A Simple Synthesis of Triangular All-Metal Aromatics Allowing Access to Isolobal All-Metal Heteroaromatics

Advances for Triangular and Sandwich-Shaped All-Metal Aromatics

Much experimental work has been contributed to all-metal σ, π and δ-aromaticity among transition metals, semimetallics and other metals in the past two decades. Before our focused investigations on the properties of triangular and sandwich-shaped all-metal aromatics, A. I. Boldyrev presented general discussions on the concepts of all-metal σ-aromaticity and σ-antiaromaticity for metallo-clusters. Schleyer illustrated that Nucleus-Independent Chemical Shifts (NICS) were among the most authoritative criteria for aromaticity. Ugalde discussed the earlier developments of all-metal aromatic compounds with all possible shapes. Besides the theoretical predictions, many stable all-metal aromatic trinuclear clusters have been isolated as the metallic analogues of either the σ-aromatic molecule's [H3]+ ion or the π-aromatic molecule's [C3H3]+ ion. Different from Hoffman's opinion on all-metal aromaticity, triangular all-metal aromatics were found to hold great potential in applications in coordination chemistry, catalysis, and material science. Triangular all-metal aromatics, which were theoretically proved to conform to the Hückel (4n + 2) rule and possess the smallest aromatic ring, could also play roles as stable ligands during the formation of all-metal sandwiches. The triangular and sandwich-shaped all-metal aromatics have not yet been specifically summarized despite their diversity of existence, puissant developments and various interesting applications. These findings are different from the public opinion that all-metal aromatics would be limited to further applications due to their overstated difficulties in synthesis and uncertain stabilities. Our review will specifically focus on the summarization of theoretical predictions, feasible syntheses and isolations, and multiple applications of triangular and sandwich shaped all-metal aromatics. The appropriateness and necessities of this review will emphasize and disseminate their importance and applications forcefully and in a timely manner.

Aryl sulfonate anion stabilized aromatic triangular cation [Pd3]+: syntheses, structures and properties

A series of sulfonate anions paired aromatic triangular palladium clusters 3-7, abbreviated as [Pd3]+[ArSO3]-, were synthesized using a simple "one pot" method, and gave excellent isolated yields (90-95%). Their structures and properties have been fully characterized and further investigated by fluorescence, single crystal X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). In varying organic solvents, they presented apparently stronger absorption and emission in MeOH, driven by the combined interactions of hydrogen bonds and polarity. The crystallographic data demonstrated that the methyl orange ion stabilized complex 7 possessed a D3h symmetric metallic core which was still coplanar and almost equilateral, jointly influenced by the giant hindrance and milder donating effect from the sulfonate. The binding energies for Pdn+ 3d5/2 and Pdn+ 3d3/2 measured by XPS presented at 336.55 and 342.00 eV, respectively. These data were much lower than that of a usual Pd2+ 3d and significantly higher than that of a Pd0 species, further proving the unified palladium valence state (+4/3) in the tri-palladium core and its aromaticity featured by the cyclic electron delocalization.

Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl4] (Cp* = ɳ5-C5Me5) with four equivalents of Li[BH2E3] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)3(μ3-Se)3(BH)(μ-Se)3] (1) and the homocubane-like cluster [(NbCp*)3(μ3-Se)3(μ-Se)3(BH)(μ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se3BH}− ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH2S3] afforded the trimetallic clusters [(NbCp*)3(μ-S)4{μ-S2(BH)}] (3) and [(NbCp*)3(μ-S)4{μ-S2(S)}] (4). Both clusters 3 and 4 have an Nb3S6 core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by 11B{1H}, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb3 framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.

Evolution of Triangular All-Metal Aromatic Complexes from Bonding Quandaries to Powerful Catalytic Platforms

This manuscript describes an overview on the literature detailing the observation of trinuclear complexes that present delocalized metal-metal bonds similar to those of regular aromatics, which are formed combining main group elements. A particular emphasis is given to the structural and electronic features of aromatic clusters that are sufficiently stable to allow their isolation. In parallel to the description of their key bonding properties, the work presents reported catalytic applications of these complexes, which already span from elaborated C-C-forming cascades to highly efficient cross-coupling methods. These examples present peculiar aspects of the unique reactivity exerted by all-metal aromatic complexes, which can often be superior to their established, popular mononuclear peers in terms of chemoselectivity and chemical robustness.

Is Aromaticity a Driving Force in Catalytic Cycles? A Case from the Cycloisomerization of Enynes Catalyzed by All-Metal Aromatic Pd3+ Clusters and Carboxylic Acids

The work details a mechanistic study based on density functional theory modeling on the cycloisomerization of polyunsaturated substrates catalyzed by all-metal aromatic tripalladium complexes and carboxylic acids. These clusters are an emerging class of catalysts for a variety of relevant transformations, including C-C forming processes that occur under mild conditions and display synthetic features complementary to those of established mononuclear complexes. This study is the first computational one devoted to the comprehension of the series of elementary steps involved in a synthetic transformation catalyzed by an all-metal aromatic complex. Present results confirm previous experimental hints on the striking mechanistic differences exerted by these clusters with respect to the usual cyclization pathways of related substrates. Moreover, the catalytic cycle involving present all-metal aromatic clusters closely parallels the mechanism of the aromatic substitution of regular arenes.

