2,5-Bis{ N -(2,6-diisopropylphenyl)iminomethyl}pyrrolyl complexes of the heavy alkaline earth metals: Synthesis, structures, and hydroamination catalysis

Article abstract of DOI:10.1021/om100937c

The heteroleptic iodo complexes [(DIP₂pyr)MI(THF)_n] (M = Ca, Sr (n = 3); Ba (n = 4); (DIP₂pyr)^- = 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl) were synthesized by reaction of [(DIP₂pyr)K] with anhydrous alkaline earth metal diiodides. All complexes are monomeric in the solid state. A κ³-coordination mode of the (DIP₂pyr)^- ligand was observed for the strontium and the barium compounds, while the analogous calcium derivative is κ²-coordinated in the solid state. However, VT-¹H NMR studies of [(DIP₂pyr)CaI(THF) ₃] indicate a symmetrically coordinated (DIP₂pyr) ^- ligand in solution. Computational studies confirm the different coordination modes in solution and in the solid state. The preferred κ²-coordination mode observed in the solid state might be a result of temperature or/and crystal-packing effects. Furthermore, the calcium and strontium amido complexes [(DIP₂pyr)M{N(SiMe₃) ₂}(THF)₂] (M = Ca, Sr) were prepared by reaction of [(DIP₂pyr)MI(THF)_n] (M = Ca, Sr (n = 3)) with [K{N(SiMe₃)₂}]. Both compounds were investigated for the intramolecular hydroamination of aminoalkenes. Both catalysts showed a good activity, and the best results were obtained for the calcium complex [(DIP ₂pyr)Ca{N(SiMe₃)₂}(THF)₂].

Full text of DOI:10.1021/om100937c

ARTICLE
pubs.acs.org/Organometallics
2,5-Bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl Complexes of
the Heavy Alkaline Earth Metals: Synthesis, Structures, and
Hydroamination Catalysis
Jelena Jenter, Ralf K€oppe, and Peter W. Roesky*
Institut f€ur Anorganische Chemie, Karlsruher Institut f€ur Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany
S Supporting Information
b
ABSTRACT: The heteroleptic iodo complexes [(DIP2pyr)MI(THF)n] (M
= Ca, Sr (n = 3); Ba (n = 4); (DIP2pyr)- = 2,5-bis{N-(2,6-diisopropy-
lphenyl)iminomethyl}pyrrolyl) were synthesized by reaction of [(DIP2pyr)
K] with anhydrous alkaline earth metal diiodides. All complexes are mono-
meric in the solid state. A κ3-coordination mode of the (DIP2pyr)- ligand was
observed for the strontium and the barium compounds, while the analogous
calcium derivative is κ2-coordinated in the solid state. However, VT-1H NMR
studies of [(DIP2pyr)CaI(THF)3] indicate a symmetrically coordinated
(DIP2pyr)- ligand in solution. Computational studies confirm the different
coordination modes in solution and in the solid state. The preferred κ2-
coordination mode observed in the solid state might be a result of tempera-
ture or/and crystal-packing effects. Furthermore, the calcium and strontium
amido complexes [(DIP2pyr)M{N(SiMe3)2}(THF)2] (M = Ca, Sr) were
prepared by reaction of [(DIP2pyr)MI(THF)n] (M = Ca, Sr (n = 3)) with
[K{N(SiMe3)2}]. Both compounds were investigated for the intramolecular hydroamination of aminoalkenes. Both catalysts
showed a good activity, and the best results were obtained for the calcium complex [(DIP2pyr)Ca{N(SiMe3)2}(THF)2].
’ INTRODUCTION
hydroamination of nonactivated substrates. Recently some re-
search groups started to investigate alkaline earth metal complexes
as catalysts for the hydroamination reaction.32-43 In 2005 Hill
et al. reported first on this topic by using β-diketiminato calcium
amide [{(DIPNC(Me))2CH}Ca{N(SiMe3)2}(THF)] (DIP =
2,6-diisopropylphenyl) as catalyst for the intramolecular hydro-
amination of aminoalkenes.36 As expected, high conversions and
short reaction times at low temperatures were observed, which is
similar to rare earth metal catalyzed hydroamination. Recently,
Hill et al. expanded their experimental and computational studies
by synthesizing analogous β-diketiminato complexes of magne-
sium, strontium, and barium.33,35 The hydroamination by using
either the barium or the strontium catalyst failed, since ligand
redistribution occurred and homoleptic complexes were formed.
Consequently, the calcium catalyst remains superior, while
computational studies predict the β-diketiminato strontium
amide to be the most active catalyst. Recently, the synthesis of
[{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] (M = Ca, n = 1; M =
Sr, n = 2, ImtBu = 1-tert-butylimidazole) as catalytically active
species for the intramolecular hydroamination of aminoalkenes
was reported.32 Different from the β-diketiminato alkaline earth
metal complexes, no ligand redistribution was observed for the
strontium compound; on the contrary, the strontium derivative
Amines have been largely applied in the chemical industry as
bulk chemicals, fine chemicals, pharmaceuticals, and dyes.1 In
order to avoid chemical waste produced in time-consuming
multistep syntheses, hydroamination was discovered as an alter-
native access to nitrogen-containing molecules. Hydroamina-
tion, the direct addition of amine N-H bonds to C-C multiple
bonds, attracted the attention of many researchers and thus was
intensely studied for the last decades.1-19 Catalytic hydroamina-
tion was investigated for a wide range of complexes of various
metals, such as alkali metals,3,5,15,20,21 early transition metals,1,9,20
late transition metals,1,7,9,12,14,15,20 and lanthanides and acti-
nides,1,7-9,11-13,15-17 and very recently aluminum compounds
were studied for hydroamination reactions.22,23 Early transition
metal and late transition metal catalysts have to be distinguished.
Complexes of early transition metals, such as group 44,6,7,12,13,18
and the rare earth elements,1,7-9,11-13,15-17 are highly efficient
catalysts for the hydroamination, but the synthetic application is
limited by their high sensitivity toward moisture and air and their
low tolerance to polar functional groups. In contrast, compounds
of the late transition metals such as nickel,24 palladium,7,9,12,15,20
platinum,9,20 copper,9 and gold25-31 show high polar functional
group tolerance, but are mostly based on relatively expensive or
toxic metals, and thus some of them have a limited use for the
synthesis of pharmaceuticals. In addition, late transition metal
catalysts show low reaction rates and moderate selectivity for the
Received: September 29, 2010
Published: March 03, 2011
r
2011 American Chemical Society
1404
dx.doi.org/10.1021/om100937c Organometallics 2011, 30, 1404–1413
|

Organometallics
ARTICLE
was proven to be the most active catalyst. Furthermore, amino-
troponiminate alkaline earth metal amides were synthesized by
our group and proved to be catalytically active for the intramo-
lecular hydroamination of aminoalkenes.37-39 While heteroleptic
complexes of calcium and strontium [{(iPr)2ATI}M{N(Si-
Me3)2}(THF)2] (M = Ca, Sr; (iPr)2ATI = N-isopropyl-
2-(isopropylamino)troponiminate) were obtained, barium forms
only the homoleptic complex [{(iPr)2ATI}2Ba(THF)2]. In our
studies, the activity of the catalysts decreases with increasing
ionic radius of the metal, and the best result was obtained for the
calcium compound. This is contrary to the results observed
previously for the alkaline earth metal complexes containing
either the β-diketiminato33,35 or the bis(imidazolin-2-ylidene-1-
yl)borate ligand,32 and consequently the reaction rates and selec-
tivities of alkaline earth metal catalysts cannot be simply related
to the ionic radius of the center metal.
