Thorium Cubanes-Synthesis, Solid-State and Solution Structures, Thermolysis, and Chalcogen Exchange Reactions

Article abstract of DOI:10.1021/acs.inorgchem.8b00836

Thorium cubanes (py)₈Th₄(μ₃-E′)₄(μ₂-EPh)₄(ν-EPh)₄ (E, E′ = S, Se) were prepared from ligand-based redox reactions of elemental E′ with Th(EPh)₄. Products with all four

Full text of DOI:10.1021/acs.inorgchem.8b00836

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Thorium Cubanes−Synthesis, Solid-State and Solution Structures,
Thermolysis, and Chalcogen Exchange Reactions
Marissa Ringgold,† David Rehe,† Peter Hrobarik,‡,§ Anna Y. Kornienko,† Thomas J. Emge,†
́
and John G. Brennan*,†
†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New
Jersey 08854-8087, United States
‡Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany
̈
̈
§Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, SK-84215 Bratislava, Slovakia
S
* Supporting Information
ABSTRACT: Thorium cubanes (py)8Th4(μ3-E′)4(μ2-EPh)4(η-
EPh)4 (E, E′ = S, Se) were prepared from ligand-based redox
reactions of elemental E′ with Th(EPh)4. Products with all four
possible E/E′ combinations (E,E′ = S,S; Se,Se; S,Se; Se,S) were
isolated and structurally characterized, ligand exchange reactions
were explored, and the heterochalcogen compounds (py)8Th4(μ3-
S)4(μ2-SePh)4(η-SePh)4 and (py)8Th4(μ3-Se)4(μ2-SPh)4(η-SPh)4
were heated to deliver solid solutions of ThSxSe2−x. NMR
spectroscopy indicated that the structure of (py)8Th4(μ3-Se)4(μ2-
SePh)4(η-SePh)4 is static in pyridine solution, with no exchange
between bridging and terminal PhE− ligands on the NMR time
scale. A computational analysis of 77Se NMR shifts provides
insight into the solution structure of both clusters and monomeric
chalcogenolates.
Compounds of thorium with chalcogen-based anions4,7a,9 are
INTRODUCTION
Actinide (An) cluster chemistry1 is a potentially rich field,
■
limited. In early work, sterically demanding ancillary ligands9c−e
were used to kinetically stabilize coordination complexes,
motivated by the unique chemical2 and physical3 properties of
leading to isolation of the first discrete compounds with An−E
the actinide elements and the possibility of harnessing multiple
metal centers to afford unique cooperative reactions. While the
bonds. More recently, a number of stable molecular thorium
chalcogenolates have been prepared,5e,6d,7a,9a showing that
possibility of cooperativity exists, rationally achieving this goal
steric saturation is not a prerequisite to form stable products.
requires a more detailed understanding of fundamental physical
Of these molecular compounds, the tetrachalcogenolates
properties of f-element compounds, as well as a deeper
Th(EPh)4 (E = S, Se)9a are convenient starting materials for
appreciation of how polynuclear actinide compounds behave in
solution. Oxide-based actinide cluster chemistry1a−g is fairly
the synthesis of nanoscale thorium clusters using either ligand-
based redox chemistry or metathesis reactions, as evidenced by
well-established, but related compounds with less electro-
negative chalcogenido ligands1j−n,4 are relatively unexplored.
a description of the first thorium cubane compounds
(py)8Th4(μ3-E)4(μ2-EPh)4(η-E′C6F5)4 (E,E′ = S, Se).1j This
Given that actinide−chalcogenide bonds are ideal subjects for
work was limited to the synthesis of compounds with the same
probing covalency and the nature of the actinide−ligand bond,
there is a strong fundamental motivation for developing the
chalcogen atom in both μ3-E and μ2-EPh moieties, otherwise
chalcogen exchange processes between E and EPh sites led to
chemistry of actinide molecules and clusters with chalcogenido
products with random distributions of chalcogen atoms.
ligands.
The present work explores actinide cubane chemistry in
Thorium has a number of intrinsic properties that make this
detail, showing that all possible combinations of E′/EPh ligands
(E′, E = S or Se) in (py)8Th4(μ3-E′)4(μ2-EPh)4(η-EPh)4 can be
realized experimentally and finding that thermal decomposition
of heterochalcogen compounds leads to the formation of a
ThSxSe2−x alloy. Cluster reactivity is explored to demonstrate
metal attractive for studying the chemical properties of actinide
clusters. Most importantly, Th(IV) is diamagnetic, and so
NMR spectroscopy can be used to explore structure and
bonding,5 ligand dynamics,5b,6 and chemical reactivity.5c,e,7 In
addition, the lack of thorium redox activity, as evidenced by the
paucity of well-defined Th(III) compounds,8 simplifies
reactivity and minimizes the tendency of compounds to
undergo undesirable side reactions.
Received: March 30, 2018
© XXXX American Chemical Society
A
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that both PhE− and E2− ligands will undergo ligand-based redox
reactions to form products with more electronegative ligands.
Solution 77Se NMR data provide insight into the dynamic
behavior of these clusters, and a computational analysis of 77Se
NMR shifts reveals that solid-state structures are maintained in
solution.
1576 (m), 1377 (s), 1221 (s), 1034 (m), 1153 (w), 1036 (m), 804
(w), 602 (s), 420 (s) cm−1. Anal. Calcd for C108H100N12Th4S12: C,
45.1; H, 3.50; N, 5.84. (C88H80N8Th4S12: C, 41.2; H, 3.15; N, 4.37).
Found: C, 41.9; H, 3.35; N, 4.70%. This compound was not
sufficiently soluble to obtain NMR data.
Synthesis of (py)8Th4(μ3-Se)4(μ2-SePh)4(η-SePh)4·6.25py (2). Th
(0.232 g, 1.00 mmol), Hg (0.018 g, 0.090 mmol), and PhSeSePh
(0.624 g, 2.00 mmol) were added to pyridine (ca. 15 mL) and stirred
at 25 °C for 12 h. Elemental selenium (0.079, 1.00 mmol) and
additional pyridine (20 mL) were added, and the mixture was stirred
for 12 h (Scheme 1) to form a yellow solution with a black powder
precipitate. The solution was filtered and concentrated to ca. 20 mL,
layered with ca. 12 mL of hexanes, and left for a week to yield pale
yellow crystals (0.37 g, 43%) that melt at 246 °C and decompose at
257 °C. IR: 2923 (w), 1629 (m), 1260 (s), 1151 (s), 1089 (m), 666
(s), 624 (s), 466 (s) cm−1. Anal. Calcd for C119.25H111.25N14.25Th4Se12:
C, 39.6; H, 3.10; N, 5.52. (C89H81N8Th4Se12: C, 34.1; H, 2.60; N,
3.57). Found: C, 38.4; H, 3.05; N, 5.01%. 1H NMR: 8.74 (bs, 2H, py),
8.17 (d, 2H, J = 5.5 Hz, SePh), 7.58 (tt, 1 H, 7.5 Hz, 2 Hz, py), 7.37
(m, 2H, SePh), 7.29 (t, 1H, J = 7.5 Hz, SePh), 7.22 (m, 2H, py), 7.08
(s, 2H, SePh), 6.60 (t, 1H, J = 7 Hz, SePh), 6.41 (t, 2H, J = 7 Hz,
SePh) 77Se NMR: 928(s, μ3-Se), 573(s, η-SePh), 460 (s, μ2-SePh).
