Synthesis and structure of tris(alkyl- and silyl-tetramethylcyclopentadienyl) complexes of lanthanum

Article abstract of DOI:10.1021/ic010554i

In efforts to make sterically crowded tris(peralkylcyclopentadienyl) complexes of lanthanum for the exploration of sterically induced reduction chemistry with a diamagnetic system, the synthesis of (C₅Me₄R)₃La complexes has been pursued with R = Me, Et, ⁱPr, and SiMe₃. The complexes were synthesized in four steps: reaction of LaCl₃ with KC₅Me₄R to form (C₅Me₄R)₂LaCl₂K(THF)₂, addition of allylmagnesium chloride to make (C₅Me₄R)_2-La(C₃H₅), protonolysis with Et₃NHBPh₄ to make [(C₅Me₄R)₂La][BPh₄], and finally the replacement of BPh₄^- with C₅Me₄R^- using KC₅Me₄R to make (C₅Me₄R)₃La. X-ray crystallographic data were obtainable on the (C₅Me₄R)₃La complexes for R = Me, Et, ⁱPr, and SiMe₃. In each complex, the three C₅Me₄R ring centroids define a trigonal planar geometry around La. The average La-(ring centroid) distances are 2.64, 2.65, 2.66, and 2.69 A for the Me, Et, ⁱPr, and SiMe₃ structures, respectively, with La-C distances ranging from 2.857 (3) to 3.029 (2) A. Despite the steric crowding, ligand exchange can be observed by NMR spectroscopy.

Full text of DOI:10.1021/ic010554i

Volume 40
Number 25
December 3, 2001
© Copyright 2001 by the American Chemical Society
Ar ticles
Synthesis and Structure of Tris(alkyl- and silyl-tetramethylcyclopentadienyl) Complexes of
Lanthanum
William J. Evans,* Benjamin L. Davis, and Joseph W. Ziller
Department of Chemistry, University of California, Irvine, California 92697-2025
ReceiVed May 25, 2001
In efforts to make sterically crowded tris(peralkylcyclopentadienyl) complexes of lanthanum for the exploration
of sterically induced reduction chemistry with a diamagnetic system, the synthesis of (C5Me4R)3La complexes
i
has been pursued with R ) Me, Et, Pr, and SiMe3. The complexes were synthesized in four steps: reaction of
LaCl3 with KC5Me4R to form (C5Me4R)2LaCl2K(THF)2, addition of allylmagnesium chloride to make (C5Me4R)2-
-
La(C3H5), protonolysis with Et3NHBPh4 to make [(C5Me4R)2La][BPh4], and finally the replacement of BPh4
with C5Me4R- using KC5Me4R to make (C5Me4R)3La. X-ray crystallographic data were obtainable on the
i
(C5Me4R)3La complexes for R ) Me, Et, Pr, and SiMe3. In each complex, the three C5Me4R ring centroids
define a trigonal planar geometry around La. The average La-(ring centroid) distances are 2.64, 2.65, 2.66, and
i
2.69 Å for the Me, Et, Pr, and SiMe3 structures, respectively, with La-C distances ranging from 2.857 (3) to
3.029 (2) Å. Despite the steric crowding, ligand exchange can be observed by NMR spectroscopy.
Introduction
Although the Sm(III), Nd(III), and U(III) ions in the
(C5Me5)3M complexes synthesized thus far are paramagnetic,
NMR spectroscopy has proven to be useful in characterizing
the existence of new products and new patterns of tris-
(cyclopentadienyl) reactivity. However, definitive characteriza-
tion of the reaction products by NMR spectroscopy was limited
by the paramagnetism, and ultimate identification of the reaction
products relied on X-ray crystallography. The paramagnetism
also precluded some types of NMR-based reactivity studies. In
efforts to obtain a diamagnetic system more amenable to NMR
spectroscopic analysis, we have attempted to synthesize steri-
cally crowded tris(peralkylcyclopentadienyl) complexes of La3+.
Since lanthanum is the largest of the lanthanides,10 it was
expected that the complex (C5Me5)3La should be sterically
accessible. However, since the high reactivity of the (C5Me5)3M
complexes prepared to date is correlated with steric crowding,
it was not clear that (C5Me5)3La, with its larger metal, would
be crowded enough to have the interesting chemistry of the other
(C5Me5)3M complexes. Previously, it had been found that the
Recent studies of the chemistry of tris(pentamethylcyclopen-
tadienyl) complexes of samarium,1-4 neodymium,5,6 and ura-
nium7,8 have shown that the extreme steric crowding present in
these molecules can lead to high reactivity of two general types.
With certain substrates, the (C5Me5)3M complexes behave like
bulky alkyl compounds in which one of the rings displays η1
behavior, i.e., (η5-C5Me5)2M(η1-C5Me5), and insertion chemistry
is observed. With other substrates, reduction chemistry is found,
which can be explained by a C5Me5-/C5Me5 redox couple as
shown in eq 1.
(C5Me5)3M f e- + [(C5Me5)2M]+ + 1/2(C5Me5)2 (1)
Since this type of reactivity apparently derives from the steric
crowding, it has been called sterically induced reduction.9
(1) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1995,
117, 12635-12636.
(2) Evans, W. J.; Forrestal, K. J.; Ansari, M. A.; Ziller, J. W. J. Am. Chem.
Soc. 1998, 120, 2180-2181.
(3) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 9273-9282.
(4) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1991,
113, 7423-7424.
(7) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Angew. Chem., Int. Ed. 2000,
39, 240-242.
(5) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 6745-6752.
(8) Evans, W. J.; Nyce, G. W.; Johnston, M. A.; Ziller, J. W. J. Am. Chem.
Soc. 2000, 122, 12019-12020.
(6) Evans, W. J.; Nyce, G. W.; Clark, R. D.; Doedens, R. J.; Ziller, J. W.
Angew. Chem., Int. Ed. 1999, 38, 1801-1803.
(9) Evans, W. J. Coord. Chem. ReV. 2000, 206, 263-283.
(10) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
10.1021/ic010554i CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/30/2001

6342 Inorganic Chemistry, Vol. 40, No. 25, 2001
Evans et al.
ansa complex Me2Si(C5Me4)2Sm(C5Me5),11 which is less crowded
than (C5Me5)3Sm, has none of the alkyl or reduction reactivity
of (C5Me5)3Sm. In addition, the less-crowded (C5Me5)3Nd was
found to be a weaker reductant than (C5Me5)3Sm.6
(C5Me4SiMe3)2LaCl2K(THF)2, 3. As described for 1, LaCl3 (0.794
g, 3.24 mmol) and K(C5Me4SiMe3) (1.50 g, 6.48 mmol) were reacted
to yield 3 (1.55 g, 62%) as a white solid. 1H NMR (C6D6): δ 3.55 (m,
9 H), 1.90 (s, 6 H), 1.80 (s, 6 H), 1.40 (m, 9 H), -0.03 (s, 9 H). Anal.
Calcd for LaSi2KCl2O2C32H58: La, 17.8. Found: La, 18.1.
