Polymorphism in hetero-metallic tri-nuclear Cu^II₂Cd^II complexes of salicylaldimine ligand: structural analysis and theoretical study

Abstract

Three new trinuclear hetero-metallic copper(II)–cadmium(II) complexes [(CuL)₂Cd (NCS) ₂] (1A and 1B), [(CuL) ₂Cd(NCO)₂] (2) have been synthesized using [CuL] as so-called “ligand complex” (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine) and structurally characterized. Crystal structure analyses reveal that all three complexes have trinuclear entity in which two square planar [CuL] units are bonded to a cadmium ion through double phenoxido bridges. The Cd (II) is in a six-coordinate distorted octahedral environment being bonded additionally to two mutually cis nitrogen atoms of terminal thiocyanate (in 1A and 1B) and cyanate (in 2). Complexes 1A and 1B have the same molecular formula but crystallize in different monoclinic unit cells (P2₁/c for 1A and P2₁/n for 1B) and can be considered as polymorphs. On the other hand, complex 2 is crystallized only in one form and its conformation is similar to that observed in 1A. A theoretical investigation shows that 1A is slightly more (2.25 kcal/mol) stable than 1B. However, in the crystal lattice, 1A establishes a C–H/π interaction while 1B interacts through a C–H•••S hydrogen bond to form dimer and the more favorable dimerization energy in the later (ca. 3 kcal/mol) compensates the difference in the relative energy of the polymorphs. Complex 2 crystallizes only in one form and conformation of the molecule is similar to 1A. Analyses of the noncovalent interactions of 2 show a weakening of the dimerization energy compared to 1A or 1B. Moreover, the H-bonding interaction that facilitates the formation of 1B is unlikely to be established in 2. Therefore this polymorph is not isolated.