High-spin ribbons and antiferromagnetic ordering of a Mn II-biradical-MnII complex

Article abstract of DOI:10.1021/ja4067783

A binuclear metal coordination complex of the first thiazyl-based biradical ligand 1 is reported (1 = 4,6-bis(1,2,3,5-dithiadiazolyl)pyrimidine; hfac =1,1,1,5,5,5,-hexafluoroacetylacetonato-). The Mn(hfac)₂-biradical- Mn(hfac)₂ complex 2 is a rare example of a discrete, molecular species employing a neutral bridging biradical ligand. It is soluble in common organic solvents and can be easily sublimed as a crystalline solid. Complex 2 has a spin ground state of S_T = 4 resulting from antiferromagnetic coupling between the S_birad = 1 biradical bridging ligand and two S_Mn = 5/2 Mn^II ions. Electrostatic contacts between atoms with large spin density promote a ferromagnetic arrangement of the moments of neighboring complexes in ribbon-like arrays. Weak antiferromagnetic coupling between these high-spin ribbons stabilizes an ordered antiferromagnetic ground state below 4.5 K. This is an unusual example of magnetic ordering in a molecular metal-radical complex, wherein the electrostatic contacts that direct the crystal packing are also responsible for providing an efficient exchange coupling pathway between molecules.

Full text of DOI:10.1021/ja4067783

Communication
pubs.acs.org/JACS
High-Spin Ribbons and Antiferromagnetic Ordering of a MnII-
Biradical-MnII Complex
Elisabeth M. Fatila,† Rodolphe Clerac,‡,∥ Mathieu Rouzieres,‡,∥ Dmitriy V. Soldatov,† Michael Jennings,§
́
̀
and Kathryn E. Preuss*,†
†Department of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada
‡CNRS, CRPP, UPR 8641, F-33600 Pessac, France
∥University of Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France
§FreeLance Crystallography, London, Ontario N6J 3J5, Canada
S
* Supporting Information
coupling due to loss of coplanarity, or twisting, that destroys
the extended π-conjugation.15 Discrete, soluble metal-biradical-
metal complexes are relatively rare with one notable
contribution, a binuclear complex of a bis(semiquinone)
biradical dianion reported by Shultz.12 Such species can act as
model systems to study magnetic interactions. They also
provide a further element with which to modify application-
driven biradical architectures.
ABSTRACT: A binuclear metal coordination complex of
the first thiazyl-based biradical ligand 1 is reported (1 =
4,6-bis(1,2,3,5-dithiadiazolyl)pyrimidine; hfac =1,1,1,5,5,5,-
hexafluoroacetylacetonato-). The Mn(hfac)2-biradical-Mn-
(hfac)2 complex 2 is a rare example of a discrete, molecular
species employing a neutral bridging biradical ligand. It is
soluble in common organic solvents and can be easily
sublimed as a crystalline solid. Complex 2 has a spin
ground state of ST = 4 resulting from antiferromagnetic
coupling between the Sbirad = 1 biradical bridging ligand
and two SMn = 5/2 MnII ions. Electrostatic contacts
between atoms with large spin density promote a
ferromagnetic arrangement of the moments of neighboring
complexes in ribbon-like arrays. Weak antiferromagnetic
coupling between these high-spin ribbons stabilizes an
ordered antiferromagnetic ground state below 4.5 K. This
is an unusual example of magnetic ordering in a molecular
metal-radical complex, wherein the electrostatic contacts
that direct the crystal packing are also responsible for
providing an efficient exchange coupling pathway between
molecules.
In devising the first thiazyl-based biradical ligand, 4,6-
bis(1,2,3,5-dithiadiazolyl)pyrimidine 1 (Figure 1), we have
rganic biradicals have an important place in the historical
Odevelopment of bond theories. In 1936, Huckel outlined
̈
a molecular orbital approach elucidating π-conjugated (non-
Figure 1. EPR spectrum of 1 in 50/50 mixture of MePh/CH2Cl2 at
RT. Inset left: line drawing of 1. Inset right: one of two π* SOMOs of
biradical ligand 1.
