Dalton Transactions Blog

This collection of articles is dedicated to Professor Malcolm L. H. Green on the occasion of his 80^th birthday.  Malcolm was born in Eastleigh, Hampshire, on the 16^th of April, 1936. He received his B. Sc. (Hons) in 1956 from the University of London (Acton Technical College) and his Ph. D. in 1959 from Imperial College of Science and Technology, where he studied under Professor Sir Geoffrey Wilkinson. Following Imperial College, Malcolm moved to Cambridge University in 1960, and finally to Oxford University in 1963, where he was appointed University Lecturer and Septcentenary Fellow in Inorganic Chemistry at Balliol College.  On the 2^nd of January 1965, he married Jennifer Green (née Bilham), with whom he has also enjoyed a long time scientific collaboration.  Malcolm became a Fellow of the Royal Society in 1985 and was appointed Statutory Professor and Head of the Inorganic Chemistry Laboratory and Professorial Fellow of St. Catherine’s College in 1989.  He became a research active Emeritus Professor in 2003 and continues to publish to this day.

	Professor Malcolm L. H. Green

Malcolm’s contributions to inorganic chemistry are numerous and varied.  His first publication, which bears the title “Bis‑cyclopentadienylrhenium hydride”,  appeared in 1958 and was based on his Ph. D. thesis which was entitled “A study of some transition metal hydrides and olefin complexes.”  To date, he has published more than 600 papers describing synthetic, structural, and mechanistic aspects of the chemistry of virtually every transition element.  In order to give the reader a flavour of the research that Malcolm has performed, the present collection provides a selection of his papers that have been published in journals of the Royal Society of Chemistry over a period of more than fifty years.  For example, these papers show how Malcolm’s research popularized the now ubiquitous molybdenocene and tungstenocene systems which provided evidence for alpha‑ and beta‑hydrogen migratory insertion reactions, and also early examples of C–H bond activation.

Malcolm is well known for his synthetic achievements, an important aspect of which was his development of multigram-scale metal vapour synthesis, in which metals are vapourized and condensed with a reactive ligand.  This technique allowed him to obtain molecules such as dibenzene titanium, zirconium and hafnium, the first zerovalent compounds of these elements.  Likewise, Malcolm also employed metal vapour synthesis to isolate Mo(PMe₃)₆, a highly reactive electron-rich molecule.

His discovery that the simple molecule (dmpe)TiCl₃(CH₂CH₃) exhibits a direct bonding interaction between the titanium and the β‑C‑H moiety, an interaction which he named agostic, must be regarded as one of the most important discoveries in the field of organometallic chemistry.  Furthermore, in terms of mechanistic studies, Malcolm proposed a mechanism for the stereospecific Ziegler-Natta polymerization of olefins (the so-called Green-Rooney mechanism) and also, together with Mingos and Davies, formulated a series of rules to predict the stereospecificity of nucleophilic addition to p-coordinated ligands attached to a transition metal.

While Malcolm is best known for molecular chemistry, he also developed the field of organometallic solid state chemistry, which included the first example of an organometallic compound with a large second order non-linear optical behaviour, and many examples of organometallic intercalation compounds.

Malcolm’s reasearch has not been restricted to organometallic chemistry.  For example, in his later years, he focused much effort into developing the foundational chemistry of C₆₀ and carbon nanotubes.  His research in heterogenous catalysis, in which he discovered an excellent metal carbide catalyst for the Fischer-Tropsch conversion of synthesis gas to hydrocarbons, also resulted in the creation of the Oxford Catalysts Group (subsequently Velocys), of which he is a co-founder.

Finally, it is important to note that Malcolm has contributed much to the community by his development of a new approach for classifying covalent compounds, namely the Covalent Bond Classification (CBC) method.  This approach, which offers considerable advantages over that employing oxidation states, has now been widely adopted, to the extent that it is also used in describing materials chemistry.

