Tuning of intramolecular charge transfer properties and charge distributions in ferrocene-appended catechol derivatives by chemical substitution

Abstract

In this study, we report intramolecular charge transfer (ICT) properties and charge distributions in a series of FcC derivatives (FcC = 4-ferrocenylcatecholate where Fc = ferrocene and C = catecholate). This series consists of a previously reported complex FcV (4-ferrocenylveratrole) and newly synthesized complexes FcA (4-ferrocenylcatechol bis(acetate) and Pt(^tBu₂bpy)(FcC) (^tBu₂bpy = 4,4′-di-tert-butyl-2,2′-dipyridyl). An electrochemical analysis of Pt(^tBu₂bpy)(FcC) using cyclic voltammetry revealed two well-defined, reversible waves which were assigned to the sequential oxidation of the Pt(^tBu₂bpy)(C) and Fc moieties. The potential splitting between the waves (524 mV) indicated that there was an electronic interaction between both moieties. ICT property and charge distribution of [Pt(^tBu₂bpy)(FcC)]˙⁺ were rationalized by comparison with the [FcV]˙⁺ and [FcA]˙⁺ (4-ferrocenylcatechol bis(acetate)). DFT calculations and UV-vis-NIR spectroscopy revealed that [Pt(^tBu₂bpy)(FcC)]˙⁺, [FcV]˙⁺, and [FcA]˙⁺ were ferrocenium (Fc⁺)-centered rather than semiquinone ligand-centered and that these complexes exhibited ICT transition bands from the catechol-derivatized framework to the Fc⁺ moiety in the near infrared (NIR) region. Both the electronic coupling parameter (H_AB) and delocalization parameter (α) increased in value as the electron-donating strength of the substituent groups in the catechol-derivatized framework increased (OCOCH₃ ([FcA]˙⁺) < OCH₃ ([FcV]˙⁺) < O⁻ ([Pt(^tBu₂bpy)(FcC)]˙⁺)). The electronic interactions between the organometallic center and the non-innocent framework were tuned by changing the substituents. The potential energy surfaces of the Fc⁺ derivatives, obtained using two-state Marcus–Hush theory, can be modulated by changing the energy level of the molecular orbitals of the appended catechol-derivatized moieties.