π-bonding encapsulation in aryl-substituted lanthanide selenolates: Monomeric compounds with apparent low-coordinate metal atoms

Article abstract of DOI:10.1039/b612182a

Reaction of LnCl₃ with KSeAr* in thf afforded the unsolvated, alkane-soluble complexes LnCl(SeAr*)₂ (Ln = Nd, Pr; Ar* = 2,6-Trip₂C₆H₃; Trip = 2,4,6-iPr₃C₆H₂) in which

Full text of DOI:10.1039/b612182a

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p-Bonding encapsulation in aryl-substituted lanthanide selenolates:
monomeric compounds with apparent low-coordinate metal atoms
Sven-Oliver Hauber and Mark Niemeyer*
Received (in Cambridge, UK) 25th August 2006, Accepted 10th October 2006
First published as an Advance Article on the web 26th October 2006
DOI: 10.1039/b612182a
coordination by two selenium atoms of the terminal selenolate
ligands and the chlorine atom, respectively. Because of their very
similar features, only the solid-state structure of 1 (Fig. 1) will be
discussed in detail in the following.
Reaction of LnCl3 with KSeAr* in thf afforded the unsolvated,
alkane-soluble complexes LnCl(SeAr*)2 (Ln = Nd, Pr; Ar* =
2,6-Trip2C6H3; Trip = 2,4,6-iPr3C6H2) in which the rare-earth
metal cations show additional g6-p-coordination by two
flanking arene rings.
6
…
The most striking structural features are the additional Ln g -
p-interactions to two ortho-2,4-6-triisopropylphenyl rings of the
terphenyl groups (Fig. 2), which are unprecedented for structurally
authenticated LnIII compounds.6 Both terphenyl selenolate ligands
are orientated in such a way that the most favorable g6-p-arene
bonding is maximized. In effect, the interplanar angles between the
Ln–Se–C planes and the central 2,6-substituted C6H3 rings are
small, as indicated by the Nd–Se1–C11–C12 and Nd–Se2–C41–
C42 torsion angles of 9.1u and 10.5u, respectively. Together with
the perpendicular arrangement of the ortho-Trip groups (A/B,
A/C, D/E, D/F = 82.4–89.3u; for the definition of the least-square
planes see Fig. 1) an almost parallel orientation of the interacting
Lanthanide (Ln) based materials are increasingly important in
photonic or magnetic applications such as lasers, electrolumines-
cent devices, light emitting phosphors, optical fibers, scintillator
detectors, spintronics and high-density optical or magnetic data
storage.1 Among these materials lanthanide chalcogenides, either
in stoichiometric or doped form, have gained considerable
attraction. Molecular compounds with bonds between rare-earth
metals and the heavier group 16 elements are potential precursors
for the synthesis of bulk or nano-scale LnE and Ln2E3 (E = S, Se,
Te). Suitable candidates for this purpose are homoleptic, mono-
nuclear complexes with the composition Ln(ER)2 or Ln(ER)3 (R =
bulky organyl group), that are very rare, however. Structurally
authenticated examples are currently restricted to a couple of LnIII
and LnII thiophenolates.2,3
In order to prepare homoleptic lanthanide selenophenolates, we
have now examined reactions between rare-earth metal trichlorides
and potassium selenolates containing sterically crowded aryl
substituents.4a Thus, an excess of NdCl3 or PrCl3 was reacted
with KSeAr* (Ar* = 2,6-Trip2C6H3 with Trip = 2,4,6-iPr3C6H2) in
tetrahydrofuran as the solvent.{ Removal of the solvent followed
by extraction with n-heptane and crystallization at 220 uC gave
donor-free LnCl(SeAr*)2 (Ln = Nd {1}, Pr {2}) as air sensitive
pale green (1) or pale yellow (2) crystalline material in good yield.§
The selective formation of the bis SeAr* complexes is explained by
ligand redistribution reactions, favoured by the low solubility of
LnCl3(thf)x in alkanes and the size of the Ar* groups. Obviously,
the selenophenolate ligands are too bulky to be accommodated
more than twice around the Ln centers.
