Reaction chemistry of sterically crowded tris(pentamethylcyclopentadienyl) samarium

Article abstract of DOI:10.1021/ja9809859

Although (C₅Me₅)₃Sm is an extremely sterically crowded molecule, it displays high reactivity with a variety of substrates including CO, THF, ethylene, hydrogen, nitriles, isonitriles, isocyanates, 1,3,5,7- cyclooctatetraene, azobenzene, and Ph₃P=E (E = O, S, Se). The reactions include polymerization, insertion, ring-opening, and reduction. Depending on the substrate, (C₅Me₅)₃Sm can react (1) as if it were a bulky alkyl complex of formula (C₅Me₅)₂SmR in which R is an η¹-C₅Me₅ group or (2) as if it were the zwitterion [(C₅Me₅)₂Sm]⁺[C₅Me₅]- in which the [C₅Me₅]^- component is a one-electron reductant. In the former mode, this compound (a) reacts with CO to form (C₅Me₅)₂Sm(O₂C₇Me₅), which has a ligand containing a nonclassical carbocationic center, (b) undergoes hydrogenolysis with H₂ to form [(C₅Me₅)₂Sm(μ-H)]₂, (c) ring-opens THF to form (C₅Me₅)2Sm[O(CH₂)₄(C₅Me₅)](THF), (d) inserts PhCN to form (C₅Me₅)₂Sm[NC(Ph)(C₅Me₅)](NCPh), 2, and (e) reacts with PhNCO to form (C₅Me₅)₂SmOC(C₅Me₅)N(Ph)C(NPh)O, 3, a product which can be rationalized by a C-N coupling between a coordinated PhNCO and a PhNCO unit inserted into a Sm(η¹-C₅Me₅) bond. On the other hand, (C₅Me₅)₃Sm reduces (a) Ph₃P=O to form PPh₃, (C₅Me₅)₂, and [(C₅Me₅)₂Sm]₂(μ-O), (b) Ph₃P=E (E = S, Se) to form PPh₃, (C₅Me₅)₂, and a complex which adds THF to form [(C₅Me₅)₂Sm(THF)]₂(μ-E), (b) cyclooctatetraene to form (C₅Me₅)Sm(C₈H₈) and (C₅Me₅)₂, (c) azobenzene to form (C₅Me₅)₂Sm(N₂Ph₂) and (C₅Me₅)₂, (d) (d)Me₃CNC to form [(C₅Me₅)₂Sm(μ-CN)(CNCMe₃)]₃, 4. (C₅Me₅)₃Sm also initiates the polymerization of ethylene. This reaction chemistry is described here as well as structural data on 2-4, each of which has a formal eight-coordinate bent metallocene geometry around samarium.

Full text of DOI:10.1021/ja9809859

J. Am. Chem. Soc. 1998, 120, 9273-9282
9273
Reaction Chemistry of Sterically Crowded
Tris(pentamethylcyclopentadienyl)samarium1
William J. Evans,* Kevin J. Forrestal, and Joseph W. Ziller
Contribution from the Department of Chemistry, UniVersity of California-IrVine, IrVine, California 92697
ReceiVed March 23, 1998
Abstract: Although (C5Me5)3Sm is an extremely sterically crowded molecule, it displays high reactivity with
a variety of substrates including CO, THF, ethylene, hydrogen, nitriles, isonitriles, isocyanates, 1,3,5,7-
cyclooctatetraene, azobenzene, and Ph3PdE (E ) O, S, Se). The reactions include polymerization, insertion,
ring-opening, and reduction. Depending on the substrate, (C5Me5)3Sm can react (1) as if it were a bulky alkyl
complex of formula (C5Me5)2SmR in which R is an η1-C5Me5 group or (2) as if it were the zwitterion
[(C5Me5)2Sm]+[C5Me5]- in which the [C5Me5]- component is a one-electron reductant. In the former mode,
this compound (a) reacts with CO to form (C5Me5)2Sm(O2C7Me5), which has a ligand containing a nonclassical
carbocationic center, (b) undergoes hydrogenolysis with H2 to form [(C5Me5)2Sm(µ-H)]2, (c) ring-opens THF
to form (C5Me5)2Sm[O(CH2)4(C5Me5)](THF), (d) inserts PhCN to form (C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh),
2, and (e) reacts with PhNCO to form (C5Me5)2SmOC(C5Me5)N(Ph)C(NPh)O, 3, a product which can be
rationalized by a C-N coupling between a coordinated PhNCO and a PhNCO unit inserted into a Sm(η1-C5-
Me5) bond. On the other hand, (C5Me5)3Sm reduces (a) Ph3PdO to form PPh3, (C5Me5)2, and [(C5Me5)2-
Sm]2(µ-O), (b) Ph3PdE (E ) S, Se) to form PPh3, (C5Me5)2, and a complex which adds THF to form
[(C5Me5)2Sm(THF)]2(µ-E), (b) cyclooctatetraene to form (C5Me5)Sm(C8H8) and (C5Me5)2, (c) azobenzene to
form (C5Me5)2Sm(N2Ph2) and (C5Me5)2, and (d) Me3CNC to form [(C5Me5)2Sm(µ-CN)(CNCMe3)]3, 4. (C5-
Me5)3Sm also initiates the polymerization of ethylene. This reaction chemistry is described here as well as
structural data on 2-4, each of which has a formal eight-coordinate bent metallocene geometry around samarium.
Introduction
allow small, cylindrical substrates to access the metal center.
These possibilities could not be fully evaluated until a more
efficient, higher yield synthesis of Sm(C5Me5)3 was developed.
This was accomplished as shown in eq 2.4
For many years it was assumed that tris(pentamethylcyclo-
pentadienyl) metal complexes, (C5Me5)3M, were too sterically
crowded to exist for any metal in the periodic table. Indeed,
when the first example, (C5Me5)3Sm, 1,2 was synthesized, as
2(C5Me5)2Sm(OEt2) + (C5Me5)2Pb f 2(C5Me5)3Sm + Pb
(2)
Subsequently, the reaction of (C5Me5)3Sm with the cylindrical
(1)
2
(3)
shown in eq 1, and crystallographically characterized, its
metrical parameters were found to be extreme. It had a smaller
ring-centroid-metal-ring-centroid angle than in any previously
characterized C5Me5-containing samarium complex and the
longest Sm-C bonds of any trivalent C5Me5/Sm system.3
The extreme steric saturation could have limited the reactivity
of 1, since substrates would not be able to approach the metal
center. On the other hand, the long Sm-C(C5Me5) distances
could provide a basis for reactivity. Neither the C5Me5 rings
nor the samarium receive the normal electrostatic stabilization
from each other at this distance. This could make the C5Me5
rings prone to removal and the metal center quite electrophilic.
Moreover, space-filling models suggested that there was an open
channel down the three-fold axis of the molecule which could
substrate carbon monoxide was examined and the nonclassical
carbocation complex, (C5Me5)2Sm(O2C7Me5), shown in eq 3
was isolated.5
This reaction was remarkable in several respects. Not only
was the product an unusual example of an alkane-soluble,
thermally stable carbocation, but the reaction of a C5Me5 ligand
with CO was also unprecedented. In the scores of previously
reported reactions involving CO and C5Me5 complexes, the C5-
Me5 group had always been an inert spectator ligand. This result
showed that 1 had access to an unusual reaction pathway which
allowed coupling of CO with C5Me5 rings.
(1) Reported in part at the Fifth Chemical Congress of North America,
November 11-15, 1997, Cancu´n Mexico.
(4) Evans, W. J.; Forrestal, K. J.; Leman, J. T.; Ziller, J. W. Organo-
metallics 1996, 15, 527.
(5) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1995,
117, 12635.
(2) Evans, W. J.; Gonzales, S. G.; Ziller, J. W. J. Am. Chem. Soc. 1991,
113, 7423.
(3) Evans, W. J.; Foster, S. E. J. Organomet. Chem. 1992, 433, 79.
S0002-7863(98)00985-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/28/1998

9274 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
EVans et al.
by Desert Analytics, Tucson, AZ. (C5Me5)3Sm/pyridine NMR experi-
ments were run with 0.03 M solutions of (C5Me5)3Sm in toluene-d8.
