X-ray crystal structure of the tetra(tert-butyl)erbate anion and attempts to prepare tetravalent organolanthanide complexes

Article abstract of DOI:10.1016/j.poly.2007.04.006

The new [Li(DME)₃⁺] salt of the previously-known tetra(tert-butyl)erbate(III) anion [Er(t-Bu)₄^-] has been prepared and structurally characterized. The erbium(III) center is ligated by four tert-butyl groups in an approximately tetrahedral arrangement. The C-Er-C angles between the tert-butyl groups range from 108.8(3)° to 111.2(3)° and the Er-C distances range from 2.352(6) to 2.395(6) A?. The lithium cation is surrounded by three DME molecules, which form a distorted octahedral coordination sphere. Attempts to oxidize the analogous terbate complex [Li(DME)₃][Tb(t-Bu)₄] and its cerium analog to electrically neutral tetra(alkyl)lanthanide(IV) compounds are described.

Full text of DOI:10.1016/j.poly.2007.04.006

Polyhedron 26 (2007) 3865–3870
www.elsevier.com/locate/poly
X-ray crystal structure of the tetra(tert-butyl)erbate anion and
attempts to prepare tetravalent organolanthanide complexes
*
Wontae Noh, Gregory S. Girolami
School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, USA
Received 1 February 2007; accepted 13 April 2007
Available online 21 April 2007
Abstract
The new [Li(DME)3+] salt of the previously-known tetra(tert-butyl)erbate(III) anion [Er(t-Bu)4ꢀ] has been prepared and structurally
characterized. The erbium(III) center is ligated by four tert-butyl groups in an approximately tetrahedral arrangement. The C–Er–C
˚
angles between the tert-butyl groups range from 108.8(3)ꢀ to 111.2(3)ꢀ and the Er–C distances range from 2.352(6) to 2.395(6) A. The
lithium cation is surrounded by three DME molecules, which form a distorted octahedral coordination sphere. Attempts to oxidize
the analogous terbate complex [Li(DME)3][Tb(t-Bu)4] and its cerium analog to electrically neutral tetra(alkyl)lanthanide(IV) compounds
are described.
ꢁ 2007 Elsevier Ltd. All rights reserved.
Keywords: Lanthanide; Organolanthanide; Crystal structure; Tetravalent; Oxidation; Lithium salt; Erbium; Organoerbium; tert-Butyl
1. Introduction
to be dominated by inorganic derivatives of cerium(IV)
[8,9]; few organocerium(IV) compounds have been
described, and few tetravalent compounds of any kind
are known for the other lanthanide elements [10–13]. The
only known formally tetravalent organolanthanide
complexes are bis(cyclooctatetraene) cerium(IV) and
related derivatives, and even these are best regarded as
complexes of cerium(III) bearing partly oxidized C8H8
ligands [14–17].
We became interested in the possibility of preparing
electrically neutral tetra(alkyl)lanthanide(IV) compounds,
which are currently unknown, by oxidation of
tetra(alkyl)lanthanate(III) compounds. Of [LnR4ꢀ] com-
pounds in which R is an alkyl group, the only ones that
have been isolated are those with R = t-butyl [18–20],
CH2SiMe3 [21,22], or CH(SiMe3)2 [21]; also known are aryl
and alkynyl compounds with R = phenyl [23], 2,6-dimeth-
ylphenyl [24], or tert-butylethynyl [25]. In this work, we
report the X-ray crystal structure of [Li(DME)3]-
[Er(t-Bu)4] (1) and describe our attempts to oxidize the
terbate complex [Li(DME)3][Tb(t-Bu)4] (2).
In recent years there has been renewed interest in orga-
nometallic compounds of the lanthanide elements, whose
structures, physical properties, and chemical reactivities
often differ significantly from organometallic compounds
of the d-block transition metals [1–3]. Although the +3 oxi-
dation state dominates throughout the series, it has long
been known that certain lanthanide elements – especially
SmII, EuII, and YbII – have stable divalent oxidation states,
and certain others – especially CeIV – have stable tetrava-
lent states [4,5]. Recently, the inorganic and organometallic
chemistry of lanthanide(II) ions has expanded consider-
ably, and now includes compounds of LaII, CeII, PrII, NdII,
DyII, and TmII [6,7]. In contrast, there have been fewer
advances in the inorganic and organometallic chemistry
of lanthanide(IV) ions. This class of compounds continues
*
Corresponding author. Tel.: +1 217 333 2729; fax: +1 217 244 3186.
