Structural Properties of the Acidification Products of Scandium

Street Southeast, Minneapolis, Minnesota 55455, United States. Inorg. Chem. , 2015, 54 (24), pp 11831–11841. DOI: 10.1021/acs.inorgchem.5b02030...
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Structural Properties of the Acidification Products of Scandium Hydroxy Chloride Hydrate Timothy J. Boyle,*,† Jeremiah M. Sears,† Michael L. Neville,† Todd M. Alam,† and Victor G. Young, Jr.‡ †

Advanced Materials Laboratory, Sandia National Laboratories, 1001 University Boulevard, Southeast, Albuquerque, New Mexico 87106, United States ‡ Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: The structural properties of a series of scandium inorganic acid derivatives were determined. The reaction of Sc0 with concentrated aqueous hydrochloric acid led to the isolation of [(H2O)5Sc(μ-OH)]24Cl·2H2O (1). Compound 1 was modified with a series of inorganic acids (i.e., HNO3, H3PO4, and H2SO4) at room temperature and found to form {[(H2O)4Sc(κ2-NO3)(μOH)]NO3}2 (2a), [(H2O)4Sc(κ2-NO3)2]NO3·H2O (2b) (at reflux temperatures), {6[H][Sc(μ-PO4)(PO4)]6}n (3), and [H][Sc(μ3SO4)2]·2H2O (4a). Additional organosulfonic acid derivatives were investigated, including tosylic acid (H-OTs) to yield {[(H2O)4Sc(OTs)2]OTs}·2H2O (4b) in H2O and [(DMSO)3Sc(OTs)3] (4c) in dimethyl sulfoxide and triflic acid (H-OTf) to form [Sc(H2O)8]OTf3 (4d). Other organic acid modifications of 1 were also investigated, and the final structures were determined to be {([(H2O)2Sc(μ-OAc)2]Cl)6}n (5) from acetic acid (HOAc) and [Sc(μ-TFA)3Sc(μ-TFA)3]n (6) from trifluoroacetic acid (H-TFA). In addition to single-crystal X-ray structures, the compounds were identified by solid-state and solution-state 45Sc nuclear magnetic resonance spectroscopic studies.

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recovery of Sc0 from a support that as a first step uses a simple solution−dissolution methodology. A search of the structure literature indicated that there is a limited amount of information available pertaining to the fundamental structure of the inorganic acid derivatives of Sc.5 Therefore, a systematic study of the dissolution properties of Sc0 and identification of the subsequent acid-ligated species was undertaken. Under the particular conditions used, [(H2O)5Sc(μ-OH)]24Cl·2H2O (1) was isolated from a (conc)HCl reaction with the Sc source. Compound 1 was modified with a series of inorganic acids (i.e., HNO3, H3PO4, and H2SO4) at room temperature and found to form {[(H2O)4Sc(κ2-NO3)(μOH)]NO3}2 (2a), [(H2O)4Sc(κ2-NO3)2]NO3·H2O (2b) (at reflux temperatures), {6[H][Sc(μ-PO4)(PO4)]6}n (3), and [H][Sc(μ3-SO4)2]·2H2O (4a). Additional organosulfonic acid derivatives were investigated, including tosylic acid (H-OTs) to yield {[(H2O)4Sc(OTs)2]OTs}·2H2O (4b) in H2O and [(DMSO)3Sc(OTs)3] (4c) in dimethyl sulfoxide (DMSO) and triflic acid (H-OTf) to form [Sc(H2O)8]OTf3 (4d). Other organic acid modifications of 1 were also investigated, and the final structures were determined to be {([(H2O)2Sc(μOAc)2]Cl)6}n (5) from acetic acid (H-OAc) and [Sc(μTFA)3Sc(μ-TFA)3]n (6) from trifluoroacetic acid (H-TFA). The synthesis, characterization, and structures of these fundamental compounds are presented in detail:

INTRODUCTION Since its discovery in 1879 by Nilson and preparation as a metal in 1937 by Fischer et al., scandium has found utility in a wide variety of fields ranging from aerospace to sports equipment (Al−Sc alloys) to fuel cells to gas-discharge lighting (ScI3) to tracing agents for crude oil and many other applications.1 While Sc’s utility has increased, the availability of this 50th most common terrestrial element has not followed suit. This is mainly due to the fact that Sc is never found simply as the metal but instead is a small fraction of complex minerals [note that the highest level of Sc in a mineral is 21% for thortveitite (Sc,Y)2(Si2O7)].1 For this reason, instead of being mined directly, Sc0 is often generated as a byproduct via mining of other metals (i.e., tailings from uranium refining).1 Given the lack of an active mining process, the increased use, and the rapid exhaustion of existing sources, it is not surprising that the cost of Sc continues to climb; thus, interest in developing methods to recover the already mined metal has mounted. Because it is often associated with the rare earth (RE) elements, the chemistry of and recycling efforts for Sc are often lumped in with this family of elements. Currently, only 1% of the RE-containing consumables are recycled,2 with the majority of RE recycling efforts focused on employing modified “PUREX” processing routes that use tributyl phosphate as the main extracting solvent.1c,2,3 This wasteful, “brute-force” method utilizes substantial quantities of organic solvent, and alternative Sc2O3 or Sc0 isolation methods are limited.1c,3c,d,4 Herein, we present some of the chemistry developed for the © 2015 American Chemical Society

