Ability of terephthalate (ta) to mediate exchange coupling in ta-bridged copper(II), nickel(II), cobalt(II) and manganese(II) dinuclear complexes
Abstract
The exchange coupling between the unpaired electrons of divalent
first-row transition-metal ions M
II
(M = Cu,
Ni, Co or Mn) bridged by the terephthalate (ta) ligand (intramolecular
metal–metal separation ca. 10 Å) has been
systematically studied. The following complexes have been synthesized:
[Cu
2
(bipy)
4
(ta)][ClO
4
]
2
1,
[Cu
2
(terpy)
2
(H
2
O)
2
(ta)]
[ClO
4
]
2
2,
[Ni
2
(bipy)
4
(ta)][ClO
4
]
2
3,
[Co
2
(bipy)
4
(ta)][ClO
4
]
2
4,
[Mn
2
(phen)
4
(H
2
O)
2
(ta)][ClO
4
]
2
5 and
[Mn
2
(phen)
4
(ta)][ClO
4
]
2
6
(bipy = 2,2′-bipyridine,
terpy = 2,2′
:6′,2″-
terpyridine, phen = 1,10-phenanthroline). Complexes 1, 2,
4 and 5 have been characterized by single-crystal X-ray analysis.
Complex 6 was obtained by thermal dehydration of 5 at 60 °C
or under vacuum at room temperature. The structures have in common the
presence of cationic terephthalate-bridged
[M
2
(ta)]
2+
dinuclear units with bidentate (1,
3–6) and terdentate (2) blocking ligands and unco-ordinated
perchlorate counter ions. A co-ordinated water molecule per metal ion is
present in complexes 2 and 5. Variable-temperature magnetic
susceptibility data for all the complexes have been measured over the
range 2.0–298 K. In 1–5 only very weak antiferromagnetic
coupling has been observed [J ca. -2.2 (1), -0.01
(2), -0.6 (3), -0.3 (4) and -0.065
cm
-1
(5), the Hamiltonian being
![[H with combining macron]](https://www.rsc.org/images/entities/char_0048_0304.gif)
; = -J![[S with combining macron]](https://www.rsc.org/images/entities/char_0053_0304.gif)
A
·
B
with
S
A
= S
B
=
½ (1, 2), 1 (3), 3/2 (4) and 5/2 (5)]. In
contrast to the lack of magnetic interaction detected for complex 5, a
significant antiferromagnetic coupling
(J = -1.6 cm
-1
; maximum of
susceptibility at 8.0 K) is observed in its dehydrated phase (6). The
magnetostructural data and theoretical calculations demonstrate the low
efficiency of terephthalate as a bridge to mediate exchange interactions
between first-row transition-metal ions. The significant exchange
coupling observed in 6 is due to the occurrence of a carboxylate bridge
between the manganese(II) ions induced by a
carboxylate-assisted loss of the co-ordinated water molecule.
:6′,2″-
terpyridine, phen = 1,10-phenanthroline). Complexes 1, 2,
4 and 5 have been characterized by single-crystal X-ray analysis.
Complex 6 was obtained by thermal dehydration of 5 at 60 °C
or under vacuum at room temperature. The structures have in common the
presence of cationic terephthalate-bridged
[M
2
(ta)]
2+
dinuclear units with bidentate (1,
3–6) and terdentate (2) blocking ligands and unco-ordinated
perchlorate counter ions. A co-ordinated water molecule per metal ion is
present in complexes 2 and 5. Variable-temperature magnetic
susceptibility data for all the complexes have been measured over the
range 2.0–298 K. In 1–5 only very weak antiferromagnetic
coupling has been observed [J ca. -2.2 (1), -0.01
(2), -0.6 (3), -0.3 (4) and -0.065
cm
-1
(5), the Hamiltonian being
; = -J
A
·
B
with
S
A
= S
B
=
½ (1, 2), 1 (3), 3/2 (4) and 5/2 (5)]. In
contrast to the lack of magnetic interaction detected for complex 5, a
significant antiferromagnetic coupling
(J = -1.6 cm
-1
; maximum of
susceptibility at 8.0 K) is observed in its dehydrated phase (6). The
magnetostructural data and theoretical calculations demonstrate the low
efficiency of terephthalate as a bridge to mediate exchange interactions
between first-row transition-metal ions. The significant exchange
coupling observed in 6 is due to the occurrence of a carboxylate bridge
between the manganese(II) ions induced by a
carboxylate-assisted loss of the co-ordinated water molecule.
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