E-mail address: girolami@scs.uiuc.edu (G.S. Girolami).
0277-5387/$ - see front matter ꢁ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.04.006

3866
W. Noh, G.S. Girolami / Polyhedron 26 (2007) 3865–3870
Table 1
2. Results and discussion
Crystal Data for [Li(DME)3][Er(t-Bu)4] (1)
Empirical formula
Formula weight
Temperature (K)
C28H66ErLiO6
673.01
193(2)
2.1. Syntheses of tetra(tert-butyl)metallate complexes of
lanthanide elements
˚
k (A)
0.71073
The preparation of the salt [Li(THF)4][Er(t-Bu)4] has
been reported previously [19]. In our hands, the reported
preparation proceeded as described, but we were not able
to obtain analytically pure material. Instead, we obtained
materials with low carbon and hydrogen analyses that
retained chloride. Attempts to prepare the terbium analog
[Li(THF)4][Tb(t-Bu)4] gave similar results.
Crystal system
Space group
monoclinic
C2/c
˚
a (A)
27.309(9)
19.177(6)
27.818(9)
94.614(6)
14521(8)
16
1.231
2.343
5648
˚
b (A)
˚
c (A)
(b) (ꢀ)
3
˚
V (A )
Z
q (Mg/m3)
In order to obtain crystalline and analytically pure sam-
Absorption coefficient (mmꢀ1
F(000)
)
+
ples, we have investigated the corresponding [Li(DME)3
]
salt, where DME is 1,2-dimethoxyethane. Thus, slow addi-
tion of a pentane solution of tert-butyllithium to a suspen-
sion of anhydrous erbium trichloride in DME/pentane at
ꢀ78 ꢀC, followed by warming to room temperature
gives a pink solution, from which pink crystals of
[Li(DME)3][Er(t-Bu)4] (1) can be obtained by layering a
saturated ether solution with pentane and cooling to
ꢀ20 ꢀC. The material so obtained is analytically pure.
The corresponding Tb complex [Li(DME)3][Tb(t-Bu)4] (2)
can be made similarly, but the cerium analog could not
be successfully isolated by the same route.
Crystal size (mm)
0.08 · 0.14 · 0.14
1.92–25.38
74312
h Range for data collection (ꢀ)
Reflections collected
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Ra[I > 2r(I)]
full-matrix least-squares on F2
72825/649/625
0.907
0.0955
0.2459
2.689 and ꢀ1.158
wRb2 (all data)
Largest peak and hole (e A
ꢀ3
˚
)
P
P
a
R = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
b
2
wR2 = { w[(Fo ꢀ Fc2)2]/ [w(Fo2)2]}1/2
.
The IR spectrum of the erbium complex 1 shows mCH
bands due to the tert-butyl groups at 2753, 2724, and
2667 cmꢀ1. In comparison, for the [Li(THF)4+] salt of this
same anion, Evans et al. reported similar bands at 2760,
2730, 2670 and 2620 cmꢀ1 [19]. The low frequencies of
these C–H modes are characteristic of tert-butyl groups
[26,27], and are not consequences of CH3–Li or agostic
interactions, as verified by the X-ray structure (see below).
The IR spectrum of the terbium analog 2 is essentially iden-
tical to that of 1. 1H NMR spectra of paramagnetic 1 and 2
contain broad resonances for the tert-butyl groups at d ꢀ11
and ꢀ76.6, respectively.
