Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.

The critical review describes the known dicopper systems mediating the aromatic hydroxylation of monophenolic substrates. Such systems are of interest as structural and functional models of the type 3 copper enzyme tyrosinase, which catalyzes the ortho-hydroxylation of tyrosine to DOPA and the subsequent two-electron oxidation to dopaquinone. Small-molecule systems involving μ-η²:η² peroxo, bis-μ-oxo and trans-μ-1,2 peroxo dicopper cores are considered separately. These tyrosinase models are contrasted to copper-dioxygen systems inducing radical reactions, and the different mechanistic pathways are discussed. In addition to considering the stoichiometric conversion of phenolic substrates, the available catalytic systems are described. The second part of the review deals with tyrosinase. After an introduction on the occurrence and function of tyrosinases, several aspects of the chemical reactivity of this class of enzymes are described. The analogies between the small-molecule and the enzymatic system are considered, and the implications for the reaction pathway of tyrosinase are discussed (140 references).

In this Communication, the preparation conditions of the molecular films were described incorrectly. During the deposition of [Fe(bpz) 2 phen], the Au(111) substrate temperature was kept at about 100 8C. This correction does not affect the conclusions of the original Communication.

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Tyrosinases are essential enzymes that occur in all organisms and belong to the class of type3 copper proteins. The first crystal structure of a tyrosinase (from Streptomyces castaneoglobisporus) has been achieved and offers important insights into the mechanism of phenol hydroxylation (see scheme; Cu blue, O red, substrate orange; trans‐axial position on CuA: gray dot), the process of activation, and the incorporation of copper.

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Spin crossover (SCO) complexes possess a bistable spin state that reacts sensitively to changes in temperature or excitation with light. These effects have been well investigated in solids and solutions, while technological applications require the immobilization and contacting of the molecules at surfaces, which often results in the suppression of the SCO. We report on the thermal and light-induced SCO of [Fe(bpz)2phen] molecules in direct contact with a highly oriented pyrolytic graphite surface. We are able to switch on the magnetic moment of the molecules by illumination with green light at T = 6 K, and off by increasing the temperature to 65 K. The light-induced switching process is highly efficient leading to a complete spin conversion from the low-spin to the high-spin state within a submonolayer of molecules. [Fe(bpz)2phen] complexes immobilized on weakly interacting graphite substrates are thus promising candidates to realize the vision of an optically controlled molecular logic unit for spintronic devices.

Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K(1S) and K(2), and for the first time also the corresponding thermodynamic parameters DeltaH(1S), DeltaH(2), DeltaS(1S), and DeltaS(2), are determined for the reaction of a nickel-porphyrin (Ni-tetrakis(pentafluorophenyl)porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K(1S) and K(2) are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO(2)Et>NO(2)) whereas the DeltaH(1S) values exhibit a completely different order (OMeCO(2)Et>NO(2)). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d(xz) and d(yz) orbitals into the pi orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro- and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K(1S) relative to K(2). For nitropyridine K(2) is 17 times larger than K(1S) and in the case of methoxypyridine K(2) and K(1S) are almost equal.