Thorium complexes possessing expanded ring N-heterocyclic iminato ligands: synthesis and applications

Tapas Ghatak , Shani Drucker , Natalia Fridman and Moris S. Eisen *
Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Technion Haifa, 32000, Israel. E-mail:

Received 11th June 2017 , Accepted 27th June 2017

First published on 27th June 2017

Six and seven membered N-heterocyclic iminato ligands (L) are introduced allowing access a new class of Th(IV) complexes of the type Cp*2Th(L)(CH3). These complexes were studied in the Tishchenko reaction. Stoichiometric reactions together with kinetic and thermodynamic studies permit us to propose a plausible mechanism.

During the last three decades, the chemistry of the early actinides has emerged as a promising research area.1 A vast progress in actinide chemistry has been obtained using complexes of the type Cp*2An(CH3)2 (An = Th, U). This is attributed to their unique structure–reactivity relationships.2 Polymerization of α-olefins,3 hydrosilylation,4 hydroamination,5 hydrothiolation, and hydroalkoxylation of alkynes,6 the isonitrile–alkyne coupling,7 and the cyclization of terminal alkynes,8 comprise some of the homogeneous processes catalysed by these organoactinides. Interestingly, organic transformations involving oxygen-containing substrates promoted by organoactinide complexes are still a challenge and remain scarce. This is recognised due to the formation of thermodynamically stable, catalytically inactive, actinide–oxo species. The first report on the dimerization of an aromatic aldehyde promoted by the actinides, Cp*2An(CH3)2 (1), appeared in 2008.9 An improvement of the catalytic activity was achieved by employing a constrained-geometry catalyst (CGC) with bridged cyclopentadienyl ligands,10 and the introduction of the isolobal N-heterocyclic iminato ligands, leads to significant enhancement of the catalytic activity.11

The N-heterocyclic iminato moiety with five-membered rings (A and B in Scheme 1) represent the common class of these type of ligands.12 Recently, we have reported that changing the size of the heterocycle moiety from five- to six-membered rings allows the development of the perimidine scaffold (C in Scheme 1), which led to highly active organoactinide complexes towards the challenging addition of alcohol to carbodiimides.13

image file: c7dt02126g-s1.tif
Scheme 1 Typical heterocyclic frame works of N-heterocyclic iminato ligand.

All 5- and 6-membered N-heterocyclic iminato ancillary ligands possess a nearly planar heterocyclic framework, and this feature generates constraints on the spatial arrangement of the wingtip substituents. To alleviate this restrain, we aimed towards a new class of N-heterocyclic iminato core with a seven-membered heterocyclic framework that would undergo a torsional twist to relax ring strain (Scheme 2).

image file: c7dt02126g-s2.tif
Scheme 2 Schematic presentation of a seven-membered N-heterocyclic iminato framework.

Here, we described the facile synthesis of two Th(IV) complexes one possessing the new seven-membered ring ligand and the second complex contains the six-membered N-heterocyclic iminato framework C. In this study, we address the fundamental question of the influence of the ring size on catalytic activity of an organoactinide complex. The goal of this investigation was to examine the scope, chemoselectivity, ancillary ligand sensitivity, kinetics, and thermodynamics of the Tishchenko reaction as the study case.

The syntheses of the actinide complexes 2 and 3 were accomplished by a one pot reaction between the Cp*2Th(CH3)2 (1) with the respective N-heterocyclic imine in toluene at room temperature, which resulted in the immediate evolution of methane (Scheme 3). X-ray quality crystals of the complexes were obtained from concentrated toluene solutions of the complexes at −35 °C. The solid-state structures of complexes 2 and 3 are depicted in Fig. 1a and b, respectively.

image file: c7dt02126g-s3.tif
Scheme 3 Syntheses of Cp*2Th(L1)(Me) (2) and Cp*2Th(L2)(Me) (3).

image file: c7dt02126g-f1.tif
Fig. 1 Molecular structure of complex 2 (a) and 3 (b), with thermal ellipsoid set at the 50% probability levels. All Hydrogen atoms are omitted for the clarity.

