Trends in Metallophilic Bonding in Pd–Zn and Pd–Cu Complexes

Investigating Recurrent Matere Bonds in Pertechnetate Compounds

In this manuscript we evaluate the X-ray structure of five new pertechnetate derivatives of general formula [M(H2O)4(TcO4)2], M=Mg, Co, Ni, Cu, Zn (compounds 1-5) and one perrhenate compound Zn(H2O)4(ReO4)2 (6). In these complexes the metal center exhibits an octahedral coordination with the pertechnetate units as axial ligands. All compounds exhibit the formation of directional Tc⋅⋅⋅O Matere bonds (MaBs) that propagate the [M(H2O)4(TcO4)2], into 1D supramolecular polymers in the solid state. Such 1D polymers are linked, generating 2D layers, by combining additional MaBs and hydrogen bonds (HBs). Such concurrent motifs have been analyzed theoretically, suggesting the noncovalent σ-hole nature of the MaBs. The interaction energies range from weak (~ -2 kcal/mol) for the MaBs to strong (~ -30 kcal/mol) for the MaB+HB assemblies, where HB dominates. In case of M=Zn, the corresponding perrhenate Zn(H2O)4(ReO4)2 complex, has been also synthesized for comparison purposes, resulting in the formation of an isostructural X-ray structure, corroborating the structure-directing role of Matere bonds.

Robustness of Threshold Collision-Induced Dissociation Simulations for Bond Dissociation Energies

The threshold collision-induced dissociation (T-CID) method is the workhorse for gas-phase bond dissociation energy (BDE) measurements. However, T-CID does not measure BDEs directly; instead, BDEs are obtained by fitting simulated data to the experimental data. We previously observed several large discrepancies between the computed and experimental BDEs. To analyze the reliability of the experimental values, we previously reported a study of the dissociation rate models in the simulation. Here, we report a study of the collision simulation part, specifically in the L-CID (ligand CID) program. We show that the BDE values are robust even to intentionally introduced mistakes in the simulations, varying in most cases by less than 3 kcal mol-1. The most significant exception is the collisional energy transfer (CET) simulation, which led to deviations larger than 10 kcal mol-1. However, we found that the BDEs obtained with explicitly simulated CET distributions deviated by only 3 kcal mol-1 from those simulated with the original model. Collectively, our results suggest that the T-CID-derived BDE values are robust and are likely to be accurate.

Palladium-Catalyzed Electrooxidative Double C-H Arylation

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

H2 , B-H, and Si-H Bond Activation and Facile Protonolysis Driven by Pt-Base Metal Cooperation

We report a series of heterobimetallic Pt/Zn and Pt/Ca complexes to study the effect of proximity of a dicationic base metal on the organometallic Pt species. Varying degrees of Pt⋅⋅⋅Zn and Zn interaction with the bridging Me group are achieved, showcasing snapshots of a hypothetical process of retrotransmetalation from Pt to Zn. In contrast, only weak interactions were observed for Ca with a Pt-bound Me group. Activation of H₂ , B-H and Si-H bonds leads to the formation of hydride-bridged Pt-H-Zn complexes, which is not observed in the absence of Zn, pointing out the importance of metal-metal cooperation. Reactivity of PtMe₂ /M²⁺ with terminal acetylene, water and methanol is also studied, leading to facile protonation of one of the Me groups at the Pt center only when Zn is present. This study sheds light on various ways in which the presence of a 2+ metal cation significantly affects the reactivity of a common organoplatinum complex.

Model Compounds for Intermediates and Transition States in Sonogashira and Negishi Coupling: d8-d10 Bonds in Large Heterobimetallic Complexes Are Weaker than Computational Chemistry Predicts

We report an experimental study, with accompanying DFT calculations, on a series of heterobimetallic complexes with Pd(II) and Cu(I), Ag(I), or Au(I). The isolable pincer complexes are models for the intermediates and transition state for the transmetalation step in Sonogashira and Negishi coupling reactions, among which, according to the DFT calculations, only the transition state has the two metal centers within bonding distance. Furthermore, we report a substituted version of an analogous heterobimetallic complex in which a competing dissociation sets an upper-bound on the strength of the d⁸-d¹⁰ metal-metal bond. Analysis of the structures in the solid state and in solution, and the competitive dissociation experiment in the gas phase, indicate that the dispersion-corrected DFT method used in the study appears to overestimate the strength of the metal-metal interaction, thus distorting the shape of the computed potential energy surface systematically for transmetalation.

