Synthesis, structural and magnetic characterisation of iron(ii/iii), cobalt(ii) and copper(ii) cluster complexes of the polytopic ligand: N-(2-pyridyl)-3-carboxypropanamide

Article abstract of DOI:10.1039/c3dt51301g

Herein we describe the synthesis, structural and magnetic characterisation of three transition metal cluster complexes that feature the polytopic ligand N-(2-pyridyl)-3-carboxypropanamide (H₂L): [Fe₃ ^IIIFe₂^II(HL)₆(O)(H ₂O)₃][ClO₄]₅·3MeCN· 4H₂O, 1, [Co₈(HL)₈(O)(OH)₄(MeOH) ₃(H₂O)]-[ClO₄]₃·5MeOH· 2H₂O, 2, and [Cu₆(L_ox)₄(MeOH)(H ₂O)₃]·MeOH, 3. Complex 1 is a mixed valence penta-nuclear iron cluster containing the archetypal {Fe₃ ^IIIO} triangular basic carboxylate cluster at its core, with two Fe(ii) ions above and below the core coordinated to three bidentate pyridyl-amide groups. The structure of the octanuclear Co(ii) complex, 2, is based upon a central Co₄ square with the remaining four Co(ii) centres at the 'wing-tips' of the complex. The cluster core is replete with bridging oxide, hydroxide and carboxylate groups. Cluster 3 contains an oxidised derivative of the ligand, L_ox, generated in situ through hydroxylation of an α-carbon atom. This hexanuclear cluster has a 'barrel-like' core and contains Cu(ii) ions in both square planar and square-based pyramidal geometries. Bridging between Cu(ii) centres is furnished by alkoxide and carboxylate groups. Magnetic studies on 1-3 reveals dominant antiferro-magnetic interactions for 1 and 2, leading to small non-zero spin ground states, while 3 shows ferro-magnetic exchange between the Cu(ii) centres to give an S = 3 spin ground state.

Full text of DOI:10.1039/c3dt51301g

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DOI: 10.1039/C3DT51301G
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ARTICLE TYPE
Synthesis, structural and magnetic characterisation of iron(II/III),
cobalt(II) and copper(II) cluster complexes of the polytopic ligand: Nꢀ(2ꢀ
pyridyl)ꢀ3ꢀcarboxypropanamide
a
a,b
a
a
b
Mark E. Russell, Chris S. Hawes, Alan Ferguson, Matthew I. J. Polson, Nicholas F. Chilton,
b
b
a
5
Boujemaa Moubaraki, Keith S. Murray and Paul E. Kruger*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Herein we describe the synthesis, structural and magnetic characterisation of three transition metal cluster
complexes that feature the polytopic ligand Nꢀ(2ꢀpyridyl)ꢀ3ꢀcarboxypropanamide (H L):
2
III
II
. .
[Fe3 Fe (HL) (O)(H O) ][ClO ] 3MeCN 4H O, 1, [Co (HL) (O)(OH) (MeOH) (H O)]ꢀ
2 6 2 3 4 5 2 8 8 4 3 2
. .
ClO ] 5MeOH 2H O, 2, and [Cu (L ) (MeOH)(H O) ]ꢁMeOH, 3. Complex 1 is a mixed valence pentaꢀ
4 3 2 6 ox 4 2 3
nuclear iron cluster containing the archetypal {Fe3 O} triangular basic carboxylate cluster at its core,
with two Fe(II) ions above and below the core coordinated to three bidentate pyridylꢀamide groups. The
1
1
2
0
5
0
[
III
structure of the octanuclear Co(II) complex, 2, is based upon a central Co square with the remaining four
Co(II) centres at the ‘wingꢀtips’ of the complex. The cluster core is replete with bridging oxide, hydroxide
4
and carboxylate groups. Cluster 3 contains an oxidised derivative of the ligand, L , generated inꢀsitu
ox
through hydroxylation of an αꢀcarbon atom. This hexanuclear cluster has a ‘barrelꢀlike’ core and contains
Cu(II) ions in both square planar and squareꢀbased pyramidal geometries. Bridging between Cu(II)
centres is furnished by alkoxide and carboxylate groups. Magnetic studies on 1ꢀ3 reveals dominant
antiferroꢀmagnetic interactions for 1 and 2, leading to small nonꢀzero spin ground states, while 3 shows
ferroꢀmagnetic exchange between the Cu(II) centres to give an S = 3 spin ground state.
