Derivatives of luminescent metal–polypyridyl complexes with pendant adenine or thymine groups: building blocks for supramolecular assemblies based on hydrogen bonding
Abstract
Alkylation of adenine or thymine with
5-bromomethyl-2,2′-bipyridine afforded bipya and bipyt, in which a
2,2′-bipyridyl (bipy) is attached to the N
9
position of
adenine or the N
1
position of thymine via a
CH
2
spacer. Attachment of the bipy site of bipya to
[Ru(bipy)
2
Cl
2
] or
[Ru(dbbipy)
2
Cl
2
]
[dbbipy = 4,4′-bis(tert-butyl)-2,2′
-bipyridine] gave the complexes
[Ru(bipy)
2
(bipya)][PF
6
]
2
and
[Ru(dbbipy)
2
(bipya)][PF
6
]
2
(Ru-Ade)
respectively, in which an adenine fragment is pendant from the
{Ru(bipy)
3
}
2+
core. Attachment of the
bipy site of bipyt to [Os(dbbipy)
2
Cl
2
] and
[Re(CO)
5
Cl] afforded
[Os(dbbipy)(bipyt)][PF
6
]
2
(Os-Thy) and
[Re(bipyt)(CO)
3
Cl] (Re-Thy) respectively, in which the
{Os(bipy)
3
}
2+
and
{Re(bipy)(CO)
3
Cl} cores have pendant thymine
groups. Recrystallisation of
[Ru(bipy)
2
(bipya)][PF
6
]
2
from wet MeCN
resulted in partial protonation to give
[{Ru(bipy)
2
(bipya)}{Ru(bipy)
2
(Hbipya)}][PF
6
]
5
·4MeCN in which
[Ru(bipy)
2
(bipya)]
2+
and protonated
[Ru(bipy)
2
(Hbipya)]
3+
complex cations are
associated by a Watson–Crick type hydrogen-bonding interaction
between the adenine groups across an inversion centre. Similarly, in
[Os(dbbipy)
2
(bipyt)][PF
6
]
2
·Me
2
CO there are two
[Os(dbbipy)
2
(bipyt)]
2+
complex cations associated
via a centrosymmetric thymine–thymine hydrogen-bonding
interaction across an inversion centre. In contrast, in
[Ru(dbbipy)
2
(bipya)][PF
6
]
2
·2MeCN
the [Ru(dbbipy)
2
(bipya)]
2+
complex cations are
associated via a Hoogsteen-type hydrogen-bonding interaction to
give a one-dimensional ‘ribbon-like’ chain of hydrogen
bonds. The electrochemical, UV/VIS spectroscopic and luminescence
properties of the complexes are very similar to those of the parent
unsubstituted complexes, indicating that the adenine or thymine
substituents do not perturb the desirable properties of the complex
cores. By monitoring the chemical shift of the thymine NH proton, NMR
titrations allowed estimation of the association constants of the
complementary Ru-Ade/Os-Thy pair in CD
3
CN and
CD
2
Cl
2
as 60 and 123 dm
3
mol
-1
respectively, and that of the Ru-Ade/Re-Thy pair
in CD
3
CN as 17.9 dm
3
mol
-1
. At
the very low concentrations used for luminescence studies, these
association constants are much too low to allow significant formation of
hydrogen-bonded associates in mixtures of complementary complexes such
as Ru-Ade/Os-Thy and Ru-Ade/Re-Thy. The requirements for observing
energy-transfer across hydrogen-bonded bridges in associates of this
type are discussed.
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