Organolanthanide-based coordination and insertion reactivity of the anion formed by deprotonation of ε-caprolactam

Article abstract of DOI:10.1021/om010403n

Methods to incorporate the strong ligation capacity of ε-caprolactam into monoanionic ligands suitable for electropositive metals such as yttrium are described. ε-Caprolactam, HN(CH₂)₅C=O, is deprotonated by (C₅H₄Me)₂Y[N(SiMe₃)₂] to form the dimeric complex [(C₅H₄Me)₂Y (μ-NC₆H₁₀O)]₂, 1. Each caprolactamate anion, (NC₆H₁₀O)^-, forms one Y-N bond and coordinates to both yttrium centers via a bridging oxygen such that yttrium has a formal coordination number of nine. ε-Caprolactam is also deprotonated by (C₅Me₅)₂Y(C₃H₅)(THF), to form the monomeric, pentamethylcyclopentadienyl analogue (C₅Me₅)₂Y(NC₆H₁₀O), 2, in which the caprolactamate anion chelates the formally eight-coordinate yttrium. CO₂ inserts into the Ln-N bond in 2 to form [(C₅Me₅)₂Y(μ-O₂CNC₆ H₁₀O)]₂, 3. The tridentate monoanionic (O₂CNC₆H₁₀O)^- ligand forms an eight-membered YOCOYOCO ring via carboxylate oxygen atoms and also coordinates to each yttrium center via the oxygen originating from ε-caprolactam. Phenyl isocyanate inserts into the Y-N bond of 2 to form (C₅Me₅)₂Y[(μ²-OC(NPh) NC₆H₁₀O)], 4, which contains a planar YOCNCO ring and eight-coordinate yttrium. tert-Butyl isocyanide does not undergo insertion chemistry with 2, but instead forms the adduct (C₅Me₅)₂Y(NC₆H₁₀O) (CNCMe₃), 5.

Full text of DOI:10.1021/om010403n

Organometallics 2001, 20, 4529-4536
4529
Or ga n ola n th a n id e-Ba sed Coor d in a tion a n d In ser tion
Rea ctivity of th e An ion F or m ed by Dep r oton a tion of
E-Ca p r ola cta m
William J . Evans,* Cy H. Fujimoto, and J oseph W. Ziller
Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025
Received May 16, 2001
Methods to incorporate the strong ligation capacity of ꢀ-caprolactam into monoanionic
ligands suitable for electropositive metals such as yttrium are described. ꢀ-Caprolactam,
HN(CH2)5CdO, is deprotonated by (C5H4Me)2Y[N(SiMe3)2] to form the dimeric complex
[(C5H4Me)2Y(µ-NC6H10O)]2, 1. Each caprolactamate anion, (NC6H10O)-, forms one Y-N bond
and coordinates to both yttrium centers via a bridging oxygen such that yttrium has a formal
coordination number of nine. ꢀ-Caprolactam is also deprotonated by (C5Me5)2Y(C3H5)(THF),
to form the monomeric, pentamethylcyclopentadienyl analogue (C5Me5)2Y(NC6H10O), 2, in
which the caprolactamate anion chelates the formally eight-coordinate yttrium. CO2 inserts
into the Ln-N bond in 2 to form [(C5Me5)2Y(µ-O2CNC6H10O)]2, 3. The tridentate monoanionic
(O2CNC6H10O)- ligand forms an eight-membered YOCOYOCO ring via carboxylate oxygen
atoms and also coordinates to each yttrium center via the oxygen originating from
ꢀ-caprolactam. Phenyl isocyanate inserts into the Y-N bond of 2 to form (C5Me5)2Y[(µ2-OC-
(NPh)NC6H10O)], 4, which contains a planar YOCNCO ring and eight-coordinate yttrium.
tert-Butyl isocyanide does not undergo insertion chemistry with 2, but instead forms the
adduct (C5Me5)2Y(NC6H10O)(CNCMe3), 5.
In tr od u ction
atoms. However, recently while this work was in
progress, it was discovered that the ꢀ-caprolactamate
anion can also bridge between two lanthanide metals:
the X-ray crystal structure of [(C5H4Me)2Yb(µ-NC6-
Recent studies of the coordination chemistry of ꢀ-ca-
prolactam in lanthanide complexes have revealed that
this is a powerful ligand capable of displacing anions
from the metal to make cationic complexes such as [Pr-
H
10O)]27 was reported along with synthetic information
on Er and Y analogues.
(C6H11NO)7]+3 1 [Pr(C6H11NO)6Cl]+2 2
, , and [Eu(C6H11-
We report here our initial studies of the coordination
chemistry of this ligand with yttrium as a representative
of the late lanthanides. Two different coordination
modes of the resonance-stabilized ꢀ-caprolactam anion
are found depending on the ancillary ligation around
the metal: chelating and bridging (Figure 1). In addi-
tion, we report the insertion reactivity of this anion with
CO2, PhNCO, and Me3CNC.8-11
NO)4Cl2]+.2 To determine if this strong ligation capacity
could be incorporated into an anionic ligand which could
provide charge balance in neutral complexes of electro-
positive metals, we have examined the coordination
chemistry and reactivity of the anion obtained by
deprotonating ꢀ-caprolactam. Although this (C6H10NO)-
caprolactamate anion is well known as a component in
the anionic ring-opening polymerization of ꢀ-caprolac-
tam to form nylon-6,3,4 prior to this study it had been
structurally characterized in only two metal com-
plexes: [(C5H5)2Ti(NC6H10O)]5 and [O[Si(iPr)2N(C5H3-
NMe)]2Ti(NMe2)(NC6H10O)].6 The ꢀ-caprolactam anion
in both cases formed monomeric complexes in which the
metal is chelated by the nitrogen and oxygen donor
Exp er im en ta l Section
Gen er a l P r oced u r es. The chemistry described below was
performed under nitrogen with rigorous exclusion of air and
water using Schlenk, vacuum line, and glovebox techniques.
