Lanthanide compounds with fluorinated OC6F5 ligands: Homo- and heterovalent complexes of Eu(II) and Eu(III)

Article abstract of DOI:10.1021/ic062395e

The fluorinated phenoxide OC₆F₅ forms the stable Eu(II) and Eu(III) derivatives (DME)₂Eu(μ-OC₆F ₅)₃Eu(μ-OC₆F₅) ₃-Eu(DME)₂ and (DME)₂Eu(OC₆F ₅)₃, as well as the heterovalent product (DME) ₂Eu(μ-OC₆F₅)₃Eu(DME)(OC ₆F₅)₂, in redox reactions of Eu with HOC ₆F₅ or in proton-transfer reactions of HOC ₆F₅ with Eu(SPh)₂. The divalent complex crystallizes as a trimer with three bridging phenoxides bridging each pair of metals, with the terminal metals coordinating DME and the central metal ion encapsulated totally by O(C₆F₅) and dative fluoride interactions. The trivalent compound is monomeric with terminal phenoxide ligands and no Eu-F interactions. The heterovalent compound has clearly localized metal valence states and coordination features that mimic the homovalent species with the terminal OC₆F₅ bound to the Eu(III) ion, three bridging OR ligands spanning the Eu(II) and Eu(III) ions, and dative Eu(II)-F bonds. At elevated temperatures, these compounds decompose to give a mixture of solid-state fluoride phases.

Full text of DOI:10.1021/ic062395e

Inorg. Chem. 2007, 46, 4060−4066
Lanthanide Compounds with Fluorinated OC6F5 Ligands: Homo- and
Heterovalent Complexes of Eu(II) and Eu(III)
Kieran Norton, Thomas J. Emge, and John G. Brennan*
Department of Chemistry and Chemical Biology, Rutgers, State UniVersity of New Jersey, 610
Taylor Road, Piscataway New Jersey 08854-8087
Received December 14, 2006
The fluorinated phenoxide OC6F5 forms the stable Eu(II) and Eu(III) derivatives (DME)2Eu(µ-OC6F5)3Eu(µ-OC6F5)3-
Eu(DME)2 and (DME)2Eu(OC6F5)3, as well as the heterovalent product (DME)2Eu( -OC6F5)3Eu(DME)(OC6F5)2, in
µ
redox reactions of Eu with HOC6F5 or in proton-transfer reactions of HOC6F5 with Eu(SPh)2. The divalent complex
crystallizes as a trimer with three bridging phenoxides bridging each pair of metals, with the terminal metals
coordinating DME and the central metal ion encapsulated totally by O(C6F5) and dative fluoride interactions. The
trivalent compound is monomeric with terminal phenoxide ligands and no Eu−F interactions. The heterovalent
compound has clearly localized metal valence states and coordination features that mimic the homovalent species
with the terminal OC6F5 bound to the Eu(III) ion, three bridging OR ligands spanning the Eu(II) and Eu(III) ions,
and dative Eu(II)
fluoride phases.
−F bonds. At elevated temperatures, these compounds decompose to give a mixture of solid-state
Introduction
Ln sources for the synthesis of LnOx thin films that have
useful dielectric properties.3
Fluorinated ligands impart unique chemical and physical
properties to metal compounds, including superior solubility/
volatility properties, unusual crystal packing motifs, and the
stabilization of elements in high oxidation states. A number
of fluorinated ligand systems have been used in lanthanide
(Ln) chemistry. Most developed is the Ln chemistry of the
fluorinated acetylacetonate1 and acetate ligands2 because
these volatile air-stable molecules can be used as volatile
Nonchelating fluorinated ligands have been investigated
less extensively. Lanthanide compounds with fluorinated
t-butoxides,4 amido ligands such as N(C6F5)2 or N(SiMe3)-
(NC6F5),5 and the arenethiolate SC6F56 have been described,
and in the amido and thiolate compounds, there are Ln-F
dative interactions that are largely absent in related transition
metal systems. The fluorinated thiolate structures invariably
contained extensitve π-π stacking interactions, a structural
motif that has also been noted in related transition metal
compounds.7 These fluorinated Ln thiolates displayed re-
markable NIR emission properties8 because the absence of
CH bonds in the anionic ligand, coupled with the low Ln-S
* To whom correspondence should be addressed. E-mail: bren@
ccbmail.rutgers.edu.
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4060 Inorganic Chemistry, Vol. 46, No. 10, 2007
10.1021/ic062395e CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/20/2007

Ln Compounds with OC6F5 Ligands
phonon energies, minimizes competitive vibrational relax-
ation pathways.
In this work, we outline our first experiments using OC6F5
to form stable Ln compounds. Our initial target is the
chemistry of redox-active Eu, where we establish the stability
and physical properties of both Eu(II) and Eu(III) compounds
with OC6F5 ligands and the high yield synthesis of a
heterovalent dimer.
