Synthesis and first structural characterization of lanthanide(II) aryls: Observation of a Schlenk equilibrium in europium(II) and ytterbium(II) chemistry [8]

Full text of DOI:10.1021/ja993494c

J. Am. Chem. Soc. 2000, 122, 4227-4228
4227
in depth, structural data is not available yet. The simplest
ytterbium diaryl YbPh2 is accessible by transmetalation from
HgPh2 and activated Yb metal in THF.17 However, this compound
was characterized only very poorly, and its true composition,
presumably YbPh2(thf)4, is not known.
Synthesis and First Structural Characterization of
Lanthanide(II) Aryls: Observation of a Schlenk
Equilibrium in Europium(II) and Ytterbium(II)
Chemistry
In this paper the synthesis and characterization of the m-
terphenyl derivatives Yb(Dpp)I(thf)3 (1b) and Eu(Dpp)2(thf)2 (3a)
(Dpp ) 2,6-Ph2C6H3) are described. In addition to the solid-state
structures of 1b and 3a, which appear to be the first structurally
authenticated lanthanoid(II) aryls, the crystal structure of EuI2-
(thf)5 (4a) is reported. Compounds 1b and 3a were obtained from
reactions of DppI with lanthanoid metals in THF.18 In the case
of the ytterbium derivative the expected monosubstituted ytter-
bium aryl 1b is isolated in good yield. Going to europium
surprisingly affords the disubstituted lanthanoid aryl 3a and
europium diodide as main products. Apparently the different
solubility of the species present in solution is responsible for the
different shift of the Schlenk-like equilibrium. This is consistent
with the isolation of the ytterbium diaryl 3b by changing the
solvent from THF to toluene.18
Gernot Heckmann and Mark Niemeyer*
Institut fu¨r Anorganische Chemie
UniVersita¨t Stuttgart
Pfaffenwaldring 55, D-70569 Stuttgart, Germany
ReceiVed September 28, 1999
The use of lanthanoid(II) reagents in organic synthesis has
developed considerably over the last two decades.1,2 Most of the
attention has been given to the unique reducing agent samarium-
(II) iodide and to other mainly π-bonded samarium species.
However, there is a growing interest in synthetic applications of
divalent σ-bonded organolanthanoid complexes of the composition
RLnX (Ln ) Sm, Eu, Yb; X ) Br, I). The first compounds of
this type were obtained almost thirty years ago by Evans and
co-workers from organic iodides and lanthanoid metals in THF.3
It was found later that these reagents show some unique reactivity
toward various electrophiles4-7 Unfortunately, little characteriza-
tion of such intermediates has been performed. As a consequence,
only limited information8,9 is available on the structure of these
molecules or the exact nature of the reactive species (type 1-5
see below) which may be present in solution.
2 Ln(Dpp)I(thf)x h Ln(Dpp)2(thf)2 + LnI2(thf)4/5
(1)
1a,b
3a,b
4a,b
Ln ) Eu (a), Yb (b)
In the case of the ytterbium compound it is possible for the
first time to detect all three species which are present in THF
solution, using 171Yb and 13C NMR spectroscopy as a probe.
Therefore, the 171Yb NMR spectrum20 of 1b in THF-d8 solution
shows a main signal at 677 ppm. Smaller resonances of equal
height at 927 and 457 ppm are assigned to the diaryl species Yb-
(Dpp)2(thf)2 3b and solvated ytterbium(II) iodide YbI2(thf)4 4b,21
(14) van den Hende, J. R.; Hitchcock, P. B.; Holmes, S. A.; Lappert, M.
F.; Tian, S. J. Chem. Soc., Dalton Trans. 1995, 3933-3939.
By using very bulky silylated alkyl groups as ligands, dimeric
complexes [Yb{C(SiMe3)2(SiMe2R′)}I(OEt2)]2 (R′ ) Me, CHd
CH2, OMe) of type 2 were synthesized and structurally character-
ized only recently by Smith and co-workers.10,11 They also
reported the crystal structures of two donor-free lanthanide alkyls
Ln{C(SiMe3)3}2 (Ln ) Eu, Yb)11 which are bent in the solid
state.10 Very recently the structural characterization of a solvated
samarium(II) alkyl12 was published. In addition, several solvated
ytterbium(II) dialkyls have been characterized by NMR spec-
troscopy.13,14 Much less structural information is available for
lanthanide(II) aryls.15 The synthesis of perfluorated ytterbium(II)
diaryls, in particular Yb(C6F5)2(thf)4 was pioneered by Deacon
et al.16 Although the reactivity of these compounds was studied
(15) For the structure of a mixed-valent Yb(II/III) aryl see: Bochkarev,
M. N.; Khramenkov, V. V.; Rad’kov, Y. F.; Zakharov, L. N.; Struchkov, Y.
T. J. Organomet. Chem. 1992, 429, 27-39.
(16) Deacon, G. B.; Raverty, W. D.; Vince, D. G. J. Organomet. Chem.