A Bis-(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi-Metallic Configurations

Carbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis-(carbone) pincer framework with a well-defined solid-state structure. This bis-(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au-Pd-Au, Au-Ni-Au, and Cu-Ni-Cu configurations. Sophisticated quantum-theoretical analyses found that the metal-metal interactions are too weak to play a significant role in upholding these multi-metallic configurations; rather, the four lone pairs of electrons within the bis-(carbone) framework are the main contributors to the stability of the complexes.

Synthesis and reactivity of heterometallic complexes containing Mg- or Zn-metalloligands

Heteronuclear metal complexes comprising main group metals and transition metals have attracted widespread attention from researchers due to their applications in stoichiometric and catalytic activation of small molecules with possible cooperative effects. Herein, the advances of heterometallic complexes containing Mg- or Zn-metalloligands over the past ten years are reviewed. They consist of two parts: (i) synthetic approaches to heterometallic complexes. Only a brief discussion is made on the different Mg/Zn precursors since they have been summarized before. (ii) Stoichiometric and catalytic reactivities of heterometallic complexes containing Mg/Zn metalloligands. The exploration of the cooperative catalytic reaction of heterometallic complexes is still in its infancy, promising but challenging; thus, further investigations are required in the future.

Noncovalent Interactions in Organometallic Chemistry: From Cohesion to Reactivity, a New Chapter

Noncovalent interactions (NCIs) have long interested a vast community of chemists who investigated their "canonical categories" derived from descriptive crystallography, e.g., H-bonds, π-π interactions, halogen/chalcogen/tetrel bonds, cation-π and C-H-π interactions, metallophilic interactions in the broad sense, etc. Recent developments in theoretical chemistry have enabled the treatment of noncovalent interactions under new auspices: dispersion-force-inclusive density functionals have emerged, which are reliable for modeling small to large molecular systems. It is possible to perform the full analysis of the contributions of London, Debye, and Keesom forces, i.e., the main components of van der Waals forces, by the DFT-D and ab initio methods at a reasonable computational cost. Our research has been focusing for now 15 years on the role of NCIs in the cohesion of organometallic complexes. NCIs are not only effective in Werner's secondary coordination sphere but also in the metal's primary one. The stabilization of electron-unsaturated transition metal complexes by hemichelation, metal-metal donor-acceptor complexes, and self-aggregation of cationic Rh(I) chromophores have indeed outlined the significance of the London dispersion force as an attractive force operating throughout the whole molecule or molecular assembly. The recent outburst of interest in C-H bond functionalization led us to address the broader question of reaction and catalyst engineering: although one can now satisfactorily analyze bonding and molecular cohesion in transition-metal-based organometallic systems, can modern theoretical methods guide reactivity exploration and the engineering of novel catalytic systems? We addressed this question by investigating the ambiphilic metal-ligand activation/concerted metalation-deprotonation mechanism involved in transition-metal-catalyzed directed C-H bond functionalization. This endeavor was initiated having in scope the construction of a rationale for the transposition of 4-5d metal chemistry to earth-abundant 3d metals. In this base-assisted mechanism of C-H bond metalation, agostic interactions are necessary but not sufficient because C-H bond breaking actually relies on the attractive NCI coding of a proton-transfer step and the minimization of metal-H repulsion. This Account introduces the recent shift of our research toward the construction of an NCI-inclusive paradigm of chemical reactivity engineering based on experimental efforts propped up by state-of-the-art theoretical tools.