Now, we were interested in extending our studies to a new
ligand system. Since the 2,5-bis{N-(2,6-diisopropylphenyl)imino-
methyl}pyrrolyl ligand ((DIP2pyr)-) was proven to stabilize rare
earth metal(III) complexes,44-48 we recently introduced the
ligand into the coordination chemistry of the divalent lantha-
nides, which resulted in new structures with interesting coordi-
nation modes.49 As a result of the same oxidation state and the
similar ionic radii,50,51 the reactivity and coordination behavior
of the divalent lanthanides and the heavier alkaline earth metals
are related.37,52-56 In this context we were interested in studying
the differences and similarities in the coordination chemistry of
the (DIP2pyr)- ligand in alkaline earth metals compared to the
divalent lanthanides.57 Furthermore, we studied the new com-
pounds as catalysts for the hydroamination and thus extended
the short list of catalytically active alkaline earth metal species.
= 6.9 Hz), 6.79 (s, 2 H, 3,4-pyr), 7.03 (dd, 2 H, p-Ph, JH,H,1 =8.7Hz, JH,H,2 =
6.6 Hz), 7.11-7.14 (m, 4 H, Ph), 8.24 (br, 2 H, NdCH) ppm. 13C{1H}
NMR (THF-d8, 100.4 MHz, 25 °C): δ 25.4 (CH(CH3)2), 28.2
(CH(CH3)2), 116.0 (3,4-pyr), 123.2 (Ph), 124.3 (br, Ph), 138.2 (Ph),
139.5 (br, 2,5-pyr), 150.9 (Ph), 161.4 (br, NdCH) ppm. Anal. Calcd for
C42H62CaIN3O3 (2) (823.94): C, 61.22, H, 7.58, N, 5.10. Found: C,
60.83, H, 7.69, N, 4.78. Variable-temperature (VT)-1H NMR studies of 2
(65 and -70 °C): 1H NMR (THF-d8, 300 MHz, 65 °C): δ 1.19 (d, 24 H,
CH(CH3), JH,H = 6.9 Hz), 3.17 (sept, 4 H, CH(CH3), JH,H = 6.9 Hz), 6.80
(s, 2 H, 3,4-pyr), 7.01 (dd, 2 H, p-Ph, JH,H,1 =8.4Hz, JH,H,2 = 6.6 Hz), 7.09-
1
7.12 (m, 4 H, Ph), 8.23 (s, 2 H, NdCH) ppm. H NMR (THF-d8,
300 MHz, -70 °C): δ 1.09-1.28 (m, 24 H, CH(CH3), 3.03-3.18
(m, 4 H, CH(CH3)), 6.59 (s, 2 H, 3,4-pyr), 7.00-7.16 (m, 6 H, Ph),
8.07 (s, 2 H, NdCH) ppm.
[(DIP2pyr)SrI(THF)3] (3). SrI2 (341 mg, 1.00 mmol). Yield 634 mg,
0.73 mmol, 73%. 1H NMR (THF-d8, 300 MHz, 25 °C): δ 1.17-1.21
(m, 24 H, CH(CH3)), 3.22 (sept, 4 H, CH(CH3), JH,H = 6.9 Hz), 6.59
(s, 2 H, 3,4-pyr), 7.05 (dd, 2 H, p-Ph, JH,H,1 = 8.8 Hz, JH,H,2 = 6.3 Hz),
7.13-7.15 (m, 4 H, Ph), 8.01 (s, 2 H, NdCH) ppm. 13C{1H} NMR
(THF-d8, 100.4 MHz, 25 °C): δ 25.4 (CH(CH3)2), 27.6 (CH(CH3)2),
117.3 (3,4-pyr), 122.5 (Ph), 123.8 (Ph), 139.4 (Ph), 142.5 (2,5-pyr),
150.9 (Ph), 161.5 (NdCH) ppm. Anal. Calcd for C46H70IN3O4Sr (3 þ
THF) (943.59): C, 58.55, H, 7.48, N, 4.45. Found: C, 58.54, H, 7.48,
N, 4.66.
[(DIP2pyr)BaI(THF)4] (4). BaI2 (391 mg, 1.00 mmol). Yield:
1
455 mg, 0.46 mmol, 46% (single crystals). H NMR (THF-d8, 400
MHz, 25 °C): δ 1.16-1.32 (m, 24 H, CH(CH3)), 3.22 (sept, 4 H,
CH(CH3), JH,H = 6.9 Hz), 6.59 (s, 2 H, 3,4-pyr), 7.07 (dd, 2 H, p-Ph, JH,
= 8.4 Hz, JH,H,2 = 6.8 Hz), 7.13-7.17 (m, 4 H, Ph), 8.03 (s, 2 H,
H,1
NdCH) ppm. 13C{1H} NMR (THF-d8, 100.4 MHz, 25 °C): δ 26.1
(CH(CH3)2), 28.4 (CH(CH3)2), 118.1 (3,4-pyr), 123.3 (Ph), 124.5
(Ph), 140.1 (2,5-pyr), 143.7 (Ph), 151.3 (Ph), 162.4 (NdCH) ppm.
Anal. Calcd for C46H70BaIN3O4 (4) (993.30): C, 55.62, H, 7.10, N,
4.23. Found: C, 55.21, H, 6.71, N, 4.53.
’ EXPERIMENTAL SECTION
[(DIP2pyr)M{N(SiMe3)2}(THF)2] (5, 6). General Procedure.
THF (15 mL) was condensed at -78 °C onto a mixture of [(DIP2p-
yr)MI(THF)3] (M = Ca (2), Sr (3)) (0.5 mmol) and [K{N(SiMe3)2}]
(100 mg, 0.5 mmol), and the resulting yellow reaction mixture was
stirred at 60 °C for 16 h. The yellow solution was filtered off and con-
centrated until a yellow precipitate appeared. The mixture was heated
carefully until the solution became clear. The solution was allowed to
stand at room temperature to obtain the product as yellow crystals after
several hours.
General Procedures. All manipulations of air-sensitive materials
were performed with the rigorous exclusion of oxygen and moisture in
flame-dried Schlenk-type glassware either on a dual-manifold Schlenk
line, interfaced to a high-vacuum (10-3 Torr) line, or in an argon-filled
MBraun glovebox. THF was distilled under nitrogen from potassium
benzophenone ketyl prior to use. Hydrocarbon solvents (toluene and
n-pentane) were dried using an MBraun solvent purification system
(SPS-800). All solvents for vacuum line manipulations were stored in
vacuo over LiAlH4 in resealable flasks. Deuterated solvents were
obtained from Aldrich (99 atom % D). NMR spectra were recorded
on a Bruker Avance 400 MHz or on a Bruker Avance II 300 MHz NMR
spectrometer. Chemical shifts are referenced to internal solvent reso-
nances and are reported relative to tetramethylsilane. IR spectra were
obtained by means of an ATR unit on a Bruker FTIR IFS 113v
spectrometer. Mass spectra were recorded at 70 eV on a Varian Mat
SM 11. Elemental analyses were carried out with an Elementar Vario EL.
(DIP2pyr)H58,59 and [(DIP2pyr)K] (1)45,60 were prepared according to
literature procedures.
[(DIP2pyr)MI(THF)n] (2-4). General Procedure. THF (20 mL)
was condensed at -78 °C onto a mixture of MI2 (M = Ca, Sr, Ba) (1.00
mmol) and [(DIP2pyr)K] (1) (480 mg, 1.00 mmol), and the resulting
yellow reaction mixture was stirred at 60 °C for 16 h (M = Ca, 60 h). The
yellow solution was filtered off and concentrated until a yellow pre-
cipitate appeared. The mixture was heated carefully until the solution
became clear. The solution was allowed to stand at room temperature to
obtain the product as yellow crystals after several hours.
