Acid and base strength variations: rationalization for cyclic amine bases and acidic aqua cations.

This perspective highlights and evaluates recent key developments in the thermodynamic approach used to analyze trends in acid and base strength variation. According to this approach, acid and base strength ranking can be interpreted by using thermodynamic or thermochemical cycles. Each cycle generally consists of three independent but well-defined steps. The modus operandi described here entails the identification of the dominant step and the rationalization of its free energy/enthalpy/energy change along a selected series in terms of known structural chemical concepts. Developments in this approach are described by focusing on two related series of bases and two series of acids. In the case of the former the protonation of a series of N-heterocyclic amine bases together with their methyl-substituted analogs receives particular attention while in the case of acids, the acidic properties of aqua dications of elements in period 4 and group 2 are probed. It is illustrated how significant progress in computational chemistry and mass spectrometric techniques can be employed to compare 'inherent' basicity or acidity in the selected families of compounds by using simple gas-phase energy cycles. Unique, dual functions for both electronegativity (element and orbital) and charge density (for aqua cations) indicators are identified and used to evaluate these cycles. Solvent effects (in aqueous solution) are accommodated by including dehydration and hydration changes in appropriately-extended, three-step free energy cycles. It is further suggested that the dominant step in the extended thermodynamic cycle for monomeric aqua cations is the transfer of M(H2O)n2+ complex hydrates from the gas-phase to bulk water. Charge density of the aqua cations again features prominently in proposed rationalizations. Finally, this article also sheds light on salient relationships that exist between empirically and quantum-chemically estimated enthalpy and entropy changes for the aforementioned transfer process.

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions.

Excimers and exciplexes are defined as assemblies of atoms or molecules A/A' where interatomic/intermolecular bonding appears only in excited states such as [A2 ]* (for excimers) and [AA']* (for exciplexes). Their formation has become widely known because of their role in gas-phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration AuI (5d10 )) through aurophilic contacts. In the corresponding excimers, a new type of Au⋅⋅⋅Au bonding arises, with bond energies and lengths approaching those of ground-state Au-Au bonds between metal atoms in the Au0 (5d10 6s1 ) and AuII (5d9 ) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.

Computational investigation of Au·H hydrogen bonds involving neutral AuI N-heterocyclic carbene complexes and amphiprotic binary hydrides.

In this computational study, we investigate the ability of various neutral R-AuI-NHC (NHC = N-heterocyclic carbene) complexes [R = H, CH3, Cl, OH] to form hydrogen bonds with the amphiprotic binary hydrides NH3, H2O and HF. Optimized geometries of the adducts calculated at various levels of theory all exhibit Au⋯HX hydrogen bonds. In adducts of complexes containing NHC ligands with α(N)H units, (NH)carbene⋯XH interactions also exist, yielding hydrogen-bonded rings with graph-set notation [Formula: see text] that correspond to pseudo chelates with κ2C,H coordination. AIM analysis at the MP2/aug-cc-pVTZ-pp level of theory indicates that the (NH)carbene⋯XH hydrogen bonds are generally stronger than the Au···HX interactions, except for those involving HF. The Au⋯HX interactions vary with the Lewis basicity of the Au(I) center as a result of the nature of the R ligand, while the (NH)carbene⋯XH hydrogen bonds are unaffected by R. Energy decomposition analysis at the BP86/TZP level of theory identifies the origin of this difference as the greater component of polarization involved in Au⋯HX interactions. Replacing the α(N)Hs with methyl groups prevents formation of a strong (NH)carbene⋯XH interaction, thus reducing the overall stabilization of the adducts. Nevertheless, the Au⋯H interactions remain largely unchanged and are strong enough to sustain the hydrogen-bonded complexes, although weak C-H⋯X interactions are often also present.

Gold(I) Hydrides as Proton Acceptors in Dihydrogen Bond Formation.

