Stabilization of unsolvated europium and ytterbium pentafluorophenyls by π-bonding encapsulation through a sterically crowded triazenido ligand

Article abstract of DOI:10.1021/ic0514602

The one-pot transmetalation/deprotonation reaction of the bulky triazene Dmp(Tph)N₃H with bis(pentafluorophenyl)mercury and europium or ytterbium affords the structurally characterized unsolvated metal(II) pentafluorophenyl triazenides [Dmp(Tph)N₃-MC₆F ₅] (M = Eu, Yb; Dmp = 2,6-Mes₂C₆H₃ with Mes = 2,4,6-Me₃C₆H₂; Tph = 2-TripC ₆H₄ with Trip = 2,4,6-ⁱPr₃C ₆H₂) or, depending on the molar ratio, the solvated complex [Dmp(Tph)-N₃YbC₆F₅(THF)].

Full text of DOI:10.1021/ic0514602

Inorg. Chem. 2005, 44, 8644−8646
Stabilization of Unsolvated Europium and Ytterbium Pentafluorophenyls
by -Bonding Encapsulation through a Sterically Crowded Triazenido
π
Ligand
Sven-Oliver Hauber and Mark Niemeyer*
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55,
70569 Stuttgart, Germany
Received August 26, 2005
The one-pot transmetalation/deprotonation reaction of the bulky
triazene Dmp(Tph)N3H with bis(pentafluorophenyl)mercury and
europium or ytterbium affords the structurally characterized
unsolvated metal(II) pentafluorophenyl triazenides [Dmp(Tph)N3-
MC6F5] (M
Me3C6H2; Tph
)
Eu, Yb; Dmp
)
2,6-Mes2C6H3 with Mes
) 2,4,6-
)
2-TripC6H4 with Trip
)
2,4,6-iPr3C6H2) or,
depending on the molar ratio, the solvated complex [Dmp(Tph)-
N3YbC6F5(THF)].
of heteroleptic pentafluorophenylmetal(II) triazenides of the
rare-earth metals Eu and Yb. These compounds are accessible
via a convenient one-pot transmetalation/deprotonation reac-
tion in tetrahydrofuran (THF) as the solvent from the triazene
Dmp(Tph)N3H (1) (Dmp ) 2,6-Mes2C6H3 with Mes ) 2,4,6-
Me3C6H2; Tph ) 2-TripC6H4 with Trip ) 2,4,6-iPr3C6H2),
bis(pentafluorophenyl)mercury,7 and the corresponding metal
(Scheme 1).
After crystallization from n-heptane/toluene mixtures,
either the deep orange, THF-free [Eu(C6F5)(N3Dmp(Tph))]
(2) or the deep red solvate [Yb(C6F5)(N3Dmp(Tph))(THF)]
(4) is isolated in good yield. Remarkably, solvate-free purple
[Yb(C6F5)(N3Dmp(Tph)] (3)8 is accessible only in the
presence of a second equivalent of the triazene 1. A possible
explanation involves the formation of a loose adduct between
4 and 1 in solution, which enables displacement of coordi-
nated THF under reduced pressure. Upon crystallization, the
adduct may dissociate in its components, and the less soluble
compound 3 is isolated. It is notable that attempts to replace
the pentafluorophenyl substituents by a second triazenide
ligand have not been successful so far. Apparently, the steric
bulk of the latter prevents further substitution or ligand
redistribution and therefore the formation of the homoleptic
complexes.9
The design and development of alternative ligand systems
capable of stabilizing monomeric metal complexes while pro-
voking novel reactivity remains one of the most intensely
studied areas of organometallic chemistry. Exploration of
this field is driven by the potential use of these compounds
in catalysis and organic synthesis.1 Examples of monoanionic
chelating N-donor ligands that have received much recent
attention include the â-diketiminate (I)2 and the amidinate
(II)3 ligand systems. Much less attention has been given to
the closely related triazenides (III).4 This may be attributed
to the lack of suitable ligands that are sterically crowded
enough to prevent undesirable ligand redistribution reactions
and allow better control of the electronic and steric properties
at the metal. In comparison to the isoelectronic amidinates
and the related â-diketiminates, triazenides are weaker donors
and should induce greater electrophilicity at a bonded metal
atom.5
We recently succeeded in the preparation of aryl-substi-
tuted sterically crowded triazenes, which were used to sta-
bilize the first examples of structurally characterized aryl
compounds of the heavier alkaline-earth metals Ca, Sr, and
Ba.6 As a continuation of this work, we report the synthesis
Solutions of 2-4 in aromatic or aliphatic solvents show
considerable thermal stability and can be stored at ambient
temperature for months with only minor decomposition. This
behavior may be contrasted with that of other europium or
ytterbium pentafluorophenyls that are much more thermally
labile.10 The IR spectra show three (4) or four (2 and 3)
strong νas absorptions in the range 1231-1295 cm-1, which
* To whom correspondence should be addressed. E-mail: mn@
lanth.de.
