Tracing Hydrogen Bonding Au···H–C at Gold Atoms: A Case Study

Investigation of the Nature of Intermolecular Interactions in Tetra(thiocyanato)corrolato-Ag(III) Complexes: Agostic or Hydrogen Bonded?

Tetra(thiocyanato)corrolato-Ag(III) complexes presented here constitute a new class of metallo-corrole complexes. The spectroscopic properties of these complexes are quite unusual and interesting. For example, the absorption spectra of these β-substituted corrolato-Ag(III) complexes are very different from those of the β-unsubstituted corrolato-Ag(III) derivatives. Single-crystal XRD analysis of a representative tetra(thiocyanato)corrolato-Ag(III) derivative reveals C-H···Ag interactions. The C-H···Ag interactions are rarely demonstrated in the crystal lattice of a discrete coordination/organometallic compound. Optimization of the hydrogen positions of the crystal structure discloses the geometrical parameters of the said interaction as a Ag···H distance of 2.597 Å and ∠C-H···Ag of 109.62°. The natural bond orbital analysis provides information about the donor-acceptor orbitals involved in the interactions and their interaction energies. It was observed that the σ_C-H orbital overlaps with the vacant d-orbital of Ag with an interaction energy of 17.93 kJ/mol. The filled d-orbital of Ag overlaps with the σ*_C-H orbital with an interaction energy of 4.79 kJ/mol. The highlights of this work are that the H···Ag distance is outside of the distance range for the typical agostic interaction but fitted with the weak H-bond distance. However, the ∠C-H···Ag angle is within the range of the agostic interaction. Both crystallographic data and electronic structure calculations reveal that these kinds of intermolecular interactions in square-planar d⁸ Ag(III) complexes are intermediate in nature. Thus, they cannot be categorically called either hydrogen bonding or agostic interaction.

Unprecedented {dz2-CuIIO4}···π-hole interactions: the case of a cocrystal of Cu(II) bis-β-diketonate complex with 1,4-diiodotetrafluoro-benzene

Cocrystallization of bis­[1-(4-pyridyl)butane-1,3-dionato]copper(II) (1) complex and 1,4-diiodoperfluorobenzene in the presence of pyridine yields to a 1:1 cocrystal where both the σ and π-holes of 1,4-diiodoperfluorobenzene play a role. The crystal...

Inorganic–Organic {dz2-MIIS4}···π-Hole Stacking in Reverse Sandwich Structures. The Case of Cocrystals of Group 10 Metal Dithiocarbamates with Electron-deficient Arenes

Cocrystallization of the dithiocarbamate complexes [M(S2CNEt2)2] (M = Ni 1, Pd 2, Pt 3) and X-substituted perfluoroarenes (X = I, Br; 1,2-dibromoperfluorobenzene FBrB and 1,2-diiodoperfluorobenzene FIB) gives isomorphous cocrystals of...

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

Hydrogen bonding to metals as a probe for an inverted ligand field

Electron-rich, late transition metals are known to act as hydrogen-bonding (HBd) acceptors. In this regard, Pt(ii) centres in square-planar environments are particularly efficient. It is however puzzling that no convincing experimental evidence is currently available for the isoelectronic neighbour Au(iii) being involved in HBd interactions. We report now on the synthesis and characterisation of two series of isoleptic and isoelectronic (d8) compounds [(CF3)3Pt(L)]- and (CF3)3Au(L), where the L ligands are based on the quinoline frame and have been selected to favour HBd with the metal centre. Strong HBd interactions were actually found in the Pt(ii) compounds, based on structural and spectroscopic evidence, and they were further confirmed by theoretical calculations. In contrast, no evidence was obtained in the Au(iii) case. In order to find the reason underlying this general disparity, we undertook a detailed theoretical analysis of the model systems [(CF3)3Pt(py)]- and (CF3)3Au(py). This study revealed that the filled dz2 orbital is the HOMO in the case of Pt(ii), but is buried in the lower energy levels in the case of Au(iii). The sharply different electronic configurations involve ligand-field inversion on going from Pt to the next element Au. This is not a gradual but an abrupt change, which invalidates Au(iii) as a HBd-acceptor wherever ligand-field inversion occurs.

