Rare-earth complexes with multidentate tethered phenoxy-amidinate ligands: Synthesis, structure, and activity in ring-opening polymerization of lactide

Article abstract of DOI:10.1021/om200786u

New multidentate tethered amidine-phenol pro-ligands {4,6-tBu ₂C₆H₂O-(2-C(N-R)=N-R}H₂ ({LON ^R}H₂, R = iPr, cyclohexyl (Cy), 2,6-iPr₂C ₆H₃ (Ar)) were synthesized from the corresponding carbodiimines and 2-bromo-2,4-(di-tert-butyl)phenol. Pro-ligands {LON ^iPr}H₂ and {LON^Ar}H₂ were metalated by 2 equiv of nBuLi to provide the corresponding dilithium salts {LON ^iPr}Li₂ (1) and {LON^Ar}Li₂ (2), which were authenticated by elemental analysis, X-ray crystallography, and NMR spectroscopy. Three different approaches were explored to coordinate these (pro)ligands onto rare earths: salt metathesis, and amine and methane elimination reactions. The salt metathesis reaction between 1 and YCl ₃ afforded the chloro complex [{LON^iPr}YCl]_n (3), which was, in turn, converted into the corresponding amide {LON ^iPr}YN(SiMe₃)₂ (6) by reaction with MN(SiMe₃)₂ (M = Li, Na). Similar reactions between 2 and YCl₃, followed by recrystallization from DME, led systematically to the isolation of the monoprotonated product, that is, phenoxy-amidino complex {LO^HN^Ar}YCl₂(DME) (5). Amine elimination reactions between {LON^iPr}H₂ or {LON^Cy}H ₂ and Ln[N(SiMe₃)₂]₃ afforded the corresponding phenoxy-amidinate amides {LON^R}LnN(SiMe ₃)₂ (Ln = Y, R = iPr, 6; R = Cy, 8; Ln = Nd, R = Cy, 9), whereas the same reaction between{LON^Ar}H₂ and Y[N(SiMe₃)₂]₃, under various conditions, always yielded the homoleptic tris(phenoxy-amidinate) complex {LO^HN ^Ar}₃Y (11). Bimetallic "ate"-complexes {LON ^R}₂Ln₂Me₄Li₂(TMEDA) ₂ of yttrium (12 and 13), neodymium (14), samarium (15), and {LON^iPr}₂Yb₂Me₂(OH) ₂Li₂(TMEDA)₂ (16) were prepared by alkane elimination of the corresponding pro-ligand and [Li(TMEDA)][LnMe₄] complex. Both amido and methyl "ate"- complexes were shown by X-ray diffraction studies to be dimeric in the solid state. The multidentate nature of the ligands in these dimeric species generates a cis/trans isomerism related to the nitrogen atoms in the nonsymmetrically coordinated amidinate fragments. Amido complexes 6, 8, and 9 are effective initiators for the ring-opening polymerization (ROP) of racemic lactide (rac-LA), giving atactic or heterotactic-enriched (P_r up to 76%) polymers with high molecular weights (M_n up to 158 800 g?mol^-1), but broad molecular weight distributions (M_w/M_n = 1.5-2.8). An effective immortal ROP of rac-LA was feasible by combining complex 6 with 5-50 equiv of isopropanol or benzyl alcohol, affording PLAs with well-controlled molecular weights and narrow polydispersities (M_w/M_n = 1.11-1.38).

Full text of DOI:10.1021/om200786u

ARTICLE
pubs.acs.org/Organometallics
Rare-Earth Complexes with Multidentate Tethered
Phenoxy-Amidinate Ligands: Synthesis, Structure, and
Activity in Ring-Opening Polymerization of Lactide
Mikhail Sinenkov,† Evgeny Kirillov,*,‡ Thierry Roisnel,‡ Georgy Fukin,† Alexander Trifonov,*,† and
Jean-Franc-ois Carpentier*,‡
†G.A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, GSP-445,
603950 Nizhny Novgorod, Russia
‡Catalyse et Organomꢀetalliques, CNRS - Universitꢀe de Rennes 1 - Laboratoire Sciences Chimiques de Rennes (UMR 6226),
Campus de Beaulieu, 35042 Rennes Cedex, France
S Supporting Information
b
ABSTRACT: New multidentate tethered amidine-phenol pro-
ligands {4,6-tBu2C6H2O-(2-C(N-R)dN-R}H2 ({LONR}H2,
R = iPr, cyclohexyl (Cy), 2,6-iPr2C6H3 (Ar)) were synthesized
from the corresponding carbodiimines and 2-bromo-2,4-(di-
tert-butyl)phenol. Pro-ligands {LONiPr}H2 and {LONAr}H2
were metalated by 2 equiv of nBuLi to provide the correspond-
ing dilithium salts {LONiPr}Li2 (1) and {LONAr}Li2 (2), which
were authenticated by elemental analysis, X-ray crystallography,
and NMR spectroscopy. Three different approaches were
explored to coordinate these (pro)ligands onto rare earths: salt
metathesis, and amine and methane elimination reactions. The
salt metathesis reaction between 1 and YCl3 afforded the chloro
complex [{LONiPr}YCl]n (3), which was, in turn, converted into the corresponding amide {LONiPr}YN(SiMe3)2 (6) by reaction
with MN(SiMe3)2 (M = Li, Na). Similar reactions between 2 and YCl3, followed by recrystallization from DME, led systematically to
the isolation of the monoprotonated product, that is, phenoxy-amidino complex {LOHNAr}YCl2(DME) (5). Amine elimination
reactions between {LONiPr}H2 or {LONCy}H2 and Ln[N(SiMe3)2]3 afforded the corresponding phenoxy-amidinate amides
{LONR}LnN(SiMe3)2 (Ln = Y, R = iPr, 6; R = Cy, 8; Ln = Nd, R = Cy, 9), whereas the same reaction between{LONAr}H2 and
Y[N(SiMe3)2]3, under various conditions, always yielded the homoleptic tris(phenoxy-amidinate) complex {LOHNAr}3Y (11).
Bimetallic “ate”-complexes {LONR}2Ln2Me4Li2(TMEDA)2 of yttrium (12 and 13), neodymium (14), samarium (15), and
{LONiPr}2Yb2Me2(OH)2Li2(TMEDA)2 (16) were prepared by alkane elimination of the corresponding pro-ligand and
[Li(TMEDA)][LnMe4] complex. Both amido and methyl “ate”- complexes were shown by X-ray diffraction studies to be dimeric
in the solid state. The multidentate nature of the ligands in these dimeric species generates a cis/trans isomerism related to the
nitrogen atoms in the nonsymmetrically coordinated amidinate fragments. Amido complexes 6, 8, and 9 are effective initiators for
the ring-opening polymerization (ROP) of racemic lactide (rac-LA), giving atactic or heterotactic-enriched (Pr up to 76%) polymers
with high molecular weights (Mn up to 158 800 g molꢀ1), but broad molecular weight distributions (Mw/Mn = 1.5ꢀ2.8). An
3
effective immortal ROP of rac-LA was feasible by combining complex 6 with 5ꢀ50 equiv of isopropanol or benzyl alcohol, affording
PLAs with well-controlled molecular weights and narrow polydispersities (Mw/Mn = 1.11ꢀ1.38).
’ INTRODUCTION
phenoxide and amidinate fragments5b,d,6 and that proved to be
efficient initiators in ROP processes (Scheme 1);5b,6a,b in fact, in these
cases, the phenoxide group has been proposed to be the true
initiating group of the ROP, or the actual nature of the latter
initiating group remains still obscure.
As exemplified exhaustively with the ubiquitous “constrained
geometry” Cp-amido dianionic ligands,7 the implementation of
new ligand assemblies incorporating tethered anionic moieties
Phenoxide1 and amidinate2 anionic ligands are among the
most ubiquitous building motifs encountered in the elaboration
of rare-earth-based initiators/catalysts that are used in the ring-
opening polymerization (ROP) of lactides and other cyclic
esters.3ꢀ5 The main advantages of these ancillaries include a
highly electron-donating character that results in remarkable
stability/robustness of the corresponding metal complexes, and
readily tunable steric and electronic features for stabilizing var-
ious geometries and oxidation states. On the other hand, there
are only few examples of rare-earth complexes that combine
Received: August 22, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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Organometallics
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Scheme 1. Examples of Rare-Earth Complexes Supported by Phenoxide and Amidinate Ligands5b,d,6
Scheme 2. Some of the Possible Coordination Modes of the
Dianionic Phenoxy-Amidinate Ligand (I and II) and a Pro-
tonated Monoanionic Form (III)
Scheme 3. Synthesis of Amidine-Phenol Pro-ligands
{LONR}H2
Figure 1. Crystal structure of [{LONiPr}Li2(THF)2]2 (THF) (1)
3
(ellipsoids are drawn at the 50% probability level; all hydrogen atoms,
carbon atoms of the coordinated THF molecules, and one THF
molecule present in the cell are omitted for clarity).
Both compounds were isolated as white solids and authenticated
1
on the basis of H and 13C NMR spectroscopies, elemental
analysis, and X-ray diffraction studies performed on single
crystals of THF adducts [{LONiPr}Li2(THF)2]2 (THF) and
3
{LONAr}Li2(THF)3, obtained by recrystallization of 1 and 2,
respectively, from THF/hexanes (1:1) mixtures.
The solid-state structure of [{LONiPr}Li2(THF)2]2 (THF)
3
is dimeric, featuring a crystallographic inversion center that
lies at the center of the Li(1)ꢀO(1)ꢀLi(1_1)ꢀO(1_1) plane
(Figure 1; for crystallographic details, see Tables S1 and S2,
Supporting Information). Two of the four Li atoms in the
dimeric structure are bound to the oxygen atoms of the bridging
phenoxides (LiꢀO = 1.832(3) and 1.979(3) Å)8 and to one
carbon and one nitrogen atom (LiꢀC(7) = 2.236(3), LiꢀN(2) =
2.172(3) Å, respectively) of the anionic amidinate group. Two
other Li atoms are also four-coordinated and bound each to the
nitrogen atoms of the amidinate groups (LiꢀN = 2.019(3) and
2.042(3) Å)9 and oxygen atoms of THF molecules (LiꢀO =
1.944(3) and 2.009(3) Å).
can be of both fundamental and practical value. Herein, we report
a new multidentate tethered phenoxy-amidinate dianionic ligand
system I (Scheme 2). The coordination chemistry of this ligand
system with rare-earth metals has been investigated. Two alter-
native coordination modes of this dianionic ligand (I and II) and
a protonated monoanionic form (III, Scheme 2) have been
evidenced in complexes of group 3 metals. Preliminary studies on
the catalytic activity of some complexes supported by this new
tethered phenoxy-amidinate ancillary in the ROP of lactide are
reported as well.
The H NMR spectrum of {LONiPr}Li2 (1) in THF-d8
1
’ RESULTS AND DISCUSSION
displayed broadened resonances in the 298ꢀ353 K temperature
range. This feature is indicative of a fluxional dynamic phenom-
enon that can be attributed to a haptotropic rearrangement
process involving the amidinate moieties, and which is relatively
slow on the NMR time scale.
Synthesis of Pro-ligands. The amidine-phenol pro-ligands
were prepared via a three-step procedure, starting from 2-bromo-
4,6-(di-tert-butyl)phenol and a set of carbodiimides bearing
substituents imparting variable bulkiness and solubility (Scheme 3).
The products were isolated in good yields and characterized by NMR
spectroscopy and elemental analysis.
Salt Metathesis Reactions. Double deprotonation of the
amidine-phenol pro-ligands {LONiPr}H2 and {LONAr}H2 with
nBuLi, conducted in either toluene or diethyl ether, resulted in
the formation of the corresponding dianionic dilithium salts,
{LONiPr}Li2 (1) and {LONAr}Li2 (2), respectively (Scheme 4).
On the other hand, the dilithium salt {LONAr}Li2(THF)3 (2)
is monomeric in the solid state (Figure 2). The amidinate moiety
in 2, despite its anionic character, is nonsymmetrically coordi-
nated with metal atoms. The differences in the CꢀN bond
lengths (N(1)ꢀC(7) = 1.368(4) Å), N(2)ꢀC(7) =1.314(4) Å)
argue that most of the negative charge of the amidinate moiety is
located mainly on the nitrogen atom N(1), onto which are actually
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ARTICLE
Scheme 4. Synthesis of the Dilithium Salts 1 and 2 and Yttrium Complexes 3ꢀ5 Derived Thereof
DME, in isolation of the unexpected complex {LOHNAr}YCl2
(DME) (5) in 32% yield (Scheme 4). Repeated experiments,
with special care to avoid adventitious hydrolysis, resulted in a
similar outcome. Apparently, the high instability of the dilithium
salt 2 and/or its yttrium complex 4 accounts for the observed
protonolysis reaction, although the exact stage and nature of the
proton source in this latter reaction could not be unambiguously
identified.
