f-Block complexes of a m-terphenyl dithiocarboxylate ligand

Michael A. Boreen a, Bernard F. Parker ab, Stephan Hohloch ab, Brighton A. Skeel a and John Arnold *ab
aDepartment of Chemistry, University of California, Berkeley, California 94720, USA. E-mail: arnold@berkeley.edu
bChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Received 28th October 2017 , Accepted 20th November 2017

First published on 27th November 2017


Straightforward syntheses are provided for the m-terphenyl dithiocarboxylic acid 2,6-(C6H4-4-tBu)2C6H3CS2H (TerphCS2H, 2) and its lithium and potassium salts, TerphCS2Li(Et2O)2 and TerphCS2K (1·Et2O and 4, respectively). These compounds can be isolated in good yields on multi-gram scales starting from Terph–I without isolating intermediates. Salt metathesis and protonolysis reactions provided access to the homoleptic actinide(IV) complexes (TerphCS2)4An (An = Th (5) and U (6)). Electrochemical and reactivity studies revealed that the dithiocarboxylate ligand is incompatible with U(III). The homoleptic lanthanum(III) complex (TerphCS2)3La and its η6-toluene adduct (7 and 7·tol, respectively) were also structurally characterized. Binding of toluene to 7 was shown to displace intramolecular La–Carene close contacts that are facilitated by a distortion from the usual geometry of bound dithiocarboxylate ligands.


Introduction

A major problem in the treatment of nuclear waste is separation of lanthanide elements from minor actinides that have high radiological activities, particularly americium and curium.1 Current processes proposed for this separation, such as the SANEX or TALSPEAK processes, aim to use soft N- or S-donor ligands.1 It is generally believed that these types of soft donor ligands achieve large Ln/An separation factors due to increased covalency in actinide-soft donor ligand bonding relative to the corresponding lanthanide systems.1–4 One of the most selective ligands that has been studied for Ln/An separations is the chelating S,S′-donor ligand, bis(2,4,4-trimethylpentyl)dithiophosphinic acid, the active extractant in Cyanex 301 (Fig. 1, left).5 A more complete understanding of the chemistry of chelating S,S′-donor ligands may assist in the design of better extractants for Ln/An separations.
image file: c7dt04073c-f1.tif
Fig. 1 Structure of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (left); general structures of dithiocarboxylic acids (middle) and dithiocarbamic acids (right).

While the chemistry of carboxylic acids with f-block elements has been studied extensively,6–10 the f-block chemistry of the sulphur analogues, dithiocarboxylic acids (Fig. 1, middle), has hardly been explored.11–20 In general, dithioacids have been studied to a much lesser extent than carboxylic acids largely because dithioacids tend to be unstable in air, undergoing slow oxidation to the corresponding carbothioyl disulphide, (RCS2)2.21 Notably, Cyanex 301 is also susceptible to oxidation in conditions used for extractions.2,22,23 Alkali and alkaline earth metal salts of smaller dithioacids are often water-soluble and stable in basic solution;21 however, systematic studies of their chemistry are lacking. The lanthanide and actinide chemistries of dithiocarbamates (Fig. 1, right) have also been studied more extensively than the closely related dithiocarboxylates.24,25

The present work aims to expand not only the f-block chemistry of dithiocarboxylates but also the Group 1 chemistry, as these species are important as precursors to metal complexes. Specifically, we chose to use an m-terphenyl substituent; these bulky groups have been used widely by Power and co-workers as well as others to stabilize low-coordinate complexes and to prevent or reduce aggregation of complexes.26–31 We hoped these ligands would enable us to isolate mononuclear, homoleptic complexes in which the bonding of the dithiocarboxylate ligand could be studied in the absence of complicating factors.

Results and discussion

Previous work by Hagadorn and co-workers showed that lithium dithiocarboxylate complexes with m-terphenyl substituents can be synthesized and used to prepare Ti(III), Ti(IV), and V(III) complexes.32 In this work, carbon disulphide was added to Terph–Li (Terph = C6H3-2,6-(C6H4-4-tBu)2), generated in situ from Terph–I,33 to yield the bis-ether adduct of the lithium dithiocarboxylate, TerphCS2Li(Et2O)2 (1·Et2O, Scheme 1). Compound 1·Et2O was isolated in 70% yield as a crystalline yellow solid after crystallization from ether. When heated under N2, compound 1·Et2O turned orange then pale pink before decomposing at 311–312 °C. When stored at room temperature for several weeks, the product was found to slowly turn orange; addition of ether caused the colour to change back to a yellow solid, which was confirmed by 1H NMR to be compound 1·Et2O with no sign of decomposition. These observations are consistent with slow desolvation of compound 1·Et2O without concurrent decomposition. Dissolving compound 1·Et2O in THF and removing solvent in vacuo afforded the THF adduct, TerphCS2Li(THF)2 (1·THF), in quantitative yield. Compound 1·THF was more resistant to desolvation than 1·Et2O, and its formulation was confirmed by NMR and elemental analysis.
image file: c7dt04073c-s1.tif
Scheme 1 Synthesis of 1·Et2O, 1·THF, 2, 3, and 4. Each compound can be synthesized from Terph–I without isolating prior intermediates.

In order to provide access to protonolysis chemistry as a potential route to form complexes with dithiocarboxylate ligands, it was necessary to isolate the dithiocarboxylic acid, TerphCS2H (2). Initial attempts to isolate 2, by quenching the reaction to form 1·Et2O with acidic aqueous solutions in air, yielded a mixture of products. The products could be separated using column chromatography, revealing the presence of significant quantities of the carbothioyl disulphide, (TerphCS2)2 (3), formed via oxidation of 2. The identity of 3 was confirmed using X-ray crystallography (Fig. 2). Bond distances and angles are consistent with previously reported structures.34,35 Column chromatography also showed that a large amount of compound 1·Et2O had not been protonated even after 16 hours of vigorous stirring with dilute HCl in saturated NH4Cl(aq) due to poor mixing between the two phases of the reaction. This issue was overcome by using a solution of HCl in dioxane,20 enabling compound 1·Et2O to be protonated to completion without exposure to air or water. Compound 2 was isolated in 79% yield (7 g) without isolating compound 1·Et2O after extraction into toluene to remove LiCl and recrystallization from ether/hexane.


image file: c7dt04073c-f2.tif
Fig. 2 X-ray crystal structure of 3 with 50% probability ellipsoids. H atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): S(1)–S(3) 2.0381(9), S(1)–C(1) 1.767(3), S(2)–C(1) 1.615(2), S(3)–C(28) 1.764(3), S(4)–C(28) 1.612(3), S(1)–C(1)–S(2) 126.0(2), S(3)–C(28)–S(4) 127.2(2), C(1)–S(1)–S(3)–C(28) 116.8(1).

