Fluxional behavior of indenyl-derived ytterbocene(II) complexes. The SiHR2 group as a NMR spectroscopic probe

Article abstract of DOI:10.1021/om000324w

Indenyl-derived ytterbocene(II) complexes [1-(SiHR₂)-2-R′-C₉H₅]₂Yb(L)₂ (R = Me, C₆H₅; R′ = H, Me; L = donor ligand) have been synthesized via silylamine elimination from Yb[N(SiMe₃)₂]₂(THF)₂ and the indene derivatives in hexane at ambient temperature. The `SiH-decorated' silyl substituents assist in the examination of the fluxional behavior of the stereoisomers in solution, the ratio and interconversion of which are affected by the size and ring position of the substituents as well as the type of donor ligand, e.g., THF vs TMEDA.

Full text of DOI:10.1021/om000324w

4666
Organometallics 2000, 19, 4666-4668
F lu xion a l Beh a vior of In d en yl-Der ived Ytter bocen e(II)
Com p lexes. Th e SiHR2 Gr ou p a s a NMR Sp ectr oscop ic
P r obe
Michael G. Klimpel, Wolfgang A. Herrmann, and Reiner Anwander*
Anorganisch-chemisches Institut, Technische Universita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85747 Mu¨nchen, Germany
Received April 18, 2000
Summary: Indenyl-derived ytterbocene(II) complexes
[1-(SiHR2)-2-R′-C9H5]2Yb(L)2 (R ) Me, C6H5; R′ ) H,
Me; L ) donor ligand) have been synthesized via sil-
ylamine elimination from Yb[N(SiMe3)2]2(THF)2 and the
indene derivatives in hexane at ambient temperature.
The “SiH-decorated” silyl substituents assist in the ex-
amination of the fluxional behavior of the stereoisomers
in solution, the ratio and interconversion of which are
affected by the size and ring position of the substituents
as well as the type of donor ligand, e.g., THF vs TMEDA.
SiHR2 group (R ) Me, Ph) is exploited as a sensitive
probe for dynamic NMR investigations.
New silyl-substituted indenes 1-(SiHMe2)C9H7 (1),12
1-[SiH(C6H5)2]C9H7 (2), and 1-(SiHMe2)-2-Me-C9H6 (3)
were prepared according to common salt metathesis
procedures from lithiated indene and the corresponding
chlorosilanes in THF/ether. Yb[N(SiMe3)2]2(THF)2 (4)13
reacts with 1 and 2 in hexane to form the dark red
compounds [1-(SiHMe2)C9H6]2Yb(THF)2 (5) and [1-{SiH-
(C6H5)2}C9H6]2Yb(THF)2 (6), respectively, in yields >90%
(Scheme 1).14-16
Ytterbocene(II) and samarocene(II) complexes gained
considerable interest in the field of rare earth-mediated
R-olefin and ring-opening polymerization.1 Various de-
rivatives comprising ansa- and nonlinked cyclopenta-
dienyl and indenyl systems as well as heteroleptic com-
plexes2-7 were ascribed high catalytic efficiency occur-
ring via a novel bisinitiator mechanism.8 It is known
that the interconversion of stereoisomers, implied chemi-
cally and thermally through rotation and epimerization
processes, significantly affects the stereospecifity of
olefin transformations.9 To better examine such flux-
ional phenomena, our interest was drawn to the design
of specially functionalized indenyl derivatives of the
diamagnetic ytterbium(II) center.10 We report here the
synthesis of ytterbocene(II) complexes via a silylamine
elimination reaction.11 Moreover, the indenyl-bonded
(10) For structurally characterized Ln(II) indenyl complexes, see:
(a) J in, J .; J in, Z.; Wie, G.; Chen, W. J . Struct. Chem. (J iegou Huaxue)
1993, 12, 24. (b) Evans, W. J .; Gummersheimer, T. S.; Boyle, T. J .;
Ziller, J . W. Organometallics 1994, 13, 1281-1284. (c) Khvostov, A.
