1 INTRODUCTION 1.1 WHY STUDY ORGANOMETALLIC CHEMISTRY? Organometallic chemists try to understand how organic molecules or groups interact with compounds of the inorganic elements, chiefl y metals. These elements can be divided into the main group, consisting of the s and p blocks of the periodic table, and the transition elements of the d and f blocks. Main-group organometallics, such as n -BuLi and PhB(OH)2 , have proved so useful for organic synthesis that their leading characteristics are usually extensively covered in organic chemistry courses. Here, we look instead at the transition metals because their chemistry involves the intervention of d and f orbitals that bring into play reaction pathways not readily accessible elsewhere in the periodic table. While main-group organometallics are typically stoichiometric reagents, many of their transition metal analogs are most effective when they act as catalysts. Indeed, the expanding range of applications of catalysis is a COPYRIGHTEDmajor reason for the continued MATERIAL rising interest in organo- metallics. As late as 1975, the majority of organic syntheses had no recourse to transition metals at any stage; in contrast, they now very often appear, almost always as catalysts. Catalysis is also a central prin- ciple of Green Chemistry 1 because it helps avoid the waste formation, The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc. 1 2 INTRODUCTION for example, of Mg salts from Grignard reactions, that tends to accom- pany the use of stoichiometric reagents. The fi eld thus occupies the borderland between organic and inorganic chemistry. The noted organic chemist and Associate Editor of the Journal of Organic Chemistry , Carsten Bolm, 2 has published a ringing endorse- ment of organometallic methods as applied to organic synthesis: In 1989, OMCOS-VI [the 6th International Conference on Organometal- lic Chemistry Directed Toward Organic Synthesis] took place in Florence and . left me with the impression that all important transformations could — now or in the future — be performed with the aid of adequately fi ne-tuned metal catalysts. Today, it is safe to say that those early fi ndings were key discoveries for a conceptual revolution that occurred in organic chemistry in recent years. Metal catalysts can be found everywhere, and many synthetic advances are directly linked to . developments in cata- lytic chemistry. Organometallic catalysts have a long industrial history in the produc- tion of organic compounds and polymers. Organometallic chemistry was applied to nickel refi ning as early as the 1880s, when Ludwig Mond showed how crude Ni can be purifi ed with CO to volatilize the Ni in the form of Ni(CO)4 as a vapor that can subsequently be heated to deposit pure Ni. In a catalytic application dating from the 1930s, Co2 (CO)8 brings about hydroformylation, in which H2 and CO add to an olefi n, such as 1- or 2-butene, to give n-pentanal or n-pentanol, depending on the conditions. A whole series of industrial processes has been developed based on transition metal organometallic catalysts. For example, there is intense activity today in the production of homochiral molecules, in which racemic reagents can be transformed into single pure enantiomers of the product by an asymmetric catalyst. This application is of most sig- nifi cance in the pharmaceutical industry where only one enantiomer of a drug is typically active but the other may even be harmful. Other examples include polymerization of alkenes to give polyethylene and polypropylene, hydrocyanation of butadiene for nylon manufacture, acetic acid manufacture from MeOH and CO, and hydrosilylation to produce silicones and related materials. Beyond the multitude of applications to organic chemistry in indus- try and academia, organometallics are beginning to fi nd applications elsewhere. For example, several of the organic light-emitting diode (OLED) materials recently introduced into cell phone displays rely on organometallic iridium compounds. They are also useful in solid-state light-emitting electrochemical cells (LECs).3 Samsung has a plant that has been producing OLED screens since 2008 that use a cyclometallated COORDINATION CHEMISTRY 3 Ir complex as the red emitter. Cyclometallated Ru complexes may have potential as photosensitizers for solar cells. 4 Organometallic drugs are also on the horizon. Bioinorganic chemistry has traditionally been concerned with classical coordination chemistry—the chemistry of metal ions sur- rounded by N- or O-donor ligands, such as imidazole or acetate— because metalloenzymes typically bind metals via such N or O donors. Recent work has identifi ed a small but growing class of metalloenzymes with organometallic ligands such as CO and CN – in hydrogenases or the remarkable central carbide bound to six Fe atoms in the active site MoFe cluster of nitrogenase. Medicinally useful organometallics, such as the ferrocene-based antimalarial, ferroquine, are also emerging, together with a variety of diagnostic imaging agents. 