Magnetic exchange coupling in chloride-bridged 5f-3d heterometallic complexes generated via insertion into a uranium(IV) dimethylpyrazolate dimer

Article abstract of DOI:10.1021/ja0725044

The homoleptic dimer complex [U(Me₂Pz)₄]₂ (Me₂Pz- = 3,5-dimethylpyrazolate) was obtained upon reacting UCl₄ with KMe₂Pz in THF, followed by extraction into toluene. The structure of the dimer consists of two U^IV centers, each coordinated in a pseudo trigonal bipyramidal geometry, connected through two bridging Me₂Pz^- ligands. Bases are capable of cleaving the dimer; for example, reaction with THF affords the mononuclear complex (Me₂Pz)₄U(THF). More importantly, the dimer can be cleaved via insertion of terminal chloride ligands, such that reactions with (cyclam)MCl₂ (M = Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane) in dichloromethane generate the linear, chloride-bridged clusters (cyclam)M[(μ-Cl)U(Me₂Pz)₄]₂. Variable-temperature magnetic susceptibility data were collected for all three clusters to probe any possible magnetic exchange coupling. The data for the cluster centered by an S = 0 Zn^II ion exhibit behavior typical of U^IV complexes, with χ_MT decreasing steadily as the temperature drops. Data for the CuU₂ cluster show a parallel variance with temperature, indicating the absence of any magnetic exchange coupling. In contrast, subtracting the ZnU₂ data from the NiU₂ data exposes a rise in χ_MT with decreasing temperature, suggesting weak ferromagnetic coupling between the Ni^II (S = 1) and U^IV centers. Employing a simple spin-only exchange model, a lower bound for the coupling constant was estimated at J = 2.3 cm^-1. Consistent with a simple superexchange mechanism for the coupling, density functional theory calculations performed on a [(Me₂Pz)₄UCl]^- fragment of the cluster show the spin to reside in 5f_xyz and 5f_z(x2-y2) orbitals, exhibiting δ symmetry with respect to the U-Cl bond. Low-temperature magnetization data collected for NiU₂ suggest the presence of a large axial zero-field splitting; however, ac magnetic susceptibility experiments gave no indication of single-molecule magnet behavior. Copyright

Full text of DOI:10.1021/ja0725044

Published on Web 08/11/2007
Magnetic Exchange Coupling in Chloride-Bridged 5f-3d Heterometallic
Complexes Generated via Insertion into a Uranium(IV) Dimethylpyrazolate
Dimer
Stosh A. Kozimor, Bart M. Bartlett, Jeffrey D. Rinehart, and Jeffrey R. Long*
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460
Received April 10, 2007; E-mail: jrlong@berkeley.edu
The observation of slow magnetic relaxation in certain high-
spin molecules has sparked a search for new systems capable of
supporting higher relaxation barriers.1 Recently, a number of such
“single-molecule magnets” have been discovered in which lan-
thanide ions contribute substantially or entirely to the magnetic
anisotropy responsible for the relaxation barrier.2 Unfortunately,
high spin ground states that are well-isolated in energy are difficult
to achieve in species of this type, because the contracted 4f orbitals
of lanthanide ions have little overlap with bridging ligand orbitals,
leading to magnetic exchange constants of only a few wavenumbers
at best.3 In contrast, the 5f orbitals of actinide ions can exhibit an
increased radial extension relative to the core electron density,4
potentially enabling stronger magnetic coupling via superexchange.
To date, however, there are only a few cases of actinide-containing
molecules for which the presence of magnetic exchange coupling
has been established.5 This is due in part to difficulties with
interpreting the magnetic susceptibility data, but also to the dearth
of approaches for generating multinuclear 5f-3d assemblies.6
Herein, we present a new strategy for synthesizing halide-bridged
5f-3d clusters, and demonstrate weak ferromagnetic coupling
within a linear UIV-Cl-NiII-Cl-UIV species.
