Self-assembly by ligand disassembly? - Formation of an unusual dodecanuclear [Co(II)6Co(III)6] cluster

Article abstract of DOI:10.1039/b405131a

A sterically encumbered 'tritopic' picolinic-dihydrazone ligand reacts with cobalt(II) nitrate in air to give a dodecanuclear [Co(II)₆Co(III) ₆] cluster, in which six ligands are hydrolyzed to mono-carboxylate analogues.

Full text of DOI:10.1039/b405131a

Self-assembly by ligand disassembly?—formation of an unusual
dodecanuclear [Co(II)6Co(III)6] cluster
Victoria A. Milway, Laurence K. Thompson* and David O. Miller
Department of Chemistry, Memorial University, St. John’s, NL, Canada A1B 3X7. E-mail: lthomp@mun.ca;
Fax: +01-709-737-3702; Tel: +01-709-737-8750
Received (in Cambridge, UK) 7th April 2004, Accepted 25th June 2004
First published as an Advance Article on the web 20th July 2004
A sterically encumbered ‘tritopic’ picolinic-dihydrazone ligand
reacts with cobalt(II) nitrate in air to give a dodecanuclear
[Co(II)6Co(III)6] cluster, in which six ligands are hydrolyzed to
mono-carboxylate analogues.
cluster (Fig. 1, viewed down the trigonal axis; phenyl rings bound
to C(6) (L2) and C(31) (L1) removed for clarity; the phenyl ring
bound to C(6) is disordered over two positions‡), with the
[Co(H2O)6]2+ cation appearing in the lattice, presumably required
for overall charge balance. Several ‘oxygen’ atom sites have been
located inside the cluster, and are assigned as water molecules (e.g.
O(17), O(18)). The dodecanuclear cluster shows three-fold sym-
metry, with two almost planar hexanuclear halves, comprising three
L2 ligands each binding two six-coordinate cobalt centres, which
are bridged by a deprotonated hydrazone oxygen atom (O(1)). Each
dinuclear subunit is bridged internally by a carboxylate in a syn-anti
conformation (O(2)–O(3)), linking the three dinuclear subunits
together via Co(2) to form the hexanuclear layer. The abbreviated
asymmetric dinuclear core subunit is shown in Fig. 2 (Co(2) is
bridged through O(3)).
‘Tritopic’ ligands based on a 2,6-dipicolinic-hydrazone core, e.g.
2poap, represent a unique ligand class, which preferentially forms
[3 3 3] nonanuclear grid complexes. Examples with Mn(II)9 and
Cu(II
)
are well documented, and provide novel multi-spin
9
platforms for magnetic, electronic and surface studies.1–8 A Mn(II
)
9
system has been shown to exhibit quantum-magneto oscillations in
the field dependence of magnetic torque at low temperature.9 2poap
and related ligands form grids with essentially square core
structures, but 2popp, which has bulky phenyl end groups, exerts a
major distortion in the complex [Mn9(2popp)6](NO3)6·12H2O,
which has a diamond shaped core.8 However, despite the
pronounced distortion, the magnetic properties are normal for grids
in this class.
The inner group of cobalt centres (Co(2); CoNO5) have Co–L
distances in the range 2.022–2.321 Å indicating them to be Co(II
)
ions. The outer group of three cobalt centres (Co(1); cis-CoN4O2)
has Co–L distances in the range 1.846–1.938 Å indicating that they
are Co(III) ions. Apical ligands to each Co(II) centre are mono-
dentate nitrate (O(5)) on the outer surface, and an inner water
molecule (O(14)). The hexanuclear subunits are cross-linked by
three full ligands (L1), which provide an N2O donor grouping from
2popp and Cl2popp are produced by condensation of the
appropriate precursor dihydrazide with phenyl pyridyl ketone, and
are classic Schiff base ligands. Hydrolytic instability of the CNN
linkages in such ligands can be promoted in the presence of metal
(Lewis acid) centres. We have discovered numerous examples of
such metal ion directed hydrolyses of vulnerable hydrazone ligand
sites, which have only been revealed from structural studies on the
reaction products.10 The present study reports a rather unusual
hydrolysis reaction of Cl2popp, in the presence of Co(II) and
Co(III), leading to a novel, mixed ligand, dodecanuclear cobalt
cluster.
Fig. 1 Structural representation for 1. Phenyl groups are removed for
clarity.
Cl2popp (L1) was added to a solution of Co(NO3)2·6H2O in
CH3CN–MeOH, and the mixture stirred in air.†A red solution
formed, which deposited dark red crystals suitable for structural
analysis on standing for several days.‡
The crystal structure reveals a complex with thirteen cobalt
centres, with the formula [(L1–2H)3(L2–2H)6Co(III)6Co(II)6-
(H2O)6(NO3)6][Co(H2O)6](NO3)8(CH3CN)3(H2O)6 (1), where L2
is a partially hydrolyzed ligand with a carboxylate group at a
pyridine 2-position. Twelve cobalt ions form a dodecanuclear
Fig. 2 Structural representation of the asymmetric dinuclear subunit in 1.
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C h e m . C o m m u n . , 2 0 0 4 , 1 7 9 0 – 1 7 9 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

