Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaldehyde (H₂L¹) and 2-hydroxyacetophenone (H₂L²) (general abbreviation, H₂L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H₂L and a bidentate NN donor [e.g., 2,2′-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [V^IVO(L)(B)]; complexes (1)-(4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [V^VO(L)(hq)]; complexes (5) and (6). Aerial oxygen is most likely the oxidant (for the oxidation of V^IV → V^V) in the synthesis of pentavalent complexes (5) and (6). [V^IVO(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [V^VO(L)(hq)] complexes are diamagnetic. The X-ray structure of [V^VO(L²)(hq)] (6) indicates that H₂L² ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V-O (enolato) bond is longer than V-O (phenolato) bond by ∼0.07 Å and is identical with V-O (carboxylate) bond. ¹H NMR spectrum of (6) in CDCl₃ solution indicates that the binding nature in the solid state is also retained in solution. Complexes (1)-(4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes (5) and (6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near −0.10 V versus SCE in CH₂Cl₂ solution. The VO³⁺-VO²⁺E_1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO³⁺ binding via phenolato oxygen. For a given bidentate ligand, E_1/2 increases in the order: (L²)²⁻ < (L¹)²⁻.     

Synthesis and structural studies of mixed-ligand rhenium(V) complexes anchored by tridentate pyrazole-based ligands

The novel oxorhenium dichlorides mer-[ReO(L1)Cl₂] (1) and fac-[ReO(L2)Cl₂] (2) (L1 = 2-[2-(pyrazol-1-yl)ethyliminomethyl]phenolate; L2 = 2-[2-(pyrazol-1-yl)ethylaminomethyl]phenolate) were synthesized by reacting [NBu₄][ReOCl₄] with L1H and L2H, respectively. X-ray structural analysis of 1 and 2 has shown that L1 and L2 act as (N,N,O)-tridentate chelators coordinating to the Re(V) centre in a meridional and in a facial fashion, respectively. The reactivity of 2 towards potential bidentate/dianionic substrates is strongly dependent on the donor atom set, being observed that the presence of sulphur favours the displacement of the ancillary ligand (L2). By contrast, complex 2 reacted with (O,O)-bidentate substrates (1,2-ethanediol and oxalic acid) providing the mixed-ligand complexes fac-[ReO(L2)(OCH₂CH₂O)] (3) and fac-[ReO(L2)(C₂O₄)] (4). Complexes 3 and 4 are air and water-stable and have been characterized by the common spectroscopic techniques (IR, ¹H and ¹³C NMR) and by X-ray diffraction analysis.     

Ternary oxovanadium(IV) complexes with amino acid-Schiff base and polypyridyl derivatives: synthesis, characterization, and protein tyrosine phosphatase 1B inhibition

To investigate the structure-activity relationship of vanadium complexes in inhibiting protein tyrosine phosphatase1B (PTP1B), eight mixed-ligand oxovanadium(IV) complexes, [V^IVO(SalAla)(NN)] (H₂SalAla for salicylidene alanine, NN for N,N′-donor heterocyclic base, namely, 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq, 3), dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 4)), [V^IVO(SalLys)(dpq)] (5), [V^IVO(SalLys)(dppz)] (6), [V^IVO(SalAsp)(dppz)], (7) and [V^IVO(SalTrp)(dppz)] (8)), of which 3-8 are new, have been prepared and characterized by elemental analysis, infrared, UV-visible, electrospray ionization mass spectrometry and conductivity. The molar conductance data confirmed the non-electrolytic nature of the complexes in DMSO solution. The coordination in [V^IVO (SalAla)(phen)] (2) was confirmed by X-ray crystal structure analysis. The oxidation state of V(IV) with d¹ configuration in 2 was confirmed by EPR. The speciation of VO-SalAla-phen in aqueous solution was investigated by potentiometric pH titrations. The results indicate that the main species are two ternary complexes at the pH range 7.0-7.4. Biochemical assays demonstrate that the mixed-ligand oxovanadium(IV) complexes are potent inhibitors of PTP1B with IC₅₀ values in the range of 62-597 nM, approximately 3-10 fold weaker in potency than those of similar mixed-ligand oxovanadium(IV) complexes of salicylidene anthranilic acid (SAA) derivative with polypyridyl ligands, except complex 8, which exhibits comparable or better inhibition activity than those of the mixed-ligand oxovanadium(IV) complexes of SAA derivative with polypyridyl ligands. The results demonstrate that the structures of vanadium complexes influence the PTP1B inhibition activity. Kinetics assays reveal that complex 2 inhibits PTP1B in a competitive manner.     