The fluorinated phenoxide OC6F5 has been used frequently
in main group9 and transition metal10 systems as a stabilizing,
solublizing, commercially available anion. This ligand has
many properties that may be useful in lanthanide chemistry:
OC6F5 has no C-H functional groups that might quench NIR
emissions, and so it is potentially valuable for forming stable,
emissive Ln complexes. The tendency of fluoro substituents
to enhance solubility properties is also important in composite
materials synthesis.11 Finally, such an electronegative
pseudochalcogenolate is also potentially useful in the syn-
thesis of chalcogenido clusters of Eu(III) because Eu(III)
reductively eliminates less electronegative EPh ligands (E
) S, Se, Te) to form PhEEPh and Eu(II) compounds.12
Highly electronegative OC6F5 could be used as cluster
capping reagents to produce soluble Eu sulfide clusters. Such
materials are sought after for possible electronics applica-
tions, that is, as soluble analogs of Eu-doped Y2O2S,13 the
red phosphor in CRT screens.
Experimental Section
General Methods. All syntheses were carried out under ultrapure
nitrogen (WELCO CGI, Pine Brook, NJ), using conventional dry
box or Schlenk techniques. Dimethoxyethane (DME), hexane, and
pyridine (Fisher Scientific, Agawam, MA) were purified with a
dual-column Solv-Tek solvent purification system (Solv-Tek Inc.,
Berryville, VA). Eu and Hg were purchased from Strem Chemicals
(Newburyport, MA). HOC6F5 was purchased from Aldrich. Melting
points were taken in sealed capillaries and are uncorrected. IR
spectra were taken on a Thermo Nicolet Avatar 360 FT-IR
spectrometer and were recorded from 4000 to 600 cm-1 as Nujol
mulls on NaCl plates. Electronic spectra were recorded on a Varian
DMS 100S spectrometer with the samples in a 1.0 mm quartz cell
attached to a Teflon stopcock. Powder diffraction spectra were
obtained from Bruker AXS D8 Advance diffractometer using Cu
KR radiation. Elemental analyses were performed by Quantitative
Technologies, Inc. (Whitehouse Station, NJ).
Synthesis of (dme)2Eu(µ2-OC6F5)3Eu(µ2-OC6F5)3Eu(dme)2 (1).
Eu metal (0.15 g, 1.0 mmol), C6F5OH (0.31 g, 1.67 mmol), and
Hg (0.025 g, 0.12 mmol) were added to DME (20 mL), and the
mixture was stirred for two weeks at room temperature to give a
green-gray solution with a black precipitate. The solution was
filtered; the filtrate was concentrated to around 8 mL and layered
with hexane (20 mL) to give pale yellow lathes (0.21 g, 40%) that
become darker yellow at 160-170 °C and melt at 220 °C. Anal.
Calcd for C52H40Eu3F30O14: C, 32.6; H, 2.10. Found: C, 32.4; H,
2.15. IR: 2923 (s), 2853 (s), 2729 (w), 2670 (w), 1653 (w), 1618
(w), 1501 (s), 1463 (s), 1377 (s), 1300 (w), 1247 (w), 1193 (w),
1173 (m), 1122 (w), 1067 (s), 1009 (s), 979 (s), 861 (m), 721 (m),
618 (m) cm-1. No UV-vis absorption maxima were observed
between 300 and 750 nm in either DME or pyridine.
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Synthesis of Eu(OC6F5)3(dme)2 (2). Method A. Eu metal (0.13
g, 0.84 mmol), PhSSPh (0.29 g, 1.3 mmol), and Hg (0.025 g, 0.12
mmol) were added to DME (20 mL), and the mixture was stirred
magnetically for 7 days to give a yellow solution with a yellow-
green precipitate. C6F5OH (0.46 g, 2.56 mmol) was added, and the
mixture was stirred for two weeks and then filtered to separate
trace gray precipitate from a bright yellow solution. The solution
was concentrated to about 4 mL and layered with hexane (∼25
mL) to give yellow rod-shaped crystals (0.24 g, 32%) that turn
orange at 95 °C and then turn red and melt at 123-125 °C. Anal.
Calcd for C26H20EuF15O7: C, 35.4; H, 2.27. Found: C, 34.9; H,
2.20. The UV-vis (DME) spectrum contained peaks at 575 (ꢀ )
9 × 10-2 L mol-1 cm-1) and 529 nm (ꢀ ) 1.5 × 10-1 L mol-1
cm-1). In pyridine, no well-defined absorption maxima
attributable to a MLCT excitation could be found. IR: 2923 (s),
2853 (s), 2727 (w), 2669 (w), 1763 (w), 1651 (w), 1622 (w), 1502
(m), 1460 (m), 1382 (s), 1307 (w), 1261 (m), 1175 (m), 1096 (m),
1049 (m), 1017 (m), 988 (w), 857 (m), 800 (m), 720 (w), 633 (w),
615 (s) cm-1
.