[(DIP2pyr)Ca{N(SiMe3)2}(THF)2] (5). [(DIP2pyr)CaI(THF)3]
(2) (411 mg, 0.5 mmol). Yield: 270 mg, 0.34 mmol, 69% (single
1
crystals). H NMR (THF-d8, 300 MHz, 25 °C): δ -0.21 (s, 18 H,
SiMe3), 1.21 (d, 24 H, CH(CH3), JH,H = 6.9 Hz), 3.07 (sept, 4 H,
CH(CH3), JH,H = 6.9 Hz), 6.71 (s, 2 H, 3,4-pyr), 7.07 (dd, 2 H, p-Ph, JH,
= 8.7 Hz, JH,H,2 = 6.0 Hz), 7.13-7.16 (m, 4 H, Ph), 8.07 (s, 2 H,
H,1
NdCH) ppm. 13C{1H} NMR (THF-d8, 100.4 MHz, 25 °C): δ 5.0
(SiMe3), 25.4 (CH(CH3)2), 27.7 (CH(CH3)2), 118.8 (3,4-pyr), 122.6
(Ph), 124.0 (Ph), 140.0 (Ph), 142.5 (2,5-pyr), 150.5 (Ph), 161.3
(NdCH) ppm. Anal. Calcd for C48H80CaN4O3Si2 (5 þ THF)
(857.42): C, 67.24, H, 9.40, N, 6.53. Found: C, 67.36, H, 9.11, N, 6.32.
[(DIP2pyr)Sr{N(SiMe3)2}(THF)2] (6). [(DIP2pyr)SrI(THF)3]
(3) (436 mg, 0.5 mmol). Yield: 300 mg, 0.36 mmol, 72% (single
1
crystals). H NMR (THF-d8, 300 MHz, 25 °C): δ -0.29 (s, 18 H,
SiMe3), 1.15-1.32 (m, 24 H, CH(CH3)), 3.06 (sept, 4 H, CH(CH3),
JH,H = 6.9 Hz), 6.62 (s, 2 H, 3,4-pyr), 7.07 (dd, 2 H, p-Ph, JH,H,1 = 9.0 Hz,
J
H,H,2 = 6.0 Hz), 7.13-7.16 (m, 4 H, Ph), 8.03 (s, 2 H, NdCH) ppm.
13C{1H} NMR (THF-d8, 100.4 MHz, 25 °C): δ 5.0 (SiMe3), 25.4
(CH(CH3)2), 27.7 (CH(CH3)2), 118.1 (3,4-pyr), 122.6 (Ph), 124.0
(Ph), 139.0 (Ph), 142.5 (2,5-pyr), 150.8 (Ph), 162.0 (NdCH) ppm.
[(DIP2pyr)CaI(THF)3] (2). CaI2 (294 mg, 1.00 mmol). Yield: 505 mg,
0.61 mmol, 61% (single crystals). 1H NMR (THF-d8, 300 MHz, 25 °C):
δ 1.20 (d, 24 H, CH(CH3), JH,H = 6.9 Hz), 3.22 (sept, 4 H, CH(CH3), JH,H
1405
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Table 1. Crystallographic Details of 2-6
2 THF
3 2THF
4 THF
5 THF
6 THF
3
3
3
3
3
chemical formula
fw
C
46H70CaIN3O4
896.03
C50H78IN3O5Sr
1015.67
15.253(3)
31.967(6)
11.330(2)
107.95(3)
5256(2)
200(2)
P21/c
C50H78BaIN3O5
1065.39
10.979(2)
31.676(6)
15.769(3)
109.07(3)
5183(2)
200(2)
P21
C48H80CaN4O3Si2
857.42
C48H80N4O3Si2Sr
904.96
a/Å
17.187(3)
15.177(3)
19.291(4)
107.76(3)
4792(2)
150(2)
P21/c
16.426(3)
10.568(2)
29.727(6)
97.67(3)
5114(2)
150(2)
P21/c
16.605(3)
10.727(2)
29.874(6)
96.87(3)
5283(2)
200(2)
P21/c
b/Å
c/Å
β/deg
unit cell volume/Å3
temperature/K
space group
no. of formula units per unit cell, Z
radiation type
absorp coeff, μ/mm-1
no. of reflns measd
no. of indep reflns
Rint
4
4
4
4
4
Mo KR
0.816
Mo KR
1.657
Mo KR
1.406
Mo KR
0.210
Mo KR
1.105
30 443
42 462
39 822
22 279
39 441
8441
9253
23 278
10 661
10 530
0.1016
0.2298
0.0492
0.0614
0.0968
final R1 values (I > 2σ(I))
final wR(F2) values (all data)
goodness of fit on F2
0.0729
0.0885
0.0397
0.0689
0.0653
0.1963
0.2350
0.0992
0.1790
0.1564
0.986
0.933
1.010
0.970
1.057
Anal. Calcd for C48H80SrN4O3Si2 (6 þ THF) (904.96): C, 63.71, H,
8.91, N, 6.19. Found: C, 64.30, H, 8.58, N, 6.23.
Positional parameters, hydrogen atom parameters, thermal parameters,
and bond distances and angles have been deposited as Supporting
Information.
Hydroamination Studies. The catalyst was weighed under argon
gas into an NMR tube. C6D6 (∼0.5 mL) was condensed into the NMR
tube, and the mixture was frozen to -196 °C. The reactant was injected
onto the solid mixture, and the whole sample was melted and mixed just
before insertion into the core of the NMR machine (t0). The ratio
between the reactant and the product was calculated by comparison
of the integrations of the corresponding signals. Ferrocene was used
as an internal standard for the kinetic measurements. The substrates 2,2-
diphenyl-pent-4-enylamine (7a),61 C-(1-allylcyclohexyl)methylamine
(8a),61 2,2-dimethylpent-4-enylamine (9a),61 and 2-amino-5-hexene
(via hex-5-en-2-one oxime62) (10a)63 were synthesized according to
’ RESULTS AND DISCUSSION
The salt metathesis reaction of [(DIP2pyr)K] (1) with
anhydrous alkaline earth metal diiodides in THF at elevated
temperature gave the corresponding complexes [(DIP2pyr)MI-
(THF)n] (M = Ca (2), Sr (3) (n = 3); Ba (4) (n = 4)), as shown
in Scheme 1. While the strontium and barium complexes 3 and 4
were obtained within 16 h, the synthesis of the calcium com-
pound 2 required a reaction time of 60 h.57
1
the literature procedures. H NMR spectra of 2-methyl-4,4-diphenyl-
Complexes 2-4 were characterized by standard analytical/
spectroscopic techniques, and the solid-state structures were
analyzed by single-crystal X-ray diffraction. Compounds 2-4 are
monomeric in the solid state (Scheme 1). Interestingly, the
(DIP2pyr)- ligand in the calcium complex 2 shows a κ2-coor-
dination mode, while the ligands in 3 and 4 coordinate in a κ3-
pyrrolidine (7b),61 3-methyl-2-aza-spiro[4.5]decane (8b),61 2,4,4-tri-
methylpyrrolidine (9b),61 and 2,5-dimethylpyrrolidine (10b)63 conform
with the literature.
Isomerization Product 10c, in Situ 1H NMR Data. Compound
10c was detected within a mixture and not further purified.
1
1
2-Amino-4-hexene (10c). H NMR (C6D6, 300 MHz, 25 °C):
fashion to the metal centers. The H NMR spectrum of com-
δ 0.73 (br, 2 H, NH2), 0.92 (d, 3 H, Me, JH,H = 6.3 Hz), 1.47-1.50 (m, 3
H, Me-CdC), 1.92 (t, 2 H, CdC-CH2, JH,H = 6.9 Hz), 2.71 (ms, 1 H,
CH), 5.28-5.38, 5.44-5.54 (2 m, 2 H, HCdCH) ppm.
pound 2 shows a broad singlet for the imino NdCH moieties
(δ = 8.24 ppm), while for the isopropyl groups one doublet (δ =
1.20 ppm) and one septet (δ = 3.22 ppm) were observed,
indicating that the 2,6-diisopropylaniline moieties of the ligand
can freely rotate in solution. These observations can be ascribed
to a similar chemical environment in solution at room tempera-
ture for both 2,6-diisopropylaniline moieties, which is in contrast
to the solid-state structures of 2 (see below). Further VT-1H
NMR studies of 2 were performed, in order to investigate the
coordination mode of the (DIP2pyr)- ligand in solution. At
65 °C the 1H NMR spectrum of compound 2 is very similar to
the one measured at room temperature, except the imino NdCH
moieties showing a clear singlet (δ = 8.23 ppm), which is
certainly caused by the thermal motion. This thermal motion
includes the rotation of the 2,6-diisopropylaniline groups as well
as flipping of the imino NdCH moieties. Upon lowering the
temperature to -70 °C, a multiplet for the isopropyl CH3 groups
(δ = 1.09-1.28) was observed, indicating a restricted rotation of
the 2,6-diisopropylaniline groups of the ligand at low temperature.
