Observation of internal electron transfer in bulky allyl ytterbium complexes with substituted terpyridine ligands

Article abstract of DOI:10.1021/ic060603x

A series of new bulky allyl terpyridyl-ytterbium complexes have been synthesized to determine the effect of allyl ligands on the internal charge-transfer process that exists in these materials. Compared to the pentamethylcyclopentadienyl-ytterbocene compound Cp*₂Yb(tpyCN) (ν_C≡N = 2172 cm^-1), the symmetrically substituted allyl complex [1,3-(SiMe₃)₂C₃H ₃]₂Yb(tpyCN) possesses a markedly lowered C≡N frequency of 2130 cm^-1. Furthermore, the electronic nature of these bulky allyl complexes can be tuned, as demonstrated by the C≡N frequency of the asymmetric derivatives [1-(SiMe₃)C₃H ₄]₂Yb(tPyCN) and [1-(SiPh₃)-3-(SiMe ₃)C₃H₃]₂Yb(tpyCN) (2171 and 2164 cm^-1, respectively). The differences in these frequencies can be attributed to differences in the ligands' steric and electronic character. Single-crystal X-ray characterization of [1,3-(SiMe₃) ₂C₃H₃]₂Yb(tpy) reveals that the allyl moiety possesses, shorter Yb-C and Yb-N bond distances than the Cp* analogue. The magnetic susceptibility data for [1,3-(SiMe₃) ₂C₃H₃]₂Yb(tpy) departs dramatically from the Curie law, with a room-temperature magnetic moment of 2.95 μ_B.

Full text of DOI:10.1021/ic060603x

Inorg. Chem. 2006, 45, 7004−7009
Observation of Internal Electron Transfer in Bulky Allyl Ytterbium
Complexes with Substituted Terpyridine Ligands
Rosemary E. White,† Christin N. Carlson,‡ Jacqueline M. Veauthier,‡ Cheslan K. Simpson,‡
J. D. Thompson,§ Brian L. Scott,‡ Timothy P. Hanusa,*,† and Kevin D. John*,‡
Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee 37235, and Chemistry and
Materials Science and Technology DiVisions, Los Alamos National Laboratory (LANL),
Los Alamos, New Mexico 87545
Received April 10, 2006
A series of new bulky allyl terpyridyl
ligands on the internal charge-transfer process that exists in these materials. Compared to the pentamethylcyclo-
pentadienyl ytterbocene compound Cp*2Yb(tpyCN) (νC
2172 cm-1), the symmetrically substituted allyl complex
[1,3-(SiMe3)2C3H3]2Yb(tpyCN) possesses a markedly lowered C
N frequency of 2130 cm-1. Furthermore, the
electronic nature of these bulky allyl complexes can be tuned, as demonstrated by the C
−ytterbium complexes have been synthesized to determine the effect of allyl
−
)
tN
t
t
N frequency of the
asymmetric derivatives [1-(SiMe3)C3H4]2Yb(tpyCN) and [1-(SiPh3)-3-(SiMe3)C3H3]2Yb(tpyCN) (2171 and 2164 cm-
,
1
respectively). The differences in these frequencies can be attributed to differences in the ligands’ steric and electronic
character. Single-crystal X-ray characterization of [1,3-(SiMe3)2C3H3]2Yb(tpy) reveals that the allyl moiety possesses
shorter Yb
[1,3-(SiMe3)2C3H3]2Yb(tpy) departs dramatically from the Curie law, with a room-temperature magnetic moment of
2.95 µB
−C and Yb−N bond distances than the Cp* analogue. The magnetic susceptibility data for
.
Introduction
Although the nature of the magnetic coupling between the
YbIII f13 metal center and the ligand radical anion is not the
same for the aforementioned Cp* adducts, the room-
temperature magnetic moments (µeff) are lower than that
predicted for a YbIII f13 metal center with a π*1 ligand.
In addition to the ytterbocene-polypyridyl complexes, we
have studied the structures and catalytic activity of a series
of bulky allyl lanthanide complexes including [1,3-(Si-
Me3)2C3H3]2Yb(THF)2 (1; THF ) tetrahydrofuran).5 The use
of trimethylsilyl-substituted allyl ligands has become a
common way to synthesize thermally stable allyl complexes
throughout the periodic table. Indeed, the symmetrically
substituted allyl ligand [1,3-(SiMe3)2C3H3]- has been incor-
porated into complexes with early-main-group metals,5-8
transition metals,9-11 lanthanides,5,12-15 and actinides.16 We
report here the application of bulky allyl ytterbium synthons
The charge-transfer behavior of cyclopentadienyl sandwich
compounds of the divalent lanthanides has been of interest
for some time.1,2 In particular, a variety of ytterbocene
N-heterocyclic adducts (e.g., Cp*2Yb(L), where Cp* ) C5-
Me5 and L ) 2,2′-bipyridine (bpy),1 10-phenanthroline
(phen),1 2,2′:6′,2′′-terpyridine (tpy),2 and 4′-cyano-2,2′:6′,2′′-
terpyridine (tpyCN)2) have been reported to display a stable
charge-transfer electronic configuration derived from a
spontaneous electron transfer from a diamagnetic YbII f14
metal center to the lowest unoccupied molecular orbital
(LUMO) on the N-heterocyclic ligand, (f14 π*0 f f13 π*1).
