A new nitrogen rich open chain diazine ligand system: Synthesis coordination chemistry and magneto-structural studies

Article abstract of DOI:10.1039/c7dt03845c

The synthesis and coordination chemistry of a new series of open chain diazine based ligands (L_3aH₂, L_3bH₂, and L_3cH₂) is reported. The ligands comprise a central disubstituted bipyridine moiety with two bridging alkoxide oxygen donors, together with diazine and pyridine terminal groups strategically located to coordinate three metal centres. Reactions of L_3aH₂ and L_3bH₂ with CuX₂ (X^- = ClO₄, Cl, NO₃) yield a trinuclear complex (1) and 1-D copper chains (2, 3). In these complexes the ligands bind copper ions via N_bipyridne, trans N_diazine, O_hydrazone, and N_pyridne donors while vacant sites are occupied by counter ions or solvent molecules (methanol, water, acetonitrile). Reaction of L_3cH₂ with MnCl₂ affords a linear trinuclear Mn complex (4), where the Mn(ii) ions are connected via μ₂-O_hydrazone linkers with no N-N bridging. Reaction of L_3aH₂ with Fe(SO₃CF₃)₂ yields a tetranuclear mixed valence Fe complex (5), in which both trans N-N and O_hydrazone bridging is observed. Magnetic studies reveal the presence of moderate to strong antiferromagnetic interactions in complexes 1-4 while a mix of ferromagnetic and antiferromagnetic interactions is observed in complex 5.

Full text of DOI:10.1039/c7dt03845c

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DOI: 10.1039/C7DT03845C
Table of Contents
A new series of open chain diazine ligands that have a central bipyridne unit with two side-arms offering
a range of ketone/alkoxide oxygen donors, diazine and pyridine terminal groups show a strong
preference for binding three metals forming a trimetallic sub-unit but sufficient ligand flexibility to
modify the coordination pockets for metal binding.

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DOI: 10.1039/C7DT03845C
Journal Name
ARTICLE
A New Nitrogen Rich Open Chain Diazine Ligand System: Synthesis
Coordination Chemistry and Magneto-Structural Studies
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
Muhammad U. Anwar,* Ahmed Al-Harrasi, Emma L. Gavey, Melanie Pilkington, Jeremy M.
c
d
Rawson,* Laurence K. Thompson.
DOI: 10.1039/x0xx00000x
The synthesis and coordination chemistry of a new series of open chain diazine base ligands (L3a
H
2
, L3b
H
2
, and L3c
H ) is
2
www.rsc.org/
reported. The ligands comprise a central disubstituted bipyridine moiety with two bridging alkoxide oxygen donors,
together with diazine and pyridine terminal groups strategically located to coordinate three metal centres. Reactions of
-
L
3a
H
2
and L3b
H
2
with CuX
2
(X = ClO
4
, Cl, NO
3
) yield a trinuclear complex (1) and 1-D copper chains (2 , 3). In these complexes
hydrazone, and Npyridne donors while vacant sites are occupied by
counter ions or solvent molecules (methanol, water, acetonitrile). Reaction of L3c with MnCl affords a linear trinuclear
Mn complex (4), where the Mn(II) ions are connected via µ -Ohydrazone linkers with no N-N bridging. Reaction of L3a with
Fe(SO CF yields a tetranuclear mixed valence Fe complex (5), in which both trans N-N and Ohydrazone bridging is observed.
the ligands bind copper ions via Nbipyridne, trans Ndiazine,
O
H
2
2
2
H
2
3
3 2
)
Magnetic studies reveal the presence of moderate to strong antiferromagnetic interactions in complexes 1-4 while a mix
of ferromagnetic and antiferromagnetic interactions is observed in complex 5.
Introduction
The controlled self-assembly of non-covalent architectures from
programmed components involves the design of suitable ligands
with well-defined binding sites coupled with the selection of metal
ions with complimentary coordination geometries. The metal ions
then recognize the binding sites of the ligands according to the
coordination algorithm defined by both the metal and the ligand.
This has led to the design of polydentate ligands with linear
arrangements of coordination pockets which have been used to
prepare square grid like architectures , tetrahedra as well as 3-D
metal organic frameworks. Alternatively, coordination chemists
degree of inherent conformational variation within the ligand
geometry, as well as the metal ion’s coordination preference. The
design of ligand systems which are not only capable of binding
metal sites in well-defined arrays but which also place them in close
proximity to facilitate effective magnetic communication has led to
fundamental studies in the field of magnetochemistry which
1
7,8
2
includes the effect of the bridge angle on exchange coupling. In
addition, design strategies towards polynuclear high spin molecules
have been developed which have led to the discovery of single
molecule magnets, individual molecules capable of storing magnetic
3
4
9
5
information below their blocking temperatures. In these systems
efficient magnetic exchange coupling is usually propagated via
superexchange interactions through the one or two atom
diamagnetic bridges of their bridging ligands.
can follow a serendipitous approach in which ligands typically offer
multiple binding modes, while the metals often exhibit flexibility in
their coordination geometries. The choice of
6
a semi-rigid
polydentate ligand which offers some degree of pre-determined
coordination preference provides an interesting compromise
between the two aforementioned strategies. In this respect,
aspects of structural control are embodied within the ligand design,
but the final outcome of the reaction still draws upon a certain Figure 1: Conformational flexibility around the open chain N-N bond of a diazine based
ligand
a.University of Nizwa, Chair of Oman’s Medicinal Plants and Marine Natural
Products, P O Box 33, PC 616, Birkat Almouz. Nizwa, Oman, Email:
usman.anwar@unizwa.edu.om
b.
Department of Chemistry, Brock University, Sir Isaac Brock Way. St Catharines,
ON, L2S3A1, Canada
Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset
c.
Ave, Windsor ON, N9B3P4, Canada, Email: jmrawson@uwindsor.ca
Department of Chemistry, Memorial University, St. John’s, Newfoundland, A1B
d.
3
X7, Canada
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Figure 2: Conformational variation in diazine ligands and their coordination modes10
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
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DOI: 10.1039/C7DT03845C
2-
1 2 3
Figure 4: Different coordination modes C , C , and C found in ligands L3a (R =
2-
2-
3 2
CH ), L3b (R = NH ), and L3c (R = H)
1 2 3 2
Figure 3: Molecular structures of ligands L H to L H .
depending on the nature of the co-ligands and the reaction
conditions employed.
