Experimental evidence for magnetic exchange in Di-and trinuclear uranium(IV) ethynylbenzene complexes

Article abstract of DOI:10.1021/ic901986w

We report the preparation andmagnetic property investigations of a structurally related family ofmono-, di-, and trinuclear U(IV) aryl acetylide complexes. The reaction between [(NN ₃)UCl] and lithiated aryl acetylides leads to the formation of the hexacoordinate complexes [(NN ₃)U(CCPh)₂(Li 3 THF)] (1) and [(NN ₃) ₂U₂(p-DEB)(THF)] (2) as red-brown and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN ₃)U] (bit-NN₃ = [N(CH₂CH ₂NSitBuMe₂)2(CH₂CH₂SitBuMeCH ₂]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN₃)U(CCPh)] (3), [(NN₃)₂U ₂-(m-DEB)] (4), [(NN₃)₂U₂(p-DEB)] (5), and [(NN₃)₃U₃(TEB)] (6), where NN ₃ = [N(CH₂CH₂NSitBuMe₂)3]. The measured magnetic susceptibilities for 1-6 trend toward non-magnetic ground states at low temperatures. Nevertheless, the di-and trinuclear pentacoordinate compounds 4-6 appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data, using the spin Hamiltonian H =-2J(S _iSi* S _j). These results are consistent with weak ferromagnetic coupling for complexes 4-6 (J = 4.76, 2.75, and 1.11 cm ^-1, respectively), while the fit for 2 is consistent with a near-negligible exchange interaction (J =-0.05 cm^-1). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes of 3-5. The mononuclear complex shows a triplet ground state with singly occupied degenerate f orbitals. The meta-and para-bridged species are computed to show very weak ferro-and antiferromagnetic coupling, respectively. All three complexs show only small net spin density on the acetylide-containing ligands. The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at-1.02 V versus [Cp₂Fe]^+/0, assignable to an oxidation of U(IV) to U(V).

Full text of DOI:10.1021/ic901986w

Inorg. Chem. 2010, 49, 1595–1606 1595
DOI: 10.1021/ic901986w
Experimental Evidence for Magnetic Exchange in Di- and Trinuclear Uranium(IV)
Ethynylbenzene Complexes
ꢀ
Brian S. Newell, Anthony K. Rappe, and Matthew P. Shores*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Received October 8, 2009
We report the preparation and magnetic property investigations of a structurally related family of mono-, di-, and trinuclear
U(IV) aryl acetylide complexes. The reaction between [(NN03)UCl] and lithiated aryl acetylides leads to the formation
of the hexacoordinate complexes [(NN03)U(CCPh)2(Li THF)] (1) and [(NN03)2U2(p-DEB)(THF)] (2) as red-brown
3
and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN03)U] (bit-NN03 =
[N(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes
affords the yellow-green pentacoordinate ar0ylacetylide complexes [(NN03)U(CCPh)] (3), [(NN03)2U2-
(m-DEB)] (4), [(NN03)2U2(p-DEB)] (5), and [(NN 3)3U3(TEB)] (6), where NN03 = [N(CH2CH2NSitBuMe2)3]. The
measuredmagnetic susceptibilitiesfor 1-6 trendtoward non-magnetic groundstatesatlowtemperatures. Nevertheless,
the di- and trinuclear pentacoordinate compounds 4-6 appear to display weak magnetic communication between
the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data,
^
^ ^
usingthe spin Hamiltonian H = -2J(Si Sj). These results are consistent withweak ferromagnetic couplingfor complexes
3
4-6 (J = 4.76, 2.75, and 1.11 cm-1, respectively), while the fit for 2 is consistent with a near-negligible exchange
interaction (J = -0.05 cm-1). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out
(spin-orbit coupling omitted) on model complexes of 3-5. The mononuclear complex shows a triplet ground state with
singly occupied degenerate f orbitals. The meta- and para-bridged species are computed to show very weak ferro- and
antiferromagnetic coupling, respectively. All three complexes show only small net spin density on the acetylide-containing
ligands. The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at -1.02 V versus [Cp2Fe]þ/0
,
assignable to an oxidation of U(IV) to U(V).
Introduction
normally associated with transition metal ions (e.g., super-
exchange) and lanthanides (e.g., spin-orbit coupling),11 and
can be used to probe electronic structure in detail. Thus,
combining magnetochemical studies with high level calcula-
tions offers a pathway for understanding this unique group of
compounds.
In addition to fundamental interest in electronic structure,
recent work in f-element magnetochemistry is motivated by
the potential for these species to contribute to the develop-
ment of single-molecule magnets (SMMs).12-15 These mono-
disperse superparamagnetic particles exhibit a thermal
barrier to magnetic spin reorientation, and may eventually
The electronic structures of actinide-containing complexes
feature a rich interplay of orbital interactions, spin-
orbit coupling, and electron correlation, whose understand-
ing is critical to using actinides in fuels or catalysis, and to
settle longstanding questions about the role of f orbitals in
metal-ligand bonding.1-10 The magnetic properties of
actinide complexes represent a mixing of characteristics
*To whom correspondence should be addressed. E-mail: shores@lamar.
colostate.edu.
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1596 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
find use in data storage,16,17 quantum computing,18-23 or
refrigeration applications.24,25 However, their exploita-
tion awaits variants that can display magnetic bistability
at more practical temperatures than the ∼4.5 K currently
observed.15 Here, incorporation of paramagnetic lantha-
nide ions have received attention, since spin-orbit coupling
and relativistic effects common to those ions can engender
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magnetism. A Th6Mn10 cluster shows that even f0 species
may contribute to the observation of frequency-dependent
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and cubic U(IV) centers has been demonstrated in molecular
species.11,47-50,53-56 Also relevant to the work to be pre-
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well-known that the 5f2 electronic configuration gives dia-
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metry is octahedral, but exhibits paramagnetic ground states
(S = 1) when the U(IV) ion is surrounded by a cubic ligand
field.48-50,52-56,58,59 We wondered if a trigonal bipyramidal
(tbp) coordination geometry may offer another way for
U(IV) to show paramagnetic ground states. In this case,
group theory predicts a doubly degenerate e00 ground state,
which should result in an S = 1 species.58 We note that
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Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1597
predicting the level of splitting of the f orbitals because of
ligand field effects alone is complicated by the substantial
spin-orbit coupling present in the actinides.60 It is also
known that many low symmetry U(IV) complexes give
“non-magnetic” ground states.61 Nevertheless, monomeric
tbp U(IV) phenylacetylide complexes in which triamido-
amine (NN03) ligands occupy the other coordination sites
offer synthetic precedent for enforcing 5-coordinate geo-
metries,62-66 and to our knowledge the magnetic properties
of these species have not been studied in detail. In addition,
ethynylbenzene ligands have been demonstrated to be effi-
cient communicators of spin information between para-
magnetic transition metal species.67,68 Thus, the combina-
tion of [(NN03)U] species with bridging aryl acetylide
ligands may be expected to give rise to di- and trinuclear
assemblies by which uranium magnetochemistry may be
tuned structurally.
pentane. The solids were collected, dried in vacuo, and used
without further characterization. All other reagents were ob-
tained from commercial vendors and used without further
purification.
Caution! Depleted uranium (primary isotope 238U) is a weak
R emitter (4.197 MeV) with a half-life of 4.47 ꢀ 109 years;
manipulations and reactions should be carried out in monitored
fume hoods or in an inert atmosphere glovebox in a radiation
laboratory equipped with R- and β-counting equipment.
[(NN03)U(CCPh)2(Li THF)] (1). Solid, recrystallized [(NN03)-
3
UCl] (0.198 g, 0.261 mmol) was combined with lithium phenyla-
cetylide (0.057 g, 0.53 mmol) and 15 mL of pentane. The yellow-
green mixture was stirred at room temperature for 1 h. Subsequent
addition of 2 mL of tetrahydrofuran (THF) resulted in a color
change to red-brown. The mixture was stirred at room tempera-
ture for 1 h, and then filtered to remove LiCl. Volatiles were
removed from the filtrate in vacuo to afford a red-brown residue.
This was extracted into 10 mL of pentane, concentrated to about
5 mL under reduced pressure, and left at ambient temperature for
8 h, at which point several red-brown crystals were observed. The
product was collected by filtration, dried in vacuo, and recrystal-
lized from hot pentane to afford a deep red crystalline solid (0.121
g, 46% yield based on [(NN03)UCl]). Single crystals suitable for
X-ray analysis were grown from a concentrated pentane solution
maintained at -34 ꢀC for 8 h. Absorption spectrum (pentane)
Herein, we describe the preparation and (magneto)-
structural characterization of di- and trinuclear penta- and
hexacoordinate U(IV) species bridged by aryl acetylides. The
experimental and theoretical assessment of exchange cou-
pling in these species provides evidence for weak exchange
coupling operative between pentacoordinate U(IV) centers.