Synthesis of (py)8Th4(μ3-S)4(μ2-SePh)4(η-SePh)4·4py (3). Th (0.232
g, 1.00 mmol), Hg (0.020 g, 0.10 mmol), and PhSeSePh (0.624 g, 2.00
mmol) were added to pyridine (15 mL) and stirred for 12 h.
Elemental sulfur (0.032, 1.00 mmol) and pyridine (20 mL) were
added, and the mixture was stirred for 12 h (Scheme 1) to give a
yellow solution with a greyish-green precipitate. The solution was
filtered, concentrated to ca. 20 mL, layered with hexanes (12 mL), and
left for a week at 25 °C to yield pale yellow crystals (0.31 g, 19%) that
melt at 254 °C and decompose at 370 °C. IR: 2957 (w), 1599 (s),
1571 (s), 1218 (s), 1153 (m), 1094 (s), 800 (m), 662 (s), 625 (s), 465
(s) cm−1. Anal. Calcd for C108H100N12Th4Se8S4: C, 39.9; H, 3.10; N,
5.16. (C88H80N8Th4Se8S4: C, 36.0; H, 2.75; N, 3.82). Found: C, 35.7;
H, 2.88; N, 4.02%. This compound was not sufficiently soluble to
obtain NMR data.
EXPERIMENTAL SECTION
■
Caution! Thorium-232 is a strong α-emitter and potentially is a health
hazard. Appropriate radioactive and personal safety trainings should be
mandatory before reproducing this work.
General Methods. All syntheses were performed under ultrapure
nitrogen (Welco Praxair), using conventional drybox or Schlenk
techniques. Pyridine and hexane (Aldrich) were purified with a dual-
column Solv-Tek solvent purification system and collected immedi-
ately prior to use. PhSeSePh (Aldrich) was purchased and recrystal-
lized from hexanes. PhSSPh (Acros), thorium chips (International
Bioanalytical Industries Inc.), and mercury (Strem Chemicals) were
purchased and used as received. (C6F5)2Se2 was prepared according to
literature procedure.10 Melting points were recorded in sealed
capillaries and are uncorrected. IR spectra were recorded on a
Thermo Nicolet Avatar 360 FTIR spectrometer from 4000 to 450
cm−1 as mineral oil mulls on CsI plates. UV−Vis absorption spectra
were recorded on a Varian DMS 100S spectrometer with the samples
dissolved in pyridine, placed in either a 1.0 mm × 1.0 cm Spectrosil
quartz cell or a 1.0 cm2 special optical glass cuvette, and scanned from
400 to 1000 nmall four compounds were found to be optically
transparent. NMR data were collected on a Varian VNMRS 500
spectrometer at 25 °C with the compounds dissolved in pyridine-d5.
1H and 77Se NMR spectra were obtained at 499 and 95 MHz,
respectively. 77Se NMR spectra were acquired with a longer relaxation
delay (7.0 s) in hydrogen-decoupled mode using (C6F5)2Se2 as an
external standard. Gas chromatography-mass spectrometry (GC-MS)
data were collected on a Varian Saturn 2100T instrument fitted with a
capillary column (30 mm length, 0.25 mm ID, 0.25 mm film
thickness). Elemental analyses were performed by Quantitative
Technologies, Inc. Because some of these compounds rapidly lose
both lattice pyridine within minutes of isolation, elemental analyses are
time-dependent. Combustion analyses were obtained on freshly
isolated materials from compounds that were desolvated fully under
vacuum. Powder X-ray diffraction (PXRD) data were obtained on a
Bruker Vantec500 area detector using Cu Kα radiation, as described
below, to confirm that single-crystal X-ray data were consistent with
bulk materials.
Synthesis of (py)8Th4(μ3-Se)4(μ2-SPh)4(η-SPh)4·2py (4). Method A.
(Scheme 2) Th (0.232 g, 1.00 mmol), Hg (0.005 g, 0.025 mmol),
PhSSPh (0.218 g, 1.00 mmol), and PhSeSePh (0.312 g, 1.00 mmol)
were added to pyridine (15 mL), and the mixture was stirred for 12 h
at 25 °C. Elemental selenium (0.079, 1.00 mmol) and toluene (14
mL) were added, and the mixture was stirred for 6 h to form a yellow
solution with a black powder. The solution was filtered, concentrated
to 20 mL, layered with 12 mL of hexanes, and left at 25 °C to yield
colorless crystals (0.13 g, 18%). The product identity was confirmed
by single-crystal analysis.
Synthesis of (py)8Th4(μ3-S)4(μ2-SPh)4(η-SPh)4·4py (1). Th (0.232 g,
1.00 mmol), Hg (0.015 g, 0.075 mmol), and PhSSPh (0.437 g, 2.00
mmol) were combined in pyridine (ca. 15 mL), and the mixture was
stirred for 12 h at 25 °C. Elemental sulfur (0.032, 1.00 mmol) and
additional pyridine (20 mL) were added to the mixture that was then
stirred for additional 12 h (Scheme 1), filtered to remove a trace
amount of air-sensitive white powder, concentrated to 20 mL, and
layered with hexanes (12 mL) to yield colorless crystals (0.25 g, 32%)
that melt at 264 °C and decompose at 313 °C. IR: 1601 (s), 1598 (s),
Method B. (Scheme 3) Th (0.232 g, 1.00 mmol), Hg (0.015 g,
0.075 mmol), and PhSeSePh (0.624 g, 2.00 mmol) were added to
pyridine (15 mL), and the mixture was stirred for 12 h. Elemental Se
(0.079 g, 1.00 mmol) was added, and the mixture was stirred for 8 h.
PhSSPh (0.218 g, 1.00 mmol) was added, and after it was stirred for 20
min the solution was filtered, concentrated to 20 mL, and layered with
hexanes (12 mL) to yield colorless crystals (0.38 g, 54%) that melt at
182 °C and decompose at 356 °C. IR: 2359 (w), 1598 (s), 1574 (m),
1220 (s), 1154 (m), 1034 (m), 1001 (m), 733 (m), 689 (s) cm−1.
Anal. Calcd for C98H90N12Th4Se4S8: C, 40.5; H, 3.12; N, 4.82.
(C88H80N8Th4Se4S8: C, 38.4; H, 2.93; N, 4.08). Found: C, 38.7; H,
Scheme 1. Synthesis of Thorium Chacogenido Cubanes 1−3
1
3.13; N, 4.45%. H NMR: 8.69 (d, 2H, J = 5.0 Hz, py), 8.07 (bs, 2H,
SPh), 7.55 (tt, 1 H, 8.0 Hz, 1.5 Hz, py), 7.47 (bs, 2H, SPh), 7.22 (bs,
1H, SPh), 7.16 (m, 2H, py), 7.03 (bs, 2H, SPh), 6.45 (s, 1H, SPh),
6.32 (s, 2H, SPh). This compound was not soluble enough to obtain
reliable 77Se NMR data.
