Self-assembly of multinuclear coordination species with chiral bipyridine ligands: Silver complexes of 5,6-CHIRAGEN(o,m,p-xylidene) ligands and equilibrium behaviour in solution

The complexation reactions between Ag+ and a series of enantiopure ligands belonging to the CHIRAGEN (from CHIRAlity GENerator) family (L1, L2, L3, ba sed on (-)-5,6-pinene bipyridine) have been studied in solution. It has bee n shown that the length of the bridge plays a fundamental role in the self- assembly processes leading to different compounds: mononuclear complexes (w ith L3), mixtures of polynuclear complexes (with L2) and circular helicates (with L1). although the absolute configuration of the chiral centres ill a ll three ligands is the same, the metal-centred chirality of L3 (Delta) is inverted with respect to that in the other two complexes with L1 and L2 (De lta). The metal configuration is thus opposite in the mononuclear complex w ith respect to the polynuclear species. Detailed thermodynamic studies were carried out for the Ag+ and LI ligand system by H-1 and Ag-109 NMR spectro scopy (as a Function of concentration, temperature and pressure). At low te mperature and high pressure, the [Ag(6)L1(6)](6+) hexa-nuclear circular hel icate forms a tetranuclear circular helicate [Ag(4)L1(4)](4+); 2[Ag(6)L1(6) ](6+)=(3)[Ag(4)L1(4)](4+). The thermodynamics parameters, obtained by tempe rature and pressure variation, have the following values: K-298 = (8.7 +/- 0.7) x 10(-5) mol kg(-1), DeltaH(o) = -15.65 +/- 0.8 kJ mol(-1), DeltaS(o) = -130.2 +/- 3 J mol(-1) K-1 and DeltaV(0)(256 K) = -160 +/- 12 cm(3)mol(-1 ). The reaction volume calculated according to Connolly's method indicates that the calculated structure of [Ag4L1(4)](4+) is plausible. Both the sign s and large magnitudes of DeltaS(o) and DeltaV(o) are counterintuitive, yet can be understood by modelling methods.