Theoretical Study of Hydrated Cd~（2＋） Interactions with Guanine

Theoretical study was performed to investigate how the hydration of cadmium ca-tion influences the structure and properties of guanine.The aqueous environment was simulated by both explicit solvent(1-5 water molecules) model and implicit solvent model.For complexes in which Cd2+ attached to the N(7) and O(6) sites of guanine,energy analysis together with the Natural Bonding Orbital(NBO) analysis were performed to elucidate the bonding characteristics in detail.The most stable structures are penta-coordinate complexes without aqua ligand located at the guanine site.Higher number of water ligands corresponds to higher stabilization energies.Average bonding energies of G-Cd increase with the number of water molecules.Bonding energies of water ligands depend on its position in the complexes.The charge distribution of guanine changed with increasing the number of water ligands,which may also influence the base-pairing pattern of guanine.There is positive charge transfer from guanine to aqua ligand as the number of the hydration waters increases.IEFPCM optimization has results comparable to the [CdG(H2O)5]2+ structure 5a.     

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid-State NMR Spectroscopy,Polarized Neutron Diffraction,and First-Principle Calculations

The local magnetic structure in the [Fe^III(Tp)(CN)₃]⁻ building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions.     

Relaxometric study of copper [15]metallacrown-5 gadolinium complexes derived from alpha-aminohydroxamic acids

Proton nuclear magnetic relaxation dispersion (NMRD) profiles were recorded between 0.24 mT and 1.4 T for lanthanum(III)- and gadolinium(III)-containing [15]metallacrown-5 complexes derived from alpha-aminohydroxamic acids and with copper(II) as the ring metal. The influence of the different R-groups on the proton relaxivity was investigated, and a linear relationship between the relaxivity and the molecular mass of the metallacrown complex was found. The selectivity of the metallacrown complexes was tested by transmetalation experiments with zinc(II) ions. The crystal structure of the copper [15]metallacrown-5 gadolinium complex with glycine hydroximate ligands is reported.     

Trip2C6H3SeF: the first isolated selenenyl fluoride

Joining the stable: The first examples of the highly instable selenenyl fluorides RSeF are prepared from the reaction on the tin selenide RSeSnMe(3) with XeF(2). Through the use of extremely large protecting groups (m-terphenyl ligands) which stabilizes the RSeF units against disproportionation, the compounds could be isolated and characterized by NMR spectroscopy and single-crystal structure analysis (see structure).     

Preparation and evaluation of 2-azinyl-2H-benzotriazoles as bidentate ligands: Synthesis and characterization of [2-(2-pyridynyl)-2H-benzotriazole](bpy)2Ru

Five 2-azinyl-2H-benzotriazoles (azinyl = 2-pyridinyl, 2-pyrazinyl, 2-pyrimidinyl, 6-methoxy-3-pyridazinyl, 5-methyl-2-pyridinyl were prepared and characterized as bidentate ligands. The electronic structure of these and related heterocycles was investigated spectroscopically and computationally (TD-DFT). They were tested at the B3LYP/6-31++G(d, p)//B3LYP/6-31G(d, p) level of theory as ligands for MgH₂, which permitted the elucidation of trends in complex formation, its geometry as a function of the ring structure, and the number and position of the nitrogen atoms in the azine ring. A Ru²⁺ complex 7a-Ru with 2-pyridinyl-2H-benzotriazole (7a) and two bpy ligands was prepared and characterized structurally, spectroscopically and electrochemically. The results were compared to those for similar complexes and discussed in the context of computational results for MgH₂ complexes.     

Oxidation state changes and electron flow in enzymatic catalysis and electrocatalysis through Wannier-function analysis

In catalysis by metalloenzymes and in electrocatalysis by clusters related in structure and composition to the active components of such enzymes transition-metal atoms can play a central role in the catalyzed redox reactions. Changes to their oxidation states (OSs) are critical for understanding the reactions. The OS is a local property and we introduce a new, generally useful local method for determining OSs, their changes, and the associated bonding changes and electron flow. The method is based on computing optimally localized orbitals (OLOs). With this method, we analyze two cases, superoxide reductase (SOR) and a proposed hydrogen-producing model electrocatalyst [FeS(2)]/[FeFe](P), a modification of the active site of the diiron hydrogenase enzymes. Both utilize an under-coordinated Fe site where a one-electron reduction (for SOR) or a two-electron reduction (for [FeFe](P)) of the substrate occurs. We obtain the oxidation states of the Fe atoms and of their critical ligands, the changes of the bonds to those ligands, and the electron flow during the catalytic cycle, thereby demonstrating that OLOs constitute a powerful interpretive tool for unraveling reaction mechanisms by first-principles computations.     

