Divalent lanthanide complexes free of coordinating anions: Facile synthesis of fully solvated dicationic [LnLx]2+ compounds

Article abstract of DOI:10.1016/S0277-5387(02)01257-3

Fully-solvated, divalent lanthanide dication complexes free of coordinating anions, [LnL_x]²⁺, in which L = MeCN, THF, have been synthesized by protonation of amide, indenyl, and pentamethylcyclopentadienyl precursors. The Sm(II) dication [Sm(THF)₇]²⁺ was initially isolated as [Sm(THF)₇][BPh₄]₂, (1), by protonolysis of the indenyl complex (C₉H₇)₂Sm(THF)₃ with [Et₃NH][BPh₄]. Compound 1 can also be obtained from (C₅Me₅)Sm[N(SiMe₃)₂] (THF)₂ and [Et₃NH][BPh₄]. The Yb analog, [Yb(THF)₆][BPh₄]₂, (2), was obtained from the reaction of the bimetallic [(C₅Me₅)Yb(THF)]₂(C₈ H₈) with AgBPh₄ and the reaction of (C₅Me₅)Yb[N(SiMe₃)₂] (THF)₂ with [Et₃NH][BPh₄], but [YbL_x]²⁺ is best obtained from the reaction of Yb[N(SiMe₃)₂]₂(THF)₂ with [Et₃NH][BPh₄]. Reaction of Yb[N(SiMe₃)₂]₂(THF)₂ with [Et₃NH][BPh₄] in THF followed by recrystallization from acetonitrile affords [Yb(MeCN)₈][BPh₄]₂, (3). The seven THF molecules in 1 form a pentagonal bipyramidal ligand environment around Sm²⁺. Complex 2 has an octahedral Yb²⁺ coordination environment and 3 has a distorted square antiprismatic arrangement of MeCN ligands around Yb²⁺.

Full text of DOI:10.1016/S0277-5387(02)01257-3

Polyhedron 22 (2003) 119ꢃ126
/
www.elsevier.com/locate/poly
Divalent lanthanide complexes free of coordinating anions:
facile synthesis of fully solvated dicationic [LnLx]2ꢀ compounds
William J. Evans ꢁ, Matthew A. Johnston, Michael A. Greci,
Tammy S. Gummersheimer, Joseph W. Ziller
Department of Chemistry, University of California, Irvine, CA 92697-2025, USA
Received 12 July 2002; accepted 25 September 2002
Abstract
Fully-solvated, divalent lanthanide dication complexes free of coordinating anions, [LnLx]2ꢀ, in which Lꢂ
/MeCN, THF, have
been synthesized by protonation of amide, indenyl, and pentamethylcyclopentadienyl precursors. The Sm(II) dication
[Sm(THF)7]2ꢀ was initially isolated as [Sm(THF)7][BPh4]2, (1), by protonolysis of the indenyl complex (C9H7)2Sm(THF)3 with
[Et3NH][BPh4]. Compound 1 can also be obtained from (C5Me5)Sm[N(SiMe3)2](THF)2 and [Et3NH][BPh4]. The Yb analog,
[Yb(THF)6][BPh4]2, (2), was obtained from the reaction of the bimetallic [(C5Me5)Yb(THF)]2(C8H8) with AgBPh4 and the reaction
of (C5Me5)Yb[N(SiMe3)2](THF)2 with [Et3NH][BPh4], but [YbLx]2ꢀ is best obtained from the reaction of Yb[N(SiMe3)2]2(THF)2
with [Et3NH][BPh4]. Reaction of Yb[N(SiMe3)2]2(THF)2 with [Et3NH][BPh4] in THF followed by recrystallization from acetonitrile
affords [Yb(MeCN)8][BPh4]2, (3). The seven THF molecules in 1 form a pentagonal bipyramidal ligand environment around Sm2ꢀ
Complex 2 has an octahedral Yb2ꢀ coordination environment and 3 has a distorted square antiprismatic arrangement of MeCN
ligands around Yb2ꢀ
.
.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Lanthanides; Dications; Divalent; Cyclopentadienyl; Amide; Protonolysis
1. Introduction
[Sm(THF)7][Zn4(m2-SePh)6(SePh)4] [4] [(DIME)3Ln]-
[M(CO)x]2 (LnꢂSm, Yb, Eu; MꢂCo, xꢂ4; Mꢂ
Mn, xꢂ5, DIMEꢂdiethylene glycol dimethyl ether)
[5], {(DIME)2Yb(NCMe)2}{Hg[Fe(CO)4]2} [5], [(DI-
ME)Yb(NCMe)5][B12H12] [5], {(C5H5N)5Yb(NCMe)2}-
{Hg[Fe(CO)4]2} [5], [{Zr2(OiPr)9}Yb(THF)2][BPh4] [6]
{[C5H3(SiMe3)2]Sm(18-crown-6)}{[C5H3(SiMe3)2]3Sm}
[7] and {[C5H3(SiMe3)2]Yb(18-crown-6)}[C5H3(SiMe3)2]
[7]. The isolation of these specific compounds depends
crucially on the choice of reaction solvent and counter-
anion.