Received: September 3, 2015 Published: December 7, 2015 11831

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

Article

Inorganic Chemistry Table 1. Data Collection Parameters for 1−6 chemical formula formula weight temp (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg/m3) μ(Mo Kα) (mm−1) R1a (%) (all data) wR2b (%) (all data) chemical formula formula weight temp (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg/m3) μ(Mo Kα) (mm−1) R1a (%) (all data) wR2b (%) (all data) a

1

2a

2b

3

4a

Cl2H13O7Sc 240.96 100 orthorhombic Pnnm 8.6423(4) 15.6534(7) 7.1018(4)

H14N4O20Sc2 480.07 100 triclinic P1̅ 7.5078(9) 7.6416(9) 8.2171(10) 71.167(2) 89.336(2) 64.608(2) 398.63(8) 1 2.000 0.968 2.64 (3.15) 6.95 (7.34) 4c

H20N6O28Sc2 642.14 100 triclinic P1̅ 6.9961(3) 8.1265(4) 19.9163(9) 82.932(2) 81.327(2) 76.927(2) 1085.70(9) 2 1.964 0.766 2.97 (4.58) 6.62 (7.46) 4d

H18O36P9Sc3 1007.75 100 trigonal R3̅c 8.2617(3) 8.2617(3) 25.957(2)

H4O10S2Sc 273.11 100 monoclinic P21/c 8.989(3) 10.686(3) 8.476(2)

1534.35(19) 2 2.181 1.245 2.78 (3.59) 7.25 (7.52) 5

C3H16F9O17S3Sc 636.30 100 trigonal R3̅ 22.3946(12) 22.3946(12) 23.0150(14)

C4H10ClO6Sc 234.53 100 trigonal P3221 8.1584(5) 8.1584(5) 12.1399(8)

C6F9O6Sc 384.02 100 orthorhombic Pbcm 8.3181(16) 9.0751(18) 14.717(3)

9996.0(12) 18 1.903 0.759 3.08 (5.43) 7.50 (8.83)

699.77(10) 3 1.670 1.065 2.06 (2.07) 5.92 (5.92)

1111.0(4) 4 2.296 0.824 2.34 (2.93) 5.41 (5.68)

960.74(8) 4 1.666 1.312 2.36 (3.01) 5.15 (5.43) 4b C21H33O15S3Sc 666.61 100 monoclinic P21/c 7.7345(5) 15.6134(10) 23.4792(13)

C27H39O12S6Sc 792.90 100 monoclinic P21/n 15.8806(9) 27.8414(15) 24.3034(14)

92.9780(19)

90.137(2)

2831.6(3) 4 1.564 0.552 2.87 (3.48) 7.78 (8.80)

10745.4(10) 12 1.470 0.612 6.64 (13.44) 19.13 (22.09)

103.217(4) 792.6(4) 4 2.289 1.490 4.57 (10.43) 9.19 (10.85) 6

R1 = ∑||Fo| − |Fc||/∑|Fo| × 100. bwR2 = [∑w(Fo2 − Fc2)2/∑(w|Fo|2)2]1/2 × 100.

