Phenolate and phenoxyl radical complexes of Cu(II) and Co(III), bearing a new redox active N,O-phenol-pyrazole ligand

The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, (pz)LH, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH(2)Cl(2) at room temperature, the cyclic voltammogram (CV) of (pz)LH exhibits a quasi-reversible one-electron oxidation process (at E(1/2) = 0.66 V vs. Fc(+)/Fc) attributed to the formation of the phenoxyl radical cation [(pz)LH]˙(+). (pz)LH reacts with M(II)(BF(4))(2) (M = Cu, Co) in a 2:1 ratio to afford the bis-Cu(pz)L(2) (1) and tris-Co(pz)L(3) (2) complexes respectively. The X-ray structure of 1 reveals a Cu(II) ion in a square-planar trans-Cu(II)-N(2)O(2) coordination environment whereas that of 2 consists of a Co(III) ion with an octahedral mer-N(3)O(3) coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate phenolate ligands respectively. Both structures are preserved in CH(2)Cl(2) solution, as revealed by their NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E(1/2): 0.43 and 0.58 V vs. Fc(+)/Fc) and three (E(1/2) = 0.12, 0.54 and 0.89 V vs. Fc(+)/Fc) reversible one-electron oxidation processes, respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1(+) and 2(+), which are stable for several hours at room temperature under inert atmosphere in CH(2)Cl(2). The UV/vis and EPR data obtained for 1(+) and 2(+) are unambiguously consistent with the latter being formulated as Cu(II)- and Co(III)-phenoxyl radical complexes, as [Cu(II)((pz)L˙)((pz)L)](+) and [Co(III)((pz)L˙)((pz)L)(2)](+) respectively.     

Formation and characterization of Co(III)-semiquinonate phenoxyl radical species

Co(III) complexes of N(3)O-donor tripodal ligands, 2,4-di(tert-butyl)-6-{[bis(2-pyridyl)methyl]aminomethyl}phenolate (tbuL), 2,4-di(tert-butyl)-6-{[bis(6-methyl-2-pyridyl)methyl]aminomethyl}phenolate (tbuL(Mepy)(2)), were prepared, and precursor Co(II) complexes, [Co(tbuL)Cl] (1) and [Co(tbuL(Mepy)(2))Cl] (2), and ternary Co(III) complexes, [Co(tbuL)(acac)]ClO(4) (3), [Co(tbuL)(tbu-cat)] (4), and [Co(tbuL(Mepy)(2))(tbu-SQ)]ClO(4) (5), where acac, tbu-cat, and tbu-SQ refer to pentane-2,4-dionate, 3,5-di(tert-butyl)catecholate, and 3,5-di(tert-butyl)semiquinonate, respectively, were structurally characterized by the X-ray diffraction method. Complexes 3 and 5 have a mononuclear structure with a fac-N(3)O(3) donor set, while 4 has a mer-N(3)O(3) structure. The cyclic voltammogram (CV) of complex 3 exhibited one reversible redox wave centered at 0.93 V (vs Ag/AgCl) in CH(3)CN. Complex 5 was converted to a phenoxyl radical species upon oxidation with Ce(IV), showing a characteristic pi-pi* transition band at 412 nm. The ESR spectrum at low temperature and the resonance Raman spectrum of 3 established that the radical species has a Co(III)-phenoxyl radical bond. On the other hand, the CVs showed two oxidation processes at E(1/2) = 0.01 and E(pa) = 0.92 V for 4 and E(1/2a) = 0.05 and E(1/2b) = 0.69 V for 5. The rest potential of 4 (-0.11 V) was lower than the E(1/2) value, whereas that of 5 (0.18 V) was higher, indicating that the first redox wave of 4 and 5 is assigned to the tbu-cat and the tbu-SQ redox process, respectively. One-electron oxidized 4 showed absorption, resonance Raman, and ESR spectra which are similar to those of 5, suggesting formation of a stable Co(III)-semiquinonate species, which has the same oxidation level of 5. The resonance Raman spectrum of two-electron oxidized 4 showed the nu(8a) bands of the semiquinonate and phenoxyl radical, which were absent in the spectrum of one-electron oxidized 5. Since both oxidized species were ESR inactive at 5 K, the former was concluded to be a biradical species containing semiquinonate and phenoxyl radicals coupled antiferromagnetically and the latter to a species with a coordinated quinone.     

Co(II) and Co(III) complexes of m-benziphthalocyanine

The syntheses and structural elucidations of three different cobalt complexes of m-benziphthalocyanine are reported; both Co(II) and Co(III) complexes can be generated, and the ring undergoes partial oxidation upon metalation with Co(OAc)2x4H2O.     

