Octahedral Cobalt Complexes Research Articles

Multi‐functional coordination crosslinks in poly(vinylamine) complexes with cobalt chloride

AbstractPoly(vinylamme) complexes with cobalt chloride hexahydrate exhibit the largest glass transition temperature enhancements that have been measured in this laboratory. Tg increases from 56°C in the undiluted polymer to 193°C when the molar concentration of cobalt is only 3%, based on the moles of polymeric repeat units. This translates to a 45°C enhancement in Tg per mol% cobalt. Higher glass transition temperatures have been measured for other polymeric complexes with d‐block salts, but this study reports the largest ΔTg=Tg(complex)‐Tg(undiluted polymer) when transition‐metal catalyzed chemical crosslinking reactions do not occur. A plausible explanation for this effect is based on the fact that cobalt chloride hexahydrate maintains pseudo‐octahedral symmetry in the undiluted crystalline state and in an amorphous glassy complex with the polymer. The lone pair on nitrogen in the amino sidegroup is a strong base and, most likely, displaces all four waters of hydration that coordinate directly to the metal center. In fact, estimates of the ligand field splitting and the ligand field stabilization energy for octahedral cobalt complexes that contain methyl amine (CH3NH2) model ligands suggest that five or six amino sidegroups from several different chains might coordinate to each metal center, drastically reducing the mobility of the polymer. Both chloride anions remain near each divalent Co2+ center, but either one or two Cl− might not reside in the first‐shell coordination sphere of the transition metal. This bonding picture suggests that octahedral cobalt acts as a multi‐functional bridge between five or six amino sidegroups. Empirical ligand field stabilization calculations support the concept that octahedral Co2+ is a multi‐functional coordination crosslinking agent, and the inorganic literature provides experimental verification that similar small‐molecule complexes (i.e., [Co(NH3)6]2+) exist. It is proposed that the onset of Tg occurs when sufficient thermal energy is supplied to remove either x or (x‐1) amino sidegroup ligands from the coordination sphere of cobalt, where x is either 5 or 6. This is an endothermic process based on bond energies, as well as calculations which focus on the relative energies of the metal d‐electrons.

The synthesis and characterization of new substituted diaminoglyoxime and its cobalt(III) complexes containing 15-membered dioxa-trithia macrocyclic moieties

A novel (E, E)-dioxime (H 2L) containing dioxa-trithia macrocyclic moieties has been synthesized from the reaction of 2,3-(4′-aminobenzo)-1,4-dioxa-7,10,13-trithia-cyclopentadecane-2-ene ( 2) which was prepared from 2,3-(4′-nitrobenzo)-1,4-dioxa-7,10, 13-trithiacyclopentadecane-2-ene ( 1) and (E, E)-dichloroglyoxime. Cobalt(III) complex of H 2L was prepared with pyridine together with chlorine ion as axial ligand. The BF 2 + -capped cobalt(III) complex is achieved by the reaction of the hydrogen bridged octahedral cobalt(III) complex with boron trifluoride etherat. This complex was used in the extraction of mercury(II) and silver(I) for a high percentage from aqueous media. The structure of the dioxime and its complexes are proposed according to elemental analysis, 1H, 13C NMR, IR and mass spectral data.

Cobalt-59 Nuclear Magnetic Relaxation Studies of Aqueous Octahedral Cobalt(III) Complexes

Variable-temperature 59Co nuclear magnetic longitudinal relaxation and line width measurements have been performed on dilute aqueous solutions of 12 octahedral cobalt(III) complexes spanning a large range of isotropic chemical shifts (−1−12 570 ppm) at two fields (4.7 and 11.7 T). A variable-concentration 59Co longitudinal relaxation study of Na[Co(ethylenediaminetetraacetate)] was performed, and the results of this study indicated a need for the use of dilute solutions. The quadrupolar mechanism of relaxation was found to be dominant for 59Co nuclei in diamagnetic octahedral complexes in aqueous solution, and values of the quadrupolar coupling constants in all 12 complexes have been determined. At temperatures above 323 K, the spin−rotation relaxation mechanism contributes to the overall rate of relaxation for 59Co in cobalt(III) complexes with high local symmetry or large moments of inertia. Previous claims that the chemical shielding anisotropy mechanism of relaxation is also a major contributor to the total 59Co relaxation in cobalt(III) complexes are apparently unjustified. We believe that the previous incorrect conclusions were due to concentration-dependent relaxation behavior that we and others have recently observed. An approximate absolute chemical shielding scale for cobalt is also presented.

