Supporting information for the manuscript. Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes

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1 Supporting information for the manuscript Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes Lucile Chatelain, Rosario Scopelliti, and Marinella Mazzanti* ISIC, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. S1

2 Table of Contents : 1. General considerations Synthesis Synthesis of Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], Synthesis of Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], Reaction of 3 with CS 2, synthesis of (Cs()) 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-s) 2 ], Reaction of 2 with CS X-ray Crystallography NMR data... 9 Ø Addition of 18C6 to complex 1, 2 and Ø Addition of 18C6 to complex Ø Addition of 18C6 to complex Ø Reduction of complex 1 with 1 equiv. of Cs 0 in the presence of 18C Ø Addition of 18C6 to complex Ø Reduction of complex 1 with excess Cs 0 in the presence of 18C Ø Reactivity of 3 with CS Ø Reactivity of 2 with CS Electrochemical studies S2

3 1. General considerations. All manipulations were carried out under an inert argon atmosphere using Schlenk techniques and an MBraun glovebox equipped with a purifier unit. The water and oxygen level were always kept at less than 1 ppm. The solvents were purchased from Aldrich in their anhydrous form conditioned under argon and were vacuum distilled from K/benzophenone (, toluene, hexane). Depleted uranium turnings were purchased from the "Société Industrielle du Combustible Nucléaire" of Annecy (France). Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 1 was synthesized as previously described. [1] Cesium ingot and anhydrous CS 2 was purchased from Sigma Aldrich and used without further purification. 18c6 was purchased from Sigma Aldrich and was dried under high vacuum for 7 days prior to use. Elemental analyses were performed under argon with a Thermo Scientific Flash 2000 Organic Elemental Analyzer. 1 H NMR experiments were carried out using NMR tubes adapted with J. Young valves. 1 H NMR spectra were recorded on Bruker 400 MHz spectrometer. NMR chemical shifts are reported in ppm with solvent as internal reference. Caution: Depleted uranium (primary isotope 238 U) is a weak α-emitter (4.197 MeV) with a half-life of years. Manipulations and reactions should be carried out in monitored fume hoods or in an inert atmosphere glovebox in a radiation laboratory equipped with α- and β-counting equipment. S3

4 2. Synthesis 2.1. Synthesis of Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 2 A cold solution of Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 1 (65mg, 0.029mmol, 1eq.) in (3mL) was added to metallic cesium (3.9mg, 0.029mmol, 1eq.). The reaction mixture was stirred for 5 hours at -40 C with a glass coated stir bar. The resulting dark brown solution was filtrated and volatiles were removed under reduced pressure, yielding 70.1mg of brown solid. This solid was recrystallized at -40 C into 3mL of cold to give 50.4 mg of crystals of (yield : 67%). 1 H NMR (400 MHz, -d 8, 298 K): δ = 0.62 (s, 162H, CH 3 ), (400 MHz, toluene-d 8, 298 K): δ = 0.81 (s, 162H, CH 3 ). Elemental analysis calcd (%) for Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)].2.9 (C 72 H 162 Cs 2 NO 24 Si 6 U (C 4 H 8 O)): C, 39.45; H, 7.33; N, Found: C, 39.28; H, 7.65; N, X-ray quality crystals of Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)].2, 2.2, were obtained after 2days, from a solution of Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] (0.031 M) in at -40 C. The complex 2 is stable in the solid state for up to a month and stable in solution at -40C for 10 days. At room temperature decomposes rapidly to give complex 1 and free siloxide as the only known decomposition products detectable by proton NMR. 2.2.Synthesis of Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 3 A cold solution of Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 1 (90.3mg, 0.041mmol, 1eq.) in 5mL of was added to metallic cesium (27.3mg, 0.205mmol, 5eq.). The reaction mixture was stirred for 3 hours at -40 C with a glass coated stir bar. The stirring time should be kept as close as possible to 3 hours to avoid decomposition of the final complex 3 by the excess Cs 0. The dark purple solution was quickly decanted or filtered at -40 to remove the excess of Cs 0 and the volatiles were removed under reduced pressure, yielding 3 as dark purple solid (83.1mg, 77%). The complex decomposes quickly in solution in the absence of Cs at -40 C (decomposition products are observed after 1 hour). At room temperature 3 decomposes immediately to give a mixture of complex 2 and free siloxide as the only known S4

