Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

Abstract

Salen-type metal complexes have been actively studied for their nonlinear optical (NLO) properties, and push-pull compounds with charge asymmetry generated by electron releasing and withdrawing groups have shown promising results. As a continuation of our research in this field and aiming at solid-state features, herein we report on the synthesis of mononuclear copper(II) derivatives bearing either tridentate N2O Schiff bases L(a−c)− and pyridine as the forth ancillary ligand, [Cu(La−c)(py)](ClO4) (1a–c), or unsymmetrically-substituted push-pull tetradentate N2O2 Schiff base ligands, [Cu(5-A-5′-D-saldpen/chxn)] (2a–c), both derived from 5-substituted salicylaldehydes (sal) and the diamines (1R,2R)-1,2-diphenylethanediamine (dpen) and (1S,2S)-1,2-diaminocyclohexane (chxn). All compounds were characterized through elemental analysis, infrared and UV/visible spectroscopies, and mass spectrometry in order to guarantee their purity and assess their charge transfer properties. The structures of 1a–c were determined via single-crystal X-ray diffraction studies. The geometries of cations of 1a–c and of molecules 2a–c were optimized through DFT calculations. The solid-state NLO behavior was measured by the Kurtz–Perry powder technique @1.907 µm. All chiral derivatives possess non-zero quadratic electric susceptibility (χ(2)) and an efficiency of about 0.15–0.45 times that of standard urea.

Studies on the Effect of Diamine Elongation in Copper(II) Complexes with NNO Tridentate Schiff Base Ligands

“…The solvated cation [Cu( G L2 H,H )(H 2 O) n ] + (with n from 1 to 3) is expected to be the predominant species due to labilization of the monodentate chloride ion by water. In support of this view, which is also in line with what was observed for A1 – A3 [4], the solvated [Cu( NO2 L1 H,H )(H 2 O)](ClO 4 )⋅H 2 O [1] and the homologous with the chiral 1,2-diphenylethylenediamine [3] could also be fully characterized in the solid state, thus confirming the existence of the solvated species.…”

“…The chemical stability of the compounds once dissolved in aqueous solution in the μmol L −1 regime is crucial for understanding the potential application as cytotoxic agents [23]. Copper(II) derivatives with Schiff base ligands absorb in the UV/Vis region with a characteristic profile given by a convolution of ligand-to-metal charge transfer (LMCT) and π → π* transitions at about 300–400 nm [3]. Thus, similarly to A1 – A3 [4], the stability of A4 and A5 in mimicked physiological conditions, 37 °C in darkness in saline phosphate-buffer solution (PBS) at pH = 7.4, was investigated by means of UV/Vis spectroscopy over a period of 72 h (Figure 3).…”

Studies on the Effect of Diamine Elongation in Copper(II) Complexes with NNO Tridentate Schiff Base Ligands

“…The solvated cation [Cu( G L2 H,H )(H 2 O) n ] + (with n from 1 to 3) is expected to be the predominant species due to labilization of the monodentate chloride ion by water. In support of this view, which is also in line with what was observed for A1 – A3 [4], the solvated [Cu( NO2 L1 H,H )(H 2 O)](ClO 4 )⋅H 2 O [1] and the homologous with the chiral 1,2-diphenylethylenediamine [3] could also be fully characterized in the solid state, thus confirming the existence of the solvated species.…”

“…The chemical stability of the compounds once dissolved in aqueous solution in the μmol L −1 regime is crucial for understanding the potential application as cytotoxic agents [23]. Copper(II) derivatives with Schiff base ligands absorb in the UV/Vis region with a characteristic profile given by a convolution of ligand-to-metal charge transfer (LMCT) and π → π* transitions at about 300–400 nm [3]. Thus, similarly to A1 – A3 [4], the stability of A4 and A5 in mimicked physiological conditions, 37 °C in darkness in saline phosphate-buffer solution (PBS) at pH = 7.4, was investigated by means of UV/Vis spectroscopy over a period of 72 h (Figure 3).…”