Triangular Palladium Cluster from Activation of the Si-Si Bond in a Disilane with Phosphine Pendants

A disilane that contains two diphenylphosphino moieties, (Ph₂PCH₂)Ph₂Si-SiPh₂(CH₂PPh), was readily synthesized from the reaction of ClPh₂Si-SiPh₂Cl with (tmeda)Li(CH₂PPh₂). Treatment of the thus-obtained disilane with the palladium(0) precursor [Pd(CN^tBu)₂]₃ led to the exclusive formation of a trinuclear palladium cluster in which three palladium atoms are arranged in a triangular fashion. Single-crystal X-ray diffraction analysis of the obtained triangular cluster revealed that novel silylphosphido chelating ligands were formed via a skeletal rearrangement of the ligand framework.

Photoelectric properties of aromatic triangular tri-palladium complexes and their catalytic applications in the Suzuki-Miyaura coupling reaction

The photoelectric properties and catalytic activities of substituted triphenylphosphine and sulfur/selenium ligand supported aromatic triangular tri-palladium complexes 1-4, abbreviated as [Pd₃]⁺, were investigated. The cyclic voltammogram of [Pd₃]⁺ in CH₃CN-ⁿBu₄NPF₆ showed a single quasi-reversible wave which was consistent with their robust property and provided preliminary proof for their electron transfer processes in catalysis. With excitation at 267 nm, [Pd₃]⁺ exhibited strong ratiometric fluorescence at 550 and 780 nm at a temperature gradient from 77 K to 287 K. These peculiar triangular tri-palladium complexes showed excellent catalytic activities and exclusive reactivity with aryl iodides over the other halogenated aromatics in the Suzuki-Miyaura coupling reaction. The electronic and steric hindrance effects of substituents on the aryl iodides and aryl boronic acids including heteroaromatics like pyridine, pyrazine and thiophenes were explored and most substrates achieved up to 99% of yields. (2-[1,1'-Biphenyl]-2-ylbenzothiazole) which was analogous to the selective cyclooxygenase-2 (COX-2) inhibitors was also synthesized with our tri-palladium catalyst and gave good isolated yield (94%). The study of the catalytic process revealed that the mechanism of the reaction may involve the replacement of the sulphur ligand on [Pd₃]⁺ by iodine from aryl iodides, which was beneficial for the matching of C-I bond energy.

"Bottled" spiro-doubly aromatic trinuclear [Pd2Ru]+ complexes

Following an ongoing interest in the study of transition metal complexes with exotic bonding networks, we report herein the synthesis of a family of heterobimetallic triangular clusters involving Ru and Pd atoms. These are the first examples of trinuclear complexes combining these nuclei. Structural and bonding analyses revealed both analogies and unexpected differences for these [Pd₂Ru]⁺ complexes compared to their parent [Pd₃]⁺ peers. Noticeably, participation of the Ru atom in the π-aromaticity of the coordinated benzene ring makes the synthesized compound the second reported example of ‘bottled’ double aromaticity. This can also be referred to as spiroaromaticity due to the participation of Ru in two aromatic systems at a time. Moreover, the [Pd₂Ru]⁺ kernel exhibits unprecedented orbital overlap of Ru d_z² AO and two Pd d_xy or d_x²−y² AOs. The present findings reveal the possibility of synthesizing stable clusters with delocalized metal–metal bonding from the combination of non-adjacent elements of the periodic table which has not been reported previously.

Synthesis of a triangular [Pd₂Ru]⁺ complex with delocalized metal–metal bonding between non-adjacent elements of the periodic table, double aromaticity and overlap of d-AOs with different angular momentum.

Diastereoselective bicyclization of enynols via gold catalysis

Steering the regiochemical outcome with a cationic gold–phosphite catalyst.

All-metal aromatic cationic palladium triangles can mimic aromatic donor ligands with Lewis acidic cations

We present that cationic rings can act as donor ligands thanks to suitably delocalized metal-metal bonds. This could grant parent complexes with the peculiar properties of aromatic rings that are crafted with main group elements. We assembled Pd nuclei into equilateral mono-cationic triangles with unhindered faces. Like their main group element counterparts and despite their positive charge, these noble-metal rings form stable bonding interactions with other cations, such as positively charged silver atoms, to deliver the corresponding tetranuclear dicationic complexes. Through a mix of modeling and experimental techniques we propose that this bonding mode is an original coordination-like one rather than a 4-centre-2-electron bond, which have already been observed in three dimensional aromatics. The present results thus pave the way for the use of suitable metal rings as ligands.

Boosting catalyst activity in cis-selective semi-reduction of internal alkynes by tailoring the assembly of all-metal aromatic tri-palladium complexes

Zwitterionic all-metal aromatic tri-palladium complexes can catalyse the cis-selective semi-reduction of internal alkynes at ppm levels.