Cavitation energies can outperform dispersion interactions.

The accurate dissection of binding energies into their microscopic components is challenging, especially in solution. Here we study the binding of noble gases (He-Xe) with the macrocyclic receptor cucurbit[5]uril in water by displacement of methane and ethane as 1H NMR probes. We dissect the hydration free energies of the noble gases into an attractive dispersive component and a repulsive one for formation of a cavity in water. This allows us to identify the contributions to host-guest binding and to conclude that the binding process is driven by differential cavitation energies rather than dispersion interactions. The free energy required to create a cavity to accept the noble gas inside the cucurbit[5]uril is much lower than that to create a similarly sized cavity in bulk water. The recovery of the latter cavitation energy drives the overall process, which has implications for the refinement of gas-storage materials and the understanding of biological receptors.

Covalent Co-O-V and Sb-N Bonds Enable Polyoxovanadate Charge Control.

The formation of [{CoII(teta)2}{CoII2(tren)(teta)2}VIV15SbIII6O42(H2O)]·ca.9H2O [teta = triethylenetetraamine; tren = tris(2-aminoethyl)amine] illustrates a strategy toward reducing the molecular charge of polyoxovanadates, a key challenge in their use as components in single-molecule electronics. Here, a V-O-Co bond to a binuclear Co2+-centered complex and a Sb-N bond to the terminal N atom of a teta ligand of a mononuclear Co2+ complex allow for full charge compensation of the archetypal molecular magnet [V15Sb6O42(H2O)]6-. Density functional theory based electron localization function analysis demonstrates that the Sb-N bond has an electron density similar to that of a Sb-O bond. Magnetic exchange coupling between the VIV and CoII spin centers mediated via the Sb-N bridge is comparably weakly antiferromagnetic.

Group 10-group 14 metal complexes [E-TM](IV): the role of the group 14 site as an L, X and Z-type ligand.

A series of new complexes of a general motif [R2E(µ-N,S)2TM-L] (E: metalloid group 14 element; TM: group 10 metal; R: Cl, Ph, pyS, OH, (N,N,O)-chelating ligands; N,S: 1-methylimidazole-2-thiolate (methimazolyl, mt(-)), pyridine-2-thiolate (pyS(-)); L: PPh3, PCy3, pyS) was synthesised and characterised by single-crystal X-ray diffraction, multi-nuclear NMR spectroscopy ((1)H, (13)C, (31)P, (119)Sn), (119)Sn Mössbauer spectroscopy and quantum chemical calculations. The E-TM bonding situation in these compounds can be described with various resonance structures which comprise E(ii)→TM(ii), E(iii)-TM(i) and E(iv)←TM(0) features. Thus, in these complexes the atoms of the group 14 based ligand sites reveal L-, X- and Z-type ligand characteristics. A systematic comparison between structural and spectroscopic parameters as well as the results from NLMO analyses of structurally related compounds provided information about the differences in the E-TM bonding situation upon alteration of the metal atoms or ligand patterns. Under investigation are the structurally related compounds [Cl2Sn(µ-pyS)2TM-PPh3] (1: TM = Pd; 2: TM = Ni; 3: TM = Pt), [Cl2Ge(µ-pyS)2Pd-PPh3] (4) and, for in silico analysis, [Cl2Si(µ-pyS)2Pd-PPh3] (5), which indicate a pronounced shift of the E-TM bond electron pair towards TM for TM = Pt. Further complexes serve as representatives of these compounds with different bridging ligands {[Cl2Sn(µ-mt)2Pd-PPh3] (8)}, different trans-E-TM-bound ligands {[Cl2Sn(µ-pyS)2Pd-PCy3] (9), [Cl2Sn(µ-pyS)2Pd]4 (10)} and with different substituents at Sn (including penta- and hexacoordinated tin compounds), i.e., [R2Sn(µ-pyS)2Pd-PPh3] with R = Ph (6) and pyS (7), [(O,N,N)Sn(µ-pyS)2Pd-PPh3] (11) and (12) having two different (O,N,N) tridentate ligands, and [(µ-OH)ClSn(µ-pyS)2Pd-PPh3]2 (13). The latter series indicates a shift of the E-TM (= Sn-Pd) bond electron pair towards Pd upon transition from penta- to hexacoordinated tin compounds.

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.

Pyridine-2-thiolate bridged tin-palladium complexes with Sn(PdN2Cl2), Sn(PdN2S2), Sn(PdN2C2) and Sn(Pd2N4) skeletons.

Reactions of tin(IV) complexes of the type Sn(PyS)2X2 (X = Cl, PyS, Ph; PyS = pyridine-2-thiolate) with Pd(PPh3)4 provide easy access to novel heterometallic complexes with Pd-Sn bonds. Electronic characteristics of this connection were analysed by X-ray crystallography, (119)Sn Mössbauer spectroscopy and quantum chemical analyses.

Disilicon complexes with two hexacoordinate Si atoms: paddlewheel-shaped isomers with (ClN4 )Si-Si(S4 Cl) and (ClN2 S2 )Si-Si(S2 N2 Cl) 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.

Phosphorus(V) complexes with acyclic monoaminocarbene ligands via oxidative addition.

(Difluoroorganyl)dimethylamines, RCF2NMe2 (R = H, Ph, tBu), can be used as carbene precursors for phosphorus trifluoride in an oxidative addition reaction. By this method, complexes of sterically nondemanding asymmetric and acyclic carbenes were obtained that are otherwise not accessible.

MFU-4 -- a metal-organic framework for highly effective H(2)/D(2) 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.

Ylenes in the M(II)→Si(IV) (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.

Hydrogen sieving and storage in fullerene intercalated graphite.

The geometrical properties of recently synthesised C60 intercalated in graphite have been confirmed by density-functional-based computer simulations. The capability of this material to store molecular hydrogen by physisorption is evaluated. While the material can sieve H2 from heavier molecular gases, our free energy calculations indicate that further tuning of the system by reducing the amount of intercalated fullerene cages is necessary to achieve H2 loadings which are interesting for technical applications.

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.

Graphene nanostructures as tunable storage media for molecular hydrogen.

Many methods have been proposed for efficient storage of molecular hydrogen for fuel cell applications. However, despite intense research efforts, the twin U.S. Department of Energy goals of 6.5% mass ratio and 62 kg/m3 volume density has not been achieved either experimentally or via theoretical simulations on reversible model systems. Carbon-based materials, such as carbon nanotubes, have always been regarded as the most attractive physisorption substrates for the storage of hydrogen. Theoretical studies on various model graphitic systems, however, failed to reach the elusive goal. Here, we show that insufficiently accurate carbon-H2 interaction potentials, together with the neglect and incomplete treatment of the quantum effects in previous theoretical investigations, led to misleading conclusions for the absorption capacity. A proper account of the contribution of quantum effects to the free energy and the equilibrium constant for hydrogen adsorption suggest that the U.S. Department of Energy specification can be approached in a graphite-based physisorption system. The theoretical prediction can be realized by optimizing the structures of nano-graphite platelets (graphene), which are light-weight, cheap, chemically inert, and environmentally benign.

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.