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Molecular and Electronic Structures of Eight-Coordinate Uranium Bipyridine Complexes: A Rare Example of a Bipy2− Ligand Coordinated to a U4+ Ion Michael W. Rosenzweig,† Frank W. Heinemann,† Laurent Maron,‡ and Karsten Meyer*,† †

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen−Nürnberg (FAU), Egerlandstrasse 1, 91058 Erlangen, Germany ‡ LPCNO, Université de Toulouse, INSA Toulouse, 135 Avenue de Rangueil, 31077 Toulouse, France S Supporting Information *

ABSTRACT: Reaction of trivalent [((Ad,tBuArO)3tacn)U] (1) with 2,2′-bipyridine (bipy) yields [((Ad,tBuArO)3tacn)U(bipy)] (2) and subsequent reduction of 2 with KC8 in the presence of Kryptofix222 furnishes [K(2.2.2-crypt)][((Ad,tBuArO)3tacn)U(bipy)] (3). Alternatively, complex 3 can be synthesized from 1 by addition of [K(bipy)] in the presence of the cryptand. New complexes 2 and 3 are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy•−) ligand in 2 and a dianionic (bipy2−) species in 3. Complex 3 represents a rare example of an isolated and unambiguously characterized bipy2− ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes 2 and 3. The electronic ground state for 2 consists of an open-shell doublet U4+(bipy•−) state (91%) and a closed-shell doublet U5+(bipy2−) state (9%). The almost degenerate multiconfigurational ground state for 3 was found to be composed of an openshell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U4+(5f2) (bipy2−) configuration.

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INTRODUCTION The concept of redox noninnocent ligands that participate in metal-mediated redox processes provides a powerful tool for chemical conversion and catalysis.1−3 Redox-active ligands (such as bipyridines,4−14 porphyrins,15−17 pyridine-2,6-diimines,18,19 etc.) can act as an electron reservoir, facilitating multielectron processes, which are not accessible utilizing the bare metal center. In uranium coordination chemistry, for instance, this has been most impressively demonstrated by Bart et al., who employed a redox-flexible pyridine(diimine) ligand for the synthesis of a unique uranium tris(imido) species.20 Additionally, redox noninnocent ligands play a prominent role as ligands for photocatalysts.6,21 Hence, the mechanism of cooperative metal−ligand interactions, which is particularly underexplored for actinides, is of substantial importance for the development of new catalysts.1,3,22 For the detailed understanding of the complexes’ reactivity, advanced spectroscopic methods and computational analyses are necessary to fully clarify the intricate electronic situation within the metal−ligand unit.17,23,24 2,2′-Bipyridine (bipy) is probably one of the most studied redox-active (redox noninnocent) ligands that can bind in its neutral, radical, or even dianionic form.9,25 Predominantly, bipy © XXXX American Chemical Society

binds as a neutral, bidentate ligand to the metal center, whereas electropositive, low-valent metal ions are capable of reducing the bipy ligand, with concomitant oxidation of the metal center.26 It has been shown that high-resolution crystallographic analyses, in combination with magnetochemical and computational studies, provide clear information on the distinct redox state of the redox-active bipy ligand, and hence, the metal ions’ oxidation state.9,27 Structurally characterized complexes with a diamagnetic dianionic bipy2− ligand, however, remain scarce,9,25,27,28 but would be of great interest for chemical transformations because the ligand reversibly stores two electrons in its bidentate chelate. This is of particular significance for lanthanide and actinide complexes, which mostly undergo one-electron redox chemistry and chemical transformations.29−31 The redox-active ligand can thus facilitate multielectron processes in f-element chemistry. Despite the increasing number of characterized bipy complexes with lanthanides and actinides in recent years, these compounds still remain less studied than their transition metal counterparts. A number of Yb,32−36 Tm,37 and Th bipy complexes,38 as well Received: December 5, 2016

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DOI: 10.1021/acs.inorgchem.6b02954 Inorg. Chem. XXXX, XXX, XXX−XXX

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SQUID magnetization, single-crystal XRD studies, and CASSCF computational studies.

