Electronic structure of ferric heme nitrosyl complexes with thiolate coordination

The effect of trans thiolate ligation on the coordinated nitric oxide in ferric heme nitrosyl complexes as a function of the thiolate donor strength, induced by variation of NH-S(thiolate) hydrogen bonds, is explored. Density functional theory (DFT) calculations (BP86/TZVP) are used to define the electronic structures of corresponding six-coordinate ferric [Fe(P)(SR)(NO)] complexes. In contrast to N-donor-coordinated ferric heme nitrosyls, an additional Fe-N(O) sigma interaction that is mediated by the dz2/dxz orbital of Fe and a sigma*-type orbital of NO is observed in the corresponding complexes with S-donor ligands. Experimentally, this is reflected by lower nu(N-O) and nu(Fe-N) stretching frequencies and a bent Fe-N-O moiety in the thiolate-bound case.     

Synthesis and spectroscopic characterization of copper(II)-nitrito complexes with hydrotris(pyrazolyl)borate and related coligands

This study focuses on the geometric (molecular) structures, spectroscopic properties, and electronic structures of copper(II)-nitrito complexes as a function of second coordination sphere effects using a set of closely related coligands. With anionic hydrotris(pyrazolyl)borate ligands, one nitrite is bound to copper(II). Depending on the steric demand of the coligand, the coordination mode is either symmetric or asymmetric bidentate, which leads to different ground states of the resulting complexes as evident from EPR spectroscopy. The vibrational spectra of these compounds are assigned using isotope substitution and DFT calculations. The results demonstrate that nu sym(N-O) occurs at higher energy than nu asym(N-O), which is different from the literature assignments for related compounds. UV-vis absorption and MCD spectra are presented and analyzed with the help of TD-DFT calculations. The principal binding modes of nitrite to Cu(II) and Cu(I) are also investigated applying DFT. Using a neutral tris(pyrazolyl)methane ligand, two nitrite ligands are bound to copper. In this case, a very unusual binding mode is observed where one nitrite is eta1-O and the other one is eta1-N bound. This allows to study the properties of coordinated nitrite as a function of binding mode in one complex. The N-coordination mode is easily identified from vibrational spectroscopy, where N-bound nitrite shows a large shift of nu asym(N-O) to >1400 cm-1, which is a unique spectroscopic feature. The optical spectra of this compound exhibit an intense band around 300 nm, which might be attributable to a nitrite to Cu(II) CT transition. Finally, using a bidentate neutral bis(pyrazolyl)methane ligand, two eta1-O coordinated nitrite ligands are observed. The vibrational and optical (UV-vis and MCD) spectra of this compound are presented and analyzed.     

New Developments in Nitrogen Fixation

The production of ammonia from atmospheric dinitrogen at room temperature and ambient pressure in analogy to nature is a long-term goal for coordination chemists. Novel reactions of N₂-containing transition metal complexes with H₂, the first side-on N₂-bridged structure of an actinide complex, and an interesting variation of synthetic N₂ fixation are the key points addressed in this contribution. The results are related to the known chemistry of N₂ complexes, and their significance is discussed with respect to enzymatic N₂ fixation.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Vibrational Properties of [FeH(N2)(depe)2]+ and [FeCl(N2)(depe)2]+]: Dinitrogen Bonding in the Low Activation Limit

The vibrational properties of the two octahedral Fe^II dinitrogen complexes [FeH(N₂)(depe)₂]⁺ ( 1 ) and [FeCl(N₂)(depe)₂]⁺ ( 2 , depe = 1, 2‐bis(diethylphosphino)ethane) are investigated with the help of infrared and Raman spectroscopies. Vibrational data are evaluated with a Quantum Chemistry Assisted Normal Coordinate Analysis (QCA‐NCA; N. Lehnert, F. Tuczek, Inorg. Chem. 1999 , 38, 1659). In agreement with high values found for ν(NN) and the corresponding force constants f(NN), the N₂ ligands in compounds 1 and 2 are non‐activated which corresponds to the observation that N₂ is not protonable in Fe^II systems. Taking into account the short Fe‐N bond lengths, the values of the Fe‐N stretching force constants (2.55mdyn/Å for 1 and 2.58mdyn/Å for 2 ) are found to be compatible with those of other Fe^II low‐spin compounds coordinated to backbonding N‐coordinating ligands. The force fields obtained for the Fe‐N₂ units of 1 and 2 are almost identical although the thermal stability of 1 and 2 with respect to loss of N₂ is different. This indicates that the zero‐point vibrational levels are unaffected by possible ground‐state level crossing processes occuring at larger Fe‐N bond lengths, as observed for 2 (O. Franke, B. E. Wiesler, N. Lehnert, C. Näther, V. Ksenofontov, J. Neuhausen, F. Tuczek, Inorg. Chem. 2002 , 41, 3491).     

On Polychalcogenides of Thallium with M2 Q11 Groups as a Structural Building Block. II. Tl4Ta2Se11: Synthesis,Crystal Structure,Properties and Spectroscopic Investigations of the First Polyselenide being Composed of a Discrete [Ta2Se11]4— Anion

The new ternary compound Tl₄Ta₂Se₁₁ was prepared in a melt of thallium polyselenides applying elemental tantalum. It crystallises in the triclinic space group P1¯ with a = 7.996(1) Å, b = 9.866(1) Å, c = 13.668(2) Å, α = 73.03(1)°, β = 89.21(2)° and γ = 85.72(1)°. Tl₄Ta₂Se₁₁ is the first polyselenide with discrete complex [M₂Se₁₁]^4— anions. Every Ta atom is in a sevenfold environment of Se atoms to form a distorted pentagonal bi‐pyramid. The two TaSe₇ polyhedra have a face in common thus yielding the [Ta₂Se₁₁]^4— unit. In the structure, the anions are well separated by the Tl¹⁺ cations. An assignment of the different vibration modes in the IR and Raman spectra is given based on density functional calculations.