Some years ago in an effort to make cationic
metallocene complexes from (C9H7)2Sm(THF)3 [8] by
protonolysis with [Et3NH][BPh4], a common route to
make cationic f-element derivatives [2,9,10] the fully
solvated [Sm(THF)7][BPh4]2 (1), was isolated and crys-
tallographically characterized [11]. The existence of this
dication free of any coordinating anions stimulated our
interest in this class of compounds in terms of their
reactivity compared with that of the popular reagent
SmI2(THF)2 [12]. An indirect route to an ytterbium
/
/
/
/
Recently, the chemistry of cationic metal complexes
has received considerable attention due to the enhanced
electrophilicity conveyed on such complexes by the
formal positive charge and the effect this can have in
polymerization and Lewis acid catalyzed reactions [1,2].
Several types of cationic complexes are known for the
lanthanide metals, almost all of which involve the metals
/
/
in their most common and stable ꢀ3 oxidation state
/
[2,3]. Cationic complexes of divalent lanthanides are
potentially more interesting since they also possess
redox reactivity. To our knowledge the only crystal-
lographically characterized cationic complexes of the
divalent lanthanides in the literature are the following
specialized examples: [Yb(THF)6][Sn(SePh)3]2 [4],
ꢁ Corresponding author. Tel.: ꢀ
2210
E-mail address: wevans@uci.edu (W.J. Evans).
/
1-949-824-5174; fax: ꢀ1-949-824-
/
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 2 5 7 - 3

120
W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ126
/
analog, [Yb(THF)6][BPh4]2 (2), was also discovered in
the reaction of [(C5Me5)Yb(THF)]2(C8H8) [13] with 1
equiv. of AgBPh4. Attempts to make the putative
[(C5Me5)Yb(THF)x]ꢀ intermediate in the latter reaction
by protonolysis of (C5Me5)Yb[N(SiMe3)2](THF)2 [14]
2.3. [Sm(THF)7][BPh4]2 (1). From
(C5Me5)Sm[N(SiMe3)2](THF)2
In a glovebox, (C5Me5)Sm[N(SiMe3)2](THF)2 (0.126
g, 0.21 mmol) was reacted with [Et3NH][BPh4] (0.067 g,
0.22 mmol) in 4 ml of THF. The clear purple solution
was stirred 12 h and then centrifuged to remove a small
amount of white insoluble material. The solvent was
provided
a reliable route to 2. Protonolysis of
(C5Me5)Sm[N(SiMe3)2](THF)2 likewise formed 1.
Although these syntheses were more reliable and they
indicated that fully solvated lanthanide dication com-
plexes could be obtained with simple counter-anions, the
synthetic route was not optimal since it was not an
efficient use of the C5Me5 ligand.
removed from the deep purpleꢃblue solution via rotary
/
1
evaporation to leave purple solids. H NMR spectro-
scopy in THF-d8 was consistent with the presence of
(C5Me5)2Sm(THF)2 [20] and two BPh4 groups per two
C5Me5 groups. Concentration of the solution at
We report here that the reaction of the bis(trimethyl-
silyl)amide complexes, Ln[N(SiMe3)2]2(THF)2 (LnꢂSm
/
ꢄ30 8C resulted in the formation of purple crystals of
/
[15] Yb [16]) with alkylammonium salts of [BPh4]ꢄ
constitutes a useful synthesis of divalent lanthanide
complexes free of any coordinating anions. Dicationic
[LnLx]2ꢀ species are isolated in which L is a common
solvent such as THF or MeCN and the anion is a non-
coordinating tetraarylborate.
1 (0.103 g, 75%). The NMR spectra of the supernatant
1
revealed only (C5Me5)2Sm(THF)2. H NMR (THF-d8,
20 8C): d 7.14 (broad s, 2H), 6.85 (broad s, 3H), 3.57
(THF), 1.58 (THF). 13C NMR (THF-d8, 20 8C): d
133.9, 122.7, 119.1 (phenyl), 67.5, 23.2 (THF). IR: 2968
w, 2361 m, 2327 w, 1620 m, 1477 w, 1426 w, 1270 m,
1157 w, 1031 m, 950 w, 871 m, 711 m cmꢄ1. Anal. Calc.
for SmB2O7C76H96: C, 70.56; H, 7.42. Found: C, 69.80;
H, 7.26%.
