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.

Mimicked: Through biomimetic iron activation of peroxides (H2O2 or tBuOOH), the shown N4O ligands in their iron(III) complexes can be oxygenated selectively at either an aromatic carbon atom or an aliphatic nitrogen atom.

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.

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)).

The first structurally characterized Fe(II)-Fe(III) complex containing a M2(mu-OH)2 diamond core is a Robin and Day class II mixed-valence complex.