1977, 135, 103-114.
(17) Starostina, T. A.; Shifrina, R. R.; Rybakova, L. F.; Petrov, E. S. J.
Gen. Chem. USSR 1987, 57, 2148-2148.
(18) All manipulations were carried out under strictly anaerobic and
anhydrous conditions using argon as inert atmosphere. (a) 1b‚(thf)0.5: Yb chips
(0.93 g, 5.37 mmol) were added at ambient temperature to a stirred solution
of DppI19 (1.42 g, 4.0 mmol) in THF (60 mL). After an induction period of
several minutes a color change to orange was observed. Stirring was continued
for 1 h, whereupon the resulting red-brown solution was carefully decanted
from the excess of Yb metal. The volume of the solution was reduced to
incipient crystallization under reduced pressure. Storage in a -30 °C freezer
overnight afforded 1b‚(thf)0.5 as thin orangered needles. yield 2.28 g (2.92
mmol, 73%); mp 110-125 °C (dec, crystals gradually turn black); NMR data
in THF-d8: values for 3b are given in brackets 1H NMR δ 1.77 (m, OCH2CH2,
14H), 3.63 (m, OCH2CH2, 14H), 6.97-7.78 (m, aryl-H, 13H); 13C NMR δ
26.4 (OCH2CH2), 68.2 (OCH2CH2), 124.4 [124.2] (m-C6H3), 125.0 [125.2]
(p-C6H3), 126.0 [126.1] (p-Ph), 128.2 [128.1] (o-Ph), 130.4 [129.9] (m-Ph),
151.5 [150.7] (i-Ph), 152.1 [151.5] (o-C6H3), 199.4 [201.4] (i-C6H3). The
assignment is based on 13C DEPT experiments and comparison to coupled
13C spectra of DppH and DppI. 171Yb NMR δ 677 [927] (w1/2 ) 8 Hz [12
Hz]). (b) Isolation of 3b:1b was treated with a 20:1 mixture of toluene and
THF. Storage of the filtered solution in a -60 °C freezer gave tiny red rhombi
of 3b in ∼10% yield. According to a preliminary X-ray structural study 3b is
essentially isostructural to 3a. (c) 3a‚(thf): The synthesis was accomplished
in a manner similar to that of the preparation of 1b with use of an Eu ingot
(4.8 g, 31.6 mmol) and DppI (1.07 g, 3.0 mmol). Crystallization from THF
yielded a mixture of orange (3a) and yellow (4a) crystals, which proved
difficult to separate. Further experimental details and additional spectroscopic
data will be reported in the full paper. Owing to the lability of the coordinated
and cocrystallized THF ligands satisfactory elemental analytical data for 1b,
3a, 3b, and 4a could not be obtained.
(1) Imamoto, T. Lanthanides in Organic Synthesis; Academic Press:
London, 1994.
(2) Kobayashi, S. Lanthanides: Chemistry and Use in Organic Synthesis;
Springer: Berlin, 1999.
(3) Evans, D. F.; Fazakerley, G. V.; Phillips, R. F. J. Chem. Soc. A 1971,
1931-1934.
(4) Jin, W.-S.; Makioka, Y.; Kitamura T.; Fujiwara, Y. Chem. Commun.
1999, 955-956.
(5) Rybakova, L. F.; Syutkina, O. P.; Novgorodova, M. N.; Petrov, E. S.
Russ. J. Gen. Chem. 1999, 69, 85-87.
(6) Syutkina, O. P.; Rybakova, L. F.; Petrov, E. S.; Beletskaya, I. P. J.
Organomet. Chem. 1985, 280, C67-C69
(7) Fukuzawa, S.; Fujinami, T.; Sakai, S. J. Chem. Soc., Chem. Commun.
