Monoalkyllanthanide complexes with new β-diketiminato derivative dianionic ligand

Article abstract of DOI:10.1021/om900040r

A new class ofβ-diketiminato derivative dianionic ligands was designed, and three ligand precursors [CH₃C(ArNH)CHC(CH ₃)(NCH₂CH₂-NHR)] (Ar = 2,6-( ⁱPr)₂C₆H₃; R =

Full text of DOI:10.1021/om900040r

2318
Organometallics 2009, 28, 2318–2324
Monoalkyllanthanide Complexes with New ꢀ-Diketiminato
Derivative Dianionic Ligands
Erli Lu,† Wei Gan,†,‡ and Yaofeng Chen*,†
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 354 Fenglin Road, Shanghai 200032, People’s Republic of China, and Department
of Chemistry, Northwest UniVersity, 229 Taibai North AVenue, Xi’an 710069, People’s Republic of China
ReceiVed January 18, 2009
A new class of ꢀ-diketiminato derivative dianionic ligands was designed, and three ligand precursors
t
[CH3C(ArNH)CHC(CH3)(NCH2CH2-NHR)] (Ar ) 2,6-(iPr)2C6H3; R ) Bu (H2L1), 2,6-(CH3)2C6H3
(H2L2), 2,6-(iPr)2C6H3 (H2L3)) were synthesized. The alkane elimination reactions between these ligand
precursors and Ln(CH2SiMe3)3(THF)n provided eight five-coordinate monoalkyllanthanide complexes,
in which the ligand serves as a tridentate dianionic donor, with one -CH2SiMe3 and one THF molecule
completing the five-coordinate center. These monoalkyl complexes exhibited low to very high catalytic
activities for intramolecular hydroamination of 2,2-dimethyl-1-aminopent-4-ene. The catalytic activity
increased with increasing metal ion size. For the Nd complex, 98% yield was obtained in 1 h at 60 °C
with 0.5 mol % catalyst loading.
Chart 1
Introduction
Due to their rich and diversified coordinating properties and
reactivities, organolanthanide complexes have received growing
attention.1-3 The most widely investigated organolanthanide
complexes are those bearing Cp-type ligands. Recently, there
is a tendency to explore “non-Cp” organolanthanide complexes.4
Among the “non-Cp” type ligands, ꢀ-diketiminato ligands are
one of the most promising ligand families. The precursors for
these ligands can be readily prepared by condensation of
ꢀ-diketones with amines. The steric and electronic properties
of these ligands can be easily tuned by an appropriate choice
of ꢀ-diketone and amine, and coordination to a lanthanide ion
can range from purely σ to a combination of σ and π donation.5
Numerous ꢀ-diketiminato lanthanide complexes have been
prepared, and some of them show good catalytic activities in
organic and polymer synthesis.6-9
For the purpose of stabilizing the reactive complexes with
larger size lanthanide ions, some modifications on ꢀ-diketimi-
nato ligands were made. Roesky and co-workers developed a
ꢀ-diketiminato derivative A (Chart 1), which contains two
dangling arms with nitrogen donors incorporated. With the
tetradentate monoanionic ligand derived from A, they prepared
a series of lanthanide complexes.10 On considering both steric
and electronic features of the ligand, we had designed tridentate
monoanionic NNN ligands derived from B and successfully
prepared highly reactive organolanthanide dialkyl complexes.11
For the organolanthanide complexes, the chelating dianionic
ligands are of great importance, as they are capable of creating
* To whom correspondence should be addressed. E-mail: yaofchen@
mail.sioc.ac.cn. Fax: (+86)21-64166128.
† Shanghai Institute of Organic Chemistry.
‡ Northwest University.
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10.1021/om900040r CCC: $40.75
2009 American Chemical Society
Publication on Web 03/13/2009

Monoalkyllanthanide Complexes with ꢀ-Diketiminato Ligands
Organometallics, Vol. 28, No. 7, 2009 2319
Scheme 1
rigid supporting environments for steric control during the
catalytic reactions, and the corresponding monoalkyl complexes
can provide the sole initiating group. Therefore, our rationale
began with our previous ligand precursors B; replacing the
tertiary amino groups by the secondary amino groups resulted
in ligand precursors C, which, upon deprotonation, are antici-
pated to yield dianionic ligands D. Herein we report the
preparation of ligand precursors C, the preparation and structures
of monoalkyllanthanide complexes containing dianionic ligands
D, and their catalytic activities in the intramolecular hydroami-
nation of 2,2-dimethyl-1-aminopent-4-ene.
H2L1 (1), H2L2 (2), and H2L3 (3) in 62%-74% yield (Scheme
1). These compounds were characterized by NMR (1H and 13C)
and HRMS spectroscopy.
The alkane elimination method was applied for the synthesis
of desired monoalkyllanthanide complexes. The reaction be-
tween H2L1 and Y(CH2SiMe3)3(THF)2 in hexane under ambient
temperature gave a complicated mixture. Thus, the reaction was
carried out at -35 °C, from which some colorless crystals
covered by red viscous oil were obtained. The crystals and the
oil were both readily soluble in hexane and toluene, and attempts
to obtain pure product from the mixture failed. Finally, the oil
was carefully removed by grease, and the crystal structure was
determined by single-crystal X-ray diffraction. It was found that
the product is the monoalkyl complex 4 (Figure 1). As the
reaction between H2L1 and Y(CH2SiMe3)3(THF)2 was compli-
cated, we turned to H2L2, which has a bulky aryl substituent
on the nitrogen atom of the pendant arm. The reactions between
H2L2 and Ln(CH2SiMe3)3(THF)2 (Ln ) Sc, Y, Lu) in hexane
proceeded well and gave the desired monoalkyl complexes
L2Ln(CH2SiMe3)(THF) (Ln ) Sc (5), Y (6), Lu (7)) in
71%-90% yields (Scheme 1). When large lanthanide ions
(Dy3+, Gd3+, Nd3+) were introduced, some modifications to the
previous procedure were adopted. Ln(CH2SiMe3)3(THF)n (Ln
) Gd, Dy, Nd) were produced by reactions of LnCl3(THF)n
with LiCH2SiMe3,15 and the in situ generated trialkyl complexes
Results and Discussion
Synthesis and Characterization. 2-((2,6-Diisopropylphe-
nyl)imido)-2-penten-4-one was prepared by condensation of
acetylacetone with 2,6-diisopropylaniline.12 This product was
subsequently treated with diamines (N1-tetrabutylethane-1,2-
diamine,13 N1-(2,6-dimethylphenyl)ethane-1,2-diamine, and N1-
(2,6-diisopropylphenyl)ethane-1,2-diamine14) in toluene, with
the presence of a catalytic amount of sulfuric acid or p-
toluenesulfonic acid, to provide the desired ligand precursors
(9) (a) Cui, C. M.; Shafir, A.; Schmidt, J. A. R.; Oliver, A. G.; Arnold,
J. Dalton Trans. 2005, 1387. (b) Sa´nchez-Barba, L. F.; Hughes, D. L.;
Humphrey, S. M.; Bochmann, M. Organometallics 2005, 24, 3792. (c)
Vitanova, D. V.; Hampel, F.; Hultzsch, K. C. Dalton Trans. 2005, 1565.
