Editorial pubs.acs.org/CR
Introduction: Metal Hydrides he first molecular “hydrides” were probably H2Fe(CO)4 (1931) and HCo(CO)4 (1937), both discovered by Hieber and co-workers.1−3 The acidity of both was mentioned in a review of metal carbonyls that was published in this journal around the time Hieber reported HCo(CO)4.4 The first “organometallic TM hydride” has been said5 to be Cp2ReH, reported by Wilkinson and Birmingham in 1955 but described more fully in 1958.6,7 This is the first thematic issue of Chemical Reviews on transition-metal hydride complexes. Very few comprehensive reviews of hydrides in general have been published. One by Kaesz and Saillant appeared in this journal in 1972,8 one by McCue appeared in Coordination Chemistry Reviews in 1973,9 one by Moore and Robinson appeared in Chemical Society Reviews in 1983,10 and one by McGrady appeared in the same journal in 2003.5 Other reviews of broad scope include one on anionic metal hydrides by Darensbourg and Ash in 198711 and one on early transition-metal hydrides by Hoskin and Stephan in 2002,12 while two reviews of dihydrogen complexes,13,14 a review of their applications in catalysis,15 a review on the computational analysis of polyhydrides,16 a review of the M−H bond,17 and a review of M−H bond strengths15 have appeared in Chemical Reviews. However, several books, all with the title Transition Metal Hydrides, have been published whose chapters allow one to track the growth of this subject since it first attracted widespread attention in the 1950s. The book edited by Muetterties (1971)18 emphasized the synthesis and stereochemistry of hydride complexes. The volume edited by Bau (Advances in Chemistry #167, 1978)19 emphasized structural and vibrational issues. The book edited by Dedieu (1992)20 took a more theoretical approach, emphasizing bonding and thermodynamic properties along with reactivity (nucleophilicity, isotope effects). A final volume, with a different title (Recent Advances in Hydride Chemistry), edited by Peruzzini and Poli, has appeared more recently (2001);21 it emphasizes the physical properties and reactivity of hydrides. In this thematic issue of Chemical Reviews we have provided up-to-date reviews on some of the topics, as well as adding reviews on topics that were not previously covered.
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comprehensive overview of their synthesis, characterization, and stoichiometric and catalytic reactions. Ni−H bonds are involved in catalyzing the oxidation of H2, and in the action of Ni-containing enzymes such as NiFe hydrogenase and methylcoenzyme M reductase. The role that cation radicals play in the chemistry of hydride complexes is attracting increasing attention, so they are reviewed in this issue by Hu, Norton, and co-workers. “Hydride” abstraction often begins with a one-electron oxidation, giving a cation radical with a pKa far lower than that of the original hydride. Transfer of protons is thus a characteristic reaction of cation radicals, although transfer of H• and reductive elimination also occur. Surface hydrides, reviewed in this issue by Copéret and coworkers, have been proposed as intermediates in numerous processes, and modern spectroscopic techniques have made it possible to characterize them on oxide supports. Surface hydrides can be generated from molecular hydrides, and by the hydrogenolysis of supported alkyl complexes.
TOPICS THAT ARE BEING RE-REVIEWED Aspects of photochemistry were reviewed in two chapters of the books mentioned above (Chapter 14 of the Bau volume, and Chapter 7 of the Dedieu one). However, the review in this issueby Perutz and Procacciis the first to treat the subject comprehensively. Hydride complexes are often good hydrogen bond acceptors, and dihydrogen complexes are often good hydrogen bond donors. Both types of hydrogen bonds were covered in Chapter 1 of Peruzzini/Poli, and detection of these hydrogen bonds by spectroscopy was covered in Chapter 14 of that book. The protonation of hydride ligands was reviewed in Chapters 2 and 14 of the Peruzzini/Poli book, and more recently (2009) in Chemical Society Reviews by Lledós and co-workers.22 In the present issue, hydrogen and dihydrogen bonds are reviewed by Shubina and co-workers. The thermodynamic acidity of hydrides was reviewed in the Dedieu book, but measurements have become more widespread (and extended to a wider variety of solvents) in recent years. Morris has reviewed the acid strengths of hydride and dihydrogen complexes in various solvents, giving a detailed account of the limitations of such measurements in solvents with low dielectric constants. He offers a simple equation that estimates hydride acidities and applies it to the analysis of stoichiometric reactions and catalytic cycles. Measurements of the thermodynamic hydricity of transitionmetal hydrides are relatively new. Several thermodynamic cycles that could be used for this purpose were described by DuBois and co-workers,23,24 and reviewed by Berke in Chapter 4 of the Peruzzini/Poli book. Hydricities are best expressed as free energies in solution, but are very much solvent dependent.