For these reasons, we sought to synthesize a series of
(C5Me4R)3La complexes in which R is an alkyl or silyl group
that is larger than methyl. Given the difficulties in synthesizing
and definitively characterizing the first four examples of tris-
(peralkylcyclopentadienyl)metal in the literature, namely,
(C5Me5)3Nd,5 (C5Me5)3U,12 (C5Me5)3Sm,4 and (C5Me4Et)3Sm,13
it was not clear if such (C5Me4R)3La complexes could be ob-
tained, much less crystallized for detailed structural analysis.
We report here the successful synthesis and structural charac-
terization of examples in which the R ) Me, Et, iPr, and SiMe3.
To our knowledge, this is the first fully characterized lanthanide
(C5Me4Et)2La(CH2CHCH2), 4. In a nitrogen glovebox, ClMg(CH2-
CHCH2) (1.35 mL of a 2.2 M solution in THF, 2.97 mmol) was added
to a stirring slurry of (C5Me4Et)2LaCl2K(THF)2 (1.71 g, 2.48 mmol)
in toluene (∼50 mL). The white slurry immediately became a yellow
solution. After 1 h, the solvent was removed by rotary evaporation to
yield a bright yellow solid. This material was triturated with 2%
dioxanes in hexanes (75 mL) and filtered through a coarse frit to yield
an orange solution. After removal of the solvent, the yellow solid was
dried under high vacuum (1 × 10-5 Torr) for 24 h at 35-45 °C to
remove coordinated THF. The resulting material was extracted with
hexanes to yield 4 (1.12 g, 94%) upon solvent removal. 1H NMR
(C6D6): δ 6.48 (m, CH2CHCH2, 1 H), 3.10 (d, JHH ) 15 Hz, anti-
CH2CHCH2, 2 H), 2.65 (d, JHH ) 9 Hz, syn-CH2CHCH2, 2 H), 2.36
(q, -CH2CH3, 4 H), 1.99 (s, 12 H), 1.90 (s, 12 H), 0.88 (t, -CH2CH3,
6 H).
complex utilizing the C5Me4SiMe3 ligand in the literature.14
-
Experimental Section
The complexes described below are extremely air and moisture
sensitive. Therefore, the syntheses and manipulations of these com-
pounds were conducted under nitrogen or argon with rigorous exclusion
of air and water by Schlenk, vacuum line, and glovebox techniques.
Since the (C5Me4R)3Ln complexes can react with THF, all manipula-
tions involving these complexes were done under THF-free conditions.
All reaction chemistry was done in glassware silylated using Siliclad
(Gelest) diluted to 1% in deionized water. Solvent drying and physical
measurements have been described previously.15,16 Anhydrous lantha-
num trichloride was used as received from Strem Chemicals. C5Me4-
i
i
(C5Me4 Pr)2La(CH2CHCH2), 5. As described for 4, (C5Me4 Pr)2-
LaCl2Li(THF)2 (1.94 g, 2.83 mmol) was reacted with ClMg(CH2-
CHCH2) (1.6 mL of a 2.2 M solution in THF, 2.6 mmol) to yield 5 as
a yellow solid (0.910 g, 64%). 1H NMR (C6D6): δ 6.29 (m, CH2CHCH2,
1 H), 2.88 (d, JHH ) 15 Hz, anti-CH2CHCH2, 2 H), 2.75 (m, -CH-
(CH3)2, 1 H), 2.69 (m, -CH-(CH3)2, 1 H), 2.59 (d, JHH ) 9 Hz, syn-
CH2CHCH2, 2 H), 2.02 (s, 6 H), 1.99 (s, 6 H), 1.77 (s, 6 H), 1.72 (s,
6 H), 0.88 (d, JHH ) 7 Hz, 6 H), 0.83 (d, JHH ) 7 Hz, 6 H). IR: 2957
s, 2922 s, 2864 s, 2725 m, 1544 s, 1444 s, 1382 s, 1258 s, 1239 s,
1100 s, 1019 s, 876 m, 802 s, 776 s cm-1
.
EtH,17 C5Me4 PrH,18 and C5Me4SiMe3H19 were prepared according to
i
(C5Me4SiMe3)2La(CH2CHCH2), 6. As described for 4, (C5Me4-
SiMe3)2LaCl2K(THF)2 (1.554 g, 2.00 mmol) was reacted with ClMg-
(CH2CHCH2) (1.5 mL of a 2.2 M solution in THF, 3.0 mmol) to yield
literature methods. KC5Me4R and LiC5Me4R (R ) Et, iPr, SiMe3) were
prepared by reaction of the respective cyclopentadienes with KH in
THF or 10% excess n-BuLi in hexanes. Allylmagnesium chloride (2.0
M in THF) was used as received from Aldrich. (C5Me5)2La(BPh4) was
prepared according to the literature method.5 1H and 13C NMR spectra
were obtained on a Bruker DRX 400 or Omega 500 MHz spectrometer
at 25 °C. Infrared analyses were acquired as thin films using an Applied
Systems ReactIR 1000 instrument. Elemental analyses were performed
by Analytische Laboratorien, Lindlar (Germany), or by complexometric
titration.20
1
6 (1.12 g, 94%) upon solvent removal. H NMR (C6D6): δ 6.48 (m,
CH2CHCH2, 1 H), 3.16 (d, JHH ) 12 Hz, anti-CH2CHCH2, 2 H), 2.76
(d, JHH ) 8 Hz, syn-CH2CHCH2, 2 H), 2.26 (s, 6 H), 2.20 (s, 6 H),
1.89 (s, 6 H), 1.85 (s, 6 H), 0.21 (s, 9 H), 0.18 (s, 9 H). 13C NMR
(C6D6): δ 151.4, 127.7, 124.3, 124.2, 116.3, 116.0, 76.3, 14.8, 14.6,
11.4, 11.2, 1.6, 1.2. IR: 2953 s, 2922 s, 2856 s, 1544 m, 1477 s, 1324
s, 1247 s, 1092 s, 1019 s, 838 m, 803 m, 753 m, 683 m cm-1
.
[(C5Me4Et)2La][BPh4], 7. In an argon-filled glovebox, 4 (0.145 g,
0.303 mmol) and Et3NHBPh4 (0.193 g, 0.454 mmol) were stirred in
benzene (∼10 mL) for 12 h. Excess Et3NHBPh4 was removed by
centrifugation, yielding a pale yellow solution. Complex 7 was isolated
as a pale yellow solid after removal of the solvent (0.212 g, 92%). 1H
(C5Me4Et)2LaCl2K(THF)2, 1. In a nitrogen glovebox, LaCl3 (0.690
g, 2.81 mmol) and K(C5Me4Et) (1.06 g, 5.63 mmol) were stirred in
THF for 24 h. The resulting white slurry was filtered to yield a pale
yellow solution. The solvent was removed under vacuum to yield 1
(1.02 g, 53%) as a white solid. 1H NMR (C6D6): δ 3.57 (m, 6 H), 2.64
(q, 4 H), 2.22 (s, 12 H), 2.18 (s, 12 H), 1.39 (m, 6 H), 1.18 (t, 6 H).
Anal. Calcd for LaKCl2O2C30H50: La, 20.0. Found: La, 19.6.