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

“…The efficacy of this method, which takes advantage of the thermodynamic template effect [67], relies on two features: (i) copper prefers a square planar geometry, and (ii) the ancillary ligand L' can saturate the coordination sites in the intermediate [Cu(G-sal)(L') 2 ] + in solution and in the final product [Cu( G L)(L')](X) in the solid state to ensure such a coordination environment (Figure 2). Indeed, the crystal structure of several derivatives has been obtained during the years [39,[65][66][67][68]70], and all of them show the copper(II) ion surrounded by the N 2 O ligand G Land a molecule of auxiliary ligand L' in the equatorial plane. The Cu-O bond distances usually settle around 1.90 Å, while the Cu-N ones are usually longer and depend on the nitrogen atom involved.…”

“…Copper (II) has revealed to be a very versatile and efficient templating metal ion for obtaining tridentate N 2 O half units, and several approaches have been described for the last four decades. The first method here described employs an auxiliary monodentate ligand L’ in the reaction process to isolate the mononuclear complexes of general formula [Cu( G L)(L’)](X) (X – = anion) [39,53,62,63,64,65,66,67,68,69,70]. The reaction occurs at room temperature in methanol, and the sequence of addition of the reagents in the reaction mixture is fundamental, usually entailing copper(II) salt, G-salH, and L’ first in a 1:1:3 ratio, and then the aliphatic diamine.…”

“…The auxiliary ligands L’ usually employed are pyridine (py) [39,53,62,63,65,66,68,69,70], imidazole (imH) [62,66], and piperidine (pip) [67,69], while the copper(II) salts are nitrate [62,63] or perchlorate [39,53,62,63,65,66,68,69,70]. The py/ClO 4 – combination is among the most applied since it probably ensures the highest purity and yield of the isolated products.…”

Studies on the Effect of Diamine Elongation in Copper(II) Complexes with NNO Tridentate Schiff Base Ligands

“…The solvated cation [Cu( G L2 H,H )(H 2 O) n ] + (with n from 1 to 3) is expected to be the predominant species due to labilization of the monodentate chloride ion by water. In support of this view, which is also in line with what was observed for A1 – A3 [4], the solvated [Cu( NO2 L1 H,H )(H 2 O)](ClO 4 )⋅H 2 O [1] and the homologous with the chiral 1,2-diphenylethylenediamine [3] could also be fully characterized in the solid state, thus confirming the existence of the solvated species.…”

“…The chemical stability of the compounds once dissolved in aqueous solution in the μmol L −1 regime is crucial for understanding the potential application as cytotoxic agents [23]. Copper(II) derivatives with Schiff base ligands absorb in the UV/Vis region with a characteristic profile given by a convolution of ligand-to-metal charge transfer (LMCT) and π → π* transitions at about 300–400 nm [3]. Thus, similarly to A1 – A3 [4], the stability of A4 and A5 in mimicked physiological conditions, 37 °C in darkness in saline phosphate-buffer solution (PBS) at pH = 7.4, was investigated by means of UV/Vis spectroscopy over a period of 72 h (Figure 3).…”

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

“…The efficacy of this method, which takes advantage of the thermodynamic template effect [67], relies on two features: (i) copper prefers a square planar geometry, and (ii) the ancillary ligand L' can saturate the coordination sites in the intermediate [Cu(G-sal)(L') 2 ] + in solution and in the final product [Cu( G L)(L')](X) in the solid state to ensure such a coordination environment (Figure 2). Indeed, the crystal structure of several derivatives has been obtained during the years [39,[65][66][67][68]70], and all of them show the copper(II) ion surrounded by the N 2 O ligand G Land a molecule of auxiliary ligand L' in the equatorial plane. The Cu-O bond distances usually settle around 1.90 Å, while the Cu-N ones are usually longer and depend on the nitrogen atom involved.…”