as the thoroughly studied Ce(COT)2 (COT = cyclooctatetraene),39,40 show complicated electronic and magnetic properties. Various experimental and theoretical studies indicate that these complexes possess a multiconfigurational ground state, which consists of different open and closed-shell configurations. Hence, the almost degenerate (fn)(bipy) and (fn−1)(bipy•−) configurations result in a nonintegral effective valence of the metal ion.34 This feature is often indicated by a reduced magnetic moment of the complexes and can be verified by CASSCF calculations.32,41,42 In contrast to uranium complexes supported by a neutral bipy ligand, uranium compounds featuring redox-active bipy ligands still remain scarce. Recently reported complexes of uranium with a bipyridine radical ligand are [Cp*2U(bipy)] (Cp* = 1,2,4-(Me3C)3C5H2),43 [Tp*2U(bipy)] (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate),44 and [(C5Me5)(C8H8)U(Me2bipy)]45 as well as [(Ar(t-Bu)N)2U(bipy)2], [(Ar(1-Ad)N)3U(bipy)] (Ar = 3,5-C6H3Me2, 1-Ad = 1adamantyl), and [(Mes(t-Bu)CN)3U(bipy)2] (Mes = 2,4,6C6H2Me3),26 in which the latter one features one neutral and one radical anionic bipy ligand. In a more recent, very elegant study, Fortier et al. reinvestigated the homoleptic bipy complex, namely, [U(bipy)4], which was claimed to be a zerovalent uranium species.27,46 A carefully conducted X-ray crystallographic analysis, supported with magnetic and computational studies, reveals the presence of a tetravalent uranium ion coordinated to four bipy•− ligands, thus, precluding the presence of a U0 compound. Interestingly, reduction of [U(bipy)4] with slight excess of elemental sodium results in the isolation of [Na(THF)6][U(bipy)4]. However, and likely due to the fact that four bipy ligands are bound to the uranium ion, neither structural analysis nor SQUID magnetization measurements could clarify the exact valence in the complex anion [U(bipy)4]−; and formulations such as [U5+(bipy•−)2(bipy2−)2]− or [U4+(bipy)(bipy•−)(bipy2−)2]− seem to be plausible. Accordingly, the authors suggest that the complex is better considered in terms of its total spin multiplicity and not by the exact valence. Regardless of the anion’s ligand’s exact redox description, [Na(THF)6][U(bipy) 4] represents the first uranium complex with a diamagnetic bipy2− ligand. Aiming at the identification of a bona fide bipy2− ligand bound to uranium, we studied the [((R,R′ArO)3tacn)U-X] system, hoping to be able to bind an additional, single bipy ligand. As previously demonstrated, the uranium(III) complexes [((R,R′ArO)3tacn)U] (with ((R,R′ArO)3tacn)3− = trianion of 1,4,7-tris(3,5-dialkyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, R = tBu, Ad, R′ = tBu) are suitable for the reduction and stabilization of redox noninnocent ligands, namely, diphenyldiazomethane,47 4,4′-di-tert-butylbenzophenone,48 and CO2,49 to yield their one-electron-reduced, coordinated radical anions. Complete characterization (XRD, UV/vis/NIR, SQUID, DFT) of these compounds unambiguously revealed [UIV−L•−] charge-separated complexes; and no intermediate valence could be detected. For [((tBu,tBuArO)3tacn)UIV(OCtBuPh2•−)], the uranium +4 oxidation state has been further corroborated by an in-depth U LIII-edge X-ray absorption near-edge structure (XANES) study.50 Here, we report the redox chemistry of the uranium(III) complex [((Ad,tBuArO)3tacn)U] with 2,2′-bipyridine and the subsequent reduction with potassium graphite (KC8). All complexes are fully characterized by CHN elemental analyses, 1 H NMR, IR, UV/vis/NIR, and EPR spectroscopy as well as

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RESULTS AND DISCUSSION Addition of one equivalent of 2,2′-bipyridine to a red-brown solution of trivalent [((Ad,tBuArO)3tacn)U] (1) in n-pentane results in formation of a brown precipitate (Scheme 1). The Scheme 1. Synthesis of Uranium Complex 2

solid was collected by filtration and dried under reduced pressure to obtain [((Ad,tBuArO)3tacn)U(bipy)] (2) as light brown solid. Single crystals of 2 suitable for an X-ray diffraction study were obtained by diffusion of n-hexane into a saturated DME solution at room temperature. The molecular structure of 2 features the uranium center in an eight-coordinate pseudododecahedral coordination sphere, with both nitrogen atoms of the 2,2′-bipyridine ligand coordinated to the metal center (Figure 1). One Nbipy atom

Figure 1. Molecular structure of 2 in crystals of 2·2DME, and the U ions’ coordination polyhedron (bottom right). Hydrogen atoms, tBu groups, and cocrystallized solvent molecules are omitted for clarity. Colors: blue, nitrogen; red, oxygen; magenta, uranium; gray, carbon.

is located in the equatorial plane defined by the three aryloxide oxygen atoms, and the second one occupies the axial position, resulting in an Nax−U−Neq angle of 64.58(9)°. Solid-state molecular structures of bipyridyl complexes have been used to define the degree of reduction of the bipy ligand, to determine whether the bipyridine acts as neutral, mono-, or dianionic ligand. In the neutral form, the two pyridine rings show character of conjugated systems, which significantly deviates from aromatic rings upon reduction. This is most pronounced for the doubly reduced bipyridine bipy2−.51−54 Thus, the most distinctive structural parameter for the characterization of a redox-active bipy ligand is the C−C distance between the carbon atoms connecting the two heterocycles. In the case of 2, the characteristic Cpy−Cpy bond distance was determined to be 1.424(5) Å (Table 1), B


