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Five mononuclear dioxidomolybdenum(VI) complexes of phloroglucinol and resorcinol derived ligands are reported. Two types of ligands are accessible through condensation of 2,4,6-trisacetyl-1,3,5-trihydroxybenzene (2,4,6-triacetylphloroglucinol) and benzoylhydrazide (H2ptk-bhz, I) or 2-furoylhydrazide (H2ptk-fah, II) and condensation of 4,6-diactylresorcinol and benzoylhydrazide (H2dar-bhz, III), 2-furoylhydrazide (H2dar-fah, IV) or nicotinoylhydrazide (H2dar-nah, V). Isolated complexes, [MoVIO2(ptk-bhz)(MeOH)] (1), [MoVIO2(ptk-fah)(MeOH)] (2), [MoVIO2(dar-bhz)(MeOH)] (3), [MoVIO2(dar-fah)(MeOH)] (4) and [MoVIO2(dar-nah)(MeOH)] (5) have been characterized by spectroscopic techniques like FT-IR, UV–Vis, 1H NMR and single crystal X-ray diffraction (for complexes 1, 3 and 5) analysis. These complexes have been tested for the epoxidation of alkenes in the presence of 30% H2O2 and NaHCO3 where latter one acts as an additive. Under optimized reaction conditions, a good conversion of alkenes along with their selective epoxidation and high turnover number was achieved.

Trinuclear cis-dioxidomolybdenum(VI) complexes of the type [{MoVIO2(MeOH)}3L1-7] (1–7) have been synthesized using tris(H2ONO) donor ligands [H6L1-7 (I–VII)] assembled from benzene-1,3,5-tricarbohydrazide (bthz) and the corresponding salicylaldehyde (sal). All the ligands and the complexes were characterized by numerous techniques, such as FT-IR, UV–visible, NMR (1H and 13C) spectroscopy, electrochemical study, elemental analysis, thermogravimetric study and single crystal X-ray diffraction of the ligand III and complexes 1 and 5. In the presence of H2O2 as an oxidant, these complexes show excellent catalytic potential towards the one-pot three-components [ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate] dynamic covalent assembly in the Hantzsch reaction. Under solvent free conditions, as high as 98% conversion along with 100% selectivity towards diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate (1,4-DHP) has been achieved in 1 h. Although solvents do not improve the conversion, they do influence the selectivity of the products. With the elapse of time, the conversion of dihydropyridine to the diethyl 2,6-dimethyl-4-phenylpyridine-3,5-dicarboxylate derivative occurs and completes in ca. 10 h with a distinct color change, showing the importance of the catalysts. Efforts have been made to provide suitable reaction pathways for the catalytic reaction based on spectroscopic and density functional theory studies.

Condensation of equimolar amount of 8-formyl-7-hydroxy-4-methylcoumarin (fhmc) and hydrazides [benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and furoylhydrazide (fah)] in methanol results in the formation of potential ONO tridentate ligands H2fhmc-bhz (I), H2fhmc-inh (II), H2fhmc-nah (III), and H2fhmc-fah (IV), respectively. Reaction of these ligands with [MoVIO2(acac)2] in refluxing methanol gives dioxidomolybdenum(VI) complexes, [MoVIO2(fhmc-bhz)(MeOH)] (1), [MoVIO2(fhmc-inh)(MeOH)] (2), [MoVIO2(fhmc-nah)(MeOH)] (3) and [MoVIO2(fhmc-fah)(MeOH)] (4), respectively. The structures of the obtained ligand and their respective metal complexes were elucidated by elemental analyses, spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermogravimetric analyses. Single crystal X-ray analysis of complex 1a confirms the chelation of the ligand to the metal ion through two oxygen and a nitrogen atoms. These metal complexes have been tested against oxidative bromination of monoterpene (thymol) to yield 2-bromothymol, 4-bromothymol and 2,4-dibromothymol using H2O2 as an oxidant therefore they act as functional models of vanadium dependent haloperoxidases. The effects of various factors, such as amounts of catalyst, oxidant, KBr, HClO4 and different solvents have been considered to optimize the reaction conditions for the maximum brominated products.

