Shigehisa Akine

Wheel-Shaped ErIIIZnII3Single-Molecule Magnet: A Macrocyclic Approach to Designing Magnetic Anisotropy

Single-molecule magnets (SMMs) are chemically and physically interesting compounds that exhibit hitherto unobserved magnetic properties. To prevent reversal of the molecular magnetic moment, the use of heavy lanthanide ions is becoming popular because of their large spin multiplicity and large magnetic anisotropies in the ground state. Lanthanide ions exhibit flexibility in magnetic anisotropy, which is another advantage of Ln-based SMMs that is attributable to the flexible design and control of the ligandfield (LF) anisotropy. These anisotropies are correlated through Stevens factor qm as B n m 1⁄4 Am r h iqm, where Bm denotes the mth-order magnetic anisotropy parameters (m is 2, 4, or 6 for lanthanide ions; n varies between 0 and m ; second-order terms of B2 and B 2 2 correspond to the axial and rhombic anisotropic parameters D and E), and Am r m h i denotes the LF anisotropy parameters. Therefore, Ln complexes have a wide scope in the synthetic design of anisotropic magnets. Although many complexes including one or more heavy lanthanide ions are reported to be SMMs, most of them were synthesized in a fortuitous manner without design of the magnetic anisotropy. We have demonstrated previously that an axial LF, whereby donor atoms with higher negative charges are located along the principal axis, induces a strong Ising-type anisotropy of Tb and Dy ions. This type of LF anisotropy is easily achieved in an accidental manner, and thus a wide variety of Tb and Dy SMMs have been reported. On the contrary, Er-based SMMs are rare. When the second-order anisotropy terms are dominant, magnetic anisotropy of the Er ion has opposite features to those of Tb and Dy ions, since the q2 parameter of the Er III

Synthesis and crystal structure of a novel triangular macrocyclic molecule, tris(H2saloph), and its water complex

Abstract Condensation of 2,3-dihydroxybenzene-1,4-dicarbaldehyde and 1,2-phenylenediamine afforded a novel 30-membered macrocyclic hexaimine bearing six hydroxyl groups, in which a water molecule is trapped in the crystalline state.

A Molecular Leverage for Helicity Control and Helix Inversion

The helical tetranuclear complex [LZn(3)La(OAc)(3)] having two benzocrown moieties was designed and synthesized as a novel molecular leverage for helicity control and helix inversion. Short alkanediammonium guests H(3)N(+)(CH(2))(n)NH(3)(+) (n = 4, 6, 8) preferentially stabilized the P-helical isomer of [LZn(3)La(OAc)(3)], while the longer guest H(3)N(+)(CH(2))(12)NH(3)(+) caused a helix inversion to give the M-helical isomer as the major isomer. The differences in the molecular lengths were efficiently translated into helical handedness via the novel molecular leverage mechanism using the gauche/anti conversion of the trans-1,2-disubstituted ethylenediamine unit.

Stepwise Helicity Inversions by Multisequential Metal Exchange

Development of artificial helical molecules that can undergo responsive helicity inversion has been a challenging research target in functional molecular chemistry. However, most reported helicity inversions are based on a single-mode transition, i.e., the conversion between right- and left-handed states. We report here the first molecular system that allows stepwise multisequential helicity inversion utilizing metal exchange of helical complexes derived from a hexaoxime ligand, H6L(1). The ligand H6L(1) underwent a four-step conversion (H6L(1) → L(1)Zn3 → L(1)Zn5 → L(1)Zn3Ba → L(1)Zn3La) upon sequential metal addition (Zn(2+), Ba(2+), then La(3+)). Associated with the conversion, three-step helicity inversion took place (L(1)Zn3, right-handed → L(1)Zn5, left-handed → L(1)Zn3Ba, right-handed → L(1)Zn3La, left-handed). This is the first example of stepwise multimode helicity inversion of a discrete molecule, which could be useful as a platform for construction of dynamic regulation systems with multiple asymmetric functions.

