Metalla-calix arenes

The chemistry associated with the metal-oxo-surfaces modeled by the metalla-calix[4]arene at the molecular level will be reviewed according to subdivisions in the following sections. 

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

The reduction of a high-valent metalla-calix[4]arene in the presence of olefins, dienes, and acetylenes is probably the best, though not unique, synthetic access to the corresponding complexes. A few significant examples have been so far reported. 

Other synthetic approaches have been explored for binding an alkylidene functionality to a metalla-calix[4]arene. Among them, the reaction of diazoalkanes with coordinatively unsaturated metalla-calix[4]arenes deserves particular mention. The synthesis of an unusual high-spin (5.2 BM at 292 K) iron(II)-carbene, 192, is displayed in Scheme 39,13 and its structure is shown in Fig. 22. 

The active species Nb=Nb, 1, can perform the six-electron reduction of N2 without the addition of any further reducing agent when the reaction is carried out in toluene. The reaction leads to the trinuclear 2-bisnitrido species, which is rather labile in the presence of solvents binding alkali cations, leading to the compounds 7 and 8 (Scheme 4). A detailed report has been published on why the solvent can drastically affect the reduction pathway of N2 mediated by metalla-calix[4]arenes. The bifunctionality of the complexes used is such that the solvation of the alkali cation can be an important driving force and at the same time the presence of tight-ion pair or separated-ion forms can affect the kinetic pathways (see Chart 3). 

Fig. 24.20 (a) Analogy between calix[4]arenes and metallacalix[4]arenes [60]. (b) Hybrids between organic (hetero)calix[4]arenes and Pt(II)-coordinated metalla-calix[4]arenes (55 and 56) [62]. (c) Platinum(II)-based metallacalix[4]arenes (57-60) interconverting between 1,3-alter-nate conformation at low temperatures and cone conformation at higher temperatures [66]... 

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