Synthesis, characterization and catalytic performances of benzimidazolin-2-iminato actinide (IV) complexes in the Tishchenko reactions for symmetrical and unsymmetrical esters

Article abstract of DOI:10.1016/j.jorganchem.2017.09.020

A new family of benzimdazolin-2-iminato actinide?(IV) complexes [(Bim_7-Me^Dipp/MeN)An(N(SiMe₃)₂)₃] (An = U (3), Th (4)) and [(Bim_4-Me^Dipp/MeN)An(N(SiMe₃)₂)₃] (An = U (5), Th (6)) were synthesized and their solid state structures were established by single-crystal X-ray diffraction analysis. The catalytic performances of complexes 3–6 towards the homo- and cross-coupling of aldehydes (Tishchenko reaction) were studied and the thorium complexes 4 and 6 displayed moderate to high activities for the production of the corresponding symmetric and unsymmetrical esters. Coupling of aldehyde and alcohols, known as the tandem proton-transfer esterification, and the intermolecular coupling reaction between aldehyde and trifluoromethylketones were also investigated by these thorium complexes, indicating a complementary method to obtain unsymmetrical esters selectively. Plausible mechanisms for these reactions are proposed based on stoichiometric studies.

Full text of DOI:10.1016/j.jorganchem.2017.09.020

Accepted Manuscript
Synthesis, characterization and catalytic performances of benzimidazolin-2-
iminato actinide (IV) complexes in the Tishchenko reactions for symmetrical and
unsymmetrical esters
Heng Liu, Maxim Khononov, Natalia Fridman, Matthias Tamm, Moris S. Eisen
PII:
S0022-328X(17)30540-5
10.1016/j.jorganchem.2017.09.020
JOM 20095
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 15 August 2017
Revised Date: 10 September 2017
Accepted Date: 11 September 2017
Please cite this article as: H. Liu, M. Khononov, N. Fridman, M. Tamm, M.S. Eisen, Synthesis,
characterization and catalytic performances of benzimidazolin-2-iminato actinide (IV) complexes in the
Tishchenko reactions for symmetrical and unsymmetrical esters, Journal of Organometallic Chemistry
(2017), doi: 10.1016/j.jorganchem.2017.09.020.
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ACCEPTED MANUSCRIPT
Synthesis, Characterization and Catalytic Performances of
Benzimidazolin-2-iminato Actinide (IV) Complexes in the
Tishchenko Reactions for Symmetrical and Unsymmetrical Esters
Heng Liu,a Maxim Khononov,a Natalia Fridman,a Matthias Tamm,*b Moris S. Eisen*a
a Schulich Faculty of Chemistry, Technion – Israel Institute of Technology. Haifa City,
32000, Israel.
b
Institut für Anorganische und Analytische Chemie, Technische Universität
Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
Corresponding authors
Moris S. Eisen: E-mail: chmoris@tx.technion.ac.il. Phone: +972-4-8292680 (M.S.E.).
Matthias Tamm: E-mail: m.tamm@tu-bs.de (M.T.).
Dedication. In honor of William J. Evans. Friend and Colleague.
ABSTRACT
A new family of benzimdazolin-2-iminato actinide (IV) complexes
[(Bim7-MeDipp/MeN)An(N(SiMe3)2)3]
(An
=
U
(3),
Th
(4))
and
[(Bim4-MeDipp/MeN)An(N(SiMe3)2)3] (An = U (5), Th (6)) were synthesized and their
solid state structures were established by single-crystal X-ray diffraction analysis. The
catalytic performances of complexes 3 - 6 towards the homo- and cross-coupling of
aldehydes (Tishchenko reaction) were studied and the thorium complexes 4 and 6
displayed moderate to high activities for the production of the corresponding
symmetric and unsymmetrical esters. Coupling of aldehyde and alcohols, known as
the tandem proton-transfer esterification, and the intermolecular coupling reaction
between aldehyde and trifluoromethylketones were also investigated by these thorium
complexes, indicating a complementary method to obtain unsymmetrical esters
selectively. Plausible mechanisms for these reactions are proposed based on
stoichiometric studies.
1

ACCEPTED MANUSCRIPT
Keywords: Actinide Catalysts, Benzimidazolin-2-iminato, Tishchenko Reaction,
Ester, Coupling Reaction
1. INTRODUCTION
The application of organometallic actinide complexes in homogeneous catalytic
processes has been flourishing, reaching high levels of sophistication in the past two
decades. This is mainly attributed to the unique characteristics of the early-actinide
metals, such as the large ionic radii and the presence of f-orbitals, which allows for
large coordination numbers and unusual coordination chemistries, and therefore
enable them to exhibit unique catalytic performance in organic transformations
[1-16]. In general, these distinctive reactivities are complementary to the ones
encountered in main group, transition metal, and lanthanide chemistry [16, 17]. For
instance, during the hydrothiolation of terminal alkynes, Markovnikov selectivity was
displayed by actinide complexes, which is in marked contrast to the
anti-Markovnikov addition products obtained by transition metal precatalysts [12, 18,
19]. Another example is the dehydrocoupling of amines with silanes promoted by
organoactinides, in which oligomeric cyclic aminosilanes are obtained [20]. Despite
these special features, it is interesting for actinide complexes to utilize
oxygen-containing substrates in catalytic reactions since the high oxophilicities of the
actinides will lead to the formation of thermodynamically stable, catalytically inactive
actinide-oxo/alkoxide species [21]. To date, a limited number of catalytic reactions are
known
that
involve
oxygenated
substrates,
including
the
hydroalkoxylation/cyclization of alkynyl alcohols [22, 23], small molecule activations
[24, 25], the Tishchenko reaction [26-28], and the polymerization of cyclic esters and
epoxides [15, 29-33]. Most of the precatalysts used in these reactions employed
strongly basic ligand skeletons, stemming from the consideration that the presence of
electron-rich ligands will decrease the oxophilicity of the actinide center, which will
render the formation of thermodynamically stable actinide-alkoxide species less
favorable, and therefore results in an enhanced activity towards oxygen-containing
molecules. This conceptual speculation was confirmed by using actinide amidinate
complexes bearing side arms for catalyzing the ring-opening polymerization (ROP) of
2

ACCEPTED MANUSCRIPT
ε-caprolactone, which resulted in much better catalytic performance as compared to
their side-arm free actinide counterparts [34, 35]. Besides amidinate ligands, our
recent focus was transferred to imidazolin-2-iminato ligands, because such ligands
possess strongly basic and highly nucleophilic properties, which should significantly
decrease the oxophilicity of the actinide centers. Moreover, imidazolin-2-iminato
ligands can be considered as 2σ,4π-electron donors, which are electronically
pseudo-isolobal with the widely used cyclopentadienyl moiety (Scheme 1) [36-40].
Being supported by the imidazolin-2-iminato ligands, the actinide complexes exhibit
very high activities in the ring opening polymerization (ROP) of ε-caprolactone and
the Tishchenko reaction [28, 33]. For instance, the imidazolin-2-iminato uranium
complexes (ImDippN)2U(NMeEt)2] showed surprisingly high activities in the ROP of
ε-caprolactone at room temperature, with productivities of 1.2×107 g (PCL) mol-1 h-1,
which represents the highest activity observed to date [29]. Generally speaking, two
positions are available in order to modify the imidazolin-2-iminato ligands (Scheme 1
(a)), i.e. site i and site ii, through which electronic and steric structures can be easily
tailored [41]. Herein, the modification on site i is performed by the benzannulation at
the backbone of the imidazolin-2-iminato ligands, and
a
series of
benzimidazolin-2-iminato actinide complexes were prepared [42, 43]. The electronic
structure of these benzimidazolin-2-iminato ligands was further studied by the
incorporation of a methyl group in different positions of the backbone (Scheme 1 (b)).
Scheme 1. (a) Modification sites of imidazolin-2-iminato ligand (b) mesomeric
structures for imidazolin-2-iminato (top) and benzimidazolin-2-iminato (middle and
bottom) ligands.
3

