Divalent samarium compounds with heavier chalcogenolate (EPh; E = Se, Te) ligands

Article abstract of DOI:10.1021/ic9913278

Crystalline coordination complexes of Sm(EPh)₂ (E = Se, Te) are described. The selenolate compound Sm(SePh)₂ is unstable in solution, but a divalent selenolate can be prepared and isolated when precisely 1 equiv of Zn(SePh)₂ is present to form heterometallic [(THF)₃Sm(μ₂-SePh)₃Zn(μ₂-SePh)](n) (1). This compound is a 1D coordination polymer with alternating Sm(II) and Zn(II) ions connected by an alternating(1,3) number of bridging selenolate ligands and three THF ligands bound to each Sm(II) ion. The tellurolate Sm(TePh)₂ forms a stable pyridine coordination compound (py)₅Sm(TePh)₂ (2) that is isostructural with known Eu and Yb benzenetellurolates. Both compounds were characterized by conventional spectroscopic methods. Polymer 1 was characterized by low-temperature single-crystal X-ray diffraction, and the unit cell of the tellurolate was determined. Crystal data (Mo Kα, 153(5) K) are as follows. 1: Monoclinic space group P2₁, a = 10.666(2) A, b = 16.270(3) A, c = 12.002(3) A, β = 114.81(2)°, Z = 2.2: orthorhombic space group Pbca, with a = 13.865(3) A, b = 16.453(5) A, c = 31.952(7) A, Z = 8.

Full text of DOI:10.1021/ic9913278

2168
Inorg. Chem. 2000, 39, 2168-2171
Divalent Samarium Compounds with Heavier Chalcogenolate (EPh; E ) Se, Te) Ligands
Deborah Freedman, Anna Kornienko, Thomas J. Emge, and John G. Brennan*
Department of Chemistry, Rutgers, the State University of New Jersey, 610 Taylor Road,
Piscataway, New Jersey 08854-8087
ReceiVed NoVember 16, 1999
Crystalline coordination complexes of Sm(EPh)2 (E ) Se, Te) are described. The selenolate compound Sm(SePh)2
is unstable in solution, but a divalent selenolate can be prepared and isolated when precisely 1 equiv of Zn(SePh)2
is present to form heterometallic [(THF)3Sm(µ2-SePh)3Zn(µ2-SePh)]n (1). This compound is a 1D coordination
polymer with alternating Sm(II) and Zn(II) ions connected by an alternating (1,3) number of bridging selenolate
ligands and three THF ligands bound to each Sm(II) ion. The tellurolate Sm(TePh)2 forms a stable pyridine
coordination compound (py)5Sm(TePh)2 (2) that is isostructural with known Eu and Yb benzenetellurolates. Both
compounds were characterized by conventional spectroscopic methods. Polymer 1 was characterized by low-
temperature single-crystal X-ray diffraction, and the unit cell of the tellurolate was determined. Crystal data (Mo
KR, 153(5) K) are as follows. 1: monoclinic space group P21, a ) 10.666(2) Å, b ) 16.270(3) Å, c ) 12.002(3)
Å, â ) 114.81(2)°, Z ) 2. 2: orthorhombic space group Pbca, with a ) 13.865(3) Å, b ) 16.453(5) Å, c )
31.952(7) Å, Z ) 8.
Introduction
Ln ions into chalcogenido matrices. These molecules are also
excellent subjects for probing the nature of the Ln-E bond. In
addition to the relatively straightforward trivalent chalcogenolate
chemistry of the redox active and inactive Ln(ER)3 com-
pounds,7-11,14-19,21-25 considerable effort has been devoted
toward determining the chemistry of Ln(ER)2 (Ln ) Sm, Eu,
Yb).1-8,12-14,20-22 The simple benzenechalcogenolate (R ) Ph)
ligands have been particularly useful for the synthesis of well-
defined Ln(II) coordination compounds, but while compounds
of Eu(EPh)21,2 and Yb(EPh)22,3 were among the first crystallo-
graphically characterized Ln(II) chalcogenolates to be described,
the corresponding compounds of Sm have been more elusive.