The X-ray structure analysis of complex 2 revealed that the tetrahedral coordinated thorium is surrounded by two pentamethylcyclopentadienyl (Cp*) ligands, one methyl group and the perimidin-2-iminato ligand. The Th–N1imine bond distance of 2.225(5) Å is comparable to those distances observed in other imidazolin-2-iminato thorium structures, indicating a high bond order for the Th–N bond.11 The Th–Cp*centroid distances are 2.57 and 2.56 Å, and the Th–CH3 = 2.488(12) Å, all marginally shorter than those observed for Cp*2Th(CH3)2.

Analysis of the crystal structure of the free ligand L2H (Fig. S1) shows as expected axial symmetry resulting from the torsional twist of the seven-membered N-heterocyclic imine rings (torsional angle between two phenyl rings is 42.2(2)°).

For complex 3, the Th–N bond (2.217(7) Å), Th–Cp*centroid (2.55 and 2.58 Å) distances are identical to those observed in complex 2, indicating that electronically those complexes are alike. Despite our repetitive attempts, the preparation of stable analogues possessing the imidazolin-2-iminato ligands with smaller wingtip substituents (iso-propyl, tert-butyl) were unsuccessful.11

Complexes 2 and 3 were utilized as precatalysts for the dimerization of aldehydes (Tishchenko reaction) towards the corresponding esters in high yields and at room temperature. Aromatic, heteroaromatic, cyclic, and aliphatic aldehydes comprise the broad scope of substrates (Table 1). In general, the activity of complex 2 is equivalent or slightly better than that of complex 3.

Table 1 Homocoupling of aldehydes mediated by complexes 2 and 3

image file: c7dt02126g-u1.tif

Entrya RCHO Precatalyst Yieldb
2 h 12 h 24 h
a Reaction conditions: Catalyst/RCHO 1[thin space (1/6-em)]:[thin space (1/6-em)]100, rt, 0.5 mL of C6D6. b NMR conversion determined from 1H NMR spectroscopy.
1 Ph 2 40 65 96
2 3 31 60 90
3 o-NO2-Ph 2 25 45 56
4 3 23 42 55
5 m-NO2-Ph 2 71 91 95
6 3 59 80 85
7 p-NO2-Ph 2 71 88 98
8 3 48 80 90
9 o-Cl-Ph 2 27 65 90
10 3 21 58 86
11 p-Cl-Ph 2 26 75 80
12 3 17 80 88
13 p-F-Ph 2 21 25
14 3 18 20
15 p-Br-Ph 2 80 >99
16 3 47 75 88
17 p-I-Ph 2 85 95
18 3 70 90
19 p-CN-Ph 2 82 >99
20 3 73 98
21 p-MeO-Ph 2 20 30
22 3 15
23 p-CH3-Ph 2 35 61 66
24 3 24 55 60
25 1-Naphthyl 2 53 85 96
26 3 49 84 90
27 2-Naphthyl 2 34 55 75
28 3 22 45 50
29 2-Pyridyl 2 33 48 78
30 3 12 43 70
31 2-Furyl 2 20
32 3 18
33 2-Thiophen 2 21 30
34 3 18
35 Cyclohexyl 2 100
36 3 100
37 Cyclopentyl 2 100
38 3 100
39 Isopropyl 2 100
40 3 100

The quantitative conversion of benzaldehyde to desired product was achieved by both complexes 2 and 3 (entries 1 and 2). However, and for comparison, only 70% conversion was achieved with the corresponding imidazolin-2-iminato-Cp*–Th complexes, even after increasing the catalyst loading to 1 mol% and extending the reaction time to 48 h. Aromatic aldehydes with electron withdrawing groups react faster than these aldehydes bearing electron donating groups. Cyclic or branched aliphatic aldehydes were found to be the most active substrates yielding quantitative products within 2 h. Interestingly, product yields depended on the position of the substituent at the phenyl ring. For example, the ortho nitrobenzaldehyde, showed the lower activity (entries 3 and 4) as compared to the corresponding para analogues (entries 7 and 8). Important to point out, that the corresponding imidazolin-2-iminato-Cp* Th(IV) complexes showed no catalytic activity with ortho nitrobenzaldehyde. By comparing the halogen position effect in the aromatic aldehydes, the ortho and para chloro benzaldehyde showed similar reactivity however, for different halogens, the trend follows I ∼ Br > Cl > F (entries 9–18). p-Cyanobenzaldehyde was found to be very active producing quantitative yields within 12 h (entries 19 and 20), while aromatic aldehydes with electron rich substrates such as p-methoxy benzaldehyde and p-methyl benzaldehyde were rather slow (entries 21–24). The similar imidazolin-2-iminato-Cp* Th(IV) complexes were found almost inactive towards the electron rich substrates such as p-methoxy benzaldehyde and p-methyl benzaldehyde.

