No abstract

The classical metallocenes, ferrocene and ruthenocene, dissolve readily in neat ethyl 2-cyanoacrylate (CA) monomer. The electronic spectra of the resulting solutions display a near-ultraviolet absorption band assigned as a charge-transfer-to-solvent (metallocene → CA) transition. Irradiation into this band causes the one-electron oxidation of the metallocene to the corresponding metallocenium cation accompanied by reduction of CA to its radical anion. Addition of the latter species to CA initiates the anionic polymerization of the electrophilic monomer. The progress of photoinitiated polymerization was monitored in real time by attenuated total reflectance infrared spectroscopy.

Resonance Raman spectroscopy has been employed to probe the excited-state distortions associated with the low-energy electronic transition of benzoylferrocene and 1,1'-dibenzoylferrocene. Resonance intensity enhancement of in-plane ligand modes, in general, and the carbonyl stretching mode, in particular, supports the proposal that the excited state populated by this transition contains appreciable metal-to-ligand charge transfer character. The redistribution of charge that occurs upon populating this state weakens the metal-ring bonding and facilitates the loss of a benzoylcyclopentadienide anion. This photochemical reaction has been investigated by an on-line electrospray ionization mass spectrometry technique that allows direct detection of primary and secondary products with solution lifetimes down to the millisecond range.

Electron affinities of methyl-2-cyanoacrylate (MCA) and ethyl-2-cyanoacrylate (ECA) were predicted using four different density functional or hybrid Hartree-Fock/density functional methods. Equilibrium structures and harmonic vibrational frequencies were computed for the neutral and anionic species of each system. Because of structural similarities to cyanoethylene and 1,1-dicyanoethylene, the results for MCA and ECA are compared to previous work at the same levels of theory on these two cyano-substituted ethylenes. Spin densities and isotropic hyperfine splitting constants (hfs) were determined for the radical anions of these four systems as a gauge of the delocalization of the unpaired electron. An experimental EA exists for 1,1-dicyanoethylene, although collisional electron transfer experiments on short-lived anions are questionable. The EAs for the four systems studied here are predicted to be 0.06 (cyanoethylene), 1.36 (1,1-dicyanoethylene), and 1.08 eV (MCA and ECA). The computed spin densities and hfs constants for these radical anions indicate that the unpaired electron is primarily localized on the C atom of the CH 2 segment of each molecule. 1,1dicyanoethylene, MCA, and ECA each have two electron withdrawing groups, which results in appreciable resonance stabilization and a bound radical anion. The radical anions of MCA and ECA are nucleophilic and can attack the neutral monomer to initiate polymerization by a newly proposed mechanism.

No abstract

Electronic absorption and resonance Raman spectral studies of benzoylruthenocene (BRc) and 1,1'-dibenzoylruthenocene (DRc) indicate that the low-energy electronic excited states of these 4d(6) metallocenes possess metal-to-ligand charge transfer (MLCT) character. While this MLCT contribution should weaken the metal-ring bonding in the excited state, neither compound is photosensitive in nonhalogenated solvents such as methanol, acetonitrile, and cyclohexane. In contrast, irradiating BRc and DRc in the good electron-accepting solvent, carbon tetrachloride, results in ring loss via a pathway that appears to originate from a charge-transfer-to-solvent excited state. Both metallocenes function as photoinitiators for the anionic polymerization of ethyl 2-cyanoacrylate, and the kinetics and mechanism of this process have been investigated. Comparing the present results on BRc and DRc with those reported earlier for the corresponding benzoyl-substituted ferrocene compounds reveals some interesting commonalities and differences between the excited-state properties of these 3d and 4d metallocenes.