Synthesis and structural characterization of a homoleptic cerium (III) propiolamidinate

Article abstract of DOI:10.1016/j.jorganchem.2010.05.009

The first potassium salt of a propiolamidinate ligand, K[PhCC(N ⁱPr)₂] (1), was prepared in 51% yield by addition of potassium phenylacetylide to N,N′-diisopropylcarbodiimide. Subsequent reaction of 1 with anhydrous cerium(III) trichloride in a molar ratio of 3:1 in THF afforded the first homoleptic lanthanide tris(propiolamidinate) derivative, [PhCC(NⁱPr)₂]₃Ce (2), in the form of bright yellow crystals in 71% yield. The first potassium salt of a propiolamidinate ligand, K[PhCC(NⁱPr)₂] (1), was obtained by addition of PhCCK to N,N′-diisopropylcarbodiimide. Reaction of 1 with anhydrous cerium(III) trichloride in a molar ratio of 3:1 in THF afforded the first homoleptic lanthanide tris(propiolamidinate) derivative, [PhCC(N ⁱPr)₂]₃Ce (2), in the form of bright yellow crystals in 71% yield.

Full text of DOI:10.1016/j.jorganchem.2010.05.009

Journal of Organometallic Chemistry 695 (2010) 1953e1956
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Synthesis and structural characterization of a homoleptic cerium
(III) propiolamidinate
*
Peter Dröse, Cristian G. Hrib, Frank T. Edelmann
Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, Germany
a r t i c l e i n f o
a b s t r a c t
The first potassium salt of a propiolamidinate ligand, K[PhC^C(NiPr)2] (1), was prepared in 51% yield by
addition of potassium phenylacetylide to N,N0-diisopropylcarbodiimide. Subsequent reaction of 1 with
anhydrous cerium(III) trichloride in a molar ratio of 3:1 in THF afforded the first homoleptic lanthanide
tris(propiolamidinate) derivative, [PhC^C(NiPr)2]3Ce (2), in the form of bright yellow crystals in 71%
yield.
Article history:
Received 18 March 2010
Received in revised form
29 April 2010
Accepted 6 May 2010
Available online 31 May 2010
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to Professor Henri Brunner on the
occasion of his 75th birthday
Keywords:
Lanthanides
Amidinate ligands
Potassium
Cerium
X-ray structure
1. Introduction
films [9]. We report here the preparation of the first potassium
propiolamidinate reagent as well as the synthesis and structural
characterization of a homoleptic cerium(III) tris(propiolamidinate)
complex.
Propiolamidines of the general type ReC^CeC(]NR0)NHRR0 [1]
are highly valuable precursors for the synthesis of various nitrogen-
and sulfur-containing heterocycles [2]. Certain propiolamidines
have also been found to be useful antitussives [3]. More recently,
propiolamidines have gained considerable attention due to their
diverse applications in biological and pharmacological systems [4].
General synthetic routes to such amidine derivatives involve the
reaction of chloroformamidine derivatives with lithium acetylides
or alkynyl Grignard reagents or, more convenient, the catalytic
addition of terminal alkynes to carbodiimides (Scheme 1) [1,5].
Recent work by Hou et al. [6,7] and others [8] has revealed that
the latter reaction is affectively catalyzed by organolanthanide
complexes. Lanthanide propiolamidinate complexes have been
shown to be key intermediates in the catalytic cycles [6e8]. These
findings motivated us to develop a straightforward synthetic route
to hitherto unknown lanthanide tris(propiolamidinate) complexes.
Lanthanide tris(amidinates) have recently been demonstrated to be
highly effective catalysts for the polymerization of polar monomers
as well as promising precursors for the production of Ln2O3 thin
2. Results and discussion
Scheme 2 illustrates the synthetic route leading to the title
compound. The new potassium propiolamidinate derivative
K[PhC^C(NiPr)2] (1) was prepared in a straightforward manner by
nucleophilic addition of potassium phenylacetylide to N,N0-diiso-
propylcarbodiimide. The starting material PheC^CeK was
obtained according to a modified literature procedure [10] by
deprotonation of phenylacetylene using potassium hydride in THF.
