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Article

Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations

1
Department of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
2
School of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK
*
Authors to whom correspondence should be addressed.
Inorganics 2021, 9(12), 86; https://doi.org/10.3390/inorganics9120086
Received: 21 October 2021 / Revised: 17 November 2021 / Accepted: 24 November 2021 / Published: 28 November 2021
(This article belongs to the Special Issue Cornerstones in Contemporary Inorganic Chemistry)

Abstract

:
Tripodal multidentate ligands have become increasingly popular in f-element chemistry for stabilizing unusual bonding motifs and supporting small molecule activation processes. The steric and electronic effects of ligand donor atom substituents have proved crucial in both of these applications. In this study we functionalized the previously reported tris-anilide ligand {tacn(SiMe2NPh)3} (tacn = 1,3,7-triazacyclononane) to incorporate substituted aromatic rings, with the aim of modifying f-element complex solubility and ligand steric effects. We report the synthesis of two proligands, {tacn(SiMe2NHAr)3} (Ar = C6H3Me2-3,5 or C6H4Me-4), and their respective group 1 transfer agents—{tacn(SiMe2NKAr)3}, M(III) complexes [M{tacn(SiMe2NAr)3}] for M = La and U, and U(IV) complexes [M{tacn(SiMe2NAr)3}(Cl)]. These compounds were characterized by multinuclear NMR and FTIR spectroscopy and elemental analysis. The paramagnetic uranium complexes were also characterized by solid state magnetic measurements and UV/Vis/NIR spectroscopy. U(III) complexes were additionally studied by EPR spectroscopy. The solid state structures of all f-block complexes were authenticated by single-crystal X-ray diffraction (XRD), together with a minor byproduct [U{tacn(SiMe2NC6H4Me-4)3}(I)]. Comparisons of the characterization data of our f-element complexes with similar literature examples containing the {tacn(SiMe2NPh)3} ligand set showed minor changes in physicochemical properties resulting from the different aromatic ring substitution patterns we investigated.

1. Introduction

Coordinatively unsaturated metal complexes with well-defined reactive sites are ideal candidates for systematic reactivity studies, and as with the rest of the periodic table, such systems can be accessed for f-elements through the use of judiciously selected ligands [1]. Given the predominantly electrostatic bonding regimes and large ionic radii of f-elements, sterically demanding ligands with hard donor atoms provide the kinetic and electronic stabilization required to give robust complexes [2]. Multidentate and macrocyclic ligands form a privileged subset over their monodentate analogues in f-element coordination chemistry, as they can impart additional thermodynamic stability [1,2]. Tripodal examples, which exhibit approximate C3 symmetry in solution, have proved particularly effective in controlling the coordination sphere of uranium by encapsulating the metal ion in a single well-defined “steric pocket” along the C3 axis. Such motifs have supported unusual uranium oxidation states and bonding regimes, and coordinatively unsaturated U(III) centers that support rich small molecule activation chemistry [3,4,5,6,7].
Over the last 25 years, the tripodal ligands that have proved most popular for generating landmark uranium complexes include the anchored tris-aryloxides {N(CH2OAr)3} (Ar = substituted aryl), {tacn(CH2OAr)3} (tacn = 1,3,7-triazacyclononane) and {Mes(CH2OAr)3} (Mes = C6H2Me3-2,4,6); and the tris-amides {N(CH2CH2NSiR3)3} (SiR3 = SiMe3, SitBuMe2, SiiPr3) [3,4,5,6,7]. Several related macrocyclic ligand systems have more recently been applied in f-element chemistry [8,9,10]. f-Element complexes of tripodal ligands that impose approximate C3 symmetry have been applied in the electrocatalytic reduction of water [11,12,13], and it has been shown that the steric and electronic effects of substituents on donor atoms of these ligands are crucial for dictating the physicochemical properties of resultant complexes. For example, the reaction of CO2 with [U{tacn(CH2ArtBu,tBuO)3}] (ArtBu,tBuO = 3,5-di-tert-butyl-2-oxybenzyl) gives the dinuclear U(IV) bridging oxo complex [{U[tacn(CH2ArtBu,tBuO)3]}2(µ-O)] [14], whereas the more sterically demanding [U{tacn(CH2ArAd,AdO)3}] (ArAd,AdO = 3-adamantyl-5-tert-butyl-2-oxybenzyl) reacts with CO2 to yield the remarkable terminal CO2 U(IV) complex [U{tacn(ArAd,AdO)3}(CO2)] [15]. Those examples showcase how the outcomes of small molecule activation reactions for tripodal U(III) complexes are dependent upon the size of the axial reactivity pocket [3,4,5,6,7].
By contrast, there have only been a handful of reports to date of the synthesis and physicochemical properties of uranium complexes of the related tris-anilido ligand {tacn(SiMe2NPh)3} [16,17,18,19,20]. However, it has already been shown that the SiMe2 linkers engender considerable flexibility compared to the more rigid substituted aryl groups in [U{tacn(CH2ArO)3}] [3,4,5,6,7]. For example, the nearly trigonal prismatic arrangement of N-donor atoms in the U(III) complex [U{tacn(SiMe2NPh)3}] [16] is lost upon oxidation by elemental sulfur to give the distorted bicapped trigonal bipyramidal dinuclear U(IV) complex [{U[tacn(SiMe2NPh)3]}2(µ-S)] [17], whilst approximate C3 symmetry is typically retained when the U(III) complexes [U{tacn(CH2ArO)3}] are converted to U(IV/V/VI) products [3,4,5,6,7].
Herein we report the synthesis of two novel tris-anilido ligands with substituted aryl rings, {tacn(SiMe2Nar)3} (Ar = C6H3Me2-3,5; C6H4Me-4). We utilized group 1 transfer agents of these ligands to synthesize U(III), La(III) and U(IV) complexes; and we characterized these complexes by a variety of techniques in order to compare these data with respective literature examples of lanthanum and uranium complexes of the unsubstituted ligand {tacn(SiMe2NPh)3}.