NMR (C6D6): δ 7.71 (d, JHH ) 6.8 Hz, m-C6H5, 8 H), 7.21 (t, JHH
)
7.2 Hz, o-C6H5, 8 H), 7.08 (t, JHH ) 7.2 Hz, p-C6H5, 4 H), 2.13 (q, 4
H), 1.66 (s, 12 H), 1.58 (s, 12 H), 0.74 (t, 6 H).
i
(C5Me4 Pr)2LaCl2K(THF)2, 2. As described for 1, LaCl3 (0.439 g,
i
i
i
[(C5Me4 Pr)2La][BPh4], 8. As described for 7, (C5Me4 Pr)2La(CH2-
CHCH2) (0.910 g, 1.80 mmol) was reacted with excess Et3NHBPh4
(0.989 g, 2.337 mmol) to yield 8 as a pale yellow solid (1.409 g, 99%).
1.79 mmol) and K(C5Me4 Pr) (0.724 g, 3.58 mmol) were reacted to
yield 2 (1.13 g, 92%) as a white solid. 1H NMR (C6D6): δ 3.56 (m, 4
H), 3.24 (m, 2 H), 2.30 (s, 14 H), 2.15 (s, 13 H), 1.40 (m, 14 H), 1.35
(m, 4 H). Anal. Calcd for LaKCl2O2C32H54: La, 19.3. Found: La, 19.5.
1H NMR (C6D6): δ 7.73 (d, JHH ) 6 Hz, m-C6H5, 8 H), 7.24 (t, JHH
)
7.6 Hz, o-C6H5, 8 H), 7.13 (t, JHH ) 7.2 Hz, p-C6H5, 4 H), 2.7.6 (m,
2 H), 1.87 (s, 12 H), 1.51 (s, 12 H), 1.01 (d, JHH ) 7.2 Hz, 12 H).
(11) Evans, W. J.; Cano, D. A.; Greci, M. A.; Ziller, J. W. Organometallics
1999, 18, 1381-1388.
[(C5Me4SiMe3)2La][BPh4], 9. As described for 7, (C5Me4SiMe3)2-
La(CH2CHCH2) (0.097 g, 0.17 mmol) was reacted with Et3NHBPh4
(0.094 g, 0.22 mmol) to yield 9 (0.145 g, 95%) as a pale yellow solid.
1H NMR (C6D6): δ 7.71 (d, JHH ) 6.0 Hz, m-C6H5, 8 H), 7.28 (t,
JHH ) 7.0 Hz, o-C6H5, 8 H), 7.13 (t, JHH ) 7.0 Hz, p-C6H5, 4 H), 1.97
(s, 12 H), 1.46 (s, 12 H), 0.23 (s, 18 H). 13C NMR (C6D6): δ 136.1,
134.3, 130.2, 129.4, 126.8, 126.2, 123.44, 15.0, 11.7, 2.7.
(12) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed.
Engl. 1997, 36, 774-776.
(13) Evans, W. J.; Forrestal, K. J.; Leman, J. T.; Ziller, J. W. Organome-
tallics 1996, 15, 527-531.
(14) Constantine, S. P.; Delima, G. M.; Hitchcock, P. B.; Keates, J. M.;
Lawless, G. A. Chem. Commun. 1996, 2421-2422.
(15) Evans, W. J.; Grate, J. W.; Doedens, R. J. J. Am. Chem. Soc. 1985,
107, 1671.
(C5Me5)3La, 10. In an argon-filled glovebox, (C5Me5)2La(BPh4)
(0.938 g, 1.29 mmol) was stirred with KC5Me5 (0.246 g, 1.42 mmol)
in benzene (∼10 mL) for 24 h. Insoluble materials were removed by
centrifugation, and the solvent was removed from the resulting yellow
solution by rotary evaporation to yield 10 (0.552 g, 79%) as a bright
(16) Evans, W. J.; Chamberlain, L. R.; Ulibarri, T. A.; Ziller, J. W. J. Am.
Chem. Soc. 1988, 110, 6423-6432.
(17) Bensley, D. M.; Mintz, E. A. J. Organomet. Chem. 1988, 353, 93-
102.
(18) Forrestal, K. J. Ph.D. Thesis, University of California, Irvine, CA,
1995.
(19) Courtot, P.; Pichon, R.; Salaun, J. Y.; Toupet, L. Can. J. Chem. 1991,
69, 661-672.
(20) Evans, W. J.; Engerer, S. C.; Coleson, K. M. J. Am. Chem. Soc. 1981,
103, 6672-6677.
1
yellow solid. H NMR (C6D6): δ 1.997. 13C NMR (C6D6): δ 122.0,
12.5. IR: 2961 s, 2907 s, 2856 s, 1478 s, 1440 s, 1378 s, 1251 s, 1154
s, 1034 s, 946 m, 926 m, 714 s, 675 m cm-1. Anal. Calcd for
LaC30H45: La, 25.51; C, 66.17; H, 8.32. Found: La, 27.25; C, 64.09;

Synthesis and Structure of Complexes of Lanthanum
Inorganic Chemistry, Vol. 40, No. 25, 2001 6343
i
Table 1. X-ray Data Collection Parameters for (C5Me5)3La, 10, (C5Me4Et)3La, 11, (C5Me4 Pr)3La, 12, and (C5Me4SiMe3)3La, 13
10
11
12
13
formula
fw
space group
crystal system
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C30H45La
544.57
P63/m
hexagonal
10.1019 (4)
C33H51La
586.65
P1h
C36H57La
628.73
C2/c
monoclinic
35.675 (3)
10.2853 (8)
18.7528 (14)
C36H63Si3La•C7H8
811.18
P1h
triclinic
10.2192 (7)
10.2734 (7)
20.4975 (14)
96.1360 (10)
98.3940 (10)
91.8110 (10)
2114.3 (3)
2
triclinic
10.0459 (5)
10.0786 (5)
16.1832 (8)
90.3550 (10)
90.1080 (10)
118.8050 (10)
1435.73 (12)
2
15.5214 (8)
112.2890 (10)
1371.73 (10)
2
6366.9 (8)
8
λ (Å)
0.71073
1.318
1.571
183
0.0206
0.0609
0.71073
1.357
1.506
158
0.0297
0.71073
1.312
1.363
163
0.0355
0.0980
0.71073
1.274
1.123
158
0.0240
F
calcd (Mg/m3)
µ (Mo KR) (mm-1
temp (K)
)
Ra (I > 2σ(I)): R1
Rb (all data): wR2
0.0722
0.0637
a R1 ) ∑||F0| - |Fc||/∑|F0|. b wR2 ) [∑[w(F0 - Fc2)2]/ ∑(w(F0 )2)]1/2
.
2
2
H, 8.12. Single crystals suitable for X-ray diffraction were obtained
by cooling a hot toluene solution to room temperature.
data was collected on complexes 10 and 11; a 30 s/frame scan time
for a hemisphere of data was collected on complex 12, and a 30 s/frame
scan time for a sphere of data was collected on complex 13. The raw
frame data were processed using SAINT22 and SADABS23 to yield the
reflection data file. Subsequent calculations were carried out using the
SHELXTL23 program. The structures were solved by direct methods
and refined on F2 by full-matrix least-squares techniques. The analytical
scattering factors24 for neutral atoms were used throughout the analysis.
Hydrogen atoms were located from a difference Fourier map and refined
(x,y,z and Uiso) or were included using a riding model.