Acyclic Bis(N2O2 chelate) Ligand for Trinuclear d-Block Homo- and Heterometal Complexes

We have synthesized a new type of acyclic bis(N2O2 chelate) ligand that affords a C-shaped O6 site by the metalation of the N2O2 salamo sites. UV-vis titration clearly showed that complexation of H4L with MII (MnII, CoII, and NiII) affords the 1:3 complex [LM3]2+ in a cooperative fashion, whereas complexation with copper(II) gave two or more complexes in a stepwise fashion. The manganese(II) complex [LMn3(OAc)2(MeOH)2] crystallizes in the triclinic system, space group P_1, with unit cell parameters a = 9.584(6) A, b = 13.666(9) A, c = 15.566(10) A, alpha = 108.702(8) degrees, beta = 95.255(4) degrees, gamma = 101.023(8) degrees, and Z = 2, and the cobalt(II) complex [LCo3(OAc)2(EtOH)2].2CHCl3 crystallizes in the triclinic system, space group P_1, with unit cell parameters a = 13.291(6) A, b = 13.913(7) A, c = 14.599(8) A, alpha = 88.27(2) degrees, beta = 67.391(15)degrees, gamma = 73.90(2) degrees, and Z = 2. In the crystal structures, three metal ions occupied both the N2O2 and O6 sites of the ligand L4-. The resultant trinuclear complexes have a C- or S-shaped structure depending on the metal employed. The different nature of the N2O2 and O6 sites of the ligand H4L leads to the site-selective introduction of two different d-block transition metals. An X-ray crystallographic analysis revealed the structures of the two heterotrinuclear complexes, [LZn2Mn(OAc)2(MeOH)2] and [LCu2Zn(OAc)2(H2O)].

Synthesis, stability, and complexation behavior of isolable salen-type N2S2 and N2SO ligands based on thiol and oxime functionalities.

The new salen-type N(2)S(2) tetradentate ligands, H(2)L(1) and H(2)L(2), which have a donor set comprising oxime and thiol groups, were synthesized. These ligands are obtained as isolable colorless crystals, whereas the imine analogues are too unstable to be isolated. The unsymmetrical N(2)SO ligands, H(2)L(3) and H(2)L(4), were also obtained as stable compounds. When ligands H(2)L(1)-H(2)L(4) are heated above the melting points, they mainly decompose via cleavage of the N-O bonds of a thiosalicylaldoxime moiety to give 1,2-benzisothiazole derivatives. The complexation of the N(2)S(2) ligands (H(2)L(1) and H(2)L(2)) with nickel(II) acetate afforded square-planar mononuclear complexes [Ni(L(1))] and [Ni(L(2))], respectively. In contrast, the complexation of the N(2)SO ligand H(2)L(3) with nickel(II) acetate resulted in cleavage of the N-O bond, giving a tetranuclear complex having a cubane-type Ni(4)O(4) core. The N-O bonds of H(2)L(1)-H(2)L(4) are more readily cleaved when the ligands are allowed to react with copper(II) acetate. In these cases, the alkoxo-bridged dinuclear complexes having a Cu-O-Cu-O four-membered ring are obtained. On the other hand, mononuclear complexes can be obtained by complexation of the ligands (H(2)L(1) or H(2)L(3)) with palladium(II) acetate without N-O bond cleavage.

Multiple folding structures mediated by metal coordination of acyclic multidentate ligand.

Four kinds of folded structures are formed upon the metal complexation of a bis(N(2)O(2)) ligand in which two oxime-type N(2)O(2) chelate ligands are connected by a flexible diethyleneoxy linker. The N(2)O(2) coordination sites are intended for d-block transition-metal ions, and the diethyleneoxy linker can interact with hard metal cations. Meso double helical, folded Omega-shaped, S-shaped helical, and single helical structures were formed depending on the metal combination. The difference in the affinity to metal cations resulted in variation of the folding modes and enabled the structural conversion between the folded structures.

