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The Vibrancy and Variety of Modern f‑Element Organometallic Chemistry 7b00633). The flexibility of this ligand, which allows it to take on multiple binding modes, plays a role in supporting uranium and thorium alkyl and alkynyl compounds. Computational studies support the observation that the most common coordination mode of the ligand in the heterobimetallic species is η5:η5 due to an energetic preference derived from orbital mixing. Thus, the versatility of this ligand promotes new organoactinide chemistry at tetravalent centers. Effective encapsulation of metal centers by new ligands offers the opportunity for small molecule activation at open coordination sites. Early actinides are suited to such chemistry due to their highly reducing natures. The oxophilicity of these elements also makes activation of oxygenated substrates a particularly rich research area. In their contribution, the Cloke laboratory has tested carbon dioxide activation by uranium(III) compounds that feature a new dianionic ligand, C6H4(pC(CH 3 ) 2 C 6 H 2 Me 2 O − ) 2 (p-Me 2 bp) (DOI: 10.1021/acs. organomet.7b00263). Spectroscopic and structural characterization shows a benzene anchor coordinated in an η6 fashion with flanking aryloxides. Treating the uranium(III) complex with excess supercritical carbon dioxide resulted in reduction of the CO bonds and extrusion of CO, with isolation of a bridging carbonate, μ-η1:η2-CO3 dimer. In contrast, 2 equiv of gaseous 13CO2 produces a mixture of this carbonate and a new bridging oxalate species, μ-η2: η2-C2O2. Further experiments support the assertion that the oxalate is the kinetic product, while the carbonate is the thermodynamic product. The impact of subtle ligand variation on small-molecule activation continues in the contribution from Nocton, Maron, and co-workers (DOI: 10.1021/acs.organomet.7b00630). This team interrogated the reactivity of the divalent Sm complexes Cptt2Sm (Cptt = 1,3-(tBu)2(C5H3)) and Cpttt2Sm (Cpttt = 1,2,4(tBu)3(C5H2)) with CO2 using experimental and computational chemistry. In both cases, reductive disproportionation to bridging carbonate and CO was noted, which was contrasted with the reported reaction of (C5Me5)2Sm with CO2 that produced oxalate as a reduction product. Similarly, Sutton and co-workers examine the reaction of CO2 with [(C5Me5)2CeH]2, which yielded the carbonato species [Ce(C5Me5)2(thf)]2(μη2:η1-CO3) (DOI: 10.1021/acs.organomet.7b00639). Two possible mechanisms were proposed for this reaction, including a reductive disproportionation or Ce−oxo formation, followed by CO2 insertion. Together, the contributions from Sutton and from Nocton and Maron demonstrate some of the varied chemistries of organolanthanide compounds toward smallmolecule carbon oxygenates. Dianionic hydrocarbyl fragments are the focus of a DFT study on a multimetallic species by the Hou group, in this case stabilized by a trinuclear yttrium−methyl complex, [L3Y3(μ2Me)3(μ3-Me)(μ3-CH2)] (L = PhC-[NC6H4(iPr-2,6)2]2) (DOI:
C
hemists have become adept at manipulating d-block metal complexes to mediate reactivity for Nobel Prize-worthy effects on society.1 This is especially true in the field of organometallic chemistry, where both early- and late-transitionmetal catalysts have been recognized. By application of fundamental molecular studies to develop processes at scale, chemical understanding has facilitated the development of new materials that have transformed modern life. It is with this enthusiasm and interest that we turn to the elements of the f block. Because they are often overlooked in the undergraduate curriculum, the elements at the bottom of the periodic table are regarded as mysterious and exotic. It is only with (hopefully) further study that students come to learn that these elements are unique and that understanding their properties, reactivity, and utility are at the forefront of inorganic chemistry. From magnets, displays, carbon-neutral power sources, and catalysts to medical applications, f-block elements have played a central role in some of the most significant technological advances. In fact, studies of the reactivity of the lanthanides and actinides have informed the development of organometallic chemistry. In an effort to highlight the activity and enthusiasm at the forefront of f-element organometallics, we have assembled examples of the latest work in the synthesis, characterization, and reactivity of f-block chemistry. We have elected to highlight progress in broad areas of organometallic chemistry, including ligand design, synthetic methods, smallmolecule activation, catalysis, electronic structure, and magnetism. We are grateful to have contributions from both new investigators and those more established, including several founders of the field. For the purposes of this special issue, we included contributions that encompass both the actinide series and the broader rare earths: Sc, Y, and La−Lu. At the heart of organometallic chemistry is ligand design toward metal-based reactivity, a principle that motivates this discipline as a subset of inorganic chemistry. For this collection, work from the Fortier laboratory describes a new ligand for uranium(III), a p-terphenyl bis(anilido) that features bulky 2,6diisopropylphenyl substituents (DOI: 10.1021/acs.organomet. 