Exploring the Redox Properties of Bench-Stable Uranyl(VI) Diamido–Dipyrrin Complexes

The uranyl complexes UO2(OAc)(L) and UO2Cl(L) of the redox-active, acyclic diamido–dipyrrin anion L– are reported and their redox properties explored. Because of the inert nature of the complexes toward hydrolysis and oxidation, synthesis of both the ligands and complexes was conducted under ambient conditions. Voltammetric, electron paramagnetic resonance spectroscopy, and density functional theory studies show that one-electron chemical reduction by the reagent CoCp2 leads to the formation of a dipyrrin radical for both complexes [Cp2Co][UO2(OAc)(L•)] and [Cp2Co][UO2Cl(L•)].


■ INTRODUCTION
Redox-active ligands, also referred to as redox-noninnocent ligands, continue to fascinate and perplex chemists. While the ability of these ligands to adopt multiple stable oxidation states often hinders analysis of the electronic structures of metal complexes, the reactivity of metals can be expanded by their action as electron reservoirs, altered Lewis acids, and reactive ligand radicals and in enabling ligand-to-substrate electron transfer. 1−3 Although the chemistry with transition metals has been vastly explored, there has only recently been a rise in interest of actinide complexes of redox-active ligands, in particular those of uranium. 4−6 Uranium is most commonly present as the uranyl(VI) dication UO 2 2+ under ambient conditions. This dioxide adopts a linear [OU VI O] 2+ structure in which the axial oxygen atoms (O ax ) are strongly bound to the uranium center. 7 As a result, UO 2 2+ is very stable in terms of both kinetics and thermodynamics. Even so, the reduction of uranyl(VI) to uranium(IV) via the unstable uranyl(V) cation UO 2 + is an important aspect of uranium remediation by immobilization, and significant advances have been made in the isolation and study of reduced uranyl complexes, e.g., in oxometalated and oxosilylated uranyl(V) compounds. 8 Uranyl complexes of redox-active ligands, such as Schiff bases, 9,10 quinones, 4 and pyrroles in, for example, tetraaza [14]annulenes, 11 calix [4]pyrroles, 12 and dipyrrins, 13−15 have been reported. Because of the added redox character of these ligands, the complexes react differently under reducing conditions. For example, uranyl(VI) complexes of pentadentate N 3 O 2 -saldien ligands with various substituents all underwent one-electron uranium reduction to afford the corresponding uranyl(V) complex, regardless of the difference in the substituents. 16 In contrast, the uranyl(VI) α-di-imine diphenolate (1) ( Figure 1) and uranyl(VI) salophens undergo one-electron reduction of the ligand, leading to ligand-centered radical anions and not the expected uranyl(V) complexes. 9,10,17 Dipyrrins are popular because of their effective absorption of visible light through π−π* transitions, forming colorful and luminescent metal complexes. 18,19 Uranyl complexes of dipyrrin ligands can be readily accessed through anhydrous, salt metathesis routes. 13 We recently reported the redox behavior of the donor-expanded Schiff-base uranyl(VI) dipyrrin complex 2 ( Figure 1) and its contrasting but controlled inner-and outer-sphere redox chemistry. The use of 1 equiv of the outer-sphere reductant CoCp 2 resulted in one-electron reduction of the ligand to a dipyrrin radical. The addition of a second equiv of CoCp 2 reduced the uranium center to uranyl(V). The reaction of 2 with 1 equiv of the inner-sphere reductant [TiCp 2 Cl] 2 led to the formation of a doubly titanated uranium(IV) complex. 14 In addition, the effects of both the equatorial coordination sphere and axial oxo−ligand bonding in 2 were investigated, showing that it is possible to shift the nonaqueous uranyl(VI/V) and uranyl(V/ IV) reduction potentials to values in the range accessible to reductants that are present in uranium remediation processes and in nuclear fuel storage. 15 However, these dipyrrin complexes all display air sensitivity and therefore need to be handled accordingly.
