Dinuclear Cobalt Complexes Research Articles

Spectral assignments and molecular modeling of dinuclear cobalt(II) complexes with a bridged bidentate β-diketone and a cyclic octaamine ligand

Four new cobalt(II) complexes of the general formula [Co 2(Rac)tpmc](ClO 4) 3, where tpmc and Rac refer to N, N′, N″, N‴-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane and β-diketonato ligands i.e. 2,4-pentanedionato (acac), 1,3-diphenyl-1,3-propanedionato (dibzac), 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato (hfac) or 2,2,6,6-tetramethyl-3,5-heptanedionato (tmhd) ions, respectively, have been prepared. Elemental analyses, conductometric measurements, UV/VIS, IR, EPR and mass spectroscopy were used for the complex characterization. Each cobalt(II) ion is coordinated with four nitrogen atoms in an exo arrangement of tpmc groups while the additional β-diketone bridges metal-ion centers through the both enolate oxygens. The presence of different R-groups on the diketone influence the ν( C ▪ O) and ν( C ▪ C) vibrations in the IR spectra. These frequencies decrease in the order of the complexes with coordinated hfac>dibzac>acac>tmhd ligands. The mechanism of the mass spectral fragmentation of the complexes entails a multi-step decomposition process. The g eff-factors estimated from EPR spectra suggest a similar molecular character of the complexes. Molecular modeling calculations were used to characterize the complex species.

Synthesis, crystal structure and magnetic properties of a novel dinuclear cobalt(II) complex based on the mixed N- and O-donor ligands: [Co 2(pyrazine)(fumarato) 2(H 2O) 8]·7H 2O

The novel dinuclear complex [Co 2(pyrazine)(fumarato) 2(H 2O) 8]·7H 2O which contains the mixed N- and O-donor ligands has been prepared and structurally characterized. The structure is made up of the symmetrical pyrazine-bridged [Co 2(pyrazine)(fumarato) 2(H 2O) 8] moiety and 7 lattice water molecules. The X-ray analysis reveals that it is the first dinuclear complex in which the fumarato dianion acts as a unidentate ligand instead of a bridge. An extended two-dimensional structure was formed through hydrogen bonds. Magnetic susceptibility measurements reveal that a very weak antiferromagnetic interaction exists between the high-spin Co(II) ions within the binuclear unit with J=−0.16 cm −1 based on the spin Hamiltonian H=−2 JS 1 S 2.

Magnetic susceptibility calculation of the dinuclear cobalt complex [Co2(ox)tpmc](ClO4)2·3H2O

Abstract Magnetic properties of the dinuclear cobalt complex originate from the magnetically isolated “asymmetric dimers” where each Co(II) ion within the dimer is situated in the differently distorted octahedral ligand field. Calculated values of magnetic susceptibility, obtained by use of the anisotropic Heisenberg-asymmetric dimer model, are found to be in excellent agreement with experimental χ ( T ) data over a wide range of temperature (4–295 K). Values of fitted parameters, the exchange integral J and both g -factors of the dimer, are in a very good agreement with the literature data.

Dinuclear 1,4,7-triazacyclononane (tacn) complexes of cobalt(III) with amido and tacn bridges. Synthesis, characterization and reversible acid-accelerated bridge cleavage.

Amido-bridged dinuclear cobalt(III) complexes with 1,4,7-triazacyclononane (tacn) were synthesized from [Co(tacn)(O3SCF3)3] by treatment with potassium amide in liquid ammonia at 100 degrees C. Two isomeric triply bridged complexes, [(tacn)Co(mu-NH2)3Co(tacn)]3+ and [(tacn)Co(mu-NH2)2[mu-tacn(-H)]Co(NH3)]3+, were isolated as perchlorates, and the crystal structure of the perrhenate of the latter complex was determined by X-ray diffraction. In this compound a nitrogen atom (deprotonated) from one of the tacn ligands forms a third bridge together with two amido bridges. In 1.0 M (Na,H)ClO4 ([H+] 0.1-1.0 M) the two isomers undergo acid-accelerated amido bridge cleavage, as earlier found for chromium(III) analogues, in spite of the fact that such bridges are co-ordinatively saturated. The triamido-bridged isomer is in this acid medium in equilibrium with [(H2O)(tacn)Co(mu-NH2)2Co(tacn)(NH3)]4+. An isolated perchlorate of this complex appeared to be the salt of the trans-ammineaqua isomer as determined by X-ray diffraction. Equilibration from both sides fits the first-order rate constant dependence k(obs)=6.2(3) x 10(-5)[H+] + 2.1(2) x 10(-5)(s(-1)) at 40 degrees C. Prolonged treatment of the two triply bridged isomers in 1.0 M HClO4 at elevated temperature produces primarily triply bridged dinuclear species where one or two amido bridges have been replaced by hydroxo bridges.