The molecular structure of Cr[(CH2)2PMe2]3: dimethylphosphonium-bis-methylide chromium compounds as inner-phosphonium-alkyl-ate- or 2-phospha-allyl-complexes?

The molecular structure of the chromium(III) complex Cr[(CH₂)₂PMe₂]₃ (1a) was determined by X-ray crystallography. The bonding mode of the chelating dimethylphosphonium-bis-methylide ligand is discussed either as a part of an inner-phosphonium alkyl-ate-complex or as a 2-phospha-allyl system. In principle it seems possible to extend this consideration also to the bridging dimethylphosphonium-bis-methylide ligands in the chromium(II) complex Cr₂[(CH₂)₂PMe₂]₄. Regarding the electron delocalization in such ligands the structural parameters of 1a are compared to the results of recently described quantum chemical calculations.     

The Molecular and Electronic Structure of Nickelatetrazole – A Theoretical Study

Nickelatetrazoles have been proposed as intermediates in the course of the photoreaction of Ni^II complexes of [NiP₂(N₃)₂] constitution (P₂: mono‐ or diphosphane ligands). However, any metallatetrazoles as well as their organic analogue, 5 H‐carbatetrazole, could neither be prepared nor identified up to now. Based on density functional theory (DFT) calculations, predictions are given concerning the molecular and electronic structure of tetrazoles. While 5 H‐tetrazole is indeed a rather unstable species, metallatetrazole moieties in square‐planar d⁸ transition metal complexes should be experimentally accessible.     

Four New Cu（Ⅱ） Complexes with Benzotriazole-based Ligands： Syntheses,Crystal Structures and Theoretical Investigations

Four new Cu（Ⅱ） complexes with two benzotriazole-based ligands, [Cu2（L^1）2（NO3）2]· 2H2O （1）, [Cu2（L^1）2]·2ClO4·2H2O （2）, [Cu2（HL^2）2（NO3）4]·2CH3COCH3 （3） and [Cu（HL^2）2（Cl）]·Cl·2CH2Cl2 （4）, where HL^1 = 1,3-bis（benzotriazol-2-yl）-2-propanol and HL^2 = 1,3-bis（benzotriazol-1-yl）-2-propanol, were synthesized and structurally characterized by elemental analyses, IR and single-crystal X-ray diffraction analyses. It is revealed that complexes 1～3 have dinuclear structures, while 4 possesses a one-dimensional （1-D） chain structure, which extends in two orthogonal orientations. In 1～4, the coordination numbers of Cu（Ⅱ） centers range from four to six, which may be attributed to the different geometries and coordination abilities of the ligands and anions. The L^1 ligand in complexes 1 and 2 adopts a tridentate di-chelating coordination mode, whereas ligand HL^2 in complexes 3 and 4 has a bidentate bridging coordination mode. The different coordination modes of these two ligands may be explained by the different charges of nitrogen donor atoms in the benzotriazole ring, which has been investigated by density functional theory （DFT） calculations.     

103Rh NMR chemical shifts in organometallic complexes: a combined experimental and density functional study

Experimental 103Rh NMR chemical shifts of mono- and binuclear rhodium(I) complexes containing s- or as-hydroindacenide and indacenediide bridging ligands with different ancillary ligands (1,5-cyclooctadiene, ethylene, carbonyl) are presented. A protocol, based on density functional theory calculations, was established to determine 103Rh NMR shielding constants in order to rationalise the effects of electronic and structural variations on the spectroscopic signal, and to gain insight into the efficiency of this computational method when applied to organometallic systems. Scalar and spin-orbit relativistic effects based on the ZORA (zeroth order regular approximation) level have been taken into account and discussed. A good agreement was found for model compounds over a wide range of chemical shifts of rhodium (approximately 10,000 ppm). This allowed us to discuss the experimental and calculated delta(103Rh) in larger complexes and to relate it to their electronic structure.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

On the Electronic Structure of Distorted Cubic Rhodium Cluster Complexes Containing Bridging Germanium or Phosphorus Ligands

DFT calculations show that the optimal metal valence electron (MVE) count of omnicapped cubic rhodium clusters containing more than eight terminal ligands, is 114. For such a count, a closed-shell configuration is computed with a substantial HOMO-LUMO gap. The presence of more than eight terminal ligands in the clusters favors highly distorted cubic architectures with capping ligands asymmetrically bound to the distorted metallic square faces. Removal of terminal ligands leads to the replacement of bonding M–L electron pairs by nonbonding electron pairs localized on the metal atoms, giving rise to unchanged MVE count.     