Four dioxidomolybdenum(VI) complexes, [Mo VI O 2 (dha-bhz)(MeOH)]·(H 2 dha-bhz) ( 1 ), [MoO 2 (dha-inh)(MeOH)] ( 2 ), [MoO 2 (dha-nah)(MeOH)] ( 3 ) and [MoO 2 (dha-fah)(MeOH)] ( 4 ), and their corresponding oxidoperoxidomolybdenum(VI) complexes, [MoO(O 2 )(dha-bhz)(MeOH)] ( 5 ), [MoO(O 2 )(dha-inh)(MeOH)] ( 6 ), [MoO(O 2 )(dha-nah)(MeOH)] ( 7 ) and [MoO(O 2 )(dha-fah)(MeOH)] ( 8 ), with ONO tridentate Schiff base ligands derived from 3-acetyl-6-methyl-(2H)-pyran-2,4(3H)-dione (dehydroacetic acid, Hdha) and aromatic hydrazides, {benzoyl hydrazide (bhz), isonicotinoyl hydrazide (inh), nicotinoyl hydrazide (nah) and furoyl hydrazide (fah)} have been synthesized and characterized by elemental analysis, spectroscopic techniques (infrared, UV–Vis, 1 H and 13 C NMR) and thermogravimetric analyses. The structures of complexes 1 and 2 , confirmed by single crystal X-ray study, reveal that the tridentate ligand binds to the metal centers through the enolic oxygen (of the pyrone group), azomethine nitrogen and enolic oxygen (of the hydrazide group) atoms; methanol is coordinated at one site through the oxygen atom. The dioxidomolybdenum(VI) complexes have been tested as catalysts for the homogeneous oxidation of secondary alcohols (1-phenyl ethanol, 2-propanol and 2-butanol), using 30% H 2 O 2 as an oxidant. Both microwave and conventional liquid phase oxidation methods have been tested for catalytic reactions. Various parameters, such as amount of catalyst, oxidant, solvent and temperature, of the reaction mixture have been taken into consideration for the maximum conversion of the substrates. Under the atmospheric optimized reaction conditions, secondary alcohols give high yields of the respective ketones selectively. Addition of an N-based additive reduces the time and increases the conversion of alcohols. Amongst the two methods studied, the microwave technique proves to be a time efficient system.

The reaction of three Schiff bases, H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III) [derived from the condensation of 2,6-diformyl-4-methylphenol (dfmp) with S-benzyldithiocarbazate (sbdt), S-methyldithiocarbazate (smdt) and thiosemicarbazide (tsc)], with [MoVIO2(acac)2] (Hacac = acetylacetone) on stirring in methanol results in the formation of the dioxidomolybdenum(VI) complexes [MoVIO2{Hdfmp(sbdt)2}(H2O)] (1), [MoVIO2{Hdfmp(smdt)2}(H2O)] (2) and [MoVIO2{Hdfmp(tsc)2}(H2O)] (3), respectively. All these complexes have been characterized in the solid state and in solution by elemental analyses, spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermogravimetric analyses. The structures of complexes 1, 2 and 3, confirmed by a single crystal X-ray study, reveal that only one set of azomethine nitrogen, enthiolate sulfur and phenolate oxygen atoms of the ligands are coordinated to the molybdenum centre. All the complexes form an antiparallel dimer through different π–π interactions. These molybdenum complexes catalyze the oxidation of styrene and cyclohexene using 30% H2O2 as an oxidant in the presence of sodium bicarbonate. Styrene, under optimized reaction conditions, gave two reaction products, namely phenylacetaldehyde as the major product and styrene oxide as the minor product. Oxidation of cyclohexene selectively gave 2-cyclohexene-1-ol.

