Chelation-Controlled Regioselectivity in the Synthesis of Substituted Pyrazolylpyridine Ligands. 3. Unsymmetrically Substituted Tridentates and Ditopic Bis(tridentates) en Route to Singly Stranded Helicates†

A strategy for the synthesis of helical multinuclear complexes (helicates) of any length is outlined, making use of ditopic ligands to bridge between metals and monotopic ligands to cap the chain's termini. We present the syntheses of both ditopic and monotopic ligands with tridentate binding units for octahedral metals, in liposoluble and functionalized varieties. Of special interest are ligands bearing benzoic acid or toluic acid side-chains which can ionize and serve as counteranions. Several singly N(1)-substituted 2,6-bis(4,5,6,7-tetrahydroindazol-3-yl)pyridines were prepared by regioselective, chelation-controlled mono-N-alkylation or -arylation of the tautomeric N,N‘-H2 parent compound. These were then either coupled with CH2 linkages to provide the ditopic ligands or were further N-functionalized to give unsymmetrically substituted capping ligands. The regiochemistry of the N-substitutions and the ring-to-ring conformations were ascertained by 1H NMR spectra with and without added complexands (H+,...