Alkylyttrium complexes of amidine-amidopyridinate ligands. Intramolecular C(sp3)-H activation and reactivity studies

Article abstract of DOI:10.1021/om400027d

The new multidentate amidine-aminopyridine proligand {N ^Me2NN^Me2C^MeN^Me2}H (1) was used in σ-bond metathesis reactions with the trialkyl precursors Y(CH ₂SiMe₃)₃(THF)₂ and Y(CH ₂C₆H₄-o-NMe₂)₃. These reactions generate the dialkyl complexes {N^Me2NN^Me2C ^MeN^Me2}Y(CH₂SiMe₃)₂(THF) (2a) and {N^Me2NN^Me2C^MeN^Me2}Y(CH ₂C₆H₄-o-NMe₂)₂ (2b), respectively, which were characterized by NMR spectroscopy and microanalysis (for 2b). Complex 2a is unstable at -30 C in toluene and selectively undergoes intramolecular C(sp³)-H activation to give {N^Me2NN ^Me2C^MeN^Me-CH₂}Y(CH ₂SiMe₃)(THF) (2a′), authenticated by X-ray crystallography, NMR spectroscopy, and elemental analysis. The reactivity of 2a′ toward a number of Lewis and Bronsted acids and bases as well as oxidants and reductants was explored. In the course of these studies, the new complexes {N^Me2NN^Me2C^MeN^MeCH ₂B(C₆F₅)₃}Y(CH₂SiMe ₃)(THF) (3), [{N^Me2NN^Me2C^MeN ^MeCH₂}Y(CH₂SiMe₃)(THF) _x]⁺[BPh₄]^- (4), {N ^Me2NN^Me2C^MeN^MeCH₂} Y(iPrNC(PPh₂)NiPr) (5), [{N^Me2NN^Me2C ^MeN^Me2}Y{(μ₂-BH₃) (μ₂-NH)}]₂ (6), and [{N^Me2NN ^Me2C^MeN^MeCH₂(μ-O)}Y(CH ₂SiMe₃)]₂ (7) were isolated and characterized by NMR spectroscopy and X-ray crystallography and microanalysis (for 6 and 7). Complex 6 was found to be active in the ROP of rac-lactide, affording slightly heterotactic-enriched PLAs.

Full text of DOI:10.1021/om400027d

Article
pubs.acs.org/Organometallics
Alkylyttrium Complexes of Amidine−Amidopyridinate Ligands.
Intramolecular C(sp3)−H Activation and Reactivity Studies
Vasily Rad’kov,†,§ Vincent Dorcet,‡ Jean-Franco
̧
is Carpentier,*,† Alexander Trifonov,*,§
and Evgeny Kirillov*,†
†Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
Rennes, France
́
de Rennes 1,
‡Centre de Diffractomet
́ ́
rie X, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite de Rennes 1, Rennes, France
§G. A. Razuvaev Institute of Organometallic Chemistry of RAS, Nizhny Novgorod, Russia
S
* Supporting Information
ABSTRACT: The new multidentate amidine−aminopyridine
proligand {NMe2NNMe2CMeNMe2}H (1) was used in σ-bond
metathesis reactions with the trialkyl precursors Y(CH2-
SiMe3)3(THF)2 and Y(CH2C6H4-o-NMe2)3. These reactions
generate the dialkyl complexes {NMe2NNMe2CMeNMe2}Y-
(CH2SiMe3)2(THF) (2a) and {NMe2NNMe2CMeNMe2}Y-
(CH2C6H4-o-NMe2)2 (2b), respectively, which were charac-
terized by NMR spectroscopy and microanalysis (for 2b).
Complex 2a is unstable at −30 °C in toluene and selectively
undergoes intramolecular C(sp3)−H activation to give
{NMe2NNMe2CMeNMe−CH2}Y(CH2SiMe3)(THF) (2a′), au-
thenticated by X-ray crystallography, NMR spectroscopy, and
elemental analysis. The reactivity of 2a′ toward a number of Lewis and Brønsted acids and bases as well as oxidants and
reductants was explored. In the course of these studies, the new complexes {NMe2NNMe2CMeNMeCH2B(C6F5)3}Y(CH2SiMe3)-
(THF) (3), [{NMe2NNMe2CMeNMeCH2}Y(CH2SiMe3)(THF)x]+[BPh4]− (4), {NMe2NNMe2CMeNMeCH2}Y(iPrNC(PPh2)NiPr)
(5), [{NMe2NNMe2CMeNMe2}Y{(μ2-BH3)(μ2-NH)}]2 (6), and [{NMe2NNMe2CMeNMeCH2(μ-O)}Y(CH2SiMe3)]2 (7) were
isolated and characterized by NMR spectroscopy and X-ray crystallography and microanalysis (for 6 and 7). Complex 6 was
found to be active in the ROP of rac-lactide, affording slightly heterotactic-enriched PLAs.
an auxiliary pendant donating group to the bis(phenylamino)-
pyridine ligand skeleton should give rise to a new perfectly
chelating ligand system capable of stabilizing monometallic
monoligand species, which would feature valuable reactivity.
The coordination chemistry of the new amidine−amidopyr-
idinate ligand system with yttrium, starting from yttrium tris-
carbyl precursors, has been investigated. Special emphasis has
been placed on (i) elucidation of intramolecular C−H
activation reactions occurring in these yttrium alkyl complexes
and (ii) exploration of the reactivity of these C−H activation
products with different substrates. In addition, the efficacy of
some complexes obtained during this study in representative
polymerization reactions (vinyl polymerization of styrene and
isoprene; ROP of rac-lactide) has been preliminarily inves-
tigated.
INTRODUCTION
■
Nitrogen-based donor ligands play a crucial role in modern
non-metallocene polymerization catalysis.1 Since the pioneering
implementation of late-transition-metal catalysts incorporating
α-diimine and bis(imino)pyridine ligand systems for the
polymerization of olefins,2 a stupendous amount of research
has been directed toward the design of new polydentate ligand
systems, and new efficient and stereoselective polymerization
catalysts have been eventually created and intensely studied.3
Several new classes of early-transition-metal polymerization
catalysts have recently flourished, which incorporate N-based
ligands with steric and electronic properties tunable almost at
will, such as β-diketiminate,4 tris(pyrazolyl)borate5 and related
ligands,6 amidinate/guanidinate7 and related aminopyridinates,8
pyridyl−amido,9 and some other scaffolds.10
Herein we report a new multidentate amidine−amidopyr-
idinate ligand system derived from the parent bis-
(phenylamino)pyridine platform.11 Due to the linear geometry
and intrinsic multidentate coordination character, both the
monoanionic and dianionic ligands, derived from the original
bis(phenylamino)pyridine platform, essentially lead to di- and
polymetallic bis(ligand) complexes. We anticipated that adding
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: January 13, 2013
Published: February 19, 2013
© 2013 American Chemical Society
1517
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Scheme 1. Synthesis of Proligand 1
the 1:1 reaction of Y(CH2SiMe3)3(THF)2 and proligand 1 was
conducted in toluene (Scheme 2). Unexpectedly, the standard
workup led to the isolation in high yield of product 2a′, which
results from an intramolecular C−H activation of a methyl
group of the amidine phenyl fragment.15−17
RESULTS AND DISCUSSION
■
Synthesis of Proligand {NMe2NNMe2CMeNMe2}H (1). The
amidine−aminopyridine proligand 1 was obtained straightfor-
wardly by the one-step condensation of 2,6-bis(2,6-
dimethylphenylamino)pyridine with N-(2,6-dimethylphenyl)-
acetimidoyl chloride (Scheme 1). The product was isolated as
an off-white solid in 72% yield and characterized by NMR
spectroscopy, elemental analysis, and X-ray crystallography.
Suitable crystals for X-ray diffraction studies of compound 1
were isolated by successive recrystallizations from diethyl ether.
Figure 1 shows the molecular structure of 1, along with selected
1
In situ monitoring by H NMR spectroscopy showed that
Y(CH2SiMe3)3(THF)2 reacts very slowly with 1 equiv of
proligand 1 at −50 °C in toluene-d8. Raising the temperature of
the reaction to −30 °C led to its completion within several
minutes, giving quantitatively {NMe2NNMe2CMeNMe2}Y-
(CH2SiMe3)2(THF) (2a), with concomitant release of 1
equiv of SiMe4 (Scheme 3). Due to its high instability, complex
2a was rapidly characterized at this temperature by 1D and 2D
(COSY, HMQC, and HMBC) 1H NMR spectroscopic
1
techniques. The H NMR spectrum of 2a (Figure S3; see the
Supporting Information) contains two doublets of doublets (δ
−0.50 and −0.36 ppm; 2JH−H = 11.5 Hz, 2JY−H = 3.0 Hz) for the
diastereotopic CHHSiMe3 hydrogens and a series of singlet
resonances for each of the SiMe3 groups, the Me group of the
imino unit, and the methyl groups of three 2,6-dimethylanilinic
units, as well as two broadened signals for the α-CH2 and β-
CH2 THF hydrogens. The aromatic region of the spectrum
features characteristic doublets and one triplet (3JH−H = 8.0 Hz)
for the pyridine linker and multiplets for the aniline hydrogens.
This pattern indicates an apparent Cs-symmetric coordination
sphere around yttrium in 2a.
Figure 1. Molecular structure of {NMe2NNMe2CMeNMe2}H (1). All
hydrogen atoms, except that of the N−H group, are omitted for
clarity; thermal ellipsoids are drawn at the 50% probability level.
Selected bond distances (Å) and angles (deg): C(2)−N(1), 1.373(3);
C(3)−N(3), 1.406(3); C(4)−N(3), 1.406(3); C(4)−N(4), 1.276(3);
N(3)−C(4)−N(4), 117.3(2); N(1)−C(2)−N(2), 113.4(2); N(2)−
C(3)−N(3), 114.6(2).
Complex 2a slowly undergoes intramolecular C−H
activation at −30 °C following zero-order kinetics (kapp
=
(7.5
0.4) × 10−3 M s−1). The resulting product 2a′ is
thermally quite robust and does not decompose even upon
prolonged heating at elevated temperatures (up to 60 °C, days,
benzene or toluene). Also, 2a′ is stable in pyridine-d5 at room
temperature; however, it rapidly reacts in this solvent at 70 °C
to give a mixture of unidentified products.
