Aryl gold isocyanide complexes are found to have interesting photoluminescence properties in the solid state, being referred to as luminescence mechanochromism and molecular domino. A gold complex containing two gold atoms, {[pentafluorophenylgold]₂(μ-1,4-diisocyanobenzene)}, shows significant change in its luminescence property when mechanical stimulus such as grinding or pressing is applied on its solid sample. This “luminescence mechanochromism” is most probably attributed to a ground state structure change from the microcrystalline to the amorphous state accompanied to the optical properties alternations. Different feature observed in phenyl(phenyl isocyanide)gold(I) complex is “molecular domino”, where even a small mechanical stimulus can trigger the structure change of the entire crystal. This spontaneous structure change proceeded in a single-crystal-to-single-crystal fashion with the drastic emission color alternation. These features enable sub-molecular-level structure investigation with single X-ray crystallographic analysis and visual observation of the phase transition under UV light irradiation during the mechano-induced phase change.

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In this review, we focus on the integrated assemblies of metal complexes and their functional emergence generated by the synergy effects of π-π stacking interaction and metal complexation. Programmable construction of integrated molecular assemblies is the best measure for bringing out the emergent functions which can be evolved from the intermolecular communications of functional molecular components. Combination of π-π stacking interaction and metal complexation allows not only to reinforce the mutual interactions but also to generate the emergent intermolecular communications. This review introduces the recent studies including the synergy effects on electrical, magnetic, catalytic and molecular recognition functions in soft, crystalline and single molecular materials.

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Hydrogen is a promising potential source of environmentally-clean energy. However, the transport, storage, and handling of hydrogen gas have some problems because of its physical properties. Transition metal-catalyzed hydrogen generation has attracted significant attention as a resolution method for these problems and can be classified into three major types, including (i) decomposition of formic acid, (ii) dehydrogenation of ammonia borane, (iii) dehydrogenation of methanol. In this topics, the recent researches on hydrogen generation catalyzed by group 8 transition-metal complex are described, and the plausible reaction pathway of these reactions is also described.

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In nature, hydrogenase enzymes catalyze H₂ production from water with the turnover frequency (TOF) of 9000 s^-1. In this context, studies on the water-splitting reactions with use of earth-abundant metal complexes have attracted great attention, and some nickel complexes have recently been reported to show efficient catalysis in hydrogen production. Dubois et al. demonstrated that a nickel(II) bis(diphosphine) complex with tertiary amines serves as an electrocatalyst for H₂ production with the TOF of 106,000 s^-1. The pendant amine was proposed to act as a proton relay promoting the nickel hydride formation together with H-H bond fixation, mimicking the reaction field of [FeFe] hydrogenase. In addition, visible-light-driven hydrogen production in the presence of the analogous complex is also promoted with 2700 turnover numbers (TONs) after 150 h, as reported by Holland et al. On the other hand, Eisenberg et al. showed that a biomimetic pyridinethiolate nickel(II) complex gives 5500 TONs over 40 h, employing the photocatalytic H₂-evolving system. In the active form of the complex, coordinated pyridine is suggested to dissociate from nickel center to serve as a proton shuttle instead. These results indicate that a nickel(II) compound has considerable potential to become an alternative to platinum catalyst in the future.

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