Self-assembled monolayer of a peroxo-bridged dinuclear cobalt(III) complex on Au(111).

Densely packed Self-Assembled Monolayers (SAMs) of a peroxide-bridged dicobalt complex, [Co2(O2)(bpbp)(O2CCH2CH2S)]2+, 3, (bpbp- = 2,6-bis((N,N-bis-(2-picolyl)amino)-methyl)-4-tert-butylphenolato) have been prepared on atomically planar Au(111) surfaces. Surface voltammetric and interfacial capacitance data, along with electrochemical scanning tunnelling microscopy (in situ STM) imaging, support the formation of a densely packed adlayer of 3 attached via a gold-thiolate bond. In solution, the disulfide linked precursor for 3 reversibly binds dioxygen with high affinity. Electrochemical measurements show that the redox potential of the O22-/O2*- couple of the monolayer of 3 is cathodically shifted by nearly 500 mV compared to the precursor in solution. This is attributed to the close proximity of the O2 binding site to the gold surface. Since the redox potential of the O22-/O2*- couple reflects tentatively the binding affinity of O2 to the deoxygenated CoII2 binding site, the potential of the O22-/O2*- couple of the SAM of 3 suggests a much higher affinity towards O2 compared to the solution precursor.

Models for proton-coupled electron transfer in photosystem II.

The coupling of proton and electron transfers is a key part of the chemistry of photosynthesis. The oxidative side of photosystem II (PS II) in particular seems to involve a number of proton-coupled electron transfer (PCET) steps in the S-state transitions. This mini-review presents an overview of recent studies of PCET model systems in the authors' laboratory. PCET is defined as a chemical reaction involving concerted transfer of one electron and one proton. These are thus distinguished from stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT). Hydrogen atom transfer (HAT) reactions are one class of PCET, in which H(+) and e (-) are transferred from one reagent to another: AH + B --> A + BH, roughly along the same path. Rate constants for many HAT reactions are found to be well predicted by the thermochemistry of hydrogen transfer and by Marcus Theory. This includes organic HAT reactions and reactions of iron-tris(alpha-diimine) and manganese-(mu-oxo) complexes. In PS II, HAT has been proposed as the mechanism by which the tyrosine Z radical (Y(Z)*) oxidizes the manganese cluster (the oxygen evolving complex, OEC). Another class of PCET reactions involves transfer of H(+) and e (-) in different directions, for instance when the proton and electron acceptors are different reagents, as in AH-B + C(+) --> A-HB(+) + C. The oxidation of Y(Z) by the chlorophyll P680 + has been suggested to occur by this mechanism. Models for this process - the oxidation of phenols with a pendent base - are described. The oxidation of the OEC by Y(Z)* could also occur by this second class of PCET reactions, involving an Mn-O-H fragment of the OEC. Initial attempts to model such a process using ruthenium-aquo complexes are described.

Squeezing the [Cu-OH...H2O-Cu]3+ bridge by cryptate encapsulation.

Treatment of cryptand L(1) with Cu(II) generates a H3O2(-)-bridged dicopper(II) cryptate, 2, where the guest anion has responded to steric constraint by a significant shortening of the O-O distance to 2.325(9) A; computational optimization at the B3LYP/6-31(d) level suggests that the bridging O-H...O H-bond is bent (approximately 157 degrees) but that the barrier to interchange of the bridging H atom is low (<4 kJ mol(-1)). This cryptate, rather than the [Cu2L(1)muCN]3+ species recently claimed to derive from cleavage of the C-C bond of the solvent, is the product of acetonitrile recrystallization of the initially formed reaction product, 1.

2:2 Fe(III):ligand and "adamantane core" 4:2 Fe(III):ligand (hydr)oxo complexes of an acyclic ditopic ligand.

A bis-hydroxo-bridged diiron(III) complex and a bis-mu-oxo-bis-mu-hydroxo-bridged tetrairon(III) complex are isolated from the reaction of 2,6-bis((N,N'-bis-(2-picolyl)amino)methyl)-4-tert-butylphenol (Hbpbp) with iron perchlorate in acidic and neutral solutions respectively. The X-ray structure of the dinuclear complex [{(Hbpbp)Fe([mu-OH)}(2)](ClO(4))(4).2C(3)H(6)O (1.2C3H6O) shows that only one of the metal-binding cavities of each ligand is occupied by an iron(III) atom and two [Fe(Hbpbp)]3+ units are linked together by two hydroxo bridging groups to form a [Fe(III)-(mu-OH)](2) rhomb structure with Fe...Fe = 3.109(1)A. The non-coordinated tertiary amine of Hbpbp is protonated. Magnetic susceptibility measurements show a well-behaved weak antiferromagnetic coupling between the two Fe(III) atoms, J= -8 cm(-1). The tetranuclear complex [(bpbp)(2)Fe(4)(mu-O)(2)(mu-OH)(2)](ClO(4))(4)(2) was isolated as two different solvates .4CH(3)OH and .6H(2)O with markedly different crystal morphologies at pH ca. 6. Complex .4CH(3)OH forms red cubic crystals and .6H(2)O forms green crystalline platelets. The Fe(4)O(6) core of shows an adamantane-like structure: The six bridging oxygen atoms are provided by the two phenolato groups of the two bpbp(-) ligands, two bridging oxo groups and two bridging hydroxo groups. The hydroxo and oxo ligands could be distinguished on the basis of Fe-O bond lengths of the two different octahedral iron sites: Fe-mu-OH, 1.953(5), 2.013(5)A and Fe-mu-O, 1.803(5), 1.802(5)A. The difference in ligand environment is too small for allowing Mossbauer spectroscopy to distinguish between the two crystallographically independent Fe sites. The best fit to the magnetic susceptibility of .4CH(3)OH was achieved by using three coupling constants J(Fe-OPh-Fe)= 2.6 cm(-1), J(Fe-OH-Fe)=-0.9 cm(-1), J(Fe-O-Fe)=-101 cm(-1) and iron(III) single ion ZFS (|D|= 0.15 cm(-1)).