The subsequent reaction with N,N0-diisopropylcarbodiimide was
carried out in DME solution at room temperature (Scheme 2).
Crystallization of the product directly from the filtered and
concentrated reaction mixture afforded bright yellow, block-like
crystals of the DME solvate of 1. These crystals were found to lose
the DME solvent very easily upon drying. Thus thorough drying in
vacuo afforded the unsolvated potassium salt K[PhC^C(NiPr)2] (1)
as a pale green, very air- and moisture-sensitive solid in 51% iso-
lated yield. All spectroscopic data were in agreement with the
formation of the desired propiolamidinate salt. Unfortunately,
* Corresponding author. Tel.: þ49 391 6718327; fax: þ 49 391 6712933.
E-mail address: frank.edelmann@ovgu.de (F.T. Edelmann).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.05.009

1954
P. Dröse et al. / Journal of Organometallic Chemistry 695 (2010) 1953e1956
NHR'
NR'
Cat.
R C C
H
+
R'
N
C
N R'
R C C C
Scheme 1. Catalytic formation of propiolamidines from terminal alkynes and carbodiimides.
attempted recrystallization from various solvents or solvent
mixtures such as toluene, pentane, cyclopentane, diethyl ether,
THF, DME/toluene or DME/pentane did not provide single-crystals
suitable for X-ray diffraction. Only on one occasion one of the well-
formed block-like crystals obtained directly from the concentrated
reaction mixture could be successfully subjected to X-ray diffrac-
tion, but the crystal quality was too poor to allow full refinement of
the crystal structure of 1. However, the main structural features
could clearly be derived showing a dimeric solid state structure for
the DME adduct of 1 in which two potassium ions are symmetri-
cally bridged by the propiolamidinate ligands. Each potassium
becomes hexacoordinated through addition of a DME ligand
(Scheme 3).
The preparation of a homoleptic cerium(III) tris(propiolamidi-
nate) complex was carried out in THF solution according to Scheme
2 by treatment of anhydrous CeCl3 with three equivalents of 1. The
reaction proceeded smoothly under formation of a golden-yellow
solution and precipitaion of finely divided KCl. Extraction of the
crude product with pentane followed by crystallization at 5 ꢀC
afforded well-formed, bright yellow, block-like single-crystals of
[PhC^C(NiPr)2]3Ce (2) in high yield (71%). Like many other cerium
(III) amides and organocerium(III) complexes [11], crystals of 2 are
very sensitive to traces of air and moisture. Spectroscopic charac-
terization of 2 was straightforward. In the IR spectrum, a medium
strong band at 2207 cmꢁ1 could be assigned to the C^C stretching
vibration. The mass spectrum showed the molecular ion with 20%
relative intensity. Despite the paramagnetic nature of the Ce3þ ion,
meaningful 1H and 13C NMR spectra could also be obtained for 2.
Both were in agreement with the formation of an unsolvated,
homoleptic cerium(III) propiolamidinate. This was further verified
by a single-crystal X-ray diffraction study of 2. Large block-like
single-crystals were obtained by slow cooling of a saturated solu-
tion in pentane to 5 ꢀC. Crystal data and structure refinement
details are listed in Table 1, and selected bond lengths and angles for
2 are summarized in Table 2. Fig. 1 depicts the molecular structure
of 2.
