Van der Waals complexes of Cu, Ag, and Au with hydrogen sulfide. The bonding character

The electronic and structural features of the Cu...SH2, Ag...SH2, and Au...SH2 complexes are investigated by using the spin-adapted restricted open-shell HF coupled cluster CCSD(T) method combined with the second-order spin-free Douglas-Kroll-Hess (DKH) relativistic approach. M...SH2 complexes are nonplanar with bonding energies -5.99, -1.99, and -9.08 mHartree, respectively. Comparison with analogous M...OH2 and M...NH3 complexes allows us to establish general features of the bonding between coinage metal atoms and ligand molecules with the participation of their lone electron pairs. Consistent interpretation of the interaction effects can be obtained by using the molecular orbital picture of the M...L region. The bonding character is explained by stressing the importance of the charge transfer from the lone pair of the ligand to the metal atom. Relativistic changes of the metal element electron affinity and polarizability facilitate the understanding of major trends in the pattern of interactions between the coinage metal atoms and different lone pair donating ligands.     

Source of Cooperativity in Halogen‐Bonded Haloamine Tetramers

Inspired by the isostructural motif in α‐bromoacetophenone oxime crystals, we investigated halogen–halogen bonding in haloamine quartets. Our Kohn–Sham molecular orbital and energy decomposition analysis reveal a synergy that can be traced to a charge‐transfer interaction in the halogen‐bonded tetramers. The halogen lone‐pair orbital on one monomer donates electrons into the unoccupied σ*_N?X orbital on the perpendicular N?X bond of the neighboring monomer. This interaction has local σ symmetry. Interestingly, we discovered a second, somewhat weaker donor–acceptor interaction of local π symmetry, which partially counteracts the aforementioned regular σ‐symmetric halogen‐bonding orbital interaction. The halogen–halogen interaction in haloamines is the first known example of a halogen bond in which back donation takes place. We also find that this cooperativity in halogen bonds results from the reduction of the donor–acceptor orbital‐energy gap that occurs every time a monomer is added to the aggregate.     

Metallomacrocycle (MacM) complex with cyanide as bridged ligand: Electronic structures of [MacMCN]n

The electronic structures of complexes and one‐dimensional metallomacrocycles with cyanide as bridged ligand, such as [MacM(CN)₂]^? and [MacM(CN)]_n [Mac=phthalocyanine, tetrabenzoporphyrine; M=Co(III), Rh(III)] have been investigated using density functional theory. The results of this study show that the intrinsic semiconductivity properties depend on the frontier bands. The valence band is composed by the π‐macrocycle orbital. The conduction band for the cobalt polymers is a mixture of orbitals between this metal and the cyanide ligand along of the stacking direction. However, in the rhodium polymers such a band is exclusively composed of the π* system of the macrocycles. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)

The d-orbital contribution from the transition metal centers of phthalocyanine brings difficulties to understand the role of the organic ligands and their molecular frontier orbitals when it adsorbs on oxide surfaces. Here we use zinc phthalocyanine (ZnPc)/TiO(2)(110) as a model system where the zinc d-orbitals are located deep below the organic orbitals leaving room for a detailed study of the interaction between the organic ligand and the substrate. A charge depletion from the highest occupied molecular orbital is observed, and a consequent shift of N1s and C1s to higher binding energy in photoelectron spectroscopy (PES). A detailed comparison of peak shifts in PES and near-edge X-ray absorption fine structure spectroscopy illustrates a slightly uneven charge distribution within the molecular plane and an inhomogeneous charge transfer screening between the center and periphery of the organic ligand: faster in the periphery and slower at the center, which is different from other metal phthalocyanine, e.g., FePc/TiO(2). Our results indicate that the metal center can substantially influence the electronic properties of the organic ligand at the interface by introducing an additional charge transfer channel to the inner molecular part.     

Optical and redox properties of ruthenium phthalocyanine complexes tuned with axial ligand substituents

The optical and electrochemical properties of the ruthenium phthalocyanine complexes [[(t-Bu)4Pc]Ru(4-Rpy)2], where R = NO2, Me, NH2, and NMe2, are reported. The electron density at the macrocycle may be adjusted using the axial ligand substituents, which have varying electron-donating/withdrawing strengths. Electrochemical data show that the axial pyridine ligand substituents exert significant influence over the phthalocyanine ring-based redox processes. The axial ligands also influence the electronic absorption properties of the complexes with influence also being observed in the electrogenerated oxidized and reduced species.     