primarily with its potential biological activity or medicinal
6
Introduction
application and in the formation of hydrogels. A few studies
have reported the spectral characterisation of mononuclear Cu(II)
and Ni(II) complexes of H L, although no solidꢀstate structures
of these species are known. Only one structurally characterised
coordination compound has been reported: a discrete monoꢀ
nuclear complex, {[MnHL(phen) ]ClO 1.25H O}, where HL
coordinates via a chelating carboxylate group to Mn(II) with
,10ꢀphenanthroline (phen) present as a coꢀligand. Surprised by
the dearth of structural reports featuring H L, and driven by the
recognition of its potentially potent coordination capabilities, we
begun an investigation into its coordination chemistry with a
variety of 1 row transition metal ions. Here, we report the
synthesis and structural characterisation of the first multiꢀnuclear
cluster complexes of H L: a pentanuclear Fe(II/III), an octaꢀ
The pyridyl, amide and carboxylate functional groups are
commonly used ligand donor sets in coordination chemistry as
they offer a broad range of potential coordination modes to metal
2
7
5
5
6
0
5
0
2
3
3
4
4
5
0
5
0
5
1
ions from across the periodic table. While the monodentate
.
2
4
2
pyridyl group is limited in the way it might coordinate to a metal
ion, the amide and carboxylate functionalities offer a vast array of
coordination capacity ranging from monoꢀ and multiꢀ dentate
modes to bridging multiple metal ions in the formation of clusters
and polymers. Carboxylate ligands have been used as key
building blocks in supramolecular chemistry due to the many
8
1
2
st
2
potential binding modes available to the carboxylate group.
These various binding modes are observed often within metalloꢀ
clusters that may range in size from two to an impressive eightyꢀ
2
nuclear Co(II) and a hexanuclear Cu(II) species along with details
of their magnetic properties.
3
four metal ions. Many of these carboxylate containing clusterꢀ
4
complexes behave as singleꢀmolecule magnets (SMMs). In these
rapidly evolving areas of research, the search for new polydentate
ligands is an ongoing challenge. This has led increasingly to the
design of ligands containing multiple functional groups that are
5
capable of binding several metal ions. In this respect, the Nꢀ(2ꢀ
pyridyl)ꢀ3ꢀcarboxyꢀpropanamide ligand, H L, is an excellent
2
candidate for the synthesis of new polynuclear transition metal
complexes, Fig. 1.
1
Fig. 1 Structure of the ligand H
2
L with the labelling scheme used for H
Previous work with H L is limited and has been concerned
13
2
65
and C NMR spectroscopy shown.
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DOI: 10.1039/C3DT51301G
Experimental
suitable for a single crystal Xꢀray diffraction study were isolated
after 1 week following the slow evaporation of the reaction
mixture. A sample for analysis was dried in vacuo overnight to
Materials and methods
All reagents were used as received from BDH or Sigma Aldrich. 60 generate the partially desolvated phase of 1 as its monoꢀhydrate.
1
13
H and C NMR spectroscopy was carried on a Varian INOVA
Yield: 52 mg, 21%. Found: C, 32.20; H, 3.38; N, 8.08%;
C H Cl Fe N O requires: C, 32.07; H, 2.99; N 8.32%;
max/cm (KBr): 3322 br, 1677 s, 1601 s, 1540 s, 1478 s, 1319 s,
1219 s, 1095 s, 884 m, 788 s, 628 m.
1
5
0
5
0
5
500 spectrometer operating at 500 MHz for H and 125 MHz for
ν
5
4
60
−1
5
5
12 43
1
3
C. High resolution mass spectrometry was recorded on a Bruker
maXis 3G UHRꢀTOF mass spectrometer. Infrared spectra were
recorded on a Perkin Elmer Spectrum One FTIR spectrometer in
65
Synthesis and characterisation of the octanuclear complex:
−1
the range 400ꢀ4000 cm . Samples were analyzed using diffuse
reflectance in ground KBr. Elemental analyses (C, H, N) were
conducted at Campbell Microanalytical Laboratories, Dunedin.
The magnetic susceptibilities were measured using a Quantum
Design Squid magnetometer, MPMS 5, at a dc field of 1 T, with
the samples (ca. 20 mg) contained in gelatin capsules held at the
centre of a drinking straw that was fixed to the end of the sample
rod. Ligand diamagnetic corrections were obtained using Pascal’s
constants. Magnetization isotherms were measured using the
same instrument under dc fields of 0 to 5 T at temperatures 2, 3,
[
Co (HL) (O)(OH) (MeOH) (H O)][ClO ] .5MeOH.2H O, 2
8 8 4 3 2 4 3 2
1
1
2
2
To a MeOH solution (30 ml) of H L (0.13 g, 0.67 mmol) was
added Co(ClO ) .6H O (0.245 g, 0.67 mmol) followed by the
dropwise addition of a 10% Et N solution (MeOH, 1 ml). Pink
block crystals suitable for a single crystal Xꢀray diffraction study
were isolated after 1 week following the slow evaporation of the
reaction mixture. A sample for analysis was dried in vacuo
overnight to generate the desolvated phase of 2. Yield: 23 mg,
2
4 2
2
3
7
0
1
0%. Found: C, 34.50; H, 3.38; N, 9.18%; C H Cl Co N O
72 90 3 8 16 45
−1
75
requires: C, 34.89; H, 3.66; N 9.05%; ν max/cm (KBr): 3318 w
br, 1674 s, 1602 m, 1581 m, 1540 s, 1479 s, 1438 s, 1311 s, 1240
m, 1106 s, 783 m, 626 m, 512 m.