This spontaneous electron transfer has been examined
extensively with electrochemical and spectroscopic methods.2-4
* To whom correspondence should be addressed. E-mail:
t.hanusa@vanderbilt.edu (T.P.H.), kjohn@lanl.gov (K.D.J.).
† Vanderbilt University.
(3) Kuehl, C. J.; Da Re, R. E.; Scott, B. L.; Morris, D. E.; John, K. D.
Chem. Commun. 2003, 2336-2337.
(4) Da Re, R. E.; Kuehl, C. J.; Brown, M. G.; Rocha, R. C.; Bauer, E.
D.; John, K. D.; Morris, D. E.; Shreve, A. P.; Sarrao, J. L. Inorg.
Chem. 2003, 42, 5551-5559.
(5) Simpson, C. K.; White, R. E.; Carlson, C. N.; Wrobleski, D. A.; Kuehl,
C. J.; Croce, T. A.; Steele, I. M.; Scott, B. L.; Hanusa, T. P.;
Sattelberger, A. P.; John, K. D. Organometallics 2005, 24, 3685-
3691.
‡ Chemistry Division, Los Alamos National Laboratory.
§ Materials Science and Technology Division, Los Alamos National
Laboratory.
(1) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R.
A. Organometallics 2002, 21, 460-472.
(2) Veauthier, J. M.; Schelter, E. J.; Kuehl, C. J.; Clark, A. E.; Scott, B.
L.; Morris, D. E.; Martin, R. L.; Thompson, J. D.; Kiplinger, J. L.;
John, K. D. Inorg. Chem. 2005, 44, 5911-5920.
7004 Inorganic Chemistry, Vol. 45, No. 17, 2006
10.1021/ic060603x CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/22/2006

Bulky Allyl Ytterbium Complexes
temperature. Anhydrous solvents, tetrahydrofuran (THF), toluene,
and methylene chloride were purchased from Aldrich or Acros and
stored in the glovebox over activated 4 Å molecular sieves overnight
and passed the ketyl test before use. Deuterated solvents (C6D6,
THF-d8, and toluene-d8) were sparged with Ar and stirred over a
Na/K (1:2) alloy, from which they were transferred under vacuum;
CDCl3 (Acros) was used as received.
in the development of new magnetic materials with two key
objectives: (1) to determine the difference in the magnetic
behavior between terpyridyl-ytterbium complexes with Cp*
and bulky allyl ligands so as to understand the nature of the
internal charge transfer and its impact on the magnetism of
these molecular species and (2) to tune the electronic/
magnetic behavior of these complexes through allyl substitu-
tion. To facilitate the latter, we have synthesized an
asymmetrically substituted allyl ligand, [1-(SiPh3)-3-(SiMe3)-
C3H3]-, through a preparation similar to that of the symmetric
trimethylsilylated allyl ligand.5,17 In addition to these two
allyl ligands, the previously reported,17 less bulky asymmetric
allyl ligand, [1-(SiMe3)C3H4]-, has also been used for
comparison.
Preparation of 1-(SiPh3)-3-(SiMe3)C3H4. Allyltrimethylsilane
(3.55 g, 31.07 mmol) and hexanes (100 mL) were added to a 250-
mL Schlenk flask equipped with a stirring bar. After the solution
was cooled to 0 °C, n-BuLi (12.4 mL, 31.10 mmol) was added
dropwise over 20 min. After the solution was allowed to warm to
room temperature overnight, it was brought into the glovebox.
Chlorotriphenylsilane (9.16 g, 31.07 mmol) was added slowly over
10 min. Chlorotriphenylsilane was not soluble in the reaction
mixture; therefore, THF (40 mL) was added to the solution, which
immediately turned light orange and cloudy. The reaction was
allowed to stir for 8 h. The solution was extracted with ∼25 mL of
deionized water and ∼10 mL of diethyl ether three times each.
The organic layers were combined, dried with magnesium sulfate,
and filtered. Solvent was removed under vacuum, yielding 7.58 g
of a white powder (20.3 mmol, 65% yield). 1H NMR (25 °C, 300
MHz, CDCl3): δ 7.51 (m, SiPh3, 6H), 7.38 (m, SiPh3, 9H), 6.07
(dt, J1 ) 18.5 Hz, J2 ) 7.7 Hz, CHCHCH2, 1H), 5.48 (d, J ) 18.5
Hz, CHCHCH2, 1H), 2.48 (d, J ) 7.7 Hz, CHCHCH2, 2H), -0.08
(s, SiMe3, 9H). MS: m/e 372 (M+), 259 (SiPh3+), 105 (SiPh+).