Initial attempts to prepare simple coordination complexes of
Sm(SePh)2 in either THF or pyridine were unsuccessful. Related
Lewis base adducts of Sm(SeR)2 (R ) Si(SiMe3)3;20 2,4,6,tri-
iprC6H213) have been isolated successfully, but the analogous
Sm(SePh)2 was found to decompose thermally (at room tem-
perature) to give selenido clusters,26,27 presumably by reduction
of the Se-C(Ph) bond and oxidation of the Sm(II) ion. In
weakly basic DME, the mixed valent, tetranuclear cluster
(DME)4Sm4(Se)(SePh)8 has been isolated, while in more basic
solvents, the larger octanuclear cluster (THF)8Sm8Se6(SePh)12
has been isolated. There is no information related to the stability
of the analogous SPh and TePh compounds.
Lanthanide (Ln) chalcogenolate (Ln(ER)x: E ) S, Se, Te; x
) 2, 3; R ) Ph,1-11 aryl,12-17 SiR3,18-20 2-E-NC5H4,21-23
alkyl24,25) chemistry has developed rapidly in the past few years,
motivated by the potential utility of such compounds for doping
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10.1021/ic9913278 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/06/2000

Samarium Compounds
Inorganic Chemistry, Vol. 39, No. 10, 2000 2169
precursors to Ln chalcogenido clusters,37-41 the Sm(EPh)2
compounds are desirable synthetic targets. In this work we
describe the high-yield synthesis of crystalline coordination
compounds containing Sm(II) ions coordinated to the heavier
benzenechalcogenolate EPh (E ) Se, Te) ligands.
Table 1. Summary of Crystallographic Details for
[(THF)3Sm(µ-SePh)3Zn(µ-SePh)]n (1)
empirical formula
fw
space group (No.)
a (Å)
b (Å)
C36H44O3Se4SmZn
1056.27
P21 (4)
10.666(2)
16.270(3)
12.002(3)
90.00(2)
114.81(2)
90.00(2)
1891(1)
2
Experimental Section
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
General Methods. All syntheses were carried out under ultrapure
nitrogen (JWS), using conventional drybox or Schlenk techniques.
Solvents (Fisher) were refluxed continuously over molten alkali metals
or K/benzophenone and collected immediately prior to use. Anhydrous
pyridine (Aldrich) was purchased and refluxed over KOH. PhSeSePh
was purchased from either Aldrich or Strem and recrystallized from
hexane. PhTeTePh was prepared according to literature procedures.42,43
Ln and Hg were purchased from Strem. Melting points were taken in
sealed capillaries and are uncorrected. IR spectra were taken on a Mattus
Cygnus 100 FTIR spectrometer and recorded from 4000 to 450 cm-1
as a Nujol mull on KBr 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. Elemental analyses were performed
by Quantitative Technologies, Inc. (Whitehouse NJ). These compounds
are sensitive to the thermal dissociation of neutral donor ligands at
room temperature, and so the experimentally determined elemental
analyses are often found to be lower than the computed analyses.
Products appear homogeneous, and for each compound, several crystals
were examined by single-crystal X-ray diffraction and consistently gave
the same unit cell.
D(calcd) (g/cm-3
temp (K)
)
1.856
153
λ (Å)
0.710 73
6.056
0.037
abs coeff (mm-1
)
R(F)a [I > 2σ(I)]
Rw(F2)a [I > 2σ(I)]
0.081
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|; Rw(F2) ) {∑[w(Fo - Fc2)2]/
2
∑[w(Fo )2]}1/2. Additional crystallographic details are given in the
2
Supporting Information.
(m), 2342 (w), 1579 (m), 1461 (s), 1378 (s), 1314 (w), 1261 (w), 1216
(m), 1145 (w), 1067 (w), 1031 (m), 801 (w), 732 (m), 701 (s), 634 (s)
cm-1. λmax (THF): 614 (w1/2 ) 292 nm, ꢀ ) 550 L mol-1 cm-1), 307
nm. Unit cell (Mo KR, -120 °C): space group Pbca, with a )
13.865(3) Å, b ) 16.453(5) Å, c ) 31.952(7) Å, V ) 7289(3) Å3.