Metallomacrocycles as ligands: synthesis and characterisation of aluminium-bridged bisglyoximato complexes of palladium and iron

Dialuminiummacrocycles based on bisglyoximato moieties were prepared and their coordination chemistry with Fe(II) and Pd(II) was investigated. The bridging aluminium centers were supported by several types of tetradentate diphenoxide diamine ligands. The nature of the ancillary ligands bound to aluminium was found to affect the overall geometry and symmetry of the metallomacrocycles. Enantiopure, chiral diphenoxide ligands based on the (R,R)-trans-1,2-diaminocyclohexane backbone afforded cleanly one metallomacrocycle isomer. The size and electronic properties of remote substituents on aluminium-bound ligands affected the binding mode and electronic properties of the central iron. A structurally characterized iron complex shows trigonal prismatic coordination mode, with phenoxide bridges between iron and aluminium. Increasing the size of the phenoxide substituents led to square bipyramidal coordination at iron. Employing p-NO(2)- instead of p-tBu-substituted phenoxide as supporting ligands for aluminium caused a 0.27 V positive shift of the Fe(III)/Fe(II) reduction potential. These results indicate that the present synthetic approach can be applied to a variety of metallomacrocycles based on bisglyoximato motifs to affect the chemistry at the central metal.     

Iodide Recognition and Sensing in Water by a Halogen‐Bonding Ruthenium(II)‐Based Rotaxane

The synthesis and anion‐recognition properties of the first halogen‐bonding rotaxane host to sense anions in water is described. The rotaxane features a halogen‐bonding axle component, which is stoppered with water‐solubilizing permethylated β‐cyclodextrin motifs, and a luminescent tris(bipyridine)ruthenium(II)‐based macrocycle component. ¹H NMR anion‐binding titrations in D₂O reveal the halogen‐bonding rotaxane to bind iodide with high affinity and with selectively over the smaller halide anions and sulfate. The binding affinity trend was explained through molecular dynamics simulations and free‐energy calculations. Photo‐physical investigations demonstrate the ability of the interlocked halogen‐bonding host to sense iodide in water, through enhancement of the macrocycle component’s Ru^II metal–ligand charge transfer (MLCT) emission.     

Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process.     

Mutual control of axial and equatorial ligands: model studies with [Ni]-bacteriochlorophyll-a

Modification of the metal's electronic environment by ligand association and dissociation in metalloenzymes is considered cardinal to their catalytic activity. We have recently presented a novel system that utilizes the bacteriochlorophyll (BChl) macrocycle as a ligand and reporter. This system allows for charge mobilization in the equatorial plane and experimental estimate of changes in the electronic charge density around the metal with no modification of the metal's chemical environment. The unique spectroscopy, electrochemistry and coordination chemistry of [Ni]-bacteriochlorophyll ([Ni]-BChl) enable us to follow directly fine details and steps involved in the function of the metal redox center. This approach is utilized here whereby electro-chemical reduction of [Ni]-BChl to the monoanion [Ni]-BChl(-) results in reversible dissociation of biologically relevant axial ligands. Similar ligand dissociation was previously detected upon photoexcitation of [Ni]-BChl (Musewald, C.; Hartwich, G.; Lossau, H.; Gilch, P.; Pollinger-Dammer, F.; Scheer, H.; Michel-Beyerle, M. E. J. Phys. Chem. B 1999, 103, 7055-7060 and Noy, D.; Yerushalmi, R.; Brumfeld, V.; Ashur, I.; Baldridge, K. K.; Scheer, H.; Scherz, A. J. Am. Chem. Soc. 2000, 122, 3937-3944). The electrochemical measurements and quantum mechanical (QM) calculations performed here for the neutral, singly reduced, monoligated, and singly reduced, monoligated [Ni]-BChl suggest the following: (a) Electroreduction, although resulting in a pi anion [Ni]-BChl(-) radical, causes electron density migration to the [Ni]-BChl core. (b) Reduction of nonligated [Ni]-BChl does not change the macrocycle conformation, whereas axial ligation results in a dramatic expansion of the metal core and a flattening of the highly ruffled macrocycle conformation. (c) In both the monoanion and singly excited [Ni]-BChl ([Ni]-BChl*), the frontier singly occupied molecular orbital (SOMO) has a small but nonnegligible metal character. Finally, (d) computationally, we found that a reduction of [Ni]-BChl*imidazole results in a weaker metal-axial ligand bond. Yet, it remains weakly bound in the gas phase. The experimentally observed ligand dissociation is accounted for computationally when solvation is considered. On the basis of the experimental observations and QM calculations, we propose a mechanism whereby alterations in the equatorial pi system and modulation of sigma bonding between the axial ligands and the metal core are mutually correlated. Such a mechanism highlights the dynamic role of axial ligands in regulating the activity of metal centers such as factor F430 (F430), a nickel-based coenzyme that is essential in methanogenic archea.     

