Low-temperature spectral observation of the first six-coordinate nitrosyl complexes of cobalt(II) meso-tetratolylporphyrin with trans nitrogen base ligands

Low-temperature interaction of nitrogen base ligands with layered Co(TTP)(NO) (TTP = meso-tetratolylporphyrinato dianion) as well as its toluene solution leads to the formation of the first six-coordinate species of the general formula (B)Co(TTP)(NO) (where B = piperidine and pyridine). The nu(NO) stretching bands of these species appear at lower frequencies compared with the five-coordinate nitrosyl derivative and depend on the nature of the trans axial ligand. The equilibrium constants and enthalpies of formation of these new species are determined. Fairly stable at low-temperature conditions in the solid state, they slowly dissociate the nitrogen base ligands upon warming to restore the five-coordinate nitrosyl complex Co(TTP)(NO).     

Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO2) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO2 groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO2) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue.     

Conducting dimerized cobalt complexes with tetrathiafulvalene dithiolate ligands

To obtain novel single-component molecular metals, we attempted to synthesize several cobalt complexes coordinated by TTF (tetrathiafulvalene)-type dithiolate ligands. We succeeded in the syntheses and structure determinations of ((n)Bu(4)N)(2)[Co(chdt)(2)](2) (1), ((n)Bu(4)N)(2)[Co(dmdt)(2)](2) (2), [Co(dmdt)(2)](2) (3), and [Co(dt)(2)](2) (4) (chdt = cyclohexeno-TTF-dithiolate, dmdt = dimethyl-TTF-dithiolate, and dt = TTF-dithiolate). Structure analyses of complexes 1-4 revealed that two monomeric [Co(ligand)2]- or [Co(ligand)(2)](0) units are connected by two Co-S bonds resulting in dimeric [Co(ligand)(2)](2)(2-) or [Co(ligand)(2)](2) molecules. Complex 1 has a cation-anion-intermingled structure and exhibited Curie-Weiss magnetic behavior with a large Curie constant (C = 2.02 K x emu x mol(-1)) and weak antiferromagnetic interactions (theta = -8.3 K). Complex 2 also has a cation-anion-intermingled structure. However, the dimeric molecules are completely isolated by cations. Complexes 3 and 4 are single-component molecular crystals. The molecules of complex 3 form two-dimensional molecular stacking layers and exhibit a room-temperature conductivity of sigmart = 1.2 x 10(-2) S.cm(-1) and an activation energy of E(a) = 85 meV. The magnetic behavior is almost consistent with Curie-Weiss law, where the Curie constant and Weiss temperature are 8.7 x 10(-2) K x emu x mol(-1) and -0.85 K, respectively. Complex 4 has a rare chair form of the dimeric structure. The electrical conductivity was fairly large (sigmart = 19 S.cm(-1)), and its temperature dependence was very small (sigma(0.55K)/sigma(rt) = ca. 1:10), although the measurements were performed on the compressed pellet sample. Complex 4 showed an almost constant paramagnetic susceptibility (chi(300) (K) = 3.5 x 10(-4) emu x mol(-1)) from 300 to 50 K. The band structure calculation of complex 4 suggested the metallic nature of the system. Complex 4 is a novel single-component molecular conductor with a dimeric molecular structure and essentially metallic properties down to very low temperatures.     

Some binuclear complexes of iron and cobalt with isocyanide ligands

Whereas Co₂(CO)₈ and RNC (R= Me, Et, and Cy) react to give mixtures of [(RNC)₅Co] [Co(CO)₄] and the covalent, carbonyl-bridged [(RNC)_mCo₂(CO)_8?m] derivatives (m = 1–3), [(π-dienyl)Fe(CO)₂]₂ give only [(π-dienyl)₂Fe₂(CO)_4?n(CNR)_n] complexes (dienyl = C₅H₅, MeC₅H₄ and C₉H₇; n = 1–2) that exist in solution as mixtures of cis- and trans-CO- and RNC-bridged tautomers with the μ-RNC species decreasing in importance as the bulk of R increases.     