Reaction of Compound 2 with PhSSPh (Scheme 4).
Crystalline 2 (1.31 g, 0.42 mmol) and PhSSPh (0.550 g, 2.52
mmol) were combined in 30 mL of pyridine and stirred for 12 d at 25
°C. The solution was filtered to remove a trace amount of yellow
powder, concentrated to ca. 14 mL, and layered with hexane (28 mL)
to give colorless crystals (0.32 g, 27%), which were identified as 4 by
single-crystal X-ray diffraction.
B
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Scheme 2. Synthesis of the Thiolate-Capped Selenido Cubane 4 from Heterochalcogenolate Starting Materials
initially empty “cold” end of the glass tube was held at −196 °C by
immersion in liquid nitrogen. The samples were heated to 850 °C at a
ramp rate of 10 °C/min and then held at 850 °C for 6 h, at which time
it was cooled to 25 °C at a rate of 3.5 °C/min. The gray-black powder
that was formed at the sample end of the quartz tube was placed on
Kapton tape and identified by PXRD as a solid-state mixture, or alloy,
of isomorphic ThS2 and ThSe2, namely, ThSxSe2−x, with approximately
equal amounts of Se and S based upon the resulting unit cell
dimensions from whole pattern fitting routine (JADE7) after using as
starting point either the ThS2 phase9 (Pmnb (62), Z = 4, PDF No. 75-
1967), or the ThSe2 phase10 (Pmnb (62), Z = 4, PDF No. 74-0978).
However, the nearly equal amounts of Se and S determined from the
calculated unit cell alone may not be accurate due to the considerable
asymmetrical broadening of peaks as a result of low-dimensional and
restacking effects of the crystal packing on the nano scale, similar to
those observed for MoS2,14 TaS2,15 and NbS2.16 GC-MS analysis of the
volatile products identified py (m/z 79), SPh2 (m/z 186), and SePh2
(m/z 234).
Scanning Electron Microscopy (SEM). Field-emission scanning
electron microscopy (FE-SEM, ZEISS Sigma) and energy-dispersive
X-ray spectroscopy (EDS, silicon drift detector from Oxford
Instruments) were employed to observe the morphological character-
istics of the pyrolysis materials. Imaging and EDS were completed
using 5 and 15 keV electron beam voltages correspondently. All
samples were covered with 20 nm Au coating using rotary-pumped
sputter coating system (EMS150R, Electron Microscopy Sciences) and
placed on carbon adhesive tapes with specimen holders (Ted Pella).
Powder X-ray Diffraction (PXRD). PXRD data for compounds
1−4 were obtained with a Bruker Vantec-500 area detector and a
Bruker FR571 rotating-anode X-ray generator operating at 40 kV and
50 mA and equipped with a three-circle Azlan goniometer. The system
used 0.5 mm pinhole collimation and a Rigaku Osmic parallel-mode
Scheme 3. Ligand Exchange on a Cluster Surface to Form
Compound 4
NMR Scale. Crystalline 2 (0.110 g, 0.03 mmol) was mixed with
PhSSPh (0.039 g, 0.0.18 mmol) in 0.8 mL of pyridine-d5. The mixture
was stirred for ∼1 min and then allowed to settle at 25 °C for 45 min.
77Se NMR spectrum was collected within the first 12 h and contained
resonances at 520 (s, PhSSePh)11 and 456 (s, PhSeSePh)12 ppm. After
6 d, yellow crystals had formed in the NMR tube. These were
identified as PhSeSePh13 by X-ray diffraction. GC-MS of the solution
revealed a number of products including PhSeSeSeSPh (m/z, 424),
PhSSSSePh (m/z, 330), PhSSPh (m/z, 218), PhSePh (m/z, 234), and
PhSPh (m/z, 186).
Reaction of 2 with Excess PhSSPh. (Scheme 4) Crystalline 2
(1.56 g, 0.50 mmol) and PhSSPh (0.873 g, 4.00 mmol) were
combined in 40 mL of pyridine and stirred for 4 d at 90 °C. The deep
yellow solution was filtered from a small amount of yellow powder,
concentrated to ca. 26 mL, and layered with hexane (14 mL) to give
colorless crystals (0.65 g, 69%) that were identified as (py)4Th-
9a
(SPh)4 by single-crystal X-ray diffraction. The purity of the final
monomeric product was verified by comparing experimental and
calculated PXRD of the bulk materials, as reported in the Supporting
Information. The residue of the deep yellow solution was analyzed by
GC-MS to reveal the presence of PhSSePh (m/z, 266), PhSSPh (m/z,
218), PhSeSePh (m/z, 314), PhSePh (m/z, 234), and PhSPh (m/z,
186).
Thermolysis (Scheme 5). Ground samples of 3 or 4 (∼20 mg)
were placed in a quartz thermolysis tube under nitrogen atmosphere
that was evacuated and purged with argon three times, sealed under
vacuum, and placed into a model 847 Lindberg tube furnace. The
Scheme 4. Reactivity of the All-Selenium Compound 2 with Diphenyl Disulfide under Different Conditions
C
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(e.g., primary beam dispersion less than 0.016° in 2θ) mirror
monochromator (Cu Kα; λ = 1.5418 Å). Data were collected at 20
°C with a sample-to-detector distance of ∼26 cm. Spatial calibration
and flood-field correction for the area detector were performed prior
to data collection. The 2048 × 2048 pixel images were collected at the
fixed detector (2θ) angles of 0 and 15° for ∼10 min with ω fixed and
continuous rotation in ϕ (capillary axis) of ∼2° per min. For the
intensity versus 2θ plots (see Supporting Information figures),17
background levels, modeled as amorphous scattering from air,
capillary, and sample, were subtracted, and integrations in χ (Bruker
GADDS)18 were performed. No other corrections to the PXRD data
were made. PXRD of bulk samples were compared with the calculated
diffraction pattern of the respective compound. To account for the
temperature difference in the PXRD (T = 293 K) and single-crystal (T
= 153 K) patterns, a wavelength of 1.51 Å was used for the calculated
PXRD patterns. Also, it appeared that the intensity and d-spacings of
low-angle PXRD data were much less affected by evaporation of lattice
solvent. Thus, the 4° < 2θ < 12° PXRD data provided an excellent
region for the phase verification performed here. Diagrams of the
PXRD data are available in the Supporting Information.