Wavefunction and DFT calculations indicate that anionic dihydride complexes of AuI form strong to moderate directed Au-H⋅⋅⋅H bonds with one or two HF, H2 O and NH3 prototype proton donor molecules. The largely electrostatic interaction is influenced by relativistic effects which, however, do not increase the binding energy. Very weak Au⋅⋅⋅H associations-exhibiting a corresponding bond path-occur between neutral AuH and HF units, although ultimately F becomes the preferred donor atom in the most stable structure. Increasing the hydridicity of AuH by attachment of an electron donating NHC ligand effects Au-H⋅⋅⋅H bonding of moderate strength only with HF, whereas competing Au⋅⋅⋅H interactions dominate for H2 O and NH3 . Rare η2 coordinated and HX (X=F or OH) associated H2 complexes are produced during interaction with a single ion of stronger acidity, H2 F+ or H3 O+ . Theoretically, reaction of excess [AuH2 ]- as proton acceptor with H3 O+ or NH4+ in 3:1 or 4:1 ionic ratios, respectively, affords H⋅⋅⋅H bonded analogues of Eigen-type adducts. Outstanding analytical relationships between selected bonding parameters support the integrity of the results.

Gold setting the "gold standard" among transition metals as a hydrogen bond acceptor - a theoretical investigation.

The Au(i) atom of dimethylaurate (DMA) is shown to behave as a hydrogen-bond acceptor, providing theoretical evidence that it can act as a Lewis base. Calculations at the MP2/aug-cc-pVTZ-pp level of theory confirm that DMA forms hydrogen bonds decreasing in strength from -16.2 kcal mol-1 to -2.4 kcal mol-1 in the order HCN ≈ HF > H2O > HCCH > NH3 > CH4, i.e. following the trend of decreasing proton acidity of the hydrogen-bond donor. The geometrical and Atoms in Molecules (AIM) parameters of the hydrogen-bonded adducts compare well to those obtained with the auride anion, a known hydrogen-bond acceptor. Relativistic effects are shown to play a dominant role in the formation of the hydrogen bonds with DMA: omission of these effects (confirmed using two different approaches) results in the loss of the hydrogen bond. Instead, the hydrogen-bond donor interacts with the carbon atom on one of the methyl ligands, yielding an adduct that is closely comparable to those found with the Cu and Ag analogues of DMA.

Preparing Gold(I) for Interactions with Proton Donors: The Elusive [Au]⋠⋠⋠HO Hydrogen Bond.

MP2 and DFT calculations with correlation consistent basis sets indicate that isolated linear anionic dialkylgold(I) complexes form moderately strong (ca. 10 kcal mol(-1) ) Au⋅⋅⋅H hydrogen bonds with single H2 O molecules as donors in the absence of sterically demanding substituents. Relativistic effects are critically important in the attraction. Such bonds are significantly weaker in neutral, strong σ-donor N-heterocyclic carbene (NHC) complexes (ca. 5 kcal mol(-1) ). The overall association (>11 kcal mol(-1) ), however, is strengthened by co-operative, synergistic classical hydrogen bonding when the NHC ligands bear NH units. Further manipulation of the interaction by ligands positioned trans to the carbene, is possible.

The gold-hydrogen bond, Au-H, and the hydrogen bond to gold, Au∙∙∙H-X.

In the first part of this review, the characteristics of Au-H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au∙∙∙H-X. Such interactions have been proposed for gold atoms in the Au(-I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H-X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au∙∙∙H-X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au∙∙∙H-X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au∙∙∙H-X can also be advanced in most cases. Although numerous examples of short Au∙∙∙H-X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.

Amides of gold(I) diphosphines prepared from N-heterocyclic sources and their in vitro and in vivo screening for anticancer activity.

A series of new neutral mononuclear or dinuclear gold(I) complexes and a cyclic cationic tetranuclear amidogold(I) complex comprising of the phosphines 1,2-bis(dimethylphosphino)ethane (dmpe), µ-1,2-bis(diphenylphosphino)ethane (dppe), µ-1,3-bis(diphenylphosphino)propane (dppp), µ-1,5-bis(diphenylphosphino)pentane (dpppe), µ-1,6-bis(diphenylphosphino)hexane (dpph) or trimethylphosphine, and several N-heterocyclic ring systems (imidazolate, pyrazolate, 1,2,3-triazolate, 1,2,4-triazolate, pyrrolate, 9H-purine-9-ate or 9H-purine-6-amine-9-ate) as ligands, reveal intermolecular aurophilic interactions and 2D channels available for solvent molecules in some of their crystal structures. The antitumour activity of the acyclic gold(I) compounds is highly dependent on the substituents on the phosphorus atoms being highest for phenyl groups and lower for methyl groups. The activity of these compounds against selected cell lines is linked to the length of the carbon bridge between the two phosphorus atoms being highest with a bridge consisting of 5 or 6 carbons. Two compounds with the highest tumour specifities that contain dpppe and pyrazolate (a lipophilic compound) or 1,2,4-triazolate (a hydrophilic compound) induce an apoptotic cell death pathway and a maximum dose to Balb/C mice is tolerated.