Variation of oxo-transfer reactivity of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands

Derivatives of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands have been prepared in solution and in the solid state. Crystal structures of two of these derivatives, (H2O)CoTpivPP(NO2) and (CH3OH)CoTpivPP(NO2), have been determined. The ethanol complex (C2H5OH)Co(TPP)(NO2) has been obtained and spectrally characterized using sublimed layers methodology. The formation constant and the DeltaH degrees value of the association reaction with ethanol have been determined by FTIR measurements in CCl4 solution. Catalytic oxygen activation and oxo-transfer reactions of these derivatives have been assessed in solution. Correlations between the oxo-transfer reactivity, thermodynamics, and characteristics of the nitro ligand show that although calculated and observed ONO vibrational spectra and bond lengths suggest activation of the NO2 ligand and enhanced oxo-transfer reactions as seen in the analogous five-coordinate complexes, density functional theory calculations support that thermodynamics limits oxo-atom transfer reactions in these six-coordinate systems.     

Catalytic systems for reduction of nitro compounds involving cerium complexes with organic ligands

Conclusions Effective catalytic systems were developed for the reduction of aromatic nitro compounds, which consist of a heterogeneous palladium catalyst and cerium complexes with organic ligands.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp.1104–1107, May, 1979.     

Redox properties of cobalt(II) complexes with isoindoline-based ligands

Cobalt bis-(N-arylimino)isoindolinates undergo electrostatic interactions with DNA or react with alkyl hydroperoxides to form ketones and alcohols. Redox behavior of the metal center should affect such reactivities; therefore, six neutral Co^II(L)₂ complexes with L = bis-(N-arylimino)isoindolinates have been synthesized to elucidate the effect of the aryl substituents on the redox potential of the metal center. Redox properties of various M^II(L)₂ complexes (M = Mn, Fe, Co, Ni) are compared. Moreover, data are presented on the dismutation rates of superoxide radical anion (a knowingly sensitive reagent on the redox properties of the metal center) in the presence of the various Co^II(L)₂ complexes among identical conditions.     

Formations and structures of cobalt dithiolene complexes with nitrogen group-substituted cyclopentadienyl ligands

The cobalt dithiolene complex with the sulfonylamide-substituted Cp ligand [(C₅H₄-NHTs)Co{S₂C₂(COOMe)₂}] (1, Ts = p-SO₂C₆H₄Me) reacted with TsOH · H₂O to give [(C₅H₄-NH₂)Co{S₂C₂(COOMe)(H)}] (2), [(C₅H₄-NHTs)Co(S₂C₂H₂)] (3) and [(C₅H₄-NHTs)Co{S₂C₂(COOMe)(H)}] (4). Complex 1 was dissolved in a basic aqueous solution, and the anion reacted with Me₂SO₄ to form the N-methylated product [{C₅H₄-N(Me)Ts}Co{S₂C₂(COOMe)₂}] (5); the carboxylic acid complex [{C₅H₄-N(Me)Ts}Co{S₂C₂(COOMe)(COOH)}] (6) formed by a base hydrolysis. The X-ray crystal structures of complexes 4-6 and the methylsulfonylamide-substituted Cp complex [(C₅H₄-NHMs)Co{S₂C₂(COOMe)₂}] (7, Ms = SO₂Me) were determined. In the crystal structures of complexes 4 and 7, intermolecular hydrogen bondings of NH?O (ca. 2.1 Å) and NH?S (ca. 3.1 Å) were observed. Complex 6 showed the OH?O intermolecular hydrogen bonding (ca. 1.6 Å) of COOH moiety between two molecules, and these two molecules were assembling each other. Complexes 5 and 6 showed an intramolecular π-π interaction between the aromatic cobaltadithiolene and benzene rings, and complex 5 also has intermolecular π-π interactions between two benzene rings, and between two cobaltadithiolene rings.     

Reductive carbonylation of cobalt(II) complexes with phosphorus-sulfur bidentate ligands

The synthesis of the low-spin cationic complexes [Co(P-SR)₂] (BF₄)₂ (P-SR = Ph₂PCH₂SR) and its reaction with CO to form the cobalt(I) carbonyl complexes [Co(CO)₂(P-SR)₂]BF₄ are described. The chemical and spectroscopic properties of the complexes are presented and the stoichiometry and mechanism of the carbonylation reaction discussed.     

Direct synthesis of heterometallic complexes

The idea of direct synthesis of coordination compounds – use of elemental metals or their oxides as starting material – has been developed to synthesize heterometallic compounds. The advantages of this approach in obtaining polynuclear, mixed-anion and mixed-valence coordination compounds are shown, taking complexes with aminoalcohols as examples. The preparation of heterobimetallic Cu/M (Pb, Zn, Co) complexes with aminoalcohols using zerovalent copper as a starting material is presented. The main principles for the formation of heterobimetallic complexes and the most interesting crystal structures are briefly reviewed.     