2. Results

2.1. Synthesis

The ligand precursors {tacn(SiMe2NHAr)3} (Ar = C6H4Me-4, 1-H3; C6H3Me2-3,5, 2-H3) were prepared by modifications of the reported synthesis of {tacn(SiMe2NHPh)3} [21] (Scheme 1). The substituted anilines NH2C6H4Me-4 and NH2C6H3Me2-3,5 were separately reacted with excess NaH in THF to provide the sodium anilides NaNHC6H4Me-4 and NaNHC6H3Me2-3,5. These salts were filtered from the remaining NaH and were separately added dropwise to toluene solutions of {tacn(SiMe2Cl)3} [21] to give 1-H3 and 2-H3, respectively, as pale yellow oils in very good yields after removal of volatiles under reduced pressure. The proligands were both reacted with excess KH in THF without further purification to afford the group 1 transfer agents {tacn(SiMe2NKC6H4Me-4)3}·2THF (1-K3) and {tacn(SiMe2NKC6H3Me2-3,5)3}·THF (2-K3), respectively, in fair–good yields as off-white powders (Scheme 1). Despite repeated attempts, crystals of 1-K3 and 2-K3 suitable for analysis by single-crystal X-ray diffraction could not be obtained, but elemental analysis values obtained from powders dried in vacuo for 1 h indicated that bound THF molecules remained, as is the case for the Li and Na salts of {tacn(SiMe2NPh)3} [21].
With 1-K3 and 2-K3 in hand, we adapted the syntheses of [La{tacn(SiMe2NPh)3}(THF)] and [U{tacn(SiMe2NPh)3}] [16] to perform separate salt metathesis reactions with [La(I)3(THF)4] [22] and [U(I)3(THF)4] [23] in THF to give [M{tacn(SiMe2NC6H4Me-4)3}] (M = La, 3-La, U, 3-U) and [M{tacn(SiMe2NC6H3Me2-3,5)3}] (M = La, 4-La, U, 4-U) in fair yields following work-up and recrystallization (Scheme 2). We first performed the lanthanum reactions, as these have no radiological hazard and La(III) is diamagnetic, providing NMR spectra that are often more easily interpreted than 5f3 U(III) [3,4,5,6,7]. Additionally, La(III) is a reasonable surrogate for U(III) due to their comparable ionic radii [6-coordinate, La(III) = 1.032 Å, U(III) = 1.025 Å] [24]. It is noteworthy that on one occasion during the synthesis of 3-U, we observed several crystals of the U(IV) complex [U{tacn(SiMe2NC6H4Me-4)3}(I)] (5), which we attribute to the presence of traces of UI4, but the low yield precluded solution phase characterization (see below). Instead of optimizing conditions for the synthesis of 5, we decided to target analogous U(IV) chloride complexes, as straightforward salt metathesis routes to [U{tacn(SiMe2NPh)3}(Cl)] have been reported previously [10]. By adapting these procedures, the separate reactions of UCl4 [25,26] with 1-K3 and 2-K3 gave the U(IV) complexes [U{tacn(SiMe2NC6H4Me-4)3}(Cl)] (6) and [U{tacn(SiMe2NC6H3Me2-3,5)3}(Cl)] (7) in poor yields, following work-up and recrystallization from THF or DME, respectively (Scheme 2). We found that 6 and 7 could also be synthesized by the respective oxidation of 3-U or 4-U with excess tBuCl. This oxidative route gave improved yields of 6 and 7 over salt metathesis from UCl4. We anticipate that the U(III) complexes 3-U and 4-U could give rich reactivity with small molecules and unsaturated organic compounds, as previously seen for similar tripodal U(III) complexes [3,4,5,6,7]; however, we were unable to isolate any uranium-containing products from the separate 1:1 reactions of 4-U with either benzophenone or 4,4′-bipyridine in toluene. Since [U{tacn(SiMe2NPh)3}] has been shown to react with elemental sulfur [17], and the 2e oxidation reactions of other tripodal U(III) complexes have been shown to give U(V) products [3,4,5,6,7], we attempted the synthesis of terminal U(V) oxo complexes by the separate reactions of 3-U with pyridine N-oxide in toluene and 4-U with 4-methyl morpholine N-oxide in THF; no products could be identified from either of these reaction mixtures.

2.2. NMR and IR Spectroscopy

NMR spectra are presented in the ESI Figures S13–S42. The 1H and 13C{1H} NMR spectra of 1-H3 in C6D6 indicate that approximate C3 symmetry was present on the NMR timescale at 298 K. One signal was observed for each proton environment in the 1H NMR spectrum apart from the para-Me resonances, where two signals were observed in a 3:6 ratio (δH: 2.11 and 2.20 ppm). The three NH protons were obscured by the broad multiplets of the tacn protons (δH: 2.98–3.19 ppm). Similar features were observed for 2-H3, though with the meta-Me resonances in a 12:6 ratio (δH: 2.23 and 2.24 ppm) in the 1H NMR spectrum. In both 1-H3 and 2-H3 there are also more Ar-H resonances than would be expected from ideal C3 symmetry, and this deviation is tentatively attributed to conformational rigidity of the tacn ring in solution rendering the CH2 protons diastereotopic. The low solubility of 1-K3 and 2-K3 in benzene and toluene necessitated the employment of deuterated THF for NMR spectroscopy, precluding accurate integration of the proton THF resonances to validate elemental analysis data. The addition of more polar solvents to 1-K3 and 2-K3, such as pyridine, lead to decomposition. The doubling of the number of resonances in the 13C and 29Si NMR spectra of 1-K3Si: −18.48 and −15.33 ppm) and 2-K3Si: −17.88 and −14.74 ppm) compared to the parent anilines 1-H3Si: −8.02 ppm) and 2-H3Si: −7.97 ppm) indicates that either coordinated THF reduced these systems to approximate Cs symmetry or that two different approximately C3 symmetric complexes formed in solution. The former interpretation is the most plausible, as coordinating solvents should disfavor oligomerization.
The most noteworthy difference between the 1H NMR spectra of 3-M and 4-M and their parent proligands is that the tacn methylene group signals split into two multiplets with AA′BB′ patterns and a relative intensity of 1:1 upon complexation. This pattern of signals is due to a significant deviation from ideal C3 symmetry and fluxional behavior, and was previously seen for [M{tacn(SiMe2NPh)3}] (M = Y, Eu, Yb, U) and [La{tacn(SiMe2NPh)3}(THF)] by Marques and co-workers [16]. Following variable temperature 1H NMR spectroscopy experiments on those literature complexes, the authors postulated that the two N3 vertices of an approximate trigonal prism twist with respect to each other with a concomitant inversion of the tacn chelate rings; thus, we infer that similar processes are in operation for 3-M and 4-M. As expected, the 1H NMR spectra of 3-U and 4-U are paramagnetically shifted, with signals from −30 to +10 ppm that show significant line broadening [27]. The 1H NMR spectra of 6 and 7 displayed similar features to those of 3-U and 4-U, but the signals were broadened to a greater extent, with some FWHM values being as high as 1200 Hz for 6, and the aromatic ring protons of 7 were not observed. The bulk features of these spectra are similar to those previously seen for [U{tacn(SiMe2NPh)3}(Cl)] [16]. Only one resonance was seen for the methylene groups in the 13C{1H} NMR spectra of 3-La and 4-La, but no signals could be observed in the 13C{1H} NMR spectra of 3-U, 4-U, 6 and 7 due to paramagnetism. The 29Si{1H} NMR spectra of 3-LaSi: −5.10 ppm) and 4-LaSi: −5.18 ppm) are deshielded compared to 1-H3 and 2-H3; whilst no signals were seen for the U(IV) complexes 6 and 7, the U(III) complexes exhibited remarkably shielded resonances (δSi: −263.80 ppm, 3-U; −270.90 ppm, 4-U) that to the best of our knowledge are the most negative values reported for any U(III) complex to date [28].
The FTIR spectra of all complexes containing {tacn(SiMe2NAr)3} scaffolds exhibit strong absorptions at ν ˜ ~ 1600 cm−1 due to the substituted anilide groups, as has been seen by other authors for f-block complexes containing {tacn(SiMe2Nar)3} [16,17,18,19,20]. In addition, 1-H3 ( ν ˜ = 3406 cm−1) and 2-H3 ( ν ˜ = 3404 and 3377 cm−1) exhibit N–H stretching absorptions, as does {tacn(SiMe2HNHPh)3} ( ν ˜ = 3381 cm−1) [16].

