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.

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.

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

A series of new tris(2-pyridyl) bismuthine ligands of the type [Bi(2-py')3] have been prepared, containing a range of substituents at various positions within their pyridyl rings (py'). They can act as intact ligands or, as a result of the low C-Bi bond energy, exhibit noninnocent reactivity in the presence of metal ions. Structural studies of Li+ and Ag+ complexes show that the coordination to metal ions using their pyridyl-N atoms and to anions using the Lewis acidity of their Bi(III) centers can be modified by the presence of substituents within the 2-pyridyl rings, especially at the 6- or 3-positions, which can block the donor-N or Lewis acid Bi sites. Electron withdrawing groups (like CF3 or Br) can also severely reduce their ability to act as ligands to metal ions by reducing the electron donating ability of the pyridyl-N atoms. Noninnocent character is found in the reactions with Cu+ and Cu2+, resulting in the coupling of pyridyl groups to form bipyridines, with the rate of this reaction being dependent on the anion present in the metal salts. This leads to the formation of Bi(III)/Cu(I) complexes containing hypervalent [X2Bi(2-R-py)]- (X = Cl, Br) anions. Alternatively, the tris(2-pyridyl) bismuthine ligands can act as 2-pyridyl transfer reagents, transferring 2-py groups to Au(I) and Fe(II).

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

The systematic assembly of supramolecular arrangements is a persistent challenge in modern coordination chemistry, especially where further aspects of complexity are concerned, as in the case of large molecular mixed-metal arrangements. One targeted approach to such heterometallic complexes is to engineer metal-based donor ligands of the correct geometry to build 3D arrangements upon coordination to other metals. This simple idea has, however, only rarely been applied to main group metal-based ligand systems. Here, we show that the new, bench-stable tris(3-pyridyl)stannane ligand PhSn(3-Py)3 (3-Py=3-pyridyl) provides simple access to a range of heterometallic SnIV /transition metal complexes, and that the presence of weakly coordinating counter anions can be used to build discrete molecular arrangements involving anion encapsulation. This work therefore provides a building strategy in this area, which parallels that of supramolecular transition metal chemistry.

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

Postfunctionalization of the aluminate anion [EtAl(6-Me-2-py)3 ]- (1) (2-py=2-pyridyl) with alkoxide ligands can be achieved by the selective reactions of the lithium salt 1 Li with alcohols in the appropriate stoichiometry. This method can be used to introduce 3- and 4-py functionality in the form of 3- and 4-alkoxymethylpyridyl groups, while maintaining the integrity of the aluminate framework, thereby giving entry to new supramolecular chemistry. Chirality can be introduced either by using a chiral alcohol as a reactant or by the stepwise reaction of 1 Li with two different nonchiral alcohols. The latter route has allowed the synthesis of a rare example of a chiral-at-aluminium aluminate.

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.

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.

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.

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

Difficulties in the preparation of neutral ligands of the type [RSi(2-py)3] (where 2-py is an unfunctionalised 2-pyridyl ring unit) have thwarted efforts to expand the coordination chemistry of ligands of this type. However, simply switching the pyridyl substituents to 6-methyl-pyridyl groups (6-Me-2-py) in the current paper has allowed smooth, high-yielding access to the [PhSi(6-Me-2-py)3] ligand (1), and the first exploration of its coordination chemistry with transition metals. The synthesis, single-crystal X-ray structures and solution dynamics of the new complexes [{PhSi(6-Me-2-py)3}CuCH3CN][PF6], [{PhSi(6-Me-2-py)3}CuCH3CN][CuCl2], [{PhSi(6-Me-2-py)3}FeCl2], [{PhSi(6-Me-2-py)3}Mo(CO)3] and [{PhSi(6-Me-2-py)3}CoCl2] are reported. The paramagnetic Fe2+ and Co2+ complexes show strongly shifted NMR resonances for the coordinated pyridyl units due to large Fermi-contact shifts. However, magnetic anisotropy also leads to considerable pseudo-contact shifts so that both contributions have to be included in the paramagnetic NMR analysis.