) ppm for the C5Me5 groups of (C5Me5)2YbMe(py) and
the Me groups of Me3Al(py), respectively. No resonance due
to the ytterbium-bound methyl group was observable.
(b) F r om (C5Me5)2Yb(OSiMe3)(NH3). Trimethylaluminum
(1.8 mL of 1.3 M solution, 2.3 mmol) was added by syringe to
a stirred solution of (Me5C5)2Yb(OSiMe3)(NH3) (0.43 g, 0.76
mmol) in 150 mL of hexane. An immediate color change from
deep orange to blue-violet occurred. After being stirred for 2
h, the solution was filtered and the filtrate was concentrated
to 80 mL. Cooling at -10 °C overnight produced violet blocks
of 1. Yield: 0.26 g (64%).
(c) F r om (C5Me5)2Yb(TeP h )(NH 3). Trimethylaluminum
(1.6 mmol) as a pentane solution was added to a stirred
solution of (Me5C5)2Yb(TePh)(NH3) (0.36 g, 0.54 mmol) in 100
mL of toluene by syringe. The resulting deep blue solution was
stirred for 1 h and filtered and the filtrate was concentrated
to 30 mL. Cooling at -10 °C overnight produced large blue
crystals of 1 that rapidly lost solvent and became opaque on
exposure to vacuum. Yield: 0.26 g (64%).
[(Me5C5)2Yb(AlMe3)2(S-p-C6H4Me)]2‚C7H8 (3). Compound
3 was prepared using the procedure described for 2 starting
from 1.00 g (1.56 mmol) of (Me5C5)2Yb(S-p-C6H4Me)(OEt2).
Several crops of 3 were isolated as blue prisms by cooling the
mother liquor at -10 °C. Yield: 0.98 g (78%). Mp 200-202
°C. IR (Nujol, CsI): 2729 (w), 1491 (m), 1208 (w), 1187 (vs),
1087 (m), 1065 (w), 1018 (m), 933 (br s), 897 (br s), 813 (vs),
796 (br s), 695 (br vs), 652 (vs), 585 (vs), 527 (m), 497 (w), 385
(w), 378 (m), 360 (vw), 309 (vs), 290 (w), 281 (vw) cm-1 1H
.
NMR (toluene-d8, 32 °C): δ 247 (6H, ν1/2 > 500 Hz), 78.4 (4H,
ν1/2 ) 285 Hz), 27.0 (3H, ν1/2 ) 89 Hz), 12.0 (5H, ν1/2 ) 37 Hz),
6.2 (53H, ν1/2 ) 72 Hz), 5.1 (30H, ν1/2 ) 55 Hz), -0.2 (1H, ν1/2
) 12 Hz), -13.2 (4H, ν1/2 ) 42 Hz), -26.8 (6H, ν1/2 ) 35 Hz)
ppm. Relative integrated intensities change with temperature.
Anal. Calcd for C73H118Al4S2Yb2: C, 57.9; H, 7.86; S, 4.24.
Found: C, 55.2; H, 7.65; S, 4.78.
The stoichiometry of this complex was also established by
1
examining the H NMR spectrum of a toluene-d8 solution of 3
to which a few drops of pyridine were added. The resulting
orange solution clearly showed resonances at δ 1.6 (30H) and
-0.2 (18H) ppm for the C5Me5 groups of (C5Me5)2Yb(S-p-
C6H4Me)(py) and the Me groups of Me3Al(py), respectively.
[(Me5C5)2Yb(AlMe3)2(SeP h )]2 (4). Compound 4 was pre-
pared using the procedure described for 2 starting from 0.60
g (0.97 mmol) of (Me5C5)2Yb(SePh)(NH3). Several crops of 4
were isolated as lavender crystals by cooling the mother liquor
at -10 °C. The crystals became opaque on exposure to vacuum.
Yield: 0.45 g (62%). Mp 230-232 °C. IR (Nujol, CsI): 2720
[(Me5C5)2Yb(AlMe3)2(SP h )]2 (2). A solution of trimethyl-
aluminum in pentane (1.4 mmol) was added to a stirred
solution of (Me5C5)2Yb(SPh)(NH3) (0.29 g, 0.46 mmol) in 50
mL of toluene by syringe. The blue solution was stirred 1 h
(27) (a) Coates, G. E. J . Chem. Soc. 1951, 2003. (b) Tsvetkov, V. G.;
Kozyrkin, B. I.; Fukin, K. K.; Galiullina, R. F. J . Gen. Chem. USSR
1977, 47, 1966.
(28) Grimm, D. H.; Bartmess, J . E. J . Am. Chem. Soc. 1992, 114,
1227.

632 Organometallics, Vol. 22, No. 4, 2003
Berg and Andersen
(w), 1577 (m), 1189 (s), 1072 (vw), 1022 (m), 1000 (vw), 936
(br vs), 795 (sh w), 760 (sh m), 737 (vs), 694 (vs), 650 (vs), 586
(vs), 527 (br m), 486 (m), 390 (br w), 318 (vs), 303 (vw), 280
inside a thin-walled quartz capillary under argon and mounted
on a modified Picker FACS-1 automated diffractometer equipped
with graphite-monochromated Mo radiation. A set of θ-2θ
scans was collected and corrected for absorption (analytical
method),29 Lorentz, and polarization effects. The Yb, S, and
Al atoms were located in the three-dimensional Patterson map
and subsequent least-squares refinements and electron density
maps revealed the location of all other non-hydrogen atoms.
The structure was refined by full-matrix least squares on F.
