A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction in [Re(η6-C6H5Br)(η6-C6R6)](PF6).

The contraction of coordinated aromatic hydrocarbons is a rare reactivity pattern in organometallic chemistry. We describe the conversion of a bromobenzene coordinated to a ReI center into a cyclopentadienyl aldehyde. Under mildly alkaline conditions, the expected phenol complex is formed with Re and 99Tc but under strong basic conditions; ring contraction occurs in close to quantitative yields for Re only. A mechanism for this unprecedented reaction is proposed based on 1H and 2H NMR spectra and density functional theory calculations.

Sn(IV) Metalloporphyrin/Co(III) Complex: An All-Abundant-Element System for the Photocatalytic Production of H2 in Aqueous Solution.

A new, molecular system for the light-driven production of hydrogen in aqueous solution was developed by combining a water-soluble tin porphyrin ([Sn(IV)Cl2TPPC], A) acting as photosensitizer with a cobalt-based proton-reduction catalyst ([Co(III)Cl(dmgH)2(py)], C). Under visible light illumination and with triethanolamine (TEOA) as electron source, the system evolves H2 for hours and is clearly catalytic in both dye and catalyst. A detailed analysis of the relevant redox potentials in combination with time-resolved spectroscopy resulted in the development of a Z-scheme type model for the flow of electrons in this system. Key intermediates of the proposed mechanism for the pathway leading to H2 are the porphyrin dye's highly oxidizing singlet excited state (1)A* (E ∼ +1.3 V vs NHE), its strongly reducing isobacteriochlorin analogue (E ∼ +0.95 V), and the Co(I) form of C (E ∼ -0.8 V), acting as catalyst for H2 formation. Among other results, the suggested reaction sequence is supported by the detection of a shortened excited-state lifetime for singlet (1)A* (τ ∼ 1.75 ns) in the presence of TEOA and the ultraviolet-visible detection of the Sn(IV) isobacteriochlorin intermediate at λ = 610 nm. Thus, a molecular, conceptually biomimetic, and precious-metal-free reaction chain was found which photocatalytically generates H2 in a 100% aqueous system from an electron donor with a high oxidation potential (E(TEOA) ∼ +1.1 V). On the other hand, at identical conditions, this photoreaction chain yields H2 markedly slower than a system using the photosensitizer [Re(I)(CO)3(bpy) (py)](+), probably due to the much longer excited-state lifetime (τ ∼ 120 ns) of the rhenium dye and better electron-transfer rates caused by its simple single-electron photoreduction chemistry.

[1,2,4]Triazolo[4,3-a]pyridines as bridging ligands--magnetism of azole-bridged dinuclear copper(II) complexes influenced by the trigonal distortion parameter τ.

The dinuclear doubly azole-bridged copper(II) complexes [Cu(II)2(L)2(MeCN)4](ClO4)4·3.73MeCN·0.80H2O and [Cu(II)2(L)6](ClO4)4·solvent (solvent = 2MeCN·H2O; 2MeCN·2H2O; 1.5MeOH·3.5H2O) were prepared [L = 3-(6-methyl-2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine]. Structural characterizations revealed very different local geometries about the copper(II) ions, being trigonal bipyramidal for the former (τ = 0.76) and square pyramidal for the latter (τ = 0.07, 0.15, 0.07) complex. Magnetic measurements of bulk material [Cu(II)2(L)2(H2O)4](ClO4)4 and [Cu(II)2(L)6](ClO4)4·2H2O revealed antiferromagnetic coupling in both complexes, however, of very different strengths. Electron paramagnetic resonance (EPR) spectroscopy was applied to investigate magnetic properties of the complexes in detail. These experimental findings were supported by broken-symmetry DFT calculations. Systematic magneto-structural correlations are discussed.