Synthesis of calcium and ytterbium complexes supported by a tridentate imino-amidinate ligand and their application in the intermolecular hydrophosphination of alkenes and alkynes

Article abstract of DOI:10.1021/om201223j

Well-defined calcium and ytterbium complexes [{2-NC(Ph)NArC ₆H₄CHNAr}M{N(SiMe₃)₂}(THF)] (M = Ca, Yb; Ar = 2,6-iPr₂C₆H₃) have been synthesized and characterized. They catalyze the

Full text of DOI:10.1021/om201223j

Note
pubs.acs.org/Organometallics
Synthesis of Calcium and Ytterbium Complexes Supported by a
Tridentate Imino-Amidinate Ligand and Their Application in the
Intermolecular Hydrophosphination of Alkenes and Alkynes
Hongfan Hu and Chunming Cui*
State Key Laboratory and Institute of Element-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
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* Supporting Information
ABSTRACT: Well-defined calcium and ytterbium complexes [{2-NC(Ph)-
NArC6H4CHNAr}M{N(SiMe3)2}(THF)] (M = Ca, Yb; Ar = 2,6-iPr2C6H3)
have been synthesized and characterized. They catalyze the intermolecular
hydrophosphination of alkenes, dienes, and alkynes with high activity and
selectivity under mild conditions. Highly selective 1,4-additions (94−100%)
for the conjugated dienes examined have been observed with both catalysts.
The calcium complex exclusively catalyzes anti addition to alkynes, including
terminal alkynes, while the ytterbium, in most cases, catalyzes syn addition.
The calcium catalyst could promote hydrophosphination of hindered alkenes
such as stilbene under relatively mild conditions.
ince calcium is a cheap and environmentally benign element,
the development of calcium-based homogeneous catalytic
RESULTS AND DISCUSSION
The ligand [2-{NHC(Ph)NAr}C6H4CHNAr] (1, Ar = 2,6-
iPr2C6H3; Scheme 1) was synthesized by a salt metathesis
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systems has received considerable attention in the past few
years.1 For example, they have been successfully used as
polymerization,2 hydroamination,3 and hydrosilylation4 catalysts.
In 2007, Hill and co-workers reported that the hydrophosphina-
tion of unhindered activated alkenes and diphenylacetylene with
diphenylphosphine was catalyzed by 10−20 mol % of the β-
diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N-
(SiMe3)2}(THF)] (A).5 However, their attempts to catalyze
hydrophosphination reactions with the homoleptic calcium
amide [Ca{N(SiMe3)2}(THF)2] met with very limited success.
Subsequently, Westerhausen and co-workers reported that the
calcium diphosphide complex [Ca(PPh2)2(THF)4] catalyzes the
intermolecular hydrophosphination of phenyl-substituted alkynes
and butadiynes.6 Intermolecular hydrophosphination of alkynes
and olefins catalyzed by late-transition-metal complexes7 and
rare-earth-metal complexes8 has also been reported.
Scheme 1. Synthesis of 2 and 3
Amidinate and β-diketiminato ligands have been extensively
used as supporting ligands for the synthesis of rare-earth metal
complexes and alkaline-earth-metal complexes.1,9 In order to tune
the catalytic performance and reaction patterns of the complexes
of these highly electropositive metals, we have designed a new type
of tridentate amidinate ligand featuring an imine arm with the
expectation of stabilizing large ionic radii metal complexes. Herein
we report on the synthesis, characterization, and catalytic hydro-
phosphination reactions of the well-defined calcium complex
[{2-NC(Ph)NArC6H4CHNAr}Ca{N(SiMe3)2}(THF)] (2; Ar =
2,6-iPr2C6H3) with a modified tridentate amidinate ligand. The
catalyst is highly efficient, with selectivity distinct from that of the
reported calcium catalyst supported by a β-diketiminato ligand. In
addition, the corresponding ytterbium complex has also been pre-
pared for comparison.
reaction of [2-{CHNAr}C6H4NHLi] with [ArNC(Ph)Cl] (Ar =
2,6-iPr2C6H3) in good yield and was fully characterized by
standard analytical and spectroscopic techniques. The calcium
complex 2 was obtained as a light yellow solid in 78% yield by the
silylamine elimination reaction of the calcium diamide [Ca{N-
(SiMe3)2}(THF)2] with 1. The corresponding ytterbium complex
[{2-NC(Ph)NArC6H4CHNAr}Yb{N(SiMe3)2}(THF)] (3, Ar =
2,6-iPr2C6H3) was prepared similarly and obtained as dark green
blocks in ca. 71% yield. Complexes 2 and 3 have been fully
characterized by 1H and 13C NMR spectroscopy, IR spectroscopy,
and elemental analysis.