The solid-state structure of 5 has been determined (Figure 3;
see Tables 1 and 2 for main bond distances and angles) and
revealed the yttrium atom in a distorted octahedral coordination
environment, with the ligand nitrogen and DME oxygen atoms
occupying the axial positions, while two oxygen atoms of the
ligand and DME as well as two terminal chloro ligands lie in the
equatorial positions. The Y(1)ꢀO(1) bond length in 5
(2.129(2) Å) is similar to the values of covalent bonds reported
for related six- and seven-coordinated yttrium complexes with
phenoxide and salicylaldimine ligands (2.114ꢀ2.166 Å).10 The
Y(1)ꢀN(1) bond (2.300(3) Å) is much shorter than the YꢀN
coordination bonds (2.515ꢀ2.661 Å) in the latter compounds10
and is comparable to the YꢀN distances reported for yttrium
amidinate and guanidinate complexes (2.284ꢀ2.469 Å).11 The
YꢀCl bond lengths in 5 (2.584(1) and 2.614(1) Å) fall into the
regular range for terminal YꢀCl bonds in six-coordinated
complexes.12 Simple electroneutrality considerations imply the
existence of a protonated amidinate fragment (i.e., RꢀNdC-
(R0)ꢀNHR) in 5, although the X-ray diffraction study did not
allow us to localize this amino hydrogen, most likely due to its
proximity to the heavy metal atom. However, the geometric
features of the NCN fragment differ considerably from that
described for anionic amidinate moieties with similar monoden-
tate coordination13 and are indicative of its protonated form at
N(2). In fact, the NꢀC bonds within the NCN fragment are quite
inequivalent (N(1)ꢀC(7) = 1.389(4) Å vs N(2)ꢀC(7) =
1.306(4) Å) and differ noticeably from the corresponding dis-
tances in the related free amidine (1.321(3) and 1.352(3) Å).14
Figure 2. Crystal structure of {LONAr}Li2(THF)3 (2) (ellipsoids are
drawn at the 50% probability level; all hydrogen atoms and carbon atoms
of the coordinated THF molecules, as well as those of the disordered tBu
and iPr groups, are omitted for clarity).
bound the two Li atoms. Both Li atoms are four-coordinated and
bound to the same oxygen and nitrogen atoms of the dianionic
ligand, however, in a rather dissymmetric manner. For instance, the
Li(1)ꢀO(1) distance (1.926(6) Å) is longer than the Li(2)ꢀO(1)
one (1.818(6) Å),8 whereas the Li(1)ꢀN(1) distance (2.018(6) Å)
is shorter as compared with the Li(2)ꢀN(1) one (2.182(6) Å).9
Also, a dynamic behavior similar to the one observed for 1 was
1
evidenced for 2 by H NMR spectroscopy in THF-d8 in the
temperature range of 298ꢀ353 K.
The reaction of dilithium salt 1 with YCl3 in THF gave the
corresponding chloro complex [{LONiPr}YCl]n (3) in 56%
isolated yield (Scheme 4). This compound was characterized
by 1H and 13C NMR spectroscopies and elemental analysis. All
the efforts to obtain crystals of 3 suitable for X-ray diffraction
study failed. A similar reaction between 2 and YCl3 resulted, after
workup and a subsequent recrystallization of the product from
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To access complexes potentially useful for ROP catalysis, the
chloro complex 3 was treated with MN(SiMe3)2 (where M = Li,
Na); the corresponding amido complex {LONiPr}YN(SiMe3)2
(6) was thus recovered in 78% yield (Scheme 5). Despite its very
high solubility in aliphatic hydrocarbons (pentane, hexanes),
single crystals of 6 suitable for X-ray diffraction studies were
successfully grown. The complex is dimeric in the solid state, and
its structure is depicted in Figure 4. The molecule of 6 features a
crystallographic C2-symmetry axis that passes through the center
of the Y(1)ꢀN(2_1)ꢀY(1_1)ꢀN(2) core (this makes the
molecule chiral, and the unit cell of 6 contains the two enantio-
meric molecules). Each one of the two yttrium atoms is in a
distorted trigonal bipyramidal environment, with one oxygen and
two nitrogen atoms of the phenoxide and chelating amidinate
moieties, respectively, lying in the equatorial plane, and two axial
nitrogen atoms of the (Me3Si)2N and amidinate groups. Unlike
in mono- and bisamidinate yttrium complexes,6a the coordina-
tion of the amidinate moiety to the metal center in 6 is non-
symmetric, as indicated by inequivalent YꢀN(amidinate) bonds
(2.363(4), 2.437(4), 2.371(3), and 2.459(4) Å) and CꢀN bonds
(C(7_1)ꢀN(1_1) = 1.304(5) Å, C(7_1)ꢀN(2_1) = 1.384(6)
Å, C(7)ꢀN(1) = 1.287(5) Å, C(7)ꢀN(2) = 1.394(6) Å) in the
amidinate fragment. The YꢀN(amido) bond length (2.228(4),
2.238(4) Å) is in agreement with the values reported for yttrium
amido complexes.15
The 1H and 13C NMR spectra of 6, in C6D6 or in THF-d8 at
room temperature, were consistent with an average C2-symmetric
structure in solution on the NMR time scale (Figures S10ꢀS12,
1
respectively; see the Supporting Information). Key H NMR
Table 2. Selected Bond Distances (Å) and Angles (°) for Com-
plexes 5, 7, and 10
5
7
10
M1ꢀCl2
N1ꢀC7
N2ꢀC7
M1ꢀO2
M1ꢀN3
Li1ꢀN4
Li1ꢀN2
2.5842(11)
1.389(4)
1.306(4)
1.393(3)
1.297(3)
2.1488(18)
2.411(2)
1.983(6)
1.337(4)
1.352(4)
Figure 3. Crystal structure of {LOHNAr}YCl2(DME) (5) (ellipsoids
are drawn at the 50% probability level; H atoms are omitted for clarity).
2.118(5)
Table 1. Selected Bond Distances (Å) and Angles (°) for Dimeric Complexes 6, 9, 12, 13, and 16
6
9
12
13
16
M(1)ꢀO(1)
2.124(3)
2.363(4)
2.437(4)
2.473(4)
2.113(3)
2.372(3)
2.459(4)
2.447(4)
2.196(3)
2.445(4)
2.571(4)
2.528(4)
2.216(3)
2.471(4)
2.551(4)
2.589(4)
2.1723(11)
2.4967(13)
2.4575(13)
2.5525(13)
2.1723(11)
2.4967(13)
2.4575(13)
2.5525(13)
2.4943(18)
2.4746(18)
2.159(2)
2.398(3)
2.474(3)
2.518(3)
2.168(2)
2.423(3)
2.453(3)
2.525(3)
2.532(4)
2.491(4)
2.1123(14)
2.3720(17)
2.4183(16)
2.4648(16)
2.1173(15)
2.3671(16)
2.4228(16)
2.4694(17)
2.474(2)
M(1)ꢀN(1_1)
M(1)ꢀN(2_1)
M(1)ꢀN(2)
M1_1ꢀO1_1
M(1_1)ꢀN(1)
M(1_1)ꢀN(2)
M(1_1)ꢀN(2_1)
M(1)ꢀC(22)
M(1)ꢀC(23)
M(1)ꢀN(3)
2.228(4)
2.238(4)
2.344(4)
2.316(4)
M(1_1)ꢀN(3_1)
M(1)ꢀO(2)
2.5481(16)
1.312(3)
N(1)ꢀC(7)
N(2)ꢀC(7)
1.286(6)
1.394(6)
1.311(6)
1.404(5)
1.313(2)
1.375(2)
1.315(5)
1.392(5)
1.393(3)
N(1_1)ꢀC(7_1)
N(2_1)ꢀC(7_1)
N(1_1)ꢀM(1)ꢀN(2_1)
N(1_1)ꢀC(7_1)ꢀN(2_1)
C(7_1)ꢀM(1)ꢀO(1)
C(7_1)ꢀM(1)ꢀN(3)
O(1)ꢀM(1)ꢀN(3)
C(22)ꢀM(1)ꢀC(23)
Li(1)ꢀC(22)ꢀM(1)
C(7_1)ꢀM(1)ꢀLi(1)
O(1)ꢀM(1)ꢀLi(1)
1.304(5)
1.314(6)
1.313(2)
1.299(4)
1.306(3)
1.384(6)
1.381(6)
1.375(2)
1.382(5)
1.393(3)
56.48(13)
115.3(4)
53.66(12)
114.5(4)
54.22(4)
55.18(10)
114.7(3)
56.36(5)
114.28(14)
159.84(4)
113.82(18)
120.92(6)
122.86(13)
111.52(14)
109.34(13)
121.26(12)
115.42(13)
112.88(13)
118.62(10)
91.70(6)
80.25(10)
105.78(7)
92.99(6)
89.88(13)
79.96
80.80(13)
120.41(9)
107.67(8)
121.28
110.49
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Scheme 5. Synthesis of Amido Yttrium Complex
{LONiPr}YN(SiMe3)2 (6) by Salt Metathesis
Figure 5. Molecular structure of {LONAr}2YLi(THF)2 (7) (ellipsoids
are drawn at the 50% probability level; all hydrogen atoms and carbon
atoms of the coordinated THF molecules, as well as those of the
disordered tBu groups, are omitted for clarity).
a one-pot reaction, to minimize protonolysis problems encoun-
tered in the case of 4/5 (vide supra) (Scheme 6). However, this
procedure led to the heterobimetallic bis-ligand “ate”-complex
{LONAr}2YLi(THF)2 (7), isolated in 33% yield, which was
authenticated by elemental analysis, NMR spectroscopy, and
an X-ray diffraction study.
Figure 4. Molecular structure of {LONiPr}YN(SiMe3)2 (6) (ellipsoids
are drawn at the 50% probability level; H atoms and Me groups of Me3Si
units are omitted for clarity).
The solid-state structure of 7 revealed a five-coordinate
species, in which the yttrium center lies in a distorted tetragonal
pyramidal environment (Figure 5. Two oxygen and two nitrogen
atoms of the two chelating ligands occupy the equatorial plane of
this pyramid, while the oxygen atom of the coordinated THF
molecule is in the axial position (Y(1)ꢀO(3) = 2.287(2) Å).
Each phenoxy-amidinate ligand in 7 is connected to the yttrium
center by one YꢀO and one YꢀN bond, while the second
amidinate nitrogen does not bind to yttrium. The two dianionic
ligands are not equivalent since the second nitrogen of one of
them (N(4)) is also bound with a lithium atom. Despite this fact,
both ligands display quite similar YꢀO bond distances (2.154(2)
and 2.149 (2) Å) and their bonding situation is consistent with
phenoxy-amido character.16 However, the geometric parameters
of the NCN fragments in the two ligands differ noticeably: in one
moiety, the C(7)ꢀN(2) bond is significantly shorter than the
C(7)ꢀN(1) (1.297(3) vs 1.393(3) Å), suggesting a double
character for the former bond. On the other hand, in the moiety
that binds both yttrium and lithium atoms, the Y(1)ꢀN(3) bond
(2.411(2) Å) is almost 0.1 Å longer than the Y(1)ꢀN(1) one. In
the amidinate group of this ligand, the double bond is partially
delocalized within the N(3)ꢀC(57)ꢀN(3) fragment, which is in
agreement with the corresponding bond lengths C(57)ꢀN(3)
(1.370(3) Å) and C(57)ꢀN(4) (1.329(3) Å). The lithium atom
in this molecule is bound with one of the two nitrogen atoms of
the amidinate group (Li(1)ꢀN(4) = 1.983(6) Å) and also
accommodates a THF molecule (Li(1)ꢀO(4) = 1.887(6) Å).