(d) Liu, X. L.; Shang, X. M.; Tang, T.; Hu, N. H.; Pei, F. K.; Cui, D. M.;
Chen, X. S.; Jing, X. B. Organometallics 2007, 26, 2747.
(10) (a) Neculai, D.; Roesky, H. W.; Neculai, A. M.; Magull, J.; Schmidt,
H. G.; Noltemeyer, M. J. Organomet. Chem. 2002, 643, 47. (b) Neculai,
D.; Roesky, H. W.; Neculai, A. M.; Magull, J.; Herbst-Irmer, R.; Walfort,
B.; Stalke, D. Organometallics 2003, 22, 2279. (c) Nikiforov, G. B.; Roesky,
H. W.; Labahn, T.; Vidovic, D.; Neculai, D. Eur. J. Inorg. Chem. 2003,
433. (d) Neculai, A. M.; Neculai, D.; Roesky, H. W.; Magull, J.; Baldus,
M.; Andronesi, O.; Jansen, M. Organometallics 2002, 21, 2590. (e) Neculai,
A. M.; Neculai, D.; Roesky, H. W.; Magull, J. Polyhedron 2004, 23, 183.
(11) Xu, X.; Xu, X. Y.; Chen, Y. F.; Sun, J. Organometallics 2008, 27,
758.
(12) He, X. H.; Yao, Y. Z.; Luo, X.; Zhang, J.; Liu, Y.; Zhang, L.; Wu,
Q. Organometallics 2003, 22, 4952.
(13) Assef, G.; Kister, J.; Metzger, J. Bull. Soc. Chim. Fr. 1979, 3-4,
172.
(14) Marshall, C.; Ward, M. F.; Skakle, J. M. S. Synthesis 2006, 6, 1040.
(15) (a) Luo, Y. J.; Nishiura, M.; Hou, Z. M. J. Organomet. Chem. 2007,
692, 536. (b) Arndt, S.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed.
2003, 42, 5075. (c) Vollershtein, E. L.; Yakovlev, V. A.; Tinyakova, E. I.;
Dolgoplosk, B. A. Dokl. Acad. Nauk SSSR 1980, 250, 365. (d) Schumann,
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2422.
Figure 1. Molecular structure of 4 with thermal ellipsoids at the
30% probability level.

2320 Organometallics, Vol. 28, No. 7, 2009
Lu et al.
Table 1. Selected Bonds Lengths (Å) and Angles (deg) for 4, 6, and
11
4 (Ln ) Y)
6 (Ln ) Y)
11 (Ln ) Nd)
Ln-N1
2.377(3)
2.317(3)
2.212(3)
2.377(3)
2.444(2)
1.338(4)
1.385(4)
1.402(5)
1.310(4)
0.911(4)
76.07(9)
133.51(10)
74.96(10)
92.69(11)
124.4(3)
128.0(3)
123.0(3)
126.3(2)
127.9(2)
2.350(4)
2.309(4)
2.258(4)
2.411(5)
2.379(3)
1.323(6)
1.399(7)
1.407(7)
1.318(6)
0.645(8)
76.93(15)
128.67(14)
74.65(15)
100.93(15)
123.4(5)
130.0(5)
121.4(5)
129.4(3)
131.7(3)
2.397(3)
2.451(3)
2.274(3)
2.474(4)
2.497(3)
1.343(5)
1.377(5)
1.400(6)
1.312(5)
0.792(8)
72.98(11)
133.66(12)
71.72(12)
95.18(12)
122.2(4)
129.4(4)
123.6(4)
132.7(3)
128.7(3)
Ln-N2
Ln-N3
Ln-C51
Ln-O
N1-C2
C2-C3
C3-C4
C4-N2
N2C3 plane-Ln
N1-Ln-N2
N1-Ln-N3
N2-Ln-N3
C51-Ln-O1
N1-C2-C3
C2-C3-C4
C3-C4-N2
Ln-N1-C2
Ln-N2-C4
Figure 2. Molecular structure of 6 with thermal ellipsoids at the
30% probability level.
center is completed by one -CH2SiMe3 and one THF molecule.
The geometry at the metal center is best described as a distorted
square pyramid, with -CH2SiMe3 taking the apical position.
The ꢀ-diketiminato backbone is bonded to the metal ion through
two Ln-N bonds, the length of which varies from 2.31 to 2.45
Å, falling in the range 2.04-2.49 Å observed for Ln-N bonds
in other ꢀ-diketiminato lanthanide complexes.6-9 The Ln-N3
bond lengths of the pendant arm in complexes 4, 6, and 11
(2.21-2.27 Å) are significantly shorter than their counterparts
in our previously reported dialkyl complexes (2.50 to 2.65 Å);11
this is in accordance with the anionic nature of N3 in 4, 6, and
11. The C-N and C-C bond lengths of the ꢀ-diketiminato
backbone are between those of typical single and double bonds,
and N1, C2, C3, C4, and N2 atoms are coplanar, indicating
that there is a delocalized electronic structure. The metal ions
sit above the C3N2 plane (C3N2 plane-Ln ) 0.65 to 0.91 Å);
the Ln-C2, Ln-C3, and Ln-C4 distances (>3.3 Å) are too
long for effective interaction. Thus, the bonding mode of the
ꢀ-diketiminato backbone is best described as a 2σ electron
donor. The alkyl ligand (-CH2SiMe3) is coordinated with Ln-C
bond lengths of 2.377(3) Å (4), 2.411(5) Å (6), and 2.474(4) Å
(11). The spatial arrangement of the two aromatic rings and
the THF molecule in 6 and 11 is interesting (Figure 4). The
THF is bound by two phenyl rings, to adopt a sandwich-like
arrangement. In the Nd complex 11, the THF between the two
phenyl rings has a larger inclination than that in 6; this can be
attributed to the larger radii of the Nd3+ ion.