Preparation of K[1-(SiPh3)-3-(SiMe3)C3H3]. Hexanes (75 mL)
was added to a 125-mL Schlenk flask containing vacuum-dried
1-(SiPh3)-3-(SiMe3)C3H4 (5.00 g, 13.41 mmol) and a stirring bar.
After the solution was cooled to 0 °C, n-BuLi (5.4 mL, 13.50 mmol)
was added dropwise over 10 min. After the solution was stirred
overnight while warming to room temperature, KO-t-Bu (1.52 g,
13.51 mmol) was added slowly. The solution became sticky and
dark orange, and THF (25 mL) was added to increase the solubility
of the product. The solution was stirred for 10 h. THF was removed
under vacuum, and additional hexanes was added. The yellow solid
that remained was filtered over a medium-porosity glass frit. The
solid was washed with hexanes and dried under vacuum, yielding
Experimental Section
General Considerations. All manipulations were performed with
the rigorous exclusion of air and moisture using Schlenk or
glovebox techniques. 1H NMR spectra were obtained on a Bruker
DPX 300-MHz or an Avance 400-MHz spectrometer and were
referenced to the residual resonances of C6D6 (δ 7.16), THF-d8 (δ
3.58), or toluene-d8 (δ 2.09). IR spectra were recorded on a Thermo-
Nicolet FT-IR module instrument Magna 760 spectrometer at 4
cm-1 resolution as mineral oil mulls. Elemental analysis (C and
H) was performed by Desert Analytics (Tucson, AZ); complexo-
metric methods were used for the analysis of Yb.18
Materials. [1,3-(SiMe3)2C3H3]2Yb(THF)2 (1) and Yb(OTf)2-
(THF)3 were prepared as previously described.5 K[1-(SiMe3)C3H4]
was prepared as a lithium salt according to the literature procedure17
and transmetalated with potassium t-butoxide (KO-t-Bu). Allyl-
trimethylsilane was purchased from Gelest and degassed prior to
use. Chlorotriphenylsilane (Gelest), n-butyllithium (n-BuLi; 2.5 M
in hexane, Acros), KO-t-Bu (Strem), anhydrous YbI2 (Aldrich), and
2,2′-6′,2′′-terpyridine (tpy, Aldrich) were used as received. 4′-
Cyano-2,2′:6′,2′′-terpyridine (tpyCN) and 6,6′′-dicyano-2,2′:6′,2′′-
terpyridine (tpy(CN)2) were prepared according to literature pro-
cedures.19 Potassium 6,6′′-dicyano-2,2′:6′,2′′-terpyridine (K+[tpy-
(CN)2]-) was prepared by adding 1 equiv of freshly cut K metal to
1 equiv of tpy(CN)2 in THF and stirring overnight at room
1
2.08 g of yellow powder (6.14 mmol, 46% yield). H NMR (25
°C, 300 MHz, C6D6): δ 7.66 (m, SiPh3, 6 H), 7.20 (m, SiPh3, 9H),
6.90 (t, J ) 16.1 Hz, CHCHCH, 1H), 3.39 (d, J ) 16.1 Hz,
CHCHCH, 1H), 3.17 (d, J ) 16.1 Hz, CHCHCH, 1H), 0.15 (s,
SiMe3, 9H).
(6) Harvey, M. J.; Hanusa, T. P.; Young, V. G., Jr. Angew. Chem., Int.
Ed. 1999, 38, 217-219.
(7) Quisenberry, K. T. Ph.D. Dissertation, Vanderbilt University, Nashville,
TN, 2005.
(8) Gren, C. K.; Hanusa, T. P.; Brennessel, W. W. Polyhedron 2006, 25,
286-292.
Preparation of [1-(SiMe3)C3H4]2Yb(THF)2 (2). In a scintillation
vial, Yb(OTf)2(THF)3 (0.55 g, 0.80 mmol) was suspended in 10
mL of THF and cooled to -30 °C. In a separate vial, K[1-(SiMe3)-
C3H4] (0.27 g, 1.77 mmol) in 5 mL of THF was also cooled to
-30 °C. The latter solution was added to the stirring Yb(OTf)2-
(THF)3 suspension dropwise over 10 min. The resulting solution
turned red-brown immediately and was allowed to stir overnight
while warming to room temperature. The solvent was removed
under vacuum, hexanes (30 mL) was added, and the resulting
solution was filtered through a fine-porosity glass frit. The filtrate
was dried under vacuum, yielding 0.35 g of a red-brown solid
powder (0.61 mmol, 76% yield). 1H NMR (25 °C, 300 MHz,
toluene-d8): δ 0.25 (s, SiMe3). Other allylic peaks either were not
observed or were obscured by solvent peaks. Anal. Calcd for
C20H42O2Si2Yb: Yb, 31.82. Found: Yb, 31.79.