ACCEPTED MANUSCRIPT
The Tishchenko reaction, which has been known for more than a century, is a
waste-free and atom-efficient strategy to prepare symmetrical esters by the coupling
of aldehydes [44-47]. Despite the intensive investigations using main group, transition
metal or lanthanide complexes [48, 49], this reaction was only very recently studied in
the presence of organoactinide catalysts by our group [50]. Previous endeavors had
revealed few catalytic systems that displayed high catalytic activities to afford the
symmetrical esters [26, 28, 50]. In contrast, the formation of unsymmetrical esters
remains challenging, because of a crucial limitation: there are only very few methods
known that lead to a highly selective crossed-Tishchenko reaction with the formation
of a single cross-coupled unsymmetrical ester from the four possible products [51, 52].
Herein, we report the homocoupling of aldehydes to afford symmetrical esters in the
presence of a new family of benzimidazolin-2-iminato actinide complexes. In addition,
in order to tap alternative methods to prepare unsymmetrical esters, some other
reactions, including the crossed Tishchenko reaction, the tandem proton-transfer
esterification (TPTE), the coupling reactions between aldehyde and trifluoromethyl
ketones, were also investigated using the actinide complexes, and detailed catalytic
behaviors are herein disclosed.
2. EXPERIMENTAL SECTION
2.1 General considerations
All manipulations of air sensitive materials were performed with rigorous
exclusion of oxygen and moisture in flamed Schlenk-type glassware on a high
vacuum line (10-5 torr), or in nitrogen filled MBraun and Vacuum Atmospheres
gloveboxes with a medium capacity recirculator (1-2 ppm oxygen). Argon and
nitrogen were purified by passage through an MnO oxygen removal column and a
Davison 4 Å molecular sieve column. Analytically pure solvents were dried and
stored with Na/K alloy and degassed by three freeze-pump-thaw cycles prior to use
(hexane, toluene, and benzene-d6). Benzaldehyde, 4-trifluromethylbenzaldehyde,
4-methylbenzaldehyde,
2-pyridinylaldehyde,
4-methoxylbenzaldehyde,
2-thiophencarboxaldehyde, fufural,
1-naphthaldehyde,
isobutyraldehyde,
cyclohexyladehyde, α,α,α-trifluoroacetophenone, α,α,α,4’’-tetrafluoroacetophenone,
4’-bromo-α,α,α-trifluoroacetophenone, methanol (MeOH), ethanol (EtOH),
4

ACCEPTED MANUSCRIPT
isopropanol (iPrOH), were distilled under sodium bicarbonate (or CaH2) and stored
over
4
Å
molecular sieves. 4-Nitrobenzaldehyde, 3-nitrobenzaldehyde,
4-cyanobenzaldehyde, 2-naphthaldehyde, phthaldehyde were dried for 12 hours on a
high vacuum line (10-5 torr) and stored in a glovebox prior to use. The actinide
metallacycles
2-Bromo-3-nitrotoluene,
tris(dibenzylideneacetone)dipalladium(0)
were
prepared
according
to
previous
reports.[53]
(97%),
2,6-diisopropylaniline
Pd2(dpa)3,
(±)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (±BINAP), Caesium carbonate
(Cs2CO3), palladium on activated charcoal Pd/C (5 Pd), methyl
%
trifluoromethanesulfonate CF3SO3CH3, 3-bromo-2-nitrotoluene were purchased from
Sigma Aldrich and used as received.
NMR spectra were recorded on Bruker Avance 300 and Avance 400 Bruker
1
spectrometers. The chemical shifts for H NMR and 13C NMR measurements are
reported in ppm and are referenced using residual proton or carbon signals of the
deuterated solvents relative to tetramethylsilane. Elemental analyses were carried out
by the microanalysis laboratory at the Hebrew University of Jerusalem. MS
experiments were performed at 200 °C (source temperature) on a Maxis Impact
(Bruker) mass spectrometer with an APCI solid probe method. For X-ray
crystallographic measurements, the single crystals were immersed in
perfluoropolyalkylether oil and was quickly mounted on a Kappa CCD diffractometer
under a cold flow of nitrogen. Data collection was performed using monochromated
Mo Kα radiation using φ and ω scans to cover the Ewald sphere [54]. Accurate cell
parameters were obtained with the amount of indicated reflections [55]. The structures
were solved by SHELXS-97 direct methods [56], and refined by the SHELXL-97
program package [57]. The atoms were refined anisotropically. Hydrogen atoms were
included using the riding model. The structural data was deposited at the Cambridge
Center under the numbers CCDC 1565109-1565112 for complexes 3 – 6,
respectively.
2.2 Synthesis of (2,6-diisopropylphenyl)-(2-methyl-6-nitro-phenyl)amine (a1):
An oven-dried 250 ml Schlenk flask was charged with 2-bromo-3-nitrotoluene
(4.32 g, 20 mmol) and 50ml of toluene. Then to this mixture, 2,6-diisopropylaniline
(5.32 g, 30 mml), Pd2(dpa)3 (0.45 g, 0.5 mmol), ±BINAP (0.46 g, 0.75 mmol) and
Cs2CO3 (3.26 g, 10 mmol) were added, the mixture was degassed, backfilled with
5

ACCEPTED MANUSCRIPT
o
nitrogen three times and sealed afterwards. The flask was heated to 110 C for 24 h.
After the solution was cooled to room temperature, the resulting slurry was filtered
through a pad of Celite followed by removing the solvent on a rotatory evaporator,
and the final product was purified by chromatography on silica gel with the eluent of
n-hexane/ethyl
acetate
(15/1),
affording
(2,6-diisopropylphenyl)-(2-methyl-6-nitro-phenyl)amine as yellow powders. Yield:
1
4.6 g (74 %). H NMR (400 MHz, CDCl3) δ 9.48 (s, 1H, NH), 8.12 – 7.97 (m, 1H,
HAr), 7.26 – 7.14 (m, 1H, HAr), 7.12 – 7.00 (m, 3H, HAr), 6.65 – 6.50 (m, 1H, HAr),
3.04 (septet, J = 6.9 Hz, 2H, CH(CH3)2), 1.56 (s, 3H, CH3), 1.09 (d, J = 6.9 Hz, 6H,
CH(CH3)2), 1.06 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3) δ
146.02, 144.01, 139.08, 135.93, 134.82, 127.54, 127.15, 125.25, 123.25, 116.87,
28.63, 24.62, 21.91, 20.95. MS (APCI, m/z): calcd. 313.1838 (M+H)+; found
313.1743 (M+H)+.
2.3 Synthesis 1-(2,6-diisopropylphenyl)-7-methyl-1H-benzoimidazol-2-ylamine
(a2):
To
a
100
ml
round
bottom
flask
was
added
(2,6-diisopropylphenyl)-(2-methyl-6-nitro-phenyl)amine (2 g, 6.4 mmol), THF (30
ml), 5 % Pd/C (0.2 g) were reacted carefully under 1 atm H2. The mixture was stirred
vigorously overnight at room temperature (TLC was used to monitor the reduction
process), and then filtered through a pad of Celite followed by removing the solvent
on a rotatory evaporator, affording the product a1 quantitatively. Then the flask was
charged with cyanogen bromide (1.06 g, 10 mmol) and 30 ml absolute ethanol, and
the mixture was stirred at room temperature for 30 min and then refluxed for 2 h.
Afterwards, to complete the reaction, the temperature was increased to 150 oC for 2 h
and then decreased and maintained at 90 oC for overnight. After the solution was cool
to room temperature, the solvent was removed completely and 50 ml diethyl ether was
added. The mixture was stirred for 1 h and the precipitate filtered off, washed with
diethyl ether, and then redissolved in 50 ml of CHCl3. 50 ml 1M NaOH aqueous
solution was then added to the above solution, leaving the mixture stirring for 2 hours,
and then the organic layer separated. The remaining aqueous phase was extracted with
CHCl3 (3×30 ml), and the combined organic fractions were dried over MgSO4, and
concentrated
in
vacuo
to
afford
1-(2,6-diisopropylphenyl)-7-methyl-1H-benzoimidazol-2-ylamine as the target
product. Yield: 1.7 g (87 %). 1H NMR (400 MHz, CDCl3) δ 7.45 (d, J = 7.8 Hz, 1H,
6