X-ray Crystallographic Studies of 2-6. A suitable crystal was
covered in mineral oil (Aldrich) and mounted on a glass fiber. The
crystal was transferred directly to either the -73 or -123 °C cold stream
of a STOE IPDS 2T diffractometer. Subsequent computations were
carried out on an Intel Pentium Core2Duo PC.
All structures were solved by the Patterson method (SHELXS-9765).
The remaining non-hydrogen atoms were located from successive dif-
ference in Fourier map calculations. The refinements were carried out by
using full-matrix least-squares techniques on F, minimizing the function
2
2
(Fo - Fc)2, where the weight is defined as 4Fo /2(Fo ) and Fo and Fc are
the observed and calculated structure factor amplitudes using the
program SHELXL-97.65 Carbon-bound hydrogen atom positions
were calculated. The hydrogen atom contributions were cal-
culated, but not refined. The final values of refinement parameters
are given in Table 1. The locations of the largest peaks in the final
difference Fourier map calculation as well as the magnitude of the
residual electron densities in each case were of no chemical significance.
1406
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Scheme 1
In addition, the pyrrolyl hydrogen atoms as well as the imino
NdCH moieties show a clear singlet (δ = 6.59 ppm and δ =
8.07 ppm), which can be attributed to a similar chemical envi-
ronment for both 2,6-diisopropylaniline moieties of the ligand. In
solution even at a low temperature, there is no evidence of an
asymmetrical coordination mode of the (DIP2pyr)- ligand in the
1
calcium complex 2. The H NMR spectra of 3 and 4 show a
multiplet (δ = 1.17-1.21 (3), 1.16-1.32 (4) ppm) and a clear
septet (δ = 3.22 (3), 3.22 (4) ppm) for the isopropyl groups of
the (DIP2pyr)- ligands, which indicates a restricted rotation of
the 2,6-diisopropylaniline groups. In addition, the sharp singlet
for the imino NdCH units (δ = 8.01 (3), 8.03 (4) ppm)
confirms the symmetrical coordination mode of the ligands in
3 and 4.
Compound 2 crystallizes in the monoclinic space group P21/c
with four molecules of the complex and four THF molecules in
the unit cell. The (DIP2pyr)- ligand of 2 coordinates with one
Schiff-base function and the pyrrolyl moiety (Ca-N1 2.510(5) Å
and Ca-N2 2.408(5) Å) to the metal center (Figure 1). As
expected, the coordinated imino NdCH unit exhibits a slightly
larger N-C bond length (N1-C1 1.296(8) Å) than the non-
coordinated NdCH moiety (N3-C6 1.262(7) Å). The 6-fold
coordination sphere around the metal center is formed by the κ2-
coordinated (DIP2pyr)- ligand, the iodine atom, and three THF
molecules, showing a distorted octahedral geometry. The iodine
atom and the coordinated Schiff-base moiety are in trans position
and show a nearly linear setup (N1-Ca-I 178.34(12)°). The
remaining square area of the distorted octahedron is formed by
the pyrrolyl moiety and the three THF molecules, and the almost
planar arrangement of the CaO3N fragment is confirmed by the
sum of the four corresponding valence angles (360.2°). The Ca-
I bond distance (3.0782(13) Å) in 2 is similar to the Yb-I bond
length (3.100(2) Å) of the analogous ytterbium compound
[(DIP2pyr)YbI(THF)3]49 and to those in [{(Me3SiNPPh2)2C-
H}CaI(THF)2] or [{(iPr)2ATI}CaI(THF)3] ((iPr)2ATI = N-
isopropyl-2-(isopropylamino)troponiminate).56,66 Usually, ana-
logous calcium and ytterbium derivatives tend to form isostructural
complexes.37,53-56 In the present case, the calcium complex 2
exhibits a κ2-coordinated (DIP2pyr)- ligand, which is coordinated
Figure 1. Perspective ORTEP view of the molecular structure of 2.
Thermal ellipsoids are drawn to encompass 30% probability. Hydrogen
atoms are omitted for clarity. Selected distances [Å] and angles [deg]:
Ca-N1 2.510(5), Ca-N2 2.408(5), Ca-O1 2.367(4), Ca-O2
2.371(5), Ca-O3 2.368(4), Ca-I 3.0782(13), N1-C1 1.296(8),
N3-C6 1.262(7); N1-Ca-N2 71.6(2), N1-Ca-O1 90.31(15),
N1-Ca-O2 87.8(2), N1-Ca-O3 89.74(15), N2-Ca-O1 92.0(2),
N2-Ca-O2 159.4(2), N2-Ca-O3 85.13(15), N1-Ca-I
178.34(12), N2-Ca-I 107.21(11), O1-Ca-I 90.84(11), O2-Ca-I
93.41(13), O3-Ca-I 89.05(11), O1-Ca-O2 88.0(2), O2-Ca-O3
95.1(2), O1-Ca-O3 176.9(2).
only by one Schiff-base function and the pyrrolyl moiety, while
the analogous ytterbium compound [(DIP2pyr)YbI(THF)3]49
additionally shows a long interaction with the second Schiff-
base unit.
Compound 3 crystallizes in the monoclinic space group P21/c
with four molecules of the complex and eight THF molecules in
the unit cell. As a result of “twining” problems, the data quality is
low but the connectivity of the atoms could be clearly assigned.
Compound 3 is isostructural to the analogous samarium and
1407
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Figure 2. Perspective ORTEP view of the molecular structure of 3.
Thermal ellipsoids are drawn to encompass 30% probability. Hydrogen
atoms are omitted for clarity. Selected distances [Å] and angles [deg]:
Sr-N1 2.964(7), Sr-N2 2.478(7), Sr-N3 2.966(7), Sr-O1 2.566(6),
Sr-O2 2.589(6), Sr-O3 2.562(6), Sr-I 3.2282(12); N1-Sr-N2
61.1(2), N1-Sr-N3 121.7(2), N2-Sr-N3 60.7(2), N1-Sr-O1
89.0(2), N1-Sr-O2 76.0(2), N1-Sr-O3 152.1(2), N2-Sr-O1
82.4(2), N2-Sr-O2 136.2(2), N2-Sr-O3 143.3(2), N3-Sr-O1
80.3(2), N3-Sr-O2 158.6(2), N3-Sr-O3 83.6(2), N1-Sr-I
98.44(14), N2-Sr-I 104.2(2), N3-Sr-I 98.59(13), O1-Sr-I
171.8(2), O2-Sr-I 89.7(2), O3-Sr-I 88.04(15), O1-Sr-O2
88.7(2), O2-Sr-O3 77.0(2), O1-Sr-O3 83.7(2).
Figure 3. Perspective ORTEP view of the molecular structure of 4.