Synthesis, characterization, and protein tyrosine phosphatases inhibition activities of oxovanadium(IV) complexes with Schiff base and polypyridyl derivatives

Seven new mixed-ligand vanadyl complexes, [V^IVO(5-Br-SAA)(NN)] and [V^IVO(2-OH-NAA)(NN)] (1-7) (5-Br-SAA for 5-bromosalicylidene anthranilic acid, 2-OH-NAA for 2-hydroxy-1-naphthaldehyde anthranilic acid and NN for N,N′-donor heterocyclic base, namely, 2,2′-bipyridine (bpy, 1 and 5), 1,10-phenanthroline (phen, 2 and 6), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq, 3 and 7), dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 4)), were synthesized and characterized. X-ray crystal structure of [V^IVO(5-Br-SAA)(phen)] revealed a distorted octahedral geometry with the Schiff base ligand coordinated in a tridentate ONO-fashion and the phenanthroline ligand in a bidentate fashion. Density-functional theory (DFT) calculations suggest a similar structure and the same coordination mode for all the other oxovanadium complexes synthesized. Biochemical assays demonstrate that the mixed-ligand oxovanadium(IV) complexes are potent inhibitors of protein tyrosine phosphatase 1B (PTP1B), with IC₅₀ values approximately 41-75 nM. Kinetics assays suggest that the complexes inhibit PTP1B in a competitive manner. Notably, they had moderate selectivity of PTP1B over T-cell protein tyrosine phosphatase (TCPTP) (about 2-fold) and good selectivity over Src homology phosphatase 1 (SHP-1) (about 4∼7-fold). Thus, these mixed-ligand complexes represent a promising class of PTP1B inhibitors for future development as anti-diabetic agents.     

Structure and properties of a macrocyclic silver(II) supramolecular polymer

A new silver(II) complex, {[Ag(L1)](NO₃)₂·4H₂O}_n (1) (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0^1.18,0^7.12] docosane) has been synthesized and structurally characterized by a combination of analytical, spectroscopic, electrochemical and X-ray diffraction methods. The complex 1 exhibits a 1D supramolecular polymer with the silver(II) macrocycle L1 and nitrate ions, where 1D chain is formed by hydrogen bonds between the two sets of pre-organized N-H groups of the macrocycle and nitrate ions. The lattice water molecules mediate to interconnect each 1D chain to form the 2D supramolecular sheet. In 1 the unusual high oxidation state of Ag(II) is stabilized by the tetraazamacrocyclic ligand L1. The cyclic voltammogram for 1 indicates that the electrochemical oxidation of [Ag(L1)]²⁺ is an irreversible process.     

Structural variety and stereochemical features of bis(diphenylphosphinyl)[3]ferrocenophane gold(I) complexes

