Helical Structures Featuring Thiolato Donors - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 164

bonding, which apparently does not exist in the electron-poorer Ni III complex. Cyclic
voltammetry studies show quasi-reversible two electron transfer waves for the reduction
Ni III Ni II with a peak potential recorded at ! 1160mV for (NEt 4 ) 2 [Ni 2 ( 2 ) 2 ]and

785mV for Na 4 [Ni 2 ( 3 ) 2 ] (vs Fc/Fc þ ). The reduction of complex anion [Ni 2 ( 3 ) 2 ] 2 is
facilitated by electron withdrawing diamide bridges while the complex anion with alkyl
spacers is more difficult to reduce.
Reduction of the length of the spacer in H 4 - 2 from an ethylene group to a methylene
group leads to the bis(benzene- o -dithiol) ligand H 4 - 1 [38]. The analogous dicatechol lig-
and has been described to react with [TiO(acac) 2 ] with formation of a triple stranded
meso -helicate [5d]. Reaction of H 4 - 1 with CoCl 2

6H 2 O in the presence of Li 2 CO 3 as a
base followed by aerial oxidation and cation exchange with PNPCl did not yield the
expected helicate but instead the salt (PNP) 4 [Co 4 ( 1 ) 4 ] featuring a tetranuclear complex
anion. The complex anion (Figure 5.8) is composed of four Co III ions each coordinated
by two benzene- o -dithiolato groups from two different 1 4 ligands, leading to a metal-
lamacrocycle. Two different types of cobalt centers are observed in the complex anion
with the alkyl spacers arranged in an anti conformation (Co1, Co3) or syn conformation
(Co2, Co4). While the analogous dicatechol ligand H 4 - 1 forms triple-standed meso -heli-
cates with Ti IV [5d], helicate formation is not possible with 1 4 since the longer MS
bonds and the short bridging unit between the bdt 2 donor groups would lead to short
intrastrand S S contacts. Rotation of one bdt 2 group about the Ar CH 2 bond mini-

mizes these interactions, leading to the metallamacrocycle.
In the search for dinuclear triple-stranded helicates with bis(benzene- o -dithiolato)
ligands, compound H 4 - 5 (Figure 5.3), featuring a larger spacer between the donors than
H 4 - 1 , was deprotonated with Li 2 CO 3 and reacted with [Ti(OEt) 4 ]followedbycation
exchange with (Ph 4 As)Cl. Again, the reaction did not yield a triple-standed dinuclear com-
plex but instead gave the dinuclear double-stranded complex anion [Ti 2 ( 5 ) 2 (m-OCH 3 ) 2 ] 2
(Figure 5.9) [39]. The two metal centers are connected by two ligand strands 5 4 in
addition to two bridging methoxo ligands which are part of a central four-membered
Ti 2 (m-OCH 3 ) 2 ring. The coordination geometry at the metal centers is best described
as strongly distorted octahedral. The two metal centers assume different configurations
(L and D) which leads to a meso -complex anion. As can be seen from Figure 5.9, the
amide groups are not oriented in a coplanar fashion with the benzene- o -dithiolate groups

Figure 5.8 Molecular structure of the complex anion [Co III 4 (1) 4 ] 4 .