Initial salt metathesis reactions with complexes 1·Et2O and 1·THF and actinide salts led to intractable, oily mixtures of products. The potassium analogue of 1·Et2O, TerphCS2K (4), was thus targeted as an alternative starting material for salt metathesis reactions. Compound 4 was isolated by deprotonation of the free dithioacid (2) with potassium bis(trimethylsilyl)amide (K[N(SiMe3)2]) in toluene. While 1·Et2O is weakly soluble in toluene, 4 is completely insoluble in toluene and could be isolated as a pure pink solid after filtration in 76% yield (4.5 g). Notably, no isolation of any intermediate starting from Terph–I was necessary. Additionally, it was found that 3 could be reduced to 4 on a small scale using KC8.

With the dithioacid (2) and its lithium and potassium salts (1·Et2O, 1·THF, and 4) in hand, we set out to isolate homoleptic thorium and uranium complexes. The salt metathesis reaction of 4 with ThCl4(DME)2 in toluene led to the isolation of (TerphCS2)4Th (5) in 44% yield after extraction with dichloromethane and crystallization from THF (Scheme 2). All THF, including the THF molecule coordinated to the thorium centre (vide infra), was removed from the product by heating in vacuo at 100 °C; a colour change from yellow to orange-yellow was observed during this process. Complex 5 exhibits high thermal stability in the solid-state, decomposing at 306–310 °C under N2.


image file: c7dt04073c-s2.tif
Scheme 2 Synthesis of 5 and 6 by salt metathesis and protonolysis, respectively.

The protonolysis reaction between 2 and U[N(SiMe3)2]2(CH2SiMe2NSiMe3) led to the isolation of (TerphCS2)4U (6) as orange crystals in 75% yield. Complex 6 exhibits lower solid-state stability than 5, decomposing between 277–281 °C under N2.

The solid-state structures of the thorium and uranium complexes were determined using X-ray crystallography (Fig. 3 and 4). Single crystals of both molecules were grown by vapour diffusion of hexane into THF solutions of each. However, while a THF molecule was found to be bound to the thorium complex to form (TerphCS2)4Th(THF) (5·THF), the structure of 6 revealed no bound THF. This can be explained by the smaller ionic radius of U(IV) relative to Th(IV) but contrasts with the results found by Walensky and co-workers with related m-terphenyl dithiocarboxylate ligands wherein the complexes [2,6-(Mes)2C6H3CS2]4An(THF) (An = Th, U) each have a coordinated THF molecule that could not be removed by heat or vacuum.20 The average Th–S distance of 2.942(1) Å in 5·THF is very close to the average Th–S distance of 2.934(3) Å in [2,6-(Mes)2C6H3CS2]4Th(THF). The average U–S distance of 2.831(2) Å in 6 is shorter than the average U–S distance of 2.8775(17) in [2,6-(Mes)2C6H3CS2]4U(THF); this can be explained on the basis of the lower coordination number of the metal centre in 6.


image file: c7dt04073c-f3.tif
Fig. 3 X-ray crystal structure of 5·THF with 50% probability ellipsoids. H atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): Th–Savg 2.942(1), Th–O(1) 2.531(3), C–Savg 1.687(7), S–Th–Savg(in single ligand) 59.91(3), S–C–Savg 121.0(3).

image file: c7dt04073c-f4.tif
Fig. 4 X-ray crystal structure of one of the two molecules of 6 in the asymmetric unit with 50% probability ellipsoids. H atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): U–Savg 2.831(2), C–Savg 1.684(6), S–U–Savg(in single ligand) 62.23(5), S–C–Savg 120.6(4).

To see if it would be possible to isolate the uranium complex 6 in different oxidation states, we performed cyclic voltammetry studies in THF (see Fig. S19–S22 and Table S1 in ESI). As references, we also examined the potassium (4) and thorium (5) complexes. Compound 4 was irreversibly oxidized with Epa = −0.30 V versus ferrocene. The irreversible nature of this oxidation is most likely due to the formation of the disulphide species 3. Similar redox behaviour was found in 5 and 6. For both complexes, no oxidation and three irreversible reduction features were observed in the first scan. In the second scans, new oxidation features occurred at Epa = −0.59 V for 5 and Epa = −0.68 V for 6. Upon further investigation, this oxidation was dependent on the first reduction taking place at Epc = −2.08 V for 5 and Epc = −2.16 V for 6. Notably, the potassium salt 4 showed a reduction feature at a similar value, Epc = −2.17 V, so we therefore assume that this reduction takes place on one of the ligands in 5 and 6 and is followed by decomposition. Furthermore, attempts to isolate a U(III) dithiocarboxylate species by the reaction of U[N(SiMe3)2]3 with three equivalents of 2 led only to isolation of a U(IV) product, 6. Based on these studies, dithiocarboxylate ligands seem to be incompatible with the strongly reducing U(III) ion. Other reports of U(IV) tetrakis dithiocarboxylate,20 dithiophosphinate,36 and diselenophosphinate37 complexes isolated from protonolysis or salt metathesis reactions with U(III) precursors suggest many types of soft chalcogen donor ligands may be incompatible with U(III).

