Coordination Chemistry of N‐Heterocyclic Nitrenium‐Based Ligands

Comprehensive studies on the coordination properties of tridentate nitrenium‐based ligands are presented. N‐heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh^I, Rh^III, Mo⁰, Ru⁰, Ru^II, Pd^II, Pt^II, Pt^IV, and Ag^I complexes based on these unusual ligands. Formation of nitrenium–metal bonds is unambiguously confirmed both in solution by selective ¹⁵N‐labeling experiments and in the solid state by X‐ray crystallography. The generality of N‐heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second‐row transition and post‐transition metals (Y–Cd) in terms of the corresponding bond‐dissociation energies.     

Synthesis,Structure and Reactivity of Sulfur-Rich Cyclopentadienyl-Transition Metal Complexes: Sulfur Chemistry from an Organometallic Point of View

Metal-sulfur centers play an important role in the activity of metalloproteins in enzymatic catalysis and the activity of metal sulfides as heterogeneous catalysts. The systematic search for M? S model compounds led to the discovery of an interesting and novel structural chemistry, which stems from the numerous coordination possibilities of sulfur ligands. The intention of this review article is to present and outline new approaches to sulfur chemistry from the organometallic point of view. Reactive cyclopentadienyl-transition metal fragments incorporate elemental sulfur to give polynuclear sulfur-rich complexes, which can contain either mono-, di- or polysulfido ligands or several such ligands in combined form. The versatile structural chemistry of the complexes formed and their reactivity towards organic, inorganic and organometallic compounds are discussed, and examples of some simple and rational procedures for their synthesis starting from cyclopentadienylcarbonyl- and cyclopentadienylhydrido-complexes are outlined. Their reactivity is manifested in numerous metal- and ligandcentered reactions. Finally the, albeit far less extensive, complex chemistry of the other chalcogens (O, Se, Te) is also considered for comparison, thus providing a more detailed survey of particular aspects of this area of chemistry.     

Synthesis and Coordination Chemistry of Iminophosphanes

Iminophosphanes are a new group of 1,3‐P,N‐ligands, readily obtainable from secondary phosphanes and nitrilium ions, having a tunable N‐donor site by means of varying the imine substituents. These ligands give, in high yields, monodentate gold complexes and bidentate rhodium and iridium complexes. Crystal structures are reported for both the ligands and the complexes.     

The Trigonal Prism in Coordination Chemistry

Herein we analyze the accessibility of the trigonal‐prismatic geometry to metal complexes with different electron configurations, as well as the ability of several hexadentate ligands to favor that coordination polyhedron. Our study combines i) a structural database analysis of the occurrence of the prismatic geometry throughout the transition‐metal series, ii) a qualitative molecular orbital analysis of the distortions expected for a trigonal‐prismatic geometry, and iii) a computational study of complexes of several transition‐metal ions with different hexadentate ligands. Also the tendency of specific electron configurations to present a cis bond‐stretch Jahn–Teller distortion is analyzed.     

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.     

Expanding the Chemistry of Cationic N‐Heterocyclic Carbenes: Alternative Synthesis,Reactivity, and Coordination Chemistry

A new synthetic route to complexes of the cationic N‐heterocyclic carbene ligand 2 has been developed by the attachment of a cationic pentamethylcyclopentadienylruthenium ([RuCp*]⁺) fragment to a metal‐coordinated benzimidazol‐2‐ylidene ligand. The coordination chemistry and the steric and electronic properties of the cationic carbene were investigated in detail by experimental and theoretical methods. X‐ray structures of three carbene–metal complexes were determined. The cationic ligand 2 is a poorer overall electron donor relative to the related neutral carbene, which is evident from cyclic voltammetry (CV) and IR measurements.     

Ligands with Two Monoanionic N,N-Binding Sites: Synthesis and Coordination Chemistry

Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono- and polynuclear complexes of transition metals as well as rare-earth and main-group elements. Nitrogen-based ditopic ligands, in which two monoanionic N,N-binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β-diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N-chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).     

