Highly fluorinated tris(indazolyl)borate silylamido complexes of the heavier alkaline earth metals: Synthesis, characterization, and efficient catalytic intramolecular hydroamination

Article abstract of DOI:10.1002/chem.201405454

Heteroleptic silylamido complexes of the heavier alkaline earth elements calcium and strontium containing the highly fluorinated 3-phenyl hydrotris(indazolyl)borate {F₁₂-Tp^4Bo,3Ph}^- ligand have been synthesized by using salt metathesis reactions. The homoleptic precursors [Ae{N-(SiMe₃)₂}₂] (Ae=Ca, Sr) were treated with [Tl(F₁₂-Tp^4Bo,3Ph)] in pentane to form the corresponding heteroleptic complexes [(F₁₂-Tp^4Bo,3Ph)Ae{N(SiMe₃)₂}] (Ae=Ca (1); Sr (3)). Compounds 1 and 3 are inert towards intermolecular redistribution. The molecular structures of 1 and 3 have been determined by using X-ray diffraction. Compound 3 exhibits a Sr...MeSi agostic distortion. The synthesis of the homoleptic THF-free compound [Ca{N(SiMe₂H)₂}₂] (4) by transamination reaction between [Ca{N(SiMe₃)₂}₂] and HN(SiMe₂H)₂ is also reported. This precursor constitutes a convenient starting material for the subsequent preparation of the THF-free complex [(F₁₂-Tp^4Bo,3Ph)Ca{N(SiMe₂H)₂}] (5). Compound 5 is stabilized in the solid state by a Ca...β-Si-H agostic interaction. Complexes 1 and 3 have been used as precatalysts for the intramolecular hydroamination of 2,2-dimethylpent-4-en-1-amine. Compound 1 is highly active, converting completely 200 equivalents of aminoalkene in 16 min with 0.50 mol% catalyst loading at 25°C.

Full text of DOI:10.1002/chem.201405454

DOI: 10.1002/chem.201405454
Full Paper
&
Alkaline Earth Metals |Hot Paper|
Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of
the Heavier Alkaline Earth Metals: Synthesis, Characterization,
and Efficient Catalytic Intramolecular Hydroamination
[
a, b]
[c]
[c]
[c]
Nuria Romero,[
Laure Vendier,
Sorin-Claudiu Ro s¸ ca, Yann Sarazin, Jean-FranÅois Carpentier,
a, b]
[d]
[a, b]
[a, b]
Sonia Mallet-Ladeira, Chiara Dinoi,*
and Michel Etienne*
Abstract: Heteroleptic silylamido complexes of the heavier
alkaline earth elements calcium and strontium containing
the highly fluorinated 3-phenyl hydrotris(indazolyl)borate
compound [Ca{N(SiMe H) } ] (4) by transamination reaction
2 2 2
between [Ca{N(SiMe ) } ] and HN(SiMe H) is also reported.
3
2 2
2
2
This precursor constitutes a convenient starting material for
the subsequent preparation of the THF-free complex
4Bo,3Ph ꢀ
{
F -Tp
}
ligand have been synthesized by using salt
1
2
4Bo,3Ph
metathesis reactions. The homoleptic precursors [Ae{N-
[(F -Tp
)Ca{N(SiMe H) }] (5). Compound 5 is stabilized in
2 2
1
2
4
Bo,3Ph
(
SiMe ) } ] (Ae=Ca, Sr) were treated with [Tl(F -Tp
)] in
the solid state by
a
Ca···b-SiꢀH agostic interaction.
3
2 2
12
pentane to form the corresponding heteroleptic complexes
Complexes 1 and 3 have been used as precatalysts for the
intramolecular hydroamination of 2,2-dimethylpent-4-en-1-
amine. Compound 1 is highly active, converting completely
200 equivalents of aminoalkene in 16 min with 0.50 mol%
catalyst loading at 258C.
4Bo,3Ph
[
(F -Tp
)Ae{N(SiMe ) }] (Ae=Ca (1); Sr (3)). Compounds
3 2
1
2
1
and 3 are inert towards intermolecular redistribution. The
molecular structures of 1 and 3 have been determined by
using X-ray diffraction. Compound 3 exhibits a Sr···MeSi
agostic distortion. The synthesis of the homoleptic THF-free
2
+
2+
Introduction
radii and cation charge densities (Mg =0.72 ꢀ; Ca =1.00 ꢀ;
2
+
2+
[9]
Sr =1.18 ꢀ, Ba =1.35 ꢀ for six-coordinate ions). Their
chemistry is therefore largely governed by electrostatic and
steric factors, featuring highly ionic and essentially non-direc-
tional bonding. One consequence is the deleterious Schlenk
equilibrium that redistributes ligands in a heteroleptic complex
The synthesis of nitrogen-containing molecules through
metal-catalyzed hydroamination of CꢀC multiple bonds offers
a perfectly atom-economic route to the production of fine
[
1]
chemicals. Whereas the catalytic activity of transition metal
and rare earth complexes for the addition of a NꢀH bond
across CꢀC multiple bonds has been largely described in the
[(L X)AeX’] yielding a mixture of the two homoleptic species
n
[(L X) Ae] and [AeX’ ] (Scheme 1). This equilibrium is often
n
2
2
[
2–6]
literature,
the use of alkaline earth (Ae) metal complexes as
problematic for catalysis, and its control by the appropriate
choice of ligands remains a challenge in this chemistry.
[7,8]
hydroamination catalysts is only now starting to emerge.
Ae
metals are abundant, oxophilic, and redox-inactive in their cat-
ionic form. They are characterized by a wide range of ionic
Scheme 1. Schlenk-type equilibrium with heteroleptic Ae complexes.
[
a] N. Romero, Dr. L. Vendier, Dr. C. Dinoi, Prof. M. Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination)
BP 44099, 205 route de Narbonne, 31077 Toulouse Cedex 4 (France)
E-mail: michel.etienne@lcc-toulouse.fr
Intermolecular hydroamination reactions are extremely
chiara.dinoi@lcc-toulouse.fr
challenging and only a limited number of reports involving Ae
[b] N. Romero, Dr. L. Vendier, Dr. C. Dinoi, Prof. M. Etienne
[10–13]
catalysts are available in the literature.
By contrast, the
Universitꢀ de Toulouse, UPS, INPT, LCC
entropically favored intramolecular version of this process has
3
1077 Toulouse Cedex 4 (France)
[13–22]
been investigated in more detail.
Both homo and hetero-
[
c] S.-C. Ro s¸ ca, Dr. Y. Sarazin, Prof. J.-F. Carpentier
Institut des Sciences Chimiques de Rennes
UMR 6226 CNRS-Universitꢀ de Rennes 1
leptic alkaline earth complexes are competent cyclohydroami-
nation precatalysts and, in general, the activity trend observed
for such reactions is Ca>Sr>Ba; on the other hand, no defini-
tive trend in their reactivity with respect to the nature of the
ancillary ligands emerges so far. With the readily available ho-
Campus de Beaulieu, 35042 Rennes Cedex (France)
[d] S. Mallet-Ladeira
Institut de Chimie de Toulouse (FR 2599), Universitꢀ Paul Sabatier
118 route de Narbonne, 31062 Toulouse Cedex 9 (France)
moleptic compounds [M{N(SiMe ) } (thf) ] (M=Ca, Sr, Ba; x=0
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405454.
3 2 2
x
or 2), the THF-free complexes displayed lower catalytic activity
Chem. Eur. J. 2015, 21, 4115 – 4125
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Full Paper
than their THF-containing analogues, except in the case of
substrates with secondary amine functionalities; moreover, the
strontium systems were less active than their calcium
Earlier reports have shown that sterically encumbered,
differently substituted tris(pyrazolyl)borates (Tp’) coordinate
2
+
[32]
Ca with the formation of heteroleptic Tp’CaX complexes.
[
15]
tBu
counterparts.
A four-coordinate amido complex [Tp Ca{N(SiMe ) }] was
3 2
tBu
For the heteroleptic Ae species, very bulky ancillary donating
ligands, capable of stabilizing the oxophilic metal center while
providing the steric protection needed to prevent the Schlenk
equilibrium, were employed. The catalytic activities of the
heteroleptic silylamide b-diketiminate derivatives [(BDI)M{N(Si-
Me ) }(thf) ] ((BDI)H=H C[C(Me)N-2,6-(iPr) C H ] ; M=Ca, x=0
obtained with Tp , whereas a five-coordinate THF adduct
iPr
iPr [33]
[Tp Ca{N(SiMe ) }(thf)] was obtained with Tp . These amido
3
2
complexes were shown to catalyze the ring-opening polymeri-
[33]
zation of lactide, but to our knowledge they have not been
tested for hydroamination reactions.