Solvents were purified and dried over sodium or potassium
benzophenone ketyl or over sodium/potassium alloy. [(C5H4-
Me)2Y(µ-Cl)]2 was prepared according to the literature.12 (C5-
Me5)2Y(C3H5)(THF) was prepared as a yellow solid in 77% yield
(1) Isolani, P. C.; Alvarez, H. A.; Matos, J . R.; Vicentini, G.;
Castellano, E.; Zukerman-Schpector, J . J . Coord. Chem. 1998, 43, 346-
360.
(2) Evans, W. J .; Fujimoto, C. H.; Greci, M. A.; Ziller, J . W. Eur. J .
Inorg. Chem. 2001, 3, 745-749.
(7) Shen, Q.; Wang, Y.; Wu, L.; Zhang, Y.; Sun, J . J . Organomet.
Chem. 2001, 626, 176-180.
(3) Vasiliu-Oprea, C.; Dan, F. J . Appl. Polym. Sci. 1996, 62, 1517-
1527, and references therein.
(8) Evans, W. J .; Seibel, C. A.; Ziller, J . W. Inorg. Chem. 1998, 37,
770-776.
(4) Cerny, Z.; Kriz. O.; Fusek, J .; Casensky, B.; Bernat, P.; Brozek,
J .; Roda. J . Organomet. Chem. 1998, 555, 237-245.
(5) Arndt, P.; Lefeber, C.; Kempe, R.; Tillack, A.; Rosenthal, U.
Chem. Ber. 1996, 129, 1281-1285.
(6) Kempe, R.; Arndt, P.; Oberthur, M.; Hillebrand, G. Chem. Ber.
1997, 130, 789-794.
(9) Evans, W. J .; Forrestal, K. J .; Ziller, W. J . J . Am. Chem. Soc.
1998, 120, 9273-9282.
(10) Evans, W. J .; Meadow, J . H.; Hunter, W. E.; Atwood, J .
Organometallics 1983, 2, 1252-1254.
(11) Evans, W. J .; Hanusa, T. P.; Meadows, J . H.; Hunter, W. E.;
Atwood, J . L. Organometallics 1987, 6, 295-301.
10.1021/om010403n CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/02/2001

4530 Organometallics, Vol. 20, No. 22, 2001
Evans et al.
s, 1664 s, 1606 m, 1556 w, 1478 w, 1436 w, 1386 m, 1347 w,
1305 w, 1278 m, 1258 s, 1235 w, 1204 m, 1154 w, 1127 m,
1085 m, 1031 s, 984 m, 961 m, 930 w, 830 s, 803 m, 760 s
cm-1
.
Syn th esis of (C5Me5)2Y(NC6H10O), 2. In
a glovebox,
ꢀ-caprolactam (0.24 g, 2.1 mmol) was added to a yellow solution
of (C5Me5)2Y(C3H5)(THF) (1.0 g, 2.1 mmol) in 10 mL of benzene,
which turned clear upon addition. The reaction was stirred
for 12 h, and removal of solvent by rotary evaporation gave a
white solid (1.8 g, 88%). Colorless crystals of 2 suitable for
X-ray diffraction were grown in hexanes/toluene (80:20) at
room temperature. Anal. Calcd for C26H40NOY: Y, 18.9.
Found: Y, 18.2. 1H NMR (C6D6): δ 2.93 (m, 2H), 2.32 (m, 2H),
1.94 (s, 30H), 1.47 (m, 4H), 1.41 (m, 2H). 13C NMR (C6D6): δ
182.4 (CO), 116.7 (C5Me5), 47.1 (CH2), 37.7 (CH2), 32.0 (CH2),
30.5 (CH2), 24.0 (CH2), 11.2 (C5Me5). IR: 2961 s, 2922 s, 2856
s, 1656 s, 1575 w, 1532 m, 1444 s, 1409 w, 1378 w, 1351 w,
F igu r e 1. Delocalization in the caprolactamate anion (I)
and two possible binding modes (II).
following literature procedures13 from the reaction of (C5Me5)2-
YCl2K(THF)214 (0.96 g, 1.6 mmol) with ClMg(CH2CHCH2) (0.78
mL, 1.6 mmol) in THF. Research grade CO2 was purchased
from Airgas and used without further purification. Phenyl
isocyanate and tert-butyl isocyanide were purchased from
Aldrich, dried over activated 3 Å molecular sieves, and put
1262 s, 1200 m, 1150 w, 1123 m, 1089 s, 1019 s, 799 s cm-1
.
Syn th esis of [(C5Me5)2Y(µ-O2CNC6H10O)]2, 3. A flask
fitted with a high-vacuum greaseless stopcock, which contained
2 (290 mg, 0.62 mmol) in 10 mL of benzene, was attached to
a high-vacuum line and freeze-pump-thawed three times.
Excess CO2 at 1 atm was admitted to the flask, and within 1
h, colorless crystalline solids appeared. After 3 h, the flask
was evacuated to the pressure of the solvent and returned to
the glovebox, where solvent was decanted to leave 3 as a
colorless crystalline solid (264 mg, 89%). Anal. Calcd for
1
through three freeze-pump-thaw cycles before use. H and
13C NMR spectra were obtained on an Omega 500 MHz or a
Bruker DRX-400 MHz spectrometer at 25 °C. X-ray crystal-
lographic data were obtained on a Bruker CCD platform
diffractometer. IR spectra were taken on thin films obtained
from benzene solutions (except where noted) using an ASI
ReactIR 1000 spectrometer. Complexometric analyses were
obtained as previously described.15 Complete elemental analy-
ses were performed by Desert Analytics.
C
54H80N2O6Y2: C, 62.91; H, 7.82; N, 2.72. Found: C, 62.92;
1
H, 7.99; N, 2.97. H NMR (C5D5N): δ 4.17 (m, 2H), 2.82 (m,
2H), 1.82 (s, 30H), 1.58 (br, 6H, ∆ν1/2 ) 29.1 Hz). 13C NMR
(C5D5N): δ 187.7 (CO), 180.3 (CO), 128.3 (C5Me5), 45.7 (CH2),
39.9 (CH2), 31.3 (CH2), 29.0 (CH2), 28.0 (CH2), 11.3 (C5Me5).
