Homoleptic Chiral Benzamidinate Complexes of the Heavier Alkaline Earth Metals and the Divalent Lanthanides

Article abstract of DOI:10.1021/acs.organomet.6b00373

Reaction of the chiral amidine N,N′-bis(1-phenylethyl)benzamidine ((S)-HPEBA), KCH(SiMe₃)₂, and MI₂ (M = Ca, Sr, Ba) or LnI₂ (Ln = Eu, Yb) in a 2:2:1 stoichiometric ratio resulted in the chiral homoleptic monome

Full text of DOI:10.1021/acs.organomet.6b00373

Article
pubs.acs.org/Organometallics
Homoleptic Chiral Benzamidinate Complexes of the Heavier Alkaline
Earth Metals and the Divalent Lanthanides
Meng He, Michael T. Gamer, and Peter W. Roesky*
Institut fur Anorganische Chemie, Karlsruher Institut fur Technologie (KIT), Engesserstr. 15, 76131 Karlsruhe, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Reaction of the chiral amidine N,N′-bis(1-phenylethyl)benzamidine
((S)-HPEBA), KCH(SiMe3)2, and MI2 (M = Ca, Sr, Ba) or LnI2 (Ln = Eu, Yb) in a
2:2:1 stoichiometric ratio resulted in the chiral homoleptic monomeric alkaline earth
metal compounds [Ca(PEBA)2(THF)2] (1) and [Sr(PEBA)2(THF)2] (2), the
dimeric barium complex [Ba(PEBA)2]2 (3), and the monomeric divalent lanthanide
compounds [Eu(PEBA)2(THF)2] (4), and [Yb(PEBA)2(THF)2] (5). The solid-state
structures of all compounds were established by single-crystal X-ray diffraction. Three
different structures are observed in the solid state. Compounds 1, 2, 4, and 5 form
distorted coordination octahedra. For the alkaline earth element complexes 1 and 2,
the two THF molecules are located in a trans-position, whereas, for the lanthanide
compounds 4 and 5, they are arranged in a cis-position. In contrast, the barium
complex 3 is dimeric with two amidinate ligands in an unusual “side-on” bridging
mode. All five complexes were used as catalysts for hydrophosphination reactions of styrene and substituted analogues.
Scheme 1. Chiral Amidinate (S)-PEBA
INTRODUCTION
■
Amidinates and the closely related guanidinates are a very
popular class of ligands in organometallic and coordination
chemistry. These monoanionic heteroallylic ligands coordinate
to almost all metals of the periodic table.1−3 In most cases, a
N,N′-chelating binding mode is observed.4−11 The broad
application of amidinates is a result of the good adjustability
of these ligands. The amidinate anions of the general formula
[RC(NR′)2]− allow an effective tuning of the steric and
electronic requirements by varying the substituents R and R′.
There are several synthetic strategies for the access of amidinate
complexes reported,9 but for the lanthanides, either salt
metathesis reactions of an alkali metal amidinate with a metal
halide or deprotonation of an amidine with metal alkyls or
amides is most common. However, other routes such as the
insertion of carbodiimides into a metal−carbon bond are also
known.
corresponding chiral amidine N,N′-bis(1-phenylethyl)-
benzamidine (HPEBA) was reported about 35 years ago by
Brunner for the first time.29 In our work, we developed an
improved synthesis of HPEBA, including its single crystal
structure and its corresponding lithium and potassium salts
(LiPEBA and KPEBA).33
In trivalent lanthanide chemistry, we synthesized the
tris(amidinate) complex [{(S)-PEBA}3Sm], the bis(amidinate)
complexes [Ln(PEBA)2(μ-Cl)]2 (Ln = Sm, Er, Yb, Lu), and the
mono(amidinate) compounds [Ln(PEBA)(Cl)2(THF)n] (Ln =
Sm, Yb, Lu).34,35 Furthermore, the chiral mono(amidinate)
bisborohydride rare-earth complexes [Ln{(S)-PEBA}(BH4)2-
(THF)2] (Ln = Sc, La, Nd, Sm, Yb), Lu) were prepared.36 The
chiral bis(amidinate) amido complexes [{(S)-PEBA}2Ln{N-
(SiMe3)2}] (Ln = Y, Lu) were used as precatalysts in the
enantioselective hydroamination/cyclization reaction. Good
catalytic activities and high enantioselectivities were ob-
served.34,35
Since the pioneering work of Edelmann et al.4−7,9−12 and
later by Deacon and Junk et al.,13−16 amidinates are very well
established as ligands in rare-earth metal chemistry. In contrast,
