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Linear and flexible N4 ligand trien (N′-{2-(2-aminoethylamino)-ethyl}-ethane-1,2-diamine) has been used to show octahderal coordination around NiII ions and cis coordination from two ancillary bridges from azide ions. Room temperature reaction of trien with Ni(NO3)·6H2O and NaN3 has led to an end-to-end (EE) azido bridged one-dimensional NiII based helical coordination chain of formula [NiII(μ-N3)2 (trien)]n (NO3)·H2O {1n·(NO3)·H2O}. Propagation of the chain in one dimension in a helical fashion is continued from the azido bridges in cis directions. Comparative reaction protocol using Co(NO3)2·6H2O on the other hand provided a discrete mononuclear octahedral cobalt (III) complex cis-[CoIII(N3)2 (trien)](NO3)·2H2O {2·(NO3)·2H2O} due to different type of ligand wrapping having only terminal coordination from azide ions.

The dinuclear copper(II) complex [Cu2(µ–L2NH)2(OH2)2](ClO4)4 (1) was obtained following one imine side arm hydrolysis of HL1 (2,6-bis-[{3-(diethylamino)propylimino}methyl]-4-methylphenol). Introduction of carboxylates inhibited the hydrolysis and resulted in two tetranuclear copper(II) complexes [Cu4(µ4–O)(µ–L1)2(µ1,3–O2CCH3)4]·H2O (2a) and [Cu4(µ4–O)(µ–L1)2(µ1,3–O2CC2H5)4]·H2O (2b). Use of perchlorate and carboxylate salts of copper(II) for the reactions under study with HL1 has directed these reactions. Perchlorate anions in presence of H2O molecules, as hydrolytic nucleophiles, triggered imine arm hydrolysis and bridging carboxylate anions allowed transformation of copper(II) bound H2O molecule to μ4-oxido central nucleating group via hydroxido-bridge. The complexes were obtained from simultaneous chelation and bridging by the hydrolyzed and as used ligand anions L2NH− and HL1− respectively. These complexes have been characterized in the solid state by X-ray crystallography and FT-IR spectroscopy. Solution phase stability of Cu2-based fragments was studied by UV–vis absorption spectroscopy. Solution properties have been examined for substrate binding in catechol oxidase activity and binding affinity for biomacromolecules like DNA and Human Serum Albumin.

The Schiff base ligand, 2-(((2-hydroxybenzyl)imino)methyl)phenol (H2L) having ONO donor centres are utilized to synthesize {Ni4Dy3} coordination aggregate following the support of six benzoate bridging groups. Sequential addition of Dy(NO3)3·6H2O and NiCl2·6H2O to H2L followed by PhCO2Na completes the coordination driven aggregation of heptanuclear Na[Ni4Dy3(L)4(μ3-OH)4(μ1,1,3,3-PhCO2)2(μ1,3-PhCO2)4(CH3OH)4]∙CH2Cl2·9H2O (1) having 2,3,6M7 − 1 topology. Two adjacent perpendicular inverse Ni2Dy2 partial di-cubane units sharing a common vertex through the central DyIII ion resulted a new structural arrangement within the Ni − Dy family of coordination aggregates. The complex shows no slow magnetic relaxation under zero applied fields and only shows a very weak field dependent magnetization. The DFT calculations revealed weak ferromagnetic exchange between benzoato-bridged NiII ions, whereas the CASSCF calculations were used to identify magnetic anisotropy in NiII and DyIII ions resulting in low-lying excited states of DyIII ions with significant probability for the quantum tunneling of the magnetization, which rationalize the observed fast dynamics in the magnetic properties.