(C5Me5)3Sm was also unusual in that it could initiate the
polymerization of ethylene, eq 4.6
Reaction with PhCN. In the glovebox, addition of a solution of
PhCN (37 mg, 0.36 mmol) in toluene (5 mL) to a brown solution of
(C5Me5)3Sm (100 mg, 0.18 mmol) in toluene (5 mL) caused an
immediate color change to yellow-orange. After 5 min, the solvent
was removed in vacuo leaving (C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh),
4a, (130 mg, 95%) as a yellow solid. 1H NMR (C6D6) δ 6.16 (t, JHH
(C5Me5)3Sm + CH2dCH2 f polyethylene
(4)
This was unexpected since (C5Me5)3Sm is a sterically
saturated molecule and the approach of the olefin to the metal
center appeared to be hindered. Furthermore, lanthanide-based
olefin polymerization generally requires alkyl or hydride func-
tionalities or a reactive divalent complex such as (C5Me5)2Sm.7,8
The CO insertion and ethylene polymerization reactions both
could be rationalized if an intermediate containing a Sm-C
single bond was present, for example in an η1-C5Me5 complex
such as (C5Me5)2Sm(η1-C5Me5). No spectroscopic evidence for
an η1-C5Me5 ring was observed by NMR spectroscopy, even at
-80 °C, to support such an intermediate, but (C5Me5)3Sm
reacted with hydrogen in a manner analogous to that of σ-bound
alkyl complexes to form the expected hydrogenolysis products,
) 7.1 Hz, C6H5, 1H) 5.92 (t, JHH ) 7.5 Hz, C6H5, 2H) 5.19 (d, JHH
)
7.5 Hz, C6H5, 2H) 2.88 (s, CH3, 6 H), 2.22 (s, CH3, 3 H), 2.18 (s, CH3,
6 H), 1.65 (s, C5Me5, 30 H). 13C{1H} NMR (C6D6) δ 170.4, 144.4,
135.6, 133.6, 131.5, 128.5, 126.7, 112.4, 104.4, 60.8, 25.0, 15.7, 13.0,
12.0. IR (KBr) 2908 s, 2250 m, 1630 s, 1448 m, 1376 w, 1023 w,
758 m, 694 w cm-1
Found: Sm 20.8.
. Anal. Calcd for C44H55N2Sm: Sm, 19.8.
Reaction with PhNCO. In the glovebox, PhNCO (12.8 mg, 0.108
mmol) dissolved in toluene (5 mL) was added to (C5Me5)3Sm (30 mg,
0.054 mmol) in toluene. The solution turned yellow immediately upon
addition of PhNCO. The solvent was removed in vacuo, leaving (C5-
Me5)2SmOC(C5Me5)N(Ph)C(NPh)O, 6, (42 mg, 98%) as a yellow solid.
1H NMR (C6D6) δ 9.13 (d, C6H5, 2 H), 7.61 (t, C6H5, 2 H), 7.40 (t,
C6H5, 1 H), 2.14 (s, CH3, 6 H), 1.30 (s, CH3, 6 H), 0.97 (s, C5Me5, 30
H), -3.61 (s, CH3, 3 H). 13C{1H} NMR (C6D6) δ 140.2, 138.5, 138.0,
132.1, 128.9, 126.7, 126.1, 125.6, 122.5, 120.1, 118.0, 117.6, 64.2,
31.9, 23.0, 17.6, 14.3, 13.5, 12.2, 10.9. IR (KBr) 2910 m, 1686 w,
1636 s, 1589 m, 1554 m, 1438 m, 1289 w, 764 w, 694 w cm-1. Anal.
Calcd for C44H55N2O2Sm: Sm, 18.93; C, 66.53; H, 6.98; N, 3.53.
Found: Sm, 20.6; C, 66.11; H, 7.25; N, 3.09.
2 (C5Me5)3Sm + 2 H2 f [(C5Me5)2Sm(µ-H)]2 + 2 C5Me5H
(5)
[(C5Me5)2Sm(µ-H)]2 and C5Me5H, eq 5.6
9
As a consequence of the unusual reactivity observed for (C5-
Me5)3Sm, we have expanded the investigation of its reactivity.
We report here chemical evidence that an η1-intermediate is
accessible in the (C5Me5)3Sm system, but we also describe an
extensive one electron-reduction chemistry for this trivalent
complex. This is unexpected since a Sm(III)/Sm(IV) redox
couple has never been observed.10 Since the reductive reactivity
does not require an η1-intermediate, the reactivity of (C5Me5)3-
Sm appears to be variable depending on the nature of the
substrate available.
Reaction with THF. In the glovebox, THF (2.91 mL, 0.36 mmol)
was added to (C5Me5)3Sm (100 mg, 0.18 mmol) in toluene (5 mL),
and the solution turned yellow immediately. Removal of the solvent
by rotary evaporation yielded the previously characterized (C5Me5)2-
1
Sm[O(CH2)4(C5Me5)](THF)12 (120 mg, 95%) identified by H NMR
spectroscopy (C6D6).
Reaction with E-Caprolactone. In a glovebox, ꢀ-caprolactone (628
mg, 5.5 mmol) was added dropwise to (C5Me5)3Sm (31 mg, 0.055
mmol) in toluene (10 mL). A yellow color change was observed
immediately. The reaction was allowed to proceed at room temperature
for 18 h during which time an increase in viscosity was observed. The
solution was removed from the glovebox, diluted with toluene (15 mL),
and washed with 5% HCl. The toluene solution was then added to
hexane (100 mL), and a white solid precipitated. This material was
collected by filtration and dried for 20 h on a vacuum line (500 mg,
80% conversion). The white solid was identified as polycaprolactone
by 13C NMR spectroscopy. No evidence for C5Me5 groups was
observed by 13C NMR spectroscopy.
Experimental Section
The chemistry described below was performed under nitrogen or
argon with rigorous exclusion of air and water using Schlenk, vacuum
line, and glovebox techniques. Solvents were dried over sodium/
benzophenone ketyl and distilled prior to use. (C5Me5)3Sm was
prepared according to the literature procedure.4 PhCN, PhNCO, and
Me3CNC were dried over molecular sieves and degassed prior to use.
Ph3PdE (E ) O, S, Se) and azobenzene were used as received
(Aldrich). 1,3,5,7-Cyclooctatetraene was purified by drying over CaH2,
followed by vacuum distillation. 2,5-Dimethyltetrahydrofuran was dried
over sodium/benzophenone ketyl and distilled prior to use. ꢀ-Capro-
lactone was dried over molecular sieves and vacuum transferred prior
to use. Pyridine was dried over CaH2 and vacuum transferred prior to
use. Complexometric analyses and physical measurements were
obtained as previously described.11 Elemental analysis was provided
Reaction with SedPPh3. In the glovebox, SedPPh3 (15 mg, 0.043
mmol) in toluene (5 mL) was added to (C5Me5)3Sm (48 mg, 0.086
mmol) in toluene (5 mL). After 15 min, the color of the toluene solution
had turned to orange, and a white precipitate had formed. This
precipitate was removed by centrifugation and identified as PPh3 (10
mg, 90%) by 1H NMR spectroscopy (C6D6). The reaction solvent was
removed in vacuo to give a tacky orange solid which was washed with
hexane to give an orange powder (36 mg, 90%). This orange powder
was recrystallized from THF and identified as the previously character-
(6) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed.
Engl. 1997, 109, 774.
(7) (a) Ballard, D. G. H.; Courtis, A.; Holton, J.; McMeeking, J.; Pearce,
R. J. Chem. Soc., Chem. Commun. 1978, 994. (b) Jeske, G.; Schock, L. E.;
Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985,
107, 8103. (c) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.;
Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (d) Evans,
W. J.; Chamberlain, L. R.; Ziller, J. W. J. Am. Chem. Soc. 1987, 109, 7209.
(e) Evans, W. J.; Chamberlain, L. R.; Ulibarri, T. A.; Ziller, J. W. J. Am.
Chem. Soc. 1988, 110, 6423. (f) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W.
J. Am. Chem. Soc. 1990, 112, 2314. (g) Hoff, S. M.; Novak, B. M.
Macromolecules 1993, 26, 4067. (h) Schaverien, C. J. Organometallics 1994,
13, 69. (i) Yasuda, H.; Tamai, H. Prog. Polym. Sci. 1993, 18, 1097. (j)
Evans, W. J.; DeCoster, D. M.; Greaves, J. Macromolecules 1995, 28, 7929.
(8) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem.
Soc. 1981, 103, 6507. Watson, P. L.; Herskovitz, T. ACS Symp. Ser. 1983,
212, 459. Evans, W. J.; DeCoster, D. M.; Greaves, J. Organometallics 1996,
15, 3210-3221.
1
ized [(C5Me5)2Sm(THF)]2(µ-Se)13 by H NMR spectroscopy (C6D6).
Rotary evaporation of solvent from the hexane wash yielded a tacky
material which was identified by 1H NMR spectroscopy (C6D6) as the
known, oxidatively coupled dimer, (C5Me5)2 (12 mg, 97%).14
Reaction with SdPPh3. In the glovebox, SdPPh3 (13 mg, 0.045
mmol) was reacted with (C5Me5)3Sm (50 mg, 0.091 mmol) as described
above. After 3 days, the color had turned to yellow-orange, and a white
precipitate was observed. The reaction workup was performed in the
(11) Evans, W. J.; Grate, J. W.; Levan, K. R.; Bloom, I.; Peterson, T.
T.; Doedens, R. J.; Zhang, H.; Atwood, J. L. Inorg. Chem. 1986, 25, 3614.
(12) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.
Organometallics 1990, 9, 2124.
(13) Evans, W. J.; Rabe, G. W.; Ziller, J. W.; Doedens, R. J. Inorg.
Chem. 1994, 33, 2719.
(9) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem.
Soc. 1983, 105, 1401.
(10) Morss, L. R. Chem. ReV. 1976, 76, 827-841.
(14) Jutzi, P.; Kohl, F. J. Organomet. Chem. 1979, 164, 141.