Moreover, 1-napththaldehyde was found to be more active than 2-naphthaldehyde (entries 25–28) and almost quantitative conversion was achieved with 1-napththaldehyde, whereas for the imidazolin-2-iminato-Cp* Th(IV) complexes, 85% conversion was obtained after increasing the catalyst loading to 1 mol%. Regarding heteroaromatic aldehydes, pyridine-2-carbaldehyde, showed moderate conversion to the corresponding ester (entries 29 and 30) whereas furfural and thiophene-2-carbaldehyde exhibited lower reactivities (entries 31–34). The decrease in reactivity is observed for those substrates interacting (plausible via a dative coordination) with the highly electrophilic thorium catalyst, inhibiting the coupling of the aldehyde and the metal-alkoxo moiety (vide infra in the catalytic cycle). To annul any coordination effect, cyclic and acyclic aliphatic aldehydes were also studied. Cyclopentanecarboxaldehyde, cyclohexanecarboxaldehyde and isobutyraldehyde produced quantitatively the corresponding esters, in short amounts of time, with both complexes (entries 35–40).

The reactivity of complexes 2 and 3 was also studied towards the intramolecular Tishchenko reaction of phthalaldehyde yielding the quantitative lactone within 10 min (Scheme S2, Fig. S6). We were encouraged to perform the cross-Tishchenko reactions (Table 2) with complex 2, which in principle may produce a mixture of four different esters. Hence, the reaction between equimolar amounts of benzaldehyde and 1-naphthaldehyde resulted, as expected, in the four possible esters, however with an excess of the asymmetrically substituted ester III (entry 1, Table 2). Longer reaction times led to the formation of the symmetrically substituted ester I and II with trace amount of the asymmetrically substituted ester IV. The chemoselective formation of III over other ester was achieved by increasing the ratio among the aldehydes to RCHO[thin space (1/6-em)]:[thin space (1/6-em)]R′CHO 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5 (entry 2). The formation of ester III indicates that a naphthylic thorium alkoxo species is formed first, which reacted preferentially with benzaldehyde, and the subsequent hydride transfer to another 1-naphthaldehyde released the product and closed the catalytic cycle (vide infra). In a similar fashion, in the equimolar cross Tishchenko reaction between benzaldehyde and pyridine-2-carbaldehyde a mixture of esters are obtained with a clear preference towards compound III. Increasing the ratio among the aldehydes to 1.5 allows obtaining ester III, as the sole reaction product (entry 4, Table 2). The selectivity in this reaction is presumably due to hydride acceptor and donor properties of the aldehydes; with benzaldehyde being the better donating substrate whereas pyridine-2-carbaldehyde as the better acceptor compound.

Table 2 Cross-Tishchenko Reaction Catalyzed by Complex 2a

image file: c7dt02126g-u2.tif

Entry RCHO R′CHO Yield (%)
a Reaction conditions: Catalyst 2/RCHO 1[thin space (1/6-em)]:[thin space (1/6-em)]100, rt, 0.5 mL of C6D6. Yield was determined by 1H NMR spectroscopy of the crude reaction mixture after 1.5 h. The yield is based on the moles of the aromatic aldehyde. b RCHO[thin space (1/6-em)]:[thin space (1/6-em)]R′CHO = 1[thin space (1/6-em)]:[thin space (1/6-em)]1. c RCHO[thin space (1/6-em)]:[thin space (1/6-em)]R′CHO = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5. d RCHO[thin space (1/6-em)]:[thin space (1/6-em)]R′CHO = 1.5[thin space (1/6-em)]:[thin space (1/6-em)]1.
1b PhCHO 1-Naphthyl 17 16 62 4
2c PhCHO 1-Naphthyl 95
3b PhCHO 2-Pyridyl 3 6 80 10
4c PhCHO 2-Pyridyl 100
5c PhCHO Cyclohexyl 27 46 26
6d PhCHO Cyclohexyl 20 35 40
7b PhCHO Isopropyl 18 55 6 25
8d PhCHO Isopropyl 46 28 11 15
9c PhCHO Isopropyl 77 23