In the present work, a discrete dinuclear cadmium(II) complex Cd2(μ−H2L)(μ−OH)Cl2⋅H2O (1) and a dinuclear zinc(II) based 1D chain complex ({Zn2(L')(O2CCH3)}ClO4⋅3H2O)n (2) (where H3L = 2,6-bis{2-(2-hydroxyethylamino)ethylimino}methyl-4-methylphenol and H2L′ = bis-oxazoline (BOX) ligand), have been synthesized and characterized by single-crystal X-ray crystallography. The Cd2+ ions in the first case showed straight cut incorporation of metal ions within the ligand pockets. In case of zinc(II), the coordination induced oxazolidine ring formation resulted in macrocyclic ligand formation having two adjacent coordination pockets. In complex 2, the acetate ion acts as an O,O linker and joins the Zn2 nodes bound within the newly formed macrocyclic ligand bearing bis-oxazolidine rings to form 1D polymeric chain at the end. The H3L was next screened for its efficacy as selective fluorescence sensor for Mg2+ ions in aqueous MeOH medium. Steady state absorption and emission behaviour of H3L, 1 and 2 have been investigated. Time resolved fluorescence properties of H3L and the complexes 1 and 2 have been examined, and their lifetimes have been determined in 0.038–0.257 ns range. The cell viability in presence of H3L makes it a suitable candidate for application in live cell imaging studies.

Two new tetranuclear CuII complexes [Cu4(L2)2(µ2-OH)2(H2O)2]·(ClO4)2·DMF·2H2O (1) and [Cu4(L3)2(µ2-OH)2(H2O)2]·(ClO4)2·DMF·H2O (2) have been synthesized containing piperazine based Schiff base ligands H2L2 (asymmetric) and H2L3 (symmetric). H2L2 contains one o-vanillin and a salicylaldehyde unit while H2L3 has two o-vanillin units. An interesting synthetic methodology involving transition metal ion induced aldehyde exchange reaction between two Schiff base ligands has been explored for the synthesis of H2L2 and H2L3. While the latter can be obtained by direct condensation of the respective amine and aldehyde, the former can only be synthesized by this procedure developed. Both 1 and 2 contain a piperazine ring bridging two CuII centers in chair conformation in the rare axial-equatorial mode of coordination. Two piperazine bridged Cu2 moieties are further connected via two hydroxido and two phenoxido bridges. Of the two Ophenoxido–Cu bonds one is short (basal) and the other is long (apical) while the hydroxido O bridges the two CuII centers symmetrically in basal–basal manner giving rise to a unique [Cu2O2] moiety where, of the four bonds linking the metals, three are basal (short) and one axial (long). Magnetic studies reveal weak antiferromagnetic interactions across the two piperazine bridges with an average exchange constant value of −4.007 cm−1 and −4.507 cm−1 for 1 and 2 respectively. The antiferromagnetic exchanges across the phenoxido–hydroxido bridges were found to be stronger with the average value being −43.036 cm−1 for 1 and −38.031 cm−1 for 2. Computational studies further support the experimental findings and the spin density plots in the HS configuration reveal that the exchange across the piperazine bridge takes place via through space mechanism. The spin density analysis in the HS and BS configurations also reveal that the exchange interaction is propagated mainly through the hydroxido bridge. The unsymmetrical nature of H2L2 leads to greater spin density at the hydroxido bridge in 1 leading to slightly stronger exchange interaction.

The reactions of phenol-based ligand, 2,6-bis-{(2-hydroxy-propylimino)-methyl}-4-methylphenol (H3L), with three different CuII salts provided completely different copper-ligand aggregates [{Cu2(OH)(H2L)(H2O)(μ-ClO4)n}ClO4·2H2O]n (1), [Cu4(μ3-OH)2(H2L)2(μ1,3-NO3)2](NO3)2 (2) and [Cu4(μ4-O)(H2L)2(μ1,3-OOCH)4]·3H2O (3·3H2O). These complexes are examined by single crystal X-ray analysis and by magnetic study. The perchlorate bearing complex 1, when separately reacted in solution with nitrate and formate anions, provided complexes 2 and 3, respectively. The relative binding potential of the anions as NO3− ≅ HCOO− > ClO4− has been demonstrated from the two core-conversion and ligand exchange reactions. A strong phenoxido/hydroxido mixed bridge mediated antiferromagnetic coupling was observed for 1 with J = −204.9 cm−1. For 2 the [Cu4O4] structure involving phenoxido/μ3-hydroxido bridges provides J1 = −114.2 cm−1 and J2 = −1.0 cm−1. In case of [Cu4O] arrangement in 3 the overall J value lies near −420 cm−1.

