Rare earth and alkaline earth metal complexes with Me2Si-bridged cyclopentadienyl-imidazolin-2-imine ligands and their use as constrained-geometry hydroamination catalysts

Article abstract of DOI:10.1002/ejic.200800606

The imidazolin-2-imino-functionalized tetramethylcyclopentadiene, 3-H, has been prepared by the reaction of two equivalents of 1,3-diisopropyl-4,5- dimethylimidazolin-2-imine (1) with 5-(chlorodimethylsilyl)-1,2,3,4-tetramethyl- 1,3-cyclopentadiene (2). T

Full text of DOI:10.1002/ejic.200800606

FULL PAPER
DOI: 10.1002/ejic.200800606
Rare Earth and Alkaline Earth Metal Complexes with Me2Si-Bridged
Cyclopentadienyl-Imidazolin-2-Imine Ligands and Their Use as Constrained-
Geometry Hydroamination Catalysts
Tarun K. Panda,[a] Cristian G. Hrib,[a] Peter G. Jones,[a] Jelena Jenter,[b] Peter W. Roesky,[b]
and Matthias Tamm*[a]
Keywords: Rare earth metals / Calcium / Cyclopentadienyl ligands / Constrained-geometry complexes / Hydroamination
The imidazolin-2-imino-functionalized tetramethylcyclopen-
tadiene, 3-H, has been prepared by the reaction of two
equivalents of 1,3-diisopropyl-4,5-dimethylimidazolin-2-
imine (1) with 5-(chlorodimethylsilyl)-1,2,3,4-tetramethyl-
1,3-cyclopentadiene (2). The reactions of 3-H with the tri-
methylsilylmethyl (neosilyl) [Ln(CH2SiMe3)3(THF)2] (Ln = Sc,
Y, Lu) complexes afford tetramethylsilane and the con-
strained-geometry [(η5:η1-3)Ln(CH2SiMe3)2] (Ln = Sc, 4a; Ln
= Y, 4b; Ln = Lu, 4c) complexes, whereas the analogous
bis(trimethylsilyl)amido (HMDS) calcium complex, [(η5:η1-3)-
Ca(HMDS)(THF)] (5), can be obtained from 3-H and
[Ca(HMDS)2(THF)2] with loss of hexamethyldisilazane,
(Me3Si)2NH. The molecular structures of 4a–4c and 5 have
been established by X-ray diffraction analyses, which con-
firm in all cases the formation of complexes with a chelating
Me2Si-bridged cyclopentadienyl-imidazolin-2-imine ligand.
The presence of short Ln–N and Ca–N bonds is indicative
of the strong electron-donating capacity of the imidazolin-
2-imino nitrogen atom, which is based on the ability of the
imidazolium ring to effectively stabilize a positive charge.
The rare earth metal complexes 4a–4c can be used as ef-
ficient catalysts for the hydroamination/cyclization reaction
of terminal aminoalkenes and aminoalkynes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
ligands.[6] These characteristics have been exploited for the
development of imidazolin-2-iminato tungsten alkylidyne
complexes as catalysts for efficient room-temperature al-
kyne metathesis,[7] and also for the preparation of lantha-
nide complexes with very short Ln–N bonds, which can
serve as accurate models for elusive mononuclear lantha-
nide imido complexes.[8]
Introduction
Imidazolin-2-imines (ImNH) and imidazolin-2-imides
(ImN–) can be described by two limiting resonance struc-
tures, IA and IB, which indicates that the ability of the imid-
azolium ring to stabilize a positive charge leads to highly
basic ligands with a strong electron-donating capacity
towards transition metals (Scheme 1).[1–3] We have recently
established a general method for the preparation of these
ligands from stable carbenes of the imidazolin-2-ylidene
type and trimethylsilyl azide. This procedure resembles the
Staudinger reaction[4] between phosphanes and azides and
furnishes 2-trimethylsilyliminoimidazolines (X = SiMe3),[3]
which are suitable precursors for the synthesis of transition
metal complexes incorporating ancillary imidazolin-2-imin-
ato ligands (X = N–).[5] Owing to their ability to act as
2σ,4π-electron donors, these ligands can be regarded as
monodentate analogues of cyclopentadienyl derivatives
(C5R5) and also as monoanionic imido ligands in a similar
fashion to that described for related phosphoraneiminato
[a] Institut für Anorganische und Analytische Chemie, Technische
Universität Carolo-Wilhelmina zu Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-391-5309
E-mail: m.tamm@tu-bs.de
[b] Institut für Anorganische Chemie, Universität Karlsruhe,
Engesserstr. 15, 76128 Karlsruhe, Germany
Fax: +49-721-608-4854
Scheme 1. Mesomeric structures of imidazolin-2-imines and imides,
and schematic representation of silylene-linked cyclopentadienyl-
amido and cyclopentadienyl-imino complexes.