Tris(pentamethylcyclopentadienyl)samarium
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9275
manner described above to isolate a yellow-orange powder (37 mg,
93%). This yellow-orange powder was recrystallized from THF and
identified as the previously characterized [(C5Me5)2Sm(THF)]2(µ-S)13
by H NMR spectroscopy (C6D6). (C5Me5)2 (12 mg, 97%) and PPh3
(11 mg, 93%) were identified as described above.
Table 1. Experimental Data for the X-ray Diffraction Studies of
(C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh), 2, (C5Me5)2SmOC(C5Me5)N-
1
(Ph)C(NPh)O, 3, and [(C5Me5)2Sm(µ-CN)(CNCMe3)]3(THF)2,
4‚2THFa
Reaction with OdPPh3. In the glovebox, OdPPh3 (11 mg, 0.045
mmol) was reacted with (C5Me5)3Sm (50 mg, 0.091 mmol) as described
above. After 30 min, the color had turned to yellow, and a white
precipitate was observed. The reaction workup was performed in the
manner described above to isolate a yellow powder (35 mg, 94%). This
yellow powder was recrystallized from THF and identified as the
compd
2
3
4‚2THF
formula
C
44H55N2Sm
C44H55N2O2Sm
794.25
C86H133N6O2Sm3
1734.03
fw
762.25
crystal dimen 0.20 × 0.23 × 0.30 0.17 × 0.20 × 0.20 0.27 × 0.30 × 0.36
(mm)
crystal color yellow
yellow
yellow
Temp (K)
163
158
163
1
previously characterized [(C5Me5)2Sm]2(µ-O)15 by H NMR spectro-
crystal system triclinic
triclinic
P1
triclinic
P1
scopy (C6D6). (C5Me5)2 (11 mg, 95%) and PPh3 (10 mg, 89%) were
identified as described above.
space group P1
a (Å)
b (Å)
c (Å)
10.416(2) Å
11.2894(11)
11.442(2)
17.509(2)
100.280(9)
97.755(8)
115.222(9)
1955.9(4)
2
14.290(2)
16.540(2)
18.288(2)
78.823(8)
84.929(7)
86.441(7)
4219.5(7)
2
10.506(2)
18.565(3)
77.06(2)
82.60(2)
85.31(2)
1960.7(7)
2
Reaction with PhNdNPh. In the glovebox, azobenzene (17 mg,
0.092 mmol) was added to (C5Me5)3Sm (50 mg, 0.091 mmol) in toluene
(5 mL). After 15 min, the mixture had turned dark green. Toluene
was removed in vacuo to afford a tacky, dark-green material which
was washed with hexane to give a dark-green powder (53 mg, 93%).
1H NMR spectroscopy identified the green solid as the previously
characterized (C5Me5)2Sm(N2Ph2).16 (C5Me5)2 (11 mg, 89%) was
identified as described above. Two equivalents of (C5Me5)3Sm were
also reacted with azobenzene in an NMR tube experiment. No evidence
for the organosamarium compound containing the dianionic azobenzene
moiety, [(C5Me5)2Sm]2(µ-N2Ph2),16,17 was observed.
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (mg/m3) 1.291
1.349
1.365
diffractometer Siemens P4
Siemens P4
rotating anode
1.538
Siemens P4
µ (mm-1
)
1.528
0.1557
2.107
refinement,
0.0921
0.0770
wR2b
refinement, R1 0.0718
goodness of fit 1.052
0.0390
1.195
0.0430
1.046
Reaction with 1,3,5,7-C8H8. In an argon-filled glovebox, cyclooc-
tatetraene (1.5 µL, 0.013 mmol) was added to (C5Me5)3Sm (15 mg,
0.027 mmol) in C6D6 (1 mL). No color change was observed upon
addition. After the solution was allowed to stir at room temperature
for 18 h, it was transferred to an NMR tube, and the 1H NMR spectrum
was obtained. (C8H8)Sm(C5Me5),2 (C5Me5)2, and (C5Me5)3Sm were
a Radiation: Mo KR (µ ) 0.710 730 Å). Monochromator: highly
oriented graphite. b wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
absorption and for Lorentz and polarization effects and were placed
on an approximately absolute scale. There were no systematic absences
nor any diffraction symmetry other than the Friedel condition. The
centrosymmetric triclinic space group P1 was assigned and later
determined to be correct.
The structure was solved by direct methods and refined on F2 by
full-matrix least-squares techniques. The analytical scattering factors
for neutral atoms were used throughout the analysis.22 Hydrogen atoms
were included, using a riding model. The cyclopentadienyl ring defined
by atoms C(11)-C(15) is disordered. Two complete rings, each with
site-occupancy-factors ) 0.50, were included to account for the
observed rotational disorder.
1
identified by H NMR spectroscopy.
Reaction with Me3CNC. In the glovebox, addition of Me3CNC
(30 mg, 0.36 mmol) in toluene (5 mL) to (C5Me5)3Sm (100 mg, 0.18
mmol) in toluene (5 mL) caused an immediate color change to red.
The red toluene solution was transferred to a flask fitted with a high
vacuum greaseless stopcock. The flask was removed from the
glovebox, attached to a vacuum line, and the system was evacuated to
the vapor pressure of the solvent. The flask was heated to 80 °C for
1 h, during which time the solution began to turn yellow with
concomitant formation of a yellow precipitate. The yellow, toluene-
insoluble material was isolated by filtration (83 mg). Rotary evapora-
tion of the toluene filtrate yielded a tacky material which exhibited an
extremely complex 1H NMR spectrum in C6D6 containing several peaks
between 1.0 and 3.0 ppm. The toluene-insoluble material was
recrystallized from hot THF (51 mg, 54%) over several days and
identified as the previously reported [(C5Me5)2Sm(µ-CN)(CNCMe3)]3,
4,18 by IR spectroscopy and X-ray crystallography.
X-ray Data Collection, Structure Determination, and Refinement
for (C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh), 2. The determination of
Laue symmetry, crystal class, unit cell parameters, and the crystal’s
orientation matrix were carried out according to standard procedures.19
Intensity data were collected using a 2θ/ω scan technique. Details are
given in Table 1. The raw data were processed with a local version of
CARESS20 which employs a modified version of the Lehman-Larsen
algorithm to obtain intensities and standard deviations from the
measured 96-step peak profiles. Subsequent calculations were carried
out using the SHELXTL21 program. All 5438 data were corrected for
X-ray Data Collection, Structure Determination, and Refinement
for (C5Me5)2SmOC(C5Me5)N(Ph)C(NPh)O, 3. Intensity data were
collected as described above. All 5360 data were corrected for decay
(5.0%), absorption, and for Lorentz and polarization effects and were
placed on an approximately absolute scale. There were no systematic
absences nor any diffraction symmetry other than the Friedel condition.
The centrosymmetric triclinic space group P1 was assigned and later
determined to be correct. The structure was solved as described above.
X-ray Data Collection, Structure Determination, and Refinement
for [(C5Me5)2Sm(µ-CN)(CNCMe3]3, 4. Intensity data were collected
as described above. All 13 819 data were corrected for absorption and
for Lorentz and polarization effects and were placed on an ap-
proximately absolute scale. There were no systematic absences nor
any diffraction symmetry other than the Friedel condition. The
centrosymmetric triclinic space group P1 was assigned and later
determined to be correct.
The structure was solved as described above for 2. There are two
THF molecules present per formula unit. The bridging CN ligands
appear to be disordered. It was not possible to distinguish between
the carbon and nitrogen positions on the basis of either thermal
parameters or interatomic distances. Refinement of the ligands as
discrete nitrogen and carbon atoms led to lower than expected thermal
parameters for the atoms designated as carbon and higher than expected
thermal parameters for the atoms designated as nitrogen. The same
trend was observed upon reassignment of the carbon atoms as nitrogen
and of the nitrogen atoms as carbon. The conclusion is that the nitrogen
(15) Evans, W. J.; Grate, J. W.; Bloom, I.; Hunter, W. E.; Atwood, J. L.
J. Am. Chem. Soc. 1985, 107, 405.
(16) Evans, W. J.; Drummond, D. K.; Chamberlain, L. R.; Doedens, R.
J.; Bott, S. G.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1988, 110,
4983.
(17) Evans, W. J.; Drummond, D. K.; Bott, S. G.; Atwood, J. L.
Organometallics 1986, 5, 2389.
(18) Evans, W. J.; Drummond, D. K. Organometallics 1988, 7, 797.
(19) XSCANS Software Users Guide, Version 2.1, Siemens Industrial
Automation, Inc., Madison, WI, 1994.
(20) Broach, R. W. Argonne National Laboratory, Argonne, Illinois,
1978.
(21) Sheldrick, G. M., Siemens Analytical X-ray Instruments, Inc.;
Madison, WI, 1990.
(22) International Tables for X-ray Crystallography; Dordrecht: Kluwer
Academic Publishers, 1992; Vol. C.

9276 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
EVans et al.
and carbon atoms are equally disordered over the two sites. Refinement
of the ligands assuming an equal electron distribution over the two
sites resulted in nearly equal thermal parameters for each atom.
