1-phenyl-3-(pyrid-2-yl)benzo[e][1,2,4]triazinyl: The first blatter radical for coordination chemistry

Article abstract of DOI:10.1021/ic402954p

A neutral air- and moisture-stable N,N′-chelating radical ligand, 1-phenyl-3-(pyrid-2-yl)benzo[e][1,2,4]triazinyl (1) was synthesized and characterized by electron paramagnetic resonance spectroscopy, X-ray crystallography, and magnetic measurements. Subsequent reaction of 1 with Cu(hfac)₂·2H₂O (hfac = hexafluoroacetylacetonate) under ambient conditions afforded the coordination complex Cu(1)(hfac) ₂ in which the radical binds to the metal in a bidentate fashion. Magnetic susceptibility data collected from 1.8 to 300 K indicate a strong ferromagnetic metal-radical interaction in the complex and weak antiferromagnetic radical···radical interactions between the Cu(1)(hfac)₂ units. Detailed computational investigations support this assignment. Radical 1 is a new addition to the growing library of 1,2,4-triazinyl radicals and the first member of this family of paramagnetic species synthesized specifically for coordination purposes.

Full text of DOI:10.1021/ic402954p

Communication
pubs.acs.org/IC
1‑Phenyl-3-(pyrid-2-yl)benzo[e][1,2,4]triazinyl: The First “Blatter
Radical” for Coordination Chemistry
Ian S. Morgan,*,† Anssi Peuronen,† Mikko M. Hanninen,† Robert W. Reed,‡ Rodolphe Clerac,§,⊥
́
̈
and Heikki M. Tuononen*,†
†Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland
̈
̈
̈
̈
‡Department of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada
§CNRS, CRPP, UPR 8641, F-33600 Pessac, France
⊥Universite
́
Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France
S
* Supporting Information
behavior.13−18 Curiously, until now, no coordination attempts
of this radical or modifications of the radical architecture for
coordination purposes have been reported.
ABSTRACT: A neutral air- and moisture-stable N,N′-
chelating radical ligand, 1-phenyl-3-(pyrid-2-yl)benzo[e]-
[1,2,4]triazinyl (1) was synthesized and characterized by
electron paramagnetic resonance spectroscopy, X-ray
crystallography, and magnetic measurements. Subsequent
reaction of 1 with Cu(hfac)2·2H2O (hfac = hexafluor-
oacetylacetonate) under ambient conditions afforded the
coordination complex Cu(1)(hfac)2 in which the radical
binds to the metal in a bidentate fashion. Magnetic
susceptibility data collected from 1.8 to 300 K indicate a
strong ferromagnetic metal-radical interaction in the
complex and weak antiferromagnetic radical···radical
interactions between the Cu(1)(hfac)2 units. Detailed
computational investigations support this assignment.
Radical 1 is a new addition to the growing library of
1,2,4-triazinyl radicals and the first member of this family
of paramagnetic species synthesized specifically for
coordination purposes.
The preparation of 1 begins with formation of the
corresponding pyridylthioamide species via the reaction of
picoline, sodium sulfide, and aniline, followed by a reaction with
phenyl hydrazine to afford pyridyl amidrazone and H2S (see S1
in the Supporting Information, SI). Ring closure and oxidation of
the purified amidrazone can then be accomplished using a
combination of Pd/C and 1,8-diazabicycloundec-7-ene in air.13
The purity of the product is ensured using column chromatog-
raphy followed by recrystallization. None of the above steps
requires any specific precautions, and 1 can be stored under
ambient conditions, as reported for other derivatives of 2.19
The X-band electron paramagnetic resonance (EPR) spec-
trum of 1 in CH2Cl2 (Scheme 1 and S2 in the SI) shows a septet
pattern consistent with the coupling of the unpaired electron to
ew magnetic materials are becoming increasingly
Nimportant as technology advances toward the ultimate
size barrier, the molecular limit. A quintessential requirement for
the development of “molecular spin science” is the ability to
rationally manipulate the magnetic couplings within these
materials. One possible technique is to utilize the metal-radical
approach in which organic radical ligands are used to mediate the
magnetic coupling between paramagnetic metal centers.1 While
there exists a plethora of open-shell ligands that can coordinate to
metals (e.g., semiquinones,2,3 nitroxides,4 thiazyls,5,6 and
verdazyls7,8), many of them are not air- and moisture-stable
and, hence, cannot be used as building blocks for practical
magnetic materials.9,10 In this contribution, we report the
synthesis of a new stable coordinating radical, 1-phenyl-3-(pyrid-
2-yl)benzo[e][1,2,4]triazinyl (1), and demonstrate its desired
coordinating properties via complete characterization of its
copper(II) complex by experimental and theoretical methods.