Cationic Gold(II) Complexes: Experimental and Theoretical Study**

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [Au^II(L)(X)]⁺ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [Au^II(L)(X)]⁺ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.     

The "invisible" 13C NMR chemical shift of the central carbon atom in [(Ph3PAu)6C]2+: a theoretical investigation

The experimental 13C NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)6C]2+ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the 13C NMR properties of the related complex [C{Au[P(C6H5)2(p-C6H4NMe2)]}6]2+, Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.     

X-ray structure of [Re(NO)2.09Br1.91(PPh3)2] and DFT studies of [Re(NO)2Br2(PPh3)2]

The crystal and molecular structure of [Re(NO)_2.09Br_1.91(PPh₃)₂] and DFT studies of [Re(NO)₂Br₂(PPh₃)₂] are reported. The linearly bonded nitrosyl ligands adopt cis geometry, and two bulky triphenylphosphine molecules occupy axial positions of a distorted octahedral coordination sphere. The cis-nitrosyl grouping with respect to PPh₃ molecules (π-acid ligands) is the result of the electronic influence of the multiply bonded ligand, which forces the metal nonbonding d electrons to lie in the plane perpendicular to the M–NO bond axis.     

Prediction of the Efficiency of Phosphorescent Emitters: A Theoretical Analysis of Triplet States in Platinum Blue Emitters

DFT methods are routinely used to predict the excited-state structure of phosphorescent triplet emitters. However, sometimes they fail: different functionals predict diverse lowest adiabatic emissive states. An evaluation is undertaken to determine whether it is possible to use DFT methods to investigate the triplet emitter's hypersurfaces and to explain the experimental observation that similar ligands lead to remarkably diverse phosphorescence quantum yields.     

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

We describe herein computational studies on the unusual ability of Pd(PtBu₃)₂ to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu₃ that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR₃)Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu₃ favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium‐catalyzed carbonylation reactions with simple phosphine ligands, including the key rate‐determining steps and palladium(0) catalyst resting state in carbonylations.     

A highly tunable family of chiral bisphospholanes for rh-catalyzed enantioselective hydrogenation reactions

A set of 16 new and closely related bisphospholane ligands have been prepared by using a highly flexible and convergent approach. Each synthesis can be performed on an industrially relevant scale. The bisphosphines differ in the nature of the bridge connecting both phospholane units. Bridges are formed by three-, four-, five- and six-membered heterocyclic or alicyclic rings. Bisphospholanes and their Rh-precatalysts have been investigated by using results of theoretical calculations (DFT) and analytic measurements ((31)P and (103)Rh NMR spectroscopy, X-ray structure analysis). The studies showed that catalysts based on ligands with maleic anhydride or maleimide bridges give constantly superior enantioselectivities in methanol as the solvent. This may account for optimised steric and electronic effects. However, by changing the solvent catalysts with other backbones can give rise to excellent results. This gives proof that simple correlations between steric and electronic properties and results in the enantioselective hydrogenation frequently claimed in literature are not general.     

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts**

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the ⁷⁷Se liquid state NMR of Se-NHC adducts. We demonstrate that ⁷⁷Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via ⁷⁷Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state ⁷⁷Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the ¹³C isotropic chemical shift from the liquid state NMR and the ¹³C tensor components are also discussed, and compared with their ⁷⁷Se counterparts. ⁷⁷Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state ⁷⁷Se NMR can provide an in-depth outlook on the properties of NHC ligands.     

Phthalocyanoiron complex with bridged ligands. Electronic structure of monomers and polymers

Electronic structure aspects related to the semiconducting properties of monomers and polymers of phthalocyanoiron with bidentate bridging ligands, PcFe–L₂ and ? [PcFe(L)]_n, have been investigated from density functional calculations [L = pyrazine, triazine, tetrazine, pyridine, 4,4′‐bipyridine, bipyridyacetylene, and bis(4‐pyridyl)bencene]. The following relevant results have been obtained: (a) an energy analysis in terms of electrostatic interaction, Pauli repulsion, and occupied/virtual orbital interactions show that the Pauli repulsion is the origin that the axial ligands (L) prefer be located toward the aza positions of the macrocycle, and (b) the intrinsic semiconducting properties depend of the frontier band. The valence band is composed largely by the transition metal d_xy orbital. The conduction band is composed of a mixture between the metallomacrocycle and bridged ligand orbitals for systems formed by pyrazine, bipyridine, and bipyridyacetylene. However, this composition is different when the ligands are triazine and tetrazine, which show a band composed of π* orbitals. These systems are predicted to show the higher conductivity within the series, in agreement with experimental results. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 170–181, 2001