Diamine bis(phenolate) as supporting ligands in organoactinide(IV) chemistry. Synthesis, structural characterization, and reactivity of stable dialkyl derivatives

Article abstract of DOI:10.1021/om3010806

The homoleptic compounds [U(salan-R₂)₂] (R = Me (1), ^tBu (2)) were prepared in high yield by salt-metathesis reactions between UI₄(L)₂ (L = Et₂O, PhCN) and 2 equiv of [K₂(salan-R₂)] in THF. In contrast, the reaction of the tetradentate ligands salan-R₂ with UI₃(THF)₄ leads to disproportionation of the metal and to mixtures of U(IV) [U(salan-R₂)₂] and [U(salan-R₂)I₂] complexes, depending on the ligand to M ratio. The reaction of K ₂salan-Me₂ ligand with U(IV) iodide and chloride salts always leads to mixtures of the homoleptic bis-ligand complex [U(salan-Me ₂)₂] and heteroleptic complexes [U(salan-Me ₂)X₂] in different organic solvents. The structure of the heteroleptic complex [U(salan-Me₂)I₂(CH₃CN)] (4) was determined by X-ray studies. Heteroleptic U(IV) and Th(IV) chloride complexes were obtained in good yield using the bulky salan-^tBu ₂ ligand. The new complexes [U(salan-^tBu ₂)Cl₂(bipy)] (5) and [Th(salan-^tBu ₂)Cl₂(bipy)] (8) were crystallographically characterized. The salan-^tBu₂ halide complexes of U(IV) and Th(IV) revealed good precursors for the synthesis of stable dialkyl complexes. The six-coordinated alkyl complexes [Th(salan-^tBu₂)(CH ₂SiMe₃)₂] (9) and [U(salan-^tBu ₂)(CH₂SiMe₃)₂] (10) were prepared by addition of LiCH₂SiMe₃ to the chloride precursor in toluene, and their solution and solid-state structures (for 9) were determined by NMR and X-ray studies. These complexes are stable for days at room temperature. Preliminary reactivity studies show that CO₂ inserts into the An-C bond to afford a mixture of carboxylate products. In the presence of traces of LiCl, crystals of the dimeric insertion product [Th ₂Cl(salan-^tBu₂)₂(μ- η¹:η¹-O₂CCH₂SiMe ₃)₂(μ-η¹:η²-O ₂CCH₂SiMe₃)] (11) were isolated. The structure shows that CO₂ insertion occurs in both alkyl groups and that the resulting carboxylate is easily displaced by a chloride anion.

Full text of DOI:10.1021/om3010806

Article
pubs.acs.org/Organometallics
Diamine Bis(phenolate) as Supporting Ligands in Organoactinide(IV)
Chemistry. Synthesis, Structural Characterization, and Reactivity of
Stable Dialkyl Derivatives
Elsa Mora,† Leonor Maria,*,† Biplab Biswas,‡ Clement Camp,‡ Isabel C. Santos,† Jacques Pecaut,‡
́
́
Adelaide Cruz,† Jose M. Carretas,† Joaquim Marcalo,† and Marinella Mazzanti*,‡
̧
́
†Unidade de Cien
2686-953 Sacavem
̂
cias Químicas e Radiofarmaceu
, Portugal
̂
ticas, IST/ITN, Instituto Superior Tec
́
nico, Universidade Tec
́
nica de Lisboa,
́
‡Laboratoire de Reconnaissance Ionique et Chimie de Coordination, SCIB, UMR-E CEA/UJF-Grenoble 1, INAC, CEA-Grenoble,
38054 Grenoble Cedex 09, France
S
* Supporting Information
ABSTRACT: The homoleptic compounds [U(salan-R2)2] (R
t
= Me (1), Bu (2)) were prepared in high yield by salt-
metathesis reactions between UI4(L)2 (L = Et2O, PhCN) and
2 equiv of [K2(salan-R2)] in THF. In contrast, the reaction of
the tetradentate ligands salan-R2 with UI3(THF)4 leads to
disproportionation of the metal and to mixtures of U(IV)
[U(salan-R2)2] and [U(salan-R2)I2] complexes, depending on the ligand to M ratio. The reaction of K2salan-Me2 ligand with
U(IV) iodide and chloride salts always leads to mixtures of the homoleptic bis-ligand complex [U(salan-Me2)2] and heteroleptic
complexes [U(salan-Me2)X2] in different organic solvents. The structure of the heteroleptic complex [U(salan-Me2)I2(CH3CN)]
(4) was determined by X-ray studies. Heteroleptic U(IV) and Th(IV) chloride complexes were obtained in good yield using the
bulky salan-tBu2 ligand. The new complexes [U(salan-tBu2)Cl2(bipy)] (5) and [Th(salan-tBu2)Cl2(bipy)] (8) were
crystallographically characterized. The salan-tBu2 halide complexes of U(IV) and Th(IV) revealed good precursors for the
synthesis of stable dialkyl complexes. The six-coordinated alkyl complexes [Th(salan-tBu2)(CH2SiMe3)2] (9) and
[U(salan-tBu2)(CH2SiMe3)2] (10) were prepared by addition of LiCH2SiMe3 to the chloride precursor in toluene, and their
solution and solid-state structures (for 9) were determined by NMR and X-ray studies. These complexes are stable for days at
room temperature. Preliminary reactivity studies show that CO2 inserts into the An−C bond to afford a mixture of carboxylate
products. In the presence of traces of LiCl, crystals of the dimeric insertion product [Th2Cl(salan-tBu2)2(μ-η1:η1-
O2CCH2SiMe3)2(μ-η1:η2-O2CCH2SiMe3)] (11) were isolated. The structure shows that CO2 insertion occurs in both alkyl
groups and that the resulting carboxylate is easily displaced by a chloride anion.
reactivity and catalytic efficiency of organoactinide com-
plexes.13,25,26
INTRODUCTION
■
Due to the large coordination numbers of actinides and the
possible participation of 5f electrons in bonding and reactivity,
actinide complexes can promote unusual reactivity and perform
as more effective catalysts in comparison to lanthanides and d-
block metals.1−7
In particular, alkyl complexes of actinides can act as versatile
catalysts in many transformations or participate as intermedi-
ates in catalytic transformations.8−11
The chemistry of alkyl complexes of An(IV) has been
extensively investigated since the early 1970s using cyclo-
pentadienyl as an effective supporting ligand.12−19 The
insertion reactions of substrates into the actinide−carbon
bond have a key role in actinide-promoted transformation and
functionalization of organic molecules, and accordingly,
insertion reactions are also well documented in actinide
metallocene chemistry,15,20−24 although only very few insertion
products have been crystallographically characterized.
Previous studies show that the geometry and electronic
properties of the ancillary ligand play an important role in the
However, only a few non-cyclopentadienyl supporting
ligands have been used to prepare stable An(IV) alkyl
complexes and to investigate their chemistry,27−29 which
includes three dianionic ancillary ligands.30−35 Some of these
systems have demonstrated high stability,34 unprecedented
reactivity such as C−C coupling36 and C−H activation,37 and
catalytic activity in alkene polymerization or hydroamina-
tion.38−40 The use of non-cyclopentadienyl ligands has also
recently led to the characterization of a rare monoalkyl U(III)
species that can insert CO2 and CS2 into An−C bonds to yield
the corresponding carboxylate and dithiocarboxylate com-
plexes.41 These results highlight the interest in identifying
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: November 9, 2012
Published: December 18, 2012
© 2012 American Chemical Society
1409
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
few days. Complex 1 was obtained as a green microcrystalline solid in
new supporting ligands capable of stabilizing An alkyl
complexes and in tuning their reactivity.
73% yield (39 mg, 0.041 mmol). Anal. Calcd for C44H60N4O4U: C,
1
55.80; H, 6.38; N, 5.91. Found: C, 55.48; H, 6.28; N, 5.88. H NMR
salans are tetradentate dianionic diamine bis(phenolate)
ligands which are being increasingly used as supporting ligands
in coordination chemistry and catalytic transformations
promoted by transition metals42,43 and lanthanides.44 Tetra-
dentate salan ligands should provide a robust coordination
environment for reactivity studies on uranium alkyl complexes
and are good candidates for the development of uranium-based
catalysts. Moreover, the bulk and electronic properties of these
ligands can be easily tuned to optimize the stability and
reactivity of the final complex. However, in uranium chemistry
they have only been used to prepare stable uranyl(V)
complexes and their uranyl(VI) analogues.45−47 Here we have
(200 MHz, THF-d8, 298 K): δ 41.2 (s, 4H), 15.5 (s, 12H), 12.6 (s,
4H), 7.4 (s, 4H), 3.2 (s, 12H), −4.3 (s, 4H), −12.0 (s, 12H), −33.2 (s,
4H), −52.4 (s, 4H).
Synthesis of [U(salan-tBu2)2] (2). Method 1. A green solution of
UI3(THF)4 (50 mg, 0.055 mmol) in 1 mL of acetonitrile was added to
a suspension of [K2(salan-tBu2)] (33 mg, 0.055 mmol) in acetonitrile
(1 mL). The resulting mixture was stirred for 1 h at room temperature.
The KI that formed was removed by filtration, and the resulting brown
solution was transferred to a crystallization tube. After 2 days at room
temperature, green crystals suitable for X-ray diffraction of compound
2·1.25CH3CN were obtained. Yield: 65% (23 mg, 0.018 mmol).
Crystals of 2·THF of better quality were obtained from a THF
solution. Anal. Calcd for C68H108N4O4U.CH3CN: C, 63.47; H, 8.45;
N, 5.29. Found: C, 63.21; H, 8.59; N, 5.40.
investigated the ability of the tetradentate ligands salan-tBu2
2−
(H2salan-tBu2 = N,N′-bis(2-hydroxybenzyl-3,5-di-tert-butyl)-
Method 2. Complex 2 can also be obtained from the addition of a
brown solution of UI4(EtO2)2 (37 mg, 0.042 mmol) in 1 mL of THF
to a suspension of [K2(salan-tBu2)] (50 mg, 0.083 mmol) in THF (1.5
mL). The resulting suspension was stirred for 3 h at room temperature
and then filtered to afford the final compound. Yield: 78% (42 mg,
0.033 mmol). Crystals of 2·THF were grown from a tetrahydrofuran
solution at room temperature and confirmed the presence of the bis-
2−
1,2-dimethylaminomethane) and salan-Me2 (H2salan-Me2 =
N,N′-bis(2-hydroxybenzyl-3,5-methyl)-1,2-dimethylaminome-
thane) to form complexes with uranium in lower oxidation
states. We show that the bulky salan-tBu2 ligand provides a
2−
suitable environment for the synthesis of heteroleptic An(IV)
halide complexes, [An(salan-tBu2)X2], which provide versatile
precursors for the synthesis of robust alkyl complexes. The
reaction of CO2 insertion with these alkyl complexes is also
described.
1
ligand complex. The H NMR of 2 in THF at room temperature
1
displays a series of broad signals. H NMR (400 MHz, THF-d8, 233
K): 80.99 (s, 2H), 53.55 (s, 6H, NCH3), 31.51 (s, 18H. C(CH3)3),
30.99 (s, 2H), 12.88 (s, 2H), 6.99 (s, 18H, C(CH3)3), −1.51 (s, 18H,
C(CH3)3), −2.24 (s, 2H), −2.92 (s, 2H), −5.74 (s, 18H, C(CH3)3),
−7.53 (s, 2H), −10.49 (s, 2H), −26.81 (s, 2H), −38.54 (s, 2H),
−43.59 (s, 2H), −57.32 (s, 2H), −59.09 (s, 6H, C(CH3)3), −72.52 (s,
2H). 1H NMR (300 MHz, C7D8, 233 K): 78.95 (s, 2H), 51.19 (s, 6H,
NCH3), 30.89 (s, 18H, C(CH3)3), 12.64 (s, 2H), 8.60 (s, 2H), 6.82 (s,
18H, C(CH3)3), −1.14 (s, 18H, C(CH3)3), −1.50 (s, 2H), −2.85 (s,
2H), −5.32 (s, 18H, C(CH3)3), −7.02 (s, 2H), −9.68 (s, 2H), −26.00
(s, 2H), −38.45 (s, 2H), −42.99 (s, 2H), −55.79 (s, 2H), −56.82 (s,
6H, NCH3), −70.24 (s, 2H).
