Mixed-valent polynuclear cobalt complexes incorporating tetradentate phenoxyamine ligands

Article abstract of DOI:10.1071/CH09256

The new potentially tetradentate ligand precursor 2-[(bis(2-hydroxyethyl) amino)methyl]-4,6-bis-tert-butylphenol (H₃L) has been synthesized and characterized. The reaction of H₃L with cobalt(ii) acetate has afforded the novel mixed-v

Full text of DOI:10.1071/CH09256

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2009, 62, 1124–1129
www.publish.csiro.au/journals/ajc
Mixed-Valent Polynuclear Cobalt Complexes Incorporating
Tetradentate Phenoxyamine Ligands
Joo Chuan Ang,A Yanyan Mulyana,A Chris Ritchie,A Rodolphe Clérac,B,C
and Colette BoskovicA,D
ASchool of Chemistry, University of Melbourne, Vic. 3010, Australia.
BCentre National De La Recherche Scientifique (CNRS), UPR 8641, Centre de Recherche Paul Pascal
(CRPP), Equipe ‘Matériaux Moléculaires Magnétiques’, 115 avenue du Dr Albert Schweitzer,
Pessac, F-33600, France.
CUniversité de Bordeaux, UPR 8641, Pessac, F-33600, France.
DCorresponding author. Email: c.boskovic@unimelb.edu.au
The new potentially tetradentate ligand precursor 2-[(bis(2-hydroxyethyl)amino)methyl]-4,6-bis-tert-butylphenol (H3L)
has been synthesized and characterized. The reaction of H3L with cobalt(ii) acetate has afforded the novel mixed-valent
tetra- and pentanuclear cobalt complexes [CoI2ICo2III(OAc)2(L)2(HL)] (1) and [CoIICoI4II(OAc)2(L)4] (2). Single-crystal
X-ray diffraction studies of these complexes indicate different coordination geometries for the divalent cobalt centres in
each complex, with distorted trigonal bipyramidal and distorted tetrahedral coordination evident in 1 and 2, respectively.
The variable temperature magnetic susceptibility data for complex 1 reveal behaviour dominated by antiferromagnetic
coupling between the two cobalt(ii) centres. Their approximate trigonal bipyramidal coordination geometries give rise to
a 4Aꢀ2 ground term, allowing a spin-only treatment assuming local spin quantum numbers of Si = 3/2. Fitting the data to
the Heisenberg exchange Hamiltonian (Hex = −2JS1·S2) results in J = −6.9(1) cm−1 and g = 2.30(5).
ˆ
Manuscript received: 29 April 2009.
Manuscript accepted: 2 June 2009.
OH
N
D
N
Introduction
The synthesis of new polynuclear complexes of paramagnetic
metal centres is driven in part by their potential to display
novel magnetic properties that may ultimately give rise to new
molecularmagneticmaterials.[1–3] Thedevelopmentofsuchspin
clusters relies on the exploration of new families of organic
ligands that can bridge the metal centres, or act as capping
ligands that prevent agglomeration beyond a certain size by
encapsulating the metal-containing core. An ongoing focus of
our research is the exploration of organic ligands that can simul-
taneously chelate and bridge metal centres for the preparation
of new spin clusters. In previous work, we have investigated the
use of phenoxide-containing Schiff base ligands for the syn-
thesis of new polynuclear Mn, Fe, Co, and Ni complexes.[4]
We have separately explored the use of a tripodal bridging and
chelating ligands with an (N,O,O)-donor set for the formation
of polynuclear Fe, Co, Ni, and mixed metal Cu–Ln (Ln = Gd,
Tb, and Dy) complexes.[5] The obvious extension of this was
to combine the two functionalities in the one ligand, exploit-
ing their differing coordination tendencies to obtain polynuclear
complexes with novel structural features and resulting prop-
erties. Herein we report the synthesis and characterization of
the new proligand 2-[(bis(2-hydroxyethyl)amino)methyl]-4,6-
bis-tert-butylphenol (H3L, Scheme 1), which contains both
phenol and tripodal amine-alcohol functionalities, and its use
in the synthesis of two novel mixed-valent polynuclear cobalt
complexes.
