Heterometallic Molecular and Ionic Isomers

Numerous descriptions\nof structural isomerism in metal complexes\ndo not list any molecular vs ionic isomers. At the same time, one\nof the most striking examples of structural isomerism in organic chemistry\nis molecular urea, which has the same atomic composition as the chemically\ndistinct ionic ammonium cyanate. This iconic organic couple now meets\nits inorganic heterometallic counterpart. We introduce a new class\nof structural isomers, molecular vs ionic, that can be consummated\nin complex and coordinatively unsaturated polynuclear/heterometallic\ncompounds. We report inorganic molecular and ionic isomers of the\ncomposition [NaCrFe (acac)<sub>3</sub>(hfac)<sub>3</sub>] (acac =\nacetylacetonate; hfac = hexafluoroacetylacetonate). Heterometallic\nmolecular [Cr<sup>III</sup>(acac)<sub>3</sub>-Na-Fe<sup>II</sup>(hfac)<sub>3</sub>] (<b>1m</b>) and ionic {[Cr<sup>III</sup>(acac)<sub>3</sub>-Na-Cr<sup>III</sup>(acac)<sub>3</sub>]<sup>+</sup>[Fe<sup>II</sup>(hfac)<sub>3</sub>-Na-Fe<sup>II</sup>(hfac)<sub>3</sub>]<sup>−</sup>} (<b>1i</b>) isomers have been isolated in pure\nform and characterized. While both ions are hetero<i>bi</i>metallic trinuclear entities, the neutral counterpart is a hetero<i>tri</i>metallic trinuclear molecule. The two isomers exhibit\ndistinctly different characteristics in terms of solubility, volatility,\nmass spectrometry ionization, and thermal behavior. Unambiguous assignment\nof the positions and oxidation/spin states of the Periodic Table neighbors,\nFe and Cr, in both isomers have been made by a combination of characterization\ntechniques that include synchrotron X-ray resonant diffraction, synchrotron\nX-ray fluorescence spectroscopy, Mössbauer spectroscopy, and\nDART mass spectrometry. The transformation between the two isomers\nthat does take place in solutions of noncoordinating solvents has\nalso been tested.