High magnetic fields study of asymmetric Co(II) dimer in octahedral ligand environment

Magnetic properties of the dinuclear cobalt complex [Co 2(ox)tpmc](ClO 4) 2·3H 2O have been studied from the magnetization data obtained in the high magnetic field-low temperature experiment ( B≤20 T, T=4.2 K ). An excellent agreement between experimental and calculated data was achieved within the framework of the isolated Ising Co(II) pairs in the ground state, where each cobalt ion from the pair was situated in a differently distorted octahedral environment (‘asymmetric dimer’). The obtained values for g-factors, g a =(4.5±0.1) and g b =(3.3±0.1), and for the intracluster exchange integral J=(−16.0±0.5) K confirm both the previous results on the crystal structure of the complex under consideration and the assumption on the strong antiferromagnetic coupling between cobalt ions realized by the superexchange interaction over the asymmetric oxalato bridge.

An altered mechanism of hydrolysis for a metal-complexed phosphate diester.

Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 +/- 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 +/- 0.002, 15N = 1.0026 +/- 0.0002) indicate significant fission of the P-O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition-elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.

Cobalt(II) and manganese(II) complexes of unsymmetric compartmental ligands bearing adjacent {N 2O} and {N,S,O} donor sets

The crystal structures of the dinuclear cobalt(II) complex, [ Co 2( L 5)( OAc)( NCS) 2( CH 3 OH) ], and the trinuclear manganese(II) complex, [ Mn 3( L 7) 2( OAc) 2( NCS) 2 ], are reported.

Synthesis, structure, and magnetic properties of dinuclear cobalt(II) complexes with a new phenol-based dinucleating ligand with four hydroxyethyl chelating arms.

A new end-off type acyclic ligand with four hydroxyethyl arms, 2,6-bis[bis(2-hydroxyethyl)aminomethyl]-4-methylphenol [H(bhmp)], formed dinuclear cobalt(II) complexes [Co(2)(bhmp)(OAc)(2)]BPh(4) (1) and [Co(2)(bhmp)(OBz)(2)]BPh(4) (2). The complex 1.2.5CH(3)CN (C(50)H(62.5)BCo(2)N(4.5)O(9)) crystallizes in the monoclinic space group C2/c with dimensions a = 25.424(5) A, b = 13.376(2) A, c = 29.913(6) A, beta = 105.930(3) degrees, and V = 9781(3) A(3) and with Z = 8. X-ray diffraction analysis revealed a mu-phenoxo-bis(mu-acetato)dicobalt(II) core structure containing two octahedral cobalt(II) ions. Electronic spectra were investigated for 1 and 2 in the range 400-1800 nm, and the data were typical for the octahedral high-spin cobalt(II) complexes. Magnetic susceptibility was measured for 1 and 2 over the temperature range 4.5-300 K, and the data were analyzed well using our theoretical method. The best fitting parameters were kappa = 0.77, lambda = -116 cm(-1), Delta = 572 cm(-1), and J = -0.44 cm(-1) for complex 1 and kappa = 0.96, lambda = -93 cm(-1), Delta = 616 cm(-1), and J = -0.33 cm(-1) for complex 2.

