Introduction to the “Recent Advances in f-Element Organometallic

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Introduction to the “Recent Advances in f‑Element Organometallic Chemistry” Special Issue of Organometallics groups.1,5b,10 Highlights include the synthesis of the first uranium−carbon multiple bonds, reported by Cramer and Gilje,11 and the first uranium carbonyl complex by Carmona,12a with the bonding ultimately elucidated by Eisenstein and Andersen.12b Beyond simple f-element cyclopentadienyl complexes, the next major advance came several years later with the implementation in both lanthanide and actinide organometallic chemistry of the Me5C5 (Cp*) ligand by Andersen,13 Watson,14 Evans,15 Marks,16 Schumann,16 Ephritikhine,17 and many others. This new ancillary ligand provides highly reactive organometallic complexes having good hydrocarbon solubility, which eliminates the need to use more coordinating/ deactivating solvents, while opening the metal coordination spheres for greater diversity in reactivity. What followed was a dazzling array of new reactivity patterns, lanthanide and actinide molecular structures, and f-element bonding modes.18 For example, new classes of metal hydrocarbyls, hydrides, amides, phosphides, alkoxides, etc. were reported.18−20 New reactivity modes included CO activation to form carbonyls, carbene-like acyls, and formyls and coupling of these species to form enediolates and other oxygenates, the rapid coordinative polymerization of olefins, and the coupling of olefin insertion with σ-bond metathesis processes to effect extremely rapid olefin hydrogenation and hydrosilation. Stunning examples of saturated hydrocarbon activation at 4f and 5f centers were also reported.14,21 At about the same time, in an effort to better understand metal−ligand bonding trends, Marks undertook measurements of both relative and absolute metal−ligand bond disruption enthalpies for a wide variety of organo-4f, -5f, and related -d0 complexes. Instructive congruencies in the bonding energetic patterns within the two f and d0 series were observed (e.g., in D(M−H) vs D(M−CH3)) along with marked differences vis à vis established bonding energetic patterns in late-transition-metal complexes.22 To first order, these differences could be attributed to metal orbital occupancy and/or electronegativity differences. Not only do these data provide insights into the driving forces for a plethora of known transformations but they also provide suggestions useful in inventing completely new catalytic processes. Parallel to the rapid development of lanthanide and actinide Cp* chemistry came important advances in other ancillary ligand systems, which enabled further advances. Thus, Ernst developed the novel f-element chemistry of alkylated “open” pentadienyl ligands,23 Cotton exploited the venerable “reducing Friedel−Crafts” reaction to prepare and structurally characterize unusual new cationic mononuclear and polynuclear uranium(III) and uranium(IV) arene complexes,24 and binuclear X3U(μ-η6:η6-arene)UX3 complexes were reported by

T

he f elements, meaning the lanthanides and actinides, are the elements in the periodic table in which the 4f and 5f shells, respectively, are filled. The electronic and chemical properties of these metals portend unusual organometallic chemistry: very large ionic radii and large, flexible coordination geometries, relatively polar metal−ligand bonding, with the lanthanides and later actinides exhibiting minimal 4f/5f covalency, high Lewis acidity, generally labile metal−ligand bonding, predominantly fixed oxidation states, with interesting exceptions, and many oxidation states having unpaired f electrons. Note that the “rare earths” are not rare in terms of relative abundance in the earth’s crust and find numerous applications in magnetic materials, laser materials, phosphors, and organic synthesis. Actinides form the basis of current generation nuclear fuels production processes, and the early actinides require minimal (but prudent) laboratory precautions in terms of radiation safety. Regarding the history of f-element organometallic chemistry, Organometallics Founding Editor Dietmar Seyferth published a detailed and engaging 2004 Organometallics essay on the early history of organoactinide chemistry, celebrating the anniversary of the discovery of uranocene.1 The present introduction is intended to serve as a slightly personalized complement to that article, touching upon some of the highlights over the past decades, and offering a prelude to the diverse topics presented in this special issue. The earliest published attempts to prepare organouranium alkyl compounds date back to unsuccessful efforts by Gilman in the 1940s as part of the Manhattan Project and led to a general feeling that such species were intrinsically unstable.2 The rapid development of ferrocene and other cyclopentadienyl “sandwich” compounds in the early 1950s next led to the synthesis of a number of actinide and lanthanide cyclopentadienyl compounds initially by the groups of Wilkinson and Fischer (e.g., Cp 3 AnX, Cp 4 An, Cp 3 Ln complexes, An = actinide, Ln = lanthanide).3,4 The discovery of the sandwich compound uranocene ((C8H8)2U) by Streitwieser in 1968 created excitement in this field and illustrated the unusual organometallic chemistry and bonding possibilities of these elements. What followed was the rapid development of (η8-C8H8)An synthetic, structural, and electronic structural information by the Streitwieser, Raymond, and Edelstein research groups at Berkeley, Cummins at MIT, Cloke at Sussex, and at many other places.1,5 Stimulating questions arose about the role of 5f orbitals in metal−ligand bonding and a great deal of research activity took place. Next, the question of whether stable complexes with metal−carbon σ bonds were accessible was answered by the work of Marks,6 Tsutsui,7 and Mazzei,8 who reported a wide range of isolable Cp3UR and Cp3ThR complexes and explorations of their reactivity, molecular structures, and electronic structures. However, it was not until 1989 that the first well-defined homoleptic uranium hydrocarbyl was reported by Sattelberger and Burns.9 This was followed by a rapid expansion of 4f and 5f cyclopentadienyl chemistry by many other talented © 2013 American Chemical Society

