New chromogens of the ferroin type-VI: some derivatives of 1- and 3-cyanoisoquinoline and substituted 2-cyanopyridines

The chromogenic reactions with iron(II), copper(I) and cobalt(II) of 32 recently synthesized pyridazines, triazines, tetrazines and certain other compounds containing the ferroin grouping have been examined spectrophotometrically. Some interesting examples were found of steric and donor-atom selectivities in chelation of iron by ligands containing several ferroin groups. Of the various chromogens studied, the pyridyl and isoquinolyl derivatives of phenyl or pyridyl substituted 1,2,4-triazine show the most promising analytical utility.     

Synthesis and electrochemical investigation of a series of iron(II) complexes with bidentate,tridentate and hexadentate ligands

Iron(II) complexes of a bidentate, a hexadentate and a series of tridentate ligands have been prepared and characterized. Electrochemical properties have shown that of the tridentate ligands, those capable of dπ-pπ bonding between the pyridyl rings and central ligand atom can stabilize formal low oxidation states (I,O, −I) of the metal. A bidentate ligand, 2-(2-pyridyl) imidazole although resembling 2,2′-bipyridine in structure, does not stabilize any low oxidation states and its iron(II) complex is reduced irreversibly to iron(O). The iron(II) complex of a hexadentate ligand, tetra(2-picolyl)ethylenediamine, is reduced in two, one electron steps to yield formal Fe(I) and Fe(O) oxidation states. However, both of these reductions are totally irreversible at a Pt and HMDE electrode.     

Electrochemical Synthesis and Characterization of Nickel(II) Complexes with N‐2‐Pyridyl‐sulfonamide Ligands

Heteroleptic nickel(II) complexes [NiL₂L′] of a series of monoanionic and potentially bidentate N‐2‐pyridyl‐sulfonamide ligands [HL] and 2,2′‐bipyridine or 1,10‐Phenanthroline (L′) have been prepared by electrochemical oxidation of a nickel anode in an acetonitrile solution of the ligands. The complexes have been characterized by microanalysis, IR and electronic spectroscopy, magnetic measurements and LSI mass spectrometry. The crystal structure of [Ni(Ms6mepy)₂(bipy)] has been determined by x‐ray diffraction and shows the metal in an octahedral NiN₆ environment. Octahedral structures are also proposed for the other complexes with the N‐2‐pyridyl‐sulfonamide ligands acting as N,N′ or N, O bidentate systems, depending on the position of the methyl substituent on the pyridine ring.     

New chromogens of the ferroin-type--II. Pyrido and pyridyl derivatives of phenazine and quinoxaline

Eleven pyrido and pyridyl derivatives of phenazine (6) and quinoxaline (5) have been examined as chromogens of the ferroin type for iron(II) and copper(I). Three of the quinoxaline derivatives show promise as reagents for iron(II) but are expensive and difficult to prepare.     

Rigid bis-zinc(II) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.     

Ligand displacement reactions of dimer and trimer pyridyl ligand/transition metal complexes

Ligand exchange reactions of pyridyl ligand/transition metal complexes are examined in a quadrupole ion trap mass spectrometer to evaluate the ability of multidentate ligands to displace other pyridyl ligands in complexes where the charge is highly delocalized and there is a great degree of ligand repulsions. Partially or fully coordinated transition metal ions in dimer or trimer species involving small mono- or bidentate pyridyl ligands undergo ligand displacement reactions with larger bi- and tridentate pyridyl ligands. Larger ligands with greater chelation abilities, such as 1,10-phenanthroline and 2,2′:6,2″-terpyridine, are often able to simultaneously displace two nonchelating ligands from a partially coordinated metal ion. However, the analogous reactions involving displacement of bidentate chelating ligands from more fully coordinated transition metal ion complexes are nearly quenched. In other cases, mixed-ligand dimer and trimer complexes are observed, indicating step-wise displacement of the initially complexed ligands.     