The Coordination Chemistry of Multidentate Isocyanide Ligands

In spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the M? C?N? R group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate fashion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono- or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR) ₆ ]⁺. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR^*)₃ unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.     

A Flexible,Extremely Sterically Demanding Triazenide Ligand: Synthesis and Coordination Chemistry

Seeking to further the investigation of extremely sterically congested main group metals, the synthesis of a new oligoaryl triazene (Dipp*₂N₃H) is reported. This was accessed by diazotization of Dipp*NH₂ to access both Dipp*I and Dipp*N₃ in high yield and with scalable preparations. These compounds are used as precursors to Dipp*₂N₃H, via a magnesium aryl, due to the difficulties in obtaining pure triazene via an aryl‐lithium species. Deprotonation of the triazene with both potassium and thallium tert‐butoxides gives the respective metal triazenides in high yield. The solid‐state structures of these complexes have been determined and three high hapticity metal arene‐π interactions, which sterically shield the metal centers, were observed in each case; spectroscopic C_2v symmetry suggests these interactions are fluxional in solution. The variability of these interactions in the solid state as well as the changes in calculated steric demand of the ligand between metals point to an adaptive, flexible coordination mode.     

Synthesis and Complexing Properties of Phosphorus-Substituted Pyridine-2-Carboxylic Acid Anilides

Abstract

Novel phosphorus-containing ligands have been synthesized by the condensation of 2-(thio)phosphorylated anilines with 2,6-dichloro- and 4-chloropyridine-2-carbonyl chlorides in the presence of Et₃N and unsubstituted pyridine-2-carboxylic acid using P(OPh)₃ as a coupling agent. The compounds derived readily form complexes with a range of transition metals (M=Re(I), Pd(II), and Cu(II)), serving as bi- or tridentate monoanionic ligands. The composition and structure of the resulting metallocycles depend both on the presence/amount of the Cl atoms in the pyridine ring and on the nature of the donor atom X in the P=X moiety. The novel compounds were characterized by multinuclear NMR and IR spectroscopy as well as X-ray crystallography.     

Macrocyclic Metal Complexes Bearing Rigid Polyaromatic Ligands: Synthesis and Catalytic Activity

We synthesised palladium and platinum complexes possessing cyclic and acyclic pincer‐type polyaromatic ligands and investigated their structural effect on the catalysis. The pincer‐type bis(6‐arylpyridin‐2‐yl)benzene skeleton was constructed via Kröhnke pyridine synthesis under transition metal‐free conditions on gram‐scale quantity. Ligand structure significantly influenced catalytic activity toward the platinum‐catalysed hydrosilylation of diphenyl acetylenes, despite the ligand‐independence of the conformations and electronic properties of these complexes.     

True Boundary for the Formation of Homoleptic Transition‐Metal Hydride Complexes

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition‐metal hydride complexes are limited to the Groups 7–12. Here we present evidence for the formation of Mg₃CrH₈, containing the first Group 6 hydride complex [CrH₇]^5?. Our theoretical calculations reveal that pentagonal‐bipyramidal H coordination allows the formation of σ‐bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3–5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition‐metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen‐rich materials that are of both technological and fundamental interest.     

Synthesis, structure and magnetic properties of chiral and nonchiral transition-metal malates

