New chemistry from an old reagent: Mono- and dinuclear macrocyclic uranium(III) complexes from [U(BH4)3(THF)2]

Article abstract of DOI:10.1021/ja504835a

A new robust and high-yielding synthesis of the valuable U^III synthon [U(BH₄)₃(THF)₂] is reported. Reactivity in ligand exchange reactions is found to contrast significantly to that of uranium triiodide. This is exemplified by the synthesis and characterization of azamacrocyclic U^III complexes, including mononuclear [U(BH ₄)(L)] and dinuclear [Li(THF)₄][{U(BH₄)} ₂(?-BH₄)(L^Me)] and [Na(THF) ₄][{U(BH₄)}₂(?-BH₄)(L ^A)(THF)₂]. The structures of all complexes have been determined by single-crystal X-ray diffraction and display two new U ^III₂(BH₄)₃ motifs.

Full text of DOI:10.1021/ja504835a

Communication
pubs.acs.org/JACS
New Chemistry from an Old Reagent: Mono- and Dinuclear
Macrocyclic Uranium(III) Complexes from [U(BH4)3(THF)2]
Polly L. Arnold,*,† Charlotte J. Stevens,‡,† Joy H. Farnaby,‡,† Michael G. Gardiner,§ Gary S. Nichol,†
and Jason B. Love*,†
†EaStCHEM School of Chemistry, University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JJ, U. K.
§School of Physical Sciences (Chemistry), University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia
S
* Supporting Information
ABSTRACT: A new robust and high-yielding synthesis of
the valuable UIII synthon [U(BH4)3(THF)2] is reported.
Reactivity in ligand exchange reactions is found to contrast
significantly to that of uranium triiodide. This is
exemplified by the synthesis and characterization of
azamacrocyclic UIII complexes, including mononuclear
[U(BH4)(L)] and dinuclear [Li(THF)4][{U(BH4)}2(μ-
BH4)(LMe)] and [Na(THF)4][{U(BH4)}2(μ-BH4)(LA)-
(THF)2]. The structures of all complexes have been
determined by single-crystal X-ray diffraction and display
two new UIII2(BH4)3 motifs.
Figure 1. Azamacrocyclic ligands used in this work.
dinuclear [U2I4(L)], respectively,12 but the chemistry is highly
dependent on reaction conditions.
he highly reducing nature of UIII and its many accessible
Tfrontier bonding orbitals enable a wide range of
impressive small molecule activation chemistry with substrates
including N2, NO, CO, CO2, arenes, and alkynes.1 The
encapsulation of one or more of these large ions in a defined
macrocyclic microenvironment would be expected to result in
significantly better control over the activation and subsequent
substrate reactivities compared with simple, flexible supporting
ligand sets. An illustrative UIII example is that the tetradentate
triamido tren-based ligand enabled the isolation of the first
uranium dinitrogen complex,2 while a complex of a more rigid
polypyrrolic framework enabled the only f-block example of the
complete cleavage of the triple bond in N2.3
Pyrrolic macrocyclic ligands are well-known in uranium
chemistry, and the coordination chemistry of uranyl and
neptunyl cations4 relevant to separations chemistry5 and
actinide sensing applications6 has been extensively studied.
The majority of our work in this area using the octadentate,
tetraanionic Pacman calixpyrroles (H4LR/A, Figure 1) has
explored the reduction and functionalization of the uranyl
dication.7 However, there are very few examples of low
oxidation state complexes of these ligands known.8 In the last
10 years, we have been able to synthesize the mononuclear
complex [UIV(LEt)]9 and the magnetically interesting oligo-
meric UIII and NpIII iodides [(AnIIII)2(LEt)]n, for which no
structural information was obtainable from standard crystallo-
graphic techniques.10 Very recently, we also reported the first
actinide complexes of the conformationally flexible, small-cavity
macrocycle trans-calix[2]benzene[2]pyrrolide (L)2− (H2L,
Figure 1).11 This ligand is competent for the stabilization of
one or two UIII cations as [UI(THF)(L)] and the very unusual
The nontrivial role of solvent,13 choice of alkali metal salt,14
and U-precursor15 in UIII chemistry is well precedented if not
well understood and often yields dramatic differences in
reaction chemistry. We have found considerable synthetic
difficulties in the reactions of UI3 with (L)2−, (LR)4− R = Me,
Et, and (LA)4−, observing the formation of multiple products,
unstable complexes, and poorly soluble materials that were
difficult to characterize (see below). This led us to investigate
borohydrides as alternative UIII precursors.
