Synthesis and magnetic properties of a 4-(2′-pyrimidyl)-1,2,3,5- dithiadiazolyl dimanganese complex

Article abstract of DOI:10.1039/b512312g

A spin-bearing bis-bidentate ligand, designed from a pyrimidyl-substituted R-CN₂S₂ neutral radical, is used to co-ordinate two Mn(ii) metal centres yielding a thermally stable complex with antiferromagnetic coupling between the ligand-centred spin and the metal-centred spins, and thus an overall ferrimagnetic coupling scheme with a ground state S = 9/2. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512312g

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Synthesis and magnetic properties of a
4-(29-pyrimidyl)-1,2,3,5-dithiadiazolyl dimanganese complex{
Michael Jennings,a Kathryn E. Preuss*b and Jian Wub
Received (in Cambridge, UK) 1st September 2005, Accepted 31st October 2005
First published as an Advance Article on the web 25th November 2005
DOI: 10.1039/b512312g
coupling between the metals cannot be achieved in the reported
A spin-bearing bis-bidentate ligand, designed from a pyrimidyl-
substituted R-CN2S2 neutral radical, is used to co-ordinate two
Mn(II) metal centres yielding a thermally stable complex with
antiferromagnetic coupling between the ligand-centred spin and
the metal-centred spins, and thus an overall ferrimagnetic
coupling scheme with a ground state S = 9/2.
S-coordination geometry.
1,2,3,5-Dithiadiazolyl (DTDA) radicals, with a wide variety of
substituents in the 4- position, have been studied for many years.7
They are known to be 7 p electron planar rings with the unpaired
electron occupying a p* molecular orbital that has high spin
density on the S and N atoms and is nodal at the C atom. This
spin distribution can be predicted by ab initio calculations and is
borne out by solution EPR spectra.8 We chose the DTDA radical
as our building block based on these attributes. The p* singly
occupied molecular orbital (SOMO) with high spin density on the
nitrogen atoms facilitates strong magnetic coupling to a coordi-
nated metal ion and the node at the carbon atom diminishes the
electronic influence of any substituent at the 4-position. A further
consideration in our choice of radical building block is the report
that p-NC–C6F4–CN2S2 exhibits the onset of weak ferromagnet-
ism at low temperature arising from intermolecular interactions in
the solid state.9 This bodes well for the future of this project since
the incorporation of intermolecular magnetic interactions may
prove possible.
The coordination of paramagnetic metal ions to organic radicals is
a promising design for the development of molecule-based
magnets.1,2 There is, however, a limited number of organic radical
building blocks from which suitable ligands can be developed.
There is a greater dearth of designs incorporating a spin-bearing
organic radical capable of coordinating two or more metal ions to
ligand atoms of high spin density such that magnetic coupling
between the metal centres is mediated via coupling to the ligand
spin. Successful ligand designs of this nature include nitronyl
nitroxide3 and verdazyl4 radical building blocks. We recently
reported a metal complex of 4-(29-pyridyl)-1,2,3,5-dithiadiazole,
demonstrating for the first time the use of the R-CN2S2 radical as
an N-coordinating ligand.5 Herein, we report the first sulfur–
nitrogen heterocyclic radical ligand capable of N-coordinating two
metal ions 1, and we evince the nature of the magnetic coupling
between the unpaired electron on the ligand and those on two
coordinated high-spin Mn(II) metal centres of a dimanganese
complex 2 (Scheme 1). It is worth noting that coordination of Ph–
CN2S2 to two nickel ions and to two iron atoms via the S atoms
has previously been reported,6 however the mediation of magnetic
The synthesis of 4-(29-pyrimidyl)-1,2,3,5-dithiadiazole 1 was
undertaken following standard preparatory procedures for such
heterocycles10 and the physical properties of 1 were found to
be comparable to those of other known DTDA radicals. The
room temperature EPR of a solution of 1 in CH2Cl2 shows a
1 : 2 : 3 : 2 : 1 five line pattern consistent with coupling of one
unpaired electron to two equivalent nitrogen atoms (aN = 5.12 g,
g = 2.010(7)). The cyclic voltammetry of 1 in dry CH2Cl2 (0.087 M
n-Bu4NPF6) shows a reversible oxidation wave at E1/2(ox) =
+1.22 V (vs. SCE) and a reversible reduction wave at E1/2(red) =
20.79 V (vs. SCE). There is also a temperature- and concentration-
dependent irreversible reduction wave at Ered = 20.54 V (vs. SCE),
probably associated with the redox properties of a small quantity
of a dimer of 1 in solution.