(9) Quisenberry, K. T.; Smith, J. D.; Voehler, M.; Stec, D. F.; Hanusa,
T. P.; Brennessel, W. W. J. Am. Chem. Soc. 2005, 127, 4376-4387.
(10) Carlson, C. N.; Smith, J. D.; Hanusa, T. P.; Brennessel, W. W.; Young,
V. G., Jr. J. Organomet. Chem. 2003, 683, 191-199.
(11) Smith, J. D.; Quisenberry, K. T.; Hanusa, T. P.; Brennessel, W. W.
Acta Crystallogr., Sect. C. 2004, 60, m507-m508.
(12) Kuehl, C. J.; Simpson, C. K.; John, K. D.; Sattelberger, A. P.; Carlson,
C. N.; Hanusa, T. P. J. Organomet. Chem. 2003, 683, 149-154.
(13) Woodman, T. J.; Schormann, M.; Bochmann, M. Isr. J. Chem. 2003,
42, 283-293.
(14) Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M.
Organometallics 2003, 22, 3028-3030.
(15) Ihara, E.; Koyama, K.; Yasuda, H.; Kanehisa, N.; Kai, Y. J.
Organomet. Chem. 1999, 574, 40-49.
(16) Carlson, C. N.; Hanusa, T. P.; Brennessel, W. W. J. Am. Chem. Soc.
2004, 126, 10550-10551.
Preparation of [1-(SiPh3)-3-(SiMe3)C3H3]2Yb(THF) (3). In a
scintillation vial, Yb(OTf)2(THF)3 (0.21 g, 0.31 mmol) was
suspended in 10 mL of THF and cooled to -30 °C. In a separate
vial, K[1-(SiPh3)-3-(SiMe3)C3H3] (0.26 g, 0.63 mmol) in 5 mL of
THF was cooled to -30 °C. The latter solution was added to the
(17) Fraenkel, G.; Chow, A.; Winchester, W. R. J. Am. Chem. Soc. 1990,
112, 1382-1386.
(18) Schwarzenbach, G.; Flaschka, H. Complexometric Titrations, 2nd ed.;
Methuen: London, 1969.
(19) Veauthier, J. M.; Carlson, C. N.; Collis, G. E.; Kiplinger, J. L.; John,
K. D. Synthesis 2005, 2683-2686.
Inorganic Chemistry, Vol. 45, No. 17, 2006 7005

White et al.
stirring Yb(OTf)2(THF)3 suspension dropwise over 10 min. The
resulting solution turned red-brown immediately and was allowed
to stir overnight while warming to room temperature. The solvent
was removed under vacuum, hexanes (30 mL) was added, and the
resulting solution was filtered through a fine-porosity glass frit.
The filtrate was dried under vacuum, yielding 0.24 g of a red-brown
solid powder (0.23 mmol, 75% yield). Dissolution of the product
in a small amount of hexanes and cooling to -30 °C allowed for
the growth of X-ray-quality crystals. 1H NMR evidence and
elemental analysis both indicate the coordination of two THF
molecules in 3, while crystallographic data of 3 indicate the presence
of one THF molecule. This disagreement may result from differ-
as that for 1‚tpy, using 0.26 g (0.24 mmol) of 3 and 0.06 g (0.24
mmol) of tpyCN. The reaction mixture turned dark blue upon the
addition of 3 in solution, and a dark-blue powder (0.23 g) was
isolated in 81% yield. Terpyridyl and C3 allyl NMR resonances
were not observed. Anal. Calcd for C64H64N4Si4Yb: Yb, 14.73.
Found: Yb, 14.59. IR (mineral oil): 2164 cm-1 (νC≡N).
Magnetic Measurements. Magnetic measurements over the
temperature range 2-300 K were made using a Quantum Design
Superconducting Quantum Interference Device (SQUID) magne-
tometer. The microcrystalline samples were sealed in borosilicate
NMR tubes along with a small amount of quartz wool, which held
the sample near the tube center. Contributions to the magnetization
from quartz wool and the tube were measured independently and
subtracted from the total measured signal. The magnetic susceptibil-
ity, defined as the sample magnetization M divided by the applied
magnetic field H, was measured for 1‚tpy as a function of the
temperature at an applied field of 0.1 T. Diamagnetic corrections
were made using Pascal’s constants.
1
ences in solution and solid-state environments. H NMR (25 °C,
300 MHz, THF-d8): δ 7.61 (m, SiPh3, 10H), 7.28 (m, SiPh3 and
CHCHCH, 22H), 4.00 (br s, THF, 8H), 3.56 (d, J ) 14.6 Hz,
SiPh3CHCHCHSiMe3, 2H), 3.50 (d, J ) 14.6 Hz, SiPh3CH-
CHCHSiMe3, 2H), 2.4 (br s, THF, 8H), 0.15 (s, SiMe3, 18H). Anal.
Calcd for C56H70O2Si4Yb: Yb, 16.32. Found: Yb, 16.87.