Single-Crystal X-ray Structure Determination. Data for 1−4
were collected on a Bruker Smart APEX CCD diffractometer with
graphite monochromatized Mo Kα radiation (λ = 0.710 73 Å) at 100
or 120 K.19 Crystals were immersed in mineral oil and examined at low
temperatures. The data were corrected for Lorenz effects, polarization,
and absorption, the latter by a face-based numerical method.19 The
structures were solved by direct methods.20 All non-hydrogen atoms
were refined20 based upon Fobs.2 All hydrogen atom coordinates were
calculated with idealized geometries. All structures were drawn using
the Mercury 3.8 program.21 Crystallographic data and final R indices
for 1−4 are given in Table S1 of the Supporting Information. In
separate syntheses, compound 2 was found to also crystallize in three
other phases: the same monoclinic phase noted for 1 and 3, space
group C2/c, with formula = C108H100N12Se12Th4, fw = 3441.67, a =
29.565(2) Å, b = 12.6321(9) Å, c = 31.612(2) Å, β = 111.613(1)°, α =
γ = 90.00°, V = 10976.2(13) Å3, Z = 4, D(calcd) = 2.083 g/cm−3, T =
120(2) K; in an orthorhombic phase, space group P212121, with
formula = C98H90N10Se12Th4, fw = 3283.47, a = 15.8657(7) Å, b =
16.2476(8) Å, c = 38.7871(18) Å, α = β = γ = 90.00°, V = 9998.5(8)
Å3, Z = 4, D(calcd) = 2.181 g/cm−3, T = 100(2) K; and in a triclinic
regular approximation (ZORA).30 Bulk solvent effects were simulated
by the conductor-like screening model (COSMO) as implemented
self-consistently in ADF.31 The computed 77Se nuclear shieldings were
converted to chemical shifts (δ, in ppm) relative to the shielding of
Me2Se, computed at the same level. Where applicable, the calculated
shieldings were averaged over the magnetically equivalent nuclei. The
calculated NMR shieldings were broken down into molecular orbital
(MO) contributions using the analysis tools in the ADF code.
Natural population analyses (NPA), natural bond analysis (NBO),
and analysis of natural localized molecular orbitals (NLMOs)32 were
performed at the PBE0/def2-TZVP/ECP level using the NBO6 code,
interfaced with Gaussian 09.33,34 The latter was also used for
evaluation of CM5 (charge model 5)35 partial atomic charges, which
are essentially independent of a basis set. Bader’s quantum theory of
atoms-in-molecules (QTAIM)36 analyses of the Kohn−Sham wave
functions (generated in Gaussian at the same level as used for NLMO
analysis and stored as .wfx files) were performed using the Multiwfn
program.37
RESULTS AND DISCUSSION
■
In situ prepared Th(EPh)4 (E = S, Se)9a undergoes ligand-
based redox reactions with elemental E′ (E′ = S, Se) in
pyridine. Elemental E′ is reduced to E′2−, and the PhE− from
Th(EPh)4 is oxidized to PhEEPh, leading to the formation of
(py)8Th4(μ3-S)4(μ2-SPh)4(η-SPh)4 (1), (py)8Th4(μ3-Se)4(μ2-
SePh)4(η-SePh)4 (2), and (py)8Th4(μ3-S)4(μ2-SePh)4(η-
SePh)4 (3) (Scheme 1). Elemental mercury expedites the
reaction either by forming a reactive Th/Hg amalgam or by
insertion of Hg into the E−E bond, followed by transmetalation
with elemental Th. If Hg contamination of Th waste is
problematic, these reactions can be reproduced without Hg, but
the rates will be slower and dependent on the level of Th
surface oxidation.
While E′ is reduced by PhE− when the electronegativity of E′
is equal to or greater than the electronegativity of E, the
reaction does not proceed when E′ is less electronegative than
E. To establish the synthetic feasibility of all possible E/E′
combinations, the final cubane target (py)8Th4(μ3-Se)4(μ2-
SPh)4(η-SPh)4 (4) was approached in two ways. First, Th metal
was used to reduce a mixture of PhSSPh and PhSeSePh, and
elemental Se was then added to oxidize the PhSe− ligand
(Scheme 2), and second, the selenido cubane 1 can be prepared
in situ and reacted further with PhSSPh, resulting in
displacement of the surface PhSe moieties with PhS (Scheme
3).
Compounds 1−4 were characterized by spectroscopic
methods and by low-temperature single-crystal X-ray diffrac-
tion, with POVRAY diagrams for 1−4 given in Figures 1−4,
respectively. Unlike the recently described Th4(μ3-E)4(μ2-
EPh)4(η-E′C6F5)4 cubanes1j with fluorinated chalcogenolate
ligands that crystallize only in the tetragonal I4- cell, or in
lanthanide chemistry, where surface ER ligands can have a
dramatic impact on cluster structure,38 compounds 1−4
crystallize in a number of lower symmetry unit cells, as
shown in Table S1 (Supporting Information), with all having
the same basic structure. PXRD data for bulk isolated products
show that it is possible to have more than one crystalline phase
present for 2, but the P21/n phase listed in Table S1 is
reproducibly the dominant phase. Upon the basis of PXRD
measurements on bulk samples of compounds 1, 3, and 4, the
only phase present is the corresponding phase listed in Table
S1.
phase, space group P1, with formula = C125.5H117.5N15.5Se12Th4, fw =
̅
3718.52, a = 15.282(2) Å, b = 16.885(2) Å, c = 25.893(4) Å, α =
84.915(3), β = 77.498(3)°, γ = 80.255(3)°, V = 6419.6(15) Å3, Z = 2,
D(calcd) = 1.823 g/cm−3, T = 100(2) K. The CIF files for the above
C2/c (2b) and P212121 (2c) phases of 2 and the four compounds
reported in this paper were deposited in the Cambridge Structural
Database with CCDC numbers 1580072, 1580077, 1580076, 1580075,
1580073, and 1580074 for compounds 1, 2, 3, 4, 2b, 2c, respectively.
Thermal ellipsoid diagrams for 1−4 are shown in Figures 1−4,
respectively. Complete crystallographic details for 1, 2, 3, and 4, and
fully labeled thermal ellipsoid plots, are given in the Supporting
Information
Computational Details. All structures were fully optimized at the
PBE0 level of theory,22 including an atom-pairwise correction for
dispersion forces via Grimme’s D3 model23 with Becke-Johnson (BJ)24
damping in the Turbomole program.25 Quasirelativistic energy-
consistent small-core pseudopotential (effective-core potential,
ECP)26 along with (14s13p10d8f1g)/ [10s9p5d4f1g] Gaussian-type
orbital valence basis set was used for thorium, while ligand atoms were
treated with an all-electron def2-TZVP basis set.27 Relativistic all-
electron density functional theory (DFT) calculations of the nuclear
shielding were performed using the Amsterdam Density Functional
(ADF) program suite,28 employing a customized PBE0 exchange-
correlation functional with 40% exact-exchange admixture (PBE0−
40HF)29 in conjunction with Slater-type orbital basis sets of triple-ζ
doubly polarized (TZ2P) quality for thorium, selenium, and sulfur
atoms and double-ζ polarized (DZP) quality for lighter ligand atoms
(C, H, N, F). Because of the large size of tetrametallic clusters, an
integration accuracy of 4 was used. Both scalar and spin−orbit
relativistic effects were treated by the two-component zeroth-order
All four structures contain the same (py)8Th4(μ3-E)4(μ2-
E′Ph)4(η-E′Ph)4 tetrameric core, with different numbers and
D
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Figure 3. Thermal ellipsoid diagram of (py)8Th4(μ3-S)4(μ2-SePh)4(η-
SePh)4 (3) with the H atoms removed for clarity and ellipsoids at the
50% probability level. Significant bond-length averages are given in
Table 1.