Sc 0 + xs(conc)HCl → {[(H 2O)5 Sc(μ‐OH)]2Cl}2

broadband NMR probe and a standard one-pulse sequence. Solution NMR samples were the original crystalline material dissolved in D2O. The 45Sc chemical shift was referenced to the external standard 0.2 M ScCl3 in H2O [Sc(OH2)63+ δ 0].7 Yields reported are for the first batch of crystals isolated and were not optimized further. [(H2O)5Sc(μ-OH)]24Cl·2H2O (1).8 (conc)HCl (∼75 mL) was added to a Sc0 source (∼6.8 g, 0.14 mol) in a beaker. Significant bubbling, gas evolution, and a significant increase in solution temperature were noted. The reaction mixture was allowed to stir overnight, forming a clear, yellow to green colored solution. Slow evaporation, in air, of the sample, over days, led to the isolation of colorless crystals that proved to be 1: yield 91% (33 g); 45Sc NMR (D2O, 0.01 M) δ 35.1. {[(H2O)4Sc(κ2-NO3)(μ-OH)]NO3}2 (2a). In a 25 mL beaker, (conc)HNO3 (∼10 mL) was pipetted onto 1 (1.50 g, 3.11 mmol) and the mixture stirred for 12 h. After this time, the reaction mixture was set aside, open to the atmosphere, to allow the volatile components to slowly evaporate. After an extended period of time (on the order of days), colorless crystals of 2a were isolated: yield 42.3% (0.63 g). [(H2O)4Sc(κ2-NO3)2]NO3·H2O (2b). In a 25 mL beaker, (conc)HNO3 (∼10 mL) was pipetted onto 1 (1.50 g, 3.11 mmol), and the reaction mixture was then heated to boiling, held for 5 min, and then placed in an oven (140 °C) for 12 h. The resulting colorless crystals proved to be 2b: yield 86.4% (1.72 g); 45Sc NMR [(conc)HNO3] δ −15.0. {6[H][Sc(μ-PO4) (PO4)]6}n (3). In a 25 mL beaker, (conc)H3PO4 (1.33 g, 11.6 mmol) was added to a solution of 1 (1.00 g, 2.08 mmol)

(1)

{[(H 2O)5 Sc(μ‐OH)]2Cl}2 + (conc)Hx‐L →[Sc(L)x (OH)w (Cl)y (H 2O)z ] L = NO3 , SO3‐R, PO4 , OAc, TFA

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(2)

EXPERIMENTAL SECTION

All compounds described below were handled in a benchtop hood. Analytical data were collected on freshly air-dried or oven-dried (140 °C) crystalline samples. All chemicals were used as received (from Aldrich and Alfa Aesar) without further purification, including dimethyl sulfoxide (DMSO), (conc)HCl, (conc)HNO3, (conc)H3PO4, (conc)H2SO4, H-OAc, H-OTs, H-OTf, and H-TFA. Crystalline materials were used for all analytical analyses. The solid-state 45Sc MAS nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 600 NMR instrument using a 2.5 mm broadband MAS probe, at an observed frequency of 145.78 MHz, using a rotor spinning frequency of 25 kHz (unless otherwise noted). The one-dimensional NMR spectra were recorded using a single-pulse Bloch decay, with selective π/12 pulses, 64−1024 scan averages, and a 1 s recycle delay. Simulations and extractions of the 45Sc quadrupolar coupling parameters and chemical shifts were obtained using DMFIT.6 Solution-state 45Sc NMR spectra were recorded on a Bruker Avance III 500 NMR instrument using a 5 mm 11832

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

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Inorganic Chemistry Table 2. Select Average Bond Distances (angstroms) and Angles (degrees) for 1−6 (L refers to ligand) distance (Å) Sc−OH2O

angle (deg) Sc−L

OL−Sc−OL

O−L−O

4.91 (Cl) 4.41 (H2O) 4.52 (NO3)

69.64 (OH)

110.36 (OH)

55.8 (NO3)

114.1 (NO3)

Sc−OL

1

2.18

2.06 (OH)

2a

2.31

2b 3 4a 4b 4c

2.14 − − 2.10 −

4d 5 6

2.21 2.12 −

2.05 2.30 2.28 2.06 2.05 2.06 2.07 2.07 − 2.06 2.06

(OH) (NO3) (NO3) (PO4) (SO4) (OTs) (DMSO) (OTs) (OAc) (TFA)

4.87 (NO3) − − 4.19 (OTs) − 4.79 (OTf) 4.71 (Cl) −

56.1 88.2 176.6 94.2 89.1

(NO3) (PO4) (SO4) (OTs) (OTs)

− 90.0 (OAc) 89.5 (TFA)

114.1 107.4 109.6 112.0 112.5

(NO3) (PO4) (SO4) (OTs) (OTs)

114.3 (OTf) 122.0 (OAc) 128.2 (TFA)