When the cross Tishchenko reaction was performed with benzaldehyde and an electron-rich aliphatic aldehyde, such as cyclohexanecarboxaldehyde or isobutyraldehyde (entries 5–9), as expected, the ester III was either not observed or obtained in minimal amounts (depending on the ratio among the aldehydes). In these reactions the aliphatic aldehyde will be the better hydride donor tilting the selectivity towards esters II and IV unless an excess of the aromatic aldehyde is utilized, which produces also the symmetrical ester I.

Equimolar poisoning experiments were performed with the secondary alcohol isopropanol in order to determine the percentage of active catalyst. A reduced catalytic activity of 25% was observed using complex 2[thin space (1/6-em)]:[thin space (1/6-em)]iso-propanol ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]0.25. When the ratio was changed to 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5, the catalytic activity was reduced by 50%. This result indicates that all the methyl groups are active in the catalytic cycle. In addition, 1H NMR experiments, after the poisoning experiments, confirmed that the perimidin-2-iminato ligand remains coordinated to the metal centre. An additional stoichiometric reaction with complex 2 and one equivalent benzyl alcohol resulted in the formation of the mono alkoxo complex 2a which showed identical catalytic efficiency in similar kinetic manner to complex 2 (Scheme 4).

image file: c7dt02126g-s4.tif
Scheme 4 Stoichiometric reaction with complex 2 and one equivalent benzyl alcohol.

Kinetic studies of the initial rates of the reaction with complexes 2 or 3 were measured to determine that the reaction follows a first order dependence in aldehyde and complexes, thus apparent rate law for the dimerization of aldehyde promoted by precatalysts 2 and 3 can be formulated as depicted in eqn (1).

dp/dt = kobs × [2 or 3]1 × [PhCHO]1.(1)

When the reaction was studied using α-deuterated benzaldehyde a primary KIE of 2.7 (0.2) was measured. This result indicates that the hydride transfer is involved at the rate-determining step.14 Thermodynamic studies reveal an energy of activation (Ea), enthalpy of activation (ΔH) and entropy of activation (ΔS) of 3.48(4) kcal mol−1, 2.63(4) kcal mol−1, and −68.4(3) e.u., respectively. The high negative entropy value corroborates with a highly organized transition state at the RDS.

A plausible mechanism for the dimerization of aldehydes by complex 2 is presented in Scheme 5.

image file: c7dt02126g-s5.tif
Scheme 5 Proposed mechanism for the Tishchenko reaction mediated by complex 2.

In the first step, an aldehyde inserts rapidly into the Th–CH3 bond via a four-centered transition state, to obtain the thorium alkoxo intermediate a. A second insertion of an aldehyde into the Th–O bond results in intermediate b and the subsequent hydride transfer to an incoming aldehyde, via a chair like six-membered transition state (c), produces the catalytically active intermediate d and 1 equiv. of the α-methylated ester. The insertion of an aldehyde into the Th–O bond of d forms the intermediate complex e, which upon hydride transfer reaction (step 5, RDS) with an additional aldehyde close the catalytic cycle towards the product and the regeneration of the active complex d.

In conclusion, here we disclosed the first example of an organometallic complex with this new family of six and seven membered N-heterocyclic iminato ligands. The seven-membered N-heterocyclic iminato ligand exhibits a torsional twist resulting in a C2 symmetry. Both of the ligands with small substituent allow us to access a new class of complexes of type Cp*2Th(L)(CH3), as very active catalyst for the catalytic dimerization of aldehydes.


This work was supported by the Israel Science Foundation administered by the Israel Academy of Science and Humanities under Contract No. 78/14; and by the PAZY Foundation Fund (2015) administered by the Israel Atomic Energy Commission.


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  14. Detailed analysis of kinetic rate law can be found in ESI..


This manuscript is dedicated to Prof. Evamarie Hey Hawkins (a great friend and a colleague) for the occasion of her 60th birthday.
Electronic supplementary information (ESI) available. CCDC 1548353–1548355. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02126g

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