STRUCTURAL STUDIES ON PYRAZOLYLPYRIDINE LIGANDS AND COMPLEXES. 2. COMPARISONS BETWEEN BISPYRAZOLYLPYRIDINE LINKAGE ISOMERS AND WITH 2,2′,6′,2″-TERPYRIDINE JERZY ZADYKOWICZ

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Abstract
Crystal structures were obtained for the 3(C),2′;6′,3″(C)-linked bispyrazolylpyridines 2,6-di(2H-4,5,6,7-tetrahydroindazol-3-yl)pyridine (1), 2,6-di(l-methyl-4,5,6,7-tetrahydroindazol-3-yl)pyridine (2), 2,6-di(1 -(4-ethoxycarbonylphenyl)-4,5,6,7-tetrahydroindazol-3-yl)pyridine (3) and for the homoleptic RuII complex of 2, [Ru(2)2]Cl2, which crystallized with 7 molecules of CHCl3. Ligand 1 adopts the inter-and intramolecularly hydrogen-bonded syn,syn rotameric conformation, while 2 and 3 were in the anti,anti forms. Relative to the latter, iigand distortions were assessed in 1 (considered as a H+ complex) and [Ru(2)2]Cl2. Comparisons were drawn with other tridentate ligands containing a pyridine nucleus, specifically the 1(N),2′;6′,1″(N″) linkage isomers and 2,2′;6′,2″-terpyridine, in both free and RuII complexed forms, as well as with their bidentate analogues. Unlike with bidentate ligands, the bonds to the pyridine moiety are shortest, the outer heterocyclic rings are drawn inward and, overall, the ligands remain fairly planar. Flanking substituents remain well splayed out in the 1,2′;6′,1″-linked bispyrazolylpyridines, are more parallel in the 3,2′;6′,3″ linkage isomers and are unfavorably compressed in terpyridines.

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