Covalent tripod

The capping of both termini of three tridentate chelating units with covalent tripods provides highly rigid macrobicyclic receptors (i.e., cryptands), which are ideally suited for investigating variations of crystal-field parameters within isostructural series. Platas et al. (1999) have iso-... 

The two-compartment ligand 5 consists of three strands, each containing a hydroxamate subunit and a bpy subunit, covalently linked to a tripodal platform. In the system illustrated in Fig. 2.3, the platform is provided by the tetramine tren. 

The perylene chromophotes were modified by covalently attaching different side groups. Two bulky groups (tertiary-butyl), indicated by a prime, prevented dimerization of neighbouring perylene chromophores when attached to the surface. Linkage to the semiconductor was achieved by different anchor-cum-spacer groups carbonic acid (-COOH) [4], propionic acid (-CH2-CH2-COOH) [2], phosphonic acid (-P(0)(OH)2) [4], methyl-phosphonic acid (-CH2-PO(OH)2) [2] and a -Tripod (see inset in Fig.2) [5], The preparation and sensitization of the nano-structured TiC>2 film and further experimental details have been described before [2]. 

In order to limit flexibility, three tridentate binding units similar to L3 have been connected to a covalent tri(2-aminoethyl)amine (TREN) tripod to give the nonadentate podand L4. Upon reaction with /J(III) (R = La-Eu, Yb, Lu, Y) in acetonitrile/nitromethane (1 1), the NMR spectra confirm the formation of the expected C3-symmetrical complexes [/ (L4)]3+ (Re-naud et al., 1999). The methylene protons are systematically diastereotopic at 298 K. For the diamagnetic cations R = Y, Lu, no coalescence is observed in the accessible temperature range (233-343 K), but the rate of helical interconversion increases with increasing ionic radii and [La(L4)]3+ shows coalescence at 323 K, thus pointing to a fast /J- Ea(L4)]3+ = M-... 

La(L4)]3+ isomerisation process on the NMR time scale. Compared to the parent complexes [La(L3)3]3+ which display enantiotopic methylene protons for T > 233 K (see sect. 3.1.3), the introduction of the covalent TREN tripod in [La(L4)]3+ significantly slows down the helical interconversion process. Protonation of the apical nitrogen atom produces the C3-symmetrical podates [R(L4+H)]4+ which display only marginally faster racemisation processes. Although some steric constraints induced by the tripod limit the thermodynamic stability of the podates [f (L4)]3+ and [/J(L4+H)]4+, stability constants confirm their quantitative formation for a total ligand concentration of 0.05 mol dm-3 and a stoichiometric ratio R L4 = 1.0 (Renaud etal., 1999 fig. 17). 

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