Synthesis, structure and magnetic properties of phenylhydroxamate-based coordination clusters

Article abstract of DOI:10.1039/c4dt02405b

The strategic recombination of preformed coordination clusters in the presence of polymodal bridging ligands has successfully led to the characterisation of five new compounds of structural and magnetic interest. Indeed using the dinuclear complex [M₂(H₂O)(piv)₄(Hpiv)₄] (M = Co, Ni; Hpiv = pivalic acid) as starting material and reacting it with phenylhydroxamic acid (H₂pha) has yielded the four tetrametallic coordination clusters [Co₄(Hpha)₂(piv)₆(Hpiv)₄] (1), [Ni₄(Hpha)₂(piv)₆(Hpiv)₂(DMF)₂] (2), [Co₄(Hpha)₂(piv)₆(EtOH)₂(H₂O)₂] (3), [Ni₄(Hpha)₂(piv)₆(EtOH)₂(H₂O)₂] (4) and the hexanuclear complex [Co₆(Hpha)₄(piv)₈(EtOH)₂]·EtOH (5). All the compounds have been structurally characterised revealing a particular binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed and the modelling of these properties has been done using the appropriate hamiltonians for each compound. The experimental data and their modelling show non-zero spin ground states for compounds 4 and 5.

Full text of DOI:10.1039/c4dt02405b

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Synthesis, structure and magnetic properties of
phenylhydroxamate-based coordination clusters†‡
Cite this: Dalton Trans., 2014, 43,
16805
Rémi Tirfoin,a Lise-Marie Chamoreau,a,b,c Yanling Li,a,b Benoit Fleury,a
Laurent Lisnarda,b and Yves Journaux*a,b
The strategic recombination of preformed coordination clusters in the presence of polymodal bridging
ligands has successfully led to the characterisation of five new compounds of structural and magnetic
interest. Indeed using the dinuclear complex [M2(H2O)(piv)4(Hpiv)4] (M = Co, Ni; Hpiv = pivalic acid) as
starting material and reacting it with phenylhydroxamic acid (H2pha) has yielded the four tetrametallic
coordination clusters [Co4(Hpha)2(piv)6(Hpiv)4] (1), [Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2] (2), [Co4(Hpha)2(pi-
v)6(EtOH)2(H2O)2] (3), [Ni4(Hpha)2(piv)6(EtOH)2(H2O)2] (4) and the hexanuclear complex [Co6(Hpha)4(pi-
v)8(EtOH)2]·EtOH (5). All the compounds have been structurally characterised revealing a particular
binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed
and the modelling of these properties has been done using the appropriate hamiltonians for each com-
pound. The experimental data and their modelling show non-zero spin ground states for compounds 4
and 5.
Received 7th August 2014,
Accepted 25th September 2014
DOI: 10.1039/c4dt02405b
www.rsc.org/dalton
One strategy to achieve the synthesis of paramagnetic
coordination clusters lies in the use of specific ligands that
Introduction
Molecule-based magnetic materials have attracted consider- can display multiple binding modes through distinct coordi-
able interest for the past two decades.1 Indeed the existence of nation sites and thus promote magnetic exchange between
molecular compounds that can retain their magnetisation several metal ions. Indeed ligands containing O- and/or
below a blocking or a critical temperature or else switch their N-donors such as carboxylates, poly-ols, poly-azoles or phos-
magnetic state under external stimuli has motivated both che- phonates to name a few, have efficiently led to numerous para-
mists and physicists to further explore this area of research magnetic clusters6 including single-molecule magnets
and its potential applications in data storage, quantum com- (SMM).4a,c,d To this end, hydroxamic acids constitute excellent
puting or spintronics.2 One particular focus has been on dis- candidates for the preparation of coordination clusters as
crete polymetallic compounds – coordination clusters – where demonstrated by V. Pecoraro’s group work on metallacrowns
fundamental quantum magnetic phenomena can be which includes examples of SMMs.7,8 Another known strategy
observed.3 Convincing results have been obtained and ever- for the preparation of magnetic coordination compounds is
growing efforts are currently being made towards the compre- based on the use of pre-formed clusters. Used as reactants,
hension of such phenomena, the structuring of the molecular specific clusters can be assembled into higher nuclearity com-
systems and of course the synthesis of novel coordination plexes or into extended structures, they can be used to incor-
clusters.4,5
porate co-ligands or additional metal ions or they can simply
represent handy and/or reactive precursors in comparison to
commercial sources. There are now several examples of pre-
formed carboxylato clusters that have been used either as start-
ing materials or building blocks to generate macromolecular,
supramolecular or multidimensional assemblies.9 Similarly,
arrays of topologically SMM-like molecules have been obtained
from in situ preparation of the polymetallic building blocks
and further linking with organic or inorganic spacers.10
aSorbonne Universités, UPMC Univ Paris 06, UMR 8232, Institut Parisien de Chimie
Moléculaire, F-75005 Paris, France. E-mail: yves.journaux@upmc.fr;
Fax: +33 1 44 27 38 41; Tel: +33 1 44 27 55 62
bCNRS, UMR 8232, Institut Parisien de Chimie Moléculaire, F-75005 Paris, France
cCentre de résolution structurale, Institut Parisien de Chimie Moléculaire, Paris,
75005, France
†Dedicated to Pr Marius Andruh in celebration of his 60th birthday.
‡Electronic supplementary information (ESI) available: XRD powder data for 3,
additional structural data for 3, additional magnetic data (χM vs. T and 1/χM vs.