Preparation of [1,3-(SiMe3)2C3H3]2Yb(tpy) (1‚tpy). In a scintil-
lation vial, 1 (0.10 g, 0.15 mmol) was suspended in 15 mL of
toluene. This solution was added dropwise to a second vial
containing tpy (0.04 g, 0.19 mmol). The reaction mixture im-
mediately turned dark green and was allowed to stir overnight (16
h). The solvent was then removed under vacuum, approximately
15 mL of hexanes was added to the dark-green residue, and the
solution was filtered through Celite and glass microfiber filter paper.
The solvent was removed under vacuum, resulting in a dark-green
powder (0.09 g, 75% yield). X-ray-quality crystals were grown from
X-ray Crystallography of 1‚tpy. Single-crystal X-ray diffraction
experiments for 1‚tpy were performed on a Bruker P4/CCD/PC
diffractometer. Diffraction data were refined using SHELXTL.20
Crystals were coated in mineral oil and mounted on a glass fiber
at 203 K. Data collection and initial indexing and cell refinement
were performed using SMART21 software. Frame integration and
final cell parameter calculation were carried out using SAINT22
software. The data were corrected for absorption using the
SADABS23 program. Decay of the reflection intensity was not
observed. The structures were solved using difference Fourier
techniques. The initial solutions revealed the metal center and the
majority of all other non-H positions. The remaining atomic
positions were determined from subsequent Fourier syntheses. All
H atoms were placed in ideal positions and refined using a riding
model. The crystal and refinement parameters for 1‚tpy are listed
in Table 1. Selected bond distances and angles are listed in Table
2.
1
a concentrated solution of hexanes. H NMR (25 °C, 400 MHz,
THF-d8): δ -0.13 (s, SiMe3). 1H NMR spectra at room temperature
and -20 °C were identical; terpyridyl and C3 allyl resonances were
not observed. Anal. Calcd for C33H52N3Si4Yb: Yb, 22.27. Found:
Yb, 21.97. λmax (nm): 404, 610, 951.
Preparation of [1,3-(SiMe3)2C3H3]2Yb(tpyCN) (1‚tpyCN).
Complex 1‚tpyCN was prepared using the same method as that for
1‚tpy, using 0.09 g (0.14 mmol) of 1 and 0.05 g (0.18 mmol) of
tpyCN. The reaction mixture turned dark blue upon the addition
of 1 in solution, and the isolated product was a dark-blue powder
(0.09 g, 80% yield). 1H NMR (25 °C, 400 MHz, THF-d8): δ 0.01
(br s, SiMe3), -0.02 (br s, SiMe3). Terpyridyl and C3 allyl
resonances were not observed. Anal. Calcd for C34H52N4Si4Yb: Yb,
21.57. Found: Yb, 21.16. IR (mineral oil): 2130 cm-1 (νC≡N). λmax
(nm): 356, 573, 923.
Results and Discussion
Synthesis and Structural Characterization. The lithium
salt of the asymmetric allyl ligand [1-(SiMe3)C3H4]- was
synthesized as described by Fraenkel;17 subsequent trans-
metalation with KO-t-Bu yielded the potassium salt. The K
complex of the symmetric allyl [1,3-(SiMe3)2C3H3]- was
prepared as described in the literature,5,17 and the trimeth-
ylsilyl- and triphenylsilyl-substituted ligand [1-(SiPh3)-3-
(SiMe3)C3H3]- was prepared through a similar method.
Diallyl ytterbium complexes with each of these three allyl
ligands were synthesized by treatment of 2 equiv of the
corresponding potassium allyl complex with YbI2 or Yb-
(OTf)2(THF)3 in THF at -30 °C (Scheme 1, reaction I). The
resulting Yb complexes consist of two η3-bound allyl ligands
and one (3) or two (1 and 2) THF molecules, as indicated
Preparation of [1,3-(SiMe3)2C3H3]2Yb(tpy(CN)2) [1‚tpy(CN)2].
Complex 1‚tpy(CN)2 was prepared using the same method as that
for 1‚tpy, using 0.10 g (0.15 mmol) of 1 and 0.06 g (0.20 mmol)
of tpy(CN)2. The reaction mixture turned dark red-brown upon the
addition of 1 in solution, and a dark-red-brown powder (0.11 g)
1
was isolated in 86% yield. H NMR (25 °C, 400 MHz, THF-d8):
δ -0.02 (br s, SiMe3), -0.04 (br s, SiMe3). Terpyridyl and C3 allyl
resonances were not observed. IR (mineral oil): 2125 cm-1 (νC≡N).
λmax (nm): 411, 577.
Preparation of [1-(SiMe3)C3H4]2Yb(tpyCN) (2‚tpyCN). Com-
plex 2‚tpyCN was prepared using the same method as that for 1‚
tpy, using 0.88 g (1.62 mmol) of 2 and 0.42 g (1.64 mmol) of
tpyCN. The reaction mixture turned dark blue upon the addition
of 2 in solution, and a dark-blue powder (0.86 g) was isolated in
1
by the crystal structures (1 and 3) and H NMR data.
