Actinide Amidinate Complexes with a Dimethylamine Side Arm

Article pubs.acs.org/Organometallics

Actinide Amidinate Complexes with a Dimethylamine Side Arm: Synthesis, Structural Characterization, and Reactivity Isabell S. R. Karmel, Natalia Fridman, and Moris S. Eisen* Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology, Technion City, 32000 Israel S Supporting Information *

ABSTRACT: The reactivity of monoanionic amidinate ligands containing a dimethylamine side arm with variable lengths of the linker chain and aromatic substituents of the ipso carbon atom was investigated for the early actinides thorium and uranium. The bis(amidinate) actinide complexes obtained were structurally characterized, displaying a coordination of both dimethylamine nitrogen atoms to the respective metal center, allowing for a fine tuning of the reactivity of the complex by manipulation of the coordination environment around the metal center. The reactivity of the actinide amidinate complexes was studied in the catalytic ring-opening polymerization of ε-caprolactone.

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INTRODUCTION The coordination chemistry of the early actinides uranium and thorium has been a major subject of investigation over the past century, reaching a high level of sophistication in the last two decades.1 Special attention has been attributed to nitrogen containing ancillary ligands, such as the amido,2 imido,3 ketimido,4a−c nitrido,5 and amidinate moieties,6 as well as to macrocyclic systems.7 Coordination complexes of the actinides have been applied in a variety of catalytic and stoichiometric organic transformations,8 such as the hydroamination,9 hydrosilylation,9 hydroalkoxylation, and hydrothiolation of terminal alkynes,10 ring-opening polymerization of cyclic esters,11 coupling of acetylenes with nitriles,12 and polymerization of olefins.6d,13 The reactivity of the actinide compounds is often complementary to the regio- and chemoselectivity observed for transition-metal complexes, expanding the scope of accessible products.14 The distinctive reactivity of actinide complexes toward organic substrates can be attributed to the presence of the 5f orbitals and large ionic radii, which in turn enables large coordination numbers and unusual coordination geometries of the respective actinide compounds.15b Recently, we have shown that the reactivity of coordinatively unsaturated uranium(IV) complexes can be increased by manipulating the electronic properties of the ligands. The use of the monodentate, strongly nucleophilic imidazolin-2-iminato ligand system leads to an increased electron density on the electrophilic metal center, allowing an impressive catalytic activity in the ROP of ε-caprolactone of 7 × 103 kg mol−1 h−1.11d Consequently, a conceptual question concerns the tailoring of the reactivity of actinide complexes, by adjusting the steric and the electronic environment around the metal center. Hence, we have decided to study the reactivity of amidinate salts containing an additional side arm with a donor functionality toward thorium and uranium © 2015 American Chemical Society

motifs. The amidinates exemplify an attractive ancillary ligand system, due to the simplicity of modification of their steric and electronic properties by variation of the substituents at the nitrogen and ipso carbon atoms. Moreover, amidinate ligands are considered as steric analogues of the cyclopentadienyl moiety, although the NCN heteroallylic core is only an ionic fourelectron donor, rendering the respective metal center more electron deficient.16 The addition of a dimethylamine side arm functionality to the amidinate core increases, on one hand, the steric bulk around the metal complex formed. On the other hand, the electron lone pair of the amine functionality is expected to be donated into the empty orbitals of the metal center, increasing the electron density of the metal. Hence, in this work we present the synthesis of thorium(IV) and uranium(IV) bis(amidinate) complexes, containing a pendant dimethylamine side arm with different chain lengths. Due to the unique steric properties of the actinide complexes obtained, as well as the increased electron density on the metal, actinide bis(amidinate) complexes represent an interesting substance class for catalytic processes involving oxygen-containing substrates. The catalytic activity of the actinide(IV) bis(amidinate) complexes was therefore studied in the ROP of ε-caprolactone.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The bis(amidinate) actinide(IV) complexes 1−5 are obtained in high yields by the slow addition of a toluene solution of the amidinate ligand to a suspension of the respective AnCl4·xTHF (An = Th, x = 3; An = U, x = 0) at −78 °C (Scheme 1). Single Received: November 22, 2014 Published: January 29, 2015 636

DOI: 10.1021/om501179e Organometallics 2015, 34, 636−643

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Organometallics Scheme 1. Synthesis of Complexes 1−5

angle values of 19.88 and 22.22° for U−N1−C1−N2 and U−N4−C15−N5, respectively. Interestingly, of all the possible isomers that could be expected, the reaction produces only one isomer (complex 1), having the two chlorides disposed in a cis position that are trans to the amine groups, and two trans amidinate nitrogens bonded to the TMS moieties. The uranium(IV) complex 2 crystallizes in the in the crystallographic space group C2/c as green needle-shaped crystals (Figure 2). The U−N bond distances are similar to