Borohydride compounds of uranium were targeted during
the Manhattan project because of their volatility, and extensive
U(BH4)4 chemistry was reported in spite of its nontrivial
synthesis.16 The thermal instability of these UIV complexes with
respect to UIII underlines the attractiveness of borohydride as a
supporting ligand for UIII chemistry:17 Ephritikhine and co-
workers demonstrated the thermal decomposition of U(BH4)4
to [U(BH4)3(η-C6H3Me3)]18 and the subsequent protonation
to [U(BH4)2(THF)5][BPh4], a rare example of a UIII cation.19
Prior to this work the only report of U(BH4)3 from a UIII
precursor was the reaction of UH3 with diborane.20 This
reaction allowed the structural characterization of the tris-
(THF) adduct [U(BH4)3(THF)3] but in a 4% yield. The
reaction between UCl3 and 3 equiv of NaBH4 yielded a red
solid which was not fully characterized.21
In contrast, we have found that the reaction between UI3 and
3 equiv of NaBH4 reproducibly yields [U(BH4)3(THF)2] 1 (eq
1) as an analytically pure, red microcrystalline material in >90%
Received: May 14, 2014
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
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3NaBH4, THF
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ U(BH4)3(THF)2
UI3(THF)4
−3NaI
1 (90%)
(1)
yield after removal of volatiles and Soxhlet extraction of the
crude reaction product with Et2O. The degree of solvation has
been determined by NMR spectroscopy (see SI) and elemental
analysis. Unfortunately, crystals of 1 isolated from a variety of
solvents could not be analyzed by X-ray diffraction due to rapid
and destructive desolvation during crystal mounting. The
formal low coordination number of UIII in [U(BH4)3(THF)2]
may well be augmented in the solid state by oligomerization.
Figure 2. Solid-state structure of 2 (displacement ellipsoids are drawn
at 50% probability). For clarity all H atoms are omitted. Selected bond
distances (Å) and angles (deg) for 2: U1−N1 2.452(4), U1−N2
1
The H and 11B NMR spectra of 1 in d8-THF at ambient
2.476(4), U1···B1 2.927(7), N1−U1−N2 118.94(14), [aryl1]cent
−
temperature exhibit very broad resonances assigned to BH4− at
85 and 230 ppm, respectively.
U1− [aryl2]cent 174.73.
The development of a routine and reproducible route to 1
has allowed us to probe the low oxidation state uranium
chemistry of the mono- and dinucleating macrocycles shown in
Figure 1. The reaction between 1 and K2L in THF at ambient
temperature gives [U(BH4)(L)] 2 as a dark-brown solid in 77%
yield after workup (eq 2). Unlike the synthesis of the iodide
While the bridging mode of the borohydride in 2 could not be
determined in the solid-state structure, it is likely to be (μ-
H)2BH2 based on the U1···B1 separation,24 which is consistent
with the IR data.
The Pacman family of Schiff-base calixpyrroles was
developed to provide bimetallic ligands for the cooperative
activation of small molecules within the intermetallic cleft.25 In
light of this, we have long sought to exploit the combined
reducing power of two UIII centers bound within such a
framework. However, reactions of UI3 as the UIII source with
the ortho-phenylene hinged macrocycle H4LEt result in poorly
soluble, oligomeric materials and reactions with the anthracenyl
macrocycle H4LA form unstable species which could not be
isolated cleanly (see SI).10
The reactions between 2 equiv of 1 and the alkali metal salts
of the Pacman macrocycles H4LMe and H4LA in THF yield the
ionic dinuclear products [M(THF)4][{U(BH4)}2(μ-BH4)(L)-
(THF)x] 3 and 4, respectively, where M = Li, L = LMe, x = 0,
(3), and M = Na, L = LA, x = 2, (4) as crystalline solids in
moderate isolated yields of 24% and 30% (Scheme 1). These
are highly unusual dinuclear UIII/UIII complexes of a single
ligand, the only previous example of which is [U2I4(L)].12
While the solid-state structures of UIV(BH4)4 are oligomeric
with bridging BH4 groups,26 3 and 4 are molecular and the first
compounds to contain the {UIII-(μ-BH4)-UIII} moiety. Indeed,
the ability of BH4 to effectively bridge two UIII centers in the
intermetallic cleft appears to provide important stabilization to
these dinuclear complexes and may explain their improved
kinetic stability compared to the iodide analogues.
derivatives [UI(THF)(L)] and [U2I4(L)], this reaction is high
yielding and selective, and the mono(borohydride) 2 has
improved thermal stability and solubility compared to the
mono(iodide) analogue. It also does not require the use of Li2L
(which generates unwanted, soluble byproducts) or a reduction
step.