Crystallographic data for 1 have been reported previously.5
X-ray-quality crystals of 3 were grown from a concentrated
(20) SHELXTL, version 5.1; Bruker Analytical X-ray Systems: Madison,
1
80% yield. H NMR (25 °C, 400 MHz, THF-d8): δ -0.02 (br s,
WI, 1997.
SiMe3). Terpyridyl and C3 allyl resonances were not observed. Anal.
Calcd for C28H36N4Si2Yb: Yb, 26.30. Found: Yb, 26.40. IR
(mineral oil): 2171 cm-1 (νC≡N).
(21) SMART, version 4.210; Bruker Analytical X-ray Systems: Madison,
WI, 1996.
(22) SAINT, version 4.05; Bruker Analytical X-ray Systems: Madison, WI,
1996.
Preparation of [1-(SiPh3)-3-(SiMe3)C3H3]2Yb(tpyCN) (3‚
(23) Sheldrick, G. M. SADABS; University of Go¨ttingen: Go¨ttingen,
tpyCN). Complex 3‚tpyCN was prepared using the same method
Germany, 1996.
7006 Inorganic Chemistry, Vol. 45, No. 17, 2006

Bulky Allyl Ytterbium Complexes
Table 1. Crystal Data and Summary of X-ray Data Collection for 1‚tpy
empirical formula
fw
C33H49N3Si4Yb
773.15
space group
a, Å
b, Å
P1h
10.202(4)
12.018(4)
17.226(6)
71.618(5)
77.715(5)
71.934(5)
1889.8(12)
1.359
c, Å
Figure 1. Terpyridine ligands: tpy, tpyCN, and tpy(CN)2.
R, deg
â, deg
γ, deg
V, Å3
F
calcd, g cm-3
Z
2
µ, mm-1
2.625
λ(Mo KR), Å
T, K
0.710 73
203
1.069
0.1065
0.2859
0.1278
0.2957
8.844 and -2.044
GOFa
R1 [I > 2σ(I)]b
wR2 [I > 2σ(I)]c
R1 (all data)b
wR2 (all data)c
largest diff peak/hole, e Å-3
2
a GOF ) [∑[w(Fo2 - Fc )]2/(n - p)]1/2; n ) number of reflections, and
p ) total number of parameters refined. b R1 ) ∑|Fo| - |Fc|∑|Fo|. c wR2
) [∑[w(Fo - Fc )]2]/∑[w(Fo )2]1/2
.
2
2
2
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1‚tpy
atoms
distance
atoms
angle
Yb-N1
2.39(2)
2.27(1)
2.38(1)
2.60(4)
C-C-C(allyl, ave)
N1-C-C-N2
N3-C-C-N2
121.4(2)
-5.8
Figure 2. Thermal ellipsoid representation of 1‚tpy (50% probability
Yb-N2
ellipsoids). H atoms have been omitted for clarity.
Yb-N3
Yb-C(ave)
-3.4
a
Acent-Yb-Acent
135.9
and terpyridyl resonances of these complexes were not
a Acent is defined as the centroid comprising three allyl C atoms.
1
evident in their H NMR spectra. This is indicative of the
expected paramagnetic f13 π*1 configuration. In contrast, the
SiMe3 proton resonances are observed, presumably because
of the fact that they are spatially isolated from the YbIII center
and the ligand radical anion. The absorption bands in the
UV-vis-near-IR spectra of adducts of 1 are listed in the
Experimental Section. They show π-π* and π*-π* transi-
tions that demonstrate electron transfer between the ytterbium
(donor) and terpyridyl ligand (acceptor). Neither the parent
allyl complexes (1-3) nor their terpyridyl derivatives display
the reversible redox behavior exhibited by their Cp* ana-
logues. Chemical isolation of cationic complexes was at-
tempted using common oxidizing agents (e.g., AgOTf);
however, these reactions provide intractable solids. We
believe that this redox instability is due to the propensity of
the allyl groups to reductively eliminate, providing hexadiene
products.9,24
Scheme 1. Reaction I: Synthesis of 1-3 (in THF at -30 °C) and
Reaction II: Synthesis of 1‚tpy, 1‚tpyCN, 1‚tpy(CN)2, 2‚tpyCN, and
3‚tpyCN (in Toluene at Room Temperature)
X-ray-quality crystals of 1‚tpy were grown in a concen-
trated solution of hexanes at -30 °C overnight; the structure
is shown in Figure 2. The two allyl ligands in 1‚tpy are in
an anti configuration with an average C-C-C angle of
121.4(2)° (see Table 2). This value has contracted relative
to that of 1 (128.9°), which suggests a slight rehybridization
of the allyl moiety that is consistent with a greater extent of
electron donation to the Yb center. The allyl ligands are
π-bound to the Yb center, with Yb-C bond lengths ranging
from 2.52(2) to 2.62(2) Å (Table 2). These distances are
shorter than the analogous distances for 1 (Yb-C 2.741-
(9)-2.754(9) Å).5 This is to be expected given that the
hexanes solution at -30 °C. The structure exhibited severe
disorder, and only the atom connectivity could be established
(see the Supporting Information).