only a few structurally characterized amidinate compounds of
the heavier alkaline earth metals (Ca, Sr, Ba) have been
reported.17−24 Most of them were synthesized by Westerhausen
et al.17−19,21,22 and Junk et al.23,24
In contrast to the vast number of amidinate complexes, there
are surprisingly few compounds with a chiral version of this
ligand known.25−32 Before we entered the field, only relatively
few complexes of group 4 metals,25−28 molybdenum,29,32
nickel,29,30 and rhodium31,32 with chiral amidinate ligands
were reported. We recently published the first rare-earth metal
complexes using the chiral amidinate N,N′-bis(1-phenylethyl)-
benzamidiate ((S)-PEBA) as ligand (Scheme 1).33−36 The
The addition of a P-H function across an unactivated
carbon−carbon double or triple bond (hydrophosphination) is
an attractive route for the synthesis of phosphines.37−40 It is
Received: May 9, 2016
© XXXX American Chemical Society
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related to the above-mentioned hydroamination. Intramolecu-
lar hydrophosphination of alkynes catalyzed by trivalent
lanthanides was reported 15 years ago by Marks et al.41−44
The first intermolecular version using divalent lanthanides
followed soon thereafter.45 Ever since, there is an increasing
interest in the hydrophosphination catalyzed by di- and
trivalent lanthanide complexes.46−52 The recent development
was covered by some review articles.53−57
As reported earlier, the reactivity and coordination behavior
of divalent lanthanide complexes and the heavier alkaline earth
metals is in some cases similar.58−61 This relationship in
coordination chemistry originates from the similar ion radii (for
CN 6 (pm): Ca2+ 100, Yb2+ 102, Sr2+ 118, Eu2+ 117).62 In
terms of σ-bond metathesis, there is also a certain similarity
between group 2 elements, which have a d0 electronic
configuration and the trivalent redox inactive d0 lanthanide
complexes.63 As a result of this chemical relationship, the
heavier alkaline earth metals have also been employed recently
as catalysts for the hydrophosphination reaction.46,50,63−66
Usually, the order of reactivity was found to be Ca < Sr <
Ba.46,63,66
the larger barium atoms in 3 each are coordinated by one
terminal and two metal bridging (S)-PEBA ligands.
Compounds 1−3 were fully characterized by standard
analytical/spectroscopic techniques, and their solid-state
structures were established by single-crystal X-ray diffraction.
1
In the H NMR spectra, the signal of the methyl group, which
split into a doublet, and the methine group, which appears as
multiplet, are characteristic. For the calcium compound 1
(Figure S1), the signals are well resolved (δ = 1.47 (CH3), 4.29
(NCH)), whereas they are broadened for compound 2 (δ =
1.44 (CH3), 4.29 (NCH)) (Figure S3). These signals are
slightly shifted in comparison to those of KPEBA (δ = 1.30
(CH3) and 4.22 ppm (NCH)).33 For the barium complex, two
different amidinate ligands (terminal and metal bridging) are
seen. Therefore, two sets of signals for the amidinate ligands
1
were observed in the H and 13C{1H} NMR spectra (Figures
S5 and S6), e.g., at δ(1H) = 1.14 and 1.25 for the CH3 groups.
In the IR spectrum, a characteristic strong peak at 1636 cm−1,
which is assigned to the carbon−nitrogen stretching vibration,
is observed for each compound.
The monomeric complexes 1 and 2 are isostructural (Figure
1). They crystallize in the chiral tetragonal space group P41212
Herein, we describe the synthesis and characterization of
chiral homoleptic (S)-PEBA complexes of divalent lanthanide
and the heavier alkaline earth elements as well as their
application as catalyst in the hydrophosphination reaction.
RESULTS AND DISCUSSION
■
Reaction of (S)-HPEBA, KCH(SiMe3)2, and MI2 (M = Ca, Sr,
Ba) in a 2:2:1 stoichiometric ratio resulted in the chiral
homoleptic monomeric alkaline earth metal compounds
[Ca(PEBA)2(THF)2] (1) and [Sr(PEBA)2(THF)2] (2) and
in the dimeric barium complex [Ba(PEBA)2]2 (3) (Scheme 2).