In addition, the lithium atom exhibits short interactions with the
π-system of the adjacent aromatic ring of the di(iso-propyl)-
Scheme 6. Attempted “One-Pot” Salt Metathesis Reaction
Towards Complex {LONAr}YN(SiMe3)2 and Formation of
{LONAr}2YLi(THF)2 (7)
resonances include (a) two singlets for H3 and H5 hydrogens
of the phenoxide groups, (b) one broadened multiplet for the
CH(CH3) isopropyl groups, (c) two sharp singlets for the tBu
groups, (d) one doublet for the CH(CH3) isopropyl groups, and
(e) a unique singlet resonance for the SiMe3 groups.
anilinic moiety (Li(1)ꢀCipso = 2.382(6) Å and Li(1)ꢀOortho
=
2.415(6) Å). These distances are in good agreement with
LiꢀC(arene) distances (2.15ꢀ2.651 Å) observed in arene-
substituted amido and amidinate17 and in arene18 complexes of
lithium.
Attempts to synthesize an amido complex {LONAr}YN(SiMe3)2
supported by a bulkier LONAr ligand system were undertaken via
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Scheme 7. Synthesis of Amide Complexes
{LONR}LnN(SiMe3)2 by Amine Elimination Reactions
Amine Elimination Approach. Alternatively, amine elimina-
tion reactions were investigated as an efficient one-step route
toward the compounds targeted for ROP catalysis. The NMR-
scale reactions between Y[N(SiMe3)2]3 and {LONiPr}H2 or
{LONCy}H2 pro-ligands in C6D6 were sluggish at room tem-
perature and yielded in 24 h mixtures of at least two unidentified
products containing unreacted ꢀNHR amino groups (R = iPr or
Cy) of the amidinate moieties. Further heating of these mixtures
over prolonged reaction times (ca. 50 h) did not improve either
the selectivity or the yield of these reactions. On the other hand,
the same reactions, carried out in THF-d8 at room temperature
or at 50 °C over 48 h, resulted in the selective formation of the
corresponding amido complexes {LONR}YN(SiMe3)2 (R = iPr,
6; Cy, 8) (Scheme 7). Following a similar approach, the neo-
dymium complex {LONCy}NdN(SiMe3)2 (9) was isolated
in 61% yield. The solution structure of yttrium complex 8 was
Figure 6. Molecular structure of {LONCy}NdN(SiMe3)2 (9) (ellip-
soids are drawn at the 50% probability level; H atoms and Me groups of
Me3Si units are omitted for clarity).
Scheme 8. Formation of Yttrium “ate”-Complex
{LONCy}2YLi(DME) (10)
1
established by H and 13C NMR spectroscopies, whereas
the NMR data obtained for 9 were not informative due to
extremely broad and unresolved resonances that resulted from
the strong paramagnetism of neodymium. In C6D6 solution,
8 behaves as an average C2-symmetric species, similarly to
6 (Figures S15 and S16; see the Supporting Information).
A moderate fluxional behavior, assigned to hindered motion of
the cyclohexyl groups, was observed for 8 in C6D6 solution at
room temperature.
The solid-state structure of neodymium compound 9 was
established by X-ray diffraction (Figure 6). Two crystallographi-
cally independent enantiomeric molecules featuring very similar
geometrical parameters were found in the crystal cell, and only
one of these is discussed hereafter. The overall geometry of 9
is quite similar to that of 6 in terms of relative atom placements
and bonding. That is, the dimeric structure is composed of two
metal centers grasped into a core by bridging N,N0-dihapto-
amidinate groups of two phenoxy-amidinate ligands. The values
of the NdꢀO(1) and NdꢀO(1_1) bond lengths (2.196(3) and
2.216(3) Å, respectively) in 9 are close to those formerly reported
for neodymium phenoxides,2d,19 whereas the NdꢀN(amidinate)
bond distances (2.445(4), 2.571(4), 2.551(4), and 2.471(4) Å)
fall in the range characteristic for those in neodymium amidi-
nates and guanidinates.7e,20 Expectedly, the bond lengths
between the central carbon atom of the amidinate fragment
and the μ-bridging nitrogen atom (C(7)ꢀN(2) = 1.405(5) Å
and C(7_1)ꢀN(2_1) = 1.382(6) Å, respectively) are longer
than the terminal ones (C(7)ꢀN(1) = 1.311(6) Å and C(7_1)ꢀ
N(1_1) = 1.314(6) Å).
In the course of purification of complex 8, the treatment of its
hexane solution with DME resulted in the formation of colorless
crystals of heterobimetallic complex {LONCy}2YLi(DME) (10)
(Scheme 8), which was isolated in 5% yield. Complex 10 is hardly
soluble in DME and THF and is insoluble in toluene and hexane.
Obviously, the presence of residual LiN(SiMe3)2 in the starting
Y[N(SiMe3)2]3 accounted for the formation of 10. Complex 10
was prepared purposely in 45% yield via the reaction between
YCl3 and 2 equiv of {LONCy}Li2.21
An X-ray diffraction study of suitable crystals of 10 revealed
that this compound is a heterobimetallic complex containing
lithium and yttrium atoms connected together by two dianionic
phenoxy-amidinate ligands, which are coordinated to the yttrium
center in a bidentate k3-fashion (Figure 7). The molecule of 10
features C2 symmetry in the solid state, with the corresponding
axis passing through the yttrium and lithium centers and the
midpoint of the CꢀC bond of the coordinated DME molecule.
As a result, 10 exists as a racemic mixture of two enantiomers;
both of them are found in the unit cell. In the series of phenoxy-
amidinate rare-earth complexes reported in this paper, com-
pound 10 is the sole example where all three donor atoms (N,N,
O) are bound to the same metal atom. At the same time, one of
the nitrogen atoms of each amidinate moiety is coordinated to
the lithium ion, thus bridging it with yttrium. The Y(1)ꢀO(1)
bond in 10 (2.174(2) Å) is somewhat longer than the corre-
sponding bond in heterobimetallic complex 7. The four YꢀN
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bonds are inequivalent, and the distance between the yttrium
atom and terminal nitrogen (Y(1)ꢀN(21) = 2.362(3) Å) is
expectedly shorter as compared with the one μ-bridging between
the yttrium and lithium atoms (Y(1)ꢀN(31) = 2.521(3) Å). The
CꢀN bond lengths within the amidinate fragments are also
slightly different (C(7)ꢀN(21) = 1.337(4) Å vs C(7)ꢀN(31_2) =
1.352(4) Å).
methyl “ate”-complexes[Li(TMEDA)][LnR4] (R=Me,CH2SiMe3)
have been scrutinized in our previous studies as convenient
starting materials for preparing highly reactive lanthanide com-
plexes being able to promote enantioselective intramolecular
olefin hydroamination.25
Accordingly, a series of methyl “ate”-complexes {LONR}2Ln2-
Me4Li2(TMEDA)2 of yttrium (12 and 13), neodymium (14),
and samarium (15) were prepared in high yields via a one-step
methane-elimination reaction between the pro-ligands {LONiPr}
H2 and {LONCy}H2 with the in situ generated [Li(TMEDA)]
[LnMe4]25b (Scheme 10). These compounds are readily soluble
in diethyl ether and aromatic hydrocarbons (toluene and benzene).
They were characterized by elemental analysis, NMR spectros-
copy (for diamagnetic species), and crystal structure deter-
minations for 12 and 13.
When the bulkier pro-ligand {LONAr}H2 was used in the
amine elimination reaction with Y[N(SiMe3)2]3, the sole pro-
duct observed was the homoleptic complex {LOHNAr}3Y (11)
(Scheme 9). Tuning the reaction conditions (more elevated
temperatures, extended reaction times, nature of solvents) did
not affect the final outcome.22 The 1H and 13C NMR data for 11
(C6D6, room temperature) were consistent with an average
symmetric species on the NMR time scale in which all the three
ligands are monoanionic, chelating the metal center with the imino
nitrogen atoms. Diagnostic 1H NMR resonances include (a) six
multiplets for six different types of aromatic hydrogens, (b) one
singlet for the amino protons of the pendant NH(iPr2C6H3)
groups, (c) two septets for the methine protons of the iPr groups,
(d) two singlets for the tBu groups, and (e) four doublets for the
methyl hydrogens of the iPr groups.
Crystals of 12 and 13 suitable for X-ray diffraction studies were
obtained by cooling concentrated solutions of the complexes in
diethyl ether at ꢀ20 °C. Both compounds crystallize as solvates
containing one molecule of diethyl ether per unit cell. These
complexes adopt dimeric structures in the solid state (Figures 8
and 9, respectively). The dimers formed via μ-bridging of the
dianionic phenoxy-amidinate ligands, with the amidinate and
alkoxo units of a given ligand bound to two different yttrium atoms.
One of the two nitrogen atoms of each amidinate fragments is
μ-bridging between the two yttrium atoms, while the second one
is terminal. In addition, two μ-bridging methyl groups connect
the yttrium center with Li(TMEDA). Each six-coordinate yttrium
atom in 12 (Et2O) and 13 (Et2O) adopts a trigonal prismatic
Alkane Elimination Approach. Recent studies in the chem-
istry of groups 3 and 4 metals have shown the alkane elimination
approach, using homoleptic MR3 and MR4 precursors, respec-
tively, and amidine-based pro-ligands, to be a versatile route
toward different classes of amidinate complexes.23,24 Heteroleptic
3
3
geometry. The molecule of 12 has a crystallographic inversion
center lying at the center of the Y(1)ꢀN(2)ꢀY(1_1)ꢀN(2_1)
plane. On the other hand, the molecule of 13 is C2-symmetric
with the corresponding axis passing perpendicularly through the
Scheme 10. Synthesis of Heterometallic Methyl “ate”-Com-
plexes {LONR}2Ln2Me4Li2(TMEDA)2 (12ꢀ15)
Figure 7. Molecular structure of {LONCy}2YLi(DME) (10) (H atoms
and cyclohexyl rings are omitted for clarity; ellipsoids are drawn at the
50% probability level).
Scheme 9. Amine Elimination Reaction between {LONAr}H2 and Y[N(SiMe3)2]3
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Scheme 11. Synthesis of Heterometallic Methyl-Hydroxo
“ate”-Complex {LONiPr}2Yb2Me2(OH)2Li2(TMEDA)2 (16)
Figure 8. Molecular structure of {LONiPr}2Y2Me4Li2(TMEDA)2
(Et2O) (12) (ellipsoids are drawn at the 50% probability level; H atoms,
except those of the bridging methyl groups, and a molecule of Et2O are
omitted for clarity).
3
Figure 10. Molecular structure of {LONiPr}2Yb2Me2(OH)2Li2(TM-
EDA)2 (Et2O) (16) (ellipsoids are drawn at the 50% probability level;
3
all hydrogen atoms are omitted for clarity).
Unexpectedly, the reaction of [Li(TMEDA)][YbMe4] with
{LONiPr}H2, carried out under conditions identical to those
used for other elements (Scheme 10), led to the isolation in 67%
yield of {LONiPr}2Yb2Me2(OH)2Li2(TMEDA)2 (16) as pale-
yellow crystals (Scheme 11). Suitable crystals of 16 were studied
by X-ray crystallography (Figure 10). The molecule of 16 is very
similar to those of 12 and 13, exhibiting the same organization of
the coordination spheres at the ytterbium and lithium centers.
The presence of adventitious water and the extreme sensitivity of
the putative intermediate {LONiPr}2Yb2Me4Li2(TMEDA)2 are
the most probable reasons that can be envisioned for the origin of
the OH ligands in this compound.
It is worth noting that the multidentate coordination of
phenoxy-amidinate ligands in the amido and dimethyl complexes
gives rise to the formation of dimeric chiral molecules that feature
cis/trans stereoisomerism (Scheme 12). For instance, complexes 6,
9, 13, and 16 were found to adopt a Ci-symmetric cis configuration
in the solid state. They can be regarded as cis heterochiral dimeric
molecules according to the existing classification.27 On the other
hand, 12 adopts in the solid state a C2-symmetric homochiral trans
form. These differences may be accounted for by possible dynamic
interconversion processes between the cis and trans forms of these
dimeric molecules. Such a process is apparently fairly rapid on the
NMR time scale, as shown by the completely symmetric sets of
signals in the NMR spectra of dimeric diamagnetic complexes 6, 12,
and 16 (vide supra).
Figure 9. Molecular structure of {LONCy}2Y2Me4Li2(TMEDA)2
(Et2O) (13) (ellipsoids are drawn at the 50% probability level; H atoms,
except those of the bridging methyl groups, cyclohexyl groups, and a
molecule of Et2O are omitted for clarity).