The 1H NMR spectrum of complex 2a′, recorded in toluene-
d8 at room temperature (Figure S6; see the Supporting
Information), is quite different from that of 2a. Key resonances
in the aliphatic region include (i) two doublets of doublets (δ
−1.18 and −1.02 ppm, 2JH−H = 11.3 Hz, 2JY−H = 3.0 Hz) for the
diastereotopic hydrogens of the CHHSiMe3 group and two
doublets (δ 1.67 and 1.97 ppm; 2JH−H = 6.0 Hz, 2JY−H = 2.0 Hz)
for those of the CHHC6H3(CH3)N group, (ii) one singlet for
the SiMe3 group, (iii) six singlets for methyl groups of the
imino, the two 2,6-dimethylaniline, and one 6-methylaniline
units, and (iv) two broad signals for the α-CH2 and β-CH2
THF hydrogens. In the 13C{1H} NMR spectrum, the
CH2SiMe3 and CH2C6H3(CH3)N groups appear as two
metrical data. In the solid state, 1 adopts a geometry in which
the amidine fragment deviates ca. 50° from the plane defined by
the pyridine ring and the two adjacent nitrogen atoms (the
dihedral angle between the plane of the Me−C(4)−N(3)−
N(4) fragment and that of the pyridine ring is 50.0°). The
C(2)−N(1), C(3)−N(3), and C(4)−N(3) bonds in 1
(1.373(3), 1.406(3), and 1.406(3) Å, respectively) are
noticeably shorter than regular single C−N bonds (1.47−1.49
Å) observed in various organic compounds.12 This is indicative
of substantial n−π delocalization within the N(1)C(2)N(2)-
C(3)N(3)C(4) fragment of 1. The C(4)−N(4) distance of
1.276(3) Å is in agreement with double-bond character.
Synthesis of Dialkylyttrium Complexes. Recent studies
of the chemistry of rare-earth metals have shown the alkane
elimination approach, using homoleptic MR3 precursors and
amidine-based or related protio ligands, to be a straightforward
route toward different classes of amidinate complexes.13,14 In an
1
characteristic doublets (δ 23.1 ppm, JY−C = 38.9 Hz; δ 43.7
1
ppm, JY−C = 21.6 Hz, respectively).
On the other hand, the reaction of 1 with Y(CH2C6H4-o-
NMe2)3, carried out under conditions identical with those used
for the preparation of 2a′ (Scheme 3), resulted in the isolation
in 85% yield of {NMe2NNMe2CMeNMe2}Y(CH2C6H4-o-NMe2)2
attempt to obtain the corresponding bis(alkyl) complex
−
incorporating a monoanionic {NMe2NNMe2CMeNMe2
}
ligand,
1518
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Scheme 2. Synthesis of Dialkylyttrium Complexes 2a′ and 2b
Scheme 3. Intramolecular C−H Activation of Complex 2a
(2b). Product 2b is a yellow solid which appeared quite stable
upon heating at elevated temperatures (up to 60 °C, days,
aromatic hydrocarbons), not undergoing either C−H activation
or other decomposition pathways. We assume that this stability
may arise from the lower basicity of the benzyl group as
compared to that of CH2SiMe3 and/or from the less sterically
crowded coordination sphere in 2b, notably due to the absence
1
of coordinated THF molecule(s). The H NMR spectrum of
2b, in C6D6 at room temperature, is consistent with a
dissymmetric structure on the NMR time scale (Figure S8;
see the Supporting Information). In the 13C{1H} NMR
spectrum (Figure S9; see the Supporting Information), two
1
Figure 2. Molecular structure of {NMe2NNMe2CMeNMeCH2}Y-
(CH2SiMe3)(THF) (2a′). All hydrogen atoms are omitted for clarity;
thermal ellipsoids are drawn at the 30% probability level. Selected
bond distances (Å) and angles (deg): Y(1)−N(1), 2.353(3); Y(1)−
N(2), 2.344(3); Y(1)−N(4), 2.417(3); Y(1)−O(1), 2.389(3); Y(1)−
C(11), 2.475(4); Y(1)−C(12), 2.845(5); Y(1)−C(13), 2.907(5);
Y(1)−C(1), 2.418(4); C(2)−N(1), 1.343(5); C(2)−N(2), 1.378(5);
C(3)−N(2), 1.338(5); C(3)−N(3), 1.434(5); C(4)−N(3), 1.388(6);
C(4)−N(4), 1.280(5); N(1)−Y(1)−C(1), 109.9(1); Y(1)−C(11)−
C(12), 89.4(3); N(3)−C(4)−N(4), 121.6(4).
doublets are observed (δ 42.4 ppm, JY−C = 32.0 Hz; δ 42.9
1
ppm, JY−C = 26.3 Hz), which were assigned to the
nonequivalent methylenic moieties. All attempts to obtain
crystals of 2b suitable for X-ray analysis failed.
Crystals of 2a′ suitable for X-ray diffraction studies were
grown at room temperature from a toluene solution. The unit
cell of 2a′ contains two crystallographically independent
monomeric molecules, but those feature very similar geometries
and overall organizations, and therefore the structural details of
only one of them will be discussed. The molecular structure of
complex 2a′ is shown in Figure 2. The coordination
environment of the yttrium center in 2a′ is completed by
three nitrogen atoms of the {NMe2NNMe2CMeNMe}2− ligand
fragment, one oxygen atom from a THF molecule, one sp3
carbon atom of the residual trimethylsilylmethyl group, and the
sp3 carbon atom of the “benzylic” group resulting from the C−
H activation process. In addition, two close contacts
(Y(1)···C(12), 2.845(5) Å; Y(1)···C(13), 2.907(5) Å) between
the metal center and the sp2 carbon atoms of the aromatic
system of this “benzylic” group were identified. This may
increase the formal coordination number of yttrium in 2a′ up
to 8 (vide infra). The amidopyridinato−amidinate ligand
system in 2a′ is not strictly planar. The dihedral angle formed
between the Y(1)N(1)N(2) plane and one of the pyridine rings
is 168.2(4)°, while the dihedral angle between the NCN planes
of the amidopyridinate and amidinate fragments is 161.9(4)°.
The Y−C(carbyl) bond length in 2a′ (2.418(4) Å) is
comparable with those reported for related yttrium systems
(2.410(8)−2.439(3) Å).18 These Y−C(alkyl) and the Y−
C(“benzyl”) distances (2.418(4) and 2.475(4) Å, respectively)
1519
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Scheme 4. Formation of Complexes 3−7
are quite comparable to those previously reported for an
yttrium alkyl−benzyl species (2.4139(17) and 2.4520(18) Å,
respectively) resulting from a related intramolecular C−H bond
activation reaction.17c Among the Y−N bonds in 2a′, that
involving the amidinate nitrogen N(4) is the longest (2.417(3)
Å). The bonding situation within the Y(1)N(1)N(2) fragment
in 2a′ is quite different from those previously described in
related yttrium amidopyridinate species, in which the two Y−N
bond lengths are significantly different (ca. 0.1 Å).17d,19 Here,
the Y(1)−N(1) and Y(1)−N(2) bonds exhibit very similar
lengths (2.353(3) and 2.344(3) Å, respectively).20 The
geometric parameters of the amidinate fragment in 2a′ prove
its neutral state.21 Also, the noticeable difference in the C(4)−
N(3) and C(4)−N(4) bond lengths (1.281(5) and 1.389(5) Å,
respectively) argues for the absence of charge delocalization
within the NCN fragment.
comparable with those observed in the solid state (see
Computational Details in the Supporting Information). A
natural population analysis (NPA) performed on the optimized
structure of 2a′ revealed that no significant changes of the
computed Wiberg bond indexes occurred for the C−N bonds
in the dianionic {NMe2NNMe2CMeNMeCH2}2− ligand skeleton,
in comparison with those for the corresponding bonds and
charges in the neutral proligand 1 (Table S2; see the
Supporting Information). This result argues against distribution
of the two negative charges over the entire ligand skeleton and,
in particular, against considerable resonance of the nitrogen
lone pair located on the N(3) atom into the π system of the
ligand.22 Very weak interactions between the sp2 carbons C(12)
and C(13) and the metal center were attested by the
corresponding Wiberg bond indexes (0.06 and 0.05,
respectively). A second-order perturbation NBO analysis also
indicated weak interactions (highest stabilization energies of 2.5
and 3.9 kcal mol−1, respectively) between these carbons and
yttrium in 2a′.23 Thus, the coordination number at yttrium
The electronic structure of 2a′ was examined in more detail
on the basis of DFT calculations. The overall geometry of this
complex and that of proligand 1 thus computed are quite
1520
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
should be considered as more 6 than 8 (vide supra). Finally, the
HOMO in 2a′ is localized mostly at the Y(1)−C(11) bond
(Figure S21a; see the Supporting Information), while the
HOMO-1 is predominantly the σ bond Y(1)−C(1) (Figure
S21b; see the Supporting Information). On the basis of ground-
state orbital control arguments, an electrophile (or an oxidant)
would be predicted to attack initially at the HOMO: that is, via
cleavage of the Y(1)−C(“benzyl”) bond.
Reactivity Studies of Complex 2a′. Given the dissimilar
nature of the two alkyl groups in 2a′, they may reveal different
reactivity patterns toward appropriate substrates.24 Hence, we
set out to explore the reactivity of 2a′ with a variety of Lewis
and Brønsted acids and bases, as well as oxidants and
reductants (Scheme 4).
methyl hydrogens of the nonequivalent iPr groups of the
phosphaguanidinate moiety. As in complex 4, the Y−CH2
1
group in 5 appears in the H NMR spectrum as two doublets
2
of doublets (δ 2.53 and 1.99 ppm, JHH = 5.9 Hz) and as a
1
doublet (δ 47.0 ppm, JY−C = 24.3 Hz) in the 13C{1H} NMR
spectrum.
With the aim of synthesizing hydrido species with a
−
monoanionic {NMe2NNMe2CMeNMe2
}
ligand scaffold, efforts
were made to explore the reactivity of 2a′ toward reagents such
as H2, PhSiH3, and Ph3SiH.24,28 The reaction of 2a′ with H2 (1
1
bar, 25 °C) in toluene-d8 or in C6D6 (monitored by H NMR
spectroscopy) proceeded slowly over 5 days and afforded
mixtures of unidentified products. Similarly, the reaction
between equimolar amounts of 2a′ and PhSiH3 in toluene-d8
at room temperature resulted in rapid consumption of the
initial reagents, however, giving rise to mixtures of unidentified
products; no hydride resonance could be unambiguously
identified. No reaction was observed between 2a′ and the
more bulky Ph3SiH in toluene-d8 at room temperature over 1
day.