Phenolate complexes of iron(III) in dimethylsulphoxide solution

Summary The formation of complexes between Fe³⁺ and 2,4-dinitrophenol, 4-nitrophenol and 4-methylphenol is studied in dimethylsulphoxide solution. The reaction proceeds almost to completion and the occurrence, in solution, of complexes with higher stoichiometry than 11 is reported for the first time. The following stability constants are determined (25 °C, 0.1 M KClO₄): Fe^III-2,4-dinitrophenolate ₁=1.8×10³, ₂=4.4×10⁵; Fe^III-4-nitrophenolate ₁=1.10×10⁷, ₂=2.5×10¹² ₃=3.9×10¹⁶; Fe^III-4-methylphenolate =1.7×10¹².The binding of a phenolate ligand to iron(III) strongly decreases its affinity for other phenolate ligands. The superior -donor, 4-methylphenolate (OC₆H₄Me)^–, only forms a 11 complex, Fe(OC₆H₄Me)²⁺. The e.s.r. spectrum of this species clearly prove it is a true iron(III) complex and does not contain iron(II), in opposition to recent claims. However, the complex Fe(OC₆H₄Me)²⁺ easily undergoes a ligand-to-metal electron transference. Addition of an excess of 1,10 phenanthroline yields the iron(II) complex [Fe(o-phen)₃]²⁺ instantaneously.     

A phenoxyl radical complex of copper(II)

A new N,O-bidentate pro-ligand (HL), [ML2] (M = Cu, Zn) and [CuL2][BF4] have been synthesised; [CuL2].4DMF and [CuL2][BF4].2CH2Cl2 have been crystallographically and spectroscopically characterised; these data indicate that [CuL2]+ cations are constituted as [Cu2+(L.)(L-)]+ and involve the phenoxyl radical L..     

Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues.

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L(1)](3)(-) the third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L(2)](3)(-) it is 2-mercaptobenzyl, and in [L(3)](3)(-) it is 3,5-di-tert-butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes [M(III)L(1)] (M = Ga, Fe, Co), [M(III)L(2)] (M = Ga, Fe, Co), and [M(III)L(3)] (M = Ga, Co) where the gallium and cobalt complexes possess an S = 0 and the iron complexes an S = (5)/(2) ground state. Complexes [CoL(1)].CH(3)OH.1.5H(2)O, [CoL(3)].1.17H(2)O, [FeL(1)].H(2)O, and [FeL(2)] have been characterized by X-ray crystallography. Cyclic voltammetry shows that all three [M(III)L(1)] complexes undergo a reversible, ligand-based, one-electron oxidation generating the monocations [M(III)L(1)(*)](+) which contain a coordinated phenoxyl radical as was unambiguously established by their electronic absorption, EPR, and M?ssbauer spectra. In contrast, [M(III)L(2)] complexes in CH(3)CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [M(III)L(2)-L(2)M(III)](2+). [GaL(3)] behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate, but [Co(III)L(3)] in CH(2)Cl(2) at -20 to -61 degrees C displays a reversible one-electron oxidation yielding a relatively stable monocation [Co(III)L(3)(*)](+). Its electronic spectrum displays intense transitions in the visible at 509 nm (epsilon = 2.6 x 10(3) M(-)(1) cm(-)(1)) and 670sh, 784 (1.03 x 10(3)) typical of a phenylthiyl radical. The EPR spectrum of this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (g(iso) = 2.0226; A(iso)((59)Co) = 10.7 G).     

Dinuclear Co(II)Co(III) mixed-valence and Co(III)Co(III) complexes with N- and O-donor ligands: characterization and water oxidation studies

The tridentate ligand N-methyl-N,N-bis(2-pyridylmethyl)amine (L) has been employed to synthesize a dinuclear Co(II)Co(III) mixed-valence complex containing μ-methoxo and μ-carboxylato bridging ligands, [LCo(II)(μ-carboxylato)bis(μ-methoxo)Co(III)L](ClO(4))(2). In this complex, the two pseudo-octahedral Co centers have an identical ligand environment, yet the average Co-N and Co-O bond distances at the two Co ions differ significantly. Electrochemical, spectroscopic, and magnetic susceptibility measurements confirm that it belongs to a localized Class II mixed-valence system, despite the presence of a short Co···Co distance of 3.021 ?. Oxidation of this Co(II)Co(III) complex leads to formation of the corresponding Co(III)Co(III) complex that was characterized structurally and spectroscopically. In addition, dinuclear and trinuclear μ-hydroxo Co(III) complexes have been obtained in the presence of phosphate anions and absence of methanol, respectively, suggesting that an additional bridging ligand is needed to stabilize the Co(III)bis(μ-hydroxo)Co(III) fragment. Moreover, the ability of the mixed-valence Co(II)Co(III) complex and the three related Co(III) complexes to electrocatalytically oxidize water was also investigated. The observed limited water oxidation catalytic ability for these systems suggests that a multinuclear Co cluster and/or presence of O-rich ligands may be needed for the generation of efficient molecular Co-based water oxidation catalysts.     

