Experimental and theoretical studies on the complexes of [Pb(m)-pyridyl]- (m = 1-4)

The pyridyl-lead complexes [Pb(m)-C5H4N](-) (m = 1-4), which are produced from the reactions between lead clusters formed by laser ablation and the pyridine molecules seeded in argon carrier gas, are studied by photoelectron (PE) spectra and density functional theory. The adiabatic electron affinity (EA) of [Pb(m)C5H4N](-) is obtained from PE spectra at photon energies of 308 and 193 nm. Theoretical calculation is carried out to elucidate their structures and bonding modes. A variety of geometries for the isomers are optimized to search for the lowest-energy geometry. By comparing the theoretical results, including the EA and simulated density of state spectra, with the experimental determination, the lowest-energy structures for each species are obtained. The following analysis of the molecular orbital composition provides the evidence that the pyridyl binds on lead clusters through the Pb-C sigma bond. Moreover, there is an apparent spin-state transition from triplet state toward singlet state for the ground-state structure of [Pb(m)C5H4N](-) with an increase of lead cluster.     

Photophysical, spectroscopic, and computational studies of a series of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide and 5- and 6-derivatized phenanthroline ligands

The ligand 2,6-dimethylphenylisocyanide (CNx) forms six complexes of the formula [Re(CO)3(CNx)(L)]+, where L = 1,10-phenanthroline (1), 5-chloro-1,10-phenanthroline (2), 5-nitro-1,10-phenanthroline (3), 5-methyl-1,10-phenanthroline (4), 5,6-dimethyl-1,10-phenanthroline (5), and 1,10-phenanthrolinopyrrole (6). The lowest-energy absorption peaks of the complexes red-shift in the order 1 < 2 < 3 < 4 < 5 < 6. The time-dependent density functional theory (TDDFT) and conductor-like polarizable continuum model (CPCM) computed singlet excited states in ethanol deviate by 1000 cm(-1) or less from the experimental UV-vis peaks. The complexes undergo reversible reductions and irreversible oxidations. The electronic energy gap increases in the order 3 < 2 < 1 < 4 < 5 < 6, which is the order of increasing electron-donating power of the phen substituents. The reduction potentials linearly correlate with the B3LYP calculated LUMO energies for 1-6. The complexes emit at room temperature and at 77 K except 3, which emits only at 77 K. The calculated (3)MLLCT energies are within 1100 cm(-1) from the experimental emission energies at 77 K. The 77 K emission curve-fitting analysis results agree with the computational assignment of the emitting state as 3MLLCT for 1-5 and 3LC for 6. The experimental 77 K emission energies and the calculated 3MLLCT state energies increase in the order 6 < 5, 3 < 2 < 4, 1. The 77 K emission lifetimes increase upon addition of substituents from 65 micros for 1 to 171 micros for 2, to 230 micros for 4 and 5, and to 322 micros for 3. The emission quantum yields at room temperature in solution are 0.77, 0.78, 0.83, 0.56, and 0.11 for complexes 1, 2, 4, 5, and 6, respectively.     

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.     

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

A density functional theory study of atomic oxygen and nitrogen adsorption over alpha-alumina (0001)

The interaction of atomic oxygen and nitrogen on the (0001) surface of corundum (alpha-alumina) is investigated from first-principles by means of periodic density functional calculations within the generalized gradient approximation. A large Al(2)O(3) slab model (18 layers relaxing 10) ended with the most stable aluminium layer is used throughout the study. Geometries, adsorption energies and vibrational frequencies are calculated for several stationary points for two spin states at different sites over an 1 x 1 unit cell. Two stable adsorption minima over Al or in a bridge between Al and O surface atoms are found for oxygen and nitrogen, without activation energies. The oxygen adsorption (e.g., E(ad) = 2.30 eV) seems to be much more important than for nitrogen (e.g., E(ad) = 1.23 eV). Transition states for oxygen surface diffusion are characterized and present not very high-energy barriers. The computed geometries and adsorption energies are consistent with similar adsorption theoretical studies and related experimental data for O, N or alpha-alumina. The present results along with our previous results for beta-cristobalite do not support the assumption of an equal E(ad) for O and N over similar oxides, which is commonly used in some kinetic models to derive catalytic atomic recombination coefficients for atomic oxygen and nitrogen. The magnitude of O and N adsorption energies imply that Eley-Rideal and Langmuir-Hinshelwood reactions with these species will be exothermic, contrary to what happens for beta-cristobalite.     