3
center of the Y(1)ꢀN(2_1)ꢀY(1_1)ꢀN(2) core. The aver-
age YꢀC(methyl) bond length in 12 (2.484 Å) is somewhat
shorter than that in 13 (2.515 Å),26 whereas on the contrary,
the average YꢀN distance is longer in 12 (2.476 Å) than in 13
(2.437 Å). The YꢀO bond distances in 12 (2.172(1) Å) and
13 (2.168(2) and 2.159(2) Å) are longer with respect to those
in the five-coordinate complex 6 (2.113(3) and 2.124(3) Å)
and are also close to those in the six-coordinate 7 (2.154(2)
and 2.149 (2) Å).
The composition and symmetry of dimeric 12 and 13 found in
the solid state are retained in solution, as judged by 1H and 13
C
NMR spectroscopies. Both the 1H and the 13C NMR spectra of
12 and 13, recorded in C6D6 at room temperature, exhibited only
one set of resonances for each type of nucleus (Figures S21,
S22 and Figures S23, S24, respectively; see the Supporting
Information). Also, as a result of fast fluxional dynamics on the
NMR time scale, the methyl groups appeared as a single
resonance in the 1H and 13C NMR spectra of both 12 and 13.
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ARTICLE
Scheme 12. cis and trans Isomerism in Dimeric Phenoxy-
Amidinate Complexes 6, 9, 13, and 16, and Interconversion
Processes
polydispersity and molecular weights values—are indicative of rela-
tively slow initiation as compared to chain-propagation reaction. It
must be noted that this general trend was observed with rare-earth
amido complexes in the ROP of lactide.3ꢀ5
On the other hand, the ROP of rac-LA proceeded in a
much better controlled fashion when alcohols (i.e., iPrOH or
PhCH2OH) were added in the reaction medium as external co-
initiator/chain transfer agents (Table 3, entries 14ꢀ22). For
instance, the ROP of 500ꢀ1000 equiv of rac-LA was performed
using 6 in the presence of 5ꢀ50 equiv (vs Y) of iPrOH, even-
tually yielding PLAs with accordingly decreased molecular weights.
The much narrower polydispersities (Mw/Mn = 1.11ꢀ1.34)
observed under those conditions, as compared with those
obtained without alcohol added, and the good match between
calculated and experimental molecular weights argue for fast and
reversible transfer reactions. 1H NMR spectroscopic analysis of
the low-molecular-weight PLA produced with 50 equiv of iPrOH
under such conditions (entry 19) was in agreement with selective
capping of the PLA chains with iPrOC(O)ꢀ and HOCH-
(CH3)COꢀ end-groups, thus supporting the transfer process of
an effective immortal polymerization.29 However, under those
immortal conditions, the catalyst systems were not stereoselective
any longer as essentially atactic PLAs were recovered.
’ CONCLUSIONS
We have prepared new multidentate ligand systems combin-
ing phenoxy and amidinate fragments into a dianionic chelating
platform. These systems have been successfully used to prepare
phenoxy-amidinate complexes of group 3 and lanthanide metals.
Ligands with relatively less sterically demanding isopropyl and
cyclohexyl N,N0-disubstituted amidinate fragments lead to di-
meric complexes, whereas the bulkier di(2,6-isopropyl)phenyl N,
N0-disubstituted version of this ligand system apparently favors
the formation of monomeric complexes in which the ligand
acts as a monoanionic phenoxy-amidine. Dimeric complexes of
isopropyl and cyclohexyl N,N0-disubstituted ligands exhibit
stereoselective coordination onto the metal centers, giving rise
to cis/trans isomerism. These isomers have been identified in the
solid state by X-ray diffraction studies, and they rapidly inter-
convert in solution, as observed by NMR spectroscopy. Amido
complexes of these phenoxy-amidinate ligands are active initia-
tors or catalysts, when combined with iPrOH or PhCH2OH used
as co-initiators/chain transfer agents, in the ROP of racemic
lactide, giving atactic to heterotactic-enriched PLAs.
Ring-Opening Polymerization Studies. We next evaluated the
performances of the prepared compounds {LONR}Ln(N;(SiMe3)2)
that possess a potentially reactive nucleophilic amido group, that
is, 6, 8, and 9, as initiators/catalysts in the ROP of racemic lactide
(rac-LA) (Scheme 13).28 Representative results are summarized
in Table 3.
Use of these amido complexes as initiators (i.e., in the absence
of any external co-initiator, vide infra) revealed they are moder-
ately active at room temperature. Compounds 6 and 8, which
incorporate, respectively, isopropyl and cyclohexyl substituents
at amidinate groups, featured similar activities equally in THF
and toluene solvents. Also, the yttrium and neodymium com-
pounds 8 and 9 displayed about the same activities. Interestingly,
’ EXPERIMENTAL SECTION
General Conditions. All manipulations requiring an anhydrous
atmosphere were performed under a purified argon atmosphere using
standard Schlenk techniques or in a glovebox. Anhydrous LnCl3
precursors (99.99%) were purchased from Strem Chemicals and used
as received. TMEDA was purchased from Acros and dried over 3A
1
the homodecoupled H NMR spectra of the PLAs formed
showed significantly heterotactic-enriched microstructures. The
Pr (probability of racemic linkage) values were in the range of
0.64ꢀ0.76, depending on the nature of the initiator and solvent.
The PLA samples obtained under these conditions (i.e., in the
absence of any external co-initiator) all had monomodal, but
broad, molecular weight distributions (Mw/Mn = 1.5ꢀ2.6). The
relatively poor degree of control provided by these systems in
terms of molecular weights is also illustrated by the experimental
Mn values that were quite often significantly higher than the
theoretical ones; a noticeable exception is the good match observed
between Mn,calc and Mn,exptl values for polymers prepared from 6 in
THF (Table 3, entries 5 and 6). Both features—relatively high
molecular sieves before use. The metal precursors, LiN(SiMe3)2
3
(Et2O),30 Ln[N(SiMe3)2]3,31 and [Li(TMEDA)][LnMe4],25b were
prepared by the corresponding literature procedures. Solvents were
freshly distilled from Na/benzophenone (THF, toluene, Et2O, DME) or
Na/K amalgam (pentane, hexanes) under argon and degassed thor-
oughly by freezeꢀpumpꢀthaw cycles prior to use. Deuterated solvents
(THF-d8, benzene-d6, toluene-d8, pyridine-d5) were freshly distilled
from Na/K amalgam under argon and degassed prior to use. Racemic
lactide (Aldrich) was recrystallized from iPrOH and twice from dry
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Scheme 13
Table 3. Ring-Opening Polymerization of racemic Lactide Promoted by Amido Phenoxy-Amidinate Rare-Earth Complexes
{LONR}LnN(SiMe3)2
a
[LA]/[Ln]/[ROH] solvent time (min)b conv (%)c Mn,calcd/103 g molꢀ1 Mn,exptle/103 g molꢀ1 Mw/Mn
Prf,g
e
entry comp.
ROH
1
2
6
6
6
6
6
6
8
8
9
9
9
9
9
6
6
6
6
6
6
6
6
9
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
350:1:0
500:1:0
100:1:5
100:1:5
100:1:5
100:1:5
500:1:5
500:1:50
1000:1:5
1000:1:5
100:1:5
toluene
toluene
toluene
toluene
THF
25
80
10
83
98
99
96
98
99
98
30
100
99
79
74
20
95
53
94
63
70
25
100
100
1.44
12.00
14.11
14.90
13.82
14.11
14.26
14.11
4.32
14.33
28.42
20.02
23.54
15.50
16.24
23.58
24.40
8.26
1.54
2.63
1.91
2.24
2.23
2.01
2.60
2.55
1.5o
1.60
1.71
1.68
1.76
1.27
1.20
1.20
1.19
1.11
1.25
1.13
1.34
1.38
nd
nd
3
240
720
240
720
720
720
120
720
720
840
1500
80
0.68
nd
4
5
0.69
nd
6
THF
7
toluene
THF
0.64
0.76
nd
8
9
toluene
toluene
THF
10
11
12
13
14
15
16
17
18
19
20
21
22
14.40
14.26
38.68
53.28
0.58
49.00
58.12
105.20
158.77
0.33
nd
0.72
nd
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
0.67
nd
PhCH2OH
PhCH2OH
iPrOH
180
80
2.70
1.51
0.50
nd
1.53
1.31
iPrOH
180
240
240
720
1800
120
2.70
1.64
0.50
0.52
0.50
nd
iPrOH
9.07
5.52
0.82 (1.04)f
iPrOH
1.01
iPrOH
7.21
6.83
23.78
2.36
iPrOH
28.80
0.52
iPrOH
2.88
nd
a General conditions: [rac-LA] = 1.0 mol Lꢀ1, T = 20 °C, unless otherwise stated. b Reactions times were not optimized. c Conversion of lactide as
determined by 1H NMR on the crude reaction mixture. d Mn values calculated considering one polymer chain per metal center from the relation: Mn,calc
=
Conv. ꢁ [LA]/[Ln] ꢁ 144. e Experimental Mn and Mw/Mn values determined by GPC in THF vs PS standards and corrected with a factor of 0.58.
f Determined by 1H NMR. g Pr is the probability of racemic linkage, as determined by 1H NMR homodecoupling experiments.
toluene, and dried in vacuum. Other starting materials were purchased
from Acros, Strem, and Aldrich and used as received.
(Mw/Mn) of the polymers were calculated with reference to a universal
calibration versus polystyrene standards. The Mn values of PLAs were
corrected with a factor of 0.58 to account for the difference in
hydrodynamic volumes between polystyrene and polylactide.4c,32 The
microstructure of PLAs was determined by homodecoupling 1H NMR
spectroscopy at 25 °C in CDCl3 with a Bruker AC-500 spectrometer
operating at 500 MHz.
Instruments and Measurements. NMR spectra were recorded
on Bruker AC 200, Bruker ACP 200, Bruker AC 300, Bruker Avance
DRX 400, and Bruker AC 500 spectrometers in Teflon-valved NMR
tubes. 1H and 13C NMR chemical shifts are reported in parts per million
versus SiMe4 and were determined by reference to the residual solvent
resonances. Assignment of signals was carried out via multinuclear 1D
{(iPrN)2CC6H2(tBu)2O}H2 ({LONiPr}H2). To a solution of 2-bro-
mo-4,6-(di-tert-butyl)phenol (3.90 g, 13.67 mmol) in diethyl ether
(30 mL) was added n-butyllithium (11.48 mL of a 2.50 M solution in
hexane, 28.71 mmol) at 0 °C. The reaction mixture was stirred for 0.5 h
at room temperature and a solution of N,N0-diisopropylcarbodiimide
(2.23 mL, 14.31 mmol) in diethyl ether (10 mL) was added dropwise.
The reaction mixture was stirred under reflux overnight and then cooled
to room temperature. The white precipitate formed was separated by
filtration under argon. Water (50 mL) and Et2O (50 mL) were added
stepwise. The organic layer was separated and dried over NaSO4, and
volatiles were removed in vacuo. The residue was dried in vacuo to give
{LONiPr}H2 as a pale yellow solid (3.82 g, 11.48 mmol, 84%). mp =
158 °C. 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.35 (d, J = 2.5, 1H,
13
(1H, 13C{1H}) and 2D (1Hꢀ C HMBC and HMQC) NMR experi-
7
ments. Coupling constants are reported in hertz. Li NMR chemicals
shifts are reported versus LiCl(aq).
C, H, and N elemental analyses were performed by the microanaly-
tical laboratory of IOMC. Lanthanide metal analysis was carried out by
complexometric titration. IR spectra were recorded either as Nujol mulls
or using ATR module on a Bruker-Vertex 70 spectrophotometer.