Instantaneous reaction of 2a′ with B(C6F5)3 in toluene-d8
1
solution was observed at −40 °C, as evidenced by H NMR
spectroscopy.25 Formation of the zwitterionic complex 3 took
place through chemoselective abstraction of the “benzylic”
group −CH2C6H3(Me)N−, while the CH2SiMe3 moiety
1
remained intact. This is evidenced by the presence in the H
NMR spectrum of 3 (Figure S10; see the Supporting
Information) of two doublets of doublets (δ −0.88 and
Amine−boranes RNH2·BH3 (R = H, alkyl) have recently
emerged as nonconventional sources of hydrogen for the
synthesis of alkaline-earth-metal complexes having M−NH-
(R)·(μ-H)xB(H)3−x bridging moieties.29 In this study, the
reaction of 1 equiv of NH3·BH330 and 2a′ in toluene-d8 at room
temperature was monitored by 1H NMR spectroscopy.
Immediate release of Me4Si was observed, while the product
of this reaction, complex 6, precipitated as a pale yellow
crystalline solid. Compound 6 was resynthesized on a
preparative scale by following this procedure. The very poor
solubility of 6 in toluene and THF hampered its character-
ization by 13C and 11B NMR spectroscopy. Complex 6 was
identified by 1H NMR spectroscopy (Figure S19; see the
Supporting Information), elemental analysis, and an X-ray
crystallographic study.
2
2
−0.70 ppm, JH−H = 11.3 Hz, JY−H = 3.3 Hz) for the
diastereotopic hydrogens of the CHHSiMe3 group and two
broad signals integrating each for one hydrogen for the
(C6F5)3BCHHC6H3(Me)N− moiety (δ 3.28 and 2.42 ppm; the
latter overlaps with a methyl resonance). In the 13C{1H} NMR
spectrum (Figure S13; see the Supporting Information), these
two methylene groups appeared as a characteristic doublet (δ
1
39.6 ppm, JY−C = 61.5 Hz) and a broadened signal (δ 23.5
ppm), respectively. The 19F{1H} NMR spectrum of 3 recorded
at −40 °C (Figure S12; see the Supporting Information)
contains a set of three broadened signals for the o-, p-, and m-F
groups, which suggests possible dynamic processes. No
evidence for nonvalent F···Y interactions was found.26 Complex
3 rapidly decomposes in toluene solution above 0 °C, affording
unidentified products.
Complex 6 crystallized as a toluene solvate (6·C7H8); its
molecular structure and selected geometric parameters are
given in Figure 3. The molecular structure of 6 consists of a
centrosymmetric dimer, in which two [(NMe2NNMe2CMeNMe2)-
Y] moieties are bound by two μ-bridging dianionic NH·BH3
−
Reaction of 2a′ with 1 equiv of [Et3NH]+[BPh4] in THF-
d8 does not take place below −20 °C.25 Monitoring of the
1
reaction by H NMR spectroscopy at −10 °C showed that it
follows first-order kinetics (kapp = (1.1 0.1) × 10−3 s−1) and
proceeds with concomitant evolution of Me4Si. The major
(>95%) product of this reaction, complex 4, was characterized
by a combination of NMR spectroscopic methods at 0 °C
(Figures S14 and S15; see the Experimental Section and the
Supporting Information) and appears overall as the proto-
nolysis product of the CH2SiMe3 group in 2a′.27 The
fragments. The yttrium atoms are coordinated by three
−
nitrogen atoms of the monoanionic {NMe2NNMe2CMeNMe2
}
ligands and two nitrogens and two hydrogens of the NH·BH3
groups, thus resulting in a formal coordination number of 7. In
the BH3 groups, two hydrogen atoms bridge both yttrium and
boron atoms while the third remains terminal. The
amidopyridinate fragment in 6 coordinates nearly symmetri-
cally to the metal center, which is evidenced by the quite similar
Y(1)−N(1) and Y(1)−N(2) bond lengths (2.403(2) and
2.377(2) Å, respectively), those being in the range reported for
yttrium amidopyridinate complexes (2.289−2.554 Å).8 The
Y(1)−B(1) distance in 6 (2.767(3) Å) is longer than those
reported for the borohydride complexes of yttrium incorporat-
1
corresponding Y−CHH(“benzyl”) group appeared in the H
NMR spectrum as an AB system (δ 1.24 and 1.93 ppm, 2JH−H
=
8.0 Hz) and as a doublet (δ 48.2 ppm, 1JY−C = 26.0 Hz) in the
13C{1H} NMR spectrum. Product 4 was found to be stable in
THF solution at room temperature. However, all attempts to
crystallize this compound have failed so far.
ing anionic BH4 moieties (2.496−2.643 Å).31
−
Reaction between 2a′ and 1 equiv of the phosphaguanidine
iPrNC(PPh2)NHiPr in C6D6 rapidly proceeded via selective
protonolysis of the Y−CH2SiMe3 bond.27 The corresponding
Furthermore, 2a′ was found to be totally unreactive toward
ethylene, propylene, and diphenylacetylene even upon
prolonged heating at 60−80 °C in toluene-d8. On the other
hand, complex mixtures of products were obtained in the
reactions of equimolar amounts of 2a′ and carbodiimide
iPrNCNiPr or isocyanate 3,4,5-(CH3O)3C6H2NCO,
respectively, conducted in toluene at −50 °C.
1
product 5 formed quantitatively and was authenticated by H,
13C, and 31P NMR spectroscopic studies (Figures S16−S18; see
the Supporting Information). Complex 5 exists as a
dissymmetric species in C6D6 solution, as evidenced by the
1
presence in the H NMR spectrum of six resonances from
methyl groups of the {NMe2NNMe2CMeNMeCH2} ligand, two
multiplets for the methine hydrogens, and four doublets for the
During several independent attempts to recrystallize 2a′ from
benzene or toluene solutions, crystals of the dimeric complex
1521
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Figure 4. Molecular structure of [{NMe2NNMe2CMeNMeCH2(μ-O)}Y-
(CH2SiMe3)]2·C7H8 (7·C7H8). Only the asymmetric unit is given. All
hydrogen atoms and the toluene molecule are omitted for clarity;
thermal ellipsoids are drawn at the 50% probability level. Selected
bond distances (Å) and angles (deg): Y(1)−N(1), 2.397(2); Y(1)−
N(2), 2.379(2); Y(1)−N(4), 2.493(2); Y(1)−O(1), 2.236(19);
Y(1)−C(1), 2.396(3); C(5)−O(1), 1.406(3); C(2)−N(1),
1.342(3); C(2)−N(2), 1.379(3); C(3)−N(2), 1.342(3); C(3)−
N(3), 1.417(3); C(4)−N(3), 1.397(4); C(4)−N(4), 1.292(3); N(4)
−Y(1)−O(1), 76.93(7); N(1)−C(2)−N(2), 111.5(2).
are only slightly different (2.396(3) and 2.362(4) Å,
respectively) and compare well with that observed in 2a′
(2.418(4) Å). Also, the Y(1)−N(1), Y(1)−N(2), and Y(1)−
N(4) bond distances in 7·2C6H6 (2.469(3), 2.414(3), and
2.509(3) Å, respectively) are noticeably longer than those
found in 7·C7H8 (2.397(2), 2.379(2), and 2.4493(2) Å,
respectively), but these distances remain comparable with the
corresponding distances in 2a′ and 6.
Figure 3. (a) Molecular structure of [{NMe2NNMe2CMeNMe2}Y{(μ2-
BH3)(μ2-NH)}]2·C7H8 (6·C7H8). Only the asymmetric unit is given.
All hydrogen atoms and the solvent molecule are omitted for clarity;
thermal ellipsoids are drawn at the 50% probability level. (b) Central
bimetallic core of 6·C7H8. Selected bond distances (Å) and angles
(deg): Y(1)−N(1), 2.403(2); Y(1)−N(2), 2.377(2); Y(1)−N(4),
2.520(2); Y(1)−N(5), 2.243(2); Y(1)−B(1), 2.767(3); C(1)−N(1),
1.343(3); C(1)−N(2), 1.379(3); C(2)−N(2), 1.331(3); C(2)−N(3),
1.418(3); C(3)−N(3), 1.392(3); C(4)−N(4), 1.298(3); N(1)−
C(1)−N(2), 111.2(2); N(2)−Y(1)−N(5), 101.40(8); N(5)−Y(1)−
B(1), 82.78(9).
The exogenous source of oxygen that resulted in the
formation of 7 remained eventually unclear.32 All attempts to
generate in a controlled fashion a thio compound counterpart
of complex 7 by reaction of 2a′ with both stoichiometric and
substoichiometric amounts of S8 in toluene-d8 failed to afford
the target product and led to the formation of intractable
mixtures of unidentified materials.
Preliminary Polymerization Investigations. Due to the
ongoing success of group 3 metal complexes applied in various
catalytic polymerization processes,1a,33 we sought to assess the
abilities of some of the complexes obtained in this study in
representative reactions.
First, no activity in isoprene and styrene polymerizations
(bulk or toluene solutions, [monomer]/[Y] = 3000, Tpol = 25−
60 °C) was observed with dialkyl complex 2a′, employed as
such or upon activation to generate in situ cationic systems: i.e.,
2a′/B(C6F5)3/iBu3Al (1/1/200) and 2a′/[Ph3C][B(C6F5)4]/
iBu3Al (1/1/200). The inactivity of these systems may stem
from either (i) the presence of a coordinated THF molecule in
2a′ which may impede monomer coordination or (ii) high
instability of the cationic species under the given conditions.