4
, 5.5, 10, 20 K with samples 1 and 2 dispersed in a Vaseline
mull to prevent torquing effects.
The structural analysis at 120 K was performed on an Agilent
dual wavelength SuperNova with monochromated CuꢀKa (λ =
Synthesis and characterisation of the hexanuclear complex:
[Cu (L ) (MeOH)(H O) ].MeOH, 3
1
.5418 Å) (1 and 3) or MoꢀKα (λ = 0.71073 Å) (2) radiation.
6
ox 4
2
3
9
CrysAlisPro was used for the data collection and data
processing. The structure was solved using direct methods with
SHELXS and refined on Olex2 using all data by full matrix
leastꢀsquares procedures with SHELXLꢀ97.
absorption correction using SCALE3 ABSPACK. CCDC
80
To a MeOH solution (30 ml) of H L (0.13 g, 0.67 mmol) was
2
added Cu(OAc)
dropwise addition of a 10% Et
3
green blockꢀlike crystals suitable for a single crystal Xꢀray
diffraction study were isolated after 2 weeks following the slow
evaporation of the reaction mixture. A sample for analysis was
dried in vacuo overnight to generate the desolvated phase of 3.
Yield: 16 mg, 11%. Found: C, 33.80; H, 2.97; N, 8.63%;
1
0
11
2
.H
2
O (0.133 g, 0.67 mmol) followed by the
1
2
N solution (MeOH, 1 ml). Large
Multiꢀscan
1
3
numbers: 939536 (1); 939537 (2); 939538 (3).
85
30
Synthesis of Nꢀ(2ꢀpyridyl)ꢀ3ꢀcarboxypropanamide, H L
2
This synthesis was adapted from the method of Godina and coꢀ
−1
14
C H Cu N O requires: C, 34.19; H, 2.71; N 8.87%; ν max/cm
36 34 6 8 19
workers, full details and spectroscopic data are reported here for
ꢀ
2
(KBr): 3340 w br, 2831 s, 1569 s, 1480 s, 1455 s, 1389 s, 1297 s,
90 1196 s, 1091 s, 1042 m, 969 s, 916 s, 886 s, 773 s, 746 m, 657 s,
81 m, 527 m.
the first time. Succinic anhydride (2.73 g, 2.73 × 10 mol) and 2ꢀ
ꢀ
2
aminopyridine (2.57 g, 2.73 × 10 mol) were dissolved in ethyl
acetate (150 ml) and heated at reflux temperature for 24 hours to
produce a white solid, which was isolated by filtration, washed
with hot ethyl acetate (50 ml) and air dried. Recrystallisation
from methanol by slow evaporation afforded colourless needle
crystals suitable for a single crystal Xꢀray diffraction study. Yield
5
35
40
45
50
Table 1. Xꢀray crystallography data for 1, 2 and 3.
1
2
3
Temperature/K
Chemical formula
120(1)
120(1)
120(1)
C
60
H77Cl
5
Fe
5
C
79.5
H113Cl Co
3 8
38 6
C H42Cu
1
4
2.08 g, 39%; mp. 180ꢀ182 °C (lit. 181ꢀ182 °C) ; HRMSꢀESI m/z
N
15
O
46
N
16
O
52
8 21
N O
Formula Mass
Crystal System
2200.82
2702.65
1328.04
(
[
MeOH): Found: 195.0766; C H N O requires: 195.0764
9 11 2 3
+
Monoclinic
16.2556(2)
22.8571(4)
25.5874(4)
90
Monoclinic
25.9059(4)
14.9866(2)
27.3422(4)
90
Monoclinic
41.7229(8)
13.7165(3)
15.3592(3)
90
M+H ]; δH (500 MHz, CD OD): 2.66 (2H, t, J = 6.1 Hz, 12.7
3
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å
Space group
Z
Hz, H ), 2.72 (2H, t, J = 6.1 Hz, 12.7 Hz, H ), 7.08 (1H, at, J =
8
7
6
.1 Hz, 1.0 Hz, H ), 7.74 (1H, at, J = 8.1 Hz, 1.0 Hz, H ), 8.06
2 3
(1H, d, J = 8.1 Hz, H ), 8.26 (IH, d, J 6.1 Hz, 1.0 Hz, H ); δ (125
4
1
C
102.0441(15)
90
9297.9(2)
91.2539(15)
90
10612.8(3)
C2/c
91.5503(18)
90
8786.7(3)
C2/c
MHz, CD OD): 28 (C ), 31 (C ), 114 (C ), 119 (C ), 138 (C ),
3
7
8
4
2
3
−1
3
1
3
1
48 (C ), 152 (C ), 172 (C ), 175 (C ); ν max/cm (KBr): 3320 br,
1 5 6 9
260 br, 3071 m, 2475 m, 1696 s br, 1590 m, 1438 m, 1357 m,
313 m, 1248 s, 1168 s, 1006 s, 963 m, 916 w, 845 m, 784 s, 745
1
P2 /c
4
4
8
λ/
ꢁ
Å
/mm
1.5418
8.284
0.7107
1.398
1.5418
4.009
m, 634 s.
