F. Ekkehardt Hahn

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Evidence for an Equilibrium between an N‐heterocyclic Carbene and Its Dimer in Solution

Evidence for the Wanzlick equilibrium between carbene 1 and dibenzotetraazafulvalene (1)(2) at ambient temperature has been found (see scheme). Enetetramines of type (1)(2) can also be cleaved by coordinatively unsaturated transition metal compounds to form dicarbene complexes.

Self-Assembly of Molecular Cylinders from Polycarbene Ligands and AgI or AuI

Tris- and tetrakis(imidazolium) salts react with Ag(2)O to give cylindrical polynuclear silver(I) complexes such as the tetrasilver derivative [Ag(4)(1)(2)](PF(6))(4). The silver(I) complexes undergo transmetalation with [AuCl(SMe(2))] to yield homonuclear gold(I) complexes with retention of the metallosupramolecular assembly.

Synthesis of Silver(I) and Gold(I) Complexes with Cyclic Tetra- and Hexacarbene Ligands

Cyclic polyimidazolium salts have been investigated with respect to their function as anion receptors and cyclophanelike tetraimidazolium tetracations have been shown to bind anions inside the ring cavity. Cyclic polyimidazolium cations can also serve as precursors for N-heterocyclic carbenes (NHCs) and their complexes. Complexes of cyclic di-NHC ligands have been generated from bisazolium cyclophanes like A. However, such dicarbene ligands coordinate in a cisfashion to the metal center. They act as classical bidentate ligands rather than encapsulating the metal in a manner typical for macrocyclic ligands. Only one cyclic tetradentate dicarbene ligand is known to form a saddle-shaped macrocyclic complex B with nickel(II). We have described the first complex with a macrocyclic tetracarbene ligand C. The rhenium complex of a tridentate, facially coordinated [11]ane-P2C NHC macrocycle has also been reported. Since the preparation of complex C in a metal-template controlled domino-reaction is cumbersome we became interested in the preparation of complexes with macrocyclic poly-NHC ligands from cyclic polyimidazolium cations. This approach has previously been applied successfully for the preparation of some mono (Pd) and dinuclear (Cu, Ag) carbene complexes from the cyclic tetraimidazolium salt D. We became particularly interested in the lutidine-bridged tetraimidazolium salt H4-1 ACHTUNGTRENNUNG(Br4)[8] which upon C deprotonation would yield a potentially hexadentate ligand possessing two C-N-C pincer-type binding sites. Here we describe the mono and dinuclear silver(I) complexes with the carbene ligands derived from H4-1(Br)4 and the transfer of the carbene ligand to yield the mono and dinuclear gold(I) complexes (Scheme 1). A slight modification in the synthesis protocol employed for the preparation of H4-1(Br)4 yields the cyclic hexaimidazolium salt H6-6Br ACHTUNGTRENNUNG(BPh4)5 (Scheme 2). The preparation and molecular structure of the hexanuclear silver(I)-NHC complex derived from this ligand is also ACHTUNGTRENNUNGdescribed. The tetraimidazolium salt H4-1(Br)4 is soluble in water. [8]

Reactions of β-functional phenyl isocyanides

Abstract Aspects of the coordination chemistry of β-functional phenyl isocyanides are reviewed, with emphasis on research involving 2-hydroxyphenyl isocyanide and derivatives thereof, which has been carried out in our Berlin group during the last few years. The driving force of coordinated 2-hydroxyphenyl isocyanide to form carbene complexes with 2,3-dihydrobenzoxazol-2-ylidene ligands depends on the attached metal fragment and can be used as a probe for the π-electron release ability of the metal center. Related work on N- and C-functional phenyl isocyanides aimed towards the synthesis of 2,3-dihydro-1H-imidazol-2-ylidene and 2,3-dihydro-1H-indol-2-ylidene complexes is also described.

Heterometallic complexes, tandem catalysis and catalytic cooperativity

This minireview reports the most recent advances in the use of heterometallic catalysts based on single-frame N-heterocyclic carbene ligands. The article describes the synthetic strategies for the preparation of heterometallic catalysts, and their applications in the design of tandem processes by combining the catalytic properties associated with the two (or more) different metal centers. Several examples are discussed in which the use of heterometallic complexes results in a clear enhancement of the catalytic outcome compared to the results provided by mixtures of related homometallic complexes. The field constitutes a research area that is full of potential and is at its very earliest stage.

Metal template controlled formation of [11]ane-P2CNHC macrocycles

The synthesis of N-heterocyclic carbene-diphosphine macrocycles by metal template assisted cyclization reactions has been explored. Attempts to prepare the facial tungsten tricarbonyl precursor complex containing an NH,NH-functionalized carbene and a suitable diphosphine resulted in displacement of the coordinated carbene and the isolation of the corresponding diphosphine tungsten tetracarbonyl [3]. The Re(I) chloro tetracarbonyl complex bearing an NH,NH-functionalized carbene ligand [5] can be prepared and is a suitable precursor for the subsequent formation of the carbene-diphosphine tricarbonyl intermediate [H(2)-6]Cl bearing reactive 2-fluoro substituents at the phosphine-phenyl groups. Two of these fluoro substituents are displaced by a nucleophilic attack upon deprotonation of the coordinated NH,NH-functionalized carbene resulting in new C-N bonds resulting in the partially coupled intermediate, [10], followed by the desired complex with the macrocyclic ligand [8]Cl. Compounds [H-7]Cl and [8]Cl are also formed during the synthesis of [H(2)-6]Cl as a result of spontaneous HF elimination. Complex [8](+) may be converted to the neutral dicarbonyl chloro analog [11] by action of Me(3)NO. Related chemistry with analogous manganese complexes is observed. Thus, from the NH,NH-functionalized carbene manganese bromo tetracarbonyl [12], the diphosphine manganese carbene tricarbonyl cation [H(2)-13] may be readily prepared which provides the macrocyclic carbene-diphosphine tricarbonyl cation [14](+) following base promoted nucleophilic intramolecular displacement of fluoride. Again, [14](+) is converted to the neutral bromo dicarbonyl upon reaction with Me(3)NO. All complexes with the exception of the reaction intermediate [10] have been characterized by spectroscopic and analytical methods in addition to X-ray crystallographic structure determinations for complexes [3], [5], [H(2)-6]Cl, [H(2)-6][9], [8]Cl, [10], [11], [12], and [14]Br.

