The new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)]
(H2
L
3,5
), has been
prepared and the reactions of its dianion (Na2
L
3,5
) with M3+ ions (M = Ga or
In) yield a new class of “3d-np block” heterometallic
triple-stranded helicates, M2(L
3,5
)
3, by the self-assembly
process. The X-ray structural analysis of the new ligand shows that
it is in the enolic form with each enolic carbon bonded to the pyridine
ring and each carbonyl carbon connected to a ferrocene moiety; overall,
the nonferrocenyl part of the molecule is nearly planar. The M2(L
3,5
)
3 (M = Ga or In) complexes are helicates with three ligand
strands, each of which is twisted into an S-shape, coordinating to
two metal ions, each of which is in a distorted octahedral geometry.
The new helicates are observed as a racemic mixture in the solid state
by single-crystal X-ray analysis, and in solution by NMR, with both
the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers
present. Variable-temperature 1H NMR study of the Ga2(L
3,5
)
3 helicate indicates that the right-handed Δ,Δ-isomer
and left-handed Λ,Λ-isomer equilibrate through a heterochiral
Λ,Δ-intermediate by a concerted twist motion of one-half
of the dinuclear complex through a trigonal prismatic transition state,
according to the Bailar twist mechanism. Electrochemical properties
of the ligand (H2
L
3,5
) and the M2(L
3,5
)
3 helicates were investigated through cyclic
voltammetry, and the results indicate the lack of communication between
the ferrocene units, because the separation between any two ferrocene
units is greater than the 5–6 Å range in both the free
ligand and the helicates.