5 decomposition products detectable by proton NMR. 1 H NMR (400 MHz, -d 8, 233 K): δ = 1.42 (s, 162H, CH 3 ), (400 MHz, toluene-d 8, 233 K): δ = 1.52 (s, 162H, CH 3 ). X-ray quality crystals of Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] were obtained in a concentrated solution containing Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] in at -40 C. Elemental analysis calcd (%) for Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)].2 (C 72 H 162 Cs 2 NO 24 Si 6 U 2. 2(C 4 H 8 O)): C, 36.76; H, 6.87; N, Found: C, 36.77; H, 7.13; N, The elemental analysis of the isolated crude product is in agreement with the presence of pure complex 3. In view of the high reactivity of complex 3 and of its quick decomposition in solutions even at -40, reactivity studies were carried out without isolating the complex using freshly prepared solutions of 3 according to the following procedure: Complex 3 is prepared by reacting a cold solution of 1 (30.0mg, 0.014mmol, 1eq.) in 1mL of -d 8 with metallic cesium (9.7mg, 0.068mmol, 5eq.). The reaction mixture is stirred for 3 hours at -40 C with a glass coated stir bar and the resulting dark purple solution is quickly decanted at low temperature to remove the excess of Cs 0. The proton NMR of this solution shows only the signal assigned to complex Reaction of 3 with CS 2, synthesis of (Cs()) 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-s) 2 ], 4 Complex 3 was prepared in situ by reacting a cold solution of 1 (30.0mg, 0.014mmol, 1eq.) in 1mL of with metallic cesium (9.7mg, 0.068mmol, 5eq.). The reaction mixture was stirred for 3 hours at -40 C with a glass coated stir bar. The resulting dark purple solution of 3 was quickly decanted at low temperature to remove the excess of Cs 0 and 22 µl (0.014 mmol) of a 0.64 M solution of 13 CS 2 were added. Immediately the color of the solution changed from dark purple to yellowish brown. After 3 hours at -40 C, light brown crystals of 4 were isolated (8.6mg, 25%). Volatiles of the supernatant were removed under vacuum. The residue was dissolved in D 2 O and N 13 CS - was identified using 13 C NMR spectroscopy [2] ((400 MHz, D 2 O, 298 K): δ = 133.7ppm). A quantitative 13 C NMR was performed in presence of labeled sodium acetate as internal standard, to determine the amount of NCS - formed from the reaction of 3 with CS 2 and a yield of 32% was obtained. S5

6 Crystals of (Cs()) 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-s) 2 ], 4 suitable for X-ray diffraction were grown letting stand the crude reaction mixture at -40 C for 3 hours after addition of CS 2. 1 H NMR (400 MHz, -d 8, 298 K): δ = (s, 162H, CH 3 ), (400 MHz, toluene-d 8, 298 K): δ = -0.7 (s, 162H, CH 3 ). Elemental analysis calcd (%) for (Cs()) 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-s) 2 ] (C 72 H 162 Cs 2 S 2 O 24 Si 6 U 2. C 4 H 8 O): C, 37.13; H, 6.97; N, Found: C, 37.07; H, 6.92; N, Elemental analysis shows the presence of only one molecule indicating that one of the Cs-bound molecules was lost while drying under vacuum the crystalline material Reaction of 2 with CS µl (0.011 mmol) of a 0.64 M Toluene-D8 solution of 13 CS 2 were added to a cold dark brown solution of 2 (26.6mg, 0.011mmol, 1eq.) in 0.5mL of Toluene-D 8. The 1 H NMR spectrum measured immediately after addition shows that all the starting complex 2 reacted to afford complex 4 and unidentified products. The formation of 4 was also confirmed by X-ray diffraction studies. Volatiles of the supernatant were removed under vacuum. The residue was dissolved in D 2 O and N 13 CS - was identified using 13 C NMR spectroscopy [2] ((400 MHz, D 2 O, 298 K): δ = 133.7ppm). 3. X-ray Crystallography. X-ray Experimental Part: The diffraction data data of 2, 3 and 4 were measured at low temperature [100(2) or 120(2) K] using Mo Kα radiation on a Bruker APEX II CCD diffractometer equipped with a kappa geometry goniometer. The data sets were reduced by EvalCCD [3] and then corrected for absorption. [4] The solution and refinement for structure 2, 3 and 4 was performed by SHELX. [5] The crystal structures were refined using full-matrix least-squares based on F 2 with all non hydrogen atoms anisotropically defined. Hydrogen atoms were placed in calculated positions by means of the riding model. Additional electron density found in the difference Fourier map (due to highly disordered solvent) of compound 3 and 4 was treated by the SQUEEZE algorithm of PLATON. [6] Pseudomerohedral twinning was treated successfully in the case of compound 2, by applying the classical twin law for monoclinic simulating S6

