Uranium pyrrolylamine complexes featuring a trigonal binding pocket and interligand noncovalent interactions

Article abstract of DOI:10.1021/ic202411f

The syntheses of tri- and tetravalent uranium complexes of the Ar ^F₃TPA^3- ligand [Ar^F = 3,5-bis(trifluoromethyl)phenyl; TPA = tris(pyrrolyl-α-methylamine)] are described. Interligand noncovalent interactions between arene groups within the complexes are detected both in the solid state and in solution.

Full text of DOI:10.1021/ic202411f

Communication
pubs.acs.org/IC
Uranium Pyrrolylamine Complexes Featuring a Trigonal Binding
Pocket and Interligand Noncovalent Interactions
Andrew J. Lewis,† Ursula J. Williams,† James M. Kikkawa,‡ Patrick J. Carroll,† and Eric J. Schelter*,†
†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 & Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
20 mL of THF and 0.5 mL of pyridine at −25 °C (Scheme 1).
The product was obtained in 55% yield following crystal-
ABSTRACT: The syntheses of tri- and tetravalent
uranium complexes of the ArF TPA3− ligand [ArF = 3,5-
3
bis(trifluoromethyl)phenyl; TPA = tris(pyrrolyl-α-methyl-
amine)] are described. Interligand noncovalent interac-
tions between arene groups within the complexes are
detected both in the solid state and in solution.
Scheme 1. Syntheses of Complexes 1 and 2
he incorporation of noncovalent interactions into the
T
secondary coordination sphere of metal complexes is a
proven strategy for stabilizing reactive fragments and effecting
new chemistry.1,2 Because the metal−ligand bonding in many f-
block complexes is largely ionic and nondirectional,3 we
postulated that noncovalent interactions between ligands
could promote directed coordination chemistry at f-block
ions by providing collective secondary structures.4 The use of
collective secondary structures could contribute to several
research themes including the stabilization of uranium(III) and
uranium(V) complexes that are prone to disproportionation,5,6
the isolation of complexes of reactive small-molecule fragments
at f-block ions,7 and the mitigation of kinetic factors, such as
large inner-sphere electron-transfer reorganization energy,
which impede the oxidation of cerium(III) compounds.8
As a first step to probe these ideas, we set out to prepare
complexes showing attractive noncovalent interactions between
ligands in the coordination sphere of a uranium ion that could
be detected in the solid state using X-ray crystallography and in
solution using NMR spectroscopy. We reasoned that a well-
defined molecular pocket at a metal ion picketed by electron-
deficient arene groups could bind aryl-based ligands coopera-
tively through dative and arene−arene interactions. The C3-
symmetric Ar3TPA3− and other tris(pyrrolyl) ligands have
recently been used in transition-metal chemistry for small-
molecule activation9 and magnetism studies.10,11 Herein we
lization. The moderate yield of the complex is due to its
solubility in organic solvents. Attempts to synthesize a THF
analogue of 1 failed, generating only mixtures of intractable
products in the absence of pyridine. The X-ray structure of 1
(Figure 1) displays a seven-coordinate, C3-symmetric geometry
at the UIII ion, where the ArF TPA3− ligand binds in a
3
tetradentate fashion and three pyridine molecules complete the
coordination sphere. The UIII ion in 1 is displaced ∼0.86 Å
above the plane of the pyrrolyl nitrogen atoms, in contrast to
the ∼0.4−0.7 Å displacements observed in transition-metal
TPA complexes.9−11,13
The U−Npyrrolyl bond lengths in 1 are long at an average
distance of 2.602(6) Å. Reported UIII−Npyrrolyl bonds in
calix[4]pyrrole complexes are ∼2.52 Å.14,15 The long U−
Npyrrolyl distances are presumably the result of a poor fit of the
UIII ion with three bound pyridines into the pocket of the
ArFTPA3− ligand. Notably, each pyridine ligand is found in an
offset face-to-face orientation with respect to an adjacent ArF
report the synthesis of uranium complexes of the ArF TPA3−
3
ligand [ArF = 3,5-bis(trifluoromethyl)phenyl; TPA = tris-
(pyrrolyl-α-methylamine)]. In contrast to related transition-
metal complexes of the ArF TPA3− ligand, the uranium
3
complexes form trigonal frameworks with three open-
coordination sites. The uranium complexes show evidence for
substituent on the ArF TPA3− ligand at an observed pyridine−
arene−arene interactions12 between the ArF TPA3− ligand and
3
ArF interaryl−centroid distance of ∼3.5 Å. These metrics
3
pyridyl ligands in the solid state and in solution.