The P,N-[3]ferrocenophane ligand 3 forms a (κP-ligand)AuCl complex (5) upon treatment with (Me₂S)AuCl. The corresponding P,P-[3]ferrocenophane system 4 yields a binuclear (κP,κP-chelate ligand)(AuCl)₂ complex (6) when reacted with 2 equivalents of the (Me₂S)AuCl reagent. Complex 6 features an intramolecular aurophilic Au?Au interaction. Treatment of the P,P-[3]ferrocenophane 4 with 1.0 equiv. of (PPh₃)AuCl gives the tetra-coordinated mono-gold(I) complex (P,P-ligand)(PPh₃)AuCl (7), whereas the cationic [(P,P-ligand)₂Au]⁺[Cl⁻] system is obtained from 4 and 0.5 equivalents of (Me₂S)AuCl. The [(P,P-ligand)₂Au]⁺ system is obtained in different diastereoisomeric forms (8 and 9) depending on the stereochemistry of the pair of P,P-[3]ferrocenophane chelate ligand used. Examples of the complexes 5, 6, 7 and 8 were characterized by X-ray diffraction.     

Unusual reactivity of copper(I) complexes of functionalised calix[4]arenes

Two functionalised calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)calix[4]arene (L¹) and 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)-26,28-dibutoxycalix[4]arene (L³), were prepared and characterised. The copper(I) complexes of both calix[4]arenes were synthesised and their reactivities were analysed and compared. The presence of the metal induced a radical in the case of L¹ whereas no such radical was observed in the metal complex of ligand L³.     

Synthesis, characterisation and crystal structures of a few coordination complexes of nickel(II), cobalt(III) and zinc(II) with N′-[(2-pyridyl)methylene]salicyloylhydrazone Schiff base

Four new coordination complexes, Ni^II(L)₂ (1), [Co^III(L)₂]ClO₄ (2), [Zn(HL)(L)]ClO₄ · H₂O (3) and [Zn(L)₂][Zn(L)(HL)]ClO₄ · 7H₂O (4) (where L is a monoanion of a Schiff base ligand, N′-[(2-pyridyl)methylene]salicyloylhydrazone (HL) with NNO tridentate donor set), have been synthesised and systematically characterised by elemental analysis, spectroscopic studies and room temperature magnetic susceptibility measurements. Single crystal X-ray diffraction analysis reveals that 1 is a neutral complex, while 2-4 are cationic complexes. Among them, 4 is a rare type of cationic complex with two molecules in the asymmetric unit. The ligand chelates the metal centre with two nitrogen atoms from the pyridine and imino moieties and one oxygen atom coming from its enolic counterpart. All the reported complexes show distorted octahedral geometry around the metal centres, with the two metal-N (imino) bonds being significantly shorter than the two metal-N (Py) bonds.     

Synthesis, characterization and supramolecular structures of two Ni(II) complexes with potentially heptadentate ligand N,N-bis(2-hydroxybenzyl)-1,3-bis[(2-aminoethyl)amino]-2-propanol

A potentially heptadentate ligand H₃L (N,N^′-bis(2-hydroxybenzyl)-1,3-bis[(2-aminoethyl)amino]-2-propanol) and its two Ni(II) complexes, [Ni(H₂L)H₂O](H₂O)₃ClO₄ (1) and [Ni(H₂L)(H₂O)](H₂O)Cl (2) were prepared and characterized. X-ray structural analyses indicate that complex 1 has a distorted octahedral coordination geometry, with four amine N atoms of H₂L⁻ defining the equatorial plane, one aqua O atom and one phenoxo O atom of the ligand occupying two axial positions, respectively. The Ni(II) center of 2 has coordination geometry similar to that of 1. IR and electronic spectra of 1 and 2 are in agreement with their crystal structural features. Approximately along the ab plane, 2D supramolecular structure of 1 is assembled through multiple hydrogen bonds between hydroxy groups of the ligands, coordinated and crystal lattice H₂O and π-π stacking interactions between adjacent phenyl rings of the ligands, while for that of 2, probably along the a axis, 1D chain structure is also formed by multiple hydrogen bonds, but lack of π-π stacking interactions.     