The homoleptic dimer complex [U(Me2Pz)4]2 (Me2Pz- ) 3,5-
dimethylpyrazolate) was obtained from a reaction of UCl4 with
KMe2Pz in THF. Its formation proceeds under mild conditions, in
stark contrast to the more demanding synthetic procedures required
for generating homoleptic lanthanide pyrazolates.7 Depending on
Figure 1. Insertion of (cyclam)MCl2 complexes into the uranium(IV)
3,5-dimethylpyrazolate dimer to give linear, chloride-bridged clusters.
Orange, yellow, green, gray, and blue spheres represent U, M, Cl, C, and
N atoms, respectively; H atoms are omitted for clarity. The trinuclear clusters
reaction conditions, two additional products were observed: a THF
adduct, (Me2Pz)4U(THF), and a salt of the pentapyrazolate uranium-
(IV) complex, K[U(Me2Pz)5]‚THF. Despite the propensity of
uranium(IV) for binding THF,8 the dimer can be isolated in high
yield by repeated extractions into toluene. The pentapyrazolate
byproduct is insoluble in toluene, and its yield was minimized by
adjusting the reaction stoichiometry to precisely four equivalents
of unsolvated KMe2Pz per equivalent of UCl4.
reside upon inversion centers within the crystal structures. Selected
interatomic distances (Å) and angles (deg) for [U(Me2Pz)4]2: mean
U-Nterminal 2.37(3), U-Nbridging 2.489(7)-2.719(7), U‚‚‚U 3.9485(5), mean
U-N-U 99.1(7), U-Nbridging-C 116.5(6)-119.8(5), 140.9(6)-144.2(6),
and for (cyclam)M[(µ-Cl)U(Me2Pz)4]2 with M ) Ni, Cu, Zn, respectively:
U-Cl 2.838(2), 2.785(2), 2.822(2), mean U-N 2.39(1), 2.40(1), 2.40(2),
M-Cl 2.564(2), 2.774(2), 2.680(2), mean M-N 2.061(2), 2.020(3), 2.085-
(3), U‚‚‚M 5.0687(8), 5.1822(6), 5.1419(5), U-Cl-M 139.47(9), 137.61-
(7), 138.31(6).
Green needle-shaped crystals of [U(Me2Pz)4]2‚0.5PhMe formed
upon cooling a saturated toluene solution of the dimer, and X-ray
analysis revealed the structure depicted at the top of Figure 1. Here,
considering each pyrazolate ligand as a single entity, the UIV centers
adopt a trigonal bipyramidal coordination geometry. Two of the
ligands, one equatorial and one axial, form asymmetric bridges,
with the latter exhibiting the more elongated U-N bonds of 2.616-
(7) and 2.719(7) Å. To our knowledge, this is the first example of
a homoleptic pyrazolate complex of an actinide element. Related
dimeric structures in which each metal center bears just two terminal
pyrazolate ligands have been observed, for example, in [Ln(tBu2-
Pz)3]2 (Ln ) La, Nd, Yb, Lu).9 In contrast to the tetravalent uranium
ions, the trivalent lanthanide ions exhibit a coordination geometry
intermediate between square planar and tetrahedral, with cases
of both symmetric and asymmetric pyrazolate bridging modes
arising.
