Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

Preparations of a series of face-capped octahedral hexarhenium(III) clusters having two N-heterocyclic ligands, [Bu4N]2[trans-[Re6(mu 3-S)8Cl4(L)2]] (Bu4N+ = tetra-n-butylammonium cation; L = pyrazine (1a), 4,4'-bipyridine (3a), 4-methylpyridine (5a), 4-(dimethylamino)pyridine (6a)) and their cis analogues (1b, 3b, 5b, and 6b, respectively), and their electrochemical and photophysical properties have been reported. An X-ray crystal structure determination has been carried out for 1a to confirm the trans configuration (C40H80N6S8Cl4Re6, orthorhombic, space group Cmca (No. 64), a = 19.560(5) A, b = 19.494(4) A, c = 18.592(4) A, beta = 115.76(2) degrees, Z = 4). The redox potential of the reversible ReIII6/ReIII5ReIV process of these complexes and previously reported [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(4-cyanopyridine)2]] (2a and 2b, respectively) and [Bu4N]2[trans- and cis-[Re6(mu 3-S)8Cl4(pyridine)2]] (4a and 4b, respectively) in acetonitrile depends linearly on the pKa of the N-heterocyclic ligands, with the potentials being more negative with basic ligands. The ligand-centered-redox waves for 1a, 1b, 2a, and 2b were observed as split waves (delta E1/2 = 90-140 mV), the extent of the splitting being larger for the cis isomer and largest for the pyrazine complexes. Electronic interaction between the two ligands through the [Re6(mu 3-S)8]2+ core has been suggested. The second ligand-reduction wave was also observed for 3a and 3b, the potential being shifted positively to coalesce with the first reduction wave on addition of the weak proton donor imidazole. This is accounted for by the proton-coupled redox reaction at the free pyridyl site of the 4,4'-bipyridine ligands. All of the complexes show luminescence in acetonitrile at room temperature. While the complexes of pyridine and 4-methylpyridine show photophysical characteristics (lambda em 740-750 nm, phi em 0.031-0.057, tau em 4.2-6.2 microseconds) similar to those (770 nm, 0.039, and 6.3 microseconds, respectively) of [Re6(mu 3-S)8Cl6]4-, emissions of other complexes are significantly weak with lambda em, phi em, and tau em values in the ranges 763-785 nm, 0.0010-0.0017, and 0.013-0.029 microsecond, respectively. Suggestions are given for the excited states localized on the cluster core and the ligand pi* orbitals.

A bent fluorenone-based dipyridyl ligand L A reacts with Pd II cations to a solvent-dependent dynamic library of [Pd n L 2 n ] assemblies, constituted by a [Pd 3 L A 6 ] ring and a [Pd 4 L A 8 ] tetrahedron as major components, and a [Pd 6 L A 12 ] octahedron as minor component. Introduction of backbone steric hindrance in ligand L B allows exclusive formation of the [Pd 6 L B 12 ] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd 4 L A 4 L B 4 ] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd 4 L 8 ] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent L A is maintained in both homoleptic [Pd 3 L A 6 ] and heteroleptic [Pd 4 L A 4 L B 4 ].

A series of &mgr;(3)-sulfido Re-Re bonded octahedral hexarhenium(III) clusters having mixed chloride-pyridine (py) or -4-cyanopyridine (cpy) terminal ligands, [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(py)(2)}] (Bu(4)N(+) = tetra-n-butylammonium cation) (1a), [Bu(4)N](2)[cis-{Re(6)S(8)Cl(4)(py)(2)}] (1b), [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a), [Bu(4)N](2)[cis-{Re(6)S(8)Cl(4)(cpy)(2)}] (2b), and [Bu(4)N][mer-{Re(6)S(8)Cl(3)(py)(3)}] (3), and their one-electron-oxidized Re(III)(5)Re(IV) species, [Bu(4)N][trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a'), [Bu(4)N][trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a'), and mer-[Re(6)S(8)Cl(3)(py)(3)] (3'), have been prepared and characterized by several physical methods. X-ray crystallographic studies for 1a, 2a, and 3 showed that the Re(6)S(8) core structures are not significantly affected by the type and number of pyridyl ligands. The mixed valent cluster 1a' is of a structurally delocalized type, structural parameters being very similar to those of 1a. Cyclic voltammograms in acetonitrile showed that there is no distinct difference in the redox potentials (Re(III)(6)/Re(III)(5)Re(IV)) between the cis and the corresponding trans isomers. Both 1a and 1b show a reversible redox wave at 0.77 V vs Ag/AgCl. Redox potentials are more positive for 2a and 2b (0.83 V) and 3 (0.97 V). Clusters 2a and 2b show two-step ligand-centered redox waves at -1.19 and -1.28 V, and -1.18 and -1.29 V, respectively. Temperature-dependent magnetic susceptibility measurements have revealed that 1a' and 3' have an S = 1/2 ground state. The electron self-exchange rate constant for the reaction of 2a with 2a' in dichloromethane as obtained by (1)H NMR line-broadening method is 1.2 x 10(9) M(-)(1) s(-)(1) (298.2 K) with DeltaH() = 30.2 +/- 2.1 kJ mol(-)(1) and DeltaS() = 30 +/- 8 J mol(-)(1) K(-)(1). It has been suggested that the previously reported protonated species [Re(6)S(7)(SH)Cl(6)](3)(-) would actually be a one-electron-oxidized [Re(6)S(8)Cl(6)](3)(-). Crystal data: [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a), monoclinic, space group C2/c, a = 24.693(8) Å, b = 19.494(4) Å, c = 18.592(4) Å, beta = 115.76(2) degrees, Z = 4; [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a), orthorhombic, space group Cmca, a = 19.304(3) Å, b = 17.894(7) Å, c = 18.773(4) Å, Z = 4; [Bu(4)N][mer-{Re(6)S(8)Cl(3)(py)(3)}] (3), monoclinic, space group P2(1)/n, a = 16.156(5) Å, b = 19.760(5) Å, c = 18.895(4) Å, beta = 108.94(2) degrees, Z = 4; [Bu(4)N][trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a'), monoclinic, space group C2/c, a = 20.524(5) Å, b = 13.794(4) Å, c = 16.399(4) Å, beta = 109.72(2) degrees, Z = 4.