Abstract
Three new tetradentate imine ligands, H
L
HBr
, H
L
ClH
and H
L
BrH
(H
L
R1R2
) were synthesised by 2 : 1 condensation of the appropriately
n
-halo substituted pyridine-2-carboxaldehyde (5-bromo-
4a
, 6-bromo-
4b
or 6-chloro-
4c
) with 1,3-diaminopropan-2-ol (
5
). Reactions of each of these three ligands with one equivalent of cobalt(
ii
) tetrafluoroborate resulted in the formation of three N
4
O
2
coordinated cobalt(
ii
) complexes: the anticipated mononuclear complex [Co
II
(H
L
HBr
)(MeOH)
2
](BF
4
)
2
(
1
), and two unexpected dinuclear complexes, [Co
II
2
(
L
BrH-BF2OMe
)]
2
(BF
4
)
2
(
2
) and [Co
II
2
(
L
ClH-BF2OMe
)]
2
(BF
4
)
2
(
3
). Dinuclear
2
and
3
result from complexation of cobalt(
ii
) to the ligands derived from the sterically demanding 6-halo substituted pyridine-2-carboxaldehydes (
4b
and
4c
) undergoing rearrangement, reacting with MeOH and a BF
4
anion, resulting in a pair of borate ester bridges between the two cobalt(
ii
) centres. A similar type of rearrangement is proposed for the PF
6
analogues. Cyclic voltammetry in acetonitrile reveals that cobalt(
ii
) complexes
1-3
undergo a quasi-reversible oxidation:
E
m
= 0.57, 0.38 and 0.29 V
vs.
0.01 AgNO
3
/Ag, respectively. The observed
E
m
value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest
E
m
value being observed for the corresponding cobalt complex,
3
, rather than for either of the complexes of the
n
-bromo-substituted ligands (
n
= 6 or 5),
2
and
1
.
Sterically encumbered 6-halo-substituents lead to BF
4
/MeOH/ligand rearrangement, forming doubly bridged {Co1-[
O(Me)-B(F
2
)-O(R)
]
2
-Co1A} dinuclear complexes (
6-Cl
,
6-Br
) of the modified ligands.