Facile formation of a homoleptic Ce(IV) amide via aerobic oxidation

Article abstract of DOI:10.1039/b607429d

Oxidation of [Ce(NCy₂)₃(thf)] or [Ce(NCy ₂)₄Li(thf)] with dry air produced the first homoleptic Ce(IV) amide [Ce(NCy₂)₄]. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b607429d

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Facile formation of a homoleptic Ce(IV) amide via aerobic oxidation{
Peter B. Hitchcock, Michael F. Lappert* and Andrey V. Protchenko*
Received (in Cambridge, UK) 25th May 2006, Accepted 28th June 2006
First published as an Advance Article on the web 20th July 2006
DOI: 10.1039/b607429d
The thf-solvated cerium triamide 1 was stable in solution in
hydrocarbon solvents and was thermally robust up to its melting
point (ca. 235 uC) in contrast to the bulkier analogue
[Ce(TMP)3(thf)],5 which can be desolvated at room temperature.
The lithium tetraamidocerate(III) 2 decomposed slowly in aromatic
solvents and much faster in thf, apparently due to thf C–O bond
cleavage similar to that observed in the synthesis of [Ce(TMP)3].6
Complexes 1 and 2 were extremely air-sensitive in solution and if
the smallest amount of air was accidentally introduced during the
work-up, a blue colouration appeared in the solution immediately,
later yielding black microcrystals. When a measured amount of
dry air was added to toluene or thf solutions of 1 or 2 the
diamagnetic Ce(IV) amide [Ce(NCy2)4] (3){ was isolated in a
moderate yield (Scheme 1).
Oxidation of [Ce(NCy2)3(thf)] or [Ce(NCy2)4Li(thf)] with dry
air produced the first homoleptic Ce(IV) amide [Ce(NCy2)4].
Cerium is often considered as an analogue of a group 4 metal due
to its ability to form compounds in the oxidation state +4.
However, homoleptic dialkyl amides, which are common and
widely used for Zr and Hf,1 are unknown for Ce. Related are
Ce(IV) bis(trifluoromethanesulfonyl)amide2 and porphyrinato- or
phthalocyaninato-Ce complexes,3 the latter often being considered
as having the Ce atom in an intermediate (between +3 and +4)
oxidation state due to the high redox activity of the ligands. Each
of the three heteroleptic Ce(IV) amides reported up to date,
[CeI{N(SiMe2But)CH2CH2}3N]4a and [Ce{N(SiMe3)2}3X] (X =
Cl or Br),4b,c contain silyl substituents on the amido N atoms.
The initial attempts to prepare a Ce(IV) dialkylamido complex
using [Ce(TMP)3] (TMP = 2,2,6,6-tetramethylpiperidinate) failed,5
apparently due to the high steric demands of the TMP ligand.
Thus, dicyclohexylamide (NCy2) was chosen as a less bulky
alternative to TMP for stabilisation of the Ce +4 oxidation state.
Here we report the synthesis and characterisation of two new
Ce(III) dicyclohexylamides and their oxidation to produce the first
homoleptic Ce(IV) amide.
Complex 3 was only sparingly soluble in hexane, benzene,
toluene or thf, producing deep blue solutions, which were stored in
a vacuum-sealed tube without decomposition for several months
at room temperature (in contrast to the solution instability of
[Ce{N(SiMe3)2}3X] [X = Cl, Br]4c). The solubility increased upon
heating to 70 uC thus allowing recrystallisation of 3 without
noticeable decomposition even at this temperature.
To confirm the oxidation state of Ce in these complexes the
structures of 1, 2 and 3 were determined by single crystal X-ray
diffraction.§ The molecular structures of complexes 1 and 2 are
similar to those of their Sm analogues;7 details are given in ESI{.
In the structure of complex 3 there are two independent molecules
The starting Ce(III) amides [Ce(NCy2)3(thf)] (1) and
[Ce(NCy2)4Li(thf)] (2) were prepared via the salt elimination route
(Scheme 1) using 2.7 or 4 equivalents of Li amide, respectively.{
Both compounds demonstrate paramagnetically shifted 1H NMR
signals in C6D6 with one set of the Cy group signals for 1 and two
sets (terminal and bridging NCy2 ligands) for 2 (see ESI{). In a
coordinating solvent (thf-d8) complex 2 apparently dissociates into
the tetrahedral anion [Ce(NCy2)4]2 (which gives only one set of
slightly paramagnetically shifted Cy signals) and the [Li(thf)x]+
7
cation (which gives a sharp signal in the Li NMR spectrum; no
such signal was observed in C6D6 solvent due to paramagnetic
broadening).
Scheme 1
The Chemistry Department, University of Sussex, Brighton,
UK BN1 9QJ E-mail: m.f.lappert@sussex.ac.uk;
a.protchenko@sussex.ac.uk; Fax: 44 1273 677196; Tel: 44 1273 678316
{ Electronic supplementary information (ESI) available: details of
synthesis, characterisation and crystallographic data for compounds 1, 2
and 3. See DOI: 10.1039/b607429d
Fig. 1 Molecular structure of [Ce(NCy2)4] (3) (50% probability ellip-
soids) showing one of the three independent molecules in the crystal.