Mono cationic palladium(II): Synthesis, characterization and catalytic activity in Suzuki coupling

Monocationic five and four-coordinate Pd^II complexes have been synthesized in a facile manner involving the reaction of (COD)PdCl₂ with AgPF₆ and a nitrogen or phosphorus ligand. Complexes synthesized via this route are five coordinate [PdCl(2,9-Me₂Phen)₂]PF₆2 and four coordinate [L₃PdCl]PF₆ (L = PPh₃, 3; P(p-Tol)₃, 4; P(OEt)₃; 5). ESI-MS studies at varying cone-voltage indicated ligand dissociation depending upon ligand cone-angle. A halo-bridged complex [PdCl(μ-Cl){P(p-Tol)₃}]₂7 has been isolated from 4 in methanol. The cationic complexes have been screened as catalyst in Suzuki coupling involving aryl halide and phenylboronic acid. Complex 4 showed appreciable turnover.     

Synthesis and characterization of Cu(II) and Ni(II) complexes of a tripodal ligand containing imidazoles

Two complexes of the formula [MH₃L](ClO₄)₂ [M = Cu(II) (1), Ni(II) (2)] have been prepared by the reaction of M(ClO₄)₂ · 6H₂O with the ligand (H₃L) formed by the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 4-methyl-5-imidazolecarboxaldehyde and structurally and magnetically characterized. The structures of 1 and 2 are isomorphous with each other and with the iron(II) complex of H₃L which has been reported previously. The ligand, while potentially heptadentate, forms six coordinate complexes with both metal centers forming three M-N_imine and three M-N_imidazole bonds. The tren central N atom is at a nonbonded distance from M of 3.261 Å for 1 and 3.329 Å for 2. The neutral complex CuHL 3 was prepared by reaction of H₃L with Cu(OCH₃)₂ and the ionic complex Na[NiL] 4 was prepared by deprotonation of 2 with aqueous sodium hydroxide. Magnetic measurements of 1-3 are consistent with the spin-only values expected for S = 1/2 (d⁹, Cu(II)) and S = 1 (d⁸, Ni (II)) systems.     

Synthesis, crystal structures and magnetic characterization of heterodinuclear CuGd and CuTb Schiff base complexes

Preparation, crystal structures and magnetic properties of new heterodinuclear Cu^IIGd^III (1) and Cu^IITb^III (2) complexes [CuLn(L)(NO₃)₂(H₂O)₃MeOH]NO₃·MeOH (where Ln = Gd, Tb) with the hexadentate Schiff-base compartmental ligand N,N′-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine (H₂L = C₁₉H₂₀N₂O₄Br₂) (0) have been described. Crystal structure analysis of 1 and 2 revealed that they are isostructural and form discrete dinuclear units with dihedral angle between the O1Cu1O2 and O1Gd1/Tb1O2 planes equal to 2.5(1)° and 2.6(1)°, respectively. The variable-temperature and variable-field magnetic measurements indicate that the metal centers in 1 and 2 are ferromagnetically coupled (J = 7.89 cm⁻¹ for 1). Crystal and molecular structure of the Schiff base ligand (0) has been also reported. The complex formation changes the conformation of Schiff base ligand molecule.     

The first pincer-aryl [M-(NCN)] complexes {M = Pd(II); Pt(II)} with chiral pyridine donors: Synthesis and catalytic activity in C-C bond formation

The first chiral bis(pyridine) N-C(H)-N pincer ligand, (5S,7S)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanquinolin-2-yl)benzene (HL) has been synthesized and characterized by a thorough ¹H NMR analysis. Reaction of HL with K₂[PtCl₄] in acetic acid gives [Pt(L)Cl] (1), where L acts as a tridentate N-C-N pincer ligand. The analogous palladium(II) derivatives [Pd(L)Cl] (2), and [Pd(L)(OAc)] (3), were first prepared through a transmetalation reaction between Pd(OAc)₂ and the organomercury compound [Hg(L)Cl] (4). The structures of compounds 1 (Pt) and 2 (Pd), as determined by X-ray diffraction, are reported and compared. Compound 2 can also be obtained from Na₂[PdCl₄] and HL in refluxing acetic acid, i.e., under the same conditions used for compound 1. Apparently, this is the first palladium pincer derivative of a 1,3-bis(pyridyl)benzene ligand synthesized by direct C-H activation.The neutral complexes 1-3 are catalysts of modest activity, but devoid of enantioselectivity in the Heck reaction between iodobenzene and methyl acrylate and in the aldol condensation of benzaldehyde with methyl isocyanoacetate.     