Kekule)
́
biradicals1 and laid the foundation for a simple
topological model used to predict the ground state of biradical
alternant hydrocarbons.2 From a synthetic standpoint, note-
worthy contributions include those of Tschitschibabin,3
Schlenk,4 and Yang,5 however Ullman’s biradical, bis(nitronyl)
nitroxide,6 changed the field by demonstrating unprecedented
stability of a light-atom π-biradical species. More recent
developments have been primarily application driven, including
biradical designs for dynamic nuclear polarization (DNP)
agents,7 possible spin-pump devices,8 and conductive9 and
magnetic materials.10 While monoradicals have been employed
as paramagnetic bridging ligands between metal ions (the so-
called “metal-radical approach” to molecular magnets),11 similar
use of biradicals is less common.12 As noted by Rajca,13 designs
featuring biradicals as bridging ligands typically result in
coordination polymers14 and often suffer from weak exchange
two goals. One is to create a discrete, molecular π-biradical-
bridged metal complex designed to maintain coplanarity of the
extended π-system. To this end, we use chelation of two
Mn(hfac)2 fragments to anchor the bridging biradical ligand in
complex 2 in a roughly planar geometry (hfac =1,1,1,5,5,5-
hexafluoroacetylacetonato-) (Figure 2). Planarity is important
because it stabilizes the triplet ground state of the meta-
substituted pyrimidine biradical ligand. It is worth noting that
the architecture used to maintain planarity in 2 is different from
Received: July 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4067783 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
reactions employing 1 as a ligand but also facilitates
characterization of this biradical species. The formation of π-
stacked “dimers” of DTDA radicals is known to be reversible,
such that a monomer/dimer equilibrium exists in solution,21
making it possible to measure the EPR spectrum of a very
dilute solution of 1 in MePh/CH2Cl2 at ambient temperature
(Figure 1). A 9-line pattern centered at g = 2.0104 is evident,
with a peak-to-peak distance of roughly 2.55 G. This is
consistent with a biradical species wherein the exchange
coupling between the two unpaired electrons is significantly
larger than the hyperfine interactions with four approximately
equivalent14N nuclei (aN ≈ 5.1 G)22 and suggests a preference
for planarity of the ring systems in solution.
Using a simple topological model, a triplet ground state is
predicted for meta-substituted biradical alternant hydro-
carbons.2 Density functional theory calculations (uB3LYP/6-
31g(d,p)) on the gas-phase geometry-optimized biradical ligand
1 predict a planar structure and also assign an Sbirad = 1 ground
state to biradical 1 (ΔES‑T = 28.33 cm−1). The singly occupied
molecular orbitals (SOMOs) are π* in nature, with coefficients
on the S and N atoms of the DTDA heterocycles (Figure 1)
and closely resemble the SOMO of typical DTDA radicals.23
The result is large alpha spin density on the DTDA sulfur and
nitrogen atoms (Figure S1).
Coordination complex 2 is prepared by reacting biradical
ligand 1 and Mn(hfac)2(THF)2 in a mixture of toluene and
CH2Cl2. Despite its high molecular weight, complex 2 is
significantly more volatile than ligand 1, and sublimation is the
most facile method of purification and crystal growth for
structural analysis.
Figure 2. A ribbon-like assembly of molecules in 2 governed by the
close Sδ+···Oδ− contacts; H atoms have been omitted and only O
atoms of the hfac are shown on all but one molecule. Inset: line
drawing of 2.
that of the bis(semiquinone) analog,12 wherein planarity must
be maintained as a function of crystal packing rather than by
molecular design. The second goal is to achieve magnetic
ordering of a discrete, molecular metal-radical complex by
harnessing the properties of thiazyl radicals, particularly the
large spin density and partial positive charge at unencumbered
peripheral sulfur atoms. We have previously shown that strong
electrostatic contacts between Sδ+ atoms of a thiazyl radical and
Oδ− atoms of a neighboring hfac ligand direct the solid-state
packing of thiazyl-metal complexes and provide an efficient
pathway for exchange-coupling between molecules.16,17 By
using a biradical complex, we aim to increase the dimensionality
of these contacts and create an exchange-coupled network.