2. Experimental
2.1. General
2.4. [Yb(THF)6][BPh4]2 (2). From
[(C5Me5)Yb(THF)]2(C8H8)
The chemistry described below was performed under
nitrogen with rigorous exclusion of air and water by
using Schlenk, vacuum line, and glovebox techniques.
Red [(C5Me5)Yb(THF)]2(C8H8) (0.072 g, 0.08 mmol)
reacts with AgBPh4 (0.036 g, 0.08 mmol) in 5 ml of THF
Ln[N(SiMe3)2]2(THF)2,
[14,15]
to form a brownꢃ
material over 8 h. Solids were removed by centrifugation
and the resulting redꢃbrown supernatant was dried via
rotary evaporation to yield a purpleꢃred solid. Extrac-
tion of this solid with 4 ml of toluene yielded a redꢃ
/
yellow solution and black insoluble
[(C5Me5)Yb(THF)]2(C8H8), [13] (C9H7)2Sm(THF)3, [8]
and (C5Me5)Ln[N(SiMe3)2](THF)2 [14] were prepared
by the literature method. Solvents were purified as
previously described [17]. NMR spectra were recorded
and magnetic moments were measured by the method of
Evans [18] using a Bruker DRX400 or a General Electric
GE500 spectrometer. Infrared spectra were recorded on
a ReactIR system (Applied Systems Inc.) as thin films
[6]. Complexometric analyses for Yb and Sm were
performed as previously described [19]. C and H
elemental analyses were carried out by Desert Analytics,
Tucson, AZ.
/
/
/
purple toluene solution and black insoluble material.
1
The H and 13C NMR spectra of the toluene soluble
product showed only the presence of (C5Me5)Yb(C8H8)
[13]. When the toluene insoluble material was treated
with THF, a bright yellow solution formed from which 2
crystallizes from over the period of 48 h at room
temperature (r.t.; 0.037 g, 36%). These crystals were
suitable for X-ray analysis. The NMR spectrum of the
supernatant revealed only (C5Me5)2Yb(THF) [21] (1.89
1
ppm). H NMR (THF-d8, 20 8C): d 7.35 (m, C6H5),
2.2. [Sm(THF)7][BPh4]2, (1). From
(C9H7)2Sm(THF)3
6.91 (m, C6H5), 6.78 (m, C6H5), 3.65 (s, THF), 1.75 (s,
THF) 13C NMR: d 137.7, 125.8, 121.9. (C6H5). IR: 3033
m, 2991 w, 2949 m, 2869 w, 2243 w, 1644 s, 1513 s, 1463
s, 1278 s, 1089 s, 1046 m, 940 s, 907 m, 837 w, 772 s, 683
m cmꢄ1. Anal. Calc. for YbB2O6C72H88: C, 69.51; H,
7.13. Found: C, 69.14; H, 6.71%. Addition of MeCN to
2 quantitatively generates 3, as determined by NMR
spectroscopy and by indexing the crystals of 3, which
formed.
In a glovebox, (C9H7)2Sm(THF)3 (27 mg, 0.055
mmol) and [Et3NH][BPh4] (24 mg, 0.057 mmol) were
stirred in 5 ml of THF for 20 h. The initially bright
purple mixture became greenꢃ
this THF solution at ꢄ35 8C for 48 h resulted in the
/
brown. Concentration of
/
isolation of 1 as purple crystals suitable for X-ray
crystallographic analysis.

W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ
/126
121
2.5. [Yb(THF)6][BPh4]2 (2). From
(C5Me5)Yb[N(SiMe3)2](THF)2
Table 1
X-ray collection parameters for [Sm(THF)7][BPh4]2×THF (1) THF,
[Yb(THF)6][BPh4]2×THF (2) THF, and [Yb(MeCN)8][BPh4]2×2MeCN
(3) 2MeCN
In a glovebox, (C5Me5)Yb[N(SiMe3)2](THF)2 (0.267
g, 0.44 mmol) reacts with [Et3NH][B(C6H5)4] (0.137 g,
0.46 mmol) in 4 ml of THF over 12 h to form a deep
Compound
Formula
Fw
1 THF
C
1356.6
2 THF
80H114O8B2Sm C76H96B2O7Yb C68H70B2N10Yb
3 2MeCN
1316.18
163
1222.00
158
orangeꢃyellow solution and a small amount of white
/
Temperature (K) 296
Crystal system
Space group
insoluble material which was removed by centrifugation.