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the cryptand (Figure 2). Comparison of the bipy intraligand’s structural metrics in 2 and 3 clearly reveals a further ligand

Table 1. Selected Structural Parameters (Å) of Complexes 2 and 3 structural param

2

3

Cpy−Cpy U−Nbipy (axial) U−Nbipy (equat) U−OAr

1.424(5) 2.503(2) 2.512(3) 2.197(2) 2.216(2) 2.230(2) 2.728(2) 2.740(2) 2.770(2) −0.365(2)

1.379(10) 2.373(4) 2.406(6) 2.257(5) 2.269(6) 2.289(5) 2.787(5) 2.886(6) 2.906(6) −0.298(6)

U−Ntacn

Uoop

clearly indicating a monoanionic bipyridyl radical species, reduced by the metal center (Figure 4, left). The U−Nbipy bond distances in 2 were determined to be 2.512(3) Å for the equatorial Nbipy, and 2.503(2) Å for the axial Nbipy, respectively, which is in the range of other structurally characterized UIV(bipy•−) complexes (2.429(2)−2.609(7) Å).26,45 The two pyridyl rings of the bipy ligand remain planar, but the two torsion angles N−C−C−N (8.2(4)°) and C−C−C−C (8.8(5)°) between the planes of the rings show that the two pyridyl moieties are no longer coplanar. The average U−Ntacn bond length is 2.746 Å, and the average U−OAr distance was determined to be 2.214 Å. These bond lengths are in good agreement with other UIV complexes supported by the ((R,R′ArO)3tacn) ligand system.55−57 Subsuently, complex 2 was treated with KC8 to study the reduction chemistry of the compound. This raises the question of whether the reduction takes places at the metal center (back to U3+) or the redox-active ligand (to yield the targeted bipy2− at U4+). Upon adding a stoichiometric amount of KC8 and 2.2.2-cryptand to a brown solution of 2 in benzene, a black powder was obtained, which was identified as [K(2.2.2cryptand)][((Ad,tBuArO)3tacn)U(bipy)] (3) (Scheme 2). Similar to 2, crystals of 3, suitable for a single-crystal X-ray diffraction study, were obtained by diffusion of n-pentane into a saturated solution of 3 in DME at room temperature. In the solid state, complex 3 exhibits the same 8-coordinate pseudododecahedral coordination sphere as 2, with an additional outer sphere potassium counterion encapsulated by

Figure 2. Molecular structure of 3 in crystals of 3·2(n-pentane), and the U ions’ coordination polyhedron (bottom left). Hydrogen atoms, tBu groups, and cocrystallized solvent molecules are omitted for clarity. Colors: blue, nitrogen; light blue, potassium; red, oxygen; magenta, uranium; gray, carbon.

reduction to a doubly reduced, dianionic bipy2− species for complex 3 (Figure 4, right). The Cpy−Cpy distance, linking the two pyridine rings, contracts from 1.424(5) in 2 to 1.379(10) Å in 3, which is in the range of a typical CC double bond now. Comparable transition metal and lanthanide complexes featuring dianionic bipy2− ligands exhibit Cpy−Cpy distances ranging from 1.35 to 1.40(1) Å.9,12,52,58,59 Similarly, the only two reported examples of actinide ions coordinated by dianionic bipy2− ligands, namely, [Cp*2Th(bipy2−)]28 and [Na(THF)6][U(bipy)4], display Cpy−Cpy bond distances of 1.379(8) to 1.382(8) Å. In addition, and due to the coordination of a dianionic ligand to U4+, the U−Nbipy bond distances are significantly shortened to 2.373(4) Å and 2.406(6) Å, respectively, which is noticeably shorter than the only other characterized uranium complex with a doubly reduced bipy2− ligand (2.425(4)−2.510(4) Å).27 It has been pointed out that bipy2− exhibits π-donating properties and may facilitate π-backbonding to an empty metal-centered orbital with π-symmetry.9 It is suggested that the shortened U−Nbipy bond distances in 3 are due to a certain degree of multiple bonding, resulting from electron donation of the ligand to the π−accepting uranium center. In this regard, an additional and notable structural feature is the U out-of-plane shift (Uoop), which decreases from −0.365(2) Å in 2 to −0.298(6) Å in 3. This is in line with observations in terminal uranium(IV) chalcogenido complexes [((Ad,MeArO)3tacnUS···K(db-18-c6)] and [K(2.2.2-crypt)][((Ad,MeArO)3tacnUS], in which a reduced Uoop is indicative for a significant degree of covalency in the uranium−sulfur bond.55 However, an increased electro-