Figure 1. Molecular structure of complex 1 (50% probability ellipsoids). Color code: U, blue; Si, yellow; Cl, green; N, light blue; C, gray. Hydrogen atoms are omitted for clarity.

crystals of the uranium(IV) bis(amidinate) complex 1 (Figure 1) were obtained from a concentrated toluene solution layered with hexane at −6 °C. Complex 1 crystallizes in the triclinic crystallographic space group P1.̅ The uranium center is chelated by two amidinate ligands and two chloro ligands, and additional coordination of the dimethylamine nitrogen atoms leads to a coordination number of 8. The two amidinate moieties display very similar bond lengths and angles, and the chloro ligands display nearly identical U−Cl bond distances of 2.644(5) and 2.642(5) Å for U−Cl1 and U−Cl2, respectively. The U−N bond distances display values of 2.347(13), 2.523(11), and 2.804(14) Å for the U−N1, U−N2, and U−N3 bonds, respectively, and values of 2.390(12), 2.542(10), and 2.826(14) Å for the U−N4, U−N5, and U−N6 bonds, respectively. The U−N3 and U−N6 bond distances to the dimethylamine nitrogen atoms of the side arm are elongated in comparison to the U−Namidinate bond distances, as expected. The electron delocalization along the central N−C−N linkage of the amidinate ligand can be further described by the Häflinger and Kuske parameter ΔCN = d(C−N) − d(CN) (d = bond distance in Å), which ranges from 0 to 0.178 Å for highly conjugated to nonconjugated amidinate systems, respectively.17 In the case of complex 1 ΔCN displays values of 0.061 and 0.025 Å for the N1−C1−N2 and N4−C15−N5 linkages, respectively. Hence, the higher electron density of the N2−C1 bond leads toward a weaker electron donation to the uranium center and therefore to an elongation of the U−N2 bond. This result confirms that the amidinate is coordinated in an unsymmetrical fashion. The N−U−N linkages display values of 54.6(5), 54.7(4), 94.2(4), and 61.8(4)° for N1−U−N2, N4−U−N5, N1−U−N4, and N4−U−N6, respectively, which are comparable to values for other uranium(IV) bis(amidinate) complexes,6f and dihedral

Figure 2. Molecular structure of complex 2 (50% probability ellipsoids). Color code: U, blue; Si, yellow; Cl, green; N, light blue; C, gray. Hydrogen atoms are omitted for clarity.

those of complex 1 with values of 2.393(3), 2.461(3), and 2.910(4) Å for U−N1, U−N2, and U−N4 and 2.462(3), 2.381(3), and 2.833(3) Å for U−N5, U−N6, and U−N8, respectively. The U−N bond distances to the dimethylamine nitrogen atoms are elongated in comparison to the U−Namidinate bond distances, similarly to complex 1. The Häflinger and Kuske parameter, ΔCN,17 displays very small values (within the error range of the C−N bonds) for the N1−C1−N2 and N5− C15−N6 linkages, corroborating a higher electron delocalization along the N5−C15−N6 linkage. The larger values for the N1−U−N4 and N6−U−N8 angles can be explained by the longer linker chain between the amidinate core and the dimethylamine group, leading toward the formation of a sixmembered ring. It is noteworthy that complex 2 is also the sole isomer obtained from all possibilities. While the Cl−U−Cl bond angle in complex 1 is 89.7°, which is comparable to the Cl−U−Cl angle observed in Cp*2UCl2 (97.9°),15a in complex 2 the value reaches 128.0°. This opening is a result of the larger side arm disposing the amine nitrogen trans to the amidinate nitrogen containing the side arm of the second ligand. The U−N1−C1−N2 torsion angle displays a small value of 3.27° (U−N6−C15−N5 = 5.57°), forming almost an ideal plane as expected for a symmetrical N−C−N core. 637

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Figure 3. Comparison of UV−visible and near-IR electronic absorption spectral intensities for the uranium complexes 1 and 2. The measurements were carried out in 0.004 M toluene solutions of 1 and 2, respectively.