ꢀ1
Nobs
unique
int
56674
20971
9391
17949
8599
0.0283
0.0338
0.0940
Synthesis and characterisation of the pentanuclear complex:
III
II
.
.
[
Fe3 Fe2 (HL) (O)(H O) ][ClO ] 3MeCN 4H O, 1
N
1
6567
6
2
3
4 5
2
R
0.0518
0.0998
0.3250
1.195
0.0220
0.0481
0.1029
1.070
a
To a MeCN solution (30 ml) of H L (0.13 g, 0.67 mmol) was
R
1
wR
2
a
added Fe(ClO ) .6H O (0.25 g, 0.67 mmol) followed by the
2
4
2
2
Goodness of fit
1.028
55
dropwise addition of a 10% Et N solution (MeCN, 1 ml). The
3
a
2
2 2
c
2 2 1/2
R = Σ||F | ꢀ |F ||/Σ|F | wR = [Σ[w(F – F ) ]/Σ[w(F ) ]]
1
o
c
o
2
o
o
mixture was stirred for 12 h at 40 °C. Orange plate crystals
2
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Results and Discussion
The ligand, H L, was synthesised through the condensation of
2
succinic anhydride with 2ꢀaminopyridine which, following
recrystallisation from MeOH, produced colourless crystals. The
ꢀ
1
5
IR spectrum of H L displays broad absorbance around 3300 cm
2
consistent with the presence of an amide (NH) and a stronger
ꢀ
1
absorbance at around 2475 cm , indicating the formation of a
hydrogen bond between pyridyl and carboxyl groups. The strong
ꢀ
1
broad peak at 1696 cm can be attributed to C=O of free
carboxylic groups and C=O of both amide and hydrogen bonded
carboxylic acid groups, respectively. These spectroscopic features
are consistent with the interactions observed within the solid state
structure as determined through Xꢀray crystallography (ESI).
Complexes 1ꢀ3 were obtained by adding M(II) salts to either
1
1
2
0
5
0
MeCN or MeOH solutions of H L in a 1:1 stoichiometry,
2
followed by the addition of Et N base, and allowing the resultant
3
solutions to evaporate slowly at ambient temperature. Although
isolated with modest yields, multiple crystalline batches were
cropped from the mother liquor on further standing. The crystals
of 1ꢀ3 were of sufficient quality to enable a single crystal Xꢀray
diffraction study to be carried out from which their structures
were determined unambiguously.
Xꢀray crystal structure of 1
The atomic numbering scheme and atom connectivity for 1 are
2
3
3
4
5
0
5
0
shown in Fig. 2. The structure was refined in the monoclinic
P2 /c space group and crystallographic data are detailed in Table
1
1
. Complex 1 is a pentanuclear mixed valence iron cluster and
displays unusual core geometry. Fe(II/III) oxidation states were
assigned on the basis of bond valence sum calculations (ESI),
considering the bond lengths and charge balance for the cluster.
III
At the centre of the complex resides the archetypal {Fe3 O}
triangular basic carboxylate cluster. The coordination sphere
around the Fe(III) ions is comprised of the six carboxylate groups
of the ligand tail, which bind in a typical 1,3ꢀbridging mode. The
distorted octahedral geometry about each Fe(III) is completed by
Fig. 3 Molecular structure and partial atom labelling scheme for 2 (top)
and the coordination core of 2 showing the potential pathways for
magnetic exchange between the Co(II) centres (bottom). Hydrogen atoms
and solvent molecules of crystallisation omitted for clarity.
