Zinc(ii), iron(ii/iii) and ruthenium(ii) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Abstract

Reactions of benzoyl pyridine, o-phenylenediamine and anhydrous ZnX₂ in methanol afford imine complexes [Zn(L₁)X₂] (X = Cl, 1; X = Br, 2) in good yields (L₁ = (E)-N¹-(phenyl(pyridin-2-yl)methylene)benzene-1,2-diamine). The reduction of 1 with NaBH₄ affords (E)-N¹-(phenyl(pyridine-2-yl)methylene)benzene-1,2-diamine (L₂H). The reaction of L₂H with [Ru^II(PPh₃)₃Cl₂] results in the oxidative dehydrogenation to L₁ generating cis-[Ru^II(L₁)(PPh₃)Cl₂] (3). The reaction of L₂H with salicylaldehyde affords (E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol (L₃H₂). The reaction of L₃H₂ with anhydrous FeCl₃ in CH₃OH affords cis-[Fe^III(L₃H⁻)Cl₂] (4). Reaction of L₃H₂ with [Ru^II(PPh₃)₃Cl₂] results in the oxidative dehydrogenation to diimine, L₄H, affording trans-[Ru^II(L₄⁻)(PPh₃)₂]^+, which is isolated as trans-[Ru^II(L₄⁻)(PPh₃)₂]PF₆ (5⁺PF₆⁻) (L₄H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino)methyl)phenol). The reduction of L₃H₂ with NaBH₄ produces 2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)amino)methyl)phenol (L₅H₃). With iron(III) L₅H₃ undergoes oxidative dehydrogenation to L₃H₂ affording 4, while with [Ru^II(PPh₃)₃Cl₂], L₅H₃ undergoes 4e + 4H⁺ transfer giving 5⁺. A fluid solution of L₃H₂ at 298 K exhibits an emission band at 470 nm (λ_ex = 330 nm, τ₁ = 3.70 ns) and a weaker band at 525 nm (λ_ex = 330, 390 nm, τ₁ = 1.1 ns) at higher concentrations due to molecular aggregation, which are temperature dependent. 4 is brightly emissive (λ_ex = 330 nm, λ_em = 450 nm, Φ = 0.586, τ₁ = 3.70 ns). Time resolved emission spectra (TRES) and lifetime measurements confirm that the lower energy absorption band of L₃H₂ at 390 nm, which is absent in complex 4, has a larger non-radiative rate constant (k_nr). The redox innocent Al^III adduct of L₃H₂ is fluorescent (λ_ex = 330 nm, λ_em = 450 nm, τ₁ = 3.70 ns). On the contrary, the cis-[Fe^II(L₃H⁻)Cl₂]⁻ and cis-[Co(L₃H⁻)Cl₂]⁻ analogues are non emissive. Density function theory (DFT) calculations, redox potentials and the near infra-red (NIR) absorption data prove that 4 is emissive due to the stable [Fe^III(L₃H⁻*)] state, while 3, 5⁺, cis-[Fe^II(L₃H⁻)Cl₂]⁻ and cis-[Co(L₃H⁻)Cl₂]⁻ are non-emissive due to transformations of the [M^II(L*)] to [M^III(L˙⁻*)] states.