Experimental and density functional theoretical investigations of linkage isomerism in six-coordinate FeNO(6) iron porphyrins with axial nitrosyl and nitro ligands

A critical component of the biological activity of NO and nitrite involves their coordination to the iron center in heme proteins. Irradiation (330 < lambda < 500 nm) of the nitrosyl-nitro compound (TPP)Fe(NO)(NO(2)) (TPP = tetraphenylporphyrinato dianion) at 11 K results in changes in the IR spectrum associated with both nitro-to-nitrito and nitrosyl-to-isonitrosyl linkage isomerism. Only the nitro-to-nitrito linkage isomer is obtained at 200 K, indicating that the isonitrosyl linkage isomer is less stable than the nitrito linkage isomer. DFT calculations reveal two ground-state conformations of (porphine)Fe(NO)(NO(2)) that differ in the relative axial ligand orientations (i.e., GS parallel and GS perpendicular). In both conformations, the FeNO group is bent (156.4 degrees for GS parallel, 159.8 degrees for GS perpendicular) for this formally {FeNO}(6) compound. Three conformations of the nitrosyl-nitrito isomer (porphine)Fe(NO)(ONO) (MSa parallel, MSa perpendicular, and MSa(L)) and two conformations of the isonitrosyl-nitro isomer (porphine)Fe(ON)(NO(2)) (MSb parallel and MSb perpendicular) are identified, as are three conformations of the double-linkage isomer (porphine)Fe(ON)(ONO) (MSc parallel, MSc perpendicular, MSc(L)). Only 2 of the 10 optimized geometries contain near-linear FeNO (MSa(L)) and FeON (MSc(L)) bonds. The energies of the ground-state and isomeric structures increase in the order GS < MSa < MSb < MSc. Vibrational frequencies for all of the linkage isomers have been calculated, and the theoretical gas-phase absorption spectrum of (porphine)Fe(NO)(NO(2)) has been analyzed to obtain information on the electronic transitions responsible for the linkage isomerization. Comparison of the experimental and theoretical IR spectra does not provide evidence for the existence of a double linkage isomer of (TPP)Fe(NO)(NO(2)).     

Direct electrochemical synthesis and characterization of cobalt and nickel complexes with 2-pyridinone and 2-pyridinemethanethiol-1-oxide

Summary The electrochemical oxidation of anodic metals (nickel and cobalt) in MeCN solutions of 2-pyridinone (HOpy) or 2-pyridine methanethiol-1-oxide (HPMTO) or its dimer, 2,2′-dithiodimethyldipyridine-1,1′-dioxide (DPMTO), gives the simple complexes [Ni(Opy)₂]·2H₂O, [Co(Opy)₂]·2H₂O, [Ni(PMTO)₂]·2H₂O and [Co(PMTO)₂]·3H₂O. When 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) was added to the electrolytic phase, only three mixed complexes were obtained: [Ni(Opy)₂bipy]·H₂O, [Co·(Opy)₂phen]·H₂O and [Ni(Opy)₂phen]·5H₂O. The complexes were characterized by their elemental analyses, i.r. spectra and magnetic properties.     

Conformational isomers of neutral trans-di­nitro­cobalt(III) complexes

The reaction of Co(acac)₃ with N-(2-amino­ethyl)-1,3-propane­di­amine in the presence of NaNO₂ results in the preparation of an unexpected di­nitro­cobalt(III) compound, (11-amino-4-methyl-5,8-di­aza­undeca-2,4-dien-2-olato-κ⁴­N^5,8,11,O)-di­nitrocobalt(III), [Co(C₁₀H₂₀N₃O)(NO₂)₂], containing the tetra­dentate anion of 11-amino-4-methyl-5,8-diazaundeca-2,4-dien-­2-ol. Two isomers of the compound were obtained by recrystallization of the crude product. In one isomer, the two trans nitro groups are staggered, and in the other they are eclipsed.     

