Coordination Sphere Of Cobalt Research Articles

Catalytic Synthesis of 1H-2-Benzoxocins: Cobalt(III)-Carbene Radical Approach to 8-Membered Heterocyclic Enol Ethers.

The metallo-radical activation of ortho-allylcarbonyl-aryl N-arylsulfonylhydrazones with the paramagnetic cobalt(II) porphyrin catalyst [CoII(TPP)] (TPP = tetraphenylporphyrin) provides an efficient and powerful method for the synthesis of novel 8-membered heterocyclic enol ethers. The synthetic protocol is versatile and practical and enables the synthesis of a wide range of unique 1H-2-benzoxocins in high yields. The catalytic cyclization reactions proceed with excellent chemoselectivities, have a high functional group tolerance, and provide several opportunities for the synthesis of new bioactive compounds. The reactions are shown to proceed via cobalt(III)-carbene radical intermediates, which are involved in intramolecular hydrogen transfer (HAT) from the allylic position to the carbene radical, followed by a near-barrierless radical rebound step in the coordination sphere of cobalt. The proposed mechanism is supported by experimental observations, density functional theory (DFT) calculations, and spin trapping experiments.

Catalytic Synthesis of 8-Membered Ring Compounds via Cobalt(III)-Carbene Radicals.

The metalloradical activation of o‐aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)‐carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium‐sized ring structures. Herein we make use of the intrinsic radical‐type reactivity of cobalt(III)‐carbene radical intermediates in the [CoII(TPP)]‐catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8‐membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8‐membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis‐allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring‐closure to the two different types of 8‐membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o‐quinodimethanes (o‐QDMs) which undergo a non‐catalyzed 8π‐cyclization, DFT calculations suggest that ring‐closure to the monobenzocyclooctadienes involves a radical‐rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring‐closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt‐porphyrin catalyst.

Catalytic Synthesis of 8‐Membered Ring Compounds via Cobalt(III)‐Carbene Radicals

AbstractThe metalloradical activation of o‐aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)‐carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium‐sized ring structures. Herein we make use of the intrinsic radical‐type reactivity of cobalt(III)‐carbene radical intermediates in the [CoII(TPP)]‐catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8‐membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8‐membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis‐allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring‐closure to the two different types of 8‐membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o‐quinodimethanes (o‐QDMs) which undergo a non‐catalyzed 8π‐cyclization, DFT calculations suggest that ring‐closure to the monobenzocyclooctadienes involves a radical‐rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring‐closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt‐porphyrin catalyst.

DNA Complexes with Cobalt(II) Phthalocyanine Disodium Disulfonate.

The interaction of cobalt phthalocyanine disodium disulfonate (CoPc) with calf thymus DNA in solutions was investigated by UV/vis spectrophotometry, circular dichroism (CD), and hydrodynamic methods (viscosity and flow birefringence). Two types of CoPc binding to DNA were observed. Fast CoPc interactions with DNA via external binding to phosphates were accompanied by the formation of stack-type phthalocyanine structures on the periphery of the DNA helix. The optical absorption spectra of such CoPc complexes with DNA were analyzed in order to obtain a binding constant K = (4.8 ± 0.4) × 104 M–1. CD spectra show the increasing optical activity of phthalocyanines bonded to DNA. DNA plays the role of a matrix, contributing to an increase in their stacking interactions. The CD spectrum of DNA varies slightly. The second type of cobalt-to-DNA binding manifests itself over a certain time. It can be associated with the reorganization of ligands in the cobalt coordination sphere by introducing DNA atoms. In our experiments, such binding was observed after storage of solutions for approximately 20 h at a temperature of 4 °C. It was shown that the minor groove of DNA remains free in CoPc–DNA complexes. CoPc does not bind with the most important group for metal coordinating to DNA in the major groove (N7 guanine). We completely excluded the intercalation binding model. The planes of phthalocyanines in CoPc–DNA complexes are oriented predominantly normal to the axis of the DNA helix. DNA rigidity (persistent length) does not change. This follows from the data on the measurement of the optical anisotropy and intrinsic viscosity of DNA in complexes.

