BoronâBoron Multiple Bonding: From Charged to Neutral and Back

Review pubs.acs.org/Organometallics

Boron−Boron Multiple Bonding: From Charged to Neutral and Back Again Holger Braunschweig* and Rian D. Dewhurst Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ABSTRACT: The area of boron−boron multiple bonding has fascinated many researchers over the years. However, the apparent difficulty of this chemistry meant that for decades the number of compounds in this category was very limited. In 2007, Robinson and co-workers presented a synthesis of a neutral compound with a boron−boron double bond. In our laboratories we have since extended this by adding four new, selective routes to these diborene compounds, as well as isolating the first compound with a boron−boron triple bond. Herein a history of accomplishments in noncarbon main-group multiple bonding and the base stabilization of allotropes of main-group atoms is given. This is followed by an account of the recent progress made in the area of boron−boron multiple bonding.

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HOMOATOMIC MULTIPLE BONDING BEYOND C, N, AND O The classical double bond rule was an informal assumption that arose around half a century ago on the basis of the observed absence of multiple bonding in the heavier main-group elements. This “rule” can be understood by considering the clear distinctions between the bonding of multiply bound species CO2, N2 and O2 and that of singly bound heavier analogues SiO2, P4, and S8 (among many other examples). In his 1975 review, Peter Jutzi formalized the rule: “...elements having a principal quantum number greater than 2 should not be able to form (p-p)π bonds with themselves or with other elements”. However, he then proceeded to conclusively discredit the rule by documenting its violations, which even in 1975 were numerous.1 Since this review, landmark achievements in main-group multiple bonding accompanied the realization that inert, sterically bulky groups can kinetically stabilize otherwise reactive groups. The synthesis of divalent group 14 species,2 such as a distannene by Lappert and Thomas2a and a disilene by West, Fink, and Michl,2b the synthesis of the heavier group 14 alkyne analogues REER (E = Si, Ge, Sn, Pb), containing varying degrees of multiple bonding by the groups of Sekiguchi and Power,3 and the synthesis of diphosphenes by Yoshifuji, Inamoto, and others,4 repeatedly defied the classical doublebond rule. Nevertheless, it retains some relevance, as maingroup elements tend to undergo increasing trans-bending as the group is descended in order to avoid unfavored π bonding in the heavier elements. In this account, we attempt to chart the chronology of the breakthroughs in multiply bound compounds of groups 13 and 14. Homoatomic, multiply bound compounds of group 15 elements are also known in the literature;4 however, this chemistry is somewhat distinct from that of the relatively © 2014 American Chemical Society

electron-poor elements of groups 13 and 14 and will not be discussed at length. Arguably, progress in the field of main-group homoatomic multiple bonding in the heavier group 13 and 14 elements began with the isolation of the first distannene in 1976 by Lappert and Thomas.2a Interestingly, the field of noncarbon multiple bonding in groups 13 and 14 can be broken up into three chronological eras on the basis of the types of compounds discovered, with only very small overlap. During the first era, from 1976 to 1998 (the Tricoordinate Era, Figure 1), the first examples of compounds containing homoatomic multiple bonding between group 13 and 14 elements (denoted “E”) were discovered. This era is characterized by the fact that the element E in each newly discovered class of compounds was tricoordinate. This included (a) group 14 “-enes”, heavier analogues of olefins (blue timeline, Figure 1),2 (b) radical anion derivatives of diboranes(4), dialumanes(4), and digallanes(4) with a formal E−E bond order of 1.5 (orange timeline, Figure 1),5 and (c) the first diboranyl dianion from Power and coworkers with a formal bond order of 2 (green timeline, Figure 1).6 Having only a small overlap with the Tricoordinate Era, a second era began in 1997, with Robinson’s synthesis of a digallyne dianion.7a This was followed nine years later by the synthesis, by Power and co-workers, of a dialuminyne dianion.7b Both of these dianions showed short E−E distances, suggesting significant multiple bonding between the atoms. These two firsts ([REER]2−; yellow timeline, Figure 2) bookend the Dicoordinate Era, in which the major breakthroughs in the synthesis of homoatomic group 13 and 14 multiply bound species with dicoordinate E atoms occurred. Received: August 27, 2014 Published: November 4, 2014 6271

dx.doi.org/10.1021/om500875g | Organometallics 2014, 33, 6271−6277

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Figure 1. Timeline of the Tricoordinate Era (1976−1998), where the major breakthroughs in multiply bound group 13 and 14 compounds containing tricoordinate E atoms were made. Only the first examples of each class are depicted.