With the goal of finding differences between the chemistry of lanthanides and actinides with dithiocarboxylate ligands, we extended the coordination chemistry of these ligands to lanthanum. The homoleptic lanthanum tris(dithiocarboxylate) complex, (TerphCS2)3La (7), was isolated in 60% yield from the reaction of 2 with La[N(SiMe3)2]3 in hexane (Scheme 3). X-ray quality crystals of 7 were grown from a mixture of dichloromethane and hexane; the solid-state structure of 7 is shown in Fig. 5.


image file: c7dt04073c-s3.tif
Scheme 3 Synthesis of 7 by protonolysis.

image file: c7dt04073c-f5.tif
Fig. 5 X-ray crystal structure of 7 with 50% probability ellipsoids. H atoms and tBu groups have been omitted for clarity. Selected bond distances (Å) and angles (°): La–S(1) 2.9066(7), La–S(2) 2.9067(7), La–S(3) 2.9089(8), La–C(34) 3.263(2), La–C(35) 3.403(3), C(34)–C(35) 1.390(4), C–Savg 1.690(2), S–C–Savg 123.8(2), La–C(1)–C(2) 180, La–C(16)–C(17) 141.2(2).

Complex 7 crystallizes in the space group C2/c on a crystallographic 2-fold axis passing through La(1), C(1), C(2), and C(5), making the asymmetric unit one half of a molecule of 7. Therefore, the ligand on the 2-fold axis is unique, and the other two ligands are identical to each other by symmetry. The average La–S distance in 7 of 2.9074(7) Å is between the average Th–S and U–S distance in 5 and 6, consistent with ionic radii predictions for 6-coordinate La(III), 9-coordinate Th(IV), and 8-coordinate U(IV).38 For comparison, the average La–S distances in Cp*2La(S2CC5Me5) and Cp*2La(S2CC5Me5)(OPPh3) are 2.9439(6) and 3.012(1) Å, respectively.13

The two unique ligands in the solid-state structure of 7 have dramatically different bonding geometries. Ordinarily, the metal centre and the CS2 unit of a dithiocarboxylate ligand are all expected to lie within the same plane; in complex 7, this is the case only for the ligand on the 2-fold axis with a crystallographically imposed 180° La(1)–C(1)–C(2) angle. The other two ligands are severely distorted with 141° La(1)–C(16)–C(17) angles. This distortion facilitates close contacts of 3.263(2) and 3.403(3) Å between the lanthanum centre and C(34) and C(35), respectively. Similar close contacts between lanthanide centres and carbon atoms on flanking phenyl rings of m-terphenyl substituents have been observed with a range of hapticities.37 The room temperature 1H NMR spectrum of 7 contains only five resonances, indicating that all three ligands are equivalent on the NMR timescale. Furthermore, variable temperature 1H NMR experiments on 7 in CD2Cl2 showed no splitting of the five resonances down to −70 °C (see Fig. S18 in ESI), demonstrating that any interaction between the lanthanum centre and the flanking arene rings of the terphenyl group is weak in solution despite apparently causing a considerable distortion in the solid-state.

The structure of 7 suggests the lanthanum centre is sterically unsaturated. However, the solution-state NMR studies indicate that any intramolecular interactions driven by the strongly Lewis acidic metal centre should be displaced readily. While 7 is orange as a solid and in CH2Cl2 solution, 7 was found to form yellow solutions in toluene. When solutions of 7 in toluene were dried in vacuo, a yellow solid was observed; furthermore, 1H NMR spectroscopy in C6D6 revealed approximately one equivalent of toluene. However, the toluene signals appeared at the chemical shift values expected for free toluene in solution. These observations suggest that 7 binds toluene molecules but that the bound toluene molecules are displaced by benzene in solution.

Lanthanide complexes with neutral, π-bonded arene ligands have attracted considerable interest since the first isolated example, Sm(η6-C6Me6)(κ2-AlCl4)3, was reported by Cotton and Schwotzer in 1986.39,40 Typically, these adducts have been formed in one-pot reactions, precluding analysis of the species without the bound arene; however, Schelter and co-workers found that Ce[N(C6F5)2]3 as well as its toluene and mesitylene adducts could be isolated.41 In the case of Ce[N(C6F5)2]3, intramolecular C–F → Ce dative interactions are displaced upon arene coordination. Similarly, we hypothesized that the intramolecular arene contacts in 7 were displaced upon coordination of toluene.

This hypothesis was confirmed by X-ray crystallography; X-ray quality crystals of the η6-bound toluene adduct of 7, 7·tol, were formed by vapour diffusion of hexane into a toluene solution of 7. Complex 7·tol has a distorted pentagonal bipyramidal structure formed by three κ2-S,S′-donors and an η6-toluene (Fig. 6); related η6-arene Ln(III) complexes with κ2-tetrachloroaluminate or κ2-tetrachlorogallate ligands adopt a similar geometry at the Ln centre.40,42–45 The La–centroid distance in 7·tol is 2.7401(1) Å, longer than the La–centroid distance in [La(η6-toluene)(κ2-AlCl4)3], 2.633(7) Å.42 The range of La–Carene bond lengths, 3.051(5)–3.102(3) Å, is consistent with a η6-arene.41,42 Also, the bound arene ring is planar, and the Carene–Carene bond length range (1.376(5)–1.402(5) Å) shows no significant bond elongations or contractions.


image file: c7dt04073c-f6.tif
Fig. 6 X-ray crystal structure of 7·tol with 50% probability ellipsoids. H atoms and tBu groups have been omitted for clarity. Selected bond distances (Å) and angles (°): La–S(1) 2.9622(8), La–S(2) 2.8959(8), La–S(3) 2.9644(9), La–S(4) 2.9413(8), La–S(5) 2.930(1), La–S(6) 2.9240(9), La–Cavg 3.073(4), C–Savg 1.668(5), S(1)–La–S(2) 61.15(2), S(3)–La–S(4) 60.74(2), S(5)–La–S(6) 61.04(2), S–C–Savg 124.2(3).

Importantly, all of the CS2 units in 7·tol lie in the same plane as the La centre with no major distortion, and there are no close contacts between the La centre and any carbon atoms on the terphenyl groups. To see if the Lewis acidic La centre of 7 could bind other even less common ligands for lanthanides, we added 10 equivalents of 3-hexyne, 2-butyne, and cyclohexene separately to CD2Cl2 samples of 7, but we observed no shifting of resonances at room temperature.

Conclusions

An m-terphenyl dithiocarboxylic acid and its lithium and potassium salts were prepared on multi-gram scales in high yields from the m-terphenyl iodide without isolating intermediates. These compounds were shown to be useful precursors for the synthesis of f-block dithiocarboxylate species. Homoleptic tetrakis(dithiocarboxylate) complexes of thorium and uranium were isolated. Considering the electrochemical and reactivity studies discussed in this work, dithiocarboxylate ligands seem to be incompatible with the strongly reducing U(III) ion.

