Chelating Borohydrides for Lanthanides and Actinides

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Chelating Borohydrides for Lanthanides and Actinides: Structures, Mechanochemistry, and Case Studies with Phosphinodiboranates Taylor V. Fetrow,† Rina Bhowmick,‡ Andreas J. Achazi,‡ Anastasia V. Blake,† Francesca D. Eckstrom,† Bess Vlaisavljevich,*,‡ and Scott R. Daly*,† †

Department of Chemistry, The University of Iowa, E331 Chemistry Building, Iowa City, Iowa 52242, United States Department of Chemistry, The University of South Dakota, 414 East Clark Street, Vermillion, South Dakota 57069, United States

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ABSTRACT: In this Forum Article, we review the development of chelating borohydride ligands called aminodiboranates (H3BNR2BH3−) and phosphinodiboranates (H3BPR2BH3−) for the synthesis of trivalent f-element complexes. The advantages and history of using mechanochemistry to prepare molecular borohydride complexes are described along with new results demonstrating the mechanochemical synthesis of M2(H3BPtBu2BH3)6, where M = U, Nd, Tb, Er, and Lu (1−5). Multinuclear NMR, IR, and singlecrystal X-ray diffraction data are reported for 1−5 alongside complementary density functional theory calculations to reveal differences in their structure and reactivity with and without tetrahydrofuran. The results demonstrate how mechanochemistry can be used to access f-element complexes with chelating borohydrides in improved and reproducible yields, which is an important step toward investigating the properties of lanthanide and actinide phosphinodiboranate complexes with different phosphorus substituents. The relevance of these results is contextualized by a discussion of structural factors known to influence the volatility of f-element borohydrides and applications that require the development of volatile f-element complexes.

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INTRODUCTION f-element borohydrides are lanthanide and actinide complexes containing M−H−B bonds. The first f-element borohydride complex, U(BH4)4, was discovered during the Manhattan project as part of the hunt for volatile uranium complexes to separate U-235 from U-238 and isotopically enrich nuclear fuel.1−4 Following World War II, when the veil of secrecy began to lift on some atomic research efforts in the United States, the syntheses of U(BH4)4, U(BH3Me)4, and U(BH4)3(BH3Me),5−7 as well as related Th(BH4)4,8 were reported. Synthetic investigations of lanthanide tetrahydroborates followed shortly thereafter,9 and the field of f-element borohydride chemistry rapidly expanded after the first structural investigations of U(BH4)4 using IR spectroscopy,10,11 X-ray diffraction (XRD),12 and neutron diffraction13 revealed exceptionally high coordination numbers and inner coordination spheres comprised exclusively of hydrogen atoms. The unusual structure of U(BH4)4 led to the synthesis and study of homoleptic BH4− and BH3Me− complexes with the more radioactive protactinium,14−16 neptunium,15−21 and plutonium,15,16 as well as indepth investigations of heteroleptic thorium, uranium, and lanthanide complexes with Lewis-base adducts and other ancillary ligands.2,22 One of the most fascinating properties of some f-element borohydrides (which will be discussed in more detail in the sections that follow) is their remarkably high volatility. In collaboration with Girolami and co-workers at the University of Illinois, we recently set out to investigate whether differences in © XXXX American Chemical Society

the volatility of borohydride complexes could be used to separate various f-elements by selective volatilization or thermochromatography.23−25 The impetus for this work is the desire to develop f-element separation methods that minimize the production of new radioactive solvent waste, which is often generated in significant quantities by conventional separation processes such as solvent extraction. Ideally, these new methods could also reduce stocks of existing radioactive solvent waste, and this need is perhaps best exemplified by the estimated 56 million gallons of highly radioactive mixed liquid waste that is currently being stored in Hanford, WA.26,27 The separation of trivalent actinides (An3+) and lanthanides (Ln3+), proposed for several waste separation and remediation strategies such as partitioning and transmutation,28,29 also remains particularly challenging because of their similar oxidation states, size, and limited redox chemistry.30−32 Motivated by these challenges, our desired approach aims to synthesize borohydride complexes and exploit differences in their volatility to drive chemical separations with f elements. Achieving this goal requires an understanding of the chemical factors that give rise to volatility in borohydride complexes and the development of new borohydride ligands that can be used to Special Issue: Innovative f-Element Chelating Strategies Received: June 2, 2019

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DOI: 10.1021/acs.inorgchem.9b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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In contrast to tetravalent actinides (An4+ ), trivalent lanthanide complexes with BH4− yield nonvolatile Ln(BH4)3 because the ligands are too small to saturate the larger coordination sphere of Ln3+.3,36,37 Trivalent Ln3+ ions are not only larger but they also require fewer ligands to offset the lower charge on the metal than do tetravalent An4+ ions. As a result, Ln(BH3R)3 complexes have large voids in the coordination sphere that yield numerous intermolecular B−H−M bridging coordination modes,3 which dramatically attenuates their volatility.2,22,38 This problem can be circumvented to some extent by adding neutral Lewis bases to fill voids in the coordination sphere and form complexes that can be dissolved in organic solvents.2,4,22,39−41 The same is true for trivalent An(BH4)3 complexes,42,43 which are often formed by borohydride-induced reduction. U(BH4)4 can be reduced to U(BH4)3 by heating or photolysis,6,44−46 and Np(BH4)4 and Pu(BH4)4 are even more thermally unstable with respect to reduction, decomposing readily at room temperature to what is presumed to be Np(BH4)3 and Pu(BH4)3.15

modify the volatility of complexes containing different f elements. In this Forum Article, we outline previous efforts aimed at developing chelating borohydride ligands to prepare volatile complexes with f elements in 3+ oxidation states and present new results that demonstrate the mechanochemical synthesis of trivalent lanthanide and uranium complexes with a new class of chelating borohydrides called phosphinodiboranates (Chart 1). Chart 1. Chelating Borohydride Ligands