2.3. NIR/Vis/UV Spectroscopy

The electronic absorption spectra of 3-U, 4-U, 6 and 7 were recorded as 0.5 mM toluene solutions (Figure 1). The similar spectra of the pale green U(IV) complexes 6 and 7 are dominated by intense charge transfer bands in the UV region that tail into the visible region, and are otherwise essentially featureless. The charge transfer bands are less prominent at higher energy for red 3-U and 4-U solutions, as they both exhibit a series of strong absorptions with shoulders in the visible region ( ν ˜ max, cm−1 (ε, M−1 cm−1): ~17,000 (~628, 3-U; ~692, 4-U); ~20,000 (~1,444, 3-U; ~1,758, 4-U); ~23,000 (~1,308, 3-U; ~1,612, 4-U)) that are mainly assigned as 5f26d1 ← 5f3 transitions, consistent with U(III) centers [6]. In the NIR region, 3-U and 4-U exhibit nearly identical series of weak absorptions (ε < 250 M−1 cm−1) that are assigned as 5f ← 5f transitions with intensities that are typical of U(III) complexes [29].

2.4. Structural Characterization

The molecular structures of 3-M·0.5C7H8, 4-M, 5, 6·THF and 7 were determined by single-crystal X-ray diffraction (selected bond lengths and angles are listed in Table 1, and supporting crystallographic information is compiled in the ESI Tables S1 and S2). The structures of 3-U·0.5C7H8, 4-U, 6·THF and 7 are shown in Figure 2 and Figure 3. As 3-La·0.5C7H8 and 4-La are structurally analogous with 3-U·0.5C7H8 and 4-U, respectively, these are discussed together with U(III) congeners for brevity; and their structures are depicted in the ESI together with 5, which shows similar features to 6·THF (Figures S1–S3).
There are two independent complex molecules in the unit cells of 3-M·0.5C7H8. This was seen previously for [U{tacn(SiMe2NPh)3}]·0.5C7H8 and was attributed to the presence of two isomers [16]. As the metrical parameters of the isomers are similar, we describe one of the molecules of 3-M here. In each case, the metal centers exhibit approximate trigonal prismatic geometries defined by approximately parallel and near-eclipsed amine (N1-N3) and anilide (N4-N6) planes, and lie much closer to the anilide [M···N4-N6 (Å): 0.468(2) (3-La); 0.513(4) (3-U)] than the amine [M···N1-N3 (Å): 2.064(2) (3-La); 2.030(4) (3-U)] planes. Despite differences in 6-coordinate ionic radii (La(III) = 1.032 Å, U(III) = 1.025 Å) [18], significant disorder in the dataset for 3-U led to poor resolution of M–N distances, to the extent that these were within statistical error of the corresponding distances in 3-La. The bulk features of 4-M are similar to those of their 3-M analogs; for example, in 4-U the position of the uranium center within the N6 prism [U···N4-N6: 0.509(8) Å; U···N1-N3: 2.002(8) Å; mean U–Namine: 2.63(2) Å; U–Nanilide: 2.36(2) Å] is almost identical to those seen for [U{tacn(SiMe2NPh)3}] [16] and 3-U (see Figure S4 for space-filling representations of [U{tacn(SiMe2NPh)3}], 3-U and 4-U viewed along the approximate C3 axes above the N-aryl rings).
Although 5–7 all crystallize in different space groups and 6·THF additionally incorporates a molecule of THF in the unit cell, the metrical parameters of 5–7 are similar to those of [U{tacn(SiMe2NPh)3}(Cl)]. Whilst [U{tacn(SiMe2NPh)3}(I)] was also synthesized previously by oxidation of [U{tacn(SiMe2NPh)3}] with iodine, its solid state structure was not reported [16]. As with [U{tacn(SiMe2NPh)3}(Cl)] [16], the positions of the donor atoms in 5–7 can be described as bicapped trigonal bipyramids with one anilide and two amine donors forming the equatorial plane; the halide and remaining amine at the axial positions [N(1)–U(1)–X(1): 169.7(5)° (5); 171.7(2)° (6); 171.7(2)° (7)]; and the other two anilides capping two faces near the halide apex. The uranium centers are situated above the N3-tacn plane [U···N1-N3 (Å): 1.04(2) (5); 1.021(5) (6); 1.011(6) (7)] such that for 6 and 7 the apical U–N distances [2.683(7) Å (6); 2.667(9) Å (7)] are similar to the U–Cl distances [2.706(3) Å (6); 2.687(2) Å (7)].

2.5. Magnetism and EPR Spectroscopy

Solution magnetic susceptibility measurements of U(III) and U(IV) complexes, measured in C6D6 at 298 K using the Evans method [30], gave similar χT values: 3-U (1.05 cm3 K mol−1), 4-U (0.95 cm3 K mol−1), 6 (0.98 cm3 K mol−1) and 7 (1.13 cm3 K mol−1). Whilst these molar susceptibilities are all lower than the free ion values [U(III) 5f3 4I9/2 = 1.70 cm3 K mol−1; U(IV) 5f2 3H4 = 1.60 cm3 K mol−1], this was expected, as not all crystal field levels are thermally occupied at this temperature [31]. Indeed, the values obtained are around the middle of the previously reported ranges for monometallic U(III) (0.38–1.81 cm3 K mol−1) and U(IV) (0.48–1.34 cm3 K mol−1) complexes [32]. Variable temperature solid state magnetic susceptibility measurements for powdered samples of 3-U, 4-U, 6 and 7 suspended in eicosane were recorded using a SQUID from 300–2 K (ESI Figures S5–S9). At 300 K, the magnetic susceptibility temperature products [χT (cm3 K mol−1): 0.84 (3-U); 0.70 (4-U); 0.60 (6); 0.80 (7)] were consistently lower than solution values by 0.2–0.4 cm3 K mol−1, but such discrepancies should be expected from changes in phase, together with variable errors associated with sample masses and diamagnetic corrections. At 298 K, the solid state χT value for 3-U is similar to that previously reported for [U{tacn(SiMe2NPh)3}] (0.94 cm3 K mol−1) [19]. U(IV) complexes 6 and 7 showed χT temperature dependence that correlates with metal oxidation state, tending towards zero at low temperatures due to a non-magnetic singlet ground state [χT (cm3 K mol−1) at 2 K: 0.07 (6); 0.10 (7)]. The U(III) complexes, 3-U and 4-U, reached values that are higher than those of 6 and 7 [χT (cm3 K mol−1) at 2 K: 0.20 (3-U); 0.15 (4-U)], but did not reach their low temperature limit, which suggests occupation of low lying excited states [33,34,35]. This is consistent with the magnetization data (see below).
The U(III) formulations of 3-U and 4-U were confirmed by X- and Q-band EPR spectroscopy on powdered samples at 5 K (X-band spectrum of 4-U shown in Figure 4a; see ESI Figures S10–S12 for other spectra). The spectra of 4-U are characteristic of an effective spin ½, consistent with a Kramers doublet, with geff = 2.80, 2.37 and 1.17. Complex 3-U shows greater rhombicity in the geff values, but they are complicated by what appears to be an impurity, and we were unable to model these spectra. As U(III) complexes can exhibit single-molecule magnet (SMM) behavior in small applied magnetic fields [33,34,35], we investigated the dynamic magnetic properties of 3-U and 4-U. Both complexes exhibited hysteresis at 1.8 K, but this was more pronounced for 3-U (Figure 4b) than for 4-U (see ESI Figure S6d); in both cases, magnetic saturation was not achieved at 7 T, the physical limit of the magnetometer. Ac susceptibility measurements were performed on 3-U and 4-U under an applied dc field of 600 G at 1.8 K. Whilst 3-U did not exhibit in-phase or out-of-phase components, 4-U displayed clear frequency dependent behavior (Figure 4c,d). An Arrhenius plot of the data gave an effective energy barrier to magnetic reversal, Ueff, of 16.2 (±0.8) K (see ESI Figure S9).