Axial ligand influence on geometries, charge distributions and electronic structures of iron tetraazamacrocycle [FeTIM(X)(Y)] complexes assessed by Density Functional Theory

We employed the Density Functional Theory along with small basis sets, B3LYP/LANL2DZ, for the study of FeTIM complexes with different pairs of axial ligands (CO, H₂O, NH₃, imidazole and CH₃CN). These calculations did not result in relevant changes of molecular quantities as bond lengths, vibrational frequencies and electronic populations supporting any significant back-donation to the carbonyl or acetonitrile axial ligands. Moreover, a back-donation mechanism to the macrocycle cannot be used to explain the observed changes in molecular properties along these complexes with CO or CH₃CN. This work also indicates that complexes with CO show smaller binding energies and are less stable than complexes with CH₃CN. Further, the electronic band with the largest intensity in the visible region (or close to this region) is associated to the transition from an occupied 3d orbital on iron to an empty π^∗ orbital located at the macrocycle. The energy of this Metal-to-Ligand Charge Transfer (MLCT) transition shows a linear relation to the total charge of the macrocycle in these complexes as given by Mulliken or Natural Population Analysis (NPA) formalisms. Finally, the macrocycle total charge seems to be influenced by the field induced by the axial ligands.     

Poly(2-thienyl)borates: An Investigation into the Coordination of Thiophene and Its Derivatives

The synthesis, characterization, and reactivity of a new sulfur-rich tridentate ligand, tetrakis(2-thienyl)borate (1(-)()), are reported along with a molecular orbital analysis of its coordination to a metal center. Unlike the analogous tetrakis((methylthio)methyl)borate (2(-)()), 1(-)() does not coordinate Mo(CO)(3) when reacted with (C(7)H(8))Mo(CO)(3). The sulfur atoms in both ligands are oriented to coordinate the metal in a pyramidal eta(1) sulfur-bound mode. Approximate molecular orbital calculations are used to compare the metal-ligand interactions in these related species, and the results indicate that the magnitude and polarizability of the electronic charge density of the lone pairs on the sulfur atoms dictate the coordination strength of the ligands. Simple Mulliken atomic charges and orbital occupation numbers are used to determine the extent of charge delocalization. While the conjugation of the sulfur lone pair electrons with adjacent pi bonds in the ligands decreases the corresponding Lewis basicity, the contribution from the aromaticity in the thienyl groups is negligible. During the course of these studies, the structure of K[1] was determined by X-ray diffraction. K[1]: monoclinic space group C2/c, with a = 16.00(2) ?, b = 7.680(7) ?, c = 16.22(2) ?, beta = 118.520(7) degrees, V = 1750(3) ?(3), Z = 4, R(F) = 0.0494, and R(w)(F(2)()) = 0.122. The crystal lattice contains one-dimensional chains of 1(-)() bridged by K ions.     

Theoretical and spectroscopic study of nickel(II) porphyrin derivatives

A set of substituted (sulfonate, amino) nickel porphyrin derivatives such as phthalocyanine and phenylporphyrin was studied by spectroscopic (UV-vis, FTIR, XPS) and quantum-chemical methods. The Q and Soret bands were identified in the UV-vis spectra of aquo solutions of the tetrasulfo-substituted complexes and in DMF and ACN solutions of the amino-substituted phenylporphyrin and phthalocyanine Ni(II) complexes, respectively. In all the complexes the frontier molecular orbitals predict that the oxidation and reduction sites are localized on the ligand rather than in the metal atom. A natural bonding orbital (NBO) analysis of all the complexes showed that a two-center bond NBO between the pyrrolic nitrogens (Npyrr) and the nickel atom does not exist, the Npyrr...Ni interaction occurring instead by a delocalization from one lone pair of each Npyrr toward one lone pair of the nickel atom, as estimated by second-order perturbation theory. The calculated values of electronic transitions between the frontier molecular orbitals are in good agreeement with the UV-vis data. At the theoretical level, we found that while the ligand effect is more important in the Q-band (approximately 16 kcal/mol), the substituent effect is more significant in the Soret band (approximately 9 kcal/mol). A good agreement was also found between the experimental and calculated infrared spectra, which allowed the assignment of many experimental bands. The XPS results indicate that the Ni(II) present in the phenylporphyrin structure is not affected by a change of the substituent (sulfonate or amino).     

