Halogen Bonding Involving Gold Nucleophiles in Different Oxidation States

A single-crystal X-ray diffraction (XRD) study of diaryliodonium tetrachloroaurates (or, in the recent terminology, tetrachloridoaurates), [(p-XC₆H₄)₂I][AuCl₄] (X = Cl, 1; Br, 2), was performed for 1 (the structure is denoted as 1a to show similarity with the isomorphic structure 2a) and two polymorphs─2a (obtained from MeOH) and 2b (from 1,2-C₂H₄Cl₂). Examination of the XRD data for these three structures revealed 2-center C-X···Au^III (X = Cl and Br) and 3-center bifurcated C-Br···(Cl-Au) halogen bonding (abbreviated as XB) between the p-Cl or p-Br atoms of the diaryliodonium cations and the gold(III) atom of [AuCl₄]^-. The noncovalent nature of Au^III-involving interactions, the nucleophilicity of the gold(III) atoms, and the electrophilic role of p-X atoms of the diaryliodonium cations in the XBs were studied by a set of complementary computational methods. Combined experimental and theoretical studies allowed the recognition of the d-nucleophilicity of the [d⁸Au^III] atom which, regardless of its rather substantial formal 3+ charge, can function as a d-nucleophilic partner of XB. This conclusion was also supported by theoretical calculations performed for the structures' refcodes BINXOM and ICSD 62511; the obtained data verified the nucleophilicity of Au^III toward a K⁺ ions or a σ-(Cl)-hole, respectively. All our results, together with consideration of relevant literature, indicate that gold atoms in the three oxidation states (0, I, and even III) exhibit nucleophilicity in XBs.

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d‐orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron‐rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d‐[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d‐[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Metal-Involving Chalcogen Bond. The Case of Platinum(II) Interaction with Se/Te-Based σ-Hole Donors

Platinum(II) complexes exhibiting an expressed d_z²-nucleophilicity of the positively charged metal centers, namely, [Pt(ppy)(acac)] (1; acacH is acetylacetone; ppyH is 2-Ph-pyridine) and [Pt(ppy)(tmhd)] (2; tmhdH is 2,2,6,6-tetramethylheptanedione-3,5), were cocrystallized with the chalcogen bond donors (4-NC₅F₄)₂Ch (Ch = Se, Te) to form two isostructural cocrystals 1·¹/₂(4-NC₅F₄)₂Ch, and 2·²/₃(4-NC₅F₄)₂Se and 2·(4-NC₅F₄)₂Te. The X-ray data for these cocrystals and appropriate theoretical DFT calculations (PBE0-D3BJ) allowed the recognition of the metal-involving chalcogen bond, namely, Ch···d_z²-Pt^II (its energy spans from -7 to -12 kcal/mol). In 1·¹/₂(4-NC₅F₄)₂Ch, Ch···d_z²-Pt^II bonding is accompanied by the C···d_z²-Pt^II interaction, representing a three-center bifurcate, whereas in 2·(4-NC₅F₄)₂Te the chalcogen bond Te···d_z²-Pt^II is purely two-centered and is stronger than that in 1·¹/₂(4-NC₅F₄)₂Ch because of more efficient orbital overlap. The association of 2 with (4-NC₅F₄)₂Te and the structure of the formed adduct in CDCl₃ solutions was studied by using ¹H, ¹³C, ¹⁹F, ¹⁹⁵Pt, ¹²⁵Te NMR, ¹⁹F-¹H HOESY, and diffusion NMR methods. The ¹⁹⁵Pt and ¹²⁵Te NMR titration and the isothermal titration calorimetry results revealed a 1:1 association of 2 with (4-NC₅F₄)₂Te.

Studies of Nature of Uncommon Bifurcated I-I···(I-M) Metal-Involving Noncovalent Interaction in Palladium(II) and Platinum(II) Isocyanide Cocrystals

Two isostructural trans-[MI2(CNXyl)2]·I2 (M = Pd or Pt; CNXyl = 2,6-dimethylphenyl isocyanide) metallopolymeric cocrystals containing uncommon bifurcated iodine···(metal-iodide) contact were obtained. In addition to classical halogen bonding, single-crystal X-ray diffraction analysis revealed a rare type of metal-involved stabilizing contact in both cocrystals. The nature of the noncovalent contact was studied computationally (via DFT, electrostatic surface potential, electron localization function, quantum theory of atoms in molecules, and noncovalent interactions plot methods). Studies confirmed that the I···I halogen bond is the strongest noncovalent interaction in the systems, followed by weaker I···M interaction. The electrophilic and nucleophilic nature of atoms participating in I···M interaction was studied with ED/ESP minima analysis. In trans-[PtI2(CNXyl)2]·I2 cocrystal, Pt atoms act as weak nucleophiles in I···Pt interaction. In the case of trans-[PdI2(CNXyl)2]·I2 cocrystal, electrophilic/nucleophilic roles of Pd and I are not clear, and thus the quasimetallophilic nature of the I···Pd interaction was suggested.

