How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

Abstract

Although am ultitude of studies have explored the coordination chemistry of classical tripodal ligands containing ar ange of main-group bridgehead atoms or groups,i ti sn ot clear howperiodic trends affect ligand character and reactivity within as ingle ligand family.Acase in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py) 3 (E = C À R, N, P), whicha re closely related to archetypal tris-pyrazolyl borates.W itht he 6-methyl substituted ligands E(6-Me-2-py) 3 (E = As,Sb, Bi)inhand, the effects of bridgehead modification alone on descending as ingle 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, whichn ot only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.Facially coordinating chelate ligands have been extensively applied in organometallic and bioinorganic chemistry. [1][2][3][4][5][6] Preeminent in this area are tris-pyrazolyl borates (Figure 1a), which have found countless applications as ancillary ligands over the last three decades in catalytic and biomimetic systems. [7] Thesuccess and broad-ranging use of these ligands is in large part due to the ease with which the steric and electronic environment of the coordinated metal centres can be tuned by incorporating substituents on the pyrazolyl ring units.B uilding tripodal ligands that contain heavier maingroup element bridgeheads (beyond the second period) potentially enables the systematic modification of donor properties,s uch as ligand bite and s-donor/p-acceptor character,a sw ell as the potential introduction of novel reactivity at the bridgehead atom (such as redox chemistry, where metallic or semi-metallic bridgeheads are involved).Although less well studied than their tris-pyrazolyl borate relatives,G roup 14 and 15 tris-2-pyridyl ligands containing non-metallic bridgeheads have been known for an umber of years ( Figure 1b). [8][9][10] Our focus in this area has been the exploration of the reactivity and coordination chemistry of Group 13 and 14 tris-2-pyridyl ligands in which metallic or semi-metallic elements are present (like the aluminate shown in Figure 1c). [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Despite the ongoing interest in expanding classical tripodal ligand sets,s uch as tris-pyrazolyl borate and tris-2pyridyl ligands,n os tudies have managed to decipher the actual effects of changing the size and electronic character of the bridgehead on structure and reactivity.The preparation of the neutral valence-isoelectronic tris-pryridyl ligands [E(6-Me-2-py) 3 ]( E= As,S b, Bi)h as allowed us to explore this issue for the first time on asingle class of main-group ligands of this type.Surprisingly,t here are no reports of the synthesis or coordination chemistry of tris-2-pyridyl ligands of Sb or Bi. Our attempts to obtain the unsubstituted ligands E(2-py) 3 (E = As,S b, Bi)u sing...

The coordination chemistry of the neutral tris-2-pyridyl silicon ligand [PhSi(6-Me-2-py)3]

“…Thus, for example, whereas in situ reaction of unsubstituted 2-lithio-py with BiCl 3 or SbCl 3 could not be used to prepare the Bi(2-py) 3 or Sb(2-py) 3 ligands, the reaction involving 6-Me-2-py gave Bi(6-Me-2-py) 3 and Sb(6-Me-2-py) 3 in good yields. 12 In the current study, we were able to obtain the new Si(IV) ligand [PhSi(6-Me-2-py) 3 ] (1) in high yield (82%) from the reaction of PhSiCl 3 with 6-Me-2-Li-py in thf (Scheme 1b), providing gram-quantities for the further investigation of coordination chemistry. In contrast, the reaction between unmethylated 2-lithio-pyrdine and PhSiCl 3 yields a mixture of products, as is apparent from the 1 H NMR spectrum of the crude reaction mixture.…”

“…The majority of studies in the past three decades have concerned neutral frameworks containing lighter, non-metallic bridgehead atoms Y(2-py') 3 (Y = CR, COR, CH, N, P, P=O; 2-py' = an unsubstituted or substituted 2-pyridyl group) (e.g., Figure 1b). 8 More recently, however, attention has turned to the effects of incorporating more metallic Group 13, 9 14 10,11 and 15 12 bridgeheads. 13 These isoelectronic metallic relatives now span almost the entire range of p-block elements, from anionic aluminate ligands (e.g., inset to Figure 1, A), 9 through to the heaviest counterpart containing a Bi III bridgehead (inset to Figure 1, C).…”

“…13 These isoelectronic metallic relatives now span almost the entire range of p-block elements, from anionic aluminate ligands (e.g., inset to Figure 1, A), 9 through to the heaviest counterpart containing a Bi III bridgehead (inset to Figure 1, C). 12 We have shown recently that changing the bridgehead atom can have significant effects on both the coordination chemistry and reactivity of tris(2-pyridyl) ligands. For example, the increasing Lewis acidity of the elements as Group 15 is descended can impact dramatically on the ligand coordination mode as well as the -donor character of the ligand frameworks, introducing the potential for modulation of the structures and catalytic activity of metal complexes.…”

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

“…[11] However, analogues containing heavier, metallic or semi-metallic bridgehead atoms have only been introduced relatively recently, [10] with intensive work in this area in the past few decades establishing this class of ligands across the p-block. [12][13][14][15][16] While the orientation of the donor-N atoms in tris(2-pyridyl) ligands leads to chelation of metal ions in the majority of complexes, changing the N-atom to the 3-or 4-positions introduces the prospect of forming supramolecular assemblies, as a result of bridging of the ligands between metal centers. Coordination studies in this area have, however, so far been limited to only a few E(3-py) 3 (E = P, [17] MeSi, [18] PhSn [19] ) and E(4py) 3 (E = CR, [20] P, [21] MeSi [22] ) ligands.…”