Figure 2. Timeline of the Dicoordinate Era (1997−2006), where the major breakthroughs in multiply bound group 13 and 14 compounds containing dicoordinate E atoms were made. Only the first examples of each class are depicted.

Power and co-workers presented the synthesis of a diplumbyne in 2000,3a which, although strongly trans-bent, contained two connected dicoordinate lead atoms. This breakthrough was followed by syntheses of lighter neutral group 14 “ynes”, where E = Si, Ge, Sn (REER; pink timeline, Figure 2),3 and neutral group 13 “enes”, where E = Ga, In, Tl (REER; olive green timeline, Figure 2) by the groups of Power and Sekiguchi.8 Similarly, a monoanion of a distannyne and dianionic derivatives of digermynes and distannynes were also prepared by Power et al. by overreduction of the corresponding divalent compound RECl (E = Ge, Sn; not shown in Figure 2).9 Notable omissions in this list were, however, diborenes

(RBBR) and dialumenes (RAlAlR), although a reactive compound described as a “probable” dialuminene was generated and isolated as its toluene cycloaddition product by Power et al. in 2003.10 A third conceptual wave in this chemistry was the concept of stabilization of low-valent main-group species using strong Lewis bases, most frequently persistent carbenes (the Lewis Base Era; Figure 3). In 2007, two ground-breaking reports in this direction, from the groups of Bertrand and Robinson, founded the new subfields of base-stabilized zerovalent elements (sometimes also described as “base-stabilized allotropes”; purple timeline, Figure 3), and base-stabilized 6272


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situations. The empty p orbital of sp2-hybridized boron means that it often forms multiple bonds with elements that are electron-pair π donors, particularly sp2-hybridized nitrogen (in aminoboranes, R2BNR′2, as well as the relatively rare iminoboranes,13 RBNR′, and iminoboryl complexes,14 MBNR12) and to some extent oxygen (in alkoxyboranes, R2BOR′, and oxoboryl complexes, MBO15). A small but diverse field has also arisen around boron−carbon multiple bonding (in alkylideneboranes, RBCR′2,1a,16 and related alkylideneboryl complexes, MBCR217), despite carbon’s inability to provide a lone pair of electrons to boron in most cases. While boron appears willing to participate in heteroatomic multiple bonding, it is reluctant to undergo multiple bonding with itself. This reluctance is exemplified by the case of the simplest boron-only multiply bound species: diborenes of the form RBBR. Diborenes have been generated under matrix conditions18 and constructed as ligands for transition metals19 but have never been isolated as stable compounds. Furthermore, even if such a compound could be isolated, it is predicted to avoid multiple bonding altogether, instead being a triplet diradical (i.e., RB•B•R) in its ground state. More confounding still, between 1997 and 2006, the groups of Robinson and Power reported the synthesis of doubly and triply bound gallium and aluminum species containing dicoordinate group 13 atoms.7,8 These results suggested that homoatomic mutiple bonding may in fact be easier in the heavier group 13 elements than with boronthe exact inverse of the double bond rule. The first steps toward the controlled formation of B−B multiple bonds came via reduction. In 1981 Berndt and coworkers chemically reduced a diborane(4) (R2BBR2) by a single electron, partially populating the empty, bonding π(BB) orbital and leading to a radical anion with a formal B−B bond order of 1.5 (see orange timeline, Figure 1).5a This radical anion was characterized by EPR; however, the first structurally authenticated example was prepared in 1996 by Power.5b In a similar manner, the groups of Power5b,c and Nöth5d were able to isolate dianions of diboranes(4), corresponding to a formal B−B bond order of 2. In 2007, Robinson and co-workers presented the synthesis of a neutral diborene with a boron− boron double bond ((IDip)HBBH(IDip); IDip =1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene), by the reduction of an NHC adduct of boron tribromide, a process involving adventitious hydrogen abstraction, presumably from the solvent.12a This result was one of the first signifiers of the extraordinary stabilizing ability that persistent carbenes are able to exert on main-group compounds, eventually leading to the now-vibrant subdiscipline of carbene-stabilized low-valent compounds.20 Our first forays into the area of boron−boron multiple bonding began due to two findings. The first was that tetrabromodiborane(4), unstable at room temperature, could be conveniently converted to its stable bis(NHC) adduct B2Br4IDip2 at low temperature. This allowed the reduction of boron−bromide bonds in the presence of stabilizing Lewis bases and a preformed boron−boron bond. This is in contrast to the reaction of Robinson et al., where monoboranes were reduced, leading to boron−boron bond formation but also (presumably radical-based) hydrogen abstraction. The second finding was that NHC-stabilized aryldihaloboranes could be very simply reduced with lithium to form diborenes without hydrogen abstraction. These two findings together have opened

