Synthesis and structure of a rare unsolvated bis(amidinato) lanthanide chloride, (Piso)2CeCl (Piso = [tBuC(NAr)2]-, Ar = C6H3iPr2-2,6)

Article abstract of DOI:10.1002/zaac.201000338

Use of the very bulky amidinate ligand [tBuC(NAr)₂] ^-(= Piso, Ar = C₆H₃iPr₂-2,6) allowed the synthesis and structural characterization of the chloro-functional complex (Piso)₂CeCl (1). Complex 1 represents a rare example of an unsolvated bis(amidinato) lanthanide chloride. An X-ray diffraction study confirmed the presence of a pentacoordinate cerium(III) complex. Copyright

Full text of DOI:10.1002/zaac.201000338

SHORT COMMUNICATION
DOI: 10.1002/zaac.201000338
Synthesis and Structure of a Rare Unsolvated Bis(amidinato) Lanthanide
Chloride, (Piso)2CeCl (Piso = [tBuC(NAr)2]–, Ar = C6H3iPr2–2,6)
Peter Dröse,[a] Cristian G. Hrib,[a] Liane Hilpert,[a] and Frank T. Edelmann*[a]
Keywords: Amidinate ligands; Lanthanide amidinates; Bis(amidinato) complex; Cerium; Rare earths
Abstract. Use of the very bulky amidinate ligand [tBuC(NAr)2]– plex 1 represents a rare example of an unsolvated bis(amidinato) lan-
(= Piso, Ar = C6H3iPr2–2,6) allowed the synthesis and structural char- thanide chloride. An X-ray diffraction study confirmed the presence
acterization of the chloro-functional complex (Piso)2CeCl (1). Com- of a pentacoordinate cerium(III) complex.
ligands (= Fiso, Ar = C6H3iPr2–2,6) [5b]. We report here the
Introduction
Amidinate ligands of the general type [RC(NR')2]– (R = H,
synthesis and structural characterization of a rare example of
an unsolvated bis(amidinato) lanthanide chloride complex,
(Piso)2CeCl (1, Piso = [tBuC(NAr)2]–, Ar = C6H3iPr2–2,6).
alkyl, aryl; R' = alkyl, cycloalkyl, aryl, SiMe3) have made an
astonishing career as alternatives for the ubiquitous cyclopen-
tadienyl ligands [1] in organolanthanide chemistry. Closely re-
lated are the guanidinate anions of the general type
[R2NC(NR')2]– (R = alkyl, SiMe3; R' = alkyl, cycloalkyl, aryl,
SiMe3). Two amidinate or guanidinate ligands can coordinate
to a lanthanide ion to form a metallocene-like coordination
environment which allows the isolation and characterization of
stable though very reactive amide, alkyl, and hydride species
[2, 3]. Of particular interest with respect to applications in ho-
mogeneous catalysis are lanthanide mono- and bis(benzamidi-
nato) complexes [4]. Especially the mono(amidinates) are of-
ten accessible only with the use of very bulky amidinate and
guanidinate ligands. Sterically very demanding amidinate and
guanidinate ligands containing 2,6-diisopropylphenyl substitu-
ents were introduced mainly by Jones and Junk et al. [5]. They
include the very bulky anionic ligands [RC(NAr)2]– (Ar =
C6H3iPr2–2,6; Fiso: R = H, Aiso: R = Me, Piso: R = tBu) and
[R2NC(NAr)2]– (Ar = C6H3iPr2–2,6; Priso: R = iPr, Giso: R =
cyclohexyl).
Results and Discussion
The title compound (Piso)2CeCl (1, Piso = [tBuC(NAr)2]–,
Ar = C6H2iPr2–2,6) was prepared according to Scheme 1 by
treatment of anhydrous cerium trichloride with K(Piso) in a
molar ratio of 1:2 in THF solution. The potassium intermediate
K(Piso) was made in situ by deprotonation of the free amidine
tBu(= NAr)NHAr [6] with an excess of potassium hydride in
THF. This method allowed the preparation of the title com-
pound in a very simple manner.
Trivalent rare-earth metal amidinate complexes of the type
(amidinate)2LnCl are formally pentacoordinate. Due to this rel-
atively low coordination number, such complexes show a
strong tendency to form solvated derivatives or “ate” com-
plexes in which e.g. LiCl is incorporated in order to increase
the coordination number [3e]. Even the monofluoro sama-
rium(III) bis(amidinate) (Fiso)2SmF(THF), which was made
by oxidative fluorination of (Fiso)2Sm(THF)2, was isolated as
a THF adduct, although it contains two bulky [HC(NAr)2]–
Scheme 1. Synthesis of the title compound (Piso)2CeCl (1).
The product was isolated in 39 % yield in the form of a
yellow-green crystalline solid. All attempts to increase the
yield by running the reaction at higher temperatures (e.g.
60 °C) failed and led to the formation of brown insoluble sol-
ids and much lower yields. This is why the reaction is best
carried out at room temperature over prolonged periods of time
(72 h). In the crystalline state, the compound is very air-sensi-
tive, although not as exceedingly as previously reported ce-
rium(III) amides and amidinates [1, 4, 7]. The slightly higher
stability of 1 towards air and moisture can be traced back to
* Prof. Dr. F. T. Edelmann
Fax: +49-391-67-18327
E-Mail: frank.edelmann@ovgu.de
[a] Chemisches Institut
Otto-von-Guericke-Universität
Universitätsplatz 2
39106 Magdeburg, Germany
Z. Anorg. Allg. Chem. 2011, 637, 31–33
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
31