Group 11 m-Terphenyl Complexes Featuring Metallophilic Interactions

A series of group 11 m-terphenyl complexes have been synthesized via a metathesis reaction from the iron(II) complexes (2,6-Mes₂C₆H₃)₂Fe and (2,6-Xyl₂C₆H₃)₂Fe (Mes = 2,4,6-Me₃C₆H₂; Xyl = 2,6-Me₂C₆H₃). [2,6-Mes₂C₆H₃M]₂ (1, M = Cu; 2, M = Ag; 6, M = Au) and [2,6-Xyl₂C₆H₃M]₂ (3, M = Cu; 4, M = Ag) are dimeric in the solid state, although different geometries are observed depending on the ligand. These complexes feature short metal-metal distances in the expected range for metallophilic interactions. While 1-4 are readily isolated using this metathetical route, the gold complex 6 is unstable in solution at ambient temperatures and has only been obtained in low yield from the decomposition of (2,6-Mes₂C₆H₃)Au·SMe₂ (5). NMR spectroscopic measurements, including diffusion-ordered spectroscopy, suggest that 1-4 remain dimeric in a benzene-d₆ solution. The metal-metal interactions have been examined computationally using the Quantum Theory of Atoms in Molecules and by an energy decomposition analysis employing natural orbitals for chemical valence.

Reactivity of [Pt(P t Bu3)2] with Zinc(I/II) Compounds: Bimetallic Adducts, Zn-Zn Bond Cleavage, and Cooperative Reactivity

Metal-only Lewis pairs (MOLPs) based on zinc electrophiles are particularly interesting due to their relevance to Negishi cross-coupling reactions. Zinc-based ligands in bimetallic complexes also render unique reactivity to the transition metals at which they are bound. Here we explore the use of sterically hindered [Pt(P t Bu3)2] (1) to access Pt/Zn bimetallic complexes. Compounds [(P t Bu3)2Pt → Zn(C6F5)2] (2) and [Pt(ZnCp*)6] (3) (Cp* = pentamethylcyclopentadienyl) were isolated by reactions with Zn(C6F5)2 and [Zn2Cp*2], respectively. We also disclose the cooperative reactivity of 1/ZnX2 pairs (X = Cl, Br, I, and OTf) toward water and dihydrogen, which can be understood in terms of bimetallic frustration.

Modeling Gas-Phase Unimolecular Dissociation for Bond Dissociation Energies: Comparison of Statistical Rate Models within RRKM Theory

The Rice-Ramsperger-Kassel-Marcus (RRKM) theory provides a simple yet powerful rate theory for calculating microcanonical rate constants. In particular, it has found widespread use in combination with gas-phase kinetic experiments of unimolecular dissociations to extract experimental bond dissociation energies (BDEs). We have previously found several discrepancies between the computed BDE values and the respective experimental ones, obtained with our empirical rate model, named L-CID. To investigate the reliability of our rate model, we conducted a theoretical analysis and comparison of the performance of conventional rate models and L-CID within the RRKM framework. Using the previously published microcanonical rate data as well as reaction cross-section data, we show that the BDE values obtained with the L-CID model agree with the ones from the other rate models within the expected uncertainty bounds. Based on this agreement, we discuss the possible rationalization of the good performance of the L-CID model.

Metal-only Lewis Pairs of Rhodium with s, p and d-Block Metals

Metal-only Lewis pairs (MOLPs) in which the two metal fragments are solely connected by a dative M→M bond represent privileged architectures to acquire fundamental understanding of bimetallic bonding. This has important implications in many catalytic processes or supramolecular systems that rely on synergistic effects between two metals. However, a systematic experimental/computational approach on a well-defined class of compounds is lacking. Here we report a family of MOLPs constructed around the RhI precursor [(η5 -C5 Me5 )Rh(PMe3 )2 ] (1) with a series of s, p and d-block metals, mostly from the main group elements, and investigate their bonding by computational means. Among the new MOLPs, we have structurally characterized those formed by dative bonding between 1 and MgMeBr, AlMe3 , GeCl2 , SnCl2 , ZnMe2 and Zn(C6 F5 )2, as well as spectroscopically identified the ones resulting from coordination to MBArF (M=Na, Li; BArF - =[B(C6 H2 -3,5-(CF3 )2 )4 ]- ) and CuCl. Some of these compounds represent unique examples of bimetallic structures, such as the first unambiguous cases of Rh→Mg dative bonding or base-free rhodium bound germylene and stannylene species. Multinuclear NMR spectroscopy, including 103 Rh NMR, is used to probe the formation of Rh→M bonds. A comprehensive theoretical analysis of those provides clear trends. As anticipated, greater bond covalency is found for the more electronegative acids, whereas ionic character dominates for the least electronegative nuclei, though some degree of electron sharing is identified in all cases.