Figure 3. Molecular structure of 11 with thermal ellipsoids at the
30% probability level.
Figure 4. Space-filling views of 6 and 11.
1
The H NMR spectra of the diamagnetic complexes 5, 6, 7,
in hexane or THF were subsequently added to the ligand.
However, repeated attempts to synthesize monoalkyl complexes
with Dy3+, Gd3+, and Nd3+ ions gave only complicated mixtures
either at room temperature or at -35 °C. When H2L3, which
has the more bulky 2,6-diisopropylphenyl on the pendant arm,
was introduced, first L3Y(CH2SiMe3)(THF) (8) was synthesized
in 59% yield by the reaction of Y(CH2SiMe3)3(THF)2 with H2L3
in hexane. In addition, L3Ln(CH2SiMe3)(THF) (Ln ) Dy (9),
Gd (10), Nd (11)) were obtained by reactions between
LnCl3(THF)n, LiCH2SiMe3, and H2L3 in hexane or THF at 0
°C (Scheme 1).
Single crystals of complexes 6 and 11 were grown from
hexane solutions at -35 °C and characterized by X-ray
diffraction. ORTEP diagrams are shown in Figures 2 and 3,
and selected bond lengths and angles are given in Table 1.
Complexes 4, 6, and 11 all are five-coordinate monomers. L1,
L2, and L3 serve as tridentate ligands, and the five-coordinate
and 8 in C6D6 at 25 °C showed AB systems for the Ln-CH2
methylene protons, which indicated a Cs-symmetric structure
in solution. In complex 5, the two methyl groups on the phenyl
ring are diastereotopic and display two distinct singlets (Figure
5, a), revealing a large rotation barrier of the N3-CAr bond.
Increasing the temperature from 25 to 80 °C results in
broadening and coalescence of the ArCH3 resonances followed
by sharpening of the resulting coalesced signal (Figure 5). The
coalescence temperature (ca. 335 K) and the ∆δ for the
1
individual H NMR resonances due to ArCH3 groups (ca. 160
Hz) have been used to estimate a ∆Gq value of ca. 66.0 kJ/mol
for the rotation of the N-CAr bond.16 The THF of 5 has some
interesting 1H NMR features: the R-H’s appeared as four
multiplets in the range 3.05 to 4.25 ppm,17 and the ꢀ-H’s
appeared as two significantly upfield shifted broad peaks at 0.68
and 0.58 ppm, respectively (Figure 5, a). Therefore, the THF is
strongly bonded and the rotation restricted. Upon increasing the

Monoalkyllanthanide Complexes with ꢀ-Diketiminato Ligands
Organometallics, Vol. 28, No. 7, 2009 2321
1
Figure 5. Variable-temperature H NMR spectra for 5 (400 MHz, C6D6).
Table 2. Hydroamination of 2,2-Dimethylpent-4-ene-1-amine
Catalyzed by 5-11a
entry
catalyst
[cat.]/[sub.] (%)
time (h)
yield (%)b
1
2
3
4
5
6
7
8
5
6
7
8
1
1
1
1
1
1
1
0.5
24.0
4.0
12.0
2.0
1.0
1.0
<5
98
96
97
97
98
98
98
9
Figure 6. Comparison of ArCH3 signals between 5(L2Sc), 6(L2Y),
and 7(L2Lu) (300 MHz, C6D6, 25 °C).
10
11
11
0.5
1.0
temperature, the resonances due to R-H’s broaden, and those
for ꢀ-H’s coalesced and shifted downfield. The 1H NMR spectra
of other diamagnetic complexes (6, 7, and 8) at 25 °C showed
similar behavior for the THF. The restricted rotation of THF
observed in solution is in agreement with the arrangement of
THF in the solid state, where the THF is bound by two phenyl
rings. It is noteworthy that the size of the central metal ion18
has a significant influence on the rotation barrier of the N-CAr
bond; 1H NMR resonances for the methyl groups on the phenyl
ring at 25 °C varied from two singlets in the Sc complex 5 to
one broad singlet near coalescence in the Lu complex 7 and to
one singlet in the Y complex 6 (Figure 6).
Catalytic Behaviors for Intramolecular Hydroamination.
Intramolecular hydroamination offers an efficient and atom-
economical method to construct nitrogen heterocycles that are
important for fine chemicals and pharmaceuticals. Various metal
complexes, including those of alkali metals, early transition
metals, and late transition metals, have been investigated for
this transformation; the lanthanide complexes are among the
most promising.2,7f,19 The catalytic behaviors of lanthanide
complexes 5-11 for intramolecular hydroamination were briefly
tested by employing 2,2-dimethyl-1- aminopent-4-ene as sub-
a 10 mmol L-1 [cat.], 60 °C, C6D6 as the solvent. b NMR yield
determined relative to p-xylene internal standard.
strate (Table 2). A solution of the lanthanide complex and
substrate in C6D6 was loaded into a NMR tube, and the reaction
process was monitored by 1H NMR spectroscopy. The 1H NMR
spectrum of the mixture showed a rapid protonolysis of the
lanthanide alkyl by amine with a release of SiMe4, and the
pendant group remains coordinated to the metal. A clean
formation of 2,4,4-trimethylpyrrolidine, the Markovnikov-
selective product, was observed in several minutes, and no traces
of other heterocyclic regioisomers were detected through the
proceeding in all cases. Catalytic activity increased with
increasing ionic radii, as observed in other lanthanide complex-
catalyzed intramolecular hydroaminations.20 Complex 5, which
with the smallest Sc3+ ion, gave less than 5% yield in 24 h.
When Lu complex 7 was used, the reaction was nearly complete
(19) (a) Gagne, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 4108.