Synthesis and stability of new hexaazamacrocyclic dinuclear cobalt complexes and its oxidation reactions

Homodinuclear cobalt complexes with a new 26-membered unsymmetric macrocyclic ligand: 3,6,10,18,22,25,-hexaaza-31,32-dihydroxy-14,29-dimethyl-tricyclo[25,3,1,1 11,17] dotriaconta-l(30),12,14,16(32),27,28-hexaene (L or BTBP), have been synthesized and characterized with elemental analysis, FAB-MS and X-ray. The ligand maintains preorganized dinuclear integrity for cobalt(II) complexes, while facilitating the formation of non-bridged dicobalt centers. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear complexes are formed with Co(II) from pH 2 through 12 in aq. solution. The stability constants and species distribution as a function of pH of the 1:1 and 1:2 [ligand: Co(II)] complexes were determined in KCI supporting electrolyte ( μ=0.100 M) at 25 °C. Dinuclear cobalt(II) complex could form a μ-peroxo adduct at 1 atm oxygen atmosphere with the oxygenation constant log K O 2 =3.19. The neutral homodinuclear complexes: [Co 2C 28H 44N 6O 2(NO 3) 2]·12H 2O has been synthesized and its crystal structure has been reported with triclinic system, a=8.3376(5), b=9.4501(6), c=14.1028(9) Å, α=73.9100(2), β=73.4940(2), γ=87.7310(2)° and Z=1. Each cobalt(II) ion is complexed by half of the coordination sites of the dinucleating macrocycle, and the anionic nitrate enters into the coordination sphere and completes a six-coordinated octahedron.

Synthesis of novel mononuclear and dinuclear cobalt(III) complexes with cyanamide derivative ligands

Synthesis of novel mononuclear and dinuclear cobalt(III) complexes with cyanamide derivative ligands

Synthesis and Absolute Configuration of Novel Mono- and Dinuclear Cobalt(III) Complexes Containing S-Phenylalanine

Two diastereomers of bis(1,3-diaminopropane)(S-phenylalaninato)cobalt(III) were prepared by reaction of S-phenylalanine with carbonatobis(1,3-diaminopropane)cobalt(III). The diastereomers were separated on an optically active Sephadex QAE column and their absolute configurations assigned by means of circular dichroism. In addition, 1HNMR spectra of the diastereomers were analyzed in terms of the population of the three predominant rotamers of the coordinated S-phenylalaninato ligand. One out of 24 theoretically possible diastereomers of the dinuclear species di- µ -hydroxo- tetrakis(S-phenylalaninato)dicobalt(III) was obtained by direct synthesis and its absolute configuration deduced from CD spectra.

Phosphate diester hydrolysis within a highly reactive dinuclear cobalt(iii) complex. Ligand effect on reactivity, transition state and dissociation

The hydrolysis of methyl aryl phosphate diesters coordinated to a dinuclear Co(III) complex ([Co2(tame)2(OH)2{O2P(OAr)(OMe)}]3+; tame = 1,1,1-tris(aminomethyl)ethane; 2) has been studied in aqueous solution at 25 °C. Hydrolysis of the phosphate diester is base catalysed and occurs 30 to 60 fold faster than in analogous complexes where tame is replaced by 1,4,7-triazacyclononane (tacn) (1). The second order rate constants for base catalysed hydrolysis of 2 are highly sensitive to the basicity of the aryloxy leaving group with βlg = −1.29 ± 0.03. This leaving group dependence is similar to that of 1 (βlg = −1.38 ± 0.01), showing that the ligand affects reactivity without greatly altering the transition state at phosphorus. The slight decrease in βlg is consistent with previous rationalisations of this high sensitivity. Dimethyl phosphate coordinated to both types of complex (3, tame; 4, tacn) only dissociates from the complex, with no hydrolysis. Base catalysed dissociation is slower with tame (3 20 fold slower than 4) but the pH independent reaction is faster (3 10 fold faster than 4). These data suggest that the reactivity and turnover properties of these dinuclear complexes may be tuned rationally and independently.

Dinuclear Cobalt(II) Complexes of an Acyclic Phenol-Based Dinucleating Ligand with Four Methoxyethyl Chelating Arms − First Magnetic Analyses in an Axially Distorted Octahedral Field

An end-off type acyclic ligand with four methoxyethyl chelating arms, 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)], formed dinuclear cobalt(II) complexes [Co2(bomp)(MeCO2)2]BPh4 (1) and [Co2(bomp)(PhCO2)2]BPh4 (2). X-ray analysis revealed that complex 1·CHCl3 contains two cobalt ions bridged by a phenolic oxygen and two acetate groups, forming a μ-phenoxo-bis(μ-acetato)dicobalt(II) core. Magnetic susceptibility was measured for 1 and 2 over the temperature range 1.8−300 K. The data were successfully analyzed using a new theoretical method assuming an axially distorted octahedral field (Δ = 753 and 775 cm−1, respectively) for each cobalt(II) ion to reveal weak antiferromagnetic interactions (J = −0.96 and −1.74 cm−1, respectively).