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Inorganic Chemistry, Vol. 46, No. 10, 2007 4061

Norton et al.
Table 1. Summary of Crystallographic Details for 1-3
1
2
3
empirical formula C52H40Eu3F30O14 C26H20EuF15O7 C42H27Eu2F25O11
fw
1914.72
C2/c
881.38
P21/n
1486.56
Cc
space group
a (Å)
b (Å)
c (Å)
20.6837(12)
13.6138(8)
22.0265(13)
92.989(1)
6193.9(6)
4
10.4086(5)
12.8244(6)
23.480(1)
96.058(1)
3116.6(3)
4
11.9347(6)
20.997(1)
20.660(1)
104.768(1)
5006.4(4)
4
â (deg)
V (Å3)
Z
D
calcd (g cm-3
)
2.053
1.878
1.972
temp (K)
100(2)
100(2)
100(2)
λ (Å)
0.71073
3.153
0.0320
0.71073
2.145
0.0212
0.0237
0.71073
2.632
0.0424
0.0760
abs coeff (mm-1
)
R(F)a [I > 2σ(I)]
Rw(F2)b [I > 2σ(I)] 0.0797
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(F2) ) {∑[w(Fo - Fc )2]/
2
2
∑[w(Fo )2]}1/2
.
2
Method B. Eu metal (0.15 g, 1.0 mmol), C6F5OH (0.55 g, 3.0
mmol), and Hg (0.010 g, 0.05 mmol) were added to DME (20 mL),
and the reaction mixture was stirred for 5 days to give a yellow
solution with brown precipitate. The solution was filtered, concen-
trated to about 8 mL, and layered with hexane (15 mL) to give 2
(0.36 g, 42%).
Figure 1. ORTEP diagram of trimetallic (DME)4Eu3(OC6F5)6: green, F;
red, O; blue, Eu; gray, C. The H atoms were removed for clarity.
Synthesis of (dme)(OC6F5)2Eu(µ2-OC6F5)3Eu(dme)2 (3). Method
A. Eu metal (0.14 g, 0.92 mmol), (SPh)2 (0.205 g, 0.94 mmol),
and Hg (0.03 g, 0.15 mmol) were added to DME (20 mL), and the
mixture was stirred at room temperature for 2 days to give a yellow
solution and green precipitate. C6F5OH (0.33 g, 1.79 mmol) was
added to this mixture, and it was stirred for an additional 6 days.
The mixture was filtered to separate a yellow-orange solution
from the red-orange precipitate. The filtrate was concentrated
to ∼8 mL and layered with 15 mL of hexanes to give yellow-
orange crystals (0.34 g, 63%) that darken at 132 °C and melt at
142-146 °C. Anal. Calcd for C42H30Eu2F25O11: C, 33.9; H, 2.03.
Found: C, 33.6; H, 2.25. The UV-vis (DME) spectrum contained
peaks at 575 (ꢀ ) 5 × 10-2 L mol-1 cm-1) and 529 nm (ꢀ ) 1 ×
10-1 L mol-1 cm-1). In pyridine, no well-defined absorption
maxima attributable to a MLCT excitation could be defined. IR:
2923 (s), 2853 (s), 2670 (w), 2457 (w), 1651 (m), 1504 (s), 1455
(s), 1382 (s), 1311 (m), 1246 (m), 1177 (m), 1164 (m), 1113 (m),
1068 (m), 1016 (m), 860 (m), 834 (w), 721 (m), 634 (m),
Figure 2. ORTEP diagram of molecular (DME)2Eu(OC6F5)3: green, F;
red, O; blue, Eu; gray, C. The H atoms were removed for clarity. Significant
distances (Å) and angles (deg): Eu(1)-O(1) ) 2.205(1), Eu(1)-O(2) )
2.245(1), Eu(1)-O(3) ) 2.232(1), Eu(1)-O(5) ) 2.456(1), Eu(1)-O(6)
) 2.482(1), Eu(1)-O(7) ) 2.488(1), Eu(1)-O(4) ) 2.492(1), C(1)-O(1)-
Eu(1) ) 164.22(12), C(7)-O(2)-Eu(1) ) 132.23(10), C(13)-O(3)-Eu-
(1) ) 149.21(11).
The structures were solved by direct methods (SHELXS86).15 All
non-hydrogen atoms were refined (SHELXL97)16 on the basis of
Fobs2. All hydrogen atom coordinates were calculated with idealized
geometries (SHELXL97). Scattering factors (fo, f′, f′′) are as
described in SHELXL97. Crystallographic data and final R indices
for 1-3 are given in Table 1. ORTEP diagrams17,18 for 1-3 are
shown in Figures 1-3, respectively. Complete crystallographic
details are given in the Supporting Information.
618 (m) cm-1
.
