Two Supramolecular Isomers of Molecular Squares and 1D Helical Chains with Alternating Right- and Left-Handed Chirality

The reaction of [Ni(α-rac-L)](ClO4)2 with K2[Ni(CN)4] gives a cyanide bridged [2+2] type of molecular square, {cis-[Ni(f-rac-L)][Ni(CN)4]}2 (1). By slightly changing the reaction conditions, the reaction of [Ni(α-rac-L)](ClO4)2 with
KCN leads to a metastable compound, cis-[Ni(f-rac-L)(CN)2] (2), and an unexpected 1D helical chain, {cis-[Ni(f-rac-L)][Ni(CN)4]}n (3). In 3, the 1D helical chains are packed in an alternating right- and left-handed chirality due
to the oppositely twisted arrangements of two adjacent [Ni(CN)4]2- anions. The metastable compound 2 can be
converted to 3 in a CH3CN/CH3OH solution. Compounds 1 and 3 are classified as supramolecular isomers, and
isomer 3 can be considered to be formed by the ring-opening polymerization of the square precursor 1. Magnetic
susceptibility measurements of 1 and 3 show that the adjacent six-coordinated Ni(II) atoms are antiferromagnetically
coupled through the bent −NC−Ni−CN− bridges of the diamagnetic [Ni(CN)4]2- anions, with g = 2.08 and J =
−0.426 cm-1 for 1 and g = 2.08 and J = − 0.278 cm-1 for 3. The correlation between the structures and the
J values is discussed.