Not surprisingly, his research has been widely recognized by numerous awards, some of which include:  The Royal Society of Chemistry Corday‑Morgan Medal and Prize in Inorganic Chemistry (1974); the Chemistry Society Medal in Transition Metal Chemistry (1978); the Royal Society of Chemistry Tilden Lectureship and Prize (1982); The J. C. Bailar Lecture and Medal, University of Illinois (1983); the American Chemical Society Award in Inorganic Chemistry (1984); the Royal Society of Chemistry Medal in Organometallic Chemistry (1986); the Royal Society of Chemistry Sir Edward Frankland Prize (1989); The Karl‑Ziegler Prize of the Gesellschaft Deutscher Chemiker (1992); the Davy medal of the Royal Society (1995) ; American Chemical Society award in Organometallic Chemistry (1997) and The Royal Society of Chemistry Sir Geoffrey Wilkinson Medal and Prize (2000).  Other recognitions of his work include:  University of Western Ontario, Visiting Professor (1971); Ecole de Chimie and Institute des Substances Naturalles, Paris, Visiting Professor (1972); Harvard University, A. P. Sloan Visiting Professor (1973); Pacific West Coast Lecturer in Inorganic Chemistry (1977); Sherman Fairchild Visiting Scholar at the California Institute of Technology (1981); Karl Ziegler Gastprofessor, Max Plank Institute, Mulheim (1983); Hutchinson Lectureship, University of Rochester (1983); The University Lecturer in Chemistry, University of Western Ontario (1984); Debye Lecturer, Cornell University (1985); Wuhan University, PRC, Visiting Professor (1985); Julius Stieglitz Lecturer, University of Chicago (1986); Frontiers of Science Lecturer, Texas A & M University (1987); The DuPont Lecturer, Indiana University (1989/90); The Ida Beam Lecturer, University of Iowa (1990); The Glenn T. Seaborg Lecturer in Inorganic Chemistry at the University of California, Berkeley (1991); The South‑East Lecturer in Inorganic Chemistry, USA (1991); The Walter Heiber Gastprofessor, University of Munich (1991); The Pacific Coast Lecturer (1994); The Rayson Huang Visiting Lecturer, Hong Kong (1995); The A. D. Little Lecturer, Massachusetts Institute of Technology (1995); The Stauffer Lecturer, University of Southern California (1996); the Dow Lecturer at the University of Ottowa (1996); and the James Walker Memorial Lecture, University of Edinburgh (1996); Doutor Honoris Causa, University of Lisbon, Portugal (1997); The Frank Dyer Medal, University of New South Wales (1997); The Fred Basolo Medal and Lecture., Northwestern University (1998); Ernest H. Swift Lectureship, California Institute of Technology (1998); Lewis Lecture, Cambridge UK (2001); FMC Lecturer, Princeton (2001); Distinguished Visiting Professor, Hong Kong University (2002);  Eastman Company Distinguished Lecturer in Inorganic Chemistry, University of North Carolina (2002); 34^th Camille and Henry Dreyfus Lecturer, Dartmouth College (2002); Raymond Siedle Lecturer, Indiana University (2004); Bert Vallee Visiting Professor at Harvard University (2004); Falk-Plaut Lecturer, Columbia University, New York (2006); Prix Franco-Briitannique, Societé,  Francaise de Chemie (2007); and, most recently, the European Prize for Organometallic Chemistry (2015).

The breadth and originality of Malcolm’s work collected here underscores his fearless and often iconoclastic approach to chemistry. Armed with his pipe and cigarette lighter (essential for checking  new products in the lab) Malcolm infected new students with his enthusiastic approach by asking them to prepare starting materials on big scales, whether it was 3 moles of trimethylphosphine, a kilogram of molybdenum pentachloride, or more than 100 grams of  tungstenocene dihydride.  Many students encountered some of his more ambitious ideas in the King’s Arms, where original experiments and apparatus were dreamed up on the back of a beer mat.  Malcolm’s seminars became legendary for the anthropomorphism of his chemical intuition, where both mechanism and apparatus were often described in mime.  Every student of Malcolm went away not only with a broad training in inorganic chemistry, from organometallic to solid state, but also with a wealth of often hilarious stories from their time in the group.

In closing, Malcolm has been at the forefront of organometallic chemistry for more than a half-century and we hope that this collection serves as a simple means to highlight some of his significant achievements.

This themed collection of articles in Dalton Transactions is dedicated to Ken Wade, who very unexpectedly passed away on 16^th March 2014.  While this sad loss is the reason for this commemoration, it is an honour for me to write this preface.

I had the pleasure and privilege of getting to know Ken personally during my student days at Durham, a relationship that continued when, much more recently, I became a colleague in the same Department.

