Synthesis, structure and magnetism of homodinuclear complexes of Co, Ni and Cu supported by a novel bitriazine scaffold

Article abstract of DOI:10.1039/c1dt00018g

Btzn (1), an amine-functionalized bi(1,3,5-triazine) 4,4′-(NH ₂)₂-6,6′-(NHC₆H₅) ₂-2,2′-(1,3,5-C₃N₃)₂, is reported, and its coordination with Co, Ni and Cu is explored. Reactions of metal salts (2 equiv) with Btzn (1 equiv) result in dimeric species [(Btzn)Co₂(NCS)₄(EtOH)₂(DMF)₂], (2), [(Btzn)Ni₂(η¹-ONO₂)₂(MeOH) ₄(DMF)₂]·2[NO₃], (3), [(Btzn)Cu ₂Cl₄(DMF)₂], (4), and [(Btzn)Cu ₂(η²-O₂NO)₂(OH₂) ₂(DMF)₂]·2[NO₃], (5). These complexes are the first examples of the coordination of transition metals with bi(1,3,5-triazine) ligands. Their structures display a bridging bis-bidentate coordination mode for Btzn. Variable-temperature magnetic susceptibility of the complexes reveals antiferromagnetic exchange between the spin carriers, with calculated exchange coupling values (J) of -4.7 cm^-1 for 3, -18.2 cm^-1 for 4, and -5.5 cm^-1 for 5. An in-depth evaluation of the metal geometry highlights the inefficient overlap of the magnetic d-orbitals through the bridging ligand, most likely leading to reduced delocalization and coupling.

Full text of DOI:10.1039/c1dt00018g

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PAPER
Synthesis, structure and magnetism of homodinuclear complexes of Co, Ni
and Cu supported by a novel bitriazine scaffold†
Derek J. Reid,a John E. W. Cull,a Kimberley D. S. Chisholm,a Alexandre Langlois,a Po-Heng Lin,b
Je´roˆme Long,b Olivier Lebel,a Ilia Korobkov,b Ruiyao Wang,c James D. Wuest,d Muralee Murugesub and
Jennifer Scott*a
Received 5th January 2011, Accepted 24th February 2011
DOI: 10.1039/c1dt00018g
Btzn (1), an amine-functionalized bi(1,3,5-triazine) 4,4¢-(NH2)2-6,6¢-(NHC6H5)2-2,2¢-(1,3,5-C3N3)2, is
reported, and its coordination with Co, Ni and Cu is explored. Reactions of metal salts (2 equiv) with
Btzn (1 equiv) result in dimeric species [(Btzn)Co2(NCS)4(EtOH)2(DMF)2], (2),
1
[(Btzn)Ni2(h -ONO2)2(MeOH)4(DMF)2]·2[NO3], (3), [(Btzn)Cu2Cl4(DMF)2], (4), and
2
[(Btzn)Cu2(h -O2NO)2(OH2)2(DMF)2]·2[NO3], (5). These complexes are the first examples of the
coordination of transition metals with bi(1,3,5-triazine) ligands. Their structures display a bridging
bis-bidentate coordination mode for Btzn. Variable-temperature magnetic susceptibility of the
complexes reveals antiferromagnetic exchange between the spin carriers, with calculated exchange
coupling values (J) of -4.7 cm-1 for 3, -18.2 cm-1 for 4, and -5.5 cm-1 for 5. An in-depth evaluation of
the metal geometry highlights the inefficient overlap of the magnetic d-orbitals through the bridging
ligand, most likely leading to reduced delocalization and coupling.
Introduction
Of the various isomers of triazine, 1,3,5-triazine (or s-triazine)
is the most prominent. Derivatives of this compound have
many noteworthy applications, such as serving as herbicidal
and antibacterial agents,1 components of resins,2 complexants in
analytical chemistry3 and reagents in organic synthesis.4 Recently,
triazine-based monomers and polymers have been used as physical
building blocks or templates in crystal engineering,5 combinatorial
chemistry6 and peptidomimetics.7 The opto-electronic properties
of p-conjugated systems are attracting increased attention for their
potential as components of diverse materials, including liquid
crystals,8 OLEDs9,10 and advanced magnetic materials.11
Unlike other coupled heterocyclic systems such as bipyridine
(A, Chart 1) and bipyrimidine (B), which are commonplace,
triazines containing two or more consecutive rings are rare.12–16
Chart 1
Several bi(1,2,4-triazines) have been reported,12 including one
involved in the formation of an extended Cu(I) network.13
To date, however, the only structurally-characterized bi(1,3,5-
triazines) in the literature are 4,4¢,6,6¢-tetracyano-2,2¢-bi(1,3,5-
triazine) (C, R = CN),14 generated by reduction of 2,4,6-
tricyano-1,3,5-triazine and isolated as a charge-transfer complex,
and 4,4¢,6,6¢-tetraphenyl-2,2¢-bi(1,3,5-triazine) (C, R = C6H5),15
produced by metal-catalyzed coupling of 2-chloro-4,6-diphenyl-
1,3,5-triazine. Syntheses involving Ni-15 or Cu-catalyzed16 self-
coupling reactions of chloro- or iodotriazines have been re-
ported, resulting in the isolation of bitriazines with conjugated
substituents. A comprehensive investigation of the electronic
aDepartment of Chemistry and Chemical Engineering, Royal Military
College of Canada, Kingston, Canada. E-mail: Jennifer.Scott@rmc.ca;
Fax: 1-613-542-9489; Tel: 1-613-541-6000 ¥ 3562
bDepartment of Chemistry, University of Ottawa, Ottawa, Ontario, Canada