Size exclusion chromatography (SEC) of PLAs was performed in
THF (1 mL minꢀ1) at 20 °C using a Polymer Laboratories PL50
apparatus equipped with PLgel 5 μm MIXED-C 300 ꢁ 7.5 mm columns,
and combined RI and Dual angle LS (PL-LS 45/90°) detectors. The
number-average molecular masses (Mn) and polydispersity index
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C6H2), 7.30 (s, 1H, C6H2), 3.78 (hept, J = 6.3, 2H, CH(CH3)), 1.48
(s, 9H, C(CH3)), 1.34 (s, 9H, C(CH3)), 1.16 (d, J = 6.3, 12H,
CH(CH3)). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 160.8
(CN2), 159.1 (O-C6H2), 137.3, 137.5, 125.9, 122.5, 114.7, 47.2
(CH(CH3)2), 35.2 (Ph-C(CH3)3), 34.2 (Ph-C(CH3)3), 31.6 (Ph-C-
(CH3)3), 29.7 (Ph-C(CH3)3), 24.3 (CH(CH3)2). Anal. Calcd for
C21H36N2O: C, 75.85; H, 10.91; N, 8.42. Found: C, 75.34; H, 10.25;
N, 8.35.
hexane, 5.28 mmol). Workup afforded 2 as an off-white solid (2.11 g,
3.63 mmol, 98%). Crystals of {LONAr}Li2(THF)3 suitable for X-ray
diffraction studies were obtained from a THF/hexane (1:1 v/v) solution
1
at room temperature. H NMR (THF-d8, 500 MHz, 25 °C): δ 6.99
(br m, 2H, arom), 6.87 (br m, 2H, Ar), 6.67 (br m, 2H, arom), 6.55ꢀ6.45
(br m, 2H, arom), 3.33 (br m, 2H, CH(CH3)), 3.00 (br m, 2H,
CH(CH3)), 1.45 (s, 9H, C(CH3)), 1.21 (br m, 6H, CH(CH3)), 1.15
(br m, 6H, CH(CH3)), 0.80 (s, 9H, C(CH3)), 0.77 (br s, 6H, CH-
(CH3)), 0.64 (br s, 6H, CH(CH3)). 13C{1H} NMR (THF-d8, 125
MHz, 25 °C): δ 160.9 (CN2), 160.2 (O-C6H2), 148.2, 141.2, 135.8,
135.5, 129.9, 124.6, 119.8, 119.5, 119.0, 115.3, 39.5 (Ph-C(CH3)3),
33.2 (Ph-C(CH3)3), 31.3 (Ph-C(CH3)3), 29.3 (Ph-CH(CH3)2), 27.7
(Ph-C(CH3)3), 26.8 (Ph-CH(CH3)2), 25.8 (Ph-CH(CH3)2), 23.5
(Ph-CH(CH3)2), 22.9 (Ph-CH(CH3)2), 20.3 (Ph-CH(CH3)2). Anal.
Calcd for C39H54Li2N2O: C, 80.66; H, 9.37; N, 4.95. Found: C, 80.45; H,
9.43; N, 4.55.
{(CyN)2CC6H2(tBu)2O}H2 ({LONCy}H2). A protocol similar to
that described above for 1-H2 was used, starting from 2-bromo-4,6-(di-
tert-butyl)phenol (7.87 g, 27.6 mmol), n-butyllithium (23.20 mL of a
2.50 M solution in hexane, 57.9 mmol), and N,N0-dicyclohexylcarbodii-
mide (5.98 g, 29.0 mmol). Workup afforded {LONCy}H2 as a yellowish
solid (9.79 g, 23.7 mmol, 86%). mp = 121 °C. 1H NMR (CDCl3, 500
MHz, 25 °C): δ 7.35 (d, J = 2.5, 1H, C6H2), 7.31 (d, J = 2.5, 1H, C6H2),
3.45 (br m, 2H, CH(CH2), Cy), 1.99 (br m, 4H, Cy), 1.81 (br m, 4H,
Cy), 1.67 (br m, 2H, Cy), 1.49 (s, 9H, C(CH3)), 1.35 (br m, 10H, Cy),
1.34 (s, 9H, C(CH3)). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ
160.9 (CN2), 160.6 (O-C6H2), 137.8, 136.9, 126.0, 122.2, 113.9, 54.7
(NCH(CH2)5), 35.2 (Ph-C(CH3)3), 34.8 (CH(CH2)5), 34.2 (Ph-C-
(CH3)3), 31.6 (Ph-C(CH3)3), 29.6 (Ph-C(CH3)3), 25.5 (CH(CH2)5),
25.0 (CH(CH2)5). Anal. Calcd for C27H44N2O: C, 78.59; H, 10.75; N,
6.79. Found: C, 78.16; H, 10.56; N, 6.55.
[{(iPrN)2CC6H2(tBu)2O}YCl]2 ([{LONiPr}YCl]n, 3). To a solu-
tion of {LONiPr}H2 (0.92 g, 2.78 mmol) in toluene (10 mL) was added
n-butyllithium (6.0 mL of a 0.93 M solution in hexane, 5.56 mmol) at
0 °C, and the reaction mixture was stirred during 20 min at 0 °C and then
during 30 min at room temperature. Volatiles were evaporated in vacuo,
the solid residue was dissolved in THF (10 mL), and the solution was
added to a suspension of yttrium chloride (0.54 g, 2.78 mmol) in THF
(10 mL) at room temperature. The reaction mixture was stirred during
12 h. THF was evaporated in vacuo, and the solid residue was extracted
with toluene (20 mL). The clear solution was concentrated at 0 °C to
{(iPr2C6H3N)2CC6H2(tBu)2O}H2 ({LONAr}H2). A protocol simi-
lar to that described above for {LONiPr}H2 was used, starting from
2-bromo-4,6-di(tert-butyl)phenol (15.7 g, 55.0 mmol), n-butyllithium
(46.2 mL of a 2.50 M solution in hexane, 115.6 mmol), and N,N0-di(bis-
2,6-diisopropylphenyl)carbodiimide (20.95 g, 57.8 mmol). Workup
afforded {LONAr}H2 as a yellowish oil. The latter was purified by
removing volatiles in vacuo using a Kugelrohr apparatus (2 ꢁ 10ꢀ2 Torr,
120 °C) to afford a brownish solid (9.85 g, 17.3 mmol, 31%). mp =
1
afford 3 (0.70 g, 1.54 mmol, 56%) as a white crystalline powder. H
NMR (THF-d8, 500 MHz, 25 °C): δ 7.29 (br s, 2H, C6H2), 7.06 (br s,
2H, C6H2), 3.48 (br sept, 4H, CH(CH3)2), 1.48 (br s, 18H, C(CH3)3),
1.29 (br s, 18H, C(CH3)3 that overlapped with 12H, CH(CH3)2), 1.21
(br s, 12, CH(CH3)2. 13C{1H} NMR (THF-d8, 125 MHz, 25 °C): δ
184.8 (CN2), 163.7 (O-C6H2), 137.5 (C6H2tBu), 136.5 (C6H2-tBu),
125.1 (C6H2), 123.4 (C6H2), 122.4 (C6H2), 46.9 (CH(CH3)2), 33.0
(Ph-C(CH3)3), 31.8 (Ph-C(CH3)3), 29.3 (CH(CH3)2), 27.3 (Ph-C-
(CH3)3), 24.1 (Ph-C(CH3)3). IR (Nujol, KBr) ν (cmꢀ1): 1609 s, 1571
s, 1530 w, 1366 m, 1294 w, 1259 m, 1226 m, 1200 w, 1181 w, 1168 w,
1146 w, 1128 w, 1119 w, 1073 w, 1049 m, 1032 m, 963 w, 918 w, 888 m,
844 s, 786 m, 757 w, 646 w, 614 m, 530 m. Anal. Calcd for
C42H68Cl2N4O2Y2: C, 55.45; H, 7.53; N, 6.16; Y, 19.55. Found: C,
55.80; H, 7.72; N, 6.31; Y, 19.65.
1
151 °C. H NMR (CD2Cl2, 500 MHz, 25 °C): δ 7.30ꢀ7.18 (m, 6H,
C6H2), 7.10 (m, 2H, C6H2), 6.67 (d, J = 2.4, 1H, OH or NH), 5.89
(s, 1H, OH or NH), 3.25 (hept, J = 6.7, 2H, CH(CH3)), 3.06 (hept, J = 6.7,
2H, CH(CH3)), 1.50 (s, 9H, C(CH3)), 1.36 (d, J = 6.7, 6H, CH(CH3)),
1.24 (d, J = 6.7, 6H, CH(CH3)), 1.06 (d, J = 6.7, 6H, CH(CH3)), 0.90
(s, 9H, C(CH3)), 0.87 (d, J = 6.7, 6H, CH(CH3)). 13C{1H} NMR
(CDCl3, 75 MHz, 25 °C): δ 158.0 (CN2), 156.5 (O-C6H2), 144.6, 140.8,
137.2, 135.0, 126.2, 124.3, 124.1, 123.4, 112.7, 35.3 (Ph-C(CH3)3), 33.8
(Ph-C(CH3)3), 31.2 (Ph-C(CH3)3), 29.6 (Ph-C(CH3)3), 24.9 (Ph-CH-
(CH3)2), 24.8 (Ph-CH(CH3)2), 22.3 (Ph-CH(CH3)2), 22.1 (Ph-CH-
(CH3)2). Anal. CalcdforC27H44N2O: C, 82.34; H, 9.92; N, 4.92. Found: C,
82.88; H, 10.05; N, 4.89.
{(iPr2C6H3N)2(H)CC6H2(tBu)2O}YCl2 DME ({LOHNAr}YCl2-
3
(DME), 5). To a solution of {LOHNAr}H2 (0.33 g, 0.578 mmol) in
hexanes (10 mL) was added n-butyllithium (1.0 mL of a 1.16 M solution
in hexane, 1.16 mmol) at 0 °C. The reaction mixture was stirred during
20 min at 0 °C and then during 30 min at room temperature. Volatiles
were evaporated in vacuo, the solid residue was dissolved in THF
(10 mL), and the solution was added to a suspension of yttrium chloride
(0.133 g, 0.578 mmol) in THF (10 mL) at room temperature. The
reaction mixture was stirred during 24 h. THF was evaporated in vacuo,
and the solid residue was extracted with toluene (20 mL). Toluene was
evaporated, and the residue was dissolved in a DME/hexane (1:10 v/v)
mixture. The clear solution was concentrated at 0 °C to afford 5 as a
white crystalline powder (0.15 g, 32%). Colorless crystals suitable for
X-ray diffraction studies were obtained by slow concentration of a
solution in a DME/hexane (1:10 v/v) mixture at room temperature.
1H NMR (C6D6, 500 MHz, 60 °C): δ 7.36 (d, 4J = 2.5, 1H, C6H2), 7.27
(d, 3J = 7.5, 2H, C6H3), 7.17 (t, 3J = 7.5, 1H, C6H3), 7.11 (d, 3J = 7.5, 2H,
C6H3), 7.07 (d, 4J = 2.5, 1H, C6H2), 6.92 (t, 3J = 7.5, 1H, C6H3), 3.88 (br
sept, 2H, CH(CH3)2), 3.33 (br sept, 2H, CH(CH3)2), 3.14 (br s, 6H,
(CH3)2 DME), 3.10 (br s, 4H, CH2, DME), 1.78 (br s, 9H, C(CH3)3),
1.51 (d, 3J = 6.4, 6H, CH(CH3)2), 1.44 (d, 3J = 6.4, 6H, CH(CH3)2),
1.14 (br s, 9H, C(CH3)3 overlapped with 6H of CH(CH3)2), 1.03 (d, 3J =
6.4, 6H, CH(CH3)2).33 13C{1H} NMR (C6D6, 125 MHz, 25 °C): δ
162.3 (CN2), 157.0 (O-C6H2), 146.8 (C6H3-iPr), 137.6 (C6H2-tBu),
{[(iPrN)2CC6H2(tBu)2O]Li}2 ({LONiPr}Li2(Et2O)0.5, 1). To a
solution of {LONiPr}H2 (2.00 g, 6.02 mmol) in diethyl ether (20 mL)
was added n-butyllithium (4.81 mL of a 2.50 M solution in hexane, 12.04
mmol) at 0 °C under stirring. The resulting yellowish reaction mixture
was stirred at room temperature overnight. Volatiles were then evapo-
rated in vacuo to give 1 as a white powder (2.25 g, 5.89 mmol, 98%).
Crystals of [{LONiPr}Li2(THF)2]2 (THF) suitable for X-ray diffrac-
3
tion studies were obtained from a THF/hexane (1:1 v/v) solution. 1H
NMR (THF-d8, 500 MHz, 25 °C): δ 7.35 (s, 1H, C6H2), 6.60 (s, 1H,
C6H2), 3.38 (q, J = 7.0, 2H, OCH2CH3), 3.15 (br m, 2H, CH(CH3)),
1.45 (br s, 9H, C(CH3)), 1.24 (br s, 9H, C(CH3)), 1.11 (t, J = 7.0, 2H,
OCH2CH3), 0.84 (br m, 12H, CH(CH3)). 13C{1H} NMR (THF-d8,
125 MHz, 25 °C): δ 176.8 (CN2), 158.6 (O-C6H2), 132.9, 129.2, 125.9,
120.9, 118.5, 118.4, 63.5 (CH2, Et2O), 44.8 (CH(CH3)2), 31.9 (Ph-
C(CH3)3), 31.5 (Ph-C(CH3)3), 29.7 (Ph-C(CH3)3), 28.2 (Ph-C-
(CH3)3), 24.9 (CH(CH3)2), 24.7 (CH(CH3)2), 12.8 (CH3, Et2O).