3. Discussion

Comparisons of the multinuclear NMR spectra of 3-M and 4-M with each other and analogous M(III) {tacn(SiMe2NPh)3} complexes [16] indicate that minor variations in anilide substitution patterns do not considerably affect dynamic solution behavior. The solubility and solution stability of complexes also did not appear to vary significantly upon ligand substitution. In the solid state, the single-crystal XRD data show that most of the metrical parameters of 3-U (e.g., mean U–Namine: 2.651(13) Å; U–Nanilide: 2.38(2) Å) are nearly identical to those reported previously for [U{tacn(SiMe2NPh)3}] [U···N1–N3: 0.52 Å; U···N4–N6: 2.02 Å; mean U–Namine: 2.66(3) Å; U–Nanilide: 2.35(3) Å] [10]. However, the ranges of Nanilide–U–Nanilide [3-U: 110.8(3)–119.0(3)°; 4-U: 112.9(5)–117.9(6)°] and U–Nanilide–Cipso [3-U: 115.8(6)–127.0(7)°; 4-U: 117.7(10)–125.9(9)°] angles are greater for 3-U than for 4-U, consistent with nearer-axial EPR spectra of 4-U, and showing that substitution of the N-aryl groups can influence both the shape and size of the apical channel in the solid state. Structural differences are even more pronounced for the La(III) homologs 3-La and 4-La, which do not contain bound THF molecules, in contrast with the 7-coordinate La(III) unsubstituted analog [La{tacn(SiMe2NPh)3}(THF)] [16]. This change in coordination number leads to more significant changes in metrical parameters; for example, the mean La–Namine [2.685(5) Å] and La–Nanilide [2.415(5) Å] distances for 3-La differ markedly from [La{tacn(SiMe2NPh)3}(THF)] [mean La–Namine: 2.751(8) Å; La–Nanilide: 2.453(8) Å] [16].
The main differences between the solid state structures of the U(IV) and U(III) complexes reported herein are that the approximate C3 symmetry is broken upon oxidation of the metal center, with the seventh coordination site being occupied by a halide ion. Despite the increase in metal oxidation state, the respective mean U–Nanilide and U–Namine distances of 5–7 were similar to those seen for 3-U and 4-U, which we attribute to the considerable rearrangement of metal coordination spheres. This facile reorganization is a distinguishing feature of the flexible SiMe2 linkers in {tacn(SiMe2NAr)3} frameworks [16,17,18,19,20] compared to the more rigid tethered aryloxides in {tacn(CH2ArO)3)}, where oxidation of U(III) starting materials to U(IV/V/VI) products has been shown to proceed with retention of approximate C3 symmetry [3,4,5,6,7]. As {tacn(SiMe2NAr)3} scaffolds readily reorganize to accommodate an additional donor atom, we posit that THF may bind to 3-M and 4-M in solution, but that this molecule is readily displaced upon exposure to vacuum during work-up and recrystallization. A detailed reactivity study would be required to determine if the fluxionality of {tacn(SiMe2NAr)3} frameworks in solution reduces the steric effect of anilide substituents compared to analogous R-group variation in more rigid tripodal ligands.
A comparison of the characterization data for the U(III) complexes 3-U, 4-U and [U{tacn(SiMe2NPh)3}] [16,19] indicates that minor changes in the anilide substituents can lead to subtle changes in the physicochemical properties of complexes. The UV/Vis/NIR electronic absorption spectrum of 4-U shows more intense absorption intensities than that of 3-U. To the best of our knowledge, the corresponding data for [U{tacn(SiMe2NPh)3}] have not been published. Similarly, as we were only able to model the EPR spectra of 4-U (geff = 2.80, 2.37 and 1.17) and we could not find literature EPR data for [U{tacn(SiMe2NPh)3}], we cannot make a detailed comparison of structurally similar complexes, though we note that geff =3.54(5), 2.042(4) and 1.66(5) were determined for [U{tacn(SiMe2NPh)3}(OPPh3)], where the phosphine oxide occupies the apical position [20]. We previously reported the EPR spectra of the planar U(III) tris-amide [U{N(SiBuMe2)2}3] [36]. They gave geff (=3.55, 2.97, 0.55) that approach those expected for the |mJ| = 1/2 doublet of a 4I9/2 term (3.65, 3.65, 0.73), which is stabilized by the in-plane crystal field. For 4-U, the uranium center is close to the anilide N3 plane, which likewise would stabilize the |mJ| = 1/2 state, but the out-of-plane crystal field arising from the three amine donors leads to significant mixing. Assuming three-fold symmetry, the mJ = ±1/2 state can mix with both the ±5/2 and ±7/2 states (assuming a 4I9/2 term) leading to very different g-values. A similar result was concluded for the parent [U{tacn(SiMe2NPh)3}], where crystal field calculations gave a very mixed ground state comprising almost equal fractions of |mJ| = 5/2 and 1/2 (EPR spectra were not reported) [19]. Finally, we note that the SMM behavior of U(III) {tacn(SiMe2NAr)3} complexes appears to be quite sensitive to anilide substituent variation. No effective barrier to magnetic reversal was seen for [U{tacn(SiMe2NPh)3}] [19] and 3-U, whereas Ueff = 16.2 (±0.8) K for 4-U is similar to values reported previously for U(III) complexes [33,34,35]; e.g., [U{tacn(SiMe2NPh)3}(OPPh3)], Ueff = 21.9(7) K [20].

