Mareike C. Jahnke

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.

Chemistry of N-Heterocyclic Carbene Ligands

N-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes.

A Versatile Methodology for the Regioselective C8-Metalation of Purine Bases

Purine nucleobases are excellent ligands for metal ions, forming normally coordinative Werner-type bonds by utilizing the N donor atoms of the nucleobase skeleton. Here we show that purines such as 8-chlorocaffeine and 8-bromo-9-methyladenine react with [Pt(PPh3)4] under oxidative addition of the C(8)-halogen bond to the metal center. The resulting Pt(II) complexes feature a C(8)-bound ylidene ligand. Protonation of the ylidene at the N(7/9)-atom yields complexes bearing a protic N-heterocyclic carbene ligand derived from the purine base with an NMe,NH-substitution pattern.

Platinum Complexes with Picoline-functionalized Benzimidazolin-2-ylidene Ligands

The dicarbene platinum complexes of the type [Pt(L)2]Br2 [5]Br2 - [8]Br2 (L = N-alkyl-N´- picolylbenzimidazolin-2-ylidene) have been prepared by two different methods. The in situ deprotonation of picoline-functionalized benzimidazolium salts 1 - 4 with platinum acetylacetonate gave the platinum complexes [5]Br2 - [8]Br2 in good yields. Complex [8]Br2 has also been obtained by a ligand transfer reaction from the silver dicarbene complex [9][AgBr2]. Attempts to cystallize [8]Br2 obtained from the carbene transfer reaction led to the isolation of the previously undetected monocarbene complex [Pt(Cl)2L] (10) which contains only one picoline-functionalized carbene ligand coordinating in a chelating fashion to the metal center Graphical Abstract Platinum Complexes with Picoline-functionalized Benzimidazolin-2-ylidene Ligands

Molecular Structures and Catalytic Activity of Palladium Complexes Derived from Lutidine-bridged Bis(benzimidazolin-2-ylidene) Ligands

Reaction of lutidine-bridged dibenzimidazolium dibromides 1 - 4 with palladium acetate gives pincer-type palladium complexes of the type [Pd(L)Br]Br [5]Br-[8]Br. Crystals suitable for an X-ray diffraction study have been obtained by slow evaporation of the solvent from dichloromethane/methanol solutions of [7]Br and [8]Br. The crystal structure of [7]+ reveals a pincer topology of the cationic complex with a distorted square-planar coordination geometry at the metal center. From a solution of complex [8]Br, a dinuclear byproduct [9]+ was obtained with two bis(benzimidazolin- 2-ylidene) ligands coordinating in a bridging fashion. The pincer-type palladium complexes [5]Br- [8]Br were tested as precatalysts in Suzuki coupling reactions.

Oxidative addition of N-ether-functionalized 2-chlorobenzimidazole to d10 metals

Abstract Reaction of 2-chloro-N-(methoxymethyl)benzimidazole 1 with zerovalent group 10 metal complexes in the presence of an additional proton source yielded, via an oxidative addition of the C2–Cl bond, complexes with a protic NH,NR-substituted (R=methoxymethyl) benzimidazolin-2-ylidene ligand. The oxidative addition of 1 to Ni0 and Pd0 complexes proceeded with the exclusive formation of the trans-configured complexes trans-[2]BF4 and trans-[3]BF4, respectively. Contrary to this observation, the reaction of 1 with a more substitution inert Pt0 complex leads, depending on the reaction temperature, to a mixture of cis-/trans-[4]BF4 or exclusively to trans-[4]BF4. The molecular structures of all three trans-configured complexes were determined.

Digold(I) Complexes Derived from 5,5′-Bibenzimidazolin-2-ylidene Ligands

Abstract The 5,5′-bibenzimidazolium dibromide salts 2 and 3 have been prepared by fourfold N-alkylation of 5,5′-bibenzimidazole (2: R=Pr; 3: R=Bu). The diazolium salts were treated with silver oxide, and the in situ-formed silver complexes were subsequently reacted with [AuCl(SMe2)] to give the dinuclear gold complexes 4 and 5. The molecular structure of complex 5 has been determined by X-ray diffraction showing linearly coordinated gold(I) centers and, most likely due to steric crowding around the metal centers, no aurophilic interactions.

Pincer complexes derived from benzimidazolin-2-ylidene ligands

Benzimidazolin-2-ylidene ligands are the type of N-heterocyclic carbines (NHC) that exhibit a different behavior compared to imidazolin-2-ylidenes because of its intermediate position between the saturated and unsaturated N-heterocyclic carbenes. Various procedures for the synthesis of complexes with benzimidazolin-2-ylidene ligands have been reported. Among those are the reaction of the stable carbene and the cleavage of a dibenzotetraazafulvalene with coordinatively unsaturated transition metals. In addition, the reaction of benzimidazolium salts with complexes that bear basic ligands, such as metal acetates or silver oxide, affords the corresponding carbene complexes. The template-controlled cyclization of β-functionalized phenyl isocyanides results in complexes with NH- and NH -stabilized benzimidazolin-2-ylidene ligands. The N-alkylation of such carbene ligands allows the introduction of various functional groups. Based on the variety of synthetic methods for the preparation of benzimidazolium salts and stable benzimidazolin-2-ylidene, the chapter illustrates the preparation of complexes with pincer topology. These complexes contain benzimidazolin-2-ylidene donors.