Synthesis, Characterization, and Electrochemistry of Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles

Abstract

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.

Synthesis and Electrochemical Behavior of Ferrocenyl β‐Ketoamines FcC(O)CH=C(NH(C6H4‐4‐R‘))R

“…The appropriate events of 5 a – d are in the same order. The redox separations Δ E°′ with 215 mV ( 5 a ), 220 mV ( 5 b , d ) and 205 mV ( 5 c ) are similar and are, compared to 2 (Δ E°′ =180 mV) only somewhat larger, but correspond to different functionalized diferrocenyl β ‐diketones, which generally show no or less electronic communication in the respective mixed‐valent species [16] . In addition, 5 b and 5 c display an irreversible N ‐oxidation at higher potentials (Figures S6 and S7, see the ESI).…”

“…That the oxidations of 5 a – d are all reversible was verified by subsequent electrochemical reduction of [ 5 a – d ] n+ (n=1, 2) showing the initial UV‐vis/NIR spectra. In contrast, the starting material 2 showed a very weak coupling between the Fc/[Fc] + redox couple ( ϵ max <50 L mol −1 cm −1 ) [16] . This means that the substitution of one β ‐keto functionality with an amine group decreases the electronic interaction between the ferrocenyl moieties.…”

“…In the following years the number of mono‐ferrocenyl substituted β ‐ketoamine compounds increased by using different primary amine derivatives, for example, aniline, [5l] o ‐methylaniline, [5l] p ‐fluoroaniline, [7a] 1,2‐diaminoethane, [10e] p ‐hydroxyaniline [14] and p ‐anisidine [15] . There are only a few examples [15] where both carbonyl functionalities reacted with amines, usually these are cyclic compounds [16,17] . However β ‐ketoamines of type FcC(O)CH=C(NHR)Fc (Fc=Fe( η 5 ‐C 5 H 4 )( η 5 ‐C 5 H 5 ) have not been reported so far.…”

Recent Developments in the Synthesis of β-Diketones

“…Diferrocenyl β-diketones 43 have also been prepared using a Claisen approach (Scheme 7b) [69]. These compounds have been complexed with different metals and converted into the corresponding pyrazoles, and were characterized and analyzed.…”

“…One example was shown in 2019, employing acid chlorides as electrophiles in the condensation process for the preparation of a set of sterically hindered 1,3-diketones 70 containing the dipivaloyl methane group [82], as shown in Scheme 10a. Initial studies were performed in the reaction of the electrophiles pivaloyl chloride or methyl pivalate (68) with sodium pinacolonate (69) to yield the corresponding 1,3-diketone. Different solvents were tested, achieving similar yields (around 65% after 2 h at room temperature) when using pivaloyl chloride in the presence of toluene, DCM, or THF, although toluene was more effective if the reaction was left overnight.…”