Green-black UIII(ArFTPA)(py)3 (1) was produced from the
slow addition of a dilute tetrahydrofuran (THF) solution of
Received: November 9, 2011
Published: December 13, 2011
K3ArF TPA(THF)1.5 to a stirred solution of UI3 in a mixture of
3
© 2011 American Chemical Society
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Inorganic Chemistry
Communication
atoms on the ArF substituents are inequivalent on the NMR
time scale. The 19F NMR spectrum supports this assignment. It
displays two broad peaks in a 1:1 integration due to hindered
rotation of the ArF rings nestled into the cleft of the adjacent
ligand. It is evident that the self-complementary arrangement of
arene groups in the structure of 2 contributes to the formation
of the 1:2 complex. Noting the tendency of 2 to form from UI3,
we attempted to prepare it directly from UCl4.
The addition of K3ArF TPA(THF)1.5 to a solution of UCl4 in
3
THF/pyridine at −25 °C produced red UIV(ArF TPA)(py)2Cl
3
(3; Scheme 2). Surprisingly, performing this reaction at room
Scheme 2. Syntheses of Complexes 3 and 4 and Failed
Synthesis of 2 from UCl4
Figure 1. Thermal ellipsoid plot of 1 at 50% probability. Hydrogen
atoms and interstitial solvents are omitted for clarity. Bond lengths (Å)
and angles (deg): U−N(1) 2.605(6), U−N(2) 2.595(5), U−N(3)
2.605(6), U−N(4) 2.561(5), U−N(5) 2.641(6), U−N(6) 2.653(6),
U−N(7) 2.632(6); N(4)−U−N(1) 71.1(2), N(4)−U−N(2) 70.8(2),
N(4)−U−N(3) 69.9(2), N(4)−U−N(5) 130.0(2), N(4)−U−N(6)
128.7(2), N(4)−U−N(7) 128.5(2).
support the presence of face-to-face interarene interactions
between the pyridine ligands and ArF substituents. Face-to-face
arene−arene interactions are typically ∼3.5 Å16 but can be
expected to be longer with more bulky substituents. The
1
assignment of the H NMR spectrum of 1 confirms the C3
symmetry of the complex in solution at room temperature
because eight resonances are observed as expected. A single,
sharp resonance is observed in the 19F NMR spectrum at −65.8
ppm, consistent with a symmetric ligand environment.
Magnetism measurements carried out on 1 displayed a room
temperature magnetic moment of 2.91 μB, typical for the 5f3
electronic configuration of a UIII ion.17 Variable-temperature
and low-temperature/variable-field magnetism studies also
supported the assignment of 1 as a uranium(III) complex
(see Figures S17 and S18 in the Supporting Information).
Electronic structure calculations performed on the optimized
geometry of 1 demonstrate that the valence 5f orbitals are
largely uninvolved in bonding in this complex (see Supporting
Information).
temperature led to no observable formation of 2 even when 2
equiv of K3ArF TPA(THF)1.5 was added (Scheme 2). In our
3
hands, complex 2 was only accessible through a disproportio-
nation reaction of the uranium(III) starting material. As in the
study of 1, attempts to prepare the analogous THF adduct of 3
in the absence of pyridine were unsuccessful.
The structure of 3 reveals a seven-coordinate geometry
(Figure S3 in the Supporting Information). The average U−
Npyrrolyl bond length in 3 is 2.375(9) Å, ∼0.23 Å shorter than
the related distances in 1 and slightly shorter than the
previously reported UIV−pyrrolyl bond lengths in calix[4]-
pyrrole complexes.14 In complex 3, one pyridine ligand is
sandwiched between two ArF rings at an average ArF centroid to
pyridine centroid distance of ∼3.6 Å. The second pyridine
ligand shows no short interarene distances with the ArF groups.