(b) Molander, G. A.; Dowdy, E. D. J. Org. Chem. 1998, 63, 8983. (c)
Molander, G. A.; Dowdy, E. D. J. Org. Chem. 1999, 64, 6515. (d) Burgstein,
M. R.; Berberich, H.; Roesky, P. W. Organometallics. 1998, 17, 1452. (e)
Rastatter, M.; Zulys, A.; Roesky, P. W. Chem.-Eur. J. 2007, 13, 3606. (f)
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(16) ∆Gq ) aTc(9.972 + log(Tc/∆δ)), a is a constant of 1.914 × 10-2
kJ/mol, Tc (K) is the coalescence temperature, and ∆δ (Hz) is the frequency
difference of the coalescing signals. Sandstro¨m, J. Dynamic NMR Spec-
troscopy; Academic Press Inc.: London, 1982; Chapter 7.
(17) 4.24 (1H), 4.04 (1H), 3.60 (1H, overlapped with ArCHMe2), 3.06
(1H).
(18) Six-coordinate ionic radii: Sc3+ (0.745 Å), Lu3+ (0.861 Å), Y3+
(0.900 Å), Dy3+ (0.912 Å), Gd3+ (0.938 Å), Nd3+ (0.983 Å). Shannon,
R. D. Acta Crystallogr., Sect. A 1976, A32, 751.

2322 Organometallics, Vol. 28, No. 7, 2009
Lu et al.
for organolanthanide complexes. The precursors for these
ligands can be readily prepared by condensation of ketoarylimi-
nato with diamines, and the alkane elimination reactions between
these ligand precursors and Ln(CH2SiMe3)3(THF)n gave five-
coordinate monoalkyllanthanide complexes. The substituent on
the nitrogen atom of the pendant arm has a significant influence
on the synthesis of monoalkyllanthanide complexes. With a
bulky 2,6-iPr2-C6H3 substituent, complexes of larger size lan-
thanide ions, such as Nd3+ and Gd3+, were accessible. These
monoalkyl complexes are active for intramolecular hydroami-
nation of 2,2-dimethyl-1-aminopent-4-ene, and the catalytic
activity increases with increasing size of the metal ion. Kinetic
studies of the reaction showed the rate to be zero-order in
substrate and first-order in catalyst. Future studies will strive
to prepare the related ligand precusors with chiral pendent arms
and corresponding metal complexes.
Figure 7. Concentration of substrate versus time for the intramo-
lecular hydroamination catalyzed by 8 ([cat.] ) 40 mmol L-1) at
25 °C.
Experimental Section
General Procedures. All operations were carried out under an
atmosphere of nitrogen using standard Schlenk techniques or in a
nitrogen-filled glovebox. THF was distilled from Na-benzophe-
noneketyl and degassed by freeze-thaw-vacuum prior to use.
Toluene, hexane, and C6D6 were dried over Na/K alloy, distilled
under vacuum, and stored in the glovebox. 2,6-Diisopropylaniline,
2,6-dimethylaniline, 2-aminoethyl chloride hydrochloride, and 2,4-
pentanedione were purchased from Aldrich and used without further
purification. 2-((2,6-Diisopropylphenyl)imido)-2-penten-4-one,12 N1-
tetrabutylethane-1,2-diamine,13 N1-(2,6-dimethylphenyl)ethane-1,2-
diamine, and N1-(2,6-diisopropylphenyl)ethane-1,2-diamine14 were
synthesized according to literature procedures. LiCH2SiMe321 and
Ln(CH2SiMe3)3(THF)2 (Ln ) Sc, Y, Lu)22 were synthesized
following the standard procedures. 2,2-Dimethyl-1-aminopent-4-
ene was synthesized following the literature procedure,20,23 dried
Figure 8. Observed reaction rate versus concentration of catalyst
for the intramolecular hydroamination catalyzed by 8 at 1.0 mol
L-1 [sub.]0 and 25 °C.
1
over CaH2, and distilled under vacuum prior to use. H and 13C
NMR spectra were recorded on a Varian Mercury 300 MHz or a
Varian 400 MHz spectrometer. All chemical shifts were reported
in δ units with references to the residual solvent resonance of the
deuterated solvents for proton and carbon chemical shifts. Elemental
analysis was performed by the Analytical Laboratory of Shanghai
Institute of Organic Chemistry. Melting points of the compounds
were determined on a SWG X-4 digital melting point apparatus in
a sealed capillary and are uncorrected.
in 12 h. For the Y complexes (6 and 8), the Dy complex (9),
and the Gd complex (10), the time for completing the reaction
was 4, 2, 1, and 1 h, respectively. When complex 11, which
has the largest, Nd3+, ion, was introduced, the reaction was
completed in only 30 min. Even with a very small amount of
11, 0.5 mol % catalyst loading, the reaction could be completed
in 1 h.
H2L1 (1). 2-((2,6-Diisopropylphenyl)imido)-2-penten-4-one (11.06
g, 42.6 mmol), N1-tetrabutylethane-1,2-diamine (5.00 g, 43.0 mmol),
10 drops of concentrated sulfuric acid, and toluene (100 mL) were
introduced into a 250 mL flask equipped with a Dean-Stark
apparatus. After refluxing for 24 h, the solvent was removed under
vacuum. Distillation of the crude product under reduced pressure
(bp 135-137 °C, 5 Pa) gave 1 as a yellow viscous oil (9.51 g,
62% yield). 1H NMR (300 MHz, C6D6, 25 °C): δ (ppm) 11.03 (br,
1H, MeC(NH)CH), 7.06-7.18 (m, 3H, ArH), 4.67 (s, 1H, MeC-
Kinetic studies of this class of lanthanide complex-catalyzed
intramolecular hydroamination of 2,2-dimethyl-1-aminopent-
4-ene were carried out at 25 °C by in situ 1H NMR spectroscopy.