Special Issue: Recent Advances in Organo-f-Element Chemistry Published: March 11, 2013 1133

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Cummins25 and pentalene complexes of thorium and urnanium by Cloke.26 Highlights of the next phase of organo-f-element development include the exploitation of coupled C−C insertion and σbond metathesis processes for catalysis, new ancillary ligand systems building upon Cp* and its variants, transformations involving metal redox processes, remarkable zerovalent lanthanide and actinide complexes,27 and catalytic hydroelementation processes in which heteroatom−H bonds are added to C−C unsaturation.28 In the former work, Cloke reported the synthesis of a number of stable, crystallographically characterizable, formally zerovalent Ln(alkylated arene)2 and An(alkylated arene)2 complexes, formally analogous to classical transition-metal sandwich complexes such as bis(ibenzene)chromium. Detailed thermochemical studies of these curious complexes by Cloke and Marks as well as experimental and theoretical electronic structure studies by Cloke, Fragalà, and Marks argued that these are indeed zerovalent complexes, that 5d bonding to the arene π system is important, and that the metal−ligand bond dissociation enthalpies are much larger (the bonding is “stronger” than in bis(benzene)chromium).29 Regarding hydroelementation, Marks and then later Roesky, Anwander, Hultzsch, Livinghouse, and many other active research groups developed a number of classes of organolanthanide ancillary ligands for both achiral and asymmetric hydrogenations, hydrosilations, hydroaminations, and hydrophosphinations.28−33 In the optimum cases, ee’s and de’s greater than 90% were reported, although in many cases trends in selectivity are not clearly understood and may reflect the large f-element ionic radii (less predictable steric constraints) and nondirectional bonding (less covalency). Cycles in which C−C unsaturation (olefins, alkynes, allenes, dienes) insertion into Ln−NR2 or An−NR2 linkages are coupled with protonolysis/σ bond methathesis of the resulting Ln−C or An−C bonds were shown to yield broad classes of hydroamination reactions. Examples of natural product/ pharmaceutical skeleton construction were also reported. In general, rates far exceed those possible with analogous delement catalysts.30−33 In related work it was shown that these hydroelementation processes could be coupled to olefin polymerization processes to produce amine-, phosphine-, and silane-capped polyolefins.33 This f-element catalytic hydroelementation strategy was later extended to analogous catalytic hydroalkoxylation and hydrothiolation processes by Marks.34 Using organoactinide catalysts, Eisen developed an extensive chemistry of alkyne and alkene oligomerization, hydroamination, and hydrosilation processes.35 In this same time period, Evans developed an extensive and highly informative chemistry of divalent Cp*2Ln complexes. A plethora of unprecedented coupling processes was demonstrated, including those involving C−C unsaturated substrates, CO, and dinitrogen.12,36 In roughly the same time period, an important feature in the next phase of organo-f-element chemistry evolution was the exploration of fascinating new metal−ligand multiple bonds and the push to higher metal oxidation states. Thus, Burns, Kiplinger, Andrews, Hayton, Liddle, and others developed the chemistry of organo-5f U NR, UNR, UO, UC/UC, and U(chalcogen) complexes.37 In the area of novel ligands, there was a virtual explosion of activity in the area of novel non-Cp supporting ligands of all types, from nacnac and other multidentate nitrogenous ligands to diverse chiral systems such as lanthanide binaphtholato and bis(oxazolinato) complexes. Using these