The reaction of tridentate Schiff base {2-[(2-piperazine-1-yl-ethylimino)-methyl]-phenol} ligand (HOPhL) with copper(II) salts resulted three new compounds (i) [Cu(HNL)(NO3)(H2O)](NO3) (1), (ii) [Cu(HNL)(N3)(ClO4)] (2) and (iii) [Cu2(HNL)(μ1,3-NCS)2](ClO4)2·2H2O (3) where the used ligand transforms to its zwitterionic form (HNL) during complexation. The complexes are characterized by elemental analysis, IR and UV–Vis spectroscopy and single crystal X-ray diffraction. The tridentate NNO donor ligand coordinates copper ion in square pyramidal environment where rest of coordination sites around Cu(II) are occupied by water, nitrate, azide or thiocyanate anion. In complexes 1 or 2 the presence of NO3− and N3− ions do not reveal any kind of aggregation reaction whereas in 3 binding of NCS− ions trigger the dimerization reaction. The interaction of these compounds with bovine serum albumin suggests a static quenching mechanism of fluorescence. The quenching constant, binding constant and number of binding sites are calculated using Stern–Volmer and Scatchard equation. The anti-proliferative and anti-migratory properties of these compounds are established in human breast cancer cells. The observed IC50 value by MTT assay is 30 μM after 6 h for the compounds 1–3 irrespective of their chemical structures. The findings indicate the anti-cancerous therapeutic potential of zwitterionic ligand based copper(II) complexes.

Bis - and tris -chelates ( 1 – 5 ) of Ni(II), Cu(II), Fe(III) and Co(II) with two O,S donor substituted benzoylthiourea based ligands 3,3-diethyl-1-benzoylthiourea (HL 1 ) and 3-methyl-3-phenyl-1-benzoylthiourea (HL 2 ) have been synthesized and structurally characterized by X-ray crystallography. The keto-thione form of the free ligands transform to the keto-thiol form during coordination through neutral O and anionic S atoms. Crystallographic characterizations with supportive spectroscopic signatures confirm the cis -conformation in bis -chelates and facial binding to the tris -chelates.

The heptanuclear and tetracationic nickel(II) complex [Ni7(μ3-OH)6(μ3-H2L)4](NO3)4·2MeOH (3·2MeOH); (H3L is 2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methyl-phenol), featuring four defective cubanes around a pivotal nickel(II) ion, has been obtained via hydroxido-bridge-induced assemblage of two Ni3 fragments. Presence of azido (N3−) and bromido (Br−) ions in reactions on the contrary lead to one ligand imine arm hydrolysis and formations of [Ni2(N3)2(μ-HL1)2(OH2)2] (1) and [Ni2Br2(μ-HL1)2(OH2)2] (2); where H2L1 is 2-hydroxy-3-[(3-hydroxy-propylimino)-methyl]-5-methyl-benzaldehyde. X-ray structural analyses of 3 show six symmetrically positioned NiII around a central NiII and bridged by six in situ generated hydroxido groups from solvent water molecules. Density functional theory (DFT) calculations have also been performed to predict magnetic behavior of the complexes. The spin density surface of compounds 1 and 2 has been also obtained.

Dinuclear [Cu2(μ-OH)] fragment and its self-assembled tetranuclear part [Cu2(μ4-O)Cu2] are found in two copper(II) complexes of ligand H3bpmp: [Cu2(μ2-OH)(μ-H2bpmp)(μ1,3-NO3)][Cu2(μ2-OH)(μ-H2bpmp)(NO3)(H2O)](NO3)2·2H2O (1·2H2O) and [Cu4(μ4-O)(μ-H2bpmp)2(μ1,3-O2CCH2Ph)4] (2) [H3bpmp = 2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methylphenol]. These complexes have been synthesized and characterized by single crystal X-ray crystallography. Complex 1 showed unique reactivity with phenylacetate anions to provide 2 in which two [Cu2(μ2-OH)] units available within same crystal lattice collapsed to give [Cu4(μ4-O)] hub. Formation of the [Cu4] compound is also achieved from individual components and phenylacetate ligands, which allows an effective tuning of the self-assembly of two ligand bound [Cu2] fragments and removal of H+ from hydroxido bridges to provide oxido nucleus.

