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

The consecutive synthesis of 1,3,5-triferrocenyl-2,4,6-tris(ethynylferrocenyl)benzene (6c) is described using 1,3,5-Cl-3-2,4,6-I-3-C-6 (2) as starting compound. Subsequent Sonogashira C,C cross-coupling of 2 with FcC CH (3) in the molar ratio of 1:4 afforded solely 1,3,5-Cl-3-2,4,6-(FcC C)(3)-C-6 (4c) (Fc = Fe(eta(5)-C5H4)-(eta(5)-C5H5)). However, when 2 is reacted with 3 in a 1: 3 ratio a mixture of 1,3,5-Cl-3-2-(FcC C)-4,6-I-2-C-6 (4a) and 1,3,5-Cl-3-2,4-(FcC C)(2)-6-I-C-6 (4b) is obtained. Negishi C,C cross-coupling of 4c with FcZnCl (5) in the presence of catalytic amounts of [Pd(CH2C(CH3)(2)P((C4H9)-C-t)(2))(mu-Cl)(2) gave 1,3-Cl-2-5-Fc-2,4,6-(FcC C)(3)-C-6 (6a), 1-Cl-3,5-Fc(2)-2,4,6-(FcC C)(3)-C-6 (6b) and 1,3,5-Fc(3)-2,4,6-(FcC C)(3)-C-6 (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 pi-pi interacting dimer (4b) and a complex pi-pi 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 FcC C-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 degrees' = 190, Delta E-p = 71; 4b, E-1 degrees' = 195, Delta E-p = 59; 4c, E-1 degrees' = 390, Delta E-p = 59 mV). In case of 4c, the oxidation states 4c(n+) (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 delta- interactions. When ferrocenyl units are directly attached to the benzene C-6 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 6c(n+) (n = 1, 2) (n = 1: nu(max) = 7860 cm(-1), epsilon(max) = 405 L mol(-1) cm(-1), Delta nu(1/2) = 7070 cm(-1); n = 2: nu(max) = 9070 cm(-1), epsilon max = 620 L mol(-1) cm(-1), Delta nu(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.