IR (thin film from THF): 2926 s, 2856 s, 1695 s, 1637 s, 1455
m, 1378 w, 1309 w, 1258 m, 1227 w, 1173 w, 1143 w, 1089 s,
Syn th esis of (C5H4Me)2Y[N(SiMe3)2)]. In a glovebox,
NaN(SiMe3)2 (1.5 g, 8.0 mmol) was added to a stirred slurry
of [(C5H4Me)2Y(µ-Cl)]2 (2.3 g, 8.1 mmol) in 10 mL of THF, and
the mixture was stirred for 12 h. The solvent was removed by
rotary evaporation to give a white solid. Hexanes were added,
and the slurry was heated to boiling. The hot slurry was
centrifuged, and the colorless solution was collected and cooled
to room temperature followed by removal of solvent by rotary
evaporation to yield (C5H4Me)2Y[N(SiMe3)2] (2.0 g, 63%). Anal.
Calcd for C18H32NSi2Y: Y, 21.8. Found: Y, 22.2. 1H NMR
(C6D6): δ 6.01 (m, 4H), 5.86 (m, 4H), 2.08 (s, 6H), 0.07 (s, 18H).
13C NMR (C6D6): δ 113.9 (C5H4Me), 112.3 (C5H4Me), 15.3
(C5H4Me), 3.1 (SiMe3). IR: 2949 s, 2899 s, 1478 w, 1243 s, 1019
1023 s, 973 w, 857 w, 803 s cm-1
.
Syn th esis of (C5Me5)2Y(P h NCOC6H10NO), 4. In a glove-
box, PhNCO (27 mg, 0.23 mmol) was slowly added to a stirring
solution of 2 (109 mg, 0.23 mmol) in 10 mL of benzene. Upon
addition, the solution developed a yellow color. After the
reaction was stirred for 12 h, the solvent was removed by
rotary evaporation to leave 4 as a yellow powder (134 mg,
98%). Crystals of 4 were grown from a hexanes/toluene mixture
(90:10) at -32 °C. Anal. Calcd for C33H45N2O2Y: C, 67.11; H,
7.68; N, 4.74. Found: C, 66.78; H, 7.97; N, 4.49. 1H NMR
(C6D6): δ 7.50 (d, 2H, J H-H 10 Hz), 7.34 (t, 2H, J H-H 10 Hz),
7.02 (t, 1H, J H-H 10 Hz), 4.07 (m, 2H), 2.14 (m, 2H), 1.90 (s,
30H), 1.55 (br, 2H), 1.36 (br, 2H), 1.23 (br, 2H). 13C NMR
(C6D6): δ 180.3 (CO), 151.7 (CO), 149.9 (C6H5), 128.4 (C6H5),
124.6 (C6H5), 122.1 (C6H5), 116.9 (C5Me5) 48.2 (CH2), 40.3
(CH2), 28.7 (CH2), 27.5(CH2), 23.0 (CH2), 11.4 (C5Me5). IR: 2934
m, 2910 m, 2860 m, 1640 s, 1590 s, 1548 w, 1502 w, 1447 m,
1309 m, 1258 w, 1170 w, 1139 w, 1085 w, 1023 w, 977 w, 820
s, 869 m, 834 s, 807 m, 772 m cm-1
.
Syn th esis of [(C5H4Me)2Y(µ-NC6H10O)]2, 1. In a glovebox,
ꢀ-caprolactam (0.55 g, 4.8 mmol) was added to a stirring,
colorless solution of (C5H4Me)2Y[N(SiMe3)2] (2.0 g, 4.9 mmol)
in 10 mL of hexanes. A white solid immediately precipitated,
and the solution was stirred for 12 h. Removal of solvent by
rotary evaporation yielded 1 as a white solid (1.6 g, 87%).
Crystals suitable for X-ray diffraction were grown in toluene
at room temperature as colorless blocks. Anal. Calcd for
w, 753 w cm-1
.
Syn th esis of (C5Me5)2Y(NC6H10O)(CNCMe3), 5. In a
glovebox, CNCMe3 (0.05 mL, 1.06 mmol) was added to a white
slurry of 2 (0.50 g, 1.06 mmol) in 5 mL of hexanes. The slurry
immediately became clear, and the reaction was stirred for 1
h. Solvent was removed by rotary evaporation to give 5 as a
colorless solid (0.48 g, 92%). X-ray quality crystals were
obtained by concentrating a toluene solution of 5 and storing
it overnight at -35 °C. Anal. Calcd for C31H49N2OY: Y, 16.0.
Found: Y, 15.2. 1H NMR (C6D6): δ 2.98 (m, 2H), 2.38 (m, 2H),
2.10 (s, 30H), 1.53 (b, 6H, ∆ν1/2 ) 30.4 Hz), 0.91 (s, 9H). 13C
NMR (C6D6): δ 180.3 (CO), 150.7 (CNCMe3), 114.6 (C5Me5)
55.2 (CNCMe3), 47.5 (CH2), 38.1 (CH2), 32.0 (CH2), 31.0 (CH2),
29.5 (CNCMe3), 24.3 (CH2), 11.7 (C5Me5). IR: 2904 m, 2858
m, 2186 m, 1637 s, 1545 s, 1444 s, 1375 m, 1205 m, 1128 w,
C
36H48N2O2Y2: Y, 12.4. Found: Y, 13.0. Isopiestic molecular
weight (in THF vs (C5H5)2Fe): Calcd for C36H48N2O2Y2: 719.
Found: 710. 1H NMR (C6D6): δ 6.02 (dd, 2H, J H-H ) 5 Hz,
J H-H ) 3 Hz), 5.95 (dd, 2H, J H-H ) 5 Hz, J H-H ) 3 Hz), 5.90
(dd, 2H, J H-H ) 4.5 Hz, J H-H ) 2 Hz), 5.84 (dd, 2H, J H-H
)
4.8 Hz, J H-H ) 2 Hz), 2.92 (m, 2H), 2.21 (s, 6H), 2.08 (m, 2H),
1.42 (m, 4H), 1.32 (m, 2H). 13C NMR (C6D6): δ 180.8 (CO),
121.7 (C5H4CH3), 111.9 (C5H4CH3), 111.0 (C5H4CH3), 110.4
(C5H4CH3), 109.7 (C5H4CH3), 48.8 (CH2), 38.0 (CH2), 32.6
(CH2), 30.1 (CH2), 24.8 (CH2), 17.0 (C5H4Me). IR: 2922 s, 2853
(12) Evans, W. J .; Meadows, J . H.; Wayda, A. L.; Hunter, W. E.;
Atwood, J . L. J . Am. Chem. Soc. 1982, 104, 2008-2014.
