Redox properties of cobalt(II) complexes with isoindoline-based ligands

Cobalt bis-(N-arylimino)isoindolinates undergo electrostatic interactions with DNA or react with alkyl hydroperoxides to form ketones and alcohols. Redox behavior of the metal center should affect such reactivities; therefore, six neutral Co^II(L)₂ complexes with L = bis-(N-arylimino)isoindolinates have been synthesized to elucidate the effect of the aryl substituents on the redox potential of the metal center. Redox properties of various M^II(L)₂ complexes (M = Mn, Fe, Co, Ni) are compared. Moreover, data are presented on the dismutation rates of superoxide radical anion (a knowingly sensitive reagent on the redox properties of the metal center) in the presence of the various Co^II(L)₂ complexes among identical conditions.     

Gaseous complexes of cobalt(II) bromide and pyridine

The stepwise decomposition of CoBr₂py₂(s) has been investigated on a thermobalance by the “modified entrainment” method yielding Δ₁H=88.6 kJ mol¹, Δ₁S=156.6 JK^?1 mol^?1 and Δ₂H=119.0 kJ mol^?1, Δ₂S=211.8 JK^?1 mol^?1 for the dissociation of the first and second pyridine. The evaporation of CoBr₂py₂(l) and the association of gaseous pyridine to CoBr₂py(l) forming CoBr₂py₂(g) has been studied by vis spectroscopy at 250?420°C. By combining the new results with literature values, a complete thermodynamic cycle for the solid-liquid-gas equilibria in the CoBr₂-pyridine system could be established. It shows that in solution the formation of CoBr₂py₂ is not determined by the cobalt-pyridine bond energy but by the solvation energy of the rectants.     

Dinuclear rhodium(II) pivalate complexes with N-donor ligands

Eight dinuclear rhodium(II) complexes containing various, (primarily, polyfunctional) N-donor ligands in the trans position with respect to the Rh-Rh bond were synthesized and characterized by X-ray diffraction. In the Chinese-lantern dinuclear rhodium(II) pivalates, Rh^II ₂ (μ-OOCCMe₃)₄(L)₂ (L is 2,3-diaminopyridine (2), 7,8-benzoquinoline (4), 2,2′:6′,2″-terpyridine (5), N-phenyl-o-phenylenediamine (7)), and Rh^II ₂ (μ-OOCCMe₃)₄L¹L² (3, L¹ is 2-phenylpyridine, L² = MeCN), the steric effects of the axial ligands are most strongly reflected in the Rh-N(L) and Rh-Rh bond lengths. The introduction of chelating ligands containing a conformationally rigid chelate ring leads to the cleavage of two carboxylate bridges to form the dinuclear double-bridged structure Rh^II ₂ (μ- OOCCMe₃)₂(OCCMe₃)₂(η²-L³)₂, where L³ is 8-amino-2,4-dimethylquinoline (6). The reaction of complex 7 containing coordinated N-phenyl-o-phenylenediamine with pyrrole-2,5-dialdehyde afforded the new Rh^II ₂(μ-OOCCMe₃)₄(L⁴)₂ complex (8) containing 5-(1-phenyl-1-H-benzimidazol-2-yl)-1H-pyrrole-2-carbaldehyde (L⁴) in the axial positions of the dirhodium tetracarboxylate fragment. The coordinated diamine differs in reactivity from the free diamine. The reaction of the former with the above dialdehyde affords the [1+1]-condensation product, viz., 5-{(E)-[(2-anilinophenyl)imino]methyl}-1-H-pyrrole-2-carbaldehyde, whereas the reaction of unsubstituted o-phenylenediamine gives 5-{(E)-[(2-aminophenyl)imino]methyl}-1-H-pyrrole-2-carbaldehyde (L⁵) . The reaction of the latter with Rh^II ₂(μ-OOCCMe₃)₄(H₂O)₂ affords the dinuclear complex Rh^II ₂(μ-OOCCMe₃)₂(OOCCMe₃)₂(η²-L⁵)₂ (9), which is an analog of complex 6 containing only two bridging carboxylate groups.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 581–591, March, 2005.     