Disorder in the crystalline state{ suggests two possible modes of
dimerization of the radical 1. The first is cofacial dimerization (as
˚
shown in Fig. 1) with intermolecular S–S contacts of 3.035 A and
˚
3.148 A (at 100 K). This mode of dimerization is common for
RCN2S2 molecules in the solid state11 and, given that the S–S
contact distance is smaller than the sum of the van der Waals radii
for two S atoms, it is expected that cofacial dimers of 1 in the
crystalline state are diamagnetic. The second mode of dimerization
that may be discerned from the disordered crystal structure is one
in which the DTDA ring of one molecule is p-stacked with the
pyrimidyl ring of the other molecule of a dimer pair. This mode of
dimerization has no close intermolecular contacts between sites of
high spin density and therefore is expected to give rise to a
Scheme 1 Radical ligand 1 and dimanganese complex 2.
aDepartment of Chemistry, The University of Western Ontario, London,
Ontario, Canada N6A 5B7
bDepartment of Chemistry, University of Guelph, Guelph, Ontario,
Canada N1G 2W1. E-mail: kpreuss@uoguelph.ca;
Fax: +1-519-766-1499; Tel: +1-519-824-4120
{ Electronic Supplementary Information (ESI) available: Experimental
details and CIF files for 1 and 2 are included. MAGMUN modeling of the
magnetic data is also available. See DOI: 10.1039/b512312g
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 341–343 | 341

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Fig. 1 An ORTEP drawing of a cofacial dimer of 1 representing one
part of the 50/50 disordered structure. Data collected at T = 100 K.
˚
Selected bond distances (A): S(13a)–S(14a), 2.093(3); S(23b)–S(24b),
2.086(3).
Fig. 3 Measured xT versus Tdata for complex 2 (#), collected at 1000 Oe.
Magnetic susceptibility was measured from 2 K to 300 K and back to 2 K. As
there was no variation between the field cooled and zero-field cooled
measurements, a representative cycle (300 K to 2 K) is shown. The data were
modeled as described in the text and the best fit is shown (—).
measurable amount of paramagnetism in the solid state. We are
currently investigating the magnetic and conductive properties of 1.
The dimanganese complex 2 was prepared by stirring two molar
equivalents of Mn(hfac)2?2THF with one equivalent of 1 in dry
THF at room temperature for 1 h. The crude product was
recovered from the solution as a dark purple powder and sublimed
under dynamic vacuum (1022 Torr) in a gradient-temperature tube
furnace (120 uC) yielding small green crystals suitable for elemental
analysis and magnetometry. Crystals suitable for X-ray analysis
were grown by sublimation under static vacuum (1022 Torr,
120 uC).
The single crystal X-ray structure{ of 2 (Fig. 2) confirms the
coordination of two manganese ions, with two hexafluoroacetyl-
acetonato-ligands apiece, to the pyrimidine and DTDA nitrogen
atoms of ligand 1. The ligand remains intact and approximately
planar (N53–C54–C55–N56 torsion angle 2.4(16)u). There is no
significant change in the intramolecular S–S and S–N distances as
compared with the free ligand 1. The complex does not form
dimers in the solid state.
optimized fit parameters were found to be: g = 2.03, J1
265.7 cm21, J2 = 23.5 cm21, zJ9 = 0.034 cm21 and xTIP
=
=
0.0043 cm21 where zJ9 accounts for the intermolecular coupling
in the solid state and xTIP is the temperature independent
paramagnetism.