Dinuclear Co(III)/Co(III) and Co(II)/Co(III) mixed-valent complexes: synthetic control of the cobalt oxidation level

The reactivity of cobalt(II) salts towards H(3)L (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) was studied in different reaction conditions. Accordingly, the interaction of cobalt(II) acetate with H(3)L in methanol gives rise to the discrete complex [Co(III)(2)L(OAc)(2)(OMe)]*1.5H(2)O.MeOH, 1. Reaction of cobalt(II) acetylacetonate with H(3)L in the presence of dicarboxylic acids was also investigated. Thus, when cobalt(II) acetylacetonate and H(3)L are mixed with terephthalic or malonic acid in 4 : 2 : 1 molar ratios, the mixed valent [Co(II/III)(2)L(acac)(p-O(2)CC(6)H(4)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*2H(2)O*2MeOH, 2 and [Co(II/III)(2)L(acac)(O(2)CCH(2)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*7H(2)O, complexes are isolated. Decreasing the pH of the medium, by addition of a second mol of dicarboxylic acid, leads to [Co(II/III)(2)L(O(2)CCH(2)CO(2))(MeOH)]*2MeOH, 4, while the reaction with terephthalic acid does not proceed. 1, 2 and 4 were crystallographically characterised and all the complexes are dinuclear, with hydrogen bonds that expand the initial nodes. The magnetic characterisation, as well as the NMR spectroscopy, indicates a diamagnetic nature for 1, in agreement with the presence of Co(III), showing the aerial oxidation suffered by the cobalt(II) ions. Nevertheless, are paramagnetic. Temperature variable magnetic measurements were recorded for the crystallographically characterised complexes 2 and 4 and these studies confirm the mixed valence Co(II)/Co(III) nature of the compounds. The best fits of the magnetic data give an axial distortion parameter Delta = 628.7 cm(-1) for 2 and 698.8 cm(-1) for 4, and spin-orbit coupling constant lambda = -117.8 cm(-1) for 2 and -107.0 cm(-1) for 4. Therefore, this study shows that the oxidation degree of the initial cobalt(ii) salt by atmospheric oxygen can be controlled according to the pH of the medium.     

H-bonding and steric effects on the properties of phenolate and phenoxyl radical complexes of Cu(II)

Herein, the N-R substituted N,O-phenol-pyrazole redox-active pro-ligands, (R)LH (R = Me, Pr) are reported together with their corresponding bis-Cu(R)L(2) complexes (2 and 3, respectively). The latter are reversibly oxidised to the corresponding stable Cu(II)-phenoxyl radical complexes 2(+) and 3(+). The properties of the tetrahedrally distorted complexes 2 and 3 (and those of 2(+) and 3(+)) are being compared to those of the square-planar H-bonded complex 1 (bis-Cu(H)L(2)) and those of 1(+). These studies have permitted H-bonding and steric effects on the redox, spectroscopic and chemical properties of Cu(II)-phenolate and Cu(II)-phenoxyl radical species to be established.     

Reverse-phase chromatographic separation of Co(II) and Co(III) as the Co(DEDTC)3 and Co(acac)3 complexes

A reverse-phase chromatographic method is described for the simultaneous determination of Co(II) and Co(III) using gradient elution of the Co(DEDTC)₃ and Co(acac)₃ complexes, respectively, with phosphate buffered acetonitrile water mixtures. The separated Co species are detected spectrophotometrically at 322?nm. The analytical range of the method is 0.1 to 1.25?μg/ml for Co(II) and 0.1 to 1.5?μg/ml for Co(III).     

Co(II) and Co(III) macrocyclic boron-bearing dioximates

Syntheses are reported for CoD₃(BF)₂ and [CoD₃(BF)₂]BF₄,where H ₂ D is dimethylglyoxime, -benzyldioxime, or cyclohexanedione dioxime. The IR spectra at 400–4000 cm ^–1 have been measured, as have the electronic absorption spectra and the¹H,¹³C,and ¹¹BNMR spectra; a comparison is made with the spectra of the analogous iron(II) complexes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 375–377, May–June, 1990.     