ideal geometry and refined using SHELX software. The final refinement of each compound included anisotropic thermal parameters for all non-hydrogen atoms. All final CIF files were checked using CheckCIF (http://www.iucr.org/). Additional information concerning the data collection and final structural solutions can be found in the Supporting Information or by accessing CIF files through the Cambridge Crystallographic Data Base. Table 1 lists the unit cell parameters, and Table 2 lists select metrical data for 1−6. Specific issues that arose with the various crystal structure solutions are discussed below. For 4c, the data were metrically evaluated in the orthorhombic and monoclinic crystal system, where the latter point group was found to yield the better solution. Additionally, 4c exhibits warning signs of a pseudomerohedral twin and refines with a 2:1 ratio but was solved without issues. Further details concerning any twinning issues are described in the Supporting Information and the CheckCIF report. The refinement for 4d exhibited the warning signs of reticular merohedral twinning, whereby obverse and reverse settings were present simultaneously. The R1 terminated at ∼9% before being corrected for twinning. The correction required a conversion to SHELX HKLF 5 format. The data for the obverse setting were merged prior to correction with the Make HKLF5 utility of WinGX version 2014.1.9 All reflections within zones hkl: l = 3n are affected by the twinning. The twin law used was a 2-fold rotation parallel to the c-axis. The twin law matrix applied was (by rows) [−1 0 0 0 −1 0 0 0 1]. The refinement continued by accurately placing all water hydrogen atoms by difference Fourier, with a final R1 of 3.1% at completion.

dissolved in H2O (∼1.5 mL). After being stirred for 48 h, the reaction mixture was placed in an oven (140 °C) for 12 h. The resulting white solid proved to be 3: yield 41.2% (1.29 g); 45Sc NMR [(conc)H3PO4] δ −8.4. [H][Sc(μ3-SO4)2]·2H2O (4a). In a 25 mL beaker, (conc)H2SO4 (0.80 g, 7.7 mmol) was added to a solution of 1 (1.0 g, 2.07 mmol) dissolved in H2O (∼1.5 mL). After being stirred for 48 h, the reaction mixture was placed in an oven (140 °C) for 12 h. The resulting colorless crystals proved to be 4a: yield 62% (0.78 g); 45Sc NMR [(conc)H2SO4] δ −1.1. {[(H2O)4Sc(OTs)2]OTs}·2H2O (4b). In a 25 mL beaker, H-OTs (2.20 g, 11.6 mmol) was added to a solution of 1 (1.00 g, 2.07 mmol) dissolved in H2O (∼1.5 mL). After being stirred for 48 h, the reaction mixture was placed in an oven (140 °C) for 12 h. The resulting white solid proved to be 4b: yield 58.3% (1.58 g). [(DMSO)3Sc(OTs)3] (4c). In a 25 mL beaker, DMSO (∼2 mL) was added to a vial containing 4b (0.50 g, 0.75 mmol) and stirred for 10 min. The reaction mixture was set aside, and the volatile components slowly evaporated. After an extended period of time (on the order of weeks), colorless crystals of 4c were isolated: yield 42% (0.25 g). [Sc(H2O)8]OTf3 (4d). In a 25 mL beaker, H-OTf (25.4 g, 169.5 mmol) was added to a solution of 1 (1.00 g, 2.07 mmol) dissolved in H2O (∼1.5 mL). After being stirred for 24 h, the mixture was heated at reflux temperatures for 8 h in air, allowed to cool, and then left to slowly evaporate until small colorless crystalline rods of 4d were obtained: yield 55.7% (1.47 g). {([(H2O)2Sc(μ-OAc)2]Cl)6}n (5). In a 25 mL beaker, H-OAc (∼10 mL) was pipetted onto 1 (1.50 g, 3.11 mmol). After this, the reaction mixture was heated to reflux temperatures and then set aside to allow the volatile components to slowly evaporate. After an extended period of time (on the order of days), colorless crystals of 5 were isolated: yield 46.0% (0.670 g); 45Sc NMR (HOAc) δ −116.2. [Sc(μ-TFA)3Sc(μ-TFA)3]n (6). In a 25 mL beaker, H-TFA (5.27 g, 46.3 mmol) was added to a solution of 1 (3.00 g, 6.22 mmol) in H2O (∼1.5 mL). After being stirred for 7 days, the reaction mixture was placed in an oven (140 °C) for 12 h. The resulting white solid proved to be 6: yield 49.0% (2.34 g). General X-ray Crystal Structure Information. Single crystals were mounted onto a loop from a pool of Fluorolube and immediately placed in a cold N2 vapor stream, on a Bruker Kappa Apex II CCD diffractometer employing an incident-beam graphite monochromator, Mo Kα radiation (λ = 0.71073 Å), and a SMART APEX CCD detector. Lattice determination and data collection were conducted using SMART version 5.054. Data reduction was performed using SAINTPLUS version 6.01 and corrected for absorption using the SADABS program within the SAINT software package. Structures were determined by direct methods or by using the PATTERSON method that yielded the heavy atoms, along with a number of the lighter atoms. Subsequent Fourier syntheses yielded the remaining light atom positions. The hydrogen atoms were fixed in positions of

