Tris(2-pyridyl) Bismuthines: Coordination Chemistry, Reactivity, and Anion-Triggered Pyridyl Coupling

Structural and dimensional control of porphyrin capsules using Group 15 tris(3-pyridyl) linkers

While supramolecular chemistry involving organic and metallo-organic host assemblies is a well-established and important field with applications in gas-storage, drug-delivery and the regio- and stereo-control of organic reactions, the use of main group elements in this setting (beyond the second row of the p-block) has been little explored. In this paper we show how periodic trends in the p-block can provide the means for systematic size and structural control in an important class of supramolecular porphyrin-based capsules. The formation of molecular and extended 2D capsule arrangements between the heavier Group 15 tris(3-pyridyl) linkers Sb(3-py)₃ and Bi(3-py)₃ and the metallo-porphyrins MTPP (M = Zn, Mg; TPP = tetraphenylporphyrin, 3-py = 3-pyridyl) is the first study involving heavier Group 15 pyridyl linkers. The increase in C-E bond length in the E(3-py)₃ linkers moving down Group 15 (from E = P, to Sb, to Bi) can be used to alter the dimensions and structural preference of the capsules, as can oxidation of the Group 15 bridgehead atoms themselves. The subtle changes in the dimensions and Lewis acidity of the encapsulates have a dramatic effect on the rate and selectivity of the catalytic oxidative cleavage of organic diols and catalytic oxidation of α-hydroxyketones. By providing simple tools for modulating the chemical and steric properties of the capsules this work should have direct applications for the tuning of the activity and specificity of a range of catalytic systems based on main-group-based capsules of this type.

Highly Adaptive Nature of Group 15 Tris(quinolyl) Ligands─Studies with Coinage Metals

The substitution of heavier, more metallic atoms into classical organic ligand frameworks provides an important strategy for tuning ligand properties, such as ligand bite and donor character, and is the basis for the emerging area of main-group supramolecular chemistry. In this paper, we explore two new ligands [E(2-Me-8-qy)₃] [E = Sb (1), Bi (2); qy = quinolyl], allowing a fundamental comparison of their coordination behavior with classical tris(2-pyridyl) ligands of the type [E'(2-py)₃] (E = a range of bridgehead atoms and groups, py = pyridyl). A range of new coordination modes to Cu⁺, Ag⁺, and Au⁺ is seen for 1 and 2, in the absence of steric constraints at the bridgehead and with their more remote N-donor atoms. A particular feature is the adaptive nature of these new ligands, with the ability to adjust coordination mode in response to the hard-soft character of coordinated metal ions, influenced also by the character of the bridgehead atom (Sb or Bi). These features can be seen in a comparison between [Cu₂{Sb(2-Me-8-qy)₃}₂](PF₆)₂ (1·CuPF₆) and [Cu{Bi(2-Me-8-qy)₃}](PF₆) (2·CuPF₆), the first containing a dimeric cation in which 1 adopts an unprecedented intramolecular N,N,Sb-coordination mode while in the second, 2 adopts an unusual N,N,(π-)C coordination mode. In contrast, the previously reported analogous ligands [E(6-Me-2-py)₃] (E = Sb, Bi; 2-py = 2-pyridyl) show a tris-chelating mode in their complexes with CuPF₆, which is typical for the extensive tris(2-pyridyl) family with a range of metals. The greater polarity of the Bi-C bond in 2 results in ligand transfer reactions with Au(I). Although this reactivity is not in itself unusual, the characterization of several products by single-crystal X-ray diffraction provides snapshots of the ligand transfer reaction involved, with one of the products (the bimetallic complex [(BiCl){ClAu₂(2-Me-8-qy)₃}] (8)) containing a Au₂Bi core in which the shortest Au → Bi donor-acceptor bond to date is observed.

The study of a novel high selectivity pyrenyl-based fluorescence probe with aggregation-induced emission characteristics for Fe3+ detection designed by a structure modulation strategy

The past decades have witnessed the feat of fluorescent probes for Fe³⁺ detection, where eliminating the interference by other metal ions plays a pivotal role in its detection by probes in complex environments. Herein, by taking advantage of the substituent effects, the electron-withdrawing group (EWG) -CF₃ and electron-donating group (EDG) -CH₃ were introduced to 2-(1-pyrenyl)pyridine (pypyr) to prepare two turn-off fluorescence probes, 5-trifluoromethyl-2-(1-pyrenyl)pyridine (pypyr-CF3) and 5-methyl-2-(1-pyrenyl)pyridine (pypyr-CH3). Intriguingly, both probes displayed novel aggregation-induced emission (AIE) characteristics in MeCN/H₂O mixtures and the size and morphology of the aggregated particles were studied via DLS and TEM. By the modulation strategy, pypyr-CF3 can detect Fe³⁺ in the presence of 29 different metal ions without interference. Comparatively, pypyr-CH3 experienced serious interference from other metal ions such as Hg²⁺ and Zr⁴⁺. Besides, pypyr-CF3 not only demonstrated a higher photoluminescence quantum yield (PLQY) of 65.25% and wider pH adaptability but is also capable of Fe³⁺ detection over a wide pH range of 2-11 with a short response time (5 seconds). A plausible quenching mechanism based on the inner filter effect has also been demonstrated. More importantly, the versatile applications of pypyr-CF3, such as the quantitative analysis of Fe³⁺ in actual water samples, anti-forgery ink, fingerprint identification, etc., further corroborate its superb capabilities. This study aims to lend concrete support to the design and selectivity modulation of probes.

