3d Transition Metal Complexes Research Articles

Sequence-Selective Cleavage of Oligoribonucleotides by 3d Transition Metal Complexes of 1,5,9-Triazacyclododecane-Functionalized 2‘-O-Methyl Oligoribonucleotides

2'-O-Methyl oligoribonucleotides bearing a 3'-[2,6-dioxo-3,7-diaza-10-(1,5,9-triazacyclododec-3-yl)decyl phospate conjugate group have been shown to cleave in slight excess of Zn(2+) ions complementary oligoribonucleotides at the 5'-side of the last base-paired nucleotide. The cleavage obeys first-order kinetics and exhibits turnover. The acceleration compared to the monomeric Zn(2+) 1,5,9-triazacyclododecane chelate is more than 100-fold. In addition, 2'-O-methyl oligoribonucleotides having the 1,5,9-triazacyclododec-3-yl group tethered to the anomeric carbon of an intrachain 2-deoxy-beta-d-erythro-pentofuranosyl group via a 2-oxo-3-azahexyl, 2,6-dioxo-3,7-diazadecyl, or 2,9-dioxo-3,10-diazatridecyl linker have been studied as cleaving agents. These cleave as zinc chelates a tri- and pentaadenyl bulge opposite to the conjugate group approximately 50 times as fast as the monomeric chelate and show turnover. The cleavage rate is rather insensitive to the length of linker. Interestingly, a triuridyl bulge remains virtually intact in striking contrast to a triadenyl bulge. Evidently binding of the zinc chelate to a uracil base prevents its catalytic action. Replacement of Zn(2+) with Cu(2+) or Ni(2+) retards the cleaving activity of all the cleaving agents tested.

Photoluminescence and Electroluminescence of d6 Metal−Organic Conjugated Oligomers: Correlation of Photophysics and Device Performance

The photophysical and electroluminescent device properties of a series of d6 transition metal complexes that feature an oligo(aryleneethynylene) (OAE) “ligand” have been investigated. The metals are coordinated η2- to the OAE via a 2,2‘-bipyridine moiety that is in the “core” of the oligomer, and the metals used are η2-Ir(ppy)2+, η2-Ru(bpy)22+, and η2-Os(bpy)22+ (Ir-2, Ru-2, and Os-2, respectively, where ppy = 2-phenylpyridine and bpy = 2,2‘-bipyridine). The photoluminescence spectra and excited-state decay parameters for the metal−OAEs indicate that at ambient temperature Ru-2 and Os-2 feature a lowest 3MLCT excited state; however, in Ir-2 the lowest excited state is 3π,π* localized on the OAE. Electroluminescent (EL) devices that contain the metal−OAEs dispersed at 5 wt % in a poly(vinyl carbazole):2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole matrix exhibit electroluminescence at wavelengths that correlate well with the photoluminescence from the complexes in a rigid glass at low temperature. The EL efficiency of the devices varies in the order Ir-2 > Ru-2 ≫ Os-2. Factors that may contribute to the difference in EL efficiencies are discussed.

Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution

Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO 2(B), Cr 2O 3 were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline γ-Mn 2O 3 was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single crystal nickel nanowhiskers were synthesized by direct reduction without post-annealing process. All the products were characterized on their structure and micro-morphology. The reaction details and features were described and discussed.

Calculation of electronic g-tensors for transition metal complexes using hybrid density functionals and atomic meanfield spin-orbit operators.

We report the first implementation of the calculation of electronic g-tensors by density functional methods with hybrid functionals. Spin-orbit coupling is treated by the atomic meanfield approximation. g-Tensors for a set of small main group radicals and for a series of ten 3d and two 4d transition metal complexes have been compared using the local density approximation (VWN functional), the generalized gradient approximation (BP86 functional), as well as B3-type (B3PW91) and BH-type (BHPW91) hybrid functionals. For main group radicals, the effect of exact-exchange mixing is small. In contrast, significant differences between the various functionals arise for transition metal complexes. As has been shown previously, local and in particular gradient-corrected functionals tend to underestimate the "paramagnetic" contributions to the g-tensors in these cases and thereby recover only about 40-50% of the range of experimental g-tensor components. This is improved to ca. 60% by the B3PW91 functional, which also gives slightly reduced standard deviations. The range increases to almost 100% using the half-and-half functional BHPW91. However, the quality of the correlation with experimental data worsens due to a significant overestimate of some intermediate g-tensor values. The worse performance of the BHPW91 functional in these cases is accompanied by spin contamination. Although none of the functionals tested thus appears to be ideal for the treatment of electronic g-tensors in transition metal complexes, the B3PW91 hybrid functional exhibited the overall most satisfactory performance. Apart from the validation of hybrid functionals, some aspects in the treatment of spin-orbit contributions to the g-tensor are discussed.