cDepartment of Chemistry, Queen’s University, Kingston, Ontario, Canada
dDe´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec,
Canada
† Electronic supplementary information (ESI) available: Crystallographic
information files in the form of CIF files, pictorial depictions of NMR
spectra of compound 1, models of various orientations of complexes 2–5,
and tables of selected bond distances and angles of complexes 2–5. CCDC
reference numbers 806769–806772. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt00018g
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properties revealed improved electron-accepting abilities in the
heterocycle upon coupling and conjugation to a second triazine
ring,15 leading Ren et al. to explore the capabilities of these novel
bitriazines as electron-transport layers for organic light-emitting
diodes.9
To the best of our knowledge, there are no reported examples
of transition metal complexes supported by bi(1,3,5-triazine)
scaffolds. This is in contrast to the closely-related cases of 2,2¢-
bipyridine and 2,2¢-bipyrimidine, which have enjoyed widespread
use as ligands in coordination chemistry.17 Although not as
ubiquitous as bipyridine, bipyrimidine can nevertheless assemble
multinuclear species by bridging two metal centers in a bis-
bidentate fashion. This behaviour has attracted attention as a
way to form dinuclear species, coordination polymers and ex-
tended structures.18–22 In addition, bipyrimidine complexes display
enhanced participation in certain photophysical processes in
comparison to bipyridines. This can permit a higher degree of in-
tramolecular energy/electron transfer between two bridged metal
centers, making bipyrimidine complexes of interest in the areas
of advanced magnetic materials and electro- and photocatalysis,
and suggesting that bitriazine complexes may have similar useful
behaviour.19
There are extensive references to transition metal complexes
of pyridyl-substituted 1,3,5-triazines, such as 2,4,6-tris(2-pyridyl)-
1,3,5-triazine (D), which may adopt bipyridine- or terpyridine-like
coordination motifs to bind one, two or three metal centers.23–27
Available commercially as an analytical reagent for the detection
of Fe, Ru and Co,3 compound D has been widely studied in
coordination chemistry. These studies have revealed interesting
spectroscopic properties and redox behaviours in the assembled
transition metal systems, with potential applications in electro-
and photocatalytic reduction reactions.23 However, in certain
cases of metal coordination, ligand-to-metal s-donation enhances
the electron deficiency of the triazine ring, thereby increasing
its susceptibility towards hydrolysis by nucleophilic attack.23,27
Similar behaviour is witnessed in the coordination chemistry of
the closely-related congener, 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine
(E),28–30 which has not yielded crystallographically characterized
transition metal complexes.31 When this compound reacts with Cu
salts, the triazine ring tends to undergo hydrolytic cleavage, leading
to complexes derived from products of degradation.30,32
Chart 2
In this report, we present the first examples of a transition metal
complex supported by a novel amino-functionalized bi(1,3,5-
triazine), which has proven to be a novel bridging ligand for
the assembly of metallic dimers of Co, Ni and Cu. Structural
characterization of the resulting complexes is presented, as well as
an analysis of their magnetic properties.
Results and discussion
Unlike previously reported bi(1,3,5-triazines), which have been
prepared by coupling reactions of preformed triazines, the novel
amino-functionalized bitriazine scaffold, 1, was synthesized by
constructing the triazine rings using a procedure similar to that
described by Shapiro and Overberger.36 Condensation of dimethyl
oxalate with 2 equiv of phenylbiguanide (Scheme 1) gave ligand 1
as a colourless solid in good yield. Its 1H NMR spectrum displays
the expected peaks, with the aminophenyl proton shifted downfield
to 9.73 ppm. The 13C NMR spectrum includes the 2,2¢-C peak at
170.7 ppm, with the other two carbon atoms of the triazine ring
at 167.5 and 164.9 ppm.
The coordinating abilities of compound 1 were explored with
several first-row transition metal salts. Notably, under the circum-
stances presented herein, the ligand does not undergo hydrolytic
cleavage upon coordination such as that observed in electron-
deficient tris(pyrimidyl)triazine systems.30,32 Complexes 2–5 were
synthesized by the reaction of 2 equiv of the transition metal
salt with 1 equiv of the ligand to obtain homodinuclear species,
in which the ligand bridges two metal centers in a bis-bidentate
fashion.
