Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

Situated in the middle of the second row of the transition-metal series, ruthenium lies at the heart of the periodic table. This central location bestows upon ruthenium properties that are common to both its early- and late-transition-metal cousins. Harnessing the best of both worlds, ruthenium benefits from a confluence of desirable properties, amounting to a winning combination for catalysis. Borrowing the high reactivity of elements to its left and the less oxophilic and Lewis acidic nature of those to its right results in a special array of properties, which led Geoffrey Wilkinson to refer to ruthenium as an element for the connoisseur.

Early in the pursuit of its preparative chemistry, an unexpected capacity of Ru(II) for backbonding was uncovered by Henry Taube. Ultimately, the optimization of reactivity, multiple metal bonding, and functional group tolerance culminated in the creation of a family of olefin metathesis catalysts that are finding broad use in organic and polymer synthesis--the main focus of my research.

I first became fascinated with olefin metathesis when I was a postdoc. At that time, the catalysts were ill-defined and had a very narrow substrate scope. The presence of air, water, or any basic organic functional groups would poison these early catalysts, limiting their application to the metathesis of hydrocarbon substrates. Furthermore, nothing was known about the structure of the active complex. Known metal carbene/ alkylidene complexes--once implicated in the mechanism of the reaction--were screened for catalytic activity. The high-oxidation-state early-metal complexes, prepared by Fred Tebbe and Richard R. Schrock, were active and well-defined. Despite the continued acute sensitivity to air and functional groups, subsequent members of Schrock's tungsten and molybdenum catalyst family paved the way for well-defined olefin metathesis as we know it today.

I should have listened to Harry; it would have saved a lot of time. Harry Gray, who has exploited ruthenium redox chemistry in his work on electron tunneling in proteins, always told me that ruthenium was a great metal and was particularly good for Kentucky kids! Instead, we wandered across the periodic table before finally discovering the magic of ruthenium.

Our relationship with ruthenium began when Bruce Novak found that none of the well-defined catalysts based on titanium and tungsten known at that time would polymerize a particular functionalized monomer. A literature search revealed that ruthenium chloride would polymerize related monomers in protic solvents. After considerable effort, Novak determined that Ru(II) and a strained olefin were essential ingredients for the preparation of an ill-defined, yet very active, catalyst--a result that would lead SonBinh Nguyen to the first member of a family of highly active, well-defined ruthenium metathesis catalysts.

With the basic structure in hand, a series of ligand modifications led to much more active catalysts. We developed and optimized new synthetic routes, allowing for the commercialization of this family of catalysts. The most recent additions to the family use N-heterocyclic carbene ligands and have activities on a par with the best early-metal complexes. Most significantly, they retain the functional group tolerance observed with the parent ruthenium complexes, while allowing for metathesis of highly functionalized double bonds.

These catalysts will react selectively with olefins in the presence of water, alcohols, and sulfur-containing compounds, all of which were poisons to earlier catalysts. Furthermore, these ruthenium complexes are not particularly sensitive to oxygen and can therefore be handled under normal synthetic organic conditions. In fact, a number of new drugs now in Phase II trials employ ruthenium-mediated olefin metathesis as a key step in their synthesis.

The extraordinary tolerance of these catalysts to both functional groups and impurities is also providing new opportunities in the polymer chemistry arena, leading to new families of functional polymers and polymer composites. Their stability in concert with the ability to precisely control their activity has resulted in the synthesis of "living," block, and cyclic polymers.

The special properties of ruthenium have opened up the area of olefin metathesis to synthetic organic and polymer chemists and are finally allowing this powerful reaction to realize its considerable promise.

Robert H. Grubbs is Victor & Elizabeth Atkins Professor of Chemistry at California Institute of Technology. He was the recipient of the 2002 Arthur C. Cope Award.

RUTHENIUM AT A GLANCE

Name: From the Latin Ruthenia, Russia.

Atomic mass: 101.07.

History: Ruthenium was discovered by Karl Karlovich Klaus, a Russian chemist, in 1844.

Occurrence: Found in platinum and other ores.

Appearance: Silvery white, solid metal.

Behavior: RuO4 is toxic and explosive.

Uses: Used as a catalyst in many industrial processes and to increase the corrosion resistance of titanium.

OSMIUM

IAN SHOTT

Osmium, used primarily in pen nibs and armor-piercing weapon shells, is the densest metal in the periodic table. It can, however, typify an often encountered chemical conundrum. On the one hand, it is very useful as the metal in a highly stereospecific catalyst, which is both nonvolatile and nontoxic. In the development world, it has enjoyed widespread use in directly synthesizing a range of valuable pharmaceutical intermediates. On the other hand, its oxide is both highly toxic and very volatile, with high physical penetration of a variety of otherwise inert materials. Commercially, the oxide is used as a stain in microscopy.

Thus, scale-up in conventional multipurpose plants poses three significant issues: waste containment and treatment; product purification, as even parts-per-billion concentrations cannot be tolerated; and decontamination of the plant and equipment where the oxide will have penetrated surfaces, particularly PTFE (polytetrafluoroethylene) linings and joints.

This combination can result in the necessity to make an otherwise multipurpose plant dedicated, with the consequential negative impact on capacity utilization and overall production economics. A frustrating cycle then occurs, whereby industrious process researchers find an elegant solution to synthesize an otherwise difficult-to-make intermediate, and they successfully produce a sample that is purified and analyzed. This is well received by the customer, who then wants larger quantities and detailed large-scale pricing. It proves impossible to fit it into existing equipment, and a substantial investment is required for a dedicated, segregated, highly specified plant. The market is not yet established, cost is an issue, and price sensitivity is critical. High capital expenditure and low occupacity mean production costs are prohibitive, and the opportunity is consequently lost. The activation energy to convert an intellectual curiosity into a commercial reality has not been overcome.

The specialty chemical company Rhodia does have access to intellectual property in this area and has undertaken some semitechnical trials and scale-up pilot-plant activity. To my knowledge, only two or three other companies also have limited operational experience but no fully commercial activity.

The cycle can be broken only if one single opportunity is so certain, so large, so cost insensitive it can support the investment. Today, this type of opportunity is a rare animal and more probably in danger of extinction.

Standing back from this issue, we could be facing a huge generic problem with the combination of today's increasingly complex chemistry and existing state-of-the-art multipurpose technology. The latter is, in general, based on a simplified flow sheet of large stirred-tank reactors that has essentially existed for the past five centuries! Modernization has brought a plethora of new materials and instrumentation that have increased the cost of fundamentally low productivity equipment trains. Thus, no discernable benefit in terms of unit cost reduction has resulted.

Most organic synthesis routes to complex pharmaceuticals have numerous stages with high dilution and slow kinetics offering miserable overall yields and appalling productivity. These routes under batch conditions are very complex and involve inherently dirty chemistry with numerous side reactions. A new vision for tomorrow could involve a type of process "circuit board" of modules encompassing fewer stages, inherently clean and direct routes, rapid reaction kinetics, high throughputs, high overall yield, lower waste, and substantially improved productivity.

In today's world of process intensification, supercritical fluid technology, nanotechnology, and "multidisciplinarianism," there is considerable scope for innovation.

POINTED One of osmium's major uses is in fountain pen tips.