This study presents the formation of easy-to-synthesize and bench-stable uranyl complexes of a diamidodipyrrin ligand and an evaluation of their reduction properties. A similar ligand has previously been exploited in the formation of boron and transition-metal complexes, such as nickel, copper, and cobalt, although these studies mainly focused on the rich coordination chemistry of these ligands. 20−22 We rationalized that the use of these ligands would deliver a uranyl complex that would potentially be resistant toward oxidation reactions and hydrolysis, while maintaining its redox properties.

■ RESULTS AND DISCUSSION
Synthesis and Structures of Uranyl(VI) Complexes. The synthesis of HL was achieved using a modification of previously reported procedures (Scheme 1). 20 The amination of (trichloroacetyl)pyrrole was conducted in neat, boiling tertbutylamine; however, because of the steric demand of tertbutylamine, the pyrrole amide 4 was synthesized in lower yield compared with the literature derivatives. The second step was an acid-catalyzed condensation that led to formation of the dipyrromethane 5 in 36% yield. In contrast to acyclic Schiffbase dipyrrin ligands made previously in our group, 5 did not spontaneously oxidize during its synthesis and required additional oxidant (2,3-dichloro-5,6-dicyano-1,4-benzoquinone, DDQ) to form the dipyrrin HL, which was readily purified using silica chromatography. 23 The formation of HL was indicated not only by the disappearance of the mesoproton singlet at 5.86 ppm in the 1 H NMR spectrum but also by the intensely orange solid obtained, typical of the dipyrrin chromophore (see the Supporting Information, SI).
The reaction between HL, triethylamine, and 1 equiv of uranyl acetate [UO 2 (OAc) 2 ·2H 2 O] or uranyl chloride [UO 2 Cl 2 (THF) 2 ] (THF = tetrahydrofuran) in a mixture of methanol (MeOH) and CHCl 3 (1:3, v/v) in air led to rapid color changes from an orange to a dark-pink solution (Scheme 2). The acetate complex UO 2 (OAc)(L) was obtained in 77% yield as a dark-pink solid, and the chloride UO 2 Cl(L) was obtained in 91% yield as a dark-reddish-pink solid after aqueous workups. While no additional purification steps were required for UO 2 (OAc)(L), UO 2 Cl(L) was heated in chloroform to ensure the formation of a single product. The second product is likely the ion pair [UO 2 (solvent)(L)][Cl] formed through ready dissociation of the chloride anion. 15 The chloride complex UO 2 Cl(L) may also be prepared via KL using air-sensitive methods.
Formation of the uranyl complexes was indicated by the disappearance of the pyrrole N−H proton at 12.69 ppm for HL and the downfield shift of the pyrrole peaks in the 1 H NMR spectra (see the SI). 24 Both complexes adopt C 2h symmetry in solution, which is also seen in the 19 F NMR spectra, with three resonances indicating horizontal planar symmetry. In addition, the 1 H NMR spectrum of UO 2 (OAc)-(L) contains a broad singlet at 2.17 ppm with an integration of 3H that is assigned to the coordinated acetate ion; this fluxionality of the acetate means that it is not easily identified in the 13 C{ 1 H} NMR spectrum. The chloride complex UO 2 Cl(L) was also prepared under nonaqueous conditions: the reaction between KL (formed in situ by the reaction of HL and KH in THF) and UO 2 Cl 2 (THF) 2 in THF formed UO 2 Cl(L) in high yield.
Crystals suitable for X-ray analysis were grown for HL, UO 2 (OAc)(L) and UO 2 Cl(L) (Figures 2 and 3). Weakly diffracting orange plates of HL were crystallized from a concentrated dimethyl sulfoxide (DMSO) solution, and so the X-ray structure is reported to show connectivity only. HL did not display any intermolecular hydrogen bonding and instead displayed hydrogen bonding between the amide N4-H and the O3 atom of the DMSO solvate molecule.