The uranium complexes 1 and 2 were further characterized by electronic absorption spectroscopy (Figure 3), displaying molar absorptivity values in the f-f region (ε = 44.0−93.0 M−1 cm−1 for complex 1 and ε = 47−171 M−1 cm−1 for complex 2), which are consistent with the values observed by Kiplinger et al. for uranium(IV) complexes with nitrogen donor ligands.4c The higher intensity of the absorption bands for complex 2 can be attributed to the ligand-localized π−π* transitions, as well as to U−L ligand to metal charge transfer transitions, which are increased by the presence of the pyridine unit. In addition, electronic absorption spectra in the near-IR region show a similar trend for the uranium complexes 1 and 2 with molar absorptivity values in the region characteristic for f−f transitions (ε = 117−19 M−1 cm−1 and ε = 1240−253 M−1 cm−1 for complexes 1 and 2, respectively). The high intensity of the absorption bands for the uranium(IV) bis(amidinate) compounds 1 and 2 is consistent with the observations made by Kiplinger et al. for uranium(IV) bis(pentamethylcyclopentadienyl) complexes containing nitrogen-bearing ligands, such as ketimido, imido, and hydrazonato systems. The increased intensity of the electronic absorption bands suggests an “intensity-stealing” mechanism, in which the La Porte rule is broken by coupling of an electronically forbidden state with an energetically nearby electronically fully allowed state, leading to an enhanced intensity of the resulting absorption band.4c,d The observed increased intensity of the f−f bands can therefore be attributed to a coupling of f−f electronic states to metal to ligand charge transfer (MLCT) states of the respective uranium(IV) bis(amidinate) complex. In general, an augmented intensity of the f−f transitions can be correlated with a more covalent M−L bond, as first described by Henrie et al.,4e and further adapted for uranium compounds by Kiplinger et al.4c The thorium(IV) bis(amidinate) complex 4 (Figure 4) was crystallized from a concentrated toluene solution at −35 °C as colorless prisms. Complex 4 crystallizes in the triclinic crystallographic space group P1̅ with half a molecule of toluene per unit cell. The thorium center is coordinated by two amidinate ligands and two chloro ligands, and additional coordination of the amidinate side arms allows an eight-coordinated complex. The Th−N bond distances display values of 2.525(5), 2.440(6), and 2.934(6) Å for Th−N1, Th−N2, and Th−N3 and 2.436(6), 2.525(5), and 2.871(6) Å for Th−N5, Th−N6, and Th−N4, respectively, which are similar to the corresponding values in complexes 1 and 2. The electron distribution along the central N−C−N linkage of the amidinate core is slightly uneven (ΔCN = 0.014 Å for N1−C4−N2 and 0.007 Å for N5−C21−N6). The N−Th−N linkages display values of

Figure 4. Molecular structure of complex 4 (50% probability ellipsoids). Color code: Th, blue; Si, yellow; Cl, green; N, light blue; C, gray. Hydrogen atoms are omitted for clarity.

53.83(18), 53.95(17), 67.49(18), and 65.70(18)° for N2− Th−N1, N6−Th−N5, N5−Th−N4, and N2−Th−N3, respectively, which are consistent with the values observed for thorium(IV) bis(benzamidinate) complexes.6c Similarly to the uranium complexes 1 and 2 (vide supra), complex 4 is obtained as a single isomer, displaying a value of 132.2° for the Cl− Th−Cl bond angle, which is slightly larger than the value observed for the uranium complex 2, and clearly displays a value higher than the Cl−Th−Cl angle in Cp*2ThCl2 of 100.1°.15a In order to investigate the behavior of complex 3 in solution, the salt metathesis reaction was followed by 1H NMR spectroscopy as a function of time. When the reaction was carried out at room temperature, only the starting material and the complex 3 were observed, without any further byproducts or intermediate complexes. When the salt metathesis reaction was performed at elevated temperatures (70 °C), several byproducts formed; however, complex 3 could be isolated in 60% yield. Therefore, we carried out variable-temperature 1H NMR experiments with complex 3. In the temperature range between −45 and +80 °C, no further byproducts or dynamic entities were observed. Thus, it seems that complex 3 is the sole and fastest product formed, among all other possible products. Similar results were observed for the diamagnetic thorium complexes 4 and 5, indicating that compounds 4 and 5 are the unique products formed in the salt metathesis reaction. Variable-temperature 1H NMR measurements with the paramagnetic uranium complex 1 also exhibited no dynamic processes, indicative of the exclusive formation of the uranium(IV) species in solution. Interestingly, at −45 °C the signal assigned to the protons of the dimethylamine group of the amidinate side arm displayed a slight paramagnetic shift to higher values. 638