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RESULTS AND DISCUSSION Surprisingly, the majority of fundamental structural information pertaining to the “simple” and routinely used inorganic acids of Sc has not been widely disseminated.5 It is of note that in some instances, structural properties of acid Sc components are available, but the structures typically possess an organic component.5 Therefore, this report presents the synthesis, structure, and subsequent characterization of the simple acid derivatives as obtained from the reaction of 1 with a variety of inorganic acids (eq 2). Compound 1 was used as it was isolated in high yield from reaction 1 in contrast to the original report by Hubert-Pfalzgraph and co-workers8 as a byproduct of an airexposed ScCl3(diglyme) sample. Synthesis and Single-Crystal Structures. A survey study of a variety of standard inorganic and organic acids with Sc0 indicated that (conc)HCl would be an effective method for solubilizing Sc0. This was favored because it was also in line with the existing processes for the production of the rare earth ores that are converted to chlorides.3c Reaction 1 was run under 11833

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

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Inorganic Chemistry

hydroxides act as the bridge with the remaining sites on the metal filled by H2O molecules. The four Cl atoms reside in the outer sphere. The hydrogen bonding that links the dinuclear moieties results in a chain. The metrical data (Table 2) noted for 1 were found to be consistent with the literature structure.8 The reported H2O−Sc distances range from 2.079 to 2.285 Å, and that in [Sc2(μ-OH)2] moieties ranged from 2.056 to 2.102 Å.5a The comparable Sc−O distances of 1 (averages of 2.18 and 2.06 Å) fall within this range. Inorganic Acids. To modify the compound, a sample of 1 was reacted with an excess of the various acids and stirred. The samples were allowed to slowly evaporate, and 2−4a were isolated from (conc) (a) HNO3, (b) H3PO4, and (c) H2SO4, respectively. The structures of the complexes isolated are discussed below. Nitrate. There are several structure reports for NO3 derivatives of Sc; however, these all involve the cocrystallization of the complex with an organic species. These compounds were found to adopt three general types: (a) solvated monomers, [(solv)xSc(κ2-NO3)3], where x = 1 for terpyridyl,11 x = 2 for triphenyl phosphine oxide [or OP(C6H3)3],12 4-amino-2,6bis(2-pyridyl)-1,3,5-triazine (or bpt),13 [(OP(C6H3)3)4Sc(κ2NO3)2](NO3)·CH2Cl2,12a and x = 4 for urea14 and [(bpt)Sc(κ2-NO3)2(κ-NO3)];13 (b) organic macrocyle monomers,

ambient conditions, and the product proved to be 1. This structure, shown in Figure 1, is consistent with [Sc(μ-

Figure 1. Structure plot of 1. Thermal ellipsoids of heavy atoms drawn at 30%.

OH)(H2O)5]24Cl reported by a number of researchers;8,10 however, the final structure noted here is significantly improved compared to the previous structures and/or has differing crystal data parameters. For the two heptacoordinated Sc atoms, two

Figure 2. Structure plots of (a) 2a and (b) 2b. Thermal ellipsoids of heavy atoms drawn at 30%. 11834

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

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Inorganic Chemistry

Figure 3. Structure plots of 3: (a) unit cell, (b) single-chain diagram, and (c) packing diagram. Thermal ellipsoids of heavy atoms drawn at 30%. Hydrogen atoms omitted for the sake of clarity.

[(H2O)xSc(κ2-NO3)3]·crown ether (where x = 3 and crown = 18-crown-615 or 15-crown-516 and x = 2 and crown = benzo15-crown-517 or 12-crown-4);16 and (c) hydroxy dimers, [(H2O)3(κ2-NO3)Sc(μ-OH)]2(NO3)2·12-crown-4,16 [(pyridine-carbaidimino-ethane)Sc(κ 2 -NO 3 )(μ-OH)] 2 (NO 3 )· CH3CN,11 and [(H2O)4(cucurbit[6]uril)Sc(κ2-NO3)](NO3)· H2O.18 For the room-temperature mixture of 1 with nitric acid, 2a (Figure 2a) was isolated and crystallographically characterized. The structure was found to retain the symmetric -OH dimer structure with one chelating η2-NO3, three H2O molecules, and an outer-sphere nitrate. This forces the Sc metal centers into a pentagonal bipyramidal geometry. While this moiety has been noted previously, all other species reported with this fragment also had an organic moiety present.11,16 The distances and angles of 2a are consistent with the 12-crown-4 species,16 with similar asymmetric nitrate coordination and central core with a

range of bound H2O distances. The Sc−OH distance is consistent with those noted for 1. The retention of the μ-OH in the presence of the HNO3 was surprising, and attempts were made to remove it by repeating the reaction at higher temperatures. The crystal structure isolated upon cooling was determined to be the mononuclear species of 2b (Figure 2b). The 8-coordination (CN-8) environment of the monomeric Sc metal center was filled by two chelating NO3 ligands and four H2O molecules. This results in a distorted dodecahedron structure. The necessary charge balancing additional NO3− was located in the outer sphere along with another H2O molecule. This is the first fully substituted nitrate with no organic species present. As noted in Table 2, the Sc−O distances and angles of the bound nitrates for 2b and 2a are consistent with each other. The 2.13 Å Sc− OH2 distance of 2b is also in agreement with that of 2a and falls within the range of distances previously reported.5a,8 11835