Heterovalent trinuclear Co(III)-Co(II)-Co(III) complexes with N-(2-Carboxyethyl)salicylaldimines

The heterovalent trinuclear cobalt complexes [Co₂^IIIL₄ ⁱ · Co^II(H₂O)₄] · nXmY (L ⁱ are deprotonated Schiff bases derived from substituted salicylaldehydes and β-alanine; i = 1–3) were obtained and characterized. An X-ray diffraction study of the trinuclear cobalt complex with N-(2-carboxyethyl)salicylaldimine showed that the central Co(II) ion and the terminal Co(III) ions are linked by bridging carboxylate groups. Either terminal Co(III) atom is coordinated to two ligand molecules. They form an octahedral environment consisting of two azomethine N atoms, two phenolate O atoms, and two O atoms of two carboxylate groups. The central Co(II) atom is coordinated to four water molecules and to two O atoms of two bridging carboxylate ligands involved in the coordination sphere of the terminal Co(III) atoms.     

Dinuclear triple helical Mn(III), Fe(III) and Co(III) complexes

Synthesis, characterization and physical properties of the dinuclear triple helical complexes [Mn₂(μ-L)₃] (1), [Fe₂(μ-L)₃] (2) and [Co₂(μ-L)₃] (3) with the tetradentate Schiff base (H₂L) derived from 1 mol equiv. of hydrazine and 2 mole equiv. of 2-hydroxy-1-naphthaldehyde are described. Triple helical molecular structures of 2 and 3 have been confirmed by X-ray crystallography. Magnetic susceptibility measurements reveal complex 3 is diamagnetic while a weak antiferromagnetic interaction is operative between the metal centres in both 1 and 2.     

Synthesis and physicochemical studies on Co(III) complexes

Complexes of Co(III) with 2-hydroxyacetophenone-thiosemicarbazone, 2-hydroxy-3-methylacetophenonethiosemicarbazone and 2-hydroxy-4-methyl-acetophenonethiosemicarbazone, and the addition complexes of 2-hydroxy-acetophenone thiosemicarbazone with ammonia, pyridine, aniline,o-toluidine,m-toluidine andp-toluidine have been synthesized and characterized on the basis of their conductivities, electronic and infrared spectral data. All complexes are low-spin octahedral in nature. Various parameters have been obtained using ligand field theory.     

Synthesis,characterization, and electrochemical studies of Co(II,III) dithiocarbamate complexes

Cobalt(II) complexes of N-methyl phenyl, 1-phenylpiperazyl, and morpholinyl dithiocarbamates have been synthesized and characterized by UV–Visible, FTIR, ¹H-, ¹³C-NMR, and mass spectrometry. The spectroscopic data indicated that two ligands coordinated in bidentate chelating to the metal ion to form four-coordinate cobalt(II) complexes (1–3), which was confirmed by mass analysis (TOF MS ES⁺) of the complexes with m/z [M]⁺ = 450.98, 382.94, and 382.94 for 1, 2, and 3, respectively. Single crystal analysis of 2A and 3A show centrosymmetric mononuclear cobalt(III) bonded to three dithiocarbamate ligands forming a distorted octahedral geometry, indicating the cobalt(II) undergoes aerial oxidation to cobalt(III) during recrystallization. In addition, 2A crystallized with one solvated molecule of toluene. The redox behaviors of the complexes were studied by cyclic and square wave voltammetry in dichloromethane; the result revealed a metal centered redox process consisting of a one-electron quasi-reversible process assigned to Co(III)/Co(IV) oxidation and a corresponding Co(IV)/Co(III) reduction. Randles–Sevcik plots (anodic peak current versus the square root of the scan rate (I_p,a versus ν^1/2)) for the redox couples revealed diffusion-controlled behavior.     

Potentiometric titration of Co(II) in presence of Co(III)

A potentiometric titration for cobalt(II) determination in the presence of Co(III) based on the oxidation of Co(II) with Na₂CrO₄ in ethylenediamine medium and back-titration of the oxidant excess with (NH₄)₂Fe(SO₄)₂ in acid medium is described. The titration is monitored with a Pt indicator electrode and carried out until the greatest jump of potential from one drop of titrant appears. A RSD smaller than 1.5% has been obtained for 50–300 μmol Co(II). The method proposed was applied in the analysis of a new type electroless copper plating solutions containing Co(II)-ethylenediamine complex compounds as reducing agents. Cu(II), Co(III) and Cr(III) do not interfere in the determination of Co(II).     

