σ-donor versus η6-π-arene interactions in monomeric europium(II) and ytterbium(II) thiolates - An experimental and computational study

Article abstract of DOI:10.1002/1099-0682(200108)2001:8<1969::AID-EJIC1969>3.0.CO;2-0

The base-free, hydrocarbon-soluble complexes M(SAr*)₂ (4a: M = Eu, orange; 4b: M = Yb, purple; Ar* = 2,6-Trip₂C₆H₃; Trip = 2,4,6-iPr₃C₆H₂) have been prepared by a protolysis reac

Full text of DOI:10.1002/1099-0682(200108)2001:8<1969::AID-EJIC1969>3.0.CO;2-0

FULL PAPER
σ-Donor versus η6-π-Arene Interactions in Monomeric Europium(II) and
Ytterbium(II) Thiolates ؊ An Experimental and Computational Study
Mark Niemeyer[a]
Dedicated to Professor Glen B. Deacon on the occasion of his 65th birthday
Keywords: Ab initio calculations / Lanthanides / π interactions / S ligands / Ytterbium
The base-free, hydrocarbon-soluble complexes M(SAr*)2 (4a:
M = Eu, orange; 4b: M = Yb, purple; Ar* = 2,6-Trip2C6H3;
thiolate ligands [av. M−S: 281.7 pm (4a)/269.1 pm (4b)]. Addi-
tional η6-π-arene interactions are observed to two C6H2 rings
Trip = 2,4,6-iPr3C6H2) have been prepared by a protolysis re- of the Trip substituents [av. M···C: 306.5 pm (4a)/297.3 pm
action of the Grignard-analogous compound RLnI (R = 2- (4b)]. The binding energy for these interactions has been de-
F3CC6H4) with HSAr*. Compound 4b has been characterized
by melting point, elemental analysis, 1H, 13C, and 171Yb
NMR, IR spectroscopy, UV/Vis spectrophotometry, mass
spectrometry, and X-ray crystallography. Additionally, the
termined by variable-temperature 1H NMR spectroscopy to
be 54.3 kJ mol−1 (in methylcyclohexane) and 57.1 kJ mol−1
(in toluene). The nature and strength of the η6-π-arene bond-
ing has been studied by ab initio calculations on the model
purple YbIII complex YbI2(SAr*)(THF)3 (3) and the yellow systems [M(arene)]2+ (arene = C6H6, C6F6, 1,3,5-Me3C6H3,
DME adduct Yb(SAr*)2(dme)2 (5) have been synthesized and 1,3,5-iPr3C6H3) and Yb(SH)2(C6H6)n (n = 1, 2). The results are
characterized by X-ray crystallography. The solid-state struc-
compared with the binding energies in the σ-donor adducts
tures of 4a and 4b (as THF or benzene hemisolvates) show [M(THF)]2+, [M(dme)]2+, and [M(imidazol-2-ylidene)]2+
.
monomeric units with the metal atom bonded to two terminal
Introduction
Results and Discussion
Syntheses
The use of charged π-donor ligands and the principle of
steric saturation by associated neutral σ-donors or the addi-
tion of anionic ligands have dominated organolanthanoid[1]
chemistry over the past decades. More recently, however,
there has been a growing interest in low-coordinate σ-bon-
ded lanthanoid complexes with bulky substituted or-
ganyls,[2,3] Group 14, Group 15, or Group 16 heteroele-
ments.[4] Moreover, the avoidance of coordinating solvents
often leads to novel structures and unusual
lanthanoidϪligand interactions. Examples of the latter in-
clude agostic-type interactions[5] with alkyl-substituted
ligands[6Ϫ11] or metalϪπ-arene interactions[12Ϫ15] in aryl-
substituted ligand systems. In this paper, the synthesis and
characterization of the m-terphenyl-based[16] metal thiolates
YbI2(SAr*)(THF)3 (3) (Ar* ϭ 2,6-Trip2C6H3; Trip ϭ 2,4,6-
iPr3C6H2), M(SAr*)2 [M ϭ Eu (4a), Yb (4b)], and
Yb(SAr*)2 (dme)2 (5) are described. The σ-donor-free com-
pounds 4a and 4b show two metalϪη6-π-arene interactions
in the solid state. According to NMR-spectroscopic results,
these interactions persist in solution. The nature and
strength of these interactions has been studied by ab initio
calculations on simple model systems.
The syntheses of compounds 1Ϫ5 are summarized in
Scheme 1. Initially, the ytterbium(II) thiolate 4b was ob-
tained from the reaction of Yb[N(SiMe3)2]2 with HSAr*.
Mixing the two compounds in n-pentane resulted in a color
change from orange to purple. Removal of the solvent fol-
lowed by recrystallization of the residue from a mixture of
n-heptane and benzene gave 4b as an extremely air-sensitive
crystalline material. However, a much easier route to 4b,
which circumvents the somewhat cumbersome prepara-
tion[17] of donor-free Yb[N(SiMe3)2]2, is a direct synthesis
starting from ytterbium metal and 2-iodobenzotrifluoride.
The reaction of lanthanide metals with organyl iodides to
give Grignard-analogous compounds RLnI (Ln ϭ Sm, Eu,
Yb) has been known for some time.[18] The isolation and
structural characterization of such compounds has, how-
ever, only recently been achieved.[15,19Ϫ21] The initially
formed arylytterbium(II) iodide 1b is instantly protolysed
by the thiol HSAr* to give the mixed (thiolato)ytter-
bium(II) iodide 2b. Replacing THF by n-pentane as the
solvent causes the precipitation of YbI2(THF)4.[22] After re-
moval of the solvent and recrystallization of the residue
from an n-heptane/toluene mixture, purple 4b is obtained in
66% yield. The preparation of the donor-free europium(II)
thiolate Eu(SAr*)2 4a was accomplished in a similar man-
ner. Attempts to isolate the intermediate mixed (thiolato)-
lanthanide(II) iodides 2a and 2b did not prove to be success-
ful. Thus, in the case of the ytterbium derivative, crystalliza-
[a]
Institut für Anorganische Chemie der Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax: (internat.) ϩ 49-(0)711/685-4241
E-mail: niemeyer@iac.uni-stuttgart.de
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0808Ϫ1969 $ 17.50ϩ.50/0
1969

M. Niemeyer
FULL PAPER
tion at Ϫ15 °C only afforded YbI2(THF)4. Nevertheless, the X-ray Crystal Structures
formation of 2b was confirmed by its 171Yb NMR signal.
Compounds 3, 4a, 4b, and 5 were examined by X-ray
crystallography. Their molecular structures are shown in
Figures 1, 2, 3 and 4, while salient structural parameters are
given in Tables 1, 2 and 3. The solid-state structure of
purple YbI2(SAr*)(THF)3 (3) consists of monomeric units
(Figure 1). The central Yb atom shows a distorted octahed-
ral coordination with two iodo ligands in apical positions
[I(1)ϪYbϪI(2) ϭ 176.74(3)°]. Despite the presence of the
sterically crowded SAr* ligand, the YbϪS bond length of
258.1(2) pm seems to be the shortest ytterbiumϪsulfur dis-
tance yet reported for a molecular ytterbium compound.[4]
It may be compared with the average YbϪS bond lengths
in the six-coordinate complexes Yb(STrip)3(py)3 and mer-
Yb(SPh)3(py)3, which were determined as 264.8 pm and
264.7 pm, respectively (Table 4). The shortest YbϪS dis-
tance of 260.9(4) pm is found trans to one pyridine ligand
in the latter. In 3, the YbϪI bond lengths of 291.0(1) pm
[YbϪI(1)] and 298.4(1) pm [YbϪI(2)] are close to the
values seen in the previously reported dimeric complex [(µ-
OMe)YbI2(dme)]2 (av. 293.2 pm)[25] and the formally eight-
coordinate compound Yb(Cp)2I(THF) (293.2 pm).[26] The
different YbϪI bond lengths in 3 can be attributed to the
significant deviation of the Yb atom from the
SϪO(41)ϪO(51)ϪO(61) plane by 10.6 pm (Table 1). A not-
able feature of the structure of 3 is that the YbϪO distances
to the coordinated THF ligands are rather different. With
a value of 239.0(6) pm, the YbϪO(41) bond trans to the
SAr* group is unusually long, whereas the others (av.
YbϪO ϭ 227.3 pm) are in the normal range for YbIII com-
plexes with six-coordinate metal centers.[27] The long
YbϪO(41) bond may be compared with the average YbϪO
distance of 239.5 pm trans to the C6H5 groups in fac-
YbPh3(THF)3.[28] The trans influence of THF ligands in
lanthanoid(III) chemistry has been noted previously.[29]
Moreover, it was possible to synthesize the diiodo(thiola-
to)ytterbium(III) complex 3 in good yield by oxidation of
2b with iodine. A remarkable feature of the preparation of
4a and 4b is the ease with which the coordinated tetrahy-
drofuran can be removed. Although in the case of 4b coor-
dination of THF is observed at low temperatures (vide
infra), it can be completely removed by pumping at 10Ϫ3
mbar at ambient temperature. In the case of the europium
compound, it is even possible to crystallize donor-free 4a
from an n-heptane/THF mixture to give the packing com-
plex 4a·(THF)0.5, in which no EuϪO(THF) interactions are
observed. This unusual behavior is in contrast to the pre-
paration of other unsolvated lanthanide(II) compounds,
e.g. LnCp*2 or Ln(OAr)2, from solvated complexes, which
normally requires repeated sublimation at elevated temper-
atures.[23] Using dimethoxyethane as a more strongly chelat-
ing ligand, it is possible to synthesize the more stable yellow
complex Yb(SAr*)2(dme)2 (5). Even in this case, complete
removal of the coordinated solvent can be achieved by
pumping overnight at 20 °C/10Ϫ3 mbar.
Scheme 1. Syntheses of compounds 1Ϫ5
The protolysis of the aryllanthanide(II) iodides 1a and 1b
seems to be of general interest for the preparation of other
lanthanide(II) complexes. In contrast, the usual metathesis
reactions often lead to alkali metal halide contaminated
products or to the formation of unwanted ate complexes,
while the related one-pot transmetallation/protolysis route
of Deacon et al.[17] relies on poisonous Ph2Hg. Using our
method, we were able to synthesize the known com-
Figure 1. Molecular structure of 3, showing the atom numbering
pounds[23]
Yb[N(SiMe3)2]2(THF)2,
Yb(OArЈ)2(THF)2
scheme; hydrogen atoms have been omitted for clarity
(ArЈ ϭ 2,6-tBu2-4-MeC6H2), and YbCp*2(THF)2 in pure
form and good yields.[24] Although the 2-iodobenzotrifluor-
Because of their very similar features, the solid-state
ide used here can be replaced by other aryl iodides, we have structures of the compounds Eu(SAr*)2 [4a·(THF)0.5] and
found it to be superior, giving shorter reaction times and Yb(SAr*)2 [4b·(C6H6)0.5] will be discussed together. Both
improved purity of the products.[24]
complexes crystallize as monomers. Their molecular struc-
1970
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981

