Model iron(II) porphyrin complexes for the active site of myoglobin and oxymyoglobin have been synthesized and fully characterized by Mossbauer, electronic, and infrared spectral analysis, magnetic susceptibility, and X-ray crystallography. The synthesis is reported for the "picket fence porphyrin," mero-tetra(a,a,a',a-o-pivalamidophenyl)porphyrin, which has great steric bulk creating a nonprotic cavity on one side of the porphyrin. The unsaturated ferrous complex, 1, reacts with strong field ligands to give six-coordinate, diamagnetic complexes, 2. The complexes, 2, react reversibly with oxygen, in solution at 25°. Crystalline diamagnetic dioxygen complexes, 3, having 1-methylimidazole or 1-n-butylimidazole as axial ligands were isolated and characterized. The complex, 3, contains O2 bound "end-on" with an Fe-O-O angle of 136 (4)°. A dioxygen complex with tetrahydrofuran as an axial base, 4, was also prepared and appears to be paramagnetic (2.4 BM). Carbonyl complexes with imidazoles (i-co = 1965 cm-1) and THF (veo = 1955 cm-1) as axial ligands are all diamagnetic.The dioxygen complexes, 3 and 4, show no vo2 at 25°but a strong, sharp band is seen at 1385 cm-1 at -175°. The complexes are thus described as containing coordinated singlet ( ) oxygen. The remarkable stability of the complexes, 3 and 4, and their Mossbauer and spectral properties are discussed.

Contents 1. Introduction 561 2. Biomimetic Analogues of Hemoglobin and Myoglobin 563 2.1. The Proteins 563 2.2. Synthetic Analogues of Mb 565 2.2.1. The Molecular Origin of CO vs O 2 Discrimination by Mb and Hb 565 2.2.2. Electrostatic and H-Bonding Effects on Heme's Affinity for Small Molecules 570 2.2.3. Reversible Oxygen Carriers in Protic Media 572 2.3. Reversible Cooperative O 2 Carriers: Biomimetic Analogues of Hb 573 3. Functional Analogues of the Heme/Cu B Site of Cytochrome c Oxidase 574 3.1. The Enzyme 574 3.2. Methodology of Electrocatalytic Studies of Heme/Cu Analogues 576 3.3. General Considerations for the Design of Biomimetic Heme/Cu Analogues for Electrocatalytic Studies 577 3.4. Electrocatalytic O 2 Reduction by Simple Fe Porphyrins 578 3.5. Biomimetic Electrocatalytic Studies Prior to 2000 582 3.6. Role(s) of Cu B Based on Biomimetic Electrocatalytic Studies 583 4. Conclusions 585 5. Acknowledgments 586 6. Supporting Information Available 586 7. References 586

We studied the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fe(a3), Cu(B), and Tyr(244). To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer-coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Thc role o f metalloenzymes in important biological transformations has attracted incrcasing attention over the past sever-;II decades. Of the many chemical transformations mediated by enzymes, few are a s challenging ;is multielectron redox reactions. Kcccnt studies have revealed ;I partial structiirnl and mechanistic dcscription o f these redox-active metallocn/ymes. but thcrc is much still to be Icarned regarding the mechanisms of substrate transformation. Due to the complexity of the metalloenzyme systems, simplified model systems are employed to mimic structural or functional features of the enzyme. In multielectron redox enzymes, several metals are probably involved in both substrate binding and the subsequent redox reactions. Thus, functional mimics of multielectron redox enzymes might also need two or more metal centers to be efficacious. The roles of multiple metal centers are to 1) increase the substrate's affinity for the catalyst, 2) increase the rate of electron transfer to the bound substrate, 3 ) increase the reactivity of the bound substrate, and 4) inhibit deleterious side reactions. Determining the importance of each factor may help in the development of these catalysts. Cofacial metallodiporphyrins, because of the control they provide over the geometric and electronic properties of the synthetic reaction center, are ideal bimetallic model complexes. The knowledge gained from model studies will help in understanding the mechanisms of metalloenzymes and can be used to design new homogeneous catalysts to effect multielectron transformations.

We have prepared and characterized mixed self-assembled monolayers (SAM) on gold electrodes from azido alkane thiols and various omega-functionalized alkane thiols. In the presence of copper(I) catalysts, these azide-modified surfaces are shown to react rapidly and quantitatively with terminal acetylenes forming 1,2,3-triazoles, via "click" chemistry. The initial azide substituents can be identified and monitored using both grazing-angle infrared (IR) and X-ray photoelectron spectrosopies. Acetylenes possessing redox-active ferrocene substituents react with the azide-terminated mixed SAMs and electrochemical measurements of the ferrocene-modified SAM electrodes have been used to quantify the redox centers attached to these platforms. Time-resolved electrochemical measurements have enabled us to follow the formation of these ferrocene centers and thus to measure the rate of the surface "click" reaction. Under optimal conditions this well-behaved second-order reaction takes place with a rate constant of 1 x 10(3) M(-)(1) s(-)(1). Typical reaction times of several minutes were realized using micromolar concentrations of acetylene. These techniques have been used to construct well-characterized, covalently modified monolayers that can be employed as functional electrode surfaces.