Results
Reactions with Lewis Bases. The initially studied reactions
of (C5Me5)3Sm with CO,5 CH2dCH2,6 and H2,6 eqs 3-5, were
consistent with the existence of an intermediate of the type (C5-
Me5)2Sm(η1-C5Me5). Since no evidence was obtained by low-
temperature NMR spectroscopy for such a structure, reactions
with Lewis bases were examined in efforts to trap an adduct
such as (C5Me5)2Sm(η1-C5Me5)L (L ) Lewis base). THF was
not useful in this regard since it is ring-opened as described
below. Likewise, Ph3PdO and its sulfur and selenium conge-
ners were not useful because they were reduced as described
below. Pyridine proved to be the most informative.
Addition of 1 equiv of pyridine to (C5Me5)3Sm in toluene-d8
results in a color change from brown to red. Two peaks were
observed in the 1H NMR spectrum in the C5Me5 region, one at
-1.24 ppm, characteristic for (C5Me5)3Sm, and one at 1.20 ppm.
Addition of 3 equiv of pyridine to a toluene solution of (C5-
Me5)3Sm gives a red solution with only the 1.20 ppm resonance
in the C5Me5 region plus three very broad signals between 2.0
and 5.5 ppm. Low-temperature NMR experiments were per-
formed to determine if the peak located at 1.20 ppm would split
into a pattern indicative of (C5Me5)2Sm(η1-C5Me5)(py). At -20
°C, the 1H NMR spectrum contained peaks at 5.39, 3.06, 2.41,
and 1.66 ppm of relative intensity 3:6:6:30 consistent with the
presence of a (C5Me5)2Sm(η1-C5Me5)(pyridine) structure. How-
ever, the peaks due to coordinated pyridine could not be
definitively located in this paramagnetic system. The spectrum
provided no additional information at lower temperatures, and
attempts to crystallize a pyridine adduct have been unsuccessful.
Insertion Chemistry. Reactions with Nitriles. In contrast
to the pyridine reactions, brown toluene solutions of (C5Me5)3-
Sm react with nitriles to form yellow-orange solutions. The
yellow product, 2, obtained from (C5Me5)3Sm and PhCtN had
Figure 1. Thermal ellipsoid plot of (C5Me5)2Sm[NC(Ph)(C5Me5)]-
(NCPh), 2, with ellipsoids drawn at the 50% probability level.
structural evidence for the availability of an intermediate with
η1-C5Me5-like reactivity in the (C5Me5)3Sm system. For
example, (C5Me5)(C2B9H11)TiMe reacts analogously with MeCN
to form (C5Me5)(C2B9H11)Ti(NCMe2)(MeCN), which has simi-
lar spectroscopic and structural features.25a The spectroscopic
properties of (C5Me5)2Y{NC(CMe3)[CH(SiMe3)2]}(NCCMe3)
generated from (C5Me5)2Y[CH(SiMe3)2] and Me3CCN are also
similar.25b
Complex 2 has an eight-coordinate geometry around sa-
marium which is typical of trivalent (C5Me5)2Sm-containing
complexes3 and the metrical parameters for the trivalent
decamethylsamarocene component are in the normal range,3
Table 2. The 2.225(8) Å Sm-N(1) distance in 2 is considerably
shorter than the 2.533(8) Å Sm-N(2) bond length, Table 3.
This is consistent with N(1) being part of a formally anionic,
imine ligand, [NdC(Ph)(C5Me5)]-, while N(2) is part of a
neutral nitrile ligand.26 The C(37)-N(1) distance of 1.261(12)
Å is longer than the C(38)-N(2) distance of 1.137(11) Å, which
is consistent with the CdN double bond character of the imine
ligand versus the CtN triple bond of the coordinated nitrile.25,27
Hence, the structural data match the IR data presented above.
The pentamethylcyclopentadienyl substituent attached to C(37)
has a single bond between C(37) and C(21) [1.57(1) Å],
localized CdC double bonds between C(22) and C(23) [1.33-
(2) Å] and C(24) and C(25) [1.29(2) Å], and single bonds
between C(21) and C(25) [1.51(2) Å] and C(21) and C(22)
[1.49(2) Å].27 The 1.40(2) Å C(23)-C(24) bond distance is
intermediate. Both the coordinated nitrile and the imine ligand
are bound in a very nearly linear fashion to Sm; the Sm-N(2)-
C(38) angle is 174.3(7)° while the Sm-N(1)-C(37) angle is
173.7(7)°.
1
a H NMR spectrum containing a singlet at 1.65 ppm in the
region where resonances for C5Me5 groups attached to Sm(III)
are found, as well as three other peaks at 2.88, 2.12, and 1.47
ppm. The 2:2:1 integration ratio of these latter three peaks was
consistent with the presence of a σ-bound C5Me5 moiety. The
infrared spectrum of 2 contained an absorption at 1630 cm-1
in the region characteristic of CdN stretches and a band at 2250
cm-1 (cf. 2232 cm-1 for free PhCtN) which could be
attributed to PhCtN coordinated to samarium.23,24
X-ray crystallography definitively confirmed the implications
of the spectroscopic data and showed that 2 was (C5Me5)2Sm-
[NC(Ph)(C5Me5)](NCPh), Figure 1, in which one benzonitrile
unit had been inserted into a Sm-C5Me5 linkage while another
coordinated to samarium. This reaction, eq 6, presented strong
Reaction with PhNCO. PhNCO also reacts immediately
with a toluene solution of (C5Me5)3Sm to form a yellow product,
3. The 1H NMR spectrum of 3 contained resonances attributable
to a trivalent decamethylsamarocene unit and an σ-bound C5-
Me5 unit; however, as usual for paramagnetic Sm(III) com-
plexes, it was not structurally definitive. X-ray crystallography
(6)
(25) (a) Kreuder, C.; Jordan, R. F.; Zhang, H. Organometallics 1995,
14, 2993. (b) den Haan, K. H.; Luinstra, G. A.; Meetsma, A.; Teuben, J.
H. Organometallics 1987, 6, 1509.
(26) Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1986, 108, 7440.
Evans, W. J.; Kociok-Ko¨hn, G.; Leong, V. S.; Ziller, J. W. Inorg. Chem.
1992, 31, 3592. Evans, W. J.; Ansari, M. A.; Khan, S. I.; Ziller, J. W.
Inorg. Chem. 1996, 35, 5435.
(23) Storhoff, B. N.; Lewis, H. C., Jr. Coord. Chem. ReV. 1977, 23, 1.
(24) Adam, M. Schultze, H.; Fischer, R. D. J. Organomet. Chem. 1992,
429, C1. Heeres, H. J.; Meetsma, A.; Teuben, J. H. Angew. Chem., Int. Ed.
Engl. 1990, 29, 420. Li, X. F.; Eggers. S.; Kopf, J.; Jahn, W.; Fischer, R.
D.; Apostolidis, C.; Kanellakopulos, B.; Benetollo, F.; Polo, A.; Bombieri,
G. Inorg. Chim. Acta 1985, 100, 183. Deacon, G. B.; Forsyth, C. M.;
Newnham, R. H.; Tuong, T. D. Aust. J. Chem. 1987, 40, 895.
(27) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.