We were also interested in assessing the performance of 6 as
initiator/precatalyst of the ROP of rac-lactide, considering that
this complex possesses original, potentially active nucleophilic
NHBH3 groups. Complex 6 actually proved to be moderately
active at room temperature, especially in toluene, possibly
reflecting solubility issues (Table 1, entry 1). Indeed, much
better activity was observed in THF (entry 2). However,
increasing the [rac-LA]/[Y] ratio appeared to be detrimental
[{NMe2NNMe2CMeNMeCH2(μ−O)}Y(CH2SiMe3)]2 (7) were
recurrently isolated in significant amounts. They were separated
and authenticated by X-ray diffraction analyses, NMR spec-
troscopy, and microanalysis. Complex 7 crystallized as solvates
containing either one molecule of toluene (7·C7H8) or two
molecules of benzene (7·2C6H6). The molecular structure of
7·C7H8 is depicted in Figure 4. Different space groups and cell
parameters were determined for these two adducts, but the
general atom connectivity and bonding features were similar. In
both cases, 7 adopts centrosymmetric dimeric structures
d i ff e r i n g i n t h e m u t u a l d i s p o s i t i o n o f t h e
[(NMe2NNMe2CMeNMeCH2O)Y(CH2SiMe3)] fragments. Inter-
estingly, in 7·2C6H6 the [(NMe2NNMe2CMeNMeCH2O)Y-
(CH2SiMe3)] moieties are located in trans positions relative
to the Y2O2 plane, while in 7·C7H8 they are oriented cis. Two
identical [{NMe2NNMe2CMeNMeCH2O}Y(CH2SiMe3)] frag-
ments are linked by two bridging OCH2 groups. In 7·2C6H6,
the Y2O2 core is nearly equilateral, with very similar Y−O
distances (2.282(2) and 2.287(2) Å; compare with the
corresponding distances in 7·C7H8, 2.236(2) and 2.263(2)
Å). The yttrium centers in both 7·C7H8 and 7·2C6H6 lie in a
distorted-octahedral environment, coordinated by three nitro-
gens of the {NMe2NNMe2CMeNMeCH2O} ligands, two oxygens
of the bridging OCH2 groups, and one carbon of the carbyl
group. The Y−C(carbyl) bond lengths in 7·C7H8 and 7·2C6H6
1522
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Table 1. Ring-Opening Polymerization of rac-Lactide Promoted by Complex 6
b
c
d
e
e
f
entry compd [LA]/[Ln]/[iPrOH]
solvent
time (min)
conversn (%)
Mn(calcd) (103 g mol−1
)
Mn(exptl) (103 g mol−1
)
Mw/Mn
Pr
1
2
3
4
6
6
6
6
100/1/0
100/1/0
500/1/0
1000/1/1
toluene
THF
1140
5
71
62
6
10.2
8.9
10.9
13.0
nd
1.3
1.3
nd
nd
0.58
0.56
nd
THF
120
4.3
THF
1020
0
nd
nd
nd
a
b
c
General conditions: [rac-LA] = 2.0 mol L−1, T = 25 °C. Reaction times were not optimized. Conversion of lactide as determined by 1H NMR of
d
the crude reaction mixture. Mn values calculated considering one polymer chain per metal center from the relation: Mn(calcd) = conversion ×
e
[LA]/[Ln] × 144. Experimental Mn and Mw/Mn values determined by GPC in THF vs PS standards and corrected with a factor of 0.58.
f
1
1
Determined by H NMR; Pr is the probability of racemic linkage, as determined by H NMR homodecoupling experiments.
hydride and vacuum-transferred before use. The precursors N-(2,6-
and resulted in a significantly less active system (entry 3). The
polymers formed had relatively narrow distributions and
experimental molecular weights in good agreement with
calculated values, assuming initiation of one polymer chain
per NHBH3 moiety (Table 1).34 This observation hints at
rather good initiation efficiency. The recovered PLAs were
essentially atactic with a slight bias toward heterotacticity (Pr =
0.56−0.58). Surprisingly, addition of 1 equiv of iPrOH, a
reagent often used as coinitiator to generate in situ alkoxide
species,35 did not improve the outcome of polymerization
(entry 4).
dimethylphenyl)acetimidoyl chloride,36 Me3SiCH2 Li,37
39
((CH3)3SiCH2)3Y(THF)2,38 and Y(CH2C6H4-o-NMe2)3 were pre-
pared according to literature protocols. Styrene (99%, Acros) and
isoprene (99%, Acros) were dried over CaH2 and distilled under
reduced pressure prior to experiments or stored at −30 °C under
argon in the glovebox. rac-Lactide (Aldrich) was recrystallized once
from iPrOH and twice from dry toluene and then dried under vacuum.
Other starting materials were purchased from Acros, Strem, and
Aldrich and used as received.
Instruments and Measurements. NMR spectra of complexes
were recorded on Bruker AM-400 and AM-500 spectrometers in
Teflon-valved NMR tubes at 25 °C, unless otherwise indicated. 1H and
13C chemical shifts are reported in ppm vs SiMe4 (δ 0.00), as
determined by reference to the residual solvent peaks. 19F and 31P
chemical shifts were determined by external reference to aqueous
solutions of NaBF4 and H3PO4, respectively. 11B NMR spectra were
referenced to external BF3·OEt2. Assignment of resonances was made
from 2D 1H−1H COSY, 1H−13C HMQC, and HMBC NMR
experiments. Coupling constants are given in hertz. Elemental analyses
(C, H, N) were performed using a Flash EA1112 CHNS Thermo
Electron apparatus and are the average of two independent
determinations. Size exclusion chromatography (SEC) of PLAs was
performed in THF (1 mL min−1) 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 indexes (Mw/Mn) of the polymers were calculated with
reference to a universal calibration vs 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. The microstructure of PLAs was determined by
CONCLUSIONS
■
A new multidentate ligand system combining amidopyridinate
and amidine fragments into a chelating platform has been
prepared. The reaction between the proligand
{NMe2NNMe2CMeNMe2}H (1) and Y(CH2SiMe3)3(THF)2 pro-
ceeds through the formation of dialkyl complex 2a, which bears
−
a monoanionic {NMe2NNMe2CMeNMe2
}
ligand. The latter
compound rapidly undergoes intramolecular C−H activation
at a methylphenyl group, yielding selectively the hetero dialkyl
s p e c i e s 2 a ′ , w h i c h b e a r s
a
d i a n i o n i c
{NMe2NNMe2CMeNMeCH2}2− ligand. On the other hand, a
similar σ-bond metathesis reaction between 1 and Y(CH2C6H4-
o-NMe2)3 selectively afforded the stable dibenzyl species 2b.
The latter proved to be robust and does not undergo either
intramolecular C−H activation reactions or other decom-
position pathways.
Reactivity studies of hetero dialkyl 2a′ with various substrates
revealed different patterns. In particular, 2a′ was found to react
with Lewis and Brønsted acids as well as with (probably)
dioxygen preferentially at the Y−(“benzyl”) bond. Thus, new
complexes incorporating both monoanionic and dianionic
multidentate ligands (products 3 and 6 and 4, 5, and 7,
respectively) have been synthesized and authenticated. The
product of the reaction between 2a′ and NH3·BH3, complex 6,
represents the first structurally characterized amidoborate
derivative of group 3 metals. Complex 6 allowed the ring-
opening polymerization of rac-lactide under mild conditions.
Ongoing studies in this field are focused on modifications of
this amidine−amidopyridinate ligand system and elaboration of
new polymerization catalysts derived thereof.
1
homodecoupling H NMR spectroscopy at 25 °C in CDCl3 with a
Bruker AC-500 spectrometer operating at 500 MHz.
N,N′-Bis(2,6-dimethylphenyl)pyridine-2,6-diamine. This com-
pound was prepared according to a modified literature procedure.11b
2,6-Dibromopyridine (8.00 g, 33.8 mmol) and 2,6-dimethylaniline
hydrochloride (21.43 g, 136.0 mmol) were added in a 250 mL two-
neck flask equipped with a stirring bar and a condenser. The mixture
was heated to 220 °C. As the mixture melted, it turned dark orange.
After 4 h of stirring under argon at this temperature, the reaction
mixture was cooled. An aqueous solution of sodium carbonate (10%,
100 mL) was added to the mixture followed by extraction with Et2O
(100 mL). The organic phase was reduced to ca. 40 mL under reduced
pressure, and then pentane (50 mL) was added. The crude precipitate
was recrystallized from pentane to afford the analytically pure
1
compound as an off-white solid (6.40 g, 20.2 mmol, 60%). H NMR
(400 MHz, CDCl3, 298 K):40 δ 7.09 (m, 7H, p-Py and Ar), 6.19 (s,
3
EXPERIMENTAL SECTION
2H, NH), 5.35 (d, JH−H = 7.9, 2H, m-Py), 2.26 (s, 12H,
■
(CH3)2C6H3N).
General Considerations. All manipulations were performed
under a purified argon atmosphere using standard Schlenk techniques
or in a glovebox. Solvents were distilled from Na/benzophenone
(THF, Et2O) and Na/K alloy (toluene, hexane, and pentane) under
nitrogen, degassed thoroughly, and stored under nitrogen prior to use.
Deuterated solvents (benzene-d6, toluene-d8, THF-d8; >99.5% D,
Deutero GmbH and Euroisotop) were vacuum-transferred from Na/K
alloy into storage tubes. CDCl3 and CD2Cl2 were kept over calcium
(NMe2NNMe2CMeNMe2)H (1). To a solution of N,N′-bis(2,6-
dimethylphenyl)pyridine-2,6-diamine (2.38 g, 7.50 mmol) in toluene
(30 mL) was added Et3N (1.04 mL, 7.50 mmol) and N-(2,6-
dimethylphenyl)acetimidoyl chloride (1.32 mL, 7.50 mmol) with
rigorous stirring. The reaction mixture was stirred at 80 °C for 3 days.
Then the solvent was removed in vacuo. The resulting solid residue
was dissolved in diethyl ether (100 mL) and washed with Na2CO3.
1523
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
The organic layer was separated and dried with MgSO4, the solvent
was removed, and the solid residue was recrystallized from diethyl
ether. Compound 1 was isolated as a pale yellow powder (2.58 g, 5.40
0.84 mmol, 85%). 1H NMR (500 MHz, C6D6, 298 K): δ 7.09 (d, 3JH−H
= 7.5, 3H, (CH3)2C6H3N), 7.03 (m, 2H, C6H4-o-NMe2), 6.99−6.87
(m, 14H, (CH3)2C6H3N and C6H4-o-NMe2), 6.79 (t, 3JH−H = 8.0, 2H,
C6H4-o-NMe2), 6.72 (d, 3JH−H = 8.0, 1H, C6H4-o-NMe2), 6.61 (m, 2H,
1
mmol, 72%). H NMR (400 MHz, C6D6, 298 K): δ 7.03 (m, 5H,
3
(CH3)2C6H3N), 6.90 (m, 5H, p-Py and (CH3)2C6H3N), 6.27 (d, 3JH−H
C6H4-o-NMe2, p-Py), 6.45 (d, JH−H = 8.5, 1H, C6H4-o-NMe2), 5.34
3
3
3
(d, JH−H = 8.0, 1H, m-Py), 4.79 (d, JH−H = 8.0, 1H, m-Py), 2.44 (s,
9H, (CH3)2C6H3N), 2.33 (s, 3H, (CH3)2C6H3N), 2.28 (br s, 6H,
(CH3)2C6H3N), 2.24 (s, 3H, (CH3)2C6H3N), 2.23 (br m, 1H,
CHHC6H4-o-NMe2), 2.18 (s, 3H, C6H4-o-NMe2), 1.89 (s, 3H, C6H4-o-
NMe2),1.85 (br d, 2JH−H = 10.2, 1H, CHHC6H4-o-NMe2), 1.83 (br m,
1H, CHHC6H4-o-NMe2), 1.77 (s, 3H, C6H4-o-NMe2), 1.66 (s, 3H,
= 7.7, 1H, m-Py), 5.57 (d, JH−H = 7.9, 1H, m-Py), 5.06 (s, 1H, NH),
2.26 (s, 6H, (CH3)2C6H3N), 2.08 (s, 6H, (CH3)2C6H3N), 2.01 (s, 6H,
(CH3)2C6H3N), 1.82 (s, 3H, NC(CH3)N). 13C{1H} NMR (100 MHz,
C6D6, 298 K): δ 156.4 (CN), 156.1, 155.1 (o-Py), 148.4, 142.5,
138.8, 137.4, 137.2, 136.5, 128.4, 128.2, 127.3, 127.0, 126.4, 122.0 (p-
Py), 104.4 (m-Py), 99.5 (m-Py), 18.6 (NC(CH3)N), 18.5
((CH3)2C6H3N), 18.2 ((CH3)2C6H3N), 18.0 ((CH3)2C6H3N). Anal.