Complex 2 was produced via the reaction of Co(NCS)2,
generated in situ, with ligand 1 in DMF/EtOH. Bright orange
paramagnetic crystals of complex 2 were grown from a concen-
trated solution in EtOH and the structure was elucidated by X-ray
diffraction (Fig. 1). The molecular unit consists of a symmetry-
generated dimer, in which the ligand bridges two Co centers in a
bis-bidentate nature via the adjacent nitrogen atoms of the triazine
rings [Co–N1 = 2.188(4) A, Co–N3 = 2.180(4) A, N1–Co–N3 =
75.48(13)◦]. Two of the six coordination sites on each cobalt are
occupied by the ligand, and the coordination is completed by
Herein, we describe the preparation of novel amino-
functionalized bi(1,3,5-triazine) 1 (Btzn, Chart 2). Btzn bears
resemblance to 2-appt (2-appt = 2-amino-4-phenylamino-6-(2-
pyridyl)-1,3,5-triazine, Chart 2),33 which has been studied in crys-
tal engineering for its ability to associate by hydrogen bonds and
p–p interactions.34 The donor–acceptor–donor motif of the amine-
functionalized triazine has been shown to help Ru complexes bind
to the minor groove of DNA as potential metal-based therapeutic
agents.35
˚
˚
˚
two nitrogen-bound thiocyanate anions [Co–N7 = 2.056(5) A,
Scheme 1
5010 | Dalton Trans., 2011, 40, 5009–5017
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Table 1 Crystal data and structure analysis results for complexes 2–5
2
3
4
5
formula
Mw
crystal system
C36H54Co2N16O6S4
1053.05
monoclinic
P21/c
9.8096(18)
17.481(3)
14.825(3)
90
96.962(3)
90
2523.5(8)
2
0.71073
223(2)
1.386
0.880
1096
C28H46N16Ni2O18
1012.23
C26H38Cl4Cu2N12O4
851.56
C26H37Cu2N17O16
970.81
monoclinic
triclinic
triclinic
¯
¯
space group
P1
P1
C2/c
˚
a (A)
9.10730(10)
9.48280(10)
13.0297(2)
106.2700(10)
92.7070(10)
93.2830(10)
1076.09(2)
1
10.3414(2)
10.4430(2)
10.5451(2)
66.8280(10)
89.8480(10)
62.3640(10)
904.60(3)
1
25.2640(4)
10.8761(2)
19.8923(6)
90
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
127.5490(10)
90
4333.53(17)
4
0.71073
296(2)
1.488
1.065
1992
0.0456, 0.1309
1.024
3
˚
V (A )
Z
˚
radiation (Ka, A)
T (K)
0.71073
223(2)
1.562
0.965
0.71073
296(2)
1.563
1.521
436
0.0270, 0.0712
1.043
Dcalcd (g cm-3)
mcalcd (mm-1)
F000
526
R, wR2a
0.0590, 0.1353
0.987
0.0278, 0.0707
1.038
GoF
ꢀ
ꢀ
ꢀ
ꢀ
2
1/2
a R = |F0| - |Fc|/ |F|, wR = [ (|F0| - |Fc|)2/ wF0
}
.
Reaction of 2 equiv of Ni(NO3)2·6H2O with bitriazine, 1, led
to the formation of complex 3 as a bright emerald-green solid.
Crystals suitable for X-ray diffraction were grown by the slow
evaporation of a methanol solution, and a model of the determined
structure appears in Fig. 2. The structure is composed of a
symmetry-generated dimer in which the ligand bridges two Ni
centers. The geometry about each Ni center is distorted octahedral,
˚
completed by the ligand system [Ni1–N1 = 2.1231(13) A, Ni–
˚
N2 = 2.0992(14) A], an oxygen atom from a molecule of DMF
˚
[Ni1–O3 = 2.0462(12) A], two molecules of methanol located trans
˚
˚
to each other [Ni1–O1 = 2.0561(15) A, Ni1–O2 = 2.0603(14) A,
˚
O1–Ni1–O2 = 173.50(6) A], and an oxygen-bound monodentate
˚
nitrate moiety [Ni–O4 = 2.0943(13) A], which displays a non-
coplanar arrangement with the metal center [Ni1–O4–N7–O5 =
101.39(19)◦]. The ligand has a bite angle of 78.39(5)◦ and separates
Fig. 1 Thermal ellipsoid plot for complex 2 with the ellipsoids drawn at
the 50% probability level. Hydrogen atoms and amine substituents have
been removed for clarity.
˚
the two Ni centers by 5.645 A. The distance between the two
carbon atoms joining the triazine rings is slightly shorter than
˚
that seen in compound 2 [C1–C1A = 1.500(3) A], and the
˚
nickel atoms are located approximately 0.095 A above and below
the plane of the ligand. Charge balance is maintained by the
presence of two additional nitrate ions per molecule, located above
˚
˚
Co–N8 = 2.071(4) A], one molecule of DMF [Co–O1 = 2.158(3) A]
˚
and a molecule of ethanol [Co–O2 = 2.058(4) A] in a distorted
octahedral geometry. The thiocyanate anions are located cis to
each other [N7–Co–N8 = 93.57(17)◦] and have slightly different
˚
bond distances and angles [N7–C15 = 1.142(6) A vs. N8–C16 =
˚
˚
˚
1.159(6) A, C15–S1 = 1.630(6) A vs. C16–S2 = 1.592(6) A, Co–
N7–C15 = 164.9(4)◦ vs. Co–N8–C16 = 157.9(4)◦, N7–C15–S1 =
177.3(5)◦ vs. N8–C16–S2 = 179.0(5)◦] as well as CN stretching
frequencies in the IR spectrum [u(CN) = 2110, 2058 cm-1]. One
molecule of ethanol per Co center can be found in the lattice.
