Synthesis and co-ordination chemistry of 2,2′:4′,2″:6″,2‴-quaterpyridine, an asymmetric bridging ligand with inequivalent bipyridyl binding sites

Abstract

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 ¹H 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]₂1 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(µ-L)][PF₆]₄2 and [{Ru(terpy)Cl}₂(µ-L)][PF₆]₂4 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}(µ-L){Ru(terpy)Cl}][PF₆]₃5 was prepared, and undergoes two well separated one-electron oxidations (E_½=+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 (λ_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.