Construction of alkyl-substituted pentaphosphido ligands in the coordination sphere of cobalt.

Rare mono- and diorganopentaphosphido cobalt complexes are accessible by P-P condensation using the unprecedented, reactive cobalt-gallium tetraphosphido complex [K(dme)2{(MesBIAN)Co(μ-η4:η2-P4)Ga(nacnac)}] (2). Compound 2 was prepared in good yield by reaction of [K(Et2O){(MesBIAN)Co(η4-1,5-cod)}] [1, BIAN = bis(mesitylimino)acenaphthene diimine, cod = 1,5-cyclooctadiene] with [Ga(nacnac)(η2-P4)] (nacnac = CH[CMeN(2,6-iPr2C6H3)]2). Reactions with R2PCl (R = iPr, tBu, and Cy) selectively afford [(MesBIAN)Co(cyclo-P5R2)] (3a-c), which feature η4-coordinated 1,1-diorganopentaphosphido ligands. The mechanism of formation of these species has been studied by 31P{1H} NMR spectroscopy and DFT calculations. In the case of 3a (R = iPr), it was possible to identify the intermediate [(MesBIAN)Co(μ-η4:η2-P5iPr2)Ga(nacnac)] (4) by single-crystal X-ray diffraction. A related, monosubstituted organopentaphosphido cobalt complex [(MesBIAN)Co(μ-η4:η1-P5 tBu)GaCl(nacnac)] (5) was isolated by reacting dichloroalkylphosphane tBuPCl2 with 2. Heterobimetallic complexes such as 2 thus may enable the targeted construction of a range of new metal-coordinated polyphosphorus frameworks by P-P condensation.

New complexes with 2-pyridyl ketone Schiff bases

Seven new complexes of 2-pyridyl ketone Schiff bases N,N′-bis-(pyridin-2-yl-benziliden)-propan-1,3-diamine (bpbpd) and N,N′-bis-[1-(pyridin-2-yl)-etiliden]-propan-1,3-diamine (bpepd) with Co(II), Ni(II), Zn(II) and Cd(II) nitrates and perchlorates were synthesized and investigated using elemental analysis, spectroscopic techniques (IR and UV–Vis–NIR) as well as magnetic and conductometric measurements. The structure of a cadmium(II) complex was accomplished by single-crystal X-ray diffraction. The Schiff base ligands act as NNNN tetradentate ligands coordinating through pyridine nitrogens and azomethine nitrogens to transition metal ions. The metal ions are six-coordinate for Co(II) and Ni(II), four-coordinate in the case of Zn(II) and eight-coordinate for Cd(II) ions. The coordination sphere of cobalt and nickel complexes is completed by two water molecules, resulting a distorted octahedron. In the case of zinc(II) complexes has been proposed a distorted tetrahedral geometry. The Cd(II) ions are surrounded by a tetradentate Schiff base ligand and two bidentate nitrate anions, leading to a dodecahedral coordination environment. Thermal decomposition of these compounds is a multistage process. Generally, water elimination occurs in the first step and the decomposition of nitrate groups and organic ligands in the next stages. The last step of decomposition for all complexes is strong exothermic and corresponds to oxidative combustion of the Schiff base ligand.