“…Copper (II) has revealed to be a very versatile and efficient templating metal ion for obtaining tridentate N 2 O half units, and several approaches have been described for the last four decades. The first method here described employs an auxiliary monodentate ligand L’ in the reaction process to isolate the mononuclear complexes of general formula [Cu( G L)(L’)](X) (X – = anion) [39,53,62,63,64,65,66,67,68,69,70]. The reaction occurs at room temperature in methanol, and the sequence of addition of the reagents in the reaction mixture is fundamental, usually entailing copper(II) salt, G-salH, and L’ first in a 1:1:3 ratio, and then the aliphatic diamine.…”

“…The auxiliary ligands L’ usually employed are pyridine (py) [39,53,62,63,65,66,68,69,70], imidazole (imH) [62,66], and piperidine (pip) [67,69], while the copper(II) salts are nitrate [62,63] or perchlorate [39,53,62,63,65,66,68,69,70]. The py/ClO 4 – combination is among the most applied since it probably ensures the highest purity and yield of the isolated products.…”

Studies on the Effect of Diamine Elongation in Copper(II) Complexes with NNO Tridentate Schiff Base Ligands

“…The solvated cation [Cu( G L2 H,H )(H 2 O) n ] + (with n from 1 to 3) is expected to be the predominant species due to labilization of the monodentate chloride ion by water. In support of this view, which is also in line with what was observed for A1 – A3 [4], the solvated [Cu( NO2 L1 H,H )(H 2 O)](ClO 4 )⋅H 2 O [1] and the homologous with the chiral 1,2-diphenylethylenediamine [3] could also be fully characterized in the solid state, thus confirming the existence of the solvated species.…”

“…The chemical stability of the compounds once dissolved in aqueous solution in the μmol L −1 regime is crucial for understanding the potential application as cytotoxic agents [23]. Copper(II) derivatives with Schiff base ligands absorb in the UV/Vis region with a characteristic profile given by a convolution of ligand-to-metal charge transfer (LMCT) and π → π* transitions at about 300–400 nm [3]. Thus, similarly to A1 – A3 [4], the stability of A4 and A5 in mimicked physiological conditions, 37 °C in darkness in saline phosphate-buffer solution (PBS) at pH = 7.4, was investigated by means of UV/Vis spectroscopy over a period of 72 h (Figure 3).…”

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

“…The efficacy of this method, which takes advantage of the thermodynamic template effect [67], relies on two features: (i) copper prefers a square planar geometry, and (ii) the ancillary ligand L' can saturate the coordination sites in the intermediate [Cu(G-sal)(L') 2 ] + in solution and in the final product [Cu( G L)(L')](X) in the solid state to ensure such a coordination environment (Figure 2). Indeed, the crystal structure of several derivatives has been obtained during the years [39,[65][66][67][68]70], and all of them show the copper(II) ion surrounded by the N 2 O ligand G Land a molecule of auxiliary ligand L' in the equatorial plane. The Cu-O bond distances usually settle around 1.90 Å, while the Cu-N ones are usually longer and depend on the nitrogen atom involved.…”

“…Copper (II) has revealed to be a very versatile and efficient templating metal ion for obtaining tridentate N 2 O half units, and several approaches have been described for the last four decades. The first method here described employs an auxiliary monodentate ligand L’ in the reaction process to isolate the mononuclear complexes of general formula [Cu( G L)(L’)](X) (X – = anion) [39,53,62,63,64,65,66,67,68,69,70]. The reaction occurs at room temperature in methanol, and the sequence of addition of the reagents in the reaction mixture is fundamental, usually entailing copper(II) salt, G-salH, and L’ first in a 1:1:3 ratio, and then the aliphatic diamine.…”

“…The auxiliary ligands L’ usually employed are pyridine (py) [39,53,62,63,65,66,68,69,70], imidazole (imH) [62,66], and piperidine (pip) [67,69], while the copper(II) salts are nitrate [62,63] or perchlorate [39,53,62,63,65,66,68,69,70]. The py/ClO 4 – combination is among the most applied since it probably ensures the highest purity and yield of the isolated products.…”