Bases are capable of cleaving the uranium(IV) dimer via
displacement of the axial bridging pyrazolate ligands. For example,
reaction with THF yields the pseudo trigonal bipyramidal complex
(Me2Pz)4U(THF), wherein THF occupies an axial coordination site
(see Figure S6 in the Supporting Information). More interestingly,
the dimer can be cleaved via insertion of a terminal chloride ligand
of a transition metal complex. In particular, reactions with (cyclam)-
MCl2 (M ) Ni, Cu, Zn; cyclam ) 1,4,8,11-tetraazacyclotetrade-
cane) in dichloromethane generate the trinuclear species (cyclam)M-
[(µ-Cl)U(Me2Pz)4]2. Note that this is consistent with the established
halophilicity of uranium(IV),10 and that, fortunately, in these cases,
complete chloride abstraction does not occur. Although there are
examples of molecular compounds with halide ligands bridging
between uranium(IV) and alkali metal cations,11 to our knowledge
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C O M M U N I C A T I O N S
K/mol at 5 K.12 This behavior is typical of 5f2 UIV complexes, and
the reduction in moment at lower temperatures is consistent with a
poorly isolated singlet ground state arising from crystal field
effects.13 For the CuU2 cluster, the data show a nearly identical
temperature dependence, but with increased øMT values. By
subtracting the ZnU2 data from the CuU2 data, the individual UIV
contributions are removed, leaving the magnetism of the central S
1
) /2 CuII ion, together with any vestiges of magnetic exchange
coupling. The subtracted data, shown in red, reveal a moment that
is essentially invariant with temperature, affording a mean value
of øMT ) 0.36(2) emu‚K/mol (µeff ) 1.70(4) µB). The results are
consistent with a CuII ion in the absence of any magnetic exchange
coupling, albeit with a moment that is slightly below the usual value
for a pseudooctahedral complex. For comparison, (cyclam)CuCl2
was observed to display a mean value of øMT ) 0.43(1) emu‚K/
mol (µeff ) 1.84(1) µB).14 The slight difference may be associated
with the change in ligand field arising upon attachment of a UIV
center to each chloride ligand, and perhaps with the presence of a
small amount of diamagnetic impurity. The sole unpaired electron
II
2
2
of the Cu center of the CuU2 cluster resides in the dx -y orbital,
which has δ symmetry with respect to the Cu-Cl bonds. Thus,
the lack of exchange coupling can be explained as arising from
the orthogonality of the CuII spin orbital with σ- and π-type orbitals
of the chloride bridges. The complete invariance of the subtracted
data affirms that the ZnU2 cluster serves well in partitioning out
the magnetism of the UIV centers.
As shown at the bottom of Figure 2, the øMT data for the NiU2
cluster display a quite different temperature dependence. Subtracting
the ZnU2 data affords a øMT value of 1.19 emu‚K/mol at 298 K,
consistent with the presence of an S ) 1 NiII ion having g ) 2.18.
Below 100 K, the subtracted data increases, achieving a maximum
of 1.26 emu‚K/mol at 30 K. This rise in moment is not expected
for an isolated NiII center and indicates the presence of ferromag-
netic exchange interactions. To assess the strength of the exchange
coupling, a simple spin-only model was adopted. A temperature-
invariant contribution of øMT ) 1.00 emu‚K/mol for each UIV center
was added back into the subtracted data (see Figure S2, Supporting
Information). Utilizing MAGFIT 3.115 and a spin Hamiltonian of
the form Hˆ ) -2JˆNi‚(SˆU(1) + SˆU(2)) + gµBS‚B, the data above 40
K were then fit to give the parameters J ) 2.3 cm-1, g ) 1.86, and
TIP ) 8.25 × 10-4 emu/mol. To our knowledge, this represents
the first estimate of a 5f-3d coupling constant within a molecular
complex. Note, however, that the treatment provides only a lower
bound for J. The actual J value is likely substantially higher, since
we have not accounted for the gradual loss of spin on the UIV centers
with decreasing temperature.
Figure 2. Variable temperature magnetic data for the linear clusters
(cyclam)M[(µ-Cl)U(Me2Pz)4]2 (M ) Ni (green), Cu (blue), Zn (black)).12
The data sets shown in red were obtained upon subtracting the Zn data
from the Cu data (upper) and Ni data (lower). The solid line represents a
fit to the latter data, as discussed in the text.
these are the first examples of halide-bridged species involving
uranium(IV) and transition metal ions.
Cooling saturated solutions of the products afforded a set of
isostructural crystals, featuring the intended linear, chloride-bridged
clusters (see Figure 1, lower). Here, a U(Me2Pz)4 unit is appended
to each of the trans chloride ligands of a central (cyclam)MCl2
complex. In comparing the three structures, the most obvious
difference lies in the variation of the MII-Cl distances, with a Jahn-
Teller distortion leading to a significantly longer CuII-Cl separation.
