The Diversity of the Metal-Ligand Interplay in Coordination Chemistry†

On the occasion of the 63th Conference of the Japan Society of Coordination Chemistry, held in Okinawa from November 2-4, 2013, it appears appropriate to recall that this year marks the 100th anniversary of the award of the Nobel Prize to Alfred Werner (1866-1919) "in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry". His pioneering achievements laid the foundation of Coordination Chemistry. Elucidating the nature of the metal-ligand interactions and recognizing their central role in determining the structures and properties of coordination complexes soon became subjects of major endeavour. The complexity of some of the ligands used today contrasts with those found in Wernerʼs chemistry (halides, water, ammonia, ethylenediamine,...) and they often require multistep organic synthesis. Interestingly, in the days of this chemistry giant, the notion that metal-metal bonding could occur within molecules, and not only within bulk metals, was non-existent. Later on, it was recognized that such bonding could exist and be responsible for holding together dior trinuclear complexes, as in [Mn2(CO)10] 2 or [Ru3(CO)12]. 3 The new and fast growing field of cluster chemistry became coined a “post-Wernerian chemistry”. Already in 1999, a 3 volumes book was not sufficient to cover all the aspects of the synthetic, structural and theoretical chemistry of metal clusters and their diverse applications in numerous fields of modern science. In this Account, we shall examine selected examples of complexes in which the ligands play an essential role in determining the overall molecular properties. We will limit ourselves to those relevant to catalytic applications in ethylene oligomerization. Some polynuclear complexes and metal clusters, where the nuclearity and the nature of the metals present are of primary concern, will also be discussed. The molecular chemistʼs toolbox allows the combination of ligands, which can range from a single atom to complex biorelevant molecules, and metals to form monoor polynuclear metal complexes. When metal centres are associated within the same molecule through metal-metal interactions, clusters This Account will focus on some recent aspects of the research performed in the authorʼs laboratory. Selected examples of complexes in which the ligands play an essential role in determining the molecular properties will be examined. Concerning the applications of these complexes, we will limit ourselves to some aspects related to the catalytic oligomerization of ethylene. In the search for such catalysts, we found that deprotonation of N,OH ligands chelated to Ni(II) with NaH can give rise to a range of heterometallic Na-Ni polynuclear complexes. In addition to their catalytic relevance, their magnetic properties were also investigated and have revealed in some cases Single Molecule Magnet behaviour. The diverse functions that co-catalysts may exert will be illustrated by the characterization of Cr(III) complexes obtained by reaction of fac-[Cr(NpyPNpy)Cl3] with typical cocatalysts such as MAO, EtAlCl2, AlMe3 and AlEt3. We will also discuss some polynuclear and cluster chemistry where a prediction of the nuclearity and composition of the metal cores remains difficult or impossible. This emphasizes the importance of basic research and the need to recognize the contribution of serendipity. The relevance of cluster chemistry to various fields of molecular synthesis and activation, catalysis and nanosciences, will also be mentioned.