Received: December 8, 2011
Published: January 18, 2012
© 2012 American Chemical Society
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Organometallics
Note
Single crystals of 2 suitable for X-ray single-crystal analysis
were obtained from n-hexane at 25 °C, and the structure is
shown in Figure 1 along with selected bond parameters.
substrate affect the catalytic activity: more hindered alkenes are
less active, and electron-donating groups on the substrates lower the
activity (entries 2, 6, and 7). These observations are in accordance
with those for calcium-catalyzed intermolecular hydroamination
reactions.10 For every substrate, the loading of the catalyst and
reaction temperatures have been optimized; only the lowest load-
ings and optimized temperatures are given in Table 1.
The calcium complex 2 shows a significantly higher activity than
the reported [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}-
(THF)] (A) (entries 1, 3, and 8). For example, the hydrophos-
phination of styrene with only 2 mmol % of 2 is complete (100%
conversion) in 3 h at 25 °C, while the same reaction employing
10 mmol % of the β-diketiminato complex as catalyst occurred at
75 °C with only 95% conversion in 20 h. Interestingly, 2 can
catalyze the hydrophosphination reactions of bulky substrates such
as α-methylstyrene and cis-stilbene (entries 6 and 7). In contrast,
the reported β-diketiminato complex is inactive for these substrates.
2 also catalyzes the hydrophosphination of the terminal alkynes
PhCCH and PyCCH (entries 9 and 13). It is well-known that
terminal alkynes tend to undergo σ-bond metathesis and
subsequent dimerization in the presence of this type of catalyst.11
To the best of our knowledge, this is the first calcium-mediated
intermolecular hydrophosphination reaction of terminal alkynes.
The regioselectivity of 2 is somewhat different from the repor-
ted β-diketiminato complex. The hydrophosphination of isoprene
catalyzed by the latter mainly results in the 1,2-addition product,
while the same reaction catalyzed by 2 predominately gives the 1,4-
addition product (entry 3). Similarly, other 1,3-dienes catalyzed by
2 also yield 1,4-addition products with high selectivity (entries 4
and 5). Different stereoselectivities for the hydrophosphination of
alkynes have also been observed with 2 and the β-diketiminato
complex. With the β-diketiminato complex as catalyst, a syn addi-
tion of diphenylphosphine to alkynes is favorable, which leads to E
isomers (entry 8), while reactions mediated by 2 mainly lead to anti
addition products and Z isomers are preferable (entries 8−13). All
these observations reveal the impact of the ligand geometry on cata-
lytic performance. It is noted that 1,4-hydrophosphination of 1,3-
dienes has been rarely reported with early-transition-metal
catalysts.12,8a
Figure 1. Ortep drawing of 2 with 30% probability ellipsoids. Hydro-
gen atoms and isopropyl groups have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ca1−N1 = 2.335(4), Ca1−N2 =
2.519(4), Ca1−N3 = 2.652(4), Ca1−N4 = 2.306(4), Ca1−C1 =
2.888(4), Ca1−O1 = 2.400(4); N1−Ca1−N2 = 54.84(11), N1−Ca1−
N3 = 69.29(12), N1−C1−N2 = 112.9(4), N4−Ca1−O1 = 96.14(14).
Complex 2 is monomeric in the solid state with a five-coordi-
nate calcium center. Most of the bond distances and angles are
as expected, but there are exceptions. Unlike most amidinate
metal complexes with two almost equal M−N bond lengths,9a,c
the Ca1−N2 bond distance (2.519(4) Å) is noticeably longer
than the Ca1−N1 distance (2.335(4) Å), indicating a relatively
low degree of electron delocalization over the N1−C1−N2
amidinate backbone. The long Ca1−N3 bond length (2.652(4) Å)
is indicative of a dative bond. Complexes 2 and 3 proved to
be thermally robust, as no decomposition and ligand distribu-
tion were observed even in solution at 160 °C, indicating the
unusual stabilizing effects of the tridentate ligand.