Figure 3. Timeline of the Lewis Base Era (2007 onward), showing breakthroughs in doubly carbene stabilized zerovalent elements and group 13 doubly bonded species. Only the first examples of each class are depicted.

diborenes (yellow timeline, Figure 3), respectively. The family of bis(carbene)-stabilized allotropes of the form L→Ex←L now include a range of examples such as Ex = B2, Si2, Ge2, P1, P2, P4, P12, and As2.11 Meanwhile, the field of doubly base stabilized diborenes has grown to include examples with both Nheterocyclic carbenes (NHCs) and phosphines as Lewis bases, as well as H, Br, aryl and heteroaryl groups as substituents.12

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BORON−BORON MULTIPLE BONDING Like its neighboring elements carbon, nitrogen, and oxygen, boron forms strong, stable multiple bonds in a number of 6273


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a number of exciting possibilities in the field of boron−boron multiple bonding.

orbital of diborenes is the HOMO in each case, which is unsurprising, given the harsh conditions needed to fill this orbital with electrons. One of the first investigations made in our group into the reactivity of the prepared diborenes was their chemical oxidation. Addition of 1 equiv of the tropylium salt [C7H7][BArF4] (ArF = 3,5-bis(trifluoromethyl)phenyl) to either NHC- or phosphine-stabilized diborenes resulted in isolation of single-electron-oxidation products, the radical cations [B2Dur2IMe2][BArF4] and [B2Mes2(PEt3)2][BArF4] (Figure 5A).12e These cations were characterized by single-

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BASE-STABILIZED DIBORENES Since Robinson’s initial breakthrough in the field of diborenes (route A, Figure 4),12a,b we have reported four independent,

Figure 5. Reactivity of doubly base stabilized diborenes (Mes = 2,4,6trimethylphenyl, Dur = 2,3,5,6-tetramethylphenyl).

crystal X-ray crystallography and their radical nature confirmed by EPR spectroscopy. Concurrently, the same two diborene precursors were also studied electrochemically, each showing a reversible oxidation wave (B2Mes2(PEt3)2, E1/2 = −1.05 V; B2Dur2IMe2, E1/2 = −1.55 V; both vs Fc/Fc+) indicating that they are strong reductants and that the B−B π bond may be able to act as a useful electron reservoir. The first published reactivity pattern of diborenes was the reaction of B2Dur2IMe2 with silver(I) chloride, leading to the π complex [AgCl(η2-B2Dur2IMe2)] (Figure 5B).12c While this complex outwardly resembles a conventional π-olefin complex and was calculated to contain both σ donation (B2→Ag) and π back-donation (Ag→B2) in a conventional Dewar−Chatt− Duncanson manner, the total orbital interaction between the B2 ligand and the Ag atom was found to be very low in comparison to electrostatic interactions. This stark contrast between π olefin and π diborene complexes was speculated to be due to the large negative charges on the boron atoms and the orbital energy mismatch between the very high lying diborene HOMO and the orbitals of the Ag fragment. A very recently uncovered diborene reactivity pattern is the hydroboration of the BB double bond (Figure 5C).12d The