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were performed using standard Schlenk techniques or in an
inert-atmosphere glovebox filled with nitrogen or argon. Tetrahy-
drofuran, dimethoxyethane, toluene, and n-hexane were predried using
4A molecular sieves, distilled under nitrogen from sodium−
benzophenone, and degassed prior to use. Tetrahydrofuran-d8,
benzene-d6, and toluene-d8 were dried over sodium-benzophenone
and distilled under argon. Acetonitrile and acetonitrile-d3 were distilled
from P2O5 or CaH2 under argon and maintained in contact with 3A
molecular sieves several days before use.
Isolation of Crystals of [U(salan-tBu2)I2(bipy)] (3). To a
solution of UI3(THF)4 (332 mg, 0.38 mmol) in THF (10 mL) was
added a solution of [K2(salan-tBu2)] (228 mg, 0.38 mmol) in THF (10
mL), and the mixture was stirred at room temperature for 2 h. A
solution of 2,2′-bipyridine (60 mg, 0,38 mmol) in THF (0.5 mL) was
then added, and the mixture was stirred overnight at room
temperature. After centrifugation, the solvent was removed under
vacuum, giving a dark green solid, which was further extracted with
toluene. Green crystals adequate for X-ray diffraction analysis were
obtained by slow diffusion of n-hexane into a concentrated toluene
UI3(THF)4, UCl4, UI4(L)n (L = Et2O, PhCN), and ThCl4(dme)2
were prepared using previously reported procedures.48−52 The
dianionic salan-R2 ligand precursors and the potassium salts were
synthesized as previously reported.53 2,2′-Bipyridine was sublimed
before use. Pentane was removed under vacuum from LiCH2Si(CH3)3
(1.0 M, Aldrich) prior to use.
The NMR experiments (1H, 13C, COSY, HSQC) were performed
on Varian 300 and 400 MHz and Bruker 200 MHz spectrometers; all
1
solution of the mixture. H NMR (300 MHz, C6D6, 298 K): 169.90
1
(2H, br), 48.26 (18H, s, C(CH3)3), 43.50 (2H, s), 22.03 (2H, s), 17.13
(2H, s), 10.34 (18H, s, C(CH3)3)), 3.30 (2H, s), −22.03 (2H, s),
−30.31 (2H, br), −44.79 (2H, br), −50.16 (2H + 2H, s), −63.85 (6H,
s, NCH3).
This complex could not be obtained analytically pure in large
quantity because the formation of 3 is always accompanied by the
presence of [U(salan-tBu2)2] (2).
chemical shifts are reported relative to the peak for SiMe4 using H
and 13C chemical shifts of residual solvents. CHN elemental analyses
were performed using a CE Instruments EA1110 automatic analyzer or
by Analytische Laboratorien GMBH at Lindlar, Germany. Electrospray
ionization mass spectrometry (ESI/MS) was performed using a Bruker
HCT quadrupole ion trap equipped with an electrospray interface; the
samples were prepared under N2 with dry solvents, and the airtight
syringe was filled inside an inert-atmosphere glovebox and carried in a
closed vessel under N2 before injection.
Isolation of Crystals of [U(salan-Me2)I2(CH3CN)] (4). A brown
solution of UI4(PhCN)4 (50 mg, 0.043 mmol, 1 equiv) in acetonitrile
(1.5 mL) was added to a suspension of [K2(salan-Me2)] (18.7 mg,
0.043 mmol, 1 equiv) in acetonitrile (1.5 mL). The resulting
suspension was stirred for 3 h until it became green, and then the
white precipitate of KI was removed by filtration. The resulting green
solution was transferred to a crystallization tube, and after 3 days X-
Synthesis of [U(salan-Me2)2] (1). Method 1. A green solution of
UI3(THF)4 (49 mg, 0.054 mmol, 1 equiv) in acetonitrile (1 mL) was
added to a suspension of [K2(salan-Me2)] (23.3 mg, 0.054.0 mmol, 1
equiv) in acetonitrile (1 mL). The resulting suspension was stirred for
30 min, and the solids were removed by filtration. The resulting green
solution was left to stand at room temperature for 1 day to yield X-ray-
quality crystals of 1 in 55% yield (15 mg, 0.015 mmol).
Method 2. A brown solution of UI4(Et2O)2 (50 mg, 0.056 mmol) in
1.5 mL of THF was added to a suspension of [K2(salan-Me2)] (48.3
mg, 0.111 mmol) in THF (1.5 mL). The resulting mixture was stirred
for 0.5 h, at room temperature, until the suspension became green.
The white solid that formed was removed by filtration, and the
solution was concentrated and left standing at room temperature for
1
ray-quality crystals of 4 were formed. H NMR (200 MHz, THF-d8,
298 K): δ 81.4 (s, 6H), 47.6 (s, 2H), 20.6 (s, 2H), 19.8 (s, 6H), −26.0
(s, 2H), −33.0 (s, 2H), −53.5 (s, 4H), −76.4 (s, 6H).
This complex could not be obtained analytically pure in large
quantity because the formation of 4 is always accompanied by the
presence of [U(salan-Me2)]2 (1). Although complex 4 is more
insoluble, it was impossible to obtain an analytically pure complex by
crystallization.
1410
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
Synthesis of [U(salan-tBu2)Cl2(bipy)] (5). To a green solution of
UCl4 (289 mg, 0.76 mmol) in THF (10 mL) was slowly added a
solution of [K2(salan-tBu2)] (457 mg, 0.76 mmol) in THF (15 mL),
and the mixture was stirred for 30 min to afford the mono-ligand
complex [U(salan-tBu2)Cl2(THF)x] (1H NMR (200 MHz, THF-d8,
298 K): δ 52.6 (s, 18H, C(CH3)3), 47.3 (s, 2H), 20.4 (s, 2H), 11.5 (s,
18H, C(CH3)3), −23.9 (s, 2H), −33.6 (s, 2H), −52.2 (s, 2H), −53.3
(s, 2H), −76.9 (s, 6H, NCH3)). Then it was added to a solution of
2,2′-bipyridine (119 mg, 0,76 mmol) in THF (0.5 mL). After the
reaction mixture was stirred overnight at room temperature, the
potassium chloride was separated by centrifugation and the solvent
was removed under vacuum. After the residue was washed three times
with 2 mL of acetonitrile and once with 3 mL of n-hexane and dried
under vacuum, a green solid was obtained and identified as 5 (459 mg,
0.464 mmol, 61%). Green crystals of 5 suitable for X-ray diffraction
analysis were obtained by slow diffusion of n-hexane into a saturated
toluene solution. Anal. Calcd for C44H62N4O2Cl2U: C, 53.49; H, 6.33;
N, 5.67. Found: C, 53.75; H, 6.70; N, 5.80. MS (ESI/MS, CH3CN/
THF): m/z 853.4 [U(salan-tBu2)Cl2Na]+, 869.5 [U(salan-tBu2)Cl2K]+,
140.3 (Ar-C), 136.9 (Ar-C), 126.5 (Ar-C), 125.6 (Ar-CH), 71.9
(OCH2CH2O), 64.4 (ArCH2N), 63.0 (CH3O), 53.0 (NCH2CH2N),
47.7 (NCH3), 35.5 (C(CH3)), 34.3 (C(CH3)), 31.9 (C(CH3)), 31.2
(C(CH3)).
Synthesis of [Th(salan-tBu2)Cl2(bipy)] (8). A 2,2′-bipyridine (34
mg, 0.22 mmol) toluene solution (1 mL) was added to a solution of 7
(200 mg, 0.22 mmol) in toluene (10 mL). After 1 h of stirring at room
temperature the volatiles were removed under vacuum. The residue
solid was redissolved in 4 mL of toluene, and addition of 8 mL of n-
hexane afforded a white crystalline solid formulated as [Th-
(salan-tBu2)Cl2(bipy)] (170 mg, 0.173 mmol, 79%). X-ray-quality
crystals were obtained by slow diffusion of n-hexane into a toluene
solution of 8. Anal. Calcd for C44H62Cl2N4O2Th: C, 53.82; H, 6.36; N,
5.71. Found: C, 53.41; H, 6.25; N, 5.45. MS (ESI/MS, CH3CN): m/z
863.5 [Th(salan-tBu2)Cl2K]+, 945.6 [Th(salan-tBu2)Cl(bipy)]+, 861.5
[Th(salan-tBu2)Cl3]−. 1H NMR (300 MHz, C6D6, 298 K): δ 9.17 (2H,
d, 5.1 Hz, bipy), 7.78 (2H, d, JHH = 2,4 Hz, Ar-H), 7.05 (2H, d, JHH
=
2,4 Hz, Ar-H), 6.92 (2H, d, 7.8 Hz, bipy), 6.85 (2H, m, bipy), 6.35
(2H, m, bipy), 5.56 (2H, d, JHH = 13,7 Hz, ArCH2N), 3.18 (2H, d, JHH
= 9.9 Hz, NCH2CH2N), 2.93 (2H, d, JHH = 13.9 Hz, ArCH2N), 2.69
(6H, s, CH3), 1.78 (18H, s, C(CH3)3) 1.38 (18H, s, C(CH3)3), 1.33
(2H, d, JHH = 9,07 Hz, NCH2CH2N). 13C NMR (75.4 MHz, C6D6,
298 K): δ 159.9 (Ar-CO), 154.8 (bipy-C), 152.3 (bipy-CH), 140.4
(Ar-C), 138.2 (bipy-CH), 137.6 (Ar-C), 127.2 (Ar-C), 126.1 (Ar−
CH), 125.7 (Ar-C), 125.3 (Ar-CH), 123.8 (bipy-CH), 122.0 (bipy-
CH), 64.6 (ArCH2N), 53.1 (NCH2CH2N), 47.8 (NCH3), 35.9
(C(CH3)3), 34.4 (C(CH3)3), 32.1 (C(CH3)3), 32.0 (C(CH3)3).
Synthesis of [Th(salan-tBu2)(CH2SiMe3)2] (9). To a solution of 7
(312 mg, 0.342 mmol) in toluene (10 mL) was added a solution of
LiCH2SiMe3 (65 mg, 0.69 mmol) in toluene (1 mL). After 1 h of
stirring at room temperature the resulting mixture was centrifuged and
the solids were discarded. The volatiles were removed under reduced
pressure, and the residue was washed with ca. 0.4 mL of n-hexane. The
resulting white solid was redissolved in 3 mL of a 1/1 toluene/n-
hexane mixture. Concentration of the solution led to the formation of
white crystals of 8 suitable for X-ray diffraction analysis (210 mg, 0.226
mmol, 66%). Anal. Calcd for C42H76N2O21Si2Th: C, 54.28; H, 8.24; N,
3.01. Found: C, 54.33; H, 8.31; N, 3.08. H NMR (300 MHz, C6D6,
298 K): δ 7.62 (d, JHH = 2.4 Hz, 2H, Ar-H), 6.86 (d, JHH = 2.4 Hz, 2H,
Ar-H), 4.13 (d, JHH = 13.2 Hz, 2H, ArCH2N), 2.66 (d, JHH = 9.3 Hz,
2H, NCH2CH2N), 2.56 (d, JHH = 13.2 Hz, 2H, ArCH2N), 1.75 (s,
18H, C(CH3)3), 1.69 (s, 6H, NCH3), 1.31 (s, 18H, C(CH3)3), 1.05 (d,
JHH = 9.3 Hz, 2H, NCH2CH2N), 0.36 (s, 18H, Si(CH3)3), 0.30 (d, JHH
= 10.2 Hz, 2H, Th-CH2), 0.00 (d, JHH = 10.2 Hz, 2H, Th-CH2). 13C
NMR (75.4 MHz, C6D6, 298 K): δ 161.0 (Ar-CO), 140.6 (Ar-C),
136.9 (Ar-C), 125.6 (Ar-C), 125.0 (Ar-CH), 124.8 (Ar-CH), 87.2
(CH2SiMe3), 63.0 (ArCH2N), 51.3 (NCH2CH2N), 42.1 (NCH3), 35.5
(C(CH3)), 34.4 (C(CH3)), 31.9 (C(CH3)), 31.4 (C(CH3)), 4.21
(Si(CH3)3).