Scheme 2. Synthesis of Complex 3

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EPR SPECTROSCOPY In order to prove the radical character of the bipyridine ligand in 2, the compound was studied by continuous-wave X-band EPR spectroscopy. Complex 2 reveals an axial signal with g∥ = 2.035 and g⊥ = 2.002 at 80 K, in frozen pentane glass (Figure 5), which is persistent at room temperature (giso(298 K) = 2.02

static interaction of the U(IV) ion with a dianionic ligand may also explain the (Uoop) and shortened bond lengths as well. Alternatively, 3 can also be synthesized by adding one equivalent of potassium 2,2′-bipyridine and 2.2.2-cryptand to a solution of trivalent 1 in THF (Scheme 2). This is an interesting observation as it shows that trivalent 1 still is reducing enough to donate one electron from the metal ion into the π* orbital of the bipy•− ligand, thus forming the closed-shell dianionic species. It is worth mentioning that the conversion of 2 into 3 is fully reversible. Addition of 0.5 equivalent of iodine to complex 3 in THF results in the formation (recovery) of single reduced bipy complex 2. The latter observation is perfectly in line with the results of an electrochemical study on 2. The cyclic voltammogram, recorded in THF vs Fc+/Fc, shows one reversible reduction wave at a half-wave potential, E1/2, of −2.70 V (Figure 3). This feature is assigned to a ligand-based reduction

Figure 5. Continuous-wave X-band EPR spectrum (perpendicular mode) of 2, recorded in pentane glass at 80 K. Experimental conditions: microwave frequency = 8.9769 GHz; power = 0.499 mW, modulation width = 0.4 mT at 100 kHz. The spectrum was simulated with g⊥ = 2.002 and g∥ = 2.035, and Gaussian lines with W⊥fwhm = 1.4 and W∥fwhm = 1.5 mT.

Figure 3. Reversible reduction of 2 at different scan rates. The scans were recorded in THF with 0.1 M [N(nBu)4][PF6].

and Wfwhm(298 K) = 1.6 mT; see the Supporting Information). Since EPR signals originating from uranium-centered electrons can only be detected at very low temperatures (typically below 20 K), the unpaired electron ought to originate from an organic radical, namely, bipy•−. Additionally, the narrow line width, Wfwhm, simulated to be 1.5 (g∥) and 1.4 mT (g⊥), respectively, further strongly suggests that the unpaired electron is located on the reduced bipy•− ligand; uranium-centered EPR signals usually display much larger line width (10−40 mT).55,60,61 However, the slight anisotropy of the signal indicates a minor delocalization of the unpaired spin on the metal center. It is noteworthy that no hyperfine coupling of the unpaired electron to the two 14N (I = 1, 99.6%) nuclei of the bipy ligand can be observed. This is possibly due to additional (unresolved) coupling to 8 bipy Hs and/or due to broadening of the signal as a result of large spin−orbit coupling of the uranium ion. In contrast, the X-band EPR spectrum of 1 exhibits an exceedingly broad, isotropic signal at g = 2.005 (Wfwhm = 40 mT), which is only apparent at temperatures below 14 K.61 These observations are in line with the doublet ground state of a trivalent UIII ion (f3). In contrast, and as expected, complex 3 is EPR-silent over the entire temperature range. This is consistent with the formulation of a diamagnetic bipy2− ligand coordinated to a uranium(IV) ion.

Figure 4. Uranium−bipyridine core structures with the corresponding bond distances in Å of complexes 2 (left) and 3 (right).

from the coordinated (bipy•−) to the (bipy2−) moiety, which allows for the isolation of complex 3. Scanning the entire electrochemical window from 0 to −3.5 V (Figure S20), an additional reduction at −3.37 V appears. This redox event, likely due to the U(IV/III) redox couple, partly overlaps with solvent decomposition and subsequently leads to a number of irreversible redox processes. D