The solvent was removed via rotary evaporation to leave
1
a yellow solid. H NMR data (THF-d8) indicated the
monoclinic
C2/c
monoclinic
P21/n
tetragonal
I41cd
˚
a (A)
20.742(3)
19.319(3)
18.556(3)
90
11.9661(6)
13.3457(7)
21.0776(10)
90
18.3318(6)
18.3318(6)
39.9872(17)
90
˚
b (A)
presence of (C5Me5)2Yb(THF) [21] and phenyl signals
consistent with the presence of two BPh4 groups per two
C5Me5 groups. Concentration of the solution at
˚
c (A)
a (8)
b (8)
g (8)
95.085(11)
90
94.580(1)
90
90
90
ꢄ30 8C resulted in the formation of yellow crystals of
/
3
˚
2 (0.210 g, 77%), while the NMR spectrum of the
supernatant showed only peaks for (C5Me5)2Yb(THF).
V (A )
7406(2)
4
1.225
3355.3(3)
2
1.303
13 437.9(8)
8
1.208
Z
rcalc (Mg mꢄ3
)
Diffractometer
m (mmꢄ1
Final R indicies 0.080
R1[I ꢀ2s(I)] 0.072
Siemens P4 RA Siemens CCD Siemens CCD
2.6. [Yb(MeCN)8][BPh4]2, (3)
)
0.846
1.447
0.1558
0.0693
1.437
0.1307
0.0470
In a glovebox, red Yb[N(SiMe3)2]2(THF)2 (71 mg,
0.11 mmol) reacts with [Et3NH][BPh4] (94 mg, 0.23
mmol) in THF over 12 h to form a pale yellow
supernatant which was separated from yellow insoluble
material via centrifugation. Addition of 5 ml of MeCN
to the insoluble product generates a bright orange
Crystallographic calculations were carried out using
either a locally modified version of the UCLA Crystal-
lographic Computing Package [24] or the SHELXTL PLUS
program set [25]. The analytical scattering factors for
neutral atoms were used throughout the analysis; [26]
both the real (Df?) and imaginary (iDfƒ) components of
anomalous dispersion were included. The structure was
solution. Concentration of this solution at ꢄ30 8C
/
1
resulted in the isolation of 3 (86 mg, 68%). H NMR
(CD3CN, 20 8C): d 7.25 (m, 2H, C6H5), 6.98 (m, 2H,
C6H5), 6.82 (m, 1H, C6H5), 1.96 (MeCN). 13C NMR: d
136.8, 126.6, 122.8 (C6H5). IR: 3057 m, 3003 w, 2930 s,
2304 m, 2274 m, 1583 w, 1482 w, 1268 m, 1185 m, 1067
m, 1031 s, 988 m, 926 s, 849 m, 737 s, 706 s cmꢄ1. Anal.
Calc. for C64H64N8B2Yb: C, 67.43; H, 5.66. Found: C,
68.09; H, 5.63%. Crystals suitable for X-ray analysis
were obtained from a concentrated solution of 3 at r.t.
solved via an automated Patterson routine (SHELXTL)
and refined by full-matrix least-squares techniques.
Hydrogen atoms were included using a riding model
2
0.08 A . The molecule
˚
0.96 A and Uiso
˚
with d(CÃ
/
H)ꢂ
/
ꢂ
/
is located about a twofold rotation axis. Atoms Sm(1)
and O(2) lie directly on the twofold axis and were each
assigned site-occupancy-factors of 1/2. There are two
molecules of tetraphenylborate and one molecule of
THF per formula unit. The THF molecule is located
about the twofold rotation axis and is disordered. The
five atoms that define the THF are disordered over six
positions as a result of the molecule lying about the
twofold. Each of the three symmetry-independent atoms
(C(39), C(40), C(41)) were refined as carbon atoms with
site-occupancy-factors of 0.88889 to account for an
equal disorder of the oxygen and carbon atoms over
the six sites. The hydrogen atoms associated with the
disordered THF were not included in the refinement.
Refinement of positional and thermal parameters led to
2.7. X-ray data collection, structure determination and
refinement for [Sm(THF)7][BPh4]2×
/THF
A dark red crystal of approximate dimensions 0.13ꢅ
/
0.23ꢅ0.30 mm was mounted in a thin-walled capillary
/
under nitrogen and transferred to a Siemens P4 rotating-
anode diffractometer. The determination of Laue sym-
metry, crystal class, unit cell parameters, and the
crystal’s orientation matrix were carried out by standard
procedures [22]. Intensity data were collected at 296 K
using a u ꢃ2u scan technique with Mo Ka radiation
/
under the conditions described in Table 1. All 5176 data
were corrected for absorption [23] and for Lorentz and
polarization effects and were placed on an approxi-
mately absolute scale. The diffraction symmetry was 2/
convergence with RFꢂ
1.20 for 416 variables refined against those 3439 data
with F ꢀ3.0s(F). A final difference-Fourier synthesis
yielded r(max)ꢂ
and angles are given in Table 2.