ACCEPTED MANUSCRIPT
HAr), 7.32 – 7.23 (m, 3H, HAr), 6.98 (d, J = 7.8 Hz, 1H, HAr), 6.67 (d, J = 7.8 Hz, 1H,
HAr), 4.42 (br, 2H, NH2), 2.52 (septet, J = 6.9 Hz, 2H, CH(CH3)2), 1.74 (s, 3H, CH3),
13
1.08 (d, J = 6.9 Hz, 6H, CH(CH3)2), 1.02 (d, J = 6.9 Hz, 5H, CH(CH3)2). C{1H}
NMR (101 MHz, CDCl3) δ 151.07, 145.89, 140.19, 130.52, 128.72, 128.14, 122.03,
120.37, 119.16, 117.23, 112.09, 25.85, 21.95, 20.78, 13.76. MS (APCI, m/z): calcd.
308.2048 (M+H)+; found 308.2010 (M+H)+.
2.4
Synthesis
of
1-(2,6-diisopropylphenyl)-3,7-dimethyl-1,3-dihydro-benzimidazolin-2-imine
(Bim7-MeDipp/MeNH) (a):
The precursor a2 obtained above (0.77 g, 2.5 mmol) was redissolved in CH2Cl2
o
(30 ml), and the solution cool down to -40 C. CF3SO3CH3 (1.1 equiv.) was added
dropwise and the reaction mixture was stirred at this temperature for 1 h. The mixture
was allowed to warm up to room temperature, and stirred overnight. After removing
the solvent in vacuo, the remaining solids were washed with diethyl ether (2×20 ml),
dried in vacuo and redissolved in 30ml THF. KH (0.12 g, 3 mmol) was added into the
solution slowly (gas was evolved immediately), and then the mixture was stirred at
room temperature for 30 min and refluxed for 2 h. The solvent was completely
removed and 30 ml diethyl ether was added to extract the target ligand that after
1
solvent evaporation affords the target. Yield: 0.72 g (91 %). H NMR (400 MHz,
CDCl3) δ 7.40 (d, J = 7.8 Hz, 1H, HAr), 7.25 – 7.18 (m, 2H, HAr), 6.84 (d, J = 7.7 Hz,
1H, HAr), 6.70 (d, J = 7.6 Hz, 1H, HAr), 6.55 (d, J = 7.6 Hz, 1H, HAr), 3.40 (s, 3H,
CH3), 2.61 (septet, J = 6.8 Hz, 2H, CH(CH3)2), 1.57 (s, 3H, CH3), 1.06 (d, J = 6.8 Hz,
6H, CH(CH3)2), 0.99 (d, J = 6.8 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz,
CDCl3) δ 155.83, 149.29, 132.88, 131.08, 130.33, 129.59, 124.33, 123.78, 120.71,
118.19, 104.41, 28.49, 28.23, 24.61, 23.14, 16.38. MS (APCI, m/z): calcd. 322.2205
(M+H)+; found 322.2144 (M+H)+.
2.5 Synthesis of (2,6-diisopropylphenyl)-(5-methyl-6-nitro-phenyl)amine (b1):
The product was prepared using similar method as to prepare a1. Yield: 5.4 g
1
(89 %). H NMR (400 MHz, CDCl3) δ 7.85 (s, 1H, NH), 7.32 – 7.20 (m, 1H, HAr),
7.21 – 7.13 (m, 2H, HAr), 6.98 (t, J = 7.9 Hz, 1H, HAr), 6.48 (d, J = 7.9 Hz, 1H, HAr),
6.12 (d, J = 7.9 Hz, 1H, HAr), 2.96 (septet, J = 6.8 Hz, 2H, CH(CH3)2), 2.48 (s, 3H,
CH3), 1.09 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.04 (d, J = 6.8 Hz, 6H, CH(CH3)2).
13C{1H} NMR (101 MHz, CDCl3) δ 147.22, 144.29, 135.69, 135.38, 133.23, 133.07,
7

ACCEPTED MANUSCRIPT
128.22, 124.17, 120.23, 113.03, 28.40, 24.48, 23.00, 21.58. MS (APCI, m/z): calcd.
313.1838 (M+H)+; found 313.1957 (M+H) +.
2.6 Synthesis of 1-(2,6-diisopropylphenyl)-4-methyl-1H-benzoimidazol-2-ylamine
(b2):
The product was prepared using similar method as to prepared a2. Yield: 1.6 g
(83 %). 1H NMR (400 MHz, CDCl3) δ 7.45 (d, J = 7.8 Hz, 1H, HAr), 7.28 (d, J = 7.8
Hz, 2H, HAr), 6.87 (d, J = 7.6 Hz, 1H, HAr), 6.80 (d, J = 7.6 Hz, 1H, HAr), 6.46 (d, J =
7.6 Hz, 1H, HAr), 5.37 (br, 2H, NH2), 2.56 – 2.35 (m, 5H, CH(CH3)2 and CH3), 1.09 (d,
J = 6.9 Hz, 6H, CH(CH3)2), 0.94 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101
MHz, CDCl3) δ 152.50, 147.55, 139.99, 134.06, 129.47, 127.92, 124.70, 123.76,
121.51, 118.81, 105.14, 27.27, 23.34, 22.92, 15.65. MS (APCI, m/z): calcd. 308.2048
(M+H)+; found 308.2179 (M+H) +.
2.7
Synthesis
of
1-(2,6-diisopropylphenyl)-3,4-dimethyl-1,3-dihydro-benzimidazolin-2-imine
(Bim4-MeDipp/MeNH) (b):
The product was prepared using similar method as for the synthesis of compound
a. Yield: 0.65 g (82 %). 1H NMR (400 MHz, CDCl3) δ 7.51 (d, J = 7.8 Hz, 1H, HAr),
7.36 (d, J = 7.8 Hz, 2H, HAr), 6.79 – 6.73 (m, 2H, HAr), 6.26 – 6.20 (m, 1H, HAr), 3.80
(s, 3H, CH3), 2.81 – 2.54 (m, 5H, CH(CH3)2 and CH3), 1.18 (d, J = 6.9 Hz, 6H,
CH(CH3)2), 1.05 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3) δ
156.25, 149.27, 133.05, 130.29, 124.79, 124.43, 120.30, 118.32, 105.32, 31.06, 28.44,
24.14, 18.49. MS (APCI, m/z): calcd. 322.2205 (M+H)+; found 322.2298 (M+H) +.
2.8
Synthesis
of
1-(2,6-diisopropylphenyl)-3,7-dimethyl-1,3-dihydro-benzimidazolin-2-imine
uranium (IV) complex (1) [(Bim7-MeDipp/MeN)U(N(SiMe3)2)3] (3)
A solution of uranium metallacycle (0.11 g, 1.5 mmol) in 2ml toluene was treated
with 1.0 equiv of ligand a in 3ml toluene. The reaction mixture was stirred at room
temperature for 12 h, and the solvent was removed partially. Recrystallization from a
concentrated toluene solution afforded the target complex in high yield. Yield: 0.14 g,
8

ACCEPTED MANUSCRIPT
91 %. 1H NMR (300 MHz, C6D6) δ 96.09 (s, 2H, CH(CH3)2), 26.65 (d, J = 7.7 Hz, 1H,
HAr), 15.27 (d, J = 7.7 Hz, 1H, HAr), 14.41 (b, 9H, CH(CH3)2 and CH3), 12.62 (d, J =
7.7 Hz, 1H, HAr), 2.39 (s, 3H, CH3), 0.51 (s, 6H, CH(CH3)2), -0.16 – -0.44 (m, 1H,
HAr), -2.13 – -2.53 (m, 1H, HAr), -13.09 (br, 42H, N(Si(CH3)3)2), -21.17 (s, 12H,
N(Si(CH3)3)2). 13C{1H} NMR (75 MHz, C6D6) δ 171.53, 158.62, 133.89, 132.85,
132.48, 127.94, 124.72, 124.56, 124.16, 119.21, 111.80, 34.66, 21.64, 20.05, 19.16,
6.46. Anal. Calcd. for C39H80N6Si6U: C, 45.06; H, 7.76; N, 8.08. Found: C, 45.11; H,
7.90; N, 8.04.
2.9
Synthesis
of
1-(2,6-diisopropylphenyl)-3,7-dimethyl-1,3-dihydro-benzimidazolin-2-imine
thorium (IV) complex (2) [(Bim7-MeDipp/MeN)Th(N(SiMe3)2)3] (4)
A solution of thorium metallacycle (0.11g, 1.5mmol) in 2ml toluene was treated
with 1.0 equiv ligand a in 3ml toluene. The reaction mixture was stirred at room
temperature for 12 h, and the solvent was removed partially. The recrystallization from
a concentrated toluene solution afforded the target complex in high yield. Yield: 0.13 g,
87 %. 1H NMR (300 MHz, C6D6) δ 7.42 – 7.10 (m, 3H, HAr), 6.99 (d, J = 7.7 Hz, 1H,
HAr), 6.74 (d, J = 7.7 Hz, 1H, HAr), 6.62 (d, J = 7.7 Hz, 1H, HAr), 3.57 (s, 3H, CH3),
3.16 – 2.94 (m, 2H, CH(CH3)2), 1.63 (s, 3H, CH3), 1.41 (d, J = 6.8 Hz, 6H,
CH(CH3)2), 1.02 (d, J = 6.8 Hz, 6H, CH(CH3)2), 0.54 (s, 54H, N(Si(CH3)3)2). 13C{1H}
NMR (75 MHz, C6D6) δ 148.10, 144.71, 134.43, 131.97, 129.62, 129.39, 125.19,
124.20, 121.20, 119.54, 105.13, 29.97, 28.57, 24.73, 23.62, 16.85, 5.08. Anal. Calcd.
for C39H80N6Si6Th: C, 45.32; H, 7.80; N, 8.13. Found: C, 45.22; H, 7.94; N, 8.19.
2.10
Synthesis
of
1-(2,6-diisopropylphenyl)-3,4-dimethyl-1,3-dihydro-benzimidazolin-2-imine
uranium (IV) complex (3) [(Bim4-MeDipp/MeN)U(N(SiMe3)2)3] (5)
Complex 5 was prepared using a similar method as for the synthesis of 3. Yield:
1
0.14 g, 93 %. H NMR (300 MHz, C6D6) δ 104.55 (s, 2H, CH(CH3)2), 18.28 (s, 3H,
CH3), 15.44 (s, 9H CH(CH3)2, CH3), 14.82 – 14.65 (m, 1H, HAr), 12.28 – 11.92 (m,
1H, HAr), 9.57 – 9.31 (m, 1H, HAr), 0.12 (s, 2H, HAr), -0.01 – -0.06 (m, 6H,
CH(CH3)2), -0.78 – -0.89 (m, 1H, HAr), -14.61 (s, 42H, N(Si(CH3)3)2), -21.64 (s, 12H,
N(Si(CH3)3)2). 13C{1H} NMR (75 MHz, C6D6) δ 168.33, 160.36, 138.68, 136.66,
131.63, 128.95, 128.88, 120.81, 118.54, 116.55, 111.42, 37.05, 36.40, 23.24, 18.78,
9

