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.     

Main group pyridyl-based ligands; strategies to mixed metal complexes

Tris-2-pyridyl ligands of the type [Y(2-py)(3)](n-) containing metallic or semi-metallic bridgehead atoms (Y) are a relatively new innovation in the area of main group coordination chemistry. This review highlights the major classes of ligands currently available, compares their coordination chemistry to their more well known non-metallic counterparts and explores the way in which they can be used to form heterometallic complexes in a targeted way.     

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 Sn^IV/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.     

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes

Phosph(III)azanes, featuring the heterocyclobutane P₂N₂ ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]₂. The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-N^tBu)]₂ (2-py=2-pyridyl) not only pre-organises the N−H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-^tNBu)]₂ (and of the new quinolyl derivative [(8-Qu)NHP(μ-N^tBu)]₂ (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.     

Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry

Polydentate ligands with benzene-o-dithiolato donor groups are useful building blocks in supramolecular coordination chemistry. The coordination chemistry of bis- and tris(benzene-o-dithiolato) ligands and mixed benzene-o-dithiolato/catecholato ligands is reviewed. These ligands exhibit a versatile coordination chemistry both in solution and in the solid state.     

Dinuclear versus trinuclear complex formation in zinc(II) benzoate/pyridyl oxime chemistry depending on the position of the oxime group

The initial employment of pyridine-3-carbaldehyde oxime, (3-py)C(H)NOH, and pyridine-4-carbaldehyde oxime, (4-py)C(H)NOH, in zinc(II) carboxylate chemistry is reported. The syntheses, crystal structures and IR characterization are described for [Zn₃(O₂CPh)₆{(3-py)C(H)NOH}₂] (1) and [Zn₂(O₂CPh)₄{(4-py)C(H)NOH}₂] (2). The trinuclear molecule of 1 has a linear structure, with one monoatomically bridging η¹:η²:μ and two syn, syn-η¹:η¹:μ benzoate groups spanning each pair of Zn^II ions; the terminal metal ions are each capped by one (3-py)C(H)NOH ligand coordinating through its pyridyl nitrogen. Complex 2 exhibits a dinuclear paddle–wheel structure with a Zn···Zn distance of 2.990(2) Å; each Zn^II ion has a square pyramidal geometry with four carboxylate oxygens in the basal plane and the pyridyl nitrogen of one monodentate (4-py)C(H)NOH ligand at the apex. Both complexes form 1D architectures by virtue of hydrogen bonding interactions involving the free oxime group as donor and the oxime nitrogen (1) or carboxylate oxygens (2) as acceptors. IR data are discussed in terms of the known structures and coordination modes of the ligands.     

Novel aspects in the coordination chemistry of mixed group 15/16 cage molecules

Cage molecules with mixed group 15 (E = P, As) and group 16 elements (Q = S, Se) are versatile ligands in coordination chemistry. The introduction of solid-state techniques allows one to extend the coordination flexibility of the cage molecules and to realize new coordination modes. The use of P₄S₃ or As₄S₃ as building blocks in copper(I) halide networks has initiated a new kind of “soft” solid-state chemistry, in which solutions of binary inorganic cage molecules are transferred into one-, two- or three-dimensional polymers. This strategy has been stimulated by the existence of competitive phosphorus, arsenic and sulfur coordination sites in the cage molecules.     

A Simple,Transition Metal Catalyst-Free Method for the Design of Complex Organic Building Blocks Used to Construct Porous Metal–Organic Frameworks

The design of metal–organic frameworks (MOFs) having large pore sizes and volumes often requires the use of complex organic ligands, currently synthesized using costly and time-consuming palladium-catalyzed coupling chemistry. Thus, in the present work, a new strategy for ligand design is reported, where piperazine and dihydrophenazine units are used as substitutes for benzene rings, which are the basic building block of most MOF ligands. This chemistry, which is based on simple, nucleophilic aromatic substitution (S_NAr) reactions, is used for the transition metal catalyst-free construction of 21 new, carboxylate-based ligands with varying sizes, shapes, and denticity and 15 linear di- and tetra-nitriles. Moreover, to demonstrate the utility of the ligands as building blocks, 16 new structurally diverse MOFs having surface areas up to 3100 m² g⁻¹ were also synthesized.     

