Acid–Base Behavior of Substituted Hydrazone Complexes Controlled by the Coordination Geometry

Abstract

A new series of substituted hydrazone complexes, [Cu(Hpbph)I] (1), [Cu(Hpbph)PPh3]PF6 (2-PF6), [NiCl(Hpbph)]Cl (3-Cl), [PtCl(Hpbph)]ClO4 (4-ClO4), and [PtCl(pbph)] (4b) (Hpbph = 2-(diphenylphosphino)benzaldehyde 2-pyridylhydrazone) have been synthesized and characterized. X-ray crystallography revealed that the copper(I) complexes adopt pseudo-tetrahedral geometry, while the nickel(II) and platinum(II) complexes provide square-planar forms. All the complexes exhibit distinct color changes on the basis of the deprotonation/protonation on the ligand although their acid/base behaviors are largely different. The acidity constants (pKa) in methanol were determined to be 11.4 (1), 12.5 (2), 7.7 (3), and 6.7 (4). The results indicate that the dissociation of the proton on the ligand strongly depends on the ligand deformation controlled by the coordination geometry of the complexes and ancillary ligands also somewhat affect the acidity.

A family of readily synthesised phosphorescent platinum(ii) complexes based on tridentate N^N^O-coordinating Schiff-base ligands

“…It is well-established that deprotonation of free hydrazones occurs only under highly basic conditions, but that coordination to a metal ion causes a significant increase in the acidity of the N-H proton. 40 Complexes of substituted hydrazones R 1 -NH-N=CH-R 2 are known as colorimetric reagents and their acid-base behaviour is accompanied by remarkable colour changes. 41 Protonated and deprotonated forms can co-exist even in the crystalline state, for example, as reported for the Pd(II) complex of 2-(diphenylphosphine)benzaldehyde 2-pyridylhydrazone.…”

Coordination Structure Conversion of Hydrazone–Palladium(II) Complexes in the Solid State and in Solution

“…It was suggested that the absorption band in the visible region was mainly attributed to the intraligand π−π* transition of the coordinating hydrazone ligand ( Figure S3) The related copper(II), nickel(II), and platinum(II) complexes containing the deprotonated anionic hydrazonato ligand 2-[2-(diphenylphosphino)benzylidene]-1-(pyridin-2-yl)hydrazin-1-ide showed a characteristic absorption band in the visible region (420−506 nm), which were also attributed to intraligand transition of the hydrazone ligand. 27,28 Thermal and Photochemical Coordination Structure Conversion in Solution. Previously, copper(II) complexes containing similar hydrazone ligands of N′-[(pyridine-2-yl)-methylene]pyrazine-2-carbohydrazide were prepared, and three different forms of crystals that contain tetranuclear, 1D chiral chain, and 1D decker structures of the complexes were obtained.…”

Photoinduced Dimerization Reaction Coupled with Oxygenation of a Platinum(II)–Hydrazone Complex

“…As mentioned in the Introduction, we have previously reported the variation of the UV−vis spectral profile of Pt1 under various pH conditions in methanol and found that the absorption band centered around 530 nm, attributed to the ILCT transition, disappeared upon protonation of the hydrazone moiety. 20 It was also found that the color of the solution and the emission of Pt1 gradually changed under photoirradiation without the addition of acids. This change motivated us to investigate the photoreactivity of metal(II)− hydrazone complexes.…”

“…As previously reported, the Ni1 and Pt1 complexes showed relatively intense intraligand charge transfer (ILCT) transition bands at 533 and 530 nm, respectively, whereas the spectrum of hydrazone ligand HL showed only a strong absorption band in the UV region at 335 nm. 20 This difference suggests that the complexation reaction involving the deprotonation of the hydrazone moiety had a large effect on the electronic state of the hydrazone ligand. In addition, a weak absorption band at ∼645 nm was observed only in the case of the Ni1 complex and could be assigned as the d−d transition of the Ni 2+ center.…”