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REVIEW OF THE VOLATILITY/STRUCTURE RELATIONSHIPS IN F-ELEMENT BOROHYDRIDE COMPLEXES To provide context as to why chelating borohydrides are necessary for preparing volatile complexes with trivalent f metals, it is useful to discuss the structural factors known to influence volatility in borohydride complexes. The volatility of felement borohydride complexes depends first and foremost on the size and charge of the metal ion in relation to the size of the borohydride ligand. For example, all An(BH4)4 complexes are volatile, but their volatility increases as the size of the An4+ ion decreases. Starting with the largest metal ion Th4+ (1.05 Å; coordination number = 8),33 Th(BH4)4 sublimes at 150 °C under vacuum,8,11 but the sublimation temperature decreases abruptly to 55 °C for Pa(BH4)4;15 Pa4+ has a radius of 1.01 Å. Continuing across the series, U(BH4)4 with U4+ (1.00 Å) sublimes at room temperature and has a vapor pressure of 0.30 Torr at 34 °C,6 whereas Np(BH4)4 and Pu(BH4)4 are room temperature liquids with a much higher vapor pressure of 10 Torr at 25 °C (Np4+ = 0.98 Å and Pu4+ = 0.96 Å).15 Differences in the An(BH4)4 volatility can be rationalized by considering their solid-state structures. In general, the volatility of An(BH4)4 complexes depends on the presence and strength of intermolecular B−H−M bonds and the ability of BH4− to saturate the coordination sphere of 4+ metals. Th(BH4)4, Pa(BH4)4, and U(BH4)4 are 14-coordinate coordination polymers in the solid state with two κ3-BH4 ligands and four κ2-BH4 ligands that bridge to adjacent metals.12,13,15,16 The relatively high temperatures at which they sublime can be accounted for by the energy needed to depolymerize the 14coordinate solid-state structure to form monomeric and sublimable 12-coordinate complexes. The 12-coordinate gasphase structure of U(BH4)4 has indeed been observed by IR spectroscopy and electron diffraction.10,34,35 In contrast, Np(BH4)4 and Pu(BH4)4 are already 12-coordinate monomers with four κ3-BH4 ligands in the solid state.15,16,18 They require no additional energy to reorganize before sublimation, which can account for their remarkable jump in vapor pressure relative to U(BH4)4 (from 0.30 to 10 Torr). Further evidence of this structure/volatility relationship is observed with Th(BH4)4 and Th(BH3Me)4. The addition of the methyl group in Th(BH3Me)4 eliminates intermolecular bridging B−H modes to give a 12-coordinate, monomeric structure in the solid state.19 As a result, the 150 °C sublimation temperature for 14coordinate Th(BH4)4 decreases to 50 °C when 12-coordinate Th(BH3Me)4 is sublimed under the same conditions.19

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CHELATING BOROHYDRIDES: REVIEW OF AMINODIBORANATES Accessing volatile complexes with trivalent lanthanides and actinides has required the development of new borohydride ligands that can saturate the coordination sphere of these larger metals, preferably without forming solid-state coordination polymers that reduce volatility. The first success with this strategy came with the use of aminodiboranates, a class of chelating borohydrides that have been known since at least the late 1960s.47,48 As the name implies, aminodiboranates are chelating, anionic ligands best described as two BH3 groups bridged by an amido linker (Chart 1). Although numerous derivatives of aminodiboranate salts with different substituents attached to nitrogen are known,49 most of the lanthanide and actinide complexes that have been reported contain N,Ndimethylaminodiboranate (DMADB).47 It has been shown that the increased size and chelating nature of DMADB relative to BH4− and BH3Me− allow it to saturate a larger portion of a metal coordination sphere. Girolami and co-workers reported homoleptic DMADB complexes with Th4+,50 U3+,51,52 and almost all of the trivalent lanthanides,53,54 in addition to some of their Lewis-base adducts with tetrahydrofuran (THF) and 1,2dimethoxyethane.52,54 Molecular etherate adducts of divalent Eu2+ and Yb2+ DMADB complexes have also been prepared.55 Lanthanide DMADB complexes demonstrate the utility of using chelating borohydrides to prepare volatile complexes: Ln(H3BNMe2BH3)3 complexes are among the most volatile lanthanide complexes ever reported, subliming at temperatures of 65−125 °C at 10−2 Torr,54 and their high volatility has proven useful in the synthesis of lanthanide oxide and lanthanide boride thin films via chemical vapor deposition (CVD).53,56 Like An(BH4)4 complexes, Ln(H3BNMe2BH3)3 complexes become more volatile as the size of the Ln3+ ion decreases across the series,33,57,58 and this again appears to follow the change in the Ln(H3BNMe2BH3)3 structure in response to the size of the Ln3+ ion. As shown in Chart 2, the DMADB ligand can chelate to metals or bridge adjacent metals to form dimeric and polymeric structures. As demonstrated by the structure of Pr(H3BNMe2BH3)3 (Pr3+ = 1.126 Å), the largest lanthanides form linear coordination polymers in the solid state. Each metal is bound by two chelating DMADB ligands and two DMADB ligands that bridge to adjacent metals with an overall Ln−H coordination number of 14 (Chart 3).53 As the size of the metal B

DOI: 10.1021/acs.inorgchem.9b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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CHELATING BOROHYDRIDES: REVIEW OF PHOSPHINODIBORANATES The enhanced volatility of lanthanide DMADB complexes suggests that it may be possible to access volatile U3+ complexes and increase the volatility of Ln3+ further by developing larger chelating borohydride ligands that are less likely to form coordination polymers in the solid state. The most straightforward way to achieve borohydrides larger than aminodiboranates is by swapping nitrogen for phosphorus. Phosphinodiboranates are congeners of aminodiboranates and differ only in that they contain a bridging phosphido linker between the two BH3 groups instead of an amido linker (Chart 1). Like aminodiboranates, phosphinodiboranate salts with different substituents attached to phosphorus have been known for over half a century,64−73 but f-element coordination complexes with these ligands were unknown until the first compounds were reported with P,P-di-tert-butylphosphinodiboranate (H3BPtBu2BH3−, t Bu-PDB) by us in a communication last year.74 Phosphinodiboranates join a series of phosphinate and related ligands with other second-row donor groups (i.e., O,75,76 NR,77−82 and CH283−90) that have been used in f-element separations and coordination chemistry (Chart 4). They are also similar to soft-

Chart 2. Aminodiboranate Coordination Modes

Chart 3. Structures of Lanthanide DMADB Complexes as a Function of the Metal Sizea

a

The subscript on boron indicates the denticity of the associated BH3 group. Reprinted with permission from ref 53. Copyright 2010 American Chemical Society.

Chart 4. Comparison of Phosphinodiboranates to Known Phosphinate Ligands Used To Prepare f-Element Complexes (R = Alkyl, Aryl, or SiMe3)

ion decreases to Sm3+ (1.079 Å), the solid-state structures formed contain three chelating DMADB ligands, yet the structure remains a coordination polymer held together by one intermolecular Sm−H−B linkage from a terminal B−H group on an adjacent complex.53 As the ionic radii continues to decrease to Dy3+ and Er3+ (1.027 and 1.004 Å, respectively), the structures that form are 12-coordinate molecular dimers. Each metal has two chelating DMADB ligands, and the dimer is held together with two DMADB ligands, each with coordinating κ2BH3 groups (Chart 3).53 The fact that polymeric Ln(H3BNMe2BH3)3 complexes are volatile indicates that the DMADB ligands can rearrange to form sublimable molecular complexes. However, the structure does not appear to be the sole determinant of volatility in DMADB complexes. Previous studies show that U(H3BNMe2BH3)3 is not volatile despite forming coordination polymers that are isostructural with volatile Pr(H3BNMe2BH3) 3 and Sm(H3BNMe2BH3)3 (Chart 3).51 Rather than subliming, U(H3BNMe2BH3)3 appears to decompose when heated to 150 °C at 10 −2 Torr. Relevant to this observation, La(H3BNMe2BH3)3 sublimes at 125 °C at 10−2 Torr,59 despite La3+ being slightly larger than U3+ (1.032 vs 1.025 Å, respectively).33 Density functional theory (DFT) calculations by Gagliardi and co-workers suggest that the lack of volatility for U(H3BNMe2BH3)3 is attributed to subtle differences in covalent metal−hydride bonding with uranium versus similarly sized lanthanides.60 This result suggests that, in addition to the structure, the small differences between trivalent actinide (such as U3+) and lanthanide metal−ligand covalency can yield appreciable differences in the volatility of f-element borohydride complexes. This is notable because differences in trivalent lanthanide and actinide metal−ligand covalency are thought to be a key driver in the selectivity of soft-donor extractants in more conventional solution extraction methods,61,62 as proposed initially by Seaborg in 1954.63