The homoleptic tris(dithiocarboxylate) complex of lanthanum was also isolated and found to display an unusual bonding geometry with two of the three CS2 units; these distortions facilitate intramolecular La–Carene close contacts that are displaced upon η6-coordination of a toluene molecule. These findings expand the field of non-aqueous f-block chemistry with soft donor ligands and demonstrate that dithiocarboxylate ligands can form robust complexes with hard f-block cations.

Experimental section

General considerations

Unless otherwise noted, all reactions were performed using standard Schlenk line techniques under an atmosphere of nitrogen or argon or in an MBraun inert atmosphere glove box under an atmosphere of nitrogen. Glassware and Celite® were stored in an oven at ca. 150 °C for at least 3 h prior to use. Molecular sieves (4 Å) were activated by heating to 200 °C overnight under vacuum prior to storage in a glovebox. NMR spectra were recorded on Bruker AV-600, AV-500, DRX-500, AVB-400, AVQ-400, and AV-300 spectrometers. 1H and 13C{1H} chemical shifts are given relative to residual solvent peaks and are recorded in units of parts per million (ppm). FT-IR samples were prepared as Nujol mulls pressed between KBr plates, with data collected with a Nicolet iS10 FT-IR spectrometer. Melting points were determined using sealed capillaries prepared under nitrogen on an OptiMelt automated melting point system. Elemental analyses were determined at the Microanalytical Facility at the College of Chemistry, University of California, Berkeley or at the School of Human Sciences, Sciences Centre, London Metropolitan University. UV-Vis measurements were performed on a Varian Cary® 50 UV-Vis spectrophotometer. A one mm path length quartz cell was used with a blank measured before each run.

Materials

Diethyl ether, n-hexane, dichloromethane, toluene, dimethoxyethane (DME), and THF were purified by passage through columns of activated alumina and degassed by sparging with nitrogen. Deuterated solvents were vacuum-transferred from flasks containing sodium/benzophenone (C6D6) or calcium hydride (CDCl3, CD2Cl2, and pyridine-d5), degassed with three freeze–pump–thaw cycles, and stored over molecular sieves. Carbon disulphide (CS2) was distilled from a flask containing P2O5. K[N(SiMe3)2] was recrystallized from toluene. 2,6-Bis(4-tert-butylphenyl)iodobenzene (Terph–I),46,47 KC8,48 ThCl4(DME)2,49 U[N(SiMe3)2]2(CH2SiMe2NSiMe3),50 U[N(SiMe3)2]3,51 and La[N(SiMe3)2]352 were synthesized according to literature procedures. All other chemicals were purchased from commercial sources and used as received.

Electrochemistry

Cyclic voltammetry (CV) experiments were performed with a Gamry potentiostat using a three electrode set-up (working electrode: platinum, counter electrode: platinum, quasi-reference electrode: silver). The measurements were conducted in 0.2 M solutions of [N(nBu)4][PF6] in THF at room temperature. The electrolyte [N(nBu)4][PF6] was purchased from Sigma-Aldrich in electrochemical grade and dried at 50 °C under vacuum prior to use. All measurements were referenced to ferrocene (Fc).

X-ray crystallography

In a dry nitrogen glovebox, samples of single crystals of 3, 5·THF, 6, 7, and 7·tol were coated in paratone–N oil prior to transport to the diffraction facility, where they were evaluated by polarized light microscopy and mounted on a Kaptan loop. Diffraction data were collected at CheXray, Berkeley, CA, using a Bruker SMART APEX or APEX II QUAZAR instrument outfitted with a monochromated Mo-Kα radiation source (λ = 0.71073 Å). All data collections were conducted at 100 K, with the crystals cooled by a stream of dry nitrogen. Absorption corrections were carried out by a multi-scan method utilizing the SADABS program.53 Bruker APEX2 and APEX3 software were used for data collections, and Bruker SAINT V8.37A software was used to conduct the cell refinement and data reductions procedures.54 Using the WinGX software package,55 initial structure solutions were found using direct methods (SHELXT), and refinements were carried out using SHELXL-2014.56 Thermal parameters for all non-hydrogen atoms were refined anisotropically. Thermal ellipsoid plots were made using Mercury.57 Compound 3 possessed a mixture of hexane isomers in the crystal lattice that could not be modelled accurately, and compound 5·THF possessed highly disordered outer sphere solvent molecules; the data for these structures were treated with the SQUEEZE routine included in PLATON.58