Zn-Promoted C-H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru-Zn Intermediate

Reaction of [Ru(PPh₃)₃HCl] with LiCH₂TMS, MgMe₂, and ZnMe₂ proceeds with chloride abstraction and alkane elimination to form the bis-cyclometalated derivatives [Ru(PPh₃)(C₆H₄PPh₂)₂H][M′] where [M′] = [Li(THF)₂]⁺ (1), [MgMe(THF)₂]⁺ (3), and [ZnMe]⁺ (4), respectively. In the presence of 12-crown-4, the reaction with LiCH₂TMS yields [Ru(PPh₃)(C₆H₄PPh₂)₂H][Li(12-crown-4)₂] (2). These four complexes demonstrate increasing interaction between M′ and the hydride ligand in the [Ru(PPh₃)(C₆H₄PPh₂)₂H]⁻ anion following the trend 2 (no interaction) < 1 < 3 < 4 both in the solid-state and solution. Zn species 4 is present as three isomers in solution including square-pyramidal [Ru(PPh₃)₂(C₆H₄PPh₂)(ZnMe)] (5), that is formed via C–H reductive elimination and features unsaturated Ru and Zn centers and an axial Z-type [ZnMe]⁺ ligand. A [ZnMe]⁺ adduct of 5, [Ru(PPh₃)₂(C₆H₄PPh₂)(ZnMe)₂][BAr^F₄] (6) can be trapped and structurally characterized. 4 reacts with H₂ at −40 °C to form [Ru(PPh₃)₃(H)₃(ZnMe)], 8-Zn, and contrasts the analogous reactions of 1, 2, and 3 that all require heating to 60 °C. This marked difference in reactivity reflects the ability of Zn to promote a rate-limiting C–H reductive elimination step, and calculations attribute this to a significant stabilization of 5 via Ru → Zn donation. 4 therefore acts as a latent source of 5 and this operational “dual unsaturation” highlights the ability of Zn to promote reductive elimination in these heterobimetallic systems. Calculations also highlight the ability of the heterobimetallic systems to stabilize developing protic character of the transferring hydrogen in the rate-limiting C–H reductive elimination transition states.

Structural, Spectroscopic, and Chemical Bonding Analysis of Zn(II) Complex [Zn(sal)](H2O): Combined Experimental and Theoretical (NBO, QTAIM, and ELF) Investigation

The Zn(II) complex of salen-like scaffold [Zn(sal)](H2O) was synthesized and characterized by elemental analysis, IR, UV–Vis, and 1H-NMR spectroscopic techniques. The structure of complex was confirmed by single crystal X-ray diffraction studies. In the complex, Zn (II) was placed in the inner N2O2 compartment of the salen scaffold in square planar geometry and crystallized in the monoclinic space group P21/n. DFT and TDDFT calculations were performed to reproduce the experimentally observed structural and spectroscopic (IR and UV–vis) findings. The bonding of the Zn(II) framework in the [Zn(sal)](H2O) complex was explored in depth. The theoretical approaches employed were perturbation theory within the context of the natural bond orbital (NBO) framework, and quantum theory of atoms in molecule (QTAIM) and electron localization function (ELF) analysis. The study begins by delineating the difference between the NBO and QTAIM approaches. This paper thus exhibits the supportive nature of NBO theory and QTAIM in discussion of the bonding in the [Zn(sal)](H2O) complex, when both the methodologies are used in combination.

The mutual noncovalent interactions based on metallophilic cluster and anions: A theoretical investigation of the molecular structure and spectroscopic properties of Host–Guest complexes

The d[Formula: see text] metallophilic host clusters [Au(NHC)2][Formula: see text] [M(CN)2][Formula: see text] [Au(NHC)2][Formula: see text](NHC [Formula: see text] N-heterocyclic carbene, [Formula: see text], Ag) with high phosphorescence are synthesized recently and their phosphorescent modulation by solvents is investigated in theory. In this paper, the guest anions (F−, Cl−, Br−, NO[Formula: see text], and BF[Formula: see text] are used to elucidate their effects on metallophilic interactions and phosphorescence of hosts, and also they served as the probes to study the recognition characters of metallophilic hosts. The calculation shows that the guest anions can mutually interact with the host clusters and further, which can modulate the metallophilic Au[Formula: see text]M distances and the phosphorescence spectra of the hosts.