donor dithiophosphinates and diselenophosphinate ligands,91−96 which include dithiophosphinates renowned for their excellent selectivity for trivalent minor actinides (e.g., Am3+) over lanthanides in solvent extraction studies.62,75,97 The first f-element phosphinodiboranate complex, U2(H3BPtBu2BH3)6 (1), was prepared by treating UI3(1,4dioxane)1.5 with 3 equiv of K(H3BPtBu2BH3)70 in Et2O (Figure 1),74 albeit in relatively poor yields (≤15%). Similar methods were used to prepare the first lanthanide complexes, Nd2(H3BPtBu2BH3)6 (2) and Er2(H3BPtBu2BH3)6 (3), from LnI3 starting materials.74 As expected, the fact that phosphorus in phosphinodiboranates is larger than nitrogen in aminodiboranates results in a larger chelate size: the B···B distance in H3BNMe2BH3− is ∼2.6 Å, whereas H3BPtBu2BH3− is ∼3.1 Å. The larger chelate leads to obvious differences between the solid-state structures of some phosphinodiboranate and aminodiboranate complexes. As mentioned above, U(H3BNMe2BH3)3 exists as a coordination polymer in the solid state because the DMADB ligand is too small to fully saturate the relatively large coordination sphere of U3+.51 In contrast, 1 exists as a dimer. Also unlike aminodiboranates, whose solid-state structures vary depending on the size of the metal ion (Chart 3), both 2 and 3 adopt the same structure as 1 despite significant differences in the metal size. However, size-dependent structural differences are observed in solution. Both 1 and 2 with similarly sized U3+ (1.025 Å) and Nd3+ (0.983 Å) exist as an equilibrium mixture of monomer and dimer in solution (as observed by NMR spectroscopy), whereas the dimeric solid-state structure of 3 with smaller Er3+ (0.89 Å) appears as a monomer (Chart 5).33,74 C

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Figure 1. Ball-and-stick comparison of polymeric U(H3BNMe2BH3)3 (left) and dimeric 1 (right) from single-crystal XRD data.51,74

proceed to completion by using less-coordinating Et2O instead of THF as the reaction solvent,52,98 whereas reactions with K(H3BPtBu2BH3) and metal iodides do not (Figure S25). The poor salt elimination reactivity with K(H3BPtBu2BH3) and our desire to investigate the volatility of phosphinodiboranate complexes led us to investigate their synthesis using mechanochemical methods. In the following sections, we report how mechanochemical reactions helped us to overcome the attenuated solution-phase reactivity with K(H3BPtBu2BH3) and allowed us to compare the structure and THF-binding properties of tBu-PDB complexes with those of DMADB. Except where noted, these results have yet to be published and are reported here for the first time.

Free-energy calculations further corroborate these differences (vide infra).74 Chart 5. Comparison of Monomeric and Dimeric Phosphinodiboranate Complexes Postulated To Exist in Solution

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Despite that tBu-PDB complexes are not polymeric like U(H3BNMe2 BH3) 3 and DMADB complexes with early lanthanides, none of the tBu-PDB complexes that we have tested so far appear to be appreciably volatile during sublimation attempts up to 150 °C under vacuum (10−2 Torr). It is possible that this is attributed in part to the larger tert-butyl substituents in tBu-PDB, which can give rise to increased intermolecular interactions that suppress volatility (especially relative to smaller methyl groups in DMADB). However, our struggle to this point has been preparing phosphinodiboranate complexes in adequate amounts for follow-on sublimation experiments. Thus, we first needed to address the poor synthetic yields of tBu-PDB complexes before proceeding with more in-depth volatility studies and expanding our efforts to other phosphinodiboranates containing different phosphorus substituents. We recently deduced that the poor and irreproducible yields of 1−3 are due to attenuated salt metathesis reactivity between K(H3BPtBu2BH3) and the trivalent lanthanide or uranium iodides. 11B NMR spectra of the reaction mixtures, for example, show that the salt elimination reactions do not proceed to completion in Et2O after 24 h, and similar reactions in THF do not appear to proceed to any appreciable extent (Figure S25). The attenuated solution reactivity for phosphinodiboranates mirrors recent mechanistic reports for the synthesis of DMADB complexes. Notably, it was discovered that treating UCl4 or LnCl3 with the required equivalents of Na(H3BNMe2BH3) in THF does not yield the desired M(H3BNMe2BH3)3(THF) complexes (where M = U or Ln) until the final extraction and crystallization step with pentane.98 Mixing NdCl3 with 3 equiv of Na(H3BNMe2BH3) in THF, for example, resulted in the substitution of one chloride to form Nd(H3BNMe2BH3)Cl2(THF)x.98 Elimination of the final two chlorides only occurs once THF is removed and pentane is added to the residue for extraction. Interestingly, the DMADB salt elimination reactions

MECHANOCHEMICAL SYNTHESIS OF PHOSPHINODIBORANATE COMPLEXES Mechanochemical reactions are reactions that use mechanical energy to achieve a desired chemical transformation.99−102 Mechanochemistry has proven to be especially useful for preparing complexes that cannot be isolated using solution methods and is often considered to be a more environmentally friendly synthetic method because it requires little to no solvent. Although mechanochemistry has experienced a renaissance in recent years103,104 and is now widely recognized as a useful method to prepare metal complexes,105−107 materials,108−110 and organic chemicals,111−113 it is not new in the preparation of metal borohydride complexes.114−116 The first credited mechanochemical synthesis of a metal borohydride complex was reported by Brenner and co-workers in 1957.117 Zr(BH4)4 was prepared by grinding ZrCl4 and 4 equiv of LiBH4 with nickel spheres and subsequently isolated from the mixture by sublimation. In 1976, Volkov and Myakishev demonstrated that U(BH4)4 can be prepared by the same method using UCl4 instead of ZrCl4.118 Similar methods have also been used to prepare aminodiboranate complexes. Homoleptic lanthanide DMADB complexes, for example, can be prepared by mechanically grinding 3 equiv of Na(H3BNMe2BH3) with LnCl3 and subliming the volatile Ln(H3NMe2BH3)3 complexes out of the solid reaction mixture.53,54 We discovered that grinding ErI3 with 3 equiv of K(H3BPtBu2BH3) with a mortar and pestle for as little as 15 min yielded ca. 50% conversion compared to only ca. 25% after mixing for 24 h in Et2O (Figure S25). Building on these promising results, we used a shaker mill to develop a more reproducible mechanochemical synthesis of tBu-PDB complexes starting with 1. Where our previous attempts to prepare 1 by solution methods resulted in consistently poor yields (≤15%), D

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Figure 2. Molecular structure of 4 with thermal ellipsoids drawn at the 35% probability level. Hydrogen atoms attached to carbon atoms were omitted from the figure. Bond distances (Å): Tb1−B1 = 3.018(7), Tb1−B2 = 2.780(6), Tb1−B3 = 2.829(7), Tb1−B4 = 2.812(6), Tb1− B5 = 2.615(5), Tb1−B6 = 2.610(6). Bond angles: B1−P1−B2 = 104.6(3)°, B3−P2−B4 = 102.7(3)°, B5−P3−B6 = 110.1(2)°.