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

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Inorganic Chemistry

Organic Acids. Organosulfates. Exploring other organosulfonic acid derivatives resulted in additional structure types. For instance, the introduction of the toluene pendant moiety to the sulfate functional group yielded 4b, shown in Figure 4b. For this salt derivative, a CN-6 coordinated monomeric species was isolated with four bound H2O molecules and two κ1-OTs ligands. The OTs ligands are arranged cis to each other. Another charge-balancing OTs− moiety was located in the lattice and H-bonded to the H2O molecules of other Sc moieties. In addition, two H2O molecules were isolated in the lattice. This is consistent with that noted by Ohki et al.;28 however, the structure reported here is substantially improved in comparison to the original solution. Isolation of the same mixture from DMSO also led to a monomeric structure, 4c (Figure 4c). Each Sc is CN-6 coordinated by three κ1-OTs and three DMSO solvent ligands. One additional alkyl derivative was investigated using triflic acid (H-OTf). The isolated monomeric species 4d possessed three outer-sphere OTf and eight H2O ligands bound to the CN-8 square antiprismatic Sc metal center (Figure 4d). This structure is identical to that reported by Abassi et al.,1c who synthesized it from a heated solution of Sc2O3 in H-OTf; however, a slightly better structure was determined in this effort. There appears to be an interaction of the F of the CF3 with the H of the H2O molecules on the Sc metal. The (ScO)−S−(O-Sc) angles for the literature bridging sulfates range from 109.4° to 112.6°, a range in line with those noted for 4a− d. The average Sc−Osulfate distances are 2.06 Å (Table 2), which fall within the short end of the range of Sc−O-S distances of 2.03−2.21 Å. Acetic Acid. Figure 5 shows the structure plot of the OAc derivative, 5. Each CN-6 Sc has four μ-OAc ligands that link to four different Sc atoms. Furthermore, there are two coordinated trans H2O solvates and one outer-sphere Cl ion per Sc atom. The hydrogen bonding between coordinated H2O molecules from two Sc moieties and a Cl atom forms two orthogonal lines of Cl atoms down the a- and c-axes. Down the b-axis there is an alternating Cl, H2O arrangement. The connection of two of the “[-Sc-(μ-OAc)-]” moieties leads to the formation of a sixmembered ring. Another ring is formed by the other two “[-Sc(μ-OAc)-]” moieties ultimately forming a polymer of interlinked six-membered “[Sc-μ-OAc]” rings. This structure varies from the homoleptic [Sc2(μ-OAc)6]n polymer,29 which employs three bridging OAc ligands between the Sc metals forming a long chain. The average Sc−O distance (2.08 Å) and bite angle of the OAc (124.0°) for this model compound are similar to those noted for 5 (2.06 Å and 122.0°, respectively). The Sc−OH2 distance for 5 (2.09 Å) is within the range previously discussed for the literature compounds.5a Trifluoroacetic Acid. The structure of 6 (Figure 6) was determined as a chain of Sc atoms bridged by three μ-TFA molecules. These chains do not appear to interact, which leaves the Sc metal centers in a fairly regular CN-6 geometry. The Sc− O distance of 6 (TFA) and that of 5 (OAc) are consistent with each other and literature reports.5a Bulk Powder Characterization. Numerous attempts to identify the bulk powders in comparison to the single crystal structures were made. For the majority of the analytical tools available, little pertinent information was collected. The FTIR data were consistent with the various ligands present, but identification of the purity could not be determined. The mass spectroscopy, PXRD, thermal analysis (TGA/DSC), and additional data did not yield sufficient information to

