Oxidovanadium(iv), oxidomolybdenum(vi) and cobalt(iii) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Abstract

Reactions of o-phenylenediamine derivatives (L₃H₂) incorporating a (Ph)(Py)(H)C–N(H)– function with the oxidovanadium(IV) and oxidomolybdenum(VI) ions afford amide complexes of types [V^IVO(L₃²⁻)] (3), [V^IVO(L₃^{t-Bu 2−})] (4) and cis-[Mo^VIO₂(L₃²⁻)] (5) (L₃H₂ = ((E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol); L₃^t-BuH = ((E)-2,4-di-tert-butyl-6-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol)), while the similar reaction of L₃H₂ with the anhydrous CoCl₂ in air results in oxidative dehydrogenation (OD) of the (Ph)(Py)(H)C–N(H)– function, affording a cobalt(III) diimine complex, trans-[Co^III(L₄⁻)Cl₂] (6) (L₄H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino) methyl)phenol), contradicting the participation of the higher oxidation states of the metal ions in OD reaction of amines. 3–6 are characterized by elemental analyses and mass, IR, ¹H NMR and EPR spectra. The molecular geometries of 4·CH₃OH, 5 and 6 were confirmed by single crystal X-ray structure determinations. The V^IV–O_phenolatocis to the VO bond and the V^IVO lengths in 4·CH₃OH are 1.925(2) and 1.612(2) Å. Two cis MoO lengths are 1.710(2) Å and 1.720(2) Å in 5. The aliphatic –C–N– lengths in 4·CH₃OH and 5 are 1.448(3) and 1.479(2) Å, while the same is 1.285(4) Å in 6. DFT calculations on 3 and 6 inferred a significant mixing among d_M and NN-ligand backbone favoring a t₂⁶ state of the metal ion for the OD of the amine fragment to have stronger d_M → π_ketimine^* back-bonding. The π_NHPh → π_aldimine^* transition of L₃H₂ is red shifted in 3 and 4 quenching the emissive π_Phenolato → π_aldimine^* transitions, elucidated by the TD DFT calculations on 3 (and 3⁺). The π_NPh → π_aldimine^* transitions are blue shifted in the oxidovanadium(V) analogues, [V^VO(L₃²⁻)]⁺ (3⁺) and [V^VO(L₃^{t-Bu 2−})]⁺ (4⁺), which are fluorescent (3⁺, λ_ex = 331, λ_em = 444 nm; 4⁺, λ_ex = 339, λ_em = 490 nm) recorded by the fluorescence-spectroelectrochemical measurements in CH₂Cl₂. 5 and 6 emit weakly at 466 and 473 nm (5, λ_ex = 336 nm, ϕ = 0.003; 6, λ_ex = 324 nm, ϕ = 0.027).