7 orthorhombic (TWIN ) and obtaining a BASF parameter of (8). Compound 3 was refined as an inversion twin because of its non-centrosymmetric space group, obtaining a BASF parameter of 0.388(11). Table S1. Crystallographic data of 2.2, 3 and 4 Formula Crystal size (mm) C 80 H 178 Cs 2 NO 26 Si 6 U 2 C 72 H 162 Cs 3 NO 24 Si 6 U 2 C 80 H 178 Cs 2 O 26 S 2 Si 6 U x 0.18 x x x x x cryst syst Monoclinic Orthorhombic Triclinic space group P 21/n Pnn2 P-1 volume (Å 3 ) 11440(4) (13) (9) a (Å) (3) (2) (3) b (Å) (7) (14) (3) c (Å) (5) (3) (2) α (deg) (10) β (deg) (16) (11) γ (deg) (14) Z formula weight (g/mol) density (g cm -3 ) absorption coefficient (mm -1 ) F(000) temp (K) 120(2) 100(2) 120(2) total no. reflections unique reflections [R(int)] Final R indices [I > 2σ(I)] Largest diff. peak and hole (e.å -3 ) [R(int) = ] R1 = , wr2 = [R(int) = ] R1 = , wr2 = [R(int) = ] R1 = , wr2 = and and and GOF S7

8 Figure S1. Space-filling representations for 2 (left) and 3 (right). Hydrogen atoms and solvent molecules are omitted for clarity. Atoms: C (grey), O (red), Si (light yellow), N (light blue), Cs (purple) and U (green). S1# U1# Cs1 # U1 # Cs1# S1 # Figure S2. Mercury representation for 4. Hydrogen atoms, methyl groups and solvent molecules are omitted for clarity. Atoms: C (grey), O (red), Si (light yellow), N (light blue), Cs (purple) and U (green). Description of the solid state structure of 4: The X-ray crystal structure of 4 shows the presence of a tetranuclear complex containing two uranium and two cesium atoms. The two uranium cations are held together by two bridging sulfido S2- ligands (U S U angle, 98.34(10) ). An inversion center is present in the middle of the parallelogram defined by the two uranium ions and the two S2-, meaning that only half complex is present in the asymmetric unit. The U S bond distances (U1 S1, 2.639(3) Å, U1S (3) Å) are in the range of those reported for sulfide-bridged U(IV) complexes[7]. Both uranium atoms are coordinated by three siloxides ligands and two sulfides with a distorted trigonal bipyramidal geometry. On each uranium, the three siloxide act as tridentate bridging ligands forming an O6 coordination pocket for Cs+ binding (mean Cs-U 4.012Å). S8

9 4. NMR data Cs[U IV N U IV ] Toluene Figure S3. Evolution over time of the 1 H NMR (400MHz, Toluene-D8, 298 K) of the complex Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 1 at room temperature. Cs 2 [U III N U IV ] Siloxide ligand Figure S4. 1 H NMR (400MHz, -D8, 298 K) of the crude reaction mixture of the reaction between Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] and 1 equiv. Cs 0 to form Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2 after 3 hours of stirring. S9

10 Cs[U IV N U IV ] Cs 2 [U III N U IV ] Siloxide ligand Figure S5. 1 H NMR (400MHz, -D8, 298 K) of the crude reaction mixture of the reaction between Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] and 1 equiv. Cs 0 to form Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2 after 3 hours of stirring and after storing the crude mixture overnight at -40 C. Cs 2 [U III N U IV ] Toluene Figure S6. 1 H NMR (400MHz, Toluene-D8, 298 K) spectrum of the crystallized Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2. S10

11 Cs 2 [U III N U IV ] Cs[U IV N U IV ] 58% of Cs 2 [U III N U IV ] remaining 77% of Cs 2 [U III N U IV ] remaining Figure S7. Evolution over time of the 1 H NMR (400MHz, -D8, 298 K) spectrum of Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2 at room temperature. Cs 3 [U III N U III ] Figure S8. 1 H NMR (400MHz, -D8, 233 K) at -40 C of the crude reaction mixture after reacting Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] with 5 equiv. of Cs 0 for 3 hours and decanting excess of Cs 0 to yield Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3. S11

