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

The sequential ligand 5 -H 3, 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 ) 3 Ti 2 ] - and [( 5 ) 3 Ga 2 ] 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 ) 3 GaTi] 2 - is formed in a cooperative process. In this coordination compound the three ligands are orientated in one direction (type I structure). For K 2 [( 5 ) 3 GaTi]·6DMF·ether, space group P 2 1 / c, a = 12.868(3) Å, b = 19.902(2) Å, c = 27.063(2) Å, β = 102.22(1)°, V = 6774(2) Å 3, 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.