Synthetic methods

TerphCS2Li(Et2O)2 (1·Et2O). Compound 1·Et2O was synthesized by modification of a previously reported procedure for a similar compound.32 Terph–I (3.00 g, 6.40 mmol, 1.00 equiv.), 30 mL of hexane, and 20 mL of ether were added to a 100 mL Schlenk flask. After cooling to −78 °C, n-butyllithium (4.4 mL, 7.0 mmol, 1.1 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 16 hours. Then reaction was then cooled to −78 °C, and CS2 (0.92 mL, 15 mmol, 2.3 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 24 hours, resulting in an orange solution with a yellow precipitate. Volatiles were removed in vacuo, and the product was extracted into 110 mL of ether, concentrated, and cooled to −40 °C. The product was isolated as yellow crystals and dried in vacuo. Concentration and cooling of the supernatant yielded a second crop of product in a similar fashion (2.57 g combined, 4.49 mmol, 70% yield); mp 311–312 °C (decomp.); 1H NMR (600 MHz, CDCl3) δ 7.57 (d, J = 8.3 Hz, 4H, ArH), 7.30 (d, J = 8.3 Hz, 4H, ArH), 7.22 (dd, J = 8.2, 6.9 Hz, 1H, ArH), 7.15 (d, J = 7.4 Hz, 2H, ArH), 3.46 (q, J = 7.0 Hz, 8H, Et2O), 1.21 (s, 18H, C(CH3)3), 1.14 (t, J = 7.0 Hz, 12H, Et2O); 13C NMR (126 MHz, CDCl3) δ 153.8, 148.8, 140.0, 135.9, 129.8, 129.1, 125.8, 124.3, 66.0 (Et2O), 34.5 (C(CH3)3), 31.6 (C(CH3)3), 15.0 (Et2O) (note: the CS2 carbon peak was not observed between 314 and −14 ppm); IR: 1910 (w), 1694 (w), 1609 (w), 1572 (w), 1509 (m), 1300 (m), 1268 (m), 1221 (w), 1196 (w), 1178 (m), 1152 (w), 1125 (m), 1091 (m), 1063 (m), 1017 (s), 967 (m), 914 (m), 843 (s), 833 (m), 800 (m), 751 (s), 723 (m), 687 (w), 626 (w), 608 (w), 573 (s), 536 (w), 512 (w) cm−1. Elemental analysis was not performed on 1·Et2O due to the observation of partial desolvation at room temperature.
TerphCS2Li(THF)2 (1·THF). Compound 1·Et2O was dissolved in minimal THF. Removal of solvent in vacuo yielded 1·THF in quantitative yield as an orange-yellow solid; mp 244 °C (decomp.); 1H NMR (500 MHz, C6D6) δ 7.97 (d, J = 8.0 Hz, 4H, ArH), 7.31 (d, J = 7.5 Hz, 2H, ArH), 7.22 (d, J = 8.0 Hz, 4H, ArH), 7.13 (t, 7.5 Hz, 1H, ArH), 3.44 (m, 8H, THF), 1.37 (m, 8H, THF), 1.27 (s, 18H, C(CH3)3); 13C NMR (126 MHz, C6D6) δ 148.4, 141.1, 136.7, 130.11, 130.08, 125.8, 124.3, 68.2 (THF), 34.5 (C(CH3)3), 31.7 (C(CH3)3), 25.6 (THF) (note: the CS2 carbon peak was not observed between 314 and −14 ppm); IR: 1917 (w), 1572 (w) 1510 (w), 1270 (m), 1220 (w), 1201 (w), 1177 (w), 1120 (w), 1098 (w), 1045 (s), 1018 (s), 960 (w), 914 (m), 888 (m), 842 (m), 825 (m), 799 (m), 751 (m), 722 (w), 687 (w), 626 (w), 607 (w), 574 (m), 537 (w) cm−1; Anal. calc. (%) for C35H45LiO2S2 (1 + 2 THF): C, 73.91; H, 7.97. Found: C, 74.01; H, 8.05.
TerphCS2H (2). Terph–I (10.0 g, 21.3 mmol, 1.00 equiv.), 60 mL of hexane, and 40 mL of ether were added to a 250 mL Schlenk flask. After cooling to −78 °C, n-butyllithium (14.7 mL, 23.5 mmol, 1.10 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 16 hours. The reaction was then cooled to −78 °C, and CS2 (2.6 mL, 43 mmol, 2.0 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 24 hours. Volatiles were removed in vacuo, and the solid was suspended in 80 mL of ether. The reaction was cooled to 0 °C, and a solution of HCl in 1,4-dioxane (6.7 mL, 27 mmol, 1.3 equiv.) was added dropwise by syringe, resulting in a pink solution. The reaction was stirred at 0 °C for 1 hour, then volatiles were removed in vacuo. The product was triturated with 50 mL of hexane then extracted into 80 mL of toluene and filtered to remove LiCl. The solvent was removed in vacuo, and the product was dissolved in 150 mL of ether and 30 mL of hexane. The resulting solution was concentrated to approximately 60 mL and cooled to −40 °C. The product was isolated as pink crystals and dried in vacuo. Concentration and cooling of the supernatant yielded a second crop of product in a similar fashion (7.11 g combined, 17.0 mmol, 79% yield); mp 187–190 °C; 1H NMR (600 MHz, C6D6) δ 7.54 (d, J = 8.2 Hz, 4H, ArH), 7.29 (d, J = 8.2 Hz, 4H, ArH), 7.20 (d, J = 7.6 Hz, 2H, ArH), 7.07 (t, J = 7.6 Hz, 1H, ArH), 5.73 (s, 1H, SH), 1.22 (s, 18H, C(CH3)3); 13C NMR (126 MHz, C6D6) δ 231.8 (CS2), 150.4, 147.2, 138.6, 138.4, 130.1, 129.4, 128.8, 125.3, 34.6 (C(CH3)3), 31.4 (C(CH3)3); IR: 2525 (w, νSH), 1574 (w), 1268 (m), 1240 (w), 1201(w), 1103 (m), 1062 (m), 1017 (w), 927 (m), 834 (s), 801 (m), 748 (m), 734 (m), 687 (w), 671 (w), 619 (w), 575 (s) cm−1; UV/vis (THF): λmax (nm) 317. Anal. calc. (%) for C27H30S2 (2): C, 77.46; H, 7.22. Found: C, 77.36; H, 7.18.