λ
max (εM):686nm (95 L mol-1 cm-1). 1HNMR (293K, C6D6):δ
3
3
8.09 (br, 6H, CH2), 7.39 (d, 4H, THF), 7.06 (br, 4H, aryl), 6.90 (m,
6H, aryl), 5.22 (s, 27H, Bu), 4.01 (s, 18H, Me2Si), 1.50 (d, 4H,
Experimental Section
t
Preparation of Compounds. Allmanipulations were carried out
either inside a dinitrogen-filled glovebox (MBRAUN Labmaster
130) or via standard Schlenk techniques on a N2 manifold.
Pentane was distilled over sodium metal, degassed (freeze-
pump-thawed 3 ꢀ 20 min) and stored under an atmosphere of
dinitrogen. All other solvents were reagent grade, passed through
alumina, degassed, and stored under dinitrogen. The compounds
UCl4,69 [Li3(NN03)(THF)3] (where NN03 = [N(CH2CH2NSitBu-
Me2)3]),70 [(NN03)UCl],71 [(bit-NN03)U] (where bit-NN03 = [N-
(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]),63 [(NN03)U-
(CCPh)] (3),63 and 1,3,5-triethynylbenzene72 (H3TEB) were
prepared according to the literature, except that sublima-
tion was not carried out on the [(NN03)UCl] complex. The
acetylene ligands 1,4- and 1,3-diethynylbenzene (p-H2DEB
and m-H2DEB, respectively) were purchased from Sigma
and were sublimed or distilled, respectively, before use. The
lithiated acetylides, lithium phenylacetylide and Li2(p-DEB),
were synthesized by reacting the appropriate stoichiometric
amount of n-BuLi with the corresponding free acetylene in
THF), -16.39 ppm (br, 6H, CH2). IR (mineral oil): νCtC 2044
cm-1. Magnetic susceptibility (SQUID, 300 K): μeff = 2.14 μB.
Anal. Calcd for C44H75N4OSi3ULi: C, 52.57; H, 7.52; N, 5.57.
Found: 0C, 52.38; H, 7.76; N, 5.52.
[(NN 3)2U2(p-DEB)(THF)] (2). Solid [(NN03)UCl] (0.500 g,
0.659 mmol) was combined with Li2(p-DEB) (0.045 g, 0.330
mmol) and 5 mL of toluene. The resulting brown-green mixture
was stirred at ambient temperature for 8 h, filtered to remove
LiCl, concentrated to about 2 mL under reduced pressure, and
then cooled to -34 ꢀC. After 8 h, a yellow-green crystalline
precipitate was observed. The crude product was collected by
filtration, dried in vacuo, and recrystallized from hot pentane to
afford a yellow crystalline solid (0.152 g, 28% yield based on
[(NN03)UCl]). Single crystals of [(NN03)2U2(p-DEB)(THF)2]
3
C5H12 (2 THF C5H12) suitable for X-ray analysis were grown
3
3
from a concentrated pentane solution maintained at -34 ꢀC
for 8 h. Absorption spectrum (pentane) λmax (εM): 501 (406), 528
(326), 587 (212), 606 (176), 687 nm (320 L mol-1 cm-1). IR
3
3
(mineral oil): νCtC 2061 cm-1. Magnetic susceptibility (SQUID,
300 K): μeff = 4.73 μB. Anal. Calcd for C62H126N8OSi6U2: C,
45.29; H, 7.72; N, 6.81. Found: C, 45.07; H, 7.22; N, 6.81.
[(NN03)U(CCPh)] (3). A solution of phenylacetylene in 1 mL
of pentane (80 μL, 0.73 mmol) was added dropwise to a stirred
solution of [(bit-NN03)U] (0.539 g, 0.745 mmol) in 10 mL of
pentane at -78 ꢀC, and the resulting yellow-green solution was
warmed to room temperature and stirred for 3 h. The solution
was filtered, concentrated to about 2 mL under reduced pres-
sure, and then cooled to -34 ꢀC. After 8 h, a yellow-green
microcrystalline precipitate was observed. The crude product
was collected by filtration, dried in vacuo, and recrystallized
from hot pentane to afford a yellow-green crystalline solid
(0.400 g, 65% based on [(bit-NN03)U]). Single crystals suitable
for X-ray analysis were grown from a concentrated pentane
solution maintained at -34 ꢀC for 8 h. Absorption spectrum
(pentane) λmax (εM): 281 (4800), 485 (42), 503 (44), 529 (48), 587
(26), 614 (19), 621 (19), 629 (18), 650 (16), 654 (16), 658 (17), 687
(75), 691 (70), 719 (20), 803 (12), 828 (13), 880 (12), 924 (12),
961 nm (10 L mol-1 cm-1). 1H NMR (293 K, C6D6): δ 8.09 (s,
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1997, 36, 5716–5721.
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Organometallics 1998, 17, 5569–5579.
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3436.
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I. J.; Sanders, C. J.; Scott, P. Inorg. Chem. 1999, 38, 3651–3656.
(72) Royles, B. J. L.; Smith, D. M. J. Chem. Soc., Perkin Trans. 1 1994,
355–358.
3
3
6H, CH2), 5.23 (s, 27H, tBu), 4.02 (s, 18H, Me2Si), 3.37 (m, 2H,
aryl), 1.52 (d, 2H, aryl), 1.51 (s, 1H, aryl), -16.35 ppm (s, 6H,
CH2). IR (mineral oil): νCtC 2054 cm-1. Magnetic susceptibility

1598 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
Table 1. Crystallographic Dataa for Compounds [(NN03)U(CCPh)2(Li THF)] (1), [(NN03)2U2(p-DEB)(THF)2] C5H12 (2 THF C5H12), [(NN03)U(CCPh)] (3),
3
3
3
3
[(NN03)2U2(m-DEB)] C5H12 (4 C5H12), and [(NN03)2U2(p-DEB)] C5H12 (5 C5H12
)
3
3
3
3
1
2 THF C5H12
3
3
4 C5H12
3
5 C5H12
3
3
formula
formula wt
color, habit
T, K
space group
Z
a, A
b, A
c, A
R, deg
β, deg
C44H75N4OSi3ULi
1005.32
red/brown needle
100(2)
P21/c
C66H134N8O2Si6U2
1788.56
yellow/green block
100(2)
P1
C32H62N4Si3U
825.16
yellow/green rod
100(2)
P1
C63H130N8Si6U2
1644.35
yellow/green rod
100(2)
P21/c
C63H130N8Si6U2
1644.35
yellow/green cube
100(2)
P21212
4
2
4
4
4
14.6808(3)
18.0721(4)
18.8132(3)
16.6289(5)
16.8054(4)
17.4480(4)
75.127(2)
78.361(2)
67.296(2)
4317.19(19)
1.376
12.4841(16)
17.7695(8)
18.0989(9)
89.375(3)
89.013(3)
77.136(3)
3913.5(3)
1.401
21.6288(13)
17.3104(10)
22.1171(13)
23.2529(10)
18.4727(8)
19.1547(8)
96.3540(10)
107.924(4)
γ, deg
V, A3
4960.72(17)
1.346
0.99
7878.8(8)
1.386
1.06
8227.8(6)
1.269
1.22
dcalc, g/cm3
GOF
1.06
4.35(10.17)
1.01
3.42(9.38)
R1(wR2)b, %
3.36(6.29)
3.17(6.70)
10.54(25.78)
P
P
P
P
a Obtained with graphite-monochromated Mo KR (λ = 0.71073 A) radiation. b R1 = ||Fo| - |Fc||/ |Fo|, wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2
for all data.
(SQUID, 300 K): μeff = 3.12 μB. Anal. Calcd for C32H62N4Si3U:
pentane at -78 ꢀC, and the resulting yellow-green solution was
warmed to room temperature and stirred for 3 h. The solution
was filtered, concentrated to about 2 mL under reduced pres-
sure, and then cooled to -34 ꢀC. After 8 h, a yellow-green
microcrystalline precipitate was observed. The crude product
was collected by filtration, dried in vacuo, and recrystallized
from hot pentane to afford a yellow-green crystalline solid
(0.243 g, 79%, based on H3TEB). Single crystals suitable for
X-ray analysis were grown from a concentrated pentane solu-
tion maintained at -34 ꢀC for 8 h. Absorption spectrum
(pentane) λmax (εM): 292 (21000), 503 (11), 529 (114), 587 (44),
614 (23), 621 (23), 629 (20), 650 (16), 658 (17), 687 (198), 691
(177), 719 (33), 803 (8), 828 (13), 880 (12), 924 (13), 961 nm
(9 L mol-1 cm-1). IR (mineral oil): νCtC 2054 cm-1. Magnetic
C, 46.58; H, 7.57; N, 6.79. Found: C, 46.50; H, 7.21; N, 6.83.