In recent years, diazine-based ligand systems (Fig. 1 & 2) have
attracted attention due to their versatility and strong coordinating
13,14
In this paper, we present an extension to the
1 2 2
L H and L H
11
ability to a broad range of 3d- and 4f-metal ions. Among diazines,
the open chain ligands have shown a variety of geometrical
conformations that together with appropriate metal (II/III) salts
have afforded a number of interesting structural topologies
comprising mononuclear through to polynuclear coordination
family of diazine-based ligands and report herein a new series
of ligands
pyridine of
L
3a
H
2 2
2,
L
3b
H
2
and
3c 2
L H in which the central 2’,6’-
L H is replaced by a 6,6’-bipyridine (bipy) unit and
the functional groups attached to the diazine moiety are CH
NH and H for to respectively (Fig 3). In its
family of ligands offers two
3
,
2
L
3a
H
2
L
3c
2−
H
2
12
complexes. Interestingly, magnetic studies on binuclear Cu(II) deprotonated form this
L
3
systems have identified a linear relationship between the rotational potential bridging alkoxide oxygen donors, alongside diazine
7
and pyridine terminal groups designed specifically to
coordinate three metal centres. Bipyridine based ligands are
very commonly employed in 3d-metal chemistry due to their
angle around the N-N bond and the magnetic exchange integral. In
this respect, the torsional angle (α) about the N-N single bond, the
bond angles at the two nitrogen atoms (β, β'), and the nature of
functional groups attached to the diazine moiety typically dictate
15
predictable and rich coordination chemistry. They are often
referred to as non-innocent due to their potential to
6,7
the sign and magnitude of the exchange interaction (Fig. 1).
16
participate in redox reactions. We report preliminary results
from the reactions of these ligands with MX salts (where M =
CF ) which offer a
Previously reported diazine bridged metal complexes have shown
2
7
predominantly antiferromagnetic interactions. In earlier studies,
-
Cu(II), Mn(II), Fe(II); X = Cl, NO
3
, ClO
4
, SO
2
3
1
we reported the reactivity of L H (Fig. 3) with 3d-metal salts, where
M = Cu(II), Ni(II), Co(II), Mn(II), Fe(II), which afforded a non- different binding modes of L3aH , L3bH2, and L3cH2 identified in
range of both coordinating and non-coordinating anions. The
2
homoleptic [2 × 2] squares, together with a series of tetranuclear these studies are highlighted in Fig. 4. Despite the conjugated
alkoxo-bridged M
a second family of larger L
several [3 × 3] M (µ-O)12) (M = Mn(II), Fe(II/III), Co(II), Cu(II)), square
grids. Like the original series of [L (µ-O) complexes,
antiferromagnetic coupling was observed in the [L (µ-O)12
4 4 1
(µ-O) grids. Extension of the L
H series to include nature of the general backbone of the ligands, on coordination
-
3
to 3d-metals some variation in the ligand geometry is observed
based on the orientation of the amide side-arms in relation to
the central 2,2’-bipyridine core. For example, in coordination
2
based ligand systems (Fig. 1) afforded
9
M
1 4
4
]
mode
C
1
(Fig. 4), the carbonyl groups are both located exo to
they are both endo and in
2
M
9
]
the central cavity, whereas in
C
2
C
3
compounds with the exception of the Cu(II) complex, where
they adopt an intermediate coordination geometry with one
7
ferromagnetic coupling was dominant. Such grids are generally
exo and one endo carbonyl group. Detailed structural and
–
formed by employing weakly coordinating anions such as ClO
4
and
II
magnetic studies on the resulting trinuclear Cu complexes (
1
–
-
BF
4
, where the polydentate L
2
H
2
ligands complete the metal
II
3
), a trinuclear Mn compound (
4
) and a tetranuclear mixed
coordination environments. In contrast, more strongly coordinating valence FeII2FeII
anionic ligands tend to inhibit grid formation. For example, linear
2
(5) system are also presented.
Experimental
trinuclear Cu(II) complexes were isolated from reactions of L
2 2
H
together with copper salts containing relatively strong coordinating
Commercially available solvents and chemicals were purchased and
13
counter ions such as Cu(CH
3
COO)
2
.
1 2 2
Although both L H and L H
used without further purification. 6,6’-dicyano-2,2’-bipyridine (a)
possess extended conjugation, structural studies reveal that a
variety of geometrical conformations can be accommodated
17
was prepared according to the literature method.
2
| J. Name., 2012, 00, 1-3
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Physical Measurements
(C26
H
22
N
8
O
2
)(C12
H
12
N
6
O
2
).H
2
O C, 59.37; H, 4.72; N, 25.51%. Found:
View Article Online
DOI: 10.1039/C7DT03845C
C, 59.56; H, 4.43; N, 25.73%. The analytical data and the mass
spectrum of L3a
bipyridine-6,6'-dicarbohydrazide and the product (L3a
inability to separate these two species, the impure compound was
used “as is” for the subsequent coordination chemistry reactions.
NMR spectra were recorded on a Bruker 500 MHz spectrometer
with a Broadband AX Probe using CDCl
internal reference point relative to Me Si (δ = 0 ppm). IR spectra
were obtained using a Bruker Alpha FT-IR spectrometer equipped
with a Platinum single reflection diamond ATR module. Elemental
H
2
indicated formation of a 1:1 cocrystal of 2,2'-
). Due to the
3
(δ = 7.26 ppm, s) as an
H
2
4
compositions were determined on a PerkinElmer 2400 Series II Preparation of ligand L3b
Elemental Analyzer. Variable temperature magnetic data (2–300 K) mmol) was added to
were obtained using Quantum Design MPMS SQUID dicarbohydrazide (1.7 g, 6.28 mmol) in MeOH (50 mL). The mixture
2
H : 2-pyridinecarboxaldehyde (2.0 g, 18.67
a
suspension of 2,2'-bipyridine-6,6'-
a
magnetometer using a field strength of 0.1 T. Polycrystalline was refluxed overnight and the resultant precipitate was filtered,
samples were optically screened and a random selection of crystals washed with excess MeOH/ether and then dried in air. Yield 2.2 g,
-1
from each batch examined by unit cell determination to check 78 %. IR data (cm ): 3497 (br), 3287 (br), 1698 (s), 1661 (m), 1584
phase purity. Background corrections for the sample holder (m), 1511 (s), 1465 (s), 1435 (s), 1348 (m), 1295 (m), 1243 (s), 1138
assembly and sample diamagnetism were applied. No specific (s), 1105 (w), 1077 (s), 1055 (m), 995 (s), 929 (s), 825 (s), 778 (s),
approaches were employed to avoid torqueing of the sample but 736 (m), 709 (m), 633 (m), 621 (m), 589 (s), 523 (w), 472 (w). NMR
+
+
data were recorded in warming and cooling modes for 4 and 5 (insoluble). Mass spectrum (m/z, EI ): 451 [M+H] . Anal. Calcd. for
which are likely to be most susceptible and there were no evident
discontinuities in the data which might reflect sample movement.
24 18 8 2 2
C H N O ·2H O: C, 59.25; H, 4.56%. N, 23.03. Found: C, 59.94; H,
4.26; N, 23.15%.