Figure 1. Thermal ellipsoid diagram of (py)8Th4(μ3-S)4(μ2-SPh)4(η-
SPh)4 (1) with the H atoms removed for clarity and ellipsoids at the
50% probability level. The view direction for Figures 1−4 is along the
4 bar or pseudo 4 bar axis (see text). Significant bond-length averages
are given in Table 1.
Figure 4. Thermal ellipsoid diagram of (py)8Th4(μ3-Se)4(μ2-SPh)4(η-
SPh)4 (4) with the H atoms removed for clarity and ellipsoids at the
30% probability level. There is only one crystallographically
independent set of (py)2ThSe(SPh)2 atoms. Significant bond-length
averages are given in Table1.
Figure 2. Thermal ellipsoid diagram of the first of two molecules in
crystals of (py)8Th4(μ3-Se)4(μ2-SePh)4(η-SePh)4 (2) with the H
atoms removed for clarity and ellipsoids at the 50% probability level.
The second unique molecule in compound 2 differs from the first only
by the minor rotations of py and EPh ligands, and its diagram can be
found in Supporting Information. Significant bond-length averages are
given in Table 1.
orientations of lattice pyridine molecules. Cluster cores as
shown in Figure 5 are defined by a significantly distorted Th4E4
cube, with the four equivalent “side” Th2E2 square faces
containing μ2-EPh ligands that bridge the two Th atoms on
each side face. Unlike the slightly distorted Th2E2 squares on
the sides, the “top” and “bottom” Th2E2 faces are highly
distorted rhomboids, as shown in Figure 5. Comparisons of key
interatomic geometries in 1−4 are given in Table 1.
Individual bond distances are all consistent with prior
literature, with narrow ranges of Th−N,1j,9a,d,39 Th−E,1j,40 or
Th−EPh1j,7a,9a,d,39b and values that reflect the atomic sizes,41 as
shown in Table 1. A very good agreement of the X-ray
determined Th−E/Th−E′ bond lengths with those optimized
at the DFT (PBE0-D3(BJ)/ECP/def2-TZVP) level (cf. Table 2
and Table 3) is observed.
Figure 5. Generalized side and top views of the common core
Th4E12N8 region for thorium cubanes 1−4. The view directions are
normal to the squarelike and rhomboidlike faces, respectively, for the
unique side and top views.
of possible weak H-bonding interactions but also of the
conformational flexibility of μ-E′Ph, η-E′Ph, and py ligands.
The magnitude and consistency of these values, as well as those
of the torsion angles about Th−N and Th−E′, indicate that
there is limited flexibility in the ligands for any one of these
compounds. Additionally, to locate significant conformational
We considered the close intramolecular H atom contact
distances in 1−4 as given in Table 2 to be a measure not only
E
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Table 1. Ranges of Selected Distances (Å), Angles and Torsions (degrees) in X-ray Structures of 1−4
E,E′ = S, S
E,E′ = Se, Se
E,E′ = S, Se
E,E′ = Se, S
distance
Th−N
Th−μ3E (side)
Th−μ3E (top)
Th−μ2E′Ph
1
2
3
4
a
2.628−2.691(5)
2.763−2.772(2)
2.786−2.854(2)
2.990−3.046(2)
2.860−2.876(2)
4.011−4.024(<1)
4.522−4.525(<1)
1
2.633−2.697(9)
2.894−2.913(1)
2.923−2.973(1)
3.069−3.150(1)
2.970−3.015(1)
4.139−4.164(<1)
4.652−4.722(<1)
2
2.630−2.709(3)
2.760−2.778(1)
2.798−2.849(1)
3.107−3.164(1)
2.986−3.000(<1)
4.037−4.051(<1)
4.508−4.517(<1)
3
2.651−2.697(<1)
2.909(<1)
2.943−2.952(1)
2.970−3.020(1)
2.884(1)
Th−ηE′Ph
Th···Th′ (side)
Th···Th″ (top)
bond angle
4.116(<1)
4.708(<1)
4
Th−μ3E−Th (side)
Th−μ3E−Th (top)
Th−μ2E′−Th (side)
Th−μ2E′−C
91.2−92.6(<1)
106.7−106.7(<1)
83.3−84.2(<1)
117.1−119.8(2)
114.3−115.6(2)
1
89.21−90.54(3)
104.75−107.33(3)
83.15−84.12(3)
112.4−118.8(4)
112.0−114.0(4)
2
92.06−93.17(3)
106.15−106.23(3)
80.15−80.93(1)
113.7−117.4(1)
112.8−114.0(1)
3
89.22−89.39(1)
105.98(2)
86.81(3)
117.5−118.4(2)
113.8(2)
Th−ηE′−C
b
torsion angle
4
Th−μ2E′−C−C
μ3E−Th−ηE′−C
Th···Th−N−C
μ3E···μ2E′−C−C
44−53
46−47
4−5, 102−106
1.8, 5.3
42−59
25−53
1−14, 92−99
0.4−9.8, 0.1−5.4
2
37−49
45−48
2−8, 104−107
1.2, 8.7
48
39
0, 93
3.1
c
intramolecular
1
3
4
Cβ(ηE′)···μ3E
3.51, 3.53
3.64, 3.66
6.39, 6.46
6.43, 6.44
3.38−3.70, 3.42−3.47
3.73−3.86, 3.77−3.79
6.14−6.48, 6.19−6.44
3.65, 3.64
3.59
3.73
6.41
6.48
Cβ(μ2E′)···μ3E
3.72, 3.73
cen(ηE′-Ph)···core
cen(μ2E′-Ph)···core
6.54, 6.58
6.61−6.66, 6.61−6.63
6.56, 6.59
a
b
Average ESD for values contributing to the range are given in parentheses. Absolute values are used here, and all ESDs are less than 1.0°. For Th−
Th−N−C, both Th atoms are on the same side, as defined in Figure 5, and the two ranges separated by a comma correspond to the two py bonded
to Th atoms. For compound 2, ranges for both unique molecules are given. Complete atomic labeling, symmetry operations, and values for specific
geometric parameters can be found in the Supporting Information. These intramolecular distances relate to the conformation of the rotatable E′-Ph
group and the distance between the centroids of its phenyl ring and the centroid of the “core” Th4E4E′8N4 region.
c
a
Table 2. Selected H···E Contact Geometries in 1−4
were clearly located about the terminal E′Ph groups, as
depicted in Figure 6 and listed in Table 4.