Photosensitized generation of singlet oxygen from rhenium(I) and iridium(III) complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer ((3)MLCT) or a ligand centred ((3)LC) state. Rate constants, k(q), for quenching of the lowest excited states by molecular oxygen are in the range 1.5 x 10(8) to 1.4 x 10(10) M(-1) s(-1). Efficiency of singlet oxygen production, f(Delta)(T), following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27-1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution p(CT) of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of pi-pi* triplet states by oxygen.     

Chiral diamines 4: a computational study of the enantioselective deprotonation of Boc-pyrrolidine with an alkyllithium in the presence of a chiral diamine

The enantioselective deprotonation of N-Boc-pyrrolidine (1) with i-PrLi-(-)-sparteine has been studied at theoretical levels up through B3P86/6-31G. Four low-energy intermediate complexes involving i-PrLi-(-)-sparteine and 1 were located via geometry optimizations; two of these complexes would lead to abstraction of the pro-S hydrogen from 1, and the other two complexes would lead to loss of the pro-R hydrogen. The lowest-energy intermediate complex was found to lead to loss of the pro-S hydrogen as observed experimentally. Transition states for the deprotonations were located using the synchronous transit-guided quasi-Newton method. The calculated activation enthalpy for transfer of the pro-S hydrogen within the lowest-energy intermediate complex, 10.8 kcal/mol, is reasonable for a reaction that occurs at a relatively low temperature, and the calculated kinetic hydrogen isotope effect is in agreement with experimental data. The lower enantioselectivity observed experimentally for deprotonation of 1 using t-BuLi-(-)-sparteine is attributed to a transition-state effect due to increased steric interaction engendered by the bulky t-BuLi. Replacement of the tert-butoxycarbonyl group in 1 by a methoxycarbonyl is predicted to result in a slower deprotonation with somewhat decreased enantioselectivity. Asymmetric deprotonation of 1 using i-PrLi in combination with the C(2)-symmetric diamine, (S,S)-1,2-bis(N,N-dimethylamino)cyclohexane, was calculated to be much less selective than is the deprotonation mediated by (-)-sparteine as observed experimentally. The relative energies of the intermediate complexes were fairly well-reproduced by ONIUM calculations in which the sparteine ligand less its nitrogen atoms was treated by molecular mechanics and the remainder of the complex was treated by quantum mechanics.     

Tetraphenylporphyrin-cobalt(III) bis(1,2-dicarbollide) conjugates: from the solution characteristics to inhibition of HIV protease

Tetraphenylporphyrin conjugates with one (PB1) and four (PB4) cobalt(III) bis(1,2-dicarbollide) substituents were synthesized and the physicochemical and photophysical properties as well as inhibition of HIV-1 protease were described. In methanol, both PB1 and PB4 were monomeric producing the triplet states and singlet oxygen after excitation. The triplet states of PB4 were quickly protonated. Porphyrins exhibited a small decrease of the quantum yields of the singlet oxygen formation (17% for PB4 and 13% for PB1) as compared with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin. On the contrary, no singlet oxygen was detected in aqueous solutions because of strong aggregation. Light scattering and atomic force microscopy (AFM) measurements documented that the behavior of aggregates in aqueous solutions is fairly complex and depends on pH, concentration, and aging. The aggregation started from spherical particles in neutral solutions. In acidic solutions, extended aggregation occurred because of slow protonation of the porphyrin pyrrole nitrogen atoms. Both PB1 and PB4 are new representatives of nonpeptide HIV-1 protease inhibitors. Their activity increased with the increasing number of the cobalt(III) bis(1,2-dicarbollide) substituents and was characterized with the IC50 values of 290+/-44 nM for PB1 and 77+/-13 nM for PB4.     