The First Example of Dinuclear Co(III) Complexes Containing Crown Ether Moieties

AbstractThe new dinuclear, octahedral cobalt(III) complexes have been synthesized and then their ability to interact with 4,4′‐trimethylene dipyridine investigated. The 1H and 13C‐NMR, IR and elemental analysis data indicated that the cobalt(III) complexes are dinuclear.

Generation of an optically active octahedral cobalt complex by a chiral autocatalysis

Abstract Optically active cis-[CoBr(NH3)(en)2]Br2 could be readily obtained by the reaction of optically inactive [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in a water suspension. The optically active cis-[CoBr(NH3)(en)2]Br2 was not readily racemized in its aqueous solution, indicating it not to be produced by total spontaneous resolution but by asymmetric synthesis. This synthesis was exclusively observed when the weight ratio of[Co(H2O)2{(OH)2Co(en)2}2](SO4)2 to water was above 0.31. Since the crystal of each enantiometer of cis-[CoBr(NH3)(en)2]Br2 introduced into the reaction system lead to the preferential production of the same enantiomer, chiral autocatalysis was shown to be performed during the reaction. In the concentrated suspension, crystallization of cis-[CoBr(NH3)(en)2]Br2 that was produced by the reaction would be started during the relatively early stage of the reaction. If several crystals to be initially produced are enantiomerically unequivalent by statistical fluctuation, it may cause the spontaneous generation of the enantiomeric predominance.

Effect of Changes in Structure of Phosphoric Acid Esters on Solvent Extraction of Cobalt (II) and Nickel (II)

Complexation behaviors of phosphoric acid esters with cobalt (II) and nickel (II) in solvent extraction have been assessed to elucidate the relationship between the structure of phosphoric acid esters and the extraction phenomena. By using phosphoric acid esters with aryl and alkyl groups, it is concluded that the structure of the organic groups in the phosphates plays an important role in the formation of the complexes and the extraction behaviors. In the case of di (nonylphenyl) phosphoric acid, the octahedral complexes of cobalt (II) and nickel (II) form in the extraction, and the compound extracts the metal ions at the lowest pH among the phosphates used. By changing the phenyl group with an alkyl group, the extraction curve shifts to higher pH, and the formation of the tetrahedral complex of cobalt increases in the extraction. Furthermore, the increase of the steric hindrance of the alkyl groups in the phosphates causes the shift of the extraction curves to higher pH. In addition, the selectivity of cobalt over nickel increases with increasing the steric hindrance of the phosphates.

Trans-Tetraaquabis(p-nitrohippurato)cobalt(II) dihydrate

Crystals of [Co(4-NO 2 -hip) 2 (H 2 O) 4 ].2H 2 O (4-NO 2 -hip=p-NO 2 .C 6 H 4 CO.NHCH 2 CO 2 - ) contain molecules of the centrosymmetric octahedral cobalt(II) complex and lattice water molecules which are interlinked by hydrogen bonds. The Co atom lies on a crystallographic symmetry centre and has a slightly elongated octahedral environment arising from coordination by the carboxylic O atoms of two monodentate 4-nitrohippurate anions and the O atoms of four water molecules

Spectroscopic investigation of cobalt(III) complexes of 1,10-diamino-4,7-diazadecane (3,2,3-tet)

A series of cis and trans octahedral cobalt(III) complexes of the general formula [Co(3,2,3-tet)XY] +/3+ have been synthesized, together with some other new members, where XY = Cl 2, Br 2, (NO 2) 2, (N 3) 2, (CN) 2, (OCN) 2, (SCN) 2, (NH 3) 2, (NO 2)Cl, (NO 2)Br, (NO 2)SCN), (CH 3COO) 2, (C 6H 5COO) 2, CO 3 2−, C 2O 4 2− and en. The spectroscopic properties of these complexes are recorded. The IR and UV-vis spectra were employed to characterize the synthesized complexes and to confirm the cis-geometry for the carbonato, oxalato and ethylenediamine series, and the trans-geometry for the rest. In general the IR spectra for the CH 2 rocking in the 800-910 cm −1 region exhibit four weak absorption bands for the cis complexes and only two or three more intense bands for the trans complexes. The average ligand field strength Dq av for a number of quadridentate amines increases in the order; trpn < cyclen < 3,2,3-tet ∼ 2,2,3-tet < 2,3,2-tet < tren ≤ 2,2,2-tet.