NEW TOPICS Interest in the “coinage metal” hydrides has recently soared, so a review of them by Lalic, Jordan, and Sadighi is included. Binary copper hydrides and hexacopper hydrides L6Cu6H6 have been known for some years, although recent structural (neutron) and theoretical work has raised questions about the earlier proposal that the hydrides in L6Cu6H6 are “face capping”. Larger copper hydrides, from Cu8 through the “Chinese puzzle ball” Cu28, are becoming common, and a significant number of silver and gold hydrides have recently been prepared. All of these hydrides are increasingly used in organic synthesis and catalysis. Interest in nickel hydrides has also grown in the past few years. The review by Eberhardt and Guan provides a © 2016 American Chemical Society
Special Issue: Metal Hydrides Published: August 10, 2016 8315
DOI: 10.1021/acs.chemrev.6b00441 Chem. Rev. 2016, 116, 8315−8317
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We are grateful to Guy Bertrand for suggesting this thematic issue, and to Michele Soleilhavoup for holding all of us to our schedulealbeit a more lenient one than the originaland assembling the referee reports, the revisions, and the final version. We thank the authors who have worked so hardand so effectivelyon their reviews, not only for completing them but for putting up with much cajolery from the two of us as Guest Editors. On behalf of the authors, we thank the reviewers for many helpful suggestions. We thank Yue (Rachel) Hu for help in getting this project started, and Edgar Schilter for the cover drawing (the hydride form of NiFe hydrogenase). We hope that readers will learn as much from reading this issue as we have from assembling it.
As explained by Miller, Appel, and co-workers in their review, thermodynamic hydricities predict the ability of various complexes to catalyze a variety of transformations, including the reduction of CO2 and the production and use of H2. Metalloenzymes, particularly those that catalyze the evolution or uptake of hydrogen, often contain hydride ligands. They have been previously reviewed in Chapter 16 of the Peruzzini/Poli book. Hydrogenases contain either an FeFe, a NiFe, or an Fe core; all three classes are reviewed in this issue by Schilter and co-workers. The NiFe hydrogenase catalytic cycle features intermediates in which a hydride ligand bridges the nickel and iron centers (one such form is drawn on the cover of this issue). However, little evidence exists for the involvement of hydrides in the FeFe or Fe enzyme mechanisms. The chemistry behind the native catalysts and their synthetic models is described here. Dihydrogen complexes have, as mentioned above, been the subject of several reviews. Early theoretical studies were summarized in Chapter 5 of the Dedieu book. The coordination chemistry of H2 was discussed in several chapters of the Peruzzini/Poli book: Chapter 1 on the heterolytic splitting of dihydrogen, Chapter 9 on the catalytic applications of dihydrogen complexes, and Chapters 2 and 14 on their generation by the protonation of ordinary hydride ligands. In this thematic issue, Crabtree reviews recent developments in the chemistry of dihydrogen complexes. It has now become apparent that the distinction between dihydrogen complexes and dihydrides is not strict, but a matter of continuous variation, from H2 complexes through “elongated dihydrogen” to “compressed dihydride” and eventually dihydride complexes. Polyhydrides offer exciting conceptual challenges and should have applications in energy storage, pharmaceutical synthesis, and materials science. They are reviewed here by Esteruelas and co-workers. The storage of hydrogen, and its release so that it can serve as a source of energy, has become an important area of research. The use of hydride complexes for hydrogen storage was covered in Chapter 18 of the Peruzzini/Poli book. The dehydrogenation of amine/borane adducts, lightweight enough to be particularly attractive as hydrogen carriers, is reviewed here by Rossin and Peruzzini. Hydride ligands readily coordinate with Lewis acids, and that coordination generally decreases their reactivity. However, substrate reactivity (e.g., that of aldehydes, ketones, and carbonyl ligands) is often accelerated by a Lewis acid, as is the heterolytic cleavage of H2. These reactions and others