Ruthenium-(II)/-(III) terpyridine complexes incorporating imine functionalities. Synthesis, structure, spectroscopic and electrochemical properties

Abstract

A new class of ruthenium terpyridine complexes of the type [Ru^II(trpy)(L^1–6)Cl] 1–6 (trpy = 2, 2′∶6′, 2′′-terpyridine; L^1–3 = o-⁻OC₆H₃(R)C(R′)NCH₂C₆H₅ and L^4–6 = o-⁻OC₆H₃(R)CH(R′)NNC₆H₅; where R = H or p-NO₂ and R′ = H or CH₃) have been synthesized. The “free” ligands incorporating a NH spacer o-⁻OC₆H₃(R)C(R′)NNHC₆H₅ (HL^4–6) have undergone imine to azo tautomerism on co-ordination to the ruthenium terpyridine moiety in the complexes 4–6, whereas those having a CH₂ spacer (HL^1–3) remain unaltered on co-ordination. The diamagnetic, neutral complexes 1–6 exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region. A significant shift in MLCT band energy has been observed depending on the ligand field strength of the co-ordinated L. The complexes display a reversible ruthenium(III)–ruthenium(II) couple in the potential range of 0.12–0.63 V and a quasi-reversible ruthenium(IV)–ruthenium(III) couple in the range of 1.21–1.85 V versus SCE. The higher ligand field strength of the co-ordinated L^4–6 compared to the co-ordinated L^1–3 is reflected in the observed metal redox processes. The reduction of the co-ordinated terpyridine has been observed near −1.3 V. The complexes exhibit moderately strong emissions from the lowest energy MLCT bands in the range 661–690 nm in EtOH–MeOH (4∶1 v/v) at 77 K. The quantum yields of the complexes (Φ = 0.006–0.09) are found to be reasonably sensitive to the nature of the co-ordinated L. The oxidised complexes 3⁺ and 6⁺ have been isolated in the solid state as their perchlorate salts. The crystal structure of 3⁺ exhibits pseudo-octahedral trans geometry with regard to the relative disposition of the imine nitrogen (N4) of L³ and the central pyridyl group of the trpy ligand. The one-electron paramagnetic complexes show 1∶1 conductivity and display ligand-to-metal charge transfer bands near 600 and 400 nm and intraligand transitions in the UV region. The observed rhombic EPR spectra at 77 K corresponding to the distorted octahedral geometry have been analysed to furnish values of axial (Δ) and rhombic (V) distortion parameters as well as the energies of the two expected ligand field transitions (ν₁ and ν₂) within the t₂ shell.