SYNTHESIS AND COORDINATION CHEMISTRY OF 2,2'/4',2''/6'',2'''-QUATERPYRIDINE, AN ASYMMETRIC BRIDGING LIGAND WITH INEQUIVALENT BIPYRIDYL BINDING-SITES

Reductive coupling of the radical anion of 2,2'-bipyridine (bipy) with lithium diisopropylamide as reducing agent resulted in the unexpected product 2,2': 4',2'': 6'',2''-quaterpyridine (L) via a 2,4-coupling; its structure was confirmed unambiguously by H-1 NMR spectroscopy. The new asymmetric ligand L has two chelating bipyridyl binding sites which are inequivalent, one being much more sterically hindered than the other. By reaction with 1 equivalent of [Ru(bipy)2Cl2] or [Ru(terpy)Cl3] (terpy = 2,2':6',2''-terpyridine) the mononuclear complexes [Ru(bipy)2L][PF6], 1 and [Ru(terpy)Cl(L)][PF6] 3 could be prepared, in which the unhindered binding site only is occupied; their cyclic voltammograms show one-electron oxidations at +0.90 and +0.42 V respectively vs. the ferrocene-ferrocenium couple, consistent with the presence of {Ru(bipy)3}2+ and {Ru(terpy)(bipy)Cl}+ groups. With 2 equivalents of the same reagents under more forcing conditions the binuclear analogues [{Ru(bipy)2}2(mu-L)][PF6]4 2 and [{Ru(terpy)Cl}2(mu-L)] [PF6]2 4 could be prepared; cyclic voltammetry shows two closely spaced oxidations indicative of a weak electrostatic interaction between the two metal centres. By reaction of 1 (which has a vacant bipyridyl binding site) with [Ru(terpy)Cl3], the 'mixed' binuclear complex [{Ru(bipy)2}(mu-L){Ru(terpy)Cl}] [PF6]3 5 was prepared, and undergoes two well separated one-electron oxidations (E1/2 = +0.89 and +0.40 V vs. ferrocene-ferrocenium) consistent with the presence of the different separate metal centres. Whereas 1 and 2 are luminescent at room temperature in MeCN solution (lambda(em) = 594 and 615 nm respectively), luminescence of the {Ru(bipy)3}2+ group in 5 is completely quenched by the {Ru(terpy)(bipy)Cl}+ group.