Heterometallic Molecular and Ionic Isomers

Numerous descriptions of structural isomerism in metal complexes do not list any molecular vs ionic isomers. At the same time, one of the most striking examples of structural isomerism in organic chemistry is molecular urea, which has the same atomic composition as the chemically distinct ionic ammonium cyanate. This iconic organic couple now meets its inorganic heterometallic counterpart. We introduce a new class of structural isomers, molecular vs ionic, that can be consummated in complex and coordinatively unsaturated polynuclear/heterometallic compounds. We report inorganic molecular and ionic isomers of the composition [NaCrFe (acac)₃(hfac)₃] (acac = acetylacetonate; hfac = hexafluoroacetylacetonate). Heterometallic molecular [Cr^III(acac)₃-Na-Fe^II(hfac)₃] (1m) and ionic {[Cr^III(acac)₃-Na-Cr^III(acac)₃]⁺[Fe^II(hfac)₃-Na-Fe^II(hfac)₃]⁻} (1i) isomers have been isolated in pure form and characterized. While both ions are heterobimetallic trinuclear entities, the neutral counterpart is a heterotrimetallic trinuclear molecule. The two isomers exhibit distinctly different characteristics in terms of solubility, volatility, mass spectrometry ionization, and thermal behavior. Unambiguous assignment of the positions and oxidation/spin states of the Periodic Table neighbors, Fe and Cr, in both isomers have been made by a combination of characterization techniques that include synchrotron X-ray resonant diffraction, synchrotron X-ray fluorescence spectroscopy, Mossbauer spectroscopy, and DART mass spectrometry. The transformation between the two isomers that does take place in solutions of noncoordinating solvents has also been tested.