E-mail: roesky@chemie.uni-karlsruhe.de
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Eur. J. Inorg. Chem. 2008, 4270–4279

Constrained-Geometry Complexes as Hydroamination Catalysts
Imidazolin-2-imines (X = NH, Scheme 1) are not only diene (2) with two equivalents of 1,3-diisopropyl-4,5-di-
valuable ligands or ligand precursors in their own right, but methylimidazolin-2-imine, ImiPrNH (1). One equivalent of
can also serve as useful building blocks for the design and the imine serves as a hydrochloric acid scavenger and pre-
preparation of novel multidentate poly(imidazolin-2-imine) cipitates from the reaction mixture as the corresponding 2-
ligands. Yet again, these ligands attain their unique proper- aminoimidazolium chloride, 1·HCl (Scheme 2).[13] The for-
ties from their ability to effectively delocalize a positive mation of cyclopentadiene 3-H from imine 1 can easily be
1
charge over the imidazole moiety to produce compounds followed by H NMR spectroscopy since four additional
with considerably enhanced basicity and N-nucleophilic- resonances are observed for the (tetramethylcyclopen-
ity.[9–12] For instance, the unusual stability of coordinatively tadienyl)dimethylsilyl moiety, while the imine NH reso-
unsaturated 16-electron ruthenium and molybdenum half- nance disappears. The resonances of the imidazolin-2-imine
sandwich complexes supported by ethylene-bridged bis- moieties are only slightly shifted in comparison to those of
(imidazolin-2-imine) ligands can be ascribed to the strongly imine 1 and are almost identical to those reported for the
π-basic nature of the novel bidentate ligands,[10] which also corresponding 2-(trimethylsilylimino)imidazoline mole-
holds true for the high reactivity of copper(I) bis(imidazo- cule.[3,5b] No fluxional behaviour is observed for the cyclo-
1
lin-2-imine) complexes and allows for effective CO2 and O2 pentadiene moiety, and the H NMR spectrum exhibits a
fixation, C–Cl bond activation and CuI disproportiona- singlet at δ = 3.20 ppm for the CH hydrogen atom along
tion.[11] Recently, the pronounced electron donor properties with two signals at about 2.22 and 1.97 ppm for the 2,4-
of these ligands have also been demonstrated by the synthe- and 3,5-methyl groups, respectively, which reveals that com-
sis and structural characterization of lanthanide complexes pound 3-H adopts Cs symmetry with the silylene bridge in
containing a bis(imidazolin-2-imino)pyridine pincer ligand, the allylic position. In the 13C NMR spectra, product for-
which confirms that the imine moieties act as ylidic amido- mation is indicated by a characteristic high-field shift of
type ligands upon metal complexation.[12]
about 10 ppm for the imidazoline C2 (NCN) resonance
In a similar fashion, tethering of an imidazolin-2-imine (from 153.4 ppm in 1 to 144.2 ppm in 3-H). The cyclopenta-
unit to a cyclopentadienyl ring through a Me2Si bridge diene CH carbon resonance is observed at δ = 59.8 ppm,
leads to cyclopentadienyl-imidazolin-2-imine ligands, which whereas the resonances for the tertiary carbon atoms in the
can be expected to form functional “constrained-geometry” 2,5- and 3,4-positions appear at δ = 134.5 and 133.9 ppm,
ansa-cyclopentadienyl-amido complexes, despite the overall respectively.
neutral charge of the imine moiety. This concept has been
realized by the preparation of chromium(III) complexes[13]
that are structurally closely related to well-established and
commercialized type II ansa-cyclopentadienyl-amido
complexes containing tetravalent group
4
metals
(Scheme 1).[14–16] The introduction of other trivalent metal
atoms such as rare earth elements (group 3 metals and lan-
thanides) should also lead to isostructural complexes, as the
resulting cyclopentadienyl-imidazolin-2-imine complexes,
[(η5:η1-C5R4-SiMe2-NIm)LnX2] (III), require two mono-
anionic X ligands to attain an overall neutral charge.[17] In
contrast, “conventional” silylene-linked cyclopentadienyl-
amido complexes of the [(η5:η1-C5R4-SiMe2-NR)LnX] (Ln
= rare earth metal) type contain only one additional an-
ionic ligand,[18] and ever since their introduction in the early
1990s,[19] these systems have attracted considerable interest
because of their remarkable activity in various stoichiomet-
ric and catalytic processes.[18,20–25] Monoalkyl complexes
have been of particular importance, and this contribution
establishes the corresponding type III rare earth metal di-
alkyl complexes and compares their properties with those
of their silylene-linked cyclopentadienyl-amido counter-
parts.
Scheme 2. Synthesis of cyclopentadienyl-imidazolin-2-imine com-
plexes.
Results and Discussion
Ligand Synthesis
Synthesis and Characterization of Rare Earth Metal
Complexes
The imidazolin-2-imino-tetramethylcyclopentadiene li-
gand, 3-H, was isolated as a yellow oil from the reaction of
5-(chlorodimethylsilyl)-1,2,3,4-tetramethyl-1,3-cyclopenta-
Following the salt-free route developed for the prepara-
tion of trimethylsilylmethyl (“neosilyl”) complexes of the
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[(η5:η1-C5Me4-SiMe2-NR)Ln(CH2SiMe3)(THF)n] (n = 1, 2) eters of the three complexes are very similar, and pertinent
type,[20,21] the acid–base reaction between the rare earth data are summarized in Table 1. As a representative exam-
metal [Ln(CH2SiMe3)3(THF)2] (Ln = Sc, Y, Lu) trialkyl ple, two different views of the molecular structure of the
complexes and cyclopentadiene 3-H in hexane at ambient lutetium complex 4c are shown in Figure 1. Coordination
temperature afforded the solvent-free [(η5:η1-C5Me4-SiMe2- of the Cp-imine ligand, 3, in a chelating η5:η1-fashion is
NImiPr)Ln(CH2SiMe3)2] (Ln = Sc, 4a; Ln = Y, 4b; Ln = confirmed for all complexes, and 4a–4c exhibit a three-leg-
Lu, 4c) complexes, which can be isolated in good yields as ged piano-stool geometry with a distorted pseudotetrahe-
colourless crystalline solids from THF/pentane solutions at dral environment around the metal atoms. The solid-state
–30 °C (Scheme 2). As expected for time-averaged Cs-sym-
1
metric structures in solution, the H NMR spectra of 4a–
4c in C6D6 exhibit four characteristic singlet resonances in
a 6:6:6:6 ratio for four different types of methyl groups pres-
ent in the cyclopentadienyl-imine ligand along with a low-
field septet resonance at about 4.8 ppm for the isopropyl
CH hydrogen atoms. The corresponding isopropyl CH3
groups give rise to broad signals in the 1.23 to 1.08 ppm
range, which indicates dynamic behaviour at room tempera-
ture on the NMR timescale. For the neosilyl (CH2SiMe3)
2
groups, one singlet (SiMe3) and two doublets with J1H,1H
= 11 Hz are observed at high fields for each complex, which
reveals the presence of diastereotopic CH2 hydrogen atoms.