ligands, the application of organolanthanides to achiral and chiral hydroamination catalysis blossomed with contributions from numerous investigators such as Roesky, Anwander, Hultzsch,21 Carpentier,38 Trifonov,38 Livinghouse,30b Piers,39 and Schaefer, 40 to name just a few. In the area of polymerization catalysis, new work was reported on lanthanide-mediated polymerization/copolymerization of dienes and polar monomers such as lactides and acrylates.29a,b,38 Further examples of hydrocarbon activation emerged from the work of Tilley, along with detailed descriptions of the reaction mechanisms.41 In the area of heterogeneous catalysis, organoactinide molecules supported on dehydroxylated alumina and MgO were shown to be very effective catalysts for the rapid, selective hydrogenation and polymerization of olefins, while solid-state 13C CPMAS NMR spectroscopy showed that only small percentages of the surface species (present as cations) are catalytically significant.42 Subsequently, Anwander showed that chemisorption of lanthanide complexes on mesoporous silicas yields very active and recyclable hydroamination catalysts.43 In addition to the aforementioned experimental advances, the sophistication of heavy-element electronic structure and reaction pathway computations continued to advance, tackling questions of metal−ligand covalency and catalytic reaction pathways that were impossible to answer a decade before, with important contributions from Bursten, Martin, Eisenstein, Tobisch, Fragalà, Pyykkko, Pitzer, and others.44 The present Organometallics special issue is intended to offer an update on recent advances in organo-f-element chemistry. As such, it features both lanthanide and actinide chemistry, as well as closely related group 3 chemistry, with contributions from both established and, gratifyingly, many young investigators. Many new types of ligands and their ability to modify and control f-element reactivity is one focal area reported. Another area concerns unusual metal oxidation states and the activation of small molecules by highly unsaturated metal complexes. Catalytic chemistry is also well represented here by articles on polymerization, copolymerization, hydroelementation, and σ-bond metathesis processes. Physicochemical studies reported include investigations of f-orbital bonding and unusual f-element complex magnetic properties.

Tobin J. Marks, Associate Editor

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Northwestern University

AUTHOR INFORMATION

Biography

Tobin Marks is Vladimir N. Ipatieff Professor of Catalytic Chemistry and Professor of Materials Science and Engineering at Northwestern University. He received a B.S. degree from the University of Maryland 1134

