Insight and Control of the Crystal Growth of Zeolitic Imidazolate Framework ZIF-67 by Atomic Force Microscopy and Mass Spectrometry

Combination of electrospray ionization mass spectrometry (ESI-MS) and in situ atomic force microscopy (AFM) are applied to provide the first nanoscopic study of the crystal growth of zeolitic imidazolate framework ZIF-67. ZIF-67 is found to form through a process of nucleation and spreading of metastable unenclosed substeps to form stable surface steps of the enclosed framework structure and demonstrates that isostructural MOFs, ZIF-67 and ZIF-8, undergo identical crystal growth mechanisms. The information on the crystal growth species obtained from the AFM experiments correlates well with the solution species identified by ESI-MS indicating that the species involved in the growth under low supersaturation conditions are methylimidazole/methylimidazolate, monomeric nonmethylimidazole/methylimidazolate complexed Co²⁺ ions and monomeric complexed [Co­(methylimidazole/methylimidazolate)_1‑2] ions. Combination of the use of low supersaturation growth solutions and in situ AFM has also allowed the successful extraction of the synthetic conditions necessary for formation of ZIF-67 nanodots that possesses a maximum vertical dimension of 1.2 nm which is the smallest dimension reported for a stable ZIF-67 entity. This methodology may be expanded to understand the formation of, and to form, complex crystal forms of other MOFs for new or improved functionality

Thermally Stable Terbium(II) and Dysprosium(II) Bis-amidinate Complexes

The thermostable four-coordinate divalent lanthanide (Ln) bis-amidinate complexes [Ln(Piso)2] (Ln = Tb, Dy; Piso = {(NDipp)2CtBu}, Dipp = C6H3iPr2-2,6) were prepared by the reduction of parent five-coordinate Ln(III) precursors [Ln(Piso)2I] (Ln = Tb, Dy) with KC8; halide abstraction of [Ln(Piso)2I] with [H(SiEt3)2][B(C6F5)] gave the respective Ln(III) complexes [Ln(Piso)2][B(C6F5)]. All complexes were characterized by X-ray diffraction, ICP-MS, elemental analysis, SQUID magnetometry, UV–vis-NIR, ATR-IR, NMR, and EPR spectroscopy and ab initio CASSCF-SO calculations. These data consistently show that [Ln(Piso)2] formally exhibit Ln(II) centers with 4fn5dz21 (Ln = Tb, n = 8; Dy, n = 9) valence electron configurations. We show that simple assignments of the f–d coupling to either L–S or J–s schemes are an oversimplification, especially in the presence of significant crystal field splitting. The coordination geometry of [Ln(Piso)2] is intermediate between square planar and tetrahedral. Projecting from the quaternary carbon atoms of the CN2 ligand backbones shows near-linear C···Ln···C arrangements. This results in strong axial ligand fields to give effective energy barriers to magnetic reversal of 1920(91) K for the Tb(II) analogue and 1964(48) K for Dy(II), the highest values observed for mononuclear Ln(II) single-molecule magnets, eclipsing 1738 K for [Tb(C5iPr5)2]. We tentatively attribute the fast zero-field magnetic relaxation for these complexes at low temperatures to transverse fields, resulting in considerable mixing of mJ states