intramolecular
1
2
3
4
Cubanes 1−4 and the recently described Th clusters with
fluorinated chalcogenolates1j afford an opportunity to establish
the impact of ring fluorination on the Th−E bond length for
thiolate and selenolate ligands. Both Th−S and Th−Se bond
lengths are elongated when the aromatic ring is fluorinated. For
thiolates, the terminal Th−S(C6F5) bonds [2.904(2) Å in
H(μ2E′)···μ3E
2.82, 2.84
2.72, 2.96
3.01, 2.94
3.39, 3.33
3.39, 3.37
3.41, 3.16
3.23, 3.33
3.88, 2.98
3.74, 3.16
2.85, 2.78
2.88, 2.84
3.07, 2.97
2.95, 2.90
3.05, 3.05
3.12, 3.15
2
2.88, 2.89
2.89
3.17
3.43
3.33
2.66
2.93
3.19
4
H(py2)···μ3E
H(py1)···μ3E
H(ηE′)···μ3E
H(py1)···μ2E′
H(py1)···ηE′
H(py2)···ηE′
3.09, 2.98
3.24, 3.52
3.04, 3.08
2.69, 2.74
2.89, 2.91
3.36, 3.20
3.12, 3.01
3.26, 3.48
3.19, 3.21
2.80, 2.80
2.98, 2.98
3.33, 3.21
(py)8 Th4 S4 (SPh)4 (SC6 F5 )4 and 2.899(2)
Å in
(py)8Th4Se4(SePh)4(SC6F5)4]1j are longer than the terminal
Th−S(Ph) bonds in the present work [2.860−2.876(1) Å in 1
and 2.884(1) Å in 4] by ∼0.03 Å. Similarly for selenolates, the
terminal Th−Se(C6 F5 ) bonds [3.020(1)
Å in
(py)8Th4Se4(SePh)4(SeC6F5)4 and 3.030(1) Å in
(py)8Th4S4(SPh)4(SeC6F5)4]1j are also longer than the
terminal Th−Se(Ph) bonds in 2 and 3 [2.989−3.005(1) Å in
2 and 2.986−3.000(1) Å in 3] by ∼0.03 Å. This lengthening is
consistent with the polarizing ability of the fluorine
substituents, namely, the withdrawal of negative charge from
the chalcogen atom E in the EC6F5 group resulting in less
electrostatic attraction and covalent bonding between Th and
E. This polarization effect is reflected in the atomic partial
charges and delocalization indices given in Table 3. A similar
impact on M−E bond lengths was noted in analogous
lanthanide systems.42
intermolecular
H(py)···ηE′
H(μ2E′)···ηE′
solvent···ηE′
1
3
2.92, 2.88
>3.6
3.07, 2.97
3.13, 3.12
3.21, 3.11
2.96, 2.95
>3.6
3.01, 3.41
3.54
3.03, 3.16
3.15
>3.6
a
Italicized values are those less than the van der Waals radii sum
(Bondi), 2.8 Å for H···N, 3.0 Å for H···S, and 3.1 Å for H···Se. All H
atoms are at calculated positions for sp2 geometry and C−H distance
of 0.95 Å (SHELXL). Average ESDs are 0.01−0.02 Å for distances and
0.05−0.10° for angles. Note that, because of molecular site symmetry,
there are two, four, two, and one unique values for named contacts for
compounds 1, 2, 3, and 4, respectively.
A comparison of Th−ligand and U−ligand bond lengths is
also possible. A related uranium cubane, (py)8U4(μ3-Se)4(μ2-
SePh)4(η-SePh)4, has been described,1k crystallizing in the
monoclinic space group C2/c as observed for 1, 2, and 3, but
not for 4. The P21/n phase of 2 was the most commonly
variations on going from 1 to 4, molecular overlay calculations
(CCDC Mercury) were performed, and the largest deviations
F
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Table 3. DFT Optimized Th−E Bond Lengths (in Å), QTAIM Delocalization Indices, DI(Th−E), as a Measure of the Th−E
a b
,
Bond Covalency, and CM5 Atomic Charges at Thorium and Chalcogen Centers in Selected Systems
Th−E bond lengths
DI(Th−E)
CM5 atomic charges
compound
Th−μ3E Th−μ2E Th−ηE Th−μ3E Th−μ2E Th−ηE
Th
μ3E
μ2E
ηE
(py)8Th4(μ3-S)4(μ2-SPh)4(η-SPh)4 (1)
(py)8Th4(μ3-Se)4(μ2-SePh)4(η-SePh)4 (2)
(py)8Th4(μ3-S)4(μ2-SePh)4(η-SePh)4 (3)
(py)8Th4(μ3-Se)4(μ2-SPh)4(η-SPh)4 (4)
(py)8Th4(μ3-Se)4(μ2-SePh)4(η-SeC6F5)4
(py)8Th4(μ3-Se)4(μ2-SePh)4(η-SC6F5)4
2.786
2.922
2.791
2.920
2.934
2.934
2.871
2.987
3.114
3.109
2.995
3.093
3.095
3.040
2.836
2.974
2.982
2.827
3.008
2.870
2.943
0.531
0.532
0.533
0.528
0.525
0.523
0.565
0.338
0.352
0.344
0.337
0.366
0.363
0.445
0.519
0.529
0.516
0.529
0.488
0.484
0.712
0.626
0.666
0.662
0.635
0.558
0.546
0.627
−0.403
−0.428
−0.372
−0.462
−0.316
−0.323
−0.402
−0.182
−0.240
−0.261
−0.163
−0.248
−0.252
−0.214
−0.277
−0.338
−0.338
−0.279
−0.308
−0.253
−0.256
Th4(μ3-Se)4(μ2-SePh)4(η-SePh)4 (fully desolvated
cluster of 2)
(py)3Th(SeC6F5)4
(py)4Th(SeC6F5)4
(py)3Th(SePh)4
(py)4Th(SePh)4
Ph2Se
2.920
2.980
2.915
2.967
0.589
0.511
0.600
0.530
0.759
0.790
0.757
0.775
−0.269
−0.287
−0.310
−0.321
−0.038
+0.028
(C6F5)2Se
a
b
PBE0-D3(BJ)/ECP/def2-TZVP results (cf. Computational Details). Only averaged values over chemically equivalent atoms/bonds are given here.
While the U4Se4 cluster1k and the fully characterized P21/n
phase of 2 presented here are not isostructural, they are
isomorphous, and so a comparison of average bond lengths is
appropriate. There is a significant difference in the ionic radii of
eight-coordinate Th(IV) and U(IV), with the uranium ion
smaller by 0.05 Å,41b and this difference should be reflected
systematically in all metal−ligand interatomic separations.
These expected differences are observed: the average differ-
ences between Th−L and U−L bond lengths (Δ) are all within
0.02 Å of that predicted by using appropriate ionic radii41b for
all four ligands: An−N(py) (Th, 2.671 Å; U−N; 2.619 Å,
Δ
Th−U = 0.052 Å), An−Se2− (Th, 2.940 Å; U, 2.868 Å; ΔTh−U
=
0.072 Å), terminal An−Se(Ph) (Th, 2.994 Å; U, 2.927 Å;
ΔTh−U = 0.067 Å), and bridging An−Se(Ph) (Th, 3.108 Å; U,
3.061 Å; ΔTh−U = 0.047 Å).
