A new heterometallic terbium(iii)–ruthenium(ii) complex and its terbium(iii)–zinc(ii) analog: syntheses, characterization, luminescence, and electrochemical properties

Abstract

Synthesis of a d–f heterometallic trinuclear complex [Tb(NO₃)₂(L²){Ru(ttpy)}₂](PF₆)₅ (6) (L² = N¹,N²-bis(2-((Z)-2-(4-(2,2′:6′,2′′)-terpyridin-4′-yl-benzyloxy)benzylideneamino)ethyl)ethane-1,2-diamine; ttpy = 4′-tolyl-2,2′:6′,2′′-terpyridine) and intramolecular energy transfer from terbium(III) to ruthenium(II) at room temperature are reported. 2-(4-(2,2′:6′,2′′)-Terpyridin-4′-yl-benzyloxy)benzaldehyde (L¹) is characterized by X-ray diffraction: triclinic, P. The terbium(III) complex [Tb(NO₃)₂(L²)]NO₃·5H₂O (2) is synthesized by Schiff base condensation of triethylenetetraamine with L¹ in the presence of Tb(NO₃)₃·5H₂O as the template as well as by the reaction of the preformed Schiff base L² with Tb(NO₃)₃·5H₂O. The dinuclear complex [{Ru(ttpy)}₂(L²)](PF₆)₄ (5) is synthesized by Schiff base condensation of triethylenetetraamine with the mononuclear complex [Ru(L¹)(ttpy)](PF₆)₂ (4). The terbium(III) complex 2 exhibits a set of seven emission bands at 490, 545, 585, 622, 651, 668, and 684 nm characteristic of the ⁵D₄ → ⁷F_0–6 transitions of terbium(III). The Tb^III–Ru^II d–f heterometallic complex 6 exhibits emission at 687 nm characteristic of the ruthenium(II) tolylterpyridine molecular components. Ruthenium(II) luminescence is sensitized at room temperature by intramolecular intercomponent energy transfer from the luminescent ⁵D₄ metal centered excited state of terbium(III) to the ³MLCT state of ruthenium(II) with a concomitant increase in the ruthenium(II) luminescence lifetime of 1.20 ns. The Tb^III–Zn^II heterometallic assembly [Tb(NO₃)₂(L²){Zn(ttpy)}₂](PF₆)₅ (7) exhibits emission bands characteristic of the terbium(III) center. The results indicate photoinduced intramolecular energy transfer from the ttpy moieties of the [Zn(ttpy)₂]²⁺ molecular components to the terbium(III) center. This study demonstrates the use of the terpyridine derivative appended terbium(III) complex as a synthon for the construction of heterometallic d–f assemblies. The two ruthenium(II) centers of 6 undergo electrochemical oxidation at the same potential (the E_1/2 for the Ru(II)/Ru(III) redox couple is 0.93 V) indicating their electrochemical equivalence.