Ligands and Complex Synthesis
2
Preparation of ligand L3cH : 2-Pyridinecarbonitrile (1.0 g, 9.60
Preparation of 6,6'-bis(methoxycarbonyl)-2,2'-bipyridine:18 An
mmol) was added to a dilute solution of sodium methoxide in
MeOH (100 mL) and then refluxed overnight. The resultant solution
was adjusted to neutral pH by the addition of glacial acetic acid. To
this solution, 2,2'-bipyridine-6,6'-dicarbohydrazide (1.0 g, 3.62
mmol) was added in portions. The mixture was refluxed overnight
and the resultant yellow precipitate obtained was then filtered,
washed with excess MeOH/ether and dried in air. Yield 1.76 g, 100
excess of sodium methoxide (3.5 g, 64.79 mmol) was added to a
suspension of 6,6'-dicyano-2,2'-bipyridine (2.0 g, 9.69 mmol) in
MeOH (100 mL) and the mixture was refluxed for 5 hours. The off-
white precipitate of the intermediate imine-diester was then
dissolved in a MeOH/H
min at rt. The resulting solution was adjusted to pH 1 by the
addition of dilute H SO and stirred for 3 hours at rt. The solution
2
O mixture (50 mL, 50:50) and stirred for 10
-1
%
. IR data (cm ): 3410 (m), 3313 (m), 3168 (br), 1688 (s), 1615 (s),
2
4
1
1
582 (s), 1565 (s), 1519 (s), 1474 (s), 1438 (s), 1383 (s), 1310 (w),
296 (w), 1253 (m), 1161 (s), 1102 (w), 1051 (w), 1080 (s), 1051 (m),
was then neutralized with aq. NaOH solution. The resultant white
precipitate was filtered, washed with water (100 mL) and dried in
1
994 (s), 929 (s), 821 (s), 795 (s), 759 (s), 741 (w), 720 (w), 705 (m),
air. Yield 1.4 g (53 %). The HNMR and melting point matched the
18
680 (w), 660 (w), 630 (m), 622 (m), 575 (w), 535 (w), 509 (w), 468
literature data.
1
(
w). H NMR (ppm, DMSO-d
6
) = 10.79 (2H, s), 9.06 (2H, d), 8.66 (2H,
Preparation of 2,2'-bipyridine-6,6'-dicarbohydrazide: Hydrazine
hydrate (1.5 g, 30 mmol) was added dropwise to a suspension of
d), 8.25 (4H, dd), 8.17 (2H, d), 7.97 (2H, dd), 7.55 (2H, t), 7.10 (4H,
+
+
s). Mass spectrum (m/z, EI ): 481 [M+H] . Anal. Calcd. for
: C, 59.99; H, 4.20; N, 29.15%. Found: C, 59.35; H, 4.01;
6
,6'-bis(methoxycarbonyl)-2,2'-bipyridine (2.1 g, 7.7 mmol). The
24 20 10 2
C H N O
reaction mixture was refluxed overnight. The off-white precipitate
was collected, washed with excess MeOH/ether and dried in air.
N, 28.65%.
Preparation of [Cu (CH
4 2 2
.52 mmol) and Cu(ClO ) ·6H O (0.06 g, 0.16 mmol) were stirred
3
L
3a(ClO
4
)
4
3 2 2 2 2
CN) (H O) ] (1): L3aH (0.025 g,
-1
Yield: 1.7 g, 81% , IR data (cm ): 3550 (br), 3292 (m), 1674 (s), 1621
0
(
(
7
m), 1582 (s), 1495 (s), 1431 (s), 1329 (br), 1260 (m), 1196 (w), 1117
together in MeCN (10 mL) until a clear green solution was obtained.
This was then filtered and the solution was layered with diethyl
ether. Dark green crystals suitable for single crystal X-ray diffraction
w), 1102 (m), 1071 (m), 996 (s), 962 (s), 832 (s), 765 (m), 737 (s),
1
05 (m), 671 (s), 632 (s), 586 (w), 545 (w), 476 (m). H NMR (ppm,
DMSO-d
6
) 10.31 (2H, s), 9.11 (2H, d), 8.13 (2H, t), 8.08 (2H, d), 4.71
-1
formed over a few days. Yield = 0.01 g, 16%. IR data (cm ): 3527
br), 3437 (br), 1631 (br), 1601 (w), 1573 (w), 1548 (m), 1483 (m),
+
(
4H, s). Mass spectrum (m/z, EI ): 273 (M + 1), Anal. Calcd. for
(
12
C H
12
6
N O
2
·0.5H
2
O: C, 51.24; H, 4.66; N, 29.88%. Found: C, 51.91; H,
1
1
472 (m), 1445 (w), 1431 (w), 1415 (w), 1366 (m, br), 1328 (m),
271 (m), 1195 (m), 1172 (w), 1061 (s, br), 946 (w), 930 (w), 920
4
.41; N, 29.95%.
Preparation of ligand L3a
was added to a suspension of 2,2'-bipyridine-6,6'-dicarbohydrazide
0.5 g, 1.81 mmol) in MeOH (50 mL) and the mixture was refluxed
overnight. The resultant precipitate was filtered, washed with
excess MeOH/ether and dried in air. Yield 0.45 g, 3 % based on
. IR data (cm ) 3327 (m), 3292 (w), 1699 (s), 1674 (m),
621 (m), 1581 (s), 1519 (s), 1470 (s),1431 (s), 1364 (m), 1233 (w),
260 (m), 1240 (m), 1163 (m), 1117 (m), 1102 (m), 1077 (m), 1044
2
H : 2-Acetyl pyridine (0.5 mL, 0.45 mmol)
(
w), 823 (m), 780 (m), 762 (m), 747 (w), 725 (m), 619 (s), 589 (m),
5
70
(w),
)Cu
440
(H
(m). Anal.
Calcd.
for
(
(
C
26
H
20
N
8
O
2
3
(ClO
4
)
4
2
O)
2
(CH CN) : C, 30.00; H, 2.69; N, 11.66%.
3
2
Found: C, 29.72; H, 2.57; N, 11.28%.