In the solid state, [PhC^C(NiPr)2]3Ce (2) crystallizes in the
hexagonal space group P3c1 with two molecules in the asymmetric
unit. The X-ray study clearly established the presence of the first
homoleptic, unsolvated lanthanide(III) tris(propiolamidinate)
complex. The central cerium(3þ) ion is coordinated by three
chelating propiolamidinate ligands in a heavily distorted octahe-
dral fashion. The effective shielding of the cerium by six isopropyl
groups obviously prevents the coordination of additional solvent
molecules. The CeeN distances are in the narrow range of 2.487
(5)e2.502(5) Å and are in good agreement with those found e.g. in
the
homoleptic
acetamidinates
[MeC(NtBu)2]3La
(LaeN:
2.530e2.553 Å) and [MeC(NtBu)2]3Eu (EueN: 2.432e2.469 Å) [12].
Thus the overall bond lengths and angles follow the trends which
are expected taking into account the ionic radius of six-coordinated
Ce(III) of 1.15 Å. Amidinate ligands generally have small NeMeN
bite angles typically in the range of 63e65ꢀ. In lanthanide amidi-
nates these bite angles are even smaller by ca. 10ꢀ due to the large
ionic radii of the lanthanide ions. With values of 54.16(15)ꢀ and
54.31(17)ꢀ the NeCeeN angles in 2 are virtually identical with
those reported for [MeC(NtBu)2]3Eu (NeEueN: 54.1(3)ꢀ and 54.7
(3)ꢀ) [12]. Quite remarkable are the dihedral angles between the
phenyl ring planes and the NeCeN units of the chelating amidinate
ligands. With 55.9ꢀ and 60.6ꢀ, respectively, the dihedral angles are
surprisingly large despite the presence of the eC^Ce spacer
groups between the two planes. They are also large enough to
minimize potential conjugation between the two p-systems.
In summarizing the results reported here, the first potassium
propiolamidinate derivative, K[PhC^C(NiPr)2] (1), was readily
prepared through nucleophilic addition of potassium phenyl-
acetylide to N,N0-diisopropylcarbodiimide. A preliminary X-ray
diffraction study revealed a dimeric solid state structure for the
DME adduct of 1. Compound 1 can be expected to be a valuable
precursor for the synthesis of lanthanide propiolamidinates as
exemplified by the reaction with anhydrous cerium trichloride
which produced the homoleptic cerium(III) tris(propiolamidi-
nate) [PhC^C(NiPr)2]3Ce (2) in high yield. Future work in this
N
C
N
C
C
K
+
r. t.
DME
24 h
Ph
N
N
N
N
N
CeCl3
K
Ph
Ce
N
THF
N
- 3 KCl
N
1
Ph
2
Scheme 2. Synthetic route to K[PhC^C(NiPr)2] (1) and [PhC^C(NiPr)2]3Ce (2).

P. Dröse et al. / Journal of Organometallic Chemistry 695 (2010) 1953e1956
1955
Table 2
Selected bond lengths [Å] and angles [ꢀ] for [PhChC(NiPr)2]3Ce (2).
O
O
O
O
K
K
Ce(1)eN(1)
2.499(6)
2.487(5)
1.332(8)
1.343(8)
54.16(15)
54.31(17)
116.8(5)
117.6(5)
Ce(1)eN(2)
Ce(2)eN(4)
N(2)eC(1)
N(4)eC(21)
2.502(5)
N
N
N
N
Ce(2)eN(3)
N(1)eC(1)
2.500(5)
1.341(8)
1.317(7)
N(3)eC(21)
N(1)eCe(1)eN(2)
N(3)eCe(2)eN(4)
N(1)eC(1)eN(2)
N(4)eC(21)eN(3)
Scheme 3. Schematic representation of the dimeric structure of the DME adduct of K
[PhC^C(NiPr)2] (1) in the solid state.
(SHELXS-97) and refined by full-matrix least-squares methods on
F2 using SHELXL-97 [15].
area should be direct towards the synthesis of other lanthanide
propiolamidinates which could exhibit interesting catalytic
activities.