V.; Bulychev, B. M.; Belsky, V. K.; Sizov, A. I. J . Organomet. Chem.
1999, 584, 164-170. (d) Qian, C.; Li, H.; Sun, J .; Nie, W. J . Organomet.
Chem. 1999, 585, 59-62.
(11) Further nonsalt metathesis routes to lanthanidocene(II) com-
plexes comprise: (a) Fischer, E. O.; Fischer, H. Angew. Chem. 1964,
76, 52. (b) Hammel, H.; Weidlein, J . J . Organomet. Chem. 1990, 388,
75-87. (c) Watson, P. L.; Whitney, J . F.; Harlow, R. L. Inorg. Chem.
1981, 20, 3271-3278. (d) Deacon, G. B.; Newnham, R. H. Aust. J .
Chem. 1985, 38, 1757-1765. (e) Wang, S.; Yu, Y.; Ye, Z.; Qian, C.;
Huang, X. J . Organomet. Chem. 1994, 464, 55-58. (f) Qian, C.; Zou,
G.; Sun, J . J . Organomet. Chem. 1998, 566, 21-28.
(12) Eppinger, J .; Spiegler, M.; Hieringer, W.; Herrmann, W. A.;
Anwander, R. J . Am. Chem. Soc. 2000, 122, 3080-3096.
(13) (a) Andersen, R. A.; Boncella, J . M. Organometallics 1985, 4,
205-207. (b) Boncella, J . M. Ph.D. Thesis, University of California,
Berkeley, 1982.
(14) Complexes 5-9 were synthesized analogously (see Supporting
Information). Complex 5: in a glovebox, indene 1 (349 mg, 2.00 mmol)
and complex 4 (638 mg; 1.00 mmol) were dissolved in hexane (20 mL)
and stirred at ambient temperature. The initially orange solution
turned dark red within 3 min. After 3 h the reaction mixture was
centrifugated, leaving a small amount (<5%) of dark brown, insoluble
residue. Product 5 was obtained as dark red needles by crystallization
of the hexane fraction at -40 °C (500 mg, 75%). IR (Nujol): 2081 s
(Si-H), 1244 m, 1156 m, 1027 m, 968 m, 878 m, 722 s, 661 w, 440 w
cm-1. Anal. Calcd for C30H42O2Si2Yb: C, 54.28; H, 6.38. Found: C,
53.50; H, 6.36. 1H NMR (C6D6, 25 °C): Major isomer (67%): δ 7.59
(dd, 3J H,H ) 6.9 Hz, 2H, indenyl), 7.36 (dd, 3J H,H ) 7.7 Hz, 2H, indenyl),
6.99 (m, 2H, Indenyl), 6.81 (m, 2H, indenyl), 6.28 (m, 2H, indenyl),
* Corresponding author. Fax: +49 89 289 13473. E-mail: reiner.
anwander@ch.tum.de.
(1) For a recent review, see: Yasuda, H. Top. Organomet. Chem.
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Y.; Kasai, N. Chem. Lett. 1988, 1963-1966. (b) Yasuda, H.; Yamamoto,
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Kanehisa, N. Macromolecules 1993, 26, 7134-7143.
3
5.14 (sp, J H,H ) 3.6 Hz, 2H SiH), 2.99 (s, 8H, THF I), 1.24 (s, 8H,
3
3
THF II), 0.71 (d, J H,H ) 3.6 Hz, 6H, SiMe I), 0.54 (d, J H,H ) 3.6 Hz,
3
6H, SiMe II). Minor isomer (33%): δ 7.64 (dd, J H,H ) 8.0 Hz, 2H,
3
(4) J iang, T.; Shen, Q.; Lin, Y.; J in, S. J . Organomet. Chem. 1993,
450, 121-124.