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MAGNETISM AND ELECTRONIC PROPERTIES OF 2 AND 3 Since bipyridine is one of the most commonly used redox− active ligands, the electronic properties of its neutral and reduced species have been studied comprehensively.5,62,63 Whereas neutral 2,2′-bipy features two major electronic absorption bands in the UV region, singly reduced bipy shows additional bands at lower energies, due to Laporteforbidden π*−π* transitions. The UV/vis spectrum of 2 exhibits two strong absorptions in the UV region at 234 and 280 nm, respectively (Figure S11, top). These spin- and parityallowed transitions originate from bipy-centered transitions, as well as from phenyl-based π−π* transitions of the tacn ligand. A prominent feature of the UV/vis spectrum of 2 is the bathochromic shift of the bipy-centered π−π* transition (λ = 398 nm) compared to free bipy (λ = 312 nm), and the appearance of π*−π* transitions in the visible region of the spectrum (Figure S11, bottom). The latter absorptions, which are absent in free bipy, are responsible for the intense color of complex 2.5 In the NIR region, low-intense absorption bands (ε = 40−115 M−1 cm−1) are detectable from 800 to 2200 nm, which originate from Laporte-forbidden f−f transitions of the U4+ ion (Figure S12).31 The pattern of absorption bands in the UV/vis region provides a valuable “fingerprint” for bipy•−containing species, and is in good accordance with other characterized Mn+(bipy•−) complexes.5,26 Additionally, the absorption bands in the NIR region of the spectrum are comparable to other UIV complexes of the [((R,R′ArO)3tacn)UIV(L•−)] system.47,48 Hence, UV/vis/NIR spectroscopy is consistent with the formulation of a (UIV)(bipy•−) species. Upon reduction, complex 3 features four absorption bands in the UV/vis region (λ1 = 234 nm, λ2 = 245 nm, λ3 = 334 nm, λ4 = 517 nm, see Figure S13). The former two absorption bands originate from spin- and parity-allowed π−π* transitions of the bipy ligand and the aryloxide moieties of the triazacyclononane supporting ligand. The latter derive from bipy2− centered π*−π* transitions. The NIR region of 3 features nine low intense absorption bands (800−2300 nm), which is in line with the presence of a U4+ ion (Figure S14). Another approach to study the oxidation and spin states of metal complexes is temperature-dependent SQUID magnetometry. For data reproducibility, SQUID magnetization data of 2 and 3 were recorded on three independently synthesized samples, in a temperature range from 2 to 300 K, with an applied magnetic field of 1 T. The below discussed magnetic moments are averaged values. The magnetic moment of 2 is strongly temperature-dependent and was determined to be 0.76 μB at 2 K and 2.86 μB at 300 K, respectively, which is in good agreement with a uranium(IV) ion with a nonmagnetic ground state.31,64 However, it seems particularly noteworthy that, according to the SQUID data, the ligand centered radical does not seem to significantly contribute to the overall magnetic moment of 2 (Figure 6). In the past, several comparable charge-separated UIV compounds with radical anionic ligands were synthesized and characterized, namely, [(( Ad,tBu ArO) 3 tacn)U IV (CO 2 •− )], [(( tBu,tBu ArO) 3 tacn)U I V (OC t B u Ph 2 • − )], and [(( t B u , t B u ArO) 3 tacn)U I V (η 2 NNCPh2•−)]. All of these examples exhibit considerably higher magnetic moments than simple U4+ ions at low temperatures (1.51−1.75 μB, 5 K), due to magnetic contributions from the single unpaired electron of the radical ligand [UIV−(L•−)] as well as from the UIII/neutral ligand resonance structure [UIII−

Figure 6. Temperature-dependent SQUID magnetization data of complexes [((Ad,tBuArO)3tacn)U] (1), [((Ad,tBuArO)3tacn)U(bipy)] (2), and [K(2.2.2cryptand)][((Ad,tBuArO)3tacn)U(bipy)] (3). The depicted data trace for each compound is the average of three independently synthesized and studied samples (see the Supporting Information).

(L)].48,60,61 For comparison, trivalent 1 with a 5f3 configuration exhibits a low-temperature magnetic moment of 1.58 μB (2 K). It is noteworthy that complex 2 shows no maximum in the χM versus T plot between 2 and 300 K, which excludes antiferromagnetic coupling (see the Supporting Information). Moreover, the χM−1 versus T plot of complex 2 shows an almost linear slope over the whole range of T, which is indicative of a paramagnet with no cooperative magnetic effects. Thus, at first sight, the observed low temperature magnetic moment, and its temperature dependency with increasing temperatures, appears to be contradictory to the EPR spectrum of 2. Regardless, such reduced magnetic moments were already observed and reported for different lanthanide bipy complexes, which exhibit multiconfigurational ground states.41,42,65 To gain further insight into the electronic structure of 2, CASSCF calculations have been applied to verify the proposed multiconfigurational ground state. Thereby, two different active spaces have been considered to determine the electronic ground state, a methodology already employed by Booth et al. on ytterbium−bipy complexes.36,41 Initially, the 3 valence electrons of the complex were distributed in 8 frontier orbitals (7 uranium 5f orbitals + bipy π* orbital) to calculate the electronic ground state. To validate the results, the active space was reduced to 5 frontier orbitals (the four energetically lowest 5f orbitals + bipy π* orbital) and examined if both calculations converge. In both cases, the electronic ground state was found to be a multiconfigurational open-shell doublet. In principle, the lowest optimized wave function can be described as mixing of a U4+(5f2)(bipy•−) configuration (91%) with a U5+(5f1)(bipy2−) configuration (9%). The pure doublet state, namely, the uranium(V) complex, is found to be only 0.15 eV higher in energy, in line with the coupling found in the ground state. Analyzing the nature of the ground state wave function in more detail, it appears that the major configuration arises from two unpaired electrons with opposed spin on the uranium center. However, the metal-centered electrons do not couple with the single unpaired electron located in the bipy π* orbital. Thus, CASSCF calculations provide further inside into the complicated electronic structure of complex 2; however, it E


