Ac-Phg(4-OAc)-OH

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ~94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ~72 wt % mass loss during the decomposition at 280-325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC-CH3, OH-CO, H3C-CO-CH3, and H3C-O-CO-CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.

Mechanochemical synthesis is emerging as an environmentally friendly yet efficient approach to preparing metal-organic frameworks (MOFs). Herein, we report our systematic investigation on the mechanochemical syntheses of Group 4 element-based MOFs. The developed mechanochemistry allows us to synthesize a family of Hf4O4(OH)4(OOC)12-based MOFs. Integrating [Zr6O4(OH)4(OAc)12]2 and [Hf6O4(OH)4(OAc)12]2 under the mechanochemical conditions leads to a unique family of cluster-precise multimetallic MOFs that cannot be accessed by the conventional solvothermal synthesis. Extensive efforts have not yielded an effective pathway for preparing TiIV-derived MOFs, tentatively because of the relatively low Ti-O bond dissociation energy.

Predictive and mechanistically driven access to polynuclear oxo clusters and related materials remains a grand challenge of inorganic chemistry. We here introduce a novel strategy for synthetic control over highly sought-after transition metal {M4O4} cubanes. They attract interest as molecular water oxidation catalysts that combine features of both heterogeneous oxide catalysts and nature's cuboidal {CaMn4O5} center of photosystem II. For the first time, we demonstrate the outstanding structure-directing effect of straightforward inorganic counteranions in solution on the self-assembly of oxo clusters. We introduce a selective counteranion toolbox for the controlled assembly of di(2-pyridyl) ketone (dpk) with M(OAc)2 (M = Co, Ni) precursors into different cubane types. Perchlorate anions provide selective access to type 2 cubanes with the characteristic {H2O-M2(OR)2-OH2} edge-site, such as [Co4(dpy-C{OH}O)4(OAc)2(H2O)2](ClO4)2. Type 1 cubanes with separated polar faces [Co4(dpy-C{OH}O)4(L2)4] n+ (L2 = OAc, Cl, or OAc and H2O) can be tuned with a wide range of other counteranions. The combination of these counteranion sets with Ni(OAc)2 as precursor selectively produces type 2 Co/Ni-mixed or {Ni4O4} cubanes. Systematic mechanistic experiments in combination with computational studies provide strong evidence for type 2 cubane formation through reaction of the key dimeric building block [M2(dpy-C{OH}O)2(H2O)4]2+ with monomers, such as [Co(dpy-C{OH}O)(OAc)(H2O)3]. Furthermore, both experiments and DFT calculations support an energetically favorable type 1 cubane formation pathway via direct head-to-head combination of two [Co2(dpy-C{OH}O)2(OAc)2(H2O)2] dimers. Finally, the visible-light-driven water oxidation activity of type 1 and 2 cubanes with tuned ligand environments was assessed. We pave the way to efficient design concepts in coordination chemistry through ionic control over cluster assembly pathways. Our comprehensive strategy demonstrates how retrosynthetic analyses can be implemented with readily available assembly directing counteranions to provide rapid access to tuned molecular materials.