Philip Power at 65: an icon of organometallic chemistry

Professor Philip P. Power turns 65 this month. His contributions to organometallic and inorganic chemistry over the last four decades are expansive, and he has been instrumental in the definition of several new subfields in the discipline. To date, Professor Power has contributed over 500 publications to the literature. His lab remains an active source of exciting chemistry and has been a way point in numerous successful chemists’ careers.

Power was born in the Republic of Ireland and undertook a BA in Chemistry at Trinity College, Dublin, completed in 1974. He subsequently crossed the Irish Sea, completing a PhD at the University of Sussex in 1977. This was followed by a postdoctoral stay at Stanford University and his appointment to the University of California, Davis in 1981. During his career Power has been the recipient of awards too numerous to list here, perhaps most notably both the Ludwig Mond Medal of the RSC and F. A. Cotton Award of the ACS in 2005. He was appointed a Fellow of the Royal Society in this same year. Power has been committed in the service of the chemical community, acting as an Associate Editor of Inorganic Chemistry, editor of volume 37 of Inorganic Syntheses, and having served on the advisory boards of twelve journals including Dalton Transactions.

Over the course of his career, a significant proportion of Power's oeuvre has been published in RSC journals, charting a course from his very earliest contribution to some of his most recent enterprises. In honour of Professor Power's birthday we have put together a web collection of his highlights in RSC journals. These publications are landmarks and continue to have outstanding impact on the chemical community. In the following, we refer to these key publications in RSC journals to revisit some milestones in his career.

Alongside these, we have invited previous co-workers, collaborators and chemists whose work shares an affinity with Professor Power's to submit innovative research close in theme to his foci in Dalton Transactions and Chemical Communications. This collection reveals a chemist with wide-ranging interests and a propensity for timely research, coupled with a cerebral dedication to conceptual frameworks which have helped to unify organometallic chemistry's vibrant cutting edges.

Early contributions

Power's first paper in the chemical literature, originating from his PhD studies under Professor Michael F. Lappert FRS was entitled “Ready oxidative addition of an alkyl or aryl halide to a tin(II) alkyl or amide – evidence for a free-radical pathway”.¹ It is the first in a swathe of papers from Power investigating the reactivity of low-coordinate tin compounds. This first work utilised the bulky alkyl and amide ligands –E(SiMe₃)₂ (E = CH, N), which Power has regularly revisited throughout the last 4 decades. Thus began a stream of related papers with Lappert,² including Power's first exposure to stable radicals, the pnictyl species [E{CH(SiMe₃)₂}₂] (E = P, As).³ Later work as Lappert's collaborator would identify these as “Jack-in-the-box molecules”, reflecting Power's ability to succinctly frame research areas.⁴ After graduation in 1977, Power went on to work with Professor Richard H. Holm, yielding his first forays into the chemistry of cluster compounds.

A focus on low-coordinate, low-valent and low-oxidation state compounds defines Power's career and molecules in this class generally feature a small energy difference between frontier orbitals. This results in highly sensitive compounds, initially providing Power a synthetic challenge and subsequently allowing him to report unprecedented reactivity. Within this theme, Power has worked in a wide range of areas as summarised hereafter.

Low coordinate species

In 1981, Power took up a position at the University of California, Davis where he remains to this day. Work in collaboration with Lappert continued, with the X-ray crystallographic characterisation of the group 14 bisamides [M{N(SiMe₃)₂}₂] (M = Ge, Sn).⁵ This is representative of much of Power's earlier work at Davis; the crystallographic characterisation of highly air- and moisture-sensitive compounds, requiring exquisite practical skill. Power started to explore new ligand systems as in his paper entitled “Syntheses and X-ray crystal structures of lithium and chromium(II) complexes of the tri-t-butylmethoxide ligand”.⁶ Subsequently, Power began working with the terphenyl ligands, –C₆H₃-2,6-Ar₂ (Ar = 2,4,6-Me₃C₆H₂, Ar^Me₆; 2,6-Prⁱ₂C₆H₃, Ar^Pri₄; 2,4,6-Prⁱ₃C₆H₂, Ar^Pri₆; in general Ar*), which have been the platform for many of his most remarkable discoveries. Occasional work with other ligand systems has provided a further range of striking compounds, including the group 13 carbene analogue [HC{C(Me)N(2,6-iPr₂C₆H₃)}₂]Ga.⁷