We report here the synthesis of heteroleptic calcium and
3
2
x
2
2
6
3 2
4Bo,3Ph
or 1; M=Sr, x=1) provided useful insight into the mechanism
strontium silylamido complexes [(F -Tp
)Ae{N(SiMe3)2}]
12
[
14,15]
of the reaction.
The Ca precatalyst exhibited better catalyt-
(Ae=Ca; Sr), which are inert to intermolecular redistribution in
ic performances than its Sr analogue. In contrast, the hydrobis
and tris(imidazol-2-ylidene-1-yl)borates, containing C-based
donors instead of N-based donors in the tripodal ligand, result-
ed in the Sr silylamido complexes performing better than their
Ca counterparts. With the N-bound aminotroponiminate or
solution. The synthesis of the homoleptic THF-free compound
[Ca{N(SiMe H) } ] by
a
transamination reaction between
2
2 2
[Ca{N(SiMe ) } ] and HN(SiMe H)2 is also reported. This
3
2 2
2
precursor has then been used for the preparation of
4
Bo,3Ph
[(F -Tp
)Ca{N(SiMe H) }], which exhibits a Ca···b-SiꢀH agos-
1
2
2
2
2,5-bis-[N-(aryl)iminomethyl]-pyrrolyl Ca and Sr silylamido com-
tic distortion. The catalytic activity of these amido complexes
plexes, the activity of the catalysts decreased with increasing
for the intramolecular hydroamination of 2,2-dimethylpent-4-
[
16–18]
4Bo,3Ph
atomic number of the metal.
Asymmetric versions of these
en-1-amine is described. [(F -Tp
)Ca{N(SiMe ) }] is highly
3 2
12
[
19–21]
reactions have also been reported.
active, converting completely 200 equivalents of aminoalkene
Some of us have recently described new Ca, Sr, and Ba
in 16 min at 258C.
heteroleptic [(LO)Ae{N(SiMe R) }(thf) ] (R=H, Me) complexes
2
2
x
bearing different types of N- and O-based amino-phenolate
ꢀ
ligands (LO ). The rates for the cyclohydroamination reactions Results and Discussion
[22]
decreased with increasing atomic number of the metal. For
Synthesis and characterization of
a given ligand framework and metal center, complexes that
4Bo,3Ph
[
(F -Tp
12
)Ae{N(SiMe ) }] (Ae=Ca, Sr)
ꢀ
3 2
contain {N(SiMe ) } were much better precatalysts than those
3
2
ꢀ
[22]
4Bo,3Ph
containing {N(SiMe H) } . Heteroleptic silylamido complexes
Treatment of [Tl(F -Tp
)] with an excess of [Ca{N(SiMe ) } ]
2
2
12
3 2 2
Bo,3Ph
4
[
{N^N}Ae{N(SiMe ) }(thf) ] (Ae=Ca, x=1; Sr, Ba, x=2) contain-
in pentane provided the heteroleptic [(F -Tp
)-
3
2
x
12
ꢀ
ing the bulky imino-anilide ligand {N^N} also catalyzed the
cyclohydroamination reaction efficiently, with rates increasing
Ca{N(SiMe ) }] (1) in 67% yield. Compound 1 was characterized
3 2
by standard analytical/spectroscopic techniques, and the solid-
state structure was analyzed by single-crystal X-ray diffraction
[
13]
in the order Ba<Sr<Ca.
19
We became interested in stabilizing electron-deficient heter-
(see below). The F NMR spectrum in [D ]benzene shows four
6
oleptic alkaline earth complexes [(L X)AeX’] while maintaining
signals (d=ꢀ144.30, ꢀ151.59, ꢀ153.83, and ꢀ163.15 ppm)
n
their reactivity. Our strategy is based on the use of highly fluo-
corresponding to the four benzo fluorine atoms of
4
Bo,3R ꢀ [25–29]
4Bo,3Ph ꢀ
1
rinated hydrotris(indazolyl)borate ligands ({F -Tp
} )
{F -Tp
12
} . The H NMR spectrum displays one singlet for
n
ꢀ
that combine two fundamental properties: 1) The steric
hindrance, which would avoid Schlenk-type redistributions and
the Me groups of the {N(SiMe ) } amido ligand (d=
3
2
ꢀ0.34 ppm) and three multiplets (d=7.54, 7.32, and 7.23 ppm)
for the aromatic protons of {F -Tp } . From the equiva-
4Bo,3Ph ꢀ
2
) the electron-withdrawing ability, which would enhance the
12
[
30]
4Bo,3Ph ꢀ
polarity, hence the reactivity of the AeꢀX bond. Two differ-
ently substituted ligands, {F -Tp } (R=CF , n=21, and R=
lence of the three indazolyl groups of {F -Tp
} in solu-
12
4Bo,3R ꢀ
tion, a time-averaged C3v symmetry can be inferred for 1. The
synthesis and purification of 1 proved to be challenging due
to the involvement of an equilibrium process as summarized
in Scheme 3 and Figure 1.
n
3
Ph, n=12), have been tested on Ca, the phenyl-based ligand
providing the best compromise in terms of steric protection
[
31]
and electron-withdrawing properties (Scheme 2).
The homoleptic compound [Ca{N(SiMe ) } ] was treated with
3
2 2
4Bo,3Ph
[
Tl(F -Tp
)] in [D ]benzene affording 1 quantitatively over
6
1
2
a period of 30 h (Figure 1a–c). After evaporation of the solvent
and re-dissolution in [D ]benzene, however, we observed the
6
4
Bo,3Ph
re-appearance of the characteristic signals of [Tl(F -Tp
)]
12
together with those of 1 (Figure 1d), evidencing that the
reverse reaction had occurred according to the equilibrium
4Bo,3Ph
process in Scheme 3. [Tl(F -Tp
)] is less soluble than 1 in
12
[
D ]benzene and its precipitation displaced the equilibrium
6
towards the formation of the reactants.
The isolation of complex 1 was therefore achieved by carry-
ing out the reaction in pentane. Due to their poor solubility,
Scheme 2. The 3-Ph substituted hydrotris(indazolyl)borate ligand
X=reactive ligand).
(
Chem. Eur. J. 2015, 21, 4115 – 4125
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Scheme 3. Equilibrium process involved in the synthesis of 1 and 3.
4
Bo,3Ph
Figure 2. ORTEP drawing of [(F12-Tp
3 2
)Ca{N(SiMe ) }] 1. Selected bond
lengths [ꢀ] and bond angles [8]: Ca1ꢀN7 2.2342(15), Ca1ꢀN2 2.5084(14),
Ca1ꢀN4 2.4989(16), Ca1ꢀN6 2.4566(15); N7-Ca1-N2 146.79(6), N7-Ca1-N4
126.94(5), N7-Ca1-N6 121.82(5), N2-Ca1-N4 71.79(5), N2-Ca1-N6 83.13(5),
N4-Ca1-N6 85.49(5).
4Bo,3Ph
we suggest is [(F -Tp
)Ca{N(SiMe ) }(thf)] (see below) as
3 2
12
the major compound together with free indazole.
An ORTEP drawing of the solid state molecular structure of
1
is given in Figure 2 along with selected bond lengths and
angles. Compound 1 has a monomeric structure where the Ca
atom is four-coordinate in a distorted tetrahedral geometry.
The B1, Ca1, and N7 (amido) atoms are almost collinear (1658).
1
9
Figure 1. F NMR spectroscopic monitoring of the reaction of
4
Bo,3Ph
4Bo,3Ph
The distortion of the tetrahedron is such that Ca1ꢀN bond
[
Ca{N(SiMe
3
)
2
}
2
] with [Tl(F12-Tp
)];
)Ca{N(SiMe ) }]); c) Reaction mixture over a period of 30 h;
)] in [D
6
]benzene; a) Pure [Tl(F12-Tp
)];
indz
4
Bo,3Ph
b) 1:1 Reaction mixture over a period of 3 h (~ =[Tl(F12-Tp
=[(F -Tp
12
lengths are longer for N2 and N4 (2.5084(14) and 2.4989(16) ꢀ)
than for N6 (2.4566(15) ꢀ) and the angle N2-Ca1-N4 (71.79(5)8)
is smaller than those involving N6 (83.13(5) and 85.49(5)8).
4
Bo,3Ph
*
3 2
6
d) 1:1 Reaction mixture after evaporation and re-dissolution in [D ]benzene.
These parameters are comparable to those observed for the
4
Bo,3Ph
4Bo,3Ph
[
Tl(F -Tp
)] and complex
1
precipitated leaving
homoleptic complex [(F -Tp
*) Ca], even though the latter
2
12
12
[31]
[
Ca{N(SiMe ) } ] and [Tl{N(SiMe ) }] in solution. Following a
exhibits one inverted indazolyl ring. Despite this distortion
and the low coordination number, complex 1 does not show
any agostic-type interaction involving the hexamethyldisilyl-
amido group.