Chiral recognition in the gas phase: mass spectrometric studies of diastereomeric cobalt complexes

Results of mass analyzed ion kinetic energy (MIKE) spectra and kinetic energy release (KER) measurements of diastereomeric octahedral cobalt complexes indicate that these diastereomers can be distinguished in the gas phase. Four alkyl tartrate esters were complexed to cobalt trisacetylacetonate (Co(acac) 3) in the presence of a chiral auxiliary, RR- and SS-threohydrobenzoin. Different KER values of the product ion generated from [Co(acac) 2/D- or L-diisopropyl tartrate] + reflect differences in the precursor ion structure. The dissociation pathway resulting in this product ion is believed to arise via a hydride transfer from the acetylacetonate ligand to the metal center with subsequent loss of neutral organic species. It has been established that two conditions are necessary for observation of chiral recognition in this system; (1) the cobalt complex must be octahedral and (2) a chemical kinetic resolving agent must be present during formation of the complex.

Efficient stereochemical regulation of octahedral cobalt(III) complexes by a chiral bidentate ligand. Part 1. Effect of N-alkyl substitutions

A series of octahedral cobalt(III) complexes involving a tetradentate diaminedicarboxylate and a chiral diamine was prepared: [CoL(am)]+ where L = ethylenediamine-N,N′-diacetate (edda2–), or its N,N′-dimethyl (dmedda2–) and N,N′-diethyl (deedda2–) derivatives; am =(1R),(2R)-cyclohexane-1,2-diamine (R,R-chxn), or its N,N′-dimethyl (R,R′-dmchxn) and N,N′-diethyl (R,R-dechxn) derivatives. The complexes were separated into their diastereoisomers using cation-exchange column chromatographies, and characterized by elemental analyses, electronic absorption, circular dichroism and 13C NMR spectroscopies The crystal structure of a dominant product in the [Co(edda)(R,R-dmchxn)]+ system, (–)550[Co(edda)(R,R-dmchxn)]ClO4·H2O, has been established, and the stereochemistry determined to be Δ-sym-cis; orthorhombic, space group P212121, a= 13.469(2), b= 19.184(3), c= 7.917(1)Å, Z= 4, R= 0.067, R′= 0.055, N-Alkyl substitutions on the edda ligand and those on the R,R-chxn ligand influence the stereochemistry of the cobalt(III) complexes in different ways. The former caused stereoselective formation of ∧-sym-cis forms, the latter preferential formation of Δ-sym-cis forms. High stereoselectivity was observed in the formation of [Co(deedda)(R,R-chxn)]+, yielding a final product distribution of 96%∧-sym-cis and 4%Δ-sym-cis forms, whereas [Co(edda)(R,R-dechxn)]+ yielded 2%∧-sym-cis and 98%Δ-sym-cis forms. The latter is due to steric interaction between N-alkyl substituents in the R,R-chxn chelate and acetate rings in the edda chelate, while the former is due to steric interaction between N-alkyl substituents in the edda chelate and NH in the R,R-chxn chelate. Such interactions are efficient in steric regulation of octahedral cobalt(III) complexes.

Rhodium(III) complexes with cyanide and sulfur-donor ligands: rhodium-103 nuclear magnetic resonance chemical shift correlations

The 103Rh NMR chemical shifts for hexa(cyano-κC)-, hexa(thiocyanato-κS)-rhodate(III) and tris(O,O′-diethyl dithiophosphato-κ2S,S′)rhodium(III) have been determined in aqueous solution. Two-bond spin–spin coupling is observed between 103Rh and 31P in the 103Rh NMR spectrum of [Rh{S2P(OEt)2}3], 2J(Rh–P)= 13 Hz. A value for the nephelauxetic ratio (β= 0.29 ± 0.05) was obtained for the [Rh(SCN)6]3– ion from a correlation between the 103Rh NMR chemical shifts of octahedral rhodium(III) complexes and the ligand-field parameter ratio, β/ΔE(1A1g–1T1g). Comparison with the nephelauxetic ratios of other S- and N-bonded complexes shows S-bonding between rhodium(III) and the thiocyanate ligand. An empirical correlation is demonstrated between the metal NMR chemical shift and the logarithm of the overall thermodynamic formation constant (log β6), for a range of octahedral rhodium(III) and cobalt(III) complexes, thus allowing the formation constants log β6= 47 ± 4 and 35 ± 3 to be estimated for [Rh(CN)6]3– and [Rh(SCN)6]3–, respectively. The stability of the aquafluororhodium(III) species is discussed. The thermodynamic basis for the dissolution of rhodium metal in aqueous solution using cyanide or thiocyanate is examined.