Table 2
˚
Selected bond distances (A) and angles (ꢀ) for [Li(DME)3][Er(t-Bu)4] (1)
Distances
Er(1)–C(1)
Er(1)–C(5)
Er(1)–C(9)
Er(1)–C(13)
C(1)–C(2)
C(1)–C(3)
C(1)–C(4)
C(5)–C(6)
C(5)–C(7)
C(5)–C(8)
C(9)–C(10)
2.378(6)
2.352(6)
2.395(6)
2.378(6)
1.509(14)
1.500(14)
1.491(14)
1.471(14)
1.485(14)
1.478(14)
1.497(14)
C(9)–C(11)
C(9)–C(12)
C(13)–C(14)
C(13)–C(15)
C(13)–C(16)
Li(1)–O(41)
Li(1)–O(42)
Li(1)–O(51)
Li(1)–O(52)
Li(1)–O(61)
Li(1)–O(62)
1.493(14)
1.480(14)
1.477(14)
1.501(14)
1.488(14)
2.089(14)
2.077(14)
2.144(15)
2.083(15)
2.113(14)
2.066(15)
2.2. Molecular structure of [Li(DME)3][Er(t-Bu)4] (1)
Angles
C(1)–Er(1)–C(5)
C(1)–Er(1)–C(9)
C(1)–Er(1)–C(13)
C(5)–Er(1)–C(9)
C(5)–Er(1)–C(13)
O(41)–Li(1)–O(42)
O(41)–Li(1)–O(51)
O(41)–Li(1)–O(52)
O(41)–Li(1)–O(61)
O(41)–Li(1)–O(62)
108.8(3)
O(42)–Li(1)–O(51)
O(42)–Li(1)–O(52)
O(42)–Li(1)–O(61)
O(42)–Li(1)–O(62)
O(51)–Li(1)–O(52)
O(51)–Li(1)–O(61)
O(51)–Li(1)–O(62)
O(52)–Li(1)–O(61)
O(52)–Li(1)–O(62)
O(61)–Li(1)–O(62)
90.5(6)
163.4(8)
93.8(6)
100.26(6)
77.4(5)
91.2(6)
164.8(7)
97.8(6)
93.9(6)
77.6(5)
General crystallographic data, selected bond lengths
and angles for 1 are listed in Tables 1 and 2, respectively;
views of one of the cations and one of the anions are
given in Fig. 1. In the asymmetric unit there are two
anions, one full cation, and two half-cations. Except for
some disordering of the tert-butyl groups on Er(2), the
two crystallographically inequivalent anions have very
similar metric parameters. The DME ligands about one
of the lithium centers are also disordered, but again the
coordination geometries of all three independent cations
are very similar.
109.3(3)
109.6(3)
111.2(3)
108.4(3)
77.7(5)
100.5(6)
93.2(6)
165.5(8)
92.3(6)
C angles range from 107.3ꢀ to 109.8ꢀ and the Lu–C
distances range from 2.32(2) to 2.43(2) A. In Cp2Lu-
(t-Bu)(THF) [28], the Lu–C distance to the tert-butyl
group was much longer at 2.47(2) A, probably because
the coordination number of this compound is higher.
Each lithium cation in 1 is surrounded by three DME
molecules, which form a distorted octahedral coordination
˚
The erbium(III) center in 1 is ligated by four tert-butyl
groups in an approximately tetrahedral arrangement. The
C–Er–C angles between the tert-butyl groups range from
108.4(3)ꢀ to 111.2(3)ꢀ and the Er–C distances range from
˚
˚
2.352(6) to 2.395(6) A. These values are similar to those
seen for the [Lu(t-Bu)4ꢀ] anion [18], in which the C–Lu–

W. Noh, G.S. Girolami / Polyhedron 26 (2007) 3865–3870
3867
Fig. 1. Molecular structure of [Li(DME)3][Er(t-Bu)4] (1); anion at left, cation at right.
sphere. The Li–O distances, which lie between 2.07(2) and
ꢀ40 ꢀC. No color change occurred. Filtration and removal
of the solvent gave an oily liquid, from which no organo-
metallic species could be isolated.