Preparation of [Cu (2): L3a
CuCl ·2H O (0.03 g, 0.17 mmol) were stirred together in MeOH (10
4
-1
3
L
3a(Cl)
4
]
n
2
H (0.025 g, 0.52 mmol) and
3 2
pure L aH
2
2
1
1
mL) until a clear green solution obtained. It was then filtered and
left undisturbed to generate a green microcrystalline solid whose IR
and elemental analysis composition were consistent with a
(
(
m), 945 (s), 963 (s), 912 (m), 833 (s), 819 (s), 793 (s), 752 (s), 703
s), 650 (br), 620 (s), 601 (s), 558 (s), 477 (w). NMR (insoluble). Mass
-1
+
+
methanol solvate (Yield = 0.013 g, 28 %). IR data (cm ): 3375 (br),
spectrum (m/z, EI ): 272, 479 [M+H] . Anal. Calcd. for
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Journal Name
3
1
074 (w), 1626 (w), 1569 (w), 1538 (s), 1532 (s), 1505 (m), 1480 (m), [Fe
4
(C26
H
20
N
8
O
2
)
2
(H
2
O)
6
][CF
3
SO
3
]
6
·(CH
3
CN): C, 32.46; H, 2.50; N,
View Article Online
471 (m), 1443 (m), 1415 (m), 1377 (s), 1324 (m), 1298 (m), 1275 10.73%. Found: C, 30.80; H, 2.75; N, 10.72 %D. OI: 10.1039/C7DT03845C
(
1
s), 1183 (s), 1167 (m), 1145 (m)< 1124 (m), 1108 (m), 1071 (s),
040 (m), 1025 (s), 945 (w), 933 (w), 903 (w), 825 (s), 782 (s), 765
X-ray Crystallography
Single crystals of 1 – 5 were mounted on a cryoloop with paratone
oil and examined on a Bruker D8 Venture APEX diffractometer
equipped with Photon 100 CCD area detector and Oxford
Cryostream cooler at 173(2) K, using graphite-monochromated Mo-
Kα radiation (λ = 0.71073 Å). Data were collected using the APEX-II
(
(
(
m), 764 (s), 743 (m), 719 (m), 710 (s), 688 (s), 648 (s), 605 (w), 590
m), 568 (w), 513 (m), 489 (w), 466 (m), Anal. Calcd. for
26
C H
20
N
8
O
2 3 4
)Cu Cl ·2MeOH: C, 38.52; H, 3.23; N, 12.84. Found: C,
3
7.81; H, 2.91; N, 12.77%. A second crop of dark green solvent-free
crystals suitable for single crystal X-ray diffraction formed over a
few days.
19
20
software, integrated using SAINT and corrected for absorption
21
using a multi-scan approach (SADABS). Final cell constants were
determined from full least squares refinement of all observed
reflections. All non-H atoms were located in subsequent difference
Preparation of [Cu
added to a solution of Cu(NO
(
solution obtained. It was then filtered and left undisturbed. Green
crystals suitable for single crystal X-ray diffraction formed
overnight. (Yield = 0.012 g, 22%) IR data (cm ): 3381 (br), 3216 (br),
1
1
3
L
3b(NO
3
4
) ]
n
(3): L3b
2
H (0.025 g, 0.55 mmol) was
3
)
2
·3H O (0.06 g, 0.25mmol) in MeCN
2
10 mL). The mixture was gently heated for 5 min until a clear green
22
maps and refined anisotropically with SHELXL-97 using full least
2
squares refinement against F . H atoms were added at calculated
positions and refined with a riding model. O-H atoms were located
in the difference map and refined with U(iso) riding on the O with
DFIX constraints applied. For compound 5 all triflate anions and
-1
659 (w), 1618 (w), 1560 (s), 1508 (s), 1471 (s), 1469 (br), 1449 (w),
364 (br), 1344 (br), 1321 (br), 1302 (br), 1265 (s, br), 1213 (s), 1187
some solvent molecules (two MeCN and one Et
the difference map. Four of the six triflate anions and most solvent
molecules exhibited disorder. One particularly problematic Et
molecule was omitted in the latter stages of refinement and the
2
O) were located in
(
1
(
(
3
s), 1161 (m), 1100 (br), 1081 (m), 1074 (m), 1041 (m), 1028 (m),
015 (w), 991 (s, br), 868 (m), 830 (m), 816 (s), 802 (m), 784 (s), 757
s), 733 (m), 711 (s), 700 (s), 645 (m), 520 (s). Anal. Calcd. for
)Cu (NO : C, 32.49; H, 1.82; N, 18.95%. Found: C,
2.19; H, 1.80; N, 18.61%.
Preparation of [Mn
mmol) was added to a solution of MnCl
2
O
C
24
H
16
N
8
O
2
3
3
)
4
23
24
residual electron density treated with SQUEEZE within PLATON.
A summary of crystallographic data for complexes 1 – 5 is presented
3
L
4
3c(Cl) (CH
3
2
OH) (H
2
O)] (4): L3c
H
2
(0.025 g, 0.52 in Table S1 of the ESI and the structures have been deposited with
(0.06 g, 0.47 mmol) in the CCDC (CSD deposition numbers 1579185-1579189).
2
MeOH (10 mL) followed by the addition of one drop of
triethylamine. The mixture was stirred at rt for 10 min until a clear
orange solution was obtained. It was then filtered and left
undisturbed. Light yellow crystals suitable for single crystal X-ray
Results and Discussion
Ligand Synthesis
-
diffraction appeared in a week. (Yield = 0.017 g, 42 %) IR data (cm
1
)
: 3140 (br), 3317 (br), 1651 (m), 1645 (s), 1615 (w), 1599 (m), 1563
(
(
s), 1553 (s), 1538 (s), 1532 (s), 1506 (m), 1479 (m), 1464 (s), 1435
m), 1407 (s), 1538 (s), 1296 (m), 1268 (s, br), 1183 (m, br) 1163 (m),
1
8
091 (w), 1079 (w), 1047 (w), 1005 (m), 964 (w), 930 (w), 841 (w),
21 (s), 791 (s), 762 (s), 742 (s), 676 (s), 638 (s), 632 (s), 565 (w), 532
(
(
w), 524 (w), 498 (w), 461 (s). Anal. Calcd. for (C26
28 10 5 4 3
H N O )Cl Mn ·
H
2
O)2/3·(MeOH)2/3 (estimated lattice solvent ratio from X-ray
diffraction): C, 35.80; H, 3.53; N, 15.65%. Found: C, 35.65; H, 3.40;
N, 15.79%.
3a 2 3b 2 3c 2
Scheme 1: Reaction route to L H , L H , and L H
Ligands L3a
reacting 6,6'-dicarboxylicacid-2,2'-dihydrazide with their respective
terminal pyridyl groups (Scheme 1). While ligands L3b
were recovered in pure form, L3a proved more problematic
precipitating as an insoluble 1:1 mixture of L3a with unreacted
starting material, despite multiple adjustments to the synthetic
protocol. In this case preliminary coordination chemistry studies
were undertaken using this mixture. Fortuitously in the case of their
2 2 2
H , L3bH , and L3cH were prepared in 50 – 80 % yields by
Preparation of [Fe
4
(L3a
)
2
(H
SO
2
O)
6 3 3 6 2
][CF SO ] (5): Ligand L3aH (0.025 g,
0
.52 mmol) and Fe(CF
3
3
)
2
·2MeCN (0.06 mg, 0.057 mmol) were
2 2
H and L3cH
stirred together in MeCN (10 mL) until a clear purple solution was
obtained. This was then filtered and layered with diethyl ether.