Although a few calcium catalysts for the hydrophosphination
of alkynes and alkenes have been reported, the scope of the
substrates and selectivity are not satisfactory.5a,6 Since catalytic
performances are largely related to the ligand sets, compounds
2 and 3 with the new tridentate ligand have been employed as
catalysts for the intermolecular hydrophosphination of alkenes
and alkynes (Scheme 2). The hydrophosphination reactions of
The same reactions have also been examined with the
ytterbium complex 3 as catalyst. It can be concluded that 3 has
activity comparable to that of 2 for almost all of the substrates.
For the hydrophosphination of 1,3-dienes, 2 and 3 show
extremely similar regioselectivities and catalyze 1,4-addition to
the dienes (entries 3−5). However, differences emerge in the
hydrophosphination of alkynes. Unlike the anti addition fashion
catalyzed by 2, the reactions catalyzed by 3 are somewhat
substrate dependent but, in most cases, lead to syn addition
products to form E isomers (entries 9−12).
Scheme 2. Catalysis of Intermolecular Hydrophosphination
The reaction of 2 with the same amount of diphenylphos-
phine has been investigated at room temperature on an NMR
1
scale (Scheme 3). After 2 h, a singlet at 0.09 ppm in the H
1,3-conjugated dienes, alkynes and sterically hindered olefins
have been examined. The results for these catalytic reactions are
given in Table 1. The catalysts are not active for unactivated
substrates such as 1-hexene, norbornene, and 4-hexyne but
showed a moderate activity for activated alkenes/alkynes. The
hydrophosphination products are exclusively consistent with
anti-Markovnikov rules, and 2,1-insertions have been observed
as a result of the stabilizing effect of the aryl/alkyl group to the
highly electropositive calcium center of the four-membered-ring
intermediate that has been previously proposed for activated
alkene/alkynes.3a Both steric and electronic factors of the
NMR spectra indicates the protonolysis of HN(SiMe3)2 from 2.
A new peak at −21.1 ppm in the 31P NMR spectrum may be
attributed to the formation of a calcium phosphide complex.
Both of these peaks can be observed in all of the catalytic reac-
tions on the NMR scale, suggesting the formation of a reactive
calcium phosphide intermediate. However, the stoichiometric
reaction cannot achieve a complete transformation to the cor-
responding phosphide, even at 80 °C. Increasing the amount of
diphenylphosphine to 5 equiv also led to a mixture of 2 and the
corresponding calcium phosphide. Attempts to isolate the
putative intermediate have been unsuccessful to date.
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Organometallics
Note
Table 1. Intermolecular Hydrophosphination of Alkenes and Alkynes
a
b
c
On the basis of the consumption of phosphine from integration of signals in the 1H and 31P NMR. 5 mol % of catalyst used. 2 mol % of catalyst
d
e
f
g
h
i
used. 10 mol % of catalyst used. 20 mol % of catalyst used. Z:E ratio 20:80. Z:E ratio 22:78. Data taken from ref 5a. 10% of Markovnikov
addition product.
Scheme 3. Stoichiometric Reaction between 2 and HPPh2
characterized. They are efficient catalysts for the intermolecular
hydrophosphination of alkenes and alkynes. The complexes
show high activity and regioselectivity distinct from that of the
reported β-diketiminato calcium complex. Efforts to tune the
ligand system for the development of highly active calcium
catalysts with high selectivity are currently in progress.
EXPERIMENTAL SECTION
■
All manipulations of air-sensitive materials were carried out
under an atmosphere of dry argon by using modified Schlenk
line and glovebox techniques. All the liquid alkenes and alkynes
were dried over CaH2, freshly distilled, and freeze−thaw degassed
CONCLUSION
■
1
prior to use. The H, 13C, and 31P NMR spectroscopic data were
Well-defined calcium and ytterbium complexes supported by a
tridentate amidinate-based ligand have been synthesized and
recorded on Bruker Mercury Plus 400 MHz NMR spectrometers.
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Organometallics
Note
NMR-Scale Catalytic Reactions. In a glovebox, diphenyl-
phosphine (43 μL, 0.25 mmol) was added to a solution of the
catalyst (0.012 mmol, 5 mol %) in C6D6 and the alkene/alkyne
(0.3 mmol, 1.2 equiv) was added either as a solid or a solution
in the same solvent. The solution was then loaded into a Young
AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
Notes
■
The authors declare no competing financial interest.
1
tap NMR tube. The reaction was monitored by H and 31P
spectroscopy.