σ-Donor versus η6-π-Arene Interactions in Monomeric EuII and YbII Thiolates
FULL PAPER
spectively. The most striking structural features of 4a and
4b are the additional η6-π-interactions to two ortho-2,4,6-
triisopropylphenyl rings of the terphenyl groups (Figure 3).
Although the occurrence of two η6-π-bonded arene rings is
unprecedented for structurally authenticated di- or trivalent
lanthanide species,[12] such interactions are well-known in
some bis(arene)lanthanide(0) derivatives. Structurally char-
acterized examples include the compounds M(η6-1,3,5-
tBu3C6H3)2 (M ϭ Y, Gd, Ho).[30] It should be noted that
the cyclometallated scandium(II) complex ScH(η6-
tBu3C6H3)[η6,η1-(CH2CMe2)tBu2C6H3], which was formed
by CϪH insertion from Sc(η6-1,3,5-tBu3C6H3)2, may also
possess two metalϪπ-arene interactions, although this was
based purely on ESR evidence.[31]
Figure 2. Molecular structure of 4a, showing the atom numbering
scheme for 4a and 4b; hydrogen atoms, the minor parts of the disor-
dered isopropyl groups, and the co-crystallized THF molecule have
been omitted for clarity
The two terphenylthiolate ligands in 4a and 4b are orient-
ated in such a way that the most favorable η6-π-arene bond-
ing is maximized. In effect, the interplanar angles between
the MϪSϪC planes and the central 2,6-substituted phenyl
rings, i.e. A/D and E/H (Table 2), are in the range 0.3° to
6.7°. Together with the perpendicular arrangement of the
ortho-Trip groups (A/B, A/C, E/F, E/G ϭ 82.0Ϫ89.5°), an
almost parallel orientation of the C(7)ϪC(12) and
C(43)ϪC(48) rings to the central S(1)ϪMϪS(2) plane is
achieved (B/I, F/I ϭ 6.3Ϫ13.6°). Despite the formal coor-
dination number of eight, the observed europiumϪsulfur
and ytterbiumϪsulfur distances are surprisingly short. In
fact, they are ca. 8Ϫ13 pm (4a) and 7Ϫ19 pm (4b) pm
shorter than the metalϪsulfur distances reported for other
europium(II) and ytterbium(II) thiolates (Table 4). In 4a,
the europium ion interacts essentially equally with both
Trip rings with EuϪC distances in the relatively narrow
range of 298.8(3)Ϫ315.0(3) pm for C(7)ϪC(12) and
296.7(3)Ϫ316.1(3) pm for C(43)ϪC(48), with average values
of 307.0 pm and 305.9 pm, respectively. In 4b, the YbϪC
interactions to the C(7)ϪC(12) ring were found to have dis-
tances between 287.0(8) and 305.2(8) pm (av. 296.3 pm), with
a somewhat greater variation of 282.4(8)Ϫ313.9(8) pm (av.
298.3) for the bonding to the second C(43)ϪC(48) arene ring.
The average metalϪcentroid distances are 272.8 pm (4a) and
263.7 pm (4b), whereas the X(1)ϪMϪX(2) angles, where
X(1) and X(2) define the centroids of the coordinated arene
rings, are 166.9° and 164.5° respectively. Salient structural
parameters in known compounds with europiumϪ and
ytterbiumϪη6-π-arene coordination are summarized in
Table 5. The YbϪC interactions in 4b are comparable to the
Figure 3. Molecular structure of 4b, showing the two η6-coordin-
ated arene rings and part of the atom numbering scheme; hydrogen
atoms, the minor parts of the disordered isopropyl groups, and the
co-crystallized benzene molecule have been omitted for clarity
Figure 4. Molecular structure of 5, showing the atom numbering
scheme; hydrogen atoms and the minor parts of the disordered iso- reported average YbIIϪC distances, which fall in the range
296Ϫ304 pm. In accordance with the smaller radius of YbIII
,
propyl groups have been omitted for clarity
the complex Yb(C6Me6)(AlCl4)3 shows shorter YbϪC dis-
tures are illustrated in Figure 2 (4a) and Figure 3 (4b), while tances of 286 pm (av.). The nine-coordinate Eu center in the
salient bond parameters are listed in Table 2. Despite the tetrameric compound [Eu(C6Me6)(AlCl4)2]4 exhibits an aver-
difference in the ionic radii of europium(II) and ytter- age EuϪC interaction of 300 pm. The shortening by 6Ϫ7 pm
bium(II), both complexes crystallize in isomorphous cells. compared to 4a may be explained in terms of a stronger polar
They differ in the type of solvent that is packed in the cavit- attraction between the Eu ion and the arene ring caused by
ies of the structure. There are, however, no significant inter- the more electronegative AlCl4Ϫ ligands.
actions between the metal centers and the co-crystallized
The molecular structure of the yellow complex
tetrahydrofuran or benzene molecules. Each metal atom is Yb(SAr*)2(dme)2 (5) is shown in Figure 4. The Yb atom
coordinated to the sulfur atoms of the thiolate ligands with has a distorted octahedral coordination, in which the thiol-
SϪMϪS angles of 141.89(4)° (4a) and 142.73(8)° (4b), re- ate groups and each DME ligand occupy a cis position. The
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
1971

