N-Heterocyclic Carbenes in Late Transition Metal Catalysis

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 β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

2.3.5. Multi-NHCs Linked by Spacers 3568 2.4. The Ag2O Route 3570 2.4.1. Feasibility 3570 2.4.2. Complications 3571 2.4.3. Theoretical Consideration 3572 2.5. Applications 3572 2.5.1. Ag(I)-NHCs in NHC Transfer 3572 2.5.2. Ag(I)-NHCs in Catalysis 3572 2.5.3. Ag(I)-NHCs in Medicine 3572 2.5.4. Ag(I)-NHCs in Nanomaterials 3573 3. Au(I)and Au(III)-NHCs 3573 3.1. Historical Background 3573 3.2. General Synthetic Methods 3573 3.3. Formation of Au(I)and Au(III)-NHCs 3574 3.3.1. Neutral [Au(NHC)L] 3574 3.3.2. Ionic [Au(NHC)L][Anion] 3576 3.3.3. Multinuclear Au(I)-NHCs 3578 3.3.4. Other Classes of Au(I)-NHCs 3578 3.3.5. Au(III)-NHC Complexes 3579 3.4. Applications 3579 3.4.1. Au(I)and Au(III)-NHCs in Catalysis 3579 3.4.2. Au(I)-NHCs in Medicine 3580 4. Cu(I)and Cu(II)-NHCs 3581 4.1. Historical Background 3581 4.2. General Synthetic Methods 3582 4.3. Formation of Cu(I)and Cu(II)-NHCs 3583 4.3.1. Complexes Containing the Cu(NHC)2 Core 3583 4.3.2. [Cu(NHC)(Halide)] 3583 4.3.3. [Cu(NHC)(Ligand)] 3584 4.3.4. Multinuclear Cu(I)and Cu(II)-NHCs 3589 4.4. Catalysis 3591 4.4.1. Past Events 3591 4.4.2. Recent Advancements 3591 5. Photoluminescence 3592 6. Conclusions 3594 7. Abbreviations 3594 8. Acknowledgments 3595 9. References 3595

Coinage Metal−N-Heterocyclic Carbene Complexes

ion of the chloride ligands, compound 2 was further reacted with MeMgCl, yielding the bimetallic compound 3 in which the biscarbene ligand is bridging two Cr(II) atoms, also bridged by two methyl ligands. Recent studies by Arnold and co-workers showed that the incorporation of an anionic functional group (alkoxide, amido, or amino) in an NHC unit allows the preparation of tridentate ligands capable of coordinating to d0 early transition metals such as yttrium, titanium, and zirconium.87 The lithium aminodicarbene chloride complex 5 reacts with Y[N(SiMe3)2]3 affording the yttrium(III) complex 6 (Scheme 10). Hexacoordinated titanium (7) and zirconium (8) complexes were synthesized by reaction of 4 with M(NEt2)4 (M ) Ti, Zr) (Scheme 10). Complexes 7 and 8 constituted the first examples of group 3 and 4 metal complexes with monoanionic biscarbene ligands. Another interesting example of early transition metal NHC-based complexes was described by Smith and coworkers.88 By the reaction shown in Scheme 11, the tris(NHC)borate 989,90 afforded a tricarbonyl complex of Mn(I) with a tripodal tris-NHC ligand, 10. The analysis of the ν(CO) bands on the IR spectrum of this complex showed that 9 is the most electron-donating tripodal ligand compared with all others that have been bound to the same Mntricarbonyl fragment. Compound 10 is air-sensitive and is easily oxidized to a homoleptic Mn(IV) complex (11, Scheme 11), which is the first example of a Mn(IV)-NHC complex reported to-date. 4.2. Catalytic Applications Although many catalytic applications of late transition metal complexes bearing poly-NHC ligands have been described, those of early transition metals are restricted to a few examples. In 2003, Gibson and co-workers reported Cr(III)-based ethylene oligomerization precatalysts that incorporate a CNC-pincer carbene ligand.91,92 More recently, McGuinness shed some light on the mechanism of the reaction with the mentioned pincer complexes in combination with MAO.93 The Cr(II) and Cr(III) complexes described by Theopold were tested for polymerization of ethylene with a MAO cocatalyst, showing low activities and a broad weight distribution of the polymers.85 In particular, Cr(II) compounds such as 2 (Scheme 9) were unreactive to ethylene when exposed to MAO, showing that the more Lewis acidic Cr(III) performs better in this reaction. The authors pointed out the ease with which the Cr(III) complexes are reduced to Cr(II), arguing that very strong σ-donating but soft NHC ligands may have a stronger affinity for the softer Cr(II) rather than the harder Cr(III). Thus, they concluded that this ligand system may be better suited for lower oxidation state chromium chemistry and that, due to the inactivity of the Cr(II) compounds tested, it would not lead to successful results in ethylene polymerization. 5. Poly-NHC Ligands in Complexes of Group 8 Metals

Complexes with poly(N-heterocyclic carbene) ligands: structural features and catalytic applications.