Scheme 2. Synthesis of 1−3
Figure 1. Molecular structure of 1 in the solid state, omitting the
hydrogen atoms for clarity. Selected bond lengths [Å] or angles [deg]
(data for the isostructural compound 2 are also given). 1: Ca−N1
2.443(2), Ca−N2 2.448(2), Ca−C1 2.849(2), Ca−O1 2.421(3), Ca−
O2 2.373(3), N1−C1 1.325(3), N1−C9 1.42(2), N2−C1 1.323(3),
C1−C2 1.476(8); N1−Ca−N2 55.28(6), N1−Ca−N1′ 177.37(12),
N1−Ca−O1 88.68(6), N1−Ca−O2 91.32(6), N1−C1−N2 117.9(2),
N2−Ca−O1 90.41(6), N2−Ca−O2 89.59(6), N2−Ca−N2′
179.18(12), O1−Ca−C1 89.81(6), O2−Ca−C1 90.19(6), O1−Ca−
O2 180.00(4). 2: Sr−N1 2.567(3), Sr−N2 2.578(3), Sr−C1 2.995(3),
Sr−O1 2.546(4), Sr−O2 2.507(5), N1−C1 1.326(4), N1−C9
1.44(2), N2−C1 1.329(4), C1−C2 1.496(7); N1−Sr−N2 52.40(9),
N1−Sr−N1′ 177.0(2), N1−Sr−O1 88.50(8), N1−Sr−O2 91.50(8),
N1−C1−N2 117.6(3), N2−Sr−O1 90.54(9), N2−Sr−O2 89.46(9),
N2−Sr−N2′ 179.0(2), O1−Sr−C1 89.65(8), O2−Sr−C1 90.35(8),
O1−Sr−O2 180.00(3).
with half of a molecule in the asymmetric unit. A crystallo-
graphic C2 axis is located through the center metal. Although
some disorder is observed, the structures were fully refined.
Each metal atom is 6-fold coordinate by two chelating
amidinate ligands and two molecules of THF. A distorted
coordination octahedron around the alkaline earth metal is
observed. The two THF molecules are located in a trans-
position with an ideal angle of 180°. As a result of the tight
NCN bite angle of 117.9(2)° (1) and 117.6(3)° (2), the
In the first step, (S)-HPEBA is treated with KCH(SiMe3)2 to
give (S)-KPEBA, which is then reacted in a salt metathesis with
MI2 to give the desired complexes in moderate yields as single
crystalline material. These single crystals were obtained from a
concentrated n-pentane solution. Compounds 1 and 2 are
monomeric complexes, in which two (S)-PEBA ligands and two
molecules of THF coordinate to the metal center. In contrast,
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octahedron is distorted. The M−N bond distances of each
ligand are almost identical, showing Ca−N bond lengths of
2.443(2) and 2.448(2) Å and Sr−N bond lengths of 2.567(3)
and 2.578(3) Å. A similar distorted octahedral arrangement is
observed in other achiral amidinate complexes of the alkaline
earth metals, e.g., in calcium and strontium bis[N,N′-
bis(trimethylsilyl)benzamidinate]·2THF, also a trans-coordina-
tion of the THF molecules was seen.17,19
In a similar approach as that used for the synthesis of 1−3,
the homoleptic divalent lanthanide complexes [Eu(PEBA)2-
(THF)2] (4) and [Yb(PEBA)2(THF)2] (5) were obtained.
Reaction of (S)-HPEBA, KCH(SiMe3)2, and LnI2 (Ln = Eu,
Yb) in a 2:2:1 stoichiometric ratio resulted in the desired chiral
compounds (Scheme 3). Although the synthesis is similar to
Scheme 3. Synthesis of 4 and 5
Compound 3 crystallizes in the chiral orthorhombic space
group P212121 with one molecule of 3 and one molecule of n-
pentane in the asymmetric unit (Figure 2). The bimetallic
Figure 2. Molecular structure of 3 in the solid state, omitting the
hydrogen atoms and the solvent molecules for clarity. Selected bond
lengths [Å] or angles [deg]: Ba1−N1 2.740(3), Ba1−N2 2.711(3),
Ba1−N3 2.900(3), Ba1−N4 2.782(3), Ba1−C1 3.120(4), Ba1−C24
3.154(3), Ba1−C47 3.301(3), Ba1−Ba2 3.8231(3), Ba2−N3 2.876(3),
Ba2−N4 2.905(3), Ba2−N7 2.764(3), Ba2−N8 2.706(3), Ba2−C70
3.158(4); N1−Ba1−N2 49.68(10), N3−Ba1−N4 46.81(9), N5−Ba2−
N6 45.36(9), N7−Ba2−N8 49.57(9), C1−Ba1−Ba2 162.55(8), Ba1−
Ba2−C70 168.70(8).
the alkaline earth compounds, the solid-state structures of 4 and
5 are different. Compounds 4 and 5 crystallize in the
orthorhombic space group P212121 with one molecule of the
complex in the asymmetric unit (Figure 3). Both compounds
compound shows an unusual coordination mode of the
amidinate ligands. Whereas two of those ligands coordinate
in the expected chelating mode, the other two amidinate
ligands bridge the barium atoms in a “side-on” fashion in a
μ2,κ2-N,N′-bridging mode. This coordination mode is very rare.