/
7.2%; RwF
ꢂ
/
8.0% and GOFꢂ
/
m with systematic absences hkl for hꢀ
/
kꢂ
/
2nꢀ
/
1 and
h0l for hꢀlꢂ2nꢀ1. The two possible monoclinic space
/
/
/
/
groups are the noncentrosymmetric Cc or the centro-
symmetric C2/c. It was later determined that space
group C2/c was correct.
/
0.90 e Aꢄ3. Selected bond distances
˚

122
W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ126
/
2.8. X-ray data collection, structure determination, and
refinement for [Yb(THF)6][BPh4]2×THF
Table 3
Selected bond lengths and angles for [Yb(MeCN)8][BPh4]2 (3)
/
Bond lengths
A yellow crystal of dimensions 0.33ꢅ
/
0.11ꢅ0.11 mm
/
YbÃN(1)
YbÃN(2)
YbÃN(3)
YbÃN(4)
2.566(10)
2.537(10)
2.552(10)
2.570(9)
was mounted on a glass fiber and transferred to a
Siemens CCD platform diffractometer. The SMART [27]
program package was used to determine the unit-cell
parameters and for data collection (30 s per frame scan
time for a hemisphere of diffraction data). The raw
frame data were processed using SAINT [28] and SADABS
[29] to yield the reflection data files. Subsequent
calculations were carried out using the SHELXTL [30]
program. The diffraction symmetry was 2/m and the
systematic absences were consistent with the centrosym-
metric monoclinic space group P21/n which was later
determined to be correct. The structure was solved by
direct methods and refined on F2 by full-matrix least-
squares techniques. One molecule of THF co-crystal-
lized with the ytterbium complex. Disorder in the BPh4
and THF groups was modeled by assigning partial
occupancy to disordered components. The analytical
scattering factors [26] for neutral atoms were used
throughout the analysis. Hydrogen atoms were included
Bond angles
N(1)ÃYbÃN(2)
N(1)ÃYbÃN(3)
N(1)ÃYbÃN(4)
N(2)ÃYbÃN(3)
N(2)ÃYbÃN(4)
N(3)ÃYbÃN(4)
N(2?)ÃYbÃN(2)
N(2?)ÃYbÃN(3)
N(3?)ÃYbÃN(1?)
N(2)ÃYbÃN(4?)
73.6(3)
74.1(3)
86.44(17)
119.51(18)
149.4(4)
74.9(3)
82.1(4)
132.42(16)
150.0(4)
69.8(3)
structure was solved as described in the previous
paragraph. Two molecules of acetonitrile co-crystallized
with the compound. Disorder in the acetonitrile was
modeled by assigning partial occupancy to disordered
components. At convergence, wR2ꢂ
1.188 for 363 variables refined against 7426 unique data.
Selected bond distances are given in Table 3.
/
0.1307 and GOFꢂ
/
using a riding model. At convergence, wR2ꢂ
/
0.1558 and
GOFꢂ1.151 for 355 variables refined against 8004
/
unique data. Selected bond distances are given in Table
2.
3. Results
2.9. X-ray data collection, structure determination, and
2MeCN
3.1. Synthetic results. Initial isolation of
[Sm(THF)7]2ꢀ (1), and [Yb(THF)6]2ꢀ (2)
refinement for [Yb(MeCN)8][BPh4]2×
/
A yellow crystal of dimensions 0.40ꢅ
/
0.27ꢅ0.30 mm
/
As part of a study of the reaction chemistry of the
bis(indenyl) Sm(II) metallocene, (C9H7)2Sm(THF)3 [8]
this complex was treated with [Et3NH][BPh4] to see if
Sm(II) reduction or ligand protonation would occur
first. Surprisingly, the only isolated product was
[Sm(THF)7][BPh4]2 (1), Fig. 1, which contained a fully
solvated Sm(II) dication free of any coordinating
anions. Although this reaction provided material suita-
ble for crystallographic studies and demonstrated the
existence of this dication, it had neither the yield nor the
reliability to be useful synthetically.