5
0
a central ꢂ ꢀoxoꢀligand and a coordinated water molecule. This
3
The separation between the peripheral Fe(II) centres and the
central cluster Fe(III) atoms ranges from 5.932(1) to 6.348(1) Å;
intraꢀtriangular cluster separations range from 3.269(1) to
{
Fe O} unit is common in ironꢀoxo chemistry and has served as a
3
15
building block for many higher nuclearity clusters. At the
periphery of the complex, above and below the triangular cluster,
are two Fe(II) centres with pseudoꢀoctahedral geometry.
Coordination about these peripheral metal centres is furnished by
three pyridyl nitrogen atoms and three amido oxygen atoms from
3
.304(1) Å and the distance between peripheral Fe(II) centres is
5
6
6
5
0
5
11.535 Å through the central cluster. These Fe(II) ions are offset
from the centre of the triangular cluster toward the Fe2ꢀFe3 axis
i.e. they are not positioned directly above/below the central ꢂ3ꢀ
oxoꢀligand, O1A. Interestingly, the trisꢀchelated nature of the
HL ligands bestows handedness upon the terminal metal ions,
which is ꢂ and Λ about Fe4 and Fe5, respectively. The cluster is
therefore a clusterꢀmesocate; related to the cluster helicates which
differ from 1 in that they contain peripheral metal centres that
three chelating HL ligands to yield an N O donor set.
3
3
16
possess the same handedness. To the best of our knowledge this
is only the second example of a clusterꢀmesocate to be
17
structurally characterised.
Xꢀray crystal structure of 2
The atomic numbering scheme and atom connectivity for 2 are
shown in Fig. 3. The structure was refined in the monoclinic C2/c
space group and crystallographic data are detailed in Table 1.
Fig. 2 Molecular structure and atomic labelling scheme for 1. Hydrogen
4
5
atoms, perchlorate anions and solvent molecules of crystallisation omitted 70 Compound 2 consists of an octanuclear Co(II) cluster that is built
for clarity.
around a {Co OH} square assembly, with the four remaining
4
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DOI: 10.1039/C3DT51301G
Co(II) centres forming the ‘wingꢀtips’ of the complex. The
Co(II) centres in the {Co OH} unit (Co2, Co2', Co3, Co3')
4
display a distorted octahedral geometry composed of two ꢂ ꢀOH
3
(
O1H and O2H), one ꢂ ꢀOH (O1A) and three carboxylate O
4
5
0
5
0
5
0
atoms (O3, O5 and O11 for Co2; O2, O6 and O8 for Co3). The
octahedral geometry of the remaining Co(II) ions (Co1, Co1',
Co4, Co4') is also distorted. Co4 and Co4' are bound by one ꢂ3ꢀ
OH (O1H), two carboxylate O atoms (O9 and O6), one amido O
atom (O4), a pyridyl N atom (N3) and a coordinated MeOH
molecule (O14). Co1 and Co1' also possess a similar donor set:
1
1
2
2
3
one ꢂ ꢀOH (O2H), two carboxylate O atoms (O3 and O12), one
3
amido O atom (O1), a pyridyl N atom (N1); however, the
coordinated MeOH molecule (O19) is present with only 25%
occupancy with the remaining occupancy filled by a coordinated
water molecule.
The HL ligand is present in two distinct binding modes.
2
1
1
1
0
Firstly, in the ꢂ :η :η :η :η :η mode the ligand chelates to one
3
Co(II) centre at the wingꢀtip of the complex through the pyridyl
N and amido O atoms, while the carboxylate group bridges three
Co(II) ions: two from the central core and one from the wingꢀtip.
1
1
0
0
0
Secondly, the ꢂ :η :η :η :η :η bonding mode is unbound at the
2
pyridyl end and links a central Co(II) with a wingꢀtip Co(II)
through the 1,3ꢀbridging mode of the carboxylate group. The
unbound pyridyl ring is stabilised by both intraꢀ and interꢀ
molecular interactions. Hydrogen bond interactions between the
pyridyl nitrogen atom and a ꢂ ꢀOH ligand (N5 to O2H and N7 to
3
O1H) and from the adjacent amido NH to a carboxylate O atom
(N6 to O11 and N8 to O8) act to stabilise the flexible unbound
arm of the ligand. The pyridyl ring is stabilised further by offset
intermolecular faceꢀtoꢀface πꢁꢁꢁπ interaction (N5ꢀC23 to N3ꢀC14
=
3.475(13) Å, N5ꢀC23 to N3''ꢀC14'' = 3.664(14), N7ꢀC32 to N1ꢀ
C5 = 3.454(13) Å and N7ꢀC32 to N7''ꢀC32'' = 3.452(14) Å).
