A general and modular synthesis of monoimidouranium(IV) dihalides

Article abstract of DOI:10.1021/ic200377b

The conproportionation reaction between the dimeric diimidouranium(V) species [U(N^tBu)₂(I)(^tBu₂bpy)] ₂ (^tBu₂bpy = 4,4′-di-tert-butyl-2, 2′-bipyridyl) and UI₃(THF)₄ in the presence of additional ^tBu₂bpy yields U(N^tBu)(I) ₂(^tBu₂bpy)(THF)₂ (2), an unprecedented example of a monoimidouranium(IV) dihalide complex. The general synthesis of this family of uranium(IV) derivatives can be achieved more readily by adding 2 equiv of MN(H)R (M = Li, K; R = ^tBu, 2,6- ⁱPrC₆H₃, 2-^tBuC₆H ₄) to UX₄ in the presence of coordinating Lewis bases to give complexes with the general formula U(NR)(X)₂(L)_n (X = Cl, I; L = ^tBu₂bpy, n = 1; L = THF, n = 2). The complexes were characterized by ¹H NMR spectroscopy and single-crystal X-ray diffraction analysis of compounds 2 and {U[N(2,6-ⁱPrC ₆H₃)](Cl)₂(THF)₂}₂ (4). (The X-ray structures of 5 and 6 are reported in the Supporting Information.)

Full text of DOI:10.1021/ic200377b

COMMUNICATION
pubs.acs.org/IC
A General and Modular Synthesis of Monoimidouranium(IV) Dihalides
Robert. E. Jilek,† Liam P. Spencer,† David L. Kuiper,† Brian L. Scott,† Ursula J. Williams,‡ James M. Kikkawa,§
Eric J. Schelter,‡ and James M. Boncella*,†
†MPA Division, Los Alamos National Laboratory, MS J514, Los Alamos, New Mexico 87545, United States
‡P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia,
Pennsylvania 19104, United States
§Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S Supporting Information
b
Our foray into the area of monoimidouranium(IV) chemistry
ABSTRACT: The conproportionation reaction between
the dimeric diimidouranium(V) species [U(NtBu)2(I)(tBu2-
bpy)]2 (tBu2bpy = 4,40-di-tert-butyl-2,20-bipyridyl) and
began by investigating the conproportionation reaction between
the diimidouranium(V) complex [U(NtBu)2(I)(tBu2bpy)]2 (1)5
and UI3(THF)4. A similar approach has proven successful in the
synthesis of a uranium(IV) metallocene complex from uranium-
(III) and uranium(V) reagents.6 In the presence of 2 equiv of
tBu2bpy, the monoimidouranium(IV) species U(NtBu)(I)2-
(tBu2bpy)(THF)2 (2) is produced in 78% yield (Scheme 1).
t
UI3(THF)4 in the presence of additional Bu2bpy yields
U(NtBu)(I)2(tBu2bpy)(THF)2 (2), an unprecedented ex-
ample of a monoimidouranium(IV) dihalide complex. The
general synthesis of this family of uranium(IV) derivatives
can be achieved more readily by adding 2 equiv of MN(H)R
(M = Li, K; R = tBu, 2,6-iPrC6H3, 2-tBuC6H4) to UX4 in the
presence of coordinating Lewis bases to give complexes with
the general formula U(NR)(X)2(L)n (X = Cl, I; L = tBu2bpy,
n = 1; L = THF, n = 2). The complexes were characterized by
1H NMR spectroscopy and single-crystal X-ray diffraction
analysis of compounds 2 and {U[N(2,6-iPrC6H3)](Cl)2-
(THF)2}2 (4). (The X-ray structures of 5 and 6 are reported
in the Supporting Information.)
1
The H NMR spectrum of 2 is consistent with a paramagnetic
uranium(IV) complex, containing a broad resonance at 92.2
ppm attributable to the tert-butyl group of the imido ligand and
bipyridyl aryl resonances at 7.37, 8.79, and 8.87 ppm.