7b00429). Notably, the ligand stabilizes a trivalent uranium cation that forms an unsupported U−Fe bond with the [Fp]− (Fp = (η6-C5H5)Fe(CO)2) anion. Analyzing this interesting heterobimetallic compound using X-ray crystallography establishes a M−M bond that is within the range of the sum of the covalent radii for iron and uranium, indicating a single bond. Computational evaluation from the Vlaisavljevich group highlights that this bond is polarized toward the iron, and is primarily electrostatic. In a second collaboration between experimentalists and theorists, the groups of Polly Arnold, Jason Love, and Nik Kaltsoyannis report on organometallic thorium and uranium derivatives of a new trans-calix[2]benzene[2]pyrrolide ligand that they leverage to develop actinide−transition-metal heterobimetallic species, featuring the shortest reported distance between Th and Ni (DOI: 10.1021/acs.organomet. © 2017 American Chemical Society
Special Issue: Organometallic Actinide and Lanthanide Chemistry Published: December 11, 2017 4507
DOI: 10.1021/acs.organomet.7b00829 Organometallics 2017, 36, 4507−4510
Organometallics
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1021/acs.organomet.7b00452). Notably, the 9-BBN reaction induces C(sp3)−H bond borylation of the supporting ligand, generating a Sc−borohydride. This result is in contrast to the reported reaction of the DMAP-stabilized imido complex [(NNN)ScNDIPP(DMAP)], where addition of the B−H bond to the ScN double bond was noted. Reaction with CatBH showed B−O cleavage to afford a scandium catecholate. In both cases, the imino−borane side products were reported to be highly reactive. Subtle structural effects leading to significant and tunable reactivity outcomes in this part of the periodic table require new and varied synthetic methods. Andersen, Walter, Maron, and co-workers disclose oxidative group transfer chemistry with organo−zinc and −copper reagents to synthesize base-free (C5Me5)2Yb(μ-CH3)Yb(CH3)(C5Me5)2 (DOI: 10.1021/acs. organomet.7b00384). Key here is the description of the monomeric congener “(C5Me5)2Yb(CH3)” and its comparison with the “textbook” compound (C5Me5)2Lu(CH3) that exhibits exchange with free 13CH4, first reported by Watson in 1983.2 Watson’s discoveries formed a basis for the ensuing articulation of the σ-bond metathesis mechanism by Bercaw and coworkers.3 What is striking in Andersen’s contribution here is that, despite their careful isolation of (C5Me5)2Yb(CH3), the complex behaves very differently from its Lu (and Sc) congener for electronic reasons, evidently due to low-lying electronic excited states in the former. These still obscure behaviors are compared with the related hydrides in an intellectual feast of a paper. Given the sensitivities of f-block organometallic compounds, the application of new synthetic methods is crucial to access new and important congeners. Continuing on this theme, Evans and co-workers contribute with a worthy application of mechanochemistry, using it to obtain sterically crowded and reactive (C5Me5)3Ln (Ln = Tb, Dy, Ho, Er) complexes (DOI: 10.1021/acs.organomet.7b00385). Ball milling of suitable precursors in an arene (and coordinating) solvent-free drybox followed by extraction of the solid product into hexane neatly produced the compounds of interest. The results expand the library of reactive (C5Me5)3M species, allowing for identification of an unusual (C5Me5)− tilt angle. This tilt angle was found to be a key structural feature that correlated with the tendency for these compounds to activate arene C−H bonds. Flowers and co-workers make an important contribution in their mechanistic study of the reduction of unactivated ketones with SmI2(H2O)n (DOI: 10.1021/acs.organomet.7b00392). Important here is the recognition that the first elementary step of this process is not reversible electron transfer to coordinated substrates but rather an initial, irreversible, outersphere PCET from SmI2(H2O)n. This postulate has the potential to revise mechanistic dogma for this widely used reagent. Flowers notes that the strong thermodynamic driving force for hydrogen atom transfer from SmI2(H2O)n implies that the ketyl−Lewis acid adduct may not be a relevant intermediate and that product distributions are potentially misleading in developing mechanistic proposals for this type of reduction reaction. Extending the knowledge gained from stoichiometric bond activation reactions leads to the development of fundamental processes related to catalytic chemistry. Moris Eisen’s laboratory demonstrates the ability of uranium and thorium to mediate addition of alcohols to carbodiimides catalytically (DOI: 10.1021/acs.organomet.7b00432). Using benzimidazolin-2-iminato ligands, uranium and thorium catalyze addition of a variety of commercially available alcohols to