Addition of 0.5 equiv of PhTeTePh to a solution of 2 results in no
color change, and extraction of the reaction mixture with hexane
followed by evaporation of the hexane reclaims the 0.5PhTeTePh.
X-ray Structure Determination of 1. Data for 1 were collected on
an Enraf-Nonius CAD4 diffractometer with graphite monochromatized
Mo KR radiation (λ ) 0.710 73 Å) at -120 °C. The check reflections
measured every hour showed less than 3% intensity variation. The data
were corrected for Lorenz effects and polarization, and absorption, the
latter by a numerical (SHELX76)44 method. The structure was solved
by direct methods (SHELXS86).45 All non-hydrogen atoms were refined
(SHELXL97) on the basis of Fobs2. All hydrogen atom coordinates were
calculated with idealized geometries (SHELXL97).46 Scattering factors
(fo, f ′, f ′′) are as described in SHELXL97. Crystallographic data and
final R indices for 1 are given in Table 1. Significant bond distances
and angles for 1 are given in Table 2. Complete crystallographic details
are given in the Supporting Information. An ORTEP diagram47 for the
common repeating unit of 1 is shown in Figure 1.
Synthesis of [(THF)3Sm(µ2-SePh)3Zn(µ2-SePh)]n (1). Zinc metal
(0.13 g, 2.0 mmol), diphenyl diselenide (1.25 g, 4.0 mmol), and mercury
(0.05 g, 0.25 mmol) were combined in THF (50 mL). After all the
zinc had reacted (ca. 1 day) to give a pale-yellow solution/precipitate,
samarium metal (0.30 g, 2 mmol) was added. After 2 days the resultant
dark-green solution was filtered, concentrated (35 mL), and allowed
to sit at room temperature (RT) to give dark-green crystals (1.0 g, 47%;
the compound melts with decomposition and turns red at ca. 183 °C,
then slowly turns orange at 220 °C). Anal. Calcd for C36H44O3Se4-
1
SmZn: C, 40.9; H, 4.21. Found: C, 40.1; H, 4.06. H NMR (OC4D8,
20 °C) revealed phenyl resonances at 9.03 (2H), 7.50 (1H), and 6.83
ppm (2H). IR: 2924 (s), 2861 (s), 2361 (w), 1571(m), 1464 (s), 1382
(s), 1262 (w), 1069 (m), 1033 (m), 1020 (m), 875 (w), 825 (w), 800
(w), 739 (m), 729 (m), 691 (m), 665 (w) cm-1. λmax (THF): 612 (w1/2
) 153 nm), 398 (w ) 125 nm). Reliable molar absorptivities were
1/2
not obtainable because the compound does not redissolve readily.
Synthesis of (py)5Sm(TePh)2 (2). Sm (0.30 g, 2.0 mmol) and
diphenylditelluride (0.82 g, 2.0 mmol) were added to pyridine (50 mL),
and the mixture was stirred for 2 days. The resultant black-orange
solution was filtered and cooled (-6 °C) to give black (orange) crystals
of 3 (1.27 g, 67%; mp 130 °C). Anal. Calcd for C37H35N5SmTe2: C,
Results
The divalent selenolate Sm(SePh)2 can be stabilized by
coordination of Zn(SePh)2. Whereas solutions of green Sm-
(SePh)2 turn red-brown within days at room temperature
(reaction 1),26 when Sm metal is oxidized by PhSeSePh in the
presence of Zn(SePh)2, the resultant green solution color that
is characteristic of Sm(II) chalcogenolates13,20 is maintained for
extended periods. Saturation of this green solution gives the
heterometallic polymer [(THF)3Sm(µ2-SePh)3Zn(µ2-SePh)]n (1)
(reaction 2) that was characterized by conventional spectroscopic
methods and low-temperature single-crystal X-ray diffraction.