HO
HO
OH
D
tBu
tBu
NO2
H3L (D ϭ OH)
H3L4
HL1 (D ϭ OMe)
HL2 (D ϭ NEt2)
HL3 (D ϭ CH2NMe2)
Scheme 1. Proligands.
Although the proligand H3L has not been reported previ-
ously, ligands similar to H3L but bearing amine or ether donors
on the pendant arms of the ligand (Scheme 1: HL1, HL2, and
HL3), in place of alcohol groups, have been synthesized, together
with their mononuclear Mg, Ca, Al, Zn, Cr(iii), Co(ii), and Zr
complexes.[6,7] A dinuclear dichloride-bridged Fe(ii) complex
of the ligand (L1)− has been structurally characterized, as has a
dinuclear K complex with the ligand (L2)−. In the latter com-
plex, the phenoxide oxygen atoms of the ligand serve to bridge
the K centres. Tertiary amine and ether donors are unlikely to be
able to bridge metal centres; thus their replacement with alco-
hol groups in the present work was pursued in order to obtain
© CSIRO 2009
10.1071/CH09256
0004-9425/09/091124

RESEARCH FRONT
Mixed-Valent Polynuclear Cobalt Complexes
1125
OH
N
Table 1. Selected interatomic distances [Å] and angles [◦] for 1·2MeCN
and 2
HO
HO
OH
Parameter
1·2MeCN
3.427(1)
3.043(1)–3.075(1)
2.855(1)
1.948(3)–2.074(3)
2.204(4)
1.874(3)–1.969(3)
1.935(4)–1.939(4)
122.8(2)
90.4(1)–105.0(1)
94.6(1)–96.3(1)
112.1(1)–121.4(1)
167.4(1)
2
OH
tBu
tBu
CH2O
CoII· · ·CoII
CoII· · ·CoIII
CoIII· · ·CoIII
CoII–O
HN
ϩ
3.188(1)
2.825(1)
1.988(8)–2.022(8)
MeOH
tBu
tBu
OH
CoII–N
CoIII–O
1.796(9)–2.054(7)
1.924(4)
Scheme 2. Synthesis of proligand H3L.
CoIII–N
CoII–O–CoII
CoII–O–CoIII
CoIII–O–CoIII
O–CoII–OA
O–CoII–OB
O–CoII–NB
O–CoII–OC
O–CoIII–OD
O–CoIII–OE
O–CoIII–NCD
O–CoIII–NE
polynuclear complexes. The promise in this regard of the pro-
ligand H3L was also inferred from the previously reported use
of the related polyalcohol-containing proligand H3L4 (Scheme
1) for the synthesis of di- and heptanuclear Mn complexes.[8,9]
107.7(3)–112.3(3)
92.6(3)–97.7(3)
173.7(1)
Results and Discussion
90.8(3)–119.8(3)
74.0(3)–98.0(3)
169.7(2)–175.8(2)
81.8(2)–96.8(2)
163.4(3)–168.5(2)
81.7(1)–94.5(1)
175.2(1)–178.5(1)
85.9(1)–95.1(1)
165.0(2)–166.9(2)
Synthesis and Characterization
The ligand precursor H3L was prepared by a Mannich con-
densation between 2,3-di-tert-butylphenol, diethanolamine, and
paraformaldehyde in methanol (Scheme 2). The product was
obtained as a white crystalline material in reasonable yield, the
purity of which was established by elemental analysis and 1H and
13C NMR spectroscopy. A similar preparation has been reported
previously for the related ligand precursors HL1, HL2, and HL3
(Scheme 1).[6,7]
ATrigonal bipyramidal, equatorial.
BTrigonal bipyramidal, axial.
CTetrahedral.
DOctahedral, cis.
EOctahedral, trans.