1
951.4 [U(salan-tBu2)Cl(bipy)]+, 867.4 [U(salan-tBu2)Cl3]−. H NMR
(300 MHz, C6D6, 298 K): δ 154.39 (2H, br), 45.90 (s, 18H,
C(CH3)3), 41.96 (s, 2H), 24.14 (s, 2H), 19.07 (s, 2H), 17.51 (s, 2H),
17.17 (s, 2H), 10.15 (s, 18H, C(CH3)3), −24.08 (s, 2H,), −27.20 (s,
2H), −38.79 (br, 2H,), −48.91 (s, 2H+2H), −64.80 (s, 6H, NCH3).
Isolation of Crystals of [Th(salan-tBu2)2] (6). To a suspension of
ThCl4(dme)2 (158 mg, 0.285 mmol, 1 equiv) in tetrahydrofuran (10
mL) was added a solution of [K2(salan-tBu2)] (344 mg, 0.572 mmol, 2
equiv) in THF (5 mL). The resulting suspension was stirred overnight
at room temperature, and then the white precipitate of KCl was
removed by centrifugation. Evaporation under reduced pressure
produced a white solid. Complex 6 could not be obtained analytically
pure because the formation of 6 is always accompanied by the
presence of [Th(salan-tBu2)Cl2] and other species, and separation of 6
by crystallization proved to be difficult. A few crystals adequate for X-
ray diffraction analysis were grown from a saturated hexane solution.
1H NMR (300 MHz, C6D6, 298 K): δ 7.61 (d, 4H, Ar-H), 6.73 (d, JHH
= 2.7 Hz, 4H, Ar-H), 5.85 (d, JHH = 12.7 Hz, 4H, ArCH2N), 3.11 (d,
JHH = 9 Hz, 4H, NCH2CH2N), 2.82 (d, JHH = 4H, 12.96 HZ,
ArCH2N), 2.67 (s, 6H, NCH3), 1.82 (s, 36H, C(CH3)3), 1.27 (d, 4H,
NCH2CH2N), 1.18 (s, 36H, C(CH3)3).
Synthesis of [Th(salan-tBu2)Cl2(dme)] (7). A solution of
[K2(salan-tBu2)] (560 mg, 0.932 mmol) in 1,2-dimethoxyethane (10
mL) was added dropwise to a solution of ThCl4(dme)2 (516 mg, 0.932
mmol) in 1,2-dimethoxyethane (10 mL), and the mixture was stirred
overnight at room temperature. The mixture was centrifuged, and the
volatiles were removed under reduced pressure. The residue was
extracted with a mixture of 10 mL of toluene and 0.4 mL of dme, and
the resulting solution was concentrated under vacuum. A white
microcrystalline solid was isolated by centrifugation, washed with n-
hexane, and dried under vacuum to give 6 (632 mg, 0.690 mmol,
74%). Anal. Calcd for C38H64Cl2N2O4Th·0.2(toluene): C, 50.65; H,
7.08, N, 3.30. Found: C, 50.77; H, 7.44; N, 3.08. MS (ESI/MS,
CH3CN): m/z 863.2 [Th(salan-tBu2)Cl2K]+, 861.4 [Th(salan-tBu2)-
Synthesis of [U(salan-tBu2)(CH2SiMe3)2] (10). To a solution of
UCl4 (217 mg, 0.57 mmol) in THF (10 mL) was added a solution of
[K2(salan-tBu2)] (344 mg, 0.57 mmol) in THF (10 mL), and the
mixture was stirred overnight at room temperature. The resulting
mixture was centrifuged, and the solvent was removed under reduced
pressure to give [U(salan-tBu2)Cl2(THF)x]. Toluene was added to the
solid residue, and to the resulting suspension was added a solution of
LiCH2SiMe3 (108 mg, 1.14 mmol) in toluene (1 mL). The mixture
was stirred for 1 h at room temperature and centrifuged, and the LiCl
that formed was discarded. The volatiles were removed under reduced
pressure, giving a dark green solid, which was further extracted with n-
hexane. The first crop of the hexane solution (0.5 mL) was discarded,
and the following crops were evaporated under reduced pressure,
giving compound 10 as a green solid (247 mg, 0.264 mmol, 46%). An
analytically pure sample was obtained from concentration of a
saturated THF solution. Anal. Calcd for C42H76N2O2Si2U: C, 53.94;
1
Cl3]−. H NMR (300 MHz, C6D6, 298 K): δ 7.68 (2H, d, JHH = 2.5
Hz, Ar), 6.92 (2H, d, JHH = 2.5 Hz, Ar), 5.29 (2H, d, JHH = 13.8 Hz,
ArCH2N), 2.78 (2H, d, JHH = 13.8 Hz, ArCH2N), 3.78 (br,
OCH2CH2O), 3.31 (br, OCH3). 2.99 (2H, d, JHH = 10.4 Hz,
NCH2CH2N), 2.55 (6H, s, NCH3), 1.82 (18H, s, C(CH3)3), 1.34
(18H, s, C(CH3)3), 1.20 (2H, d, JHH = 10.4 Hz, NCH2CH2N). 13C
NMR (75.4 MHz, C6D6, 298 K): δ 159.6 (Ar-C-O), 140.6 (Ar-C),
137.2 (Ar-C), 126.7 (Ar-C), 125.7 (Ar-CH), 125.1 (Ar-CH), 72.1
(OCH2CH2O), 64.5 (ArCH2N), 63.1 (CH3OCH2CH2OCH3), 53.1
(NCH2CH2N), 47.8 (NCH3), 34.6 (C(CH3)), 34.4 (C(CH3)), 31.9
(C(CH3)), 31.5 (C(CH3)). 1H NMR (300 MHz, C7D8, 263 K): δ 7.66
(2H, d, J = 2.4 Hz; Ar), 6.91 (2H, d, J = 2.4 Hz; Ar), 5.24 (2H, d, J =
13.5 Hz, ArCH2N), 3.76 (2H, d, J = 6.9 Hz, OCH2CH2O), 3.30 (6H,
s, CH3O), 2.99 (2H, d; 9.9 Hz, NCH2CH2N), 2.77 (2H, d, 13.8 Hz,
ArCH2N), 2.65 (2H, d, J = 6.9 Hz, OCH2CH2O), 2.52 (6H, s, CH3N),
1.81 (18H, s, C(CH3)3, 1.35 (18H, s, C(CH3)3), 1.20 (2H, d; J =10.2,
NCH2CH2N). 13C NMR (75.4 MHz, C7D8, 263 K): δ 159.5 (Ar-CO),
1
H, 8.19, N, 3.00. Found: C, 53.64; H, 8.62; N, 2.91. H NMR (300
MHz, C6D6, 298 K, ppm): δ 46.621 (s, 18H, C(CH3)3), 42.67 (s, 2H),
19.21 (s, 2H), 10.46 (s, 18H, C(CH3)3), −10.894−10. (s, 18H,
Si(CH3)3), −17.68 (s, 2H), −26.55 (s, 2H), −47.41 (s, 2H), −50.90
1411
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
Table 1. X-ray Crystal Data and Collection Parameters for the Uranium Complexes
2·1.25CH3CN
2·C4H8O
3·2C7H8
4
5·C7H8
empirical formula
fw
C70.50H111.75N5.25O4U
1334.93
C72H116N4O5U
1355.72
150(2)
monoclinic
C2/c
C58H78I2N4O2U
1355.07
150(2)
C24H33I2N3O2U
887.36
C51H67Cl2N4O2U
1077.02
150(2)
temp (K)
cryst syst
space group
a (Å)
150(2)
150(2)
tetragonal
P43212
triclinic
monoclinic
P21/c
monoclinic
C2/c
P1
̅
15.1010(4)
15.3382(5)
36.8413(6)
85.198(2)
78.323(2)
60.699(3)
7286.3(4)
4
18.2767(3)
24.4463(4)
16.6782(3)
90
10.1592(9)
31.130(3)
18.5587(17)
90
8.52423(14)
8.52423(14)
38.5227(12)
90
34.0855(8)
16.1899(3)
18.7980(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
99.620(1)
90
91.005(3)
90
90
105.919(2)
90
90
7347.0(2)
4
5868.4(10)
4
2799.15(11)
4
9975.7(4)
8
Z
ρcalcd (Mg/m3)
1.217
1.226
1.534
2.106
1.434
μ (mm−1
)
2.273
2.256
3.859
8.028
3.402
a
R1 (I > 2σ(I))
0.0635
0.0309
0.0719
0.0439
0.0363
a
wR2 (all data)
0.1459
0.0838
0.1571
0.0845
0.0900
a
2
2
Definitions: R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
Table 2. X-ray Crystal Data and Collection Parameters for the Thorium Complexes
6·C6H14
8·3C7H8
9
11·4C7H8
empirical formula
fw
C74H122N4O4Th
1363.80
150(2)
C109H148Cl4N8O4Th2
2240.23
C42H76N2O2Si2Th
929.27
C111H173ClN4O10Si3Th2
2307.33
temp (K)
cryst syst
space group
a (Å)
150(2)
150(2)
150(2)
monoclinic
C2/c
trigonal
monoclinic
C2/c
triclinic
R3
̅
P1
̅
18.2062(4)
24.5461(4)
16.5539(3)
90
41.4321(13)
41.4321(13)
19.3575(7)
90
20.0165(4)
10.3489(2)
21.9123(5)
90
18.1779(3)
18.2607(3)
20.3703(4)
109.6670(10)
97.926(2)
108.813(2)
5792.2(2)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
98.9980(10)
90
90
92.1110(10)
90
120
7306.8(2)
4
28777.5(16)
9
4536.03(16)
4
Z
ρcalcd (Mg/m3)
1.240
1.163
1.361
1.323
μ (mm−1
)
2.088
2.450
3.373
2.672
a
R1 (I > 2σ(I))
0.0247
0.0364
0.0792
0.0252
0.0414
a
wR2 (all data)
0.0524
0.0501
0.1056
a
2
2
Definitions: R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
(s, 2H), −86.19 (s, 6H, NCH3), −236.43 (s, 2H, CH2SiMe3), −238.53
(s, 2H, CH2SiMe3).
NMR Tube Scale Reaction of [An(salan-tBu2)(CH2SiMe3)2] (An
= Th (9), U (10)) with CO2. A 10−30 mg portion of the bis(alkyl)
actinide was dissolved in C7D8 (0.4 mL), and the solution was freeze−
pump−thawed three times. The solution was then exposed to 1 atm of
CO2 (2 equiv or excess) at room temperature. The reaction was
Isolation of [Th2Cl(salan-tBu2)2(μ-η1:η1-O2CCH2SiMe3)2(μ-
η1:η2-O2CCH2SiMe3)] (11). A Schlenk tube was charged with 9 (98
mg, 0.106 mmol) and toluene (5 mL). The solution was degassed by
three freeze−pump−thaw cycles, and CO2 (2.3 mmol) was expanded
into the tube (1 atm). The reaction mixture was stirred for 1 h at room
temperature, and then the solvent was removed under vacuum. The
white solid obtained was analyzed by 1H NMR and ESI/MS, showing
that a mixture of compounds was formed. A few crystals of 11 were
isolated by slow evaporation of a toluene solution of the bulk product.