At 300 K, the measured xT is 9.4 cm3 K mol21 (Fig. 3). This
compares well to the predicted high temperature limit of xT which
is equal to the sum of the contributions of two isolated Mn(II) ions
and one isolated organic radical (calculated to be 9.1 cm3 K mol21
for g = 2.) A small decrease in xT is observed as the temperature is
decreased from 300 K to about 150 K, followed by a significant
increase as the temperature is lowered further. This is consistent
with the irregular spin state structure predicted for a system with
antiferromagnetic coupling between the central site and the outer
Mn(II) sites, resulting in a ground state ferrimagnetic coupling
scheme with spin S = 9/2.2 As the temperature is initially lowered,
the first excited state to be depopulated is the highest spin state (S =
11/2) which results in a drop in the value of xT. Upon further
cooling, excited states with lower spin than the ground state are
depopulated resulting in an increase in xT. This is reminiscent of
previously reported linear Mn(II)–Cu(II)–Mn(II) systems12 and the
Mn(II)–verdazyl–Mn(II) system.4
The DC-SQUID magnetic susceptibility measurements were
collected on a homogeneous solid sample of 2 (purity confirmed by
elemental analysis) between 2 K and 300 K at a field strength of
1000 Oe. The magnetic data have been modeled as a three-spin
system using an isotropic spin Hamiltonian (H
=
2J1(2SMnSDTDA) 2 J2(SMnSMn) + gbSH 2 zJ9 + xTIP).§ The
We have demonstrated that the 4-(29-pyrimidine)-1,2,3,5-
dithiadiazolyl neutral radical 1 can be used as a bis-bidentate
ligand for the coordination of two metal cations. The resulting
complex 2, when Mn(II)(hfac)2 is chelated to both ligand sites, is
thermally stable and volatile, such that it can be purified by
sublimation at elevated temperatures under dynamic vacuum.
Furthermore, the unpaired electron associated with the p* SOMO
of the ligand is antiferromagnetically coupled to the unpaired
electrons of both Mn(II) centres such that all the metal unpaired
electrons are spin-aligned, generating a spin ground state S = 9/2.
Fig. 2 An ORTEP drawing representing one part of the disorder for one
of the two molecules of complex 2 observed in the single crystal X-ray
˚
data. Data collected at T = 100 K. Selected bond distances (A): S(51)–
Notes and references
{ Crystal data for 1. C5H3N4S2, M = 183.23, monoclinic, a = 7.1561(4), b =
3
˚
˚
10.6746(5), c = 10.1407(7) A, b = 116.994(3)u, U = 690.24(7) A , T =
100(2) K, space group P21/c, Z = 4, m(Mo-Ka) = 0.696 mm21, 6572
reflections measured, 1216 unique (Rint = 0.041) which were used in
S(52), 2.073(5); N(50)–Mn(1), 2.380(12); N(53)–Mn(1_2), 2.308(13);
N(53_2)–Mn(1), 2.308(13); N(56)–Mn(1_2), 2.26(3).
342 | Chem. Commun., 2006, 341–343
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calculations. The compound was refined as a 50/50 disorder, verified by
free-variable refinement. The 6-membered ring had three distinct distances
refined as free variables [C(apical)–N 1.337(11); N–C 1.325(11); C–C
1.399(10)]. All non-hydrogen atoms were refined with anisotropic
displacement parameters under ‘‘soft’’ restraints (ISOR, DELU, SIMU).
The final wR(F2) was 0.0882.
3 A. Caneschi, D. Gatteschi, P. Rey and R. Sessoli, Inorg. Chem., 1988,
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Crystal data for 2. C25H7F24Mn2N4O8S2, M = 1121.35, triclinic, a =
˚
8.9148(14), b = 12.209(2), c = 18.562(3) A, a = 96.563(10), b = 91.913(10),
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3
˚
¯
c = 110.860(9)u, U = 1869.5(5) A , T = 100(2) K, space group P1, Z = 2,
m(Mo-Ka) = 0.696 mm21, 18090 reflections measured, 6570 unique (Rint
=
0.103) which were used in calculations. In both of the crystallographically
unique manganese complexes, the DTDA was refined at half-occupancy (it
was located on a centre of symmetry.) The 6-membered ring was refined as
a regular hexagon. The displacement parameters of the atoms of DTDA
were refined isotropically with the symmetry related atoms having their
displacement parameters restrained to be equal. The final wR(F2) was
0.0962. CCDC 283236 and 283237.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b512312g
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§ This modeling was performed using ChiT Pro 1.0 E 2004 developed by
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Chem. Commun., 2006, 341–343 | 343