3.2. Synthesis of potassium propiolamidinate, K[PhC^C(NiPr)2] (1)
A 250 ml Schlenk flask was charged with 2.71 g (19.3 mmol)
potassium phenylacetylide and 120 ml of DME. To the resulting
suspension were added with stirring 2.43 g (21.2 mmol, slight excess)
of N,N0-diisopropylcarbodiimide, causing a yellow-green color to
develop. Stirring was continued for 24 h at room temperature, and
insoluble materialwas removed by filtration. The clear, yellow-brown
filtratewasconcentratedinvacuotoatotalvolumeof30ml.Coolingto
5 ꢀC for another 24 h afforded bright yellow, block-like crystals of the
DME solvate of 1. These crystals easily lose DME upon drying. Thus, in
order to obtain a well-defined material, the product was thoroughly
dried under vacuum to give unsolvated 1 as a pale green powder in
51% yield (2.61 g). M.p. 175 ꢀC (dec.). Analysis (C15H19KN2,
Mw ¼ 266.42 g/mol): C 67.55 (calcd. 67.62), H 7.44 (7.19), N 9.49
(10.51) %. IR (KBr): nmax 3055 (w), 3032 (w), 2965 (vs), 2857 (s), 2605
3. Experimental section
3.1. General procedures
The reactions were conducted in flame-dried glassware under
an inert atmosphere of dry argon employing standard Schlenk and
glovebox techniques. All solvents were distilled from sodium/
benzophenone under nitrogen atmosphere prior to use. All glass-
ware was oven-dried at 140 ꢀC for at least 24 h, assembled while
hot, and cooled under vacuum prior to use. The starting materials
potassium phenylacetylide [10] and anhydrous CeCl3 [13] were
prepared according to the literature procedures. NMR spectra were
recorded in THF-d8 or C6D6 solutions on a Bruker DPX 400 spec-
trometer at 25 ꢀC. Chemical shifts were referenced to TMS. Micro-
analyses were performed using a Leco CHNS 923 apparatus. The
intensity data of 2 were collected on a Stoe IPDS 2T diffractometer
with MoKa radiation. The data were collected with the Stoe XAREA
(w), 2197 (w,
n C^C), 1596 (s, n C]N), 1501 (vs), 1441 (s), 1373 (vs),
1354 (vs), 1328 (vs), 1243 (w), 1226 (w), 1164 (s), 1119 (s), 1069 (w),
1028 (s), 998 (w), 929 (w), 847 (w), 756 (vs), 720(m), 690 (st), 540 (w),
529 (w), 445 (w) cmꢁ1. 1H NMR (THF-d8, 400.1 MHz):
d
¼ 7.40 (d,
[14] program using u-scans. The space group was determined with
XRED32 [14] program. The structure was solved by direct methods
3J ¼ 6.5 Hz, 2H, C6H5), 7.31e7.22 (m, 3H, C6H5), 3.93 (sept, 3J ¼ 6.2Hz,
Table 1
Crystal data and structure refinement for [PhC^C(NiPr)2]3Ce (2).
Identification code
Empirical formula
Formula weight
Temperature
ip83
C45H57CeN6
822.09
133(2) K
Wavelength
Crystal system
Space group
0.71073 Å
Hexagonal
P3c1
Unit cell dimensions
a ¼ 16.074(2) Å
b ¼ 16.074(2) Å
c ¼ 19.577(4) Å
4380.3(12) Å3
4
a
b
g
¼ 90ꢀ
¼ 90ꢀ
¼ 120ꢀ
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 28.28ꢀ
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.247 Mg/m3
1.075 mmꢁ1
1708
0.60 ꢂ 0.17 ꢂ 0.15 mm3
2.53e28.28ꢀ
ꢁ19 ꢃ h ꢃ 16, ꢁ21 ꢃ k ꢃ 21, ꢁ20 ꢃ l ꢃ 26
11715
6246 [R(int) ¼ 0.0623]
99.8%
None
0.8554 and 0.5648
Full-matrix least-squares on F2
6246/1/321
1.029
R1 ¼ 0.0463, wR2 ¼ 0.0941
R1 ¼ 0.0763, wR2 ¼ 0.1047
0.03(3)
Absolute structure parameter
Largest diff. peak and hole
Fig. 1. ORTEP view of the molecular structure of [PhC^C(NiPr)2]3Ce (2) with thermal
ellipsoids at the 50% probability level (H atoms are not shown for clarity).
