Uncovering the Hidden Landscape of Tris(4-pyridyl) Ligands: Topological Complexity Derived from the Bridgehead

Supramolecular main group chemistry is a developing field which parallels the conventional domain of metallo-organic chemistry. Little explored building blocks in this area are main group metal-based ligands which have the appropriate donor symmetry to build desired molecular or extended arrangements. Tris(pyridyl) main group ligands (E(py)₃ , E=main group metal) are potentially highly versatile building blocks since shifting the N-donor arms from the 2- to the 3-positions and 4-positions provides a very simple way of changing the ligand character from mononuclear/chelating to multidentate/metal-bridging. Here, the coordination behaviour of the first main group metal tris(4-pyridyl) ligands, E(4-py)₃ (E=Sb, Bi, Ph-Sn) is explored, as well as their ability to build metal-organic frameworks (MOFs). The complicated topology of these MOFs shows a marked influence on the counter anion and on the ability of the E(4-py)₃ ligands to switch coordination mode, depending on the steric and donor character of the bridgehead. This structure-directing influence of the bridgehead provides a potential building strategy for future molecular and MOF design in this area.

Pentafluoroethylated Compounds of Silicon, Germanium and Tin

In this contribution we present an account on pentafluoroethylated compounds of silicon, germanium and tin. The pronounced electron-withdrawing effect of the pentafluoroethyl group leads to a markedly increased Lewis acidity at the central atom which results in the stabilization of hypervalent complexes, anionic element(II) species as well as remarkable reactivities of element-element and element-hydrogen bonds. By addition to unsaturated C-C bonds or by reaction with organic halides as well as transition-metal complexes the molecules bearing a pentafluoroethyl-element group are readily accessible. Moreover, the utilization of pentafluoroethyl groups facilitates the formation of donor-stabilized germylenes and stannylenes. A series of such compounds serves as suitable pentafluoroethylation reagents. Conversely to the well-studied trifluoromethyl derivatives these compounds frequently exhibit a higher thermostability, which allows a more convenient handling.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main-group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py)₃ (E=C-R, N, P), which are closely related to archetypal tris-pyrazolyl borates. With the 6-methyl substituted ligands E(6-Me-2-py)₃ (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

Penta- and hexaorganostannate(iv) complexes based on O-heterocyclic ligands

In the present study, the synthesis of homoleptic five- and six-coordinate heteroaryl tin(iv) compounds using two O-heterocyclic substituents, 2-furyl (2-fu) and 2-benzofuryl (2-fu^Bz) ligands is described. The compounds were obtained as their lithium salts [Li₂(OEt₂)₂Sn(2-fu)₆] (1), [Li(tmeda)₂][Sn(2-fu)₅] (tmeda = N,N,N',N'-tetramethylethylenediamine) (2) and [Li(thf)₄][Sn(2-fu^Bz)₅] (3), featuring both an intramolecular coordination of the counterions by the anionic hypercoordinate tin(iv) species found in 1 as well as solvent separated cation/anion pairs for compounds 2 and 3. In addition, the co-crystalline complex [K₂(thf)₆Sn(2-fu^Bz)₆]·[K₂(thf)₄Sn(2-fu^Bz)₆] (4a·4b) was achieved. The characterization of all compounds by various spectroscopic methods and X-ray structural analysis complemented by density functional computations provided insights into the diversity of complex formation.