Synthetic, electrochemical, spectroscopic and computational aspects of ferrocene derivatives coordinated to subphthalocyanines

Abstract

A series of six ferrocene derivatives, namely the ferrocenylcarboxylic acid dyads (Fc(CH2)nCO2H (n = 0 – 3), Fc(CH)2CO2H and FcCO(CH2)2CO2H) were synthesized and coordinated to subphthalocyanines (SubPcs) in the axial position to form seven novel (Fc(CH2)nCO2BSubPc(H)12 (n = 1 – 3), FcCO(CH2)2CO2BSubPc(H)12, (Fc-CH=CHCOO)BSubPc(H)12 and Y-BSubPc(F)12 with Y = (Fc-CH2-CH2-COO) or (Fc-CH=CH-COO)) and one known (Fc(CH2)nCO2BSubPc(H)12) ferrocenylcarboxylic acid subphthalocyanines dyads. In addition a novel subphthalocyanine HOBSubPc(C12H25)6(H)6 with a hydroxy group in the axial position and alkyl ligands (C12H25) on the non-peripheral positions, as well as the known ClBSubPc(F)12 and the mother compound ClBSubPc(H)12 were synthesized. UV/vis analysis of the SubPcs revealed that both the donor (C12H25) non-peripheral substituents and the acceptor (F) ring substituents shifted the Q band of SubPcs toward longer wavelengths compared to the unsubstituted ClBSubPc. However, the different ferrocenylcarboxylic acid dyads in the axial position did not have any influence on the position of the Q band of the SubPcs. An electrochemical study of the SubPcs showed that peripheral substitution generally has a smaller influence (ca 0.1 V shift) on the shift of the oxidation and reduction potential of a SubPc than non-peripheral substitution (ca 0.3 V shift). It was found that the novel SubPc HOBSubPc(C12H25)6(H)6 had the lowest macrocycle-based oxidation potential and that novel SubPc Fc(CH2)3CO2BSubPc(H)12 has the lowest first ring-based reduction potential reported for SubPcs to date. The experimental conditions used for the electrochemical study made it possible for the first time to obtain chemically reversible ring-based oxidation with peak current ratios approaching 1 and peak current separation ΔEp < 0.086 V for SubPcs. The cyclic voltammetry data of the seven new ferrocenylsubphthalocyanine dyads, in agreement with the character of DFT calculated molecular orbitals, showed that Fe group of the ferrocenyl-containing axial ligand is involved in the first reversible oxidation process, followed by a second ring based oxidation. All reductions are localised on the subphthalocyanine ligand. The fluorine ring substituents of the electron-withdrawing SubPcs Y-BSubPc(F)12, caused the ferrocenyl oxidation to shift with ca. 0.1 V more positive compared to electron-rich Y-BSubPc(H)12. Furthermore, the CV data revealed that the formal reduction potential of Fe(II/III) of the axial ferrocenyl moiety of the eight ferrocenylcarboxylic acid containing SubPc dyads followed the same trend than the free ferrocenylcarboxylic acid dyads, but shifted to a lower oxidation potential compared to free ferrocenylcarboxylic acid dyads. The carboxylic group in the free ferrocenylcarboxylic acid dyads is directly connected to ferrocenyl, or by an ethene group, or by an alkyl chain of varying length. The length of the alkyl chain separating the two groups affected the formal reduction potential of Fe of the ferrocenyl group. The formal reduction potential of Fe was also affected by the electron withdrawing carbonyl group. The extent of the effect depended on whether the carbonyl group was directly bound to a ferrocenyl moiety or isolated by an sp3 hybridised carbon atom backbone (-CH2-CH2- with no π-communication) or a non-isolated sp2 hybridised carbon atom backbone (-CH=CH- with π-communication between Fc and the carbonyl group possible). Density functional theory (DFT) calculations gave further insight into the redox properties of the novel dyads. A linear relationship between the formal reduction potential of these ferrocenyl carboxylic acid derivatives and DFT calculated HOMO (highest occupied molecular orbital) energies were obtained. The DFT study also provided linear relationships between the first oxidation potential and HOMO energies, as well as between the first reduction potential and LUMO (lowest unoccupied molecular orbital) energies for a series of non-ferrocene-containing SubPcs with peripheral and non-peripheral substituents = H, F or (C12H25) and axial substituent = Cl or an alkoxy group. The neutral ferrocenylcarboxylic acid subphthalocyanines dyads have LUMOs and HOMOs of mainly -ring and iron-d character respectively, confirming ring-based reduction and Fe(II) to Fe(III) oxidation. Optimisation of the cation (oxidised) species was essential to verify the locus of the second ring-based oxidation, since the frontier orbitals rearranged upon oxidation.