Tris(pentamethylcyclopentadienyl)samarium
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9277
Table 2. Bond Distances (Å) and Angles (°) in Metallocene Components of (C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh), 2,
(C5Me5)2SmOC(C5Me5)N(Ph)C(NPh)O, 3, and [(C5Me5)2Sm(µ-CN)(CNCMe3)]3, 4
2
3
4
Sm-C(C5Me5), range
2.69(2)-2.77(2)
2.666(5)-2.747(5)
2.697(5)-2.806(5)
Sm-C(C5Me5), average
2.73(3)
2.71(3)
2.75(3)
(ring centroid)-Sm-(ring centroid) angle
137.6
136.5
132.5
Table 3. Selected Bond Distances (Å) and Angles (deg) for (C5Me5)2Sm[NC(Ph)(C5Me5)](NCPh), 2, (C5Me5)2SmOC(C5Me5)N(Ph)C(NPh)O,
3, and [(C5Me5)2Sm(µ-CN)(CNCMe3)]3, 4
2
3
4
Sm(1)-N(1)
Sm(1)-N(2)
N(1)-C(37)
N(2)-C(38)
C(31)-C(37)
C(38)-C(39)
C(21)-C(37)
C(21)-C(22)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(21)-C(25)
2.225(8)
2.533(8)
1.261(12)
1.137(2)
1.509(14)
1.430(13)
1.570(14)
1.49(2)
Sm(1)-O(1)
Sm(1)-O(2)
O(1)-C(31)
N(1)-C(31)
N(1)-C(32)
N(1)-C(38)
N(2)-C(38)
O(2)-C(38)
N(2)-C(39)
C(31)-C(21)
C(21)-C(22)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(21)-C(25)
2.386(3)
2.259(3)
1.252(6)
1.354(7)
1.456(7)
1.483(7)
1.278(7)
1.278(6)
1.409(7)
1.541(8)
1.523(8)
1.342(8)
1.447(8)
1.344(8)
1.526(8)
Sm(1)-N(1)
Sm(1)-C(63)
Sm(1)-C(64)
Sm(2)-N(2)
Sm(2)-C(61)
Sm(2)-C(69)
Sm(3)-N(3)
Sm(3)-C(62)
Sm(3)-C(74)
2.561(4)
2.604(5)
2.651(5)
2.589(4)
2.606(5)
2.645(5)
2.576(5)
2.612(5)
2.634(5)
1.33(2)
1.40(2)
1.29(2)
N(1)-Sm(1)-C(63)
N(2)-Sm(2)-C(61)
N(3)-Sm(3)-C(62)
Sm(1)-N(1)-C(61)
Sm(2)-N(2)-C(62)
Sm(3)-N(3)-C(63)
Sm(1)-C(63)-N(3)
Sm(2)-C(61)-N(1)
Sm(3)-C(62)-N(2)
72.91(13)
71.33(13)
74.27(13)
175.1(4)
168.5(4)
171.4(4)
171.1(4)
175.6(4)
176.7(4)
1.51(2)
N(1)-Sm(1)-N(2)
95.3(3)
Sm(1)-N(1)-C(37)
Sm(1)-N(2)-C(38)
N(1)-C(37)-C(31)
N(1)-C(37)-C(21)
C(31)-C(37)-C(21)
N(2)-C(38)-C(39)
173.7(7)
174.3(7)
119.8(9)
123.4(9)
116.7(8)
178.9(10)
O(1)-Sm(1)-O(2)
69.53(12)
139.6(3)
137.3(3)
121.3(5)
117.6(5)
121.1(5)
122.5(5)
123.4(4)
113.9(5)
112.8(5)
116.6(5)
130.5(5)
118.7(5)
Sm(1)-O(1)-C(31)
Sm(1)-O(2)-C(38)
O(1)-C(31)-N(1)
O(1)-C(31)-C(21)
C(21)-C(31)-N(1)
C(31)-N(1)-C(32)
C(31)-N(1)-C(38)
C(32)-N(1)-C(38)
N(1)-C(38)-N(2)
N(1)-C(38)-O(2)
O(2)-C(38)-N(2)
C(38)-N(2)-C(39)
Scheme 1
by a C-N bond and form the metallacylic structure shown in
Scheme 1. If the reaction proceeds as observed for the PhCN
reaction, an intermediate of the type (C5Me5)2Sm[OC(C5Me5)N-
(Ph)](OCNPh) could initially form. Since this contains a
nucleophilic nitrogen adjacent to the carbon atom of the
isocyanate, a position which is generally electrophilic,28 it is
reasonable to expect that a samarium-mediated coupling oc-
curred to form the observed product.
Figure 2. Thermal ellipsoid plot of (C5Me5)2SmOC(C5Me5)N(Ph)C-
(NPh)O, 3, with thermal ellipsoids drawn at the 50% probability level.
was used to identify the product as (C5Me5)2SmOC(C5Me5)N-
(Ph)C(NPh)O, Figure 2. This complex is analogous to 2 in that
it is composed of one (C5Me5)2Sm unit, two molecules of
substrate, and pentamethylcyclopentadiene. However, it differs
in that neither of the substrate molecules are simply coordinated
to samarium. Instead, the two substrate molecules are joined
(28) Patai, S., Ed.; The Chemistry of Cyanates and Their Thio DeriVa-
tiVes; Wiley: New York, 1977.

9278 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
EVans et al.
Scheme 2
Me5)2Sm[O(CH2)4(C5Me5)](THF), eq 7, was that previously
(C5Me5)3Sm + THF f
(C5Me5)2Sm[O(CH2)4(C5Me5)](THF) (7)
identified from the reaction of [(C5Me5)2Sm(THF)2][BPh4] with
KC5Me5, eq 8.12 In fact, this THF ring-opening reactivity is a
[(C5Me5)2Sm(THF)2][BPh4] + KC5Me5 f
The coordination environment around samarium in complex
3 is typical of trivalent (C5Me5)2Sm-containing complexes, and
the complex has no unusual distances or angles in the metal-
locene unit,3 Table 2. The 69.52(12)° O(1)-Sm-O(2) angle
in 3 is less than typical donor-atom-metal-donor-atom angles
in compounds of this type due to the fact that the two donor
atoms are part of a chelating ligand system.
The σ-bound C5Me5 portion of the molecule is much like
that in 2, i.e., it has the characteristics of a localized diene
substituent, Table 3. This C5Me5 ring is attached by a 1.541-
(8) Å C(21)-C(31) single bond, and localized double bonds
are present for C(25)-C(24), 1.344(8) Å, and C(22)-C(23),
1.342(8) Å. The 1.523(8) Å C(21)-C(22) and 1.526(8) Å
C(21)-C(25) bonds have typical single bond lengths and the
1.447(8) Å C(23)-C(24) bond distance is between the single
and double bond distance ranges as in 2.
(C5Me5)2Sm[O(CH2)4(C5Me5)](THF) (8)
major obstacle in the isolation of (C5R5)3M complexes, and
reaction products of this type have been found for a variety of
lanthanides including La, Nd, Tm, and Lu.29
Reaction 8 was understandable since THF is activated to
nucleophilic ring opening by Lewis acids,30 and coordination
of THF to the cationic electrophilic samarium center should
activate it with respect to ring opening by (C5Me5)-. The
reaction of (C5Me5)3Sm with THF was more difficult to
rationalize since the nucleophile rather than THF was attached
to the metal. However, this reactivity can be explained if an
η1-C5Me5 intermediate is accessible as shown in eqs 9 and 10.
(C5Me5)3Sm + THF f (C5Me5)2Sm(η1-C5Me5)(THF) (9)
The bond distances in the rest of the chelating ligand indicate
considerable delocalization. The bond angles around C(31),
C(38), and N(1) are consistent with sp2 hybridization, although
Sm(1), O(1), O(2), C(31), C(38), and N(1) are not rigorously
coplanar. O(2) is out of the plane of these atoms by 0.17 Å,
and N(2) is out of the plane by 0.12 Å in the other direction.
The ipso phenyl carbon atoms C(32) and C(39) and the C5Me5
C(21) are out of from this plane by 0.04, 0.06, and 0.17 Å,
respectively. The C(32)-C(37) phenyl ring plane is almost
perpendicular to this plane with a 93.6° dihedral angle. The
C(39)-C(44) phenyl ring plane is tilted at a 44.6° angle to the
SmO2C2N plane. Within this extended ligand, there are four
short bonds, C(31)-O(1), C(38)-O(2), C(31)-N(1), and
C(38)-N(2). The 1.278(6) Å C(38)-O(2) and 1.252(6) Å
C(31)-O(1) distances are equivalent within their error limits
and are between the 1.192-1.256 Å range of C(sp2)dO and
the 1.293-1.407 Å range of C(sp2)-O distances in the
literature.27 The 1.278(7) Å C(38)-N(2) bond is near the low
end of the 1.279-1.329 Å range of reported C(sp2)dN
distances.27 The 1.354(7) Å C(31)-N(1) bond is close to the
1.279-1.329 Å range of C(sp2)dN distances in the literature
but is in the middle of the 1.321-1.416 Å range of C(sp2)-N
distances which overlaps with the other range. The 1.483(7)
Å C(38)-N(1) bond is clearly longer than this typical single
bond range. The N-C(ispo) bonds, 1.456(7) Å N(1)-C(32)
and 1.409(7) Å N(2)-C(39), also have lengths indicative of
single bonds. The 2.386(3) Å Sm-O(1) and 2.259(3) Å Sm-
O(2) distances are within the 2.08(1) Å to 2.39(1) Å range
observed for (C5Me5)2Sm-OR bonds involving anionic oxygen
donor ligands such as alkoxides and are shorter than the 2.44-
(2) Å to 2.72(2) Å range of (C5Me5)2ZSm-(OR2) bonds (Z )
monoanionic ligand) for neutral oxygen donor ligands.3,12,15 Two
resonance structures which correlate with these crystallographic
data are shown in Scheme 2. Both reflect the important reaction
chemistry, namely that the isocyanate has inserted into a Sm-
C5Me5 moiety and a second isocyanate has coupled to this unit.
Ring-Opening Reactions. Like benzonitrile, THF reacts
immediately with (C5Me5)3Sm to form yellow solutions. It was
discovered during a study on the development of new synthetic
routes to (C5Me5)3Sm4 that the product of this reaction, (C5-
(C5Me5)2Sm(η1-C5Me5)(THF)f
(C5Me5)2Sm[O(CH2)4(C5Me5)](THF) (10)
If THF could coordinate to make a (C5Me5)2Sm(η1-C5Me5)-
(THF) intermediate, this species would have an activated THF
adjacent to a nucleophile. Analogous chemistry has been
observed for (C5Me5)2Sm(PPh2)(THF) and (C5Me5)2Sm(AsPh2)-
(THF), which can be isolated but subsequently form ring-opened
products analogous to (C5Me5)2Sm[O(CH2)4(C5Me5)](THF), i.e.,
(C5Me5)2Sm[O(CH2)4PPh2](THF) and (C5Me5)2Sm[O(CH2)4-
AsPh2](THF).31
The fact that increasing the steric bulk of the furan interferes
with this reactivity is consistent with this scenario. Hence, 2,5-
dimethyltetrahydrofuran does not react with (C5Me5)3Sm, even
at 75 °C. This sterically more bulky furan would not be able
to coordinate as easily to the sterically crowded (C5Me5)2Sm-
(η1-C5Me5) unit.