Effect of histidine on the sensitized generation of singlet oxygen in the complexes with chlorophyll

The energies of the intermolecular interactions of an O₂ molecule in the ground and excited states with the electron-excited and non-excited model complexes of chlorophyll were calculated using the DFT, CASSCF, SA-CASSCF, MCQDPT2, and XMCQDPT2 methods. The activation energies of formation and dissociation of the oxygen complexes were estimated. The radiative electric dipole moments of (0 → 0) spin-allowed S → S, T → T, and spin-forbidden S → T transitions were calculated taking into account the spin-orbit coupling, and rate constants of nonradiative transitions that determine the generation and deactivation of the O₂ molecule (¹Δ_g) were evaluated. The effect of histidine on the probability of singlet oxygen generation sensitized by the model chlorophyll complex was considered in detail.     

Organic triplet emissions of arylacetylide moieties harnessed through coordination to [Au(PCy(3))]+. Effect of molecular structure upon photoluminescent properties

A family of mono- and binuclear Cy(3)P-supported gold(I) complexes containing various pi-conjugated linear arylacetylide ligands, including the two homologous series (Cy(3)P)Au(Ctbd1;CC(6)H(4))(n)()(-)(1)(Ctbd1;CPh) and (Cy(3)P)Au(Ctbd1;CC(6)H(4))(n)()Ctbd1;CAu(PCy(3)) (n = 1-4), have been prepared. X-ray crystal analyses revealed no intermolecular aurophilic interactions in their crystal lattice. The lowest-energy singlet transitions are predominately intraligand in nature and exhibit both phenyl and acetylenic (1)(pipi) character. Strong photoluminescence is detected in solid and solution states under ambient conditions, with lifetimes in the microsecond regime. For complexes with a single arylacetylide group, only phosphorescence from the arylacetylide (3)(pipi) state is observed. Vibrational spacings in the solid-state emission spectra can be attributed to a combination of phenyl ring deformation and symmetric phenyl ring and Ctbd1;C stretches. Additional delayed-fluorescence emission is recorded for complexes with multiple p-arylacetylide units, and this is attributed to a triplet-triplet annihilation process. The phosphorescence energy of these complexes are readily modified by altering the length of the conjugated arylacetylide system, while the intensity of phosphorescence relative to fluorescence decreases when the p-arylacetylide chain is elongated. Information regarding the nature and relative energies of arylacetylide singlet and triplet excited states has been derived from the two homologous series and extrapolated to polymeric arylacetylide species. The (3)(pipi) excited-state reduction potentials E degrees [Au(+)/Au] (Au = 1a, 2, and 4) are estimated to be -1.80, -1.28, and -1.17 V versus SSCE, respectively.     

Coordination chemistry of amine bis(phenolate) cobalt(II), nickel(II), and copper(II) complexes

Cobalt(II), nickel(II), and copper(II) (1, 2, and 3) complexes of the dianionic form of the bis(phenolate) ligand N,N-bis(3,4-dimethyl-2-hydroxybenzyl)-N',N'-dimethylethylenediamine (H2L) have been synthesized by electrochemical oxidation of the appropriate metal in an acetonitrile solution of the ligand. When copper is used as the anode, the addition of 1,10-phenanthroline to the electrolytic phase gave rise to a different compound [CuL]2.2CH3CN (4). The compounds [CoL]2.2CH3CN (1), [Ni2L2(H2O)].H2O (2), [CuL]2.3H2O (3), and [CuL]2.2CH3CN (4) were characterized by microanalysis, IR, electronic spectroscopy, FAB mass spectrometry, magnetic measurements and by single-crystal X-ray diffraction. The crystal structures show that the complexes have a dinuclear structure. In compounds 1, 3, and 4, two metal ions are coordinated by the two amine nitrogens and the two phenol oxygen atoms of a deprotonated pendant phenol ligand, with one phenolic oxygen atom from ligand acting as a bridge. In compounds 1 and 3, each metal center has a geometry that is closest to trigonal bipyramidal. Magnetic susceptibility data for both compounds show an antiferromagnetic coupling with 2J = -15 cm(-1) for the cobalt(II) complex and a strong antiferromagnetic coupling with 2J = -654 cm(-1) for the copper(II) complex. However, in 4 the geometry around the metal is closer to square pyramidal and the compound shows a lower antiferromagnetic coupling (2J = -90 cm(-1)) than in 3. The nickel atoms in the dimeric compound 2 are hexacoordinate. The NiN2O4 chromophore has a highly distorted octahedral geometry. In this structure, a dianionic ligand binds to one nickel through the two amine nitrogen atoms and the two oxygen atoms and to an adjacent nickel via one of these oxygen atoms. The nickel atoms are linked through a triple oxygen bridge involving two phenolic oxygens, each from a different ligand, and an oxygen atom from a water molecule. The two nickel ions in 2 are ferromagnetically coupled with 2J = 19.8 cm(-1).     