The diamagnetic yttrium complex 8, which allows convenient
NMR monitoring of reactions, was used as the catalyst. The
concentration of substrate was held constant (1.0 mol/L), and
the catalyst concentration was varied over a 12-fold range from
120 to 10 mmol/L. Figure 7 presents the typical relevance of
the substrate concentration versus time, and Figure 8 shows rate
3
(N)CH), 3.14 (sp, JHH ) 6.6 Hz, 2H, ArCHMe2), 2.97 (q, 2H,
1
3
dependence on catalyst concentration. The in situ H NMR
NCH2), 2.45 (t, JHH ) 6.3 Hz, 2H, NCH2), 1.68 (s, 3H, MeC),
3
studies indicated the rate to be zero-order in substrate and first-
order in catalyst, which is in accordance with the generally
accepted mechanism of intramolecular hydroamination catalyzed
by organolanthanide complex, in which the alkyl complex
undergoes a rapid protonolysis by amine, and the olefin insertion
is rate-determining.20
1.64 (s, 3H, MeC), 1.24 (d, JHH ) 6.9 Hz, 6H, ArCHMe2), 1.20
(d, 3JHH ) 6.9 Hz, 6H, ArCHMe2), 0.88 (s, 9H, NCMe3), 0.45 (br,
1H, tBu-NH). 1H NMR (300 MHz, CDCl3, 25 °C): δ (ppm) 10.83
(br, 1H, MeC(NH)CH), 6.99-7.12 (m, 3H, ArH), 4.65 (s, 1H,
MeC(N)CH), 3.34 (br, 2H, NCH2), 2.87 (sp, 2H, ArCHMe2), 2.68
(br, 2H, NCH2), 2.02 (s, 3H, MeC), 1.62 (s, 3H, MeC), 1.17 (d,
3
3JHH ) 6.9 Hz, 6H, ArCHMe2), 1.13 (d, JHH ) 6.9 Hz, 6H,
Conclusions
A new class of ꢀ-diketiminato derivative dianionic ligands
was designed, which provided versatile tridentate ligand sets
(21) Tessier-Youngs, C.; Beachley, O. T. Inorg. Synth. 1986, 24, 95.
(22) Estler, F.; Eickerling, G.; Herdtweck, E.; Anwander, R. Organo-
metallics 2003, 22, 1212.
(20) Gagne, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992,
114, 275.
(23) Tamaru, Y.; Hojo, M.; Higashimura, H.; Yoshida, Z. J. Am. Chem.
Soc. 1988, 110, 3994.

Monoalkyllanthanide Complexes with ꢀ-Diketiminato Ligands
Organometallics, Vol. 28, No. 7, 2009 2323
t
ArCHMe2), 1.03 (s, 9H, CMe3), 0.82 (br, 1H, Bu-NH). 13C NMR
thin-walled glass capillaries and monitored on a single-crystal X-ray
diffractometer. The X-ray analysis showed the complex is 4.
L2ScCH2SiMe3(THF) (5). H2L2 (260 mg, 0.641 mmol) and
Sc(CH2SiMe3)3(THF)2 (288 mg, 0.642 mmol) were mixed in 5 mL
of hexane at -35 °C. The reaction mixture was gradually warmed
to room temperature and stirred for 1 h. The volatiles were removed
under vacuum, and the residue was extracted by 5 mL of hexane.
The extract was concentrated to approximately 2 mL, and the
residue was cooled to -35 °C to afford 5 as a white solid (276
mg, 71% yield). Mp: 76-78 °C without decomposition. Anal. Calcd
for C35H55N3OSiSc: C, 69.27; H, 9.13; N, 6.92. Found: C, 69.29;
H, 9.07; N, 7.05. 1H NMR (300 MHz, C6D6, 25 °C): δ (ppm) 7.12
(d, 3JHH ) 7.2 Hz, 1H, ArH3or5), 7.03-6.96 (m, 4H, ArH), 6.85 (t,
3JHH ) 7.6 Hz, 1H, ArH4), 5.05 (s, 1H, MeC(N)CH), 4.24 (m, 1H,
THF-HR), 4.04 (m, 1H, THF-HR), 3.64-3.55 (m, 2H, THF-HR and
ArCHMe2), 3.21-3.12 (m, 3H, NCH2 and ArCHMe2), 3.06 (m,
1H, THF-HR), 2.72 (s, 3H, ArMe), 2.58 (br, 2H, NCH2), 2.32 (s,
(75 MHz, CDCl3, 25 °C): δ (ppm) 165.8, 155.5 (imine C), 146.6,
137.7, 122.4, 122.3(ArC), 93.1 (MeC(N)CH), 49.9, 44.2, 43.1(NCH2
and NCMe3), 28.8, 27.8, 23.6, 22.6, 21.4, 19.4 (AriPr, MeC and
NCMe3). HRMS (EI): calcd for C23H39N3 (M+) 357.3144; found
357.3145.
H2L2 (2). 2-((2,6-Diisopropylphenyl)imido)-2-penten-4-one (13.01
g, 50.2 mmol), N1-(2,6-dimethylphenyl)ethane-1,2-diamine (8.24
g, 50.2 mmol), a catalytic amount of p-toluenesulfonic acid (250
mg), and toluene (100 mL) were introduced into a 250 mL flask
equipped with a Dean-Stark apparatus. After refluxing for 24 h,
the solvent was removed under vacuum. Recrystallization of the
crude product from methanol gave 2 as a pale yellow solid (13.01
1
g, 64% yield). Mp: 58-60 °C without decomposition. H NMR
(300 MHz, C6D6, 25 °C): δ (ppm) 11.18 (br, 1H, MeC(NH)CH),
7.13-7.22 (m, 3H, ArH), 6.87-6.96 (m, 3H, ArH), 4.71 (s, 1H,
MeC(NH)CH), 3.17 (sp, 3JHH ) 7.2 Hz, 2H, ArCHMe2), 3.15 (br,
1H, NH), 2.92 (br, 2H, NCH2), 2.86 (br, 2H, NCH2), 2.14 (s, 6H,
ArMe), 1.67 (s, 3H, MeC), 1.58 (s, 3H, MeC), 1.22 (d, 3JHH ) 6.9
3H, ArMe), 1.91 (s, 3H, MeC), 1.63 (s, 3H, MeC), 1.25 (d, 3JHH
)
3
6.9 Hz, 3H, ArCHMe2), 1.17 (d, JHH ) 6.9 Hz, 3H, ArCHMe2),
3
3
3
1
1.13 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 1.01 (d, JHH ) 6.6 Hz,
3H, ArCHMe2), 0.67 (br, 2H, THF-Hꢀ), 0.58 (br, 2H, THF-Hꢀ),
0.42 (s, 9H, CH2SiMe3), 0.19 (d, 2JHH ) 11.1 Hz, 1H, CH2SiMe3),
Hz, 6H, ArCHMe2), 1.21 (d, JHH ) 6.9 Hz, 6H, ArCHMe2). H
NMR (300 MHz, CDCl3, 25 °C): δ (ppm) 10.99 (br, 1H,
3
MeC(NH)CH), 7.00-7.12 (m, 3H, ArH), 6.97 (d, JHH ) 7.2 Hz,
-0.42 (d, JHH ) 11.1 Hz, 1H, CH2SiMe3). 13C NMR (75 MHz,
2
2H, ArH3or5), 6.81 (t, JHH ) 7.5 Hz, 1H, ArH4), 4.69 (s, 1H,
3
C6D6, 25 °C): δ (ppm) 166.1, 164.8 (imine C), 157.9, 147.0, 145.7,
143.7, 136.0, 134.0, 126.2, 125.0, 124.2, 122.4 (ArC), 99.7
(MeC(N)CH), 71.0 (THF-CR), 56.9, 54.7 (NCH2), 32.6 (CH2SiMe3),
29.3, 28.0, 25.9, 25.6, 25.3, 25.2, 25.1, 24.9, 22.6, 21.0, 20.5 (ArMe,
AriPr, THF-Cꢀ and MeC), 4.7 (CH2SiMe3).