[(NN03)2U2(m-DEB)] (4). A solution of m-H2DEB in 1 mL of
pentane (37 μL, 0.28 mmol) was added dropwise to a stirred
solution of [(bit-NN03)U] (0.407 g, 0.563 mmol) in 10 mL of
pentane at -78 ꢀC, and the resulting yellow-green solution was
warmed to room temperature and stirred for 3 h. The solution
was filtered, concentrated to about 2 mL under reduced pres-
sure, and then cooled to -34 ꢀC. After 8 h, a yellow-green
microcrystalline precipitate was observed. The crude product
was collected by filtration, dried in vacuo, and recrystallized
from hot pentane to afford a yellow-green crystalline solid
(0.336 g, 76% based on m-H2DEB). Single crystals suitable for
X-ray analysis were grown from a concentrated pentane solu-
tion maintained at -34 ꢀC for 8 h. Absorption spectrum
(pentane) λmax (εM): 281 (11400), 485 (73), 503 (74), 529 (78),
587 (35), 614 (21), 629 (20), 650 (17), 658 (18), 687 (138), 691
(124), 719 (26), 803 (11), 828 (14), 881 (13), 925 (14), 961 nm
(11L mol-1 cm-1). 1HNMR(293K, C6D6):δ7.90 (s, 12H, CH2),
3
3
susceptibility (SQUID, 300 K): μeff = 5.45 μB. Anal. Calcd for
C84H184N12Si9U3: C, 43.50; H, 7.56; N, 7.22. Found: C, 43.14;
H, 7.44; N, 6.82.
X-ray Structure Determinations. Structures were determined
for the compounds listed in Table 1. Single crystals were coated
with Paratone-N oil in the glovebox and mounted under a cold
stream of dinitrogen gas. Single crystal X-ray diffraction data
were acquired on a Bruker Kappa APEX II CCD diffractometer
with MoKR radiation (λ=0.71073 A) and a graphite mono-
chromator. Initial lattice parameters were obtained from a least-
squares analysis of more than 100 reflections; these parameters
were later refined against all data. None of the crystals showed
significant decay during data collection. Data were integrated
and corrected for Lorentz and polarization effects using SAINT,
and semiempirical absorption corrections were applied using
SADABS.73 Space group assignments were based on systematic
absences, E statistics, and successful refinement of the structures.
Structures were solved by direct methods or Patterson maps and
were refined with the aid of successive Fourier difference maps
against all data using the SHELXTL 6.14 software package.74
Thermal parameters for all atoms with Z> 3 were refined
anisotropically, except for those disordered over multiple par-
tially occupied sites in the structures of 4 C5H12 and 5 C5H12
3
3
4.77 (s, 54H, tBu), 4.62 (br, 2H, aryl), 3.44 (s, 36H, Me2Si), -0.52
(t, 1H, aryl), -3.96 (s, 1H, aryl), -16.36 ppm (s, 12H, CH2). IR
(mineral oil): νCtC 2053 cm-1. Magnetic susceptibility (SQUID,
300 K): μeff = 4.49 μB. Anal. Calcd for C58H118N8Si6U2: C,
44.31; H, 7.57; N, 7.12. Found: C, 44.72; H, 7.65; N, 6.69.
[(NN03)2U2(p-DEB)] (5). A solution of p-H2DEB in pentane
(0.033 g, 0.26 mmol) was added dropwise to a stirred solution of
[(bit-NN03)U] (0.402 g, 0.556 mmol) in 10 mL pentane at -78 ꢀC,
resulting in the precipitation of a yellow solid. This mixture was
warmed to room temperature and stirred for 3 h. The yellow
precipitate was collected by filtration, dried in vacuo, and
recrystallized from hot toluene to afford a yellow-green crystal-
line solid (0.362 g, 89% based on p-H2DEB). Single crystals
suitable for X-ray analysis were grown from a concentrated
toluene solution maintained at -34 ꢀC for 8 h. Absorption
spectrum (toluene) λmax (εM): 312 (20700), 329 (15800), 363
(4200), 503 (77), 529 (116), 587 (47), 614 (30), 621 (31), 629 (28),
650 (24), 658 (25), 687 (150), 691 (135), 719 (34), 803 (13), 828
(16), 880 (15), 924 (15), 961 nm (12 L mol-1 cm-1). 1H NMR
3
3
and solvate molecules in 5 C5H12. All hydrogen atoms were
3
3
3
assigned to ideal positions and refined using a riding model with
an isotropic thermal parameter 1.2 times that of the attached
carbon atom (1.5 times for methyl hydrogens).
(293 K, C6D6): δ 8.07 (s, 12H, CH2), 7.70 (br, 2H, aryl), 5.54
(s, 54H, tBu), 4.65 (s, 36H, Me2Si), 2.70 (br, 2H, aryl), -16.45
(br, 2H, aryl), -18.04 ppm (s, 12H, CH2). IR (mineral oil) νCtC
2060 cm-1. Magnetic susceptibility (SQUID, 300 K): μeff
=
Data for 4 C5H12 were truncated to 1.0 A resolution during
3
integration because of weak scattering. In the structure of
4.45 μB. Anal. Calcd for C58H118N8Si6U2: C, 44.31; H, 7.57; N,
7.12. Found: C, 44.24; H, 7.53; N, 6.96.
[(NN03)3U3(TEB)] (6). A solution of H3TEB in 1 mL of
pentane (0.020 g, 0.13 mmol) was added dropwise to a stirred
solution of [(bit-NN03)U] (0.306 g, 0.423 mmol) in 10 mL of
(73) Sheldrick, G. M. SADABS, A program for area detector absorption
corrections; Bruker AXS: Madison, WI.
(74) Sheldrick, G. M. SHELXTL, v 6.14; Bruker AXS: Madison, WI, 2004.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1599
Table 2. Selected Bond Distances (A) and Angles (deg) for Crystallographically (1-5) and Computationally Determined (3-5) Structures of the New Mono- and Dinuclear
U(IV) Complexes
1
2
3
3 (calc)
4
4 (calc)
5
5 (calc)
U-C
2.604(3)
2.562(2)
2.6597(19)
2.479(7)
2.475(7)
2.668(5)
2.653(5)
2.285(5)
2.260(5)
2.257(5)
2.284(5)
2.254(5)
2.263(5)
1.210(9)
1.219(9)
176.6(6)
173.0(6)
165.06(19)
161.08(19)
68.03(17)
70.19(17)
69.86(17)
70.20(16)
68.70(16)
69.89(17)
100.7(2)
96.71(18)
124.96(19)
105.16(18)
121.86(19)
97.16(19)
13.0415(5)
2.503(4)
2.571(4)
2.480(4)
2.457
2.490(9)
2.443(9)
2.693(6)
2.673(6)
2.207(6)
2.214(6)
2.230(6)
2.211(6)
2.223(6)
2.229(6)
1.215(10)
1.210(11)
158.2(7)
170.2(7)
174.7(2)
177.4(2)
69.5(2)
69.41(12)
69.4(2)
69.7(2)
70.2(2)
69.9(2)
107.0(2)
107.1(2)
111.0(2)
107.2(2)
111.6(2)
107.8(2)
9.2837(9)
n/a
2.447
2.450
2.664
2.667
2.223
2.226
2.226
2.224
2.227
2.227
1.235
2.31(2)
2.48(2)
2.64(2)
2.73(2)
2.26(2)
2.24(2)
2.26(2)
2.18(2)
2.12(2)
2.28(2)
1.22(3)
1.42(3)
177(2)
2.450
U-Nax (amino)
U-Neq (amido)
2.702(3)
2.668
2.222
2.224
2.225
1.229
179.5
178.0
67.7
2.667
2.223
2.226
2.226
1.236
2.2799(19)
2.293(2)
2.214(3)
2.220(3)
2.2437(19)
2.245(3)
CtC
1.219(3)
1.222(3)
177.8(2)
169.1(2)
109.81(7)
167.32(7)
70.03(6)
1.212(5)
U-C-C
Nax-U-C
Nax-U-Neq
160.9(4)
179.6
179.5
179.6
177.3
177.4
67.5
161(2)
174.92(12)
69.06(11)
69.26(11)
68.90(11)
108.42(12)
106.64(12)
108.87(12)
176.8
177.2
67.5
178.2(10)
177.4(8)
69.5(7)
72.0(8)
68.1(8)
69.5(7)
69.4(7)
66.5(7)
111.4(8)
107.4(8)
107.7(8)
105.7(8)
106.2(7)
110.2(8)
12.9499(11)
n/a
70.06(6)
69.43(7)
94.80(7)
129.65(7)
98.30(7)
67.9
68.0
68.0
68.0
68.0
67.9
Neq-U-Neq
106.2
106.8
107.0
106.0
106.7
107.2
106.0
107.5
107.6
11.297
n/a
106.0
107.3
106.7
106.7
106.0
107.1
13.065
n/a
U
U
n/a
n/a
n/a
n/a
n/a
n/a
3 3 3
U-O
4 C5H12, one of the SitBuMe2 groups is disordered over two
microcrystalline samples (10-20 mg, ∼6-20 μmol) were loaded
into gelatin capsules in the glovebox, inserted into a straw, and
transported to the SQUID instrument under dinitrogen. AC
magnetic susceptibility data were collected at temperatures
ranging from 2 to 5 K at an applied field of 0.1 T with various
AC frequencies. Powdered microcrystalline samples were
loaded into gelatin capsules in the glovebox and suspended in
Eicosane to prevent crystallites from torquing at high and/or
alternating magnetic fields. Contributions to the magnetization
from the gelatin capsule and the straw were measured indepen-
dently and subtracted from the total measured signal. Data were
corrected for diamagnetic contributions using Pascal’s con-
stants. Susceptibility data were fit with theoretical models
using a relative error minimization routine (MAGFIT 3.1).76
Reported coupling constants are based on exchange Hamilto-
3
positions and refined to a 71:29 ratio. The methylene carbons
(C29 and C30) of the ligand with the disordered Si group as well
as all of the carbon atoms of the pentane solvate molecule were
refined anisotropically but restrained to have the same Uij
parameters.