As with the related LnII compounds Ln(SAr*)2 (Ln = Eu, Sm,
Yb),3 1 and 2 also exhibit a remarkably low tendency to
coordinate s-donor solvents such as tetrahydrofuran, which was
used in their synthesis. This unusual behavior is in contrast to the
preparation of other unsolvated lanthanide compounds, e.g.
Ln(OAr)3, in the presence of coordinating solvents, that normally
requires repeated sublimations at elevated temperatures.5
Fig. 1 Molecular structure of 1 with thermal ellipsoids set to 30%
probability. Hydrogen atoms have been omitted and carbon atoms of iso-
˚
propyl groups are shown as lines for clarity. Selected bond lengths (A),
angles (u) and interplanar angles (u) for 1 and 2 (in brackets): M–Cl
2.629(2) {2.652(3)}, M–Se1 2.8805(11) {2.8963(14)}, M–Se2 2.8863(10)
…
{2.9059(13)}, M C21 3.167(6) {3.176(9)}, M C22 3.176(6) {3.199(9)},
…
… … …
C23 3.131(6) {3.156(10)}, M C24 3.133(7) {3.140(9)}, M C25
M
…
…
3.104(6) {3.109(9)},
…
M
C26 3.160(6) {3.179(9)},
…
M
C51 3.187(7)
{3.195(9)}, M C52 3.164(7) {3.177(9)}, M C53 3.108(7) {3.132(9)},
…
… …
C54 3.125(7) {3.123(10)}, M C55 3.125(7) {3.139(10)}, M C56
M
… …
3.195(7) {3.183(10)}, M X6 2.816 {2.831}, M X69 2.822 {2.822}, Cl–
M–Se1 117.55(5) {117.89(7)}, Cl–M–Se2 119.88(5) {120.07(7)}, Se1–M–
…
In the solid state," complexes 1 and 2 crystallize as monomers in
which the metal atoms show a slightly distorted trigonal planar
…
Se2 122.56(3) {122.05(4)}, X6
M
X69 174.6, {173.9}. A/B 82.9, A/C
88.2, D/E 82.4, D/F 89.3, B/G 2.2, E/G 3.2, B/E 4.6. X6 and X69 define the
centroids of the coordinated C21 A C26 and C51 A C56 arene rings,
respectively. Least square planes defined by the following atoms: A: C11
A C16; B: C21 A C26; C: C31 A C36; D: C41 A C46; E: C51 A C56; F:
C61 A C66; G: M, Se1, Se2, Cl.
Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart,
Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
E-mail: niemeyer@iac.uni-stuttgart.de; Fax: +49 711 685 64241;
Tel: +49 711 685 64217
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Chem. Commun., 2007, 275–277 | 275

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˚
2.57 to 2.61 A were observed for a number of tetrachloroaluminate
complexes of the composition [Nd(g6-arene)(AlCl4)3].10 Taking
2+
into account the different ionic radii for Eu2+ (+0.187 A) and Yb
˚
8
(+0.037 A) much stronger Ln g -arene interactions are found in
6
…
˚
…
the thiophenolates [Eu(SAr*)2] (av. Eu X6 2.728 A) and
˚
3a
…
˚
[Yb(SAr*)2] (av. Yb X6 2.637 A).
6
…
The considerably weaker Ln g -arene interactions in com-
plexes 1 and 2 compared to the [Ln(SAr*)2] compounds are
somewhat surprising. Since the bonding between a g6-coordinated
arene ring and a Ln2+ or Ln3+ cation is dominated by polar
contributions (ion–quadrupol interaction) stronger interactions
III
…
and shorter Ln C contacts would be expected for the Ln
compounds.11 The weaker Ln C interactions are most probably
…
a result of increased steric crowding due to the replacement of S by
Se and the introduction of a third substituent. However, additional
experimental data such as the preparation and characterization of
Ln(SeAr*)2 or LnCl(SAr*)2 derivatives is needed to prove this fact.