[MoVIO2(acac)2] (Hacac = acetylacetone) reacts with the tribasic pentadentate Schiff bases H3sal-dahp (I), H3clsal-dahp (II) and H3brsal-dahp (III) (sal = salicylaldehyde, clsal = 5-chlorosalicylaldehyde, brsal = 5-bromosalicylaldehyde, dahp = 1,3-diamino-2-hydroxypropane) in methanol under refluxing conditions to yield the dioxidomolybdenum(VI) complexes [MoVIO2(Hsal-dahp)(H2O)] (1), [MoVIO2(Hclsal-dahp)(H2O)] (2) and [MoVIO2(Hbrsal-dahp)(H2O)] (3), respectively. In these complexes, only one set of phenolic oxygen and azomethine nitrogen atoms, along with the alcoholic oxygen atom of the ligands coordinate to the molybdenum. The reactions of these complexes with pyridine result in the formation of [MoVIO2(Hsal-dahp)(py)] (4), [MoVIO2(Hclsal-dahp)(py)] (5) and [MoVIO2(Hbrsal-dahp)(py)] (6). A single crystal X-ray study of [MoVIO2(Hbrsal-dahp)(DMSO)] (3a), grown by slow evaporation of a DMSO solution of 3, confirms the non-participation of one set of phenolic oxygen and azomethine nitrogen atoms of the ligand. The reaction of a methanolic solution of the in situ generated H3clsal-dahp with [MoVIO2(acac)2] in DMSO gave [MoVIO2(Hclsal-hdap)(DMSO)]4[Mo8O26]·6DMSO (7) (H2clsal-hdap = Schiff base obtained by the 1:1 condensation of 5-chlorosalicylaldehyde and 1,3-diamino-2-hydroxypropane), where the non-coordinated azomethine nitrogen hydrolyzes giving a free protonated amine group, as confirmed by a single crystal X-ray study. Complexes 1, 2 and 3 catalyze the oxidative bromination of styrene to yield 1,2-dibromo-1-phenylethane, 1-phenylethane-1,2-diol and 2-bromo-1-phenylethane-1-ol; therefore they act as functional models of vanadium dependent haloperoxidases. It has also been demonstrated that complexes 1, 2 and 3 are catalyst precursors for the oxidation of methyl phenyl sulfide and benzoin.

The monobasic tridentate ONN donor ligand 1-(2-pyridylazo)-2-naphthol [Hpan (I)] reacts with [VIVO(acac)2] in dry methanol to yield the oxidovanadium(IV) complex [VIVO(acac)(pan)] (1). The dioxidovanadium(V) complex [{VVO(pan)}2(μ-O)2] (2) is obtained by aerial oxidation of 1 in methanol. Complex 2 can also be prepared directly by reacting [VIVO(acac)2] with I followed by aerial oxidation in methanol. Treatment of 1 or 2 in methanol with H2O2 yields the oxidomonoperoxidovanadium(V) complex [VVO(O2)(pan)(MeOH)] (3). Reaction of imidazolomethylpolystyrene cross-linked with 5% divinylbenzene (PS-im) with 2 in DMF resulted in the formation of the polymer-grafted dioxidovanadium(V) complex PS-im[VVO2(pan)] (4). All these complexes are characterized by various spectroscopic techniques (IR, electronic, NMR (1H and 51V) and electron paramagnetic resonance (EPR)), thermal, field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDX) studies. The crystal and molecular structure of 3 has been determined, confirming the ONN binding mode of I. The polymer-grafted complex 4 has been used for the oxidative bromination of styrene, salicylaldehyde and trans-stilbene. Various parameters, such as amounts of catalyst, oxidant (aqueous 30% H2O2), KBr and aqueous 70% HClO4 have been optimized to obtain the maximum oxidative bromination of the substrates. Under the optimized reaction conditions, styrene gave a maximum of 99% conversion after 2 h of reaction, with the main products having a selectivity order of: 1-phenylethane-1,2-diol (75%) > 2-bromo-1-phenylethane-1-ol (20%) > 1,2-dibromo-1-phenylethane (1.2%). With nearly same conversion in same time, the oxidative bromination of salicylaldehyde gave three products with the selectivity order: 5-bromosalicylaldehyde > 2,4,6-tribromophenol > 3,5-dibromosalicylaldehyde. A maximum of 91% conversion of trans-stilbene has been obtained in 2 h of reaction time, where selectivity of the obtained reaction products varied in the order: 2,3-diphenyloxirane (trans-stilbene oxide) > 1,2-dibromo-1,2-diphenylethane > 2-bromo-1,2-diphenylethanol. The catalytic activity of the non-polymer grafted complex 2 is lower than that of the polymer-grafted one. In addition, the recycling ability of the grafted complex makes it better over the neat complex.