4. Materials and Methods

General Information

Caution—uranium-238 (t1/2 = 4.47 × 109 years) is a weak α-emitter; therefore, all manipulations should be performed in suitable laboratories that have been designated for radiochemical use, and α-counting equipment should be available. All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of dry argon. THF, toluene, n-hexane and DME were dried by refluxing over potassium. All solvents were stored over K mirrors (with the exception of THF and DME, which were stored over activated 4 Å molecular sieves), and were degassed prior to use. Deuterated solvents were dried over K, degassed by three freeze-pump-thaw cycles and stored under Ar. tacn(SiMe2Cl)3 [21], [La(I)3(THF)4] [22], [U(I)3(THF)4] [23] and UCl4 [25,26] were prepared according to literature procedures, and all other reagents were purchased. p-Toluidine was dried for 4 h under vacuum before use, whilst 3,5-dimethylaniline was refluxed over CaH2 and distilled. KH and NaH were obtained as suspensions in mineral oil and were washed three times with n-hexane before use.
1H, 13C{1H} and 29Si{1H} NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer operating at 400.1, 100.6 and 79.5 MHz, respectively; chemical shifts are quoted in ppm and are relative to TMS. FTIR spectra were recorded as Nujol mulls in KBr discs on a Perkin Elmer Spectrum RX1 spectrometer (Perkin Elmer, Waltham, MA, USA). Elemental microanalyses were carried out by Stephen Boyer at the Microanalysis Service, London Metropolitan University, or by Martin Jennings and Anne Davies at The University of Manchester. Low carbon values were consistently obtained in elemental analyses of 2-K3, 3-La and 4-La. We attribute this observation to silicon carbide formation, as <5% protic impurities were observed by 1H NMR spectroscopy, and we note that low C values were intermittently obtained in microanalysis experiments for f-element complexes of the related ligand {tacn(SiMe2NPh)3} [16]. UV-Vis-NIR spectroscopy was performed on samples in Youngs tap appended 10 mm pathlength quartz cuvettes on an Agilent Technologies Cary Series UV-Vis-NIR Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) from 175–3300 nm. X- and Q-band EPR spectroscopy was performed on powdered samples in quartz tubes at 5 K sealed under vacuum. Magnetic measurements were made using a Quantum Design MPMS-XL7 SQUID magnetometer on ground crystalline samples suspended in eicosane in borosilicate tubes sealed under vacuum.
Crystals of 3-M·0.5C7H8, 4-M (M = La, U) and 5–8 were examined on a Rigaku Oxford Diffraction SuperNova CCD area detector diffractometer (Rigaku, Tokyo, Japan) using mirror-monochromated Mo Kα radiation (λ = 0.71073 Å). Intensities were integrated from data recorded on 1° frames by ω rotation. Cell parameters were refined from the observed positions of all strong reflections in each data set. A Gaussian grid face-indexed absorption correction with a beam profile correction was applied [37]. The structures were solved variously by direct and heavy atom methods using SHELXS [38] or SHELXT [39] and were refined by full-matrix least-squares on all unique F2 values [38], with anisotropic displacement parameters for all non-hydrogen atoms, and with constrained riding hydrogen geometries; Uiso(H) was set at 1.2 (1.5 for methyl groups) times Ueq of the parent atom. The largest features in final difference syntheses were close to those of heavy atoms and were of no chemical significance. CrysAlisPRO [37] was used for control and integration, SHELX [38] and was employed through OLEX2 [40] for structure solution and refinement and ORTEP-3 [41] and POVRAY [42] were used for molecular graphics.
Synthesis of {tacn(SiMe2NHC6H4Me-4)3} (1-H3): p-toluidine (5.36 g, 50.0 mmol) was added to a slurry of NaH (2.16 g, 90.0 mmol) in THF (30 mL) and was heated to 50 °C for 18 h. The resultant brown suspension was allowed to cool to room temperature and filtered. The dark brown supernatant was added dropwise to a solution of tacn(SiMe2Cl)3 (6.30 g, 15.5 mmol) in toluene (50 mL) at −78 °C, allowed to slowly warm to room temperature and stirred for 18 h. Volatiles were removed under reduced pressure and the resultant yellow oil was extracted with hexane (3 × 50 mL). The solvent was removed under reduced pressure to give crude 1-H3 as a viscous yellow oil (7.56 g, 79%), which was used without further purification. Anal. Calcd. for C33H54N6Si3: C, 64.02; H, 8.79; N, 13.58. Found: C, 64.22; H, 8.94; N, 13.44. 1H NMR (C6D6, 298 K): δ = 0.21 (18H, s, Si(CH3)2), 2.11 (3H, s, Ar-CH3), 2.20 (6H, s, Ar-CH3), 2.98–3.19 (15H, br m, NCH2 and NH), 6.61 (d, 4H, JHH = 8.0 Hz, Ar-H), 7.01 (m, 6H, JHH = 8.0 Hz, Ar-H), 7.13 (2H, br m, JHH = 8.0 Hz, Ar-H). 13C{1H} NMR (C6D6, 298 K): δ = –0.69 (Si(CH3)2), 20.99 (Ar-CH3), 50.96 (NCH2), 117.25 (p-Ar-CH), 127.25 (Ar-CH), 130.45 (Ar-CH), 145.33 (ipso-Ar-C). 29Si{1H} NMR (C6D6, 298 K): δ = –8.02 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 3406 (m), 1614 (s), 1512 (w), 1285 (s), 1167 (m), 1148 (m), 1109 (m), 995 (s), 968 (s), 895 (s), 692 (m), 638 (m).
