Coordinated transition metal redox-active macrocycles

Coordinated transition metal redox-active macrocycles, 39 108-124 ammonium cation, 39 128-133 crown ether and bis crown ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithiolene complexes, 39 123-124 metalloporphyrin crown ether compounds, 39 108-109... 

Redox-active macrocycles, 39 79-151, see also Coordinated transition metal redox-active macrocycles... 

C. Coordinated Transition Metal Redox-Active Macrocycles... 

A series of polyether macrocycles (40)-(47) that contain a coordinated reducible transition metal redox-active 16-electron molybdenum nitro-syl Mo(NO) 3+ group have been prepared by us in collaboration with McCleverty and Jones (86,87) (Scheme 14). Compounds (40)-(44) were synthesized from the reactions between [Mo(NO)LX2] (L = tris(3,5-dimethylpyrazolyl)hydroborate X = Cl or I) and the appropriate amine-substituted benzo crown ether. Compounds (45)—(47) were prepared from reactions between [Mo(NO)LI2] and tetra-, penta-, or hexa-ethylene glycol, respectively, in the presence of triethylamine. A crystal structure determination of (45) (88) is shown in Fig. 14. 

A convenient way to append a transition metal to any organic molecule is making use of a cyclam ring. This 14-membered tetramine macrocycle incorporates 3d metal ions, to give complex species that are very stable both in a thermodynamic and a kinetic sense. Moreover, the firmly encircled cation keeps or enhances the interesting features typically presented by transition metals redox activity through fast and reversible one-electron transfer processes, affinity towards ligands (which will go to occupy the two axial positions left available by the co-planarly coordinated tetra-aza macrocycle). Thus, on reaction of 9-bromomethylanthracene with an excess of cyclam, the two-component system 4 was obtained (cyAn). 

Macrocyclic polythioethers form coordination compounds with many transition metal ions [55] and, owing to their moderate rr-acidity (intermediate between that of amines and phosphines), can exert a stabilizing effect on lower oxidation states of the encircled metal [56]. Copper complexes of thiacrowns have been widely investigated, even in view of the fact that they can be considered convenient models in the study of redox properties of cuproproteins (systems whose active site is a copper center bound to the thioether groups of methionine residues [57]). 

The electrochemical process in substituted porphyrins can be more or less localized at the peripheral groups, as demonstrated with wero-pyridyporphyrins coordinated to one to four transition metal complexes [236]. A study involving porphyrin derivatives substituted with diones and tetraones demonstrated that these redox-active sites can function independently of the central macrocycle [237]. The reduction of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin-2,3-diones, 2,3,-12,13-tetraones [L-(P-tetraone)M] and 2,3,7,8-tetraones [C-(P-tetraone)M], (where M is 2H, Cu ", Zn, Ni " and Pd " ) was investigated and the products characterized by ESR and thin-layer UV-Visible spectroelectrochemistry. In diones, the first two reduction processes occur at the peripheral units whereas in the... 

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