Formation and reactivity of cobalt(III)-dioxygen complexes promoted by 2-methylpropanal: Oxidation of alkenes and alcohols

2-Methylpropanal promotes the formation of cobalt(III)-dioxygen species from catalysts1 or2 and dioxygen. The cobalt(III)-dioxygen complexes efficiently catalyse the oxidation of various alcohols to carbonyl compounds in the presence of dioxygen and 2-methylpropanal. Similarly substituted alkenes are smoothly transformed to the corresponding monoepoxides under these reaction conditions. EPR study of these reactions indicates that different activated dioxygen species are formed in the presence of catalysts1 and2.     

Synthesis, characterisation and X-ray structures of diorganotin(IV) and iron(III) complexes of dianionic terdentate Schiff base ligands

Diorganotin(IV) complexes, [SnR₂L] (1)-(4), (R = Me, Ph), of the terdentate Schiff bases N-[(2-pyrroyl)methylidene]-N′-tosylbenzene-1,2-diamine (H₂L¹) and N-[(2-hydroxyphenyl)metylidene]-N′-tosylbenzene-1,2-diamine (H₂L²) have been synthesised. The complexes were obtained by addition of the appropriate ligand to a methanol suspension of the corresponding diorganotin(IV) dichloride in the presence of triethylamine. However, the reaction between the precursor [η⁵-C₅H₅Fe(CO)₂]₂SnCl₂ and the Schiff bases in the presence of triethylamine gave (5) and (6), respectively. The crystal structures of the ligands and complexes have been studied by X-ray diffraction. The structure of [SnR₂L] complexes shows the tin to be five-coordinate in a distorted square pyramidal environment with the dianionic ligand acting in a terdentate manner. In 5 and 6, the iron atom is in a slightly distorted octahedral environment and is meridionally coordinated by two ligands. Spectroscopic data for the ligands and complexes (IR, ¹H, ¹³C and ¹¹⁹Sn NMR and mass spectra) are discussed and related to the structural information.     

Dinuclear cobalt(II) complexes with double phosphate ester bridges and tetradentate ligands having anisole or quinoline appendages

Phosphate esters provide a rigid and stable polymeric backbone in nucleic acids. Metal complexes with phosphate ester groups have been synthesized as structural and spectroscopic models of phosphate‐containing enzymes. Dinucleating ligands are used extensively to synthesize model complexes since they provide the support required to stabilize such complexes. The crystal structures of two dinuclear Co^II complexes, namely bis(μ‐diphenyl phosphato‐κ²O :O ′)bis({2‐methoxy‐N ,N‐bis[(pyridin‐2‐yl)methyl]aniline‐κ⁴N ,N ′,N ′′,O }cobalt(II)) bis(perchlorate), [Co(C₁₂H₁₀O₄P)₂(C₁₉H₁₉N₃O)₂](ClO₄)₂, and bis(μ‐diphenyl phosphato‐κ²O :O ′)bis({N ,N‐bis[(pyridin‐2‐yl)methyl]quinolin‐8‐amine‐κ⁴N ,N ′,N ′′,O }cobalt(II)) bis(perchlorate), [Co(C₁₂H₁₀O₄P)₂(C₂₁H₁₈N₄)₂](ClO₄)₂, with tetradentate 2‐methoxy‐N ,N‐bis[(pyridin‐2‐yl)methyl]aniline (L ¹) and N ,N‐bis[(pyridin‐2‐yl)methyl]quinolin‐8‐amine (L ²) ligands are reported. The complexes have similar structures, with distorted octahedral geometries around the metal centres. Both are centrosymmetric (Z ′ = 0.5), with the Co^II centres doubly bridged by diphenyl phosphate ester groups. A number of aromatic–aromatic interactions are present and differ between the two complexes as the anisole group in L ¹ is replaced by a quinoline group in L ². A detailed study of these interactions is presented.     