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Yamagashira, M.; Hashimoto, H.; Kanehisa, N.; Kai, Y. Organome-
tallics 1998, 17, 3945-3956.
indenyl), 7.33 (dd, J H,H ) 5.9 Hz, 2H, indenyl), 6.88 (m, 2H indenyl),
3
6.80 (m, 2H indenyl), 6.33 (dd, 2H, indenyl), 4.94 (sp, J H,H ) 3.3 Hz,
2H, SiH), 2.99 (s, 8H, THF I), 1.24 (s, 8H, THF II), 0.51 (d, 3J H,H ) 3.3
Hz, 6H, SiMe I), 0.44 (s, 3J H,H ) 3.3 Hz, 6H, SiMe II). 13C NMR (C6D6,
25 °C): δ 134.9, 131.8, 127.3, 121.9, 121.7, 118.4, 99.7, 99.2, 97.6, 69.4,
25.3, 1.3, -1.2, -2.5. Although well-shaped crystals of complex 5 could
be obtained, attempts to mount the crystals for diffraction studies were
frustrated by the fact that they readily redissolved upon warming to
ambient temperature. For the synthesis of the N-MeIm and TMEDA
derivatives 10 and 11 a slight excess of the free donor ligand was added
to the reaction mixtures.
(6) (a) Knjazhanski, S. Ya.; Nomerotsky, I. Yu.; Bulychev, B. M.;
Belsky, V. K.; Soloveichik, G. L. Organometallics 1994, 13, 2075-2078.
(b) Knjazhanski, S. Ya.; Kalyuzhnaya, E. S.; Elizalde Herrera, L. E.;
Bulychev, B. M.; Khvostov, A. V.; Sizov, A. I. J . Organomet. Chem.
1997, 531, 19-25. (c) Knjazhanski, S. Ya.; Elizalde Herrera, L. E.;
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Y. Macromolecules 1998, 31, 8650-8652. (b) Zhang, Y.; Hou, Z.;
Wakatsuki, Y. Macromolecules 1999, 32, 939-941.
(15) The ytterbocene(II) complexes reported here can also be syn-
thesized according to our extended silylamide route starting from Yb-
[N(SiHMe2)2]2(THF)x. The latter complex constitutes the preferred
synthetic precursor compound to ansa-ytterbocene(II) complexes
(Klimpel, M. G.; Anwander, R. A., 12. Tage der Seltenen Erden,
December 3-5, 1999, Hamburg, Germany).
(8) Boffa, l. S.; Novak, B. M. Macromolecules 1994, 27, 6993-6995.
(9) For
a recent review, see: Brintzinger, H.-H.; Fischer, D.;
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1255-1283; Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(16) Anwander, R. Top. Organomet. Chem. 1999, 2, 1-61.
10.1021/om000324w CCC: $19.00 © 2000 American Chemical Society
Publication on Web 10/12/2000

Communications
Sch em e 1. Syn th esis of Ytter biu m In d en yl
Organometallics, Vol. 19, No. 23, 2000 4667
Com p lexes via Silyla m in e Elim in a tion
F igu r e 2. Eyring plot for complexes 5 ([), 10 (9), and 11
(2) (cf. Table 1).
Ta ble 1. Activa tion P a r a m eter s of Com p ou n d s 5,
10, a n d 11 Obta in ed fr om Lin e Sh a p e An a lysis17,a
Tc
(K) (kcal mol-1
∆Gq(T)
∆Hq (kcal ∆Sq (cal
complexb
)
mol-1 K-1 mol-1
)
)
(R-Ind)2Yb(THF)2 (5)
313 15.4 ( 1.6 19.9 ( 0.8 14.1 ( 2.4
(R-Ind)2Yb(N-MeIm)2 (10) 318 15.7 ( 1.4 20.0 ( 0.7 13.4 ( 2.4
(R-Ind)2Yb(TMEDA) (11) 343 17.1 ( 1.6 19.8 ( 0.8 7.9 ( 2.1
a
NMR spectra recorded on a J EOL J NM-GX-270 instrument.
b
R-Ind ) 1-(SiHMe2)C9H6.
F igu r e 1. Selected resonances of the variable-temperature
400 MHz 1H NMR spectra for 5 (toluene-d8, coalescence
temperature 40 °C).
Table 1) support a dissociative mechanism, involving
initial separation of a donor ligand (e.g., THF) followed
by a hindered rotation of the indenyl ligands about the
Yb-centroid (Cp) axis.