Dinuclear Cobalt(II) Complexes of an Acyclic Phenol-Based Dinucleating Ligand with Four Methoxyethyl Chelating Arms − First Magnetic Analyses in an Axially Distorted Octahedral Field

An end-off type acyclic ligand with four methoxyethyl chelating arms, 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)], formed dinuclear cobalt(II) complexes [Co2(bomp)(MeCO2)2]BPh4 (1) and [Co2(bomp)(PhCO2)2]BPh4 (2). X-ray analysis revealed that complex 1·CHCl3 contains two cobalt ions bridged by a phenolic oxygen and two acetate groups, forming a μ-phenoxo-bis(μ-acetato)dicobalt(II) core. Magnetic susceptibility was measured for 1 and 2 over the temperature range 1.8−300 K. The data were successfully analyzed using a new theoretical method assuming an axially distorted octahedral field (Δ = 753 and 775 cm−1, respectively) for each cobalt(II) ion to reveal weak antiferromagnetic interactions (J = −0.96 and −1.74 cm−1, respectively).

Synthesis and Coordination Chemistry of Novel Binucleating Macrocyclic Ligands with Amine-thioether and Amine-thiophenolate Donor Functions

Abstract The ability of the aromatic tetraaldehyde l,2-bis(4-tert-butyl-2,6-diformylphenylsulfanyl)-ethane (1) to function as a precursor in the preparation of binucleating hexaamine-dithiolate ligands has been investigated. Reductive amination of compound 1 with bis(aminoethyl)amine under medium-dilution conditions affords the macrobicyclic hexaamine-dithioether compound L1. Deprotection of the [1+2] condensation product gives the corresponding 24-membered hexa-amine-dithiophenol ligand H2L2. The formulation of L1 as a macrobicyclic amine-thioether was confirmed by an X-ray crystal structure determination of the tetranuclear nickel(II) complex of L1, [{(L1)Ni2Cl2}2(μ-Cl)3](BPh4) (2b). The formulation of the doubly deprotonated form (L2)2- of H2L2 as a 24-membered amine-thiophenolate ligand was confirmed by an X-ray crystal structure determination of the dinuclear cobalt(III) complex, [(L2)CoIII(μ-OH)](ClO4)2 · Cl (3). The preparation and the crystal structures of the new compounds are described.

Dinuclear Cobalt(II) Complexes of an Acyclic Phenol-Based Dinucleating Ligand with Four Methoxyethyl Chelating Arms − First Magnetic Analyses in an Axially Distorted Octahedral Field

An end-off type acyclic ligand with four methoxyethyl chelating arms, 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)], formed dinuclear cobalt(II) complexes [Co2(bomp)(MeCO2)2]BPh4 (1) and [Co2(bomp)(PhCO2)2]BPh4 (2). X-ray analysis revealed that complex 1·CHCl3 contains two cobalt ions bridged by a phenolic oxygen and two acetate groups, forming a μ-phenoxo-bis(μ-acetato)dicobalt(II) core. Magnetic susceptibility was measured for 1 and 2 over the temperature range 1.8−300 K. The data were successfully analyzed using a new theoretical method assuming an axially distorted octahedral field (Δ = 753 and 775 cm−1, respectively) for each cobalt(II) ion to reveal weak antiferromagnetic interactions (J = −0.96 and −1.74 cm−1, respectively).

Mononuclear and Dinuclear Cobalt Complexes and its Dioxygen Adduct with a New 24-membered Hexaaza Macrocyclic Ligand

Dinucleating 24-membered hexaazadiphenol macrocyclic ligand 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyl-tricyclo[23,3,1,111,15] triaconta-1(29), 11,13,15(30),25,27-hexaene (L or BDBPH), is prepared by the NaBH4 reduction of the Schiff base obtained from [2+2] template condensation of 2,6-diformyl-p-cresol with diethylenetriamine. The ligand maintains dinuclear integrity for cobalt (II) while facilitating the formation of bridging phenolate dicobalt cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear cobalt (II) complexes form from p[H] 2 through 11 in aqueous solution. The protonation constants of this ligand and stability constants of the 1:1, 1:2 ligand: cobalt(II) complexes were determined in KCl supporting electrolyte (μ = 0.100 M) at 25°C. The mechanism for the formation of dinuclear dioxygen cobalt(II) complexes is also described. The stability constants of mononuclear and dinuclear cobalt complexes were determined under nitrogen. Preliminary results show that the dinuclear dioxygen cobalt (II) complexes do not catalyze hydroxylation of adamantane in the presence of H2S as two-electron reductant.