Anal. Calcd for C92H156Li8N8O6: C, 72.42; H, 10.71; N, 7.34. Found: C,
72.16; H, 10.56; N, 7.26.
{(iPr2C6H3N)2CC6H2(tBu)2O}Li2 ({LONAr}Li2, 2). A protocol
similar to that described above for 1 was used, starting from {LONAr}H2
(1.50 g, 2.64 mmol) and n-butyllithium (2.11 mL of a 2.50 M solution in
5519
dx.doi.org/10.1021/om200786u |Organometallics 2011, 30, 5509–5523

Organometallics
ARTICLE
127.8 (C6H2), 126.2 (N-C6H3), 123.8 (C6H3), 122.8 (C6H3), 122.5
(C6H2), 119.6 (C6H3), 71.0 (CH2, DME), 59.1 (CH3, DME), 35.2
(Ph-C(CH3)3), 33.8 (Ph-C(CH3)3), 31.4 (Ph-C(CH3)3), 30.6 (Ph-C-
(CH3)3), 28.7 (Ph-CH(CH3)2), 27.9 (Ph-CH(CH3)2), 26.6 (Ph-CH-
(CH3)2), 24.5 (Ph-CH(CH3)2), 22.8 (Ph-CH(CH3)2). IR (Nujol, KBr) ν
(cmꢀ1): 3341 w, 3171 w, 1600 m, 1544 s, 1189 m, 1080 s, 1036 s, 968 m,
859 s, 780 m, 766 m. Anal. Calcd for C43H65Cl2N2O3Y: C, 63.15; H, 8.01;
N, 3.43; Y, 10.87. Found: C, 62.93; H, 8.11; N, 3.67; Y, 10.80.
volatiles were evaporated in vacuo, and the solid residue was extracted
with toluene (20 mL). The clear solution was concentrated at 0 °C to
afford 7 as colorless crystals (0.18 g, 0.13 mmol, 33%). Colorless crystals
suitable for X-ray diffraction studies were obtained by slow concentra-
tion of a toluene solution at room temperature. 1H NMR (pyridine-d5,
400 MHz, 25 °C): δ 7.94 (s, 1H, C6H3), 7.44 (d, 4J = 2.5, 1H, C6H2),
7.27 (several mult, 11H, C6H3, C6H2), 7.03 (d, 4J = 2.5, 1H, C6H2), 6.67
4
3
(d, J = 2.5, 1H, C6H2), 6.38 (s, 1H, C6H3), 3.70 (sept, J = 6.8, 2H,
CH(CH3)2), 3.48 (sept, 3J = 6.8, 2H, CH(CH3)2), 3.41 (sept, 3J = 6.8,
3J = 6.8, 2H, CH(CH3)2), 3.35 (sept, 3J = 6.8, 2H, CH(CH3)2), 1.59 (br
[{(iPrN)2CC6H2(tBu)2O}YN(SiMe3)2]2 ({LONiPr}YN(SiMe3)2,6).
Method A. To a solution of 3 (0.67 g, 2.78 mmol) in THF (10 mL) was
added a solution of Na[N(SiMe3)2] (0.27 g, 2.78 mmol) in THF
(10 mL) at room temperature, and the reaction mixture was stirred
during 48 h. Volatiles were evaporated in vacuo, and the solid residue
was extracted with hexanes (30 mL). Hexanes were evaporated, and the
solid residue was dried in vacuo during 12 h to afford 6 as a pale yellow
crystalline powder (0.64 g, 1.09 mmol, 74%). Crystals suitable for
X-ray diffraction studies were obtained from a Et2O solution at room
temperature.
3
s, 9H, C(CH3)3), 1.58 (br s, 9H, C(CH3)3), 1.30 (d, J = 6.8, 6H,
CH(CH3)2), 1.26 (d, 3J = 6.8, 6H, CH(CH3)2), 1.21 (d, 3J = 6.8, 6H,
CH(CH3)2), 1.12 (br s, 9H, C(CH3)3 that overlapped with 6H, CH-
(CH3)2), 0.99 (br s, 9H, C(CH3)3 that overlapped with 6H, CH-
(CH3)2), 0.96 (d, 3JH,H = 6.8, 12H, CH(CH3)2), 0.71 (d, 3J = 6.8, 6H,
CH(CH3)2). 7Li NMR (pyridine-d5, 156 MHz, 25 °C): δ 2.60. 13C{1H}
NMR (pyridine-d5, 100 MHz, 25 °C): δ 169.4 (CN2), 158.9 (O-C6H2),
157.6 (O-C6H2), 145.2 (C6H3-iPr), 144.0 (C6H3-iPr), 143.6 (C6H3-iPr),
141.7 (C6H3-iPr), 141.0 (C6H3-iPr), 140.0 (C6H3-iPr), 137.2 (C6H2-
tBu), 136.1 (C6H2-tBu), 129.2 (C6H2), 128.4 (C6H2), 128.0 (C6H2),
127.8 (C6H2), 126.0 (N-C6H3), 125.2 (N-C6H3), 124.6 (C6H3), 124.4
(C6H3), 124.2 (C6H2), 118.6 (C6H3), 35.7 (Ph-C(CH3)3), 35.4
(Ph-C(CH3)3), 33.9 (Ph-C(CH3)3), 33.6 (Ph-C(CH3)3), 31.9 (Ph-C-
(CH3)3), 31.2 (Ph-C(CH3)3), 29.9 (Ph-C(CH3)3), 29.6 (Ph-C(CH3)3),
28.9 (Ph-CH(CH3)2), 28.7 (Ph-CH(CH3)2), 28.5 (Ph-CH(CH3)2), 28.1
(Ph-CH(CH3)2), 25.4 (Ph-CH(CH3)2), 24.8 (Ph-CH(CH3)2), 24.3
(Ph-CH(CH3)2), 24.1 (Ph-CH(CH3)2), 22.6 (Ph-CH(CH3)2), 22.5
(Ph-CH(CH3)2), 22.3 (Ph-CH(CH3)2). IR (Nujol, KBr) ν (cmꢀ1):
1612 s, 1577 s, 1560 m, 1435 m, 1361 m, 1323 m, 1283 m, 1260 s, 1223
m, 1202 w, 1188 m, 1143 m, 1098 m, 1057 m, 1041 w, 964 m, 934 m, 886
m, 869 m, 833 m, 801 m, 786 m, 777 w, 768 w, 750 s, 694 m, 662 w, 642
m, 527 s. Anal. Calcd for C86H124LiN4O4Y: C, 75.19; H, 9.10; N, 4.08; Y,
6.47. Found: C, 75.48; H, 9.21; N, 4.15; Y, 6.13.
Method B. To a solution of {LONiPr}H2 (0.303 g, 0.91 mmol) in
toluene (10 mL) was added n-butyllithium (1.33 mL of a 1.37 M solution
in hexane, 1.82 mmol) at 0 °C, and the reaction mixture was stirred during
20 min at 0 °C and then during 30 min at room temperature. Volatiles
were evaporated in vacuo, the solid residue was redissolved in THF
(10 mL), and the resulting solution was then added to a suspension of
yttrium chloride (0.178 g, 0.91 mmol) in THF (10 mL) at room
temperature. The reaction mixture was stirred during 12 h, and a solution
of Li[N(SiMe3)2] Et2O (0.219 g, 0.91 mmol) in THF (10 mL) was
3
added in at room temperature. The reaction mixture was stirred during
12 h, and then volatiles were evaporated in vacuo. The solid residue was
extracted with hexanes (30 mL), and the clear solution was concentrated
at 0 °C to afford 6 as a white crystalline powder (0.410 g, 0.354 mmol,
78%). 1H NMR (C6D6, 400 MHz, 25 °C): δ 7.57 (br s, 2H, C6H2), 7.16
(br s, 2H, C6H2), 3.65 (br m, 4H, CH(CH3)2), 1.66 (s, 18H, C(CH3)3),
1.34 (s, 18H, C(CH3)3), 1.24 (br m, 24 H, CH(CH3)2), 0.46 (br s, 36H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 187.0
(CN2), 161.3 (O-C6H2), 137.5 (C6H2-tBu), 137.2 (C6H2-tBu), 125.5
(C6H2), 124.0 (C6H2-CN2), 122.5 (C6H2), 49.4 (CH(CH3)2), 35.3
(Ph-C(CH3)3), 33.9 (Ph-C(CH3)3), 31.6 (Ph-C(CH3)3), 29.5 (Ph-C-
(CH3)3), 24.6 (CH(CH3)2), 5.3 (N(Si(CH3)3)2). IR (Nujol, KBr) ν
(cmꢀ1): 1600 s, 1546 w, 1508 s, 1437 s, 1408 m, 1365 m, 1288 s, 1259 w,
1246 m, 1223 m, 1194 m, 1171 m, 1147 s, 1126 s, 1094 w, 1026 m, 1015
w, 967 s, 872 s, 842 s, 829 m, 779 m, 756 s, 695 w, 667 s, 617 s, 597 m, 572
w, 549 s, 529 m. Anal. Calcd for C54H104N6O2Si4Y2: C, 55.93; H, 9.04;
N, 7.25; Y, 15.33. Found: C, 56.05; H, 9.08; N, 7.14; Y, 15.10.
[{(CyN)2CC6H2(tBu)2O}YN(SiMe3)2]2 ({LONCy}Y(N(SiMe3)2), 8).
To a solution of Y[N(SiMe3)2]3 (0.180 g, 0.313 mmol) in a toluene/
THF mixture (1:10 v/v) (10 mL) was added a solution of {LONCy}H2
(0.129 g, 0.313 mmol) in toluene (10 mL). The reaction mixture was
stirred at 50 °C during 120 h, and then volatiles were evaporated
in vacuo. Hexamethyldisilazane was detected by chromatography, and
its amount (0.64 mmol) was close to the theoretic one (0.62 mmol).
The solid residue was recrystallized from DME to afford byproduct
{LONCy}2YLi(DME) (10) as colorless crystals (ca. 0.05 g). The super-
natant solution was separated and concentrated in vacuo to afford 8
1
(0.180 g, 0.136 mmol, 87%) as a yellow crystalline powder. H NMR
Method C. In the glovebox, a Teflon-valved NMR tube was charged
with {LONiPr}H2 (0.012 g, 0.036 mmol) and Y[N(SiMe3)2]3 (0.0206 g,
0.036 mmol). To this mixture was transferred in THF-d8 (ca. 0.6 mL)
under vacuum, and the tube was sealed. The tube was shaken over 48 h
before NMR was recorded, after which time period the quantitative
formation of 6 was observed. 1H NMR (THF-d8, 200 MHz, 25 °C): δ
7.09 (d, 4J = 2.2, 2H, C6H2), 6.93 (d, 4J = 2.2, 2 H, C6H2), 3.08 (sept, 3J =
6.2, 4H, CH(CH3)2), 1.40 (s, 18H, C(CH3)3), 1.28 (s, 18H, C(CH3)3),
1.01 (d, 3J = 6.2, 24H, CH(CH3)2), 0.17 (br s, 36H, N(Si(CH3)3)2).
Synthesis of {LONAr}2YLi(THF)2 (7) by Salt Metathesis. To a
solution of {LONAr}H2 (0.45 g, 0.79 mmol) in toluene (10 mL) was
added n-butyllithium (1.15 mL of a 1.37 M solution in hexane, 1.58
mmol) at 0 °C. The reaction mixture was stirred during 20 min at 0 °C
and then during 30 min at room temperature. Volatiles were evaporated
in vacuo, the solid residue was redissolved in THF (10 mL), and the
obtained solution was added to a suspension of yttrium chloride (0.16 g,
0.79 mmol) in THF (10 mL) at room temperature. The reaction
mixture was stirred during 12 h, after which time period a solution of
Li[N(SiMe3)2](Et2O) (0.19 g, 0.79 mmol) in THF (10 mL) was added
in at room temperature. The reaction mixture was stirred during 12 h,
(C6D6, 400 MHz, 25 °C): δ 7.58 (br s, 2H, C6H2), 7.27 (br S, 2H,
C6H2), 3.60 (br m, 4H, CH(CH2)2), 2.1ꢀ1.1 (several br m, 40H,
CH(CH2)5, 1.71 (s, 18H, C(CH3)3), 1.34 (s, 18H, C(CH3)3), 0.51
(br s, 36H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 25 °C):
δ 187.4 (CN2), 161.6 (O-C6H2), 137.3 (C6H2-tBu), 137.0 (C6H2-tBu),
125.3 (C6H2-CN2), 124.3 (C6H2), 123.1 (C6H2), 59.2 (CH(CH2)2),
35.2 (C(CH3)3), 34.7 (C(CH3)3), 31.5 (C(CH3)3), 29.5 (C(CH3)3),
26.0ꢀ25.5 (CH(CH2)5), 5.3 (N(Si(CH3)3)2). IR (Nujol, KBr) ν
(cmꢀ1): 1617 s, 1594 m, 1362 m, 1303 s, 1258 s, 1238 w, 1219 m,
1201 m, 1145 m, 1124 m, 1090 w, 1067 m, 1027 w, 970 m, 889 s, 845 s, 824
w, 784 m, 677 m, 645 w, 617 w, 526 w. Anal. Calcd for C66H120N6O2Si4Y2:
C, 60.06; H, 9.16; N, 6.37; Y, 13.47. Found: C, 60.20; H, 9.05; N,
6.25; Y, 13.2.