Dark purple crystals suitable for single crystal X-ray diffraction
H
2
H
2
-1
formed over a few days. (Yield = 0.011 g, 16%). IR data (cm ): 3192
(
(
br), 3078 (br), 1652 (m), 1645 (m), 1634 (m), 1600 (m, br), 1575
m), 1558 (w), 1532 (m), 1565 (m), 1471 (w), 1435 (m, br), 1373 (m,
reactivity with CuCl
2
·2H
2 4 2 2
O and Cu(ClO ) ·6H O, the complexes of
br), 1274 (s), 1235 (s), 1221 (s), 1159 (s, br), 1083 (w), 1024 (s), 820
m), 776 (m), 759 (m), 726 (w), 702 (w), 681 (w), 631 (s), 598 (w),
73 (m, br), 555 (w), 516 (s), 493 (m), 485 (m), 475 (m), 467 (m).
Crystallographic studies indicated two MeCN molecules in the
asymmetric unit (at least one part occupancy) and two Et
2-
L
3a crystallized selectively (vide infra).
(
5
Syntheses of complexes 1 – 5
Reactions of ligands L3a
2
O
H
2
, L3b
H
2
, and L3c
, SO CF
2 2
H with MX (M = Cu(II),
molecules (again some part occupancy indicated) both of which
were disordered and the crystals showed a strong tendency to de-
solvate when removed from the mother liquor. Anal. Calcd. for
Mn(II), Fe(II); X = Cl, NO
3
, ClO
4
2
3
) afforded the trinuclear
II
II
II
complex Cu (1) and 1-D Cu chains (2 , 3), a trinuclear Mn complex
II
III
(
2
4) and a tetranuclear Fe Fe complex (5). All five complexes were
4
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ARTICLE
Figure 6: Crystal Structure of [Cu
3
L
3aCl
4
]
n
(2) with additional symmetry related chloride
View Article Online
DOI: 10.1039/C7DT03845C
anions included to emphasise the coordination geometry at each Cu centre.
3 4 4 3 2 2 2
Figure 5: Crystal Structure of [Cu L3a(ClO ) (CH CN) (H O) ] (1)
Figure 7: Polymeric structure of 2 highlighting the Cu-Cl-Cu bridges
characterized by a variety of analytical techniques including single
crystal X-ray diffraction, IR and elemental analysis. Single crystals
were grown either directly from the mother liquor or layering with
diethyl ether. The crystals of all five complexes proved slightly
hygroscopic, absorbing small quantities of water upon standing, but
otherwise appeared air stable.
Cu1–O8s distances of 2.748 (2) Å. Cu1 is connected to Cu2 and
Cu2A by trans N–N diazine bridges with Cu1–N2–N–Cu2 torsional
o
angle (α) of 157.7 , slightly less than that previously reported for
6
similar structures. The Cu∙∙∙Cu separation is 4.769 Å which is
6
consistent with other related diazine bridged complexes. Given the
II
Description of Crystal Structures
+6 charge on the three Cu ions and the presence of four
perchlorate ions, the charge balance is consistent with the
[
Cu
were isolated from the reaction of L3a
:1 mixture of acetonitrile/diethyl ether solution. The molecular
3
L
3a(ClO
4
)
4
(CH
3
CN)
2
(H
2
O)
2
] (1): Dark green single crystals of 1
2-
polydentate ligand being doubly-deprotonated L3a . The diazine
N2–N3 (1.380(3) Å) and C6–O4 (1.260(3) Å) bond distances imply
single and double bond character respectively, consistent with
deprotonation of the azine N-H. The torsional angle (α) around the
H
2
with Cu(ClO ·6H O in a
4
)
2
2
1
structure of 1 is shown in Fig. 5 and selected interatomic distances
and angles are listed in Table S2 of the ESI. The complex crystalizes
in the monoclinic space group C2/c, with half a molecule of 1 in the
asymmetric unit. The ligand binds three Cu(II) centres in a chain
o
diazine N–N bond is 157.7(1) and the bond angles (β, β′) are
o
o
1
08.9(2) and 124.0(2) .
(2): Reaction of L3a
by two symmetry equivalent terminal divalent copper ions (Cu2 and coordination polymer with formula [Cu
Cu2A). These two peripheral copper ions exhibit distorted were grown via the slow evaporation of MeOH at rt. The complex
arrangement with Cu1 located on a two-fold rotation axis, flanked [Cu
3
L
3a(Cl)
]
4 n
H
2
2
with CuCl ·2H
2
O afforded a 1-D
L3aCl ] . Single crystals of 2
3 4 n
2-
octahedral coordination geometry, with the dianionic ligand L3a
acting as an N
O donor in the equatorial plane (Cu–N distances in molecule in the asymmetric unit. Selected bond lengths and angles
the range 1.942(2) – 2.017(2) Å) and an acetonitrile solvent are listed in Table S3 of the ESI. The fundamental repeat unit in 2 is
crystalizes in the monoclinic space group I2/a with half of the
2
4+
molecule adopting the fourth position in the equatorial plane. A a similar trinuclear [Cu
perchlorate anion and a water molecule were found in the apical coordination geometry around Cu1 is octahedral with L3a
positions. The Cu-Owater distance is 2.215(2) Å while The Cu2– coordinating via the bipyridine N (N1, N1A) and diazine N (N2,N2A)
perchlorate distances is 2.617(3) Å, reflecting the Jahn-Teller atoms with bridging -chloride ions (Cl1 and Cl1A) in the axial
3 3a
L ] motif to that observed in 1 (Fig. 6). The
2-
O
µ
2
elongation axis. The central Cu1 ion, located on the two-fold positions forming a Cu1-Cl1-Cu2A link between trimers. The Cu–N
rotation axis, exhibits a marked Jahn-Teller distortion. The distances are in the range 1.9753(3) – 2.0138(3) Å, normal for
2
-
II 3
equatorial plane comprises four N-donors derived from L3a with equatorial coordination to Cu . The symmetry equivalent Cu–Cl
Cu1–N distances of 1.937(2) Å and 2.003(2) Å. Two symmetry distances are 2.7314(16) Å. The peripheral pair of symmetry-related
2+
equivalent perchlorates adopt the axially elongated positions with