High-spin ribbons of complex 2 are formed in the solid state
and weak antiferromagnetic (AF) interactions between these
ribbons are sufficient to generate an AF ordered ground state.
Thus, 2 is a rare example of a magnetically ordered,
nonpolymeric, metal-radical complex.
Biradical ligand 1 is prepared starting from 4,6-dicyano-
pyrimidine18 which is converted to N4,N4,N4,N6,N6,N6-hexakis-
(trimethylsilyl)pyrimidine-4,6-dicarboxamidine. Ring closure is
effected by reaction with excess S2Cl2 generating the dication
chloride [12+][Cl−]2. An efficient reduction to the biradical
ligand 1 is achieved using triphenylantimony, a common
reducing agent for generating 1,2,3,5-dithiadiazolyl (DTDA)
radicals.19 Sublimation of 1 results in high-purity, microcrystal-
line material, however neither sublimation nor recrystallization
produces crystals suitable for structural analysis. In the solid
state, 1 is diamagnetic, which is readily explained by the
propensity of DTDA radicals to form diamagnetic π-stacked
“dimers”.20 Although insoluble in most organic solvents, 1 is
moderately soluble in a 1:1 mixture of toluene and methylene
chloride. Solubility is important for subsequent coordination
Complex 2 crystallizes in P21/c with two molecules per
asymmetric unit. The molecular structure of 2 (Figure 2) can
be described as N,N-chelation of two Mn(hfac)2 fragments by
the roughly planar biradical ligand 1. However, the most
important feature is the abundance of close intermolecular
contacts between sulfur and oxygen atoms. Electrostatic
interactions between the Sδ+ atoms of DTDA rings and
electronegative atoms of neighboring molecules, such as Oδ−,
Nδ−, and Brδ−,24 are known to influence the packing of DTDA
radicals. In the present case, multiple short Sδ+···Oδ− contacts
between the two distinct molecules in the asymmetric unit are
apparent. Further short Sδ+···Oδ− contacts between the
symmetry-unrelated molecules in adjacent asymmetric units
define ribbon-like structures propagating along the c-axis in the
(010) plane (Figure 2).
The magnetic properties of 2 were investigated between 300
and 1.8 K with an applied field of 1000 Oe (Figure 3). At 300
K, the χT product is 9.7 cm3 K mol−1, which is close to the
expected value (9.75 cm3 K mol−1) for two noninteracting hs-d5
ions (SMn = 5/2) plus one organic biradical (Sbirad = 1),
assuming g ≈ 2. As the temperature is lowered, the χT product
decreases to 8.4 cm3 K mol−1 at 33 K and then increases rapidly
with further cooling to a maximum value of 13.9 cm3 K mol−1
at 5.5 K. This magnetic behavior is consistent with AF coupling
between the MnII ions and the Sbirad = 1 biradical ligand,
generating a spin ground state of ST = 4 for a single molecule of
2. It should be noted that AF exchange coupling is anticipated
by simple orbital overlap arguments (nonorthogonal overlap
between the π* magnetic orbitals of the ligand and at least one
of the 3d orbitals of each hs-d5 MnII).25 The magnetic data can
be numerically fitted down to 9 K using an isotropic
̂
̂
̂
̂
Heisenberg spin Hamiltonian, H = −2J(SMn1 + SMn2)·Sbirad
,
=
treating the molecule as a spin trimer composed of two SMn
B
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Journal of the American Chemical Society
Communication
Figure 3. Temperature dependence of χT product for 2 at 1000 Oe (χ
defined as M/H per complex); open circles indicate measured data
points; red line represents best fit as described in text. Inset: (T, H)
magnetic phase diagram determined from data shown in Figure 4.
5/2 and one Sbirad = 1 and including intercomplex magnetic
interactions zJ′ in the frame of the mean-field approximation.26
The best values for this model are J/kB = −5.9(1) K, zJ′/kB =
+0.17(3) K, and g = 2.03(5).