Synthesis of {tacn(SiMe2NKC6H4Me-4)3}·2THF (1-K3): 1-H3 (7.56 g, 12.2 mmol) was dissolved in THF (20 mL) and added dropwise to a suspension of KH (2.79 g, 70 mmol) in THF (10 mL) at −78 °C. The reaction mixture was allowed to warm to room temperature, heated to 50 °C overnight, allowed to cool and filtered. Volatiles were removed under reduced pressure and the resultant solid was washed with hexane (20 mL) and dried in vacuo to yield 1-K3 as an off-white powder (6.10 g, 62%). Anal. Calcd. for C41H67K3N6O2Si3: C, 56.11; H, 7.70; N, 9.52. Found: C, 55.95; H, 7.80; N, 9.47. 1H NMR (C4D8O, 298 K): δ = 0.01 (s, 12H, Si(CH3)2), 0.11 (s, 6H, Si(CH3)2), 1.78 (m, 4H, OCH2CH2), 2.05 (s, 9H, Ar-CH3), 2.72–2.91 (m, 12H, NCH2), 3.62 (m, 4H, OCH2CH2), 6.23 (d, 4H, JHH = 8.0 Hz, Ar-H), 6.56 (d, 6H, JHH = 8.0 Hz, Ar-H), 6.63 (d, 2H, JHH = 8.0 Hz, Ar-H). 13C{1H} NMR (C4D8O, 298 K): δ = 0.22 (Si(CH3)2), 0.39 (Si(CH3)2), 20.74 (Ar-CH3), 20.83 (Ar-CH3), 26.55 (OCH2CH2), 49.19 (NCH2), 49.32 (NCH2), 68.39 (OCH2CH2), 117.46 (p-Ar-C), 118.82 (p-Ar-C), 122.23 (Ar-CH), 122.82 (Ar-CH), 130.66 (Ar-CH), 130.90 (Ar-CH), 160.09 (ipso-Ar-C), 160.94 (ipso-Ar-C). 29Si DEPT NMR (C4D8O, 298 K): δ = –18.48, –15.33 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1595 (s), 1172 (m), 1057 (m), 995 (s), 955 (m), 893 (m), 870 (m), 756 (m), 683 (w), 635 (w).
Synthesis of {tacn(SiMe2NC6H3Me-3,5)3} (2-H3): 3,5-dimethylaniline (7.23 g, 59.7 mmol) was added to a slurry of NaH (3.46 g, 144.0 mmol) in THF (30 mL) and heated to 50 °C for 18 h. The resultant brown suspension was allowed to cool to room temperature and filtered. The dark brown supernatant was added dropwise to a solution of tacn(SiMe2Cl)3 (7.51 g, 18.5 mmol) in toluene (50 mL) at –78 °C, allowed to slowly warm to room temperature and stirred for 18 h. Volatiles were removed under reduced pressure and the resultant yellow oil was extracted with hexane (3 × 50 mL). The solvent was removed under reduced pressure to give crude 2-H3 as a viscous yellow oil, which was used without further purification (10.40 g, 85%). Anal. Calcd. for C36H60N6Si3: C, 65.40; H, 9.15; N, 12.71. Found: C, 64.94; H, 9.14; N, 12.38. 1H NMR (C6D6, 298 K): δ = 0.23 (s, 12H, s, Si(CH3)2), 0.27 (s, 6H, s, Si(CH3)2), 2.23 (s, 12H, Ar-CH3), 2.24 (6H, s, Ar-CH3), 3.02–3.25 (15H, br m, NCH2 and NH), 6.32 (s, 4H, Ar-H), 6.35 (s, 2H, Ar-H), 6.45 (s, 3H, Ar-H). 13C{1H} NMR (C6D6, 298 K): δ = 0.68 (Si(CH3)2), 22.08 (Ar-CH3), 51.03 (NCH2), 115.35 (Ar-CH), 120.62 (Ar-CH), 138.93 (m-Ar-C), 147.70 (ipso-Ar-C). 29Si{1H} NMR (C6D6, 298 K): δ = −7.97 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 3404 (m), 3377 (m), 1599 (s), 1406 (m), 1325 (m), 11776 (s), 1120 (s), 1109 (m), 1057 (s), 966 (m), 821 (br, s), 691 (s).
Synthesis of {tacn(SiMe2NKC6H3Me2-3,5)3}·THF (2-K3): 2-H3 (10.40 g, 15.7 mmol) was dissolved in THF (20 mL) and added dropwise to a suspension of KH (2.16 g, 54.0 mmol) in THF (10 mL) at −78 °C. The reaction mixture was allowed to warm to room temperature and heated to 50 °C overnight, allowed to cool and filtered. Volatiles were removed under reduced pressure, and the resultant solid was washed with hexane (20 mL) and dried in vacuo to yield 2-K3 as an off-white powder (12.50 g, 86%). Anal. Calcd. for C40H65K3N6OSi3: C, 58.69; H, 7.73; N, 9.92. Found: C, 54.96; H, 7.72; N, 9.95. 1H NMR (C4D8O, 298 K): δ = 0.04 (s, 12H, s, Si(CH3)2), 0.13 (s, 6H, s, Si(CH3)2), 1.78 (m, OCH2CH2), 2.00 (s, 12H, Ar-CH3), 2.06 (6H, s, Ar-CH3), 2.72-2.93 (12H, br m, NCH2), 3.62 (m, OCH2CH2), 5.70 (s, 2H, Ar-H), 5.78 (s, 1H, p-Ar-H), 6.00 (s, 4H, Ar-H), 6.29 (s, 2H, Ar-H). 13C{1H} NMR (C4D8O, 298 K): δ = 0.35 (Si(CH3)2), 0.43 (Si(CH3)2), 22.31 (Ar-CH3), 22.36 (Ar-CH3), 26.55 (OCH2CH2), 49.16 (NCH2), 49.39 (NCH2), 68.39 (OCH2CH2), 112.52 (Ar-CH), 113.67 (Ar-CH), 120.32 (Ar-CH), 120.91 (Ar-CH), 138.34 (m-Ar-C), 138.74 (m-Ar-C), 162.82 (ipso-Ar-C), 163.25 (ipso-Ar-C). 29Si DEPT NMR (C4D8O, 298 K): δ = –17.88, –14.74 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1557 (s), 1352 (m), 1240 (m), 1194 (m), 1173 (m), 1117 (m), 1057 (m), 978 (w), 916 (m), 893 (m), 843 (m), 760 (m), 654 (m).
Synthesis of [La{tacn(SiMe2NC6H4Me-4)3}] (3-La): THF (20 mL) was added to a pre-cooled (−78 °C) mixture of [La(I)3(THF)4] (0.40 g, 0.5 mmol) and 1-K3 (0.44 g, 0.5 mmol). The pale yellow reaction mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed under reduced pressure and the residue was extracted with toluene (2 × 10 mL), reduced in volume to ca. 2 mL and stored at −30 °C overnight to give colorless crystals of 3-La·0.5C7H8 (0.13 g, 32%). Anal. Calcd. for C33H51LaN6Si3·C7H8: C, 56.71; H, 7.02; N, 9.92. Found: C, 51.64; H, 6.93; N, 9.54. 1H NMR (C6D6, 298 K): δ = 0.27 (s, 18H, Si(CH3)2), 2.21 (s, 9H, Ar-CH3), 2.23-2.34 (m, 6H, NCH2), 2.81-2.89 (m, 6H, NCH2), 6.64 (d, 6H, JHH = 8.0 Hz, Ar-H), 6.98 (d, 6H, JHH = 8.0 Hz, Ar-H). 13C{1H} NMR (C6D6, 298 K): δ = 0.80 (Si(CH3)2), 21.13 (Ar-CH3), 48.65 (NCH2), 121.04 (Ar-CH), 126.14 (p-Ar-C), 131.33 (Ar-CH), 151.10 (ipso-Ar-C). 29Si{1H} NMR (C6D6, 298 K): δ = –5.10 (s, SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1606 (s), 1503 (s), 1297 (m), 1178 (w), 1041 (m), 1017 (m), 958 (m), 894 (m), 816 (s), 771 (s).