Figure 1 is an ORTEP diagram of the repeating polymeric unit,
and Table 2 gives a listing of significant bond lengths and angles
for the compound. The compound contains an alternating
1
46.5; H, 3.74; N, 7.33. Found: C, 46.1; H, 3.47; N, 7.60. H NMR
(OC4D8, 20 °C) showed only the pyridine resonances at 8.54 (2H),
7.65 (1H), 7.26 (2H) ppm. IR: 2932 (s), 2724 (w), 2673 (w), 2361
(31) Hughes, A. D.; Price, D. A.; Simpkins, N. S. J. Chem. Soc., Perkins
Trans. 1999, 1295.
(32) Keck, G. E.; Wager, C. A.; Wager, T. T. J. Org. Chem. 1999, 64,
2172.
(33) Nakamura, Y.; Takeuchi, S.; Mikami, K. Tetrahedron 1999, 55, 4595.
(34) Molander, G. A.; Machrouhi, F. J. Org. Chem. 1999, 64, 4119.
(35) Molander, G. A.; McKie, J. A.; Curran, D. P. Chemtracts: Org. Chem.
1994, 7, 351.
(36) Kagan, H. B. New J. Chem. 1990, 14, 453.
(37) Melman, J. H.; Emge, T. J.; Brennan, J. G. Chem. Commun. 1997,
2269.
(38) Freedman, D.; Melman, J.; Emge, T. J.; Brennan, J. G. Inorg. Chem.
1998, 37, 4162.
(39) Melman, J. H.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1999, 38,
2117.
(40) Freedman, D.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1999, 38,
4400.
(41) Melman, J. H.; Fitzgerald, M.; Freedman, D.; Emge, T. J.; Brennan,
J. G. J. Am. Chem. Soc. 1999, 121, 10247.
(44) Sheldrick, G. M. SHELX76, Program for Crystal Structure Determi-
nation; University of Cambridge: England, 1976.
(45) Sheldrick, G. M. SHELXS86, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(46) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(47) (a) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976. (b) Zsolnai, L. XPMA and
ZORTEP, Programs for InteractiVe ORTEP Drawings; University of
Heidelberg: Germany, 1997.
(42) Haller, W. S.; Irgolic, K. J. J. Organomet. Chem. 1973, 38, 97.
(43) Petragnani, N.; DeMoura, M. Chem. Ber. 1963, 96, 249.

2170 Inorganic Chemistry, Vol. 39, No. 10, 2000
Table 2. Significant Distances (Å) and Angles (deg) for 1
Freedman et al.
Eu(TePh)22 and Yb(TePh)2.3 Molecule 2 is considerably more
soluble than the selenolate polymer, redissolves readily in
pyridine, does not react further with additional PhTeTePh, and
is thermally unstable in THF. The compound is intensely colored
because of two intense electronic absorptions at 307 and 614
nm that have tentatively been assigned as an f 6-to-f 5d1
promotion and a Sm(II)-to-pyridine charge-transfer absorption,
respectively. The presence of additional PhTeTePh does not
result in further oxidation of the Sm(II) ion.