These observations are in agreement with the assumption
that the dynamic behaviour of complexes 4 in solution is
mainly a result of rotation around the N–Cimine bond axes
and does not involve cleavage of the Ln–N bonds. It should
also be noted that the 1H NMR spectrum of yttrium deriva-
tive 4b in [D8]THF shows a well-resolved doublet of dou-
blets with a 2J89Y, 1H coupling constant of 2.7 Hz, which falls
in the range previously observed for neosilyl yttrium com-
plexes.[20b,21e]
Single crystals of complexes 4a–4c could be obtained
from THF/hexane solutions, and their molecular structures
could be established by X-ray diffraction analyses. 4a (Ln
= Sc) and 4c (Ln = Lu) are isostructural and crystallize as
hexane hemisolvates in the space group P21/c, whereas yt-
trium compound 4b crystallizes without solvent incorpora-
tion in the space group P21/n with two independent mole-
cules in the asymmetric unit. The overall structural param- rameters drawn at 50% probability.
Figure 1. ORTEP drawings of 4c with thermal displacement pa-
Table 1. Selected bond lengths [Å] and angles [°] in complexes 4 and 5.
4a [M = Sc]
4b [M = Y][a]
4c [M = Lu]
5 [M = Ca]
M–N1
2.2135(16)
2.228(2)
2.253(2)
2.410(2)–2.651(2)
2.221
2.354(2)/2.352(2)
2.403(3)/2.400(3)
2.417(3)/2.416(3)
2.568(2)/2.562(2)–2.711(3)/2.738(2)
2.356/2.360
2.298(2)
2.365(3)
2.341(3)
2.515(3)–2.735(3)
2.327
2.448(2)
M–C23
M–C27
M–C(Cp)
2.601(3)–2.887(3)
2.471
[b]
M–Cpct
M–Si1
N1–Si1
N1–C1
3.0480(7)
1.7425(15)
1.345(2)
100.11(7)
130.97(12)
128.83(13)
96.83(8)
101.1
23.6
104.56(8)
105.09(6)
107.04(7)
3.1931(8)/3.1715(8)
1.755(2)/1.748(2)
1.337(3)/1.342(3)
100.98(9)/100.31(10)
132.36(16)/130.13(17)
126.59(18)/129.51(18)
99.23(11)/98.54(11)
96.1/95.4
3.1378(8)
1.746(2)
1.342(3)
100.90(9)
130.48(17)
128.47(18)
97.96(11)
97.3
3.2263(10)
1.731(2)
1.330(3)
99.62(9)
132.39(16)
127.02(18)
102.07(12)
94.6
M–N1–Si1
M–N1–C1
C1–N1–Si1
N1–Si1–C14
Cpct–M–N1[b]
Cp–C14–Si1[c]
C23–M–C27
N1–M–C23
N1–M–C27
23.5/23.6
22.8
18.2
105.41(10)/109.60(11)
114.68(9)/110.86(9)
108.07(9)/105.49(9)
106.42(11)
108.13(10)
105.40(9)
[a] Two independent molecules in the asymmetric unit. [b] Cpct = centroid of the cyclopentadienyl ring. [c] Dihedral angle between the
Cp plane and the C14–Si bond.
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Constrained-Geometry Complexes as Hydroamination Catalysts
structures of all of the complexes deviate quite significantly 3, would potentially lead to complexes that are isostructural
from Cs symmetry, and the Si1–N1–C1 planes subtend with silylene-linked cyclopentadienyl-amido complexes of
angles of 14.6° (4a), 11.1/19.3° (4b) and 15.3° (4c), respec- the [(η5:η1-C5R4-SiMe2-NR)LnX] type. In view of the diag-
tively, with the best plane containing the centroid of the Cp onal relationship between calcium and yttrium and of their
ring, the N1 atom and the metal atom. This distortion is comparable ionic radii,[26] we aimed towards the synthesis
presumably a result of the syn and anti orientation of the of a CaII complex of the [(η5:η1-C5R4-SiMe2-NIm)CaX]
neosilyl SiMe3 groups towards the Cp ring, which leads to type containing one additional monoanionic ligand, X. Cal-
a twisting of the imidazole ring in order to accommodate cium
bis(hexamethyldisilazide),
[Ca(HMDS)2(THF)2]
the isopropyl groups. Despite this imidazole slippage, the [HMDS = N(SiMe3)2], seemed to be a promising starting
near perpendicular orientation of the heterocycles towards material for developing a salt-free route,[27] and its reac-
the C1–N1–Si1 plane [interplanar angles of 85.1° (4a), 80.1/ tion with 3-H in hexane afforded the THF complex,
76.3° (4b) and 83.6° (4c)] rules out any significant π-interac- [(η5:η1-C5Me4-SiMe2-NImiPr)Ca(HMDS)(THF)] (5), in
tion between the exocyclic nitrogen atom and the imidazole good yield as a colourless solid after recrystallization from
system. In comparison to the molecular structures of unco- a THF/pentane solution at –30 °C (Scheme 3). The 1H
ordinated N-silylated imidazolin-2-imines,[3] this orientation NMR spectrum of 5 in C6D6 is very similar to the spectra
leads to a pronounced elongation of the C1–N1 bonds and recorded for complexes 4 (vide supra) and reveals the time-
to more obtuse N2–C1–N3 angles (Table 1). This reveals an averaged Cs symmetry of the molecule in solution despite
increase of the imidazolium-amide character, as indicated the presence of one additional THF ligand, as indicated by
by mesomeric form IB and the ylidic presentation of com- the observation of two multiplets at 3.61–3.54 and 1.44–
plexes 4 in Scheme 1. As a consequence of this charge sepa- 1.37 ppm. The bis(trimethylsilyl)amide ligand gives rise to
ration, relatively short Ln–N1 bonds are observed, which one high-field resonance at δ = 0.18 ppm.