ꢀ-Caprolactone is polymerized by (C5Me5)3Sm, eq 11, a
(C5Me5)3Sm + caprolactone f poly-caprolactone (11)
reaction which again can be rationalized with the intermediacy
of a (C5Me5)2Sm(η1-C5Me5)L unit since ꢀ-caprolactone is known
to coordinate lanthanides32 and is readily polymerized by alkyl
lanthanide complexes.33
Reduction Chemistry. Ph3PdE Reactions. Triphenylphos-
phine oxide and its congeners, Ph3PdE (E ) S, Se), were
initially examined as possible trapping agents for a (C5Me5)2-
Sm(η1-C5Me5) intermediate. (C5Me5)3Sm reacts with these
(29) Schumann, H.; Glanz, M.; Hemling, H.; Go¨rlitz, F. H. J. Organomet.
Chem. 1993, 462, 155.
(30) Dreyfuss, P.; Dreyfuss, M. P. In Ring Opening Polymerization;
Frisch, K. C.; Reegen, S. L., Eds.; Marcel Dekker: New York, 1969; Chap
2. Penczek, S.; Kubisa, P.; Matyjaszewski, K. AdV. Polym. Sci. 1980, 37,
1. Woodhouse, M. E.; Lewis, F. D.; Marks, T. J. J. Am. Chem. Soc. 1982,
104, 5586.
(31) Evans, W. J.; Leman, J. T.; Ziller, J. W. Inorg. Chem. 1996, 35,
4238.
(32) Evans, W. J.; Shreeve, J. L.; Doedens, R. J. Inorg. Chem. 1993,
32, 245. Evans, W. J.; Shreeve, J. L.; Boyle, T. J.; Ziller, J. W. J. Coord.
Chem. 1995, 34, 229.

Tris(pentamethylcyclopentadienyl)samarium
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9279
reagents but not in the manner originally expected. Addition
of THF to the Ph3PdS and Ph3PdSe reaction products gives
the previously characterized trivalent organosamarium com-
pounds [(C5Me5)2Sm(THF)]2(µ-E).13 The conversion of Ph3Pd
E to (E)2- is a two-electron reduction equivalent to that
previously observed with (C5Me5)2Sm(THF)2 according to eq
12.13 In the case of reaction 12, however, the reduction is done
1,3,5,7-Cyclooctatetraene is reduced at -1.83 V (vs SCE)35
and is converted by (C5Me5)2Sm to (C5Me5)3Sm and (C5-
Me5)Sm(C8H8) according to eq 1.2 (C5Me5)3Sm also reduces
1,3,5,7-C8H8, as shown in eq 18, in a reaction which is parallel
2(C5Me5)3Sm + 1,3,5,7-C8H8 f
(C5Me5)3Sm + (C5Me5)Sm(C8H8) + (C5Me5)2 (18)
2(C5Me5)2Sm(THF)2 + Ph3PdE f
to eq 1.
In contrast to (C5Me5)2Sm, (C5Me5)3Sm is not a strong
enough reducing agent to reduce anthracene (-1.98 V vs SCE35)
or pyrene (-2.10 V vs SCE35). (C5Me5)2Sm reduces these
substrates to form the crystallographically characterized trivalent
complexes, [(C5Me5)2Sm]2(µ-C14H10) and [(C5Me5)2Sm]2(µ-
C16H10).36
[(C5Me5)2Sm(THF)]2(µ-E) + Ph3P (12)
with 2 equiv of the strongly reducing divalent (C5Me5)2Sm-
(THF)2, whereas the reaction starting with (C5Me5)3Sm involves
2 equiv of a trivalent samarium complex. Since a Sm(III) to
Sm(IV) conversion is not thermodynamically reasonable,10 the
reduction in this (C5Me5)3Sm-based reaction must be arising
from a C5Me5 ligand. Indeed, isolated as a byproduct along
with triphenylphosphine was (C5Me5)-(C5Me5), the product
known to form by oxidative coupling of (C5Me5)-.14 The
overall reaction is shown in eq 13. (C5Me5)3Sm also reduces
Me3CNC Reactions. The reactivity of (C5Me5)3Sm with
isocyanides was examined since (C5Me5)3Sm readily reacts with
CO5, and isocyanides often have chemistry similar to that of
CO. (C5Me5)3Sm reacts instantly with Me3CNC in a 1:1 molar
ratio in toluene at room temperature, but in contrast to the CO
reaction which gives a yellow product, an intensely colored red
solution is obtained. Removal of the solvent yields a red powder
that exhibits two singlets in the 1H NMR spectrum at 0.37 ppm
and -0.10 ppm which integrate in a 5:1 ratio, respectively,
consistent with formation of a (C5Me5)3Sm(CNCMe3) adduct.
The signal at 0.37 ppm, which corresponds to three C5Me5
ligands, is shifted 1.61 ppm downfield from the -1.24 ppm
resonance of (C5Me5)3Sm, while the resonance corresponding
to the Me3C group is shifted 1.90 ppm upfield from that of free
Me3CNC. Infrared spectroscopy reveals a strong band at 2155
cm-1 in the CtN bond stretching region which is 20 cm-1
higher than that of free Me3CNC.37 Hence, the spectroscopic
data imply that an isocyanide adduct had formed without causing
an η5 to η1-(C5Me5) shift and that no insertion had taken place.
Crystals suitable for structural characterization were not isolated
from this product, but crystallographic data were obtainable from
this system when the Me3CNC/(C5Me5)3Sm reaction was heated
to 80 °C in toluene for 1 h. The solution turned pale yellow,
and a yellow precipitate was isolated in 87% yield. Yellow
crystals were obtained from a concentrated THF solution and
identified by IR spectroscopy and X-ray crystallography as [(C5-
Me5)2Sm(µ-CN)(CNCMe3)]3, 4 (Figure 3), reaction 19. This
2(C5Me5)3Sm + Ph3PdE f THF8
Ph3P + (C5Me5)2 + [(C5Me5)2Sm(THF)]2(µ-E) (13)
Ph3PdO to make the previously characterized oxide complex,
[(C5Me5)2Sm]2(µ-O),15 as shown in eq 14.
2(C5Me5)3Sm + Ph3PdO f
Ph3P + (C5Me5)2 + [(C5Me5)2Sm]2(µ-O) (14)
Azobenzene, Cyclooctatetraene, and Pyrene Reactions. To
determine if the reductive reactivity of (C5Me5)3Sm observed
in the Ph3PdE reactions was general and to estimate the
effective reduction potential of (C5Me5)3Sm, reactions with other
reducible substrates with known reduction potentials were
examined. Substrates which had been previously reduced with
divalent (C5Me5)2Sm were preferred because their organosa-
marium reduction products had already been fully characterized.
Azobenzene, which has a first reduction potential of -1.35
to -1.41V and a second reduction potential of -1.75 to -2.03V
(vs SCE),34 can be reduced by 1 and 2 equiv of (C5Me5)2Sm-
(THF)2 as shown in eqs 15 and 16.16 (C5Me5)3Sm reacts with
2(C5Me5)3Sm + Me3CNC f
[(C5Me5)2Sm(µ-CN)(CNCMe3)]3 (19)
(C5Me5)2Sm(THF)2 + PhNdNPh f
(C5Me5)2Sm(Ph2N2)(THF) (15)
product was previously obtained by reduction of Me3CNC with
(C5Me5)2Sm,18 reaction 20.
2(C5Me5)2Sm(THF)2 + PhNdNPh f
[(C5Me5)2Sm]2(Ph2N2) (16)
(C5Me5)2Sm + Me3CNC f
[(C5Me5)2Sm(µ-CN)(CNCMe3)]3 (20)
azobenzene to form only the one-electron reduction product,
(C5Me5)2Sm(Ph2N2), as shown in eq 17. This reaction estab-
Since only a low quality crystal structure was obtainable on
4 previously, the full structural details are included here. The
structure of 4 is very similar to that of [(C5Me5)2Sm(µ-CN)-
(CNC6H11)]3, 5, obtained from (C5Me5)2Sm and CNC6H11,18 i.e.,
three isocyanide-solvated decamethylsamarocene units are
located at the corners of a triangle connected by bridging cyanide
ligands along the edges. As is common for X-ray analyses of
2(C5Me5)3Sm + 2Ph2N2 f
2(C5Me5)2Sm(Ph2N2) + (C5Me5)2 (17)
lished that (C5Me5)3Sm is a reducing agent weaker than (C5-
Me5)2Sm.
(33) Evans, W. J.; Katsumata, H. Macromolecules 1994, 27, 2330. Evans,
W. J.; Katsumata, H. Macromolecules 1994, 27, 4011. McLain, S. J.; Ford,
T. M.; Drysdale, N. E. Polym. Prepr., (Am. Chem. Soc., DiV. Polym. Chem.)
1992, 33, 463.