Do Gold(III) Complexes Form Hydrogen Bonds? An Exploration of AuIII Dicarboranyl Chemistry

The reaction of 1,1'-Li₂ [(2,2'-C₂ B₁₀ H₁₀ )₂ ] with the cyclometallated gold(III) complex (C^N)AuCl₂ afforded the first examples of gold(III) dicarboranyl complexes. The reactivity of these complexes is subject to the trans-influence exerted by the dicarboranyl ligand, which is substantially weaker than that of non-carboranyl anionic C-ligands. In line with this, displacement of coordinated pyridine by chloride is only possible under forcing conditions. While treatment of (C^N)Au{(2,2'-C₂ B₁₀ H₁₀ )₂ } (2) with triflic acid leads to Au-C rather than Au-N bond protonolysis, aqueous HBr cleaves the Au-N bond to give the pyridinium bromo complex 7. The trans-influence of a series of ligands including dicarboranyl and bis(dicarboranyl) was assessed by means of DFT calculations. The analysis demonstrated that it was not sufficient to rely exclusively on geometric descriptors (calculated or experimental) when attempting to rank ligands for their trans influence. Complex (C^N)Au(C₂ B₁₀ H₁₁ )₂ containing two non-chelating dicarboranyl ligands was prepared similar to 2. Its reaction with trifluoroacetic acid also leads to Au-N cleavage to give trans-(Hpy^C)Au(OAc^F )(C₂ B₁₀ H₁₁ )₂ (8). In crystals of 8 the pyridinium N-H bond points towards the metal centre, while in 7 it is bent away. The possible contribution of gold(III)⋅⋅⋅H-N hydrogen bonding in these complexes was investigated by DFT calculations. The results show that, unlike the situation for platinum(II), there is no evidence for an energetically significant contribution by hydrogen bonding in the case of gold(III).

Proof of Concept for Hydrogen Bonding to Gold, Au⋅⋅⋅H-X

Convincing and consistent evidence for the existence of hydrogen bonding to gold has been obtained. An ammonium or pyridinium group has been shown to be an efficient hydrogen bond donor unit for gold(I) coordination centers, and the assembly leads to the structural pattern typical for standard hydrogen bonds. This constitutes the first rigorous, scrutinizing, and comprehensive study of hydrogen bonds to a metal atom, with gold being an ideal model element because of relativistic effects.

Evidence for genuine hydrogen bonding in gold(I) complexes

The ability of gold to act as proton acceptor and participate in hydrogen bonding remains an open question. Here, we report the synthesis and characterization of cationic gold(I) complexes featuring ditopic phosphine-ammonium (P,NH+) ligands. In addition to the presence of short Au∙∙∙H contacts in the solid state, the presence of Au∙∙∙H-N hydrogen bonds was inferred by NMR and IR spectroscopies. The bonding situation was extensively analyzed computationally. All features were consistent with the presence of three-center four-electron attractive interactions combining electrostatic and orbital components. The role of relativistic effects was examined, and the analysis is extended to other recently described gold(I) complexes.

Organometallic gold(iii) complexes with hybrid SNS-donating thiosemicarbazone ligands: cytotoxicity and anti-Trypanosoma cruzi activity

Stable organogold(iii) compounds of the composition [Au^III(Hdamp)(L1)]Cl are formed from reactions of [AuCl₂(damp)] with H₂L1 (damp^- = dimethylaminomethylphenyl; H₂L1 = N'-(diethylcarbamothioyl)benzimidothiosemicarbazides). The cationic complexes can be neutralized by reactions with weak bases under the formation of [Au^III(damp)(L1)] compounds. The structures of the products show interesting features like relatively short AuH contacts between the methylene protons of the Hdamp ligand and the gold(iii) ions. Preliminary biological studies on the uncoordinated compounds H₂L1 and their gold complexes indicate considerable cytotoxicity for the [Au^III(Hdamp)(L1)]Cl complexes against MCF-7 cells. The in vitro trypanocidal activity was evaluated against the intracellular form of Trypanosoma cruzi. The organometallic complexes display a remarkable activity, which is dependent on the alkyl substituents of the thiosemicarbazone building blocks of the ligands. One representative of the cationic [Au^III(Hdamp)(L1)]Cl complexes, where H₂L1 contains a dimethylthiosemicarbazide building block, shows a trypanocidal activity against the intracellular amastigote form in the same order of magnitude as that of the standard drug benznidazole. Furthermore, no appreciable toxicity to mice spleen cells is observed for this compound resulting in a therapeutic index of about 30, which strongly recommends it as a promising candidate for the development of a future antiparasitic drug.