Whilst Power has regularly returned to p-block compounds, he has worked with all blocks of the periodic table, and the terphenyls have been ably applied to transition metal chemistry. This approach yielded unusual univalent Cr(I), Mn(I) and Fe(I) complexes⁸ as well as a two-coordinate heteroleptic Co(II) species, Ar^Pri₄CoN(SiMe₃)₂,⁹ amongst others. Through one of Power's multitude of fruitful collaborations Ar^Pri₄CoN(SiMe₃)₂ was shown to have unusual magnetic properties and a subsequent analysis of low coordinate, highly anisotropic systems contributed to a new perspective on single molecule magnetism.¹⁰

As well as describing many new compounds in the literature, Power has shown a yen for chasing down chemical mysteries from the past, most recently through his reinvestigation of the nickel bisamide Ni{N(SiMe₃)₂}₂ first reported by Bürger and Wannagat in the early 1960s.^11a–c This willingness to challenge accepted conclusions has been a theme throughout Power's career, and even his own theories are not immune from re-evaluation. Power has contributed to a range of useful reviews over the years, including the seminal text on [M(NR₂)_n] compounds, Metal Amide Chemistry, with Lappert, Protchenko and Seeber.¹²

Beyond homoleptic species, the terphenyls have provided an excellent co-ligand for a variety of substituents on main group compounds. This has allowed the interrogation of divalent group 14 halides and hydrides, Ar*MX (M = Ge, Sn, Pb; X = H, Cl, Br), the latter of which show fascinating structural isomerism.¹³ These compounds have provided a source of much interesting reactivity, and Power has also used them as starting materials for clusters¹⁴ and multiply bonded species (vide infra). Power has also implicated tin(II) hydrides as catalytic intermediates in the isohypsic dehydrogenative coupling of amines and boranes, emphasising his willingness to confront any challenge in the field.¹⁵

Multiple bonding

Much work by Lappert and others on the group 14 alkyls [M{CH(SiMe₃)₂}₂]₂ (M = Ge, Sn) centred on the rationalisation of their bonding, and attempts to address the existence (or lack thereof) of an M=M double bond. A fascination with main group multiple bonding has permeated Power's work throughout the years, and yielded the compounds with which he is probably most closely associated. Initial work in this area interrogated the possibility of π-bonds in reduced diicosagen(4) radicals, exemplified by “Comparison of B–B π-bonding in singly reduced and neutral diborane(4) derivatives: isolation and structure of [{Li(Et₂O)₂}{MeO(mes)BB(mes)OMe}]”.¹⁶ In the hunt for multiple bonding in group 14, Power then investigated reduction of the aforementioned heteroleptic terphenyl tetrylene halides, but the reaction of Ar^Me₆GeCl with potassium instead yielded radical and anionic clusters.¹⁷

Power's focus on multiply bonded species undiminished, the dipnictenes provided a rich seam of compounds such as in his report “Unsymmetric dipnictenes—synthesis and characterization of MesP = EC₆H₃-2,6-Trip₂ (E = As or Sb; Mes = C₆H₂-2,4,6-Me₃, Trip = C₆H₂-2,4,6-Prⁱ₃)”.¹⁸ “Homonuclear multiple bonding in heavier main group elements”¹⁹ demonstrates Power's provision of a range of useful and often field-defining reviews. These articles, in general, are more than a mere compilation of compounds, synthetic pathways and structural data, but instead draw attention towards unifying concepts in structure and reactivity.

Power's attack on unsaturated heavier group 14 compounds was, however, not over and at this point the utility of the terphenyl system became apparent. Judicious, and readily accessible, alteration of the ligand coupled with Power's commitment to synthetic excellence allowed access to the heavier alkyne analogues Ar*MMAr* (M = Ge, Sn, Pb). Subsequently, group 13 analogues were similarly accessed with reports of Ar*MMAr* (M = Ga, In, Tl), the gallium congener of which is the source of an oft heard comment in Power's lab; “green is good”. Power then reviewed this area²⁰ and went on to modify the enduringly modular terphenyl system to analyse ligand effects on the metal–metal interaction in these species.²¹ These compounds, landmarks in organometallic synthesis, have made it into textbooks and contributed much to change the field's perception of multiple bonding. Thus, Power's syntheses of beautiful compounds are not simply stamp collecting, but an opportunity to interrogate unique molecular systems. This fact is best displayed in his assessment of the reactivity of heavier main group multiple bonds.