Anionic Bipyridyl Ligands for Applications in Metallasupramolecular Chemistry

The facile synthesis of anionic bipyridyl ligands with dinuclear clathrochelate cores is described. These metalloligands can be obtained in high yields by the reactions of M(ClO₄)₂(H₂O)₆ (M: Zn, Mn, or Co) with 4‐pyridylboronic acid and 2,6‐diformyl‐4‐methylphenol oxime or 2,6‐diformyl‐4‐tert‐butylphenol oxime, followed by deprotonation. The ligands are interesting building blocks for metallasupramolecular chemistry, as evidenced by the formation of a Pt‐based molecular square and four coordination polymers with 2D or 3D network structures. Competition experiments reveal that the utilization of anionic bipyridyl ligands can result in significantly more stable assemblies.     

Self-assembly of 3p-Block Metal-based Metal-Organic Frameworks from Structural Perspective

Metal-organic frameworks(MOFs) are a class of porous inorganic-organic hybrid materials, which are constructed from diverse inorganic building units and multi-functional organic ligands. Highly ordered pore structures and tailored functionalization have made MOF materials potential for applications in many fields. Among various MOF materials, 3p-block metal(Al, Ga, and In)-based MOFs exhibit higher chemical stability than divalent transition metal-based MOFs due to their higher valence. In this review, Al-MOFs and In-MOFs were mainly discussed from the perspective of categories of inorganic building blocks, coordination types, and numbers of organic ligands. This review will give intuitive guidance to the design and synthesis of novel 3p-block metal-based MOFs with potential applications.     

Photoisomerization of a Maleonitrile‐Type Salen Schiff Base and Its Application in Fine‐Tuning Infinite Coordination Polymers

Strategically designed salen ligand 2,3‐bis[4‐(di‐p‐tolylamino)‐2‐hydroxybenzylideneamino]maleonitrile ( 1 ), which has pronounced excited‐state charge‐transfer properties, shows a previously unrecognized form of photoisomerization. On electronic excitation (denoted by an asterisk), 1Z *→ 1E isomerization takes place by rotation about the C2? C3 bond, which takes on single‐bond character due to the charge‐transfer reaction. The isomerization takes place nonadiabatically from the excited‐state ( 1Z ) to the ground‐state ( 1E ) potential‐energy surface in the singlet manifold; 1Z and 1E are neither thermally inconvertible at ambient temperature (25–30 °C), nor does photoinduced reverse 1E *→ 1Z (or 1Z *) isomerization occur. Isomers 1Z and 1E show very different coordination chemistry towards a Zn^II precursor. More prominent coordination chemistry is evidenced by a derivative of 1 bearing a carboxyl group, namely, N,N′‐dicyanoethenebis(salicylideneimine)dicarboxylic acid ( 2 ). Applying 2Z and its photoinduced isomer 2E as building blocks, we then demonstrate remarkable differences in morphology (sphere‐ and needlelike nanostructure, respectively) of their infinite coordination polymers with Zn^II.     

Transition Metal Borate Complexes Containing κ0-κ6 Denticity of Scorpionate Borate Ligands

The recent developments in the field of transition metal (TM) borate complexes have been a landmark in modern coordination chemistry. The structural diversities of these complexes play an important role in several catalytic processes. Generally, polypyrazolyl borate ligands, [BH_n(pz)_4-n]⁻ (n=1, 2; pz=pyrazolyl), popularly known as scorpionates have been used extensively for the preparation of TM borate complexes. The presence of multiple donor atoms in the flexible borate proligands led to several coordination modes. Based on the electronic and steric properties of these ligands and the metals, the denticity of borate ligands in TM complexes varied from κ⁰ to κ⁶. The presence of different bonding modes of these borate ligands made them very interesting in main group organometallic chemistry. In addition, cooperative activation of boranes by TM complexes containing metal-nitrogen or metal-sulfur bonds has become an alternative to the utilization of borate proligands for the synthesis of TM borate complexes. This review summarizes the advancements of the chemistry of TM borate complexes focusing exclusively on the synthetic methods and various bonding scenarios.     

Surface chemistry of metal-organic frameworks at the liquid-solid interface

Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.     