The ability to alternate between these two compounds was further explored chemically. Not surprisingly, it was found that 2a could be converted to 2b by simply heating a reaction mixture of 2a in (conc)HNO3. The opposite conversion was also found to be fairly simple by merely dissolving 2b in H2O, followed by crystallization. Phosphate. Transformation of 1 with phosphoric acid was found to yield 3 (Figure 3). Initially, the slow evaporation of the volatile component made isolation of the crystals of 3 very difficult as the reaction mixture tends to remain an oil; however, slow thermal drying in an oven (140 °C) led to high-yield, Xray-quality crystals. The majority of previously reported structures with a Sc−PO4 moiety5a were polymeric, including a variety of ammonium19 or alkali metal20 salts, and an inorganic scandium phosphate21 derivative. Of these, a so-called “monosubstituted” acid scandium phosphate [Sc(H2PO4)3] structure was disseminated with similar unit cell parameters noted for 3; however, details of the synthesis and final structure are not readily accessible.21a Because the final structure solution and model parameters are substantially better for 3 in comparison to the available information about “Sc(H2PO4)3”21a and the preparative route is most likely different, the full discussion of the structure is presented. The structure of 3 was determined to be a polymer with CN6 ScO6 centers that are linked together by μ-PO4 fragments forming 3 (Figure 3a). There are six PO4 moieties per Sc that link to other ScO6 centers, forming a chain of interlocked fourmembered rings of [Sc(η2-O)P(O)2(η2-O)]4 (Figure 3b). Only two of the O atoms per PO4 moiety act as the bridge. This results in a cycle of four Sc atoms with 18 PO4 ligands, which by charge balance requires six protons. The various chains are interlinked and form an extended three-dimensional network with channels through the molecule [looking down the c-axis (Figure 3c)]. The literature distances for the Sc-OPO3 moiety ranged from 2.05 to 2.14 Å, which are in line with the distances noted for 3 at 2.063 Å (see Table 2). The (Sc-O)−P−(O-Sc) angles range from 109° to 112°, which are significantly smaller than the value of 115.46° reported for 3. This leaves a 103.79° angle for the terminal O−P−O angle, which, not surprisingly, falls at the short end of the literature range (102.6−109.9°). Sulfate. The SO42− derivatization of 1 was attempted, and the product was isolated as 4a (a). Of the 40 Sc structures reported that contain a “SO3” moiety, all possess an organic fragment and the majority are salts, either (i) a fully hydrated Sc cation with an outer-sphere [RSO3]− anion {i.e., [Sc(H2O)n][L] [L = C(SO3CF3)3, n = 7;22 O3SCF3, n = 81c n = 923]} or (ii) a -OH bridge dinuclear hydrated cation with an outer-sphere [RSO3]− counteranion {i.e., [(H2O)5Sc(μOH)]2[SO3R]·H2O where R = C6H5,15b,24 (O3SCF2)2CF2,24b or bipyridinium10a}. Other structures are also reported, including a methylsulfonate species [Sc(μ-OH)(O3SCH3)2]n,25 polymeric chains of Sc2(SO4)3 with organic cations (guanidinium or hexamethylenediammonium),26 and the homosulfate [Sc2(SO4)3]n27 isolated from a NaCl melt. For 4a, the complex was found to adopt a polymeric structure with CN-6 Sc bound by four bridging SO4 moieties and two H2O solvent moieties. The general structure is a series of interlocking, asymmetric, four-membered rings of [Sc4(μSO4)2(μ3-SO4)2]2. Because of charge balance, the [Sc2(κ2SO4)4] unit must have two protons, which were subsequently located in the final structure solution. The resulting formula makes 4a a hydrosulfate derivative. 11836

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

Article

Inorganic Chemistry

Figure 5. Structure plots of 5: (a) asymmetric unit thermal ellipsoids of heavy atoms drawn at 30% and hydrogen atoms omitted for the sake of clarity and (b) expansion of the unit cell [Sc (white), O (red), Cl (green), C (gray), and H (white)].

spin (I) of 7/2, high observed frequency, with a moderate quadrupole moment (Q) of 0.22 × 10−28 m2 and a chemical shift range of 250 ppm.7a Furthermore, Shchurko et al. indicated, in their comprehensive report on solid-state 45Sc NMR, that a correlation between the NMR tensor characteristics and the coordination of the Sc metal center exists.7a With this unique set of compounds available, the samples discussed above were analyzed by both solution- and solid-state 45Sc NMR spectroscopy. The collected spectral data are listed in Table 3, with solid-state spectra shown in Figure 7 and solution spectra detailed in the Supporting Information. Solid-State NMR. On the basis of the solid-state structures obtained, a single 45Sc MAS NMR resonance was expected in the solid state for all of the precursors. The observed chemical shifts ranged over 90 ppm (from 44.0 to −40.6 ppm), consistent with a ScO6 and ScO7 coordination sphere. These data are summarized in Table 3 and shown in Figure 7. For 1 and 4a, a single peak was observed with a classic second-order quadrupolar broadened powder pattern. The quadrupolar coupling constants (QCC) were measured to be 13.7 and 6.2

Figure 4. Structure plots of (a) 4a, (b) 4b, (c) 4c, and (d) 4d. Thermal ellipsoids of heavy atoms drawn at 30%. Hydrogen atoms omitted for the sake of clarity.

