Quasi-Relativistic Density Functional Study of Aurophilic Interactions

A first principles study of the electronic structure and optical response of heterobimetallic M-dicyanoaurate-based coordination polymers (M = Mn, Co, Ni, Zn and Cd)

The electronic structure and derived optical properties of five synthesized metal-dicyanoaurate(I), (K)M[Au(CN)2], (M = Mn, Co, Ni, Zn and Cd), coordination polymers are described from a combined experimental analysis and theoretical study based on density functional theory. In this sense, the topological features that influence the electronic structure, which in turn give rise to electronic transitions associated with the band gap energy, are studied from first principles calculations (with hybrid HSE06 and GGA-PBE density functionals) and electronic spectroscopy. The impact of gold (through spin-orbit coupling) and aurophilic interactions on the electronic transitions that gives rise to optical properties is described. The calculated projected density of states and band dispersion diagrams shed light on the molecular orbital distribution and the role of a dicyanoaurate(I) molecular block as the origin of the optical properties. Infrared, Raman and ultraviolet-visible spectroscopic analyses reveal the effect that charge transfer interactions, of a metal → ligand and metal → metal nature, have on the electronic behavior within the solids through association with the polarizing power of transition metals and gold atoms.

Molecular Flexibility in Solvated Crystals of the Dimer, Au2(μ-1,2-bis(diphenylphosphino)ethane)2I2, with Three-Coordinate Gold(I)

We report the ability to trap the dimer Au2(μ-dppe)2I2 (dppe is 1,2-bis(diphenylphosphino)ethane) with different separations between the three-coordinate gold ions in crystalline solvates. All of these solvates ((Au2(μ-dppe)2I2·4(CH2Cl2) (1), Au2(μ-dppe)2I2·2(CH2Cl2) (2), the polymorphs α-Au2(μ-dppe)2I2·2(HC(O)NMe2) (3) and β-Au2(μ-dppe)2I2·2(HC(O)NMe2) (4), and Au2(μ-dppe)2I2·4(CHCl3) (5)) along with polymeric {Au(μ-dppe)I}n·n(CHCl3) (6)) originated from the same reaction, only the solvent system used for crystallization differed. In the different solvates of Au2(μ-dppe)2I2, the Au···Au separation varied from 3.192(1) to 3.7866(3) Å. Computational studies undertaken to understand the flexible nature of these dimers indicated that the structural differences were primarily a result of crystal packing effects with aurophillic interactions having a minimal effect.

Anisotropic Metal-Metal Pauli Repulsion in Polynuclear d10 Metal Clusters

Metallophilicity has been widely considered to be the driving force for self-assembly of closed-shell d10 metal complexes, but this view has been challenged by recent studies showing that metallophilicity in linear d10-d10 dimers is repulsive. This is due to strong metal-metal (M-M') Pauli repulsion (Wan, Q., Proc. Natl. Acad. Sci. U. S. A. 2021, 118, e2019265118). Here, we study M-M' Pauli repulsion in d10 metal clusters. Our results show that M-M' Pauli repulsion in d10 polynuclear clusters is 6-52% weaker than in similar linear d10 complexes due to the anisotropic shape of (n+1)s-nd hybridized orbitals. The overall M-M' interactions in closed-shell d10 polynuclear metal clusters remain repulsive. The effects of coordination geometry, relativistic effects, and the ligand's electronegativity on M-M' Pauli repulsion in polynuclear d10 clusters have been explored. These findings provide valuable guidance for the design and development of ligands and coordination geometries that alleviate M-M' Pauli repulsion in d10 metal cluster systems.

Modulating the Extent of Anisotropic Cuprophilicity via High Pressure with Piezochromic Luminescence Sensitization

Metallophilicity has been widely studied as a fundamental supramolecular interaction. However, the extent and directionality thereof remain controversial. A major obstacle lies in the difficulty to separately control the geometry and chemical composition. Herein, we address this challenge by modulating metallophilicity with mechanical pressure. Using a multinuclear Cu(I) complex as model system, we report anomalous anisotropies of (supra)molecular structures, vibrations, and interaction energies upon isotropic compression as well as concomitant (essentially turn-on) piezochromic luminescence enhancement with ∼10³ modulation. The in situ characterizations indicate opposite behaviors of contact distances and cuprophilic interactions for intermolecular vs intramolecular Cu-Cu pairs under pressure. Theoretical calculations break down the attractive and repulsive forces associated with cuprophilicity, its spontaneous 4p-3d hybridization origin, and direction-dependent interaction strength. The use of isotropic mechanical force reveals the intrinsic anisotropy of metallophilicity in multinuclear systems.