Attempts to synthesize complex 11 in analytically pure form have
failed, which is consistent with the NMR and ESI/MS analysis. Even in
the presence of an excess of LiCl the ESI/MS and NMR analysis
shows the presence of a mixture of compounds. In the presence of
excess LiCl, the mono-chloride [Th(salan-tBu)(O2CCH2SiMe3)2Cl]−
and bis-chloride [Th(salan-tBu)(O2CCH2SiMe3)Cl2]− species are
both present in solution. The neutral bis-carboxylate is also present
in solution, even in the presence of excess chloride. The presence of
multiple species and of excess LiCl renders difficult the synthesis and
characterization of 11.
1
monitored by H NMR spectroscopy and ESI/MS.
X-ray Crystallography. Crystallographic and experimental details
of data collection and crystal structure refinement are summarized in
Tables 1 and 2. Suitable crystals of complexes were selected and
coated in FOMBLIN oil under an inert atmosphere. Crystals were
then mounted on a loop, and the data were collected using graphite-
monochromated Mo Kα (λ = 0.71073 Å) on a Bruker AXS-KAPPA
APEX II area detector or an Oxford Diffraction X-Calibur S
diffractometer equipped with an Oxford Cryosystem open-flow
nitrogen cryostat, and data were collected at 150 K. Cell parameters
were retrieved using Bruker SMART or CrysalisPro CCD software and
refined using Bruker SAINT or CrysalisPro Red on all observed
reflections. Absorption corrections were applied using SADABS or
ABSPACK.54 The structures were solved by direct methods using
either SHELXS-9755 or SIR 9756 or SHELXTL 6.10 and refined using
full-matrix least-squares refinement against F2 using SHELXL-97.55 In
1412
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
the former case, all programs are included in the package of programs
WINGX-version 1.64.05.57All non-hydrogen atoms were refined
anisotropically, unless it was mentioned in the cif files of the
structures, and all hydrogen atoms were placed in idealized positions
and allowed to refine riding on the parent carbon atom.
Increasing the metal to ligand ratio to 1/2 enhanced the
formation of the bis-ligand complex [U(salan-tBu2)2] (2),
which is the only species present in solution. Crystals of 2
suitable for X-ray diffraction analysis were grown by letting
stand at room temperature a saturated acetonitrile solution (or
tetrahydrofuran solution). Complex 2 can be obtained in 33%
yield analytically pure from the reaction of UI3(THF)4 and
K2(salan-tBu2), in a 1/1 ratio.
The salan uranium(IV) complexes 1 and 2 were probably
formed by a disproportionation process promoted by the salan
ligands, although the formation of U(0) metal could not be
detected. An analogous behavior was reported by the
Diaconescu group, showing that stoichiometric reactions
between UI3(THF)4 and the dianionic ligands 2,6-bis(2,6-
diisopropylanilidomethyl)pyridine (NNpy)31 and ferrocene-
diamide (NNfc)30 give the complexes [U(NNpy)2], [U(NNfc)2],
and [U(NNfc)I2(THF)], respectively, in reproducible yields.
The authors indicated that the uranium(IV) complexes were
presumably formed by the disproportionation of uranium(III)
intermediates to U(IV) and some form of U(0). This
disproportionation pathway (UIII → UIV + U0) has also been
reported by other authors.58,59
RESULTS AND DISCUSSION
■
Reactivity of U(III) with salan-R2. The reaction of
UI3(THF)4 with 1 equiv of [K2(salan-Me2)] in CH3CN led
to the isolation of the homoleptic uranium(IV) complex
[U(salan-Me2)2] (1) (Scheme 1). Proton NMR spectra of 1/1
Scheme 1. Reaction of [UI3(THF)4] with salan-R2 Ligands
t
(R = Me, Bu)
Disproportionation also occurs when the reaction of
UI3(THF)4 with 1 equiv of [K2(salan-tBu2)] is carried out in
tetrahydrofuran in the presence of 2,2′-bipyridine. The proton
NMR spectrum in toluene-d8 of the reaction mixture at −40 °C
shows the presence of two groups of NMR resonances, which
were assigned to mono-ligand species and to the bis-ligand
complex [U(salan-tBu2)2] (2). Due to the similar solubilities of
2 and 3, it was not possible to isolate the bis-iodide 3 complex
as an analytically pure compound. However, it was possible to
isolate crystals suitable for X-ray diffraction analysis and
confirm the structure of [U(salan-tBu2)(bipy)I2] (3).
and 1/2 reaction mixtures of UI3(THF)4 and K2(salan-Me2)
show the presence of only one set of signals assigned to
complex 1. X-ray diffraction studies on crystals isolated from
both solutions confirmed the presence of the bis-ligand
complex. While the crystals were not of sufficient quality to
discuss the metrical parameters, the connectivity clearly shows
the presence of a bis-ligand complex (see Figure S1 in the
Supporting Information). This indicates that independently of
the metal/ligand ratio the oxidation of the metal occurs, leading
to the formation of the bis-ligand complex.
The 1H NMR spectrum of 3 recorded in benzene-d8 at 20 °C
(Figure S4, Supporting Information) shows that the ONNO
ligand protons feature an integration pattern consistent with an
overall C2-symmetric molecule in solution.
The 1H NMR spectrum of 1 at 25 °C in THF-d8 (Figure S2,
Supporting Information) displays nine paramagnetically shifted
peaks and is consistent with the presence of D2h-symmetric
solution species, with three resonances for the four aromatic
methyl and four amine methyl groups, two signals for the
aromatic protons, and four signals for the two diastereotopic
benzylic and the two diastereotopic methylenic bridge protons.
In order to probe the effect of bulky phenolate substituents
in stabilizing a reduced species, the reaction between
UI3(THF)4 and the more bulky ligand [K2(salan-tBu2)], in a
1/1 ratio, was performed in different coordinating solvents
(THF, CH3CN, dme). In all the cases, the reaction led to the
formation of a mixture of U(IV) species which were assigned
from the NMR studies to the monoligand and bis-ligand
Uranium(IV) salan-R2 Complexes. The homoleptic
t
compounds [U(salan-R2)2] (R = Me (1), Bu (2)) can also
be prepared in high yields by the salt-metathesis reactions
between UI4(L)2 (L = Et2O, PhCN) and 2 equiv of [K2(salan-
R2)] in THF (Scheme 2).
Scheme 2. Synthesis of the Homoleptic [U(IV)(salanR2)2]
t
Complexes (R = Me, Bu)
1
complexes. The H NMR spectra of the crude mixtures in
toluene-d8 at 25 °C display two distinct groups of resonances
(Figure S3, Supporting Information): one group of narrow
resonances, with a characteristic pattern of a C2-symmetrical
complex in solution, and one group of broad resonances. The
first group was assigned to a mono-salan uranium(IV) complex.
The broad resonances were attributed to the bis-ligand complex
2, and lowering the temperature of the sample in toluene-d8 or
We have also investigated the possibility of preparing
heteroleptic U(IV) complexes of salan-R2 containing labile
halide ligands, with the objective of identifying new precursors
for the synthesis of U(IV) alkyl complexes.
The reaction of UI4(Et2O)2 with K2(salan-Me2) in
tetrahydrofuran, at room temperature, in a metal to ligand
ratio of 1/1 (or less) always gave a mixture of the mono- and
bis-ligand complexes (Figure S5, Supporting Information). The
lower solubility of [U(salan-Me2)I2(CH3CN)] (4) in compar-
ison to 1 allowed the isolation of X-ray-quality crystals of 4
from a saturated acetonitrile solution of the reaction mixture.
The proton NMR spectrum of crystals of 4 shows the presence
1
THF-d8 to −40 °C allowed the observation in the H NMR
spectrum of 4 resonances for the tert-butyl groups and 2
resonances for the methyl groups in the intensity ratio of 18/
18/18/18/6/6 and 12 resonances with identical intensity (2H)
that could be assigned to the methylenic and aromatic protons
of the salan ligands of the complex [U(salan-tBu2)2] (2).
1413
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
of C2-symmetric solution species (Figure S6, Supporting
Information). However, the reproducible synthesis of complex
4 in an analytically pure form turned out to be difficult.
In order to identify synthetic routes to heteroleptic salan
complexes suitable for reactivity studies, we have used UCl4 as
an alternative starting material for the preparation of these
complexes. We expected that the chloride atoms would be less
easily displaced during the salt-metathesis reactions, therefore
leading to the stabilization of the mono-ligand complex.
Nevertheless, in the case of the less bulky ligand (salan-
Me2)2− the reactions in a 1/1 ligand to metal ratio still gave a
mixture of the mono-ligand and bis-ligand complexes (Figure
S7, Supporting Information). To favor the formation of the bis-
chloride complex, we then performed the reaction with the
bulky K2(salan-tBu2) ligand. The proton NMR in THF-d8 at 25
°C of the reaction mixture of UCl4 and 1 equiv of
K2(salan-tBu2) shows only one set of paramagnetically shifted
resonances for the diamine bis(phenolate) ligand with an
integration pattern consistent with a C2-symmetric compound
in solution (Figure S8, Supporting Information). Several efforts
were made to obtain crystals for X-ray diffraction analysis, but
without success. However, the addition of 2,2′-bipyridine to a
toluene solution of the isolated product led to the formation of
[U(salan-tBu2)Cl2(bipy)] (5), confirming the formation of a
monosubstituted species of the type [U(salan-tBu2)-
Cl2(THF)x]. Complex 5 was isolated in 61% yield by reacting
UCl4 with 1 equiv of [K2(salan-tBu2)] in THF and in the
presence of 2,2′-bipyridine (Scheme 3). Compound 5 is soluble
in aromatic solvents and THF and insoluble in acetonitrile and
n-hexane.
Figure 1. ORTEP view of the solid-state molecular structure of
[U(salan-tBu)2] in 2·THF (50% probability ellipsoids). Hydrogen
atoms and solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg): U−O(1) = 2.221(2), U−O(2) =
2.263(2), U−N(1) = 2.767(3), U−N(2) = 2.792(3); O(1)−U−
O(1A) = 148.60(12), O(2)−U−O(2A) = 144.93(12), N(1)−U−
N(1A) = 65.51(13), N(2)−U−N(2A) = 62.69(13).
bond distances of 2.219(2) and 2.263(2) Å are within the
values reported for U(IV) phenolates.61 The U−N bond
lengths of 2.790(2) and 2.766(2) Å are slightly longer than
those found in the uranium(IV) bis(phenolate) complex
[U(ONO′O)2] (2.654(16) and 2.707(13) Å) (ONO′O =
MeOCH2CH2N(CH2-(2-O-C6H2-But2-3,5))2).62
The complex [U(salan-tBu2)I2(bipy)] (3) crystallized by
slow diffusion of n-hexane into a saturated toluene solution
with one independent molecule of 3 and two toluene molecules
in the unit cell. The structure of 3 is shown in Figure 2 with
selected bond lengths and angles.
Scheme 3. Synthesis of the Heteroleptic U(IV) salan-R2/
Halide Complexes
The molecular structure shows that the uranium(IV) is eight-
coordinated by the ONNO donors of the tetradentate
[salan-tBu2]2− ligand, by the two nitrogens of the bipyridine
chelate, and by two iodine atoms. Although the arrangement of
the donor atoms around the uranium atom in 3 results in the
absence of crystallographic symmetry, the structure displays a
Crystal and Molecular Structures of the Uranium
salan-R2 Complexes. The molecular structures of the
uranium salan complexes 2−5 were determined by single-
crystal X-ray diffraction analysis.