12 Cs[U IV N U IV ] Cs 2 [U III N U IV ] Cs 3 [U III N U III ] Siloxide ligand Figure S9. Evolution over time of 1 H NMR (400MHz, -D8, 233 K) at -40 C of the crude reaction mixture after reacting Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] with 5 equiv. of Cs 0 for 3 hours and decanting excess of Cs 0 to yield Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3. Cs 2 [U III N U IV ] Cs 3 [U III N U III ] Siloxide ligand Figure S10. 1 H NMR (400MHz, -D8, 298 K) spectrum of a solution of Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3 stored 30 minutes at room temperature. S12

13 Ø Addition of 18C6 to complex 1, 2 and 3 1 H NMR studies in deuterated indicate that the Cs + cations in 1 and 2 can be removed by adding stoichiometric amounts of crown 18C6. In contrast, we have no evidence of Cs + displacement by 18C6 for complex 3. Ø Addition of 18C6 to complex 1 Upon addition of 18C6 (1.5mg, mmol, 1eq.) to a red-brown solution of 1 (12.5mg, mmol, 1eq.) in 0.5mL of -D8 no color change was perceived, but a shift of 0.4 ppm was observed in the 1 H NMR spectrum suggesting that the coordinated Cs + cation can be removed. If 18C6 was added to a solution of 1 in toluene-d8, no shift was seen in the 1 H NMR spectrum suggesting that 18C6 does not remove Cs + in toluene. Cs[U IV N U IV ] (Cs18c6)[U IV N U IV ] 18C6 In presence of 18C6 Figure S11. 1 H NMR (400MHz, -D8, 298 K) of complex Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 1 in presence of 18C6. S13

14 (Cs18c6)[U IV N U IV ] 18C6 Figure S12. Evolution overtime of the 1 H NMR (400MHz, -D8, 298 K) spectra of complex Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 1 in presence of 18C6 at room temperature. Ø Addition of 18C6 to complex 2 Upon addition of 18C6 (1.5mg, mmol, 1eq.) to a dark brown solution of 2, prepared in situ from the reaction of 1 (12.0mg, mmol, 1eq.) and Cs 0 (0.8mg, , 1eq.) in 0.5mL of -D8 no color change was perceived, but a shift of 1 ppm was observed by 1 H NMR. A second equivalent of 18C6 (1.5mg, mmol, 1eq.) was added to this solution and an additional shift of 0.2 ppm was again observed in the 1 H NMR spectrum suggesting that removal of the two Cs + has occurred. The evolution of the proton NMR spectra of the complex 2 reacted with crown ether shows that decomposition to afford the complex 1 occurs much more rapidly in the absence of the Cs + cations. S14

15 Cs(Cs18c6)[U III N U IV ] (Cs18c6) 2 [U III N U IV ] 18C6 Figure S13. 1 H NMR (400MHz, -D8, 298 K) of complex Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2 in presence of 18C6. (Cs18c6) 2 [U III N U IV ] (Cs18c6)[U IV N U IV ] 18c6 28% of (Cs18c6) 2 [U III N U IV ] remaining 41% of (Cs18c6) 2 [U III N U IV ] remaining Figure S14. Evolution overtime of the 1 H NMR (400MHz, -D8, 298 K) spectra of complex Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 2 in presence of 18C6 at room temperature. Ø Reduction of complex 1 with 1 equiv. of Cs 0 in the presence of 18C6 A cold red brown solution of 1 (16.6mg, mmol, 1eq.) and 18c6 (3.4mg, 0.015, 2eq.) in 0.5mL of -d8 was added to Cs 0 (1.0mg, mmol, 1eq.) and the mixture was stirred 3 S15

16 hours at -40 C. The color became dark brown. The 1 H NMR spectra reveals only two signals corresponding to (Cs18c6) 2 [U III N U IV ]. (Cs18c6) 2 [U III N U IV ] 18C6 Figure S15. 1 H NMR (400MHz, -D8, 298 K) of the crude mixture of the reaction between Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 2 equivalents of 18C6 and 1 equivalent of Cs 0. Ø Addition of 18C6 to complex 3 1 (5.6mg, mmol, 1eq.) and Cs 0 (1.7mg, mmol, 5eq.) were stirred 3 hours at -40 C in 0.5mL of -D8. The dark purple solution was filtered and a cold solution of 18C6 (2.8mg, 0.011mmol, 4eq.) in 0.2 ml of -d8 was added. No obvious change in the 1 H NMR spectrum was observed. Cs 3 [U III N U III ] (Cs18c6) 2 [U III N U IV ] Siloxide ligand 18C6 S16