(C5Me5)3La, 10. The systematic absences were consistent with the
hexagonal space groups P63 and P63/m. It was later determined that
the centrosymmetric space group P63/m was correct. The molecule was
located on a site of 6h symmetry. At convergence, wR2 ) 0.0609 and
GOF ) 1.201 for 52 variables refined against 1164 unique data. As a
comparison for refinement on F, R1 ) 0.0206 for those 1133 data
with I > 2.0σ(I).
(C5Me4Et)3La, 11. As described for 10, [(C5Me4Et)2La][BPh4] (0.196
g, 0.26 mmol) was stirred with KC5Me4Et (0.083 g, 0.440 mmol) to
form 11 (0.129 g, 85%). 1H NMR (C6D6): δ 2.53 (q, -CH2CH3, 6 H),
2.15 (s, 14 H), 2.03 (s, 14 H), 1.10 (t, 2 H), 0.91 (t, 7 H). 13C NMR
(C6D6): δ 126.3, 124.2, 122.0, 121.0, 118.5, 117.5, 19.7, 19.5, 15.4,
14.1, 13.8, 12.4, 11.4, 11.3. IR: 2960 s, 2910 s, 2856 s, 2721 m, 1449
s, 1378 s, 1368 s, 1310 s, 1258 s, 1050 s, 1019 s, 972.5 m, 800 m
cm-1. Anal. Calcd for LaC33H51: La, 23.67; C, 67.55; H, 8.76. Found:
La, 22.55; C, 67.85; H, 8.29. Single crystals suitable for X-ray
diffraction were obtained by cooling a hot toluene solution to room
temperature.
i
i
(C5Me4 Pr)3La, 12. As described for 10, [(C5Me4 Pr)2La][BPh4]
i
(0.724 g, 0.93 mmol) was reacted with KC5Me4 Pr (0.225 g, 1.11 mmol)
to yield 12 as a yellow solid (0.499 g, 85%). 1H NMR (C6D6): δ 3.11
(s, JHH ) 7.2 Hz, -CH-(CH3)2, 3 H), 2.29 (s, 17.6 H), 2.27 (s, 0.7 H),
2.07 (s, 0.6 H), 2.02 (s, 17.6 H), 1.26 (d, 0.8 H), 1.15 (d, 17.8 H). 13
C
(C5Me4Et)3La, 11. There were no systematic absences or any
diffraction symmetry other than the Friedel condition. The centrosym-
metric triclinic space group P1h was assigned and later determined to
be correct. At convergence, wR2 ) 0.0722 and GOF ) 1.233 for 511
variables refined against 6754 unique data. As a comparison for
refinement on F, R1 ) 0.0297 for those 6383 data with I > 2.0σ(I).
NMR (C6D6): δ 129.2, 123.8, 121.2, 27.0, 22.0, 14.4, 12.0. IR: 2961
s, 2914 s, 2868 s, 1444 s, 1363 s, 1382 s, 1380 s, 1262 s, 1100 s, 1031
s, 803 s, 745 s, 714 s, 676 s cm-1. Anal. Calcd for LaC36H57: La,
22.09; C, 68.78; H, 9.13. Found: La, 22.05; C, 69.02; H, 8.54. Single
crystals suitable for X-ray diffraction were obtained by cooling a hot
toluene solution to room temperature.
i
(C5Me4 Pr)3La, 12. The diffraction symmetry was 2/m, and the
(C5Me4SiMe3)3La, 13. As described for 10, [(C5Me4SiMe3)2La]-
[BPh4] (0.948 g, 1.12 mmol) reacted with KC5Me4SiMe3 (0.260 g, 1.12
mmol) to yield 13 as a light yellow solid (0.473 g, 59%). H NMR
systematic absences were consistent with the monoclinic space groups
Cc and C2/c. It was later determined that the centrosymmetric space
group C2/c was correct. At convergence, wR2 ) 0.0980 and GOF )
1.111 for 562 variables refined against 7596 unique data. As a
comparison for refinement on F, R1 ) 0.0355 for those 6698 data
with I > 2.0σ(I).
(C5Me4SiMe3)3La, 13. There were no systematic absences or any
diffraction symmetry other than the Friedel condition. The centrosym-
metric triclinic space group P1h was assigned and later determined to
be correct. There was one molecule of toluene solvent present per
formula unit. At convergence, wR2 ) 0.0637 and GOF ) 1.100 for
676 variables refined against 9905 data. As a comparison for refinement
on F, R1 ) 0.0226 for those 9502 data with I > 2.0σ(I).
1
(C6D6): δ 2.36 (s, 18 H), 2.01 (s, 18 H), 0.36 (s, 27 H). 13C NMR
(C6D6): δ 134.0, 126.2, 118.4, 17.4, 13.0, 3.1. IR: 2953 s, 2914 s,
2860 s, 1444 s, 1324 s, 1247 m, 1127 s, 1019 m, 984 s, 837 s, 753 m,
676 m cm-1. Anal. Calcd for LaSi3C36H63: La, 19.32; Si, 11.72; C,
60.14; H, 8.23. Found: La, 19.50; Si, 10.15; C, 61.60; H, 8.63. Single
crystals suitable for X-ray diffraction were obtained by cooling a hot
toluene solution to room temperature.
Exchange Reaction of 10 and 13. Complexes 10 (0.0065 g, 0.013
1
mmol) and 13 (0.0096 g, 0.012 mmol) were dissolved in C6D6. H
NMR spectroscopy indicated no reaction after mixing. After 12 h, two
new sets of peaks were observed, set A at δ 2.327, 2.050, 1.984, and
0.329 ppm and set B at 2.314, 2.020, 1.934, and 0.307 ppm, along
with the starting materials. Six days of reaction left 5% of 10 and 10%
of 13. 1H NMR spectroscopy indicated a 1.6:1 ratio of A:B. See Figure
6 in Supporting Information for a series of representative spectra
displaying this exchange.
X-ray Data Collection, Structure Determination, and Refinement
for the (C5Me4R)3La Complexes 10-13. In all cases, a yellow crystal,
of dimensions reported in Table 1, was mounted on a glass fiber and
transferred to a Bruker CCD platform diffractometer. The SMART21
program package was used to determine the unit-cell parameters and
for data collection. A 20 s/frame scan time for a sphere of diffraction
Results
Synthetic Background. Although it was originally thought
that (C5Me5)3Sm complexes were too sterically crowded to exist,
(21) SMART Software User’s Guide, version 4.21; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 1997.
(22) SAINT Software User’s Guide, version 4.05; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 1997.
(23) Sheldrick, G. M. SHELXTL, version 5.10; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 1997.
(24) International Tables for X-ray Crystallography; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1992.

6344 Inorganic Chemistry, Vol. 40, No. 25, 2001
Evans et al.
there now exist four separate syntheses (eqs 2-5).4,5,12,13 Since
two of these syntheses rely on the special reduction chemistry
of Sm(II) (eqs 2 and 3), only two precedented routes were
available for a trivalent lanthanide such as La(III).
reagent. Prior work on the synthesis of (C5Me5)3Nd showed
that the allyl complex (C5Me5)2Nd(CH2CHCH2) was a better
synthetic precursor to form the cationic complex, [(C5Me5)2-
Nd][BPh4], than the commonly used hydrocarbyl species (C5-
Me5)2Nd[CH(SiMe3)2].5 Solvent-free (C5Me4R)2La(CH2CHCH2)
complexes were made from the (C5Me4R)2LaCl2K(THF)2
complexes using allylmagnesium chloride as shown in eq 7.
