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

In her own niobium work, deBeixedon uses a technique called anodic painting that allows exquisite control of color. She dips a paintbrush to which a wire is attached into the electrolyte, sets her apparatus to the voltage corresponding to the color she wants, and "paints" on the surface of the jewelry with the paintbrush electrode.

Like many other jewelry artists working with niobium today, deBeixedon first learned of its potential from Seeley. Recently, Seeley shared his expertise with a different group of professionals. At a workshop at the North Carolina School of Science & Mathematics in Durham, he taught high school chemistry teachers to anodize niobium and titanium.

The project was the idea of Myra J. Halpin, a chemistry teacher at the school, a statewide magnet for students with high aptitudes for science and math. She received a Toyota Tapestry grant to purchase anodizers and supplies and brought in Seeley to work with teachers from the local area. She's already used what they learned with her own students.

"I've always been fascinated by chemistry and colors," Halpin says. She introduced the unit after her class had tackled electrochemistry. The students designed, shaped, and anodized earrings and other small pieces. In a more advanced research class, Halpin's students investigated the effect of different variables on the colors produced.

"The kids get something real and tangible from chemistry to take home," Seeley notes. "Electrochemistry makes beautiful things."

Beautiful, indeed. For my next birthday, I'm hoping for more niobium jewelry.

Pamela S. Zurer, C&EN's managing editor, has been with the magazine for 22 years. She's still amazed she gets paid to talk with people about the interesting chemistry they do.

NIOBIUM AT A GLANCE

Name: Named for the Greek mythological figure Niobe, daughter of Tantalus, because of its position above tantalum.

Atomic mass: 92.91.

History: Discovered in 1801 by Charles Hatchett while working with a sample of columbite.

Appearance: Shiny gray metal that takes on a bluish tinge when exposed to air at room temperatures for a long time. Soft and ductile.

Behavior: Oxidizes in air at high temperatures.

Uses: Used in various superconductor applications and in welding rods, cutting tools, and pipelines. Niobium-stabilized steel is very heat resistant.

TANTALUM

RICHARD R. SCHROCK, MASSACHUSETTS INSTITUTE OF TECHNOLOGY

At the beginning of my independent research career in 1972, I received an offer from the central research department at DuPont where a talented group of organometallic chemists was exploring the synthesis of new early-transition-metal organometallic species. Of greatest interest were compounds with some potential application in homogeneous catalysis. Fascinating work was being carried out by Fred Tebbe (among others) on titanocene dihydride and dialkyl complexes and by Ulrich Klabunde on tantalocene trihydrides.

Tantalum! Now there was a tough metal, not only literally, but also in terms of its organometallic chemistry; put simply, little organometallic chemistry was known. The prospect of new tantalum chemistry, a far cry from rhodium in my graduate student days, was attractive to me as an initial project.

THROUGH THE LOOKING GLASS A heat- tinted, fusion-welded tantalum tube, shown here magnified through a 35x bright field.

Some of the simplest early organo-transition-metal species are binary alkyl complexes, MR_x, where the metal M is in group x. For example, compounds such as M(CH₂SiMe₃)₄ and M(CH₂CMe₃)₄ (M = Ti, Zr, or Hf) were prepared by Geoffrey Wilkinson and/or Michael F. Lappert between 1970 and 1973, while Wilkinson reported the isolation of W(CH₃)₆ in 1972. (Compounds in which the alkyl contains one or more protons on the carbon that is with respect to the metal, such as M(CH₂CH₃)₄, are still unknown.) G. W. A. Fowles, D. A. Rice, and J. D. Wilkins (following work by Juvinall in 1964) showed that TaMe₃Cl₂ could be prepared readily from TaCl₅ and ZnMe₂ and that it was an isolable and well-behaved species (but, of course, sensitive to moisture and air). I was delighted to find that TaMe₅ could be prepared, although it was highly volatile and unstable, decomposing intermolecularly, sometimes explosively. In contrast, red crystalline Ta(CH₂Ph)₅ proved to be stable at room temperature.

It was natural to attempt to complete the series by preparing Ta(CH₂CMe₃)₅. Fortunately, the products of an attempt to synthesis Ta(CH₂CMe₃)₅ from Ta(CH₂CMe₃)₃ Cl₂ and LiCH₂CMe₃ were neopentane and (Me₃CCH₂)₃Ta=5CHCMe₃, the first example of a "carbene" complex that contains a proton on the carbene carbon atom. This compound is quite stable thermally, melting at about 70 °C and distilling readily in a good vacuum. This event in 1974 marked the beginning of high-oxidation-state alkylidene chemistry. It also dramatically illustrated that four bulky covalently bound ligands could stabilize pseudotetrahedral species against bimolecular decomposition.

The reaction between (Me₃CCH₂)₃ Ta=CHCMe₃ and butyllithium to give a lithium salt of [(Me₃CCH₂)₃Ta [CCMe₃]^- further suggested that group 6 high-oxidation-state alkylidyne species could be made, as was illustrated four years later at MIT with the synthesis of (Me₃CCH₂)₃ WCCMe₃, another distillable, thermally stable compound. Deprotonation of [(⁵-C₅H₅)₂TaMe₂]⁺ to yield (⁵-C₅H₅)₂Ta (CH₂)Me, the first isolable methylene complex, further emphasized the relationship between tantalum and phosphorus. Furthermore, reactions of tantalum alkylidene complexes suggested that the metal is in its highest possible oxidation state and that the alkylidene carbon is a strong nucleophile. Therefore, they became known as "nucleophilic" alkylidene or carbene complexes.

One of the most mysterious reactions in the 1970s that captured the interest of chemists was olefin metathesis, a catalytic reaction that involved unknown Mo, W, or Re catalysts. The reaction consists of "chopping up" carbon-carbon double bonds in olefins and redistributing the alkylidenes (usually =CHR moieties) to give a mixture containing all possible olefins formed in that manner. My question upon my move to MIT in 1975 was, "Do the high-oxidation-state species that have been discovered have anything to do with olefin metathesis?"

It took five years to prove that they do. For example, we were able to show that (PMe₃)(t-BuO)₂ClTa=CH-t-Bu reacts with styrene in the presence of PMe₃ to provide the isolable benzylidene complex, (PMe₃)₂(t-BuO)₂ClTa=CHPh, and that species of this type would metathesize cis-2-pentene to 2-butenes and 3-hexenes in the presence of PMe₃ to the extent of 25-30 turnovers. This was the first time that an alkylidene analogous to the initial alkylidene could be isolated upon reaction with an olefin and the species actually responsible for metathesis identified conclusively. An important message was that bulky alkoxide ligands are beneficial to sustained metathesis reactions involving tantalum or niobium alkylidenes. Although tantalum was a reluctant player in the metathesis game, the principles learned from studies of tantalum alkylidene complexes were key to the later development of well-defined tungsten, molybdenum, and rhenium alkylidene complexes for the metathesis of olefins.

Although I maintain only a small interest in tantalum organometallic chemistry today, tantalum started virtually all of the areas in which I am active; it still tantalizes.

Richard R. Schrock is the Frederick G. Keyes Professor of Chemistry at MIT. He is the discoverer of high-oxidation-state organometallic compounds that contain metal-carbon double or triple bonds that are now used as catalysts for alkene or alkyne metathesis, respectively.