M. Niemeyer
FULL PAPER
Table 1. (a) Selected interatomic distances [pm], (b) angles [°], (c) distances from least-square planes [pm], and (d) interplanar angles [°]
in YbI2(SAr*)(THF)3 (3)
(a)
YbϪS
YbϪI(1)
YbϪI(2)
YbϪO(41)
258.1(2)
YbϪO(51)
YbϪO(61)
SϪC(1)
227.0(6)
227.5(6)
177.8(9)
291.00(9)
298.40(10)
239.0(6)
(b)
I(1)ϪYbϪI(2)
SϪYbϪO(41)
SϪYbϪO(51)
SϪYbϪO(61)
SϪYbϪI(1)
176.74(3)
175.9(2)
93.7(2)
100.0(2)
96.81(6)
86.45(6)
86.8(2)
93.6(2)
91.9(2)
90.0(2)
I(2)ϪYbϪO(51)
I(2)ϪYbϪO(61)
O(41)ϪYbϪO(51)
O(41)ϪYbϪO(61)
O(51)ϪYbϪO(61)
YbϪSϪC(1)
SϪC(1)ϪC(6)
SϪC(1)ϪC(2)
C(1)ϪC(2)ϪC(7)
C(1)ϪC(6)ϪC(22)
86.2(2)
87.5(2)
83.9(2)
81.9(2)
164.5(2)
125.1(3)
120.9(7)
120.2(7)
124.8(8)
124.5(8)
SϪYbϪI(2)
I(1)ϪYbϪO(41)
I(1)ϪYbϪO(51)
I(1)ϪYbϪO(61)
I(2)ϪYbϪO(41)
(c)[a]
C(1)*
ϩ2.1
S*
C(2)*
Ϫ0.6
O(41)*
ϩ3.3
C(3)*
Ϫ1.0
O(51)*
Ϫ2.9
C(4)*
ϩ1.1
O(61)*
Ϫ2.8
C(5)*
ϩ0.4
Yb
C(6)*
Ϫ2.0
I(1)
S
C(7)
Ϫ14.1
C(22)
Ϫ12.4
Ϫ14.6
I(2)
Ϫ287.7
ϩ2.4
ϩ10.6
ϩ300.4
(d)[b]
A/B
74.5
A/C
73.0
A/D
88.8
[a]
[b]
Atoms marked with an asterisk define the least-square plane. Ϫ
Least-square planes defined by the following atoms: A: C(1) to
C(6); B: C(7) to C(12); C: C(22) to C(27); D: C(1), S, Yb.
rather different S(1)ϪYbϪS(2), O(82)ϪYbϪO(85), and compounds 2b, 4b, and 5 have been examined by 171Yb
O(92)ϪYbϪO(95) angles of 141.3(1)°, 64.8(2)°, and NMR spectroscopy (Table 6). The 171Yb NMR spectrum of
65.1(2)°, respectively, reveal a severe distortion from the an in situ prepared solution of 2b in [D8]THF shows a main
ideal octahedral environment. The YbϪS bond length of signal at δ ϭ 351. Smaller resonances of equal height at
273.9 pm and the average YbϪO distances of 248.0 pm may δ ϭ 281 and δ ϭ 457 can be assigned to the dithiolato
be compared with the corresponding values of 275.6(8) pm species Yb(SAr*)2(THF)x (6) and solvated ytterbium(II)
and 253.5 pm in the related complex Yb(SMes*)2(dme)2.[32] iodide YbI2(THF)4 (4b).[33] In the 171Yb NMR spectrum
Apparently, the slightly shorter distances in 5 are compens- of 4b in [D8]toluene or [D6]benzene, a sharp resonance is
ated by the more acute SϪYbϪS angle of 124.4(2)° in the observed at δ ϭ 768. A different chemical shift is found if
latter.
the aromatic solvent is replaced by an alkane. Thus, a 171Yb
Although the Ar* and Mes* substituents are of similar NMR signal at δ ϭ 837 is observed in [D14]methylcyclohex-
size, they often induce different oligomerization and solvent ane at 233 K, whereas no 171Yb resonance is detectable at
complexation behavior, as has been noted previously.[16] In ambient temperature. The reaction of purple 4b with more
contrast to the structures of 4a and 4b, the interplanar strongly coordinating solvents such as THF or DME results
angles between the MϪSϪC planes and the central C6H3 in a color change to yellow. In the 171Yb NMR spectrum,
arene rings, i.e. A/D and E/H (Table 3), are close to 90°, a shift to higher field is observed with resonances at δ ϭ 204
which is a consequence of repulsive forces between the (at 233 K) and δ ϭ 281 for the solvated complexes Yb(SAr*
DME ligands and the ortho-Trip groups.
)2(dme)2 5 and Yb(SAr*)2(THF)x 6, respectively. These
It is interesting to compare the molecular structures of 3, values may be compared with the 171Yb NMR chemical
4a, 4b, and 5 in terms of the different degrees of steric shifts of the ytterbium(II) thiolate Yb(SMes*)2(dme)2 and
crowding. Thus, the average MϪSϪC angle increases from related selenolates and tellurolates, which are found at lower
115° and 116° in 4b and 4a, through 125° in 3, up to 146° field, specifically at δ ϭ 485[32] and δ ϭ 430Ϫ526,[34] re-
in 5. Furthermore, the average displacement of the ipso-Trip spectively. It is notable that no 171Yb NMR signal can be
carbon atoms [i.e. C(7), C(22), C(43), and C(58)] from the detected for 5 at ambient temperature. Apparently, dynamic
central C6H3 plane increases in the order 4a (8.2 pm), 4b processes such as solvent exchange, intermolecular arene
(9.6 pm), 3 (13.3 pm), and 5 (22.9 pm). Apparently, the coordination, or equilibria due to hindered rotation com-
maximum steric congestion is observed for compound 5.
plicate the interpretation of solution NMR-spectroscopic
data for compounds 4b and 5. In order to gain more insight
into these different processes, variable-temperature 1H
NMR experiments were performed. At room temperature,
the 1H NMR spectrum of 4b in [D8]toluene shows only one
type of thiolate ligand environment with equivalent Trip
Spectroscopic Characterization
The diamagnetic complexes 4b and 5 have been charac-
terized by 1H and 13C NMR spectroscopy. In addition,
1972
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981

σ-Donor versus η6-π-Arene Interactions in Monomeric EuII and YbII Thiolates
FULL PAPER
Table 2. (a) Selected interatomic distances [pm], (b) angles [°], (c) distances from least-square planes [pm], (d) and interplanar angles [°]
in Eu(SAr*)2 (4a) and Yb(SAr*)2 (4b)
4a·(THF)0.5
4b·(C6H6)0.5
(a)
MϪS(1)
MϪS(2)
M···C(7)
M···C(8)
M···C(9)
M···C(10)
M···C(11)
M···C(12)
M···C(43)
M···C(44)
M···C(45)
M···C(46)
M···C(47)
M···C(48)
M···X(1)[b]
M···X(2)
281.6(1)
281.8(1)
303.7(3)
309.4(3)
312.8(3)
315.0(3)
302.4(3)
298.8(3)
301.1(3)
306.9(3)
311.1(3)
316.1(3)
303.3(3)
296.7(3)
273.4
268.5(2)
269.6(2)
292.2(8)
297.1(8)
302.8(8)
305.2(8)
293.3(9)
287.0(8)
287.7(8)
311.8(8)
313.9(8)
301.7(8)
292.3(8)
282.4(8)
262.3
{281.7}[a]
{307.0}
{269.1}
{296.3}
{305.9}
{272.8}
{298.3}
{263.7}
272.2
265.0
(b)
S(1)ϪMϪS(2)
141.89(4)
115.85(11)
115.40(11)
166.9
122.3(2)
119.5(2)
122.3(3)
120.4(2)
120.3(3)
119.6(3)
119.9(3)
119.6(3)
142.73(8)
115.7(3)
115.0(3)
164.5
121.7(6)
119.7(6)
120.4(6)
121.2(6)
119.4(7)
120.4(7)
120.9(7)
121.1(7)
YbϪS(1)ϪC(1)
YbϪS(2)ϪC(37)
X(1)ϪMϪX(2)
S(1)ϪC(1)ϪC(2)
S(1)ϪC(1)ϪC(6)
S(2)ϪC(37)ϪC(38)
S(2)ϪC(37)ϪC(42)
C(1)ϪC(2)ϪC(7)
C(1)ϪC(6)ϪC(22)
C(37)ϪC(38)ϪC(43)
C(37)ϪC(42)ϪC(58)
(c)[c]
C(1)*
C(2)*
Ϫ0.9
Ϫ1.5
C(38)*
Ϫ0.7
Ϫ1.5
C(3)*
Ϫ1.7
Ϫ1.7
C(39)*
Ϫ0.6
ϩ0.4
C(4)*
ϩ2.2
ϩ3.1
C(40)*
ϩ1.1
ϩ0.7
C(5)*
ϩ0.0
Ϫ1.3
C(41)*
Ϫ0.4
Ϫ0.8
C(6)*
Ϫ2.6
Ϫ1.9
C(42)*
Ϫ0.9
Ϫ0.2
S(1)
C(7)
C(22)
Ϫ16.2
Ϫ20.1
C(58)
Ϫ6.2
4a:
4b:
ϩ3.0
ϩ3.2
C(37)*
ϩ1.4
ϩ1.4
ϩ15.0
ϩ17.5
S(2)
ϩ3.0
ϩ1.0
Ϫ2.8
Ϫ10.8
C(43)
Ϫ7.6
Ϫ1.1
4a:
4b:
Ϫ6.3
(d)[d]
A/B
83.6
82.0
A/C
89.5
89.0
E/F
85.2
84.2
E/G
85.0
82.7
B/I
11.5
13.6
F/I
6.3
7.3
B/F
16.0
19.5
A/D
4.7
6.7
E/H
1.7
0.3
4a:
4b:
[a]
[b]
Average values are given in brackets. Ϫ
X(1) and X(2) define the centroids of the C(7) to C(12) and C(43) to C(48) arene rings,
[c]
[d]
respectively. Ϫ Atoms marked with an asterisk define the least-square plane. Ϫ Least-square planes defined by the following atoms:
A: C(1) to C(6); B: C(7) to C(12); C: C(22) to C(27); D: C(1), S(1), M; E: C(37) to C(42); F: C(43) to C(48); G: C(58) to C(63); H:
C(37), S(2), M; I: S(1), M, S(2).
substituents, although the signals are very broad. If the persistence of the metalϪπ-arene interactions in solution.
temperature is gradually increased, the linewidth decreases In order to exclude additional interactions with the aro-
and sharp signals are observed at 358 K. At lower temper- matic solvent toluene, further low-temperature 1H NMR
atures, two coalescence points were detected at 284 K and experiments were performed in [D14]methylcyclohexane. In
227 K, corresponding to energy barriers of 57.1 kJ molϪ1 this solvent, a single energy barrier of 54.3 kJ molϪ1 is
and 46.5 kJ molϪ1, respectively. A full interpretation of the found and two signals due to the aromatic protons of the
low-temperature 1H NMR spectra has not yet been possible Trip substituents are observed below 263 K. In contrast, the
due to their complex nature and overlapping signals. It is 1H NMR spectrum of 5 shows only one set of signals due
clear, however, that below 284 K two different Trip substitu- to the Trip substituents and the coordinated DME molec-
ent environments exist, as is evident from the appearance ules in the temperature range 223Ϫ300 K.
of two different resonances for the m-Trip protons. This ob-
The weak tendency of 4b to coordinate THF was exam-
servation can be interpreted in terms of hindered rotation ined by additional 171Yb NMR experiments. As mentioned
about the S(1)ϪC(1) or S(2)ϪC(37) bonds and hence the above, a yellow solution of 4b in neat [D8]THF shows a
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
1973