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Actinide Elements, 2nd ed.; Katz, J. J., Seaborg, G. T., Eds.; Chapman and Hall: London, 1986, Chapter 23. (b) Marks, T. J., Fragalà, I. L. Eds. Fundamental and Technological Aspects of Organo-f-Element Chemistry; Reidel: Dordrecht, Holland, 1985 (a book; see the chapters therein). (c) Barnea, E.; Eisen, M. S. Coord. Chem. Rev. 2006, 250, 855. (d) Tamer, A.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550. (e) Patel, D.; Liddle, S. T. Rev. Inorg. Chem. 2012, 32, 1. (19) (a) Manriquez, J. M.; Fagan, P. J.; Marks, T. J. J. Am. Chem. Soc. 1978, 100, 3939. (b) Manriquez, J. M.; Fagan, P. J.; Marks, T. J.; Day, C. S.; Day, V. W. J. Am. Chem. Soc. 1978, 100, 7112. (c) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, V. W.; Vollmer, S. H.; Day, C. S. J. Am. Chem. Soc. 1980, 102, 5393. (d) Fagan, P. J.; Manriquez, J. M.; Vollmer, S. H.; Day, C. S.; Day, V. W.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 2206. (20) (a) Evans, W. J.; Meadows, J. J.; Wayda, A. L.; Hunter, W. J.; Atwood, J. L. J. Am. Chem. Soc. 1982, 104, 2008. (b) Evans, W. J.; Bloom, I.; Hunter, W. J.; Atwood, J. L. J. Am. Chem. Soc. 1983, 105, 1401. (c) Evans, W. J.; Grate, J. W.; Choi, H. W.; Bloom, I.; Hunter, W. J.; Atwood, J. L. J. Am. Chem. Soc. 1985, 107, 941. (21) (a) Bruno, J. W.; Duttera, M. R.; Fendrick, C. M.; Smith, G. M.; Marks, T.J. C. M.; Fendrick; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 2214. (b) Fendrick, C. M.; Marks, T. M. J. Am. Chem. Soc. 1986, 108, 425. (22) (a) Bruno, J. W.; Morss, L.; Marks, T. J. J. Am. Chem. Soc. 1983, 105, 6824. (b) Sonnenberger, D. C.; Bruno, J. W.; Stecher, H. A.; Morss, L. R.; Marks, T. J. J. Am. Chem. Soc. 1986, 108, 7275. (c) Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 7844. (c) Nolan, S. P.; Porchia, M.; Marks, T. J. Organometallics 1991, 10, 1450. (23) (a) Ernst, R. D.; Cymbaluk, T. H. Organometalllics 1982, 1, 708. (b) Cymbaluk, T. H.; Liu, J.-Z.; Ernst, R. D. J. Organomet. Chem. 1983, 255, 311. (24) (a) Cotton, F. A.; Schwotzer, W.; Simpson, C. Q. Angew. Chem., Int. Ed. Engl. 1986, 25, 637. (b) Cotton, F. A.; Schwotzer, W. Organometallics 1985, 4, 921. (c) Campbell, G. C.; Haw, J. F.; Cotton, F. A.; Schwotzer, W. Organometallics 1985, 4, 921. (25) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.; Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 78446108. (26) (a) Cloke, F. G. N.; Hitchcock, P. B. J. Am. Chem. Soc. 1997, 119, 7899. (b) Cloke, F. G. N.; Green, J. C.; Jardine, C. N. Organometallics 1999, 18, 1080. (27) (a) Brennan, J. G.; Cloke, F. G. N.; Sameh, A. A.; Zalkin, A. J. Chem. Soc., Chem. Commun. 1987, 1668. (b) King, W. A.; DiBella, S.; Lanza, G.; Khan, K.; Duncalf, D. J.; Cloke, F. G. N.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 627. (28) (a) Gagné, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 4108. (b) Gagné, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275. (c) Li, Y. W.; Fu, P.-F.; Marks, T. J. Organometallics 1994, 13, 439. (d) Douglass, M. R.; Marks, T. J. J. Am. Chem. Soc. 2001, 123, 10221. (e) Hong, S.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 7886. (f) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (29) (a) Roesky, P. W., Ed. Molecular Catalysis of Rare-Earth Elements (Structure and Bonding); Springer: Berlin, 2010, and chapters therein. (b) Sharma, M.; Eisen, M. S. In Organometallic and Coordination Chemistry of the Actinides (Structure and Bonding); Albrecht-Schmitt, T. E., Ed.,; Springer: Berlin, 2008;, pp 1−85. (c) Roesky, P. W.; Muller, T. E. Angew. Chem., Int. Ed. 2003, 42, 2708. (d) Le Roux, E.; Liang, Y.; Storz, M. P.; Anwander, R. J. Am. Chem. Soc. 2010, 132, 16368. (e) Müller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795. (30) (a) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. J. Am. Chem. Soc. 2006, 128, 3748. (b) Kim, Y. K.; Livinghouse, T. J. Am. Chem. Soc. 2003, 125, 9560. (31) (a) Conticello, V. P.; Brard, L.; Giardello, M. A.; Stern, C. L.; Tsuji, Y.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 2761. (b) Gagné, M. R.; Brard, L.; Conticello, V. P.; Giardello, M. A.; Stern, C. L.; Marks, T. J. Organometallics 1992, 11, 2003. (32) (a) Douglass, M. R.; Ogasawara, M.; Hong, S.; Metz, M. V.; Marks, T. J. Organometallics 2002, 21, 283−292. (b) Hong, S.; Tian,

(1966) and a Ph.D. degree from MIT (1971). Among his recognitions, he received the 2006 U.S. National Medal of Science, the 2008 Principe de Asturias Prize in Science and Technology, the 2009 MRS Von Hippel Award, the 2011 Dreyfus Prize in the Chemical Sciences, and the 2012 U.S. National Academy of Sciences Award in the Chemical Sciences. He is an elected member of the U.S., German, and Indian National Academies of Sciences, an elected member of the U.S. National Academy of Engineering, and a Fellow of the Royal Society of Chemistry and of the American Academy of Arts and Sciences. He has published 1065 peer-reviewed articles and has 215 issued U.S. patents.