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Organometallics Table 1. Polymerization Results for the ROP of ε-Caprolactone Mediated by Complexes 1 and 3 entrya

catalyst

time/min

activity/g mol‑1 h‑1

Mw/Da

Mn/Da

PD

b

1 1 1 1 1 3 3 3 3 3 3

30 60 120 300 720 30 60 120 300 720 60

0 1699 10 768 6678 3895 6623 11507 13658 10629 4484 48675

43200 47900 64300 89800 24900 49700 177600 202100 299500 311400

22500 26500 39800 54400 14900 29500 126000 128700 148200 199300

1.92 1.81 1.61 1.65 1.67 1.68 1.40 1.57 2.02 1.56

1 2b 3b 4b 5b 6c 7c 8c 9c 10c 11b a

Ri/mol h‑1 8.76 5.65 3.50 2.04 3.05 6.09 7.23 5.62 2.40 2.58

× × × × × × × × × ×

10−5 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−3

Rt/mol h‑1 4.44 2.43 1.00 4.26 2.68 2.36 6.55 4.99 1.85 1.48

× × × × × × × × × ×

10−7 10−6 10−6 10−7 10−6 10−6 10−7 10−7 10−7 10−6

Polymerization conditions: 5 mL of toluene, 6.00 μmol of catalyst 1 or 3, catalyst/ε-caprolactone 1/500. b70 °C. cRoom temperature.

Scheme 2. Proposed Mechanism for the Cationic Ring-Opening Polymerization of ε-Caprolactone Mediated by Complexes 1 and 3

Catalytic Ring-Opening Polymerization of ε-Caprolactone. The biodegradable polymer polycaprolactone has found a wide range of applications, such as in the biomedical field (e.g. in tissue engineering19 and long-term drug delivery systems20) as well as in environmentally friendly packaging materials,21 microelectronics,22 and adhesives,23 leading to an increasing demand for polycaprolactone over the last two decades.24 Therefore, numerous studies involving maingroup,25 transition-metal,26 and lanthanide27 catalysts have been carried out, among which lanthanide amidinate complexes have shown high catalytic activities.28 Despite the large variety of compounds studied as catalysts for the ROP of ε-caprolactone, only a few examples involving actinide complexes have been previously investigated.6f,11,29 This can be attributed to the high oxophilicity of the early actinides, which leads to the formation of thermodynamically stable, catalytically inactive actinide−oxo species, impeding the use of the actinide coordination complexes as catalysts in processes involving oxygen-containing substrates; therefore, their use remains a challenge in the field of homogeneous catalysis. The bis(amidinate) actinide complexes 1−5 were investigated in the catalytic ROP polymerization of ε-caprolactone, displaying a low to moderate activity for the uranium complexes 1 and 2 at elevated temperatures (70 °C) and moderate to high activities for the thorium compounds 3−5 at room temperature, among which complexes 1 and 3 exhibited the highest catalytic activity and were therefore applied for all further catalytic studies (Table 1). The polymerization results

exhibit an increase in the molecular weight of the polymer as well as an increase in catalytic activity over time, until the monomer is completely consumed: after 5 h when the ROP is mediated by complex 1 and 2 h when mediated by complex 3. Furthermore, when the ROP catalyzed by the thorium bis(amidinate) compound 3 is performed at elevated temperatures, a higher catalytic activity and molecular weight of the polymer obtained are observed, as expected. The increase of the molecular weight over time, as well as the narrow polydispersity values of the polymer obtained, are indicative of a polymerization process, performed via a single-site catalyst. The rates of monomer insertion and chain termination were calculated using eqs 1 and 2, respectively, displaying a reduced rate of insertion monomer insertion rate (R i) =

m(polymer) [g] M w (monomer) [g/mol] × time [h]

(1) chain termination rate (R t) =

m(polymer) [g] M n(polymer) × time [h]

(2)

as the monomer is consumed in the course of the reaction, after an induction period (Table 1), suggesting a precatalyst activation (Scheme 2). After the induction period, the rate of monomer insertion (Ri) decreases as a function of time, while the molecular weight of the obtained polymer increases only slightly for the ROP of ε-caprolactone mediated by complex 1 (entries 3−5) and 3 (entries 8−10). These results can be 639

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Figure 5. Plot of rate of polymerization ∂p/∂t against concentration of catalyst 1.