Structures and stabilities of small lead oxide clusters PbmOn (m=1-4,n=1-2m)

The structures and stabilities of small lead oxide clusters PbmOn with m=1-4, n=1-2m are systematically studied using density functional theory. It is found that the lowest-energy structures of all these clusters can be obtained by the sequential oxidation of small "core" lead clusters. For Pb-rich clusters (oxygen-to-lead ratio<1), oxygen atoms favor bridge sites for Pb2On and Pb3On and surface sites for Pb4On. The lead-monoxide-like clusters (PbO)i (i=1-4) have great stability because of their significant dissociation energies and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps. This suggests that they could be adopted as the building blocks of cluster-assembled materials. For O-rich clusters (oxygen-to-lead ratio>1), the grouping of oxygen atoms usually appears. It is found that the structures with a grouping of more than two oxygen atoms are unstable.     

Quenching of I(2P1/2) by O3 and O(3P)

Oxygen-iodine lasers that utilize electrical or microwave discharges to produce singlet oxygen are currently being developed. The discharge generators differ from conventional chemical singlet oxygen generators in that they produce significant amounts of atomic oxygen. Post-discharge chemistry includes channels that lead to the formation of ozone. Consequently, removal of I(2P1/2) by O atoms and O3 may impact the efficiency of discharge driven iodine lasers. In the present study, we have measured the rate constants for quenching of I(2P1/2) by O(3P) atoms and O3 using pulsed laser photolysis techniques. The rate constant for quenching by O3, (1.8 +/- 0.4) x 10(-12) cm3 s-1, was found to be a factor of 5 smaller than the literature value. The rate constant for quenching by O(3P) was (1.2 +/- 0.2) x 10(-11) cm3 s-1.     

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

Spectroscopic characterization of chloride and pseudohalide ruthenium(II) complexes with 4-(4-nitrobenzyl)pyridine

Chloride, isocyanate and isothiocyanate hydride carbonyl ruthenium(II) complexes of 4-(4-nitrobenzyl)pyridine were synthesized from the precursor complex [RuHCl(CO)(PPh₃)₃] and characterized by IR, NMR, UV–Vis spectroscopy and X-ray crystallography. The electronic structures of the complexes were investigated by means of DFT calculations, based on their crystal structures. The spin-allowed singlet–singlet electronic transitions of the complexes were calculated by time-dependent DFT, and the UV–Vis spectra are discussed on this basis. The emission properties of the complexes were studied at ambient temperature, and the quantum yields of fluorescence, the lifetimes and nature of the excited states are discussed. The chloride and isothiocyanate complexes are practically nonemissive, with quantum yields under 0.01 %. Interpretation of spectra, supported by TD-DFT calculations, indicates that in this energy region, the transitions have MLCT character with admixture of LLCT (chloride and isothiocyanate complexes). The dominant LLCT character was visible in the case of the most emissive (isocyanate) complex. The low values of the lifetimes and quantum yields for these complexes indicate the influence of the metal center in the emission process.     

Homoleptic trimethylsilylacetylide complexes of chromium(III), iron(II), and cobalt(III): syntheses, structures, and ligand field parameters