5 The scientifi c community is increasingly being urged to tackle prob- lems of practical interest. 6 In this context, alternative energy research, driven by climate change concerns, 7 and green chemistry, driven by environmental concerns, are rising areas that should also benefi t from organometallic catalysis.8 Solar and wind energy being intermittent, conversion of the resulting electrical energy into a storable fuel is pro- posed. Splitting water into H2 and O2 is the most popular suggestion for converting this electrical energy into chemical energy in the form of H–H bonds, and organometallics are currently being applied as cata- lyst precursors for water splitting. 9 Storage of the resulting hydrogen fuel in a convenient form has attracted much attention and will prob- ably require catalysis for the storage and release steps. The recent extreme volatility in rare metal prices has led to “earth-abundant” metals being eagerly sought 10 as replacements for the precious metal catalysts that are most often used today for these and other practically important reactions. 1.2 COORDINATION CHEMISTRY Even in organometallic compounds, N- or O-donor coligands typical of coordination chemistry are very often present along with C donors. With the rise of such mixed ligand sets, the distinction between coor- dination and organometallic chemistry is becoming blurred, an added reason to look at the principles of coordination chemistry that also underlie the organometallic area. The fundamentals of metal–ligand bonding were fi rst established for coordination compounds by the founder of the fi eld, Alfred Werner (1866–1919). He was able to identify the octahedral geometric preference of CoL6 complexes without any of the standard spectroscopic or crystallographic techniques.11 4 INTRODUCTION Central to our modern understanding of both coordination and organometallic compounds are d orbitals. Main-group compounds either have a fi lled d level that is too stable (e.g., Sn) or an empty d level that is too unstable (e.g., C) to participate signifi cantly in bonding. Partial fi lling of the d orbitals imparts the characteristic properties of the transition metals. Some early-transition metal ions with no d elec- trons (e.g., group 4 Ti 4 + ) and some late metals with a fi lled set of 10 (e.g., group 12 Zn 2 + ) more closely resemble main-group elements. Transition metal ions can bind ligands (L) to give a coordination 2 + compound, or complex MLn , as in the familiar aqua ions [M(OH2 )6 ] (M = V, Cr, Mn, Fe, Co, or Ni). Together with being a subfi eld of organic chemistry, organometallic chemistry can thus also be seen as a subfi eld of coordination chemistry in which the complex contains an M–C bond (e.g., Mo(CO)6 ). In addition to M–C bonds, we include M–L bonds, where L is more electropositive than O, N, and halide (e.g., M–SiR3 and M–H). These organometallic species tend to be more covalent, and the metal more reduced, than in classical coordination compounds. Typical ligands that usually bind to metal ions in their more reduced, low valent forms are CO, alkenes, and arenes, as in Mo(CO)6 , Pt(C2 H4 )3 , and (C6 H6 )Cr(CO)3 . Higher valent states are beginning to play a more important role, however, as in hexavalent WMe6 and pentavalent O = Ir(mesityl)3 (Chapter 15 ). 1.3 WERNER COMPLEXES 3 + In classical Werner complexes , such as [Co(NH3 )6 ] , a relatively high valent metal ion binds to the lone pairs of electronegative donor atoms, typically, O, N, or halide. The M–L bond has a marked polar covalent character, as in Ln M–NH3 , where Ln represents the other ligands present. The M–NH3 bond consists of the two electrons present in lone pair of free NH3 , but now donated to the metal to form the complex. Stereochemistry The most common type of complex, octahedral ML6 , adopts a geometry (1.1 ) based on the Pythagorean octahedron. By occupying the six ver- tices of an octahedron, the ligands can establish appropriate M–L bonding distances, while maximizing their L···L nonbonding distances. For the coordination chemist, it is unfortunate that Pythagoras decided to name his solids after the number of faces rather than the number of vertices. The solid and dashed wedges in 1.1 indicate bonds that point toward or away from us, respectively: WERNER COMPLEXES 5 The assembly of metal and ligands that we call a complex may have a 2 − net ionic charge, in which case it is a complex ion (e.g., [PtCl 4 ] ). Together with the counterions, we have a complex salt (e.g., K2 [PtCl4 ]). In some cases, both cation and anion may be complex, as in the pictur- esquely named Magnus ’ green salt [Pt(NH3 )4 ][PtCl4 ], where the square brackets enclose the individual ions.