Calcd for C31H34N4: C, 80.48; H, 7.41; N, 12.11. Found: C, 80.35; H,
7.53; N, 12.17.
2
C6H4-o-NMe2), 1.35 (s, 3H, NC(CH3)N), 0.91 (d, JH−H = 10.2, 1H,
CHHC6H4-o-NMe2). 13C{1H} NMR (125 MHz, C6D6, 298 K): δ
167.9 (o-Py), 158.7 (NCN), 152.8, 149.8, 146.1, 144.8 (C4aro), 141.6
(p-Py), 139.2, 138.9, 137.2, 136.9, 136.8, 134.4, 132.7, 132.2, 132.1
(C4, (CH3)2C6H3N)), 131.1 ((CH3)2C6H3N), 130.7 (C4, C6H4-o-
NMe2), 129.2, 129.1, 129.1, 128.8 (C6H4-o-NMe2), 128.2, 126.4,
124.5, 123.4, 122.7 ((CH3)2C6H3N), 120.1, 119.0, 118,5, 118.2 (C6H4-
o-NMe2), 99.8 (m-Py), 93.2 (m-Py), 45.4 ((CH3)2C6H3N), 43.8
Reaction of 1 with Y(CH2SiMe3)3(THF)2. NMR Scale Synthesis
of 2a. In the glovebox, a Teflon-valved NMR tube was charged with 1
(0.050 g, 0.11 mmol) and Y(CH2SiMe3)3(THF)2 (0.061 g, 0.11
mmol). To this mixture, toluene-d8 (0.6 mL) was vacuum-transferred
in and the tube was shaken for 15 min at −50 °C. Quantitative
1
1
((CH3)2C6H3N), 42.9 (d, JY−C = 26.3, CH2C6H4-o-NMe2), 42.4
formation of 2a was observed. H NMR (500 MHz, toluene-d8, 243
(1JY−C = 32.0, CH2C6H4-o-NMe2), 41.8 ((CH3)2C6H3N), 18.9
((CH3)2C6H3N), 18.8 (NC(CH3)N), 18.4 ((CH3)2C6H3N), 18.1,
17.4, 17.0, 16.9 (C6H4-o-NMe2). Anal. Calcd for C49H57N6Y: C, 71.87;
H, 7.02; N, 10.26; Y, 10.86. Found: C, 71.65; H, 7.15; N, 10.30; Y,
10.95.
3
K): δ 7.11 (d, JH−H = 7.8, 3H, (CH3)2C6H3N), 6.98 (m, 2H,
3
(CH3)2C6H3N), 6.92 (d, JH−H = 7.5, 2H, (CH3)2C6H3N), 6.73 (d,
3JH−H = 7.7, 2H, (CH3)2C6H3N), 6.44 (t, 3JH−H = 8.0, 1H, p-Py), 5.39
(d, 3JH−H = 8.0, 1H, m-Py), 4.73 (d, 3JH−H = 8.0, 1H, m-Py), 3.59 (br. s,
12H, α-CH2,THF), 2.52 (s, 6H, (CH3)2C6H3N), 2.22 (s, 6H,
(CH3)2C6H3N), 1.84 (s, 6H, (CH3)2C6H3N), 1.39 (br s, 12H, β-
CH2, THF), 0.95 (s, 3H, NC(CH3)N), 0.21 (s, 18H, SiMe3), −0.34
Reaction of 2a′ with B(C6F5)3. NMR Scale Synthesis of 3. In the
glovebox, a Teflon-valved NMR tube was charged with 2a′ (0.040 g,
0.056 mmol) and B(C6F5)3 (0.032 g, 0.056 mmol). To this mixture,
toluene-d8 (0.6 mL) was vacuum-transferred in and the tube was
shaken for 15 min at −50 °C. The progress of the reaction was
2
2
2
(dd, JH−H = 11.4, JY−H = 3.0, 2H, CHHSiMe3), −0.50 (dd, JH−H
=
11.4, JY−H = 3.0, 2H, CHHSiMe3). 13C{1H} NMR (125 MHz,
toluene-d8, 243 K): δ 167.5 (o-Py), 161.6 (NC(CH3)N), 145.6, 142.0
(p-Py) 141.5, 138.9, 135.7, 132.3, 131.4, 129.2, 128.4, 101.1 (m-Py),
93.4 (m-Py), 67.8 (α-CH2 THF), 36.9 (CH2SiMe3), 25.1 (β-CH2
THF), 19.5, 19.2 ((CH3)2C6H3N), 18.7 ((NC(CH3)N), 17.3
((CH3)2C6H3N), 3.6 (CH2SiMe3).
2
1
monitored at −40 °C by NMR spectroscopy. H NMR indicated that
1
3 formed quantitatively after 1−2 min. H NMR (400 MHz, toluene-
3
d8, 233 K): δ 7.62 (d, JH−H = 8.0, 1H, (CH3)2C6H3N), 7.14 (s, 1H,
CH2(CH3)C6H3N), 7.06 (s, 1H, CH2(CH3)C6H3N), 6.99 (s, 1H,
CH2(CH3)C6H3N), 6.92 (t, 3JH−H = 7.6, 1H, (CH3)2C6H3N), 6.71 (m,
2H, (CH3)2C6H3N), 6.58 (t, 3JH−H = 8.4, 2H, (CH3)2C6H3N), 6.33 (t,
3JH−H = 8.2, 1H, p-Py), 5.29 (d, 3JH−H = 8.2, 1H, m-Py), 4.75 (d, 3JH−H
= 8.2, 1H, m-Py), 3.50 (br m, 2JH−H = 6.4, 2H, α-CH2 THF), 3.37 (br
m, 2JH−H = 6.4, 2H, α-CH2 THF), 3.28 (br s, 1H, CHHB(C6F5)), 2.42
(br s, 1H, CHHB(C6F5) and 3H, (CH3)2C6H3N), 2.36 (s, 3H,
CH2(CH3)C6H3N), 2.00 (s, 3H, (CH3)2C6H3N), 1.89 (s, 3H,
(CH3)2C6H3N), 1.74 (s, 3H, (CH3)2C6H3N), 1.25 (s, 3H, NC(CH3)-
(NMe2NNMe2CMeNMe-CH2)Y(CH2SiMe3)(THF) (2a′). To a solution
of 1 (1.30 g, 2.80 mmol) in toluene (10 mL) was added a solution of
Y(CH2SiMe3)3(THF)2 (1.40 g, 2.80 mmol) in toluene (25 mL). The
reaction mixture was stirred at room temperature overnight. All
volatiles were evaporated in vacuo, and the crude product was
recrystallized from toluene to give yellow crystals of 2a′ (1.60 g, 2.20
mmol, 80%). 1H NMR (500 MHz, toluene-d8, 298 K): δ 7.07 (m, 2H,
3
(CH3)2C6H3N), 7.04 (d, JH−H = 7.4, 1H, (CH3)2C6H3N), 7.00 (s,
2
N), 1.11 (br m, JH−H = 6.0, 4H, β-CH2 THF), 0.08 (s, 9H, SiMe3),
1H, CH2(CH3)C6H3N), 6.92 (m, 2H, CH2(CH3)C6H3N), 6.86 (d,
2
2
−0.70 (dd, JH−H = 11.3, JY−H = 3.3, 1H, CHHSiMe3), −0.88 (dd,
2JH−H = 10.6, 2JY−H = 2.2, 1H, CHHSiMe3). 13C{1H} NMR (100 MHz,
toluene-d8, 233 K) (signals from carbons of the C6F5 moieties in the
aromatic region were not identified): δ 168.5 (NCN), 159.6
((NC(CH3)N), 147.8 NCN, 144.7 (Caro), 144.6 (p-Py), 136.8,
135.5, 133.2, 132.1, 131.9, 131.3, 130.8, 130.5, 130.1, 130.0, 129.5
((CH3)2C6H3N), 129.3, 129.1, 128.4 (CH2(CH3)C6H3N), 126.5,
126.2, 125.2 ((CH3)2C6H3N), 102.7, 95.4 (m-Py), 71.9 (α-CH2,THF),
3
3JH−H = 7.5, 1H, (CH3)2C6H3N), 6.78 (d, JH−H = 7.4, 1H,
3
(CH3)2C6H3N), 6.73 (d, JH−H = 7.3, 1H, (CH3)2C6H3N), 6.63 (t,
3JH−H = 8.2, 1H, p-Py), 5.37 (d, 3JH−H = 8.2, 1H, m-Py), 4.84 (d, 3JH−H
= 8.2, 1H, m-Py), 3.56 (br s, 9H, α-CH2 THF), 2.46 (s, 3H,
(CH3)2C6H3N), 2.26 (s, 3H, (CH3)2C6H3N), 2.25 (s, 3H, CH22(CH3)-
2
C6H3N), 2.18 (s, 3H, (CH3)2C6H3N), 1.97 (dd, JH−H = 6.0, JY−H
=
0.5, 1H, CHH(CH3)C6H3N), 1.86 (s, 3H, (CH3)2C6H3N), 1.67 (dd,
2
2JH−H = 6.0, JY−H = 2.0, 1H, CHH(CH3)C6H3N), 1.39 (s, 3H,
1
39.6 (d, JY−C = 61.5, CH2SiMe3), 25.7 (β-CH2,THF), 23.5 (br s,
NC(CH3)N), 1.34 (br s, β-CH2 THF), 0.12 (s, 9H, SiMe3), −1.02
CH2B(C6F5)), 21.8 ((CH3)2C6H3N), 21.5 (CH2(CH3)C6H3N), 20.2
(NC(CH3)N), 19.6 ((CH3)2C6H3N), 18.6 ((CH3)2C6H3N), 17.0
((CH3)2C6H3N), 5.00 (SiMe3). 11B NMR (128 MHz, toluene-d8, 233
K): δ −12.6. 19F{1H} NMR (376 MHz, toluene-d8, 233 K): δ −133.1
(br m, 6F, o-F), −160.1 (br m, 3F, p-F), −164.2 (br m, 6F, m-F).
