Metal clusters consisting of fewer than 100 atoms (diameter <2 nm) are highly promising as a new class of building units for functional materials because of their novel and size-dependent properties. Nevertheless, basic and applied studies of metals clusters have been hampered by the lack of specific guidelines for design and precise synthetic methods. This account surveys recent investigations of gold clusters focusing on our effort toward an atomic-level understanding and control of their size-specific properties. We have developed a size-controlled method for synthesizing gold clusters protected by ligands, stabilized by polymers, and supported on solids. Remarkable size-effects on stabilities and various properties including catalysis were observed. Their mechanisms are discussed based on fundamental knowledge of bare gold clusters in the gas phase.
Metal clusters are highly promising as a new class of building units for functional materials because of their novel and size-dependent properties. This account surveys recent investigations of gold clusters focusing on our effort toward an atomic-level understanding and control of their size-specific properties.
In spite of a long history of quasi-one-dimensional halogen-bridged complexes (MX-chains), all Pt and Pd complexes form charge-density-wave (CDW) state (–X···M2+···X–M4+–X···), while all Ni complexes form Mott–Hubbard (MH) state (–X–Ni3+–X–Ni3+–X–), without exception. Although an interesting charge dynamics are expected at the boundary between the two states, no compound thus far reported forms both states. Thus, we have sought a phase boundary in bromo-bridged Pd compounds by decreasing Pd···Pd distance. In this article, we introduce the following two methods to decrease Pd···Pd distances: introduction of long alkyl chains which can afford attractive force between them, and partial substitution with smaller Ni ions, [Ni1−xPdx(chxn)2Br]Br2. In both systems, we have succeeded in realizing Pd3+ MH state. It has been revealed that the electronic state of bromo-bridged Pd compounds is determined by the Pd···Pd distances, in other words, CDW and MH states are stabilized when Pd···Pd distances are longer and shorter than 5.26 Å, respectively.
We have quested for phase boundary in bromo-bridged Pd compounds by decreasing Pd···Pd distance by using chemical pressure. It has been revealed that CDW and MH states are stabilized when Pd···Pd distances is longer and shorter than 5.26 Å.
Electron-transport properties of cyclophanes are investigated with qualitative Hückel molecular orbital analysis for better understanding of the intermolecular interaction in molecular devices. Charge and electron transfers often take place via through-space interactions, which are observed both in large biological molecules and in organic molecular crystals. Since the intermolecular electronic coupling in π-stacked structures plays an important role in total device performance, in this work [2,2]paracyclophane is studied to investigate the effect of the intermolecular interactions in aromatic hydrocarbons on its electron-transport properties. According to the orbital symmetry rule, the symmetry-allowed and symmetry-forbidden connections for electron transport between the benzene rings are predicted just from the phase and amplitude of the frontier orbitals. The meta connection is symmetry allowed for electron transport while the para and ortho connections are symmetry forbidden. The qualitative predictions made with the Hückel approximation are found consistent with the calculation results obtained with density functional theory. The qualitative but essential understanding in the orbital views would extend the application of the rule from a single molecule to a crystal structure for the development of high-performance molecular devices.
The applicability of a derived concept for orbital control of electron transport is tested on the molecule with intermolecular interactions to estimate its predictive power. Connection meta for effective electron transport can be predicted by the molecular orbital analysis.
NMR spin–lattice relaxation times (T1) for nucleus D of D2O in FA-d3–D2O and DMF–D2O mixtures were measured up to 80 mol % of amide at 5, 25, and 45 °C under atmospheric pressure. To discuss the dynamic behavior of water molecules in the mixtures in detail, the reorientational correlation times (τc) of D2O molecules were estimated from the quadrupole coupling equation. In FA-d3–D2O mixtures, τc increased monotonously with increasing FA content at 25 and 45 °C, while we can recognize a shallow minimum at 5 °C in the D2O-rich region. In DMF–D2O mixtures, τc showed a remarkable maximum in the composition dependence at each temperature. In order to investigate the effect of the microscopic environments on the reorientational dynamics of water molecules, we calculated τc for individual environments through the MD simulations. From these results, we discussed the effects of hydrophilic and hydrophobic interactions on the molecular reorientational dynamics.