10.1021/acs.organomet.7b00443). The methyl/methylidene cluster was shown to activate E−H (E = N, P) bonds of PhEH with elimination of methane, and a mechanism was proposed through computational means. The process was described to occur through three primary mechanistic sequences along the reaction coordinate, including E−H bond activation at the basic methylidene ligand, intramolecular isomerization, and a second E−H bond activation. The results give information on the differences between mono- and multimetallic complexes in advancing organometallic reactions. Actinide−carbon bonds are explored in the contribution from the Hayton group, which demonstrates the synthesis of a phosphorano-stabilized thorium(IV) carbene complex, [Th(CHPPh3)(NR2)3] (R = SiMe3), which features thorium− carbon multiple-bond character. Synthesized from treatment of a common thorium cyclometalated species, [Th(CH2SiMe2NSiMe3)(NR2)2], with the organic ylide Ph3P CH2 (DOI: 10.1021/acs.organomet.7b00202), the short Th−C bond and assignment of its multiple-bond character are supported using a combination of structural, computational, and spectroscopic analyses. Appropriate to this special issue is the broader context of this work, which demonstrates how 13C NMR spectroscopy can be used to compare covalency among compounds in a series that feature metal−carbon multiplebond interactions. Metal−element multiple bonding continues for group 3, where characterization of interactions, the shaping of coordination spheres with anionic ligands, and the influence of both on magnetic properties motivate the contribution from the Mills and Liddle groups in their study of Y(III)−BIPM methanediide complexes (BIPM = [C(PPh2NSiMe3)2]) (DOI: 10.1021/acs.organomet.7b00394). Key here is the question of cyclometalation in the pursuit of targets with interesting magnetic properties: e.g., [Ln(BIPM)(Nn)] (Ln = Dy, Y; Nn = N(SiMe3)2, N(SitBuMe2)2, N(SitBuMe2)(SiiPr3), N(SiiPr3)2). With Mills’ and Liddle’s work, we can again see how subtle structural variation makes for rich and sometimes challenging chemistry in the pursuit of specific structural targets. A collaboration from the groups of Guofu Zi and Marc Walter highlights the parallels between thorium−nitrogen multiple bonds and their early-transition-metal counterparts with the thorium(IV) aryl imido species (C5Me5)2Th NMes(DMAP) (Mes = 2,4,6-trimethylaniline; DMAP = 4dimethylaminopyridine) (DOI: 10.1021/acs.organomet. 7b00212). This extensive piece of work shows the rich chemistry of ThN multiple bonds, detailing their reactions with a number of small molecules, including thiazoles, silanes, acetylenes, nitriles, ketones, CS2, isothiocyanates, carbodiimides, lactides, organoazides, and diazoalkanes. Activation occurs through established organometallic pathways, including Lewis base ligand substitution, [2 + 2] cycloaddition, 1,2addition, insertion, and multiple-bond metathesis, forming an array of new organometallic derivatives. Exploring the preparation and reactivity of new group 3 metal−ligand multiple bonds continues to be an important and demanding topic. These species show unusual reactivity patterns in comparison to their d-block congeners due to their internal charge polarization, which is more extreme than for d-block transition metal multiply bonded moieties. Chen and co-workers report on the reactivity of base-free [(NNN)ScNDIPP] (NNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMe2)]−, DIPP = 2,6-iPr2C6H3 with 9-borabicyclononane (9-BBN) and catecholborane (CatBH)) (DOI: 10. 4508
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[[(NNTBS)M](μ-C10H8)] react with 1,2-diphenylacetylene to instead furnish the C−C bond coupled, metallacyclopentadiene products, [K(THF)x][(NNTBS)Y(η2-C4Ph4)]. It is evident from these results that the capabilities of rare-earth metals are only expanded when they are combined with redox-active ligands. With the unusual electronic structures imparted by redox flexible ligand frameworks and the large numbers of unpaired electrons characteristic of the lanthanides, magnetic studies are a key characterization technique that help to elucidate electronic structures of f-block elements. The design of felement organometallic complexes with groundbreaking magnetic properties, namely high-barrier/high-blocking-temperature single-molecule magnets, has emerged as a red-hot area of chemistry.4,5 In a contribution from the Murugesu group to this special issue, we again observe that SMM behavior is extremely sensitive to the symmetry of the ligand fields, in a comparison of the Kramers ion complexes [Tm(COT)I(THF)2] and [K(18-crown-6)(THF)2][Tm(COT)2] (DOI: 10.1021/acs.organomet.7b00449). From magnetism to spectroscopy, full characterizations of group 2 and group 3 metal silyl compounds are the focus of the contribution from the Sadow group (DOI: 10.1021/acs. organomet.7b00383). Structural and characteristic spectroscopic features including one-bond silicon−hydrogen coupling constants (1JSiH) and infrared stretching frequencies (νSiH) revealed metal−poly(hydrosilyl) bonding through classical twocenter−two-electron modes, ruling out side-on secondary βSiH interactions. Among other reactions, the silyl compounds undergo β-H abstraction rather than silyl group abstraction. The results speak to the potential of group 2 and group 3 metal silyls as useful precatalysts or as nucleophilic silylating agents. The exciting work presented in this special issue is multifaceted, spanning a variety of fundamental areas in organometallic chemistry. It is our hope that featuring this diverse content will help to not only recognize the contributions of the organometallic f-block chemistry community but also highlight the great opportunities apparent in the field. Chemists working in organic and inorganic synthesis and molecular characterization, including magnetism, ligand design, and stoichiometric and catalytic small-molecule activation, will all benefit from the rich intellectual framework of f-element organometallic chemistry.