ACCEPTED MANUSCRIPT
6.61, 2.27. Anal. Calcd. for C39H80N6Si6U: C, 45.06; H, 7.76; N, 8.08. Found: C,
45.14; H, 7.83; N, 8.00.
2.11
Synthesis
of
1-(2,6-diisopropylphenyl)-3,4-dimethyl-1,3-dihydro-benzimidazolin-2-imine
uranium (IV) complex (4) [(Bim4-MeDipp/MeN)Th(N(SiMe3)2)3] (6)
Complex 6 was prepared using a similar method as for the synthesis of 4. Yield:
0.14 g, 91 %. 1H NMR (300 MHz, C6D6) δ 7.27 – 7.17 (m, 1H, HAr), 7.16 – 7.09 (m,
2H, HAr), 6.67 – 6.48 (m, 2H, HAr), 6.13 – 6.01 (m, 1H, HAr), 3.82 (s, 3H, CH3), 2.99
– 2.71 (septet, J = 6.8 Hz, 2H, CH(CH3)2), 2.13 (s, 3H, CH3), 1.28 (d, J = 6.8 Hz, 6H,
CH(CH3)2), 0.87 (d, J = 6.8 Hz, 6H, CH(CH3)2), 0.55 – 0.06 (m, 54H, N(Si(CH3)3)2).
13C{1H} NMR (75 MHz, C6D6) δ 147.91, 144.71, 132.14, 131.63, 129.40, 128.96,
128.84, 124.57, 124.47, 120.42, 118.29, 107.83, 32.56, 28.23, 25.10, 23.94, 18.17,
4.89, 3.53 Anal. Calcd. For C39H80N6Si6Th: C, 45.32; H, 7.80; N, 8.13. Found: C,
45.29; H, 7.91; N, 8.06.
2.12 General procedure for the Tishchenko reactions
A J. Young Teflon sealed NMR tube was loaded with a certain amount of the
precatalyst from a stock solution in C6D6. Then the respective aldehyde (100
equivalents) was added, and immediately diluted until 600 µL with C6D6. The tube
was sealed and taken out of the glovebox; the reaction progress was monitored by 1H
NMR. The ratio between the ester products and aldehyde was calculated by
integration of the methylene signals and aldehyde signals. Then the tube was opened
to the air, and the reaction was quenched with methanol and submitted to MS analysis.
1
The H NMR of ester products from homocoupling of benzaldehyde [28],
4-trifluromethylbenzaldehyde
4-methoxylbenzaldehyde [27], 1-naphthaldehyde [28], 2-pyridinylaldehyde [28],
2-thiophencarboxaldehyde [28], fufural [28], isobutyraldehyde [28],
[58],
4-methylbenzaldehyde
[26],
cyclohexylaldehyde [28], 4-nitrobenzaldehyde [28], 3-nitrobenzaldehyde [27],
4-cyanobenzaldehyde [27], 2-naphthaldehyde [28], phthaldehyde [28], were
compared with previous reports.
2.13 General procedure for the crossed Tishchenko reactions
A J. Young Teflon sealed NMR tube was loaded with a certain amount of
precatalyst from a stock solution in C6D6. The respective aldehydes (50 equivalents)
10

ACCEPTED MANUSCRIPT
were added, and immediately diluted until 600 µL with C6D6. The tube was sealed
and taken out of the glovebox. The reaction progress was monitored by 1H NMR. The
ratio between the ester and aldehyde was calculated by integration of the methylene
signals and aldehyde signals. Then the tube was opened to the air, and the reaction
1
was quenched with methanol and submitted to MS analysis. The H NMR of ester
products from coupling of benzaldehyde with 2-pyridinylaldehyde [28],
isobutyraldehyde [28], cyclohexyladehyde [28], 2-naphthaldehyde [28],
3-nitrobenzadehyde [27, 59], were compared with previous reports.
2.14 General procedures for tandem-proton transfer esterification (TPTE)
In a typical experiment, into a J. Young Teflon sealed NMR tube the desired
amount of the catalyst from a stock solution of C6D6 was added. Aldehyde (150 equiv)
and alcohol (50 equiv) (TFMAP was added if necessary) were added and the reaction
mixture was diluted with C6D6. Samples were then sealed and placed in an oil bath
preheated to 70 °C. The progress of reaction was monitored at regular intervals using
1H NMR spectroscopy for up to 24 hours. The yield was calculated from the ratio of
1
1
esters and alcohols from the crude H NMR spectra. The H NMR spectra of ester
products, including methyl benzoate, ethyl benzoate, isopropyl benzoate, methyl
4-nitrobenzoate, methyl 4-cyanobenzoate, methyl 3-nitrobenzoate, methyl
4-methylbenzoate, methyl 2-furoate, methyl naphthalene-1-carboxylate, were
compared with previous reports [60].
2.15 General procedures for coupling aldehyde and ketones
In a typical experiment, into a J. Young Teflon sealed NMR tube aldehyde (50
equiv) and trifluoromethylketone (150 equiv) were added followed by adding the
desired amount of the catalyst in C6D6. Samples were then sealed and placed in an oil
bath preheated to 70 °C. The progress of the reaction was monitored at regular
1
intervals using H NMR spectroscopy for up to 48 hours. The yield was calculated
1
from the ratio of aldehyde and corresponding ester from the crude H NMR spectra
(see an example in Figure S1). After completion of the reaction, the pure product was
obtained by flash column chromatography on silica gel (15:1 EtOAc: Hexane) and
compared with the literature.
2.16 Coupling reaction between 4-NO2PhCHO and PhCOCF3
11

ACCEPTED MANUSCRIPT
Reaction of 4-nitrobenzaldehyde with α,α,α-trifluoroacetophenone was carried
out following the general procedure described above using complex 2 as precatalysts.
Yield: 96 %.1H NMR (300 MHz, CDCl3) δ 8.30 (m, 4H), 7.49 (m, 5H), 6.36 (q, 3JH,F
13
= 6.8 Hz, 1H). C{1H} NMR (75 MHz, CDCl3) δ 162.50, 150.91, 133.86, 131.07,
130.49, 130.19, 128.81, 127.93, 124.79, 123.68, 21.08, 73.83, 73.39, 72.95, 72.50.
19F{1H} NMR (188 MHz, CDCl3) δ -75.76. MS (APCI, m/z): calcd. 326.0562
(M+H)+; found 326.0654 (M+H)+.
2.17 Coupling reaction between 3-NO2PhCHO and PhCOCF3 [61]
Reaction of 3-nitrobenzaldehyde with α,α,α-trifluoroacetophenone was carried out
following the general procedure described above using complex 2 as precatalysts.
Yield: 93 %. 1H NMR (300 MHz, CDCl3) δ 8.95 – 8.86 (m, 1H), 8.50 – 8.37 (m, 2H),
7.73 – 7.67 (m, 1H), 7.58 – 7.52 (m, 2H), 7.46 – 7.38 (m, 3H), 6.38 (q, 3JH,F = 6.8 Hz,
1H). 13C{1H} NMR (75 MHz, CDCl3) δ 162.33, 148.29, 135.47, 130.48, 130.32,
130.18, 129.89, 128.81, 128.17, 127.96, 124.84, 124.80, 121.08, 73.80, 73.36, 72.91,
19
72.47. F{1H} NMR (188 MHz, CDCl3) δ -75.72. MS (APCI, m/z): calcd. 326.0562
(M+H)+; found 326.0655 (M+H) +.
2.18 Coupling reaction between 4-CNPhCHO and PhCOCF3 [62]
Reaction of 4-cyanobenzaldehyde with α,α,α-trifluoroacetophenone was carried
out following the general procedure described above using complex 2 as precatalysts.
Yield: 92 %. 1H NMR (300 MHz, CDCl3) δ 8.24 – 8.18 (m, 1H), 7.80 – 7.75 (m, 2H),
7.55 – 7.49 (m, 2H), 7.45 – 7.39 (m, 3H), 6.35 (q, 3JH,F = 6.8 Hz, 1H). 13C{1H} NMR
(75 MHz, CDCl3) δ 162.74, 132.34, 130.54, 130.39, 130.15, 128.79, 127.90, 124.81,
121.09, 117.60, 117.20, 73.72, 73.27, 72.83, 72.39. 19F{1H} NMR (188 MHz, CDCl3) δ
-75.79. MS (APCI, m/z): calcd. 306.0664 (M+H)+; found 306.0757 (M+H) +.
2.19 Coupling reaction between 4-CF3PhCHO and PhCOCF3
Reaction of 4-trifluoromethylbenzaldehyde with α,α,α-trifluoroacetophenone was
carried out following the general procedure described above using complex 2 as
precatalysts. Yield: 83 %. 1H NMR (300 MHz, CDCl3) δ 8.27 – 8.17 (m, 2H), 7.81 –
13
7.67 (m, 2H), 7.52 (m, 2H), 7.46 – 7.35 (m, 3H), 6.36 (q, J = 6.8 Hz, 1H). C{1H}
12