Phosphorescent Modulation of Metallophilic Clusters and Recognition of Solvents through a Flexible Host-Guest Assembly: A Theoretical Investigation

MP2 (Second order approximation of Møller–Plesset perturbation theory) and DFT/TD-DFT (Density functional theory/Time-dependent_density_functional_theory) investigations have been performed on metallophilic nanomaterials of host clusters [Au(NHC)₂]⁺⋅⋅⋅[M(CN)₂]⁻⋅⋅⋅[Au(NHC)₂]⁺ (NHC = N-heterocyclic carbene, M = Au, Ag) with high phosphorescence. The phosphorescence quantum yield order of clusters in the experiments was evidenced by their order of μ_S1/ΔE_S1−T1 values (μS1: S₀ → S₁ transition dipole, ∆ES1−T1: splitting energy between the lowest-lying singlet S₁ and the triplet excited state T₁ states). The systematic variation of the guest solvents (S1: CH₃OH, S2: CH₃CH₂OH, S3: H₂O) are employed not only to illuminate their effect on the metallophilic interaction and phosphorescence but also as the probes to investigate the recognized capacity of the hosts. The simulations revealed that the metallophilic interactions are mainly electrostatic and the guests can subtly modulate the geometries, especially metallophilic Au⋅⋅⋅M distances of the hosts through mutual hydrogen bond interactions. The phosphorescence spectra of hosts are predicted to be blue-shifted under polar solvent and the excitation from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) was found to be responsible for the ³MLCT (triplet metal-to-ligand charge transfer) characters in the hosts and host-guest complexes. The results of investigation can be introduced as the clues for the design of promising blue-emitting phosphorescent and functional materials.

Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

The accurate calculation of ligand dissociation (or equivalently, ligand binding) energies is crucial for computational coordination chemistry. Despite its importance, obtaining accurate ab initio reference data is difficult, and density-functional methods of uncertain reliability are chosen for feasibility reasons. Here, we consider advanced coupled-cluster and multiconfigurational approaches to reinvestigate our WCCR10 set of 10 gas-phase ligand dissociation energies [ J. Chem. Theory Comput. 2014, 10, 3092]. We assess the potential multiconfigurational character of all molecules involved in these reactions with a multireference diagnostic [ Mol. Phys. 2017, 115, 2110] in order to determine where single-reference coupled-cluster approaches can be applied. For some reactions of the WCCR10 set, large deviations of density-functional results including semiclassical dispersion corrections from experimental reference data had been observed. This puzzling observation deserves special attention here, and we tackle the issue (i) by comparing to ab initio data that comprise dispersion effects on a rigorous first-principles footing and (ii) by a comparison of density-functional approaches that model dispersion interactions in various ways. For two reactions, species exhibiting nonnegligible static electron correlation were identified. These two reactions represent hard problems for electronic structure methods and also for multireference perturbation theories. However, most of the ligand dissociation reactions in WCCR10 do not exhibit static electron correlation effects, and hence, we may choose standard single-reference coupled-cluster approaches to compare with density-functional methods. For WCCR10, the Minnesota M06-L functional yielded the smallest mean absolute deviation of 13.2 kJ mol^-1 out of all density functionals considered (PBE, BP86, BLYP, TPSS, M06-L, PBE0, B3LYP, TPSSh, and M06-2X) without additional dispersion corrections in comparison to the coupled-cluster results, and the PBE0-D3 functional produced the overall smallest mean absolute deviation of 4.3 kJ mol^-1. The agreement of density-functional results with coupled-cluster data increases significantly upon inclusion of any type of dispersion correction. It is important to emphasize that different density-functional schemes available for this purpose perform equally well. The coupled-cluster dissociation energies, however, deviate from experimental results on average by 30.3 kJ mol^-1. Possible reasons for these deviations are discussed.