Cu ions (Cu2/Cu2A) exhibit square pyramidal coordination
2-
geometry with one chloride ion and an N
forming the basal plane and an apical chloride which offers a µ
bridge to Cu1 in a neighbouring molecule. The Cu2–N distances are
.993(3) Å and 1.966(3), while the Cu2–Cl distances are 2.227(15) Å
2
O donor set from L3a
2
-
1
and 2.5488 (14) Å respectively for the basal and apical Cl atoms and
the Cu2–O distance is 1.9992(3) Å. The diazine N2–N3 (1.387(4) Å)
and C–O (1.251(5) Å) bond distances are similar to those found in 1
implying single and double bond character respectively. The
torsional angle (α) around the diazine N–N bond is
o
o
o
1
65.4(2) and the (β, β′) bond angles are 107.5 (3) and 112.9(4)
similar to those found in 1. Each trinuclear subunit of 2 is linked via
apical
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The axial positions are occupied by two symmetry equivalent O
View Article Online
DOI: 10.1039/C7DT03845C
atoms (O7) of a bridging nitrate anion. The Cu–O distance is
2
.572(16) Å. The peripheral Cu1 (Cu1A) ions exhibit a geometry
intermediate between 5 and 6-coordinate. In the equatorial plane
2-
2-
ligand L3b offers an N
2
O donor set from L3b which is completed by
an O atom from a nitrate ligand (Cu1-O4). The Cu1–N distances are
1
1
.935(3) and 2.056(3) Å, while the Cu1–O distances are in range
.940(2) – 2.122(2) Å. The axial position is taken up by a nitrate O
atom (O5) with a Cu-O distance of 2.204(3) Å. An additional O atom
from the equatorially bound nitrate is also located in the vicinity of
Cu(1) with a Cu-O distance of 2.626(3) Å but the O-Cu-O angle to
o
o
O(5) (136.2(1) ) lies a long way from the idealised 180 expected
from a Jahn-Teller distorted octahedron and Cu(1) is probably
better considered as square pyramidal. Within these trimeric units
Cu2 is connected to the symmetry equivalent Cu1/Cu1A atoms via
o
trans N–N bridges (Cu–N–N–Cu torsional angle of 151.5 ), leading
Figure 8: Crystal Structure of [Cu
for clarity
3
L
3
3b(NO )
4
] in 3. Selected H atoms have been removed to a Cu∙∙∙Cu separation of 4.738 Å consistent with other related
diazine bridging complexes.11 The diazine N2–N3 (1.387(4) Å) and
C–O (1.250(4) Å) bond distances are similar to those found in 1 and
2
implying single and double bond character respectively. The
torsional angle (α) around the diazine C6–N3–N2–C7 bond is
o
o
o
1
65.3(3) and the bond angles (β, β′) are 109.4(2) and 122.5(3) ,
again comparable with those found in 1 and 2. One of the two
crystallographically independent nitrates adopts a 1,3-µ -bridging
2
mode between trimer units linking a terminal Cu(1) to the central
Cu(2) of a neighbouring molecule such that each trimer exhibits two
bridging nitrates to an adjoining trimer unit which propagates
through the structure to form a polymeric chain arrangement. (Fig.
9
).
Mn
complex was isolated from the reaction of L3c
[
3
L
3c
H
2
(Cl)
4
3
(CH O)
2
(H
2
O)] (4): A unique trinuclear manganese
with MnCl in
H
2
2
Figure 9: Nitrate bridged polymer chain in
3
methanol. Suitable single crystals for X-ray diffraction were grown
from slow evaporation of MeOH solution. The molecular structure
of complex 4 is shown in Fig. 10 and selected interatomic distances
and angles are listed in Table S5 of the ESI. The complex crystalizes
2
connections between Cu1 and Cu2 via a µ -chloride bridge affording
a polymeric 1-D chain topology in which each trimer unit is
connected to the next via two bridging chloride ions (Fig. 7). This
arrangement resembles that of complex 3 described below, as well
as a previously reported complex where hexanuclear subunits are
2-
in the triclinic space group P-1. The ligand L3c adopts an alternative
2-
2-
coordination environment to L3a and L3b previously described. In
this case the carbonyl groups are located ‘endo’ (geometry C , Fig.
4). The bond lengths within L3c are significantly different from
2
2-
11
linked to form chains via carboxylate bridges.
Cu (3): Dark green single crystals of 3 were isolated
from the reaction of L3b and Cu(NO ·3H O via the evaporation of
2-
2-
those in the closely related L3a and L3b ligands. The latter were
[
3 3 3 n
L3b(NO ) ]
consistent with C=O character and a C-N single bond whereas the
H
2
3
)
2
2
a hot solution of acetonitrile. The molecular structure of complex 3
is shown in Fig. 8 and selected interatomic distances and angles are
listed in Table S4 of the ESI. The complex crystalizes in the
monoclinic space group C2/c, with a half a crystallographically
unique molecule of 3 in the asymmetric unit. The structure closely
resembles complex 2 with respect to its coordination geometry and
polymeric structural topology, the major difference being the
replacement of the chlorides by nitrate ions (cf. Figs. 7 and 9). Once
4+
again the fundamental repeat unit in 3 is a trinuclear [Cu
3 3b
L ]
moiety. The coordination geometry around Cu1 is octahedral with
2-
L
3b offering an N
two bipyridine N atoms (N4, N4A) and the deprotonated diazine N
N2, N3) atoms. The Cu–N distances are 1.937(3) and 1.970(3) Å
4
donor set in the equatorial plane comprising the
(
which are slightly less than the distance in complex 2 but can still be
considered in the normal range for equatorial coordination to Cu .