Like their C5Me5 analogue (C5Me5)2La(CH2CHCH2), com-
plexes 4-6 are very soluble in alkanes, arenes, and coordinating
solvents. However, the (C5Me4Et)2La(CH2CHCH2) and (C5Me4-
iPr)2La(CH2CHCH2) complexes lost THF with greater facility,
requiring only mild temperatures (35-45 °C) and desolvation
times (24 h). In comparison, (C5Me5)2La(CH2CHCH2) required
temperatures of 55-65 °C and 48 h. In the R ) SiMe3 case,
isolation of a solvent-free allyl complex was facile: complex
6, (C5Me4SiMe3)2La(CH2CHCH2), did not readily coordinate
THF and was easily crystallized from hexanes as a solvent-
free complex. Evidently, the increased steric bulk around the
metal center in 6 prevents effective coordination of the solvent.
The third step of the synthesis involved protonation of the
allyl units in (C5Me4R)2La(CH2CHCH2) to form the cations [(C5-
Me4R)2La][BPh4] (eq 8). This was readily accomplished in
benzene at room temperature yielding complexes 7-9. The 1H
NMR spectra for these complexes contained one set of
resonances for the tetraphenylborate protons and one set of peaks
for the equivalent C5Me4R ligands. The identities of complexes
7 and 9 were confirmed by X-ray crystallographic analysis31
and found to be similar to the structure of [(C5Me5)2Sm][(µ-
Ph2)BPh2].5
2(C5Me5)2Sm + C8H8 f (C5Me5)3Sm + (C5Me5)Sm(C8H8)
(2)
2(C5Me5)2Sm(OEt2) + (C5Me5)2Pb f
2(C5Me5)3Sm + Pb + 2OEt2 (3)
[(C5Me5)2SmH]2 + 2C10H14 f 2(C5Me5)3Sm
[(C5Me5)2Sm][BPh4] + K(C5Me5) f
(4)
(C5Me5)3Sm + KBPh4 (5)
Given the limited stability of the [(C5Me5)2LnH]2 lanthanide
hydrides,25-28 the best of the trivalent syntheses of (C5Me5)3-
Ln complexes appears to be the reaction of the unsolvated
cation, [(C5Me5)2Ln][BPh4], with K(C5Me5) shown in eq 5.
To make (C5Me4R)3La complexes starting from LaCl3 by this
route,5 four separate reactions are required, as shown in eqs
6-9.
2K(C5Me4R) + LaCl3 f (C5Me4R)2LaCl2K(THF)2 + KCl
(6)
(C5Me4R)2LaCl2K(THF)2 + (allyl)MgCl f
(C5Me4R)2La(CH2CHCH2) + MgCl2 + KCl (7)
The final step in the syntheses was the reaction of KC5Me4R
with the cationic complexes 7-9 (eq 9). The (C5Me4R)3La
(C5Me4R)2La(CH2CHCH2) + Et3NHBPh4 f
[(C5Me4R)2La][BPh4] + Et3N + CH3CHdCH2 (8)
i
(R ) Et, Pr, SiMe3) complexes, 11-13, were formed by this
route in benzene after the solutions were stirred for 24 h at room
temperature. Each was isolated as a bright yellow powder and
characterized by 1H NMR, 13C NMR, and IR spectroscopy and
X-ray crystallography.
[(C5Me4R)2La][BPh4] + KC5Me4R f
(C5Me4R)3La + KBPh4 (9)
Although both (C5Me5)3Sm and (C5Me5)3Nd were made by this
route, initial attempts to make the lanthanum analogue, the least
sterically crowded member of the (C5Me5)3Ln series, failed. It
was not until these reactions were conducted in silylated
glassware, as described below, that the synthesis was a success.
(C5Me4R)3La Syntheses. In the first step of the synthesis,
eq 6, formation of the (C5Me4R)2LaCl2K(THF)2 “ate” salts from
LaCl3 and KC5Me4R was accomplished following the traditional
procedure for introducing a C5R5 ligand into a lanthanide ion
coordination sphere.27,29,30 The synthesis of the (C5Me4R)2La-
Cl2K(THF)2 complexes was nearly identical to the route used
for formation of (C5Me5)2LaCl2Li(Et2O)2,27 except that heating
the reaction mixture at reflux was found to be unnecessary. The
use of the KC5Me4R salts rather than LiC5Me4R gave slightly
better yields (53-62%) than those previously reported for (C5-
Me5)2LaCl2Li(Et2O)2 (49%). The 1H NMR spectra of complexes
1-3 contained the pattern expected for the C5Me4R ligand and
coordinated THF.
Special Aspects of (C5Me5)3La. In contrast, the synthesis
of the R ) Me complex (C5Me5)3La was more difficult.
Although a bright yellow product, 10, similar to 11-13, could
be isolated and displayed a single 1H NMR resonance as
expected for (C5Me5)3La (1.997 ppm), initial attempts to confirm
the existence of (C5Me5)3La by X-ray crystallography failed.
Crystallization attempts with this yellow product provided,
instead, the bridged oxide, [(C5Me5)2La]2(µ-O), which could
be fully characterized by X-ray diffraction studies.31 The
structure of this oxide is similar to that of [(C5Me5)2Sm]2O,32
which is also a common byproduct of reactions involving
(C5Me5)2Sm units.
This (C5Me5)3La system was complicated by the fact that
the oxide had a single 1H NMR resonance at 2.02 ppm, which
was very close to that of (C5Me5)3La at 1.997 ppm. Even in a
sealed NMR tube, the 1.997 ppm product was observed to
convert to the 2.02 ppm oxide contaminant. For example, 16%
conversion was observed after 3 days at room temperature.
However, when the [(C5Me5)2La][BPh4]/KC5Me5 reaction was
run in silylated glassware and the product was handled in
silylated glassware, the formation of the oxide decomposition
product was minimized. Following this discovery, reactions 6-9
were conducted in silylated glassware, and this resulted in the
full characterization of (C5Me5)3La. It is interesting to note that
the least sterically crowded of the (C5Me5)3Ln complexes
The second step of the synthesis involved conversion of salts
1-3 to complexes that could be readily transformed into solvent-
free cations via protonolysis with [Et3NH][BPh4] or a similar
(25) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem.
Soc. 1983, 105, 1401-1403.
(26) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51-66.
(27) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann,
H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091-8103.
(28) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10,
134-142.
(31) Evans, W. J.; Ziller, J. W.; Davis, B. L. Manuscript in preparation.
(32) Evans, W. J.; Grate, J. W.; Bloom, I.; Hunter, W. E.; Atwood, J. L.
J. Am. Chem. Soc. 1985, 107, 405-409.
(29) Evans, W. J.; Wayda, A. Inorg. Chem. 1980, 19, 2190-2191.
(30) Andersen, R. A.; Tilley, T. D. Inorg. Chem. 1981, 20, 3267-3270.