Symmetrical P4 Cleavage at Cobalt: Characterization of Intermediates on the Way from P4 to Coordinated P2 Units

Degradation of white phosphorus (P(4)) in the coordination sphere of transition metals is commonly divided into two major pathways depending on the P(x) ligands obtained. Consecutive metal-assisted P-P bond cleavage of four bonds of the P(4) tetrahedron leads to complexes featuring two P(2) ligands (symmetric cleavage) or one P(3) and one P(1) ligand (asymmetric cleavage). A systematic investigation of the degradation of white phosphorus P(4) to coordinated μ,η(2:2)-bridging diphosphorus ligands in the coordination sphere of cobalt is presented herein as well as isolation of each of the decisive intermediates on the reaction pathway. The olefin complex [Cp*Co((i)Pr(2)Im)(η(2)-C(2)H(4))], 1 (Cp* = η(5)-C(5)Me(5), (i)Pr(2)Im = 1,3-di-isopropylimidazolin-2-ylidene), reacts with P(4) to give [Cp*Co((i)Pr(2)Im)(η(2)-P(4))], 2, the insertion product of [Cp*Co((i)Pr(2)Im)] into one of the P-P bonds. Addition of a further equivalent of the Co(I) complex [Cp*Co((i)Pr(2)Im)(η(2)-C(2)H(4))], 1, induces cleavage of a second P-P bond to yield the dinuclear complex [{Cp*Co((i)Pr(2)Im)}(2)(μ,η(2:2)-P(4))], 3, in which a kinked cyclo-P(4)(4-) ligand bridges two cobalt atoms. Consecutive dissociation of the N-heterocyclic carbene with concomitant rearrangement of the cyclo-P(4) ligand and P-P dissociation leads to complexes [Cp*Co(μ,η(4:2)-P(4))Co((i)Pr(2)Im)Cp*], 4, featuring a P(4) chain, and [{Cp*Co(μ,η(2:2)-P(2))}(2)], 5, in which two isolated P(2)(2-) ligands bridge two [Cp*Co] fragments. Each of these reactions is quantitative if performed on an NMR scale, and each compound can be isolated in high yields and large quantities.

Ethenedithione (SCCS): Trapping and Isomerization in a Cobalt Complex

Swing bridge: The triplet species ethenedithione has been generated within the coordination sphere of cobalt, leading to a dinuclear μ-η(2)-η(2)-C(2)S(2) complex (see picture: C gray, Co blue, P purple, S yellow). Depending on the solvent, the C(2)S(2) moiety displays a transoid or a cisoid geometry. This isomerization step changes the sign of the magnetic coupling between the cobalt centers.

Synthesis, Crystal Structure and Electrochemical Property of New Rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)bis(azido)cobalt(III) Nitrate Methanol Solvate Complex

The new cobalt compound, [Co(C16H35N10]+(NO3)−·CH3OH (1), have been synthesized and characterized. It’s structure and electrochemical property were investigated using single-crystal X-ray diffraction and cyclic voltammetry in solutions of dimethyl sulfoxide (DMSO) and methanol (CH3OH). The X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group P21/c with a = 9.8339 (10) A, b = 16.4048 (17) A, c = 15.3628 (16) A, β = 104.96 (9)° and Z = 4. The complex 1 consists of one [Co(C16H35N10)]+ cation, one NO3 − anions and one solvent CH3OH molecule. The metal center of complex 1 is six-coordinated, with the four nitrogens of the macrocycle defining an equatorial plane and two axially ligated [N3]− ions completing the cobalt coordination sphere. Moreover, hydrogen bonds of O–H⋯N, O–H⋯O and N–H⋯O and static force connect two components of the compound and maintain the crystal structure. Additionally, in situ spectro-electrochemical measurements support Co(III)/Co(II) reversible reduction processes with the observation of the corresponding spectral changes with the applied potentials E app = −1.5 and 0.5 V. The results obtained by the method are in agreement with those by the CV methods. The paper is about synthesis, crystal structure and electrochemical property of a new complex.