MeC(N)CH), 3.37 (br, 2H, NCH2), 3.27 (br, 1H, NH), 3.12 (br,
2H, NCH2), 2.89 (sp, 3JHH ) 6.9 Hz, 2H, ArCHMe2), 2.22 (s, 6H,
ArMe), 1.94 (s, 3H, MeC), 1.65 (s, 3H, MeC), 1.16 (d, 3JHH ) 6.9
Hz, 6H, ArCHMe2), 1.10 (d, JHH ) 6.9 Hz, 6H, ArCHMe2). 13C
3
NMR (75 MHz, CDCl3, 25 °C): δ (ppm) 166.3, 155.8 (imine C),
146.6, 145.2, 138.0, 129.5, 128.8, 122.7, 122.6, 121.9 (ArC), 93.7
(MeC(N)CH), 48.6, 43.5 (NCH2), 28.0, 23.8, 22.8, 21.7, 19.4, 18.3
(ArMe, AriPr and MeC). HRMS (EI): calcd for C27H39N3 (M+)
405.3144; found 405.3146.
L2YCH2SiMe3(THF) (6). Following the procedure described
for 5, reaction of Y(CH2SiMe3)3(THF)2 (277 mg, 0.562 mmol) and
H2L2 (228 mg, 0.562 mmol) gave 6 as a white crystalline solid
(275 mg, 75% yield). Mp: 118-120 °C without decomposition.
Anal. Calcd for C35H55N3OSiY: C, 64.59; H, 8.52; N, 6.46. Found:
H2L3 (3). The procedure described for 2 was followed, but with
2-((2,6-diisopropylphenyl)imido)-2-penten-4-one (6.32 g, 24.4 mmol)
and N1-(2,6-diisopropylphenyl)ethane-1,2-diamine (5.37 g, 24.4
mmol). Recrystallization of the crude product from methanol gave
3 as a pale yellow solid (7.32 g, 74% yield). Mp: 72-74 °C without
decomposition. 1H NMR (300 MHz, C6D6, 25 °C): δ (ppm) 11.29
(br, s, 1H, MeC(NH)CH), 7.24-7.25 (br, m, 6H, ArH), 4.80 (s,
1H, MeC(NH)CH), 3.36 (sp, 3JHH ) 7.2 Hz, 2H, ArCHMe2), 3.23
(sp, 3JHH ) 6.9 Hz, 2H, ArCHMe2), 3.15 (br, 2H, NCH2), 2.98 (br,
1
C, 64.17; H, 8.02; N, 6.22. H NMR (300 MHz, C6D6, 25 °C): δ
3
(ppm) 7.08 (d, JHH ) 7.5 Hz, 2H, ArH3or5), 6.99-6.94 (m, 3H,
ArH), 6.83 (t, 3JHH ) 7.3 Hz, 1H, ArH4), 5.00 (s, 1H, MeC(N)CH),
4.24 (m, 2H, THF-HR), 3.63-3.55 (m, 2H, THF-HR and ArCHMe2),
3.16-3.08 (m, 2H, THF-HR and ArCHMe2), 2.95 (q, 2H, NCH2),
2.54 (br, 6H, ArMe), 2.43 (q, 2H, NCH2), 1.92 (s, 3H, MeC), 1.64
3
(s, 3H, MeC), 1.23 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 1.16 (d,
3
3JHH ) 6.9 Hz, 3H, ArCHMe2), 1.14 (d, JHH ) 6.9 Hz, 3H,
3
ArCHMe2), 1.03 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 0.73 (m, 2H,
2H, NCH2), 1.75 (s, 3H, MeC), 1.74 (s, 3H, MeC), 1.29 (d, 3JHH
)
THF-Hꢀ), 0.65 (m, 2H, THF-Hꢀ), 0.45 (s, 9H, CH2SiMe3), -0.21
7.2 Hz, 12H, ArCHMe2), 1.25 (d, 3JHH ) 6.9 Hz, 12H, ArCHMe2).
1H NMR (300 MHz, CDCl3, 25 °C): δ (ppm) 10.99 (br, 1H,
MeC(NH)CH), 7.01-7.11 (m, 6H, ArH), 4.70 (s, 1H, MeC(N)CH),
2
2
(dd, JHH ) 10.8 Hz, JYH ) 3.3 Hz, 1H, CH2SiMe3), -0.66 (dd,
2JHH ) 10.8 Hz, JYH ) 3.3 Hz, 1H, CH2SiMe3). 13C NMR (75
2
MHz, C6D6, 25 °C): δ (ppm) 166.3, 162.6 (imine C), 156.9, 145.1,
144.0, 143.8, 134.1, 125.4, 124.2, 123.9, 120.7 (ArC), 98.5
3
3.46 (br, 2H, NCH2), 3.20 (sp, JHH ) 6.6 Hz, 2H, ArCHMe2),
3.06 (br, 1H, NH), 3.00 (br, 2H, NCH2), 2.87 (sp, JHH ) 6.6 Hz,
3
(MeC(N)CH), 69.4 (THF-CR), 56.7, 55.5 (NCH2), 32.1 (d, 1JYC
)
2H, ArCHMe2), 2.04 (s, 3H, MeC), 1.65 (s, 3H, MeC), 1.14-1.18
45.9 Hz, CH2SiMe3), 28.2, 27.8, 25.4, 25.1, 24.5, 23.9, 23.8, 22.4,
(m, 18H, ArCHMe2), 1.09 (d, JHH ) 6.6 Hz, 6H, ArCHMe2). 13C
3
20.6 (ArMe, AriPr, THF-Cꢀ and MeC), 4.3 (CH2SiMe3).