1,3-di-p-tolylcarbodiimide under mild conditions at high rates of conversion. Kinetic experiments give insight into the mechanism, with the rate-determining step involving carbodiimide CN bond insertion into a strong actinide−alkoxide bond, formed from alcohol activation. From small molecules to large, catalytic polymerization of isoprene from Anwander and co-workers demonstrates how exquisitely tuning lanthanide complexes in terms of cation size, supporting ligand, and cocatalyst influences polymer selectivity (DOI: 10.1021/acs.organomet.7b00543). Cation size is shown to selectively generate trans- or cis-1,4 polyisoprene. In addition, polymerization reactions are also of interest to the Hou group in their disclosure of cationic, half-sandwich scandium complexes of anisole and N,N-dimethyl-o-toluidine that are pertinent in the controllable, syndiospecific chain-transfer polymerization of styrene. In this context, anisole and N,Ndimethyl-o-toluidine function critically as chain transfer agents (DOI: 10.1021/acs.organomet.7b00526). Furthermore, the discovery of N,N-dimethyl-o-toluidine as a chain transfer agent has provided new structural and mechanistic insight into the polymerization process. Insight into fundamental organometallic processes is on display in the contribution from John Arnold and co-workers, who show the importance of understanding how redox properties of f-block elements can be tuned (DOI: 10.1021/ acs.organomet.7b00301). Reductive elimination is a key twoelectron transformation in important catalytic cycles, but it is typically thought that actinides are incapable of this due to their propensity for single-electron chemistry. Arnold’s example of P−P reductive elimination from a thorium(IV) N-heterocyclic carbene bis(phosphido) complex is noteworthy. In this case, an exogenous redox-active ligand, 2,2′-bipyridine, accepts reducing equivalents subsequent to the elimination process, creating a stable Th(IV) product and circumventing what would be an unstable Th(II) intermediate. From catalysis to stabilization of unusual electronic structures, redox-active ligands are playing a burgeoning role in chemistry at the bottom of the periodic table. The contribution to this special issue from the Gorden group shows that combining redox-noninnocent Ar-BIAN (Ar-BIAN = bis[(aryl)imino]acenaphthene) ligands with mixed donor O− N−N−O salen-type ligands generates a new tetradentate ligand with tunable redox properties (DOI: 10.1021/acs.organomet. 7b00454). Using this ligand with uranyl, a largely redox inactive moiety, imparts interesting redox chemistry at the uranium, as probed by electrochemistry. Evidence for mixed-valence species with unusual U(V) and U(IV) centers is presented. Interestingly, structural characterization with X-ray diffraction shows short contacts between the oxo moieties and the chlorinated solvents used for crystallization, indicating that this unique ligand framework causes solvent association for this typically inert moiety. Perhaps even more so than for the actinides, an important strategy for multielectron chemistry with rare earths in the activation of small molecules is the use of ligands that act as reservoirs for reducing equivalents. Using a ferrocene−diamide supporting ligand, NNTBS (=fc(NSitBuMe2)2; fc = 1,1′ferrocenediyl), Diaconescu and co-workers describe the reactive compound [K(solvent)x][[(NNTBS)M](μ-C10H8)] (M = Sc, Y, La, Lu; solvent = THF, toluene), wherein the reduced naphthalene, stabilized by metal coordination, stores two electrons (DOI: 10.1021/acs.organomet.7b00541). In pursuit of the first rare-earth cyclobutane complexes, [K(solvent)x]-
Suzanne C. Bart*,† Eric J. Schelter*,‡ †
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H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ‡ P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States
AUTHOR INFORMATION
Corresponding Authors
*S.C.B.: e-mail,
[email protected]; tel, (+1) 765-494 5451. *E.J.S.: e-mail,
[email protected]; tel, (+1) 215-8988633. ORCID
Suzanne C. Bart: 0000-0002-8918-9051 Eric J. Schelter: 0000-0002-8143-6206 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. 4509
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REFERENCES
(1) Thayer, A. M. Chem. Eng. News 2013, 91, 68. (2) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491−6493. (3) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203−219. (4) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (5) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Nature 2017, 548, 439.
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