Crystal structure, spectroscopy and magnetism of trinuclear nickel(II), cobalt(II) complexes and their solid solution

Four new complexes [Ni₃(μ-L)₆(H₂O)₆](NO₃)₆·6H₂O (1), [Co₃(μ-L)₆(H₂O)₆](NO₃)₆·6H₂O (2), [Ni₃(μ-L)₆(H₂O)₄(CH₃OH)₂](NO₃)₆·4H₂O (3), [Co₃(μ-L)₆(H₂O)₄(CH₃OH)₂](NO₃)₆·4H₂O (4) (L = 4-amino-3,5-dimethanyl-1,2,4-triazole) were synthesized and structurally characterized by X-ray single-crystal diffraction. The structural analyses show that complex 1 and 2 are isomorphous; complex 3 and 4 are isomorphous. Four complexes all consist of the linear trinuclear cations ([M₃(μ-L)₆(H₂O)₆]⁶⁺ (M = Ni,Co) for 1 and 2; [M₃(μ-L)₆(H₂O)₄(CH₃OH)₂]⁶⁺ (M = Ni,Co) for 3 and 4), NO₃⁻ anions and crystallized water molecules. In the trinuclear cations, the central M(II) ions and two terminal M(II) ions are bridged by three triazole ligands. Other eleven solid solution compounds which are isomorphous with complex 3 and 4 were obtained by using different ratio of Ni(II) and Co(II) ions as reactants and ICP result indicates that ligand L has higher selectivity of Ni(II) ions than that of Co(II) ions. The magnetic analysis was carried out by using the isotropic spin Hamiltonian ? = −2J(?₁?₂ + ?₂?₃) (for complexes 1 and 3) and simultaneously considering the temperature dependent g factor (for complexes 2 and 4). Both the UV-Vis spectra and the magnetic properties of the solid solutions can be altered systematically by adjusting the Co(II)/Ni(II) ratio.     

Reactions of 2-pyridinethiolate cobalt(III) complexes with pyridinethiolate or benzenethiolate ligands

Four half-sandwich cobalt complexes, Cp^∗Co(2-PyS)₂ (2), Cp^∗Co(2-PyS)₂ · HI (3), Cp^∗Co(2-PyS) (4-PyS) (4), (Cp^∗Co)₂(μ-PhS)₂(μ-2-PyS)I (5) [Cp^∗ = pentamethylcyclopentadienyl, 2-PyS = 2-pyridinethiolate, 4-PyS = 4-pyridinethiolate, PhS = benzenethiolate] were successfully synthesized by the reactions of 2-pyridinethione, lithium 4-pyridinethiolate and lithium benzenethiolate with Cp^∗Co(2-PyS)I (1), respectively. Complexes 2 and 3 have the structures with two 2-pyridinethiolates ligands coordinated to the cobalt atom. Two different pyridinethiolates ligands can be identified in complex 4. The molecular structure of 5 consists of two Cp^∗-Co fragments, which are triply bridged by three sulfur atoms from different ligands. The molecular structures of 3 and 5 were determined by X-ray crystallographic analysis. All the complexes have been well characterized by elemental analysis, NMR and IR spectra.     

Flash photolysis studies of charge-transfer photochemistry of nickel(II) and cobalt(III) complexes

The photochemical behavior of cobalt(III) and nickel(II) complexes on excitation in the charge-transfer bands is reviewed in this article with particular reference to the study of intermediates. Investigations on the photoredox reactions of cobalt(III) and nickel(II) complexes using flash kinetic spectroscopic methods reveal details on the characteristics of the intermediates produced from the charge-transfer excited states of these metal complexes. The reactive species produced on photolysis of cobalt(III)-amine complexes activate molecular oxygen, producing mononuclear and dinuclear dioxygen species coordinated as superoxo and peroxo forms. Cobalt(III)-amino-acid complexes on photolysis lead to the formation of cobalt(III)-alkyl complexes which are identified as transients. The spectra and the decay kinetics are described with the view to elucidate mechanistic details. Nickel(II) macrocyclic complexes on excitation in the charge-transfer bands lead to oxidation of the metal centre. Scavenging experiments using dioxygen, alcohols and acids were carried out to understand the mechanistic details.     