Thermal ellipsoids are drawn to encompass 30% probability. Only one of
the two independent molecules is shown. Hydrogen atoms are omitted
for clarity. Selected distances [Å] and angles [deg]: Ba1-N1 3.196(4),
Ba1-N2 2.690(3), Ba1-N3 3.145(3), Ba1-O1 2.812(3), Ba1-O2
2.801(3), Ba1-O3 2.806(3), Ba1-O4 2.877(3), Ba1-I1 3.4950(8),
Ba2-N4 3.214(4), Ba2-N5 2.679(3), Ba2-N6 3.156(3), Ba2-O5
2.819(3), Ba2-O6 2.774(4), Ba2-O7 2.865(3), Ba2-O8 2.776(3),
Ba2-I2 3.4922(7); N1-Ba1-N2 57.13(10), N1-Ba1-N3 115.04(9),
N2-Ba1-N3 57.94(10), N1-Ba1-O1 79.94(9), N1-Ba1-O2
147.02(10), N1-Ba1-O3 74.46(10), N1-Ba1-O4 77.66(10), N2-
Ba1-O1 74.37(9), N2-Ba1-O2 119.27(10), N2-Ba1-O3
126.22(11), N2-Ba1-O4 71.75(9), N3-Ba1-O1 84.92(9), N3-
Ba1-O2 72.28(10), N3-Ba1-O3 156.64(9), N3-Ba1-O4
81.52(10), N1-Ba1-I1 128.59(6), N2-Ba1-I1 148.40(7), N3-
Ba1-I1 106.04(7), O1-Ba1-I1 134.84(7), O2-Ba1-I1 73.56(7),
O3-Ba1-I1 79.99(8), O4-Ba1-I1 79.31(7), O1-Ba1-O2
68.43(10), O1-Ba1-O3 75.58(10), O1-Ba1-O4 145.75(9), O2-
Ba1-O3 88.44(11), O2-Ba1-O4 134.71(11), O3-Ba1-O4
121.82(10) N4-Ba2-N5 56.92(9), N4-Ba2-N6 115.20(9), N5-
Ba2-N6 58.28(10), N4-Ba2-O5 85.62(9), N4-Ba2-O6
159.73(11), N4-Ba2-O7 79.68(10), N4-Ba2-O8 73.89(10), N5-
Ba2-O5 74.66(9), N5-Ba2-O6 126.08(10), N5-Ba2-O7
147.29(9), N5-Ba2-O8 118.65(11), N6-Ba2-O5 78.57(9), N6-
Ba2-O6 71.77(10), N6-Ba2-O7 81.55(10), N6-Ba2-O8
143.67(9), N4-Ba2-I2 106.90(6), N5-Ba2-I2 149.15(7), N6-
Ba2-I2 128.84(7), O5-Ba2-I2 133.86(6), O6-Ba2-I2 78.85(8),
O7-Ba2-I2 78.65(7), O8-Ba2-I2 74.35(7), O5-Ba2-O6
77.00(11), O5-Ba2-O7 147.29(9), O5-Ba2-O8 66.78(10), O6-
Ba2-O7 120.59(12), O6-Ba2-O8 89.53(12), O7-Ba2-O8
134.24(10).
europium compounds [(DIP2pyr)LnI(THF)3] (Ln = Sm, Eu).49
The (DIP2-pyr)- ligand coordinates as a tridentate chelate
ligand in an almost symmetrical fashion to the metal center, as
shown in Figure 2. The Sr-N distances reflect the differences in
bond strength and character. Accordingly, the Sr-N(Schiff-
base) bonds display a rather weak σ-donor character, while the
short Ln-N(pyrrole) bonds signal a strong interaction as result
of the negatively charged nitrogen atom. Thus, the Sr-N bond
distances to the pyrrole ring (Sr-N1 2.964(7) Å, Sr-N2
2.478(7) Å, Sr-N3 2.966(7) Å) are about 0.5 Å shorter than
the distance to the Schiff-base nitrogen atoms. The coordination
sphere of the metal center of 3 is formed by the (DIP2pyr)-
ligand, the iodine atom, and three THF molecules, which resulted in
a distorted pentagonal bipyramidal geometry. The iodine atom
and one THF molecule are arranged in a nearly linear setup
(O1-Sr-I 171.8(2)°) and form the apexes of the distorted
pentagonal bipyramid. The almost planar arrangement of the
SrN3O2 fragment is confirmed by the sum of the corresponding
five valence angles (358.4°). The iodine atom occupies a cisoid
position to all three nitrogen atoms of the (DIP2pyr)- ligand
(N1-Sr-I 98.44(14)°, N2-Sr-I 104.2(2)°, N3-Sr-I
98.59(13)°).
Compound 4 crystallizes in the monoclinic space group P21
with four molecules of the complex and four THF molecules in
the unit cell. The asymmetric unit contains two independent
molecules of 4 and two THF molecules. Similarly to 3, the
(DIP2pyr)- ligand of 4 is coordinated almost symmetrically to
the metal center (Figure 3). The metal-imine nitrogen bond
distances (Ba1-N1 3.196(4) Å, Ba1-N3 3.145(3) Å, Ba2-N4
3.214(4) Å, Ba2-N6 3.156(3) Å) are longer than the Ba-
Npyrrolyl distance (Ba1-N2 2.690(3) Å, Ba2-N5 2.679(3) Å).
Remarkably, the coordination sphere of the metal center is sat-
isfied by four additionally coordinated THF molecules, instead of
forming a dimer. Consequently, the 8-fold coordination sphere is
formed by the (DIP2pyr)- ligand, the iodine atom, and four
THF molecules. The iodine atom of 4 is not arranged linearly
with respect to any of the nitrogen atoms of the (DIP2pyr)-
ligand (e.g., N1-Ba1-I1 128.59(6)°, N2-Ba1-I1 148.40(7)°,
N3-Ba1-I1 106.04(7)°). The Ba-I bond distances (Ba1-I1
3.4950(8) Å and Ba2-I2 3.4922(7) Å) are similar to that
observed for the monomeric barium iodo complex stabilized
by the very bulky tris[3-(2-methoxy-1,1-dimethyl)pyrazolyl]hy-
droborate (TpC*) ligand (Ba-I 3.5008(2) Å).67
Similarly to the previously described divalent lanthanide com-
pounds,49 all complexes 2-4 are monomeric in the solid state,
which is remarkable since strontium and barium tend to form
dimeric complexes, bridged by iodine atoms.56,66,68-71 Only few
monomeric heteroleptic iodo complexes of strontium and bar-
ium are known. For example, [TpC*SrI(THF)] and [TpC*BaI],
which contain a very bulky hexadentate ligand, form monomers
in the solid state.67 In addition, the monomeric cluster com-
pounds [IM(OtBu)4{Li(THF)}4(OH)] of calcium, strontium,
and barium are known.72 All these compounds contain very
bulky ligands, which saturate the coordination sphere of the
1408
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Figure 4. Energy diagram of compounds 2 and 4 based on computational studies.
corresponding metal center to form monomeric structures and
thus cannot be compared to complexes 3 and 4, in which the
coordination sphere is satisfied by additionally coordinated THF
molecules. Furthermore, it is very interesting that the solid-state
structure of the calcium complex 2 shows a κ2-coordinated
(DIP2pyr)- ligand, while the VT-1H NMR studies indicate a
symmetrical coordination of the ligand in solution even at low
temperatures. In order to obtain more insight into the coordina-
tion behavior of the (DIP2pyr)- ligand in the calcium complex 2,
computational studies of 2 and the barium derivative 4 were
performed.
Computational Studies. The possible local minima on the
corresponding energy hyperfaces were calculated within the
framework of density functional theory (RI-DFT)73,74 at the
BP86-level75-78 (basis set of def2-SVP quality for each atom),79-81
as this theoretical level provides reliable data on this kind of
highly ionic complexes. Due to the huge size of the molecules,
frequency calculations to check on imaginary frequencies as
indicators for a negative curvature of the energy hypersurface
were not possible. Instead the calculations were performed without
symmetry constraints to ensure that minima calculated are “real”.
All of the calculations entailed the use of the TURBOMOLE
package.82 As the size of the ligand may be of importance for the
complexation of the unsymmetrical or symmetrical isomer of the
ligand, the molecular structures of the most different com-
pounds, which are 2 and 4, were calculated without simplification
of the ligands (Figure 4).