Synthesis of [U{tacn(SiMe2NC6H4Me-4)3}] (3-U) and [U{tacn(SiMe2NC6H4Me-4)3}(I)] (5): THF (20 mL) was added to a pre-cooled (−78 °C) mixture of [U(I)3(THF)4] (0.45 g, 0.5 mmol) and 1-K3 (0.44 g, 0.5 mmol). The dark red reaction mixture was allowed to warm to room temperature and stirred for 16 h. The volatiles were removed in vacuo and the residue was extracted with toluene (2 × 10 mL) and filtered. The solution was reduced in volume to ca. 2 mL and stored at −30 °C overnight to give dark red crystals of 3-U∙0.5C7H8 (0.25 g, 56%). On one occasion, several crystals of 5 formed. The solid state structure of 5 was determined, but no other characterization data could be obtained due to the low yield. Data for 3-U: Anal. Calcd. for C33H51N6Si3U·0.5C7H8: C, 48.70; H, 6.16; N, 9.34. Found: C, 48.53; H, 6.09; N, 9.47. µeff = 2.97 µB (Evans method). 1H NMR (C6D6, 298 K): δ = −30.29 (br, ν½ ~ 150 Hz, 6H, NCH2), −7.50 (br, ν½ ~ 150 Hz, 6H, NCH2), −0.65 (br, ν½ ~ 20 Hz, 6H, Ar-CH), −0.40 (br, ν½ ~ 30 Hz, 9H, Ar-CH3), 2.67 (br, ν½ ~ 30 Hz, 6H, Ar-CH), 7.74 (br, ν½ ~ 50 Hz, 18H, Si(CH3)2). 13C{1H} NMR (C6D6, 298 K): not observed. 29Si{1H} NMR (C6D6, 298 K): δ = −263.80 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1605 (s), 1506 (m), 1290 (s), 1260 (m), 1177 (m), 1034 (m), 951 (s), 912 (w), 895 (m), 862 (m), 816 (s), 770 (s).
Synthesis of [La{tacn(SiMe2NC6H3Me2-3,5)3}] (4-La): THF (20 mL) was added to a pre-cooled (−78 °C) mixture of [La(I)3(THF)4] (0.40 g, 0.5 mmol) and 2-K3 (0.46 g, 0.5 mmol). The pale yellow reaction mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed under reduced pressure and the residue was extracted with toluene (2 × 10 mL), reduced in volume to ca. 1 mL and stored at −30 °C overnight to give colorless crystals of 4-La (0.27 g, 68%). Anal. Calcd. for C36H57LaN6Si3: C, 54.25; H, 7.21; N, 10.54. Found: C, 53.06; H, 7.23; N, 10.03. 1H NMR (C6D6, 298 K): δ = 0.30 (s, 18H, s, Si(CH3)2), 2.13 (s, 18H, Ar-CH3), 2.23-2.33 (br m, 6H, NCH2), 2.79-2.87 (br m, 6H, NCH2), 6.39 (s, 6H, o-Ar-H), 6.42 (s, 3H, p-Ar-H). 13C{1H} NMR (C6D6, 298 K): δ = 0.82 (Si(CH3)2), 21.84 (Ar-CH3), 48.66 (NCH2), 118.08 (Ar-CH), 119.95 (Ar-CH), 140.01 (m-Ar-C), 153.33 (ipso-Ar-C). 29Si{1H} NMR (C6D6, 298 K): δ = –5.18 (s, SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1585 (s), 1350 (m), 1246 (m), 1196 (s), 1070 (m), 1041 (m), 966 (m), 988 (m), 896 (s), 866 (s), 769 (s).
Synthesis of [U{tacn(SiMe2NC6H3Me-3,5)3}] (4-U): THF (20 mL) was added to a pre-cooled (−78 °C) mixture of [U(I)3(THF)4] (0.45 g, 0.5 mmol) and 2-K3 (0.46 g, 0.5 mmol). The dark red reaction mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed under reduced pressure and the residue was extracted with toluene (2 × 10 mL) and filtered. The solution was reduced in volume to ca. 2 mL and stored at −30 °C overnight to give dark red crystals of 4-U (0.29 g, 65%). Anal. Calcd. for C36H57N6Si3U: C, 48.25; H, 6.64; N, 9.38. Found: C, 47.74; H, 5.96; N, 9.53. µeff = 2.61 µB (Evans method). 1H NMR (C6D6, 298 K): δ = −30.84 (br, ν½ ~ 60 Hz, 6H, NCH2), −9.53 (br, ν½ ~ 60 Hz, 6H, NCH2), –3.64 (br, ν½ ~ 20 Hz, 18H, Ar-CH3), −0.75 (br, ν½ ~ 40 Hz, 6H, o-Ar-H), 0.51 (s, 3H, p-Ar-H), 8.09 (br, ν½ ~ 40 Hz, 18H, Si(CH3)2). 13C{1H} NMR (C6D6, 298 K): not observed. 29Si{1H} NMR (C6D6, 298 K): δ = −270.90 (SiMe2). FTIR (Nujol, ν ˜ /cm−1): 1584 (s), 1344 (s), 1289 (w), 1069 (m), 1038 (m), 989 (w), 966 (m), 912 (m), 897 (s), 866 (s), 772 (s), 673 (m).
Synthesis of [U{tacn(SiMe2NC6H3Me-4)3}(Cl)] (6): Method 1: THF (20 mL) was added to a pre-cooled (−78 °C) mixture of UCl4 (0.38 g, 1 mmol) and 1-K3 (0.88 g, 1 mmol). The dark green reaction mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed in vacuo and the residue was extracted with THF (2 × 30 mL) and filtered. The solution was reduced in volume to ca. 2 mL and stored at −30 °C overnight to give green crystals of 6·THF (0.16 g, 16%). Method 2: An excess of tBuCl (0.023 g, 0.25 mmol) was added to a pre-cooled (−10 °C) solution of 3-U (0.050 g, 0.056 mmol) in toluene. Upon warming to room temperature, a color change to green was observed. Volatiles were removed under reduced pressure to yield 6 in quantitative crude yield as a green solid. Anal. Calcd. for C33H51ClN6Si3U: C, 44.56; H, 5.78; N, 9.45. Found: C, 44.56; H, 5.73; N, 9.33. µeff = 2.76 µB (Evans method). 1H NMR (C4D8O, 298 K): δ = −46.31 (br, ν½ ~ 70 Hz, 6H, Ar-CH), −15.17 (br, ν½ ~ 1,200 Hz, 6H, NCH2), 5.86 (br, ν½ ~ 100 Hz, 9H, Ar-CH3), 12.34 (br, ν½ ~ 150 Hz, 6H, Ar-CH), 16.08 (br, ν½ ~ 600 Hz, 18H, Si(CH3)2), 23.82 (br, ν½ ~ 1,000 Hz, 6H, NCH2). 13C{1H} and 29Si{1H} NMR (C4D8O, 298 K): not observed. FTIR (Nujol, ν ˜ /cm−1): 1605 (s), 1501 (m), 1247 (m), 1176 (w), 1109 (w), 1085 (m), 1045 (s), 1013 (s), 945 (m), 921 (m), 905 (m), 831 (s), 776 (m), 730 (s), 708 (m).
Synthesis of [U{tacn(SiMe2NC6H3Me2-3,5)3}(Cl)] (7): Method 1: THF (20 mL) was added to a pre-cooled (−78 °C) mixture of UCl4 (0.38 g, 0.5 mmol) and 2-K3 (0.92 g, 1 mmol). The dark green reaction mixture was allowed to warm to room temperature and stirred for 16 h. Volatiles were removed in vacuo and the residue was extracted with DME (2 × 20 mL) and filtered. The solution was reduced in volume to ca. 2 mL and stored at −30 °C overnight to give green crystals of 7 (0.09 g, 10%). Method 2: An excess of tBuCl (0.079 g, 0.85 mmol) was added to a pre-cooled (−10 °C) solution of 4-U (0.150 g, 0.16 mmol) in toluene. Upon warming to room temperature, a color change to green was observed. Volatiles were removed under reduced pressure and the green solid was dissolved in DME (1 mL) and stored −30 °C for 8 h to give green needles of 7 (0.063 g, 42%). Anal. Calcd. for C36ClH57N6Si3U: C, 46.41; H, 6.17; N, 9.02. Found: C, 46.34; H, 6.30; N, 8.93. µeff = 3.20 µB (Evans method). 1H NMR (C6D6, 298 K): δ = −50.42 (br, ν½ ~ 200 Hz, 2H, Ar-CH), 1.15 (br, ν½ ~ 300 Hz, 18H, Si(CH3)2), 12.67 (br, ν½ ~ 1,000 Hz, 12H, NCH2), most Ar-CH and Ar-CH3 not observed. 13C{1H} and 29Si{1H} NMR (C6D6, 298 K): not observed. FTIR (Nujol, ν ˜ /cm−1): 1587 (s), 1317 (s), 1184 (s), 1042 (m), 962 (w), 920 (m), 903 (m), 889 (m), 679 (m).