Bifurcated Halogen Bonding Involving Two Rhodium(I) Centers as an Integrated σ-Hole Acceptor

The complexes [RhX(COD)]2 (X = Cl, Br; COD = 1,5-cyclooctadiene) form cocrystals with σ-hole iodine donors. X-ray diffraction studies and extensive theoretical considerations indicate that the d z 2-orbitals of two positively charged rhodium(I) centers provide sufficient nucleophilicity to form a three-center halogen bond (XB) with the σ-hole donors. The two metal centers function as an integrated XB acceptor, providing assembly via a metal-involving XB.

Electron belt-to-σ-hole switch of noncovalently bound iodine(i) atoms in dithiocarbamate metal complexes

The nature of metals in the isostructural series of dithiocarbamate complexes affects the electron belt-to-σ-hole switch of noncovalently bound iodine(i) leading to either semicoordination, or halogen bonding.

Interaction between Trinuclear Regium Complexes of Pyrazolate and Anions, a Computational Study

The geometry, energy and electron density properties of the 1:1, 1:2 and 1:3 complexes between cyclic (Py-M)3 (M = Au, Ag and Cu) and halide ions (F-, Cl- and Br-) were studied using Møller Plesset (MP2) computational methods. Three different configurations were explored. In two of them, the anions interact with the metal atoms in planar and apical dispositions, while in the last configuration, the anions interact with the CH(4) group of the pyrazole. The energetic results for the 1:2 and 1:3 complexes are a combination of the specific strength of the interaction plus a repulsive component due to the charge:charge coulombic term. However, stable minima structures with dissociation barriers for the anions indicate that those complexes are stable and (Py-M)3 can hold up to three anions simultaneously. A search in the CSD confirmed the presence of (Pyrazole-Cu)3 systems with two anions interacting in apical disposition.

Excimer Formation of Aryl Iodides Chemisorbed on Gold Nanoparticles for the Significant Enhancement of Photoluminescence

The photoluminescence properties of aryl iodides chemisorbed on gold nanoparticles were examined. Chemisorption of a series of aryl iodides onto poly(N-vinylpyrrolidone)-protected Au nanoparticles (Au:PVP) in solution resulted in highly enhanced luminescence. Kinetic studies revealed that the photoluminescence is derived from the excimer formation of aryl iodides on Au:PVP, and the process is significantly faster than intersystem crossing by the heavy atom.

Metal-involving halogen bond Ar–I⋯[dz2PtII] in a platinum acetylacetonate complex

The observed and confirmed theoretically metal-involving halogen bond Ar–I⋯[d_z2Pt^II] provides experimental evidence favoring a XB formation step upon oxidative addition of ArI to Pt^II.

Understanding Regium Bonds and their Competition with Hydrogen Bonds in Au2 :HX Complexes

A theoretical study of the regium and hydrogen bonds (RB and HB, respectively) in Au₂ :HX complexes has been carried out by means of CCSD(T) calculations. The theoretical study shows as overall outcome that in all cases the complexes exhibiting RB are more stable that those with HB. The binding energies for RB complexes range between -24 and -180 kJ ⋅ mol^-1, whereas those of the HB complexes are between -6 and -19 kJ ⋅ mol^-1 . DFT-SAPT also indicated that HB complexes are governed by electrostatics, but RB complexes present larger contribution of the induction term to the total attractive forces. ¹⁹⁷ Au chemical shifts have been calculated using the relativistic ZORA Hamiltonian.

Ligand-driven formation of halogen bonds involving Au(i) complexes

A theoretical investigation shows that the Au(i) centre in a variety of complexes can behave as a halogen bond acceptor.

Halogen bond triggered aggregation induced emission in an iodinated cyanine dye for ultra sensitive detection of Ag nanoparticles in tap water and agricultural wastewater

Aggregation induced emission (AIE) has emerged as a powerful method for sensing applications. Based on AIE triggered by halogen bond (XB) formation, an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs) is reported. The dye (CyI) has an iodine atom in its skeleton which functions as a halogen bond acceptor, and aggregates on the Ag NP plasmonic surfaces as a halogen bond donor or forms halogen bonds with the vacant π orbitals of silver ions (Ag⁺). Formation of XB leads to fluorescence enhancement, which forms the basis of the Ag NPs or Ag⁺ sensor. The sensor response is linearly dependent on the Ag NP concentration over the range 1.0–8.2 pM with an LOD of 6.21 pM (σ = 3), while for Ag⁺ it was linear over the 1.0–10 μM range (LOD = 2.36 μM). The sensor shows a remarkable sensitivity for Ag NPs (pM), compared to that for Ag⁺ (μM). The sensor did not show any interference from different metal ions with 10-fold higher concentrations. This result indicates that the proposed sensor is inexpensive, simple, sensitive, and selective for the detection of Ag NPs in both tap and wastewater samples.