was handled as described in the previous paragraph. The
diffraction symmetry was 4/mmm and the systematic
absences were consistent with the tetragonal space group
I41cd which was later determined to be correct. The
Table 2
Selected bond lengths and angles for [Sm(THF)7][BPh4]2 (1), and
[Yb(THF)6][BPh4]2 (2)
[Sm(THF)7][BPh4]2
[Yb(THF)6][BPh4]2
Bond lengths
SmÃO(1)
SmÃO(2)
SmÃO(3)
SmÃO(4)
2.646(10) YbÃO(1)
2.388(4)
2.392(4)
2.350(4)
An ytterbium analog of 1 was also obtained in an
indirect manner. In this case, the synthesis arose from
efforts to prepare a mixed valent bimetallic species
2.555(8)
2.569(8)
2.621(8)
YbÃO(2)
YbÃO(3)
such
as
{[(C5Me5)YbIII(THF)x](m-C8H8)[(C5Me5)-
Bond angles
YbII(THF)x]}ꢀ from the reaction of [(C5Me5)Yb-
(THF)]2(C8H8) with 1 equiv. of AgBPh4. As shown in
Eq. (1), the
O(1)ÃSmÃO(2)
O(2)ÃSmÃO(4)
O(4)ÃSmÃO(2?)
O(1)ÃSmÃO(3)
O(4)ÃSmÃO(4?)
O(2)ÃSmÃO(3)
O(3?)ÃSmÃO(2)
O(2)ÃSmÃO(2?)
O(3)ÃSmÃO(4)
87.8(2)
77.1(3)
106.6(3)
75.3(2)
72.9(4)
99.0(3)
79.9(3)
175.5(4)
74.6(3)
O(1)ÃYbÃO(2)
O(1)ÃYbÃO(3)
O(2)ÃYbÃO(3)
O(1)ÃYbÃO(1?)
O(3?)ÃYbÃO(1)
O(1?)ÃYbÃO(2?)
O(3)ÃYbÃO(2?)
88.14(15)
88.13(15)
89.50(16)
180.0(30
91.87(15)
91.86(15)
90.50(16)
[(C5Me5)Yb(THF)]2(C8H8)ꢀAgBPh4
THF
toulene(C5Me5)Yb(C8H8)ꢀA
(1)
ꢄAg
reaction, initially led to the formation of toluene soluble
(C5Me5)Yb(C8H8) and a black toluene insoluble mate-

W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ
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123
Fig. 1. Thermal ellipsoid plot of [Sm(THF)7][BPh4]2 (1), with ellipsoids drawn at the 50% probability level.
rial, A. Subsequent treatment of A with THF gave a
3.2. Improved synthesis of [Yb(THF)6]2ꢀ
mixture from which [Yb(THF)6][BPh4]2, (2), preferen-
tially crystallized leaving (C5Me5)2Yb(THF) [21] in the
mother liquor according to Eq. (2).
The products of Eq. (2) suggested that the black
insoluble A contained a mono(pentamethylcyclopenta-
dienyl) Yb(II) cation, [(C5Me5)Yb(THF)x]ꢀ. In efforts
to make such a complex directly, (C5Me5)Yb[N(Si-
Me3)2](THF)2 [14] was treated with 1 equiv. of
[Et3NH][BPh4] in THF. This resulted in the formation
A T0HF[Yb(THF)6][BPh4]2 ꢀ(C5Me5)2Yb(THF)
(2)
1
of a yellow solution which had a H NMR spectrum
containing a resonance at 1.93 ppm in THF-d8 different
from (C5Me5)2Yb(THF) or starting materials. Attempts
to crystallize this product again generated [Yb(THF)6]-
[BPh4]2 and (C5Me5)2Yb(THF) in a combined yield of
77% based on Yb, according to Eq. (3).
Compound 2 was characterized by NMR, IR, ele-
mental analysis and X-ray crystallography, Fig. 2. The
1H NMR spectrum of 2 shows only resonances for the
tetraphenylborate groups at 7.35, 6.91, and 6.78 ppm
and coordinated THF appears at 3.65 and 1.75 ppm.
The 13C NMR spectrum has tetraphenylborate peaks at
137.7, 125.8, and 121.9 ppm as well as peaks for the
associated THF molecules.
(C5Me5)Yb[N(SiMe3)2](THF)2 ꢀ[Et3NH][BPh4]
THF
0 [Yb(THF)6][BPh4]2 ꢀ(C5Me5)2Yb(THF)
(3)
The [Sm(THF)7]2ꢀ dication, 1, could also be gener-
ated in good yield (75%) by an analogous route, Eq. (4).