2-(2′-Hydroxyphenyl) imidazoline ring grafted dinucleating diimine-diamine-tris-phenol ligand (H3aeas) has been obtained from a two-step reaction of 2-hydroxy acetophenone, N,N′-bis-(2-aminoethyl)ethylenediamine and 2-hydroxy benzaldehyde. Reaction of the ligand with Co(ClO4)2·6H2O and NEt3 in MeOH–DCM solvent mixture yielded the monometallic complex [Co(aea)]ClO4·H2O (1) of imidazolidine ring hydrolyzed hexadentate proligand H2aea. Any solvent derived MeO− bridged Co2 complex could not be obtained due to facile cobalt coordination assisted hydrolytic cleavage of substituted imidazolidine ring. When the reaction is carried out with Co(NO3)2·4H2O and CoCl2·6H2O in presence of NH4NCS and NaN3 in MeOH–DCM and MeOH under aerobic conditions, preassembly of bimetallic [Co2(μ-OMe)]5+ and [Co2(μ-N3)]5+ cores takes place on the solvent derived methoxido and azido clips through non-hydrolytic pathways in [(SCN)2Co2(μ-OMe)(μ-aeas)]·DMF (2), and cocrystals of [(N3)2Co2(μ-OMe)(μ-aeas)] (3a) and [(N3)2Co2(μ-N3)(μ-aeas)] (3b), respectively.

Two mixed ligand complexes trans-[Ni(NCS)4(PpzH)2] (1) and trans-[Ni(NCO)2(Imd)4] (2) (where Ppz = piperazine; Imd = imidazole) were synthesized from one-pot reactions, and their crystal and molecular structures were determined. The reaction of NiCl2·4H2O with piperazine and KSCN in a 1:2:4 mole ratio in aqueous MeOH at room temperature under stirring condition affords 1. Whereas the reaction of NiCl2·6H2O with imidazole and NaOCN in a 1:4:2 mole ratio in aqueous MeOH at room temperature under stirring condition yields 2 quantitatively. The nickel ions are hexacoordinated by six nitrogen donor atoms of two different origin, thiocyanate and piperazinium ions in case of 1 and cyanate ion and neutral imidazole in case of 2, to form tetragonally elongated and quite regular octahedral arrangements. In 1, the average Ni–Nppz distance is 2.198 A is longer than the average Ni–Nthc distance at 2.070 A, whereas, in case of 2, the average Ni–Nimd distance at 2.112 A and is close to the average Ni–Ncyt distance at 2.107 A. The reactions in other varying stoichiometries failed to provide the hitherto unknown complexes [Ni(NCS)2(Ppz)4] and [Ni(NCO)4(ImdH)2] confirming the ease of protonation on non-bonding of piperazine is more compared to that of imidazole.