3
2 2
3 2
simple filtration of the reaction mixture, complex 1 was then
separated from [Tl(F -Tp
4Bo,3Ph
)] by extraction with a toluene/
12
pentane (1:3) mixture. The whole process was facilitated by
using an excess of [Ca{N(SiMe ) } ] that further displaced the
The Ca1ꢀN7 bond (2.2342(15) ꢀ) is shorter than the
3
2 2
equilibrium towards the products. Compound 1 was finally
isolated in 67% yield. Complex 1 is very air and moisture
sensitive; small traces of air or water lead to immediate
decomposition, with the formation of an unidentified white
precipitate and HN(SiMe ) . As a result of the presence of the
CaꢀN
bonds of related 4-coordinate complexes
amido
[33,34]
[12,13]
[{BDI}Ca{N(SiMe ) }(thf)],
[{N^N}Ca{N(SiMe ) }(thf)],
and
3
2
3 2
[35]
[(LX )Ca{N(SiMe ) }]
(LX =CH C(2,6-(iPr) C H N)CHC(CH )-
1
3 2
1
3
2
6
3
3
(NCH CH NMe ) (2.313(1), 2.286(3) and 2.303(4) ꢀ, respectively).
2
2
2
This can be ascribed to the presence of the electron-
3
2
4Bo,3Ph ꢀ
4Bo,3Ph ꢀ
bulky {F -Tp
} ligand, compound 1 was found to be inert
withdrawing fluorinated {F -Tp
} ligand in 1.
1
2
12
towards the Schlenk equilibrium. Indeed, heating a [D ]benzene
Four-coordinate Ca complexes are not common. Only two
6
3
solution of 1 at 608C over a period of 2 h did not result in any
four-coordinate Ca complexes bearing k -N-bound ligands
apparent decomposition nor in ligand redistribution.
have been previously described: The alkoxide species
tBu
Addition of THF to a [D ]benzene solution of 1 suggested
[Tp Ca(O-2,6-(iPr) C H )], with the tBu-substituted tris
6
2
6
3
[
33,34]
the formation of
a
mixture of THF amido species
(pyrazolyl)borate ligand,
Ca{N(SiMe3)2}] complex,
b-diketiminate ligand LX1.
and the abovementioned [(LX1)-
containing the tridentate
4Bo,3Ph
[
(F -Tp
)Ca{N(SiMe ) }(thf) ]. Concentration under vacuum
12
3 2
x
1
19
[35]
yielded, on the basis of H and F NMR spectroscopies, what
Chem. Eur. J. 2015, 21, 4115 – 4125
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1
The one-pot procedure employed for the synthesis of sever-
H NMR resonance for the Si-CH groups was observed at all
3
[
33]
al tris(pyrazolyl)borate Ca amido complexes was not success-
temperatures between 298 and 183 K in [D ]toluene. An X-ray
8
ful for the synthesis of 1. Addition of THF or Et O to a 1:1:1
crystal structure has been obtained for 3 for which an ORTEP
drawing is given in Figure 4 along with selected bond lengths
2
4Bo,3Ph
mixture of [KN(SiMe ) ], CaI , and [K(F -Tp
)] provided the
3
2
2
12
Bo,3Ph
4
heteroleptic compounds [(F -Tp
)Ca{N(SiMe ) }L ] in ex-
12
3 2
x
tremely low yields (L=THF, x=2, 1a; L=Et O, x=1, 1b, 4%
2
4
Bo,3Ph ꢀ
yield). Several byproducts either containing the {F -Tp
}
12
ligand or resulting from BꢀN bond cleavage were observed in
all cases. After extraction with a 1:3 toluene/pentane mixture,
4Bo,3Ph
compound 1a yielded [(F -Tp
)Ca{N(SiMe ) }(thf)] (1c) in
3 2
12
7
% yield. Attempts at crystallizing 1a resulted in crystals of
4Bo,3Ph
the indazolate complex [(F -Tp
)Ca(3-PhIndF )(thf) ] (2).
4 2
12
The X-ray molecular structure of 2 (Figure 3) features two coor-
4
Bo,3Ph
Figure 4. ORTEP drawing of [(F12-Tp
3 2
)Sr{N(SiMe ) }] (3). Selected bond
lengths [ꢀ] and bond angles [8]: Sr1ꢀN7 2.4219(18), Sr1ꢀN1 2.6129(16), Sr1ꢀ
N3 2.5784(16), Sr1ꢀN5 2.7629(16); N7-Sr1-N1 104.69(6), N7-Sr1-N3 106.91(6),
N7-Sr1-N5 166.96(6), N1-Sr1-N3 78.20(5), N3-Sr1-N5 62.36(5), N1-Sr1-N5
66.97(5).
and angles. Complex 3 has a monomeric structure and the
metal center is only four-coordinate, a truly remarkable feature
for such a large metal as strontium. Only two heteroleptic
four-coordinate Sr amido complexes have been previously
4
Bo,3Ph
Figure 3. ORTEP drawing of [(F12-Tp
4 2
)Ca(3-PhIndF )(thf) ] (2). Selected
bond lengths [ꢀ] and bond angles [8]: Ca1ꢀN1 2.584(3), Ca1ꢀN3 2.532(3),
Ca1ꢀN5 2.600(3), Ca1ꢀN7 2.400(3), Ca1ꢀN8 2.469(3); N1-Ca1-N5 75.78(8),
N3-Ca1-N5 85.24(9), N3-Ca1-N1 76.69(8), N7-Ca1-N8 32.39(9), N7-Ca1-N3
[39]
described: The [{BDI}Sr{N(SiMe ) }(thf)] and the [{N^N}Sr{N(Si-
3
2
[13]
2
Me ) }(thf)]
amides, containing the k -N-bound b-diketimi-
3
2
nate and imino-anilide ligand, respectively. Compound 3 is
8
3.30(9), N7-Ca1-N5 88.55(9), N7-Ca1-N1 155.40(10), N8-Ca1-N3 110.10(9),
N8-Ca1-N5 108.14(9), N8-Ca1-N1 172.16(9).
therefore the first heteroleptic four-coordinate Sr complex
3
bearing a tripodal k -N-bound ligand. Unlike the Ca analogue
1
, the coordination sphere in 3 is strongly distorted and the
2
[36]
dinated THF molecules and an h –indazolate ligand arising
geometry around Sr is best described as a trigonal bipyramid
with a seemingly vacant equatorial coordination site (Figure 5).
N5, one of the indazolyl nitrogen, and N7, the amido nitrogen,
occupy the apical positions (N7-Sr1-N5 166.96(6)8), and N1 and
N3 define two of the equatorial sites. Remarkably, the vacant
4Bo,3Ph
from fragmentation of the [(F -Tp )] group by BꢀN bond
cleavage. The 3-phenylindazolate ligand directs its phenyl
12
4
Bo,3Ph ꢀ
group syn to the boron of {F -Tp
}
with CaꢀN7
12
(
2.400(3) ꢀ) significantly shorter than CaꢀN8 (2.469(3) ꢀ).
Coordination-induced BꢀN bond cleavage of tris(pyrazolyl)-
equatorial site is occupied by one of the CH groups of the
3
[
[
37]
ꢀ
borates
borates
and hydrobis- and tris-(imidazol-2-ylidene-1-yl)-
{N(SiMe ) } ligand suggesting that an agostic distortion is
3
2
23,24]
have been observed previously. CꢀN bond
present in 3. The two Sr-N-Si angles in 3 are indeed quite dif-
ferent with Sr1-N7-Si1 (106.84(10)8) being significantly less
obtuse than Sr1-N7-Si2 (126.74(10)8). The distance Sr1···C64
(2.996(3) ꢀ) is slightly longer than the sum of the covalent radii
cleavage in a tris(3-phenylpyrazolyl)methanide complex of
[
38]
calcium has also been reported.
The strontium analogue of 1 has also been similarly synthe-
4Bo,3Ph
[40]
sized. [(F -Tp
)Sr{N(SiMe ) }] (3) was isolated in 25% yield
of Sr and C (2.71 ꢀ) and is significantly shorter than Sr1···C62
(ca., 3.81 ꢀ).
1
2
3 2
4
Bo,3Ph
upon reaction of [Tl(F -Tp
)] with an excess of
A
similar barium complex of formula
12
1
19
Me2
[41]
[
Sr{N(SiMe ) } ] in pentane (Scheme 3). The H and F NMR
[(Tp )Ba{N(SiMe ) }(thf) ] has been also described. It shows
3 2 2
3
2 2
3
spectra for 3 are qualitatively similar to those for 1. A single
the favored k (N,N,N) coordination mode of the Tp ligand and
Chem. Eur. J. 2015, 21, 4115 – 4125
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2 2 2
Scheme 4. Synthesis of [Ca{N(SiMe H) } ] 4.