NMR (75 MHz, CDCl3, 25 °C): δ (ppm) 166.2, 155.6 (imine C),
146.6, 142.6, 142.5, 137.9, 123.9, 123.5, 122.7, 122.6 (ArC), 93.7
(MeC(N)CH), 52.1, 43.8 (NCH2), 28.1, 28.0, 27.6, 24.2, 23.7, 22.8,
21.6, 19.3 (AriPr and MeC). HRMS (EI): calcd for C31H47N3 (M+)
461.3770; found 461.3768.
L1YCH2SiMe3(THF) (4). A solution of H2L1 (166 mg, 0.464
mmol in 5 mL of hexane) was added to Y(CH2SiMe3)3(THF)2 (252
mg, 0.511 mmol) in 5 mL of hexane at -35 °C. The color of the
solution turned from colorless to red immediately. After standing
at -35 °C for 2 h, the reaction solution was concentrated to
approximately 2.5 mL, and the residue was cooled to -35 °C to
give some colorless crystals, which were surrounded by a red,
viscous oil. The crystals and the oil were both readily soluble in
hexane and toluene; attempts to obtain pure product from the
mixture failed. Therefore, the single crystals covered by the red
oil were put into grease, and the red oil was carefully wiped using
grease and a needle. The single crystals with grease were sealed in
L2LuCH2SiMe3(THF) (7). Following the procedure described
for 5, reaction of Lu(CH2SiMe3)3(THF)2 (150 mg, 0.258 mmol)
and H2L2 (104 mg, 0.258 mmol) gave 7 as a white crystalline solid
(175 mg, 90% yield). Mp: 125-127 °C without decomposition.
Anal. Calcd for C35H55N3OSiLu: C, 57.05; H, 7.52; N, 5.70. Found:
1
C, 56.58; H, 6.75; N, 5.65. H NMR (300 MHz, C6D6, 25 °C): δ
(ppm) 7.08 (br, 2H, ArH3 or 5), 6.98 (m, 3H, ArH), 6.83 (t, 3JHH
)
7.2 Hz, 1H, ArH4), 4.98 (s, 1H, MeC(N)CH), 4.15 (m, 2H, THF-
HR), 3.63-3.55 (m, 2H, THF-HR and ArCHMe2), 3.22-3.14 (m,
2H, THF-HR and ArCHMe2), 3.00 (q, 2H, NCH2), 2.57 (br, 6H,
ArMe), 2.45 (q, 2H, NCH2), 1.89 (s, 3H, MeC), 1.63 (s, 3H, MeC),
3
3
1.24 (d, JHH ) 6.6 Hz, 3H, ArCHMe2), 1.18 (d, JHH ) 6.9 Hz,
3
3H, ArCHMe2), 1.16 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 1.04 (d,
3JHH ) 6.6 Hz, 3H, ArCHMe2), 0.72 (m, 2H, THF-Hꢀ), 0.59 (m,
2H, THF-Hꢀ), 0.46 (s, 9H, CH2SiMe3), -0.36 (d, 2JHH ) 11.1 Hz,
1H, CH2SiMe3), -0.80 (d, JHH ) 11.1 Hz, 1H, CH2SiMe3). 13C
2
NMR (75 MHz, C6D6, 25 °C): δ (ppm) 166.7, 163.7 (imine C),

2324 Organometallics, Vol. 28, No. 7, 2009
Lu et al.
157.9, 145.0, 144.9, 143.6, 125.4, 124.2, 123.9, 120.9 (ArC), 99.0
(MeC(N)CH), 69.8 (THF-CR), 56.4, 55.0 (NCH2), 38.0 (CH2SiMe3),
28.4, 27.6, 25.3, 25.0, 24.8, 24.4, 24.3, 24.1, 22.3, 20.5 (ArMe,
AriPr, THF-Cꢀ and MeC), 4.4 (CH2SiMe3).
mmol in 10 mL of THF) at room temperature and stirred for 2 h.
The reaction mixture was cooled to 0 °C, and H2L3 (305 mg, 0.661
mmol) in 5 mL of THF was added. After stirring at 0 °C for 2 h,
the volatiles were removed under vacuum. The residue was
extracted by 20 mL of hexane. The extract was concentrated to
approximately 5 mL and cooled to -35 °C to afford 11 as a green
crystalline solid (393 mg, 78% yield). Mp: 137-139 °C without
decomposition. Anal. Calcd for C39H63N3OSiNd: C, 61.45; H, 8.33;
N, 5.51. Found: C, 61.30; H, 8.88; N, 5.22. 11 is paramagnetic.
General Procedure for Intramolecular Hydroamination. The
lanthanide complex, 2,2-dimethyl-1-aminopent-4-ene, and the
standard p-xylene were mixed in C6D6 and transferred into a NMR
tube. The NMR tube was heated at 60 °C, and the process of the
L3YCH2SiMe3(THF) (8). Following the procedure described
for 5, reaction of Y(CH2SiMe3)3(THF)2 (150 mg, 0.305 mmol) and
H2L3 (142 mg, 0.306 mmol) gave 8 as a white crystalline solid
(127 mg, 59% yield). Mp: 148-150 °C without decomposition.