The reaction of Cu(ClO4)2·6H2O with dimethylglyoxime (H2dmg) in a 1:1 mole ratio in aqueous methanol at room temperature affords the dinuclear complex [Cu2(μ-Hdmg)4] (1). Reaction of 1 with [Cu(bpy)(H2O)2](ClO4)2 (bpy = 2,2′-bipyridine) in a 1:1 mole ratio in aqueous methanol at room temperature yields the tetranuclear complex [Cu4(μ-Hdmg)2(μ-dmg)2(bpy)2(H2O)2](ClO4)2 (2). The direct reaction of Cu(ClO4)2·6H2O with H2dmg and bpy in a 2:2:1 mole ratio in aqueous methanol at room temperature also yields 2 quantitatively. The complexes 1 and 2 were structurally characterized by X-ray crystallography. Unlike the binding in Ni/Co-dmg, two different types of N−O bridging modes during the oxime based metallacycle formation and stacking of square planar units have been identified in these complexes. The neutral dinuclear complex 1 has CuN4O coordination spheres and complex 2 consists of a dicationic [Cu4(μ-Hdmg)2(μ-dmg)2(bpy)2(H2O)2]2+ unit and two uncoordinated ClO4− anions having CuN4O and CuN2O3 coordination spheres. The two copper(II) ions are at a distance of 3.846(8) A in 1 for the trans out of plane link and at 3.419(10) and 3.684(10) A in 2 for the trans out of plane and cis in plane arrangements, respectively. The average Cu–Noxime distances are 1.953 and 1.935 A, respectively. The average basal and apical Cu−Ooxime distances are 1.945, 2.295 and 2.429 A. The UV–Vis spectra of 2 is similar to the spectrum of the reaction mixture of 1 and [Cu(bpy)(H2O)2]2+. Variable temperature magnetic properties measurement shows that the interaction between the paramagnetic copper centers in complex 1 is antiferromagnetic in nature. The EPR spectra of frozen solution of the complexes at 77 K consist of axially symmetric fine-structure transitions (ΔMS = 1) and half-field signals (ΔMS = 2) at ca. 1600 G, suggesting the presence of appreciable Cu–Cu interactions.

A new Ni4 distorted cubane complex [Ni4(μ3-OMe)4Q4(MeOH)4] (1) (where Q− is the anion of 8-quinolinol) is obtained from the reaction of NaQ with Ni(OAc)2 · 4H2O in refluxing MeOH via solvent derived μ3-OMe assisted self-assembly of four nickel(II) centres. The periphery of [Ni4(OMe)4] cubane is covered by four Q− and four MeOH molecules. This methanol specific reaction is not supported in solvent glycinol (Hgl; NH2(CH2)2OH), an amine substituted ethanol, producing monomeric [NiQ2(Hgl)2] · 2H2O (2 · 2H2O) instead and is able to cleave 1 to yield 2 · 2H2O. The cryomagnetic susceptibility data of powdered 1 can be modeled by a two J equation yielding J1 = −1.8(1) cm−1, J2 = 3.9(1) cm−1 and g = 2.24.

The double μ3-OMe supported tetrameric Co4 complex [Co4(μ3-OMe)2(μ-bemp)2(N3)2(OH2)2](ClO4)2 · 4H2O {1 · (ClO4)2 · 4H2O} was isolated from the reaction of Co(ClO4)2 · 6H2O with H3bemp (where H3bemp = 2,6-bis-[(2-hydroxy-ethylimino)-methyl]-4-methyl-phenol) and NaN3 in presence of NEt3 in aqueous methanol. The X-ray structure analysis reveals a cationic tetrameric arrangement consisting of a defective dicubane-like core with two missing vertexes. The four Co(III) ions are connected through in situ generated μ3-OMe and central phenolate plus terminal ethanolate bridges of the ligand. Terminal aqua and azido coordinations finally fulfil the octahedral geometries around the CoIII ions within this Co4 aggregate.

Reactions of Co(OAc)2 · 4H2O with NaQ (sodium salt of 8-quinolinol) in methanol in absence and presence of NaN3 give the mononuclear complex mer-CoQ3 · MeOH (1) and a linear trinuclear mixed valence complex [Co3(N3)4Q4(OH2)2] · 2H2O (2). Presence of N 3 - ions prevents the formation of tris chelate leading to an aggregation to 2. Four N 3 - anions bridges three cobalt ions all in μ-1,1 coordination mode, resulting in a trinuclear structure with a center of inversion. The complex contains a high-spin CoII and two terminal low-spin CoIII.