Compound 4 is fluxional as revealed by a variable-tempera-
1
ture H NMR spectroscopic study in the 358–183 K range
(
Figure 6). According to this dynamic process, we propose that
complex 4 is a trinuclear Ca species of the type [Ca{N(Si-
Me H) } ] , displaying a linear array of three calcium atoms,
Figure 5. Different geometrical environments for complexes 1 (left) and 3
right) highlighting the coordination polyhedra (distorted tetrahedron versus
(
2
2 2 3
ꢀ
trigonal bipyramid, respectively) in shaded gray.
bridged by four {N(SiMe H) } units and framed by two termi-
2 2
ꢀ
nal symmetrical {N(SiMe H) } ligands as depicted in Figure 7.
2
2
differently from compound 3, it is six-fold coordinated with
two molecules of THF completing the coordination sphere of
the barium center.
For comparison purposes, the diffusion coefficients of 4 and
[Ca{N(SiMe ) } ] were measured by using DOSY NMR spectra in
3
2 2 2
ꢀ
9
[D ]toluene at 298 K. The values obtained (D =0.6ꢀ10 and
8
t
ꢀ
9
2
ꢀ1
Other Sr complexes featuring an asymmetrical arrangement
1.0ꢀ10 m s for 4 and [Ca{N(SiMe ) } ] , respectively) indi-
3 2 2 2
ꢀ
of the {N(SiMe ) }
ligand have been previously de-
cate that 4 has a higher nuclearity than [Ca{N(SiMe ) } ] . A dif-
3
2
3 2 2 2
[
23,39,42–44]
scribed,
although the difference between the two Sr-
fusion molecular weight analysis against a series of reference
ꢀ
1
N-Si angles is in all cases smaller than that observed in 3. The
Sr1ꢀN5 bond trans to the amido is significantly longer
compounds (D-fw) yielded a molecular weight of 906 gmol
ꢀ1
fully consistent with the trinuclear structure (913 gmol )
proposed for 4 (see the Supporting Information).
(
2.7629(16) ꢀ) than Sr1ꢀN1 and Sr1ꢀN3 (2.6129(16) and
2
.5784(16) ꢀ), those two being in a range typical of [Tp’ Sr]
In the fast exchange limit (358 K, Figure 6), two resonances
2
[
45,46]
complexes.
The Sr1ꢀN7 bond (2.4219(18) ꢀ) is slightly
for the SiH and the SiCH groups were observed at d=4.48
3
shorter than the related bond length reported for the four-co-
ordinate complexes [{BDI}Sr{N(Si-
and 0.37 ppm, respectively (1:6 ratio, label A’’). A first exchange
[
39]
Me ) }(thf)]
and [{N^N}Sr{N(Si-
(2.446(2) and
3
2
[
13]
Me ) }(thf)]
3
2
2.463(2) ꢀ, respectively), outlin-
ing the electron-withdrawing
properties of the fluorinated {F -
12
4
Bo,3Ph ꢀ
Tp
} ligand.
Synthesis and characterization
of bis(dimethylsilylamido)
calcium complexes [{LX}Ca{N
(
SiMe H) }] ({LX}=N(SiMe H)
2
4
2
2
2,
Bo,3Ph
F -Tp
)
12
[
47–51]
As reported for lanthanides
[
52]
and Ae complexes,
the re-
1
ꢀ
8
Figure 6. Variable-temperature H NMR spectra of 4 in [D ]toluene. Low field enlargement=SiH region; high field
placement of {N(SiMe ) } by
3
2
ꢀ
3 2 2
enlargement=SiCH region. *=residual HN(SiMe H) .
{
N(SiMe H) } can impart a stabi-
2 2
lizing effect by the formation of
internal b-SiꢀH agostic interactions. Whereas [Ca{N(Si-
Me H) } (thf)] was known, its THF-free analogue [Ca{N(Si-
2
2 2
Me H) } ] (4), a potentially very attractive starting material for
2
2 2
Ca chemistry, was not. We first describe the synthesis and
properties of
4
before presenting those of [(F -
12
4
Bo,3Ph
Tp
)Ca{N(SiMe H) }] (5).
2
2
Following a similar procedure to that used for the synthesis
[
52]
of [Ca{N(SiMe H) } (thf)],
transamination between HN(Si-
2
2 2
[
53–55]
Me H) and [Ca{N(SiMe ) } ]
in pentane at 08C yielded the
2
2
3 2 2
pure homoleptic [Ca{N(SiMe H) } ] (4) in 87% yield (Scheme 4).
2
2 2
Compound 4 is a highly air and moisture-sensitive white solid
for which satisfactory elemental analyses were obtained.
Despite numerous attempts, we were unable to crystallize 4.
Figure 7. Schematic view of the proposed trinuclear structure for
[Ca{N(SiMe
2
H)
2
}
2
] (4) with labels according to the NMR spectra.
Chem. Eur. J. 2015, 21, 4115 – 4125
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process was revealed upon cooling the sample below 338 K in
1:1:1:1 (B–E) reflects the four different types of bridging
ꢀ
which each of the SiH and SiCH3 split to give each two
{N(SiMe H) } groups with significant b-SiꢀH agostic interac-
2
2
°
3
resonances (A and A’, Figure 6) in a 1:2 ratio (DG
tions for the E sites (Figure 7). From the NMR spectroscopic
data, there is no way to assign which resonance corresponds
to the non-equivalent B, C, and D sites in Figure 7.
30
ꢀ
1 [56]
ꢁ
69 kJmol ).
Upon further cooling, line-broadening
occurred only for each of the more shielded signals A’ charac-
terizing a second exchange process; below 253 K, the A’ sig-
With the exception of the agostic interactions, the only
previous example of a trinuclear homoleptic Ca complex is
nals of the SiH and SiCH groups split into four and eight sepa-
3
°
[61]
rate resonances (B–E, Figure 6), respectively (DG 2
[Ca (tBu pz) ], which exhibits a similar variable-temperature
53
3
2
6
ꢀ
1
[57]
1
ꢁ
40 kJmol ).
In the slow exchange limit (183 K), the
H NMR behavior. Trinuclear homoleptic Mg compounds
displaying two different ligand environments in a 1:2 ratio for
1
H NMR spectrum showed five types of SiMe H signals, (A and
2
[62–65]
B–E, Figure 6) in a 2:1:1:1:1 ratio. They correspond to five
the terminal and bridging positions are more common.
Trinuclear divalent lanthanide complexes of composition
inequivalent SiMe H groups, likely dependent on the presence
2
[66]
[67]
of different b-SiꢀH agostic interactions with Ca.
Ln{[m-N(SiHMe ) ] Ln[N(SiHMe ) ](thf)} (Ln=Sm,
Yb,
and
2
2 2
2 2
2
1
[68]
Evidence for b-SiꢀH agostic interactions in 4 came from a H-
Eu ) have been also described. As in the case of complex 4,
the coordinatively unsaturated and electron-deficient Ln(II)
centers favor the formation of close Ln···b-SiꢀH agostic interac-
29
Si HMQC 2D NMR experiment at 183 K. Figure 8 shows the
29
correlations between five different Si signals at d=ꢀ23.7,
tions involving all of the SiHMe groups. Extensive Eu···b-SiꢀH
2
secondary interactions, finally, have been also reported very re-
cently for the donor-free, mixed-valence, trinuclear complex
II
III
[68]
Eu {[m-N(SiHMe ) ]Eu [N(SiHMe ) ] } .
2
2
2 2 3 2
Complex 4 has been used as a precursor for the synthesis of
4
Bo,3Ph
the heteroleptic compound [(F -Tp
)Ca{N(SiMe H) }] (5).
2 2
12
4Bo,3Ph
Treatment of [Tl(F -Tp
)] with an excess of 4 in pentane
12
over a period of 2 days, followed by extraction with a 1:3
toluene/pentane mixture, provided 5 in 37% yield (Scheme 5).
1
29
Figure 8. SiꢀH region enlargement of the H- Si HMQC 2D spectrum of 4 in
]toluene at 183 K.
[D
8
ꢀ
15.3, ꢀ14.0, ꢀ13.9, and ꢀ11.8 ppm and the H , H , H , H ,
A
C
B
D
1
and H protons, respectively ( J =152, 146, 143, 147, and
Scheme 5. Synthesis of complex 5.
E
SiH
1
1
1
22 Hz, respectively). J coupling constants in the range 140–
SiH
[
52,58]
60 Hz suggest mild agostic interactions.
For SiH , a stron-
E
1
ger Ca···b-SiꢀH interaction is reflected by the lower J value
Compound 5 was characterized by elemental analysis, NMR,
and IR spectroscopies. In the H NMR spectrum at 298 K, the
SiH
[
52,59,60]
1
of 122 Hz.
