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PINCH AND STING: THE SCORPIONATES Metal binding by pyrazolylborates and related tripodal ligands has generated diverse chemistry in the past 35 years STEPHEN K. RITTER, C&EN WASHINGTON Picture how a scorpion uses its clawlike pincers to grab its prey--usually a juicy insect--as its curved tail arches forward to inflict a venomous, disabling sting. That is the image Swiatoslaw (Jerry) Trofimenko has chemists use to visualize how anionic tris(pyrazolyl)borate ligands bind to metals. Like the pincers of a scorpion, these versatile tripodal ligands that Trofimenko discovered bind a metal with nitrogen heteroatoms from two pyrazole rings attached to a central boron atom. The third pyrazole ring attached to the boron rotates forward like a scorpion's tail to "sting" the metal. Trofimenko first prepared the pyrazolylborate ligands in the mid-1960s while a researcher at DuPont. He now continues his work on the ligands as a visiting scholar in the department of chemistry at the University of Delaware. Trofimenko and his "scorpionate" ligands recently were guests of honor at a symposium to celebrate 35 years of chemistry accomplished with the pyrazolylborate and related tripodal ligands. Sponsored by the Division of Inorganic Chemistry, the symposium brought together 31 speakers from 10 countries over three days during the American Chemical Society national meeting last month in New Orleans. "Pyrazolylborate ligands have given rise to a tremendous amount of chemistry with an amazing diversity," noted chemistry professor Daniel L. Reger of the University of South Carolina, Columbia, a symposium co-organizer. Pyrazolylborates are among the complex ligands used most often, he said, because they provide a large steric shielding of the metal center and serve as reliable spectator ligands. Complexes with all the metals or metalloids in the periodic table have been prepared over the years, he added, with the tris ligands considered analogs of cyclopentadienyl ligands and the bis ligands analogs of acetylacetone. "What the pyrazolylborate ligands have allowed us to do is grab the top half of a metal so we can mess around with the other half," Reger explained. "That provides a considerable amount of flexibility to do interesting chemistry." Among the areas that continue to be explored are catalysis for C–H bond activation, models for enzymatic reactions, metal extraction, and biomedical applications. Trofimenko has championed the ligands by helping many others to understand and use them, Reger added. Another part of Trofimenko's contribution has been to keep track of the more than 2,000 research papers published on scorpionates through the years and to write a series of review articles and a book. The tris(pyrazolyl)borate ligand and its 3,5-dimethyl analog are the most commonly used members of the class and have been given the chemical abbreviations Tp and Tp*, respectively. Ligands that have a 3-substituent other than methyl or have 3,5-substituents are abbreviated as TpR or TpR,R', respectively, where R and R' are the substituent groups. GO GATORS A patterned polymer near-infrared light-emitting diode made by University of Florida chemists has an active layer containing an ytterbium tetraphenylporphyrin complex topped by a tris(pyrazolyl)borate ligand (shown, Ph = phenyl). The complex luminesces at 980 nm under an applied voltage, revealing the "UF chem" pattern when viewed with a near-IR camera. -------------------------------------------------------------------------------- THE PINCH-AND-STING concept of scorpionates applies specifically to the tris(pyrazolyl)borates. In general, however, the pyrazolylborates are tripodal ligands that include a diverse set of structures. The ligands also can be tetradentate, bidentate, or monodentate, depending on the number of donor substituents on boron and the steric congestion around the metal. Nitrogen heterocycles other than pyrazole can be used, such as pyrrole, imidazole, indole, and indazole. In addition, tripodal ligands can have central atoms other than boron, such as carbon, phosphorus, or gallium. The ligands also can bind metals through sulfur, phosphorus, or other donor atoms. Other modifications include having a methylene between the central atom and the donor group. For example, Reger's work has included improved synthesis of tris(pyrazolyl)methane ligands in which carbon is the central atom rather than boron. His group has used these charge-neutral ligands to make highly symmetrical supramolecular silver complexes. More than 100 attendees heard Trofimenko share background information on the ligands and lesser