Method B. Eu metal (0.076 g, 0.50 mmol), C6F5OH (0.230 g,
1.25 mmol), and Hg (0.015 g, 0.075 mmol) were added to DME
(20 mL), and the reaction mixture was stirred for 3 days to give a
yellow solution with brown precipitate. The solution was filtered.
The filtrate was concentrated to ∼8 mL and layered with hexanes
(15 mL) to give 3 (0.096 g, 26%).
Thermolysis and X-ray Powder Diffraction Measurements.
Samples of 1-3 were placed in quartz tubes that were then sealed
under vacuum. The end of the tubes with the samples were placed
Results
side by side in an oven; the temperature was raised (20 °C min-1
)
Divalent and trivalent Eu complexes with OC6F5 ligands
are most easily prepared by direct oxidation of the metal
to 650 °C and held for 5 h. The other end of the thermolysis tubes
were kept in liquid nitrogen during the experiment. Black powders
formed and X-ray powder diffraction patterns were obtained by
scanning from 20 to 80°.
X-ray Structure Determination. Data for 1-3 were collected
on a Bruker Smart APEX CCD diffractometer with graphite-
monochromatized Mo KR radiation (λ ) 0.71073 Å) at 100 K.
Crystals were immersed in Paratone oil and examined at low
temperatures. The data were corrected for Lorenz effects, polariza-
tion, and absorption, the latter by a multiscan (SADABS)14 method.
(14) SADABS, Bruker Nonius Area Detector Scaling and Absorption
Correction, version 2.05; Bruker-AXS Inc.: Madison, WI, 2003.
(15) Sheldrick, G. M. SHELXS86, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(16) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(17) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(18) Sheldrick, G. M. SHELXTL (XP), version 6.14; Bruker-AXS, Inc.:
Madison, WI, 2000.
4062 Inorganic Chemistry, Vol. 46, No. 10, 2007

Ln Compounds with OC6F5 Ligands
Table 3. Significant Distances (Å) and Angles (deg) for 3
Eu(1)-O(1)
Eu(1)-O(2)
Eu(1)-O(3)
Eu(1)-O(6)
Eu(1)-O(8)
Eu(1)-O(9)
Eu(1)-F(11)
Eu(1)-F(1)
Eu(2)-O(11)
2.591(4) Eu(2)-O(1)
2.551(4) Eu(2)-O(2)
2.564(4) Eu(2)-O(3)
2.643(4) Eu(2)-O(4)
2.657(4) Eu(2)-O(5)
2.663(4) Eu(1)-O(7)
2.796(4) Eu(1)-F(6)
3.109(5) Eu(2)-O(10)
2.537(4) Eu(2)-F(5)
2.373(4)
2.410(4)
2.348(4)
2.223(4)
2.221(4)
2.696(5)
2.835(4)
2.482(4)
3.072(4)
Eu(2)-O(1)-Eu(1)
Eu(2)-O(3)-Eu(1)
C(7)-O(2)-Eu(2)
C(7)-O(2)-Eu(1)
96.60(14) Eu(2)-O(2)-Eu(1)
97.99(14) C(1)-O(1)-Eu(2)
96.77(13)
129.1(4)
130.5(4)
126.4(4)
122.8(4)
C(1)-O(1)-Eu(1)
C(13)-O(3)-Eu(2) 126.8(4)
C(13)-O(3)-Eu(1) 120.4(4)
C(19)-O(4)-Eu(2) 161.0(4)
C(25)-O(5)-Eu(2) 164.3(4)
bridging phenoxides (with Eu-O bond lengths that range
from 2.457(2) to 2.569(2) Å) and two DME ligands (with
Eu-O bond lengths that range from 2.638(2) to 2.757(3)
Å) to give a seven-coordinate environment with three
additional remote Eu-F interactions to the R-C-F atoms at
3.004(2), 3.013(2), and 3.32(1) Å.
Pyridine solutions of this yellow compound are deep
orange, but an absorption maximum that would correspond
to an Eu-to-pyridine charge-transfer excitation could not be
obtained.
Figure 3. ORTEP diagram of heterovalent (DME)3Eu2(OC6F5)5: green,
F; red, O; blue, Eu; gray, C. The H atoms were removed for clarity.
Trivalent 2 is monomeric with three terminal OC6F5 (three
Eu-O in the range of 2.205(1)-2.245(1) Å) and two
chelating DME ligands (four Eu-O in the range of 2.456-
(1)-2.492(1) Å). Figure 2 shows an ORTEP diagram of the
complex with significant distances and angles given in the
figure caption. Within the three phenoxides in 2, there is a
wide range of Eu-O-C angles (C(1)-O(1)-Eu(1) )
164.22(12)°, C(7)-O(2)-Eu(1) ) 132.23(10)°, C(13)-
O(3)-Eu(1) ) 149.21(11)°), with the larger angles found
for the ligands with the shortest Eu-O bond lengths. There
are no significant Eu-F interactions present because the six
distances to R-C-F atoms range from 3.54(1) to 4.63(1) Å.