To the best of our knowledge, such a structural motif was not
observed earlier in barium amidinate chemistry. However, an
amidinate bridging a barium and potassium atom was reported
in [{PhCCC(NiPr)2}3BaK]n.67 A structure related to 3 was
observed in the guanidinate complex [(L)Ba(μ2-L)2Ba(L)]
(HL = iPrNC(N(iPr)2)N(H)iPr).68 In 3, each barium atom
is 6-fold coordinate. The coordination geometry around the Ba
atom is best described as a distorted trigonal prism. No solvent
molecule is bound to the metal center. The Ba−N bond lengths
of the bridging ligands are not equivalent (Ba1−N3 2.900(3) Å,
Ba2−N3 2.876(3) Å, Ba1−N4 2.782(3) Å, Ba2−N4 2.905(3)
Å, Ba1−N5 3.130(3) Å, Ba2−N5 2.871(3) Å, Ba1−N6
2.837(3) Å, Ba2−N6 3.025(3) Å). They are slightly longer
than the Ba−N bond lengths of terminal ligands (Ba1−N1
2.740(3) Å, Ba1−N2 2.711(3) Å, Ba2−N7 2.764(3) Å, Ba2−
N8 2.706(3) Å). The bite angles of the bridging ligands (N3−
Ba1−N4 46.81(9)°, N5−Ba1−N6 44.55(9)°, N3−Ba2−N4
46.81(9)°, N5−Ba2−N6 45.36(9)°) are smaller than those of
the terminal ones (N1−Ba1−N2 49.68(10)°, N7−Ba2−N8
49.57(9)°).
Figure 3. Molecular structure of 4 in the solid state, omitting the
hydrogen atoms for clarity. Selected bond lengths [Å] or angles [deg]
(data for the isostructural compound 5 are also given). 4: Eu−N1
2.582(4), Eu−N2 2.567(4), Eu−N3 2.557(4), Eu−N4 2.580(3), Eu−
C1 2.995(5), Eu−C24 2.987(5), Eu−O1 2.600(3), Eu−O2 2.613(4);
N1−Eu−N2 52.67(13), N1−Eu−N3 107.45(13), N1−Eu−N4
148.33(14), N1−Eu−O1 102.00(13), N1−Eu−O2 101.70(14), N3−
Eu−N4 52.74(15) N3−Eu−O1 83.7(2), N3−Eu−O2 148.18(14),
N4−Eu−O1 99.89(12), N4−Eu−O2 105.1(2), O1−Eu−O2 77.9(2).
5: Yb−N1 2.475(3), Yb−N2 2.470(3), Yb−N3 2.452(3), Yb−N4
2.483(2), Yb−C1 2.881(4), Yb−C24 2.871(3), Yb−O1 2.491(3), Yb−
O2 2.505(3); N1−Yb−N2 55.02(9), N1−Yb−N3 108.36(10), N1−
Yb−N4 152.31(10), N1−Yb−O1 101.61(10), N1−Yb−O2
100.10(10), N3−Yb−N4 55.07(10), N3−Yb−O1 83.23(10), N3−
Yb−O2 148.35(10), N4−Yb−O1 98.22(9), N4−Yb−O2 102.89(10),
O1−Yb−O2 77.65(10).
C
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are isostructural. As observed for 1 and 2, monometallic
complexes were formed, in which the metal center is
surrounded by two (S)-PEBA ligands and two molecules of
THF. In contrast to 1 and 2, the stereochemistry of the thus
obtained distorted coordination octahedron is different. The
THF molecules in 4 and 5 are arranged in a cis-position with an
O1−Ln−O2 angle of 77.9(2)° (4) and 77.65(10)° (5). There
are surprisingly only very few structurally characterized
bis(amidinate) complexes of the divalent lanthanides known.
In ytterbium bis[N,N′-bis(trimethylsilyl)benzamidinate]·2THF,
a trans-coordination of the THF molecules was seen.12 In
contrast, the lanthanide(II)bis(amidinate) complexes [LnL2-
(THF)2] (Ln = Sm, Eu; L = PhC(NSiMe3)(NC6H3iPr2-2,6))
show a cis-configuration for samarium and both cis- and trans-
configurations for europium.69 For the lanthanide(II)bis-
(guanidinate) complexes, [Ln(Giso)2] (Ln = Sm, Eu; Giso =
[(ArN)2CN(C6H11)2]−, Ar = 2,6-diisopropylphenyl), planar 4-
coordinate samarium and europium atoms and distorted
tetrahedral coordinate ytterbium atoms were reported.70 In
contrast, the homoleptic four-coordinate lanthanide(II) com-
plexes [Ln(Priso)2] (Ln = Sm, Eu, Yb; Priso = [(ArN)2-
CNiPr2]−, Ar = 2,6-diisopropylphenyl) show coordination
geometries midway between tetrahedral and planar.71 The Ln−
N bond distances in 4 and 5 are in the expected ranges of
2.557(4)−2.582(4) Å (compound 4) and 2.452(3)−2.483(2)
Å (compound 5).