[(DIP2pyrSYM)MI(THF)3] (with one further unbound THF
molecule) as realized for M = Ca. For M = Ca we found that
[(DIP2pyrSYM)MI(THF)4] is not formed and spontaneously
“decomposes” into [(DIP2pyrSYM)MI(THF)3] plus an unbound
THF during the relaxation process of the calculation. The
coordination sphere of Ca is obviously not large enough to allow
the aggregation of four THF donor molecules. [(DIP2pyrASYM
)
CaI(THF)3] and [(DIP2pyrSYM)CaI(THF)3] are of comparable
energy at this level of theory so that the shape of the realized
isomer is obviously due to temperature or/and crystal-packing
effects.57 For M = Ba we confirmed the experimental finding:
[(DIP2pyrSYM)BaI(THF)4] is energetically favored by 31 kJ/
mol with respect to [(DIP2pyrASYM)BaI(THF)3] plus an un-
bound THF. The theoretical species [(DIP2pyrASYM)BaI(THF)4]
is 11 kJ/mol less stable than the experimentally obtained com-
plex [(DIP2pyrSYM)BaI(THF)4].
Amido Complexes. In order to form catalytically active spe-
cies based on 2 and 3 for the intramolecular hydroamination
catalysis, the {N(SiMe3)2}- ligand was introduced as a leaving
group.35-37,39 Since our previous studies resulted in decreased
catalytic activity by increasing the ionic radius of the alkaline
earth metal, we were not interested in synthesizing the analogous
barium compound. The reaction of [(DIP2pyr)MI(THF)3] (M
= Ca (2), Sr (3)) with [K{N(SiMe3)2}] in THF at elevated
temperature gave [(DIP2pyr)M{N(SiMe3)2}(THF)2] (M = Ca
(5), Sr (6)), as shown in Scheme 2.57
Complexes 5 and 6 were characterized by standard analytical/
spectroscopic techniques, and the solid-state structures were
analyzed by single-crystal X-ray diffraction. The 1H NMR spectra
of 5 and 6 show a singlet for the imino NdCH groups (δ = 8.07
(5), 8.03 (6) ppm) of the (DIP2pyr)- ligand, which confirms the
κ3-coordination mode. While for 6 a multiplet and a septet are
observed for the isopropyl groups of the (DIP2pyr)- ligand
The energy differences between the symmetrically (SYM)
coordinated (DIP2pyrSYM -
)
ligand and the asymmetrically
(ASYM) coordinated (DIP2pyrASYM -
)
ligand for both the
barium and the calcium compound were calculated. In both
cases, the symmetrically coordinated solvent-free species
[(DIP2pyrSYM)MI] exhibit a lower energy than the asymmetri-
cally coordinated ones [(DIP2pyrASYM)MI] by 57 kJ/mol (M = Ba)
and 49 kJ/mol (M = Ca), respectively.
1
(1.15-1.32, 3.06 ppm), the H NMR spectrum of 5 shows a
doublet and a septet (δ = 1.21, 3.07 ppm), indicating a free
1
We compared the energy of the possible isomer [(DIP2p-
rotation of the 2,6-diisopropylaniline moieties in 5. In the H
yrSYM)MI(THF)4] as realized for M = Ba with that of
NMR spectra of 5 and 6 one singlet for the {N(SiMe3)2}- ligand
1409
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Scheme 2
Figure 6. Perspective ORTEP view of the molecular structure of 6.
Thermal ellipsoids are drawn to encompass 30% probability. Hydrogen
atoms are omitted for clarity. The THF molecule around O2 is
disordered, and bond lengths and angles are given for both positions
O2A and O2B. Selected distances [Å] and angles [deg]: Sr-N1
3.034(3), Sr-N2 2.515(3), Sr-N3 2.994(3), Sr-N4 2.516(3), Sr-
O1 2.527(3), Sr-O2A 2.539(9), Sr-O2B 2.509(9), Sr-C31 3.299(5);
N1-Sr-N2 59.79(9), N1-Sr-N3 120.63(8), N1-Sr-N4 125.43(9),
N2-Sr-N3 60.85(9), N2-Sr-N4 174.35(10), N3-Sr-N4
113.93(9), N1-Sr-O1 85.85(10), N1-Sr-O2A 82.9(2), N1-Sr-
O2B 92.7(2), N2-Sr-O1 86.37(10), N2-Sr-O2A 84.5(2), N2-Sr-
O2B 88.9(2), N3-Sr-O1 91.66(11), N3-Sr-O2A 90.6(2), N3-Sr-
O2B 85.1(2), N4-Sr-O1 91.75(10), N4-Sr-O2A 98.0(2), N4-Sr-
O2B 92.9(2), O1-Sr-O2A 168.1(2), O1-Sr-O2B 175.1(2), Sr-
N4-Si1 112.23(15), Sr-N4-Si2 121.7(2).
Figure 5. Perspective ORTEP view of the molecular structure of 5.
Thermal ellipsoids are drawn to encompass 30% probability. Hydrogen
atoms are omitted for clarity. Selected distances [Å] and angles [deg]:
Ca-N1 3.022(3), Ca-N2 2.374(3), Ca-N3 2.888(3), Ca-N4
2.372(3), Ca-O1 2.368(2), Ca-O2 2.363(3); N1-Ca-N2
61.08(8), N1-Ca-N3 124.98(7), N1-Ca-N4 123.60(9), N2-
Ca-N3 63.90(8), N2-Ca-N4 174.96(11), N3-Ca-N4 111.42(9),
N1-Ca-O1 85.23(9), N1-Ca-O2 87.42(10), N2-Ca-O1
86.85(9), N2-Ca-O2 86.75(10), N3-Ca-O1 92.98(9), N3-Ca-
O2 88.55(10), N4-Ca-O1 91.62(9), N4-Ca-O2 95.17(10), O1-
Ca-O2 171.96(11), Ca-N4-Si1 113.86(15), Ca-N4-Si2
122.55(14).
(δ = -0.21 (5), -0.29 (6) ppm) is observed. The 13C{1H}
NMR data are consistent with the information obtained from the
1H NMR spectra.
6-fold coordination sphere of each complex is formed by the
(DIP2pyr)- ligand, the {N(SiMe3)2}- ligand, and two THF
molecules, showing a distorted octahedral geometry. The apexes
of each polyhedron are formed by the two THF molecules, which
are arranged linearly (O1-Ca-O2 171.96(11)° (5) and O1-
Sr-O2B 175.1(2)° (6)). The planar arrangement of the CaN4
fragment is confirmed by the sum of the four corresponding
valence angles (360.0° in 5 and 6)). The M-N bond distances to
the pyrrolyl units of the (DIP2pyr)- ligands in 5 and 6 (Ca-N2
2.374(3) Å (5) and Sr-N2 2.515(3) Å (6)) are similar to the
M-N bond length observed for the {N(SiMe3)2}- ligands
(Ca-N4 2.372(3) Å (5) and Sr-N4 2.516(3) Å (6)). Further-
more, the different M-N4-Si angles (Ca-N4-Si1 113.86(15)°
Compounds 5 and 6 crystallize in the monoclinic space group
P21/c with four molecules of the corresponding complex and
four THF molecules in the unit cell. As shown in Figures 5 and 6,
the (DIP2pyr)- ligand coordinates in both compounds 5 and 6
in a tridentate fashion. In compound 6 an almost symmetrical
coordination of the ligand to the metal center (Sr-N1 3.034(3)
Å, Sr-N3 2.994(3) Å) is seen, while due to the smaller ionic
radius of Ca2þ, the Schiff-base functions of the ligand in 5
coordinate in an asymmetrical fashion (Ca-N1 3.022(3) Å,
Ca-N3 2.888(3) Å). In contrast to the different coordination
modes of the (DIP2pyr)- ligand in 2 and 3, in both products 5
and 6 the ligand is thus κ3-coordinated in the solid state. The
1410
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Table 2. Hydroamination Reactions of Terminal Aminoalkenes and Alkynes Catalyzed by 5 and 6a
1
c
a Conditions: cat. 15-20 mg (5 mol %), C6D6. b Calculated by H NMR, ferrocene as internal standard. Products were accompanied by alkene
isomerization byproduct 10c (see text for details). d Ratio cis:trans. e Cis/trans signals obscured by other signals.
