Lyuben Zhechkov

An Efficient a Posteriori Treatment for Dispersion Interaction in Density-Functional-Based Tight Binding.

The performance of density functional theory (DFT) (VWN-LDA, PBE-GGA, and B3LYP hybrid functionals), density-functional-based tight binding (DFTB), and ab initio methods [HF, MP2, CCSD, and CCSD(T)] for the treatment of London dispersion is investigated. Although highly correlated ab initio methods are capable of describing this phenomenon, if they are used with rather large basis sets, DFT methods are found to be inadequate for the description of H2/PAH (polycyclic aromatic hydrocarbon) interactions. As an alternative approach, an a posteriori addition of a van der Waals term to DFTB is proposed. This method provides results for H2/PAH interactions in close agreement with MP2 and higher-level ab initio methods. Bulk properties of graphite also compare well with the experimental data.

Hydrogen storage by physisorption on nanostructured graphite platelets

The physisorption energy of molecular hydrogen (H2) on flat carbon nanoparticles (graphitic platelets) and polycyclic aromatic hydrocarbons (PAHs) is determined to be attractive between 3.5 and 7.2 kJ mol−1, depending on the orientation of H2 and on the particle size. Entropy, estimated from experimental data, reduces the interaction energy by 3.4 kJ mol−1 at room temperature. Therefore, nanostructured graphitic platelets might be suitable for hydrogen storage. Computations have been carried out for PAHs from benzene to coronene using second order Moller–Plesset (MP2) theory at the basis set limit, and the results are extrapolated to graphene layers.

MFU‐4 – A Metal‐Organic Framework for Highly Effective H2/D2 Separation

The metal-organic framework, MFU-4, possessing small cavities and apertures, is exploited for quantum sieving of hydrogen isotopes. Quantum mechanically, a molecule confined in a small cavity shows an increase in effective size depending on the particle mass, which leads to a faster deuterium adsorption from a H(2)/D(2) isotope mixture.

DFTB Parameters for the Periodic Table: Part 1, Electronic Structure

A parametrization scheme for the electronic part of the density-functional based tight-binding (DFTB) method that covers the periodic table is presented. A semiautomatic parametrization scheme has been developed that uses Kohn-Sham energies and band structure curvatures of real and fictitious homoatomic crystal structures as reference data. A confinement potential is used to tighten the Kohn-Sham orbitals, which includes two free parameters that are used to optimize the performance of the method. The method is tested on more than 100 systems and shows excellent overall performance.

(NHCMe)SiCl4: a versatile carbene transfer reagent – synthesis from silicochloroform

A new synthetic pathway for the N-heterocyclic carbene adduct (NHCMe)SiCl4 (2) (NHCMe = 1,3-dimethylimidazolidin-2-ylidene) using silicochloroform is presented. Supported by DFT calculations, the energy for dissociation of 2 into the carbene and the SiCl4 fragment was found to be comparable to carbene transfer reagents based on silver(I) chloride. Compound 2 was used to transfer the NHC ligand to three different phosphorus(III) chloro compounds, resulting in the neutral complexes (NHCMe)PCl3 (3a), (NHCMe)PCl2Ph (3b) and (NHCMe)PCl2Me (3c). The sterically non-demanding NHC ligand allowed the phosphorus(III) in complex 3a to be oxidized to phosphorus(V) without loss of the NHC ligand, and afford (NHCMe)PF4H (4). Furthermore, bis-carbene complexes of Ni(II) (5) and Pd(II) (6) were obtained by reacting 2 with the respective metal chlorides.

Stannylene or Metallastanna(IV)ocane: A Matter of Formalism

The s basicity of electron-rich transition metals (TMs) plays a crucial role in Brønsted acid–base reactions of TM complexes, such as [H2Fe(CO)4] and [HCo(CO)4] (strong acids, poor s-basicity of the corresponding conjugate bases) and was shown to increase upon coordination of good donor ligands L, such as phosphines; that is, lowered acidity of [H2Fe(CO)3(PPh3)] or [HCo(CO)3(PPh3)]. [2] Thus, P and/or S donors bearing electron-rich TM centers have been shown to support s donation towards other main-group-element (E) Lewis acidic centers, for example in the so-called metallaboratranes I and II and Be, Al, and Ga compounds of type III (Scheme 1). Very recently, we have described compounds IV–VII comprising {L5TM(d )} moieties that exhibit s donation towards electronically saturated Lewis acidic centers E, that is, Si and Sn. Gabba et al. have reported similar intermetallic interactions in the heterobimetallic complexes VIII–X (Scheme 1), which comprise d TM donor sites with an almost square-planar coordination sphere. Whereas compounds IV–X were obtained by a straightforward route starting from sources that comprise TM and E in the desired oxidation states, herein we present a (formal) redox approach, which involves a reaction sequence starting from a stannylene (SnCl2) and yielding hypercoordinate tin compounds that can be regarded as palladastanna(IV)ocanes. In a convenient one-pot synthesis, [PdCl2(PPh3)2] was treated with the potassium salt of 1-methyl-2-mercaptoimidazole (methimazole, Hmt) and [SnCl2(dioxane)] (Scheme 2) to afford compound 1. Substitution of the tin-bound chlorine atoms with a dianionic tridentate ligand afforded compound 2, which comprises a hexacoordinate tin atom (Scheme 2). Reference compounds 3 and 4 (comprising Sn and Sn, respectively, and the same tridentate ONN ligand as 2) were prepared as references for spectroscopic properties. The molecular structures of 1–4 were confirmed crystallographically (see Figure 1 and the Supporting Information). Scheme 1. Selected examples of TM–base complexes with electrophilic main-group-element sites (“Z-type ligands”). Cy = cyclohexyl.