Synthesis and chemistry of tris(2-pyridyl)phosphine and bis(2-pyridyl)phenylphosphine complexes of mercury(II) X (X = Br,Cl) and X-ray crystal structural determination of [HgBr2(PPh(2-py)2)2]

Mercury(II) halide complexes [HgX₂(P(2-py)₃)₂] (X?=?Br (1), Cl (2)) and [HgX₂(PPh(2-py)₂)₂] (X?=?Br (3), Cl (4)) containing P(2-py)₃ and PPh(2-py)₂ ligands (P(2-py)₃ is tris(2-pyridyl)phosphine and PPh(2-py)₂ is bis(2-pyridyl)phenylphosphine) were synthesized in nearly quantitative yield by reaction of corresponding mercury(II) halide and appropriate ligands. The synthesized complexes are fully characterized by elemental analysis, melting point determination, IR, ¹H, and ³¹P-NMR spectroscopies. Furthermore, the crystal structure of [HgBr₂(PPh(2-py)₂)₂] determined by X-ray diffraction is also reported.     

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.     

Coordination Chemistry of Aluminum,Gallium, and Indium at Transition Metals

The current upswing in the interest in organoelement chemistry of Group 13 metals is attributed not least to the establishment of the coordination chemistry of R_aE fragments (E=Al, Ga, In; a=1, 2) at d-block metals (M). Recently the availability of low-valent organoelement compounds as building blocks for synthesis has substantially enriched the structure chemistry of this class of compounds. The M–E bonding conditions and the question of the significance of M(d_π)-E(p_π) backbonding as well as potential applications in materials science, for example, as single-source precursors for the deposition of thin intermetallic films by chemical vapor deposition, are discussed.     

Electronically Driven Regioselective Iridium-Catalyzed C−H Borylation of Donor-π-Acceptor Chromophores Containing Triarylboron Acceptors

We observed a surprisingly high electronically driven regioselectivity for the iridium-catalyzed C−H borylation of donor-π-acceptor (D -π-A) systems with diphenylamino ( 1 ) or carbazolyl ( 2 ) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl ( B(^FXyl)₂ ) as the acceptor, and 1,4-phenylene as the π-bridge. Under our conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C−C coupling, functional group transformations) the C−H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D -π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated ( 1-(Bpin)₂ ) and unborylated ( 1 ) diphenylamino-substituted D -π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.     

A Mixed Molecular Building Block Strategy for the Design of Nested Polyhedron Metal–Organic Frameworks

A mixed molecular building block (MBB) strategy for the synthesis of double‐walled cage‐based porous metal–organic frameworks (MOFs) is presented. By means of this method, two isostructural porous MOFs built from unprecedented double‐walled metal–organic octahedron were obtained by introducing two size‐matching C₃‐symmetric molecular building blocks with different rigidities. With their unique framework structures, these MOFs provide, to the best of our knowledge, the first examples of double‐walled octahedron‐based MOFs.     

Imido Analogues of Common Oxo Anions: A New Episode in the Chemistry of Cluster Compounds

Oxo anions of p- and d-block elements, for example, SiO(4)(4-), PO(4)(3-), SO(4)(2-), and CrO(4)(2-), are commonly encountered species. The full or partial replacement of the oxo ligands by isoelectronic imido (NR) groups generates homoleptic polyimido anions of the type [E(NR)(x)](z-) or heteroleptic imidooxo anions with the general formula [O(y)E(NR)(x-y)](z-) (where E=main group element or transition metal). The alkali metal derivatives of this new class of anions form ternary or quaternary cluster systems, respectively. The structures of these clusters can be rationalized in terms of the self-assembly of fundamental building blocks. An understanding of the factors that control this process may allow the design of functional materials with specific properties. In addition, these anions are attracting attention as multidentate ligands with unique electronic and stereochemical properties that may engender novel metal-centered chemistry.     

Review: the multicolored coordination chemistry of violurate anions

This review provides a comprehensive overview on the coordination chemistry of violuric acid, C₄H₃N₃O₄ (= H₃Vio), and its derivatives (e.g. 1,3-diorganovioluric acids and thiovioluric acid). The most remarkable property of these colorless compounds is the formation of brightly colored (pantochromic/polychromic) salts with colorless cations such as alkali metal and alkaline earth metal ions and organoammonium ions. These magnificent colors have fascinated chemists for more than a century. Only in recent years it has been fully recognized that the structural chemistry of violurates is rather interesting and diverse. Violurate anions are excellent building blocks for new supramolecular assemblies in the crystalline state. Various organoammonium violurates and transition metal violurate complexes have been structurally characterized through single-crystal X-ray diffraction. Highly characteristic for these structures is the formation of 1D, 2D, or 3D hydrogen-bonded assemblies in the crystalline state. This review provides a comprehensive overview on the multicolored coordination chemistry of violurate anions, with the focus being on structurally characterized species.