Anion directed structural diversity in zinc complexes with conformationally flexible quinazoline ligand: structural, spectral and theoretical studies

Abstract

In this paper, we report the synthesis, structure and photophysical studies of four new complexes of conformationally flexible 6-chloro-4-phenyl-2-(pyridin-2-yl)quinazoline ligand (L) with Zn(II). The coordinating ability of the ligand and geometrical preferences of the resultant complexes are tuned by varying the anion of the metal salt as confirmed by structural and DFT studies. The choice of the metal salt (especially anion) directs the stabilisation of different conformations of the ligand arising out of the twisting of pyridyl ring with respect to quinazoline ring, resulting in complexes with different nuclearities (monomer/dimer) as well as different coordination geometries (tetrahedral/trigonal bipyramidal/octahedral). Photophysical properties are also found to be tuned due to conformational change on complexation. DFT studies on the ligand establish the conformationally stable forms as observed in the reported structures. IntroductionDesign and synthesis of transition metal complexes with structural diversities have become a fascinating area of contemporary research due to their possible applications in various fields such as heterogeneous catalysis, 1 nonlinear optical activity, 2 molecular sensors, 3 magnetic switches, 4 and micro porous materials. 5 The design of these structures is highly influenced by several key factors among others, the coordination geometry, the size of the metal ion, the structural characteristics of the ligand and the nature of the counter anion, for the creation of new functional materials with solvent inclusion or gas-adsorption characteristics and special optical, electronic, magnetic and catalytic properties. 6The d 10 configuration of zinc allows architecting flexible coordination environments and forms metal complexes with tetrahedral to octahedral geometries, which is a very important criterion for designing a wide variety of metal-organic frameworks, 7 fluorescent frameworks being one lead application. The presence of electronically saturated Zn(II) ion is advantageous for the design of fluorescent systems, because it imposes conformational rigidity to the ligand and prevents energy loss via bond vibration and electron transfer processes, whereas paramagnetic transition metals may quench fluorescence by different electron/energy transfer mechanisms. [8][9][10] Hence, zinc complexes with variable fluorescence properties have attracted much attention due to their potential applications in biological systems, 11 photophysical properties 12 and in emitting materials of organic light emitting diodes. 13 Moreover, inorganic anions can be considered as : Zn(NO 3 ) 2 ·6H 2 O (0.5949 g, 2.00 mmol) was used instead of Zn(CH 3 COO) 2 ·2H 2 O, the remaining procedure being the same as reported for complex 1. Single crystals, suitable for x-ray diffraction, were obtained after 2 days by a slow layer diffusion of methanol into a DMSO solution of the complex at room temperature (25 ºC). Yield: 0.9883 g Zn-salt directed complex formation is monitored in all the complexes 1-4. It is inte...

Conformationally Restricted Triarylmethanes: Synthesis, Photophysical Studies, and Applications

“…[7] Various condensed polynuclear aromatic resins are prepared based on this molecular scaffold. [9,10] To study the effect of this decrement in the conformational freedom and the properties that arise due to this restriction, scientists term the molecules with such reduced conformational mobility as "conformationally restricted analogues". Extended network of hydrogen bonds and other noncovalent interactions, steric interactions, cycles/polycycles present in a molecule, restrict the rotation of groups attached to a mol-ecule and hence decrease the conformational mobility enjoyed by the molecules.…”

“…[9,112,113] Due to an electrophilic mercuration (in presence of Hg 2+ in acetate buffer) of the aromatic rings in the 4, 5 position of (11a), the absorption bands of (11a) at 475 nm and 505nm gets red shifted to 483 nm and 533 nm respectively. [9,112,113] Due to an electrophilic mercuration (in presence of Hg 2+ in acetate buffer) of the aromatic rings in the 4, 5 position of (11a), the absorption bands of (11a) at 475 nm and 505nm gets red shifted to 483 nm and 533 nm respectively.…”

Photophysics and Photochemistry of Conformationally Restricted Triarylmethanes: Application as Photoredox Catalysts

“…Through various non-covalent interactions, extended networks of hydrogen bonds, steric interactions, cycles/polycycles present in a molecule, the rotation of groups attached to the central carbon are restricted thereby decreasing the conformational mobility enjoyed by the molecules. This change in the conformation of the molecule in general not only modulates different physical properties of the molecule but also influences different photophysical and photochemical properties of the molecule [16,17]. For example in open chain molecules due to rapid rotation of various bonds the vibrational relaxation of these molecules from the excited state to the ground state through non-radiative pathways are very rapid and these Chart 1.…”