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

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.

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.