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 K-2[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(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(ll) 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.