Data for 5 C5H12 were truncated to 0.9 A resolution during
integration because of weak scattering. In the structure of
3
5 C5H12, two of the SitBuMe2 groups are disordered over two
3
positions and refined to 65:35 and 73:27 site occupancy ratios. All
chemically equivalent atoms were restrained to have the same Uij
parameters. The space between the uranium complexes shows
severe solvent disorder. One pentane solvate molecule was found
in Fourier difference maps, and the thermal parameters of the
carbon atoms were refined isotropically. SQUEEZE75 was used to
remove the remaining disordered components; approximately
0.25 equiv of pentane (per formula unit) are estimated to be
present in the void space. The final residual structure factors for
^
^
^
nians of the form H = -2J(Si Sj).
3
Other Physical Measurements. UV-visible absorption spec-
tra were obtained in pentane or toluene solutions in an airtight
glass cell of path length 1 cm on an Agilent 8453 spectrometer.
1H NMR spectra were recorded using a Varian INOVA 500
MHz instrument, and the spectra were referenced internally
using residual protio solvent resonances relative to tetramethyl-
silane (δ = 0 ppm). Infrared spectra were collected on a Thermo
Nicolet 380 FTIR spectrometer as mineral oil mulls pressed
between sodium chloride plates. EPR spectra were obtained on
solid samples at ambient temperature using a continuous wave
X-band Bruker EMX 200U instrument. Electrochemical mea-
surements were conducted with a CH Instruments 1232A po-
tentiostat/galvanostat, and the data were processed with CHI
software (version 7.20). All experiments were performed in a
glovebox using a 20 mL glass scintillation vial as the cell. The
electrodes consisted of platinum wire microelectrode (0.250 mm
diameter), platinum wire mesh counter, and Ag/Agþ reference
electrodes. Solution concentrations employed during CV studies
the structure of 5 C5H12 are high owing to extensive disorder and
3
the accompanying poor quality of the data.
Refinement of matrix scans for crystals of 6 give a primi-
tive orthorhombic cell with the following unit cell parameters:
a = 18.5219(7), b = 22.2851(8), c = 28.0242(10) A, and V =
11567(1) A3. A preliminary refinement of 6 confirms the ex-
pected cluster connectivity, but the diffraction data are not of
sufficient quality to afford a complete X-ray analysis.
Selected bond distances and angles for crystals of compounds
1-5 are collected in Table 2. All other metric parameters can be
found in the Supporting Information.
Magnetic Susceptibility Measurements. Magnetic susceptibi-
lity measurements were collected using a Quantum Design
MPMS XL SQUID magnetometer. Direct current (DC) mag-
netic susceptibility data were collected at temperatures ranging
from 2 to 300 K at an applied field of 0.1 T. Powdered
(76) Schmitt, E. A. Ph.D. Thesis, University of Illinois at Urbana-
(75) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
Champaign, 1995.

1600 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
were typically 3 mM for the uranium complex and 0.1 M for the
[TBA][B(ArF)4] electrolyte. All potentials are reported versus
the [Cp2Fe]þ/0 couple. Elemental analyses were performed by
Columbia Analytical Services, Tucson, AZ (compounds 2-6) or
the University of California, Berkeley (compound 1).
Electronic Structure Calculations. Spin unrestricted B3LYP
hybrid density functional studies77 were carried out on model
complexes of 3, 4, and 5 where the SitBuMe2 substituents are
replaced by H atoms and geometries are optimized. Singlet
states were described with broken symmetry representations.
In a broken symmetry treatment, R and β orbitals of a given
molecular orbital are allowed to be different, permitting the
differential localization of R and β spin sets.61 For MS =0
“singlet” states this model is not an eigenfunction of spin but
is an admixture of spin states. The standard Noodleman spin
projection formula (J = (EHS - EBS)/ÆSæ2) can be used to esti-
mate spin-spin coupling constants, J.78 This treatment has been
demonstrated to reproduce spin-spin (J) coupling in transition
metal complexes within a factor of 2.79 The Stuttgart RSC 1997
basis and effective core potential was employed for U, which
incorporates scalar relativistic effects and replaces 60 core
electrons.78-81 Linear dependency issues and SCF convergence
was improved by deletion of the outermost zeta = 0.05 S, P, D,
and F exponents. The 6-31g* basis sets were used for C, H, and
N atoms.82-84 All calculations were carried out in the G03 suite
of electronic structure codes.85 Selected bond distances and
angles for the calculated structures are presented in Table 2.
Coordinates for the calculated structures are provided in the
Supporting Information.
Figure 1. Crystal structures of the U(IV) arylacetylide complexes in
compounds 1 (left) and 3 (right), rendered with 40% ellipsoids. Green,
dark blue, light blue, red, purple, and gray ellipsoids represent U, N, Si,
O, Li, and C atoms, respectively. Hydrogen atoms are omitted for
clarity, and the tBuMe2 groups have been removed from the Si atoms in
1 for a clearer display of the coordination geometry about the uranium
center.
THF allows for a greater isolated yield of the hexacoordi-
nate U(IV) bis-arylacetylide complex.
The X-ray analyses of single crystals of 1 reveal two
different polymorphs depending on the reaction condi-
tions (P1 from 1:1 and P21/c from 1:2 stoichiometry).
Metric parameters for the complexes in both polymorphs
are essentially identical; the structures differ only in the
relative orientation of the complexes within the unit cells.
The thermal ellipsoid representation of the monoclinic
polymorph of 1 is shown in Figure 1; see the Supporting
Information for the triclinic structure. The uranium is
ligated by three amido nitrogens, one amine nitrogen, and
two phenylacetylide carbon atoms in η1 fashion. The
(NN03) fragment is unsymmetrically oriented with respect
to the metal center, resulting in a wider range of “flap”
dihedral angles Nax-U-Neq-Si (137-163ꢀ) than is nor-
mally observed for virtually all other compounds contain-
ing the [(NN03)U] fragment (131-137ꢀ).63 However, the
range of dihedral angles in 1 is similar to that reported by
Scott and co-workers for the U(V) oxo-bridged complex
[(bit-NN03)2U2(μ-O)] (132-177ꢀ).63 The ligands form a
Results and Discussion
Syntheses and Characterizations of [(NN03)U] Acetylide
Complexes. Several monomeric synthons avail themselves
for the preparation of pentacoordinate U(IV) species.
Scott and co-workers have shown that the [(NN03)UCl]
complex can undergo ligand substitution with a variety of
lithiated ligands via salt metathesis.66 However, in our
hands the apparent 1:1 combination of [(NN03)UCl] with
lithium phenylacetylide does not yield the expected penta-
coordinate complex, but instead produces a hexacoordi-
nate species, [(NN03)U(CCPh)2(Li THF)] (1) as the only
3
isolable product (eq 1).
½ðNN03ÞUClꢁ xTHF þ 2LiðCCPhÞ f
P
distorted octahedral first coordination sphere about the
3
metal center, as evidenced by the
which is the sum of the deviations from 90ꢀ of the twelve
cis j angles in the coordination sphere (
parameter (177.71),
½ðNN03ÞUðCCPhÞ2ðLi THFÞꢁ þ LiCl
ð1Þ
3
P
P
12
=
i=1|90 -
ji|).86 The two acetylide bridges are nearly linear, with
U-C-C angles of 169.1(2) and 177.8(2)ꢀ. This contrasts
with the only other structurally characterized U(IV) aryl
acetylide complex, [(NN03)U(CCPhMe)], a pentacoordi-
nate U(IV) complex that shows a U-C-C angle of
156.4ꢀ.63 In the structure of 1, the lithium ion is coordi-
nated by THF in an η1 mode, and by the acetylides in a π
fashion; the latter coordination mode may help explain
the observed linearity of the U-C-C linkages.