Atmospheric CO2 fixation in basic medium (initial pH 8.5) leads to a novel hydrogencarbonate single-oxygen bridged dicopper(II) complex [ Cu 2 II ( L ) ( HCO 3 ) ] · 4 H 2 O ( 1 · 4 H 2 O ) from the dinucleating ligand H3L (2-methyl-2-(2′-hydroxyphenyl)-1,3-bis[4-methyl-4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine). The same reaction in neutral medium (initial pH 6) gives the monocopper(II) complex [CuII(HL1)] · 2ClO4 (2) of the zwitterionic ligand (HL1), following 2-(2′-hydroxyphenyl) imidazolidine ring and terminal (2-hydroxyphenyl)methylimine hydrolysis. Both 1 · 4H2O and 2 have been characterized by IR and UV–Vis spectroscopy, conductivity measurements and X-ray crystallography. Variable temperature (2–300 K) magnetic measurements show that complex 1 · 4H2O exhibits weak intramolecular antiferromagnetic coupling.

Two dinucleating ligands H 3 L I x (subscript I for imidazolidine; x = 1, 2) having pendant imidazolidine rings have been reacted with cobalt salts in aqueous methanol at room temperature to obtain the mononuclear complexes [CoIIIL1]Cl · 2H2O (1 · 2H2O) and [CoIIIL2]ClO4 · H2O (2 · H2O) of hydrolyzed ligands H2Lx. Expected dicobalt complexes were not obtained due to the coordination induced hydrolytic cleavage of the central substituted imidazolidine rings. The complexes 1 · 2H2O and 2 · H2O were structurally characterized by X-ray crystallography. Both mer- and fac-ON2 binding modes of the two halves of the hexadentate ligands have been identified in these complexes. The mononuclear complexes have CoN4O2 coordination spheres with a N4O2 donor atom set of the two hexadentate ligands, forming isomeric distorted octahedral coordination geometries around the metal atoms. As a result, the imine nitrogen pairs coordinated trans to each other in [CoL1]+, whereas the phenolic oxygens are trans in [CoL2]+. In complex 1 · 2H2O the average Co–N(imine), Co–N(amine), and Co–O(phenolate) distances are 1.910, 1.959, 1.894 A, respectively, and in 2 · H2O the corresponding average distances are 1.944, 1.991 and1.887 A, respectively. Both the cobalt(III) complexes are diamagnetic. The UV–Vis spectra of the end products, after the imidazolidine ring hydrolysis, is different from the spectrum of the initial reaction mixture containing the μ-bis(tetradentate) ligands, H 3 L I x ( x = 1 , 2 ) and metal salts.

The reaction of triethylenetetramine, salicylaldehyde and benzaldehyde in 1:2:1 mole ratio in methanol at room temperature affords a novel μ-bis(tridentate) ligand H 2 L′ through the formation of an imidazolidine ring within the parent hexadentate precursor in a two step reaction. The ligand H 2 L′ reacts with Fe(ClO 4 ) 2 · 6H 2 O in aqueous methanol in the presence of triethylamine to form the mononuclear [Fe III L](ClO 4 ) complex, (where L 2− is the anion of the parent hexadentate H 2 saltrien ligand) after the cleavage of the imidazolidine ring. The mononuclear complex has a structure with an N 4 O 2 donor atom set of the hexadentate ligand forming a distorted octahedral coordination geometry around the metal atom as established from a crystal structure determination. The Fe–N(imine) distances are 1.934(10) and 1.948(9) A, Fe–N(amine) distances are 2.062(8) and 2.076(9) A and Fe–O(phenol) distances are 1.864(8) and 1.872(7) A. The terminal oxygen donor atoms occupy cis positions and the remaining four nitrogen atoms (two cis amine and two trans imine) complete the coordination sphere. The mononuclear complex has a magnetic moment 1.89 μ B corresponding to the low-spin 3d 5 configuration. The UV–Vis spectrum of the end product, after the imidazolidine ring hydrolysis, is different from the spectrum of the initial reaction mixture containing the μ-bis(tridentate) ligand H 2 L′.

First N,N-dimethylformamide coordinated chair-piperazine bridged dicopper(II/II) complex [ Cu 2 II ( μ - L ) ( DMF ) 2 ] ( ClO 4 ) 2 was obtained following a solution transformation of the heterocyclic imidazolidine spacer of the new μ-bis(tridentate) ligand (H2L′).