Greenish-pink blocks of UO 2 (OAc)(L) were grown through the slow evaporation of a concentrated THF solution. The asymmetric unit comprises two molecules that differ primarily in the orientation of the monodentate acetate group, supporting the fluxionality of this anion seen in solution by NMR spectroscopy. In the solid state, the complex adopts a distorted pentagonal-bipyrimidal coordination geometry, in which the ONNO donor set of the expanded dipyrrin ligand   This shows ONNO coordination geometry similar to that of Cu(DADP ph,ipr )Cl (DADP ph,ipr = 1,1′-isopropylamide-5-phenyl-4,6-dipyrrinato) in which the equatorial position is occupied by a chloride ligand. 20 The uranium coordinates to the oxygen atoms of the amide groups, as seen with other uranyl(VI) amide complexes. 25 The  8 In addition, the U−O amide bond distance is similar to those found in other uranyl(VI) amide complexes (typically 2.34−2.40 Å). 25 Electrochemistry. The cyclic voltammograms (CVs) of HL, UO 2 (OAc)(L), and UO 2 Cl(L) were recorded in acetonitrile (MeCN) at a scan rate of 100 mV s −1 ( Figure  4). The CV of HL features a quasi-reversible reduction at E 1/2 = −1.15 V versus ferrocene/ferrocenium (Fc/Fc + ) and an irreversible reduction at E p = −1.99 V versus Fc/Fc + . The first reduction appears reversible when isolated in the CV ( Figure  4, dotted line). This feature is significantly less negative than that of the analogous diimine−dipyrrin ligand (seen in 2), which displays a reversible reduction at E 1/2 = −1.51 V versus Fc/Fc + in CH 2 Cl 2 . 14 Although the diamide ligand is more easily reduced than the diimine analogue, this is not true of their corresponding complexes. The CV of UO 2 (OAc)(L) features four different redox processes upon cathodic scanning. The first is a quasi-reversible reduction process at E 1/2 = −1.10 V versus Fc/Fc + , followed by irreversible reduction processes at E p = −1.97, −2.31, and −2.53 V versus Fc/Fc + . The CV of UO 2 Cl(L) also features four different redox processes, the first a quasi-reversible reduction process at E 1/2 = −0.88 V versus Fc/Fc + , followed by irreversible reduction processes at E p = −1.72, −2.15, and −2.50 V versus Fc/Fc + . In contrast, the diimine−dipyrrin analogue 2 has two consecutive quasireversible reduction processes that are both more accessible at E 1/2 = −0.97 and −1.18 V versus Fc/Fc + compared with UO 2 (X)(L) (X = OAc, Cl). This variation may be due to an increase of the electron density from the amide oxygen atoms to the uranium in the uranyl complexes of L, making them less susceptible to reduction. In addition, the solution of UO 2 Cl(L) required additional stirring after each measurement because of the formation of a second species with a similar reduction pattern (see the SI), which may arise from chloride dissociation to form the ion pair [UO 2 (MeCN)(L)][Cl].
One-Electron Reduction. Colbaltocene (CoCp 2 ) is a strong outer-sphere reductant with a formal cobalt(III)/ cobalt(II) redox potential of −1.33 V versus Fc/Fc +26 but could only be used to study the first reduction of UO 2 (OAc)-(L) and UO 2 Cl(L) because of the significantly more negative second reduction potentials. Reactions between either UO 2 (OAc)(L) or UO 2 Cl(L) and 1 equivalent of CoCp 2 in pyridine-d 5 lead to a dark-red, NMR-silent compound (Scheme 2). Scale-ups were carried out in dry THF, causing the products to precipitate as greenish-brown solids, which are characterized as the ligand-reduction products [Cp 2 , respectively. These complexes are highly sensitive toward air and react rapidly to form new compounds; unfortunately, we have been unable to identify the products of these reactions. Both reduced complexes were successfully characterized by elemental analysis, but attempts to obtain single crystals for X-ray structural characterization were unsuccessful.