T for compounds 1 to 5) and informations on the modelilling; BVS calculations
for 1, 3 and 5. CCDC 915714–915717. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt02405b
Aiming at the synthesis of novel magnetic coordination
clusters we have combined the two strategies mentioned above
in studying the reactivity of phenylhydroxamic acid, H2pha,
with the preformed bimetallic complex, [M2(H2O)(piv)4(Hpiv)4]
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{M2piv} (Hpiv: trimethylacetic acid, M = Co,11 Ni12). Surpris- recrystallised from hot water. Yield: 29 g (78%). RMN 1H
1
ingly the H2pha ligand has scarcely been used in the synthesis (400 MHz, DMSO, 298 K, δ): 7.45 (2 H, t, J = 7.4 Hz, Ph); 7.51
1
1
of coordination clusters and only a limited number of clusters (1 H, t, J = 7.2 Hz, Ph); 7.76 (2 H, d, J = 7.76 Hz, Ph); 11.23
containing paramagnetic early transition metal ions have been (1 H, s, NH). RMN 13C (400 MHz, DMSO, 298 K, δ): 126.82;
reported.13 Both the cobalt(II) and nickel(II) pivalate dimers 128.33; 131.09; 132.73; 164.20. Elemental analysis (%) calcu-
have been successfully used as starting materials to build lated for C7H10NO3,5 (Mr = 164.16 g mol−1): C, 51.2; H, 6.1; N,
coordination clusters.9h,i,k,11,12,14–16 In particular, the cobalt(II) 8.5. Found: C, 51.1; H, 5.9; N, 8.5. ATR/FT-IR (cm−1): 3283s;
dimer alone is known to recombine in solution yielding homo- 3030w; 2693br; 1642m; 1622w; 1601s; 1554s; 1488m; 1452m;
and hetero-valent species with nuclearities ranging from three 1433m; 1326w; 1314s; 1302sh; 1185w; 1160m; 1077w; 1039w;
to fourteen cobalt ions.11 Its use in the presence of additional 1020w; 930w; 896m; 796m; 786m; 704w; 687s; 674m; 618w;
bridging ligands has also led to single-molecule magnets14e as 513s; 486s; 426s; 368s; 332m; 291sh; 282m.
well as remarkable nanosized clusters.14f Moreover the unique
[Co4(Hpha)2(piv)6(Hpiv)4] (1). H2pha·1.5H2O (0.041 g,
and appealing magnetic properties of the cobalt(II) ion have in 0.25 mmol) and {Co2piv} (0.237 g, 0.25 mmol) were dissolved
recent years motivated numerous research on the synthesis of in MeOH (20 mL) and the mixture was stirred overnight. The
cobalt-based coordination clusters.17,18 Besides, cobalt-based resulting purple solution was left to slowly evaporate at room
coordination clusters are also receiving considerable attention temperature. After eight weeks purple crystals of 1 were col-
in water oxidation catalysis.19
lected by filtration and dried in air. Yield: 0.073 g (38% based
Herein we report the synthesis, the structural characteris- on Co). Elemental analysis (%) calculated for C64H106Co4N2O24
ation and the magnetic properties of five coordination clusters (Mr = 1523.2 g mol−1): C, 50.4; H, 7.0; N, 1.8; Co, 15.5. Found:
we have obtained reacting H2pha with either {Co2piv} or C, 50.3; H, 7.0; N, 1.8; Co, 15.3. ATR/FT-IR (cm−1): 3372w; 2960
{Ni2piv}. Coordination of the H2pha ligand has led to a recom- (m); 2931w; 2872w; 2579br; 1686m; 1614s; 1604sh; 1572sh;
bination of the dinuclear building blocks which, upon varying 1519m; 1480s; 1460w; 1410s; 1357s; 1315m; 1223sh; 1199s;
solvent and base, has afforded the four tetrametallic clusters 1157w; 1078w; 1025w; 936sh; 914w; 897w; 872w; 796sh; 786m;
[Co4(Hpha)2(piv)6(Hpiv)4] (1), [Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2] 766w; 712m; 697w; 683m; 603m; 574w; 538m; 482m; 412m;
(2), [Co4(Hpha)2(piv)6(EtOH)2(H2O)2] (3), [Ni4(Hpha)2(piv)6- 393m; 371w; 302w.
(EtOH)2(H2O)2] (4) and the hexanuclear complex [Co6(Hpha)4-
(piv)8(EtOH)2]·EtOH (5).
[Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2]
(2). H2pha·1.5H2O
(0.041 g, 0.25 mmol) and {Ni2piv} (0.237 g, 0.25 mmol) were
dissolved in DMF (15 mL) followed by the addition of triethyl-
amine (0.036 mL, 0.25 mmol). The mixture was stirred over-
night and the resulting green solution left to slowly evaporate
at room temperature. After six weeks green crystals of 2
were collected by filtration and dried in air. Yield: 0.086 g
Experimental
Synthesis
All reagents were used as purchased with no further (47% based on Ni). Elemental analysis (%) calculated for
purification.
C60H100N4Ni4O22 (Mr = 1464.2 g mol−1): C, 49.2; H, 6.9; N, 3.8;
[Co2(H2O)(O2CCMe3)4(HO2CCMe3)4], {Co2piv}, was prepared Ni, 16.0. Found: C, 48.5; H, 6.8; N, 4.4; Ni, 16.2. ATR/FT-IR
according to the literature procedure.11 Elemental analysis (%) (cm−1): 3326w; 2956m; 2926w; 2906sh; 2869w; 1689sh; 1664m;
calculated for C40H78Co2O17 (Mr = 948.9 g mol−1): C 50.63, H 1615s; 1586w; 1572w; 1555sh; 1524m; 1481s; 1461w; 1448w;
8.29. Found: C 50.46, H 8.25.
1416s; 1371w; 1354w; 1324w; 1260w; 1228m; 1208m; 1158m;
[Ni2(H2O)(O2CCMe3)4(HO2CCMe3)4], {Ni2piv}, was prepared 1100m; 935w; 915w; 894w; 873w; 797w; 787m; 771w; 713m;
according to the literature procedure.12 Elemental analysis (%) 685s; 606m; 587w; 566w; 538w; 504w; 455w; 414m; 401w;
calculated for C40H78Ni2O17 (Mr = 948.4 g mol−1): C 50.65, H 371w; 318m; 275w; 229m.