I first met Ken when, as a second year undergraduate, he introduced us to aspects of structure and bonding.  Only after looking up the subject in the textbooks, did I realise that the relationships he was describing were in fact ‘Wade’s rules’.

It is a vivid reflection of Ken’s humble and self-effacing character that he himself only ever referred to these rules as polyhedral skeletal electron pair theory (PSEPT) or even occasionally during his lectures or in discussions as “Blogg’s Rules”!

Ken’s pioneering work exploring and explaining the structures of deceivingly unrelated clusters and ring systems, which developed primarily as a result of his insightfulness, logical thinking and intuition, now means that his name is familiar to every inorganic chemist, from undergraduate student to senior research professor.

This work lead to the formalised relationship between structure and electron counting in polyhedral cluster systems: relationships that are known universally as Wade’s Rules (J. Chem. Soc. D, 1971, 792).

This set of deceptively simple rules has been developed and expanded into what are known today as the Wade-Mingos Rules, which continue to be taught to chemistry students worldwide as a fundamental tool of Inorganic Chemistry.

Ken’s research career started in 1954 when he began his Ph.D. at the University of Nottingham with Norman Greenwood, probing the addition compounds of gallium and boron trichlorides.  This was followed by two postdoctoral positions: first at Cambridge with Harry Emeléus, working on “assorted reactions of diborane”, and second, at Cornell University with Albert Laubengayer, where he investigated various aspects of organonitrogen-aluminium chemistry.

In 1960, Ken returned to the UK to take up his first independent academic position as a lecturer at Derby College of Technology, before moving north to take up a lectureship at the University of Durham a year later, where his interests in synthetic main group chemistry continued to expand.

Although universally known for his electron counting rules, Ken’s innovative and creative character also led him to be major contributor to the areas of azomethine, lithium amide, organophosphorus, transition metal and organolithium chemistry, to name but a few.  Ken officially retired in 1997 and, for the sixteen years leading up to his untimely passing, he remained in the Department holding an Emeritus chair of Inorganic Chemistry.

Throughout his sixty-year research career Ken maintained a tireless enthusiasm and curiosity for chemistry, teaching, writing books and original papers, reviewing articles, attending chemistry meetings, and giving invited lectures around the world.  Ken’s numerous contributions to the broad field of chemistry were recognised by his election to Fellow of the Royal Society in 1989, and by his election to President of the Dalton Division of The Royal Society of Chemistry in 1996.

On top of all these achievements, Ken’s enormous abilities as a teacher and leader must not be forgotten.  His lectures were always inspiring, and were characterised by being both simple and clear, while delivering a broad spectrum of inorganic chemistry topics.

Every lecture was punctuated by anecdotes and amusing stories – who can forget the colour-blind English spy in Russia – and always demonstrated Ken’s ability for clear, logical thinking, something he continually encouraged and inspired in others, be it through undergraduate tutorials, questions after seminars, or chats in the corridor or around the whiteboard.

This themed collection of papers of 60 papers in Dalton Transactions covers a wide and diverse spectrum of topics spanning inorganic chemistry, something that very clearly demonstrates Ken’s significant and wide-ranging contributions to inorganic chemistry in its broadest context.

The rapidity with which all of the invited authors agreed to contribute their work to this special collection of articles unmistakeably reflects the great esteem and friendship with which Ken was held, and provides a fitting tribute to the man himself, his huge role as a teacher and mentor, and his important and extensive contributions to chemistry…  Without a doubt, Ken is very sorely missed.

Philip W. Dyer
Department of Chemistry
Durham University, UK

(Left – cover art for the themed collection kindly provided by Professor Jeremy Rawson, University of Windsor. It features the an image of seminal ChemComm paper from 1971 – click to zoom)

This themed collection on the carbon‒metal bond and C‒H metalation is devoted to the career of Professor William (Bill) Kaska, who will celebrate his 80^th birthday on May the 13^th, 2015. Bill was a faculty member at The University of California at Santa Barbara for the entirety of his 41-year independent career (1964‒2004). During this time, he supervised many graduate students and post-doctoral scholars, a large number of whom are active in teaching, research, and in industry positions today.