Xꢀray crystal structure of 3
The partial atomic numbering scheme and atom connectivity for
3 are shown in Fig. 4. The structure was solved and refined in the
monoclinic C2/c space group and the crystallographic data are
detailed in Table 1. The data identifies a hexanuclear Cu(II)
complex and reveals that an in situ oxidation of the ligand has
taken place via hydroxylation of the αꢀcarbon atom adjacent to
the amide carbonyl group, Fig. 4. Hydroxylation of this kind,
although not common, is not without precedent and has been
shown to proceed through metal directed oxidation involving
Fig. 4 Molecular structure and partial atomic numbering scheme for 3
35
40
45
50
55
(top); the coordination core of 3 showing the potential pathways for
6
0
magnetic exchange (middle) and the structure of the hydroxylated ligand
generated in situ (bottom). Hydrogen atoms and solvent of crystallisation
removed for clarity.
and O2 for Cu2; O6 and O7 for Cu5) from two different ligands
to form two N,Oꢀchelate rings around each Cu centre (four
ligands in total). Each of the pyridyl rings adjacent to the
coordinated amido groups are rotated by ca. 50° to the amidoꢀ
alkoxy chelate ring plane and coordinate to squareꢀpyramidal
Cu(II) ions (Cu1, Cu3, Cu4 and Cu6). The basal plane of the
square pyramidal Cuꢀcentres is comprised of a pyridyl nitrogen
atom, a bridging alkoxy oxygen atom and two bridging
carboxylate oxygen atoms with either a coordinated water
molecule (Cu1, Cu4 and Cu6) or methanol molecule (Cu3)
located at the apex of the pyramid. The bridging and linking
between each Cuꢀion in 3 is such that the square or basal planes
of the coordination spheres around adjacent Cuꢀcentres subtend
angles to each other between ca. 98ꢀ102° for the twelve angles
formed by the six intersecting planes around the barrel.
6
7
7
8
5
0
5
0
18
molecular oxygen. When the reaction was performed under
oxygen free conditions, and the solution left to stand under an
inert atmosphere in an ‘open’ vial for several weeks, crystals of 3
failed to form. This is consistent with molecular oxygen being
responsible for the hydroxylation, as no precaution was made to
exclude water, and is in agreement with the previously proposed
18
mechanism. The resultant ligand is in its triply deprotonated
state and the [Cu:L] ratio of 6:4 generates a neutral hexanuclear
cluster that is best described as having a distorted ‘barrelꢀlike’
topology.
Two square planar Cu(II) centres (Cu2 and Cu5) reside at the
ends of the barrel, while four squareꢀpyramidal Cu(II) ions (Cu1,
Cu3, Cu4 and Cu6) occupy its sides. Square planar Cu2 and Cu5
are each bound by two amido nitrogen atoms (N11 and N16 for
Cu2; N9 and N10 for Cu5) and two alkoxy oxygen atoms (O1
Each hexanuclear complex is linked to four neighbouring
clusters through eight hydrogen bond interactions that involve
each amido oxygen atom and the neighbouring coordinated
solvent molecules bound to the squareꢀpyramidal Cu(II) ions.
4
| Journal Name, [year], [vol], 00–00
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These connections create a 2D sheet in the crystallographic bc
plane. Weak offset faceꢀtoꢀface πꢁꢁꢁπ interactions between each
pyridyl ring and its adjacent symmetry related partner (ring
centroidꢀcentroid distances range from 3.416(1) ꢀ 3.763(1) Å)
link the cluster to a further three neighbouring clusters along the
crystallographic aꢀaxis. The shortest intermolecular metalꢀmetal
separation is between Cu5 and Cu5' at 3.683(1).
5
Magnetic properties
The bulk magnetic properties of 1ꢀ3 were probed via variable
1
1
2
0
5
0
temperature, DC and AC susceptibility measurements on
3
polycrystalline samples. For 1 the χ T value at 300 K is 10.1 cm
M
ꢀ
1
mol K, which is in good agreement with the expected value of
10.5 cm mol K for a basic carboxylate {Fe O} (S = 5/2
3
trimer) plus two nonꢀcoupled highꢀspin Fe(II) S = 2 centres, Fig.
3
ꢀ1
~
19
25
ꢀ1
Fig. 6 Fitting of the magnetic data for 3 with PHI, using J
1
= 15.1 cm ,
ꢀ1
5. As the temperature is lowered the χ T product decreases
J = 5.18 cm and g = 2.10 with proposed coupling scheme (inset).