Small molecule activation

While working in Power's lab, it was not unusual to hear the question “what would you do with ten grams of this compound?” Having access to multigram amounts of the heavier alkyne analogues, Power has done much to answer this question for such species. In doing so he has contributed to a renaissance in main group chemistry which has done much to inspire the authors.

Power's work with the ditetrylynes showed reactivity towards activated alkenes²² and nitrosyl compounds²³ amongst others. However, it is for their reactivity towards small molecules, otherwise considered inert to main group species under ambient conditions, that Power's heavier alkyne analogues are most well-known. In the paper “Addition of H₂ to distannynes under ambient conditions”,²⁴ Power contributed to a recognition that certain main group compounds could be “transition metal mimetic”, an understanding which has helped push back the frontiers of organometallic chemistry in the s- and p-blocks. The aforementioned multiply bonded group 13 compounds also readily activate olefins.²⁵

The marked ability of terphenyl ligands to support multiple bonding is by no means isolated to the p-block. The synthesis of a Cr–Cr quintuple bond applying these ligands is one of Power's most well-known contributions, and this bond was shown to react with unsaturated nitrogen compounds.²⁶ Beyond multiple bonded species, Power has done much to assess the reactivity of homoleptic diaryls of both main group and transition metal species with small molecules, for example in his paper “Insertion reactions of a two-coordinate iron diaryl with dioxygen and carbon monoxide”.²⁷

London dispersion forces

Sterically bulky ligands have not just provided Power with a collection of compounds which must be the envy of any synthetic organometallic chemist. It is apparent that the unique characteristics of these ligands have both directed Power's thinking and provided surprises demanding re-interrogation of results. Power's aforementioned habit of chasing down chemical mysteries has not been isolated to results of other workers but encompasses an eager willingness to reassess his compounds in the light of new findings.

Power has recently begun to evaluate the importance of C–H⋯H–C dispersion interactions in rationalising the structure and reactivity of main group and d-block compounds. It is thus fitting that this editorial closes with reference to a paper from Power which addresses a compound he first encountered during his PhD, and reflects his collaborative approach to science. In 2015, Power, in collaboration with Professor Shigeru Nagase, reassessed the question of M–M bonding in [M{CH(SiMe₃)₂}₂]₂ (M = Ge, Sn, Pb) and concluded “although much effort has been expended in development of bonding models for the multiple bonds between heavier main group elements, it seems both probable and ironic that the dispersion force attraction forces exceed those of the multiple bonds in many instances and that a variety of more subtle interactions including packing effects are of key importance for the understanding of their bonding”.²⁸

Final thoughts

Phil's contributions to chemistry are expansive, dramatic and sometimes overlooked. He continues to be, and we are pretty sure he hates the term, an influencer in chemistry. Disinclined to tout his work, it is instead for an abundance of truly ground-breaking, stunning experimental discoveries and seminal review articles and comments that he is recognised. Thus, we do not claim this is in any way a thorough assessment of Phil's work, but feel that these highlighted papers draw attention to an approach to chemistry which has been, without a doubt, defining. Nevertheless, two of us (RCF, DJL) have been fortunate enough to work with Phil and one (MSH) has been inspired by him, and enjoyed the opportunity to host him as a visiting colleague. Phil, as ever, was a gentleman and, alongside enjoying the odd mixed grill, contributed greatly to the spirit of the department. We hope that this collection, and related events around Phil's birthday are a suitable tribute. We thank the contributors to the collection for their hard work and willing, and we hope interested readers will join us in celebrating this anniversary of a titan of organometallic chemistry. Finally we, and doubtlessly you, look forward to the ongoing contributions from Phil's lab to the ever exciting field of organometallic chemistry.

