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The reaction of PPh2(C≡CFc) (Fc = Fe(η5-C5H4)(η5-C5H5)) (1) with Fe2(CO)9 (2) and Co2(CO)8 (3) afforded Fe(CO)4(PPh2(C≡CFc)) (4), Fe(CO)3(PPh2(C≡CFc))2 (5), Fe2(CO)6(μ,η2-C≡CFc)(μ-PPh2) (6) (reaction of 1 with 2), PPh2((η2-C≡CFc)Co2(CO)6) (7) and Co2(CO)7(PPh2((η2-C≡CFc)Co2(CO)6)) (8) (reaction of 1 with 3). Treatment of 4 with one equiv of 3 produced Fe(CO)4(PPh2((η2-C≡CFc)Co2(CO)6)) (9). This compound was also obtained when 7 was reacted with 2. All compounds were characterized by NMR, UV–Vis and IR spectroscopy, high resolution ESI-TOF mass-spectrometry and elemental analysis. The structures of 4 and 6–8 in the solid state were determined by single crystal X-ray structure analysis. These studies verified that 1 acts as a κP-ligand to a Fe(CO)4 fragment in 4. In 7 and 8 the C≡C unit is μ-κC:κC′ coordinated to a Co2(CO)6 fragment, and additionally for 8 the PPh2 unit is datively-bonded to a Co2(CO)7 moiety. By the reaction of 1 with 2 at elevated temperature, the P C bond in 1 is cleaved and hence 6 is formed in which the Ph2P unit is μ-1:2κ2P coordinated and the FcC≡C moiety is bonded in a μ-1:2κ2C1,κC2 fashion to a Fe2(CO)6 (Fe Fe) entity. The electronic properties of 4–6 and 9 were studied by cyclic and square-wave voltammetry. The replacement of a CO ligand in 4 with a 2nd PPh2(C≡CFc) ligand induces electrochemical reversibility of the Fe(CO)3 moiety in 5. As compared to non-coordinated 1, a cathodic shift of the Fc redox potentials is characteristic for 6 and 9, respectively, whereas an anodic shift is observed for 4 and 5.

The reaction of FcC≡C–C≡CFc [Fc = Fe(η5-C5H4)(η5-C5H5)] (1) with Co(η5-C5H5)(CO)2 (2) afforded ferrocenyl-functionalized cyclobutadiene and cyclopentadienone cobalt(I) compounds as well as multiferrocenyl benzene derivatives. The synthesis procedures are described. Eleven products could be separated by column chromatography and were characterized by NMR, UV/Vis, and IR spectroscopy, high-resolution ESI-TOF mass spectrometry and elemental analysis. For five representatives, the structure in the solid state was determined by single X-ray structure analysis. The electronic properties of the appropriate compounds were studied by using cyclic and square-wave voltammetry. Further investigation of the interaction between FeII/FeIII centers in the mixed-valent species was achieved by in situ UV/Vis/NIR spectroelectrochemistry. These measurements demonstrated that weak electronic metal–metal interactions through the cobalt-coordinated cyclobutadiene building block occur (weakly coupled class II systems according to the classification of Robin and Day), whereas the cyclopentadienone core acts as an insulator (class I), and hence only electrostatic interactions are characteristic.

Multiferrocenyl-substituted benzenes could be obtained in a “one-pot” reaction of 1,4-diferrocenylbutadiyne with substoichiometric amounts of [CoCp(CO)2] (Cp = cyclopentadienyl) by combining cobalt-assisted formal [2+2+2] cycloaddition with C–H bond activation at a single catalyst. A mechanism for the reaction is discussed. The newly synthesized hexaferrocenyl species belongs to the rather scarcely examined family of multimetallic aromatic compounds. It is one of the few examples in which the ferrocenyl units are either single bonded or 1,2-substituted to give rise to planar chirality at three of the ferrocenyl units. Electrochemical studies showed that up to six consecutive ferrocenyl-based redox events are observed. Within in situ spectro-electrochemical measurements of both compounds it is shown that two intervalence charge-transfer pathways are possible. At different excitation energies either a charge transfer through the peripheric ethylene bridging unit or through the central benzene unit occurs.

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 FcC[triple bond, length as m-dash]CH (3) in the molar ratio of 1 : 4 afforded solely 1,3,5-Cl3-2,4,6-(FcC[triple bond, length as m-dash]C)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-(FcC[triple bond, length as m-dash]C)-4,6-I2-C6 (4a) and 1,3,5-Cl3-2,4-(FcC[triple bond, length as m-dash]C)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-(FcC[triple bond, length as m-dash]C)3-C6 (6a), 1-Cl-3,5-Fc2-2,4,6-(FcC[triple bond, length as m-dash]C)3-C6 (6b) and 1,3,5-Fc3-2,4,6-(FcC[triple bond, length as m-dash]C)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 FcC[triple bond, length as m-dash]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 = 190, ΔEp = 71; 4b, E°′1 = 195, ΔEp = 59; 4c, E°′1 = 390, ΔEp = 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+–Clδ− 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: νmax = 7860 cm−1, εmax = 405 L mol−1 cm−1, Δν1/2 = 7070 cm−1; n = 2: νmax = 9070 cm−1, εmax = 620 L mol−1 cm−1, Δν1/2 = 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. Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

The luminescence arising from lanthanide cations offers several advantages over organic fluorescent molecules: sharp, distinctive emission bands allow for easy resolution between multiple lanthanide signals; long emission lifetimes (μs - ms) make them excellent candidates for time-resolved measurements; and high resistance to photo bleaching allow for long or repeated experiments. In order to obtain luminescence from lanthanide cations, the cation must be located at close distance to a suitable sensitizer ("antenna"). Two similar methods have been used in our group to develop more efficient lanthanide complexes based on a polymetallic approach to obtain lanthanide compounds with improved luminescence efficiency. The first method involves using dendrimers to combine multiple antennae groups and several lanthanide cations into the same discrete molecule. The second approach involves doping CdSe semiconductor nanocrystals with luminescent terbium cations to use the nanocrystal electronic structure as an antenna to sensitize lanthanide cations. Using nanocrystals as antennae provides a superior way to protect the lanthanide cations from non-radiative deactivations, while providing a variety of controlled donating energy levels. In both methods, it is possible to incorporate several lanthanide metal cations into each dendrimer or nanocrystal, thus increasing the number of emitters and the resulting luminescence intensity of the species.