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Inorganic Chemistry

oven overnight (>8 h) at temperature 125 °C. NMR solvents were obtained from Euriso-top GmbH. [((Ad,tBuArO)3tacn)U] was prepared according to literature procedure.61 All other reagents were acquired from commercial sources and used as received. 1 H NMR spectra were recorded on a JEOL ECX 400 or JEOL ECX 270 instrument at a probe temperature of 23 °C. Chemical shifts, δ, are reported relative to residual 1H resonances of the solvent in ppm. Electronic absorption spectra were recorded from 220 to 2500 nm (Shimadzu, UV-3600) in the indicated solvent at room temperature. Infrared (IR) spectra were recorded on a Shimadzu Affinity-1 CE FTIR instrument from 400 to 4000 cm−1. Solid samples of the compounds were homogenized with excess KBr, and a pellet was measured at room temperature. Elemental analyses were obtained using Euro EA 3000 (EuroVector) and EA 1108 (Carlo-Erba) elemental analyzers in the Chair of Inorganic Chemistry at the University Erlangen-Nürnberg (Erlangen, Germany). SQUID magnetization data of powdered samples were recorded with a SQUID magnetometer (Quantum Design) at 10 kOe between 2 and 300 K for all samples. Values of the magnetic susceptibility were corrected for the underlying diamagnetic increment by using tabulated Pascal constants.67 Diamagnetic corrections (χdia [10−6 cm3 mol−1]) used for the complexes: 1 (−502.03), 2 (−970.05), and 3 (−1106.99). Samples used for magnetization measurements were checked for purity by 1H NMR spectroscopy. Data reproducibility was also checked by obtaining SQUID data on three independently synthesized samples. EPR spectra were recorded on a JEOL continuous wave spectrometer JES-FA200 equipped with an X-band Gunn oscillator bridge, a cylindrical mode cavity, and a helium cryostat. Spectra simulation was performed using the program W95EPR written by F. Neese.68 Electrochemical measurements have been investigated using a threeelectrode setup with rotating glassy carbon working electrode and platinum rods as counter electrode and reference electrode. The potentiostat was an Autolab Type-III. The electrochemical cell was placed inside an inert-gas glovebox under nitrogen atmosphere, and the samples were measured in 0.1 M electrolyte solutions of [N(nBu)4][PF6] (purchased from Sigma-Aldrich and used without further purification) in THF at room temperature. The reported halfwave potentials are referenced to the Fc+/Fc redox couple, which was measured by adding ferrocene to the sample solution. Synthesis of 2. [((Ad,tBuArO)3tacn)U(bipy)]. A solution of 2,2′bipyridine (0.05 g, 0.32 mmol) in n-pentane (2 mL) was added to a red-brown solution of [((Ad,tBuArO)3tacn)U] (0.40 g, 0.32 mmol) in npentane (2 mL) and stirred for 12 h. The solution turned dark brown, and a solid precipitated out of solution. The brown solid was filtered off and thoroughly washed with cold pentane (T = −34 °C). Diffusion of n-hexane into a DME solution of the complex yielded brown crystals suitable for a single-crystal X-ray diffraction analysis. Yield: 0.34 g (75%). 1H NMR (400 MHz, benzene-d6): δ (ppm) = 118.69 (s, fwhm = 80 Hz), 110.95 (s, fwhm = 72 Hz), 44.64 (s, fwhm = 60 Hz), 38.78 (s, fwhm = 60 Hz), 36.64 (s, fwhm = 44 Hz), 26.97 (s, fwhm = 48 Hz), 24.89 (s, fwhm = 92 Hz), 24.17 (s, fwhm = 44 Hz), 23.27 (s, fwhm = 64 Hz), 22.67 (s, fwhm = 44 Hz), 21.81 (s, fwhm = 72 Hz), 20.30 (s, fwhm = 64 Hz), 14.48 (s, fwhm = 56 Hz), 13.78 (s, fwhm = 44 Hz), 11.22 (s, fwhm = 68 Hz), 10.54 (s, fwhm = 80 Hz), 10.25 (s, fwhm = 64 Hz), 9.76 (s, fwhm = 44 Hz), 8.25 (s, fwhm = 52 Hz), 7.76 (s, fwhm = 56 Hz), 5.88 (s, fwhm = 56 Hz), 5.11 (s, fwhm = 68 Hz), 4.91 (s, fwhm = 44 Hz), 1.93 (s, fwhm = 52 Hz), 0.95 (s, fwhm = 60 Hz), −3.70 (s, fwhm = 60 Hz), −4.72 (s, fwhm = 44 Hz), −9.67 (s, fwhm = 44 Hz), −13.28 (s, fwhm = 48 Hz), −23.09 (s, fwhm = 60 Hz), −26.67 (s, fwhm = 60 Hz), −31.54 (s, fwhm = 52 Hz), −36.01 (s, fwhm = 60 Hz), −36.59 (s, fwhm = 68 Hz), −40.75 (s, fwhm = 56 Hz), −46.11 (s, fwhm = 56 Hz), −49.46 (s, fwhm = 60 Hz), −51.99 (s, fwhm = 56 Hz), −57.50 (s, fwhm = 60 Hz), −64.38 (s, fwhm = 72 Hz), −89.48 (s, fwhm = 84 Hz), −105.73 (s, fwhm = 88 Hz), −134.77 (s, fwhm = 220 Hz). Anal. Calcd for C79H104N5O3U: C 67.31, H 7.44, N 4.97. Found: C 67.58, H 7.16, N 4.74. Synthesis of 3. [K(2.2.2-cryptand)][((Ad,tBuArO)3tacn)U(bipy)] (Method A). A solution of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane (Kryptofix 222, 41 mg, 0.11 mmol) in benzene