The UIV ion lies ∼0.87 Å above the plane of the three pyrrolyl
nitrogen atoms, nearly identical with 1. Magnetic susceptibility
measurements carried out on 3 displayed a room temperature
magnetic moment of 3.32 μB and variable-temperature and
-field responses typical for a 5f2 ion.17
Following the same reaction conditions for the synthesis of 1
but performing the addition at room temperature resulted in
the formation of the bright-yellow complex [K-
(py)5]2[UIV(ArF TPA)2] (2; Scheme 1). Heating at 50 °C
3
rapidly produced 2 as the major product. Generation of this
uranium(IV) product from a uranium(III) starting material
implicates a disproportionation pathway (4UIII → 3UIV + U0).18
Crystallization from diethyl ether provided 2 in 52% yield. The
crystal structure of 2 (Figure S2 in the Supporting Information)
shows tetradentate coordination of the UIV ion by each of the
1
The number of H and 19F NMR resonances of 3 is highly
dependent on the temperature as well as the solvent (Figure 3).
In pyridine-d5, five ArF TPA ligand resonances are observed for
3
3 in the 1H NMR spectrum between −23 and +62 °C,
indicating that the complex is effectively C3-symmetric in a
pyridine-d5 solution at room temperature. These peaks appear
sharp at higher temperature and broaden at lower temperature,
until they can no longer be observed below −23 °C (Figure
S14 in the Supporting Information). The absence of resonances
observed for bound pyridine and the effective C3 symmetry is
consistent with fast exchange of the pyridine ligands with
pyridine-d5. Similar behavior is observed in 19F NMR, where a
single sharp peak broadens at lower temperature. Spectra
collected for 3 in toluene-d8 exhibit more complicated features.
At room temperature, the 1H NMR spectrum displays a
number of broad, overlapping peaks, consistent with an
two ArF TPA3− ligands in an eight-coordinate, pseudocubic
3
geometry. This structure also contains interarene interactions
between all of the ArF and pyrrolyl groups. The arms of the two
ArF TPA3− ligands interdigitate such that each ArF ring is
3
aligned with an adjacent pyrrolyl ring with an average ArF
centroid to pyrrolyl centroid distance of ∼3.7 Å. The UIV ion in
2 is displaced ∼1.0 Å above each plane of three pyrrolyl
nitrogen atoms.
The 1H NMR spectrum of 2 in pyridine-d5 at room
temperature displays six resonances in a 2:1:1:1:1:1 ratio. Five
resonances are expected based on the equivalency of the six ArF
pyrrolylmethylene arms of the two ligands. The appearance of a
sixth resonance is rationalized on the basis that the o-hydrogen
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Inorganic Chemistry
Communication
centers. We are currently expanding upon these initial findings
in the context of small-molecule activation.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic files (CIFs), full experimental details,
computational details and data, H and 19F NMR spectra, and
1
magnetism data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: schelter@sas.upenn.edu.
■
ACKNOWLEDGMENTS
■
E.J.S. acknowledges the University of Pennsylvania for financial
support. We thank the NSF for support of the X-ray
diffractometer (Grant CHE-0840438) and computing cluster
(Grant CHE-0131132). E.J.S. and J.M.K. are grateful for partial
support from the NSF MRSEC program (Grant DMR-
0520020). We thank Johnson Matthey for donation of the
palladium reagents, Prof. Donald H. Berry (Penn) and Dr.
Thibault Cantat (CEA-Saclay) for assistance with the
calculations, and Prof. Christopher R. Graves (Albright
College) for helpful discussions.
Figure 3. Variable-temperature 19F NMR data for complexes 3 (top)
and 4 (bottom). Lines are provided as a guide for the eye. Dotted lines
indicate the region around coalescence.
unsymmetric ligand environment. Below room temperature,
multiple decoalescence processes are evident in the 19F NMR
spectrum (Figure S16 in the Supporting Information). The
appearance of multiple decoalescence processes in the variable-
temperature NMR experiment suggests that the molecule
adopts multiple low-energy conformations that readily
equilibrate. These conformations likely correspond to inter-
actions between the ArF rings and the pyridine ligands. To
further investigate the role of interarene interactions in 3, we
prepared a second Lewis base adduct of 3.
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UIV(ArF TPA)(py-O)2Cl (4). Complex 4 was obtained by
3
crystallization at −25 °C from a concentrated solution in
toluene layered with hexane in 90% yield, and the structure was
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in 4 as in 3 (Figure S4 in the Supporting Information), but
there is an elongation (∼0.07 Å) of the U−Npyrrolyl bonds in 4
consistent with the increased electron-donating nature of the
py-O ligand relative to pyridine. The major difference in the
solid-state structures of 3 and 4 is that the py-O ligands in 4 are
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3
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applicable strategy for manipulation of reactive uranium
39
dx.doi.org/10.1021/ic202411f | Inorg. Chem. 2012, 51, 37−39