After recrystallization from toluene/THF, the room-
1
temperature H NMR spectra of these complexes show
The line shape treatment led to a free energy param-
eter ∆Gq of 15.4 ( 1.6 kcal/mol for complex 5. The high
coalescence temperature of 80 °C of the SiHPh2-sub-
stituted derivative 6 excluded an adequate line shape
analysis. Surprisingly, the activation enthalpy and en-
tropy barriers ∆Hq and ∆Sq, respectively, are markedly
higher than expected for an exclusive rotation of the
1-functionalized indenyl ligand about the Yb-centroid
(Cp) axis. For comparison, a ∆Gq rotation of 8.3 kcal/
mol (-70 °C) and 13.1 kcal/mol (-27.5 °C) were obtained
for bulky U(C8H6tBu2-1,4) and Fe(C5H3tBu2-1,3),18 while
the energy barrier ∆Hq for the cyclopentadienyl ring
rotation in Fe(Cp)(C5H4CMe2Et) was determined as 1.39
( 0.23 kcal/mol.19 However, a dissociation enthalpy of
20 kcal/mol for complex 5 seems to be far too low for
indenyl dissociation according to a classical epimeriza-
tion process. For comparison, the dissociation enthalpy
for the cyclopentadienyl ligands in Yb(Cp)3 derived from
a mass spectrometric study is ca. 55 kcal/mol.20 There-
fore, we postulate the following scenario. Generation of
a sterically unsaturated transition state [(R-Ind)2Yb-
(THF)x, x < 2] due to donor ligand displacement should
require less than 10 kcal/mol (cf., the bond disruption
enthalpy for the Sm(II)-THF bond was determined as
7.3 kJ /mol from a iodonolytic titration calorimetry21).
Subsequently, the decreased steric crowding of the
the presence of two isomers in the ratio of 2:1 (5) and
3:2 (6), readily detectable by their SiH signals in ben-
zene-d6 and tetrahydrofuran-d8 (δSiH: 5, 5.14-4.94; 6,
6.02-5.88). Signal integrals indicate an R-Ind:THF ratio
of 1:1, consistent with the composition established by
microanalysis. The SiH stretching vibrations were de-
tected at 2081 (5) and 2111 (6) cm-1. A variable-tem-
perature NMR study in toluene-d8 revealed the coale-
scense of these doubled sets of signals at elevated tem-
perature depending on the steric bulk of the silyl sub-
stituents. This dynamic behavior is representatively
shown for complex 5 in Figure 1, reinforcing the role of
the SiH moiety as a powerful NMR spectroscopic probe.
The importance of a silyl group in the 1-position for the
occurrence of two complex isomers at ambient temper-
1
ature was further shown by the H NMR spectra of [1-
(SiHMe2)-2-Me-C9H5]2Yb(THF)2 (7), (C9H7)2Yb(THF)2
(8), and [2-Me-C9H6]2Yb(THF)2 (9), synthesized analo-
gously. While the latter two complexes do not show any
signal coalescence in toluene-d8 within the temperature
range of -100 to 90 °C, the 1,2-substituted derivative
7, like the complexes 5 and 6, diplays two signals in
the ratio of 1:1 at ambient temperature, which coalesce
at 50 °C.
Such a fluxional behavior can alternatively be inter-
preted as a pseudo rac/meso isomerism or an intercon-
version of rotational isomers. To get more information
about this isomerization process, we performed a line
shape analysis according to the procedures of Shanan-
Atidi and Piette/ Anderson for unequally populated
isomers.17 The Eyring plots and the activation param-
eters ∆Hq, ∆Gq, and ∆Sq derived herefrom (Figure 2,
(17) (a) Shanan-Atidi, H.; Bar-Eli, U. H. J . Phys. Chem. 1970, 74,
961-963. (b) Piette, L. H.; Anderson, W. A. J . Chem. Phys. 1959, 30,
899-908.