in all cases confirm the strong donating capability of the
planar, three-coordinate N1 nitrogen atom. The angle sums
at N1 in 4a–4c lie within 0.1° of 360.0° (Table 1).
The Y–N1 and Lu–N1 distances of 2.354(2)/2.352(2) Å
(4b) and 2.298(2) Å (4c) fall in the range observed for the
related
[(η5:η1-C5Me4-SiMe2-N-C6H5)Ln(CH2SiMe3)-
(THF)2] neosilyl complexes containing two THF ligands
[Y–N 2.327(5) Å, Lu–N 2.283(5) Å];[21e] they are longer,
however, than the metal–nitrogen bond lengths observed for
mono(tetrahydrofuran) complexes such as [(η5:η1-C5Me4-
SiMe2-NtBu)Y(CH2SiMe3)(THF)] [2.208(6) Å][20b] and
[(η5:η1-C5Me4-SiMe2-NXy)Lu(CH2SiMe3)(THF)] [2.188(2)
Å, Xy = 2,6-dimethylphenyl].[21a] The Sc–N1 distance of
2.2135(16) Å is also longer than previously observed for
other scandium complexes of the Cp-amido type, which
range from 2.072 to 2.104 Å.[19b,25a] This elongation might
be a consequence of a slightly reduced electron-donating
ability of the neutral imine moiety in comparison with the
anionic amido group; however, a particularly strong steric
interaction of the N-isopropyl groups and two neosilyl
Scheme 3. Synthesis of a cyclopentadienyl-imidazolin-2-imine cal-
cium complex.
Although there has been ongoing interest in alkaline
groups must also be taken into account. Finally, it should earth organometallics[28] and particularly in the cyclopen-
be noted that the angles at the N1 and Si1 atoms, which tadienyl chemistry of these elements,[28b] 5 representsϪto
are associated with the constrained geometry of the ligand the best of our knowledgeϪthe first structurally charac-
system, are very similar to those in structurally charac- terized constrained-geometry (CG) alkaline earth metal
terized silylene-linked cyclopentadienyl-amido rare earth complex containing a silylene-bridged Cp–N ligand, despite
metal complexes (see for instance ref.[21f] for X-ray crystal the fact that Grignard reagents have been employed fre-
structures of half-sandwich complexes over the full size quently as intermediates for the synthesis of other CG com-
range of group 3 and lanthanide metals).
plexes (CGC).[29] Therefore, its molecular structure was de-
termined by X-ray diffraction analysis, and an ORTEP
presentation is shown in Figure 2. In agreement with the
NMR spectroscopic data, the solid-state molecular struc-
ture of 5 contains one coordinated THF ligand, which leads
to a three-legged piano-stool geometry with a strongly dis-
torted pseudotetrahedral environment around the metal
Synthesis and Characterization of a Constrained-Geometry
Calcium Complex
In a similar fashion to that described for the structural atom. This distortion can be mainly attributed to the steric
analogy between complexes of type II and III (Scheme 1), requirements of the N(SiMe3)2 ligand, as indicated by the
complexation of a divalent metal by the Cp-imine ligand, large N1–Ca–N4 angle of 118.10(7)°. The Cp-imine ligand,
Eur. J. Inorg. Chem. 2008, 4270–4279
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M. Tamm et al.
FULL PAPER
however, is coordinated in a relatively symmetric fashion, Catalytic Hydroamination Studies
and the angles of 11.0° and 89.4° between the Si1–N1–C1
plane and the planes containing either the centroid of the
The catalytic addition of an organic amine N–H bond to
Cp ring, the N1 atom and the metal atom or the imidazole alkenes or alkynes (hydroamination) to give nitrogen-con-
ring indicate approximately coplanar or almost perfectly taining molecules is of great interest.[34] Currently, most
perpendicular orientations, respectively. The N1 atom re- amines are made in multistep syntheses that produce signifi-
sides in an irregular trigonal-planar environment, and the cant amounts of waste. Therefore, hydroamination offers an
sum of the angles at N1 is 359.0°.