The complex [U(salan-tBu2)2] (2) crystallized from a THF
solution as a tetrahydrofuran solvate and consists of discrete
molecules in which the uranium(IV) center is eight-coordinated
by two κ4-ONNO ligands (Figure 1). The coordination around
the metal is best described as dodecahedral60 with the two
orthogonal trapezoids defined by the atoms O1−O1A−N2−
N2A and O2−O2A−N1−N1A with a dihedral angle of
87.78(12)° between the two planes. The arrangement of the
donor atoms around the uranium atom presents a crystallo-
graphically imposed C2 symmetry that makes the two phenolate
groups of the same diamine bis(phenolate) ligand identical and
the two ONNO ligands different, which is consistent with the
solution NMR data obtained at low temperature. The U−O
Figure 2. ORTEP view of the solid-state molecular structure of
[U(salan-tBu2)I2(bipy)] (3) in 3·2C7H8 (50% probability ellipsoids).
Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths (Å) and angles (deg): U−O(1) = 2.128(8), U−
O(2) = 2.118(8), U−N(1) = 2.705(9), U−N(2) = 2.702(9), U−
N(3)(bipy) = 2.673(10), U−N(4)(bipy) = 2.663(10), U−I(1) =
3.2032(9), U−I(2) = 3.2020(9); O(1)−U−O(2) = 150.9(3), I(1)−
U−I(2) = 142.83(3), N(1)−U−N(2) = 66.9(3), N(3)−U−N(4) =
61.2(3).
1414
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
pseudo-C2 axis that crosses the midpoint of the C−C bond
linking the two pyridyl rings, which is consistent with the
solution NMR data. The coordination geometry for the
diiodide salan complex is irregular.
The U−O (2.118(8) and 2.128(8) Å) and U−N(salan)
(2.704(9) and 2.702(9) Å) bond distances are slightly shorter
than those found for compound 2, certainly due to an increased
steric congestion around the uranium center in complex 2
imposed by the coordination of two tetradentate salan ligands.
The average U−N(bipy) bond distance of 2.668(15) Å is
similar to U−N(bipy) bond distances reported for the U(IV)
complexes [UCl4(dmpe)(Me2bipy)] (dmpe = 1,2-bis-
(dimethylphosphino)ethane; average 2.642(6) Å)63 and [U-
(OSiMe3)2I2(bipy)2] (average 2.658(5) Å).64 The C−C bond
distance linking the two pyridyl rings of 1.515(18) Å is
comparable with that of 1.490(3) Å in free bipyridine and in
other reported bipyridine uranium complexes,65−67 confirming
the C−C single-bond character. The bipyridine ligand is not
planar, as expected for a neutral bipyridine upon coordination
to a metal,68 presenting a torsion angle between the two pyridyl
rings of 17.9(8)°.
Figure 4. ORTEP view of the solid-state molecular structure of
[U(salan-tBu2)Cl2(bipy)] (5) (50% probability ellipsoids; one of the
tBu groups could not be refined anisotropically). Hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths (Å)
and angles (deg): U−O(1) = 2.163(3), U−O(2) = 2.151(3), U−N(1)
= 2.702(4), U−N(2) = 2.674(4), U−N(3)(bipy) = 2.677(4), U−
N(4)(bipy) = 2.677(4), U−Cl(1) = 2.7356(11), U−Cl(2) =
2.6985(12); O(1)−U−O(2) = 151.63(12), Cl(1)−U−Cl(2) =
144.37(4), N(1)−U−N(2) = 67.58(12), N(3)−U−N(4) = 61.07(14).
The complex [U(salan-Me2)I2(CH3CN)] (4) crystallizes
with one independent monomeric complex in the asymmetric
unit. The molecular structure is presented in Figure 3.
coordination sphere around the uranium center is highly
distorted.
The average U−O, U−N, and U−N(bipy) bond distances
are almost identical with those found for complex 3. The
average U−Cl bond lengths of 2.717(1) Å in 5 is within the
values for other uranium(IV) chlorides reported in the
literature (2.617(3)−2.719(1) Å).69,70 The C−C bond distance
linking the two pyridyl rings of 1.466(8) Å and the torsion
angle between the two pyridyl rings of 15.9(3)° are also
characteristic of the coordination of a neutral bipyridine ligand
to the uranium center.
Thorium(IV) salan-R2 Complexes. On the basis of the
information obtained with uranium, we have also investigated
the possibility of preparing analogous thorium(IV) salan
complexes.
Figure 3. ORTEP view of the solid-state molecular structure of
[U(salan-Me)I2(CH3CN)] (4) (50% probability ellipsoids). Hydrogen
atoms and solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg): U−O(1) = 2.078(5), U−N(1) =
2.700(5), U−N(2) = 2.591(7), U−I(1) = 3.0754(5); O(1)−U−
O(1)A = 153.2(2), I(1)−U−I(1)A = 173.55(2), O(1)−U−N(2) =
76.58(12), O(1)−U−N(1) = 69.42(16), N(1)−U−N(1)A = 68.1(2),
O(1)−U−I(1) = 91.46(12).
The reaction of ThCl4(dme)2 with K2(salan-tBu2) in
tetrahydrofuran, at room temperature, in a metal to ligand
ratio of 1/2 gave a mixture of the mono- and bis-ligand
complexes. It was not possible to isolate the complex
[Th(salan-tBu2)2] (6) in an analytically pure form. Only a
few crystals suitable for X-ray diffraction analysis were obtained
from a saturated n-hexane solution, which allowed us to
confirm the presence in the reaction mixture of the bis-ligand
thorium complex 6.
The structure shows that the metal is six-coordinated in a
pentagonal-bipyramidal geometry, with the two iodide atoms
occupying the apical positions of the bipyramid (I1−U−I1A =
173.55(2)°). The acetonitrile nitrogen atom and the ONNO-
salan donors lie in the equatorial plane with X−M−X angles
ranging between 68.1(2) and 76.58(12)°. The U−O (2.078(5)
Å), U−N(salan) (2.700(5) Å), and U−I bond lengths
(3.0754(5) Å) are shorter than those observed for 2 and 3,
as expected for a less sterically crowded complex with a lower
coordination number.
As depicted in Scheme 4, the reaction of ThCl4(dme)2 with 1
equiv of K2(salan-tBu2) in dme, at room temperature, afforded
the monosubstituted thorium complex [Th(salan-tBu)-
Cl2(dme)] (7) as a white solid in good yield.
The thorium diamine bis(phenolate) complex 7 was fully
1
characterized by ESI/MS and H and 13C NMR spectroscopy.
The 1H NMR spectra of 7 in benzene-d6, at room temperature,
display chemical shifts in the range of 0−8 ppm with well-
resolved coupling patterns, consistent with a diamagnetic
compound and also reflecting the presence of C2-symmetric
complexes in solution, as observed for the dihalide uranium-
(IV) salan complexes. The ancillary ligand displays one set of
resonances for the phenolate groups and one resonance for the
methyl protons, while the diastereotopic methylenic bridge
protons appear as two doublets and the diastereotopic benzylic
The complex [U(salan-tBu2)Cl2(bipy)] (5) crystallized by
slow diffusion of n-hexane into a saturated toluene solution
with one toluene molecule in the asymmetric unit. The X-ray
diffraction structure shows that compound 5 is isomorphous
with 3 (Figure 4), with a pseudo-C2 axis crossing the C−C
bond linking the two pyridyl rings of the bipyridine. As
observed for the diiodide uranium salan complex, the
1415
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
Scheme 4. Synthesis of Mono-salan Thorium(IV) Chloride
Complexes
Figure 5. ORTEP view of the solid-state molecular structure of
[Th(salan-tBu2)2] (6) (50% probability ellipsoids) in 6·C6H14.
Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Th−O(1) = 2.2755(18),
Th−O(2) = 2.2950(15), Th−N(1) = 2.8194(19), Th−N(2) =
2.807(2); O(1)−Th−O(1A) = 147.03(8), O(2)−Th−O(2A) =
144.37(8), N(1)−Th−N(1A) = 60.17(8), N(2)−Th−N(2A) =
62.38(9).
protons appear as a typical AB discrete doublet at 5.24 and 2.77
ppm. The difference in chemical shifts observed for the
diastereotopic benzyl protons is remarkable and more
pronounced than those observed for salan−transition-metal
complexes.53,71 An analogous observation was reported for the
amine bis(phenolate) complex [Th(ONOO)Cl2] (ONOO =
MeOCH2CH2N(CH2-(2-O-C6H2-But2-3,5))2,72 indicating that
one set of the benzylic protons is closer to the electropositive
metal center. The signals corresponding to the methylene
groups of the coordinated dme in [Th(salan-tBu2)Cl2(dme)]
(7) are very broad. Decreasing the temperature of the NMR
sample to −10 °C, in toluene-d8, allowed the observation of
two doublets at 3.76 and 2.65 ppm for the diastereotopic CH2
protons of the symmetrically coordinated dme molecule. The
C2 symmetry in solution was also confirmed by the 13C NMR
data. Although the NMR data and the other analyses were
consistent with the formulation proposed, the solid-state
structures of the dme adduct could not be determined.
(salan-tBu2) complex 2, which can be explained by the larger
ionic radius of Th(IV).73 The mean value of the Th−O bond
distance (2.285(2) Å) is similar to that found in the amine
bis(phenolate) complex [Th(ONOO)2] (2.266(4) Å).72
Crystals suitable for X-ray diffraction analysis were obtained
in the case of [Th(salan-tBu2)Cl2(bipy)] (8) by slow diffusion
of n-hexane into a saturated toluene solution. Complex 8
crystallizes in the trigonal space group R3 as a toluene solvate.
The arrangement of the donor atoms around the thorium atom
in 8 results in a crystallographic C1 symmetry but approximates
a C2 symmetry; small variations of the angles in solution could
be enough to result in a symmetrical compound in solution, as
observed by NMR analysis.
Compound 8 is isomorphous with the uranium complex 5
(Figure 6) and, as expected, has an identical molecular structure
1H NMR studies in benzene-d6 showed that the coordinated
dme molecule in 7 is quantitatively replaced by the 2,2′-
bipyridine chelate. The same reaction carried out in toluene
(Scheme 4) led to the isolation of the complex [Th-
(salan-tBu2)Cl2(bipy)] (8) as a white solid in 79% yield. The
1H NMR spectrum of 8 is consistent with an overall C2-
symmetric structure in solution, in agreement with the pseudo-
C2 symmetry found in the solid state for 8 (see below). The 13C
1
NMR spectra confirmed all the conclusions drawn from H
NMR spectral analysis. The proton and carbon chemical shift
assignments for compounds 7 and 8 were made on the basis of
1
1H−1H COSY and H−13C HSQC experiments.
Crystal and Molecular Structures of the Thorium
salan Complexes. The complex [Th(salan-tBu2)2] (6)
crystallizes in the monoclinic space group C2/c as an n-hexane
solvate and is isostructural with [U(salan-tBu2)2] (2) (Figure
5). Complex 6 exhibits a distorted-dodecahedral geometry, in
which the two orthogonal trapezoids are defined by O2−O2A−
N1−N1A and O1−O1A−N2−N2A with a dihedral angle of
87.85(5)°. As observed for the uranium complex, the
thorium(IV) complex presents a crystallographically imposed
C2 symmetry, which is consistent with the solution NMR data.
The Th−O (2.276(2) and 2.295(2) Å) and Th−N (2.807(2)
and 2.819(2) Å) bond lengths are slightly longer than the
corresponding distances observed for the uranium bis-
Figure 6. ORTEP view of the solid-state molecular structure of
[Th(salan-tBu2)Cl2(bipy)] (8) in 8·1.5C7H8 (50% probability
ellipsoids). Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Th−O(1) =
2.227(3), Th−O(2) = 2.225(3), Th−N(1) = 2.748(4), Th−N(2) =
2.707(4), Th−N(3)(bipy) = 2.717(4), Th−N(4)(bipy) = 2.724(4),
Th−Cl(1) = 2.7647(11), Th−Cl(2) = 2.7342(12); O(1)−Th−O(2) =
150.03(11), Cl(1)−Th−Cl(2) = 145.54(4), N(1)−Th-N(2) =
66.30(11), N(3)−Th−N(4) = 59.42(11).