The geometry of nonmetal hydrides and the ligand radius of hydrogen

The aim of this paper was to investigate why the geometries of nonmetal hydrides are often not in accordance with the VSEPR model. From a consideration of interligand distances in a variety of BX(4), CX(4), and NX(4) molecules where X is a ligand or a lone pair and in which there are at least two H ligands we have shown that the hydrogen ligands are essentially close-packed. For each of the central atoms we have obtained a value for the ligand radius of hydrogen. These radii decrease with decreasing negative charge and increasing positive charge of the hydrogen ligand as the electronegativity of the central atom increases, as has been found previously for other ligands such as F and Cl. We show that ligand-ligand intractions are an important factor in determining bond angles in hydrides and that the ligand close-packing (LCP) model gives a better explanation of bond angles than the VSEPR model according to which bond angles depend on the electronegativity of the ligand rather than on its size. For example, although the very small angles in PH(3) and SH(2) are not in accord with the VSEPR model, they are consistent with the LCP model in that they are a consequence of the small size of hydrogen ligands which are pushed together by the lone pairs until they are almost close-packed.     

Synthesis of novel phthalocyanine-tetrathiafulvalene hybrids; intramolecular fluorescence quenching related to molecular geometry

A number of silicon phthalocyanine bis-esters have been synthesized and characterized, with axial ligands containing one or more tetrathiafulvalene groups. Variations in the substitution positions around a central aromatic "hinge" within the ligands lead to different molecular geometries, and the fluorescence of the macrocyclic core is subsequently quenched to varying degrees by the electron-rich tetrathiafulvalene moiety, the magnitude of this effect being dependent upon both the relative separation of the two units and the flexibility of the linking group. Pc derivative 24, with a highly flexible linker group, and pc derivative 28, with a dendritic axial ligand, have the intensity of the macrocycle emission reduced by 99% and 96%, respectively, relative to a similar silicon pc reference compound lacking the TTF moieties. Molecular modeling studies of a series of such hybrids allow the degree of this fluorescence quenching to be related to the intramolecular spacing. Additionally, the potential for rapid electrochemical switching of the phthalocyanine fluorescence by oxidation of the appended tetrathiafulvalene units is explored.     

Dendron-functionalized bis(terpyridine)-iron(II) or -cadmium(II) metallomacrocycles: synthesis, traveling-wave ion-mobility mass spectrometry, and photophysical properties

The synthesis, purification, structural analysis, and photophysical properties of a series of five-, six-, and seven-sided Fe(II) macrocycles and the corresponding hexameric Cd(II) macrocycle, all prepared by self-assembly of a 120° bis(terpyridine) ligand modified with first- and second-generation 1→3 C-branched dendrons, are reported. All metallomacrocycles were fully characterized by (1)H and (13)C NMR spectroscopy, traveling-wave ion-mobility mass spectrometry (TWIM MS), molecular modeling, UV/Vis absorption spectroscopy, photoluminescence, and cyclic voltammetry. A gradual increase of the collision cross sections of the Fe(II) metallomacrocycles was observed with a successive increase of the number and molecular size of the ligands. The combination of ion-mobility mass spectrometry and NMR techniques unveils structural features that agree well with calculations. Extinction coefficients and emission are significantly modulated by increasing the ring size and changing the metal ion center from Fe(II) to Cd(II) .     

The effect of pH on the photophysics and photochemistry of disulphonated aluminum phthalocyanine

The results of a study of the effect of pH on the photophysics and photochemistry of di-sulphonated aluminum phthalocyanine (AlPcS₂) in aqueous solution are presented. The pH dependence of the triplet quantum yield, fluorescence quantum yield, singlet-oxygen quantum yield, triplet lifetime, fluorescence lifetime and apparent dimerization constants is investigated and the results interpreted in terms of the pH dependence of the nature of the axial ligands. Evidence that the aluminum–axial ligand bond strength, rather than dimer binding energy that determines the extent of dimerization is provided by semi-empirical and ab initio calculations. Possible dimer structures obtained using ab initio calculations are discussed.     

咔咯锰(Ⅲ)与锰(V)-氧配合物与含氮配体的轴向配位性质

采用密度泛函理论(DFT)的BP86方法对含氮配体咪唑、甲基咪唑、异丙基咪唑和吡啶与5,10,15-三(五氟苯基)咔咯锰[(TPFC)Mn^Ⅲ]和5,10,15-三(五氟苯基)咔咯锰氧[(TPFC)Mn^VO]的轴向配位性质进行理论研究.计算结果表明配体能与五重态下的(TPFC)Mn^Ⅲ形成有效的轴向配位作用,结合能绝对值次序为：咪唑>4-甲基咪唑>吡啶,与实验结果一致. 另外,结合能和轴向配位键长数据显示,这些配体不能与基态(单重态)或三重态(TPFC)Mn^VO中的Mn^V原子形成有效的轴向配位作用,自然键轨道(NBO)分析表明其Mn^V没有空的3d 轨道来接受配体的孤对电子,但配体可与三重态下的(TPFC)Mn^VO形成弱的配位作用.     