The two triazine rings of the ligand are co-planar and joined by
˚
a C–C single bond [C3–C3A = 1.521(9) A] and the other bond
distances and angles of the triazine rings are as expected. The
˚
two Co centers are separated by 5.828 A across the ligand and
˚
lie alternately 0.135 A above and below the plane of the ligand.
Fig. 2 Thermal ellipsoid plot of complex 3 with the ellipsoids drawn at
the 50% probability level. Hydrogen atoms and amine substituents have
been removed for clarity.
Crystallographic parameters can be found in Table 1, and selected
bond distances and angles can be found in the ESI.†
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and below the plane of the ligand and stabilized by hydrogen
bonding with neighbouring metal-bound methanol groups [H2H–
be found in the lattice. A striking feature of the complex, not
found in others described herein, is the degree of non-coplanarity
of the metal centers with the ligand backbone. The Cu atoms in
complex 4 are oriented above and below the plane by 13.32◦ (or by
˚
˚
˚
O9¢¢ = 1.985 A, H2H–O8¢¢ = 2.606 A, H1H–O7 = 2.196 A,
˚
H1H–O8 = 2.502 A]. One oxygen atom of the nitrate anion is also
˚
˚
hydrogen-bonded to an amine group from a separate ligand [O9–
a distance of 0.374 A), leading to a separation of 5.584 A between
˚
H4NA = 2.135 A], assembling a zig-zag chain structure in which
the two Cu centers. The molecules are arranged in planes and the
˚
the ligand planes are parallel, but staggered from one another. The
intermolecular distance between Cu centers is 6.757 A.
˚
˚
distance between layered planes is 3.454 A, leading to the shortest
The solid-state structure of complex 5 was solved by X-ray
diffraction, using crystals grown from a concentrated DMF
solution layered with acetonitrile. The structure (Fig. 4) shows
two identical hexa-coordinate Cu centers bridged across the bis-
intermolecular Ni–Ni distance of 9.107 A.
Dinuclear copper complexes 4 and 5 were synthesized via
reaction of ligand 1 with 2 equiv of the hydrated copper salts, CuCl2
and Cu(NO3)2, respectively. Yellow crystals of complex 4 were
grown from a MeOH–layered DMF solution at room temperature,
and the molecular unit can be seen in Fig. 3. Compound 4 consists
of two identical 5-coordinate Cu centers bridged by the bis-
bidentate ligand system. The dimer is symmetry-generated, with a
center of inversion located in the middle of the C–C bond between
the two triazine rings. The geometry of each Cu center can best be
described as a distorted trigonal bipyramid (t = 0.74),37 in which
the chloride ions [Cu–Cl1 = 2.3214(6) A, Cu–Cl2 = 2.2907(7) A]
and one nitrogen donor of the ligand [Cu–N2 = 2.1805(15) A]
occupy the equatorial plane. The adjacent N-donor atom of the
ligand [Cu–N1 = 2.0163(16) A, N1–Cu–N2 = 78.38(6) ] and an
˚
bidentate bitriazine ligand [Cu–N1 ◦= 2.2086(19) A, Cu–N2 =
˚
2.0129(19) A, N1–Cu–N2 = 78.22(7) ]. The distorted octahedral
geometry is completed by the presence of a molecule of water
˚
[Cu–O5 = 1.9739(17) A], an oxygen-bound molecule of DMF
˚
[Cu–O4 = 2.0117(18) A] and a nitrate anion that is asymmetrically
˚
˚
bidentate [Cu–O1 = 2.429(4) A, Cu–O2 = 2.071(3) A, O1–Cu–O2 =
55.43(10)◦]. The longer Cu–O1 distance reflects participation in
weak off-axis semi-coordination with the metal center.38 Elonga-
tion of the Cu–N bond trans to the elongated Cu–O bond [O1–
Cu–N1 = 160.22(8)◦] results in an axially-elongated “4 + 2”-type
geometry. Two nitrate anions are held in proximity to the molecule
via short hydrogen bonds to the coordinated water [O8¢¢–H5C =
˚
˚
˚
◦
˚
˚
˚
oxygen-bound molecule of DMF [Cu–O1 = 1.9607(15) A] occupy
1.653 A]. The other hydrogen atom of water binds a second anion
˚
the axial positions. Spanning the axial and equatorial positions,
the ligand–metal bond distances are notably different, resulting
in an axially-compressed distortion to the trigonal bipyramidal
geometry. Alternatively (and on the other extreme), the geometry
of Cu may be described as a “4 + 1” apically-elongated square
pyramid, in which the nitrogen atom of the ligand with the longest
ligand–metal bond occupies the apical position and all other atoms
are basally located. Due to the geometrical constraints enforced
by the ligand coordination, the most accurate description lies
somewhere between the two, closer to the former, but with a
small, but not insignificant, contribution from the latter. The bond
distances and angles of the ligand are within expected values and
of nitrate [O6–H5D = 1.811 A], thereby assembling a network of
molecules in which the shortest intermolecular Cu–Cu distance
˚
is 6.357 A. The two triazine rings are not perfectly coplanar
and are both bent down by approximately 2◦ and held together
˚
by a carbon–carbon bond of 1.497(4) A. The molecular unit
can best be described as having a C2 axis located equidistant
between the two carbon atoms joining the two triazine rings and
perpendicular to the idealized ligand plane (in contrast to the other
complexes described herein, which contain an inversion center
midway between the two C atoms). As a result of this symmetry,
˚
the Cu centers are both located about 0.15 A above the idealized
˚
plane of the ligand and separated by 5.634 A. In addition, the
˚
the C–C distance between the two triazine rings is 1.500(4) A.