(13) Evans, W. J .; Seibel, C. A.; Ziller, J . W. J . Am. Chem. Soc. 1998,
120, 6745-6752.
(14) Evans, W. J .; Peterson, T. T.; Rausch, M. D.; Hunter, W. E.;
Zhang, H.; Atwood, J . L. Organometallics 1985, 4, 554-560.
(15) Taylor, M. D.; Carter, C. D. J . Inorg. Nucl. Chem. 1962, 24,
387.
1089 w, 1027 w, 811 w, 703 w cm-1
.
X-r a y Da ta Collection , Str u ctu r e Deter m in a tion , a n d
Refin em en t for [(C5H4Me)2Y(µ-NC6H10O)]2, 1. A colorless
crystal of approximate dimensions 0.23 × 0.26 × 0.27 mm was

Anion Formed by Deprotonation of ꢀ-Caprolactam
Organometallics, Vol. 20, No. 22, 2001 4531
mounted on a glass fiber and transferred to a Bruker CCD
platform diffractometer. The SMART16 program package was
used to determine the unit-cell parameters and for data
collection (30 s/frame scan time for a hemisphere of diffraction
data). The raw frame data were processed using SAINT17 and
SADABS18 to yield the reflection data file. Subsequent calcula-
tions were carried out using the SHELXTL19 program. The
diffraction symmetry was 2/m, and the systematic absences
were consistent with the centrosymmetric monoclinic space
group P21/n that was 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 factors20 for neutral atoms were used throughout
the analysis. The molecule was a dimer that was located about
an inversion center. Hydrogen atoms were located from a
difference Fourier map and refined (x,y,z and Uiso). At conver-
gence, wR2 ) 0.0580 and GOF ) 1.104 for 287 variables
refined against 3926 unique data (as a comparison for refine-
ment on F, R1 ) 0.0246 for those 3572 data with I > 2.0σ(I)).
Subsequent complexes were handled as described for 1.
(C5Me5)2Y(C6H10NO), 2. The space group was determined
to be P21/n, and the pentamethylcyclopentadienyl ligand
defined by atoms C(11)-C(20) was disordered. The penta-
methylcyclopentadienyl was included using multiple compo-
nents with partial site-occupancy factors. At convergence, wR2
) 0.1585 and GOF ) 1.066 for 252 variables refined against
4191 data (0.85 Å resolution). As a comparison for refinement
on F, R1 ) 0.0536 for those 3299 data with I > 2.0σ(I).
[(C5Me5)2Y(µ-O2CNC6H10O)]2, 3. The space group was
determined to be P21/c, and the molecule was located about
an inversion center. There was one molecule of toluene solvent
present per formula unit. The toluene molecule was disordered
and included using multiple components with partial site-
occupancy factors. At convergence, wR2 ) 0.1495 and GOF )
1.009 for 330 variables refined against 6838 data. As a
comparison for refinement on F, R1 ) 0.0537 for those 4016
data with I > 2.0σ(I).
F igu r e 2. Thermal ellipsoid plot of [(C5H4Me)2Y(µ-
NC6H10O)]2, 1, drawn at the 50% probability level with
hydrogen atoms omitted for clarity.
C5H4Me methyl resonance is observed for 1, but four
separate ring proton signals are observed for the (C5H4-
Me)2Y unit. The IR spectrum lacked the NH stretch of
ꢀ-caprolactam and differed from coordinated ꢀ-capro-
lactam-lanthanide complexes in the νCO region. Al-
though the spectroscopic data showed that a new
product had formed cleanly, definitive identification
required an X-ray crystallographic study. 1 was found
to be the dimer, [(C5H4Me)2Y(µ-NC6H10O)]2, in the solid
state as shown in Figure 2. Evidently, deprotonation of
the NH proton of ꢀ-caprolactam was cleanly effected by
the yttrium amide precursor, giving 1 in 88% yield
according to eq 1. Isopiestic solution molecular weight
studies of 1 in THF vs ferrocene indicated that the
complex exists as a dimer in solution as well.
(C5Me5)2Y(P h NCOC6H10NO), 4. The space group was
determined to be P21/c. Hydrogen atoms were either located
from a difference Fourier map and refined (x,y,z and Uiso) or
included using a riding model. At convergence, wR2 ) 0.0971
and GOF ) 1.034 for 423 variables refined against 7474 unique
data (as a comparison for refinement on F, R1 ) 0.0367 for
those 5628 data with I > 2.0σ(I)).
(C5Me5)2Y(NC6H10O)(CNCMe3), 5. The space group was
determined to be P21/n, and the pentamethylcyclopentadienyl
ligand defined by atoms C(11)-C(15) exhibited rotational
disorder. The disorder was modeled using two components for
each atom with site-occupancy factors of 0.60 for the major
component and 0.40 for the minor component. At convergence,
wR2 ) 0.1422 and GOF ) 1.020 for 306 variables refined
against 7415 unique data (as a comparison for refinement on
F, R1 ) 0.0550 for those 4911 data with I > 2.0σ(I)).
Dimeric 1 is isomorphous with the recently published
Yb/ꢀ-caprolactamate complex [(C5H4Me)2Yb(µ-NC6H10
-
O)]2.7 It is presumably identical with the yttrium
analogue reported in that paper. In 1, each bis(meth-
ylcyclopentadienyl)yttrium metallocene moiety is ligated
by the nitrogen of the deprotonated ꢀ-caprolactam and
the two metallocenes are connected by bridging oxygen
atoms. Each yttrium is formally nine-coordinate. The
2.388 and 2.393 Å Y-C (ring centroid) distances and
the 127° (ring centroid)-Y-(ring centroid) angle are in
the normal range for yttrium metallocenes with this
degree of ring substitution.12,21 The 3.908 Å Y‚‚‚Y
separation is longer than the 3.664(1) Å distance in
[(C5H4Me)2Y(µ-H)(THF)]2,12 which is consistent with the
larger size of the bridging ligand in 1.