Aurophilic Interactions Studied by Quantum Crystallography

This is the first use of a wave-function-based crystallographic method to characterize aurophilic interactions from X-ray diffraction data. Theoretical calculations previously suggested the importance of electron correlation and dispersion forces, but no influence of relativistic corrections to the Au...Au interaction energy was found. In this study, we confirm the importance of relativistic corrections in the characterization of aurophilic interactions in addition to electron correlation and dispersion.

Hirshfeld atom refinement was used to characterize aurophilic interactions from X-ray diffraction data. An intermediate closed-shell type of aurophilic interaction with some features of covalency was identified when both electron correlation and relativistic corrections were applied. Relativistic correction changes the electron density distribution more than electron correlation. Relativistic effects strongly dominate the metal core region also in the direction of the noncovalent interactions and all of the valence and bonding regions with regard to the Au···Au interaction.

Metallophilic interactions in silver(I) dicyanoaurate complexes

This manuscript reports four new gold(I)−silver(I) complexes with 2-(2-pyridyl)-1,8-naphthyridine (pyNP) and terpyridine (terpy) as ancillary ligands, of formulation [Ag(pyNP)(Au(CN)2)]2 (1), [Ag2Au2(μ-CN)2(CN)2(pyNP)2] (2), [Ag2Au(μ-CN)2(terpy)2][Au(CN)2] (3) and [Ag4Au4(μ-CN)8(terpy)2(py)] (4). Complexes 1 and...

Reaction dynamics studied via femtosecond X-ray liquidography at X-ray free-electron lasers

X-ray free-electron lasers (XFELs) provide femtosecond X-ray pulses suitable for pump–probe time-resolved studies with a femtosecond time resolution. Since the advent of the first XFEL in 2009, recent years have witnessed a great number of applications with various pump–probe techniques at XFELs. Among these, time-resolved X-ray liquidography (TRXL) is a powerful method for visualizing structural dynamics in the liquid solution phase. Here, we classify various chemical and biological molecular systems studied via femtosecond TRXL (fs-TRXL) at XFELs, depending on the focus of the studied process, into (i) bond cleavage and formation, (ii) charge distribution and electron transfer, (iii) orientational dynamics, (iv) solvation dynamics, (v) coherent nuclear wavepacket dynamics, and (vi) protein structural dynamics, and provide a brief review on each category. We also lay out a plausible roadmap for future fs-TRXL studies for areas that have not been explored yet.

Femtosecond X-ray liquidography using X-ray free-electron lasers (XFELs) visualizes various aspects of reaction dynamics.

A dynamic tetranuclear gold(i)-cyclophane - gold(i)-centred chirality and fluxionality arising from an intramolecular shift of Au-S bonds

A tetranuclear gold(i) complex [Au₄(μ-PAnP)₂(μ-L)₂] (PAnP = 9,10-bis(diphenylphosphino)anthracene and L = benzene-1,2-dithiolate) has been synthesized and characterised by multinuclear NMR and X-ray crystallography. The molecule has a cyclophane-like structure which can be considered to be composed of two [Au₂(μ-PAnP)(μ-L)] units held together by Au-S bonds and aurophilic interactions (Au-Au = 3.0712(2) Å). L acts as a chelating and bridging ligand with one of its S atoms bonded to two Au ions as sulfonium ions and there are two Au₂S₂ cores on each side of the cyclophane. A sulfur atom in each Au₂S₂ core is a chiral sulfonium ion, being bonded to two chemically distinct Au ions. Two Au ions are bonded to four atoms (2S, P and Au) in an asymmetric environment, making them a rare example of gold(i)-centred chirality. The two Au₂S₂ cores have R_Au, R_S and S_Au, S_S configurations, and the chiralities of the sulfonium ion and the gold ion are correlated. Variable-temperature NMR spectroscopy showed that the metallacyclophane undergoes rapid exchange in solution. A bond shift mechanism involving simultaneous cleavage and formation of Au-S bonds is proposed for the exchange.

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.

Coinage-Metal-Based Cyclic Trinuclear Complexes with Metal-Metal Interactions: Theories to Experiments and Structures to Functions

Among the d¹⁰ coinage metal complexes, cyclic trinuclear complexes (CTCs) or trinuclear metallocycles with intratrimer metal-metal interactions are fascinating and important metal-organic or organometallic π-acids/bases. Each CTC of characteristic planar or near-planar trimetal nine-membered rings consists of Au(I)/Ag(I)/Cu(I) cations that linearly coordinate with N and/or C atoms in ditopic anionic bridging ligands. Since the first discovery of Au(I) CTC in the 1970s, research of CTCs has involved several fundamental areas, including noncovalent and metallophilic interaction, excimer/exciplex, acid-base chemistry, metalloaromaticity, supramolecular assemblies, and host/guest chemistry. These allow CTCs to be embraced in a wide range of innovative potential applications that include chemical sensing, semiconducting, gas and liquid adsorption/separation, catalysis, full-color display, and solid-state lighting. This review aims to provide a historic and comprehensive summary on CTCs and their extension to higher nuclearity complexes and coordination polymers from the perspectives of synthesis, structure, theoretical insight, and potential applications.

Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity

In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. Although it has been extensively investigated in various reactions, the regioselectivity of hyperconjugative aromaticity on either main group systems or transition metal ones remains elusive due to the challenge of synthesizing the target products. Here we report a joint theoretical and experimental study on this issue. Theoretical calculations predicted that electron-withdrawing groups prefer an attachment to the sp³-hybridized carbon atom rather than the nitrogen atom in indoliums. For the electron-donating groups, the two isomers bonded to the sp³-hybridized carbon or nitrogen atom are almost isoenergetic. When both sp²- and sp³-hybridized carbon and nitrogen atoms in the five-membered ring of indoliums are considered, the isomer with the polyaurated substituents bonded to the sp³-hybridized carbon atom is thermodynamically more stable than that with the polyaurated substituents bonded to the sp³-hybridized nitrogen atom. This prediction is reasonably verified by experimental observation. Bond dissociation energy is found to be more important than aromaticity in rationalizing such a preference. Our findings could help experimentalists to design and realize more novel hyperconjugative aromatics.

d8⋯d10 RhI⋯AuI interactions in Rh 2,6-xylylisocyanide complexes with [Au(CN)2]−: bond analysis and crystal effects

The well-known [RhL₄]_n(anion)_n structures, with Rh^I⋯Rh^I d⁸⋯d⁸ interactions, are replaced by others with Rh^I⋯Au^I d⁸⋯d¹⁰ interactions such as [{RhL₄}{Au(CN)₂}] (L = 2,6-xylylisocyanide) or [{RhL₄}{Au(CN)₂}{RhL₄}{Au₂(CN)₃}·4(CHCl₃)]_∞ when the anion is [Au(CN)₂]⁻.

Relevance of Orbital Interactions and Pauli Repulsion in the Metal-Metal Bond of Coinage Metals

The importance of relativity and dispersion in metallophilicity has been discussed in numerous studies. The existence of hybridization in the bonding between closed shell d¹⁰–d¹⁰ metal atoms has also been speculated, but the presence of attractive MO interaction in the metal–metal bond is still a matter of an ongoing debate. In this comparative study, a quantitative molecular orbital analysis and energy decomposition is carried out on the metallophilic interaction in atomic dimers (M⁺···M⁺) and molecular perpendicular [H₃P–M–X]₂ (where M = Cu, Ag, and Au; X = F, Cl, Br, and I). Our computational studies prove that besides the commonly accepted dispersive interactions, orbital interactions and Pauli repulsion also play a crucial role in the strength and length of the metal–metal bond. Although for M⁺···M⁺ the orbital interaction is larger than the Pauli repulsion, leading to a net attractive MO interaction, the bonding mechanism in perpendicular [H₃P–M–X] dimers is different due to the larger separation between the donor and acceptor orbitals. Thus, Pauli repulsion is much larger, and two-orbital, four-electron repulsion is dominant.

Attract and repel: Our computational studies on the metallophilic interaction in atomic dimers (M⁺···M⁺) and molecular perpendicular [H₃P−M−X]₂ (where M = Cu, Ag, and Au; X = F, Cl, Br, and I) prove that besides the commonly accepted dispersive interactions attractive and repulsive MO interactions play a crucial role in metallophilicity.

Influence of Relativistic Effects on Bonding Modes in M(II) Dinuclear Complexes (M = Au, Ag, and Cu)

The stability and bonding in dinuclear group 11 metal complexes (M = Au, Ag, and Cu) in their +2 oxidation state has been investigated by quantum chemical methods. Two model complexes were selected as representatives of different bonding situations in the dinuclear M(II) complexes, a direct metal-metal bond between two ligand stabilized monomers and ligand-mediated bridged dimer system, making them interesting for a direct comparison and to study the influence of relativistic effects. Relativity substantially stabilizes the direct metal-metal bonded system obtaining the sequence in M-M bond stability Au > Ag > Cu. In the ligand-bridged structure, an asymmetric bonding situation is obtained for gold, resulting in two stronger/covalent and two weaker/ionic bonds per gold atom. Here we observe the opposite trend in stability Cu > Ag > Au. Our analysis nicely corroborates with what is known from experimental observation.