1,1,2,2-Tetra-kis(1,3-benzoxazol-2-yl)ethene.

The title compound, C(30)H(16)N(4)O(4), reveals [Formula: see text] crystallographic and mol-ecular symmetry and accordingly the asymmetric unit comprises one half-mol-ecule. The dihedral angle between the planes of the two geminal benzoxazole rings is 74.39 (5)°. The packing features weak C-H⋯N and π-π inter-actions [centroid-centroid distance = 3.652 (1) Å].

[Bis-(4-methyl-1,3-thia-zol-2-yl-κN)methane]-tricarbonyl-dichlorido-tungsten(II).

The title compound, [WCl(2)(C(9)H(10)N(2)S(2))(CO)(3)], is a hepta-coordinate tungsten(II) complex with a capped-octa-hedral coordination sphere in which one CO ligand caps a face formed by a chloro ligand and the two other carbonyls. The chloro ligands are mutually trans positioned at an angle of 156.98 (7)°. The chelating bis-(4-methyl-1,3-thia-zol-2-yl)methane ligand coordinates with the imine N atoms. In the crystal, mol-ecules are linked into chains parallel to [201] by weak C-H⋯O contacts between the CH(2) group of the bis-(4-methyl-thia-zol-2-yl)methane ligand and the O atom of the capping CO group.

(Acetonitrile-κN)penta-carbonyl-tungsten(0).

The acetonitrile ligand in the title compound, [W(CH(3)CN)(CO)(5)], is coordinated end-on to a penta-carbonyl-tungsten(0) fragment with a W-N bond length of 2.186 (4) Å, completing an octa-hedral coordination environment around the W atom.

2,2'-(Sulfanediyldimethyl-ene)bis-(1,3-benzothia-zole).

In the title compound, C(16)H(12)N(2)S(3), the two benzothia-zole groups are oriented differently with respect to the -CH(2)- groups, one being approximately staggered and one nearly eclipsed. A sulfur-π inter-action of 3.3627 (11) Šis observed between the bridging thio-ether S atom and a thia-zole ring. The crystal packing is further stabilized by inter-molecular C-H⋯N and C-H⋯π inter-actions.

Novel N-heterocyclic ylideneamine gold(I) complexes: synthesis, characterisation and screening for antitumour and antimalarial activity.

Ylideneamine functionalised heterocyclic ligands, 1,3-dimethyl-1,3-dihydro-benzimidazol-2-ylideneamine (I), 3-methyl-3H-benzothiazol-2-ylideneamine (II) or 3,4-dimethyl-3H-thiazol-2-ylideneamine (III), were employed in the preparation of a series of both charged and neutral gold(I) complexes consisting either of a Au(C(6)F(5)) fragment (1-3), a [Au(PPh(3))](+) unit (4-6) or a [Au(NHC)](+) unit (7) coordinated to the imine nitrogen of the neutral ylideneamine ligand. These complexes were fully characterised by various techniques including X-ray diffraction. In addition, the antitumour and antimalarial potential of selected compounds were assessed in a preliminary study aimed at determining the medicinal value of such compounds. Complexation of the azol-2-ylideneamine ligands with [Au(PPh(3))](+) increases their antitumour as well as antimalarial activity.

{1,5,9-Tris[(2S)-2-hydroxy-prop-yl]-1,5,9-triaza-cyclo-dodeca-ne}zinc(II) dinitrate monohydrate.