8.29. Found: C 50.44, H 8.24.
[Co4(Hpha)2(piv)6(EtOH)2(H2O)2]
(3). H2pha·1.5H2O
Hydrate phenylhydroxamic acid, H2pha·1.5H20, was pre- (0.041 g, 0.25 mmol) and {Co2piv} (0.237 g, 0.25 mmol) were
pared by a modification of the literature procedure.20 Sodium dissolved in EtOH (20 mL) followed by the addition of
hydroxide (43.7 g, 1.093 mol) dissolved in 100 mL of water was tetrabutylammonium hydroxide (1 M in MeOH, 0.25 mL,
added to an aqueous solution (100 mL) of hydroxylamine 0.25 mmol). The mixture was stirred overnight and the result-
hydrochloride (38 g, 0.546 mol). The mixture was stirred for ing purple solution left to slowly evaporate at room tempera-
15 min and then added to an ethanolic solution (200 mL) of ture. After eight weeks pink crystals of 3 were collected by
ethyl benzoate (41 g, 0.273 mol). The resulting solution was filtration and dried in air. Yield: 0.071 g (43% based on Co).
stirred for 72 h at 40 °C under inert atmosphere and left to Elemental analysis (%) calculated for C48H82Co4N2O20 (Mr =
cool to room temperature. pH was adjusted to 2 with 37% HCl 1242.9 g mol−1): C, 46.4; H, 6.6; N, 2.2;. Found: C, 46.3; H, 6.7;
and a yellow precipitate was obtained after evaporation of the N, 2.3. ATR/FT-IR (cm−1): 3345,84(f); 2959m; 2928w; 2904w;
solvents under vacuum. The crude product was dissolved in 2868w; 1674w; 1599s; 1566sh; 1539w; 1521w; 1482s; 1457w;
250 mL of absolute ethanol and the solution filtered to remove 1417s; 1374sh; 1361m; 1324w; 1259w; 1226s; 1207sh; 1157w;
the NaCl precipitate that was formed. Further evaporation of 1088m; 1051m; 1026w; 910m; 893m; 805sh; 786m; 708w;
the solvent under vacuum has yielded a white powder that was 684w; 603m; 556w; 491m; 409m; 392m.
16806 | Dalton Trans., 2014, 43, 16805–16817
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[Ni4(Hpha)2(piv)6(EtOH)2(H2O)2] (4). H2pha·1.5H2O (0.041 g, Physical measurements
0.25 mmol) was dissolved in EtOH (10 mL) followed by the
addition of triethylamine (0.072 mL, 0.5 mmol) and of a solu-
tion of {Ni2piv} (0.237 g, 0.25 mmol) in dichloromethane
(5 mL). The mixture was stirred overnight and the resulting
green solution left to slowly evaporate at room temperature.
After ten days green crystals of 4 were collected by filtration
and dried in air. Yield: 0.101 g (62% based on Ni). Elemental
analysis (%) calculated for C48H82N2Ni4O20 (Mr = 1241.9 g
mol−1): C, 46.4; H, 6.6; N, 2.2; Ni, 18.9. Found: C, 46.3; H, 6.7;
N, 2.4; Ni, 18.8. ATR/FT-IR (cm−1): 3442w; 2956m; 2929w;
2902w; 2869w; 1603s; 1569w; 1539w; 1523w; 1482s; 1485w;
1417s; 1372w; 1362m; 1324w; 1225s; 1207sh; 1156m; 1090m;
1051m; 1026w; 916w; 893w; 801sh; 787m; 708w; 687w; 607m;
561w; 495m; 419m; 399w.
Crystallographic data were collected on a Bruker Kappa-APEX II
CCD diffractometer for 1, 2, 3 and 5 (MoKα, λ = 0.71069 Å). Crys-
tals were mounted on a Hamilton cryoloop using Paratone-N oil
and placed in the cold flow produced with an Oxford Cryo-
cooling device. Partial hemispheres of data – preselected with
the APEX 2 software21 – were collected using φ and ω scans (25
s per frame for 1–2 40 s per frame for 3 and 5). Integrated inten-
sities were obtained with SAINT and were corrected for absorp-
tion with SADABS.22 The structures were solved by direct
methods and completed by iterative cycles of ΔF syntheses and
full-matrix least-squares refinement against F2 using SHELXL
software.21 Crystallographic data and refinements parameters
for 1–3 and 5 are given in Table 1. Crystallographic details are
available in CIF format. CCDC numbers 915714–915717.
[Co6(Hpha)4(piv)8(EtOH)2]·EtOH (5). H2pha·1.5H2O (0.041 g,
0.25 mmol) was dissolved in EtOH (10 mL) followed by the
addition of triethylamine (0.072 mL, 0.5 mmol) and of a solu-
tion of {Co2piv} (0.237 g, 0.25 mmol) in dichloromethane
(5 mL). The mixture was stirred overnight and the resulting
purple solution left to slowly evaporate at room temperature.
After six days pink crystals of 5 were collected by filtration and
dried in air. Yield: 0.095 g (69% based on Co). Elemental ana-
lysis (%) calculated for C74H114Co6N4O27 (Mr = 1845.3 g mol−1):
C, 48.1; H, 6.2; N, 3.0; Co, 19.1. Found: C, 47.8; H, 6.3; N, 2.9;
Co, 18.9. ATR/FT-IR (cm−1): 3353w; 3261br; 2963m; 2929w;
2901w; 2871w; 1623sh; 1613s; 1573s; 1523w; 1481s; 1458w;
1418s; 1374m; 1359m; 1319w; 1226s; 1154s; 1091w; 1043w;
1025w; 912s; 888w; 787s; 707sh; 685s; 595w; 494w; 422m; 387w.