The presence of SiꢀH agostic interactions was
further corroborated by solid-state IR spectroscopy: Whereas
SiH resonance of 5 (d=4.17 ppm) is significantly upfield with
respect to the free amine (d=4.92 ppm). In the H- Si HMQC
ꢀ
1
1
29
nSiH of HN(SiMe H) is found at 2122 cm , three n absorp-
2
2
SiH
ꢀ
1
tions are observed for 4 in the region 1900 to 2030 cm . The
lowest nSiH at 1905 cm , significantly lower than that for
2D NMR spectrum, the SiH resonance is observed at d=
ꢀ
1
1
ꢀ24.1 ppm as a doublet with J =159 Hz. As observed above
SiH
[
52]
[Ca{N(SiMe H) } (thf)], most likely reflects a stronger Ca···b-
for 4, this time-average value is indicative of a mild (140–
2
2 2
[
52][58]
SiꢀH interaction. The presence of three n frequencies in the
160 Hz) to weak (160–170 Hz) agostic interaction.
No
SiH
SiꢀH stretching modes region has been also reported for ho-
decoalescence of the SiH signal was observed by variable-
1
moleptic TMEDA- or THF-coordinated Ca complexes containing
temperature H NMR from 298 to 193 K in [D ]toluene. The
8
ꢀ
2
[60]
the {C(SiMe H) } ligand.
solid-state IR data confirmed the presence of a b-SiꢀH agostic
2
Overall, exchange of terminal and bridging amido groups in
affords a single set of resonances in the fast exchange limit
interaction. Compound 5 exhibited one n absorption at
SiH
ꢀ
1
ꢀ1
4
2044 cm and a second remarkably low band at 1888 cm ,
characteristic of substantial Ca···b-SiꢀH interaction.
(A’’, Figure 6). By decreasing the temperature, the resonances
ꢀ
for the terminal and bridging groups of the {N(SiMe H) }
The solid-state structure of 5 was determined by X-ray
diffraction (Figure 9). The metal center is four-coordinate, an
2
2
ligands split into two signals in a 1:2 ratio (A and A’). At lower
temperature, finally, the separation of the bridging SiH and
SiCH3 signals (A’) into four well-defined peaks with a ratio
uncommon feature for Ca complexes, with a short CaꢀN
amido
distance (CaꢀN7=2.261(2) ꢀ), barely longer than that of 1. Ex-
Chem. Eur. J. 2015, 21, 4115 – 4125
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1
ꢀ
24.1 ppm with J =160 Hz. Addition of some drops of THF
SiH
to a [D ]benzene solution of 5 led to similar spectra and
6
behavior.
Cyclohydroamination catalysis
Our efforts towards the synthesis of 1 were rewarded upon
examination of its catalytic performance for the cyclohydro-
amination of 1-amino-2,2-dimethyl-4-pentene (S) (Scheme 6),
a common model aminoalkene for which abundant data are
available in the literature. Representative results are gathered
in Table 1.
Scheme 6. Cyclohydroamination reaction of S catalyzed by 1.
4
Bo,3Ph
Figure 9. ORTEP drawing of [(F12-Tp
2 2
)Ca{N(SiMe H) }] 5. Selected bond
lengths [ꢀ] and bond angles [8]: Ca1ꢀN7 2.261(2), Ca1ꢀN2 2.481(2), Ca1ꢀN4
Table 1. Representative data for the cyclohydroamination of S catalyzed
by 1 at 258C.
2
.474(2), Ca1ꢀN6 2.458(2) Ca1ꢀH2a 2.41(2); N7-Ca1-N2 141.26(8), N7-Ca1-N4
133.31(8), N7-Ca1-N6 125.26(8), N2-Ca1-N4 75.97(7), N2-Ca1-N6 75.80(7),
N4-Ca1-N6 82.40(7).
Entry
[1]/[S]
t
Conversion
[%]
[
d]
[
min]
[
[
[
[
a]
a]
b]
c]
1
2
3
4
1:50
6
99
99
85
57
amination of the Ca{N(SiMe H) } fragment strongly supports
1:200
1:400
1:600
16
27
18
2
2
the presence of an agostic Ca···b-SiꢀH contact, as the two
SiMe H moieties are clearly non-equivalent. This is reflected by
2
the difference between the obtuse Ca1-N7-Si1 and much more
acute Ca1-N7-Si2 angles (134.99(13) and 97.92(10)8, respective-
ly), corresponding to a much shorter Ca1···Si2 distance
[a] Reaction conditions: 3.0 mmol of precatalyst, 1.2 mL of [D
6
6
6
]benzene.
]benzene.
]benzene.
[
[
[
b] Reaction conditions: 3.0 mmol of precatalyst, 1.8 mL of [D
c] Reaction conditions: 3.0 mmol of precatalyst, 1.7 mL of [D
1
d] The conversions were determined by H NMR spectroscopic
(
3.0003(9) ꢀ) with respect to Ca1···Si1 (3.655(1) ꢀ). Interestingly,
the agostic hydrogen H2a has been unambiguously localized
Ca1···H2a=2.41(2) ꢀ), affording a nearly planar Ca1N7Si2H2a
core (torsion angle 6.58). Similar geometric features have been
monitoring
(
In all cases, the ring-closure of S proceeded according to
Baldwin’s rules (5-exo-trig) leading to the exclusive formation
of 2,4,4-trimethylpyrrolidine. Virtually complete conversion of
50 and 200 equivalents of S was observed within 6 and
16 min, respectively, at room temperature (turnover number,
[
69]
noted for other heteroleptic silylamido and tris(dimethylsi-
[
60,70]
lyl)methyl
Ca complexes that also exhibit b-SiꢀH agostic
interactions. The presence of this interaction is likely to confer
a certain degree of stability, since 5 appeared significantly less
reactive towards air and moisture than 1.
TON=50 and 200; apparent turnover frequency, TOF =8
app
ꢀ
1
Attempts at synthesizing the THF adduct of 5 were
and 12 min ). This reactivity highlights the highly effective
4Bo,3Ph
less successful. The treatment of [Tl(F -Tp
)] with
catalytic activity of 1, which, although comparable with the
[{NN}Ca(X)(thf)] systems (X=N(SiMe3)2 or CH(SiMe3)2),
12
[13]
[
Ca{N(SiMe H) } (thf)] in pentane over a period of 2 days pro-
out-
2
2 2
vided, after extraction with a 1:3 toluene/pentane mixture,
performs that of the homoleptic complexes [Ca{N(Si-
4Bo,3Ph
[15]
[
(F -Tp
)Ca{N(SiMe H) }(thf)] (6) as the major compound in
Me ) } (thf) ] (x=0 or 2) or [Ca{CH(SiMe ) } (thf) ], as well as
1
2
2
2
3 2 2
x
3 2 2
2
[
23]
36% yield. Despite numerous attempts, it proved impossible
that of the heteroleptic bis(imidazolin-2-ylidene-1-yl)borate-,
[22]
[16]
[18]
to fully purify and analytically characterize 6; crystals of 6 suita-
ble for X-ray diffraction could not be obtained. In addition to
amino-phenolate-,
b-diketiminate-based
aminotroponiminate-,
14,15]
pyrrolyl-,
and
[
amido species. Therefore, compound
1
19
6
, H and F NMR spectroscopies revealed the presence of sev-
1 represents one of the most efficient precatalysts reported to
date for the cyclohydroamination of S. When large loadings of
400 and 600 equivalents of aminoalkene were used, the
conversion peaked at 85 and 57% after 27 and 18 min,
4Bo,3Ph ꢀ
eral unidentified species containing {F -Tp
} . The
12
19
F NMR spectrum of 6 (and the other species) shows four sig-
nals for the benzo fluorines of {F -Tp
4
Bo,3Ph ꢀ
1
} . The H NMR
12
ꢀ
1
spectrum of 6 shows SiH and SiCH resonances at d=4.69 and
respectively (TON=340 and 342, TOFapp =12 and 19 min ).
These results were found quite reproducible and can be
ascribed to catalyst decomposition, consistently with the ob-
served formation of an unidentified white precipitate during
3
0
.01 ppm, respectively, as well as THF signals whose integra-
1
29
tion is in accord with a mono adduct. In the H- Si HMQC 2D
NMR spectrum, the SiꢀH resonance is observed at d=
Chem. Eur. J. 2015, 21, 4115 – 4125
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[
72]
the catalytic runs. Such catalyst decay at higher substrate load-
ings may be due to either the very high sensitivity of these Ae
species towards residual impurities (moisture or other uniden-
tified factors) contained in the substrate/solvent or intrinsic
decomposition pathways of complex 1 (or species derived
thereof) under the catalytic conditions. Kinetic monitoring was
phenoxy-amine ligands. This rate law is consistent with a
mechanism implying a fast s-insertive pathway followed by
rate-limiting aminolysis, as recently computed for both intra-
[73–75]
and intermolecular hydroamination.
