1,3,5-Triferrocenyl-2,4,6-tris(ethynylferrocenyl)-benzene – a new member of the family of multiferrocenyl-functionalized cyclic systems

Abstract

The consecutive synthesis of 1,3,5-triferrocenyl-2,4,6-tris(ethynylferrocenyl)benzene (6c) is described using 1,3,5-Cl₃-2,4,6-I₃-C₆ (2) as starting compound. Subsequent Sonogashira C,C cross-coupling of 2 with FcCCH (3) in the molar ratio of 1 : 4 afforded solely 1,3,5-Cl₃-2,4,6-(FcCC)₃-C₆ (4c) (Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)). However, when 2 is reacted with 3 in a 1 : 3 ratio a mixture of 1,3,5-Cl₃-2-(FcCC)-4,6-I₂-C₆ (4a) and 1,3,5-Cl₃-2,4-(FcCC)₂-6-I-C₆ (4b) is obtained. Negishi C,C cross-coupling of 4c with FcZnCl (5) in the presence of catalytic amounts of [Pd(CH₂C(CH₃)₂P(^tC₄H₉)₂)(μ-Cl)]₂ gave 1,3-Cl₂-5-Fc-2,4,6-(FcCC)₃-C₆ (6a), 1-Cl-3,5-Fc₂-2,4,6-(FcCC)₃-C₆ (6b) and 1,3,5-Fc₃-2,4,6-(FcCC)₃-C₆ (6c) of which 6b is the main product. Column chromatography allowed the separation of these organometallic species. The structures of 4a,b and 6a in the solid state were determined by single crystal X-ray diffractometry showing a π–π interacting dimer (4b) and a complex π–π pattern for 6a. The electrochemical properties of 4a–c and 6a–c were studied by cyclic voltammetry (=CV) and square wave voltammetry (=SWV). It was found that the FcCC-substituted benzenes 4a–c show only one reversible redox event, indicating a simultaneous oxidation of all ferrocenyl units, whereby 4c is most difficult to oxidise (4a, E°′₁ = 190, ΔE_p = 71; 4b, E°′₁ = 195, ΔE_p = 59; 4c, E°′₁ = 390, ΔE_p = 59 mV). In case of 4c, the oxidation states 4cⁿ⁺ (n = 2, 3) are destabilised by the partial negative charge of the electronegative chlorine atoms, which compensates the repulsive electrostatic Fc⁺–Fc⁺ interactions with attractive electrostatic Fc⁺–Cl^δ− interactions. When ferrocenyl units are directly attached to the benzene C₆ core, organometallic 6a shows three, 6b five and 6c six separated reversible waves highlighting that the Fc units can separately be oxidised. UV-Vis/NIR spectroscopy allowed to determine IVCT absorptions (=Inter Valence Charge Transfer) for 6cⁿ⁺ (n = 1, 2) (n = 1: ν_max = 7860 cm⁻¹, ε_max = 405 L mol⁻¹ cm⁻¹, Δν_1/2 = 7070 cm⁻¹; n = 2: ν_max = 9070 cm⁻¹, ε_max = 620 L mol⁻¹ cm⁻¹, Δν_1/2 = 8010 cm⁻¹) classifying these mixed-valent species as weakly coupled class II systems according to Robin and Day, while for 6a,b only LMCT transitions (=ligand to metal charge transfer) could be detected.