The overnight treatment of a solution of cobalt(ii) acetate in
acetonitrile with one equivalent of the proligand H3L afforded a
solution from which crystals of [Co4(OAc)2(L)2(HL)]·2MeCN
(1·2MeCN) were obtained on standing. A similar reaction
involving pretreatment of the proligand with two equiva-
lents of triethylamine and recrystallization of the product
from dichloromethane/hexane afforded a crystalline sample of
[Co5(OAc)2(L)4] (2). Both reactions involved partial aerial oxi-
dation of the cobalt starting material and can be summarized as
follows:
N3
Co4
O8
O9
O13
O7
Co3
O1
O2
N1
O4
4 Co(OAc)2 + 3 H3L → [CoI2ICo2III(OAc)2(L)2(HL)]
+ 6 AcOH + 2 H+ + 2 e− (1)
O12
Co1 O6
Co2
O5
O3
5 Co(OAc)2 + 4 H3L → [CoIICo4III(OAc)2(L)4]
N2
O11
+ 8 AcOH + 4 H+ + 4 e− (2)
O10
Both reactions appear to be driven by protonation of the
acetate ligands of the starting material and their replacement
by the incoming chelating ligands. This process is accompanied
by structural rearrangement and agglomeration, which is com-
monly observed in the synthesis from acetate-containing starting
materials of polynuclear complexes bearing ligands that simulta-
neously chelate and bridge the metal centres.[1,5] It is noteworthy
that addition of base to the reaction mixture afforded complex 2,
with complete deprotonation of the proligand, as opposed to the
partial deprotonation observed in 1, which was synthesized with-
out added base.Although both complexes could only be obtained
in modest yield, the syntheses are reproducible. The low yield is
largely due to the very high solubility of both complexes in most
organic solvents, which in turn results from the presence of the
solubilizing tert-butyl substituents on the aromatic ring.
Fig. 1. Structural representation of complex 1 in 1·2MeCN. Colour code:
CoII (cyan), CoIII (green), O (red), N (blue).
molecules in the asymmetric unit. A summary of the impor-
tant interatomic distances and angles is provided in Table 1.
The asymmetric neutral tetranuclear complex 1 (Fig. 1) has a
{Co2IICo2III(µ2-O)4(µ3-O)} core where the five oxygen atoms
are provided by the ethoxo arms of the single HL2− and two
L3− ligands. Bond valence sum calculations indicate that cobalt
centres Co1 and Co2 are trivalent, whereas Co3 and Co4 are
divalent.[10] The trivalent centres both display distorted octahe-
dral coordination geometries. However, the divalent centres are
both five-coordinate, with distorted trigonal bipyramidal coor-
dination geometries, where the axial coordination of the trigonal
bipyramids is defined by O5 and O7 for Co3 and N3 and O13
for Co4. A distortion parameter, τ, can be calculated for five-
coordinate complexes, such that the parameter τ is 100% and
Structure Descriptions
Compound 1·2MeCN crystallizes in the monoclinic space
group P21/c with one tetranuclear complex and two acetonitrile

RESEARCH FRONT
1126
J. C. Ang et al.
(a)
(b)
O4
Co2
O3
Co1
O1
N1
O2
Fig. 2. Binding modes of (a) L3−, and (b) HL2− and in complexes 1 and
2. Colour code: CoII (cyan), CoIII (green), O (red), N (blue).
Fig. 3. Structural representation of complex 2. Colour code: CoII (cyan),
CoIII (green), O (red), N (blue).
0% for trigonal bipyramidal and square pyramidal coordina-
tion geometries, respectively.[11] This parameter is calculated
as ∼86% and ∼91% for Co3 and Co4, respectively, indicating
slightly distorted trigonal bipyramidal coordination in each case.
In addition to the bridging oxygen atoms, the remaining coor-
dination sites on the cobalt centres are completed by the other
N and O donor atoms on the L3− and HL2− ligands and by the
two acetate ligands, each binding in a µ2-bridging mode. The
two L3− ligands each bridge three cobalt centres, whereas the
HL2− ligand bridges two metal centres (Fig. 2). Bond valence
sum calculations indicate that the terminally bound oxygen atom
O8 of the HL2− ligand is protonated,[12] although the hydrogen
atom could not be detected crystallographically. Intermolecu-
lar O8–H· · ·O9 hydrogen bonds are observed between pairs
of complexes, with an O· · ·O separation of 2.568(5) Å. Each
pair of hydrogen-bonded complexes is held together by two
intermolecular hydrogen bonds, giving rise to an intermolecular
Co4· · ·Co4ꢀ separation of 4.488(1) Å.