ACCEPTED MANUSCRIPT
NMR (75 MHz, CDCl3) δ 163.11, 135.42, 134.98, 131.77, 130.75, 130.33, 130.05,
128.74, 128.60, 127.89, 125.66, 125.61, 125.56, 125.51, 125.14, 124.88, 121.52,
121.17, 73.51, 73.06, 72.62, 72.18. 19F{1H} NMR (188 MHz, CDCl3) δ -63.27, -75.83.
MS (APCI, m/z): calcd. 349.0585 (M+H)+; found 349.0622 (M+H)+.
2.20 Coupling reaction between 4-ClPhCHO and PhCOCF3
Reaction of 4-chlorobenzaldehyde with α,α,α-trifluoroacetophenone was carried
out following the general procedure described above using complex 2 as precatalysts.
Yield: 61 %.1H NMR (300 MHz, CDCl3) δ 8.10 – 7.98 (m, 2H), 7.59 – 7.48 (m, 2H),
7.48 – 7.37 (m, 5H), 6.33 (q, J = 6.8 Hz, 1H). 13C{1H} NMR (75 MHz, CDCl3) δ 163.43,
140.38, 131.28, 130.96, 129.94, 128.93, 128.69, 127.88, 126.99, 124.95, 121.23, 73.20,
72.76, 72.32, 71.88. 19F{1H} NMR (188 MHz, CDCl3) δ -75.83. MS (APCI, m/z): calcd.
315.0321 (M+H)+; found 315.0320 (M+H)+.
2.21 Coupling reaction between PhCHO and PhCOCF3 [62]
Reaction of benzaldehyde with α,α,α-trifluoroacetophenone was carried out
following the general procedure described above using complex 2 as precatalysts.
Yield: 36 %.1H NMR (300 MHz, CDCl3) δ 8.16 – 8.07 (m, 2H), 7.66 – 7.36 (m, 8H),
6.36 (q, J = 6.8 Hz, 1H).13C{1H} NMR (75 MHz, CDCl3) δ 164.24, 133.78, 131.20,
129.93, 129.83, 128.64, 128.56, 128.53, 127.91, 125.05, 121.33, 72.99, 72.55, 72.11,
71.67. 19F{1H} NMR (188 MHz, CDCl3) δ -75.84. MS (APCI, m/z): calcd. 281.0711
(M+H)+; found 281.0769 (M+H)+.
2.22 Coupling reaction between 4-MePhCHO and PhCOCF3 [63]
Reaction of 4-methylbenzaldehyde with α,α,α-trifluoroacetophenone was carried
out following the general procedure described above using complex 2 as precatalysts.
Yield: 6 %.1H NMR (300 MHz, CDCl3) δ 8.00 (m, 2H), 7.53 (m, 2H), 7.40 (m, 2H),
7.34 – 7.19 (m, 3H), 6.33 (q, J = 6.5 Hz, 1H), 2.42 (d, J = 7.4 Hz, 3H). 3C{1H} NMR
(75 MHz, CDCl3) δ 164.43, 144.85, 131.60, 130.18, 130.02, 129.45, 128.74, 128.10,
126.09, 123.45, 73.02, 72.59, 72.14, 71.70, 21.80. 19F{1H} NMR (188 MHz, CDCl3) δ
-75.86. MS (APCI, m/z): calcd. 295.0868 (M+H)+; found 295.0923 (M+H)+.
2.23 Coupling reaction between 4-CNPhCHO and 4-FPhCOCF3
Reaction of 4-cyanobenzaldehyde with α,α,α,4’-tetrafluoroacetophenone was
carried out following the general procedure described above using complex 2 as
precatalysts. Yield: 74 %. 1H NMR (300 MHz, CDCl3) δ 8.22 – 8.13 (m, 2H), 7.83 –
13

ACCEPTED MANUSCRIPT
13
7.70 (m, 2H), 7.52 (m, 2H), 7.17 – 7.07 (m, 2H), 6.30 (q, J = 6.4 Hz, 1H). C{1H}
NMR (75 MHz, CDCl3) δ 165.29, 162.68, 161.98, 132.37, 132.15, 130.38, 130.02,
129.90, 126.45, 124.67, 120.95, 117.54, 117.31, 116.14, 115.85, 73.09, 72.64, 72.19,
19
71.75. F{1H} NMR (188 MHz, CDCl3) δ -76.13, -110.19. MS (APCI, m/z): calcd.
324.0569 (M+H)+; found 324.0640 (M+H)+.
2.24 Coupling reaction between 4-CNPhCHO and 4-BrPhCOCF3
Reaction of 4-cyanobenzaldehyde with 4’-bromo-α,α,α-trifluoroacetophenone
was carried out following the general procedure described above using complex 2 as
1
precatalysts. Yield: 96 %. H NMR (300 MHz, CDCl3) δ 8.19 (d, J = 8.2 Hz, 1H),
7.78 (d, J = 8.2 Hz, 1H), 7.56 (d, J = 8.1 Hz, 1H), 7.40 (d, J = 8.1 Hz, 1H), 6.29 (q, J
13
= 6.4 Hz, 1H). C{1H} NMR (75 MHz, CDCl3) δ 162.64, 132.39, 132.10, 130.39,
19
129.50, 124.62, 124.53, 120.81, 117.53, 117.36, 73.18, 72.73, 72.28, 71.84. F{1H}
NMR (188 MHz, CDCl3) δ -75.79. MS (APCI, m/z): calcd. 383.9769 (M+H)+; found
383.9815 (M+H)+.
2.25 Coupling reaction between 1,4-phthalaldehyde with PhCOCF3
Reaction of 1,4-phthalaldehyde with α,α,α-trifluoroacetophenone was carried out
following the general procedure described above using complex 2 as precatalysts. For
ester 19, yield, 64 %. 1H NMR (400 MHz, CDCl3) δ 10.07 (s, 1H), 8.22 (d, J = 8.3 Hz,
2H), 7.93 (d, J = 8.3 Hz, 2H), 7.55 – 7.45 (m, 2H), 7.41 – 7.32 (m, 3H), 6.31 (q, J =
13
6.8 Hz, 1H). C{1H} NMR (101 MHz, CDCl3) δ 190.39, 162.40, 138.72, 132.52,
129.85, 129.62, 129.14, 128.65, 127.84, 127.01, 123.51, 120.67, 72.47, 72.13, 71.80,
71.44. 19F{1H} NMR (377 MHz, CDCl3) δ -75.75. For ester 20, yield, 28 %. 1H NMR
(400 MHz, CDCl3) δ 8.16 (s, 4H), 7.52 – 7.45 (m, 4H), 7.39 – 7.32 (m, 6H), 6.30 (q, J
13
= 6.7 Hz, 2H). C{1H} NMR (101 MHz, CDCl3) δ 163.40, 133.21, 130.88, 130.21,
19
130.17, 128.87, 128.02, 125.93, 120.36, 114.78, 73.95, 73.29, 72.63, 71.96. F{1H}
NMR (188 MHz, CDCl3) δ -75.79. MS (APCI, m/z): calcd. 483.0953 (M+H)+; found
483.1003 (M+H)+.
2.26 Coupling reaction between isophthalaldehyde with PhCOCF3
Reaction of isophthalaldehyde with α,α,α-trifluoroacetophenone was carried out
following the general procedure described above using complex 2 as precatalysts For
ester 21, , yield, 58 %. 1H NMR (300 MHz, CDCl3) δ 10.09 (s, 1H), 8.58 (m, 1H),
8.39 – 8.32 (m, 1H), 8.18 – 8.08 (m, 1H), 7.67 (t, J = 7.7 Hz, 1H), 7.59 – 7.52 (m,
14

ACCEPTED MANUSCRIPT
2H), 7.49 – 7.38 (m, 3H), 6.38 (q, J = 6.8 Hz, 1H). 13C{1H} NMR (75 MHz, CDCl3) δ
190.97, 163.35, 136.86, 135.52, 134.02, 131.55, 130.95, 130.13, 129.89, 129.63,
128.90, 128.10, 124.57, 121.84, 73.47, 73.14, 72.81, 72.48. 19F{1H} NMR (377 MHz,
CDCl3) δ -75.72. For ester 22, yield, 34%. 1H NMR (300 MHz, CDCl3) δ 8.82 (s, 1H),
8.35 (dd, J = 7.8, 1.7 Hz, 2H), 7.63 (t, J = 7.8 Hz, 1H), 7.59 – 7.49 (m, 4H), 7.49 –
13
7.35 (m, 6H), 6.36 (q, J = 6.7 Hz, 2H). C{1H} NMR (75 MHz, CDCl3) δ 163.35,
134.95, 131.62, 130.93, 130.15, 129.49, 129.26, 128.86, 128.04, 125.02, 121.29,
19
117.58, 73.58, 73.15, 72.71, 72.25. F{1H} NMR (377 MHz, CDCl3) δ -75.79. MS
(APCI, m/z): calcd. 483.0953 (M+H)+; found 483.0998 (M+H)+.
3. RESULTS AND DISCUSSION
3.1 Synthesis and structure characterization
Scheme 2. Synthetic procedures for the synthesis of complexes 3 - 6.
15