Not only electronic but also steric effects of 4,4′-substituents in BIPHEP derivatives and metal (Pd, Pt, and Au) complexes are shown to influence the stability of the biphenyl single bond rotation. While electron-donating or sterically demanding substituents on the 4,4′-positions destabilize the axial chirality of BIPHEP derivatives, electron-withdrawing or sterically less demanding ones on the 4,4′-positions stabilize the axis chirality. Particularly, the axial chirality of palladium dichloride complexes bearing BIPHEP with t-Bu and CF3 substituents on the 4,4′-positions is most labile and stable, respectively (ΔG≠ = 29.22 and 30.49 kcal mol−1 at 300 K; t1/2 = 7 and 56 years at 300 K). These enantiopure dicationic BIPHEP–Pd complexes can be employed for catalytic enantioselective arylation, alkenylation, and ene reactions to give the corresponding products in good-to-excellent yields and enantioselectivities. Significantly, in the carbonyl-ene reaction of trifluoropyruvate with isobutene, the turnover frequency (TOF) reached 58200 h−1. The remarkable effects of 4,4′-substituents in BIPHEP derivatives can be employed as a guiding principle in the design of versatile and efficient ligands.
We recently found new reaction conditions with which [Ir(acac)3] reacts with ppyH (2-phenylpyridine) to produce fac-[Ir(ppy)3] in high yield. It is desirable that Brønsted acids and water molecules exist in the reaction mixture to achieve both the high yield and the selectivity of fac-[Ir(ppy)3]. In this study, the reaction mechanism was investigated theoretically to find reasons of the observed effect of the acid and the water molecule at the B3LYP/SDD level of theory. Although a transition state was located without taking a hydron in the mechanism into account, an IRC calculation confirmed that the TS leads to form a by-product, in which acac makes a bond with ppyH. A neighboring ppyH molecule is involved in the removal of the hydron. A hydron of ppyH was extracted by acac when acac leaves the complex and the orthometalation reaction proceeds. Theoretical calculations showed that the trans effect of the Ir–C bond is key to understanding the selectivity of our reaction.
We study the dynamical electron mechanism of “double proton transfer” in formic acid dimer, which is a prototype of many double proton transfers in chemical and biological systems. Since the present double proton transfer takes place in a cyclic molecular system (a ring structure of dimerizing formic acids), and since the molecular structures before and after the double proton transfer are isomorphic to each other, the pathway of electron flow associated with the reaction cannot be determined in principle within the adiabatic electronic structures based on the Born–Oppenheimer approximation. It is also hard to determine whether “protons” are shifted in formic acid dimer or “hydrogen atoms” are migrated. The general distinction between them is often controversial in experimental studies, too. We identify the mechanism of such dynamics by highlighting the electron current and its pathway. To extract such information of dynamical electrons, we apply the semiclassical Ehrenfest theory, which takes an explicit account of all nuclei nonadiabatic electron wave packet dynamics. It turns out that the reaction proceeds in a concerted manner so that the chance of radical separation is very low and as though the net electronic flow is kept as low as possible to minimize the charge separation within a molecular complex.
Quantitative elemental analysis method of a bulk material by examining the relation between intensity of muonic X-rays and elemental composition was developed. Using this new calibration method, we demonstrated quantitatively determination of the elemental composition inside of an ancient Chinese coin without sample destruction.
Strecker-type reaction of nitrones using acetone cyanohydrin as a cyanide source was developed. By treating nitrones with acetone cyanohydrin in the presence of n-BuMgCl, transcyanation from the cyanohydrin to the nitrones smoothly proceeded in THF at 35 °C. The amount of n-BuMgCl could be reduced to 0.2 equiv to give the corresponding α-cyanohydroxylamines in up to 98% yields.
3,4,5-Trialkoxyphenyl isocyanate derivatives, in which alkoxy was dodecyloxy or methoxy, reacted with 4,6-decadiyn-1,10-diol and 5,7-dodecadiyn-1,12-diol, and four butadiyne derivatives with (N-trialkoxyphenyl)urethane groups were successfully synthesized. Their solid-state polymerization stimulated by UV or γ-ray irradiation was investigated. All monomers in crystals were found to be polymerizable. However, conversion was different depending on the compounds. The derivatives from 5,7-dodecadiyn-1,12-diol showed better conversion, suggesting that they have more favorable monomer arrangement for the solid-state polymerization. Polymers of dodecyloxy derivatives could be partially dissolved in chloroform and they showed solvatochromism when hexane was added to the solution. As was expected from the structure of tridodecyloxyphenyl groups introduced, dodecyloxy derivatives gave organogels in various organic solvents in the concentration less than 2 wt %. However, these gels could not be polymerized by UV irradiation, and the monomer alignment was found to be different between crystalline and gel states.
The synthesis and properties of novel subphthalocyanines having a phosphorus group as an axial substituent are reported. The excellent solubility of these materials may provide a good production method for optical films without impairing the excellent properties peculiar to the original subphthalocyanines.