a

The three circles arranged in a triangle are used to indicate ball milling, as proposed by Rightmire and Hanusa.105

2.829(7) Å are consistent with bidentate κ2-BH3, whereas the Tb1−B1 distance is significantly longer at 3.018(7) Å, consistent with monodentate κ1-BH3. The bridging Tb−B bond distances for the bridging tBu-PDB ligands are significantly shorter at 2.615(5) and 2.610(6) Å and suggest κ3-BH3 groups. However, one Tb−H distance on both sides of the bridging tBuPDB ligand is significantly longer than the other two, as shown in Chart 2. As described previously for 3,74 we tentatively assign the Tb−H−B coordination number for 4 as 13 based on the Tb−B distances, but we note that the actual coordination number for 3 and 4 could be lower depending on the denticity of the BH3 groups on the bridging tBu-PDB ligands. Single-crystal XRD data collected for 3-THF−5-THF revealed monomeric complexes with a new type of coordination mode for tBu-PDB, where only one BH3 group from each ligand binds to the metal in a tridentate fashion (Figure 3). The other BH3 groups are referred to as dangling because they do not interact with any neighboring complexes like the bridging DMADB ligands do in the structure of U(H3BNMe2BH3)3 (Figure 1). If one considers the arrangement of the boron and oxygen atoms, the coordination geometry can be considered distorted trigonal-prismatic with fac THF and tBu-PDB ligands. The three largest B−Ln−O angles in the structure of 4-THF range from 161.71(9) to 162.98(9)° and are similar in the structures of 3-THF and 5-THF (Table 1). The average Ln−B bond distances are typical of κ3-BH3 groups and show a stepwise decrease from 2.59(1) Å (Tb) to 2.56(2) Å (Er) to 2.54(2) Å (Lu) consistent with the decrease in the ionic radius from Tb3+ to Lu3+ (Table 1). The average Ln−O bonds show a similar stepwise decrease of 2.40(1), 2.36(1), and 2.33(1) Å, which are on par with those observed in Ln(H3BNMe2BH3)3(THF) complexes with DMADB.54 The unconstrained B−P−B angles for the dangling tBu-PDB ligands in the structures of 3-THF and 4-THF allowed us to evaluate how the B−P−B angles change in 3 and 4 when tBuPDB ligands chelate or bridge metals. Using the unconstrained B−P−B angles of ca. 111° in 3-THF and 4-THF as references, the bridging tBu-PDB ligands in 3 and 4 revealed no significant change at 111.0(5)° and 110.1(2)°, respectively.74 In contrast, the chelating tBu-PDB ligands are more acute at 104.6(3)° and

The isolation of 1 and 2 revealed another notable difference compared to DMADB complexes. Contacting base-free U(H3BNMe2BH3)3 and Ln(H3BNMe2BH3)3 complexes with THF yielded U(H3BNMe2BH3)3(THF) and Ln(H3BNMe2BH3)3(THF),52,54 whereas no evidence of THF binding to U3+ or Nd3+ was observed with 1 and 2 despite using THF as a crystallizing solvent. In contrast, using the same methods to prepare Ln2(H3BPtBu2BH3)6 complexes with smaller lanthanide ions such as Tb3+, Er3+, and Lu3+ resulted in several THF molecules binding to the metals during workup (Scheme 1). Crystallizing mechanochemically prepared Ln2(H3BPtBu2BH3)6, where Ln = Er (3), Tb (4), and Lu (5), with THF/pentane as described for 1 and 2 yielded new Lewisbase adducts with the formula Ln(H3BPtBu2BH3)3(THF)3, where Ln = Er (3-THF), Tb (4-THF), and Lu (5-THF). Repeating the mechanochemical synthesis used to prepare 3THF−5-THF and crystallizing the products from concentrated pentane solutions (i.e., no THF) yielded the aforementioned base-free complex 3, as well as Tb2(H3BPtBu2BH3)6 (4) and Lu2(H3BPtBu2BH3)6 (5) in crystalline yields ranging from 56 to 71%. These values are on par with crystalline yields previously reported for lanthanide DMADB complexes.54 Molecular Structures. Single-crystal XRD studies revealed that 4 (Tb) and 5 (Lu) are dimers in the solid state, as reported previously for 1−3.74 The modeled XRD data for 5 were not of sufficient quality for quantifying bond distances and angles120 but allowed us to confirm the dimeric structure (Figure S2). Overall, unlike DMADB complexes that adopt different structures depending on the size of the trivalent f metals (Chart 3), the tBu-PDB complexes appear to maintain their dimeric structure for metal ions both large (U3+ and Nd3+) and small (Lu3+). Each terbium ion in the structure of 4 is coordinated by two chelating tBu-PDB ligands and two bridging ligands that hold the dimer together (Figure 2). The M−B bond distances often provide a good estimate of the borohydride denticity,1 which is useful considering the challenges with accurately locating hydrogen-atom positions in the XRD data. Three of the Tb−B distances on the chelating tBu-PDB range from 2.780(6) to E

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Figure 3. Molecular structures of 3-THF (left) and 5-THF (right). Thermal ellipsoids are shown at 35% probability. Hydrogen atoms attached to carbon atoms were omitted from the figure.

Table 1. Selected Bond Distances (Å) and Angles (deg) for 3-THF−5-THF 4-THF (Tb)

3-THF (Er)

5-THF (Lu)

Ln−B

Ln−O

B···B

B−P−B

B−Ln−B

2.600(5) 2.599(3) 2.577(3) 2.571(5) 2.575(4) 2.547(4) 2.519(3) 2.551(5) 2.556(3)

2.389(2) 2.396(2) 2.411(2) 2.358(3) 2.359(2) 2.375(3) 2.342(2) 2.326(2) 2.328(2)

3.192(9) 3.182(4) 3.183(7) 3.19(1) 3.180(5) 3.197(8) 3.200(7) 3.194(8) 3.192(4)

111.1(2) 110.6(2) 111.2(2) 111.2(2) 111.0(2) 111.6(2) 111.8(2) 111.5(2) 111.6(2)

98.2(1) 98.0(1) 98.3(1) 98.0(1) 97.8(1) 97.9(1) 96.8(1) 97.0(1) 97.2(1)

102.7(3)° in 4 and 104.7(6)° and 100.8(5)° in 3.74 The 10° range of the B−P−B angles reveals the inherent flexibility in tBuPDB that helps to accommodate chelation to different sized metal ions. The B−P−B angles are also significantly smaller than those observed in related DMADB complexes. For comparison, the B−N−B angles in the isomorphous dimer Er2(H3BNMe2BH3)6 are 107.5(13)−110.4(13)° for the chelating DMADB ligands and 111.5(13)° and 115.5(12)° for the bridging ligands.54 Spectroscopic Analysis. 1H and 11B NMR data were collected on all of the tBu-PDB complexes to evaluate their speciation in solution (Table 2). As shown in Chart 5,74 1 (U3+)

1 (dimer) 1 (monomer) 2 (dimer) 2 (monomer) 3 3-THF 4 4-THF 5 5-THF

H (tBu)