It is know from studies on lanthanide amide, phenolate and
C21 A C26 and C51 A C56 rings to the central NdClSe2 plane is
achieved (B/G = 2.2u, E/G = 3.2u).
Taking into account the additional p-contacts, a formal
thiophenolate complexes that infrared spectroscopy is a valuable
tool for the identification of intra-3 or intermolecular9a,12 Ln g -
6
coordination number of nine is calculated. However, the observed
˚
Nd–Se distances (av. 2.883 A) are surprisingly short. In fact, they
˚
are approx. 0.09–0.25 A shorter than the corresponding values
…
arene interactions. IR spectroscopy shows distinct differences in
aromatic CLC vibrational modes for p-bound and non-coordinat-
ing arene rings in these compounds.
reported for the polymer [Nd(m-SeTrip)3(thf)]‘ that contains seven-
coordinate neodymium atoms and represents the only neodymium
selenolate available for comparison.7a Moreover, significantly
Table 1 shows the IR vibration frequencies for the aromatic
stretching modes in the LnIII compounds 1, 2, in the LnII
complexes Eu(SAr*)2, Yb(SAr*)2 and in the thiol Ar*SH or the
selenol Ar*SeH, recorded as Nujol mulls. There are two bands in
the regions 1602–1606 cm21 and 1563–1569 cm21 which are visible
in all spectra. Additional stretching modes in the range 1591–
1592 cm21 and 1541–1545 cm21 are observed only in the IR
spectra of the metal complexes. They clearly indicate a weakening
of the aromatic CLC bonds, consistent with the presence of
longer or comparable mean distances are found in the compounds
7b
˚
7, Sm–Se 2.948 A)
[Sm(Me2pzBH)2(SePh)] (cn
=
and
7a
[Er(SePh)3(thf)3] (cn = 6, Er–Se 2.776 A). Allowing for the
˚
larger size of Nd3+ than Sm3+ or Er3+ (difference +0.025 A and
8
+0.093 A for cn = 6, respectively) Nd–Se distances of 2.973 A and
˚
˚
˚
˚
2.869 A are extrapolated for seven- and six-coordinate metal
atoms, respectively.
From these data a lower effective coordination number of
approximately 5 may be estimated for complex 1 (and 2), assuming
that every g6-bonded arene ring occupies only one coordination
site. In 1, the neodymium cation interacts essentially equally with
both Trip rings with Nd–C distances in the relatively narrow range
…
metal p-arene interactions as observed by X-ray crystallography.
In conclusion, the following features of 1 and 2 are remarkable:
(i) The compounds contain apparent low coordinated metal
6
…
atoms. (ii) The observation of two additional metal g -p-arene
interactions is unprecedented for structurally authenticated LnIII
compounds. These interactions are responsible for the low
tendency of 1 and 2 to coordinate s-donor solvents such as
tetrahydrofuran. (iii) 1 and 2 are the first monomeric unsolvated
bis(organylselenato) lanthanide compounds. (iv) Both compounds
are also the first examples of monomeric halogeno-functionalized
lanthanide chalcogenolates of the heavier group 16 elements that
are stable with respect to ligand redistribution. (v) Finally, it is
notable that both complexes crystallize in isomorphous cells to the
previously reported Eu(SAr*)2,13 thus allowing in principle doping
experiments with lanthanide ions in different oxidation states.
We thank the Deutsche Forschungsgemeinschaft (SPP 1166) for
financial support.
˚
˚
3.104(6)–3.176(6) A for C21 A C26 and 3.108(7)–3.195(7) A for
˚
˚
C51 A C56 with average values of 3.145 A and 3.151 A,
˚
respectively. The metal–centroid distances average to 2.819 A,
whereas the X6–Sm–X69 angle, where X6 and X69 define the
centroids of the coordinated arene rings, is 174.6u. Significantly
…
˚
˚
shorter metal centroid distances of 2.696 A and 2.714 A, have
been reported for the phenolates [{Nd(ODip)3}2]9a (Dip = 2,6-
iPr2C6H3) and [Nd(ODpp)3]9b (Dpp = 2,6-Ph2C6H3), respectively.