Femtosecond X-ray solution scattering reveals that bond formation mechanism of a gold trimer complex is independent of excitation wavelength

The [Au(CN)2 (-)]3 trimer in water experiences a strong van der Waals interaction between the d(10) gold atoms due to large relativistic effect and can serve as an excellent model system to study the bond formation process in real time. The trimer in the ground state (S0) exists as a bent structure without the covalent bond between the gold atoms, and upon the laser excitation, one electron in the antibonding orbital goes to the bonding orbital, thereby inducing the formation of a covalent bond between gold atoms. This process has been studied by various time-resolved techniques, and most of the interpretation on the structure and dynamics converge except that the structure of the first intermediate (S1) has been debated due to different interpretations between femtosecond optical spectroscopy and femtosecond X-ray solution scattering. Recently, the excitation wavelength of 267 nm employed in our previous scattering experiment was suggested as the culprit for misinterpretation. Here, we revisited this issue by performing femtosecond X-ray solution scattering with 310 nm excitation and compared the results with our previous study employing 267 nm excitation. The data show that a linear S1 structure is formed within 500 fs regardless of excitation wavelength and the structural dynamics observed at both excitation wavelengths are identical to each other within experimental errors.

Direct observation of bond formation in solution with femtosecond X-ray scattering

The making and breaking of atomic bonds are essential processes in chemical reactions. Although the ultrafast dynamics of bond breaking have been studied intensively using time-resolved techniques, it is very difficult to study the structural dynamics of bond making, mainly because of its bimolecular nature. It is especially difficult to initiate and follow diffusion-limited bond formation in solution with ultrahigh time resolution. Here we use femtosecond time-resolved X-ray solution scattering to visualize the formation of a gold trimer complex, [Au(CN)2(-)]3 in real time without the limitation imposed by slow diffusion. This photoexcited gold trimer, which has weakly bound gold atoms in the ground state, undergoes a sequence of structural changes, and our experiments probe the dynamics of individual reaction steps, including covalent bond formation, the bent-to-linear transition, bond contraction and tetramer formation with a time resolution of ∼500 femtoseconds. We also determined the three-dimensional structures of reaction intermediates with sub-ångström spatial resolution. This work demonstrates that it is possible to track in detail and in real time the structural changes that occur during a chemical reaction in solution using X-ray free-electron lasers and advanced analysis of time-resolved solution scattering data.

The thermochemistry of london dispersion-driven transition metal reactions: getting the 'right answer for the right reason'

Reliable thermochemical measurements and theoretical predictions for reactions involving large transition metal complexes in which long-range intramolecular London dispersion interactions contribute significantly to their stabilization are still a challenge, particularly for reactions in solution. As an illustrative and chemically important example, two reactions are investigated where a large dipalladium complex is quenched by bulky phosphane ligands (triphenylphosphane and tricyclohexylphosphane). Reaction enthalpies and Gibbs free energies were measured by isotherm titration calorimetry (ITC) and theoretically 'back-corrected' to yield 0 K gas-phase reaction energies (ΔE). It is shown that the Gibbs free solvation energy calculated with continuum models represents the largest source of error in theoretical thermochemistry protocols. The ('back-corrected') experimental reaction energies were used to benchmark (dispersion-corrected) density functional and wave function theory methods. Particularly, we investigated whether the atom-pairwise D3 dispersion correction is also accurate for transition metal chemistry, and how accurately recently developed local coupled-cluster methods describe the important long-range electron correlation contributions. Both, modern dispersion-corrected density functions (e.g., PW6B95-D3(BJ) or B3LYP-NL), as well as the now possible DLPNO-CCSD(T) calculations, are within the 'experimental' gas phase reference value. The remaining uncertainties of 2-3 kcal mol(-1) can be essentially attributed to the solvation models. Hence, the future for accurate theoretical thermochemistry of large transition metal reactions in solution is very promising.

Fischer-type gold(I) carbene complexes stabilized by aurophilic interactions

The synthesis and structure of rare acyclic alkoxy- and aminocarbene complexes of gold(i) are reported, including a novel ferrocenophane dinuclear biscarbene complex. X-Ray diffraction analyses and DFT calculations reveal that these complexes are stabilized by genuine aurophilic interactions.

Synthesis of gold(I) fluoroalkyl and fluoroalkenyl-substituted phosphine complexes and factors affecting their crystal packing

A series of gold(I) phosphine complexes of the type [AuCl{PR(2)(R(f))}] (R = Et, i-Pr, Cy; R(f) = CF = CF(2); R = Ph, R(f) = C = CFH, CCl = CF(2), C ≡ CCF(3), CF(3), i-C(3)F(7), s-C(4)F(9)) have been prepared and most have been structurally characterised. All of the complexes are monomeric in the solid state, and a number of secondary interactions are observed--including short intramolecular AuF distances, metal-bound Au-ClH non-classical hydrogen bonds, fluorous domains and phenyl embraces. Only in the case of [AuCl{PEt(2)(CF = CF(2))}] is an aurophilic interaction with an AuAu contact less than the sum of the van der Waals radii observed. Even then, the distance, 3.3458(10) Å, is longer than that previously observed for the related complex with R = Ph; R(f) = CF = CF(2).