The X-ray powder diffraction diagram of 4 was collected on
a Philips X’pert Pro diffractometer using CuKα1 monochroma-
tised radiation (λ = 1.54060 Å) and equipped with a Pixcel
detector. The comparison of the experimental powder pattern
of 4 with the calculated one for 3 shows that the compounds
are isostructural (Fig. S1, ESI‡).
Magnetic measurements in dc mode were performed on a
Quantum Design MPMS SQUID on a crushed crystalline
sample restrained in a plastic film, drops of Paratone-N oil
were added to prevent torquing of the crystallites. Data were
corrected for the diamagnetism contributions of the samples
using Pascal constants. The sample holder and Paratone-N oil
diamagnetism were measured and subtracted from the raw
data. To model the magnetic properties we used a homemade
Table 1 Crystallographic data for compounds 1–3 and 5
1
2
3
5
Formulaa
C64H106Co4N2O24
1523.2
C60H100N4Ni4O22
1464.2
C48H82Co4N2O20
1242.9
C74H114Co6N4O27
1845.3
Monoclinic
C2/c
26.774(2)
13.963(1)
25.509(2)
90
102.481(5)
90
9311(1)
4
F.W. [g mol−1
]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1
P1
P1
12.1420(5)
13.3720(5)
14.1654(6)
62.550(2)
80.916(2)
71.263(2)
1932.70(14)
1
11.8753(4)
13.2048(4)
13.8854(5)
62.403(1)
82.791(1)
69.928(1)
1810.99(10)
1
10.9832(2)
12.7170(2)
13.1082(1)
114.643(1)
98.680(1)
106.364(1)
1519.21(4)
1
γ [°]
V [Å3]
Z
T [K]
200(2)
0.71073
1.309
200(2)
0.71073
1.343
200(2)
0.71073
1.358
200(2)
0.71073
1.349
λ [Å]
ρcalc [g cm−3
]
μ (MoKα) [mm−1
]
0.914
1.095
1.141
1.117
Measured reflections
Unique reflections
Rint
35 172
8772
0.0271
7532
33 162
10 630
0.0264
8011
23 018
8871
0.0188
7250
49 173
13 563
0.0401
9251
Reflections I > 2σ(I)
Parameters
540
219
0.0528
0.1549
1.042
420
0
0.0446
0.1185
1.023
391
155
0.0309
0.0795
1.015
590
102
0.0398
0.0971
1.001
Restraints
R1 b [I > 2σ(I)]
wR2 c [I > 2σ(I)]
GOF
Largest residuals [e Å3]
−0.652; 1.175
−0.835; 0.765
−0.405; 0.536
−0.383; 0.442
a Including solvate molecules. b R1 = ∑||Fo| − |Fc||/∑|Fo|. c wR2 = [∑(ω(Fo2 − Fc2)2)/∑(ω(Fo2)2]1/2
.
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Mathematical code which establishes the hamiltonian matrix
and calculates the partition function allowing the derivation of
the physical properties. The best parameters were found using
the Neldear–Mead algorithm.23
1H and 13C NMR spectra were collected on a 400 MHz
Bruker Avance spectrometer at 298 K.
ATR/FT-IR spectra were collected on a Bruker TENSOR 27
equipped with a simple reflexion ATR diamond plate of the
Harrick MPV2 series.
Results and discussion
Structures and synthesis
The tetranuclear compounds 1–4 [M4(Hpha)2(piv)6Lb2Lw2
(1: M = CoII, Lb = Lw = Hpiv; 2: M = NiII, Lb = DMF, Lw = Hpiv;
3: M = CoII, Lb = EtOH, Lw = H2O; 4: M = NiII, Lb = EtOH, Lw
]
=
H2O) are structurally closely related and all adopt a centrosym-
metric butterfly topology with two edge-sharing {MO6} octa-
hedra constituting the butterfly’s body positions and two
{MO6} octahedra defining the wingtip positions, each con-
nected to the body via a vertex (Fig. 1a).
In each cluster, two (Hpha)− ligands and six pivalate groups
are found bridging the metal ions. The two body sites (M(2)
and M(2)#) are bridged to one wingtip position (M(1)) by a μ3-
oxygen atom from the hydroxamate group and M(1) and M(2)
are further bridged by one or two pivalate ligands alternatively
(Fig. 2). The oxygen atom from the carbonyl group of the
phenylhydroxamate ligand binds M(1). In 1 the coordination
spheres of the cobalt ions are completed by pivalic acid both
on the body and wingtip positions (Lb = Lw) where in 2, DMF
and pivalic acid molecules are found on the body and wingtip
nickel ions, respectively.
In the isostructural compounds 3 and 4 ethanol acts as
terminal ligand for the metal ions located on the body posi-
tions and water is found on the wingtip metal centres. In com-
pounds 1 to 4 the metal ions are all six-coordinate with
distorted octahedral geometries. Charge balance consider-
ations, elemental analysis and BVS calculations support the
presence of cobalt(II) ions in 1 and 3. There are numerous but-
terfly-type clusters described in the literature although only
one example is known with hydroxamate as a constitutive
ligand.24 Yet the coordination mode of the ligand differs from
the mode observed here with a μ hydroxamate oxygen atom
instead of the μ3 binding mode seen in compounds 1 to 4.
Fig. 2 Structure representations of (a) 1, (b) 2 and (c) 3. H atoms have
been omitted for clarity and (Hpha)− ligands represented in black.
Regarding cobalt(II) and nickel(II) butterflies clusters, dis-
tinguishing {M4(μ3-O)2} from {M4(μ3-O)2(μ-O,N,X)4} (Fig. 1) one
can notice that the former structural type—to which 1 to 4
belong—is less common and almost systematically obtained in
the presence of bridging carboxylate ligands.25–28 There are no
clear signs of intermolecular interactions in the crystal
packing of 1 and 2 however the presence of water molecules as
Fig. 1 Polyhedra representation of the butterfly topologies: (a) {M4(μ3-O)2}
and (b) {M4(μ3-O)2(μ-O,N,X)4}.