M. Niemeyer
FULL PAPER
Table 3. (a) Selected interatomic distances [pm], (b) angles [°], (c) distances from least-square planes [pm], and (d) interplanar angles [°]
in Yb(SAr*)2(dme)2 (5)
(a)
YbϪS(1)
YbϪS(2)
YbϪO(82)
2.739(2)
2.739(3)
2.478(6)
YbϪO(85)
YbϪO(92)
YbϪO(95)
2.477(5)
2.510(6)
2.461(5)
(b)
S(1)ϪYbϪS(2)
S(1)ϪYbϪO(82)
S(1)ϪYbϪO(85)
S(1)ϪYbϪO(92)
S(1)ϪYbϪO(95)
S(2)ϪYbϪO(82)
S(2)ϪYbϪO(85)
S(2)ϪYbϪO(92)
S(2)ϪYbϪO(95)
O(82)ϪYbϪO(85)
O(82)ϪYbϪO(92)
O(82)ϪYbϪO(95)
O(85)ϪYbϪO(95)
141.25(11)
77.18(16)
100.22(13)
136.93(17)
88.61(13)
138.08(14)
87.19(14)
79.95(14)
102.00(14)
64.8(2)
O(85)ϪYbϪO(92)
O(92)ϪYbϪO(95)
YbϪS(1)ϪC(1)
91.9(2)
65.1(2)
149.0(4)
142.8(3)
121.0(6)
120.9(6)
120.4(6)
120.9(6)
122.6(7)
121.5(7)
124.1(7)
122.6(7)
YbϪS(2)ϪC(37)
S(1)ϪC(1)ϪC(6)
S(1)ϪC(1)ϪC(2)
S(2)ϪC(37)ϪC(42)
S(2)ϪC(37)ϪC(38)
C(1)ϪC(2)ϪC(7)
C(1)ϪC(6)ϪC(22)
C(37)ϪC(38)ϪC(43)
C(37)ϪC(42)ϪC(58)
71.1(2)
92.7(2)
152.8(2)
(c)[a]
C(1)*
Ϫ3.1
C(37)*
ϩ5.1
C(2)*
ϩ2.0
C(38)*
Ϫ3.5
C(3)*
ϩ0.7
C(39)*
Ϫ1.5
C(4)*
Ϫ2.4
C(40)*
ϩ4.7
C(5)*
ϩ1.2
C(41)*
Ϫ3.2
C(6)*
ϩ1.6
C(42)*
Ϫ1.8
S(1)
Ϫ1.7
S(2)
C(7)
C(22)
ϩ21.1
C(58)
Ϫ20.9
ϩ22.1
C(43)
Ϫ27.6
ϩ11.1
(d)[b]
A/B
89.6
A/C
89.1
E/F
71.7
E/G
78.6
A/D
82.1
E/H
87.3
[a]
[b]
Atoms marked with an asterisk define the least-square plane. Ϫ
Least-square planes defined by the following atoms: A: C(1) to
C(6); B: C(7) to C(12); C: C(22) to C(27); D: C(1), S(1), Yb; E: C(37) to C(42); F: C(43) to C(48); G: C(58) to C(63); H: C(37), S(2), Yb.
Table 4. Salient structural parameters for selected europium and ytterbium thiolates
Compound
Oxid.
c. n.[a]
av. YbϪS [pm]
av. YbϪSϪC [°][b]
Ref.
[55]
[56]
[56]
Eu2(STrip)4(THF)6
Eu(SC5H4N-2)2(py)4
Eu(SC5H4N-2)2(THF)2(bipy)
4a
ϩII
ϩII
ϩII
ϩII
ϩII
ϩII
ϩII
ϩII
ϩII
ϩII
ϩIII
ϩIII
ϩIII
ϩIII
ϩIII
6
8
8
289.8[c]/301.6[d]
304.6
300.6
281.7
275.6
282.7
282.6
288.4
273.9
125[c]
2 (ϩ 6)
6
6
7
7
6
116
125
102
[32]
[57]
[58]
[59]
Yb(SMes*)2(dme)2
Yb(SPh)2(py)4
[Yb(µ-SPh)(η5-C4Me4P)(THF)2]2
Yb(SC5H4N-2)2(py)3
5
146
115
4b
2 (ϩ 6)
269.1
264.7
[56]
[55]
[60]
[59]
Yb(SPh)3(py)3
Yb(STrip)3(py)3
Yb2(StBu)6(bipy)2
Yb(SC5H4N-2)3(py)2
3
6
6
6
8
6
264.8
117
124
262.3[c]/274.7[d]
276.1
258.1
125
[a]
[b]
[c]
[d]
Coordination number. Ϫ
Bridging ligand.
Selected values for monomeric compounds with monodentate ligands. Ϫ
Terminal ligand. Ϫ
171Yb NMR resonance of δ ϭ 281. In contrast, there is no temperatures. The temperature dependence of the chemical
detectable signal in the 171Yb NMR spectrum at ambient shift for 6 (Ϫ0.85 ppm KϪ1) may be compared with other
temperature if a 4:1 mixture of toluene and THF is used as reported values, which typically vary between ϩ2 and Ϫ0.5
solvent. However, if this purple solution is cooled to 253 K, ppm KϪ1 [34]
.
a color change to yellow is observed and a broad resonance
appears at δ ϭ 316 (w1/2 ϭ 700 Hz). At 233 K, the signal is
In the mass spectra of 4a and 4b, intense signals due to
shifted to δ ϭ 336 and the linewidth is reduced to 180 Hz. the molecular ions were found (base peak for 4b; rel. intens-
Apparently, if the concentration of THF is too low, the ity of 48% for 4a). In addition, solutions of 4b in n-pentane
solvated complex Yb(SAr*)2(THF)x (6) is only stable at low (purple) and THF (yellow) were studied by UV/Vis spectro-
1974
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981