Synthesis and coordination chemistry of macrocyclic ligands featuring NHC donor groups

Poly-NHC (NHC = N-heterocyclic carbene) ligands emerged almost immediately after the first stable NHCs had been described. Macrocyclic ligands, featuring NHC donor groups and their metal complexes, however, remained rare until recently. This perspective highlights modern developments in the fields of synthesis and coordination chemistry of macrocyclic poly-NHC ligands. These include the synthesis of tetracarbene ligands which were obtained from complexes of β-functionalized isocyanides followed by cyclization of the coordinated iscocyanide ligands to NH,NH-functionalized NHCs and the subsequent metal template controlled bridging alkylation of the NH,NH-NHCs to yield the macrocycle. The template synthesis of ligands featuring a mixed NHC/phosphine donor set like [11]ane-P(2)C(NHC) and [16]ane-P(2)C(NHC)(2) by linkage of NH,NH-NHCs to different phosphines is also presented. Finally, methods for the preparation of cyclic polyazolium salts, their deprotonation and metalation and the different modes of coordination of such macrocyclic poly-NHC ligands are discussed.

Stepwise preparation of a molecular square from NR,NR- and NH,O-substituted dicarbene building blocks.

Metallosupramolecular self-assembly has been a field of intensive research ever since Lehn et al. first demonstrated the spontaneous self-assembly of dinuclear helicates from bipyridine and copper(I). Thereafter, different metallosupramolecular structures have been synthesized. Some of these feature internal cavities suitable for the encapsulation of small molecules. Reactive intermediates have been stabilized in such cavities and selected catalytic transformations have been carried out and accelerated inside metallosupramolecular assemblies. The molecular square [A] (Scheme 1) by Fujita et al. built up from end-capped Pd ions and 4,4’-bipyridine building blocks was among the first metallosupramolecular structures to be studied in detail. Related molecular squares were subsequently studied by Stang and Olenyuk and other research groups. Complex [A] and most other metallosupramolecular assemblies described to date are derived from polydentate ligands featuring either nitrogen or oxygen donor atoms coordinating to the metal centers. Supramolecular assemblies built from polydentate ligands with carbon donors are quite rare, although some examples with bridging diisocyanide, acyclic diaminocarbene, remote-NHC, or a,w-dicarbanion ligands have been described. The preparation of the molecular rectangle [B], which features rigid linear dicarbenes and 4,4’-bipyridine linkers, as well as cylindrical structures synthesized from macrocyclic and other poly-NHC ligands, have only recently been reported, while N-heterocyclic carbenes have developed into an important class of ligands in organometallic chemistry over the last 30 years. Herein we introduce a novel molecular square that is built up from four platinum(II) corners that are held together by bonds formed between platinum and the carbon atoms of two classical bis(diaminocarbene) and two di(NH,O-NHC) ligands. In contrast to the known metallosupramolecular structures featuring bridging ligands that form M C bonds, only Pt Ccarbene bonds are employed for the formation of the molecular square. Previous studies have established that the reaction of square-planar platinum(II) with sterically demanding diphosphines and rigid linear dicarbenes, obtained by double deprotonation of benzobisimidazolium salts, yielded a mixture of dinuclear dicarbene-bridged syn (minor) and anti complexes (major). Once formed, these isomers do not interconvert with the anti isomer being geometrically unfit for the construction of a molecular square. We have now found that a reduction of the steric demand of the ligands at platinum(II) leads to a lower barrier of rotation about the Pt Ccarbene bond, thereby allowing the formation of the dinuclear complex with syn geometry by interconversion of the potential geometrical isomers. The tetramethyl-substituted benzobisimidazolium salt 1 I2 was reacted with NaOAc and [PtCl2(dmpe)] instead of the previously used [PtCl2(dppe)] [16] to yield complex [2]I2 (Scheme 2; see Supporting Information; dmpe = bis(dimethylphosphino)ethane, dppe = bis(diphenylphosphino)ethane). The complex was identified by the characteristic chemical shift for the carbene carbon atom at d = 183.9 ppm, which was recorded as a multiplet owing to coupling with the phosphorus atoms. The high-resolution mass spectrometry (ESI, positive ions) shows the mass of the cationic complex ion [2] as peak of highest intensity. Only one multiplet was observed in the P{H} NMR spectrum at d = 30.9 ppm for the two chemically different phosphorus atoms. Scheme 1. Molecular square [A] and molecular rectangle [B] (R=n-butyl).

Nachweis des Gleichgewichts zwischen einem N‐heterocyclischen Carben und seinem Dimer in Lösung

Der direkte Nachweis des Wanzlick-Gleichgewichts zwischen dem Carben 1 und dem Dibenzotetraazafulvalen (1)2 bei Raumtemperatur (siehe Schema) wurde erbracht. Entetramine des Typs (1)2 lassen sich aber auch durch koordinativ ungesattigte Ubergangsmetallverbindungen unter Bildung von Dicarbenkomplexen spalten.