the metal–oxygen core are very similar. The µ3- and µ2-binding
modes observed for L3− and HL2− in complexes 1 and 2, respec-
tively (Fig. 2), differ from the binding modes observed for the
related ligands (H2L4)− and (L4)3− (Scheme 1) in polynuclear
manganese complexes. A dinuclear manganese complex incor-
porates(H2L4)− bindingina µ2-mode, withthephenoxooxygen
atom the only bridging donor atom,[8] while a heptanuclear man-
ganese complex features (L4)3− binding in a µ4-mode, with the
ethoxooxygenatomsbridgingtwoorthreemanganesecentres.[9]
Magnetic Measurements
The magnetic susceptibility data for compound 1·2H2O were
measured in the temperature range 1.8–300 K with an applied
magnetic field of 0.1T and are plotted in Fig. 4 as χMT
versus T. As the temperature is decreased, the χMT product
decreases gradually (above 100 K) and then more rapidly (below
100 K) from a room temperature value of 4.4 to a value of
0.17 cm3 mol−1 K at 2.0 K. The coordination evident in 1 is con-
sistent with two low-spin distorted octahedral cobalt(iii) centres
and two high-spin distorted trigonal bipyramidal cobalt(ii) cen-
tres. As the low-spin cobalt(iii) centres are diamagnetic, from a
magnetic perspective, the complex can be treated as a dinuclear
cobalt(ii) system. The room-temperature value of χMT is higher
than the spin-only value of 3.8 cm3 mol−1 K calculated for two
non-interacting cobalt(ii) centres with S = 3/2 and g = 2.0, but is
consistent with the values typically observed for complexes with
high-spin trigonal bipyramidal cobalt(ii) centres, for which the
g parameter is generally found to be in the range 2.2–2.4.[14–17]
In the weak field of D3h symmetry associated with trigonal
bipyramidal coordination, the lowest-energy 4F term of the free
cobalt(ii) ion splits into five terms, with a 4A2ꢀ ground term.[18]
As this ground term does not have any orbital angular momen-
tum, a spin-only treatment is possible. Thus, the susceptibility
data in the range 1.8–300 K were fitted to the simple isotropic
Heisenberg exchange Hamiltonian:
Compound 2 crystallizes in the tetragonal space group I41/a
with no solvent molecules of crystallization. A summary of the
importantinteratomicdistancesandanglesisprovidedinTable1.
The neutral pentanuclear complex 2 (Fig. 3) has S4 point sym-
metry. Complex 2 has a {CoIICo4III(µ2-O)8}2− core where the
eight oxygen atoms are provided by the ethoxo arms of the four
L3− ligands. Bond valence sum calculations indicate that the
four cobalt centres Co1 are trivalent, whereas the central cobalt
atom Co2 is divalent.[10] The trivalent centres display distorted
octahedral coordination geometries, whereas the divalent cen-
tre is four-coordinate, with a distorted tetrahedral coordination
geometry. The remaining coordination sites on the cobalt cen-
tres are completed by the other N and O donor atoms on the
L3− ligands and by the two acetate ligands, each bridging two
cobalt(iii) centres in a µ2-manner. The four L3− ligands each
bridge the single cobalt(ii) centre and two cobalt(iii) centres in
a manner similar to that observed in 1 (Fig. 2a).
A search of the Cambridge Crystallographic Database yields
noothertetranuclearcobaltcomplexesdisplayingasimilarstruc-
tural core to 1 and only one other pentanuclear cobalt complex
with a core resembling that of 2.[13] The cobalt valencies in
this pentanuclear complex are the same as those observed in
2, while the interatomic distances and angles associated with
ˆ
Hex = −2J(SCo3·SCo4),
(3)
where J is the coupling constant between the two cobalt(ii) cen-
tres Co3 and Co4 and Si are the spin operators for each of the

RESEARCH FRONT
Mixed-Valent Polynuclear Cobalt Complexes
1127
5
4
3
2
1
0
deprotonated forms, respectively. The coordinative flexibility of
this ligand suggests that it and its derivatives have considerable
promiseforthesynthesisofnewspinclusters.Thedivalentcobalt
centres in the reported complexes manifest relatively uncommon
trigonal bipyramidal and tetrahedral coordination geometries.