Two disordered molecules of methanol per molecular unit can
bidentate nitrate moieties are both located on the same side of
the ligand plane and canted by 57.49◦ from each other, and both
molecules of DMF are located on the opposite side. One molecule
of acetonitrile per ligand completes the lattice.
Fig. 4 Thermal ellipsoid plot of complex 5 with the ellipsoids drawn at the
50% probability level. Selected hydrogen atoms and the amine substituents
of the ligand have been removed for clarity.
Fig. 3 Thermal ellipsoid plot of complex 4 with the ellipsoids drawn at
the 50% probability level. Hydrogen atoms have been removed for clarity.
5012 | Dalton Trans., 2011, 40, 5009–5017
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Various structural features of ligand 1 in its coordination
complexes deserve attention. In particular, the triazine rings are
largely coplanar with respect to each other and separated by a
˚
normal single C–C bond, ranging from 1.521(9) A in complex
˚
2 to 1.497(4) A in complex 5. The degree of non-coplanarity of
the metal centers with the ligand backbone varies between the
complexes, as highlighted in Fig. S3–6 in the ESI,† in which the
complexes are displayed in two different orientations (along the
C–C bond of the ligand and along the line connecting the two
metal centers). The amine substituents are strongly conjugated
with the rings, as suggested not only by the short exocyclic N–C
bond distances (shorter than the majority of the endocyclic C–
N bonds) but also by the planar sp2 hybridization of the amino
nitrogen atoms, based on the positions of hydrogen atoms located
from difference Fourier maps. In addition, the aromatic planes
of the –NHPh substituents are close to the plane of the triazine
rings (tilting of 9.11◦–16.85◦). Both –NH2 and –NHPh hydrogen
atoms are involved in hydrogen bonding with cis-coordinated
groups. Notably, in the majority of cases, DMF is bound in a
meridional position with respect to ligand 1, highlighting the
previously reported substrate-directing capabilities of pendant
amino groups.39
Fig. 5 Temperature dependence of cT at 0.1 T (with c = M/H normalized
per mol) for complexes 2–5. Solid lines represent the fitted curves (see text
for details).
describing the exchange interactions for a dinuclear system can
be written as H = -2J(S1S2), where S1 = S2 = 1 for complex 3
Magnetic measurements
1
and S1 = S2 = for complexes 4 and 5. The cT equation can
2
Variable temperature dc magnetic susceptibility data were col-
lected on freshly filtered crystalline samples of complexes 2–5.
The dc magnetic susceptibility (c) data were collected in the
temperature range of 1.8–300 K in a 1000 Oe magnetic field and
are plotted as cT vs. T in Fig. 5.
⎡
⎢
⎢
⎢
⎢
⎢
⎤
⎛
⎜
⎞
⎟
⎟
⎛
⎜
⎞
⎟
⎟
2J
6J
⎥
exp
+5exp
⎜
⎜
⎝
⎜
⎜
⎝
2Ng2b2
⎟
⎠
⎟
⎠
⎥
kT
kT
⎥
cMT =
+TIP
for
be written as:
⎛
⎞
⎟
⎟
⎛
⎞⎥
2J
6J
k
⎟
⎜
⎜
⎥
1+3exp
+5exp
⎟
⎜
⎜
⎝
⎜
⎜
⎝
⎟
⎠
⎟
⎢
⎣
⎥
⎠
kT
kT
⎦
a dinuclear Ni system with S1 = S2 = 1 (Temperature-
Independent Paramagnetism (TIP) = 200 ¥ 10-6 cm3 mol-1), and
For each sample, the product of cT decreases with decreasing
temperature to a value close to zero at 1.8 K, indicating a
ground state configuration of S = 0 and displaying behaviour
characteristic of antiferromagnetic exchange between the spin
carriers. In each case, the room temperature values of cT are in
agreement with those expected for two uncoupled divalent metal
ions (Table 2). For complex 2, a definitive interpretation of the
negative deviation of cT is complicated by the presence of spin–
orbit coupling of Co(II) and may not necessarily be the result
of antiferromagnetic exchange, but rather magnetic anisotropy,
depopulation of low-lying excited states or a combination of
all three factors. Due to the complexity of Co(II) magnetism, it
is difficult to evaluate the strength of the magnetic interaction.