Electron Paramagnetic Resonance (EPR) Spectroscopy. The room temperature (RT) EPR spectra of [Cp 2 Co]-   ion not only has instigated the g shift but also broadened the line, obscuring all hyperfine splitting from the various spin-active nuclei in the dipyrrin. 14 No signal for a uranyl(V) complex (f 1 ) would be expected to be seen at RT. Electronic Spectroscopy. The absorbance spectra of HL, acetate, chloride uranyl complexes UO 2 (X)(L), and reduced complexes [Cp 2 Co][UO 2 (X)(L • )] were recorded ( Figure 5).
HL has a maximum absorbance of 470 nm (ε = 27280 M −1 cm −1 ) and is similar to the previously synthesized derivatives. 20 Upon metalation to form the uranyl complexes UO 2 (X)(L), the easy-to-visualize color change is reflected in the UV−vis spectrum with significant red shifts observed relative to HL; the absorbance is independent of the anion, and both complexes exhibit a maximum absorbance at 546 nm (ε = 82316 M −1 cm −1 ) along with a second, weaker band at 510 nm and a shoulder at 478 nm. The reduced compounds [Cp 2 Co][UO 2 (X)(L • )] are poorly soluble in THF, and measurements were therefore carried out in pyridine. Both compounds exhibit near-identical spectra. The intense absorption of the dipyrrin chromophore in the UV and visible regions that took place before 300 nm has now shifted dramatically and can be seen just before 400 nm. The maximum absorbance has also shifted to 500 nm (ε = 45700 M −1 cm −1 ).
Density Functional Theory (DFT) Calculations. The occurrence of one-electron reduction of the diamido−dipyrrin ligand and not the uranium center in the uranyl complexes is supported by computational analysis. DFT calculations were undertaken on both UO 2 (OAc)(L) and UO 2 Cl(L) and their one-electron-reduction products. The former experiments reveal that the lowest unoccupied molecular orbitals (LUMOs) of both complexes are located entirely on the ligand, whereas in contrast, the LUMOs+1 are metal-based, indicating that one-electron reductions should indeed lead to ligand-based radicals ( Figure 6). Furthermore, the LUMOs+1 suggest that the second reduction should lead to uranium reduction, i.e., to the formation of uranyl(V) complexes. The singly occupied molecular orbitals (SOMOs) of [UO 2 (OAc)-(L • )] − and [UO 2 Cl(L • )] − are also ligand-based, and the unpaired spin-density maps of both show that the electron density is located entirely on the meso-carbon of the ligand, furthermore confirming the radical character of the ligand after one-electron reduction.
As shown previously, the CV of UO 2 Cl(L) exhibits another similar set of reductions, and it was concluded that this was due to the lability of the chloride, forming the ion pair [UO 2 (MeCN)(L)][Cl] in solution. A study conducted previously in the group, however, demonstrated that the cation of 2, [UO 2 (L 2 )][BAr F ], first undergoes uranyl(VI)/ uranyl(V) reduction rather than the formation of a ligand radical (L 2 = dipyrrin−diimine ligand). 15

■ CONCLUSIONS
The diamido−dipyrrin ligand acts as a tetradentate chelate for the uranyl dication and, because of its low-lying π* molecular orbitals, is a redox-noninnocent partner in the reduction chemistry of its uranyl complexes. The uranyl complexes UO 2 (OAc)(L) and UO 2 Cl(L) are both insensitive toward hydrolysis and could therefore be easily prepared and stored on the bench. In addition, both complexes undergo one-  Inorganic Chemistry pubs.acs.org/IC Article electron reduction when reacted with CoCp 2 , leading to ligand radicals rather than uranyl(V) complexes. Although attempts to crystallize the singly reduced complexes were unsuccessful, EPR, cyclic voltammetry, and DFT studies support the presence of a ligand radical. Our current investigations are focused on manipulation of the redox behavior of similar dipyrrin ligands in order to form air-stable uranyl(V) dipyrrin complexes.