Table 1. Intermolecular Hydrophosphination of Styrenes
with Diphenylphosphine
The diamagnetic complex 5 was also characterized by NMR
1
spectroscopy (Figures S7−S9). In the H NMR spectrum, the
signals of the methyl (δ = 4.40 ppm) and the methine group (δ
1
= 1.39 ppm) of (S)-PEBA are well resolved. A 2D H, 171Yb-
gHMBC NMR spectrum (gradient-selected Heteronuclear
Multiple-Bond Coherence) shows a 171Yb peak at 792 ppm,
which couples to the NCH group of the ligand (Figure S9).
Compounds 1−5 were used as precatalysts in the hydro-
phosphination reaction of differently substituted styrenes and
Ph2PH (Table 1). In contrast to the well-established hydro-
phosphination catalysts of the alkaline earth metals, compounds
1−5 do not have a leaving group.46,50,63−66,72 Attempts to
synthesize compounds with a leaving group, e.g., [M((S)-
PEBA)CH(SiMe3)2], failed. We presume that this is a result of
the Schlenk equilibrium. Nevertheless, we used 1−5 as Lewis
acidic catalysts. As a result of the low pKa values of phosphines,
we do not anticipate a substitution of one (S)-PEBA ligand by a
phosphine during the catalytic transformation. To support this
assumption, we reacted 5 and Ph2PH in a 1:1 ratio (Figure
S24). There is no sign of ligand displacement or oxidation of
the ytterbium atom, which would result in a paramagnetic
1
sample. Moreover, a 2D H, 171Yb-gHMBC NMR spectrum of
5 in the presence of Ph2PH was recorded (Figure S25). As
expected, no 171Yb,31P coupling, and thus no tight coordination,
was observed.
a
Based on the consumption of diphenylphosphine from integration of
b
c
signals in the 31P{1H} NMR. 2.5 mol% catalyst used. Ambient
temperature. Decomposition of the catalyst.
The products of the catalytic hydrophosphination were
obtained in moderate to high yield and high purity. With one
exception, all reactions were conducted with 2.5 or 5 mol%
catalyst loading in benzene at 60 °C in NMR scale. The
transformation of p-chlorostyrene in the presence of 3 was also
performed at room temperature (Table 1, entry 10). The
reaction was monitored by NMR spectroscopy. Conversions
were calculated based on the integration area of product and
d
NMR spectra, the peak at δ = −40 ppm was assigned to the
starting material diphenylphosphine, while the new peak at
around δ = −20 ppm belongs to the hydrophosphination
products. Blank reactions without catalysts have been
performed for comparison (Table 1, entries 19−22). Even
without a catalyst, formation of the hydrophosphination
products was observed at high temperature over a long period.
It can be clearly seen from Table 1 that the yields and the
reaction times are substrate and catalyst dependent. Most of the
1
starting material signals in the H and 31P{1H} NMR spectra.
Although the 31P NMR spectrum was recorded in a proton
decoupled mode, the results do not deviate significantly from
1
the results obtained by H NMR spectroscopy. In 31P{1H}
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catalytic reactions stopped before being completed. Within the
series of the heavier alkaline earth metals 1−3, the barium
compound 3 showed the highest activity (entries 9−12). Since
compound 3 is bimetallic, we used in comparison to the other
catalyst only half of the stoichiometric amount to have the same
molar concentration of the metal atoms. By using a catalyst
loading of 2.5 mol% of 3 and p-chlorostyrene as substrate, the
corresponding product even formed at room temperature in 36
h, achieving a conversion of 77% (entry 10). At 60 °C, the p-
chlorostyrene can be converted within 3 h to a conversion of
94%. In contrast, the same reaction occurred with conversions
of only 58% (118 h) and 78% (94 h) by using 1 and 2 as
catalysts, respectively (entries 2 and 6). As observed earlier in
hydrophosphination reaction, the catalyst activity increased
with increasing ion radius of the center metal.46,63,66 In fact,
there is, in some cases, not a significant improvement by using
1 as a catalyst (entries 1−4) in comparison to the blank
reaction (entries 19−22). Cooperative effects of the bimetallic
compound 3 do not seem to play a significant role.
Next, we investigated the divalent lanthanide compounds 4
and 5 as catalysts. For styrene, the catalytic activities of 4 and 5
were comparable to those of 2 (entries 5, 13, and 16).