Anal. Calcd for C39H63N3OSiY: C, 66.26; H, 8.98; N, 5.94. Found:
1
C, 66.15; H, 9.22; N, 5.59. H NMR (300 MHz, C6D6, 25 °C): δ
3
3
(ppm) 7.23 (t, JHH ) 4.8 Hz, 1H, ArH4), 7.06 (d, JHH ) 4.5 Hz,
2H, ArH3 or 5), 6.96 (m, 3H, ArH), 5.03 (s, 1H, MeC(N)CH), 4.79
(sp, JHH ) 6.6 Hz, 1H, ArCHMe2), 4.38 (m, 2H, THF-HR),
3
1
3.67-2.57 (m, 3H, THF-HR and ArCHMe2), 3.12-2.99 (m, 4H,
THF-HR, ArCHMe2 and N-CH2), 2.40 (q, 2H, N-CH2), 1.94 (s, 3H,
MeC), 1.61 (s, 3H, MeC), 1.55 (d, 3JHH ) 7.2 Hz, 3H, ArCHMe2),
reaction was monitored by H NMR.
Kinetic Studies of Intramolecular Hydroamination. The Y
complex 8, 2,2-dimethyl-1-aminopent-4-ene, the standard ferrocene,
and C6D6 were loaded into a NMR tube. The tube was immediately
inserted into the probe of the Varian 400 MHz spectrometer, which
had been previously set to 25 ( 0.2 °C. Data were corrected using
four scans per time interval with a 10 s delay to ensure accurate
integration. The substrate concentration was measured from the
olefin peak area standardized to the area of Cp2Fe (4.00 ppm in
C6D6).
X-ray Crystallography. Single crystals of 4, 6, and 11 were
sealed in thin-walled glass capillaries, and data collection was
performed at 20 °C on a Bruker SMART diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The
SMART program package was used to determine the unit-cell
parameters. The absorption correction was applied using SADABS.
The structures were solved by direct methods and refined on F2 by
full-matrix least-squares techniques with anisotropic thermal pa-
rameters for non-hydrogen atoms. Hydrogen atoms were placed at
calculated positions and were included in the structure calculation
without further refinement of the parameters. All calculations were
carried out using the SHELXS-97 program. The software used is
listed in ref 24. Crystallographic data and refinement for 4, 6, and
11 are listed in Table S-1 (Supporting Information).
3
3
1.40 (d, JHH ) 6.6 Hz, 3H, ArCHMe2), 1.39 (d, JHH ) 6.6 Hz,
3
3H, ArCHMe2), 1.25 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 1.19 (d,
3JHH ) 6.9 Hz, 3H, ArCHMe2), 1.16 (d, JHH ) 6.6 Hz, 3H,
3
3
3
ArCHMe2), 1.06 (d, JHH ) 6.9 Hz, 3H, ArCHMe2), 1.04 (d, JHH
) 6.9 Hz, 3H, ArCHMe2), 0.77 (m, 2H, THF- Hꢀ), 0.64 (m, 2H,
THF- Hꢀ), 0.46 (s, 9H, CH2SiMe3), -0.13 (dd, JHH ) 10.8 Hz,
2
2JYH ) 3.3 Hz, 1H, CH2SiMe3), -0.68 (dd, JHH ) 10.8 Hz, JYH
) 3.3 Hz, 1H, CH2SiMe3). 13C NMR (75 MHz, C6D6, 25 °C): δ
(ppm) 167.1, 163.3 (imine C), 155.8, 146.7, 145.9, 145.8, 144.6,
144.5, 126.2, 124.9, 124.6, 124.1, 123.7, 122.9 (ArC), 99.3
2
2
(MeC(N)CH), 70.2 (THF-CR), 61.0, 56.4 (NCH2), 32.5 (d, 1JYC
)
47.4 Hz, CH2SiMe3), 28.9, 28.6, 28.5, 28.4, 28.0, 26.2, 25.9, 25.8,
25.3, 24.7, 24.4, 24.1, 23.5, 23.2 (AriPr, THF-Cꢀ and MeC), 4.8
(CH2SiMe3).
L3DyCH2SiMe3(THF) (9). DyCl3 (225 mg, 0.837 mmol) was
suspended in 10 mL of THF and stirred overnight. The THF solvent
was removed under vacuum, and 5 mL of hexane was added. To
the above suspension was added a solution of LiCH2SiMe3 (232
mg, 2.47 mmol in 5 mL of hexane) at room temperature and stirred
for 1 h. The precipitate was separated by centrifugation, and the
clear solution was added to a solution of H2L3 (318 mg, 0.689
mmol in 10 mL of hexane) at 0 °C. After stirring at 0 °C for 2 h,
the reaction mixture was gradually warmed to room temperature.
The precipitate was removed by centrifugation, and the clear
solution was concentrated to approximately 5 mL and cooled to
-35 °C to afford 9 as a pale yellow crystalline solid (485 mg,
90% yield). Mp: 149-151 °C without decomposition. Anal. Calcd
for C39H63N3OSiDy: C, 60.01; H, 8.14; N, 5.38. Found: C, 60.20;
H, 8.07; N, 5.21. 9 is paramagnetic.
L3GdCH2SiMe3(THF) (10). Following the procedure described
for 9, reaction of GdCl3 (187 mg, 0.709 mmol), LiCH2SiMe3 (197
mg, 2.09 mmol), and H2L3 (278 mg, 0.602 mmol) afforded 10 as
a white crystalline solid (251 mg, 54% yield). Mp: 131-133 °C
without decomposition. Anal. Calcd for C39H63N3OSiGd: C, 60.42;
H, 8.19; N, 5.42. Found: C, 61.48; H, 8.72; N, 5.18. 10 is
paramagnetic.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Grant Nos.
20672134, 20872164, and 20821002), Chinese Academy of
Sciences, and Shanghai Municipal Committee of Science and
Technology (Grant No. 07JC14063).
Supporting Information Available: Crystallographic data and
refinement for 4, 6, and 11, and CIF files giving X-ray crystal-
lographic data for 4, 6, and 11. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM900040R
(24) (a) Sheldrick, G. M. SADABS, An Empirical Absorption Correction
Program for Area Detector Data; University of Goettingen: Germany, 1996.
(b) Sheldrick, G. M. SHELXS-97; University of Goettingen: Germany, 1997.
(c) SMART Version 5.628; Bruker ASX Inc. (d) SAINT+ Version 6.22a;
Bruker AXS Inc. (e) SHELXTL NT/2000, Version 6.1.
L3NdCH2SiMe3(THF) (11). NdCl3 (230 mg, 0.918 mmol) was
suspended in 10 mL of THF and stirred overnight. To the above
suspension was added a solution of LiCH2SiMe3 (255 mg, 2.708