1

1

1.73, 0.42 2.15 1.25, 1.67 2.71 −1.18 −1.07 17.94 14.49 1.23 1.26

72.6, 94.0 99.2 76.1, 78.2 83.5 −180.7 −180.7 −371.4 −370.9 2.39 2.22

H (BH3)

1

H (THF)

1.46, 1.00 6.38, 2.81 3.77, 1.38

161.9(1) 161.71(9) 162.98(9) 162.2(1) 161.8(1) 162.9(1) 163.52(9) 162.9(1) 162.6(1)

98.7(1) 99.39(9) 99.24(9) 99.0(1) 99.5(1) 99.7(1) 100.2(1) 99.4(1) 99.73(9)

84.5(1) 82.47(9) 83.43(9) 84.4(1) 82.8(1) 83.5(1) 83.81(9) 84.6(1) 83.20(9)

and 2 (Nd3+) exist primarily as dimers in solution, whereas 3 with the smaller Er3+ ion appears to break up into monomers. 4 and 5 also appear to break up into monomers in solution, but several unresolved resonances are observed in the baseline of 4 (Figure S13). We tentatively assign these resonances to the dimeric structure presumed to exist in equilibrium with the monomer, as observed for 1 and 2 (Chart 5). This interpretation is supported by DFT calculations presented in the following section (vide infra). The 1H NMR spectrum of diamagnetic 5 revealed a doublet assigned to the tBu resonance at δ 1.26 ppm with a JH−P coupling of 13 Hz. A broad quartet assigned to BH3 was observed at δ 2.39 ppm because of coupling to quadripolar 11B (I = 3/2). The 11B NMR spectrum revealed a single BH3 resonance with resolved coupling to both 31P and the three B−H hydrogen atoms. The JB−H and JB−P coupling constants were nearly identical, yielding a doublet of overlapping quartets that appeared as a quintet. The terminal and coordinated B−H hydrogen atoms often appear equivalent in the 1H and 11B NMR spectra because they rapidly exchange on the NMR time scale, as is typically observed for felement borohydride complexes.1 As for 1−3, the paramagnetism of the Tb3+ ion in 4 causes dramatic shifts and broadening of the NMR resonances relative to those in diamagnetic 5 (Table 2). The paramagnetic shifts are especially pronounced for the BH3 groups because of their close proximity to the metal and large Fermi contact shifts;54 the 1H NMR spectrum of 4 revealed a broad resonance at δ −371.4 ppm and the corresponding 11B resonance was located at δ

Table 2. Room Temperature 1H and 11B NMR Data (δ, ppm) for tBu-PDB Complexes in C6D6 complex

B−Ln−O

11

B

120.8, 301.3 180.5 68.8, 176.3 94.1 −278.0 −279.9 −536.2 −537.0 −28.7 −28.8 F

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Figure 4. 1H NMR spectra of 3 in C6D6 as a function of increasing THF equivalents. The asterisk symbol indicates the resonance assigned to residual pentane and small amounts of hydrolysis impurities, and the dotted line is included for reference.

−536.2 ppm. In contrast, the single tert-butyl 1H NMR resonance was less shifted by comparison at δ 17.9 ppm because it is further removed from the metal. The 1H NMR data collected on 3-THF−5-THF revealed that crystals of all three complexes lose THF when isolated from the mother liquor. For example, crystals of diamagnetic 5-THF were left under dynamic vacuum for several hours and analyzed by 1H NMR spectroscopy in C6D6 to reveal tBu-PDB-to-THF ratios of ca. 3:1, which was corroborated by microanalysis. Similar 3:1 t Bu-PDB-to-THF ratios were revealed for paramagnetic 3-THF and 4-THF when they were left under dynamic vacuum for extended periods. However, allowing the crystals to evaporate to dryness in the glovebox atmosphere for ca. 15 min prior to analysis provided tBu-PDB-to-THF ratios closer to one-to-one according to 1H NMR integrations and elemental analysis. The loss of THF from 3-THF−5-THF suggests that it is only weakly bound to the metal, which is corroborated by a comparison of the NMR peak positions. For example, the 1H NMR spectrum for the most paramagnetic complex 4-THF revealed THF resonances at δ 6.38 and 2.81 ppm, which are relatively close to those observed in the 1H NMR resonances observed in diamagnetic 5-THF at δ 3.77 and 1.28 ppm. Furthermore, the tert-butyl and BH3 NMR resonances for all three THF complexes are remarkably close to their base-free complexes 3−5 (Table 2). As a final comparison, we performed an NMR-scale titration study starting with the base-free dimer 3 (Figure 4). Only small stepwise shifts in the 1H and 11B NMR resonances were observed as 1−3 equiv of THF was added (Figures 4, S23, and S24). Even the addition of a 20-fold excess of THF had only subtle effects on the position of the BH3 and tert-butyl resonances. The solid-state IR spectra for the base-free complexes 3−5 were collected and compared to their corresponding THF complexes to investigate the influence of THF binding on the B−H stretching modes. Borohydrides have distinct B−H stretching vibrations that occur between 2200 and 2500 cm−1 and are sensitive to the borohydride coordination modes. The IR spectrum of 5 (which is nearly identical and representative of

those for 3 and 4) revealed a sharp B−H stretching absorption at 2434 cm−1 that was assigned to terminal B−H stretches on the t Bu-PDB ligands and a broad absorption centered at 2234 cm−1 that was assigned to different bridging B−H−M stretches (Figure 5). A weaker absorption was observed at 2358 cm−1.

Figure 5. Comparison of the solid-state IR spectra (ATR) for 3, 3THF, 5, and 5-THF. A dashed line was added at 2350 cm−1 for reference. The asterisk symbols are assigned to the symmetric O−C−O stretches of bound THF in 3-THF and 5-THF.

Unexpectedly, IR spectra collected on 3-THF−5-THF as KBr pellets yielded spectral profiles similar to those of 3−5. This suggested that THF was being lost from the complexes as the KBr pellets were being prepared. To test this hypothesis, we collected the IR spectrum of each compound using an attenuated total reflectance (ATR) accessory to avoid grinding the samples with KBr, which allowed the expected differences in the B−H stretching modes to be observed (Figure 5). The IR spectra of 3-THF and 5-THF each revealed intense absorptions G

DOI: 10.1021/acs.inorgchem.9b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at 2349 and 2354 cm−1, respectively, that were not observed in the spectra of 3 and 5. As such, we assign this new feature to B− H stretches associated with the unbound (i.e., dangling) BH3 group. New absorption bands were also observed in the spectra of 3-THF and 5-THF at 834 and 853 cm−1 and are assigned to the symmetric O−C−O stretch on THF, as described previously.54 DFT Calculations. As shown in Chart 5, the dimeric solidstate structures of 1−5 all appear to break up to some extent in solution depending on the size of the trivalent metal ion: the largest U3+ and Nd3+ ions in 1 and 2 exist primarily as dimers, whereas smaller Er3+, Tb3+, and Lu3+ in 3−5 appear to exist as monomers. To evaluate the energetic preference for the dimeric structure relative to the proposed monomeric M(H3BPtBu2BH3)3 structures (which will be referred to as 1a− 5a), DFT was used to calculate the ΔG values for the dimerization reaction 2M(H3BPtBu2BH3)3 → M2(H3BPtBu2BH3)6. We previously reported that the calculated ΔG values were slightly exergonic for uranium and neodymium (−1.5 and −2.0 kcal/mol, respectively) but endergonic for erbium (+1.0 kcal/mol), in excellent agreement with the solution NMR data.74 In line with this trend, we see that the values for terbium and lutetium fall as expected based on the size of the Ln3+ cation. The reaction with terbium is almost thermoneutral at −0.1 kcal/mol, while lutetium is endergonic by +2.8 kcal/mol (Table 3).