known details about their development. "It was standard policy at DuPont Central Research for each chemist to explore on a part-time basis some new area of chemistry unrelated to his project," Trofimenko said. Based on his previous research on pyrazole and on polyhedral boranes, Trofimenko decided to work on boron-pyrazole chemistry as his independent project. "I thought it would be interesting to explore BR2 bridging," he noted. After some preliminary experiments, Trofimenko made the bis-, tris-, and tetrakis(pyrazolyl)borates by reacting pyrazole with alkali-metal borohydrides [J. Am. Chem. Soc., 88, 1842 (1966); C&EN, Aug. 28, 1967, page 72]. He then began to explore the coordination chemistry of the ligands with first-row transition-metal ions. DuPont became interested in developing the ligands, Trofimenko noted, and he was allowed to work on them full time. In 1973, Trofimenko's work on the ligands ended abruptly when he was transferred to other DuPont departments, including international offices in Switzerland and Poland, he said. He returned to the U.S. in 1980 to work on nylon and fluorocarbon research, but he eventually had some spare time to return to the Tp ligands. SCORPIONATE KING Co-organizers Reger (left) and Parkin flank Trofimenko as they present him with a copy of his book on pyrazolylborate ligands signed by each symposium speaker. PHOTO BY STEVE RITTER INTEREST AMONG the inorganic community had waned during the years that Trofimenko was not working on the ligands. But he rejuvenated the field in 1986 when he developed a second generation of Tp ligands that have bulky substituents such as tert-butyl or phenyl in the 3-position [Chem. Commun., 1986, 1122]. "It was gratifying to see how the steric effects introduced a new dimension in the coordination chemistry of pyrazolylborates," Trofimenko said. These second-generation ligands have led to a number of new coordination compounds that are important for catalysis and for studying the bioinorganic chemistry of enzymatic reactions. "Jerry discovered that nearly any type of substituent group desired could be incorporated into the pyrazole rings," noted Gerard Parkin, a chemistry professor at Columbia University and a symposium co-organizer. "That meant that one could completely tailor the chemistry of the ligand system. At that point, many others started to become interested in the ligands and the field just blossomed. "Symposia where people get together to celebrate the work of a researcher are not uncommon," Parkin added. "However, these events usually involve a group of colleagues and former students getting together when someone wins an award. But Jerry has never had any students, and in this case he hasn't won an award. Still, he has had a very important impact on the inorganic community." In the late 1980s, Trofimenko helped Parkin, a new assistant professor, get started working with Tp ligands by providing him with samples of ligands and reagents. Parkin was one of the first researchers to explore the second-generation TptBu ligands, which he used to stabilize magnesium alkyl compounds for use as Grignard reagents. Parkin described his group's work on tris[(2-indole)methyl]amine, which is a trianionic tetradentate ligand that, like the Tp ligands, coordinates to metals through heterocyclic nitrogen atoms. Much work has been done to study strong -donor ligands such as tris(amidoethyl)amine for enhanced stabilization of high-oxidation-state transition metals, Parkin noted. But he wanted to find a heterocycle-containing ligand system that had a slightly reduced -donor ability. He showed that the indole ligands are efficient for forming tantalum amido and imido complexes [Inorg. Chem., 42, 264 (2003)]. VERSATILE BINDING Uranium iodide complex with two tris[3-(2-pyridyl)pyrazolyl]borate ligands (boron, orange) has the remarkable feature of 12 nitrogen atoms (blue) coordinated to a single uranium atom, forming a symmetrically beautiful icosahedral structure that shows the versatility of pyrazolylborate ligand systems [Chem. Commun., 1995, 1881]. IMAGE COURTESY OF ARNOLD RHEINGOLD IN ADDITION to presenting their latest research results, many of the symposium speakers told of their interactions with Trofimenko over the years. For example, James M. Mayer, now a chemistry professor at the University of Washington, remembered visiting Trofimenko in the 1980s. "I recall meeting with Jerry at his desk in the back of this huge lab, with shelves everywhere filled with bottles of Tp ligands. At some point, while still listening to you describe the research you were doing, Jerry would get up from his desk and would decide which of the various Tp