(34) Thomas, F. G.; Boto, K. G. In The Chemistry of the Hydrazo, Azo,
and Azoxy Groups; Patai, S., Ed.; Wiley: New York, 1975; Chapter 12.
(35) de Boer, E. AdV. Organomet. Chem. 1964, 2, 115.
(36) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1994,
116, 2600.
(37) Dolphin, D.; Wick, A. E. Tabulation of Infrared Spectral Data;
Wiley: New York, 1977. Malatesta, L.; Bonati, R. Isocyanide Complexes
of Metals; Wiley: New York, 1969.

9280 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
EVans et al.
Scheme 3
Figure 3. Thermal ellipsoid plot of [(C5Me5)2Sm(µ-CN)(CNCMe3)]3,
4, with ellipsoids drawn at the 50% probability level.
in eq 5 is quite reasonable, assuming a (C5Me5)2Sm(η1-C5Me5)
intermediate.
cyanide groups,38 it was not possible to differentiate between
carbon and nitrogen atoms for these ligands. Hence, the three
independent values of the Sm-C(isocyanide donor atom) distance
are 2.651(5) Å, 2.645(5) Å, and 2.634(5) Å and are equivalent
within experimental error to the analogous distances found in
5. The 1.165(6) Å, 1.162(6) Å, and 1.156(6) Å C-N distances
also compare favorably to the cyanide distances in 5 and are
consistent with the presence of a triple bond in a cyanide
ligand.27 The Sm-C-N (Sm-N-C) angles in these sides
approach linearity, ranging from 171.1(4)° to 176.7(4)°. The
Sm-C(ring) distances and ring-centroid-Sm-ring-centroid angles
are typical for trivalent decamethylsamarocene units, Table 2.3
The (C5Me5)3Sm insertion chemistry of CO, eq 3, PhCN, eq
6, and PhCNO, Scheme 1, can also be explained using this (C5-
Me5)2SmR model since insertion reactions are also common for
this class of complexes. It is well established that CO inserts
into lanthanide alkyl bonds to make reactive intermediates which
can add 1 equiv of CO.41 Hence, a sequence of the type shown
in Scheme 3 can be envisioned for the reaction of CO with
(C5Me5)3Sm.
The steps in Scheme 2 are strongly supported by the insertion
reactions with PhCN and PhNCO. The PhCN reaction, eq 6,
models the first insertion of a substrate into a Sm(η1-C5Me5)
unit, and the isolation of the coupled ligand in 3 shows that a
second equivalent of substrate can be incorporated into the first
insertion product (Scheme 1).
Discussion
The reactions of (C5Me5)3Sm investigated so far appear to
fall into two categories, depending on the substrate encountered.
One class of reactions can be explained in terms of access to
an η1-pentamethylcyclopentadienyl intermediate such as (C5-
Me5)2Sm(η1-C5Me5) and the other in terms of one-electron
reduction in which one of the (C5Me5)- ligands is functioning
as the reductant. These are unusual reaction pathways for C5-
Me5 ligands attached to metals, but both are reasonable in this
sterically crowded (C5Me5)3Sm complex since they lead to relief
of the steric strain. Since the first class of reactions is the easiest
to rationalize, this will be discussed first.
(C5Me5)2Sm(η1-C5Me5) Reactivity. Several of the reactions
of (C5Me5)3Sm are typical of metallocene alkyl complexes such
as (C5Me5)2SmR, and in the case of these (C5Me5)3Sm reactions,
R is η1-C5Me5. The general class of (C5Me5)2SmR complexes
is well-documented in the literature with crystallographically
characterized examples known for the solvated (C5Me5)2SmR-
(THF) analogues with R ) Ph,39 CH2Ph,40 and Me7e and for
the closely related (C5Me5)2Nd[CH(SiMe3)2].7c Since hydro-
genolysis reactivity is well-known for these (C5Me5)2SmR
complexes,7c,9,40 the analogous reaction by (C5Me5)3Sm shown
Although the reaction patterns and products are consistent
with a (C5Me5)2Sm(η1-C5Me5) intermediate, there is no spec-
troscopic or structural evidence for this species in the solid state
or in solution at low temperature. However, pyridine appears
to form a Lewis base adduct with (C5Me5)3Sm which has an
NMR spectrum consistent with a (C5Me5)2Sm(η1-C5Me5)-
(pyridine) structure. This suggests that, in the presence of the
appropriate Lewis base, (C5Me5)3Sm can react as if it were an
alkyl complex such as (C5Me5)2Sm(R)L.
The ring-opening reactivity of (C5Me5)3Sm with THF is also
consistent with a (C5Me5)2Sm(η1-C5Me5)L intermediate. A (C5-
Me5)2Sm(η1-C5Me5) complex alone would not be expected to
ring-open THF, since the THF is not activated for nucleophilic
attack. However, in a complex such as (C5Me5)2Sm(η1-C5Me5)-
(THF), the THF could be Lewis acid-activated to nucleophilic
attack by the adjacent C5Me5 group. Such a situation has been
demonstrated crystallographically with (C5Me5)2Sm(AsPh2)(THF)
and (C5Me5)2Sm[O(CH2)4AsPh2](THF).31 Consistent with the
scenario that substrate coordination must occur to access the
η1-C5Me5 intermediate is the fact that the sterically more
crowded ether, 2,5-dimethyltetrahydrofuran, is not ring-opened
by (C5Me5)3Sm presumably because it is less likely to coordi-
nate.
(38) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition
Metals; Academic Press: New York, 1976.
(39) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organome-
tallics 1985, 4, 112.
(40) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics, 1991,
10, 134.
(41) Evans, W. J.; Wayda, A. L.; Hunter, W. E.; Atwood, J. L. J. Chem.
Soc., Chem. Commun. 1981, 706 and references therein. Evans, W. J.;
Drummond, D. K. J. Am. Chem. Soc. 1988, 110, 2772.

Tris(pentamethylcyclopentadienyl)samarium
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9281
The fact that hydrogen can also react with (C5Me5)3Sm, eq
5, suggests that the formation of a base adduct may not be
necessary to access an η1-C5Me5 intermediate. Furthermore, it
Alternatively, the reduction chemistry of (C5Me5)3Sm can be
viewed as an electron-transfer involving heterolytic separation
of a (C5Me5)- anion. In this case, (C5Me5)3Sm can be viewed
as having a zwitterionic intermediate, [(C5Me5)2Sm]+(C5Me5)-,
in which one pentamethylcyclopentadienide ligand is arbitrarily
written separately to show it as the reducing agent. If the
(C5Me5)- moiety which is doing the reduction is still interacting
with the metal, it could have either an η5-form or an η1-form
like that discussed above. As in the case of the (C5Me5)2Sm-
(η1-C5Me5) intermediates discussed above, there is no physical
evidence for this zwitterionic form.
Reactivity consistent with a zwitterionic form may happen
as a result of the extreme steric crowding around (C5Me5)3Sm.
The 2.82(5) Å average Sm-C(ring) distance observed for (C5-
Me5)3Sm is the longest distance known for a trivalent samarium
complex. This means that the (C5Me5)- anions are not as
stabilized by the cationic center as in typical organometallic
pentamethylcyclopentadienyl compounds. As such, a (C5Me5)-
ring could act as a separate entity and do reduction chemistry.
This zwitterionic explanation means that (C5Me5)3Sm can be
viewed as providing a reactive, solubilized form of (C5Me5)-.
Simple (C5Me5)- salts do not exhibit this type of reductive
reactivity; neither KC5Me5 nor ClMg(C5Me5) nor Pb(C5Me5)2
reduces SedPPh3 and 1,3,5,7-C8H8.43 This zwitterionic analysis
also means that (C5Me5)3Sm is providing a soluble form of the
unsolvated cation [(C5Me5)2Sm]+.
Implications of the Reduction Chemistry. Reduction
Chemistry for Trivalent Lanthanides Which Do Not Have
a Divalent State. The fact that trivalent (C5Me5)3Sm can do
one-electron reduction chemistry suggests that one-electron
reduction chemistry can be extended to lanthanides beyond those
with accessible divalent states. This would be quite valuable
since an extensive amount of chemistry has been developed with
the divalent lanthanides that is not readily accessible for the
other lanthanides.44,45
In the lanthanide metallocene area in general, (C5Me5)2Sm-
containing complexes have been much more extensively inves-
tigated than any other type of C5Me5/Ln combination because
(C5Me5)2Sm and (C5Me5)2Sm(THF)2 provide facile reductive
routes unavailable to the other metals. For example, the first
[(C5Me5)2LnH]2 complex was prepared with LndSm via a Sm-
(II) route.9 Direct reduction of substrates by (C5Me5)2Sm or
(C5Me5)2Sm(THF)2 to complexes of the form [(C5Me5)2Ln]2-
(substrate) is much easier than attempting an ionic metathesis
between a lanthanide metallocene halide adduct such as (C5-
Me5)2LnCl2M(solvent)2 and a substrate which has been previ-
ously reduced by an alkali metal. The facile formation of
adducts containing alkali metals and anions in the system make
these ionic metathesis reactions complicated.46 For example,
there are no routes to trivalent azobenzene complexes of the
1
is interesting to note that the H NMR spectrum of (C5Me5)3-
Sm in the presence of Me3CNC is consistent with the formation
of a Lewis base adduct of the type (η5-C5Me5)3SmL in which
all of the rings retain their η5-orientation. This is consistent
with the channel available down the S3 axis in (C5Me5)3Sm and
with the cylindrical nature of the donor portion of Me3CNC. It
is therefore conceivable that other cylindrical molecules such
as CO, PhCN, and PhNCO could also form such intermediates.