(C5Me5)Sm[N(SiMe3)2](THF)2 ꢀ[Et3NH][BPh4]
THF
0 [Sm(THF)7][BPh4]2 ꢀ(C5Me5)2Sm(THF)2
(4)
3.3. Reactions of Ln[N(SiMe3)2]2(THF)2 with
ammonium reagents
Since Eqs. (1)ꢃ(4) were neither intended as routes to
/
[LnLx]2ꢀ cations nor were they efficient routes since the
cyclopentadienyl and cyclooctatetraenyl byproducts
were not the primary synthetic targets, a more direct
route to the fully solvated cations was pursued. Eq. (3)
suggested that protonolysis of Yb[N(SiMe3)2]2(THF)2
would be a good route to 2. Surprisingly, protonolysis of
Yb[N(SiMe3)2]2(THF)2 in THF gave a yellow powder
which was insoluble. This material analyzed for
Fig. 2. Thermal ellipsoid plot of the [Yb(THF)6]2ꢀ dication in 2,
drawn at the 50% probability level.

124
W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ126
/
[Yb(THF)6][BPh4]2, but could not be recrystallized for
full structural and spectroscopic confirmation of its
composition. Further evidence supporting the analytical
data was the fact that addition of a small amount of
MeCN generated [Yb(MeCN)8][BPh4]2 (3), quantita-
tively. Similar results were obtained with the reaction
of Sm[N(SiMe3)2]2(THF)2 with ammonium reagents.
Since 3 could be readily isolated and crystallized from
MeCN, the direct reaction of Yb[N(SiMe3)2]2(THF)2
with 2 equiv. of [Et3NH][BPh4] in MeCN was examined.
This formed 3 in 68%, Eq. (5).
3.4.2. [Yb(THF)6][BPh4]2 (2)
As shown in Fig. 2, the dication [Yb(THF)6]2ꢀ has a
well-defined octahedral geometry of oxygen atoms
around the Yb center. The cis O(THF)Ã
angles range from 88.13(15)8 to 91.87(13)8 while the
O angles have an average of 180.03(10)8.
/
YbÃ/O(THF)
trans OÃ
/
YbÃ
/
In this structure, the O(1) and O(3) THF rings were
disordered and were modeled with partial occupancy.
The YbÃ
/
O(THF) distances range from 2.350(4) to
˚
2.392(2) A. In comparison, the ytterbium dication
˚
2.402(11) A
[Yb(THF)6][Sn(SePh)3]2 [4] has 2.354(11)ꢃ
/
YbÃ
/
O(THF) distances and similar O(THF)Ã
/
YbÃ
/
Yb[N(SiMe3)2]2(THF)2 ꢀ2Et3NHBPh4
O(THF) angles.
THF
0 [Yb(MeCN)8][BPh4]2
(5)
3.4.3. [Yb(MeCN)8][BPh4]2 (3)
Compound 3 was characterized by NMR and IR
spectroscopy, elemental analysis, and X-ray crystallo-
The eight acetonitrile ligands coordinated to Yb2ꢀ in
3 define a distorted square antiprismatic structure, Fig.
3. The dihedral angle between the N(1), N(2), N(3),
N(4a) plane and the N(1a), N(2a), N(3a), and N(4a)
1
graphy. The H NMR and 13C spectra of 3 contained
only the resonances of the anion and MeCN. Complex 3
displayed a nCN stretch at 2274 cmꢄ1 in the infrared
spectrum which can be compared with the 2260 cmꢄ1
absorption in free MeCN. Higher nCN stretching fre-
quencies for MeCN solvated to lanthanides are common
[3a,3b,31].
plane is 179.2(1)8. The YbÃ
/
N(MeCN) bond lengths
˚
range from 2.537(10) to 2.570(9) A. These distances can
˚
be compared the analogous 2.44(2) and 2.46(3) A
distances in the seven coordinate [(py)5Yb(MeCN)2]2ꢀ
˚
[5] and a range of 2.513(6)ꢃ
/
2.597(7) A YbÃ
/
N(MeCN)
distances in the eight coordinate [(DIME)Yb-
(MeCN)5]2ꢀ [5].
3.4. Structural results
4. Discussion
3.4.1. Structure of [Sm(THF)7][BPh4]2 (1)
The formation of fully solvated divalent lanthanide
dications free of anionic ligands appears to be facile via
a number of synthetic pathways. Initial attempts to
make the cyclopentadienyl-ligated cations [(C9H11)Sm-
(THF)x]ꢀ and [(C5Me5)Yb(THF)x](m-C8H8)[Yb(C5-
Me5)(THF)x]ꢀ generated the dications, [Sm(THF)7]2ꢀ
and [Yb(THF)6]2ꢀ instead. Likewise, in the deliberate
As shown in Fig. 1, the seven THF molecules adopt a
distorted pentagonal bipyramidal geometry around
Sm(II). This geometry is the same as is found in other
divalent, seven coordinate Sm structures, such as
SmI2(THF)5, [32] SmI2(DME)(THF)3, [32] and SmI2(D-
ME)2(THF) [32]. The irregularity of the structure can be
seen from the O(2)Ã
which is not quite linear, and the 77.1(3)8ꢃ
O(axial)ÃSmÃO(equatorial) angles, which would be 908
in an idealized pentagonal bipyramid. The
O(equatorial)ÃSmÃO(equatorial) angles in 1, which
/
SmÃ
/
O(2?) angle in 1, 175.5(4)8,
reaction
of
(C5Me5)Yb[N(SiMe3)2](THF)2
and
/
106.6(3)8
/
/
/
/
should be 728 in an idealized pentagonal bipyramid,
range from 72.9(4)8 to 75.3(2)8. The equatorial THF
ligands do not line up in a regular canted arrangement.