A straightforward method for synthesizing soluble homoleptic trimethylsilylacetylide complexes of first-row transition metal ions is presented. Reaction of anhydrous CrCl2 with an excess of LiCCSiMe3 in THF at -25 degrees C affords orange Li3[Cr(CCSiMe3)6].6THF (1), while analogous reactions employing M(CF3SO3)2 (M = Fe or Co) generate pale yellow Li4[Fe(CCSiMe3)6].4LiCCSiMe3.4Et2O (2) and colorless Li3[Co(CCSiMe3)6].6THF (3). Slightly modified reaction conditions lead to Li8[Cr2O4(CCSiMe3)6].6LiCCSiMe3.4glyme (4), featuring a bis-mu-oxo-bridged binuclear complex, and Li3[Co(CCSiMe3)5(CCH)].LiCF3SO3.8THF (5). The crystal structures of 1-3 show the trimethylsilylacetylide complexes to display an octahedral coordination geometry, with M-C distances of 2.077(3), 1.917(7)-1.935(7), and 1.908(3) angstroms for M = Cr(III), Fe(II), and Co(III), respectively, and nearly linear M-C[triple bond]C angles. The UV-visible absorption spectrum of [Cr(CCSiMe3)6]3- in hexanes exhibits one spin-allowed d-d transition (4T2g <-- 4A1g) and three lower-energy spin-forbidden d-d transitions. The spectra of [Fe(CCSiMe3)6]4- and [Co(CCSiMe3)6]3- in acetonitrile display high-intensity charge-transfer bands, which obscure all d-d transitions except for the lowest-energy spin-allowed band (1T1g <-- 1A1g) of the latter complex. Time-dependent density functional theory (TD-DFT) calculations were employed as an aide in assigning the observed transitions. Taken together, the results are most consistent with the ligand field parameters delta(o) = 20,200 cm(-1) and B = 530 cm(-1) for [Cr(CCSiMe3)6]3-, delta(o) = 32 450 cm(-1) and B = 460 cm(-1) for [Fe(CCSiMe3)6]4- and delta(o) = 32 500 cm(-1) and B = 516 cm(-1) for [Co(CCSiMe3)6]3-. Ground-state DFT calculations support the conclusion that trimethylsilylacetylide acts as a pi-donor ligand.     

Luminescence properties of Pt(II) complexes containing polypyridine ligands with extended aromatic moieties

The luminescence properties of eleven Pt(ii) complexes containing polypyridine ligands with extended aromatic moieties have been studied, both in acetonitrile fluid solution at 298 K and in butyronitrile rigid matrix at 77 K. For comparison purposes, also the phosphorescence properties of three free ligands at 77 K in butyronitrile have been investigated. The absorption spectra of all the compounds exhibit intense bands (epsilon in the range 10(4)-10(5) M(-1) cm(-1)) in the UV region, which are attributed to spin-allowed ligand-centered (LC) transitions, and moderately intense bands (epsilon in the range 10(3)-10(4) M(-1) cm(-1)) in the visible region, which receive contribution from both spin-allowed LC transitions and spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. At low energy, less intense spin-forbidden MLCT bands are also present. At 77 K in rigid matrix, all the studied compounds exhibit structured and long-lived (lifetimes from 840 mus on the millisecond timescale) luminescence, which is attributed to triplet LC states in all cases. At room temperature in fluid solution the luminescence lifetime of all the compounds is largely shortened (nanosecond timescale), and most of the emission spectra are unstructured and red-shifted. For species exhibiting structured emission spectra even at room temperature, low luminescence quantum yields are always obtained (Phi < 10(4)), and their emission is assigned to triplet LC states, which mainly deactivate to the ground state by thermal-activated surface crossing to a closely-lying metal-centered (MC) triplet state. Compounds exhibiting unstructured emission show relatively high emission quantum yields (about 0.1) and their emission is assigned to a mixed LC/MLCT state.     

Coordination complexes of 2-(4-quinolyl)nitronyl nitroxide with M(hfac)(2) [M = Mn(II), Co(II), and Cu(II)]: syntheses, crystal structures, and magnetic characterization