Bimetallic 3 is a localized heterovalent material. An
ORTEP diagram of the molecule is given in Figure 3, and
significant distances and angles are given in Table 3. The
divalent Eu(1) ion bound to a pair of chelating DME ligands
(four Eu-O in the range of 2.643(4)-2.696(5) Å), three
bridging phenoxide oxygen atoms (three Eu-O in the range
of 2.551(4)-2.591(4) Å), and three short Eu(1)-F interac-
tions (2.796(4), 2.835(4), and 3.109(5) Å), of which the first
two are closer to the “expected” distances for a formal
Eu2+-F dative interaction. The trivalent Eu(2) ion coordi-
nates the three bridging phenoxides (three Eu-O in the range
of 2.348(4)-2.410(4) Å), a pair of terminal phenoxide
ligands (2.223(4) and 2.221(4) Å), and one chelating DME
ligand (Eu-O bond lengths of 2.482(4) and 2.537(4) Å).
The nearest Eu(2)3+-F contact interaction is at 3.072(4) Å,
and the other three are in the range of 3.82-4.14 Å.
In 3, the metal oxygen bond lengths are clearly indicative
of localized oxidation states. The three bridging Eu-O(C6F5)
bond lengths average 2.57 Å for divalent Eu(1) and 2.37 Å
for trivalent Eu(2). Similarly, for the neutral donors, the Eu-
O(DME) average for Eu(1) is 2.66 Å, while that for Eu(2)
is 2.51 Å. Given the smaller ionic radius of Eu3+, there does
Table 2. Significant Distances (Å) and Angles (deg) for 1
Eu(1)-O(2)
Eu(1)-O(3)
Eu(1)-Eu(2)
Eu(2)-O(2)
Eu(2)-O(4)
Eu(2)-O(7)
Eu(2)-F(15)
2.533(2)
2.551(2)
3.7528(2) Eu(2)-O(1)
2.488(3)
2.638(2)
2.674(2)
3.004(2)
Eu(1)-O(1)
Eu(1)-F(1)
2.550(2)
2.928(2)
2.457(2)
2.569(2)
2.667(2)
2.757(3)
3.013(2)
Eu(2)-O(3)
Eu(2)-O(5)
Eu(2)-O(6)
Eu(2)-F(10)
C(1)-O(1)-Eu(2)
C(7)-O(2)-Eu(2)
136.0(2)
129.4(2)
C(13)-O(3)-Eu(1) 129.9(2)
C(1)-O(1)-Eu(1)
C(7)-O(2)-Eu(1)
C(13)-O(3)-Eu(2)
Eu(2)-O(2)-Eu(1)
127.0(2)
133.5(2)
120.6(2)
96.74(8)
Eu(2)-O(1)-Eu(1)
Eu(1)-O(3)-Eu(2)
97.07(8)
94.27(7)
Eu(2)-Eu(1)-Eu(2′) 166.33(1)
with HOC6F5 (reaction 1), and the trivalent product can also
be prepared in high yields from proton-transfer reactions of
Eu(SPh)2 with HOC6F5 (reaction 2).
Eu + xHOC6F5 f Eu(OC6F5.)x + x/2H2 (x ) 2, 3) (1)
“Eu(SPh)2” + 3HOC6F5 f Eu(OR)3 + 2HSPh + 1/2H2
(2)
From DME, both divalent (DME)4Eu3(OC6F5)6 (1) and
trivalent (DME)2Eu(OC6F5)3 (2) were isolated and character-
ized completely. Trimer 1 contains a linear arrangement of
Eu2+ ions. Figure 1 shows an ORTEP diagram of 1, and
Table 2 gives a listing of significant distances and angles
for the complex. The central eight-coordinate Eu(1) is
encapsulated by two sets of three µ2-OC6F5 ligands and a
symmetry-related pair of dative Eu(1)-F interactions to the
F atom on the R-carbon, at 2.982(2) Å. The other Eu‚‚‚F
distances of this type are 3.21 and 4.26 Å and are outside
the realm of dative interactions. The Eu(1)-O(C6F5) bond
lengths range from 2.533(2) to 2.551(2) Å. The two end Eu2+
ions are symmetry related, and each coordinates three
Inorganic Chemistry, Vol. 46, No. 10, 2007 4063

Norton et al.
not appear to be a significant Eu(III)-F interaction, including
the Eu(2)-F(5) separation at 3.072(4) Å. While both 1 and
2 are yellow crystalline solids, heterovalent 3 is deep orange.
When they are heated at 650 °C for 5 h, these compounds
give a variety of solid-state EuF3 phases, with the trivalent
compound 2 giving a single LnF3 phase and compounds 1
and 3 showing a pair of LnF3 phases.19 CGMS analysis of
the volatile products did not reveal the identity of the organic
fluoride abstraction product.