Professor William (Bill) Charles Kaska

Throughout his career, Bill has been a true pioneer and adventurer in organometallic chemistry, bearing the innate synthetic flair and expertise to make unusual molecules where others had tried and failed. He has made several important (and perhaps under-recognized) impacts in the field of metal-mediated C‒H activation, as well as in several other areas of organometallic chemistry (see below). Perhaps most notably, Bill was one of the first researchers to realize the power of C‒H bond metalation using transition metals and his early work demonstrated some of the first reported instances of such chemistry.

During his career, Bill collaborated with many groups around the World, and was a visiting scientist at Monsanto in Zurich, at the Universities of Cambridge, Erlangen, Tübingen and Utrecht, and at the Max-Planck Institute in Mülheim. We celebrate the achievements of Bill, his mentors and co-workers in this themed collection: the publications kindly dedicated to this celebratory themed collection serve to illustrate how far the field has come since the publication of a number of seminal results over 40 years ago.

Beginnings

Bill Kaska was born, raised and educated in Ancón, a suburb of Panama City, located in the then U.S.-controlled Canal Zone. He graduated from Loyola University in Los Angeles in 1957 and subsequently joined the group of Professor John J. Eisch at St. Louis University. Eisch had just returned to the United States from a post-doctoral position with Professor Karl Ziegler in Mülheim, Germany. Bill was one of Eisch’s first graduate students, charged with the task of investigating the then unexplored reactivity of alkyl and aryl aluminium compounds with unsaturated systems as alternatives to alkyl lithium reagents. Two years later, the Eisch group moved to The University of Michigan in Ann Arbor, and it was around this time that Kaska and Eisch published the first bona fide example of sp² C‒H metalation using triphenyl aluminium.

After graduating from Michigan in 1963, Bill studied organoboron and organoberyllium compounds as a post-doctoral scholar in the laboratory of Professor Thomas Wartik at Penn State University. He was hired only one year later as an assistant professor at the newly-formed University of California at Santa Barbara, in the fall of 1964.

Notable contributions

The training Bill had gained during his formative graduate and post-doctoral years set him on the path to become a die-hard synthetic organometallic chemist; the type that refuses to believe that any structure that can be imagined and scribbled on the back of an envelope cannot also be made and isolated…somehow.

In his early years in Santa Barbara, Bill presented the first example of an organometallic Wittig reaction, demonstrated by the reaction of MnBr(CO)₅ with (Ph₃P₂)C to afford a Mn=C=C=PPh₃ ylide. Bill and his group later used (Ph₃P)₂C to prepare several other examples of acetylenic organometallic complexes. Notably, in a 1974 collaborative paper with R. F. Reichelderfer, it was shown that treatment of (COD)IrPF₆ with (Ph₃P)₂C resulted in the oxidative addition of an sp³ C‒H bond of the coordinated cyclooctadiene ligand directly to the metal centre. This was the first example of a C‒H bond insertion of a coordinated ligand by a transition metal; it was not until the following decade that elegant work by Crabtree and several others elevated the profile of this powerful reaction type.

The hexaphenylcarbodiphosphorane species (Ph₃P)₂C used in Bill’s early chemistry had been discovered a decade earlier by Ramirez. The compound itself was found to exhibit triboluminescence, the mechanism and origin of which was not well understood at the time. Large yellow crystals of (Ph₃P)₂C grown from diglyme were found to emit a bright yellow-green light when touched. In 1977, Bill published a paper in collaboration with Jeffrey Zink (UCLA) that presented a detailed spectroscopic analysis of (Ph₃P)₂C and other aromatic triboluminescent materials. The conclusions of this study suggested that light emission was caused by a combination of frictional electrification, piezoelectrification, and internal electrification at shear planes within individual crystals.

In the late 1970s, Bill and his group had gained an interest in the use of what would come to be known as pincer ligands for the formation of coordinatively unsaturated complexes with bulky groups around the metal atom, as a way to promote C−H metalation of hydrocarbons. In 1980, at the Biennial Inorganic Chemistry Symposium in Guelph, Canada, Bill presented the X-ray structure and reactivity of a 14-electron Rh(I) PCP-pincer complex. The dehalogenated Rh(I) centre readily formed adducts with both aromatic and aliphatic hydrocarbons. The subsequent publication of this work included Craig Jensen as a co-author (now on the faculty at the University of Hawaii), who was an undergraduate student in Bill’s group at that time. A number of additional reports of the reactivity of other pincer complexes were published by Bill’s group shortly after, and this body of work forms an integral part of the early history of pincer chemistry.

