Structural Diversity of Copper(I)-N-Heterocyclic Carbene Complexes; Ligand Tuning Facilitates Isolation of the First Structurally Characterised Copper(I)-NHC Containing a Copper(I)-Alkene Interaction

The preparation of a series of imidazolium salts bearing N-allyl substituents, and a range of substituents on the second nitrogen atom that have varying electronic and steric properties, is reported. The ligands have been coordinated to a copper(I) centre and the resulting copper(I)–NHC (NHC=N-heterocyclic carbene) complexes have been thoroughly examined, both in solution and in the solid-state. The solid-state structures are highly diverse and exhibit a range of unusual geometries and cuprophilic interactions. The first structurally characterised copper(I)–NHC complex containing a copper(I)–alkene interaction is reported. An N-pyridyl substituent, which forms a dative bond with the copper(I) centre, stabilises an interaction between the metal centre and the allyl substituent of a neighbouring ligand, to form a 1D coordination polymer. The stabilisation is attributed to the pyridyl substituent increasing the electron density at the copper(I) centre, and thus enhancing the metal(d)-to-alkene(π*) back-bonding. In addition, components other than charge transfer appear to have a role in copper(I)–alkene stabilisation because further increases in the Lewis basicity of the ligand disfavours copper(I)–alkene binding

Electrochemistry in continuous systems

The use of continuous flow conditions for synthetic electrochemical reactions exhibits many benefits over more traditional batch conditions. This has resulted in many research groups developing continuous electrochemical reactors and their application. The benefits of continuous flow include increased Faradaic efficiencies resulting in reduced energy consumption, higher selectivities, and lower electrolyte loadings which decreases waste streams and simplifies purification. These improvements are largely derived from the smaller interelectrode gaps employed in continuous systems compared to batch protocols. While this may be perceived as a small change in terms of practical reaction setup, it presents many challenges associated with reactor design and development. This perspective will highlight reactor layouts designed to address some of these challenges

Making electrochemistry easily accessible to the synthetic chemist

A significantly renewed interest in synthetic electrochemistry is apparent in the increasing number of publications over the last few years. Electrochemical synthesis offers a mild, green and atom efficient route to interesting and useful molecules, thus avoiding harsh chemical oxidising and reducing agents used in traditional synthetic methods. As such, encouraging broader application of electrochemistry by synthetic chemists should be a priority. Despite the renewed interest there remains a barrier to widespread adoption of this technology derived from the extra knowledge and specialised equipment required. This has led to a knowledge gap between experienced electrochemists and those new in the field. In this tutorial we will bridge the knowledge gap by providing an easily accessible introduction which will enable synthetic chemists new to the field to explore electrochemistry. We will discuss mechanistic considerations, the setup of an electrochemical reaction with all its components, trouble shooting and selected examples from the literature

Alternating polarity for enhanced electrochemical synthesis

Synthetic electrochemistry has recently become an exciting technology for chemical synthesis. The majority of reported syntheses use either constant current or constant potential, however a few use nonlinear profiles – mostly alternating polarity – to maintain efficiency throughout the process, such as controlling deposits on electrodes or ensuring even use of electrodes. However, even though parameters that are associated with such profiles, such as the frequency, can have a major impact on the reaction outcome, they are often not investigated. Herein, we report the crucial impact that the applied frequency of the alternating polarity has on the observed reaction rate of Cu(I)–NHC complex formation and demonstrate that this can be manipulated to give enhanced yield that is stable over extended reaction times

A Straightforward Electrochemical Approach to Imine- and Amine-bisphenolate Metal Complexes with Facile Control Over Metal Oxidation State

Synthetic methods to prepare organometallic and coordination compounds such as Schiff-base complexes are diverse, with the route chosen being dependent upon many factors such as metal–ligand combination and metal oxidation state. In this work we have shown that electrochemical methodology can be employed to synthesize a variety of metal–salen/salan complexes which comprise diverse metal–ligand combinations and oxidation states. Broad application has been demonstrated through the preparation of 34 complexes under mild and ambient conditions. Unprecedented control over metal oxidation state (MII/III/IV where M=Fe, Mn) is presented by simple modification of reaction conditions. Along this route, a general protocol-switch is described which allows access to analytically pure FeII/III–salen complexes. Tuning electrochemical potential, selective metalation of a Mn/Ni alloy is also presented which exclusively delivers MnII/IV–salen complexes in high yield

Tethered N-Heterocyclic Carbene-Carboranyl Silver Complexes for Cancer Therapy

Silver complexes of tethered N-heterocyclic carbene-carboranyl ligands have been prepared and fully characterized. The first example of silver bonded directly to the cage of o-carborane has been identified in the solid state. The presence of a carboranyl N substituent on the N-heterocyclic carbene significantly enhances the in vitro cytotoxicity of the silver complex against HCT116 p53+/+ and HCT116 p53–/– colon cancer cells in comparison to a phenyl derivative. Conversely, the presence of a carboranyl on the backbone of a xanthine-derived N-heterocyclic carbene decreases the in vitro cytotoxicity of the silver complex in comparison to its phenyl derivative. Stability studies on the xanthine-derived ligands and complexes show that decomposition via deboronation occurs in hydrous dimethyl sulfoxide, which may attribute to the contrasting in vitro behaviors of the carborane-containing complexes

Mapping the properties of bidentate ligands with calculated descriptors (LKB-bid)

We have extended the Ligand Knowledge Base (LKB) approach to consider a broad range of bidentate ligands, varying donors, substituents and backbones, which gives rise to a diverse set of 224 ligands in a new database, LKB-bid. Using a subset of steric and electronic parameters described previously for bidentate P,P-donor ligands (LKB-PP), here this approach has been applied to a wider set of bidentate ligands, to explore how these modifications affect the properties of organometallic complexes. The resulting database has been processed with Principal Component Analysis (PCA), generating a “map” of ligand space which highlights the contribution of donor atoms and bridge length to the variation in ligand properties. This mapping of bidentate ligand space with DFT-calculated steric and electronic parameters has demonstrated that the properties of ligands with different donor atoms can be captured within a single computational approach, providing both an overview of ligand space and scope for the more detailed investigation and comparison of different ligand classes