Two supramolecular isomers of molecular squares and 1D helical chains with alternating right- and left-handed chirality

The reaction of Ni(alpha-rac-L)](ClO4)2 with K2Ni(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(alpha-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.