ACCEPTED MANUSCRIPT
Methyl substituted benzimidazolin-2-imine ligands (Bim7-MeDipp/MeNH (a),
Bim4-MeDipp/MeNH (b)) were prepared according to Scheme 2. Coupling reactions
between nitro- methyl- substituted 2-nitrobromobenzene and 2,6-diisopropylaniline
afforded a1 and b1, which were then reduced and subsequently underwent ring
closure in the presence of BrCN to give rise to the ligand precursor a2 and b2 in high
yields. Methylation of a2 and b2 using methyl trifluoromethanesulfonate and
subsequent deprotonation furnished the target ligands a and b. Reacting these ligands
with
1.0
equiv
of
the
actinide
metallacycles
[(Me3Si)2N]2An[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)] (An = U(1), Th(2)) in toluene for
12h furnished the target complexes [(Bim7-MeDipp/MeN)An(N(SiMe3)2)3] (An = U (3),
Th (4)) and [(Bim4-MeDipp/MeN)An(N(SiMe3)2)3] (An = U (5), Th (6)) in high yields.
The benzimidazolin-2-iminato actinide complexes [(BimDipp/MeN)An(N(SiMe3)2)3]
(An = U (7), Th (8)) without the methyl substituents on the phenyl rings were
prepared in a similar methodology according to previous reports [42]. Single crystals
for X-ray diffraction analyses of the complexes 3-6 were obtained by recrystallization
of the complexes from concentrated toluene solutions. The resulting molecular
structures, selected bond distances and angles, are presented in Figures 1 and 2, and
crystallographic details are presented in Table 1.
Complexes 3 and 4 (Figure 1) crystallize as brown and colorless crystals,
respectively, and display the same monoclinic P21/c space group. Both complexes
adopt a pseudo- tetrahedral geometry around the actinide center, with much shorter
An-N(1) bonds (2.132(6) Å in 3, 2.188(8) Å in 4) as compared to the other three
An-Namido bonds. In addition, the An-N(1)-C(1) angles of 159.0(5)o, 166.3(7)o for
complexes 3 and 4, which are slightly smaller than other imidazolin-2-iminato and
ketimido actinide complexes [50], perhaps due to the asymmetric nature of the ligand
with the sterically more encumbered diisopropylphenyl group repelling the adjacent
N(SiMe3)2 substituents. The solid structures of complexes 5 and 6 are shown in Figure
2, from which it was observed that both of them display isomorphous pseudo
tetrahedral geometries as in complexes 3 and 4. As before, shorter An-N(1) bond
lengths in comparison with An-Namido bonds are also observed in complexes 5 and 6,
with values of 2.144(5) Å and 2.201(3) Å, respectively. The An-N(1)-C(1) angles of
complexes 5 and 6 are 159.2(4)o, 158.2(2)o, respectively, which are similar to other
imidazolin-2-iminato or ketimino transition metal, lanthanide and actinide complexes
[38, 40, 64, 65].
16

ACCEPTED MANUSCRIPT
Figure 1. ORTEP drawing of complex 3 (left) and 4 (right) with thermal displacement
parameters at 50 % probability. Hydrogen atoms are omitted for clarity. (Bond length:
An-N1: 2.132(6) Å, 2.188(8) Å; An-N4, 2.307(5) Å, 2.365(9) Å; An-N5: 2.300(6) Å,
2.375(9) Å; An-N6: 2.292(6) Å, 2.359(8) Å; C1-N1: 1.284(8) Å, 1.290(13) Å. Bond
angles: An-N1-C1: 159.0(5)o, 166.3(7)o; N1-An-N4: 117.0(2)o, 111.0(3)o; N1-An-N5:
106.5(2)o, 108.6(3)o; N1-An-N6: 95.3(2)o, 103.6(3)o; N4-An-N5: 107.31(19)o,
110.0(3)o; N4-An-N6: 116.5(2)o, 108.6(3)o; N5-An-N6: 113.8(2)o, 114.9(3)o. cone
angles: 140.0o, 140.4o. All the values are in the order for complexes 3 and 4)
In order to investigate the influence of the backbone methyl group on the
structural properties, a comparison between complexes 3 - 6 and previously reported
benzimidazolin-2-iminato actinide complexes without the methyl groups
((BimDipp/MeN)An(N(SiMe3)2)3, An = U (7), Th (8)) was carried out. In complexes 3
and 4, the presence of the methyl groups do not induce a significant repulsion,
displaying similar C(1)-N(3)-CDipp bond angles, 123.5(5)o and 123.7(9)o, respectively,
which are also similar to those in the benzimidazolin-2-iminato actinide complexes
(125.6(2)o for 7, and 125.1(4)o for 8) [42]. Alike observations were obtained in
complexes 5 and 6. The cone angles in the complexes were calculated for the
benzimidazolin-2-iminato ligands in order to describe the proximity between the
ligand and the metal center. Based on the data obtained from the crystal structures the
complexes 3 - 6 displayed cone angles of 140o, 140o, 138o and 136o, respectively. This
angles are also similar to the values observed in benzimidazolin-2-iminato actinide
complexes for complexes 7 and 8; 138o, 140o, respectively [42]. Hence, it seems
plausible that the presence of the backbone methyl groups do not present any impact
on the steric environment around the metal center. It is important to point out that in
17

ACCEPTED MANUSCRIPT
comparison with imidazolin-2-iminato complexes bearing two bulky groups on both
sides of the ligands, much smaller cone angle values are obtained in the present four
complexes, providing much more open space for the incoming substrates to access the
metal center [50].
Figure 2. ORTEP drawing of complex 5 (left) and 6 (right) with thermal displacement
parameters at 50 % probability. Hydrogen atoms are omitted for clarity. (Bond length:
An-N1: 2.144(5) Å, 2.201(3)Å; An-N4, 2.313(4) Å, 2.362(3) Å; An-N5: 2.295(4) Å,
2.359(3)Å; An-N6: 2.293(5) Å, 2.339(3) Å; C1-N1: 1.281(7) Å, 1.282(5) Å. Bond
angles: An-N1-C1: 159.2(4)o, 158.2(2)o; N1-An-N4: 113.85(17)o, 112.18(10)o;
N1-U-N5: 97.48(17)o, 97.87(11)o; N1-An-N6: 105.16(16)o, 105.77(11)o; N4-An-N5:
119.13(16)o, 118.39(11)o; N4-An-N6: 106.50(16)o, 107.15(11)o; N5-An-N6:
113.87(17)o, 114.63(11)o. cone angles: 137.6o, 136.0o. All the values are in the order
for complexes 5, 6).
18

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Table 1. Crystallographic data of complexes 3 - 6.
3
4
5
6
Formula
Fw/g mol-1
crystal System
space group
a, (Å)
C39H80N6Si6U
C39H80N6Si6Th
1033.67
monoclinic
P 21/c
C39H80N6Si6U
1039.66
triclinic
P -1
C39H80N6Si6Th
1033.67
triclinic
P -1
1039.66
monoclinic
P 21/c
11.7600(17)
21.467(10)
22.282(4)
90.00
20.2000(4)
11.9970(3)
27.3540(6)
90.00
12.190(4)
13.442(3)
16.240(4)
84.309(7)
88.480(14)
80.535(6)
2611.7(13)
2
12.190(4)
13.442(3)
16.240(4)
84.309(7)
88.480(14)
80.535(6)
2611.8(12)
2
b, (Å)
c, (Å)
α, (deg)
β, (deg)
γ, (deg)
V, Å3
112.846(12)
90.00
128.5221(9)
90.00
5184(3)
4
5186.3(2)
4
Z
T/K
200(2)
200(2)
200(2)
200(2)
D
calcd, (g cm3)
1.332
1.324
1.322
1.314
µ(Mo Ka), (mm-1)
3.301
3.045
3.276
3.023
F(000)
2128
2120
1064
1060
θ
range (deg)
1.75-24.18
7697/7697/
0.0890
1.7-24.14
7708/7708/
0.0980
1.54-24.67
8798/8798/
0.0770
1.54-24.71
8898/8898/0.05
60
relefection collected/ unique/(Rint)
No. of data/ restraints/params
Goodness of fit
7697/0/493
1.029
7708/0/493
1.111
8798/0/493
1.025
8898/0/493
1.058
R1, wR2 (I>2σ(I))
R1, wR2 (all)
0.0462/0.0838
0.0783/0.0912
0.0691/0.1656
0.0897/0.1753
0.0448/0.0849
0.0638/0.0908
0.0294/0.0645
0.0371/0.0667
3.2 Catalytic Tishchenko reactions
The Tishchenko reaction, i.e. the dimerization of two aldehydes, represents a
facile strategy for the preparation of ester compounds. In our previous study,
Cp*2ThMe2 have showed good reactivities towards aromatic aldehydes and high
tolerance to functional groups [26, 27]. To mimic the ligand skeleton structure of the
Cp* moiety, mono(imidazolin-2-iminato) actinide (IV) complexes were designed and
found to possess also high activities in the conversion of both aromatic and aliphatic
aldehydes with good selectivity in the cross Tishchenko reaction [50]. In the
following studies, the catalytic performance of the newly synthesized
19