Within the structures, bond lengths can be predicted with
ionic radius summation rules,24 where the difference between
Eu(II) and Eu(III) Eu-O bond lengths is 0.20 Å, the ionic
radii difference.25 Differences between the dative Eu-
O(DME) bond lengths are slightly less than the predicted
value, with a difference between Eu(II) and Eu(III) of 0.15
Å, and are probably more affected by steric considerations.
The constancy of the Eu-F contacts is even less ideal. In
divalent 1, there are short Eu-F separations for both the
internal (Eu(1)-F(1) ) 2.928(2) Å) and external (Eu(2)-
F(15) ) 3.004(2) Å, Eu(2)-F(10) ) 3.013(2) Å) metal ions,
and the distances are similar to the 3.006(6) Å separation
found in the related thiolate polymer [(THF)2Eu(SC6F5)2]n.
There are no significant Eu(III)-F bonds in 2. Similarly, in
3, there are no significant Eu(III)-F interactions, whereas
there are Eu-F bonds to the divalent ion that are within the
range found in 1. One possible explanation for the different
behavior between Ln(II) and Ln(III) is that electrostatic bonds
to the former are weaker, and thus entropy considerations
determine whether a phenoxide chelates or a neutral molecule
coordinates to saturate a metal center. It should also be
pointed out that such interactions with Ln(III) are not
impossible, given similar distances in Sm(SC6F5)3 coordina-
tion compounds (i.e., 2.582(7) and 2.641(6) Å in (THF)4Sm2-
(SC6F5)6),6b but the electronegativity of O and the tendency
for OR ligands to adopt more linear Ln-O-R geometries
certainly disfavors the formation of Ln-F bonds.
Obvious π-stacking interactions are prevalent throughout
the structures of 1-3, as found in both the analogous
thiolates, as well as transition metal phenoxides. Monomeric
2 contains intra- and intermolecular π-π interactions along
the crystallographic b+c direction. The intramolecular π-π
interaction between the O(2) and O(3) C6F5 rings is quite
close with a dihedral angle of 6.5° and a center-to-center
distance of about 3.5 Å. This intramolecular π-π interaction
links up with an adjacent coplanar pair (inversion related
O(2′) and O(3′) with a dihedral angle of 0.0° and a 3.6 Å
center-to-center distance) in one direction (b+c) and with a
somewhat tilted O(1′) alkoxide C6F5 ring (with a dihedral
angle of 18° and a 3.8 Å center-to-center distance) in the
opposite direction (b-c) to yield true 1-D π-stacking overall.
The dimeric complex 3 has a remote π-π interaction (e.g.,
only 2 C atoms overlap, with a separation of >4 Å) between
aryloxides at O(1) and O(3)′ along the unit cell body
diagonal. Within one dimer, no pair of C6F5 planes are even
remotely coplanar, but the possibility of some π-π interac-
tion does exist, namely, between rings O(1) and O(5) with
a dihedral of 19°, because the center-to-center distance is
short at 3.3 Å.
Discussion
Direct reduction of HOR with elemental Ln is a particu-
larly facile synthetic route to divalent and trivalent fluorinated
phenoxides of Eu. Similar reactivity was noted in the
preparation of Eu(OR)2/3 (R ) C6H4Me2-2,6),20 although
these reactions required highly basic (NH3, MeCN, N-
methylimidazole) solvents. This chemistry is not necessarily
extendable to the other R groups previously employed in
Eu alkoxide syntheses, that is, the analogous isopropoxide
system is considerably more complicated. Reactions of Eu
with HOiPr gave the heterovalent compound [Eu4(OiPr)10]
from reactions that were expected to yield Eu(OiPr)2,21c,22
and exposure to oxygen led to the isolation of [Eu5O(Oi-
Pr)12] and [Eu5O(OiPr)13].21a,b Additional reaction chemistry
of in situ-prepared Eu(OiPr)2 with HOC6H3Me2-2,6 led to
both straightforward proton-transfer products (i.e., Eu3(O-
2,6-Me2C6H3)(THF)6, another linear trimer with an alkoxide
encapsulated central Eu(II) ion), as well as a breathtaking
array of polymetallic clusters, including H10[Eu8O8(OR)10(Oi-
Pr)2(THF)6] and H18{[Eu9O8(OR)10(THF)7][Eu9O9(OR)10-
(THF)6]}.22
The compounds that contain Eu(III) (2 and 3) can also be
prepared in proton-transfer reactions of HOC6F5 with in situ-
prepared Eu(SPh)2 (reaction 2). Attempts to isolate hetero-
leptic compounds (i.e., Eu(SPh)x(OC6F5)3-x) from these
reactions led only to the isolation of the phenoxide species.