Symmetry transformations used to generate equivalent atoms: 9 1 2 y, x,
2z; 0 y, 1 2 x, 2z; - 1 2 y, 1 2 x, z.
3546 | Chem. Commun., 2006, 3546–3548
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microcrystalline solvate. To this suspension a solution of LiNCy2 (0.945 g,
5.05 mol) in thf (20 mL) was slowly added at 0 uC producing a yellow
solution. The mixture was stirred for 1 h at room temperature and the
solvent was removed in a vacuum. The residue was extracted with toluene
(2 6 10 mL), filtered, and the filtrate was concentrated to ca. 2 mL and
covered with hexanes. Storing at 227 uC for 2 days gave 0.811 g (57%) of
1?PhMe as large yellow crystals; mp 235–240 uC. 1H-NMR (C6D6): d 13.33,
7.00–7.13 (aromatic CH, toluene), 2.47, 2.11 (sharp s, CH3 toluene), 1.71,
1.49 (and 1.47 sh), 1.19, 1.05, 0.27, 24.07, 27.83.
Synthesis of [Ce(NCy2)4Li(thf)] (2): CeCl3 (0.383 g, 1.55 mmol) was
solvated with thf as described above and a solution of LiNCy2 (1.132 g,
6.05 mol) in thf (20 mL) was slowly added at 0 uC. The mixture was stirred
for 1 h at room temperature and the solvent was removed in a vacuum (if it
was left as the THF solution overnight, the yield of crystalline products
decreased significantly). The residue was extracted with toluene (2 6
10 mL), the combined toluene solution was concentrated to ca. 5 mL and
covered with hexanes. Storing at 5 uC overnight gave 0.892 g (61%) of 2 as
bright pink crystals; mp 146–148 uC (decomp.). 1H-NMR (C6D6): d 12.73,
11.75, 6.05, 5.50, 3.34, 1.50, 0.29, 20.35, 20.87, 21.38, 22.06, 23.44,
24.85, 26.58, 210.52, 229.74.
Table 1 Ce–N (terminal amido ligand) and Ce–O (thf) bond lengths
˚
(A) in selected Ce(III) and Ce(IV) amides
Compound
Ce–N
Ce–O
[Ce(NCy2)3(thf)] (1)
[Ce(NCy2)4Li(thf)] (2) 2.320(2), 2.330(2)
2.299(2), 2.317(2), 2.336(2)
2.582(2)
[Ce(TMP)3]6
2.332(7), 2.346(7), 2.291(7)
[Ce(TMP)3(thf)]5
[Ce{N(SiMe3)2}3]8
[Ce(NCy2)4] (3)a
2.374(2), 2.346(2), 2.363(2)
2.320(3)
2.247(6), 2.242(6), 2.238(5), 2.240(5)
2.810(2)
[CeCl{N(SiMe3)2}3]4b 2.217(3)
[CeBr{N(SiMe3)2}3]4c 2.219(7)
a
The four Ce–N bond lengths for 3 are due to the presence of 3
different molecules in the crystal.
¯
lying on 4 inversion centers and one lying on a two-fold rotation
axis. Fig. 1 shows a higher-symmetry molecule of 3, and important
bond lengths for these compounds (together with the data for
selected Ce(III) and Ce(IV) complexes) are given in Table 1.
Each of 1, 2 and 3 has the four-coordinate Ce atom in a
distorted tetrahedral environment, which facilitates the following
bond length comparisons. In complex 1 the three Ce–N distances
are very similar to those in [Ce(TMP)3]6 but slightly shorter than in
the overcrowded thf solvate [Ce(TMP)3(thf)],5 while the Ce–O
distance is much shorter than in the latter (Table 1) paralleling the
difference in the thermal stability of these complexes.
Synthesis of [Ce(NCy2)4] (3): A Schlenk tube containing a degassed
solution of 1?PhMe (0.303 g, 0.36 mmol) in toluene (30 mL) was connected
via a short rubber tubing to an ampoule containing dry air (10 mL, 0.093
mmol of O2). After stirring for 30 min at 25 uC a dark blue-violet solution
was formed; the Schlenk tube was closed and stored at 5 uC overnight
yielding a greenish-yellow solution with black crystals and some light
brown amorphous precipitate. The crystals were washed by decantation
with cold toluene and dried, yielding 0.108 g (35% based on Ce) of 3.
1
Complex 3 decomposed at 90–100 uC without melting. H-NMR (C6D6):
d 4.13 (m, 1 H, NCH), 2.04 (d, 2 H), 1.88 (m, 2 H), 1.79 (m, 1 H), 1.70 (m,
2 H), 1.55 (m, 2 H), 1.30 (m, 1 H). 13C-NMR (C6D6): d 57.44 (NCH), 39.60
(CH2), 27.22 (CH2), 26.08 (CH).
