Synthesis, Characterization, Electrochemistry, and Computational Studies of Ferrocenyl-Substituted Siloles

Abstract

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.

Ferrocenyl‐Pyrenes, Ferrocenyl‐9,10‐Phenanthrenediones, and Ferrocenyl‐9,10‐Dimethoxyphenanthrenes: Charge‐Transfer Studies and SWCNT Functionalization

“…The diferrocenyl‐substituted compounds 5 b , 7 b , 10 , and 12 exhibit an anti ‐positioning of the Fc groups towards each other with regard to the aromatic planes . Furthermore, the Fe⋅⋅⋅Fe distances were calculated, ranging from 9.0374(13) ( 10 ) to 12.7288(8) Å ( 7 b ).…”

“…Ferroceneb oronic acid FcB(OH) 2 (1;F c= Fe(h 5 -C 5 H 4 )(h 5 -C 5 H 5 )) was treated with the respective 1-Br-(2), 1,6-Br 2 -( 4), 2,7-Br 2pyrene (6), or 3,6-Br 2 -9,10-phenanthrenedione (9)i nt he presence of [PdCl 2 (dppf)] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) as the catalysti nt he molar ratio of 1:1o r2:1 in boiling toluene. After appropriate work-up, compounds 3, 5a,b, 7a, and 10 could be isolated as orange (3, 5a,b, 7a)o rg reen (10) solids in ay ield of 3-65 %( Experimental Section). For comparison, compound 9-ferrocenylphenanthren wass ynthesized accordingt ot he Suzuki CÀCc ross-coupling protocol used within the synthesis of 3, 5a,b, 7a,a nd 10 (for more details, see the Supporting Information, Reaction SI1).…”

“…They are easy to functionalize at the heterocyclic or aromatic hydrocarbon core and/or the redox‐active moiety, enabling the straightforward modification of the electronic properties. In this respect, we recently focused on the synthesis and (spectro)electrochemical behavior of, in particular, ferrocenyl‐substituted heterocycles featuring SiR 2 , NR, PR, O, S, ‐ c C 2 N 2 S, TiCp 2 , or ZrCp 2 (R=H, organic group; Cp=η 5 ‐C 5 H 5 ) constituents. With the example of 2,5‐Fc 2 ‐ c C 4 H 2 E (Fc=Fe(η 5 ‐C 5 H 4 )(η 5 ‐C 5 H 5 ); E=O, S, NR), it could be shown that similar geometries and hence comparable electrostatic interactions enable a correlation between electrochemical and spectroscopic properties …”

Transition-Metal Carbonyl Complexes of 2,5-Diferrocenyl-1-phenyl-1H-phosphole

“…Anal. Calcd for C 35 Synthesis of Pentacarbonyl(2,5-diferrocenyl-1-phenyl-1H-phosphole)tungsten (3c). Using the general synthesis procedure described above, 0.11 g (0.20 mmol) of 1 was reacted with 0.07 g (0.20 mmol) of 2c to afford 3c.…”

Multiferrocenyl Cobalt‐Based Sandwich Compounds

“…The electrochemical properties of 3a , 5a , b , 5c , 6a – c , and 7 were studied by using cyclic voltammetry (CV) and square‐wave voltammetry (SWV) and by in situ UV/Vis/NIR spectroelectrochemistry ( 3a and 6a – c ) (Figures , , , , and S68–S70; Tables and ). The electrochemical measurements were performed under argon in dichloromethane solutions containing [ n Bu 4 N][B(C 6 F 5 ) 4 ] (0.1 mol L –1 ) as supporting electrolyte , , . The weak coordinating counterion [B(C 6 F 5 ) 4 ] – is known to stabilize highly charged species in solution.…”