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ACKNOWLEDGMENTS T.J.M. thanks the NSF for generous support of his f-element research program under grant CHE-11213235 during the time in which this overview was written.

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REFERENCES

(1) Seyferth, D. Organometallics 2004, 23, 3562. (2) For an overview of early attempts, see: Marks, T. J. Acc. Chem. Res. 1976, 9, 223. (3) Reynolds, L. T.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 275. (4) (a) Fischer, E. O.; Hristidu, Y. Z. Naturforsch. 1962, 176, 275. (b) Fischer, E. O.; Treibner, A. Z. Naturforsch. 1962, 176, 276. (5) (a) Müller-Westerhoff, U.; Streitweiser, A. D., Jr. J. Am. Chem. Soc. 1968, 90, 7364. (b) Marks, T. J.; Streitwieser, Jr., A. Organoactinide Chemistry. Properties of Compounds Having πBonded Ligands. In The Chemistry of the Actinide Elements, 2nd ed.; Katz, J. J., Seaborg, G. T., Eds.; Chapman and Hall: London, 1986; Chapter 22. (c) Parry, J. A.; Cloke, F. G. N.; Coles, S. J.; Hursthouse, M. G. J. Am. Chem. Soc. 1999, 121, 6867. (d) Diaconescu, P. L.; Cummins, C. C. J. Am. Chem. Soc. 2002, 124, 7660. (6) (a) Marks, T. J.; Seyam, A. M. J. Am. Chem. Soc. 1972, 94, 6545. (b) Marks, T. J.; Wachter, W. A. J. Am. Chem. Soc. 1976, 98, 703. (7) Gebala, A. E.; Tsutsui, M. Chem. Lett. 1972, 775. (8) Brandi, G.; Brunelli, M.; Lugli, G.; Mazzie, A. Inorg. Chim. Acta 1978, 7, 319. (9) Vandersluys, W. G.; Burns, C. J.; Sattelberger. Organometallics 1989, 8, 855. (10) For overviews, see: (a) Marks, T. J. Science 1982, 217, 989. (b) Marks, T. J., Fischer, R. D., Eds. Organometallics of the f-Elements; Reidel: Dordrecht, Holland, 1979 (a book; see the chapters therein). (11) (a) Cramer, R. E.; Maynard, R. B.; Paw, J. C.; Gilje, J. W. J. Am. Chem. Soc. 1981, 103, 3589. (b) Cramer, R. E.; Maynard, Panchanatheswaran, K.; Gilje, J. W. J. Am. Chem. Soc. 1984, 106, 1853. (12) (a) Parry, J.; Carmona, E.; Coles, S.; Hursthouse, M. J. Am. Chem. Soc. 1995, 117, 2469. (b) Maron, L.; Eisenstein, O.; Andersen, R. A. Organometallics 2009, 28, 3629. (13) (a) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin, A.; Templeton, D. H. Inorg. Chem. 1980, 19, 2999. (b) Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514. (14) (a) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (b) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (15) (a) Evans, W. J. Adv. Organomet. Chem. 1985, 24, 131. (b) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 211. (c) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1988, 110, 6877. (16) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. J.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 809. (b) Jeske, G.; Schock, L. E.; Swepston, P. J.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103. (c) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111. (17) Boisson, D.; Berthet, J.-C.; Ephritikhine, M.; Lance, H.; Nierlich, M. J. Organomet. Chem. 1997, 533, 7. (18) (a) Marks, T. J. Organoactinide Chemistry. Properties of Compounds with Actinide-to-Carbon, Actinide-to-Hydrogen, and Actinide-to-Transition Metal Sigma Bonds. In The Chemistry of the 1135