involving transition-metal hydrides and main group Lewis acids are reviewed in this thematic issue by Maity and Teets. Finally, transition-metal hydrides are increasingly used in organic synthesis. They can generate radicals by H• transfer, usually with Markovnikov selectivity. The radical hydrofunctionalization of olefins, reviewed here by Shenvi and coworkers, can be accomplished with Mn, Fe, or Co catalysts; H2 gas, alcohols, or silanes generate the hydrides. Given the primary position of hydrogen in the periodic table, its bond to transition metals will remain central to organometallic chemistry. The synthesis of hydride complexes has been easier with second- and third-row metals because of the strength of their M−H bonds, but future applications of hydride complexes to energy storage, materials chemistry, organic synthesis, and catalysis will profit from the weakness of M−H bonds with first-row transition metals.
Jack R. Norton*
Columbia University
John Sowa
Governors State University
AUTHOR INFORMATION Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. Biographies
Jack R. Norton is Professor of Chemistry at Columbia University. He was born in Dallas, Texas, and received his B.A. in chemistry from Harvard University in 1967 and his Ph.D. in chemistry from Stanford University in 1972 (under the mentorship of J. P. Collman); he was a postdoctoral fellow in Cambridge, England (under J. Lewis), in 1972. He began his academic career as Assistant Professor at Princeton University in 1973 and became Associate Professor at Colorado State University in 1979 and Professor at CSU in 1981; he moved to Columbia in 1997. He has been named a Dreyfus Foundation Teacher Scholar (1976), a Sloan Fellow (1977), and a Guggenheim Fellow (1989), and has received the ACS award in organometallic chemistry (2005) and the Arthur C. Cope Scholar Award (2013). From 1992 to 2003, he served as Associate Editor of the Journal of the American Chemical Society. He is interested in the mechanisms of organometallic reactions and their applications to catalysis and organic synthesis. In particular he and his group have studied the transfer of protons, hydrides, and hydrogen atoms from transition-metal hydrides, and have used H• transfer to effect radical cyclizations. Other reactions of recent interest include (1) the use of polynuclear hydrides to catalyze the generation of electricity from hydrogen; (2) the insertion of isonitriles into the M−C bonds of titanacycles, and the resulting synthesis of α-methylene cyclopentenimines. They are also exploring the ability of coordinated aldimines and aldehydes to exchange 8316
DOI: 10.1021/acs.chemrev.6b00441 Chem. Rev. 2016, 116, 8315−8317
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(13) Heinekey, D. M.; Oldham, W. J. Coordination chemistry of dihydrogen. Chem. Rev. (Washington, DC, U. S.) 1993, 93, 913−926. (14) Kubas, G. J. Fundamentals of H2 binding and reactivity on transition metals underlying hydrogenase function and H2 production and storage. Chem. Rev. 2007, 107, 4152−4205. (15) Esteruelas, M. A.; Oro, L. A. Dihydrogen complexes as homogeneous reduction catalysts. Chem. Rev. 1998, 98, 577−588. (16) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Transition metal polyhydrides: From qualitative ideas to reliable computational studies. Chem. Rev. 2000, 100, 601−636. (17) Pearson, R. G. The transition-metal-hydrogen bond. Chem. Rev. 1985, 85, 41−49. (18) Muetterties, E. L. Transition Metal Hydrides; Marcel Dekker: New York, 1971. (19) Bau, R. Transition Metal Hydrides; American Chemical Society: Washington, DC, 1978. (20) Dedieu, A. Transition Metal Hydrides; VCH Publishers: New York, 1992. (21) Peruzzini, M.; Poli, R. Recent Advances in Hydride Chemistry; Elsevier: New York, 2001. (22) Besora, M.; Lledos, A.; Maseras, F. Protonation of transitionmetal hydrides: a not so simple process. Chem. Soc. Rev. 2009, 38, 957−966. (23) Berning, D. E.; Noll, B. C.; DuBois, D. L. Relative hydride, proton, and hydrogen atom transfer abilities of HM(diphosphine)(2) PF6 complexes (M = Pt, Ni). J. Am. Chem. Soc. 1999, 121, 11432− 11447. (24) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. Measurement of the Hydride Donor Abilities of [HM(diphosphine)2]+ Complexes (M = Ni, Pt) by Heterolytic Activation of Hydrogen. J. Am. Chem. Soc. 2002, 124, 1918−1925.