…
Even shorter metal centroid distances within the narrow range
Table 1 Infrared vibrational wavenumbers [cm21] for the n(CLC)
stretching modes in Ln complexes with Ar*-substituted thiolato and
selenolato ligands
1
2
1606
1606
1602
1606
1606
1605
1592
1592
1568
1568
1563
1569
1568
1567
1541
1541
this work
this work
4a
3a
3a
4b
Ar*SeH
Eu(SAr*)2
Yb(SAr*)2
Ar*SH
1591
1592
1545
1544
Fig. 2 Molecular structure of 2, showing a view perpendicular to the
PrClSe2 plane. Hydrogen atoms and carbon atoms of iso-propyl groups
have been omitted for clarity.
276 | Chem. Commun., 2007, 275–277
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2 (a) B. C¸ etinkaya, P. B. Hitchcock, M. F. Lappert and R. G. Smith,
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Notes and references
{ The use of an excess of LnCl3 is based on the earlier observation (see ref.
3) that heteroleptic complexes of the composition Ln(SAr*)I(thf)x cleanly
rearrange in alkanes as solvent to homoleptic Ln(SAr*)2 with elimination
of LnI2(thf)x.
4 (a) M. Niemeyer and P. P. Power, Inorg. Chim. Acta, 1997, 263,
201; (b) M. Niemeyer and P. P. Power, Inorg. Chem., 1996, 35,
7264.
5 F. T. Edelmann, Lanthanides and Actinides, in Synthetic Methods of
Organometallic and Inorganic Chemistry (Herrmann/Brauer), ed. W. A.
Herrmann, Georg Thieme Verlag, Stuttgart, 1997.
§ 1: A mixture of NdCl3 (1.81 g, 7.22 mmol) and KSeAr*?(C7H8) (1.66 g,
2.77 mmol) in thf (40 ml) was stirred for 64 h, after which the solvent was
removed under reduced pressure. The residue solid was extracted with 40 ml
of n-heptane and solid by-products were separated by centrifugation. The
solution was concentrated and stored overnight at 216 uC to give pale
green crystals (1.48 g, 1.14 mmol, 82%). Mp: 208 uC. Anal.: Calc. for
C72H98ClNdSe2: C, 66.6; H, 7.6%. Found: C, 66.4; H, 7.7%. EI-MS: m/z
(%) 1300 (M+, 20), 1265 [(M 2 Cl)+, 2], 740 [(M 2 Trip2C6H3Se)+, 57], 562
[(Trip2C6H3Se)+, 100]. IR (CsBr, Nujol, cm21): 1606s, 1592m, 1568s,
1541w, 1336m, 1316s, 1252m, 1169m, 1104s, 1082m, 1070m, 1052m, 1028s,
939s, 927m, 898s, 875s, 795s, 739s, 713m, 650m. 2: The synthesis was
accomplished in a similar manner to that of 1. Pale yellow crystals. Yield:
67%. Mp: 195 uC. Anal.: Calc. for C72H98ClPrSe2: C, 66.6; H, 7.6%.
Found: C, 66.7; H, 7.6%. EI-MS: m/z (%) 1297 (M+, 8), 1262 [(M 2 Cl)+,
1], 737 [(M 2 Trip2C6H3Se)+, 57], 562 [(Trip2C6H3Se)+, 84]. IR (CsBr,
Nujol, cm21): 1607s, 1591s, 1568s, 1541m, 1335m, 1317s, 1252m, 1189m,
1169s, 1151m, 1104s, 1082m, 1070m, 1053m, 1028s, 939s, 927m, 897s, 875s,
793s, 773m, 755m, 739s, 713m, 658m, 650m, 507m.
6 M. N. Bochkarev, Chem. Rev., 2002, 102, 2089.
7 (a) A. C. Hillier, S.-Y. Liu, A. Sella and M. R. J. Elsegood, Inorg.