Figure 10: Crystal Structure of [Mn
3 4 3 2 2
L3cCl (CH OH) (H O)] (4) [Disordered
II 11
methanol/water solvent omitted for clarity]
6
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn sa rgins
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2-
geometric parameters for L3c show a marked lengthening of the C- of the ligand away from the nearly co-planar structureVsiewobArsteicrlevOednlinine
DOI: 10.1039/C7DT03845 C2 -
O distances at 1.295(5) – 1.303(6) Å accompanied by a shortening complexes 1 – 4. In this context one of the side chains of L3a
2-
of the C-N bond such that L3c has more alkoxide character. The remains planar with the central bipy plane (torsional angle (α)
o
light yellow colour of complex 4 and bond lengths around around C33–N11–N10–C32 is 178.5(5) and bond angles (β, β′) are
o
o
manganese atoms (vide infra) suggest all Mn ions carry a +2 charge 107.7(4) and 120.3(4) ), while the other side chain twists around
2-
and so the charge on the ligand is formally L3c . The central Mn1 ion the diazine bond with the torsional angle (α) around C44–N14–
o
o
adopts an unusual pentagonal bipyramidal coordination geometry N15–C45 = -87.5 (6) and the bond angles (β, β′) are 116.0(4) and
comprising an N
2-
o
2 2
O donor set from L3c and an equatorially 117.7(4) respectively. The loss of delocalization in the ligand
coordinated water molecule and two axially coordinated methanol framework occurs via rotation about the diazine N-N bond. Fe1 and
25
solvent molecules. The methanol solvent molecules form well- Fe3, occupy the central bipy pocket of the two crystallographically
defined intermolecular O-H···Cl hydrogen bonds (O···Cl distances of independent units with pentagonal bipyramidal geometries. In the
2
-
3
1
.135(3) and 3.206(3) Å and C-O···Cl angles of 101.6(3) and equatorial plane this comprises an N
3
O donor from one L3a ligand
o
2-
17.9(3) ) which clearly define each as a neutral coordinating and a diazine N atom from a second L3a ligand. The axial positions
methanol rather than a methoxide anion. The Mn1–N distances are in each case comprise a terminal pyridyl unit from the second L3a
2-
2
2
.297(3) and 2.298(3) Å, while Mn1–O distances are in range anion and coordinated water, providing each Fe centre with an
.202(3) – 2.306(3) Å. The peripheral manganese atoms Mn2 and coordinated geometry. The Fe1/Fe3–O and Fe1/Fe3–N
5 2
N O
Mn3 both exhibit distorted square pyramidal geometries with distances are in the range 2.097(4) – 2.474(4) Å and 2.154(6) –
26
Addison τ values close to 0.203 for Mn2 and 0.375 for Mn3 with 2.384(4) Å respectively. The remaining two Fe centres (Fe2 and Fe4)
2-
Mn-N bond lengths in the range 2.117(4) – 2.323(4) Å, Mn-Cl adopt a distorted N
distances of 2.368(1) – 2.389(1) Å and Mn-O bond lengths of acting as an N O donor, the second L3a ligand acting as an O donor
and two axially ligated water molecules completing the
which link metal centers via a two-atom diazine bridge, the central coordination sphere. The Fe2/Fe4–O and Fe2/Fe4–N distances fall
and peripheral Mn ions in 4 are linked via μ -O bridges leading to in range 1.902(4) – 2.044(5) Å and 2.069(4) – 2.145(5) Å
Mn1···Mn2 and Mn1···Mn3 separation of 4.176(1) and 4.0652(9) Å respectively. Fe1 and Fe2 are linked via a μ -O atom as are Fe3 and
2 4
O octahedral geometry with one L3a ligand
2-
2
2-
2-
2
.203(3) – 2.227(3) Å. Unlike the copper complexes of L3a and L3b
2
2
respectively, reflecting the harder oxophilic nature of the Mn Fe4, while the central Fe1 and Fe3 atoms are bridged via two cis-
o
centres. Unlike complexes 2 and 3, there are no covalent bonds diazine bridges (Fe1–N14–N15–Fe3; torsional angle of 66.0 (4) and
o
between molecules.
Fe (H O) ][CF
complexes derived from the copper and manganese reactions, the
reaction of L3a with Fe(CF SO ·2MeCN in acetonitrile under
aerobic conditions unexpectedly afforded the tetranuclear complex
Fe (H O) ][CF SO , 5. The molecular structure of 5 is shown
in Fig. 11 and selected interatomic distances and angles are listed in
Table S6 of the ESI. The compound crystallizes in the triclinic space
group, P–1, with two ligands, four iron centres, six coordinated
water molecules and six triflates in the asymmetric unit. In addition
Fe1–N6–N7–Fe3 torsional angle of 62.5(4) ) leading to an Fe1···Fe3
separation of 4.482(1) Å. (Fig. 9). Fe1 and Fe4 are linked via the 3-
atom NCO bridge of an L3a ligand as are Fe2 and Fe3. The six
coordinated water molecules form a web of hydrogen bonded
[
4
(L3a
)
2
2
6
3
SO
3
]
6
(5): In contrast to the trimetallic
2-
H
2
3
3 2
)
-
interactions to the CF
3
SO
3
counterions, but these tetranuclear units
are otherwise well-separated in the crystal lattice with the closest
hydrogen-bonded contact between complexes being 8.561 Å. Bond
lengths around Fe1/Fe3 (BVS = 1.90) and Fe2/Fe4 (BVS = 3.036)
suggest +2 and +3 oxidation states respectively for these four iron
centres. The resulting +10 charge on these four ions is subsequently
[
4
(L3a
)
2
2
6
3
3 6
]
2-
charge balanced by two L3a and six triflate anions.
Magnetic Properties
Variable temperature dc susceptibility data (χT vs T, 5 – 300 K) for 1
(
Fig. 12), 2 (Fig. 1S) and 3 (Fig. 2S) indicate room temperature χT
-1
-1
-1
values of 1.16 emu·K·mol , 1.11 emu·K·mol and 1.02 emu·K·mol
respectively. These are in reasonable agreement with the
theoretical value expected for three magnetically isolated Cu ions
2+
-1
(
3 × S = ½ with g ~ 2.1 gives an expected χT = 1.24 emu·K·mol ). The
χT value continuously decreases on cooling reaching a plateau at
-1
-1
-1
0
.42 emu·K·mol , 0.33 emu·K·mol and 0.40 emu·K·mol for
complexes 1 and 2 and 3 respectively at the base temperature of 5
Figure 11: Crystal Structure of [Fe
4 2 2 6 3 3 6
(L3a) (H O) ][CF SO ] (5) with counter ions and
K. This behaviour is typical of antiferomagnetically coupled
solvent molecules removed for clarity.
II
trinuclear Cu complexes where an S
expected (χT = 0.41 emu·K·mol for S
T
= ½ spin ground state is
= ½ and g ~ 2.1). The
trinuclear cores of 1 – 3 are all similar but 2 and 3 differ from 1 in
that they form coordination polymers via µ -chloride or 1,3-bridging
-1
T
poorly refined lattice solvent suggest a maximum of two MeCN and
two Et O solvent molecules per formula unit but the crystals
2
2
showed a strong propensity for desolvation when removed from
the mother liquor. In 5, there is a significant geometrical distortion
nitrate counterions respectively. Here the Jahn-Teller axes of the
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DOI: 10.1039/C7DT03845C
Figure 12: Temperature dependence of χT for 1 with the solid line representing the fit
to the linear trimer model (g = 2.096(3) and J/k = -35.5(3) K)
Figure 13: Temperature dependence of χT for 4 with the solid line representing the fit
to the linear trimer model (g = 1.963(2) and J/k = -0.66(1)( K)
Cu2+ ions clearly permit us to delineate the directionality of the dx2-
y2 magnetic orbitals which help define the dominant exchange path-
way(s). In all three complexes these magnetic orbitals are located in
the basal plane providing efficient communication via the trans-N–
N diazine bridges but the exchange coupling between trimeric units
is therefore expected to be weak. It is therefore unsurprising that
the three complexes exhibit similar magnetic responses and all
elsewhere.6,7 The nature of the dx -y magnetic orbitals with diazine
2
2
N atoms located in the basal plane appears to afford efficient
exchange coupling through the σ-framework of the trans diazine
unit.
three systems were modelled by the simple isotropic spin Variable temperature dc susceptibility data for 4 indicates a room
-
1
Hamiltonian described in Eq. (1).