(TerphCS2)2 (3). Terph–I (10.2 g, 21.8 mmol, 1.00 equiv.), 100 mL of hexane, and 75 mL of ether were added to a 250 mL Schlenk flask. After cooling to −78 °C, n-butyllithium (15.0 mL, 24.0 mmol, 1.10 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 16 hours. Then reaction was then cooled to −78 °C, and CS2 (2.6 mL, 43 mmol, 2.0 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 24 hours. The reaction was cooled to 0 °C, and a degassed 60 mL solution of dilute aqueous HCl in saturated aqueous NH4Cl was added by cannula. The reaction was allowed to warm to room temperature and was stirred vigorously for an additional 16 hours. Work up was performed in air. The products were extracted into ether, dried with MgSO4 and vacuum filtered. Solvent was removed in vacuo to yield a crude red-pink solid, which was purified using acid washed alumina column chromatography (100% hexanes, Rf = 0.17) to yield compound 3 as a crystalline pink solid in 20% yield (1.8 g, 2.2 mmol). X-ray quality crystals of 3 were obtained by slow cooling of a saturated boiling hexanes solution of 3 to room temperature; mp 215–216 °C (decomp.); 1H NMR (600 MHz, CDCl3) δ 7.41 (t, J = 7.7 Hz, 2H, ArH), 7.30–7.22 (m, 20H, ArH), 1.32 (s, 36H, C(CH3)3); 13C NMR (126 MHz, CDCl3) δ 150.1, 142.4, 139.8, 137.4, 130.1, 129.5, 129.0, 125.0, 34.7 (C(CH3)3), 31.6 (C(CH3)3) (note: the CS2 carbon peak was not observed between 314 and −14 ppm); IR: 1582 (w), 1510 (m), 1269 (w), 1243 (w), 1201 (w), 1095 (m), 1045 (m), 1018 (w), 834 (s), 800 (m), 754 (m), 734 (w), 687 (w), 663 (w), 614 (w), 573 (m) cm−1. Anal. calc. (%) for C54H58S4 (3): C, 77.65; H, 7.00. Found: C, 77.56; H, 7.08.
TerphCS2K (4). Method A. Terph–I (6.02 g. 2.9 mmol, 1.00 equiv.), 30 mL of hexane, and 20 mL of ether were added to a 100 mL Schlenk flask. After cooling to −78 °C, n-butyllithium (8.8 mL, 14 mmol, 1.1 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 16 hours. Then reaction was then cooled to −78 °C, and CS2 (1.5 mL, 25 mmol, 1.9 equiv.) was added dropwise by syringe. The reaction was allowed to warm to room temperature and was stirred for an additional 24 hours. Volatiles were removed in vacuo, and the solid was suspended in 50 mL of ether. The reaction was cooled to 0 °C, and a solution of HCl in 1,4-dioxane (3.9 mL, 16 mmol, 1.2 equiv.) was added dropwise by syringe. The reaction was stirred at 0 °C for 1 hour, then volatiles were removed in vacuo. The product was triturated with 40 mL of hexane then extracted into 100 mL of toluene and filtered to remove LiCl. The toluene solution was cooled to 0 °C, and a solution of K[N(SiMe3)2] (3.07 g, 15.4 mmol, 1.19 equiv.) in 30 mL of toluene was added by cannula with stirring, resulting in a red solution. The reaction was slowly allowed to warm to room temperature and was stirred for an additional 20 hours. Compound 4 was isolated as a pink power by vacuum filtration over a medium-porosity fritted filter, washed with 3 × 20 mL of toluene, and dried in vacuo (4.47 g, 9.79 mmol, 76% yield); mp 291–295 °C (decomp.); 1H NMR (600 MHz, Pyr-d5) δ 8.28 (d, J = 8.2 Hz, 4H, ArH), 7.40 (d, J = 7.6 Hz, 2H, ArH), 7.34 (d, J = 8.2 Hz, 4H, ArH), 7.30 (t, J = 7.6 Hz, 1H, ArH), 1.16 (s, 18H, C(CH3)3); 13C NMR (126 MHz, Pyr-d5) 258.2 (CS2), 157.6, 149.0, 141.9, 136.7, 130.6, 130.0, 125.2, 125.1, 34.7 (C(CH3)3), 31.7 (C(CH3)3); IR: 1506 (m), 1268 (m), 1217 (w), 1200 (w), 1115 (w), 1021 (s), 970 (w), 961 (w), 907 (m), 838 (s), 798 (m), 745 (s), 680 (w), 629 (w), 582 (s) cm−1; Anal. calc. (%) for C27H29KS2 (4): C, 71.00; H, 6.40. Found: C, 70.85; H, 6.51.