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terminal ligands in 3 and 4 leads to intermolecular H-bonds wingtip-located metal ions Co(1) and Co(2). The connectivity
with pivalate and hydroxamate oxygen atoms from adjacent within the cluster is further supported by two kinds of brid-
clusters (O(–H)⋯O distances of 2.72 and 2.88 Å) resulting in ging pivalate groups. Six carboxylates adopt a μ:η1:η1 coordi-
the formation of chains in the solid (Fig. S2, ESI‡).
nation mode connecting Co(1) with Co(3), Co(2) with Co(3)
Compound 5 is a hexametallic cluster made of four brid- and Co(1) with Co(4) (and symmetry equivalents respectively).
ging hydroxamate ligands and eight bridging pivalate groups. The remaining two pivalate groups show a μ3:η2:η1 binding
The inorganic skeleton can be described as a butterfly with mode linking Co(3) with Co(1) and Co(4) (and symmetry
two additional cobalt centres extending the butterfly wings, equivalents resp.). Oxygen atoms from the carbonyl function
formally resulting in the oxo-cluster {Co6(μ3-O)4(μ-O)2} (Fig. 3).
of the phenylhydroxamate ligands complete the coordination
The core butterfly is defined by Co(2) and Co(4) on the body spheres of Co(1) and Co(3). The latter is also coordinated to a
positions, Co(1) and its symmetry equivalent being located on terminal ethanol molecule. All the cobalt ions in 5 are six-
the wingtips. The additional cobalt atoms (Co(3) and Co(3)#) coordinate with distorted octahedral geometries. Charge
are attached to two adjacent edges of the butterfly and situated balance considerations, elemental analysis and BVS calculations
on each side of its metallic plane (Fig. 3b). The four (Hpha)− support cobalt ions in the +II oxidation state. To the best of our
ligands have similar coordination modes all with μ3 hydroxa- knowledge the topology of 5 has never been encountered to
mate oxygen atoms but with two distinct environments. One date in early transition metal ions coordination clusters.
consist in bridging Co(1) with both Co(2) and Co(4) within the
In analogy to the observations made by Winpenny et al.11 it
internal butterfly and the second has a μ3-oxo group binding is likely that the tetrametallic cluster observed for 1 to 4 results
the external cobalt atoms Co(3) to the butterfly’s body- and from the condensation of two dimeric sub-unit that occurs
when substituting the bridging water molecule in the metal-
pivalate starting material by the hydroxamato group, hence
forming a μ3-oxo bridge. Similarly the substitution of the brid-
ging water molecules in the cobalt(II) dimer by the hydroxamate
ligands should be driving the condensation of the metal ions in
5, however arguing whether an intermediate cluster of lower
nuclearity is involved in a multi-step self-assembly mechanism
seems rather inappropriate with no further solution studies.
Structurally all compounds display a μ3-hydroxamate ligand
which is a striking feature since there are only a limited
number of hydroxamate-based coordination compounds dis-
playing such a bonding mode.29 This in turn would explain the
relatively high nuclearity of the clusters described here. Indeed
the phenylhydroxamate-based coordination clusters reported to
date are dinuclear or trinuclear species with μ hydroxamate
oxygen atoms only.13,30 There is only one recent example of
high nuclearity compound based on phenylhydroxamate dis-
playing a μ3 oxygen atom, a {Co16} cluster.13d This cluster was
prepared solvothermally. Compounds 1 to 5 represent then
some of the highest nuclearities obtained so far in the presence
of phenylhydroxamic acid. However, the incomplete deprotona-
tion of the ligand has most definitely restrained the nuclearity
of the formed species. The synthesis of 1 when compared to 2–5
tends to indicate that in the absence of base proton exchange
between the carboxylate groups and the hydroxamic acid is
sufficient to generate the μ3-hydroxamate bridge. A look at the
synthesis of 4 and 5 shows that the addition of two equivalents
of base per ligand does not lead to the formation of the hydroxi-
mate bridge but results, in the case of cobalt(II), in a slight
increase of the hydroxamate to metal ratio.
Magnetic properties
DC magnetic susceptibility measurements were performed for
1 to 5 at 1 kOe in the 2–300 K temperature range (Fig. 4). At
300 K, χMT values for 1 and 3 are equal to 13.1 cm3 K mol−1
Fig. 3 Structure representations of 5, (a) top view and (b) side view. H
and 12.7 cm3 K mol−1, values compatible with the expected
ones for four non-interacting cobalt(II) ions.32 Upon cooling,
atoms have been omitted for clarity and (Hpha)− ligands highlighted in
lime.
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the χMT products steadily decrease to reach values of 0.6 cm3 K
mol−1 and 0.44 cm3 K mol−1 at 2.5 K. At 300 K, 5 has a χMT
value of 20.6 cm3 K mol−1 which is in agreement with the
expected one for six non-interacting cobalt(II) ions.32
The product continuously decreases when lowering the
temperature, reaches a minimum of 10.4 cm3 K mol−1 at
ca. 16 K then increases to a local maximum of 12.25 cm3
K
mol−1 at 3 K and finally drops to 12.1 cm3 K mol−1 at 2 K.
Besides spin–orbit coupling, the rise of χMT at low temperature
for 5 tends to indicate the co-existence within the cluster of anti-
ferro- and ferromagnetic interactions. At room temperature the
χMT values for 2 and 4 are equal to 5.8 and 5.43 cm3 K mol−1
respectively and are relatively higher than the expected value for
four non-interacting nickel(II) ions (4.8 cm3 K mol−1, assuming
g = 2.2). The χMT product for 2 is almost constant from 300 to
22 K and then rapidly falls close to zero at 2 K. This behaviour
would be consistent with weak antiferromagnetic interactions
along with zero-field splitting. The χMT curve for 4 shows no sig-
nificant variation down to 100 K, slowly increases to 6.3 cm3 K
mol−1 at 6 K and then drops to 5.3 at 2 K, which indicates weak
ferromagnetic coupling as well as zero-field splitting.