Oxidative condensation reactions of (diethylenetriamine)cobalt(III) complexes with substituted bis(pyridin-2-yl)methane ligands

The synthesis and characterisation of Co(III) complexes derived from a condensation reaction with a central or terminal nitrogen of a dien ligand and the -carbon of a range of substituted bis(pyridin-2-yl)methane ligands are described. Aerial oxidation of bpm {bis(pyridin-2-yl)methane with Co(II)/dien or direct reaction with Co(dien)Cl₃ provided in low yield a single C–N condensation product 1 (at the primary terminal NH₂) after the pyridyl –CH₂– is formally oxidised to –CH⁺–. The methyl substituted ligand bpe {1,1-bis(pyridin-2-yl)ethane} behaves likewise, except both terminal (prim) and central (sec) amines condense to yield isomeric products 2 and 3. Two of these three materials have been characterised by single crystal X-ray crystallography. The corresponding reactions for the bis(pyridyl) ligand bpk {bis(pyridin-2-yl)ketone} provided C–N condensation products without the requirement for oxidation at the -C center; two carbinolamine complexes in different geometrical configurations resulted, mer-anti-[Co(dienbpc)Cl]ZnCl₄, 5, and unsym-fac-[Co(dienbpc)Cl]ZnCl₄, 6, {dienbpc=[2-(2-aminoethylamino)-ethylamino]-di-pyridin-2-yl-methanol}. In addition, a novel complex, [Co(bpk)(bpd-OH)Cl]ZnCl₄, 4, in which one bidentate N, N-bonded bpk ligand and one tridentate N, O, N-bonded bpd (the diol from bpk+OH⁻) were coordinated, was obtained via the Co(II)/O₂ synthetic route. When the bpc ligand (bpc=bis(pyridin-2-yl)methanol) was employed directly as a reagent along with dien, no condensation reactions were observed, but rather a single isomeric complex [Co(dien)(bpc)]Cl.ZnCl₄, 7, in which the ligand bpc acted as a N,N,O-bonded tridentate ligand rather than as a N,N-bidentate ligand was isolated. ¹³C, 1D and 2D ¹H NMR studies are reported for all the complexes; they establish the structures unambiguously.     

Studies of thermal decomposition of palladium(II) complexes with olefin ligands

Thermal decomposition kinetics of ML₂ (M = Ni(II) and Co(II); L = 5-(2-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)hydrazono)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione) complexes were investigated by thermogravimetric analysis (TGA). The first decomposition process of the NiL₂ and CoL₂ complexes occurs in the temperature range of 320–350 °C. Kinetics parameters corresponding to this step, such as activation energy, E_α, and apparent pre-exponential factor, ln A_aap, were calculated from the thermogravimetric data at the heating rates of 5, 10, 15 and 20 K min⁻¹ by differential (Friedman's equation) and integral (Flynn–Wall–Ozawa's equation) methods. The results show that the activation energy evidently depends on the extent of conversion. As far as their activation energy is concerned, NiL₂ complex shows a higher thermal stability than the CoL₂ complex.     

Structural characterization of dinuclear Ti(IV) complexes of rigid tetradentate dianionic diamine bis(phenolato) ligands; effect of steric bulk on coordination features

A small difference in diamine bis(phenolato) ligands, namely an additional single methylene unit, directs formation of dinuclear Ti(IV) complexes rather than mononuclear ones as characterized by X-ray crystallography. Varying steric bulk of the ligand affects the coordination number in the dinuclear complexes and the ligand to metal ratio. A ligand with reduced steric bulk leads to a L₂Ti₂(OiPr)₄ type complex featuring two octahedral metal centers bridged only by the two phenolato ligands, whereas a bulky ligand leads to a Ti₂(μ-L¹)(μ-OiPr)₂(OiPr)₄ type complex with a single chelating ligand, two bridging isopropoxo ligands, and two terminal isopropoxo groups on each of the two metal centers, which are of trigonal bi-pyramidal geometry.     