Adventitious THF present in the unsublimed U(IV) start-
ing material changes the stoichiometry of the reaction, and
lithium ion coordination to the phenyl acetylide ligands
likely drives formation of 1 over the expected monoaryl-
acetylide compound. Rationalization of the reaction con-
ditions by doubling the amount of added lithium phenyl-
acetylide and performing the reaction with an excess of
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1245–1263.
The absorption spectrum of 1 (Supporting Information,
Figure S7) contains only one feature at 686 nm. While
spectral features which would normally mark the presence
of a U(IV) ion in solution are absent, the position (686 nm)
and molar absorptivity (95 L mol-1 cm-1) of the singu-
(82) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724-728.
3
3
lar absorption maximum observed are similar to other
(83) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257-2261.
(84) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102,
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Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1601
Scheme 1. Synthesis of Complexes 3-6a
Figure 2. Crystal structure of the dinuclear complex in compound
2 THF C5H12, rendered with 40% ellipsoids. Green, dark blue, light
3
3
blue, red, and gray ellipsoids represent U, N, Si, O, and C atoms,
respectively. tBuMe2 groups have been removed from the Si atoms for a
clearer display of the coordination geometry about the uranium center.
Hydrogen atoms and solvent molecules are omitted for clarity.
a a=phenylacetylene, b=m-H2DEB, c=p-H2DEB, and d=H3TEB.
All reactions were carried out in pentane at -78 ꢀC.
pentacoordinate U(IV) complexes containing the (NN03)
ligand.66,71,87
Whereas the reaction of [(NN03)UCl] with lithium
phenylacetylide yields a bis-phenylacetylide complex, its
combination with 0.5 equiv of a ditopic aryl acetylide
such as Li2(p-DEB) results in the formation of a dinuclear
U(IV) complexes via eq 2:
analysis data obtained for bulk 2 indicate that approxi-
mately one THF molecule is absent in the bulk samples.
As will be discussed in more detail below, the “hexacoor-
dinate” 2 and the pentacoordinate 5 are found to have
virtually identical spectroscopic properties.
Alternatively, the monodeprotonated complex [(bit-
NN03)U] can serve as an excellent precursor for reactions
with free acetylenes, also previously demonstrated by
Scott and co-workers.63 As shown in Scheme 1, the
triamidoamine ligand can be reprotonated by the acet-
ylene, and the acetylide anion formed in situ can bind to
the cationic U(IV) center. In our hands, the combination
of the orange-brown [(bit-NN03)U] with 1 equiv of phenyl-
acetylene allows for the preparation of the yellow-green
pentacoordinate U(IV) monoacetylide complex (3) in
good yield.
2½ðNN03ÞUClꢁ xTHF þ Li2ðp-DEBÞ f
3
½ðNN03Þ2U2ðp-DEBÞðTHFÞꢁ þ 2LiCl
ð2Þ
Unlike the formation of 1, only one acetylide interacts
with each U(IV) ion; however, the presence of adventi-
tious THF nevertheless provides at least some hexacoor-
dinate U(IV) species in the final product. Uranium
complexes are quite oxophilic,88 and consistent with the
formation of complex 1, the triamidoamine groups are
not sufficiently sterically encumbering to prevent the
coordination of a sixth ligand. Thus, hexacoordinate
geometry is observed in the solid state structure of
The crystal structure of monomeric 3, determined from
crystals grown at -34 ꢀC from a saturated pentane solu-
tion (Figure 1), is very similar to the previously reported
[(NN03)U(CCPhMe)] complex.93 The triamidoamine li-
gand adopts a typical trigonal pyramidal geometry around
the uranium center in 3. Although not imposed crystallo-
graphically, the ligand is essentially 3-fold symmetric
about the U center, as measured by the dihedral angles
Nax-U-Neq-Si (131-137ꢀ). The acetylide ligand binds
the U(IV) ion in an η1 fashion, but shows a bent config-
uration unlike those of the nearly linear acetylides in the
hexacoordinate complex 2 (U-C-C angle 160.9(4)ꢀ). This
bending is similar to Scott’s pentacoordinate complex,
where it was suggested that the alkynyl uranium fragment
bends to allow for increased U-C π-overlap.63 Density
functional theory (DFT) calculations (discussed below)
reveal that bending the U-C-C bond angle from 180ꢀ to
160ꢀ only slightly perturbs the energy of the complex,
implying that intermolecular packing forces may represent
significant contributors to the observed bond angles.
Utilizing the same revision to the synthetic procedure
as described in the synthesis of 3, we find that mixing [(bit-
NN03)U] with the appropriate acetylenes leads to di- and
trinuclear complexes in which the U(IV) center is penta-
coordinate (Scheme 1). In this manner, we have prepared
the di- and trinuclear U(IV) ethynylbenzene complexes
[(NN03)2U2(m-DEB)] (4), [(NN03)2U2(p-DEB)] (5), and
[(NN03)3U3(TEB)] (6) in good yields. Crystal structures
for the dinuclear compounds 4 and 5 are depicted in
[(NN03)2U2(p-DEB)(THF)2] (2 THF), as determined by
3
X-ray analysis (Figure 2). Again, the [(NN03)U] fragment
is asymmetrically oriented, as measured by the dihedral
angles Nax-U-Neq-Si ranging from 132 to 176ꢀ.63 The
U-O distances (2.583(4) and 2.571(4) A for U1-O1 and
U2-O2, respectively) are similar to those reported for
other crystallographically characterized U(IV) THF ad-
ducts.87,89-92 The THF solvent molecules are rotated by
approximately 90ꢀ with respect to each other. The η1-
bound acetylide in 2 links the uranium centers in a nearly
linear fashion with U-C-C angles of 173.0(6) and
176.6(6)ꢀ. The uranium centers in 2 THF sit in distorted
3
P
octahedrons as measured by their respective
para-
meters (175.21ꢀ for U1 and 191.78ꢀ for U2).86 The (NN03)
fragments in 2 THF are rotated by approximately 90ꢀ
with respect to each other.
3
Whereas the crystal structure of 2 THF C5H12 clearly
shows two THF molecules per complex, the elemental
3
3
(87) Ball, R. G.; Edelmann, F.; Matisons, J. G.; Takats, J.; Marques, N.;
Marcalo, J.; Dematos, A. P.; Bagnall, K. W. Inorg. Chem. Acta 1987, 132,
137–143.
(88) Gorden, A. E. V.; Xu, J. D.; Raymond, K. N.; Durbin, P. Chem. Rev.
2003, 103, 4207–4282.
(89) Charpin, P.; Nierlich, M.; Vigner, D.; Lance, M.; Baudin, C. Acta
Crystallogr. 1988, 44, 255–257.
(90) Gaunt, A. J.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2006, 45, 7401–
7407.
(91) Salmon, L.; Thuery, P.; Asfari, Z.; Ephritikhine, M. Dalton Trans.
2006, 3006–3014.
(92) Salmon, L.; Thuery, P.; Ephritikhine, M. Chem. Commun. 2006,
856–858.
(93) Boaretto, R.; Roussel, P.; Kingsley, A. J.; Munslow, I. J.; Sanders,
C. J.; Alcock, N. W.; Scott, P. Chem. Commun. 1999, 1701–1702.

1602 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
Figure 3. Crystal structures of the dinuclear complexes in compounds
4 C5H12 (top) and 5 C5H12 (bottom), rendered with 40% ellipsoids.
Figure 4. Electrochemical behavior for 3 in static solution recorded in
0.1 M solution of [TBA][BArF4] in o-difluorobenzene at ambient tem-
perature with a 0.250 mm diameter platinum wire microelectrode.
3
3
Green, dark blue, light blue, and gray ellipsoids represent U, N, Si, and C
atoms, respectively. tBuMe2 groups have been removed from the Si atoms
for a clearer display of the coordination geometry about the uranium
center. Hydrogen atoms and solvent molecules are omitted for clarity.
infrared spectra. While these could indicate that the co-
ordination environment of the uranium center does not
have a discernible effect on electronic properties, more
likely these results point to THF solvate loss in solution.
Thus, crystals of 2 THF C5H12 show two THF molecules,
t
One and two of the SiMe2 Bugroupsare disorderedovertwo positionsfor
4 C5H12 and 5 C5H12, respectively; only one orientation is shown for
3
3
clarity. See Supporting Information, Figures S24 and S25 for disordered
components.