New iron bis(imino)pyridyl complexes containing dendritic wedges for alkene oligomerisation

The synthesis, characterisation and catalytic behaviour of new iron bis(imino)pyridyl complexes containing dendritic wedges, as well as the synthesis of bis(para-hydroxyphenylimino)pyridines is described. The hydroxyl functionality of the bis(para-hydroxyphenylimino)pyridines was used to attach dendritic wedges of the carbosilane type as well as the benzylphenyl ether type. After attachment of the dendritic wedges, complexation of these ligands to iron(II) chloride was achieved. The resulting dendritically functionalised bis(imino)pyridyl iron complexes were tested in the catalytic oligomerisation of ethene.     

The effect of methyl substitution on the spin-state in solid bis(methyl-substituted 2-pyridinalphenylimine)di(thiocyanato)iron(II)

Summary A series of iron(II) complexes of the type [FeL₂(NCS)₂] have been prepared and characterized, where L denotes the bidentate diimine ligands 2-pyridinalphenylimine orN-phenyl-2-pyridinaldimine (ppi) and its methyl-substituted derivatives. The electronic ground spin-state of iron(II) in these complexes has been studied by means of Mössbauer spectroscopy and magnetic susceptibility measurements.     

Iron(II)–ethylene polymerization catalysts bearing 2,6-bis(imino)pyrazine ligands: Part II. Catalytic behaviour, homogeneous and heterogeneous insights

A series of new iron(II) complexes bearing tridentate pyrazine-bis(2,6-arylimino) ligands where the aryl groups are 1-naphthyl, 2,6-dimethylphenyl, and 2,6-diisopropylphenyl have been used as ethylene polymerization catalysts after activation with alkylaluminiums. The new complexes display a lesser catalytic activity than those bearing the corresponding pyridine-bis(2,6-arylimino) ligands. Varying the steric bulkiness of the aromatic groups in the tridentate ligands and the polymerization conditions affects the catalytic productivity.     

Relative axial ligand orientation in bis(imidazole)iron(II) porphyrinates: are "picket fence" derivatives different?

The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm) 2] 1-RIm = 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric requirements of the four alpha,alpha,alpha,alpha-o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole planes are 77.2 degrees , 62.4 degrees , and 78.5 degrees . All three derivatives have nearly planar porphyrin cores. M?ssbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.     

Synthesis of some benzimidazole‐, benzothiazole‐ and pyridine‐derived chelating agents

Procedures are given for the preparation of new linear bidentate, tetradentate and tripodal heptadentate ligands incorporating benzimidazole, benzothiazole and pyridyl groups. The compounds were characterized by their nmr, uv and mass spectra. The crystal and molecular structure is reported for a chiral benzothiazole derived from camphoric acid.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n -butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N * ) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC * ) mesophase. The metal complexes with n -butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N * phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C * phase of the two ligands.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.     

Understanding atom transfer radical polymerization: effect of ligand and initiator structures on the equilibrium constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.     

On the stability of metal–aminoacid complexes in water based on water–ligand exchange reactions and electronic properties: Detailed study on iron–glycine hexacoordinated complexes

Thermodynamic stability of metal–aminoacid complexes in water is discussed in terms of the Gibbs free energy of water–ligand exchange processes, and the electronic stabilizing factors thoroughly investigated by means of 1‐electron and 2‐electron density properties. Hexacoordinated complexes formed between iron cations and glycine molecules acting as monodentate or bidentate ligands have been chosen as targets for the current study. Results agree with experimental findings, and complexes formed with bidentate ligands are found to be more stable than those formed with monodentate ones. The larger the number of the coordinated glycine molecules the more stable is the complex. Fe(III) complexes are more stable than Fe(II) ones, but differences are small and the Fe³⁺/Fe²⁺ exchange process appears to be energetically feasible for these complexes. Formation of the second glycine–iron interaction involving the amino nitrogen in the bidentate ligands is enthalpycally unfavorable but takes place due to the large entropy rise of the process. The larger stability of Fe(III) complexes is due however to the balance between energetic and solvation terms, which is favorable to these complexes. Electron density properties account satisfactorily for the electronic energy changes along the complex formation in terms of ligand–metal electron transfer and covalent bond orders. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