versus Ca-N4-Si2 122.55(14)° (5); Sr-N4-Si1 112.23(15)°
versus Sr-N4-Si2 121.7(2)° (6)) of the {N(SiMe3)2}- ligands
indicate a distorted geometry. The Si1-N4-Si2 angles are
123.5(2)° (5) and 126.1(2)° (6). In contrast to [Me2Si(Flu)-
(C5Me5)DyN(SiMe3)2] (Flu = C13H8, fluorenyl), in which
agostic interactions between the dysprosium ion and one methyl
group of the {N(SiMe3)2}- ligand were observed,83 the differ-
ence betweenthetwoanglesissmallerinthepresentcase. Asareason
for the slightly asymmetrical arrangement of the {N(SiMe3)2}-
ligands in 5 and 6, crystal packing effects are suggested.57
Recently, Hill et al. reported the β-diketiminato complexes
[{(DIPNC(Me))2CH}M{N(SiMe3)2}(THF)] (DIP = 2,6-dii-
sopropylphenyl; M = Ca, Sr, Ba) and on the problem of ligand
redistribution, observed for the heavier homologues strontium
and barium.35 The desired products were accompanied by the
homoleptic complexes, and obviously the ligand was not able to
stabilize the heteroleptic compounds in the case of strontium and
barium. In the present work, no problematic ligand redistribution
occurred and the desired heteroleptic compounds 5 and 6 were
obtained without byproducts. In contrast to the different co-
ordination modes of the (DIP2pyr)- ligand in 2 and 3, in both
products 5 and 6 the ligand is κ3-coordinated in the solid state.
Hydroamination Studies. The catalytic activity of complexes
5 and 6 was investigated for the intramolecular hydroamination
of nonactivated aminoalkenes. All experiments were carried out
under rigorously anaerobic reaction conditions by using dry,
degassed substrates and a catalyst loading of 5 mol % and were
monitored by 1H NMR spectroscopy, by using ferrocene as an
internal standard.57 As presented in Table 2, both complexes 5
and 6 are catalytically active. The calcium species 5 was more
active than the strontium compound 6, which is consistent with
the results from our previously studies by using aminotroponi-
minate alkaline earth metal amides as catalysts.37 As shown in
Table 2, substrates 7a, 8a, and 9a were cyclized selectively to the
corresponding pyrrolidines 7b, 8b, and 9b (entries 1-6) in high
yields under mild conditions, while the reactions of the R-
branched aminoalkene 10a catalyzed by either 5 or 6 were
accompanied by a side reaction (entries 7-9). The side product
10c illustrated in Scheme 3 was determined via 1H NMR
spectroscopy.
The cyclization of 10a by using either catalyst 5 or 6 afforded
the desired product 10b and additionally 10c, which was formed
in a competitive alkene isomerization reaction (Table 2, entries
7-9). 10c was not further cyclized by the catalysts, since internal
1411
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
Scheme 3
structure determinations of 2-6 are available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: roesky@kit.edu.
alkenes are less reactive than terminal aminoalkenes. A similar
alkene isomerization was observed by Hill et al. by using
[{(DIPNC(Me))2CH}Ca{N(SiMe3)2}(THF)] as a catalyst.35
The alkene isomerization depends on the reaction temperature
and the type of catalyst. By using 5 as a catalyst, the cyclization of
10a at an elevated temperature of 60 °C within 5 h led to 40% of
the rearrangement product 10c (Table 2, entry 8). However, by
performing the same reaction at room temperature within 46 h,
the selectivity could be improved, and only 18% of 10c was
detected (Table 2, entry 7). Furthermore, by lowering the
temperature the cis/trans selectivity of the product 10b was
improved from cis:trans = 1:8 at 60 °C to 1:13 at room
temperature. By using 6 as a catalyst, the alkene isomerization
reaction is favored even at room temperature, and 71% of 10c
was determined (Table 2, entry 9).
However, substrates 7a, 8a, and 9a were cyclized without the
formation of undesired side products. On the basis of the
Thorpe-Ingold effect,88-91 the reaction rate increases with
increasing steric bulk of the substituents in β-position to the
amino group, and thus aminoalkenes 7a and 8a were converted
rapidly to their cyclic products at room temperature. Substrate
9a, which contains less bulky substituents, was cyclized by the
more active catalyst 5 within 0.5 h at room temperature (Table 2,
entry 5), whereas by using 6 as a catalyst an elevated temperature
of 60 °C was required and the conversion was complete after two
hours (Table 2, entry 6). The best result was obtained by using 5
as the catalyst and 7a as the substrate, and 99% of the corre-
sponding product 7b was formed at room temperature within 2
min (Table 2, entry 1).
’ ACKNOWLEDGMENT
This work was supported by the Deutsche Forschungsgemein-
schaft (SPP 1166) and the Baden-W€urttemberg Stiftung gGmbH.
’ REFERENCES
(1) Brunet, J.-J.; Neibecker, D. In Catalytic Heterofunctionalization;
Togni, A., Gr€utzmacher, H., Eds.; 2001; pp 91-141.
(2) Johannsen, M.; Jorgensen, K. A. Chem. Rev. 1998, 98, 1689–1708.
(3) Roundhill, D. M. Chem. Rev. 1992, 92, 1–27.
(4) Nobis, M.; Driessen-H€olscher, B. Angew. Chem., Int. Ed. 2001,
40, 3983–3985.
(5) Seayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. Adv. Synth.
Catal. 2002, 344, 795–813.
(6) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 935–946.
(7) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104–114.
(8) Roesky, P. W.; M€uller, T. E. Angew. Chem., Int. Ed. 2003,
42, 2708–2710.
(9) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004,
104, 3079–3160.
(10) Hartwig, J. F. Pure Appl. Chem. 2004, 76, 507–516.
(11) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673–686.
(12) Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367–391.
(13) Hultzsch, K. C. Org. Biomol. Chem. 2005, 3, 1819–1824.
(14) Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem. 2006,
2006, 4555–4563.
(15) Aillaud, I.; Collin, J.; Hannedouche, J.; Schulz, E. Dalton Trans.
2007, 5105–5118.
(16) Hunt, P. A. Dalton Trans. 2007, 1743–1754.
(17) M€uller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M.
Chem. Rev. 2008, 108, 3795–3892.
’ SUMMARY
(18) Roesky, P. W. Angew. Chem., Int. Ed. 2009, 49, 4892–4894.
(19) Li, T.; Jenter, J.; Roesky, P. W. Struct. Bonding (Berlin) 2010,
137, 165–228.
(20) M€uller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675–704.
(21) Hill, M. S. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 2007,
103, 39–53.
(22) Koller, J.; Bergman, R. G. Chem. Commun. 2010, 46, 4577–4579.
(23) Koller, J.; Bergman, R. G. Organometallics 2010, 29, 3350–3356.
(24) Fadini, L.; Togni, A. Chem. Commun. 2003, 30–31.
(25) Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem.
2006, 4555–4563.
The heteroleptic iodo complexes [(DIP2pyr)MI(THF)n] (M
= Ca (2), Sr (3) (n = 3); Ba (4) (n = 4); (DIP2pyr)- = 2,5-
bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl) were syn-
thesized by reaction of [(DIP2pyr)K] (1) with anhydrous alka-
line earth metal diiodides. Further treatment of 2 and 3 with
[K{N(SiMe3)2}] resulted in the amido complexes [(DIP2pyr)-
M{N(SiMe3)2}(THF)2] (M = Ca (5), Sr (6)). Both compounds
were investigated for the intramolecular hydroamination of
aminoalkenes. Both complexes 5 and 6 are catalytically active
systems, although the formation of rearrangement products by
alkene isomerization led to a limited reaction scope. In particular,
the calcium complex 5 showed high activity and can be compared
to the β-diketiminato calcium amide [{(DIPNC(Me))2CH}-
Ca{N(SiMe3)2}(THF)], the aminotroponiminate calcium amide
[{(iPr)2ATI}Ca{N(SiMe3)2}(THF)2] ((iPr)2ATI = N-isopro-
pyl-2-(isopropylamino)troponiminate), and the bis(imidazolin-
2-ylidene-1-yl)borate alkaline earth metal amides [{H2B-
(ImtBu)2}M{N(SiMe3)2}(THF)n] (M = Ca, n = 1; M = Sr, n
= 2; ImtBu = 1-tert-butylimidazole).32,35,37
(26) Kang, J.-E.; Kim, H.-B.; Lee, J.-W.; Shin, S. Org. Lett. 2006,
8, 3537–3540.