Method B. KC8 (14 mg, 0.10 mmol, 2.0 equiv.) was suspended in 2 mL THF. A solution of compound 3 (42 mg, 0.050 mmol, 1.0 equiv.) in 3 mL THF was added dropwise at room temperature. The reaction was stirred for an additional 16 hours then filtered through Celite to remove graphite. Solvent was removed in vacuo. The product was triturated with toluene to remove the remaining THF, yielding compound 4 as a pink solid. The purity of compound 4 was confirmed by 1H NMR and melting point.

(TerphCS2)4Th (5). Compound 4 (911 mg, 1.99 mmol, 4.00 equiv.) and 275 mg (0.497 mmol, 1.00 equiv.) of ThCl4(DME)2 were added to a 100 mL Schlenk flask. Toluene (50 mL) was added, and the reaction was stirred at room temperature for 18 hours, resulting in a cloudy orange mixture. Volatiles were removed in vacuo, and the product was extracted into 160 mL of CH2Cl2 and filtered through Celite. Volatiles were again removed in vacuo. The product was then dissolved in THF, filtered, concentrated, and cooled to −40 °C, resulting in the formation of yellow-orange crystals. Concentration and cooling of the supernatant yielded a second crop of product in a similar fashion. Both crops were heated at 100 °C in vacuo for 18 hours to remove all remaining THF molecules, yielding complex 5 as an orange powder (412 mg combined, 0.216 mmol, 44% yield). X-ray quality crystals of 5·THF were obtained by vapour diffusion of hexane into a THF solution of 5. Mp 306–310 °C (decomp.); 1H NMR (600 MHz, C6D6) δ 7.38 (d, J = 8.1 Hz, 16H, ArH), 7.23–7.17 (m, 24H, ArH), 7.10 (d, J = 7.7 Hz, 4H, ArH), 1.15 (s, 72H, C(CH3)3); 13C NMR (126 MHz, C6D6) δ 260.2 (CS2), 150.2, 150.0, 138.4, 138.1, 130.5, 129.3, 128.8, 126.1, 34.6 (C(CH3)3), 31.5 (C(CH3)3); IR: 1580 (w), 1511 (m), 1268 (m), 1223 (m), 1128 (w), 1099 (w), 1017 (s), 992 (m), 917 (m), 832 (s), 800 (m), 751 (s), 722 (w), 682 (w), 607 (w), 574 (m), 557 (w) cm−1. Anal. calc. (%) for C108H116S8Th (5): C, 68.18; H, 6.15. Found: C, 68.01; H, 6.29.
(TerphCS2)4U (6). A solution of U[N(SiMe3)2]2(CH2SiMe2NSiMe3) (105 mg, 0.146 mmol, 1.0 equiv.) in 3 mL of hexane was layered onto a solution of compound 2 (245 mg, 0.584 mmol, 4.0 equiv.) in 6 mL of ether. The reaction quickly turned red and began to precipitate crystalline solid. After sitting at room temperature for 24 hours, the solvent was decanted, and orange crystals of 6 were washed with 5 × 1 mL of ether and dried in vacuo (210 mg, 0.110 mmol, 75% yield). X-ray quality crystals of 6 were grown by vapour diffusion of hexane into a THF solution of 6 at room temperature. Mp 277–281 °C (decomp.); 1H NMR (400 MHz, CDCl3) δ 6.70 (t, J = 7.7 Hz, 4H, ArH), 6.36 (d, J = 7.7 Hz, 8H, ArH), 5.99 (d, J = 7.6 Hz, 16H, ArH), 4.35 (s, 16H, ArH), 1.18 (s, 72H, C(CH3)3). IR: 1580 (w), 1511 (m), 1269 (m), 1223 (m), 1128 (w), 1099 (w), 1016 (s), 919 (m), 872 (m), 832 (s), 799 (m), 750 (s), 683 (w), 573 (m) cm−1; Anal. calc. (%) for C108H116S8U (6): C, 67.96; H, 6.13. Found: C, 68.04; H, 6.33.
(TerphCS2)3La (7). A solution of La[N(SiMe3)2]3 (28 mg, 0.046 mmol, 1.0 equiv.) in 1 mL of hexane was added to a solution of compound 2 (58 mg, 0.14 mmol, 3.0 equiv.) in 8 mL of hexane. The reaction quickly turned orange and began to precipitate crystalline solid. After sitting at room temperature for 24 hours, the solvent was decanted, and orange crystals of complex 7 were washed with 3 × 1 mL of hexane and dried in vacuo (47 mg, 0.034 mmol, 74% yield). X-ray quality crystals of 7 were grown by adding 10 mL of hexane to a saturated solution of 7 in 0.5 mL of CH2Cl2 and allowing the mixture to sit at room temperature. X-ray quality crystals of 7·tol were grown by vapour diffusion of hexane into a toluene solution of 7 at room temperature. Mp 262–270 °C (decomp.); 1H NMR (600 MHz, C6D6) δ 7.53 (d, J = 8.4 Hz, 12H, ArH), 7.31 (d, J = 8.4 Hz, 12H, ArH), 7.22 (d, J = 7.6 Hz, 6H, ArH), 7.08 (dd, J = 7.9, 7.4 Hz, 3H, ArH), 1.20 (s, 18H, C(CH3)3); 13C NMR (126 MHz, C6D6) δ 260.6 (CS2), 151.9, 149.5, 139.4, 137.3, 130.5, 129.8, 127.6, 125.5, 34.5 (C(CH3)3), 31.6 (C(CH3)3); IR: 1579 (w), 1508 (m), 1268 (m), 1223 (w), 1118 (w), 1098 (w), 1004 (s), 961 (w), 914 (m), 854 (m), 835 (s), 799 (s), 751 (s), 681 (w), 605 (w), 571 (s), 533 (w) cm−1. Anal. calc. (%) for C81H87LaS6 (7): C, 69.90; H, 6.30. Found: C, 69.72; H, 6.38.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences Heavy Element Chemistry Program of the U.S. Department of Energy (DOE) at LBNL under Contract DE-AC02-05CH11231. M. A. B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 1106400. M. A. B. also thanks UC Berkeley for a graduate research fellowship. S. H. acknowledges the German Academic Exchange Service (DAAD) for a postdoctoral scholarship.