Competition between Intra and Intermolecular Pnicogen Bonds. Complexes between Naphthalene Derivatives and Neutral or Anionic Bases

The PnF 2 (Pn=P,As,Sb,Bi) on a naphthalene scaffold can engage in an internal pnicogen Pn···N bond (PnB) with a NH 2 group placed close to it on the adjoining ring. An approaching neutral NH 3 molecule can engage in a second PnB with the central Pn, which tends to weaken the intramolecular bond. The presence of the latter in turn weakens the intermolecular PnB with respect to that formed in its absence. Replacement of the external NH 3 by a CN - anion causes a fundamental change in the bonding pattern, producing a fourth covalent bond with Pn, which rearranges into a trigonal bipyramidal motif. This addition disrupts the internal Pn···N pnicogen bond, recasting the PnF 2 ···NH 2 interaction into a NH···F H-bond.

Cation- and Anion-Mediated Supramolecular Assembly of Bismuth and Antimony Tris(3-pyridyl) Complexes

The use of antimony and bismuth in supramolecular chemistry has been largely overlooked in comparison to the lighter elements of Group 15, and the coordination chemistry of the tripodal ligands [Sb(3-py)₃] and [Bi(3-py)₃] (L) containing the heaviest p-block element bridgehead atoms has been unexplored. We show that these ligands form a common hybrid metal–organic framework (MOF) structure with Cu(I) and Ag(I) (M) salts of weakly coordinating anions (PF₆^–, SbF₆^–, and OTf^–), composed of a cationic substructure of rhombic cage (M)₄(L)₄ units linked by Sb/Bi–M bonding. The greater Lewis acidity of Bi compared to Sb can, however, allows anion···Bi interactions to overcome Bi–metal bonding in the case of BF₄^–, leading to collapse of the MOF structure (which is also seen where harder metals like Li⁺ are employed). This study therefore provides insight into the way in which the electronic effects of the bridgehead atom in these ligand systems can impact their supramolecular chemistry.

The Lewis acidity of the Group 15 bridgehead atoms (E = Sb vs Bi) proves to be a decisive structural directing factor in the coordination of tris(3-pyridyl) ligands E(3-py)₃, being responsible for promoting or disfavoring E−metal or E···anion interactions.

Synthesis of tris(3-pyridyl)aluminate ligand and its unexpected stability against hydrolysis: revealing cooperativity effects in heterobimetallic pyridyl aluminates

We report the elusive metallic anion [EtAl(3-py)₃]^- (3-py = 3-pyridyl) (1), the first member of the anionic tris(3-pyridyl) family. Unexpectedly, the lithium complex 1Li shows substantial protic stability against water and alcohols, unlike related tris(2-pyridyl)aluminate analogues. This stability appears to be related to the inability of the [EtAl(3-py)₃]^- anion to chelate Li⁺, which precludes a decomposition pathway involving Li/Al cooperativity.

Reversing Lewis acidity from bismuth to antimony

Investigations on the boundaries between the neutral and cationic models of (Mesityl)₂EX (E = Sb, Bi and X = Cl^-, OTf^-) have facilitated reversing the Lewis acidity from bismuth to antimony. We use this concept to demonstrate a higher efficiency of (Mesityl)₂SbOTf over (Mesityl)₂BiOTf in the catalytic reduction of phosphine oxides to phosphines. The experiments supported with computations described herein will find use in designing new Lewis acids relevant to catalysis.

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.

Coordination chemistry of the bench-stable tris-2-pyridyl pnictogen ligands [E(6-Me-2-py)3] (E = As, As[double bond, length as m-dash]O, Sb)

Tripodal ligands with main group bridghead units are well established in coordination chemistry and single-site organometallic catalysis. Although a large number of tris(2-pyridyl) main group ligands [E(2-py)₃] (E = main group element, 2-py = 2-pyridyl) spanning across the whole p-block are now synthetically acessible, only limited work has been done on the coordination chemistry on the tris(2-pyridyl) group 15 ligands for the heavier elements (As, Sb). In the current study we investigate the coordination chemistry of the ligand family E(6-Me-2-py)₃ (E = As, Sb) and of the As(v) ligand O[double bond, length as m-dash]As(6-Me-2-py)₃. The air- and mositure-stability of all of these main group ligands makes them especially attractive in future catalytic applications.