3
3
but bulk 2 contains only one, and dissolved 2 is spectro-
scopically identical to 5, which contains no THF solvate.
Oxidation of the Pentacoordinate U(IV) Arylacetylide
Complexes. Efforts to produce unambiguously paramag-
netic U-containing assemblies, either by oxidations or
reductions of 2 and 4-6 that may lead to U(V) or U(III)
species, respectively, yield mixed results. Cyclic voltam-
metry experiments performed on the monomeric phenyl-
acetylide complex 3 in o-difluorobenzene show a well-
defined, reversible wave centered at -1.02 V versus Fcþ/
Fc (Figure 4). This process is assignable to an oxidation of
the neutral compound to a formally U(V) species, and is
supported by an agitation experiment whereby the vol-
tammogram is collected while stirring the sample
(Supporting Information, Figure S13). This is compar-
able to results reported by Kiplinger and co-workers
for [(C5Me5)2U(=N-Ar)(X)] (X = F, Cl, Br, I), where
reversible couples ranging between -1.21 and -1.84 V
versus Fcþ/Fc are observed.96 While the cyclic voltam-
mograms suggest a reversible U(IV/V) redox couple on
the time scale of the experiment (scan rate of 50 mV/s),
initial attempts to isolate oxidized complexes by chemical
oxidation with [FeCp*2](BArF4) have not been successful.
Infrared spectra obtained on the products of oxidation
attempts show no shift in the acetylide resonance, con-
trary to what would be expected if a change in uranium
oxidation state occurred. In addition, crystals isolated
from the oxidation attempts were determined to be
[(C5Me5)Fe(C5Me4CH2)](BArF4).97
Figure 3. Crystals of trinuclear 6 diffract sufficiently to
confirm the expected cluster connectivity, but the diffrac-
tion data are not of sufficient quality to provide a
completeX-rayanalysis. As with compound 3(see Table 2
for comparisons of bond distances and angles), the
[(NN03)U] adopts its usual orientation, with dihedral
angles of 132-142ꢀ and 136-145ꢀ for meta- and para-
bridged 4 and 5, respectively. Interestingly, one of the
U-acetylide linkages in meta-bridged 4 is significantly
more linear than the other (U-C-C angle of 158.2(7)ꢀ
versus 170.2(7)ꢀ). Fourier difference maps do not indicate
any evidence for crystallographic disorder present in the
structure of 4. Rather, the different U-C-C bond angles
observed could be due to a competition between (NN03)
sterics, which would favor linear U-C-C linkages, and
π-overlap of the acetylide and U(IV) ion, similar to that
observed in the structure of the monomeric complex 3.
The (NN03) fragments in meta-bridged 4 are rotated by
approximately 60ꢀ with respect to each other. In para-
bridged 5, the (NN03) fragments are not rotated with
respect to each other and can be related by a non-crystallo-
graphic mirror plane.
We have characterized all the ethynylbenzene-bridged
species by FT-IR and UV-visible spectroscopic techni-
ques (Supporting Information, Figures S1-S12). The
fingerprint region of the IR is nearly identical to those
reported for most of the structurally characterized com-
pounds containing the [(NN03)U] fragment.63,64,66,94 The
electronic absorption spectra of uranium compounds
represent a good indicator for the oxidation state of the
metal ion; and the spectra of complex 2 and compounds
4-6 are consistent with an assignment of U(IV), in
agreement with other reported [(NN03)U]-containing
compounds.95 Interestingly, the UV-visible spectra of 2
and 5 in toluene are similar; they also display similar
Attempts to obtain cyclic voltammograms on com-
pounds 2, 4, 5, and 6 under similar conditions have proven
more difficult. Although experimental conditions have
been systematically varied (including solvents, scan rates,
and working electrodes), in all instances, only ill-defined
waves are observed (Supporting Information, Figure S14),
(94) Roussel, P.; Boaretto, R.; Kingsley, A. J.; Alcock, N. W.; Scott, P.
J. Chem. Soc., Dalton Trans. 2002, 1423–1428.
(95) Schelter, E. J.; Veauthier, J. M.; Graves, C. R.; John, K. D.; Scott,
B. L.; Thompson, J. D.; Pool-Davis-Tournear, J. A.; Morris, D. E.;
Kiplinger, J. L. Chem.;Eur. J. 2008, 14, 7782–7790.
(96) Graves, C. R.; Vaughn, A. E.; Schelter, E. J.; Scott, B. L.; Thompson,
J. D.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2008, 47, 11879–11891.
(97) Kreindlin, A. Z.; Dolgushin, F. M.; Yanovsky, A. I.; Kerzina, Z. A.;
Petrovskii, P. V.; Rybinskaya, M. I. J. Organomet. Chem. 2000, 616,
106–111.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1603
Figure 5. Temperature dependence of the magnetic susceptibility for
compounds [(NN03)U(CCPh)2(Li THF)] (1) and [(NN03)U(CCPh)] (3),
obtained at a measuring field of 1000 G.
3
suggesting the occurrence of multielectron processes and/
or the decomposition of the original species. It is also
possible that the complicated nature of the cyclic voltam-
mograms could be due to electronic communication be-
tween the uranium centers via the bridging ligand.
Magnetic Properties of the U(IV) Complexes. The
temperature dependence of the magnetic susceptibility
(2-300 K) for each uranium-acetylide complex was char-
acterized by SQUID magnetometry (Figures 5 and 6, also
Supporting Information, Figures S15-S17), and MAG-
FIT76 was used to fit the subtracted paramagnetic suscepti-
bility data (vide infra) to a simple spin Hamiltonian with
one exchange parameter J (black traces in Figure 6 and
Supporting Information, Figures S15-S17). Fitted para-
meters are listed in Table 3.
Figure 6. Top: temperature dependence of the magnetic susceptibility
for compounds 3 and 4, obtained at a measuring field of 1000 G; and fit of
the data obtained from the subtraction method for 4, see text for details of
the fitting procedures. Bottom: solid lines give best fits to the data
obtained from the subtraction method for complexes 2, 4, 5, and 6; see
text for details of the data correction procedures.
Magnetic Susceptibilities of Monomeric Complexes 1
and 3. The temperature dependencies of the magnetic
susceptibility, χMT, for the monomeric U(IV) arylacety-
lide complexes 1 and 3 are shown in Figure 5. The room
temperature χMT values for 1 and 3 (1.37 and 1.18
emu K mol-1, respectively) are comparable to those of
Table 3. Tabulated MAGFIT Results for Compounds [(NN03)]2U2(p-DEB)(THF)]
(2), [(NN03)2U2(m-DEB)] (4), [(NN03)2U2(p-DEB)] (5), and [(NN03)3U3(TEB)] (6)
2
4
5
6
J (cm-1
)
-0.05
1.99
5
4.76
1.80
1435
0.18
2.75
1.89
860
1.11
1.84
1473
0.17
3
3
other reported complexes containing U(IV) in a low
symmetry ligand field and are consistent with the
presence of paramagnetic state(s) at room tempera-
ture.49,94,98 Upon decreasing the temperature, higher-
energy Stark sublevels begin to depopulate, resulting in
a subsequent decrease in the magnitude of the total
angular momentum vector. This phenomenon leads to a
variation in the thermal population of the many states
that are energetically comparable to the ground state.11
The physical manifestation of this decrease in the angular
momentum is evident by the decrease in the observed
magnetic susceptibility. As can be seen in a plot of χMT
versus T for 1, a gradual decrease to 0.93 emu K mol-1
g
TIP (ꢀ10-6 emu)
relative error
0.19
0.08
magnetism (TIP), and thermal population of paramag-
netic excited states. A poorly isolated singlet ground state
is not atypical for complexes with 5f2 valence config-
urations;99-101 further, it is well-known that an octahedral
ligand field will produce a diamagnetic ground state for
a 5f2 electronic configuration.59 Although the pentacoor-
dinate species 3 also displays magnetic properties which
appear to be consistent with a non-magnetic ground
state,102-104 there seems to be less influence from TIP than
3
3
at 120 K occurs, followed by a sharper decrease to 0.13
emu K mol-1 at 8 K. Similarly, as the temperature is
(99) Siddall, T. H. Theory and Applications of Molecular Paramagnetism;
Wiley: New York, 1976.
(100) Kanellakopulos, B. Organometallics of the f-Elements; D. Reidel
Pub. Co.: Dordrecht, The Netherlands, 1978.
(101) Edelstein, N. M.; Lander, G. H. The Chemistry of the Actinide and
Transactinide Elements; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.; Springer:
Dordrecht, The Netherlands, 2006; Vol. 4, Chapter 20.