Ylenes in the MII→SiIV (M=Si, Ge, Sn) Coordination Mode

The reaction of the methimazolyl (mt, i.e., 2-mercapto-1-methylimidazolide) substituted silane Si(mt)(4) with SnCl(2) and GeCl(2) in dioxane affords the paddlewheel-shaped complexes [ClSi(μ-mt)(4)MCl] (M=Sn (1) and Ge (2), respectively). These compounds represent the first crystallographically characterized hexacoordinate silicon complexes comprising a Sn or Ge atom in the Si coordination sphere. An attempt to synthesize the related silicon compound 3 [ClSi(μ-mt)(4)SiCl] instead afforded the trisilane [ClSi(μ-mt)(4)Si-SiCl(3)] (3a), which provides the first crystallographic evidence for the feasibility of oligosilanes with adjacent hexacoordinate Si atoms. One of the hexacoordinate Si atoms of 3a features the unprecedented (Si(2)S(4))Si skeleton. Natural bonding orbital (NBO) analyses of compounds 1, 2, 3a (and the target compound 3) revealed characteristics of M(II)→Si(IV) (for 2 and 3) or M(I)→Si(IV) (for 3a) dative bonding in the systems with M=Si and Ge, whereas compound 1 exhibits a covalent Sn(III)-Si(III) bond.

C28 fullerites—structure, electronic properties and intercalates

Mechanical and electronic properties of hypothetical carbon nanostructures, on the basis of C28 building blocks, hyperdiamond and hyperlonsdaleite, have been investigated with DFT based methods. The low mass density and large internal surface suggest applications as catalyst, nanosieve and gas storage material. We estimate the active volume accessible by H2. Special emphasis is given to the possibility to tune their properties by endo- and exohedral intercalation with Zn, Ti and K. While endohedral intercalation with Zn does not affect the overall structure, endohedral Ti intercalation has different consequences on the structural stability of the two allotropes. Exohedral intercalation with K leads to an ionic fullerite phase with metallic conductivity.

Disilicon Complexes with Two Hexacoordinate Si Atoms: Paddlewheel‐Shaped Isomers with (ClN4)SiSi(S4Cl) and (ClN2S2)SiSi(S2N2Cl) Skeletons

The reaction of 1-methyl-3-trimethylsilylimidazoline-2-thione with hexachlorodisilane proceeds toward substitution of four of the disilane Cl atoms during the formation of disilicon complexes with two neighboring hexacoordinate Si atoms. The N,S-bidentate methimazolide moieties adopt a buttressing role, thus forming paddlewheel-shaped complexes of the type ClSi(μ-mt)4 SiCl (mt=methimazolyl). Most interestingly, three isomers (i.e., with (ClN4 )SiSi(S4 Cl), (ClN3 S)SiSi(S3 NCl), and (ClN2 S2 )SiSi(S2 N2 Cl) skeletons as so-called (4,0), (3,1), and cis-(2,2) paddlewheels) were detected in solution by using (29) Si NMR spectroscopic analysis. Two of these isomers could be isolated as crystalline solids, thus allowing their molecular structures to be analyzed by using X-ray diffraction studies. In accord with time-dependent NMR spectroscopy, computational analyses proved the cis-(2,2) isomer with a (ClN2 S2 )SiSi(S2 N2 Cl) skeleton to be the most stable. The compounds presented herein are the first examples of crystallographically evidenced disilicon complexes with two SiSi-bonded octahedrally coordinated Si atoms and representatives of the still scarcely explored class of Si coordination compounds with sulfur donor atoms.

Metallophilic Contacts in 2-C6F4PPh2 Bridged Heterobinuclear Complexes: A Crystallographic and Computational Study

Treatment of the bis(chelate) complex trans-[Pd(κ(2)-2-C6F4PPh2)2] (7) with PMe3 gave trans-[Pd(κC-2-C6F4PPh2)2(PMe3)2] (13) as a mixture of syn- and anti-isomers. Reaction of 13 with CuCl, AgCl, or [AuCl(tht)] (tht = tetrahydrothiophene) gave the heterobinuclear complexes [(Me3P)2Pd(μ-2-C6F4PPh2)2MCl] [M = Cu (14), Ag (15), Au (16)], from which the corresponding salts [(Me3P)2Pd(μ-2-C6F4PPh2)2M]PF6 [M = Cu (17), Ag (18), Au (19)] could be prepared by abstraction of the chloro ligand with TlPF6; 18, as well as its triflato (20) and trifluoroacetato (21) analogues, were also prepared directly from 13 and the appropriate silver salt. Reaction of 13 with [AuCl(PMe3)] gave the zwitterionic complex [(Me3P)PdCl(μ-2-C6F4PPh2)2Au] (24) in which the 2-C6F4PPh2 ligands are in a head-to-head arrangement. In contrast, the analogous reaction with [AuCl(PPh3)] gave [(Ph3P)PdCl(μ-2-C6F4PPh2)2Au] (25) with a head-to-tail ligand arrangement. Single crystal X-ray diffraction studies of complexes 14-21 show short metal-metal separations [2.7707(11)-2.9423(3) Å] suggestive of attractive noncovalent (dispersion) interactions, a conclusion that is supported by theoretical calculations of the electron localization function and the noncovalent interactions descriptor.