Synthesis and Structure of Complexes of Lanthanum
Inorganic Chemistry, Vol. 40, No. 25, 2001 6345
i
Figure 3. Thermal ellipsoid plot of (C5Me4 Pr)3La with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Figure 1. Thermal ellipsoid plot of (C5Me5)3La with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity.
Figure 4. Thermal ellipsoid plot of (C5Me4SiMe3)3La with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms and
toluene are omitted for clarity.
Figure 2. Thermal ellipsoid plot of (C5Me4Et)3La with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
(Ln ) La, Nd, Sm)4,5 was the most difficult to isolate in a pure
crystalline form. Silylated glassware was not necessary in the
synthesis of the (C5Me5)3Nd and (C5Me5)3Sm analogues.
Structural Data. Although the spectroscopic data supported
the formation of the (η5-C5Me4R)3La products, the rather simple
1H NMR spectra of the complexes were not definitive. Only
the simple resonance patterns of the C5Me4R moieties were
observed. This made it difficult to distinguish the desired (η5-
C5Me4R)3La complexes and the bimetallic oxide-bridged de-
composition products [(η5-C5Me4R)2La]2O. For this reason, we
undertook X-ray diffraction studies to confirm the existence of
these molecules. In addition, it seemed prudent to obtain the
crystallographic data, given that for decades it was assumed
that it was not possible to put three C5Me5 rings around any
metal. The structural data were also needed to provide critical
information on the orientation of the R substituent and the
relative amount of steric congestion as the R group increases
in size.
(C5Me5)3La with the two other (C5Me5)3Ln structures in the
literature and shows that (C5Me5)3La has the metrical parameters
expected on the basis of the larger size of La3+. Subtraction of
each average Ln-C(ring) distance from the nine coordinate
metal ion radius gives a nearly constant difference in the 1.697-
1.688 Å range as is typical of isomorphous organo-f-element
complexes.33
In all of these (C5Me5)3Ln structures, the three rings are
crystallographically equivalent and the three ring centroids
define a trigonal planar geometry around the metal with 120°
(ring centroid)-M-(ring centroid) angles. Within each ring,
there are just three crystallographically unique ring carbon
positions. In (C5Me5)3La, these carbon atoms have La-C
distances of 2.975 (3), 2.896 (2), and 2.873 (2) Å for C(1), C(2),
and C(3), respectively. Each of the methyl carbons attached to
these ring carbons lies out of the plane of the ring carbons. The
displacements are 0.501, 0.160, and 0.309 Å for the methyl
carbon atoms attached to C(1), C(2), and C(3), respectively.
Hence, the size of the displacement does not correlate with the
length of the La-C bond. These displacements can be compared
to the analogous displacements of 0.521, 0.175, and 0.362 Å in
(C5Me5)3Sm. Hence, the larger La system leads to smaller out
of plane displacements than for Sm. Displacements of 0.09-
0.31 Å have been observed in other types of less-crowded
(C5Me5)3Ln complexes.34-36
Fortunately, good single crystals could be obtained for
i
(C5Me5)3La (10), (C5Me4Et)3La (11), (C5Me4 Pr)3La (12), and
(C5Me4SiMe3)3La (13), as shown in Figures 1-4, respectively.
Crystallographic cell parameters are provided in Table 1, and a
comprehensive summary of structural features of the (C5Me4-
R)3La complexes is given in Table 2. Comparisons with related
complexes are given in Tables 3 and 4. (C5Me5)3La, 10, Figure
1, is isomorphous with (C5Me5)3Nd,5 (C5Me5)3Sm,4 and
(C5Me5)3U.12 (C5Me4Et)3La, 11, Figure 2, is isomorphous with
(C5Me4Et)3Sm.13
Table 4 compares the (C5Me4R)3La structures with those of
(C5Me5)3La and (C5Me4Et)3Sm. Complexes 11 (R ) Et) and
(33) Raymond, K. N.; Eigenbrot, C. W. Acc. Chem. Res. 1980, 13, 276-
The structure of (C5Me5)3La will be discussed first, since it
provides a basis for comparing the effect of introducing the
substituent R in the other C5Me4R structures. Table 3 compares
283.
(34) Watson, P. L.; Whitney, J. F.; Harlow, R. L. Inorg. Chem. 1981, 20,
3271-3278.

6346 Inorganic Chemistry, Vol. 40, No. 25, 2001
Table 2. Selected Bond Distances (Å) and Angles (deg) for (C5Me5)3La, 10, (C5Me4Et)3La, 11, (C5Me4 Pr)3La, 12, and (C5Me4SiMe3)3La, 13
10 11 12 13
Evans et al.
i
La-C(1)
La-C(2)
La-C(3)
La-Cnt
2.975 (3)
2.896 (2)
2.873 (2)
2.642
La-C(1)
2.973 (3)
2.932 (3)
2.913 (3)
2.879 (3)
2.878 (3)
2.876 (3)
2.921 (3)
2.948 (3)
2.960 (3)
2.857 (3)
2.869 (3)
2.899 (3)
2.907 (3)
2.966 (3)
2.909 (3)
2.654
2.652
2.654
3.56
3.64
119.8
120.0
120.2
La-C(1)
2.928 (3)
2.893 (3)
2.921 (3)
2.933 (3)
2.894 (3)
2.971 (3)
2.874 (3)
2.877 (3)
2.921 (3)
2.928 (3)
2.968 (3)
2.886 (3)
2.908 (3)
2.960 (3)
2.953 (3)
2.652
2.653
2.674
4.01
4.10
119.5
120.5
120.0
La-C(1)
3.029 (2)
2.962 (2)
2.941 (2)
2.940 (2)
2.957 (2)
3.018 (2)
2.986 (2)
2.923 (2)
2.890 (2)
2.906 (2)
2.988 (2)
2.952 (2)
2.948 (2)
2.925 (2)
2.925 (2)
2.706
2.685
2.687
3.42
4.10
120.3
119.5
119.7
La-C(2)
La-C(2)
La-C(2)
La-C(3)
La-C(3)
La-C(3)
La-C(4)
La-C(4)
La-C(4)
Cnt-La-Cnt
120.0
La-C(5)
La-C(5)
La-C(5)
La-C(12)
La-C(13)
La-C(14)
La-C(15)
La-C(16)
La-C(23)
La-C(24)
La-C(25)
La-C(26)
La-C(27)
La-Cnt(1)
La-Cnt(2)
La-Cnt(3)
La‚‚‚(11)
La‚‚‚C(22)
La-C(13)
La-C(14)
La-C(15)
La-C(16)
La-C(17)
La-C(25)
La-C(26)
La-C(27)
La-C(28)
La-C(29)
La(1)-Cnt(1)
La-Cnt(2)
La-Cnt(3)
La‚‚‚C(23)
La‚‚‚C(35)
La-C(13)
La-C(14)
La-C(15)
La-C(16)
La-C(17)
La-C(25)
La-C(26)
La-C(27)
La-C(28)
La-C(29)
La-Cnt(1)
La-Cnt(2)
La-Cnt(3)
La‚‚‚C(24)
La‚‚‚C(35)
Cnt(1)-La-Cnt(2)
Cnt(1)-La-Cnt(3)
Cnt(2)-La-Cnt(3)
Cnt(1)-La-Cnt(2)
Cnt(1)-La-Cnt(3)
Cnt(2)-La-Cnt(3)
Cnt(1)-La-Cnt(2)
Cnt(1)-La-Cnt(3)
Cnt(2)-La-Cnt(3)
Table 3. Comparison of Metrical Data for (C5Me5)3Ln Complexes (M ) La, Nd, and Sm)