borohydride ligands, which are better suited to saturate the coordination sphere of large f metals in lower oxidation states. DMADB yields highly volatile lanthanide complexes that have proven useful for the synthesis of lanthanide thin films via CVD.53,54,56 It has also allowed for side-by-side comparisons of covalent M−H−B bonding in isomorphous uranium and lanthanide complexes and yielded the highest f-metal coordination number known (15).50,60 The more recent discovery of trivalent lanthanide and uranium complexes with tBu-PDB shows that even larger chelating borohydride ligands are possible. These phosphinodiboranate ligands afford significant differences in solution and solid-state structures with and without THF compared to compositionally similar DMADB complexes. A key observation from the work outlined in this Forum Article is that chelating amino- and phosphinodiboranates display limited salt elimination reactivity in organic solvents. THF attenuates salt elimination with Na(H3BNMe2BH3) and fmetal chlorides, 98 and similar reactions between K(H3BPtBu2BH3) and f-metal iodides in THF do not proceed to any appreciable extent. Even reactions with K(H3BPtBu2BH3) in less-coordinating Et2O are severely attenuated. More remarkably, it has been shown that the addition of NaCl to Nd(H3BNMe2BH3)3(THF) in THF causes it to revert to NdCl2(H3BNMe2BH3)(THF)x and Na(H3BNMe2BH3),98 and similar reversibility is likely to occur with phosphinodiboranates. This may account for why lanthanide and actinide complexes with these chelating borohydrides were only recently discovered despite aminodiboranate and phosphinodiboranate salts having been known for over 50 years.47,64 Fortunately, mechanochemical reactions, which have long been used to prepare borohydride complexes,117 can be used to overcome paltry metathesis reaction yields with chelating borohydrides in solution, as demonstrated here in reactions with K(H3BPtBu2BH3). Preliminary attempts to sublime tBu-PDB complexes suggest that they are not appreciably volatile like lanthanide DMADB complexes even though they do not form coordination polymers in the solid state. However, the attenuated volatility may be attributed to differences in the phosphorus and nitrogen substituents (tert-butyl vs methyl) in addition to electronic differences associated with the phosphorus and nitrogen atoms themselves. In general, there is still much we do not know about these nonstructural factors such as a metal−ligand covalency that appears to influence the solution chemistry and volatility of chelating borohydrides with different f elements. This is especially true for transuranic metals that prefer to adopt trivalent oxidation states in the presence of reducing borohydride ligands. For example, trivalent Np(BH4)3 and Pu(BH4)3, believed to form via thermal reduction of tetravalent Np(BH4)4 and Pu(BH4)4 at ambient temperatures, have not been thoroughly characterized because they are intractable solids that are difficult to handle.15 Amino- and phosphinodiboranates should form molecular complexes with these trivalent ions (as they do for similarly sized lanthanides), so they can be studied by single-crystal XRD and other means. We are now pursuing the mechanochemical synthesis of these and other trivalent f-element complexes with different phosphinodiboranate ligands to evaluate their properties in support of our broader goal of separating f elements by selective volatilization.

Table 3. Calculated ΔG Values for the Dimerization Reaction 2M(H3BPtBu2BH3)3 → M2(H3BPtBu2BH3)6 at 298 K metal (M)

ΔG (kcal/mol)

metal (M)

ΔG (kcal/mol)

U Nd Tb

−1.5a −2.0a −0.1

Er Lu

+1.0a +2.8

a

Obtained from ref 74.

Next, we computed the gas-phase Gibbs free energy for the reaction of 1a−5a with three THF molecules to form M(H3BPtBu2BH3)3(THF)3 for comparison to experimental differences in solution THF binding (Table 4). Again, the DFT Table 4. Calculated ΔG Values for the Reaction M(H3BPtBu2BH3)3 → M(H3BPtBu2BH3)3(THF)3 at 298 K metal (M)

ΔG (kcal/mol)

metal (M)

ΔG (kcal/mol)

U Nd Tb

+2.8 +2.4 −2.6

Er Lu

−5.6 −3.8

calculations showed remarkably good agreement with our experimental observations. The addition of THF is exergonic for the smaller lanthanides terbium, erbium, and lutetium (−2.6, −5.6, and −3.8 kcal/mol, respectively), which accounts for our ability to isolate and characterize 3-THF−5-THF. In contrast, the reaction to form 1-THF and 2-THF with uranium and neodymium is endergonic (+2.8 and +2.4 kcal/mol, respectively), consistent with the fact that we were unable to isolate THF complexes with these metals.

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CONCLUSION AND OUTLOOK Over half a century of chemical exploration with the simplest felement borohydride complexes containing BH4− and BH3Me− paved the way for the development of larger chelating H

