The monomeric ferric hemepeptide from cytochrome c, N-acetylmicroperoxidase-8 (NAcMP8), retains the proximal His ligand and has a coordinated water molecule in the second axial site. It provides an opportunity for quantitatively studying in aqueous solution the coordination chemistry of an iron porphyrin with a single accessible coordination site. An examination of the dependence of the spectrophotometrically determined equilibrium constants for coordination of imidazole and cyanide as a function of pH provides a procedure for correcting conditional equilibrium constants for the ionization of coordinated H2O in NAcMP8 and protonation of the entering ligand, L. An examination of the coordination by NAcMP8 of L = primary amines and imidazoles (25 °C, μ = 0.1 M) and pyridines (as a function of temperature, μ = 0.1 M) has shown that the process of binding L to NAcMP8 may be complex. With some ligands such as pyridines and some primary amines there is evidence for formation of an outer-sphere complex before displacement of H2O occurs. In some cases, a third process occurs at high concentrations of L which may entail substitution of the proximal His ligand. Pyridines and imidazoles fall into different classes in their basicity−affinity for Fe(III) relationships; primary amines fall into two distinctive classes, with one class having a higher affinity for Fe(III) than expected from basicity alone. It is suggested that neutral or cationic functional groups in the side chain of these ligands provide a second site of attachment to the iron porphyrin by hydrogen bonding. A compensating effect between ΔH and ΔS values for coordination of pyridines to NAcMP8 is demonstrated; hence log K values, rather than the individual thermodynamics parameters, were used to explore the possible influence of the electronic structure of the ligands on the magnitude of their affinity for Fe(III). Log K for coordination of pyridines and imidazoles correlates linearly, and statistically meaningfully, with the energies of the frontier orbitals of σ and π symmetry (determined by ab initio or semiempirical molecular orbital methods at the RHF level of theory using, respectively, the 6-31G* split-valence polarized basis set and the PM3 model), which have large amplitude on the donor N atom. There is only a weak correlation between log K and the energy of the frontier σ orbital for primary amines. These observations suggest that both σ bonding and π bonding between planar aromatic ligands and Fe(III) play a significant role in determining the magnitude of the binding constant of these ligands to a ferric porphyrin.

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Since the discovery of the stable N-heterocyclic carbenes (NHCs) in the early 1990s, research on this important class of compounds has boomed. Their structural versatility and functionalisation enables them to display an array of exploitable and tunable properties. Their strong coordination to metal centres, especially to late transition metals, makes NHCs not only suitable as organocatalysts, but also as organometallic catalysts. This has had far-reaching implications for the entire chemistry community.Although the role of the NHCs in organometallic chemistry is now well established, these compounds continue to inspire researchers to delve further into their vast chemistry. As a leading society journal for Inorganic Chemistry, EurJIC aims to serve the needs of its broad-based inorganic chemistry audience. With this in mind, the idea to present a cluster of papers in one issue, which focuses on the exciting developments in this topical field but covers the broadest variety of aspects as possible, was born. Together with Guest Editor Steven Nolan, it is our pleasure to offer you this Cluster Issue, which captures some of the many notable advances in NHC chemistry by many of the key players in the field. The application of these metal-based NHCs as catalysts and precatalysts in cross-couplings, polymerisations, aminations, hydrogenations and hydrosilylations, amongst others, is highlighted in this issue. Their use as precursors for the activation of inert bonds and the use of chiral NHCs in asymmetric catalysis are also featured. A range of metals are employed, from the lanthanides, to ruthenium and rhenium, to rhodium and iridium, to nickel and palladium, and to copper through to gold. Functionalisation of NHCs towards increasing their catalytic potential, examination of catalytic pathways, investigation of structureϪactivity relationships, all are presented to give you a taste of what the NHCs have to offer. In addition to the preparative and application-based catalysis offered in this issue, there are a number of contributions that emphasise the importance of theoretical investigations on NHCs, be it by the determination of the buried volume of the NHC ligands or by the study of tautomerism in NHC complexes. Moreover, the versatility of NHC complexes is further exemplified by the antimicrobial activity of silver carbene complexes.This issue is a compilation of contributions from all corners of the globe, which makes this Cluster Issue truly universal. We certainly strive to live by EurJIC's motto "Made in Europe for the World". The cover picture of this issue aims to showcase the widerange scope of the NHCs, and we therefore exhibit a composite cover that encompasses the different aspects of NHC chemistry presented in this issue. Contributors to this cover are Christopher Bielawski, Luigi Cavallo, Cathleen Crudden, Lionel Delaude and Wiley Youngs et al.; with their designs they hope to entice you into the world of the NHCs. We trust that you will find this issue stimulating, one that helps spark off many great i...