ligands you should be using in your chemistry. He would reach up on these shelves and then serve you a sample of these ligands," he said. Mayer said the episodes remind him of the big friendly giant in the popular Roald Dahl children's book "The BFG." The giant keeps nice dreams for children bottled on large shelves in his cave, much like Trofimenko keeps Tp ligands in his lab. "The success of the Tp ligands and the joy many of us have had using them is due not only to the wonderful science Jerry continues to do, but also to his generosity, support, and cheerfulness," Mayer commented. He went on to describe chemistry his group is doing using Tp ligands to stabilize strongly oxidizing osmium oxo and nitrido complexes [Inorg. Chem., 42, 605 (2003)]. Another fruitful collaboration Trofimenko has had is with chemistry professor Arnold L. Rheingold at the University of Delaware, who recently moved to the University of California, San Diego. Over the years, Rheingold and his group have determined the X-ray crystal structures of nearly 400 scorpionate ligand complexes in work involving 67 students and faculty at 19 different laboratories, he said. Rheingold described the structural features of a few of the compounds he had most enjoyed working with. One was a uranium iodide complex with two tris[3-(2-pyridyl)pyrazolyl]borate ligands that has the remarkable feature of 12 nitrogen atoms coordinated to a single uranium atom [Chem. Commun., 1995, 1881]. "This particular structure is one that is rewarding for its symmetrical beauty alone," Rheingold said. "The geometry is almost perfectly icosahedral. The ability of the pyrazolylborates to accommodate such an extreme environment, as well as virtually all of the more common ones, is a powerful demonstration of the unprecedented versatility of this ligand system." After retiring from DuPont in 1996, Trofimenko's relationship with researchers at Delaware allowed him to join the chemistry department there as a visiting scholar. A small enclave of researchers who each work at least in part with Tp ligands was thus formed at the university. Besides Trofimenko and Rheingold, this group includes chemistry professors Charles G. Riordan and Klaus H. Theopold, who also gave talks at the symposium. Trofimenko's most recent work includes functionalization of aliphatic C–H bonds with a brominated scorpionate copper(I) catalyst, TpBr3Cu(NCCH3). The ligand has nine bromine atoms--three on each pyrazolyl ring--and is the only Tp ligand devoid of C–H bonds, he told C&EN. The electron-withdrawing bromine atoms have been proposed to favor C–H bond insertion reactions by enhancing the electrophilicity of the metal-carbene intermediate. With chemistry professor Pedro J. Pérez of the University of Huelva, in Spain, Trofimenko has shown that the catalyst provides good yields and high selectivity and is an improvement over other copper catalysts for inserting ethyl diazoacetate into tertiary C–H bonds of alkanes [J. Am. Chem. Soc., 125, 1446 (2003)]. Some 200 pyrazolylborate ligands have been prepared over the years, Trofimenko estimates, and he has had a hand in making more than 100 of them. "I consider myself a gunsmith rather than a hunter," Trofimenko said, preferring to make the ligands while others explore the chemistry that can be done with them. -------------------------------------------------------------------------------- "I consider myself a gunsmith rather than a hunter." -------------------------------------------------------------------------------- ONE EXAMPLE is a materials application presented by chemistry professor James M. Boncella of the University of Florida. Boncella's group has been working with the groups of Florida chemistry professors John R. Reynolds and Kirk S. Schanze to prepare electroluminescent lanthanide complexes containing Tp ligands. The researchers are incorporating the complexes in the active layer of polymer light-emitting diodes (PLEDs). In a new series of lanthanide complexes described by Boncella, a Tp ligand binds the top half of the lanthanide metal and a tetraphenylporphyrin ligand binds the lower half, encapsulating the lanthanide(III) ion and shielding it from outside interactions. The researchers are finding that the seven-coordinate complexes with Pr, Nd, Ho, Er, Tm, and Yb have enhanced luminescence efficiencies compared with mono- or bidentate ligands, he said. The Florida chemists have constructed multilayer PLEDs in which the active layer is a 2:1 blend by weight of a lanthanide complex and polystyrene or an alkoxy-substituted poly(p-phenylene), Boncella explained. When a voltage is applied to the PLED, the conjugated polymer transfers