Validation study of meta-GGA functionals and of a model exchange–correlation potential in density functional calculations of EPR parameters

We report the first implementation of the DFT calculation of electronic g-tensors and hyperfine coupling constants using meta-GGA exchange–correlation functionals (both Laplacian and kinetic-energy-density dependent) and statistical average of orbital model exchange–correlation potentials (SAOP). The g-tensors for a set of eleven small main group radicals and for a series of ten 3d and two 4d transition metal complexes have been compared using two meta-GGA functionals (FT98 and PKZB), SAOP, as well as the BP86 GGA, and B3PW91 and BHPW91 hybrid functionals. The same functionals have been compared in the calculation of hyperfine coupling constants for thirteen small main group radicals and twelve 3d transition metal compounds. The results for main group radicals are satisfactory, both with the newly validated and previously tested functionals. In contrast, the accurate evaluation of the EPR parameters for 3d transition metal complexes remains a challenge for “pure” density functional theory: the new functionals (potentials) do not improve the results compared to GGA functionals, whereas hybrid functionals tend to provide superior results. SAOP leads to large spin contamination for a number of transition metal complexes. This has been traced back to the orbital-energy dependence of the potential.

Mechanisms of EPR Hyperfine Coupling in Transition Metal Complexes

A detailed quantum chemical analysis of the underlying principles of hyperfine coupling in 3d transition metal complexes has been carried out. The explicit evaluation of one- and two-electron integrals for some atomic systems has been used to understand the spin polarization of the core shells. While spin polarization enhances the exchange interaction of the 2s and 2p shells with the singly occupied orbitals, the opposite spin polarization of the 3s and 3p shells arises from the required orthogonality to the 2s and 2p shells, respectively. Core-shell spin polarization in molecules is found to be proportional to the spin population in the valence 3d orbitals but to depend little on other details of bonding. In contrast, the spin polarization of the valence shell depends crucially on the overlap between the singly occupied and certain doubly occupied valence orbitals. Large overlap leads to pronounced spin polarization of these orbitals and, among other things, likely to spin contamination when using UHF wave functions or hybrid density functionals. The role of core- and valence-shell spin polarization for dipolar hyperfine couplings in transition metal complexes is discussed. It is demonstrated that great care should be exercised in deriving spin populations or even orbital compositions from dipolar couplings alone.

Synthesis, Characterization and Scavenger Effects on O2 − of 3d Transition Metal Complexes of Isonicotinoyl Hydrazone Derived from Isoniazid with PMBP

The 3d transition metal complexes of isonicotinoyl hydrazone derived from isoniazid with l-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) have been synthesized and characterized on the basis of elemental analyses, IR UV spectra and thermal analyses. The general formula of the complexes is ML nH2O (where M(II) = Cr, Mn, Fe, Co, Ni and Cu, n = 0, 1). It has been discovered that the complexes possess certain scavenger effects on O2 − radicals.

Partial Degradation of the Newexo-Heterodisubstituted Carborane Derivatives with d10Transition Metal Ions (Cu, Au)

Reaction of the closo heterodisubstituted [1-PPh2-2-SR-1,2-C2B10H10] compounds with d10 transition metal complexes, [MCl(PPh3)n] (M = Cu, n = 2; M = Au, n = 1), in ethanol affords the complexes [M(...

Luminescent polynuclear d10 metal complexes

A number of polynuclear d10 transition metal complexes have been found to exhibit interesting luminescence properties. The photoluminescence properties of polynuclear d10 metal complexes are highly diversified. In the presence of a wide range of bridging and ancillary ligands, the excited states of such complexes have been suggested to range from metal-to-ligand charge-transfer, ligand-to-metal charge-transfer, metal-centred to ligand-centred in nature. Recent work on the photophysical and photochemical properties, as well as the applications of this class of luminescent polynuclear d10 metal complexes will be described in this review article.

Bond Characterization of Metal Squarate Complexes [MII(C4O4)(H2O)4; M = Fe, Co, Ni, Zn

