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Figure 5.19 Molecular structure of dinuclear complexes [M
2
(N,S^N,S)
2
] viewed along the
MM axis (M¼Ni
II
: 16, left, M¼Pd
II
: 18, right).
intensity (ca. 10%) was observed in the
1
H NMR spectrum of
17
, which can be assigned
to the corresponding trinuclear [Zn
3
(N,S^N,S)
3
] species, as indicated by MALDI MS
data. 2D EXSY NMR experiments also confirmed the presence of an equilibrium between
the dinuclear and trinuclear zinc species. A similar equilibrium between dimeric and tri-
meric copper complexes bearing related Schiff base ligands has been reported [57].
The subcomponent self-assembly protocol is not applicable for the synthesis of the
dipalladium helicate
18
, most likely due to the reduced reactivity of the mononuclear
palladium bis(2-thiolatobenzaldehyde) complex towards diamines. Fortunately, both
dinuclear helicates
16
and
17
undergo complete metal exchange with palladium(II) via
the simple addition of two equivalents of palladium acetate to solutions of the com-
plexes at ambient temperature, affording [Pd
2
(N,S^N,S)
2
]
18
(Scheme 5.8). These
transmetallations can be followed visually by the fast color change from either dark
brown (
16
, absorption of the {NiN
2
S
2
} chromophore at l
max
¼
501 nm) or bright yellow
(
17
, l
max
¼
463 nm).
A single-crystal X-ray diffraction analysis of
18
(Figure 5.19, right) confirmed the for-
mation of a dinuclear double-stranded palladium complex, which shows structural fea-
tures comparable to the nickel analogue
16
. Both palladium atoms are coordinated in a
slightly distorted square-planar fashion and the helical twist angle measures 152.1
. The
chelate rings are not planar, but bent along the N
426 nm) to deep orange for dipalladium species
18
(l
max
¼
S vector. Although both complexes
16
and
18
are stabilized by intra- and intermolecular interactions, the latter play a more
important role for the dipalladium complex
18
. The metallosupramolecular transmetalla-
tion protocol was completed by the reaction of the dizinc complex
17
with nickel acetate,
leading to the dinickel complex
16
.
The thermodynamic driving force for these metal exchange reactions can be attributed
to the preferred coordination of the soft (N,S) binding groups to the softer metal center
available [58], as shown earlier for transmetallations involving mononuclear zinc, nickel
[59] or palladium [60] complexes with sulphur-nitrogen ligation. In order to gain more
insight into the principles of the transmetallation, exchange reactions involving helicates
16
,
17
and
18
were investigated.
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