˚
˚
2.14(2) A, agree with the 2.07(1)–2.17(1) A range found in
˚
[Li(DME)3][(Cp3Sm)2(l-Cl)] [29] and the 2.12(1) A value
in [Li(DME)3][Zr(SCMe3)5] [30]. Some [Li(DME)3+] cat-
The significance of this result is unclear, in part because
the organocerium(III) starting material was not isolated
but prepared in situ. To complement this result, we investi-
gated whether pure samples of the anionic terbium(III)
compound [Li(DME)3][Tb(t-Bu)4] (2) react with oxidants
to generate Tb(t-Bu)4. Terbium (along with Pr) has the
most accessible +4 oxidation state after cerium, its stability
deriving in part from its f7 electronic configuration.
Injection of 4 equivalents of dioxygen into solutions of 2
immediately caused a color change from colorless to yel-
low. When this reaction was followed by NMR spectros-
ions are known in which the Li–O distances vary over a
˚
wider range: from 2.03(2) to 2.37(2) A in [Li(D-
ME)3][(C5H4Me)La(NPh2)3] [31] and from 2.07(2) to
˚
2.34(2) A in [Li(DME)3][Cp2Nd(NPh2)2] [32]. The O–Li–
O angles in 1 range from 77.4(5)ꢀ to 77.7(5)ꢀ, a range that
is consistent with previous values for this cation.
Colorless crystals of the terbium analog 2 were also
obtained by diffusion of pentane into an ether solution at
ꢀ20 ꢀC. The crystals were twinned and the diffraction data
set, which approximated to cubic symmetry, could not be
solved.
1
copy, the broad H NMR resonance at d ꢀ76.6 due to
the starting material disappeared after the addition of oxy-
gen. Two new resonances appeared at d 0.88 and 4.62,
which are due to iso-butane (1.67 mol/mol Tb), and iso-
butene (0.80 mol/mol Tb), respectively [36–38]; these
assignments were confirmed by GC analysis. No new
NMR resonances appeared that could be ascribed to an
organolanthanide product. Similar addition of O2 to the
erbium complex 1 caused an exotherm but did not result
in a change in the solution color. The broad 1H NMR peak
at d ꢀ11 due to the starting material disappeared after
dioxygen was added, but no peaks due to new organolanth-
anide species were apparent.
We also examined the action of one-electron chemical
oxidants on the terbium compound 2. Addition of silver
acetate or silver chloride to a solution of 2 in diethyl ether
resulted in the immediate precipitation of silver metal and
a color change similar to that seen upon the addition of oxy-
gen. Neither of these reactions resulted in the formation of
pentane-soluble organolanthanide species. Addition of one
equivalent of 1,10-dimethylferrocenium tetrakis[3,5-bis(tri-
fluoromethyl)phenyl]borate to 2 in diethyl ether caused
the solution to change color from blue (the color of the ferr-
ocenium salt) to yellow (the color of dimethylferrocene),
2.3. Attempts to prepare tetra(alkyl)lanthanide(IV) species
Alkyl groups are reasonably strongly donating and are
well-known to stabilize higher oxidation states of transition
metals [33]. Among alkyl groups, tert-butyl substituents are
among the most strongly donating [34]. In addition,
because four tert-butyl groups are able to saturate the
coordination sphere of a Ln3+ metal – as the existence of
[Ln(t-Bu)4ꢀ] anions shows – they should also be able to sat-
urate the coordination sphere of a somewhat smaller Ln4+
metal center. Such Ln4+ centers are still large enough to
accommodate four tert-butyl substituents, as shown by
the existence of a tetra(tert-butyl) complex of an even smal-
ler ion: the Cr4+ center in Cr(t-Bu)4 [35].
Of all the lanthanide elements, cerium has the most
accessible tetravalent oxidation state and is the natural
starting point for efforts to prepare a tetra(alkyl) derivative.
Although we were unable to isolate pure salts of the tetra-
(tert-butyl)cerate(III) ion (see above), we attempted to pre-
pare this species and oxidize it in situ. After treatment of
CeCl3 with 4 equivalents of tert-butyllithium in THF/ pen-
tane at ꢀ78 ꢀC, the mixture was exposed to dry O2 at

3868
W. Noh, G.S. Girolami / Polyhedron 26 (2007) 3865–3870
but again there was no evidence from the NMR spectra of
the formation of a new organolanthanide species.