the ends of each ligand (the central pyridine remains uncoordi-
nated). This can be seen more clearly in Fig. 3 (phenyl rings
removed and axially bonded nitrates abbreviated to bound oxygen),
which is viewed across the hexacobalt subunits, and reveals the
overall structure of the triangular based prism (dimensions 20 Å
across and 17.6 Å deep).
centres. The drop in moment below 100 K might signal the
presence of intramolecular antiferromagnetic exchange, but if
present it would be very weak.
The conclusion from this and related studies is that novel and
unexpected results can occur through hydrolytic chemical reactions
at vulnerable ligand sites in the presence of metal ions, and the
products are frequently not fully revealed without a structural
analysis. The fortuitous combination of the heptadentate ligand L1,
and the pentadentate ligand L2 in the present case, has produced a
novel dodecanuclear mixed oxidation state cobalt [6 + 6] cluster. A
full report on this and other ligand hydrolysis reactions will be
presented elsewhere.10
Cl2popp is synthesized in a two step process from the diethyl
ester of 4-chloro-2,6-dipicolinic acid, by reaction with hydrazine,
followed by reaction with phenyl 2-pyridyl ketone. Two different
hydrolytically susceptible CNN linkages exist, with the hydrazone
being the most vulnerable in the present case. Since ligands of this
type typically produce trinuclear and nonanuclear complexes, with
three ligand compartments occupied by metal ions, it is reasonable
to assume that this occurs initially in the present case. The unusual
grid distortion in the case of complex [Mn9(2popp-
)6](NO3)6·12H2O,8 would suggest a similar distortion in the present
case in a putative grid, and in combination with the cobalt aerial
oxidation to produce Co(III) may provide the right circumstances
for metal ion promoted hydrazone hydrolysis to the carboxylic acid.
A similar hydrolysis of Cl2popp occurs in the presence of copper
acetate, with the formation of L2, 4-chloro-pyridine-2,6-dicarbox-
ylate, and 3-phenyl-triazolo[1,5-a]pyridine, all in the same dinu-
clear copper complex structure.10 It is also of interest to note that
the reaction of Cl2popp with NiCO3 in CH3CN–CH3OH gives a red
square planar, neutral mononuclear complex [Ni(Cl2popp)], in
high yield, with no ligand decomposition.10
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support.
Notes and references
† Cl2popp was prepared by reacting 4-chloro-pyridine-2,6-dihydrazide
with phenyl pyridyl ketone as reported for 2popp.8 It was characterized by
MS, NMR and elemental analysis. Reaction of Cl2popp (0.10 g, 0.018
mmol) with Co(NO3)2·6H2O (0.17 g, 0.06 mmol) in warm CH3CN–MeOH
(1 : 1) (25 mL), without exclusion of air, produced a clear dark red solution.
Dark red crystals (30 mg) formed on prolonged standing.
‡ Crystal
(NO3)8(CH3CN)3(H2O)6. 193 K, hexagonal, P63/m, a = 24.2682(6), c =
29.470(2) Å, V = 15030.9(8) Å3, Z = 2, Dc = 1.332 g cm23, 2qmax
data:
[(L1)3(L2)6Co(III)6Co(II)6(H2O)6(NO3)6][Co(H2O)6]
=
52.8°, m(0.71073 Å) = 8.61 cm21. 96973 collected reflections, 10478
unique were used for refinement (570 variables). The disordered phenyl
group was modeled using a rigid group for the 25% occupancy component.
The 75% component was refined isotropically. The final R values were R1
= 0.093, wR2 = 0.326. The highest electron density on the final difference
map was 1.63 e Å23. CCDC 236722. See http://www.rsc.org/suppdata/cc/
b4/b405131a/ for crystallographic data in .cif format.
The Co(II) centres are separated by 5.180 Å, and arranged within
each hexanuclear layer in a triangular array, bridged by three syn–
anti carboxylate groups. Such an arrangement would not be
expected to lead to significant magnetic exchange between the
metal centres, partly because of the long distance of separation, but
also because a symmetry mis-match of magnetic orbitals would
result.11,12 This is reflected in the variable temperature magnetic
properties, which show an essentially constant magnetic moment
(per mole) between 300 and 100 K (10.2–11.0 mB), which then
drops slightly to 8.5 mB at 2 K. The room temperature moment is
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Fig. 3 Core structural representation for 1 (Povray J); magenta = Co, blue
= nitrogen, red = oxygen, black = carbon, green = chlorine).
C h e m . C o m m u n . , 2 0 0 4 , 1 7 9 0 – 1 7 9 1
1791