enantiofaces, and thus to undergo dynamic kinetic asymmetric transformations.
John R. Sowa is Professor of Chemistry and Chair of the Division of Chemistry and Biological Sciences at Governors State University. He was born in Philadelphia, Pennsylvania, and was raised in Schenectady, New York. He received his B.S. in Chemistry from Manhattan College in 1985 and worked with Prof. John D. Mahony on environmental chemistry. He received his Ph.D. in Chemistry from Iowa State University in 1991 under the mentorship of Prof. Robert J. Angelici. It was through this experience that he became interested in transitionmetal hydrides and developed a method for measuring the basicity of transition metals via calorimetry. From 1991 to 1994, he was a postdoctoral associate with Profs. Paul G. Gassman and Kent R. Mann, and a chemistry instructor at the University of Minnesota. John started his academic career at Seton Hall University (1994−2014), and then served as Associate Principal Scientist at Gibraltar Laboratories (2014) and as a visiting scientist in the research group of Prof. Donna G. Blackmond (2015) at the Scripps Research Institute.
REFERENCES (1) Heiber, W.; Leutert, F. Ethylenediamine substituent carbonyls and a new method of formation of iron carbonyl hydrogen (XI. Announcement on metal carbonyls). Ber. Dtsch. Chem. Ges. B 1931, 64, 2832−2839. (2) Hieber, W.; Leutert, F. Information on the coordinative bonded carbon oxide - The formation of iron carbonyl hydrogen. Naturwissenschaften 1931, 19, 360−361. (3) Hieber, W.; Schulten, H. Concerning metal carbonyls XXIV. Method of formation and metal salt reactions of cobalt carbonyl hydrides. Z. Anorg. Allg. Chem. 1937, 232, 17−28. (4) Blanchard, A. A. The volatile metal carbonyls. Chem. Rev. 1937, 21, 3−38. (5) McGrady, G. S.; Guilera, G. The multifarious world of transition metal hydrides. Chem. Soc. Rev. 2003, 32, 383−392. (6) Wilkinson, G.; Birmingham, J. M. Biscyclopentadienylrhenium Hydride - a New Type of Hydride. J. Am. Chem. Soc. 1955, 77, 3421− 3422. (7) Green, M. L. H.; Pratt, L.; Wilkinson, G. Biscyclopentadienylrhenium Hydride. J. Chem. Soc. 1958, 3916−3922. (8) Kaesz, H. D.; Saillant, R. B. Hydride Complexes of the Transition Metals. Chem. Rev. 1972, 72, 231−281. (9) McCue, J. P. Transition-Metal Hydrides. Coord. Chem. Rev. 1973, 10, 265−333. (10) Moore, D. S.; Robinson, S. D. Hydrido Complexes of the Transition-Metals. Chem. Soc. Rev. 1983, 12, 415−452. (11) Darensbourg, M. Y.; Ash, C. E. Anionic Transition-Metal Hydrides. Adv. Organomet. Chem. 1987, 27, 1−50. (12) Hoskin, A. J.; Stephan, D. W. Early transition metal hydride complexes: synthesis and reactivity. Coord. Chem. Rev. 2002, 233, 107−129. 8317
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