(102) Almond, P. M.; Deakin, L.; Porter, M. J.; Mar, A.; Albrecht-Schmitt,
T. E. Chem. Mater. 2000, 12, 3208–3213.
(103) Kiplinger, J. L.; Pool, J. A.; Schelter, E. J.; Thompson, J. D.; Scott,
B. L.; Morris, D. E. Angew. Chem., Int. Ed. 2006, 45, 2036–2041.
(104) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.;
Kiplinger, J. L. Inorg. Chem. 2007, 46, 5528–5536.
3
3
reduced to 160 K, χMT for 3 reveals a gradual decrease
to 0.99 emu K mol-1, followed by a sharper decrease to
3
3
0.03 emu K mol-1 at 2 K. The behavior of the hexacoor-
3
3
dinate 1 can be interpreted as a ground state diamagnetic f2
species, which is paramagnetic at room temperature be-
cause of spin-orbit coupling, temperature-independent
(98) Spirlet, M. R.; Rebizant, J.; Apostolidis, C.; Dornberger, E.; Kanel-
lakopulos, B.; Powietzka, B. Polyhedron 1996, 15, 1503–1508.

1604 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
observed for 1. Overall, the foregoing results imply that
coordination geometry differences impart only minor im-
pacts on the magnetic properties of these [(NN03)U]-
containing complexes.
for g=2.00). In the case where no communication
between the metal centers is occurring, a plot of the
obtained values versus temperature is expected to form a
line with zero slope at a χMT value of 2.00 emu
3
Magnetism of Di- and Trinuclear Species 2, 4-6. The
temperature dependence of the magnetic susceptibility
for dinuclear “hexacoordinate” complex 2 is shown in
Supporting Information, Figure S15. The room tempera-
ture χMT value for 2 (1.40 emu K mol-1 per U(IV) ion) is
K mol-1 (assuming g = 2.00).50,53,65,101 However, the
3
resulting blue traces (Figure 6, Supporting Information,
Figures S15-S17) do possess some curvature, suggesting
the presence of U-U magnetic interactions. While this
method of data treatment only allows an estimation of
the lower limit to any exchange interactions (since
spin-orbit interactions have been removed), MAGFIT
estimates the magnetic exchange in meta-bridged 4 to
be weakly ferromagnetic, with J = 4.76 cm-1. There are
scant comparisons available in the literature. The cou-
pling in [(MeC5H4)6U2(μ-1,4-N2C6H4)] was reported by
Andersen and co-workers to be significantly stronger and
antiferromagnetic, (J = -19 cm-1);57 however it must be
noted that this represents coupling between U(V) centers.
Meanwhile, coupling between U(IV) and Ni(II) ions in
{(cyclam)Ni[(μ-Cl)U(Me2Pz)4]2} using the above-men-
3
3
comparable to that of other reported complexes with
U(IV) in a low symmetry ligand field.49,94,98 It is impor-
tant to note that χMT drops as the temperature ap-
proaches zero to a minimum of 0.04 emu K mol-1 at
3
3
2 K. Again, this may be due to “octahedral” geometry,
similar to the description of the magnetic properties of 1.
However, the drop is not linear like 1, perhaps related to
the fact that the 6-coordinate geometry is quite distorted
from a perfect octahedron. The construction of a Weiss
plot for 2 (Supporting Information, Figure S18) yields a
θ value of -180 K with a Curie constant (C) of 4.44
cm3 K mol-1. A complicating factor in magnetic inter-
tioned subtraction scheme yields a J value of 2.8 cm-1 50
;
3
3
pretation for the compound is the potential loss of some
THF from ground up bulk samples of this compound. In
fact, if the mass of 1 equiv of THF is removed from 2, the
adjusted susceptibility data virtually overlay the data for
para-bridged 5 (vide infra).
although the structure is not similar to the compounds
presented here, the result shows that coupling in the single
wavenumber range is not unexpected. Finally, coupling of
Fe(III) ions through the m-DEB bridge is ferromagnetic,
albeit significantly stronger than that observed in the meta-
bridged dinuclear complex 4 (J = 65 cm-1).67
Although complexes 4-6 display quite different coor-
dination geometries from the crystal of 2 THF, their
Interestingly, fits to the data for para-bridged 5 and
trinuclear complex 6 (Table 3) also indicate weak ferro-
magnetic coupling, although the J couplings are weaker
(2.75 and 1.11 cm-1, respectively) than that determined
for 4. Note that g values determined from the fitting
procedure are consistent with other reported U(IV) com-
plexes.49,50 In an attempt to compare experimental and
fitted g values, preliminary room temperature electron
paramagnetic resonance (EPR) data were collected for
meta-bridged 4; however, no signal was obtained, which
is not unexpected for an integer spin system (S = 1 or 0).
Data derived from best fits for “hexacoordinate” dinu-
clear complex 2 are also presented in Table 3, but the
determined parameters are less reliable owing to the lack
of a suitable monomeric hexacoordinate U(IV) complex
for use in the data adjustment scheme, as well as uncer-
tainty about the coordination geometry in bulk samples
of 2.
3
magnetic properties appear to be quite similar on a per
U(IV) basis. The temperature dependence of the magnetic
susceptibility, χMT, for the meta-bridged dinuclear com-
plex 4 is shown in Figure 6; those for para-bridged 5
and TEB-bound 6 can be found in the Supporting In-
formation (Figures S16, and S17, respectively; Support-
ing Information, Figure S18 reveals Weiss constant
determinations). The room temperature χMT value for 4
(1.26 emu K mol-1 per U(IV) ion) is in the range of other
3
3
literature values for paramagnetic U(IV) ions in low
symmetry ligand fields.49,94,98 Similar to the behavior of
2, χMT approaches zero as the temperature is reduced,
which appears to be consistent with a non-magnetic
ground state.102-104 This is inconsistent with the simple
ligand-field diagram for an f2 ion in trigonal bipyramidal
complex geometries; however, it must be noted that
spin-orbit coupling was not included in the group theo-
retical analysis.
Indeed, this exemplifies a general concern about the
potential for measurement errors to propagate in the
course of applying the subtraction scheme. Regarding
the reproducibility of data, we have analyzed multiple
samples of the mono- and multinuclear complexes, both
within a batch and between different preparations, and
obtain the same raw data in all cases. With respect to
electronic differences between the mono- and multi-
nuclear complexes (3 and 4-6, respectively) it is possi-
ble that the observed curvature in the χMT versus T
plots is an artifact, but the structural similarities be-
tween the complexes argues against this. Finally, fits to
the corrected data give the same values, even when the
initial guesses for J and g are varied significantly. Thus,
we argue that the temperature dependence of the cor-
rected susceptibilities represent real albeit qualitative
evidence of magnetic coupling operative between U(IV)
centers.
However, when we perform a precedented subtraction
scheme11 on the susceptibility data for complexes 2, 4, 5,
and 6, the adjusted data reveal evidence of net intra-
molecular exchange interactions.50,105 Here, the discussion is
focused on the data interpretation for meta-bridged 4, but
is applicable to the magnetic interpretations for the other
multinuclear complexes 2, 5, and 6. At each temperature,
two times the paramagnetic susceptibility of the mono-
acetylide species 3 (three times in the case of trinuclear 6)
are subtracted from the corresponding paramagnetic
susceptibility of the dinuclear meta-bridged complex 4 to
remove any contribution from the spin-orbit coupling
present in the U(IV) ions. To this value is added the
contribution expected for two S=1 ions (i.e., χMT=1.00
(105) Salmon, L.; Thuery, P.; Riviere, E.; Girerd, J. J.; Ephritikhine, M.
Chem. Comm. 2003, 762–763.

Article
On the basis of analogy with transition metal ana-
Inorganic Chemistry, Vol. 49, No. 4, 2010 1605
logues,67,68 we would expect antiferromagnetic coupling
for paramagnetic species bridged by p-DEB and ferro-
magnetic exchange for di- and trinuclear complexes
bridged by m-DEB and TEB, respectively. In support of
this, antiferromagnetic coupling is observed in Andersen’s
para-substituted imido bridged UV2 species.57 Thus, while
the results of the subtraction procedure may appear
reasonable for the meta-linked complexes 4 and 6, we
might expect antiferromagnetic coupling for the p-DEB-
bridged 5. That this is not operative suggests that the
particular bridging geometry may only have a small effect
on the type of coupling in these U(IV) complexes. We
note, however, that the geometry and nuclearity do appear
to have an effect on the strength of the coupling (Table 3).
First, the meta-linked 4 exhibits a J value twice as large as
that determined for 5. Second, comparison of the mag-
netic data for 4 and 6 shows that increasing the number of
uranium centers results in a smaller coupling constant.