P. Dröse, C. G. Hrib, F. T. Edelmann
SHORT COMMUNICATION
Table 2. Selected bond lengths /Å and angles /° for (Piso)2CeCl 1.
the effective steric shielding of the central cerium ion by the
two very bulky Piso ligands. However, solutions of 1 in n-
pentane are decomposed within seconds upon contact with air
(color change to dark brown).
1
Ce–N(1)
2.5116(19)
2.437(2)
2.6470(10)
1.330(3)
1.355(3)
53.27(7)
111.5(2)
Ce–N(2)
Interpretable NMR spectroscopic data for 1 could not be ob-
tained due to the paramagnetic nature of the Ce3+ ion. The
IR spectrum of 1 closely resembles those of other lanthanide
amidinate complexes [8]. IR bands attributable to the symmet-
ric and asymmetric C–H stretching vibrations of the methyl
groups appear at 2962 and 2871 cm–1, respectively, while the
band typical for the C-H deformation vibration is observed at
1384 cm–1 [8]. Characteristic bands at 1655 and 1390 cm–1 can
be assigned to the vibrations of the N–C–N units, whereas C–
H ring vibrations give rise to bands at 1157, 1027, and
758 cm–1. The mass spectrum shows mainly peaks originating
from fragmentation of the Piso ligand (e.g. elimination of
methyl and isopropyl groups). A low-intensity (1 %) peak at
m/z = 970.2 corresponds to the ion [M – iPr]+.
Ce–Cl
N(1)–C(1)
N(2)–C(1)
N(1)–Ce–N(2)
N(1)–C(1)–N(2)
N(2)–Ce–Cl
N(2A)–Ce–Cl
N(2)–Ce–N(2A)
N(1)–Ce–Cl
N(1A)–Ce–Cl
N(2)–Ce–N(1)
N(2A)–Ce–N(1A)
N(2A)–Ce–N(1)
N(2)–Ce–N(1A)
N(1)–Ce–N(1A)
120.30(5)
120.30(5)
119.41(10)
95.11(5)
95.11(5)
53.27(7)
53.27(7)
120.55(7)
120.55(7)
169.78(10)
Symmetry transformations used to generate equivalent atoms:
A –x+1, y, –z+3/2
The molecular structure of 1 was determined by X-ray dif-
fraction. After numerous unsuccessful attempts, block-like yel-
low-green single crystals of 1 were grown by slow cooling
of a saturated solution in n-pentane to 5 °C. The compounds
crystallizes in the monoclinic space group C2/c. Table 1 sum-
marizes the crystallographic data; selected bond lengths and
angles are listed in Table 2. Figure 1 depicts the molecular
structure of 1. The X-ray analysis confirmed the presence of
monomeric, unsolvated, chloro-functionalized cerium(III)
bis(amidinato) complex (Figure 1).
Table 1. Crystallographic Data for (Piso)2CeCl 1.
1
Identification code
Empirical formula
Formula weight
Temperature
ip63a
C58H86CeClN4
1014.88
133(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Unit cell dimensions
monoclinic
C2/c
a = 24.2724(8) Å, α = 90°
b = 10.6507(3) Å, β = 11.704(2)°
c = 22.4285(7) Å, γ = 90°
5387.1(3) Å3
4
Volume
Z
Figure 1. Molecular structure of (Piso)2CeCl (1).
Density (calculated)
Absorption coefficient
F(000)
1.251 Mg·m–3
0.934 mm–1
2148
The arrangement around the central Ce3+ ion can be best
described as distorted trigonal bipyramidal with Cl, N2 and
N2A forming the equatorial plane. Both four-membered
CeNCN chelate rings are nearly planar. With Ce–N distances
of 2.437(2) and 2.5116(19) Å, a slight asymmetry in the che-
late coordination is observed. Typical for rare-earth metal ami-
dinato complexes is the narrow bite angle of the chelating ami-
dinate ligands. In the case of 1, the N–Ce–N angle is 53.27(7)°,
which is favorably similar to the corresponding N–Ln–N an-
gles in related lanthanide amidinates [3, 4]. With 2.647(1) Å,
the Ce–Cl distance is almost identical with Ce–Cl bond lengths
reported in the literature (e.g. 2.672(7) Å in (C5Me5)2CeCl [9]
and 2.6973(16) Å in [{N(SiMe3)C(Ph)}CH]2CeCl [10].
Crystal size
0.46 × 0.38 × 0.22 mm
Theta range for data collection 2.19 to 29.21°
Index ranges
–33 ≤ h ≤ 33
–14 ≤ k ≤ 14
–30 ≤ l ≤ 30
Reflections collected
32199
Independent reflections
Completeness to θ = 29.23°
Absorption correction
Refinement method
7258 [R(int) = 0.0653]
99.4 %
None
Full-matrix least-squares on F2
7258 / 0 / 301
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.203
R1 = 0.0464, wR2 = 0.0756
R1 = 0.0568, wR2 = 0.0779
0.989 and –2.858 e·Å–3
Largest diff. peak and hole
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 31–33