§ Crystal data. For 1?PhMe (yellow prism 0.25 6 0.20 6 0.20 mm3):
[C40H74CeN3O]?(C7H8), M = 845.28, monoclinic, space group P21/c, a =
In complex 2, two Ce–N distances to the terminal ligands are
close to those in 1 and in the silylamide [Ce{N(SiMe3)2}3],8 while
˚
10.3004(2), b = 23.4770(4), c = 19.3077(3) A, b = 101.189(1), V =
4580.29(14) A , Z = 4, T = 173(2) K, m = 1.03 mm21, 8044 independent
3
˚
the two bridging Ce–N distances are longer at 2.472(2) and
IV
2.497(2) A. The Ce –N distances in 3 are shorter than the
˚
reflections [Rint = 0.060], final R1 = 0.032 [for 6438 reflections with I .
2s(I)], wR2 = 0.069 (all data). For 2 (pink prism 0.25 6 0.25 6 0.20 mm3):
[C52H96CeLiN4O], M = 940.39, monoclinic, space group P21/n, a =
III
˚
terminal Ce –N distances in 1 or 2 by ca. 0.1 A, but are slightly
longer than the CeIV–N bonds in the less sterically hindered
heteroleptic compounds [Ce{N(SiMe3)2}3X] (X = Cl or Br).4b,c
Thus the structural study (along with the NMR data) confirms the
+4 cerium oxidation state in the homoleptic amide 3.
˚
15.1681(3), b = 19.1024(4), c = 18.1727(3) A, b = 96.822(1), V =
5228.21(17) A , Z = 4, T = 173(2) K, m = 0.91 mm21, 10277 independent
3
˚
reflections [Rint = 0.078], final R1 = 0.035 [for 7866 reflections with I .
2s(I)], wR2 = 0.073 (all data). For 3 (black needle 0.10 6 0.05 6
3
0.05 mm ): [C48H88CeN4], M = 861.34, tetragonal, space group P4, a = b =
¯
3
Possible intermediates along the route to complex 3 may include
CeIV superoxo-, peroxo- and oxo-complexes; the latter would
disproportionate into 3 and a polymeric amido-oxo-compound of
the type {Ce(m-O)(NCy2)2}x. While organolanthanides and low-
valent Ln compounds with N-centred ligands are generally
considered as decomposing completely on exposure to air, some
examples of oxygen-containing products resulting from aerial
oxidation have been reported, including superoxo- [Sm{HB(3,5-
Me2pz)3}2(g2-O2)],9 peroxo- [{Yb(C5H9C5H4)2(thf)}2(m-O2)]10 and
[{Yb(N(SiMe3)2)2(thf)}2(m-O2)],11 and complexes with oxygenated
ligands.12 The present study has demonstrated the suitability of
this approach to the synthesis of oxygen-free organoamides of
high-valent cerium. This oxidative route to complex 3 parallels that
reported for the aerial oxidation of Ce(k2-S2CNEt2)3 to [Ce(k2-
S2CNEt2)4].13
˚
˚
21.1876(5), c = 10.3198(3) A, V = 4632.7(2) A , Z = 4, T = 173(2) K, m =
1.02 mm21, 8991 independent reflections [Rint = 0.054], final R1 = 0.048
[for 7005 reflections with I . 2s(I)], wR2 = 0.106 (all data). CCDC
numbers: 1?PhMe 609099, 2 609100, 3 609101. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b607429d
1 E. Hollink and D. W. Stephan, in Comprehensive Coordination
Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier,
Amsterdam, 2004, vol. 4, p. 105.
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3 A. G. Montalban, S. L. J. Michel, S. M. Baum, B. J. Vesper,
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M. T. M. Choi, R. Li, A. C. H. Ng, P. Zhu, N. Pan, X. Sun,
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4 (a) C. Morton, N. W. Alcock, M. R. Lees, I. J. Munslow, C. J. Sanders
and P. Scott, J. Am. Chem. Soc., 1999, 121, 11255; (b) O. Eisenstein,
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In conclusion, two new CeIII amides 1 and 2 containing bulky
dicyclohexylamido ligand were prepared by the salt metathesis
reaction; their oxidation by air provided a route to the remarkably
stable homoleptic CeIV amide [Ce(NCy2)4] (3).
5 P. B. Hitchcock, Q.-G. Huang, M. F. Lappert and X.-H. Wei, J. Mater.
Chem., 2004, 14, 3266.
6 S. D. Daniel, J.-S. M. Lehn, J. D. Korp and D. M. Hoffman,
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We thank the EPSRC for continued support.
7 R. K. Minhas, Y. Ma, J.-I. Song and S. Gambarotta, Inorg. Chem.,
1996, 35, 1866.
Notes and references
{ Synthesis of [Ce(NCy2)3(thf)] (1): CeCl3 (0.453 g, 1.84 mmol) was
stirred in thf (15 mL) at room temperature for 2 days to give a copious
8 W. S. Rees, Jr., O. Just and D. S. Van Derveer, J. Mater. Chem., 1999,
9, 249.
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