Ligand Adducts of Bis(acetylacetonato)iron(II): A 1H NMR Study

We report here a thorough 1H NMR study of Fe­(acac)2 solutions in a wide variety of noncoordinating and coordinating solvents, as well as the interaction of this complex with Et3N, pyridine, PMe2Ph, and R2PCH2CH2PR2 [R = Ph (dppe), Et (depe)] in C6D6. The study reveals that Fe­(acac)2 is readily transformed into Fe­(acac)3 in solution under aerobic conditions and that the commercial compound is usually contaminated by significant amounts of Fe­(acac)3. The 1H NMR resonances of Fe­(acac)2 are rather solvent-dependent and quite different than those reported in the literature. The compound is unstable in CDCl3 and stable in CD2Cl2, C6D6, CD3CN, acetone-d6, DMSO-d6, THF-d8, and CD3OD. The addition of the above-mentioned ligands (L) reveals only one paramagnetically shifted band for each type of acac and L proton, the position of which varies with the L/Fe ratio, consistent with rapid ligand exchange equilibria on the NMR time scale. A fit of the NMR data at a high L/Fe ratio allows the calculation of the expected resonances for all protons in the Fe­(acac)2L2 molecules. The system with the bidentate depe ligand shows evidence for a slow ligand exchange at low depe/Fe ratios, proposed to involve a species with the cis-chelated mononuclear Fe­(acac)2(depe) structure, whereas the fast exchange at a higher ratio is proposed to involved a trans-Fe­(acac)2(κ1-depe)2 complex. Complex Fe­(acac)2(dppe) cannot be investigated in solution because of low solubility in a noncoordinating solvent and because of the poor dppe competition for binding in coordinating solvents. The compound was crystallized, and its X-ray structure reveals a 1-dimensional polymeric structure with dppe-bridged Fe centers having the trans-octahedral Fe­(acac)2(κ1-dppe)2 coordination environment.