Heterovalent lanthanide compounds are relatively uncom-
mon,23 with all well-defined systems thus far behaving as
localized valence materials, as is dimer 3. The absence of
any crystallographic disorder in 3 is fortunate because the
asymmetry of the complex allows for immediate assignment
of oxidation states for divalent Eu(1) and trivalent Eu(2).
(19) (a) Greis, O.; Petzel, T. Z. Anorg. Allg. Chem. 1973, 401, 1. (b) Zalkin,
A.; Templeton, D. H. J. Am. Chem Soc. 1953, 75, 2453.
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J. Solid State Inorg. Chem. 1996, 33, 145. (b) Evans, W. J.; Greci,
M. A.; Ziller, J. W. J. Chem. Soc., Dalton Trans. 1997, 3035. (c)
Evans, W. J.; Greci, M. A.; Ziller, J. W. J. Chem. Soc., Chem.
Commun. 1998, 2367.
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41, 3249. (b) Moustiakimov, M.; Kritikos, M.; Westin, G. Inorg. Chem.
2005, 44, 1499. (c) Evans, W. J.; Greci, M. A.; Ziller, J. W. Inorg.
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Am. Chem. Soc. 1999, 121, 11713. (b) Deacon, G. B.; Gitlits, A.;
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2752.
For the trimeric complex 1, the only π-π interaction is
found within one trimer subunit, between the central bridging
alkoxide and its 2-fold symmetry mate [e.g., O(3) and O(3′)].
The two C6F5 rings for this π-π interaction are nearly
coparallel; although they are not crystallographically con-
strained to be so, with a dihedral angle of only 4.4°.
(24) Raymond, K. N.; Eigenbrot, C. W. Acc. Chem. Res. 1980, 13, 276.
(25) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
4064 Inorganic Chemistry, Vol. 46, No. 10, 2007

Ln Compounds with OC6F5 Ligands
The effects of fluorination on the Eu-O bond length can
be assessed, given the relative abundance of Eu-O-Ar bond
lengths in the literature. Most straightforward would be
comparisons with terminal phenoxide linkages, which
seem to have a narrower range of separations relative to
bridging interactions. Of the reported phenoxide com-
pounds, three are reasonable for comparative purposes: in
(diglyme)2(DME)Ba2Eu(OC6H4-4-Me)7,26 the six coordinate
Eu ion coordinates two terminal OAr with Eu-O bond
lengths of 2.16(1) and 2.19(1) Å; in (pyrazolylborate)Sm-
(OC6H4-4-tBu),27 the seven coordinate Sm ion bonds to a
single phenoxide with a 2.16(1) Å Sm-O bond length;
and in (DME)2TmI2(OC6H5),28 the seven coordinate Tm(III)
ion bonds to a single phenoxide with a 2.025(7) Å Tm-O
bond length (the lanthanide contraction here is responsible
for the Tm-O distances being shorter by ∼0.06 Å). When
we compare the terminal Eu(III)-O distances in 2 (av 2.22-
(1) Å) and 3 (2.221(4) Å), it is clear that the bond lengths
are significantly longer. This is attributed to the polarizing
effect of the fluorine substituents, delocalizing the anionic
charge and weakening the Ln-O bond.
In contrast, bridging Eu-O bond lengths for divalent 1
fall within the range of bridging distances found in the similar
set of DME, THF, and N-methylimidazole20 coordination
complexes of Eu(OC6H3Me2-2,6)2. These bridging bonds are
both weaker electrostatically because of the lower charge
on the Eu ion and more sensitive to steric influences with
repulsions from two primary coordination spheres contribut-
ing to the observed distances.
The electronic properties of these compounds are readily
understood. For Eu(II), there are allowed f-d transitions that
fall in the UV spectrum when the ligands are relatively
electronegative.29 Divalent Eu compounds also exhibit al-
lowed MLCT transitions when the ligand has accessible π*
orbitals. Divalent 1 is yellow, both in solution and as a DME
adduct, because the only visible chromophore is the high-
energy f-d process that tails into the visible portion of the
spectrum. The addition of pyridine results in displacement
of DME by the more basic nitrogen donor ligand, and upon
coordination, there exists an allowed Eu-to-py CT process30
that generates a deep orange color, which unfortunately could
not be defined quantitatively by a maximum in the absorption
spectrum.
3 are dissolved in DME, the visible spectra are essentially
identical, but this is likely caused by the general insensitivity
of f-f transitions to coordination environment and not by
the solution state environments for the Eu(II) ions in 2 and
3 being identical. It should be noted that the absorption
maxima are within 1-2 nm of the maxima found for a series
of Eu(III) isopropoxide compounds.
Still, trivalent Eu compounds can be intensely colored, if
the anions are sufficiently electropositive such that LMCT
absorptions are in the visible, rather than UV portion of the
spectrum. Examples of intensely colored Eu(III) compounds
are those that contain sulfur-based anions such as S-2-
NC5H430c,31 or S2CR32 or organometallic complexes such as
(C5H5)3Eu(THF).30 The fluorinated phenoxide anions are
apparently electronegative enough to shift any phenoxide-
related LMCT absorption into the UV spectrum, and so 2 is
also light yellow.