1
The H NMR spectrum of 2 in d8-toluene is consistent with
a C2v symmetric macrocyclic environment and η6:κ1:η6:κ1
metal−ligand binding, with the geminal methyl groups
observed as two magnetically nonequivalent, contact-shifted
singlets of equal intensity at 3.14 and −0.69 ppm. The (BH4)−
protons are seen as a very broad resonance at 113 ppm (W1/2
=
994 Hz) in the 1H NMR spectrum and as a singlet at 170 ppm
in the 11B{1H} NMR spectrum, consistent with an averaged
BH4 proton environment on the NMR time scale; there was no
resolution of the coupling in the 11B spectrum (Figure 3). The
IR spectrum of 2 displays strong absorptions in the region of
2500−2000 cm−1 consistent with (μ-H)2BH2 binding: ν(B−
Ht) 2414 and 2384 cm−1 and ν(B−Hμ) 2120 cm−1.22
The 1H NMR spectra of 3 and 4 (Figure 3) display
paramagnetically contact-shifted resonances corresponding to
symmetric ligand environments in which the two N4 metal
binding pockets are identical. The alkyl groups at the meso
carbons, which are equivalent in the spectra of the free
macrocycles, become desymmetrized in the metalated products
and appear as two sets of resonances corresponding to the two
groups oriented toward the intermetallic cavity and the two
pointing away from the cavity. Broad resonances are observed
in the 1H NMR spectra of 3 at 63 ppm (W1/2 = 702 Hz) and 4
at −73 ppm (W1/2 = 498 Hz) which are assigned to the protons
of the terminal borohydride ligands. No resonances are
observed for the bridging borohydride protons of either
Single crystals of 2 were grown by vapor diffusion of hexanes
into a saturated THF solution at ambient temperature. The
uranium macrocycle binding in the molecular structure (Figure
2) of 2 is directly analogous to that seen in the unsolvated
iodide complex [UI(L)].12 The interplanar arene angle, which
gives a measure of uranium−arene interaction, of 14.32° is even
smaller than the 15.52° found in [UI(L)]. However, the U1···
B1 separation of 2.927(7) Å is much longer than those found in
U(BH4)3(THF)3 (2.625−2.699 Å)20 and other examples of UIII
borohydrides.16c In [U(BH4)3(DMPE)2] (DMPE = dimethyl-
phospinoethane),23 two distinct U···B separations were seen,
with the shorter (2.68(4) Å) axial ligand assigned as (μ-H)3BH
and the longer (2.84(3) Å) equatorial ligands as (μ-H)2BH2.
1
complex, even in the H{11B} spectra.
The 11B NMR spectra show two different boron environ-
ments in a 1:2 ratio (Figure 3). In the spectrum of 3 broad
resonances are observed at 325 ppm (μ-BH4) and 212 ppm
(terminal BH4), while for 4 they appear at 212 ppm (μ-BH4)
B
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Scheme 1. Synthesis of [Li(THF)4][(UBH4)2(μ-BH4)(LMe)] 3 and [Na(THF)4][(UBH4)2(μ-BH4)(LA)(THF)2] 4
Table 1. Selected Distances (Å) and Angles (deg) for 3 and
4py
3
4py
5.9243(3)
U···U
4.7884(3)
U···Bt
U···Bμ
mean U−Npyr
mean U−Nim
Bt−U−Bμ
U−Bμ−U
2.630(9), 2.640(9)
2.915(7), 2.911(7)
2.59
2.681(9), 2.723(8)
2.949(7), 2.977(7)
2.49
Figure 3. 11B NMR spectra (d8-THF) of the complexes 2 (lower,
blue), 3 (upper, red), and 4 (middle, gray). Spectra of 3 and 4 were
acquired over the range 60−340 ppm; that of 2 was acquired over the
range −20 to 200 ppm.
2.50
2.62
158.6(3), 158.7(3)
110.5(3)
175.5(2), 169.3(2)
177.8(3)
Single crystals of the pyridine solvate 4py were grown from a
pyridine/hexane solution of 4 at ambient temperature. In
contrast to 3, in the solid-state the anthracenyl macrocycle
adopts the classic Pacman geometry (Figure 5) since the
and 207 ppm (terminal BH4). Coupling to the H atoms is not
resolved on the NMR time scale. The IR spectra of 3 and 4
display several overlapping bands in the region from 2500 to
2100 cm−1 consistent with multiple BH4 binding modes.22
Single crystals of 3 were grown from a saturated THF
solution at ambient temperature. In the solid state (Figure 4)
Figure 4. Solid-state structure of the anionic portion of 3
(displacement ellipsoids are drawn at 50% probability). For clarity,
H atoms, lattice solvent and the [Li(THF)4]+ cation are omitted.