Blatter’s radical (2), developed in 1968, has received limited
attention despite its stability toward both air and water.11 It has
been cocrystallized with tetracyanoquinodimethane to make
pressure-sensitive charge-transfer complexes,12 while its deriva-
tives have been extensively studied for their magnetic
Scheme 1. Synthesis of the radical 1 along with its EPR and CV
signatures (in CH2Cl2)
Received: November 29, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/ic402954p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
three distinct nitrogen atoms (g = 2.0040, aN1 = 6.43 G, aN2
=
copper(II) salts and the stability of 1 toward water, we opted to
use copper(II) hexafluoroacetylacetonate dihydrate, Cu(hfac)2·
2H2O, as the starting material. Thus, the complex Cu(1)(hfac)2
(3) was prepared by treating 1 with Cu(hfac)2·2H2O in CH2Cl2
under ambient conditions. Crystalline material of the product
was grown by slow diffusion with pentane to afford large purple
blocks. The single-crystal X-ray structure of 3 displays an
octahedral copper(II) center with a typical Jahn−Teller distorted
geometry (Figure 3). The radical binds to the metal in a
bipyridine-like fashion, as designed, with a nearly planar
coordination pocket.
4.29 G, aN3 = 3.92 G, and lw = 2.01 G). The EPR signature of 1
can be easily understood on the basis of density functional theory
(DFT) calculations (S6 in the SI). While there are four nitrogen
atoms in the molecule, the singly occupied molecular orbital
(SOMO) of 1 possesses a node on the carbon atom bearing the
pyridyl substituent (Figure 1), as calculated for other 1,2,4-
Figure 1. Isosurface plots of the SOMO (left) and spin density (right) of
1.
triazinyls.19 The spin density of 1 is absent on the pyridyl group
(and thus N22), while it shows an excess α density on the fused
triazine and benzo rings, with a large percentage at the two
nitrogen atoms N2 and N4, which could coordinate to metal
centers. Cyclic voltammetry experiments of 1 in CH2Cl2
(Scheme 1 and S3 in the SI) indicate reversible 0/1+ and 1−/
0 couples with E1/2 = 6 and −1140 mV and ΔEpp = 140 and 160
mV, respectively (vs standard calomel electrode).20
Figure 3. Single-crystal X-ray structure of 3 (left) and its packing to
weakly interacting π dimers in the solid state at 123 K (right). Fluorine
and hydrogen atoms are removed for clarity, and thermal ellipsoids are
drawn at the 50% probability level.
Complex 3 exhibits a reversible single crystal-to-single crystal
phase transition, whereupon cooling the sample from 298 to 123
K results in an approximate doubling of the crystallographic a
axis. This temperature-related phase transition is accompanied
by resolution of the positions of the CF3 groups as well as minor
(ca. 0.1 Å) strengthening of the radical···radical interaction. It
should be noted that, in contrast to 1, complex 3 is unable to
effectively π-stack in the solid state because of the steric bulk of
the hfac ligands. Consequently, the Cu(1)(hfac)2 units interact
very weakly in the crystal structure even at 123 K [closest N···N
distance of 3.745(2) Å in 3 compared to 3.215(1) Å for 1], and
the observed phase transition has no apparent effect on the
magnetic properties of 3 [cf. related “breathing” crystals of
composition Cu(LR)(hfac)2].22
Crystals of 1 belong to monoclinic space group P21/n. There
are four molecules per unit cell that form radical pairs with close
N···N [3.215(1) Å] and C···C [3.294(2) Å] contacts (Figure 2).