Bulky guanidinates for the stabilization of low oxidation state metallacycles

This article summarizes the development of a new class of very bulky guanidinate ligands. These have been used to prepare unprecedented examples of heterocycles containing groups 2, 13, 14 or 15 elements in the +1 oxidation state. The ligands have also been harnessed in the preparation of the only examples of guanidinato, and/or closely related amidinato, complexes of iron(I), cobalt(I) and planar four-coordinate lanthanide(II) metals. Preliminary studies of the further chemistry of these very reactive complexes are also reviewed. Throughout, the tendency of the bulky guanidinate ligands to exhibit ligating and stabilizing properties more akin to those of bulky β-diketiminate ligands than less bulky amidinates or guanidinates, will be discussed.     

Highly cytotoxic iron(II) complexes with pentadentate pyridyl ligands as a new class of anti-tumor agents

The iron(II) complexes and with pentadentate pyridyl ligands are stable under physiological conditions and exhibit higher cytotoxicities toward a series of human carcinoma cell lines than cisplatin; can significantly increase intracellular oxidant levels, cleave supercoiled plasmid DNA in vitro without addition of a reductant and induce apoptotic cell death in human cervical epithelioid carcinoma cells (HeLa) as observed in flow cytometric studies.     

Rollover‐Assisted C(sp2)C(sp3) Bond Formation

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a “rollover”/“retro‐rollover” reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR₃)] resulted in a rare C(sp²)?C(sp³) bond formation to give the bidentate nitrogen ligands 3‐methyl‐2,2′‐bipyridine, 3,6‐dimethyl‐2,2′‐bipyridine, and 3‐methyl‐2‐(2′‐pyridyl)‐quinoline, which were isolated and characterized. The nature of the phosphane PR₃ is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C?H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize.     

Structural and spectroscopic studies of nickel(II) complexes with a library of Bis(oxime)amine-containing ligands

A library of tripodal amine ligands with two oxime donor arms and a variable coordinating or noncoordinating third arm has been synthesized, including two chiral ligands based on l-phenylalanine. Their Ni(II) complexes have been synthesized and characterized by X-ray crystallography, UV-vis absorption, circular dichroism, and FTIR spectroscopy, mass spectrometry, and room-temperature magnetic susceptibility. At least one crystal structure is reported for all but one Ni/ligand combination. All show a six-coordinate pseudo-octahedral coordination geometry around the nickel center, with the bis(oxime)amine unit coordinating in a facial mode. Three distinct structure types are observed: (1) for tetradentate ligands, six-coordinate monomers are formed, with anions and/or solvent filling out the coordination sphere; (2) for tridentate ligands, six-coordinate monomers are formed with Ni(II)(NO(3))(2), with one monodentate and one bidentate nitrate filling the remaining coordination positions; (3) for tridentate ligands, six-coordinate, bis(mu-Cl) dimers are formed with Ni(II)Cl(2), with one terminal and two bridging chlorides filling the coordination sphere. The UV-vis absorption spectra of the complexes show that the value of 10 Dq varies according to the nature of the third arm of the ligand. The trend based on the third arm follows the order alkyl/aryl < amide < carboxylate < alcohol < pyridyl < oxime.     

Synthesis and structures of 5-(pyridyl)tetrazole complexes of Mn(II)

New Mn(II) complexes containing 5-(2-pyridyl)tetrazole, 5-(3-cyano-4-pyridyl)tetrazole or 5-(4-pyridyl)tetrazole ligands are described. The complexes are prepared by reaction of the corresponding cyanopyridines with sodium azide in the presence of Mn(II) salts. All the complexes have been characterized by X-ray crystallography, which reveals that 5-(pyridyl)tetrazole ligands can coordinate to Mn through either type of nitrogen atom in the tetrazole residue or via the pyridyl group. In the solid state, extended 2D and 3D structures are produced through networks of hydrogen bonding (involving water molecules and the tetrazolate residue). Acidification of the complexes produces the corresponding free 5-(pyridyl)-1H-tetrazole.