For complexes 3–5, the application of the van Vleck equation40
to Kambe’s vector coupling method41 enabled the fitting of
the susceptibility data to determine the exchange interaction
parameter (J) between the metal centers. The spin Hamiltonian
⎡
⎢
⎢
⎢
⎢
⎢
⎤
⎛
⎜
⎞
⎟
⎟
2J
⎥
exp
⎜
⎜
⎝
2Ng2b2
⎟
⎠
⎥
kT
⎥
c T =
+TIP
for a dinuclear Cu system
as:
M
⎛
⎞⎥
2J
k
⎟
⎜
⎥
1+3exp
⎟
⎜
⎜
⎝
⎟
⎢
⎣
⎥
⎠
kT
⎦
1
2
with S1 = S2 = (TIP = 120 ¥ 10-6 cm3 mol-1). The best set of fitting
parameters obtained is given in Table 2.
In complexes 2–5, the metal centers interact antiferromag-
netically across the bitriazine ligand, albeit to varying extents.
As shown above, the ligand features in each complex are very
similar, with the triazine rings connected by normal single C–C
bonds. The clear presence of a C–C single bond in this position
suggests a lack of delocalization between the triazine rings,
contrary to the related dinuclear complexes of bipyrimidine,20
in which the C–C bond decreases in length by approximately
Table 2 Selected data from magnetic susceptibility measurements
Room temperature magnetic
susceptibility (cm3 K mol-1)
Calculated parameters
-J (cm-1)
g
˚
Complex
M–M (A)
Expected
Experimental
2
3
4
5
5.828
5.645
5.584
5.634
5.40
2.46
0.77
0.77
5.53
2.418
0.73
0.77
—
—
4.7(1)
18.2(2)
5.5(1)
2.22(1)
2.03(1)
2.03(1)
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˚
0.08 A from that observed in the free ligand. The extent of
made to constrain the final geometry of the metal center using a
variety of supplemental ligands and anions.
magnetic interaction correlates well with the distance between
the metal centers, with the shortest distance (4) displaying the
largest coupling constant. The calculated exchange constant J for
complex 3 is similar to previously reported values for dinuclear Ni
complexes of bipyrimidine.21 However, bipyrimidine-bridged Cu
complexes commonly display coupling constants much greater
than those reported here (>100 cm-1).20,22 Although neither of
the calculated values for complexes 4 and 5 can be considered
strong, the value of -18.2 cm-1 for 4 is still significant. It is
common for Cu complexes to display greater exchange than that
shown by other first-row transition metal bivalent ions due to
the shorter metal–ligand bond distances and the lower energy d-
orbitals. Consequently, the enhanced spin-density delocalization
throughout the bridge facilitates the magnetic interaction between
the two metal centers. However, the magnitude of J is controlled in
part by the orientation of the metal center and its d-orbitals with
respect to the bridging ligand. Stronger couplings are observed
when the ligand is located in the equatorial position and co-planar
with the metal, thereby achieving good overlap of the d(x2 - y2)
orbitals and enhancing the s in-plane exchange pathway.20 The
non-coplanarity of the metal centers with the ligand plane in
complex 4 may therefore help explain why the exchange values
for Cu are lower than expected.20,22 However, the Cu complex
displaying the largest degree of non-coplanarity of the metal
(see ESI†) is also the complex with the greatest exchange. The
decreased antiferromagnetic coupling between the Cu centers in
complexes 4 and 5 may therefore be clarified by an in-depth
exploration of the stereochemistry of the metal centers.
The geometry about copper in complex 4 is best represented
as an axially-compressed trigonal bipyramid with a degree of
square-pyramidal distortion in which the ligand system spans axial
and equatorial positions. The asymmetric binding of the ligand
therefore ensures that the d(z2) ground state overlaps at only one
site on the ligand, that site being opposite and located on the other
triazine ring in the second Cu center due to the Ci symmetry of
the molecule. Similarly, the ligand system in complex 5 also spans
the axial and equatorial sites on the axially-elongated distorted
octahedral Cu center. As such, the overlap between the orbitals
of the Cu centers is once again not ideal due to the fact that the
d(x2 - y2) ground states are opposite and overlap with separate
triazine rings of the ligand (C2 symmetry). Given that overlap is
not ideal for complexes 4 and 5, the greater exchange interaction
observed in 4 may in fact be due to the shorter Cu ◊ ◊ ◊ Cu and Cu–N
bond distances, most likely imposed by the reduced coordination
number.
Experimental section
General considerations
Co(NO3)2·6H2O, Ni(NO3)2·6H2O and CuCl2·2H2O were pur-
chased from Fisher Scientific Company, Cu(NO3)2·2.5H2O was
purchased from Caledon Laboratory Chemicals, KSCN was pur-
chased from BDH Chemicals and dimethyl oxalate was purchased
from Sigma Aldrich. All of these materials were used without
further purification. Phenylbiguanide was prepared following
published procedures.42 All operations were conducted under
standard conditions. NMR spectra were recorded at 20 ◦C with a
300 MHz Mercury plus Varian Instrument with an Oxford magnet.
Elemental analyses were performed on an EAS 1108 apparatus
from Fisons Instruments SPA. The IR spectra were recorded in
Nujol mulls with a Perkin Elmer FT-IR System Spectrum GX
spectrometer (4000–600 cm-1).
