Ruthenium monoterpyridine complexes incorporating α,α'-diimine based ancillary functions. Synthesis, crystal structure, spectroelectrochemical properties and catalytic aspect

Abstract

Ruthenium monoterpyridine complexes of the type [Ru^II(trpy)(L')(X)](ClO₄)_m.2H₂O (1-2) [trpy=2,2':6',2"-terpyridine; L'=NC₅H₄C(H)=N(C₆H₄)_nNH₂ (n=1 and 2); X=Cl^-, m=1 (1); X=H₂O, m=2 (2)] have been synthesized via the selective hydrolysis of one of the imine functions present in the preformed stable dinucleating bridging functions, NC₅H₄C(H)=N(C₆H₄)_nN=C(H)H₄C₅N (L) (n=1, 2). The single crystal X-ray structures of the dinucleating bridging function (L¹) and the chloro complex (1a) (in both cases n=1) have been determined. The complexes stabilize preferentially in one particular isomeric form where the Cl^- or H₂O molecule is in the trans configuration with respect to the N(imine) center. The chloro complexes (1) exhibit strong MLCT bands near 500 nm whereas in the case of the aqua complexes (2) the MLCT bands are blue shifted near 470 nm. The chloro complexes (1) exhibit weak emissions in EtOH-MeOH (4:1 v/v) glass at 77 K near 600 nm (quantum yield, φ_em=0.015-0.03). In acetonitrile solvent 1 display a ruthenium(III)-ruthenium(II) couple near 0.8 V and terpyridine based reduction near 1 V versus SCE. The aqua complexes (2) exhibit a concerted 2e^-/2H⁺ oxidation process in the acidic region and in the alkaline region, the complexes display a 2e^-/H⁺ oxidation process. The potentials are observed to decrease linearly with the increase in pH. The proton coupled redox processes in the acidic and basic regions correspond to [Ru^II(trpy)(L')(H₂O)]²⁺-[Ru^IV(trpy)(L')(O)]²⁺ and [Ru^II(trpy)(L')(OH)]⁺-[Ru^IV(trpy)(L')(O)]²⁺ couples, respectively. The chemical oxidation of 2 by excess Ce^IV solution in 1 (N) H₂SO₄ also leads to the formation of the corresponding [Ru^IV(trpy)(L')(O)]²⁺ (3). The oxo complexes (3) are stable only in the presence of excess Ce^IV ion, otherwise they slowly catalyze the oxidation of water to dioxygen and return back to the parent aqua species. The electrochemically generated oxo-species are found to catalyze the benzyl alcohol oxidation process.