Sm(1)-O(1)
Sm(1)-O(3)
Sm(1)-Se(1)
Sm(1)-Se(2)
Zn(1)-Se(2)
Zn(1)-Se(1)
Se(2)-C(7)
Se(4)-C(19)
2.504(8)
2.526(8)
Sm(1)-O(2)
Sm(1)-Se(3)
Sm(1)-Se(4)
Zn(1)-Se(4)′
Zn(1)-Se(3)
Se(1)-C(1)
2.505(7)
3.1898(13)
3.2010(13)
2.4299(17)
2.4749(16)
1.942(12)
1.897(13)
2.4299(17)
3.1923(13)
3.4470(13)
2.4725(17)
2.4942(17)
1.934(11)
1.940(11)
Se(3)-C(13)
Se(4)-Zn(1)′′
O(1)-Sm(1)-O(2)
O(2)-Sm(1)-O(3)
95.5(2)
125.1(3)
O(1)-Sm(1)-O(3) 120.2(3)
O(1)-Sm(1)-Se(3)
79.2(2)
90.0(2)
92.65(17)
Discussion
O(2)-Sm(1)-Se(3) 140.00(17) O(3)-Sm(1)-Se(3)
O(1)-Sm(1)-Se(1) 151.7(2)
O(2)-Sm(1)-Se(1)
Measurements of electronic spectra in previous heterometallic
Ln/main group chalcogenolate chemistry led to the conclusion
that the interaction of Ln(ER)x with the more covalent M(ER)2
of the group 12 (M ) Zn, Cd, Hg)4,6,7 and group 14 (M ) Sn,
Pb)5 metals resulted in polarization of electron density away
from the Ln ion. This polarization prompted the preparation of
Sm(SePh)2 in the presence of Zn(SePh)2. If the Sm ion were
electron-depleted, then any reductive cleavage process involving
oxidation of the Sm would be less favorable. While an excess
of Zn(SePh)2 in solution leads to the formation of the salt
[Sm(THF)7]2+[Zn4(µ2-SePh)10]2- that does not contain Sm-
Se(Ph) bonds,7 the use of a 1:1 Zn:Sm ratio gives the 1:1
heterometallic polymer 1 in high yield. Such 1D polymers are
important reference points for understanding the fundamental
physical properties of Ln ions doped into chalcogenido-based
matrixes48 and because Ln-doped polymeric materials have
potential applications in upconversion processes.49-54
O(3)-Sm(1)-Se(1)
O(1)-Sm(1)-Se(4)
O(3)-Sm(1)-Se(4)
75.2(2)
85.6(2)
69.7(2)
Se(3)-Sm(1)-Se(1) 77.22(3)
O(2)-Sm(1)-Se(4) 73.66(17)
Se(3)-Sm(1)-Se(4) 143.80(3)
Se(1)-Sm(1)-Se(4) 122.68(3) O(1)-Sm(1)-Se(2)
O(2)-Sm(1)-Se(2)
Se(3)-Sm(1)-Se(2)
85.64(19)
74.41(17) O(3)-Sm(1)-Se(2) 141.4(2)
65.69(3) Se(1)-Sm(1)-Se(2) 70.56(3)
Se(4)-Sm(1)-Se(2) 145.83(3) Se(4)′-Zn(1)-Se(2) 117.96(6)
Se(4)′-Zn(1)-Se(3) 117.47(6) Se(2)-Zn(1)-Se(3) 93.59(6)
Se(4)′-Zn(1)-Se(1) 116.60(6) Se(2)-Zn(1)-Se(1) 101.29(6)
Se(3)-Zn(1)-Se(1) 106.53(6) C(1)-Se(1)-Zn(1) 106.3(3)
C(1)-Se(1)-Sm(1)
C(7)-Se(2)-Zn(1)
Zn(1)-Se(2)-Sm(1)
C(13)-Se(3)-Sm(1) 119.5(4)
C(19)-Se(4)-Zn(1)′′ 104.4(3)
Zn(1)′′-Se(4)-Sm(1) 124.86(5)
106.4(4)
108.9(3)
Zn(1)-Se(1)-Sm(1) 76.14(4)
C(7)-Se(2)-Sm(1) 119.4(3)
71.53(4) C(13)-Se(3)-Zn(1) 112.9(4)
Zn(1)-Se(3)-Sm(1) 76.44(4)
C(19)-Se(4)-Sm(1) 103.4(3)
a Symmetry transformations used to generate equivalent atoms.
1
1
′ indicates -x, y + /2, -z, and ′′ indicates - x, y - /2, -z.
Surprisingly, the structure of heterometallic 1 differs from
the analogous Eu derivative (THF)4Eu(µ2-SePh)3Zn(SePh),7
which crystallizes as a molecular bimetallic product with a
terminal SePh ligand coordinated to the Zn and an additional
THF ligand bound to the Eu. These differences in structure could
in theory be explained by the slightly larger (0.01 Å) Sm(II)
ion increasing the tendency of chalcogenolate ligands to bridge
Ln ions,11 but there are also changes in the synthetic strategies
that are far more likely to account for the difference. Changes
in concentration or the presence of trace Hg, Hg(SePh)2, or
Sm(SePh)2 thermolysis products could conceivably have a
significant influence on the structure of the final isolated product.
