Exploring the coordinative adaptation and molecular shapes of trinuclear Cu^II₂M^II (M = Zn/Cd) complexes derived from salen type Schiff bases: structural and theoretical studies

Abstract

Three new trinuclear hetero-metallic complexes [(CuL) ₂Zn(NCS) ₂] (1), [(CuL^R)₂Zn(NCS)(μ_1,1-NCS)] (2) and [(CuL^R) ₂Cd(μ_1,3-NCS) ₂] (4) have been synthesized using [CuL] and [CuL^R] as “metalloligands” (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine and H₂L^R = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine). All three complexes are characterized by elemental analysis, spectroscopic methods and single crystal XRD. Complex 1 is an angular trinuclear species, in which two terminal four-coordinate square planar “metalloligands” [CuL] are coordinated to a central Zn(II) through double phenoxido bridges along with two mutually cis nitrogen atoms of terminal isothiocyanate ions as is usually found in such complexes. In contrast, in complex 2, the two terminal “metalloligands” [CuL^R] are square pyramidal, as one of the SCN⁻ ions makes an unusual μ_1,1-NCS bridge between copper centers while the other one coordinates to Zn(II) through a N atom in a usual fashion making its geometry also square pyramidal. For 4 which possesses an angular trinuclear structure, in addition to double phenoxido bridges from two terminal [CuL^R], both the SCN⁻ ions are S-bonded to Cd(II) and form a bridge (cis-μ_1,3-SCN) between Cd(II) and each of the terminal Cu(II) ions. This structure is different from its unreduced analogue in which NCS⁻ was N-terminal coordinated to Cd(II) (3/3′). All the structures have been optimized using Density Functional Theory (DFT) calculations. It has been found that for H₂L, optimized structures like 1 and 2 differ only by 0.4 kcal mol⁻¹ but the H₂L^R structure 2 is more stable by 5.5 kcal mol⁻¹ than the structure resembling 1. For Cd(II) complexes also, H₂L optimized structures such as 3 and 4 do not differ significantly in energy (1.0 kcal mol⁻¹) but the H₂L^R structure 4 is more stable than that of 3 by 4.6 kcal mol⁻¹. In fact, structure 4 has been found to be the most stable one among the other possible isomers of H₂L^R.