Self-assembly of pentanuclear mesocate versus octanuclear helicate: size effect of the [M(II)3(μ3-O/X)]n+ triangle core.

The first cluster mesocate (H(3)O)[{Fe(2)(μ-L)(3)}{Fe(3)(μ(3)-O)(μ-Cl)(3)}]·3EtOH (1) and a new series of cluster helicates, [{Mn(μ-L)(3)}(2){Mn(3)(μ(3)-Cl)}(2)](ClO(4))(2)·2MeOH·6H(2)O (2), [{Cd(μ-L)(3)}(2){Cd(3)(μ(3)-Br)}(2)]Br(2)·2DMF·14H(2)O (3), and [{Cd(μ-L)(3)}(2){Cd(3)(μ(3)-I)}(2)](CdI(4))·3H(2)O (4), have been synthesized by the self-assembly of a C(2)-symmetric tritopic ligand, 2,6-bis[5-(2-pyridinyl)-1H-triazol-3-yl]pyridine (H(2)L) with different metal halogen salts. Single-crystal X-ray diffraction and electrospray ionization mass spectrometry measurements were carried out on these complexes. 1 was crystallized as a triple-stranded pentanuclear mesocate in which a [Fe(II)(3)(μ(3)-O)](4+) triangle core was wrapped by a [Fe(II)(2)(μ-L)(3)](2-) shell. 2-4 have similar octanuclear helicate structures in which two propeller-shaped [M(II)(μ-L)(3)](4-) units embrace two [M(II)(3)(μ(3)-X)](5+) triangles inside. The [M(II)(3)(μ(3)-O/X)](n+) triangle core were found to play an important role in the selective synthesis of the two architectures: the smaller [Fe(II)(3)(μ(3)-O)](4+) triangle core prefers a mesocate structure because it matches the small cavity imposed by the [Fe(II)(2)(μ-L)(3)](2-) shell, while the bigger [M(II)(3)(μ(3)-X)](5+) induces a screwed arrangement of the ligands, thus stabilizing the helicate structure. Variable-temperature magnetic susceptibility measurements indicate that both 1 and 2 display an overall antiferromagnetic coupling. Density functional theory calculations for 1 confirm the strong antiferromagnetic interaction in the central [Fe(II)(3)(μ(3)-O)](4+), while interaction through the triazole bridging ligands is slightly ferromagnetic. For 2, three interaction pathways were considered and all sets of J values reveal the presence of weak antiferromagnetic interaction.