Group 12 metal complexes of (2-piperazine-1-yl-ethyl)-pyridin-2-yl-methylene-amine: rare participation of terminal piperazine N in coordination leads to structural diversity

Abstract

By using a potential tridentate ligand L ((2-piperazine-1-yl-ethyl)-pyridin-2-yl-methylene-amine), a series of group 12 metal complexes namely, [ZnLHCl][ZnLCl]·2HO (1), [CdL(SCN)(CHOH)] (2), and [Hg(l-pyCO)Cl] (3), were synthesized and structurally characterized. In all the complexes the piperazine nitrogen of the ligand takes part in coordination and leads to the complexes of group 12 metal ions having structural diversity. The X-ray diffraction analysis of complex 1 indicates for one Zn(ii) ion a geometry in between trigonal bipyramidal/square pyramidal and for the second a distorted tetrahedral sphere. In the polymeric complex 2 the Cd(ii) ion shows a distorted octahedral environment, while in the mononuclear complex 3, where Hg(ii) exhibits a square-pyramidal geometry, an unexpected condensation between the uncoordinated NH piperazine fragment with 2-pyridinecarboxaldehyde was detected. The M-N bond lengths in all the complexes are in accordance with the metal ionic radius. Continuous shape measures through a DFT approach provide the coordination environment around each metal centre that is comparable with the experimental observations. We have also investigated the importance of hydrogen bonding of methanol in the generation of the polymeric Cd complex 2 along with the rearrangement of the tridentate ligand to generate an octahedral complex. The photoluminescence properties of the complexes as well as of the ligand were investigated in solution at ambient temperature. The low quantum yield of the ligand was ascribed due to a very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated pyridine moiety. Complexation prevents the electron transfer, and consequently an increase in quantum yield was observed in the complexes. Among the three complexes the highest photoluminescence was exhibited by a Zn complex, being lower in Cd and Hg complexes as a consequence of the heavy atom perturbation effect.

Hexanuclear Co4Dy2, Zn4Dy2, and Co4Y2 Complexes with Defect Tetracubane Cores: Syntheses, Structures, and Magnetic Properties

“…The relaxations of the Orbach, Raman, and Quantum tunneling processes were taken into account for 2 and Orbach and Quantum tunneling processes were considered for 1 (Figure S15) [65–66] . The most accurate fit of Equation (2) across the whole temperature range provided U eff =25.92 K, τ 0 =5.23×10 −8 s for 1 .…”

A Chemodosimetric Approach for Fluorimetric Detection of Hg2+ Ions by Trinuclear Zn(II)/Cd(II) Schiff Base Complex: First Case of Intermediate Trapping in a Chemodosimetric Approach

“…The ligand framework becomes rigid because of complexation with metal ions that reduces the nonradiative channels, thereby showing an increment in fluorescence intensity as well as quantum yield. The difference in quantum yield of the three complexes can be rationalized considering the heavy atom perturbation effect. , …”

“…The difference in quantum yield of the three complexes can be rationalized considering the heavy atom perturbation effect. 48,49 To get details on the nature of interaction between the ligand and cations, fluorescence titration has been performed. Upon addition of Zn 2+ and Cd 2+ ions separately to the methanolic solution of the ligand, emission intensity gradually increases with a blue shift in emission maxima (Figure S18, Supporting Information).…”

“…The group 12 metals (Zn, Cd, and Hg) have the d 10 configuration and generally exist as divalent ions but not only also as monovalent Hg 2 2+ ion via a Hg–Hg bond (2.5 Å). However, most of the mercury complexes exist with +2 metal oxidation state because of the lower reactivity and poor purity of the Hg­(I) complexes, and only few Hg 2 2+ complexes have been structurally reported to date. − There are several reports on the photophysical properties and relative binding affinity of some Zn­(II) and a few examples of Cd­(II) and Hg­(II) − complexes in other type of ligand studies. Rational design of fluorescent chemosensors remains a challenge in terms of selectivity and sensitivity.…”

Influence of a series of pyridine ligands on the structure and photophysical properties of Cd(ii) complexes

“…19,20 In this scenario, group 12 metal ions emerge as good candidates for the synthesis of efficient fluorescent complexes owing to the absence of potential quenching processes derived from d-d transitions and their zero-crystal field stabilization energy (CFSE), which offer a wide variety of possible geometries, making them ideal building blocks. 21,22 Among them, Cd(II) stands out as the most promising metal ion in this group given that it presents a similar ionic radius to that of Hg(II), but its electronegativity resembles that of Zn(II). 23,24 These characteristics permit it to maximize CHEF, while preventing the heavy atom effect.…”