Chloroplast biogenesis—47. Spectroscopic study of net spectral shifts induced by axial ligand coordination in metalated tetrapyrroles

Net absorption and fluorescence spectral shifts, directly induced by coordination of metalated tetrapyrroles to axial ligands, were calculated for the Soret and visible regions of the electromagnetic spectrum. An examination of the calculated net spectral shifts confirmed the conclusions of several other investigators and revealed that the axial coordination potential of a metalated tetrapyrrole is strongly influenced (a) by functional group distribution around the periphery of the tetrapyrrole macrocycle; (b) by the temperature of the sample; (c) by the availability of adventitious ligands in the immediate environment of the metalated tetrapyrroles and (d) by the nature of the central metal atom of the metallotetrapyrrole. In general, electron withdrawing peripheral groups, low temperatures and the availability of unhindered Lewis bases all enhanced the formation of hexacoordinated complexes in Mg-tetrapyrroles. For example, in ether at room temperature, all Mg-tetrapyrroles coordinated to one axial ligand thus forming pentacoordinated complexes. At 77 K, all Mg-porphyrins with electron withdrawing side chains occurred mainly in the pentacoordinated state and to a much lesser extent in the hexacoordinated state. On the other hand in ether at 77 K, Mg-chlorins, such as monovinyl and divinyl chl(ide) a, coordinated to two axial ligands and occured predominantly in the hexacoordinated state. The relevance of these observations to the positive charge density on the central metal atom of metallotetrapyrroles and to the orientation and organization of Mg-tetrapyrroles in biological membranes is discussed.     

(CH3)2S与HOCl分子间的卤键和氢键相互作用

在DFT-B3LYP/6-311++G**水平上分别求得(CH3)2S…ClOH卤键复合物和(CH3)2S…HOCl氢键复合物势能面上的稳定构型. 频率分析表明, 与单体HOCl相比, 在两种复合物中, 10Cl—11O和12H—11O键伸缩振动频率发生显著的红移. 经MP2/6-311++G**水平计算的含基组重叠误差(BSSE)校正的气相中相互作用能分别为-11.69和-24.16 kJ·mol-1. 自然键轨道理论(NBO)分析表明, 在(CH3)2S…ClOH卤键复合物中, 引起10Cl—11O键变长的因素包括两种电荷转移: (i) 孤对电子LP(1S)1→σ*(10Cl—11O); (ii) 孤对电子LP(1S)2→σ*(10Cl—11O), 其中孤对电子LP(1S)2→σ*(10Cl—11O)转移占主要作用, 总的结果是使σ*(10Cl—11O)的自然布居数增加0.14035e, 同时11O原子的再杂化使其与10Cl成键时s成分增加, 即具有与电荷转移作用同样的“拉长效应”; 在(CH3)2S…HOCl氢键复合物中也存在类似的电荷转移, 但是11O原子的再杂化不同于前者. 自然键共振理论(NRT)进行键序分析表明, 在卤键复合物和氢键复合物中, 10Cl—11O和12H—11O键的键序都减小. 通过分子中原子理论(AIM)分析了复合物中卤键和氢键的电子密度拓扑性质.     

Abilities of different electron donors (D) to engage in a P···D noncovalent interaction

Previous work has documented the ability of the P atom to form a direct attractive noncovalent interaction with a N atom, based in large measure on the charge transfer from the N lone pair into the σ* antibonding orbital of the P-H that is turned away from the N atom. The present work considers whether other atoms, namely, O and S, can also participate as electron donors, and in which bonding environments. Also considered are the π-systems of multiply bonded C atoms. Unlike an earlier observation that the interaction is unaffected by the nature of the electron-acceptor atom, there is strong sensitivity to the donor. The P···D binding energy diminishes in the order D = NH(3) > H(2)CO > H(2)CS > H(2)O > H(2)S, different from the patterns observed in both H and halogen bonds. The P···D interactions are comparable to, and in some cases stronger than, the analogous H-bonds formed by HOH as proton donor. The carbon π systems form surprisingly strong P···D complexes, augmented by the back-donation from the P lone pair to the C-C π* antibond, which surpass the strengths of H-bonds, even some with HF as proton donor.