Reaction of 2a′ with [HNEt3]+[BPh4]−. NMR Scale Synthesis of
4. In the glovebox, a Teflon-valved NMR tube was charged with 2a′
(0.040 g, 0.056 mmol) and [HNEt3]+[BPh4]− (0.024 g, 0.056 mmol).
To this mixture, THF-d8 (0.6 mL) was vacuum-transferred in and the
tube was shaken for 15 min at −70 °C. The progress of the reaction
2
2
2
(dd, JH−H = 11.3, JY−H = 3.0, 1H, CHHSiMe3), −1.18 (dd, JH−H
=
11.3, JY−H = 3.0, 1H, CHHSiMe3). 13C{1H} NMR (125 MHz,
toluene-d8, 298 K): δ 167.5 (o-Py), 158.4 (NC(CH3)N), 149.9
(NCN), 147.0 ((CH3)2C6H3N), 141.7 (p-Py), 140.8, 139.3, 137.1,
136.9, 136.8, 133.3, 132.8, 131.9, 130.6, 129.2, 128.8, 128.1, 123.1,
123.0, 119.3, 99.9 (m-Py), 93.6 (m-Py), 63.9 (α-CH2 THF), 43.7 (d,
2
1JY−C = 21.6, CH2(CH3)C6H3N), 25.1 (β-CH2 THF), 23.1 (1JY−C
=
38.9, CH2SiMe3), 19.5, 19.2, 19.1(CH3)2C6H3N), 18.9 (NC(CH3)N),
17.4 ((CH3)2C6H3N), 17.0 (CH2(CH3)C6H3N), 4.7 (SiMe3). Anal.
Calcd for C78H102N8O2Si2Y2: C, 65.99; H, 7.38; N, 7.89. Found: C,
65.73; H, 7.50; N, 7.96.
1
was monitored at −50 °C by NMR spectroscopy. H NMR indicated
that 4 formed quantitatively after 2 min. 1H NMR (400 MHz, THF-d8,
273 K): δ 7.36 (m, 2H, (CH3)2C6H3N), 7.31 (br m, 8H, B(C6H5)),
7.11−7.04 (m, 4H, p-Py and (CH3)2C6H3N), 6.95−6.91 (m, 4H,
(NMe2NNMe2CMeNMe2)Y(CH2C6H4-o-NMe2)2 (2b). To a solution of
1 (0.47 g, 0.99 mmol) in toluene (10 mL) was added a solution of
Y(CH2C6H4-o-NMe2)3 (0.50 g, 0.99 mmol) in toluene (10 mL). The
reaction mixture was stirred at room temperature overnight. All
volatiles were evaporated in vacuo, and the crude product was
recrystallized from toluene to afford pure 2b as a yellow solid (0.69 g,
3
CH2(CH3)C6H3N and (CH3)2C6H3N), 6.88 (t, JH−H = 7.4, 8H,
3
3
B(C6H5)), 6.74 (t, JH−H = 7.1, 4H, B(C6H5)), 6.58 (d, JH−H = 7.0,
1H, CH2(CH3)C6H3N), 5.48 (d, 3JH−H = 8.6, m-Py), 5.10 (d, 3JH−H
7.8, m-Py), 2.49 (q, JH−H = 7.1, 6H, CH2, Et3N), 2.37 (s, 3H,
=
3
1524
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
(CH3)2C6H3N), 2.23 (s, 3H, (CH3)2C6H3N), 2.20 (s, 3H, CH2(CH3)-
C6H3N), 2.17 (s, 3H, (CH3)2C6H3N), 2.05 (s, 3H, (CH3)2C6H3N),
Isolation of [{NMe2NNMe2CMeNMeCH2(μ-O)}Y(CH2SiMe3)]2 (7).32
To a solution of 1 (0.43 g, 0.93 mmol) in toluene (25 mL) was added
a solution of Y(CH2SiMe3)3(THF)2 (0.46 g, 0.93 mmol) in hexane
(10 mL). The reaction mixture was stirred for 1 h at 0 °C. All volatiles
were evaporated in vacuo, and a mixture of toluene/hexane (3/1) was
vacuum-transferred onto the crude solid product. Yellow crystals of 7
rapidly grew from this solution at room temperature (0.55 g, 0.74
2
1.93 (d, JH−H = 8.0, 1H, CH2(CH3)C6H3N), 1.87 (s, 3H,
2
NC(CH3)N), 1.24 (d, JH−H = 8.0, 1H, CH2(CH3)C6H3N), 1.02 (t,
3JH−H = 7.1, 9H, CH3, Et3N). 13C{1H} NMR (100 MHz, THF-d8, 273
K): δ 166.9 (o-Py), 164.2 (B(C6H5), 161.8 (NC(CH3)N), 147.2
((CH3)2C6H3N), 144.8 (CH2(CH3)C6H3N), 141.8 (p-Py), 138.8
((CH3)2C6H3N), 137.5 (CH2(CH3)C6H3N), 136.7, 135.8, 132.5,
129.7, 128.8, 128.2, 127.6, 124.5 ((CH3)2C6H3N), 122.6 (CH2(CH3)
C6H3N), 120.1 ((CH3)2C6H3N), 101.8 (o-Py), 93.4 (o-Py), 48.2 (d,
1JY−C = 26.0, CH2(CH3)C6H3N), 47.3 (CH2, Et3N), 20.6 (NC(CH3)-
1
mmol, 80%). H NMR (500 MHz, toluene-d8, 333 K): δ 7.32 (d,
3
3JH−H = 7.3, 1H, (CH3)2C6H3N), 7.14 (d, JH−H = 7.6, 1H,
3
(CH3)2C6H3N), 7.11 (s, 2H, C6H5CH3), 7.06 (d, JH−H = 7.4, 1H,
CH2(CH3)C6H3N), 7.03 (s, 2H, CH2(CH3)C6H3N), 7.01 (s, 3H,
C6H5CH3), 6.96 (t, 3JH−H = 6.0, 2H, (CH3)2C6H3N), 6.92 (d, 3JH−H
=
N), 18.6 (CH2(CH3)C6H3N), 17.6 ((CH3)2C6H3N), 17.0
((CH3)2C6H3N), 11.5 (CH3, Et3N). 11B NMR (128 MHz, THF-d8,
273 K): δ −6.6.
3
6.7, 2H, (CH3)2C6H3N), 6.79 (t, JH−H = 7.5, 1H, (CH3)2C6H3N),
6.65 (t, 3JH−H = 8.2, 1H, p-Py), 5.38 (d, 3JH−H = 8.2, 1H, m-Py), 4.8 (d,
2
3JH−H = 8.2, 1H, m-Py), 4.62 (d, JH−H = 12.1, 1H, OCH2), 4.28 (d,
Reaction of 2a′ with iPrNC(PPh2)N(H)iPr. NMR Scale
Synthesis of 5. In the glovebox, a Teflon-valved NMR tube was
charged with 2a′ (0.040 g, 0.057 mmol) and iPrNC(PPh2)-N(H)iPr
(0.017 g, 0.057 mmol). To this mixture, C6D6 (0.6 mL) was vacuum-
transferred in and the tube was shaken for 15 min at room
temperature. 1H NMR indicated that 5 formed quantitatively. 1H
NMR (500 MHz, C6D6, 298 K): δ 7.46 (m, 4H, P(C6H5)2), 7.29 (d,
3JH−H = 7.8, 1H, (CH3)2C6H3N), 7.24−7.18 (m, 4H, (CH3)2C6H3N
2JH−H = 12.1, 1H, OCH2), 2.64 (s, 3H, (CH3)2C6H3N), 2.34 (s, 3H,
(CH3)2C6H3N), 2.31 (s, 3H, CH2(CH3)C6H3N), 2.19 (s, 6H,
(CH3)2C6H3N), 2.13 (m, 3H, C6H52CH3), 1.10 (s, 3H, NC(CH3)N),
2
0.28 (s, 9H, SiMe3), −0.39 (dd, JH−H = 11.7, JY−H = 3.1, 1H,
2
2
CHHSiMe3), −1.02 (dd, JH−H = 11.7, JY−H = 3.1, 1H, CHHSiMe3).
13C{1H} NMR (125 MHz, toluene-d8, 333 K): δ 167.8 (o-Py), 159.8
(NC(CH3)N), 147.7 ((CH3)2C6H3N), 145.1 (CH2(CH3)C6H3N),
141.0 (p-Py), 140.4 ((CH3)2C6H3N), 137.3 (CH2(CH3)C6H3N),
136.7 (C6H5CH3), 130.7 ((CH3)2C6H3N), 128.7 (C6H5CH3), 127.9
(CH2(CH3)C6H3N), 127.7 ((CH3)2C6H3N), 124.9 ((CH3)2C6H3N),
122.8 ((CH3)2C6H3N), 100.4 (m-Py), 92.9 (m-Py), 65.2 (OCH2), 29.4
(CH2SiMe3), 19.6 (NC(CH3)N), 19.4, 18.9 (CH3)2C6H3N), 18.2
(CH2(CH3)C6H3N), 18.1 (C6H5CH3), 17.6 ((CH3)2C6H3N), 4.6
(SiMe3). Anal. Calcd for C84H102N8O2Si2Y2: C, 67.63; H, 7.03; N,
7.51; Y, 11.92. Found: C, 67.45; H, 7.14; N, 7.57; Y, 11.95.
Crystal Structure Determination of 1, 2a′, 6, and 7.
Diffraction data were collected at 150(2) or 250(2) K using a Bruker
APEX CCD diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). The crystal structures were solved by direct
methods; the remaining atoms were located from difference Fourier
synthesis followed by full-matrix least-squares refinement based on F2
(programs SIR97 and SHELXL-97).41 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 Table S1 (see the
Supporting Information). Detailed crystallographic data (excluding
structure factors) are available as Supporting Information, as CIF files.