Chem., 2000, 39, 2635; (b) J. Lee, D. Freedman, J. H. Melman,
M. Brewer, L. Sun, T. J. Emge, F. H. Long and J. G. Brennan, Inorg.
Chem., 1998, 37, 2512.
8 R. D. Shannon, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 1976,
A32, 751.
9 (a) D. M. Barnhart, D. L. Clark, J. C. Gordon, J. C. Huffman,
R. L. Vincent, J. G. Watkin and B. D. Zwick, Inorg. Chem., 1994, 33,
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J. Chem., 1995, 48, 741.
10 (a) B. Fan, Q. Shen and Y. Lin, J. Organomet. Chem., 1989, 377, 51; (b)
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Q.-C. Liu, Q.-J. Meng and Y.-H. Lin, Chin. J. Chem., 2001,
19, 588.
" Crystallographic data. 1: C72H98ClNdSe2, M = 1301.1, crystal size 0.40 6
3
¯
0.20 6 0.06 mm , triclinic, space group P1, a = 13.423(3), b = 14.363(3),
˚
c = 18.700(4) A, a = 103.421(17), b = 93.755(18), c = 99.332(17)u, V =
3440.4(13) A , Z = 2, Dcalc = 1.256 g cm23, m(Mo Ka) = 1.89 mm21, T =
3
˚
173(2) K, 14143 collected (3 ¡ 2H ¡ 50u) and 12097 unique reflections
(Rint = 0.061), 712 parameters, 8 restraints, R1 = 0.065 for 7925 reflections
with I . 2s(I), wR2 = 0.117 (all data), GOF = 1.20. 2: C72H98ClPrSe2, M =
11 According to quantum-chemical calculations and low-temperature
NMR experiments, metal–p-arene interactions to flanking arene rings
in some LnII compounds or related complexes of the heavier alkaline-
earth metals Ca and Sr are in the range 40–55 kJ mol21. See ref. 3a and:
(a) S.-O. Hauber and M. Niemeyer, Inorg. Chem., 2005, 44, 8644; (b)
S.-O. Hauber, F. Lissner, G. B. Deacon and M. Niemeyer, Angew.
Chem., Int. Ed., 2005, 44, 5871.
12 (a) R. J. Butcher, D. L. Clark, S. K. Grumbine, R. L. Vincent-Hollis,
B. L. Scott and J. G. Watkin, Inorg. Chem., 1995, 34, 5468; (b)
G. R. Giesbrecht, J. C. Gordon, D. L. Clark, P. J. Hay, B. L. Scott and
C. D. Tait, J. Am. Chem. Soc., 2004, 126, 6387.
3
¯
1297.8, crystal size 0.20 6 0.20 6 0.05 mm , triclinic, space group P1, a =
˚
13.439(3), b = 14.392(3), c = 18.751(4) A, a = 103.497(17), b = 93.751(19),
c = 99.288(17)u, V = 3460.2(13) A , Z = 2, Dcalc = 1.246 g cm23
,
3
˚
m(Mo Ka) = 1.83 mm21, T = 173(2) K, 11404 collected (3 ¡ 2H ¡ 48u)
and 10864 unique reflections (Rint = 0.075), 712 parameters, 8 restraints,
R1 = 0.079 for 6132 reflections with I . 2s(I), wR2 = 0.137 (all data),
GOF = 1.10. CCDC 619008 and 619009. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b612182a.
13 Cell parameters for Eu(SAr*)2: a = 13.371(3), b = 14.372(4), c =
˚
3
˚
1 (a) S. Cotton, Lanthanide and Actinide Chemistry; Wiley & Sons:
Chichester, 2006; (b) Pacifichem 2005, Honolulu, Hawaii, USA,
Symposium 176: New Photonic Lanthanide-based Materials.
18.535(5) A, a = 104.17(2), b = 94.08(2), c = 98.99(2)u, V = 3388.7(15) A .
These parameters are close to those reported earlier3a for the packing
complex Eu(SAr*)2?(thf)0.5
.
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Chem. Commun., 2007, 275–277 | 277