attractive alternative route to nitrogen-containing molecules
that are important for the preparation of fine chemicals and
pharmaceuticals. It has been shown that hydroamination
can be catalyzed by d- and f-block transition metals[22,35,36]
and s-block metals,[37] and the scope of catalytic hydro-
amination has been recently reviewed.[34] In lanthanide
chemistry, it was shown by T. J. Marks et al. that amido and
alkyl metallocene complexes represent efficient catalysts for
the hydroamination/cyclization of primary aminoalkenes,
allenes and alkynes.[22,34k] Moreover, “constrained-geome-
try” organolanthanides of [(η5:η1-C5Me4-SiMe2-NtBu)-
LnE(SiMe3)2] (Ln = Sm, Nd, Yb, Lu; E = CH, N) composi-
tion were used as precatalysts for aminoalkene hydroamin-
ation/cyclization and are significantly more active than the
corresponding metallocenes [(η5-C5Me5)2LnE(SiMe3)2].[22a]
We have used compounds 4a–4c in the intramolecular hy-
droamination/cyclization reaction of terminal aminoalkenes
and of one aminoalkyne (Table 2). Our intention was to
evaluate the catalytic activity of compounds 4a–4c in com-
parison to other rare earth metal complexes. In our substrate
screening, we focused on challenging nonactivated aminoal-
kenes. The rigorously anaerobic reaction of the catalyst with
dry, degassed substrates proceeds regiospecifically. All sub-
strates are converted into the cyclic product under mild con-
ditions using compounds 4a–4c as catalysts with low catalyst
Figure 2. ORTEP drawing of 5 with thermal displacement param-
eters drawn at 50% probability. The THF ligand is disordered over
two positions, and only one position is shown. Selected bond
lengths [Å] and angles [°]: Ca–N4 2.357(2), N4–Si2 1.683(3), N4–
Si3 1.692(2), N1–Ca–N4 118.10(7), Ca–N4–Si2 118.69(12), Ca–
N4–Si3 116.96(12), Si2–N4–Si3 123.77(13); see also Table 1 for ad-
ditional data.
As expected from the larger atomic radius of Ca2+ [26]
,
the metal–N1 and metal–C(Cp) distances are elongated in loadings of 2 to 6 mol-%. Kinetic studies were undertaken
1
comparison with the corresponding values determined for by in situ H NMR spectroscopy. The reaction of a 20-fold
the LnIII complexes, 4 (Table 1). The Ca–C(Cp) bond molar excess of the substrates was monitored with constant
lengths are in satisfactory agreement with the correspond- catalyst concentration until substrate consumption was com-
ing distances reported for other cyclopentadienyl-calcium plete. The decrease of the substrate peak was integrated vs.
complexes;[30] their variation from 2.601(3) to 2.887(3) Å, the product signals. As observed for metallocene catalysts of
however, indicates a slightly more pronounced deviation the lanthanides,[34k] the rates increase with increasing ionic
from an ideal η5-coordination mode than observed for radius, which shows yttrium compound 4b to be the most
complexes 4. The calcium–nitrogen distances are Ca–N1 active. In rate, we observe significant deviations from ideal
2.448(2) Å and Ca–N4 2.357(2) Å, which is in agreement zero-order kinetics. By using compound 4b, all substrates
with a slightly more tightly bound HMDS ligand; the Ca– were converted at room temperature (entries 2, 5, 8 and 11),
N4 bond length is slightly longer than the analogous dis- whereas for the scandium compound a reaction temperature
tances found in [Ca(HMDS)2]2 [2.282(6) and 2.267(7) Å], of 60 °C was required (entries 1, 4, 7 and 10). Substrates
[Ca(HMDS)2(DME)] [2.271(3) Å][31] and [Ca(HMDS)2- 6a, 7a, and 8a, bearing bulky geminal substituents in the β-
(THF)2] [2.294(3) and 2.309(3) Å].[32] The Ca–N4–Si2 and position to the amino group (Thorpe–Ingold effect), could
Ca–N4–Si3 angles of 118.69(12)° and 116.96(12)°, respec- be cyclized within a few minutes (entries 1–3 and 4–6).[38] By
tively, show that the HMDS ligand is symmetrically bound using less bulky substituents in the β-positions, significantly
and that there is no indication of a structural distortion as lower rates are observed for all catalysts (entries 10–12). A
a result of an “agostic” interaction, as observed for the re- comparison of compound 4b using substrate 9a with the CG
lated half-sandwich complex, [(C5iPr4H)Ca(HMDS)- [(η5:η1-C5Me4-SiMe2-NtBu)LnE(SiMe3)2] compounds[22a]
(THF)].[33] Analogous Ln···H3C–Si contacts have also been shows a slower turnover frequency for 4b by a factor of
observed in the unsolvated [(η5:η1-C5R4-SiMe2-NtBu)- about three. In contrast, a comparison of 4b with CG actin-
Ln(HMDS)] (Ln = Y, Sm) complexes,[22a,25b] which repre- ide complexes using substrate 7a shows a similar turnover
sent the most closely structurally related rare earth metal frequency to [(η5:η1-C5Me4-SiMe2-NtBu)U(NMe2)2] but a
analogues to the calcium complex, 5, presented in this pub- lower turnover frequency than [(η5:η1-C5Me4-SiMe2NtBu)-
lication.
Th(NMe2)2].[39]
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4270–4279

Constrained-Geometry Complexes as Hydroamination Catalysts
Table 2. Hydroamination reaction of terminal aminoalkenes and alkynes catalyzed by 4a–4c.[a]
1
[a] Conditions: Cat. 10 mg, C6D6. [b] Calculated by H NMR spectroscopy.
Conclusions
Experimental Section
General: All manipulations of air-sensitive materials were per-
formed with rigorous exclusion of oxygen and moisture in flame-
dried Schlenk-type glassware either on a dual manifold Schlenk
line, interfaced to a high vacuum (10–4 Torr) line, or in an argon-
filled MBraun glove box. All solvents were dried using an
M. Braun solvent purification system. Deuterated solvents were
obtained from Sigma Aldrich (all Ն 99 atom-% D) and were de-
gassed, dried, and stored in the argon-filled M. Braun glove box.
NMR spectra were recorded with Bruker DPX 200 and 400 MHz
spectrometers. Chemical shifts are referenced to internal solvent
resonances and are reported relative to tetramethylsilane. Elemen-
tal analyses (C, H, N) were carried out with an Elementar vario
EL. The 2-iminoimidazoline 1 and [Ln(CH2SiMe3)3(THF)2] (Ln =
Sc, Y, Lu) were prepared according to published procedures.[3,40]
[Ca(HMDS)2(THF)2] was prepared from CaI2 and NaN(SiMe3)2
according to a modified literature procedure;[27] a detailed protocol
is given below. 5-(Chlorodimethylsilyl)-1,2,3,4-tetramethyl-1,3-cy-
clopentadiene (2) was purchased from Boulder Scientific Company
and distilled prior to use.