Complexes 1-3 were treated with terpyridine derivatives
tpy, tpyCN, and tpy(CN)2 (Figure 1) in toluene at room
temperature as shown in Scheme 1, reaction II. In each case,
a color change from purple occurred immediately (to dark
green for tpy, dark blue for tpyCN, and dark red-brown for
tpy(CN)2). Terpyridyl adducts of 1-3 are air- and moisture-
sensitive and are thermally unstable above room temperature.
Despite the use of large frequency windows, the C3 allyl
(24) John, K. D.; Salazar, K. V.; Scott, B. L.; Baker, R. T.; Sattelberger,
A. P. Organometallics 2001, 20, 296-304.
Inorganic Chemistry, Vol. 45, No. 17, 2006 7007

White et al.
Table 3. IR CtN Stretching Frequenciesa
compound
ν
CtN (cm-1
)
compound
ν
CtN (cm-1
)
tpyCN
22382
21492
2130
2171
2164
Cp*2Yb(tpyCN)
tpy(CN)2-
21722
2238
2130
2125
tpyCN-
1‚tpyCN
2‚tpyCN
3‚tpyCN
tpy(CN)2
1‚tpy(CN)2
a All spectra were obtained in mineral oil except that of Cp*2Yb(tpyCN),
which was measured in toluene.
Figure 3. Possible binding modes for 1‚tpy(CN)2.
lanthanide in 1 is divalent, whereas for 1‚tpy, the metal center
is effectively trivalent. The tpy ligand is η3-bound to the
metal center with Yb-N bond lengths from 2.27(2) to 2.39-
(2) Å. The analogous distances in Cp*2Yb(tpy) (Yb-C 2.41-
(1)-2.42(1) Å) are slightly longer.3 Evidently, the more
compact allyl ligand allows for closer Yb-C and Yb-tpy
binding than is possible with the sterically bulky penta-
methylcyclopentadienyl ligands.
IR Spectroscopy. The CtN moieties of tpyCN and
tpy(CN)2 provide an excellent means to gauge the extent of
charge transfer for the allyl derivatives. The CtN stretching
frequencies for 1-3‚tpyCN, 1‚tpy(CN)2, and the neutral and
anionic forms of the free ligands (tpyCN and tpy(CN)2) are
presented in Table 3.2 The tpyCN adducts of the asymmetri-
cally substituted allyl complexes (2‚tpyCN and 3‚tpyCN)
have stretching frequencies very close to that reported for
Cp*2Yb(tpyCN) (Table 3) with CtN stretches between 2164
and 2172 cm-1. This correspondence is not unexpected
because allyls are believed to have electron-donating ability
similar to that of cyclopentadienyl ligands.6
Surprisingly, however, the CtN stretching frequency for
the symmetric allyl complex 1‚tpyCN (2130 cm-1) is ∼40
cm-1 lower than that of the analogous Cp* derivative and is
even lower than the CtN stretching frequency for the anionic
form of the free ligand (tpyCN-; νC≡N ) 2149 cm-1). It
seems that, compared to 2‚tpyCN (with 1 SiMe3 per allyl),
the additional trimethylsilyl group on the allyl ligand of 1‚
tpyCN increases the electron-density donation to the metal
center, which in turn increases electron transfer to the tpyCN
ligand. A similar trend is observed for allylcarbonyl com-
plexes of transition metals. For example, the bis(1,3-
trimethylsilyl)-substituted allylmetal complexes (e.g., [1,3-
(SiMe3)2C3H3]2Fe(CO)2) have lower CO stretching frequencies
than their unsubstituted analogues.7 The relatively high Ct
N stretching frequency of 3‚tpyCN can be explained based
on steric considerations, because the allyl groups are farther
away from the Yb center, which should inhibit a stronger
electron-donating interaction. Furthermore, the electron-
withdrawing ability of SiPh3 should also lead to a smaller
degree of electron donation.25
Figure 4. ø-1 vs T and øT vs T plots of 1‚tpy. The data were measured
in an applied field of 0.1 T.
provide a great deal of information about the connectivity
of the tpy(CN)2 ligand, where three bonding modes are
plausible for the ligand (Figure 3). Coordination in an η1
fashion to the nitrile group (Figure 3a) is unlikely because
we would anticipate an increase in the CtN stretch, as has
been observed for Cp*2YbI(NCtpy) and other η1-nitrile
complexes.2,26,27 Furthermore, asymmetric binding of the type
shown in Figure 3a,b would provide two distinct CtN
stretches. Therefore, the motif in Figure 3c is the most
reasonable binding mode for tpy(CN)2 in 1‚tpy(CN)2. To
substantiate this binding motif, the geometry of a model of
1‚tpy(CN)2 was optimized using density functional theory
(B3PW91/SDD; see the Supporting Information). The result-
ing structure is similar to that of the crystal structure of 1‚
tpy, where the tpy(CN)2 ligand is η3-bound to the Yb center
(Figure 3c). In the optimized structure, the distance between
the metal and nitrile groups (Yb-C(N)) averages to 3.575
Å, which indicates that steric crowding around the Yb does
not prevent the terpyridyl ligand from being η3-bound to the
Yb center.