The present studies suggest that Ln(t-butyl)4 species, if
they exist, are not readily obtained by direct oxidation of
their [Ln(t-butyl)4ꢀ] analogs with common oxidizing
agents, possibly because the alkyl groups are more easily
oxidized than the metal center.
3.2. Tris(1,2-dimethoxyethane)lithium
tetra(tert-butyl)terbate, [Li(DME)3][Tb(t-Bu)4] (2)
The procedure is the same as that for the preparation of
1 except that the reagents used were TbCl3 (0.30 g,
1.13 mmol) and tert-butyllithium (2.9 mL of a 1.7 M solu-
tion in pentane). The product was isolated as colorless
prisms. Yield: 0.31 g (41%). Mp: 80 ꢀC (dec). Anal. Calc.
for C28H66LiO6Tb: C, 50.6; H 10.0; Li, 1.04; Tb, 23.9.
3. Experimental
1
Found. C, 49.8; H, 9.78; Li, 1.02; Tb, 23.4%. H NMR
All syntheses were carried out under an argon atmo-
sphere, using a glove box and Schlenk techniques. Sol-
vents were dried over Na/benzophenone and distilled
under nitrogen immediately before use. The starting mate-
rials ErCl3 and TbCl3 (99.9% pure, Alfa-Aesar), pentane
solutions of tert-butyllithium (Aldrich), and oxygen gas
(S. J. Smith) were used as received. Microanalyses were
performed by the University of Illinois Microanalytical
Laboratory. The IR spectra were recorded on a Nicolet
Impact 410 instrument as Nujol mulls between KBr
plates. The gas chromatographs were recorded on an Agi-
lent 6890/5973 GCMS equipped with FID and TCD
detectors; the inlet temperature was 250 ꢀC, the oven tem-
perature was 275 ꢀC, the carrier gas was He (20.0 mL/
min), and the separations were carried out on a 30 m
long, 0.32 mm diameter, 1.5 lm thick carbon PLOT col-
umn with a deactivated SiO2 guard column (J&W Scien-
tific). The 1H NMR data were collected on a Varian
Unity U400 instrument at 400 MHz. Chemical shifts are
reported in d units (positive shifts to high frequency) rel-
ative to tetramethylsilane.
(d8-THF): d ꢀ76.6 (s, fwhm = 300 Hz, t-Bu) d 3.3 (s,
MeO), 3.4 (s, OCH2). IR (cmꢀ1): 2752 s, 2722 s, 2668 s,
1246 s, 1195 s, 1130 s, 1088 s, 1030 m, 985 m, 871 s, 777 s.
3.3. Attempts to oxidize tetra(tert-butyl)terbate,
[Li(DME)3][Tb(t-Bu)4] (2)
Colorless crystals of 2 (0.31 g, 0.47 mmol) were dis-
solved in diethyl ether (30 mL) and O2 gas (45 mL,
1.85 mmol) was injected by syringe into the solution at
room temperature. The solution color changed from col-
orless to yellow and a small amount of a flocculent pre-
cipitate formed. An aliquot was removed for analysis by
GCMS, which showed the presence of iso-butene and
iso-butane, in addition to peaks for the solvent and 1,2-
dimethoxyethane.
In a separate experiment, a sample of 2 was placed in a
5 mm diameter NMR tube capped with a rubber septum,
and the crystals were dissolved in d8-THF (1 mL) to give
a colorless solution. O2 gas was bubbled into the solution
1
until the solution color became yellow. H NMR spectra
1
were taken before and after the introduction of O2. H
3.1. Tris(1,2-dimethoxyethane)lithium
tetra(tert-butyl)erbate, [Li(DME)3][Er(t-Bu)4] (1)
NMR (d8-THF): d 0.88 (s, CHMe3), d 4.62 (s,
CH2@CMe2). The methine peak of iso-butane and the
methyl peak of iso-butene were obscured by the resonance
at d 1.72 due to the solvent; peaks at d 3.44 and 3.28 due to
1,2-dimethoxyethane were also present. By assuming that
the 1,2-dimethoxyethane peaks corresponded to 3 mol/
(mol Tb), the peak integrations suggested that 1.67 mol
of iso-butane and 0.80 mol of iso-butene were formed per
mol of Tb.