For the next two decades, Bill worked alongside other major researchers, notably including Alan Goldman (Rutgers) and Gerard van Koten (Utrecht University) to further advance the chemistry of pincer complexes, which continues to attract significant attention to this day. The utility of pincer catalysts continues to break new ground. In 2001, the Kaska group published a communication in collaboration with Michael Hall (Texas A&M University) and Matthias Haenel (Max-Planck Institute) showing how a PCP-Ir(I) complex constructed using an “anthraphos” (1,8-substituted anthracene) ligand led to a thermally stable catalyst for alkane dehydrogenation.

In the later phase of Bill’s career, he gained an interest in the synthesis of proton sponge materials and super bases, using fused N-heterocyclic ligands based on quino[7,8-h]quinolines. In collaborative work with his long-time colleague at UCSB, Galen Stucky, and Ferdi Schüth (Max-Planck Institute), the first examples of transition metal coordination complexes of these so-called proton sponges were reported in 2001. These complexes were unusually thermally and chemically stable, due to the unique conformational bending of the qunioline backbone.

Over the past five decades, the Kaska lab has produced many highlight results in a number of fields of molecular inorganic chemistry. The works contributed to this celebratory compilation build upon the legacy of Bill Kaska’s work, his teaching, and his friendships with others in the community.

Articles in this themed collection

While this narrative is intended to summarise just a few of the highlights of Bill’s research career, his former Ph.D. mentor John Eisch has authored a terrific editorial entitled, “Emergence of electrophilic alumination as the counterpart of established nucleophilic lithiation: an academic sojourn in organometallics with William Kaska as a fellow traveler,” which provides a comprehensive, personal account of their seminal work in organoaluminium chemistry.

Research in the arena of C‒H bond activation has continued to attract much interest. This interest is driven not only by a fundamental curiosity to discover new chemical reactivity, but also to solve important energy-related problems involving hydrocarbon activation, in drug development, and in a host of other homogeneously-catalysed processes. This is abundantly clear from the new work on display in this themed collection.

In close relation to some of the seminal work published by Bill and his co-workers on the reaction chemistry of Group 9 PCP-pincer complexes, this themed collection features new work by Professors Alan Goldman, Karsten Krogh-Jesperson (University of Rochester) and co-workers, who present an elegant combined experimental and theoretical study of the C‒H versus C‒C bond activation selectivity observed between a PCP-Ir catalyst and biphenylene. They describe how and why biphenylene tends to initially undergo C‒H oxidative addition to the PCP-Ir centre, but upon heating can undergo a rearrangement that results in C‒C activation to yield a less sterically-hindered, cyclometallated species.

A collaborative experimental and theoretical study of an unusual PCP-Ir system is reported by the groups of Professors Johannes Wielandt (Karl-Franzens University) and Hermann Mayer (University of Tuebingen). They employ a cycloheptatriene-based PCP ligand scaffold; cyclometallation of this ligand with Ir(CO)₃Cl requires activation of an sp³ C‒H bond. Upon standing in tetrahydrofuran, the complex undergoes isomerisation via transfer of the remaining sp³-H onto the ligand backbone. This results in three new isomers, each containing a more common sp²-metalated arrangement at Ir, and a partially saturated ligand backbone. Continuing the theme of cyclometallated Ir-based complexes, Professors Roy Periana, Brian Hashiguchi and co-workers from the Scripps Research Institute describe the use of a robust NNC-Ir complex that is active for the oxidation of methane, benzene and other hydrocarbons in the presence of trifluoroacetic acid.

Professor Gerard van Koten and co-workers describe a series of new NCN-Pt pincer complexes that feature 4-(E)-[(4-R-phenyl)imino] methyl substituents, which induce important electronic effects on the Pt(II) sites. A combination of multinuclear NMR studies have been used to elucidate the electronically-tuneable behaviour of this unique family of Pt-pincer complexes. The group of Professor Paul Hayes at the University of Lethbridge describe how Y and Sm complexes of their bis(phosphinimie) carbazolate and pyrrolate NNN-pincers undergo varying patterns of ortho-metalation toward N– and P-aryl substituents accompanied by reductive elimination of silanes.

