Synthesis and characterization of linear and heterocyclic bis(ferrocenyl) containing compounds

Abstract

The new and unexpected organometallic mercury compound has been identified in the palladium-catalyzed reaction between bis(ferrocenyl)mercury and dimethylacetylene-dicarboxylate along with the expected dimethyl-(Z)-2,3-di(ferrocenyl)-2-butenedioate and biferrocene. Dimethyl-(Z)-2,3-di(ferrocenyl)-2-butenedioate resulted from syn-addition of 1 to 2 equivalents of dimethylacetylene-dicarboxylate. Dimethyl-(Z)-2,3-di(ferrocenyl)-2-butenedioate has been characterized using NMR spectroscopy, while its structure was determined by X-ray crystallography. Dimethyl-(Z)-2,3-di(ferrocenyl)-2-butenedioate represents a rare example of an organomercury compound in which a mercury ion is coordinated to two vinylic fragments. X-ray study of the mercury-containing reaction intermediate reveals new insight on the palladium catalyzed reaction mechanism. Accurate, low-temperature experimental data for the crystal structure of biferrocene is also reported. In addition, structural and spectroscopic study of a newly synthesized 3,4-diferrocene substituted pyrrole derivative was characterized. Electronic structures of dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate and 3,4-diferrocene substituted pyrrole were investigated by UV-vis spectroscopy and density functional theory (DFT) as well as time-dependent DFT (TDDFT) approaches. The calculated vertical excitation energies are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand charge-transfer bands in the visible region of the UV-Vis spectra for dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate. Redox properties of dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate and 3,4-diferrocene substituted pyrrole were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical oxidation approaches. In a DCM/(NBu4)[B(C6F5)4] system, ferrocene-centered oxidation processes are separated by 300 mV (dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate) and 298 mV (3,4-diferrocene substituted pyrrole), respectively. Stepwise spectroelectrochemical oxidation of dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate and 3,4-diferrocene substituted pyrrole allowed us to obtain spectroscopic signatures of the mixed-valence dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate and 3,4-diferrocene substituted pyrrole cations. Hush analysis of the intervalence charge-transfer bands in dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate cation is suggestive of class II (in the Robin and Day classification) behavior.