1416
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
Scheme 5. Synthesis of Mono-salan Thorium(IV) and Uranium(IV) Alkyl Complexes
and very similar bond distances and angles. The bond distances,
with differences of 0.07 Å for M−O, 0.04 Å for M−N(salan),
0.04 Å for M−N(bipy), and 0.05 Å for M−Cl, are in line with
the 0.05 Å difference in ionic radii for U(IV) and Th(IV).73
The average thorium−oxygen bond length is 2.228(6) Å and
compares well with the value of 2.266(4) Å found in the eight-
coordinate amine bis(phenolate) complex Th(ONOO)2.72 The
pyridyl rings of the bipyridine ligand are twisted with respect to
each other, with a torsion angle of 8.3(2)°. The value of the
bond distance (1.486(8) Å) for the C−C linking the two
pyridyl rings is nearly identical with those found in the free
bipyridine and in the recently structurally characterized
complex [{η5-1,2,4-(Me3C)C5H2)}Th(SePh)3(bipy)]
(1.484(9) Å)74 and is longer than the value found for the
Figure 7. ORTEP view of the solid-state molecular structure of
[Th(salan-tBu2)(CH2SiMe3)2 ] (9) (50% probability ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Th−O(1) = 2.202(2), Th−N(1) = 2.706(2), Th−
C(18) = 2.529(3); O(1)−Th−O(1A) = 154.57(10), C(18)−Th−
N(1A) = 155.16(10), Th−C(18)−-Si(1) = 118.3(1), C(18)−Th−
N(1) = 92.61(9), N(1)−Th−N(1A) = 64.17(11), C(18)−Th−
C(18A) = 111.34(15).
thorium complex [{η5-1,2,4-(Me3C)C5H2)}2Th(bipy)]74
(1.382(8) Å) with a dianionic bipyridine ligand. The average
Th−N(bipy) bond distance in 8 (2.720(6) Å) is much longer
than the value found in the cyclopentadienyl thorium complex
with anionic bipyridine (2.344(5) Å), as expected, and slightly
longer than the average Th−N bond distance (2.609(5) Å) in
the cyclopentadienyl complex with neutral bipyridine.
Thorium(IV) and Uranium(IV) Alkyl Derivatives. The
reaction of [Th(salan-tBu2)Cl2(dme)] (7) with 2 equiv of
LiCH2SiMe3 in toluene at room temperature led to the
formation of the Th(IV) dialkyl complex [Th(salan-tBu2)-
(CH2SiMe3)2] (9) (Scheme 5).
The dialkyl complex was obtained as a white crystalline solid
in 89% yield. Single crystals of [Th(salan-tBu2)(CH2SiMe3)]
(9) crystallizing in the centrosymmetric monoclinic C2/c space
group were grown by slow evaporation of a toluene/n-hexane
solution. The thorium center is hexacoordinated (Figure 7) and
adopts a distorted-octahedral geometry, in which the phenolate
oxygens are mutually trans with an O−Th−O angle of
154.57(10)°, as observed for group 4 salan complexes.53 The
alkyl groups are cis, as well as the amine nitrogen atoms of the
ancillary ligand, with C−Th−C and N−Th−N angles of
111.34(15) and 64.67(11)°, respectively.
Consistent with the solution NMR data, the two alkyl groups
are symmetrical, as the structure possesses an imposed
crystallographic C2 axis, and the Th−C bond distance of
2.529(3) Å is comparable to those reported for the
bis(trimethylsilylmethyl) complexes [(XA2)ThR2] (H2-XA2 =
4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethyl-
xanthene) (2.476(6) Å),34 [{Me2Si(C5Me4)2}ThR2] (2.52(2)
Å),75 [(2,6-tBu2H3C6O)2ThR2] (2.46(2) Å),27 [Cp*-
(2,6-tBu2H3C6O)ThR2] (2.474 Å),76 and [(iPr2PhNCOCN)Th-
(CH2SiMe3)2] (2.50(1) Å).77
The Th−N bond length of 2.706(2) Å and the Th−O bond
distance of 2.202(2) Å are shorter than those in complex 8, due
to the lower coordination number. The Th−C−Si angle in 9 is
118.29(1)°, which is close to the angle expected for a sp3-
hybridized carbon atom.
Structurally characterized thorium dialkyl complexes sup-
ported by noncarbocyclic ligands are rare,27,32.34,77 and to the
best of our knowledge, 9 represents the first structurally
characterized dialkyl actinide complex supported by a dianionic
tetradentate ligand.
The 1H and 13C NMR data (Figures S9 and S10, Supporting
Information) of 9 corroborate the formation of the dialkyl
complex, and as found for the other thorium complexes, one set
of resonances is observed for the diamine bis(phenolate)
ligand, consistent with a C2-symmetric molecular structure in
solution. As a result of the ligand coordination, the benzylic and
methylenic protons are diastereotopic, giving rise to one 1H AB
spin system and two doublets, respectively. The Th−CH2
resonances give rise to two doublets at 0.30 and 0 ppm with
1417
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
Scheme 6. Reaction of the Dialkyl An(IV) Complexes with 1 atm of CO2 at Room Temperature
dialkyl complexes.75−77,79 and for dialkyl complexes of Th(IV)
containing monoanionic monodentate tert-butylphenoxides as
ancillary ligands.27
2J(H,H) = 10.2 Hz in the 1H NMR and one signal at 87.2 ppm
in the carbon spectrum. The chemical shifts are in conformity
with other thorium bis(trimethylsilylmethyl) complexes
reported.27,32,34,75,77 The H and 13C resonances of 9 were
1
Reactivity of An−Bis(alkyl) Complexes with CO2.
Insertion of small molecules into metal−carbon bonds is an
important step in many metal-mediated chemical trans-
formations. Carbon dioxide insertion into metal−carbon
bonds has been extensively explored for transition metals and
lanthanides and yields carboxylates.80−85 The insertion of CO2
into an actinide−carbon bond has been observed in the
presence of carbocyclic ancillary ligands,4,22,23,41,86−89 the first
example being reported by Marks and Moloy in 1985,23 but is
more rare and only four examples of structurally characterized
insertion products have been reported.20,22,41,89 Preliminary
experiments on the reaction of the [An(salan-tBu2)-
(CH2SiMe3)2] complexes (An = Th (9), U (10)) with CO2
assigned with the support of 1H−1H COSY and 1H−13C
HSQC spectra.
The analogous complex [U(salan-tBu2)(CH2SiMe3)2] (10)
was synthesized by reacting [U(salan-tBu2)Cl2(thf)x], prepared
in situ from the reaction of UCl4 with 1 equiv of
[K2(salan-tBu2)] in tetrahydrofuran, with 2 equiv of
LiCH2SiMe3 in toluene (Scheme 5). Crystals of 10 were
obtained by slow evaporation of an n-hexane solution. The X-
ray diffraction analysis showed that the uranium bis(alkyl) is
isostructural78 with 9. However, the crystallographic data were
not of very good quality, and therefore the structure will not be
discussed in detail. Nevertheless, the structure possesses
enough quality to indicate atom connectivity, showing that
the uranium(IV) center is six-coordinated by the κ4-ONNO
salan ligand and by two trimethylsilylmethyl groups.
1
were carried out on an NMR scale and followed by H NMR
and ESI/MS. Treatment of a toluene-d8 solution of 9 with
excess CO2 at room temperature, over 1 h, led to disappearance
1
1
of the H NMR resonances corresponding to the bis(alkyl)
The H NMR spectrum of 10 is paramagnetically shifted,
species 9 and the emergence of broad resonances in a very
simple spectrum (Figure S12-a, Supporting Information),
where the Si(CH3)3 protons appear as a broad singlet at 0.81
ppm and the aromatic protons of the salan-tBu phenolate
groups appear as two broad signals at 7.48 and 6.89 ppm. When
the sample temperature was lowered, it was possible to observe
at −50 °C the split of the resonance at 0.81 ppm into various
singlets with different integrations (Figure S12-b, Supporting
Information). However, the identification and assignment of
and the resonances were assigned on the basis of the signal
integration (Figure S11, Supporting Information) is consistent
with a C2-symmetric molecular structure in solution. The
diastereotopic U−CH2 protons are significantly upfield at
−236.4 and −238.5 ppm, probably due to the close proximity
of the CH2 protons to the paramagnetic center.
Both An−dialkyl complexes are stable at room temperature
for several weeks in benzene-d6 or toluene-d8 solution but
decompose 50% and 25% for [Th(salan-tBu2)(CH2SiMe3)2]
(9) and [U(salan-tBu2)(CH2SiMe3)2] (10), respectively, over 9
h at 70 °C in toluene-d8. The small differences found in the
stabilities of 9 and 10 probably result from the different degrees
of steric saturation of the coordination spheres of these actinide
metals. Higher stabilities have been reported for thorium alkyls
of highly rigid dianionic bidentate noncarbocyclic ligands,34 but
the stability of complexes 9 and 10 is comparable to the
thermal stability reported for actinide(IV) cyclopentadienyl
1
the H NMR spectrum proved to be difficult due to signal
overlapping. The NMR analysis indicates that the reaction
[Th(salan-tBu2)(CH2SiMe3)2]/excess CO2 produces complex
product mixtures, which might be involved in fluxional
behavior.
Treating the thorium bis(alkyl) complex with 2 equiv of CO2
led to the observation of 1H NMR spectra, at 20 °C and at low
temperature, with profiles similar to those observed in the
1418
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
reaction with excess CO2. The 13C NMR at 20 °C also
presented broad resonances (Figure S13, Supporting Informa-
tion), but it was possible to observe resonances at 190.3 and
−1.26 ppm that could be attributed to carboxylate90 and
Si(CH3)3 carbon resonances, respectively. The complexity of
the NMR spectra indicates the presence of an equilibrium
between different products or isomers in solution, which could
result from different coordination modes of the CO2 fragment
(Scheme 6).91
The ESI/MS analysis of the CO2 reactions (Figure S14,
Supporting Information), in the positive ion mode, showed two
peaks at m/z 1039.9 and 2056.1 corresponding to [Th-
(salan-tBu2)(O2CCH2SiMe3)2Na]+ and [{Th(salan-tBu2)-
(O2CCH2SiMe3)2}2Na]+ (the sodium cation is present in the
electrospray source), respectively, and in the negative ion mode
one peak at m/z 1051.9 corresponding to [Th(salan-tBu2)-
(O2CCH2SiMe3)2Cl]−. These results are consistent with the
formation of monomeric and dimeric insertion products of
CO2 into the two Th−C bonds. The ESI/MS studies also
indicated that the CO2 insertion occurred into the Th−
C(alkyl) bond and not into the Th−salan ligand bonds, as
collision-induced dissociation (CID) experiments with [Th-
(salan-tBu2)(O2CCH2SiMe3)2Na]+ showed elimination of
neutral sodium carboxylate [NaO2CCH2SiMe3] (Figure S15,
Supporting Information).
Several attempts were made to obtain crystals of a bis-CO2
insertion thorium compound. In one of these attempts, a few
colorless crystals were obtained by slow evaporation of a
toluene solution of the [Th(salan-tBu2)(CH2SiMe3)2]/CO2
reaction mixture, which were identified as the dimeric complex
[Th2Cl(salan-tBu2)2(μ-η1:η1-O2CCH2SiMe3)2(μ-η1:η2-
O2CCH2SiMe3)] (11) by X-ray diffraction analysis. The
ORTEP diagram and selected bond lengths and angles are
depicted in Figure 8.
