Tris(2-pyridylborate) (Tpyb) Metal Complexes: Synthesis, Characterization, and Formation of Extrinsically Porous Materials with Large Cylindrical Channels

Abstract

Sandwich-like metal complexes (Tpyb)2M (M = Mg, Fe, Mn) that are based on the novel t-butylphenyltris(2-pyridyl)borate ligand were prepared and fully characterized by multinuclear NMR spectroscopy, high-resolution matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and single crystal X-ray crystallography. The unique steric and electronic nature of the Tpyb ligand led to structural parameters and properties that are quite different to those of previously reported tris(pyrazolyl)borate and tris(2-pyridyl)aluminate complexes. Most importantly, depending on the crystallization procedure, supramolecular structures could be generated with relatively smaller (ca. 4-5 Å) or larger (ca. 8 Å) diameter pores propagating throughout the crystal lattice. Although the supramolecular structures are held together only by weak intermolecular C-H···π interactions, the solvent in the larger channels could be completely removed without any loss of crystallinity or degradation of the framework. Surface area and gas uptake measurements on the Mg complex further confirmed the permanent porosity, while the calculated non-localized density functional theory (NLDFT) pore diameter of 8.6 Å proved to be in excellent agreement with that obtained from single crystal X-ray crystallography. Our new materials are remarkably thermally stable as degradation did not occur up to about 400 °C based on thermogravimetric analysis (TGA), and a sample of the Mg complex showed no loss of crystallinity even after heating to 140 °C under high vacuum for 72 h according to single crystal X-ray diffraction data.