Alternatively, it may also be accounted for by the interaction
between two substrate molecules and the metal center, with
the formation of a six-membered transition state involving
concerted ring-closure and proton transfer, as the turnover-lim-
1
performed by H NMR spectroscopy at 08C since at 258C com-
plex 1 was found too active for reliable measurements. Sub-
strate consumption followed first-order kinetics; no induction
period was observed (Figure 10). The semi-logarithmic plots of
[71]
M
III
iting step. DFT computations performed on {Cpo }Y alkyl,
M
II
{To }Mg alkyl, and anilido-imine-Ae complexes suggested that
the latter scenario may be significantly more energy demand-
M
ing, and therefore less realistic ({Cpo }=cyclopentadienyl-
M
[73–75]
bis(oxazolinyl)borate, {To }=tris(oxazolinyl)borate).
The strontium analogue 3 was also evaluated in the intra-
molecular hydroamination reaction of S, displaying a much
lower catalytic activity than its Ca analogue. The reaction with
50 equivalents of the aminoalkene at 258C only provided a con-
version of 10% in 7 min. Signs of catalyst decomposition were
noted with also the formation of a white precipitate at the
bottom of the NMR tube within seconds after the reactants
were mixed. Apparently, the active species in the strontium
[76]
case decomposes much faster than in the calcium case.
Conclusion
Figure 10. Semilogarithmic plot of substrate conversion versus reaction time
for the cyclohydroamination of S (0.12, 0.24, and 0.31m) catalyzed by 1.
Reaction conditions: 08C, 3.0 mmol of precatalyst in [D ]toluene (1.2 mL).
8
The synthesis and characterization of the silylamido complexes
4Bo,3Ph
[
(F -Tp
)Ae(NR )] (R=SiMe , Ae=Ca, Sr; R=SiMe H, Ae=
2 3 2
1
2
Ca) supported by the bulky, highly fluorinated, hydrotris(inda-
4
Bo,3Ph ꢀ
zolyl)borate ligand {F -Tp
}
have been detailed. These
12
monomer conversion versus reaction time were linear over 3
heteroleptic complexes, free of ethereal ligands, are inert in so-
half-lives, with the apparent rate constants k values ranging
lution, showing a remarkable inertness towards Schlenk equili-
app
ꢀ1
4Bo,3Ph
from 0.0005 to 0.0013 s for 125 and 50 equivalents of S, re-
bria. The solid-state structure of [(F -Tp
)Sr{N(SiMe ) }] (3)
3 2
12
ꢀ
1
spectively (with kapp =k[1] expressed in s , in which k is the re-
displays an agostic interaction between the electron-deficient
ꢀ
1
ꢀ1
2+
ꢀ
action rate constant in Lmol s , see below). The reaction
rates decreased when higher substrate concentrations were
used, pointing to a possible catalyst inhibition by the product.
First-order dependence upon substrate concentration has
also been observed for other heavier Ae-based catalytic
Sr center and one of the SiꢀCH groups of the {N(SiMe ) }
3
3 2
ligand. The synthesis of the new homoleptic diamido complex
[Ca{N(SiMe H) } ] (4) has been described. On the basis of its
2
2 2
fluxional behavior, a trinuclear structure with additional b-SiꢀH
agostic interactions has been proposed for 4. This precursor
[
13,22]
systems,
although reaction rates with zeroth-order in
or inversely proportional to substrate concen-
has enabled the synthesis of the heteroleptic THF-free com-
[
22]
4Bo,3Ph
substrate
pound [(F -Tp
)Ca{N(SiMe H)}] 5, also stabilized by a
2
1
2
[
14,15]
tration may also be possible.
b-SiꢀH agostic contact.
To determine the effect of the precatalyst concentration
Complex 1 proved to be particularly efficient for the catalytic
cyclohydroamination of 1-amino-2,2-dimethyl-4-pentene, rep-
resenting one of the most active precatalysts reported to date
for this reaction. However, the activity of 1 seems limited by
catalyst degradation over time with large substrate loadings
(much higher than those reported in the literature), a feature
apparently even more obvious for the strontium analogue 3.
Despite stability issues largely due to BꢀN bond cleavage,
upon the reaction rate, the cyclohydroamination of S was per-
formed at 08C with initial concentrations [1] varying from 3.60
o
to 7.14 mm. The linear plot of ln(kapp) versus ln([1] ) had
0
a slope of 1.05 suggesting first-order dependence upon pre-
catalyst concentration (see the Supporting Information). Thus,
the rate law for the cyclohydroamination of the aminoalkene S
catalyzed by 1 is given in Equation (1).
4
Bo,3Ph ꢀ
the highly fluorinated ligand {F -Tp
} forms well-defined
12
r ¼ k½1ꢂ½Sꢂ
ð1Þ
heteroleptic Ca and Sr silylamido complexes. Its electron-with-
drawing properties coupled to the bulky tripodal scaffold en-
hance the polarity and hence the reactivity of the AeꢀN bond
yet avoiding Schlenk-type redistributions. This is remarkable
since there are only very few ligands capable of preventing
scrambling equilibria during catalysis, especially in the case of
Catalytic systems based on heavier alkaline earth metals that
feature such a kinetic rate law are uncommon. Examples in-
[13]
clude the imino-anilide Ba-amido, and -alkyl precatalysts.
The same rate law was also established for Mg-based
[71]
[12,13]
precatalysts supported by tris(oxazolinyl)phenylborate
or
Sr.
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4
Bo,3Ph
One-pot synthesis of [(F -Tp
)Ca{N(SiMe ) }(thf) ] (x=2, 1a;
Experimental Section
12
3
2
x
x=1, 1c): KN(SiMe ) (0.068 g, 0.343 mmol), CaI2 (0.101 g,
3
2
4
Bo,3Ph
All operations were performed with rigorous exclusion of air and
moisture, using standard Schlenk, high-vacuum, and glovebox
techniques under Ar (O <3 ppm, H O<1 ppm). All solvents were
0.343 mmol), K(F12-Tp
) (0.290 g, 0.343 mmol), and THF (10 mL)
were added to a Schlenk flask. The mixture was stirred at room
temperature for 8 h. After removal of volatiles under vacuum, the
resulting solid was washed with pentane, extracted with toluene
and filtered, affording a solution of 1a. After removal of volatiles
under vacuum, the solid was washed with pentane and extracted
into a toluene/pentane (1:3 v/v) mixture, affording 1c in 7% yield.
2
2
dried and distilled under Ar (THF over Na/benzophenone; acetone
over drierite; toluene over Na; pentane over CaH ) and further de-
2
gassed by freeze–pump–thaw cycles. Deuterated solvents were
dried over molecular sieves, filtered, and degassed by several
freeze–pump–thaw cycles and stored in sealed ampules in the glo-
Compound 1c decomposes in solution over a period of few hours.
4
Bo,3Ph
[28]
1
vebox. [Tl(F -Tp
)] was prepared as previously reported. CaI2
Compound 1a: H NMR ([D
6
]benzene, 300 MHz): d=7.81 (dd, JHH
), 7.27 (t,
(brm, 8H, OCH ), 0.81 (brm, 8H, CH
=
1
2
3
and SrI2 (Aldrich; anhydrous beads, ꢀ10 mesh, 99.999% trace
8.1, 3.0 Hz, 6H, o-C
6
H
5
J
HH =7.6 Hz, 6H, m-C
6
H
5
), 2.89
);
metal basis) were used as received. HN(SiMe H) (97%) was dried
2
2
), ꢀ0.10 ppm (s, 18H, SiCH
3
2
2
19
over activated 3 ꢀ molecular sieves. HN(SiMe ) was distilled over
F NMR ([D
6
]benzene, 282 MHz): d=ꢀ143.82 (t, JFF =17.1 Hz, 3 F,
3
2
CaH . The substrate 1-amino-2,2-dimethyl-4-pentene (S) was pre-
F-4), ꢀ151.32 (s, 3 F, F-6), ꢀ154.39 (m, 3 F, F-7), ꢀ164.14 ppm (t,
2
[
77]
1
pared according to literature methods. KN(SiMe ) was prepared
J
FF =20.3 Hz, 3 F, F-5); 1c H NMR ([D
6
]benzene, 300 MHz): d=7.75
3
2
3
from KH and HN(SiMe ) following the same procedure used for
(dd, JHH =8.3, 2.8 Hz, 6H, o-C
2.76 (br m, 4H, OCH ), 0.69 (brm, 4H, CH
2
6
H
), 7.29 (t, JHH =7.6 Hz, 6H, m-C
5
H ),
6 5
3
2
[52]
KN(SiMe H) .
[Ca{N(SiMe ) } ] and [Sr{N(SiMe ) } ] were synthe-
2
), ꢀ0.15 ppm (s, 18H,
2
2
3 2 2
3 2 2
19
tized from KN(SiMe ) and either CaI or SrI according to literature
SiCH
3
); F NMR ([D
6
]benzene, 298 K, 282 MHz): d=ꢀ143.93 (t, JFF
=
3
2
2
2
[78,79]
procedures.
reported.