Heterovalent 3 has two metal ions that are intrinsically
light yellow derivatives of 1 and 2, and so the deeper color
of 3 in the solid state may arise from an excitation process
that transfers an electron from Eu(II) to Eu(III). Similar
highly colored combinations of colorless ions has been
observed previously in deep red heterovalent Ce4O(OiPr)13,
which is derived from yellow Ce(IV) and white Ce(III)
alkoxides.33
The presence of fluorine within the phenoxide ligand
system complicates compound thermolysis because, in ad-
dition to the formation of pure oxide phases, fluorides can
be abstracted to form either oxyfluorides or trifluoride solids.
LnOF phases have been obtained frequently from oxygen
based ligand systems, that is, in the decomposition of
fluorinated diketonates1,34 and carboxylates.2,35 The trifluo-
rides have only been noted in the thermolysis of fluorinated
thiolates, in which the formation of relatively electropositive
sulfide anions is clearly less favorable from an electrostatic
perspective. In the present compounds, rapid thermolysis of
1-3 led to the formation of a mixture of solid-state EuF3
phases with no indication of oxide materials. Apparently the
aryl transfer decomposition pathway that dominates the
thermolysis of M(EPh)x systems (E ) S, Se, Te)36 is inhibited
by the strength of the C-O bond. This reactivity is still
surprising, given the facility with which oxyfluoride solids
have been prepared from fluorinated molecular precursors.
Oxidation of the divalent Ln ions by abstraction of fluoride
Trivalent Eu(III) compounds have characteristicly weak
(ꢀ < 1) f-f transitions, and absorptions from these transitions
are noted in the visible spectra of both 2 and 3. When 2 and
(31) Lee, J.; Freedman, D.; Melman, J. H.; Brewer, M.; Sun, L.; Emge, T.
J.; Long, F. H.; Brennan, J. G. Inorg. Chem. 1998, 37, 2512.
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Chem. 1996, 38, 207. (b) Regulacio, M. D.; Tomson, N.; Stoll, S. L.
Chem. Mater. 2005, 17, 3114.
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Chem. 2002, 2, 453.
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39, 2635.
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V.; Evans, W. J.; Greci, M. A.; Ziller, J. W. Akad Nauk Ser. Khim.
1999, 9, 1804.
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1981, 5, 479. (b) Melsen, J.; Wills, J. M.; Johansson, B.; Eriksson, O.
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Opt. Spektrosk. 1962, 13, 235.
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Aust. J. Chem. 1987, 40, 895. (b) Brewer, M.; Khasnis, D.; Buretea,
M.; Berardini, M.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1994,
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W. E.; Vaarstra, B. A.; Caulton, K. G. Inorg. Chem. 1991, 30, 2317.
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127, 14008.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4065

Norton et al.
has been noted previously for Sm and Yb organometallic
complexes,37 but not for Eu(II), which has a more stable
divalent oxidation state. The oxidation of Eu(II) to Eu(III)
in the thermolysis of both 1 and 3 is presumably facilitated
by the presence of highly electronegative anions that can
stabilize the higher oxidation state. Unfortunately, there were
no organic products that could be identified in these reactions.
tions are noted with Eu(II) ions, while Eu(III) ions tend to
adopt terminal phenoxide ligations. The OC6F5 ligands bound
to both Eu(II) and Eu(III) are involved in π stacking
interactions. Because these phenoxides are soluble in apolar
media, they are potentially useful as capping reagents in
lanthanide cluster chemistry, and efforts along this direction
are in progress.
Conclusion
Acknowledgment. We would like to acknowledge the
The OC6F5 ligand forms easily isolated compounds with
support of NSF (CHE-0303075).
both divalent and trivalent europium. Dative Ln-F interac-
Supporting Information Available: X-ray crystallographic files
in CIF format for the crystal structures of 1-3 and IR spectra and
XRPD profiles of the pyrolysis products for 1-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(37) (a) Xie, Z.; Chui, K.; Yang, O.; Mak, T. C. W.; Sun, J. Organometallics
1998, 17, 3937. (b) Schumann, H.; Keitsch, M. R.; Winterfeld, J.;
Demtschuk, J. J. Organomet. Chem. 1996, 525, 279. (c) Watson, P.
L.; Tulip, T. H.; Williams, I. Organometallics 1990, 9, 1999. (d)
Deacon, G. B.; Fallon, G. D.; Forsyth, C. M.; Harris, S. C.; Junk, P.
C.; Skelton, B. W.; White, A. H. Dalton Trans. 2006, 802.
IC062395E
4066 Inorganic Chemistry, Vol. 46, No. 10, 2007