Polymorphism of cobalt(II) complex with 6-bromopicolinic acid: The influence of the solution pH value on the formation of polymorphs

Triclinic ( 1) and monoclinic ( 2) polymorphs of cobalt(II) complex with 6-bromopicolinic acid, namely [Co(6-Brpic) 2(H 2O) 2]·3H 2O were prepared and characterized by spectroscopic methods (IR), their molecular and crystal structures were determined by X-ray crystal structure analysis and their thermal stability by TGA/DTA methods. Polymorph 1 was prepared by reaction of cobalt(II) acetate tetrahydrate and 6-bromopicolinic acid in aqueous solution at pH value of 6, while 2 was obtained by the same reaction in aqueous solution at pH value of 8. The pH value was adjusted with the addition of ammonia solution. This is an example of the influence of the solution pH value on the formation of polymorphs. X-ray structural analysis revealed octahedron as a coordination polyhedron in both 1 and 2 and the same N,O-chelated coordination mode of 6-bromopicolinic acid. The coordination sphere of cobalt(II) ion is completed by two coordinated water molecules. The polymorphs differ in their crystal structures, which are dominated by intermolecular O–H ⋯ O hydrogen bonds and π– π stacking interactions, and in the crystal system (triclinic vs. monoclinic). The infrared spectra of 1 and 2 are very similar so the polymorphs cannot be unambiguously distinguished by IR spectroscopy. Thermal methods confirm number of co-crystallized water molecules in both polymorphs.

The investigation of Co(II)-dextran complexes

Co(II) ion complex compounds were synthesized with reduced dextran (RD) in alkali solutions using CoCl2, 6H2O and isolated in the solid state. UV-VIS spectrophotometric investigations of Co(II) complexes with reduced dextran showed that the complexation of Co(II) ions begins at pH over 7. The formation of Co(II) ion complexes with dextran monomer units was observed at pH 7-13. With increase in solution pH the light absorption maximum of complex solutions shifts to longer wavelengths (bat-hochromic shift) compared with [Co(H2O)6]2+ ion. Dextran complexes with Co(II) ion are formed by the displacement of water molecules from the coordination sphere of cobalt by the OH ligand groups. The spectrophotometric parameters of the investigated complexes are characteristic of a Co(II) ion in octahedral or distorted octahedral coordination with O ligand atoms.

CRYSTAL STRUCTURE AND ANTIMICROBIAL ACTIVITY OF A HIGH SPIN COBALT(II) DIMER: [Co2(OH)2 (Ac)3(Bipy)2]·K ·2(H2O)

ABSTRACT The ionic cobalt (II) title complex [Co 2 (OH) 2 (Ac) 3 (Bipy) 2 ]·K ·2(H 2 O) (where Ac: Acetate, Bipy: 2,2-bipyridine) consists of anionic bimetallicunit in which cobalt ions are bridged by hydroxo and acetate ligands, balanced by a (disordered) potassium cation. The geometry at cobalt is distortedoctahedral with the O atoms of a monodentate and a bridging acetate groups occupying the apical sites and O atoms from the hydroxo groups and Natoms from Bipy, the equatorial sites in each cobalt coordination sphere. Magnetic susceptibility data of powdered samples of the compound arepresented.The antimicrobial activities of the complex has been screened in vitro against the microorganisms Staphylococcus aureus , Pseudomonasaeruginosa , Escherichia coli .Keywords: cobalt crystal structure, antibacterial activities. INTRODUCTION In recent years a substantial amount of work has been devoted topolynuclear complexes of transition metals, covering a very wide rangeof scientific interest,

Co(II)-Mediated and microwave assisted coupling between 2,6-diaminopyridine and nitriles. A new synthetic route to N-(6-aminopyridin-2-yl)carboximidamides