2
M
3
ꢀ1
gradually to reach ~7 cm mol K at ca. 25 K, followed by a
3
ꢀ1
with a small nonꢀzero spin ground state for 1. The general shapes
are indicative of antiferromagnetically coupled Fe(III) sites
together with the two highꢀspin Fe(II) sites that have their
contributing M values reduced because of spinꢀorbit coupling and
orbital degeneracy effects. Antiferromagnetic coupling in Fe(III)ꢀ
more rapid decrease to 1.9 cm mol K at 2 K. This behaviour is
consistent with dominant antiferromagnetic interactions within
the cluster, while the sharp decrease at low temperature is due to
zeroꢀfield splitting (ZFS).
3
3
4
4
5
5
6
6
0
5
0
5
0
5
0
5
The magnetization (M) was measured as a function of the
applied field at various temperatures (ESI). It is clear the
magnetisation curves are far from saturated. Even at 2 K and 5 T,
the M vs. H curve is not saturated, but equal to 5.2 Nβ, consistent
20
oxo complexes is common,
however, larger complexes
containing {Fe O} building blocks have been shown to display
3
21
large nonꢀzero spin ground states.
3
ꢀ1
The χ T value of 2 at 300 K is 18 cm mol K, which is
M
consistent with eight uncoupled spinꢀonly S = 3/2 Co(II) centres
with g = 2.19. The χ T product decreases gradually as the
M
3
ꢀ1
temperature is lowered to reach 0.8 cm mol K at 2 K, Fig. 5.
This indicates antiferromagnetic exchange coupling between the
metal centres. Changing the field in the range 2ꢀ70 K gave the
same χ T value, thus magnetisable impurities and Zeeman
M
depopulation effects appear to be absent. The magnetisation
isotherms were measured using dc fields of 0 to 5 T and
temperatures of 2 to 20 K (ESI). The M values do not saturate and
the values at 2 and 3 K at 5 T are similar, 2 Nβ. These plots
suggest the spin ground state is not zero and the low M value at 2
K and 5 T confirms antiferromagnetic coupling probably also
4
influenced by spinꢀorbit coupling of the parent T singleꢀion
states. Measurements of outꢀofꢀphase ac susceptibilities, χ , for 2
1
g
''
M
in the range 2ꢀ10 K, gave zero values thus showing that no slow
relaxation of magnetisation occurred and no SMM behaviour was
observed, as expected for an antiferromagnetically coupled
system.
Modelling Co(II) systems in any situation is extremely
difficult due to the strong orbital moment and important crystal
field effects. This case with eight such octahedral Co(II) ions is at
present much too complicated to make any attempt at quantifying
the magnetic data. A previously reported Co8 complex has a
similar core to 2, however, it does not contain a bridging ꢃ ꢀOH
moiety and the Co(II) ions display trigonal bipyramidal geometry
compared to the pseudo octahedral geometry observed in
complex 2. Despite these differences both complexes display
similar magnetic behaviour, with antiferromagnetic interactions
dominating. Whilst Co(II)ꢀbased clusters often display antiferroꢀ
magnetic exchange interactions, there are several examples
where ferromagnetic interactions lead to the observation of SMM
4
2
2
23
24
behaviour.
2
5
Fig. 5 χ
M
T vs. T curves for 1 (top) and 2 (bottom).
3
ꢀ1
The χ T value of 3 at 300 K is 2.5 cm mol K, which is in
M
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Conclusions
This study has identified for the first time some of the potential
coordination capability possessed by the polytopic ligand Nꢀ(2ꢀ
pyridyl)ꢀ3ꢀcarboxyꢀpropanamide, H L. Reaction of H L with
3
4
4
5
5
6
5
0
5
0
5
0
2
2
either Fe(II), 1, Co(II), 2, or Cu(II), 3, led to the formation of
three new transition metal cluster complexes possessing different
and novel core geometries which were structurally and
magnetically characterised. Pentanuclear complex 1 and octaꢀ
nuclear 2 show dominant antiferromagnetic interaction that leads
to small nonꢀzero spin ground states, whereas hexanuclear 3
displays ferromagnetic interaction between the Cu(II) centres
yielding an S = 3 spin ground state. Interestingly, an in situ
oxidation of H L occurred during the synthesis of 3 via the
2
hydroxylation of the αꢀcarbon adjacent to the amide carbonyl
3
group to form Lox . H L and its oxidised derivative H L are
2
3
ox
extremely flexible in the nature of their binding to the metal
centres, which is highlighted by the presence of four distinct
binding modes across the three complexes, Fig. 7. In complexes 1
and 2, the ligand is monoꢀdeprotonated and binds via the pyridyl
group and the amidoꢀO atom with the carboxylate group in
various bridging modes. However, in complex 3 coordination
involves the pyridyl group, and now the amidoꢀN atom (which is
deprotonated) and an Oꢀatom from the hydroxyl group in
combination with a bridging carboxylate group.