Figure 5. Solid state structure of 4py (displacement ellipsoids are
drawn at 50% probability). For clarity, all H atoms except those of the
BH4 ligands, the four pyridine molecules ligating the Na+ cation, and
lattice solvent are omitted.
the complex does not adopt the classic Pacman geometry,
where the ortho-phenylene rings of the macrocycle act as
hinges, but instead the ligand flexes at the meso carbons so
adopting a bowl-shaped geometry.4b Each UIII is pseudo-
octahedral and bound in the equatorial macrocyclic plane to the
two ortho-imine nitrogens of one aryl ring and to two adjacent
pyrrolide nitrogens, while two BH4 ligands, one terminal (B1/
B2) and one bridging (B3), occupy the axial sites. The BH4
protons could not be located crystallographically, but the U1···
B1 and U2···B2 distances of 2.630(9) and 2.640(9) Å,
respectively (Table 1), are consistent with (μ-H)3BH binding
modes for the exo BH4 ligands (B1 and B2), while the endo BH4
protons bridge the two metal centers with longer U1/2···B3
separations of 2.915(7) Å and 2.911(7) Å. This would agree
with an endo-(μ-H)2BH(μ-H) interaction averaged over the
two UIII centers. By adopting the bowl conformation, the U1···
B3···U2 interaction is optimized since the two UIII centers can
achieve greater separation (U1···U2 = 4.7884(3) Å) than is
seen in archetypal Pacman complexes of this ligand (M···M
3.1−4.2 Å).27
separation between the two metal binding pockets is much
greater than for the ortho-phenylene macrocycle. For the
pentagonal bipyramidal UIII centers, four of the equatorial
donors are provided by the two imine nitrogens (attached to
different anthracene rings) and two pyrrolide nitrogens of the
ligand with the fifth site occupied by a pyridine solvent
molecule, having displaced the less strongly coordinating THF.
One terminal and one bridging BH4 occupy the axial sites. The
borohydride protons were located from the difference Fourier
map and their positions refined (see SI). The terminal BH4
groups (B1 and B2) are bound in a (μ-H)3BH fashion, while
the central bridging BH4 (B3) binds to each UIII center through
two bridging (μ-H)2B(μ-H)2 consistent with the U···B
separations (Table 1). The sodium cation is coordinated to
the remaining H of one terminal BH4 group and four pyridine
molecules in the solid-state structure, but this does not persist
in solution where the NMR spectra show a symmetric ligand
environment. The terminal and bridging Bt···U and Bμ···U
separations observed in 3 and 4py are similar (Table 1), but the
C
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Journal of the American Chemical Society
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451, 315. (b) Arnold, P. L.; Pecharman, A. F.; Love, J. B. Angew. Chem.,
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complexes differ due to the constraints imposed by the two
macrocycles. While this core is essentially linear in 4py, the
metal-borohydride unit in 3 has a significant bend at the
bridging BH4 (U1−B3−U2 = 110.5(3)°) and the {BH4-U-(μ-
BH4)} units deviate from linearity. The U···U separation is also
1.14 Å longer in 4py (5.9243(3) Å) than in 3 (4.7884(3) Å).
We have demonstrated a new and straightforward synthesis
of [U(BH4)3(THF)2], the first directly from UIII. This well-
defined UIII precursor has greater use than UI3 in the formation
of mono- and dinuclear UIII complexes. This is only the second
report of dinuclear UIII complexes of a single ligand for which
structural information has been obtained and the first
incorporating a {UIII-(μ-BH4)-UIII} unit. These complexes
combine the reducing potentials of two UIII metal cations with
the chemical versatility of the borohydride anion and present
unique opportunities to explore the chemistry of low oxidation
state uranium confined within different macrocyclic frame-
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, characterization data, and crystallographic
information. X-ray structural CCDC codes 999591−999594.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Polly.Arnold@ed.ac.uk; Jason.Love@ed.ac.uk
■
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‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Schroder, M.; Love, J. B. Angew. Chem., Int. Ed. Engl. 2007, 46, 584.
̈
The authors acknowledge the University of Edinburgh and the
EPSRC for funding, and the University of Tasmania for study
leave support for MGG.
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Placa, S. J.; Lippard, S. J.; Mayerle, J. J. Inorg. Chem. 1972, 11, 3009.
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