ACKNOWLEDGMENTS
■
LCaN(SiMe3)2·THF (2). A solution of 1 (1.63 g, 3 mmol) in
30 mL of hexane was added to a solution of Ca[N(SiMe3)2]2·2THF
(1.52 g, 3 mmol) in the same solvent. The mixture was stirred for
36 h at room temperature and then filtered. The filtrate was con-
centrated, and recrystallization at −40 °C yielded the product as
We are grateful to the National Natural Science Foundation of
China (Grant No. 20972074) and the National Basic Research
Program of China (973 Program No. 2012CB821600) for
support of this work.
1
light yellow crystals (1.90 g, 78%). Mp: 151−154 °C. H NMR
REFERENCES
(1) Harder, S. Chem. Rev. 2010, 110, 3852.
■
(C6D6, 400 MHz): δ 0.24 (s, 18H), 0.97 (d, 6H, 6.4 Hz), 1.10 (m,
4H), 1.27 (m, 6H), 1.31 (d, 12H, 6.4 Hz), 3.26 (sept, 2H, 6.8 Hz),
3.35 (m, 4H), 3.50 (sept, 2H, 6.8 Hz), 6.21 (d, 1H, 8.4 Hz), 6.44 (t,
1H, 7.2 Hz), 6.66 (t, 1H, 8.4 Hz), 6.81 (m, 3H), 6.97 (d, 1H, 7.6
Hz), 7.08 (m, 8H), 8.06 (s, 1H). 13C NMR (C6D6, 100 MHz): 6.1
(SiMe3), 24.0, 25.0, 25.5, 28.4 (CHMe2), 29.0, 68.9 (THF), 117.6,
123.6, 123.9, 124.0, 124.1, 124.4, 126.0, 128.2, 128.5, 130.3, 132.2,
134.5, 135.7, 139.7, 141.3, 144.7, 150.7, 151.8 (ArC), 169.2, 171.8
(CN). IR (KBr, cm−1): 3689, 3646, 3372, 3061, 2960, 2868,
1916, 1800, 1621, 1576, 1456, 1316, 1249, 1213, 1128, 1058, 932,
842, 697, 617, 531, 419. Anal. Calcd for C48H70CaN4OSi2 (815.34):
C, 70.71; H, 8.65; N, 6.87. Found: C, 70.16; H, 8.14; N, 6.33.
LYbN(SiMe3)2·THF (3). LYbN(SiMe3)2·THF was synthe-
sized by a method similar to that described for complex 2. The
product was obtained as dark brown crystals (0.67 g, 71%). Mp:
(2) (a) Zhong, Z.; Schneiderbauer, S.; Dijkstra, P. J.; Birg, C.;
Westerhausen, M.; Feijen, J. Macromolecules 2001, 34, 3863.
(b) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2004,
43, 6717. (c) Darensbourg, D. J.; Choi, W.; Ganguly, P.; Richers, C. P.
Macromolecules 2006, 39, 4374. (d) Harder, S.; Feil, F.; Knoll, K.
Angew. Chem., Int. Ed. 2001, 40, 4261.
(3) (a) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc.
2005, 127, 2042. (b) Datta, S.; Roesky, P. W.; Blechert, S.
Organometallics 2007, 26, 4392. (c) Buch, F.; Harder, S. Z. Naturforsch.
2008, 63b, 169.
(4) (a) Buch, F.; Brettar, J.; Harder, S. Angew. Chem., Int. Ed. 2006,
45, 2741. (b) Spielmann, J.; Harder, S. Eur. J. Inorg. Chem. 2008, 1480.
(5) (a) Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock,
P. B.; Procopiou, P. A. Organometallics 2007, 26, 2953. (b) Crimmin,
M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A.
Organometallics 2008, 27, 497.
1
(6) Al-Shboul, T. M. A.; Gorls, H.; Westerhausen, M. Inorg. Chem.
̈
160−163 °C. H NMR (C6D6, 400 MHz): δ 0.30 (s, 18H),
Commun. 2008, 11, 1419.
