Solvent effects on acetylacetonato iron complexes

Abstract

Interactions of various solvents with Fe(acac) 2+, Fe(acac) 2 + and Fe(acac) 3 (the expression represents the solvated species: acac t- is the 2,4-pentanedionate ion, acetylacetonate ion) were investigated through observing v(FeO) and v(CO) in resonance Raman spectra. In the Fe(acac) 2+ system, both v(CO) and v(FeO) correlate with donor number ( DN): the v(CO) band shifts toward high frequency with the increase in DN, while the v(FeO) band exhibits the inverse trend. The relation between v(CO) and v(FeO) in different solvents is a good example of the bond length variation rules (the donor-acceptor concept). In the Fe(acac)3 system, i.e. the non-direct solvation system, v(CO) shifts toward low frequency with increase in the relative dielectric constant (ϵ r) of the solvent, whereas v(FeO) is constant, independent of ϵ r These facts indicate that the CO bond of the acacligand is lengthened by the polarizability effect of the solvents, while the FeO bond in the inside of the complex is not influenced. On the other hand, v(CO) and v(FeO) do not correlate with ϵ r in the Fe(acac) 2+ system. They indicate that the direct effect (donor effect) of the solvent molecules on the metal is larger than macroscopic effects such as polarizability. In the Fe(acac) 2 + system, the v(CO) shift exhibits a similar trend to that in Fe(acac)3, whereas the shift of v(FeO) is similar to that in Fe(acac) 2 +. The results suggest that Fe(acac) 2 + is influenced by both the macroscopic and direct effects of the solvents. To confirm this in the Fe(acac) 2 + system, solid samples were prepared by freeze drying the solutions and measuring their resonance Raman spectra.