(27) Nishina, N.; Yamamoto, Y. Angew. Chem., Int. Ed. 2006,
45, 3314–3317.
(28) Nishina, N.; Yamamoto, Y. Synlett 2007, 1767–1770.
(29) Zhang, J.; Yang, C.-G.; He, C. J. Am. Chem. Soc. 2006,
128, 1798–1799.
(30) Zhang, Y.; Donahue, J. P.; Li, C.-J. Org. Lett. 2007, 9, 627–630.
(31) Zhang, Z.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc.
2009, 131, 5372–5373.
(32) Arrowsmith, M.; Hill, M. S.; Kociok-K€ohn, G. Organometallics
2009, 28, 1730–1738.
’ ASSOCIATED CONTENT
(33) Barrett, A. G. M.; Brinkmann, C.; Crimmin, M. R.; Hill, M. S.;
Hunt, P.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 12906–12907.
(34) Buch, F.; Harder, S. Z. Naturforsch., B: Chem. Sci. 2008,
63, 169–177.
S
Supporting Information. Details of the computational
b
studies and X-ray crystallographic files in CIF format for the
1412
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413

Organometallics
ARTICLE
(35) Crimmin, M. R.; Arrowsmith, M.; Barrett, A. G. M.; Casely, I.
J.; Hill, M. S.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 9670–9685.
(36) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005,
127, 2042–2043.
(37) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008,
27, 1207–1213.
(73) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem. Phys. 2003,
118, 9136–9148.
(74) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346–354.
(75) Perdew, J. P. Phys. Rev. B 1986, 34, 7406–7406.
(76) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824.
(77) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100.
(78) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980,
58, 1200–1211.
(38) Datta, S.; Gamer, M. T.; Roesky, P. W. Dalton Trans.
2008, 2839–2843.
(39) Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007,
26, 4392–4394.
(40) Dunne, J. F.; Fulton, D. B.; Ellern, A.; Sadow, A. D. J. Am. Chem.
(79) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065.
(80) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005,
7, 3297–3305.
Soc. 2010, 132, 17680–17683.
(41) Horrillo-Martínez, P.; Hultzsch, K. C. Tetrahedron Lett. 2009,
50, 2054–2056.
(81) Sch€afer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992,
97, 2571–2577.
(82) Ahlrichs, R.; B€ar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem.
Phys. Lett. 1989, 162, 165–169.
(42) Neal, S. R.; Ellern, A.; Sadow, A. D. J. Organomet. Chem. 2011,
696, 228–234.
(83) Qian, C.; Nie, W.; Sun, J. Organometallics 2000, 19, 4134–4140.
(84) Burkey, D. J.; Hanusa, T. P. Organometallics 1996,
15, 4971–4976.
(43) Zhang, X.; Emge, T. J.; Hultzsch, K. C. Organometallics 2010,
29, 5871–5877.
(44) Matsuo, Y.; Mashima, K.; Tani, K. Organometallics 2001,
20, 3510–3518.
(45) Meyer, N.; Kuzdrowska, M.; Roesky, P. W. Eur. J. Inorg. Chem.
2008, 1475–1479.
(46) Meyer, N.; Jenter, J.; Roesky, P. W.; Eickerling, G.; Scherer, W.
Chem. Commun. 2009, 4693–4695.
(85) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295–9306.
(86) Panda, T. K.; Hrib, C. G.; Jones, P. G.; Jenter, J.; Roesky, P. W.;
Tamm, M. Eur. J. Inorg. Chem. 2008, 2008, 4270–4279.
(87) Trambitas, A. G.; Panda, T. K.; Jenter, J.; Roesky, P. W.;
Daniliuc, C.; Hrib, C. G.; Jones, P. G.; Tamm, M. Inorg. Chem. 2010,
49, 2435–2446.
(47) Jenter, J.; K€oppe, R.; Roesky, P. W. C. R. Chim. 2010,
13, 603–607.
(48) Jenter, J.; Meyer, N.; Roesky, P. W.; Thiele, S. K.-H.; Eickerling,
G.; Scherer, W. Chem.—Eur. J. 2010, 16, 5472–5480.
(49) Jenter, J.; Gamer, M. T.; Roesky, P. W. Organometallics
2010, 4410–4413.
(88) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735–1766.
(89) Jung, M. E.; Gervay, J. J. Am. Chem. Soc. 1991, 113, 224–232.
(90) Ingold, C. K. J. Chem. Soc. Trans. 1921, 119, 305–329.
(91) Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc. Trans.
1915, 107, 1080–1106.
(50) Shannon, R. Acta Crystallogr., Sect. A 1976, 32, 751–767.
(51) Greenwood, N. N.; Earnshaw, A. In Chemistry of the Elements;
Pergamon Press: Oxford, 1984; pp 122, 1430.
(52) Harder, S. Angew. Chem., Int. Ed. 2004, 43, 2714–2718.
(53) Weber, F.; Schultz, M.; Sofield, C. D.; Andersen, R. A. Orga-
nometallics 2002, 21, 3139–3146.
(54) Schumann, H.; Schutte, S.; Kroth, H.-J.; Lentz, D. Angew.
Chem., Int. Ed. 2004, 43, 6208–6211.
(55) Wiecko, M.; Marks, S.; Panda, T. K.; Roesky, P. W. Z. Anorg.
Allg. Chem. 2009, 635, 931–935.
(56) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W.
J. Organomet. Chem. 2005, 690, 5078–5089.
(57) Jenter, J. Ph.D. Thesis, Karlsruhe Institute of Technology, 2010.
(58) Miller, R.; Olsson, K. Acta Chem. Scand. 1981, B35, 303–310.
(59) Olsson, K.; Pernemalm, P. Acta Chem. Scand. 1979, B33, 125–132.
(60) Meyer, N. Ph.D. Thesis, Freie Universit€at Berlin, 2007.
(61) Martinez, P. H.; Hultzsch, K. C.; Hampel, F. Chem. Commun.
2006, 2221–2223.
(62) Damljanoviꢀc, I.; Vukiꢀceviꢀc, M.; Vukiꢀceviꢀc, R. D. Monatsh. Chem.
2006, 137, 301–305.
(63) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 4391–4393.
(64) Rast€atter, M.; Zulys, A.; Roesky, P. W. Chem.—Eur. J. 2007,
13, 3606–3616.
(65) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.
(66) Datta, S.; Gamer, M. T.; Roesky, P. W. Dalton Trans.
2008, 2839–2843.
(67) Chisholm, M. H.; Gallucci, J. C.; Yaman, G. Dalton Trans.
2009, 368–374.
(68) Sitzmann, H.; Weber, F.; Walter, M. D.; Wolmershauser, G.
Organometallics 2003, 22, 1931–1936.
(69) Tesh, K. F.; Burkey, D. J.; Hanusa, T. P. J. Am. Chem. Soc. 1994,
116, 2409–2417.
(70) El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. Polyhedron 2006,
25, 224–234.
(71) Harvey, M. J.; Hanusa, T. P. Organometallics 2000,
19, 1556–1566.
(72) Maudez, W.; Meuwly, M.; Fromm, K. M. Chem.—Eur. J. 2007,
13, 8302–8316.
1413
dx.doi.org/10.1021/om100937c |Organometallics 2011, 30, 1404–1413