The magnetic properties of 3 were investigated over the
temperature range from 1.8 to 300 K with an applied field of 1000
Oe (Figure 4). At 300 K, the χT value is 0.96 cm3 K mol−1, with
Figure 2. Single-crystal X-ray structure of 1 (left) and its packing to π
dimers in the solid state at 123 K (right). Thermal ellipsoids are drawn at
the 50% probability level.
Magnetic susceptibility measurements performed on a poly-
crystalline sample of 1 indicate that the radical pairs are strongly
antiferromagnetically coupled and are essentially diamagnetic
below 100 K (S5 in the SI). On the basis of radical dimer
1
topology, this system can be viewed as an S = /2 spin dimer
described by the isotropic Heisenberg spin Hamiltonian H =
−2JS1·S2, where J is the radical···radical magnetic interaction. In
the weak-field approximation, the analytical expression of
magnetic susceptibility can be estimated by applying the van
Vleck equation.21 The experimental data are well fitted to this
model with g = 2.0(1) and J/kB = −412(3) K, while the broken-
symmetry DFT calculations predict J = −335 K (S6 in the SI).
After synthesis and characterization of the new radical ligand 1,
its coordinating properties with paramagnetic metal centers were
put to the fore. Owing to the intrinsic azaphilic nature of
Figure 4. Temperature dependence of the χT product for 3 at 1000 Oe
[with χ defined as the molar magnetic susceptibility equal to M/H per
mole of Cu(1)(hfac)2]: measured data (black circles); best fit (red line).
Inset: schematic view of the spin interaction topology in 3.
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Inorganic Chemistry
Communication
an increase to 1.05 cm3 K mol−1 when the temperature is lowered
to 100 K. This thermal behavior can be attributed to
ferromagnetic coupling between the radical ligand and the
copper(II) spin. Upon further cooling, the χT product decreases
to 0.2 cm3 K mol−1, which is consistent with additional
antiferromagnetic interactions likely between the radical spins
of the neighboring Cu(1)(hfac)2 complexes. The magnetic data
were fitted to the theoretical susceptibility calculated in the low-
field approximation using the isotropic Heisenberg spin
Hamiltonian H = −2J1(S1·S2 + S3·S4) − 2J2(S2·S3) and
considering the presence of both Cu−radical (J1) and radical···
radical (J2) interactions (Figure 4, inset, and S5 in the SI).23 The
model fits extremely well to the experimental data (Figure 4) and
accurately gives the relevant magnetic parameters: g = 2.10(5),
J1/kB = +144(7) K, and J2/kB = −9.5(5) K. This analysis
demonstrates the S = 1 ground state of the Cu(1)(hfac)2
complex (J1) and coupling of the pairs of these complexes to
give an overall S = 0 state (J2). We note that the Cu−radical
magnetic exchange interaction is larger in 3 than in analogous
verdazyl-copper(hfac) complexes,24 possibly because of Jahn−
Teller distortion of the bonds to the coordinated verdazyl ligand.
Both the J1 and J2 interactions observed in 3 can be rationalized
via the orbital-symmetry approach (S6 in the SI). The
ferromagnetic coupling between the copper(II) spin and the
*E-mail: heikki.m.tuononen@jyu.fi.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the funding provided by the Academy of
Finland, Technology Industries of Finland Centennial Founda-
■
tion, University of Jyvaskyla, University of Bordeaux, Reg
́
ion
Aquitaine, and CNRS. We thank Elina Hautakangas (University
of Jyvaskyla) and MHW Laboratories (Phoenix, AZ) for
̈
̈
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̈
elemental analyses.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental and computational details, X-ray crystallographic
and magnetic data of 1 and 3, and EPR spectrum and cyclic
voltammogram of 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ian.s.morgan@jyu.fi.
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dx.doi.org/10.1021/ic402954p | Inorg. Chem. XXXX, XXX, XXX−XXX