Importantly, the U-Cl-M angle and UIV coordination environment
show little variation as M changes. Thus, the species containing
the diamagnetic ZnII ion can be used as a reasonable model in
accounting for the UIV contributions to the magnetism of the other
clusters. Note that such an approach is necessitated by the lack of
a suitable theoretical construct for predicting the temperature
dependence of the magnetic moments of the constituent actinide
ions. This method has been employed previously in probing
exchange coupling in bridged uranium(IV)-transition metal
complexes5b-f and is commonly applied in evaluating compounds
containing lanthanide ions with unquenched orbital angular
momentum.3c
The drop in øMT for the subtracted data below 30 K stems in
part from this loss of UIV spin, but probably also from a zero-field
splitting contribution to the ground state. Indeed, variable-field
magnetization data collected at low temperatures reveal large
separations between isofield lines (see Figure S3, Supporting
Information), as typically associated with a significant axial zero-
field splitting. Despite this large anisotropy, ac magnetic suscep-
tibility measurements carried out at temperatures down to 1.8 K
with switching frequencies of up to 1500 Hz showed no out-of-
phase signal attributable to single-molecule magnet behavior.
Density functional theory calculations were performed on a
[(Me2Pz)4UCl]- fragment of the NiU2 cluster to determine the
symmetry of 5f orbitals involved in magnetic exchange. Assuming
the z-axis is oriented along the U-Cl bond, the spin-containing
Variable-temperature magnetic susceptibility data were measured
for all three trinuclear complexes (see Figure 2). The data for the
ZnU2 cluster exhibit a steady drop in øMT as the temperature is
lowered, decreasing from 2.16 emu‚K/mol at 298 K to 0.10 emu‚
2
2
orbitals are 5fxyz and 5fz(x -y ) (see Figure 3). Importantly, both of
these orbitals exhibit δ symmetry with respect to the U-Cl bond,
II
2
such that they are rigorously orthogonal to the Ni 3dz spin feeding
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Figure 3. Frontier energy level diagram and depictions of the spin-
containing molecular orbitals, as calculated for [(Me2Pz)4UCl]- using density
functional theory. The Me2Pz- ligands are shown as skeletal representations,
and the U-Cl axis is oriented vertically.
through σ-type Cl- orbitals. Thus, the observed ferromagnetic
coupling is consistent with a simple superexchange mechanism.
The foregoing results disclose a potentially generalizable means
of assembling 5f-3d clusters, and demonstrate weak ferromagnetic
coupling between UIV and NiII through a chloride bridge. Future
investigations will focus on utilizing this synthetic approach to
achieve higher-nuclearity clusters, as well as to enhance exchange
coupling via introduction of either bromide or iodide bridges, or
lower-valent uranium centers. We note that each of these directions
presents significant synthetic difficulties16 and that the utilization
of actinide elements in generating high-anisotropy single-molecule
magnets therefore remains a daunting challenge.
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(14) This result is consistent with the X-band EPR spectrum of the complex
at 80 K in frozen methanol, but differs significantly from the øMT value
of 0.17 emu‚K/mol reported previously: De Buysser, K.; Herman, G.
G.; Bruneel, E.; Hoste, S.; Van Driessche, I. Chem. Phys. 2005, 315, 286.
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Acknowledgment. This research was funded by NSF Grant No.
CHE-0617063. We thank the UC President’s Postdoctoral Fellow-
ship Program for support of B.M.B., Drs. Frederick J. Hollander
and Allen G. Oliver for expert advice on crystal structure
determinations, and Prof. James K. McCusker and Mr. Joel
Schrauben for assistance with collecting the EPR spectrum of
(cyclam)CuCl2.
Supporting Information Available: Complete experimental and
computational details. X-ray crystallographic files. This material is
available free of charge via the Internet at http://pubs.acs.org.
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