■ EXPERIMENTAL SECTION
General Procedure. Caution! Depleted uranium (primary isotope 238 U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years. Manipulations and reactions should be carried out in monitored fume hoods or in an inert-atmosphere glovebox in a radiation laboratory equipped with αand β-counting equipment. The syntheses of all airand moisture-sensitive compounds were carried out using standard Schlenk techniques under an atmosphere of dry argon. Vacuum Atmospheres and MBraun gloveboxes were used to manipulate and store air-and moisture-sensitive compounds under an atmosphere of dried and deoxygenated dinitrogen. The solvents pyridine-d 5 and THF-d 8 were refluxed over potassium metal overnight, trap-to-trapdistilled, and three times free-pump-thaw-degassed prior to use. All glassware was dried in an oven at 160°C, cooled under 10−3 mbar vacuum, and then purged with argon. Prior to use, all Fisherbrand R 1.2 mm retention glass microfiber filters and stainless-steel cannula were dried in an oven at 160°C overnight. All solvents for use with air-and moisture-sensitive compounds were stored in Teflon-tapped ampules containing predried 4 Å molecular sieves. Solvents were collected from a solvent purification system (Innovation Technologies), where they had been passed over a column of molecular sieves for 24 h prior to collection. They were then degassed prior to use and subsequent storage. All chemicals were used as used as received without any purification, unless otherwise specified. Tetrabutylammonium hexafluorophosphate, [ n Bu 4 N][PF 6 ], was recrystallized twice from absolute EtOH and further dried for 2 days under vacuum. 1 H NMR spectra were recorded on a Bruker AVA400 spectrometer operating at 399.90 MHz, a Bruker AVA500 or a Bruker PRO500 spectrometer operating at 500.12 MHz, or a Bruker AVA600 spectrometer operating at 599.81 MHz. 13 C{ 1 H} NMR spectra were recorded on a Bruker AVA500 or a Bruker PRO500 spectrometer operating at 125.76 MHz. 19 F{ 1 H} NMR spectra were recorded on a Bruker AVA500 spectrometer operating at 470.59 MHz. Chemical shifts are reported in parts per million. 1 H and 13 C{ 1 H} NMR spectra are referenced to residual solvent resonances calibrated against an external standard, SiMe 4 (d = 0 ppm). 19 F{ 1 H} NMR spectra are referenced to an external standard, CCl 3 F (d = 0 ppm). All spectra were recorded at 298 K unless otherwise specified. All data were processed using MestReNova 12.0.3. Full assignments are given in the Supporting Information.
Single-crystal X-ray diffraction data were collected at 120 K on an Oxford Diffraction Excalibur diffractometer using graphite-monochromated Mo Kα radiation equipped with an Eos CCD detector (λ = 0.71073 Å) or at 120 K on a Supernova Dual Cu at Zero Atlas diffractometer using Cu Kα radiation (λ = 1.5418 Å). Structures were solved using ShelXT direct methods or intrinsic phasing and refined using a full-matrix least-squares refinement on |F| 2 using ShelXL. 27−29 All programs were used within the OLEX suite. 30 All non-hydrogen atoms refined with anisotropic displacement and H parameters were constrained to parent atoms and refined using a riding model unless otherwise specified. All single-crystal X-ray structures were analyzed and illustrated using Mercury 4.3.1.
Elemental analyses were carried out by Elemental Microanalysis Ltd., measured in duplicate. All Fourier transform infrared (FTIR) spectra were recorded using a JASCO 410 or a JASCO 460 plus spectrometer. Intensities are assigned as w = weak, m = medium, and s = strong. All UV−vis absorption spectra were recorded on a Jasco V-670 spectrometer on a 10 mm quartz cuvette, fitted with a septum for air-sensitive compounds.
Full synthetic procedures, X-ray crystallography, DFT calculations, EPR spectroscopy, and electrochemical methods (PDF)