A single-crystal X-ray diffraction study of 2 confirmed its identity
as a monoimidouranium(IV) species; the solid-state molecular
structure is shown in Figure 1. Complex 2 is mononuclear and
features a distorted pentagonal-bipyramidal geometry at the ura-
nium center with the imido moiety and one iodide ligand assuming
the axial positions [N1ꢀU1ꢀI2 = 165.08(13)°]. Importantly, the
coordinated bpy ligand possesses bond lengths that are typical of a
coordinated, neutral bpy ligand and not those of a radical anion
species.7 In the case of the UdN(imido) bond distance [1.931(5)
Å], this value is similar to the metallocene imidouranium(IV)
complex (C5Me5)2U(dN-2,4,6-tBu3C6H2) [1.952(12) Å]1c and
the values reported for monoimidouranium(V) (1.940ꢀ2.019 Å)8
and monoimidouranium(VI) (1.854 Å) derivatives.9
Intrigued by the possibility of a more direct synthesis of 2, we
investigated the reaction of UI4(OEt2)2 with 2 equiv of LiNHtBu
and 1 equiv of tBu2bpy. This procedure produces 2, albeit in low
yield (13%), which appears to be related to difficulties in
removing LiI from the product. The dimeric dichloride derivative
[U(µ-NtBu)(Cl)2(tBu2bpy)]2 (3) can be obtained in 68% yield
through the reaction of 2 equiv of LiNHtBu with UCl4 in the
presence of tBu2bpy.
We next investigated whether this general approach could be
extended to include (arylimido)uranium(IV) complexes. Indeed,
the addition of 2 equiv of LiN(H)R (R = Dipp = 2,6-iPr2C6H3,
2-tBuC6H4) to UCl4 in THF provides mono(arylimido) com-
plexes with the general formula [U(μ-NR)(Cl)2(THF)2]2 [R =
Dipp (4), 2-tBuC6H4 (6)], as shown in Scheme 2. In the solid
state, 4 and 6 exhibit nearly identical dimeric structures with
bridging imido interactions (R = Dipp, Figure 1; R = 2-tBuC6H4,
Figure S5 in the Supporting Information, SI) that are reminiscent
ctinide complexes that possess multiple bonds to nitrogen
A
continue to attract significant attention as a means to explore
fundamental differences between d-block and actinide elements.
Arguably the most well-studied of these derivatives are the
monoimido metallocenes, (C5Me5)2U(dNR) (R = 2,4,6-Me3-
C6H2, 2,6-iPr2C6H3, 2,4,6-tBu3C6H2), which have provided
important contrasts with analogous transition-metal complexes
and critical insight into the unique chemical properties and
behaviors that exist in UꢀN multiple bonds.1 Surprisingly, non-
metallocene examples of monoimido derivatives are exceedingly
rare and are limited to multinuclear examples that are predomi-
nantly amide-based in nature.2,3 Noticeably missing from this
family of compounds are the dihalide derivatives U(dNR0)(X)2
(R0 = alkyl, aryl; X = halide); these species possess significant
potential to increase our general knowledge of organoimidour-
anium chemistry and provide important comparisons with analo-
gous monoimido halide transition-metal species.4 Of further
importance, this family of complexes could serve as useful
synthons for the generation of other monoimido derivatives, thus
significantly expanding our knowledge of actinide chemistry.
Herein, we report the general synthesis of monoimidouranium-
(IV) dihalide derivatives with the formula U(NR0)(X)2(L)n [R0 =
tBu, 2,6-iPrC6H3, 2-tBuC6H4; X = Cl, I; L = tBu2bpy (tBu2bpy =
4,40-di-tert-butyl-2,20-bipyridyl), n = 1, L = tetrahydrofuran
(THF), n = 2]. These interesting molecules can be prepared in
one step from uranium tetrahalide precursors.