Host–Guest Complexation of Perethylated Pillar[5]arene with Alkanes in the Crystal State

Activated perethylated pillar[5]arene crystals show an unexpected alkane-shape- and -length-selective gate-opening behavior. Activated crystals were obtained upon removing solvents from perethylated pillar[5]arene crystals by heating. The activated crystals could quantitatively take up n-alkanes with carbon chains containing more than five carbon atoms as a consequence of their gate-opening pressure. As the chain length of the n-alkanes increased, the gate pressure decreased. A transformation into a herringbone structure was induced when n-hexane was used as a guest. By contrast, cyclic and branched alkanes were not taken up and could not induce a crystal transformation because they were too large to fit in the cavities of the pillar[5]arene. Alkane-shape-selective molecular recognition of pillar[5]arenes in the solution state was translated into the vapor/crystal state.

Functional supramolecular systems with highly cooperative and responding properties

The dynamic processes of host-guest interactions contribute to the multistep regulation of various molecular functions such as the catalysis of chemical reactions, transport of materials, control of reaction pathways, and cooperative and responsive phenomena particularly in biological systems. In this review, we describe artificial metallo-supramolecular systems, which exhibit highly cooperative and responsive functions to external stimulus, by utilizing formation of the metal complexes and their characteristic properties. Pseudomacrocycles such as pseudocrown ethers and pseudocryptands have been synthesized to control macrocyclic and macrobicyclic effects on guest recognition by using a metal ion as an effector, and remarkably positive and negative allosteric effects have been achieved. Highly cooperative stepwise regulation of the affinity to anions has also been achieved by a pseudocryptand and a salt-binding host. The electrostatic interactions between the anions and cations are important for the combination specificity of the salts. We also introduce a linear bis-salamo ligand as a precursor for a novel multimetal cooperative host. A trinuclear zinc complex was formed cooperatively and only the central zinc ion was replaced by lanthanide and calcium ions in a transmetalation way.

Modulation of multimetal complexation behavior of tetraoxime ligand by covalent transformation of olefinic functionalities.

A new multimetal complexation system that can change its complexation behavior by C-C bond formation has been developed. The acyclic tetraoxime ligand H(4)L(1) having two terminal allyl groups was synthesized. The olefin metathesis of H(4)L(1) selectively produced trans-H(4)L(2) while the reaction of [L(1)Zn(2)Ca] exclusively afforded cis-H(4)L(2). The saturated analogue H(4)L(3) was synthesized by hydrogenation. The complexation of the ligands H(4)L (L = L(1), trans-L(2), cis-L(2), L(3)) with zinc(II) acetate (3 equiv) yielded the trinuclear complexes [LZn(3)] with a similar trinuclear core bridged by acetato ligands. Whereas the formation process of [L(1)Zn(3)] having an acyclic ligand was highly cooperative, the macrocyclic analogues [LZn(3)] (L = trans-L(2), cis-L(2), L(3)) were formed in a stepwise fashion via the intermediate 2:3 complex [(HL)(2)Zn(3)]. The trinuclear complexes [LZn(3)] (L = L(1), trans-L(2), cis-L(2), L(3)) can recognize alkaline earth metal ions via site-selective metal exchange. The acyclic [L(1)Zn(3)] selectively recognizes Ca(2+), while the cyclic [trans-L(2)Zn(3)] showed a Ba(2+) selectivity. The metal exchange of [LZn(3)] (L = L(1), trans-L(2), cis-L(2), L(3)) with La(3+) efficiently occurred to give [LZn(2)La], but the trans-olefin linker of the [trans-L(2)Zn(2)La] significantly deforms the structure in such a way that one of the salicylaldoxime moieties does not participate in the coordination. Consequently, the chemical transformation of the olefinic moiety significantly affects the multimetal complexation behavior of the tetraoxime ligands.