Carboxylate-bridged complexes of transition metals, M(II)=Mn(II), Fe(II), Co(II), Ni(II), Zn(II), were synthesised by reaction of M(II) salts with dl-malate and L-malate under hydrothermal conditions. These complexes form four series of compounds, which have been fully characterised structurally, thermally and magnetically. The crystal structures of the new chiral compounds, [Mn(L-mal)(H(2)O)] (1), [Fe(L-mal)(H(2)O)] (2), [Co(L-mal)(H(2)O)] (3) and [Zn(L-mal)(H(2)O)] (4) as well as those of the bimetallic analogues [Mn(0.63)Co(0.37)(L-mal)(H(2)O)] (5) and [Mn(0.79)Ni(0.21)(L-mal)(H(2)O)] (6) have been solved by single-crystal X-ray diffraction. The six L-malate monohydrates crystallise in the chiral space group P2(1)2(1)2(1) and consist in a three-dimensional network of metal(II) centres in octahedral sites formed by oxygen atoms. These structures were compared to those of the chiral trihydrate compounds [Co(L-mal)(H(2)O)]2 H(2)O (7), [Ni(L-mal)(H(2)O)]2 H(2)O (8) and [Co(0.52)Ni(0.48)(L-mal)(H(2)O)]2 H(2)O (9), which exhibit helical chains of M(II) centres, and those of dl-malate dihydrates [Co(dl-mal)(H(2)O)]H(2)O (10) and [Ni(dl-mal)(H(2)O)H(2)O (11) and trihydrate [Mn(L-mal)(H(2)O)]2 H(2)O (12) highlighting the great flexibility of the coordination by the malate ligand. UV/Vis spectroscopic results are consistent with octahedral coordination geometry of high-spin transition-metal centres. Extensive magnetic characterisation of each homologous series indicates rather weak coupling interaction between paramagnetic centres linked through carboxylate bridges. Curie-like paramagnetic, antiferromagnetic, ferromagnetic or weak ferromagnetic behaviour is observed and discussed on the basis of the structural features. The bimetallic compounds 5 and 6 represent new examples of chiral magnets.     

Facile Synthesis of Triptycene-Azolium Salts and NHC-Metal Complexes

The cycloaddition reactions of eleven substituted anthracenes with nosylated quinone imines provides a convenient route to the respective triptycenes. Following re-aromatization, selective O-butylation and cleavage of the nosyl-group the respective triptycene anilines are obtained, which are converted into the respective imidazolium salts according to established procedures. Deprotonation of the imidazolium salts provide new triptycene-NHC-metal complexes (M=AuCl, RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, PdI₂(py), PtCl₂(py), Pd(allyl)Cl) with unusual ligand sterics.     

A Germacalicene: Synthesis,Structure, and Reactivity

The synthesis of the germacalicene 7 from the reaction of the dipotassium germole dianion K₂[ 6 ] with 1,2-bis-diisopropylamino-3-chlorocyclopropenyl perchlorate is reported. Based on the crystal structure analysis and the results of DFT calculations, the germacalicene 7 can be viewed as a cyclopropenium germacyclopentadienide ylide that is isoelectronic to α-cationic phosphanes. First reactivity studies revealed its nucleophilic character and resulted in the isolation of the air- and moisture-stable carbonyl iron complex 15 and the cationic silver complex 20 . One-electron oxidation of the germacalicene 7 was achieved by its reaction with [Ph₃C][B(C₆F₅)₄] and the bis-cationic Ge−Ge-bonded dimer 22 was isolated.     

锰基配位聚合物的简易合成、结构及其在葡萄糖电化学传感中的应用

利用刚性配体6-（3-吡啶基）间苯二甲酸（H₂PIAD）,制备了一种基于Mn （Ⅱ）的配位聚合物{[Mn （PIAD）（DMF）]·H₂O}_n（1）。采用后合成Ag纳米颗粒的策略制备了复合材料（Ag@1）以提高葡萄糖传感的电催化活性。在优化的外加电位下,通过计时电流法评估了Ag@1修饰的玻碳电极（GCE）的电催化性能。配位聚合物1为在其表面的Ag纳米颗粒均匀分布提供了固定基质,而且Ag@1传感器可以最大限度地发挥Ag与1结合对葡萄糖氧化的电催化协同效应。结果表明,Ag@1修饰的GCE对葡萄糖的检测性能良好,检出限低（6.36 μmol·L^-1）,选择性和灵敏度好（166.71 μA·L·mmol^-1·cm^-2）。     

The Versatility of Tertiary Phosphane Ligands in Coordination and Organometallic Chemistry

Tertiary phosphanes form kinetically stable complexes with formal oxidation states of the metal ranging from + IV to ?I (“electronic versatility”). Variously substituted phosphanes and polydentate, chelate forming phosphanes and polyphosphanes can be synthesized (“steric versatility”). Studies on complexes containing such ligands provide, inter alia, useful information about the metal-phosphorus bond, about the steric effects of the phosphane ligand, and about reactions of coordinated phosphanes.