However, 4 showed significantly better activity within a shorter
reaction time and higher conversion (entries 6 and 14) when
using p-chlorostyrene as substrate. In contrast, electron-
withdrawing substituents (Cl, F) did not affect the reaction
rate by using the established system [{tBuC(NC6H3-iPr2-
2,6)2}YbN(SiMe3)2(THF)] as catalyst.72 The hydrophosphi-
nation reaction of p-methylstyrene with 4 and 5 as catalysts
does not show a clear tendency. The reaction of p-
chlorostyrene with the divalent Yb compound 5 resulted in
an immediate decomposition of the catalyst. We anticipate that
the chlorine function of the substrate reacts with the Yb2+ atom.
In order to evaluate the enantioselectivity of the chiral
amidinate catalysts, the hydrophosphination reaction of α-
methylstyrene with Ph2PH was investigated as a model
reaction. The results are presented in Table S2. Because of
the absence of a leaving group at the catalyst, the conversions
were low to moderate. Only compound 3 showed a significantly
higher conversion compared to the blank reaction. In the
presence of 2.5 mol% of compound 3, the reaction reached a
conversion of 60 % after 95 h at 100 °C (Table S2, entry 3). To
determine the enantiomeric excess (ee) by 31P{1H} NMR
spectroscopy, the chiral derivatizing agent bis[{dimethyl(1-(2-
naphthyl)ethyl)aminato-C2,N}palladium(II)chloride], (R)-[Pd-
{Me2NCH(Me)C10H6}(μ-Cl)]2,73,74 was added. Unfortunately,
no clear results indicating an enantioselectivity were obtained.
substitution of one amidinate ligand by a phosphine during the
catalytic transformation. Thus, a Lewis acid catalysis is
suggested. Because of the absence of a leaving group, the
catalytic activity of compounds 1−5 is lower compared to
established systems. However, significant activity for some
catalysts is observed. Within this series, the barium compound
3 showed the highest activity. This is in agreement with earlier
observations, in which the activity increased with increasing ion
radius of the center metal.46,63,66
EXPERIMENTAL SECTION
■
General Methods. All air- and water-sensitive materials were
prepared under a nitrogen atmosphere by using either a Schlenk line
or a glovebox. THF was dried by distilling from potassium
benzophenone ketyl under nitrogen before use. Toluene and n-
pentane were obtained from a MBraun solvent purification system
(SPS-800). Deuterated solvents were purchased from Carl Roth
GmbH (99.5 atom % D) and were dried and stored in vacuum with
Na/K alloy. NMR spectra were recorded on a Bruker Avance II 300
MHz spectrometer. Chemical shifts are referenced to internal solvent
resonances and are reported relative to tetramethylsilane (1H and 13C
NMR), [Yb(C5Me5)2(THF)2] (171Yb NMR), respectively. Elemental
analyses were carried out with an Elementar Vario Micro Cube. IR
spectra were performed on a Bruker TENSOR 37 spectrometer via the
attenuated total reflection method (ATR). [LnI2(THF)2]75 and
76
KCH(SiMe3)2 were prepared according to literature procedures.
[Ca(PEBA)2(THF)2] (1). (S)-HPEBA (0.40 g, 1.22 mmol) and
KCH(SiMe3)2 (0.137 g, 1.22 mmol) were dissolved in toluene (10
mL) and then stirred at room temperature for 1 h. The solution was
transferred to a suspension of CaI2 (0.179 g, 0.61 mmol) in 10 mL of
toluene. The resulting mixture was vigorously stirred overnight. Then,
the solvent was removed under reduced pressure. The residue was
dissolved in 20 mL of THF and stirred at room temperature for
another 24 h. The solvent was directly removed in vacuo, and the
residue was extracted with n-pentane (15 mL). The solution was
concentrated to dryness. Finally, the remaining powder was dried
under reduced pressure. The product was purified by recrystallization
from a concentrated n-pentane solution. Yield: 204 mg (based on
single crystals), 0.23 mmol, 38%. Anal. Calcd for C59H67N4O2Ca
(904.28): C, 78.37, H, 7.47, N, 6.2. Found: C, 79.00, H, 6.63, N, 6.00.
