Controlling the Orientation of Sequential Ligands in the Self-Assembly of Binuclear Coordination Compounds

The sequential ligand 5-H3, which contains
one catechol and one aminophenol unit, was prepared in a
four-step sequence. The two electronically different binding sites
of 5 are able to control the metal-directed
self-assembly of binuclear coordination compounds with different ligand
orientations. In coordination studies with
titanium(IV) or gallium(III) ions, this ligand 5 was used to
obtain anionic homobinuclear complexes
[(5)3Ti2]- and
[(5)3Ga2]3-
with two ligands orientated in one and the third in the opposite
direction (type II structure). On the other hand, if
the ligand 5 reacts with a 1:1 mixture of titanium(IV)
and gallium(III) ions exclusively, the heterobimetallic
complex
[(5)3GaTi]2- is
formed in a cooperative process. In this coordination compound the
three ligands are orientated in
one direction (type I structure). For
K2[(5)3GaTi]·6DMF·ether,
space group P21/c, a =
12.868(3) Å, b = 19.902(2)
Å, c = 27.063(2) Å, β = 102.22(1)°,
V = 6774(2) Å3, Z = 4, and
R = 0.071. The structural analysis shows
that
titanium(IV) binds to the catecholate moieties while
gallium(III) prefers the aminophenolate binding site of
the
ligand 5. Internal hydrogen bonding enhances the
stability of the ΛΔ configurated binuclear complex.