Regardless, the principal Sm(II) species in solution is likely to
resemble the structure of the Eu-Zn complex.
From the Sm-Se bond length distribution in 1 it is clear that
there are significant steric interactions that influence the length
of the Sm-Se bond and thus possibly the solid-state structure.
Polymer 1 has three nearly equal Sm-Se bonds (Sm-Se(1),
3.192(1) Å; Sm-Se(3), 3.190(1) Å; Sm-Se(4), 3.201(1) Å)
and a fourth bond (Sm-Se(2), 3.447(1) Å) that differs consider-
ably. A similar bond length inequivalence was found in the
structure of (THF)4Eu(µ2-SePh)3Zn(SePh), although the differ-
ences (Eu-Se ) 3.162(1) Å, 3.176(1) Å, 3.282(1) Å) were less
pronounced. There is only one other structurally characterized
Sm(II) compound with a bond to Se in the literature55 [(THF)2-
Figure 1. ORTEP diagram of the repeating unit of [(THF)3Sm(µ2-
SePh)3Zn(SePh)]n, with the thermal ellipsoids drawn at the 50%
probability level and the C/H atoms removed for clarity.
number (1,3) of doubly bridging SePh ligands connecting the
alternating series of Sm(II) and Zn(II) ions. The tetrahedral Zn
coordination sphere is comprised entirely of Se atoms, while
the larger seven-coordinate Sm(II) ion coordinates to four Se(Ph)
and three additional THF ligands. The green color of 1 results
from a broad, intense electronic absorption centered at 612 nm
that is tentatively assigned as an f 6-to-f 5d1 promotion. When
the compound is dissolved in pyridine, no absorption maxima
are noted from 350 to 750 nm.
Sm + PhSeSePh THF, RT8 (THF)8Sm8Se6(SePh)12 (1)
Zn + 2PhSeSePh THF8 Zn(SePh)2 + PhSeSePh Sm8
(48) Dos Santos, P. V.; Gouveia, E. A.; Neto, J. A. M. Appl. Phys. Lett.
1999, 74, 3607.
[(THF)3SmZn(SePh)4]n (2)
(49) Dos Santos, P. V.; de Araujo, M. T.; Sombra, A. S. B. IEEE J.
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Lett. 1998, 73, 578.
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1998, 72, 753.
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3549.
The tellurolate compound (py)5Sm(TePh)2 (2) can be prepared
in high yield from the reduction of PhTeTePh with elemental
Sm in pyridine (reaction 3). The product crystallizes in a unit
Sm + PhTeTePh PY8 (py)5Sm(TePh)2 RhTeTePh8 no reaction
(3)
(53) Davey, A. P.; Bourdin, E.; Blau, W. Appl. Phys. Lett. 1995, 67, 884.
(54) Brodin, A.; Mattsson, B.; Torell, L. M. Electrochim. Acta 1995, 40,
2393.
cell that is isostructural with the pyridine complexes of

Samarium Compounds
Inorganic Chemistry, Vol. 39, No. 10, 2000 2171
Sm(Ph2P(Se)NP(Se)Ph2)2; Sm-Se ) 3.14 Å], but the different
ligand resonance description and inequivalent Sm coordination
numbers preclude a significant comparison of Sm-Se bond
lengths.
charge-transfer excitation by establishing the influence of both
pyridine and Ln on Ln(EPh)2 electronic spectra. To date,
addition of pyridine to THF solutions of Ln(EPh)2 compounds
leads to the formation of significantly more intensely colored
products. This effect is most notable in Eu(II) chemistry,1,2
where Eu(EPh)2 compounds are essentially colorless because
the fn-to fn-1d1 promotion is not found in the visible spectrum.