0.98 (d, 6H, 5.2 Hz), 1.17 (d, 6H, 6.0 Hz), 1.23 (m, 4H), 1.29
(d, 6H, 6.4 Hz), 1.35 (d, 6H, 5.6 Hz), 3.19 (br, 4H), 3.37 (sept,
2H, 6.4 Hz), 3.56 (sept, 2H, 5.6 Hz), 6.24 (d, 1H, 8.4 Hz), 6.44
(t, 1H, 7.2 Hz), 6.72 (t, 1H, 7.6 Hz), 6.81 (m, 3H), 7.07 (m,
9H), 8.21 (s, 1H). 13C NMR (C6D6, 100 MHz): 6.0 (SiMe3),
24.1, 25.4, 25.7, 28.2 (CHMe2), 28.8, 68.8 (THF), 118.1, 123.7,
124.1, 124.3, 125.0, 126.0, 128.0, 128.2, 128.6, 130.0, 132.3,
134.8, 135.8, 139.9, 141.6, 144.7, 150.2, 151.0 (ArC), 168.3,
170.8 (CN). IR (KBr, cm−1): 3645, 3060, 3025, 2960, 2868,
1917, 1620, 1534, 1440, 1402, 1362, 1317, 1248, 1214, 1162,
1128, 1044, 932, 898, 842, 754, 696, 619, 531, 468, 415. Anal.
Calcd for C48H70YbN4OSi2 (948.31): C, 60.79; H, 7.44; N,
5.91. Found: C, 60.51; H, 7.18; N, 5.63.
(7) (a) Kazankova, M. A.; Efimova, I. V.; Kochetkov, A. N.;
Afanes’ev, V. V.; Beletskaya, I. P.; Dixneuf, P. H. Synlett 2001, 497.
(b) Shulyupin, M. O.; Kazankova, M. A.; Beletskaya, I. P. Org. Lett.
2002, 4, 761.
(8) (a) Takaki, K.; Takeda, M.; Koshoji, G.; Shishido, T.; Takehira,
K. Tetrahedron Lett. 2001, 6357. (b) Takaki, K.; Komeyama, K.;
Takehira, K. Tetrahedron 2003, 59, 10381. (c) Takaki, K.; Koshoji, G.;
Komeyama, K.; Takeda, M.; Shishido, T.; Kitani, A.; Takehira, K.
J. Org. Chem. 2003, 68, 6554.
(9) (a) Bambirra, S.; Leusen, D.; Meetsma, A.; Hessen, B.; Teuben, J.
H. Chem. Commun. 2003, 4, 522. (b) Bambirra, S.; Bouwkamp, M. W.;
Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 9182. (c) Zhang,
L.; Nishiura, M.; Yuki, M.; Luo, Y.; Hou, Z. Angew. Chem., Int. Ed.
2008, 47, 2642.
(10) Barrett, A. G. M.; Brinkmann, C.; Crimmin, M. R.; Hill, M. S.;
Hunt, P.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 12906.
(11) (a) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993,
12, 2618. (b) Forsyth, C. M.; Nolan, S. P.; Stern, C. L.; Marks, T. J.;
Rheingold, A. L. Organometallics 1993, 12, 3618. (c) Lee, L.; Berg, D.
J.; Bushnell, G. W. Organometallics 1995, 14, 5021. (d) Barrett, A. G.
M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Lomas, S. L.;
Procopioud, P. A.; Suntharalingama, K. Chem. Commun. 2009, 2299.
(12) Perrier, A.; Comte, V.; Molse, C.; Gendre, P. L. Chem. Eur. J.
2010, 16, 64.
(13) Sheldrick, G. M. SHELXS-90/96, Program for Structure
Solution. Acta Crystallogr., Sect. A 1990, 46, 467.
(14) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
X-ray Structural Determination. The X-ray data were
collected on a Rigaku Saturn CCD diffractometer using graphite-
monochromated Mo Kα radiation (λ = 0.710 73 Å) at 113 K. The
structure was solved by direct methods (SHELXS-97)13 and refined
by full-matrix least squares on F2. All non-hydrogen atoms were
refined anisotropically, and hydrogen atoms were refined by a riding
model (SHELXL-97).14 Crystal data for 2: C48H70CaN4OSi2, fw =
̈
815.34, hexagonal, space group R3, a = 50.829(8) Å, c = 11.144(2)
̅
Å, V = 24934(7) Å3, Z = 18, ρcalcd = 0.977 g cm−3, 69 723 reflections,
9754 unique reflections(Rint = 0.0793), R1 = 0.1050 (I > 2σ(I)),
wR2 = 0.2716 (all data).
Refinement; University of Gottingen, Gottingen, Germany, 1997.
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ASSOCIATED CONTENT
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* Supporting Information
Text, figures, and CIF files giving experimental details for the
synthesis and characterization of the compounds in this paper
and crystal structure data for 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
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