(1) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(2) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031.
(3) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219.
(4) (a) Moore, D. D.; Robinson, S. D. AdV. Inorg. Chem. Radiochem.
1986, 30, 1. (b) Vrieze, K.; van Koten, G. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford,
U.K., 1987; Vol. 2, p 189.
(6) Hauber, S.-O.; Lissner, F.; Deacon, G. B.; Niemeyer, M. Angew.
Chem., Int. Ed. 2005, 44, 5871.
(7) Deacon, G. B.; Forsyth, C. M.; Nickel, S. J. Organomet. Chem. 2002,
647, 50.
(5) Gantzel, P.; Walsh, P. J. Inorg. Chem. 1998, 37, 3450.
8644 Inorganic Chemistry, Vol. 44, No. 24, 2005
10.1021/ic0514602 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/22/2005

COMMUNICATION
Scheme 1. Synthesis of 2-4
Figure 1. Molecular structure of 2 with thermal ellipsoids set to 30%
probability. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg) for 2 and 3 (in brackets): M-N1, 2.5510(17)
{2.445(4)}; M-N3, 2.5448(17) {2.444(4)}; M-C51, 2.643(2) {2.527(6)};
M‚‚‚C7, 3.127(2) {3.047(5)}; M‚‚‚C8, 3.088(2) {3.003(5)}; M‚‚‚C9, 3.134-
(2) {3.062(5)}; M‚‚‚C10, 3.233(2) {3.160(6)}; M‚‚‚C11, 3.246(2) {3.189-
(6)}; M‚‚‚C12, 3.232(2) {3.160(6)}; M‚‚‚C31, 3.255(2) {3.237(5)}; M‚‚‚
C32, 3.153(2) {3.070(5)}; M‚‚‚C33, 3.179(2) {3.136(5)}; M‚‚‚C34, 3.344(2)
{3.421(6)}; M‚‚‚C35, 3.415(2) {3.535(6)}; M‚‚‚C36, 3.395(2) {3.476(5)};
M‚‚‚F56, 3.426(2) {3.489(4)}; N1-N2, 1.312(2) {1.327(6)}; N2-N3,
1.313(2) {1.299(6)}; M-C51-C52, 128.64(18) {125.4(4)}; M-C51-C56,
117.12(18) {121.5(5)}.
are indicative of the triazenides acting as chelating ligands.4a
Additional bands at 686 cm-1 (2), 687 cm-1 (3), or 683 cm-1
(4) may be assigned to δas N3 vibrations. Characteristic
absorptions of the C6F5 groups are observed at 1072/923
cm-1 (2), 1071/929 cm-1 (3), and 1072/923 cm-1 (4),
respectively. For 4, the presence of coordinated THF is
indicated by prominent bands at 1020 and 874 cm-1.
Compounds 2 and 3 were examined by X-ray crystal-
lography.11 The molecular structure of the Eu derivative and
important structural parameters for both complexes are shown
in Figure 1. Despite the relatively large ionic radii of the
M2+ cations (Yb, 1.02 Å; Eu, 1.17 Å; for coordination
number 6),12 the size of the η2-bonded triazenide ligands
enforces the formation of strictly monomeric compounds in
which the metal atoms possess apparent low coordination
numbers of 3. With a range of 1.299-1.327 Å, the N-N
distances within the triazenido ligand cores are consistent
with delocalized bonding. The coordination of the triazenide
ligand is rather symmetrical with very little variation of the
Eu-N [average 2.5510(17) Å] and Yb-N [average 2.445-
(4) Å] bond lengths. The latter distances are slightly shorter
than those of the amidinate [{PhC(NSiMe3)2}2Yb(THF)2]
(2.473 Å),13 which contains a 6-coordinate metal atom.