Figure 4. Synthetic routes to doubly base stabilized diborenes (Mes = 2,4,6-trimethylphenyl, Dur = 2,3,5,6-tetramethylphenyl).

selective routes to the compounds. Route B is thus far the most broadly applicable, involving the simple reduction of basestabilized aryldihaloboranes.12c,d While the organyl group has been varied to include aryl, furyl, and thienyl groups, so far this has only been demonstrated using the relatively small carbene IMe (1,3-dimethylimidazol-2-ylidene) as a Lewis base. Routes C and D both stem from the synthesis of B2Br4(IDip)2 described above, the former via simple two-electron reduction and the latter via a comproportionation reaction with the diboryne B2IDip2 (vide infra).11a The latest route, route E, proceeds by reduction of an unusual monophosphine-stabilized diborane(4)21 containing a bridging bromide, in the presence of an extra equivalent of phosphine.12e The strongly reducing conditions needed for the synthesis of diborenes gives one an indication of their highly energetic nature. DFT studies confirm that the filled boron−boron π 6274


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base-stabilized diborynes from the groups of Mitoraj and Frenking.23 The strongly reducing nature of the diboryne B2IDip2 was already hinted at in the first report on the topic, namely the aforementioned comproportionation reaction of the diboryne with B2Br4IDip2, leading to 2 equiv of the dibromodiborene B2Br2IDip2 (Figure 4, route D).11a Further evidence of the reducing ability of B2IDip2 was witnessed by its reaction with CO.24 Exposing a solution of the diboryne to 1 atm of CO led very quickly to decoloration of the solution and formation of a red-orange doubly base stabilized bis(boralactone) (Figure 6A, bottom right), in which four CO units have been coupled in both head-to-head and head-to-tail fashions. The reaction makes B2IDip2 the first metal-free compound to bind and couple more than one CO molecule. In an attempt to obtain information about how the bis(boralactone) framework might be constructed in this reaction, B2IDip2 was subsequently treated with a measured amount of CO (ca. 2 equiv). The solidstate structure of the very sensitive orange-brown solid obtained from this reaction exhibited a single CO unit bound unsymmetrically to a slightly bent L→B2←L axis (Figure 6A, bottom left). The boron−boron distance in the monocarbonyl adduct complies with a double-bond description (1.549(3) Å). This report also disclosed the facile electrochemical oxidation of the diboryne (E1/2 = −1.3 V vs Fc/Fc+), further indicating its strong reducing ability, which is presumably responsible for the reduction of CO in these reactions. The similar doubly base stabilized tetrabromodiborane(4) can be prepared using cyclic (alkyl)(amino)carbenes (CAACs25) as the Lewis bases, generating B2Br4CAAC2 (CAAC = 3,3,5,5-tetramethyl-1-(2′,6′-diisopropylphenyl)pyrrolidine-2-ylidene. Similarly to the NHC derivative, fourelectron reduction of B2Br4CAAC2 led cleanly to the deep purple compound B2CAAC2 (Figure 6B).11b This compound was found to be structurally very different from the NHC analogue B2IDip2, having significantly longer B−B (1.489(2) Å) but shorter B−C (1.459(2), 1.458(2) Å) bonds (in each case by 0.03−0.04 Å). With five electron pairs in total in the B2C2 axis, the compound can thus be described as an electrondeficient (4-π-electron) cumulene.

reaction of three bis(NHC) bis(heterocyclyl) diborenes with catecholborane resulted in diastereoselective, catalyst-free hydroboration at room temperature and the construction of the first doubly base stabilized triborane. While just a simple proof of concept, the reaction is a very rare example of a mild method for the formation of electron-precise boron−boron bonds.22 The extension of this reaction to include multiple hydroboration using dihydroboranes (or even BH3) could lead to novel chains of boron atoms that are otherwise very difficult to selectively prepare.