Reaction of [VIVO(acac)2] with H3hap-dahp (I: Schiff base derived from 2-hydroxyacetophenone and 1,3-diamino-2-hydroxypropane) in methanol under aerobic conditions resulted in the formation of the complex [VVO(hap-dahp)] (1). Treatment of I with vanadium(IV) exchanged zeolite Y followed by aerial oxidation gave the encapsulated oxidovanadium(V) complex [VVO(hap-dahp)]-Y (2) in the nanocavity of zeolite Y. Elemental analysis, spectroscopic (IR, electronic, 1H and 51V NMR) studies, scanning electron micrographs and X-ray diffraction patterns have been used to characterize these complexes, which have a distorted octahedral structure, confirming the tribasic O3N2 binding mode of the ligand. The encapsulated complex [VVO(hap-dahp)]-Y (2) catalyzes the oxidation of styrene, methyl phenyl sulfide and diphenyl sulfide using H2O2 as an oxidant in good yield (82%, 98% and 81%, respectively). Styrene, under optimized reaction conditions, gave five reaction products, namely styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and phenylacetaldehyde, while the organic sulfides gave the corresponding sulfoxides as the major product. The neat complex [VVO(hap-dahp)] (1) also exhibits good catalytic activity for these substrates. The encapsulated complex showed a good recycling ability.

The reaction of Mn II (ClO 4 ) 2 with 2-[2-(1H-(benzo[d]imidazol-2-yl)ethylimino)methyl]phenol (Hsal-aebmz, I ) under aerobic conditions results in the formation of [Mn III (sal-aebmz) 2 ]·ClO 4 ( 1 ). In the presence of triethylamine and using MnCl 2 under aerobic conditions, I forms [Mn III (sal-aebmz-H)(sal-aebmz)] ( 2 ). These complexes are characterised on the basis of elemental and electrochemical analyses, spectroscopic (IR and UV–Vis) data and thermogravimetric studies. Single crystal X-ray analysis of 2 shows that it is stabilized in the solid state through intermolecular hydrogen bonding between the NH groups of the two benzimidazole moieties after losing one of the hydrogen atoms; the coordination sites of the ligand are the imine nitrogen of the benzimidazole ring, the azomethine nitrogen and the deprotonated phenolic oxygen. Complex 2 catalyses the oxidative bromination of styrene by H 2 O 2 , yielding 1,2-dibromo-1-phenylethane, 1-phenylethane-1,2-diol and 2-bromo-1-phenylethane-1-ol, acting as a functional mimic of haloperoxidases.

Reaction of MnII(CH3COO)2 with dibasic tetradentate ligands, N,N′-ethylenebis(pyridoxylideneiminato) (H2pydx-en, I), N,N′-propylenebis(pyridoxylideneiminato) (H2pydx-1,3-pn, II) and 1-methyl-N,N′-ethylenebis(pyridoxylideneiminato) (H2pydx-1,2-pn, III) followed by aerial oxidation in the presence of LiCl gives complexes [MnIII(pydx-en)Cl(H2O)] (1) [MnIII(pydx-1,3-pn)Cl(CH3OH)] (2) and [MnIII(pydx-1,2-pn)Cl(H2O)] (3), respectively. Crystal and molecular structures of [Mn(pydx-en)Cl(H2O)] (1) and [Mn(pydx-1,3-pn)Cl(CH3OH)] (2) confirm their octahedral geometry and the coordination of ligands through ONNO(2-) form. Reaction of manganese(II)-exchanged zeolite-Y with these ligands in refluxing methanol followed by aerial oxidation in the presence of NaCl leads to the formation of the corresponding zeolite-Y encapsulated complexes, abbreviated herein as [MnIII(pydx-en)]-Y (4), [MnIII(pydx-1,3-pn)]-Y (5) and [MnIII(pydx-1,2-pn)]-Y (6). These encapsulated complexes are used as catalysts for the oxidation, by H2O2, of methyl phenyl sulfide, styrene and benzoin efficiently. Oxidation of methyl phenyl sulfide under the optimized reaction conditions gave ca. 86% conversion with two major products methyl phenyl sulfoxide and methyl phenyl sulfone in the ca. 70% and 30% selectivity, respectively. Oxidation of styrene catalyzed by these complexes gave at least five products namely styrene oxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde with a maximum of 76.9% conversion of styrene by 4, 76.3% by 5 and 76.0% by 6 under optimized conditions. The selectivity of the obtained products followed the order: benzaldehyde > benzoic acid > styrene oxide > phenylacetaldehyde > 1-phenylethane-1,2-diol. Similarly, ca. 93% conversion of benzoin was obtained by these catalysts, where the selectivity of the products followed the order benzil > benzoic acid > benzaldehyde-dimethylacetal. Tests for the recyclability and heterogeneity of the reactions have also been carried. Neat complexes are equally active. However, the recycle ability of encapsulated complexes makes them better over neat ones.