Magnetic Susceptibility. The magnetic susceptibility (ø)
for compound 1‚tpy was measured as a function of temper-
ature and is presented in Figure 4. The interpretation of the
magnetic data is based on the premise that the neutral
electronic configuration is f13 π*1. The ø-1 vs T plot for 1‚
tpy departs dramatically from the Curie law and exhibits a
temperature-dependent profile reminiscent of previously
examined species such as Cp*2Yb(L) (L ) bpy,1 tpy,2 and
tpyCN2). Field-dependent ø vs T measurements of 1‚tpy (see
There are fewer complexes available for comparison in
the case of tpy(CN)2 because we have been unable to isolate
the analogous Cp*Yb(tpy(CN)2) complex, but the overall
trend of the IR data is similar to that of the diallylytterbium
adducts of tpyCN. Specifically, the CtN stretching fre-
quency of 1‚tpy(CN)2 (2125 cm-1) is slightly lower (∼5
cm-1) than that of a tpy(CN)2 anion. This frequency can
(26) Coerver, H. J.; Curran, C. J. Am. Chem. Soc. 1958, 80, 3522-3523.
(27) Kubota, M.; Schulze, S. R. Inorg. Chem. 1964, 3, 853-856.
(25) Dilman, A. D.; Mayr, H. Eur. J. Org. Chem. 2005, 1760-1764.
7008 Inorganic Chemistry, Vol. 45, No. 17, 2006

Bulky Allyl Ytterbium Complexes
the Supporting Information) at 0.1 and 5 T do not diverge
above 10-15 K, suggesting that there is no ferromagnetic
impurity in the material under study.
by their Cp* analogues. Chemical isolation of cationic
complexes was attempted using common oxidizing agents;
however, these reactions provide intractable solids, presum-
ably as a result of reductive elimination of the allyl groups.
The symmetric allyl complex, 1‚tpy, displays temperature-
dependent magnetic behavior similar to that of Cp*2Yb(tpy)
with a room-temperature magnetic moment that is lower than
that predicted for an uncoupled YbIII center and a ligand
radical anion. The CtN stretching frequency of 1‚tpyCN
(2130 cm-1) is significantly lower than that of Cp*2Yb-
(tpyCN) (2172 cm-1), which appears to be a combination
of the electronic effect of the two SiMe3 groups and the
aforementioned reduction in steric pressure. Furthermore, we
have demonstrated that we can tune the electronic properties
of these complexes via allyl substitution as evidenced by
changes in the CtN stretching frequencies of the associated
terpyridylnitrile ligands.
Unfortunately, the redox instability of the allyl complexes
(as evidenced by the irreversible nature of the electrochem-
istry of 1‚tpy and our inability to isolate a chemically
oxidized analogue) prevents us from determining the room-
temperature magnetic moment of the monocationic complex.
The room-temperature magnetic moment of 1‚tpy (2.95 µB)
is lower than that reported for Cp*2Yb(tpy) (3.77 µB); both
room-temperature magnetic moments are significantly lower
than the value expected for an uncoupled YbIII ion and an
organic radical (4.85 µB). We were unable to obtain magnetic
data beyond 300 K because of the thermal instability of 1‚
tpy. The nature of these low magnetic moments is currently
being investigated using a variety of variable-temperature
techniques (UV-vis-near-IR, magnetic circular dichroism,
and NMR) to probe the nature of the magnetic coupling in
this class of materials.
Acknowledgment. We thank LANL’s Laboratory Di-
rected Research and Development (LDRD) ER program and
the Petroleum Research Fund, administered by the American
Chemical Society, for financial support. C.N.C., J.M.V., and
R.E.W. thank the Seaborg Institute for postdoctoral and
student fellowships. We thank Dr. David E. Morris for
conducting electrochemistry experiments.
Summary and Conclusions
In summary, we have prepared a new series of Yb-based,
internal charge-transfer complexes using a series of bulky
allyl ligands to explore the direct comparison of the bulky
allyl moiety with Cp*. The symmetrically substituted allyl
ligand [1,3-(SiMe3)2C3H3] in 1‚tpy is less sterically demand-
ing than Cp* because it allows for closer Yb-tpy binding
(Yb-N(ave) 2.35 Å for 1‚tpy and 2.42 Å for Cp*2Yb(tpy)).
Neither the parent allyl complexes (1-3) nor their terpyridyl
derivatives display the reversible redox behavior exhibited
Supporting Information Available: X-ray crystallographic files
in CIF format for 1-3, coordinates for the optimized structure of
1‚tpy(CN)2, and a plot of ø vs T for 1‚tpy. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC060603X
Inorganic Chemistry, Vol. 45, No. 17, 2006 7009