Resu lts
[(C5H4Me)2Y(µ-NC6H10O)]2, 1. Addition of ꢀ-capro-
lactam to (C5H4Me)2Y[N(SiMe3)2] in hexanes generates
a white solid, 1, which has a 1H NMR spectrum
containing peaks consistent with an ꢀ-caprolactam
moiety lacking the resonance of the NH proton. A single
(16) SMART Software Users Guide, Version 4.21; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1997.
(17) SAINT Software Users Guide, Version 4.05; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1997.
(18) Sheldrick, G. M. SADABS; Bruker Analytical X-Ray Systems,
Inc.: Madison, WI, 1997.
(19) Sheldrick, G. M. SHELXTL Version 5.10; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1997.
The Y-caprolactamate bond distances as well as the
distances within the caprolactamate anion, Table 2,
(20) International Tables for X-ray Crystallography; Kluwer Aca-
demic Publishers: Dordrecht, 1992; Vol. C.
(21) Evans, W. J .; Dominguez, R.; Hanusa, T. P. Organometallics
1986, 5, 1291-1296.

4532 Organometallics, Vol. 20, No. 22, 2001
Evans et al.
Ta ble 1. Exp er im en ta l Da ta for th e X-r a y Diffr a ction Stu d ies of [(C5H4Me)2Y(µ-NC6H10O)]2, 1,
(C5Me5)2Y(C6H10NO), 2, [(C5Me5)2Y(µ-O2CNC6H10O)]2, 3, (C5Me5)2Y(P h NCOC6H10NO), 4, a n d
(C5Me5)2Y(NC6H10O)(CNtBu ), 5
1
2
3
4
5
formula
fw
temp (K)
cryst syst
space group
a (Å)
C
36H48N2O2Y2
C26H40NOY
471.50
158
monoclinic
P21/n
8.6069(7)
21.0692(16)
13.587(10)
90.7210(10)
2463.7(3)
4
C66H92N2O6Y2
1123.15
163
monoclinic
P21/c
10.8178(14)
14.9661(19)
17.994(2)
105.264(2)
2810.5(6)
2
C33H45N2O2Y
590.62
163
monoclinic
P21/c
10.0802(6)
20.5779(13)
15.0401(9)
98.5670(10)
3084.9(3)
4
C31H49N2OY
554.63
162
monoclinic
P21/n
10.5610(11)
18.4443(19)
16.4911(17)
106.588(2)
3078.6(6)
4
718.58
158
monoclinic
P21/n
12.2124(6)
11.1331(5)
12.7269(6)
107.3940(10)
1651.25(13)
2
b (Å)
c (Å)
â (deg)
V (Å3)
Z
Fcalcd (Mg/m3)
1.445
3.530
0.0246
0.0580
1.271
2.383
0.0536
0.1499
1.327
2.106
0.0537
0.1206
1.272
1.921
0.0367
0.0971
1.197
1.918
0.0550
0.142
µ (mm-1
)
final R1 [I>2σ(I)]
final wR2 (all data)
Ta ble 2. Selected Bon d Dista n ces (Å) for [(C5H4Me)2Y(µ-NC6H10O)]2, 1, (C5Me5)2Y(C6H10NO), 2,
[(C5Me5)2Y(µ-O2CNC6H10O)]2, 3, (C5Me5)2Y(P h NCOC6H10NO), 4, a n d (C5Me5)2Y(NC6H10O)(CNtBu ), 5
1
2
3
4
5
Y-(C5Me5 ring centroid)
Y-O
2.391
2.360
2.288(3)
2.424
Y(1)-O(1)
2.287(3)
Y(1)-O(2)
2.359(3)
2.363
2.409
2.326(3)
Y(1)-O(1)
2.3961(12)
Y(1)-O(1′)
2.2990(12)
Y(1)-O(1)
2.2528(14)
Y(1)-O(2)
2.1733(14)
Y(1)-O(3)
2.392(3)
Y-N
C-N
2.4116(14)
C(13)-N(1)
1.296(2)
2.362(4)
C(21)-N(1)
1.280(6)
2.391(3)
C(26)-N(1)
1.303(5)
C(22)-N(1)
1.362(5)
C(21)-N(1)
1.345(3)
O-C
C(13)-O(1)
1.323(2)
C(21)-O(1)
1.309(5)
C(22)-O(2)
1.247(5)
C(21)-O(1)
1.258(2)
C(26)-O(1)
1.302(5)
suggest that the anionic charge is delocalized in 1, as
shown in Figure 1. The 2.4116(14) Å Y(1)-N(1) distance
is similar to that in the ytterbium analogue, which has
an Yb-N distance of 2.374(4) Å,7 when the difference
in the metal sizes is taken into consideration.22 Similar
to [(C5H4Me)2Yb(µ-NC6H10O)]2, the metal-nitrogen bond
in 1 is slightly longer then the Y-N bond distances in
(C5Me5)2Y[N(SiMe3)2]23 and (C5Me4Et)2Y[N(SiMe3)2],24
which range from 2.324(7) to 2.39(1) Å, but it is shorter
than dative YrNR3 interactions in the complexes
(C5H5)2Y(C6H4-2-CH2NMe2)25 and (C5Me5)Y(C6H4-2-CH2-
NMe2)2,26 which range from 2.43(2) to 2.588(5) Å. The
2.396(1) Å Y(1)-O(1) and 2.2990(12) Å Y(1′)-O(1)
distances in 1 are also similar to the analogous distances
seen in [(C5H4Me)2Yb(µ-NC6H10O)]2, 2.370(3) and 2.277(3)
Å, which show that the caprolactamate anion makes a
longer bond to the metal to which it is chelated than to
the other metal. These Y-O distances in 1 are longer
than the bridging metal-oxygen distances in the dimer-
ic ytterbium butoxide [(C5H5)2Yb(µ-OBu)]2, 2.190(5) and
2.207(6) Å,27 and the dimeric yttrium methoxide [(C5H4-
SiMe3)2Y(µ-OMe)]2, 2.217(3) and 2.233(3) Å,28 but they
are shorter than the 2.460(8) Å dative Y-O distance
found in nine-coordinate [(C5H4Me)2Y(µ-H)(THF)]2.12
The 1.323(2) Å C(13)-O(1) distance is longer than the
CdO bond found in neutral ꢀ-caprolactam (1.242(2) Å),29
while the 1.296(2) Å N(1)-C(13) distance is shorter than
the analogous N-C bond in ꢀ-caprolactam (1.327(2) Å).