Figure 8. Eyring plot for the polymerization of ε-caprolactone, mediated by complex 3.

higher activation barrier (Ea) for the uranium(IV) complex 1 than for its thorium analogue 3, with values of 18.42 and 11.36 kcal mol−1 for complexes 1 and 3 and enthalpies of activation ΔH⧧ of 17.66 and 10.67 kcal mol−1 for complexes 1 and 3, respectively. The negative values for the entropy of activation (ΔS⧧ = −23.19 and −38.35 cal mol−1 K−1 for complexes 1 and 3, respectively) indicate, as expected, an ordered transition state. Important measurements when performing catalytic transformations are poisoning experiments for determining the percentage of active catalyst. In this study, we carried out poisoning experiments with isopropyl alcohol, which indicated that all moles of complex 1 and 3 are indeed active in the catalytic ROP of ε-caprolactone. In order to investigate the mechanism for the ROP of ε-caprolactone mediated by the actinide complexes 1 and 3, reactions with the diamagnetic thorium(IV) bis(amidinate) complex 3 and stoichiometric amounts of the monomer ε-caprolactone were carried out and monitored by 1H NMR spectroscopy. These studies showed no change of the 1H NMR spectrum, even after 24 h and elevated temperatures, corroborating that there is no interaction of the monomer with the pendant amine group of the amidinate ligand in complex 3. Moreover, no incorporation of chloride into the polymer could be observed (MALDI), eliminating the possibility of a coordination−insertion mechanism as the operative polymerization pathway. Furthermore, no hydrochloric acid evolved during the polymerization reaction, ruling out the protonolysis of the chloro ligands. It is important to point out that the obtained polycaprolactone exhibited a caprolactonyl end group, as indicated by 1H and 13C NMR measurements, corroborating that no cyclic polymer is obtained. Therefore, it seems plausible that the polymerization of ε-caprolactone is catalyzed by the actinide bis(amidinate) complexes 1 and 3 following a Lewis acid mechanism as presented in Scheme 2. After a rapid activation of the monomer ε-caprolactone by the activated Lewis acidic actinide complex, the actinide alkoxocaprolate intermediate A is formed. The nucleophilic attack of a further monomer unit on A leads to the open-chain compound B, which is the rate-determining step (rds) of the reaction. The growing polymer chain C is obtained by further insertions of ε-caprolactone units at the chain of intermediate B, which upon elimination of the polymer with a caprolactonyl end group (D) regenerates the active catalyst. The activity of the actinide(IV) bis(amidinate) complexes 1−5 with a dimethylamine side arm was investigated in the catalytic ring-opening polymerization of ε-caprolactone. The thorium complexes 3−5 displayed moderate to high activities at room temperature, depending on the nature of the substituent in the ipso position, as well as on the length of the linker of the

Figure 6. Plot of rate of polymerization ∂p/∂t against concentration of catalyst 3.

attributed to the increasing viscosity of the polymer solution, which reduces the mobility of the monomers and therefore reduces the rate of monomer insertion into the growing polymer chain, (Trommsdorff−Norrish effect). In order to determine the dependence of the reaction on the catalyst and ε-caprolactone concentration, kinetic NMR experiments were carried out, displaying a first-order dependence on complex 1 (Figure 5) and 3 (Figure 6), respectively, and a first-order dependence on ε-caprolactone, therefore giving rise to the kinetic rates given in eqs 3 and 4. ∂p = kobs[complex 1][ε‐caprolactone] ∂t

(3)

∂p = kobs[complex 3][ε‐caprolactione] ∂t

(4)

The thermodynamic parameters were determined from the Eyring and Arrhenius plots (Figures 7 and 8), displaying a

Figure 7. Eyring plot for the polymerization of ε-caprolactone mediated by complex 1. 640