Tetraaqua metal squarate complexes, M(C4O4)(H2O)4, (M = Fe, Co, Ni, Zn), are known to have a polymeric chain structure with C4O42- as a bridge (μ-2) ligand between two metal ions in the trans position. Each metal ion is bonded to two C4O42- and four water molecules. They are all isostructural with space group C2/c. A complete Ewald sphere of data is measured at 120 K up to 2θ of 100−120° using Mo Kα radiation for each complex. Such carefully measured intensities are used to investigate the detailed electron density distribution in order to understand the chemical bonding and the d-orbital splitting of the metal ions subjected in such a ligand field. Results on the electron density distribution will be presented in the form of deformation density and of Laplacian maps. Deformation density will be presented in terms of experimental Δρx-x, Δρm-a (multipole model), and of theoretical Δρ derived from the HF and DFT calculations. The interesting bent bond feature on the four-membered ring ligand C4O42- is explicitly demonstrated by the deformation density distribution and the bond path of the cyclic carbon−carbon bond. The asphericity in electron density distribution around the metal ion is also clearly illustrated in these compounds both in the deformation density and in Laplacian of the density. The comparison on the series of 3d-transition metal complexes will be made not only by the deformation density distribution and by the Laplacian of the density but also by the d-orbital population and by the associated topological properties at the bond critical point. The total number of d-electrons from the experiments are 6.05, 6.88, 7.89, and 8.40, respectively, for Fe(II), Co(II), Ni(II), and Zn(II) ions in these compounds. A comparison between experiment and theory is made for the Ni complex.

Density Functional Study of Cobalt-59 Nuclear Magnetic Resonance Chemical Shifts and Shielding Tensor Elements in Co(III) Complexes

A density functional method has been used to successfully predict the isotropic 59Co nuclear magnetic resonance (NMR) chemical shifts of the following anionic, cationic and neutral Co(III) complexes: [Co(CN)6]3-, [Co(NH3)6]3+, [Co(NO2)6]3-, [Co(NH3)4CO3]+, Co(acac)3, and [Co(en)3]3+. Isotropic chemical shifts are well-reproduced by using Wachters' cobalt basis set and uniform 6-31G* basis sets on the light atoms, together with the use of the B3LYP hybrid functional. In addition, the principal elements of the 59Co shielding tensor (σ11, σ22, and σ33), the absolute shieldings of Co(CN)63- and Co(acac)3, and the Co−C bond length shielding derivative for Co(CN)63- are also in good agreement with previous experimental estimates. There are no obvious distinctions between the predicted shifts (or shielding tensor elements) of anionic and cationic complexes. The ability to successfully predict both shift trends, absolute shieldings, shielding tensor elements, and a vibrational shielding derivative for d6 transition metal complexes opens up new possibilities for probing metal ions in biological systems by using NMR spectroscopy.

Sequential Electrophile/Nucleophile Additions for η2-Cyclopentadiene Complexes of Osmium(II), Ruthenium(II), and Rhenium(I)

A series of d6 transition-metal complexes of the type ML5(η2-CpH), where ML5 = [OsII(NH3)5]2+, [RuII(NH3)5]2+, and [ReI(PPh3)(PF3)(dien)]+, were synthesized as their triflate salts and combined with electrophiles (HOTf, CH2(OMe)2) to form η3-allyl complexes. Treatment of these π-allyl complexes with the mild carbon nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)propene (MMTP) followed by decomplexation affords substituted η2-cyclopentene derivatives with excellent regio- and stereocontrol. Deuteration and NOE studies for the π-allyl complexes along with stereochemical analysis of the organic products confirm that both electrophilic and nucleophilic addition occurs exclusively from the exo face of the ring (opposite to metal coordination) for all three systems.

Theoretical study of the vibrational spectra of the transition metal carbonyls M(CO)6 [M=Cr, Mo, W], M(CO)5 [M=Fe, Ru, Os], and M(CO)4 [M=Ni, Pd, Pt

The harmonic force fields of the title compounds have been calculated at the level of Hartree–Fock (HF) theory, Mo/ller–Plesset second-order perturbation theory (MP2), and gradient-corrected density functional theory (DFT) using all-electron and effective core potential wave functions in conjunction with polarized double- and triple-zeta basis sets. The DFT results are in very good agreement with the available experimental data, whereas the HF results are inadequate and the MP2 results are satisfactory only for the 5d and (partly) the 4d transition metal complexes, but not for the 3d transition metal complexes. The calculated DFT frequencies are accurate enough to suggest reassignments in the vibrational spectrum of Fe(CO)5. In the case of Ru(CO)5, Os(CO)5, Pd(CO)4, and Pt(CO)4 where experimental data are scarce, the DFT predictions may guide future experimental work.

Differences between L3 and L2 x-ray absorption spectra of transition metal compounds

The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single-particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low-spin and the occupation of the t2g states implies a decrease of the first peak. This decrease is stronger for the L2 edge, implying an increase in the L3:L2 ratio. For 4d5 systems transitions to the t2g hole are only possible at the L3 edge due to the combined effects of 4d spin–orbit coupling and the dd multiplet effects.