The same samarium dication in [Sm(THF)7][Zn4(m2-
SePh)6(SePh)4] [4] has a pentagonal bipyramidal geo-
metry even more distorted than that in 1. The O(axial)Ã
SmÃO(axial) angle is 166.6(7)8 with the O(axial)ÃSmÃ
O(equatorial) angles ranging from 77.2(7)8 to 115.3(6)8.
/
/
/
/
˚
˚
The axial (2.55(5) A) and equatorial (2.60(6) A) SmÃ
/
O(THF) bond lengths in 1 are similar to those in
˚
SmI2(DME)2(THF) (2.530(5) A), [32] SmI2(D-
˚
ME)(THF)3 (2.578(4) A), [32] (C13H9)2Sm(THF)2
˚
(2.550(6) A), [8] (C5Me5)2Sm(THF) (2.569(3) A), [33]
˚
and (C5Me5)2Sm(THF)2 (2.62(1) A) [20].
˚
Fig. 3. Thermal ellipsoid plot of the dication [Yb(MeCN)8]2ꢀ and one
[BPh4]ꢄ anion in 3 with ellipsoids drawn at the 50% probability level.

W.J. Evans et al. / Polyhedron 22 (2003) 119ꢃ
/126
125
[Et3NH][BPh4] to make [(C5Me5)Yb(THF)x]ꢀ, the
dicationic [Yb(THF)6]2ꢀ species was isolated preferen-
tially. This situation does not arise from any inherent
incompatibility of cyclopentadienyl rings with cationic
complexes as evidenced by the many types of metallo-
cene cations, [(C5R5)2MLx]ꢀ and [(C5R5)2MRL]ꢀ, in
the literature [1] and the isolation of {[C5H3(Si-
Me3)2]Sm(18-crown-6)}{[C5H3(SiMe3)2]3Sm}, [7] and
{[C5H3(SiMe3)2]Yb(18-crown-6)}[C5H3(SiMe3)2] [7].
soluble species are to be obtained. Hence, acetonitrile
provides a soluble complex with tetraphenylborate,
[Yb(MeCN)8][BPh4]2, (3), but in THF, the [BPh4]ꢄ
salt, [Yb(THF)6][BPh4]2, (2), is formed as an insoluble
powder.
5. Conclusion
The [(C5Me5)Yb(THF)]2(C8H8)ꢃ
/
AgBPh4 reaction
These studies show that [Ln(solvent)x]2ꢀ[anion]2ꢄ are
often the end products in reactions of divalent lantha-
nide species with protic reagents. In the presence of the
appropriate solvent, soluble dications free of coordinat-
ing anions are accessible in high yields. The chemistry of
these divalent dicationic species is currently under
investigation.
suggests that a divalent cyclopentadienyl intermediate
such as [(C5Me5)YbII(THF)](m-C8H8)[YbIII(C5Me5)-
(THF)x] is formed, but its stability is marginal. The
formation of the (C5Me5)Yb(C8H8) product is the usual
result from reactions of [(C5Me5)Ln]2(C8H8) with re-
ducible substrates [13]. This leaves the [(C5Me5)Yb]ꢀ
monocation, which is usually oxidized further and
trapped by the oxidant. Clearly, this [(C5Me5)Yb]ꢀ is
a reactive species since only (C5Me5)2Yb(THF)x and
[YbLx]2ꢀ were isolated from this system. Further
evidence on this disproportionation was obtained from
the reaction of (C5Me5)Yb[N(SiMe3)2](THF)2 with 1
equiv. of [Et3NH][BPh4] in THF. In this case, a new
C5Me5 NMR resonance was observed consistent with
the presence of [(C5Me5)Yb(THF)x]ꢀ, but only
[YbLx]2ꢀ and (C5Me5)2Yb(THF) were isolable. This
‘[(C5Me5)Yb]ꢀ’ system does have the advantage that it
can provide [YbLx]2ꢀ in soluble form which could be
useful synthetically in further reactions.
Acknowledgements
We thank the National Science Foundation for
support of this research.
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