The magnetisation curves measured at 2 K support the
observations made from the χMT vs. T plots. For 1, 2 and 3, the
M vs. H curves tend to confirm the existence of moderate to
weak antiferromagnetic exchange interactions and no signs of
saturation are observed at 7 T (Fig. 4d). The magnetisation
˜
curve of 4 saturates at 8.5Nβ which would confirm the exist-
ence of ferromagnetic interactions leading to an S = 4 ground
state. For 4 M vs. H at various temperature and isofield data
have also been measured between 0 and 7 T and between 1.8 K
and 8 K (Fig. 5). The separation of the isofield curves indicates
the presence of zero field splitting or (and) the existence of
populated low lying excited states. At 7 T, the magnetisation of
5 reaches 11Nβ but no saturation is observed which is coherent
with the presence of anisotropic cobalt(II) ions.
Since the pioneering work of Lines,31 modelling the mag-
netic properties of high-spin Co(II) polynuclear complexes is
still an open challenge due to the presence of unquenched
orbital angular momentum and several approaches have been
proposed.32 In the cobalt butterfly compounds 1 and 3 there
are two different pathways between the cobalt ions of the wing
and the two cobalt ions of the body. To limit overparametrisa-
tion only one wing–body interaction has been considered
according to Scheme 1(a).
In this context, the appropriate Hamiltonian to model the
magnetic properties of 1 and 3 is: Htot ¼ Hint þ Hso þ HZe with
Hint ¼ ꢀ JbbSCo2a ꢁ SCo2b
ꢀJwbðSCo1a ꢁ SCo2a þ SCo1a ꢁ SCo2b þ SCo1b ꢁ SCo2a þ SCo1b ꢁ SCo2b
Þ
4
X
3
2
ν
Hso
¼
ꢀ
ꢁ
αλLCoiSCoi
i¼1
ꢂ ꢀ
ꢀ
ꢁ
ꢃ
4
X
1
3
3α
2
2
2
HZe
¼
Δ LZ Coi
ꢀ
LCoi
þ
ꢀ
Lν Coi þ geSν Coi β ꢁ H
Fig. 4 (a–c) Plots of χMT vs. T measured from 300 to 2 K at 1 kOe for 1
i¼1
(
), 2 ( ), 3 ( ), 4 ( ) and 5 ( ), the solid black lines are the best-fit
with ν = x,y,z and where Jbb is the coupling constant between
the two cobalt ions in the butterfly body positions and Jwb the
curves; (d) plots of M vs. H measured from 0 to 7 T at 2 K for 1 ( ), 2 ( ),
3 ( ), 4 ( ) and 5 ( ), the solid lines are a guide for the eyes.
16810 | Dalton Trans., 2014, 43, 16805–16817
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coupling constant between the body and the wing cobalt ions.
For the Hso Hamiltonian, λ is the spin–orbit coupling constant,
α the orbital reduction factor and Δ the axial distortion para-
meter. L et S are respectively the orbital and spin operators with
L = 1 and S = 3/2 in the T–P isomorphism approach.33 To sim-
plify, all the cobalt ions are assumed to be identical and the
values of α, λ and Δ are averaged values. However, even within
this idealised model, the size of the matrix (20 736 × 20 736) is
too large to have the problem treated exactly in a reasonable
amount of time. To overcome this situation we used the pertu-
bational approach developed by Lloret et al.32b where the χMT
product is expressed as:
χMT ¼ Ftetra ½GiðT; Jij; λ; α; ΔÞ; T; Jijꢂ
where Ftetra is the thermal variation of χMT calculated for a tetra-
nuclear cluster with the same topology than 1 and 3 but with a
fictitious Seff = 1/2 to mimic the ground state Kramer doublet of a
Co(II) ion. To establish the Ftetra analytical law it is necessary to
take into account a 25/9 scaling factor in the Heisenberg Hamil-
tonian Hint (S = >5/3 Seff) leading to the following Hamiltonian:
Htot ¼ Hint þ HZe
25
9
25
eff
eff
Hint ¼ ꢀ
ꢀ
J
S
Co2a ꢁ S
bb
Co2b
ꢄ
eff
Co1a
eff
Co2a
eff
eff
Co2b
Jwb
S
ꢁ S
þ S
ꢁ S
ꢁ S
Co1a
9
ꢅ
eff
Co1b
eff
Co2a
eff
Co1b
eff
Co2b
þ S
ꢁ S
þ S
4
X
ꢄ
ꢅ
HZE
¼
GCoi T; Jij βSeCfofiH
Fig. 5 (a) M vs. H measured from 0 to 7 T and 1.8 K to 7.8 K for 4; (b)
Isofield M vs. H/T from 1 T to 7 T for 4, the solid lines are the best-fit
curves.
i¼1
In Ftetra the Landé factors of Seff are replaced by the GCoi(T,
Jij, λ, α, Δ) fictitious Landé factors introduced to take into
account the influence of the excited states due to spin–orbit
coupling and exchange interaction. These GCoi factors are cal-
culated according to
ꢄ
ꢅ
ꢄ
ꢅ
4k
Nβ2
2
2
GCoi T; Jij; λ; α; Δ ¼
ðχM ꢁ TÞCo þ f Jij; T
:
(χMT)Co is the thermal variation of χMTi for an isolated aniso-
tropic cobalt ion calculated by full diagonalisation of the
hamiltonian matrix and f(Jij, T) is a pertubation term due to
excited levels (see ESI‡).