The results presented here suggest that H L has great potential
2
Fig. 7 The different binding modes adopted by the ligands in 1 (top), 2
middle) and 3 (bottom).
in the synthesis of new polynuclear complexes containing
paramagnetic metal ions and we are currently extending this
family of complexes to include different metals from across the
transition and lanthanide metal ion series and will report upon
these examples in the future.
(
good agreement with the expected value for six uncoupled S =
/2 Cu(II) centres with g = 2.10, Fig. 6. As the temperature is
1
3
ꢀ1
5
0
5
0
5
0
decreased towards 50 K, χ T increases gradually to 3.3 cm mol
M
K. Then below 50 K there is a more rapid increase in χ T to a
M
3
ꢀ1
maximum of 6.1 cm mol K at 4 K, before finally decreasing to
Acknowledgements
3
ꢀ1
4
.9 cm mol K at 2 K. Such behaviour is indicative of
The authors thank the University of Canterbury (College of
Science Scholarship to CSH), the Royal Society of New Zealand
Marsden Fund (PEK), the MacDiarmid Institute for Advanced
Materials and Nanotechnology (PEK) and the Australian
Research Council (KSM) for financial support. We also thank Dr.
Anthea Lees (UC) for helpful discussions.
ferromagnetic exchange coupling with an S = 3 ground spin state.
The magnetisation isotherms confirm this behaviour with the 2 K
data saturating at M = 6 Nβ as expected for an isolated S = 3
ground state (ESI). Simultaneous fitting of the susceptibility and
6
5
1
1
2
2
3
25
magnetisation data was performed with the program PHI, using
the exchange spin Hamiltonian ꢀ2J (Ŝ .Ŝ + Ŝ .Ŝ + Ŝ .Ŝ + Ŝ .Ŝ )
1
1
2
2
3
4
5
5
6
ꢀ2J (Ŝ .Ŝ + Ŝ .Ŝ + Ŝ .Ŝ + Ŝ .Ŝ ). The J pathways involve
2
1
4
1
6
3
4
3
6
1
70
Notes and references
{
(OR)(1,3ꢀNCN)} bridging while the J pathways involve {antiꢀ
2
a
1
,3ꢀOCO} bridging. This two J model reproduced the data
MacDiarmid Institute for Advanced Materials and Nanotechnology,
Department of Chemistry, University of Canterbury, Private Bag 4800,
Christchurch, 8041, New Zealand; Eꢀmail:paul.kruger@canterbury.ac.nz
extremely well and led to a fit of the magnetic properties giving
ꢀ
1
ꢀ1
J = 15.1 cm , J = 5.18 cm and g = 2.10, Fig. 6. It is more
b
1
2
School of Chemistry, Monash University, Clayton, Victoria 3800,
common to observe antiferromagnetic exchange interactions in
7
8
8
9
5
0
5
0
Australia; Eꢀmail: keith.murray@monash.edu
† Electronic Supplementary Information (ESI) available: [ H NMR
spectrum and single crystal Xꢀray diffraction data for H L; hydrogen
2
bonding diagrams and bonding parameters for H L; magnetisation
2
6
1
polynuclear Cu(II) complexes. Complex 3 is one of the few
examples of Cu(II) cluster displaying ferromagnetic
a
2
7
2
interactions, leading to a spin ground state of S > 1. Possible
reasons for ferromagnetic coupling occurring include the CuꢀORꢀ
Cu bridging angles being low, in the range 102ꢀ104°, with CuꢀCu
separations of ~3.04 Å, and, or combined with, the equatorial
isotherms for 1ꢀ3],. See DOI: 10.1039/b000000x/
1
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DOI: 10.1039/C3DT51301G
Graphical Abstract
5
Synthesis, structural and magnetic characterisation of iron(II/III), cobalt(II) and copper(II)
cluster complexes of the polytopic ligand: Nꢀ(2ꢀpyridyl)ꢀ3ꢀcarboxypropanamide
Mark E. Russell, Chris S. Hawes, Alan Ferguson, Matthew I. J. Polson, Nicholas F. Chilton, Boujemaa Moubaraki, Keith S.
Murray and Paul E. Kruger*
The structural and magnetic characterisation of three transition metal cluster
complexes that feature the polytopic ligand Nꢀ(2ꢀpyridyl)ꢀ3ꢀcarboxypropanamide
are described: a mixed valence pentanuclear iron cluster, an octanuclear cobalt(II)
complex and a hexanuclear copper(II) cluster. Magnetic studies reveal dominant
antiferromagnetic interactions for the iron and cobalt complexes while the copper
species shows ferromagnetic coupling and an S = 3 spin ground state.
8
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]