Intramolecular Cyclization Reactions in Haloalkyl-Cobalt Complexes with Macrocyclic Equatorial Ligands

Organocobalt complexes containing axial haloalkyl groups afford metallacycles of different size by N or O alkylation of the macrocyclic equatorial ligands. The reaction mechanism involves the intra-molecular nucleophilic attack of a negatively charged atom of the equatorial ligand on the axial XCH2 haloalkyl group with simultaneous detachment of a halide ion, X−. In imino/oxime and amino/oxime derivatives, the generation of the negatively charged nitrogen requires the abstraction of a proton and the reaction occurs only in alkaline medium. In bis(dimethylglioximato) and Schiff base complexes, a negatively charged oxygen is present in the equatorial ligand and the reaction occurs even in neutral medium. Three-, six- and seven- membered metallacycles are obtained, with the common feature that the Co–C bond is shorter and more resistant toward homolysis than in parent complexes or in closely related derivatives. Keywords

Intramolecular Cyclization Reactions in Haloalkyl-Cobalt Complexes with Macrocyclic Equatorial Ligands

Organocobalt complexes containing axial haloalkyl groups afford metallacycles of different size by N or O alkylation of the macrocyclic equatorial ligands. The reaction mechanism involves the intra-molecular nucleophilic attack of a negatively charged atom of the equatorial ligand on the axial XCH2 haloalkyl group with simultaneous detachment of a halide ion, X−. In imino/oxime and amino/oxime derivatives, the generation of the negatively charged nitrogen requires the abstraction of a proton and the reaction occurs only in alkaline medium. In bis(dimethylglioximato) and Schiff base complexes, a negatively charged oxygen is present in the equatorial ligand and the reaction occurs even in neutral medium. Three-, six- and seven- membered metallacycles are obtained, with the common feature that the Co–C bond is shorter and more resistant toward homolysis than in parent complexes or in closely related derivatives. Keywords

Synthesis, characterization, and electrochemical properties of a new series of inorganic and organometallic Co(III) complexes with a Schiff base ligand derived from tyrosine

A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand \uf07b[Z-Tyr(3-Ac)-OMe]2en\uf07d derived from the condensation of two equivalents of protected 3-acetyl- L-tyrosine with one equivalent of ethylenediamine has been synthesized. As the amino acids have L configuration, the resulting ligand is chiral. The complexes have been characterized by 1H and 13C NMR, ESI-MS spectrometry and U.V.-visible spectroscopy. The U.V.-visible spectra recorded in CH2Cl2 show an absorption band at about 650 nm, characteristic of penta-coordinated alkylcobalt(III)(salen), but an evident deviation from the Lambert Beer law in the concentration range 0.20 \uf02d 2.0 \uf0b4 10-3 M suggests some kind of association in this solvent. In coordinating solvent the complexes are hexa-coordinated. The equilibrium constants for pyridine ligation in CH3OH decrease with increasing electron-donor power of the axial alkyl group. Cyclic voltammetry studies indicate that after the reduction RCo(III)/RCo(II), the complexes decompose giving a Co(I) species that is stable on the CV time scale and can be reoxidized to Co(II) and Co(III) species. Both the Epa and the Epc values show that the peak potentials are shifted to more negative values with increasing inductive effect of the axial alkyl group

Formation and Structure of a Cobalt(III) Complex Containing a Nonstabilized Pyridinium Ylide Ligand

The reaction of [CoIII(4,4\u2032dmsalen)(CH2Cl)(S)], where 4,4\u2032dmsalen = 4,4\u2032-dimethylsalen and S = solvent, with pyridine led to the formation of [CoIII(4,4\u2032dmsalen)(CH2py)(Cl)], containing a nonstabilized pyridinium ylide as axial ligand. The complex has been unambiguously characterized by single-crystal X-ray diffraction analysis. Time-resolved 1H NMR spectra showed that the formation of [CoIII(4,4\u2032dmsalen)(CH2py)(Cl)] occurs in a two-step process involving a metallacyclized intermediate, cis-\u3b2-[CoIII(4,4\u2032dmsalenCH2)(py)(S)]+. A similar experiment carried out in the presence of different nitrogen bases having higher pKa values (4-Me-py or 4-t-Bu-py) allowed a better separation of the two consecutive reactions. The almost complete conversion of [CoIII(4,4\u2032dmsalen)(CH2Cl)(S)] in the cyclized intermediate before the formation of the ylide indicates that the ylide complex forms exclusively through the nucleophilic attack of the nitrogen base at the 12CH2O\u2013 carbon of the cyclized species, whereas a parallel direct conversion through the displacement of Cl\u2013 from the axial CH2Cl group of [CoIII(4,4\u2032dmsalen)(CH2Cl)(S)] may be ruled out