Redox-active ligand-mediated Co–Cl bond-forming reactions at reducing square planar cobalt(III) centers

Synthetic routes to new square planar cobalt complexes with redox-active amidophenolate chelates are presented. Contrary to previous reports, steric bulk on the ligands is not a prerequisite to formation of the low-coordinate materials. X-ray crystal structure metrical data of the neutral S = ½ complexes are most consistent with cobalt(III) bound to one iminobenzoseminonate(1–) radical and one amidophenolate(2–) ligand. Addition of 1e⁻ affords reduced congeners that are also square planar cobalt(III) because the redox-active ligand accepts an electron to generate bis(amidophenolate) species. The redox-activity of the ligands facilitates reactions with chlorine electrophiles to generate square pyramidal products containing new Co–Cl bonds. The bond-forming reactions all formally require oxidation of the metal fragment but there is no change in formal cobalt oxidation state. Instead, the reaction proceeds with oxidation of the amidophenolate ligands. Control of ligand oxidation state provides a mechanism for 1e⁻ versus 2e⁻ selectivity in the bond-forming redox reactions.     

Conductance behavior of some tris(ethylenediamine)cobalt(III) complexes and their ion association in aqueous solutions

The conductance behavior of some tris(ethylenediamine)cobalt(III) complexes was studied in dilute aqueous solutions at 25°C to investigate the ion-pair formation. The thermodynamic formation constants of the ion pairs [Co(en)₃]³⁺·X^– are 28 (chloride), 28 (bromide), 19 (nitrate), and 15 (perchlorate). These values were compared with theoretical values calculated by using Bjerrum's theory of ion association. The formation constant of [Co(en)₃]³⁺·Cl^– was larger than that obtained from the spectrophotometric measurement in solutions containing perchlorate ion. This difference in the formation constants was explained by considering the contribution of ion association of the complex cation with perchlorate ion.     

Preparation and structural characterization of nickel(II), cobalt(II), zinc(II) and tin(IV) complexes of the isatin Schiff bases of S-methyl and S-benzyldithiocarbazates

The molecular structures of the isatin Schiff bases of S-methyldithiocarbazate (Hisasme) and S-benzyldithiocarbazate (Hisasbz) have been determined by X-ray diffraction and their complexes of general formula [ML₂]·n(solvate) [M = Co²⁺, Ni²⁺, Zn²⁺; L = anionic forms of Hisasme or Hisasbz; solvate = DMF, DMSO; n = 1, 2] and [Sn(L)Ph₂Cl]·nMeOH (n = 0, 1) have been synthesized and characterized by a variety of physicochemical techniques and X-ray diffraction. The bis-ligand complexes, [Ni(isasbz)₂]·2DMSO and [Co(isasme)₂]·DMF have a six-coordinate, distorted octahedral geometry with the two uninegatively charged tridentate ONS ligands coordinated to the metal ions meridionally via the amide O-atoms, the azomethine nitrogen atoms and the thiolate sulfur atoms. By contrast, the crystal structure of [Zn(isasbz)₂]·2DMF shows a four-coordinate distorted tetrahedral geometry with the two Schiff bases coordinated as NS bidentate ligands via the azomethine nitrogen atoms and the thiolate sulfur atoms. Steric constraints of the rigid tridentate ligands lead to unusual ‘pseudo-coordination’ of the O-donors which occupy sites close to the metal but too distant to be considered as true coordinate bonds.The crystal structures of the tin(IV) complexes [SnLPh₂Cl]·nMeOH (L = isasme and isasbz; n = 0, 1) also show that the Schiff bases act as monoanionic bidentate NS chelating agents coordinating the tin(IV) ion via the azomethine nitrogen atoms and the thiolate sulfur atoms, the tin atom in each complex is five-coordinate with a highly distorted geometry intermediate of square pyramidal and trigonal bipyramidal. Again Sn?O contacts are weak and do not qualify as coordinate bonds.     