Preparation of 4,4¢-(NH2)2-6,6¢-(NHC6H5)2-2,2¢-(1,3,5-C3N3)2
(Btzn) [1].
Dimethyl oxalate (3.33 g, 28.2 mmol) was added to a suspension of
phenylbiguanide (10.0 g, 56.4 mmol) in MeOH (350 mL) and the
mixture was heated at reflux for two days. The mixture was filtered
and the solid was subsequently dissolved in DMF. The resulting
yellowish solution was filtered prior to the addition of distilled
water (350 mL). The cloudy off-white suspension was stirred
overnight and filtered to give a solid that was dried overnight
at 115 ◦C. Compound 1 was obtained as a colourless solid (7.24 g,
19.4 mmol, 69% yield). Anal. Calcd (found) for C18H16N10 (%):
C, 58.05 (57.99); H, 4.33 (4.28); N, 37.61 (37.65); melting point:
◦
◦
1
348.5 C; H NMR (20 C, 300 MHz, (CD3)2SO): d 9.73 (s, 2H,
NH), 7.81 (d, 4H, C6H5), 7.22 (t, 2H, C6H5 & t, 4H, NH2), 6.93
◦
(t, 2H, C6H5); 13C NMR (20 C, 75 MHz, (CD3)2SO): d 170.68
(2,2¢-C), 167.52 (4,6¢-CNH2), 164.92 (4¢,6-C(NHC6H5)), 140.20,
128.87, 122.58, 120.33 (C6H5) (ESI†); IR (Nujol mull, cm-1): n
3394, 3289, 3191, 2926, 1640, 1599, 1223, 979, 817, 752.
Preparation of {(Btzn)[Co(NCS)2(EtOH)(Me2NCHO)]2}·2EtOH
[2]
A solution of KSCN (0.10 g, 1.03 mmol) in warm EtOH (15
mL) was added dropwise to a stirred solution of Co(NO3)2·6H2O
(0.15 g, 0.51 mmol) in EtOH (15 mL). The mixture became deep
cobalt blue and a solid separated. The suspension was centrifuged
to remove the solid, and a solution of ligand 1 (0.10 g, 0.27
mmol) in DMF (10 mL) was added dropwise to the stirring
supernatant, turning it brownish yellow. Stirring was continued for
4 h, the solution was evaporated to dryness, and the residue was
re-dissolved in EtOH to give a greenish-yellow solution. Orange
crystals of 2 grew overnight when this mixture was left at room
temperature (0.20 g, 0.19 mmol, 71% yield). Anal. Calcd (found)
for C36H38Co2N16O6S4 (%): C, 41.06 (40.77); H, 5.16 (4.85); N,
21.29 (21.63); S, 12.18 (12.48); IR (Nujol mull, cm-1) n 3292, 3150,
2110, 2058, 1606, 1583, 1562, 1488, 1451, 1405, 1377, 1262, 1227,
1114, 1040, 1009, 803, 760, 679, 622.
Conclusions
In summary, the novel bitriazine ligand, 1, has been synthesized,
and its transition metal complexes have been prepared and
characterized. Variable temperature magnetic susceptibility data
display small, but significant, degrees of antiferromagnetic ex-
change between the spin carriers. Due to the geometrical distortion
about the metal centers and the decreased overlap of the ligand
with the ground state d-orbitals, conditions do not favour an
overly coupled system. Work is in progress on the preparation
of additional transition metal complexes and attempts are being
5014 | Dalton Trans., 2011, 40, 5009–5017
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Preparation of {(Btzn)[Ni(g1-ONO2)(MeOH)2(Me2NCHO)]2}·
diffractometer equipped with a sealed Mo tube source (l =
0.71073 A) and an APEX II CCD detector.43 Data for all samples
˚
2[NO3] [3]
were collected with a sequence of 0.5◦ w scans at 0, 120, and
240◦ in j. Initial unit cell parameters were determined from
60 data frames collected at the different sections of the Ewald
sphere. Semi-empirical absorption corrections based on equivalent
reflections were applied.44 Systematic absences in the diffraction
data and unit-cell parameters were consistent with monoclinic
A solution of ligand 1 (0.20 g, 0.54 mmol) in DMF (10 mL)
was added dropwise to a stirred solution of Ni(NO3)2·6H2O
(0.32 g, 1.10 mmol) in EtOH (30 mL), and the resulting emerald–
lime green solution was stirred overnight. After evaporation
of the solvent, the residual solid was dissolved in a minimal
amount of MeOH and the solution was allowed to stand at room
temperature. Green crystals of 3 grew by slow evaporation of the
solution (0.39 g, 0.38 mmol, 71% yield). Anal. Calcd (found) for
C28H46Ni2N16O18 (%): C, 33.22 (32.87); H, 4.58 (4.19); N, 22.15
(22.44); IR (Nujol mull, cm-1) n 3312, 3251, 3214, 3108, 1667,
1615, 1590, 1570, 1491, 1453, 1409, 1379, 1336, 1294, 1018, 800,
764, 690.