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were synthesized according to published literature procedures. Pyridine (Sigma-Aldrich) was distilled under reduced pressure from CaH2 and stored in the glovebox prior to use. NMR spectra were recorded on DPX200, Avance 300, and Avance 500 Bruker spectrometers. Chemical shifts for 1H NMR and 13C NMR are reported in ppm and referenced using residual proton or carbon signals of the deuterated solvent relative to tetramethylsilane. For crystal structure determinations, single crystals, immersed in Parathone-N oil, were quickly fished with a glass rod and mounted on a KappaCCD diffractometer under a cold stream of nitrogen at 230 K. Data collection was carried out with monochromized Mo Kα radiation using ω and π scans to cover the Ewald sphere.33 Accurate cell parameters were obtained with the amount of indicated reflections.34 The structure was solved by SHELXS-97 direct methods35 and refined by the SHELXL-97 program package.36 The atoms were refined anisotropically. Hydrogen atoms were included using the riding model. The software used for molecular graphics was Mercury 3.1.37 Elemental analysis was carried out by the microanalysis laboratory at the Hebrew University of Jerusalem. GPC measurements were carried out on a Waters Breeze system with a styrogel RT column and with THF (HPLC grade, T.G. Baker) as the mobile phase at 30 °C. Relative calibration was done with polystyrene standards (Aldrich, 2000−1800000 range). Mn values were multiplied by a factor of 0.58 and correlated to actual PCL values. General Procedure for the Synthesis of Bis(amidinate)uranium(IV) Complexes. A flame-dried Schlenk flask, equipped with a magnetic stirring bar and a frit, was charged with UCl4 (500 mg, 1.32 mmol) inside the glovebox. A second Schlenk flask was charged with the respective ligand (2.0 equiv, 2.64 mmol) inside the glovebox. Toluene (ca. 30 mL) was condensed into both flasks using vacuum transfer. The reaction flask containing UCl4 was cooled to −78 °C (acetone/dry ice bath), and the toluene solution of the ligand was added slowly via syringe to the UCl4 suspension under a constant stream of argon. Then, the reaction mixture was covered from light, warmed slowly to room temperature, and stirred for 48 h at room temperature. Then the reaction mixture was filtered, the toluene was evaporated, and the solid residue was washed with hexane (2 × 10 mL). The products were obtained as green powders. Bis((N-(2-dimethylamino)ethyl)-N′-(trimethylsilyl)benzamidinate) uranium(IV) Chloride (1). Yield: 87% (960.0 mg, 1.15 mmol). 1H NMR (C6D6, 300.0 MHz): δ −0.46 (br s, 2 H, (CH3)2NCH2CH2), −0.24 (s, 9 H, Si(CH3)3), −0.54 (br s, 6 H, (CH3)2NCH2CH2), 1.51 (br s, 2 H, (CH3)2NCH2CH2), 6.55−7.19 (m, 5H, Har). 13C NMR (C6D6, 50.0 MHz): δ 3.7 (Si(CH3)3), 21.5 ((CH3)2NCH2CH2), 46.7 ((CH3)2NCH2CH2), 63.1 ((CH3)2NCH2CH2), 126.1−132.0 (Car− H), 137.5 (Car-C), 165.2 (NC(Ph)N). 29Si NMR (C6D6, 59.6 MHz): δ −16.43. Anal. Calcd: C, 40.33; H, 5.80; N, 10.08; Cl, 8.50. Found: C, 40.65; H, 5.72; N, 10.12; Cl, 8.59. Bis((N-(2-dimethylamino)propyl)-N′-(trimethylsilyl)pyridylamidinate) uranium(IV) Chloride (2). Yield: 80% (913.0 mg, 1.06 mmol). 1H NMR (C7D8, 300.0 MHz): δ −0.11 (s, 9 H, Si(CH3)3), 1.12−1.36 (m, 6 H, (CH3)2NCH2CH2CH2), 2.30 (br s, 6 H, N(CH3)2), 7.23−8.16 (m, 4 H, Har). 13C NMR (C7D8, 50.0 MHz): δ 4.7 (Si(CH3)3), 31.5 ((CH3)2NCH2CH2CH2), 35.2 ((CH3)2NCH2CH2CH2), 51.2 ((CH3)2NCH2CH2CH2), 55.2 ((CH3)2NCH2CH2CH2), 121.7−147.5 (Car−H), 157.3 (Car−C), 167.5 (NC(Py)N). 29Si NMR (C7D8, 59.6 MHz): δ −11.49. Anal. Calcd: C, 38.93; H, 5.83; N, 12.97: Cl: 8.21. Found: C, 38.51; H, 5.86; N, 13.01; Cl, 8.25. General Procedure for the Preparation of Bis(amidinate)thorium(IV) Complexes. A flame-dried Schlenk flask, equipped with a magnetic stirring bar and a frit, was charged with ThCl4·3THF (500 mg, 0.85 mmol) inside the glovebox. A second Schlenk flask was charged with the respective ligand (2.0 equiv, 1.69 mmol) inside the glovebox. Toluene (ca. 30 mL) was condensed into both flasks using a vacuum transfer. The reaction flask, containing ThCl4·3THF, was cooled to −78 °C (acetone/dry ice bath), and the toluene solution of the ligand was added slowly via syringe to the ThCl4·3THF solution under a constant stream of argon. Then, the reaction mixture was warmed slowly to room temperature and stirred for 48 h at room temperature. The reaction mixture was filtered, the toluene was evaporated, and the