An investigation of the magnetic properties of the tetradentate
complex reveals a moderate antiferromagnetic coupling between
the divalent cobalt centres.
Experimental
Materials
All manipulations were performed under aerobic conditions,
using materials as received.
0
50
100
150
200
250
300
Measurements
T [K]
Infrared spectra (KBr disk) were recorded on a BioRad 175
Fourier-transform (FT)-IR spectrometer. 1H and 13C NMR spec-
tra were recorded on a Varian Unity+ 400 MHz spectrometer.
Electrospray ionization (ESI) mass spectra were performed on
a Micromass Quattro II mass spectrometer. Elemental analy-
ses were performed by Chemical and Microanalytical Services,
Belmont, Australia, and Campbell Microanalytical Laboratory,
Dunedin, New Zealand. Thermogravimetric analyses (TGA)
were performed on a Mettler Toledo thermal analyzer.
Fig. 4. Plot of χMT versus T for compound 1·2H2O at 0.1 T; the solid line
is the fitted to the data as described in the text.
centres. An excellent fit (solid line in Fig. 4) was obtained with
J = −6.9(1) cm−1, g = 2.30(5), and ρ = 0.087 (ρ corresponds
to the residual paramagnetism observed below 10 K, taking into
account the presence of monomeric S = 3/2 cobalt(ii) impurity
species), giving rise to an S = 0 ground state.
Syntheses
As mentioned above, the g value of 2.3 derived for 1·2H2O
compares well with the values obtained for other complexes
with high-spin cobalt(ii) centres in trigonal bipyramidal coor-
dination geometries. This deviation from the free-ion value
ge can be attributed to a second-order Zeeman effect associ-
2-[(bis(2-Hydroxyethyl)amino)methyl]-4,6-bis-
tert-butylphenol (H3L)
Paraformaldehyde (0.700 g, 23.2 mmol) and diethanolamine
(2.44 g, 23.2 mmol) were refluxed in MeOH (50 mL) for 90 min,
followed by the addition of a solution of 2,4-di-tert-butylphenol
(4.79 g, 23.2 mmol) in MeOH (50 mL) and refluxed for a further
24 h. After cooling to room temperature, the resulting solution
was evaporated to dryness and the residue was dissolved in
CH2Cl2 and washed with water (3 × 30 mL). The CH2Cl2 solu-
tion was dried with magnesium sulfate and n-hexane was added
to crystallize the product as white needles, yield 43%. (Anal.
calc. for H3L, C19H33NO3: C 70.55, H 10.28, N 4.33%. Found
C 70.83, H 9.23, N 4.30.) Selected νmax/cm−1 3294, 2961, 2926,
2870, 2830, 1482, 1390, 1385, 1289, 1235, 1164, 1125, 1073,
996, 757, 723. δH (400 MHz, CDCl3) 1.26 (s, 9H, C(CH3)3),
1.39 (s, 9H, C(CH3)3), 2.78 (t, 4H, CH2CH2OH), 3.74 (t, 4H,
CH2CH2OH), 3.84 (s, 2H, ArCH2N), 6.83 (s, 1H, ArH), 7.21
(s, 1H, ArH). δC (13C{1H}, 100 MHz, CDCl3) 29.6, 31.7, 34.2,
34.9, 56.5, 60.3, 60.4, 121.5, 123.3, 123.6, 136.0, 141.1, 153.7.
m/z (ESI MS, CH2Cl2) 324.4 [M + H]+.