Access to a transient η1-C5Me5 intermediate should be favored
even more from (η5-C5Me5)3SmL than from (η5-C5Me5)3Sm
since it is more crowded.
Collectively, we find that a variety of substrates with
considerable differences in size and basicity can react with (C5-
Me5)3Sm to form products consistent with the intermediacy of
an (η1-C5Me5) unit. As such, we can consider (C5Me5)3Sm as
a bulky (C5Me5)2SmR complex in disguise. It appears that it
is a rather special system, however, since it can lead to
derivatization of C5Me5 ligands which are usually inert and it
can readily insert nitriles, a reaction generally found only for
very reactive metal carbon bonds.
Reduction Chemistry. A second major class of reactions
of (C5Me5)3Sm involves reductions in which trivalent (C5Me5)3-
Sm acts as a one-electron reducing agent analogous to divalent
(C5Me5)2Sm(THF)x (x ) 0 or 2) as shown in eqs 12-18. A
variety of substrates can be reduced including OdPPh3, Sd
PPh3, SedPPh3, PhNdNPh, and 1,3,5,7-C8H8, and the orga-
nosamarium products are the same as those obtained from
reductions with (C5Me5)2Sm or (C5Me5)2Sm(THF)2.
Since Sm(III) rather than Sm(IV) products are isolated in the
(C5Me5)3Sm reductions and since the pentamethylcyclopenta-
dienyl dimer, (C5Me5)2, is a byproduct, it is the (C5Me5)- anion
and not the metal that is the reducing agent in these reactions.
These (C5Me5)3Sm reductions would be expected to make the
same products as (C5Me5)2Sm reductions since, after electron
transfer, the same cation, [(C5Me5)2Sm]+, remains with the
reduced substrate. This is shown in the half reactions 21 and
22.
(C5Me5)2Sm f [(C5Me5)2Sm]+ + e-
(21)
(C5Me5)3Sm f [(C5Me5)2Sm]+ + e- + 0.5 (C5Me5)2 (22)
One way to view the observed reductions involves a ho-
molytic cleavage of a Sm-C5Me5 bond to give a divalent [(C5-
Me5)2SmII](C5Me5 ) intermediate. In this case, one would
•
expect that (C5Me5)3Sm would have the same reduction potential
as (C5Me5)2Sm. Since (C5Me5)3Sm does not reduce anthracene
or pyrene as (C5Me5)2Sm does36 and since (C5Me5)3Sm reduces
azobenzene by only one electron whereas (C5Me5)2Sm can effect
a two-electron reduction,16 the reduction potentials of (C5Me5)3-
Sm and (C5Me5)2Sm are not the same, and a homolytic
explanation seems unlikely. In addition, homolytic cleavage
of a Sm-C5Me5 bond to give divalent (C5Me5)2Sm and the C5-
(43) Evans, W. J.; Nyce, G. W.; Seibel, C. A., unpublished results.
(44) Evans, W. J. Polyhedron 1987, 6, 803.
(45) (a) Edelmann, F. T. ComprehensiVe Organometallic Chemistry II;
Lappert, M. F., Ed.; Pergamon Press: Oxford, England, 1995; Vol. 4,
Chapter 2 and references therein. (b) Schumann, H.; Meese-Marktscheffel,
J. A.; Esser, L. Chem. ReV. 1995, 95, 865 and references therein. (c)
Schaverien, C. J. AdV. Organomet. Chem. 1994, 36, 283 and references
therein. (d) Evans, W. J. J. Alloys Compd. 1993, 192, 205 and references
therein. (e) Harrison, K. N.; Marks, T. J. J. Am. Chem. Soc. 1992, 114,
9220. (f) Wang, K.-G.; Stevens, E. D.; Nolan, S. P. Organometallics 1992,
11, 1011. (g) Forsyth, C. M.; Nolan, S. P.; Marks, T. J. Organometallics
1991, 10, 2543. (h) Recknagel, A.; Stalke, D.; Roesky, H. W.; Edelmann,
F. T. Angew. Chem., Int. Ed. Engl. 1989, 28, 445. (i) Hou, Z.; Fujita, A.;
Zhang, Y.; Miyano, T.; Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc.
1998, 120, 754. (j) Tashiro, D.; Kawasaki, Y.; Sakaguchi, S.; Ishii, Y. J.
Org. Chem. 1997, 62, 8141.
•
Me5 radical is equivalent to saying that a (C5Me5)- anion is
capable of reducing Sm(III) to Sm(II). This is not consistent
with known C5Me5 lanthanide chemistry. Of the four lanthanide
metals with the most available divalent oxidation states, Tm,
Sm, Eu, and Yb, only Eu(III) is readily reduced to Eu(II) by
the (C5Me5)- anion.42
(42) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin, A.;
Templeton, D. H. Inorg. Chem. 1980, 19, 2999. Evans, W. J.; Shreeve, J.
L.; Ziller, J. W. Organometallics 1994, 13, 731.
(46) Evans, W. J.; Boyle, T. J.; Ziller, J. W. Inorg. Chem. 1992, 31,
1120 and references therein.

9282 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
EVans et al.
type (C5Me5)2Sm(Ph2N2)(THF)16 for the lanthanides which lack
a divalent state, and complexes such as (C5Me5)2LnMe-
(THF),7e,47 (C5Me5)2Ln(allyl)7c,f [(C5Me5)2Ln(THF)2][BPh4],12,48
and [(C5Me5)2LnH]2 can be prepared much more easily for
LndSm via a divalent route than the analogues with Lndother
metals.
compounds now readily accessible from (C5Me5)2Sm(THF)x
could become available for other metals in the series via (C5-
Me5)3Sm reduction chemistry.
7c,9
Conclusion
Despite its extreme steric saturation, (C5Me5)3Sm has an
extensive and chameleon-like reaction chemistry. With some
substrates, it can behave as a bulky alkyl complex and participate
in polymerization, hydrogenolysis, insertion, and ring-opening
chemistry. With other substrates, it displays one-electron
reduction chemistry as if it were a zwitterion in which (C5Me5)-
is a reductant. In both of these reaction modes, the (C5Me5)-
ring adopts uncharacteristic reactivity patterns since it is usually
an inert spectator ligand. This is likely to be the result of the
steric crowding in the molecule and demonstrates a powerful
approach to expand both lanthanide and C5Me5 chemistry.
The fact that trivalent (C5Me5)3Sm can do one-electron
reduction chemistry suggests that reduction reactions of this type
can be extended to the other trivalent lanthanides. Hence, by
preparing the proper sterically crowded tris(peralkylcyclopen-
tadienyl) complexes of the other lanthanides, they can be
“samaricized” into the one-electron reducing systems which have
been so productive for Sm(II) complexes.
Since trivalent (C5Me5)3Sm is a one-electron reductant, this
suggests that other trivalent (C5Me5)3Ln complexes could also
be one-electron reductants. If the reduction chemistry of (C5-
Me5)3Sm arises from severe steric crowding, then the other (C5-
Me5)3Ln or related (C5R5)3Ln complexes must be similarly
crowded to have the same reduction chemistry. Since the steric
crowding can be adjusted by changing the size of the ligand,
this seems quite possible, and extension of the special chemistry
of Sm(II) to all of the lanthanides is conceivable if synthetic
routes to the other (C5R5)3Ln complexes can be developed.49
Extension of samarium-like reduction chemistry to the other
lanthanides would be quite valuable since one of the powerful
aspects of trivalent lanthanide metal chemistry is that one can
be highly selective in picking the lanthanide of just the proper
size to optimize a specific reaction.44,50 At present, many
lanthanide metallocene-based reactions can only be done
conveniently with samarium, and hence, variation of the size
of the lanthanide metal is not possible. In addition, the
lanthanide series offers a wide variety of specific physical
properties, e.g., magnetic or fluorescent properties, which may
be desirable for particular applications. Analogues of samarium
Acknowledgment. We thank the National Science Founda-
tion for support of this research, the U. S. Department of
Education Graduate Assistantship in Areas of National Needs
program for a fellowship (to K.J.F.), and Robert D. Clark and
Gregory W. Nyce for experimental assistance.
(47) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51 and
references therein. den Haan, K. H.; Wielstra, Y.; Eshuis, J. J. W.; Teuben,
J. H. J. Organomet. Chem. 1987, 323,, 181.
(48) Hazin, P. N.; Bruno, J. W.; Schulte, G. K. Organometallics 1990,
9, 416. Heeres, H. J.; Meetsma, J. H.; Teuben, J. H. J. Organomet. Chem.
1991, 414, 351.
(49) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 6745-6752.
(50) Evans, W. J.; Dominguez, R.; Hanusa, T. P. Organometallics 1986,
4, 263-270.
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