Received: February 23, 2011
Published: April 08, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 1
Scheme 2
complexes.2 Treatment of UI4(OEt2)2 with 2 equiv of KN-
(H)Dipp in THF yields the mono(arylimido)uranium(IV) pro-
duct, 5. The solid-state molecular structure of the diiodide 5
reveals a monomeric species (Figure S4 in the SI) with a short
UꢀN bond length, similar to that found in 2.10
Solution studies of 2ꢀ6 suggest that the nature of the halide
ligand in combination with the coordinating ability of the solvent
has a significant effect on the nuclearity of these complexes. In
contrast to the diiodide species 2, the 1H NMR spectrum of 3 in
pyridine-d5 is devoid of any THF resonances. In light of this detail,
we propose that complex 3 is a dimeric species in the solid state
with a structure similar to that of compound 4, having one
coordinated bpy ligand in lieu of two THF molecules. This is
consistent with the observation that crystals of the green com-
pound, 2, slowly lose THF to yield a brown product, which
remains unchanged by 1H NMR in pyridine-d5. The spectrum of 2
in CD2Cl2 is significantly altered, suggesting that this compound
may dimerize, while slowly losing coordinated THF. Similarly, 1H
NMR spectra of 4 and 6 vary dramatically depending on the
identity of the solvent. For example, the methyl resonance of 4
shifts from ꢀ8.20 ppm in THF-d8 to 13.96 ppm in pyridine-d5,
suggesting that pyridine coordinates to the dimeric complexes 3, 4,
and 6, breaking up any imido bridges and giving monomeric
complexes in solution. Interestingly, the 1H NMR spectrum of the
monomeric species 5 remains constant in both THF-d8 and
pyridine-d5, and its spectrum in pyridine-d5 is nearly identical
with that of its dichloride congener, 4, consistent with the
formation of the analogous pyridine adduct, U(N-2,6-iPr2C6H3)
(I)2(pyridine)4.11
Temperature- and field-dependent magnetic data were col-
lected for analytically pure samples of 4 and 6. The sensitivity of 2
toward the loss of THF prevented accurate measurements of this
compound in four attempts. Temperature-dependent data for 4
are shown in Figure 2, with field-dependent data shown in the
inset. Data for 6 are provided as SI (Figure S1).
At 300 K, the compounds achieve χT values of 0.88 (4) and
0.84 (6) emu K molꢀ1 per uranium ion. These χT values yield
μeff = 2.65 (4) and 2.58 (6) μB per uranium ion at 300 K. The
magnitudes of these moments are consistent with the reported
values for uranium(IV) complexes.12 The magnetic moments at
300 K reflect crystal-field splittings for 4 and 6 that are larger than
Figure 1. Solid-state molecular structures of 2 and 4 with thermal
ellipsoids drawn at the 50% probability level. Selected bond lengths (Å)
and angles (deg): 2, U1ꢀN1 = 1.931(5), U1ꢀN2 = 2.631(6), U1ꢀN3 =
2.648(6), U1ꢀI1 = 3.2111(12), U1ꢀI2 = 3.2028(11), U1ꢀO1 =
2.521(3), U1ꢀO2 = 2.531(3), N1ꢀU1ꢀI2 = 165.08(13), N1ꢀU1ꢀ
I3 = 98.79(13); 4, U1ꢀN1 = 2.125(8), U1ꢀN2 = 2.349(9), U1ꢀC21 =
2.847(10), U2ꢀN2 = 2.125(9), U2ꢀN1 = 2.328(8), U2ꢀC9 = 3.014(10),
UꢀClave = 2.657(3), UꢀOave = 2.495(8), N1ꢀU1ꢀN2 = 77.9(3),
N1ꢀU2ꢀN2 = 78.4(3), U1ꢀN1ꢀU2 = 101.2(3), U1ꢀN2ꢀU2 =
101.5(4), Cl1ꢀU1ꢀCl2 = 151.51(10), Cl3ꢀU2ꢀCl4 = 155.73(9).
of complex 1 and contain UꢀN bond lengths that are similar
to those found in other dimeric monoimidouranium(IV)
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Inorganic Chemistry
COMMUNICATION
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: boncella@lanl.gov.
’ ACKNOWLEDGMENT
R.E.J. and L.P.S. thank the Seaborg Institute (Los Alamos
National Laboratory) and the LANL LDRD program for partial
support of this work. E.J.S. acknowledges financial support from
the University of Pennsylvania. E.J.S. and J.M.K. are grateful for
partial support from the NSF MRSEC program under Award
DMR-0520020.
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In this Communication, we have demonstrated that
monoimidouranium(IV) complexes with the general formula
U(NR)(X)2(L)n (R = tBu, 2,6-iPr2C6H3, 2-tBuC6H4; X = Cl, I;
L = tBu2bpy, n = 1; L = THF, n = 2, 4) can be synthesized in a
facile manner from UX4 and 2 equiv of a primary amide. This
simple approach promises to open up a new frontier in organoi-
midouranium chemistry, allowing for the synthesis of complexes
that possess significant steric and electronic control at the metal
center. Importantly, this methodology offers tremendous poten-
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particular transactinide derivatives, which could greatly expand
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’ ASSOCIATED CONTENT
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S
Supporting Information. Complete details of the pre-
b
paration and characterization of 2ꢀ6, including X-ray crystal-
lographic details (as a CIF file) of 2 and 4ꢀ6, and magnetic data
for 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
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