ated with admixture of a higher-energy Eꢀꢀ term with orbital
4
angular momentum.[16] The two cobalt(ii) centres in complex
2 are bridged by an alkoxo and an acetate ligand, with the J
parameter reflecting net antiferromagnetic superexchange along
these pathways.Although relatively few examples of structurally
analogous complexes are available in the literature for compar-
ison, the J value obtained for 1·2H2O is consistent with the
value of −8.5 cm−1 obtained for a dinuclear cobalt(ii) com-
plex, where two alkoxo ligands bridge two trigonal bipyramidal
cobalt(ii) centres.[14] The alkoxo bridge is likely the dominant
superexchange pathway in 1, with a CoII–O–CoII bridging angle
of 123◦. Although magnetostructural correlations between the
coupling constant and the M–O–M bridging angle are not well
established for cobalt, as they are for copper and nickel,[19,20]
it is consistent with the Goodenough–Kanemori approach that
a Co–O–Co bridging angle larger than 100◦ should give rise
to an antiferromagnetic coupling.[21] Given the distorted trigo-
nal bipyramidal coordination geometries of Co3 and Co4, the
singly occupied magnetic orbitals on each cobalt centre are the
[CoI2ICo2III(OAc)2(L)2(HL)] 1
Co(OAc)2·4H2O (0.623 g, 2.5 mmol) and H3L (0.808 g,
2.5 mmol) were dissolved in MeCN (30 mL) and the solution
stirred overnight. A small amount of precipitate was removed by
filtration and the filtrate was loosely covered and left to stand.
Purple diamond-shaped crystals formed after 4 days. A sam-
ple for crystallography was maintained in contact with mother
liquor to prevent the loss of interstitial solvent.The crystals were
isolated by filtration and washed with MeCN; yield 5%. The air-
dried sample appears to be hygroscopic. (Anal. calc. for 1·2H2O,
C61H101N3Co4O15: C 54.18, H 7.53, N 3.11%. Found C 53.57,
H 7.31, N 3.43.)The degree of hydration was confirmed byTGA.
Selected νmax/cm−1 3436, 2952, 2868, 1567, 1440, 1360, 1240,
1077, 1002, 535.
dxy, d
, and dz2 orbitals.The bridging alkoxo ligand provides
x2–y2
a superexchange pathway for overlap of the dz2 orbital on Co3
with the dx2–y2 or dxy (degenerate for a strict trigonal bipyramid)
orbitals on Co4.
Conclusion
A new tetradentate phenoxyamine ligand and two new mixed-
valent polynuclear cobalt complexes bearing this ligand have
been synthesized and characterized. The phenoxyamine ligand
simultaneously chelates and bridges the metal centres, dis-
playing µ3- and µ2-bridging modes in its triply and doubly

RESEARCH FRONT
1128
J. C. Ang et al.
Table 2. Crystallographic data for 1·2MeCN and 2
magnetometer MPMS-XL. This magnetometer works between
1.8 and 400 K for applied DC fields ranging from −7 to 7T.
Measurements were performed on a polycrystalline sample of
complex 1·2H2O weighing 10.82 mg. The sample bag was pre-
pared in glove box and then immediately put into the SQUID for
measurement. The magnetic data were corrected for the sample
holder and the diamagnetic contribution.
1·2MeCN
2
Formula
Formula weight
Space group
a [Å]
C65H102Co4N5O13
1397.24
P21/c
14.603(4)
18.007(5)
27.995(7)
102.709(5)
7181(3)
4
C80H126Co5N4O16
1694.50
I41/a
22.116(2)
22.116(2)
16.765(2)
90
b [Å]
c [Å]
β [◦]
Acknowledgements
V [Å3]
Z
8200(1)
4
The present work was supported by the Australian Research Council, the
University of Melbourne, the University of Bordeaux, the CNRS, the Region
Aquitaine and MAGMANet (NMP3-CT-2005–515767). We thank Robert
Gable for helpful discussions.
T [K]
130(2)
1.292
130(2)
1.373
ρcalc [g cm−3
]
µ [mm−1
]
0.968
1.056
Reflections measured
Unique reflections
Data/restraints/parameters
Rint
43962
16312
16312/1/762
0.1429
0.0648
0.1665
0.917
1.128, −0.839
25653
4711
4711/28/224
0.1806
0.0709
0.1857
0.993
1.076, −0.577
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