With this contribution, we have introduced novel con-
strained-geometry rare earth and alkaline earth metal com-
plexes by employing the monoanionic Me2Si-bridged cyclo-
pentadienyl-imidazolin-2-imine ligand, 3, which forms
stable chelate complexes of the [(η5:η1-C5Me4-SiMe2-
NImiPr)M] type with short metal–nitrogen bonds. The
strong electron-donating capacity of the tethered nitrogen
donor atom stems from the ability of the imidazole ring to
effectively stabilize a positive charge, and this ligand can
thus be regarded as a monoanionic analogue of dianionic
silylene-linked cyclopentadienyl-amido ligands. Accord-
ingly, the use of ligands such as 3 allows the preparation of
complexes with an isolobal and isostructural relationship to
well-established Cp-amido complexes of the [(η5:η1-C5Me4-
SiMe2-NR)M] type if the charge of the metal atom is re-
duced by one (e.g., by going from M4+ to M3+),[17] and the
rare earth metal complexes, 4, presented herein are related
to Cp-amido complexes of group 4 metals. Complexes 4 are
Preparation of 3-H: A general procedure for the preparation of 3-
active catalysts for the hydroamination/cyclization of ami- H was reported in a previous publication.[13] A detailed protocol is
described as follows: a solution of the 2-iminoimidazoline 1
(2 equiv.) in hexane was treated with one equivalent of 2 at ambient
temperature and was stirred for 90 min. The solution was separated
from the cloudy precipitate of 1·HCl by filtration through a glass
frit, and the filtrate solvent was removed in vacuo. Tetramethylcy-
clopentadiene 3-H was obtained as a pale yellow oil containing
excess 2, which can be purified by heating under vacuum for several
hours at 70 °C. Yield 98%. C22H39N3Si (373.65): calcd. C 70.72, H
10.52, N 11.25; found C 69.80, H 10.59, N 11.60. 1H NMR
(400 MHz, C6D6, 25 °C): δ = 4.63 (sept, 2 H, CHCH3), 3.20 (s, 1
H, ipso-CH), 2.22 (s, 6 H, o-CCH3), 1.97 (s, 6 H, m-CCH3), 1.71
noalkenes and aminoalkynes. Future work will be aimed
towards the activation of these complexes by alkyl scaven-
gers, which should afford cationic monoalkyl complexes as
active olefin polymerization catalysts, in a similar fashion
to that described for the related group 4 systems.[15] Finally,
the use of the Cp-imine ligand 3 allowed the isolation of
the first constrained-geometry calcium complex 5, and the
full potential of 5 and other alkaline earth metal complexes
for applications in homogeneous catalysis will be further
explored.
Eur. J. Inorg. Chem. 2008, 4270–4279
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4275

M. Tamm et al.
FULL PAPER
(s, 6 H, NCCH3), 1.17 (d, 12 H, CHCH3), 0.34 (s, 6 H, SiCH3)
Preparation of [Ca(HMDS)2(THF)2]: In a 50 mL flask, CaI2
(294 mg, 1.0 mmol) was treated with NaN(SiMe3)2 (367 mg,
ppm. 13C{1H} NMR (100.62 MHz, C6D6, 25 °C): δ = 144.2
(NCN), 134.5 (m-C), 133.9 (o-C), 114.0 (NCCH3), 59.8 (ipso-C), 2.0 mmol) in THF (20 mL) at ambient temperature. The reaction
44.9 (CHCH3), 21.5 (CHCH3), 15.0 (o-CCH3), 11.6 (m-CCH3), mixture was stirred for 12 h. The solvent was evaporated under
10.0 (NCCH3), 1.7 (SiCH3) ppm.
vacuum, and the residue was dissolved in pentane. The solution
was filtered, and a fluffy white powder was obtained from the
filtrate after removal of pentane under vacuum. The 1H NMR
spectrum of this compound is in good agreement with the litera-
ture values. Yield 480 mg (95%). C20H52CaN2O2Si4 (505.1): calcd.
C 47.56, H 10.37, N 5.54; found C 47.20, H 9.98, N 5.21. 1H
NMR (200 MHz, C6D6, 25 °C): δ = 3.44 (br., 8 H, THF), 1.16
(br., 8 H, THF), 0.35 (s, 36 H, SiMe3) ppm. 13C{1H} NMR
(50.3 MHz, C6D6, 25 °C): δ = 67.2 (THF), 25.5 (THF), 6.0
(SiMe3) ppm.
Preparation of Scandium Complex 4a: In
a 50 mL flask,
[Sc(CH2SiMe3)3(THF)2] (150 mg, 0.33 mmol) was stirred for 5 min
in hexane (10 mL), after which a mixture of 3-H (125 mg,
0.33 mmol) in hexane (5 mL) was added. The mixture was stirred at
room temperature for 12 h, and then volatiles were removed under
vacuum. The residue was washed in pentane and crystallized from
THF/pentane (2:1) at –30 °C to obtain colourless single crystals.
Yield 160 mg (76%). 4a·(hexane)0.5: C30H60N3ScSi3 (592.0): calcd.