cannot explain the experimentally observed, highly reproducible reduced magnetic moment. Temperature-dependent SQUID magnetization measurements of compound 3 reveal a magnetic moment of 2.59 μB at 300 K, which decreases with decreasing temperature to 0.43 μB (2 K). The low temperature magnetic moment is consistent with the nonmagnetic ground state of the U4+ ion bound to the diamagnetic 2,2′-bipyridine dianionic ligand. Compound 3 exhibits TIP (temperature-independent paramagnetism) between 2 and 90 K, which is identifiable by the linear slope of the χMT versus T plot (see the Supporting Information). Hence the bipy2− ligand creates a crystal field that is large compared to kT at 90 K (k = Boltzmann constant), and no low-lying excited states are occupied at this temperature.66 As for 2, CASSCF calculations have been performed on 3 using a similar approach. The electronic ground state was found to be a multiconfigurational open-shell singlet, resulting from the U4+(5f2)(bipy2−) configuration, wherelike complex 2 the two unpaired electrons at the uranium center antiferromagnetically couple to a singlet state. The pure triplet state also resulting from the U4+(5f2)(bipy2−) configuration, where the two unpaired electrons at the uranium center ferromagnetically couple to a tripletis found to be almost degenerate with the open-shell singlet (0.06 eV higher in energy). Finally, the pure closed-shell singlet, where the two electrons at the uranium center are paired, is 0.36 eV higher in energy. Since the nonmagnetic states can couple with the paramagnetic state, this electronic structure results in a typical TIP behavior; similar to what is found in Yb(II) chemistry.32

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CONCLUSION To conclude, reaction of trivalent [((Ad,tBuArO)3tacn)U] with 2,2′-bipyridine results in the formation of a charge-separated U4+−(bipy•−) species determined by UV/vis/NIR and X-band EPR spectroscopy as well as single-crystal X-ray diffraction studies. Indeed, SQUID magnetometry demonstrated a far more complicated electronic structure for complex 2. CASSCF calculations reveal a multiconfigurational ground state, which consists of 91% U4+(5f2)(bipy•−) and 9% U5+(5f1)(bipy2−) character. Successive reduction of 2 with KC8 results in a ligand-centered reduction and the formation of a rare U4+− (bipy2−) species, namely, [K(2.2.2crypt)][((Ad,tBuArO)3tacn)U(bipy)] (3). Interestingly, interconversion between 2 and 3 is fully reversible. Complex 3 is an example of an unambiguously determined dianionic bipy2− ligand coordinated to a U4+ ion. In the only other reported and carefully examined complex, [Na(THF)6][U(bipy)4], the exact valence of the central ion remains open to question although it has been unequivocally shown that the redox-active ligand exists in both of its reduced bipy•− and bipy2− forms. As shown recently, uranium complexes containing redox-active ligands exhibit unique reactivity toward multielectron processes. Accordingly, the redox chemistry of complex 3 is part of our ongoing studies.