Similar effects have been observed in cyanide-bridged
transition metal complexes, where increased nuclearity
distributes spin density over a larger area, resulting in
weaker coupling.106
An important part of this discussion is that we must
fully consider the possibilities that intermolecular path-
ways (H-bonding, U-U interactions, and close contacts)
could contribute to the observed magnetic properties, and
confirm that they are not significant contributors. There
were no significant contacts in compounds 1-6, other
than weak van der Waals interactions, that would allow
for any obvious pathways for magnetic communication to
occur (see Supporting Information). In compounds 1-6,
Figure 7. (a) Average field fragment molecular orbital diagram for
[N(CH2CH2NH)3U(CCH)], relative energies are provided on an eV scale.
(b) Net spin density plot of the ground state triplet. (c) Net spin density
plot of the lowest “singlet” broken symmetry state. Blue surfaces corre-
spond to net R spin density and green to net β spin density.
observation of both bent and linear U-C-C linkages in
the isolated complexes 1-6. The calculations show that
bending the U-C-C angle from 180ꢀ to 160ꢀ only
increases the energy by 0.5 kcal/mol for both the ground
state triplet and lowest energy excited state singlet. The
harmonic curve in Supporting Information, Figure S23
demonstrates that, as is typical of sp hybridized carbon,
the bending potential is more quartic than harmonic in
character.
A conventional (spin-orbit coupling omitted) study on
the model for 3 yields a triplet ground state with two
electrons in quasi degenerate, singly occupied f orbitals;
consistent with the group theoretic analysis.58 The calcu-
lated f orbitals are mixtures of the 5f general set;107
the occupied orbitals that would have π overlap with
the acetylide ligand most closely resemble fxz2 and fyz2
(Figure 7). The lowest MS = 0 “singlet” state is one
wherein the two singly occupied orbitals are “singlet
coupled” via a broken symmetry solution. To obtain
the relative energies of the set of 7 f orbitals as well as to
obtain the character of these frontier orbitals, an average
field computation was carried out wherein the two triplet-
coupled electrons are evenly distributed over the 7 f
orbitals. Only the two lowest energy orbitals, the ones
occupied in the conventional triplet study, show net
orbital overlap with the π-type orbitals on the bound
acetylide. Nevertheless, net spin density plots for the
ground state triplet and lowest excited state singlet
(Figure 7b and c) show that negligible spin density is
found on the acetylide. As is visually evident in Figure 7,
the 7 frontier orbitals are dominantly 5f in character. The
largest d coefficient in any of the seven frontier orbitals is
only 0.118. For the two lowest energy average field
orbitals the largest d function coefficient is 0.029.
the shortest intermolecular
U
U
distance was
3 3 3
found in 4 with a distance of 8.9261(5) A. The shortest
intramolecular U U interaction was also found in 4
3 3 3
with a distance of 9.2837(9) A. None of these contacts
portend significant contributions to the observed
magnetism, thus lending further support to our assertion
that any residual magnetism in these complexes is due
to intramolecular communication between the uranium
centers.
AC susceptibility measurements carried out on meta-
bridged 4 do not show a change in the out-of-phase signal,
even at switching frequencies of ∼1500 Hz (Supporting
Information, Figure S26). Thus, regardless of coupling
considerations, at least meta-bridged 4 does not show
properties consistent with SMM behavior.
Theoretical Considerations. To gain deeper insight into
the complex magnetic behavior, we carried out geometry-
optimized Stuttgart/6-31g* B3LYP hybrid DFT calcula-
tions on model systems where the SitBuMe2 substituents
in the NN03 ligand are replaced by H atoms and relativis-
tic effects are explicitly included in the uranium effective
core potential. The structure obtained from the geometry
optimization of a mononuclear model of 3, [N(CH2-
CH2NH)3U(CCH)], compares well with the crystal struc-
ture of 3, although one difference is that the U-C-C
linkage is linear in the model complex. Computa-
tions carried out as a function of the U-C-C
angle (Supporting Information, Figure S23) address the
The triplet state is lower in energy than the broken
symmetry solution by 8.4 kcal/mol. Given a U(IV) spin-
orbit coupling parameter of roughly 6.3 kcal/mol,58
however, the triplet and singlet states should strongly
mix, resulting in a j=0 ground state. This is consistent
with the observed magnetic properties of 3, where a
paramagnetic complex at room temperature becomes
“non-magnetic” as the temperature is reduced.
For model species based on the dinuclear complexes 4
and 5, a broken symmetry model was used to construct
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1606 Inorganic Chemistry, Vol. 49, No. 4, 2010
Newell et al.
observed types of coupling, but do show that communication
through the ethynylbenzene bridge is weak for these U-
containing species, and likely subject to significant perturba-
tion by spin-orbit coupling. In turn, this may inform future
work toward utilizing actinide elements in the generation of
new SMMs.
The observed and calculated magnetic properties of this
family of U(IV)-containing complexes can be rationalized in
the following way. First, a trigonal bipyramidal ligand field
provides the potential to observe a triplet ground state for a
U(IV) ion, but spin-orbit coupling causes admixture of
excited singlet states, reducing paramagnetic contributions.
Second, although calculations point to π-type orbital overlap
between acetylide ligands and the 5f orbitals of the U(IV) ion,
negligible spin density from the metal leaks onto the bridging
ligands, leading to weak ferromagnetic coupling via applica-
tion of Hund’s rule.
The lack of delocalization for U(IV) is likely due to a
metal-bridging ligand energy mismatch. Andersen’s bis-dia-
zenylbenzene ligand59 or a dicyanobenzene species is hy-
pothesized to provide a better energy match. In addition,
because the f orbitals that can interact with acetylide π
orbitals also have σ interactions with the NN03 ligand,
substituent changes on the tetradentate ligand may also give
rise to significant changes in magnetism. Computational
studies focusing on meta and para substituted uranium
complexes with modified bridging ligands are planned, and
the results will be compared with transition-metal based
systems, both experimentally and computationally.
We have also shown that the monomeric arylacetylide
complex, 3, undergoes a reversible redox couple assignable to
a U(IV/V) process. This offers a route toward half integer
actinide-containing spin systems where the DEB ligand may
enjoy more substantial orbital overlap with U(III) or U(V)
ions. Efforts to find chemically accessible reductions or
oxidations of 1, 4-6, and related compounds to U(III) or
U(V) are underway. Precedent for this possibility is given by
the recent report of organometallic U(IV) complex oxidation
by Cu(I) phenylacetylide.112
Figure 8. Net spin density plots for m- and p-DEB-bridged dinuclear
species based on 4 and 5. Blue surfaces correspond to net R spin density
and green to net β spin density. Triplets are displayed in a and c, and
singlet (broken symmetry) wave functions are shown in b and d.
the antiferromagnetically coupled low spin state, and J
was computed to be 1.6 and -0.1 cm-1 for the meta- and
para-bridged complexes, respectively. The signs of the
computed coupling constants are not consistent with
the observed magnetic properties, but do conform to
what is expected in ethynylbenzene-bridged systems,108,68
namely, ferromagnetic coupling for meta-bridged 4 and
antiferromagnetic coupling for para-bridged 5. We note
that the magnitudes of the calculated coupling constants
for models of 4 and 5 are much smaller than those
computed for similar transition-metal based systems.68
As with the model mononuclear complex calculation, net
spin density (FR-Fβ) plots generated for models of 4 and 5
(Figure 8) show very little bridging-ligand density, no
matter what spin states are used for the U(IV) constituent
ions. We conclude that ethynylbenzene ligands such as
DEB and TEB are generally competent for mediating J-
coupling in transition metal complexes, but not for U(IV)
with the NN03 ancillary ligand set in the tbp coordination
geometry.
Summary and Outlook
We have prepared a structurally related family of penta-
and hexacoordinate U(IV) complexes bridged by anionic
ethynylbenzene ligands, and have used multiple techniques to
characterize them. Despite the fact that all the compounds
presented in this study give non-magnetic ground states at
low temperature, consistent with those described elsewhere in
the literature,94-98,100-105,109-111 fits to the adjusted mag-
netic susceptibility data point to weak ferromagnetic com-
munication between the uranium centers in the di- and
trinuclear pentacoordinate U(IV)-containing compounds 4,
5, and 6. Theoretical calculations do not reproduce the
Acknowledgment. This research was supported by Col-
orado State University and the ACS Petroleum Research
Fund (44691-G3). We thank Ms. Susie Miller and Prof.
Oren Anderson for advice on crystal structure refine-
ments, Dr. Christopher Rithner for assistance with NMR
data collection, Mr. Don Heyse for assistance with EPR
data collection, and Dr. P. Jeffrey Hay and Prof. C. M.
Elliott for helpful conversations.
Supporting Information Available: X-ray crystallographic
files (cif); full characterizations of complexes 1-6, coordinates
for calculated model complexes, and full citation for reference
85 (pdf). This material is available free of charge via the Internet
at http://pubs.acs.org.
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