Heterocyclische Carbene – Synthese und Koordinationschemie

Homoleptic carbene complexes

Modes of Reaction of 2‐ and 3‐Hydroxyalkyl Isocyanides with Metal Compounds; Oxazolidinylidene and Perhydrooxazinylidene Complexes of Palladium, Platinum, and Gold

Darstellung, Eigenschaften und IR-Spektren von L2Pd(N3)2 (L = Phosphin, Amin), [(C6H5)3P]2MN3 (M = CuI, AgI), (C6H5)3PAuN3, [(C6H5))3P]3MN3 (M = AuI, RhI), [(C6H5)3P]4Rh2(N3)2 und [(C6H5)3P]2Ir(CO)N3 werden beschrieben. Die Palladium-, Gold-, Rhodium- und Iridiumverbindungen reagieren in Chloroform- oder Benzollosung mit Kohlenmonoxid unter Bildung der entsprechenden Isocyanato-Komplexe. Auch die dimere Verbindung [(C6H5)3PPd(N3)2]2 bildet mit CO einen Komplex [(C6H5)3PPd(NCO)2]2, der vermutlich Isocyanat-Brucken enthalt. Der Mechanismus dieser Reaktionen wird aufgrund orientierender kinetischer Untersuchungen diskutiert.

Bildung von Isocyanato‐Komplexen aus komplexen Metallaziden und Kohlenmonoxid

With Sharpless' and Meldal's discovery of the immensely supportive effect that metal catalysis has on Huisgen's classical 1, 3-dipolar cycloaddition, azides (RN3) – long underappreciated in organic synthesis – suddenly got in the focus of attention as most crucial players in sensational ‘click chemistry'. Less noisy though with the same commitment and even a much broader scope of scientific topics and objectives, the inorganic azide chemistry has made just as great strides in the last few decades. This review (Part I) gives an introductory survey of the most important results, and informs about modern developments and general trends. Particular emphasis is placed on the recent successful approaches to highly unstable homoleptic azido metal complexes of the main group and early transition elements, as well as on the enormous structural versatility caused by the ‘flexidentate' N3– ligand with its unsurpassed bridging capacities. The presentation in this paper of selected compounds and reactions is meant, in a way, as a prelude to the [3+2]-cycloadditions of metal azides and related species which will be covered in-depths in Part II. A large part of the comments finally deals with applications in fields such as catalysis, high explosive performance or magnetism of metal compounds containing azide, today certainly one of the most attractive research areas world-wide.

Azide Chemistry – An Inorganic Perspective, Part I Metal ­Azides: Overview, General Trends and Recent Developments

In whatever state of bonding – whether covalent to an organic residue or a heteroatom, or polar to ionic in contact with a metal – the azide moiety N3 is characterized by its high potential of reactivity which essentially manifests itself in two basic processes: the elimination of dinitrogen and the entry into 1, 3-dipolar cycloadditions with suitable dipolarophiles, the latter of which clearly predominates the chemistry of azide, also that of its metal compounds. In a preceding review entitled “Part I – Metal Azides: Overview, General Trends and Recent Developments” which was meant to lay the foundations for the present paper, these and other reactions have already been touched upon. The present review – Part II – now focusses in great detail on the formation of five-membered heterocycles – tetrazol(at)es, triazol(at)es, triazolin(at)es, thiatriazol(at)es, etc. as well as various consecutive products – from azide and nitriles, isocyanides, alkynes, alkenes and heteroallenes (CS2, RN=C=S) in the ligand sphere of the metal. Generally, these [3+2]-cycloadditions are found to proceed under much milder conditions in comparison with the strictly organic case whose triumphant progress since the 1960s is intimately bound up with the name of Huisgen. Mechanistic considerations on the matter are presented. A secondary aspect still occupying quite a part of the discussion is concerned with the role of metals in [3+2]-cycloadditions particularly of the highly topical “click”-type, e.g. (CuAAC), (RuAAC). Likewise, a short chapter deals with the question of pentazol(at)e (N5–) which according to numerous theoretical studies could well be stabilized and isolated in combination with metals, e.g., in the form of azametallocenes. A last chapter is devoted to a cursory survey of related systems, in particular fulminato complexes, metallo nitrile ylides and metallo nitrile imines, in which the metal acts as a substituent on the 1, 3-dipole (metallo-1, 3-dipole). Other systems with a metal substituent on the dipolarophile (metallo-dipolarophile), or, with metal itself in the three- (two-) atom arrangement constituting the dipole (dipolarophile) [metalla-1, 3-dipole, metalla-dipolarophile] are only quoted by way of example.

Wolf Peter Fehlhammer

Papers

Homoleptic carbene complexes

Modes of Reaction of 2‐ and 3‐Hydroxyalkyl Isocyanides with Metal Compounds; Oxazolidinylidene and Perhydrooxazinylidene Complexes of Palladium, Platinum, and Gold

Bildung von Isocyanato‐Komplexen aus komplexen Metallaziden und Kohlenmonoxid

Azide Chemistry – An Inorganic Perspective, Part I Metal Azides: Overview, General Trends and Recent Developments

Azide Chemistry – An Inorganic Perspective, Part II[‡] [3+2]-Cycloaddition Reactions of Metal Azides and Related Systems†

Wolf Peter Fehlhammer

Papers

Homoleptic carbene complexes

Modes of Reaction of 2‐ and 3‐Hydroxyalkyl Isocyanides with Metal Compounds; Oxazolidinylidene and Perhydrooxazinylidene Complexes of Palladium, Platinum, and Gold

Bildung von Isocyanato‐Komplexen aus komplexen Metallaziden und Kohlenmonoxid

Azide Chemistry – An Inorganic Perspective, Part I Metal ­Azides: Overview, General Trends and Recent Developments

Azide Chemistry – An Inorganic Perspective, Part II[‡] [3+2]-Cycloaddition Reactions of Metal Azides and Related Systems†

Azide Chemistry – An Inorganic Perspective, Part I Metal Azides: Overview, General Trends and Recent Developments