Electronic structures of six-coordinate ferric porphyrin complexes with weak axial ligands: usefulness of 13C NMR chemical shifts

1H NMR, (13)C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L2]ClO4 with different porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled 5,10,15,20-tetraisopropylporphyrin (TiPrP), and saddled 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide, DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an essentially pure high-spin (S = 5/2) to an essentially pure intermediate-spin (S = 3/2) state depending on the field strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H chemical shift is a good probe to determine the spin state in the spin admixed S = 5/2,3/2 complexes (Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282). In this paper, we report that the chemical shifts of the alpha- and beta-pyrrole carbons can also be good probes to determine the spin state because they have shown good correlation with those of the pyrrole-H or pyrrole-C(alpha). By putting the observed or assumed pyrrole-H or pyrrole-C(alpha) chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the d(xy)-a(2u) interaction in the ruffled Fe(T(i)PrP)L2+ and d(xy)-a(1u) interaction in the saddled Fe(OETPP)L2+ are quite weak in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines has suggested that the electron configuration of an essentially pure intermediate-spin Fe(T(i)PrP)L2+ changes from (d(xy), d(yz))3(d(xy))1(d(z)2)1 to (d(xy))2(d(xz), d(yz))2(d(z)2)1 as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)(THF)2+ should adopt (d(xy), d(yz))3(d(xy))1(d(z)2)1 rather than (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the strong d(xy)-a(1u) interaction (Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312-1314), our 13C NMR study again suggests that Fe(OETPP)(THF)2+ should be represented as (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the weak d(xy)-a(1u) interaction. The contribution of the S = 3/2 state in all types of the spin admixed S = 5/2,3/2 six-coordinate complexes has been determined on the basis of the (13)C NMR chemical shifts.     

Switching on oxygen activation by cobalt complexes of pentadentate ligands

The monoanionic N(4)O ligand N-methyl-N,N'-bis(2-pyridylmethyl)ethylenediamine-N'-acetate (mebpena(-)) undergoes oxidative C-N bond cleavage in the presence of Co(II) and O(2). The two resultant fragments are coordinated to the metal ion in the product [Co(III)(2-pyridylformate)(mepena)]ClO(4) (mepena(-) = N-methyl-N'-(2-pyridylmethyl)ethylenediamine-N'-acetato). Bond cleavage does not occur in the presence of chloride ions and [Co(III)(mebpena)Cl](+), containing intact mebpena(-), can be isolated. The oxidative instability of the mebpena(-) in the presence of Co(II) and air stands in contrast to the oxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobalt complexes. Cyclic voltammetry on the matched pair [Co(III)Cl(mebpena)](+) and [Co(II)Cl(bztpen)](+), bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridine donor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in the E°(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metal oxidation state in the complexes isolated under ambient conditions. DFT calculations support a proposal that [Co(II)(mebpena)](+) reacts with O(2) to form a Co(III)-superoxide complex which can abstract an H atom from a ligand methylene C atom as the initial step towards the observed oxidative C-N bond cleavage.     

Interaction of cobalt(III) polypyridyl complexes containing asymmetric ligands with DNA

Four asymmetric cobalt(III) complexes, [Co(bpy)₂(aip)]³⁺, [Co(bpy)₂(pyip)]³⁺, [Co(phen)₂(aip)]³⁺, and [Co(phen)₂(pyip)]³⁺ (bpy = 2,2,bipyridine, phen = 1,10-phenathroline), (pyip = 2-(1-pyrenyl)-1H-imidazo[4,5-f][phen], (aip = 2-(9-anthryl)-1H-imidazo[4,5,-f][phen], have been synthesized and characterized. Their interaction with calf thymus DNA (CT-DNA) was investigated by physico-chemical methods and photocleavage. The size and shape of the ligands have a marked effect on the DNA-binding affinity of the complexes. Irradiation of pBR322 DNA with these novel cobalt(III) complexes results in nicking of the plasmid DNA. Toxicity and induced cell death investigations revealed that the complexes of pyip had higher toxicity than those of aip. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Template synthesis of tungsten complexes with saturated N-heterocyclic carbene ligands

Tungsten complex with a coordinated 2-azidoethyl isocyanide ligand reacts with PMe3 at the azido function to give a complex with a coordinated iminophosphorane which upon hydrolysis of the P=N bond yields a complex with an NH,NH-stabilized N-heterocyclic carbene ligand, 7; alkylation of the carbene ring nitrogen atoms gives a complex with an N,N'-dialkylated imidazolidin-2-ylidene ligand, 8 .