A 59Co NMR Study of the Interactions of Some Cobalt(III) Complexes in Nematic Lyomesophases

Abstract 59Co NMR spectroscopy was used to study the interactions of cationic or neutral octahedral cobalt(III) complexes such as [Co(NH3)6]3+, [Co(en)3]3+ (en = ethylenediamine), [Co(NO2)(NH3)5]2+, and [Co(acac)3] (acac = acetylacetonato) with surfactants in their nematic liquid crystalline solutions. The specific interactions of the complexes with the surfactants were studied in a replacement of one surfactant by another that had a different polar group. The extent of the binding of these complexes was monitored through the 59Co quadrupole splittings. The relationship between the extent of the binding and the magnitude of the quadrupole splitting did not hold when comparing the three cationic complexes. A comparison between the potassium dodecanoate (KDodec)/tetradecyl (or decyl)trimethylammonium bromide (TDTMABr or DTMABr) and the cesium N-dodecanoyl-L-alaninate (CsDDA)/tetradecyltrimethylammonium bromide (TDTMABr) mixed surfactant systems in the interaction with [Co(NH3)6]3+ and [Co(en)3]3+ complexes showed that these complexes bind to the alaninate polar group through a weakly interacting site in the range of the intermediate composition of the mixed surfactants; such a kind of the binding was negligible or smaller in the dodecanoate system. [Co(acac)3] complex was solubilized to liquid crystals and the interactions with the surfactant depended on their polar groups in spite of the electric neutrality of the complex.

Bis(dimethylviolurato)(phenanthroline)cobalt(II), a low-spin octahedral cobalt(II) complex. Crystal structure of [Co(dmvi)2phen].2CHCl3

The complexes of formula [Co(dmvi) 2 phen].2solv (dmvi, C 6 H 6 N 3 O 4 , dimethylviolurate anion; phen, C 12 H 8 N 2 , 1,10-phenanthroline; and solv=H 2 O, CHCl 3 ) have been synthesized, and the structure of chloroform solvate has been determined by X-ray crystallographic methods. Crystal data: monoclinic system, space group C2/c.

A detailed infrared and far-infrared spectral investigation of the alkyl- and (non-alkyl)cobalt(III) complexes of 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime and a study of ground state trans-effect in tetragonally distorted octahedral cobalt(III) complexes by far-infrared and electronic absorption spectroscopy

The tetraaza quadridentate chelate, 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime [(DOH) 2bzo], forms stable tetragonally distorted octahedral cobalt(III) complexes with monodentate ligands such as halogens, pseudohalogens and Lewis bases in the axial sites besides forming pseudooctahedral alkylcobalt(III) complexes. A detailed IR and far-IR spectral investigation has been made for nine alkyl- and nineteen (non-alkyl)cobalt(III) complexes, reported recently. These complexes exhibit IR absorptions such as ν(OH), ν(CH), ν(NO), ν(CN), ν(CC), ν(CN), ν(CC) and CCH 3 symmetric and asymmetric deformation vibrations characteristic of the (DO)(DOH)bzo ligand framework and absorptions characteristic of the axial ligands. In the case of the complexes of aromatic Lewis bases vibrations such as ring deformation, ring breathing mode, ring stretchings and ring torsion characteristic of the aromatic rings are observed. Spectral evidence is obtained for the formation of an intramolecular hydrogen bond. The effect of hyperconjugative electron release and π-backbonding are manifested in the spectra of the complexes of 4-methylpyridine and triphenylphosphine, respectively. The far-IR spectra show vibrations such as ν(CoN), ν(CoBr), ν(CoC) and ν(Co-base) (Lewis base in the axial site). The ground state trans-effect in a series of tetragonally distorted octahedral (non-alkyl)cobalt(III) complexes of the type [LCo((DO)(DOH)bzo)Br] + (where L=a Lewis base such as pyridine, imidazole, methylimidazole, 4-methylpyridine, benzimidazole, 5,6-dimethylbenzimidazole, ethylamine, diethylamine, triethylamine, 2-aminopyridine, triphenylphosphine) is studied by following the variation of the CoBr stretching frequency on the field strength, Dq z , of the Lewis base in the axial site, trans to Br −. A plot of ν(CoBr) frequency versus Dq z is linear and as the Dq z increases ν(CoBr) frequency decreases. As the electron donating power of the axial Lewis base increases it transmits more electron density to Co 3+ with a concomitant transmission of electron density to Br −, trans to the Lewis base, thereby weakening the CoBr bond. This is the first report of the ground state trans-effect being studied using a quantitative measure of the electron donating ability of a series of test ligands on the ground state property of a probe ligand involving a heavy atom in six-coordinate cobalt(III) complexes.