References

1. M. J. S. Gynane, M. F. Lappert, S. J. Miles and P. P. Power, J. Chem. Soc., Chem. Commun., 1976, 256.
2. For example: M. J. S. Gynane, D. H. Harris, M. F. Lappert, P. P. Power, P. Riviere and M. Riviere-Baudet, J. Chem. Soc., Dalton Trans., 1977, 2004.
3. M. J. S. Gynane, A. Hudson, M. F. Lappert and P. P. Power, J. Chem. Soc., Chem. Commun., 1976, 623.
4. S. L. Hinchley, C. A. Morrison, D. W. H. Rankin, C. L. B. Macdonald, R. J. Wiacek, A. H. Cowley, M. F. Lappert, G. Gundersen, J. A. C. Clyburne and P. P. Power, Chem. Commun., 2000, 2045.
5. T. Fjeldberg, H. Hope, M. F. Lappert, P. P. Power and A. J. Thorne, J. Chem. Soc., Chem. Commun., 1983, 639.
6. J. Hvoslef, H. Hope, B. D. Murray and P. P. Power, J. Chem. Soc., Chem. Commun., 1983, 1438.
7. N. J. Hardman, B. E. Eichler and P. P. Power, Chem. Commun., 2000, 1991.
8. C. Ni, B. D. Ellis, J. C. Fettinger, G. J. Long and P. P. Power, Chem. Commun., 2008, 1014.
9. C. Ni, T. A. Stich, G. J. Long and P. P. Power, Chem. Commun., 2010, 4466.
10. J. M. Zadrozny, M. Atanasov, A. M. Bryan, C.-Y. Lin, B. D. Rekken, P. P. Power, F. Neese and J. R. Long, Chem. Sci., 2013, 4, 125.
11. (a) H. Bürger and U. Wannagat, Monatsh. Chem., 1963, 94, 1007; (b) H. Bürger and U. Wannagat, Monatsh. Chem., 1964, 95, 1099; (c) C.-Y. Lin and P. P. Power, Chem. Soc. Rev., 2017, 46, 5347.
12. M. Lappert, A. Protchenko, P. P. Power and A. Seeber, Metal Amide Chemistry, John Wiley & Sons, Ltd, 2008.
13. E. Rivard and P. P. Power, Dalton Trans., 2008, 4336.
14. E. Rivard, J. Steiner, J. C. Fettinger, J. R. Giuliani, M. P. Augustine and P. P. Power, Chem. Commun., 2007, 4919.
15. J. D. Erickson, T. Y. Lai, D. J. Liptrot, M. M. Olmstead and P. P. Power, Chem. Commun., 2016, 52, 13656.
16. W. J. Grigsby and P. P. Power, Chem. Commun., 1996, 2235.
17. M. M. Olmstead, L. Pu, R. S. Simons and P. P. Power, Chem. Commun., 1997, 1595.
18. B. Twamley and P. P. Power, Chem. Commun., 1998, 1979.
19. P. P. Power, J. Chem. Soc., Dalton Trans., 1998, 2939.
20. P. P. Power, Chem. Commun., 2003, 2091.
21. Y. Peng, R. C. Fischer, W. A. Merrill, J. Fischer, L. Pu, B. D. Ellis, J. C. Fettinger, R. H. Herber and P. P. Power, Chem. Sci., 2010, 1, 461.
22. M. Stender, A. D. Phillips and P. P. Power, Chem. Commun., 2002, 1312.
23. G. H. Spikes, Y. Peng, J. C. Fettinger, J. Steiner and P. P. Power, Chem. Commun., 2005, 6041.
24. Y. Peng, M. Brynda, B. D. Ellis, J. C. Fettinger, E. Rivard and P. P. Power, Chem. Commun., 2008, 6042.
25. C. A. Caputo, Z. Zhu, Z. D. Brown, J. C. Fettinger and P. P. Power, Chem. Commun., 2011, 47, 7506.
26. C. Ni, B. D. Ellis, G. J. Long and P. P. Power, Chem. Commun., 2009, 2332.
27. C. Ni and P. P. Power, Chem. Commun., 2009, 5543.
28. J.-D. Guo, D. J. Liptrot, S. Nagase and P. P. Power, Chem. Sci., 2015, 6, 6235.

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