distance (Å)
metal-C (ring)
effective ionic radii
(9 coordinate)24
[mean metal-C(ring)] -
(C5Me5)3Ln
metal-centroid
high
low
mean
[ionic radius]
(C5Me5)3La
(C5Me5)3Nd5
(C5Me5)3Sm4
2.642
2.582
2.555
2.975 (3)
2.927 (2)
2.910 (3)
2.8732 (19)
2.8146 (13)
2.782 (2)
2.91 (5)
2.86 (6)
2.82 (5)
1.216
1.163
1.132
1.694
1.697
1.688
Table 4. Comparison of Metrical Data for (C5Me4R)3Ln Complexes (M ) La and Sm)
distance (Å)
metal-C (ring)
metal-centroid
effective ionic radius
(9 coordinate)
[mean metal-C (ring)] -
(C5Me4R)3Ln
mean
high
low
mean
[ionic radius]
(C5Me5)3La
2.642
2.653
2.659
2.693
2.568
2.975 (3)
2.973 (3)
2.971 (3)
3.029 (2)
2.900 (14)
2.8732 (2)
2.857 (3)
2.874 (3)
2.890 (2)
2.787 (12)
2.91 (5)
2.91 (3)
2.92 (3)
2.95 (3)
2.83 (4)
1.216
1.216
1.216
1.216
1.132
1.694
1.694
1.704
1.734
1.698
(C5Me4Et)3La
i
(C5Me4 Pr)3La
(C5Me4SiMe3)3La
(C5Me4Et)3Sm13
Table 5. Deviations from the C6Me5R Plane of the R-C (or Si) for
i
12 (R ) Pr) have a trigonal planar arrangement of C5Me4R
rings around the metal with the ring centroids coplanar with
La to within 0.001 Å. Complex 13 (R ) SiMe3) is distorted
slightly toward a pyramidal geometry with the La 0.111 Å out
Each Alkyl or Silyl Substituent (Atom Number Is in Brackets)
i
(C5Me5)3La, (C5Me4Et)3La, (C5Me4 Pr)3La, (C5Me4SiMe3)3La,
10
11
12
13
i
0.501 [4]
0.160 [5]
0.309 [6]
0.481 [6]
0.196 [7]
0.314 [8]
0.479 [6]
0.185 [7]
0.314 [8]
0.542 [6]
0.236 [7]
0.313 [8]
0.364 [9]
of the plane. The structures of the Et and Pr derivatives have
bond distances similar to that of (C5Me5)3La: neither the metal
centroid distances nor the bond distances are very different. Only
with the SiMe3 derivative, 13, do the absolute numbers increase,
but they are still equivalent within the error limits. The
differences between the average Ln-C(ring) distance and the
metal radius for 11 and 12, 1.694 and 1.704 Å, respectively,
are similar to the 1.679-1.688 Å values for the (C5Me5)3Ln
complexes. Only for 13 does this value increase to 1.734 Å.
Examination of the displacements of the atoms attached to
the ring carbons ( Table 5) shows that each ring in compounds
10-13 has one large displacement value around 0.5 Å. As
described above, (C5Me5)3La has two other values of 0.309 and
0.160 Å. Complexes 11-13 appear to follow a somewhat similar
0.309 [9]
0.321 [9]
0.053 [10]
0.340 [17]
0.291 [18]
0.246 [19]
0.478 [20]
0.030 [21]
0.277 [28]
0.325 [29]
0.207 [30]
0.463 [31]
0.249 [32]
0.174 [10]
0.491 [18]
0.154 [19]
0.338 [20]
0.320 [21]
0.113 [22]
0.496 [30]
0.190 [31]
0.312 [32]
0.346 [33]
0.154 [34]
0.498 [Si1]
0.458 [18]
0.271 [19]
0.279 [20]
0.369 [21]
0.161 [Si2]
0.447 [30]
0.190 [31]
0.342 [32]
0.308 [33]
0.190 [Si3]
pattern, i.e., they have two pairs in roughly this range, except
for the atoms of the R group.
In (C5Me4Et)3La, two of the ethyl carbon atoms attached
directly to the ring have very small displacements (C(10), 0.0529
Å; C(21), 0.0296 Å). Both of these ethyl groups are also oriented
so that their methyl groups point toward the La along the
(35) Teuben, J. H.; de Boer, J. L.; den Haan, K. H.; Spek, A. L.; Kojic-
Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726-
1733.
(36) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5,
1285-1291.

Synthesis and Structure of Complexes of Lanthanum
Inorganic Chemistry, Vol. 40, No. 25, 2001 6347
Figure 5. Complexes 11-13 viewed down the pseudo-C3 axis.
pseudo-C3 axis perpendicular to the plane of the metal and three
ring centroids, as was found in (C5Me4Et)3Sm and as shown in
Figure 5.
This puts the C(22) and C(11) atoms of the ethyl groups
within 3.56 and 3.64 Å, respectively, of the metal center. In
contrast, the other ethyl group points away from the metal. The
carbon attached to that ring has a more “normal” displacement
of 0.2485 Å, and C(33), the methyl carbon of the ethyl group,
is 5.34 Å away from La.
the Si(1) group is on the same side of the molecule as the Si(2)
group and has two of its methyl atoms pointed in, C(10) and
C(11). Si(1) is also the silicon with the largest out of cyclo-
pentadienyl ring plane displacement: 0.498 Å. The three
substituent Si atoms define a distorted isosceles triangle with
angles of 39.3, 67.9, and 72.9° as in the other structures. Only
one methyl group is within 4 Å of the metal center: C(24) is
3.42 Å from the metal and lies on the pseudo-C3 axis like the
methyls in 11. C(35) is close to the pseudo-C3 axis, but it is
4.10 Å from La. In contrast to complex 12, the R group with
the closest methyl is on the same side of the molecule as another
R group.
Ligand Exchange between 10 and 13. Since the diamagnetic
nature of complexes 10-13 makes them more suitable for NMR
studies than their paramagnetic analogues, a ligand exchange
study could be performed with this series of compounds, as
shown in Figure 6 in Supporting Information.
To check if the different orientations of the ethyl groups arose
due to some secondary packing, e.g., the effect of the CO-
packing effect of the ligand substituents in metal carbonyl
clusters on bridging vs terminal locations in the “ligand
polyhedral model”,37-39 the triangle defined by the three carbons
C(10), C(21), and C(32) was examined. The triangle is not
equilateral, and the lanthanum does not lie in the plane of this
triangle. It is displaced by 0.506 Å. Hence, the orientation of
the two ethyl groups toward the metal is not due to some “most
efficient packing arrangement” of the ethyl part of the C5Me4-
Et rings around the whole molecule. The angles of the C(10),
C(21), and C(32) triangle are 40.0, 68.4, and 71.8°, respectively;
i.e., it is roughly isosceles. This is consistent with the fact that
two of the ethyl groups are oriented toward the metal and one
is not.