(18) Luke, W. D.; Streitwieser, A., J r. J . Am. Chem. Soc. 1981, 103,
3241-3243.
(19) Mann, B. E.; Spencer, C.; Taylor, B. F.; Yavari, P. J . Chem.
Soc., Dalton Trans. 1984, 2027-2028.

4668 Organometallics, Vol. 19, No. 23, 2000
Communications
Sch em e 2. P ossible In d en yl-In d en yl Liga n d
Tr a n sp osition s
a
metal center favors the hindered rotation of the indenyl
ligands about the Yb-centroid (Cp) vector. The moder-
ately positive activation entropies ∆Sq are in good
agreement with an increased particle number in the
transition state, i.e., donor dissociation. To prove the
importance of initial donor dissociation, a variable-
temperature NMR study of complex 5 was also per-
formed in a mixture of toluene-d8/tetrahydrofuran-d8
(ca. 3:1): the two well-separated SiH signals (δ 5.01,
4.76) did not coalesce below 60 °C. Further evidence for
such a dissociative mechanism was obtained from the
activation entropies of complexes [1-(SiHMe2)C9H6]2Yb-
(N-MeIm)2 (10) and [1-(SiHMe2)C9H6]2Yb(TMEDA)2 (11)
synthesized from complex 5 and the corresponding
stronger donating and chelating ligands 1-methylimi-
dazole and TMEDA, respectively. Accordingly, the TME-
DA derivative exhibits a significantly smaller activation
entropy (Table 1) corresponding to the dissociation of
one donor moiety in the transition state.
b
MM2 force field calculations employing nonsubsti-
tuted (Ind)2Yb(THF)2 as the starting geometry10d are
also supportive of such a rotational process. The steric
energies of the interconversion of four highly symmetric
rotational isomers of the [1-(SiHMe2)C9H6]2Yb(THF)2
fragment were determined (Scheme 2 ; the metal center
and the THF molecules are not shown for clarity). The
rotational interconversions I f II and III f IV display
F igu r e 3. (a) Potential surface for the isomerization
process I f II. (b) Top and side views of the calculated
molecular structures of low-energy isomers “20°/180°” and
“0°/340°”. These isomers are indicated on the potential
surface in dark color.
elevated temperature in noncoordinating solvents. Al-
though this dynamic behavior cannot be unambigeously
established at this time, a reasonable fluxional process
involving donor ligand dissociation followed by hindered
rotation of the indenyl ligand is suggested. A similar
fluxional behavior in unbridged 2-arylindene zirconocene
complexes is exploited for the synthesis of elastomeric
polypropylene (“Waymouth system”).23 Finally we have
demonstrated that SiH-decorated silyl groups are valu-
able spectroscopic probes.
one global maximum and several minima [∆Esteric
-
(minima) 1-2 kcal/mol]. Figure 3 shows the potential
surface of the I f II interconversion along with the ball-
and-stick drawings of two minimum rotational isomers.
Interestingly, the high energies of the maxima do not
originate from steric hindrance between the two silyl
groups but from close contacts between silyl groups and
THF donor molecules.22
These preliminary results show that bis(indenyl)
metallocene complexes of the divalent rare earth metals
can be straightforwardly synthesized according to the
silylamide route in high yields and purity. Due to the
steric crowding of 1-functionalized silylindenyl com-
plexes, two isomers are formed which coalesce at
Su p p or tin g In for m a tion Ava ila ble: Full experimental
details for ligands 1, 2, and 3 and complexes 5-11; steric
energies for rotational isomers I-IV. This material is available
free of charge via the Internet at http://pubs.acs.org.
(20) Thomas, T. J .; Hayes, R. G. J . Organomet. Chem. 1970, 23,
OM000324W
487-489.
(21) Nolan, S. P.; Stern, D.; Marks, T. J . J . Am. Chem. Soc. 1989,
111, 7844-7853.
(23) (a) Coates, G. W.; Waymouth, R. M. Science 1995, 267, 217-
219. (b) Witte, P.; Lal, T. K.; Waymouth, R. M. Organometallics 1999,
18, 4147-4155.
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