Typical Procedure for Polymerization of rac-Lactide. In a
typical experiment (Table 1, entry 2), in the glovebox, a Schlenk flask
was charged with a solution of 6 (11.0 mg, 14.7 μmol) in toluene
(0.728 mL). rac-Lactide (0.21 g, 1.47 mmol, 100 equiv vs Y) was
added rapidly to this solution. The mixture was immediately stirred
with a magnetic stir bar at 20 °C for the appropriate time. The reaction
was quenched by adding ca. 1 mL of a 10% H2O solution in THF, and
the polymer was precipitated from CH2Cl2/pentane (ca. 2 mL/100
mL) five times. The polymer was then filtered and dried in vacuo to
constant weight.
and P(C6H5)2), 7.13−7.07 ((CH3)2C6H3N and CH2(CH3)C6H3N),
3
7.03 (d, JH−H = 6.9, 2H, CH2(CH3)C6H3N), 7.00 (m, 2H,
3
(CH3)2C6H3N), 6.93 (t, JH−H = 7.5, 2H, (CH3)2C6H3N), 6.84 (d,
3
3JH−H = 7.7, 1H, (CH3)2C6H3N), 6.77 (d, JH−H = 7.4, 1H,
3
(CH3)2C6H3N), 7.73 (d, JH−H = 7.2, 1H, (CH3)2C6H3N), 6.63 (t,
3JH−H = 8.0, 1H, p-Py), 5.45 (d, 3JH−H = 8.0, 1H, m-Py), 4.81 (d, 3JH−H
= 8.0, 1H, m-Py), 4.24 (br sept, 3JH−H = 6.2, 1H, CH(CH3)2), 3.48 (br
3
2
sept, JH−H = 5.9, 1H, CH(CH3)2), 2.53 (d, JH−H = 5.7, 1H,
CHH(CH3)C6H3N), 2.43 (s, 3H, (CH3)2C6H3N), 2.41 (s, 3H,
(CH3)2C6H3N), 2.15 (s, 3H, CH2(CH3)C6H3N), 2.12 (s, 3H,
2
(CH3)2C6H3N), 1.99 (d, JH−H = 5.7, 1H, CHH(CH3)C6H3N), 1.81
(s, 3H, (CH3)2C6H3N), 1.40 (s, 3H, NC(CH3)N), 1.09 (d, JH−H
3
=
3
6.2, 3H, CH(CH3)2), 0.95 (d, JH−H = 6.2, 3H, CH(CH3)2), 0.92 (d,
3JH−H = 5.9, 3H, CH(CH3)2), 0.24 (d, JH−H = 5.9, 3H, CH(CH3)2).
3
13C{1H} NMR (125 MHz, C6D6, 298 K): δ 171.1 (d, JP−C = 55.8,
1
NC(PPh2)N), 167.8 (o-Py), 159.7 (NC(CH3)N), 149.7 (o-Py), 146.5
((CH3)2C6H3N), 141.8, 141.5, 139.0, 137.1, 136.5, 135.3, 135.2, 134.5,
2
2
134.3, 134.1, 133.9 (d, JP−C = 19.5, PPh2), 133.1 (d, JP−C = 19.5,
2
2
PPh2), 132.2 (d, JP−C = 16.5, PPh2), 131.8 (d, JP−C = 16.5, PPh2),
133.4, 133.2, 133.1(CH3)2C6H3N, 131.3, 129.4, 129.1, 128.9, 128.4
(CH2(CH3)C6H3N), 126.6, 125.8, 123.1, 119.8 ((CH3)2C6H3N),
1
100.1 (m-Py), 93.0 (m-Py), 48.9, 48.7 (CH(CH3)2), 47.0 (d, JY−C
=
24.3, CH2(CH3)C6H3N), 26.8, 25.7, 24.6, 24.5 (CH(CH3)2), 19.7
(NC(CH3)N), 19.4 (CH2(CH3)C6H3N), 19.2 ((CH3)2C6H3N), 19.0
((CH3)2C6H3N), 17.2 ((CH3)2C6H3N), 17.1 ((CH3)2C6H3N).
31P{1H} NMR (121 MHz, C6D6, 298 K): δ −19.5 (d, JY−P = 6.6, 1P).
[{NMe2NNMe2CMeNMe2}Y{(μ2-BH3)(μ2-NH)}]2 (6). Protocol A. In the
glovebox, a Teflon-valved NMR tube was charged with 2a′ (0.040 g,
0.056 mmol) and NH3BH3 (0.0017 g, 0.056 mmol). To this mixture,
toluene-d8 (0.6 mL) was vacuum-transferred in and the tube was
shaken for 15 min at room temperature. NMR indicated quantitative
conversion of the reagents to 6.
Protocol B. To a solution of 2a′ (0.200 g, 0.28 mmol) in toluene (3
mL) was added NH3·BH3 (0.0087 g, 0.28 mmol) in toluene (2 mL).
The crude product was precipitated from toluene to give pure 6 as
yellow crystals (0.69 g, 0.84 mmol, 85%). 1H NMR (500 MHz,
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving details of the
computations, representative NMR spectra of proligand 1 and
yttrium complexes, and crystallographic data for 1, 2a′, 6, and
7. This material is available free of charge via the Internet at
http://pubs.acs.org.
3
toluene-d8, 298 K): δ 7.09 (d, JH−H = 7.4, 2H, (CH3)2C6H3N), 6.90
3
(m, 3H, (CH3)2C6H3N), 6.77 (d, JH−H = 7.7, 3H, (CH3)2C6H3N),
3
3
6.58 (t, JH−H = 8.5, 1H, p-Py), 5.39 (d, JH−H = 8.2, 1H, m-Py), 4.71
3
(d, JH−H = 7.9, 1H, m-Py), 2.44 (s, 6H, (CH3)2C6H3N), 2.26 (s, 6H,
(CH3)2C6H3N), 1.92 (s, 6H, (CH3)2C6H3N), 1.08 (s, 3H, NC(CH3)-
N), 0.20 (br m, 2H, BH), 0.12 (br m, 2H, BH). Due to the very low
solubility of 6 in toluene-d8 and THF-d8, no informative 13C NMR
spectrum could be recorded. Anal. Calcd for C38H45N5BY: C, 67.97;
H, 6.75; N, 10.43; Y, 13.24. Found: C, 67.78; H, 6.86; N, 10.47; Y,
13.59.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
1525
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
Chem. Soc., Chem. Commun. 1983, 276−277. (e) Mu, Y.; Piers, W. E.;
MacQuarrie, D. C.; Zaworotko, M. J.; Young, V. G. Organometallics
1996, 15, 2720−2726. (f) Duchateau, R.; van Wee, C. T.; Teuben, J.
H. Organometallics 1996, 15, 2291−2302. (g) Booij, M.; Kiers, N. H.;
Meetsma, A.; Teuben, J. H.; Smeets, W. J. J.; Spek, A. L.
Organometallics 1989, 8, 2454−2461. (h) Thompson, M. E.; Baxter,
S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.;
Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203−219.
(i) den Haan, K. H.; Wiestra, Y.; Teuben, J. H. Organometallics 1987,
6, 2053−2060. (j) Watson, P. L.; Parshall, G. B. Acc. Chem. Res. 1985,
18, 51−56. (k) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491−6493.
(16) For examples of C−H bond activation reactions involving Cp-
free rare-earth complexes, see: (a) Sigiyama, H.; Gambarotta, S.; Yap,
G. P. A.; Wilson, D. R.; Thiele, S. K.-H. Organometallics 2004, 23,
5054−5061. (b) Emslie, D. J. H.; Piers, W. E.; Parvez, M. Dalton
Trans. 2003, 2615−2620. (c) Emslie, D. J. H.; Piers, W. E.; Parvez, M.;
McDonald, R. Organometallics 2002, 21, 4226−4240. (d) Duchateau,
R.; Tuinstra, T.; Brussee, E. A. C.; Meetsma, A.; Van Duijnen, P. T.;
Teuben, J. H. Organometallics 1997, 16, 3511−3522. (e) Duchateau,
R.; Van Wee, C. T.; Meetsma, A.; Teuben, J. H. J. Am. Chem. Soc.
1993, 115, 4931−4932. (f) Fryzuk, M. D.; Haddad, T. S.; Rettig, S. J.
Organometallics 1991, 10, 2026−2036.
ACKNOWLEDGMENTS
■
This work was financially supported by the GDRI ‘‘CH2D’’
between RAS Chemistry and Material Science Division and
CNRS Institute of Chemistry, the Russian Foundation for Basic
Research (Grant 12-03-93109). V.R. thanks the RAS and the
French Embassy in Moscow for a Ph.D. fellowship. We thank
Prof. Abdou Boucekkine (UR1) for helpful discussions.
REFERENCES
■
(1) For reviews see: (a) Gromada, J.; Carpentier, J.-F.; Mortreux, A.
Coord. Chem. Rev. 2004, 248, 397−410. (b) Gibson, V. C.;
Spitzmesser, S. K. Chem. Rev. 2003, 103, 283−315.
(2) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169−1203. (b) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem.
Rev. 2007, 107, 1745−1776.
(3) Budagumpia, S.; Kimb, K.-H.; Kim, I. Coord. Chem. Rev. 2011,
255, 2785−2809.
(4) (a) Hamaki, H.; Takeda, N.; Nabika, M.; Tokitoh, N.
Macromolecules 2012, 45, 1758−1769. (b) Li, D.; Li, S.; Cui, D.;
Zhang, X. Organometallics 2010, 29, 2186−2193. (c) Hayes, P. G.;
Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002, 124, 2132−2133.
(d) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002,
102, 3031−3065.
(17) For recent reports on C−H bond activation reactions with
yttrium complexes, see: (a) Lu, E.; Gan, W.; Chen, Y. Dalton Trans.
2011, 40, 2366−2374. (b) Luconi, L.; Lyubov, D. M.; Bianchini, C.;
Rossin, A.; Faggi, C.; Fukin, G. K.; Cherkasov, A. V.; Shavyrin, A. S.;
Trifonov, A. A.; Giambastiani, G. Eur. J. Inorg. Chem. 2010, 608−620.
(c) Lyubov, D. M.; Fukin, G. K.; Cherkasov, A. V.; Shavyrin, A. S.;
Trifonov, A. A.; Luconi, L.; Bianchini, C.; Meli, A.; Giambastiani, G.
Organometallics 2009, 28, 1227−1232. (d) Skvortsov, G. G.; Fukin, G.
(5) Hasumi, S.; Itagaki, K.; Zhang, S.; Nomura, K. Macromolecules
2011, 44, 773−777 and references cited therein.
(6) Chen, C.; Lee, H.; Jordan, R. F. Organometallics 2010, 29, 5373−
5381 and references cited therein.
(7) (a) Epshteyn, A.; Trunkley, E. F.; Kissounko, D. A.; Fettinger, J.
C.; Sita, L. R. Organometallics 2009, 28, 2520−2526 and references
cited therein. (b) Volkis, V.; Lisovskii, A.; Tumanskii, B.; Shuster, M.;
Eisen, M. S. Organometallics 2006, 25, 2656−2666 and references cited
therein. (c) Collins, S. Coord. Chem. Rev. 2011, 255, 118−138.
(8) (a) Haas, I.; Kretschmer, W. P.; Kempe, R. Organometallics 2011,
30, 4854−4861. (b) Hafee, M.; Kretschmer, W. P.; Kempe, R. Eur. J.
Inorg. Chem. 2011, 36, 5512−5522. (c) Noor, A.; Kretschmer, W. P.;
Glatz, G.; Meetsma, A.; Kempe, R. Eur. J. Inorg. Chem. 2008, 32,
5088−5098.
