The design of biomimetic complexes for the modeling of metallo-enzyme active sites is a fruitful strategy for obtaining fundamental information and a better understanding of the molecular mechanisms at work in Nature's chemistry. The classical strategy for modeling metallo-sites relies on the synthesis of metal complexes with polydentate ligands that mimic the coordination environment encountered in the natural systems. However, it is well recognized that metal ion embedment in the proteic cavity has key roles not only in the recognition events but also in generating transient species and directing their reactivity. Hence, this review focuses on an important aspect common to enzymes, which is the presence of a pocket surrounding the metal ion reactive sites. Through selected examples, the following points are stressed: (i) the design of biomimetic cavity-based complexes, (ii) their corresponding host-guest chemistry, with a special focus on problems related to orientation and exchange mechanisms of the ligand within the host, (iii) cavity effects on the metal ion binding properties, including 1st, 2nd, and 3rd coordination spheres and hydrophobic effects and finally (iv) the impact these factors have on the reactivity of embedded metal ions. Important perspectives lie in the use of this knowledge for the development of selective and sensitive probes, new reactions, and green and efficient catalysts with bio-inspired systems.

The immobilization of a copper calix[6]azacryptand funnel complex on gold-modified electrodes is reported. Two different methodologies are described. One is based on alkyne-terminated thiol self-assembled monolayers. The other relies on the electrografting of a calix[4]arene platform bearing diazonium functionalities at its large rim and carboxylic functions at its small rim, which is post-functionalized with alkyne moieties. In both cases, the CuAAC electroclick methodology proved to be the method of choice for grafting the calix[6]azacryptand onto the monolayers. The surface-immobilized complex was fully characterized by surface spectroscopies and electrochemistry in organic and aqueous solvents. The Cu complex displays a well-defined quasi-reversible system in cyclic voltammetry associated with the Cu(II)/Cu(I) redox process. Remarkably, this redox process triggers a powerful selective detection of primary alkylamines in water at a micromolar level, based on a cavitary recognition process.

COMMUNICATIONS ence experiments, which indicate the proximity of the marked CH, groups ( Fig. 1) to the pyridine protons, and by the small chemical shift of the pyridine protons which reflect the anisotropy of the pyridine ring.

The tungsten(I1) complexes WCI2(PMePh2), (1) and WC12(CH2=CH2)2(PMePh2)2 (2) react with epoxides and aziridines to form tungsten(IV)-oxo and -imido complexes. The relative reactivities of epoxides with 2 have been determined from competition experiments. More substituted epoxides are harder to deoxygenate: the reactivities of ethylene, isobutylene, and tetramethylethylene oxides fall in the geometric progression 100: IO: I . cis-2-Butene oxide is deoxygenated faster than its trans isomer. Reaction occurs with predominant (285%) retention of configuration (e.g. cis epoxides to cis olefins). The reaction of 2 with ethylene-d4 oxide yields W(0)C12(CH2=CH2)( PMePh,), (4) and uncoordinated CD2=CD2. The data suggest that de-epoxidation occurs via oxygen atom abstraction, and not via an oxametallacyclobutane which rearranges to an oxo-ethylene complex. Similarly, the tungsten center is suggested to attack the nitrogen atom of the aziridines, rather than react by initial oxidative addition of a C-N bond. This is indicated by the observation of an N-bound complex of aziridine and by the much slower rates of reaction for aziridines with bulky substituents on the nitrogen. The reactivities of para-substituted styrene epoxides are not strongly affected by the nature of the substituent (a p + value of -0.5 is calculated with Hammett u' parameters) indicating that the transition state is not very polar, although there appears to be some conjugation between the phenyl ring and the epoxide. In sum, the data are most consistent with either concerted oxygen or nitrene transfer to tungsten or a mechanism involving a short-lived radical intermediate.The mechanism(s) of olefin epoxidation by metal-oxo compounds and the reverse reaction, t h e formation of metal-oxo complexes by deoxygenation of epoxides, have attracted a great deal of interest in recent yeaw3-I4 Epoxides a r e valuable in-( I ) PRF Summer Faculty Fellow. Present address: North Seattle Community College, (8) (a) Collman, J. P.: Kodadek, T.; Raybuck, S. A,; Brauman, J. 1.; Papazian, L. M. J . Am. Cfiem. SOC. 1985, 107,4343-5. (b) Collman, J. P.; Brauman, J. 1.; Meunier. B.; Hayashi, T.; Kodadek, T.; Raybuck, S. A. Other examples of deoxygenation of epoxides to olefins by transition-metal complexes: (a) Sharpless, K. B.; Umbreit, M. A.; Nieh, M. T.; Flood, T. C. J . Am. Cfiem. SOC. 1972, 94, 6538-40. (b) Berry, M.; Davies, S. G.; Green, M. L. H. Chem. Commun. 1978, 99-100. (c) Togashi, S.; Fulcher, J. G.; Cho, B. R.; Hasegawa, M.; Gladysz, J. A.

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The first metal complex based on the calix[6]PN3 cryptand is described. The solid-state and solution studies show a 5-coordinate Cu(II) center due to its coordination to the PN3 cap and to an exchangeable guest molecule. Spectroscopic and electrochemical studies evidence surprising properties of the metal ion, which are tentatively assigned to the unusual P-Cu(II) bond enforced by the cryptand.

The bulky, asymmetric analog of the antitumor drug cisplatin, [PtCl(2)(tmen)] (tmen = N,N,N'-trimethylethylenediamine), was used to produce crosslinks with the dinucleotide d(GpG), modeling the most frequent lesions that cisplatin and its analogs cause to DNA. The ligand tmen was chosen because it is expected to constrain the guanine cis to the NMe(2) group in the adduct [Pt(tmen){d(GpG)}](+) to an orientation perpendicular to the coordination plane and to stabilize the other guanine in an oblique orientation, thus maintaining a head-to-head geometry typical of cisplatin-d(GpG) crosslinks within single- and double-stranded DNA. Of the four possible combinations of tmen chirality (R or S symmetry of the coordinated NHMe group) and crosslink direction (5'-G bound cis to the secondary or the tertiary amino group of tmen), two isomers were preponderantly formed, [Pt(R-tmen){d(GpG)}](+) with 5'-G bound cis to NMe(2) and [Pt(S-tmen){d(GpG)}](+) with 5'-G bound cis to NHMe. The former was shown to have a right-handed R2 orientation of guanines similar to that found in duplex DNA, whereas the latter had a left-handed L1 orientation that modeled cisplatin-d(GpG) adducts within single-stranded DNA. The R2 rotamer was found to be in an equilibrium (as observed using EXSY spectroscopy) with a minor fraction (< or =4%) of a Delta-HT rotamer related to R2 by rotation of the 3'-G about the Pt-N7 bond. The major rotamers R2 and L1 were isolated using reverse-phase HPLC, and their NMR and CD signatures were compared to those of the corresponding rotamers of the less hindered adduct [Pt(dmen)(GpG)](+) (dmen = N,N-dimethylethylenediamine). From this and other comparisons with previously reported platinum dinucleotide complexes, and from molecular modeling, it could be concluded that both steric repulsion between guanine and substituents of the cis amino group and N-H...O6 hydrogen bonding are significant effects favoring the oblique orientation of one guanine base typical of the HH rotamers of [Pt(diamine){d(GpG)}](+) and [Pt(diamine)(GpG)](+) complexes.

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate Zn complex with all of the carbonyl groups of the amido arms bound and a five-coordinate Cu complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the Cu/Cu couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the Cu complex to O at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide Cu complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.