dimethylamine side arm. While complex 3 (n = 2, Ar = Ph) displayed the highest catalytic activity among the complexes, followed by the thorium compound 4 (n = 3, Ar = Ph), complex 5 (n = 3, Ar = Py) exhibited the lowest activity among the thorium(IV) complexes 3−5. In analogy to the trend observed for the thorium compounds, the uranium complex 1 (n = 2, Ar = Ph) displayed a higher catalytic activity than complex 2 (n = 2, Ar = Py). Therefore, the trend for the activity of actinide bis(amidinate) complexes follows the order 3 > 4 > 5 > 1 > 2. Thus, the thorium complexes 3−5 are more active than their uranium analogues 1 and 2 and the complexes bearing a phenyl moiety at the ipso position of the amidinate ligand (1, 3, and 4) are more active than their analogue pyridyl amidinates (5 and 2). Moreover, complexes with a shorter (n = 2) linker at the dimethylamine side arm (1 and 3) are more active in the catalytic ROP of ε-caprolactone. These results are in good agreement with our previous studies with uranium(IV) bis(imidazolin-2-iminato) complexes, in which we suggested that a higher electron density of the metal center increases the catalytic activity toward oxygen-containing substrates, such as ε-caprolactone. Hence, the amidinate complexes 2 and 5 with an electron-withdrawing pyridyl moiety at the ipso position of the amidinate ligand render the metal more electrophilic, which leads to a decreased catalytic activity (vide supra).

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CONCLUSIONS This work introduces a new family of actinide bis(amidinate) complexes with a dimethylamine nitrogen donor side arm that can be obtained in all cases as a single isomer. The thorium(IV) and uranium(IV) complexes 1−5 display a coordination of both dimethylamine side arms to the metal center, in contrast to the respective early-transition-metal complexes, which allow for the coordination of only one of the ligand side arms to the metal center.18 The additional coordination of the dimethylamine side arm to the actinide center leads to a more defined geometry around the metal, allowing for a manipulation of the reactivity by tuning the steric and electronic properties of the metal complex. Herein, we address the fundamental question of tuning the reactivity of actinide coordination complexes in the ROP of ε-caprolactone, by manipulating the steric environment around the metal, using an amidinate ligand system, with an electron-donating side arm. Mechanistic studies were carried out, revealing a single-site Lewis acid catalyzed ROP mechanism, as corroborated by the narrow polydispersity values of the polymers obtained. Furthermore, experiments with stoichiometric amounts of monomer indicated that all of the ligands stay coordinated to the metal center throughout the entire polymerization process. Kinetic 1H NMR experiments gave rise to a first-order dependence of the reaction rate on the concentration of complex 1 and 3, respectively, and ε-caprolactone.

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EXPERIMENTAL SECTION

All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flamed Schlenk-type glassware on a high-vacuum line (10−5 Torr) or in a nitrogen-filled Vacuum Atmospheres glovebox with a medium-capacity recirculator (1−2 ppm of oxygen). Argon and nitrogen were purified by passage through a 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 (THF, hexane, toluene, benzene-d6, toluene-d8). UCl4,30 ThCl4· 3THF,31 Li[Me3 SiNC(Ph)NCH2CH2 NMe2 ], Li[Me3 SiNC(Ph)NCH2CH2CH2NMe2], and Li[Me3SiNC(Py)NCH2CH2CH2NMe2]32 641