(8) All manipulations were carried out under strictly anaerobic and
anhydrous conditions using argon as an inert atmosphere. Synthetic,
spectroscopic, and analytical data for 2 and 4 are given in the
Supporting Information. [Yb(C6F5)(N3DmpTph)] (3): Bis(pentafluo-
rophenyl)mercury (0.535 g, 1.00 mmol) was added at ambient
temperature to a stirred mixture of Yb chips (0.34 g, 1.96 mmol) and
1 (1.27 g, 2.00 mmol) in THF (40 mL). Stirring was continued for 12
h, whereupon the solvent was removed under reduced pressure. The
foamy residue was treated with a mixture of 30 mL of n-heptane and
10 mL of toluene, and solid materials were separated by centrifugation.
The volume of the resulting maroon solution was reduced to incipient
crystallization under reduced pressure. Storage at ambient temperature
overnight afforded 3 as a deep red crystalline material. Yield: 0.60 g
(0.62 mmol, 62%). Mp: crystals change to blue-green between 80
and 85 °C and decompose to a black foam at 165 °C. 1H NMR (250.1
MHz, benzene-d6): δ 0.58, 0.77, 1.22 (ddd, 3JHH ) 6.7 Hz, 3 × 6H,
o- + p-CH(CH3)2), 2.06 (s, 12H, o-CH3), 2.21 (s, 6H, p-CH3), 2.52
(sep, 3JHH ) 6.7 Hz, 2H, o-CH(CH3)2), 3.04 (sep, 3JHH ) 6.7 Hz, 1H,
p-CH(CH3)2), 6.70-7.27 (m, 7H, various aryl-H), 6.79 (s, 4H, m-Mes),
7.15 (s, 2H, m-Trip). 13C NMR (62.9 MHz, toluene-d8): δ 20.9 (o-
CH3), 21.1 (p-CH3), 22.7, 23.4, 25.2 (o- + p-CH(CH3)2), 30.7 (o-
CH(CH3)2), 33.0 (p-CH(CH3)2), 116.5, 121.2, 122.2, 123.3, 128.9,
129.1, 129.2, 129.3 (aromatic CH), 129.3, 131.4, 136.0, 136.8, 139.3,
141.5, 145.6, 148.8, 149.0, 149.7 (aromatic C); signals for the C6F5
group could not be detected or assigned because of signal overlap
and their weak appearance. 19F NMR (235.4 MHz, benzene-d6): δ
(11) Shock-frozen crystals in Paratone N, diffractometer Siemens P3, T )
173 K, SHELXL-97 refinement with all data on F 2. Crystal data for
2: deep orange block, 0.70 × 0.65 × 0.55 mm, C51H52EuF5N3, M )
953.92, triclinic, space group P1h, a ) 9.043(2) Å, b ) 11.166(3) Å,
c ) 23.368(4) Å, R ) 96.206(16)°, â ) 93.326(17)°, γ ) 106.806-
(19)°, V ) 2235.6(9) Å3, Z ) 2, Fcalc ) 1.417 g cm-1, µ(Mo KR) )
1.461 mm-1, 2Θmax ) 55°, 10 905 (Rint ) 0.020) collected and 10 256
unique reflections, 594 parameters, absorption correction by Ψ scans,
R1 ) 0.025 for 9558 reflections with I > 2σ(I), wR2 ) 0.068 (all
data), GOF ) 1.376. Crystal data for 3: purple plate, 0.30 × 0.25 ×
0.08 mm, C51H52F5N3Yb, M ) 975.00, triclinic, space group P1h, a )
9.0770(18) Å, b ) 11.114(2) Å, c ) 23.416(5) Å, R ) 96.91(3)°, â
) 93.71(3)°, γ ) 106.93(3)°, V ) 2231.0(8) Å3, Z ) 2, Fcalc ) 1.451
g cm-1, µ(Mo KR) ) 2.155 mm-1, 2Θmax ) 54°, 10167 (Rint ) 0.042)
collected and 9552 unique reflections, 599 parameters, absorption
correction by Ψ scans, R1 ) 0.051 for 7365 reflections with I > 2σ-
(I), wR2 ) 0.103 (all data), GOF ) 1.136. Crystallographic data
(excluding structure factors) for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication nos. CCDC-268133 (2) and -268132 (3).
Copies of the data can be obtained free of charge upon application to
CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax +(44)-1223-
336-033; e-mail deposit@ccdc.cam.ac.uk).
