Macrocyclic Ni^II-Xanthate [Ni^II₂-µ²-bis-{(κ²S,S-S₂COCH₂CH₂)₂N(Ts)}] Complex and a Cyclic Thiocarbonate Monomer: Synthesis, Crystallography, Photophysical, TD-DFT and Investigation of Thermochromism

Potassium salt of N,N-Bis(2-dithiocarbonatoethyl)-4-methylbenzenesulfon amide (K₂xan) was synthesized and characterized prior to use in the progress of a 24-membered binuclear metallomacrocyclic compound [Ni^II₂-µ²-bis-{(κ²S,S-S₂COCH₂CH₂)₂N(Ts)}] (1) and a synthetically challenging 16-member organic functional macrocycle 6,14-ditosyl-1,3,9,11-tetraoxa-6,14-diaza-cyclohexadecane-2,10-dithione (2). Compounds have been characterized by using microanalysis, standard spectroscopic and crystallographic techniques. A plausible mechanism for the formation of compound 2 has been established based on experimental evidence. Compounds 1 and 2 fluoresces at 377 and 293 nm upon excitation at 314 and 232 nm, respectively. The single crystal X-ray diffraction (SCXRD) study revealed a distorted square planar coordination geometry around Ni^II-centre in 1 which is consistent in the solution of non-coordinating solvents such as CH₂Cl₂. In a coordinating solvent such as DMSO, however, charge transfer bands 421, 481 nm vanished and characteristic d-d transition band at 698 nm appeared which suggest a change in the coordination geometry around Ni^II centre from square planar (in CH₂Cl₂) to octahedral complex (in DMSO). This solvatochromic behaviour of Ni^II-xanthate complex 1 is supplemented by reversible thermochromic behaviour over a wide temperature ca 30–90°C. Notably, thermal degradation of Ni^II-xanthate complex 1 gave a stable residual mass that corresponds to NiS. The X-band EPR, UV–visible and TD-DFT studies and electrochemical investigations have been performed to rationalize the results and establish the structure-property correlation.