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opened to reveal a white paste. The contents was transferred to a scintillation vial with Et2O (15 mL), filtered through a fine frit, and evaporated to dryness under vacuum to reveal a colorless oil. The oil was suspended in pentane (40 mL), filtered, concentrated to 15 mL, and stored in a freezer at −30 °C. Small colorless blocks formed after 3 days (48.6 mg). The mother liquor was concentrated to 10 mL and returned to the freezer to yield an additional 28.0 mg of colorless blocks after 3 days. Yield: 61.0%. Mp: >250 °C. Anal. Calcd for C48H144B12P6Tb2: C, 42.55; H, 10.71. Found: C, 42.35; H, 10.68. 1H NMR (400 MHz, C6D6): δ 17.9 (br s, C(CH3)3, 54 H), −371.4 (br s, BH3, 18 H). 11B NMR (128 MHz, C6D6): δ −536.2 (br s, fwhm = 400 Hz, BH3), −483.8 (br s, fwhm = 800, BH3). IR (cm−1): 2983 (m), 2964 (m), 2947 (m), 2899 (m), 2867 (m), 2430 (s), 2354 (w), 2231 (vs), 2119 (vw), 1476 (s), 1391 (m), 1367 (s), 1256 (s), 1183 (s), 1147 (m), 1062 (vs), 1021 (vs), 934 (m), 903 (vw), 816 (s), 773 (vw), 730 (w), 705 (m), 668 (w), 645 (s), 608 (w). Hexakis(di-tert-butylphosphinodiboranto)dilutetium(III), Lu2(H3BPtBu2BH3)6 (5). 5 was prepared as described for 4 using LuI3 (0.101 g, 0.182 mmol) and K(H3BPtBu2BH3) (0.114 g, 0.538 mmol). Yield: 0.090 g (71.2%). Mp: >250 °C. Anal. Calcd for C48H144B12Lu2P6: C, 41.56; H, 10.46. Found: C, 41.25; H, 10.29. 1H NMR (400 MHz, C6D6): δ 1.23 (d, J = 13.0 Hz, C(CH3)3, 54 H), 2.39 (br q, J = 80 Hz, BH3, 18 H). 11B NMR (128 MHz, C6D6): δ −28.7 (dq, J = 89 and 86 Hz, BH3). IR (cm−1): 2983 (m), 2963 (m), 2947 (m), 2927 (vw), 2900 (m), 2867 (m), 2434 (s), 2358 (w), 2234 (vs, br), 2151 (vw), 2126 (vw), 1476 (vs), 1399 (m), 1367 (vs), 1272 (s), 1184 (s), 1147 (w), 1064 (vs), 1049 (w), 1021 (vs), 954 (vw), 935 (m), 903 (w), 816 (vs), 771 (m), 733 (w), 701 (m), 650 (vs), 607 (m). Tris(di-tert-butylphosphinodiboranto)tris(tetrahydrofuran)erbium(III), Er(H3BPtBu2BH3)3(THF)3 (3-THF). ErI3 (0.203 g, 0.370 mmol) and K(H3BPtBu2BH3) (0.234 g, 1.10 mmol) were loaded into a 5 mL FTS ball-milling jar with two 5 mm stainless steel balls and several drops of Et2O. The ball mill jar was hermetically sealed, transferred to an FTS shaker mill, and milled for 90 min. The jar was transferred to a glovebox and opened to reveal a light-pink paste. The contents of the jar was transferred to a 20 mL scintillation vial with Et2O (15 mL). The suspension was stirred briefly, filtered through a fine frit, and evaporated to dryness under vacuum to yield a pink oily solid. The solid was dissolved in pentane (20 mL), filtered, and evaporated to dryness to reveal a clear pink oil. The oil was dissolved in a minimal amount of THF (ca. 1−2 mL). Vapor diffusion with pentane yielded 145 mg of large pink plates after 2 days. Concentrating the mother liquor and vapor diffusion with more pentane yielded an additional 15 mg of crystals. Yield: 48.5%. Mp: >250 °C. Anal. Calcd for C24H72B6P3Er· 3C4H8O: C, 47.93; H, 10.73. Found: C, 48.44; H, 10.61. 1H NMR (400 MHz, C6D6): δ −1.07 (br s, C(CH3)3, 27 H), 1.00 (br s, THF), 1.46 (br s, THF). 11B NMR (128 MHz, C6D6): δ −280.0 (br s, fwhm = 400 Hz, BH3). IR (cm−1): 2980 (w), 2960 (m), 2947 (w), 2898 (m), 2866 (m), 2426 (w), 2349 (vs), 2281 (m), 2260 (m), 2124 (vw), 1617 (vw), 1475 (m), 1459 (m), 1388 (m), 1365 (s), 1253 (m), 1205 (m), 1184 (m), 1139 (m), 1068 (vs), 1021 (s), 1002 (s), 962 (w), 929 (w), 917 (w), 834 (s), 817 (s), 753 (m), 689 (s), 624 (m). Tris(di-tert-butylphosphinodiboranto)tris(tetrahydrofuran)terbium(III), Tb(H3BPtBu2BH3)3(THF)3 (4-THF). 4-THF was prepared as described for 3-THF using TbI3 (0.101 g, 0.187 mmol) and K(H3BPtBu2BH3) (0.118 g, 0.557 mmol). Yield: 30.2 mg (18.6%). Mp: >250 °C. Anal. Calcd for C24H72B6P3Tb·2.5C4H8O: C, 47.61; H, 10.81. Found: C, 47.81; H, 10.65. 1H NMR (500 MHz, C6D6): δ 14.49 (br s, C(CH3)3), 54 H), 2.81 (br s, THF), 6.38 (br s, THF), −370.9 (br s, BH3, 18 H). 11B NMR (160 MHz, C6D6): δ −537.0 (br s, fwhm = 400 Hz, BH3), −484.8 (br s, fwhm = 700 Hz, BH3). IR (cm−1): 2982 (m), 2963 (m), 2948 (m), 2900 (m), 2868 (m), 2428 (m), 2345 (w), 2230 (vs), 1476 (s, br), 1391 (m), 1367 (vs), 1259 (s), 1183 (s), 1047 (m), 1063 (vs), 1048 (m), 1022 (vs), 935 (m), 817 (s), 770 (w, br), 718 (w), 714 (s), 671 (w), 643 (s), 608 (m). Tris(di-tert-butylphosphinodiboranto)tris(tetrahydrofuran)lutetium(III), Lu(H3BPtBu2BH3)3(THF)3 (5-THF). 5-THF was prepared as described for 3-THF using LuI3 (0.102 g, 0.184 mmol) and K(H3BPtBu2BH3) (0.114 g, 0.538 mmol). Yield: 84.3 mg (42.1%). Mp: >250 °C. Anal. Calcd for C24H72B6P3Lu·C4H8O: C, 43.92; H, 10.53.