ACCEPTED MANUSCRIPT
benzimidazolin-2-iminato actinide (IV) complexes towards different types of
Tishchenko reactions was investigated.
3.2.1 Symmetric Tishchenko reaction
Symmetric Tishchenko reactions to afford symmetrical ester compounds were
initially investigated, and the results are summarized in Table 2. As a model reaction,
the homocoupling of benzaldehyde, by using the different precatalysts was conducted
(entries 1 - 6). It was observed that the benzimidazolin-2-iminato thorium complexes
4, 6 and 8 displayed the higher activities, affording benzyl benzoate (9a) in 87 - 89%
yields, and the presence of the different positions of the methyl group posed little
influence on the product conversions. In contrast, no turnover was observed for the
uranium (IV) counterparts 3, 5 and 7, even under heating conditions, and the specific
reasons remain unclear, besides a possible effect of the two f-electrons in the uranium,
as compared to the thorium complexes. Compound 4 was selected for the following
studies on substrate scope capabilities.
Scheme 3. Symmetric Tishchenko reaction.
From the results shown in Table 2, it can be concluded that the electronic nature
of the R substituent imposes a significant influence on the reaction rate, where
aldehydes bearing electron-withdrawing groups generally led to higher conversions.
For example, benzaldehyde functionalized with 4-NO2, 3-NO2, 4-CN, 4-CF3
20

ACCEPTED MANUSCRIPT
substituents resulted in slight higher yields than benzaldehyde even in shorter
amounts of time. On the contrary, electron-donating groups deactivate the active
species dramatically. A gradual reduction of the ester yields were obtained when
increasing the electron-donating ability of the para-substituent of the benzaldehyde
from methyl to methoxy, and the use of the electron-rich substituent 4-dimethylamino
was found to completely deactivate the thorium center. For polyaromatic substrates,
much higher yield was obtained for 2-naphthylaldehyde than for 1-naphthylaldehyde,
with values of 63% and 23%, respectively. This can be probably attributed to the
steric hindrance of 1-naphthylaldehyde, which prohibits the accessibility of the
substrate to the metal atom of the catalytically active species. This speculation was
later
proved
by
using
the
much
more
sterically
encumbered
2,6-dimethylbenzaldehyde, which did not react at all. Complex 4 was also found
suitable for the coupling of heteroaromatic aldehydes, and for 2-pyridylaldehyde, ca.
87% of the aldehyde was consumed in 36 h, which is comparable to the results
obtained with benzaldehyde. For furfural and 2-thenaldehyde, much lower
conversions to the corresponding esters were observed, presumably ascribed to a
stronger bidentate binding with the additional sulfur and oxygen bonds to the thorium
center. Similar results were also observed in previous Tishchenko processes mediated
by transition metal and lanthanide complexes [48]. Coupling of aliphatic aldehydes
was also found feasible, and 88 % and 76 % of ester conversions were obtained for
isobutyraldehyde and cyclohexyladehyde, respectively. The reactivity of complex 4
towards phthalaldehyde afforded the lactone 21 quantitatively within 3h.
Stoichiometric experiments were studied to investigate the true active species
during the symmetric Tishchenko reaction. Reactions between the precatalyst 4 and
benzaldehyde demonstrated that only two molecules of benzaldehyde are able to
insert into the Th-N(SiMe3)2) bonds, leaving one Th-N(SiMe3)2) bond unreacted
throughout the entire course of the reaction. This could be ascribed to a steric
crowding around the thorium atom, which prevents insertion of another aldehyde into
the third Th-N(SiMe3)2) bond. Based on these observations, a tentative reaction
mechanism mediated by complex 4 is proposed (Scheme 4).
21

ACCEPTED MANUSCRIPT
Table 2. Homocoupling of aldehydes mediated by complexes 3 - 8.a
Time (h)
Entry
Cat.
RCHO
Yield (%)
Product
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
36
36
36
36
36
36
24
24
24
24
36
36
36
36
36
36
36
36
36
3
0
87
0
89
0
3
4
5
6
7
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
9a
9a
9a
9a
9a
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
9k
9l
87
95
91
90
92
71
7
23
63
87
19
19
88
76
100
4-NO2Ph
3-NO2Ph
4-CNPh
4-CF3Ph
4-MePh
4-MeOPh
1-naphthyl
2-naphthyl
2-pyridyl
2-furyl
2-thiophen
iso-propyl
cyclohexyl
o-Ph(CHO)2
9
10
11
12
13
14
15
16 b
17 b
18
19
20
9m
9n
10
a Conditions: ~7 µmol catalyst, [Cat.]/[[R]CHO] = 1:100, r.t., 600 µL of C6D6, 36 h.
The yield was calculated from 1H NMR of the crude mixture. b Heating at 70 oC.
The mechanism starts with two equivalents of aldehyde that are initially inserted
into the Th-Namido bonds affording the thorium alkoxide A. Complex A subsequently
undergoes an additional insertion of another two equivalents of aldehyde giving rise
to the thorium hemiacetate species B. Intermediate B reacts with an additional
coordinating aldehydes via a six-membered transition state (C) to furnish the active
Th-O compound E. Similar to previous steps, E adds to the carbonyl group of the
incoming aldehyde substrate and undergoes a subsequent hydride transfer from the
tertiary carbon of the hemiacetate (F) to the carbonyl group of the incoming aldehyde,
through a six-membered transition state, yielding the target compound and
concomitantly regenerating the active species E, completing the catalytic cycle.
22

ACCEPTED MANUSCRIPT
R1
R2
R1
R2
N
N
Dipp
O
[Th]
N
N
Dipp
O
N
Th
O
O
N(SiMe3)2
O R
R
H
R
H
N
R
R
Th
(Me3Si)2N
O
2
O
(Me3Si)2N
N(SiMe3)2
(B)
N(SiMe3)2
R
H
N(SiMe3)2
R
N(SiMe3)2
(A)
R
R
(Me3Si)2N
R
O
[Th]
O
N(SiMe3)2
O
O
R
O
R
H
(D)
(C)
[Th]
O
R
O
R
O
(E)
R
H
O
R
R2
[Th] =
R1
R
[Th]
N
N
Dipp
O
O
R
(F)
N
Th
R
(Me3Si)2N
[Th]
O
R
O
O
O
R
H
R
H
(G)
Scheme 4. Proposed mechanism for the Tishchenko reaction.
3.2.2 Crossed Tishchenko reaction
Although the symmetric Tishchenko reaction presents an atom-efficient method
in producing the symmetric ester, it is still of great challenge to selectively produce
one unsymmetrical ester from the cross-coupling of two different aldehydes
(Scheme5) [50-52, 66]. For the present symmetric Tishchenko reactions mediated by
complex 4, it was found that ester conversions were highly dependent on the steric
and electronic properties of the aldehyde substrates. Therefore, we attempted the
cross-coupling of benzaldehyde with various aldehydes in the presence of catalytic
amounts of complex 4.
23

ACCEPTED MANUSCRIPT
Scheme 5. Crossed Tishchenko reaction products.
Table 3. Crossed Tishchenko reaction mediated by complex 4.a
Entry
R
R3
R/R3
1/1
3/1
1/3
1/1
3/1
1/3
1/1
1/1
1/1
11
16
53
5
6
24
1
3
15
24
12
26
5
32
26
26
17
17
27
27
13
21
16
8
15
17
9
-
40
9
14
23
16
11
36
8
62
1
8
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2-naphthyl
2-naphthyl
2-naphthyl
3-NO2Ph
3-NO2Ph
3-NO2Ph
2-pyridyl
iso-propyl
Cyclohexyl
48
a
Conditions: ~7 µmol catalyst, [Cat.]/[[R]CHO]/[[R3]CHO] = 1:50:50, rt, 600 µL of
C6D6, 36 h. The yield was calculated from 1H NMR of the crude mixture.
When the crossed Tishchenko reaction of equimolar amounts of benzaldehyde
(RCHO) and 2-naphthyaldehyde (R3CHO) was carried out, a nearly equal distribution
of the four possible ester (11 – 14) were obtained, indicating a competition of the
aldehydes in the reaction mixture. Increasing the molar ratio of one of the substrates
resulted in a sharp increment of the corresponding homocoupled symmetrical esters,
while it displayed little improvement on the selectivity towards the unsymmetrical
ester. For instance, when a 3:1 binary mixture of benzaldehyde and
2-naphthyaldehyde was used, the amounts of RCOOCH2R (11a) increased from 16%
to 53%, while the amount of the unsymmetrical esters 13a and 14a remains almost the
same (Table 3, entry 2). When 3-nitrobenzaldehyde (R3CHO) was used for the
coupling with benzaldehyde (RCHO), similar results were obtained as for the
24