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S.; Metz, M. V.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 14768. (c) Yu, X.; Marks, T. J. Organometallics 2007, 26, 365. (33) (a) Koo, K.; Fu, P.-F.; Marks, T. J. Macromolecules 1999, 32, 981. (b) Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 6311. (c) Amin, S. B.; Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006. (34) (a) Seo, S. Y.; Yu, X.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 263. (b) Weiss, C. J.; Wobser, S. D.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 2062. (c) Weiss, C. J.; Marks, T. J. Dalton Trans. 2010, 39, 6576. (d) Dzudza, A.; Marks, T. J. Chem. Eur. J. 2010, 16, 3403. (35) (a) Haskel, A.; Straub, T.; Eisen, M. S. Organometallics 1996, 15, 3773. (b) Haskel, A.; Straub, T.; Eisen, M. S. J. Am. Chem. Soc. 1995, 117, 6364. (c) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J. X.; Kapon, M.; Eisen, M. S. Organometallics 2001, 20, 5084. (36) (a) Evans, W. J. Polyhedron 1987, 6, 803. (b) Evans, W. J.; Grate, J. W.; Choi, H. W. J. Am. Chem. Soc. 1985, 107, 941. (c) Evans, W. J.; Ulifbaarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 2314. (37) (a) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448. (b) Thomson, R. K.; Cantat, T.; Scott, B. L.; Morris, D. E.; Batista, E. R.; Kiplinger, J. L. Nat. Chem. 2010, 2, 723. (c) Lyon, J. T.; Hu, H.-S.; Andrews, L.; Li, J. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 18919. (d) Brown, J. L.; Fortier, S.; Lewis, R. A.; Wu, G.; Hayton, T. W. J. Am. Chem. Soc. 2012, 134, 15468. (e) Hayton, T. W. Dalton Trans. 2010, 39, 1145. (f) King, D. M.; Tuna, F.; McInnes, E. J. L.; McMaster, J.; Blake, A. J.; Liddle, S. T. Science 2012, 337, 717. (38) (a) Kirillov, E.; Carpentier, J.-F. Chem. Today 2008, 26, 60. (b) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.; Cherkasov, A. V.; Thomas, C. M.; Carpentier, J.-F.; Trifonov, A. A. Chem. Eur. J. 2008, 14, 5440. (39) (a) Kenward, A. L.; Ross, J. A.; Piers, W. E.; Parvez, M. Organometallics 2009, 28, 3625. (b) Kenward, A. L.; Piers, W. E.; Parvez, M. Organometallics 2009, 28, 3012. (40) (a) Thomson, J. A.; Schafer, L. L. Dalton Trans. 2012, 41, 7897. (b) Lauterwasser, F.; Hayes, P. G.; Piers, W. E.; Schafer, L. L. Adv. Synth. Catal. 2011, 353, 1384. (41) (a) Fontaine, F. G.; Tilley, T. D. Organometallics 2006, 24, 4340. (b) Barros, N.; Eisenstein, O.; Maron, L.; Tilley, T. D. Organometallics 2007, 25, 5699. (42) (a) Finch, W. C.; Gillespie, R. D.; Hedden, D.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 6221. (b) Gillespie, R. D.; Burwell, R. L., Jr.; Marks, T. J. Langmuir 1990, 6, 1465. (c) He, M.-Y.; Burwell, R. L., Jr.; Marks, T. J. Organometallics 1983, 2, 566. (43) Leroux, E.; Liang, Y. C.; Storz, M. P.; Anwander, T. J. Am. Chem. Soc. 2010, 132, 16368. (44) (a) Sonnenberg, J. L.; Hay, P. J.; Martin, R. L.; Bursten, B. E. Inorg. Chem. 2005, 44, 2255. (b) Bursten, B. E.; Drummong, M. L. Faraday Discuss. 2003, 124, 1. (c) Werkema, E. L.; Yahia, A.; Maron, L.; Eisenstein, O.; Andersen, R. A. Organometallics 2010, 29, 5103. (d) Werkema, E. L.; Andersen, R. A.; Maron, L.; Eisenstein, O. Dalton Trans. 2010, 6648. (e) Tobsich, S. Dalton Trans. 2012, 9182. (f) Motta, A.; Fragalà, I. L.; Marks, T. J. Organometallics 2005, 24, 4995. (g) Motta, A.; Fragalà, I. L.; Marks, T. J. Organometallics 2006, 25, 5533. (h) Pyykko, P. Chem. Rev. 1988, 88, 563. (i) Chang, A.; Pitzer, R. M. J. Am. Chem. Soc. 1989, 111, 2500. (j) Pitzer, R. M.; Winter, N. W. J. Phys. Chem. 1988, 92, 3061.

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