New Multicomponent Porous Architecture of Self-Assembled Porphyrins/Calixarenes Driven by Nickel Ions

A new multicomponent material with nanoporous structure has been synthesized by co-crystallization of a mixture of cationic meso-tetrakis(4-N-methylpyridyl)porphyrin (H2T4) and meso-tri(4-N-methylpyridyl)porphyrin (H2T3py) with polyanionic 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(hydroxylcarbonylmethoxy)calix[4]arene (C4TsTc) in the presence of Ni2+ ions. The structural analysis indicates that the overall architecture is assembled by interpenetrated two-dimensional (2D) meshes where the nodes are built up by a central tetracationic H2T4 porphyrin with arms hosted in sulphonated rims of four cavitands. The approximately 2D square network is formed by Ni2+ ions bridging the calixarene carboxylate rims in a tail-to-tail fashion. The central H2T4 stacks with two external H2T3py molecules having the neutral pyridine arm N-coordinated to Ni2+ ions. These metal centers interconnect the orthogonal 2D meshes by further coordination of calixarene\u2013carboxylate groups. Self-organization of the new multicomponent material, featuring large channels (60% of volume accessible to solvent molecules) and potential readily accessible metal active sites, has been driven by both supramolecular host\u2013guest recognition and coordinative assembly. The thermal behavior of native and nickel-containing crystals was studied by hot stage microscopy and differential scanning calorimetry. The decomposition temperatures of the multicomponent materials, 465\u2013470 \ub0C, are about 100 \ub0C higher than those of the single building blocks

Cobalt(III) complexes with tridentate hydrazone ligands: protonation state and hydrogen bond competition

Cobalt(III) complexes of the type [Co(HL)(L)] were synthesized under solvothermal conditions staring from [Co(C5H7O2)3] and the corresponding ligand H2L (salicylaldehyde 4-hydroxybenzhydrazone, 3-methoxysalicylaldehyde 4-hydroxybenzhydrazone, 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone, salicylaldehyde benzhydrazone, 3-methoxysalicylaldehyde benzhydrazone, 4-methoxysalicylaldehyde benzhydrazone). The presence of differently protonated forms of the same ligand in the complexes was supported by IR and NMR spectroscopy as well as by the single crystal X-ray diffraction method. The effects of weak interactions on the supramolecular architecture and their role on the ligand form stabilization have been analysed. Molecular interactions within the unit cells were investigated and quantified by extensive quantum chemical analysis on models built from crystal structures using density functional theory and empirical dispersion. Ligands used in this study were prepared under environmentally friendly conditions by mechanochemical synthesis. Their thermal behaviour and phase transitions were investigated using TG and DSC analysis and the powder X-ray diffraction method

A Novel Series of CoIII(salen-type) Complexes Containing a Seven-Membered Metallacycle: Synthesis, Structural Characterization and Factors Affecting the Metallacyclization Rate

A series of electronically tuned trans-[CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2,2\u2032-ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis \u3b2 organometallic derivative containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the 12C\u2550N\u2013 groups of the chelate. The X-ray structures of cyclized complexes, [CoIII(chelCH2)(py)(H2O)]+, show a dependence of the conformation of the seven-membered metallacycle on the different positions of substituents in the chelate. In fact, in the complex having methyls on the 12C\u2550N\u2013 groups, the conformation is characterized by having the methylene carbon atom significantly displaced (ca. 1.26 \uc5) from the aromatic ring plane, whereas in the complex lacking methyl groups in those positions, the atoms of the Ph\u2013O\u2013CH2 fragment are coplanar. The standard Gibbs energies obtained by quantum chemical calculation reveal that the different conformations observed in the solid state are mainly the result of the energetically unfavorable setup of the methyls on the 12C\u2550N\u2013 groups and of the energetically favorable displacement of the CH2 group out of plane of the aromatic ring. 1H NMR data suggest that the different conformations of the metallacycle are, at least partially, retained in solution