Picosecond excited-state dynamics in octahedral cobalt(III) complexes: intersystem crossing versus internal conversion

The results of transient absorption experiments are reported for aqueous solutions of the low-spin d 6 complexes [Co(en) 3 ] (ClO 4 ) 3 , [Co(tpen)] (ClO 4 ) 3 , and [Co(tppn)] (ClO 4 ) 3 , where en is ethylenediamine, tpen is tetrakis(2-pyridylmethyl)ethylenediamine and tppn is tetrakis(2-pyridylmethyl)-1,2-propylenediamine. Following excitation at 314 nm with a ∼500-fs pulse, all three complexes exhibit biphasic relaxation kinetics. In the case of [Co(tpen)] 3+ excited-state decay is biphasic with relaxation time constants of τ 1 =4±2 ps and τ 2 =44±5 ps

Highest Co III-Co II redox potential in Co IIN 6 ( S = [formula omitted]) complexes of tridentate ligands. Predominance of steric over electronic effect

Using the tridentate ligand 2,6-bis(pyrazol-l-ylmethyl)pyridine and its methyl-substituted derivatives a new group of high-spin grossly octahedral cobalt(II) complexes has been synthesized and characterized. The field strength of these ligands with high-spin cobalt(II) progressively decreases with an increase in the number of methyl substituents near the donor site, as revealed from their crystal-field spectral analysis. The trends in the anodic peak potentials (Co III-Co II couple with E 1 2 / E pa values of 0.78–1.06 V vs S.C.E.) are rationalized on the basis of predominance of the steric effect over the electronic effect of the methyl groups present near the donor site.

光学活性有機コバルト錯体の合成とその液相および固相・結晶相反応

Optically active a carbon-chiral 1-substituted ethyl and 1, 2-disubstituted ethyl bis (dimethylglyoximato) cobalt complexes were prepared by resolution of chiral axial base-coordinated complexes followed by ligand displacement of the chiral axial base with achiral base. An unprecedented type of chiral octahedral cobalt complexes, trans- [Co (AB) 2 (X) (Y)], have also been synthesized enantioselectively. By taking advantage of the features of these chiral complexes, some precise information was obtained on the mechanism of Co-C bond formation and cleavage of the alkyl cobaloxime complexes, and Co-C bond dissociation energies were estimated which, in turn, made it possible to estimate steric and electronic effects on the Co-C bond cleavage quantita-tively.Unexpectedly, these complexes lead to the findings of X-ray-induced racemization of α carbon-chiral cobalt complexes in the crystalline-state, solid state-specific phenomena in the solid state photoracemization, unidirectional β to α (solid-state specific) photoisomerization. The mechanisms of these reactions are elucidated based on the X-ray structural analyses.

X-Ray Structure and Conformational Analyses of Cobalt(III) Complexes Containing 1,3-Propanediamine and 1,4-Butanediamine

Abstract Crystal structures of octahedral cobalt(III) complexes containing diamines, NH2 (CH2)n NH2 with n = 3 or 4 (abbreviations are tn and tmd, respectively), were determined by X-ray analysis. In crystals of [Co(tn)(tmd)2] Cl3, (1), the tn chelate ring takes skew-boat form, and the conformation of the complex is lel lel2. In crystals of [Co(tn)2(tmd)]Br3·2.7H2O, (2), both two independent complex cations take syn-chair2 lel configuration. Molecular mechanics calculation indicated that the six-membered tn chelate ring may take skew-boat form in a crowded system to reduce the steric repulsions between the chelate rings, although the tn chelate ring originally prefers the chair form.

Evidence of steric influences on the CoC bond lengths in vitamin B 12 model compounds

The structures of trans-(NH 2Ph)(Me)Co(DH) 2 ( 1), trans-(NH 2Ph)(CH 2C(CO 2Et) 2Me)Co(DH) 2 ( 2), and trans-(PPh( cHex) 2)(CH 2C(CO 2Et) 2Me)Co(DH) 2 ( 3), where DH = monoanion of dimethylglyoxime have been determined by X-ray diffraction studies. The variations in the geometry of these octahedral cobalt(III) complexes are interpreted in terms of steric and electronic influences. Linear regression analyses of CoC bond lengths with α and d (measures of the geometrical deformation of the equatorial moiety) in methyl cobaloximes have confirmed that steric effects play a significant role in the CoC bond stability of vitamin B 12 models.

ChemInform Abstract: Synthesis, Characterization and Spectrochemistry, Electrochemistry and Spectroelectrochemistry of Bromo (Lewis Base) Cobalt(III) Complexes of 3,8‐Dimethyl‐5,6‐benzo‐4,7‐diazadeca‐3,7‐diene‐2,9‐dione Dioxime.

AbstractThe six‐coordinate Co(III) complexes (IIIa) are prepared as shown.