Palladium(II)-allyl complexes containing chiral N-donor ferrocenyl ligands

A study of the reactivity of enantiopure ferrocenylimine (S_C)-[FcCHN-CH(Me)(Ph)] {Fc =  (η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-} (1a) with palladium(II)-allyl complexes [Pd(η³-1R¹,3R²-C₃H₃)(μ-Cl)]₂ {R¹ = H and R² = H (2), Ph (3) or R¹ = R² = Ph (4)} is reported. Treatment of 1a with 2 or 3 {in a molar ratio Pd(II):1a = 1} in CH₂Cl₂ at 298 K produced [Pd(η³-3R²-C₃H₄){FcCHN-CH(Me)(Ph)}Cl] {R² = H (5a) or Ph (6a)}. When the reaction was carried out under identical experimental conditions using complex 4 as starting material no evidence for the formation of [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(Ph)}Cl] (7a) was found. Additional studies on the reactivity of (S_C)-[FcCHN-CH(R³)(CH₂OH)] {R³ = Me (1b) or CHMe₂ (1c)} with complex 4 showed the importance of the bulk of the substituents on the palladium(II) allyl-complex (2-4) or on the ferrocenylimines (1) in this type of reaction. The crystal structure of 5a showed that: (a) the ferrocenylimine adopts an anti-(E) conformation and behaves as an N-donor ligand, (b) the chloride is in acis-arrangement to the nitrogen and (c) the allyl group binds to the palladium(II) in a η³-fashion. Solution NMR studies of 5a and 6a and [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(CH₂OH)}Cl] (7b) revealed the coexistence of several isomers in solution. The stoichiometric reaction between 6a and sodium diethyl 2-methylmalonate reveals that the formation of the achiral linear trans-(E) isomer of Ph-CHCH-CH₂Nu (8) was preferred over the branched derivative (9). A comparative study of the potential utility of ligand 1a, complex 5a and the amine (S_C)-H₂N-CH(Me)(Ph) (11) as catalysts in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with the nucleophile diethyl 2-methylmalonate (Nu⁻) is reported.     

Synthesis and characterization of cobalt(III) tetraaza macrocyclic complexes containing chloro and azido ligands

The complexes [Co(L)Cl₂]Cl · 4H₂O (1) and [Co(L)(N₃)₂]N₃ · 2H₂O (2) (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo [14,4,0^1.18,0^7.12]docosane) have been synthesized, and structurally characterized by X-ray crystallography, spectroscopy and cyclic voltammetry. The crystal structure of (1) is centrosymmetric and the cobalt(III) atom has an axially elongated octahedral geometry with four nitrogen atoms of the macrocycle and two chloride ligands. The cobalt(III) ion in (2) is coordinated to four nitrogen atoms from the macrocycle, and two azide ligands in an octahedral environment, which forms the 1D polymer through hydrogen bonding contacts involving the cation, azide anion and solvent water molecules. Electronic spectra of the complexes also exhibit a low-spin octahedral environment. Cyclic voltammetry of the complexes undergoes a one-electron wave corresponding to Co(III)/Co(II) processes. The electronic spectra and electrochemical behaviors of the complexes are significantly affected by the nature of the axial ligands.     

Fine-tuning the electronic properties of highly stable organometallic Cu(III) complexes containing monoanionic macrocyclic ligands

A family of highly stable organometallic Cu(III) complexes with monoanionic triazamacrocyclic ligands (L(i)) with general formula [CuL(i)]+ have been prepared and isolated, and their structural, spectroscopic, and redox properties thoroughly investigated. The HL(i) ligands have been designed in order to understand and quantify the electronic effects exerted by electron donor and electron-withdrawing groups on either the aromatic ring or the central secondary amine or on both. In the solid state the Cu(III) complexes were mainly characterized by single-crystal X-ray diffraction analysis, whereas in solution their structural characterization was mainly based on 1H NMR spectroscopy given the diamagnetic nature of the d(8) square-planar Cu(III) complexes. Cyclic voltammetry together with 1H NMR and UV/Vis spectroscopy have allowed us to quantify the electronic effects exerted by the ligands on the Cu(III) metal center. A theoretical analysis of this family of Cu(III) complexes has also been undertaken by DFT calculations to gain a deeper insight into the electronic structure of these complexes, which has in turn allowed a greater understanding of the nature of the UV/Vis transitions as well as the molecular orbitals involved.