ACCEPTED MANUSCRIPT
previous system in a 1:1 molar ratio, and large amounts of all four ester products were
detected. Increasing the ratio of RCHO/R3CHO to 3:1 posed little influence on the
selectivity, however, increasing the amount of the substituted aldehyde R3CHO
surprisingly increased the yield of R3COOCH2R (14b) significantly from 36 % to
62 %. Simultaneously, a dramatic decrease of the other three esters (Table 3, entry 6)
was also observed, indicating some selectivity in this reaction. Cross-coupling of
benzaldehyde with 2-pyridinylaldehyde and aliphatic aldehydes were also
investigated in an equimolar ratio, and no obvious selectivity was observed. Hence,
we concluded that the ratio among the aldehydes is not a plausible route to obtain the
selective formation of a specific ester.
3.2.3 Tandem proton-transfer esterification (TPTE) for unsymmetrical esters
The tandem proton-transfer esterification (TPTE) reaction between aldehydes and
alcohols represents a complementary methodology to the Tishchenko process in
preparing unsymmetrical esters. As the mechanism proposed in our previous work
(Scheme 6) [60, 67], it adopts a metal alkoxide moiety (I) as the active catalytic
species, which inserts into aldehydes and subsequently undergoes hydride transfer
between the metal hemiacetate and an incoming aldehyde to afford the target ester
and a metal alkoxide M. In order to complete the catalytic cycle, one more step for a
proton-transfer is necessary to regenerate the original active species, the metal
alkoxide I. Because of the fact that the TPTE process and the Tishchenko reaction
proceed via very similar catalytic cycles, with both reactions involving metal
alkoxides as the active species, the TPTE process can be viewed as an evolved
Tishchenko reaction for the preparation of unsymmetrical esters, which is of great
challenge to be achieved by traditional Tishchenko reactions. Previous studies in our
group employed actinide metallacycles as precatalysts to promote this reaction;
however, a much longer reaction time is usually required to get high conversions.
Based on the consideration that the presence of the imidazolin-2-iminato ligand will,
on one hand, reduce the electrophilicity of the metal center and on the other hand,
increase the solubility of the reaction intermediates, benzimidazolin-2-iminato
actinide complexes 3 - 8 which were expected to be better candidates for the TPTE
process were studied (Scheme 7).
25

ACCEPTED MANUSCRIPT
Scheme 6. Proposed mechanism for tandem proton-transfer esterification between
aldehyde and alcohols.
Scheme 7. Tandem proton-transfer esterification of aldehyde with alcohols.
Optimization of the reaction conditions were conducted using the catalytic system
PhCHO/CH3OH/4 as the model reaction. In the results summarized in Table 4; it was
observed that increasing the molar ratio of PhCHO/CH3OH from 1:1 to 3:1 enhanced
the yields of the corresponding methyl benzoate from 38% to 69% (Table 4, run 3),
indicating that increasing the aldehyde amount is beneficial for the isolation of the
unsymmetrical ester. In contrast, increasing the CH3OH equivalents of
PhCHO/CH3OH from 1/1 to 1/3 led to dramatic decrease of the yield from 69% to 6%
(Table 4, run 5), demonstrating its negative influence on this esterification reaction.
The TPTE process mediated by different precatalysts were also investigated, in which
the thorium complexes 6 and 8 performed in a comparable manner as for complex 4,
giving up to 70 % and 72 % yield, respectively (runs 8 and 11, Table 4). However, for
the uranium counterparts, complexes 3, 5 and 7, much lower activities were obtained
(run 6, 7 and 10, Table 4).
26

ACCEPTED MANUSCRIPT
Figure 3. Reaction progress monitored in the1H NMR for the reaction of PhCHO with
CH3OH catalysed by complex 4.
During the TPTE process, besides the target unsymmetrical ester, two additional
by products, i.e., the substituted benzyl alcohol and the symmetrical ester were also
detected, which were generated from the proton-transfer step and homocoupling of
the aldehydes, before the proton transfer, respectively (Scheme 6). As the results
shown in Table 4, using 3 equivalents of the aldehyde resulted in a minor increase of
the symmetrical ester from 11% to 19% (Table 4, run 3). In addition, it was surprising
to observe that increasing the molar ratio of MeOH/PhCHO was able to suppress the
Tishchenko reaction efficiently, and no symmetrical ester were obtained (Table 4, run
5), demonstrating a selective fashion towards the preparation of unsymmetrical esters.
Due to the presence of the symmetrical ester in the reaction mixture, in order to
rule out the possibility of trans-esterification between the benzyl benzoate and the
actinide methoxide species, to afford methyl benzoate, the progress of the reaction
1
was monitored using H NMR spectroscopy (Figure 3). It was observed that benzyl
alcohol was generated concomitantly with the methyl benzoate from the beginning of
the reaction, with a roughly 1/1 ratio. This result corroborates with the proton-transfer
step. In addition, the symmetrical benzyl benzoate can be observed only at later stages
of the reaction, when most of the alcohol was gradually consumed and the normal
Tishchenko process became gradually predominant. These observations indicate that
if a transesterification process is active, is not the main operative pathway.
27

ACCEPTED MANUSCRIPT
A comparison of the reactivities between complex 2 and the previously reported
thorium metallacycles was conducted to elucidate the influence of the
benzimidazolin-2-iminato ligand on the catalytic behaviour. Monitoring the reaction
progress (Figure 4) indicates that the rate of product formation obtained using
complex 4 is ~ 2.7 times higher than that of the thorium metallacycle. This result
indicates that the presence of the strong basic benzimidazolin-2-iminato ligand plays a
positive role in enhancing the catalytic reactivity of these actinide precatalysts. These
observations are in agreement with the results obtained for amidinate thorium
complexes, in which the introduction of a strong nucleophilic side arm resulted in
higher activities than observed for their side-arm free counterparts [34].
0.07
y = 9.21E-06x - 1.84E-03
R² = 9.92E-01
0.06
0.05
0.04
0.03
0.02
0.01
0
y = 2.54E-06x - 1.01E-03
R² = 9.87E-01
0
2000
4000
6000
8000
10000
12000
Time (s)
Figure 4. Progression of the TPTE reaction of PhCHO with CH3OH catalysed by
complex 4 (■) and the thorium metallacycle (▲).
Subsequent substrate scope investigations were carried out under the optimized
conditions and the results are summarized in Table 4. Generally, aldehydes bearing
electron-withdrawing substituents are well tolerated in the reaction and provided the
corresponding ester in high yields. These results can be mirrored from runs 10-16 in
Table 4, wherein the incorporation of nitro and cyano groups to the
para-/meta-position of the benzaldehyde gave rise to ester yields of 78% (Table 4, run
14), 85% (Table 4, run 17) and 89% (Table 4, run 19), respectively, which are much
higher than the corresponding values obtained for benzaldehyde. In contrast, the
presence of electron-donating groups or the use of heterocyclic aldehydes led to a
decrease in yields.
28

ACCEPTED MANUSCRIPT
Table 4. Tandem proton-transfer esterification between aldehydes RCHO and
alcohols R’OH. a
Yield
15 (%)[b]
38
46
69
15
6
Yield
16 (%)[c]
RCHO/ TFMAP/
Run
Cat. RCHO
R4OH
R4OH
R4OH
1
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
1/1
2/1
3/1
1/2
1/3
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
1/1
0/1
0/1
1/1
1/1
0/1
1/1
3/1
0/1
1/1
0/1
1/1
0/1
1/1
0/1
1/1
0/1
1/1
0/1
0/1
11
15
19
2
4
4
4
4
4
3
5
6
6
7
8
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2
Ph
3
Ph
4
Ph
5
Ph
-
6
Ph
14
7
-
7
Ph
-
8
Ph
70
48
12
72
63
40
78
61
47
85
72
89
86
25
9
20
-
9
Ph
10
11
12
13
14
15
16[d]
17
18
19
20
21
22
23
24
25
26
27
28
Ph
-
Ph
18
1
Ph
Ph
-
4-NO2Ph
4-NO2Ph
4-NO2Ph
4-CNPh
4-CNPh
3-NO2Ph
3-NO2Ph
4-MePh
4-MePh
2-Furanyl
2-Furanyl
25
28
5
12
-
13
3
2
-
33
7
4
-
17
3
15
-
1-Naphthyl
1-Naphthyl
4-CNPh
26
16
-
4-CNPh
iPr
-
[a] Conditions: 7 µmol catalyst, [Cat.]/[[R]CHO]/[[R4OH]= 1/150/50, 700 µL C6D6, 70
oC, 24 h, substrates were added in an order of catalyst, alcohol, aldehyde. [b] Yield was
29