[Ca{N(SiMe H) } (thf)] was prepared as previously
18.1 Hz, 3 F, F-4), ꢀ151.37 (s, 3F, F-6), ꢀ154.25 (t, JFF =17.8 Hz, 3 F,
2
2 2
[
52]
F-7), ꢀ164.14 ppm (t, JFF =20.3 Hz, 3 F, F-5).
4
Bo,3Ph
One-pot synthesis of [(F -Tp
)Ca{N(SiMe ) }(Et O)] (1b):
3 2 2
Unless stated otherwise, NMR spectra were recorded by using J.
Young valve NMR tubes at 298 K using Bruker DPX 300 ( H, 300.13;
12
1
KN(SiMe ) (0.118 g, 0.590 mmol), CaI (0.174 g, 0.590 mmol), K(F -
3 2 2 12
4Bo,3Ph
1
9
1
19
Tp
) (0.500 g, 0.590 mmol), and Et O (10 mL) were added to
2
F, 282.38 MHz) Avance III 400 ( H, 400.16;
F, 376.49 MHz),
1
19
1
a Schlenk flask. The mixture was stirred at room temperature for 3
days. After removal of volatiles under vacuum, the resulting solid
was washed with pentane, extracted with a toluene/pentane (1:3
v/v) mixture, and filtered. Removal of solvents afforded 1b in 3%
yield. 1b decomposes in solution over a period of few hours.
Avance 300 ( H, 300.13, F, 282.38 MHz), Avance 400 ( H, 400.13;
1
9
13
1
29
F, 376.48; C, 100.63 MHz), or Avance 500 ( H, 500.33; Si 79.49;
1
3
1
C 125.82 MHz) spectrometers. Chemical shifts for H NMR were
determined using residual proton signals in the deuterated sol-
vents and reported versus SiMe . Chemical shifts for C NMR spec-
13
4
1
3
1
9
29
H NMR ([D ]benzene, 400 MHz): d=7.54 (d, J =7.8 Hz, 6H, o-
6 HH
C H ), 7.31 (t, J =7.5 Hz, 6H, m-C H ), 7.22 (t, J =7.5 Hz, 3H, p-
tra were that of the solvent referenced to SiMe4. F and Si NMR
spectra were referenced versus external CFCl and SiMe , respec-
3
3
6
5
HH
6
5
HH
3
4
1
29
C H ), 3.17 (brm, 4H, OCH ), 1.02 (brm, 6H, CH ), ꢀ0.34 ppm (s,
tively. H- Si HMQC NMR experiments were carried out when
needed. DOSY NMR experiments were carried out on a Bruker
Avance 500 spectrometer equipped with a 5 mm triple resonance
6
5
2
3
19
1
1
8H, SiCH ); F NMR ([D ]benzene, 376 MHz): d=ꢀ144.28 (t, J =
3
6
FF
8.1 Hz, 3 F, F-4), ꢀ151.58 (m, 3 F, F-6), ꢀ153.78 ppm (t, J =
FF
1
31
17.7 Hz, 3 F, F-7); d=ꢀ164.09 ppm (t, J =20.0 Hz, 3 F, F-5).
inverse Z-gradient probe (TBI H, P, BB). The DOSY spectra were
acquired at 293 K with the stebpgp1s pulse program from Bruker
topspin software. All spectra were recorded with 16 K time domain
data point in the t2 dimension and 16 t1 increments. The strength
of the gradient was linearly incremented in 16 steps from 2 up to
FF
4
Bo,3Ph
4Bo,3Ph
Synthesis of [(F -Tp
)Sr{N(SiMe ) }] (3): [Tl(F -Tp
(0.200 g, 0.198 mmol), [Sr{N(SiMe ) } ] (0.121 g, 0.297 mmol), and
)]
1
2
3 2
12
3
2 2
pentane (10 mL) were combined in a Schlenk flask. The white
slurry was stirred at room temperature for 2 days. After filtration
and washing with pentane, the solid was extracted with a toluene/
pentane (1:3 v/v) mixture and filtered. After removal of volatiles
under vacuum, compound 3 was obtained as a white solid
(0.052 g, 25%, non-optimized yield). Crystals suitable for X-ray
9
5% of the maximum gradient strength. All measurements were
performed with a compromise diffusion delay D of 100 ms and
a gradient pulse length d of 1.8 ms. Infrared spectra were per-
formed on a PerkinElmer 100 FTIR spectrometer equipped with an
ATR device for measurements in the solid state into a glovebox. El-
emental analyses (London Metropolitan University or Analytical ser-
vice of the LCC) are the average of at least two independent
measurements.
diffraction were obtained from a concentrated toluene/pentane
1
solution at ꢀ408C. H NMR ([D ]benzene, 300 MHz): d=7.47 (d,
6
3
3
3
J
=7.4 Hz, 6H, o-C H ), 7.30 (t, J =7.4 Hz, 6H, m-C H ), 7.21 (t,
6 5 HH 6 5
HH
19
JHH =7.5 Hz, 3H, p-C H ), ꢀ0.34 ppm (s, 18H, SiCH ); F NMR
6
5
3
4
Bo,3Ph
4Bo,3Ph
([D ]benzene, 282 MHz): d=ꢀ144.60 (t, J =18.3 Hz, 3 F, F-4),
6
FF
Synthesis of [(F -Tp
)Ca{N(SiMe ) }] (1): [Tl(F -Tp
)]
1
2
3
2
12
ꢀ
151.36 (m, 3 F, J =11.0 Hz, F-6), ꢀ154.33 (t, J =18.0 Hz, 3 F, F-
FF
FF
(
0.500 g, 0.494 mmol) and Ca[N(SiMe ) ] (0.414 g, 1.148 mmol)
3
2 2
7
), ꢀ164.30 ppm (t, J =20.1 Hz, 4 F, F-5); elemental analysis calcd
FF
were combined in a flask and pentane (10 mL) was added. The re-
sulting white slurry was stirred at room temperature for two days.
After filtration and washing with pentane, the product was extract-
ed with a toluene/pentane (1:3 v/v) mixture. After removal of vola-
tiles under vacuum, compound 1 was obtained as a white solid
(
%) for C H N BSi F Sr: C 51.20, H 3.22, N 9.29; found: C 51.06, H
45 34 7 2 12
3
.11, N 9.20.
Synthesis of [Ca{N(SiMe H) } ] (4): [Ca{N(SiMe3)2}2] (0.410 g,
2
2 2
1.138 mmol) and pentane (20 mL) were added to a Schlenk flask
(
0.334 g, 67%). Crystals suitable for X-ray diffraction analysis were
and cooled down to 08C in an ice–water bath. HN(SiMe H)
2
2
obtained from a saturated toluene/pentane solution of 1 at
(0.494 mL, 2.844 mmol) was then added dropwise with stirring for
about 30 min. The solution was then stirred at 08C for 2.5 h. After
removal of volatiles under vacuum, pentane was added and strip-
1
3
ꢀ
408C. H NMR ([D ]benzene): d=7.54 (d,
J
=8.3 Hz, 6H, o-
HH
6
3
3
C H ), 7.32 (t, J =7.7 Hz, 6H, m-C H ), 7.23 (d, J =7.5 Hz, 3H, p-
6
5
HH
6
5
HH
19
C H ), ꢀ0.34 ppm (s, 18H, SiCH ); F NMR ([D ]benzene, 377 MHz):
ped under vacuum to afford 4 as a white solid (0.300 g, 87%).
6
5
3
6
1
d=ꢀ144.30 (t, J =18.1 Hz, 3 F, F-4), ꢀ151.59 (d, J =10.2 Hz, 3 F,
H NMR ([D ]benzene, 400 MHz): d=5.07 (s, SiH), 4.61 (s, SiH), 0.43
FF
FF
6
13
1
F-6), ꢀ153.83 (t, J =17.6 Hz, 3 F, F-7), ꢀ164.15 ppm (t, J =
(s, SiCH3), 0.35 ppm (s, SiCH3);
125.82 MHz): d=4.54 (s, SiCH ), 4.06 ppm (s, SiCH ); H NMR
C{ H} NMR ([D ]benzene,
FF
FF
6
1
2
0.7 Hz, 3 F, F-5); elemental analysis calcd (%) for C H N BSi F Ca:
45
34
7
2
12
3
3
C 53.62, H 3.38, N 9.73; found: C 53.42, H 3.21, N 9.56.
([D ]toluene 183 K, 500 MHz): d=5.25 (s, 4H, SiH ), 4.88 (s, 2H,
8
,
A
Chem. Eur. J. 2015, 21, 4115 – 4125
www.chemeurj.org
4123
ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
SiH ), 4.74 (s, 2H, SiH ), 4.41 (s, 2H, SiH ), 4.36 (s, 2H, SiH ), 0.64 (s,
perature until it was inserted into the probe of a Bruker AM 500
NMR spectrometer preset at 08C. Data points were recorded as
soon as possible after this, and it typically took altogether about
5 min to record the first of these. The reaction kinetics were moni-
tored (using the multi zgvd command; D1=0.2 s; DS=0; NS=4 or
more) over the course of 3 or more half-lives on the basis of amine
consumption, by comparing the relative intensities of resonances
diagnostic of substrate and product.