σ-Donor versus η6-π-Arene Interactions in Monomeric EuII and YbII Thiolates
FULL PAPER
Table 5. Salient structural parameters for compounds with europium and ytterbium η6-π-arene coordination
Compound
Oxid.
YbϪC [pm][a]
YbϪX [pm][b]
Ref.
[61]
[Eu(C6Me6)(AlCl4)2]4
4a
ϩII
ϩII
292(2)Ϫ307(2) {300}
298.8(3)Ϫ315.0(3) {307}
296.7(3)Ϫ316.1(3) {306}
274(4)Ϫ301(5) {286}
281.4(4)Ϫ314.8(6) {296}
275(3)Ϫ318(4) {296}
294(1)Ϫ315(1) {304}
287(1)Ϫ306(1) {297}
287.0(8)Ϫ305.2(8) {296}
282.4(8)Ϫ313.9(8) {298}
{267}
273
272
248
263
[62]
[63]
[14]
[14]
Yb(C6Me6)(AlCl4)3
ϩIII
ϩIII
ϩII
[c]
Yb(ODpp)3
[d]
Yb2(ODpp)4
[Yb2(ODpp)3]ϩ
ϩII
270
263
262
265
4b
ϩII
[d]
[a] {Av. values}. Ϫ [b] X defines the centroid of the arene rings. Ϫ [c] Dpp ϭ 2,6-Ph2C6H3. Ϫ Av. values for two independent molecules.
Table 6. 171Yb NMR parameters for compounds 1b, 2b, 4b, and 5
Compound
Solvent
T [K]
δ [ppm]
w1/2 [Hz]
[a]
1b
2b
4b
4b
[D8]THF
[D8]THF
[D6]benzene
[D14]methylcyclohexane
[D14]methylcyclohexane
[D6]toluene (80%) ϩ [D8]THF (20%)
[D6]toluene (80%) ϩ [D8]THF (20%)
[D6]toluene (80%) ϩ [D8]THF (20%)
[D8]THF
298
298
298
298
233
298
253
233
298
298
233
351[b]
28
8
768
[c]
837
70
[c]
4b
316
336
700
180
50
4b
5
281
[c]
[D10]DME
[D10]DME
204
75
[b]
[a] No 171Yb NMR signal besides the resonance for YbI2(THF)4 (δ ϭ 457) detectable. Ϫ Additional smaller signals for the homoleptic
[c]
species YbI2(THF)4 (δ ϭ 457, w1/2 ϭ 40 Hz) and Yb(SAr*)2(THF)x (δ ϭ 281, w1/2 ϭ 70 Hz) were observed. Ϫ
No 171Yb NMR
signal detectable.
photometry. Absorption maxima were observed at 475/ 1591/1545 cmϪ1 (4a) and 1592/1544 cmϪ1 (4b) are observed
546 nm and 369/417 nm, respectively. only in the IR spectra of the donor-free compounds. The
It is know from studies on dimeric lanthanide and actin- latter clearly indicate a weakening of the aromatic CϭC
ide aryl oxide complexes M2(OAr)6 (Ar ϭ 2,6-iPr2C6H3; bonds, consistent with the presence of π-arene interactions,
M ϭ La, Nd, Sm, Er, U) that infrared spectroscopy is a as observed by X-ray crystallography.
valuable tool for the identification of η6-arene-bridged di-
meric structures.[35Ϫ37] IR spectroscopy shows distinct dif-
ferences in the aromatic CϭC vibrational modes for the π-
bound and O-bridged phenoxide ligands in these com-
pounds. Table 7 shows the IR vibrational frequencies for
the aromatic stretching modes in compounds 3, 4a, 4b, 5
and the thiol Ar*SH, recorded as Nujol mulls. Two bands
in the regions 1604Ϫ1606 cmϪ1 and 1565Ϫ1569 cmϪ1 are
common to all the spectra. Additional stretching modes at
Ab initio Calculations
Previous theoretical studies of lanthanide bonding to ar-
enes have been mainly limited to complexes of the zero-
valent metals.[38,39] To gain more insight into the Eu2ϩ
Ϫ
or Yb2ϩϪarene interactions present in 4a and 4b, ab initio
calculations on suitable model systems were performed.
Quasi-relativistic pseudo-potentials of the Stuttgart/Bonn
group were employed for atoms heavier than sulfur.
(a) [Ln(donor)]2؉ Species
Table 7. Infrared vibrational wavenumbers [cmϪ1] for the ν(CϭC)
stretching modes in compounds 3, 4a, 4b, and 5 and the thiol
Ar*SH
In order to test the influence of different substituents and
to gain a qualitative impression of the magnitude of the
Ln2ϩϪη6-arene interaction, the simple complexes
[Ln(arene)]2ϩ (Ln ϭ Eu, Yb; arene ϭ C6H6, C6F6, 1,3,5-
Me3C6H3, 1,3,5-iPr3C6H3) were first examined. For com-
parison, the coordination of Eu2ϩ and Yb2ϩ to the strong
σ-donors tetrahydrofuran, dimethoxyethane, and the Ard-
uengo-type carbene imidazol-2-ylidene was also investig-
ated. Table 8 summarizes the results of these calculations,
Compound
ν(CϭC)
3
1605
1606
1606
1604
1605
1565
1569
1568
1566
1567
4a
1591
1592
1545
1544
4b
5
Ar*SH
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
1975

M. Niemeyer
FULL PAPER
Table 8. B3LYP/6Ϫ31G*-optimized bond lengths [pm] and binding energies [kJ molϪ1] for [Ln(arene)]2ϩ, [Ln(THF)]2ϩ, [Ln(dme)]2ϩ, and
[Ln(imidazol-2-ylidene)]2ϩ species (Ln ϭ Eu, Yb)
Compound
Symm.
YbϪC/O
CϪC[a]
∆Ee
∆EZPE
∆H0
[Eu(C6H6)]2ϩ
C6v
C6v
C3
C3
C2
298.9
141.0
141.2
{141.6}
{141.3}
Ϫ277.1
Ϫ115.6
Ϫ334.4
Ϫ382.3
Ϫ297.0
Ϫ428.3
Ϫ386.2
Ϫ305.3
Ϫ134.2
Ϫ366.4
Ϫ420.7
Ϫ325.8
Ϫ470.2
Ϫ425.7
3.8
2.1
3.8
Ϫ1.0
2.3
5.1
6.5
3.6
1.9
3.6
Ϫ1.4
2.4
5.4
6.5
Ϫ273.3
Ϫ113.5
Ϫ330.6
Ϫ383.3
Ϫ294.7
Ϫ423.2
Ϫ379.7
Ϫ301.7
Ϫ132.3
Ϫ362.8
Ϫ422.1
Ϫ323.4
Ϫ464.8
Ϫ419.2
[Eu(C6F6)]2ϩ
308.5
[Eu(1,3,5-Me3C6H3)]2ϩ
[Eu(1,3,5-iPr3C6H3)]2ϩ
[Eu(THF)]2ϩ
{293.1}[b]
{290.9}
231.2
[Eu(dme)]2ϩ
C2
241.0
258.8
287.9
297.1
{282.6}
{280.7}
222.5
231.6.0
249.1
[Eu(imidazol-2-ylidene)]2ϩ
[Yb(C6H6)]2ϩ
C2v
C6v
C6v
C3
C3
C2
141.1
141.4
141.7
141.3
[Yb(C6F6)]2ϩ
[Yb(1,3,5-Me3C6H3)]2ϩ
[Yb(1,3,5-iPr3C6H3)]2ϩ
[Yb(THF)]2ϩ
[Yb(dme)]2ϩ
C2
C2v
[Yb(imidazol-2-ylidene)]2ϩ
[a] For the isolated arenes, the following CϪC distances were obtained at the B3LYP/6Ϫ31G* level of theory: 139.7 (C6H6), 139.4 (C6F6),
[b]
{139.9} (1,3,5-Me3C6H3) {140.0} (1,3,5-iPr3C6H3). Ϫ {Av. values}.
Figure 5. Optimized structures for the model compounds (a) [Yb(C6H6)]2ϩ, (b) [Yb(C6F6)]2ϩ, (c) [Yb(1,3,5-Me3C6H3)]2ϩ, (d) [Yb(1,3,5-
iPr3C6H3)]2ϩ, (e) [Yb(THF)]2ϩ, (f) [Yb(imidazol-2-ylidene)]2ϩ, (g) [Yb(SH)2(C6H6)], and (h) [Yb(SH)2(C6H6)2]; calculations were either
performed at the B3LYP/6Ϫ31G* [(a)Ϫ(f)] or the MP2/6Ϫ31G* [(g), (h)] level of theory
which were performed at the B3LYP/6Ϫ31G* level of the- tween the values for [Eu(C6H6)]2ϩ and [Eu(1,3,5-
ory. The optimized structures of the Yb2ϩ adducts are Me3C6H3)]2ϩ. This is in accordance with the experimental
shown in Figure 5 (a)Ϫ(f). Obviously, the binding energy observation that intramolecular arene binding in 4a and 4b
largely depends on the electron-withdrawing or -donating is favored over the coordination of THF. It is notable that
ability of the substituents on the arene rings, as would be the binding energy for the interaction of Eu2ϩ with the Ard-
expected for a cationϪarene interaction dominated by elec- uengo-type carbene imidazol-2-ylidene is substantially
trostatics. Therefore, as a result of the strong ϪI effect of larger than that for the coordination of THF and compar-
the fluorine atoms, the smallest binding energy (∆H0) of able to that for coordination of the arene 1,3,5-iPr3C6H3.
Ϫ113.5 kJ molϪ1 is calculated for the compound Therefore, it should be possible to obtain adducts of 4a or
[Eu(C6F6)]2ϩ, whereas a value of Ϫ273.3 kJ molϪ1 is calcu- 4b with substituted carbenes of the imidazol-2-ylidene type.
lated for the benzene complex [Eu(C6H6)]2ϩ. The introduc- As one might expect, the greatest dissociation energy of
tion of electron-donating alkyl groups leads to a substantial 423 kJ molϪ1 is found for the compound [Eu(dme)]2ϩ. This
increase in ∆H0 from Ϫ330.6 kJ molϪ1 for [Eu(1,3,5- is in agreement with the isolation of the bis(DME) adduct
Me3C6H3)]2ϩ to Ϫ383.3 kJ molϪ1 for [Eu(1,3,5- 5. Compared to the corresponding Eu2ϩ compounds, the
iPr3C6H3)]2ϩ. With a value of 297 kJ molϪ1, the dissoci- binding energies of the Yb2ϩ complexes are increased by
ation energy of the adduct [Eu(THF)]2ϩ is intermediate be- 9Ϫ14%, which is attributable to the smaller ionic radius of
1976
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981