This collection also features a number of examples of complexes based on neutral pincer ligands that display a range of C‒H bond activation reactivities. Professor Dan Mindiola and his team at the University of Pennsylvania present a PNP-Ti pincer complex capable of performing catalytic dehydrogenation of cyclic and linear alkanes to cleanly yield the corresponding alkenes. They elucidate a mechanism for this surprising reactivity, which involves the formation of a Ti(III) alkylidyne intermediate that can effect a double C‒H bond activation. The group of Professor Karen Goldberg at the University of Washington present the synthesis, structures and reactivities of PtMe₂ complexes of a bidentate P(X)N ligand (X = O, NH). The N-donor pyridyl substituent is sufficiently hemilabilile to allow for cyclometallation and reductive elimination of CH₄. Professor Michael Rose and his team at the University of Texas at Austin present a family of Mn-carbonyl complexes prepared using novel neutral NNS Schiff base ligands, in which the thioether-S donors also exhibits hemilability.

C‒H activation by early transition metal complexes also features in this collection; Professors John Arnold and Robert Bergman from the University of California demonstrate the cyclometallation of a (BDI)Ta(=N^tBu)Me₂ complex, which undergoes reaction with H₂ gas to provide a dihydride intermediate by sigma-bond metathesis. Interestingly, a low-valent Ta(III) species is also generated under certain conditions, which undergoes C‒N bond cleavage of the BDI ligand to give a new Ta(V) cyclometallated species. Meanwhile, Professor Gerhard Erker and co-workers report the reaction of B(C₆F₅)₃ toward zirconacycloallenoids. They show how strongly Lewis basic B(C₆F₅)₃ species undergo insertion into the Zr metallacycles to give unusual zwitterionic allenyl/borate complexes.

Professor Manuel Soriaga and co-workers from Texas A&M University and the California Institute of Technology provide an example of heterogeneous C‒H bond activation and metalation, performed on solid Pd electrode surfaces. In this interesting and extensive study, high-resolution surface-sensitive techniques are combined with DFT calculations to elucidate the mechanism of metalation of 2,3-dimethylhydroquinone on ordered Pd(111) and polycrystalline Pd electrode surfaces. It is shown that the orientation of the quinone (side-on, or flat) as it undergoes oxidative chemisorption to the Pd surfaces is directly related to the relative quinone concentration.

Professor Carl Redshaw and collaborators from the Universities of Hull, Loughborough and East Anglia present the use of a family of new mono-, di- and tri-nuclear Zn(II)-calixarene complexes for the ring-opening polymerization of lactones and lactides. The group of Professor Dominic Wright at the University of Cambridge (where Bill Kaska spent a sabbatical in 2004) present the synthesis and crystal structures of an extended family of new ML₂ sandwich complexes (M = Ca(II), Mn(II), Fe(II)), using a tripodal NNN monoanionic donor ligand (L = tris(2-pyridyl)aluminate).

The themed collection is nicely concluded by work from the group of Professor Bruce Lipshutz – a long-time friend and colleague of Bill Kaska in the Department of Chemistry & Biochemistry at UC Santa Barbara – who report the Cu(OAc)₂-catalysed hydrophosphination of styrenes. Notably, this powerful organic conversion has been achieved under green conditions using water as the solvent, at room temperature; the reaction proceeds in high yield for a broad range of aromatic substrates.

We are very pleased to announce the launch of a commemorative article collection in memory of Lord Jack Lewis.

Lord Jack Lewis, who recently died aged 86, was the 1970 Inorganic Professor at Cambridge for 25 years from 1970 to 1995. Highly energetic and extremely talented, he was among a small band of pioneers who revolutionised inorganic chemistry and must be regarded as one of the true founding fathers of the modern field. As a tribute to Jack, Brian F. G. Johnson, William. P. Griffith, Robin J. H. Clark, John Evans, Brian H. Robinson and Paul R. Raithby have chosen a selection of his papers that they feel demonstrate Jack’s interest and contribution to inorganic chemistry over the course of his career, introducing them in a special Editorial in Dalton Transactions.

We very much hope that you will enjoy this commemorative article collection.

To access all of the Lewis articles and the Editorial, go to: http://rsc.li/lewis.