¯
P21/c for 2, triclinic P1 for 3 and 4, and monoclinic C2/c for 5.
Solutions in the centro-symmetric space groups for compounds
2–5 yielded chemically reasonable and computationally stable
results of refinement. The structures were solved by directmethods,
completed with difference Fourier synthesis, and refined with full-
matrix least-squares procedures based on F2. All structures 2–5
exhibit dimeric molecular motifs located on the inversion center
element of symmetry for 2–4 and the C2 axis for 5.
With the exceptions noted below, hydrogen atoms were treated
as idealized. All scattering factors are contained in several versions
of the SHELXTL program library, with the latest version used
being v.6.12.45 For compound 2: The two hydroxyl -H atoms of
the two EtOH molecules were located from difference Fourier
maps. The coordinated EtOH and the NH2 group are disordered.
SHELX commands, SADI, EADP, DFIX and PART were used to
resolve the disorder. For compound 3: the H atoms on –OH and
–NH were located from difference Fourier maps. For compound
4: one molecule of methanol per unit cell is disordered in the
lattice. For compound 5: one CH3 group on the coordinated DMF
molecule is disordered and there is one half-equivalent of CH3CN
per unit cell in the lattice.
Preparation of {(Btzn)[CuCl2(Me2NCHO)]2}·2MeOH [4]
A solution of ligand 1 (0.131 g, 0.352 mmol) in DMF (10 mL) was
added to a stirred solution of CuCl2·2H2O (0.120 g, 0.704 mmol)
in EtOH (15 mL). After the orange solution was evaporated to
dryness, the residue was dissolved in hot DMF and the extracts
were centrifuged and layered with MeOH. Orange crystals of
compound 4 grew when the mixture was kept for several days
at room temperature (0.162 g, 0.190 mmol, 54% yield). Anal.
Calcd (found) for C26H38Cl4Cu2N12O4 (%): C, 36.67 (36.37); H,
4.50 (4.60); N, 19.74 (19.72); IR (Nujol mull, cm-1) v 3374, 3088,
2954, 2923, 2853, 1660, 1607, 1585, 1567, 1543, 1488, 1319, 1307,
1014, 766, 694, 627, 621, 605.
Crystallographic data are reported in Table 1, and relevant bond
distances and angles are reported in Tables S1–S4 in the ESI.†
Preparation of {(Btzn)[Cu(g2-O2NO)(OH2)(Me2NCHO)]2}·
2[NO3]·CH3CN [5]
Magnetic measurements
A solution of ligand 1 (0.148 g, 0.397 mmol) in DMF (10 mL)
was added to a stirred solution of Cu(NO3)2·2.5H2O (0.188 g,
0.808 mmol) in EtOH (15 mL), and the resulting green solution
was stirred for 3 h. After the mixture was evaporated to dryness,
the residue was dissolved in a minimal amount of DMF. The
extracts were centrifuged and layered with MeCN. Olive green
crystals of compound 5 grew after several days at room tem-
perature (0.286 g, 0.274 mmol, 69% yield). Anal. Calcd (found)
for C29H44N18Cu2O17 (due to the difficulty of removing DMF
associated with the observed heat-sensitivity of this compound,
the experimental analytical values obtained with several samples
suggested contamination by 1 equiv of DMF per ligand, and this
has been taken into consideration when calculating the theoretical
values and percent yield) (%): C, 33.46 (33.42); H, 4.26 (4.22);
N, 24.23 (23.95); IR (Nujol mull, cm-1) n 3402, 3302, 3201, 3109,
2903, 1664, 1649, 1615, 1690, 1570, 1490, 1454, 1437, 1392, 1377,
1338, 1294, 1263, 1045, 1007, 772, 694.
Variable-temperature dc magnetic susceptibility data were ob-
tained using polycrystalline samples wrapped in a polyethylene
membrane and collected using a Quantum Design MPMS-XL7
SQUID magnetometer operating between 1.8 and 300 K for a dc-
applied field of 0.1 T. Experimental data were corrected for the
sample holder and for diamagnetic contributions calculated from
Pascal constants.46
Acknowledgements
JS thanks the Royal Military College of Canada for funding this
project under their Academic Research Program and Tiow-Gan
Ong for helpful discussions. OL and AL thank the Director Gen-
eral Land Equipment Program Management of the Department of
National Defence Canada. JW is grateful to the Natural Sciences
and Engineering Research Council of Canada, the Ministe`re de
´
l’Education du Que´bec, the Canada Foundation for Innovation,
Single crystal X-ray diffraction studies
the Canada Research Chairs Program, and Universite´ de Montre´al
for financial support. MM thanks CFI, ORF, NSERC-DG and
ERA, and Vision 2010.
Data used to solve the structures of complexes 2–5 were the
best sets obtained in several trials for each sample. Crystals
were mounted on thin glass fibers using paraffin oil and either
cooled to 223 K (2 and 3) or left at room temperature (4 and
5). Data were collected on a Bruker SMART APEX II X-ray
diffractometer with graphite-monochromated Mo Ka radiation
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(l = 0.71073 A) and on a Bruker AXS KAPPA single-crystal
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