In the title compound, [Zn(C(18)H(39)N(3)O(3))](NO(3))(2)·H(2)O, the coordination geometry around the central Zn(II) atom is distorted octa-hedral. The hydroxyl groups in the macrocyclic ligand and water mol-ecules are engaged in O-H⋯O hydrogen bonding, which forms two-dimensional corrugated sheets comprising 34-membered rings. Neighbouring sheets are connected by C-H⋯O inter-actions.

Bis(1,1,2,2-tetramethyldiphosphane-1,2-dithione-κS,S')gold(I) trifluoro-methane-sulfonate.

In the title compound, [Au(C(4)H(12)P(2)S(2))(2)](CF(3)SO(3)), the gold(I) atom is tightly bonded to two S atoms belonging to different ligand mol-ecules and forms two weaker contacts to the remaining S atoms. The coordination geometry around gold is inter-mediate between linear-dicoordinate and tetra-hedral with an S-Au-S angle of 161.49 (3)°.

1,1,2,2-Tetra-kis(1,3-benzothia-zol-2-yl)ethene chloro-form disolvate.

The asymmetric unit of the title solvate, C(30)H(16)N(4)S(4)·2CHCl(3), contains one half-molecule of tetrakis(2-benzothiazolyl)ethene, the complete molecule being generated by inversion symmetry, and one molecule of chloroform. Pairs of the benzothia-zole rings attached to the same carbon atom are almost perpendicular to each other, with an angle between planes of 85.74 (4)°. In the crystal, weak C-H⋯N and C-H⋯Cl interactions generate a three-dimensional network.

Ligating properties of anionic Fischer-type carbene complexes, [(CO)(5)M[double bond, length as m-dash]C(X)Y](-).

With the simplest of anionic Fischer-type carbene complexes acting as ligands, Cp(2)Zr(Cl){OCMe}M(CO)(5) compounds (M = Cr or W) promote alpha-olefin oligomerization and polymerization in the presence of MAO. Attaching an N-heterocyclic ring to the carbene carbon atom in similar precursors, allows a variety of hard metal ions and fragments to be captured by external bidentate coordination. The outcome of the attachment of a phosphorus or sulfur functionality to an alpha-carbon of an O-anionic carbene is formation of a bidentate ligand and then internal four-membered carbene-heteroatom chelate formation. alpha-Deprotonated carbene complexes are also precursors for remote, one-N, six-membered carbene complexes of various metals whereas alpha-C-, alpha-N- or alpha-O-deprotonated as well as beta-deprotonated Fischer-type carbene complexes display unique synthon properties towards Ph(3)PAu(+) and partake in unusual ensuing coordination of liberated group 6 metal carbonyl moieties to form dinuclear products.

Competitive bulk liquid membrane transport and solvent extraction of some metal ions using RC(S)NHP(X)(OiPr)(2) (X = O, S) as ionophores. Formation of the polynuclear complex of [Ag(N[triple bond]C-NP(S)(OiPr)(2))](n).

Competitive transport experiments involving metal ions from an aqueous source phase through a chloroform membrane into an aqueous receiving phase have been carried out using a series of N-(thio)phosphorylated (thio)amide and thiourea ligands as the ionophore present in the organic phase. The source phase contained equimolar concentrations of Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II) and Pb(II) with the source and receiving phases being buffered at a number of different pHs. Solvent extraction properties of the ligands towards the same metal cations under the same experimental conditions as for the transport were also studied. All ligands demonstrated 100% extraction of Ag(I). Reaction of AgNO(3) with the potassium salt of the N-thiophosphorylated thiourea NH(2)C(S)NHP(S)(OiPr)(2) gave a new supramolecular Ag(I) complex, [AgZ](n) (Z = {N[triple bond]C-NP(S)(OiPr)(2)}(-)) that contains both tri- and tetracoordinated Ag(I). The novel polynuclear Ag(I) complex [AgZ](n) described and structurally characterized by single crystal X-ray diffraction has no precedent.

Intermolecular aurophilic interactions facilitate assembly of a complex rotaxane in solution.

We describe the formation of a complex [2]rotaxane both in solution and in the solid state. The rotaxane ring is composed of a cyclic trinuclear gold complex and the Au(PMe(3))(2)(+) rod is held in place by three aurophilic interactions to the ring Au atoms. The existence of the [2]rotaxane in solution is confirmed by VT-NMR and MS analysis.