Manganese(III) complexes of N2O2 donor 5-bromosalicylideneimine ligands: Combined effects of electron withdrawing substituents and chelate ring size variations on electrochemical properties

A series of mononuclear manganese(III) complexes of formulae [Mn(L)(X)(H₂O)] (1-13) and [Mn(L)(X)] (14-17) (X = ClO₄, F, Cl, Br, I, NCS, N₃), derived from the Schiff bases of 5-bromosalicylaldehyde and different types of diamine (1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane and 1,4-diaminobutane), have been synthesized and characterized by the combination of IR, UV-Vis spectroscopies, cyclic voltammetry and by X-ray crystallography. The redox properties of all the manganese(III) complexes show grossly identical features consisting of a reversible or quasireversible Mn^III/Mn^II reduction. Besides Mn^III/Mn^II reduction, the complexes 4, 5, 10, 13 and 16 also show reversible or quasireversible Mn^III/Mn^IV oxidation. A linear correlation has been found for the complexes 5, 7, 11 and 13 [Mn(L²)(X)(H₂O)] (X = F, Cl, Br, I) when E_1/2 [Mn^III/Mn^II] is plotted against Mulliken electronegativities (χ_M). The effect of the flexibility of the ligand on redox potential has been studied. It has been observed that the manganese(II) state is stabilized with increasing flexibility of the ligand environment. The crystal structure of 6 shows an octahedral geometry.     

Synthesis, structure and solution chemistry of dioxidovanadium(V) complexes with a family of hydrazone ligands. Evidence of formation of centrosymmetric dimers via H-bonds in the solid state

[V^IVO(acac)₂] reacts with the methanol solution of tridentate ONO donor hydrazone ligands (H₂L^1-4, general abbreviation H₂L; are derived from the condensation of benzoyl hydrazine with 2-hydroxyacetophenone and its 5-substituted derivatives) in presence of neutral monodentate alkyl amine bases having stronger basicity than pyridine e.g., ethylamine, diethylamine, triethylamine and piperidine (general abbreviation B) to produce BH⁺[VO₂L]⁻ (1-16) complexes. Five of these sixteen complexes are structurally characterized revealing that the vanadium is present in the anionic part of the molecule, [VO₂L]⁻ in a distorted square pyramidal environment. The complexes 5, 6, 15 and 16 containing two H-atoms associated with the amine-N atom in their cationic part (e.g., diethylammonium and piperidinium ion) are involved in H-bonding with a neighboring molecule resulting in the formation of centrosymmetric dimers while the complex 12 (containing only one hydrogen atom in the cationic part) exhibits normal H-bonding. The nature of the H-bonds in each of the four centrosymmetric dimeric complexes is different. These complexes have potential catalytic activity in the aerial oxidation of l-ascorbic acid and are converted into the [VO(L)(hq)] complexes containing VO³⁺ motif on reaction with equimolar amount of 8-hydroxyquinoline (Hhq) in methanol.     