References

  1. M. J. Hudson, L. M. Harwood, D. M. Laventine and F. W. Lewis, Inorg. Chem., 2013, 52, 3414 CrossRef CAS PubMed .
  2. H. H. Dam, D. N. Reinhoudt and W. Verboom, Chem. Soc. Rev., 2007, 36, 367 RSC .
  3. T. W. Hayton, Chem. Commun., 2013, 49, 2956 RSC .
  4. A. J. Gaunt and M. P. Neu, C. R. Chim., 2010, 13, 821 CrossRef CAS .
  5. M. P. Jensen and A. H. Bond, J. Am. Chem. Soc., 2002, 124, 9870 CrossRef CAS PubMed .
  6. J.-C. G. Bünzli, J. Coord. Chem., 2014, 67, 3706 CrossRef .
  7. A. Ouchi, Y. Suzuki, Y. Ohki and Y. Koizumi, Coord. Chem. Rev., 1988, 92, 29 CrossRef CAS .
  8. U. Casellato, P. A. Vigato and M. Vidali, Coord. Chem. Rev., 1978, 26, 85 CrossRef CAS .
  9. S. Beer, O. B. Berryman, D. Ajami and J. Rebek Jr., Chem. Sci., 2010, 1, 43 RSC .
  10. B. Weaver and F. A. Kappelmann, J. Inorg. Nucl. Chem., 1968, 30, 263 CrossRef CAS .
  11. M. Ephritikhine, Coord. Chem. Rev., 2016, 319, 35 CrossRef CAS .
  12. T. Kanda, M. Ibi, K. Mochizuki and S. Kato, Chem. Lett., 1998, 27, 957 CrossRef .
  13. W. J. Evans, T. J. Mueller and J. W. Ziller, J. Am. Chem. Soc., 2009, 131, 2678 CrossRef CAS PubMed .
  14. W. J. Evans, C. A. Seibel, J. W. Ziller and R. J. Doedens, Organometallics, 1998, 17, 2103 CrossRef CAS .
  15. B. Fang, W. Ren, G. Hou, G. Zi, D.-C. Fang, L. Maron and M. D. Walter, J. Am. Chem. Soc., 2014, 136, 17249 CrossRef CAS PubMed .
  16. N. A. Siladke, J. Leduc, J. W. Ziller and W. J. Evans, Chem. – Eur. J., 2012, 18, 14820 CrossRef CAS PubMed .
  17. G. Bombieri, U. Croatto, E. Forsellini, B. Zarli and R. Graziani, J. Chem. Soc., Dalton Trans., 1972, 560 RSC .
  18. A. B. Ghosh, N. Saha, A. Sarkar, A. K. Dutta, P. Biswas, K. Nag and B. Adhikary, New J. Chem., 2016, 40, 1595 RSC .
  19. S. Kato, M. Wakamatsu and M. Mizuta, J. Organomet. Chem., 1974, 78, 405 CrossRef CAS .
  20. A. C. Behrle, A. J. Myers, P. Rungthanaphatsophon, W. W. Lukens, C. L. Barnes and J. R. Walensky, Chem. Commun., 2016, 52, 14373 RSC .
  21. D. Coucouvanis, Prog. Inorg. Chem., 1970, 11, 233 CAS .
  22. K. Wieszczycka and W. Tomczyk, J. Hazard. Mater., 2011, 192, 530 CrossRef CAS PubMed .
  23. K. C. Sole and J. B. Hiskey, Hydrometallurgy, 1995, 37, 129 CrossRef CAS .
  24. F. Nief, Coord. Chem. Rev., 1998, 178–180, 13 CrossRef CAS .
  25. U. Casellato, M. Vidali and P. A. Vigato, Coord. Chem. Rev., 1979, 28, 231 CrossRef CAS .
  26. E. Rivard and P. P. Power, Inorg. Chem., 2007, 46, 10047 CrossRef CAS PubMed .
  27. T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J. Long and P. P. Power, Science, 2005, 310, 844 CrossRef CAS PubMed .
  28. J. A. C. Clyburne and N. McMullen, Coord. Chem. Rev., 2000, 210, 73 CrossRef CAS .
  29. C.-S. Hwang and P. P. Power, Bull. Korean Chem. Soc., 2003, 24, 605 CrossRef CAS .
  30. M. Niemeyer and P. P. Power, Inorg. Chem., 1996, 35, 7264 CrossRef CAS PubMed .
  31. R. J. Wehmschulte, W. J. Grigsby, B. Schiemenz, R. A. Bartlett and P. P. Power, Inorg. Chem., 1996, 35, 6694 CrossRef CAS PubMed .
  32. C. Ives, E. L. Fillis and J. R. Hagadorn, Dalton Trans., 2003, 527 RSC .
  33. B. Schiemenz and P. P. Power, Organometallics, 1996, 15, 958 CrossRef CAS .
  34. R. Cea-Olivares, V. García-Montalvo, S. Hernández-Ortega, C. Rodríguez-Narváez, P. García y García, M. López-Cardoso, P. de March, L. González, L. Elias, M. Figueredo and J. Font, Tetrahedron: Asymmetry, 1999, 10, 3337 CrossRef .
  35. W. G. Weber, J. B. McLeary and R. D. Sanderson, Tetrahedron Lett., 2006, 47, 4771 CrossRef CAS .
  36. J. A. Macor, J. L. Brown, J. N. Cross, S. R. Daly, A. J. Gaunt, G. S. Girolami, M. T. Janicke, S. A. Kozimor, M. P. Neu, A. C. Olson, S. D. Reilly and B. L. Scott, Dalton Trans., 2015, 44, 18923 RSC .
  37. M. B. Jones, A. J. Gaunt, J. C. Gordon, N. Kaltsoyannis, M. P. Neu and B. L. Scott, Chem. Sci., 2013, 4, 1189 RSC .
  38. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Cryst., 1976, 32, 751 CrossRef .
  39. M. N. Bochkarev, Chem. Rev., 2002, 102, 2089 CrossRef CAS PubMed .
  40. F. A. Cotton and W. Schwotzer, J. Am. Chem. Soc., 1986, 108, 4657 CrossRef CAS .
  41. H. Yin, A. J. Lewis, P. Carroll and E. J. Schelter, Inorg. Chem., 2013, 52, 8234 CrossRef CAS PubMed .
  42. A. S. Filatov, A. Y. Rogachev and M. A. Petrukhina, J. Mol. Struct., 2008, 890, 116 CrossRef CAS .
  43. B. Fan, Q. Shen and Y. Lin, J. Organomet. Chem., 1989, 377, 51 CrossRef CAS .
  44. M. Gorlov, L. L. Hussami, A. Fischer and L. Kloo, Eur. J. Inorg. Chem., 2008, 5191 CrossRef CAS .
  45. S.-S. Liu, J. W. Ziller, Y.-Q. Zhang, B.-W. Wang, W. J. Evans and S. Gao, Chem. Commun., 2014, 50, 11418 RSC .
  46. C. J. F. Du, H. Hart and K. K. D. Ng, J. Org. Chem., 1986, 51, 3162 CrossRef CAS .
  47. N. J. Hardman, B. Twamley, M. Stender, R. Baldwin, S. Hino, B. Schiemenz, S. M. Kauzlarich and P. P. Power, J. Organomet. Chem., 2002, 643–644, 461 CrossRef CAS .
  48. M. A. Araya, F. A. Cotton, J. H. Matonic and C. A. Murillo, Inorg. Chem., 1995, 34, 5424 CrossRef CAS .
  49. T. Cantat, B. L. Scott and J. L. Kiplinger, Chem. Commun., 2010, 46, 919 RSC .
  50. A. J. Lewis, U. J. Williams, P. J. Carroll and E. J. Schelter, Inorg. Chem., 2013, 52, 7326 CrossRef CAS PubMed .
  51. L. R. Avens, S. G. Bott, D. L. Clark, A. P. Sattelberger, J. G. Watkin and B. D. Zwick, Inorg. Chem., 1994, 33, 2248 CrossRef CAS .
  52. D. C. Bradley, J. S. Ghotra and F. A. Hart, J. Chem. Soc., Dalton Trans., 1973, 1021 RSC .
  53. Bruker, SADABS, Bruker AXS Inc., Madison, WI Search PubMed .
  54. Bruker, APEX2, APEX3, and SAINT, Bruker AXS Inc., Madison, WI Search PubMed.
  55. L. J. Farrugia, J. Appl. Crystallogr., 2012, 45, 849 CrossRef CAS .
  56. G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2007, 64, 112 CrossRef PubMed .
  57. C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de Streek and P. A. Wood, J. Appl. Crystallogr., 2008, 41, 466 CrossRef CAS .
  58. A. L. Spek, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2015, 71, 9 CrossRef CAS PubMed .

Footnote

Electronic supplementary information (ESI) available: NMR spectra, electrochemistry data, and X-ray crystallographic tables. CCDC 1581848–1581852. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt04073c

This journal is © The Royal Society of Chemistry 2018