C 60.86, H 10.21, N 7.09; found C 60.16, H 9.34, N 6.89. 1H NMR
(200 MHz, C6D6, 25 °C): δ = 4.93 [sept, 2 H, CH(CH3)2], 2.47 (s,
6 H, C5Me4), 2.39 (s, 6 H, C5Me4), 1.50 (s, 6 H, CH3), 1.23 (br.,
12 H, CH3), 0.45 (s, 6 H, SiMe2), 0.34 (s, 18 H, SiMe3), –0.34 (d,
Preparation of Calcium Complex 5: In a 50 mL flask, [Ca(HMDS)2-
(THF)2] (131 mg, 0.26 mmol) was stirred for 5 min in hexane
(10 mL), after which a mixture of 3-H (97 mg, 0.26 mmol) in hex-
ane (5 mL) was added to the reaction flask. The reaction mixture
was stirred at room temperature for 20 h, and then the volatiles
were removed under vacuum. The residue was washed in pentane
several times and crystallized from THF/pentane (2:1) at –30 °C to
obtain colourless crystals of X-ray quality. Yield 120 mg (71%).
C32H64CaN4OSi3 (645.2): calcd. C 59.56, H 9.99, N 8.68; found C
58.96, H 9.70, N 8.52. 1H NMR (200 MHz, C6D6, 25 °C): δ = 4.82
[sept, 2 H, CH(CH3)2], 3.61–3.54 (m, 4 H, THF), 2.52 (s, 6 H,
C5Me4), 2.37 (s, 6 H, C5Me4), 1.49 (s, 6 H, CH3), 1.44–1.37 (m, 4
H, THF), 1.07 (d, 12 H, CH3), 0.58 (s, 6 H, SiMe2), 0.18 (s, 18 H,
SiMe3) ppm. 13C{1H} NMR (50.3 MHz, C6D6, 25 °C): δ = 152.2
(NCN), 125.8 (m-C), 122.9 (o-C), 120.4 (NCCH3), 67.5 (ipso-C),
45.3 (CHCH3), 25.3 (THF), 21.5 (CHCH3), 14.2 (o-CCH3), 9.2
(NCCH3), 5.7 (SiMe3), 4.8 (SiMe2) ppm.
2JH,H = 11 Hz, 2 H, CH2), –0.36 (d, JH,H = 11 Hz, 2 H, CH2)
2
ppm. 13C{1H} NMR (100.6 MHz, [D8]THF, 25 °C): δ = 151.2
(NCN), 126.5 (m-C), 123.7 (o-C), 118.3 (NCCH3), 100.1 (ipso-C),
46.8 (CHCH3), 30.3 (Sc-CH2SiMe3), 21.2 (CHCH3), 15.0 (o-
CCH3), 12.3 (m-CCH3), 10.6 (NCCH3), 5.7 [CH2Si(CH3)3], 4.7
[Si(CH3)2] ppm.
Preparation of Yttrium Complex 4b: In a 50 mL flask, [Y(CH2-
SiMe3)3(THF)2] (200 mg, 0.40 mmol) was stirred for 5 min in hex-
ane (10 mL), after which a mixture of 3-H (150 mg, 0.40 mmol) in
hexane (5 mL) was added. The mixture was stirred at room tem-
perature for 16 h, and then volatiles were removed under vacuum.
The residue was washed in pentane and crystallized from THF/
pentane (2:1) at –30 °C to obtain colourless single crystals. Yield
210 mg (83%). 4b·(hexane)0.5: C30H60N3Si3Y (635.9): calcd. C
56.65, H 9.50, N 6.60; found C 55.97, H 8.95, N 6.46. 1H NMR
(400 MHz, C6D6, 25 °C): δ = 4.85 [sept, 2 H, CH(CH3)2], 2.49 (s,
6 H, C5Me4), 2.36 (s, 6 H, C5Me4), 1.48 (s, 6 H, CH3), 1.08 (d, 12
Hydroamination Reactions: 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. SiMe4 was used as an internal standard for the kinetic mea-
surements. The substrates, C-(1-allylcyclohexyl)methylamine
(6a),[37b] 2,2-diphenylpent-4-enylamine (7a),[37b] [1-(pent-2-ynyl)cy-
clohexyl]methanamine (8a)[36q] and 2-amino-5-hexene (9a),[41] were
synthesized according to literature procedures. 1H NMR spectra of
3-methyl-2-azaspiro[4.5]decane (6b),[37b] 2-methyl-4,4-diphenylpyr-
rolidine (7b),[37b] 3-propyl-2-azaspiro[4.5]dec-2-ene (8b)[36q] and 2,5-
dimethylpyrrolidine(9b)[41b] conform with the literature.
H, CH3), 0.45 (s, 6 H, SiMe2), 0.36 (s, 18 H, SiMe3), –0.58 (dd,
2JH,H = 11, 2J
= 2.7 Hz, 2 H, CH2), –0.96 (dd, JH,H = 11,
2
89Y, H
2J89Y, H = 2.7 Hz, 2 H, CH2) ppm. 13C{1H} NMR (100.6 MHz, [D8]-
THF, 25 °C): δ = 151.4 (NCN), 126.2 (m-C), 123.8 (o-C), 118.9
(NCCH3), 100.4 (ipso-C), 46.4 (CHCH3), 30.7 (d, JY,C = 40.4 Hz,
Y-CH2SiMe3), 22.2 (CHCH3), 15.0 (o-CCH3), 12.1 (m-CCH3), 9.6
(NCCH3), 5.9 [CH2Si(CH3)3], 4.8 [Si(CH3)2] ppm.
Preparation of Lutetium Complex 4c: In a 50 mL flask, [Lu(CH2-
SiMe3)3(THF)2] (150 mg, 0.26 mmol) was stirred for 5 min in hex-
ane (10 mL), after which a mixture of 3-H (97 mg, 0.26 mmol) in
hexane (5 mL) was added. The reaction mixture was stirred at room
temperature for 14 h, and then the volatiles were removed under
vacuum. The residue was washed in pentane and crystallized from
THF/pentane (2:1) at –30 °C to obtain colourless single crystals.
