Pradip Bag

Synthesis and structural characterization of lanthanide(III) nitrate complexes of a tetraiminodiphenol macrocycle in the solid state and in solution

The lanthanide(III) complexes [Ln(LH2)(NO3)3] 1-11(La-Er), 15(Y) and [Ln(LH2)(NO3)2(H2O)](NO3) 12-14 (Tm-Lu) of the tetraiminodiphenolate macrocycle L2- have been prepared by the transmetallation reaction between [Pb(LH2)(NO3)2] and Ln(NO3)3.nH2O. In these compounds, the uncoordinated imino nitrogens are protonated and are hydrogen bonded to the phenolate oxygens. The X-ray crystal structures of the La (1), Ho (10) and Lu (14) compounds have been determined. Compounds 1 and 10, in which all the three nitrates are bound in bidentate fashion, are isostructural with distorted bicapped square antiprism geometry for the metal centre. In [Lu(LH2)(NO3)2(H2O)](NO3) 14, of the two metal bound nitrates one is bidentate and the other is unidentate, while the metal centre obtains a distorted square antiprism coordination environment. Proton NMR spectra of the paramagnetic lanthanide complexes have been studied in detail. Contributions of contact and pseudo-contact shifts to the lanthanide induced isotropic shifts (LIS) of the macrocycle protons have been separated and good agreement has been obtained between the calculated LIS values and the experimentally observed values. Analysis of the NMR data has led us to conclude that all the complexes in dimethyl sulfoxide solution attain similar configurations. The absorption and emission spectral characteristic of several compounds have been investigated. The complexes of samarium (5) and europium (6) on photoexcitation at 400 nm exhibit well-resolved luminescence spectra at 77 K both in the solid state and a methanol-ethanol (1 : 4) glassy matrix. For the terbium (8) and dysprosium (9) complexes, however, the observed luminescence peaks are less resolved and weak in intensity.

Mononuclear AlIII, GaIII and InIII, and heterodinuclear GaIIIMII(M = Zn, Cu, Ni, Co) complexes of a tetraiminodiphenol macrocyclic ligand

Mononuclear aluminium(III), gallium(III) and indium(III) complexes [MIII(LH2)(H2O)Cl](ClO4)2·nH2O (1–3) of the tetraiminodiphenolate macrocyclic ligand L2− have been synthesized. The two uncoordinated imine nitrogens of these compounds are protonated and hydrogen bonded to the metal-bound phenolate oxygens to stabilize them against hydrolytic cleavage. The gallium(III) compound, 2, forms the heterodinuclear complexes [GaIIIMIIL(μ-OAc)(OAc)(H2O)][ClO4] (M = Co, 4; Ni, 5) and [GaIIIMIIL(μ-OAc)(OAc)][ClO4]·2H2O (M = Cu, 6; Zn, 7). The presence of the core unit [GaIIIMIIL(μ-OAc)(OAc)]+ in all the compounds has been ascertained by ESI-MS measurements. Complexes 1–3 show luminescence at room temperature between 505 and 490 nm. Spectrofluorimetric titrations of 2 with Zn(OAc)2·2H2O and Cu(OAc)2·H2O in acetonitrile have shown that the formation of a 1 ∶ 1 GaIIIZnII complex occurs with the growth of luminescence intensity, while the formation of a GaIIICuII complex leads to the complete quenching of luminescence. Complexes 1–7 have been further characterized by their proton NMR spectra.

Synthesis, spectroscopic properties and proton-coupled redox activities of mononuclear osmium(II) mixed-chelates derived from pyrazole-3,5-bis(benzimidazole) and 2,2'-bipyridine

Abstract The mixed-ligand osmium(II) complexes [(bpy)2Os(H3pzbzim)](ClO4)2·2H2O (1) and [(bpy)2Os(Hpzbzim)]·2H2O (2), where H3pzbzim=pyrazole-3,5-bis(benzimidazole) and bpy=2,2′-bipyridine, have been synthesized and spectroscopically characterized. In complex (2), the NH deprotonation occurs at the pyrazole and at one of the metal-bound benzimidazole sites. As compared to complex (1), which undergoes reversible oxidation at 0.48 V (vs. Ag/AgCl) in acetonitrile, the redox potential of (2) is shifted to a much lower value, 0.14 V. The proton-coupled redox activities of the complexes have been studied over the pH range 1–12 in acetonitrile–water (3:2). From the pH versus E1/2 profile, the equilibrium constants of the complex species in the protonated/deprotonated forms and the metal ion in +2/+3 oxidation states have been determined. The pKa values of the complexes in the +2 state have also been determined spectrophotometrically. The spectrophotometric titration of complex (1) with cerium(IV) has revealed the disappearance of the more intense and higher energy OsIIdπ→π*(bpy) MLCT band and the appearance of the less intense and lower energy π(H3pzbzim)→OsIIIdπ LMCT band. The luminescence spectrum of (1) in methanol–ethanol at 77 K has shown it to be a weak emitter, while no emission is observed for (2).

Organized assemblies of lead(II) complexes of a tetraiminodiphenol macrocyclic ligand: manifestation of weak metal–anion interactions and the directional influence of anions

The syntheses and crystal structures of the lead(II) complexes [Pb(LH2)(ClO4)][ClO4], [Pb(LH2)(NO3)2] and [Pb2L(NO3)2] of the tetraiminodiphenol macrocyclic ligand (H2L) derived from a [2+2] condensation reaction between 2,6-diformyl-4-methylphenol and 1,3-diaminopropane are reported. In the mononuclear complexes, the two uncoordinated imino nitrogens are protonated and are hydrogen bonded to the phenolate oxygens. A supramolecular assembly occurs for [Pb(LH2)(ClO4)][ClO4], due to weak interactions between the metal and three oxygen atoms of three different symmetry-related perchlorates, thereby forming a hexameric species with a propeller structure. [Pb(LH2)(NO3)2], however, is a monomer with normal bidentate binding modes for the nitrates. By contrast, [Pb2L(NO3)2] exhibits a 2-D structural network comprising parallel chains from two independent [Pb2L]2+ units, to which the nitrate anions are associated rather unconventionally: one oxygen is coordinated to two symmetry-related metal centres, another oxygen to a single lead, while the third oxygen remains free. The structural features of the complexes in solution have been investigated by 1H NMR spectroscopy.