1H NMR (300.13 MHz, C6D6, 298 K): δ (ppm) = 7.52 (d, J = 7.0 Hz,
1H, Ph), 7.34 (d, J = 7.0 Hz, 8H, Ph), 7.23 (t, J = 7.4 Hz, 6H, Ph)
7.18−7.02 (m, 15H, Ph), 4.29 (q, J = 6.6 Hz, 4H, CH), 3.84−3.77 (m,
4H, CH2 (THF)), 3.73−3.57 (m, 4H, CH2 (THF)), 1.47 (d, J = 6.6 Hz
12H, CH3), 1.43−1.35 (m, 8H, CH2 (THF)). 13C{1H} NMR (75.48
MHz, C6D6, 298 K): δ (ppm) = 174.5 (NCN), 150.7 (i-Ph), 139.2 (i-
Ph), 127.8 (Ph), 127.0 (Ph), 126.8 (Ph), 126.7 (Ph), 125.3 (Ph), 68.4
(CH2 (THF)), 57.3 (CH), 27.2 (CH3), 25.1 (CH2 (THF)); the signals for
the quaternary carbon atoms were not observed. IR (ATR) υ (cm−1):
3406 (s), 3317 (w), 2978 (w), 2953 (w), 2919 (w), 2875 (w), 1636
(vs), 1485 (s), 1447 (m), 1304 (m), 1266 (m), 1214 (w), 1141 (w),
1071 (w), 1028 (m), 763 (s), 700 (s), 544 (m).
CONCLUSION
■
Chiral homoleptic (S)-PEBA complexes of the heavier alkaline
earth elements (M = Ca, Sr, Ba) and the divalent lanthanides
(Ln = Eu2+, Yb2+) were synthesized. Three different structures
are observed in the solid state. Compounds 1, 2, 4, and 5 form
distorted coordination octahedra. For the alkaline earth
element complexes, the two THF molecules are located in a
trans-position, whereas, for the lanthanide compounds, they are
arranged in a cis-position. In contrast, the barium complex 3 is
dimeric with two amidinate ligands in an uncommon “side-on”
bridging mode.
[Sr(PEBA)2(THF)2] (2). Following the procedure described above
for 1, the reaction of (S)-HPEBA (0.40 g, 1.22 mmol) and
KCH(SiMe3)2 (0.137 g, 1.22 mmol) and SrI2 (0.208 g, 0.61 mmol)
afforded colorless crystals of 2. Yield: 150 mg (based on single
crystals), 0.173 mmol, 28%. Anal. Calcd for C53H53N4O2Sr (865.64):
1
C, 73.54, H, 6.17, N, 6.47. Found: 73.50, H, 6.62, N, 5.49. H NMR
(300.13 MHz, C6D6, 298 K): δ (ppm) = 7.40−7.28 (m, 8H, Ph),
7.25−6.70 (m, 22H, Ph), 4.29 (q, J = 6.2 Hz, 4H, CH), 3.74−3.63 (m,
4H, CH2 (THF)), 3.61−3.47 (m, 4H, CH2 (THF)), 1.48 (d, J = 6.7 Hz,
12H, CH3), 1.41−1.36 (m, 8H, CH2 (THF)). 13C{1H} NMR (75.48
MHz, C6D6, 298 K): δ (ppm) = 174.2 (NCN), 151.3 (i-Ph), 139.5 (i-
Ph), 127.9 (Ph), 127.8 (Ph), 127.1 (Ph), 126.6 (Ph), 126.4 (Ph),
125.2 (Ph), 68.3 (CH2 (THF)), 57.6 (CH), 27.6 (CH3), 25.6
(CH2 (THF)). IR (ATR) υ (cm−1): 3406 (s), 3061 (w), 2977 (m),
2954 (m), 2921 (w), 2877 (w), 1636 (vs), 1599 (w), 1486 (s), 1448
Although compounds 1−5 do not have a leaving group, they
were used as catalysts in the hydrophosphination reaction of
differently substituted styrenes and Ph2PH. As a result of the
low pKa values of the phosphine, we do not anticipate a
E
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Article
(s), 1355 (w), 1303 (m), 1267 (m), 1211 (w), 1141 (w), 1071 (w),
1027 (m), 763 (s), 700 (s), 544 cm−1 (s).
[Ba(PEBA)2]2 (3). Following the procedure described above for 1.
The reaction of (S)-HPEBA (0.40 g, 1.22 mmol), KCH(SiMe3)2
(0.137 g, 1.22 mmol), and BaI2 (0.238 g, 0.61 mmol) afforded
colorless crystals of 3.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00373.
Yield: 137 mg (based on single crystals), 0.083 mmol, 27%. Anal.