It is extremely delicate to obtain reliable values for a
model depending on five parameters. To avoid purely math-
ematical fits, at least 20 sets of starting parameters were
used. All the optimisations lead to a dominant antiferro-
magnetic Jwb interaction around 5 cm−1 but to a relatively
wide range of Jbb values. As Jwb determines the magnetic be-
haviour of the compound and in fact creates an effective
ferromagnetic interaction between the two Co(II) ions in the
body positions, a reliable determination of the Jbb value by
magnetic measurements is difficult. Nevertheless, if for 1 the
Jbb value does not have a great influence on the quality of the
fit, there is an improvement by a factor 10 of the agreement
factor for 3 when a ferromagnetic Jbb is taken into account. It
is noteworthy that the need of a ferromagnetic Jbb interaction
Scheme 1 Topology of interaction in 1 and 3 (a) and in 5 (b).
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to model the magnetic properties of 3 is coherent with the variation of χMT is obtained by full diagonalisation of the
ferromagnetic interaction observed in the isostructural Ni(II) hamiltonian matrix established with the following hamil-
compound 4. The final fit for 1 is obtained setting Jbb to zero tonian. Htot ¼ Hint þ HZe
and the least square fit of the magnetic data gave Jwb
=
ꢄ
ꢄ
ꢄ
ꢄ
ꢅ
−4.8 cm−1, λ = −143. cm−1, α = 0.78, Δ = 429 cm−1, with R =
3.1 × 10−5. The least square fit for 3 gave Jwb = −3.7 cm−1, Jbb
= 6.2 cm−1, λ = −152. cm−1, α = 0.99, Δ = 466 cm−1, with R =
6.4 × 10−6 for 3. Almost equivalent fits are obtained with
negative Δ values. The significant antiferromagnetic Jwb inter-
action is not unexpected as the bridging angles Co(1)–O(1)–
Co(2) between the Co(II) ions of the body and the Co(II) ions
of the wings are in the 108°–120° range leading to an overlap
between the magnetic orbitals. On the other hands, the
angles Co(2)–O(1)–Co(2) between the body ions are equal to
99° and 97.4° for 1 and 3 respectively. These values are close
to the situation of accidental orthogonality of the magnetic orbi-
tals and should lead to weak ferromagnetic Jbb interactions or
25
Hint ¼ ꢀ J12
9
eff
eff
S
S
S
S
Seff þ S
Seff
Co1a Co2
Co1b′ Co2
ꢅ
25
eff
eff
ꢀ
ꢀ
ꢀ
J13
J14
J23
Seff þ S
Seff
Co1a Co3a
Co1b Co3b
9
ꢅ
25
eff
eff
Seff þ S
Seff
Co1a Co4
Co1b Co4
9
25
9
ꢅ
25
9
eff eff
Co2 Co3a
eff eff
Co2 Co3b
eff eff
S
þ S
S
ꢀ
J SCo2SCo4
24
6
X
ꢄ
ꢅ
eff
HZe
¼
GCoi T; Jij βS
H
Coi
i¼1
The mathematical form of the GCoi(T, Jij) factors for 5 are
to Jbb values close to zero since such angles also fall in the given in supplementary informations.
range where AF contribution can compensate the ferromagnetic
In a first attempt and to limit overparametrisation we fixed
one. The spin–orbit coupling constant λ is in a reasonable λ and α to sensible values (λ = −160 cm−1 and α = 0.85) and
range for both compounds but the orbital reduction parameter find the best values for the other parameters. After this first
α seems too high for 3. However it is impossible to obtain a step we blocked the Jij and Δ parameters to their best values
good fit with an α value smaller than 0.9.
The appropriate Hamiltonian to model the magnetic pro- repeated these two steps procedure several times until the
perties of 5 is (Scheme 1(b)): Htot ¼ Hint þ Hso þ HZe with process converges to stable values. The best fit curve shown in
and optimised the λ and α parameters. In an iterative way we
Fig. 4c is obtained for J12 = −1.3 cm−1, J13 = −0.28 cm−1, J14
=
,
Hint ¼ ꢀ J12ðSCo1a ꢁ SCo2 þ SCo1b ꢁ SCo2
Þ
5.9 cm−1, J23 = −9.8 cm−1, J24 = −0.19 cm−1 and Δ = 348 cm−1
λ = −159 cm−1 and α = 0.84.
ꢀ J13ðSCo1a ꢁ SCo3a þ SCo1b ꢁ SCo3b
Þ
ꢀ J14ðSCo1aCo4 þ SCo1b ꢁ SCo4
Þ
The magnetic behaviour of 5 is determined by two main
interactions: J14 and J23. The existence of one ferromagnetic
interaction, J14, was expected and is consistent with the rise of
χMT below 16 K. Magnetically, this polynuclear complex is
made up of two weakly coupled trinuclear parts. The first part
is comprised of the Co(2) ion and the two Co(3) ions. In this
trimetallic entity a large antiferromagnetic interaction is
expected between Co(2) and the Co(3) ions due to a large
Co(2)–O(1)–Co(3) angle of 120° which allows a very good
overlap between the magnetic orbitals. The second part is
composed by the Co(4) ion and the two Co(1) ions. The model-
ling gives a ferromagnetic interaction between Co(4) and the
Co(1) ions. This is not unexpected since the values of the brid-
ging angles between Co(1) and Co(4) are equal to 90.1° for
Co(1)–O(9)–Co(4) and 95.5° for Co(1)–O(5)–Co(4). These angles
are close 90° and lead to accidental orthogonality of the mag-
netic orbitals and consequently give ferromagnetic interaction
between the Co(II) ions. The weak values of J12 and J13 are more
difficult to explain, but for J12 the two bridging angles Co(1)–
O(1)–Co(2) and Co(1)–O(5)–Co(2) are equal to 99.3° and 103.3°
respectively and these values are probably not sufficiently large
to give an ample overlap between the magnetic orbitals. For J13
the bridging angle Co(1)–O(1)–Co(3) is equal to 109° and rela-
ꢀ J23ðSCo2Co3a þ SCo2Co3bÞ ꢀ J24SCo2Co4
6
X
3
2
ν
Hso
¼
ꢀ
αλLCoiSCoi
i¼1
ꢂ ꢀ
ꢁ
ꢀ
ꢁ
ꢃ
6
X
1
3
3α
2
2
HZe
¼
Δ LZ Coi
ꢀ
LCoi
þ
ꢀ
Lν Coi þ geSν Coi β ꢁ H
i¼1
where Jij are the coupling constants, the other symbols