5262725 Magnetic resonance imaging method and device for reducing image errors in a magnetic resonance image

The linear and quadratic Tg ⊗ εg Jahn–Teller effect in the Tg (Tg = 2T2g, 3T1g) ground states of low-spin octahedral cyano complexes of 3d-transition metals (M = TiIII, VIII, MnIII, FeIII, CrII, MnII) has been studied. Vibronic coupling parameters have been derived using density functional theory to calculate energies of Slater determinants, which result from various electron distributions within the t2gn configuration due to the metal based t2g(3d) molecular orbitals (n = 1, 2, 3, 4). Tetragonal elongations are found in the case of TiIII, MnIII and CrII, and compressions for VIII, FeIII and MnII, and these establish the metal–ligand π-back donation as the dominant effect and driving force for the geometry distortions and energy stabilizations towards non-degenerate 2B2g(t2g1,t2g5) and 3A2g(t2g2,t2g4) ground states. The strength of the Jahn–Teller coupling is very weak and found not to follow a monotonic trend across the transition metal series but is shown to be weakest for TiIII, VIII and FeIII, slightly larger for MnIII, increases further to MnII and is found to be the strongest but is still weak for CrII.

EXAFS and Ligand Exchange Reaction of Hydrated 3d Transition Metal Complexes

Relation between EXAFS parameters and ligand exchange reaction rate constant in solution is investigated. The EXAFS spectra of hydrated 3d metal complexes have been obtained over the temperature range of 20 K to 300 K. The temperature dependences of the mean square relative displacement σ2 and the third order coefficient C3 in cumulant expansion of EXAFS signal are examined for the first nearest metal-oxygen bond. The ligand exchange rate constant correlates not only with σ2 but also C3 both obtained from the EXAFS for solid hydrated complexes, and the higher the rate constant, the more rapidly σ2 and C3 increase with temperature.

Co-ordination chemistry of 2-phenyl-6-(2-thienyl)pyridine and 2,6-bis(2-thienyl)pyridine; new ambidentate ligands

The new ligands 2-phenyl-6-(2-thienyl)pyridine (H2pthpy) and 2,6-bis(2-thienyl)pyridine (H2bthpy) exhibit a variety of reactivities with d8 transition-metal complexes. Reaction of H2bthpy with platinum(II) gives complexes in which the ligand adopts Hbthpy-C,N and bthpy-CNC bonding modes, whereas gold(III) converts the ligand into the dimer 5,5′-bis[6-(2-thienyl)-2-pyridyl]-2,2′-bithiophene. Palladium(II) gives mixtures of cyclometallated complexes and dimerised ligand. In contrast, H2pthpy reacts with platinum(II) to give metallated complexes in which the site of metallation is the phenyl rather than the thienyl ring. The reaction of H2pthpy with gold(III) results in clean conversion into the dimer 5,5′-bis(6-phenyl-2-pyridyl)-2,2′-bithiophene and a chlorinated derivative 2-(5-chloro-2-thienyl)-6-phenylpyridine. Once again mixtures of metallated product and ligand reaction products are obtained with palladium(II).

XPS study of metal complexes with an unsymmetrical tetradentate Schiff base

Complexes of several 3d transition metals (Mn(II)-Zn(II)) with 2-(2,5,8-triaza-1-octenyl)phenol (Hsaden), [Msaden]CIO 4, have been studied using X-ray photoelectron spectroscopy (XPS). The N 1s peak is asymmetric and is an envelope of two components. A weaker component at a lower binding energy ( BE) is attributed to the azomethine nitrogen atom, the other one to the nitrogens in the two amino groups. The assignment is supported by spectra of diethylenetriamine complexes in which the N 1s peak is sharper and its BE value is similar to that of the second component. The N 1s electron BE increases from Mn to Cu, in accordance with the Irving-Williams series; planar [Nisaden]CIO 4 complex is the only exception.

EXAFS Amplitudes of Six-Coordinate Aqua and Ammine 3d Transition Metal Complexes in Solids and in Aqueous Solutions

Abstract X-Ray absorption spectra of hexaaqua and hexaammine complexes of some 3d transition metals in solids and in aqueous solutions are measured and analyzed. It is found that the peak intensities in the EXAFS Fourier transforms for aqua complexes correlate with those for ammine complexes.

Characterization of deep levels in CdTe by photo-EPR and related techniques

In photo-EPR, the EPR signature of microscopically identifiable defects is used to monitor light-induced electronic transitions. In this paper we summarize photo-EPR investigations of group IV donors, 3d transition metal impurities and complexes in CdTe. Energy levels and Huang-Rhys factors are derived for the 3d impurities by fitting a model calculation to the photo-ionization cross-section which is derived from constant photo-EPR. Comparison with results for other II–VI compounds confirm host-independent relative zero-phonon level positions for different 3d metal impurities and large lattice relaxation energies.