Calcd for (1650.57): C97H981Ba2N8: C, 70.59, H, 6.79, N, 5.98. Found:
C, 69.91, H, 5.74, N, 5.77. H NMR (300.13 MHz, C6D6, 298 K): δ
(ppm) = 7.21 (t, J = 7.7 Hz, 8H, Ph), 7.07−6.80 (m, 48H, Ph), 6.48−
6.45 (m, 4H, Ph), 4.15 (q, J = 6.5 Hz, 4H, CH), 4.05 (q, J = 6.6 Hz,
4H, CH), 1.25 (d, J = 6.7 Hz, 12H, CH3), 1.14 (d, J = 6.7 Hz, 12H,
CH3). 13C{1H} NMR (75.48 MHz, C6D6, 298 K): δ (ppm) = 177.0
(NCN), 174.8 (NCN), 151.5 (i-Ph), 148.1 (i-Ph), 139.0 (i-Ph), 136.5
(i-Ph), 129.4 (Ph), 128.2 (Ph), 127.9 (Ph), 127.6 (Ph), 126.7 (Ph),
126.6 (Ph), 126.5 (Ph), 126.3 (Ph), 126.2 (Ph), 125.1 (Ph), 57.2
(CH), 56.9 (CH), 26.5 (CH3), 25.9 (CH3); the signals for the
quaternary carbon atoms were not observed. IR (ATR) υ (cm−1):
3406 (s), 3058 (w), 2954 (m), 2921 (s), 2876 (m), 1636 (vs), 1598
(w), 1485 (s), 1447 (s), 1303 (m), 1267 (m), 1211 (w), 1141 (w),
1071 (w), 1027 (m), 762 (s), 699 (s), 543 (s).
[Eu(PEBA)2(THF)2] (4). Following the similar procedure described
above for 1. The reaction of (S)-HPEBA (0.40 g, 1.22 mmol),
KCH(SiMe3)2 (0.137g, 1.22 mmol), and EuI2(THF)2 (0.335g, 0.61
mmol) afforded 4 as orange single crystals. Yield: 156 mg (based on
single crystals), 0.164 mmol, 27%. Anal. Calcd for C54H62N4O2Eu
(951.07): C, 68.2, H, 6.57, N, 5.89. Found: C, 67.94, H, 6.05, N, 6.21.
IR (ATR) υ (cm−1): 3405 (s), 3058 (w), 2976 (m), 2956 (m), 2920
(w), 2874 (w), 1635 (vs), 1598 (w), 1485 (s), 1447 (s), 1355 (w),
1303 (m), 1266 (m), 1210 (w), 1141 (w), 1071 (w), 1026 (m), 763
(s), 699 (s), 543 (s).
[Yb(PEBA)2(THF)2] (5). Following the similar procedure described
above for 1. The reaction of (S)-HPEBA (0.40 g, 1.22 mmol),
KCH(SiMe3)2 (0.137 g, 1.22 mmol), and YbI2(THF)2 (0.348, 0.61
mmol) afforded dark red crystals of 5. Yield: 121 mg (based on single
crystals), 0.125 mmol, 20%. Anal. Calcd For: C54H62N4O2Yb (972.15):
C, 66.72, H, 6.43, N, 5.76. Found: C, 66.72, H, 6.09, N, 6.17. 1H NMR
(300.13 MHz, C6D6, 298 K): δ (ppm) = 7.37 (d, J = 7.5 Hz, 8H, Ph),
7.24−7.00 (m, 22H, Ph), 4.40 (q, J = 6.7 Hz, 4H, CH), 3.69 (br s, 4H,
CH2 (THF)), 3.60 (br s, 4H, CH2 (THF)), 1.48 (d, J = 6.6 Hz, 12H, CH3),
1.39 (br s, 8H, CH2 (THF)). 13C{1H} NMR (75.48 MHz, C6D6, 298 K):
δ (ppm) = 174.4 (NCN), 150.9 (i-Ph), 139.4 (i-Ph), 127.8 (Ph), 127.0
(Ph), 126.8 (Ph), 126.7 (Ph), 125.2 (Ph), 68.2 (CH2 (THF)), 57.2
(CH), 27.0 (CH3), 25.2 (CH2 (THF)). 171Yb{1H} NMR (52.5 MHz,
C6D6, 298 K): 792.ppm. IR (ATR) υ (cm−1): 3406 (s), 3058 (w),
3024 (w), 2955 (s), 2921 (vs), 2855 (s), 1635 (vs), 1598 (m), 1485
(s), 1448 (s), 1355 (w), 1303 (m), 1265 (m), 1211 (w), 1140 (w),
1070 (w), 1026 (m), 762 (s), 699 (s), 542 (s).
Experimental procedures and NMR spectra (PDF)
Crystallographic data of compounds 1−5 (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: roesky@kit.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the DFG-funded transregional
collaborative research center SFB/TRR 88 “3MET”. This work
was also supported by the Landesstiftung Baden-Wurttemberg
̈
gGmbH and the KIT. M.H. gratefully acknowledges a grant
from the China Scholarship Council (No. 201206250015).
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