σ-Donor versus η6-π-Arene Interactions in Monomeric EuII and YbII Thiolates
FULL PAPER
Table 9. Calculated bond lengths [pm], bond angles [°], torsion angles [°], and energies [Hartree] for Yb(SH)2 species (see below) at
different levels of theory
Compound
Symm.
YbϪS
SϪYbϪS
SϪH
HϪSϪYb
HϪSϪYbϪS
Ee
EZPE
B3LYP/6Ϫ31G*
Yb(SH)2 anti/anti
Yb(SH)2 syn/syn
C2v
C2
270.5
269.0
132.1
128.5
135.7
135.4
96.7
98.4
180
3.4
Ϫ830.601965
Ϫ830.604450
0.015359
0.015495
B3LYP/6Ϫ31ϩG**
Yb(SH)2 anti/anti
Yb(SH)2 syn/syn
C2v
C2
270.8
269.2
133.9
128.8
135.3
135.0
95.9
97.8
180
3.5
Ϫ830.612178
Ϫ830.614476
0.015359
0.015489
MP2/6Ϫ31G*
no minimum
Yb(SH)2 anti/anti
Yb(SH)2 syn/syn
C2
270.6
149.7
152.1
134.5
133.6
101.7
97.2
20.2
34.3
Ϫ829.238995
Ϫ829.267290
0.015563
0.015833
MP2/6Ϫ31ϩG**
Yb(SH)2 syn/syn
C2
271.3
the latter. In accordance with the electronic properties of
the various arene ligands, the metal···C(arene) distances
also vary considerably by 16Ϫ17 pm, being shortest in
[M(1,3,5-iPr3C6H3)]2ϩ and longest in [M(C6F6)]2ϩ. A not-
able feature of the calculated [M(1,3,5-iPr3C6H3)]2ϩ struc-
tures is the presence of three agostic-type M···H interac-
tions to CϪH bonds of the isopropyl groups (Eu···H:
296.2 pm; Yb···H: 285.5 pm). The shortest experimentally
determined M···H distances in 4a and 4b are 303 pm and
301 pm, respectively.
Figure 6. Bending potential curve for syn/syn Yb(SH)2 at the MP2/
6Ϫ31ϩG** level of theory
(b) Yb(SH)2
Figure 6 shows the rather shallow bending potential
curve for syn/syn Yb(SH)2 calculated at the MP2/6-31ϩG**
DFT calculations with two different basis sets revealed at
least two stable conformers (Table 9). The U-shaped syn,syn
species with C2 symmetry is energetically favored by ca.
6 kJ molϪ1 over the W-shaped anti,anti conformer (C2v).
According to MP2 results, only the former is a minimum
on the potential-energy surface. The bending angle at the
level of theory. With a linearization energy of 1.1 kJ molϪ1
it can be classified as a quasi-linear molecule.
,
(c) Yb(SH)2(C6H6)n Complexes (n ؍ 1, 2)
Calculated bond parameters and binding energies for
Yb center and the YbϪS distance were computed at MP2/ Yb(SH)2(C6H6) and Yb(SH)2(C6H6)2 species, both of which
6Ϫ31ϩG** level to be 152.1° and 271.3 pm, respectively. were computed at the B3LYP and MP2 levels of theory us-
The DFT calculations showed slightly shorter YbϪS bonds ing the 6Ϫ31G* basis set, are summarized in Table 10. The
and a more acute SϪYbϪS angle of ca. 130° for both con- MP2-optimized structures are shown in Figure 5 (g) and
formers. The occurrence of bent equilibrium geometries for (h). Compared to the experimentally determined distances
some lanthanide MX2 species[40,41] and related compounds in complex 4b, the calculated YbϪS bond lengths for the
of the heavier alkaline earth elements[42,43] is now well es- model compound Yb(SH)2(C6H6)2 are overestimated by 5.1
tablished. This bending can be explained by the presence of pm (MP2) and 7.1 pm (B3LYP), respectively. The difference
small σ-bonding contributions (involving metal d orbitals) between the DFT-computed and observed Yb···C distances
and/or the polarization of the metal cation by the ligands. is quite substantial, amounting to ϩ22.3 pm. A much better
According to Kaupp et al., the MX2 compounds can be description is provided by the MP2 calculation, which over-
divided into molecules with genuinely bent structures [for estimates the distance of the Yb···arene interaction by only
example, EuF2, Ba(CH3)2] and molecules with quasi-linear 5.6 pm. Replacing the coordinated benzene in
equilibrium geometries (for example, SmCp2, CaF2). The Yb(SH)2(C6H6)2 by the substituted arene 1,3,5-iPr3C6H3
latter show almost no energy change (Ͻ 1Ϫ2 kJ molϪ1
)
can be expected to result in a further reduction in the
when the equilibrium angle is increased to 180°, whereas Yb···C distances, which might, however, be partly compens-
the former exhibit considerable linearization energies of up ated by repulsion between the isopropyl groups. The disso-
to 33 kJ molϪ1
.
ciation enthalpy corresponding to the reaction
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
1977