Syntheses and structures of cobalt(III) alcoholate complexes formed by addition of a water molecule across 2-pyridyl substituted imine function

Schiff bases L1-L5 {N-[1-pyridine-2-ylethylidene]pyridine-2-amine (L1), 3-methyl-N-[1-pyridine-2-ylmethylidene]pyridine-2-amine (L2), 3-methyl-N-[1-pyridine-2-ylethylidene]pyridine-2-amine (L3), 4-methyl-N-[1-pyridine-2-ylmethylidene]pyridine-2-amine (L4), 4-methyl-N-[1-pyridine-2-ylethylidene]pyridine-2-amine (L5)} were synthesized and on reaction with Co(NO₃)₂·6H₂O, complexes having the molecular formulae [Co(L1O)₂]NO₃ (1), [Co(L2O)₂]NO₃·xH₂O (2a, x = 2; 2b, x = 3), [Co(L3O)₂]NO₃ (3), [Co(L4O)₂]NO₃·4H₂O (4), [Co(L5O)₂]NO₃ (5) were isolated from the respective imines. The salt [Co(L2O)₂]PF₆ (2c) was obtained by treating 2 with KPF₆. Complexes 1-5 were formed as a result of addition of a water molecule across the imine function and the resultant alcohol binds in its deprotonated form. The alcoholate ion remained bound in a facial tridentate fashion to the low-spin cobalt(III). X-ray crystal structure determination confirmed the presence of trans-trans-trans-N_AN_PO (A = aminopyridyl and P = pyridyl) disposition in 2a and cis-cis-trans-N_AN_PO in 2b, 2c and 4. Water dimers in 2a, 2b, 4 and water-nitrate ion network in 2a were other notable features.     

Palladium(II) complexes of quinolinylaminophosphonates: synthesis, structural characterization, antitumor and antimicrobial activity

Three types of palladium(II) halide complexes of quinolinylaminophosphonates have been synthesized and studied. Diethyl and dibutyl [α-anilino-(quinolin-2-ylmethyl)]phosphonates (L1, L2) act as N,N-chelate ligands through the quinoline and aniline nitrogens giving complexes cis-[Pd(L1/L2)X₂] (X═Cl, Br) (1-4). Their 3-substituted analogues [α-anilino-(quinolin-3-ylmethyl)]phosphonates (L3, L4) form dihalidopalladium complexes trans-[Pd(L3/L4)₂X₂] (5-8), with trans N-bonded ligand molecules only through the quinoline nitrogen. Dialkyl [α-(quinolin-3-ylamino)-N-benzyl]phosphonates (L5, L6) give tetrahalidodipalladium complexes [Pd₂(L5/L6)₃X₄] (9-12), containing one bridging and two terminal ligand molecules. The bridging molecule is bonded to the both palladium atoms, one through the quinoline and the other through the aminoquinoline nitrogen, whereas terminal ligand molecules are coordinated each only to one palladium via the quinoline nitrogen. Each palladium ion is also bonded to two halide ions in a trans square-planar fashion. The new complexes were identified and characterized by elemental analyses and by IR, UV-visible, ¹H, ¹³C and ³¹P nuclear magnetic resonance and ESI-mass spectroscopic studies. The crystal structures of complexes 1-4 and 6 were determined by X-ray structure analysis. The antitumor activity of complexes in vitro was investigated on several human tumor cell lines and the highest activity with cell growth inhibitory effects in the low micromolar range was observed for dipalladium complexes 11 and 12 derived from dibutyl ester L6. The antimicrobial properties in vitro of ligands and their complexes were studied using a wide spectrum of bacterial and fungal strains. No specific activity was noted. Only ligands L3 and L4 and tetrahalidodipalladium complexes 9 and 11 show poor activities against some Gram positive bacteria.     

Stepwise syntheses, structure and spectroscopic studies of ruthenium complexes of 2,6-bis(benzimidazol-2-yl)pyridine with o-iminoquinone ancillary ligand