1986, 475-476.
(8) Bochkarev, M. N.; Zakharov, L. N.; Kalinina, G. S. OrganoderiVatiVes
of Rare Earth Elements; Kluwer Academic: Dordrecht, 1995.
(9) Cotton, S. A. Coord. Chem. ReV. 1997, 160, 93-127.
(10) Eaborn, C.; Hitchcock, P. B.; Izod, K.; Smith, J. D. J. Am. Chem.
Soc. 1994, 116, 12071-12072.
(19) Niemeyer, M. Organometallics 1998, 17, 4649-4656.
(11) Eaborn, C.; Hitchcock, P. B.; Izod, K.; Lu, Z.-R.; Smith, J. D.
Organometallics 1996, 15, 4783-4790.
(12) Clegg, W.; Eaborn, C.; Izod, K.; O’Shaughnessy, P.; Smith, J. D.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2815-2817.
(13) Hitchcock, P. B.; Holmes, S. A.; Lappert, M. F.; Tian, S. J. Chem.
Soc., Chem. Commun. 1994, 2691-2692.
(20) Keates, J. M.; Lawless, G. A. AdVanced Applications of NMR to
Organometallic Chemistry; Gielen, M., Willem, R., Wrackmeyer, B., Eds.;
Wiley: Chichester, 1996; pp 357-370.
(21) van den Hende, J. R.; Hitchcock, P. B.; Holmes, S. A.; Lappert, M.
F.; Leung, W. P.; Mak, T. C. W.; Prashar, S. J. Chem. Soc., Dalton Trans.
1995, 1427-1433.
10.1021/ja993494c CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/13/2000

4228 J. Am. Chem. Soc., Vol. 122, No. 17, 2000
Communications to the Editor
Figure 2. Molecular structure of 3a, showing the numbering scheme.
Hydrogen atoms and the cocrystallized THF molecule have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): Eu-C(1) 2.606-
(4), Eu-C(19) 2.623(4), Eu‚‚‚C(8) 3.218(4), Eu‚‚‚C(26) 3.153(4), Eu-
O(41) 2.501(3), Eu-O(51) 2.494(4), C(1)-Eu-C(19) 135.3(1), O(41)-
Eu-O(51) 77.8(1), Eu-C(1)-C(2) 112.2(3), Eu-C(1)-C(6) 120.6(3),
Eu-C(19)-C(20) 111.9(3), Eu-C(19)-C(24) 129.5(3).
Figure 1. Molecular structure of 1b, showing the numbering scheme.
Hydrogen atoms and the cocrystallized THF molecule have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): Yb-C(1) 2.529-
(4), Yb‚‚‚C(18) 3.104(5), Yb-I 3.1148(5), Yb-O(21) 2.424(3), Yb-
O(31) 2.377(4), Yb-O(41) 2.463(4), C(1)-Yb-I 106.7(1), Yb-C(1)-
C(2) 126.5(3), Yb-C(1)-C(6) 116.5(3).
different Yb-C(1)-C(2) and Yb-C(1)-C(6) angles of 126.5-
(3)° and 116.5(3)° are observed.
respectively. All three signals are relatively sharp with line widths
<15 Hz, there is no evidence for an additional monomer dimer
equilibrium. The presence of a genuine reversible equilibrium (eq
1) which is accessible from both sides is demonstrated by further
NMR experiments. Therefore, adding YbI2 to a solution of 3b in
THF generates 1b again. Moreover, addition of an excess of YbI2
to either 1b or 3b shifts the equilibrium to the left and suppresses
the formation of 3b (see Supporting Information). The 13C NMR
of 1b at ambient temperature shows the presence of 1b and 3b
in a proportion of 67 and 33%, respectively. Due to its limited
solubility the quota of 1b decreases to 40% at 263 K and 30% at
233 K. This is in accordance with the isolation of pure 1b by
crystallization at low temperatures. If the isolated mother liquor,
which is enriched in 3b, is rewarmed to ambient temperature,
the original 1b/3b ratio is observed again. It should be noted that
1b is thermally labile in THF solution and slowly decomposes
with a half-time of ∼1 d at 293 K.