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BASE-STABILIZED DIBORYNES Similarly to the reduction of doubly base-stabilized tetrabromodiborane(4) with 2 equiv of sodium naphthalenide (Figure 4, route C), using twice the amount of reductant led to isolation of the deep green diboryne B2IDip2 (Figure 6A).11a

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CONCLUSIONS

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AUTHOR INFORMATION

The arrival of base-stabilized diborenes and diborynes has not only opened up boron−boron double and triple bonds for fundamental study but also provided a handful of neutral compounds with highly reactive and electron rich boron atoms. These compounds complement other recent results in the area of electron-rich hypocoordinate boron compounds, such as the boryl anions of Yamashita and Nozaki26 and the doubly base stabilized borylenes of Bertrand.27 Research into the reactivity and photophysical properties of these unusual molecules is ongoing in our laboratories.

Figure 6. Synthesis and reactivity of doubly base stabilized diborynes.

Single-crystal X-ray crystallography revealed the extremely short B−B distance in this compound (1.449(3) Å) and the near-linear arrangement of its L→B2←L axis, indicative of a full triple bond between the boron atoms. This linearity is in contrast to the strongly trans-bent low-coordinate compounds of the heavier group 13 elements prepared by Robinson and Power7,8 but is in accordance with prior DFT studies of similar

Corresponding Author

*E-mail for H.B.: [email protected]. Notes

The authors declare no competing financial interest. 6275


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ACKNOWLEDGMENTS

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The authors thank the talented researchers involved in the synthesis and study of these fascinating molecules, namely Dr. Philipp Bissinger, Julian Böhnke, Christina Claes, Dr. Alexander Damme, Theresa Dellermann, Dr. William C. Ewing, Annika Gackstatter, Kai Hammond, Dr. Christian Hörl, Dr. J. Oscar C. Jimenez-Halla, Thomas Kramer, Dr. Ivo Krummenacher, Dr. Thomas Kupfer, Dr. Jan Mies, Dr. Ashwini K. Phukan, Florian Pinzner, Dr. Krzysztof Radacki, Stefan Ullrich, and Dr. Alfredo Vargas. This work was generously supported by the Deutsche Forschungsgemeinschaft (DFG).

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Holger Braunschweig was born 1961 in Aachen, Germany. He obtained his Ph.D. (1991) and Habilitation (1998) from the RWTH Aachen with P. Paetzold, between these completing a postdoctoral stay with M. F. Lappert, FRS, at the University of Sussex (U.K.). After two years at Imperial College as a Senior Lecturer and Reader, he moved to a chair for inorganic chemistry at the Julius-Maximilians-University Würzburg in 2002. In 2009 he was awarded the Gottfried Wilhelm Leibniz prize of the DFG and was elected to the Bavarian Academy of Sciences. In 2011 Braunschweig was awarded a prestigious Advanced Investigator grant of the European Research Council and was elected to the German National Academy of Sciences Leopoldina. The 2012 discovery of the first molecule with a boron−boron triple bond in his laboratories attracted extensive media attention, including from The Times of London, New Scientist, Nature, Science Magazine, and Spektrum der Wissenschaft (Germany). Braunschweig has recently been awarded the 2014 Main Group Award from the Royal Society of Chemistry, based in part on this discovery. His research interests lie in the areas of boron chemistry, organometallic synthesis, and catalysis and are currently focused on borametallocenophanes, boron heterocycles, boron−boron multiple bonds, and transition-metal complexes of boron. Photo courtesy of Markus Scholz for the Leopoldina.

Rian Dewhurst obtained a B.Sc. (Hons) degree from the University of Canterbury (New Zealand) in 2002. He then completed his Ph.D. in 2006 in the research group of Prof. Anthony F. Hill at the Australian National University in Canberra, Australia, for which he was awarded the J. G. Crawford Medal of the ANU. After a postdoctoral stay in the research group of Prof. Guy Bertrand at the University of California, Riverside, he took up an Alexander von Humboldt Postdoctoral Fellowship in the group of Prof. Holger Braunschweig at the University of Würzburg. He is now a senior researcher in the Braunschweig research group. Photo courtesy of Christine Trost. 6276


Organometallics

Review

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