B
C
D
E
2
4H, SiCH ), 0.61 (s, 6H, SiCH ), 0.43 (s, 6H, SiCH ), 0.39 (s, 6H,
3A 3E 3E
SiCH ), 0.38 (s, 6H, SiCH ), 0.34 (s, 6H, SiCH ), 0.24 (s, 6H, SiCH ),
3
C
3D
3B
3C
1
29
0
.18 (s, 6H, SiCH ), 0.15 ppm (s, 6H, SiCH ); H- Si HMQC 2D
3D 3B
NMR ([D ]toluene 183 K, 79.49 MHz): d=11.8 (d, J =122 Hz SiHE),
1
8
,
SiH
1
1
ꢀ
13.9 (d, J =147 Hz, SiH ,), ꢀ14.0 (d, J =143 Hz, SiH ), ꢀ15.3
SiH
D
SiH
B
1
1
(
d, J =146 Hz, SiH ), ꢀ23.7 ppm (d, J =152 Hz, SiH ); IR: n˜
=
SiH
C
SiH
A
SiH
ꢀ
1
2
032 (sh), 1957 (s), 1095 cm (sh); elemental analysis calcd (%) for
C 31.5, H 9.2, N 9.2; found: C 31.25, H 9.37, N 9.22.
4
Bo,3Ph
4Bo,3Ph
Synthesis of [(F -Tp
)Ca{N(SiMe H) }] (5): [Tl(F -Tp
)]
1
2
2
2
12
X-ray diffraction crystallography
(
0.259 g, 0.256 mmol), Ca[N(SiMe H) ] (0.117 g, 0.384 mmol), and
2
3 2
pentane (8 mL) were combined in a flask. The white slurry was
stirred at room temperature for 2 days, during which time it
changed color to dark gray. After filtration and washing with
pentane, the solid was extracted with a toluene/pentane (1:3 v/v)
mixture and filtered. After removal of volatiles under vacuum,
compound 5 was obtained as a white solid (0.095 g, 37%, un-opti-
mized yield). Crystals suitable for X-ray diffraction analysis were ob-
tained from a saturated toluene/pentane solution at ꢀ408C.
The data for crystals of 1 and 2 were collected on a Gemini Ultra
diffractometer from Agilent equipped with an Oxford Instrument
Cooler Device. Mirror monochromatized CuKa radiation was used
for collecting diffraction data at a temperature of T=(180ꢃ2) K.
The data for crystals of 3 were collected on a Bruker D8 diffractom-
eter equipped with APEX II detector and an Oxford Cryosystem N
2
gas stream low-temperature device. Graphite-monochromatized
MoKa radiation was used for collecting diffraction data at a tempera-
ture of T=(180ꢃ2) K. The data for crystals of 5 were collected on
a Bruker D8 diffractometer equipped with APEX II detector and an
1
3
H NMR ([D ]benzene, 400 MHz): d=7.48 (d,
J =8.0 Hz, 6H, o-
HH
6
3
3
C H ), 7.31 (t, 6H, J =7.6 Hz, m-C H ), 7.21 (t, J =7.5 Hz, 3H, p-
6
5
HH
6
5
HH
3
C H ), 4.17 (brm, 2H, SiH), ꢀ0.23 ppm (d, J =2.8 Hz, 12H, SiCH3);
Oxford Cryosystem N gas stream low-temperature device. Multi-
2
6
9
5
HH
1
F NMR ([D ]benzene, 376 MHz): d=ꢀ144.39 (t, J =18.2 Hz, 3 F,
layer-monochromatized micro-focus MoKa radiation was used for
collecting diffraction data at a temperature of T=(193ꢃ2) K. The
6
FF
F-4), ꢀ151.71 (d, J =10.5 Hz, 3 F, F-6), ꢀ153.92 (t, J=18.0 Hz, 3 F,
FF
29
[80]
F-7), ꢀ164.15 ppm (t, J=20.1 Hz, 3 F, F-5); Si NMR ([D ]benzene,
structure was solved by direct methods using SIR92 and refined
6
1
2
7
1
5
9.49 MHz): d=ꢀ24.1 ppm (d, J =159 Hz, SiH); IR: n˜ =2044 (s),
by means of least-squares procedures on F with the aid of the
SiH
SiH
ꢀ
1
[81]
888 cm (s); elemental analysis calcd (%) for C H N BSi F Ca: C
program SHELX-L97
version 1.63.
included in the software package WinGX
43
30
7
2 12
[82]
2.70, H 3.06, N 10.01; found: C 52.59, H 3.25, N 9.92.
All non-hydrogen atoms were anisotropically re-
4
Bo,3Ph
4Bo,3Ph
fined. All hydrogen atoms were geometrically placed and refined
by using a riding model. In the last cycles of refinement a weight-
ing scheme was used, where weights were calculated from the fol-
Synthesis of [(F -Tp
(
)Ca{N(SiMe H) }(thf)] (6): [Tl(F -Tp
)]
1
2
2
2
12
0.179 g, 0.177 mmol), [Ca{N(SiMe H) } (thf)] (0.100 g, 0.266 mmol)
2 2 2
and pentane (8 mL) were combined in a flask. The white slurry was
stirred at room temperature for 2 days during which time it
changed color to dark gray. After filtration and washing with pen-
tane, the solid was extracted with toluene/pentane (1:3 v/v) mix-
ture. After removal of volatiles under vacuum, compound 6 was
obtained as a white solid (0.060 g, 32%). H NMR ([D ]benzene,
2
2
2
lowing formula: w=1/[s (Fo )+(0.0380P) +1.1258P] in which P=
2
2
(
Fo +2Fc )/3. For complex 2, a disorder between two molecules of
THF was treated using the PART command in SHELX-L97. For com-
plex 2, the checkcif presents one Alert level A about a low theta
full: The reason is due to incomplete scans, possibly based on
erroneously assumed higher than actual symmetry for the final
measurement strategy determination. Ellipsoids were drawn at the
30% probability level. Relevant collection and refinement data are
summarized in the Supporting Information.
1
6
3
3
3
7
2
2
00 MHz): d=7.75 (d, JHH =7.4 Hz, 6H, o-C H ), 7.31 (t, JHH =
6 5
.6 Hz, 6H, m-C H ), 7.02 (d, J =6.7 Hz, 3H, p-C H ), 4.69 (brm,
3
6
5
HH
6
5
3
H, SiH), 2.68 (t, 4H, OCH ), 0.70 (t, 4H, CH ), 0.01 ppm (d, J =
SiH
2
2
1
9
.9 Hz 12H, SiCH3); F NMR ([D ]benzene, 282 MHz): d=ꢀ143.58 (t,
6
J =18.1 Hz, 3 F, F-4), ꢀ151.43 (m, 3F, F-6), ꢀ154.56 (t, J =17.4 Hz,
CCDC-1025065 (1), CCDC-1025066 (2), CCDC-1025067 (3), and
CCDC-1025068 (5) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
FF
FF
29
3
F, F-7), ꢀ164.40 ppm (t, J =20.2 Hz, 3 F, F-5); Si NMR
FF
1
(
[D ]benzene, 79.49 MHz): d=ꢀ24.1 ppm (d,
J =160 Hz, SiH);
SiH
6
Satisfactory elemental analyses could not be obtained.
NMR-scale cyclohydroamination reactions
In a glovebox, the appropriate amount of catalyst (1 or 3) was
weighed in an NMR tube, and the substrate dissolved in
Acknowledgements
[
D ]benzene was then added. The tube was sealed, vigorously
6
Financial support from the ANR (GreenLAkE project; contract
number ANR-11-BS07-0009) is gratefully acknowledged. The as-
sistance of Dr. Remy Brousses with the X-ray diffraction crystal-
lography and Dr. Christian Bijani with the NMR spectroscopy
measurements is also acknowledged.
shaken, and immersed into an oil bath at the desired temperature.
After the required amount of time, the NMR tube was removed
from the oil bath and the H NMR spectrum of the reaction mixture
was recorded. The conversion was calculated by comparing the rel-
ative intensities of resonance signals characteristic of the substrate
and product.
1
Keywords: agostic interactions
hydroamination · ligands · metathesis
· alkaline earth metals ·
Kinetics of the cyclohydroamination reaction
In a glovebox, the appropriate amount of precatalyst was weighed
in an NMR tube. The substrate and [D ]toluene (1.6, 1.7, or 1.8 mL)
8
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25] Poly(indazolyl)borates are represented as benzopyrazolylborates, Tp ,
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