Yield 160 mg (80.4%). 4c·(hexane)0.5: C30H60LuN3Si3 (722.0):
calcd. C 49.90, H 8.37, N 5.82; found: C 49.33, H 7.91, N 5.61. 1H
Single Crystal X-ray Structure Determinations: CCDC-683442 (for
4c), -683443 (for 4a), -683444 (for 4b) and -683445 (for 5) contain
the supplementary crystallographic data for this paper. These data
NMR (200 MHz, C6D6, 25 °C): δ = 4.89 [sept, 2 H, CH(CH3)2], can be obtained free of charge from The Cambridge Crystallo-
2.52 (s, 6 H, C5Me4), 2.37 (s, 6 H, C5Me4), 1.47 (s, 6 H, CH3), 1.08
(br., 12 H, CH3), 0.37 (s, 6 H, SiMe2), 0.35 (s, 18 H, SiMe3), –0.78
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The
hydrogen atoms were included in the model at geometrically calcu-
2
2
(d, JH,H = 11 Hz, 2 H, CH2), –1.15 (d, JH,H = 11 Hz, 2 H, CH2) lated positions and refined using either a riding model or with rigid
ppm. 1H NMR (400 MHz, [D8]THF, 25 °C): δ = 4.97 [sept, 2 H, methyl groups. Special features: in 4a and 4c, the hexane molecules,
2
CH(CH3)2], 2.25 (s, 6 H, C5Me4), 2.11 (s, 6 H, C5Me4), 1.47 (d, J
= 9.4 Hz, 12 H, CH3), 0.45 (s, 6 H, CH3), 0.01 (s, 6 H, SiMe2),
–0.15 (s, 18 H, SiMe3), –1.36 (s, 2 H, CH2) ppm. 13C{1H} NMR
(50.3 MHz, C6D6, 25 °C): δ = 152.2 (NCN), 125.8 (m-C), 122.9
which lie across inversion centres, were refined isotropically. The
CH3 groups of 4a at Si3 are disordered over two positions. The
CH2 groups of the THF molecule in 5 are disordered over two
positions. The Flack parameter of 5 refined to –0.02(3). Dif-
(o-C), 120.4 (NCCH3), 100.2 (ipso-C), 47.3 (CHCH3), 37.3 (Lu- fractometer: Bruker SMART. Program system: SHELXL-97 (G.
CH2SiMe3), 22.6 (CHCH3), 14.9 (o-CCH3), 12.0 (m-CCH3), 10.0 M. Sheldrick, University of Göttingen). See Table 3 for crystallo-
(NCCH3), 5.9 [CH2Si(CH3)3], 4.7 [Si(CH3)2] ppm.
graphic data.
4276
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4270–4279

Constrained-Geometry Complexes as Hydroamination Catalysts
Table 3. Crystal data and structure refinement for 4a, 4b, 4c and 5.
4a [M = Sc]
4b [M = Y]
4c [M = Lu]
5 [M = Ca]
Empirical formula
Mr [gmol–1]
T [K]
C33H67N3ScSi3
635.13
133(2)
C30H60N3YSi3
635.99
133(2)
C33H67N3LuSi3
765.14
133(2)
C32H64N4OCaSi3
645.22
133(2)
λ [Å]
0.71073
monoclinic
P21/c
10.4951(14)
19.585(2)
19.170(2)
100.329(8)
3876.5(8)
4
0.71073
monoclinic
P21/n
10.6675(5)
21.5952(10)
32.1593(14)
95.090(1)
7379.2(6)
8
0.71073
monoclinic
P21/c
10.5398(10)
19.6595(18)
19.1864(18)
100.346(3)
3910.9(6)
4
0.71073
monoclinic
Cc
13.4917(6)
17.2912(8)
16.7587(8)
96.642(4)
3883.4(3)
4
Crystal system
Space group
a [Å]
b [Å]
c [Å]
β [°]
V [Å3]
Z
ρcalcd. [Mgm–3]
1.088
0.306
1.145
1.699
1.299
2.640
1.104
0.282
µ [mm–1]
F(000)
1396
0.50ϫ0.41ϫ0.40
61
2736
0.45ϫ0.31ϫ0.18
56.6
1596
0.29ϫ0.23ϫ0.16
61
1416
0.47ϫ0.31ϫ0.31
56.6
Crystal size [mm3]
2θmax [°]
Reflections collected
Independent reflections
R(int)
74789
11779
0.0451
0.887/0.719
1.07
0.0485
0.1487
0.596/–0.769
136526
18295
0.0739
0.750/0.595
1.04
0.0418
0.1053
1.360/–0.796
82938
11922
0.0472
0.677/0.554
1.09
0.0281
0.0668
2.025/–1.420
27034
9444
0.0399
0.918/0.879
1.02
0.0412
0.0967
0.338/–0.373
Max./min. transmission
Goodness of fit
R1[IϾ2σ(I)]
wR2
∆ρ [eÅ–3]
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Acknowledgments
[10]
[11]
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG), Schwerpunktprogramm Lanthanoidspezifische Funktiona-
litäten in Molekül und Material (SPP 1166) and the Fonds der
Chemischen Industrie.
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It should be noted that a related concept has been developed
by Sundermeyer, which is based on the replacement of Si by
P+ to lead to a lutetium cyclopentadienyl-phosphazene com-
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Eur. J. Inorg. Chem. 2008, 4270–4279
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Tamm et al.
FULL PAPER
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Received: June 16, 2008
Published Online: August 14, 2008
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