unambiguously identify the products. However, one analytical tool that did supply useful information was 45Sc NMR spectroscopy. 45Sc is an excellent nucleus for NMR experiments because of a number of properties: 100% naturally abundant, 11837

DOI: 10.1021/acs.inorgchem.5b02030 Inorg. Chem. 2015, 54, 11831−11841

Article

Inorganic Chemistry

Table 3. 45Sc NMR Chemical Shifts for 1−6 [solution (acid/ H2O) and solid-state] solution compd

acid

1

−

2a 2b

−c −15

c

H2O δ (ppm)

solid-state δ (ppm)

concentrated, 18.5 (100%) 38.7 (80%), 8.9 (20%) dilute, 35.1 (100%) 0.4 −c 0.4 −19.3 (49%), −38.3 (22%), −40.6 (29%)

3

−8.4

31.1

−11.5 (92%) −6.8 (8%) −15.8 (100%) 15.5 (87%), 44.0 (13%)

4a 4b

−1.1

9.8 −0.3

4c 4d 5

−c −c 116.2

5.2 3.7 20.8

−c −c 20.0 (19%), −4.6 (81%)

6

−c

3.0

−13.2 (19%), −15.9 (37%), −0.3 (44%)

QCCa (MHz)

ηb

13.7

0.9

−c 5.8

−c 0.8

8.5 4.4 −c

0.4 0.3 −c

6.2 5.8

0.4 0.8

−c −c −c 14.6

−c −c −c 0.5

5.4 10.1

1.0 0.1

8.1 −c

0.4 −c

a

Quadrupolar coupling constant (QCC) obtained from the 45Sc solid NMR line shape. bAsymmetry (η) obtained from the 45Sc solid NMR line shape. The relative concentrations were obtained from the integration of the DMFIT models for the overlapping resonances. c Not obtained.

Figure 6. Structure plots of 6: (a) asymmetric unit thermal ellipsoids of heavy atoms drawn at 30% and (b) expansion of the unit cell [Sc (blue), O (red), F (green), and C (gray)].

shift is most likely due to the use of a different external chemical shift reference {this work was referenced against [Sc(H2O)6]3[Cl] in H2O vs Sc(NO3)3 in HNO3 for the assignment of Petrosyants26a}. Variable-temperature NMR experiments with 1 in D2O were undertaken using both the high-concentration (∼0.2 mM) and low-concentration samples. The higher-concentration roomtemperature spectrum revealed two peaks [δ 36.3 (major) and 6.8 (minor)]. Upon heating, the two peaks coalesced and ultimately sharpened into a singlet at δ 40.6. The sample was allowed to cool to room temperature, and the spectrum again had two peaks (δ 36.9 and δ 7.4) in a similar ratio noted originally. For the low-concentration sample, the singlet at δ 35.1 shifted slightly to δ 41.3 and back to δ 34.0 upon cooling. Additionally, recrystallization of 1 from H2O led to the isolation of 1. Combined, these data indicate that an equilibrium is present in solution for 1. On the basis of the chemical shift, the peak at δ 36.3 was initially believed to be dinuclear species 1; however, it is not unreasonable to believe this undergoes a monomer−dimer equilibrium as shown in eq 3. Typically, the right-hand side products would be favored at high dilutions and temperatures. This would indicate that the peak at δ 36.3 could be the hydrate hydroxide, but this is not consistent with what was observed for the full hydrates at δ 0.00. Rehder et al. report a similar spectrum for the dissolution of ScCl3 in water as forming [(H2O)5ScCl]Cl2 (δ ∼35) and [(H2O)6Sc]Cl3 (δ 0.0).30

MHz, respectively, and reflect the subtle differences in the local bonding configuration around the Sc atom. Ab initio predictions of the QCC and asymmetry (η) based on the X-ray structures were beyond the scope of this investigation and, thus, were not pursued. The 45Sc MAS NMR spectra of the remaining products each contained multiple peaks, which were attributed to the presence of different phases, packing inequivalencies, and/or loss of crystallinity (formation of the amorphous phase with a Gaussian-shaped resonance) under the MAS conditions. Solution NMR in Water. To determine if these “extra” peaks were due to impurities or packing inequivalencies, the same powders were dissolved in D2O and the 45Sc NMR spectra collected. The results are listed in Table 3. A single peak located around δ 0.0 was noted for all the samples except 1. Understanding the behavior of 1 in H2O became of further interest. Because the solid-state spectrum revealed a single peak, it was surprising that two resonances in an 80:20 ratio were initially noted. Additional spectra recorded for 1 at different concentrations showed a change in the ratio of the very broad peaks (