The 1H NMR spectrum of an approximately equimolar
mixture of 10 and 13 in benzene showed no immediate
interchange of the ligands: only the 1.997 ppm resonance of
10 and the 2.36, 2.01, and 0.36 ppm shifts of 13 were observed.
After 24 h, two new sets of four peaks were observed, set A
(2.31, 2.02, 1.93, 0.31 ppm) and set B (2.33, 2.05, 1.98, 0.33
ppm), along with the original four peaks of the starting materials.
After 92 h, only 5% of 10 and 10% of 13 remained and the
ratio of a corresponding peak in A and B was 3.1:1 (for the
2.35 and 2.34 ppm resonances). These results are consistent with
a ligand redistribution between (C5Me5)3La and (C5Me4SiMe3)3-
La to form (C5Me4SiMe3)2La(C5Me5) (A) and (C5Me4SiMe3)-
La(C5Me5)2 (B) in a molar ratio of 1.6:1.
i
In (C5Me4 Pr)3La, 12, there are fewer options for orienting
the R groups since each R contains two methyl groups. It is
not possible to point one methyl directly toward or away from
the metal without causing some unfavorable interactions between
the other isopropyl methyl and the other ring methyls of the
i
C5Me4 Pr ring. There are no small displacements from the
cyclopentadienyl plane of the isopropyl carbon atoms bound to
the ring. These three carbon atoms, C(10), C(22), and C(34),
again define a roughly isosceles triangle like that in 11 with
angles of 41.5, 67.9, and 70.6°, respectively, but the closest
contact between an isopropyl methyl group and the metal is
C(23) at 4.01 Å. This C(23) is along the pseudo-C3 axis of the
molecule as was found for C(22) and C(11) in 11 (Figure 5).
The next closest methyl carbons in 12, C(35) and C(11), are
4.10 and 4.26 Å from La, respectively, and neither is directly
along the pseudo-C3 axis. These two R groups are on the same
side of the molecule, which may cause additional constraints
on their positions.
The structure of (C5Me4SiMe3)3La shares features with both
11 and 12. With the SiMe3 group, there are even more
constraints on R group orientation. Two of the SiMe3 groups
have one methyl carbon pointing in: C(24) on Si(2) and C(35)
on Si(3) (Figure 5). This is the sterically most favorable
orientation since two of the three methyls are pointed to the
outside of the molecule where there is more space. However,
Discussion
The synthetic route involving reaction of an unsolvated cation
[(C5Me4R)2La][BPh4], with KC5R5, which was used to make
(C5Me5)3Nd and (C5Me5)3Sm,5 can be extended to the largest
lanthanide metal, lanthanum, as well as to substituted cyclo-
i
pentadienyl ligands, C5Me4R where R ) Et, Pr, and SiMe3.
However, the synthesis of the less crowded (C5Me5)3La requires
the use of silylated glassware to obtain the complex in pure
crystalline form. It is unclear why the least sterically crowded
example of the (C5Me5)3La series requires these more stringent
reaction conditions. It suggests that the larger metal provides
reaction pathways with oxygen-containing contaminants that are
not available to the more crowded systems. Given this surprising
reactivity, it is prudent to use silylated glassware, at least
initially, when other highly reactive examples of (C5Me5)3M
complexes are pursued synthetically.
The structures of the (C5Me4R)3La complexes do not show
a large change in bonding parameters as the size of the R group
is increased. Evidently, the presence of just a single substituent
larger than methyl on each ring is not sufficient to significantly
increase the overall crowding in the molecule. Apparently there
is room for each R group to avoid the other R groups and the
other ring methyls.
(37) Johnson, B. F. G. J. Chem. Soc., Chem. Commun. 1976, 211.
(38) Johnson, B. F. G.; Lewis, J. AdV. Inorg. Chem. Radiochem. 1981, 24,
225-355.
(39) Johnson, B. F. G.; Quadrelli, E. A.; Ferrand, V.; Bott, A. W. J. Chem.
Soc., Dalton Trans. 2001, 1063-1068.

6348 Inorganic Chemistry, Vol. 40, No. 25, 2001
Evans et al.
The R ) Et complex, (C5Me4Et)3La, 11, demonstrates this
point most clearly. One ethyl group is oriented such that its
end methyl group is away from the center of the molecule and
the cyclopentadienyl ring. This orientation clearly will not affect
the amount of steric crowding in the other parts of the molecule.
Moreover, the remainder of the molecule is not too crowded to
accommodate orienting the other ethyl groups toward the center
of the molecule, presumably to access additional long distance
metal ligand interaction.
In the case of R ) Pr, (C5Me4 Pr)3La, 12, the presence of
two methyl groups on each R does not allow the simple options
taken in the R ) Et complex. If one methyl is oriented directly
in toward the metal on the pseudo-C3 axis, it will cause some
(isopropyl methyl)-(ring methyl) interactions, and this orienta-
tion is not observed. Instead, each isopropyl group appears to
be oriented such that the hydrogen of the methine carbon is in
the plane of the ring. This would put the smallest substituent
on the methine carbon in the sterically most congested region
as expected. This orientation leaves one methyl pointing in and
one methyl pointing out from the cyclopentadienyl ring plane
with each oriented at an angle consistent with a sp3 hybridization
around the methine carbon. As a result, none of the methyl
positions can tip in toward the metal and there are no especially
small out of plane displacements for the methine carbons.
Complex 12 has three isopropyl methyl groups on the outside
of the molecule, and the other three apparently can be accom-
modated without increasing the overall metal-ring centroid
distances.
methyls. Hence, in 13, one of the methyls can make a long-
range La‚‚‚Me interaction less than 4 Å despite the fact that its
silicon has two other methyl groups. Interestingly, this occurs
on the side of the molecule that has two SiMe3 groups attached
to it. Hence, even with the SiMe3 substituent, the extra steric
bulk can be accommodated.
Another aspect of the apparent ability of these (C5Me4R)3La
systems to accommodate increased steric bulk is that they can
exchange ligands. Although this process is kinetically slow
between the smallest and largest complexes, 10 and 13, it does
occur to form mixed ligand species.
i
i
Conclusion
The results of this study show that a variety of (C5Me4R)3La
complexes can be isolated and fully characterized with R groups
as large as SiMe3. The availability of these complexes in good
yield, along with their diamagnetism, will facilitate the future
reactivity investigations of sterically crowded tris(peralkylcy-
clopentadienyl) lanthanide complexes. Variation of ligand size
should be especially useful for the evaluation of sterically
induced reductions. In addition, the option to make tris(per-
alkylcyclopentadienyl) complexes with R groups that can block
the approach of substrates to the metal along the pseudo-C3 axes
may be useful with the mechanistic investigations of these
complexes.
Acknowledgment. We thank the National Science Founda-
tion for support for this research.
Supporting Information Available: Four X-ray crystallographic
files, in CIF format, and representative 1H NMR spectra for the
exchange reaction of 10 and 13. This material is available free of charge
via the Internet at http://pubs.acs.org.
In the R ) SiMe3 case, (C5Me4SiMe3)3La, 13, in which the
R group contains three methyl groups, it is very difficult to avoid
some Me(R)-Me(ring) interactions. However, since silicon-
carbon bonds are longer than carbon-carbon bonds, each of
these methyl groups can be oriented further away from the ring
IC010554I