DOI: 10.1021/om501179e Organometallics 2015, 34, 636−643

Article

Organometallics solid residue was washed with hexane (2 × 10 mL). The products were obtained as colorless powders. Bis((N-(2-dimethylamino)ethyl)-N′-(trimethylsilyl)benzamidinate) thorium(IV) Chloride (3). Yield: 92% (645 mg, 0.78 mmol). 1H NMR (C7D8, 300.0 MHz): δ −0.02 (s, 6 H, Si(CH3)3), −0.08 (s, 3 H, Si(CH3)3), 1.14−1.26 (m, 2 H, (CH3)2NCH2CH2), 1.85−1.90 (m, 2 H, (CH3)2NCH2CH2), 2.32 (br s, 6 H, (CH3)2NCH2CH2), 7.08−7.32 (m, 6 H, Har). 13C NMR (C7D8, 50.0 MHz): δ 3.9 (Si(CH3)3), 32.1 ((CH3 )2 NCH2 CH2), 41.5 ((CH3)2 NCH2 CH2 ), 61.5 ((CH3 )2 NCH2CH2), 126.5−131.5 (Car−H), 135.1 (Car-C), 168.0 (NC(Ph)N). 13 C NMR (C7D8, 59.6 MHz): δ −0.19 (Si(CH3)3), −0.30 (Si(CH3)3). Anal. Calcd: C, 40.62; H, 5.84; N, 10.15; Cl, 8.56. Found: C, 40.73; H, 5.81; N, 10.09; Cl, 8.47. Bis((N-(2-dimethylamino)propyl)-N′-(trimethylsilyl)benzamidinate)thorium(IV) Chloride·0.5C8H8 (4). Yield: 90% (652 mg, 0.76 mmol). 1 H NMR (C7D8, 300.0 MHz): δ −0.07 (s, 9 H, Si(CH3)3), 1.30 (m, 2 H, (CH3)3NCH2CH2CH2), 2.07 (br s, 6 H, (CH3)3NCH2CH2CH2), 2.09 (m, 2 H, (CH3)3NCH2CH2CH2), 2.93 (m, 2 H, (CH3)3NCH2CH2CH2), 7.17−7.21 (m, 5 H, Har). 13C NMR (C7D8, 50.0 MHz): δ 3.7 (Si(CH3)3), 31.5 ((CH3)3NCH2CH2CH2), 39.5 ((CH 3 ) 3 NCH 2 CH 2 CH 2 ), 59.5 ((CH 3 ) 3 NCH 2 CH 2 CH 2 ), 63.5 ((CH3)3NCH2CH2CH2), 121.5−128.5 (Car−H), 145.1 (Car-C), 168.5 (NC(Ph)C). 29Si NMR (C7D8, 59.6 MHz): δ −0.95 (Si(CH3)3). Anal. Calcd: C, 42.1; H, 6.12; N, 9.82; Cl, 8.28. Found: C, 42.5; H, 6.17; N, 9.77; Cl, 8.21. Bis((N-(2-dimethylamino)propyl)-N′-(trimethylsilyl)pyridylamidinate) thorium(IV) Chloride (5). Yield: 83% (596 mg, 0.69 mmol). 1H NMR (C7D8, 300.0 MHz): δ −0.25 (s, 9 H, Si(CH3)3), 1.43−1.45 (m, 2 H, (CH3)3NCH2CH2CH2), 1.89 (br s, 6 H, (CH3)3NCH2CH2CH2), 3.53−3.55 (m, 4 H, (CH3)3NCH2CH2CH2), 7.00−7.32 (m, 2 H, Har), 8.12−8.34 (m, 2 H, Har). 13C NMR (C7D8, 50.0 MHz): δ 4.9 (Si(CH 3 ) 3 ), 32.1 ((CH 3 ) 3 NCH 2 CH 2 CH 2 ), 43.2 ((CH 3 ) 3 NCH2CH2CH2), 55.5 ((CH3)3NCH2CH2CH2), 59.1 ((CH3)3NCH2CH2CH2), 122.1−133.9 (Car−H), 155.7 (Car-C), 171.9 (NC(Py)N). 29Si NMR (C7D8, 59.6 MHz): δ −32.8 (Si(CH3)3). Anal. Calcd: C, 39.20; H, 5.88; N, 13.06; Cl, 8.26. Found: C, 39.53; H, 5.92; N, 13.12; Cl, 8.34. General Procedure for the Catalytic Ring-Opening Polymerization of ε-Caprolactone. A sealable J. Young glass tube, equipped with a magnetic stirring bar, was loaded with 6.00 μmol of complex 1 or 3 from a stock solution, the required amount of ε-caprolactone, and 5 mL of dry toluene inside the glovebox. The polymerization was carried out with strong stirring for the required amount of time and temperature. Then, the reaction was quenched by the addition of methanol. After the solvent was removed under reduced pressure, the polymer was precipitated from cold methanol, isolated by filtration, washed with three portions of cold methanol (3 × 20 mL), and dried overnight under vacuum. The activity was determined as (PCL (g))/((cat. (mol))(time (h))). A sample of the obtained PCL (40 mg) was dissolved in THF and used for determination of the molecular weight. For the kinetic 1H NMR studies a J. Young NMR tube was loaded with the respective amount of complex 1 or 3 from a stock solution, ε-caprolactone and toluene-d8 were added inside the glovebox, and the tube was sealed. The reaction mixture was frozen at liquid nitrogen temperatures, until the 1H NMR measurements were started. The sample was heated (if required) inside the NMR spectrometer. Similar experiments were performed for the thermodynamic studies.

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation, administered by the Israel Academy of Science and Humanities under Contract No. 78/14.

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, and CIF files giving crystallographic data for complexes 1−5. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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DOI: 10.1021/om501179e Organometallics 2015, 34, 636−643

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DOI: 10.1021/om501179e Organometallics 2015, 34, 636−643