temperature (300 K) χT value of 12.33 emu∙K∙mol , slightly lower
-1
than the spin-only value (13.125 emu·K·mol ) expected for three
magnetically isolated high-spin Mn ions. The value of χT for 4
II
Ĥ = -2J[Ŝ
1
Ŝ
2
+ Ŝ
2 3
Ŝ ]
Eq.1
appears almost temperature independent across the temperature
range 300 – 75 K but begins to decrease rapidly below 50 K. An
.096(3) and J/k = -35.5(3) K for 1 (Fig. 12). For 2 the behaviour was excellent fit to the linear trimer model (Eq. 1) within Phi afforded
A fit to the linear trimer model (Eq. 1) within Phi27 afforded g =
2
2
7
similarly modelled with a trimer model but a small downturn in χT g = 1.963(2) and J/k = -0.66(1) K consistent with antiferromagnetic
II
was evident in the limiting low temperature region, suggesting the coupling between Mn centres, generating an S
presence of weak through bond superexchange or through space (Fig. 13).
dipolar coupling. The overall fitting parameters (g = 2.098(2), and
T
= 5/2 ground state
J/k = -38.9(2) K, θ = - 1.0 K) are similar to 1 and the mean field term
II
The Fe
4
cluster (5) comprises two seven-coordinate Fe centres (Fe1
for the inter-trimer exchange coupling an order of magnitude
smaller than the intramolecular exchange. The presence of
antiferromagnetic exchange between trimer units in 2 will generate
a diamagnetic ground state and is reflected in the downturn in χT
near the base temperature (see ESI). A somewhat larger exchange
coupling was identified for 3 evidenced by a slightly lower χT value
at room temperature and a more extended plateau in the low
temperature region, reflecting a thermally well-isolated spin ground
state. In this case the g-value was fixed at 2.03 (to replicate the low
temperature value of χT) and a single parameter fit provided J/k = -
III
III
and Fe3) and two six-coordinate Fe centres (Fe2 and Fe4). The Fe
centres are expected to exhibit a simple spin-only A term (S = 5/2,
6
II
5
g ~ 2.0,) but the pentagonal bipyramidal Fe centres exhibit a E
configuration in which the doubly degenerate ground state can
contribute significant orbital angular momentum and has been
shown to lead to very large zero field splitting (|D| < 15 cm ). In
the high temperature region these ions approximate to S = 2 with g
2.1 – 2.2. The temperature dependence of χT vs T for 5 is
presented in Fig. 14 and reveals a complex evolution of χT. In the
high temperature region (T > 250 K) a plot of 1/χ vs T follows Curie-
Weiss behaviour with C = 15.17 emu·K·mol and θ = -9.0 K (see Fig.
S, ESI), comparable with that expected for two S = 5/2 and two S =
ions with g = 2.0 (14.75 emu·K·mol ). On cooling the value of χT
initially increases reaching a maximum of 20.34 emu·K·mol before
decreasing monotonically to 1.92 emu·K·mol at 2.0 K. In the region
0 – 175 K the sample again obeys Curie-Weiss behaviour with C =
0.86 emu·K·mol and θ = -88.6 K. This value of C is broadly
-1 28
~
7
2(1) K). Data for 2 and 3 are available in the ESI. In all cases the
-1
II
presence of dominant antiferromagnetic exchange between Cu
centres leads to an S
5
2
T
= ½ ground state, reflected in a plateau in the
-1
low temperature data. Previous studies on diazine systems suggest
a linear relationship between the exchange coupling and the
rotational angle of Cu magnetic planes relative to the single N-N
-1
-1
5
3
7
bond. Cu–N–N–Cu torsion angles (α, α') and bond angles (β, β') in
-1
o
o
o
o
1
, 2, and 3 are in range 151.4 – 165.3 and 107.5 – 125.5
3+
2+
consistent with ferromagnetic coupling between Fe and Fe ions
via the µ -oxo bridge giving rise to two independent S = 9/2 spin
states (C = 24.74 emu·K·mol for g = 2.0) if other exchange
This journal is © The Royal Society of Chemistry 20xx
respectively which are quite close to the related examples studies
2
-1
8
| J. Name., 2012, 00, 1-3
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Page 10 of 11
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ARTICLE
pathways are neglected. The change of sign and large value of θ in metals forming a trimetallic sub-unit but sufficient ligVaienwdArftlieclxeiObnilliintye
this low temperature regime is consistent with the emergence of to modify the coordination pockets for meDtaOlI:b1i0n.d10in3g9./CIn7DtTh0is38c4a5sCe
additional weaker antiferromagnetic interactions plus residual the different coordination preferences of the metal play a role in
2+
orbital angular momentum arising from the Fe centers and/or determining the final outcome of the self-assembly process. For
II
significant zero field splitting. An examination of the crystal example Cu has a preference for Jahn-Teller elongated or square-
structure reveals 5 exhibits a non-crystallographic two-fold rotation based pyramidal geometry whereas they favour the formation of
II
II
axis such that a simple isotropic model of the exchange coupling pentagonal bipyramidal geometries in the case of both Mn and Fe
approximates to:
Ĥ = -2J[Ŝ
centres. These ligands support effective antiferromagnetic
II
II
exchange coupling within the Cu trimers 1 – 3 and also Mn
II
1
Ŝ
2
+ Ŝ
3 4
Ŝ ] -2J’[Ŝ
1
Ŝ
3 1
] -2J”[Ŝ Ŝ
4
+ Ŝ
2 3
Ŝ ]
Eq.2
complex 4. While the pentagonal bipyramidal Mn complex 4
6
II
5
exhibits an A ground term, the Fe center in 5 possesses a E
ground term with significant residual orbital angular momentum.
The trimetallic copper species reveal the potential to create more
complex architectures from these building units by exchanging non-
-
-
coordinating anions in 1 for bridging ligands such as Cl and NO
3
in
2
and 3 respectively. Further reactions of the series of L ligands
3 2
H
with other 3d- and 4f- metal ions are currently under investigation
and will be reported in due course.
Acknowledgements
We would like to thank NSERC and the Canada Research Chairs
program (J.M.R.) and C.F.I./O.R.F. for financial support, the NSERC
Discovery program (M.P.), Dr F. S. Razavi (University of Brock) for
access to the SQUID magnetometer and Dr N. chilton (University of
Manchester) for helpful discussions with regard to the use of Phi for
high spin pentagonal bipyramidal Fe(II).
Figure 14: Temperature dependence of χT for 5.
where J reflects exchange coupling via the Fe-O-Fe one-atom
bridge, J’ describes exchange coupling via the diazine unit and J”
describes exchange coupling via the three atom NCO linker.
Attempts to fit the magnetic data to this isotropic model failed,
presumably due to the presence of significant residual orbital
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2
7
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