Consistent with the delocalization displayed by these
bond distances, the Y(1), N(1), C(13), and O(1) atoms
are coplanar to within 0.0219 Å.
(C5Me5)2Y(NC6H10O), 2. The synthesis of a penta-
methylcyclopentadienyl analogue of 1 was explored in
order to examine how different ancillary environments
would effect the coordination of the ꢀ-caprolactam anion.
Unlike 1, however, (C5Me5)2Y[N(SiMe3)2] does not react
as readily with ꢀ-caprolactam in hexanes. After stirring
overnight, 90% of the starting material remains. To
circumvent this low reactivity, (C5Me5)2Y(C3H5)(THF)
was employed since it has been recently reported that
lanthanide allyls are convenient and reactive starting
materials.13
Addition of ꢀ-caprolactam to (C5Me5)2Y(C3H5)(THF)
in benzene causes an immediate color change from dark
yellow to colorless. 2 was isolated from the reaction in
high yield as a white solid. The 1H NMR spectrum
contained resonances similar to those of the caprolac-
tamate in 1. Unlike 1, however, the C5Me5 rings
displayed a single resonance at 1.94 ppm and the IR
spectrum contained a different pattern in the νCO region.
X-ray crystallographic analysis identified 2 as (C5-
Me5)2Y(NC6H10O) and showed that it exists as a mono-
metallic species in the solid state, Figure 3. As has been
(22) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
(23) De Haan, K. H.; De Boer, J . L.; Teuben, J . H.; Spek, A. L.; Kojic-
Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726-1733.
(24) Schumann, H.; Rosenthal, E. C.; Kociok-Ko¨hn, G.; Molander,
G. A.; Winterfeld, J . J . Organomet. Chem. 1995, 496, 233-240.
(25) Rausch, M. D.; Foust, D. F.; Rogers, R. D.; Atwood, J . L. J .
Organomet. Chem. 1984, 3, 241-248.
(26) Booij, M.; Diers, N. H.; Meetsma, A.; Teuben, J . H. Organome-
tallics 1989, 8, 2454-2461.
(27) Wu, Z.; Xu, Z.; You, X.; Zhou, X.; Xing, Y.; J in, Z. Chem.
Commun. 1993, 1494.
(28) Evans, W. J .; Sollberger, M. S.; Shreeve, J . L.; Olofson, J . H.;
Hain, J . H.; Ziller, J . W. Inorg. Chem. 1992, 31, 2492-2521.
(29) Winkler, F. K.; Duntiz, J . D. Acta Crystallogr. 1975, B31, 268-
269.

Anion Formed by Deprotonation of ꢀ-Caprolactam
Organometallics, Vol. 20, No. 22, 2001 4533
F igu r e 4. Thermal ellipsoid plot of [(C5Me5)2Y(µ-O2-
CNC6H10O)]2, 3, drawn at the 30% probability level with
hydrogen atoms omitted for clarity.
F igu r e 3. Thermal ellipsoid plot of (C5Me5)2Y(C6H10NO),
2, drawn at the 30% probability level with hydrogen atoms
omitted for clarity.
Although the NMR spectrum of 3 indicated that a new
product had formed cleanly and a carboxylate peak was
observed at 187.7 ppm in the 13C NMR spectrum, X-ray
crystallography was necessary for complete character-
ization. Complex 3 was found to be the dimer [(C5-
Me5)2Y(µ-O2CNC6H10O)]2, Figure 4. The structural data
revealed that CO2 had inserted into the Y-N bond in 2
to form an ꢀ-caprolactam-substituted carboxylate anion
in which both carboxylate oxygens as well as the
ꢀ-caprolactam oxygen coordinate to yttrium, eq 3. The
discussed previously,30 formation of bridged structures
with the (C5Me5)2Y unit is disfavored by the steric
crowding that arises with this combination of a large
ancillary ligand and a small metal. Hence, a monomeric
structure is quite reasonable. Equation 2 summarizes
the deprotonation reaction which occurs in >80% yield.
Complex 2 has normal metrical parameters in the bis-
(pentamethylcyclopentadienyl) portion of the molecule31
and there appears to be charge delocalization in the
caprolactamate anion as in 1: as shown in Table 2, the
Y-N, Y-O, C-O, and C-N distances in 2 are similar
to those in 1. The YNCO ring is planar to within 0.025
Å.
Ca r bon Dioxid e In ser tion Activity. To examine if
insertion chemistry would occur with the caprolac-
tamate anion, 1 and 2 were reacted with CO2, a
substrate that has the capacity to insert to form stable
carboxylate products.8 When 1 was put under an
atmosphere of CO2 at ambient temperature for 24 h,
no reaction was observed. However, when 2 is left
unstirred under an atmosphere of CO2 for 3 h, a white
crystalline solid, 3, precipitates from the colorless solu-
tion. The resultant solid is soluble in coordinating
solvents such as pyridine and moderately soluble in
THF, but it is only slightly soluble in arenes.
carboxylate unit bridges the two yttrium atoms through
both oxygen atoms, while the other oxygen in the newly
formed ligand chelates to just one metal to give a formal
coordination number of 9 to each yttrium metal. The
average Y-O carboxylate distance, 2.339(3) Å, is similar
to analogous distances in other organoyttrium CO2
insertion products (2.30-2.35 Å).32,33 The other Y-O
distance in 3, 2.359(3) Å, is shorter then common dative
Y-O(THF) bond lengths in (C5Me5)2Y(CH3)(THF)34 and
(30) 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,
(32) Evans, W. J .; Seibel, C. A.; Ziller, J . Organometallics 1998, 17,
2103-2112.
(33) Schumann, H.; Meese-Marktscheffel, J . A.; Dietrich, A.; Gorlitz,
F. H. J . Organomet. Chem. 1992, 430, 299-315.
3614-3619.
(31) Evans, W. J .; Foster, S. J . Organomet. Chem. 1992, 433, 79-
94.