{(CyN)2CC6H2(tBu)2O}2YLi(DME) ({LONCy}2YLi(DME), 10).
To a solution of {LONCy}H2 (0.982 g, 2.38 mmol) in toluene (10 mL)
was added n-butyllithium (4.53 mL of a 1.05 M solution in hexane,
4.76 mmol) at 0 °C, and the reaction mixture was stirred during 20 min
at 0 °C and then during 30 min at room temperature. Volatiles were
evaporated in vacuo, the solid residue was redissolved in THF (10 mL),
and the resulting solution was then added to a suspension of yttrium
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chloride (0.233 g, 1.19 mmol) in THF (10 mL) at room temperature.
The reaction mixture was stirred during 12 h, and then volatiles were
evaporated in vacuo. The solid residue was extracted with toluene
(30 mL), and toluene was evaporated in vacuo. The solid residue was
redissolved in DME (20 mL), and the clear solution was concentrated
at 0 °C to afford 10 as a white crystalline powder (0.539 g, 0.536 mmol,
extracted with diethyl ether (20 mL). The clear solution was concen-
trated at 0 °C to afford 12 as a white crystalline powder (0.25 g, 0.205
mmol, 59%). Colorless crystals suitable for X-ray diffraction studies
1
were grown by slow concentration from a diethyl ether solution. H
NMR (C6D6, 200 MHz, 25 °C): δ 7.65 (d, 4J = 2.5, 2H, C6H2), 7.39 (d,
4J = 2.5, 2H, C6H2), 4.6 (sept, 3J = 6.4, 4H, CH(CH3)2), 3.28 (q, 3J =
7.0, 4H, CH2, Et2O), 2.02 (br s, 24H, CH3, TMEDA), 1.92 (s, 18H,
C(CH3)3), 1.73 (br m, 4H, CH2, TMEDA), 1.46 (br s, 18H, C(CH3)3
1
45%). H NMR (pyridine-d5, 400 MHz, 50 °C): δ 7.58 (br m, 2H,
C6H2), 7.15 (br m, 2H, C6H2), 3.64 (br m, 4H, CH(CH2)5),), 3.49
(br s, 6H, (CH3)2 DME), 3.26 (br s, 4H, CH2, DME), 1.64 (br s, 18H,
C(CH3)3), 2.0ꢀ1.0 (several m, 40H, CH(CH2)5), 1.40 (br s, 18H,
3
that overlapped with 24H, CH(CH3)2), 1.12 (t, J = 7.0, 6H, CH3
Et2O), ꢀ0.38 (br s, 12H, YCH3). 7Li NMR (C6D6, 78 MHz, 20 °C):
δ 3.58. 13C NMR (C6D6, 50 MHz, 25 °C): δ 185.2 (CN2), 163.2
(O-C6H2), 136.3 (C6H2-tBu), 134.6 (C6H2-tBu), 126.1 (C6H2-CN2),
123.7 (C6H2), 122.3 (C6H2), 65.6 (CH2, Et2O), 56.8 (CH2, TMEDA),
49.4 (CH(CH3)2), 45.9 (CH3,TMEDA), 35.5 (C(CH3)3), 34.0
(C(CH3)3), 32.0 (CH(CH3)2), 30.2 (C(CH3)3), 25.7 (C(CH3)3),
15.3 (CH3, Et2O), 11.2 (Y(CH3)2). IR (Nujol, KBr) ν (cmꢀ1): 1601
m, 1554 s, 1492 s, 1413 m, 1360 m, 1302 s, 1270 w, 1256 w, 1229 w,
1201 w, 1181 m, 1159 w, 1151 w, 1124 s, 1067 s, 1035 s, 1018 s, 949 s,
897 w, 882 m, 860 w, 843 s, 791 s, 774 w, 758 s, 705 w, 643 w, 588 m,
580 m, 525 m, 499 m. Anal. Calcd for C62H122Li2N8O3Y2: C, 61.07; H,
10.08; N, 9.19; Y, 14.58. Found: C, 61.38; H, 9.95; N, 9.11; Y, 14.28.
[{(CyN)2CC6H2(tBu)2O}YMe2Li(TMEDA)]2Et2O ({LONCy}2-
Y2Me4Li2(TMEDA)2, 13). Using a protocol similar to that described
above for 12, complex 13 was obtained from YCl3 (0.072 g, 0.368 mmol),
TMEDA (0.22 mL, 1.47 mmol), methyllithium (1.73 mL of a 0.85 M
solution in Et2O, 1.47 mmol), and {LONCy}H2 (0.152 g, 0.368 mmol).
A similar workup afforded 13 as a pale yellow crystalline powder
(0.173 g, 0.125 mmol, 68%). Crystals suitable for X-ray diffraction
studies were grown by slow concentration of a Et2O solution at room
temperature. 1H NMR (C6D6, 200 MHz, 25 °C): δ 7.61 (d, 4J = 2.5, 2H,
C6H2), 7.41 (d, 4J = 2.5, 2H, C6H2), 3.59 (br m, 4H, CH(CH2)5), 3.25
(q, 3J = 7.0, 4H, CH2 Et2O), 2.04 (br s, 24H, CH3 TMEDA), 1.93 (br s,
18H, C(CH3)3), 1.9ꢀ1.0 (several m, 4H, CH2 TMEDA that overlapped
with 40H, CH(CH2)5), 1.44 (br s, 18H, C(CH3)3), 1.12 (tr, 3J = 7.0, 6H,
7
C(CH3)3). Li NMR (pyridine-d5, 155 MHz, 25 °C): δ 2.46. 13C{1H}
NMR (d5-py, 100 MHz, 25 °C): δ 162.6 (CN2), 162.2 (O-C6H2), 138.4
(C6H2-tBu), 136.7 (C6H2-tBu), 128.7 (C6H2-CN2), 126.2 (C6H2),
114.4 (C6H2), 72.1 (CH2, DME), 58.6 (CH3, DME), 35.6 (Ph-C(CH3)3),
34.8 (Ph-C(CH3)3), 31.8 (Ph-C(CH3)3), 30.0 (Ph-C(CH3)3), 27.1,
25.7, 25.3 (CH(CH2)5). IR (Nujol, KBr) ν (cmꢀ1): 1615 s, 1584 m,
1436 m, 1361 w, 1301 s, 1258 s, 1220 m, 1201m, 1181 w, 1148w, 1126 m,
1095 w, 1063 w, 1030 w, 939 s, 888 m, 840 s, 785 m, 753 m, 671 m, 525 m.
Anal. Calcd for C58H94LiN4O4Y: C, 69.16; H, 9.41; N, 5.56; Y, 8.83.
Found: C, 68.90; H, 9.52; N, 7.71; Y, 8.74.
[{(CyN)2CC6H2(tBu)2O}NdN(SiMe3)2}2 ({LONCy}Nd(N-
(SiMe3)2), 9). Using a protocol similar to that described above for 8,
compound 9 was obtained from Nd[N(SiMe3)2]3 (0.43 g, 0.67 mmol)
and {LONCy}H2 (0.28 g, 0.67 mmol). A similar workup afforded 9 as a
cyan-blue crystalline powder (0.29 g, 40 mmol, 61%). Crystals suitable for
X-ray diffraction studies were grown by slow concentration from a
solution in a Et2O/hexane (1:10 v/v) mixture at room temperature. IR
(Nujol, KBr) ν (cmꢀ1): 1619 s, 1411 w, 1362 m, 1300 s, 1258 s, 1201 w,
1180 m, 1149 w, 1125 w, 1094 w, 1063 m, 1028 w, 930 s, 884 m, 840 s, 753 w,
681 m, 618 w, 521 m. Anal. Calcd for C66H120N6O2Si4Nd2: C, 55.41; H,
8.46; N, 5.87; Nd, 20.17. Found: C, 55.70; H, 5.71; N, 6.01; Nd, 20.05.
Reaction between Pro-ligand {LONAr}H2 and Y[N(SiMe3)2]3.
Synthesis of {LOHNAr}3Y (11). In the glovebox, a Teflon-valved
NMR tube was charged with {LONAr}H2 (0.0225 g, 0.0393 mmol) and
Y[N(SiMe3)2]3 (0.0075 g, 0.0131 mmol). To this mixture, C6D6
(0.6 mL) was vacuum-transferred in, and the tube was shaken for 24 h
at room temperature. 1H NMR indicated that 11 formed quantitatively
1H NMR (C6D6, 500 MHz, 25 °C): δ 7.45 (d, 4J = 2.3, 3H, C6H2), 7.23
(d, 3J = 7.6, 6H, C6H3), 7.15 (t, 3J = 7.6, 3H, C6H3), 7.03 (t, 3J = 7.6, 3H,
C6H3), 6.93 (d, 3J = 7.6, 6H, C6H3), 6.91 (d, 4J = 2.3, 3H, C6H2), 6.02
(s, 3H, NH), 3.42 (hept, 3J = 6.9, 6H, CH(CH3)), 3.16 (hept, 3J = 6.9,
6H, CH(CH3)), 1.67 (s, 27H, C(CH3)), 1.29 (d, 3J = 6.9, 18H,
CH(CH3)), 1.23 (d, 3J = 6.9, 18H, CH(CH3)), 0.98 (s, 27H, C(CH3)),
0.93 (d, 3J = 6.7, 18H, CH(CH3)), 0.90 (d, 3J = 6.9, 18H, CH(CH3)).
13C{1H} NMR (C6D6, 100 MHz, 25 °C): δ 158.4 (CN2), 157.0 (O-C6H2),
144.4 (C6H3-iPr), 140.4 (C6H3-iPr), 137.5 (C6H2-tBu), 137.1 (C6H2-tBu),
135.1 (N-C6H3), 127.3 (C6H3), 126.0 (C6H2), 125.3 (C6H3), 124.1 (C6H3),
123.5 (C6H3), 113.3 (C6H2-CN2), 35.3 (Ph-C(CH3)3), 33.7 (Ph-C-
(CH3)3), 31.0 (Ph-C(CH3)3), 29.5 (Ph-C(CH3)3), 28.9 (Ph-CH-
(CH3)2), 28.7 (Ph-CH(CH3)2), 24.6 (Ph-CH(CH3)2), 24.4 (Ph-CH-
(CH3)2), 22.0 (Ph-CH(CH3)2), 21.9 (Ph-CH(CH3)2). IR (ATR
module) ν (cmꢀ1): 3420 w, 2848 w, 1665 m, 1615 m, 1472 w,
1374 s, 1360 s, 1275 s, 1169 s, 1204 s, 1031 m, 931 w, 885 m, 701 m,
682 m, 623 w. Anal. Calcd for C117H165N6O3Y: C, 78.40; H, 9.28; N,
4.69. Found: C, 78.89; H, 10.01; N, 4.80.
7
CH3 Et2O), ꢀ0.39 (br s, 12H YCH3). Li NMR (C6D6, 78 MHz,
25 °C): δ 3.64. 13C{1H} NMR (C6D6, 50 MHz, 25 °C): δ 185.5 (CN2),
163.3 (O-C6H2), 135.8 (C6H2-tBu), 134.4 (C6H2-tBu), 126.2 (C6H2-
CN2), 123.5 (C6H2), 122.8 (C6H2), 65.6 (CH2 Et2O), 59.0 (CH(CH2)2),
56.7 (CH2 TMEDA), 45.9 (CH3 TMEDA), 35.4 (Ph-C(CH3)3), 34.0
(Ph-C(CH3)3), 32.0 (Ph-C(CH3)3), 30.2 (Ph-C(CH3)3), 27.0, 26.6,
26.4 (CH(CH2)5), 15.3 (CH3 Et2O), 10.5 (Y(CH3)2). IR (Nujol, KBr)
ν (cmꢀ1): 1606 m, 1523 m, 1437 m, 1360 w, 1303 s, 1265 m, 1218 w,
1200 w, 1180 w, 1150 m, 1129 m, 1097 w, 1066 w, 1033 m, 989 w, 949 m,
885 m, 842 s, 791 m, 741 w, 660 w, 549 w, 528 m. Anal. Calcd for
C74H138Li2N8O3Y2: C, 64.42; H, 10.08; N, 8.12; Y, 12.89. Found: C,
64.07; H, 9.80; N, 8.07; Y, 12.95.