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/inorganics9120086/s1. Figures S1–S4: structures of 3-La·0.5C7H8, 4-La and 5, together with a space-filling diagrams of 3-U, 4-U and [U{tacn(SiMe2NPh)3}]. Tables S1 and S2: supplementary crystallographic data for all complexes. Figures S5–S9: solid state magnetic data for 3-U and 4-U. Figures S10–S12: EPR spectra for 3-U and 4-U. Figures S13–S42: NMR spectra for all complexes. CIF and checkCIF output files for the solid state structures of 3-La·0.5C7H8, 3-U·0.5C7H8, 4-La, 4-U, 5, 6·THF and 7.

Author Contributions

Conceptualization, A.F., L.S.N. and D.P.M.; methodology, A.F. and D.P.M.; validation, A.F. and D.P.M.; formal analysis, all authors; investigation, A.F., F.O., I.J.V.-Y. and F.T.; data curation, A.F. and D.P.M.; writing—original draft preparation, A.F. and D.P.M.; writing—review and editing, all authors; visualization, A.F., F.T. and D.P.M.; supervision, L.S.N. and D.P.M.; project administration, A.F., D.P.M. and L.S.N.; funding acquisition, A.F., E.J.L.M., D.P.M. and L.S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Manchester and the UK Engineering and Physical Sciences Research Council (EPSRC), grant numbers EP/G037140/1 (Nuclear FiRST DTC), EP/L014416/1 and EP/K039547/1; the EPSRC UK National Electron Paramagnetic Resonance Service provided access to the EPR Facility and the SQUID magnetometer (grant number EP/S033181/1).

Data Availability Statement

The data presented in this study are openly available in FigShare at doi:10.6084/m9.figshare.16680670 and the Cambridge Structural Database, deposition numbers CCDC 1833228-1833234.

Acknowledgments

We would like to thank Ian Fallis and Thomas Tatchell from Cardiff University for helpful advice on the synthesis of tacn.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Synthesis of proligands 1-H3 and 2-H3, and the group 1 transfer agents 1-K3 and 2-K3.
Scheme 1. Synthesis of proligands 1-H3 and 2-H3, and the group 1 transfer agents 1-K3 and 2-K3.
Inorganics 09 00086 sch001
Scheme 2. Synthesis of complexes 3-M, 4-M, 6 and 7 (M = La, U).
Scheme 2. Synthesis of complexes 3-M, 4-M, 6 and 7 (M = La, U).
Inorganics 09 00086 sch002
Figure 1. Electronic spectra of 3-U, 4-U, 6 and 7 in toluene (0.5 mM) at 8000–28,000 cm−1.
Figure 1. Electronic spectra of 3-U, 4-U, 6 and 7 in toluene (0.5 mM) at 8000–28,000 cm−1.
Inorganics 09 00086 g001
Figure 2. Molecular structures of (a) [U{tacn(SiMe2NC6H4Me-4)3}] (3-U·0.5C7H8) and (b) [U{tacn(SiMe2NC6H3Me2-3,5)3}] (4-U) with selective atom labelling. Displacement ellipsoids set at 30% probability level; hydrogen atoms and lattice toluene were omitted for clarity.
Figure 2. Molecular structures of (a) [U{tacn(SiMe2NC6H4Me-4)3}] (3-U·0.5C7H8) and (b) [U{tacn(SiMe2NC6H3Me2-3,5)3}] (4-U) with selective atom labelling. Displacement ellipsoids set at 30% probability level; hydrogen atoms and lattice toluene were omitted for clarity.
Inorganics 09 00086 g002
Figure 3. Molecular structures of (a) [U{tacn(SiMe2NC6H4Me-4)3}(Cl)] (6·THF) and (b) [U{tacn(SiMe2NC6H3Me2-3,5)3}(Cl)] (7) with selective atom labelling. Displacement ellipsoids set at 30% probability level; hydrogen atoms omitted for clarity.
Figure 3. Molecular structures of (a) [U{tacn(SiMe2NC6H4Me-4)3}(Cl)] (6·THF) and (b) [U{tacn(SiMe2NC6H3Me2-3,5)3}(Cl)] (7) with selective atom labelling. Displacement ellipsoids set at 30% probability level; hydrogen atoms omitted for clarity.
Inorganics 09 00086 g003
Figure 4. (a) (black) Powder X-band EPR spectrum of 4-U at 5 K, and (red) its simulation with geff = 2.80, 2.27 and 1.17. (b) Magnetization (M) hysteresis of 3-U at 1.8 K, sweep rate 13 G s−1. (c) In phase (χ′) and (d) out of phase (χ″) components of the ac susceptibility of 4-U in an applied field of 600 G and an oscillating field of 1.55 G at 1.8 K.
Figure 4. (a) (black) Powder X-band EPR spectrum of 4-U at 5 K, and (red) its simulation with geff = 2.80, 2.27 and 1.17. (b) Magnetization (M) hysteresis of 3-U at 1.8 K, sweep rate 13 G s−1. (c) In phase (χ′) and (d) out of phase (χ″) components of the ac susceptibility of 4-U in an applied field of 600 G and an oscillating field of 1.55 G at 1.8 K.
Inorganics 09 00086 g004
Table 1. Selected bond lengths (Å) and angles (°) for 3-M·0.5C7H8, 4-M, 5, 6·THF and 7 (M = La, U; X = Cl, I). Symmetry operations to generate equivalent atoms: i 1–x, +y, +z; ii = +x, ½–y, +z.
Table 1. Selected bond lengths (Å) and angles (°) for 3-M·0.5C7H8, 4-M, 5, 6·THF and 7 (M = La, U; X = Cl, I). Symmetry operations to generate equivalent atoms: i 1–x, +y, +z; ii = +x, ½–y, +z.
ComplexM–NamineM–NanilideN–M–NamineN–M–NanilideM–X
3-La·0.5C7H82.705(3)2.395(3)66.21(8)120.06(9)
2.705(3)2.414(3)68.00(9)113.46(8)-
2.654(3)2.436(3)67.54(7)115.46(10)
3-U·0.5C7H82.637(9)2.381(8)68.2(2)119.0(3)
2.659(8)2.364(7)67.4(3)116.6(3)-
2.658(6)2.396(10)67.5(3)110.8(3)
4-La2.674(2)2.421(3)67.80(7)115.74(9)
2.676(3)2.426(2)67.09(8)116.64(9)-
2.707(3)2.400(3)68.02(8)115.99(8)
4-U2.616(12)2.348(10)68.2(4)117.9(6)
2.646(10)2.31(2)68.3(4)115.6(4)-
2.63(2)2.418(14)68.6(5)112.9(5)
52.74(4)2.36(2)66.0(1)96.6(4)
2.67(2)2.32(2)64.8.(8)154.5(7)3.143(2)
2.59(4)2.32(2) i67.7(9)96.6(4) i
6·THF2.659(8)2.285(7)66.6(3)95.4(3)
2.683(7)2.295(8)67.1(2)160.3(3)2.706(3)
2.635(7)2.327(8)66.0(2)94.1(3)
72.69(2)2.305(5)66.8(5)95.77(10)
2.667(7)2.286(5)67.8(5)158.2(3)(10)2.687(2)
2.62(2)2.305(5) ii65.4(6)95.77(10) ii
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Formanuik, A.; Ortu, F.; Vitorica-Yrezabal, I.J.; Tuna, F.; McInnes, E.J.L.; Natrajan, L.S.; Mills, D.P. Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations. Inorganics 2021, 9, 86. https://doi.org/10.3390/inorganics9120086

AMA Style

Formanuik A, Ortu F, Vitorica-Yrezabal IJ, Tuna F, McInnes EJL, Natrajan LS, Mills DP. Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations. Inorganics. 2021; 9(12):86. https://doi.org/10.3390/inorganics9120086

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Formanuik, Alasdair, Fabrizio Ortu, Iñigo J. Vitorica-Yrezabal, Floriana Tuna, Eric J. L. McInnes, Louise S. Natrajan, and David P. Mills. 2021. "Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations" Inorganics 9, no. 12: 86. https://doi.org/10.3390/inorganics9120086

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