EXPERIMENTAL SECTION

General Considerations. All reactions were carried out under an atmosphere of N2 using glovebox or standard Schlenk techniques. All glassware was heated at 150 °C for at least 2 h and allowed to cool under vacuum before use. Solvents were dried and deoxygenated using a Pure Process Technologies Solvent Purification System and stored over 3 Å molecular sieves. Deuterated solvents were deoxygenated on the Schlenk line by three freeze−pump−thaw cycles and stored over 3 Å molecular sieves for at least 3 days before use. K(H3BPtBu2BH3) was prepared from commercially available starting materials using the method described by Wagner and co-workers.71 UI3(THF)4 was prepared from UI3(1,4-dioxane)1.5, as described by Kiplinger et al.121 Anhydrous LnI3 salts were purchased from Alfa Aesar and used as received. 1 H NMR data were collected on a Bruker DRX-400 instrument operating at 400 MHz, a Bruker AVANCE-400 operating at 400 MHz, or a Bruker AVANCE-500 operating at 500 MHz. The 11B NMR data were collected on a Bruker AVANCE-400 operating at 128 MHz or a Bruker AVANCE-500 operating at 160 MHz. Variable-temperature NMR data were collected on a Bruker DRX-400 instrument operating at 400 and 128 MHz for 1H and 11B, respectively. Chemical shifts are reported in δ units relative to residual NMR solvent peaks (1H) and BF3·Et2O (11B; δ 0.0 ppm). 31P NMR resonances were not observed for any of the metal complexes presumably because of unresolved peak broadening in the presence of paramagnetic metals and quadrupolar coupling with the boron nuclei (11B and 10B). Microanalytical data (CHN) were collected using an EAI CE-440 elemental analyzer at the University of Iowa. IR spectra were acquired with a Thermo Scientific Nicolet iS5 in an N2-filled glovebox as KBr pellets or using an ATR accessory. Melting points were determined in sealed capillaries using a REACH MP device. Mechanochemical synthesis was carried out on a Form-Tech Scientific (FTS) FTS1000 shaker mill operating at 1600 rpm. All mechanochemical reactions were conducted in 5 mL stainless steel “SmartSnap” (hermetic seal) grinding jars using two 5 mm stainless steel balls (304 grade) for grinding. Hexakis(di-tert-butylphosphinodiboranto)diuranium(III), U2(H3BPtBu2BH3)6 (1). 1 was prepared as described for 4 using UI3(THF)4 (0.203 g, 0.224 mmol) and K(H3BPtBu2BH3) (0.141 g, 0.663 mmol). Yield: 87.9 mg (52.6%). 1H and 11B NMR data for the dark-red crystals matched those reported previously for 1.74 1H NMR (400 MHz, toluene-d8): δ 0.38 (br d, J = 8 Hz, 18 H, C(CH3)3, 1), 1.73 (br s, 36 H, C(CH3)3, 1), 72.7 (br s, 6 H, BH3, 1), 94.6 (br s, 12 H, BH3, 1), 100.6 (br s, BH3, 1a). 11B NMR (128 MHz, toluene-d8): δ 120.8 (br s, BH3, 1), 180.5 (br s, BH3, 1a), 301.3 (br s, BH3, 1). Crystalline yields as high as 62% were achieved in separate reactions using the same approach and approximate reaction scale. Hexakis(di-tert-butylphosphinodiboranto)dineodymium(III), Nd2(H3BPtBu2BH3)6 (2). 2 was prepared as described for 4 with NdI3 (0.200 g, 0.381 mmol) and K(H3BPtBu2BH3) (0.243 g, 1.14 mmol). Yield: 0.102 g (40.6%). 1H and 11B NMR data for the purple crystals matched those reported previously for 2.74 1H NMR (400 MHz, toluene-d8): δ 1.23 (br d, J = 5.7 Hz, 18 H, C(CH3)3, 2), 1.66 (br s, 36 H, C(CH3)3, 2), 2.71 (br s, 54 H, C(CH3)3, 2a) 76.7 (br m, 12 H, BH3, 2), 78.9 (br m, 6 H, BH3, 2), 84.3 (br m, 18 H, BH3, 2a). 11B NMR (128 MHz, toluene-d8): δ 76.7 (br m, BH3, 2), 78.8 (br m, BH3, 2a), 84.1 (br m, BH3, 2). Crystalline yields as high as 65% were achieved in separate reactions using the same approach and approximate reaction scale. Hexakis(di-tert-butylphosphinodiboranto)dierbium(III), Er2(H3BPtBu2BH3)6 (3). 3 was prepared as described for 4 using ErI3 (0.202 g, 0.369 mmol) and K(H3BPtBu2BH3) (0.234 g, 1.10 mmol). Yield: 0.140 g (55.5%). 1H and 11B NMR data for the pink crystals matched those reported previously for 3.74 1H NMR (400 MHz, C6D6): δ −1.18 (br s, C(CH3)3, 54 H), −181.7 (br s, BH3). 11B NMR (128 MHz, C6D6): δ −278.0 (br s, BH3). Hexakis(di-tert-butylphosphinodiboranto)diterbium(III), Tb2(H3BPtBu2BH3)6 (4). TbI3 (0.101 g, 0.187 mmol) and K(H3BPtBu2BH3) (0.118 g, 0.557 mmol) were loaded into a 5 mL FTS ball-milling jar with two 5 mm stainless steel balls and several drops of Et2O. The jar was hermetically sealed, transferred to an FTS shaker mill, and milled for 120 min. The jar was transferred into a glovebox and I

DOI: 10.1021/acs.inorgchem.9b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Found: C, 44.00; H, 10.21. 1H NMR (400 MHz, C6D6): δ 1.26 (d, C(CH3)3, 27 H), 1.38 (br s, THF), 2.22 (br q, J = 85 Hz, BH3), 3.77 (br s, THF). 11B NMR (128 MHz, C6D6): δ −28.8 (dq, J = 85 and 85 Hz, BH3). IR (cm−1): 2979 (m), 2960 (m), 2947 (m), 2899 (m), 2867 (m), 2440 (w), 2426 (w), 2354 (s), 2262 (s), 2121 (vw), 1475 (m), 1459 (m), 1416 (vw), 1389 (m), 1365 (s), 1276 (s), 1184 (s), 1143 (w), 1068 (vs), 1020 (vs), 962 (vw), 934 (w), 853 (m), 817 (vs), 755 (w), 730 (w), 707 (m), 650 (s), 650 (m), 629 (w), 610 (vw). Single-Crystal XRD Studies. Suitable crystals grown from pentane (4) or a combination of pentane and THF (3-THF, 4-THF, and 5THF) were mounted onto a MiTeGen micromount with Paratone N oil. Crystallographic data were collected on a Bruker Nonius Kappa Apex II equipped with a charge-coupled-device detector and cooled to 150 K using an Oxford Cryostreams 700 low-temperature device. Data were collected using monochromatized Mo Kα radiation (λ = 0.71073 Å). A hemisphere of data was collected using ϕ and ω scans. The data were corrected for absorption using redundant reflections and the SADABS program. Structures were solved with SHELXT, and leastsquares refinement (SHELXL) confirmed the positions of all nonhydrogen atoms.122 All hydrogen-atom positions were idealized and allowed to ride on the attached carbon and boron atoms with B−H distances fixed at 1.20 Å. Anisotropic temperature factors for all nonhydrogen atoms were included at the last refinement. Structure solution and refinement were performed using Olex2.123 Cocrystallized solvent molecules in 4, 3-THF, 4-THF, and 5-THF were too disordered to model, so a solvent mask was applied using the SQUEEZE function in Olex2. The data collection and refinement details are provided in Table S1. DFT Calculations. Geometries were optimized, and harmonic vibrational frequencies were computed to obtain Gibbs energies by means of DFT. All calculations are minima. The complexes 1−5, 1a− 5a, and 1-THF−5-THF were studied with the TPSS functional with D3 dispersion correction and Becke−Johnson damping,124−129 the def2TZVP basis set (and the corresponding effective core potentials) for all atoms,130−135 and the grid m4 for the quadrature of exchangecorrelation terms, as implemented in the TURBOMOLE program package.136,137 For 3, the memory required to compute analytical frequencies was large with the def2-TZVP basis set; therefore, vibrational contributions for this system were computed with the def2-SV(P) basis set.136,138 Because the def2-TZVP basis was not available for uranium, the def-TZVP basis was used.136,139,140 The resolution of identity approximation was used for integral evaluation.141

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Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Separation Science program under Award DESC0019426. We thank Lou Messerle and Stosh Kozimor for gifts of uranium turnings, Dale Swenson for collecting the singlecrystal XRD data, and Timothy Hanusa for helpful discussions. We also thank Amy Charles for her assistance in organizing and revising the manuscript. T.V.F. acknowledges a postcomprehensive exam fellowship from the University of Iowa Graduate College. A.V.B. acknowledges support from a graduate fellowship from the United States Nuclear Regulatory Commission and a fellowship from the U.S. DOE, Office of Science, Graduate Student Research Fellowship Program. Computations supporting this project were performed on High-Performance Computing systems at the University of South Dakota.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01628. NMR spectra, crystallographic data, DFT-optimized structures, total electronic energies, and selected geometric parameters (PDF) Accession Codes

CCDC 1919797−1919800 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Scott R. Daly: 0000-0001-6229-0822 J

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DOI: 10.1021/acs.inorgchem.9b01628 Inorg. Chem. XXXX, XXX, XXX−XXX