4534 Organometallics, Vol. 20, No. 22, 2001
Evans et al.
F igu r e 6. Resonance structures of the anion obtained by
phenyl isocyanate insertion into caprolactamate.
phenyl isocyanate insertion product in the literature,
[(C5H4CH3)2Y(OCN(iPr)2NPh)], 2.285(2) Å.38 The other
Y(1)-O(1) distance of 2.2528(14) Å, which involves the
ꢀ-caprolactam-derived oxygen, is comparable to the
analogous lengths seen in 1 and 2.
Figu r e 5. Thermal ellipsoid plot of (C5Me5)2Y(PhNCOC6H10-
NO), 4, drawn at the 30% probability level with hydrogen
atoms omitted for clarity.
(C5Me5)2YCl(THF),35 which range from 2.435(8) to
2.466(7) Å, but it is similar to the Y-O(ꢀ-caprolactam)
bond lengths found in [Pr(C6H11NO)6Cl]2+ and [Eu-
(C6H11NO)4Cl2]+, 2.418(3) and 2.315(18) Å,2 respec-
tively, when metal radial size is taken into consider-
ation. 22
Charge delocalization is substantially diminished in
the ꢀ-caprolactam ring in 3, as measured by the rela-
tively short 1.247(5) Å C(22)-O(2) and long 1.362(5) Å
C(22)-N(1) bonds, both of which are similar to the
analogous distances in free ꢀ-caprolactam. The 1.250(5)
Å C(21)-O(3) and 1.244(5) Å C(21)-O(1) carboxylate
distances are similar to those in other carboxylate-
bridged lanthanide complexes.36,37 These distances in-
dicate that the new anionic ligand formed by this
insertion can be viewed as an ꢀ-caprolactam-substituted
carboxylate.
P h en yl Isocya n a te In ser tion Activity. Since iso-
cyanates are electronic analogues of carbon dioxide, the
insertion chemistry of phenyl isocyanate with both 1 and
2 was also examined. As in the case of the CO2 reactions,
1 did not react with PhNCO, while addition of PhNCO
to 2 immediately causes a color change to yellow. A new
product, 4, can be isolated that displays downfield shifts
of the phenyl isocyanate protons in the 1H NMR
spectrum. In the 13C NMR of 4, the carbonyl chemical
shift also moves downfield from a value of 133.7 ppm
for free PhNCO to 149.9 ppm.
To determine the coordination geometry around yt-
trium in the new product, 4 was studied by X-ray
analysis, Figure 5. PhNCO inserts into the caprolac-
tamate Y-N bond of 2 to form a new anion, as shown
in eq 4. The (PhNCONC6H10O)- anion chelates yttrium
with both oxygen atoms forming a monometallic eight-
coordinate complex. The 2.1733(14) Å Y(1)-O(2) dis-
tance involving the PhNCO-derived oxygen is shorter
than the Y-O length in the only other yttrium amide
The Y(1)-O(2)-C(27)-N(1)-C(21)-O(1) ring is pla-
nar to within 0.069 Å, and the C-O and C-N bond
distances suggest charge delocalization as shown in
Figure 6. The C(21)-O(1) and C(27)-O(2) distances of
1.258(2) and 1.283(2) Å, respectively, 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.39 The 1.345(3) Å bond
of C(21)-N(1) also lies between the typical 1.279-1.329
Å C(sp2)dN distance and 1.321-1.416 Å range of
C(sp2)-N.9 The short C(27)-N(2) bond distance of
1.277(3) Å lies in the range of C(sp2)dN and results in
a long C(27)-N(1) bond of 1.465(3) Å, which suggests
that resonance B is favored. PhNCO insertion into the
Y-N bond forms an ꢀ-caprolactam-substituted amidate
ligand, in which the ꢀ-caprolactam carbonyl oxygen
chelates to the yttrium.
(C5Me5)2Y(NC6H10O)(CNtBu ), 5. In addition to the
reactions of CO2 and PhNCO, the reactivity of 1 and 2
with tert-butyl isocyanide was examined. Consistent
with the previous examples, 1 did not react with tert-
butyl isocyanide. However, when CNtBu is added to a
hexanes solution of 2, a new product, 5, is obtained,
which displayed a new CNtBu resonance in the 1H NMR
spectrum. The IR spectrum contained an absorbance at
2186 cm-1 that is similar to the 2180 cm-1 νCN absorp-
tion of the isocyanide adduct [(C5Me5)2Sm(CNCMe3)]2-
(µ-O).40 The νCO stretch at 1637 cm-1 is similar to that
observed in 2. This suggested that simple adduct
formation could have taken place according to eq 5. This
(34) dee Haan, K. H.; de Bier, J . L.; Teuben, J . H.; Smeets, W. J . J .;
Spek, A. L. J . Organomet. Chem. 1987, 327, 31-38.
(35) Evans, W. J .; Grate, J . W.; Levan, K. R.; Bloom, I.; Peterson,
T. T.; Doedens, R. J .; Zuang, H.; Atwood, J . L. Inorg. Chem. 1986, 25,
3614-3619.
(36) Evans, W. J .; Seibel, C. A.; Ziller, J . Organometallics 1998, 17,
2103-2112.
(37) Schumann, H.; Meese-Marktscheffel, J . A.; Dietrich, A.; Gorlitz,
F. H. J . Organomet. Chem. 1992, 430, 299-315.
(38) Shen, Q.; Mao, L.; Xue, M.; Sun, J . Organometallics 1997, 16,
3711-3714.
(39) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2, 1987, 12, S1-S19.
(40) Evans, W. J .; Drummond, D. K.; Hughes, L. A.; Atwood, J . L.;
Zhang, H. Polyhedron 1988, 7, 1693-1703.

Anion Formed by Deprotonation of ꢀ-Caprolactam
Organometallics, Vol. 20, No. 22, 2001 4535
but not in (C5Me5)2Y[N(SiMe3)2]. In the bis(pentameth-
ylcyclopentadienyl) system, a more reactive ligand was
needed to form the caprolactamate. The conveniently
made13 allyl complex (C5Me5)2Y(C3H5)(THF) had the
necessary reactivity.