New trisubstituted cyclopentadienyl ligands: synthesis, characterisation and catalytic properties of mono and dinuclear cobalt, rhodium, iron and ruthenium complexes

The synthesis of a set of dialkyl 4-alkoxycarbonylcyclopenta-1,3-diene-1,2-diacetates ( 1a– e) is described. Their coordinating abilities as anions have been investigated in relation to the formation of new sandwich, half-sandwich and dinuclear complexes and their structural features. We report here the preparation and characterisation of some complexes such as a mononuclear half-sandwich cobalt(1,5-COD) complex which has shown to be a very efficient catalyst for the cyclocotrimerisation reaction of alkynes and nitriles to pyridines. Half-sandwich rhodiumdicarbonyl complexes containing trisubstituted cyclopentadienyl ligands with ester chains of different length have been employed successfully as catalysts for hydroformylation of styrene. Finally ligands 1a– e have been used for the synthesis of ferrocenes and dinuclear carbonyl complexes of iron and ruthenium. The structures of the complexes 1,5-cycloctadiene[1-methoxycarbonyl-3,4-di(methoxycarbonylmethylene)cyclopentadienyl]cobalt [Co(MDMCp)COD] ( 9), dicarbonyl[1-methoxycarbonyl-3,4-di(methoxycarbonylmethylene)cyclopentadienyl]rhodium [Rh(MDMCp)(CO) 2] ( 2a) and of a new ferrocene complex [Fe(MDMCp) 2] ( 15a) have been determined by X-ray diffraction methods.

Synthesis, crystal structure and magnetic properties of novel dinuclear complexes of manganese, cobalt and nickel with 4-acetylbispyrazolone

Five novel transition metal complexes, MnL·2C 2H 5OH·3H 2O ( 1), MnL·2.5DMF·0.5H 2O ( 2), Co(HL)(NO 3)·C 2H 5OH ( 3), CoL·2.5H 2O ( 4), and NiL·C 2H 5OH·2H 2O ( 5), have been synthesized and characterized. H 2L represents bis(1′-phenyl-3′-methyl-5′-pyrazolone-4′)-1,5-pentanedione (L 2− and HL − are the deprotonated forms of H 2L). Spectroscopic data reveal that the title complexes are dimers. The tetraketone ligand L coordinates to the metal ions through two β-diketonates. The β-diketonate is mainly in the delocalized enol form. The enol-keto form β-diketone (HL −, one β-diketone remains in keto form and the other is in deprotonated enol form) also exists in the Co complex 3. The crystal structure of MnL·2.5DMF·0.5H 2O ( 2) was determined by X-ray diffraction. The ligand L is in a bridging mode connecting two metal ions. Mn 2+ is six-coordinated with a distorted octahedral coordination sphere. A variable temperature magnetic susceptibility investigation showed that a weak antiferromagnetic exchange interaction exists between the metal centers in the complexes. The 2 J values based on H =−2 J/ S 1 S 2 are −0.71 cm −1 (Mn1), −1.96 cm −1 (Co2), −1.74 cm −1 (Co3), and −2.02 cm −1 (Ni).

A novel tetraoxime and its dinuclear and tetranuclear transition metal complexes

A new bis(dioxime) ligand (H4L) containing the diphenyl ether moiety has been prepared by reacting 3,3′,4,4′-tetraaminodiphenyl ether (1) with 2,3-butanedione monoxime (2). Dinuclear copper(II) and cobalt(III) complexes of H4L exhibit a metal–ligand ratio of 2:1 and the ligand coordinates through the 4 nitrogen atoms, as do most bis(dioximes). The [Cu2(H2L)](ClO4)2 molecule coordinates to the other two copper(II) ions through the deprotonated oximate oxygens to yield a tetranuclear structure, doubly-bridged by the oximate groups in a cis arrangement. The structure of bis(dioxime) and its complexes were identified by elemental analyses, 1H-, 13C-n.m.r, i.r and m.s. spectral data.