How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

Abstract

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) (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) (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.

A Tris(3‐pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages

“…Compound 1 is seemingly indefinitely airand moisture-stable and is readily isolated as colourless crystals in 36 %y ield after crystallisation from dichloromethane/ hexane, withoutthe need for column chromatography.I naddition to analytical and spectroscopic data (see Experimental Section), the single-crystal X-ray structure ( Figure 2b)d oes not show Mg 2 + coordination to 1 in its synthesis, as can commonly occur in this area. [7,16] This is important because the presence of ac ontaminant metal ion can be ac omplication in ligand transfer to other metals.…”

“…11 Attempts to produce as imilara rrangementt ot he latter using the same solvothermal conditions as in the previous study only produced intractable mixtureso fp roducts,r egardless of stoichiometry.T hus it appeared fromt his initial study that simply changing the bridgehead group from MeSi(3-Py) 3 to PhSn(3-Py) 3 leads to marked differences in coordination character. [16] This can be ascribed to two factors:( i) the change in ligand bite resulting from the incorporation of al arger atom at the bridging positiona nd (ii)potentially also to the differing steric influence of the Ph-group of 1 compared to the Megroup of MeSi(3-Py) 3 .…”

“…We have developed af amily of tris(2-pyridyl)l igands featuring main group bridgeheads E(2-Py) 3 (Figure 1a) [12][13][14][15] and demonstrated that the nature of the bridgehead atom can be used to modulate structure and reactivity. [16] However,t he corresponding tris(3-pyridyl) ligands, especially those featuring main groupb ridgehead atoms (limited to Si, Pa nd Bi), have seen only limited use in the literature. Notably,R eek has employed P(3-Py) 3 as al igand template to construct well-definedc atalytic cavities for regioselective hydroformylation.…”

Postfunctionalization of Tris(pyridyl) Aluminate Ligands: Chirality, Coordination, and Supramolecular Chemistry

“…For instance, as Group 15 is descended, the Lewis acidity of the bridgehead atom of the neutral ligands [E(2‐py) 3 ] (E=N−Bi) increases, changing the donor properties of the ligand and the coordination mode. Thus, changing the bridgehead atom is a method that can be used to modulate the catalytic activity of the metal complexes . We have been particularly interested in Group 13 tris(2‐pyridyl)aluminates ([RAl(2‐py) 3 ] − ; Figure B ) as they are unusual in this area in having an overall negative charge (rather than being neutral) .…”

“…[2] More recently there has been growing interest in heavier, more metallicb ridgehead ligandso fG roups 13, [3] 14, [4] and 15. [5] Changing the bridgehead can have al arge effect on the coordination and properties of the ligands. For instance, as Group 15 is descended, the Lewis acidity of the bridgehead atom of the neutral ligands [E(2-py) 3 ]( E = NÀBi)i ncreases, changing the donor properties of the ligand and the coordination mode.…”