[{(iPrN)2CC6H2(tBu)2O}NdMe2Li(TMEDA)]2 ({LONiPr}2Nd2-
Me4Li2(TMEDA)2, 14). Using a protocol similar to that described
above for 12, complex 14 was obtained from NdCl3 (0.143 g, 0.57 mmol),
TMEDA (0.34 mL, 2.28 mmol), methyllithium (2.68 mL of a 0.85 M
solution in Et2O, 2.28 mmol), and {LONiPr}H2 (0.190 g, 0.57 mmol).
A similar workup afforded 14 as a cyan-blue crystalline powder (0.265 g,
0.196 mmol, 69%). Crystals suitable for X-ray diffraction studies were
grown by slow concentration from a toluene solution at ꢀ20 °C. IR
(Nujol, KBr) ν (cmꢀ1): 1602 m, 1562 w, 1413 w, 1358 w, 1329 m, 1294 s,
1258 w, 1229 w, 1200 w, 1178 s, 1157 w, 1125 s, 1065 w, 1034 m, 1018 m,
948 m, 882 m, 839 s, 790 m, 761 m, 690 w, 582 m, 524 m. Anal. Calcd for
C65H120Li2N8O2Nd2: C, 57.91; H, 8.97; N, 8.31; Nd, 21.40. Found: C,
58.12; H, 9.01; N, 8.55; Nd, 21.80.
[{(iPrN)2CC6H2(tBu)2O}YMe2Li(TMEDA)]2Et2O ({LONiPr
}
2
Y2Me4Li2(TMEDA)2, 12). To a suspension of yttrium chloride
(0.140 g, 0.70 mmol) and TMEDA (0.42 mL, 2.80 mmol) in THF
(10 mL) was added methyllithium (3.29 mL of a 0.85 M solution in
Et2O, 2.80 mmol) at ꢀ20 °C, and the reaction mixture was stirred for
10 min at 0 °C. During this time period, all the solids dissolved. To the
resulting solution was added a solution of {LONiPr}H2 (0.23 g, 0.70 mmol)
in THF (10 mL), and the reaction mixture was stirred for 30 min
at 0 °C. Volatiles were evaporated in vacuo, and the residue was
[{(iPrN)2CC6H2(tBu)2O}SmMe2Li(TMEDA)]2 ({LONiPr}2-
3
Sm2Me2(OH)2Li2(TMEDA)2, 15). Using a protocol similar to that
described above for 12, complex 15 was obtained from SmCl3 (0.154 g,
0.60 mmol), TMEDA (0.36 mL, 2.40 mmol), methyllithium (2.84 mL
of a 0.85 M solution in Et2O, 2.40 mmol), and {LONiPr}H2 (0.195 g,
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0.60 mmol). A similar workup afforded 15 as a yellow crystalline powder
(0.261 g, 0.192 mmol, 64%). Crystals suitable for X-ray diffraction
studies were obtained by slow concentration of a toluene solution
at ꢀ20 °C. IR (Nujol, KBr) ν (cmꢀ1): 1605 m, 1495 w, 1410 w, 1358 m,
1329 m, 1292 s, 1267 m, 1256 w, 1229 w, 1200 m, 1175 m, 1157 w, 1124 m,
1065 w, 1032 s, 1018 m, 948 m, 839 s, 789 m, 760 m, 694 w. Anal. Calcd
for C65H120Li2N8O2Sm2: C, 57.39; H, 8.89; N, 8.24; Sm, 22.11. Found: C,
57.25; H, 8.75; N, 8.29; Sm, 21.8.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: evgueni.kirillov@univ-rennes1.fr (E.K.), trif@iomc.ras.
ru (A.T.), jean-francois.carpentier@univ-rennes1.fr (J.-F.C.).
’ ACKNOWLEDGMENT
[{(iPrN)2CC6H2(tBu)2O}Yb(OH)MeLi(TMEDA)]2Et2O ({LONiPr
}
2
This work was supported by the GDRI ‘‘CH2D’’ between RAS
Chemistry and Material Science Division and CNRS, the Russian
Foundation for Basic Research (Grant 11-03-00555-a), the
CNRS, and the Institut Universitaire de France (IUF).
Yb2Me2(OH)2Li2(TMEDA)2, 16). Using a protocol similar to that
described above for 12, compound 16 was obtained from YbCl3 (0.129
g, 0.46 mmol), TMEDA (0.27 mL, 1.84 mmol), methyllithium (2.16 mL
of a 0.85 M solution in Et2O, 1.84 mmol), and {LONiPr}H2 (0.153 g,
0.46 mmol). A similar workup afforded 16 as a yellow crystalline powder
(0.210 g, 0.165 mmol, 65%). Crystals suitable for X-ray diffraction
studies were grown by slow concentration of a Et2O solution at room
temperature. IR (Nujol, KBr) ν (cmꢀ1): 1601 m, 1528 m, 1411 w, 1298 s,
1266 w, 1248 m, 1222 w, 1194 w, 1172 w, 1149 m, 1130 w, 1113 m, 1068 m,
1037 m, 1022 m, 948 s, 886 s, 844 s, 790 m, 756 m, 740 m, 644 w, 602 m,
584 w. Anal. Calcd for C60H118Li2N8O5Yb2: C, 51.79; H, 8.55; N, 8.05; Yb,
24.87. Found: C, 51.85; H, 8.65; N, 8.22; Yb, 24.83.
’ REFERENCES
(1) For leading reviews on the synthesis of metal phenoxides, see:
(a) Bochkarev, M. N.; Zakharov, L. N.; Kalinina, G. S. Organoderivatives
of Rare Earth Elements; Nauka: Moscow, 1989. (b) Bradley, D. C.;
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of Metals; Academic Press: New York, 2001.
(2) For selected recent reviews on the synthesis of metal amidi-
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1347. (b) Edelmann, F. T. Chem. Soc. Rev. 2009, 38, 2253–2268.
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Soc. Rev. 2010, 39, 165–173.
(4) For selected leading references on the use of rare-earth phen-
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J. Am. Chem. Soc. 2002, 124, 1316–1326. (b) Ma, H.; Spaniol, T. P.;
Okuda, J. Dalton Trans. 2003, 4770–4780. (c) Ma, H.; Okuda, J.
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Lehmann, C. W.; Carpentier, J.-F. Chem. Commun. 2004, 330–331. (g)
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Int. Ed. 2006, 45, 2782–2784. (j) Liu, X.; Chang, X.; Tang, T.; Hu, N.; Pei,
F.; Cui, D.; Chen, X.; Jing, X. Organometallics 2007, 26, 2747–2757. (k)
Kramer, J. W.; Treiter, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.;
Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc. 2009, 131, 16042. (l)
Bouyahyi, M.; Ajellal, N.; Kirillov, E.; Thomas, C. M.; Carpentier, J.-F.
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Crystal Structure Determination of [{LONiPr}Li2(THF)2]2
3
(THF) (1),{LONAr}Li2(THF)3 (2),{LOHNAr}YCl2(DME) (5),{LONiPr}
YN(SiMe3)2 (6), {LONAr}2YLi(THF)2 (7), {LONCy}Nd(N(SiMe3)2)
(9), {LONCy}2YLi(DME) (10), {LONiPr}2Y2Me4Li2(TMEDA)2(Et2O)
(12), {LONCy}2Y2Me4Li2(TMEDA)2 (Et2O) (13), and {LONiPr}2Yb2
3
Me2(OH)2Li2(TMEDA)2 (Et2O) (16). Diffraction data were collected at
3
100(2) or 150(2) K using a Bruker APEX CCD diffractometer with graphite-
monochromatized MoKαradiation (λ= 0.71073 Å). A combination of ωand
ϕ scans was carried out to obtain a unique data set. The crystal structures were
solved by direct methods, and remaining atoms were located from difference
Fourier synthesis, followed by full-matrix least-squares refinement based on F2
(programs SIR97 and SHELXL-97).34 Many hydrogen atoms could be
located from the Fourier difference analysis. Other hydrogen atoms were
placed at calculated positions and forced to ride on the attached atom. The
hydrogen atom positions were calculated, but not refined. All non-hydrogen
atoms were refined with anisotropic displacement parameters. Crystal data and
details of data collection and structure refinement for the different compounds
are given in Tables S1 and S2 in the Supporting Information. Detailed
crystallographic data (excluding structure factors) are available in the Support-
ing Information, as CIF files.
Typical Procedure for Polymerization of rac-Lactide. In a
typical experiment (Table 3, entry 2), in the glovebox, a Schlenk flask
was charged with a solution of 6 (10.0 mg, 17.2 μmol) in toluene (0.5 mL).
rac-Lactide (0.248 g, 1.72 mmol, 100 equiv vs Y) in toluene (1.2 mL) was
added rapidly to this solution. The mixture was immediately stirred with a
magnetic stir bar at 20 °C for 1.33 h. The reaction was quenched by adding
ca. 1.0 mL of a 10% H2O solution in THF, and the polymer was precipitated
from CH2Cl2/pentane (ca. 2:100 mL) five times. The polymer was then
filtered and dried in vacuo to a constant weight.
(5) For selected leading references on the use of rare-earth amidinate
complexes in ROP catalysis, see: (a) Reference 2b. (b) Giesbrecht, G. R.;
Whitener, G. D.; Arnold, J. Dalton Trans. 2001, 923–927. (c) Kui,
S. C. F.; Li, H.-W.; Lee, H. K. Inorg. Chem. 2003, 42, 2824–2826. (d) Xu,
X.; Zhang, Z.; Yao, Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007,
46, 9379–9388. (e) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin,
G. K.; Cherkasov, A. V.; Thomas, C. M.; Carpentier, J.-F.; Trifonov, A. A.
Chem.—Eur. J. 2008, 14, 5440–5448. (f) Mahrova, T. V.; Fukin, G. K.;
Cherkasov, A. V.; Trifonov, A. A. Inorg. Chem. 2009, 48, 4258–4266.
(6) (a) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W. B.
J. Polym. Sci., Polym. Chem. Ed. 2001, 39, 284–293. (b) Wang, J.; Yao, Y.;
Shen, Q. Inorg. Chem. 2009, 48, 744–751. (c) Qin, D.; Han, F.; Yao, Y.;
Zhang, Y.; Shen, Q. Dalton Trans. 2009, 5535–5541.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data and details
b
of data collection and structure refinement for [{LONiPr}Li2
(THF)2]2 (THF) (1), {LONAr}Li2(THF)3 (2), {LOHNAr}
3
YCl2(DME) (5), {LONiPr}YN(SiMe3)2 (6), {LONAr}2YLi(THF)2
(7), {LONCy}Nd(N(SiMe3)2) (9), {LONCy}2YLi(DME) (10),
{LONiPr}2Y2Me4Li2(TMEDA)2(Et2O) (12), {LONCy}2Y2Me4-
Li2(TMEDA)2 (Et2O) (13), and {LONiPr}2Yb2Me2(OH)2Li2
3
(TMEDA)2 (Et2O) (16) as Tables S1 and S2 and CIF files, and
3
representative H and 13C NMR spectra for some complexes.
1
This material is available free of charge via the Internet at
http://pubs.acs.org.
(7) For the initial design of this ligand system, see: (a) Shapiro, P. J.;
Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867–869.
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(b) Shapiro, P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 1994, 116, 4623–4640.
R. J.; Power, P. P. J. Am. Chem. Soc. 1997, 119, 2847–2852. (d) Dinnebier,
R. E.; Behrens, U.; Olbrich, F. J. Am. Chem. Soc. 1998, 120, 1430–1433.
(19) Deacon, G. B.; Feng, T.; Junk, P. C.; Skelton, B. W.; White,
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(8) These distances are in the usual range of LiꢀO bond
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(21) Further attempt to synthesize 10 by reaction of a 1:1 mixture of
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(22) In addition to the abovementioned amine elimination synth-
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