Figure 4. (a) Temperature dependence of the magnetic susceptibility
below 15 K for applied dc field below 5500 Oe; (b) field dependence
of the magnetization below 10 K up to 7 T; (c) enlarged view of the
field dependence of the magnetization below 4.4. K up to 2 T; (d)
field dependence of the field derivative of the magnetization (dM/dH)
below 5 K up to 2 T, highlighting the antiferromagnetic-paramagnetic
phase transition. Dots represent data points, and lines provide visual
aid.
Importantly, the above model of the magnetic susceptibility
leads to effective and dominant intercomplex interactions of a
ferromagnetic nature. The closest actual intermolecular
contacts in the structure of 2 are the short Sδ+···Oδ− distances
(2.92 − 3.28 Å) between the thiazyl ring of one molecule and
the hfac oxygen atom of a neighboring molecule. We have
previously shown that contacts of this kind in Mn(hfac)2
complexes of DTDA monoradicals17 provide an efficient
pathway for AF coupling between the radical and a neighboring
MnII ion and can be understood in the context of molecular
orbital theory.16 Since the coupling between the biradical ligand
and the coordinated MnII ions is also AF, the net result in 2 is a
ferromagnetic arrangement of the Mn-biradical-Mn complex
moments in the ribbon-like networks. As described below, an
additional AF coupling between these networks is enough to
stabilize a three-dimensional ordered AF ground state.
The temperature dependence of the susceptibility χ has been
measured at various applied fields (Figure 4a) and shows a
maximum in χ below 5 K. The temperature at which this peak
occurs shifts to lower values with increasing applied field. The
field dependence of the magnetization (M vs H) has been
recorded at various temperatures (Figure 4b,c) and shows no
clear saturation up to 7 T (reaching 8.5 μB at 1.83 K). Below
4.5 K, the dM/dH vs H curves exhibit a characteristic maximum
(at 1.83 K roughly around 5000 Oe; Figure 4d). This feature is
indicative of a field-induced transition between an AF ordered
phase at low fields and a paramagnetic phase at higher fields. In
order to determine the temperature dependence of this critical
field, the maximum of the dM/dH vs H and χ vs T plots
(Figure 4d, a) has been followed, allowing the construction of
the (T, H) phase diagram shown in Figure 3 inset. This critical
allow for chelation of two Mn(hfac)2 fragments, generating a
rare example of a discrete (i.e., noncoordination polymer)
biradical-bridged coordination complex 2 that is soluble and
volatile. The judicious design of 1 provides complex 2 with a
means of engaging in multiple close electrostatic contacts
between atoms with large spin density. These contacts enable a
ferromagnetic arrangement of the moments of neighboring
complexes in a ribbon-like array. Weak antiferromagnetic
coupling between the high-spin ribbons is sufficient to stabilize
an antiferromagnetically ordered ground state (TN = 4.5 K).
Magnetic ordering of discrete molecular metal-radical com-
plexes, particularly through an established exchange mecha-
nism, is rarely observed.
ASSOCIATED CONTENT
* Supporting Information
Synthetic, computational, and crystallographic details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
kpreuss@uoguelph.ca
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
́
field extrapolates to zero at a Neel temperature (TN) of ∼4.5 K.
■
Under an applied dc field, 2 displays metamagnetic behavior
associated with a critical field extrapolated at 0 K to be HC(0) ≈
5500 Oe. From this field, the average inter-ribbon interaction is
estimated to be zJperp/kB ≈ −94 mK through the relation 2z|
K.E.P. thanks NSERC (DG), the Canadian government (CRC
Tier II), and U. Bordeaux (visiting professorship). E.M.F.
thanks NSERC (CGS-D). R.C. and M.R. thank CNRS, U.
́
Bordeaux, the Conseil Regional d’Aquitaine, and the ANR
(NT09_469563, AC-MAGnets project). D.V.S. thanks CFI,
MEDI, and U. Guelph.
2
Jperp|ST = gμBHC(0)ST.
Ligand 1 is the first biradical thiazyl-based ligand. Unlike
most bi- or multiradical ligands, 1 has been designed to
maintain coplanarity of the extended π-system when coordi-
nated to two metal ions, thereby ensuring an Sbirad = 1 spin
ground state. The two N,N-coordination “pockets” of ligand 1
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