Ferrocenylmaleimides have been synthesized from 2,5‐dibromo‐N‐methyl‐1H‐pyrrole. Bromine shift and oxidation of the pyrrole core with subsequent ferrocenylation using the Negishi C–C cross‐coupling protocol led to the formation of 3‐ferrocenyl‐N‐methylmaleimide (3), 3‐bromo‐4‐ferrocenyl‐N‐methylmaleimide (4), and 3,4‐diferrocenyl‐N‐methylmaleimide (5). The structural properties of 4 and 5 were investigated by single‐crystal X‐ray diffraction. Cyclic and square‐wave voltammetry, in situ UV/Vis/NIR and IR spectroelectrochemistry (5) highlight the electrochemical properties of these compounds. Compounds 3 and 4 exhibit one reversible ferrocenyl‐based redox event, whereas 5 shows two separate electrochemically reversible one‐electron transfer processes with remarkably high ΔE°′ values and reduction potentials of E1°′ = 50 and E2°′ = 380 mV (ΔE°′ = 330 mV), respectively, with [NBu4][B(C6F5)4] as the supporting electrolyte. The NIR measurements confirm the electronic communication between the iron centers (FeII/FeIII) as intervalence charge transfer absorptions were observed in 5+. Compound 5 was classified as a weakly coupled class II system, according to Robin and Day. UV/Vis investigations of the solvatochromic behavior of 3–5 revealed the complex solvation of these push–pull systems, which reflects that three important solvent properties (hydrogen bond formation ability, polarizability, and solvation of the carbonyl group and the C=C bond) affect $\tilde {\nu}$max.

The temperature-dependent behavior within anionic phospho-Fries rearrangements (apFr) of P(O)(OFc)(EAr) (Fc = Fe(η-CH)(η-CH); E = O; Ar = phenyl, naphthyls, (R)-BINOL, [3]ferrocenophanyl; E = N, 1H-pyrrolyl, 1H-indolyl, 9H-carbazolyl; n = 1-3) is reported. While Fc undergoes one, the Ph-based apFr depends on temperature. First, the aryls are lithiated and rearranged, followed by Fc and N-heterocycles. Addition of MeSO thus gave methylated Fc, contrary to non-organometallic aromatics giving mixtures of HO and MeO derivatives. The (R)-BINOL Fc phosphate gave Fc-rearranged phosphonate in 91% de. Exchanging O- with N-aliphatics prevented apFr, due to higher electron density at P. Also 1,2-N→C migrations were observed. X-ray analysis confirms 1D H bridge bonds for OH and NH derivatives. The differences in reactivity between N-aliphatic and N-aromatic phosphoramidates were verified by electrochemistry. The redox potentials revealed lower values for the electron-rich aliphatics, showing no apFr, preventing a nucleophilic attack at P after lithiation. Redox separations for multiple Fc molecules are based on electrostatic interactions.

Ferrocenylsiloles of the type 2,5-Fc 2-3,4-Ph 2-c C 4 SiR 2 (3a, R = Me; 3b, R = Ph) have been prepared by reductive cyclization from diethynyl silanes, followed by ferrocenylation using the Negishi C,C cross coupling protocol with the silole ring serving as either the vinyl halogenide species or as zinc organic component and the complementary functionality introduced on the ferrocenyl moiety. The electrochemical behavior of these silacyclic-bridged bis(ferrocenyl) complexes was investigated by cyclic and square wave voltammetry, and the nature of the redox products studied by in situ UV-vis/NIR spectroelectrochemical measurements. Each of 3a and 3b undergoes two sequential ferrocenyl-based redox processes, the separation of each (E°' = E 2°'-E 1°' = 300 mV (3a); 280 mV (3b)) is in the range of structural similar systems such as 2,5-diferrocenyl-1-phenyl-1H-phosphole (280 mV) and 2,5-diferrocenylfuran (290 mV). Interestingly, the more electron-rich silole 3b, compared to 3a, shows a modestly lower redox separation between the individual ferrocenyl oxidation processes, which may be due to the capacity of this group to shield the effect of an adjacent positive charge. An inter-valence charge transfer (IVCT) absorption was found in the in situ NIR measurements for [3a] + and [3b] + , the analysis of which is consistent with a moderate electronic interaction between the iron atoms through the cis-diene-like fragment of the silole bridge, and allowing description as Robin and Day class II mixedvalence systems. These conclusions are supported by results from quantum chemical calculations, which also reveal the likely presence of a range of molecular conformations in solution. RESULTS AND DISCUSSION Synthesis and Characterisation. Silacyclopentadienes 2a and 2b were synthesized by intramolecular reductive cyclization from dimethyl-bis(phenylethynyl)silane (1a) and diphenylbis(phenylethynyl)silane (1b) with lithium naphthalenide followed by bromination with elemental bromine, forming dibromide 2a, or the reaction with [ZnCl 2 •2thf] giving the zinc organic species 2b (Scheme 1). Applying Negishi-ferrocenylation conditions, the reaction of 2a with FcZnCl (Fc = Fe(5-C 5 H 4)(5-C 5 H 5)) as ferrocenyl source and [Pd(CH 2 CMe 2 P t Bu 2)(µ-Cl)] 2 as precatalyst gave silole 3a. The analog coupling of 2,5-Br 2-3,4-Ph 2-c C 4 SiPh 2 with ferrocenyl zinc chloride did not result in the formation of desired 3b. The synthesis of 3b was realized by a Negishi C,C cross-coupling reaction using iodoferrocene as ferrocenyl source, while the application of bromoferrocene was unsuccessful. After appropriate work up (Experimental Section), molecules 3a and 3b were obtained in moderate (3a) to low (3b) yield as dark red solids.

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The synthesis of FcC(O)CH(R)C(O)Fc (Fc = Fe(η5-C5H4)(η5-C5H5); R = H, 5; nBu, 7; CH2CH2(OCH2CH2)2OMe, 9), [M(κ2O,O′-FcC(O)CHC(O)Fc)n] (M = Ti, n = 3, 10; M = Fe, n = 3, 11; M = BF2, n = 1, 12), and 1-R’-3,5-Fc2-cC3HN2 (R’ = H, 13; Me, 14; Ph, 15) is discussed. The solid-state structures of 5, 7, 9, 12, 13, 15, and 16 ([TiCl2(κ2O,O′-PhC(O)CHC(O)Ph)2]) show that 7 and 9 exist in their β-diketo form. Compound 13 crystallizes as a tetramer based on a hydrogen bond pattern, including one central water molecule. The electrochemical behavior of 5–7 and 9–16 was studied by cyclic and square-wave voltammetry, showing that the ferrocenyls can separately be oxidized reversibly between −50 and 750 mV (5–7, 9, 12–15: two Fc-related events; 10, 11: six events, being partially superimposed). For complex 10, Ti-centered reversible redox processes appear at −985 (TiII/TiIII) and −520 mV (TiIII/TiIV). Spectro-electrochemical UV-Vis/NIR measurements were carried out on 5, 6, and 12, whereby only 12 showed an IVCT (intervalence charge-transfer) band of considerable strength (νmax = 6250 cm−1, Δν½ = 4725 cm−1, εmax = 240 L·mol−1·cm−1), due to the rigid C3O2B cycle, enlarging the coupling strength between the Fc groups.