The coordination mode of the caprolactamate ligand
with yttrium also apparently depends on the size of the
ancillary ligand set. Hence, with a bis(methylcyclopen-
tadienyl) ligand set, the caprolactamate functions as a
bridging ligand, Figure 2, which connects two metal
centers via bridging oxygen and provides extra coordi-
nation via the nitrogen donor atom. Interestingly, in
(C5H4Me)2Y(THF){OC[N(iPr2)]NPh}, which has a simi-
lar, but noncyclic OCN unit, coordinated to the same
(C5H4Me)2Y metallocene fragment, a monomeric THF
adduct is isolated from THF. Complex 1 evidently
prefers an unsolvated dimeric form since crystallization
of 1 prepared in THF gives the unsolvated dimer. With
larger ancillary ligands, namely, the common bis-
(pentamethylcyclopentadienyl) ligand set, the caprolac-
tamate adopts a chelating binding mode, Figure 3, as
previously observed in the titanium complexes [(C5H5)2-
Ti(NC6H10O)]5 and [O[Si(iPr)2N(C5H3NMe)]2Ti(NMe2)-
(NC6H10O)].6
F igu r e 7. Thermal ellipsoid plot of (C5Me5)2Y(NC6H10O)-
(CNtBu), 5, drawn at the 30% probability level with
hydrogen atoms omitted for clarity.
was also suggested by the 13C NMR spectrum: the 150
ppm shift of the Me3CNC carbon in 5 was very similar
to the shift of 156.2 in free tert-butyl isocyanide. To
determine how the coordination of isocyanide affected
the binding mode of the caprolactamate anion, 5 was
characterized by X-ray crystallography.
The ancillary ligand set apparently affects not only
the synthesis and structure but also the reactivity.
Insertion chemistry was not observed with [(C5H4Me)2Y-
(NC6H10O)]2, 1, under ambient conditions, whereas (C5-
Me5)2Y(NC6H10O), 2, was reactive with several sub-
strates under analogous conditions. It has been reported
elsewhere, however,7 that the [(C5H4Me)2Ln(NC6H10O)]2
(Ln ) Yb, Er, Y) complexes polymerize ꢀ-caprolactone
at or above 60 °C. The limited reactivity of 1 may be
related to the fact that 1 displays a dimeric molecular
weight even in THF. The caprolactamate anion may
form a particularly strong dimeric structure when this
is sterically possible, and this may be less reactive.
Hence, heating these dimers may be necessary to
form monomers that can participate in insertion chem-
istry.
The reactivity of 2 with CO2 and PhNCO shows that
the Y-N part of the caprolactamate anion is most
susceptible to insertion. The structure of the isocyanide
adduct, 5, suggests that prior coordination of the inser-
tion substrate to yttrium is sterically possible.
As shown in Figure 7, tert-butyl isocyanide coordi-
nates to yttrium but does not insert into the Y-N bond.
The binding mode of the caprolactamate anion is not
changed by the additional ligand, but the yttrium
caprolactamate bond distances lengthen to Y-O(1)
2.326(3) Å and Y-N(1) 2.391(3) Å compared to those in
2, 2.289(3) and 2.360(3) Å, respectively. The Y(1)-C(21)
(CNCMe3) distance of 2.578(4) is comparable to that in
the samarium isocyanide adducts [(C5Me5)2Sm(CNC-
Me3)]2(µ-O)] and [(C5Me5)2Sm(µ-CN)(CNC6H11)]3,41 2.64-
(1) and 2.67(2) Å, respectively, when the differences in
metal sizes are taken into account.22
The structures of the CO2 and PhNCO insertion
products demonstrate that this type of reaction can be
used to generate new types of anionic ligands incorpo-
rating the strong bonding of ꢀ-caprolactam. For example,
[(C5Me5)2Y(O2CNC6H10O)]2, 3, adopts a structure in
which the ꢀ-caprolactam-derived oxygen of the newly
formed (O2CNC6H10O)- anion still coordinates to yt-
trium. The alternative structure, in which the ꢀ-capro-
lactam moiety is a noncoordinating substitutent at-
tached to the carboxylate, is not observed. In the case
of (C5Me5)2Y(PhNCONC6H10O), 4, the (PhNCONC6-
Discu ssion
ꢀ-Caprolactam can be deprotonated by amide and allyl
functionalities in yttrium metallocene complexes to form
the (NC6H10O)- caprolactamate anion. This deprotona-
tion reaction is sensitive to the ancillary ligands in the
yttrium metallocene, since the [N(SiMe3)2]- anion will
accomplish this deprotonation in (C5H4Me)2Y[N(SiMe3)2],
H
10O)- formed by insertion is formally analogous to an
acetylacetonate ligand, Figure 6, but involves heteroa-
toms and the cyclic component of a ꢀ-caprolactam.
Hence, the caprolactamate anion can be used to con-
struct more complex bridging polydentate monoanionic
ligand structures in which the powerful ligating ability
of ꢀ-caprolactam can be utilized.
(41) Evans, W. J .; Drummond, D. K. Organometallics 1988, 7, 797-
802.

4536 Organometallics, Vol. 20, No. 22, 2001
Evans et al.
Con clu sion
balance the cationic metal center in complexes of
electropositive metals such as yttrium and lanthanide
metals.
ꢀ-Caprolactam can be readily deprotonated by orga-
noyttrium complexes to provide access to a chelating
monoanion that has variable coordination modes de-
pending on the ancillary ligands. The amide component
of the caprolactamate ligand is susceptible to insertion
chemistry with some substrates again depending on the
ancillary ligand coordination. Insertion chemistry with
the caprolactamate anion can provide a route to new
types of polydentate anionic ligands that contain ꢀ-cap-
rolactam components. These new ligands, as well as the
caprolactamate, incorporate the strong binding ability
of caprolactam into an anionic ligand that can charge
Ack n ow led gm en t. For support of this research, we
thank the Division of Chemical Sciences of the Office
of Basic Energy Sciences of the Department of Energy.
We also thank Dr. Matthew J ohnston for crystal-
lographic assistance.
Su p p or tin g In for m a tion Ava ila ble: This material is
available in the form of a CIF file for the structures reported
herein via the Internet at http://pubs.acs.org.
OM010403N