Figure 6. Molecular overlay of compounds 1 (red) and 2 (blue) using
the “calculate overlap” feature in CCDC graphics program Mercury.
The Th, Se, S, and N atoms are colored cyan, gold, yellow, and blue,
respectively. View highlights region of largest deviation, namely,
rotated terminal E′Ph groups and accompanying py nearest to the
E′Ph. Relevant conformational angles and interatomic distances are in
Table 2.
1
77Se NMR and Solution Structure. Both H and 77Se
NMR spectroscopy can be used to probe solution structure,
with 77Se data being particularly useful because of the large
chemical shift dispersion of this spin-half (I = 1/2) nucleus.
The 77Se NMR shifts for the mononuclear, tetraselenolate
compounds (py)nTh(SeAr)4 (Ar = Ph, C6F5; n = 3 or 4), the
Se-containing thorium cubanes, and selected, purely organic
arylselenides are summarized in Table 5. The data show an
excellent match between experimental and DFT-calculated data
(R2 = 0.996; root-mean-square deviation (RMSD) = 15 ppm),
allowing for a reliable computer-aided signal assignment and
structure elucidation. In addition, 77Se shifts in Th−Se species
are very sensitive to subtle structural changes, allowing us to
establish the number of metal-coordinated solvent molecules in
solution. This sensitivity is evident, for example, for the
(py)nTh(SeAr)4 (Ar = Ph, C6F5) series, where complexes with
four pyridines coordinated directly to thorium center are
predicted to display 77Se resonances shifted upfield by ca. 80−
90 ppm as compared to their (py)3Th(SeAr)4 congeners (cf.
Table 5). The shielding upon additional solvent coordination
could be primarily ascribed to a Th−Se bond elongation
(reduced Th−Se bond covalency) and electron-density
accumulation at the chalcogen centers (cf. Table 4). The
experimental and computed 77Se shifts for mononuclear
complexes in pyridine-d5 confirm thus the different Th
coordination-environment for systems with SePh (n = 3) and
Table 4. Short H···H Distances (Å) for the Closest py···E′Ph
a
Interactions in 1−4 (see Figure 6)
1
2
3
4
H(μ2E′)···H(py)
2.60, 2.45 2.68, 3.54, 2.52,
2.61
2.74, 2.64 2.61
outer
2.93, 2.97, 2.64,
2.56
H(μ2E′)···H(py)
2.77, 2.69 2.56, 2.62, 3.28,
2.80
2.66, 2.65 3.08
inner
2.60, 2.78, 3.05,
2.69
a
There are up to four instances of these py···E′Ph contacts per
Th4(μ3-E)4(μ2-EPh)4(η-E′Ph)4 compound, namely, one for each
unique terminal E′Ph ligand. The closest py ligand is bonded to the
same Th atom as the E′Ph; ESDs are 0.01−0.02 Å. The terms “outer”
and “inner” refer to the relative position of the contact with respect to
the molecular centroid for the C(2) versus C(6) atom of the E′Ph
ring.
produced form, and its crystal structure results were the most
precisely determined of the four phases we observed for 2.
G
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a
Table 5. Summary of Experimental and DFT-Computed 77Se NMR Shifts (in ppm vs Me2Se) for Selected Systems
calcd δ(77Se)
expt δ(77Se)
b
b
compound
μ3-Se
μ2-SePh
η-SeAr
μ3-Se
μ2-SePh
η-SeAr
Th4Se4(μ2-SePh)4(η-SePh)4(fully desolvated cluster of 2)
(py)8Th4Se4(μ2-SePh)4(η-SePh)4 (2)
1150
951
904
890
552
485
477
478
806
545
343
928
915
904
460
480
479
573
369
1j
(py)8Th4Se4(μ2-SePh)4(η-SeC6F5)4
1j
(py)8Th4Se4(μ2-SePh)4(η-SC6F5)4
(py)3Th(SeC6F5)4
490
402
644
565
517
469
415
274
98
9a
(py)4Th(SeC6F5)4
400
659
9a
(py)3Th(SePh)4
(py)4Th(SePh)4
PhSSePh11
PhSeSePh43
Ph2Se43
C6F5SePh43
(C6F5)2Se43
PhSeH43
520
456
402
265
110
155
152
a
b
NMR shifts computed at the 2c-ZORA(SO)/PBE0−40HF/TZ2P/DZP level using a COSMO solvation model (cf. Computational Details). Ar =
Ph or C6F5.
SeC6F5 (n = 4) ligands in solution, consistent with their solid-
state structures.
Se(4p) character (see Figure S17 in Supporting Information).
The latter are energetically higher for μ3-Se-centered MOs than
for those localized at μ2-Se or η-Se, that leads to lower
excitation energies (ΔE) appearing in the denominator of the
Ramsey formula and thus larger paramagnetic shielding for μ3-
Se2− atoms. The diamagnetic and spin−orbit shielding
contributions are comparable with those in mononuclear
Th−Se compounds, and the trends in NMR shifts are dictated
solely by σp shielding, consistent with previous findings.5c We
note in passing that more extreme deshielding for the μ3-Se2−
ligand, with δ(77Se) = +2006 ppm, was found for a
titanium(IV) selenido cluster, [(η-C5H5)Ti]4(μ2-Se)3(μ3-
Se)3.44
Measurements of selenido clusters are equally informative.
Solution 77Se NMR analysis of cubane 2 shows three 77Se
resonances at 928 ppm (μ3-Se), 460 ppm (μ2-SePh), and 573
ppm (η-SePh) that are assigned by comparison with both the
Se2− (904 ppm) and μ2-SePh (479 ppm) resonances in
1j
(py)4Th4(μ3-Se)4(μ2-SePh)4(η-SC6F5)4 and the resonance at
659 ppm for the terminally bound SePh in (py)3Th(SePh)4.
The 77Se NMR shifts computed for fully desolvated Th cubanes
are generally more deshielded than those in pyridine-
coordinated clusters (cf. Table 5), more so for “in-cube”, μ3-
Se, and terminal, η-Se, selenium atoms (>140 ppm), while
somewhat smaller deshielding (∼40−70 ppm) is observed for
bridging, μ2-SeAr, ligands. An excellent match between
experimental and DFT-computed 77Se shifts for pyridine-
coordinated Th cubanes supports thus the maintenance of the
tetrametallic Th4E4 structures (coordinated by two pyridine
molecules at each Th center) in solution, with no exchange
between bridging and terminal SePh sites on the NMR time
scale. Cubane 4 is also static in solution, with two sets of SPh
Generally, upfield 77Se shifts for SeC6F5-containing com-
pounds as compared to their nonfluorinated SePh congeners
were also found for some organoselenides (see Table 5),
although the former possess more positive charge at the Se
center (cf. Table 3). This somewhat counterintuitive behavior
can be ascribed to a less efficient π-conjugation of the Se
electron lone pairs with the perfluorinated aryl group(s), as
evident, for example, from the more acute C−Se−C bond
angle, the larger selenium lone-pair occupancy, and/or notably
smaller participation of Se(4p) in the HOMO−5 orbital (with
the most dominant contribution to the σp(77Se); HOMO =
highest occupied molecular orbital) when comparing (C6F5)2Se
with Ph2Se (cf. Figures S18 and S19 in Supporting
Information). The expected 77Se deshielding upon introducing
electronegative fluorine substituents is, however, observed
when comparing organoselenium compounds with Se-bound
fluorinated and nonfluorinated alkyl groups.45,43
Cluster Reactivity. Ligand-based redox chemistry outlined
in Scheme 3 was confirmed synthetically in a preparative scale
reaction of cluster 2 with PhSSPh. The reaction (Scheme 4)
was conducted at 25 °C using stoichiometric amount of
PhSSPh, and the resulting colorless crystals were verified by
single-crystal X-ray diffraction as the mixed chalcogenide
compound 4.