17 Figure S16. 1 H NMR (400MHz, -D8, 233 K) of complex Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3 in presence of 18C6. Ø Reduction of complex 1 with excess Cs 0 in the presence of 18C6 A cold light red brown solution of 1 (5.6mg, mmol, 1eq.) and 18c6 (3.4mg, , 5eq.) in 0.5mL of -d8 was added to Cs 0 (1.7mg, mmol, 5eq.). The resulting reaction mixture was stirred 3 hours at -40 C to afford a dark brown suspension. The 1 H NMR spectrum of this solution shows only two signals assigned to 18c6 and free siloxide ligand in agreement with complete decomposition of the starting material. No change occurred after 72 hours at -40 C. Siloxide(ligand( 18C6( ( Figure S17. 1 H NMR (400MHz, -D8, 233 K) of the crude mixture of the reaction at - 40 C between Cs[{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 1, 5equivalents of 18C6 and 5 equivalents of Cs 0. Ø Reactivity of 3 with CS 2 S17

18 Cs 2 [U IV -S/S-U IV ] Siloxide ligand Figure S18. 1 H NMR (400MHz, -D8, 298 K) of the crude reaction mixture between Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3 and CS 2. Cs 2 [U IV -S/S-U IV ] Siloxide ligand Figure S19. 1 H NMR (400MHz, -D8, 298 K) of (Cs()) 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-s) 2 ], 4 S18

19 NCS $% DMSO%(as%reference)% D 2 O%(solvent)%% Figure S C NMR (400MHz, D 2 O, 298 K) of the supernatant of the crude reaction mixture between Cs 3 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)] 3 and CS 2. S19

20 Ø Reactivity of 2 with CS 2 Cs 2 [U III N U IV ] Cs 2 [U IV -S/S-U IV ] UL 4 Toluene Figure S21. 1 H NMR (400MHz, Toluene-D8, 298 K) of the crude reaction mixture between Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 2 and CS 2. NCS $% DMSO%(as%reference)% D 2 O%(solvent)% Figure S C NMR (400MHz, D 2 O, 298K) of the crude reaction mixture between Cs 2 [{U(OSi(O t Bu) 3 ) 3 } 2 (µ-n)], 2 and CS 2. S20

21 5. Electrochemical studies (Cs18c6)[U IV N U IV ] Cs[U IV N U IV ] Figure S23. Cp 2 Fe + /Cp 2 Fe corrected cyclic voltammograms of a 2mM solution of 1 with or without added 18c6 in 0.1M [Bu4N][BAr F 4] at 100mV/s scan rate and 298 K. The red trace corresponds to the complex 1 (E OCV = V ) and the black trace corresponds to the complex 1 in presence of 18c6 (E OCV = V). Table S2. Measured potential (V vs Fc + /Fc) of the redox events for a solution of 1 and 1 in presence of 18c6 in 0.1M [Bu4N][BAr F 4] in 2mM c6 E pa (V vs Fc + /Fc) ; -0.63; E pc (V vs Fc + /Fc) [1] C. Camp, J. Pecaut, M. Mazzanti, J. Am. Chem. Soc. 2013, 135, [2] J. K. Brask, V. Dura-Vila, P. L. Diaconescu, C. C. Cummins, Chem. Commun. 2002, [3] A. J. M. Duisenberg, L. M. J. Kroon-Batenburg, A. M. M. Schreurs, J. Appl. Crystallogr. 2003, 36, [4] R. H. Blessing, Acta Crystallogr., Sect. A 1995, 51, [5] G. M. Sheldrick, Acta Crystallographica Section A 2008, 64, [6] PLATON, A. L. Spek, Acta Crystallogr., Sect. D 2009, 65, [7] aj. L. Brown, G. Wu, T. W. Hayton, Organometallics 2013, 32, ; bo. P. Lam, F. W. Heinemann, K. Meyer, Chem. Sci. 2011, 2, ; cc. Camp, M. A. Antunes, G. Garcia, I. Ciofini, I. C. Santos, J. Pecaut, M. Almeida, J. Marcalo, M. Mazzanti, Chem. Sci. 2014, 5, S21