0.724 and ꢁ1.032 e.Åꢁ3

1956
P. Dröse et al. / Journal of Organometallic Chemistry 695 (2010) 1953e1956
2H, {(CH3)2CHN}2CC^CPh), 1.09 (s, 6H, {(CH3)2CHN}2CC^CPh), 1.08
(s, 6H, {(CH3)2CHN}2CC^CPh) ppm. 13C NMR (100.6 MHz, THF-d8,
Acknowledgements
25 ꢀC):
d
¼ 153.62 ((iPrN)2CC^CPh), 132.27 (C6H5), 128.91 (C6H5),
This work was generously supported by the Deutsche For-
schungsgemeinschaft (SPP 1166 "Lanthanoid-spezifische Funktio-
nalitäten in Molekül und Material"). P. D. thanks the government of
Sachsen-Anhalt for a Ph.D. scholarship (Graduiertenförderung).
Financial support by the Otto-von-Guericke-Universität Magde-
burg is also gratefully acknowledged.
128.19 (C6H5), 125.48 (C6H5), 91.48 ((iPrN)2CC^CPh), 84.27 ((iPrN)2
CC^CPh), 50.51 ([(CH3)2CHN]2CC^CPh), 27.66 ([(CH3)2CHN]2
CC^CPh). MS (EI, 140Ce): m/z (%) 227.1 (12) [(iPrN)2CC^CPh]þ; 171.1
(15) [(iPrN)2CC^CPheNCH(CH3)2 þ H]þ; 151.1 (10) [(iPrN)2CC^C∙]þ,
128.0 (100) [(iPrN)2CC^CPheNCH(CH3)2eCH(CH3)2]þ, 58.0 (18)
[NiPr þ H]þ.
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(beginning dec.). Analysis (C45H57CeN6, Mw ¼ 822.09 g/mol): C
67.01 (calcd. 65.74), H 7.71 (6.99), N 10.62 (10.22) %. IR (KBr): nmax
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(m), 1189 (vs), 1136 (s), 1124 (s), 1070 (m), 1043 (st), 999 (m), 939
(w), 917 (m), 855 (m), 832 (w), 755 (vs), 704 (m), 689 (vs), 542
(m), 528 (s) cmꢁ1. 1H NMR (C6D6, 400.1 MHz):
d
¼ 12.07 (s br, 6H,
((CH3)2CHN)2CC^CPh), 9.52 (s br, 6H, AreH), 7.94 (s, 6H, AreH),
7.78 (s, 3H, AreH), ꢁ2.74 (s br, 36H, ((CH3)2CHN)2CPh) ppm. 13C
NMR (100.6 MHz, C6D6, 25 ꢀC):
d
¼ 171.30 ((iPrN)2CC^CPh),
134.37 (C6H5), 130.26 (C6H5), 129.84 (C6H5), 125.31 (C6H5), 105.22
((iPrN)2CC^CPh),
91.93
((iPrN)2CC^CPh),
56.11
([(CH3)2
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(%) 820.6 (20) [M]þ; 806.9 (10) [MeCH3 þ H]þ; 778.8 (10)
[M-iPr þ H]þ, 595.7 (35) [Me(iPrN)2CC^CPh þ 2H]þ, 593.8 (100)
[Me(iPrN)2CC^CPh]þ.
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4. Supplementary Material
Crystallographic data for the crystal structure reported in this
paper can be obtained from the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB21EZ, UK (fax: þ44-1223-
336-033; E-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.
cam.ac.uk/) by referring to the CIF deposition code CCDC
770170 (2).
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versität Göttingen, Germany, 1997.