A DFT perspective on organometallic lanthanide chemistry

Molecular Selectivity in the Binding of Alkali Metals, Alkaline Earth Metals, First-Row Transition Metals, and Lanthanides with Cyclic Depsipeptides

Beauvericin (BEA) and enniatins (ENN) are cyclic hexadepsipeptide mycotoxins known for their ionophoric activities across cell membranes. While their ability to selectively bind alkali ions to form binary complexes has been studied, their interaction with multivalent metal ions to form higher-order complexes remains less explored. We report the unique characteristics of the 1:2, Mn+:BEA or ENN complexes with monovalent, divalent, and trivalent metal ions. A thorough IMS-MS analysis underscores the substantial interplay among ionic radii, coordination numbers, and their impact on conformational selection within higher-order complexes that is pertinent to ion transport. Transition metals offer insights into the effects of ion radii and ligand side chains on conformational selection, while lanthanide complexes enable a direct evaluation of coordination chemistry. An intriguing finding concerning the lanthanide complexes involves an unexpected C-H bond activation, wherein water ligands may catalyze the deprotonation of the cyclic peptides.

Synthesis of Crystallographically Characterizable Bis(cyclopentadienyl) Sc(II) Complexes: (C5H2tBu3)2Sc and {[C5H3(SiMe3)2]2ScI}1

The synthesis of previously unknown bis(cyclopentadienyl) complexes of the first transition metal, i.e., Sc(II) scandocene complexes, has been investigated using C5H2(tBu)3 (Cpttt), C5Me5 (Cp*), and C5H3(SiMe3)2 (Cp″) ligands. Cpttt2ScI, 1, formed from ScI3 and KCpttt, can be reduced with potassium graphite (KC8) in hexanes to generate dark-red crystals of the first crystallographically characterizable bis(cyclopentadienyl) scandium(II) complex, Cpttt2Sc, 2. Complex 2 has a 170.6° (ring centroid)-Sc-(ring centroid) angle and exhibits an eight-line EPR spectrum characteristic of Sc(II) with Aiso = 82.6 MHz (29.6 G). It sublimes at 200 °C at 10-4 Torr and has a melting point of 268-271 °C. Reductions of Cp*2ScI and Cp″2ScI under analogous conditions in hexanes did not provide new Sc(II) complexes, and reduction of Cp*2ScI in benzene formed the Sc(III) phenyl complex, Cp*2Sc(C6H5), 3, by C-H bond activation. However, in Et2O and toluene, reduction of Cp*2ScI at -78 °C gives a dark-red solution, 4, which displays an eight-line EPR pattern like that of 1, but it did not provide thermally stable crystals. Reduction of Cp″2ScI, in THF or Et2O at -35 °C in the presence of 2.2.2-cryptand, yields the green Sc(II) metallocene iodide complex, [K(crypt)][Cp″2ScI], 5, which was identified by X-ray crystallography and EPR spectroscopy and is thermally unstable. The analogous reaction of Cp*2ScI with KC8 and 18-crown-6 in Et2O gave the ligand redistribution product, [Cp*2Sc(18-crown-6-κ2O,O')][Cp*2ScI2], 6, as the only crystalline product. Density functional theory calculations on the electronic structure of these compounds are reported in addition to a steric analysis using the Guzei method.