have the same meaning than in the previous formula. As for 1
and 3, overparametrisation is minimised assuming that all
the cobalt ions are identical and thus that the values of α,
λ and Δ are averaged values. Nevertheless, due to the size of
the hamiltonian matrix (2 985 984 × 2 985 984) it is almost
impossible to fit the magnetic data using this model and as
for the butterfly compounds we used the pertubational
approach developed by Lloret et al.32b where the χMT product
is expressed has:
χMT ¼ Fhexa ½GiðT; Jij; λ; α; Δ; Þ; T; Jijꢂ
where Fhexa is the thermal variation of χMT for an hexanuclear tively large antiferromagnetic interaction is expected. However,
cluster with the same topology than 5 but with an effective these two ions are also bridged by two carboxylate groups and
spin Seff = 1/2 to mimic the ground state Kramer doublet of a it is well established that such bridges can drastically reduce
Co(II) ion. For 5, it is not possible to derive an analytical the antiferromagnetic interaction due to countercomplemen-
law for the S = 1/2 hexanuclear complex and the thermal tarity effects.34
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With the lack of first order spin–orbit coupling the two integrals leading to a very weak or null value for Jbb. The null
Ni(II) complexes 2 and 4 are easier to model. The Hamiltonian value of the wing–body interactions is more puzzling but this
for 2 is the following:
interaction is already quite small in 2 and is also probably due
to the non-planar conformation of the butterfly structure and
also to the possible existence of countercomplementarity
effects due to the presence of carboxylate bridges.34
H ¼ ꢀ JbbSNi2a ꢁ SNi2b ꢀ Jwb1ðSNi2a ꢁ SNi1a þ SNi2b ꢁ SNi1b
Þ
ꢀ Jwb2ðSNi2a ꢁ SNi1b þ SNi2b ꢁ SNi1a
Þ
þ gNiβHðSNi2a þ SNi2b þ SNi1a þ SNi1b
Þ
where Jbb is the body–body interaction and Jwb1 and Jwb2 are
the two different wing–body interactions. To simplify we have
in this Hamiltonian taken the same isotropic gNi factor in the
Zeeman term for all the Ni(II) ions. To avoid overparametrisa-
tion we have set Jwb1 = Jwb2 = Jwb and the least square fit of the
Conclusions
This preliminary study shows great potential in the strategic
use of pre-formed coordination clusters to react with hydroxa-
mate ligands since it has led to five new coordination clusters.
The use of ortho-substituted phenylhydroxamic acid to favour
the formation of hydroximate bridges or else the use of polyhy-
droxamic acid should lead to higher nuclearity clusters. It
would also be relevant to study the reactivity of the phenyhy-
droxamic acid towards coordination clusters of different shape
and/or nuclearity. These new synthetic routes are currently
explored in our group. The magnetic properties of the clusters
described in this paper are less exciting than expected. Indeed
none of the compounds show slow relaxation of the magneti-
sation at low temperature. Yet compounds 4 and 5 present
magnetic ground states with some ferromagnetic interactions
hinting at the possible preparation of SMMs following the syn-
thetic strategy described here. We also would like to emphasize
that it is possible to obtain a satisfactory model of the mag-
netic properties of coordination clusters containing metal ions
with unquenched orbital angular momentum in spite of the
overparametrisation risk. However obtaining reliable results
requires caution. It can be fulfilled with a robust optimisation
method, multiple tries using several sets of starting para-
meters and common sense, of course.
magnetic data gave Jbb = 0.96 cm−1, Jwb = −1.3 cm−1 and gNi
2.38 with R = 1.6 × 10−4. An almost equivalent fit is obtained
setting Jbb to zero (Jwb = −1.28 cm−1, gNi = 2.38 R = 2.1 × 10−4
=
)
showing that as for compounds 1 and 3 the Jwb governs the
magnetic behaviour of 2 and that Jbb is weak or null. It is
worthy of note that, as in the cobalt butterfly complexes, in
spite of the large Ni(1)–O(1)–Ni(2) angles (111° and 121°) the
antiferromagnetic Jwb value is weak. This is most probably due
to the non planar conformation of the butterfly structure and
also to the presence of carboxylate bridges which could bring a
countercomplementarity effect.34
The experimental evidence of a magnetic ground state and
the strong likelihood of anisotropy in 4 lead us to add a
ZFS term in the Hamiltonian to model the magnetic data.
Indeed attempts to fit the magnetisation data by taking
into account only exchange interactions without ZFS par-
ameter failed. The anisotropy and Landé factors of all the
nickel ions are assumed to be identical and the DNi and gNi
values are averaged values. To limit overparametrisation we
also set Jwb1 = Jwb2 = Jwb and take isotropic gNi Landé factor. In
this approximation, the appropriate Hamiltonian for com-
pound 4 is:
Acknowledgements
H ¼ ꢀ Jbb SNi2a ꢁ SNi2b
ꢀ JwbðSNi2a ꢁSNi1a þ SNi2b ꢁ SNi1b þ SNi2a ꢁ SNi1b þ SNi2b ꢁ SNi1a
Þ
This work was supported by the Ministère de la Recherche et
de l’Enseignement Supérieur (MRES) and the Centre National
de la Recherche Scientifique (CNRS).
4
X
þ
DNiðS2Nið jÞz ꢀ 2=3Þ
j¼1
þ gNiβHðSNi2a þ SNi2b þ SNi1a þ SNi1b
Þ
Notes and references
Since only low temperature data give information on the
ZFS in the χMT plot we simultaneously fitted the χMT and iso-
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