energy to the lanthanide metal center through the porphyrin, which leads to emission of near-infrared light in the range of 980 to 1,700 nm, depending on the metal used. "Although the quantum yields of the devices are low at 0.1%," Boncella said, "they still produce enough light to be easily seen without specialized equipment." The researchers are tinkering with various metal-ligand-polymer combinations to tailor the performance of the PLEDs, he noted. "There is a laundry list of potential applications," Boncella stated, "ranging from telecommunications, sensors, biomedical uses, and defense applications such as night vision." TWO NEWER types of boron-based ligands described at the meeting are the phosphinomethylborates ([PhB(CH2PR2)3]–, Ph = phenyl, R = aryl or alkyl) and the aminomethylborates ([Ph2B(CH2NMe2)2]–, Me = methyl), which assistant chemistry professor Jonas C. Peters and his group at California Institute of Technology are developing. "The phosphinomethylborate ligands are hybrids inspired in part by Trofimenko's pyrazolylborates, but also by the useful donor characteristics of simple tertiary phosphine ligands," Peters said. "The aminomethylborate ligands, on the other hand, are reminiscent of tertiary diamines. These newer ligand classes are somewhat more synthetically challenging to prepare than the pyrazolylborates, perhaps explaining why they are less thoroughly developed at this stage." The phosphorus or nitrogen donor arms serve to partially insulate the negative charge of the borate unit from the metal ion, leading to complexes that are formally zwitterionic, he explained. The complexes are amenable to studying C–H activation processes (Pt), ethylene-CO copolymerization reactivity (Pd), and catalytic additions across olefins (Rh), he noted. Similar to the pyrazolylborates, the phosphinomethylborate ligands can be monodentate, bidentate, or tridentate, depending on the number of phosphorus donors attached to the central boron atom. The phosphorus atoms also are easily oxidized, he added, providing versatile sulfur- or oxygen-extended donor groups (P=S or P=O). Peters outlined structural, electronic, and reactivity properties of some of the complexes his group has prepared, so far focusing mostly on bidentate and tridentate diphenylphosphinomethylborates. For example, [PhB(CH2PPh2)3]– appears to provide a stronger ligand field strength than structurally related but neutral tripodal phosphines, he said. This property, along with the ligand's geometric constraints, has allowed Peters' group to prepare unusual low-spin, d7 cobalt(II) complexes that are "pseudotetrahedral." Structurally similar Co(II) complexes have previously exhibited high-spin ground states, he said. Peters' group has also stabilized pseudotetrahedral Fe(III) and Co(III) imides that exhibit low-spin ground states [J. Am. Chem. Soc., 124, 11238 (2002) and 125, 322 (2003)]. "These types of complexes had proven historically elusive for the later first-row transition elements," he noted. An emerging goal for the group is to use the iron template to help stabilize and mediate between simple ligands derived from nitrogen, he said. These include nitrogen (Fe–NN), hydrazido (Fe=N–NH2), nitride (FeN), imido (Fe=NH), and ammonia (Fe–NH3). The study of these types of metal-ligand complexes "will help guide mechanistic postulates concerning the role of low-coordinate, low-valent iron in nitrogen-fixing enzymes, such as the nitrogenases that convert nitrogen to ammonia." For molybdenum, many of these types of nitrogen-derived complexes and interconversions are known, Peters said, but few of these species are well established in iron chemistry. "It would be gratifying to someday establish a Chatt-type cycle with respect to nitrogen reduction chemistry, now known for molybdenum, using a trigonally coordinated iron platform," Peters concluded. "If you find some chemistry that looks beautiful and you think it's important, you should pursue your vision--even if the circumstances are not the best," Trofimenko told C&EN. "For the Tp ligands, it's not just the chemistry for me. The symmetry, the beautiful quality of the colorful crystals--it's very aesthetically pleasing." Although the pyrazolylborate ligands have been around for 35 years, Trofimenko said, there's still more to explore--including ligands that can be tridentate to hexadentate, with charges from –1 to –4, and with a range of elements as the central atom and as donor atoms. "All in all, this is a vast and promising area," Trofimenko said, "the riches of which are yet to be fully exploited by the scorpionate community." |
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