Complex 11 contains two thorium centers bridged by two
symmetrical μ-η1:η1 and one unsymmetrical μ-η1:η2 carboxylato
ligands. The Th···Th distance is 4.7536(3) Å. Each thorium
atom is eight-coordinated, however, with different coordination
spheres. One thorium atom is coordinated by the two
symmetrically bridging carboxylato oxygen atoms, by two η2
oxygen atoms of the unsymmetrically bridging μ-η1:η2
carboxylato ligand, and by the tretradentate salan ligand. The
other thorium atom is coordinated by two μ-η1:η1 bridging
carboxylato oxygen atoms, by one oxygen atom of the μ-η1:η2
carboxylato ligand, by one chloride atom, and by the
tetradentate ONNO chelate. The coordination geometry
around the two thorium centers is described as a distorted
dodecahedron, where the two nearly orthogonal trapezoids for
Th(1) (88.2(1)°) are defined by N1−N2−O2−O5 and O1−
O10−O9−O7, with deviations from planarity of 8.2(2) and
13.8(2)°, and for Th(2) (88.7(1)°) defined by N3−N4−O8−
O3 and O4−Cl1−O9−O6.
The Th−N and Th−O(Ar) distances are similar to those
found for 6 and 8. However, the O(Ph)−Th−O(Ph) angles of
104.9(2) and 98.3(1)° are significantly smaller than those
found for 6 and 8, which present values close to 150°. This
indicates that the salan-tBu2 ligand adopts a different
conformation in the dimeric complex 11, demonstrating its
flexibility and ability to adapt to diverse metal environments.
The average Th−O bond lengths (2.406(3) Å) of the
bridging (bidentate) carboxylato ligand are in agreement with
previously reported Th−O bond lengths involving η1:η1
bridging carboxylato ligands.92,93 The Th−O bond lengths of
Figure 8. ORTEP view of the solid-state molecular structure of
[Th2 Cl(salan-t Bu2 )2 (μ-η1 :η1 -O2 CCH2 SiMe3 )2 (μ-η1 :η2 -
O2CCH2SiMe3)] (11) (50% probability ellipsoids). Hydrogen atoms
and solvent molecules are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Th(1)−O(1) = 2.200(3), Th(1)−O(2) =
2.196(3), Th(2)−O(3) = 2.198(3), Th(2)−O(4) = 2.213(3), Th(1)−
N(1) = 2.778(4), Th(1)−N(2) = 2.757(4), Th(2)−N(3) = 2.751(4),
Th(2)−N(4) = 2.813(4), Th(1)−O(5) = 2.379(3), Th(1)−O(7) =
2.407(3), Th(1)−O(9) = 2.611(3), Th(1)−O(10) = 2.552(4); O(1)−
Th−O(2) = 104.94(12), O(3)−Th−O(4) = 98.29(11), average O−
C−O(carboxylato) = 123.35(75), N(1)−Th(1)−N(2) = 65.85(11),
N(3)−Th(2)−N(4) = 65.43(11), O(9)−C−O(10) = 123.8(5).
the μ-η1:η2 carboxylato ligand are longer, ranging from 2.552(4)
to 2.611(3) Å, and are comparable with the average value found
in the complex [Th(HIDA)2(C2O4)] (2.59(3) Å; H2IDA =
iminodiacetic) containing a bidentate chelate carboxylato
group.93 The similar values of the O−C bond lengths
(1.262(6) and 1.259(7) Å; 1.252(7) and 1.276(5) Å;
1.305(8) and 1.334(8) Å) indicate delocalization of an electron
over the O−C−O group in the three fragments.
While the structure of the product of the insertion of CO2
into a Th(IV)−arene bond has been reported,89 complex 11 is,
to the best of our knowledge, the first example of a
crystallographically characterized thorium(IV) carboxylate
complex prepared by insertion reaction of carbon dioxide
into a Th−C(alkyl) bond.
The structure of 11 shows that CO2 insertion into the Th−
C(alkyl) bonds occurred, with one thorium atom coordinated
to three carboxylato ligands and the other thorium atom
coordinated to three bridging carboxylates and one chloride.
The presence of the coordinated chloride ligand is ascribed to
the presence of small amounts of LiCl in the sample of complex
9 reacted with CO2. Attempt to obtain crystals from the
reaction of higher purity samples of 9 with CO2 have failed so
far. The ESI/MS analysis of the reaction mixture [Th-
(salan-tBu2)(CH2SiMe3)2]/CO2 after addition of excess LiCl
(Figure S16, Supporting Information) showed a significant
increase of the peak at m/z 955.9, corresponding to
[Th(salan-tBu)(O2CCH2SiMe3)Cl2]−, and a decrease of the
peak corresponding to [Th(salan-tBu)(O2CCH2SiMe3)2Cl]−
(m/z 1051.9), indicating that a carboxylate ligand is easily
replaced by a chloride. A complex with four bridging
carboxylates is sterically crowded, and the presence of LiCl in
the solution facilitates the displacement of a carboxylate ligand
1419
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
by a small ligand, such as the chloride. Removal of carboxylate
by a chloride ligand after CO2 insertion into the metal−carbon
bond has been previously reported and could be used to
complete a cycle by regenerating the halide precursor.20,41,94
When an analogous reaction with 2 equiv of CO2 was
performed with the uranium bis(alkyl) 10, the reaction proved
CONCLUSIONS
■
The ability of tetradentate salan-R22− (R = Me, tBu) to act as a
supporting ligand in low-valent uranium chemistry has been
explored. The salan ligands did not allow the synthesis of stable
complexes of U(III), and the reaction of U(III) with K2salan-R2
induced disproportionation to U(IV) and U(0), affording
mixtures of bis-ligand and monoligand complexes.
1
to be slower. After 1 h at room temperature, the H NMR
Stable mononuclear homoleptic bis-ligand complexes of
U(IV) can be prepared in good yield by salt metathesis by
reacting U(IV) halides with the K2salan-R2 ligands independ-
ently of the ligand bulk (R = Me, tBu). In contrast,
mononuclear heteroleptic complexes of the formula [An-
(salan-tBu2)Cl2] can only be isolated in pure form for the bulky
salan-tBu2 ligand. The reactions of U(IV) and Th(IV) halides
with K2salan-Me2 always afford a mixture of mono- and bis-
ligand complexes independently of the metal to ligand ratios,
due to the high stability of the bis-ligand complexes. These
results show that the ligand substituents can be used to favor
the formation of the targeted compound in these salan systems.
The halide complexes [An(salan-tBu2)Cl2] provide versatile
precursors for the synthesis of stable dialkyl complexes.
Preliminary studies show that CO2 inserts rapidly into the
An(IV)−C(alkyl) bond after reaction of the salan-tBu2-
supported dialkyl complexes at room temperature with 1 atm
of CO2, leading to multiple insertion products. The structure of
the insertion product isolated for thorium shows that insertion
of CO2 occurs in both alkyl groups and that the resulting
carboxylate is easily displaced by a chloride anion, probably as a
result of the important steric bulk of the salan-tBu2 ligand.
Finally, we have demonstrated that salan ligands provide a
new example of effective supporting ligands in organoactinide
chemistry. Future studies will be directed to investigate the
influence of the ligand architecture on the structure and
reactivity of dialkyl complexes of actinide(IV) using aminephe-
nolate ligands with different substituents and with more rigid
amine scaffolds.
spectrum still displayed resonances of the starting complex 10,
and several new resonances appeared. A group of relatively
sharp resonances with six distinguishable resonances, integrat-
ing for 9 protons each (Figure S17, Supporting Information),
was observed and assigned to four tBu groups, one SiMe3 of the
alkyl ligand, and one SiMe3 group of a carboxylato ligand. This
is consistent with the existence in solution of a reduced-
symmetry system, which could attributed to the formation of an
intermediate mono-CO2 insertion product of the type “[U-
(salan-tBu2)(O2CCH2SiMe3)(CH2SiMe3)]”. Additionally, the
observation of two resonances strongly shifted upfield
(−221.80 and −233.52 ppm) and typically assigned to U−
CH2 protons also supports this interpretation. After 18 h of
reaction, the signals assigned to the mono-CO2 insertion
product disappeared (Figure S19, Supporting Information).
The proton NMR after 18 h shows two sets of signals for which
the most intense peaks could be clearly identified. One set of
signals consists of two resonances at 47.23 (18H) and 10.28
ppm (18H), one resonance at −71.80 ppm (6H), and one
resonance at −12.439 ppm (18H), which could be assigned to
the tBu groups of the aromatic phenolates, to the NMe3 groups,
and to the SiMe3 groups of the carboxylato ligands, respectively.
This is consistent with the presence of a high-symmetry species,
t
with the chemical shifts of the Bu groups falling in the same
spectral region as those observed for the other C2-symmetric
monomeric uranium compounds described in this work. These
observations led us to assign these signals to a monomeric
compound of the type [U(salan-tBu2)(O2CCH2SiMe3)2]
(complex A in Scheme 6), presenting the same coordination
mode for the two carboxylate ligands. The second sets of
signals of resonances present six resonances of equal intensity,
which could be assigned to four tBu groups and two carboxylate
ligands in a dimeric compound containing two carboxylate
ligands with different coordination modes, as observed for the
thorium CO2 insertion product 11. However, an X-ray
diffraction structure is required to confirm the nuclearity and
coordination mode of the carboxylate ligand in these species.
The presence of other signals on the spectrum, corresponding
probably to other minor species and some decomposition
products which could arise from the loss of carboxylate ligands,
rendered difficult the full assignment of the 1H NMR spectrum
and the isolation of an analytically pure product. The high
solubility of the [U(salan-tBu2)(CH2SiMe3)2]/CO2 reaction
mixture has so far prevented the isolation of single crystals
adequate for X-ray diffraction analysis.
ASSOCIATED CONTENT
* Supporting Information
CIF files giving complete crystallographic data for the crystal
■
S
1
structures of 2−6, 8, 9, and 11 and figures giving H NMR
spectra for compounds 2−5, 9, and 10, and NMR and ESI-MS
spectra of the An−bis(alkyl) reactions with CO2. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*M.M.: tel, (+33)438783955; fax, (+33)438785090; e-mail,
marinella.mazzanti@cea.fr. L.M.: tel, (+351) 21-994 6197; e-
mail, leonorm@itn.pt.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
E.M., L.M., I.C.S., A.C., J.M.C., and J.M. are grateful to the
■
The ESI/MS analysis results (Figure S20, Supporting
Information) are in agreement with the formation of
monomeric and dimeric bis-CO2 insertion products. CID
experiments with [U(salan-tBu2)(O2CCH2SiMe3)2Na]+ also
showed elimination of neutral sodium carboxylate
[NaO2CCH2SiMe3] (Figure S21, Supporting Information), as
observed for the case of thorium, confirming that the CO2
insertion occurred into the U−C(alkyl) bonds.
Fundaca
̧
o para a Cien
̂
cia e a Tecnologia (FCT) for funding:
̃
research project PTDC/QUI/66187/2006, RNEM-Rede Na-
cional de Espectrometria de Massa/ITN Node (REDE/1503/
REM/2005), and “Cien
and M.M. acknowledge support from the Commissariat a
ire, RBPCH
̂
cia 2008” Programme. B.B., C.C., J.P.,
̀
l′Energie Atomique, Direction de l′Energie Nuclea
́
program and by the “Agence Nationale de la Recherche”
1420
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
(36) Monreal, M. J.; Diaconescu, P. L. J. Am. Chem. Soc. 2010, 132,
7676.
(37) Monreal, M. J.; Khan, S.; Diaconescu, P. L. Angew. Chem., Int.
(ANR-10-BLAN-0729). This work also benefited from COST
Action CM1006-EUFEN: European F-Element Network.
Ed. 2009, 48, 8352.
(38) Broderick, E. M.; Gutzwiller, N. P.; Diaconescu, P. L.