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive experiments were performed under dry nitrogen atmosphere in MBraun inert-gas gloveboxes containing an atmosphere of purified dinitrogen. The glovebox is equipped with a −35 °C freezer. Solvents were purified using a two-column solid-state purification system (Glass Contour System, Irvine, CA), transferred to the glovebox without exposure to air, and stored over activated molecular sieves (4 Å) and sodium (where appropriate). All glassware was dried by storage in an F


Article

Inorganic Chemistry (1 mL) was added to a dark-brown solution of [((Ad,tBuArO)3tacn)U(bipy)] (0.15 g, 0.11 mmol) in benzene (3 mL). A suspension of potassium graphite (KC8, 15 mg, 0.11 mmol) in benzene (1 mL) was added dropwise to the reaction solution and stirred for 5 h. The solution was filtered over a pad of Celite to remove graphite. The solvent of the filtrate was removed by reduced pressure, resulting in a black powder. Recrystallization from toluene resulted in small black crystals. Yield: 0.164 g (82%). Single crystals suitable for an X-ray diffraction study were obtained by diffusion of n-pentane into a concentrated DME solution at room temperature. Synthesis of 3. [K(2.2.2-cryptand)][((Ad,tBuArO)3tacn)U(bipy)] (Method B). A solution of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane (Kryptofix 222, 0.06 g, 0.16 mmol) was added to a red-brown solution of [((Ad,tBuArO)3tacn)U] (0.20 g, 0.16 mmol) in benzene (1 mL). A suspension of [K(bipy)] (0.03 g, 0.16 mmol) in benzene (1 mL) was added dropwise to the reaction solution. The solution turned black, and a thick black precipitate formed overnight. The solid was filtered off and dried under reduced pressure. Recrystallization from toluene resulted in small black crystals. Yield: 0.25 g (87%). 1H NMR (400 MHz, benzene-d6): δ (ppm) = 77.09 (s, fwhm = 36 Hz), 64.10 (s, fwhm = 44 Hz), 29.83 (d, J = 12.4 Hz), 27.01 (d, J = 12.4 Hz), 26.88 (s, fwhm = 8 Hz), 20.68 (d, J = 9.9 Hz), 19.82 (s, fwhm = 16 Hz), 18.09 (s, fwhm = 8 Hz), 15.24 (s, fwhm = 8 Hz), 14.29 (s, fwhm = 8 Hz), 13.60 (d, J = 14.9 Hz), 11.29 (d, J = 12.4 Hz), 10.52 (s, fwhm = 12 Hz), 10.19 (d, J = 12.4 Hz), 7.77 (s, fwhm = 8 Hz), 7.41 (d, J = 12.4 Hz), 7.32 (d, J = 12.4 Hz), 5.05 (s, fwhm = 12 Hz), 4.40 (s, fwhm = 8 Hz), 3.83 (m, fwhm = 40 Hz), 3.66 (s, fwhm = 4 Hz), 3.51 (s, fwhm = 12 Hz), 3.10 (s, fwhm = 32 Hz), 2.74 (s, fwhm = 4 Hz), 2.53 (s, fwhm = 12 Hz), 2.07 (s, fwhm = 28 Hz), −0.18 (d, J = 7.4 Hz), −0.40 (d, J = 7.4 Hz), −0.65 (s, fwhm = 12 Hz), −1.65 (d, J = 7.4 Hz), −2.18 (s, fwhm = 4 Hz), −3.62 (d, J = 11.0 Hz), −4.15 (s, fwhm = 4 Hz), −4.67 (s, fwhm = 8 Hz), −5.22 (d, J = 7.4 Hz), −7.60 (d, J = 11.0 Hz), −7.93 (s, fwhm = 8 Hz), −8.99 (s, fwhm = 24 Hz), −9.73 (s, fwhm = 40 Hz), −16.23 (s, fwhm = 20 Hz), −16.35 (d, J = 14.7 Hz), −17.11 (s, fwhm = 16 Hz), −19.24 (s, fwhm = 36 Hz), −19.99 (s, fwhm = 16 Hz), −20.78 (s, fwhm = 36 Hz), −22.59 (s, fwhm = 16 Hz), −23.65 (d, J = 14.7 Hz), −30.49 (s, fwhm = 32 Hz), −31.96 (s, fwhm = 36 Hz), −34.31 (s, fwhm = 28 Hz), −40.24 (s, fwhm = 28 Hz), −63.80 (s, fwhm = 24 Hz), −71.98 (s, fwhm = 40 Hz). Anal. Calcd for C97H140KN7O9U·C6H14: C 64.72, H 8.12, N 5.13. Found: C 64.64, H 8.30, N 5.10.

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codes 02NUK012C and 02NUK020C), the Joint DFG-ANR projects (ME1754/7-1, ANR-14-CE35-0004-01), and the FAU Erlangen-Nürnberg.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02954. Complete spectroscopic, magnetochemical, electrochemical, crystallographic, and computational data for 2 and 3 (PDF) Crystallographic data for 2 and 3 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frank W. Heinemann: 0000-0002-9007-8404 Laurent Maron: 0000-0003-2653-8557 Karsten Meyer: 0000-0002-7844-2998 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funds of the German Federal Ministry of Education and Research (BMBF 2020+ support G


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Inorganic Chemistry

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