Based on AIE triggered by halogen bond (XB) formation, we established an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs).

Halogen bonding between metal centers and halocarbons

Metal-involving halogen bonding was detected in a series of associates of CHI3 with trans-[PtX2(NCNAlk2)2] (X = Cl, Br). The HI2C-Iη(1)(Pt) halogen bonding and the bifurcated HI2C-Iη(2)(Pt-Cl) halogen bonding - the latter undergoes the thermally induced reversible HI2C-I∙∙∙η(2)(Pt-Cl) ⇄ HI2C-I∙∙∙η(1)(Pt) transformation - were observed and confirmed theoretically.

Nanocrystallinity and direct cross-linkage as key-factors for the assembly of gold nanoparticle-superlattices

We report here how the crystallinity of AuNPs and the choice of binding sites of molecular cross-linkers control their aggregation. The combination of different binding moieties (N-oxides, ArF-I) and the reactivity of the particles' facets allow control over the organization and crystallinity of the AuNP assemblies.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Self assembly of plasmonic core–satellite nano-assemblies mediated by hyperbranched polymer linkers

The morphology of plasmonic nano-assemblies has a direct influence on optical properties, such as localised surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS) intensity.

Aryl iodides as strong inhibitors in gold and gold-based bimetallic quasi-homogeneous catalysis

The strong inhibitor effect of aryl iodides on the quasi-homogenous gold-catalyzed oxidation reaction was described. Aryl iodides were adsorbed on the exposed surface of Au clusters, which affected the accessibility of the nanoclusters to the reacting species and acted as strong inhibitors in catalysis.

A method for controlling the aggregation of gold nanoparticles: tuning of optical and spectroscopic properties

Gold nanoparticles (AuNPs) have many interesting optical properties, which are derived from their surface plasmon resonance (SPR). However, the SPR of single AuNPs occurs around 520 nm, which is a limitation for biomedical imaging applications, because the maximum falls outside the tissue transparency window (∼650-1000 nm). Here the aggregation of AuNPs is mediated by balancing aggregation and steric stabilization processes. This is achieved by varying the relative amounts of hydrophobic small molecules, which act as aggregating agents, and end functional hydrophilic polymers that serve as steric stabilizing agents. This approach allows the position of the SPR shifted into the tissue transparency window, while maintaining colloidal stability. Importantly, increased depolarized scattering and surface enhanced Raman scattering (SERS) cross sections in this region are achieved compared to the single nanoparticles. By varying the structure of the aggregating agent slightly, the SERS spectra exhibit significant changes, thus demonstrating the potential to encode different aggregates. The aggregates have potential applications in biomedical imaging, as an encoding strategy for combinatorial chemistry, and for use in flow cytometry applications.

Self-assembled hyperbranched polymer-gold nanoparticle hybrids: understanding the effect of polymer coverage on assembly size and SERS performance

In the past few years, remarkable progress has been made in unveiling novel and unique optical properties of strongly coupled plasmonic nanostructures. However, the application of such plasmonic nanostructures in biomedicine remains challenging because of the lack of facile and robust assembly methods for producing stable nanostructures. Previous attempts to achieve plasmonic nanoassemblies using molecular ligands were limited by the lack of flexibility that could be exercised in forming them. Here, we report the utilization of tailor-made hyperbranched polymers (HBP) as linkers to assemble gold nanoparticles (NPs) into nanoassemblies. The ease and flexibility in tuning the particle size and number of branch ends of an HBP make it an ideal candidate as a linker, as opposed to DNA, small organic molecules, and linear or dendrimeric polymers. We report a strong correlation of polymer (HBP) concentration with the size of the hybrid nanoassemblies and "hot-spot" density. We have shown that such solutions of stable HBP-gold nanoassemblies can be barcoded with various Raman tags to provide improved surface-enhanced Raman scattering (SERS) compared to that of nonaggregated NP systems. These Raman-barcoded hybrid nanoassemblies, with further optimization of the NP shape, size, and hot-spot density, may find application as diagnostic tools in nanomedicine.