M. Niemeyer
FULL PAPER
Table 10. Calculated bond lengths [pm], bond angles [°], torsion angles [°], and binding energies [kJ molϪ1] for Yb(SH)2(C6H6) and
Yb(SH)2(C6H6)2 species at different levels of theory
Compound[a]
Symm.
YbϪC[b]
YbϪS
SϪYbϪS
HϪSϪYbϪS
∆Ee
∆EZPE
∆H0
[c]
B3LYP/6Ϫ31G*
Yb(SH)2(C6H6)
Yb(SH)2(C6H6)2
C2
{310.5}
{319.6}
272.9
276.2
139.1
124.5
0.4
44.0
Ϫ70.8
Ϫ105.2
ϩ4.4
ϩ7.3
Ϫ66.4
Ϫ97.9
C2
MP2/6Ϫ31G*
Yb(SH)2(C6H6)
Yb(SH)2(C6H6)2
C2
C2
{298.1}
{302.9}
271.8
274.2
149.9
126.6
0.02
52.4
Ϫ108.3
Ϫ185.4
ϩ4.6[d]
ϩ7.8[d]
Ϫ103.7
Ϫ177.6
[a]
[b]
[c]
[d]
syn/syn isomers only. Ϫ {Av. values}. Ϫ According to the reaction Yb(SH)2 ϩ n C6H6 Ǟ Yb(SH)2(C6H6)n. Ϫ EZPE was taken
from the DFT calculation above.
zotrifluoride (0.48 mL, 3.41 mmol) as described below for the syn-
thesis of 4b. With stirring, a solution of iodine (0.42 g, 3.31 mmol)
in THF was added dropwise at 0 °C. The mixture was allowed to
warm to ambient temperature and ca. 30 mL of n-heptane was
added. After filtration through a glass filter frit, the resulting
purple solution was concentrated until crystallization commenced
and then stored in a freezer at ca. Ϫ25 °C for 2 d to give 3 as
purple crystals (2.25 g, 57%); m.p. 120Ϫ240 °C (dec., compound
Yb(SH)2(C6H6)2 Ǟ Yb(SH)2(C6H6) ϩ C6H6 was estimated
to be 31.5 kJ molϪ1 (B3LYP) and 73.9 kJ molϪ1 (MP2).
These values may be compared with the NMR-spectroscop-
ically determined energy barriers of 57.1 kJ molϪ1 and
54.3 kJ molϪ1 found for solutions of 4b in [D8]toluene or
[D14]methylcyclohexane.
˜
gradually decolorizes). Ϫ IR (CsBr, Nujol): ν ϭ 1605 ms, 1565 m,
Conclusion
1358 s, 1345 sh, 1315 ms, 1299 s, 1247 m, 1166 ms, 1154 mw, 1130
mw, 1115 m, 1098 m, 1068 ms, 1037 s, 1000 vs, 954 w, 940 m, 916
m, 878 s, 851 sh, 835 vs, 800 s, 774 s, 745 s, 732 s, 674 m, 663
The use of the very bulky m-terphenyl-substituted Ar*S
ligand has permitted the synthesis of the first monomeric σ- mw, 650 ms, 608 w, 590 w, 526 m, 508 w, 427 mw, 410 cmϪ1 w. Ϫ
C48H73I2O3SYb (1157.0): calcd. C 49.83, H 6.36; found C 49.50,
H 6.38.
donor-free lanthanide(II) thiolates. In the solid state, steric
saturation of the europium and ytterbium centers is
achieved by two η6-π-arene interactions, as has been dem-
onstrated by X-ray structural analysis and IR spectroscopy.
Evidence that these interactions also persist in solution has
been provided by variable-temperature NMR experiments,
which show an energy barrier of 54.3 kJ molϪ1 for 4b in
methylcyclohexane solution. In contrast, the 1H NMR
spectrum of the bis(DME) adduct 5 shows no dynamic
character in the temperature range 223Ϫ300 K, despite the
more crowded environment of the Yb center. From ab initio
calculations on the model system Yb(SH)2(C6H6)2, it has
been estimated that the dissociation of one molecule of ben-
zene requires between 31.5 and 73.9 kJ molϪ1 depending on
the method used.
Eu(SAr*)2 (4a): At 0 °C, an Eu ingot (3.9 g, 25.7 mmol) was added
to a stirred solution of Ar*SH (1.08 g, 2.10 mmol) and 2-iodoben-
zotrifluoride (0.48 mL, 3.41 mmol) in THF (60 mL). Stirring was
continued for 1 h at 0 °C and for a further 1 h at 20 °C, whereupon
the resulting light-orange solution was carefully decanted from the
excess Eu metal. The volume of the solution was reduced to ca. 5
mL and n-heptane (ca. 20 mL) was added. After separation from
the precipitated EuI2(THF)5, the remaining solution was concen-
trated in vacuo until crystallization commenced. Storage in a
freezer at 5 °C overnight afforded 4a·(THF)0.5 as orange crystals
(0.89 g, 70%); m.p. 289Ϫ292 °C (compound softens Ͼ 216 °C). Ϫ
IR (CsBr, Nujol): ν˜ ϭ 1606 m, 1591 m, 1569 ms, 1545 m, 1411 m,
1363 s, 1336 m, 1314 ms, 1249 ms, 1167 ms, 1150 m, 1110 ms, 1081
m, 1069 ms, 1049 vs, 1003 m, 939 ms, 922 m, 886 vs, 870 s, 850 w,
791 s, 775 m, 756 m, 738 vs, 733 vs, 649 ms, 612 w, 587 m, 562 w,
519 ms, 426 w, 360 cmϪ1 m. Ϫ EI-MS (70 eV): m/z (%) ϭ 1179.7
(48) [Eu(SAr*)2ϩ], 666.3 (100) [EuSAr*ϩ], 514.4 (77) [SAr*ϩ]. Ϫ
C74H102EuO0.5S2 (1215.7): calcd. C 73.11, H 8.46; found C 72.91,
H 8.72.
Experimental Section
General: All manipulations were performed using standard Schlenk
techniques under purified argon. Solvents were freshly distilled un-
Yb(SAr*)2 (4b). ؊ Method A: Ar*SH (0.26 g, 0.50 mmol) was ad-
der argon from Na wire or LiAlH4. The compounds
ded through
a solid-addition tube to a stirred solution of
[17]
Yb[N(SiMe3)2]2
and Ar*SH[44] were synthesized according to
Yb[N(SiMe3)2]2 (0.12 g, 0.24 mmol) in n-pentane (ca. 15 mL). The
reaction mixture was stirred for a further 30 min and then the solv-
ent was removed under reduced pressure. The intensely colored res-
idue was redissolved in a mixture of n-heptane and a small amount
of benzene. Cooling in a freezer at 5 °C afforded 4b·(C6H6)0.5 as
purple crystals (0.19 g, 64%), which were suitable for X-ray crystal-
lographic studies. The solvate-free compound 1 was obtained by
pumping at 10Ϫ3 mbar for 2 h. Ϫ Method B: Ar*SH (2.06 g,
4.00 mmol) and Yb chips (1.1 g, 6.36 mmol) were suspended in
THF (60 mL) at 0 °C. With stirring, 2-iodobenzotrifluoride (0.56
mL, 4.00 mmol) was added dropwise and after a short induction
known literature procedures. Ϫ NMR spectra were recorded with
Bruker AM 200, AC250, and AM 400 instruments and were refer-
enced to solvent resonances (1H, 13C) or YbCp*2(THF)2
(
[34]
171Yb). Ϫ UV spectra were recorded with a Bruins Omega 10 spec-
trophotometer.
Ϫ IR spectra were recorded in the range
4000Ϫ200 cmϪ1 with a PerkinϪElmer Paragon 1000 PC spectro-
meter. Ϫ Mass spectra were measured with a Varian MAT711 in-
strument. Ϫ Melting points were determined under Ar in sealed
glass tubes.
Yb(SAr*)I2(THF)3 (3): A solution of 2b was prepared from Ar*SH
(1.76 g, 3.41 mmol), Yb chips (0.59 g, 3.41 mmol), and 2-iodoben- period a color change to orange was observed. Stirring was con-
1978 Eur. J. Inorg. Chem. 2001, 1969Ϫ1981