1
resonances observed in the H NMR spectrum at 25 °C.
We note further that, although the 77Se shifts for “in-cube”
μ3-Se atoms are close to that measured for [K(18-crown-
6)][Th(Se)(NR2)3] [δ(77Se) = 992 ppm in pyridine-d5], which
contains a highly polarized triple Th−Se bond [d(Th−Se) =
2.653 Å; DI(Th−Se) = 1.266] (DI = delocalization index),5c
both NLMO and QTAIM analyses show strongly polarized
single Th−E bonds in compounds 1−4 (cf. Table 4; note that
the DI integrates the electron density in the bonding region
between two atoms in question and is closely related to the
covalent bond order, reduced by bond polarity; that is, DI = 1.0
for a “pure” covalent single bond, and DI = 2.0 for a pure
covalent double bond, but DI = 0.0 for a pure ionic bond). This
is also reflected in notably longer Th−Se distances, which vary
from 2.894 to 2.973 Å (cf. Table 1). Despite much smaller Th−
E bond covalency, the strong 77Se deshielding of the μ3-Se2−
ligand atoms can be attributed to the presence of multiple
adjacent Th(IV) centers. These form low-lying unoccupied
MO orbitals, composed largely from Th(6d) atomic orbitals
(with a minor Se(4p) contribution), and are magnetically
coupled with suitable occupied MOs with a predominant
This chalcogenolate exchange reaction is clearly driven by
electrostatic forces, and the system affords an opportunity to
also evaluate the reversibility of the E/EPh redox reactions in
Schemes 1 and 2. In an attempt to explore the complete
replacement of Se by S in the Se/SePh cluster, 2 was mixed
H
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Scheme 5. Thermolysis of the Heterochalcogen Compounds 3 and 4
with PhSSPh (1:8) at elevated temperatures. Surprisingly, there
in any system where chalcogen exchange reactions are possible,
aryl migration could favor the elimination of PhSPh over
PhSePh, with the consequent formation of a Se-rich solid-state
product.
was no evidence for the formation of 1 or 4, but instead
9a
(Scheme 4) monometallic (py)4Th(SPh)4 was isolated in
high yield, with PXRD characterization indicating that this was
the only crystalline product formed. The remarkable range of
organic products observed is consistent with earlier work on
solution speciation of R2E2/R′2E′2 mixtures.11 While it has
previously been shown that larger chalcogenido clusters can be
fragmented46 under appropriate conditions, this is the first time
that the reversibility of these ligand-based E/PhE−/E2−/
PhEEPh redox reactions has been demonstrated. This reactivity
is consistent with previous work showing that M(SPh)x react
with S but not Se.
CONCLUSIONS
■
A family of (py)8Th4(μ3-E′)4(μ2-EPh)4(η-EPh)4 (E, E′ = S, Se)
cubane clusters were prepared for all combinations of E, E′ = S,
Se, using ligand-based redox reactions in which PhE anions
reduce elemental E or when PhS− reduces PhSeSePh. These
clusters crystallize in a variety of unit cells, always with the same
basic core structure. NMR measurements indicate that solid-
state structure is maintained in solution and that the cluster
surface EPh ligands are static on an NMR time scale. The
selenido compounds are all unstable with respect to reactions
that replace Th−Se bonds with Th−S bonds. Measured and
calculated 77Se NMR shifts are in a good agreement and
confirm the Th coordination numbers in solution. Thermolysis
of the heterochalcogen compounds suggests the formation of a
ThSxSe2−x alloy.
Thermolysis. Molecular thermolysis9a,47 is potentially useful
as a low-temperature approach to the synthesis of metastable
solid-state materials, and the heterochalcogen compounds 3
and 4 present a unique opportunity to explore the low-
temperature preparation of ternary solid-state materials.
Thermolyses of 3 and 4 were performed at the same
temperature conditions as for previously reported9a monomeric
Th(EPh)4 compounds (Scheme 5). The PXRD analyses
revealed that the products do not contain a mixture of
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00836.
■
48
49
previously reported ThS2 and ThSe2 solid-state materials,
but, instead, a new ThSxSe2−x alloy phase is observed. The
crystallinity of the thermolysis product from 3 differed slightly
from that of the product from 4, suggestive of different cooling
rates for the two thermolysis runs, rather than significantly
different S/Se ratios.
S
1
Calculated and observed PXRD profiles for 1−4, IR, H
and 77Se NMR spectra, more detailed analysis of 77Se
NMR shifts, SEM images (PDF)
According to scanning electron microscopy (SEM), the
resulting powders of the thermal residuals for compounds 3
and 4 have different surface morphologies (Figure 7). Both
Cartesian coordinates of the selected DFT-optimized
structures (XYZ)
Accession Codes
CCDC 1580072−1580077 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bren@rci.rutgers.edu.
■
Figure 7. SEM images of thermolysis products (in 500 nm scale) for
(a) compound 3 and (b) compound 4.
ORCID
Peter Hrobarik: 0000-0002-6444-8555
John G. Brennan: 0000-0002-3994-221X
Author Contributions
The manuscript was written through contributions of all
authors.
́
samples were studied by EDS. The results (Th/S/Se atomic
ratios for thermolysis products for compounds 3 and 4 are
correspondently 1:0.68:0.64 and 1:0.83:0.53) suggest that the
identity of chalcogenido ligand does not have a tremendous
impact on the Se/S ratio in the thermolysis products of mixed
chalcogen thorium compounds 3 and 4. The approximate
amount of Se is larger than the amount of S in both thermolysis
products with Se/S ratios in solid residuals of 1.56 and 2.32, for
4 and 3, respectively. The volatile products from both thermal
reactions were analyzed by GC-MS and shown to contain
pyridine, PhSPh, and PhSePh in both thermolyses. The
abundance of Se is presumably related to the fact that C−Se
bonds cleave at lower temperatures than do C−S bonds, and so
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under CHE-1464887. The computational
work has been supported by the Berlin DFG excellence cluster
on Unifying Concepts in Catalysis (UniCat) and by the Slovak
I
DOI: 10.1021/acs.inorgchem.8b00836
Inorg. Chem. XXXX, XXX, XXX−XXX

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Scientific Grant Agency VEGA (Project Nos. 1/0507/17 and
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