3
-160.5 (m, 2F, m-C6F5), -158.5 (t, JFF ) 18.3 Hz, 1F, p-C6F5),
-110.7 (m, 2F, o-C6F5). 171Yb NMR (70.1 MHz, benzene-d6): no
signal was detected. IR (Nujol): ν˜ 1624 w, 1593 m, 1533 w, 1485
sh, 1420 s, 1397 m, 1366 s, 1294 ms, 1280 ms, 1262 s, 1244 s, 1230
sh, 1208 ms, 1180 m, 1101 w, 1071 ms, 1027 vs, 1003 sh, 929 vs,
880 m, 861 m, 849 ms, 801 w, 792 w, 767 sh, 760 sh, 754 vs, 738
ms, 687 m, 658 m, 587 w, 575 w, 510 w, 471 w, 427 w, 410 w, 394
m. Anal. Calcd for C51H52YbF5N3: C, 62.82; H, 5.38; N, 4.31.
Found: C, 62.21; H, 5.43; N 4.39.
(9) A solvated phenyl-substituted tris(triazenide) complex is known for
Er: Pfeiffer, D.; Guzei, I. A.; Liable-Sands, L. M.; Heeg, M. J.;
Rheingold, A. L.; Winter, C. H. J. Organomet. Chem. 1999, 588, 167.
(10) (a) Deacon, G. B.; Forsyth, C. M. Organometallics 2003, 22, 1349.
(b) Forsyth, C. M.; Deacon, G. B. Organometallics 2000, 19, 1205.
(c) Deacon, G. B.; Forsyth, C. M. Chem.sEur. J. 2004, 10, 1798.
(12) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
Inorganic Chemistry, Vol. 44, No. 24, 2005 8645

COMMUNICATION
The pentafluorophenyls 2 and 3 appear to be the first
examples of well-characterized unsolvated aryl compounds
of the lanthanides in the oxidation state 2+.14 The Yb-C
and Eu-C distances of 2.527(6) and 2.643(2) Å are
comparable or slightly longer to those in the metal(II) aryls
[M(Dpp)2(THF)2] {Dpp ) 2,6-Ph2C6H3; M ) Yb [2.520(3)
Å], Eu [2.615(4) Å]} and [Yb(Dpp)I(THF)3] (2.520 Å).15
Considerably longer bonds have been observed in the metal-
(II) pentafluorophenyls [M(C6F5)2(THF)n] {M ) Yb, n ) 4
[2.649(3) Å], Eu, n ) 5 [2.822(2) Å]}, [Yb(C5Me5)(C6F5)-
(THF)3] (2.597(5) Å), and [M(C6F5)(THF)n][BPh4] {M )
Yb, n ) 5 [2.59(1) Å], Eu, n ) 6 [2.735(2) Å]}.10 The
deviations of the metal atoms from least-squares planes
defined by the carbon atoms of the bonded C6F5 ligands (Yb,
0.262 Å; Eu, 0.268 Å) or the nitrogen atoms of the triazenide
group (Yb, 0.374 Å; Eu, 0.533 Å) also merit comment.
Perhaps the most striking features in the solid-state
structures of 2 and 3 are the additional metal-π-arene
interactions16 with the pending arms of the biphenyl and
terphenyl groups in the triazenide ligands. Apparently, these
arene rings compete with THF to bind the MII cations and
allow the isolation of solvate-free derivatives.17,18 In both
compounds, the metal ion interacts with one mesityl (Mes)
ring of the terphenyl group in a η5 fashion,19 with M‚‚‚C
distances in the ranges 3.088(2)-3.233(2) Å (2) and 3.003-
(5)-3.160(6) Å (3). In addition, there are metal‚‚‚π-arene
interactions to the triisopropylphenyl (Trip) ring of the
biphenyl substituent, which are best described as being η4
for the smaller metal Yb and η5 in the case of the Eu
derivative. The M‚‚‚C distances considered to be bonding
are in the ranges 3.153(2)-3.395(2) Å (2) and 3.070(5)-
3.421(6) Å (3). The resulting rather distorted pseudotetra-
hedral coordination of the metal cations is reflected by the
angles N2‚‚‚M-C51 (Yb, 104.5°; Eu, 102.5°) and X1-M-
X2 (Yb, 131.2°; Eu, 139.5°), where X1 and X2 define the
centroids of the coordinated arene rings and the bidentate
triazenido ligand occupies only one coordination site.
Although we could not obtain X-ray-quality crystals of
the THF solvate 4, it is probable that this compound is
isostructural to its calcium analogue [Ca(C6F5)(N3Dmp-
(Tph))(THF)]6 because the ionic radii of Yb2+ (1.02 Å) and
Ca2+ (1.00 Å) are very similar. Here the Ca atom is
coordinated to a η2-bonded triazenide ligand, the ipso C atom
of the C6F5 group, and the O atom of a THF molecule. There
is only one additonal η5-π contact to the pending arm of a
Mes group.