The molecular structure of Eu(Dpp)2(thf)2 3a is depicted in
Figure 2. The Eu center shows a distorted tetrahedral coordination
by two aryl ligands and two THF molecules. The different size
of the ligands is clearly reflected by the rather distinct C(1)-
Eu-C(19) and O(41)-Eu-O(51) angles of 135.3(1)° and 77.8-
(1)°. With a value of 2.615 Å the average Eu-C distance is close
to the average bond length in two-coordinate Eu{C(SiMe3)3}2
(2.609 Å).11 Apparently a possible shortening in 3a caused by a
different hybridization of the σ-bonded carbon atom is compen-
sated by the lower coordination number in the dialkyl. The average
Eu-O(thf) distance of 2.498 Å is in the expected range. A notable
feature is the considerable displacement of the Eu atom from the
C(1)-C(6) and C(19)-C(24) aromatic planes by 1.503 and 0.790
Å. This can be interpreted in terms of the high ionic character of
the metal-carbon bonds and steric crowding in the molecule.
As in the case of 1b additional metal-η1-π-arene interactions
(Eu‚‚‚C(8) 3.218(4) Å, Eu‚‚‚C(26) 3.153(4) Å) are observed,
which are accompanied by different Eu-C(ipso)-C(ortho) angles.
The second product of the reaction of DppI and Eu metal was
identified by X-ray structural analysis as EuI2(thf)5 (4a). Thermally
labile crystals of 4a, which rapidly decompose above ∼0 °C, are
isomorphous to the Sm derivative SmI2(thf)5. The structure of
the latter compound was established some years ago,28 although
the refinement converged at the high conventional R value of
15%. However, in the case of 4a it was possible to obtain
reasonably accurate bond parameters29 for the pentagonal-
bipyramidal-coordinated Eu atom. Reactivity studies of 1b, 3a,
and 3b are in hand.
In the solid state22 1b consists of well-separated monomeric
units as shown in Figure 1. The ytterbium atom is bound to the
ipso-carbon atom of the terphenyl group, to the terminal iodo
substituent, and to three THF molecules. The Yb-C(1) distance
of 2.529(4) Å is the first experimentally determined value for a
Yb(II)-C(sp2) σ-bond. As expected for the different oxidation
state, this distance is ∼0.12 Å longer than the corresponding bond
length in Yb(III)-C(aryl) compounds.23-25 Similar distances are
found in Yb(II) alkyls, which cover a range of 2.47 to 2.58 Å.9
The Yb-I distance of 3.1148(5) Å may be compared with the
Yb-I bond length in hexa-coordinate YbI2(thf)4 4b which was
determined to be 3.103(1) Å.21 The average Yb-O(thf) bond
length in 1b (2.421 Å) is slightly longer than the distance (av.
2.386 Å) found in 4b. The distorted tetragonal pyramidal
coordination in 1b leaves room for an additional weak η1-π-
arene interaction26,27 (Yb‚‚‚C(18) 3.104(5) Å) to one o′-phenyl
carbon atom of the terphenyl ligand. As a consequence quite
Supporting Information Available: 171Yb NMR spectra of 1b and
3b, and X-ray structural information on 1b, 3a, and 4a (PDF). X-ray
crystallographic files (CIF) for 1b, 3a, and 4a. This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) For crystallographic data see Supporting Information.
(23) Bochkarev, L. N.; Zheleznova, T. A.; Safronova, A. V.; Drozdov, M.
S.; Zhiltsov, S. F.; Zakharov, L. N.; Fukin, G. K.; Khorshev, S. Y. Russ.
Chem. Bull. 1998, 47, 165-168.
JA993494C
(27) Deacon, G. B.; Forsyth, C. M.; Junk, P. C.; Skelton, B. W.; White,
A. H. Chem. Eur. J. 1999, 5, 1452-1459.
(24) Niemeyer, M.; Hauber, S.-O. Z. Anorg. Allg. Chem. 1999, 625, 137-
140.
(28) Evans, W. J.; Gummersheimer, T. S.; Ziller, J. W. J. Am. Chem. Soc.
1995, 117, 8999-9002.
(25) Rabe, G. W.; Strissel, C. S.; Liable-Sands, L. M.; Concolino, T. E.;
Rheingold, A. L. Inorg. Chem. 1999, 38, 3446-3447.
(26) Deacon, G. B.; Shen, Q. J. Organomet. Chem. 1996, 506, 1-17.
(29) Eu-I: 3.222(2)-3.254(2) Å (av. 3.235 Å); Eu-O: 2.587 Å (av);
I-Eu-I: 178.0° (av)