The Chinese-lantern-type Co2(O2CBut)4{2,6-(NH2)2C5H3 N}2 complex reacts with RCN (R = Me or Prn) under microwave irradiation (MWI) to give the mononuclear amidine complexes Co(O2CBut)2{H2N(C5H3 N)NHC(R)=NH} (R = Me (4a) or Prn (4c)). Under microwave irradiation, the addition of 2,6-diaminopyridine to acetonitrile in the presence of the pivalate complexes Co2(μ2-OH2)(O2CBut)4(HO2CBut)4 (1) or [Co(OH)n(O2CBut)2−n ]x (2) afforded complex 4a in higher yield compared to the corresponding reaction performed earlier without MWI. The use of MWI makes it possible to perform analogous reactions with nitriles RCN (R = Et, Prn, or Ph) giving rise to complexes 4b—d, respectively. Compounds 4a—d were characterized by elemental analysis and IR spectroscopy. The structure of complex 4c was established by X-ray diffraction. Amidines H2N(C5H3N)NHC(R)=NH, which formed in the coordination sphere of cobalt(II), were isolated in the free state from methanolic solutions of complexes 4a—d under the action of Na2S and were characterized by electrospray mass spectrometry and 1H and 13C{1H} NMR spectroscopy. The reactions of 2-aminopyridine with both complexes 1 and 2 in acetonitrile under microwave irradiation produced the Co(O2CBut)2(H2NC5H4N)2 complex.

XAS Investigation of biorelevant cobalt complexes in aqueous media

Cobalt is an essential element of biological cycles involved in numerous metallobiomolecules, but it becomes a toxic element at high concentration or a radiotoxic element because of its use in the nuclear industry. “Molecular speciation” in biological media is an essential prerequisite to evaluate its chemical behaviour as well as its toxic or beneficial effects. In this scheme, we have focused on the coordination properties of the thiol-containing amino acid cysteine (Cys) and the pseudo-peptide N-(2-mercaptopropionyl)glycine (MPG) towards the Co2+ cation in aqueous media. XAS at the Co K edge and traditional spectroscopic techniques have been coupled in order to structurally characterize the cobalt coordination sphere. Oxidation states and geometries of the bis- and tris-cysteinato Co(III) complexes are in agreement with the literature data. In addition, bond lengths between the metallic centre and the donor atoms have been determined. The structure of a new dimeric N-(2-mercaptopropionyl)glycinato Co(II) complex in solution is also reported. The coordination of MPG to Co(II) through the thiolate and carboxylate functions is ascertained. This work provides fundamental structural information about biorelevant complexes of cobalt, which will contribute to our understanding of the chemical behaviour and the biological role of this radionuclide.

Does Catalysis of Reductive Dechlorination of Tetra- and Trichloroethylenes by Vitamin B12 and Corrinoid-Based Dehalogenases Follow an Electron Transfer Mechanism?

Knowing the mechanism by which dangerous organic chloride pollutants, such as tetra- or trichloroethylene, are reductively cleaved is an important task for the establishment of remediation strategies and for a better comprehension of bacterial dehalorespiration by corrinoid-based dehalogenases. On the basis of electrochemical and thermodynamic data, application of outersphere and dissociative electron transfer theories allows the prediction of the pertinent activation/driving force relationships characterizing the electron transfer mechanism. They are validated by application of the redox catalysis method to the reaction with two typical outersphere electron donors. The kinetic gap is more than 11 and 7 orders of magnitude for the dehalogenase and for cobalamin, respectively, showing that the electron transfer mechanism is not operative. Multistep mechanisms in which the chloroethylene molecule enters the cobalt coordination sphere are preferred.

Nature of the Long-Range Structural Changes Induced by the Molecular Photoexcitation and by the Relaxation in the Prussian Blue Analogue Rb1.8Co4[Fe(CN)6]3.3·13H2O. A Synchrotron X-ray Diffraction Study

The photomagnetic properties of CoFe Prussian blue analogues arise from a photoinduced CoIII(LS)−FeII(LS) → CoII(HS)−FeIII(LS) [LS = low spin, HS = high spin] electron transfer accompanied by a spin state change of the cobalt and a significant expansion of the cobalt coordination sphere. The local bond-lengthening around the cobalt induces long-range structural changes that we studied by X-ray powder diffraction using synchrotron radiation. Beyond the structural characterization of an excited metastable state, the study shows how a photoinduced molecular bond-lengthening is made possible in a face-centered cubic Prussian blue analogue by the presence of [Fe(CN)6] vacancies: in their vicinity, the lengthening of the cobalt to ligands bonds is easier. The excitation process starts at the cobalt neighbors of the vacancies through a continuous or second-order transformation (in a single phase), which triggers a cooperative propagation through a first-order discontinuous transition (between two phases).