Anisotropic thermal parameters were refined for all non-
hydrogen atoms. A toluene of crystallization was located and
its atoms were refined isotropically with fixed C-C distances
1.4 Å (C-C) and 1.54 Å (C-Me). The six H atoms on C(28)
and C(31) were located in the electron density maps and were
refined with one single isotropic thermal parameter for all
three hydrogen atoms of each group; the C-H and H-H
distances were restrained to 1.0 Å and 1.63 Å, respectively.
Additionally, the four aryl H atoms associated with the p-tolyl
ring were located in the difference maps. These atoms were
refined isotropically using a single isotropic thermal parameter
for all four atoms. Details of the data collection and least-
squares refinements are given in Table 3. Atomic scattering
factors and anomalous dispersion terms were taken from the
International Tables for X-ray Crystallography.30 With the
exception of ORTEP,31 all computer programs used in this
work were developed by Dr. A. Zalkin.
1
(m) cm-1. H NMR (toluene-d8, 32 °C): δ 247 (6H, ν1/2 > 350
Hz), 6.0 (21H, ν1/2 ) 80 Hz), 5.1 (30H, ν1/2 ) 50 Hz), 3.9 (2H,
ν1/2 ) 35 Hz), -0.5 (4H, ν1/2 ) 30 Hz), -10.0 (3H, ν1/2 ) 54
Hz), -27.0 (6H, ν1/2 ) 21 Hz) ppm. Relative integrated
intensities change with temperature. Anal. Calcd for C64H106
Al4Se2Yb2: C, 51.7; H, 7.18. Found: C, 51.3; H, 7.19.
-
The stoichiometry of this complex was also established by
1
examining the H NMR spectrum of a toluene-d8 solution of 3
to which a few drops of pyridine were added. The resulting
orange solution clearly showed resonances at δ 1.4 (30H) and
-0.2 (18H) ppm for the C5Me5 groups of (C5Me5)2Yb(SePh)-
(py) and the Me groups of Me3Al(py), respectively.
[(Me5C5)2Yb(AlMe3)2(OCMe3)]2 (5) Compound 5 was pre-
pared from 0.20 g (0.37 mmol) of (Me5C5)2Yb(OCMe3)(NH3)
using a procedure similar to that described for 2 above but
using hexane (200 mL) rather than toluene as solvent. Several
crops of 5 were isolated as purple plates by cooling the mother
liquor at -78 °C. Yield: 0.10 g (46%). Mp 180-182 °C. IR
(Nujol, CsI): 2720 (w), 1365 (w), 1309 (vw), 1261 (vw), 1243
(w), 1180 (s), 1090 (vw), 1024 (w), 914 (s), 803 (w), 767 (w),
721 (sh vw), 693 (br vs), 628 (br s), 596 (vw), 499 (m), 472 (vw),
380 (br w), 297 (m) cm-1. 1H NMR (benzene-d6, 32 °C): δ 41.6
(9H, ν1/2 ) 313 Hz), 4.5 (30H, ν1/2 ) 150 Hz), -26.0 (9H, ν1/2
)
43 Hz) ppm. Anal. Calcd for C60H114Al4O2Yb2: C, 54.5; H, 8.70.
Found: C, 53.7; H, 8.49.
Addition of a few drops of pyridine to a solution of 5 in
benzene-d6 confirmed the complex stoichiometry: δ 9.2 (30H,
C5Me5 rings of (C5Me5)2Yb(OCMe3)(py)), -0.27 (18H, Me of
Me3Al(py)) ppm.
Ack n ow led gm en t. This work was supported by the
Director, Office of Energy Research, Office of Basic
Energy Sciences, Chemical Sciences Division of the U.S.
Department of Energy under Contract No. 76SF00098.
D.J .B. thanks NSERC (Canada) for support. We thank
Dr. A. Zalkin for collecting the X-ray data and Dr. F. J .
Hollander for helpful discussions and for assistance with
the ORTEP drawings.
Addition of Me3COAlMe2 (prepared as outlined below) to a
1
solution of 5 in toluene-d8 did not alter the H NMR spectrum
of 5. Similarly, addition of a large excess of Me3COAlMe2 to 1
1
in toluene-d8 did not produce the H NMR spectrum of 5. The
spectrum obtained in this case was a simple superposition of
1 and Me3COAlMe2 alone.
(a ) P r ep a r a tion of Me3COAlMe2. AlMe3 (10.0 mmol) was
diluted with 200 mL of pentane and the solution was cooled
to -78 °C. A solution of Me3COH (9.8 mmol) in 100 mL of Et2O
was slowly added by cannula with rapid stirring. The reaction
mixture was allowed to warm slowly to room temperature over
several hours. Removal of the solvent under reduced pres-
sure afforded a sticky white residue that was redissolved in
dry pentane and cooled to -78 °C. White microcrystals of
Me3COAlMe2 were isolated from the mother liquor by can-
nula filtration. 1H NMR (benzene-d6, 32 °C): δ 1.18 (9H, s,
CMe3), -0.46 (6H, s, AlMe2) ppm.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, complete bond distances and angles, and aniso-
tropic thermal parameters for 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM020472H
(29) Templeton, L. K.; Templeton, D. H. Abstracts, American
Crystallographic Association Proceedings; American Crystallographic
Association: Storrs, CT; Series 2, Vol. 1, p 143.
(30) International Tables for X-ray Crystallography; Kynoch: Bir-
mingham, England, 1974.
(31) J ohnson, C. K. ORTEPII; Oak Ridge National Laboratory: Oak
Ridge TN, 1976.
X-r a y Cr yst a llogr a p h y of [(Me5C5)2Yb (AlMe3)2(S-p -
C6H4Me)]2‚C7H8 (3). An air-sensitive crystal of 3 was sealed