New dodecanuclear phenylphosphonate-bridged Co(II) complexes

New dodecanuclear phenylphosphonate-bridged Co(II) complexes [(μ₂-THF)₅(η-HPiv)Co₁₂(μ₃-OH)₄(μ₆-O₃PPh)₄(μ-Piv)₁₂]·THF·2С₆Н₆ (1·THF·2С₆Н₆), [(μ₂-THF)₆Co₁₂(μ₃-OH)₄(μ₆-O₃PPh)₄(μ-Piv)₁₂]·3THF (2·3THF) and [(μ₂-Hmhp)₆Co₁₂(μ₃-OH)₄(μ₆-O₃PPh)₄(μ-Piv)₁₂]·6.5MeCN (3·6.5MeCN) have been synthesized by the reaction of the cobalt(II) pivalate polymer {Co(Piv)₂}_n (Piv is the pivalate anion), as a source of metal fragments containing cobalt(II) atoms, with potassium phenylphosphonate (K₂PO₃Ph) in the presence of neutral O-donor additional ligands like molecules of THF or 6-methyl-2-pyridone (Hmhp). The structures of the dodecanuclear complexes 1-3 were determined by using single-crystal X-ray diffraction data. The magnetic properties of the compounds showed that complexes 1 and 3 exhibit antiferromagnetic exchange interactions between the magnetic centers. The stability of compounds 1 and 3 were studied using thermogravimetric analysis.     

Sulfur-bridged Co(III)Pt(II)Co(III) trinuclear complexes with mixed aliphatic and aromatic thiolate ligands: effect of nonbridging ligands on the homochiral linkage of cobalt(III) octahedrons by platinum(II)

The reaction of [Ni[Co(aet)(2)(pyt)](2)](2+) (aet = 2-aminoethanethiolate, pyt = 2-pyridinethiolate) with [PtCl(4)](2)(-) gave an S-bridged Co(III)Pt(II)Co(III) trinuclear complex composed of two [Co(aet)(2)(pyt)] units, [Pt[Co(aet)(2)(pyt)](2)](2+) ([1](2+)). When a 1:1 mixture of [Ni[Co(aet)(2)(pyt)](2)](2+) and [Ni[Co(aet)(2)(en)](2)](4+) was reacted with [PtCl(4)](2)(-), a mixed-type S-bridged Co(III)Pt(II)Co(III) complex composed of one [Co(aet)(2)(pyt)] and one [Co(aet)(2)(en)](+) units, [Pt[Co(aet)(2)(en)][Co(aet)(2)(pyt)]](3+) ([2](3+)), was produced, together with [1](2+) and [Pt[Co(aet)(2)(en)](2)](4+). The corresponding Co(III)Pt(II)Co(III) trinuclear complexes containing pymt (2-pyrimidinethiolate), [Pt[Co(aet)(2)(pymt)](2)](2+) ([3](2+)) and [Pt[Co(aet)(2)(en)][Co(aet)(2)(pymt)]](3+) ([4](3+)), were also obtained by similar reactions, using [Ni[Co(aet)(2)(pymt)](2)](2+) instead of [Ni[Co(aet)(2)(pyt)](2)](2+). While [Pt[Co(aet)(2)(en)](2)](4+) formed both the deltalambda (meso) and deltadelta/lambdalambda (racemic) forms in a ratio of ca. 1:1, the preferential formation of the deltadelta/lambdalambda form was observed for [1](2+) (ca. deltalambda:deltadelta/lambdalambda = 1:3) and [2](3+) (ca. delta(en)lambda(pyt)/lambda(en)delta(pyt):deltadelta/lambdalambda = 1:2). Furthermore, [3](2+) and [4](3+) predominantly formed the deltadelta/lambdalambda form. These results indicate that the homochiral selectivity for the S-bridged Co(III)Pt(II)Co(III) trinuclear complexes composed of two octahedral [Co(aet)(2)(L)](0 or +) units is enhanced in the order L = en < pyt < pymt. The isomers produced were separated and optically resolved, and the crystal structures of the meso-type deltalambda-[1]Cl(2).4H(2)O and the spontaneously resolved deltadelta-[4](ClO(4))(3).H(2)O were determined by X-ray analyses. In deltalambda-[1](2+), the delta and Lambda configurational mer(S).trans(N(aet))-[Co(aet)(2)(pyt)] units are linked by a square-planar Pt(II) ion through four aet S atoms to form a linear-type S-bridged trinuclear structure. In deltadelta-[4](3+), a similar linear-type trinuclear structure is constructed from the delta-mer(S).trans(N(aet))-[Co(aet)(2)(pymt)] and delta-C(2)-cis(S)-[Co(aet)(2)(en)](+) units that are bound by a Pt(II) ion with a slightly distorted square-planar geometry through four aet S atoms.