(9) (a) Luconi, L.; Rossin, A.; Tuci, G.; Tritto, I.; Boggioni, L.;
Klosin, J. J.; Theriault, C. N.; Giambastiani, G. Chem. Eur. J. 2012, 18,
681−687 and references cited therein. (b) Zuccaccia, C.; Busico, V.;
Cipullo, R.; Talarico, G.; Froese, R. D. G.; Vosejpka, P. C.; Hustad, P.
D.; Macchioni, A. Organometallics 2009, 28, 5445−5458 and
references cited therein..
K.; Trifonov, A. A.; Noor, A.; Doring, C.; Kempe, R. Organometallics
̈
2007, 26, 5770−5773.
(18) (a) Wang, D.; Cui, D.; Miao, M.; Huang, B. Dalton Trans. 2007,
4576−4581. (b) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.;
Chen, X.; Jing, X. Organometallics 2007, 26, 2747−2757. (c) Cai, C.-
X.; Toupet, L.; Lehmann, C. W.; Carpentier, J.-F. J. Organomet. Chem.
2003, 683, 131−136. (d) Bambirra, S.; Meetsma, A.; Hessen, B.;
Teuben, J. H. Organometallics 2001, 20, 782−785.
(19) Lyubov, D. M.; Doring, C.; Fukin, G. K.; Cherkasov, A. V.;
̈
Shavyrin, A. S.; Kempe, R.; Trifonov, A. A. Organometallics 2008, 27,
2905−2907.
(20) Such an unusual situation is apparently caused by formation of
the Y−C bond, resulting in redistribution of the 2− negative charge
over the entire {NMe2NNMe2CMeNMeCH2}2− skeleton.
(21) For comparison, see: (a) Edelmann, F. T. Chem. Soc. Rev. 2009,
38, 2253−2268. (b) Edelmann, F. T. Adv. Organomet. Chem. 2008, 57,
183−352. (c) Coles, M. P. Dalton Trans. 2006, 985−1001.
(d) Edelmann, F. T. Coord. Chem. Rev. 1994, 137, 403−481.
(e) Barker, J.; Kilner, M. Coord. Chem. Rev. 1994, 133, 219−300.
(22) The planarity of the N(3) atom is most likely due to the steric
crowding provided by the bulky aryl substituents.
(23) These include donor−acceptor interactions between the core
electrons of electron-donating carbon atoms C(12) and C(13) and the
accepting LP* (one-center valence antibonding lone pair) and RY*
(one-center Rydberg) orbitals of yttrium.
(10) Zimmermann, M.; Tornroos, K. W.; Waymouth, R. M.;
Anwander, R. Organometallics 2004, 27, 4310−4317 and references
cited therein.
(11) (a) Huang, Y.-L.; Lu, D.-Y.; Yu, H.-C.; Yu, J.-S. K.; Hsu, C.-W.;
Kuo, T.-S.; Lee, G.-H.; Tsai, Y.-C. Angew. Chem., Int. Ed. 2012, 51,
7781−7785. (b) Cotton, F. A.; Daniels, L. M.; Alei, P.; Murillo, C. A.;
Wang, X. Inorg. Chem. 2001, 40, 2778−2784. (c) Polamo, M. Acta
Crystallogr. 1996, C52, 2980−2982.
(12) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1−S19.
(13) For recent references on rare-earth amidinate chemistry, see:
(a) Bambirra, S.; Perazzolo, F.; Boot, S. J.; Sciarone, T. J. J.; Meetsma,
A.; Hessen, B. Organometallics 2008, 27, 704−712. (b) Bambirra, S.;
Meetsma, A.; Hessen, B. Organometallics 2006, 25, 3454−3462.
(c) Bambirra, S.; Bouwkamp, M. W.; Meetsma, A.; Hessen, B. J. Am.
Chem. Soc. 2004, 126, 9182−9183. (d) Harder, S. Dalton Trans. 2010,
39, 6677−6681. (e) Bambirra, S.; Van Leusen, D.; Meetsma, A.;
Hessen, B.; Teuben, J. H. Chem. Commun. 2003, 522−523.
(14) Sinenkov, M.; Kirillov, E.; Roisnel, T.; Fukin, G.; Trifonov, A.
A.; Carpentier, J.-F. Organometallics 2011, 30, 5509−5523.
(15) For examples of C−H bond activation reactions involving Cp
rare-earth complexes, see: (a) Evans, W. J.; Champagne, T. M.; Ziller,
J. W. J. Am. Chem. Soc. 2006, 128, 14270−14271. (b) Evans, W. J.;
Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 1068−1069.
(c) Okuda, J. Dalton Trans. 2003, 2367−2378. (d) Watson, P. L. J.
(24) Amidopyridinate complexes of yttrium, incorporating
CH2SiMe3 and another σ-bonded (benzyl, aryl, and heteroaryl)
pendant group, exhibited different reactivities toward PhSiH3 and
H2.17d
(25)
A similar reaction between 2a′ and 1 equiv of
[Ph3C]+[B(C6F5)4]−, monitored by NMR spectroscopy in the −50
to 0 °C temperature range in toluene-d8, afforded unidentified
products.
(26) Intramolecular (ortho aryl)C−F···Zr interactions in zwitterionic
metallocene [C4H6−B(C6F5)3] complexes have been identified by 19F
NMR spectroscopy in solution from unusually strong upfield shifts of
the signals for the corresponding F groups (−208 to −212 ppm). See:
Erker, G. Acc. Chem. Res. 2001, 34, 309−317 and references cited
therein.
1526
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527

Organometallics
Article
(27) The global outcome of this protonolysis reaction does not
contradict the aforementioned orbital control rule if the reaction is
assumed to proceed stepwise: i.e., (i) protolytic cleavage of the Y(1)−
C(“benzyl”) bond in 2a′ and formation of the monoalkyl complex and
(ii) intramolecular C−H bond activation in this intermediate, resulting
in release of the final product by re-forming the Y(1)−C(“benzyl”)
bond with concomitant elimination of SiMe4. Formation of 4 via such
a stepwise process or by direct protonolysis of the CH2SiMe3 group in
2a′ still remains an open question.
Crystal Structures; University of Gottingen, Gottingen, Germany, 1997.
̈ ̈
(c) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(28) (a) Konkol, M.; Okuda, J. Coord. Chem. Rev. 2008, 252, 1577−
1591. (b) Trifonov, A. A. Russ. Chem. Rev. 2007, 76, 1051−1072.
(c) Ephritikhine, M. Chem. Rev. 1997, 97, 2193−2242. (d) Nishiura,
M.; Hou, Z. Nat. Chem. 2010, 2, 257−268. (e) Hou, Z.; Nishiura, M.;
Shima, T. Eur. J. Inorg. Chem. 2007, 2535−2545. (f) Ephritikhine, M.
Chem. Rev. 1997, 97, 2193−2242.
(29) (a) Bellham, P.; Hill, M. S.; Kociok-Kohn, G.; Liptrot, D. J.
Dalton Trans. 2013, 42, 737−745. (b) Hill, M. S.; Hodgson, M.;
Liptrot, D. J.; Mahon, M. F. Dalton Trans. 2011, 40, 7783−7790.
(c) Liptrot, D. J.; Hill, M. S.; Mahon, M. F.; McDougall, D. J. Chem.
Eur. J. 2010, 16, 8508−8515. (d) Spielmann, J.; Harder, S. J. Am.
Chem. Soc. 2009, 131, 5064−5065. (e) Spielmann, J.; Bolte, M.;
Harder, S. Chem. Commun. 2009, 6934−6936.
(30) The analogous reaction of 2a′ with tBuNH2·BH3 in toluene was
essentially not selective, giving an intractable mixture of unidentified
products.
(31) (a) Demir, S.; Siladke, N. A.; Ziller, J. W.; Evans, W. J. Dalton
Trans. 2012, 41, 9659−9666. (b) Jenter, J.; Roesky, P. W.; Ajellal, N.;
Guillaume, S. M.; Susperregui, N..; Maron, L. Chem. Eur. J. 2010, 16,
4629−4638.
(32) It is worth noting that cleavage of the THF molecule, which
could serve as a source of oxygen, would be highly unlikely. Although
the origin of the oxygen atom in complex 7 could not be conclusively
identified, it seems that its likely source is when both toluene and
hexane solvents were vacuum-transferred onto solid 2a′ in order to
eventually recrystallize it (see the Experimental Section), which
apparently contaminated the medium with trace amounts of
adventitious oxygen. At the same time, attempts to obtain selectively
complex 7 by deliberately introducing small amounts of pure oxygen in
the toluene solution of 2a′ during the recrystallization step resulted in
the formation of intractable mixtures.
(33) For leading reviews on the ROP of cyclic esters with complexes
of group 3 metals, see: (a) Dechy-Cabaret, O.; Martin-Vaca, B.;
Bourissou, D. Chem. Rev. 2004, 104, 6147−6176. (b) Wu, J.; Yu, T.-L.;
Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 205, 602. (c) Thomas,
C. M. Chem. Soc. Rev. 2010, 39, 165. (d) Buffet, J.-C.; Okuda, J. Polym.
Chem. 2011, 2, 2758.
(34) The ROP of lactide, initiated by the Y−NHBH3 functions and
followed by acidic workup, should afford PLAs bearing −CH(CH3)
OH and −C(O)NH2 end groups. Only signals from −CH(CH3)
OH termini were unambiguously identified in the 1H NMR spectra of
polymers prepared with complex 6 (Table 1, entries 1 and 2).
(35) (a) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39−48.
(b) Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S.; Helou,
M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363−
8376.
(36) (a) Hill, M. S.; Hitchcock, P. B.; Mansell, S. M. Dalton Trans.
2006, 1544−1553. (b) Whitener, G. D.; Hagadorn, J. R.; Arnold, J. J.
Chem. Soc., Dalton Trans. 1999, 1249−1255.
(37) Lewis, H. L.; Brown, T. L. J. Am. Chem. Soc. 1970, 92, 4664−
4670.
(38) (a) Lappert, M. F.; Pearce, R. J. J. Chem. Soc., Chem. Commun.
1973, 126−127. (b) Schumann, H.; Freckmann, D. M.; Dechert, S. Z.
Z. Anorg. Allg. Chem. 2002, 628, 2422−2426.
(39) Harder, S. Organometallics 2005, 24, 373−379.
(40) Faust, A.; Wolff, O.; Waldvogel, S. R. Synthesis 2009, 155−159.
(41) (a) Sheldrick, G. M. SHELXS-97, Program for the Determination
of Crystal Structures; University of Gottingen, Gottingen, Germany,
̈
̈
1997. (b) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
1527
dx.doi.org/10.1021/om400027d | Organometallics 2013, 32, 1517−1527