Hydrogen bonds to Au atoms in coordinated gold clusters

It is well known that various transition elements can form M···H hydrogen bonds. However, for gold, there has been limited decisive experimental evidence of such attractive interactions. Herein we demonstrate an example of spectroscopically identified hydrogen bonding interaction of C-H units to Au atoms in divalent hexagold clusters ([Au₆]²⁺) decorated by diphosphine ligands. X-ray crystallography reveals substantially short Au-H/Au-C distances to indicate the presence of attractive interactions involving unfunctionalized C-H moieties. Solution ¹H and ¹³C NMR signals of the C-H units appear at considerably downfield regions, indicating the hydrogen-bond character of the interactions. The Au···H interactions are critically involved in the ligand-cluster interactions to affect the stability of the cluster framework. This work demonstrates the uniqueness and potential of partially oxidised Au cluster moieties to participate in non-covalent interaction with various organic functionalities, which would expand the scope of gold clusters.Many transition metals can form hydrogen bonds to organic species, but experimental evidence for Au is still lacking. Here, the authors obtain crystallographic and NMR spectroscopic evidence of hydrogen bonding between C-H groups and Au atoms of gold clusters, suggesting that non-covalent interactions may play a role in gold cluster catalysis.

Chiral cyclometalation of 6-(1-phenylbenzyl)-2,2'-bipyridine

A new bipyridine ligand, 6-(1-phenylbenzyl)-2,2'-bipyridine, has been prepared by a multistep synthesis starting from the corresponding substituted pyridine. The coordinating properties of the new ligand have been tested with two d(8) metal ions, Pt(ii) and Pd(ii), to give the cyclometalated complexes [Pt(N,N,C)Cl] and [Pd(N,N,C)Cl], where N,N,C is a terdentate deprotonated bipyridine containing a new stereogenic carbon atom directly generated by C-H bond activation. The single-crystal of the platinum complex has been solved by X-ray diffraction. DFT calculations confirm the presence of a PtH interaction that stabilizes one of the two possible conformers by 14.7 kJ mol(-1) for Pt and 12.9 kJ mol(-1) for Pd. The energy barrier to pass from one conformer to the other is 25.4 and 23.8 kJ mol(-1) respectively. Under different reaction conditions, regioselective activation of a pyridine C-H bond gave the less common cyclometalated rollover complex [Pt(L-H)Me(DMSO)], which was isolated and characterised.

Rare intermolecular M⋯H–C anagostic interactions in homoleptic Ni(ii)–Pd(ii) dithiocarbamate complexes

New Ni(ii)–Pd(ii) dithiocarbamates have been fully characterized;1exhibits rare C–H⋯Ni intermolecular anagostic interactions, whereas2shows C–H⋯Pd hydrogen bonding–anagostic interactions.

Intermolecular Tl⋯H–C anagostic interactions in luminescent pyridyl functionalized thallium(i) dithiocarbamates

Rare intermolecular C–H⋯Tl anagostic and C–S⋯Tl interactions forming a six-membered chelate ring.

Role of size and shape selectivity in interaction between gold nanoclusters and imidazole: a theoretical study

We present a theoretical study on the structure, stability, spectra and electronic properties of imidazole (Im) adsorbed on gold nanoclusters (Aun, n = 2, 4, 6, 8, 10, and 20). These computations were performed using various density functional theories with and without inclusion of Grimme's (D3) dispersion correction. For small clusters, we also carried out wavefunction-based ab initio (MP2 and SCS-MP2) computations for comparison. Vibrational, atoms in molecules (AIM) and natural bond orbital (NBO) analyses clearly reveal the occurrence of charge transfer (CT) through covalent (N1-Au) and noncovalent interactions that play important roles in the stability of the Im@Aun complexes with anchor assisted H-bonds (Cα-H · Au). Therefore, gold clusters can act as H-bond acceptors with biomolecules for development of new materials and applications. Our study establishes also the ability and reliability of PBE0 and M05-2X functionals compared to B3LYP and PBE for an accurate description of covalent and noncovalent interactions between Im and gold clusters since they lead to close agreement with MP2. Finally, we show that the Au8 cluster may be viewed as large enough to mimic the 3D gold surface.

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.