Computational Studies of Cobalt-Substituted Aluminophosphates

Electronic structure calculations have been performed on a number of models for cobalt substituted aluminophosphates (or CoAPOs) using density functional theory. A number of different cluster models of CoAPOs were constructed and used for the calculations. Our results predict that the high-spin forms of cobalt in tetrahedral coordination are the most stable in agreement with previous electron spin resonance measurements. Also, we find that the structural parameters obtained from geometry optimizations are in good agreement with X-ray absorption experiments (EXAFS). For example, for models of CoAPO compounds with cobalt in the reduced Co(II) oxidation state we calculate the Co−O bond length to be ca. 1.94 Å from cluster calculation as compared to 1.90 Å from EXAFS measurements on reduced CoAPO-18. Our calculations also show the necessity of applying constraints when considering cluster models to correctly represent the local environment around the transition metal, and we have developed a method using atomistic shell-model calculations to obtain reasonable constraints for this purpose. We have calculated the thermochemistry of a likely initiation step for partial oxidation with a number of substrates. It is found that the reaction is more favorable when an additional ligand, H2O or hydroxyl, is introduced into the cobalt coordination sphere forming a five-coordinate local geometry, and that also considerable stabilization of the resulting alkyl radical can be achieved by the formation of a metal−carbon bond. Additionally, it can be noted that although for reproduction of EXAFS data for four-coordinated cobalt models, smaller cluster are sufficient, when considering the energetics of five-coordinated cobalt species, larger cluster are needed to avoid the formation of unrealistic fragment products. We have also investigated the interaction of small molecules (O2 and CH3CN) with the cobalt center. Whereas we find no evidence for a stable intermediate involving direct coordination of molecular oxygen, our results with acetonitrile show good agreement with previous EXAFS measurements on CoAPO-5/CH3CN.

Formation of an Association Complex without a Substitution Reaction Occurring between Methylcobalamin and Cyanide

Re-investigation of the reaction between methylcobalamin and cyanide showed that no substitution reaction in the inner coordination sphere of cobalt occurs for [CN-] ≤ 1.90 M (25.0 °C), contrary to previous reports. Association between MeCbl and CN- does occur and is clearly evident from 1H-NMR spectroscopic measurements. An association constant of 2.5 ± 0.1 M-1 (25.0 °C, I = 2.0 M (NaClO4)) was found in CD3OD.

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.

Synthesis, properties and molecular structures of iron(III), cobalt(II), nickel(II), copper(II), copper(I) and zinc(II) complexes with N,N-bis(pyrazol-1-ylmethyl)benzylamine

A newly prepared ligand, N,N-bis(pyrazol-1-ylmethyl)benzylamine (L) reacted with metal ions to form [FeLCl3], [NiL(NCS)2(MeOH)], [MLX2](M = CoII, CuII or ZnII, X = Cl–, Br– or NCS–) and [(CuLI)2]. The crystallographically determined structures of [CoLCl2] and [CoL(NCS)2] reveal the co-ordination sphere of cobalt(II) to be intermediate between distorted tetrahedral and trigonal bipyramidal. In [NiL(NCS)2(MeOH)] the co-ordination sphere of nickel(II) ion is distorted octahedral while that of copper(II) in [CuLCl2] is distorted square pyramidal. The complex [(CuLI)2] displays distorted tetrahedral geometry around copper(I), and [ZnLBr2] shows distorted tetrahedral geometry around zinc(II).