Organometallics 2010, 29, 3242.
(39) Hayes, C. E.; Platel, R. H.; Schafer, L. L.; Leznoff, D. B.
Organometallics 2012, 31, 6732.
(40) Hayes, C. E.; Leznoff, D. B. Organometallics 2010, 29, 767.
(41) Matson, E. M.; Forrest, W. P.; Fanwick, P. E.; Bart, S. C. J. Am.
Chem. Soc. 2011, 133, 4948.
(42) Cohen, A.; Yeori, A.; Kopilov, J.; Goldberg, I.; Kol, M. Chem.
Commun. 2008, 2149.
(43) Yeori, A.; Goldberg, I.; Shuster, M.; Kol, M. J. Am. Chem. Soc.
2006, 128, 13062.
(44) Liu, X. L.; Shang, X. M.; Tang, T.; Hu, N. H.; Pei, F. K.; Cui, D.
M.; Chen, X. S.; Jing, X. B. Organometallics 2007, 26, 2747.
(45) Horeglad, P.; Nocton, G.; Filinchuk, Y.; Pecaut, J.; Mazzanti, M.
Chem. Commun. 2009, 1843.
(46) Nocton, G.; Horeglad, P.; Vetere, V.; Pecaut, J.; Dubois, L.;
Maldivi, P.; Edelstein, N. M.; Mazzanti, M. J. Am. Chem. Soc. 2010,
132, 495.
(47) Vetere, V.; Maldivi, P.; Mazzanti, M. C. R. Chim. 2010, 13, 876.
(48) Clark, D. L.; Sattelberger, A. P.; Andersen, R. A. Inorg. Synth.
1997, 31, 307.
(49) Sun, Y.; McDonald, R.; Takats, J.; Day, V., W.; Eperspacher, T.
A. Inorg. Chem. 1994, 33, 4433.
(50) Hermann, J. A.; Suttle, J. F. Inorg. Synth. 1957, 5, 143.
(51) Cantat, T.; Scott, B. L.; Kiplinger, J. L. Chem. Commun. 2010,
46, 919.
REFERENCES
■
(1) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550.
(2) Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. Nature 2008,
455, 341.
(3) Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A.
L.; Meyer, K. Science 2004, 305, 1757.
(4) Lam, O. P.; Meyer, K. Polyhedron 2012, 32, 1.
(5) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.;
Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 6108.
(6) Arnold, P. L.; Turner, Z. R.; Bellabarba, R. M.; Tooze, R. P.
Chem. Sci. 2011, 2, 77.
(7) Camp, C.; Mougel, V.; Horeglad, P.; Pecaut, J.; Mazzanti, M. J.
Am. Chem. Soc. 2010, 132, 17374.
(8) Barnea, E.; Eisen, M. S. Coord. Chem. Rev. 2006, 250, 855.
(9) Stubbert, B. D.; Stern, C. L.; Marks, T. J. Organometallics 2003,
22, 4836.
(10) Barnea, E.; Andrea, T.; Berthet, J. C.; Ephritikhine, M.; Eisen,
M. S. Organometallics 2008, 27, 3103.
(11) Barnea, E.; Andrea, T.; Kapon, M.; Berthet, J. C.; Ephritikhine,
M.; Eisen, M. S. J. Am. Chem. Soc. 2004, 126, 10860.
(12) Ephritikhine, M. New J. Chem. 1992, 16, 451.
(13) Bruno, J. W.; Stecher, H. A.; Morss, L. R.; Sonnenberger, D. C.;
Marks, T. J. J. Am. Chem. Soc. 1986, 108, 7275.
(14) Marks, T. J.; Seyam, A. M. J. Am. Chem. Soc. 1972, 94, 6545.
(15) Marks, T. J.; Seyam, A. M.; Kolb, J. R. J. Am. Chem. Soc. 1973,
95, 5529.
(16) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2005,
127, 1338.
(17) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. Chem. Commun. 2005,
2591.
(18) Siladke, N. A.; Ziller, J. W.; Evans, W. J. J. Am. Chem. Soc. 2011,
(52) Enriquez, A. E.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2005, 44,
7403.
(53) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122,
10706.
(54) SADABS and ABSPACK: Area-Detector Absorption Correction;
133, 3507.
(19) Evans, W. J.; Walensky, J. R.; Furche, F.; Ziller, J. W.;
DiPasquale, A. G.; Rheingold, A. L. Inorg. Chem. 2008, 47, 10169.
(20) Webster, C. L.; Ziller, J. W.; Evans, W. J. Organometallics 2012,
31, 7191−7197.
Siemens Industrial Automation, Madison, WI, 1996.
(55) Sheldrick, G. M. SHELXS-97, 6.14 ed.; University of Gottingen,
̈
Gottingen, Germany, 2006.
̈
(56) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115.
(21) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Organometallics 2010,
29, 945.
(22) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Inorg. Chem. 2010, 49,
1743.
(57) Farrugia, L. J. J. Appl. Crystallogr. 2012, 45, 849.
(58) Odom, A. L.; Arnold, P. L.; Cummins, C. C. J. Am. Chem. Soc.
1998, 120, 5836.
(59) Lewis, A. J.; Williams, U. J.; Kikkawa, J. M.; Carroll, P. J.;
Schelter, E. J. Inorg. Chem. 2012, 51, 37.
(60) Muetterties, E. L.; Guggenberger, L. J. J. Am. Chem. Soc. 1974,
96, 1748.
(61) Vandersluys, W. G.; Sattelberger, A. P. Chem. Rev. 1990, 90,
(23) Moloy, K. G.; Marks, T. J. Inorg. Chim. Acta 1985, 110, 127.
(24) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.;
Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L.
Organometallics 2004, 23, 4682.
(25) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 4253.
(26) Fortier, S.; Melot, B. C.; Wu, G.; Hayton, T. W. J. Am. Chem.
Soc. 2009, 131, 15512.
(27) Clark, D. L.; Grumbine, S. K.; Scott, B. L.; Watkin, J. G.
Organometallics 1996, 15, 949.
(28) Turner, H. W.; Andersen, R. A.; Zalkin, A.; Templeton, D. H.
Inorg. Chem. 1979, 18, 1221.
(29) Domingos, A.; Marques, N.; Dematos, A. P.; Santos, I.; Silva, M.
Organometallics 1994, 13, 654.
(30) Duhovic, S.; Khan, S.; Diaconescu, P. L. Chem. Commun. 2010,
46, 3390.
(31) Monreal, M. J.; Diaconescu, P. L. Organometallics 2008, 27,
1702.
1027.
(62) Drew, M. G. B. Coord. Chem. Rev. 1977, 24, 179.
(63) Newell, B. S.; Schwaab, T. C.; Shores, M. P. Inorg. Chem. 2011,
50, 12108.
(64) Brown, J. L.; Mokhtarzadeh, C. C.; Lever, J. M.; Wu, G.;
Hayton, T. W. Inorg. Chem. 2011, 50, 5105.
(65) Antunes, M. A.; Pereira, L. C. J.; Santos, I. C.; Mazzanti, M.;
Marcalo, J.; Almeida, M. Inorg. Chem. 2011, 50, 9915.
(66) Maria, L.; Domingos, A.; Galvao, A.; Ascenso, J.; Santos, I. Inorg.
Chem. 2004, 43, 6426.
(67) Mehdoui, T.; Berthet, J. C.; Thuery, P.; Salmon, L.; Riviere, E.;
Ephritikhine, M. Chem. Eur. J. 2005, 11, 6994.
(68) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen,
R. A. Organometallics 2002, 21, 460.
(32) Jantunen, K. C.; Batchelor, R. J.; Leznoff, D. B. Organometallics
2004, 23, 2186.
(33) Jantunen, K. C.; Haftbaradaran, F.; Katz, M. J.; Batchelor, R. J.;
Schatte, G.; Leznoff, D. B. Dalton Trans. 2005, 3083.
(34) Cruz, C. A.; Emslie, D. J. H.; Harrington, L. E.; Britten, J. F.;
Robertson, C. M. Organometallics 2007, 26, 692.
(35) Cruz, C. A.; Emslie, D. J. H.; Jenkins, H. A.; Britten, J. F. Dalton
Trans. 2010, 39, 6626.
(69) Cantat, T.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Inorg.
Chem. 2009, 48, 2114.
(70) Thomson, R. K.; Graves, C. R.; Scott, B. L.; Kiplinger, J. L. Inorg.
Chim. Acta 2011, 369, 270.
1421
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422

Organometallics
Article
(71) (a) Shimrit Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.;
Kol, M. Inorg. Chem. 2006, 45, 4783−4790. (b) Balsells, J.; Carroll, P.
J.; Walsh, P. J. Inorg. Chem. 2001, 40, 5568−5574.
(72) Andrea, T.; Barnea, E.; Botoshansky, M.; Kapon, M.; Genizi, E.;
Goldschmidt, Z.; Eisen, M. S. J. Organomet. Chem. 2007, 692, 1074.
(73) Shannon, R. D. Acta Crystallogr. Sect. A 1976, 32, 751.
(74) Ren, W. S.; Zi, G. F.; Walter, M. D. Organometallics 2012, 31,
672.
(75) Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A.
J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40.
(76) Butcher, R. J.; Clark, D. L.; Grumbine, S. K.; Scott, B. L.;
Watkin, J. G. Organometallics 1996, 15, 1488.
(77) Hayes, C. H.; Platel, R. H.; Schafer, L. L.; Leznoff, D. B.
Organometallics 2012, 31, 6732.
(78) Crystal data for [U(salan-tBu2)(CH2SiMe3)2] (10): monoclinic,
space group Cc, a = 20.1132(8) Å, b = 10.3705(4) Å, c = 21.8851(9)
Å, β = 92.249(1)°, V = 4565.31(5) Å3.
(79) Fendrick, C. M.; Schertz, L. D.; Day, V. W.; Marks, T. J.
Organometallics 1988, 7, 1828.
(80) Correa, A.; Martin, R. Angew. Chem., Int. Ed. 2009, 48, 6201.
(81) Darensbourg, D. J.; Kudaroski, R. A. Adv. Organomet. Chem.
1983, 22, 129.
(82) Zhou, X. G.; Zhu, M. J. Organomet. Chem. 2002, 647, 28.
(83) Evans, W. J.; Rego, D. B.; Ziller, J. W.; DiPasquale, A. G.;
Rheingold, A. L. Organometallics 2007, 26, 4737.
(84) Evans, W. J.; Miller, K. A.; Ziller, J. W. Inorg. Chem. 2006, 45,
424.
(85) Liu, R. T.; Zhou, X. G. J. Organomet. Chem. 2007, 692, 4424.
(86) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Organometallics 2010,
29, 945.
(87) Evans, W. J.; Siladke, N. A.; Ziller, J. W. C. R. Chim. 2010, 13,
775.
(88) Trnka, T. M.; Bonanno, J. B.; Bridgewater, B. M.; Parkin, G.
Organometallics 2001, 20, 3255.
(89) Korobkov, I.; Gambarotta, S. Organometallics 2004, 23, 5379.
(90) Nakajima, Y.; Okuda, J. Organometallics 2007, 26, 1270.
(91) Evans, W. J.; Brady, J. C.; Ziller, J. W. J. Am. Chem. Soc. 2001,
123, 7711.
(92) Ok, K. M.; Sung, J.; Hu, G.; Jacobs, R. M. J.; O’Hare, D. J. Am.
Chem. Soc. 2008, 130, 3762.
(93) Thuery, P. Inorg. Chem. 2011, 50, 1898.
(94) Gardner, B. M.; Stewart, J. C.; Davis, A. L.; McMaster, J.; Lewis,
W.; Blake, A. J.; Liddle, S. T. Proc. Natl. Acad. Sci. U.S.A. 2012, 109,
9265.
1422
dx.doi.org/10.1021/om3010806 | Organometallics 2013, 32, 1409−1422