σ-Donor versus η6-π-Arene Interactions in Monomeric EuII and YbII Thiolates
FULL PAPER
3
tinued for 2 h, whereupon the resulting maroon solution was care-
fully decanted from the unchanged excess Yb metal. Most of the
6.8 Hz, 24 H, o/p-CH(CH3)2], 1.26 [d, JHH ϭ 6.8 Hz, 24 H, o/p-
3
CH(CH3)2], 2.86 [sept, JHH ϭ 6.8 Hz, 4 H, p-CH(CH3)2], 2.95
3
solvent was removed under reduced pressure and the concentrate [sept, JHH ϭ 6.8 Hz, 8 H, o-CH(CH3)2], 6.67 (m, 6 H, m/p-C6H3),
was treated with n-pentane (50 mL). The solution was separated
from the precipitated greenish-yellow YbI2(THF)4 and all volatile
materials were distilled off in vacuo. Recrystallization of the
6.93 (s, 8 H, m-Trip). Ϫ 13C NMR (50.33 MHz, [D8]THF, 25 °C):
δ ϭ 24.5, 24.5, 25.8 [o/p-CH(CH3)2], 31.2 [o-CH(CH3)2], 34.9 [p-
CH(CH3)2], 119.3 (p-C6H3), 121.1 (m-Trip), 129.9 (m-C6H3), 142.9,
purple-brown residue from n-heptane/toluene (20:1) followed by 144.4 (i-Trip, o-C6H3), 146.3 (p-Trip), 147.3 (o-Trip), 152.5 (i-
pumping at 10Ϫ3 mbar for 2 h gave pure 4b as a purple powder
C6H3). Ϫ 171Yb NMR (70.1 MHz, [D8]THF, 25 °C): δ ϭ 281
(1.58 g, 66%); m.p. 263Ϫ265 °C (compound softens Ͼ 220 °C). Ϫ (w1/2 ϭ 50 Hz). Ϫ 171Yb NMR {70.1 MHz, [D8]toluene (ca. 80%)
3
1H NMR (250.1 MHz, [D8]toluene, 85 °C): δ ϭ 1.06 [d, JHH
ϭ
ϩ [D8]THF (ca. 20%), Ϫ40 °C}: δ ϭ 336 (w1/2 ϭ 180 Hz).
3
6.9 Hz, 24 H, o/p-CH(CH3)2], 1.24 [d, JHH ϭ 6.9 Hz, 24 H, o/p-
3
Yb(SAr*)2(dme)2 (5): In Schlenk tube, purple 4b (0.35 g,
a
CH(CH3)2], 1.31 [d, JHH ϭ 6.9 Hz, 24 H, o/p-CH(CH3)2], 2.89
[sept, 3JHH ϭ 6.9 Hz, 8 H, o-CH(CH3)2], 3.07 [sept, 3JHH ϭ 6.9 Hz,
4 H, p-CH(CH3)2], 6.86 (m, 6 H, m/p-C6H3), 7.17 (s, 8 H, m-Trip).
0.29 mmol) was treated with two drops of DME, which immedi-
ately gave 5 as a yellow, microcrystalline material in quantitative
yield. The excess DME was carefully removed in vacuo at 0 °C.
Prolonged pumping at 10Ϫ3 mbar overnight gave 4b once more.
Crystals of 5 suitable for X-ray crystallographic studies were ob-
tained from an n-pentane/DME mixture (5:1) at Ϫ25 °C; m.p.:
compound darkens Ͼ ca. 110 °C and melts to a purple-brown li-
quid between 265 and 270 °C. After cooling to ambient temper-
ature and standing for 1 h, the upper crust of the solid turns yellow
again. Ϫ 1H NMR (250.1 MHz, [D10]DME, 25 °C): δ ϭ 0.97 [d,
Ϫ
13C NMR (62.9 MHz, [D8]toluene, 85 °C): δ ϭ 23.8, 24.3, 24.8
[o/p-CH(CH3)2], 31.2 [o-CH(CH3)2], 34.0 [p-CH(CH3)2], 120.6 (p-
C6H3), 122.0 (m-Trip), 129.0 (m-C6H3), 141.6, 142.7 (i-Trip, o-
C6H3), 148.3 (o-Trip), 149.3 (p-Trip), 152.5 (i-C6H3). Ϫ 171Yb
NMR (70.1 MHz, [D6]benzene, 25 °C): δ ϭ 768 (w1/2 ϭ 8 Hz) Ϫ
˜
IR (CsBr, Nujol): ν ϭ 1606 m, 1592 m, 1568 m, 1544 m, 1314 ms,
1249 ms, 1168 ms, 1150 m, 1110 ms, 1082 m, 1069 m, 1049 s, 1004
w, 939 s, 920 m, 886 vs, 869 vs, 849 w, 791 vs, 774 m, 755 ms, 740
vs, 733 vs, 650 s, 611 w, 587 m, 519 s, 425 w, 363 m, 306 cmϪ1 w.
Ϫ EI-MS (70 eV): m/z (%) ϭ 1200.7 (100) [Yb(SAr*)2ϩ], 687.3 (32)
[YbSAr*ϩ], 514.4 (19) [SAr*ϩ]. Ϫ C72H98S2Yb (1200.74): calcd. C
72.02, H 8.23; found C 71.41, H 8.36. Ϫ UV/Vis (n-pentane): λmax
(ε) ϭ 475 nm (110 Ϫ1 cmϪ1), 546 (90). Ϫ UV/Vis (THF): λmax
(ε) ϭ 369 nm (480 Ϫ1 cmϪ1), 417 (250).
3
3JHH ϭ 6.8 Hz, 24 H, o/p-CH(CH3)2], 1.18 [d, JHH ϭ 6.8 Hz,
3
24 H, o/p-CH(CH3)2], 1.25 [d, JHH
ϭ 6.8 Hz, 24 H, o/p-
CH(CH3)2], 2.87 [m, 12 H, o- ϩ p-CH(CH3)2], 3.27 (s, 12 H,
OCH3), 3.43 (s, 8 H, OCH2) [these two signals correspond to the
coordinated non-deuterated DME; another set of signals is ob-
served for the perdeuterated solvent], 6.67 (m, 6 H, m- ϩ p-C6H3),
6.88 (s, 8 H, m-Trip). Ϫ 171Yb NMR (70.1 MHz, [D10]DME, Ϫ40
˜
Yb(SAr*)2(THF)x (6): 1H NMR (200.13 MHz, [D8]THF, 25 °C): °C): δ ϭ 204 (w1/2 ϭ 75 Hz). Ϫ IR (CsBr, Nujol): ν ϭ 1604 m,
δ ϭ 0.95 [d, 3JHH ϭ 6.8 Hz, 24 H, o/p-CH(CH3)2], 1.16 [d, 3JHH ϭ
1566 ms, 1358 s, 1315 ms, 1276 w, 1243 ms, 1209 w, 1190 m, 1168
Table 11. Selected crystallographic data for compounds 3, 4a, 4b, and 5
Compound[a]
3
4a·(THF)0.5
4b·(C6H6)0.5
5
Empirical formula
Molecular mass
Color, habit
C48H73I2O3SYb
1157.0
C74H102S2Eu
1215.6
C75H101S2Yb
1239.7
C80H118O4S2Yb
1380.9
yellow, needle fragment
0.40 ϫ 0.30 ϫ 0.30
monoclinic
P21
13.905(3)
18.837(4)
15.329(3)
90
107.56(2)
90
3828(1)
2
1.198
13.21
3Ϫ54
purple, plate
0.45 ϫ 0.20 ϫ 0.05
orthorhombic
P212121
13.543(3)
15.984(3)
23.448(5)
90
90
90
5076(2)
4
1.514
deep orange, block
0.70 ϫ 0.40 ϫ 0.35
triclinic
purple, block
0.75 ϫ 0.65 ϫ 0.60
triclinic
Crystal size [mm]
Crystal system
Space group
¯
¯
P1
P1
˚
a [A]
13.278(2)
14.537(3)
18.804(4)
105.60(2)
95.18(1)
97.80(1)
3433(1)
2
13.028(5)
14.270(6)
18.681(8)
106.63(3)
96.02(2)
96.61(2)
3271(2)
2
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
3
˚
V [A ]
Z
dcalcd. [g cmϪ3
]
1.176
10.13
4Ϫ54
1.259
15.34
4Ϫ50
µ [cmϪ1
2θ range [°]
]
31.35
3Ϫ54
Unique data/Rint
Data with I Ͼ 2σ(I) (No)
Parameters (Np)
R1 [I Ͼ 2σ(I)][b]
wR2 (all data)[c]
GoF[d]
6727/0.0630
5499
527
0.0436
0.0848
1.213
0.62/Ϫ0.83
14752/0.0373
11454
832
0.0419
0.1018
0.956
1.10/Ϫ0.83
11176/0.0652
8311
828
0.0671
0.1818
1.225
9449/0.0466
7449
829
0.0479
0.1033
1.206
1.19/Ϫ0.60
Ϫ3
˚
Largest diff. peak and hole [e·A
]
1.78/Ϫ1.62
[a]
[b]
[c]
2 2
R1 ϭ (||Fo| Ϫ |Fc||)/|Fo|. Ϫ wR2 ϭ {[w(Fo2 Ϫ Fc ) ]/
˚
All data were collected at 173 K using Mo-Kα (λ ϭ 0.71073 A) radiation. Ϫ
[w(Fo ) ]}1/2. Ϫ GoF ϭ {[w(Fo2 Ϫ Fc ) ]/(No Ϫ Np)}.
2 2
[d]
2 2
Eur. J. Inorg. Chem. 2001, 1969Ϫ1981
1979

M. Niemeyer
FULL PAPER
m, 1128 w, 1111 ms, 1059 vs, 1051 sh, 1025 ms, 1005 m, 954 w, 941
m, 920 w, 875 s, 872 s, 868 s, 860 s, 831 m, 794 s, 775 m, 755 w,
742 vs, 736 vs, 650 ms, 608 w, 588 w, 567 w, 520 m, 425 mw,
379 cmϪ1 mw. Ϫ C80H118O4S2Yb (1381.0): calcd. C 69.58, H 8.61;
found C 69.42, H 8.71.
Acknowledgments
Support of this work by the Fonds der Chemischen Industrie is
gratefully acknowledged. The author also wishes to thank Prof. G.
Becker for generous support.
X-ray Crystallography: X-ray-quality crystals were obtained as de-
scribed above. Crystals were removed from Schlenk tubes and im-
mediately covered with a layer of viscous hydrocarbon oil (Paratone
N, Exxon). A suitable crystal was selected, attached to a glass fibre,
and instantly placed in a low-temperature N2 stream.[45] All data
were collected at 173 K using either a Syntex P21 (3, 5) or a Siemens
P4 (4a, 4b) diffractometer. Crystal data are given in Table 11. Cal-
culations were carried out with the SHELXTL PC 5.03[46] and
SHELXL-97[47] program systems installed on a local PC. The
structures were solved by direct methods and refined against F2o by
full-matrix least-squares techniques. An absorption correction was
applied by using semiempirical ψ-scans (5) or the program
XABS2[48] (3, 4b). For 4a, no absorption correction was used. An-
isotropic thermal parameters were included for all non-hydrogen
atoms with the exception of the co-crystallized THF molecule,
which was refined with a common isotropic U value. The methyl
carbon atoms of two (5) or four (4a, 4b) disordered isopropyl
groups were refined with split positions. The geometries of the dis-
ordered isopropyl groups and the co-crystallized benzene molecule
(4b) were restrained with SADI and DFIX commands. H atoms
were placed in geometrical positions and were refined using a rid-
ing model that included free rotation of the methyl groups. Their
isotropic thermal parameters were either allowed to refine or were
constrained to 1.2 (aryl H) or 1.5 (methyl groups) times Ueq of the
carbon atom to which they were bound. Final R values are listed in
Table 11. Salient bond lengths and angles are given in Tables 1Ϫ3.
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
nos. CCDC-150909 (3), -150907 (4a), -150908 (4b), and -150910 (5).
Copies of the data can be obtained free of charge on application
to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [Fax:
(internat.) ϩ 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
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Computational Details: The Gaussian 98[49] package was used for
all energy and frequency calculations. The geometries of the mole-
cules were optimized using either density functional theory (DFT)
with the functional B3LYP or second-order MøllerϪPlesset per-
turbation theory (MP2). Quasi-relativistic 11- or 10-valence-elec-
tron pseudo-potentials of the Stuttgart/Bonn group were employed
for the heavy atoms Sc,[50] Eu, and Yb.[51,52] The corresponding
(8s7p6d1f)/[6s5p3d1f] valence basis set for Sc was taken from the
literature, whereas the (7s6p5d)/[5s4p3d] valence basis sets for Eu
and Yb were augmented by one set of f-functions. The basis sets
for C, H, F, and S were either 6Ϫ31G* or 6Ϫ31ϩG** [Yb(SH)2
species only]. Frequency calculations were carried out to verify that
the geometries were true minima on the potential-energy surface
and to obtain the zero-point energy (EZPE). The sum of the elec-
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differences interpreted as reaction enthalpies (∆H0) at 0 K. For cal-
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cationϪring binding energy of [Sc(C6H6)]ϩ, which was determined
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