To a slurry of ErCl3 (0.30 g, 1.10 mmol) suspended in
1,2-dimethoxyethane (9 mL) and pentane (5 mL) at
ꢀ78 ꢀC was added
a solution of tert-butyllithium
(2.82 mL of a 1.7 M solution in pentane, diluted to
35 mL with pentane) dropwise over 90 min. The mixture
was stirred for two hours at ꢀ78 ꢀC and then slowly
warmed to room temperature. After two hours at room
temperature, the mixture had separated into a colorless
layer (pentane and DME), a pink suspension (crude
product), and a white powder (LiCl). The colorless layer
was carefully decanted and the solid was washed with
pentane (30 mL) and extracted with diethyl ether
(2 · 30 mL). The bright pink extract was filtered, concen-
trated to about 30 mL, layered with pentane (ꢁ10 mL),
and cooled to ꢀ20 ꢀC. The pink prisms were isolated
and dried in vacuum. Yield: 0.3 g (40%). Mp: 105 ꢀC
(dec). Anal. Calc. for C28H66LiO6Er: C, 49.9; H, 9.88;
Li, 1.03; Er, 24.9. Found: C, 49.1; H, 9.69; Li, 1.02;
3.4. X-ray crystallographic studies [39]
Pink crystals of [Li(DME)3][Er(t-Bu)4] (1) were grown
by diffusion of pentane into a diethyl ether solution at
ꢀ20 ꢀC. The crystal was twinned but the twinning was
non-merohedral, and the two partially overlapping diffrac-
tion patterns were indexed separately. Systematic absences
for hkl (h + k ¼ 2n), and h0l (l ¼ 2n) were consistent with
the space groups Cc and C2/c. The structure was solved
in Cc and later symmetrized to C2/c, which is correct space
group. No corrections for crystal decay were necessary
and, owing to the twinning, no absorption correction was
applied. Systematically absent reflections were deleted.
The structure was solved using direct methods by using
the SHELXTL software package. The correct positions for the
1
Er, 24.1%. H NMR (d8-THF): d ꢀ11 (s, fwhm = 680 Hz,
t-Bu), d 3.2 (br, DME). IR (cmꢀ1): 2753 s, 2724 s, 2667 s,
2615 w, 1267 m, 1194 m, 1127 m, 1085 s, 1031 m, 870 s,
784 s.

W. Noh, G.S. Girolami / Polyhedron 26 (2007) 3865–3870
3869
erbium atoms were deduced by direct-methods from an E-
map. Subsequent least-squares refinement and difference
Fourier calculations revealed the positions of the remaining
non-hydrogen atoms. Non-hydrogen atoms were refined
with independent anisotropic displacement parameters.
Hydrogen atoms were fixed in ‘‘idealized’’ positions and
their displacement parameters were tied to those of the
attached non-hydrogen atom. The Li(DME)3 cation cen-
tered on Li(3) was disordered about the two-fold axis.
The disorder involved overlapping atoms, and was mod-
eled in terms of three complete DME molecules, the atoms
of which were assigned site occupancy factors of exactly
0.5. In addition, two of the tert-butyl groups on Er(2) were
disordered over two rotameric conformations. The site
occupancy factors for these groups were initially refined,
but later set equal to exactly 0.5.
not modeled well; otherwise, there were no systematic
errors.
4. Supplementary material
CCDC 635390 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
We thank the National Science Foundation for support
of this research under Grant Nos. DMR03-54060 and
DMR04-20768, and Scott R. Wilson and Teresa Prussak-
Wieckowska for collecting the X-ray crystallographic data.
At this point, it became clear that several classes of
reflections were affected by the twinning. Reflections with
|l| = 0, 1, 11, 12, 13, 14, 24, 25, and 26 were systematically
more intense than calculated. For reflections with these
References
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0
0
0 0
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˚
˚
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