The reaction of μ-bis(tetradentate) ligand H3L′, formed from imidazolidine ring insertion within the parent hexadentate Schiff base ligand of salicylaldehyde and triethylenetetramine, with manganese(II) salt in air spontaneously transforms the binucleating ligand, with respect to the removal of the phenol substituted imidazolidine ring, affording the mononuclear manganese(III) complex of the parent hexadentate ligand (H2L). The mononuclear complex has MnN4O2 coordination sphere as established from a crystal structure determination. The imine nitrogen pairs are coordinated trans to each other in [MnIIIL]+. The Mn–N(imine), Mn–N(amine), and Mn–O(phenol) distances are, respectively, 2.001 (6), 2.246 (8) and 1.949 (6) A. The mononuclear complex has a magnetic moment corresponding to the high-spin 3d4 configuration. In dimethylformamide solution, two quasireversible couples for manganese(IV)–manganese(III) and manganese(III)–manganese(II) are observed in cyclic voltammetry with E1/2 values of 0.66 and 0.02 V versus SCE, respectively. One-electron nature of these two couples can be verified from current height and room-temperature solution coulometry data. The X-band EPR spectrum of the coulometrically oxidized and frozen methanol–toluene solution of [MnIIIL]+ consists of weak and strong resonances at g∼4 and g∼2, respectively, the latter resonance shows 55Mn hyperfine lines and forbidden lines are also resolved. The broad shoulder around 615 nm ( e ∼ 200 l mol−1cm−1) may be assigned to a d-d transition for Mn(III) in pseudotetragonal environment.

The reaction of nickel(II) acetate with RN(O)NNHC6H4SMe (HL1 : R = Me; HL2 : R = Prn; general abbreviation HL) in aqueous ethanol affords [NiL2] as brown crystalline solids. The X-ray structure of [NiL22] is reported. Each ligand acts in the tridentate meridional SNO fashion. The Ni(SNO)2 coordination sphere is severely distorted from octahedral geometry. The NiS distances, 2.519(2) and 2.549(2) A, constitute the longest nickel(II)thioether bonds reported so far. Due to low symmetry, the octahedral ν1 band of the complexes are split into components lying at ca 1400 and 1000 nm. The complexes display quasireversible cyclic voltammograms corresponding to the metal redox couple [NiIIIL2]+/[NiIIL2], E 1 2 ≈ 0.75 V (vs S.C.E.). Coulometrically generated [NiIIIL2]+ displays rhombic EPR spectra, the g values of the perpendicular components being larger than that of the parallel component corresponding to the (dz2)1 ground state.

The hexadentate ligands RN(O)NNHC6H4S(CH2)3SC6H4NHN(O)(NR (H2L1: R  Me; H2L2 : R  Prn; general abbreviation H2L) and their nickel(II) complexes, [NiL1] and [NiL2], have been synthesized. The X-ray structure of [NiL1] has revealed the presence of a severely distorted NiN2O2S2 coordination sphere with longer than normal NiS distances : 2.494(4) and 2.526(3) A. The low symmetry of the ligand field is reflected in large splittings of octahedral ν1 and ν2 bands. In dichloromethane solution the nickel(III)-nickel(II) redox couple is observable cyclic voltammetrically and the E 1 2 values are: [NiL1]  0.74 and [NiL2  0.75 V (vs SCE). Frozen (77 K) solutions of the electrogenerated nickel(III) species display rhombic EPR spectra: g1 = 2.195, g2 = 2.145, g3 = 2.038 for [NiL1+; g1 = 2.199, g2 = 2.138, g3 = 2.035 for [NiL2]+. The g1 and g2 parameters for each complex can be considered as split components of g⊥ of the idealized geometry affording the inequality g⊥ > g|, which corresponds to an effective axial elongation and (dz2)1 ground state.

The μ-bis(tetradentate) Schiff base ligand H3L (1), formed from salicylaldehyde and triethylenetetramine, affords [CuZn(OAc)(L)]·2H2O. It represents a new type of imidazolidine bridged heterodinuclear complex of copper(II)–zinc(II) as they are similar to the arrangement present in Cu,Zn superoxide dismutase.