As for the dependence of color on the identity of Ln, differences
in the aqueous Ln(II)/Ln(III) redox potentials (Sm, -1.40 V;
Yb, -1.04 V; Eu, -0.34 V)62 are nearly identical to the
differences in energy of the lowest electronic transition found
for (py)5Sm(TePh)2 (614 nm, 2.03 eV), (py)5Yb(TePh)2 (525
nm, 2.37 eV),3 and (py)5Eu(TePh)2 (398 nm, 3.12 eV).2 If this
transition is assigned correctly, then the higher energy absorption
is presumably an f6-to-f5d1 promotion. The significant shift in
this absorption position relative to the analogous transition in 1
would certainly be consistent with the extreme sensitivity of
these electronic transitions to the composition of the primary
coordination sphere.
This compound is not stable indefinitely in solution, and at
elevated temperatures it decomposes to give a variety of
products, including heterometallic selenido clusters.56 The
compound also appears to be unstable in stronger Lewis base
solvents such as pyridine. When pyridine is added to crystalline
1 to give a red-brown solution, there is no absorption maximum
in the visible spectrum. The absence of both the f-to-d promotion
and an anticipated Sm(II)-to-pyridine charge transfer implies
that Sm(II) ions are no longer present in solution. Decomposition
in the presence of a stronger base can be rationalized easily
because the stronger pyridine donor both increases electron
density on the Sm(II) ion and is more likely to fragment the
heterometallic product. Both effects will increase the tendency
of the Sm(II) ion to oxidize7 and reduce the C-Se bond.26,27
Molecule 2 is isostructural with (py)5Yb(TePh)2,3 with a larger
unit cell [7289 Å3 (Sm) vs 7178 Å3 (Yb)] that is a reflection of
larger Sm(II) ionic radius. The stability of the tellurolate
compound can be attributed to both the instability of the tellurido
ligands that would result from any reductive Te-C(Ph) bond
cleavage and the inability of Te ligands to stabilize redox active
Ln(III) ions. While tellurido clusters of the Ln elements are
viable synthetic targets,18,19,56 they are considerably more
difficult to isolate than are their S2- or Se2- analogues,26,27,37-41
and presumably the relative instability of Te2- coordinated only
to Ln(III) ions is a major factor contributing to the isolability
of 2. The instability of 2 in THF can be rationalized either by
noting that reductive cleavage of a C-Te bond would presum-
ably be facilitated by the tendency of Sm(TePh)2 to form
oligomeric structures in THF or by considering that Sm(II) can
react with THF to form an alkoxide compound.57
Conclusion
Under the appropriate conditions, Sm(EPh)2 compounds of
the heavier chalcogens (E ) Se, Te) can be isolated in high
yield. The choice of solvent is crucial; the selenolate compound
Sm(SePh)2 can be stabilized by coordination to Zn(SePh)2 in
THF. The covalent Zn ion effectively reduces the tendency of
the Sm(II) ion to oxidize further, but this heterometallic product
is unstable in pyridine. In contrast, the tellurolate compound
Sm(TePh)2 is stable in pyridine but not in THF. As such, they
are potentially useful as one-electron reducing agents for organic
synthesis or Ln cluster chemistry.
Acknowledgment. This work was supported by the National
Science Foundation under Grant CHE 9982625.
Along these lines, the thermal instability of Sm(SePh)2 and
the thermal stability of both Sm(Si(SiMe3)3 and Sm(Se-2,4,6,
tri-iprC6H2)2 coordination complexes are readily rationalized;
reductive Se-C(Ph) bond cleavage would give Sm-Se2- and
Sm-Ph bonds, whereas cleavage reactions with other ligand
systems are less favorable either because they would form rare
examples of compounds with Ln-Si bonds58-61 or sterically
congested Ln-C bonds with diorthosubstituted aryl ligands.
In the UV-visible spectrum, the lowest energy electronic
absorption for 2 is tentatively assigned as a Sm(II)-to-pyridine
Supporting Information Available: An X-ray crystallographic file
in CIF format for the crystal structure of 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC9913278
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