To clarify if the metal‚‚‚arene interactions in 2 and 3 also
persist in solution, variable-temperature NMR experiments
were performed for the diamagnetic compound 3. At 298
1
K, the H NMR spectra in toluene-d8 show only one set of
signals for the Mes substituents. This is consistent with a
fast exchange of the C7 f C12 and C16 f C21 arene rings
on the NMR time scale. Upon cooling, decoalescence of the
o-CH3 and m-CH groups is detected at 226 K with a
corresponding energy barrier of 47.1 kJ mol-1. This observa-
tion can be interpreted in terms of a hindered rotation around
the N1-C1 bond and therefore the persistence of the metal-
π-arene interactions in solution. A similar behavior with a
rotational barrier of 54.3 kJ mol-1 was recently observed
and confirmed by quantum-chemical calculations for the
unsolvated ytterbium thiophenolate [Yb(S-2,6-Trip2C6H3)2].17
Additional low-temperature 19F NMR experiments for 3 show
decoalescence of the o- and m-C6F5 fluorine atoms to two
sets of signals, respectively, below 222 K with a correspond-
ing ∆GT value of 42.9 kJ mol-1. It should be noted that
c
there are no exceptional short M‚‚‚F contacts in the solid-
state structures of 2 and 3 [M‚‚‚F56 ) 3.426(2)/3.489(4) Å],
which might be responsible for the restricted rotation around
the M-C51 bond in solution.20 Therefore, it is more plausible
that the inequivalence of the o- and m-C6F5 fluorine atoms
at low temperatures is due to the additional metal π contacts.
Further evidence for a frozen conformation in solution,
similar to the solid-state structure, is provided by the
decoalescence of the o-iPr groups (∆G216 K ) 44.8 kJ mol-1)
in the Tph substituent.
A comparison of compounds 2 and 3 with other lanthanide
pentafluorophenyls, in particular Eu(C6F5)2(THF)510b and the
dimer (Cp*2SmC6F5)2,21 allows one to estimate the steric
properties of the N3Dmp(Tph) ligand. Apparently, one
triazenide seems to offer more steric protection than two Cp*
groups and is able to substitute one C6F5 ligand and five
THF molecules.
In summary, we have prepared and characterized, with
the help of a very bulky aryl-substituted triazenide ligand,
the first structurally characterized unsolvated metal(II) aryls
of the rare-earth metals Eu and Yb. In the solid-state and in
solution, kinetic stabilization through steric and electronic
saturation of the metal centers is achieved by additional
π-arene interactions to the pendent aryl substituents. As a
result, decomposition pathways involving o-fluoride elimina-
tion are effectively blocked.
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (SPP 1166) for financial support and Prof. Glen
Deacon for fruitful discussions.
(13) Wedler, M.; Noltemeyer, M.; Pieper, U.; Schmidt, H.-G.; Stalke, D.;
Edelmann, F. T. Angew. Chem., Int. Ed. Engl. 1990, 29, 894.
(14) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. ReV.
2002, 102, 1851.
(15) (a) Heckmann, G.; Niemeyer, M. J. Am. Chem. Soc. 2000, 122, 4227.
(b) Niemeyer, M. Acta Crystallogr. 2001, E57, m578.
Supporting Information Available: Details on the preparation
and characterization of 2-4 and X-ray data (CIF) for 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(16) For a review of π-arene interactions with lanthanide ions, see:
Bochkarev, M. N. Chem. ReV. 2002, 102, 2089.
(17) Niemeyer, M. Eur. J. Inorg. Chem. 2001, 1969.
IC0514602
(18) This unusual behavior is in contrast to the preparation of other
unsolvated lanthanide(II) compounds, e.g., LnCp*2 or Ln(OAr)2, from
solvated complexes, which normally requires repeated sublimation at
elevated temperatures.
(19) The assignment of the hapticity of the metal-π-arene interactions in
compounds 2 and 3 is based on the evaluation of the M-C distances
and the angle between the M-centroid vector and the normal of the
arene plane (see Figure S3 of the Supporting Information).
(20) No decoalescence of the C6F5 signals down to 223 K was observed
for the compound Cp′2CeC6F5 (Cp′ ) 1,3,4-tBu3C5H2), which shows
a very strong Ce‚‚‚F interaction of 2.682 Å in the solid state: Maron,
L.; Werkema, E. L.; Perrin, L.; Eisenstein, O.; Andersen, R. A. J.
Am. Chem. Soc. 2005, 127, 279.
(21) Castillo, I.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10526.
8646 Inorganic Chemistry, Vol. 44, No. 24, 2005