Three new complexes of the formula M(2)L(2) derived from 2-(4-quinolyl)nitronyl nitroxide (4-QNNN) and M(hfac)(2) [M = Mn(II), Co(II), and Cu(II)], (4-QNNN)(2).[Mn(hfac)(2)](2) (1), (4-QNNN)(2).[Co(hfac)(2)](2).2H(2)O (2), and (4-QNNN)(2).Cu(hfac)(2).Cu'(hfac)(2) (3), were synthesized and characterized structurally as well as magnetically. Complexes 1 and 2 are four-spin complexes with quadrangle geometry, in which both the nitrogen atoms of quinoline rings and oxygen atoms of nitronyl nitroxides are involved in the formation of coordination bonds. For complex 3, however, the nitrogen atoms of quinoline rings are coordinated with Cu(II) ion to afford a three-spin complex, which is further linked to another molecule of Cu(hfac)(2) (referred to as Cu'(hfac)(2)) to form a 1D alternating chain. The magnetic behaviors of the three complexes were investigated. For complex 1, as the nitronyl nitroxides and Mn(II) ions are strongly antiferromagnetically coupled, consequently its temperature dependence of magnetic susceptibility was fitted to the model of spin-dimer with S = 2, yielding the intradimer magnetic exchange constant of J = -0.82 cm(-1). For complex 2, the temperature dependence of the magnetic susceptibility in the T > 50 K region was simulated with the model of two-spin unit with S(1) = 3/2 and S(2) = 1/2, leading to J = -321.9 cm(-1) for the magnetic interaction due to Co(II).O coordination bonding, D = -16.3 cm(-1) (the zero-field splitting parameter), g = 2.26, and zJ = -3.8 cm(-1) for the magnetic interactions between Co(II) ions and nitronyl nitroxides through quinoline rings and those between nitronyl nitroxides due to the short O.O short contacts. The temperature dependence of magnetic susceptibility of 3 was approximately fitted to a model described previously affording J(1) = -6.52 cm(-1) and J(2) = 3.64 cm(-1) for the magnetic interaction between nitronyl nitroxides and Cu(II) ions through the quinoline unit via spin polarization mechanism and the weak O.Cu coordination bonding, respectively.     

Exploratory studies on coordination chemistry of a redox-active bridging ligand: synthesis, properties and solid state structures of the complexes

The explorative coordination chemistry of the bridging ligand TTF-PPB is presented. Its strong binding ability to Co(II) and then to Ni(II) or Cu(II) in the presence of hexafluoroacetylacetonate (hfac(-)), forming new mono- and dinuclear complexes 1-3, is described. X-ray crystallographic studies have been conducted in the case of the free ligand TTF-PPB as well as its complexes [Co(TTF-PPB)(hfac)(2)] (1) and [Co(hfac)(2)(μ-TTF-PPB)Ni(hfac)(2)] (2). Each metal ion is bonded to two bidentate hfac(-) anions through their oxygen atoms and two nitrogen atoms of the PPB moiety with a distorted octahedral coordination geometry. Specifically, nitrogen donor atoms of TTF-PPB adopt a cis-coordination but not in the equatorial plane, which is quite rare. Electronic absorption, photoinduced intraligand charge transfer ((1)ILCT), and electrochemical behaviour of 1-3 have been investigated. UV-Vis spectroscopy shows very strong bands in the UV region consistent with ligand centred π-π* transitions and an intense broad band in the visible region corresponding to a spin-allowed π-π* (1)ILCT transition. Upon coordination, the (1)ILCT band is bathochromically shifted by 3100, 6100 and 5900 cm(-1) on going from 1 to 3. The electrochemical studies reveal that all of them undergo two reversible oxidation and one reversible reduction processes, ascribed to the successive oxidations of the TTF moiety and the reduction of the PPB unit, respectively.     

Significant effect of spin flip on the oxygen atom transfer reaction from (oxo)manganese(v) corroles to thioanisole: insights from density functional calculations

The electronic and structural features of (oxo)manganese(v) corroles and their catalyzed oxygen atom transfers to thioanisole in different spin states have been investigated by the B3LYP functional calculations. Calculations show that these corrole-based oxidants and their complexes with thioanisole generally have the singlet ground state, and their triplet forms are also accessible in consideration of the spin-orbit coupling interaction. Due to strong d-π conjugation interactions between Mn and the corrole ring arising from the π electron donation of the corrole moiety, the five-coordinated Mn approximately has the stable 18-electron configuration. The predicted free energy barriers for the singlet oxygen atom transfer reactions are generally larger than 22 kcal mol(-1), while the spin flip in reaction may remarkably increase the reactivity. In particular, the bromination on β-pyrrole carbon atoms of the meso-substituted (oxo)manganese(v) corrole strikingly enhances the spin-orbit coupling interaction and results in the dramatic increase of reactivity. The multiple spin changes are predicted to be involved in the low-energy reaction pathway. The present results show good agreement with the experimental observation and provide a detailed picture for the oxygen atom transfer reaction induced by the (oxo)manganese(v) corroles.