The ruthenium complexes [Ru^II(bbp)(L)(Cl)] (1), [Ru^II(bbp)(L)(H₂O)] (2) and [Ru^II(bbp)(L)(DMSO)] (3) {bbp = 2,6-bis(benzimidazol-2-yl)pyridine, L = o-iminoquinone} have been synthesized in a stepwise manner starting from [Ru^III(bbp)Cl₃]. The single crystal X-ray structures, except for the complex 2, have been determined. All the complexes were characterized by UV-Vis, FT-IR, ¹H NMR, Mass spectroscopic techniques and cyclic voltammetry. The Ru^III/Ru^II couple for complexes 1, 2, and 3 appears at 0.63, 0.49, 0.55 V, respectively versus SCE. It is observed that complex 2, on refluxing in acetonitrile, results into [Ru^II(bbp)(L)(CH₃CN)], 4 which has been prepared earlier in a different method. The structural, spectral and electrochemical properties of complexes 1, 2 and 3 were compared to those of earlier reported complex 4, [Ru^II(bbp)(L)(CH₃CN)].     

Reactivity studies of highly electrophilic ruthenium complexes

The 16-electron, coordinatively unsaturated, dicationic ruthenium complex [Ru(P(OH)₂(OMe))(dppe)₂][OTf]₂ (1a) brings about the heterolysis of the C-H bond in phenylacetylene to afford the phenylacetylide complex trans-[Ru(CCPh)(P(OH)₂(OMe))(dppe)₂][OTf] (2). The phenylacetylide complex undergoes hydrogenation to give a ruthenium hydride complex trans-[Ru(H)(P(OH)₂(OMe))(dppe)₂][OTf] (3) and phenylacetylene via the addition of H₂ across the Ru-C bond. The 16-electron complex also reacts with HSiCl₃ quite vigorously to yield a chloride complex trans-[Ru(Cl)(P(OH)₂(OMe))(dppe)₂][OTf] (4). On the other hand, the other coordinatively unsaturated ruthenium complex [Ru(P(OH)₃)(dppe)₂][OTf]₂ (1b) reacts with a base N-benzylideneaniline to afford a phosphonate complex [Ru(P(O)(OH)₂)(dppe)₂][OTf] (5) via the abstraction of one of the protons of the P(OH)₃ ligand by the base. The phenylacetylide, chloride, and the phosphonate complexes have been structurally characterized. The phosphonate complex reacts with H₂ to afford the corresponding dihydrogen complex trans-[Ru(η²-H₂)(P(O)(OH)₂)(dppe)₂][OTf] (5-H₂). The intact nature of the H-H bond in this species was established using variable temperature ¹H spin-lattice relaxation time measurements and the observation of a significant J(H,D) coupling in the HD isotopomer trans-[Ru(η²-HD)(P(O)(OH)₂)(dppe)₂][OTf] (5-HD).     

Divanadium(V) complexes with 4-R-benzoic acid (1-methyl-3-oxo-butylidene)-hydrazides: Syntheses, structures and properties

In acetonitrile, reactions of bis(acetylacetonato)oxidovanadium(IV) ([VO(acac)₂]) with 4-R-benzoylhydrazine in 1:1 mole ratio provide coordinatively symmetrical complexes (1-5) of the {OV(μ-O)VO}⁴⁺ motif in 40-47% yields. On the other hand, in methanol the same reactants provide complexes (6-10) containing the {OV(μ-OMe)₂VO}⁴⁺ core in 37-50% yields. In both series of complexes, the ligand is the O,N,O-donor deprotonated Schiff base system 4-R-benzoic acid (1-methyl-3-oxo-butylidene)-hydrazide formed by template condensation of acac⁻ with 4-R-benzoylhydrazine (R = H, Cl, OMe, NO₂ and NMe₂). All the complexes have been characterized by elemental analysis, magnetic and spectroscopic (IR, UV-Vis and NMR) measurements. Molecular structures of three representative complexes (4, 6 and 7) have been determined by X-ray crystallography. In each complex, the dianionic planar ligand is coordinated to the metal centre via the enolate-O, the imine-N and the O-atom of the deprotonated amide functionality. Cyclic voltammetric measurements in dichloromethane revealed that complexes 1-5 are redox inactive, while complexes 6-10 display a metal centred reduction in the potential range −0.06 to 0.0.32 V (versus Ag/AgCl).