A series of homo- and heterotrinuclear complexes
containing three face-sharing octahedra has been
synthesized by using the pendent arm macrocyclic ligands
1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L0H3, and
1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane,
LH3:
[{L0NiII}2NiII]
(1) and
[{LCoIII}2CoIII](PF6)3
(2);
[{LCoIII}2Ni]
n
+
(n = 2 (3), 3 (4), 4
(5));
[{LNi}2CoIII]
n
+
(n = 1 (6), 2 (7), 3 (8))
and its
linkage isomers
[{LNi}Ni{CoIIIL}]
n
+
(n = 1 (9), 2 (10), 3
(11)) and, finally, the complexes
[{LNi}2Ni]
n
+ (n
= 0
(12), 1 (13), 2 (14), 3
(15)). In complex 1 three octahedral
NiII ions form a linear array with two terminal
[L0NiII]-
moieties in a facial N3O3 donor set and a
central NiII ion which is connected to the terminal ions
via six phenolate
bridging pendent arms of L0. In complexes
2−15 the three metal ions are always in the
same ligand matrix yielding
an
N3M(μ-S)3M(μ-S)3MN3
first-coordination sphere regardless of the nature of the metal ions
(nickel or cobalt) or
their formal oxidation states. From temperature dependent magnetic
susceptibility measurements it has been determined
that 1 has an S = 3 ground state whereas in
12 it is S = 1. In order to understand
this difference in exchange
coupling (ferromagnetic in 1 and antiferromagnetic in
12) in two apparently very similar complexes the
magnetic
properties of 2−15 have been investigated.
Complex 3 has an S = 1 and 4 an
S = 1/2, and 5 is
diamagnetic (S =
0) as is its isoelectronic counterpart 2. This
indicates the availability of the oxidation states II, III, and IV of
the
central NiS6 unit. In the isostructural complexes
6, 7, and 8, two terminal nickel ions
are bridged by a central
diamagnetic CoIII. The exchange coupling between two
terminal paramagnetic nickel ions was studied as a
function
of their formal oxidation state. In 6 the two
NiII ions are ferromagnetically coupled (S =
2); the mixed-valent
NiIINiIII species 7 has an
S = 3/2 ground state and in
8 most probably two NiIII ions (d7
low spin) give rise to an S
= 1 ground state. In contrast, in the series 9,
10, and 11 where two nickel ions are in a
position adjacent to each
other 9 has an S = 0 (antiferromagnetic
coupling), but in the mixed-valent complex 10 an
S = 3/2 ground state
(ferromagnetic coupling) is observed. In 11 an
S = 1 ground state prevails which may be achieved by
ferromagnetic
coupling between two NiIII ions. For the trinuclear
nickel complexes 12−15 an S = 1
ground state has been determined
for 12, an S = 3/2 for
the mixed valent complex 13, and an S = 2 for
14, and 15 exhibits an S =
3/2 ground state.
The Goodenough−Kanamori rules do not provide a consistent
explanation for the observed ground states in all
cases. The concept of double exchange, originally introduced by
Zener in 1951, appears to provide a more appropriate
description for the mixed-valent species 7, 10,
13, 14, and 15. This picture is
corroborated by the electrochemistry
and EPR spectroscopy of complexes.