A Rare Unsolvated Bis(amidinato) Lanthanide Chloride, (Piso)2CeCl
Conclusions
Acknowledgement
The use of the very bulky amidinate ligand [tBuC(NAr)2]–
(= Piso, Ar = C6H3iPr2–2,6) allowed the synthesis and struc-
tural characterization of the chloro-functional complex
(Piso)2CeCl (1). Complex 1 represents a rare example of an
unsolvated bis(amidinato) lanthanide chloride. An X-ray dif-
fraction study confirmed the presence of a pentacoordinate ce-
rium(III) complex. The compound can be expected to be a
useful starting material for the synthesis of the corresponding
alkyl or hydride derivatives.
P. D. thanks the Government of Sachsen-Anhalt for a Ph.D. scholarship
(Graduiertenförderung). Financial support by the Otto-von-Guericke-
Universität Magdeburg is also gratefully acknowledged.
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Experimental Section
The reaction was conducted in flame-dried glassware in an inert atmos-
phere of dry argon employing standard Schlenk and glovebox tech-
niques. THF was distilled from sodium/benzophenone under nitrogen
prior to use. All glassware was oven-dried at 140 °C for at least 24 h,
assembled while hot, and cooled under high vacuum prior to use. The
starting materials N,N'-bis(2,6-diisopropylphenyl)pivalamidine (=
HPiso) [6] and anhydrous cerium trichloride [11] were prepared ac-
cording to published procedures and stored in a dry-box. IR spectrum:
Perkin–Elmer FT-IR 2000 (KBr plates). Mass spectrum (EI, 70 eV):
MAT 95 apparatus. Microanalysis was performed using a Leco CHNS
923 or a VARIO EL cube apparatus. The intensity data of 1 were
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structures were solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares methods on F2 using SHELXL-97 [13].
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can be obtained from the Cambridge Crystallographic Data Center, 12
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E-Mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/) by
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Preparation of (Piso)2CeCl (1): In a 250 mL Schlenk flask potassium
hydride (0.32 g, 7.98 mmol, slight excess) was suspended in THF
(100 mL), and HPiso (3.11 g, 7.40 mmol) was added as solid. The
light gray reaction mixture was stirred for 3 h at room temperature and
filtered. Afterwards it was added to anhydrous CeCl3 (0.92 g,
3.70 mmol). Stirring at room temperature was continued for 72 h. The
resulting yellow-brown solution was evaporated to dryness and the
product was extracted with n-pentane (3 × 40 mL). Concentration of
the combined extracts to a total volume of ca. 20 mL, followed by
cooling to 5 °C for 48 h afforded the crude product. Recrystallization
from n-pentane at 5 °C gave yellow-green, block-like single crystals
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of
1 in 39 % yield (1.47 g). M. p. 217.0 °C. C58H86CeClN4
(1014.9 g·mol–1): C 68.64, H 8.54, N 5.52; found: C 70.49, H 8.07, N
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5.86 %. Presumably the fairly large deviation in the found carbon
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value is due to the high air-sensitivity of 1. IR (KBr): ν = 3340 (m),
˜
3062 (w), 2962 (vs, νs CH3), 2871 (st, νas CH3), 1655 (m, N–C–N
unit), 1611 (vs), 1579 (st), 1481 (st), 1461 (st), 1432 (st), 1407 (st),
1395 (st, N–C–N unit), 1384 (st, δs CH3), 1363 (st), 1312 (vs), 1241
(vs), 1209 (vs), 1157 (vs, CH ring), 1113 (st), 1055 (m), 1027 (w, CH
ring), 985 (st), 934 (w), 923 (w), 828 (w), 807 (w), 800 (m), 770 (m),
758 (st, CH Ring), 725 (w), 638 (w), 504 (m) cm–1. MS (EI, 70 eV):
m/z (%) 970.2 (1) [M – iPr]+, 420.0 (10) [(ArN)2CtBu + H]+, 377.0
(10) [(ArN)2CtBu – iPr + H]+, 244.0 (100) [(Ar)N = CtBu]+.
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Received: September 15, 2010
Published Online: October 21, 2010
Z. Anorg. Allg. Chem. 2011, 31–33
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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