The hepta- and octa-dentate ligands N , N ′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 ( L 1 ) and N , N ′-bis(2-aminobenzyl)-1,10-diaza-18-crown-6 ( L 2 ), respectively, form stable mononuclear Mn(II) complexes. Spectrophotometric titrations performed in acetonitrile solution indicate the formation of mononuclear Mn(II) complexes with both ligands, and no evidence for the formation of binuclear complexes was obtained. The optimal architecture of L 1 allows it to impose the less usual pentagonal bipyramidal geometry on the Mn(II) guest, and the X-ray crystal structure of [Mn(L 1 )](ClO 4 ) 2 shows that the Mn(II) ion is deeply buried in the receptor cavity, coordinated to the seven available donor atoms, with the perchlorate anions remaining outside the metal coordination sphere. In spite of its higher denticity, the receptor L 2 is unable to form the expected binuclear complexes. The X-ray crystal structure of [Mn(L 2 )](NO 3 ) 2 consists of the [Mn(L 2 )] 2+ cation and nitrate anions involved in hydrogen-bonding interactions with the aniline groups. In [Mn(L 2 )] 2+ the metal ion is also placed in the crown hole, but as a result of the large size of the macrocyclic cavity only six of the eight available donor atoms of the receptor form part of the Mn(II) coordination sphere, with the Mn(II) ion found in a distorted octahedral coordination environment.

Previously we have proved that barium perchlorate is a highly effective template agent in the formation of the Schiff-base lateral macrobicycles L1 and L2 (receptors derived from the condensation of N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 with 2,6-diformylpyridine and 2,6-diformyl-4-methylphenol, respectively). Aiming to understand the effect that the nature of the counteranion present in the reaction medium has on the barium template potential, we have carried out the corresponding template synthesis in the presence of barium thiocyanate. We have found that considerably longer reaction times are required to obtain both receptors when barium thiocyanate is employed. The barium template remains trapped into the formed macrobicyclic cavity and compounds of formula Ba(L1)(SCN)2 · H2O (1) and Ba(L2)(SCN)2 · 2H2O (2) were isolated. The X-ray crystal structure of 1 shows the metal ion coordinated to the eight donor atoms of the receptor and to the nitrogen atom of a isothiocyanate group, the coordination polyhedron being best described as a monocapped distorted dodecahedron. In 1, the metal ion is nearly centred inside the macrobicyclic cavity of L1, whereas NMR studies in solution indicate that in 2 the barium ion is asymmetrically situated in the macrobicyclic cavity of L2. This asymmetric coordination is attributed to the proton transfer from the phenol group to the nitrogen atom of one of the imine groups.

Compounds of formula Pb(L1)(SCN)2·CH3CN (1) and Pb(L3)(SCN)2·0.5EtOH (2) (where L1 is the lariat crown ether N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and L3 is a Schiff-base macrobicyclic receptor derived from the condensation of L1 with 2,6-diformylpyridine) were isolated and structurally characterised both in the solid state and in acetonitrile solution. The X-ray crystal structure of 1 shows the metal ion coordinated to the seven donor atoms of the receptor and to the nitrogen atom of an isothiocyanate group. This structure is different to that found for the previously described perchlorate analogous [Pb(L1)](ClO4)2, evidencing that the nature of the anionic group present has an important effect in the conformation and fold of L1 when coordinated to lead(II). In 2, the metal ion is nearly centred inside the macrobicyclic cavity of L3 clearly bound to the three oxygen atoms, to one pivotal nitrogen atom, to one imine nitrogen atom and to the pyridine nitrogen donor of L3, while the remaining donor atoms of L3 are only weakly coordinated to the lead(II) ion. The coordination sphere of lead(II) is completed by the nitrogen atom of an isothiocyanate group, and the coordination polyhedron may be described as a distorted tricapped trigonal prism. A comparison between this structure and that previously described for [Pb(L3)(ClO4)]+ demonstrates that for the L3 complexes the different nature of the anion present (thiocyanate or perchlorate) hardly has an effect on the fold and conformation that the macrobicycle adopts when acting as a receptor for lead(II). So, the effect that the anion present has on the structure of the lead(II) complexes of L1 and L3 appears to be determined by the degree of flexibility of the organic receptor.